Patent Publication Number: US-10784936-B2

Title: Method and apparatus for transmitting and receiving signal in inter-vehicle communication system

Description:
This application is a National Stage Application of International Application No. PCT/KR2016/000854, filed on Jan. 27, 2016, which claims the benefit of U.S. Provisional Application No. 62/167,894, filed on May 29, 2015, all of which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein. 
    
    
     TECHNICAL FIELD 
     The present relates to a wireless communication system and, more particularly, to a wireless communication system applied to vehicular communication. At this time, it is possible to provide a method of feeding back channel information at a vehicle (or user equipment (UE)) in a vehicular communication system. 
     BACKGROUND ART 
     A brief description will be given of a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system as an example of a wireless communication system to which the present invention can be applied. 
       FIG. 1  illustrates a configuration of an Evolved Universal Mobile Telecommunications System (E-UMTS) network as an exemplary wireless communication system. The E-UMTS system is an evolution of the legacy UMTS system and the 3GPP is working on the basics of E-UMTS standardization. E-UMTS is also called an LTE system. For details of the technical specifications of UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, respectively. 
     Referring to  FIG. 1 , the E-UMTS system includes a User Equipment (UE), an evolved Node B (eNode B or eNB), and an Access Gateway (AG) which is located at an end of an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and connected to an external network. The eNB may transmit multiple data streams simultaneously, for broadcast service, multicast service, and/or unicast service. 
     A single eNB manages one or more cells. A cell is set to operate in one of the bandwidths of 1.25, 2.5, 5, 10, 15 and 20 Mhz and provides Downlink (DL) or Uplink (UL) transmission service to a plurality of UEs in the bandwidth. Different cells may be configured so as to provide different bandwidths. An eNB controls data transmission and reception to and from a plurality of UEs. Regarding DL data, the eNB notifies a particular UE of a time-frequency area in which the DL data is supposed to be transmitted, a coding scheme, a data size, Hybrid Automatic Repeat reQuest (HARQ) information, etc. by transmitting DL scheduling information to the UE. Regarding UL data, the eNB notifies a particular UE of a time-frequency area in which the UE can transmit data, a coding scheme, a data size, HARQ information, etc. by transmitting UL scheduling information to the UE. An interface for transmitting user traffic or control traffic may be defined between eNBs. A Core Network (CN) may include an AG and a network node for user registration of UEs. The AG manages the mobility of UEs on a Tracking Area (TA) basis. A TA includes a plurality of cells. 
     While the development stage of wireless communication technology has reached LTE based on Wideband Code Division Multiple Access (WCDMA), the demands and expectation of users and service providers are increasing. Considering that other radio access technologies are under development, a new technological evolution is required to achieve future competitiveness. Specifically, cost reduction per bit, increased service availability, flexible use of frequency bands, a simplified structure, an open interface, appropriate power consumption of UEs, etc. are required. 
     In addition, recently, interest in a vehicular communication system has increased. More specifically, interest in a method of applying wireless communication technology and position tracking technology (global positioning system (GPS)) to a vehicle and providing a service such as vehicle diagnosis, theft detection, route guidance or traffic service provision to a driver who uses the vehicle in real time has increased. At this time, there is a need for methods of efficiently performing vehicular communication in consideration of the appearance and mobility of a vehicle and interference with another vehicle in a vehicular communication system. 
     DISCLOSURE 
     Technical Problem 
     An object of the present invention is to provide a method of feeding back channel information in a vehicular communication system, and an apparatus therefor. 
     Another object of the present invention is to provide an efficient channel information feedback method for vehicular communication in consideration of an environment in which vehicles are densely distributed. 
     Technical Solution 
     The object of the present invention can be achieved by providing a method of transmitting and receiving a signal from a user equipment (UE) to a base station, the method comprising: receiving a reference signal; feeding back channel information based on the received reference signal; receiving data based on the channel information, wherein the UE includes a plurality of distributed antenna units (DUs), and wherein the channel information includes DU index set information and channel state information (CSI) when activation of each of the plurality of DUs is selectable. 
     In another aspect of the present invention, provided herein is a user equipment (UE) device for transmitting and receiving a signal, the UE comprising: a reception module configured to receive information from an external device; a transmission module configured to transmit information to an external device; and a processor configured to control the reception module and the transmission module, wherein the processor is further configured to: receives a reference signal using the reception module; feeds back channel information based on the received reference signal using the transmission module; and receives data based on the channel information using the reception module, and wherein the UE includes a plurality of distributed antenna units (DUs), and wherein the channel information includes DU index set information and channel state information (CSI) when activation of each of the plurality of DUs is selectable. 
     In addition, the following matters are commonly applicable to the method of transmitting and receiving a signal from the UE to the base station in the wireless communication system. 
     In one embodiment of the present invention, the DU index set information includes DU index information set based on activation of each of the plurality of DUs. The DU index information is set based on the number of DUs included in the UE and locations of the DUs. 
     In one embodiment of the present invention, the index set information includes 2{circumflex over ( )}N pieces of DU index information when the number of DUs included in the UE is N. 
     In one embodiment of the present invention, the DU index set information includes only the DU index information selectable by the number of activated DUs when the number of activated DUs among the DUs included in the UE is determined. 
     In one embodiment of the present invention, the data is received by the base station based on the DU index set included in the channel information and effective channel information acquired through the channel state information. 
     In one embodiment of the present invention, each of the DUs included in the UE includes a plurality of antenna ports. 
     In one embodiment of the present invention, the DU index set information includes information on a mapping relationship between the plurality of antenna ports and the plurality of DUs. 
     In one embodiment of the present invention, further comprising feeding back DU activation information. The DU activation information includes the number of DUs capable of being simultaneously activated and deactivated and combination information based on a radio frequency (RF) structure. 
     In one embodiment of the present invention, the first type of UE feeds the channel information back to the base station and receives DU selection control information from the base station when the UE is a first type of UE. 
     In one embodiment of the present invention, the second type of UE feeds the channel information back to neighboring UEs and receives channel information from the neighboring UEs to control DU selection when the UE is a second type of UE. 
     In one embodiment of the present invention, the first type of UE and the second type of UE coexist in one vehicular communication system. 
     Advantageous Effects 
     According to the present invention, it is possible to provide a method of feeding back channel information in a vehicular communication system and an apparatus therefor. 
     According to the present invention, it is possible to provide an efficient channel information feedback method for vehicular communication in an environment in which vehicles are densely distributed. 
     The effects which can be obtained by the present invention are not limited to the above-described effects and other effects which are not described herein will become apparent to those skilled in the art from the following description. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a network structure of an E-UMTS as an exemplary radio communication system according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating structures of a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on the 3GPP radio access network specification according to an embodiment of the present invention. 
         FIG. 3  is a diagram illustrating physical channels used in a 3GPP system and a general signal transmission method using the same according to an embodiment of the present invention. 
         FIG. 4  is a diagram illustrating the structure of a radio frame used in an LTE system according to an embodiment of the present invention. 
         FIG. 5  is a diagram showing the structure of a downlink radio frame used in an LTE system according to an embodiment of the present invention. 
         FIG. 6  is a diagram showing the structure of an uplink radio frame used in an LTE system according to an embodiment of the present invention. 
         FIG. 7  is a diagram showing the structure of a general multiple input multiple output (MIMO) communication system according to an embodiment of the present invention. 
         FIG. 8  is a diagram showing a vehicle including a plurality of antenna arrays according to an embodiment of the present invention. 
         FIG. 9  is a diagram showing a method of selecting a distributed antenna unit (DU) in a state in which a plurality of vehicles is concentrated, according to an embodiment of the present invention. 
         FIG. 10  is a diagram showing an example of a DU selection combination according to an embodiment of the present invention. 
         FIG. 11  is a diagram showing a DU including a plurality of antenna ports according to an embodiment of the present invention. 
         FIG. 12  is a diagram showing a method of activating a DU based on an RF structure according to an embodiment of the present invention. 
         FIG. 13  is a flowchart illustrating a method of feeding back channel information according to an embodiment of the present invention. 
         FIG. 14  is a block diagram showing a UE apparatus and a base station apparatus according to an embodiment of the present invention. 
     
    
    
     BEST MODEL 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments and is not intended to represent the only embodiments in which the concepts explained in these embodiments can be practiced. The detailed description includes details for the purpose of providing an understanding of the present invention. However, it will be apparent to those skilled in the art that these teachings may be implemented and practiced without these specific details. 
     The following embodiments are proposed by combining constituent components and characteristics of the present invention according to a predetermined format. The individual constituent components or characteristics should be considered to be optional factors on the condition that there is no additional remark. If required, the individual constituent components or characteristics may not be combined with other components or characteristics. Also, some constituent components and/or characteristics may be combined to implement the embodiments of the present invention. The order of operations to be disclosed in the embodiments of the present invention may be changed to another. Some components or characteristics of any embodiment may also be included in other embodiments, or may be replaced with those of the other embodiments as necessary. 
     It should be noted that specific terms disclosed in the present invention are proposed for the convenience of description and better understanding of the present invention, and the use of these specific terms may be changed to another format within the technical scope or spirit of the present invention. 
     In some instances, well-known structures and devices are omitted in order to avoid obscuring the concepts of the present invention and the important functions of the structures and devices are shown in block diagram form. The same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The embodiments of the present invention can be supported by the standard documents disclosed in any one of wireless access systems, such as an IEEE 802 system, a 3 rd  Generation Partnership Project (3GPP) system, 3GPP Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, and a 3GPP2 system. That is, the steps or portions, which are not described in order to make the technical spirit of the present invention clear, may be supported by the above documents. In addition, all the terms disclosed in the present document may be described by the above standard documents. 
     The following technology can be applied to a variety of wireless access technologies, for example, CDMA (Code Division Multiple Access), FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), OFDMA (Orthogonal Frequency Division Multiple Access), SC-FDMA (Single Carrier Frequency Division Multiple Access), and the like. CDMA may be embodied with wireless (or radio) technology such as UTRA (Universal Terrestrial Radio Access) or CDMA2000. TDMA may be embodied with wireless (or radio) technology such as GSM (Global System for Mobile communications)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution). OFDMA may be embodied with wireless (or radio) technology such as Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. 
     In the entire specification, when a certain portion “includes” a certain component, this indicates that the other components are not excluded, but may be further included unless specially described. The terms “unit”, “-or/er” and “module” described in the specification indicate a unit for processing at least one function or operation, which may be implemented by hardware, software and a combination thereof. 
       FIG. 2  is a diagram illustrating structures of a control plane and a user plane of a radio interface protocol between a UE and an E-UTRAN based on the 3GPP radio access network specification according to an embodiment of the present invention. 
       FIG. 2  illustrates control-plane and user-plane protocol stacks in a radio interface protocol architecture conforming to a 3GPP wireless access network standard between a User Equipment (UE) and an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is a path in which the UE and the E-UTRAN transmit control messages to manage calls, and the user plane is a path in which data generated from an application layer, for example, voice data or Internet packet data is transmitted. 
     A PHYsical (PHY) layer at Layer 1 (L1) provides information transfer service to its higher layer, a Medium Access Control (MAC) layer. The PHY layer is connected to the MAC layer via transport channels. The transport channels deliver data between the MAC layer and the PHY layer. Data is transmitted on physical channels between the PHY layers of a transmitter and a receiver. The physical channels use time and frequency as radio resources. Specifically, the physical channels are modulated in Orthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL) and in Single Carrier Frequency Division Multiple Access (SC-FDMA) for Uplink (UL). 
     The MAC layer at Layer 2 (L2) provides service to its higher layer, a Radio Link Control (RLC) layer via logical channels. The RLC layer at L2 supports reliable data transmission. RLC functionality may be implemented in a function block of the MAC layer. A Packet Data Convergence Protocol (PDCP) layer at L2 performs header compression to reduce the amount of unnecessary control information and thus efficiently transmit Internet Protocol (IP) packets such as IP version 4 (IPv4) or IP version 6 (IPv6) packets via an air interface having a narrow bandwidth. 
     A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (or L3) is defined only on the control plane. The RRC layer controls logical channels, transport channels, and physical channels in relation to configuration, reconfiguration, and release of radio bearers. A radio bearer refers to a service provided at L2, for data transmission between the UE and the E-UTRAN. For this purpose, the RRC layers of the UE and the E-UTRAN exchange RRC messages with each other. If an RRC connection is established between the UE and the E-UTRAN, the UE is in RRC Connected mode and otherwise, the UE is in RRC Idle mode. A Non-Access Stratum (NAS) layer above the RRC layer performs functions including session management and mobility management. 
     DL transport channels used to deliver data from the E-UTRAN to UEs include a Broadcast Channel (BCH) carrying system information, a Paging Channel (PCH) carrying a paging message, and a Shared Channel (SCH) carrying user traffic or a control message. DL multicast traffic or control messages or DL broadcast traffic or control messages may be transmitted on a DL SCH or a separately defined DL Multicast Channel (MCH). UL transport channels used to deliver data from a UE to the E-UTRAN include a Random Access Channel (RACH) carrying an initial control message and a UL SCH carrying user traffic or a control message. Logical channels that are defined above transport channels and mapped to the transport channels include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a Multicast Control Channel (MCCH), a Multicast Traffic Channel (MTCH), etc. 
       FIG. 3  illustrates physical channels and a general method for transmitting signals on the physical channels in the 3GPP system. 
     Referring to  FIG. 3 , when a UE is powered on or enters a new cell, the UE performs initial cell search (S 301 ). The initial cell search involves acquisition of synchronization to an eNB. Specifically, the UE synchronizes its timing to the eNB and acquires a cell Identifier (ID) and other information by receiving a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB. Then the UE may acquire information broadcast in the cell by receiving a Physical Broadcast Channel (PBCH) from the eNB. During the initial cell search, the UE may monitor a DL channel state by receiving a DownLink Reference Signal (DL RS). 
     After the initial cell search, the UE may acquire detailed system information by receiving a Physical Downlink Control Channel (PDCCH) and receiving a Physical Downlink Shared Channel (PDSCH) based on information included in the PDCCH (S 302 ). 
     If the UE initially accesses the eNB or has no radio resources for signal transmission to the eNB, the UE may perform a random access procedure with the eNB (S 303  to S 306 ). In the random access procedure, the UE may transmit a predetermined sequence as a preamble on a Physical Random Access Channel (PRACH) (S 303  and S 305 ) and may receive a response message to the preamble on a PDCCH and a PDSCH associated with the PDCCH (S 304  and S 306 ). In the case of a contention-based RACH, the UE may additionally perform a contention resolution procedure. 
     After the above procedure, the UE may receive a PDCCH and/or a PDSCH from the eNB (S 307 ) and transmit a Physical Uplink Shared Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S 308 ), which is a general DL and UL signal transmission procedure. Particularly, the UE receives Downlink Control Information (DCI) on a PDCCH. Herein, the DCI includes control information such as resource allocation information for the UE. Different DCI formats are defined according to different usages of DCI. 
     Control information that the UE transmits to the eNB on the UL or receives from the eNB on the DL includes a DL/UL ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPP LTE system, the UE may transmit control information such as a CQI, a PMI, an RI, etc. on a PUSCH and/or a PUCCH. 
       FIG. 4  illustrates a structure of a radio frame used in the LTE system. 
     Referring to  FIG. 4 , a radio frame is 10 ms (327200×T s ) long and divided into 10 equal-sized subframes. Each subframe is 1 ms long and further divided into two slots. Each time slot is 0.5 ms (15360×T s ) long. Herein, T s  represents a sampling time and T s =1/(15 kHz×2048)=3.2552×10 −8  (about 33 ns). A slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMA symbols in the time domain by a plurality of Resource Blocks (RBs) in the frequency domain. In the LTE system, one RB includes 12 subcarriers by 7 (or 6) OFDM symbols. A unit time during which data is transmitted is defined as a Transmission Time Interval (TTI). The TTI may be defined in units of one or more subframes. The above-described radio frame structure is purely exemplary and thus the number of subframes in a radio frame, the number of slots in a subframe, or the number of OFDM symbols in a slot may vary. 
       FIG. 5  illustrates exemplary control channels included in a control region of a subframe in a DL radio frame. 
     Referring to  FIG. 5 , a subframe includes 14 OFDM symbols. The first one to three OFDM symbols of a subframe are used for a control region and the other 13 to 11 OFDM symbols are used for a data region according to a subframe configuration. In  FIG. 5 , reference characters R 1  to R 4  denote RSs or pilot signals for antenna 0 to antenna 3. RSs are allocated in a predetermined pattern in a subframe irrespective of the control region and the data region. A control channel is allocated to non-RS resources in the control region and a traffic channel is also allocated to non-RS resources in the data region. Control channels allocated to the control region include a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel (PHICH), a Physical Downlink Control Channel (PDCCH), etc. 
     The PCFICH is a physical control format indicator channel carrying information about the number of OFDM symbols used for PDCCHs in each subframe. The PCFICH is located in the first OFDM symbol of a subframe and configured with priority over the PHICH and the PDCCH. The PCFICH includes 4 Resource Element Groups (REGs), each REG being distributed to the control region based on a cell Identity (ID). One REG includes 4 Resource Elements (REs). An RE is a minimum physical resource defined by one subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4 according to a bandwidth. The PCFICH is modulated in Quadrature Phase Shift Keying (QPSK). 
     The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ) indicator channel carrying an HARQ ACK/NACK for a UL transmission. That is, the PHICH is a channel that delivers DL ACK/NACK information for UL HARQ. The PHICH includes one REG and is scrambled cell-specifically. An ACK/NACK is indicated in one bit and modulated in Binary Phase Shift Keying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor (SF) of 2 or 4. A plurality of PHICHs mapped to the same resources form a PHICH group. The number of PHICHs multiplexed into a PHICH group is determined according to the number of spreading codes. A PHICH (group) is repeated three times to obtain a diversity gain in the frequency domain and/or the time domain. 
     The PDCCH is a physical DL control channel allocated to the first n OFDM symbols of a subframe. Herein, n is 1 or a larger integer indicated by the PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carries resource allocation information about transport channels, PCH and DL-SCH, a UL scheduling grant, and HARQ information to each UE or UE group. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, an eNB and a UE transmit and receive data usually on the PDSCH, except for specific control information or specific service data. 
     Information indicating one or more UEs to receive PDSCH data and information indicating how the UEs are supposed to receive and decode the PDSCH data are delivered on a PDCCH. For example, on the assumption that the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked by Radio Network Temporary Identity (RNTI) “A” and information about data transmitted in radio resources (e.g. at a frequency position) “B” based on transport format information (e.g. a transport block size, a modulation scheme, coding information, etc.) “C” is transmitted in a specific subframe, a UE within a cell monitors, that is, blind-decodes a PDCCH using its RNTI information in a search space. If one or more UEs have RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicated by “B” and “C” based on information of the received PDCCH. 
       FIG. 6  illustrates a structure of a UL subframe in the LTE system. 
     Referring to  FIG. 6 , a UL subframe may be divided into a control region and a data region. A Physical Uplink Control Channel (PUCCH) including Uplink Control Information (UCI) is allocated to the control region and a Physical uplink Shared Channel (PUSCH) including user data is allocated to the data region. The middle of the subframe is allocated to the PUSCH, while both sides of the data region in the frequency domain are allocated to the PUCCH. Control information transmitted on the PUCCH may include an HARQ ACK/NACK, a CQI representing a downlink channel state, an RI for Multiple Input Multiple Output (MIMO), a Scheduling Request (SR) requesting UL resource allocation. A PUCCH for one UE occupies one RB in each slot of a subframe. That is, the two RBs allocated to the PUCCH are frequency-hopped over the slot boundary of the subframe. Particularly, PUCCHs with m=0, m=1, and m=2 are allocated to a subframe in  FIG. 6 . 
       FIG. 7  is a diagram showing the structure of a general multiple input multiple output (MIMO) communication system according to an embodiment of the present invention. 
     MIMO refers to a method using multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Namely, a plurality of antennas is used at a transmitter or a receiver of a wireless communication system so that capacity can be increased and performance can be improved. MIMO may also be referred to as multi-antenna in this disclosure. 
     MIMO technology does not depend on a single antenna path in order to receive a whole message. Instead, MIMO technology completes data by combining data fragments received via multiple antennas. The use of MIMO technology can increase data transmission rate within a cell area of a specific size or extend system coverage at a specific data transmission rate. MIMO technology can be widely used in mobile communication terminals and relay nodes. MIMO technology can overcome a limited transmission capacity encountered with the conventional single-antenna technology in mobile communication. 
       FIG. 7  illustrates the configuration of a typical MIMO communication system. A transmitter has N T  transmit (Tx) antennas and a receiver has N R  receive (Rx) antennas. Use of a plurality of antennas at both the transmitter and the receiver increases a theoretical channel transmission capacity, compared to the use of a plurality of antennas at only one of the transmitter and the receiver. Channel transmission capacity increases in proportion to the number of antennas. Therefore, transmission rate and frequency efficiency are increased. Given a maximum transmission rate R o  that may be achieved with a single antenna, the transmission rate may be increased, in theory, to the product of R o  and a transmission rate increase rate R i  in the case of multiple antennas, as indicated by Equation 1. R i  is the smaller of N T  and N R .
 
 R   i =min( N   T   ,N   R )  [Equation 1]
 
     For example, a MIMO communication system with four Tx antennas and four Rx antennas may theoretically achieve a transmission rate four times that of a single antenna system. Since the theoretical capacity increase of the MIMO wireless communication system was verified in the mid-1990s, many techniques have been actively developed to increase data transmission rate in real implementations. Some of these techniques have already been reflected in various wireless communication standards including standards for 3rd generation (3G) mobile communications, next-generation wireless local area networks, etc. 
     Active research up to now related to MIMO technology has focused upon a number of different aspects, including research into information theory related to MIMO communication capacity calculation in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of MIMO systems, and research into space-time signal processing technologies for improving transmission reliability and transmission rate. 
     Communication in a MIMO system will be described in detail through mathematical modeling. It is assumed that N T  Tx antennas and N R  Rx antennas are present as illustrated in  FIG. 7 . Regarding a transmission signal, up to N T  pieces of information can be transmitted through the N T  Tx antennas, as expressed as the vector of Equation 2 below.
 
 s=└s   1   ,s   2   , . . . ,s   N     T   ┘ T   [Equation 2]
 
     Individual pieces of the transmission information s 1 , s 2 , . . . , s N     T    may have different transmit powers. If the individual transmit powers are denoted by P 1 , P 2 , . . . , P N     T   , respectively, then the transmission power-controlled transmission information may be given as shown in Equation 3.
 
 ŝ =[ ŝ   1   ,ŝ   2   , . . . ,ŝ   N     T   ] T =[ P   1   s   1   ,P   2   s   2   , . . . ,P   N     T     s   N     T   ] T   [Equation 3]
 
     The transmission power-controlled transmission information vector ŝ may be expressed as shown in Equation 4 below, using a diagonal matrix P of transmission power. 
     
       
         
           
             
               
                 
                   
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     Meanwhile, NT transmission signals x 1 , x 2 , . . . , x N     T    to be actually transmitted may be configured by multiplying the transmission power-controlled information vector ŝ by a weight matrix W. The weight matrix functions to appropriately distribute the transmission information to individual antennas according to transmission channel states, etc. The transmission signals x 1 , x 2 , . . . , x N     T    are represented as a vector X, as shown in Equation 5 below. Here, w ij  denotes a weight of an i-th Tx antenna and a j-th piece of information. W is referred to as a weight matrix or a precoding matrix. 
     
       
         
           
             
               
                 
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     Generally, the physical meaning of the rank of a channel matrix is the maximum number of different pieces of information that can be transmitted on a given channel. Therefore, the rank of a channel matrix is defined as the smaller of the number of independent rows and the number of independent columns in the channel matrix. Accordingly, the rank of the channel matrix is not larger than the number of rows or columns of the channel matrix. The rank of the channel matrix H (rank(H)) is restricted as shown in Equation 6 below.
 
rank( H )≤min( N   T   ,N   R )  [Equation 6]
 
     A different piece of information transmitted in MIMO is referred to as a transmission stream or stream. A stream may also be called a layer. It is thus concluded that the number of transmission streams is not larger than the rank of channels, i.e. the maximum number of different pieces of transmittable information. Thus, the channel matrix H is expressed as shown in Equation 7 below.
 
# of streams≤rank( H )≤min( N   T   ,N   R )  [Equation 7]
 
     “# of streams” denotes the number of streams. It should be noted that one stream may be transmitted through one or more antennas. 
     One or more streams may be mapped to a plurality of antennas in many ways. This method may be described as follows depending on MIMO schemes. If one stream is transmitted through a plurality of antennas, this may be regarded as spatial diversity. When a plurality of streams is transmitted through a plurality of antennas, this may be spatial multiplexing. A hybrid scheme of spatial diversity and spatial multiplexing may be contemplated. 
     It is expected that the future-generation mobile communication standard, LTE-A will support Coordinated Multi-Point (CoMP) transmission in order to increase data rate, compared to the legacy LTE standard. CoMP refers to transmission of data to a UE through cooperation from two or more eNBs or cells in order to increase communication performance between a UE located in a shadowing area and an eNB (a cell or sector). 
     CoMP transmission schemes may be classified into CoMP-Joint Processing (CoMP-JP) called cooperative MIMO characterized by data sharing, and CoMP-Coordinated Scheduling/Beamforming (CoMP-CS/CB). 
     In DL CoMP-JP, a UE may instantaneously receive data simultaneously from eNBs that perform CoMP transmission and may combine the received signals, thereby increasing reception performance (Joint Transmission (JT)). In addition, one of the eNBs participating in the CoMP transmission may transmit data to the UE at a specific time point (Dynamic Point Selection (DPS)). 
     In contrast, in downlink CoMP-CS/CB, a UE may receive data instantaneously from one eNB, that is, a serving eNB by beamforming. 
     In UL CoMP-JP, eNBs may receive a PUSCH signal from a UE at the same time (Joint Reception (JR)). In contrast, in UL CoMP-CS/CB, only one eNB receives a PUSCH from a UE. Herein, cooperative cells (or eNBs) may make a decision as to whether to use CoMP-CS/CB. 
     Hereinbelow, a description of channel state information (CSI) reporting will be given. In the current LTE standard, a MIMO transmission scheme is categorized into open-loop MIMO operated without CSI and closed-loop MIMO operated based on CSI. Especially, according to the closed-loop MIMO system, each of the eNB and the UE may be able to perform beamforming based on CSI in order to obtain multiplexing gain of MIMO antennas. To acquire CSI from the UE, the eNB transmits RSs to the UE and commands the UE to feed back CSI measured based on the RSs through a PUCCH or a PUSCH. 
     CSI is divided into three types of information: an RI, a PMI, and a CQI. First, RI is information on a channel rank as described above and indicates the number of streams that can be received via the same time-frequency resource. Since RI is determined by long-term fading of a channel, it may be generally fed back at a cycle longer than that of PMI or CQI. 
     Second, PMI is a value reflecting a spatial characteristic of a channel and indicates a precoding matrix index of the eNB preferred by the UE based on a metric of signal-to-interference plus noise ratio (SINR). Lastly, CQI is information indicating the strength of a channel and indicates a reception SINR obtainable when the eNB uses PMI. 
     An advanced system such as an LTE-A system considers additional multi-user diversity through multi-user MIMO (MU-MIMO). Due to interference between UEs multiplexed in an antenna domain in MU-MIMO, the accuracy of CSI may significantly affect interference with other multiplexed UEs as well as a UE that reports the CSI. Accordingly, more accurate CSI than in single-user MIMO (SU-MIMO) should be reported in MU-MIMO. 
     In this context, the LTE-A standard has determined to separately design a final PMI as a long-term and/or wideband PMI, W1, and a short-term and/or subband PMI, W2. 
     For example, a long-term covariance matrix of channels expressed as Equation 8 may be used for hierarchical codebook transformation that configures one final PMI with W1 and W2.
 
 W =norm( W 1 W 2)  [Equation 8]
 
     In Equation 1, W2 is a short-term PMI, which is a codeword of a codebook reflecting short-term channel information, W is a codeword of a final codebook, and norm(A) is a matrix obtained by normalizing each column of matrix A to 1. 
     Conventionally, the codewords W1 and W2 are given as Equation 9. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In Equation 9, the codewords are designed so as to reflect correlation characteristics between established channels, if cross-polarized antennas are densely arranged, for example, the distance between adjacent antennas is equal to or less than half a signal wavelength. The cross-polarized antennas may be divided into a horizontal antenna group and a vertical antenna group and the two antenna groups are co-located, each having the property of a uniform linear array (ULA) antenna. 
     Therefore, the correlations between antennas in each group have the same linear phase increment property and the correlation between the antenna groups is characterized by phase rotation. Since a codebook is quantized values of channels, it is necessary to design a codebook reflecting channel characteristics. For convenience of description, a rank-1 codeword designed in the above manner may be given as Equation 10. 
     In Equation 9, the codebook configurations are designed to reflect channel correlation properties generated when cross polarized antennas are used and when a space between antennas is dense, for example, when a distance between adjacent antennas is less than a half of signal wavelength. The cross polarized antennas may be categorized into a horizontal antenna group and a vertical antenna group. Each antenna group has the characteristic of a Uniform Linear Array (ULA) antenna and the two groups are co-located. 
     Accordingly, a correlation between antennas of each group has characteristics of the same linear phase increment and a correlation between antenna groups has characteristics of phase rotation. Consequently, since a codebook is a value obtained by quantizing a channel, it is necessary to design a codebook such that characteristics of a channel are reflected. For convenience of description, a rank-1 codeword generated by the aforementioned configurations is shown in Equation 10 below. 
     
       
         
           
             
               
                 
                   
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     In Equation 10, a codeword is expressed as an N T ×1 vector where NT is the number of Tx antennas and the codeword is composed of an upper vector X i (k) and a lower vector α j X i (k), representing the correlation characteristics of the horizontal and vertical antenna groups, respectively. X i (k) is expressed as a vector having the linear phase increment property, reflecting the correlation characteristics between antennas in each antenna group. As a representative example, a discrete Fourier transform (DFT) matrix may be used. 
     An advanced system such as an LTE-A system considers achievement of an additional multi-user diversity by the use of MU-MIMO. Due to the existence of interference channels between UEs multiplexed in an antenna domain in MU-MIMO, the accuracy of CSI may significantly affect interference with other multiplexed UEs as well as a UE that reports the CSI. Accordingly, more accurate CSI than in SU-MIMO should be reported in MU-MIMO. 
     In CoMP JT, because a plurality of eNBs transmits the same data to a specific UE through cooperation, the eNBs may be theoretically regarded as forming a MIMO system with antennas distributed geographically. That is, even when MU-MIMO is implemented in JT, highly accurate CSI is required to avoid interference between CoMP-scheduled UEs as in a single cell MU-MIMO operation. The same applies to CoMP CB. That is, to avoid interference with a serving cell caused by a neighbor cell, accurate CSI is needed. In general, a UE needs to report an additional CSI feedback in order to increase the accuracy of CSI feedback. The CSI feedback is transmitted on a PUCCH or a PUSCH to an eNB. 
     Hereinafter, a vehicular communication system based on the above-described wireless communication system will be described. 
       FIG. 8  is a diagram showing a vehicle including a plurality of antenna arrays according to an embodiment of the present invention. The number of uses of the above-described wireless communication system and service categories using the wireless communication system have increased. At this time, unlike an existing static service, need to provide high data throughput, high data rate and high quality of service (QoS) to user equipments (UEs) or users moving at a high speed has increased. 
     For example, a wireless communication system needs to support wireless services having good quality to moving UEs when a plurality of UEs or users (hereinafter, collectively referred to as a UE) desires to view multimedia content while using public transportation or when a plurality of UEs of passengers riding in a personal vehicle traveling on an expressway uses different wireless communication services. 
     However, a conventional wireless communication system has some limits in provision of a service to a UE in consideration of high-speed movement or mobility. At this time, in order to support a service, a system network needs to be revolutionized. In addition, a new system for maintaining compatibility with an existing network infrastructure without influencing the existing network infrastructure needs to be designed. 
     At this time, for example, as described below, a large-sized antenna array may be mounted in a vehicle such that the vehicle acquires large array gain, thereby providing services having good quality to UEs located in the vehicle even in a state in which the vehicle moves at a high speed. At this time, in the vehicle, data received through a central unit (hereinafter, CU) may be relayed to the UEs located in the vehicle. At this time, a vehicular MIMO system may be considered. At this time, as described above, if a large-sized antenna array is used, the vehicle can prevent communication performance from being lowered due to penetration loss having an average value of 20 dB. In addition, since the vehicle uses receive (rx) antennas greater in number than the number of UEs using a system, large array gain can be easily obtained and reception diversity can be obtained by ensuring a distance between the receive antennas. That is, it is possible to provide a service to a UE moving at a high speed without additionally designing a network through the vehicular MIMO system. 
     In spite of the above-described advantages, up to now, it has been difficult to apply a vehicular MIMO system due to problems related to vehicle appearance and manufacturing system establishment. In addition, the vehicle is significantly expensive as compared to a personal portable communication device and cannot be easily improved and updated. In addition, since the vehicle should satisfy many requirements such as design concept and aeromechanical structure in addition to communication performance, the design of the vehicle may be aesthetically/aeromechanically restricted. For example, some vehicle manufacturers have used complex antennas having quality inferior to that of a single antenna in order to eliminate visual inconvenience of an existing antenna. 
     In order to solve spatial restriction of a large-sized antenna array in an environment in which a communication system needs to be developed, installation of a distributed antenna array system for implementing a multi-antenna-array system in a vehicle has been gradually introduced in consideration of vehicle appearance. 
     At this time, for example, referring to  FIG. 8 , a vehicle may include a plurality of antennas  810 ,  820 ,  830 ,  840 ,  850  and  860  mounted therein. At this time, the locations and number of the plurality of antennas  810 ,  820 ,  830 ,  840 ,  850  and  860  may be changed according to vehicle design. At this time, the below-described configuration is equally applicable even when the locations and number of the plurality of antennas  810 ,  820 ,  830 ,  840 ,  850  and  860  mounted in the vehicle are changed, and the present invention is not limited to the below-described embodiments. That is, the present invention is applicable to antennas having various shapes and radiation patterns according to the locations of the plurality of antennas  810 ,  820 ,  830 ,  840 ,  850  and  860 . 
     At this time, signals for distributed antenna units (DUs) of the vehicle may be controlled through a central unit (CU)  870 . That is, the CU  870  of the vehicle may control the signals for the DUs  810 ,  820 ,  830 ,  840 ,  850  and  860  mounted in the vehicle to receive a signal from a base station while maximizing reception diversity and to prevent wireless connection between the base station and the vehicle in a state in which the vehicle moves at a high speed. That is, the vehicle may be a UE having a plurality of antennas or a relay for relaying a signal. The vehicle may provide a service having good quality to a plurality of UEs located in the vehicle through control and relay of the signal received through the CU  870 . 
       FIG. 9  is a diagram showing a method of selecting a distributed antenna unit (DU) in a state in which a plurality of vehicles is concentrated, according to an embodiment of the present invention. 
     As described above, a vehicle may include a plurality of DUs and a CU  870  for controlling the DUs. At this time, a plurality of vehicles  920 - 1 ,  920 - 2  and  920 - 3  may be concentrated in a narrow area. For example, the plurality of vehicles  920 - 1 ,  920 - 2  and  920 - 3  may be concentrated in a narrow area upon city driving or upon a traffic jam. At this time, if the plurality of vehicles  920 - 1 ,  920 - 2  and  920 - 3  is concentrated, it may be difficult to distinguish between beams for the DUs of the vehicles due to beam sharpness. For example, if a plurality of vehicles is close to each other, the DU located at the right side of the first vehicle  920 - 1  may be adjacent to the DU located at the left side of the second vehicle  920 - 2  and thus the beams for these DUs may not be easily distinguished. That is, since DUs located adjacent to each other receive signals undergoing similar channel environments, a plurality of DUs may be likely to receive the same beam or not to receive a signal due to blocking of obstacles. 
     Accordingly, activation of the DUs deployed in the plurality of vehicles  920 - 1 ,  920 - 2  and  920 - 3  needs to be controlled. More specifically, the vehicles  920 - 1 ,  920 - 2  and  920 - 3  may selectively control activation or deactivation of the DUs based on the density of neighboring vehicles. For example, when a beam transmitted from a first base station  910 - 1  to a first vehicle  920 - 1  is received, the first vehicle  920 - 1  may activate only the DUs located at the left side of the first vehicle  920 - 1  and deactivate the remaining DUs of the first vehicle  920 - 1 , to be distinguished from the adjacent second vehicle  920 - 2 . At this time, for example, the first vehicle  920 - 1  may determine whether vehicles are concentrated using a position information reception unit (e.g., a GPS) or a proximity sensor. In addition, for example, whether the DUs are deactivated may be determined based on a threshold value based on density of vehicles. At this time, a threshold value may be a criterion value for determining activation or deactivation. That is, a criterion for determining whether the vehicles  920 - 1 ,  920 - 2  and  920 - 3  are concentrated may be changed and is not limited to the above-described embodiment. 
     In addition, the third vehicle  920 - 3  may activate two DUs located at the front side of the third vehicle  920 - 3  in order to receive the beam from the second base station  910 - 2 . That is, the vehicles  920 - 1 ,  920 - 2  and  920 - 3  may selectively activate/deactivate the DUs thereof to distinguish the beam received through the activated DUs thereof from the beams capable of being received by adjacent vehicles. Therefore, beams passing through independent paths experiencing different clusters are received, thereby improving beam reception performance. 
     In addition, the vehicles may feed information on activation and deactivation of the DUs back to the base station as described above. At this time, for example, the above-described information may be fed back along with channel state information (CSI) fed back from the vehicles to the base station. 
     More specifically, a transmission end needs to obtain information on a channel and to accurately measure a suitable beam and gain obtained upon using the beam based on the information. At this time, in a wireless communication system, a reception end (e.g., a UE or a vehicle) may feed channel information back to the transmission end (e.g., the base station) in the form of CSI after measuring the channel. 
     At this time, for example, in a MIMO system, since a plurality of antennas is used, a plurality of channels may be present and CSI may be defined as a combination of sub-channels. At this time, as the number of antennas used in the MIMO system increases, a complicated format may be used. In consideration of such an environment, an implicit CSI reporting scheme or an explicit CSI reporting scheme may be used as a CSI reporting scheme. That is, an implicit CSI reporting scheme or an explicit CSI reporting scheme may be used as a CSI reporting scheme of a massive MIMO environment. 
     At this time, for example, the implicit CSI reporting scheme may refer to a scheme of analyzing information on a channel measured by a reception end and reporting only information substantially necessary to generate a beam, without reporting the information on the channel measured by the reception end. That is, only necessary information may be fed back based on a predefined or predetermined value. 
     In contrast, the explicit CSI reporting scheme may refer to a scheme of reporting information maximally approximating to a measured value to a transmission end without a process of analyzing a channel measured by a reception end. At this time, a method of quantizing a MIMO channel represented in a matrix or performing SVD operation may be used in the channel information. For example, the implicit CSI report information may include a precoding matrix index (PMI), a channel quality indicator (CQI), rank information (RI), etc. In addition, the explicit CSI report information may include channel coefficient quantization &amp; quantization index feedback, MIMO matrix or vector quantization &amp; quantization index feedback, channel covariance matrix feedback, Eigen matrix feedback (transmission of Eigen vectors and/or Eigen values of channel matrix), etc. At this time, the implicit CSI reporting scheme can reduce signal overhead as compared to the explicit CSI reporting scheme, since only necessary information is extracted and fed back. 
     At this time, in association with the CSI feedback method of a conventional wireless communication system, a UE receives a pilot signal (reference signal) for channel estimation from a base station and calculates and reports channel state information (CSI) to the base station. At this time, the base station transmits data to the UE based on the CSI fed back from the UE. At this time, in a wireless communication system, the CSI fed back by the UE may include a channel quality indicator (CQI), a precoding matrix index (PMI), a rank indicator (RI), etc. 
     At this time, CQI feedback may be radio channel quality information provided to the base station for the purpose of providing information regarding which modulation and coding scheme (MCS) is applied when the base station transmits data (for link adaptation). When radio quality between the base station and the UE is high, the UE may feed back a high CQI value and the base station may apply a relatively high modulation order and a low channel coding rate and transmit data. Otherwise, the UE may feed back a low CQI value and the base station may apply a relatively low modulation order and a high channel coding rate and transmit data. 
     In addition, PMI feedback may be feedback of preferred precoding matrix information provided to the base station for the purpose of providing information regarding which MIMO precoding is applied when the base station includes multiple antennas mounted therein. The UE may estimate a downlink MIMO channel between the base station and the UE from a pilot signal and provide information indicating which MIMO precoding is applied to the base station through PMI feedback. In a conventional wireless communication system, only linear MIMO precoding representable in a matrix in a configuration of a PMI was considered. At this time, the base station and the UE share a codebook composed of a plurality of precoding matrices and each MIMO precoding matrix in the codebook has a unique index. Accordingly, the UE may feed back an index corresponding to a most preferred MIMO precoding matrix in the codebook as PMI, thereby minimizing the amount of feedback information of the UE. 
     Lastly, RI feedback may be feedback of information on the number of preferred transport layers provided to the base station for the purpose of providing information on the number of transport layers preferred by the UE when each of the base station and the UE includes multiple antennas mounted therein and thus multilayer transmission through spatial multiplexing is possible. At this time, since the base station should know which precoding is applied to each layer according to the number of transport layers, the RI may be closely related with the PMI. For example, in configuration of PMI/RI feedback, a PMI codebook may be configured based on single-layer transmission and then a PMI may be defined and fed back per layer. However, in such a method, the amount of PMI/RI feedback information is significantly increased as the number of transport layers increases. Accordingly, in a conventional wireless communication system, a PMI codebook according to the number of transport layers was defined. That is, N matrices having a size of Nt×R may be defined in the codebook, for R-layer transmission (here, R denotes the number of layers, Nt denotes the number of transmit antenna ports, and N denotes the size of a codebook). At this time, the size of the codebook may be defined regardless of the number of transport layers. As a result, when the PMI/RI is defined in such a structure, the number R of transport layers becomes equal to the rank value of the precoding matrix (Nt×R matrix) and thus may be referred to as a rank indicator (RI). 
     In addition, in a conventional wireless communication system, CSI may be obtained in an overall system frequency region or some frequency regions (e.g., Wideband CSI, Subband CSI). Particularly, in a system using orthogonal frequency division multiple access (OFDMA) technology, CSI of some frequency regions (e.g., subband) preferred per UE may be obtained and fed back. 
     At this time, the below-described PMI/RI may not be limited to the index value of a precoding matrix represented in an Nt×R matrix and the rank value of a precoding matrix like a PMI/RI of a wireless communication system. In addition, the below-described PMI indicates preferred MIMO precoder information among MIMO precoders applicable to a transmission end and the precoder is not limited to a linear precoder represented in a matrix as in a conventional wireless system. In addition, the below-described RI has a broader meaning than the RI in the conventional wireless communication system and may include all feedback information indicating the number of preferred transport layers without being limited thereto. 
     In addition, for example, the PMI value may not include only one index. For example, in the conventional wireless communication system, a final PMI is divided into W1 which is a long term and/or wideband (WB) PMI and W2 which is a short term and/or subband (SB) PMI, thereby designing a PMI having a dual structure. At this time, when the final PMI is W, W=W1*W2 or W=W2*W1 may be defined. In addition, for example, in an LTE-A system, if the number of transmit antenna ports is 8 or if the number of transmit antenna ports is 4 and alternativeCodeBookEnabledFor4TX−r12=TRUE is configured through RRC signaling, a final MIMO precoding matrix may be derived by only combining two indices (WB PMI &amp; SB PMI). 
     In addition, in a wireless communication system, in single user-MIMO (SU-MIMO), only data of one UE may be scheduled in the same time/frequency domain. That is, if information is transmitted to and received from one UE by MIMO, only scheduling information of one UE may be included in one time/frequency domain. In contrast, in multiuser-MIMO (MU-MIMO), data of a plurality of UEs may be scheduled together in one time/frequency domain. At this time, in MU-MIMO, the data is multiplexed in the same time/frequency domain, thereby obtaining additional gain. However, if the plurality of UEs is scheduled together, co-channel interference is generated by the UEs, thereby deteriorating system performance. At this time, the UE may feed CSI thereof back to the base station and the base station may schedule a user based on the CSI fed back from the plurality of UEs, thereby optimizing a system. 
     However, if a new UE is further scheduled in an SU-MIMO state or an MU-MIMO state, in a conventional wireless communication system, influence of interference between users generated by scheduling the new UE in the system may not be considered. That is, since only channel information considering SU-MIMO is fed back and the base station only checks the channel state of each user and cannot acquire information on interference to be experienced by each user in MU-MIMO, it may be difficult to reduce influence of interference between UEs. Accordingly, when SU-MIMO is switched to MU-MIMO or if MU-MIMO operates, multiplexing gain capable of being obtained by supporting multiple UEs needs to be sufficiently considered. 
     At this time, in consideration of the above-described situation, if a plurality of vehicles (or UEs) (hereinafter, collectively referred to as vehicles) is present, each vehicle may feed combination information related to the number and locations of DUs activated in the vehicle and channel state information back to a base station. That is, interference with other vehicles and reception performance may be changed according to the number and locations of DUs activated in each vehicle and each vehicle needs to feed information on the activated DUs back to the base station. At this time, the base station may acquire an effective channel of each vehicle based on the received channel state information and the activated DU information and transmit data. At this time, as described above, in a state in which vehicles are concentrated, the base station needs to acquire more accurate channel information. Hereinafter, a method of feeding back activated DU information at a vehicle will be described. Although a vehicle including a DU will be described below, the method is equally applicable to a UE including a plurality of antennas or another device including a plurality of antennas and operating based on the plurality of antennas and is not limited to the below-described embodiment. 
       FIG. 10  is a diagram showing an example of a DU selection combination according to an embodiment of the present invention. As described above, an implicit CSI reporting scheme or an explicit CSI reporting scheme may be used as a CSI reporting scheme. At this time, if a plurality of DUs is included in a vehicle and activation of DUs is selectable, effective channel information may be changed according to the number and locations of activated DUs. 
     More specifically, in a conventional communication system, if a plurality of antennas is present and all antennas are activated, a UE may feed back CSI. However, as described above, if a plurality of vehicles is concentrated or if a plurality of UEs or devices is concentrated, cooperative communication may be performed in consideration of mutual interference. At this time, each UE may select DUs to be activated from among the plurality of DUs included therein, thereby reducing interference with the other vehicles or UEs. At this time, channel information to be reported by the vehicle may be changed according to the number and locations of activated DUs among the plurality of DUs and thus a channel information reporting method considering the same may be necessary. 
     At this time, for example, the channel information reported by the vehicle may include channel state information and DU index set information combined according to the number and locations of activated DUs among the plurality of DUs included in the vehicle. At this time, the channel state information may be explicitly reported in consideration of the DU index set information. At this time, for example, the explicitly reported channel state information may include at least one of the channel coefficient, quantization &amp; quantization index feedback, MIMO matrix or vector quantization &amp; quantization index feedback, channel covariance matrix feedback and Eigen matrix feedback (transmission of Eigen vectors and/or Eigen values of channel matrix). In addition, for example, the channel information fed back by the vehicle may include at least one of the DU index set information and the channel state information. That is, the channel information fed back by the vehicle may include both or one of the DU index set information and the channel state information, without being limited to the above-described embodiment. 
     In addition, the channel information fed back by the vehicle may include explicit channel state information in consideration of the DU index set as described above, without being limited thereto. For example, the vehicle may report the DU index set information and implicit channel state information to the base station and the base station may acquire final effective channel information using the implicit channel state information and the DU index set information. 
     In addition, the base station may receive the channel information of each of the plurality of vehicles. That is, the base station may receive the DU index set information and the channel state information fed back by each of the plurality of vehicles. At this time, the base station may acquire effective channel information using the received DU index set information and channel state information. The base station may transmit data to the vehicles using the acquired effective channel information. 
     At this time, the DU index set information may differ between the vehicles. As described above, the DU index set information may be differently set based on the number and locations of activated DUs among the plurality of DUs included in the vehicle. 
     For example, referring to  FIG. 10( a ) , the vehicle may include four DUs  1010 ,  1020 ,  1030  and  1040  at corner regions thereof. At this time, the number and locations of DUs included in the vehicle may be differently set and is not limited to the above-described embodiment. In addition, for example, each vehicle may feed the number and locations of DUs included therein back to the base station. In addition, for example, each vehicle may feed information on the number and locations of activated DUs among the DUs included therein back to the base station. In addition, for example, the base station may acquire the DU information of each vehicle through higher layer signaling or another path, without being limited to the above-described embodiment. 
     At this time, for example,  FIG. 10( a )  shows the case where only one of the four DUs  1010 ,  1020 ,  1030  and  1040  included in the vehicle is activated. At this time, different combinations may be configured according to selected DUs. More specifically, whether each of the DUs  1010 ,  1020 ,  1030  and  1040  included in the vehicle is activated may be indicated through a matrix or an index. At this time, if only the DU located at the front left side of the vehicle is activated, the DU index may be [1 0 0 0]. If only one DU included in the vehicle is activated using the same method, the DU index may be represented by [0 1 0 0], [0 0 1 0] or [0 0 0 1]. 
     At this time, for example, the DU index set information may include the above-described four pieces of DU index information. In addition, for example, the vehicle may feed information indicating that only one of the four DUs  1010 ,  1020 ,  1030  and  1040  is activated back to the base station. At this time, the vehicle may include the four pieces of DU index information in the DU index set information and feed the DU index set information back to the base station along with the channel state information. At this time, the base station may acquire effective channel information based on the DU index information and perform downlink data transmission. 
     In addition, for example, the vehicle may feed the DU index set information considering both the number and locations of DUs included therein back to the base station. At this time, for example, the number of cases of selecting M i (M i ≤N i ) DUs to be activated among the DUs included in one vehicle is shown in Equation 11 below.
 
 N   i   C   M   i   [Equation 11]
 
     Accordingly, the total number of cases of DU combinations selectable from one vehicle is shown in Equation 12 below. 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         
                           M 
                           i 
                         
                         = 
                         0 
                       
                       
                         N 
                         i 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       C 
                       
                         M 
                         i 
                       
                         
                         
                       
                         N 
                         i 
                       
                         
                     
                   
                   = 
                   
                     2 
                     
                       N 
                       i 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
     In addition, for example, if M i =0, the vehicle may be deactivated not to cause interference with the other vehicles. At this time, for example, the vehicle may feed the DU index set information including 2 N     i    pieces of DU index information back to the base station. The base station may determine an effective channel using the fed-back DU index set information and channel state information. 
     In addition, for example,  FIG. 10( b )  shows DU index information which is differently set according to the number and locations of activated DUs among the four DUs  1010 ,  1020 ,  1030  and  1040  of the vehicle. That is, the DU index may be determined based on the number and locations of activated DUs among the DUs included in the vehicle and the DU index information may be fed back to the base station as DU index set information. 
     At this time, for example, in  FIG. 10( a ) , only one of the four DUs may be activated in one vehicle. At this time, as described above, the DU index set may be given as {[1 0 0 0], [0 1 0 0], [0 0 1 0], [0 0 0 1]. At this time, the vehicle may transmit the channel state information to the base station as an explicit CSI report. At this time, for example, the explicit CSI report may include MIMO matrix quantization. At this time, MIMO matrix quantization is shown in Equation 13 below. 
     
       
         
           
             
               
                 
                   H 
                   = 
                   
                     [ 
                     
                       
                         
                           
                             h 
                             1 
                           
                         
                       
                       
                         
                           
                             h 
                             2 
                           
                         
                       
                       
                         
                           ⋮ 
                         
                       
                       
                         
                           
                             h 
                             
                               N 
                               i 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ] 
                 
               
             
           
         
       
     
     At this time, h i  elements configuring the matrix may be a scalar value by the number of antennas in each DU, quantization resolution, etc. In addition, for example, h i  may be a row vector or a column vector. At this time, for example, if h i  is represented in the form of a column vector, a MIMO matrix may be represented by horizontally stacking h i , not vertically stacking h i  as in Equation 13. That is, the base station, which has received the explicitly reported CSI and the selectable DU index set, may combine and use the effective channel information of the vehicle using the information for downlink transmission. At this time, for example, in Equation 13, the effective channel information capable of being extracted from explicit feedback and the DU index set [0 1 0 0] may become H eff =[h 2 ]. That is, the base station may extract some (rows or columns) of the explicit CSI based on the DU index set information fed back from each vehicle, form effective channel information of each vehicle and perform data transmission using the effective channel information. 
       FIG. 11  is a diagram showing a DU including a plurality of antenna ports according to an embodiment of the present invention. As described above, each vehicle may include DU index set information and channel state information in channel information and feed the channel information back to the base station. 
     At this time, for example, the DU index set may be configured as antenna port group (sub grouping or antenna port grouping) information per DU. 
     More specifically, referring to  FIG. 11 , one vehicle may include four DUs composed of 2/2/4/4 physical antennas. At this time, the total number of physical antennas included in each DU may be 12. At this time, for example, the vehicle may generate channel state information based on the total number of physical antennas. At the same time, the vehicle may feed information indicating the antenna port of the DU, to which each physical antenna is mapped, back to the base station through the DU index set information. 
     For example, the vehicle may include information such as DU1 (port #={0,1}, number of antennas=2), DU2 (port #={2,3}, number of antennas=2) in the DU index set and feed the information back to the base station. At this time, the base station may map the antenna port index using the received DU index set information and the channel state information and transmit data while controlling interference between the vehicles. 
       FIG. 12  is a diagram showing a method of activating a DU based on an RF structure according to an embodiment of the present invention. In addition, for example, each vehicle may transmit, to the base station, information on how many DUs are simultaneously activated/deactivated (on/off) by the vehicle based on the radio frequency (RF) structure. At this time, for example, the RF end may correspond to the below-described transmission or reception module, which will be described below. 
     At this time, for example, referring to  FIG. 12 , when a plurality of DUs is configured to be switched or is connected to one RF chain according to the structure of the RF end in each vehicle, the number of DUs activated/deactivated simultaneously and a combination thereof may have some limits. 
     At this time, the vehicle may feed the above-described information back to the base station along with the DU index set. At this time, the base station may consider a combination of selectable DUs from each vehicle in advance based on the above-described information and select a multiuser precoder in consideration of interference with a plurality of vehicles. At this time, in the above-described configuration, unlike a user in MU-MIMO of a conventional wireless communication system, DUs may be deployed in one UE to control a direction of a received beam through DU selection. 
     In addition, for example, each vehicle may differently set a method of feeding back channel information including DU index set information and channel state information. 
     More specifically, each vehicle may feed channel information of each vehicle back to the base station. In addition, for example, a first type of vehicles among the vehicles may directly feed channel information back to the base station. At this time, the base station may control DU selection of each vehicle using the DU index set information and the channel state included in the channel information received from each vehicle in consideration of interference between vehicles in a state in which a plurality of vehicles is present, and determine a precoder to control interference. 
     In addition, for example, each vehicle may transmit channel information thereof to all or some of the other vehicles. At this time, for example, a vehicle for transmitting channel information to another vehicle may be a second type of vehicle. At this time, for example, each vehicle may calculate influence of interference using the channel information thereof and the channel information received from the other vehicles. Therefore, each vehicle may perform DU selection and determine a precoder. 
     That is, the base station may determine selection of activated DUs in consideration of the number and locations of DUs included in the vehicle through the channel information of a plurality of vehicles and control vehicular communication using a method of informing the vehicle of the determined information in an environment in which a plurality of vehicles is concentrated. In addition, the plurality of vehicles may directly control DU selection by exchanging channel information to each other, thereby controlling vehicular communication. 
     In addition, for example, a combination of the above-described methods may be performed in each vehicle. More specifically, all vehicles present in one system do not need to select one of the above-described two methods and to operate using the same method. 
     For example, the first type of vehicles may transmit channel information to the base station and perform DU selection under control of the base station. In addition, the second type of vehicles may receive channel information from the other vehicles and directly control DU selection. 
     In addition, for example, specific vehicles of the plurality of vehicles may transmit channel information configured thereby to the other vehicles and vehicles which do not exchange channel information with the other vehicles may directly feed channel information thereof back to the base station. In addition, some of the vehicles which exchange channel information configured thereby may feed channel information of all or some of the vehicles that they know back to the base station. 
     Although the vehicle including the plurality of antennas is described in the above-described configuration, the present invention is not limited thereto and the same method is applicable to a UE or a device operating in a general multi-user MIMO system. 
       FIG. 13  is a flowchart illustrating a method of transmitting and receiving a signal according to an embodiment of the present invention. A vehicle (or a UE) may receive a reference signal (S 1310 ). Thereafter, the vehicle may feed back channel information based on the received reference signal (S 1320 ). Thereafter, the vehicle may receive data based on the channel information (S 1330 ). At this time, as described above with reference to  FIGS. 8 to 12 , the channel information may include DU index set information and channel state information. At this time, for example, as described above, the vehicle may include a plurality of DUs. At this time, the DU index set information may be determined based on the number and locations of activated DUs of the DUs included in the vehicle. At this time, the base station may acquire effective channel information using the DU index set information and the channel state information and perform downlink transmission. In addition, for example, the DU index set information fed back by the vehicle may further include mapping relationship information between the DUs and the antenna ports configured in the DUs. At this time, the base station may acquire information on the antenna ports using the DU index set information and the channel state information, acquire effective channel information and determine a precoder. 
     In addition, for example, DU selection for each vehicle may be determined by the base station based on the channel information transmitted from the vehicle to the base station and signaled to the vehicle. At this time, for example, as described above, DU selection may mean information on the locations and number of activated DUs among the DUs included in the vehicle. In addition, for example, DU selection for each vehicle may be directly determined by each vehicle. At this time, for example, each vehicle may acquire channel information from neighboring vehicles and calculate influence of interference. Each vehicle may perform DU selection in consideration of influence of interference and feed information on DU selection back to the base station, thereby performing communication. In addition, a combination of the above-described two methods may be used without being limited to the above-described embodiment. 
       FIG. 14  is a block diagram showing a UE apparatus and a base station apparatus according to an embodiment of the present invention. 
     At this time, the base station apparatus  100  may include a transmission module  110  for transmitting a radio frequency signal, a reception module  130  for receiving a radio frequency signal, and a processor  120  for controlling the transmission module  110  and the reception module  130 . At this time, the base station apparatus  100  may perform communication with an external device using the transmission module  110  and the reception module  130 . At this time, the external device may be a UE. That is, the base station apparatus  100  may perform communication with the UE apparatus  100  as an external device, without being limited thereto. 
     In addition, the UE apparatus  100  may include a transmission module  210  for transmitting a radio frequency signal, a reception module  230  for receiving a radio frequency signal, and a processor  220  for controlling the transmission module  210  and the reception module  230 . At this time, the UE apparatus  200  may perform communication with the base station using the transmission module  210  and the reception module  230 . That is, the UE apparatus  200  may perform communication with the base station in a wireless communication system, without being limited thereto. At this time, for example, the transmission module  210  and the reception module  220  may be the above-described RF ends. That is, the UE apparatus  200  may switch or control activation of DUs using the transmission module  210  and the reception module  220 . More specifically, the UE apparatus  200  may transmit a signal to an external device through the transmission module  210  and the reception module  220  and thus the transmission module  210  and the reception module  220  may correspond to DUs which are a plurality of antennas. 
     At this time, for example, the processor  220  of the UE apparatus  200  may receive a reference signal using the reception module  220  and feed back channel information using the transmission module  210  based on the received reference signal. In addition, the processor  220  of the UE apparatus  200  may receive data using the reception module  220  based on the channel information. At this time, if the UE apparatus  200  includes a plurality of DUs and activation of each of the plurality of DUs is selectable, the channel information may include DU index set information and channel state information (CSI). At this time, for example, the DU index set information may be determined according to the number of activated DUs and the locations of the DUs based on activation of each of the plurality of DUs. 
     In addition, for example, the above-described UE apparatus  200  may correspond to the above-described vehicle. That is, one vehicle may be one UE apparatus  200  and each element may be included in the vehicle. 
     The embodiments according to the present invention can be implemented by various means, for example, hardware, firmware, software, or combinations thereof. 
     In the case of a hardware configuration, the embodiments 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), processors, controllers, microcontrollers, microprocessors, etc. 
     In the case of a firmware or software configuration, the method according to the embodiments of the present invention may be implemented by a module, a procedure, or a function, which performs functions or operations described above. For example, software code may be stored in a memory unit and then may be executed by a processor. The memory unit may be located inside or outside the processor to transmit and receive data to and from the processor through various well-known means. 
     The detailed description of the exemplary embodiments of the present invention has been given to enable those skilled in the art to implement and practice the invention. Although the invention has been described with reference to the exemplary embodiments, those skilled in the art will appreciate that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention described in the appended claims. Accordingly, the invention should not be limited to the specific embodiments described herein, but should be accorded the broadest scope consistent with the principles and novel features disclosed herein. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Such modifications should not be individually understood from the technical spirit or prospect of the present invention. 
     Both apparatus and method inventions are mentioned in this specification and descriptions of both of the apparatus and method inventions may be complementarily applicable to each other. 
     INDUSTRIAL APPLICABILITY 
     Although an example of applying a method of transmitting and receiving a signal in a vehicular communication system and an apparatus therefor to a 3GPP LTE system is described, the present invention is applicable to various wireless communication systems in addition to the 3GPP LTE system.