METHOD AND APPARATUS FOR TERMINAL TO PERFORM FREQUENCY MEASUREMENT ON BASIS OF CELL SPECIFIC PRIORITY

[0001] The present invention relates to a wireless communication system, and more particularly, to a method for a user equipment (UE) to perform frequency measurement based on a cell-specific priority in a wireless communication system, and an apparatus supporting the same.

[0002] 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) that is an advancement of UMTS (Universal Mobile Telecommunication System) is being introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonal frequency division multiple access) is used for downlink, and SC-FDMA (single carrier-frequency division multiple access) is used for uplink. The 3GPP LTE adopts MIMO (multiple input multiple output) having maximum four antennas. Recently, a discussion of 3GPP LTE-A (LTE-Advanced) which is the evolution of the 3GPP LTE is in progress.

[0003] In a wireless communication system, when a UE moves from one cell to another cell and performs cell selection/reselection and handover, the UE should measure the strength and quality of signals of neighboring cells. In order to measure the strength and quality of such signals, the UE measures the reference signal received power (RSRP) and the reference signal received quality (RSRQ) for the reference signal in the LTE system. The UE may perform measurement using the following three methods. A UE may perform measurement using the following three methods.

[0004] 1) Reference signal received power (RSRP): RSRP indicates the average received power of all resource elements (REs) carrying CRSs transmitted over the entire band. Here, the UE may measure the average received power of all REs carrying channel state information (CSI) RSs instead of CRSs.

[0005] 2) Received signal strength indicator (RSSI): RSSI indicates received power measured over the entire band. RSSI includes all of a signal, interference, and thermal noise.

[0006] 3) Reference symbol received quality (RSRQ): RSRQ indicates a channel quality indicator (CQI) and may be determined as RSRP/RSSI depending on a bandwidth or a sub-band. That is, RSRQ refers to signal-to-interference-plus-noise-ratio (SINR). Since RSRP does not provide sufficient mobility information, RSRQ may be used instead of RSRP in a handover or cell reselection process.

[0007] RSRQ may be calculated by RSSI/RSSP. Alternatively, RSRQ may be calculated by N*RSSI/RSSP. Here, N may be a parameter (for example, the number of PRBs) or a function associated with a bandwidth in which RSSI is measured.

[0008] A cell-specific priority is considered for load balancing among individual cells. When a UE receives a cell-specific priority set for each cell from a network, the UE needs to detect a cell and to evaluate the detected cells for cell reselection. However, according to the current measurement rules for cell reselection, only a frequency-specific priority is considered in measurement but a cell-specific priority is not considered in measurement. For efficient load balancing, a new measurement procedure needs to be proposed in which not only a frequency-specific priority, which is conventionally considered, but also a cell-specific priority is considered.

[0009] According to one embodiment, there is provided a method for performing, by a UE, frequency measurement based on a cell-specific priority in a wireless communication system. The method may include: receiving a cell-specific priority and a frequency-specific priority from a network; comparing a cell-specific priority of a first cell and a frequency-specific priority of a frequency to which the first cell belongs; determining whether to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs; and performing frequency measurement based on the determined priority, wherein the first cell may be a cell that is configured with a cell-specific priority, and the same cell-specific priority may be assigned for each frequency.

[0010] When the cell-specific priority of the first cell is higher than the frequency-specific priority of the frequency to which the first cell belongs, it may be determined to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs.

[0011] When the cell-specific priority of the first cell is lower than or equal to the frequency-specific priority of the frequency to which the first cell belongs, it may be determined not to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs.

[0012] The method may further include performing a cell reselection procedure based on a result of the measurement. When the UE detects the first cell, the cell-specific priority of the first cell may be applied to the first cell in the cell reselection procedure and may not be applied to the frequency to which the first cell belongs in the cell reselection procedure. The frequency-specific priority of the frequency to which the first cell belongs, which is received from the network, may be applied to the frequency to which the first cell belongs in the cell reselection procedure. The cell reselection procedure may be performed based on the cell-specific priority applied to the first cell and the frequency-specific priority applied to the frequency to which the first cell belongs.

[0013] When the UE does not detect the first cell, the cell reselection procedure may be performed based on the frequency-specific priority received from the network. The cell-specific priority of the first cell may not be applied to the frequency to which the first cell belongs in the cell reselection procedure.

[0014] According to another embodiment, there is provided a UE for performing frequency measurement based on a cell-specific priority in a wireless communication system. The UE may include: a memory; a transceiver; and a processor to connect the memory and the transceiver, wherein the processor may be configured to: control the transceiver to receive a cell-specific priority and a frequency-specific priority from a network; compare a cell-specific priority of a first cell and a frequency-specific priority of a frequency to which the first cell belongs; determine whether to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs; and perform frequency measurement based on the determined priority, the first cell may be a cell that is configured with a cell-specific priority, and the same cell-specific priority may be assigned for each frequency.

[0015] A UE considers not only a frequency-specific priority but also a cell-specific priority in a measurement procedure, thereby efficiently distributing loads across a network.

[0031] The technology described below can be used in various wireless communication systems such as code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), etc. The CDMA can be implemented with a radio technology such as universal terrestrial radio access (UTRA) or CDMA-2000. The TDMA can be implemented with a radio technology such as global system for mobile communications (GSM)/general packet ratio service (GPRS)/enhanced data rate for GSM evolution (EDGE). The OFDMA can be implemented with a radio technology such as institute of electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc. IEEE 802.16m is evolved from IEEE 802.16e, and provides backward compatibility with a system based on the IEEE 802.16e. The UTRA is a part of a universal mobile telecommunication system (UMTS). 3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced (LTE-A) is an evolution of the LTE.

[0032] For clarity, the following description will focus on LTE-A. However, technical features of the present invention are not limited thereto.

[0033] FIG. 1 shows LTE system architecture. The communication network is widely deployed to provide a variety of communication services such as voice over internet protocol (VoIP) through IMS and packet data.

[0034] Referring to FIG. 1, the LTE system architecture includes one or more user equipment (UE; 10), an evolved-UMTS terrestrial radio access network (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers to a communication equipment carried by a user. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, etc.

[0035] The E-UTRAN includes one or more evolved node-B (eNB) 20, and a plurality of UEs may be located in one cell. The eNB 20 provides an end point of a control plane and a user plane to the UE 10. The eNB 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as a base station (BS), a base transceiver system (BTS), an access point, etc. One eNB 20 may be deployed per cell. There are one or more cells within the coverage of the eNB 20. A single cell is configured to have one of bandwidths selected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlink or uplink transmission services to several UEs. In this case, different cells can be configured to provide different bandwidths.

[0036] Hereinafter, a downlink (DL) denotes communication from the eNB 20 to the UE 10, and an uplink (UL) denotes communication from the UE 10 to the eNB 20. In the DL, a transmitter may be a part of the eNB 20, and a receiver may be a part of the UE 10. In the UL, the transmitter may be a part of the UE 10, and the receiver may be a part of the eNB 20.

[0037] The EPC includes a mobility management entity (MME) which is in charge of control plane functions, and a system architecture evolution (SAE) gateway (S-GW) which is in charge of user plane functions. The MME/S-GW 30 may be positioned at the end of the network and connected to an external network. The MME has UE access information or UE capability information, and such information may be primarily used in UE mobility management. The S-GW is a gateway of which an endpoint is an E-UTRAN. The MME/S-GW 30 provides an end point of a session and mobility management function for the UE 10. The EPC may further include a packet data network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which an endpoint is a PDN.

[0038] The MME provides various functions including non-access stratum (NAS) signaling to eNBs 20, NAS signaling security, access stratum (AS) security control, Inter core network (CN) node signaling for mobility between 3GPP access networks, idle mode UE reachability (including control and execution of paging retransmission), tracking area list management (for UE in idle and active mode), P-GW and S-GW selection, MME selection for handovers with MME change, serving GPRS support node (SGSN) selection for handovers to 2G or 3G 3GPP access networks, roaming, authentication, bearer management functions including dedicated bearer establishment, support for public warning system (PWS) (which includes earthquake and tsunami warning system (ETWS) and commercial mobile alert system (CMAS)) message transmission. The S-GW host provides assorted functions including per-user based packet filtering (by e.g., deep packet inspection), lawful interception, UE Internet protocol (IP) address allocation, transport level packet marking in the DL, UL and DL service level charging, gating and rate enforcement, DL rate enforcement based on APN-AMBR. For clarity MME/S-GW 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both the MME and S-GW.

[0039] Interfaces for transmitting user traffic or control traffic may be used. The UE 10 and the eNB 20 are connected by means of a Uu interface. The eNBs 20 are interconnected by means of an X2 interface. Neighboring eNBs may have a meshed network structure that has the X2 interface. The eNBs 20 are connected to the EPC by means of an S1 interface. The eNBs 20 are connected to the MME by means of an S1-MME interface, and are connected to the S-GW by means of S1-U interface. The S1 interface supports a many-to-many relation between the eNB 20 and the MME/S-GW.

[0040] The eNB 20 may perform functions of selection for gateway 30, routing toward the gateway 30 during a radio resource control (RRC) activation, scheduling and transmitting of paging messages, scheduling and transmitting of broadcast channel (BCH) information, dynamic allocation of resources to the UEs 10 in both UL and DL, configuration and provisioning of eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 may perform functions of paging origination, LTE_IDLE state management, ciphering of the user plane, SAE bearer control, and ciphering and integrity protection of NAS signaling.

[0041] FIG. 2 shows a control plane of a radio interface protocol of an LTE system. FIG. 3 shows a user plane of a radio interface protocol of an LTE system.

[0042] Layers of a radio interface protocol between the UE and the E-UTRAN may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. The radio interface protocol between the UE and the E-UTRAN may be horizontally divided into a physical layer, a data link layer, and a network layer, and may be vertically divided into a control plane (C-plane) which is a protocol stack for control signal transmission and a user plane (U-plane) which is a protocol stack for data information transmission. The layers of the radio interface protocol exist in pairs at the UE and the E-UTRAN, and are in charge of data transmission of the Uu interface.

[0043] A physical (PHY) layer belongs to the L1. The PHY layer provides a higher layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer, which is a higher layer of the PHY layer, through a transport channel. A physical channel is mapped to the transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. Between different PHY layers, i.e., a PHY layer of a transmitter and a PHY layer of a receiver, data is transferred through the physical channel using radio resources. The physical channel is modulated using an orthogonal frequency division multiplexing (OFDM) scheme, and utilizes time and frequency as a radio resource.

[0044] The PHY layer uses several physical control channels. A physical downlink control channel (PDCCH) reports to a UE about resource allocation of a paging channel (PCH) and a downlink shared channel (DL-SCH), and hybrid automatic repeat request (HARQ) information related to the DL-SCH. The PDCCH may carry a UL grant for reporting to the UE about resource allocation of UL transmission. A physical control format indicator channel (PCFICH) reports the number of OFDM symbols used for PDCCHs to the UE, and is transmitted in every subframe. A physical hybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement (ACK)/non-acknowledgement (NACK) signal in response to UL transmission. A physical uplink control channel (PUCCH) carries UL control information such as HARQ ACK/NACK for DL transmission, scheduling request, and CQI. A physical uplink shared channel (PUSCH) carries a UL-uplink shared channel (SCH).

[0045] A physical channel consists of a plurality of subframes in time domain and a plurality of subcarriers in frequency domain. One subframe consists of a plurality of symbols in the time domain. One subframe consists of a plurality of resource blocks (RBs). One RB consists of a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific symbols of a corresponding subframe for a PDCCH. For example, a first symbol of the subframe may be used for the PDCCH. The PDCCH carries dynamic allocated resources, such as a physical resource block (PRB) and modulation and coding scheme (MCS). A transmission time interval (TTI) which is a unit time for data transmission may be equal to a length of one subframe. The length of one subframe may be 1 ms.

[0046] The transport channel is classified into a common transport channel and a dedicated transport channel according to whether the channel is shared or not. A DL transport channel for transmitting data from the network to the UE includes a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting a paging message, a DL-SCH for transmitting user traffic or control signals, etc. The DL-SCH supports HARQ, dynamic link adaptation by varying the modulation, coding and transmit power, and both dynamic and semi-static resource allocation. The DL-SCH also may enable broadcast in the entire cell and the use of beamforming. The system information carries one or more system information blocks. All system information blocks may be transmitted with the same periodicity. Traffic or control signals of a multimedia broadcast/multicast service (MBMS) may be transmitted through the DL-SCH or a multicast channel (MCH).

[0047] A UL transport channel for transmitting data from the UE to the network includes a random access channel (RACH) for transmitting an initial control message, a UL-SCH for transmitting user traffic or control signals, etc. The UL-SCH supports HARQ and dynamic link adaptation by varying the transmit power and potentially modulation and coding. The UL-SCH also may enable the use of beamforming. The RACH is normally used for initial access to a cell.

[0048] A MAC layer belongs to the L2. The MAC layer provides services to a radio link control (RLC) layer, which is a higher layer of the MAC layer, via a logical channel. The MAC layer provides a function of mapping multiple logical channels to multiple transport channels. The MAC layer also provides a function of logical channel multiplexing by mapping multiple logical channels to a single transport channel. A MAC sublayer provides data transfer services on logical channels.

[0049] The logical channels are classified into control channels for transferring control plane information and traffic channels for transferring user plane information, according to a type of transmitted information. That is, a set of logical channel types is defined for different data transfer services offered by the MAC layer. The logical channels are located above the transport channel, and are mapped to the transport channels.

[0050] The control channels are used for transfer of control plane information only. The control channels provided by the MAC layer include a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH) and a dedicated control channel (DCCH). The BCCH is a downlink channel for broadcasting system control information. The PCCH is a downlink channel that transfers paging information and is used when the network does not know the location cell of a UE. The CCCH is used by UEs having no RRC connection with the network. The MCCH is a point-to-multipoint downlink channel used for transmitting MBMS control information from the network to a UE. The DCCH is a point-to-point bi-directional channel used by UEs having an RRC connection that transmits dedicated control information between a UE and the network.

[0051] Traffic channels are used for the transfer of user plane information only. The traffic channels provided by the MAC layer include a dedicated traffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCH is a point-to-point channel, dedicated to one UE for the transfer of user information and can exist in both uplink and downlink. The MTCH is a point-to-multipoint downlink channel for transmitting traffic data from the network to the UE.

[0052] Uplink connections between logical channels and transport channels include the DCCH that can be mapped to the UL-SCH, the DTCH that can be mapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH. Downlink connections between logical channels and transport channels include the BCCH that can be mapped to the BCH or DL-SCH, the PCCH that can be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, and the DTCH that can be mapped to the DL-SCH, the MCCH that can be mapped to the MCH, and the MTCH that can be mapped to the MCH.

[0053] An RLC layer belongs to the L2. The RLC layer provides a function of adjusting a size of data, so as to be suitable for a lower layer to transmit the data, by concatenating and segmenting the data received from an upper layer in a radio section. In addition, to ensure a variety of quality of service (QoS) required by a radio bearer (RB), the RLC layer provides three operation modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The AM RLC provides a retransmission function through an automatic repeat request (ARQ) for reliable data transmission. Meanwhile, a function of the RLC layer may be implemented with a functional block inside the MAC layer. In this case, the RLC layer may not exist.

[0054] A packet data convergence protocol (PDCP) layer belongs to the L2. The PDCP layer provides a function of header compression function that reduces unnecessary control information such that data being transmitted by employing IP packets, such as IPv4 or IPv6, can be efficiently transmitted over a radio interface that has a relatively small bandwidth. The header compression increases transmission efficiency in the radio section by transmitting only necessary information in a header of the data. In addition, the PDCP layer provides a function of security. The function of security includes ciphering which prevents inspection of third parties, and integrity protection which prevents data manipulation of third parties.

[0055] A radio resource control (RRC) layer belongs to the L3. The RLC layer is located at the lowest portion of the L3, and is only defined in the control plane. The RRC layer takes a role of controlling a radio resource between the UE and the network. For this, the UE and the network exchange an RRC message through the RRC layer. The RRC layer controls logical channels, transport channels, and physical channels in relation to the configuration, reconfiguration, and release of RBs. An RB is a logical path provided by the L1 and L2 for data delivery between the UE and the network. That is, the RB signifies a service provided the L2 for data transmission between the UE and E-UTRAN. The configuration of the RB implies a process for specifying a radio protocol layer and channel properties to provide a particular service and for determining respective detailed parameters and operations. The RB is classified into two types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting an RRC message in the control plane. The DRB is used as a path for transmitting user data in the user plane.

[0056] A Non-Access Stratum (NAS) layer placed over the RRC layer performs functions, such as session management and mobility management.

[0057] Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB on the network side) may perform functions such as scheduling, automatic repeat request (ARQ), and hybrid automatic repeat request (HARQ). The RRC layer (terminated in the eNB on the network side) may perform functions such as broadcasting, paging, RRC connection management, RB control, mobility functions, and UE measurement reporting and controlling. The NAS control protocol (terminated in the MME of gateway on the network side) may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for the signaling between the gateway and UE.

[0058] Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB on the network side) may perform the same functions for the control plane. The PDCP layer (terminated in the eNB on the network side) may perform the user plane functions such as header compression, integrity protection, and ciphering.

[0059] Hereinafter, An RRC state of a UE and RRC connection procedure are described.

[0060] An RRC state indicates whether an RRC layer of the UE is logically connected to an RRC layer of the E-UTRAN. The RRC state may be divided into two different states such as an RRC connected state and an RRC idle state. When an RRC connection is established between the RRC layer of the UE and the RRC layer of the E-UTRAN, the UE is in RRC_CONNECTED, and otherwise the UE is in RRC_IDLE. Since the UE in RRC_CONNECTED has the RRC connection established with the E-UTRAN, the E-UTRAN may recognize the existence of the UE in RRC_CONNECTED and may effectively control the UE. Meanwhile, the UE in RRC_IDLE may not be recognized by the E-UTRAN, and a CN manages the UE in unit of a TA which is a larger area than a cell. That is, only the existence of the UE in RRC_IDLE is recognized in unit of a large area, and the UE must transition to RRC_CONNECTED to receive a typical mobile communication service such as voice or data communication.

[0061] In RRC_IDLE state, the UE may receive broadcasts of system information and paging information while the UE specifies a discontinuous reception (DRX) configured by NAS, and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area and may perform public land mobile network (PLMN) selection and cell re-selection. Also, in RRC_IDLE state, no RRC context is stored in the eNB.

[0062] In RRC_CONNECTED state, the UE has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to/from the eNB becomes possible. Also, the UE can report channel quality information and feedback information to the eNB. In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE belongs. Therefore, the network can transmit and/or receive data to/from UE, the network can control mobility (handover and inter-radio access technologies (RAT) cell change order to GSM EDGE radio access network (GERAN) with network assisted cell change (NACC)) of the UE, and the network can perform cell measurements for a neighboring cell.

[0063] In RRC_IDLE state, the UE specifies the paging DRX cycle. Specifically, the UE monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle. The paging occasion is a time interval during which a paging signal is transmitted. The UE has its own paging occasion.

[0064] A paging message is transmitted over all cells belonging to the same tracking area. If the UE moves from one TA to another TA, the UE will send a tracking area update (TAU) message to the network to update its location.

[0065] When the user initially powers on the UE, the UE first searches for a proper cell and then remains in RRC_IDLE in the cell. When there is a need to establish an RRC connection, the UE which remains in RRC_IDLE establishes the RRC connection with the RRC of the E-UTRAN through an RRC connection procedure and then may transition to RRC_CONNECTED. The UE which remains in RRC_IDLE may need to establish the RRC connection with the E-UTRAN when uplink data transmission is necessary due to a user's call attempt or the like or when there is a need to transmit a response message upon receiving a paging message from the E-UTRAN.

[0066] To manage mobility of the UE in the NAS layer, two states are defined, i.e., an EPS mobility management-REGISTERED (EMM-REGISTERED) state and an EMM-DEREGISTERED state. These two states apply to the UE and the MME. Initially, the UE is in the EMM-DEREGISTERED state. To access a network, the UE performs a process of registering to the network through an initial attach procedure. If the attach procedure is successfully performed, the UE and the MME enter the EMM-REGISTERED state.

[0067] To manage a signaling connection between the UE and the EPC, two states are defined, i.e., an EPS connection management (ECM)-IDLE state and an ECM-CONNECTED state. These two states apply to the UE and the MME. When the UE in the ECM-IDLE state establishes an RRC connection with the E-UTRAN, the UE enters the ECM-CONNECTED state. When the MME in the ECM-IDLE state establishes an SI connection with the E-UTRAN, the MME enters the ECM-CONNECTED state. When the UE is in the ECM-IDLE state, the E-UTRAN does not have context information of the UE. Therefore, the UE in the ECM-IDLE state performs a UE-based mobility related procedure such as cell selection or reselection without having to receive a command of the network. On the other hand, when the UE is in the ECM-CONNECTED state, mobility of the UE is managed by the command of the network. If a location of the UE in the ECM-IDLE state becomes different from a location known to the network, the UE reports the location of the UE to the network through a tracking area update procedure.

[0068] FIG. 4 shows a procedure in which UE that is initially powered on experiences a cell selection process, registers it with a network, and then performs cell reselection if necessary.

[0069] Referring to FIG. 4, the UE selects Radio Access Technology (RAT) in which the UE communicates with a Public Land Mobile Network (PLMN), that is, a network from which the UE is provided with service (S410). Information about the PLMN and the RAT may be selected by the user of the UE, and the information stored in a Universal Subscriber Identity Module (USIM) may be used.

[0070] The UE selects a cell that has the greatest value and that belongs to cells having measured BS and signal intensity or quality greater than a specific value (cell selection) (S420). In this case, the UE that is powered off performs cell selection, which may be called initial cell selection. A cell selection procedure is described later in detail. After the cell selection, the UE receives system information periodically by the BS. The specific value refers to a value that is defined in a system in order for the quality of a physical signal in data transmission/reception to be guaranteed. Accordingly, the specific value may differ depending on applied RAT.

[0071] If network registration is necessary, the UE performs a network registration procedure (S430). The UE registers its information (e.g., an IMSI) with the network in order to receive service (e.g., paging) from the network. The UE does not register it with a network whenever it selects a cell, but registers it with a network when information about the network (e.g., a Tracking Area Identity (TAI)) included in system information is different from information about the network that is known to the UE.

[0072] The UE performs cell reselection based on a service environment provided by the cell or the environment of the UE (S440). If the value of the intensity or quality of a signal measured based on a BS from which the UE is provided with service is lower than that measured based on a BS of a neighboring cell, the UE selects a cell that belongs to other cells and that provides better signal characteristics than the cell of the BS that is accessed by the UE. This process is called cell reselection differently from the initial cell selection of the No. 2 process. In this case, temporal restriction conditions are placed in order for a cell to be frequently reselected in response to a change of signal characteristic. A cell reselection procedure is described later in detail.

[0073] FIG. 5 shows an RRC connection establishment procedure.

[0074] The UE sends an RRC connection request message that requests RRC connection to a network (S510). The network sends an RRC connection establishment message as a response to the RRC connection request (S520). After receiving the RRC connection establishment message, the UE enters RRC connected mode.

[0075] The UE sends an RRC connection establishment complete message used to check the successful completion of the RRC connection to the network (S530).

[0076] FIG. 6 shows an RRC connection reconfiguration procedure.

[0077] An RRC connection reconfiguration is used to modify RRC connection. This is used to establish/modify/release RBs, perform handover, and set up/modify/release measurements.

[0078] A network sends an RRC connection reconfiguration message for modifying RRC connection to UE (S610). As a response to the RRC connection reconfiguration message, the UE sends an RRC connection reconfiguration complete message used to check the successful completion of the RRC connection reconfiguration to the network (S620).

[0079] The following is a detailed description of a procedure of selecting a cell by a UE.

[0080] When power is turned-on or the UE is located in a cell, the UE performs procedures for receiving a service by selecting/reselecting a suitable quality cell.

[0081] A UE in an RRC idle state should prepare to receive a service through the cell by always selecting a suitable quality cell. For example, a UE where power is turned-on just before should select the suitable quality cell to be registered in a network. If the UE in an RRC connection state enters in an RRC idle state, the UE should selects a cell for stay in the RRC idle state. In this way, a procedure of selecting a cell satisfying a certain condition by the UE in order to be in a service idle state such as the RRC idle state refers to cell selection. Since the cell selection is performed in a state that a cell in the RRC idle state is not currently determined, it is important to select the cell as rapid as possible. Accordingly, when the cell provides a wireless signal quality of a predetermined level or greater, although the cell does not provide the best wireless signal quality, the cell may be selected during a cell selection procedure of the UE.

[0082] Hereinafter, a method and a procedure of selecting a cell by a UE in a 3GPP LTE is described.

[0083] A cell selection process is basically divided into two types.

[0084] The first is an initial cell selection process. In this process, UE does not have preliminary information about a wireless channel. Accordingly, the UE searches for all wireless channels in order to find out a proper cell. The UE searches for the strongest cell in each channel. Thereafter, when the UE has only to search for a suitable cell that satisfies a cell selection criterion, the UE selects the corresponding cell.

[0085] Next, the UE may select the cell using stored information or using information broadcasted by the cell. Accordingly, cell selection may be fast compared to an initial cell selection process. If the UE has only to search for a cell that satisfies the cell selection criterion, the UE selects the corresponding cell. If a suitable cell that satisfies the cell selection criterion is not retrieved though such a process, the UE performs an initial cell selection process.

[0086] After the UE selects a specific cell through the cell selection process, the intensity or quality of a signal between the UE and a BS may be changed due to a change in the mobility or wireless environment of the UE. Accordingly, when the quality of the selected cell is deteriorated, the UE may select another cell that provides better quality. If a cell is reselected as described above, the UE selects a cell that provides better signal quality than the currently selected cell. Such a process is called cell reselection. In general, a basic object of the cell reselection process is to select a cell that provides UE with the best quality from a viewpoint of the quality of a radio signal.

[0087] In addition to the viewpoint of the quality of a radio signal, a network may determine priority corresponding to each frequency, and may inform the UE of the determined priorities. The UE that has received the priorities preferentially takes into consideration the priorities in a cell reselection process compared to a radio signal quality criterion.

[0088] As described above, there is a method of selecting or reselecting a cell according to the signal characteristics of a wireless environment. In selecting a cell for reselection when a cell is reselected, the following cell reselection methods may be present according to the RAT and frequency characteristics of the cell.

[0089] Intra-frequency cell reselection: UE reselects a cell having the same center frequency as that of RAT, such as a cell on which the UE camps on.

[0090] Inter-frequency cell reselection: UE reselects a cell having a different center frequency from that of RAT, such as a cell on which the UE camps on

[0091] Inter-RAT cell reselection: UE reselects a cell that uses RAT different from RAT on which the UE camps

[0092] The principle of a cell reselection process is as follows.

[0093] First, UE measures the quality of a serving cell and neighboring cells for cell reselection.

[0094] Second, cell reselection is performed based on a cell reselection criterion. The cell reselection criterion has the following characteristics in relation to the measurements of a serving cell and neighboring cells.

[0095] Intra-frequency cell reselection is basically based on ranking. Ranking is a task for defining a criterion value for evaluating cell reselection and numbering cells using criterion values according to the size of the criterion values. A cell having the best criterion is commonly called the best-ranked cell. The cell criterion value is based on the value of a corresponding cell measured by UE, and may be a value to which a frequency offset or cell offset has been applied, if necessary.

[0096] Inter-frequency cell reselection is based on frequency priority provided by a network. A UE attempts to camp on a frequency having the highest frequency priority. A network may provide frequency priority that will be applied by UEs within a cell in common through broadcasting signaling, or may provide frequency-specific priority to each UE through UE-dedicated signaling. A cell reselection priority provided through broadcast signaling may refer to a common priority. A cell reselection priority for each UE set by a network may refer to a dedicated priority. When receiving the dedicated priority, the UE may receive a valid time associated with the dedicated priority together. When receiving the dedicated priority, the UE starts a validity timer set as the received valid time together therewith. While the valid timer is operated, the UE applies the dedicated priority in the RRC idle mode. When the valid timer is expired, the UE discards the dedicated priority and again applies the common priority.

[0097] For the inter-frequency cell reselection, a network may provide UE with a parameter (e.g., a frequency-specific offset) used in cell reselection for each frequency.

[0098] For the intra-frequency cell reselection or the inter-frequency cell reselection, a network may provide UE with a Neighboring Cell List (NCL) used in cell reselection. The NCL includes a cell-specific parameter (e.g., a cell-specific offset) used in cell reselection.

[0099] For the intra-frequency or inter-frequency cell reselection, a network may provide UE with a cell reselection black list used in cell reselection. The UE does not perform cell reselection on a cell included in the black list.

[0100] Ranking performed in a cell reselection evaluation process is described below.

[0101] A ranking criterion used to apply priority to a cell is defined as in Equation 1.

R_S=Q_meas.s+Q_hyst. R_n=Q_meas.n−Q_offset   [Equation 1]

[0102] In this case, Rs is the ranking criterion of a serving cell, Rn is the ranking criterion of a neighboring cell, Qmeas,s is the quality value of the serving cell measured by UE, Qmeas,n is the quality value of the neighboring cell measured by UE, Qhyst is the hysteresis value for ranking, and Qoffset is an offset between the two cells.

[0103] In Intra-frequency, when UE receives an offset “Qoffsets,n” between a serving cell and a neighboring cell, Qoffset=Qoffsets,n. When UE does not Qoffsets,n, Qoffset=0.

[0104] In Inter-frequency, when UE receives an offset “Qoffsets,n” for a corresponding cell, Qoffset=Qoffsets,n+Qfrequency. When UE does not receive “Qoffsets,n”, Qoffset=Qfrequency.

[0105] When the ranking criterion Rs of a serving cell and the ranking criterion Rn of a neighboring cell are changed in a similar state, ranking priority is frequency changed as a result of the change, and UE may alternately reselect the twos. Qhyst is a parameter that gives hysteresis to cell reselection so that UE is prevented from to alternately reselecting two cells.

[0106] UE measures RS of a serving cell and Rn of a neighboring cell according to the above equation, considers a cell having the greatest ranking criterion value to be the best-ranked cell, and reselects the cell. When a reselected cell is not a suitable cell, UE excludes a corresponding frequency or a corresponding cell from the subject of cell reselection.

[0107] FIG. 7 shows an RRC connection re-establishment procedure.

[0108] Referring to FIG. 7, UE stops using all the radio bearers that have been configured other than a Signaling Radio Bearer (SRB) #0, and initializes a variety of kinds of sublayers of an Access Stratum (AS) (S710). Furthermore, the UE configures each sublayer and the PHY layer as a default configuration. In this procedure, the UE maintains the RRC connection state.

[0109] The UE performs a cell selection procedure for performing an RRC connection reconfiguration procedure (S720). The cell selection procedure of the RRC connection re-establishment procedure may be performed in the same manner as the cell selection procedure that is performed by the UE in the RRC idle state, although the UE maintains the RRC connection state.

[0110] After performing the cell selection procedure, the UE determines whether or not a corresponding cell is a suitable cell by checking the system information of the corresponding cell (S730). When the selected cell is determined to be a suitable E-UTRAN cell, the UE sends an RRC connection re-establishment request message to the corresponding cell (S740).

[0111] Meanwhile, when the selected cell is determined to be a cell that uses RAT different from that of the E-UTRAN through the cell selection procedure for performing the RRC connection re-establishment procedure, the UE stops the RRC connection re-establishment procedure and enters the RRC idle state (S750).

[0112] The UE may be implemented to finish checking whether the selected cell is a suitable cell through the cell selection procedure and the reception of the system information of the selected cell. To this end, the UE may drive a timer when the RRC connection re-establishment procedure is started. The timer may be stopped when it is determined that the UE has selected a suitable cell. When the timer expires, the UE may consider that the RRC connection re-establishment procedure has failed, and may enter the RRC idle state. Such a timer is hereinafter called an RLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as an RLF timer. The UE may obtain the set value of the timer from the system information of the serving cell.

[0113] When an RRC connection re-establishment request message is received from the UE and the request is accepted, a cell sends an RRC connection re-establishment message to the UE.

[0114] The UE that has received the RRC connection re-establishment message from the cell reconfigures a PDCP sublayer and an RLC sublayer with an SRB1. Furthermore, the UE calculates various key values related to security setting, and reconfigures a PDCP sublayer responsible for security as the newly calculated security key values. Accordingly, the SRB 1 between the UE and the cell is open, and the UE and the cell may exchange RRC control messages. The UE completes the restart of the SRB1, and sends an RRC connection re-establishment complete message indicative of that the RRC connection re-establishment procedure has been completed to the cell (S760).

[0115] In contrast, when the RRC connection re-establishment request message is received from the UE and the request is not accepted, the cell sends an RRC connection re-establishment reject message to the UE.

[0116] When the RRC connection re-establishment procedure is successfully performed, the cell and the UE perform an RRC connection reconfiguration procedure. Accordingly, the UE recovers the state prior to the execution of the RRC connection re-establishment procedure, and the continuity of service is guaranteed to the upmost.

[0117] FIG. 8 shows a conventional method of performing measurement.

[0118] A UE receives measurement configuration information from a BS (S810). A message including the measurement configuration information is referred to as a measurement configuration message. The UE performs measurement based on the measurement configuration information (S820). When a measurement result satisfies a reporting condition included in the measurement configuration information, the UE reports the measurement result to the BS (S830). A message including the measurement result is referred to as a measurement report message.

[0119] The measurement configuration information may include the following information.

[0120] (1) Measurement object: The object is on which the UE performs the measurements. The measurement object includes at least one of an intra-frequency measurement object which is an object of intra-frequency measurement, an inter-frequency measurement object which is an object of inter-frequency measurement, and an inter-RAT measurement object which is an object of inter-RAT measurement. For example, the intra-frequency measurement object may indicate a neighboring cell having the same frequency as a frequency of a serving cell, the inter-frequency measurement object may indicate a neighboring cell having a different frequency from a frequency of the serving cell, and the inter-RAT measurement object may indicate a neighboring cell of a different RAT from an RAT of the serving cell.

[0121] (2) Reporting configuration: This includes a reporting criterion and a reporting format. The reporting criterion is used to trigger the UE to send a measurement report and can either be periodical or a single event description. The reporting format is a quantity that the UE includes in measurement reporting and associated information (e.g. number of cells to report).

[0122] (3) Measurement identify: Each measurement identity links one measurement object with one reporting configuration. By configuring multiple measurement identities, it is possible to link more than one measurement object to the same reporting configuration, as well as to link more than one reporting configuration to the same measurement object. The measurement identity is used as a reference number in measurement reporting. The measurement identify may be included in measurement reporting to indicate a specific measurement object for which the measurement result is obtained and a specific reporting condition according to which measurement reporting is triggered.

[0123] (4) Quantity configuration: One quantity configuration is configured per RAT type. The quantity configuration defines the measurement quantities and associated filtering used for all event evaluation and related reporting of that measurement type. One filter can be configured per measurement quantity.

[0124] (5) Measurement gaps: Measurement gaps are periods that the UE may use to perform measurements when downlink transmission and uplink transmission are not scheduled.

[0125] To perform a measurement procedure, the UE has a measurement object, a reporting configuration, and a measurement identity.

[0126] In 3GPP LTE, the BS can assign only one measurement object to the UE with respect to one frequency. Events for triggering measurement reporting are shown in the table 1. When the measurement result of the UE satisfies the determined event, the UE transmits a measurement report message to the BS.

	TABLE 1
	Event	Reporting Condition
	Event A1	Serving becomes better than threshold
	Event A2	Serving becomes worse than threshold
	Event A3	Neighboring becomes offset better than PCell/PSCell
	Event A4	Neighboring becomes better than threshold
	Event A5	PCell/PSCell becomes worse than threshold1 and
		neighboring becomes better than threshold2
	Event A6	Neighboring becomes offset better than SCell
	Event B1	Inter RAT neighboring becomes better than threshold
	Event B2	PCell becomes worse than threshold1 and inter RAT
		neighboring becomes better than threshold2
	Event C1	CSI-RS resource becomes better than threshold
	Event C2	CSI-RS resource becomes offset better than reference
		CSI-RS resource

[0127] The measurement report may include the measurement identity, a measured quality of the serving cell, and a measurement result of the neighboring cell. The measurement identity identifies a measurement object in which the measurement report is triggered. The measurement result of the neighboring cell may include a cell identity and a measurement quality of the neighboring cell. The measured quality may include at least one of reference signal received power (RSRP) and reference signal received quality (RSRQ).

[0128] FIG. 9 illustrates an example of a heterogeneous network (HetNet).

[0129] Referring to FIG. 9, the heterogeneous network is a network in which different types of cells are mixed and operated. There are many nodes overlapping in the heterogeneous network, which may include a picocell, a microcell, a femtocell or a home BS (eNB). Although the use of small cells is not limited, a picocell may generally be installed in an area with high demand for data services, a femtocell may be installed in an indoor office or home, and a wireless repeater may be installed for supplementing macrocell coverage. Small cells may be classified into a closed subscriber group (CSG) available only to a particular user according to access restriction, an open access group that general users are allowed to access, and a hybrid access group that is a combination of these two types.

[0130] FIG. 10 illustrates a problem that may occur when cell (re)selection is performed based on a frequency priority in a heterogeneous network environment.

[0131] Generally, a UE in an idle mode measures the signal level of a cell on a current frequency and also measures the signal level of a cell on another RAT or another frequency. Different frequencies and different RATs can be assigned different priorities. Such priorities may be provided to the UE as part of system information (SIB). Alternatively, such priorities may be provided to the UE via an RRC connection release message. It is assumed that the UE in the idle mode may follow these priorities when performing measurement for cell selection or reselection.

[0132] Measurement on a frequency/RAT with a high priority is always performed. Measurement on a frequency/RAT with a low priority may be performed only when the signal level of a current serving cell is lower than a defined threshold (threshServingLow). When multiple cells with the same priority are found on a frequency, a cell having the highest signal level in view of a set offset and hysteresis may be selected.

[0133] Referring to FIG. 10, it is assumed that a first cell and a second cell are macrocells and a third cell is a small cell located at the same frequency as the second cell in order to provide additional capacity in the coverage. It is assumed that F1 denotes a source frequency, and F2 denotes a target frequency. It is assumed that the UE is camping on the first cell.

[0134] In this scenario, the third cell may have very small coverage. Considering a frequency-specific priority, it is required not only to relocate a UE that is positioned both in the coverage of the third cell and in the coverage of the first cell but also to relocate a UE that is positioned outside the coverage of the third cell but is positioned in the coverage of the first cell. Thus, it may be difficult to fully utilize the capacity of the third cell by offloading the loads of the first cell and the second cell to the third cell. In order to solve such a problem, a conventional method of providing a cell-specific priority (CSP) to a UE has been proposed.

[0135] Table 2 shows an illustrative configuration of cell-specific priorities.

TABLE 2
	Cell-specific priority	Cell-specific priority	Cell-specific priority
	of first cell (F1)	of second cell (F2)	of third cell (F2)
F1	5	4	4
F2	4	5	5

[0136] This configuration may mean that in the idle mode, the frequency of a current serving cell (first cell) has a higher priority than another frequency. Thus, the UE may continue to camp on the current location and may not perform measurement on a frequency with lower priority until the signal level of the serving cell becomes lower than the defined threshold.

[0137] Table 3 shows an illustrative reconfiguration of cell-specific priorities.

TABLE 3
	Cell-specific priority	Cell-specific priority	Cell-specific priority
	of first cell (F1)	of second cell (F2)	of third cell (F2)
F1	5	4	4
F2	4 + (third cell: 6)	5	5

[0138] That is, the third cell may be assigned a higher priority than the first cell. With this configuration, the third cell (F2) can attract the UE in the idle mode camping on the first cell (F1). A static UE in the coverage of the third cell or a UE that slowly roams in the coverage of the third cell may be highly likely to be relocated.

[0139] As described above, a cell-specific priority is currently considered as a potential solution for load balancing between individual cells beyond load balancing between carrier levels. When a UE is assigned a cell-specific priority for a cell included in a cell list, the UE may detect the cell included in the cell list and may need to evaluate the detected cells for cell reselection. The detection of the cell having the cell-specific priority may be completed when the UE measures the frequency of the cell.

[0140] However, according to ‘Measurement rules for cell re-selection’ disclosed in 3GPP TS 36.304 5.2.4.2, a UE performs frequency measurement, which is controlled based on a priority associated with a serving frequency and a target frequency and the signal strength/quality of a serving cell. That is, the UE does not consider a cell-specific priority in frequency measurement. When a UE that is assigned a cell-specific priority for a frequency considers the conventional priority as a frequency reselection priority, the UE may not be able to detect and measure a cell having the cell-specific priority. Therefore, even though the cell-specific priority is provided to the UE, the UE cannot reselect the cell with the provided cell-specific priority. Hereinafter, a method for a UE to perform frequency measurement based on a cell-specific priority according to an embodiment of the present invention will be described. In addition, a method for a UE to perform cell reselection based on a cell-specific priority according to an embodiment of the present invention will be described.

[0141] FIG. 11 illustrates a method for a UE to perform frequency measurement based on a cell-specific priority according to an embodiment of the present invention.

[0142] Referring to FIG. 11, a UE may receive a cell-specific priority and a frequency-specific priority from a network (S1110). That is, the UE may be assigned the cell-specific priority and the frequency-specific priority. The cell-specific priority and the frequency-specific priority may be received via system information. The cell-specific priority and the frequency-specific priority may be broadcast by the network. The same cell-specific priority may be assigned for each frequency.

[0143] The UE may determine whether to apply the cell-specific priority instead of the frequency-specific priority (S1120). To this end, the UE may compare the cell-specific priority of a particular cell with the frequency-specific priority of a frequency to which the particular cell belongs. The particular cell may be a cell that is configured with a cell-specific priority by the network.

[0144] When the cell-specific priority of the particular cell is higher than the frequency-specific priority of the frequency to which the particular cell belongs, the frequency-specific priority of the frequency to which the particular cell belongs may be replaced with the cell-specific priority of the particular cell (S1130). That is, the cell-specific priority of the particular cell may be applied to the frequency to which the particular cell belongs. The UE may perform frequency measurement based on the changed priority of the frequency to which the particular cell belongs (that is, the cell-specific priority of the particular cell) (S1140).

[0145] When the cell-specific priority of the particular cell is lower than or equal to the frequency-specific priority of the frequency to which the particular cell belongs, the frequency-specific priority of the frequency to which the particular cell belongs may not be changed. That is, the cell-specific priority of the particular cell may not be applied to the frequency to which the specific cell belongs. The UE may perform frequency measurement based on the frequency-specific priority received from the network (S1140).

[0146] According to the embodiment of the present invention, 3GPP TS 36.304 5.2.4.2 may be newly defined as shown in Table 4.

TABLE 4
5.2.4.2 Measurement rules for cell re-selection
When evaluating Srxlev and Squal of non-serving cells for reselection
purposes, the UE shall use parameters provided by the serving cell.
When a UE is configured with cell-specific priorities included in system
information, the UE shall use the cell-specific priorities as reselection
priorities instead of frequency-specific priorities for measurements of a
frequency, if the cell-specific priority of a cell on the frequency is higher
than the frequency-specific priority (reselection priority) for the
frequency.
Following rules are used by the UE to limit needed measurements:
If the serving cell fulfils Srxlev > SIntraSearchP and Squal > SIntraSearchQ,
the UE may choose not to perform intra-frequency measurements.
Otherwise, the UE shall perform intra-frequency measurements.
The UE shall apply the following rules for E-UTRAN inter-frequencies
and inter-RAT frequencies which are indicated in system information
and for which the UE has priority provided as defined in 5.2.4.1:
For an E-UTRAN inter-frequency or inter-RAT frequency with a
reselection priority higher than the reselection priority of the current
E-UTRA frequency the UE shall perform measurements of higher
priority E-UTRAN inter-frequency or inter-RAT frequencies
according to [10].
For an E-UTRAN inter-frequency with an equal or lower reselection
priority than the reselection priority of the current E-UTRA frequency
and for inter-RAT frequency with lower reselection priority than the
reselection priority of the current E-UTRAN frequency:
If the serving cell fulfils Srxlev > SnonIntraSearchP and
Squal > SnonIntraSearchQ, the UE may choose not to perform measurements
of E-UTRAN inter-frequencies or inter-RAT frequency cells of equal or
lower priority.
Otherwise, the UE shall perform measurements of E-UTRAN inter-
frequencies or inter-RAT frequency cells of equal or lower priority
according to [10].

[0147] Referring to Table 4, when a UE is assigned a cell-specific priority and the cell-specific priority of a cell on a frequency is higher than the frequency-specific priority of the frequency, the UE may use the cell-specific priority as a reselection priority instead of the frequency-specific priority for frequency measurement. Therefore, the UE may consider the cell-specific priority in a cell reselection procedure.

[0148] Alternatively, when the UE is assigned a cell-specific priority from a network, the UE may perform measurement on a frequency by applying relaxed measurement performance requirements (for example, a low measurement performance group) according to the cell-specific priority of the frequency and the frequency-specific priority of the frequency. Specifically, when the cell-specific priority of the frequency is higher than the frequency-specific priority of the frequency, the UE may perform measurement on the frequency by applying the relaxed measurement performance requirements.

[0149] FIG. 12 illustrates a method for a UE to perform cell reselection based on a cell-specific priority according to an embodiment of the present invention.

[0150] Referring to FIG. 12, when the cell-specific priority of a particular cell is higher than the frequency-specific priority of a frequency to which the particular cell belongs, a UE may replace the frequency-specific priority with the cell-specific priority (S1210).

[0151] The UE may determine whether a cell configured with the cell-specific priority is detected (S1220).

[0152] When the UE detects the cell configured with cell-specific priority is set, the UE may apply the cell-specific priority to a cell reselection evaluation (S1221). For example, when the UE that is assigned the cell-specific priority detects the cell configured with the cell-specific priority, the UE may apply the cell-specific priority only to the detected cell. The UE may not apply the cell-specific priority to a frequency to which the detected cell belongs. The frequency-specific priority may be applied to the frequency to which the detected cell belongs. The frequency-specific priority may be a frequency-specific priority received from a network. Conventionally, a cell on a frequency having a lower reselection priority than the reselection priority of a serving frequency may be detected by applying the proposed measurement.

[0153] When the UE does not detect the cell configured with the cell-specific priority, the UE may not apply the cell-specific priority to the cell reselection evaluation (S1222). For example, when the UE that is assigned the cell-specific priority does not detect the cell configured with the cell-specific priority, the existing frequency-specific priority may be applied to the frequency. That is, the cell-specific priority may not be changed in the cell reselection evaluation.

[0154] The UE may perform a cell reselection procedure based on the priority (S1230).

[0155] FIGS. 13a and 13b illustrate a method for a UE to perform frequency measurement and cell reselection based on a cell-specific priority according to an embodiment of the present invention.

[0156] Referring to FIG. 13a, it is assumed that first to fourth UEs are in an idle mode and are currently camping on a first macrocell. It is assumed that a first small cell and a second small cell belong to a second frequency, and a third small cell and a fourth small cell belong to a third frequency. It is assumed that the first small cell and the second small cell are assigned a cell-specific priority of CP2, and the third small cell and the fourth small cell are assigned a cell-specific priority of CP3. It is assumed that the first frequency is assigned a frequency-specific priority of P1, the second frequency is assigned a frequency-specific priority of P2, and the third frequency is assigned a frequency-specific priority of P3.

[0157] Referring to FIG. 13b, the first macrocell may transmit the frequency-specific priorities and the cell-specific priorities to the first to fourth UEs (S1310). The frequency-specific priorities and the cell-specific priorities may be transmitted via system information. The frequency-specific priorities and the cell-specific priorities may be broadcast by the first macrocell. It is assumed that the priorities transmitted by the first macrocell, which are CP2, P1, P2, P3 and CP3, are arranged in descending order. That is, it is assumed that CP2 is the highest priority and CP3 is the lowest priority.

[0158] The first to fourth UEs may perform priority replacement and measurement (S1320). For the first to fourth UEs, the frequency-specific priority of the first frequency may be P1, the frequency-specific priority of the second frequency may be CP2, and the frequency-specific priority of the third frequency may be P3. In the case of the second frequency, since the cell-specific priority (CP2) of the first small cell and the second small cell is higher than the frequency-specific priority (P2) of the second frequency, the frequency-specific priority of the second frequency may be changed from P2 to CP2. In the case of the third frequency, since the cell-specific priority (CP3) of the third small cell and the fourth small cell is lower than the frequency-specific priority (P3) of the third frequency, the frequency-specific priority of the third frequency is maintained to be P3.

[0159] First UE: Since the changed frequency-specific priority (CP2) of the second frequency is higher than the frequency-specific priority (P1) of the first macerocell, the first UE may perform inter-frequency measurement on the second frequency. Accordingly, the first UE may find the second macrocell.

[0160] Second UE: Since the changed frequency-specific priority (CP2) of the second frequency is higher than the frequency-specific priority (P1) of the first macerocell, the second UE may perform inter-frequency measurement on the second frequency. Accordingly, the second UE may find the second macrocell and the second small cell.

[0161] Third UE: Since the changed frequency-specific priority (CP2) of the second frequency is higher than the frequency-specific priority (P1) of the first macerocell, the third UE may perform inter-frequency measurement on the second frequency. Accordingly, the third UE may find the second macrocell.

[0162] Fourth UE: Since the changed frequency-specific priority (CP2) of the second frequency is higher than the frequency-specific priority (P1) of the first macerocell, the fourth UE may perform inter-frequency measurement on the second frequency. Accordingly, the fourth UE may find the second macrocell and the first small cell.

[0163] The first to fourth UEs may perform cell reselection (S1330).

[0164] First UE: A cell configured with a cell-specific priority (for example, the first small cell or the second small cell) is not detected. Accordingly, the first UE may perform cell reselection based on the frequency-specific priority (P1) of the first frequency and the frequency-specific priority (P2) of the second frequency. Since P1 is assumed to be higher than P2, the first UE may continue to camp on the first macrocell.

[0165] Second UE: A cell configured with a cell-specific priority (the second small cell) is detected. Therefore, the second UE may perform cell reselection based on the frequency-specific priority (P1) of the first frequency, the frequency-specific priority (P2) of the second frequency, and the cell-specific priority (CP2) of the second small cell. Since CP2 is assumed to be higher than P1, the second UE may reselect the second small cell.

[0166] Third UE: A cell configured with a cell-specific priority (for example, the first small cell or the second small cell) is not detected. Accordingly, the third UE may perform cell reselection based on the frequency-specific priority (P1) of the first frequency and the frequency-specific priority (P2) of the second frequency. Since P1 is assumed to be higher than P2, the third UE may continue to camp on the first macrocell.

[0167] Fourth UE: A cell configured with a cell-specific priority (the first small cell) is detected. Therefore, the fourth UE may perform cell reselection based on the frequency-specific priority (P1) of the first frequency, the frequency-specific priority (P2) of the second frequency, and the cell-specific priority (CP2) of the first small cell. Since CP2 is assumed to be higher than P1, the fourth UE may reselect the first small cell.

[0168] According to the embodiment of the present invention, a cell-specific priority may be considered for conventional frequency measurement and cell reselection procedures. Thus, efficient load balancing may be possible, compared to conventional frequency measurement and cell reselection based on a frequency-specific priority.

[0169] FIG. 14 is a block diagram illustrating a method for a UE to perform frequency measurement based on a cell-specific priority according to an embodiment of the present invention.

[0170] Referring to FIG. 14, a UE may receive a cell-specific priority and a frequency-specific priority from a network (S1410). The same cell-specific priority may be assigned for each frequency.

[0171] The UE may compare the cell-specific priority of a first cell and the frequency-specific priority of a frequency to which the first cell belongs (S1420). The first cell may be a cell that is configured with a cell-specific priority.

[0172] The UE may determine whether to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs (S1430). When the cell-specific priority of the first cell is higher than the frequency-specific priority of the frequency to which the first cell belongs, the UE may determine to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs. When the cell-specific priority of the first cell is lower than or equal to the frequency-specific priority of the frequency to which the first cell belongs, the UE may determine not to apply the cell-specific priority of the first cell to the frequency to which the first cell belongs.

[0173] The UE may perform frequency measurement based on the determined priority (S1440).

[0174] The UE may perform a cell reselection procedure based on the result of the measurement.

[0175] When the UE detects the first cell, the cell-specific priority of the first cell may be applied to the first cell in the cell reselection procedure and may not be applied to the frequency to which the first cell belongs in the cell reselection procedure. The frequency-specific priority of the frequency to which the first cell belongs, which is received from the network, may be applied to the frequency to which the first cell belongs in the cell reselection procedure. The cell reselection may be performed based on the cell-specific priority applied to the first cell and the frequency-specific priority applied to the frequency to which the first cell belongs.

[0176] When the UE does not detect the first cell, the cell reselection may be performed based on the frequency-specific priority received from the network. The cell-specific priority of the first cell may not be applied to the frequency to which the first cell belongs in the cell reselection procedure.

[0177] FIG. 15 is a block diagram illustrating a wireless communication system according to the embodiment of the present invention.

[0178] ABS 1500 includes a processor 1501, a memory 1502 and a transceiver 1503. The memory 1502 is connected to the processor 1501, and stores various information for driving the processor 1501. The transceiver 1503 is connected to the processor 1501, and transmits and/or receives radio signals. The processor 1501 implements proposed functions, processes and/or methods. In the above embodiment, an operation of the base station may be implemented by the processor 1501.

[0179] A UE 1510 includes a processor 1511, a memory 1512 and a transceiver 1513. The memory 1512 is connected to the processor 1511, and stores various information for driving the processor 1511. The transceiver 1513 is connected to the processor 1511, and transmits and/or receives radio signals. The processor 1511 implements proposed functions, processes and/or methods. In the above embodiment, an operation of the UE may be implemented by the processor 1511.

[0180] The processor may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. The memory may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or other equivalent storage devices. The transceiver may include a base-band circuit for processing a wireless signal. When the embodiment is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory and may be performed by the processor. The memory may be located inside or outside the processor, and may be coupled to the processor by using various well-known means.

[0181] Various methods based on the present specification have been described by referring to drawings and reference numerals given in the drawings on the basis of the aforementioned examples. Although each method describes multiple steps or blocks in a specific order for convenience of explanation, the invention disclosed in the claims is not limited to the order of the steps or blocks, and each step or block can be implemented in a different order, or can be performed simultaneously with other steps or blocks. In addition, those ordinarily skilled in the art can know that the invention is not limited to each of the steps or blocks, and at least one different step can be added or deleted without departing from the scope and spirit of the invention.

[0182] The aforementioned embodiment includes various examples. It should be noted that those ordinarily skilled in the art know that all possible combinations of examples cannot be explained, and also know that various combinations can be derived from the technique of the present specification. Therefore, the protection scope of the invention should be determined by combining various examples described in the detailed explanation, without departing from the scope of the following claims.