Patent Publication Number: US-10778310-B2

Title: Method for transmitting or receiving channel state information in wireless communication system and device therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2016/009431, filed on Aug. 25, 2016, which claims the benefit of U.S. Provisional Application No. 62/209,870, filed on Aug. 25, 2015, No. 62/210,440, filed on Aug. 26, 2015, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless communication system and, more particularly, to a method for transmitting or receiving channel state information and an apparatus supporting the same. 
     BACKGROUND ART 
     Mobile communication systems have been developed to provide voice services, while guaranteeing user activity. Service coverage of mobile communication systems, however, has extended even to data services, as well as voice services, and currently, an explosive increase in traffic has resulted in shortage of resource and user demand for a high speed services, requiring advanced mobile communication systems. 
     The requirements of the next-generation mobile communication system may include supporting huge data traffic, a remarkable increase in the transfer rate of each user, the accommodation of a significantly increased number of connection devices, very low end-to-end latency, and high energy efficiency. To this end, various techniques, such as small cell enhancement, dual connectivity, massive Multiple Input Multiple Output (MIMO), in-band full duplex, non-orthogonal multiple access (NOMA), supporting super-wide band, and device networking, have been researched. 
     DISCLOSURE 
     Technical Problem 
     An object of the present invention is proposes a method for transmitting or receiving channel state information (CSI). 
     Furthermore, an object of the present invention proposes a method for determining a CSI-RS port for deriving CSI if a CSI-RS port number for each CSI resource for deriving CSI and a CSI-RS port number of a CSI-RS report unit are separately defined. 
     Technical objects to be achieved by the present invention are not limited to the aforementioned technical objects, and other technical objects not described above may be evidently understood by a person having ordinary skill in the art to which the present invention pertains from the following description. 
     Technical Solution 
     In an aspect of the present invention, a method for a user equipment (UE) to transmit channel state information (CSI) in a wireless communication system may include the steps of receiving the number of antenna ports in each of one or more channel state information-reference signal (CSI-RS) resources configured in the UE from an eNB, receiving, by the UE, a CSI-RS on one or more antenna ports from the eNB, deriving the CSI based on the CSI-RS by assuming a CSI-RS antenna port based on the number of antenna ports of a report unit of the CSI configured in the UE, when the number of antenna ports in each of one or more CSI-RSs configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE are different, and reporting the CSI to the eNB. 
     In another aspect of the present invention, a UE transmitting channel state information (CSI) in a wireless communication system includes a radio frequency (RF) unit for transmitting or receiving a radio signal and a processor controlling the RF unit, wherein the processor may be configured to receive the number of antenna ports in each of one or more channel state information-reference signal (CSI-RS) resources configured in the UE from an eNB, to receive a CSI-RS on one or more antenna ports from the eNB, to derive the CSI based on the CSI-RS by assuming a CSI-RS antenna port based on the number of antenna ports of a report unit of the CSI configured in the UE, when the number of antenna ports in each of one or more CSI-RSs configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE are different, and to report the CSI to the eNB. 
     Preferably, the UE may receive information of the number of antenna ports of a report unit of the CSI from the eNB. 
     Preferably, the number of antenna ports of a report unit of the CSI may be configured to be identical with the number of antenna ports within a specific CSI-RS resource of the one or more CSI-RS resources configured in the UE. 
     Preferably, the UE may sequentially index a total of antenna ports for a total of CSI-RS resources configured in the UE using the number of antenna ports of a report unit of the CSI configured in the UE. 
     Preferably, when a product value of the total of CSI-RS resources configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE is greater than a total number of antenna ports for the total number of CSI-RS resources configured in the UE, the UE may request the reconfiguration of the total number of CSI-RS resources configured in the UE and/or the number of antenna ports of a report unit of the CSI configured in the UE from the eNB. 
     Preferably, a product value of the total of CSI-RS resources configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE may be assumed to be identical with a total number of antenna ports for the total number of CSI-RS resources configured in the UE. 
     Preferably, the number of antenna ports of a report unit of the CSI may be configured to be identical with the smallest number of antenna ports of the one or more CSI-RS resources configured in the UE. 
     Preferably, the number of antenna ports of a report unit of the CSI may be configured to be identical with the greatest number of antenna ports of the one or more CSI-RS resources configured in the UE. 
     Preferably, the CSI may be derived with respect to only a CSI-RS resource that is identical with the number of antenna ports of a report unit of the CSI configured in the UE among the one or more CSI-RS resources configured in the UE. 
     Preferably, the UE may receive the number of beams configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE for each beam from the eNB. 
     Preferably, the UE may sequentially index a total of antenna ports for a total of CSI-RS resources configured in the UE using the number of antenna ports of a report unit of the CSI for each beam configured in the UE. 
     Preferably, if the number of antenna ports of a report unit of the CSI for the total of beams configured in the UE is greater than the total number of antenna ports for the total number of CSI-RS resources configured in the UE, the UE may request the reconfiguration of the total number of CSI-RS resources configured in the UE and/or the number of antenna ports of a report unit of the CSI for each beam configured in the UE from the eNB. 
     Advantageous Effects 
     In accordance with an embodiment of the present invention, a UE can smoothly derive CSI and feed it back to an eNB. 
     Furthermore, in accordance with an embodiment of the present invention, when a CSI-RS port number for each CSI resource for deriving CSI and a CSI-RS port number of a CSI-RS report unit are separately defined, an error attributable to ambiguity between the two CSI RS port numbers can be prevented. 
     Effects which may be obtained by the present invention are not limited to the aforementioned effects, and other technical effects not described above may be evidently understood by a person having ordinary skill in the art to which the present invention pertains from the following description. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included herein as a part of the description for help understanding the present invention, provide embodiments of the present invention, and describe the technical features of the present invention with the description below. 
         FIG. 1  illustrates the structure of a radio frame in a wireless communication system to which the present invention may be applied. 
         FIG. 2  is a diagram illustrating a resource grid for a downlink slot in a wireless communication system to which the present invention may be applied. 
         FIG. 3  illustrates a structure of downlink subframe in a wireless communication system to which the present invention may be applied. 
         FIG. 4  illustrates a structure of uplink subframe in a wireless communication system to which the present invention may be applied. 
         FIG. 5  shows the configuration of a known MIMO communication system. 
         FIG. 6  is a diagram showing a channel from a plurality of transmission antennas to a single reception antenna. 
         FIG. 7  illustrates reference signal patterns mapped to downlink resource block pairs in a wireless communication system to which the present invention may be applied. 
         FIG. 8  is a diagram illustrating resources to which reference signals are mapped in a wireless communication system to which the present invention may be applied. 
         FIG. 9  illustrates a 2D-AAS having 64 antenna elements in a wireless communication system to which the present invention may be applied. 
         FIG. 10  illustrates a system in which an eNB or UE has a plurality of transmission/reception antennas capable of forming a 3D beam based on the AAS in a wireless communication system to which the present invention may be applied. 
         FIG. 11  illustrates a 2D antenna system having cross-polarizations in a wireless communication system to which the present invention may be applied. 
         FIG. 12  illustrates a transceiver unit model in a wireless communication system to which the present invention may be applied. 
         FIG. 13  is a diagram illustrating a method of transmitting or receiving channel state information according to an embodiment of the present invention. 
         FIG. 14  illustrates a block diagram of a wireless communication apparatus according to an embodiment of the present invention. 
     
    
    
     MODE FOR INVENTION 
     Some embodiments of the present invention are described in detail with reference to the accompanying drawings. A detailed description to be disclosed along with the accompanying drawings are intended to describe some embodiments of the present invention and are not intended to describe a sole embodiment of the present invention. The following detailed description includes more details in order to provide full understanding of the present invention. However, those skilled in the art will understand that the present invention may be implemented without such more details. 
     In some cases, in order to avoid that the concept of the present invention becomes vague, known structures and devices are omitted or may be shown in a block diagram form based on the core functions of each structure and device. 
     In this specification, a base station has the meaning of a terminal node of a network over which the base station directly communicates with a device. In this document, a specific operation that is described to be performed by a base station may be performed by an upper node of the base station according to circumstances. That is, it is evident that in a network including a plurality of network nodes including a base station, various operations performed for communication with a device may be performed by the base station or other network nodes other than the base station. The base station (BS) may be substituted with another term, such as a fixed station, a Node B, an eNB (evolved-NodeB), a Base Transceiver System (BTS), or an access point (AP). Furthermore, the device may be fixed or may have mobility and may be substituted with another term, such as User Equipment (UE), a Mobile Station (MS), a User Terminal (UT), a Mobile Subscriber Station (MSS), a Subscriber Station (SS), an Advanced Mobile Station (AMS), a Wireless Terminal (WT), a Machine-Type Communication (MTC) device, a Machine-to-Machine (M2M) device, or a Device-to-Device (D2D) device. 
     Hereinafter, downlink (DL) means communication from an eNB to UE, and uplink (UL) means communication from UE to an eNB. In DL, a transmitter may be part of an eNB, and a receiver may be part of UE. In UL, a transmitter may be part of UE, and a receiver may be part of an eNB. 
     Specific terms used in the following description have been provided to help understanding of the present invention, and the use of such specific terms may be changed in various forms without departing from the technical sprit of the present invention. 
     The following technologies may be used in a variety of wireless communication systems, such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), and Non-Orthogonal Multiple Access (NOMA). CDMA may be implemented using a radio technology, such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented using 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 using a radio technology, such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or Evolved UTRA (E-UTRA). UTRA is part of a Universal Mobile Telecommunications System (UMTS). 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) is part of an Evolved UMTS (E-UMTS) using evolved UMTS Terrestrial Radio Access (E-UTRA), and it adopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-Advanced (LTE-A) is the evolution of 3GPP LTE. 
     Embodiments of the present invention may be supported by the standard documents disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, that is, radio access systems. That is, steps or portions that belong to the embodiments of the present invention and that are not described in order to clearly expose the technical spirit of the present invention may be supported by the documents. Furthermore, all terms disclosed in this document may be described by the standard documents. 
     In order to more clarify a description, 3GPP LTE/LTE-A is chiefly described, but the technical characteristics of the present invention are not limited thereto. 
     General System to which the Present Invention May be Applied 
       FIG. 1  shows the structure of a radio frame in a wireless communication system to which an embodiment of the present invention may be applied. 
     3GPP LTE/LTE-A support a radio frame structure type 1 which may be applicable to Frequency Division Duplex (FDD) and a radio frame structure which may be applicable to Time Division Duplex (TDD). 
     The size of a radio frame in the time domain is represented as a multiple of a time unit of T_s=1/(15000*2048). A UL and DL transmission includes the radio frame having a duration of T_f=307200*T_s=10 ms. 
       FIG. 1( a )  exemplifies a radio frame structure type 1. The type 1 radio frame may be applied to both of full duplex FDD and half duplex FDD. 
     A radio frame includes 10 subframes. A radio frame includes 20 slots of T_slot=15360*T_s=0.5 ms length, and 0 to 19 indexes are given to each of the slots. One subframe includes consecutive two slots in the time domain, and subframe i includes slot 2i and slot 2i+1. The time required for transmitting a subframe is referred to as a transmission time interval (TTI). For example, the length of the subframe i may be 1 ms and the length of a slot may be 0.5 ms. 
     A UL transmission and a DL transmission I the FDD are distinguished in the frequency domain. Whereas there is no restriction in the full duplex FDD, a UE may not transmit and receive simultaneously in the half duplex FDD operation. 
     One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in the time domain and includes a plurality of Resource Blocks (RBs) in a frequency domain. In 3GPP LTE, OFDM symbols are used to represent one symbol period because OFDMA is used in downlink. An OFDM symbol may be called one SC-FDMA symbol or symbol period. An RB is a resource allocation unit and includes a plurality of contiguous subcarriers in one slot. 
       FIG. 1( b )  shows frame structure type 2. 
     A type 2 radio frame includes two half frame of 153600*T_s=5 ms length each. Each half frame includes 5 subframes of 30720*T_s=1 ms length. 
     In the frame structure type 2 of a TDD system, an uplink-downlink configuration is a rule indicating whether uplink and downlink are allocated (or reserved) to all subframes. 
     Table 1 shows the uplink-downlink configuration. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Uplink- 
                   
                   
               
               
                 Down- 
                 Downlink-to- 
               
               
                 link 
                 Uplink 
               
               
                 configu- 
                 Switch-point 
                 Subframe number 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 ration 
                 periodicity 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 5 
                 ms 
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 S 
                 U 
                 U 
                 U 
               
               
                 1 
                 5 
                 ms 
                 D 
                 S 
                 U 
                 U 
                 D 
                 D 
                 S 
                 U 
                 U 
                 D 
               
               
                 2 
                 5 
                 ms 
                 D 
                 S 
                 U 
                 D 
                 D 
                 D 
                 S 
                 U 
                 D 
                 D 
               
               
                 3 
                 10 
                 ms 
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 4 
                 10 
                 ms 
                 D 
                 S 
                 U 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 5 
                 10 
                 ms 
                 D 
                 S 
                 U 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
                 D 
               
               
                 6 
                 5 
                 ms 
                 D 
                 S 
                 U 
                 U 
                 U 
                 D 
                 S 
                 U 
                 U 
                 D 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, in each subframe of the radio frame, ‘D’ represents a subframe for a DL transmission, ‘U’ represents a subframe for UL transmission, and ‘S’ represents a special subframe including three types of fields including a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and a Uplink Pilot Time Slot (UpPTS). 
     A DwPTS is used for an initial cell search, synchronization or channel estimation in a UE. A UpPTS is used for channel estimation in an eNB and for synchronizing a UL transmission synchronization of a UE. A GP is duration for removing interference occurred in a UL owing to multi-path delay of a DL signal between a UL and a DL. 
     Each subframe i includes slot 2i and slot 2i+1 of T_slot=15360*T_s=0.5 ms. 
     The UL-DL configuration may be classified into 7 types, and the position and/or the number of a DL subframe, a special subframe and a UL subframe are different for each configuration. 
     A point of time at which a change is performed from downlink to uplink or a point of time at which a change is performed from uplink to downlink is called a switching point. The periodicity of the switching point means a cycle in which an uplink subframe and a downlink subframe are changed is identically repeated. Both 5 ms and 10 ms are supported in the periodicity of a switching point. If the periodicity of a switching point has a cycle of a 5 ms downlink-uplink switching point, the special subframe S is present in each half frame. If the periodicity of a switching point has a cycle of a 5 ms downlink-uplink switching point, the special subframe S is present in the first half frame only. 
     In all the configurations, 0 and 5 subframes and a DwPTS are used for only downlink transmission. An UpPTS and a subframe subsequent to a subframe are always used for uplink transmission. 
     Such uplink-downlink configurations may be known to both an eNB and UE as system information. An eNB may notify UE of a change of the uplink-downlink allocation state of a radio frame by transmitting only the index of uplink-downlink configuration information to the UE whenever the uplink-downlink configuration information is changed. Furthermore, configuration information is kind of downlink control information and may be transmitted through a Physical Downlink Control Channel (PDCCH) like other scheduling information. Configuration information may be transmitted to all UEs within a cell through a broadcast channel as broadcasting information. 
     Table 2 represents configuration (length of DwPTS/GP/UpPTS) of a special subframe. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Normal cyclic prefix in downlink 
                 Extended cyclic prefix in downlink 
               
            
           
           
               
               
               
               
               
            
               
                 Special 
                   
                 UpPTS 
                   
                 UpPTS 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 subframe 
                   
                 Normal cyclic 
                 Extended cyclic 
                   
                 Normal cyclic 
                 Extended cyclic 
               
               
                 configuration 
                 DwPTS 
                 prefix in uplink 
                 prefix in uplink 
                 DwPTS 
                 prefix in uplink 
                 prefix in uplink 
               
               
                   
               
               
                 0 
                  6592 · T s   
                 2192 · T s   
                 2560 · T s   
                  7680 · T s   
                 2192 · T s   
                 2560 · T s   
               
               
                 1 
                 19760 · T s   
                   
                   
                 20480 · T s   
               
               
                 2 
                 21952 · T s   
                   
                   
                 23040 · T s   
               
               
                 3 
                 24144 · T s   
                   
                   
                 25600 · T s   
               
               
                 4 
                 26336 · T s   
                   
                   
                  7680 · T s   
                 4384 · T s   
                 5120 · T s   
               
               
                 5 
                  6592 · T s   
                 4384 · T s   
                 5120 · T s   
                 20480 · T s   
               
               
                 6 
                 19760 · T s   
                   
                   
                 23040 · T s   
               
               
                 7 
                 21952 · T s   
                   
                   
                 — 
                 — 
                 — 
               
               
                 8 
                 24144 · T s   
                   
                   
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     The structure of a radio subframe according to the example of  FIG. 1  is just an example, and the number of subcarriers included in a radio frame, the number of slots included in a subframe and the number of OFDM symbols included in a slot may be changed in various manners. 
       FIG. 2  is a diagram illustrating a resource grid for one downlink slot in a wireless communication system to which an embodiment of the present invention may be applied. 
     Referring to  FIG. 2 , one downlink slot includes a plurality of OFDM symbols in a time domain. It is described herein that one downlink slot includes 7 OFDMA symbols and one resource block includes 12 subcarriers for exemplary purposes only, and the present invention is not limited thereto. 
     Each element on the resource grid is referred to as a resource element, and one resource block (RB) includes 12×7 resource elements. The number of RBs N{circumflex over ( )}DL included in a downlink slot depends on a downlink transmission bandwidth. 
     The structure of an uplink slot may be the same as that of a downlink slot. 
       FIG. 3  shows the structure of a downlink subframe in a wireless communication system to which an embodiment of the present invention may be applied. 
     Referring to  FIG. 3 , a maximum of three OFDM symbols located in a front portion of a first slot of a subframe correspond to a control region in which control channels are allocated, and the remaining OFDM symbols correspond to a data region in which a physical downlink shared channel (PDSCH) is allocated. Downlink control channels used in 3GPP LTE include, for example, a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), and a physical hybrid-ARQ indicator channel (PHICH). 
     A PCFICH is transmitted in the first OFDM symbol of a subframe and carries information about the number of OFDM symbols (i.e., the size of a control region) which is used to transmit control channels within the subframe. A PHICH is a response channel for uplink and carries an acknowledgement (ACK)/not-acknowledgement (NACK) signal for a Hybrid Automatic Repeat Request (HARQ). Control information transmitted in a PDCCH is called Downlink Control Information (DCI). DCI includes uplink resource allocation information, downlink resource allocation information, or an uplink transmission (Tx) power control command for a specific UE group. 
     A PDCCH may carry information about the resource allocation and transport format of a downlink shared channel (DL-SCH) (this is also called an “downlink grant”), resource allocation information about an uplink shared channel (UL-SCH) (this is also called a “uplink grant”), paging information on a PCH, system information on a DL-SCH, the resource allocation of a higher layer control message, such as a random access response transmitted on a PDSCH, a set of transmission power control commands for individual UE within specific UE group, and the activation of a Voice over Internet Protocol (VoIP), etc. A plurality of PDCCHs may be transmitted within the control region, and UE may monitor a plurality of PDCCHs. A PDCCH is transmitted on a single Control Channel Element (CCE) or an aggregation of some contiguous CCEs. A CCE is a logical allocation unit that is used to provide a PDCCH with a coding rate according to the state of a radio channel. A CCE corresponds to a plurality of resource element groups. The format of a PDCCH and the number of available bits of a PDCCH are determined by an association relationship between the number of CCEs and a coding rate provided by CCEs. 
     An eNB determines the format of a PDCCH based on DCI to be transmitted to UE and attaches a Cyclic Redundancy Check (CRC) to control information. A unique identifier (a Radio Network Temporary Identifier (RNTI)) is masked to the CRC depending on the owner or use of a PDCCH. If the PDCCH is a PDCCH for specific UE, an identifier unique to the UE, for example, a Cell-RNTI (C-RNTI) may be masked to the CRC. If the PDCCH is a PDCCH for a paging message, a paging indication identifier, for example, a Paging-RNTI (P-RNTI) may be masked to the CRC. If the PDCCH is a PDCCH for system information, more specifically, a System Information Block (SIB), a system information identifier, for example, a System Information-RNTI (SI-RNTI) may be masked to the CRC. A Random Access-RNTI (RA-RNTI) may be masked to the CRC in order to indicate a random access response which is a response to the transmission of a random access preamble by UE. 
       FIG. 4  shows the structure of an uplink subframe in a wireless communication system to which an embodiment of the present invention may be applied. 
     Referring to  FIG. 4 , the uplink subframe may be divided into a control region and a data region in a frequency domain. A physical uplink control channel (PUCCH) carrying uplink control information is allocated to the control region. A physical uplink shared channel (PUSCH) carrying user data is allocated to the data region. In order to maintain single carrier characteristic, one UE does not send a PUCCH and a PUSCH at the same time. 
     A Resource Block (RB) pair is allocated to a PUCCH for one UE within a subframe. RBs belonging to an RB pair occupy different subcarriers in each of 2 slots. This is called that an RB pair allocated to a PUCCH is frequency-hopped in a slot boundary. 
     Multi-Input Multi-Output (MIMO) 
     A MIMO technology does not use single transmission antenna and single reception antenna that have been commonly used so far, but uses a multi-transmission (Tx) antenna and a multi-reception (Rx) antenna. In other words, the MIMO technology is a technology for increasing a capacity or enhancing performance using multi-input/output antennas in the transmission end or reception end of a wireless communication system. Hereinafter, MIMO is called a “multi-input/output antenna.”. 
     More specifically, the multi-input/output antenna technology does not depend on a single antenna path in order to receive a single total message and completes total data by collecting a plurality of data pieces received through several antennas. As a result, the multi-input/output antenna technology can increase a data transfer rate within a specific system range and can also increase a system range through a specific data transfer rate. 
     It is expected that an efficient multi-input/output antenna technology will be used because next-generation mobile communication requires a data transfer rate much higher than that of existing mobile communication. In such a situation, the MIMO communication technology is a next-generation mobile communication technology which may be widely used in mobile communication UE and a relay node and has been in the spotlight as a technology which may overcome a limit to the transfer rate of another mobile communication attributable to the expansion of data communication. 
     Meanwhile, the multi-input/output antenna (MIMO) technology of various transmission efficiency improvement technologies that are being developed has been most in the spotlight as a method capable of significantly improving a communication capacity and transmission/reception performance even without the allocation of additional frequencies or a power increase. 
       FIG. 5  shows the configuration of a known MIMO communication system. 
     Referring to  FIG. 5 , if the number of transmission (Tx) antennas is increased to N_T and the number of reception (Rx) antennas is increased to N_R at the same time, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike in the case where a plurality of antennas is used only in a transmitter or a receiver. Accordingly, a transfer rate can be improved, and frequency efficiency can be significantly improved. In this case, a transfer rate according to an increase of a channel transmission capacity may be theoretically increased by a value obtained by multiplying the following rate increment R_i by a maximum transfer rate R_o if one antenna is used.
 
 R   i =min( N   T   ,N   R )  [Equation 1]
 
     That is, in an MIMO communication system using 4 transmission antennas and 4 reception antennas, for example, a quadruple transfer rate can be obtained theoretically compared to a single antenna system. 
     Such a multi-input/output antenna technology may be divided into a spatial diversity method for increasing transmission reliability using symbols passing through various channel paths and a spatial multiplexing method for improving a transfer rate by sending a plurality of data symbols at the same time using a plurality of transmission antennas. Furthermore, active research is being recently carried out on a method for properly obtaining the advantages of the two methods by combining the two methods. 
     Each of the methods is described in more detail below. 
     First, the spatial diversity method includes a space-time block code-series method and a space-time Trelis code-series method using a diversity gain and a coding gain at the same time. In general, the Trelis code-series method is better in terms of bit error rate improvement performance and the degree of a code generation freedom, whereas the space-time block code-series method has low operational complexity. Such a spatial diversity gain may correspond to an amount corresponding to the product (N_T×N_R) of the number of transmission antennas (N_T) and the number of reception antennas (N_R). 
     Second, the spatial multiplexing scheme is a method for sending different data streams in transmission antennas. In this case, in a receiver, mutual interference is generated between data transmitted by a transmitter at the same time. The receiver removes the interference using a proper signal processing scheme and receives the data. A noise removal method used in this case may include a Maximum Likelihood Detection (MLD) receiver, a Zero-Forcing (ZF) receiver, a Minimum Mean Square Error (MMSE) receiver, Diagonal-Bell Laboratories Layered Space-Time (D-BLAST), and Vertical-Bell Laboratories Layered Space-Time (V-BLAST). In particular, if a transmission end can be aware of channel information, a Singular Value Decomposition (SVD) method may be used. 
     Third, there is a method using a combination of a spatial diversity and spatial multiplexing. If only a spatial diversity gain is to be obtained, a performance improvement gain according to an increase of a diversity disparity is gradually saturated. If only a spatial multiplexing gain is used, transmission reliability in a radio channel is deteriorated. Methods for solving the problems and obtaining the two gains have been researched and may include a double space-time transmit diversity (double-STTD) method and a space-time bit interleaved coded modulation (STBICM). 
     In order to describe a communication method in a multi-input/output antenna system, such as that described above, in more detail, the communication method may be represented as follows through mathematical modeling. 
     First, as shown in  FIG. 5 , it is assumed that N_T transmission antennas and NR reception antennas are present. 
     First, a transmission signal is described below. If the N_T transmission antennas are present as described above, a maximum number of pieces of information which can be transmitted are N_T, which may be represented using the following vector.
 
 s =[ s   1   ,s   2   ,Λ,s   N     T   ] T   [Equation 2]
 
     Meanwhile, transmission power may be different in each of pieces of transmission information s_1, s_2, . . . , s_NT. In this case, if pieces of transmission power are P_1, P_2, . . . , P_NT, transmission information having controlled transmission power may be represented using the following vector.
 
 ŝ =[ ŝ   1   ,ŝ   2   ,Λ,ŝ   N     T   ] T =[ P   1   s   1   ,P   2   s   2   ,Λ,P   N     T     s   N     T   ] T   [Equation 3]
 
     Furthermore, transmission information having controlled transmission power in the Equation 3 may be represented as follows using the diagonal matrix P of transmission power. 
     
       
         
           
             
               
                 
                   
                     s 
                     ^ 
                   
                   = 
                   
                     
                       
                         [ 
                         
                           
                             
                               
                                 P 
                                 1 
                               
                             
                             
                               
                                   
                               
                             
                             
                               
                                   
                               
                             
                             
                               0 
                             
                           
                           
                             
                               
                                   
                               
                             
                             
                               
                                 P 
                                 2 
                               
                             
                             
                               
                                   
                               
                             
                             
                               
                                   
                               
                             
                           
                           
                             
                               
                                   
                               
                             
                             
                               
                                   
                               
                             
                             
                               O 
                             
                             
                               
                                   
                               
                             
                           
                           
                             
                               0 
                             
                             
                               
                                   
                               
                             
                             
                               
                                   
                               
                             
                             
                               
                                 P 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         ] 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               
                                 s 
                                 1 
                               
                             
                           
                           
                             
                               
                                 s 
                                 2 
                               
                             
                           
                           
                             
                               M 
                             
                           
                           
                             
                               
                                 s 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                     = 
                     Ps 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, the information vector having controlled transmission power in the Equation 4 is multiplied by a weight matrix W, thus forming N_T transmission signals x_1, x_2, . . . , x_NT that are actually transmitted. In this case, the weight matrix functions to properly distribute the transmission information to antennas according to a transport channel condition. The following may be represented using the transmission signals x_1, x_2, . . . , x_NT. 
     
       
         
           
             
               
                 
                   x 
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               x 
                               1 
                             
                           
                         
                         
                           
                             
                               x 
                               2 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               x 
                               i 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               x 
                               
                                 N 
                                 T 
                               
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   w 
                                   11 
                                 
                               
                               
                                 
                                   w 
                                   12 
                                 
                               
                               
                                 Λ 
                               
                               
                                 
                                   w 
                                   
                                     1 
                                     ⁢ 
                                     
                                       N 
                                       T 
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   w 
                                   21 
                                 
                               
                               
                                 
                                   w 
                                   22 
                                 
                               
                               
                                 Λ 
                               
                               
                                 
                                   w 
                                   
                                     2 
                                     ⁢ 
                                     
                                       N 
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                                   w 
                                   
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                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               
                                 
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                                     i 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
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                                   w 
                                   
                                     
                                       N 
                                       T 
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               
                                 
                                   w 
                                   
                                     
                                       N 
                                       T 
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               
                                 Λ 
                               
                               
                                 
                                   w 
                                   
                                     
                                       N 
                                       T 
                                     
                                     ⁢ 
                                     
                                       N 
                                       T 
                                     
                                   
                                 
                               
                             
                           
                           ] 
                         
                         ⁡ 
                         
                           [ 
                           
                             
                               
                                 
                                   
                                     s 
                                     ^ 
                                   
                                   1 
                                 
                               
                             
                             
                               
                                 
                                   
                                     s 
                                     ^ 
                                   
                                   2 
                                 
                               
                             
                             
                               
                                 M 
                               
                             
                             
                               
                                 
                                   
                                     s 
                                     ^ 
                                   
                                   j 
                                 
                               
                             
                             
                               
                                 M 
                               
                             
                             
                               
                                 
                                   s 
                                   
                                     
                                       N 
                                       ^ 
                                     
                                     T 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       = 
                       
                         
                           W 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             s 
                             ^ 
                           
                         
                         = 
                         WPs 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
     In this case, w_ij denotes weight between an i-th transmission antenna and a j-th transmission information, and W is an expression of a matrix of the weight. Such a matrix W is called a weight matrix or precoding matrix. 
     Meanwhile, the transmission signal x, such as that described above, may be considered to be used in a case where a spatial diversity is used and a case where spatial multiplexing is used. 
     If spatial multiplexing is used, all the elements of the information vector s have different values because different signals are multiplexed and transmitted. In contrast, if the spatial diversity is used, all the elements of the information vector s have the same value because the same signals are transmitted through several channel paths. 
     A method of mixing spatial multiplexing and the spatial diversity may be taken into consideration. In other words, the same signals may be transmitted using the spatial diversity through 3 transmission antennas, for example, and the remaining different signals may be spatially multiplexed and transmitted. 
     If N_R reception antennas are present, the reception signals y_1, y_2, . . . , y_NR of the respective antennas are represented as follows using a vector y.
 
 y =[ y   1   ,y   2   ,Λ,y   N     R   ] T   [Equation 6]
 
     Meanwhile, if channels in a multi-input/output antenna communication system are modeled, the channels may be classified according to transmission/reception antenna indices. A channel passing through a reception antenna i from a transmission antenna j is represented as h_ij. In this case, it is to be noted that in order of the index of h_ij, the index of a reception antenna comes first and the index of a transmission antenna then comes. 
     Several channels may be grouped and expressed in a vector and matrix form. For example, a vector expression is described below. 
       FIG. 6  is a diagram showing a channel from a plurality of transmission antennas to a single reception antenna. 
     As shown in  FIG. 6 , a channel from a total of N_T transmission antennas to a reception antenna i may be represented as follows.
 
 h   i   T =[ h   i1   ,h   i2   ,Λ,h   iN     T   ]  [Equation 7]
 
     Furthermore, if all channels from the N_T transmission antenna to NR reception antennas are represented through a matrix expression, such as Equation 7, they may be represented as follows. 
     
       
         
           
             
               
                 
                   H 
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               h 
                               1 
                               T 
                             
                           
                         
                         
                           
                             
                               h 
                               2 
                               T 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               h 
                               i 
                               T 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               h 
                               
                                 N 
                                 R 
                               
                               T 
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       [ 
                       
                         
                           
                             
                               h 
                               11 
                             
                           
                           
                             
                               h 
                               12 
                             
                           
                           
                             Λ 
                           
                           
                             
                               h 
                               
                                 1 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         
                           
                             
                               h 
                               21 
                             
                           
                           
                             
                               h 
                               22 
                             
                           
                           
                             Λ 
                           
                           
                             
                               h 
                               
                                 2 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                         
                           
                             M 
                           
                           
                             
                                 
                             
                           
                           
                             O 
                           
                           
                             
                                 
                             
                           
                         
                         
                           
                             
                               h 
                               
                                 i 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 i 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             Λ 
                           
                           
                             
                               h 
                               
                                 iN 
                                 T 
                               
                             
                           
                         
                         
                           
                             M 
                           
                           
                             
                                 
                             
                           
                           
                             O 
                           
                           
                             
                                 
                             
                           
                         
                         
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 1 
                               
                             
                           
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 2 
                               
                             
                           
                           
                             Λ 
                           
                           
                             
                               h 
                               
                                 
                                   N 
                                   R 
                                 
                                 ⁢ 
                                 
                                   N 
                                   T 
                                 
                               
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, Additive White Gaussian Noise (AWGN) is added to an actual channel after the actual channel experiences the channel matrix H. Accordingly, AWGN n_1, n_2, . . . , n_NR added to the N_R reception antennas, respectively, are represented using a vector as follows.
 
 n =[ n   1   ,n   2   ,Λ,n   N     R   ] T   [Equation 9]
 
     A transmission signal, a reception signal, a channel, and AWGN in a multi-input/output antenna communication system may be represented to have the following relationship through the modeling of the transmission signal, reception signal, channel, and AWGN, such as those described above. 
     
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               y 
                               1 
                             
                           
                         
                         
                           
                             
                               y 
                               2 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               y 
                               i 
                             
                           
                         
                         
                           
                             M 
                           
                         
                         
                           
                             
                               y 
                               
                                 N 
                                 R 
                               
                             
                           
                         
                       
                       ] 
                     
                     = 
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     h 
                                     11 
                                   
                                 
                                 
                                   
                                     h 
                                     12 
                                   
                                 
                                 
                                   Λ 
                                 
                                 
                                   
                                     h 
                                     
                                       1 
                                       ⁢ 
                                       
                                         N 
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                                     h 
                                     21 
                                   
                                 
                                 
                                   
                                     h 
                                     22 
                                   
                                 
                                 
                                   Λ 
                                 
                                 
                                   
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                                       2 
                                       ⁢ 
                                       
                                         N 
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                                   O 
                                 
                                 
                                   
                                       
                                   
                                 
                               
                               
                                 
                                   
                                     h 
                                     
                                       i 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                                 
                                   
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                                       i 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                 
                                 
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                                     h 
                                     
                                       iN 
                                       T 
                                     
                                   
                                 
                               
                               
                                 
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                                   O 
                                 
                                 
                                   
                                       
                                   
                                 
                               
                               
                                 
                                   
                                     h 
                                     
                                       
                                         N 
                                         R 
                                       
                                       ⁢ 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     h 
                                     
                                       
                                         N 
                                         R 
                                       
                                       ⁢ 
                                       2 
                                     
                                   
                                 
                                 
                                   Λ 
                                 
                                 
                                   
                                     h 
                                     
                                       
                                         N 
                                         R 
                                       
                                       ⁢ 
                                       
                                         N 
                                         T 
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                           ⁡ 
                           
                             [ 
                             
                               
                                 
                                   
                                     x 
                                     1 
                                   
                                 
                               
                               
                                 
                                   
                                     x 
                                     2 
                                   
                                 
                               
                               
                                 
                                   M 
                                 
                               
                               
                                 
                                   
                                     x 
                                     j 
                                   
                                 
                               
                               
                                 
                                   M 
                                 
                               
                               
                                 
                                   
                                     x 
                                     
                                       N 
                                       T 
                                     
                                   
                                 
                               
                             
                             ] 
                           
                         
                         + 
                         
                           [ 
                           
                             
                               
                                 
                                   n 
                                   1 
                                 
                               
                             
                             
                               
                                 
                                   n 
                                   2 
                                 
                               
                             
                             
                               
                                 M 
                               
                             
                             
                               
                                 
                                   n 
                                   i 
                                 
                               
                             
                             
                               
                                 M 
                               
                             
                             
                               
                                 
                                   n 
                                   
                                     N 
                                     R 
                                   
                                 
                               
                             
                           
                           ] 
                         
                       
                       = 
                       
                         Hx 
                         + 
                         n 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     Meanwhile, the number of rows and columns of the channel matrix H indicative of the state of channels is determined by the number of transmission/reception antennas. In the channel matrix H, as described above, the number of rows becomes equal to the number of reception antennas N_R, and the number of columns becomes equal to the number of transmission antennas N_T. That is, the channel matrix H becomes an N_R×N_T matrix. 
     In general, the rank of a matrix is defined as a minimum number of the number of independent rows or columns. Accordingly, the rank of the matrix is not greater than the number of rows or columns. As for figural style, for example, the rank H of the channel matrix H is limited as follows.
 
rank( H )≥min( N   T   ,N   R )  [Equation 11]
 
     Furthermore, if a matrix is subjected to Eigen value decomposition, a rank may be defined as the number of Eigen values that belong to Eigen values and that are not 0. Likewise, if a rank is subjected to Singular Value Decomposition (SVD), it may be defined as the number of singular values other than 0. Accordingly, the physical meaning of a rank in a channel matrix may be said to be a maximum number on which different information may be transmitted in a given channel. 
     In this specification, a “rank” for MIMO transmission indicates the number of paths through which signals may be independently transmitted at a specific point of time and a specific frequency resource. The “number of layers” indicates the number of signal streams transmitted through each path. In general, a rank has the same meaning as the number of layers unless otherwise described because a transmission end sends the number of layers corresponding to the number of ranks used in signal transmission. 
     Reference Signal (RS) 
     In a wireless communication system, a signal may be distorted during transmission because data is transmitted through a radio channel. In order for a reception end to accurately receive a distorted signal, the distortion of a received signal needs to be corrected using channel information. In order to detect channel information, a method of detecting channel information using the degree of the distortion of a signal transmission method and a signal known to both the transmission side and the reception side when they are transmitted through a channel is chiefly used. The aforementioned signal is called a pilot signal or reference signal (RS). 
     Furthermore recently, when most of mobile communication systems transmit a packet, they use a method capable of improving transmission/reception data efficiency by adopting multiple transmission antennas and multiple reception antennas instead of using one transmission antenna and one reception antenna used so far. When data is transmitted and received using multiple input/output antennas, a channel state between the transmission antenna and the reception antenna must be detected in order to accurately receive the signal. Accordingly, each transmission antenna must have an individual reference signal. 
     In a mobile communication system, an RS may be basically divided into two types depending on its object. There are an RS having an object of obtaining channel state information and an RS used for data demodulation. The former has an object of obtaining, by a UE, to obtain channel state information in the downlink. Accordingly, a corresponding RS must be transmitted in a wideband, and a UE must be capable of receiving and measuring the RS although the UE does not receive downlink data in a specific subframe. Furthermore, the former is also used for radio resources management (RRM) measurement, such as handover. The latter is an RS transmitted along with corresponding resources when an eNB transmits the downlink. A UE may perform channel estimation by receiving a corresponding RS and thus may demodulate data. The corresponding RS must be transmitted in a region in which data is transmitted. 
     A downlink RS includes one common RS (CRS) for the acquisition of information about a channel state shared by all of UEs within a cell and measurement, such as handover, and a dedicated RS (DRS) used for data demodulation for only a specific UE. Information for demodulation and channel measurement can be provided using such RSs. That is, the DRS is used for only data demodulation, and the CRS is used for the two objects of channel information acquisition and data demodulation. 
     The reception side (i.e., UE) measures a channel state based on a CRS and feeds an indicator related to channel quality, such as a channel quality indicator (Cal), a precoding matrix index (PMI) and/or a rank indicator (RI), back to the transmission side (i.e., an eNB). The CRS is also called a cell-specific RS. In contrast, a reference signal related to the feedback of channel state information (CSI) may be defined as a CSI-RS. 
     The DRS may be transmitted through resource elements if data on a PDSCH needs to be demodulated. A UE may receive information about whether a DRS is present through a higher layer, and the DRS is valid only if a corresponding PDSCH has been mapped. The DRS may also be called a UE-specific RS or demodulation RS (DMRS). 
       FIG. 7  illustrates reference signal patterns mapped to downlink resource block pairs in a wireless communication system to which the present invention may be applied. 
     Referring to  FIG. 7 , a downlink resource block pair, that is, a unit in which a reference signal is mapped, may be represented in the form of one subframe in a time domain×12 subcarriers in a frequency domain. That is, in a time axis (an x axis), one resource block pair has a length of 14 OFDM symbols in the case of a normal cyclic prefix (CP) ( FIG. 7 a   ) and has a length of 12 OFDM symbols in the case of an extended cyclic prefix (CP) ( FIG. 7 b   ). In the resource block lattice, resource elements (REs) indicated by “0”, “1”, “2”, and “3” mean the locations of the CRSs of antenna port indices “0”, “1”, “2”, and “3”, respectively, and REs indicated by “D” mean the location of a DRS. 
     A CRS is described in more detail below. The CRS is a reference signal which is used to estimate the channel of a physical antenna and may be received by all UEs located within a cell in common. The CRS is distributed to a full frequency bandwidth. That is, the CRS is cell-specific signal and is transmitted every subframe in a wideband. Furthermore, the CRS may be used for channel quality information (CSI) and data demodulation. 
     A CRS is defined in various formats depending on an antenna array on the transmitting side (eNB). In the 3GPP LTE system (e.g., Release-8), an RS for a maximum four antenna ports is transmitted depending on the number of transmission antennas of an eNB. The side from which a downlink signal is transmitted has three types of antenna arrays, such as a single transmission antenna, two transmission antennas and four transmission antennas. For example, if the number of transmission antennas of an eNB is two, CRSs for a No. 0 antenna port and a No. 1 antenna port are transmitted. If the number of transmission antennas of an eNB is four, CRSs for No. 0˜No. 3 antenna ports are transmitted. If the number of transmission antennas of an eNB is four, a CRS pattern in one RB is shown in  FIG. 7 . 
     If an eNB uses a single transmission antenna, reference signals for a single antenna port are arrayed. 
     If an eNB uses two transmission antennas, reference signals for two transmission antenna ports are arrayed using a time division multiplexing (TDM) scheme and/or a frequency division multiplexing (FDM) scheme. That is, different time resources and/or different frequency resources are allocated in order to distinguish between reference signals for two antenna ports. 
     Furthermore, if an eNB uses four transmission antennas, reference signals for four transmission antenna ports are arrayed using the TDM and/or FDM schemes. Channel information measured by the reception side (i.e., UE) of a downlink signal may be used to demodulate data transmitted using a transmission scheme, such as single transmission antenna transmission, transmission diversity, closed-loop spatial multiplexing, open-loop spatial multiplexing or a multi-user-multi-input/output (MIMO) antenna. 
     If a multi-input multi-output antenna is supported, when a RS is transmitted by a specific antenna port, the RS is transmitted in the locations of resource elements specified depending on a pattern of the RS and is not transmitted in the locations of resource elements specified for other antenna ports. That is, RSs between different antennas do not overlap. 
     A DRS is described in more detail below. The DRS is used to demodulate data. In multi-input multi-output antenna transmission, precoding weight used for a specific UE is combined with a transmission channel transmitted by each transmission antenna when the UE receives an RS, and is used to estimate a corresponding channel without any change. 
     A 3GPP LTE system (e.g., Release-8) supports a maximum of four transmission antennas, and a DRS for rank 1 beamforming is defined. The DRS for rank 1 beamforming also indicates an RS for an antenna port index 5. 
     In an LTE-A system, that is, an advanced and developed form of the LTE system, the design is necessary to support a maximum of eight transmission antennas in the downlink of an eNB. Accordingly, RSs for the maximum of eight transmission antennas must be also supported. In the LTE system, only downlink RSs for a maximum of four antenna ports has been defined. Accordingly, if an eNB has four to a maximum of eight downlink transmission antennas in the LTE-A system, RSs for these antenna ports must be additionally defined and designed. Regarding the RSs for the maximum of eight transmission antenna ports, the aforementioned RS for channel measurement and the aforementioned RS for data demodulation must be designed. 
     One of important factors that must be considered in designing an LTE-A system is backward compatibility, that is, that an LTE UE must well operate even in the LTE-A system, which must be supported by the system. From an RS transmission viewpoint, in the time-frequency domain in which a CRS defined in LTE is transmitted in a full band every subframe, RSs for a maximum of eight transmission antenna ports must be additionally defined. In the LTE-A system, if an RS pattern for a maximum of eight transmission antennas is added in a full band every subframe using the same method as the CRS of the existing LTE, RS overhead is excessively increased. 
     Accordingly, the RS newly designed in the LTE-A system is basically divided into two types, which include an RS having a channel measurement object for the selection of MCS or a PMI (channel state information-RS or channel state indication-RS (CSI-RS)) and an RS for the demodulation of data transmitted through eight transmission antennas (data demodulation-RS (DM-RS)). 
     The CSI-RS for the channel measurement object is characterized in that it is designed for an object focused on channel measurement unlike the existing CRS used for objects for measurement, such as channel measurement and handover, and for data demodulation. Furthermore, the CSI-RS may also be used for an object for measurement, such as handover. The CSI-RS does not need to be transmitted every subframe unlike the CRS because it is transmitted for an object of obtaining information about a channel state. In order to reduce overhead of a CSI-RS, the CSI-RS is intermittently transmitted on the time axis. 
     For data demodulation, a DM-RS is dedicatedly transmitted to a UE scheduled in a corresponding time-frequency domain. That is, a DM-RS for a specific UE is transmitted only in a region in which the corresponding UE has been scheduled, that is, in the time-frequency domain in which data is received. 
     In the LTE-A system, a maximum of eight transmission antennas are supported in the downlink of an eNB. In the LTE-A system, if RSs for a maximum of eight transmission antennas are transmitted in a full band every subframe using the same method as the CRS in the existing LTE, RS overhead is excessively increased. Accordingly, in the LTE-A system, an RS has been separated into the CSI-RS of the CSI measurement object for the selection of MCS or a PMI and the DM-RS for data demodulation, and thus the two RSs have been added. The CSI-RS may also be used for an object, such as RRM measurement, but has been designed for a main object for the acquisition of CSI. The CSI-RS does not need to be transmitted every subframe because it is not used for data demodulation. Accordingly, in order to reduce overhead of the CSI-RS, the CSI-RS is intermittently transmitted on the time axis. That is, the CSI-RS has a period corresponding to a multiple of the integer of one subframe and may be periodically transmitted or transmitted in a specific transmission pattern. In this case, the period or pattern in which the CSI-RS is transmitted may be set by an eNB. 
     For data demodulation, a DM-RS is dedicatedly transmitted to a UE scheduled in a corresponding time-frequency domain. That is, a DM-RS for a specific UE is transmitted only in the region in which scheduling is performed for the corresponding UE, that is, only in the time-frequency domain in which data is received. 
     In order to measure a CSI-RS, a UE must be aware of information about the transmission subframe index of the CSI-RS for each CSI-RS antenna port of a cell to which the UE belongs, the location of a CSI-RS resource element (RE) time-frequency within a transmission subframe, and a CSI-RS sequence. 
     In the LTE-A system, an eNB has to transmit a CSI-RS for each of a maximum of eight antenna ports. Resources used for the CSI-RS transmission of different antenna ports must be orthogonal. When one eNB transmits CSI-RSs for different antenna ports, it may orthogonally allocate the resources according to the FDM/TDM scheme by mapping the CSI-RSs for the respective antenna ports to different REs. Alternatively, the CSI-RSs for different antenna ports may be transmitted according to the CDM scheme for mapping the CSI-RSs to pieces of code orthogonal to each other. 
     When an eNB notifies a UE belonging to the eNB of information on a CSI-RS, first, the eNB must notify the UE of information about a time-frequency in which a CSI-RS for each antenna port is mapped. Specifically, the information includes subframe numbers in which the CSI-RS is transmitted or a period in which the CSI-RS is transmitted, a subframe offset in which the CSI-RS is transmitted, an OFDM symbol number in which the CSI-RS RE of a specific antenna is transmitted, frequency spacing, and the offset or shift value of an RE in the frequency axis. 
     A CSI-RS is transmitted through one, two, four or eight antenna ports. Antenna ports used in this case are p=15, p=15, 16, p=15, . . . , 18, and p=15, . . . , 22, respectively. A CSI-RS may be defined for only a subcarrier interval Δf=15 kHz. 
     In a subframe configured for CSI-RS transmission, a CSI-RS sequence is mapped to a complex-valued modulation symbol a_k,l{circumflex over ( )}(p) used as a reference symbol on each antenna port p as in Equation 12. 
     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         a 
                         
                           k 
                           , 
                           l 
                         
                         
                           ( 
                           p 
                           ) 
                         
                       
                       = 
                       
                         
                           w 
                           
                             l 
                             ″ 
                           
                         
                         · 
                         
                           
                             r 
                             
                               l 
                               , 
                               
                                 n 
                                 s 
                               
                             
                           
                           ⁡ 
                           
                             ( 
                             
                               m 
                               ′ 
                             
                             ) 
                           
                         
                       
                     
                     ⁢ 
                     
                       
 
                     
                     ⁢ 
                     
                       k 
                       = 
                       
                         
                           k 
                           ′ 
                         
                         + 
                         
                           12 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           m 
                         
                         + 
                         
                           { 
                           
                             
                               
                                 
                                   
                                     
                                       - 
                                       0 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             15 
                                             , 
                                             16 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         normal 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       6 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             17 
                                             , 
                                             18 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         normal 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       1 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             19 
                                             , 
                                             20 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         normal 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       7 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             21 
                                             , 
                                             22 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         normal 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       0 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             15 
                                             , 
                                             16 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         extended 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       3 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             17 
                                             , 
                                             18 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         extended 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       6 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             19 
                                             , 
                                             20 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         extended 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       - 
                                       9 
                                     
                                   
                                   
                                     
                                       
                                         
                                           for 
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           p 
                                         
                                         ∈ 
                                         
                                           { 
                                           
                                             21 
                                             , 
                                             22 
                                           
                                           } 
                                         
                                       
                                       , 
                                       
                                         extended 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         cyclic 
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         prefix 
                                       
                                     
                                   
                                 
                               
                               ⁢ 
                               
                                 
 
                               
                               ⁢ 
                               l 
                             
                             = 
                             
                               
                                 l 
                                 ′ 
                               
                               + 
                               
                                 { 
                                 
                                   
                                     
                                       
                                         
                                           
                                             l 
                                             ″ 
                                           
                                         
                                         
                                           
                                             
                                               
                                                 
                                                   
                                                     
                                                       CSI 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       reference 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       signal 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       configurations 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       0 
                                                     
                                                     - 
                                                     19 
                                                   
                                                   , 
                                                 
                                               
                                             
                                             
                                               
                                                 
                                                   normal 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   cyclic 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   prefix 
                                                 
                                               
                                             
                                           
                                         
                                       
                                       
                                         
                                           
                                             2 
                                             ⁢ 
                                             
                                               l 
                                               ″ 
                                             
                                           
                                         
                                         
                                           
                                             
                                               
                                                 
                                                   
                                                     
                                                       CSI 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       reference 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       signal 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       configurations 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       20 
                                                     
                                                     - 
                                                     31 
                                                   
                                                   , 
                                                 
                                               
                                             
                                             
                                               
                                                 
                                                   normal 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   cyclic 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   prefix 
                                                 
                                               
                                             
                                           
                                         
                                       
                                       
                                         
                                           
                                             l 
                                             ″ 
                                           
                                         
                                         
                                           
                                             
                                               
                                                 
                                                   
                                                     
                                                       CSI 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       reference 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       signal 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       configurations 
                                                       ⁢ 
                                                       
                                                           
                                                       
                                                       ⁢ 
                                                       0 
                                                     
                                                     - 
                                                     27 
                                                   
                                                   , 
                                                 
                                               
                                             
                                             
                                               
                                                 
                                                   extended 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   cyclic 
                                                   ⁢ 
                                                   
                                                       
                                                   
                                                   ⁢ 
                                                   prefix 
                                                 
                                               
                                             
                                           
                                         
                                       
                                     
                                     ⁢ 
                                     
                                       
 
                                     
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       w 
                                       
                                         l 
                                         ″ 
                                       
                                     
                                   
                                   = 
                                   
                                     { 
                                     
                                       
                                         
                                           
                                             
                                               
                                                 1 
                                               
                                               
                                                 
                                                   p 
                                                   ∈ 
                                                   
                                                     { 
                                                     
                                                       15 
                                                       , 
                                                       17 
                                                       , 
                                                       19 
                                                       , 
                                                       21 
                                                     
                                                     } 
                                                   
                                                 
                                               
                                             
                                             
                                               
                                                 
                                                   
                                                     ( 
                                                     
                                                       - 
                                                       1 
                                                     
                                                     ) 
                                                   
                                                   
                                                     l 
                                                     ″ 
                                                   
                                                 
                                               
                                               
                                                 
                                                   p 
                                                   ∈ 
                                                   
                                                     { 
                                                     
                                                       16 
                                                       , 
                                                       18 
                                                       , 
                                                       20 
                                                       , 
                                                       22 
                                                     
                                                     } 
                                                   
                                                 
                                               
                                             
                                           
                                           ⁢ 
                                           
                                             
 
                                           
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           
                                             l 
                                             ″ 
                                           
                                         
                                         = 
                                         
                                           
                                             0 
                                             , 
                                             1 
                                             ⁢ 
                                             
                                               
 
                                             
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             m 
                                           
                                           = 
                                           0 
                                         
                                       
                                       , 
                                       1 
                                       , 
                                       … 
                                       ⁢ 
                                       
                                           
                                       
                                       , 
                                       
                                         
                                           
                                             N 
                                             RB 
                                             DL 
                                           
                                           - 
                                           
                                             1 
                                             ⁢ 
                                             
                                               
 
                                             
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             
                                               m 
                                               ′ 
                                             
                                           
                                         
                                         = 
                                         
                                           m 
                                           + 
                                           
                                             ⌊ 
                                             
                                               
                                                 
                                                   N 
                                                   RB 
                                                   
                                                     max 
                                                     , 
                                                     DL 
                                                   
                                                 
                                                 - 
                                                 
                                                   N 
                                                   RB 
                                                   DL 
                                                 
                                               
                                               2 
                                             
                                             ⌋ 
                                           
                                         
                                       
                                     
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 12, (k′,l′) (wherein k′ is a subcarrier index within a resource block and l′ indicates an OFDM symbol index within a slot.) and the condition of n_s is determined depending on a CSI-RS configuration, such as Table 3 or Table 4. 
     Table 3 illustrates the mapping of (k′,I′) from a CSI-RS configuration in a normal CP. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 Number of CSI reference signals configured 
               
            
           
           
               
               
               
               
               
            
               
                   
                 CSI reference 
                 1 or 2 
                 4 
                 8 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 signal 
                   
                 n s   
                   
                 n s   
                   
                 n s   
               
               
                   
                 configuration 
                 (k′, l′) 
                 mod 2 
                 (k′, l′) 
                 mod 2 
                 (k′, l′) 
                 mod 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Frame structure 
                 0 
                 (9, 5) 
                 0 
                 (9, 5) 
                 0 
                 (9, 5) 
                 0 
               
               
                 type 1 and 2 
                 1 
                 (11, 2)  
                 1 
                 (11, 2)  
                 1 
                 (11, 2)  
                 1 
               
               
                   
                 2 
                 (9, 2) 
                 1 
                 (9, 2) 
                 1 
                 (9, 2) 
                 1 
               
               
                   
                 3 
                 (7, 2) 
                 1 
                 (7, 2) 
                 1 
                 (7, 2) 
                 1 
               
               
                   
                 4 
                 (9, 5) 
                 1 
                 (9, 5) 
                 1 
                 (9, 5) 
                 1 
               
               
                   
                 5 
                 (8, 5) 
                 0 
                 (8, 5) 
                 0 
               
               
                   
                 6 
                 (10, 2)  
                 1 
                 (10, 2)  
                 1 
               
               
                   
                 7 
                 (8, 2) 
                 1 
                 (8, 2) 
                 1 
               
               
                   
                 8 
                 (6, 2) 
                 1 
                 (6, 2) 
                 1 
               
               
                   
                 9 
                 (8, 5) 
                 1 
                 (8, 5) 
                 1 
               
               
                   
                 10 
                 (3, 5) 
                 0 
               
               
                   
                 11 
                 (2, 5) 
                 0 
               
               
                   
                 12 
                 (5, 2) 
                 1 
               
               
                   
                 13 
                 (4, 2) 
                 1 
               
               
                   
                 14 
                 (3, 2) 
                 1 
               
               
                   
                 15 
                 (2, 2) 
                 1 
               
               
                   
                 16 
                 (1, 2) 
                 1 
               
               
                   
                 17 
                 (0, 2) 
                 1 
               
               
                   
                 18 
                 (3, 5) 
                 1 
               
               
                   
                 19 
                 (2, 5) 
                 1 
               
               
                 Frame structure 
                 20 
                 (11, 1)  
                 1 
                 (11, 1)  
                 1 
                 (11, 1)  
                 1 
               
               
                 type 2 only 
                 21 
                 (9, 1) 
                 1 
                 (9, 1) 
                 1 
                 (9, 1) 
                 1 
               
               
                   
                 22 
                 (7, 1) 
                 1 
                 (7, 1) 
                 1 
                 (7, 1) 
                 1 
               
               
                   
                 23 
                 (10, 1)  
                 1 
                 (10, 1)  
                 1 
               
               
                   
                 24 
                 (8, 1) 
                 1 
                 (8, 1) 
                 1 
               
               
                   
                 25 
                 (6, 1) 
                 1 
                 (6, 1) 
                 1 
               
               
                   
                 26 
                 (5, 1) 
                 1 
               
               
                   
                 27 
                 (4, 1) 
                 1 
               
               
                   
                 28 
                 (3, 1) 
                 1 
               
               
                   
                 29 
                 (2, 1) 
                 1 
               
               
                   
                 30 
                 (1, 1) 
                 1 
               
               
                   
                 31 
                 (0, 1) 
                 1 
               
               
                   
               
            
           
         
       
     
     Table 4 illustrates the mapping of (k′,l′) from a CSI-RS configuration in an extended CP. 
     
       
         
           
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Number of CSI reference signals configured 
               
            
           
           
               
               
               
               
               
            
               
                   
                 CSI reference 
                 1 or 2 
                 4 
                 8 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 signal 
                   
                 n s   
                   
                 n s   
                   
                 n s   
               
               
                   
                 configuration 
                 (k′, l′) 
                 mod 2 
                 (k′, l′) 
                 mod 2 
                 (k′, l′) 
                 mod 2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Frame structure 
                 0 
                 (11, 4)  
                 0 
                 (11, 4)  
                 0 
                 (11, 4) 
                 0 
               
               
                 type 1 and 2 
                 1 
                 (9, 4) 
                 0 
                 (9, 4) 
                 0 
                  (9, 4) 
                 0 
               
               
                   
                 2 
                 (10, 4)  
                 1 
                 (10, 4)  
                 1 
                 (10, 4) 
                 1 
               
               
                   
                 3 
                 (9, 4) 
                 1 
                 (9, 4) 
                 1 
                  (9, 4) 
                 1 
               
               
                   
                 4 
                 (5, 4) 
                 0 
                 (5, 4) 
                 0 
               
               
                   
                 5 
                 (3, 4) 
                 0 
                 (3, 4) 
                 0 
               
               
                   
                 6 
                 (4, 4) 
                 1 
                 (4, 4) 
                 1 
               
               
                   
                 7 
                 (3, 4) 
                 1 
                 (3, 4) 
                 1 
               
               
                   
                 8 
                 (8, 4) 
                 0 
               
               
                   
                 9 
                 (6, 4) 
                 0 
               
               
                   
                 10 
                 (2, 4) 
                 0 
               
               
                   
                 11 
                 (0, 4) 
                 0 
               
               
                   
                 12 
                 (7, 4) 
                 1 
               
               
                   
                 13 
                 (6, 4) 
                 1 
               
               
                   
                 14 
                 (1, 4) 
                 1 
               
               
                   
                 15 
                 (0, 4) 
                 1 
               
               
                 Frame structure 
                 16 
                 (11, 1)  
                 1 
                 (11, 1)  
                 1 
                 (11, 1) 
                 1 
               
               
                 type 2 only 
                 17 
                 (10, 1)  
                 1 
                 (10, 1)  
                 1 
                 (10, 1) 
                 1 
               
               
                   
                 18 
                 (9, 1) 
                 1 
                 (9, 1) 
                 1 
                  (9, 1) 
                 1 
               
               
                   
                 19 
                 (5, 1) 
                 1 
                 (5, 1) 
                 1 
               
               
                   
                 20 
                 (4, 1) 
                 1 
                 (4, 1) 
                 1 
               
               
                   
                 21 
                 (3, 1) 
                 1 
                 (3, 1) 
                 1 
               
               
                   
                 22 
                 (8, 1) 
                 1 
               
               
                   
                 23 
                 (7, 1) 
                 1 
               
               
                   
                 24 
                 (6, 1) 
                 1 
               
               
                   
                 25 
                 (2, 1) 
                 1 
               
               
                   
                 26 
                 (1, 1) 
                 1 
               
               
                   
                 27 
                 (0, 1) 
                 1 
               
               
                   
               
            
           
         
       
     
     Referring to Table 3 and Table 4, in the transmission of a CSI-RS, in order to reduce inter-cell interference (ICI) in a multi-cell environment including a heterogeneous network (HetNet) environment, a maximum of 32 different configurations (in the case of a normal CP) or a maximum of 28 different configurations (in the case of an extended CP) are defined. 
     The CSI-RS configuration is different depending on the number of antenna ports and a CP within a cell, and a neighboring cell may have a maximum of different configurations. Furthermore, the CSI-RS configuration may be divided into a case where it is applied to both an FDD frame and a TDD frame and a case where it is applied to only a TDD frame depending on a frame structure. 
     (k′,l′) and n_s are determined depending on a CSI-RS configuration based on Table 3 and Table 4, and time-frequency resources used for CSI-RS transmission are determined depending on each CSI-RS antenna port. 
       FIG. 8  is a diagram illustrating resources to which reference signals are mapped in a wireless communication system to which the present invention may be applied. 
       FIG. 8( a )  shows twenty types of CSI-RS configurations available for CSI-RS transmission by one or two CSI-RS antenna ports,  FIG. 8( b )  shows ten types of CSI-RS configurations available for four CSI-RS antenna ports, and  FIG. 8( c )  shows five types of CSI-RS configurations available for eight CSI-RS antenna ports. 
     As described above, radio resources (i.e., an RE pair) in which a CSI-RS is transmitted are determined depending on each CSI-RS configuration. 
     If one or two antenna ports are configured for CSI-RS transmission with respect to a specific cell, the CSI-RS is transmitted on radio resources on a configured CSI-RS configuration of the twenty types of CSI-RS configurations shown in  FIG. 8( a ) . 
     Likewise, when four antenna ports are configured for CSI-RS transmission with respect to a specific cell, a CSI-RS is transmitted on radio resources on a configured CSI-RS configuration of the ten types of CSI-RS configurations shown in  FIG. 8( b ) . Furthermore, when eight antenna ports are configured for CSI-RS transmission with respect to a specific cell, a CSI-RS is transmitted on radio resources on a configured CSI-RS configuration of the five types of CSI-RS configurations shown in  FIG. 8( c ) . 
     A CSI-RS for each antenna port is subjected to CDM for every two antenna ports (i.e., {15,16}, {17,18}, {19,20} and {21,22}) on the same radio resources and transmitted. For example, in the case of antenna ports 15 and 16, CSI-RS complex symbols for the respective antenna ports 15 and 16 are the same, but are multiplied by different types of orthogonal code (e.g., Walsh code) and mapped to the same radio resources. The complex symbol of the CSI-RS for the antenna port 15 is multiplied by [1, 1], and the complex symbol of the CSI-RS for the antenna port 16 is multiplied by [1 −1] and mapped to the same radio resources. The same is true of the antenna ports {17,18}, {19,20} and {21,22}. 
     A UE may detect a CSI-RS for a specific antenna port by multiplying code by which a transmitted symbol has been multiplied. That is, a transmitted symbol is multiplied by the code [1 1] multiplied in order to detect the CSI-RS for the antenna port 15, and a transmitted symbol is multiplied by the code [1 −1] multiplied in order to detect the CSI-RS for the antenna port 16. 
     Referring to  FIGS. 8( a ) to 8( c ) , in the case of the same CSI-RS configuration index, radio resources according to a CSI-RS configuration having a large number of antenna ports include radio resources having a small number of CSI-RS antenna ports. For example, in the case of a CSI-RS configuration 0, radio resources for the number of eight antenna ports include both radio resources for the number of four antenna ports and radio resources for the number of one or two antenna ports. 
     A plurality of CSI-RS configurations may be used in one cell. 0 or one CSI-RS configuration may be used for a non-zero power (NZP) CSI-RS, and 0 or several CSI-RS configurations may be used for a zero power (ZP) CSI-RS. 
     For each bit set to 1 in a zeropower (ZP) CSI-RS (‘ZeroPowerCSI-RS) that is a bitmap of 16 bits configured by a high layer, a UE assumes zero transmission power in REs (except a case where an RE overlaps an RE assuming a NZP CSI-RS configured by a high layer) corresponding to the four CSI-RS columns of Table 3 and Table 4. The most significant bit (MSB) corresponds to the lowest CSI-RS configuration index, and next bits in the bitmap sequentially correspond to next CSI-RS configuration indices. 
     A CSI-RS is transmitted only in a downlink slot that satisfies the condition of (n_s mod 2) in Table 3 and Table 4 and a subframe that satisfies the CSI-RS subframe configurations. 
     In the case of the frame structure type 2 (TDD), a CSI-RS is not transmitted in a special subframe, a synchronization signal (SS), a subframe colliding against a PBCH or SystemInformationBlockType1 (SIB 1) Message transmission or a subframe configured to paging message transmission. 
     Furthermore, an RE in which a CSI-RS for any antenna port belonging to an antenna port set S (S={15}, S={15,16}, S={17,18}, S={19,20} or S={21,22}) is transmitted is not used for the transmission of a PDSCH or for the CSI-RS transmission of another antenna port. 
     Time-frequency resources used for CSI-RS transmission cannot be used for data transmission. Accordingly, data throughput is reduced as CSI-RS overhead is increased. By considering this, a CSI-RS is not configured to be transmitted every subframe, but is configured to be transmitted in each transmission period corresponding to a plurality of subframes. In this case, CSI-RS transmission overhead can be significantly reduced compared to a case where a CSI-RS is transmitted every subframe. 
     A subframe period (hereinafter referred to as a “CSI transmission period”) T_CSI-RS and a subframe offset Δ_CSI-RS for CSI-RS transmission are shown in Table 5. 
     Table 5 illustrates CSI-RS subframe configurations. 
     
       
         
           
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 CSI-RS-SubframeConfig 
                 CSI-RS periodicity 
                 CSI-RS subframe offset 
               
               
                 I CSI-RS   
                 T CSI-RS  (subframes) 
                 Δ CSI-RS  (subframes) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0-4 
                 5 
                 I CSI-RS   
               
               
                  5-14 
                 10 
                 I CSI-RS  − 5  
               
               
                 15-34 
                 20 
                 I CSI-RS  − 15 
               
               
                 35-74 
                 40 
                 I CSI-RS  − 35 
               
               
                  75-154 
                 80 
                 I CSI-RS  − 75 
               
               
                   
               
            
           
         
       
     
     Referring to Table 5, the CSI-RS transmission period T_CSI-RS and the subframe offset Δ_CSI-RS are determined depending on the CSI-RS subframe configuration I_CSI-RS. 
     The CSI-RS subframe configuration of Table 5 may be configured as one of the aforementioned ‘SubframeConfig’ field and ‘zeroTxPowerSubframeConfig’ field. The CSI-RS subframe configuration may be separately configured with respect to an NZP CSI-RS and a ZP CSI-RS. 
     A subframe including a CSI-RS satisfies Equation 13.
 
(10 n   f   +└n   s /2┘−Δ CSI-RS )mod  T   CSI-RS =0  [Equation 13]
 
     In Equation 13, T_CSI-RS means a CSI-RS transmission period, Δ_CSI-RS means a subframe offset value, n_f means a system frame number, and n_s means a slot number. 
     In the case of a UE in which the transmission mode 9 has been configured with respect to a serving cell, one CSI-RS resource configuration may be configured for the UE. In the case of a UE in which the transmission mode 10 has been configured with respect to a serving cell, one or more CSI-RS resource configuration (s) may be configured for the UE. 
     In the current LTE standard, a CSI-RS configuration includes an antenna port number (antennaPortsCount), a subframe configuration (subframeConfig), and a resource configuration (resourceConfig). Accordingly, the a CSI-RS configuration provides notification that a CSI-RS is transmitted how many antenna port, provides notification of the period and offset of a subframe in which a CSI-RS will be transmitted, and provides notification that a CSI-RS is transmitted in which RE location (i.e., a frequency and OFDM symbol index) in a corresponding subframe. 
     Specifically, the following parameters for each CSI-RS (resource) configuration are configured through high layer signaling.
         If the transmission mode 10 has been configured, a CSI-RS resource configuration identifier   A CSI-RS port number (antennaPortsCount): a parameter (e.g., one CSI-RS port, two CSI-RS ports, four CSI-RS ports or eight CSI-RS ports) indicative of the number of antenna ports used for CSI-RS transmission   A CSI-RS configuration (resourceConfig) (refer to Table 3 and Table 4): a parameter regarding a CSI-RS allocation resource location   A CSI-RS subframe configuration (subframeConfig, that is, I_CSI-RS) (refer to Table 5): a parameter regarding the period and/or offset of a subframe in which a CSI-RS will be transmitted   If the transmission mode 9 has been configured, transmission power P_C for CSI feedback: in relation to the assumption of a UE for reference PDSCH transmission power for feedback, when the UE derives CSI feedback and takes a value within a [−8, 15] dB range in a 1-dB step size, P_C is assumed to be the ratio of energy per resource element (EPRE) per PDSCH RE and a CSI-RS EPRE.   If the transmission mode 10 has been configured, transmission power P_C for CSI feedback with respect to each CSI process. If CSI subframe sets C_CSI,0 and C_CSI,1 are configured by a high layer with respect to a CSI process, P_C is configured for each CSI subframe set in the CSI process.   A pseudo-random sequence generator parameter n_ID   If the transmission mode 10 has been configured, a high layer parameter ‘qcl-CRS-Info-r11’ including a QCL scrambling identifier for a quasico-located (QCL) type B UE assumption (qcl-ScramblingIdentity-r11), a CRS port count (crs-PortsCount-r11), and an MBSFN subframe configuration list (mbsfn-SubframeConfigList-r11) parameter.       

     When a CSI feedback value derived by a UE has a value within the [−8, 15] dB range, P_C is assumed to be the ration of PDSCH EPRE to CSI-RS EPRE. In this case, the PDSCH EPRE corresponds to a symbol in which the ratio of PDSCH EPRE to CRS EPRE is ρ_A. 
     A CSI-RS and a PMCH are not configured in the same subframe of a serving cell at the same time. 
     In the frame structure type 2, if four CRS antenna ports have been configured, a CSI-RS configuration index belonging to the [20-31] set (refer to Table 3) in the case of a normal CP or a CSI-RS configuration index belonging to the [16-27] set (refer to Table 4) in the case of an extended CP is not configured in a UE. 
     A UE may assume that the CSI-RS antenna port of a CSI-RS resource configuration has a QCL relation with delay spread, Doppler spread, Doppler shift, an average gain and average delay. 
     A UE in which the transmission mode 10 and the QCL type B have been configured may assume that antenna ports 0-3 corresponding to a CSI-RS resource configuration and antenna ports 15-22 corresponding to a CSI-RS resource configuration have QCL relation with Doppler spread and Doppler shift. 
     In the case of a UE in which the transmission modes 1-9 have been configured, one ZP CSI-RS resource configuration may be configured in the UE with respect to a serving cell. In the case of a UE in which the transmission mode 10 has been configured, one or more ZP CSI-RS resource configurations may be configured in the UE with respect to a serving cell. 
     The following parameters for a ZP CSI-RS resource configuration may be configured through high layer signaling.
         The ZP CSI-RS configuration list (zeroTxPowerResourceConfigList) (refer to Table 3 and Table 4): a parameter regarding a zero-power CSI-RS configuration   The ZP CSI-RS subframe configuration (eroTxPowerSubframeConfig, that is, I_CSI-RS) (refer to Table 5): a parameter regarding the period and/or offset of a subframe in which a zero-power CSI-RS is transmitted       

     A ZP CSI-RS and a PMCH are not configured in the same subframe of a serving cell at the same time. 
     In the case of a UE in which the transmission mode 10 has been configured, one or more channel state information-interference measurement (CSI-IM) resource configurations may be configured in the UE with respect to a serving cell. 
     The following parameters for each CSI-IM resource configuration may be configured through high layer signaling.
         The ZP CSI-RS configuration (refer to Table 3 and Table 4)   The ZP CSI RS subframe configuration I_CSI-RS (refer to Table 5)       

     A CSI-IM resource configuration is the same as any one of configured ZP CSI-RS resource configurations. 
     A CSI-IM resource and a PMCH are not configured within the same subframe of a serving cell at the same time. 
     Massive MIMO 
     A MIMO system having a plurality of antennas may be called a massive 
     MIMO system and has been in the spotlight as means for improving spectrum efficiency, energy efficiency and processing complexity. 
     In recent 3GPP, in order to satisfy the requirements of spectrum efficiency for a future mobile communication system, a discussion about the massive MIMO system has started. The massive MIMO is also called full-dimension MIMO (FD-MIMO). 
     In a wireless communication system after LTE Release (Rel)-12, the introduction of an active antenna system (AAS) is considered. 
     Unlike the existing passive antenna system in which an amplifier and antenna capable of adjusting the phase and size of a signal have been separated, the AAS means a system in which each antenna is configured to include an active element, such as an amplifier. 
     The AAS does not require a separate cable, connector and other hardware for connecting an amplifier and an antenna because the active antenna is used, and thus has a high efficiency characteristic in terms of energy and operating costs. In particular, the AAS enables an advanced MIMO technology, such as the formation of an accurate beam pattern or 3D beam pattern in which a beam direction and a beam width are considered because the AAS supports each electronic beam control method. 
     Due to the introduction of an advanced antenna system, such as the AAS, a massive MIMO structure having a plurality of input/output antennas and a multi-dimension antenna structure is also considered. For example, unlike in the existing straight type antenna array, if a two-dimensional (2D) antenna array is formed, a 3D beam pattern can be formed by the active antenna of the AAS. 
       FIG. 9  illustrates a 2D-AAS having 64 antenna elements in a wireless communication system to which the present invention may be applied. 
       FIG. 9  illustrates a common 2D antenna array. A case where N_t=N_v·N_h antennas has a square form as in  FIG. 9  may be considered. In this case, N_h indicates the number of antenna columns in a horizontal direction, and N_v indicates the number of antenna rows in a vertical direction. 
     If the antenna array of such a 2D structure is used, radio waves can be controlled both in the vertical direction (elevation) and the horizontal direction (azimuth) so that a transmission beam can be controlled in the 3D space. A wavelength control mechanism of such a type may be called 3D beamforming. 
       FIG. 10  illustrates a system in which an eNB or UE has a plurality of transmission/reception antennas capable of forming a 3D beam based on the AAS in a wireless communication system to which the present invention may be applied. 
       FIG. 10  is a diagram of the aforementioned example and illustrates a 3D MIMO system using a 2D antenna array (i.e., 2D-AAS). 
     From the point of view of a transmission antenna, if a 3D beam pattern is used, a semi-static or dynamic beam can be formed in the vertical direction of the beam in addition to the horizontal direction. For example, an application, such as the formation of a sector in the vertical direction, may be considered. 
     Furthermore, from the point of view of a reception antenna, when a reception beam is formed using a massive reception antenna, a signal power rise effect according to an antenna array gain may be expected. Accordingly, in the case of the uplink, an eNB can receive a signal from a UE through a plurality of antennas. In this case, there is an advantage in that the UE can set its transmission power very low by considering the gain of the massive reception antenna in order to reduce an interference influence. 
       FIG. 11  illustrates a 2D antenna system having cross-polarizations in a wireless communication system to which the present invention may be applied. 
     A 2D planar antenna array model in which polarization is considered may be diagrammed as shown in  FIG. 11 . 
     Unlike the existing MIMO system according to a passive antenna, a system based on an active antenna can dynamically control the gain of an antenna element by applying weight to an active element (e.g., an amplifier) to which each antenna element has been attached (or included). The antenna system may be modeled in an antenna element level because a radiation pattern depends on the number of antenna elements and an antenna arrangement, such as antenna spacing. 
     An antenna array model, such as the example of  FIG. 11 , may be represented by (M, N, P). This corresponds to a parameter that characterizes an antenna array structure. 
     M indicates the number of antenna elements having the same polarization in each column (i.e., the vertical direction) (i.e., the number of antenna elements having a +45° slant in each column or the number of antenna elements having a −45° slant in each column). 
     N indicates the number of columns in the horizontal direction (i.e., the number of antenna elements in the horizontal direction). 
     P indicates the number of dimensions of polarization. P=2 in the case of cross-polarization as in the case of  FIG. 11 , or P=1 in the case of co-polarization. 
     An antenna port may be mapped to a physical antenna element. The antenna port may be defined by a reference signal related to a corresponding antenna port. For example, in the LTE system, the antenna port 0 may be related to a cell-specific reference signal (CRS), and the antenna port 6 may be related to a positioning reference signal (PRS). 
     For example, an antenna port and a physical antenna element may be mapped in a one-to-one manner. This may correspond to a case where a single cross-polarization antenna element is used for downlink MIMO or downlink transmit diversity. For example, the antenna port 0 is mapped to one physical antenna element, whereas the antenna port 1 may be mapped to the other physical antenna element. In this case, from the point of view of a UE, two types of downlink transmission are present. One is related to a reference signal for the antenna port 0, and the other is related to a reference signal for the antenna port 1. 
     For another example, a single antenna port may be mapped to multiple physical antenna elements. This may correspond to a case where a single antenna port is used for beamforming. In beamforming, multiple physical antenna elements are used, so downlink transmission may be directed toward a specific UE. In general, this may be achieved using an antenna array configured using multiple columns of multiple cross-polarization antenna elements. In this case, from the point of view of a UE, one type of downlink transmission generated from a single antenna port is present. One is related to a CRS for the antenna port 0, and the other is related to a CRS for the antenna port 1. 
     That is, an antenna port indicates downlink transmission from the point of view of a UE not actual downlink transmission from a physical antenna element by an eN B. 
     For another example, a plurality of antenna ports is used for downlink transmission, but each antenna port may be mapped to multiple physical antenna elements. This may correspond to a case where an antenna array is used for downlink MIMO or downlink diversity. For example, each of the antenna ports 0 and 1 may be mapped to multiple physical antenna elements. In this case, from the point of view of a UE, two types of downlink transmission. One is related to a reference signal for the antenna port 0, and the other is related to a reference signal for the antenna port 1. 
     In FD-MIMO, the MIMO precoding of a data stream may experience antenna port virtualization, transceiver unit (or a transmission and reception unit) (TXRU) virtualization, and an antenna element pattern. 
     In the antenna port virtualization, a stream on an antenna port is precoded on a TXRU. In the TXRU virtualization, a TXRU signal is precoded on an antenna element. In the antenna element pattern, a signal radiated by an antenna element may have a directional gain pattern. 
     In the existing transceiver modeling, a static one-to-one mapping between an antenna port and a TXRU is assumed, and a TXRU virtualization effect is joined into a static (TXRU) antenna pattern including the effects of the TXRU virtualization and the antenna element pattern. 
     The antenna port virtualization may be performed by a frequency-selective method. In LTE, an antenna port, together with a reference signal (or pilot), is defined. For example, for precoded data transmission on an antenna port, a DMRS is transmitted in the same bandwidth as a data signal, and both the DMRS and data are precoded by the same precoder (or the same TXRU virtualization precoding). For CSI measurement, a CSI-RS is transmitted through multiple antenna ports. In CSI-RS transmission, a precoder that characterizes mapping between a CSI-RS port and a TXRU may be designed in a unique matrix so that a UE can estimate a TXRU virtualization precoding matrix for a data precoding vector. 
     A TXRU virtualization method is discussed in 1D TXRU virtualization and 2D TXRU virtualization, which are described below with reference to the following drawing. 
       FIG. 12  illustrates a transceiver unit model in a wireless communication system to which the present invention may be applied. 
     In the 1D TXRU virtualization, M_TXRU TXRUs are related to M antenna elements configured in a single column antenna array having the same polarization. 
     In the 2D TXRU virtualization, a TXRU model configuration corresponding to the antenna array model configuration (M, N, P) of  FIG. 11  may be represented by (M_TXRU, N, P). In this case, M_TXRU means the number of TXRUs present in the 2D same column and same polarization, and always satisfies M_TXRU≤M. 
     That is, the total number of TXRUs is the same as M_TXRU×N×P. 
     A TXRU virtualization model may be divided into a TXRU virtualization model option-1: sub-array partition model as in  FIG. 12( a )  and a TXRU virtualization model option-2: full connection model as in  FIG. 12( b )  depending on a correlation between an antenna element and a TXRU. 
     Referring to  FIG. 12( a ) , in the case of the sub-array partition model, an antenna element is partitioned into multiple antenna element groups, and each TXRU is connected to one of the groups. 
     Referring to  FIG. 12( b ) , in the case of the full-connection model, the signals of multiple TXRUs are combined and transferred to a single antenna element (or the arrangement of antenna elements). 
     In  FIG. 12 , q is the transmission signal vectors of antenna elements having M co-polarizations within one column. W is a wideband TXRU virtualization vector, and W is a wideband TXRU virtualization matrix. X is the signal vectors of M_TXRU TXRUs. 
     In this case, mapping between an antenna port and TXRUs may be one-to-one or one-to-many. 
     In  FIG. 12 , mapping between a TXRU and an antenna element (TXRU-to-element mapping) shows one example, but the present invention is not limited thereto. From the point of view of hardware, the present invention may be identically applied to mapping between an TXRU and an antenna element which may be implemented in various forms. 
     Method of Transmitting or Receiving Channel State Information 
     A CSI process may be configured as two classes, such as CSI report Class A and CSI report Class B. 
     In the case of Class A, a UE may report CSI according to W=W1·W2 codebook based on {8, 12, 16} CSI-RS antenna ports. 
     In the case of Class B, a UE may report L port CSI, assuming one of the following four schemes. 
     Scheme 1: An indicator for a beam selection and an L port CQI/PMI/RI for a selected beam. A total number of configured ports over all of CSI-RS resources within a CSI process are greater than L. 
     Scheme 2: An L port precoder from a codebook into which both a beam selection(s) and co-phasing are incorporated in two polarizations. A total number of configured antenna ports within a CSI process are L. 
     Scheme 3: A codebook into which an L port CSI for a selected beam and a beam selection are incorporated. A total number of configured ports is greater than L in all of CSI-RS resources within a CSI process. 
     Scheme 4: An L port CQI/PMI/RI. A total number of configured ports within a CSI process is L. If a CSI measurement restriction is supported, the L port CQI/PMI/RI is always configured. 
     In this case, the beam selection means the selection of a subset of antenna ports within one CSI-RS resource or the selection of a CSI-RS resource from a set of resources. 
     Furthermore, the CSI process is related to a K CSI-RS resource/configuration with an N_k port for a k-th CSI-RS resource (in this case, K may be equal to or greater than 1). 
     The present invention proposes a method regarding a signaling transmission or reception method for configuring a CSI-RS when multiple CSI-RS resources, such as a non-precoded CSI-RS or a beamformed CSI-RS-based technology, are configured in a multiple antenna MIMO system (e.g., FD-MIMO or a massive MIMO system). 
     For example, if a non-precoded CSI-RS (i.e., Class A) is configured in a UE, a CSI-RS configuration of 12 ports (i.e., CSI-RS resources) may be configured by aggregating four CSI-RS configurations of 3 ports. 
     Furthermore, if a precoded CSI-RS (i.e., Class B) is configured in a UE, a plurality of CSI-RS resources (e.g., corresponding to each beam) may be configured in the UE. For example, if a plurality of CSI processes is configured in a UE, a plurality of CSI-RS resources may be configured in each CSI process. Furthermore, each CSI-RS resource may correspond to one beam and may include {1,2,4,8} antenna ports. The number of CSI-RS antenna ports in each of a plurality of CSI-RS resources configured in a UE may be differently configured. 
     When multiple CSI-RS resources are configured as described above, the present invention proposes a signaling method for solving ambiguity between the number of antenna ports (i.e., L ports) as a CSI-RS report unit and the number of antenna ports (i.e., N_k) in the k-th resource of K CSI-RS resources configured in a UE and an operating method of a UE associated with the signaling method.
         In the case of L=N_k       

     First, an operation into which the case of L=N_k is taken into consideration is described. 
     In accordance with an embodiment of the present invention, an eNB may transmit information about L to a UE through high layer signaling (e.g., RRC signaling). Accordingly, the eNB may assume that the UE is a specific L-port CSI-RS and may notify the UE of a BI (or a CSI-RS resource indicator (CRI)) and/or an operation so that the UE performs the operation, such as CSI feedback. 
     In this case, the L value may be configured as an additional parameter within a CSI process or may be delivered to the UE through separate high layer signaling (e.g., RRC signaling). Alternatively, the L value may be previously defined as a specific value depending on a specific transmission mode or configuration condition. 
     In accordance with another embodiment of the present invention, L is not explicitly signaled and may be configured to be identical with an N_k port number for a CSI-RS resource. For example, the L may be implicitly defined or may be configured in a UE so that a UE recognizes that the N_k port number is the same as the L within a specific CSI-RS resource and performs an operation of performing a corresponding CSI report. Alternatively, the L may be implicitly defined or may be configured in a UE so that the UE recognizes that an N_k port number in a corresponding CSI-RS resource is the same as the L in each CSI-RS resource configured in the UE and performs an operation of performing a corresponding CSI report. 
     As described above, if the L value is explicitly signaled or implicitly signaled so that it is the same as an N_k port number within a specific CSI-RS resource, a total number of CSI-RS ports corresponding to a corresponding CSI process may correspond to L_T=N_k×K=L×K. A UE may selectively assume a CSI-RS port number, that is, the target of a CSI report, in each CSI process through the L and the CSI-RS resource number K. 
     For example, it is assumed that L=4 has been configured in a UE (explicit or implicit). In this case, the UE selectively recognizes that a unit of the number of CSI-RS ports, that is, the target of a CSI report, in each CSI process, as 4 and may assume that corresponding {15,16,17,18} has been set as corresponding ports. 
     In this case, a restriction may have been applied to the L value so that the L value is configured from among specific predetermined candidate values, such as {1,2,4,6,8,12,16, . . . }, or a restriction may have been applied to the L value so that the L value is set from among L&gt;=1.
         In the case of L≠N_k       

     If in the given L value, L≠N_k with respect to a specific CSI-RS resource k, a CSI report operation of a UE regarding L≠N_k and a method assuming a corresponding CSI-RS port for L≠N_k are as follows. 
     In this case, an eNB may notify a UE of the corresponding N_k (k=1, . . . , K) explicitly through high layer signaling (e.g., RRC signaling) or using a bitmap with respect to all of the CSI resources of K. 
     In this case, a total number of CSI-RS ports become 
               L   T     =       ∑     k   =   1     K     ⁢       N   k     .             
In this case, if the L value and a port number N_k corresponding to a specific CSI-RS resource k are different, there may be ambiguity when the UE performs port indexing and associated CSI derivation. In order to solve such a problem, the present invention proposes a method in which a UE assumes the CSI-RS port and calculates associated CSI.
 
     1. A UE first sequentially indexes a total port number L_T. For example, a case where a total number of CSI-RS resources K=4 and N_k corresponding to each CSI-RS resource is {2,4,8,4} is taken into consideration. In this case, the total port number L_T is 18, and the UE may configure or assume port indexing for each CSI-RS resource as or to be {15,16}, {15,16,17,18}, {15,16,17,18,19,20,21,22}, {15,16,17,18}. 
     However, for example, if the number of antenna ports of a CSI-RS report unit L=4 is configured in the UE, the UE may newly sequentially index the antenna ports in the total port number. For example, the UE may sequentially list the total number of 18 ports, and then may index the four CSI-RS ports of them as follows. That is, the UE may (re)index the ports as {15,16,17,18}, {15,16,17,18}, {15,16,17,18}, and {15,16,17,18} with respect to four CSI-RS resources, may derive CSI for each CSI-RS resource, and may feed it back to an eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the four CSI-RS resources and feeding it back to the eNB. 
     A. If the number of newly indexed ports is greater than the total port number (i.e., L×K&gt;L_T), the UE may perform signaling for requesting a change of the number of CSI-RS resources K and/or the CSI-RS port number L from the eNB. 
     Alternatively, the UE may not expect such signaling (i.e., signaling for L and K values in which the number of newly indexed ports is greater than the total port number). In this case, the operation of the UE may be unspecified. 
     Alternatively, the last (or first) CSI-RS resource may be limited (or bound) so that it does not exceed the total port number. Furthermore, a CSI-RS port number of less than L may be assumed with respect to only a corresponding specific CSI-RS resource, and an operation, such as a corresponding CSI report, may be defined so that the UE performs the operation or configured in the UE. 
     2. In another method, the UE may always assume 
               L   T     =         ∑     k   =   1     K     ⁢     N   k       =   L           
and that the L-port is always the same as the sum of all of CSI-RS ports within a corresponding CSI process, and may (re)index an L-port therefor. Furthermore, the UE may derive CSI for the L-port with respect to each CSI-RS resource and feed it back to the eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the total of CSI-RS resources and feeding it back to the eNB.
 
     3. Definition may be performed so that the UE performs the CSI-RS port (re)indexing on a CSI-RS resource corresponding to a CSI-RS resource, that is, L=N_k, and performs an associated CSI report operation or such a definition may be configured in the UE. 
     The above example, that is, a case where the total number of CSI-RS resources K=4 and N_k corresponding to each CSI-RS resource is {2,4,8,4} is taken into consideration. In this case, if L=4 is configured in the UE, a CSI-RS resource corresponding to L=N_k corresponds to the second and fourth resources. Accordingly, the UE may (re)index ports as {15,16,17,18} with respect to only the second and fourth CSI-RS resources, may derive CSI for each CSI-RS resource, and may feed it back to the eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the second and fourth CSI-RS resources and feeding it back to the eNB. 
     4. Alternatively, if N_k configured in each CSI-RS resource is various, definition may be performed so that the UE assumes the value of the smallest value N_k to be L, performs CSI-RS port (re)indexing, and performs an associated CSI report operation or the definition may be configured in the UE. 
     The above example, that is, a case where the total number of CSI-RS resources K=4 and N_k corresponding to each CSI-RS resource is {2,4,8,4} is taken into consideration. In this case, a total port number L_T is 18, and the UE may configure or assume port indexing for each CSI-RS resource as or to be {15,16}, {15,16,17,18}, {15,16,17,18,19,20,21,22}, and {15,16,17,18}. 
     However, as in the present embodiment, if L=2 is configured in the UE, L=2 is greater than the L in the case of the second, third and fourth CSI resources, and thus the UE may newly and sequentially index port indexing with respect to a corresponding CSI resource. For example, in the case of the second CSI resource, the UE may sequentially list the total number of 4 ports and configure them as two port indexing groups. That is, the UE may (re)index the 2 CSI-RS ports of {15, 16} and {15, 16}, may derive CSI for each port indexing group, and may feed it back to the eNB. In other words, if one CSI-RS resource includes a plurality of port indexing groups, the UE may derive CSI for each port indexing group and may feed CSI to the eNB for each port indexing group. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource of a total of CSI-RS resources and feeding it back to the eNB. 
     5. Alternatively, if N_k configured in each CSI-RS resource is various, definition may be defined so that the UE assumes the greatest value of N_k to be L, performs CSI-RS port (re)indexing, and performs an associated CSI report operation, or the definition may be configured in the UE. 
     For example, a case where K=4 and a corresponding N_k is {4,4,8,4} is taken into consideration. In this case, a total port number L_T is 20, and the UE may configure or assume port indexing for respective CSI-RS resources as or to be {15,16,17,18}, {15,16,17,18}, {15,16,17,18,19,20,21,22}, and {15,16,17,18}. 
     However, as in the present embodiment, if the UE sets L=8, L=8 is smaller than the L in the case of the first, second and fourth CSI resources, the UE may group the first and second CSI resources and (re)perform port indexing. For example, the UE may (re)perform port indexing as {15,16,17,18}+{15,16,17,18}={15,16,17,18,19,20,21,22}, may derive CSI for each indexing, and may feed it back to the eNB. In other words, if a plurality of CSI-RS resources forms one port indexing group, the UE may derive CSI for a corresponding port indexing group and feed it back to the eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the total of CSI-RS resources and feeding it back to the eNB. 
     A. In another embodiment, a case where K=4 and corresponding N_k is {2,4,8,4} is taken into consideration. In this case, a total port number L_T is 18, and the UE may configure or assume port indexing for respective CSI-RS resources as or to be {15,16}, {15,16,17,18}, {15,16,17,18,19,20,21,22}, and {15,16,17,18}. 
     However, as in the present embodiment, if the UE sets L=8, the UE may sequentially (re)perform port indexing in all of CSI resources. The UE may (re)index ports as {15,16,17,18,19,20,21,22}, {15,16,17,18,19,20,21,22}, and {15,16}, may derive CSI for each indexing, and may feed it back to the eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the total of CSI-RS resources and feeding it back to the eNB. 
     B. Alternatively, port indexing may be (re)performed on only a CSI resource corresponding to L≠N_k. The above example, that is, if K=4 and corresponding N_k is {2,4,8,4}, the UE may group the first, second and fourth CSI resources, may (re)index ports as {15,16,17,18,19,20,21,22} and {15,16}, may derive CSI for each indexing, and may feed it back to the eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the total of CSI-RS resources and feeding it back to the eNB. 
     3. If a different L is configured depending on a beam index 
     A case where a different L is configured depending on a beam index is taken into consideration. 
     That is, in the state in which a CSI resource number K and a CSI-RS port number N_k in each resource on a CSI process configuration have been configured in a UE, a separate beam number B and a CSI-RS port number L_b for each beam may be additionally explicitly signaled. In this case, a total number of CSI-RS ports is 
     
       
         
           
             
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     In this case, if the L_b value and N_k corresponding to a specific resource k are different, ambiguity is generated in the UE when the UE performs port indexing and derives associated CSI. In order to solve this problem, the present invention a method in which the UE assumes the CSI-RS port and calculates associated CSI. 
     The UE first sequentially indexes a total port number L_T. For example, a case where K=4 and the corresponding N_k is {2,4,8,4} is taken into consideration. In this case, the total port number L_T is 18, and the UE may configure or assume port indexing for each CSI-RS resource as or to be {15,16}, {15,16,17,18}, {15,16,17,18,19,20,21,22} and {15,16,17,18}. 
     However, for example, if L_b of {4,2,4,8} is configured in the UE, the UE may newly index port indexing sequentially from the total number of ports. For example, after the UE sequentially lists the total number of 18 ports, the UE may index the four CSI-RS ports of the 18 ports as follows. That is, the UE may (re)index the ports as {15,16,17,18}, {15,16}, {15,16,17,18}, {15,16,17,18}, and {15,16,17,18,19,20,21,22} with respect to the four CSI-RS resources, may derive CSI for each indexing, and may feed it back to an eNB. Furthermore, the UE may perform an operation of selecting a preferred CSI-RS resource from the total of CSI-RS resources and feeding it back to the eNB. 
     A. If the number of ports that are newly (re)indexed is greater than the total port number, the UE may signal a request for a change of the number of CSI-RS resources K or the CSI-RS port number L_b to the eNB. 
     Alternatively, the UE may not expect such signaling (i.e., signaling for the L_b and K value in which the number of newly indexed ports is greater than the total port number). In this case, the operation of the UE may be unspecified. 
     Alternatively, the last (or first) CSI-RS resource may be limited (or bound) so that it does not exceed the total port number. Furthermore, a CSI-RS port number less than the greatest L_b value may be assumed with respect to a corresponding specific CSI-RS resource, and definition may be performed so that and the UE performs an operation, such as a corresponding CSI report, or such a definition may be configured in the UE. 
       FIG. 13  is a diagram illustrating a method of transmitting or receiving channel state information according to an embodiment of the present invention. 
     Referring to  FIG. 13 , a UE receives information of one or more CSI-RS resources from an eNB (S 1301 ). 
     In this case, the CSI-RS resource information may include the number of antenna ports N_k for corresponding CSI-RS resources and resource configuration information of a CSI-RS. In this case, if a plurality of CSI processes is configured in the corresponding UE, information of one or more CSI-RS resources in each CSI process may be transmitted to the UE. 
     The UE may receive information about the number of antenna ports L of a CSI-RS report unit from the eNB (S 1302 ). 
     In this case, the UE may receive the number of beams configured in the UE and the number of antenna ports of a report unit of CSI configured in the UE for each beam from the eNB. 
     In this case, the number of antenna ports L of a CSI-RS report unit, as described above, may be configured to be identical with the number of antenna ports within a specific CSI-RS resource among the one or more CSI-RS resources configured in the UE. In this case, step S 1302  may be omitted. 
     Furthermore, the information about the number of antenna ports L of a CSI-RS report unit (or information about the number of beams and the number of antenna ports of a CSI-RS report unit for each beam configured in the UE) may be transmitted along with CSI-RS resource information in each CSI process. That is, they may be transmitted at step S 1301 . 
     The UE receives a CSI-RS on the one or more antenna ports from the eNB (S 1303 ). 
     The UE derives CSI based on the CSI-RS received through the CSI-RS resource (S 1304 ). 
     In this case, the UE may assume a CSI-RS antenna port using the method according to the embodiment of the present invention, and may derive the CSI. 
     when the number of antenna ports in each of one or more CSI-RS resources (e.g., at least any one of the number of antenna ports in each of one or more CSI-RS resources) configured in the UE and the number of antenna ports L of a report unit of the CSI configured in the UE are different, the UE may assume a CSI-RS antenna port based on the number of antenna ports L of a report unit of the CSI configured in the UE, and may derive the CSI based on the CSI-RS. 
     The UE may sequentially index a total number of antenna ports for a total of CSI-RS resources configured in the UE using the number of antenna ports L of a report unit of the CSI configured in the UE. 
     In this case, the number of antenna ports L of a report unit of the CSI may be configured to be identical with the smallest number of antenna ports or the greatest number of antenna ports of the one or more CSI-RS resources configured in the UE. 
     If the product value of a total of CSI-RS resources K configured in the UE and the number of antenna ports L of a report unit of the CSI configured in the UE is greater than the total number of antenna ports for the total of CSI-RS resources configured in the UE, the UE may request the reconfiguration of the total number of CSI-RS resources K configured in the UE and/or the number of antenna ports L of a report unit of the CSI configured in the UE from the eNB. 
     Furthermore, the UE may assume that the total number of antenna ports for the total of CSI-RS resources configured in the UE is the same as the product value of the total of CSI-RS resources configured in the UE and the number of antenna ports of a report unit of the CSI configured in the UE. 
     Furthermore, the UE may derive CSI for only a CSI-RS resource that belongs to the one or more CSI-RS resources configured in the UE and that is the same as the number of antenna ports of a report unit of the CSI configured in the UE. 
     Furthermore, the UE may sequentially index the total number of antenna ports for the total of CSI-RS resources configured in the UE using the number of antenna ports of a report unit of the CSI for each beam configured in the UE. 
     In this case, if the number of antenna ports of a CSI report unit for all of beams configured in the UE is greater than the total number of antenna ports for the total of CSI-RS resources configured in the UE, the UE may request the reconfiguration of a total number of CSI-RS resources configured in the UE and/or the number of antenna ports of a report unit of the CSI for each beam configured in the UE from the eNB. 
     The UE reports the derived CSI to the eNB (S 1305 ). 
     General Apparatus to which the Present Invention May be Applied 
       FIG. 14  illustrates a block diagram of a wireless communication apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 14 , the wireless communication system includes a base station (eNB)  1410  and a plurality of user equipments (UEs)  1420  located within the region of the eNB  1410 . 
     The eNB  1410  includes a processor  1411 , a memory  1412  and a radio frequency unit  1413 . The processor  1411  implements the functions, processes and/or methods proposed in  FIGS. 1 to 13  above. The layers of wireless interface protocol may be implemented by the processor  1411 . The memory  1412  is connected to the processor  1411 , and stores various types of information for driving the processor  1411 . The RF unit  1413  is connected to the processor  1411 , and transmits and/or receives radio signals. 
     The UE  1420  includes a processor  1421 , a memory  1422  and a radio frequency unit  1423 . The processor  1421  implements the functions, processes and/or methods proposed in  FIGS. 1 to 13  above. The layers of wireless interface protocol may be implemented by the processor  1421 . The memory  1422  is connected to the processor  1421 , and stores various types of information for driving the processor  1421 . The RF unit  1423  is connected to the processor  1421 , and transmits and/or receives radio signals. 
     The memories  1412  and  1422  may be located interior or exterior of the processors  1411  and  1421 , and may be connected to the processors  1411  and  1421  with well known means. In addition, the eNB  1410  and/or the UE  1420  may have a single antenna or multiple antennas. 
     The embodiments described so far are those of the elements and technical features being coupled in a predetermined form. So far as there is not any apparent mention, each of the elements and technical features should be considered to be selective. Each of the elements and technical features may be embodied without being coupled with other elements or technical features. In addition, it is also possible to construct the embodiments of the present invention by coupling a part of the elements and/or technical features. The order of operations described in the embodiments of the present invention may be changed. A part of elements or technical features in an embodiment may be included in another embodiment, or may be replaced by the elements and technical features that correspond to other embodiment. It is apparent to construct embodiment by combining claims that do not have explicit reference relation in the following claims, or to include the claims in a new claim set by an amendment after application. 
     The embodiments of the present invention may be implemented by various means, for example, hardware, firmware, software and the combination thereof. In the case of the hardware, an embodiment of the present invention may be implemented 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), a processor, a controller, a micro controller, a micro processor, and the like. 
     In the case of the implementation by the firmware or the software, an embodiment of the present invention may be implemented in a form such as a module, a procedure, a function, and so on that performs the functions or operations described so far. Software codes may be stored in the memory, and driven by the processor. The memory may be located interior or exterior to the processor, and may exchange data with the processor with various known means. 
     It will be understood to those skilled in the art that various modifications and variations can be made without departing from the essential features of the inventions. Therefore, the detailed description is not limited to the embodiments described above, but should be considered as examples. The scope of the present invention should be determined by reasonable interpretation of the attached claims, and all modification within the scope of equivalence should be included in the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention has been illustrated based on an example in which the present invention is applied to the 3GPP LTE/LTE-A systems, but may be applied to various wireless communication systems in addition to the 3GPP LTE/LTE-A systems.