Abstract:
Provided is a relay station that is capable of improving PDSCH channel estimation accuracy, even in a case where a DL grant is provided that gives instructions for downlink data allocation for R-PDCCH. In this device, a signal separation unit ( 203 ) receives, of the control information and data and reference signals sent from a base station ( 100 ), the reference signals disposed in a first region and a second region within a pre-coding RB group (PRG) comprising one or more PRBs of the plurality of PRBs constituting a divided frequency region; the control information disposed in the first region; and the data disposed in the second region. A demodulation unit ( 204 ) demodulates the data on the basis of the reference signals. The demodulation unit ( 204 ) demodulates the data using the reference signals disposed in the second region.

Description:
TECHNICAL FIELD 
       [0001]    The claimed invention relates to a relay station, a base station, and a communication method. 
       BACKGROUND ART 
       [0002]    In recent years, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data along with the adoption of multimedia information in cellular mobile communication systems. In order to achieve transmission of a large amount of data, studies have been actively carried out on technologies that achieve a high data rate using a high-frequency radio band. 
         [0003]    When a high-frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher data rate can be expected with a short distance. 
         [0004]    Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station” or “evolved Node B (eNB)”) decreases when a mobile communication system using a high-frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations. 
         [0005]    In order to meet such a demand, studies have been carried out on a relay transmission technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station” or “relay node (RN)”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station” or “user equipment (UE)”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station. For example, relay station  20  is installed between base station  10  and mobile station  30 , and base station  10  and mobile station  30  communicate with each other via relay station  20  in a radio relay system illustrated in  FIGS. 1A and 1B . 
         [0006]    (TD Relay) 
         [0007]    The Long Term Evolution Advanced (LTE-A) system for which the introduction of relay technique has been studied is required to maintain compatibility with Long Term Evolution (LTE) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well. In the LTE-A system, studies have been carried out on configuring MBMS single frequency network (MBSFN) subframes for downlink (hereinafter, abbreviated as “DL”) transmission from a base station to a relay station in order to achieve mutual compatibility with LTE. 
         [0008]    The base station and mobile station communicate with each other via the relay station herein using time-division relay (i.e., TD relay).  FIGS. 1A and 1B  are diagrams provided for describing the TD relay.  FIG. 1A  is a conceptual diagram for describing downlink TD relay, and  FIG. 1B  is a conceptual diagram for describing uplink TD relay. In the TD relay (also referred to as “half duplex relay” or “Type 1 relay”), transmission from a base station to a relay station and transmission from the relay station to a mobile station are divided in time. 
         [0009]    As illustrated in  FIG. 1B , transmission is performed from mobile station  30  to relay station  20  on the access link in subframe # 2  while communication from relay station  20  to base station  10  is performed on the backhaul link in subframe # 3  in uplink. In subframe # 4 , transmission is performed from mobile station  30  to relay station  20  again. 
         [0010]    Likewise, as illustrated in  FIG. 1A , transmission is performed from relay station  20  to mobile station  30  on the access link in subframe # 2  while communication is performed from base station  10  to relay station  20  on the backhaul link in subframe # 3  in downlink. In subframe # 4 , transmission is performed from relay station  20  to mobile station  30  again. 
         [0011]    As described above, dividing communication into the backhaul communication and access link communication of relay station  20  in the time domain enables dividing the time into transmission time and reception time for relay station  20 . Accordingly, relay station  20  can relay signals without being affected by coupling wave between a transmission antenna and a reception antenna. 
         [0012]    MBSFN subframes are configured for the access links in downlink. “MBSFN subframes” are subframes defined for transmitting multimedia broadcast multicast service (MBMS) data. 
         [0013]    LTE terminals are configured not to use reference signals in MBSFN subframes. 
         [0014]    In this respect, there has been proposed a technique that configures access link subframes that overlap a backhaul link subframe used by a relay station to communicate with a base station, for MBSFN subframes in LTE-A. This proposal allows LTE terminals to avoid erroneously detecting reference signals. 
         [0015]      FIG. 2  illustrates an example of control signals and a data assignment state for each of base station  10 , relay station  20 , and mobile station  30  when subframes of the LTE system are used. 
         [0016]    As illustrated in  FIG. 2 , downlink control signals transmitted or received in each station are mapped in a control signal region in the top part of a subframe (hereinafter, referred to as “Physical Downlink Control Channel (PDCCH) region”). More specifically, both of base station  10  and relay station  20  transmit control signals in the PDCCH region in the top part of the subframe. With reference to relay station  20 , downlink control signals (PDCCH) have to be transmitted to mobile station  30  even in an MBSFN subframe. Accordingly, relay station  20  transmits downlink control signals to mobile station  30  and then switches the processing to reception processing to prepare for receiving signals transmitted from base station  10 . Base station  10 , however, transmits downlink control signals intended for relay station  20  at the time relay station  20  transmits downlink control signals to mobile station  30 . For this reason, relay station  20  cannot receive the downlink control signals transmitted from base station  10 . In order to avoid such inconvenience, studies have been carried out on providing a region in which downlink control signals for relay stations are mapped (i.e., relay PDCCH (R-PDCCH) region) in a data region as illustrated in  FIG. 2  in LTE-A. 
         [0017]    (Control Signals) 
         [0018]    In LTE, a base station transmits control signals to a mobile station using a downlink control channel such as PDCCH, for example. PDCCH includes DL grant indicating DL data (i.e., Physical Downlink Shared Channel (PDSCH)) assignment and UL grant indicating UL data (i.e., Physical Uplink Shared Channel (PUSCH)) assignment. 
         [0019]    In LTE-A, studies have been carried out on including DL grant and UL grant in R-PDCCH. In addition, studies have been carried out on mapping the DL grant in the first slot and the UL grant in the second slot for R-PDCCH (see, Non-Patent Literature (hereinafter, referred to as “NPL”) 1). Mapping the DL grant only in the first slot reduces a delay in decoding the DL grant and allows relay stations to prepare for ACK/NACK transmission for DL data (e.g., transmission performed in the fourth subframe following the reception of DL grant in FDD). 
         [0020]    In addition, studies have been carried out on allocating, for each relay station, a different physical layer resource block (i.e., physical resource block (PRB)) on which an R-PDCCH region is provided as illustrated in  FIG. 3 . In  FIG. 3 , the vertical axis indicates frequency and the horizontal axis indicates time. In  FIG. 3 , the R-PDCCH for relay station RN  1  is mapped on PRB # 0  and the R-PDCCH for relay station RN  2  is mapped on PRBs # 6  and  7  in the same subframe, for example. Each relay station finds the downlink control signals intended for the relay station by performing blind-decoding on downlink control signals transmitted using an R-PDCCH region from a base station within a resource region indicated using higher layer signaling from the base station (i.e., R-PDCCH search space). 
         [0021]    (DM-RS Arrangement) 
         [0022]    In LTE-A, studies have been carried out on introduction of demodulation reference signals (DM-RS) mainly for the purpose of directing a different beam for each mobile station and relay station. DM-RS is expressed by a combination of a plurality of ports defined as mutually orthogonal resources (e.g., ports  7 ,  8 ,  9 , and  10 ) and scrambling IDs (SC-ID: SC-IDs  0  and  1 ) defined by randomization using non-orthogonal but different sequences. When DM-RS is used for channel estimation, a base station can use an optional beam by applying the same beam (i.e., same precoding) to DM-RS and control signals and data signals that pertain to the DM-RS. 
         [0023]      FIGS. 4A and 4B  are provided for describing mapping of DM-RS used for channel estimation. In  FIGS. 4A  and B, the vertical axis indicates frequency and the horizontal axis indicates time. Normally, DM-RS is mapped at the last two symbols of each slot (first slot and second slot) as illustrated in  FIG. 4A . As illustrated in  FIG. 4A , DM-RS (referred to as DM-RS ports  7  and  8  in  FIGS. 4A  and B) is mapped at OFDM symbols # 5  and # 6  as well as OFDM symbols # 11  and # 12  in a normal subframe. In addition, as illustrated in  FIG. 4B , studies have been carried out on signal arrangement without using the last symbol of the second slot (i.e., OFDM symbol # 12 ) in the abovementioned TD relay for a case where UL data assignment is present in the immediately following subframe and the restrictions on signal transmission timing from a relay station to a base station are stringent including a case where the distance between the relay station and base station is long, for example. In  FIG. 4B , DM-RS is mapped only in first slot without being mapped in the second slot for signal arrangement without using OFDM symbol # 12 . 
         [0024]    In addition, since relay stations perform R-PDCCH blind-decoding as described above, the DM-RS used on R-PDCCH is fixed to port  7  and SC-ID=0, for example. Thus, each relay station can omit blind-decoding for ports other than port  7  and SC-IDs other than SC-ID=0, thereby reducing the number of blind-decoding attempts. In this manner, a simplification of the processing is achieved. 
         [0025]    On the other hand, regarding PDSCH, each base station can explicitly report the port used for PDSCH using DL grant. 
         [0026]    Thus, each base station can perform a Single User Multiple-Input Multiple-Output (SU-MIMO) operation to transmit PDSCH intended for the same relay station using a plurality of beams, or a Multi User-MIMO (MU-MIMO) operation to transmit PDSCHs intended for a plurality of relay stations using different beams, respectively. 
         [0027]    Moreover, since no other signals are transmitted in the region of the R-PDCCH region in which DM-RS is transmitted (hereinafter, referred to as “DL grant region”), rank-1 transmission is used for DL grant regardless of the number of transmission beams for PDSCH (hereinafter, referred to as “rank”). For this reason, the accuracy in detecting DL grants in each base station can be improved by application of power boost that allocates all the power assigned to the resource on which DL grant is mapped to DM-RS and DL grant. 
         [0028]    (PRB Bundling) 
         [0029]    In addition, studies have been carried out on PRB bundling as a technique for improving the accuracy of channel estimation. PRB bundling is a technique that uses the same precoding for a plurality of PRBs adjacent to each other when a different beam is directed to each relay station and mobile station using DM-RS, thereby improving the accuracy of channel estimation (see, section 7.1.6.5 of NPL 2, for example). In PRB bundling, the receiving side performs averaging or interpolation of channel estimation values calculated using DM-RS mapped on RBs, in units of sets of adjacent PRBs to which the same precoding is applied (hereinafter, referred to as “Precoding Resource Block Groups (PRGs)”), for example. 
         [0030]    (PRG Size) 
         [0031]    The number of adjacent PRBs to which the same precoding is applied is referred to as “Precoding Resource Block Group (PRG) size.” The value configured for PRG size varies depending on the number of RBs included in the system bandwidth (hereinafter, referred to as “RBs”).  FIG. 5  illustrates a correspondence between the number of RBs in the system band, the PRG size and RBG size (Resource Block Group size). The term “RBG” as used herein refers to a unit obtained by bundling one or more RBs. As illustrated in  FIG. 5 , the PRG size and RBG size are determined according to the number of RBs in the system band. 
         [0032]    As the PRG size increases, the DM-RS to which the same precoding is applied increases. As a result, the reception performance with respect to DM-RS (e.g., Signal to Noise Ratio (SNR)) can be increased. Meanwhile, as the PRG size increases, the influence of frequency-selective fading is more likely to become uneven on each DM-RS, which in turn increases the possibility that the optimum beam is not configured even when the DM-RS is used. For this reason, even when the number of RBs in the system band is large, the PRG size is configured to be 2 or 3, considering a tradeoff between the abovementioned “effect of increasing the reception performance with respect to reference signals” and “influence of frequency-selective fading” as illustrated in  FIG. 5 . 
         [0033]    (R-PDCCH Aggregation Size) 
         [0034]    In addition, in order to adjust the coding rates of DL and UL grants according to the channel quality as in the case of PDCCH in LTE Release 8, studies have been carried out on preparing a plurality of aggregation sizes (may be referred to as “control channel element (CCE) aggregation size”).  FIG. 6  is a conceptual diagram illustrating R-PDCCH aggregation sizes. In  FIG. 6 , each vertical axis indicates frequency and each horizontal axis indicates time. As illustrated in  FIG. 6 , as the R-PDCCH aggregation size is reduced from 8, 4, 2 to 1, the coding rate is increased. In other words, a smaller aggregation size is suitable when the channel quality between a base station and a relay station is good. 
         [0035]    For example, a base station estimates the channel quality between the base station and a relay station and determines the R-PDCCH aggregation size and transmits signals in accordance with the determined aggregation size to the relay station. Meanwhile, the relay station is not previously informed of the aggregation size, which is changed for each subframe. In this respect, the relay station performs blind-decoding for each of the plurality of aggregation sizes (e.g., aggregation sizes 1, 2, 4, and 8 in  FIG. 6 ). 
       CITATION LIST 
     Non-Patent Literatures 
       [0000]    
       
         NPL 1 
         3GPP TS 36.216 v10.1.0, “Physical Layer for relaying operation (release 10),” December, 2010 
         NPL 2 
         3GPP TS 36.213 v10.0.1, “Physical Layer procedures (release 10),” December, 2010 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0040]    In an attempt to apply the PRB bundling described above to communication between a base station and a relay station without any modification, the following problems occur when DM-RS is mapped only in the first slot (see,  FIG. 4B ). 
         [0041]    For example, DM-RS to be transmitted in an R-PDCCH region (i.e., first slot of PRB # 0 ) in which DL grant is mapped has its rank=1 and becomes power-boosted signals in the PRG (PRBs # 0  to # 2 ) illustrated in  FIG. 7 . Meanwhile, as illustrated in  FIG. 7 , DM-RS to be transmitted in the data region in which PDSCH is transmitted (hereinafter, referred to as “PDSCH region”) may become signals of a plurality of ranks as in the case of data signals (i.e., PDSCH signals). 
         [0042]    During demodulation, the relay station averages the channel estimation value obtained using the DM-RS in the R-PDCCH region (i.e., DM-RS mapped on PRB # 0 ) and the channel estimation value obtained using the DM-RS in the PDSCH regions (i.e., DM-RS mapped on PRBs # 1  and # 2 ) by application of PRB bundling to PRBs # 0  to # 2  illustrated in  FIG. 7 . The relay station herein performs channel estimation for PDSCH signals using the DM-RS to which the same rank (i.e., rank-2) as that for PDSCH signals, i.e., the same precoding is applied, in PRBs # 1  and # 2  as illustrated in  FIG. 7 . Thus, the accuracy of channel estimation for PDSCH signals is good in PRBs # 1  and # 2 . On the other hand, since the rank is fixed (i.e., rank-1) in PRB # 0 , there is a possibility that the relay station may perform channel estimation for PDSCH signals using the DM-RS to which a rank different from that for PDSCH (i.e., different precoding) is applied. For this reason, it is likely that the accuracy of channel estimation for PDSCH signals in PRB # 0  in which the DL grant indicating R-PDCCH downlink data assignment is mapped becomes poor compared to PRBs # 1  and # 2 .” 
         [0043]    It is an object of the claimed invention to provide a relay station, a base station, and a communication method that are capable of improving the accuracy of channel estimation for PDSCH even when DL grant indicating R-PDCCH downlink data assignment is mapped therewith. 
       Solution to Problem 
       [0044]    A relay station according to an aspect of the claimed invention in a communication system in which a base station and a mobile station communicate with each other via the relay station, includes: a receiving section that receives control information, data, and reference signals transmitted from the base station, the reference signals being mapped in a first region and a second region in a precoding resource block (RB) group (PRG) formed of one or more resource blocks (RBs) among a plurality of RBs formed by dividing a frequency region, the control information being mapped in the first region, and the data being mapped in the second region; and a demodulation section that demodulates the data on the basis of the reference signals, in which the demodulation section demodulates the data using the reference signals mapped in the second region. 
         [0045]    A base station according to an aspect of the claimed invention in a communication system in which the base station and a mobile station communicate with each other via a relay station, includes: a mapping section that maps reference signals in a first region and a second region in a precoding resource block (RB) group (PRG), maps control information in the first region, and maps data in the second region, the PRG being formed of one or more resource blocks (RBs) among a plurality of RBs formed by dividing a frequency region; and a transmission section that transmits the mapped reference signals, control information and data to the relay station, in which the mapping section does not map the data in a region that is in the same frequency region as an RB forming the first region and that is subsequent to the RB in the time domain. 
         [0046]    A communication method according to an aspect of the claimed invention in a communication system in which a base station and a mobile station communicate with each other via a relay station, includes: receiving, in the relay station, control information, data, and reference signals transmitted from the base station, the reference signals being mapped in a first region and a second region in a precoding resource block (RB) group (PRG) formed of one or more resource blocks (RBs) among a plurality of RBs formed by dividing a frequency region, the control information being mapped in the first region, and the data being mapped in the second region; and demodulating the data on the basis of the reference signals in the relay station, in which the relay station demodulates the data using the reference signals mapped in the second region. 
       Advantageous Effects of Invention 
       [0047]    According to the claimed invention, it is possible to improve the accuracy of channel estimation for PDSCH even when DL grant indicating R-PDCCH downlink data assignment is mapped therewith. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0048]      FIGS. 1A and 1B  are diagrams provided for describing time-division relay (TD relay); 
           [0049]      FIG. 2  is a diagram illustrating an example of control signals and a data assignment state for each of a base station, a relay station, and a mobile station; 
           [0050]      FIG. 3  is a diagram illustrating an R-PDCCH mapping example; 
           [0051]      FIGS. 4A and 4B  are diagrams each illustrating a DM-RS mapping example; 
           [0052]      FIG. 5  is a diagram representing a correspondence between the number of RBs in a system band, the RPG size and RBG size; 
           [0053]      FIG. 6  is a diagram representing R-PDCCH aggregation sizes; 
           [0054]      FIG. 7  is a diagram provided for describing a difference in accuracy of channel estimation on a plurality of PRBs in the same PRG; 
           [0055]      FIG. 8  is a main configuration diagram of a relay station according to Embodiment 1 of the claimed invention; 
           [0056]      FIG. 9  is a block diagram illustrating a configuration of a base station according to Embodiment 1 of the claimed invention; 
           [0057]      FIG. 10  is a block diagram illustrating a configuration of the relay station according to Embodiment 1 of the claimed invention; 
           [0058]      FIG. 11  is a diagram provided for describing channel estimation processing according to Embodiment 1 of the claimed invention; 
           [0059]      FIG. 12  is a main configuration diagram of a base station according to Embodiment 2 of the claimed invention; 
           [0060]      FIG. 13  is a block diagram illustrating a configuration of the base station according to Embodiment 2 of the claimed invention; 
           [0061]      FIG. 14  is a block diagram illustrating a configuration of a relay station according to Embodiment 2 of the claimed invention; 
           [0062]      FIG. 15  is a diagram provided for describing data signal assignment processing according to Embodiment 2 of the claimed invention; 
           [0063]      FIG. 16  is a block diagram illustrating a configuration of a base station according to Embodiment 3 of the claimed invention; 
           [0064]      FIG. 17  is a block diagram illustrating a configuration of a relay station according to Embodiment 3 of the claimed invention; 
           [0065]      FIGS. 18A and 18B  are diagrams provided for describing data signal assignment processing according to Embodiment 3 of the claimed invention; and 
           [0066]      FIG. 19  is a diagram illustrating a variation of Embodiment 3 of the claimed invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0067]    Hereinafter, embodiments of the claimed invention will be described in detail with reference to the accompanying drawings. Throughout the embodiments, the same elements are assigned the same reference numerals, and any duplicate description of the elements is omitted. 
         [0068]    In each of the embodiments of the claimed invention, a description will be provided regarding signal arrangement on subframes without using the last symbol of the second slot (e.g., OFDM symbol # 12  in  FIG. 4B ) as illustrated in  FIG. 4B . More specifically, DM-RS is not mapped on the resources corresponding to the second slot in the R-PDCCH region and is only mapped on the resources corresponding to the first slot in the following description as illustrated in  FIG. 4B . 
       Embodiment 1 
     Overview of Communication System 
       [0069]    A communication system according to Embodiment 1 includes base station  100  and relay station  200 . Base station  100  is an LTE-A base station, and relay station  200  is an LTE-A relay station, for example. In the communication system according to Embodiment 1, base station  100  and each mobile station communicate with each other via relay station  200 . 
         [0070]    In addition, in the communication system according to Embodiment 1, base station  100  transmits, using the same precoding, data signals (i.e., PDSCH signals) in units of precoding resource block groups (PRGs) each formed of one or more resource blocks among a plurality of resource blocks formed by dividing a frequency region (i.e., PRBs), the data signals being mapped on a plurality of adjacent resource blocks (PRBs). The plurality of adjacent PRBs including the data signals (PDSCH signals) to be transmitted using the same precoding form the same PRG. Meanwhile, base station  100  improves the accuracy in detecting DL grants by application of precoding different from that for PDSCH signals (i.e., rank-1 transmission) to only the first slot of the PRB in which DL grant is mapped. 
         [0071]      FIG. 8  is a main configuration diagram of relay station  200  according to Embodiment 1. In relay station  200 , signal demultiplexing section  203  receives control information, data, and reference signals transmitted from base station  100 , and demodulation section  204  demodulates the data on the basis of the reference signals. The reference signals from base station  100  are mapped in a first region and a second region in a precoding resource block group (PRG) formed of one or more PRBs among a plurality of PRBs formed by dividing a frequency region. The control information from base station  100  is mapped in the first region and the data is mapped in the second region. Demodulation section  204  herein demodulates the data using the reference signals mapped in the second region. 
         [0072]    (Configuration of Base Station) 
         [0073]      FIG. 9  is a block diagram illustrating a configuration of base station  100  according to Embodiment 1. In  FIG. 9 , base station  100  includes antenna  101 , radio receiving section  102 , demodulation section  103 , error correction decoding section  104 , R-PDCCH search space determining section  105 , error correction coding section  106 , modulation section  107 , signal assignment section  108 , control signal coding section  109 , control signal assignment section  110 , precoding section  111 , radio transmitting sections  112 - 1  and  112 - 2 , and antennas  113 - 1  and  113 - 2 . In addition, signal assignment section  108  and control signal assignment section  110  serve as a mapping section that maps reference signals (such as DM-RS), control signals (such as DL grant), and data on the resources. 
         [0074]    Radio receiving section  102  receives, via antenna  101 , signals transmitted from relay station  200  or a mobile station, performs radio processing such as down-conversion on the signals and then outputs the processed signals to demodulation section  103 . 
         [0075]    Demodulation section  103  demodulates the signals received from radio receiving section  102  and outputs the demodulated signals to error correction decoding section  104 . 
         [0076]    Error correction decoding section  104  decodes the signals received from demodulation section  103  and outputs the resultant received signals. 
         [0077]    R-PDCCH search space determining section  105  determines the PRB bundling size (i.e., PRG size) and RBG size on the basis of system bandwidth information indicating a system bandwidth (hereinafter, referred to as “BW information”). For example, R-PDCCH search space determining section  105  may determine the PRB bundling size (i.e., PRG size) and RBG size according to the correspondence illustrated in  FIG. 5 . 
         [0078]    Moreover, R-PDCCH search space determining section  105  determines the search space (i.e., R-PDCCH search space) as a candidate for a resource region in which control signals for relay station  200  (i.e., R-PDCCH signals such as control information including DL grant and/or the like) are transmitted (i.e., R-PDCCH region). R-PDCCH search space determining section  105  outputs the search space information indicating the determined R-PDCCH search space to error correction coding section  106 . In addition, R-PDCCH search space determining section  105  outputs the PRB bundling size, RBG size, and search space information to control signal assignment section  110 . R-PDCCH search space determining section  105  generates search space information on a per RBG or PRB basis. 
         [0079]    Error correction coding section  106  takes transmission signals (mainly data signals), and the R-PDCCH search space information received from R-PDCCH search space determining section  105 , as input, and performs error correction coding on the input signals and outputs the resultant signals to modulation section  107 . 
         [0080]    Modulation section  107  modulates the signals received from error correction coding section  106  and outputs the modulated signals to signal assignment section 108. 
         [0081]    Signal assignment section  108  assigns the signals received from modulation section  107  to a resource and outputs the signals to precoding section  111 . For example, signal assignment section  108  maps the signals (including data signals) in a resource region indicated by the DL grant in the downlink control signals (i.e., R-PDCCH signals) intended for relay station  200 . For example, signal assignment section  108  maps the transmission signals (including data signals (i.e., PDSCH signals)) in a resource region other than the resource region in which R-PDCCH signals (i.e., DL grant) are mapped in the PRBs forming the PRG (or RBG) for relay station  200 . 
         [0082]    Control signal coding section  109  encodes the control signals (R-PDCCH signals). The control signals include control information on the data signals (PDSCH signals) (such as DL grant, UL grant, the number of ranks, coding rates and modulation scheme), for example. Control signal coding section  109  employs a configuration including aggregation size determining section  1091  and coding section  1092 . 
         [0083]    More specifically, aggregation size determining section  1091  determines the aggregation size for control signals according to eNB-RN channel quality information indicating the channel quality between base station  100  and relay station  200 . As illustrated in  FIG. 6 , aggregation size determining section  1091  determines one of aggregation sizes=1, 2, 4, and 8 according to the eNB-RN channel quality information, for example. Aggregation size determining section  1091  outputs the determined aggregation size to coding section  1092  and R-PDCCH PRB determining section  1101 . 
         [0084]    Coding section  1092  encodes control signals (R-PDCCH signals) according to the aggregation size determined by aggregation size determining section  1091  and outputs the coded control signals to control signal assignment section  110 . 
         [0085]    Control signal assignment section  110  assigns the control signals (R-PDCCH signals) received from control signal coding section  109  to a resource and outputs the control signals to precoding section  111 . Control signal assignment section  110  employs a configuration including R-PDCCH PRB determining section  1101  and assignment section  1102 . 
         [0086]    More specifically, R-PDCCH PRB determining section  1101  determines the PRB on which the control signals (R-PDCCH signals) are actually mapped among PRBs in the R-PDCCH search space on the basis of the eNB-RN channel quality information, the R-PDCCH aggregation size received from aggregation size determining section  1091 , and the PRB bundling size, RBG size and search space information received from R-PDCCH search space determining section  105 . For example, R-PDCCH PRB determining section  1101  selects a PRB subjected to PRB bundling among PRBs in the R-PDCCH search space and determines the PRB to be used for mapping the control signals (R-PDCCH signals). 
         [0087]    Assignment section  1102  assigns the control signals (R-PDCCH signals) to the PRB determined by R-PDCCH PRB determining section  1101 . In other words, assignment section  1102  assigns the control signals (R-PDCCH signals) for relay station  200  to the PRB subjected to PRB bundling among the PRBs in the R-PDCCH search space. The control signals (R-PDCCH) assigned to the PRB are outputted to precoding section 111. 
         [0088]    Moreover, signal assignment section  108  and control signal assignment section  110  arrange the reference signals used for channel estimation (DM-RS) on a resource. For example, signal assignment section  108  and control signal assignment section  110  map the reference signals (e.g., DM-RS) at the last two symbols of each slot (both of the first and second slots or only the first slot) as illustrated in  FIG. 4A  or  4 B. 
         [0089]    Precoding section  111  performs precoding processing on the transmission signals (data signals) assigned to the resource by signal assignment section  108 , the control signals (R-PDCCH signals) assigned to the resource by control signal assignment section  110 , and the reference signals (DM-RS) assigned to the resource by signal assignment section  108  and control signal assignment section  110 . Precoding section  111  distributes the signals that have been subjected to the precoding processing to radio transmission sections  112 - 1  and  112 - 2 . 
         [0090]    Precoding section  111  applies scheduling according to the rank configured in a configuration section (not illustrated) in the resource region in which data signals (PDSCH signals) are mapped (i.e., data region). For example, when the rank for the data signals (PDSCH signals)=2, precoding section  111  configures the power for each stream of signals to be half (½) of the previously set transmission power. Accordingly, base station  100  keeps the total transmission power constant regardless of the rank. Precoding section  111  applies the same precoding in the region in which the data signals (PDSCH signals) in the same PRG are mapped. In addition, precoding section  111  configures precoding to be applied to data signals (PDSCH signals) on a per subframe basis, for example. Meanwhile, as described above, since the rank of the resource region in which the control signals (R-PDCCH) are mapped is fixed to 1, precoding section  111  does not apply scaling to the control signals. More specifically, power boost is applied to R-PDCCH, so that the control signals (R-PDCCH) are transmitted with large power compared to the data signals (PDSCH signals). 
         [0091]    Radio transmitting sections  112 - 1  and  112 - 2  perform radio transmission processing such as up-conversion on the signals received from precoding section  111  and transmits the processed signals via antennas  113 - 1  and  113 - 2 . 
         [0092]    (Configuration of Relay Station  200 ) 
         [0093]      FIG. 10  is a block diagram illustrating a configuration of relay station  200  according to Embodiment 1. In  FIG. 10 , relay station  200  includes antennas  201 - 1  and  201 - 2 , radio receiving sections  202 - 1  and  202 - 2 , signal demultiplexing section  203 , demodulation section  204 , error correction decoding section  205 , blind-decoding determining section  206 , R-PDCCH receiving section  207 , error correction coding section  208 , modulation section  209 , signal assignment section  210 , radio transmitting section  211 , and antenna  212 . Blind-decoding determining section  206  and R-PDCCH receiving section  207  form blind-decoding section. 
         [0094]    Radio receiving sections  202 - 1  and  202 - 2  receive, respectively via antennas  201 - 1  and  201 - 2 , signals transmitted from base station  100  or a mobile station, perform radio processing such as down-conversion on the signals, and outputs the processed signals to signal demultiplexing section  203 . 
         [0095]    Signal demultiplexing section  203  demultiplexes the received signals inputted from radio receiving sections  202 - 1  and  202 - 2  into blind-decoding target signals on the basis of blind decoding information received from blind-decoding determining section  206 . The blind decoding information herein refers to information that indicates a blind-decoding target PRB among PRBs in the R-PDCCH search space and varies according to the aggregation size. The blind-decoding target PRB is a PRB on which the control signals (R-PDCCH signals) intended for relay station  200  are mapped. Signal demultiplexing section  203  outputs the blind-decoding target signals obtained by demultiplexing to R-PDCCH receiving section  207 . 
         [0096]    In addition, signal demultiplexing section  203  extracts data signals on the basis of the DL grant received from R-PDCCH receiving section  207 . Signal demultiplexing section  203  applies MIMO spatial demultiplexing processing according to the rank received from R-PDCCH receiving section  207  to data for the relay station to demultiplex the data signals into data streams. During demultiplexing, signal demultiplexing section  203  extracts the reference signals (DM-RS) mapped in each resource region and performs channel estimation for the data signals (i.e., PDSCH signals) using the extracted reference signals. Signal demultiplexing section  203  herein performs channel estimation using the reference signals (DM-RS) mapped in the resource region in which the data signals (PDSCH signals) are mapped. Signal demultiplexing section  203  then identifies the precoding applied in base station  200  on the basis of the estimated channel estimation value. 
         [0097]    Moreover, signal demultiplexing section  203  outputs the data streams obtained by demultiplexing and the channel estimation value to demodulation section  204 . The channel estimation value used in MIMO spatial demultiplexing processing in signal demultiplexing section  203  will be described in detail, hereinafter. 
         [0098]    Demodulation section  204  demodulates the data signals (PDSCH signals) on the basis of the channel estimation value received from signal demultiplexing section  203  and outputs the demodulated signals to error correction decoding section  205 . More specifically, demodulation section  204  demodulates the signals on the basis of the reference signals (DM-RS) mapped on each resource. Demodulation section  204  herein demodulates the data signals using the reference signals (DM-RS) mapped on a resource region on which the data signals (PDSCH signals) are mapped. 
         [0099]    Error correction decoding section  205  decodes the signals received from demodulation section  204  and outputs the R-PDCCH search space information in the decoded signals to blind-decoding determining section  206  and also outputs the demodulated signals to error correction coding section  208 . Relay station  200  relays the signals transmitted from base station  100 , through the transmission of signals by error correction coding section  208 , demodulation section  209 , signal assignment section  210 , and radio transmitting section  211 . 
         [0100]    Blind-decoding determining section  206  determines the RBG size and PRG size (i.e., PRB bundling size) on the basis of the system bandwidth information (i.e., BW information) when the R-PDCCH search space information (R-PDCCH assignment region candidate information) received from error correction decoding section  205  is in units of RBGs. In addition, blind-decoding determining section  206  configures a blind-decoding target PRB for each aggregation size among PRBs included in the search space and generates blind-decoding information indicating the blind-decoding target PRB. The blind-decoding information is outputted to signal demultiplexing section  203  and R-PDCCH receiving section  207 . 
         [0101]    R-PDCCH receiving section  207  performs blind-decoding on the blind-decoding target signals received from signal demultiplexing section  203  on the basis of the blind-decoding information received from blind-decoding determining section  206  to detect the R-PDCCH signals intended for relay station  200 . For example, R-PDCCH receiving section  207  performs channel estimation using the DM-RS mapped on the blind-decoding target PRB and extracts the R-PDCCH signals on the basis of the obtained channel estimation value. R-PDCCH receiving section  207  outputs DL grant and a rank included in the detected R-PDCCH signals to signal demultiplexing section  203 . In addition, R-PDCCH receiving section  207  outputs UL grant included in the R-PDCCH signals to signal assignment section  210 , for example. 
         [0102]    Error correction coding section  208  performs error correction coding on the signals received from error correction decoding section  205  (i.e., signals transmitted from base station  100 ) and outputs the resultant signals to modulation section 209. 
         [0103]    Modulation section  209  modulates the signals received from error correction coding section  208  and outputs the modulated signals to signal assignment section  210 . 
         [0104]    Signal assignment section  210  assigns the modulated signals received from modulation section  209  to a resource according to the UL grant reported from base station  100  (e.g., reported using R-PDCCH signals) and outputs the resultant signals to radio transmitting section  211 . 
         [0105]    Radio transmitting section  211  performs radio transmission processing such as up-conversion on the signals received from signal assignment section  210  and transmits the processed signals via antenna  211 . 
         [0106]    (Operations of Base Station  100  and Relay Station  200 ) 
         [0107]    A description will be provided regarding operations of base station  100  and relay station  200  respectively including the configurations described above. The processing to calculate channel estimation values will be particularly described herein. 
         [0108]    R-PDCCH search space determining section  105  determines the PRB bundling size and RBG size on the basis of BW information in base station  100 . For example, R-PDCCH search space determining section  105  determines that RBG size=3 and PRG size=3 in  FIG. 11 . More specifically, a PRG in which the same precoding is applied in units of RBGs each formed of three PRBs, namely, PRBs # 0  to # 2  is formed. 
         [0109]    In addition, R-PDCCH search space determining section  105  determines the search space which is a resource region candidate for mapping control signals (R-PDCCH signals). For example, R-PDCCH search space determining section  105  determines the R-PDCCH search space within PRB # 0  in  FIG. 11 . It should be noted that, when DL grant is mapped in first slot in the R-PDCCH region, the R-PDCCH search space is configured in the first slot of PRB # 0 . The search space information indicating the determined search space is transmitted to relay station  200 . 
         [0110]    In addition, the aggregation size of the R-PDCCH signals is set to 1 in  FIG. 11 . 
         [0111]    In  FIG. 11 , control signal assignment section  110  maps the DL grant (R-PDCCH signals) in the first slot of PRB # 0 , and signal assignment section  108  maps the data signals (PDSCH signals) to the second slot of PRB # 0 , as well as PRB # 1  and PRB # 2 . More specifically, base station  100  maps the DL grant (R-PDCCH signals) in the resource region corresponding to the first slot of PRB # 0  (i.e., first region) and the data signals (PDSCH signals) to the second slot of PRB # 0  other than the first slot of PRB # 0 , as well as PRB # 1  and PRB # 2  (i.e., second region). 
         [0112]    In  FIG. 11 , the resource region in which data signals are mapped (i.e., second region) is formed of the resource region that has the same frequency as PRB # 0  forming the resource region in which DL grant is mapped (i.e., first region) and that is subsequent to the resource region in the time domain (second slot of PRB # 0 , i.e., third region), and the resource regions, each having a frequency different from the resource region in which the DL grant is mapped (first region), (PRBs # 1  and # 2 , i.e., fourth region). 
         [0113]    In addition, signal assignment section  108  and control signal assignment section  110  map the reference signals (DM-RS) for channel estimation at the last two symbols of the first slots in PRBs # 0  to # 2  as illustrated in  FIG. 11 . In other words, the reference signals (DM-RS) used for channel estimation are mapped on both of the regions in which the DL grant is mapped (first region) and in which the data signals are mapped (second region). 
         [0114]    Next, as illustrated in  FIG. 11 , precoding section  111  performs precoding processing of rank=1 on the DL grant and DM-RS in the first slot of PRB # 0  in which the DL grant is mapped (first region) and applies power boost to the DL grant and DM-RS. Meanwhile, as illustrated in  FIG. 11 , precoding section  111  performs precoding processing of the configured rank (e.g., rank=2 in  FIG. 11 ) on the data signals and DM-RS in the second slot of PRB # 0  as well as PRBs # 1  and # 2  in which the data signals are mapped (second region). 
         [0115]    More specifically, the data signals (PDSCH signals) mapped in the PRG (in units of RBGs) are transmitted using the same precoding. In addition, the precoding (rank) to be applied to the DM-RS is different between the resource region in which the DL grant is mapped (first region) and the resource regions in which the data signals are mapped (second region). 
         [0116]    Meanwhile, in  FIG. 11 , blind-decoding determining section  206  determines that RBG size=3 and PRB bundling size=3 on the basis of BW information as in the case of base station  100 . In addition, blind-decoding determining section  206  configures the first slot of PRB # 0  to be the blind-decoding target PRB on the basis of the search space information from error correction decoding section  205 . 
         [0117]    Subsequently, signal demultiplexing section  203  demultiplexes the received signals into the signals mapped in the first slot of PRB # 0 , which is the blind-decoding target, and outputs the signals obtained by demultiplexing (i.e., blind-decoding target signals) to R-PDCCH receiving section 207. 
         [0118]    R-PDCCH receiving section  207  receives the signals mapped on the PRB # 0  (first slot), which is the blind-decoding target, from signal demultiplexing section  203  and performs blind-decoding on the signals (i.e., blind-decoding target) to detect the R-PDCCH signals intended for relay station  200 . R-PDCCH receiving section  207  then outputs the DL grant (and rank) in the detected R-PDCCH signals to signal demultiplexing section  203 . 
         [0119]    Signal demultiplexing section  203  extracts the data signals intended for relay station  200 , on the basis of the DL grant received from R-PDCCH receiving section  207 . Moreover, signal demultiplexing section  203  performs MIMO spatial demultiplexing processing in accordance with the rank received from R-PDCCH receiving section  207  on the data signals (PDSCH signals). 
         [0120]    During the processing, signal demultiplexing section  203  extracts a set of the reference signals (DM-RS) mapped in each of the resource regions and performs channel estimation for the data signals (PDSCH signals). More specifically, signal demultiplexing section  203  acquires at least one channel estimation value on a per PRB basis from at least one set of reference signals (DM-RS) that is mapped on each PRB. Signal demultiplexing section  203  herein performs channel estimation using the reference signals (DM-RS) mapped in the resource regions in which data signals (PDSCH signals) are mapped. 
         [0121]    In  FIG. 11 , signal demultiplexing section  203  performs channel estimation using only the reference signals (DM-RS) mapped on PRBs # 1  and # 2  corresponding to the resource regions in which the data signals are mapped (second region). Signal demultiplexing section  203  first calculates a channel estimation value for each PRB using the sets of DM-RS respectively mapped on PRBs # 1  and # 2 , for example. Subsequently, signal demultiplexing section  203  averages the channel estimation values obtained respectively for PRBs # 1  and # 2  to calculate an average. As illustrated in  FIG. 11 , signal demultiplexing section  203  uses the average between the channel estimation values on PRBs # 1  and # 2  not only as the channel estimation value on PRBs # 1  and # 2  but also the channel estimation value on PRB # 0 . 
         [0122]    More specifically, signal demultiplexing section  203  performs channel estimation for the data signals (PDSHC signals) mapped in the resource region that has the same frequency as PRB # 0  forming the resource region in which DL grant is mapped (first region) and that is subsequent to the resource region in the time domain (second slot of PRB # 0 , i.e., third region), using the reference signals (DM-RS) mapped in each of the resource regions (PRBs # 1  and # 2 , i.e., fourth region) having a frequency different from the resource region in which the DL grant is mapped (first region). 
         [0123]    Moreover, signal demultiplexing section  203  demultiplexes the signals into data streams on the basis of the estimated channel estimation values. 
         [0124]    Demodulation section  204  demodulates the data signals using the channel estimation values obtained by signal demultiplexing section  203 . More specifically, demodulation section  204  demodulates the data signals (PDSCH signals) mapped on PRBs # 0  to # 2  using the channel estimation value obtained using the sets of DM-RS mapped on PRBs # 1  and # 2  as illustrated in  FIG. 11 . 
         [0125]    In other words, demodulation section  204  demodulates the data signals using DM-RS mapped in the resource regions (second region) other than the resource region (first region) in which the DL grant is mapped on the PRG (in units of RBGs) configured for relay station  200 . Moreover, demodulation section  204  demodulates the data signals mapped in the resource region that is in the same frequency region as the resource region in which the DL grant is mapped (first region) and that is subsequent to the resource region in the time domain (second slot of PRB # 0 , i.e., third region), using the DM-RS mapped in each of the resource regions that have a frequency different from that of the resource region in which the DL grant is mapped (first region) (i.e., PRBs # 1  and # 2 ). 
         [0126]    To put it differently, demodulation section  204  demodulates the PDSCH signals using the DM-RS in PRBs # 1  and # 2  on which PDSCH signals are mapped (i.e., reference signals to which the same precoding as that for the PDSCH signals is applied) without using the DM-RS in the first slot of PRB # 0  in which DL grant is mapped (i.e., reference signals to which precoding different from that for the PDSCH signals is applied). 
         [0127]    Incidentally, the propagation environments of the respective PRBs in the PRG (e.g., up to three RBs per PRG in  FIG. 5 ) configured for relay station  200  as illustrated in  FIG. 11  are likely to be similar to one another. Accordingly, relay station  200  can make a copy of the channel estimation value for some PRBs in the same PRG (PRBs # 1  and # 2  in  FIG. 11 ) and divert the channel estimation value for another PRB (PRB # 0  in  FIG. 11 ). 
         [0128]    Accordingly, as illustrated in  FIG. 11 , the accuracy of channel estimation for the PDSCH signals can be improved on PRB # 0  by demodulating the PDSCH signals using the DM-RS to which the same precoding as that for the PDSCH signals on other PRBs # 1  and # 2  is applied in comparison with the case where the PDSCH signals are demodulated using the DM-RS to which precoding different from that for the PDSCH signals is applied ( FIG. 7 ). To put it differently, it is possible to avoid the situation where the accuracy of channel estimation becomes poor on PRB # 0  where the DM-RS to which precoding different from that for the PDSCH signals is applied is mapped. 
         [0129]    As described above, in Embodiment 1, relay station  200  performs reception processing, considering that the same precoding is applied to data signals (PDSCH signals) while different precoding is used only in the first slot of a PRB on which DL grant is mapped among a plurality of PRBs in units of RBGs forming a PRG. More specifically, relay station  200  does not use the DM-RS on the PRB on which DL grant is mapped (i.e., reference signals to which precoding different from that for PDSCH signals is applied). In addition, relay station  200  performs channel estimation using only the DM-RS on a PRB on which only PDSCH signals are mapped (i.e., PRB other than the PRB on which DL grant is mapped) (i.e., reference signals to which the same precoding as that for the PDSCH signals is applied). Accordingly, relay station  200  can demodulate the PDSCH signals without degrading the accuracy of channel estimation even on the PRB on which DL grant is mapped. 
         [0130]    Thus, according to Embodiment 1, the accuracy of channel estimation for PDSCH can be improved even when DL grant indicating R-PDCCH downlink data assignment is mapped therewith. 
         [0131]    In Embodiment 1, a description has been given regarding the case where relay station  200  uses, as the channel estimation value for PRB # 0 , the average between the channel estimation values calculated using the sets of DM-RS on PRBs # 1  and # 2 . However, Embodiment 1 is not limited to this case. Relay station  200  may use the channel estimation value on PRB # 1 , which is adjacent to PRB # 0 , as the channel estimation value on PRB# 0  among the channel estimation values calculated using the sets of DM-RS on PRBs # 1  and # 2 , for example. Accordingly, the channel estimation value on PRB # 1  closest to PRB # 0  (i.e., PRB the propagation environment of which is most similar to that of PRB # 0 ) is used on PRB # 0 , so that it is possible to avoid a degradation in the accuracy of channel estimation on PRB # 0  to which precoding different from that for PDSCH signals is applied. 
         [0132]    In Embodiment 1, a description has been given regarding the case where the correspondence between the system bandwidth and PRG sizes as illustrated in  FIG. 5  is used. However, the PRG sizes used in Embodiment 1 are not limited to these PRG sizes. For example, the PRG sizes may be changed with respect to the system bandwidth as deemed appropriate in order to improve the SNR and also to configure favorable beams by equalizing the influence of frequency-selective fading. 
       Embodiment 2 
       [0133]    In Embodiment 2, a base station controls assignment of signals mapped in the second slot according to signals mapped in the first slot of each PRB. 
         [0134]    In Embodiment 1, the relay station demodulates data signals using the DM-RS to which the same precoding as that for PDSCH signals is applied in the same PRG. However, as the rank increases for the precoding applied to PDSCH signals, a further problem may arise. 
         [0135]    More specifically, although the propagation environments become similar to each other on adjacent PRBs, the actual channel response gradually differs between the PRBs due to frequency-selective fading. The influence of differences in channel response between PRBs appears more obviously when the rank is higher. This is because the higher the rank is, the higher the accuracy of channel estimation will be required in spatial demultiplexing processing. In this case, data demodulation may become difficult in the relay station not only due to a degradation in demodulation performance in the demodulation section but also due to a degradation in spatial demultiplexing performance in the signal demultiplexing section. 
         [0136]    Meanwhile, extrapolation using a channel estimation value obtained from DM-RS to which the same precoding as that for PDSCH signals is applied in the same PRG may be used to avoid a degradation in the accuracy of channel estimation due to the abovementioned influence. However, the channel estimation processing based on extrapolation requires an enormous amount of processing. 
         [0137]    Incidentally, when DM-RS used for channel estimation is mapped only in the first slot (see,  FIG. 4B ), the accuracy of channel estimation degrades only on the PRB on which R-PDCCH signals are mapped in a PRG as described above. 
         [0138]    In this respect, in Embodiment 2, the base station maps data signals (PDSCH signals) in a PRB other than the PRB on which R-PDCCH signals are mapped, without mapping data signals (PDSCH signals) in the PRB on which DL grant (R-PDCCH signals) is mapped in a PRG configured for the relay station. 
         [0139]      FIG. 12  is a main configuration diagram of base station  300  according to Embodiment 2. In base station  300 , signal assignment section  301  and control signal assignment section  110 , which serve as a mapping section, map reference signals (e.g., DM-RS) in a first region and a second region of a precoding resource block group (PRG) formed of one or more PRBs among a plurality of physical resource blocks (PRBs) formed by dividing a frequency region, map control signals (e.g., DL grant) in the first region and map data (downlink signals, e.g., PDSCH signals) in the second region. Signal assignment section  301  maps no data in the region that is in the same frequency region as the PRB forming the first region and that is subsequent to the PRB in the time domain. Accordingly, the reference signals, control signals and data mapped on the respective resources are transmitted to relay station  400 . 
         [0140]      FIG. 13  illustrates a configuration of base station  300  according to Embodiment 2. In  FIG. 13 , the same elements as those in Embodiment 1 ( FIG. 9 ) are assigned the same reference numerals and any duplicate description of the elements is omitted. 
         [0141]    Base station  300  illustrated in  FIG. 13  is different from the base station in Embodiment 1 in the operation of signal assignment section  301 . 
         [0142]    More specifically, in base station  300 , signal assignment section  301  receives information indicating the PRB on which R-PDCCH signals are mapped from control signal assignment section  110 . When assigning the transmission signals (PDSCH signals) received from modulation section  107  to a resource, signal assignment section  301  excludes the PRB indicated by the information received from control signal assignment section  110  (i.e., PRB to which the R-PDCCH signals are assigned) from transmission signal assignment targets. In short, signal assignment section  301  maps no transmission signals in the resource region that is in the same frequency region as the PRB forming the resource region in which R-PDCCH signals are mapped and that is subsequent to the PRB in the time domain. In other words, signal assignment section  301  maps the transmission signals on a PRB other than the PRB on which R-PDCCH signals are mapped. 
         [0143]      FIG. 14  illustrates a configuration of relay station  400 . In  FIG. 14 , the same elements as those in Embodiment 1 ( FIG. 10 ) are assigned the same reference numerals and any duplicate description of the elements is omitted. Relay station  400  illustrated in  FIG. 14  is different from the relay station in Embodiment 1 in the operation of signal demultiplexing section  401 . 
         [0144]    More specifically, in relay station  400 , signal demultiplexing section  401  extracts data signals (PDSCH signals) according to the DL grant to be received from R-PDCCH receiving section  207  as in Embodiment 1. In addition, signal demultiplexing section  401  demultiplexes the signals into data streams by application of MIMO spatial demultiplexing processing according to the rank received from R-PDCCH receiving section  207 . Signal demultiplexing section  401  then outputs the data streams obtained by demultiplexing to demodulation section  204 . During this processing, signal demultiplexing section  401  excludes the signals mapped on the PRB to which the R-PDCCH signals are assigned (i.e., signals in the resource region that is in the same frequency region as the resource region in which the R-PDCCH signals are mapped and that is subsequent to the resource region in the time domain) and outputs the remaining signals to demodulation section  204 . 
         [0145]    (Operations of Base Station  300  and Relay Station  400 ) 
         [0146]    A description will be provided regarding operations of base station  300  and relay station  400  including the above described configurations, respectively. A description will be particularly provided regarding data signal assignment processing herein. 
         [0147]    In the following description, as in Embodiment 1 ( FIG. 11 ), the PRG is configured in units of RBGs each formed of three PRBs, which are PRBs # 0  to # 2 , with the following settings: RBG size=3 and PRG size=3 as illustrated in  FIG. 15 . PRB# 0  is configured as the R-PDCCH search space as illustrated in  FIG. 15 . 
         [0148]    Moreover, the aggregation size of R-PDCCH signals is set equal to one RB as illustrated in  FIG. 15 . 
         [0149]    As illustrated in  FIG. 15 , signal assignment section  301  in base station  300  maps no data signals (PDSCH signals) to the second slot of PRB # 0  on which DL grant (R-PDCCH signals) is mapped. More specifically, signal assignment section  301  maps no data signals (PDSCH signals) in the resource region that is in the same frequency region as PRB # 0  forming the resource region in which the DL grant (R-PDCCH signals) is mapped (first slot of PRB # 0 , i.e., first region) signals and that is subsequent to the PRB in the time domain (second slot of PRB # 0 , i.e., second region). In other words, signal assignment section  301  maps data signals on PRBs # 1  and # 2  other than PRB # 0  on which the DL grant is mapped. More specifically, the resource region on which the data signals are mapped (second region) is formed of the resource regions having a frequency different from that of the resource region in which the DL grant is mapped (first region) (i.e., PRBs # 1  and # 2 ) in  FIG. 15 . 
         [0150]    Meanwhile, in  FIG. 15 , signal demultiplexing section  401  in relay station  400  extracts data signals intended for relay station  400  on the basis of the DL grant to be received from R-PDCCH receiving section  207  as in Embodiment 1. As illustrated in  FIG. 15 , no data signals (PDSCH signals) are mapped in the second slot of PRB # 0  on which DL grant is mapped. 
         [0151]    Meanwhile, signal demultiplexing section  203  performs channel estimation using the reference signals (DM-RS) mapped on PRBs # 1  and # 2  as in Embodiment 1. Signal demultiplexing section  203  then demultiplexes the signals into data streams on the basis of the channel estimation values. Accordingly, as illustrated in  FIG. 15 , the data signals (PDSCH signals) mapped on PRBs # 1  and # 2  other than PRB # 0  to which the R-PDCCH signals are allocated among PRBs # 0  to # 2  forming the PRG are outputted to demodulation section 204. 
         [0152]    Demodulation section  204  demodulates the data signals (PDSCH signals) mapped on PRBs # 1  and # 2 , using the channel estimation values obtained using the sets of DM-RS mapped on PRBs # 1  and # 2  as illustrated in  FIG. 15 . 
         [0153]    As described, base station  300  transmits no PDSCH signals on the PRB where the DM-RS to which precoding different from that for the PDSCH signals is applied is mapped. Thus, relay station  400  does not have to perform receiving processing on the PDSCH signals using the DM-RS to which precoding different from that for the PDSCH signals is applied. Accordingly, relay station  400  can avoid a degradation in the accuracy of channel estimation for PDSCH signals and obtain good accuracy of channel estimation. In other words, it is possible to avoid the situation where data demodulation becomes difficult due to erroneous channel estimation when data is mapped on the PRB on which DL grant is mapped. 
         [0154]    As described, according to Embodiment 2, the accuracy of channel estimation for PDSCH can be improved even when the DL grant indicating R-PDCCH downlink data assignment is mapped therewith. Moreover, Embodiment 2 eliminates the need for a circuit having a large processing capacity for performing extrapolation while using a channel estimation value of an adjacent PRB for ensuring high accuracy of channel estimation when the rank is high. For this reason, it is possible to avoid an increase in the hardware cost of the relay station. 
         [0155]    In Embodiment 2, the base station does not have to map any PDSCH signals on a PRB (i.e., search space) on which DL grant may be mapped, when PRG=1. When it says PRG=1, this means that there is no fourth region (i.e., resource region having a frequency different from that of the resource region on which DL grant is mapped (first region)). In this case, there is no resource region to which the same precoding as that for the third region (resource region that has the same frequency as the PRB forming the resource region on which DL grant is mapped (first region) and that is subsequent to the PRB in the time domain) can be applied. For this reason, in order to secure high accuracy of channel estimation when the rank is high, there is required a circuit having a large processing capacity for interpolating a channel estimation value while using a channel estimation value of an adjacent PRB and estimating precoding used for the adjacent PRB and precoding used for the third region. However, since the base station employs the configuration in which no PDSCH signals are mapped on any PRB on which DL grant may be mapped, when PRG=1, the circuit having a large amount of processing is no longer required regardless of actual mapping of DL grant. Thus, it is possible to avoid an increase in the hardware cost of the relay station. 
       Embodiment 3 
       [0156]    In Embodiment 3, a base station controls assignment of PDSCH signals on a PRB forming the resource region on which R-PDCCH signals are mapped, according to the rank configured for the PDSCH signals. 
         [0157]    As described in Embodiment 2, the influence of differences in channel response between PRBs due to frequency-selective fading appears more obviously when the rank is higher. More specifically, it can be said that the influence of differences in channel response between PRBs due to frequency-selective fading is low when the rank for data signals (PDSCH signals) is low. 
         [0158]    In particular, when the rank of PDSCH signals is equal to 1, the same precoding as that for DL grant having a fixed rank (rank=1) is applied to the PDSCH signals. 
         [0159]    In this respect, the base station selects whether or not to map data signals (PDSCH signals) on the PRB on which DL grant (R-PDCCH signals) is mapped, according to the rank for the data signals (PDSCH signals). More specifically, the base station maps the data signals in the resource region that has the same frequency as the PRB forming the resource region on which DL grant is mapped and that is subsequent to the PRB in the time domain, only when the same precoding is applied to the DL grant (R-PDCCH signals) and the data signals (PDSCH signals). 
         [0160]      FIG. 16  illustrates a configuration of base station  500  according to Embodiment 3. In  FIG. 16 , the same elements as those in Embodiment 1 ( FIG. 9 ) are assigned the same reference numerals and any duplicate description of the elements is omitted. Base station  500  illustrated in  FIG. 16  is different from the base station in Embodiment 1 in the operations of signal assignment section  501  and precoding section 502. 
         [0161]    More specifically, the rank for PDSCH signals and the information indicating the PRB to which R-PDCCH signals are assigned are inputted from control signal assignment section  110  to signal assignment section  501 . When assigning the transmission signals (PDSCH signals) received from modulation section  107  to a resource, signal assignment section  501  excludes, from the assignment targets for the transmission signals, the PRB indicated by the information received from control signal assignment section  110  (i.e., PRB to which R-PDCCH signals are assigned) when a plurality of ranks are configured. Meanwhile, when the rank is one, signal assignment section  501  maps the transmission signals in the resource region including the PRB indicated by the information received from control signal assignment section  110  (i.e., PRB to which R-PDCCH signals are assigned). 
         [0162]    When a plurality of ranks are configured for the data signals (PDSCH signals), precoding section  502  applies power boost to the R-PDCCH signals and applies scaling to the data signals as in Embodiment 1. When the rank configured for the data signals (PDSCH signals) is one, precoding section  502  applies power boost to both of the R-PDCCH signals and the data signals and applies the same precoding thereto. 
         [0163]      FIG. 17  illustrates a configuration of relay station  600  according to Embodiment 3. In  FIG. 17 , the same elements as those in Embodiment 1 ( FIG. 10 ) are assigned the same reference numerals and any duplicate description of the elements is omitted. 
         [0164]    Relay station  600  illustrated in  FIG. 17  is different from the relay station in Embodiment 1 in the operation of signal demultiplexing section 601. 
         [0165]    To be more specific, signal demultiplexing section  601  in relay station  600  extracts data signals (PDSCH signals) according to the DL to be received from R-PDCCH receiving section  207  as in Embodiment 1. In addition, signal demultiplexing section  601  demultiplexes the signals into data streams by application of MIMO spatial demultiplexing processing according to the rank received from R-PDCCH receiving section  207 . Signal demultiplexing section  601  outputs the data streams obtained by demultiplexing to demodulation section 204. 
         [0166]    During this processing, when there are a plurality of ranks for the data signals (PDSCH signals), signal demultiplexing section  601  excludes the signals mapped on the PRB to which the R-PDCCH signals are assigned (i.e., signals in the resource region that is in the same frequency region as the resource region in which the R-PDCCH signals are mapped and that is subsequent to the resource region in the time domain) and outputs the remaining signals to demodulation section  204 . In addition, signal demultiplexing section  601  performs channel estimation using the reference signals (DM-RS) mapped on the PRB other than the PRB on which the R-PDCCH signals are mapped. 
         [0167]    Meanwhile, when the rank for the data signals (PDSCH signals) is one, signal demultiplexing section  601  outputs the signals including the data mapped on the PRB to which the R-PDCCH signals are assigned to demodulation section  204 . In addition, signal demultiplexing section  601  performs channel estimation using the reference signals (DM-RS) mapped on the PRB on which the data signals (PDSCH signals) and R-PDCCH signals are mapped. For example, signal demultiplexing section  601  averages the channel estimation value on the PRB on which the R-PDCCH signals are mapped and the channel estimation value on another PRB in a PRG and thus improves the accuracy of channel estimation. 
         [0168]    (Operations of Base Station  500  and Relay Station  600 ) 
         [0169]    A description will be provided regarding operations of base station  500  and relay station  600  including the above described configurations, respectively. A description will be herein provided regarding data signal assignment processing in particular. 
         [0170]    In the following description, as in Embodiment 1 (FIG.  11 ), the PRG is configured in units of RBGs each formed of three PRBs, which are PRBs # 0  to # 2 , with the following settings: RBG size=3 and PRG size=3 as illustrated in  FIGS. 18A and 18B . In addition, PRB # 0  is configured as the R-PDCCH search space as illustrated in  FIGS. 18A and 18B . In addition, the aggregation size of the R-PDCCH signals is set equal to one RB as illustrated in  FIGS. 18A and 18B . The rank for data signals (PDSCH signals) is set equal to one in  FIG. 18A  while the rank for data signals (PDSCH signals) is set equal to two in  FIG. 18B . 
         [0171]    Since the rank for data signals (PDSCH signals) is one as illustrated in  FIG. 18A , signal assignment section  501  in base station  500  maps data signals in the second slot as well in PRB # 0  on which DL grant (R-PDCCH signals) is mapped. 
         [0172]    In addition, since the rank for data signals (PDSCH signals) is one as illustrated in  FIG. 18A , precoding section  502  applies power boost to the DL grant (R-PDCCH signals) and data signals. More specifically, the DL grant (R-PDCCH signals) and data signals (PDSCH signals) are transmitted using the same precoding in  FIG. 18A . 
         [0173]    Meanwhile, since the rank for data signals (PDSCH signals) is two as illustrated in  FIG. 18B , signal assignment section  501  maps no data signals in the second slot of PRB # 0  on which DL grant (R-PDCCH signals) is mapped. 
         [0174]    In addition, since the rank for data signals (PDSCH signals) is two as illustrated in  FIG. 18B , precoding section  502  performs the precoding processing of the configured rank as in Embodiment 1. More specifically, the DL grant (R-PDCCH signals) and data signals are transmitted using different precoding in  FIG. 18B . 
         [0175]    In other words, signal assignment section  501  maps the data signals (PDSCH signals) in the resource region that is in the same frequency region as the first slot of PRB # 0  on which the DL grant is mapped (first region) and that is subsequent to the resource region in the time domain (i.e., second slot of PRB # 0 ) only when the same rank is configured for the DL grant and data signals (i.e., transmitted using the same precoding) (see,  FIG. 18A ). 
         [0176]    Meanwhile, signal demultiplexing section  601  in relay station  600  extracts data signals intended for relay station  600  on the basis of the DL grant received from R-PDCCH receiving section  207  as in Embodiment 1. 
         [0177]    As illustrated in  FIG. 18A  (rank for the data signals=1), the data signals (PDSCH signals) are mapped in the second slot of PRB # 0  on which the DL grant is mapped. Meanwhile, as illustrated in  FIG. 18B  (rank for the data signals=2), no data signals (PDSCH signals) are mapped in the second slot of PRB # 0  on which the DL grant is mapped. 
         [0178]    In  FIG. 18A  (rank for data signals=1), signal demultiplexing section  203  performs channel estimation for the PDSCH signals using the DM-RS mapped on the first slot of PRB # 0  (first region) and the DM-RS mapped on PRBs # 1  and # 2  (second region). More specifically, demodulation section  204  demodulates the data signals (PDSCH signals) mapped on PRBs # 0  to # 2  using the channel estimation values obtained using the DM-RS mapped in the first slot of PRB # 0  and the DM-RS mapped on PRBs # 1  and # 2  in  FIG. 18A . 
         [0179]    Meanwhile, in  FIG. 18B  (rank for data signals=2), signal demultiplexing section  203  performs channel estimation for the PDSCH signals using only the DM-RS mapped on PRBs # 1  and # 2  (second region). More specifically, demodulation section  204  demodulates the data signals (PDSCH signals) mapped on PRBs # 1  and # 2  using the channel estimation values obtained using the DM-RS mapped on PRBs # 1  and # 2  in  FIG. 18B . 
         [0180]    As described above, base station  500  transmits no PDSCH signals on the PRB where the DM-RS (R-PDCCH signals) to which precoding different from that for PDSCH signals is applied is mapped. Thus, relay station  600  can avoid a degradation in the accuracy of channel estimation for PDSCH signals and obtain good accuracy of channel estimation as in Embodiment 2. 
         [0181]    Moreover, base station  500  transmits PDSCH signals on the PRB where the DM-RS (R-PDCCH signals) to which the same precoding as that for the PDSCH signals is applied is mapped. More specifically, base station  500  transmits PDSCH signals on the PRB on which R-PDCCH signals are mapped, only when the rank for the PDSCH signals is the same as the rank for the R-PDCCH signals (fixed; rank=1). Accordingly, relay station  600  can use all the channel estimation values on PRBs # 0  to # 2  in the PRG. Thus, it is possible to improve the accuracy of channel estimation for PDSCH signals in relay station  600  while preventing a decrease in transmission efficiency of the PRB on which R-PDCCH signals are allocated. More specifically, it is possible to minimize a decrease in transmission efficiency due to a decrease in the number of data assignment PRBs without causing a degradation in the accuracy of channel estimation even when the rank for PDSCH signals is high. 
         [0182]    Thus, according to Embodiment 3, it is possible to improve the accuracy of channel estimation for PDSH while preventing a decrease in transmission efficiency even when DL grant indicating the downlink data assignment of R-PDCCH is mapped therewith. Moreover, as with Embodiment 2, Embodiment 3 eliminates the need for a circuit having a large processing capacity for performing extrapolation while using a channel estimation value of an adjacent PRB for ensuring high accuracy of channel estimation when the rank is high. Thus, it is possible to avoid an increase in the hardware cost of the relay station. 
         [0183]    It should be noted that, although Embodiment 1 has been described regarding the case where the aggregation size of R-PDCCH=1, the claimed invention can be applied to a case where the aggregation size is equal to two or higher (i.e., resource region in which DL grant is mapped (first region) is formed of a plurality of PRBs). More specifically, the base station may map data signals (PDSCH signals) in the resource region that is in the same frequency region as the PRB forming the resource region in which DL grant is mapped (first region) and that is subsequent to the resource region in the time domain only when the DL grant (R-PDCCH signals) mapped in the first slot of a plurality of PRBs (first region) and the data signals (PDSCH signals) mapped in the resource region different from the first region (second region) are transmitted using the same precoding. With this configuration, even if aggregation is applied to the R-PDCCH (DL grant), it is possible to improve the accuracy of channel estimation for the PDSCH while preventing a decrease in transmission efficiency even when the DL grant indicating R-PDCCH downlink data assignment is mapped therewith, as in Embodiment 3. 
         [0184]    In addition, Embodiment 3 has been described regarding the case where the base station selects whether or not to map data signals (PDSCH signals) on the PRB on which R-PDCCH signals are mapped, according to the rank for the PDSCH signals. However, Embodiment 3 is not limited to this case, and the base station may select whether or not to map data signals (PDSCH signals) on the PRB on which R-PDCCH signals are mapped, according to another index associated with the rank for the PDSCH signals. For example, the aggregation size of R-PDCCH (i.e., CCE aggregation size) may be used as another index associated with the rank. Aggregation is likely to be applied to R-PDCCH when the rank for PDSCH signals is one. Accordingly, when the aggregation size is at least two (i.e., when aggregation is applied), the base station may map data signals (PDSCH signals) on the PRB on which R-PDCCH signals are mapped. Thus, it is possible to avoid a decrease in transmission efficiency. 
         [0185]    Meanwhile, when the aggregation size is one (i.e., when no aggregation is applied), the base station maps no data signals (PDSCH signals) on any PRB on which R-PDCCH signals are mapped. In this manner, it is possible to avoid a degradation in accuracy of channel estimation. In other words, adopting this configuration enables adaptively selecting a degradation factor to be avoided, between “degradation in accuracy of channel estimation” and “decrease in transmission efficiency” while allowing for flexibility in configuration of the rank. 
         [0186]    The embodiments of the claimed invention have been described above. 
       Other Embodiments 
       [0187]    (1) The abovementioned embodiments have been described regarding the cases where relay stations  200 ,  400  and  600  receive the R-PDCCH signals. However, the claimed invention is not limited to these cases, and the same effects obtained according to the claimed invention can be obtained when a mobile station (not illustrated) performs the same processing as relay stations  200 ,  400  and/or  600  upon reception of R-PDCCH signals transmitted from base stations  100 ,  300  and/or  500 . 
         [0188]    (2) In addition, the odd/even numbering of PRBs or RBGs in  FIG. 11 ,  FIG. 15 ,  FIG. 18 , and  FIG. 19  of the abovementioned embodiments may be reversed. 
         [0189]    (3) In addition, the abovementioned embodiments have been described regarding the case where two slots (i.e., first slot and second slot) are configured in a single PRB. 
         [0190]    However, each of the slots (first slot and second slot) may be defined as a single PRB and the first and second slots of a frequency according to a predetermined combination may be called a PRB pair. 
         [0191]    (4) Each of the embodiments has been described with antennas, but the claimed invention can be applied to antenna ports in the same manner. 
         [0192]    The term “antenna port” refers to a logical antenna including one or more physical antennas. In other words, the term “antenna port” does not necessarily refer to a single physical antenna, and may sometimes refer to an antenna array formed of a plurality of antennas, and/or the like. 
         [0193]    For example, LTE does not specify the number of physical antennas forming an antenna port, but specifies an antenna port as a minimum unit allowing each base station to transmit different reference signals. 
         [0194]    In addition, an antenna port may be specified as the minimum unit for multiplication of precoding vector weighting. 
         [0195]    (5) The above-noted embodiments have been described by examples of hardware implementations, but the claimed invention can be also implemented by software in conjunction with hardware. 
         [0196]    In addition, the functional blocks used in the descriptions of the embodiments are typically implemented as LSI devices, which are integrated circuits. The functional blocks may be formed as individual chips, or a part or all of the functional blocks may be integrated into a single chip. The term “LSI” is used herein, but the terms “IC,” “system LSI,” “super LSI” or “ultra LSI” may be used as well depending on the level of integration. 
         [0197]    In addition, the circuit integration is not limited to LSI and may be achieved by dedicated circuitry or a general-purpose processor other than an LSI. After fabrication of LSI, a field programmable gate array (FPGA), which is programmable, or a reconfigurable processor, which allows reconfiguration of connections and settings of circuit cells in LSI may be used. 
         [0198]    Should a circuit integration technology replacing LSI appear as a result of advancements in semiconductor technology or other technologies derived from the technology, the functional blocks could be integrated using such a technology. Another possibility is the application of biotechnology and/or the like. 
         [0199]    The disclosure of the specification, the drawings, and the abstract included in Japanese Patent Application No. 2011-035591, filed on Feb. 22, 2011, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0200]    The claimed invention is useful in that it improves the accuracy of channel estimation for PDSCH. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100 ,  300 ,  500  Base station 
           200 ,  400 ,  600  Relay station 
           101 ,  113 ,  201 ,  212  Antenna  102 ,  202  Radio receiving section 
           103 ,  204  Demodulation section 
           104 ,  205  Error correction decoding section 
           105  R-PDCCH search space determining section  106 ,  208  Error correction coding section 
           107 ,  209  Modulation section 
           108 ,  210 ,  301 ,  501  Signal assignment section 
           109  Control signal coding section  110  Control signal assignment section 
           111 ,  502  Precoding section 
           112 ,  211  Radio transmitting section 
           203 ,  401 ,  601  Signal demultiplexing section 
           206  Blind-decoding determining section 
           207  R-PDCCH receiving section 
           1091  Aggregation size determining section 
           1092  Coding section 
           1101  R-PDCCH PRB determining section 
           1102  Assignment section