Method and apparatus for performing effective feedback in wireless communication system supporting multiple antennas

[0001] This application claims the benefit of the Korean Patent Application No. 10-2011-0098949, filed on Sep. 29, 2011, U.S. Patent Application Nos. 61/387,472 filed on Sep. 29, 2010; 61/388,615 filed on Oct. 1, 2010; 61/409,535 filed on Nov. 2, 2010; 61/410,355 filed on Nov. 5, 2010; 61/440,714 filed on Feb. 8, 2011; 61/441,616 filed on Feb. 10, 2011; 61/443,219 filed on Feb. 15, 2011 and 61/446,059 filed on Feb. 24, 2011 which are hereby incorporated by reference as if fully set forth herein.

[0002] 1. Field of the Invention

[0003] The present invention relates to a wireless communication system, and more particularly to a method and apparatus for performing effective feedback in a wireless communication system supporting multiple antennas.

[0004] 2. Discussion of the Related Art

[0005] Generally, Multiple-Input Multiple-Output (MIMO) technology will hereinafter be described in detail. In brief, MIMO is an abbreviation for Multi-Input Multi-Output. MIMO technology uses multiple transmission (Tx) antennas and multiple reception (Rx) antennas to improve the efficiency of transmission/reception (Tx/Rx), whereas the conventional art generally uses a single transmission (Tx) antenna and a single reception (Rx) antenna. In other words, MIMO technology allows a transmitting end and a receiving end to use multiple antennas so as to increase capacity or improve performance. If necessary, MIMO technology may also be called multi-antenna technology. In order to correctly perform multi-antenna transmission, the MIMO system has to receive feedback information regarding channels from a receiving end designed to receive multi-antenna channels.

[0006] Various feedback information fed back from the receiving end to the transmitting end in the legacy MIMO wireless communication system may be defined, for example, a rank indicator (RI), a precoding matrix index (PMI), channel quality information (CQI), etc. Such feedback information may be configured as information appropriate for legacy MIMO transmission.

[0007] There is a need for a new system including the extended antenna configuration as compared to the legacy MIMO wireless communication system to be developed and introduced to the market. For example, although the legacy system can support a maximum of 4 transmission antennas, new systems have an extended antenna configuration that supports MIMO transmission based on 8 transmission antennas, resulting in increased system capacity.

[0008] Accordingly, the present invention is directed to a method and apparatus for performing efficient feedback in a wireless communication system supporting multiple antennas that substantially obviates one or more problems due to limitations and disadvantages of the related art. The new system supporting the extended antenna configuration is designed to perform more complicated MIMO transmission than the legacy MIMO transmission operation, such that it is impossible to correctly support the MIMO operation for the new system only using feedback information defined for the legacy MIMO transmission operation.

[0009] An object of the present invention is to provide a method and apparatus for configuring and transmitting feedback information used for correctly and efficiently supporting MIMO operation based on an extended antenna configuration.

[0010] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0011] To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for transmitting channel status information (CSI) of downlink (DL) multi-carrier transmission includes: generating the CSI including at least one of a rank indicator (RI), a first precoding matrix index (PMI), a second PMI and a channel quality indicator (CQI) for one or more DL carriers, wherein the CQI is calculated based on precoding information determined by a combination of the first and second PMIs; determining, when two or more CSIs collide with one another in one uplink (UL) subframe of one UL carrier, a CSI to be transmitted on the basis of priority; and transmitting the determined CSI over a UL channel, wherein the CSI is classified into a first group including an RI, a second group including a wideband (WB) first PMI, a third group including a WB CQI, and a fourth group including an SB CQI, and in association with the priority, if a CSI of the first group or a CSI of the second group collides with a CSI of the third group or a CSI of the fourth group, the CSI of the third or fourth group has low priority and is dropped.

[0012] In another aspect of the present invention, a user equipment (UE) for transmitting channel status information (CSI) of downlink multi-carrier transmission includes a reception module for receiving a downlink signal from an eNode B; a transmission module for transmitting an uplink signal to the eNode B; and a processor for controlling the user equipment (UE) including the reception module and the transmission module. The processor includes generating the CSI including at least one of a rank indicator (RI), a first precoding matrix index (PMI), a second PMI and a channel quality indicator (CQI) for one or more DL carriers, wherein the CQI is calculated based on precoding information determined by a combination of the first and second PMIs, determining, when two or more CSIs collide with one another in one uplink (UL) subframe of one UL carrier, a CSI to be transmitted on the basis of priority, transmitting, through the transmission module, the determined CSI over a UL channel, wherein the CSI is classified into a first group including an RI, a second group including a wideband (WB) first PMI, a third group including a WB CQI, and a fourth group including an SB CQI, and in association with the priority, if a CSI of the first group or a CSI of the second group collides with a CSI of the third group or a CSI of the fourth group, the CSI of the third or fourth group has low priority and is dropped.

[0013] The CSI of the first group and the CSI of the second group may have the same priority.

[0014] Provided that the priorities are identical to one another, if a CSI of a DL transmission mode for 8 transmission (Tx) antennas collides with a CSI of a different DL transmission mode, the CSI of the different DL transmission mode may have low priority and be dropped.

[0015] Provided that the priorities are identical to one another, a CSI of a high-priority DL carrier configured by a higher layer in each of the one or more DL carriers may be transmitted over the UL channel.

[0016] If the CSI of the third group collides with the CSI of the fourth group, the CSI of the fourth group may have low priority and be dropped.

[0017] If the first PMI is dropped, channel status information (CSI) subsequent to the dropped first PMI may be generated on the basis of a predefined first PMI.

[0018] The UL channel may be a physical uplink control channel (PUCCH).

[0019] The UL channel may be a physical uplink shared channel (PUSCH), and the priority may be applied to collision of CSIs of different carriers.

[0020] It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0021] As apparent from the above description, exemplary embodiments of the present invention have the following effects. The embodiments of the present invention provide a method and apparatus for configuring and transmitting feedback information used for correctly and efficiently supporting MIMO operation based on an extended antenna configuration.

[0022] It will be appreciated by persons skilled in the art that the effects that can be achieved with the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings.

[0057] The following embodiments are proposed by combining constituent components and characteristics of the present invention according to a predetermined format. The individual constituent components or characteristics should be considered to optional factors on the condition that there is no additional remark. If required, the individual constituent components or characteristics may not be combined with other components or characteristics. Also, some constituent components and/or characteristics may be combined to implement the embodiments of the present invention. The order of operations to be disclosed in the embodiments of the present invention may be changed. Some components or characteristics of any embodiment may also be included in other embodiments, or may be replaced with those of the other embodiments as necessary.

[0058] The embodiments of the present invention are disclosed on the basis of a data communication relationship between a base station and a terminal. In this case, the base station is used as a terminal node of a network via which the base station can directly communicate with the terminal. Specific operations to be conducted by the base station in the present invention may also be conducted by an upper node of the base station as necessary.

[0059] In other words, it will be obvious to those skilled in the art that various operations for enabling the base station to communicate with the terminal in a network composed of several network nodes including the base station will be conducted by the base station or other network nodes other than the base station. The term “Base Station (BS)” may be replaced with a fixed station, Node-B, eNode-B (eNB), or an access point as necessary. The term “relay” may be replaced with a Relay Node (RN) or a Relay Station (RS). The term “terminal” may also be replaced with a User Equipment (UE), a Mobile Station (MS), a Mobile Subscriber Station (MSS) or a Subscriber Station (SS) as necessary.

[0060] It should be noted that specific terms disclosed in the present invention are proposed for convenience of description and better understanding of the present invention, and the use of these specific terms may be changed to other formats within the technical scope or spirit of the present invention.

[0061] In some instances, well-known structures and devices are omitted in order to avoid obscuring the concepts of the present invention and the important functions of the structures and devices are shown in block diagram form. The same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0062] Exemplary embodiments of the present invention are supported by standard documents disclosed for at least one of wireless access systems including an Institute of Electrical and Electronics Engineers (IEEE) 802 system, a 3^rd Generation Project Partnership (3GPP) system, a 3GPP Long Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system, and a 3GPP2 system. In particular, the steps or parts, which are not described to clearly reveal the technical idea of the present invention, in the embodiments of the present invention may be supported by the above documents. All terminology used herein may be supported by at least one of the above-mentioned documents.

[0063] The following embodiments of the present invention can be applied to a variety of wireless access technologies, for example, CDMA (Code Division Multiple Access), FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA (Single Carrier Frequency Division Multiple Access), and the like. CDMA may be embodied through wireless (or radio) technology such as UTRA (Universal Terrestrial Radio Access) or CDMA2000. TDMA may be embodied through wireless (or radio) technology such as GSM (Global System for Mobile communications)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may be embodied through wireless (or radio) technology such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). UTRA is a part of UMTS (Universal Mobile Telecommunications System). 3GPP (3rd Generation Partnership Project) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlink and employs SC-FDMA in uplink. LTE-Advanced (LTE-A) is an evolved version of 3GPP LTE. WiMAX can be explained by an IEEE 802.16e (WirelessMAN-OFDMA Reference System) and an advanced IEEE 802.16m (WirelessMAN-OFDMA Advanced System). For clarity, the following description focuses on 3GPP LTE and 3GPP LTE-A systems. However, technical features of the present invention are not limited thereto.

[0064] FIG. 1 exemplarily shows a radio frame structure for use in a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system. A downlink (DL) radio frame structure will hereinafter be described with reference to FIG. 1. In a cellular Orthogonal Frequency Division Multiplexing (OFDM) radio packet communication system, uplink/downlink data packet transmission is performed in subframe units. One subframe is defined as a predetermined time interval including a plurality of OFDM symbols. The 3GPP LTE standard supports a type 1 radio frame structure applicable to Frequency Division Duplexing (FDD) and a type 2 radio frame structure applicable to Time Division Duplexing (TDD).

[0065] FIG. 1(a) is a diagram showing the structure of the type 1 radio frame. A downlink radio frame includes 10 subframes, and one subframe includes two slots in a time region. A time required for transmitting one subframe is defined in a Transmission Time Interval (TTI). For example, one subframe may have a length of 1 ms and one slot may have a length of 0.5 ms. One slot may include a plurality of OFDM symbols in a time region and include a plurality of Resource Blocks (RBs) in a frequency domain. Since the 3GPP LTE system uses OFDMA in downlink, the OFDM symbol indicates one symbol duration. The OFDM symbol may be called an SC-FDMA symbol or a symbol duration. RB is a resource allocation unit and includes a plurality of contiguous carriers in one slot.

[0066] The number of OFDM symbols included in one slot may be changed according to the configuration of a Cyclic Prefix (CP). The CP includes an extended CP and a normal CP. For example, if the OFDM symbols are configured by the normal CP, the number of OFDM symbols included in one slot may be seven. If the OFDM symbols are configured by the extended CP, the length of one OFDM symbol is increased, the number of OFDM symbols included in one slot is less than that of the case of the normal CP. In case of the extended CP, for example, the number of OFDM symbols included in one slot may be six. If the channel state is unstable, for example, if a User Equipment (UE) moves at a high speed, the extended CP may be used in order to further reduce interference between symbols.

[0067] In case of using the normal CP, since one slot includes seven OFDM symbols, one subframe includes 14 OFDM symbols. At this time, the first two or three OFDM symbols of each subframe may be allocated to a Physical Downlink Control Channel (PDCCH) and the remaining OFDM symbols may be allocated to a Physical Downlink Shared Channel (PDCCH).

[0068] The structure of a type 2 radio frame is shown in FIG. 1(b). The type 2 radio frame includes two half-frames, each of which is made up of five subframes, a downlink pilot time slot (DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS), in which one subframe consists of two slots. That is, one subframe is composed of two slots irrespective of the radio frame type. DwPTS is used to perform initial cell search, synchronization, or channel estimation. UpPTS is used to perform channel estimation of a base station and uplink transmission synchronization of a user equipment (UE). The guard interval (GP) is located between an uplink and a downlink so as to remove interference generated in the uplink due to multi-path delay of a downlink signal. That is, one subframe is composed of two slots irrespective of the radio frame type.

[0069] The structure of the radio frame is only exemplary. Accordingly, the number of subframes included in the radio frame, the number of slots included in the subframe or the number of symbols included in the slot may be changed in various manners.

[0070] FIG. 2 is a diagram showing a resource grid in a downlink slot. Although one downlink slot includes seven OFDM symbols in a time domain and one RB includes 12 subcarriers in a frequency domain in the figure, the scope or spirit of the present invention is not limited thereto. For example, in case of a normal Cyclic Prefix (CP), one slot includes 7 OFDM symbols. However, in case of an extended CP, one slot may include 6 OFDM symbols. Each element on the resource grid is referred to as a resource element. One RB includes 12×7 resource elements. The number N^DL of RBs included in the downlink slot is determined based on downlink transmission bandwidth. The structure of the uplink slot may be equal to the structure of the downlink slot.

[0071] FIG. 3 is a diagram showing the structure of a downlink subframe. A maximum of three OFDM symbols of a front portion of a first slot within one subframe corresponds to a control region to which a control channel is allocated. The remaining OFDM symbols correspond to a data region to which a Physical Downlink Shared Channel (PDSCH) is allocated. The basic unit of transmission becomes one subframe. Examples of the downlink control channels used in the 3GPP LTE system include, for example, a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), a Physical Hybrid automatic repeat request Indicator Channel (PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of a subframe, and includes information about the number of OFDM symbols used to transmit the control channel in the subframe. The PHICH includes a HARQ ACK/NACK signal as a response to uplink transmission. The control information transmitted through the PDCCH is referred to as Downlink Control Information (DCI). The DCI includes uplink or downlink scheduling information or an uplink transmit power control command for a certain UE group. The PDCCH may include resource allocation and transmission format of a Downlink Shared Channel (DL-SCH), resource allocation information of an Uplink Shared Channel (UL-SCH), paging information of a Paging Channel (PCH), system information on the DL-SCH, resource allocation of a higher layer control message such as a Random Access Response (RAR) transmitted on the PDSCH, a set of transmit power control commands for individual UEs in a certain UE group, transmit power control information, activation of Voice over IP (VoIP), etc. A plurality of PDCCHs may be transmitted within the control region. The UE may monitor the plurality of PDCCHs. The PDCCHs are transmitted on an aggregation of one or several contiguous control channel elements (CCEs). The CCE is a logical allocation unit used to provide the PDCCHs at a coding rate based on the state of a radio channel. The CCE corresponds to a plurality of resource element groups. The format of the PDCCH and the number of available bits are determined based on a correlation between the number of CCEs and the coding rate provided by the CCEs. The base station determines a PDCCH format according to a DCI to be transmitted to the UE, and attaches a Cyclic Redundancy Check (CRC) to control information. The CRC is masked with a Radio Network Temporary Identifier (RNTI) according to an owner or usage of the PDCCH. If the PDCCH is for a specific UE, a cell-RNTI (C-RNTI) of the UE may be masked to the CRC. Alternatively, if the PDCCH is for a paging message, a paging indicator identifier P-RNTI) may be masked to the CRC. If the PDCCH is for system information (more specifically, a system information block (SIB)), a system information identifier and a system information RNTI (SI-RNTI) may be masked to the CRC. To indicate a random access response that is a response for transmission of a random access preamble of the UE, a random access-RNTI (RA-RNTI) may be masked to the CRC.

[0072] FIG. 4 is a diagram showing the structure of an uplink frame. The uplink subframe may be divided into a control region and a data region in a frequency region. A Physical Uplink Control Channel (PUCCH) including uplink control information is allocated to the control region. A Physical uplink Shared Channel (PUSCH) including user data is allocated to the data region. In order to maintain single carrier characteristics, one UE does not simultaneously transmit the PUCCH and the PUSCH. The PUCCH for one UE is allocated to an RB pair in a subframe. RBs belonging to the RB pair occupy different subcarriers with respect to two slots. Thus, the RB pair allocated to the PUCCH is “frequency-hopped” at a slot edge.

[0073] Carrier Aggregation

[0074] Although downlink and uplink bandwidths are different from each other, a wireless communication system typically uses one carrier. For example, a wireless communication system having one carrier for each of the downlink and the uplink and symmetry between the downlink and uplink bandwidths may be provided based on a single carrier.

[0075] The International Telecommunication Union (ITU) requests that IMT-Advanced candidates support wider bandwidths, compared to legacy wireless communication systems. However, allocation of a wide frequency bandwidth is difficult throughout most of the world. Accordingly, a technology for efficiently using small segmented bands, known as carrier aggregation (bandwidth aggregation) or spectrum aggregation, has been developed in order to aggregate a plurality of physical bands to a wider logical band.

[0076] Carrier aggregation was introduced to support increased throughput, prevent cost increase caused by introduction of wideband RF devices, and ensure compatibility with legacy systems. Carrier aggregation enables data exchange between a UE and an eNB through a group of carriers each having a bandwidth unit defined in a legacy wireless communication system (e.g. 3GPP LTE Release-8 or Release-9 in case of 3GPP LTE-A). The carriers each having a bandwidth unit defined in the legacy wireless communication system may be called Component Carriers (CCs). Carrier aggregation using one or more CCs may be applied to each of downlink and uplink. Carrier aggregation may support a system bandwidth of up to 100 MHz by aggregating up to five CCs each having a bandwidth of 5, 10 or 20 MHz.

[0077] A downlink CC and an uplink CC may be represented as a DL CC and a UL CC, respectively. A carrier or CC may be represented as a cell in terms of function in the 3GPP LTE system. Thus, a DL CC and a UL CC may be referred to as a DL cell and a UL cell, respectively. Hereinbelow, the terms ‘carriers’, ‘component carriers’, ‘CCs’ or ‘cells’ will be used to signify a plurality of carriers to which carrier aggregation is applied.

[0078] While the following description exemplarily uses an eNB (BS) or cell as a downlink transmission entity and exemplarily uses a UE as an uplink transmission entity, the scope or spirit of the present invention is not limited thereto. That is, even when a relay node (RN) may be used as a downlink transmission entity from an eNB to a UE and or be used as an uplink reception entity from a UE to an eNB, or even when the RN may be used an uplink transmission entity for a UE or be used as a downlink reception entity from an eNB, it should be noted that the embodiments of the present invention can be applied without difficulty.

[0079] Downlink carrier aggregation may be described as an eNB supporting downlink transmission to a UE in frequency resources (subcarriers or physical resource blocks [PRBs]) of one or more carrier bands in time resources (allocated in units of a subframe). Uplink carrier aggregation may be described as a UE supporting uplink transmission to an eNB in frequency resources (subcarriers or PRBs) of one or more carrier bands in time resources (allocated in units of a subframe).

[0080] FIG. 5 shows a physical layer (first layer, L1) and a MAC layer (second layer, L2) of a multi-carrier supported system. Referring to FIG. 5, an eNB or BS of the legacy wireless communication system supporting a single carrier includes one physical layer (PHY) entity capable of supporting one carrier, and one medium access control (MAC) entity for controlling one PHY entity may be provided to the eNB. For example, baseband processing may be carried out in the PHY layer. For example, the L1/L2 scheduler operation including not only MAC PDU (Protocol Data Unit) creation of a transmitter but also MAC/RLC sub-layers may be carried out in the MAC layer. The MAC PDU packet block of the MAC layer is converted into a transport block through a logical transport layer, such that the resultant transport block is mapped to a physical layer input information block. In FIG. 5, the MAC layer is represented as the entire L2 layer, and may conceptually cover MAC/RLC/PDCP sub-layers. For convenience of description and better understanding of the present invention, the above-mentioned application may be used interchangeably in the MAC layer description of the present invention.

[0081] On the other hand, a multicarrier-supporting system may provide a plurality of MAC-PHY entities. In more detail, as can be seen from FIG. 5(a), the transmitter and receiver of the multicarrier-supporting system may be configured in such a manner that one MAC-PHY entity is mapped to each of n component carriers (n CCs). An independent PHY layer and an independent MAC layer are assigned to each CC, such that a PDSCH for each CC may be created in the range from the MAC PDU to the PHY layer.

[0082] Alternatively, the multicarrier-supporting system may provide one common MAC entity and a plurality of PHY entities. That is, as shown in FIG. 5(b), the multicarrier-supporting system may include the transmitter and the receiver in such a manner that n PHY entities respectively correspond to n CCs and one common MAC entity controlling the n PHY entities may be present in each of the transmitter and the receiver. In this case, a MAC PDU from one MAC layer may be branched into a plurality of transport blocks corresponding to a plurality of CCs through a transport layer. Alternatively, when generating a MAC PDU in the MAC layer or when generating an RLC PDU in the RLC layer, the MAC PDU or RLC PDU may be branched into individual CCs. As a result, a PDSCH for each CC may be generated in the PHY layer.

[0083] PDCCH for transmitting L1/L2 control signaling control information generated from a packet scheduler of the MAC layer may be mapped to physical resources for each CC, and then transmitted. In this case, PDCCH that includes control information (DL assignment or UL grant) for transmitting PDSCH or PUSCH to a specific UE may be separately encoded at every CC to which the corresponding PDSCH/PUSCH is transmitted. The PDCCH may be called a separate coded PDCCH. On the other hand, PDSCH/PUSCH transmission control information of several CCs may be configured in one PDCCH such that the configured PDCCH may be transmitted. This PDCCH may be called a joint coded PDCCH.

[0084] To support carrier aggregation, connection between a BS (or eNB) and a UE (or RN) needs to be established and preparation of connection setup between the BS and the UE is needed in such a manner that a control channel (PDCCH or PUCCH) and/or a shared channel (PDSCH or PUSCH) can be transmitted. In order to perform the above-mentioned connection or connection setup for a specific UE or RN, measurement and/or reporting for each carrier are needed, and CCs serving as the measurement and/or reporting targets may be assigned. In other words, CC assignment means that CCs (indicating the number of CCs and indexes of CCs) used for DL/UL transmission are established in consideration of not only capabilities of a specific UE (or RN) from among UL/DL CCs constructed in the BS but also system environment.

[0085] In this case, when CC assignment is controlled in third layer (L3) Radio Resource Management (RRM), UE-specific or RN-specific RRC signaling may be used. Alternatively, cell-specific or cell cluster-specific RRC signaling may be used. Provided that dynamic control such as a series of CC activation/deactivation settings is needed for CC assignment, a predetermined PDCCH may be used for L1/L2 control signaling, or a dedicated physical control channel for CC assignment control information or an L2 MAC-message formatted PDSCH may be used. On the other hand, if CC assignment is controlled by a packet scheduler, a predetermined PDCCH may be used for L1/L2 control signaling, a physical control channel dedicated for CC assignment control information may be used, or a PDSCH configured in the form of an L2 MAC message may be used.

[0086] FIG. 6 is a conceptual diagram illustrating downlink (DL) and uplink (UL) component carriers (CCs). Referring to FIG. 6, DL and UL CCs may be assigned from an eNB (cell) or RN. For example, the number of DL CCs may be set to N and the number of UL CCs may be set to M.

[0087] Through the UE's initial access or initial deployment process, after RRC connection is established on the basis of one certain CC for DL or UL (cell search) (for example, system information acquisition/reception, initial random access process, etc.), a unique carrier setup for each UE may be provided from a dedicated signaling (UE-specific RRC signaling or UE-specific L1/L2 PDCCH signaling). For example, assuming that a carrier setup for UE is commonly achieved in units of an eNB (cell or cell-cluster), the UE carrier setup may also be provided through cell-specific RRC signaling or cell-specific UE-common L1/L2 PDCCH signaling. In another example, carrier component information for use in an eNB may be signaled to a UE through system information for RRC connection setup, or may also be signaled to additional system information or cell-specific RRC signaling upon completion of the RRC connection setup.

[0088] While DL/UL CC setup has been described, centering on the relationship between an eNB and a UE, to which the present invention is not limited, an RN may also provide DL/UL CC setup to a UE contained in an RN region. In addition, in association with an RN contained in an eNB region, the eNB may also provide DL/UL CC setup of the corresponding RN to the RN of the eNB region. For clarity, while the following description will disclose DL/UL CC setup on the basis of the relationship between the eNB and the UE, it should be noted that the same content may also be applied to the relationship between the RN and the UE (i.e., access uplink and downlink) or the relation between the eNB and the RN (backhaul uplink or downlink) without departing from the scope or spirit of the present invention.

[0089] When the above-mentioned DL/UL CCs are uniquely assigned to individual UEs, DL/UL CC linkage may be implicitly or explicitly configured through a certain signaling parameter definition.

[0090] FIG. 7 shows an exemplary linkage of DL/UL CCs. In more detail, when an eNB configures two DL CCs (DL CC #a and DL CC #b) and two UL CCs (UL CC #i and UL CC #j), FIG. 6 shows a DL/UL CC linkage defined when two DL CCs (DL CC #a and DL CC #b) and one UL CC (UL CC #i) are assigned to a certain UE.

[0091] In a DL/UL CC linkage setup shown in FIG. 7, a solid line indicates a linkage setup between DL CC and UL CC that are basically constructed by an eNB, and this linkage setup between DL CC and UL CC may be defined in “System Information Block (SIB) 2”. In the DL/UL CC linkage setup shown in FIG. 7, a dotted line indicates a linkage setup between DL CC and UL CC configured in a specific UE. The above-mentioned DL CC and UL CC linkage setup shown in FIG. 7 is disclosed only for illustrative purposes, and the scope or spirit of the present invention is not limited thereto. That is, in accordance with various embodiments of the present invention, the number of DL CCs or UL CCs configured by an eNB may be set to an arbitrary number. Thus, the number of UE-specific DL CCs or the number of UE-specific UL CCs in the above-mentioned DL CCs or UL CCs may be set to an arbitrary number, and associated DL/UL CC linkage may be defined in a different way from that of FIG. 7.

[0092] Further, from among DL CCs and UL CCs configured or assigned, a primary CC (PCC), or a primary cell (P-cell) or an anchor CC (also called an anchor cell) may be configured. For example, a DL PCC (or DL P-cell) aiming to transmit configuration/reconfiguration information on RRC connection setup may be configured. In another example, UL CC for transmitting PUCCH to be used when a certain UE transmits UCI that must be transmitted on uplink may be configured as UL PCC (or UL P-cell). For convenience of description, it is assumed that one DL PCC (P-cell) and one UL PCC (P-cell) are basically assigned to each UE. Alternatively, if a large number of CCs is assigned to UE or if CCs can be assigned from a plurality of eNBs, one or more DL PCCs (P-cells) and/or one or more UL PCCs (P-cells) may be assigned from one or more eNBs to a certain UE. For linkage between DL PCC (P-cell) and UL PCC (P-cell), a UE-specific configuration method may be considered by the eNB as necessary. To implement a more simplified method, a linkage between DL PCC (P-cell) and UL PCC (P-cell) may be configured on the basis of the relationship of basic linkage that has been defined in LTE Release-8 (LTE Rel-8) and signaled to System Information Block (or Base) 2. DL PCC (P-cell) and UL PCC (P-cell) for the above-mentioned linkage configuration are grouped so that the grouped result may be denoted by a UE-specific P-cell.

[0093] SC-FDMA Transmission and OFDMA Transmission

[0094] FIG. 8 is a conceptual diagram illustrating an SC-FDMA transmission scheme and an OFDMA transmission scheme for use in a mobile communication system. The SC-FDMA transmission scheme may be used for UL transmission and the OFDMA transmission scheme may be used for DL transmission.

[0095] Each of the UL signal transmission entity (e.g., UE) and the DL signal transmission entity (e.g., eNB) may include a Serial-to-Parallel (S/P) Converter 801, a subcarrier mapper 803, an M-point Inverse Discrete Fourier Transform (IDFT) module 804, and a Parallel-to-Serial Converter 805. Each input signal that is input to the S/P converter 801 may be a channel coded and modulated data symbol. However, a user equipment (UE) for transmitting signals according to the SC-FDMA scheme may further include an N-point Discrete Fourier Transform (DFT) module 802. The influence of IDFT processing of the M-point IDFT module 804 is considerably offset, such that a transmission signal may be designed to have a single carrier property. That is, the DFT module 802 performs DFT spreading of an input data symbol such that a single carrier property requisite for UL transmission may be satisfied. The SC-FDMA transmission scheme basically provides good or superior Peak to Average Power ratio (PAPR) or Cubic Metric (CM), such that the UL transmitter can more effectively transmit data or information even in the case of the power limitation situation, resulting in an increase in user throughput.

[0096] FIG. 9 is a conceptual diagram illustrating maximum transmission power for single antenna transmission and MIMO transmission. FIG. 9(a) shows the case of single antenna transmission. As can be seen from FIG. 9(a), one power amplifier (PA) may be provided to one antenna. In FIG. 9(a), an output signal (P_max) of the power amplifier (PA) may have a specific value, for example, 23 dBm. In contrast, FIGS. 9(b) and 9(c) show the case of MIMO transmission. As can be seen from FIGS. 9(b) and 9(c), several PAs may be mapped to respective transmission (Tx) antennas. For example, provided that the number of transmission (Tx) antennas is set to 2, 2 PAs may be mapped to respective transmission (Tx) antennas. The setting of output values (i.e., maximum transmission power) of 2 PAs may be configured in different ways as shown in FIGS. 9(b) and 9(c).

[0097] In FIG. 9(b), maximum transmission power (P_max) for single antenna transmission may be divisionally applied to PA1 and PA2. That is, if a transmission power value of x [dBm] is assigned to PA1, a transmission power value of (P_max−x) [dBm] may be applied to PA2. In this case, since total transmission power (P_max) is maintained, the transmitter may have higher robustness against the increasing PAPR in the power limitation situation.

[0098] On the other hand, as can be seen from FIG. 9(c), only one Tx antenna (ANT1) may have a maximum transmission power value (P_max), and the other Tx antenna (ANT2) may have a half value (P_max/2) of the maximum transmission power value (P_max). In this case, only one transmission antenna may have higher robustness against increasing PAPR.

[0099] MIMO System

[0100] MIMO technology is not dependent on one antenna path to receive one message, collects a plurality of data pieces received via several antennas, and completes total data. As a result, MIMO technology can increase a data transfer rate within a specific range, or can increase a system range at a specific data transfer rate. Under this situation, MIMO technology is a next-generation mobile communication technology capable of being widely applied to mobile communication terminals or RNs. MIMO technology can extend the range of data communication, so that it can overcome the limited amount of transmission (Tx) data of mobile communication systems reaching a critical situation.

[0101] FIG. 10(a) is a block diagram illustrating a general MIMO communication system. Referring to FIG. 10(a), if the number of transmission (Tx) antennas increases to N_t, and at the same time the number of reception (Rx) antennas increases to N_R, a theoretical channel transmission capacity of the MIMO communication system increases in proportion to the number of antennas, differently from the above-mentioned case in which only a transmitter or receiver uses several antennas, so that transfer rate and frequency efficiency can be greatly increased. In this case, the transfer rate acquired by the increasing channel transmission capacity can theoretically increase by a predetermined amount that corresponds to multiplication of a maximum transfer rate (R_o) acquired when one antenna is used and a rate of increase (R_i). The rate of increase (R_i) can be represented by the following equation 1.

R_i=min(N_T,N_R)  [Equation 1]

[0102] For example, provided that a MIMO system uses four transmission (Tx) antennas and four reception (Rx) antennas, the MIMO system can theoretically acquire a high transfer rate which is four times higher than that of a one antenna system. After the above-mentioned theoretical capacity increase of the MIMO system was demonstrated in the mid-1990s, many developers began to conduct intensive research into a variety of technologies which can substantially increase data transfer rate using the theoretical capacity increase. Some of the above technologies have been reflected in a variety of wireless communication standards, for example, third-generation mobile communication or next-generation wireless LAN, etc.

[0103] A variety of MIMO-associated technologies have been intensively researched by many companies or developers, for example, research into information theory associated with MIMO communication capacity under various channel environments or multiple access environments, research into a radio frequency (RF) channel measurement and modeling of the MIMO system, and research into a space-time signal processing technology.

[0104] Mathematical modeling of a communication method for use in the above-mentioned MIMO system will hereinafter be described in detail. As can be seen from FIG. 10(a), it is assumed that there are N_T transmission (Tx) antennas and N_R reception (Rx) antennas. In the case of a transmission (Tx) signal, a maximum number of transmission information pieces is N_T under the condition that N_T transmission (Tx) antennas are used, so that the transmission (Tx) information can be represented by a specific vector shown in the following equation 2.

s=└s₁,s₂, . . . ,s_NT┘^T  [Equation 2]

[0105] In the meantime, individual transmission (Tx) information pieces (s₁, s₂, . . . , s_NT) may have different transmission powers. In this case, if the individual transmission powers are denoted by (P₁, P₂, . . . , P_NT), transmission (Tx) information having an adjusted transmission power can be represented by a specific vector shown in the following equation 3.

ŝ=[ŝ₁,ŝ₂, . . . ,ŝ_NT]^T=[P₁s₁,P₂s₂, . . . ,P_NTs_NT]^T  [Equation 3]

[0106] In Equation 3, ŝ is a transmission vector, and can be represented by the following equation 4 using a diagonal matrix P of a transmission (Tx) power.

$\begin{array}{cc}\hat{s}=\left[\begin{array}{cccc}{P}_1& & & 0\\ & P_2& & \\ & & \ddots & \\ 0& & & P_{N_T}\end{array}\right]\left[\begin{array}{c}{s}_1\\ s_2\\ ⋮\\ s_{N_T}\end{array}\right]=\mathrm{Ps}& \left[\mathrm{Equation}4\right]\end{array}$

[0107] In the meantime, the information vector Ŝ having an adjusted transmission power is applied to a weight matrix (W), so that N_T transmission (Tx) signals (x_l, x₂, . . . , x_NT) to be actually transmitted are configured. In this case, the weight matrix (W) is adapted to properly distribute transmission (Tx) information to individual antennas according to transmission channel situations. The above-mentioned transmission (Tx) signals (x₁, x₂, . . . , x_NT) can be represented by the following equation 5 using the vector (X).

$\begin{array}{cc}\begin{array}{c}x=\left[\begin{array}{c}{x}_1\\ x_2\\ ⋮\\ x_i\\ ⋮\\ x_{N_T}\end{array}\right]\\ =\left[\begin{array}{cccc}{w}_{11}& w_{12}& \dots & w_{1N_T}\\ w_{21}& w_{22}& \dots & w_{2N_T}\\ ⋮& & \ddots & \\ w_{i1}& w_{i2}& \dots & w_{{\mathrm{iN}}_T}\\ ⋮& & \ddots & \\ w_{N_T1}& w_{N_T2}& \dots & w_{N_TN_t}\end{array}\right]\left[\begin{array}{c}{\hat{s}}_1\\ {\hat{s}}_2\\ ⋮\\ {\hat{s}}_j\\ ⋮\\ {\hat{s}}_{N_T}\end{array}\right]\\ =W\hat{s}\\ =\mathrm{WPs}\end{array}& \left[\mathrm{Equation}5\right]\end{array}$

[0108] Next, if N_R reception (Rx) antennas are used, reception (Rx) signals (y₁, y₂, . . . , y_NR) of individual antennas can be represented by a specific vector (y) shown in the following equation 6.

y=[y₁,y₂, . . . ,y_NR]^T[Equation 6]

[0109] In the meantime, if a channel modeling is executed in the MIMO communication system, individual channels can be distinguished from each other according to transmission/reception (Tx/Rx) antenna indexes. A specific channel passing the range from a transmission (Tx) antenna (j) to a reception (Rx) antenna (i) is denoted by h_ij. In this case, it should be noted that the index order of the channel h_ij is located before a reception (Rx) antenna index and is located after a transmission (Tx) antenna index.

[0110] Several channels are tied up, so that they are displayed in the form of a vector or matrix. An exemplary vector is as follows. FIG. 10(b) shows channels from N_T transmission (Tx) antennas to a reception (Rx) antenna (i).

[0111] Referring to FIG. 10(b), the channels passing the range from the N_T transmission (Tx) antennas to the reception (Rx) antenna (i) can be represented by the following equation 7.

h_i^T└h_i1,h_i2, . . . ,h_iNT┘  [Equation 7]

[0112] If all channels passing the range from the N_T transmission (Tx) antennas to N_R reception (Rx) antennas are denoted by the matrix shown in Equation 7, the following equation 8 is acquired.

$\begin{array}{cc}H=\left[\begin{array}{c}{h}_1^T\\ h_2^T\\ ⋮\\ h_i^T\\ ⋮\\ h_{N_R}^T\end{array}\right]=\left[\begin{array}{cccc}{h}_{11}& h_{12}& \dots & h_{1N_T}\\ h_{21}& h_{22}& \dots & h_{2N_T}\\ ⋮& & \ddots & \\ h_{i1}& h_{i2}& \dots & h_{{\mathrm{iN}}_T}\\ ⋮& & \ddots & \\ h_{N_R1}& h_{N_R2}& \dots & h_{N_RN_T}\end{array}\right]& \left[\mathrm{Equation}8\right]\end{array}$

[0113] Additive white Gaussian noise (AWGN) is added to an actual channel which has passed the channel matrix (H) shown in Equation 8. The AWGN (n₁, n₂, . . . , n_NR) added to each of N_R reception (Rx) antennas can be represented by a specific vector shown in the following equation 9.

n=[n₁,n₂, . . . ,n_NR]^T  [Equation 9]

[0114] A reception signal calculated by the above-mentioned equations can be represented by the following equation 10.

$\begin{array}{cc}\begin{array}{c}y=\left[\begin{array}{c}{y}_1\\ y_2\\ ⋮\\ y_i\\ ⋮\\ y_{N_R}\end{array}\right]\\ =\left[\begin{array}{cccc}{h}_{11}& h_{12}& \dots & h_{1N_T}\\ h_{21}& h_{22}& \dots & h_{2N_T}\\ ⋮& & \ddots & \\ h_{i1}& h_{i2}& \dots & h_{{\mathrm{iN}}_T}\\ ⋮& & \ddots & \\ h_{N_R1}& h_{N_R2}& \dots & h_{N_RN_T}\end{array}\right]\left[\begin{array}{c}{x}_1\\ x_2\\ ⋮\\ x_j\\ ⋮\\ x_{N_T}\end{array}\right]+\left[\begin{array}{c}{n}_1\\ n_2\\ ⋮\\ n_i\\ ⋮\\ n_{N_R}\end{array}\right]\\ =\mathrm{Hx}+n\end{array}& \left[\mathrm{Equation}10\right]\end{array}$

[0115] In the meantime, the number of rows and the number of columns of a channel matrix H indicating a channel condition are determined by the number of Tx/Rx antennas. In the channel matrix H, the number of rows is equal to the number (N_R) of Rx antennas, and the number of columns is equal to the number (N_T) of Tx antennas. Namely, the channel matrix H is denoted by an N_R×N_T matrix. Generally, a matrix rank is defined by a smaller number between the number of rows and the number of columns, in which the rows and the columns are independent of each other. Therefore, the matrix rank cannot be higher than the number of rows or columns. The rank of the channel matrix H can be represented by the following equation 11.

rank(H)≦min(N_T,N_R)  [Equation 11]

[0116] A variety of MIMO transmission/reception (Tx/Rx) schemes may be used for operating the MIMO system, for example, frequency switched transmit diversity (FSTD), Space Frequency Block Coding (SFBC), Space Time Block Coding (STBC), Cyclic Delay Diversity (CDD), time switched transmit diversity (TSTD), etc. In case of Rank 2 or higher, Spatial Multiplexing (SM), Generalized Cyclic Delay Diversity (GCDD), Selective Virtual Antenna Permutation (S-VAP), etc. may be used.

[0117] The FSTD scheme serves to allocate subcarriers having different frequencies to signals transmitted through multiple antennas so as to obtain diversity gain. The SFBC scheme efficiently applies selectivity of a spatial region and a frequency region so as to obtain diversity gain and multiuser scheduling gain. The STBC scheme applies selectivity of a spatial domain and a time region. The CDD scheme serves to obtain diversity gain using path delay between transmission antennas. The TSTD scheme serves to temporally divide signals transmitted through multiple antennas. The spatial multiplexing scheme serves to transmit different data through different antennas so as to increase a transfer rate. The GCDD scheme serves to apply selectivity of a time region and a frequency region. The S-VAP scheme uses a single precoding matrix and includes a Multi Codeword (MCW) S-VAP for mixing multiple codewords among antennas in spatial diversity or spatial multiplexing and a Single Codeword (SCW) S-VAP using a single codeword.

[0118] In case of the STBC scheme from among the above-mentioned MIMO transmission schemes, the same data symbol is repeated to support orthogonality in a time domain so that time diversity can be obtained. Similarly, the SFBC scheme enables the same data symbol to be repeated to support orthogonality in a frequency domain so that frequency diversity can be obtained. An exemplary time block code used for STBC and an exemplary frequency block code used for SFBC are shown in Equation 12 and Equation 13, respectively. Equation 12 shows a block code of the case of 2 transmission (Tx) antennas, and Equation 13 shows a block code of the case of 4 transmission (Tx) antennas.

$\begin{array}{cc}\frac{1}{\sqrt{2}}\left(\begin{array}{cc}{S}_1& S_2\\ -S_2^{*}& S_1^{*}\end{array}\right)& \left[\mathrm{Equation}12\right]\\ \frac{1}{\sqrt{2}}\left(\begin{array}{cccc}{S}_1& S_2& 0& 0\\ 0& 0& S_3& S_4\\ -S_2^{*}& S_1^{*}& 0& 0\\ 0& 0& -S_4^{*}& S_3^{*}\end{array}\right)& \left[\mathrm{Equation}13\right]\end{array}$

[0119] In Equations 12 and 13, S_i (i=1, 2, 3, 4) means a modulated data symbol. In addition, each row of the matrixes of Equation 12 and 13 may indicate an antenna port, and each column may indicate time (in case of STBC) or frequency (in case of SFBC).

[0120] On the other hand, the CDD scheme from among the above-mentioned MIMO transmission schemes mandatorily increases delay spread so as to increase frequency diversity. FIG. 11 is a conceptual diagram illustrating a general CDD structure for use in the MIMO system. FIG. 11(a) shows a method for applying cyclic delay to a time domain. If necessary, the CDD scheme based on the cyclic delay of FIG. 11(a) may also be implemented as phase-shift diversity of FIG. 11(b).

[0121] In association with the above-mentioned MIMO transmission techniques, the codebook-based precoding method will hereinafter be described with reference to FIG. 12. FIG. 12 is a conceptual diagram illustrating codebook-based precoding.

[0122] In accordance with the codebook-based precoding scheme, a transceiver may share codebook information including a predetermined number of precoding matrixes according to a transmission rank, the number of antennas, etc. That is, if feedback information is infinite, the precoding-based codebook scheme may be used. The receiver measures a channel status through a reception signal, so that an infinite number of preferred precoding matrix information (i.e., an index of the corresponding precoding matrix) may be fed back to the transmitter on the basis of the above-mentioned codebook information. For example, the receiver may select an optimum precoding matrix by measuring an ML (Maximum Likelihood) or MMSE (Minimum Mean Square Error) scheme. Although the receiver shown in FIG. 12 transmits precoding matrix information for each codeword to the transmitter, the scope or spirit of the present invention is not limited thereto.

[0123] Upon receiving feedback information from the receiver, the transmitter may select a specific precoding matrix from a codebook on the basis of the received information. The transmitter that has selected the precoding matrix performs a precoding operation by multiplying the selected precoding matrix by as many layer signals as the number of transmission ranks, and may transmit each precoded Tx signal over a plurality of antennas. If the receiver receives the precoded signal from the transmitter as an input, it performs inverse processing of the precoding having been conducted in the transmitter so that it can recover the reception (Rx) signal. Generally, the precoding matrix satisfies a unitary matrix (U) such as (U*U^H=I), so that the inverse processing of the above-mentioned precoding may be conducted by multiplying a Hermit matrix (P^H) of the precoding matrix H used in the precoding of the transmitter by the reception (Rx) signal.

[0124] Physical Uplink Control Channel (PUCCH)

[0125] PUCCH including UL control information will hereinafter be described in detail.

[0126] A plurality of UE control information pieces may be transmitted through a PUCCH. When Code Division Multiplexing (CDM) is performed in order to discriminate signals of UEs, a Constant Amplitude Zero Autocorrelation (CAZAC) sequence having a length of 12 is mainly used. Since the CAZAC sequence has a property that a constant amplitude is maintained in a time domain and a frequency domain, a Peak-to-Average Power Ratio (PAPR) of a UE or Cubic Metic (CM) may be decreased to increase coverage. In addition, ACK/NACK information for DL data transmitted through the PUCCH may be covered using an orthogonal sequence.

[0127] In addition, control information transmitted through the PUCCH may be discriminated using cyclically shifted sequences having different cyclic shift values. A cyclically shifted sequence may be generated by cyclically shifting a basic sequence (also called a base sequence) by a specific cyclic shift (CS) amount. The specific CS amount is indicated by a CS index. The number of available CSs may be changed according to channel delay spread. Various sequences may be used as the basic sequence and examples thereof include the above-described CAZAC sequence.

[0128] PUCCH may include a variety of control information, for example, a Scheduling Request (SR), DL channel measurement information, and ACK/NACK information for DL data transmission. The channel measurement information may include a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), and a Rank Indicator (RI).

[0129] PUCCH format may be defined according to the type of control information contained in a PUCCH, modulation scheme information thereof, etc. That is, PUCCH format 1 may be used for SR transmission, PUCCH format 1a or 1b may be used for HARQ ACK/NACK transmission, PUCCH format 2 may be used for CQI transmission, and PUCCH format 2a/2b may be used for HARQ ACK/NACK transmission.

[0130] If HARQ ACK/NACK is transmitted alone in an arbitrary subframe, PUCCH format 1a or 1b may be used. If SR is transmitted alone, PUCCH format 1 may be used. The UE may transmit the HARQ ACK/NACK and the SR through the same subframe, and a detailed description thereof will hereinafter be described in detail.

[0131] PUCCH format may be summarized as shown in Table 1.

TABLE 1
		Number of
PUCCH	Modulation	bits per
format	scheme	subframe	Usage	etc.
1	N/A	N/A	SR(Scheduling
			Request)
1a	BPSK	1	ACK/NACK	One codeword
1b	QPSK	2	ACK/NACK	Two codeword
2	QPSK	20	CQI	Joint Coding
				ACK/NACK
				(extended CP)
2a	QPSK +	21	CQI +	Normal CP only
	BPSK		ACK/NACK
2b	QPSK +	22	CQI +	Normal CP only
	BPSK		ACK/NACK

[0132] FIG. 13 shows a PUCCH resource mapping structure for use in a UL physical resource block (PRB). N_RB^UL is the number of resource blocks (RBs) for use in uplink (UL), and n_PRB is a physical resource block (PRB) number. PUCCH may be mapped to both edges of a UL frequency block. CQI resources may be mapped to a PRB located just after the edge of a frequency band, and ACK/NACK may be mapped to this PRB.

[0133] PUCCH format 1 may be a control channel used for SR transmission. SR (Scheduling Request) may be transmitted in such a manner that SR is requested or not requested.

[0134] PUCCH format 1a/1b is a control channel used for ACK/NACK transmission. In the PUCCH format 1a/1b, a symbol modulated using the BPSK or QPSK modulation scheme is multiplied by a CAZAC sequence of length 12. Upon completion of the CAZAC sequence multiplication, the resultant symbol is blockwise-spread as an orthogonal sequence. A Hadamard sequence of length 4 is applied to general ACK/NACK information, and a DFT (Discrete Fourier Transform) sequence of length 3 is applied to the shortened ACK/NACK information and a reference signal. A Hadamard sequence of length 2 may be applied to the reference signal for the extended CP.

[0135] The UE may also transmit HARQ ACK/NACK and SR through the same subframe. For positive SR transmission, the UE may transmit HARQ ACK/NACK information through resources allocated for the SR. For negative SR transmission, the UE may transmit HARQ ACK/NACK information through resources allocated for ACK/NACK information.

[0136] PUCCH format 2/2a/2b will hereinafter be described in detail. PUCCH format 2/2a/2b is a control channel for transmitting channel measurement feedback (CQI, PMI, RI).

[0137] The PUCCH format 2/2a/2b may support modulation based on a CAZAC sequence, and a QPSK-modulated symbol may be multiplied by a CAZAC sequence of length 12. Cyclic shift (CS) of the sequence may be changed between a symbol and a slot. For a reference signal (RS), orthogonal covering may be used.

[0138] FIG. 14 shows a channel structure of a CQI information bit. The CQI bit may include one or more fields. For example, the CQI bit may include a CQI field indicating a CQI index for MCS decision, a PMI field indicating an index of a precoding matrix of a codebook, and an RI field indicating rank.

[0139] Referring to FIG. 14(a), a reference signal (RS) may be loaded on two SC-FDMA symbols spaced apart from each other by a predetermined distance corresponding to 3 SC-FDMA symbol intervals from among 7 SC-FDMA symbols contained in one slot, and CQI information may be loaded on the remaining 5 SC-FDMA symbols. The reason why two RSs may be used in one slot is to support a high-speed UE. In addition, each UE may be discriminated by a sequence. CQI symbols may be modulated in the entire SC-FDMA symbol, and the modulated CQI symbols may then be transmitted. The SC-FDMA symbol is composed of one sequence. That is, a UE performs CQI modulation using each sequence, and transmits the modulated result.

[0140] The number of symbols that can be transmitted to one TTI is set to 10, and CQI modulation is extended up to QPSK. If QPSK mapping is applied to the SC-FDMA symbol, a CQI value of 2 bits may be loaded on the SC-FDMA symbol, so that a CQI value of 10 bits may be assigned to one slot. Therefore, a maximum of 20-bit CQI value may be assigned to one subframe. A frequency domain spreading code may be used to spread CQI in a frequency domain.

[0141] CAZAC sequence (for example, a ZC sequence) may be used as a frequency domain spread code. In addition, another sequence having superior correlation characteristics may be used as the frequency domain spread code. Specifically, CAZAC sequences having different cyclic shift (CS) values may be applied to respective control channels, such that the CAZAC sequences may be distinguished from one another. IFFT may be applied to the frequency domain spread CQI.

[0142] FIG. 14(b) shows the example of PUCCH format 2/2a/2b transmission in case of the extended CP. One slot includes 6 SC-FDMA symbols. RS is assigned to one OFDM symbol from among 6 OFDM symbols of each slot, and a CQI bit may be assigned to the remaining 5 OFDM symbols. Except for the six SC-FDMA symbols, the example of the normal CP of FIG. 14(a) may be used without change.

[0143] Orthogonal covering applied to the RS of FIGS. 14(a) and 14(b) is shown in Table 2.

	TABLE 2
	Normal CP	Extended CP
	[11]	[1]

[0144] Simultaneous transmission of CQI and ACK/NACK information will hereinafter be described with reference to Table 15.

[0145] In case of the normal CP, CQI and ACK/NACK information can be simultaneously transmitted using PUCCH format 2a/2b. ACK/NACK information may be transmitted through a symbol where CQI RS is transmitted. That is, a second RS for use in the normal CP may be modulated into an ACK/NACK symbol. In the case where the ACK/NACK symbol is modulated using the BPSK scheme as shown in the PUCCH format 1a, CQI RS may be modulated into the ACK/NACK symbol according to the BPSK scheme. In the case where the ACK/NACK symbol is modulated using the QPSK scheme as shown in the PUCCH format 1b, CQI RS may be modulated into the ACK/NACK symbol according to the QPSK scheme. On the other hand, in case of the extended CP, CQI and ACK/NACK information are simultaneously transmitted using the PUCCH format 2. For this purpose, CQI and ACK/NACK information may be joint-coded.

[0146] For details of PUCCH other than the above-mentioned description, the 3GPP standard document (e.g., 3GPP TS36.211 5.4) may be referred to, and detailed description thereof will herein be omitted for convenience of description. However, it should be noted that PUCCH contents disclosed in the above-mentioned standard document can also be applied to a PUCCH used in various embodiments of the present invention without departing from the scope or spirit of the present invention.

[0147] Channel Status Information (CSI) Feedback

[0148] In order to correctly perform MIMO technology, the receiver may feed back a rank indicator (RI), a precoding matrix index (PMI) and a channel quality indicator (CQI) to the transmitter. RI, PMI and CQI may be generically named Channel Status Information (CSI) as necessary. Alternatively, the term “CQI” may be used as the concept of channel information including RI, PMI and CQI.

[0149] FIG. 16 is a conceptual diagram illustrating a feedback of channel status information.

[0150] Referring to FIG. 16, MIMO transmission data from the transmitter may be received at a receiver over a channel (H). The receiver may select a preferred precoding matrix from a codebook on the basis of the received signal, and may feed back the selected PMI to the transmitter. In addition, the receiver may measure a Signal-to-Interference plus Noise Ratio (SINR) of the reception (Rx) signal, calculate channel quality information (CQI), and feed back the calculated CQI to the transmitter. In addition, the receiver may measure a Signal-to-Interference plus Noise Ratio (SINR) of the reception (Rx) signal, calculate a CQI, and feed back the calculated SINR to the transmitter. In addition, the receiver may feed back a rank indicator (RI) of the Rx signal to the transmitter. The transmitter may determine the number of layers suitable for data transmission to the receiver and time/frequency resources, MCS (Modulation and Coding Scheme), etc. using RI and CQI information fed back from the receiver. In addition, the receiver may transmit the precoded Tx signal using the precoding matrix (W_l) indicated by a PMI fed back from the receiver over a plurality of antennas.

[0151] Channel status information will hereinafter be described in detail.

[0152] RI is information regarding a channel rank (i.e., the number of layers for data transmission of a transmitter). RI may be determined by the number of allocated Tx layers, and may be acquired from associated downlink control information (DCI).

[0153] PMI is information regarding a precoding matrix used for data transmission of a transmitter. The precoding matrix fed back from the receiver may be determined considering the number of layers indicated by RI. PMI may be fed back in case of closed-loop spatial multiplexing (SM) and large delay cyclic delay diversity (CDD). In the case of open-loop transmission, the transmitter may select a precoding matrix according to predetermined rules. A process for selecting a PMI for each rank (rank 1 to 4) is as follows. The receiver may calculate a post processing SINR in each PMI, convert the calculated SINR into the sum capacity, and select the best PMI on the basis of the sum capacity. That is, PMI calculation of the receiver may be considered to be a process for searching for an optimum PMI on the basis of the sum capacity. The transmitter that has received PMI feedback from the receiver may use a precoding matrix recommended by the receiver. This fact may be contained as a 1-bit indicator in scheduling allocation information for data transmission to the receiver. Alternatively, the transmitter may not use the precoding matrix indicated by a PMI fed back from the transmitter. In this case, precoding matrix information used for data transmission from the transmitter to the receiver may be explicitly contained in the scheduling allocation information. For details of PMI, the 3GPP standard document (e.g., 3GPP TS36.211) may be referred to.

[0154] CQI is information regarding channel quality. CQI may be represented by a predetermined MCS combination. CQI index may be given as shown in the following table 3.

	TABLE 3
	CQI index	modulation	code rate × 1024	efficiency
	0	out of range
	1	QPSK	78	0.1523
	2	QPSK	120	0.2344
	3	QPSK	193	0.3770
	4	QPSK	308	0.6016
	5	QPSK	449	0.8770
	6	QPSK	602	1.1758
	7	16QAM	378	1.4766
	8	16QAM	490	1.9141
	9	16QAM	616	2.4063
	10	64QAM	466	2.7305
	11	64QAM	567	3.3223
	12	64QAM	666	3.9023
	13	64QAM	772	4.5234
	14	64QAM	873	5.1152
	15	64QAM	948	5.5547

[0155] Referring to Table 3, CQI index may be represented by 4 bits (i.e., CQI indexes of 0-15). Each CQI index may indicate a modulation scheme and a code rate.

[0156] A CQI calculation method will hereinafter be described. The following assumptions (1) to (5) for allowing a UE to calculate a CQI index are defined in the 3GPP standard document (e.g., 3GPP TS36.213).

[0157] (1) The first three OFDM symbols in one subframe are occupied by control signaling.

[0158] (2) Resource elements (REs) used by a primary synchronization signal, a secondary synchronization signal or a physical broadcast channel (PBCH) are not present.

[0159] (3) CP length of a non-MBSFN subframe is assumed.

[0160] (4) Redundancy version is set to zero (0).

[0161] (5) PDSCH transmission method may be dependent upon a current transmission mode (e.g., a default mode) configured in a UE.

[0162] (6) The ratio of PDSCH EPRE (Energy Per Resource Element) to a cell-specific reference signal EPRE may be given with the exception of ρ_A. (A detailed description of ρ_A may follow the following assumption. Provided that a UE for an arbitrary modulation scheme may be set to a Transmission Mode 2 having four cell-specific antenna ports or may be set to a Transmission Mode 3 having an RI of 1 and four cell-specific antenna ports, ρ_A may be denoted by ρ_A=P_A+Δ_offset+10 log₁₀(2) [dB]. In the remaining cases, in association with an arbitrary modulation method and the number of arbitrary layers, ρ_A may be denoted by ρ_A=P_A+Δ_offset [dB]. Δ_offset is given by a nomPDSCH-RS-EPRE-Offset parameter configured by higher layer signaling.)

[0163] Definition of the above-mentioned assumptions (1) to (5) may indicate that a CQI includes not only a CQI but also various information of a corresponding UE. That is, different CQI indexes may be fed back according to a throughput or performance of the corresponding UE at the same channel quality, so that it is necessary to define a predetermined reference for the above-mentioned assumption.

[0164] The UE may receive a downlink reference signal (DL RS) from an eNB, and recognize a channel status on the basis of the received DL RS. In this case, the RS may be a common reference signal (CRS) defined in the legacy 3GPP LTE system, and may be a Channel Status Information Reference Signal (CSI-RS) defined in a system (e.g., 3GPP LTE-A system) having an extended antenna structure. The UE may satisfy the assumption given for CQI calculation at a channel recognized through a reference signal (RS), and at the same time calculate a CQI index in which a Block Error Rate (BLER) is not higher than 10%. The UE may transmit the calculated CQI index to the eNB. The UE may not apply a method for improving interference estimation to a CQI index calculation process.

[0165] The process for allowing the UE to recognize a channel status and calculate an appropriate MCS may be defined in various ways in terms of UE implementation. For example, the UE may calculate a channel status or an effective SINR using a reference signal (RS). In addition, the channel status or the effective SINR may be measured on the entire system bandwidth (also called ‘Set S’) or may also be measured on some bandwidths (specific subband or specific RB). The CQI for the set S may be referred to as a Wideband WB CQI, and the CQI for some bandwidths may be referred to as a subband (SB) CQI. The UE may calculate the highest MCS on the basis of the calculated channel status or effective SINR. The highest MCS may indicate an MCS that satisfies the CQI calculation assumption without exceeding a transport block error rate of 10% during the decoding. The UE may determine a CQI index related to the calculated MCS, and may report the determined CQI index to the eNB.

[0166] Further, CQI-only transmission may be considered in which a UE transmits only a CQI. Aperiodic CQI transmission may be event-triggered upon receiving a request from the eNB. Such request from the eNB may be a CQI request defined by one bit of DCI format 0. In addition, for CQI-only transmission, MCS index (I_MCS) of 29 may be signaled as shown in the following table 4. In this case, the CQI request bit of the DCI format 0 is set to 1, transmission of 4 RBs or less may be configured, Redundancy Version 1 (RV1) is indicated in PUSCH data retransmission, and a modulation order (Q_m) may be set to 2. In other words, in the case of CQI-only transmission, only a QPSK (Quadrature Phase Shift Keying) scheme may be used as a modulation scheme.

	TABLE 4
		Modulation	TBS	Redundancy
	MCS Index	Order	Index	Version
	IMCS	Q'm	ITBS	rvidx
	0	2	0	0
	1	2	1	0
	2	2	2	0
	3	2	3	0
	4	2	4	0
	5	2	5	0
	6	2	6	0
	7	2	7	0
	8	2	8	0
	9	2	9	0
	10	2	10	0
	11	4	10	0
	12	4	11	0
	13	4	12	0
	14	4	13	0
	15	4	14	0
	16	4	15	0
	17	4	16	0
	18	4	17	0
	19	4	18	0
	20	4	19	0
	21	6	19	0
	22	6	20	0
	23	6	21	0
	24	6	22	0
	25	6	23	0
	26	6	24	0
	27	6	25	0
	28	6	26	0
	29	reserved	1
	30		2
	31		3

[0167] The CQI reporting operation will hereinafter be described in detail.

[0168] In the 3GPP LTE system, when a DL reception entity (e.g., UE) is coupled to a DL transmission entity (e.g., eNB), a Reference Signal Received Power (RSRP) and a Reference Signal Received Quality (RSRQ) that are transmitted via downlink are measured at an arbitrary time, and the measured result may be periodically or event-triggeredly reported to the eNB.

[0169] In a cellular OFDM wireless packet communication system, each UE may report DL channel information based on a DL channel condition via uplink, and the eNB may determine time/frequency resources and MCS (Modulation and Coding Scheme) so as to transmit data to each UE using DL channel information received from each UE.

[0170] In case of the legacy 3GPP LTE system (e.g., 3GPP LTE Release-8 system), such channel information may be composed of Channel Quality Indication (CQI), Precoding Matrix Indicator (PMI), and Rank Indication (RI). All or some of CQI, PMI and RI may be transmitted according to a transmission mode of each UE. CQI may be determined by the received signal quality of the UE. Generally, CQI may be determined on the basis of DL RS measurement. In this case, a CQI value actually applied to the eNB may correspond to an MCS in which the UE maintains a Block Error Rate (BLER) of 10% or less at the measured Rx signal quality and at the same time has a maximum throughput or performance. In addition, such channel information reporting scheme may be divided into periodic reporting and aperiodic reporting upon receiving a request from the eNB.

[0171] Information regarding the aperiodic reporting may be assigned to each UE by a CQI request field of 1 bit contained in uplink scheduling information sent from the eNB to the UE. Upon receiving the aperiodic reporting information, each UE may transmit channel information considering the UE's transmission mode to the eNB over a physical uplink shared channel (PUSCH). If necessary, RI and CQI/PMI need not be transmitted over the same PUSCH.

[0172] In case of the aperiodic reporting, a cycle in which channel information is transmitted via an upper layer signal, an offset of the corresponding period, etc. may be signaled to each UE in units of a subframe, and channel information considering a transmission (Tx) mode of each UE may be transmitted to the eNB over a physical uplink control channel (PUCCH) at intervals of a predetermined time. In the case where UL transmission data is present in a subframe to which channel information is transmitted at intervals of a predetermined time, the corresponding channel information may be transmitted together with data over not a PUCCH but a PUSCH together. In case of the periodic reporting over a PUCCH, a limited number of bits may be used as compared to PUSCH. RI and CQI/PMI may be transmitted over the same PUSCH. If the periodic reporting collides with the aperiodic reporting, only the aperiodic reporting may be performed within the same subframe.

[0173] In order to calculate a WB CQI/PMI, the latest transmission RI may be used. In a PUCCH reporting mode, RI may be independent of another RI for use in a PUSCH reporting mode. RI may be effective only at CQI/PMI for use in the corresponding PUSCH reporting mode.

[0174] The CQI/PMI/RI feedback type for the PUCCH reporting mode may be classified into four feedback types (Type 1 to Type 4). Type 1 is CQI feedback for a user-selected subband. Type 2 is WB CQI feedback and WB PMI feedback. Type 3 is RI feedback. Type 4 is WB CQI feedback.

[0175] Referring to Table 5, in the case of periodic reporting of channel information, a reporting mode is classified into four reporting modes (Modes 1-0, 1-1, 2-0 and 2-1) according to CQI and PMI feedback types.

TABLE 5
		PMI Feedback Type
		No PMI (OL, TD,
		single-antenna)	Single PMI (CL)
CQI	Wideband	Mode 1-0	Mode 1-1
Feedback		RI (only for Open-	RI
Type		Loop SM)	Wideband CQI (4bit)
		One Wideband	Wideband spatial CQI
		CQI (4bit)	(3bit) for RI > 1
		when RI > 1, CQI	Wideband PMI (4bit)
		of first codeword
	UE	Mode 2-0	Mode 2-1
	Selected	RI (only for Open-	RI
		Loop SM)	Wideband CQI (4bit)
		Wideband CQI (4bit)	Wideband spatial CQI
		Best-1 CQI (4bit)	(3bit) for RI > 1
		in each BP	Wideband PMI (4bit)
		Best-1 indicator(L-	Best-1 CQI (4bit) 1
		bit label)	in each BP
		when RI > 1, CQI of	Best-1 spatial CQI
		first codeword	(3bit) for RI > 1
			Best-1 indicator
			(L-bit label)

[0176] The reporting mode is classified into a wideband (WB) CQI and a subband (SB) CQI according to a CQI feedback type. The reporting mode is classified into a No-PMI and a Single PMI according to transmission or non-transmission of PMI. As can be seen from Table 5, ‘NO PMI’ may correspond to an exemplary case in which an Open Loop (OL), a Transmit Diversity (TD), and a single antenna are used, and ‘Single PMI” may correspond to an exemplary case in which a closed loop (CL) is used.

[0177] Mode 1-0 may indicate an exemplary case in which PMI is not transmitted but only WB CQI is transmitted. In case of Mode 1-0, RI may be transmitted only in the case of Spatial Multiplexing (SM), and one WB CQI denoted by 4 bits may be transmitted. If RI is higher than ‘1’, a CQI for a first codeword may be transmitted. In case of Mode 1-0, Feedback Type 3 and Feedback Type 4 may be multiplexed at different time points within the predetermined reporting period, and then transmitted. The above-mentioned Mode 1-0 transmission scheme may be referred to as Time Division Multiplexing (TDM)-based channel information transmission.

[0178] Mode 1-1 may indicate an exemplary case in which a single PMI and a WB CQI are transmitted. In this case, 4-bit WB CQI and 4-bit WB PMI may be transmitted simultaneously with RI transmission. In addition, if RI is higher than ‘1’, 3-bit WB Spatial Differential CQI may be transmitted. In case of transmission of two codewords, the WB spatial differential CQI may indicate a differential value between a WB CQI index for Codeword 1 and a WB CQI index for Codeword 2. These differential values may be assigned to the set {−4, −3, −2, −1, 0, 1, 2, 3}, and each differential value may be assigned to any one of values contained in the set and be represented by 3 bits. In case of Mode 1-1, Feedback Type 2 and Feedback Type 3 may be multiplexed at different time points within the predetermined reporting period, and then transmitted.

[0179] Mode 2-0 may indicate that no PMI is transmitted and a CQI of a UE-selected band is transmitted. In this case, RI may be transmitted only in case of open loop spatial multiplexing (OL SM), a WB CQI denoted by 4 bits may be transmitted. In each Bandwidth Part (BP), Best-1 CQI may be transmitted, and Best-1 CQI may be denoted by 4 bits. In addition, an indicator of L bits indicating Best-1 may be further transmitted. If RI is higher than ‘1’, CQI for a first codeword may be transmitted. In case of Mode 2-0, the above-mentioned feedback type 1, feedback type 3, and feedback type 4 may be multiplexed at different time points within a predetermined reporting period, and then transmitted.

[0180] Mode 2-1 may indicate an exemplary case in which a single PMI and a CQI of a UE-selected band are transmitted. In this case, WB CQI of 4 bits, WB spatial differential CQI of 3 bits, and WB PMI of 4 bits are transmitted simultaneously with RI transmission. In addition, a Best-1 CQI of 4 bits and a Best-1 indicator of L bits may be simultaneously transmitted at each bandwidth part (BP). If RI is higher than ‘1’, a Best-1 spatial differential CQI of 3 bits may be transmitted. During transmission of two codewords, a differential value between a Best-1 CQI index of Codeword 1 and a Best-1 CQI index of Codeword 2 may be indicated. In Mode 2-1, the above-mentioned feedback type 1, feedback 2, and feedback type 3 may be multiplexed at different time points within a predetermined reporting period, and then transmitted.

[0181] In the UE selected SB CQI reporting mode, the size of BP (Bandwidth Part) subband may be defined by the following table 6.

TABLE 6
System Bandwidth	Subband Size k	Bandwidth Parts
NRBDL	(RBs)	(J)
6-7	NA	NA
8-0	4	1
11-26	4	2
27-63	6	3
64-110	8	4

[0182] Table 6 shows a bandwidth part (BP) configuration and the subband size of each BP according to the size of a system bandwidth. A UE may select a preferred subband within each BP, and calculate a CQI for the corresponding subband. In Table 6, if the system bandwidth is set to 6 or 7, this means no application of both the subband size and the number of bandwidth parts (BPs). That is, the system bandwidth of 6 or 7 means application of only WB CQI, no subband state, and a BP of 1.

[0183] FIG. 17 shows an example of a UE selected CQI reporting mode.

[0184] N_RB^DL the number of RBs of the entire bandwidth. The entire bandwidth may be divided into N CQI subbands (1, 2, 3, . . . , N). One CQI subband may include k RBs defined in Table 6. If the number of RBs of the entire bandwidth is not denoted by an integer multiple of k, the number of RBs contained in the last CQI subband (i.e., the N-th CQI subband) may be determined by the following equation 14.

N_RB^DL−k·└N_RB^DL/k┘  [Equation 14]

[0185] In Equation 14, └ ┘ represents a floor operation, and └X┘ or floor(x) represents a maximum integer not higher than ‘x’.

[0186] In addition, N_J CQI subbands construct one BP, and the entire bandwidth may be divided into J BPs. UE may calculate a CQI index for one preferred Best-1 CQI subband in contained in one BP, and transmit the calculated CQI index over a PUCCH. In this case, a Best-1 indicator indicating which a Best-1 CQI subband is selected in one BP may also be transmitted. The Best-1 indicator may be composed of L bits, and L may be represented by the following equation 15.

L=┌log₂ N_J┐  [Equation 15]

[0187] In Equation 15, ┌ ┐ may represent a ceiling operation, and ┌x┐ or ceiling(x) may represent a minimum integer not higher than ‘x’.

[0188] In the above-mentioned UE selected CQI reporting mode, a frequency band for CQI index calculation may be determined. Hereinafter, a CQI transmission cycle will hereinafter be described in detail.

[0189] Each UE may receive information composed of a combination of a transmission cycle of channel information and an offset from an upper layer through RRC signaling. The UE may transmit channel information to an eNB on the basis of the received channel information transmission cycle information.

[0190] FIG. 18 is a conceptual diagram illustrating a method for enabling a UE to periodically transmit channel information. For example, if a UE receives combination information in which a channel information transmission cycle is set to 5 and an offset is set to 1, the UE transmits channel information in units of 5 subframes, one subframe offset is assigned in the increasing direction of a subframe index on the basis of the 0^th subframe, and channel information may be assigned over a PUCCH. In this case, the subframe index may be comprised of a combination of a system frame number (n_f) and 20 slot indexes (n₃, 0˜19) present in the system frame. One subframe may be comprised of 2 slots, such that the subframe index may be represented by 10×n_f+floor(n_s/2).

[0191] One type for transmitting only WB CQI and the other type for transmitting both WB CQI and SB CQI may be classified according to CQI feedback types. In case of the first type for transmitting only the WB CQI, WB CQI information for the entire band is transmitted at a subframe corresponding to each CQI transmission cycle. The WB periodic CQI feedback transmission cycle may be set to any of 2, 5, 10, 16, 20, 32, 40, 64, 80, or 160 ms or no transmission of the WB periodic CQI feedback transmission cycle may be established. In this case, if it is necessary to transmit PMI according to the PMI feedback type of Table 5, PMI information is transmitted together with CQI. In case of the second type for transmitting both WB CQI and SB CQI, WB CQI and SB CQI may be alternately transmitted.

[0192] FIG. 19 is a conceptual diagram illustrating a method for transmitting both WB CQI and SB CQI according to an embodiment of the present invention. FIG. 19 shows an exemplary system comprised of 16 RBs. If a system frequency band is comprised of 16 RBs, for example, it is assumed that two bandwidth parts (BPs) (BP0 and BP1) may be configured, each BP may be composed of 2 subbands (SBs) (SB0 and SB1), and each SB may be composed of 4 RBs. In this case, as previously stated in Table 6, the number of BPs and the size of each SB are determined according to the number of RBs contained in the entire system band, and the number of SBs contained in each BP may be determined according to the number of RBs, the number of BPs and the size of SB.

[0193] In case of the type for transmitting both WB CQI and SB CQI, the WB CQI is transmitted to the CQI transmission subframe. In the next transmission subframe, a CQI of one SB (i.e., Best-1) having a good channel state from among SB0 and SB1 at BP0 and an index (i.e., Best-1 indicator) of the corresponding SB are transmitted. In the further next transmission subframe, a CQI of one SB (i.e., Best-1) having a good channel state from among SB0 and SB1 at BP1 and an index (i.e., Best-1 indicator) of the corresponding SB are transmitted. After transmitting the WB CQI, CQIs of individual BPs are sequentially transmitted. In this case, CQI of a BP located between a first WB CQI transmitted once and a second WB CQI to be transmitted after the first WB CQI may be sequentially transmitted one to four times. For example, if the CQI of each BP is transmitted once during a time interval between two WB CQIs, CQIs may be transmitted in the order of WB CQI→BP0 CQI→BP1 CQI→WB CQI. In another example, if the CQI of each BP is transmitted four times during a time interval between two WB CQIs, CQIs may be transmitted in the order of WB CQI→BP0 CQI→BP1 CQI→BP0 CQI→BP1 CQI→BP0 CQI→BP1 CQI→BP0 CQI→BP1 CQI→WB CQI. Information about the number of sequential transmission times of BP CQI during a time interval between two WB CQIs is signaled through a higher layer. Irrespective of WB CQI or SB CQI, the above-mentioned information about the number of sequential transmission times of BP CQI may be transmitted through a PUCCH in a subframe corresponding to information of a combination of channel information transmission cycle signaled from the higher layer of FIG. 18 and an offset.

[0194] In this case, if PMI also needs to be transmitted according to the PMI feedback type, PMI information and CQI must be simultaneously transmitted. If PUSCH for UL data transmission is present in the corresponding subframe, CQI and PMI can be transmitted along with data through PUSCH instead of PUCCH.

[0195] FIG. 20 is a conceptual diagram illustrating an exemplary CQI transmission scheme when both WB CQI and SB CQI are transmitted. In more detail, provided that combination information in which a channel information transmission cycle is set to 5 and an offset is set to 1 is signaled as shown in FIG. 18, and BP information between two WB CQI/PMI parts is sequentially transmitted once, FIG. 20 shows the example of channel information transmission operation of a UE.

[0196] On the other hand, in case of RI transmission, RI may be signaled by information of a combination of one signal indicating how many WB CQI/PMI transmission cycles are used for RI transmission and an offset of the corresponding transmission cycle. In this case, the offset may be defined as a relative offset for a CQI/PMI transmission offset. For example, provided that an offset of the CQI/PMI transmission cycle is set to 1 and an offset of the RI transmission cycle is set to zero, the offset of the RI transmission cycle may be identical to that of the CQI/PMI transmission cycle. The offset of the RI transmission cycle may be defined as a negative value or zero.

[0197] FIG. 21 is a conceptual diagram illustrating transmission of WB CQI, SB CQI and RI. In more detail, FIG. 21 shows that, under CQI/PMI transmission of FIG. 20, an RI transmission cycle is one time the WB CQI/PMI transmission cycle and the offset of RI transmission cycle is set to ‘−1’. Since the RI transmission cycle is one time the WB CQI/PMI transmission cycle, the RI transmission cycle has the same time cycle. A relative difference between the RI offset value ‘−1’ and the CQI offset ‘1’ of FIG. 20 is set to ‘−1’, such that RI can be transmitted on the basis of the subframe index ‘0’.

[0198] In addition, provided that RI transmission overlaps with WB CQI/PMI transmission or SB CQI/PMI transmission, WB CQI/PMI or SB CQI/PMI may drop. For example, provided that the RI offset is set to ‘0’ instead of ‘−1’, the WB CQI/PMI transmission subframe overlaps with the RI transmission subframe. In this case, WB CQI/PMI may drop and RI may be transmitted.

[0199] By the above-mentioned combination, CQI, PMI, and RI may be transmitted, and such information may be transmitted from each UE by RRC signaling of a higher layer. The BS (or eNB) may transmit appropriate information to each UE in consideration of a channel situation of each UE and a distribution situation of UEs contained in the BS (or eNB).

[0200] Meanwhile, payload sizes of SB CQI, WB CQI/PMI, RI and WB CQI in association with the PUCCH report type may be represented by the following table 7.

TABLE 7
PUCCH		PUCCH Reporting Modes
Report			Mode 1-1	Mode 2-1	Mode 1-0	Mode 2-0
Type	Reported	Mode State	(bits/BP)	(bits/BP)	(bits/BP)	(bits/BP)
1	Sub-band	RI = 1	NA	4 + L	NA	4 + L
	CQI	RI > 1	NA	7 + L	NA	4 + L
2	Wideband	2 TX Antennas RI = 1	6	6	NA	NA
	CQI/PMI	4 TX Antennas RI = 1	8	8	NA	NA
		2 TX Antennas RI > 1	8	8	NA	NA
		4 TX Antennas RI > 1	11	11	NA	NA
3	RI	2-layer spatial multiplexing	1	1	1	1
		4-layer spatial multiplexing	2	2	2	2
4	Wideband	RI = 1 or RI > 1	NA	NA	4	4
	CQI

[0201] Aperiodic transmission of CQI, PMI and RI over a PUSCH will hereinafter be described.

[0202] In case of the aperiodic reporting, RI and CQI/PMI may be transmitted over the same PUSCH. In case of the aperiodic reporting mode, RI reporting may be effective only for CQI/PMI reporting in the corresponding aperiodic reporting mode. CQI-PMI combinations capable of being supported to all the rank values are shown in the following table 8.

	TABLE 8
	PMI Feedback Type
	No PMI (OL, TD, single-antenna)	with PMI (CL)
PUSCH CQI	Wideband		Mode 1-2: Multiple PMI
Feedback Type	(Wideband CQI)		RI
			1st Wideband CQI (4 bit)
			2nd Wideband CQI (4 bit) if RI > 1
			subband PMIs on each subband
	UE Selected	Mode 2-0	Mode 2-2: Multiple PMI
	(Subband CQI)	RI (only for Open-Loop SM)	RI
		Wideband CQI (4 bit) + Best-M CQI (2 bit)	1st Wideband CQI (4 bit) + Best-M CQI(2 bit)
		Best-M index	2nd Wideband CQI (4 bit) + Best-M CQI(2 bit)
		when RI > 1, CQI of first codeword	if RI > 1
			Wideband PMI + Best-M PMI
			Best-M index
	Higher layer-configured	Mode 3-0	Mode 3-1: Single PMI
	(subband CQI)	RI (only for Open-Loop SM)	RI
		Wideband CQI (4 bit) + subband CQI (2 bit)	1st Wideband CQI (4 bit) + subband CQI
		when RI > 1, CQI of first codeword	(2 bit)
			2nd Wideband CQI (4 bit) + subband CQI
			(2 bit) if RI > 1
			Wideband PMI

[0203] Mode 1-2 of Table 8 may indicate a WB feedback. In Mode 1-2, a preferred precoding matrix for each subband may be selected from a codebook subset on the assumption of transmission only in the corresponding subband. The UE may report one WB CQI at every codeword, and WB CQI may be calculated on the assumption that data is transmitted on subbands of the entire system bandwidth (Set S) and the corresponding selected precoding matrix is used on each subband. The UE may report the selected PMI for each subband. In this case, the subband size may be given as shown in the following table 9. In Table 9, if the system bandwidth is set to 6 or 7, this means no application of the subband size. That is, the system bandwidth of 6 or 7 means application of only WB CQI and no subband state.

	TABLE 9
	System Bandwidth	Subband Size
	NRBDL	(k)
	6-7	NA
	8-10	4
	11-26	4
	27-63	6
	64-110	8

[0204] In Table 8, Mode 3-0 and Mode 3-1 show a subband feedback configured by a higher layer (also called an upper layer).

[0205] In Mode 3-0, the UE may report a WB CQI value calculated on the assumption of data transmission on the set-S (total system bandwidth) subbands. The UE may also report one subband CQI value for each subband. The subband CQI value may be calculated on the assumption of data transmission only at the corresponding subband. Even in the case of RI>1, WB CQI and SB CQI may indicate a channel quality for Codeword 1.

[0206] In Mode 3-1, a single precoding matrix may be selected from a codebook subset on the assumption of data transmission on the set-S subbands. The UE may report one SB CQI value for each codeword on each subband. The SB CQI value may be calculated on the assumption of a single precoding matrix used in all subbands and data transmission on the corresponding subband. The UE may report a WB CQI value for each codeword. The WB CQI value may be calculated on the assumption of a single precoding matrix used in all the subbands and data transmission on the set-S subbands. The UE may report one selected precoding matrix indicator. The SB CQI value for each codeword may be represented by a differential WB CQI value using a 2-bit subband differential CQI offset. That is, the subband differential CQI offset may be defined as a differential value between a SB CQI index and a WB CQI index. The subband differential CQI offset value may be assigned to any one of four values {−2, 0, +1, +2}. In addition, the subband size may be given as shown in the following table 7.

[0207] In Table 8, Mode 2-0 and Mode 2-2 illustrate a UE selected subband feedback. Mode 2-0 and Mode 2-2 illustrate reporting of best-M averages.

[0208] In Mode 2-0, the UE may select the set of M preferred subbands (i.e., best-M) from among the entire system bandwidth (set S). The size of one subband may be given as k, and k and M values for each set-S range may be given as shown in the following table 10. In Table 10, if the system bandwidth is set to 6 or 7, this means no application of both the subband size and the M value. That is, the system bandwidth of 6 or 7 means application of only WB CQI and no subband state.

[0209] The UE may report one CQI value reflecting data transmission only at the best-M subbands (i.e., M selected subbands). This CQI value may indicate a CQI for Codeword 1 even in the case of RI>1. In addition, the UE may report a WB CQI value calculated on the assumption of data transmission on the set-S subbands. The WB CQI value may indicate a CQI for Codeword 1 even in the case of RI>1.

TABLE 10
System Bandwidth
NRBDL	Subband Size k (RBs)	M
6-7	NA	NA
8-10	2	1
11-26	2	3
27-63	3	5
64-110	4	6

[0210] In Mode 2-2, the UE may select the set of M preferred subbands (i.e., best-M) from among the set-S subbands (where the size of one subband is set to k). Simultaneously, one preferred precoding matrix may be selected from among a codebook subset to be used for data transmission on the M selected subbands. The UE may report one CQI value for each codeword on the assumption that data transmission is achieved on M selected subbands and selection precoding matrices are used in each of the M subbands. The UE may report an indicator of one precoding matrix selected for the M subbands. In addition, one precoding matrix (i.e., a precoding matrix different from the precoding matrix for the above-mentioned M selected subbands) may be selected from among the codebook subset on the assumption that data transmission is achieved on the set-S subbands. The UE may report a WB CQI, that is calculated on the assumption that data transmission is achieved on the set-S subbands and one precoding matrix is used in all the subbands, at every codeword. The UE may report an indicator of the selected one precoding matrix in association with all subbands.

[0211] In association with entirety of UE-selected subband feedback modes (Mode 2-0 and Mode 2-2), the UE may report the positions of M selected subbands using a combination index (r), where r may be represented by the following equation 16.

$\begin{array}{cc}r=\sum _{i=0}^{M-1}〈\begin{array}{c}N-s_i\\ M-i\end{array}〉& \left[\mathrm{Equation}16\right]\end{array}$

[0212] In Equation 16, the set {s_i}_i=0^M−1, (1≦s_i≦N, s_i<s_i+1) may include M sorted subband indexes. In Equation 14,

$〈\begin{array}{c}x\\ y\end{array}〉\hspace{1em}$

may indicate an extended binomial coefficient, which is set to

$\left(\begin{array}{c}x\\ y\end{array}\right)\hspace{1em}$

in case of x≧y and is set to zero (0) in case of x<y. Therefore, r may have a unique label and may be denoted by

$r\in \left\{0,\dots ,\left(\begin{array}{c}N\\ M\end{array}\right)-1\right\}.$

[0213] In addition, a CQI value for M selected subbands for each codeword may be denoted by a relative differential value in association with a WB CQI. The relative differential value may be denoted by a differential CQI offset level of 2 bits, and may have a value of ‘CQI index−WB CQI index’ of M selected subbands. An available differential CQI value may be assigned to any one of four values {+1, +2, +3, +4}.

[0214] In addition, the size(k) of supported subbands and the M value may be given as shown in Table 10. As shown in Table 10, k or M may be given as a function of a system bandwidth.

[0215] A label indicating the position of each of M selected subbands (i.e., best-M subbands) may be denoted by L bits, where L is denoted by

$L=\lceil {\log }_2\left(\begin{array}{c}N\\ M\end{array}\right)\rceil .$

[0216] Feedback Information for Multiple MIMO Transmission Modes

[0217] As described above, Channel Status Information (CSI) is required for MIMO transmission. The CSI may be fed back from the receiver to the transmitter. The transmitter may acquire a precoding weight, that is capable of being adaptively used in response to a channel state, from the CSI. In addition, the transmitter may acquire signal transmission information from the CSI modified by the precoding weight to be used for MIMO transmission. For example, the signal transmission information may include a modulation order, a coding rate, a transport block size, a scheduling band, etc.

[0218] The receiver may obtain channel status information (CSI) between the transmitter and the receiver using a reference signal (RS) received from the transmitter, and may feed back (or report) the obtained CSI to the transmitter. In this case, a variety of methods may be used to reduce the amount of feedback CSI. For example, a channel quality information/index (CQI), a precoding matrix index (PMI), a rank indicator (RI), etc. can be represented by quantized bits, such that the amount of feedback information is reduced, resulting in the implementation of efficient transmission.

[0219] Specifically, information regarding a rank suitable for MIMO transmission is changed according to long term fading, such that the above-mentioned rank information is not changed for a relatively longer term as compared to other CSI. On the other hand, PMI or CQI reflects a channel state abruptly changed by short-term fading, such that it is changed for a relatively short time. Accordingly, RI may be reported for a relatively longer term as compared to PMI/CQI, and the PMI and CQI may be reported for a relatively short term as compared to the RI. In addition, PMI and CQI are determined according to a rank used for transmission, such that the PMI and CQI are calculated on the basis of the determined RI until reaching the next RI report period.

[0220] As described above, there is a need for a rank value to be first determined when calculating channel status information (CSI). The rank value may be determined considering a MIMO transmission scheme. The MIMO transmission method may be classified into a Multi-User MIMO (MU-MIMO) and a Single-User MIMO (SU-MIMO). If a spatial channel capable of being created through multiple antennas is assigned to multiple users, this means the MU-MIMO. In contrast, if all spatial channels are assigned to a single user, this means SU-MIMO.

[0221] The MU-MIMO transmission scheme can be classified into a method for using a non-unitary matrix such as Dirty Paper Coding (DPC), Zero Forcing, etc. and a method for using a unitary precoding weight such as a Per-User Unitary Rate Control (PU2RC). The two methods are characterized in that the precoding weight calculated on the basis of the limited transmission rank is reported to the transmitter from the viewpoint of a single user. For example, a multi-antenna transmitter including M transmission (Tx) antennas can generate a maximum of 8 spatial channels and can transmit signals. The number of spatial channels capable of being assigned to the receiver participating in MU-MIMO transmission may be set to M or less. In this case, a maximum number of spatial channels assigned to each user is limited to N (where N<M), such that spatial channels of N or less may be received. Assuming that a maximum of N transport spatial channels can be assigned to the UE, the UE selects a rank that is most appropriate for transmission. That is, the UE may select the most appropriate rank from among the N or less ranks (i.e., 1 to N ranks). The precoding weight and the channel quality information (CQI) can be calculated according to the selected rank.

[0222] For example, if the number of spatial channels assigned to one receiver is limited to 2, the receiver can measure channel state information (CSI) on the assumption that one or two spatial channels can be assigned to the receiver. In this case, the amount of channel status information that must be measured and reported by the receiver can be greatly reduced. That is, rank information is limited to N or 2, such that the number of required bits is reduced from log 2(N) to log 2(2).

[0223] PMI amount is determined according to the defined codebook set. Assuming that L codebook sets from Rank 1 to Rank N are defined and K (where K<L) codebook sets from Rank 1 to Rank 2 are defined, the amount of feedback information requisite for PMI reporting in case of a maximum rank limited to N˜2 is reduced.

[0224] CQI must be calculated according to each codeword (CW). Provided that a system having multiple codewords (MCW) includes a maximum of 2 CWs on Rank-2 transmission, 2 CQIs should be reported for transmission of Rank-2 or higher. Provided that a maximum of 2 spatial channels are assigned, the same amount of CQI (i.e., 2 CQIs) may be reported.

[0225] The transmitter can calculate CQI in consideration of the number of transmission layers. Provided that Rank-2 is used in MCW transmission, the second layer calculates SINR in consideration of interference when calculating a CQI of a CW transmitted through a first layer. Similarly, the number of spatial channels simultaneously created by the transmitter is recognized by the receiver, the receiver measures channels status information (CSI) appropriate for the maximum number of spatial channels created by the transmitter. In this case, the accuracy of CQI may be increased. For example, provided that a maximum of 2 spatial channels are formed by the transmitter and each spatial channel is assigned to two users, the receiver may calculate the CQI on the assumption that there is an interference layer in CQI calculation.

[0226] On the other hand, SU-MIMO transmission is characterized in that one user uses all spatial channels created by the transmitter. The receiver may report rank information appropriate for transmission to a base station (BS), and the receiver may report PMI and CQI calculated on the basis of the rank information. For example, provided that a maximum number of spatial channels created by the transmitter is set to N, the receiver selects a transmission rank capable of obtaining the highest transmission efficiency from among 1 to N ranks, and reports the selected rank to an eNode B.

[0227] The transmitter can simultaneously support SU-MIMO transmission and MU-MIMO transmission. Specialized control signals may be requested for individual SU-MIMO and MU-MIMO transmission. For example, a maximum of N ranks may be received in SU-MIMO transmission, and the transmitter for MU-MIMO transmission may generate a maximum of N spatial channels. If the receiver considers a maximum of N spatial channels as the effective spatial channels corresponding to individual users, a control signal optimized for each transmission mode may be transmitted. In this case, the transmitter transmits indication information regarding the transmission mode to the receiver, such that the receiver may pre-recognize which transmission mode is to be used for signal transmission of the transmitter. Thereafter, a control signal suitable for the pre-recognized information is transmitted, such that SU-MIMO transmission and MU-MIMO transmission can be simultaneously supported.

[0228] On the other hand, the transmitter does not provide indication messages of the SU-MIMO transmission mode and the MU-MIMO transmission mode to the receiver, such that the transmitter may allow the receiver to recognize any one of the two transmission modes and decode corresponding data. In this case, the transmitter may inform the receiver of the number of layers that must be received by a current UE. In this case, it is impossible for the UE to identify the SU-MIMO mode and the MU-MIMO mode. Therefore, it is possible to support MIMO transmission using the same control signal. However, there is a need for the receiver to report different feedback information to the transmitter so as to support SU-MIMO and MU-MIMO. For example, in order to support SU-MIMO transmission, a transmission rank most appropriate for transmission may be reported in consideration of a maximum number of spatial channels capable of being generated in the transmitter. To support MU-MIMO transmission, a rank most appropriate for transmission may be selected and reported from among the restricted ranks in consideration of reception of a limited number of layers from the viewpoint of the receiver.

[0229] Multi-Rank PMI Feedback

[0230] In a feedback method for allowing a system supporting the extended antenna configuration to smoothly support multiple MIMO modes, multi-rank PMI feedback may be used.

[0231] For example, PMI may be determined on the assumption that a receiver is scheduled to receive r layers from an eNode B during SU-MIMO rank-r transmission and performs rank-r SU-MIMO transmission. On the other hand, although one receiver can receive one layer during MU-MIMO transmission, the transmitter may actually transmit multiple layers.

[0232] The multi-rank PMI feedback may indicate that a PMI of Rank-r is used for SU-MIMO mode transmission and the restricted rank (for example, Rank-1 or Rank-2) is used for MU-MIMO mode transmission. For example, it should be noted that a Rank-r PMI based on SU-MIMO can be fed back during Rank-r SU-MIMO transmission. Otherwise, PMI/CQI having the restricted rank (for example, Rank-1 or Rank-2) based on the SU-MIMO assumption may be fed back to MU-MIMO pairing. A method of using a PMI having the restricted rank (or low rank) will hereinafter be described in detail.

[0233] A restricted PMI of a low rank value (Rank-1 or Rank-2) is appended to the regular rank-r PMI so as to facilitate dynamic switching between the SU-MIMO mode and the MU-MIMO mode. In order to support dynamic SU-MIMO/MU-MIMO switching in all ranks from Rank-1 to Rank-8, a single transmission mode for Ranks 1 to 8 has to support dynamic SU-MIMO and MU-MIMO switching on a per subframe basis. In other words, the same UE feedback (up to a Rank-8 PMI/CQI) shall be used in both SU-MIMO scheduling and MU-MIMO scheduling.

[0234] Since the UE does not recognize the actual transmission mode or the actual transmission rank, a natural question is how to schedule a UE in low-rank MU-MIMO transmission mode (for example, Rank-1 or Rank-2) when the UE reports a high rank PMI/CQI (for example, Ranks 3 to 8). One possible solution is to extract the first two columns of the high rank PMI (for example, any of Rank-3 to Rank-8 PMIs) fed back from the UE for MU-MIMO scheduling. However, such “truncated PMI” are sometimes not used as the optimal rank-1/2 PMI computed under low rank (for example, Rank-1 or Rank-2) hypothesis. Of course, although the truncated PMI may adversely impact MU-MIMO performance, it is possible to use the sub-optimal “truncated PMI”. In addition, due to the low mobility setup (i.e., low rank adaptation) typically seen in a scenario suitable for the MU-MIMO transmission mode, once a UE may report a rank-r PMI, the UE may continue to report a Rank-r PMI for a long period without any Rank-1 PMI. Hence, the benefit of the multi-rank PMI proposal is to allow the UE to supplement the optimal low-rank PMI such that sufficient CSI accuracy for Rank-1 or Rank-2 MU-MIMO pairing is achieved. From this perspective, multi-rank PMI enhances CSI accuracy in addition to facilitating dynamic SU-MIMO and MU-MIMO switching.

[0235] Precoder for 8 Tx Antennas

[0236] In the system (e.g., 3GPP LTE Release-10 system) for supporting the extended antenna structure, for example, MIMO transmission based on 8 Tx antennas may be carried out, such that it is necessary to design a codebook for supporting MIMO transmission.

[0237] In order to report a CQI of a channel transmitted through 8 antenna ports, the use of codebooks shown in Tables 11 to 18 may be considered. 8 CSI antenna ports may be represented by indexes of antenna ports 15˜22. Table 11 shows an example of the codebook for 1-layer CSI reporting using antenna ports 15 to 22. Table 12 shows an example of the codebook for 2-layer CSI reporting using antenna ports 15 to 22. Table 13 shows an example of the codebook for 3-layer CSI reporting using antenna ports 15 to 22. Table 14 shows an example of the codebook for 4-layer CSI reporting using antenna ports 15 to 22. Table 15 shows an example of the codebook for 5-layer CSI reporting using antenna ports 15 to 22. Table 16 shows an example of the codebook for 6-layer CSI reporting using antenna ports 15 to 22. Table 17 shows an example of the codebook for 7-layer CSI reporting using antenna ports 15 to 22. Table 18 shows an example of the codebook for 8-layer CSI reporting using antenna ports 15 to 22.

[0238] In Tables 11 to 18, φ_n and v_n can be represented by the following equation 17.

φ_n=e^jπn/2

v_m=[e^j2πm/32 e^j4πm/32 e^j6πm/32]^T  [Equation 17]

	TABLE 11
		i2
	i1	0	1	2	3
	0-15	W2i1,0(1)	W2i1,1(1)	W2i1,2(1)	W2i1,3(1)
		i2
	i1	4	5	6	7
	0-15	W2i1+1,0(1)	W2i1+1,1(1)	W2i1+1,2(1)	W2i1+1,3(1)
		i2
	i1	8	9	10	11
	0-15	W2i1+2,0(1)	W2i1+2,1(1)	W2i1+2,2(1)	W2i1+2,3(1)
		i2
	i1	12	13	14	15
	0-15	W2i1+3,0(1)	W2i1+3,1(1)	W2i1+3,2(1)	W2i1+3,3(1)
	where   W m , n ( 1 ) = 1 8  [ ν m ϕ n  ν m ]

TABLE 12
	i2
i1	0	1	2	3
0-15	W2i1,2i1,0(2)	W2i1,2i1,1(2)	W2i1+1,2i1+1,0(2)	W2i1+1,2i1+1,1(2)
	i2
i1	4	5	6	7
0-15	W2i1+2,2i1+2,0(2)	W2i1+2,2i1+2,1(2)	W2i1+3,2i1+3,0(2)	W2i1+3,2i1+3,1(2)
	i2
i1	8	9	10	11
0-15	W2i1,2i1+1,0(2)	W2i1,2i1+1,1(2)	W2i1+1,2i1+2,0(2)	W2i1+1,2i1+2,1(2)
	i2
i1	12	13	14	15
0-15	W2i1,2i1+3,0(2)	W2i1,2i1+3,1(2)	W2i1+1,2i1+3,0(2)	W2i1+1,2i1+3,1(2)
where   W m , m ′ , n ( 2 ) = 1 4  [ ν m ν m ′ ϕ n  ν m - ϕ n  ν m ′ ]

TABLE 13
	i2
i1	0	1	2	3
0-3	W8i1,8i1,8i1+8(3)	W8i1+8,8i1,8i1+8(3)	{tilde over (W)}8i1,8i1+8,8i1+8(3)	{tilde over (W)}8i1+8,8i1,8i1(3)
	i2
i1	4	5	6	7
0-3	W8i1+2,8i1+2,4i1+10(3)	W8i1+10,8i1+2,8i1+10(3)	{tilde over (W)}8i1+2,8i1+10,8i1+10(3)	{tilde over (W)}8i1+10,8i1+2,8i1+2(3)
	i2
i1	8	9	10	11
0-3	W8i1+4,8i1+4,8i1+12(3)	W8i1+12,8i1+4,8i1+12(3)	{tilde over (W)}8i1+4,8i1+12,8i1+12(3)	{tilde over (W)}8i1+12,8i1+4,8i1+4(3)
	i2
i1	12	13	14	15
0-3	W8i1+6,8i1+6,8i1+14(3)	W8i1+14,8i1+6,8i1+14(3)	{tilde over (W)}8i1+6,8i1+14,8i1+14(3)	{tilde over (W)}8i1+14,8i1+6,8i+6(3)
where   W m , m ′ , m ″ ( 3 ) = 1 24  [ ν m ν m ′ ν m ″ ν m - ν m ′ - ν m ″ ] ,	W ~ m , m ′ , m ″ ( 3 ) = 1 24  [ ν m ν m ′ ν m ″ ν m ν m ′ - ν m ″ ]

TABLE 14
	i2
i1	0	1	2	3
0-3	W8i1,8i1+8,0(4)	W8i1,8i1+8,1(4)	W8i1+2,8i1+10,0(4)	W8i1+2,8i1+10,1(4)
	i2
i1	4	5	6	7
0-3	W8i1+4,8i1+12,0(4)	W8i1+4,8i1+12,1(4)	W8i1+6,8i1+14,0(4)	W8i1+6,8i1+14,1(4)
where   W m , m ′ , n ( 4 ) = 1 32  [ ν m ν m ′ ν m ν m ′ ϕ n  ν m ϕ n  ν m ′ - ϕ n  ν m - ϕ n  ν m ′ ]

	TABLE 15
		i2
	i1	0
	0-3	W i 1 ( 5 ) = 1 40  [ ν 2  i 1 ν 2  i 1 ν 2  i 1 + 8 ν 2  i 1 + 8 ν 2  i 1 + 16 ν 2  i 1 - ν 2  i 1 ν 2  i 1 + 8 - ν 2  i 1 + 8 ν 2  i 1 + 16 ]

TABLE 16
	i2
i1	0
0-3	W i 1 ( 6 ) = 1 48  [ ν 2  i 1 ν 2  i 1 ν 2  i 1 + 8 ν 2  i 1 + 8 ν 2  i 1 + 16 ν 2  i 1 + 16 ν 2  i 1 - ν 2  i 1 ν 2  i 1 + 8 - ν 2  i 1 + 8 ν 2  i 1 + 16 - ν 2  i 1 + 16 ]

TABLE 17
	i2
i1	0
0-3	W i 1 ( 7 ) = 1 56  [ ν 2  i 1 ν 2  i 1 ν 2  i 1 + 8 ν 2  i 1 + 8 ν 2  i 1 + 16 ν 2  i 1 + 16 ν 2  i 1 + 24 ν 2  i 1 - ν 2  i 1 ν 2  i 1 + 8 - ν 2  i 1 + 8 ν 2  i 1 + 16 - ν 2  i 1 + 16 ν 2  i 1 + 24 ]

TABLE 18
	i2
i1	0
0	W i 1 ( 8 ) = 1 8  [ ν 2  i 1 ν 2  i 1 ν 2  i 1 + 8 ν 2  i 1 + 8 ν 2  i 1 + 16 ν 2  i 1 + 16 ν 2  i 1 + 24 ν 2  i 1 + 24 ν 2  i 1 - ν 2  i 1 ν 2  i 1 + 8 - ν 2  i 1 + 8 ν 2  i 1 + 16 - ν 2  i 1 + 16 ν 2  i 1 + 24 - ν 2  i 1 + 24 ]

[0239] DCI format 0

[0240] DCI format 0 is used for the PUSCH scheduling. Control information transmitted by the DCI format 0 will hereinafter be described in detail.

[0241] A ‘Flag for format 0/format 1A differentiation’ field assigned to one bit is used to differentiate between DCI format 0 and DCI format 1A. DCI format 1A is used to schedule downlink (DL) transmission and has the same payload size as that of DCI format 0, such that there is needed a field for allowing the same format to be assigned to DCI format 0 and DCI format 1A in such a manner that DCI format 0 and DCI format 1A can be distinguished from each other. If the ‘Flag for format 0/format 1A differentiation’ field is set to 0, this indicates DCI format 0. If the ‘Flag for format 0/format 1A differentiation’ field is set to 1, this indicates DCI format 1A.

[0242] A ‘Frequency hopping flag’ field is given by one bit and indicates application or non-application of PUSCH frequency hopping. If the ‘Frequency hopping flag’ field is set to 0, this means the non-application of PUSCH frequency hopping. If the ‘Frequency hopping flag’ field is set to 1, this indicates application of PUSCH frequency hopping.

[0243] A ‘Resource block assignment and hopping resource allocation’ field indicates resource block allocation information in a UL subframe according to the presence or absence of PUSCH frequency hopping. The ‘Resource block assignment and hopping resource allocation’ field is comprised of ┌log₂(N_RB^UL(N_RB^UL+1)/2)┐ bits. N_RB^UL is a UL bandwidth configuration value, and is represented by the number of resource blocks (RBs). In case of the application of PUSCH hopping, N_UL—hop Most Significant Bits (MSBs) are used to obtain the value of ñ_PRB(i) (physical resource block index), and (┌log₂(N_RB^UL(N_RB^UL+1)/2)┐−N_UL—hop) bits provide resource allocation of the first slot in the UL subframe. In this case, N_UL—hop indicates hopping information of 1 or 2 bits according to system bandwidth. On the other hand, in case of non-application of PUSCH hopping, (┌log₂(N_RB^UL(N_RB^UL+1)/2)┐) bits provide resource allocation of a UL subframe.

[0244] A ‘Modulation and coding scheme and redundancy version’ field is given by 5 bits, and indicates a PUSCH modulation order and a PUSCH redundancy version (RV). In case of retransmission, RV indicates information as to which subpacket is retransmitted. 0^th to 28^th states from among 32 states each denoted by 5 bits are used to indicate the modulation order, and 29^th to 31^th states may indicate RV indexes (1, 2, and 3).

[0245] A ‘New data indicator’ field is given by one bit, and indicates UL scheduling information related to new data or retransmission. If much more toggling is performed as compared to previous transmission NDI, this means new data transmission. If no toggling occurs, this means retransmission.

[0246] A ‘TPC command for scheduled PUSCH’ (Transmission Power Control (TPC) command for scheduled PUSCH) field is given by 2 bits, and indicates a specific value capable of determining PUSCH transmission power.

[0247] A ‘Cyclic shift for DMRS’ field is given by 3 bits and indicates a cyclic shift value used to generate a sequence for a Demodulation Reference Signal (DMRS). DMRS is a reference signal (RS) to estimate a UL channel either for each antenna port or for each layer.

[0248] A ‘UL index (for TDD)’ [UL index (in case of TDD)] field is given by 2 bits, and may indicate a subframe index, etc. for UL transmission in a specific UL-DL configuration when a radio frame is configured by a TDD scheme.

[0249] A ‘Downlink Assignment Index (for TDD)’ [DL index (in case of TDD)] field is given by 2 bits, and may indicate a total number of subframes for PDSCH transmission in a specific UL-DL configuration in a radio frame configured by a TDD scheme.

[0250] A ‘CQI request’ field is given by one 2 bit, and can aperiodically report a Channel Quality Information (CQI), a Precoding Matrix Indicator (PMI) and a Rank Indicator (RI) over a PUSCH.

[0251] If the ‘CQI request’ field is set to 1, a UE aperiodically transmits the CQI, PMI, and RI reporting information over a PUSCH.

[0252] A ‘Modulation and coding scheme and redundancy version’ field may perform signaling of an MCS index (I_MCS) that represents 32 states using 5 bits as shown in Table 4. If I_MCS=29 is signaled for 29≦I_MCS≦31, a ‘CQI request’ bit of the DCI format 0 is set to 1, transmission of 4 RBs or less (N_PRB≦4) is configured, a redundancy version 1 (RV1) is indicated in PUSCH data retransmission, and a modulation order Q_m is set to 2 (Q_m=2). In other words, when transmitting only the CQI, only QPSK may be used as the modulation scheme.

[0253] The 3GPP LTE system can transmit a maximum of 8 layers for SU-MIMO, and can transmit signals using a maximum of 2 layers for MU-MIMO. From the viewpoint of the receiver, signals can be demodulated using the same operation irrespective of SU-MIMO and MU-MIMO.

[0254] The receiver provides signal transmission information (for example, CSI) to the transmitter. Generally, it is assumed that SU-MIMO transmission is used for CSI reporting. Generally, CSI based on SU-MIMO is calculated without considering intra-cell interference, such that performance degradation may occur due to CQI mismatch on the assumption that the MU-MIMO transmission is attempted using the SU-MIMO based CSI. Therefore, in order to increase MU-MIMO transmission performance, a method for reporting the precoder appropriate for MU-MIMO transmission can be considered.

[0255] If the UE informs the eNode B of a transmission rank capable of acquiring maximum efficiency during the SU-MIMO transmission, for example, if the Rank-8 based codebook index and CQI are calculated and reported, feedback information may be used as information appropriate for Rank-8 transmission, however, the above-mentioned feedback information may be inappropriate for MU-MIMO transmission in which UEs each having Rank-1 and Rank-2 are multiplexed and transmitted. Accordingly, not only CSI for SU-MIMO but also CSI for MU-MIMO transmission should be reported to prevent performance deterioration.

[0256] Generally, two methods may be considered for reporting the CSI to the transmitter by the receiver. One of the two methods is a method for reporting CSI using promised resources at a promised time, and the other is a method for reporting CSI at a specific time upon receiving an indication message from the transmitter. As an exemplary method for reporting CSI at a promised time, a method for reporting the periodic CQI reporting information over a PUCCH in 3GPP LTE Release 8 may be used (However, if PUSCH data is transmitted at a periodic CSI report time point, CSI is multiplexed with data, and the multiplexed result is transmitted). As a method for reporting CSI at a specific time upon receiving an indication message from the transmitter, the aperiodic CSI reporting request field is established in uplink transmission control information contained in a downlink control channel, such that the established result can be reported over a PUSCH.

[0257] The present invention relates to a method for reporting CSI information that is capable of effectively supporting SU-MIMO and MU-MIMO transmission in aperiodic CSI reporting reported through a PUSCH. Embodiment 1 can be largely classified into Embodiment 1-A that simultaneously reports a UE recommended CSI and a restricted rank CSI and Embodiment 1-B that reports one of the UE Recommended CSI and the restricted rank CSI.

[0258] Embodiment 1-A relates to a method for simultaneously reporting the UE recommended CSI and the restricted rank CSI.

[0259] Assuming that the rank range capable of being measured by the receiver (i.e., UE) is set to Rank-N, the receiver calculates CQIs of Rank-1 to Rank-N such that it can select one rank capable of maximizing a throughput. However, assuming that the receiver selects a rank of Rank-M or higher (for example, M=3), information of a rank less than Rank-M needed for MU-MIMO transmission may be additionally reported. In this case, if the restricted rank is configured and rank adaptation is possible in the maximum rank, the rank indicator (RI) is requested. If the restricted rank is set to a restricted rank such as Rank-1 or Rank-2, only the PMI and CQI values may be reported without the rank indicator (RI).

[0260] If the UE recommended rank is set to M or less (for example, M=2), only the UE recommended CSI is reported. If the UE recommended rank is set to M or higher (for example, M=2), not only the UE recommended CSI but also the restricted rank CSI may be reported.

[0261] Embodiment 1-B relates to a method for reporting only one of the UE recommended CSI and the restricted rank CSI.

[0262] Provided that the rank range capable of being measured by the receiver is set to Rank-N, the receiver calculates CQIs of Rank-1 to Rank-N such that it can select one rank capable of maximizing throughput. However, provided that the receiver selects a rank of Rank-M or higher (for example, M=3), the transmitter may request a CQI (for example, CSI of Rank of M or less) needed for MU-MIMO transmission. The transmitter may request information of a rank lower than the rank range capable of being calculated and reported by the receiver. For this operation, a variety of embodiments of the present invention will hereinafter be described in detail.

[0263] There may be used an indication method in which an indicator is defined in a DCI format of a PDCCH so that rank information desired by the eNode B can be reported.

[0264] An indicator for indicating CQI attributes is defined in UL transmission control information, such that the receiver may report a CQI of a rank contained in a range indicated by the transmitter.

[0265] A CQI request field is defined in a DCI format 0 of the 3GPP LTE Release 8. If the CQI request field is set to 1, the UE transmits CSI. In this case, the transmitted CSI information includes a Rank, a PMI, and a CQI. Generally, rank information is selected as a UE preferred value.

[0266] For example, in a DCI format (for example, DCI format 4) newly defined for UL transmission in a system having the extended antenna configuration (for example, a 3GPP LTE Release 10 system), a UE preferred Rank is primarily reported when the eNode B (or eNB) reports a CQI. If the eNB indicates a rank, the UE may report a CQI in response to an eNB configured rank. The rank indicated by the eNB may designate a specific rank value, may indicate a maximum rank value, may indicate an index for the maximum rank value, may be an indicator for using the promised rank value (or the predetermined rank value), or may be an indicator for using the promised maximum rank value.

[0267] The rank for using the eNB configured rank (or the restricted rank) may be contained in a DCI format. For example, if the CQI request field from among the fields defined in the DCI format is activated, an unused field of the corresponding DCI format may also be interpreted as an indicator for using the eNB configured rank. Alternatively, the above-mentioned indicator may also be used as an indicator for using the eNB configured rank as a combination of other fields.

[0268] For example, the bit field of the DCI format 4 may be defined as follows.

	TABLE 19
	Contents	Number of bit
	Resource block assignment	N
	1st TB	MCS and RV	5
		NDI	1
	2nd TB	MCS and RV	5
		NDI	1
	Precoding information	M
	TPC command for scheduled PUSCH	2
	Cyclic shift for DMRS	3
	UL index (for TDD)	2
	Downlink Assignment Index (for TDD)	2
	CQI request	1
	Aperiodic SRS request	1

[0269] In Table 19, if the CQI request field is activated, the MCS and RV fields for a 2^nd transport block (TB) may be unused. In this case, the MCS and RV fields for the 2^nd TB may be used to indicate the eNB configured rank (or restricted rank).

[0270] A method for establishing the report rank range according to PDCCH DCI format types may be used.

[0271] Control information for UL transmission may be classified into a DCI format for supporting single layer transmission and a DCI format for supporting multi-layer transmission. For example, single antenna transmission is single layer transmission. For this operation, DCI format 0 is defined. In addition, in order to support a specific allocation method in spite of single layer transmission, a new DCI format 0A (for example, DCI format 0A) may be defined. In spite of single layer transmission, a new DCI format (for example, DCI format 0B) including a single layer precoder indicator may be defined. In addition, a DCI format for MIMO transmission may be defined, and a new DCI format (for example, DCI format 4) may be defined for multi-TB transmission.

[0272] A CQI request field may be defined in each DCI format. In this case, if the CQI request field of the DCI format supporting one transport block (TB) is activated, the UE calculates and reports a CQI in the restricted rank. In addition, if the CQI request field of the DCI format supporting multiple transport blocks is activated, the UE calculates and reports a CQI in a rank that can be measured and received by the UE.

[0273] In other words, if the CQI request indication is received from the DCI format supporting the single TB in the same manner as in the DCI format ‘0’, ‘0A’, or ‘0B’, a CQI is calculated in the restricted rank. If the CQI request indication is received from a DCI format supporting multiple TBs in the same manner as in the DCI format ‘4’, the UE calculates and reports the CQI at a rank that can be measured and received by the UE.

[0274] In this case, the restricted rank may be established as a value independent of the UE measurable rank. The restricted rank may be informed through the RRC signaling or may be set to a fixed value. For example, the restricted rank may be set to a maximum of Rank 2.

[0275] A method for establishing a reported information type according to a number of transmission PUSCH may be used.

[0276] When a CQI request field of the DCI format 0 is set to 1 in the 3GPP LTE Release-8, PUSCH is transmitted at a (n+k)-th time point corresponding to k subframes from a time at which a DCI is received over a PDCCH. In case of FDD, k is set to 4 (k=4).

[0277] The reported CSI may be changed according to whether ‘n’ is an even or odd number on the basis of the n-th subframe at which a DCI format having an activated CQI request field requesting the aperiodic CQI is received. For example, if the n-th subframe is an even subframe, CSI of the UE recommended rank may be reported. Additionally, if the n-th subframe is an odd number, CSI of the restricted rank may be reported. Alternatively, if the n-th subframe is an odd subframe, CSI of the UE recommended rank is reported. Additionally, if the n-th subframe is an even number, CSI of the restricted rank may be reported.

[0278] At the n-subframe corresponding to a reception time of the DCI format in which a CQI request field requesting the aperiodic CQI is requested is activated, the reported CSI may be changed according to whether the value of (n+k) is an even or odd number. For example, if the (n+k)^th subframe is an even subframe, CSI of the UE recommended rank may be reported. Additionally, if the (n+k)^th subframe is an odd number, CSI of the restricted rank may be reported. Alternatively, if the (n+k)^th subframe is an odd subframe, CSI of the UE recommended rank may be reported. Additionally, if the (n+k)^th subframe is an odd subframe, CSI of the restricted rank may be reported.

[0279] The CSI reporting method for effectively supporting the SU-MIMO and MU-MIMO transmission proposed in Embodiment 1 may be applied to a transmission mode newly defined in a system (for example, 3GPP LTE-A) supporting the extended antenna configuration.

[0280] A method for selecting a precoder suitable for the restricted rank using the precoder selected in response to the UE recommended rank on the condition that the UE recommended rank is higher than the restricted rank will hereinafter be described.

[0281] The precoder having Rank-N is composed of a combination of N precoding vectors. A low rank may be transmitted using some vectors from among N vectors. Using some vectors of the precoder may be referred to as subset selection.

[0282] A variety of methods for performing subset selection for the precoder that is reported from the UE to the eNB may be used. For example, 1) a method for selecting arbitrary vectors at random, 2) a method for selecting a subset according to a predetermined or promised rule, and 3) a method for reporting a vector preferred by the reporting side (i.e., UE) may be used. In this case, the above-mentioned methods (1) (Method for selecting arbitrary vectors) and (2) (Method for selecting a subset according to a promised rule) need not use additional signals. On the other hand, according to the method (3) for reporting the UE preferred vector, the reporting side (IE) has to provide subset selection information to the reported side (eNB).

[0283] Rules and examples applicable to the above-mentioned method (2) for selecting a subset according to the promised rule will hereinafter be described.

[0284] For example, a specific rule for sequentially selecting columns starting from the first column of the precoder may be used. In case of Rank-1, a first column may be selected. In case of Rank-2, first and second columns may be selected.

[0285] In another example, another rule for selecting a subset in consideration of a layer mapped to a transport block (TB) may be used. A precoder corresponding to the M^th layer from among layers mapped to the TB may be selected. For example, assuming that 2 TBs (TB1 and TB2) are mapped to 4 layers (Layer 1, Layer 2, Layer 3, Layer 4), the 1^st TB (TB1) is mapped to the 1^st and 2^nd layers (Layer 1 and Layer 2), and the 2^nd TB (TB2) is mapped to the 3^rd and 4^th layers. In this case, if M=1 is given in precoder subset selection, a subset corresponding to a first layer (Layer 1) mapped to a TB1 and a subset corresponding to a first layer (Layer 3) mapped to a TB2 may be selected as two precoders.

[0286] Exemplary signaling methods capable of being applied to the method for reporting the preferred vector at the reporting side will hereinafter be described in detail.

[0287] For example, the precoder subset may be reported as a bitmap format. In case of a Rank-N, by means of a bitmap composed of N bits for displaying N vectors, information as to what the UE preferred precoder vector is can be reported to the eNB.

[0288] In another example, provided that preferring one vector from among the precoder subsets is reported, information as to which one of the precoder vectors is preferred by the UE may be reported to the eNB using log₂(N) bits (where N=Rank).

[0289] If the precoder is selected according to the above-mentioned schemes, a CQI corresponding to the selected precoder may be calculated and reported. For SU-MIMO transmission, the Rank-N precoder may be selected, and a CQI may be calculated in response to the selected precoder. In this case, some precoder vectors are selected from among the Rank-N precoder, a CQI corresponding to the selected subset may be re-calculated. For example, if the precoder for a Rank-4 is selected, a CQI for Rank-4 may be calculated by the precoder. In addition, if two precoder vectors are selected on the basis of the Rank-4 precoder, a Rank-2 CQI may be calculated.

[0290] A variety of exemplary feedback methods applicable to the above-mentioned precoder selection will hereinafter be described in detail.

[0291] According to a first feedback method, a method for feeding back “RI-PMI1-CQI1-PMI2-CQI2” may be used. In the first feedback method, RI is rank information corresponding to PMI1 (or Precoder1), and CQI1 is calculated on the basis of PMI1. PMI1 is precoder(s) selected from among the PMI1, and CQI2 is calculated on the basis of the PMI2. In this case, each of PMI1, PMI2, CQI1 and CQI2 may be transmitted one or more times.

[0292] According to a second feedback method, a method for feeding back “RI-PMI1-CQI1-CQI2” may be used. In the second feedback method, RI is rank information corresponding to PMI1 (or Precoder1), and CQI1 is calculated on the basis of PMI1. CQI2 may be calculated on the basis of PMI2 (or Precoder2). PMI2 is precoder(s) selected from among the PMI1. If PMI2 is selected by the promised rule, PMI2 is not reported. In this way, each of PMI1, CQI1, and CQI2 may be transmitted one or more times.

[0293] According to a third feedback method, a method for feeding back “RI-PMI1-CQI1-PSI (Precoder Selection Indicator)-CQI2” may be used. In the third feedback method, RI is rank information corresponding to PMI1 (or Precoder1), and CQI1 is calculated on the basis of PMI1. PMI2 (or Precoder2) is precoder(s) selected from among the PMI1, and may report a PSI to indicate which value was selected as the PMI2. In this case, each of PMI1, CQI1, and CQI2 may be transmitted one or more times.

[0294] In case of the application of the above-mentioned feedback methods, feedback information may be simultaneously reported according to the reported channel (e.g., PUSCH or PUCCH), or may be reported at different cycles. For example, in case of feedback reporting over a PUSCH, an RI, a PMI and a CQI may be reported over one channel. If a PMI2 selected as some subsets of the PMI1 is reported, PMI2 and CQI2 may be simultaneously reported over only one channel. Alternatively, in case of the feedback reporting over a PUCCH, RI, PMI and CQI may be reported at different cycles. If a PMI2 selected as some subsets of a PMI1 is reported as described above, PMI2 and CQI2 may also be reported at different cycles.

[0295] A method for determining a transmission time point of feedback information when precoder information for the restricted rank is transmitted will hereinafter be described in detail.

[0296] Generally, a low rank precoder is used from the viewpoint of one user to implement MU-MIMO transmission, and it is preferable that users having low spatial correlation be generally multiplexed and transmitted. Even in the case of MU-MIMO transmission, the UE assumes SU-MIMO without distinction between MU-MIMO transmission and SU-MIMO transmission, and then determines and reports a rank value at which the maximum throughput is expected. Under the condition that the UE recommended rank and the precoder in response to the corresponding rank are selected and reported, a high rank precoder and associated CQI may be calculated and reported. When the high rank precoder is reported, a low rank precoder may be configured for MU-MIMO transmission using a subset of the reported precoder, or a method for additionally reporting the low rank precoder may be used.

[0297] First, the method for allowing an eNB to select a precoder subset reported from the UE so as to perform MU-MIMO transmission will hereinafter be described in detail. If the precoder subset depending on the UE recommended rank is used for MU-MIMO transmission, a CQI is needed for the MU-MIMO transmission of the eNB. Since the eNB receives report information of a CQI calculated on the basis of the precoder of the UE recommended rank, this CQI may be used as a MU-MIMO CQI. However, a channel state of the CQI calculated on the basis of the UE recommended rank precoder may be different from a channel state requisite for data that is transmitted using the corresponding precoder subset. Accordingly, in the case in which the eNB uses the CQI calculated on the basis of the UE recommended rank precoder as the MU-MIMO CQI, a CQI mismatch may occur. Preferably, it is preferable that the CQI calculated based on the precoder subset be reported to improve a MU-MIMO throughput.

[0298] Second, the method for allowing a UE to additionally report the low rank precoder will hereinafter be described. When the low rank precoder is reported, the CQI calculated on the basis of the precoder is preferably reported simultaneously with the low rank precoder.

[0299] PUCCH resources allocated from the eNB to the UE so as to report channel information using the conventional scheme are limited, and the precoder of the UE recommended rank and associated CQI may be reported over a PUCCH. Accordingly, in order to report the precoder subset and associated CQI or in order to report the restricted rank precoder and associated CQI, there is needed a method for newly defining a report time point and/or resources of such additional feedback information.

[0300] A method for establishing an offset at which the restricted rank based precoder and CQI is reported will hereinafter be described in detail.

[0301] In case of periodic PUCCH feedback reporting, a time point for RI and PMI/CQI transmission is defined. Generally, rank and PMI/CQI information are reported at different subframes. Specifically, RI is reported at a cycle longer than that of the PMI/CQI. If a rank is reported, the PMI/CQI information corresponding to the previously reported rank is reported in response to the corresponding transmission cycle until reaching the next rank reporting time.

[0302] If a higher rank is reported as described above, it is necessary to report the low rank PMI and associated CQI or the subset of a higher rank precoder and associated CQI. The low-rank precoder/CQI information may be referred to as PMI/CQI information of the restricted rank.

[0303] The time point at which the restricted rank PMI/CQI is reported may be some parts of another timing at which the higher rank PMI/CQI is reported. That is, a specific time from among a high-rank PMI/CQI report timing between rank reporting periods may be considered a report timing of the restricted rank PMI/CQI. The restricted rank PMI/CQI may be reported at a cycle longer than the reporting timing of the UE recommended rank PMI/CQI (that is, the restricted rank PMI/CQI may be reported less frequently than the UE recommended rank PMI/CQI), and may be reported with a predetermined offset at a UE recommended rank PMI/CQI reporting time point. Especially, an offset of the restricted rank PMI/CQI transmission time point may be reported later than the UE recommended rank PMI/CQI transmission time point.

[0304] On the other hand, a timing offset of a subsframe for rank information transmission may be the same subframe on the basis of the UE recommended rank PMI/CQI subframe, or may be transmitted at a subframe located before the above-mentioned subframe. In order to prevent the restricted rank PMI/CQI transmission from colliding with a subframe at which rank information is transmitted (i.e., in order to prevent the restricted rank PMI/CQI from being transmitted at the same subframe as the rank information), the restricted rank PMI/CQI may be reported later than a reference subframe at which the UE recommended rank PMI/CQI is transmitted. In addition, the offset for a transmission time point of the restricted rank PMI/CQI may be set to a positive (or negative) integer excluding ‘0’.

[0305] FIGS. 22 and 23 illustrate examples of the restricted rank PMI/CQI transmission timing and offset.

[0306] Examples of the restricted rank PMI/CQI transmission timing and offset will hereinafter be described with reference to FIGS. 22 and 23. In FIGS. 22 and 23, N_s is a slot index, and may be 0, 1, . . . , Ns. That is, one radio frame composed of 10 subframes is shown in FIGS. 22 and 23, and └Ns/2┘ is a subframe index.

[0307] In FIG. 22, a CQI/PMI based on the UE recommended rank is transmitted at intervals of Np, RI is transmitted at intervals of an integer multiple (Np×M_RI) of the CQI/PMI cycle based on the UE recommended rank, and RI is transmitted at a time point located ahead of the CQI/PMI transmission time point of the UE recommended rank by a predetermined offset (N_offset,RI). As previously stated in the above-mentioned embodiments, the restricted rank PMI/CQI may be transmitted at a time point located behind a CQI/PMI transmission time point of the UE recommended rank by a predetermined offset (N_offset,CQI), and may be transmitted with a cycle longer than the UE recommended rank CQI/PMI transmission cycle.

[0308] As can be seen from FIG. 23, a WB CQI/PMI and an SB CQI are transmitted as the UE recommended rank CQI/PMI. WB CQI/PMI and SB CQI may be alternately transmitted at intervals of Np, and the transmission cycle of the WB CQI/PMI may be set to (H×Np). RI may be transmitted at intervals of an integer multiple of the WB CQI/PMI cycle of the UE recommended rank (i.e., at intervals of (H×Np×M_RI)). RI is transmitted at a time point located ahead of a CQI/PMI transmission time point of the UE recommended rank by a predetermined offset (N_offset,RI). As previously sated in the above-mentioned embodiments, the restricted rank PMI/CQI may be transmitted at a time point located behind a CQI/PMI transmission time point of the UE recommended rank by a predetermined offset (N_offset,RI), and may be transmitted with a cycle longer than the UE recommended rank CQI/PMI transmission cycle.

[0309] An example of a feedback mode of the restricted rank PMI/CQI will hereinafter be described in detail.

[0310] In accordance with Embodiment 3-A-1, a feedback mode of the restricted rank PMI/CQI may be based on a feedback mode of the UE recommended rank PMI/CQI. For example, if the feedback mode of the UE recommended rank PMI/CQI is a mode for transmitting a WB PMI and a WB CQI, the restricted rank PMI/CQI may also be transmitted as the WB PMI and WB CQI. Alternatively, if the feedback mode of the UE recommended rank PMI/CQI is a mode for transmitting the WB PMI and SB CQI, the restricted rank PMI/CQI may also be transmitted as the WB PMI and SB CQI.

[0311] In addition, if the SB CQI is reported in the same manner as in band cycling, there may be used one cycle in which a WB CQI is reported and all SB CQIs for individual bandwidth parts (BPs) are reported. The restricted rank PMI/CQI may be reported during one cycle in which a WB CQI and SB CQIs of individual BPs are reported. That is, at least one cycle from among the band cyclic reporting period having one or more periods in the RI report cycle may be set to the report period of the restricted rank PMI/CQI.

[0312] An example of the restricted rank PMI/CQI reporting period will hereinafter be described with reference to FIG. 24. In FIG. 24, one of the band cyclic reporting periods, each of which is used to report a WB CQI and a CQI of each BP, from among the RI reporting period (H×Np×M_RI) may correspond to └Ns/2┘ of 1˜4. In FIG. 24, as previously stated in the above-mentioned embodiments, the restricted rank PMI/CQI may be transmitted in one of the band cyclic reporting periods.

[0313] Another example of a feedback mode of the restricted rank PMI/CQI will hereinafter be described in detail.

[0314] In accordance with Embodiment 3-A-2, a feedback mode of the restricted rank PMI/CQI is established to have a constant feedback mode irrespective of the feedback mode of the UE recommended rank PMI/CQI. For example, the restricted rank PMI/CQI may be established to have a WB PMI and a WB CQI.

[0315] Embodiment 3-B-1 relates to a feedback method on the condition that a multiple-granular precoder is defined. The multiple-granular precoder may be comprised of a combination of two codebooks (W1 and W2). W1 and W2 may be composed of various codebooks. Therefore, the eNB may receive report information of different feedback indicators (W1 and W2) of the precoder and then select the entire precoder. Different information (W1 and W2) for the precoder may be reported at different time points. For example, W1 may be reported at long term and W2 may be reported at short term. When W1 is reported at long term, W1 may be reported along with rank information. Alternatively, W1 and W1 may be simultaneously reported. That is, in case of using the multiple-granular precoder, a transmission time point of feedback information may be established as shown in the following table 20.

	TABLE 20
	T1	T2
Mode (1)	Rank + W1(wideband)	W2(wideband) + CQI(wideband)
Mode (2)	Rank	W1(wideband) + W2(wideband) +
		CQI(wideband)

[0316] As can be seen from Mode (1) of Table 20, rank information (RI) and a WB W1 may be transmitted at the same time point (T1), and a WB W2 and a WB CQI may be transmitted at an arbitrary time T2 lagging the T1 time. Alternatively, as can be seen from Mode (2), rank information (Ri) may be transmitted at the time T1, and a WB W1, a WB W2 and a WB CQI may be transmitted at an arbitrary time T2 lagging the T1 time.

[0317] In this way, under the condition that the indicators (W1 and W1) for the precoder are reported at the same or different time points, the restricted rank PMI/CQI may be fed back. If the restricted rank PMI/CQI is reported, W1 and W2 suitable for the restricted rank may be selected and fed back. In addition, a CQI calculated on the basis of the selected W1 and W2 may be fed back. In this case, W1, W2 and CQI may be reported at the same time point (i.e., at one subframe).

[0318] A feedback method including the restricted rank PMI/CQI under the multiple-granular precoder will hereinafter be described with reference to FIGS. 25 and 26.

[0319] In FIG. 25, RI and PMI1 (i.e., WB W1) are simultaneously transmitted at one time point, and WB PMI2 (i.e., WB W2) and WB CQI are transmitted at a later time point. In this case, PMI1, PMI2, and CQI may be feedback information that is selected and calculated according to the UE recommended rank. In addition, the restricted rank PMI/CQI may be transmitted at a time point located behind the UE recommended rank CQI/PMI transmission time point by a predetermined offset (N_offset,CQI). In FIG. 25, PMI1, PMI2, and CQI of the restricted rank are transmitted at a time point corresponding to └Ns/2┘=2.

[0320] In FIG. 26, after RI is transmitted, WB PMI1 (i.e. WB W1), WB PMI2 (i.e., WB W2) and WB CQI are simultaneously transmitted. In this case, the transmitted PMI1, PMI2, and CQI may be feedback information that is selected and calculated according to the UE recommended rank. In addition, the restricted rank PMI/CQI may be transmitted at a time point located behind the UE recommended rank CQI/PMI transmission time point by a predetermined offset (N_offset,CQI). In FIG. 26, PMI1, PMI2, and CQI of the restricted rank are transmitted at a time point corresponding to └Ns/2┘=2.

[0321] Embodiment 3-B-2 relates to a feedback method on the condition that a multiple-granular precoder is defined.

[0322] If multiple-granular precoder indicators (i.e., W1 and W2) are reported to the eNB, different feedback modes may be indicated using a Precoder Type Indication (PTI) bit.

[0323] In one feedback mode, R1, W1 and W2/CQI are transmitted at different subframes, and W1, W2 and CQI may be established as WB information. In the other feedback mode, W2 and CQI are reported at the same subframe, a W2/CQI frequency granularity may be WB or SB according to the reported subframe. That is, feedback modes of Table 21 can be defined.

	TABLE 21
	T1	T2	T3
Mode (1)	PTI(0) + Rank	W1(wideband)	W2(wideband) +
			CQI(wideband)
Mode (2)	PTI(1) + Rank	W2(wideband) +	W2(subband) +
		CQI(wideband)	CQI(subband)

[0324] In Table 21, if PTI is set to zero (0), RI is transmitted at a time point T1, and a WB W1 is transmitted at an arbitrary time point T2. Thereafter, a feedback function may be carried out at an arbitrary time point T3 according to a mode for WB W2 and WB CQI transmission. In Table 21, if PTI is set to 1, RI is transmitted at a time point T1, and WB W1 and WB CQI are then transmitted at an arbitrary time point T2. After that, a feedback function may be carried out at an arbitrary time point T3 according to a mode for SB W2 and SB CQI transmission.

[0325] Mode (1) or Mode (2) may be determined according to a feedback cycle of the rank information (RI). After Mode (1) or Mode (2) is determined by a PTI, WB W1, WB W2, and WB CQI may be reported in response to a CQI cycle (See Mode (1)) or WB W2/WB CQI and SB W2/SB CQI may be reported in response to a CQI cycle (See Mode (2)). A reference of the reported cycle may be set to a transmission time point of WB W2 and WB CQI. Transmission timing of other feedback information may be determined as an offset for WB W2/WB CQI transmission timing.

[0326] In the feedback method of this embodiment, a method for establishing the cycle and offset for such WB W1 feedback will hereinafter be described in detail.

[0327] According to a first method, a WB W1 transmission cycle may be established at intervals of a specific time longer than a PTI/RI transmission cycle. That is, the WB W1 transmission cycle may be established less frequently than the PTI/RI transmission cycle. In addition, the WB W1 cycle may be an integer multiple of a transmission cycle of WB W2 and WB CQI. In addition, the WB W1 transmission timing may be established as an offset value of the reference timing (i.e., a transmission subframe of WB W2 and WB CQI).

[0328] According to a second method, the WB W1 transmission timing may be established as an offset value of the reference timing (i.e., a transmission subframe of WB W2 and WB CQI). In addition, if a PTI of PTI/RI feedback information is set to a specific value (0 or 1), the WB W1 may be immediately transmitted once after the PTI/RI transmission.

[0329] In the feedback method of this embodiment, a method for feeding back the restricted rank PMI/CQI will hereinafter be described in detail. The above-mentioned WB W1, WB W2, WB CQI, SB W2 and SB CQI are used as feedback information that is selected and calculated according to the UE recommended rank, and the restricted rank PMI/CQI may be further transmitted.

[0330] If PTI reported along with RI is set to 0, WB PMI and WB CQI may be reported as the restricted rank PMI/CQI. WB W1, WB W2 and WB CQI of the restricted rank may be reported at the same time point. In some subframes from among subframes in which (WB W2+WB CQI) of the UE recommended rank is reported, WB W1, WB W2 and WB CQI of the restricted rank may be simultaneously reported.

[0331] Alternatively, if PTI reported along with RI is set to 1, the restricted rank PMI/CQI may be reported. In this case, the following two methods may be used to report the restricted rank PMI/CQI.

[0332] In the first method, only WB W1, WB W2 and WB CQI of the restricted rank may be reported as the restricted rank PMI/CQI.

[0333] In the second method, WB W1, WB W2 and WB CQI of the restricted rank may be reported at one subframe, and SB W1 and SB CQI of the restricted rank may be reported at a different subframe. Transmission time points of the above-mentioned factors may be established according to the band cyclic reporting period.

[0334] Embodiment 4 discloses a method for deciding the number of bits of different precoder indexes constructing the entire precoder.

[0335] Tables 11 to 18 show codebooks for enabling a BS (or eNB) having 8 Tx antennas in the 3GPP LTE system to report a CSI. The CSI reporting codebooks shown in Tables 11 to 18 may decide the codebook element according to two kinds of feedback reports. Although Tables 11 to 18 represent two feedback report values as i1 and i2, i1 and i2 correspond to one precoder index W1 (or PMI1) and another precoder index W2 (or PMI2), respectively. Two report values may have different time points, and may be established to have different frequency granularities. For data transmission, the number (# of element) of constituent elements of the codebook may have different values according to the number of UE-recommended ranks, as represented by the following Table 22.

	TABLE 22
	Rank
	1	2	3	4	5	6	7	8
# of element for i1	16	16	4	4	4	4	4	1
# of element for i2	16	16	16	8	1	1	1	1

[0336] In Table 22, i1 may be defined to have an element of 16, 4 or 1 according to the rank, and i2 may be defined to have an element of 16, 8 or 1 according to the rank. For feedback, i1 may be represented by 0 to 4 bits, and i2 may be represented by 0 to 4 bits. A maximum number of bits capable of expressing i1 and i2 according to the rank can be represented by the following Table 23.

	TABLE 23
	Rank
	1	2	3	4	5	6	7	8
Maximum bits for i1	4	4	2	2	2	2	2	0
Maximum bits for i2	4	4	4	3	0	0	0	0

[0337] Due to limitation of control channel capacity defined to report feedback information, bits capable of representing i1 and i2 for CSI reporting may be restricted. That is, the i1 and i2 values must be transmitted to report a CSI. If an indicator for the i1 value and/or an indicator for the i2 value may be transmitted along with RI or CQI, an error rate similar to that of a channel that reports RI or CQI defined in the legacy 3GPP LTE Release-8 or Release-9 may be implemented and at the same time feedback information may be transmitted as necessary.

[0338] In the case where the indicator for the i1 value and/or the indicator for the i2 value are simultaneously transmitted along with RI or CQI, for example, RI may be reported through one subframe, and an indicator for the i1 value, an indicator for the i2 value and a CQI may be simultaneously reported through another subframe. In another example, RI and the indicator for i1 may be simultaneously reported through one subframe, and the indicator for i2 and a CQI may be simultaneously transmitted through another subframe.

[0339] The legacy 3GPP LTE Release-8 or Release-9 assumes transmission of a maximum of 2 bits for the RI. In case of RI transmission through a PUCCH, the same coding method as in ACK/NACK transmission may be used. In addition, it is assumed that a maximum of 11 bits is transmitted to report CQI/PMI, such that the coding may be carried out using a Reed-Muller (RM) code that is capable of supporting a maximum of 13 bits.

[0340] If it is assumed that the system (e.g., 3GPP LTE Release-10 system) supporting the extended antenna structure simultaneously reports i1, i2, and CQI (i1/i2/CQI), a maximum of 15 (=4+4+7) bits may be requisite for Rank-1 or Rank-2. To transmit 15 bits, the coding method for extending the legacy RM code may be used, or a control signal may be reported using the conventional convolution code. In addition, in order to implement the same level as that of maximum bits defined in the legacy system, a method for reducing the size of indicator bits for i1 and i2 may be used as necessary.

[0341] Table 24 shows numbers of bits needed to simultaneously report i1, i2 and CQI (i1/i2/CQI). If the indicator bits for i1 and i2 are set to 0˜4, the number of bits transmitted in one subframe is shown in Table 24. In addition, according to the rank, the number of indicator bits for i1 or i2 may be a fullset or subset. For example, if the i1 indicator bit is set to 4 and the i2 indicator bit is set to 4, all the fullsets of a codebook may be used to transmit Rank-1 and Rank-2. Alternatively, in the case where 2 bits are used for i1 or (W1) and 4 bits are used for i2 (or W2), the subset of i1 may be used in Rank-1 or Rank-2, the fullset of i2 may be used, and all fullsets of i1 and i2 may be used in Rank-3. In Table 24, F may represent the fullset, and S may represent the subset. In addition, in association with each expression (F/F, F/S, S/F or S/S) of Table 21, a number located in front of a specific symbol (/) represents bits for i1 and another number located behind the symbol (/) represents bits for i2.

	TABLE 24
	Rank
R	i1	i2	i1 + i2 + CQI	1	2	3	4	5	6	7	8
1/2/3	4	4	4 + 4 + 7	F/F	F/F	—	—	—	—	—	—
		3	4 + 3 + 7	F/S	F/S	—	—	—	—	—	—
		2	4 + 2 + 7	F/S	F/S	—	—	—	—	—	—
		1	4 + 1 + 7	F/S	F/S	—	—	—	—	—	—
		0	4 + 0 + 7	F/S	F/S	—	—	—	—	—	—
	3	4	3 + 4 + 7	S/F	S/F	—	—	—	—	—	—
		3	3 + 3 + 7	S/S	S/S	—	—	—	—	—	—
		2	3 + 2 + 7	S/S	S/S	—	—	—	—	—	—
		1	3 + 1 + 7	S/S	S/S	—	—	—	—	—	—
		0	3 + 0 + 7	S/S	S/S	—	—	—	—	—	—
	2	4	2 + 4 + 7	S/F	S/F	F/F	—	—	—	—	—
		3	2 + 3 + 7	S/S	S/S	F/S	F/F	—	—	—	—
		2	2 + 2 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		1	2 + 1 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		0	2 + 0 + 7	S/S	S/S	F/S	F/S	F/F	F/F	F/F	—
	1	4	1 + 4 + 7	S/F	S/F	S/F	—	—	—	—	—
		3	1 + 3 + 7	S/S	S/S	S/F	S/F	—	—	—	—
		2	1 + 2 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		1	1 + 1 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		0	1 + 0 + 7	S/S	S/S	S/S	S/S	S/F	S/F	S/F	—
	0	0	0 + 0 + 7	S/S	S/S	S/S	S/S	S/S	S/S	S/S	F/F

[0342] To apply the legacy coding method to PUCCH feedback transmission or to obtain an error rate similar to that of the conventional feedback channel, 13 bits or less may be transmitted within one subframe. In this case, when using a subset including only too small a number of codebook elements, the probability that the codebook element for expressing a CSI appropriate for an actual channel state is contained in the corresponding subset is gradually decreased, resulting in a reduction in transmission throughput. Therefore, the number of feedback bits must be reduced and a subset of an appropriate level must be used.

[0343] For example, for Rank-1 and Rank-2, a maximum of 4 bits may be requested for each of i1 and i2. The subset of an index in which ‘(bits for the i1 indicator/bits for the i2 indicator)’ is set to any of (4/3), (4/2), (3/3), (3/2), (2/3), (2/2), etc. may be used as necessary.

[0344] In addition, the fullset or subset of index may be used according to rank. For example, in order to implement the level corresponding to a maximum of 11 bits, ‘2 bits/2 bits’ may be used for i1 and i2 (i1/i2). In this case, ‘2 bits/2 bits’ may be used at Ranks 1 to 4, ‘2 bits/0 bit’ may be used at Ranks 5 to 7, and ‘0 bit/0 bit’ may be used at Rank-8. Alternatively, in order to implement the level corresponding to a maximum of 13 bits, ‘3 bits/2 bits’ may be used for i1 and i2 (i1/i2). In this case, ‘3 bits/2 bits’ may be used at Ranks 1 and 2, ‘2 bits/4 bits’ may be used at Rank-3, ‘2 bits/3 bits’ may be used at Rank-4, ‘2 bits/0 bit’ may be used at Ranks 5 to 7, and ‘0 bit/0 bit’ may be used at Rank-8. Table 25 shows exemplary bit numbers capable of being used for i1 and i2 (i1/i2) for each rank.

TABLE 25
Rank	(i1/i2)
1	(4/2), (3/3), (3/2), (2/3), (2/2)
2	(4/2), (3/3), (3/2), (2/3), (2/2)
3	(2/4), (2/3), (2/2), (2/1), (2/0), (1/4), (1/3), (1/2), (1/1), (1/0)
4	(2/3), (2/2), (2/1), (2/0), (1/3), (1/2), (1/1)
5	(2/0)
6	(2/0)
7	(2/0)
8	(0/0)

[0345] Table 26 shows preferable combination of number of bits of i1/i2 of Table 25.

	TABLE 26
	Rank
i1/i2	1	2	3	4	5	6	7	8
	4/2	4/2	2/4	2/3	2/0	2/0	2/0	0/0
	3/3	3/3	2/4	2/3	2/0	2/0	2/0	0/0
	3/2	3/2	2/3	2/3	2/0	2/0	2/0	0/0
	2/2	2/2	2/2	2/2	2/0	2/0	2/0	0/0

[0346] Table 27 shows bits required either when the RI and the i1 index are simultaneously transmitted within one subframe or when the i2 index and the CQI are simultaneously transmitted within another subframe.

	TABLE 27
	Rank
RI	i1	i2	RI + i1	i2 + CQI	1	2	3	4	5	6	7	8
3	4	4	3 + 4	4 + 7	F/F	F/F	—	—	—	—	—	—
		3		3 + 7	F/S	F/S	—	—	—	—	—	—
		2		2 + 7	F/S	F/S	—	—	—	—	—	—
		1		1 + 7	F/S	F/S	—	—	—	—	—	—
		0		0 + 7	F/S	F/S	—	—	—	—	—	—
	3	4	3 + 3	4 + 7	S/F	S/F	—	—	—	—	—	—
		3		3 + 7	S/S	S/S	—	—	—	—	—	—
		2		2 + 7	S/S	S/S	—	—	—	—	—	—
		1		1 + 7	S/S	S/S	—	—	—	—	—	—
		0		0 + 7	S/S	S/S	—	—	—	—	—	—
	2	4	3 + 2	4 + 7	S/F	S/F	F/F	—	—	—	—	—
		3		3 + 7	S/S	S/S	F/S	F/F	—	—	—	—
		2		2 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		1		1 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		0		0 + 7	S/S	S/S	F/S	F/S	F/F	F/F	F/F	—
	1	4	3 + 1	4 + 7	S/F	S/F	S/F	—	—	—	—	—
		3		3 + 7	S/S	S/S	S/F	S/F	—	—	—	—
		2		2 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		1		1 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		0		0 + 7	S/S	S/S	S/S	S/S	S/F	S/F	S/F	—
	0	0	3 + 0	0 + 7	S/S	S/S	S/S	S/S	S/S	S/S	S/S	F/F
2	4	4	2 + 4	4 + 7	F/F	F/F	—	—	—	—	—	—
		3		3 + 7	F/S	F/S	—	—	—	—	—	—
		2		2 + 7	F/S	F/S	—	—	—	—	—	—
		1		1 + 7	F/S	F/S	—	—	—	—	—	—
		0		0 + 7	F/S	F/S	—	—	—	—	—	—
	3	4	2 + 3	4 + 7	S/F	S/F	—	—	—	—	—	—
		3		3 + 7	S/S	S/S	—	—	—	—	—	—
		2		2 + 7	S/S	S/S	—	—	—	—	—	—
		1		1 + 7	S/S	S/S	—	—	—	—	—	—
		0		0 + 7	S/S	S/S	—	—	—	—	—	—
	2	4	2 + 2	4 + 7	S/F	S/F	F/F	—	—	—	—	—
		3		3 + 7	S/S	S/S	F/S	F/F	—	—	—	—
		2		2 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		1		1 + 7	S/S	S/S	F/S	F/S	—	—	—	—
		0		0 + 7	S/S	S/S	F/S	F/S	—	—	—	—
	1	4	2 + 1	4 + 7	S/F	S/F	S/F	—	—	—	—	—
		3		3 + 7	S/S	S/S	S/F	S/F	—	—	—	—
		2		2 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		1		1 + 7	S/S	S/S	S/S	S/S	—	—	—	—
		0		0 + 7	S/S	S/S	S/S	S/S	—	—	—	—
	0	0	2 + 0	0 + 7	S/S	S/S	S/S	S/S	—	—	—	—
1	4	4	1 + 4	4 + 7	F/F	F/F	—	—	—	—	—	—
		3		3 + 7	F/S	F/S	—	—	—	—	—	—
		2		2 + 7	F/S	F/S	—	—	—	—	—	—
		1		1 + 7	F/S	F/S	—	—	—	—	—	—
		0		0 + 7	F/S	F/S	—	—	—	—	—	—
	3	4	1 + 3	4 + 7	S/F	S/F	—	—	—	—	—	—
		3		3 + 7	S/S	S/S	—	—	—	—	—	—
		2		2 + 7	S/S	S/S	—	—	—	—	—	—
		1		1 + 7	S/S	S/S	—	—	—	—	—	—
		0		0 + 7	S/S	S/S	—	—	—	—	—	—
	2	4	1 + 2	4 + 7	S/F	S/F	—	—	—	—	—	—
		3		3 + 7	S/S	S/S	—	—	—	—	—	—
		2		2 + 7	S/S	S/S	—	—	—	—	—	—
		1		1 + 7	S/S	S/S	—	—	—	—	—	—
		0		0 + 7	S/S	S/S	—	—	—	—	—	—
	1	4	1 + 1	4 + 7	S/F	S/F	—	—	—	—	—	—
		3		3 + 7	S/S	S/S	—	—	—	—	—	—
		2		2 + 7	S/S	S/S	—	—	—	—	—	—
		1		1 + 7	S/S	S/S	—	—	—	—	—	—
		0		0 + 7	S/S	S/S	—	—	—	—	—	—
	0	0	1 + 0	0 + 7	S/S	S/S	—	—	—	—	—	—

[0347] If the maximum number of Ranks reported by a UE is determined according to either a maximum rank capable of being received at the UE or a maximum rank to be transmitted from an eNB, a bit for Rank indication may be determined. Provided that RI and i1 are combined and simultaneously transmitted, a maximum number of bits requisite for feedback may be 7 (=3+4) bits, and a minimum number of bits may be 5 (=1+4) bits.

[0348] Rank information is basically used to select/calculate other feedback information, such that it is necessary to robustly transmit the rank information. Thus, it is preferable that the number of bits contained in a subframe corresponding to rank transmission be reduced as much as possible. For such transmission, a method for reducing the number of bits of the i1 indicator may be used as necessary. Considering the above-mentioned condition, Table 28 exemplarily shows bit numbers capable of being used for i1 and i2 (i1/i2) for each rank.

TABLE 28
Rank	(i1/i2)
1	(3/4), (3/3), (3/2), (2/4), (2/3), (2/2)
2	(3/4), (3/3), (3/2), (2/4), (2/3), (2/2)
3	(2/4), (2/3), (2/2), (2/1), (2/0), (1/4), (1/3), (1/2), (1/1), (1/0)
4	(2/3), (2/2), (2/1), (2/0), (1/3), (1/2), (1/1)
5	(2/0), (1/0)
6	(2/0), (1/0)
7	(2/0), (1/0)
8	(0/0)

[0349] In case of setting the subset of the i1/i2 indicators, for example, the i1 and i2 subsets may be designed to have different sizes according to a preferred rank. In another example, the i1 and i2 subsets may be designed to have different sizes according to UE category. The UE category may be classified according to UE capability.

[0350] A method for setting the codebook subset through different precoder indexes (i1/i2) according to the present invention will hereinafter be described in detail.

[0351] Table 29 shows another example of a codebook appropriate for Rank-1 CSI reporting shown in Table 11. A Rank-1 codeword may be configured on the basis of a 4 Tx DFT vector (v_m) and may be represented by a combination of the 4Tx DFT vector (v_m) and a phase (φ_n). If the i1 index is defined as 0 to 15 and the i2 index is defined as 0 to 15, the codebook may be configured by both v_m having a 32PSK (Phase Shift Keying) phase and φ_n having a QPSK (Quadrature PSK) phase. In this case, the same element may be repeated between contiguous indexes of the i1 value.

	TABLE 29
	i2
i1	0	1	2	3	4	5	6	7
0	V0	V0	V0	V0	V1	V1	V1	V1
	V0	jV0	−V0	−jV0	V1	jV1	−V1	−jV1
1	V2	V2	V2	V2	V3	V3	V3	V3
	V2	jV2	−V2	−jV2	V3	jV3	−V3	−jV3
2	V4	V4	V4	V4	V5	V5	V5	V5
	V4	jV4	−V4	−jV4	V5	jV5	−V5	−jV5
3	V6	V6	V6	V6	V7	V7	V7	V7
	V6	jV6	−V6	−jV6	V7	jV7	−V7	−jV7
4	V8	V8	V8	V8	V9	V9	V9	V9
	V8	jV8	−V8	−jV8	V9	jV9	−V9	−jV9
5	V10	V10	V10	V10	V11	V11	V11	V11
	V10	jV10	−V10	−jV10	V11	jV11	−V11	−jV11
6	V12	V12	V12	V12	V13	V13	V13	V13
	V12	jV12	−V12	−jV12	V13	jV13	−V13	−jV13
7	V14	V14	V14	V14	V15	V15	V15	V15
	V14	jV14	−V14	−jV14	V15	jV15	−V15	−jV15
8	V16	V16	V16	V16	V17	V17	V17	V17
	V16	jV16	−V16	−jV16	V17	jV17	−V17	−jV17
9	V18	V18	V18	V18	V19	V19	V19	V19
	V18	jV18	−V18	−jV18	V19	jV19	−V19	−jV19
10	V20	V20	V20	V20	V21	V21	V21	V21
	V20	jV20	−V20	−jV20	V21	jV21	−V21	−jV21
11	V22	V22	V22	V22	V23	V23	V23	V23
	V22	jV22	−V22	−jV22	V23	jV23	−V23	−jV23
12	V24	V24	V24	V24	V25	V25	V25	V25
	V24	jV24	−V24	−jV24	V25	jV25	−V25	−jV25
13	V26	V26	V26	V26	V27	V27	V27	V27
	V26	jV26	−V26	−jV26	V27	jV27	−V27	−jV27
14	V28	V28	V28	V28	V29	V29	V29	V29
	V28	jV28	−V28	−jV28	V29	jV29	−V29	−jV29
15	V30	V30	V30	V30	V31	V31	V31	V31
	V30	jV30	−V30	−jV30	V31	jV31	−V31	−jV31
	i2
i1	8	9	10	11	12	13	14	15
0	V2	V2	V2	V2	V3	V3	V3	V3
	V2	jV2	−V2	−jV2	V3	jV3	−V3	−jV3
1	V4	V4	V4	V4	V5	V5	V5	V5
	V4	jV4	−V4	−jV4	V5	jV5	−V5	−jV5
2	V6	V6	V6	V6	V7	V7	V7	V7
	V6	jV6	−V6	−jV6	V7	jV7	−V7	−jV7
3	V8	V8	V8	V8	V9	V9	V9	V9
	V8	jV8	−V8	−jV8	V9	jV9	−V9	−jV9
4	V10	V10	V10	V10	V11	V11	V11	V11
	V10	jV10	−V10	−jV10	V11	jV11	−V11	−jV11
5	V12	V12	V12	V12	V13	V13	V13	V13
	V12	jV12	−V12	−jV12	V13	jV13	−V13	−jV13
6	V14	V14	V14	V14	V15	V15	V15	V15
	V14	jV14	−V14	−jV14	V15	jV15	−V15	−jV15
7	V16	V16	V16	V16	V17	V17	V17	V17
	V16	jV16	−V16	−jV16	V17	jV17	−V17	−jV17
8	V18	V18	V18	V18	V19	V19	V19	V19
	V18	jV18	−V18	−jV18	V19	jV19	−V19	−jV19
9	V20	V20	V20	V20	V21	V21	V21	V21
	V20	jV20	−V20	−jV20	V21	jV21	−V21	−jV21
10	V22	V22	V22	V22	V23	V23	V23	V23
	V22	jV22	−V22	−jV22	V23	jV23	−V23	−jV23
11	V24	V24	V24	V24	V25	V25	V25	V25
	V24	jV24	−V24	−jV24	V25	jV25	−V25	−jV25
12	V26	V26	V26	V26	V27	V27	V27	V27
	V26	jV26	−V26	−jV26	V27	jV27	−V27	−jV27
13	V28	V28	V28	V28	V29	V29	V29	V29
	V28	jV28	−V28	−jV28	V29	jV29	−V29	−jV29
14	V30	V30	V30	V30	V31	V31	V31	V31
	V30	jV30	−V30	−jV30	V31	jV31	−V31	−jV31
15	V0	V0	V0	V0	V1	V1	V1	V1
	V0	jV0	−V0	−jV0	V1	jV1	−V1	−jV1

[0352] Accordingly, in order to configure the subset of a codebook, a method for limiting a phase of a DFT matrix constructing the vector of or the phase of φ_n, and a method for constructing the i1 value using different codebook elements at different i1 indexes of codebook elements contained in one i1 value may be considered. In this way, the codebook subset may be constructed.

[0353] According to whether the i1 or i2 subset is used, DFT vector of v_m and a phase of φ_n may be determined. For example, it is assumed that, in order to indicate the i1 value, 3 bits may be used and 8 even indexes (0, 2, 4, 6, 8, 10, 12, 14) may be used. It is also assumed that, in order to indicate the i1 value, 3 bits may be used and 8 indexes (0, 1, 2, 3, 8, 9, 10, 11) may be used. Under these assumptions, a 4Tx DFT vector having a 16PSK phase for the v_m value and a QPSK for the phase (φ_n) may be configured.

[0354] As described above, when deciding the indication bit for the i1 value and the indication bit for the i2 value, one phase of the 4Tx DFT vector for constructing the v_m value and the other phase for constructing the phase (φ_n) according to a combination of indexes appropriate for individual bits may be represented by the following table 30.

	TABLE 30
	Bit for i1	Bit for i2
	(elements	(elements
	number)	number)	vm	φn
1	2 (4n, n: 0~3)	1 (0, 1)	QPSK	{1, j}
2	2 (4n, n: 0~3)	1 (0, 2)	QPSK	BPSK
3	2 (4n, n: 0~3)	2 (0~3)	QPSK	QPSK
4	2 (4n, n: 0~3)	2 (2m, m: 0~3)	QPSK + QPSK(2pi/32)	BPSK
5	2 (4n, n: 0~3)	3 (0~7)	QPSK + QPSK(2pi/32)	QPSK
6	2 (4n, n: 0~3)	3 (0~3, 8~11)	QPSK + QPSK(2 ×	QPSK
			2pi/32)
7	2 (4n, n: 0~3)	3 (2m, m: 0~7)	QPSK + QPSK(2pi/32) +	BPSK
			QPSK(2 × 2pi/32) +
			QPSK(2 × 3pi/32)
8	2 (4n, n: 0~3)	4 (0~15)	QPSK + QPSK(2pi/32) +	QPSK
			QPSK(2 × 2pi/32) +
			QPSK(2 × 3pi/32)
9	3 (2n, n: 0~7)	1 (0, 1)	8 PSK	{1, j}
10	3 (2n, n: 0~7)	1 (0, 2)	8 PSK	BPSK
11	3 (2n, n: 0~7)	2 (0~3)	8 PSK	QPSK
12	3 (2n, n: 0~7)	2 (2m, m: 0~3)	8 PSK + 8	BPSK
			PSK(2pi/32)
13	3 (2n, n: 0~7)	3 (0~7)	8 PSK + 8	QPSK
			PSK(2pi/32)
14	3 (2n, n: 0~7)	3 (0~3, 8~11)	16 PSK	QPSK
15	3 (2n, n: 0~7)	3 (2m, m: 0~7)	32 PSK	BPSK
16	3 (2n, n: 0~7)	4 (0~15)	32 PSK	QPSK
17	4 (0~15)	1 (0, 1)	16 PSK	{1, j}
18	4 (0~15)	1 (0, 2)	16 PSK	BPSK
19	4 (0~15)	2 (0~3)	16 PSK	QPSK
20	4 (0~15)	2 (2m, m: 0~3)	32 PSK	BPSK
21	4 (0~15)	3 (0~7)	32 PSK	QPSK
22	4 (0~15)	3 (0~3, 8~11)	16 PSK (Overraped)	QPSK
23	4 (0~15)	3 (2m, m: 0~7)	32 PSK (Overraped)	BPSK
24	4 (0~15)	4 (0~15)	32 PSK (Overraped)	QPSK

[0355] Table 31 shows another example of a codebook appropriate for Rank-2 CSI reporting shown in Table 12. In the Rank-2 CSI report, 16 indexes (0 to 15) are defined for each of the i1 and i2 values.

	TABLE 31
	i2
	0	1	2	3
	1st	2nd	1st	2nd	1st	2nd	1st	2nd
i1	0	2	1	3	4	6	5	7
0	V0	V0	V0	V0	V1	V1	V1	V1
	V0	−V0	jV0	−jV0	V1	−V1	jV1	−jV1
1	V2	V2	V2	V2	V3	V3	V3	V3
	V2	−V2	jV2	−jV2	V3	−V3	jV3	−jV3
2	V4	V4	V4	V4	V5	V5	V5	V5
	V4	−V4	jV4	−j V4	V5	−V5	jV5	−j V5
3	V6	V6	V6	V6	V7	V7	V7	V7
	V6	−V6	jV6	−j V6	V7	−V7	jV7	−j V7
4	V8	V8	V8	V8	V9	V9	V9	V9
	V8	−V8	jV8	−j V8	V9	−V9	jV9	−j V9
5	V10	V10	V10	V10	V11	V11	V11	V11
	V10	−V10	jV10	−j V10	V11	−V11	jV11	−j V11
6	V12	V12	V12	V12	V13	V13	V13	V13
	V12	−V12	jV12	−j V12	V13	−V13	jV13	−j V13
7	V14	V14	V14	V14	V15	V15	V15	V15
	V14	−V14	jV14	−j V14	V15	−V15	jV15	−j V15
8	V16	V16	V16	V16	V17	V17	V17	V17
	V16	−V16	jV16	−j V16	V17	−V17	jV17	−j V17
9	V18	V18	V18	V18	V19	V19	V19	V19
	V18	−V18	jV18	−j V18	V19	−V19	jV19	−j V19
10	V20	V20	V20	V20	V21	V21	V21	V21
	V20	−V20	jV20	−j V20	V21	−V21	jV21	−j V21
11	V22	V22	V22	V22	V23	V23	V23	V23
	V22	−V22	jV22	−j V22	V23	−V23	jV23	−j V23
12	V24	V24	V24	V24	V25	V25	V25	V25
	V24	−V24	jV24	−j V24	V25	−V25	jV25	−j V25
13	V26	V26	V26	V26	V27	V27	V27	V27
	V26	−V26	jV26	−j V26	V27	−V27	jV27	−j V27
14	V28	V28	V28	V28	V29	V29	V29	V29
	V28	−V28	jV28	−j V28	V29	−V29	jV29	−j V29
15	V30	V30	V30	V30	V31	V31	V31	V31
	V30	−V30	jV30	−j V30	V31	−V31	jV31	−j V31
	i2
	4	5	6	6
	1st	2nd	1st	2nd	1st	2nd	1st	2nd
i1	8	10	9	11	12	14	13	15
0	V2	V2	V2	V2	V3	V3	V3	V3
	V2	−V2	jV2	−jV2	V3	−V3	jV3	−jV3
1	V4	V4	V4	V4	V5	V5	V5	V5
	V4	−V4	jV4	−j V4	V5	−V5	jV5	−j V5
2	V6	V6	V6	V6	V7	V7	V7	V7
	V6	−V6	jV6	−j V6	V7	−V7	jV7	−j V7
3	V8	V8	V8	V8	V9	V9	V9	V9
	V8	−V8	jV8	−j V8	V9	−V9	jV9	−j V9
4	V10	V10	V10	V10	V11	V11	V11	V11
	V10	−V10	jV10	−j V10	V11	−V11	jV11	−j V11
5	V12	V12	V12	V12	V13	V13	V13	V13
	V12	−V12	jV12	−j V12	V13	−V13	jV13	−j V13
6	V14	V14	V14	V14	V15	V15	V15	V15
	V14	−V14	jV14	−j V14	V15	−V15	jV15	−j V15
7	V16	V16	V16	V16	V17	V17	V17	V17
	V16	−V16	jV16	−j V16	V17	−V17	jV17	−j V17
8	V18	V18	V18	V18	V19	V19	V19	V19
	V18	−V18	jV18	−j V18	V19	−V19	jV19	−j V19
9	V20	V20	V20	V20	V21	V21	V21	V21
	V20	−V20	jV20	−j V20	V21	−V21	jV21	−j V21
10	V22	V22	V22	V22	V23	V23	V23	V23
	V22	−V22	jV22	−j V22	V23	−V23	jV23	−j V23
11	V24	V24	V24	V24	V25	V25	V25	V25
	V24	−V24	jV24	−j V24	V25	−V25	jV25	−j V25
12	V26	V26	V26	V26	V27	V27	V27	V27
	V26	−V26	jV26	−j V26	V27	−V27	jV27	−j V27
13	V28	V28	V28	V28	V29	V29	V29	V29
	V28	−V28	jV28	−j V28	V29	−V29	jV29	−j V29
14	V30	V30	V30	V30	V31	V31	V31	V31
	V30	−V30	jV30	−j V30	V31	−V31	jV31	−j V31
15	V0	V0	V0	V0	V1	V1	V1	V1
	V0	−V0	jV0	−jV0	V1	−V1	jV1	−jV1
	i2
	8	9	10	11
	1st	2nd	1st	2nd	1st	2nd	1st	2nd
i1	0	6	1	7	4	10	5	9
0	V0	V1	V0	V1	V1	V2	V1	V2
	V0	−V1	jV0	−jV1	V1	−V2	jV1	−jV2
1	V2	V3	V2	V3	V3	V4	V3	V4
	V2	−V3	jV2	−jV3	V3	−V4	jV3	−j V4
2	V4	V5	V4	V5	V5	V6	V5	V6
	V4	−V5	jV4	−j V5	V5	−V6	jV5	−j V6
3	V6	V7	V6	V7	V7	V8	V7	V8
	V6	−V7	jV6	−j V7	V7	−V8	jV7	−j V8
4	V8	V9	V8	V9	V9	V10	V9	V10
	V8	−V9	jV8	−j V9	V9	−V10	jV9	−j V10
5	V10	V11	V10	V11	V11	V12	V11	V12
	V10	−V11	jV10	−j V11	V11	−V12	jV11	−j V12
6	V12	V13	V12	V13	V13	V14	V13	V14
	V12	−V13	jV12	−j V13	V13	−V14	jV13	−j V14
7	V14	V15	V14	V15	V15	V16	V15	V16
	V14	−V15	jV14	−j V15	V15	−V16	jV15	−j V16
8	V16	V17	V16	V17	V17	V18	V17	V18
	V16	−V17	jV16	−j V17	V17	−V18	jV17	−j V18
9	V18	V19	V18	V19	V19	V20	V19	V20
	V18	−V19	jV18	−j V19	V19	−V20	jV19	−j V20
10	V20	V21	V20	V21	V21	V22	V21	V22
	V20	−V21	jV20	−j V21	V21	−V22	jV21	−j V22
11	V22	V23	V22	V23	V23	V24	V23	V24
	V22	−V23	jV22	−j V23	V23	−V24	jV23	−j V24
12	V24	V25	V24	V25	V25	V26	V25	V26
	V24	−V25	jV24	−j V25	V25	−V26	jV25	−j V26
13	V26	V27	V26	V27	V27	V28	V27	V28
	V26	−V27	jV26	−j V27	V27	−V28	jV27	−j V28
14	V28	V29	V28	V29	V29	V30	V29	V30
	V28	−V29	jV28	−j V29	V29	−V30	jV29	−j V30
15	V30	V31	V30	V31	V31	V0	V31	V0
	V30	−V31	jV30	−j V31	V31	−V0	jV31	−jV0
	i2
	12	13	14	15
	1st	2nd	1st	2nd	1st	2nd	1st	2nd
i1	0	14	1	13	4	14	5	15
0	V0	V3	V0	V3	V1	V3	V1	V3
	V0	−V3	jV0	−jV3	V1	−V3	jV1	−jV3
1	V2	V5	V2	V5	V3	V5	V3	V5
	V2	−V5	jV2	−j V5	V3	−V5	jV3	−j V5
2	V4	V7	V4	V7	V5	V7	V5	V7
	V4	−V7	jV4	−j V7	V5	−V7	jV5	−j V7
3	V6	V9	V6	V9	V7	V9	V7	V9
	V6	−V9	jV6	−j V9	V7	−V9	jV7	−j V9
4	V8	V11	V8	V11	V9	V11	V9	V11
	V8	−V11	jV8	−j V11	V9	−V11	jV9	−j V11
5	V10	V13	V10	V13	V11	V13	V11	V13
	V10	−V13	jV10	−j V13	V11	−V13	jV11	−j V13
6	V12	V15	V12	V15	V13	V15	V13	V15
	V12	−V15	jV12	−j V15	V13	−V15	jV13	−j V15
7	V14	V17	V14	V17	V15	V17	V15	V17
	V14	−V17	jV14	−j V17	V15	−V17	jV15	−j V17
8	V16	V19	V16	V19	V17	V19	V17	V19
	V16	−V19	jV16	−j V19	V17	−V19	jV17	−j V19
9	V18	V21	V18	V21	V19	V21	V19	V21
	V18	−V21	jV18	−j V21	V19	−V21	jV19	−j V21
10	V20	V23	V20	V23	V21	V23	V21	V23
	V20	−V23	jV20	−j V23	V21	−V23	jV21	−j V23
11	V22	V25	V22	V25	V23	V25	V23	V25
	V22	−V25	jV22	−j V25	V23	−V25	jV23	−j V25
12	V24	V27	V24	V27	V25	V27	V25	V27
	V24	−V27	jV24	−j V27	V25	−V27	jV25	−j V27
13	V26	V29	V26	V29	V27	V29	V27	V29
	V26	−V29	jV26	−j V29	V27	−V29	jV27	−j V29
14	V28	V31	V28	V31	V29	V31	V29	V31
	V28	−V31	jV28	−j V31	V29	−V31	jV29	−j V31
15	V30	V1	V30	V1	V31	V1	V31	V1
	V30	−V1	jV30	−jV1	V31	−V1	jV31	−jV1