Patent Publication Number: US-9838175-B2

Title: Method and apparatus for controlling inter-cell interference in wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2012/008862, filed on Oct. 26, 2012, which claims the benefit of U.S. Provisional Application Ser. No. 61/551,449, filed on Oct. 26, 2011, 61/596,130, filed on Feb. 7, 2012, 61/600,704, filed on Feb. 19, 2012, and 61/601,517, filed on Feb. 21, 2012, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The following description relates to a wireless communication system and, more particularly, to a method and apparatus for controlling and mitigating inter-cell interference in a wireless communication system. 
     BACKGROUND ART 
       FIG. 1  illustrates a heterogeneous network wireless communication system  100  including a macro eNB eNB1 and a micro eNB eNB2. In the specification, a heterogeneous network refers to a network in which a macro eNB  110  and a micro eNB  120  coexist while using the same radio access technology (RAT). 
     The macro eNB  110  refers to a normal eNB of a wireless communication system and has wide coverage (service provision area) and high transmit (Tx) power. The macro eNB  110  may be referred to as a macro cell. The micro eNB  120  may be referred to as a micro cell, a pico cell, a femto cell, a home eNB (HeNB), a relay, etc. The micro eNB  120  is a small version of the macro eNB and may independently operate while performing most functions of the macro eNB. The micro eNB is a non-overlay type eNB that may be overlaid in the coverage area of the macro eNB or in a shadow area which is not covered by the macro eNB. The micro eNB  120  may serve fewer UEs with narrower coverage and lower Tx power than the macro eNB  110 . 
     A UE  130  may be directly served by the macro eNB  110  (the UE being referred to as a macro-UE in this case) or served by the micro eNB  120  (the UE being referred to as a micro-UE in this case). The UE  130  located in the coverage of the micro eNB  120  may be served by the macro eNB  110 .  FIG. 1  shows a case in which the UE  130  is connected to the micro eNB  120 . 
     The micro eNB may be classified into two types: a closed subscriber group (CSG) micro eNB and an open access (OA) or open subscriber group (OSG) micro eNB according to whether UE access is restricted. The CSG micro eNB can serve only an authorized UE and the OSG micro eNB can serve all UEs without access restriction. 
     DISCLOSURE 
     Technical Problem 
     As illustrated in  FIG. 1 , when the UE  130  served by the micro eNB  120  in the heterogeneous network receives a desired signal from the micro eNB  120 , the signal received by the UE  130  may be interfered by a strong signal from the macro eNB  110 . Otherwise, when a UE served by the macro eNB is located in proximity to the micro eNB, a signal received by the UE from the macro eNB may be interfered by a strong signal from the micro eNB. This interference may be referred to as inter-cell interference. The aforementioned example relates to inter-cell interference generated on downlink from an eNB to a UE. Similarly, inter-cell interference may be generated on uplink from a UE to an eNB. 
     In a conventional wireless communication system, a cell that causes interference does not perform downlink data transmission in a specific subframe to mitigate interference applied to a neighboring cell. A method of improving system performance while reducing interference applied to a neighboring cell by performing downlink data transmission with reduced Tx power rather than not-performing downlink data transmission is under discussion. However, a signaling scheme for setting a subframe corresponding to reduced Tx power is not provided to the conventional wireless communication system. 
     An object of the present invention devised to solve the problem lies in a method through which an eNB dynamically signals various pieces of subframe Tx power information to a UE. 
     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. 
     Technical Solution 
     The object of the present invention can be achieved by providing a method for transmitting transmit (Tx) power information on a subframe from an eNB to a user equipment (UE), the method including: transmitting downlink control information to the UE through a physical downlink control channel (PDCCH), wherein the Tx power information on the subframe is indicated by information related to the PDCCH, wherein the information related to the PDCCH is determined based on a predetermined mapping relationship between the Tx power information on the subframe and the information related to the PDCCH. 
     In another embodiment of the present invention, provided herein is a method for receiving, by a UE, Tx power information on a subframe from an eNB, the method including: receiving downlink control information through a PDCCH from the eNB, wherein the Tx power information on the subframe is indicated by information related to the PDCCH, wherein the information related to the PDCCH is determined based on a predetermined mapping relationship between the Tx power information on the subframe and the information related to the PDCCH. 
     In another embodiment of the present invention, provided herein is an eNB transmitting Tx power information on a subframe to a UE, including: a reception module for receiving an uplink signal from the UE; a transmission module for transmitting a downlink signal to the UE; and a processor for controlling the eNB including the reception module and the transmission module, wherein the processor is configured to transmit downlink control information to the UE through a PDCCH using the transmission module, wherein the Tx power information on the subframe is indicated by information related to the PDCCH, wherein the information related to the PDCCH is determined based on a predetermined mapping relationship between the Tx power information on the subframe and the information related to the PDCCH. 
     In another embodiment of the present invention, provided herein is a UE receiving Tx power information on a subframe from an eNB, including: a reception module for receiving a downlink signal from the eNB; a transmission module for transmitting an uplink signal to the eNB; and a processor for controlling the UE including the reception module and the transmission module, wherein the processor is configured to receive downlink control information through a PDCCH from the eNB using the reception module, wherein the Tx power information on the subframe is indicated by information related to the PDCCH, wherein the information related to the PDCCH is determined based on a predetermined mapping relationship between the Tx power information on the subframe and the information related to the PDCCH. 
     The following may be commonly applied to the above-described embodiments of the present invention. 
     The eNB may transmit information indicating the mapping relationship to the UE through higher layer signaling. 
     The Tx power information may include first Tx power information and second Tx power information, and the second Tx power information may have a Tx power value lower than that of the first Tx power information. 
     The Tx power information may include first Tx power information and second Tx power information. Here, the first Tx power information may include at least one of a UE-specific parameter P A  used to determine the ratio of cell-specific reference signal (CRS) energy per resource element (EPRE) to physical downlink shared channel (PDSCH) EPRE in an orthogonal frequency division multiplex (OFDM) symbol period in which a CRS is not transmitted in a normal subframe, ρ A , an offset value δ power-offset  used to determine ρ A , and a cell-specific parameter P B  used to determine a ratio of ρ A  to a ratio of CRS EPRE to PDSCH EPRE in an OFDM symbol period in which a CRS is transmitted in the normal subframe, ρ B . The second Tx power information may include at least one of a UE-specific parameter P A ′ used to determine a ratio of CRS EPRE to PDSCH EPRE in an OFDM symbol period in which a CRS is not transmitted in an r-ABS (reduced power-almost blank subframe), ρ A ′, an offset value δ power-offset ′ used to determine ρ A ′, and a cell-specific parameter P B ′ used to determine the ratio of ρ A ′ to a ratio of CRS EPRE to PDSCH EPRE in an OFDM symbol period in which a CRS is transmitted in the r-ABS, ρ B ′. 
     The PDCCH related information indicating the Tx power information on the subframe may correspond to one of a control channel element (CCE) aggregation level of the PDCCH, modulation order information included in the downlink control information, a hybrid automatic repeat request (HARQ) process number included in the downlink control information, Tx power information on the PDCCH and an identifier of the UE masking a cyclic redundancy check (CRC) of the PDCCH. 
     The mapping relationship may be set such that a low CCE aggregation level is mapped to a high subframe Tx power value and a high CCE aggregation level is mapped to a low subframe Tx power value when the PDCCH related information corresponds to the PDCCH CCE aggregation level. 
     The mapping relationship may be set such that a low modulation order is mapped to a low subframe Tx power value and a high modulation order is mapped to a high subframe Tx power value when the PDCCH related information corresponds to the modulation order. 
     The mapping relationship may be set such that the number of HARQ processes mapped to a high subframe Tx power value is greater than the number of HARQ processes mapped to a low subframe Tx power value when the PDCCH related information corresponds to the HARQ process number. 
     The mapping relationship may be set such that Tx power of the PDCCH is mapped to a high subframe Tx power value when the Tx power of the PDCCH exceeds a predetermined threshold and mapped to a low subframe Tx power value when the Tx power of the PDCCH is less than the threshold when the PDCCH related information corresponds to the PDCCH Tx power information. 
     When the PDCCH related information corresponds to the identifier of the UE, a plurality of UE identifiers may be allocated to the UE, and the mapping relationship may be set such that a first UE identifier is mapped to a high subframe Tx power value and a second UE identifier is mapped to a low subframe Tx power value. 
     The UE may demodulate a physical downlink shared channel (PDSCH) transmitted in the subframe based on the information related to the PDCCH and the subframe Tx power information determined by the mapping relationship. 
     The above description and the following detailed description of the present invention are exemplary and are for additional explanation of the invention disclosed in the claims. 
     Advantageous Effects 
     According to the present invention, it is possible to provide a method through which an eNB dynamically signals various pieces of subframe Tx power information to a UE. 
     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. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  illustrates a heterogeneous network wireless communication system; 
         FIG. 2  illustrates a downlink radio frame structure; 
         FIG. 3  illustrates a resource grid in a downlink slot; 
         FIG. 4  illustrates a downlink subframe structure; 
         FIG. 5  illustrates an uplink subframe structure; 
         FIG. 6  illustrates configurations of wireless communication systems including multiple antennas; 
         FIG. 7  illustrates CRS and DRS patterns defined in 3GPP LTE; 
         FIG. 8  illustrates an uplink subframe structure including SRS symbols; 
         FIG. 9  illustrates examples of implementation of functions of a transmitter and a receiver of an FDD mode relay; 
         FIG. 10  illustrates transmission from a relay to a UE and downlink transmission from an eNB to a relay; 
         FIG. 11  illustrates an example of power allocation to resource elements included in a downlink subframe; 
         FIG. 12  illustrates another example of power allocation to resource elements included in a downlink subframe; 
         FIG. 13  illustrates another example of power allocation to resource elements included in a downlink subframe; 
         FIG. 14  is a flowchart illustrating a method for dynamically signaling subframe Tx power information according to an embodiment of the present invention; and 
         FIG. 15  illustrates configurations of an eNB and a UE according to an embodiment of the present invention. 
     
    
    
     BEST MODE 
     The embodiments of the present invention described hereinbelow are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. 
     In the embodiments of the present invention, a description is made, centering on a data transmission and reception relationship between a base station (BS) and a user equipment (UE). The BS is a terminal node of a network, which communicates directly with a UE. In some cases, a specific operation described as performed by the BS may be performed by an upper node of the BS. 
     Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with a UE may be performed by the BS, or network nodes other than the BS. The term ‘BS’ may be replaced with the term ‘fixed station’, ‘Node B’, ‘evolved Node B (eNode B or eNB)’, ‘Access Point (AP)’, etc. The term ‘UE’ may be replaced with the term ‘terminal’, ‘Mobile Station (MS)’, ‘Mobile Subscriber Station (MSS)’, ‘Subscriber Station (SS)’, etc. 
     Specific terms used for the embodiments of the present invention are provided to help the understanding of the present invention. These specific terms may be replaced with other terms within the scope and spirit of the present invention. 
     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. 
     The embodiments of the present invention can be supported by standard documents disclosed for at least one of wireless access systems, Institute of Electrical and Electronics Engineers (IEEE) 802, 3rd Generation Partnership Project (3GPP), 3GPP Long Term Evolution (3GPP LTE), LTE-Advanced (LTE-A), and 3GPP2. Steps or parts that are not described to clarify the technical features of the present invention can be supported by those documents. Further, all terms as set forth herein can be explained by the standard documents. 
     Techniques described herein can be used in various wireless access 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. CDMA may be implemented as a radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented as a radio technology such as Global System for Mobile communications (GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may be implemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved-UTRA (E-UTRA) etc. UTRA is a part of Universal Mobile Telecommunication System (UMTS). 3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA for downlink and SC-FDMA for uplink. LTE-A is an evolution of 3GPP LTE. WiMAX can be described by the IEEE 802.16e standard (Wireless Metropolitan Area Network (WirelessMAN-OFDMA Reference System) and the IEEE 802.16m standard (WirelessMAN-OFDMA Advanced System). For clarity, this application focuses on the 3GPP LTE/LTE-A system. However, the technical features of the present invention are not limited thereto. 
     A description will be given of a downlink radio frame structure with reference to  FIG. 2 . 
     In a cellular OFDM wireless packet communication system, uplink/downlink data packet transmission is performed on a subframe-by-subframe basis and one subframe is defined as a predetermined time interval including a plurality of OFDM symbols. 3GPP LTE supports type-1 radio frame applicable to FDD (frequency division duplex) and type-2 radio frame applicable to TDD (time division duplex). 
       FIG. 2( a )  illustrates a type-1 radio frame structure. A downlink radio frame includes 10 subframes. Each subframe is further divided into two slots in the time domain. A unit time during which one subframe is transmitted is defined as transmission time interval (TTI). For example, one subframe may be 1 ms in duration and one slot may be 0.5 ms in duration. A slot may include a plurality of OFDM symbols in the time domain and a plurality of resource blocks (RBs) in the frequency domain. Since 3GPP LTE adopts OFDMA for downlink, an OFDM symbol represents one symbol period. An OFDM symbol may be referred to as an SC-FDMA symbol or symbol period. A resource block (RB) is a resource allocation unit including a plurality of contiguous subcarriers in a slot. 
     The number of OFDM symbols included in one slot may depend on cyclic prefix (CP) configuration. CPs include an extended CP and a normal CP. When an OFDM symbol is configured with the normal CP, for example, the number of OFDM symbols included in one slot may be 7. When an OFDM symbol is configured with the extended CP, the duration of one OFDM symbol increases, and thus the number of OFDM symbols included in one slot is smaller than that in case of the normal CP. In case of the extended CP, the number of OFDM symbols allocated to one slot may be 6. When a channel state is unstable, such as a case in which a UE moves at a high speed, the extended CP can be used to reduce inter-symbol interference. 
     When the normal CP is used, one subframe includes 14 OFDM symbols since one slot has 7 OFDM symbols. The first two or three OFDM symbols in each subframe can be allocated to a PDCCH and the remaining OFDM symbols can be allocated to a PDSCH. 
       FIG. 2( b )  illustrates a type-2 radio frame structure. The type-2 radio frame includes 2 half frames. Each half frame includes 5 subframes, a downlink pilot time slot (DwPTS), a guard period (GP) and an uplink pilot time slot (UpPTS). One subframe consists of 2 slots. The DwPTS is used for initial cell search, synchronization or channel estimation in a UE. The UpPTS is used for channel estimation in a BS and UL transmission synchronization acquisition in a UE. The GP eliminates UL interference caused by multi-path delay of a DL signal between a UL and a DL. One subframe includes 2 slots irrespective of radio frame type. 
     The radio frame structure is purely exemplary and thus the number of subframes in a radio frame, the number of slots in a subframe, or the number of OFDM symbols in a slot may vary. 
       FIG. 3  illustrates a resource grid in a downlink slot. While one downlink slot includes 7 OFDM symbols in the time domain and one RB includes 12 subcarriers in the frequency domain in  FIG. 2 , the present invention is not limited thereto. For example, one slot includes 7 OFDM symbols in the case of normal CP whereas one slot includes 6 OFDM symbols in the case of extended CP. Each element on the resource grid is referred to as a resource element (RE). One RB includes 12×7 REs. The number NDL of RBs included in the downlink slot depends on a downlink transmit bandwidth. The structure of an uplink slot may be same as that of the downlink slot. 
       FIG. 4  illustrates a downlink subframe structure. A maximum of three OFDM symbols located in a front portion of a first slot within a subframe correspond to a control region to which a control channel is allocated. The remaining OFDM symbols correspond to a data region to which a physical downlink shared chancel (PDSCH) is allocated. 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. Control information transmitted through the PDCCH is referred to as downlink control information (DCI). The DCI includes uplink or downlink scheduling information or uplink Tx power control commands for an arbitrary UE group. The PDCCH may carry a transport format and a resource allocation of a downlink shared channel (DL-SCH), resource allocation information of an uplink shared channel (UL-SCH), paging information on a paging channel (PCH), system information on the DL-SCH, information on resource allocation of an upper-layer control message such as a random access response transmitted on the PDSCH, a set of Tx power control commands on individual UEs within an arbitrary UE group, a Tx power control command, information on activation of a voice over IP (VoIP), etc. A plurality of PDCCHs can be transmitted within a control region. The UE can monitor the plurality of PDCCHs. The 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). A format of the PDCCH and the number of bits of the available PDCCH are determined by the number of CCEs. The BS determines a PDCCH format according to DCI to be transmitted to the UE, and attaches a cyclic redundancy check (CRC) to control information. The CRC is masked with an identifier referred to as a radio network temporary identifier (RNTI) according to an owner or usage of the PDCCH. If the PDCCH is for a specific UE, a cell-RNTI (C-RNTI)) of the UE may be masked to the CRC. Alternatively, when the PDCCH is for a paging message, a paging indicator identifier (P-RNTI) may be masked to the CRC. When the PDCCH is for system information (more specifically, a system information block (SIB)), a system information identifier and system information RNTI (SI-RNTI) may be masked to the CRC. To indicate a random access response corresponding to a response to transmission of a random access preamble of the UE, a random access-RNTI (RA-RNTI) may be masked to the CRC. 
       FIG. 5  illustrates an uplink subframe structure. An uplink subframe may be divided into a control region and a data region in the frequency domain. The control region is allocated a PUCCH including uplink control information. The data region is allocated a PUSCH including user data. To maintain single carrier property, one UE cannot simultaneously transmit a PUCCH and a PUSCH. A PUCCH for a UE is allocated to an RB pair. RBs belonging to an RB pair occupy different subcarriers in 2 slots. That is, an RB pair allocated to a PUCCH is frequency-hopped at a slot boundary. 
     MIMO System Modeling 
       FIG. 6  illustrates configurations of wireless communication systems including multiple antennas. 
     As shown in  FIG. 6( a ) , when the number of transmit (Tx) antennas increases to NT, and at the same time the number of receive (Rx) antennas increases to NR, a theoretical channel transmission capacity of the MIMO communication system increases in proportion to the number of antennas, differently from a case in which only a transmitter or receiver uses several antennas, so that transmission rate and frequency efficiency can be greatly increased. In this case, the transfer rate acquired by the increasing channel transmission capacity can theoretically increase by a predetermined amount that corresponds to multiplication of a maximum transfer rate (Ro) acquired when one antenna is used and a rate of increase (Ri).
 
 R   i =min( N   T   ,N   R )  [Equation 1]
 
     For example, provided that a MIMO system uses four Tx antennas and four Rx antennas, the MIMO system can theoretically acquire a high transfer rate which is four times higher than that of a single antenna system. After the above-mentioned theoretical capacity increase of the MIMO system was demonstrated in the mid-1990s, many developers began to conduct intensive research into a variety of technologies which can substantially increase data transfer rate using the theoretical capacity increase. Some of the above technologies have been reflected in a variety of wireless communication standards, for example, third-generation mobile communication or next-generation wireless LAN, etc. 
     A variety of MIMO-associated technologies have been intensively researched by many companies or developers, for example, research into information theory associated with MIMO communication capacity under various channel environments or multiple access environments, research into a radio frequency (RF) channel measurement and modeling of the MIMO system, and research into a space-time signal processing technology. 
     Mathematical modeling of a communication method for use in the above-mentioned MIMO system will hereinafter be described in detail. As can be seen from  FIG. 10( a ) , it is assumed that there are NT Tx antennas and NR Rx antennas. In the case of a transmission signal, a maximum number of transmission information pieces is NT under the condition that NT Tx antennas are used, so that the transmission information can be represented by a specific vector shown in the following equation 2.
 
 s=└s   1   ,s   2   , . . . ,s   N     T   ┘ T   [Equation 2]
 
     Individual transmission information pieces s1, s2, . . . , sNT may have different transmission powers. In this case, if the individual transmission powers are denoted by P1, P2, . . . , PNT, then transmission information having an adjusted transmission power can be represented by a specific vector shown in the following equation 3.
 
 ŝ=[ŝ   1   ,ŝ   2   , . . . ,ŝ   N     T   ] T   =[P   1   s   1   ,P   2   s   2   , . . . ,P   N     T     s   N     T   ] T    [Equation 3]
 
     In Equation 3, ŝ is a transmission vector, and can be represented by the following equation 4 using a diagonal matrix P of a transmission power. 
     
       
         
           
             
               
                 
                   
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                           W 
                           ⁢ 
                           
                             s 
                             ^ 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         WPs 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, Wij denotes a weight corresponding to i-th Tx antenna and j-th information. W represents a weight matrix or precoding matrix. 
     When NR Rx antennas are used, received signals y 1 , y 2 , . . . , yNR of individual antennas can be represented by a specific vector (y) shown in the following equation 6.
 
 y=[y   1   ,y   2   , . . . ,y   N     R   ] T   [Equation 6]
 
     When a channel modeling is executed in the MIMO communication system, individual channels can be distinguished from each other according to Tx/Rx antenna indexes. A specific channel passing the range from a Tx antenna j to a Rx antenna i is denoted by hij. In this case, it should be noted that the index order of the channel hij is located before a Rx antenna index and is located after a Tx antenna index. 
       FIG. 6( b )  shows channels from NT Tx antennas to a Rx antenna i. Several channels are tied up, so that they are displayed in the form of a vector or matrix. Referring to FIG.  10 ( b ), the channels passing the range from the NT Tx antennas to the Rx antenna i can be represented by the following equation 7.
 
 h   i   T   =└h   i1   ,h   i2   , . . . ,h   iN     T   ┘  [Equation 7]
 
     If all channels passing the range from the NT Tx antennas to NR Rx antennas are denoted by the matrix shown in Equation 7, the following equation 8 is acquired. 
     
       
         
           
             
               
                 
                   H 
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               h 
                               1 
                               T 
                             
                           
                         
                         
                           
                             
                               h 
                               2 
                               T 
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               h 
                               i 
                               T 
                             
                           
                         
                         
                           
                             ⋮ 
                           
                         
                         
                           
                             
                               h 
                               
                                 N 
                                 R 
                               
                               T 
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             
                               h 
                               11 
                             
                           
                           
                             
                               h 
                               12 
                             
                           
                           
                             … 
                           
                           
                             
                               h 
                               
                                 1 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         
                           
                             
                               h 
                               21 
                             
                           
                           
                             
                               h 
                               22 
                             
                           
                           
                             … 
                           
                           
                             
                               h 
                               
                                 2 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         
                           
                             ⋮ 
                           
                           
                             
                                 
                             
                           
                           
                             ⋱ 
                           
                           
                             
                                 
                             
                           
                         
                         
                           
                             
                               h 
                               
                                 i 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 i 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             … 
                           
                           
                             
                               h 
                               
                                 iN 
                                 T 
                               
                             
                           
                         
                         
                           
                             ⋮ 
                           
                           
                             
                                 
                             
                           
                           
                             ⋱ 
                           
                           
                             
                                 
                             
                           
                         
                         
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             … 
                           
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     Additive white Gaussian noise (AWGN) is added to an actual channel which has passed the channel matrix H shown in Equation 8. The AWGN n1, n2, . . . , nNR added to each of NR Rx antennas can be represented by a specific vector shown in the following equation 9.
 
 n=[n   1   ,n   2   , . . . ,n   N     R   ] T   [Equation 9]
 
     A reception signal calculated by the above-mentioned equations can be represented by the following equation 10. 
     
       
         
           
             
               
                 
                   
                     
                       
                         y 
                         = 
                           
                         ⁢ 
                         
                           [ 
                           
                             
                               
                                 
                                   y 
                                   1 
                                 
                               
                             
                             
                               
                                 
                                   y 
                                   2 
                                 
                               
                             
                             
                               
                                 ⋮ 
                               
                             
                             
                               
                                 
                                   y 
                                   i 
                                 
                               
                             
                             
                               
                                 ⋮ 
                               
                             
                             
                               
                                 
                                   y 
                                   
                                     N 
                                     R 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               [ 
                               
                                 
                                   
                                     
                                       h 
                                       11 
                                     
                                   
                                   
                                     
                                       h 
                                       12 
                                     
                                   
                                   
                                     … 
                                   
                                   
                                     
                                       h 
                                       
                                         1 
                                         ⁢ 
                                         
                                           N 
                                           T 
                                         
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       h 
                                       21 
                                     
                                   
                                   
                                     
                                       h 
                                       22 
                                     
                                   
                                   
                                     … 
                                   
                                   
                                     
                                       h 
                                       
                                         2 
                                         ⁢ 
                                         
                                           N 
                                           T 
                                         
                                       
                                     
                                   
                                 
                                 
                                   
                                     ⋮ 
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     ⋱ 
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     
                                       h 
                                       
                                         i 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   
                                     
                                       h 
                                       
                                         i 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                   
                                   
                                     … 
                                   
                                   
                                     
                                       h 
                                       
                                         iN 
                                         T 
                                       
                                     
                                   
                                 
                                 
                                   
                                     ⋮ 
                                   
                                   
                                     
                                         
                                     
                                   
                                   
                                     ⋱ 
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     
                                       h 
                                       
                                         
                                           N 
                                           R 
                                         
                                         ⁢ 
                                         1 
                                       
                                     
                                   
                                   
                                     
                                       h 
                                       
                                         
                                           N 
                                           R 
                                         
                                         ⁢ 
                                         2 
                                       
                                     
                                   
                                   
                                     … 
                                   
                                   
                                     
                                       h 
                                       
                                         
                                           N 
                                           R 
                                         
                                         ⁢ 
                                         
                                           N 
                                           T 
                                         
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       x 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     
                                       x 
                                       2 
                                     
                                   
                                 
                                 
                                   
                                     ⋮ 
                                   
                                 
                                 
                                   
                                     
                                       x 
                                       j 
                                     
                                   
                                 
                                 
                                   
                                     ⋮ 
                                   
                                 
                                 
                                   
                                     
                                       x 
                                       
                                         N 
                                         T 
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             [ 
                             
                               
                                 
                                   
                                     n 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     n 
                                     2 
                                   
                                 
                               
                               
                                 
                                   ⋮ 
                                 
                               
                               
                                 
                                   
                                     n 
                                     i 
                                   
                                 
                               
                               
                                 
                                   ⋮ 
                                 
                               
                               
                                 
                                   
                                     n 
                                     
                                       N 
                                       R 
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           Hx 
                           + 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     In the meantime, the number of rows and the number of columns of a channel matrix H indicating a channel condition are determined by the number of Tx/Rx antennas. In the channel matrix H, the number of rows is equal to the number (NR) of Rx antennas, and the number of columns is equal to the number (NT) of Tx antennas. Namely, the channel matrix H is denoted by an NR×NT matrix. 
     A matrix rank is defined by a smaller number between the number of rows and the number of columns, in which the rows and the columns are independent of each other. Therefore, the matrix rank cannot be higher than the number of rows or columns. The rank of the channel matrix H can be represented by the following equation 11.
 
rank( H )≦min( N   T   ,N   R )  [Equation 11]
 
     The rank may be defined as the number of non-zero Eigen values when Eigen value decomposition is performed on the matrix. Similarly, the rank may be defined as the number of non-zero singular values when singular value decomposition is performed on the matrix. Accordingly, the rank of the channel matrix refers to a maximum number of information pieces that can be transmitted on a given channel. 
     Reference Signal (RS) 
     Since a packet is transmitted through a radio channel in a wireless communication system, a signal may be distorted during transmission. A receiver needs to correct the distorted signal using channel information in order to correctly receive the distorted signal. To detect channel information, a signal known to both the receiver and a transmitter is transmitted and channel information is detected using a degree of distortion of the signal when the signal is received through a certain channel. This signal is called a pilot signal or a reference signal. 
     When multiple antennas are used to transmit and receive data, a correct signal can be received only when channel state between each Tx antenna and each Rx antenna is detected. Accordingly, a reference signal is required for each Tx antenna. 
     A downlink reference signal defines a common reference signal (CRS) shared by all UEs in a cell and a dedicated reference signal (DRS) dedicated to a specific UE. Information for channel estimation and demodulation can be provided according to these reference signals. 
     A receiver (UE) can estimate channel state from the CRS and feed back an indicator related to channel quality, such as a channel quality indicator (CQI), a precoding matrix index (PMI) and/or a rank indicator (RI), to a transmitter (eNB). The CRS may be called a cell-specific reference signal. An RS related to feedback of channel state information (CSI) such as CQI/PMI/RI may be defined as a CSI-RS. 
     The DRS can be transmitted through a corresponding RE when data demodulation is needed. Presence or absence of the DRS may be signaled to the UE by a higher layer. In addition, the fact that the DRS is valid only when a corresponding PDSCH is mapped may be signaled to the UE. The DRS may be called a UE-specific reference signal or a demodulation reference signal (DMRS). 
       FIG. 7  illustrates a pattern of mapping a CRS and a DRS defined in 3GPP LTE (e.g. release-8) to a downlink resource block (RB) pair. A downlink RB pair as a reference signal mapping unit may be represented as one subframe in the time domain×12 subcarriers in the frequency domain. That is, an RB pair has a length of 14 OFDM symbols in the case of normal CP ( FIG. 7( a ) ) and has a length of 12 OFDM symbols in the case of extended CP ( FIG. 7( b ) ) in the time domain. 
       FIG. 7  shows RS positions in RB pairs in a system in which an eNB supports 4 transmit antennas. In  FIG. 7 , REs indicated by ‘0’, ‘1’, ‘2’ and ‘3’ respectively represent CRS positions with respect to antenna port indices 0, 1, 2 and 3. An RE indicated by ‘D’ represents a DRS position. 
     CRSs will be described in detail hereinafter. 
     The CRS is used to estimate a channel of a physical antenna and can be commonly received by all UEs in a cell. The CRS is distributed in the entire band. The CRS can be used for CSI acquisition and data demodulation. 
     The CRS is defined in various forms according to antenna configuration of a transmitter (eNB). 3GPP LTE (e.g. release-8) supports various antenna configurations and a downlink signal transmitter (eNB) may have three antenna configurations of a single antenna, 2 Tx antennas and 4 Tx antennas. When the eNB performs single antenna transmission, an RS for a single antenna port is provided. When the eNB performs 2-antenna transmission, RSs for 2 antenna ports are provided through time division multiplexing (TDM) and/or frequency division multiplexing (FDM). That is, the R0Ss for the 2 antenna ports can be discriminated from each other by being present in different time resources and/or different frequency resources. Furthermore, when the eNB performs 4-antenna transmission, RSs for 4 antenna ports are provided through TDM/FDM. Channel information estimated by a signal receiver (UE) using the CRS can be used to demodulate data transmitted through single antenna transmission, transmit diversity, closed-loop spatial multiplexing, open-loop spatial multiplexing, multi-user MIMO (MU-MIMO), etc. 
     In case of multi-antenna transmission, when an RS is transmitted through a specific antenna port, the RS is transmitted in an RE designated according to RS pattern and no signal is transmitted in REs designated for other antenna ports. 
     A rule of mapping the CRS to an RB conforms to Equation 12. 
     
       
         
           
             
               
                 
                   
                     k 
                     = 
                     
                       
                         6 
                         ⁢ 
                         m 
                       
                       + 
                       
                         
                           ( 
                           
                             v 
                             + 
                             
                               v 
                               shift 
                             
                           
                           ) 
                         
                         ⁢ 
                         mod 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         6 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     l 
                     = 
                     
                       { 
                       
                         
                           
                             
                               
                                 
                                   
                                     0 
                                     , 
                                     
                                       
                                         N 
                                         symb 
                                         DL 
                                       
                                       - 
                                       3 
                                     
                                   
                                 
                                 
                                   
                                     
                                       if 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       p 
                                     
                                     ∈ 
                                     
                                       { 
                                       
                                         0 
                                         , 
                                         1 
                                       
                                       } 
                                     
                                   
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     
                                       if 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       p 
                                     
                                     ∈ 
                                     
                                       { 
                                       
                                         2 
                                         , 
                                         3 
                                       
                                       } 
                                     
                                   
                                 
                               
                             
                             ⁢ 
                             
                               
 
                             
                             ⁢ 
                             m 
                           
                           = 
                           0 
                         
                         , 
                         1 
                         , 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           
                             
                               2 
                               · 
                               
                                 N 
                                 RB 
                                 DL 
                               
                             
                             - 
                             
                               1 
                               ⁢ 
                               
                                 
 
                               
                               ⁢ 
                               
                                 m 
                                 ′ 
                               
                             
                           
                           = 
                           
                             
                               m 
                               + 
                               
                                 N 
                                 RB 
                                 
                                   max 
                                   , 
                                   DL 
                                 
                               
                               - 
                               
                                 
                                   N 
                                   RB 
                                   DL 
                                 
                                 ⁢ 
                                 
                                   
 
                                 
                                 ⁢ 
                                 v 
                               
                             
                             = 
                             
                               { 
                               
                                 
                                   
                                     
                                       
                                         0 
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           
                                             
                                               0 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               and 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               l 
                                             
                                             = 
                                             0 
                                           
                                         
                                       
                                     
                                     
                                       
                                         3 
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           
                                             
                                               0 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               and 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               l 
                                             
                                             ≠ 
                                             0 
                                           
                                         
                                       
                                     
                                     
                                       
                                         3 
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           
                                             
                                               1 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               and 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               l 
                                             
                                             = 
                                             0 
                                           
                                         
                                       
                                     
                                     
                                       
                                         0 
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           
                                             
                                               1 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               and 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               l 
                                             
                                             ≠ 
                                             0 
                                           
                                         
                                       
                                     
                                     
                                       
                                         
                                           3 
                                           ⁢ 
                                           
                                             ( 
                                             
                                               
                                                 n 
                                                 s 
                                               
                                               ⁢ 
                                               mod 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                             ) 
                                           
                                         
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           2 
                                         
                                       
                                     
                                     
                                       
                                         
                                           3 
                                           + 
                                           
                                             3 
                                             ⁢ 
                                             
                                               ( 
                                               
                                                 
                                                   n 
                                                   s 
                                                 
                                                 ⁢ 
                                                 mod 
                                                 ⁢ 
                                                 
                                                     
                                                 
                                                 ⁢ 
                                                 2 
                                               
                                               ) 
                                             
                                           
                                         
                                       
                                       
                                         
                                           
                                             if 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             p 
                                           
                                           = 
                                           3 
                                         
                                       
                                     
                                   
                                   ⁢ 
                                   
                                     
 
                                   
                                   ⁢ 
                                   
                                     v 
                                     shift 
                                   
                                 
                                 = 
                                 
                                   
                                     N 
                                     ID 
                                     cell 
                                   
                                   ⁢ 
                                   mod 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   6 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 12, k is a subcarrier index, 1 is a symbol index, p is an antenna port index. In addition, N symb   DL  denotes the number of OFDM symbols of a downlink slot, N RB   DL  denotes the number of RBs allocated to downlink, n s  is a slot index, N ID   cell  is a cell ID and mod denotes modulo operation. An RS position in the frequency domain depends on Vshift. Since Vshift depends on cell ID, the RS position has different frequency shift values per cell. 
     Specifically, to improve channel estimation performance through the CRS, a CRS position in the frequency domain is shifted per cell such that cells have different frequency shift values. For example, when an RS is present for every 3 subcarriers, the RS can be present in a subcarrier  3   k  in a cell and in a subcarrier  3   k +1 in another cell. For an antenna port, an RS is distributed at an interval of 6 REs (i.e. 6 subcarriers) in the frequency domain and spaced apart from REs in which an RS for another antenna port is present in the frequency domain. 
     Power boosting may be applied to the CRS. Power boosting is a method for transmitting an RS with higher power using power corresponding to REs of an OFDM symbol, other than REs allocated for the RS. 
     An RS is disposed at a specific interval starting from symbol index (1) 0 of each slot in the time domain. The interval is defined based on CP length. RSs are present in symbols corresponding to symbol indices 0 and 4 in a slot in the case of normal CP and present in symbols corresponding to symbol indices 0 and 3 in the slot in the case of extended CP. Only RSs for up to 2 antenna ports are defined in a single OFDM symbol. Accordingly, in the case of 4-Tx antenna transmission, RSs for antenna ports 0 and 1 are present in symbols corresponding to symbol indices 0 and 4 (symbol indices 0 and 3 in the extended CP case) in a slot and RSs for antenna ports 2 and 3 are present in a symbol corresponding to symbol index 1 in the slot. However, the frequencies of the RSs for antenna ports 2 and 3 are switched in the second slot. 
     To provide higher spectral efficiency than 3GPP LTE (e.g. release-8), a system (e.g. LTE-A) having an extended antenna configuration may be designed. The extended antenna configuration may be an 8Tx antenna configuration. The system having the extended antenna configuration needs to support UEs operating in a conventional antenna configuration. That is, the system needs to support backward compatibility. Accordingly, it is necessary to support a reference signal pattern according to the conventional antenna configuration and to design a new reference signal pattern for an additional antenna configuration. Here, when a CRS for a new antenna port is added to a system having a conventional antenna configuration, RS overhead abruptly increases, decreasing throughput. In consideration of this, LTE-A evolved from 3GPP LTE introduces an additional RS (CSI-RS) for CSI measurement for a new antenna port. 
     A DRS will now be described in detail hereinafter. 
     The DRS (or UE-specific RS) is used for data demodulation. A precoding weight used for a specific UE is used for an RS in multi-antenna transmission such that the UE can estimate an equivalent channel obtained by combining the precoding weight transmitted through each Tx antenna and a transport channel when receiving the RS. 
     3GPP LTE (e.g. release-8) supports transmission through up to 4 Tx antennas and defines a DRS for rank-1 beamforming. The DRS for rank-1 beamforming is also used as an RS for antenna port index 5. A rule of mapping the DRS to an RB conforms to Equations 13 and 14. Equation 13 relates to the normal CP case and Equation 14 relates to the extended CP case. 
     
       
         
           
             
               
                 
                   
                     k 
                     = 
                     
                       
                         
                           ( 
                           
                             k 
                             ′ 
                           
                           ) 
                         
                         ⁢ 
                         mod 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           N 
                           sc 
                           RB 
                         
                       
                       + 
                       
                         
                           N 
                           sc 
                           RB 
                         
                         · 
                         
                           n 
                           PRB 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       k 
                       ′ 
                     
                     = 
                     
                       { 
                       
                         
                           
                             
                               
                                 
                                   
                                     4 
                                     ⁢ 
                                     
                                       m 
                                       ′ 
                                     
                                   
                                   + 
                                   
                                     v 
                                     shift 
                                   
                                 
                               
                               
                                 
                                   
                                     if 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     l 
                                   
                                   ∈ 
                                   
                                     { 
                                     
                                       2 
                                       , 
                                       3 
                                     
                                     } 
                                   
                                 
                               
                             
                             
                               
                                 
                                   4 
                                   ⁢ 
                                   
                                     
                                       m 
                                       ′ 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       
                                         2 
                                         + 
                                         
                                           v 
                                           shift 
                                         
                                       
                                       ) 
                                     
                                   
                                   ⁢ 
                                   mod 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   4 
                                 
                               
                               
                                 
                                   
                                     if 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     l 
                                   
                                   ∈ 
                                   
                                     { 
                                     
                                       5 
                                       , 
                                       6 
                                     
                                     } 
                                   
                                 
                               
                             
                           
                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           l 
                         
                         = 
                         
                           { 
                           
                             
                               
                                 
                                   
                                     3 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       = 
                                       0 
                                     
                                   
                                 
                                 
                                   
                                     6 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       = 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     2 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       = 
                                       2 
                                     
                                   
                                 
                                 
                                   
                                     5 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       = 
                                       3 
                                     
                                   
                                 
                               
                               ⁢ 
                               
                                 
 
                               
                               ⁢ 
                               
                                 l 
                                 ′ 
                               
                             
                             = 
                             
                               { 
                               
                                 
                                   
                                     
                                       
                                         
                                           
                                             0 
                                             , 
                                             1 
                                           
                                         
                                         
                                           
                                             
                                               if 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               
                                                 n 
                                                 s 
                                               
                                               ⁢ 
                                               mod 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                             = 
                                             0 
                                           
                                         
                                       
                                       
                                         
                                           
                                             2 
                                             , 
                                             3 
                                           
                                         
                                         
                                           
                                             
                                               if 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               
                                                 n 
                                                 s 
                                               
                                               ⁢ 
                                               mod 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               2 
                                             
                                             = 
                                             1 
                                           
                                         
                                       
                                     
                                     ⁢ 
                                     
                                       
 
                                     
                                     ⁢ 
                                     
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                                       ′ 
                                     
                                   
                                   = 
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                                       ⁢ 
                                       
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                                   = 
                                   
                                     
                                       N 
                                       ID 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     3 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     k 
                     = 
                     
                       
                         
                           ( 
                           
                             k 
                             ′ 
                           
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                         ⁢ 
                         mod 
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                           N 
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                           N 
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                         · 
                         
                           n 
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                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       k 
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                                     shift 
                                   
                                 
                               
                               
                                 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
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                                   = 
                                   4 
                                 
                               
                             
                             
                               
                                 
                                   
                                     3 
                                     ⁢ 
                                     
                                       m 
                                       ′ 
                                     
                                   
                                   + 
                                   
                                     
                                       ( 
                                       
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                                           shift 
                                         
                                       
                                       ) 
                                     
                                     ⁢ 
                                     mod 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
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                                   = 
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                           ⁢ 
                           
                             
 
                           
                           ⁢ 
                           l 
                         
                         = 
                         
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                                     4 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       ∈ 
                                       
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                                           0 
                                           , 
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                                         } 
                                       
                                     
                                   
                                 
                                 
                                   
                                     1 
                                   
                                   
                                     
                                       
                                         l 
                                         ′ 
                                       
                                       = 
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                               ⁢ 
                               
                                 
 
                               
                               ⁢ 
                               
                                 l 
                                 ′ 
                               
                             
                             = 
                             
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                                                 n 
                                                 s 
                                               
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                                               ⁢ 
                                               2 
                                             
                                             = 
                                             0 
                                           
                                         
                                       
                                       
                                         
                                           
                                             1 
                                             , 
                                             2 
                                           
                                         
                                         
                                           
                                             
                                               if 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               
                                                 n 
                                                 s 
                                               
                                               ⁢ 
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       m 
                                       ′ 
                                     
                                   
                                   = 
                                   0 
                                 
                                 , 
                                 1 
                                 , 
                                 … 
                                 ⁢ 
                                 
                                     
                                 
                                 , 
                                 
                                   
                                     
                                       4 
                                       ⁢ 
                                       
                                         N 
                                         RB 
                                         PDSCH 
                                       
                                     
                                     - 
                                     
                                       1 
                                       ⁢ 
                                       
                                         
 
                                       
                                       ⁢ 
                                       
                                         v 
                                         shift 
                                       
                                     
                                   
                                   = 
                                   
                                     
                                       N 
                                       ID 
                                       cell 
                                     
                                     ⁢ 
                                     mod 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     3 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equations 13 and 14, k is a subcarrier index, 1 is a symbol index, p is an antenna port index. In addition, N SC   RB  denotes an RB size in the frequency domain and is represented by the number of subcarriers, n PRB  denotes a PRB number, N RB   PDSCH  denotes the bandwidth of an RB in which a corresponding PDSCH is transmitted, n s  is a slot index, N ID   cell  is a cell ID and mod denotes modulo operation. An RS position in the frequency domain depends on Vshift. Since Vshift depends on cell ID, the RS position has different frequency shift values per cell. 
     LTE-A, the next generation of 3GPP LTE, considers MIMO, multi-cell transmission, enhanced MU-MIMO, etc. of a high order and also considers DRS based data demodulation in order to support efficient RS operation and an improved transmission scheme. That is, a DRS for two or more layers can be defined to support data transmission through an added antenna, separately from the DRS (antenna port index 5) for rank-1 beamforming, defined in 3GPP LTE (e.g. release-8). 
     Cooperative Multi-Point: CoMP 
     CoMP transmission/reception scheme (which is also referred to as co-MIMO, collaborative MIMO or network MIMO) is proposed to meet enhanced system performance requirements of 3GPP LTE-A. CoMP can improve the performance of a UE located at a cell edge and increase average sector throughput. 
     In a multi-cell environment having a frequency reuse factor of 1, the performance of a UE located at a cell edge and average sector throughput may decrease due to inter-cell interference (ICI). To reduce ICI, a conventional LTE system uses a method for allowing a UE located at a cell edge in an interfered environment to have appropriate throughput using a simple passive scheme such as fractional frequency reuse (FFR) through UE-specific power control. However, it may be more preferable to reduce ICI or reuse ICI as a signal that a UE desires rather than decreasing frequency resource use per cell. To achieve this, CoMP can be applied. 
     CoMP applicable to downlink can be classified into joint processing (JP) and coordinated scheduling/beamforming (CS/CB). 
     According to the JP, each point (eNB) of a CoMP coordination unit can use data. The CoMP coordination unit refers to a set of eNBs used for a coordinated transmission scheme. The JP can be divided into joint transmission and dynamic cell selection. 
     The joint transmission refers to a scheme through which PDSCHs are simultaneously transmitted from a plurality of points (some or all CoMP coordination units). That is, data can be transmitted to a single UE from a plurality of transmission points. According to joint transmission, quality of a received signal can be improved coherently or non-coherently and interference on other UEs can be actively erased. 
     Dynamic cell selection refers to a scheme by which a PDSCH is transmitted from one point (in a CoMP coordination unit). That is, data is transmitted to a single UE from a single point at a specific time, other points in the coordination unit do not transmit data to the UE at the time, and the point that transmits the data to the UE can be dynamically selected. 
     According to the CS/CB scheme, CoMP coordination units can collaboratively perform beamforming of data transmission to a single UE. Here, user scheduling/beaming can be determined according to coordination of cells in a corresponding CoMP coordination unit although data is transmitted only from a serving cell. 
     In case of uplink, coordinated multi-point reception refers to reception of a signal transmitted according to coordination of a plurality of points geographically spaced apart from one another. A CoMP reception scheme applicable to uplink can be classified into joint reception (JR) and coordinated scheduling/beamforming (CS/CB). 
     JR is a scheme by which a plurality of reception points receives a signal transmitted over a PUSCH and CS/CB is a scheme by which user scheduling/beamforming is determined according to coordination of cells in a corresponding CoMP coordination unit while one point receives a PUSCH. 
     Sounding Reference Signal (SRS) 
     An SRS is used for an eNB to measure channel quality and perform uplink frequency-selective scheduling based on the channel quality measurement. The SRS is not associated with data and/or control information transmission. However, the usages of the SRS are not limited thereto. The SRS may also be used for enhanced power control or for supporting various start-up functions of non-scheduled UEs. The start-up functions may include, for example, an initial modulation and coding scheme (MCS), initial power control for data transmission, timing advance, and frequency non-selective scheduling (in which a transmitter selectively allocates a frequency resource to the first slot of a subframe and then pseudo-randomly hops to another frequency resource in the second slot of the subframe). 
     The SRS may be used for measuring downlink channel quality on the assumption of the reciprocity of a radio channel between the downlink and the uplink. This assumption is valid especially in a time division duplex (TDD) system in which the downlink and the uplink share the same frequency band and are distinguished by time. 
     A subframe in which a UE within a cell is supposed to transmit an SRS is indicated by cell-specific broadcast signaling. A 4-bit cell-specific parameter ‘srsSubframeConfiguration’ indicates 15 possible configurations for subframes carrying SRSs in each radio frame. These configurations may provide flexibility with which SRS overhead can be adjusted according to network deployment scenarios. The other one configuration (a 16th configuration) represented by the parameter is perfect switch-off of SRS transmission in a cell, suitable for a cell serving high-speed UEs, for example. 
     As shown in  FIG. 8 , an SRS is always transmitted in the last SC-FDMA symbol of a configured subframe. Therefore, an SRS and a DMRS are positioned in different SC-FDMA symbols. PUSCH data transmission is not allowed in an SC-FDMA symbol designated for SRS transmission. Accordingly, even the highest sounding overhead (in the case where SRS symbols exist in all subframes) does not exceed 7%. 
     Each SRS symbol is generated for a given time unit and frequency band, using a base sequence (a random sequence or Zadoff-Chu (ZC)-based sequence set), and all UEs within a cell use the same base sequence. SRS transmissions in the same time unit and the same frequency band from a plurality of UEs within a cell are distinguished orthogonally by different cyclic shifts of the base sequence allocated to the plurality of UEs. Although the SRS sequences of different cells may be distinguished by allocating different base sequences to the cells, orthogonality is not ensured between the different base sequences. 
     Relay 
     A relay is considered in order to improve the coverage of high data rates, group mobility, temporary network deployment, cell edge throughput and/or to provide coverage in new areas. 
     The relay forwards transmission and reception between an eNB and a UE, and two links (backhaul link and access link) having different attributes are applied to each carrier frequency band. The eNB may include a donor cell. The relay wirelessly accesses a wireless access network through the donor cell. 
     A backhaul link between the eNB and the relay may be represented as a backhaul downlink when the backhaul link uses a downlink frequency band or a downlink subframe resource and represented as a backhaul uplink when the backhaul link uses an uplink frequency band or an uplink subframe resource. Here, a frequency band is a resource allocated in frequency division duplex (FDD) mode and a subframe is a resource allocated in time division duplex (TDD) mode. Similarly, an access link between the eNB and the relay may be represented as an access downlink when the access link uses a downlink frequency band or a downlink subframe resource and represented as access uplink when the access link uses an uplink frequency band or an uplink subframe resource. 
     The eNB needs to have uplink reception and downlink transmission functions and the UE needs to have uplink transmission and downlink reception functions. The relay needs to have functions of backhaul uplink transmission to the eNB, access uplink reception from the UE, backhaul downlink reception from the eNB and access downlink transmission to the UE. 
       FIG. 9  illustrates examples of implementation of functions of a transmitter and a receiver of an FDD mode relay. A reception function of the relay is descried. A downlink signal received from an eNB is transmitted to a fast Fourier transform (FFT) module  912  via a duplexer  911  and an OFDMA baseband receiving process  913  is performed. An uplink signal received from a UE is delivered to an FFT module  922  via a duplexer  921  and a discrete Fourier transform-spread-OFDMA (DFT-s-OFDMA) baseband receiving process  923  is performed. The process of receiving the downlink signal from the eNB and the process of receiving the uplink signal from the UE may be performed in parallel. A transmission function of the relay is described. An uplink signal transmitted to the eNB is delivered through a DFT-s-OFDMA baseband transmission process  933 , an inverse FFT (IFFT) module  932  and a duplexer  931 . A downlink signal transmitted to the UE is delivered through an OFDM baseband transmission process  943 , an IFFT module  942  and duplexer  941 . The process of transmitting the uplink signal to the eNB and the process of transmitting the downlink signal to the UE may be performed in parallel. The duplexers arranged in one direction may be implemented as a single bidirectional duplexer. For example, the duplexer  911  and the duplexer  931  can be implemented as a single bidirectional duplexer and the duplexer  921  and the duplexer  941  can be implemented as a single bidirectional duplexer. A bidirectional duplexer may be implemented such that IFFT module and baseband process module lines related to transmission and reception in a specific carrier frequency band are branched from the bidirectional duplexer. 
     In relation to the use of a bandwidth (or spectrum) of a relay, the case where a backhaul link operates in the same frequency band as an access link is referred to as in-band, and the case where the backhaul link operates in different frequency bands from the access link is referred to as out-band. In both in-band and out-band, UEs operating according to LTE (e.g., Release-8) should be able to access a donor cell. 
     The relay may be classified into a transparent relay and a non-transparent relay depending upon whether or not the UE recognizes the relay. In the transparent relay, the UE is not aware that it is communicating with a network via the relay, and in the non-transparent relay, the UE is aware that it is communicating with the network via the relay. 
     In relation to control of the relay, the relay may be divided into a relay as part of a donor cell and a relay for controlling a cell of its own. 
     The relay as part of the donor cell may have a relay ID but does not have a cell ID of its own. If at least part of Radio Resource Management (RRM) is controlled by an eNB to which the donor cell belongs (while parts of the RRM may be located in the relay), this may be called a relay as part of the donor cell. Desirably, such a relay may support legacy UEs. Smart repeaters, decode-and-forward relays, different types of L2 (second layer) relays, and type-2 relays are examples of this type of relay. 
     In the case where a relay is in control of its own cells, the relay controls one or several cells and a unique physical-layer cell ID is provided to each of the cells controlled by the relay. The same RRM mechanism is available and, in terms of the UE, there is no difference in accessing cells controlled by a relay and cells controlled by a normal eNB. The cells controlled by the relay may support the legacy UEs. Self-backhauling relays, L3 (third layer) relays, type-1 relays, and type-1a relays are examples of this type of relay. 
     A type-1 relay is an in-band relay and controls a plurality of cells, each of which appears as a separate cell, distinct from the donor cell, to UEs. The plurality of cells has its own physical cell ID (defined in LTE Release-8) and the relay may transmit its own synchronization channels, reference signals, etc. In the context of single-cell operation, the UE may receive scheduling information and HARQ feedback directly from the relay and may transmit its own control channels (SR, CQI, ACK/NACK, etc.) to the relay. The type-1 relay appears as a legacy eNB (an eNB operating according to LTE Release-8) to legacy UEs (UEs operating according to LTE Release-8). Namely, the type-1 relay has backward compatibility. Meanwhile, to UEs operating according to an LTE-A system, the type-1 relay appears as an eNB different from the legacy eNB to allow for performance enhancement. 
     A type-1a relay has the same characteristics as the above-mentioned type-1 relay except that it operates in out-band. The operation of the type-1a relay may be configured to minimize the influence on the operation of an L1 (first layer) or to eliminate such influence. 
     A type-2 relay, which is an in-band relay, does not have a separate physical cell ID and thus does not create any new cells. The type-2 relay is transparent to the legacy UEs, and the legacy UEs are not aware of the presence of the type-2 relay. The type-2 relay may transmit a PDSCH but does not transmit a Common Reference Signal (CRS) and a PDCCH. 
     Meanwhile, in order to allow in-band operation of the relay, some resources in the time-frequency space should be reserved for the backhaul link and may be set not to be used for the access link. This is called resource partitioning. 
     A general principle for resource partitioning in the relay is as follows. The backhaul downlink and access downlink may be time division multiplexed in a single carrier frequency (namely, only one of the backhaul downlink and access downlink is activated at a specific time). Similarly, the backhaul uplink and access uplink may be time division multiplexed in a single carrier frequency (namely, only one of the backhaul uplink and access uplink is activated at a specific time). 
     In multiplexing the backhaul links for FDD, backhaul downlink transmission and backhaul uplink transmission are carried out in a downlink frequency band and an uplink frequency band, respectively. In multiplexing the backhaul links for TDD, backhaul downlink transmission and backhaul uplink transmission are carried out in downlink subframes of the eNB and relay and uplink subframes of the eNB and relay, respectively. 
     In the case of an in-band relay, for example, if reception of the backhaul downlink from the eNB and transmission of the access downlink to the UE are simultaneously performed in a predetermined frequency band, a signal transmitted from a transmitting end of the relay may be received in a receiving end of the relay and thus signal interference or Radio Frequency (RF) jamming may occur at an RF front end of the relay. Similarly, if reception of the access uplink from the UE and transmission of the backhaul uplink to the eNB are simultaneously performed in a predetermined frequency band, signal interference may occur at the RF front end of the relay. Accordingly, in the relay, simultaneous transmission and reception in a single frequency band is difficult to achieve unless sufficient separation between a transmission signal and a reception signal is provided (e.g., unless a transmission antenna and a reception antenna are sufficiently separated from each other geographically (for example, by installation thereof above/below ground)). 
     One method for solving the problem of signal interference is to allow the relay not to transmit a signal to UEs while receiving a signal from the donor cell. That is, a gap may be generated in transmission to the UEs from the relay and the UEs (including the legacy UEs) may be set not to expect any transmission from the relay during the gap. In  FIG. 10 , a first subframe  1010  is a normal subframe in which a downlink (i.e. access downlink) control signal and data are transmitted from the relay to the UE and a second subframe  1020  is a multicast broadcast single frequency network (MBSFN) subframe. A control signal is transmitted from the relay to the UE in a control region  1021  of the downlink subframe, whereas no signal is transmitted from the relay to the UE in the remaining region  1022  of the downlink subframe. Here, in the case of a legacy UE, since transmission of a physical downlink control channel (PDCCH) is expected in all downlink frames (in other words, the relay needs to support legacy UEs belonging to the coverage thereof such that the legacy UEs receive and measure PDCCHs in every subframe), it is necessary to transmit a PDCCH in all downlink subframes for correct operation of legacy UEs. Accordingly, even in a subframe (second subframe  1020 ) configured for downlink (i.e. backhaul downlink) transmission from the eNB to the relay, the relay needs to perform access downlink transmission instead of backhaul downlink reception in a period corresponding to first N (N=1, 2 or 3) OFDM symbols of the subframe. For this, since a PDCCH is transmitted from the relay to the UE in the control region  1021  of the second subframe, backward compatibility for legacy UEs served by the relay can be provided. The relay can receive a signal transmitted from the eNB in the remaining region  1022  of the second subframe since no signal is transmitted from the relay to the UE in the region  1022 . Accordingly, through such a resource partitioning scheme, access downlink transmission and backhaul downlink reception can be prevented from being simultaneously performed in an in-band relay. 
     The second subframe  1022  using an MBSFN subframe will be described in detail hereinafter. The MBSFN subframe is a subframe for multimedia broadcast and multicast service (MBMS) for simultaneously transmitting the same signal in multiple cells. The control region  1021  of the second subframe may be regarded as a relay non-listening interval. The relay non-listening interval refers to a period in which a relay transmits an access downlink signal instead of receiving a backhaul downlink signal. The relay non-listening interval can be set to a length of 1, 2 or 3 OFDM symbols, as described above. The relay can perform access downlink transmission to the UE in the relay non-listening interval  1021  and receive a backhaul downlink signal from the eNB in the remaining region  1022 . Here, since the relay cannot simultaneously perform transmission and reception in the same frequency band, time is taken for the relay to switch from transmission mode to reception mode. Accordingly, a guard time GT needs to be set to the first part of the backhaul downlink reception region  1022  such that the relay can perform transmission/reception mode switching in the region  1022 . Similarly, even when the relay receives a backhaul downlink signal from the eNB and transmits an access downlink signal to the UE, a guard time (GT) for reception/transmission mode switching of the relay can be set. The duration of the GT may be set to a value in the time domain. For example, the duration of the GT can be set to k (k=1) time sample (Ts) or one or more OFDM symbols. A GT of the last part of the subframe may not be defined or set when relay backhaul downlink subframes are continuously configured or according to a predetermined subframe timing alignment relationship. This GT may be defined only in a frequency region set for backhaul downlink subframe transmission in order to maintain backward compatibility (legacy UEs cannot be supported when a GT is set in an access downlink period). The relay can receive a PDCCH and a PDSCH from the eNB in the backhaul downlink reception period  1022  other than the GT. Particularly, a PDCCH for the relay can be represented as a relay-PDCCH (R-PDCCH) in the sense of a relay dedicated physical channel. 
     Inter-Cell Interference Coordination (ICIC) 
     When coverages of two eNBs eNB1 and eNB2 overlap since the two eNBs are located adjacent to each other, a strong downlink signal from one eNB may cause interference for a UE served by the other eNB. For example, in  FIG. 1 , interference may be generated in the UE  130  served by the micro eNB  120  by a signal from the macro eNB  110 . When inter-cell interference is generated, the two eNBs may reduce inter-cell interference through inter-cell cooperation. 
     It is assumed that signal transmission and reception are smoothly performed between two eNBs which interfere each other in the following embodiments of the present invention. For example, it is assumed that a wired/wireless link (e.g. backhaul link or Un interface) having satisfactory transmission conditions such as transmission bandwidth, time delay, etc. is present between the two eNBs and thus reliability of transmission and reception of a cooperative signal between the two eNBs is high. In addition, it is assumed that time synchronization between the two eNBs is within an allowable error range (when boundaries of downlink subframes of the two eNBs which interfere each other are aligned, for example) or the eNBs clearly recognize an offset between the subframe boundaries of the eNBs. 
     Referring back to  FIG. 1 , eNB1  110  may be a macro eNB that covers a wide area with high Tx power and eNB2  120  may be a micro eNB that covers a narrow area with low Tx power. As shown in  FIG. 1 , when eNB2  120  is located at a cell edge and strong interference from eNB1  110  is applied to the UE  130  served by eNB2  120 , effective communication may not be performed without appropriate inter-cell cooperation. 
     Particularly, when a large number of UEs are connected to micro eNB2  120  having low power to distribute loads on which macro eNB1  110  provides services, the possibility that the aforementioned inter-cell interference occurs is high. 
     For example, when a UE selects a serving eNB, the UE can calculate and compare Rx powers of downlink signals from a micro eNB and a macro eNB by adding a predetermined adjustment value (bias value) to the Rx power of the micro eNB and adding no adjustment value to the Rx power of the macro eNB. Consequently, the UE can select an eNB that provides highest downlink Rx power as the serving eNB. Accordingly, a larger number of UEs can be connected to the micro eNB. 
     The micro eNB can be selected as the serving eNB even though the intensity of the downlink signal received by the UE from the macro eNB is much higher than the downlink signal from the micro eNB, and strong interference from the macro eNB may be applied to UEs connected to the micro eNB. In this case, UEs located at the boundary of the micro eNB may not perform correct operations due to strong interference from the macro eNB if inter-cell cooperation is not provided. 
     The aforementioned inter-cell interference generation situation is exemplary and embodiments of the present invention are equally applicable to different inter-cell interference generation situations (e.g. a case in which inter-cell interference occurs between a CSG type HeNB and an OSG type macro eNB, a case in which a micro eNB causes interference and a macro eNB is interfered by the micro eNB or a case in which inter-cell interference occurs between micro eNBs or between macro eNBs). 
     The following description is based on the assumption that an interfering cell is eNB1, an interfered cell is eNB2, eNB1 is a macro eNB and eNB2 is a micro eNB. However, the present invention is not limited thereto and the principle of the present invention is applicable to various inter-cell interference cases. 
     To perform effective operations even when inter-cell interference is present between two eNBs, appropriate cooperation between the two eNBs needs to be achieved and a signal that enables cooperation can be transmitted/received through a link (e.g. X2 interface) between the two eNBs. For example, when inter-cell interference is generated between a macro eNB and a micro eNB, the macro eNB may control inter-cell cooperation and the micro eNB may perform an appropriate operation according to a cooperative signal signaled by the macro eNB. In addition, an eNB may provide signaling (e.g. OTA (over the air) signaling) related to inter-cell interference coordination to a UE affected by inter-cell interference independently or simultaneously with inter-cell interference coordination operation. 
     Inter-cell interference coordination may be performed in frequency resources and/or time resources. In one embodiment of an inter-cell interference coordination scheme, eNB1 may not perform transmission (i.e. transmit a null signal or perform silencing) in a specific resource region or reduce Tx power for a UE connected to eNB2. Here, the specific resource region in which silencing is performed may be represented as a time resource and/or a frequency resource. 
     For example, the position of a silenced time resource may be determined by a combination of at least one of the entire time region, a specific subframe, a specific slot and a specific OFDM symbol. In addition, the position of a silenced frequency resource may be determined by a combination of at least one of the entire frequency band, a specific carrier (in case of carrier aggregation using a plurality of carriers), a specific resource block and a specific subcarrier. Accordingly, a resource region in which silencing is performed can be correctly specified. 
     Cell range extension (CRE) of the micro cell can be achieved according to silencing or Tx power reduction of the macro cell. For example, the macro cell can de-boost Tx power by a maximum of 9 dB in a specific resource region or transmit no signal to reduce interference applied to the micro cell in the specific resource region and extend the range that can be served by the micro cell. 
     A description will be given of a detailed inter-cell interference coordination scheme. 
     Inter-Cell Interference Coordination in Time Domain 
     Inter-cell interference coordination with respect to a time resource will now be described. For example, inter-cell interference coordination with respect to a time resource in a 3GPP LTE (release-10) wireless communication system can be performed in such a manner that the time resource is segmented into a plurality of subframes and information representing whether silencing or transmission with reduced Tx power is performed for each subframe is indicated. That is, inter-cell interference coordination with respect to the time resource refers to cooperation between the interfering cell (eNB1) and the interfered cell (eNB2) with respect to how the subframes will be used. Here, information on silencing or Tx power information for each subframe may be exchanged between the interfering cell and the interfered cell through X2 signaling, backhaul signaling or OAM (operations administration and maintenance). A subframe to which silencing is applied may be referred to as an almost blank subframe (ABS). 
     For example, eNB1 can set a specific subframe (or subframe set) to an ABS such that strong interference from eNB1 is not applied to eNB2 in the specific subframe. That is, downlink Tx power, traffic or activity of eNB1 is reduced or no signal is transmitted (i.e. a null signal is transmitted) in the specific subframe (or subframe set). 
     Specifically, an ABS may refer to a subframe in which only a CRS is transmitted in a control region and a data region and other control information and data are not transmitted. However, important downlink channels and downlink signals, such as PBCH (physical broadcast channel), PSS (primary synchronization signal), SSS (secondary synchronization signal), etc., may be transmitted in the ABS. In addition, a CRS of the data region may not be transmitted in the ABS. 
     When eNB1 sets the ABS, information related to the ABS may be exchanged through a link (e.g. X2 interface) between eNB1 and eNB2. For example, an ABS information element (IE) and/or an ABS status IE defined in 3GPP LTE standard document (e.g. TS 36.423) can be used. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 IE type and 
                   
               
               
                 IE/Group Name 
                 Presence 
                 Range 
                 reference 
                 Semantics description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 CHOICE ABS Information 
                 M 
                 — 
                 — 
               
               
                 &gt;FDD 
                   
                 — 
                 — 
               
               
                 &gt;&gt;ABS Pattern Info 
                 M 
                 BIT STRING 
                 Each position in the bitmap 
               
               
                   
                   
                 (SIZE(40)) 
                 represents a DL subframe, 
               
               
                   
                   
                   
                 for which value “1” 
               
               
                   
                   
                   
                 indicates ‘ABS’ and value 
               
               
                   
                   
                   
                 “0” indicates ‘non ABS’. 
               
               
                   
                   
                   
                 The first position of the 
               
               
                   
                   
                   
                 ABS pattern corresponds 
               
               
                   
                   
                   
                 to subframe 0 in a radio 
               
               
                   
                   
                   
                 frame where SFN = 0. The 
               
               
                   
                   
                   
                 ABS pattern is 
               
               
                   
                   
                   
                 continuously repeated in all 
               
               
                   
                   
                   
                 radio frames. 
               
               
                   
                   
                   
                 The maximum number of 
               
               
                   
                   
                   
                 subframes is 40. 
               
               
                 &gt;&gt;Number Of Cell- 
                 M 
                 ENUMERATED 
                 P (number of antenna ports 
               
               
                 specific Antenna Ports 
                   
                 (1, 2, 4, . . .) 
                 for cell-specific reference 
               
               
                   
                   
                   
                 signals) defined in TS 
               
               
                   
                   
                   
                 36.211 [10] 
               
               
                 &gt;&gt;Measurement Subset 
                 M 
                 BIT STRING 
                 Indicates a subset of the 
               
               
                   
                   
                 (SIZE(40)) 
                 ABS Pattern Info above, 
               
               
                   
                   
                   
                 and is used to configure 
               
               
                   
                   
                   
                 specific measurements 
               
               
                   
                   
                   
                 towards the UE. 
               
               
                 &gt;TDD 
                   
                 — 
                 — 
               
               
                 &gt;&gt;ABS Pattern Info 
                 M 
                 BIT STRING 
                 Each position in the bitmap 
               
               
                   
                   
                 (1 . . . 70, . . .) 
                 represents a DL subframe 
               
               
                   
                   
                   
                 for which value “1” 
               
               
                   
                   
                   
                 indicates ‘ABS’ and value 
               
               
                   
                   
                   
                 “0” indicates ‘non ABS’. 
               
               
                   
                   
                   
                 The maximum number of 
               
               
                   
                   
                   
                 subframes depends on 
               
               
                   
                   
                   
                 UL/DL subframe 
               
               
                   
                   
                   
                 configuration. 
               
               
                   
                   
                   
                 The maximum number of 
               
               
                   
                   
                   
                 subframes is 20 for UL/DL 
               
               
                   
                   
                   
                 subframe configuration 
               
               
                   
                   
                   
                 1-5; 60 for UL/DL 
               
               
                   
                   
                   
                 subframe configuration 6; 
               
               
                   
                   
                   
                 70 for UL/DL subframe 
               
               
                   
                   
                   
                 configuration 0. 
               
               
                   
                   
                   
                 UL/DL subframe 
               
               
                   
                   
                   
                 configuration defined in TS 
               
               
                   
                   
                   
                 36.211 [10]. 
               
               
                   
                   
                   
                 The first position of the 
               
               
                   
                   
                   
                 ABS pattern corresponds 
               
               
                   
                   
                   
                 to subframe 0 in a radio 
               
               
                   
                   
                   
                 frame where SFN = 0. The 
               
               
                   
                   
                   
                 ABS pattern is 
               
               
                   
                   
                   
                 continuously repeated in all 
               
               
                   
                   
                   
                 radio frames, and restarted 
               
               
                   
                   
                   
                 each time SFN = 0. 
               
               
                 &gt;&gt;Number Of Cell- 
                 M 
                 ENUMERATED 
                 P (number of antenna ports 
               
               
                 specific Antenna Ports 
                   
                 (1, 2, 4, . . .) 
                 for cell-specific reference 
               
               
                   
                   
                   
                 signals) defined in TS 
               
               
                   
                   
                   
                 36.211 [10] 
               
               
                 &gt;&gt;Measurement Subset 
                 M 
                 BIT STRING 
                 Indicates a subset of the 
               
               
                   
                   
                 (1 . . . 70, . . .) 
                 ABS Pattern Info above, 
               
               
                   
                   
                   
                 and is used to configure 
               
               
                   
                   
                   
                 specific measurements 
               
               
                   
                   
                   
                 towards the UE 
               
               
                 &gt;ABS Inactive 
                 M 
                 NULL 
                 Indicates that interference 
               
               
                   
                   
                   
                 coordination by means of 
               
               
                   
                   
                   
                 almost blank sub frames is 
               
               
                   
                   
                   
                 not active 
               
               
                   
               
            
           
         
       
     
     Table 1 shows an ABS IE transmitted from eNB1 that sets the ABS to eNB2. The ABS IE may include ABS pattern information in TDD and FDD, information on the number of cell-specific antenna ports for CRS, measurement subset information, etc. 
     Specifically, the ABS pattern information indicates a subframe to be used as an ABS and is composed of a 40-bit bitmap in case of FDD and composed of a maximum of a 70-bit bitmap according to downlink/uplink subframe configuration in case of TDD. 
     For example, in case of FDD, 40 bits indicate 40 subframes, a bit value “1” indicates an ABS and a bit value “0” indicates a non-ABS. 
     The information on the number of cell-specific antenna ports is used for CRS measurement such that a served UE can perform restricted measurement only in the ABS. 
     The measurement subset information is a subset of the ABS pattern information and is composed of a 40-bit bitmap in case of FDD and composed of a maximum of 70-bit bitmap in case of TDD. A measurement subset refers to a restricted measurement set recommended by eNB1 to set restricted measurement in the UE served by eNB2. A subframe set to the measurement subset may be regarded as a subframe that is more statically set to an ABS from among subframes set to ABSs. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 IE type and 
                   
               
               
                 IE/Group Name 
                 Presence 
                 Range 
                 reference 
                 Semantics description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 DL ABS status 
                 M 
                 INTEGER 
                 Percentage of used ABS 
               
               
                   
                   
                 (0 . . . 100) 
                 resources. The numerator of 
               
               
                   
                   
                   
                 the percentage calculation 
               
               
                   
                   
                   
                 consists of resource blocks 
               
               
                   
                   
                   
                 within the ABS indicated in the 
               
               
                   
                   
                   
                 Usable ABS Pattern Info IE 
               
               
                   
                   
                   
                 allocated by the eNB 2  for UEs 
               
               
                   
                   
                   
                 needing protection by ABS 
               
               
                   
                   
                   
                 from inter-cell interference for 
               
               
                   
                   
                   
                 DL scheduling, or allocated by 
               
               
                   
                   
                   
                 the eNB 2  for other reasons 
               
               
                   
                   
                   
                 (e.g. some control channels). 
               
               
                   
                   
                   
                 The denominator of the 
               
               
                   
                   
                   
                 percentage calculation is the 
               
               
                   
                   
                   
                 total quantity of resource 
               
               
                   
                   
                   
                 blocks within the ABS 
               
               
                   
                   
                   
                 indicated in the Usable ABS 
               
               
                   
                   
                   
                 Pattern Info IE. 
               
               
                 CHOICE Usable ABS 
                 M 
                 — 
                 — 
               
               
                 Information 
                   
                   
                   
               
               
                 &gt;FDD 
                   
                 — 
                 — 
               
               
                 &gt;&gt;Usable ABS Pattern 
                 M 
                 BIT STRING 
                 Each position in the bitmap 
               
               
                 Info 
                   
                 (SIZE(40)) 
                 represents a subframe, for 
               
               
                   
                   
                   
                 which value “1” indicates ‘ABS 
               
               
                   
                   
                   
                 that has been designated as 
               
               
                   
                   
                   
                 protected from inter-cell 
               
               
                   
                   
                   
                 interference by the eNB 1 , and 
               
               
                   
                   
                   
                 available to serve this purpose 
               
               
                   
                   
                   
                 for DL scheduling in the eNB 2 ’ 
               
               
                   
                   
                   
                 and value “0” is used for all 
               
               
                   
                   
                   
                 other subframes. 
               
               
                   
                   
                   
                 The pattern represented by the 
               
               
                   
                   
                   
                 bitmap is a subset of, or the 
               
               
                   
                   
                   
                 same as, the corresponding 
               
               
                   
                   
                   
                 ABS Pattern Info IE conveyed 
               
               
                   
                   
                   
                 in the LOAD INFORMATION 
               
               
                   
                   
                   
                 message from the eNB 1 . 
               
               
                 &gt;TDD 
                   
                 — 
                 — 
               
               
                 &gt;&gt;Usable ABS Pattern 
                 M 
                 BIT STRING 
                 Each position in the bitmap 
               
               
                 Info 
                   
                 (1 . . . 70) 
                 represents a subframe, for 
               
               
                   
                   
                   
                 which value “1” indicates ‘ABS 
               
               
                   
                   
                   
                 that has been designated as 
               
               
                   
                   
                   
                 protected from inter-cell 
               
               
                   
                   
                   
                 interference by the eNB 1 , and 
               
               
                   
                   
                   
                 available to serve this purpose 
               
               
                   
                   
                   
                 for DL scheduling in the eNB 2 ’ 
               
               
                   
                   
                   
                 and value “0” is used for all 
               
               
                   
                   
                   
                 other subframes. 
               
               
                   
                   
                   
                 The pattern represented by the 
               
               
                   
                   
                   
                 bitmap is a subset of, or the 
               
               
                   
                   
                   
                 same as, the corresponding 
               
               
                   
                   
                   
                 ABS Pattern Info IE conveyed 
               
               
                   
                   
                   
                 in the LOAD INFORMATION 
               
               
                   
                   
                   
                 message from the eNB 1 . 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the ABS status IE. The ABS status IE may be transmitted from eNB2 to eNB1 and signals whether eNB1 needs to correct the ABS pattern. The ABS status IE may include usable ABS pattern information and downlink (DL) ABS status information. 
     The usable ABS pattern information is bitmap information indicating whether a subframe set to an ABS has been correctly used for interference mitigation and corresponds to a subset of the ABS pattern IE included in a load information message and transmitted from the interfering cell (eNB1). Specifically, each position in the bitmap represents a subframe, bitmap information corresponding to a subframe set to “1” indicates an ABS that has been designated as protected from inter-cell interference by eNB1, and eNB2 mitigates inter-cell interference by performing downlink scheduling in the subframe set to “1”. 
     The DL ABS status information indicates the percentage of used ABS resources. Specifically, the DL ABS status information refers to the percentage of resource blocks allocated by the interfered cell (eNB2) for UEs that need protection from inter-cell interference from among the total number of downlink resource blocks in the ABS indicated by the usable ABS pattern information. That is, the DL ABS status information represents whether eNB2 efficiently uses the ABS for the purpose of interference elimination. 
     Inter-Cell Interference Coordination in Frequency Domain 
     A description will be given of inter-cell interference coordination with respect to frequency resources. For example, inter-cell interference coordination with respect to a frequency resource in a 3GPP LTE (release-8) wireless communication system can be performed by dividing the frequency resource into specific resource (e.g. physical resource block (PRB) or subband) units and transmitting/receiving information on the specific resource units through the link between the two eNBs. Specifically, information on the specific resource units may include relative narrowband transmission power (RNTP), interference overload indication (IOI), high interference indication (HII), etc. 
     Here, the RNTP refers to information representing downlink Tx power that the interfering cell (eNB1) uses in a specific resource (e.g. PRB or subband). The RNTP may be determined using a bitmap in the specific resource unit and determined bitmap information may be transmitted to an interfered cell (eNB2) through the link between the eNBs. For example, RNTP set to “0” in the specific resource unit represents that downlink Tx power does not exceed a defined threshold value and RNTP set to “1” represents that it is impossible to ensure whether the downlink Tx power does not exceed the defined threshold value. 
     The IOI refers to information indicating the quantity of uplink interference in the specific resource unit, which is caused by the interfering cell (eNB1). That is, the IOI is information indicating a specific resource which is severely interfered. For example, when the IOI represents severe interference in a specific resource, the IOI indicates presence of severe uplink interference in the specific resource. In this case, the interfered cell (eNB2) can mitigate interference present between eNB1 and eNB2 by scheduling a UE using low uplink Tx power from among UEs served thereby in the specific resource. 
     The HII indicates the sensitivity of uplink interference from the interfering cell (eNB1) with respect to a specific resource. For example, when the HII is set to ‘1’ in the specific resource, the HII represents that there is a possibility that eNB1 schedules a UE having high uplink Tx power (i.e. causing storing inter-cell interference) in the specific resource. When the HII is set to ‘0’ in the specific resource, the HII represents that there is a possibility that eNB1 schedules a UE having low uplink Tx power in the specific resource. In this case, the interfered cell (eNB2) may use a specific resource to which interference is less applied for which the HII is set to ‘0’, to schedule UEs served thereby and schedule UEs that can operate even in case of serous interference in a specific resource to which strong interference is applied, for which the HII is set to ‘1’, thereby mitigating inter-cell interference. 
     Restricted Measurement 
     For a measurement resource set by an eNB, a UE can measure the intensity of a signal from the eNB and the intensity of an interference signal from another eNB and feed back the measurement result to the eNB. The eNB can use the measurement result for scheduling of the UE. 
     If eNB1 reduces Tx power in a specific resource region, a variation in an interference signal per resource region, which is received by a UE belonging to eNB2 increases. That is, the intensity of the interference signal is low in the specific resource region and is high in other resource regions. In this case, when the UE calculates the simple average of interference signal intensity in the measurement resource without taking into account the specific resource region and feeds back the simple average, it is difficult for eNB2 to correctly use a subframe in which eNB1 reduces Tx power. Accordingly, when eNB1 sets an ABS, interference applied to eNB2 depends on the ABS pattern and thus eNB2 sets resources having similar interference characteristics as measurement resources in consideration of the ABS pattern of eNB1 to enable measurement of the UE. Setting of measurement in a restricted resource for the UE may be referred to as restricted measurement. 
     For example, to set a measurement resource for CSI, an eNB can signal a plurality of subframe sets for channel measurement to a UE through higher layer signaling. If two subframe sets are configured for CSI measurement, the first subframe set may be called C0 and the second subframe set may be called C1. Subframes belonging to one subframe set preferably have similar interference characteristics. For example, C0 can be selected from subframes set by a neighboring cell to ABSs and C1 can be selected from other subframes. C0 and C1 do not overlap and a subframe which does not belong to C0 and C1 may be present. The UE for which the plurality of subframe sets for channel measurement has been set can perform channel measurement and reporting per subframe set. For example, the UE can determine CSI with respect to C0 on the basis of channel and interference signal averages in subframes belonging to C0 and determine CSI with respect to C1 on the basis of channel and interference signal averages in subframes belonging to C1. 
     To set measurement resources for RLM (radio link monitoring) or RRM (radio resource management), the eNB can signal a specific subframe set. RRM measurement may include measurement of reference signal received power (RSRP), reference signal received quality (RSRQ), received signal strength indicator (RSSI), etc. RLM measurement may include measurement for detection of radio link failure (RLF) such as no reception of a downlink control signal or remarkable decrease in received signal quality. Measurement resources for RLM/RRM may be set separately from the measurement resource for CSI. For example, a subframe set for RLM/RRM measurement in a UE belonging to eNB2 can be selected from subframes set to ABSs. 
     Improved ICIC 
     The ABS can be classified into a zero power-ABS (z-ABS) through which data (or PDSCH signal) in a data region of a downlink subframe is not transmitted and a reduced power-ABS (r-ABS) through which data (or PDSCH signal) in a data region of a downlink subframe is transmitted with very low Tx power. 
     When the r-ABS is used, a UE served by eNB1 can receive data even in an ABS to improve system throughput. That is, eNB1 may perform downlink transmission while interference applied to eNB2 is reduced by decreasing Tx power of eNB1. To achieve this, an r-ABS pattern (i.e. information indicating a subframe set to the r-ABS) and information on power applied to the r-ABS need to be exchanged between cells that cause mutual interference. In addition, it is necessary to inform an interfered UE of the r-ABS pattern and Tx power information. The present invention defines information related to r-ABS configuration and provides a method of exchanging the information between cells and a method of signaling the information to a UE. 
     Setting of r-ABS Tx Power 
     Wireless communication systems such as 3GPP LTE define EPRE (energy per resource element) that indicates an energy value per resource element for downlink resource Tx power allocation.  FIG. 11  illustrates exemplary power allocation to resource elements included in a downlink subframe. 
     In  FIG. 11 , X axis represents OFDM symbol, Y axis represents subcarrier and Z axis represents Tx power. 
     An eNB determines Tx power allocated to downlink resources as an energy value per resource element. Downlink resource Tx power allocation is based on EPRE with respect to a cell-specific reference signal (CRS), and EPRE for a resource region of a physical downlink shared channel (PDSCH) on which data is transmitted is represented as a percentage of EPRE of the CRS. 
     For example, the ratio of ERPE of the PDSCH to EPRE of the CRS in an OFDM symbol period in which the CRS is not present in the time domain in a downlink subframe is defined as ρ A  and as ρ B  in an OFDM symbol period in which the CRS is present. 
     Here, ρ A  may be determined by a power offset δ power-offset  and a UE-specific parameter PA according to whether multiuser-MIMO (MU-MIMO) transmission scheme is applied. δ power-offset  may be set to 0 dB for all PDSCH transmission schemes other than MU-MIMO and the UE-specific parameter PA may be signaled to a UE through a higher layer. In addition, ρ B /ρ A  may be determined by the number of antenna ports, which is cell-specifically set, and a cell-specific parameter PB signaled by a higher layer and given as shown in Table 3. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 ρ B /ρ A   
                   
               
            
           
           
               
               
               
            
               
                   
                 One Antenna 
                 Two and Four 
               
               
                 P B   
                 Port 
                 Antenna Ports 
               
               
                   
               
               
                 0 
                 1 
                 5/4 
               
               
                 1 
                 4/5 
                 1 
               
               
                 2 
                 3/5 
                 3/4 
               
               
                 3 
                 2/5 
                 1/2 
               
               
                   
               
            
           
         
       
     
     A power control dynamic range with respect to each RE in a wireless communication system such as LTE may be restricted as shown in Table 4 according to requirements (e.g. error vector magnitude (CVM), out-of-band emission, etc.) with respect to radio frequency. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                   
                 Modulation scheme 
                 RE power control dynamic range (dB)    
                   
               
            
           
           
               
               
               
               
            
               
                   
                 used on the RE    
                 (down)   
                 (up)   
               
               
                   
               
               
                   
                 QPSK (PDSCH)    
                 −6    
                 +4    
               
               
                   
                 QPSK (PDSCH)    
                 −6    
                 +3    
               
               
                   
                 16QAM (PDSCH)    
                 −3    
                 +3    
               
               
                   
                 64QAM (PDSCH)    
                  0    
                  0    
               
               
                   
               
            
           
         
       
     
     Table 4 shows an RE power control dynamic range according to a modulation scheme of an E-UTRA eNB. The Re power control dynamic range refers to a difference between Tx power of each RE and average Tx power of REs when transmission is performed with maximum Tx power of the eNB under specified reference conditions. The RE power control dynamic range may be restricted by requirements such as out-of-band emission, EVM, etc. Out-of-band emission is unwanted emission and refers to immediate emission to the outside of a channel bandwidth as a result of a modulation process and non-linearity of a transmitter. EVM refers to a difference between an ideal symbol and a measured symbol after quantization. EVM is defined as a square root of the ratio of mean reference power to mean error vector power and may be represented as a percentage. 
     The RE power control dynamic range shown in Table 4 can be determined as the upper limit and the lower limit of Tx power with respect to each RE on the basis of Tx power of REs when the eNB performs transmission with maximum Tx power under specified reference conditions. Specifically, the upper limit (up) of Tx power with respect to each RE can be determined by the aforementioned out-of-band emission according to a modulation process and transmitter non-linearity and the lower limit (down) thereof can be determined by the aforementioned EVM. 
     As described above, power allocation to a downlink subframe may be determined by ρ A  and ρ B  configuration information about power allocation may be provided from the eNB to the UE through higher layer signaling. 
     When the lower limit of the RE power control dynamic range is defined as shown in Table 4, RE Tx power cannot be set to lower than the lower limit. For example, supporting of the r-ABS through which downlink transmission is performed with very low power, as described above, may be restricted. In other words, the aforementioned power allocation scheme for normal downlink subframes may not be valid for the r-ABS. 
       FIG. 12  illustrates another example of allocating power per RE included in a downlink subframe. In  FIG. 12 , X axis represent OFDM symbol, Y axis represents subcarrier and Z axis represents Tx power. 
       FIG. 12  shows exemplary Tx power for each RE in an r-ABS. In the case of the r-ABS, the ratio of EPRE of PDSCH to EPRE of CRS may differ from ρ B /ρ A ≠1 which is determined based on higher layer signaling. In this case, downlink channel measurement and/or downlink data demodulation of the UE may not be correctly performed due to a difference between actual Tx power and Tx power determined by the UE through signaling from the eNB. 
     Specifically, in 3GPP LTE, allocation of Tx power of normal downlink subframes is determined on the basis of ρ A  (i.e. the ratio of EPRE of PDSCH to EPRE of CRS in an OFDM symbol period in which no CRS is present) and ρ B  (i.e. the ratio of EPRE of PDSCH to EPRE of CRS in an OFDM symbol period in which a CRS is present) and ρ A  and ρ B  are respectively determined by the UE-specific parameter PA and cell-specific parameter PB. However, the r-ABS Tx power allocation scheme in which Tx power lower than Tx power for a PDSCH resource region of a normal downlink subframe is allocated to a PDSCH resource region of the r-ABS differs from the normal downlink subframe Tx power allocation scheme. That is, Tx power allocation information about the r-ABS cannot be supported by the conventionally defined Tx power information. Accordingly, it is necessary to define downlink Tx power allocation information for supporting the r-ABS with r-ABS pattern information and to define a method for signaling the information to eNBs and/or UEs. 
     Dynamic Tx Power Information Signaling 
     As described above in relation to Table 1, an eNB (eNB1) that configures ABSs may set some subframes, which belong to a subframe set (i.e. subframes indicated by ‘1’ in the ABS pattern information bitmap of Table 1) set to ABSs, as a measurement subset and signal the measurement subset to a neighboring eNB (eNB2). As described above, the subframe set to the measurement subset may correspond to subframes more statically set to ABSs by eNB1 from among ABSs. That is, eNB1 can autonomously determine whether to use subframes that do not belong to the measurement subset as ABSs or normal subframes for subframes. Accordingly, subframes other than the subframes belonging to the measurement subset may be dynamically used as ABSs or normal subframes by eNB1. In other words, the subframes belonging to the measurement subset are subframes that eNB1 promises eNB2 to set as ABSs, and eNB2 may set measurement resources for a UE (i.e. UE2) served thereby on the assumption that enB1 sets the corresponding subframes to ABSs. 
     As described above, eNB2 may set two measurement resource sets C0 and C1 for UE2 for CSI measurement. C0 and C1 do not overlap and subframes that do not belong to C0 and C1 may be present. UE2 may perform restricted measurement for separate measurement for C0 and C1. While eNB2 sets C0 and C1 for restricted measurement of UE2 in consideration of the measurement subset of eNB1, UE2 cannot be aware of ABS configuration information of eNB1 from configuration of C0 and C1. In addition, UE2 cannot recognize an ABS pattern of eNB2 from the measurement resource sets (C0 and C1) set therefor. Similarly, a UE (i.e. UE1) served by eNB1 cannot be aware of the ABS configuration information of eNB1 and ABS configuration information of eNB2. In this manner, ABS pattern information is exchanged only between eNBs and direct/indirect signaling of ABS configuration information (particularly, subframe Tx power information) from an eNB to a UE is not provided in the conventional wireless communication system. 
     Furthermore, semi-statically configured CSI measurement sets (C0, C1) may be discriminated according to eNB Tx power or interference characteristics. For example, if C0 refers to subframes in which an eNB transmits a signal with normal Tx power, then C1 may refer to subframes (e.g. r-ABSs) in which the eNB transmits a signal with reduced Tx power. Here, for information on Tx power of the eNB with respect to the normal subframe, such as C0, the signaling scheme as described with reference to  FIG. 11  has been defined. However, a method for signaling Tx power information about a subframe in which a signal is transmitted with reduced Tx power, such as C1, has not been provided. 
     According to operation of a wireless communication system that supports only the S, eNB1 need not signal ABS configuration information to UE1 because it can be assumed that the eNB has no UL/DL unicast scheduling intention in the z-ABS and restricted scheduling is applied to the z-ABS even when the eNB performs scheduling in the z-ABS. Restricted scheduling refers to scheduling based on QPSK (quadrature phase shift keying) modulation scheme capable of scheduling even when a Tx power level is not signaled (e.g. a UE can attempt data demodulation even without Tx power information in the z-ABS even if downlink data is transmitted in the z-ABS). Accordingly, a signaling scheme in which the eNB signals ABS configuration information (particularly, Tx power information in an ABS) to the UE is not defined in the conventional wireless communication system supporting only the z-ABS. 
     However, when the aforementioned r-ABS is introduced, the eNB has scheduling intention in the corresponding r-ABS. When the eNB performs scheduling for a UE in the r-ABS, the UE needs to know Tx power of the eNB for correct demodulation of a PDSCH from the eNB. In this regard, an eNB (eNB1) that sets ABSs dynamically determines which subframe will be used as a normal subframe, z-ABS or r-ABS from among subframes other than subframes set to a measurement subset and signaled to a neighboring eNB (eNB2). Accordingly, the eNB needs to dynamically signal Tx power information about the r-ABS to the UE. 
     A description will be given of particulars commonly applied to embodiments of the present invention for dynamically signaling subframe power information. 
     The present invention assumes that an eNB and a UE share a Tx power value P normal  applied to a normal subframe and a Tx power value P r-ABS  applied to the r-ABS. 
     Here, P normal  and P r-ABS  may be defined as the ratio of PDSCH Tx power to CRS Tx power. Specifically, P normal  may be defined as one of P A , P B  and δ power-offset  or a combination thereof. In addition, P r-ABS  may be defined as one of a UE-specific parameter P A ′ used to determine the ratio of CRS EPRE to PDSCH EPRE on OFDM symbols in which a CRS is not transmitted in the r-ABS, ρ A ′, an offset value δ power-offset ′ used to determine ρ A ′, and a subframe-specific parameter P B ′ used to determine the ratio of ρ A ′ to the ratio of CRS EPRE to PDSCH EPRE on OFDM symbols in which a CRS is transmitted in the r-ABS, ρ B ′ or a combination thereof. 
     Alternatively, P normal  may be defined as a value that directly indicates eNB Tx power (e.g. PDSCH Tx power) in a normal subframe and P r-ABS  may be defined as a value that directly indicates eNB Tx power (e.g. PDSCH Tx power) in the r-ABS. 
     The eNB may pre-signal values of P normal  and P r-ABS  the UE through higher layer signaling. Accordingly, the eNB may perform dynamic subframe power information signaling by notifying only whether Tx power applied to the corresponding subframe is P normal  or P r-ABS . When the UE recognizes Tx power (the ratio of PDSCH power to CRS power or PDSCH Tx power) applied to the corresponding subframe according to dynamic signaling, the UE can perform PDSCH demodulation in the corresponding subframe on the basis of the Tx power. 
     A description will be given of embodiments of the method for dynamically signaling subframe Tx power information, proposed by the present invention. 
     Embodiment 1 
     The present embodiment relates to an explicit signaling method for newly defining a field for subframe Tx power information in downlink control information (DCI). Specifically, a bit indicating subframe Tx power information can be additionally defined in a DCI format for DCI. For example, the added bit indicates that P normal  is applied as Tx power of a corresponding subframe when the bit is 0 and indicates that P r-ABS  is applied as the Tx power of the corresponding subframe when the bit is 1. The explicit bit may have a size of 1 bit or 2 bits. 
     The DCI format has a predetermined size and the UE performs blind decoding based on the DCI format size. When the bit is added to the conventional DCI format size, blinding decoding overhead of the UE for the new DCI format size is increased. To prevent blind decoding overhead of the UE from increasing, it is possible to consider dynamic signaling of subframe Tx power information in an implicit manner rather than adding the explicit bit. 
     Embodiment 2 
     The present embodiment relates to an implicit signaling method using a PDCCH CCE aggregation level. Specifically, a PDCCH CCE aggregation level is mapped to a subframe Tx power value such that the UE indirectly recognizes the subframe Tx power value through the PDCCH CCE combination level. 
     The number of CCEs used for PDCCH transmission is referred to as a CCE aggregation level. A CCE is a logical allocation unit used to provide a PDCCH at a coding rate based on radio channel state and corresponds to a plurality of resource element groups. The CCE aggregation level is defined by the number of neighboring CCEs. For example, the CCE aggregation level may be 1, 2, 4 or 8. 
     The r-ABS is a subframe in which a signal is transmitted with reduced Tx power and UEs located at the center of a cell can be scheduled in the r-ABS, in general. To compensate for PDCCH transmission to a UE located at the center of the cell with reduced Tx power, the number of resources (i.e. CCE aggregation level) used for a PDCCH may be increased. Accordingly, the PDCCH can be transmitted at a high CCE aggregation level (e.g. 4 or 8) for the r-ABS and transmitted at a low CCE aggregation level (e.g. 1 or 2) for a normal subframe. That is, a high CCE aggregation level can be mapped to the r-ABS and a low CCE aggregation level can be mapped to the normal subframe. In other words, the high CCE aggregation level is mapped to P normal  whereas the low CCE aggregation level is mapped to P r-ABS . 
     Table 5 shows the mapping relationship between the CCE aggregation level and subframe Tx power information. The mapping relationship as shown in Table 5 may be previously signaled by the eNB to the UE through higher layer signaling. 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 PDCCH CCE 
                 Subframe Tx power 
               
               
                 aggregation level 
                 information 
               
               
                   
               
             
            
               
                 1 
                 P normal   
               
               
                 2 
                   
               
               
                 4 
                 P r-ABS   
               
               
                 8 
               
               
                   
               
            
           
         
       
     
     The scope of the present invention is not limited to the mapping relationship between the PDCCH CCE aggregation level and subframe Tx power information of Table 5 and includes various mapping relationships. 
     The UE performing PDCCH blind decoding can implicitly recognize that Tx power of a corresponding subframe is P r-ABS  (i.e. the corresponding subframe is the r-ABS) when a detected PDCCH corresponds to a high CCE aggregation level. When the detected PDCCH corresponds to a low CCE aggregation level, the UE can implicitly recognize that the Tx power of the corresponding subframe is P normal  (i.e. the corresponding subframe is a normal subframe). 
     In addition, since the method of mapping subframe Tx power to a CCE aggregation level and implicitly signaling the same is a UE-specific operation, a PDCCH search space to which the present embodiment is applied can be limited to a UE-specific search space. 
     Embodiment 3 
     The present embodiment relates to an implicit signaling scheme using a downlink transmission modulation order. Specifically, a modulation order is mapped to a subframe Tx power value such that the UE can indirectly determine a subframe Tx power value through modulation order information. 
     When the eNB schedules the UE, the eNB can provide scheduling information to the UE through a specific DCI format. The UE can detect a modulation order applied to scheduled transmission through a specific field (e.g. field with respect to MCS information) in the DCI format. 
     A range within which the eNB can change Tx power depends on the modulation order. For example, the Tx power variation range of the eNB is wide at a low modulation order (e.g. QPSK) whereas the Tx power variation range of the eNB is narrow at a high modulation order (e.g. 64QAM (quadrature amplitude modulation)). Accordingly, a low modulation order (e.g. QPSK and 16QAM) can be mapped to the r-ABS and a high modulation order (e.g. 64QAM) can be mapped to the normal subframe. In other words, the low modulation order is mapped to P r-ABS  whereas the high modulation order is mapped to P normal . 
     Table 6 shows the mapping relationship between the modulation order and subframe Tx power information. The mapping relationship as shown in Table 6 may be previously signaled by the eNB to the UE through higher layer signaling. 
     
       
         
           
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                 DCI modulation 
                 Subframe Tx power 
               
               
                   
                 order of PDCCH 
                 information 
               
               
                   
               
             
            
               
                   
                 QPSK 
                 P r-ABS   
               
               
                   
                 16QAM 
                   
               
               
                   
                 64QAM 
                 P normal   
               
               
                   
               
            
           
         
       
     
     The scope of the present invention is not limited to the mapping relationship between the DCI modulation order and subframe Tx power information of Table 6 and includes various mapping relationships. 
     Upon acquisition of modulation order information included in the DCI format, the UE can implicitly recognize that Tx power of a corresponding subframe is P r-ABS  (i.e. the corresponding subframe is the r-ABS) in the case of low modulation order and implicitly recognize that the Tx power of the corresponding subframe is P normal  (i.e. the corresponding subframe is a normal subframe) in the case of high modulation order. 
     As described above, a range in which the eNB can reduce r-ABS Tx power dynamically varies with DCI modulation order. Specifically, a range in which Tx power applied to a specific subframe can be reduced decreases as the DCI modulation order increases. Accordingly, as the eNB reduces the r-ABS Tx power, the eNB uses low modulation order. Accordingly, high throughput cannot be achieved. Furthermore, when the eNB uses a high modulation order (e.g. 64QAM) in the r-ABS, interference at the transmitter of the eNB increases due to a difference between CRS Tx power and PDSCH Tx power in an OFDM symbol period in which a CRS is transmitted. 
     Therefore, use of 64QAM in the r-ABS can be restricted. When the eNB restricts use of 64QAM in the r-ABS, the UE needs to be aware of such restriction to correctly operate. That is, when the UE performs data demodulation in the r-ABS on the assumption that 64QAM is applied while the eNB does not use 64QAM in the r-ABS, an error may be generated. Accordingly, information about a modulation order that is not used by the eNB needs to be provided to the UE. To achieve this, the eNB may signal information about a modulation order which is not used in the r-ABS to the UE through higher layer signaling. The restricted modulation order may be used as a virtual CRC. That is, when a modulation order that the UE obtains from DCI is a modulation order that is not supported by the eNB, it can be determined that the DCI has an error. 
     Embodiment 4 
     The present embodiment relates to an implicit signaling method using a HARQ process number (or HARQ process ID). Specifically, a HARQ process number is matched to a subframe Tx power value such that the UE indirectly finds out a subframe Tx power value through HARQ process number information. 
     Downlink HARQ retransmission in 3GPP LTE/LTE-A may be adaptively performed in an asynchronous manner. Asynchronous retransmission refers to retransmission which is performed at a time designated by an eNB rather than a time according to a predetermined rule. Adaptive operation refers to selection of an MCS based on a channel state of retransmission time rather than utilization of the MCS used for initial transmission. When the eNB retransmits a PDSCH, the eNB needs to indicate data which is retransmitted due to asynchronous characteristics. In 3GPP LTE/LTE-A, a maximum of 8 HARQ processes can be present in case of FDD. When the eNB schedules a PDSCH, the eNB can include information, which indicates which one of the 8 HARQ processes relates to PDSCH scheduling, in DCI. For example, a HARQ process number field in a DCI format is defined as 3 bits in case of FDD and indicates a HARQ process related to scheduled downlink transmission. 
     In the present embodiment, the state of the HARQ process number field is mapped to a subframe Tx power value. For example, 0 to 4 from among HARQ process number filed states 0 to 7 are mapped to normal subframes (or P normal ) and 5 to 7 are mapped to r-ABSs (or P r-ABS ). This mapping relationship may be determined and changed according to ABS operation and Tx power level change plan of the eNB. 
     Table 7 shows an example of the mapping relationship between the HARQ process number field and subframe Tx power information. The mapping relationship may be previously signaled to the UE by the eNB through higher layer signaling. 
     
       
         
           
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 HARQ Process 
                 Subframe Tx power 
               
               
                 number 
                 information 
               
               
                   
               
             
            
               
                 0 
                 P normal   
               
               
                 1 
                   
               
               
                 2 
                   
               
               
                 3 
                   
               
               
                 4 
                   
               
               
                 5 
                 P r-ABS   
               
               
                 6 
                   
               
               
                 7 
               
               
                   
               
            
           
         
       
     
     The scope of the present invention is not limited to the mapping relationship between the HARQ process number field and subframe Tx power information of Table 7 and includes various mapping relationships. 
     According to the present embodiment, when retransmission with respect to downlink data transmission scheduled in an r-ABS is performed, retransmission needs to be carried out in an r-ABS. 
     In an example extended from the present embodiment, different Tx power levels may be set for HARQ process numbers. For example, subframe Tx power may be dynamically changed by mapping different values of P A  and/or P B  to HARQ processes 1 to 7. In addition, Tx power may be set per subframe or HARQ process by setting common P A  and P B  for the HARQ processes 1 to 7 and mapping different values of power offset parameter δ power-offset  to the HARQ processes 1 to 7. 
     Furthermore, since different HARQ process numbers may be allocated to UEs even in one subframe, the eNB may implicitly signal Tx power information independently allocated per UE in one subframe using the HARQ process number field. This scheme may be applied as a method of allocating different Tx power values to UEs in a normal subframe rather than an r-ABS. 
     Embodiment 5 
     The present embodiment relates to an implicit signaling method using PDCCH Tx power. Specifically, the UE may indirectly recognize a subframe Tx power value on the basis of a result obtained by comparing PDCCH Tx power detected by the UE with a predetermined threshold. 
     The eNB can transmit a PDCCH as well as a PDSCH in the r-ABS without reduced Tx power compared to the normal subframe. Since the PDCCH is modulated according to QPSK and transmitted, the UE can demodulate the PDCCH even when the eNB does not signal PDCCH Tx power to the UE. Accordingly, the UE can measure the ratio of PDCCH EPRE to CRS EPRE on the basis of received signal strength. The UE can determine the corresponding subframe as a normal subframe when the measurement result exceeds a predetermined threshold and as an r-ABS when the measurement result is less than the predetermined threshold. 
     In other words, the UE can measure the ratio of PDCCH EPRE to CRS EPRE without being signaled Tx power by the eNB, determine that a PDSCH has been transmitted using a high Tx power value (e.g. P normal ) when the measurement result exceeds a predetermined threshold and demodulate the PDSCH according to the determination result. When the measurement result is less than the predetermined threshold, the UE can determine that the PDSCH has been transmitted using a low Tx power value (e.g. P r-ABS ) and demodulate the PDSCH according to the measurement result. The threshold, P normal  and P r-ABS  may be signaled by the eNB to the UE through higher layer signaling. 
     In addition, since different PDCCH Tx power values of the eNB can be set for UEs even in one subframe, the eNB may implicitly signal different PDSCH Tx power values for UEs in one subframe without additional signaling. This scheme may be applied as a method of allocating different Tx power values to respective UEs in a common frame rather than an r-ABS. 
     Embodiment 6 
     The present embodiment relates to a method of signaling a subframe Tx power value using a UE identifier. Specifically, a plurality of identifiers (e.g. RNTIs) may be allocated to one UE and respectively mapped to different subframe Tx power values such that the UE can indirectly detect a subframe Tx power value through an RNTI. 
     A 16-bit CRC can be added to each PDCCH payload for PDCCH error detection, as described above, and the CRC is masked (or scrambled) with a UE identifier. That is, when the UE receives a PDCCH, the UE can perform CRC check using the RNTI thereof and determine that the PDCCH is destined for the UE only when an error is not detected. 
     In a conventional wireless communication system, the UE is allocated an identifier (e.g. C-RNTI) for discriminating the UE from other UEs in a cell. According to the present embodiment, an additional identifier other than the identifier of the UE to the UE and the identifiers are mapped to different subframe Tx power levels. 
     For example, RNTI_A and RNTI_B may be allocated to one UE, RNTI_A may be mapped to higher Tx power value (or P normal ) and RNTI_B may be mapped to lower Tx power value (or P r-ABS ). The mapping relationship between the multiple identifiers and subframe Tx power levels may be previously signaled by the eNB to the UE through higher layer signaling. 
     Accordingly, when the eNB schedules the UE and a CRC of a PDCCH transmitted to the UE is masked with RNTI_A, the UE can perform CRC check for RNTI_A and RNTI_B upon reception of the PDCCH. When the UE confirms that the PDCCH is masked with RNTI_A, the eNB can recognize that high Tx power value (or P normal ) has been applied to a PDSCH scheduled by the PDCCH. That is, the UE can recognize that a subframe related to the PDCCH masked with RNTI_A is a normal subframe. If the UE confirms that the PDCCH is masked with RNTI_B, the eNB can recognize that low Tx power value (or P r-ABS ) has been applied to the PDSCH scheduled by the PDCCH. That is, the UE can recognize that a subframe related to the PDCCH masked with RNTI_B is an r-ABS. 
     The present embodiment in which plural identifiers related to different Tx power values are allocated to one UE and the UE recognizes a Tx power value through an identifier masking a PDCCH CRC is not limited to the normal subframe and the r-ABS and is applicable to various cases in which a PDSCH is scheduled with different Tx power values.  FIG. 14  illustrates a method of dynamically signaling subframe Tx power information according to an embodiment of the present invention. 
     In step S 1100 , an eNB may transmit, to a UE, information about the mapping relationship between a plurality of subframe Tx power values (e.g. P normal  and P r-ABS ) and information indicating subframe Tx power values. Signaling of step S 1100  may be carried out through higher layer signaling or an appropriate scheme in which the eNB and the UE share the mapping relationship. 
     In step S 1200 , the eNB may dynamically signal information indicating a subframe Tx power value to the UE. For example, dynamic signaling may be performed through a PDCCH. The information indicating the subframe Tx power value may be defined as PDCCH-related information. The various examples described in embodiments 1 to 6 may be applied as the PDCCH-related information. That is, the PDCCH-related information that indicates the subframe Tx power value may correspond to one of the explicit Tx power indication bit in DCI, the PDCCH CCE aggregation level, modulation order information in DCI, the HARQ process number in DCI, PDCCH Tx power information and the UE identifier that masks a PDCCH CRC. 
     In step S 1300 , the UE may determine the subframe Tx power value indicated by the PDCCH-related information received in step S 1200  on the basis of the mapping relationship information acquired in step S 1100 . 
     In step S 1400 , the UE may receive downlink data (e.g. a PDSCH) or transmit uplink data (e.g. a PUSCH) on the basis of the determined subframe Tx power value. 
     The aforementioned embodiments of the present invention may be applied independently or two or more thereof may be simultaneously applied. 
     While r-ABS configuration is exemplified in the above description for clarity, the principle of the present invention is equally applicable to a new transmission resource setting method to which the conventional Tx power allocation scheme is not applied. That is, the scope of the present invention is not limited to the aforementioned r-ABS configuration and includes a method for dynamically signaling Tx power allocation information by an eNB to a UE with respect to a new transmission resource pattern and/or Tx power allocation applied to a new transmission resource. 
       FIG. 15  illustrates configurations of an eNB and a UE according to an embodiment of the present invention. 
     Referring to  FIG. 15 , an eNB  10  may include a reception module  11 , a transmission module  12 , a processor  13 , a memory  14  and a plurality of antennas  15 . The plurality of antennas refers to an eNB supporting MIMO transmission/reception. The reception module  11  may receive signals, data and information from an external device and the transmission module  12  may transmit signals, data and information to the external device. The processor  13  may control overall operation of the eNB 10 . 
     The eNB  10  according to an embodiment of the present invention may be configured to transmit subframe Tx power information to the UE  20 . The processor  13  of the eNB  10  may be configured to transmit downlink control information to the UE through a PDCCH using the transmission module  12 . Here, the subframe Tx power information may be indicated by the aforementioned PDCCH-related information. The PDCCH-related information may be determined on the basis of a predetermined mapping relationship between the subframe Tx power information and the PDCCH-related information. 
     The processor  13  of the eNB  10  may process information received by the eNB  10 , information transmitted from the eNB  10  to the outside, etc. The memory  14  may store processed information for a predetermined time and may be replaced by a component such as a buffer (not shown). 
     Referring to  FIG. 15 , the UE  20  according to an embodiment of the present invention may include a reception module  21 , a transmission module  22 , a processor  23 , a memory  24  and a plurality of antennas  25 . The plurality of antennas  25  refers to a UE supporting MIMO transmission/reception. The reception module  21  may receive signals, data and information from an external device and the transmission module  22  may transmit signals, data and information to the external device. The processor  23  may control overall operation of the UE  20 . 
     The UE  20  according to an embodiment of the present invention may be configured to receive subframe Tx power information from the eNB 10 . The processor  23  of the UE  20  may be configured to receive downlink control information from the eNB through a PDCCH using the reception module  21 . Here, the subframe Tx power information may be indicated by the aforementioned PDCCH-related information. The PDCCH-related information may be determined on the basis of a predetermined mapping relationship between the subframe Tx power information and the PDCCH-related information. 
     The processor  23  of the UE  20  may process information received by the UE  20 , information transmitted from the UE  20  to the outside, etc. The memory  24  may store processed information for a predetermined time and may be replaced by a component such as a buffer (not shown). 
     In the aforementioned configurations of the eNB and the UE, the above-described various embodiments of the present invention are independently applicable or two or more thereof are simultaneously applicable and redundant description is omitted for clarity. 
     The above description of the eNB  10  in  FIG. 15  may be equally applied to a relay corresponding to a downlink transmitting entity or an uplink reception entity and the description of the UE  20  may be equally applied to a relay corresponding to a downlink reception entity or an uplink transmission entity. 
     The embodiments of the present invention may be achieved by various means, for example, hardware, firmware, software, or a combination thereof. 
     In a hardware configuration, the methods according to the embodiments of the present invention may be achieved by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, etc. 
     In a firmware or software configuration, the embodiments of the present invention may be implemented in the form of a module, a procedure, a function, etc. For example, software code may be stored in a memory unit and executed by a processor. The memory unit is located at the interior or exterior of the processor and may transmit data to and receive data from the processor via various known means. 
     The detailed description of the preferred embodiments of the present invention is given to enable those skilled in the art to realize and implement the present invention. While the present invention has been described referring to the preferred embodiments of the present invention, those skilled in the art will appreciate that many modifications and changes can be made to the present invention without departing from the spirit and essential characteristics of the present invention. For example, the structures of the above-described embodiments of the present invention can be used in combination. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. Therefore, the present invention is not intended to limit the embodiments disclosed herein but to give a broadest range matching the principles and new features disclosed herein. 
     Those skilled in the art will appreciate that the present invention may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present invention. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. Therefore, the present invention intends not to limit the embodiments disclosed herein but to give a broadest range matching the principles and new features disclosed herein. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by a subsequent amendment after the application is filed. 
     INDUSTRIAL APPLICABILITY 
     The above-described embodiments of the present invention are applicable to various mobile communication systems.