METHOD FOR INTERFERENCE CANCELLATION IN WIRELESS COMMUNICATION SYSTEM AND APPARATUS THEREFOR

[0001] The present invention relates to a wireless communication system and, more particularly, to a method for receiving downlink signal, and an apparatus therefor.

[0002] Recently, various devices requiring machine-to-machine (M2M) communication and high data transfer rate, such as smartphones or tablet personal computers (PCs), have appeared and come into widespread use. This has rapidly increased the quantity of data which needs to be processed in a cellular network. In order to satisfy such rapidly increasing data throughput, recently, carrier aggregation (CA) technology which efficiently uses more frequency bands, cognitive ratio technology, multiple antenna (MIMO) technology for increasing data capacity in a restricted frequency, multiple-base-station cooperative technology, etc. have been highlighted. In addition, communication environments have evolved such that the density of accessible nodes is increased in the vicinity of a user equipment (UE). Here, the node includes one or more antennas and refers to a fixed point capable of transmitting/receiving radio frequency (RF) signals to/from the user equipment (UE). A communication system including high-density nodes may provide a communication service of higher performance to the UE by cooperation between nodes.

[0003] A multi-node coordinated communication scheme in which a plurality of nodes communicates with a user equipment (UE) using the same time-frequency resources has much higher data throughput than legacy communication scheme in which each node operates as an independent base station (BS) to communicate with the UE without cooperation.

[0004] A multi-node system performs coordinated communication using a plurality of nodes, each of which operates as a base station or an access point, an antenna, an antenna group, a remote radio head (RRH), and a remote radio unit (RRU). Unlike the conventional centralized antenna system in which antennas are concentrated at a base station (BS), nodes are spaced apart from each other by a predetermined distance or more in the multi-node system. The nodes can be managed by one or more base stations or base station controllers which control operations of the nodes or schedule data transmitted/received through the nodes. Each node is connected to a base station or a base station controller which manages the node through a cable or a dedicated line.

[0005] The multi-node system can be considered as a kind of Multiple Input Multiple Output (MIMO) system since dispersed nodes can communicate with a single UE or multiple UEs by simultaneously transmitting/receiving different data streams. However, since the multi-node system transmits signals using the dispersed nodes, a transmission area covered by each antenna is reduced compared to antennas included in the conventional centralized antenna system. Accordingly, transmit power required for each antenna to transmit a signal in the multi-node system can be reduced compared to the conventional centralized antenna system using MIMO. In addition, a transmission distance between an antenna and a UE is reduced to decrease in pathloss and enable rapid data transmission in the multi-node system. This can improve transmission capacity and power efficiency of a cellular system and meet communication performance having relatively uniform quality regardless of UE locations in a cell. Further, the multi-node system reduces signal loss generated during transmission since base station(s) or base station controller(s) connected to a plurality of nodes transmit/receive data in cooperation with each other. When nodes spaced apart by over a predetermined distance perform coordinated communication with a UE, correlation and interference between antennas are reduced. Therefore, a high signal to interference-plus-noise ratio (SINR) can be obtained according to the multi-node coordinated communication scheme.

[0006] Owing to the above-mentioned advantages of the multi-node system, the multi-node system is used with or replaces the conventional centralized antenna system to become a new foundation of cellular communication in order to reduce base station cost and backhaul network maintenance cost while extending service coverage and improving channel capacity and SINR in next-generation mobile communication systems.

[0007] An object of the present invention devised to solve the problem lies in a method for efficiently supporting interference cancellation in a wireless communication system.

[0008] Another object of the present invention is to provide a method for using information on a specific reference signal from a cell that may cause interference for interference cancellation.

[0009] The technical problems solved by the present invention are not limited to the above technical problems and those skilled in the art may understand other technical problems from the following description.

[0010] In an aspect of the present invention, provided herein is a method for interference cancellation for a UE having interference cancellation capability in a wireless communication system, the method comprising: receiving information on a reference signal transmitted from an interfering cell having the possibility of causing inter-cell interference; and attempting to detect the reference signal using the received information on the reference signal, wherein the information on the reference signal is received when the interfering cell and a serving cell of the UE have the same cyclic prefix (CP) length and are synchronized with each other.

[0011] Alternatively or additionally, the information on the reference signal may include an information set about a first reference signal and information sets about one or more second reference signals having a specific relationship with the first reference signal, and each of the information sets about the one or more second reference signals includes information indicating that a second reference signal defined by a corresponding information set has the specific relationship with the first reference signal.

[0012] Alternatively or additionally, the specific relationship may correspond to a relationship in which an antenna port through which the second reference signal is transmitted and an antenna port through which the first reference signal is transmitted are quasi co-located.

[0013] Alternatively or additionally, the method may further include detecting the first reference signal using the information on the first reference signal; estimating reception power of the one or more second reference signals using reception power of the detected first reference signal and power correction values included in the information sets about the one or more second reference signals; and attempting to detect only a second reference signal having estimated reception power higher than a predetermined value.

[0014] Alternatively or additionally, at least one of the information sets about the one or more second reference signals may include information indicating another information set which the at least one of the information sets refers to, the method further comprising if the at least one of the information sets includes the information indicating the another information set, attempting to detect the second reference signal defined by the at least one of the information sets only when a second reference signal defined by the another information set have been successfully detected.

[0015] Alternatively or additionally, the information set about the first reference signal and the information set about the second reference signal may further include values indicating transmission possibility of data channels based on the corresponding reference signals.

[0016] Alternatively or additionally, the information set about the first reference signal may be an information set about a cell-specific reference signal (CRS), the information set including an ID for identifying the information set, a physical cell ID of the interfering cell, the number of antenna ports for the CRS and an MBSFN (Multicast Broadcast Single Frequency Network) configuration

[0017] Alternatively or additionally, the each of the information sets about the one or more second reference signals may be an information set about a channel state information-reference signal (CSI-RS), the information set including at least one of an ID for identifying the information set, the number of antenna ports for the CSI-RS, configuration of an RE position and subframe in which the CSI-RS is transmitted, a scrambling ID for sequence generation for the CSI-RS, an ID of an information set about a CRS having a specific relationship with the CSI-RS, a type associated with the specific relationship or the transmission power ratio of the CSI-RS to the CRS having the specific relationship with the CSI-RS

[0018] Alternatively or additionally, the each of the information sets about the one or more second reference signals is an information set about a demodulation-reference signal (DM-RS), the information set including at least one of an ID for identifying the information set, a scrambling ID for sequence generation for the DM-RS, a scrambling identity field value, an antenna port index for the DM-RS, the transmission power ratio of the DM-RS to a CRS or a CSI-RS having a specific relationship with the DM-RS, a type associated with the specific relationship, an ID of an information set about the CRS having the specific relationship with the DM-RS or an ID of an information set about the CSI-RS having the specific relationship with the DM-RS

[0019] In another aspect of the present invention, provided herein is a UE having interference cancellation capability in a wireless communication system, comprising: a radio frequency (RF) unit; and a processor configured to control the RF unit, wherein the processor may be configured to receive information on a reference signal transmitted from an interfering cell having the possibility of causing inter-cell interference, to attempt to detect the reference signal using the received information on the reference signal, wherein the information on the reference signal may be received when the interfering cell and a serving cell of the UE have the same cyclic prefix (CP) length and are synchronized with each other.

[0020] The above description corresponds to part of embodiments of the present invention and various embodiments reflecting technical characteristics of the present invention are derived and understood by those skilled in the art on the basis of the following detailed description of the present invention.

[0021] According to an embodiment of the present invention, it is possible to efficiently perform interference cancellation in a wireless communication system.

[0022] According to an embodiment of the present invention, it is possible to perform more efficient interference cancellation by informing a UE of information on a specific reference signal from a cell that may cause interference.

[0023] The effects of the present invention are not limited to the above-described effects and other effects which are not described herein will become apparent to those skilled in the art from the following description.

[0033] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The accompanying drawings illustrate exemplary embodiments of the present invention and provide a more detailed description of the present invention. However, the scope of the present invention should not be limited thereto.

[0034] In some cases, to prevent the concept of the present invention from being ambiguous, structures and apparatuses of the known art will be omitted, or will be shown in the form of a block diagram based on main functions of each structure and apparatus. Also, wherever possible, the same reference numbers will be used throughout the drawings and the specification to refer to the same or like parts.

[0035] In the present invention, a user equipment (UE) is fixed or mobile. The UE is a device that transmits and receives user data and/or control information by communicating with a base station (BS). The term ‘UE’ may be replaced with ‘terminal equipment’, ‘Mobile Station (MS)’, ‘Mobile Terminal (MT)’, ‘User Terminal (UT)’, ‘Subscriber Station (SS)’, ‘wireless device’, ‘Personal Digital Assistant (PDA)’, ‘wireless modem’, ‘handheld device’, etc. A BS is typically a fixed station that communicates with a UE and/or another BS. The BS exchanges data and control information with a UE and another BS. The term ‘BS’ may be replaced with ‘Advanced Base Station (ABS)’, ‘Node B’, ‘evolved-Node B (eNB)’, ‘Base Transceiver System (BTS)’, ‘Access Point (AP)’, ‘Processing Server (PS)’, etc. In the following description, BS is commonly called eNB.

[0036] In the present invention, a node refers to a fixed point capable of transmitting/receiving a radio signal to/from a UE by communication with the UE. Various eNBs can be used as nodes. For example, a node can be a BS, NB, eNB, pico-cell eNB (PeNB), home eNB (HeNB), relay, repeater, etc. Furthermore, a node may not be an eNB. For example, a node can be a radio remote head (RRH) or a radio remote unit (RRU). The RRH and RRU have power levels lower than that of the eNB. Since the RRH or RRU (referred to as RRH/RRU hereinafter) is connected to an eNB through a dedicated line such as an optical cable in general, cooperative communication according to RRH/RRU and eNB can be smoothly performed compared to cooperative communication according to eNBs connected through a wireless link. At least one antenna is installed per node. An antenna may refer to an antenna port, a virtual antenna or an antenna group. A node may also be called a point. Unlink a conventional centralized antenna system (CAS) (i.e. single node system) in which antennas are concentrated in an eNB and controlled an eNB controller, plural nodes are spaced apart at a predetermined distance or longer in a multi-node system. The plural nodes can be managed by one or more eNBs or eNB controllers that control operations of the nodes or schedule data to be transmitted/received through the nodes. Each node may be connected to an eNB or eNB controller managing the corresponding node via a cable or a dedicated line. In the multi-node system, the same cell identity (ID) or different cell IDs may be used for signal transmission/reception through plural nodes. When plural nodes have the same cell ID, each of the plural nodes operates as an antenna group of a cell. If nodes have different cell IDs in the multi-node system, the multi-node system can be regarded as a multi-cell (e.g. macro-cell/femto-cell/pico-cell) system. When multiple cells respectively configured by plural nodes are overlaid according to coverage, a network configured by multiple cells is called a multi-tier network. The cell ID of the RRH/RRU may be identical to or different from the cell ID of an eNB. When the RRH/RRU and eNB use different cell IDs, both the RRH/RRU and eNB operate as independent eNBs.

[0037] In a multi-node system according to the present invention, which will be described below, one or more eNBs or eNB controllers connected to plural nodes can control the plural nodes such that signals are simultaneously transmitted to or received from a UE through some or all nodes. While there is a difference between multi-node systems according to the nature of each node and implementation form of each node, multi-node systems are discriminated from single node systems (e.g. CAS, conventional MIMO systems, conventional relay systems, conventional repeater systems, etc.) since a plurality of nodes provides communication services to a UE in a predetermined time-frequency resource. Accordingly, embodiments of the present invention with respect to a method of performing coordinated data transmission using some or all nodes can be applied to various types of multi-node systems. For example, a node refers to an antenna group spaced apart from another node by a predetermined distance or more, in general. However, embodiments of the present invention, which will be described below, can even be applied to a case in which a node refers to an arbitrary antenna group irrespective of node interval. In the case of an eNB including an X-pole (cross polarized) antenna, for example, the embodiments of the preset invention are applicable on the assumption that the eNB controls a node composed of an H-pole antenna and a V-pole antenna.

[0038] A communication scheme through which signals are transmitted/received via plural transmit (Tx)/receive (Rx) nodes, signals are transmitted/received via at least one node selected from plural Tx/Rx nodes, or a node transmitting a downlink signal is discriminated from a node transmitting an uplink signal is called multi-eNB MIMO or CoMP (Coordinated Multi-Point Tx/Rx). Coordinated transmission schemes from among CoMP communication schemes can be categorized into JP (Joint Processing) and scheduling coordination. The former may be divided into JT (Joint Transmission)/JR (Joint Reception) and DPS (Dynamic Point Selection) and the latter may be divided into CS (Coordinated Scheduling) and CB (Coordinated Beamforming). DPS may be called DCS (Dynamic Cell Selection). When JP is performed, more various communication environments can be generated, compared to other CoMP schemes. JT refers to a communication scheme by which plural nodes transmit the same stream to a UE and JR refers to a communication scheme by which plural nodes receive the same stream from the UE. The UE/eNB combine signals received from the plural nodes to restore the stream. In the case of JT/JR, signal transmission reliability can be improved according to transmit diversity since the same stream is transmitted from/to plural nodes. DPS refers to a communication scheme by which a signal is transmitted/received through a node selected from plural nodes according to a specific rule. In the case of DPS, signal transmission reliability can be improved because a node having a good channel state between the node and a UE is selected as a communication node.

[0039] In the present invention, a cell refers to a specific geographical area in which one or more nodes provide communication services. Accordingly, communication with a specific cell may mean communication with an eNB or a node providing communication services to the specific cell. A downlink/uplink signal of a specific cell refers to a downlink/uplink signal from/to an eNB or a node providing communication services to the specific cell. A cell providing uplink/downlink communication services to a UE is called a serving cell. Furthermore, channel status/quality of a specific cell refers to channel status/quality of a channel or a communication link generated between an eNB or a node providing communication services to the specific cell and a UE. In 3GPP LTE-A systems, a UE can measure downlink channel state from a specific node using one or more CSI-RSs (Channel State Information Reference Signals) transmitted through antenna port(s) of the specific node on a CSI-RS resource allocated to the specific node. In general, neighboring nodes transmit CSI-RS resources on orthogonal CSI-RS resources. When CSI-RS resources are orthogonal, this means that the CSI-RS resources have different subframe configurations and/or CSI-RS sequences which specify subframes to which CSI-RSs are allocated according to CSI-RS resource configurations, subframe offsets and transmission periods, etc. which specify symbols and subcarriers carrying the CSI RSs.

[0040] In the present invention, PDCCH (Physical Downlink Control Channel)/PCFICH (Physical Control Format Indicator Channel)/PHICH (Physical Hybrid automatic repeat request Indicator Channel)/PDSCH (Physical Downlink Shared Channel) refer to a set of time-frequency resources or resource elements respectively carrying DCI (Downlink Control Information)/CFI (Control Format Indicator)/downlink ACK/NACK (Acknowlegement/Negative ACK)/downlink data. In addition, PUCCH (Physical Uplink Control Channel)/PUSCH (Physical Uplink Shared Channel)/PRACH (Physical Random Access Channel) refer to sets of time-frequency resources or resource elements respectively carrying UCI (Uplink Control Information)/uplink data/random access signals. In the present invention, a time-frequency resource or a resource element (RE), which is allocated to or belongs to PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH, is referred to as a PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH RE or PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH resource. In the following description, transmission of PUCCH/PUSCH/PRACH by a UE is equivalent to transmission of uplink control information/uplink data/random access signal through or on PUCCH/PUSCH/PRACH. Furthermore, transmission of PDCCH/PCFICH/PHICH/PDSCH by an eNB is equivalent to transmission of downlink data/control information through or on PDCCH/PCFICH/PHICH/PDSCH.

[0041] FIG. 1 illustrates an exemplary radio frame structure used in a wireless communication system. FIG. 1(a) illustrates a frame structure for frequency division duplex (FDD) used in 3GPP LTE/LTE-A and FIG. 1(b) illustrates a frame structure for time division duplex (TDD) used in 3GPP LTE/LTE-A.

[0042] Referring to FIG. 1, a radio frame used in 3GPP LTE/LTE-A has a length of 10 ms (307200 Ts) and includes 10 subframes in equal size. The 10 subframes in the radio frame may be numbered. Here, Ts denotes sampling time and is represented as Ts=1/(2048*15 kHz). Each subframe has a length of 1 ms and includes two slots. 20 slots in the radio frame can be sequentially numbered from 0 to 19. Each slot has a length of 0.5 ms. A time for transmitting a subframe is defined as a transmission time interval (TTI). Time resources can be discriminated by a radio frame number (or radio frame index), subframe number (or subframe index) and a slot number (or slot index).

[0043] The radio frame can be configured differently according to duplex mode. Downlink transmission is discriminated from uplink transmission by frequency in FDD mode, and thus the radio frame includes only one of a downlink subframe and an uplink subframe in a specific frequency band. In TDD mode, downlink transmission is discriminated from uplink transmission by time, and thus the radio frame includes both a downlink subframe and an uplink subframe in a specific frequency band.

[0044] Table 1 shows DL-UL configurations of subframes in a radio frame in the TDD mode.

TABLE 1
	Downlink-
	to-Uplink
DL-UL	Switch-
config-	point	Subframe number
uration	periodicity	0	1	2	3	4	5	6	7	8	9
0	5 ms	D	S	U	U	U	D	S	U	U	U
1	5 ms	D	S	U	U	D	D	S	U	U	D
2	5 ms	D	S	U	D	D	D	S	U	D	D
3	10 ms	D	S	U	U	U	D	D	D	D	D
4	10 ms	D	S	U	U	D	D	D	D	D	D
5	10 ms	D	S	U	D	D	D	D	D	D	D
6	5 ms	D	S	U	U	U	D	S	U	U	D

[0045] In Table 1, D denotes a downlink subframe, U denotes an uplink subframe and S denotes a special subframe. The special subframe includes three fields of DwPTS (Downlink Pilot TimeSlot), GP (Guard Period), and UpPTS (Uplink Pilot TimeSlot). DwPTS is a period reserved for downlink transmission and UpPTS is a period reserved for uplink transmission. Table 2 shows special subframe configuration.

TABLE 2
	Normal cyclic prefix in downlink	Extended cyclic prefix in downlink
Special		UpPTS		UpPTS
subframe		Normal cyclic	Extended cyclic		Normal cyclic	Extended cyclic
configuration	DwPTS	prefix in uplink	prefix in uplink	DwPTS	prefix in uplink	prefix in uplink
0	6592 · Ts	2192 · Ts	2560 · Ts	7680 · Ts	2192 · Ts	2560 · Ts
1	19760 · Ts			20480 · Ts
2	21952 · Ts			23040 · Ts
3	24144 · Ts			25600 · Ts
4	26336 · Ts			7680 · Ts	4384 · Ts	5120 · Ts
5	6592 · Ts	4384 · Ts	5120 · Ts	20480 · Ts
6	19760 · Ts			23040 · Ts
7	21952 · Ts			12800 · Ts
8	24144 · Ts			—	—	—
9	13168 · Ts			—	—	—

[0046] FIG. 2 illustrates an exemplary downlink/uplink slot structure in a wireless communication system. Particularly, FIG. 2 illustrates a resource grid structure in 3GPP LTE/LTE-A. A resource grid is present per antenna port.

[0047] Referring to FIG. 2, a slot includes a plurality of OFDM (Orthogonal Frequency Division Multiplexing) symbols in the time domain and a plurality of resource blocks (RBs) in the frequency domain. An OFDM symbol may refer to a symbol period. A signal transmitted in each slot may be represented by a resource grid composed of N_RB^DL/UL*N_sc^RB subcarriers and N_symb^DL/UL OFDM symbols. Here, N_RB^DL denotes the number of RBs in a downlink slot and N_RB^UL denotes the number of RBs in an uplink slot. N_RB^DL and N_RB^UL respectively depend on a DL transmission bandwidth and a UL transmission bandwidth. N_symb^DL denotes the number of OFDM symbols in the downlink slot and N_symb^UL denotes the number of OFDM symbols in the uplink slot. In addition, N_sc^RB denotes the number of subcarriers constructing one RB.

[0048] An OFDM symbol may be called an SC-FDM (Single Carrier Frequency Division Multiplexing) symbol according to multiple access scheme. The number of OFDM symbols included in a slot may depend on a channel bandwidth and the length of a cyclic prefix (CP). For example, a slot includes 7 OFDM symbols in the case of normal CP and 6 OFDM symbols in the case of extended CP. While FIG. 2 illustrates a subframe in which a slot includes 7 OFDM symbols for convenience, embodiments of the present invention can be equally applied to subframes having different numbers of OFDM symbols. Referring to FIG. 2, each OFDM symbol includes N_RB^DL/UL*N_sc^RB subcarriers in the frequency domain. Subcarrier types can be classified into a data subcarrier for data transmission, a reference signal subcarrier for reference signal transmission, and null subcarriers for a guard band and a direct current (DC) component. The null subcarrier for a DC component is a subcarrier remaining unused and is mapped to a carrier frequency (f0) during OFDM signal generation or frequency up-conversion. The carrier frequency is also called a center frequency.

[0049] An RB is defined by N_symb^DL/UL (e.g. 7) consecutive OFDM symbols in the time domain and N_sc^RB (e.g. 12) consecutive subcarriers in the frequency domain. For reference, a resource composed by an OFDM symbol and a subcarrier is called a resource element (RE) or a tone. Accordingly, an RB is composed of N_symb^DL/UL*N_sc^RB REs. Each RE in a resource grid can be uniquely defined by an index pair (k, l) in a slot. Here, k is an index in the range of 0 to N_symb^DL/UL*N_sc^RB−1 in the frequency domain and 1 is an index in the range of 0 to N_symb^DL/UL−1.

[0050] Two RBs that occupy N_sc^RB consecutive subcarriers in a subframe and respectively disposed in two slots of the subframe are called a physical resource block (PRB) pair. Two RBs constituting a PRB pair have the same PRB number (or PRB index).

[0051] FIG. 3 illustrates a downlink (DL) subframe structure used in 3GPP LTE/LTE-A.

[0052] Referring to FIG. 3, a DL subframe is divided into a control region and a data region. A maximum of three (four) OFDM symbols located in a front portion of a first slot within a subframe correspond to the control region to which a control channel is allocated. A resource region available for PDCCH transmission in the DL subframe is referred to as a PDCCH region hereinafter. The remaining OFDM symbols correspond to the data region to which a physical downlink shared chancel (PDSCH) is allocated. A resource region available for PDSCH transmission in the DL subframe is referred to as a PDSCH region hereinafter. Examples of downlink control channels used in 3GPP LTE include a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), a physical hybrid ARQ indicator channel (PHICH), etc. The PCFICH is transmitted at a first OFDM symbol of a subframe and carries information regarding the number of OFDM symbols used for transmission of control channels within the subframe. The PHICH is a response of uplink transmission and carries an HARQ acknowledgment (ACK)/negative acknowledgment (NACK) signal.

[0053] Control information carried on the PDCCH is called downlink control information (DCI). The DCI contains resource allocation information and control information for a UE or a UE group. For example, the DCI includes a transport format and resource allocation information of a downlink shared channel (DL-SCH), a transport format and resource allocation information of an uplink shared channel (UL-SCH), paging information of a paging channel (PCH), system information on the DL-SCH, information about resource allocation of an upper layer control message such as a random access response transmitted on the PDSCH, a transmit control command set with respect to individual UEs in a UE group, a transmit power control command, information on activation of a voice over IP (VoIP), downlink assignment index (DAI), etc. The transport format and resource allocation information of the DL-SCH are also called DL scheduling information or a DL grant and the transport format and resource allocation information of the UL-SCH are also called UL scheduling information or a UL grant. The size and purpose of DCI carried on a PDCCH depend on DCI format and the size thereof may be varied according to coding rate. Various formats, for example, formats 0 and 4 for uplink and formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 3 and 3A for downlink, have been defined in 3GPP LTE. Control information such as a hopping flag, information on RB allocation, modulation coding scheme (MCS), redundancy version (RV), new data indicator (NDI), information on transmit power control (TPC), cyclic shift demodulation reference signal (DMRS), UL index, channel quality information (CQI) request, DL assignment index, HARQ process number, transmitted precoding matrix indicator (TPMI), precoding matrix indicator (PMI), etc. is selected and combined based on DCI format and transmitted to a UE as DCI.

[0054] In general, a DCI format for a UE depends on transmission mode (TM) set for the UE. In other words, only a DCI format corresponding to a specific TM can be used for a UE configured in the specific TM.

[0055] A PDCCH is transmitted on an aggregation of one or several consecutive control channel elements (CCEs). The CCE is a logical allocation unit used to provide the PDCCH with a coding rate based on a state of a radio channel. The CCE corresponds to a plurality of resource element groups (REGs). For example, a CCE corresponds to 9 REGs and an REG corresponds to 4 REs. 3GPP LTE defines a CCE set in which a PDCCH can be located for each UE. A CCE set from which a UE can detect a PDCCH thereof is called a PDCCH search space, simply, search space. An individual resource through which the PDCCH can be transmitted within the search space is called a PDCCH candidate. A set of PDCCH candidates to be monitored by the UE is defined as the search space. In 3GPP LTE/LTE-A, search spaces for DCI formats may have different sizes and include a dedicated search space and a common search space. The dedicated search space is a UE-specific search space and is configured for each UE. The common search space is configured for a plurality of UEs. Aggregation levels defining the search space is as follows.

TABLE 3
Search Space
	Aggregation		Number of PDCCH
Type	Level L	Size [in CCEs]	candidates M(L)
UE-specific	1	6	6
	2	12	6
	4	8	2
	8	16	2
Common	4	16	4
	8	16	2

[0056] A PDCCH candidate corresponds to 1, 2, 4 or 8 CCEs according to CCE aggregation level. An eNB transmits a PDCCH (DCI) on an arbitrary PDCCH candidate with in a search space and a UE monitors the search space to detect the PDCCH (DCI). Here, monitoring refers to attempting to decode each PDCCH in the corresponding search space according to all monitored DCI formats. The UE can detect the PDCCH thereof by monitoring plural PDCCHs. Since the UE does not know the position in which the PDCCH thereof is transmitted, the UE attempts to decode all PDCCHs of the corresponding DCI format for each subframe until a PDCCH having the ID thereof is detected. This process is called blind detection (or blind decoding (BD)).

[0057] The eNB can transmit data for a UE or a UE group through the data region. Data transmitted through the data region may be called user data. For transmission of the user data, a physical downlink shared channel (PDSCH) may be allocated to the data region. A paging channel (PCH) and downlink-shared channel (DL-SCH) are transmitted through the PDSCH. The UE can read data transmitted through the PDSCH by decoding control information transmitted through a PDCCH. Information representing a UE or a UE group to which data on the PDSCH is transmitted, how the UE or UE group receives and decodes the PDSCH data, etc. is included in the PDCCH and transmitted. For example, if a specific PDCCH is CRC (cyclic redundancy check)-masked having radio network temporary identify (RNTI) of “A” and information about data transmitted using a radio resource (e.g. frequency position) of “B” and transmission format information (e.g. transport block size, modulation scheme, coding information, etc.) of “C” is transmitted through a specific DL subframe, the UE monitors PDCCHs using RNTI information and a UE having the RNTI of “A” detects a PDCCH and receives a PDSCH indicated by “B” and “C” using information about the PDCCH.

[0058] A reference signal (RS) to be compared with a data signal is necessary for the UE to demodulate a signal received from the eNB. A reference signal refers to, a predetermined signal having a specific waveform, which is transmitted from the eNB to the UE or from the UE to the eNB and known to both the eNB and UE. The reference signal is also called a pilot. Reference signals are categorized into a cell-specific RS shared by all UEs in a cell and a modulation RS (DM RS) dedicated for a specific UE. A DM RS transmitted by the eNB for demodulation of downlink data for a specific UE is called a UE-specific RS. Both or one of DM RS and CRS may be transmitted on downlink. When only the DM RS is transmitted without CRS, an RS for channel measurement needs to be additionally provided because the DM RS transmitted using the same precoder as used for data can be used for demodulation only. For example, in 3GPP LTE(-A), CSI-RS corresponding to an additional RS for measurement is transmitted to the UE such that the UE can measure channel state information. CSI-RS is transmitted in each transmission period corresponding to a plurality of subframes based on the fact that channel state variation with time is not large, unlike CRS transmitted per subframe.

[0059] FIG. 4 illustrates an exemplary uplink subframe structure used in 3GPP LTE/LTE-A.

[0060] Referring to FIG. 4, a UL subframe can be divided into a control region and a data region in the frequency domain. One or more PUCCHs (physical uplink control channels) can be allocated to the control region to carry uplink control information (UCI). One or more PUSCHs (Physical uplink shared channels) may be allocated to the data region of the UL subframe to carry user data.

[0061] In the UL subframe, subcarriers spaced apart from a DC subcarrier are used as the control region. In other words, subcarriers corresponding to both ends of a UL transmission bandwidth are assigned to UCI transmission. The DC subcarrier is a component remaining unused for signal transmission and is mapped to the carrier frequency f0 during frequency up-conversion. A PUCCH for a UE is allocated to an RB pair belonging to resources operating at a carrier frequency and RBs belonging to the RB pair occupy different subcarriers in two slots. Assignment of the PUCCH in this manner is represented as frequency hopping of an RB pair allocated to the PUCCH at a slot boundary. When frequency hopping is not applied, the RB pair occupies the same subcarrier.

[0062] The PUCCH can be used to transmit the following control information.

• • [0063] Scheduling Request (SR): This is information used to request a UL-SCH resource and is transmitted using On-Off Keying (OOK) scheme.
  • [0064] HARQ ACK/NACK: This is a response signal to a downlink data packet on a PDSCH and indicates whether the downlink data packet has been successfully received. A 1-bit ACK/NACK signal is transmitted as a response to a single downlink codeword and a 2-bit ACK/NACK signal is transmitted as a response to two downlink codewords. HARQ-ACK responses include positive ACK (ACK), negative ACK (HACK), discontinuous transmission (DTX) and NACK/DTX. Here, the term HARQ-ACK is used interchangeably with the term HARQ ACK/NACK and ACK/NACK.
  • [0065] Channel State Indicator (CSI): This is feedback information about a downlink channel. Feedback information regarding MIMO includes a rank indicator (RI) and a precoding matrix indicator (PMI).

[0066] The quantity of control information (UCI) that a UE can transmit through a subframe depends on the number of SC-FDMA symbols available for control information transmission. The SC-FDMA symbols available for control information transmission correspond to SC-FDMA symbols other than SC-FDMA symbols of the subframe, which are used for reference signal transmission. In the case of a subframe in which a sounding reference signal (SRS) is configured, the last SC-FDMA symbol of the subframe is excluded from the SC-FDMA symbols available for control information transmission. A reference signal is used to detect coherence of the PUCCH. The PUCCH supports various formats according to information transmitted thereon. Table 4 shows the mapping relationship between PUCCH formats and UCI in LTE/LTE-A.

TABLE 4
		Number of
		bits per
PUCCH	Modulation	subframe,
format	scheme	Mbit	Usage	Etc.
1	N/A	N/A	SR (Scheduling
			Request)
1a	BPSK	1	ACK/NACK or	One
			SR + ACK/NACK	codeword
1b	QPSK	2	ACK/NACK or	Two
			SR + ACK/NACK	codeword
2	QPSK	20	CQI/PMI/RI	Joint coding
				ACK/NACK
				(extended
				CP)
2a	QPSK + BPSK	21	CQI/PMI/RI +	Normal CP
			ACK/NACK	only
2b	QPSK + QPSK	22	CQI/PMI/RI +	Normal CP
			ACK/NACK	only
3	QPSK	48	ACK/NACK or
			SR + ACK/
			NACK or
			CQI/PMI/RI +
			ACK/NACK

[0067] Referring to Table 4, PUCCH formats 1/1a/1b are used to transmit ACK/NACK information, PUCCH format 2/2a/2b are used to carry CSI such as CQI/PMI/RI and PUCCH format 3 is used to transmit ACK/NACK information.

[0068] Coordinated Multi-Point (CoMP) Transmission and Reception

[0069] According to improved system performance requirements of a 3GPP LTE-A system, CoMP transmission/reception technology (co-MIMO, collaborative MIMO or network MIMO) is proposed. CoMP technology may increase performance of a UE located at a cell edge and increase average sector throughput.

[0070] In general, in a multi-cell environment in which a frequency reuse factor is 1, performance and average sector throughput of a UE located at a cell edge may be reduced due to inter-cell interference (ICI). In order to reduce ICI, in a legacy LTE system, a method of enabling a UE located at a cell edge to have appropriate throughput and performance in an environment restricted by interference using a simple passive method such as fractional frequency reuse (FFR) via specific power control was applied. However, ICI is reduced or reused by a UE as a desired signal more preferably than reduction of frequency resources used per cell. In order to accomplish the above-described object, a CoMP transmission scheme is applicable.

[0071] The CoMP scheme applicable to downlink may be largely divided into a joint processing (JP) scheme and a coordinated scheduling/beamforming (CS/CB) scheme.

[0072] In the JP scheme, each point (BS) of a CoMP unit may use data. The CoMP unit refers to a set of BSs used in the CoMP scheme. The JP scheme may be classified into a joint transmission scheme and a dynamic cell selection scheme.

[0073] The joint transmission scheme refers to a scheme for simultaneously transmitting a PDSCH from a plurality of points (a part or the whole of the CoMP unit). That is, data transmitted to a single UE may be simultaneously transmitted from a plurality of transmission points. According to the joint transmission scheme, it is possible to coherently or non-coherently improve the quality of the received signals and to actively cancel interference with another UE.

[0074] The dynamic cell selection scheme refers to a scheme for transmitting a PDSCH from one point (of the CoMP unit). That is, data transmitted to a single UE at a specific time is transmitted from one point and the other points in the coordinated unit do not transmit data to the UE at that time. The point for transmitting the data to the UE may be dynamically selected.

[0075] According to the CS/CB scheme, the CoMP units may cooperatively perform beamforming of data transmission to a single UE. Although only a serving cell transmits the data, user scheduling/beamforming may be determined by the coordination of the cells of the CoMP unit.

[0076] In uplink, coordinated multi-point reception refers to reception of a signal transmitted by coordination among a plurality of geographically separated points. CoMP schemes applicable to uplink may be classified into Joint Reception (JR) and Coordinated Scheduling/Beamforming (CS/CB).

[0077] JR indicates that a plurality of reception points receives a signal transmitted through a PUSCH, the CS/CB scheme indicates that only one point receives a PUSCH, and user scheduling/beamforming is determined by the coordination of the cells of the CoMP unit.

[0078] A plurality of UL points (that is, receiving points (RPs)) is referred to as UL CoMP and a plurality of DL points (that is, transmitting points (TPs)) is referred to as DL CoMP.

[0079] Enhanced-PDCCH (EPDCCH)

[0080] In an LTE system of LTE Release 11 or later, as a solution for PDCCH capacity lack due to CoMP, multi user-multiple input multiple output (MU-MIMO) and PDCCH performance reduction due to inter-cell interference, an enhanced-PDCCH (EPDCCH) which may be transmitted via a conventional PDSCH region is considered. In the EPDCCH, in order to obtain precoding gain, unlike to an existing CRS based PDCCH, channel estimation may be performed based on a DMRS.

[0081] EPDCCH transmission may be divided into localized EPDCCH transmission and distributed EPDCCH transmission according to the configuration of a PRB pair used for EPDCCH transmission. Localized EPDCCH transmission means the case in which enhanced control channel elements (ECCEs) used to transmit one piece of DCI are contiguous in the frequency domain and specific precoding is applicable in order to obtain beamforming gain. For example, localized EPDCCH transmission may be based on consecutive ECCEs corresponding in number to an aggregation level. In contrast, distributed EPDCCH transmission means that one EPDCCH is transmitted on a PRB pair separated in the frequency domain and has frequency diversity gain. For example, distributed EPDCCH transmission may be based on ECCEs each including enhanced resource element groups (EREGs) included each PRB pair separated in the frequency domain.

[0082] A UE may perform blind decoding similarly to an existing LTE/LTE-A system, in order to receive/acquire control information (DCI) via an EPDCCH. More specifically, the UE may attempt (monitor) decoding of a set of EPDCCH candidates per aggregation level, for DCI formats corresponding to a configured transmission mode. Here, the set of EPDCCH candidates to be monitored may be referred to as an EPDCCH UE-specific search space and this search space may be set/configured per aggregation level. In addition, differently from the above-described existing LTE/LTE-A system, {1, 2, 4, 8, 16, 32} is possible as an aggregation level according to subframe type, CP length, the amount of available resources in a PRB pair, etc.

[0083] A UE, in which an EPDCCH is configured, indexes REs included in a PRB pair set in EREG units and indexes the EREGs in ECCE units. The UE may determine EPDCCH candidates configuring a search space based on the indexed ECCEs to perform blind decoding, thereby receiving control information. Here, the EREG corresponds to an REG of the existing LTE/LTE-A system and the ECCE corresponds to a CCE. One PRB pair may include 16 EREGs.

[0084] In addition, for each serving cell, one UE may configure one or two EPDCCH PRB sets for PDCCH monitoring via higher layer signaling.

[0085] In 3GPP LTE Rel-11, a UE, to which a CoMP scheme is applied, may estimate channels of TPs, which may potentially participate in CoMP, using channel state information-reference signal (CSI-RS) resources defined as a CoMP measurement set and feed CSI such as precoding matrix indicator (PMI), channel quality indicator (CQI) or rank indicator (RI) back to a serving cell thereof based on the estimated channel values. A network may configure a dynamic point selection (DPS) scheme for selecting a TP having relatively excellent channel quality based on the fed-back CSI information to enable the UE to perform data transmission, a coordinated scheduling/coordinated beamforming (CS/CB) scheme for, at TPs participating in CoMP, controlling scheduling and beamforming to reduce mutual interference and a joint transmission (JT) scheme for, at TPs participating in CoMP, transmitting the same data to the UE.

[0086] The present invention relates to information provided by a network (or an eNB) and an inter-network coordination scheme in order to improve received signal performance of a UE including a high-performance receiver having interference cancellation (IC) capabilities.

[0087] In general, a cellular mobile communication system is an interference-limited system due to inter-cell interference in an urban environment and reaches system capacity limit. In addition, if a MIMO scheme, that is, an SU-MIMO scheme or a MU-MIMO scheme, is applied such that one eNB transmits a multi-layer signal of multiple beams, inter-layer interference in a cell is also a main factor for deciding system capacity limit. Therefore, in order to reduce inter-cell interference and intra-cell interference, standardization and development of a coordinated transmission and high-performance receiver scheme becomes important and numerous effects in that direction have been made.

[0088] A downlink CoMP scheme configures transmission beams such that inter-cell interference and intra-cell interference are minimized in a transmitter based on channel state information received from a receiver. In the downlink CoMP scheme, complexity of the UE in a data reception process is not increased, but performance of the CoMP scheme largely depends on accuracy of a channel state information report. The high-performance receiver scheme obtains better reception performance using properties of an interference signal at a receiver. In the high-performance receiver scheme, how the UE acquires information on the interference signal transmitted along with a signal (that is, a desired signal) scheduled thereto becomes important. Representative examples of the high-performance receiver scheme may include:

• • [0089] linear MMSE IRC receiver,
  • [0090] maximum likelihood detection receiver, and
  • [0091] interference cancellation receiver.

[0092] As performance increases, information on a larger number of interference signals is required. For example, in an iterative decoding interference cancellation receiver known to have highest performance, since a UE decodes an interference signal and regenerates an interference signal in order to cancel interference, all information for decoding the interference signal is necessary.

[0093] In the present specification, a method for cancelling an interference signal from a received signal after demodulating the interference signal without decoding will be focused upon. In particular, as a method for cancelling a co-scheduled interference signal, an interference cancellation method using a DM-RS of an interference signal on the assumption that a PDSCH is transmitted based on the DM-RS will be focused upon.

[0094] If a PDSCH co-scheduled to an RB scheduled to a specific UE is an interference signal, in order to cancel the interference signal, an eNB must provide information on the interference signal to a UE. In order to estimate an interference level using a DM-RS, the UE must know a sequence of the DM-RS of the interference signal. Therefore, the eNB must provide seed information of the sequence of the DM-RS of the interference signal to the UE and the UE estimates/cancels the interference signal using the seed information of the sequence of the DM-RS.

[0095] First, as interference received by a specific UE, there may be a signal of another layer scheduled to the specific UE. As shown in FIGS. 5(a) and 5(b), in case of single-cell SU-MIMO and multi-cell SU-MIMO, interference from another layer co-scheduled to a specific RB must be cancelled. In this case, all necessary information for interference cancellation is included in a DL control channel transmitted to a UE.

[0096] Unlike FIGS. 5(a) and 5(b), in FIGS. 5(c) and 5(d), when a UE receives control information of a PDSCH, control information of an interference signal is not received. FIGS. 5(c) and 5(d) show examples of single-cell SU-MIMO and multi-cell SU-MIMO, respectively. In the present specification, a method for improving interference cancellation performance of a receiver of a UE in the examples shown in FIGS. 5(c) and 5(d) is proposed.

[0097] Hereinafter, embodiment(s) of the present invention will be described. In the present specification, assume that a signal received by a UE includes a desired signal and an interference signal. That is, the desired signal is a downlink signal scheduled to the UE and the interference signal corresponds to a downlink signal scheduled to UE(s) different from the UE.

[0098] In addition, in the present specification, UE(s) scheduled to receive the interference signal is referred to as an “interference UE”.

[0099] First, in case of single-cell MU-MIMO of FIG. 5(c), since one eNB performs scheduling, coordination between eNBs is not necessary. Since both a desired signal (that is, a signal scheduled to a UE) and an interference signal are transmitted from a single eNB, if these two signals are based on a DM-RS, a seed value of a DM-RS sequence of the signals is selected from a single set {n_ID^DRMR,0, n_ID^DRMS,1, N_ID^cell}. Accordingly, when the UE knows nSCID={0, 1} used for the interference signal, the DM-RS sequence may be generated and a channel value of the interference signal may be estimated using the generated DM-RS sequence. In addition, the eNB must signal all ranks of PDSCHs scheduled in a corresponding subframe in addition to the rank of the desired signal for the purpose of indicating DM-RS density used for scheduled PDSCH transmission PRBs. Additionally, the eNB may inform the UE of a modulation order of the interference signal transmitted to the interference UE in addition to the rank used per nSCID in the corresponding subframe (or the number of scheduled layers) and information on the desired signal to the specific UE. When the interference signal is transmitted using two codewords and the modulation orders of the two codewords are the same, the eNB may inform the UE of one modulation order value and that the modulation orders are the same, thereby reducing signaling overhead.

[0100] In summary, in the embodiment shown in FIG. 5(c), the eNB may provide the following information to the UE such that the UE estimates interference using the RS of the interference signal.

• • [0101] When the interference signal is a DM-RS based PSDCH,
    • [0102] whether nSCID used to generate the DM-RS sequence of the interference or nSCID which is not used for the desired signal is used for the interference signal
    • [0103] number of layers (ranks) per nSCID
    • [0104] total number of ranks scheduled in the corresponding subframe
    • [0105] modulation order per codeword (nSCID)
  • [0106] when the interference signal is a CRS based PDSCH,
    • [0107] seed value (physical cell identifier) information of a CRS sequence of the interference signal, the number of CRS ports, CRS frequency shift and MBSFN configuration information
    • [0108] transmitted PMI (TPMI) information of the interference signal
    • [0109] PMI restriction information: This enables the interference eNB to use only a specific PMI set to aid interference estimation. In single-cell operation, codebook restriction information may be delivered. In multi-cell operation, such information should be transmitted between eNBs and should be delivered to the UE. Alternatively, this indicates that a specific TPMI is not used. The UE may blindly detect PMI of the interference UE in the restricted set.

[0110] Information on the interference signal may be dynamically transmitted in a state of being included in downlink control information (DCI) of the desired signal. The eNB transmits control information of the desired signal and control information of the interference signal when transmitting control information to the UE. For example, as shown in Table 5 below, the eNB provides additional information (DCI) for interference cancellation to the UE and the UE estimates an interference level using the additional information and cancels interference from a received signal.

TABLE 5
	Additional DCI (control
	information for interference
Existing DCI	cancellation)
Control information of UE	Information on interference
(RB allocation, TPC, HARQ,	UE(s)/layer(s) (nSCID, number of
{MCS, NDI, RV″ for CW1,	layers (ranks) per nSCID, modulation
{MCS, NDI, RV} for CW2}	order per codeword, all ranks)

[0111] Hereinafter, an embodiment of network signaling for aiding interference cancellation of a UE when a network performs multi-cell MU-MIMO scheduling as shown in FIG. 5(d) will be described. In the scenario shown in FIG. 5(d), a plurality of UEs is scheduled to the same PRB and a target UE receives supplementary information (or control information) for cancelling an interference signal from a serving cell thereof in order to increase reception performance of a desired signal.

[0112] Prior to a detailed description, in order to provide the supplementary information for cancelling the interference signal to the target UE in the scenario shown in FIG. 5(d), since scheduling information of a neighbor eNB should be known, network coordination between eNBs is necessary. A coordination degree between eNBs and the type of information signaled to the UE may be changed according to speed and latency of a backhaul link between eNBs.

[0113] The backhaul link may roughly be divided into three types.

[0114] Ideal backhaul (non-X2) link: As considered in existing LTE Rel.11 CoMP, coordinated eNBs establishes a CoMP cluster and cells in the same CoMP cluster are connected via a backhaul link such as an optical fiber having high capacity and low latency for coordinated scheduling and coordinated data transmission and reception, such that coordinated scheduling is possible, and accurate time synchronization is performed such that coordinated data transmission is possible. In addition, when signals transmitted from cells in the CoMP cluster participating in coordinated transmission are received, it may be assumed that a difference between reception times of signals transmitted from cells due to a propagation delay difference between the cells is in a cyclic prefix (CP) length of an OFDM symbol. In this case, in order to aid interference cancellation of the UE in every subframe, necessary information including dynamic information which may be changed according to subframe may be more accurately provided to the UE via dynamic signaling.

[0115] Slow backhaul: This is a general backhaul link having latency of several ms to several tens of ms, in which dynamic information transmission for inter-eNB coordination is impossible. In this link, only coordination such as deliver of semi-static information to neighbor eNB is possible as inter-eNB coordination.

[0116] Fast backhaul: This is between the ideal backhaul link and the slow backhaul link and certain fast inter-eNB coordination (for example, latency is equal to or less than 1 ms) may be possible. In addition to information on semi-static neighbor eNBs, restrictive information may be provided to the UE via dynamic signaling to aid interference cancellation of the UE.

[0117] As shown in FIG. 5(d), in multi-cell MU-MIMO, the target UE receives the following information from the network in order to cancel the interference signal. Similarly to FIG. 5(c), a method for demodulating the interference signal without decoding to cancel and/or suppress interference will be focused upon.

[0118] 2-1. Case in which Interference Signal is DM-RS Based Signal

[0119] When the amount of interference of the interference signal is estimated and cancelled using a DM-RS, a seed value of a DM-RS sequence is necessary. This seed value includes a cell ID value (a vertical cell ID and a physical cell ID) and nSCID information used when a neighbor cell generates a DM-RS sequence used for UE scheduling upon interfering with the neighbor cell. In addition, in order to increase DM-RS estimation performance, a channel estimate of a CSI-RS and a CRS having density higher than that of a DM-RS may be used and information thereon is necessary. In LTE Rel. 11, this is defined as quasi co-location assumption and is signaled to the UE. That is, the eNB may provide, to the target UE, with which CSI-RS or CRS the QCL assumption of an interference DM-RS sequence is possible. In order to check the density of the DM-RS in a corresponding subframe, the target also requires all rank information in a PRB scheduled in the corresponding subframe. The same effect can be obtained by signaling rank information per (VCID and nSCID) pair to the UE. In addition, a modulation order per codeword of the interference signal may be signaled to the target UE.

[0120] Hereinafter, a method for signaling the above-described information will be described. Although a detailed signaling scheme may be changed according to the speed and latency of the backhaul link, a signaling scheme considering a slow backhaul link to a signaling scheme considering an ideal backhaul link are proposed in the present specification.

[0121] Even when it is difficult for eNBs to exchange dynamic information such as UE scheduling information changed according to subframe, it is possible to semi-statically provide supplementary information for interference signal cancellation to the UE. That is, the eNB may select a candidate group of cells which may interfere with the target UE, receive DM-RS sequence information and QCL information used by the cells from the cells, configure a set of supplementary information for interference signal cancellation and provide the set of supplementary information to the target UE via higher layer signaling. In addition, the eNB may explicitly inform the UE of one of the set of supplementary information (as “bit value” of Table 6 below). The target UE may estimate and cancel the interference signal based on one of the set of supplementary information.

[0122] When the eNBs may exchange dynamic information such as UE scheduling information changed according to subframe, the eNB may check the properties of the interference signal to be transmitted on the current subframe via information exchange with the neighbor cell and select and transmit some of the set of supplementary information already provided via physical layer signaling to the target UE, and the target UE may detect the DM-RS sequence of the interference signal in some of the set to perform interference estimation.

[0123] In the above-described two cases, the target UE uses the set of supplementary information received from the eNB to detect whether the interference signal (that is, an interference PDSCH signal) in the candidate set is present in the corresponding subframe. That is, the supplementary information indicated by the PDCCH may be used in the former case and some (subset) of the set of the supplementary information may be used in the latter case. The target UE uses the received supplementary information (e.g., CRS or DM-RS sequence information, QCL information, etc.) to determine whether a reference signal exceeding predetermined reception energy is detected in the corresponding subframe. The target UE estimates an interference channel from the detected reference signal exceeding the predetermined reception energy, detects an interference PDSCH transmitted therewith, and cancels the detected reference signal and an interference PDSCH signal from the entire received signal.

[0124] Table 6 below shows an example of information delivered via semi-static signaling of such information.

TABLE 6
	DM-RS information		QCL assumption	Rate Matching information
	VCID(virtual			(non-zero		CRS information			PDSCH
	cell ID)			power)	PCID	(CRSPortsNumber,	(NZP)	ZP	starting	Modulation
Bit	∈{0, 1,	nSCID	Rank	CSI-RS	(Physical	FrequencyShift,	CSI-RS	CSI-RS	Symbol	order
value	2, . . . , 503}	∈{0, 1}	Restriction	index	Cell ID)	MBSFNsubframe pattern)	index	index	index	restriction
0	nVCID(0)	{0, 1}	—	CSI-RS	nPCID(0)	PortNum0, Freqshift0,	2	—
				index(0)		MBSFN_subframe_0
1	nVCID(1)	{0}	1	CSI-RS		PortNum1, Freqshift1,	1	2
				index(1)		MBSFN_subframe_1
2	nVCID(2)	{0}	2		nPCID(2)	PortNum2, Freqshift2,	3	4
						MBSFN_subframe_2
						. . .

[0125] [Seed Value Set of DM-RS Sequence]

[0126] The eNB determines a candidate set which may cause dominant interference with the target UE and transmits the candidate set to the target UE as shown in Table 6. First, a VCID having a value in a range from 0 to 503, which is a seed value of a DM-RS sequence, and an nSCID having any one of 0 or 1 are provided to the target UE. The VCID and the nSCID are referred to as “DM-RS related information”. nSCID may have a value of 0 or 1. If both nSCID values 0 and 1 corresponding to the VCID are used, the UE may be informed of these two values. Alternatively, the UE may not be informed of the nSCID value corresponding to the VCID. That is, when the UE is informed of both the nSCID values 0 and 1 corresponding to nVCID(0) in Table 1 above, the UE may generate a DM-RS sequence using nVCID(0) and nSCID=0 to estimate interference and generate a DM-RS sequence using nVCID(0) and nSCID=1 to estimate interference.

[0127] If the nVCID(0) value is signaled and the nSCID field value is omitted, the UE generates the DM-RS sequence with respect to all nSCID values (e.g., 0 and 1) corresponding to nVCID(0) to estimate interference. However, in Table 6, if the nSCID value corresponding to a specific VCID is restricted to one value, that is, bit value=1, the UE assumes that the DM-RS sequence of the interference signal using the VCID is generated using the specified nSCID only and performs interference estimation of the DM-RS sequence generated using the specified nSCID only. For example, since only nSCID={0} is signaled with respect to nVCID(1), interference estimation using the DM-RS sequence is performed with respect to nVCID(1) in consideration of nSCID=0 only.

[0128] [Rank Restriction]

[0129] Rank restriction information may indicate restriction of the rank of the interference signal using the DM-RS. If information on this field is omitted, the UE may blindly detect rank information of the interference signal corresponding to the DM-RS seed value and use the rank information for interference cancellation. However, if a specific rank value is signaled in a rank restriction field, the UE is informed that a maximum rank is restricted to the specific rank value. Accordingly, the UE assumes that the rank greater than the value signaled with respect to the DM-RS sequence (the DM-RS sequence by the signaled VCID value and nSCID) corresponding to the seed value is not used and does not detect the interference DM-RS sequence with respect to the rank greater than the signaled value. For example, in Table 6 above, in case of bit value=0, nSCID=O and nSCID=1 are used with respect to nVCID(0) to inform the UE that the interference DM-RS sequence may be used. If both nSCID values {0, 1} are signaled without rank restriction, the DM-RS sequence may be blindly detected from rank=1 to rank=8 with respect to nVCID(0) and nSCID0 and may be detected from rank=1 to rank=2 with respect to nSCID1, thereby performing IC. When the rank is restricted with respect to each nSCID value, a maximum rank value supported by each value is predetermined between the eNB and the UE and, when the rank restriction field value is not signaled, the UE blindly detects the DM-RS sequence up to the predetermined maximum rank. In 3GPP Rel.11, the maximum rank is restricted to 8 in case of nSCID=0 and is restricted to 2 in case of nSCID=1.

[0130] When bit value=1 in Table 6 above, nSCID is restricted to 0 with respect to nVCID(1) and, at this time, the rank restriction field is 1. In this case, the UE is informed that the rank is restricted to 1 with respect to the interference DM-RS sequence in case of nVCID(1) and NSCID=0. Thus, the UE performs the IC operation with respect to the DM-RS sequence in consideration of rank=1 only without considering interference in case of rank=2. In order to provide such rank restriction information to the UE, rank coordination between eNBs is necessary. That is, information indicating that a rank greater than a predetermined rank is not scheduled with respect to a specific DM-RS sequence, that is, that a specific rank or less is scheduled, during a predetermined time should be exchanged between eNBs.

[0131] [Modulation Order or MCS Restriction]

[0132] Modulation order restriction information indicates whether the modulation order of the interference signal using the DM-RS sequence is restricted. If information on this field is omitted, the UE blindly detects the modulation order of the interference signal corresponding to the seed value of the DM-RS sequence and uses the modulation order for interference cancellation. However, when a specific modulation order value is signaled in the modulation order restriction field, the UE is informed that a maximum modulation order is restricted to the specific modulation order and the UE blindly demodulates the DM-RS of the interference signal using the specific modulation order value or less. Here, modulation order={2, 4, 6} indicate QPSK, 16QAM and 64QAM, respectively. Of course, a higher modulation order value may be signaled. For example, with respect to the DM-RS having a modulation order restriction value of 4, the UE indicates that the DM-RS is modulated with QPSK or 16QAM. The UE performs demodulation on the assumption that that the DM-RS is modulated with QPSK to detect coordinates in constellation, performs demodulation on the assumption that the DM-RS is modulated with 16QAM to detect coordinates in constellation, determines with which of the two modulation order values modulation is performed, and estimates a channel of an interference signal using the modulation order, thereby cancelling interference.

[0133] As another embodiment, the maximum modulation order is set but, instead, a modulation order may be accurately specified to 2, 4 or 6. A specific modulation order may be indicated. In this case, the UE performs modulation using the specific modulation order only, thereby increasing interference cancellation efficiency. Additionally, blind decoding may be performed using the indicated modulation order.

[0134] Similarly, instead of modulation order restriction, modulation and coding scheme (MCS) information may be signaled to the UE. The UE is not informed of the modulation order only, but is informed of the modulation order and a code rate. That is, the UE is informed that the MCS is restricted to a specific value or less. Accordingly, the UE is aware that the interference signal corresponding to the seed value of the DM-RS sequence is modulated and coded at the specific MCS level or less, blindly detects the modulation and code rate of the interference DM-RS sequence in a given range only, and estimates the channel of the interference signal using the DM-RS, thereby cancelling interference. The MCS table for the PDSCH defined in current LTE Rel.11 is a combination of a modulation order and a transport block size (TBS) index.

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

[0135] That is, the MCS index indicates the modulation order and the TBS index, the modulation order has values of {2, 4, 6}, which indicate QPSK, 16QAM and 64QAM, respectively. The TBS index is an indirect code rate indicator and the code rate may be determined according to PDSCH RB allocation and the number of layers. Accordingly, in the present invention, when MCS restriction information is provided, the MCS is restricted to less than specific value, that is, IMCS<10, or the MCS restriction such as 10≦IMCS≦16 is provided, such that the modulation order is precisely restricted. Accordingly, it is possible to perform blind decoding within the indicated modulation order. Such MCS restriction information may be used not only for a demodulation based IC receiver but also for a code based IC receiver.

[0136] In order to provide the modulation order or MCS restriction information to the UE, coordination for modulation order or MCS between eNBs is necessary. That is, information indicating that a greater than predetermined modulation order or MCS level is not scheduled with respect to a specific DM-RS sequence, that is, that a less than predetermined modulation order or MCS level is scheduled, or indicating that a predetermined modulation order or MCS level is scheduled during a predetermined time should be exchanged between eNBs. Information on the modulation order or MCS level restriction is equally applicable to a CRS based PDSCH interference signal. When the modulation order and MCS level restriction information is signaled with respect to a PDSCH modulated with a specific CRS, the UE may perform the above-described operation on the assumption that the modulation order or MCS level of the interference signal is restricted to the signaled value or less.

[0137] [DM-RS Sequence with Restrictions]

[0138] For interference cancellation of the UE, rank restriction or modulation order (or MCS level) restriction between eNBs is set in order to enable the target UE to accurately demodulate and/or decode the interference signal. In general, since the reception SNR of the interference signal is lower than the reception SNR of the desired signal, the UE may not detect and cancel the interference signal by providing information on the interference signal only. In order for the target UE to demodulate and/or decode the interference signal, IC is accurately performed by decreasing the rank and the modulation order or the MCS level, thereby increasing reception performance of the desired signal. As a result, in a structure in which the UE is enabled to perform IC via network coordination, a maximum rank or a high modulation order (or MCS) may not be used even when the signal of the UE is scheduled. Accordingly, the eNB may separately manage DM-RS sequences and resources in order to improve overall system throughput and UE throughput. That is, a PDSCH may be scheduled to UEs, which are located at the center of a cell to have good geometry, using a DM-RS sequence without rank or modulation order restriction and may be scheduled to UEs, which are located at the edge of the cell to interfere with a neighbor cell, using a DM-RS sequence with rank or modulation order restriction.

[0139] [QCL Assumption]

[0140] In order to increase performance of interference channel estimation using a DM-RS sequence, the eNB provides QCL assumption. QCL assumption is to use a channel property value of another RS having a channel property equal to or similar to that of a specific DM-RS sequence while the density of an RS is higher than that of the DM-RS to increase channel estimation performance of the DM-RS sequence in interference channel estimation of the specific DM-RS sequence. QCL for each antenna port is defined in 3GPP LTE Rel.11. With respect to QCL, two behaviors are defined: Behavior A in which a CRS, a DM-RS and a CSI-RS are transmitted from a serving cell and all antenna ports have the same channel property and Behavior B in which a DM-RS has the same channel property as a specific CSI-RS in modulation of the PDSCH. In Behavior B, QCL between the DM-RS and the CSI-RS and QCL with a specific CRS in a frequency property may be signaled to the UE.

[0141] Accordingly, QCL assumption may be delivered with respect to each sequence in a DM-RS sequence candidate group and, for example, a DM-RS sequence and a specific (non-zero power) CSI-RS index may be mapped and delivered. The method of one embodiment of the present invention is not limited to the CoMP structure defined in 3GPP LTE Rel. 11 and thus the CSI-RS index is not limited to the CSI-RS configured for CSI feedback of the UE in the CoMP scheme. Since the UE may not feed CSI back in response to the signaled CSI-RS index, the number of CSI-RSs to be measured by the UE is unnecessarily increased in order to obtain the channel property of the CSI-RS as information aiding estimation of the specific DM-RS sequence. Accordingly, as QCL information for estimating the interference channel of the specific DM-RS sequence, information from a CRS of a specific cell is preferably referred to in addition to the CSI-RS. That is, for QCL assumption, a CSI-RS index or a PCID of a specific cell may be signaled to obtain the channel property from the CRS of the cell.

[0142] In case of CoMP scenario 4, since TPs having different locations share the same PCID, signaling only the PCID for QCL assumption is inappropriate. Accordingly, in this case, a CSI-RS index is signaled to the UE. In summary, for QCL assumption, a CSI-RS index or a PCID may be signaled and the UE refers to a channel property from the CSI-RS using the CSI-RS index or the PCID signaled in QCL assumption information only or refers to a channel property using a CRS corresponding to the PCID. If both the CSI-RS index and the PCID are signaled in the QCL information for the specific DM-RS sequence, it is indicated that the specific DM-RS sequence is in a QCL relationship with the signaled CSI-RS index and the CRS corresponding to the signaled PCID. That is, the DM-RS may be in a QCL relationship with the signaled CSI-RS index and, in some cases, may be in a QCL relationship with the CRS corresponding to the signaled PCID. Accordingly, the UE estimates interference, detects an interference signal and cancels interference with respect to each case.

[0143] When the CSI-RS index is signaled to the UE for QCL assumption as shown in Table 6 above, the eNB should separately signal a CSI-RS configuration for long-term measurement and QCL assumption to the UE in addition to a CSI-RS configuration for CSI measurement. The CSI-RS configuration may include CSI-RSs for CSI measurement.

[0144] [Rate Matching Information]

[0145] For the purpose of signaling PDSCH RE mapping of the interference signal and occurrence of CRS interference in a specific RE, CRS information of a neighbor cell is also transmitted. The CRS information includes the number of CRS antenna ports, CRS frequency shift (={0, 1, 2, . . . , 5}) and MBSFN subframe pattern, etc. If the UE is informed that CRS interference occurs to cancel the CRS interference, a physical cell ID of the CRS is signaled instead of CRS frequency shift. That is, by signaling the physical cell ID, the number of CRS antenna ports and the MBSFN subframe pattern as CRS information, the UE is enable to cancel the CRS interference at the CRS location if the UE has CRS interference control capabilities.

[0146] In addition, for the purpose of signaling whether interference is caused from an interference signal in a specific RE, CSI-RS information and ZP CSI-RS information of a neighbor cell may also be signaled to the UE. That is, since the PDSCH of the interference signal is not mapped, the UE is prevented from performing IC with respect to the corresponding RE, thereby preventing performance deterioration.

[0147] In addition, a CRS-RS index for signaling a PDSCH rate matching pattern of the interference signal may be separately signaled. As another method, a CSI-RS index indicated for QCL assumption may be used. For example, in Table 6 above, since the PDSCH of the interference signal using the DM-RS sequence generated using nVCID(0) and nSCID={0, 1} when the bit value is 0 is in a QCL relationship with the CSI-RS index (0), the UE may obtain the channel property from the CSI-RS index (0) and recognize that the PDSCH of the interference signal corresponds to the CSI-RS index (0) and is transmitted after subjected to rate matching. In this case, separate CSI-RS index signaling for rate matching information is not necessary. If the CSI-RS index for QCL and the separate CSI-RS index for rate matching information are separately signaled, rate matching of the interference signal assumed by the UE follows the separately transmitted CSI-RS index for rate matching information. In Table 6 above, a plurality of CSI-RS indices for providing rate matching information may be signaled. According to UE capabilities, a CSI-RS index received as rate matching information may be used or a QCL CSI-RS index may be used to perform interference cancellation with respect to the CSI-RS in the RE on which the corresponding CSI-RS is transmitted.

[0148] Further, a ZP CSI-RS index used for interference signal transmission may also be transmitted to the UE. The signaled ZP CSI-RS may be muted or mapped to the PDSCH depending on to which UE the interference eNB schedules a PDSCH. Through such signaling, the target UE may be informed that the PDSCH may be mapped to the ZP CSI-RS or that the ZP CSI-RS may be muted. The UE is disabled to perform IC with respect to the corresponding REs using the ZP CSI-RS or is enabled to determine whether IC is performed with respect to the REs in the ZP CSI-RS via energy detection. Except for the signaled CRS index and CSI-RS index, unless there is separate signaling and restriction (e.g. PSS/SSS/PBCH transmission, positioning RS transmission, MBSFN subframe, etc.), the UE is informed that the PDSCH is transmitted in a region other than the signaled ZP CSI-RS. The UE, which has received this information, assumes that the PDSCH is transmitted in the region except for the ZP CSI-RS.

[0149] In general, the ZP CSI-RS is configured to cover a configured CSI-RS. Accordingly, as rate matching information of a separate interference signal, information indicating that the ZP CSI-RS covers the CSI-RS transmission location may be signaled to the UE without transmitting the CSI-RS information. That is, information indicating that the PDSCH may or may not be transmitted is included in the signaled ZP CSI-RS.

[0150] If the ZP CSI-RS of the interference is not separately signaled, the UE may consider that the PDSCH of the interference signal may not be mapped to one or a plurality of ZP CSI-RSs configured therefor and may not unconditionally perform IC with respect to the corresponding REs. Detailed ID operation of the corresponding REs follows UE implementation.

[0151] [PDSCH Starting Symbol Index]

[0152] When interference received from another eNB is cancelled, PDSCH starting symbols of an interference signal and a desired signal should be aligned between eNBs in advance. Cancellation of interference due to a CRS based PDCCH of a neighbor cell using a DM-RS based PDSCH is not preferable. Since the PDSCH starting OFDM symbol is changed according to the amount of control information transmitted via a PDCCH and load of a corresponding eNB in every subframe, dynamic signaling of the PDSCH starting symbol of the neighbor cell is very burdensome. Accordingly, the eNB may signal the PDSCH starting symbol index to the UE via semi-static signaling per DM-RS sequence. Here, the PDSCH starting symbol index means that the interference signal using the corresponding DM-RS sequence does not have a PDCCH of the interference signal after the signaled PDSCH starting symbol. For example, when the bit value is 0 in Table 6, PDSCH starting symbol index=2 is signaled, which means the PDCCH of the interference signal transmitted by the eNB using the DM-RS sequence corresponding to nVCID(0) is not transmitted after the signaled PDSCH starting symbol index=2. The UE is informed that PDCCH may be transmitted (the PDSCH may be transmitted) when the OFDM symbol index=0 or 1 but may not be transmitted when the OFDM symbol index≧2. Then, when the desired signal of the UE starts from the OFDM symbol index=1 and a signal using a DM-RS sequence having nVCID(0) as a seed value of the DM-RS sequence is received as strong interference, the UE may perform demodulation without performing IC with respect to the OFDM symbol index=1 and may perform. IC and perform demodulation with respect to the subsequent OFDM symbol indices thereof. In other words, when the PDSCH starting symbol index information is received, the UE performs IC only in a region in which the PDSH is ensured to be transmitted and performs modulation without IC in the other region. At this time, a weight of the region in which modulation is performed without IC may be lowered to reduce loss.

[0153] As another embodiment of the present invention, the UE may assume that a PDSCH of a neighbor cell always starts after a symbol to which a PDSCH transmitted thereto is mapped, without separate signaling for the PDSCH starting symbol index shown in Table 6. eNBs should exchange information on the PDSCH starting symbol index in advance and, as described above, information indicating that the PDCCH is not transmitted after a specific OFDM symbol index should be exchanged along with subframe information (e.g., a subframe pattern) having valid information.

[0154] As another embodiment of the present invention, each eNB maximally conservatively assumes a PDSCH starting symbol index. The UE may assume that neighbor cells use a maximum number of OFDM symbols which may be used by a serving cell thereof for PDCCH transmission and perform IC for the PDSCH at subsequent symbols thereof. At this time, the UE assumes that bandwidths and frame structure types of the neighbor cells which interfere with the serving cell thereof are the same.

[0155] Similarly, coordination for transmission locations of the PDSCH and the EPDCCH may be exchanged between eNBs in advance. The EPDCCH may be demodulated using a DM-RS similarly to a PDSCH. In the EPDCCH, since control information of a maximum of four UEs may be multiplexed in one RB, it is difficult to perform interference cancellation in PRB units or bundled PRB units. Accordingly, when the UE cancels the interference signal in order to improve reception performance of the desired signal thereof, the UE assumes that a PDSCH of another UE or another layer is transmitted as the interference signal unless separate information is received and performs interference cancellation. A region in which the PDSCH is transmitted and a region in which the EPDCCH is transmitted are preset and information thereon is exchanged between eNBs in advance. In a specific frequency domain or a specific time-frequency domain, information indicating that the EPDCCH of the eNB is transmitted is exchanged.

[0156] 2-1-1. Signaling Optimization

[0157] In case of fast backhaul link, the eNB may transmit, to the UE, information dynamically aiding interference cancellation in every subframe along with semi-static signaling. For example, even when the UE receives information on eight candidate seed values 1 to 8 of the sequence of an interference DM-RS, the number of candidates may be reduced by providing information indicating that only the seed values 2 to 4 are used via dynamic signaling. In some cases, the eNB may transmit only the seed value of one DM-RS sequence to the UE to accurately inform the UE of the DM-RS sequence of the interference signal. Alternatively, rank information (information on the number of layers of the interference signal) may be indicated via dynamic signaling. At this time, rank information signaled to the UE may be changed according to the number of layers of the interference signal which may be cancelled by the UE.

[0158] 2-1-2. DM-RS Sequence Detection Procedure—Using Received Signal Power of RSs Used as QCL Assumption

[0159] As another embodiment of the present invention, although the UE, which has received information on the interference signal shown in Table 6 above, may blindly detect the DM-RS sequence of the interference signal, the UE may use a CRS of a physical cell ID or a CSI-RS index which is signaled to be used as QCL assumption when the DM-RS sequence of the interference signal is detected. That is, using the CSI-RS or CRS as QCL assumption means the UE periodically or aperiodically measure the CSI-RS or CRS. At this time, only when the intensity of the received CSI-RS or CRS is equal to or greater than a predetermined level, it may be determined the DM-RS associated with the CSI-RS and the CRS may act as interference. Accordingly, the DM-RS acting as interference may be preferentially detected to estimate an interference channel and then IC may be performed.

[0160] 2-1-3. PRB Bundling Alignment

[0161] When information aiding interference cancellation is received to cancel an interference signal from a received signal, PRB allocation of the interference signal should be additionally considered. A frequency domain occupied by a desired signal and a frequency domain occupied by an interference signal should be considered. That is, even when a DM-RS sequence for an interference signal is detected and interference is estimated using the DM-RS sequence, whether estimated interference can be cancelled with respect to all PRBs to which the desired signal is allocated should be considered.

[0162] For example, when the UE detects a DM-RS sequence A as an interference signal with respect to a specific interference signal A and cancels the interference signal from the received signal using the DM-RS sequence A, if the desired signal is allocated two consecutive PRBs and the interference signal A is allocated four consecutive PRBs, the UE cancels the interference signal from all PRBs allocated thereto. However, if the desired signal is allocated four consecutive PRBs and the interference signal A is allocated two consecutive PRBs, since it is not possible to ensure that the interference signal A is equally present in all the PRBs allocated to the UE without receiving information thereon, the UE may not cancel the interference signal.

[0163] Accordingly, unless resource allocation information of the interference is directly signaled, granularity for interference cancellation of the UE should be predetermined between eNBs and should be used for resource allocation of one UE. Preferably, a PRB bundling size and an interference cancellation unit should be the same. That is, on the assumption that the resource allocation of the serving cell is performed in PRB bundling units and resource allocation of the neighbor cell is performed in the same PRB bundling units, it is possible to cancel interference using allocated resources. In this case, the PRB bundling size and resource allocation unit should be predetermined between eNBs.

[0164] A specific RS sequence may assume PRB bundling and another RS sequence may not assume PRB bundling. In other words, whether PRB bundling is assumed per RS sequence is determined. That is, as shown in Table 6 above, when information on the DM-RS of the interference signal is provided, the eNB may signal information indicating whether PRB bundling is assumed per DM-RS sequence to the UE.

[0165] In order for the UE to perform interference cancellation in PRB bundling units, the channel of the interference signal is equally maintained within the bundled PRB. Accordingly, resource allocation in the bundled PRB is not of a distributed type but is of a localized type. The UE which knows such information may perform IC in a specific bundled PRB in which localized resource allocation is performed. The UE may not perform IC in a specific bundled PRB in which distributed resource allocation is performed. Accordingly, in resource allocation between eNBs, coordination between eNBs that distributed resource allocation is performed in a specific frequency domain and localized resource allocation is performed in a specific frequency domain is performed in advance.

[0166] In addition, a PRB bundling size may be changed. Resources may be allocated in a specific subframe in M PRB bundling units and may be allocated in another specific subframe in N PRB bundling units. Alternatively, resources may be allocated in a specific subframe in M PRB bundling units and may be allocated in another specific subframe in PRB units.

[0167] 2-1-4. Heterogeneous Network (HetNet) Support

[0168] In a heterogeneous network (HetNet), when the information described in the present specification is provided to the UE in order to cancel the interference signal, additional information for interference cancellation may be provided. That is, when a macro eNB manages an almost blank subframe (ABS), a specific macro eNB may determine whether a PDSCH is transmitted according to an ABS pattern of the macro eNB. If the PDSCH is transmitted in the ABS with low transmit power, the signal is unlikely to be act as a dominant interference source of neighbor pico UEs. Accordingly, an interference candidate group shown in Table 6 above may be distributed and signaled to the UE according to the ABS pattern.

[0169] For example, the pico UE is informed of a plurality of subframe sets and an interference candidate group distributed to each subframe set. The interference candidate group means information shown in Table 6 above. The information shown in Table 6 above may be signaled per subframe set. Alternatively, the macro eNB may signal the information shown in Table 6 above with respect to the subframe set corresponding to the ABS only.

[0170] All or some of the information described in 2.1 and subsequent chapters thereof may be transmitted to the target UE.

[0171] 2-2. Case in which Interference Signal is CRS Based Signal

[0172] If the interference signal is a CRS based PDSCH, information signaled by the network in order to enable the target UE to cancel the CRS based interference signal from the received signal will now be described. Since the interference signal is a CRS based PDSCH, a seed value of a CRS sequence is first obtained. This seed value is a physical cell ID of a cell for transmitting the CRS causing interference. Further, the UE needs to know a MBSFN subframe configuration in order to know the number of CRS ports, the locations of the CRS ports, information on presence/absence of the CRS, a transmission scheme in a subframe in which the CRS is not present and RS information.

[0173] Since transmit power of the CRS may be different from actual PDSCH transmit power, a ratio of PDSCH transmit power to the CRS transmit power needs to be signaled to the target UE. As the ratio of PDSCH transmit power to the CRS transmit power, both a ratio of a symbol on which the CRS is transmitted and a ratio of a symbol on which the CRS is not transmitted need to be signaled.

[0174] When the physical cell ID of the cell for transmitting the CRS, the number of CRS ports, the locations of the CRS ports and the information on presence/absence of the CRS are referred to as “CRS related information”, the target UE may estimate an interference signal channel using the signaled CRS related information and determine whether CRS based PDSCH interference is present using the signaled ratio of PDSCH transmit power to the CRS transmit power.

[0175] If the interference signal is a CRS based PDSCH, the UE for cancelling the interference signal should know a transmitted precoding matrix index (TPMI) used to transmit the interference signal. However, it is difficult to provide the TPMI which is dynamically changed according to a channel state in every subframe. However, when the TPMI of a specific UE is fixed, performance of the UE may deteriorate. In addition, enabling the target UE to blindly detecting PMI which is actually used by the interference UE (that is, which is used to transmit the interference signal) among all PMI is burdensome. Accordingly, the PMI of the interference UE may be restricted to an extent that performance of the interference UE is not reduced.

[0176] A codebook subset may be restricted. For example, a specific UE may be enabled to use some of a total of 16 PMs for a report. In addition, such information may be delivered to the target UE such that the target UE blindly detects the TPMI with respect to the remaining PM(s) except for the PM, the use of which by the interference UE is restricted, (that is, the set of PMs which may be used by the interference UE) to cancel the interference signal.

[0177] That is, by providing codebook subset restriction information, the target UE is informed of the candidates of the TPMIs used by the interference UE. Accordingly, the rank of the interference UE may be restricted and, separately, rank restriction information of the interference UE (or the interference signal) may be explicitly provided. The rank restriction information, the PDSCH starting symbol index, the modulation order (or the MCS level) restriction information and HetNet support are equal to those of the DM-RS based signal of 2-1.

[0178] Additionally, even when the interference signal is a CRS based PDSCH, transmission mode (TM) information of the dominant interference UE may also be delivered for the purpose of indicating the PDSCH transmission method.

[0179] Next, information which may be signaled to the target UE when the interference signal is a CRS based signal will be described.

TABLE 8
	CRS information
			CRS information	CRS to	TPMI or		PDSCH
	TM (Transmission	PCID	(CRSPortsNumber,	PDSCH	codebook		starting	Modulation
Bit	Mode) of	(Physical	FrequencyShift,	power	subset	Rank	Symbol	order
value	interfering UE	Cell ID)	MBSFNsubframe pattern)	ratio	restriction	Restriction	index	restriction
0	TM2	PCID (0)	_PortNum0, _Freqshift0,	- ρA(0),			2	—
			_MBSFN_subframe_0	ρB(0),
1	TM3	PCID (1)	_PortNum1, _Freqshift1,	ρA(1),	01	1	1	2
			_MBSFN_subframe_1	ρB(1),
2	TM4	PCID (2)	_PortNum1, _Freqshift1,	ρA(2),	0000 . . . 1111	2	3	4
			_MBSFN_subframe_1	ρB(2),
. . .

[0180] 2-2-1. CRS Sequence Detection Procedure

[0181] Even when the CRS based interference signal is cancelled, the UE should measure and monitor the CRS of a neighbor cell for a long term. As shown in Table 8 above, when detecting the CRS based interference signal, the UE, which has received the CRS-related information of a specific cell, may determine that the PDSCH using the CRS is received as interference only when the intensity of the received CRS based signal is equal to or greater than a predetermined level and perform channel estimation and cancellation of the interference signal using the received CRS based signal, the intensity of which is equal to or greater than the predetermined level, only.

[0182] 2-2-2. Subband Size Alignment

[0183] When information aiding interference cancellation is received to cancel the interference signal, subband size alignment of the interference signal should be further considered. A frequency domain to which a desired signal is allocated and a frequency domain to which an interference signal is allocated need to be considered. Unless resource allocation information of the interference signal is directly indicated, granularity for interference cancellation of the target UE should be predetermined between eNBs and should be used for resource allocation of one UE.

[0184] In addition, when the CRS based PDSCH interference is cancelled, the TPMI of the interference signal should not be changed in the unit of the frequency domain in which the target UE cancels interference. Preferably, the subband size used for CQI report by the target UE and the size of the frequency domain in which interference cancellation will be performed should be the same. That is, the PMI is determined in subband units when the serving cell allocates resources to the target UE and the unit in which the PMI is equally maintained when the neighbor cell allocates resources to the interference UE should be determined to the subband. Accordingly, on the premise that the subband size of the target UE and the subband size of the interference UE are the same, the target UE may perform interference cancellation in the frequency domain allocated thereto. In this case, the subband size and resource allocation should be predetermined between eNBs.

[0185] Similarly, PRB grouping is possible as a unit in which the TPMI is equally maintained. Within PRB grouping, the TPMI of the interference UE and the TPMI of the target UE are equally maintained. Information indicating how many PRBs are grouped and used for this purpose is also predetermined between eNBs.

[0186] All or some of the information described in 2.1 and subsequent chapters thereof may be transmitted to the target UE.

[0187] 2-3. Case in which DM-RS Based Signal and the CRS Based Signal are Mixed

[0188] Practically, in the interference signals of the target UE, the DM-RS based PDSCH and the CRS based PDSCH are mixed. In order to cancel the interference signal from the received signal, the target UE should determine whether the signal is a DM-RS based PDSCH or a CRS based PDSCH, estimate the amount of interference in the received signal based on the determination, and cancel interference from the received signal.

[0189] Accordingly, when the DM-RS based PDSCH and the CRS based PDSCH are mixed, information provided by the network and UE operation will be described. Table 9 below includes the information described in Tables 6 and 8 above and, for a description thereof, refer to Tables 6 and 8 above.

TABLE 9
	CRS information		DM-RS information
			CRS information	CRS to	TPMI or		VCID(virtual
	TM (Transmission	PCID	(CRSPortsNumber,	PDSCH	codebook		cell ID)
Bit	Mode) of	(Physical	FrequencyShift,	power	subset	Rank	∈{0, 1,	nSCID
value	interfering UE	Cell ID)	MBSFNsubframe pattern)	ratio	restriction	Restriction	2, . . . , 503}	∈{0, 1}
0	TM 4	PCID (0)	_PortNum0, _Freqshift0,	ρA(0),	0000 . . . 1111	2	—	—
			_MBSFN_subframe_0	ρB(0),
1	TM 10		_PortNum1, _Freqshift1,	ρA(1),	—	4	nVCID(1)	{0}
			_MBSFN_subframe_1	ρB(1),
2	TM 10	PCID (2)	_PortNum2, _Freqshift2,	ρA(2),			nVCID(2)	{0}
			_MBSFN_subframe_2	ρB(2),
. . .							. . .	. . .
	QCL assumption	Rate-matching information
		(non-zero				CRS information	PDSCH
		power)	PCID			(CRSPortsNumber,	starting	Modulation
	Bit	CSI-RS	(Physical	NZ PCSI-RS	ZP CSI-RS	FrequencyShift,	Symbol	order
	value	index	Cell ID)	index	index	MBSFNsubframe pattern)	index	restriction
	0	—	nPCID(0)	—	—	PortNum0, Freqshift0,	2	—
						MBSFN_subframe_0
	1	qcl-CSI-RS		CSI-RS	ZP CSI-RS	PortNum1, Freqshift1,	1	2
		index(1)		(1)	(1)	MBSFN_subframe_1
	2		nPCID(2)			PortNum2, Freqshift2,	3	4
						MBSFN_subframe_2
	. . .	. . .			. . .

[0190] In Table 9, information provided to the target UE when it is not determined which RS is used to modulate the interference signal is shown. First, the eNB signals information indicating whether the interference signal is a CRS based PDSCH or a DM-RS based PDSCH using a signaled TM. The transmission method of the interference signal may be identified according to the signaled TM.

[0191] In the CRS based TM (e.g., TM4), the eNB provides CRS related information. At this time, DM-RS related information is not provided. That is, in the CRS based TM, QCL assumption is not necessary and thus may be omitted.

[0192] In the DM-RS based TM (e.g., TM10), the eNB may provide DM-RS related information as well as CRS related information.

[0193] When the target UE receives the information shown in Table 9 and cancels the interference signal, since the CRS is always transmitted regardless of transmission of the PDSCH, the target UE assumes that the DM-RS based PDSCH is present using the given TM and blindly detects the DM-RS sequence. Thereafter, if it is determined that the DM-RS interference signal is not present or after the DM-RS based interference signal is cancelled, the CRS based interference signal is estimated.

[0194] In the following description, a PDSCH transmitted to a target UE having interference cancellation capability is referred to as “desired PDSCH” and a PDSCH transmitted to another UE in the same time/frequency resource is referred to as “interfering PDSCH” for convenience. In addition, a DM-RS transmitted when the interfering PDSCH is transmitted in a DM-RS mode is referred to as “interfering DM-RS” and a CRS transmitted when the interfering PDSCH is transmitted in a CRS mode is referred to as “interfering CRS”.

[0195] In the present embodiment, dependency between assistance information sets is designated for efficient blind decoding of a UE in a scheme in which assistance information for interference signal removal is semi-statically provided to the UE.

[0196] CRS Based Interference

[0197] A BS selects a candidate group of cells (referred to as interference cells hereinafter) that may cause interference for a specific UE, configures interfering CRS assistance information sets composed of cell IDs, the numbers of antenna ports and MBSFN subframe patterns with respect to interfering CRSs transmitted in the interfering cells and semi-statically informs the UE of the interfering CRS assistance information sets. When the candidate group includes N interfering cells, the BS informs the UE of N interfering CRS assistance information sets.

[CRS assistance information set]
	CRS-AssistanceInfo ::= SEQUENCE {
	crs-AssistanceInfoId	CRS-AssistanceInfoId,
	physCellId	PhysCellId,
	antennaPortsCount	MERATED {an1, an2, an4, spare1},
	mbsfn-SubframeConfigList	MBSFN-SubframeConfigList,
	...
	}

[0198] The CRS assistance information set is notified through CRS-AssistanceInfo-r12, discriminated form CRS-AssistanceInfo-r11 for FeICIC (further enhanced inter-cell interference coordination). While synchronization and CP length with respect to a CRS of a neighbor cell notified through CRS-AssistanceInfo-r11 cannot be set as compared to a CRS of a serving cell, a CRS of a neighbor cell notified through CRS-AssistanceInfo-r12 is limited to a case in which transmission synchronization is acquired and/or the same CP length is used as compared to the CRS of the serving cell. That is, interference of a PSS/SSS, CRS and PBCH from the serving cell is eliminated for the neighbor cell notified through CRS-AssistanceInfo-r11 and a PSS/SSS and CRS transmitted from the neighbor cell are detected to determine transmission time and CP length of the neighbor cell. In this case, the UE attempts to detect the PSS/SSS signal of the neighbor cell twice in consideration of normal CP case and extended CP case. However, since the UE is informed of the neighbor cell notified through CRS-AssistanceInfo-r12 only when the same CP length as that of the serving cell is used and subframe synchronization is acquired, the UE attempts to detect the PSS/SSS of the neighbor cell in a transmission position corresponding to the CP length of the serving cell at a specific transmission time.

[0199] CRS-AssistanceInfo-r12 may include information on the transmission bandwidth of the neighbor cell. The transmission bandwidth of the neighbor cell informed through CRS-AssistanceInfo-r12 may be equal to or greater than the transmission bandwidth of the serving cell. That is, the UE detects the CRS transmitted from the neighbor cell notified through CRS-AssistanceInfo-r12 on the assumption that the CRS has the transmission bandwidth corresponding to the transmission bandwidth of the serving cell.

[0200] The BS configures an interfering CSI-RS assistance information set with respect to a non-zero power (NZP) CSI-RS that may be transmitted from an interfering cell (or interfering transmission point) and semi-statically notifies the UE of the interfering CSI-RS assistance information set. The interfering CSI-RS assistance information set is composed of the number of CSI-RS antenna ports, information on configuration of CSI-RS transmission subframe and RE positions and scrambling ID for RS sequence generation, as described below.

[CSI-RS assistance information set]
NZP-CSI-RS-AssistanceInfo ::=	SEQUENCE {
nzp-CSI-RS-AssistanceInfoIdNZP-CSI-RS-AssistanceInfoId,
antennaPortsCount	ENUMERATED {an1, an2, an4, an8},
resourceConfig	INTEGER (0..31),
subframeConfig	INTEGER (0..154),
scramblingIdentity	INTEGER (0..503),
qcl-CRS-Info	CRS-AssistanceInfoId,
qcl-Operation	ENUMERATED {type1, type2},
p-D	INTEGER (−8..15),
...
}

[0201] In this case, a CRS having a QCL relationship with the CSI-RS is indicated. That is, when the interfering CSI-RS assistance information includes qcl-CRS-Info, the UE is semi-statically notified of a CRS having a qcl relationship with the CSI-RS and assistance information of the CRS. In addition, when the interfering CSI-RS assistance information includes qcl-Operation, the UE is notified of the QCL behavior type of the CSI-RS and the CRS. The QCL type may be type 1 or type 2 defined as follows.

[0202] Type 1: The UE may assume the CRS antenna ports associated with qcl-CRS-Info and CSI-RS antenna ports corresponding to the CSI-RS assistance information are quasi co-located with respect to delay spread, Doppler spread, Doppler shift, and average delay.

[0203] Type 2: The UE may assume that the CRS antenna ports associated with qcl-CRS-Info and CSI-RS antenna ports corresponding to the CSI-RS assistance information are quasi co-located with respect to Doppler shift, and Doppler spread.

[0204] When the QCL type of the interfering CSI-RS assistance information is type 1, the UE uses corresponding values estimated in the quasi-co-located CRS to obtain delay spread, Doppler spread, Doppler shift and average delay of the channel on which the CSI-RS is transmitted. When the QCL type of the interfering CSI-RS assistance information is type 2, the UE uses corresponding values estimated in the quasi-co-located CRS to obtain Doppler spread and Doppler shift of the channel on which the CSI-RS is transmitted and estimates average delay and delay spread of the channel through the received CSI-RS signal.

[0205] When the QCL type is type 1 or type 2, the index of the CSI-RS transmission subframe and the index of the quasi-co-located CRS transmission subframe are set to the same value. When the index of the CSI-RS transmission subframe and the index of the quasi-co-located CRS transmission subframe are different from each other, the difference between the indexes is included in the CSI-RS assistance information and indicated to the UE.

[0206] Since a cell may transmit a plurality of CRS-RSs at different transmission points or in different beam directions, more than N CSI-RS assistance information sets may be present even when the candidate group of interfering cells includes N cells. On the contrary, the number of CSI-RS assistance information sets may be less than N since a certain cell may not transmit a CSI-RS.

[0207] QCL type may be independently designated per CSI-RS. That is, some CSI-RSs may be set to type 1 and other CSI-RSs may be set to type 2. Furthermore, CSI-RSs quasi-co-located with respect to the same CRS may have different QCL types. That is, when a cell transmitting CRS1 transmits two CSI-RSs (CSI-RS1 and CSI-RS2), CSI-RS1 and CRS2 are transmitted using the same beam at the same transmission point and CSI-RS2 is transmitted using a different beam at a different transmission point, CSI-RS1 can be set to QCL type 1 and CSI-RS2 can be set to QCL type 2 with respect to CRS1 and the QCL types of CSI-RS1 and CSI-RS2 can be indicated.

[0208] In addition, whether the CP length of the CSI-RS corresponds to normal CP or extended CP may be included in the CSI-RS assistance information and indicated. Otherwise, the CP length of the CSI-RS is always set to the CP length of the CRS quasi-co-located with respect to the CSI-RS such that the UE detects the CSI-RS on the assumption that the CP length of the CSI-RS is identical to the CP length of the CRS.

[0209] DM-RS Based Interference

[0210] The BS configures an interfering DM-RS assistance information set with respect to an interfering DM-RS that can be transmitted from an interfering cell and semi-statically notifies a target UE of the interfering DM-RS assistance information set. The interfering DM-RS assistance information set is composed of scrambling ID for RS sequence generation and information on a CRS or CSI-RS having a QCL relationship with the DM-RS as described below. Since the DM-RS is transmitted along with a PDSCH, a process for detecting the interfering DM-RS is performed for interference cancellation of the interfering PDSCH.

[DM-RS assistance information set 1]
DMRS-AssistanceInfo ::=	SEQUENCE {
dmrs-AssistanceInfoId	DMRS-AssistanceInfoId,
scramblingIdentity	INTEGER (0..503),
n_SCID	INTEGER (0,1),
antennaPort	INTEGER (7,8..14),
p-C	INTEGER (−8..15),
qcl-CRS-Info	CRS-AssistanceInfoId,
qcl-CSI-RS-Info	NZP-CSI-RS-AssistanceInfoId,
...
}

[0211] When the aforementioned interfering DM-RS assistance information set corresponds to QCL type B of TM10 in a cell that transmits the corresponding DM-RS, qcl-CRS-Info is omitted from the interfering DM-RS assistance information set and only qcl-CSI-RS-Info is set therein. When the DM-RS is transmitted in TM8 and thus a CSI-RS is not transmitted in the cell, qcl-CSI-RS-Info can be omitted and only qcl-CRS-Info can be set.

[0212] The UE assumes QCL type 3 defined below when the DM-RS assistance information includes qcl-CRS-Info and assumes QCL type 4 when the DM RS assistance information includes qcl-CSI-RS-Info. When the DM-RS assistance information includes both qcl-CRS-Info and qcl-CSI-RS-Info, the UE can simultaneously assume QCL type 3 and QCL type 4.

[0213] Type 3: The UE may assume that the CRS antenna ports associated with qcl-CRS-Info and DM-RS antenna ports associated with the PDSCH corresponding to the DM-RS assistance information are quasi co-located with respect to delay spread, Doppler spread, Doppler shift, and average delay.

[0214] Type 4: The UE may assume that the CSI-RS antenna ports associated with qcl-CSI-RS-Info and DM-RS antenna ports associated with the PDSCH corresponding to the DM-RS assistance information are quasi co-located with respect to delay spread, Doppler spread, Doppler shift, and average delay.

[0215] The UE uses corresponding values estimated in the quasi-co-located CRS or CSI-RS to obtain delay spread, Doppler spread, Doppler shift and average delay of the channel on which the DM-RS is transmitted according to QCL type information of the DM-RS assistance information.

[0216] When the QCL type is type 3, the index of the DM-RS transmission subframe and the index of the quasi-co-located CRS transmission subframe are set to the same value. When the index of the DM-RS transmission subframe and the index of the quasi-co-located CRS transmission subframe are different from each other, the difference between the indexes is included in the DM-RS assistance information and indicated to the UE.

[0217] When the QCL type is type 4, the index of the DM-RS transmission subframe and the index of the quasi-co-located CSI-RS transmission subframe are set to the same value. When the index of the DM-RS transmission subframe and the index of the quasi-co-located CSI-RS transmission subframe are different from each other, the difference between the indexes is included in the DM-RS assistance information and indicated to the UE.

[0218] Alternatively, the QCL type of the DM-RS is directly indicated as shown in the interfering DM-RS assistance information set, only qcl-CRS-info is transmitted in case of QCL type 3, only qcl-CSI-RS-info is transmitted in case of QCL type 4, and both qcl-CRS-info and qcl-CSI-RS-info are transmitted in case of QCL type 34.

[DM-RS assistance information set 2]
DMRS-AssistanceInfo ::=	SEQUENCE {
dmrs-AssistanceInfoId	DMRS-AssistanceInfoId,
scramblingIdentity	INTEGER (0..503),
n_SCID	INTEGER (0,1),
antennaPort	INTEGER (7,8..14),
p-C	INTEGER (−8..15),
qcl-Operation	ENUMERATED {type3, type4,
type34},
qcl-CRS-Info	CRS-AssistanceInfoId,
qcl-CSI-RS-Info	NZP-CSI-RS-AssistanceInfoId,
...
}

[0219] Whether the CP length of the DM-RS corresponds to normal CP or extended CP may be included in the DM-RS assistance information and indicated. Otherwise, the CP length of the DM-RS is always set to the CP length of the CRS or CSI-RS quasi-co-located with respect to the DM-RS such that the UE detects the DM-RS on the assumption that the CP length of the DM-RS is identical to the CP length of the CRS or CSI-RS.

[0220] While the aforementioned DM-RS assistance information is provided per nSCID and antenna port index (antennaPort) in the above description, the DM-RS assistance information may be indicated through a single DM-RS assistance information set for DM-RS sequences having the same QCL relationship. In this case, nSCID and an antenna port index that may be used in the corresponding cell are indicated through a bitmap as shown in the following DM-RS assistance information set.

[DM-RS assistance information set 3]
	DMRS-AssistanceInfo ::=	SEQUENCE {
	dmrs-AssistanceInfoId	DMRS-AssistanceInfoId,
	scramblingIdentity	INTEGER (0..503),
	n_SCID	BIT STRING (SIZE(2)),
	antennaPort	BIT STRING (SIZE(8))
	p-C	INTEGER (−8..15),
	qcl-CRS-Info	CRS-AssistanceInfoId,
	qcl-CSI-RS-Info	NZP-CSI-RS-AssistanceInfoId,
	...
	}

[0221] Dependency Between RSs

[0222] FIG. 6 illustrates relationship between individual pieces of RS assistance information according to an embodiment of the present invention. FIG. 6 shows a QCL relationship between RSs according to QCL information of the RSs when two pieces of CRS assistance information, three pieces of CSI-RS assistance information and four pieces of DM-RS assistance information are transmitted.

[0223] Upon reception of assistance information about RSs that may be transmitted from interfering cells, the UE may attempt to preferentially detect a CRS that is periodically transmitted and has high frequency. In addition, the UE may attempt to detect a CSI-RS having a QCL relationship with respect to the detected CRS. The UE may attempt to detect a DM-RS having a QCL relationship with respect to the detected CRS or CSI-RS. Since the DM-RS is aperiodically transmitted although the CRS and CSI-RS are periodically transmitted, blind decoding needs to be performed on a transmittable DM-RS sequence per subframe.

[0224] The UE may limit a candidate group according to blind decoding performance thereof. Upon reception of N pieces of CRS assistance information, the UE attempts to detect corresponding CRSs and considers only signals having high signal intensity, transmitted from some cells, as an interference cancellation target on the basis of blind decoding performance thereof. The UE may select only CRSs having high received signal intensity, attempt to detect CSI-RSs having a QCL relationship with respect to the selected CRSs, select CSI-RSs having high received signal intensity and attempt to detect only DM-RSs having a QCL relationship with respect to the selected CRSs or CSI-RSs. This procedure is correctly performed when the RSs (CRS, CSI-RS and DM-RS) have the same power but is otherwise incorrect. Accordingly, the BS can inform the UE of a correct transmit power difference between RSs such that the UE can correct the transmit power difference and correctly calculate candidate RSs to be decoded.

[0225] To achieve this, the BS may include information on CSI-RS transmit power in CSI-RS assistance information and inform the UE of the CSI-RS assistance information. The aforementioned [CSI-RS assistance information set] includes “p-D” as the information on CSI-RS transmit power. The information on CSI-RS transmit power can be defined as the ratio of CSI-RS EPRE to CRS-ERPE. Here, CRS is quasi-co-located with respect to the corresponding CSI-RS. The UE may estimate CSI-RS reception intensity by applying “p-D” to CRS reception intensity for CSI-RSs having QCL relationship with respect to detected CRSs and attempt to sequentially detect the CSI-RSs from CSI-RS having highest reception intensity.

[0226] The BS may include information on DM-RS transmit power in DM-RS assistance information and inform the UE of the DM-RS assistance information. [DM-RS assistance information set 1], [DM-RS assistance information set 2] and [DM-RS assistance information set 3] include “p-C” in DM-RS assistance information as the information on DM-RS transmit power. The information on DM-RS transmit power can be defined as the ratio of DM-RS EPRE to quasi-co-located CSI-RS-ERPE when a quasi-co-located CSI-RS is present. The information on DM-RS transmit power can be defined as the ratio of DM-RS EPRE to CRS-ERPE when a quasi-co-located CSI-RS is not present. The UE may estimate DM-RS reception intensity by applying “p-C” to quasi-co-located CRS or CSI-RS reception intensity for DM-RSs having QCL relationship with respect to detected CRSs or CSI-RSs and attempt to sequentially detect the DM-RSs from DM-RS having highest reception intensity.

[0227] That is, while the serving BS informs the UE of CRS information and CSI-RS information about a plurality of cells included in an interfering candidate group and quasi-co-located DM-RS information through semi-static RRC signaling, the UE can attempt to detect only DM-RSs associated with detected CRSs having high received signal intensity. That is, DM-RS detection depends on detection of quasi-co-located CSI-RSs or DM-RSs. In blind detection, complexity of blind detection for interfering signals can be reduced through dependency between RSs to be detected. This dependency can be extended and applied to relationship between DM-RSs.

[0228] The number of DM-RS sequences that can be transmitted in a single cell is 4 since the cell can take nSCID (0 or 1) and antenna port (7 or 8), considering only TM9, when 1 layer is transmitted in the cell. Accordingly, the UE needs to perform blind detection on a total of 4 DM-RS sequences.

[0229] However, when the interfering cell uses antenna port 8 for rank-2 transmission and uses nSCID=1 only in case of MU-MIMO pairing, it is inefficient for the target UE to perform 4 blind detections on the 4 DM-RS sequences without priority. Accordingly, in an embodiment of the present invention, the BS informs the UE of dependency between DM-RSs such that the UE can perform efficient blind detection. When antenna port 8 of a specific cell has dependency on antenna port 7 of the same cell, the UE blind-detects antenna port 7 first, attempts to detect antenna port 8 when antenna port 7 has been detected and does not perform blind detection on antenna port 8 when antenna port 7 has not been detected.

[0230] To achieve this, reference DM-RS ID(ref-DMRS-AssistanceInfoId) is added to the interfering DM-RS assistance information set as follows.

[DM-RS assistance information set 4]
DMRS-AssistanceInfo ::=	SEQUENCE {
dmrs-AssistanceInfoId	DMRS-AssistanceInfoId,
scramblingIdentity	INTEGER (0..503),
n_SCID	INTEGER (0,1),
antennaPort	INTEGER (7,8..14),
qcl-CRS-Info	CRS-AssistanceInfoId,
qcl-CSI-RS-Info	NZP-CSI-RS-AssistanceInfoId,
ref-DMRS-AssistanceInfoId DMRS-AssistanceInfoId,
...
}

[0231] When ref-DMRS-AssistanceInfoId is set, blind detection on a DM-RS corresponding to the aforementioned DM-RS assistance information set is performed upon detection of the DM-RS of the corresponding ID. The BS transmits the DM-RS only when the DM-RS of the reference DM-RS ID is transmitted.

[0232] In an embodiment of the present invention, the UE is notified of transmission possibility of the corresponding RS, that is, priority in blind detection when informed of RS assistance information for interfering signal cancellation to perform efficient blind detection of an interfering PDSCH.

[0233] In one embodiment, the UE may add priority notified by the BS per DM-RS to receive power of CRSs or CSI-RSs quasi-co-located with respect to DM-RSs, calculate priority for final blind detection and perform blind detection according to priority. The following example corresponds to a case in which a DM-RS assistance information set is given priority values of 0 to 100 and signaled to the UE.

[DM-RS assistance information set 5]
DMRS-AssistanceInfo ::=	SEQUENCE {
dmrs-AssistanceInfoId	DMRS-AssistanceInfoId,
scramblingIdentity	INTEGER (0..503),
n_SCID	INTEGER (0,1),
antennaPort	INTEGER (7,8..14),
priority	INTEGER (0..100)
qcl-CRS-Info	CRS-AssistanceInfoId,
qcl-CSI-RS-Info	NZP-CSI-RS-AssistanceInfoId,
ref-DMRS-AssistanceInfoId DMRS-AssistanceInfoId,
...
}

[0234] The priority can be included in a CRS assistance information set to represent CRS-based PDSCH transmission possibility as follows. The UE compares priority included in CRS assistance information with priority of DM-RS assistance information on a DM-RS quasi-co-located with the corresponding CRS to obtain information on CRS-based PDSCH and DMRS-based PDSCH transmission possibility of the corresponding cell. The UE determines which one of blind detection of CRS-based PDSCH and blind detection of DMRS-based PDSCH is performed first based on the information on CRS-based PDSCH and DMRS-based PDSCH transmission possibility.

[CRS assistance information set 2]
CRS-AssistanceInfo ::= SEQUENCE {
crs-AssistanceInfoId	CRS-AssistanceInfoId,
physCellId	PhysCellId
antennaPortsCount	ENUMERATED {an1, an2, an4,
spare1},
priority	INTEGER (0..100)
p-b	INTEGER (0..3)
p-a	ENUMERATED {dB-6, dB-4dot77,
dB-3, dB-1dot77, dB0, dB1, dB2, dB3}
mbsfn-SubframeConfigList	MBSFN-SubframeConfigList,
...
}

[0235] The above-described CRS assistance information set includes both p-a and p-b to recognize PDSCH EPRE and CRS EPRE when the CRS-based PDSCH is transmitted.

[0236] In addition, CRS-based PDSCH transmission may be classified into TM2 (diversity transmission scheme) and TM4 (PMI transmission scheme) and priorities thereof may be indicated through the CRS assistance information set. That is, priority 1 indicating TM2 transmission possibility and priority 2 indicating TM4 transmission possibility are respectively indicated. This scheme can be extended to a scheme in which priority is indicated per available TPMI.

[0237] FIG. 7 illustrates a method according to an embodiment of the present invention. A UE 1 may receive information on a reference signal transmitted from an interfering cell that may cause inter-cell interference (S710). The information on the reference signal is received from eNB1 2 in FIG. 7 and eNB1 2 may be a serving cell, the interfering cell or a third cell of the UE 1. The UE 1 may attempt to detect the reference signal using the received information on the reference signal. The information on the reference signal can be received when the interfering cell and the serving cell of the UE 1 have the same CP length and are synchronized with each other. This restriction is for the purpose of processing complexity of the UE 1.

[0238] The information on the reference signal may include an information set about a first reference signal and information sets about one or more second reference signals having a specific relationship with the first reference signal. Each information set about the second reference signal may include information indicating that the second reference signal defined by the corresponding information set has the specific relationship with the first reference signal. Specifically, the specific relationship may be a relationship in which an antenna port through which the second reference signal defined by the corresponding information set is transmitted and an antenna port through which the first reference signal is transmitted are quasi co-located.

[0239] The aforementioned method may include the steps of: detecting the first reference signal using the information on the first reference signal; calculating estimated receive power of the second reference signals defined by the corresponding information sets using receive power of the detected first reference signal and power correction values included in the information sets about the one or more second reference signals; and attempting to detect only a second reference signal having estimated receive power higher than a predetermined value. In this manner, detection of the second reference signal is dependent on the first reference signal to reduce processing load of the UE 1 and to promote more efficient interference cancellation.

[0240] At least one of the information sets about the one or more second reference signals may include information indicating other information sets that need to be referred for the at least one information set. In this case, the aforementioned method may include the step of attempting to detect the second reference signal defined by the at least one information set only when second reference signals defined by the other information sets that need to be referred to have been successfully detected if the at least one information set includes the information indicating the other information sets. This can restrict reference signals to be used by a transmitter, that is, the eNB. For example, priority is set and a specific reference signal is transmitted only when another reference signal referred to by the specific reference signal is transmitted, to thereby control reception processing load of a receiver, that is, UE 1.

[0241] The information set about the first reference signal and the information sets about the second reference signals may further include values for indicating transmission possibility of data channels based on the corresponding reference signals. The UE 1 can determine which reference signal will be attempted to be blind-detected through transmission possibility.

[0242] The information set about the first reference signal may be an information set about a CRS and may include an ID for identifying the information set, a physical cell ID of the interfering cell, and the number of antenna ports for the CRS and MBSFN (Multicast Broadcast Single Frequency Network) configuration.

[0243] The information set about the second reference signal may be an information set about a CSI-RS and may include at least one of ID for identifying the information set, the number of antenna ports for the CSI-RS, configuration of RE position and subframe in which the CSI-RS is transmitted, scrambling ID for sequence generation for the CSI-RS, ID of an information set about a CRS having a specific relationship with the CSI-RS, a type associated with the specific relationship and the transmit power ratio of the CSI-RS to the CRS having the specific relationship with the CSI-RS.

[0244] The information set about the second reference signal may be an information set about a DM-RS and may include at least one of ID for identifying the information set, scrambling ID for sequence generation for the DM-RS, a scrambling ID field value, antenna port index for the DM-RS, the transmit power ratio of the DM-RS to a CRS or CSI-RS having a specific relationship with the DM-RS, a type associated with the specific relationship, ID of an information set about the CRS having the specific relationship with the DM-RS and ID of an information set about the CSI-RS having the specific relationship with the DM-RS.

[0245] While embodiments of the present invention according to the present invention have been described with reference to FIG. 7, the embodiment related to FIG. 7 may include at least part of the aforementioned embodiments alternatively or additionally.

[0246] FIG. 8 is a block diagram of a transmitting device 10 and a receiving device 20 configured to implement exemplary embodiments of the present invention. Referring to FIG. 12, the transmitting device 10 and the receiving device 20 respectively include radio frequency (RF) units 13 and 23 for transmitting and receiving radio signals carrying information, data, signals, and/or messages, memories 12 and 22 for storing information related to communication in a wireless communication system, and processors 11 and 21 connected operationally to the RF units 13 and 23 and the memories 12 and 22 and configured to control the memories 12 and 22 and/or the RF units 13 and 23 so as to perform at least one of the above-described embodiments of the present invention.

[0247] The memories 12 and 22 may store programs for processing and control of the processors 11 and 21 and may temporarily storing input/output information. The memories 12 and 22 may be used as buffers. The processors 11 and 21 control the overall operation of various modules in the transmitting device 10 or the receiving device 20. The processors 11 and 21 may perform various control functions to implement the present invention. The processors 11 and 21 may be controllers, microcontrollers, microprocessors, or microcomputers. The processors 11 and 21 may be implemented by hardware, firmware, software, or a combination thereof. In a hardware configuration, Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), or Field Programmable Gate Arrays (FPGAs) may be included in the processors 11 and 21. If the present invention is implemented using firmware or software, firmware or software may be configured to include modules, procedures, functions, etc. performing the functions or operations of the present invention. Firmware or software configured to perform the present invention may be included in the processors 11 and 21 or stored in the memories 12 and 22 so as to be driven by the processors 11 and 21.

[0248] The processor 11 of the transmitting device 10 is scheduled from the processor 11 or a scheduler connected to the processor 11 and codes and modulates signals and/or data to be transmitted to the outside. The coded and modulated signals and/or data are transmitted to the RF unit 13. For example, the processor 11 converts a data stream to be transmitted into K layers through demultiplexing, channel coding, scrambling and modulation. The coded data stream is also referred to as a codeword and is equivalent to a transport block which is a data block provided by a MAC layer. One transport block (TB) is coded into one codeword and each codeword is transmitted to the receiving device in the form of one or more layers. For frequency up-conversion, the RF unit 13 may include an oscillator. The RF unit 13 may include Nt (where Nt is a positive integer) transmit antennas.

[0249] A signal processing process of the receiving device 20 is the reverse of the signal processing process of the transmitting device 10. Under the control of the processor 21, the RF unit 23 of the receiving device 10 receives RF signals transmitted by the transmitting device 10. The RF unit 23 may include Nr receive antennas and frequency down-converts each signal received through receive antennas into a baseband signal. The RF unit 23 may include an oscillator for frequency down-conversion. The processor 21 decodes and demodulates the radio signals received through the receive antennas and restores data that the transmitting device 10 wishes to transmit.

[0250] The RF units 13 and 23 include one or more antennas. An antenna performs a function of transmitting signals processed by the RF units 13 and 23 to the exterior or receiving radio signals from the exterior to transfer the radio signals to the RF units 13 and 23. The antenna may also be called an antenna port. Each antenna may correspond to one physical antenna or may be configured by a combination of more than one physical antenna element. A signal transmitted through each antenna cannot be decomposed by the receiving device 20. A reference signal (RS) transmitted through an antenna defines the corresponding antenna viewed from the receiving device 20 and enables the receiving device 20 to perform channel estimation for the antenna, irrespective of whether a channel is a single RF channel from one physical antenna or a composite channel from a plurality of physical antenna elements including the antenna. That is, an antenna is defined such that a channel transmitting a symbol on the antenna may be derived from the channel transmitting another symbol on the same antenna. An RF unit supporting a MIMO function of transmitting and receiving data using a plurality of antennas may be connected to two or more antennas.

[0251] In embodiments of the present invention, a UE serves as the transmission device 10 on uplink and as the receiving device 20 on downlink. In embodiments of the present invention, an eNB serves as the receiving device 20 on uplink and as the transmission device 10 on downlink.

[0252] The transmitting device and/or the receiving device may be configured as a combination of one or more embodiments of the present invention.

[0253] The detailed description of the exemplary embodiments of the present invention has been given to enable those skilled in the art to implement and practice the invention. Although the invention has been described with reference to the exemplary embodiments, those skilled in the art will appreciate that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention described in the appended claims. For example, those skilled in the art may use each construction described in the above embodiments in combination with each other. Accordingly, the invention should not be limited to the specific embodiments described herein, but should be accorded the broadest scope consistent with the principles and novel features disclosed herein.

[0254] The present invention is applicable to a wireless communication device such as a user equipment (UE), relay, or base station (BS).