Abstract:
A wireless terminal, includes: an antenna; and, a processor, coupled to the antenna, the processor to receive, through the antenna, a common reference signal transmitted from a base station at a first timing at a frequency which is selected in accordance with identification information of a cell, to receive, through the antenna, a wireless-terminal-specific reference signal and a control signal both of which are concurrently transmitted by the base station at different frequencies, at a second timing that is different from the first timing; and, to demodulate the received control signal, based on the received wireless-terminal-specific reference signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of application Ser. No. 14/615,648, filed Feb. 6, 2015, now pending, which is a continuation of International Application PCT/JP2012/070774, filed on Aug. 15, 2012 and designating the U.S., the entire contents of each are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiment discussed herein is related to a communications system, a wireless base station, a wireless terminal, and a communications method. 
       BACKGROUND 
       [0003]    The 3rd Generation Partnership Project (3GPP), a standard-setting organization that develops specifications for wireless communications systems, is studying and developing Long Term Evolution-Advanced (LTE-Advanced), which is an advancement of LTE, a mobile communications system. Although work for specifying basic functions of LTE-A has been completed, with an aim to further improve performance and improve the capacity to handle diversified system operation scenarios, the introduction of new functions has been proposed, discussion continues, and the system continues to advance. Under the LTE-A specifications thus far, a wireless control signal for transmitting radio parameters (the position of the frequency domain, the modulation scheme, the code rate, etc. for a data signal) directly related to the wireless transmission of a data signal that is to be transmitted, has been transmitted at a time domain that differs from that of the data signal. In other words, the radio resource domain used in the transmission of the wireless control signal and the radio resource domain used in the transmission of a data signal have been time-division multiplexed. The latest discussions study a way to also map wireless control signals in the domain used for the transmission of data signals to enable the amount of radio resources that can be used in the transmission of a wireless control signal to be increased as circumstances demand. Nonetheless, if a portion of the domain for the transmission of data signals are made available for the transmission of wireless control signals as well, the amount of radio resources that can be used for data signal transmission decreases. Therefore, the use of a highly efficient transmission method and a minimal amount of radio resources is desirable for wireless control signals that are to be transmitted on the domain used for data signal transmission. One such known method is the application of a modulation scheme of a high-order modulation degree to the wireless signals and in which a high-order modulation scheme is applied to downlink control signals in a vicinity of a reference signal (for example, refer to Published Japanese-Translation of PCT Application, Publication No. 2007/052767). In the reception and demodulation of a wireless signal to which a high-order modulation scheme of using not only 16QAM, 64QAM, etc. phase components but also amplitude components and mapping information bits is applied, demodulation characteristics have to be enhanced. Normally, the demodulation characteristics of signals in the vicinity of a reference signal improves when data signal demodulation is performed by interpolating channel estimation results for multiple reference signals to obtain channel estimation information for demodulating data signals. 
         [0004]    Nonetheless, for example, as with LTE and LTE-A, if the transmission frequency of a common reference signal among wireless cells is shifted, even if a wireless control signal to which a modulation scheme of a high-order modulation degree is applied is in the vicinity of the reference signal, reception characteristics of the control signal may deteriorate consequent to interference from the common reference signal of an adjacent cell, and the benefit of improved demodulation characteristics by placing in the vicinity of a reference signal, a signal that is to be demodulated cannot be sufficiently obtained. 
       SUMMARY 
       [0005]    According to an aspect of an embodiment, a communications system in which, in each cell, a common reference signal to wireless terminals of the cell is transmitted at a first timing at a frequency that is based on identification information of the cell, and the communications system includes a wireless base station that transmits the common reference signal at the first timing, and transmits at a second timing that is different from the first timing, a wireless-terminal-specific reference signal to the wireless terminals of the cell of the wireless base station and a control signal to the wireless terminals of the cell of the wireless base station, concurrently at different frequencies; and a wireless terminal that based on the wireless-terminal-specific reference signal transmitted by the wireless base station, demodulates the control signal transmitted by the wireless base station. 
         [0006]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a diagram depicting one example of a communications system according to an embodiment; 
           [0009]      FIG. 2  is a diagram depicting one example of a downlink radio resource; 
           [0010]      FIG. 3A  is a diagram depicting one example of a structure of a communications unit of a wireless base station; 
           [0011]      FIG. 3B  is a diagram depicting one example of signal flow in the communications unit of the wireless base station depicted in  FIG. 3A ; 
           [0012]      FIG. 4A  is a diagram depicting one example of a structure of a communications unit of a wireless terminal; 
           [0013]      FIG. 4B  is a diagram depicting one example of signal flow in the communications unit of the wireless terminal depicted in  FIG. 4A ; 
           [0014]      FIG. 5  is a flowchart depicting an example of operation of an arrangement selecting unit of the wireless base station; 
           [0015]      FIG. 6  is a diagram depicting one example of cell arrangement; 
           [0016]      FIG. 7A  is a diagram depicting one example of frequency shifting of a common reference signal; 
           [0017]      FIG. 7B  is a diagram depicting one example of frequency shifting of the common reference signal; 
           [0018]      FIG. 7C  is a diagram depicting one example of frequency shifting of the common reference signal; 
           [0019]      FIG. 7D  is a diagram depicting one example of frequency shifting of the common reference signal; 
           [0020]      FIG. 7E  is a diagram depicting one example of frequency shifting of the common reference signal; 
           [0021]      FIG. 7F  is a diagram depicting one example of frequency shifting of the common reference signal; 
           [0022]      FIG. 8  is a diagram depicting one example of a hardware structure of the wireless base station; and 
           [0023]      FIG. 9  is a diagram depicting one example of a hardware structure of the wireless terminals. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    An embodiment of a communications system, a wireless base station, a wireless terminal, and a communications method will be described in detail with reference to the accompanying drawings. 
         [0025]      FIG. 1  is a diagram depicting one example of a communications system according to the embodiment. As depicted in  FIG. 1 , a communications system  100  includes wireless base stations  111 ,  112 , and wireless terminals  131 ,  132 . A cell  121  is the cell of the wireless base station  111 ; and a cell  122  is the cell of the wireless base station  112 . 
         [0026]    The wireless terminal  131  is located in the cell  121  and connected to the wireless base station  111 . However, the wireless terminal  131  is also in the cell  122  and subject to interference by a wireless signal transmitted from the wireless base station  112 . The wireless terminal  132  is located in the cell  122  and connected to the wireless base station  112 . 
         [0027]    The wireless base station  111  transmits downlink wireless signals to the wireless terminal  131 . The wireless base station  112  transmits downlink wireless signals to the wireless terminal  132 . Orthogonal frequency division multiplexing (OFDM) can be used with respect to the wireless signals transmitted by the wireless base stations  111 ,  112 . 
         [0028]    A signal map  141  depicts the signal map of wireless signals transmitted in the cell  121  by the wireless base station  111 . A signal map  142  depicts the signal map of wireless signals transmitted in the cell  122  by the wireless base station  112 . In the signal maps  141 ,  142 , “C” represents a radio resource to which a downlink common reference signal is mapped. “D” represents a radio resource to which a downlink wireless-terminal-specific reference signal is mapped. Further, an x mark represents a radio resource to which a downlink control signal is mapped. 
         [0029]    The common reference signal and the wireless-terminal-specific reference signal, for example, are symbols used for wireless communication synchronization and channel equalization; and are signals transmitted by the wireless base stations  111 ,  112  at a given cycle, to the respective cells thereof. The common reference signal is a reference signal commonly transmitted to wireless terminals in a cell. The wireless-terminal-specific reference signal is a reference signal discriminately transmitted to a wireless terminal in a cell. 
         [0030]    In the communications system  100 , for each cell, a frequency shift is performed on the frequency, which is based on the identification information of the cell and which is the frequency at which a common reference signal is transmitted to wireless terminals in the cell, at a first timing t 1 . For example, the identification information of the wireless base stations  111 ,  112  is assumed to be identification information #1, #2, respectively. Further, frequencies based on the identification information #1, #2 are assumed to be frequencies f 1 , f 2  (f 1 ≠f 2 ), respectively. 
         [0031]    In this case, at the first timing t 1 , the wireless base station  111  transmits a common reference signal at the frequency f 1 , which is based on the identification information #1 of the cell of the wireless base station  111 . Meanwhile, the wireless base station  112  transmits at the same first timing t 1  as the wireless base station  111 , a common reference signal at the frequency f 2 , which is based on the identification information #2 of the cell of the wireless base station  112 . 
         [0032]    Further, at a second timing t 2  that differs from the first timing t 1 , the wireless base station  111  transmits a wireless-terminal-specific reference signal to a wireless terminal in the cell thereof (e.g., the wireless terminal  131 ) and concurrently transmits a control signal to a wireless terminal in the cell thereof, at respectively different frequencies. More specifically, the wireless base station  112  transmits wireless-terminal-specific reference signals to a wireless terminal in the cell thereof at the frequencies f 1 , f 3 . Further, the wireless base station  111  transmits a control signal to a wireless terminal (e.g., the wireless terminal  132 ) in the cell thereof at the frequency f 2 . 
         [0033]    Thus, the wireless base station  111  transmits a downlink control signal at the second timing t 2  that differs from the first timing t 1  at which a common reference signal is transmitted in the cells. As a result, even if a common reference signal is transmitted at some frequency in adjacent cells of the cell  122 , interference of the control signal of the cell  122  by the common reference signal of an adjacent cell can be suppressed. Therefore, control signal reception characteristics at the wireless terminal  131  can be improved. 
         [0034]    Further, the second timing t 2  at which a control signal is transmitted to the wireless terminal  131  is also the timing at which a wireless-terminal-specific reference signal is transmitted to the wireless terminal  131 . As a result, channel characteristics of the wireless-terminal-specific reference signal and the control signal received by the wireless terminal  131  become close and demodulation accuracy of the control signal that is based on channel estimation results of the wireless-terminal-specific reference signal is improved. Therefore, control signal reception characteristics at the wireless terminal  131  can be improved. 
         [0035]    In the example depicted in  FIG. 1 , description of a case where the cells  121 ,  122  are formed by the wireless base stations  111 ,  112 , respectively, is given. Nonetheless, the cells  121 ,  122  may be cells (sectors) formed by one wireless base station. Further, the communications system  100  may include three or more cells. 
         [0036]    Further, in the example depicted in  FIG. 1 , description of a case where a control signal and a wireless-terminal-specific reference signal are concurrently transmitted in the cell  121  is given. Nonetheless, a control signal and a wireless-terminal-specific reference signal may be concurrently transmitted in the cell  122  as well. As a result, control signal reception characteristics in the wireless terminal  132  can also be improved. 
         [0037]      FIG. 2  is a diagram depicting one example of a downlink radio resource. A physical resource block  200  depicted in  FIG. 2 , for example, is one physical resource block (PRB) in a wireless downlink interval of the cell  121 . 
         [0038]    Along the vertical axis of the physical resource block  200 , time resources in units of 1-ms subframes are indicated. One subframe includes 14 (or 12) OFDM symbols. Along the horizontal axis of the physical resource block  200 , frequency resources in units of 12 subcarriers are indicated. 
         [0039]    In the physical resource block  200 , in the head N OFDM symbols (head interval SF 1 ), a response signal (e.g., ACK or NACK) for an uplink data signal and/or a downlink control signal are arranged. The downlink control signal includes, for example, notification information of a downlink data signal, instruction information for transmission of an uplink data signal, etc. 
         [0040]    For the downlink control signal, for example, a physical downlink control channel (PDCCH) is used. A PDCCH is a downlink control signal of the physical layer (Layer 1) level and includes information related to data signal transmission. 
         [0041]    A parameter related to the transmission of a data signal is stored in a downlink control signal correlated with the data signal and using a downlink subframe of the same data signal, is transmitted to a wireless terminal. The parameter related to the transmission of the data signal, for example, is information indicating the portion of the frequency domain by which a data signal is to be transmitted to the wireless terminals, the modulation scheme and/or the code rate applied to the data signal, and a hybrid automatic repeat request (HARQ) parameter related to data transmission. 
         [0042]    A common reference signal for demodulation of the downlink control signal and data signal is used. The position (subcarrier) on the frequency axis of the common reference signal is frequency shifted based on a value uniquely determined by a value of the identification information (identification number) of the cell that transmits the common reference signal. As a result, collisions of common reference signals between adjacent cells can be prevented from occurring. The identification information of a cell, for example, is a physical cell ID (PCI). 
         [0043]    In the example depicted in  FIG. 2 , although N=3, N may be 1 or 2. Further, the value of N may be a value that differs for each downlink subframe. The value of N, for example, is notified from the wireless base station  111  to the wireless terminal  131  by using a control signal such as a physical control format indicator channel (PCFICH) transmitted on the head OFDM symbol of the downlink subframe. 
         [0044]    In the physical resource block  200 , “C” represents a resource element to which a common reference signal is mapped. “D” represents a resource element to which a wireless-terminal-specific reference signal is mapped. “d” represents a resource element to which a data signal such as a physical downlink shared channel (PDSCH) is mapped. 
         [0045]    In portions other than the head interval SF 1  of the physical resource block  200 , downlink data signals such as a PDSCH are mapped. Control information of an upper layer (e.g., Layer 2 or Layer 3) level is stored in the data signal and transmitted. A downlink control signal under LTE is subject to QPSK, etc. as a modulation scheme, and using a transmit diversity scheme, is transmitted from a transmission antenna of the wireless base station. 
         [0046]    An enhanced-physical downlink control channel (E-PDCCH) is a downlink control signal mapped to and transmitted by a portion other than the head interval SF 1  of the physical resource block  200 . As a result, the capacity of a radio resource to which a downlink control signal can be mapped can be expanded. Further, the interference control technique between cells can be applied to not only data signals, but also downlink control signals. 
         [0047]    Further, an E-PDCCH can be subject to a high-order modulation scheme or a spatial multiplexing scheme. A high-order modulation scheme is, for example, quadrature phase shift keying (QPSK), quadrature amplitude modulation (16QAM), 64QAM, and 256QAM. A spatial multiplexing scheme is, for example, multiple input multiple output (MIMO). As a result, an E-PDCCH can be transmitted by fewer radio resources and therefore, a decrease in the capacity of radio resources to which a data signal can be mapped can be suppressed. 
         [0048]    In the demodulation of an E-PDCCH, a wireless-terminal-specific reference signal is used, not a common reference signal. Therefore, for each wireless terminal or for each E-PDCCH, a different interantenna spatial matrix can be computed. 
         [0049]    Resource elements  211  to  214  appended with an x mark represent a resource element to which an E-PDCCH is mapped. As indicated by the resource elements  211  to  214 , subframes SF 2  SF 3  to which an E-PDCCH is mapped, are time resources to which a common channel signal in the cell is not mapped. As a result, irrespective of which becomes a frequency resource of a common reference signal of an adjacent cell consequent to an application of a frequency shift, interference of the E-PDCCH by the common reference signal from an adjacent cell can be prevented. 
         [0050]    The subframes SF 2  SF 3  to which an E-PDCCH is mapped are time resources to which a wireless-terminal-specific reference signal in the cell is commonly mapped. Subcarriers SC 1 , SC 2  to which an E-PDCCH is mapped, are frequency resources that are different from the frequency resources to which a wireless-terminal-specific reference signal in the cell is mapped. 
         [0051]    As a result, the same time resource of an E-PDCCH is also the time resource of a wireless-terminal-specific reference signal and the accuracy of channel estimation for an E-PDCCH based on a wireless-terminal-specific reference signal can be improved. Therefore, even if an E-PDCCH is subject to a high-order modulation scheme, such as 16QAM, 64QAM, etc., or a spatial multiplexing transmission scheme, the demodulation characteristics of the E-PDCCH can be improved. 
         [0052]    Further, the wireless base station  111  can map a wireless-terminal-specific reference signal and a portion of a control signal (e.g., E-PDCCH) to an adjacent frequency and transmit the wireless-terminal-specific reference signal and the portion of the control signal. As a result, the accuracy of channel estimation for an E-PDCCH based on a wireless-terminal-specific reference signal can be improved. 
         [0053]    In the cells, wireless-terminal-specific reference signals are not subject to frequency shifting. Therefore, in a cell, by staggering the frequency resources of the E-PDCCH and the wireless-terminal-specific reference signal, interference of the E-PDCCH by the wireless-terminal-specific reference signal of an adjacent cell can be prevented. 
         [0054]    Thus, the wireless base station  111  can map an E-PDCCH to the resource elements  211  to  214 , which are not affected by interference by a common reference signal or a wireless-terminal-specific reference signal from an adjacent cell. As a result, the reception characteristics of a downlink control signal by an E-PDCCH at the wireless terminal  131  can be improved. 
         [0055]    According to the example depicted in  FIG. 2 , there are 24 of the resource elements  211  to  214  for an E-PDCCH in the physical resource block  200 . For example, when an E-PDCCH is subject to 16QAM, information bits after 96-bit-length encoding are mapped to the 24 resource elements  211  to  214  and become transmittable. 
         [0056]    Under LTE, the size of a PDCCH includes four types, the smallest of which has been shown by analysis results to be used at the highest frequency. By the smallest PDCCH, information bits after 72-bit-length encoding are transmitted and one downlink control signal subject to 16QAM can be mapped to the 24 resource elements  211  to  214 . 
         [0057]    Further, the interval of domains to which a wireless-terminal-specific reference signal is mapped, may be about ½ of the coherence bandwidth. For example, in the physical resource block  200  depicted in  FIG. 2 , the interval of domains to which a wireless-terminal-specific reference signal is mapped, is three subcarriers. 
         [0058]      FIG. 3A  is a diagram depicting one example of a structure of a communications unit of the wireless base station.  FIG. 3B  is a diagram depicting one example of signal flow in the communications unit of the wireless base station depicted in  FIG. 3A . As depicted in  FIGS. 3A and 3B , the wireless base station  111  includes an encoding unit  301 , a modulating unit  302 , an arrangement selecting unit  303 , an encoding unit  304 , a modulating unit  305 , an encoding unit  306 , a modulating unit  307 , a frequency multiplexing unit  308 , and a time division multiplexing unit  309 . Further, the wireless base station  111  includes a wireless transmitting unit  310 , a transmission antenna  311 , a reception antenna  312 , a wireless receiving unit  313 , a reference signal processing unit  314 , a demodulating unit  315 , a decoding unit  316 , and a control signal extracting unit  317 . 
         [0059]    The encoding unit  301  encodes an input data signal. The encoding unit  301  transmits the encoded data signal to the modulating unit  302 . The modulating unit  302  modulates the data signal output from the encoding unit  301 . The modulating unit  302  transmits the modulated data signal to the frequency multiplexing unit  308 . 
         [0060]    The arrangement selecting unit  303  selects a domain to arrange (map) an input control signal. The arrangement selecting unit  303  selects the domain based on DL_wireless characteristics information output from the control signal extracting unit  317 . Based on the domain selection result, the arrangement selecting unit  303  outputs the input control signal to the encoding units  304 ,  306 . 
         [0061]    More specifically, the arrangement selecting unit  303  outputs to the encoding unit  304 , the control signal that has been mapped to the same time domain as the data signal; and outputs to the encoding unit  306 , the control signal that has been mapped to a time domain different from that of the data signal. Further, the arrangement selecting unit  303  configures the modulation scheme for the control signal, based on the DL_wireless characteristics information. Operation of the arrangement selecting unit  303  will be described hereinafter (for example, refer to  FIG. 5 ). 
         [0062]    The encoding units  304 ,  306  respectively encode the control signal output from the arrangement selecting unit  303 . The encoding units  304 ,  306  respectively output the encoded control signals to the modulating units  305 ,  307 . The modulating units  305 ,  307  respectively modulate the control signals output from the encoding units  304 ,  306 . The modulating units  305 ,  307  respectively output the modulated control signals to the frequency multiplexing unit  308  and the time division multiplexing unit  309 . 
         [0063]    The frequency multiplexing unit  308  frequency multiplexes the data signal output from the modulating unit  302  and the control signal output from the modulating unit  305 . The frequency multiplexing unit  308  outputs the frequency multiplexed signal to the time division multiplexing unit  309 . 
         [0064]    The time division multiplexing unit  309  time division multiplexes the signal output from the frequency multiplexing unit  308  and the control signal output from the modulating unit  307 . The time division multiplexing unit  309  outputs the time division multiplexed signal to the wireless transmitting unit  310 . Via the transmission antenna  311 , the wireless transmitting unit  310  wirelessly transmits to the wireless terminal of the cell  121 , the signal output from the time division multiplexing unit  309 . 
         [0065]    Via the reception antenna  312 , the wireless receiving unit  313  receives signals wirelessly transmitted from the wireless terminal of the cell  121 . The wireless receiving unit  313  outputs a received signal to the reference signal processing unit  314  and the demodulating unit  315 . 
         [0066]    The reference signal processing unit  314  extracts a reference signal that is included in the signal output from the wireless receiving unit  313 , and outputs the extracted reference signal to the demodulating unit  315 . Based on the reference signal output from the reference signal processing unit  314 , the demodulating unit  315  demodulates the signal output from the wireless receiving unit  313 . The demodulating unit  315  outputs the demodulated signal to the decoding unit  316 . 
         [0067]    The decoding unit  316  decodes the signal output from the demodulating unit  315 . The decoding unit  316  outputs a data signal and a control signal included in the decoded signal. The control signal output from decoding unit  316  is input to the control signal extracting unit  317 . 
         [0068]    The control signal extracting unit  317  outputs the control signal output from the decoding unit  316 . Further, the control signal extracting unit  317  extracts DL_wireless characteristics information included in the control signal output from the decoding unit  316 . The control signal extracting unit  317  outputs the extracted DL_wireless characteristics information to the arrangement selecting unit  303 . 
         [0069]      FIG. 4A  is a diagram depicting one example of a structure of a communications unit of the wireless terminal.  FIG. 4B  is a diagram depicting one example of signal flow in the communications unit of the wireless terminal depicted in  FIG. 4A . As depicted in  FIGS. 4A and 4B , the wireless terminal  131  includes a reception antenna  401 , a wireless receiving unit  402 , a reference signal processing unit  403 , a wireless characteristics measuring unit  404 , a time division demultiplexing unit  405 , a control signal detecting/demodulating/decoding unit  406 , and a frequency demultiplexing unit  407 . The wireless terminal  131  further includes a control signal detecting/demodulating/decoding unit  408 , a data signal demodulating/decoding unit  409 , an encoding unit  410 , a modulating unit  411 , an encoding unit  412 , a modulating unit  413 , a switching unit  414 , a wireless transmitting unit  415 , and a transmission antenna  416 . 
         [0070]    The wireless receiving unit  402  receives via the reception antenna  401 , signals wirelessly transmitted from the wireless base station  111 . The wireless receiving unit  402  outputs a received signal to the reference signal processing unit  403  and the time division demultiplexing unit  405 . 
         [0071]    The reference signal processing unit  403  extracts a reference signal that is included in the signal output from the wireless receiving unit  402 . The reference signal processing unit  403  outputs the extracted reference signal to the wireless characteristics measuring unit  404 , the control signal detecting/demodulating/decoding unit  406 , the control signal detecting/demodulating/decoding unit  408 , and the data signal demodulating/decoding unit  409 . 
         [0072]    Based on the reference signal output from the reference signal processing unit  403 , the wireless characteristics measuring unit  404  measures the downlink wireless characteristics from the wireless base station  111  to the wireless terminal  131 . The signal to interference and noise ratio (SINR), etc., for example, can be used as the wireless characteristics measured by the wireless characteristics measuring unit  404 . The wireless characteristics measuring unit  404  outputs to the encoding unit  410 , DL_wireless characteristics information that indicates the measured downlink wireless characteristics. 
         [0073]    The time division demultiplexing unit  405  time division demultiplexes the signal output from the wireless receiving unit  402 . The time division demultiplexing unit  405  outputs each of the signals obtained by the time division demultiplexing, to the control signal detecting/demodulating/decoding unit  406  and the frequency demultiplexing unit  407 , respectively. More specifically, among the obtained signals, the time division demultiplexing unit  405  outputs to the control signal detecting/demodulating/decoding unit  406 , a signal of a frequency to which only a control signal is mapped. Further, among the obtained signals, the time division demultiplexing unit  405  outputs to the frequency demultiplexing unit  407 , a signal of a frequency to which a data signal and a control signal are mapped. 
         [0074]    Based on the reference signal output from the control signal detecting/demodulating/decoding unit  406 , the reference signal processing unit  403  detects a control signal from the signal output from the time division demultiplexing unit  405  and, demodulates and decodes the detected control signal. The control signal detecting/demodulating/decoding unit  406  outputs the decoded control signal. The control signal output from the control signal detecting/demodulating/decoding unit  406  is input to the data signal demodulating/decoding unit  409 . 
         [0075]    The frequency demultiplexing unit  407  frequency demultiplexes the signal output from the time division demultiplexing unit  405 . The frequency demultiplexing unit  407  outputs to the control signal detecting/demodulating/decoding unit  408 , a control signal obtained by the frequency demultiplexing. Further, the frequency demultiplexing unit  407  outputs to the data signal demodulating/decoding unit  409 , a data signal obtained by the frequency demultiplexing. 
         [0076]    Based on the reference signal output from the reference signal processing unit  403 , the control signal detecting/demodulating/decoding unit  408  detects the control signal output from the frequency demultiplexing unit  407  and, demodulates and decodes the detected control signal. The control signal detecting/demodulating/decoding unit  408  outputs the decoded control signal. The control signal output from the control signal detecting/demodulating/decoding unit  408  is input to the data signal demodulating/decoding unit  409 . 
         [0077]    The data signal demodulating/decoding unit  409  demodulates and decodes the data signal output from the frequency demultiplexing unit  407 . More specifically, the data signal demodulating/decoding unit  409  performs the demodulation and decoding based on the reference signal output from the reference signal processing unit  403  and the control signals output from the control signal detecting/demodulating/decoding units  406 ,  408 . The data signal demodulating/decoding unit  409  outputs the decoded data signal. 
         [0078]    The encoding unit  410  encodes the input control signal. The encoding unit  410  further stores to the control signal to be encoded, the DL_wireless characteristics information output from the wireless characteristics measuring unit  404 . The encoding unit  410  outputs the encoded control signal to the modulating unit  411 . 
         [0079]    The modulating unit  411  modulates the control signal output from the encoding unit  410 . The modulating unit  411  outputs the modulated control signal to the switching unit  414 . The encoding unit  412  encodes the input data signal. The encoding unit  412  outputs the encoded data signal to the modulating unit  413 . The modulating unit  413  modulates the data signal output from the encoding unit  412 . The modulating unit  413  outputs the modulated data signal to the switching unit  414 . 
         [0080]    The switching unit  414  switches between outputting to the wireless transmitting unit  415 , the control signal output from the modulating unit  411  and the data signal output from the modulating unit  413 . Via the transmission antenna  416 , the wireless transmitting unit  415  wirelessly transmits to the wireless base station  111 , the signal output from the switching unit  414 . 
         [0081]      FIG. 5  is a flowchart depicting an example of operation of the arrangement selecting unit of the wireless base station. The arrangement selecting unit  303  of the wireless base station  111 , for example, executes the following steps with respect to a downlink to the wireless terminal  131 . The arrangement selecting unit  303  obtains the downlink wireless characteristics of the wireless terminal  131  (step S 501 ). The downlink wireless characteristics of the wireless terminal  131 , for example, can be obtained from the DL_wireless characteristics information included in a control signal received from the wireless terminal  131 . 
         [0082]    The arrangement selecting unit  303  determines if the wireless characteristics obtained at step S 501  are at least a first base value (step S 502 ). If the wireless characteristics are greater than or equal to the first base value (step S 502 : YES), the arrangement selecting unit  303  determines if the wireless characteristics are at least a second base value (step S 503 ). The second base value is greater than the first base value. 
         [0083]    At step S 503 , if the wireless characteristics are greater than or equal to the second base value (step S 503 : YES), the arrangement selecting unit  303  configures the modulation scheme of the control signal to be 64QAM (step S 504 ), and transitions to step S 506 . If the wireless characteristics are less than the second base value (step S 503 : NO), the arrangement selecting unit  303  configures the modulation scheme of the control signal to be 16QAM (step S 505 ). 
         [0084]    The arrangement selecting unit  303  maps the control signal to a specific portion of the control signal resources (step S 506 ), and ends a series of mapping operations. The specific portion is a time when in the cells, no common reference signal is transmitted and a wireless-terminal-specific reference signal of the cell of the wireless base station is transmitted, and a radio resource of a frequency that differs from that of the wireless-terminal-specific reference signal. The specific portion, for example, is the resource elements  211  to  214  depicted in  FIG. 2 . 
         [0085]    At step S 502 , if the wireless characteristics are less than the first base value (step S 502 : NO), the arrangement selecting unit  303  configures the modulation scheme of the control signal to be QPSK (step S 507 ). The arrangement selecting unit  303  determines whether all mapping processing has been completed for other control signals for which the wireless characteristics are greater than or equal to the first base value (step S 508 ), and stands by until all the mapping processing has been completed (step S 508 : NO). 
         [0086]    At step S 508 , when all the mapping processing for the other control signals has been completed (step S 508 : YES), the arrangement selecting unit  303  determines whether the specific portion of the control signal resources is available (step S 509 ). If any of the specific portion is available (step S 509 : YES), the arrangement selecting unit  303  transitions to step S 506 . 
         [0087]    At step S 509 , if none of the specific portion is available (step S 509 : NO), the arrangement selecting unit  303  maps the control signal to a portion other than the specific portion of control signal resources (step S 510 ), and ends a series of mapping operations. A portion other than the specific portion, for example, is a radio resource of the time when in the cells, a common reference signal is transmitted; a radio resource of the time when in the cells, no common reference signal is transmitted and the wireless-terminal-specific reference signal of the cell of the wireless base station is also not transmitted. 
         [0088]    By the operations above, the arrangement selecting unit  303  configures based on the DL_wireless characteristics information, the modulating scheme performed on the control signal by the modulating unit  307 . Further, the arrangement selecting unit  303  preferentially maps a control signal that has been subject to, for example, 64QAM or 16QAM, which are higher order modulation schemes compared to QPSK and can suppress deterioration of the reception characteristics of a control signal subject to a higher order modulation scheme. 
         [0089]    Thus, the wireless base station  111  transmits a control signal, using a modulation scheme according to the wireless characteristics between the wireless base station  111  and the wireless terminal  131 . The higher the order of the modulation scheme used for a control signal, the more preferentially the wireless base station  111  transmits the control signal by a radio resource of the same time as the wireless-terminal-specific reference signal during the second timing when no common reference signal is transmitted. As a result, a high-order modulation scheme can be used on a control signal for the wireless terminal  131  that has favorable wireless characteristics, resource utilization efficiency is improved, and deterioration of the reception characteristics of the control signal can be suppressed. 
         [0090]      FIG. 6  is a diagram depicting one example of cell arrangement. A communications system  600  depicted in  FIG. 6  includes the wireless base stations  610 ,  620 ,  630 ,  640 . The wireless base station  610  forms cells  611  to  613  respectively of the identification information #1 to #3. A wireless base station  620  forms cells  621  to  623  respectively of identification information #4 to #6. A wireless base station  630  forms cells  631  to  633  respectively of the identification information #1 to #3. A wireless base station  640  for cells  641  to  643  respectively of identification information #4 to #6. 
         [0091]      FIGS. 7A, 7B, 7C, 7D, 7E, and 7F  are diagrams depicting one example of frequency shifting of the common reference signal. Physical resource blocks  701  to  706  depicted in  FIGS. 7A to 7F  represent downlink physical resource blocks of the cells respectively of the identification information #1 to #6. For example, the physical resource block  701  represents a physical resource block of the cells  611 ,  631  depicted in  FIG. 6 . The physical resource block  702  represents a physical resource block of the cells  612 ,  632  depicted in  FIG. 6 . 
         [0092]    In the physical resource blocks  701  to  706 , “C” represents a radio resource to which a common reference signal is mapped. The mapping of signals other than the common reference signal in the physical resource blocks  701  to  706  is not depicted. 
         [0093]    As depicted by the physical resource blocks  701  to  706 , the cells (the cells  611  to  613 ,  621  to  623 ,  631  to  633 ,  641  to  643 ) of the communications system  600  transmit a common reference signal by the same time resource. Further, in each of the cells of the communications system  600 , the common reference signal is transmitted by a frequency resources based on the identification information of the cell. 
         [0094]    For example, in the communications system  600 , frequency resources are correlated with the remainder that results when the cell ID (identification information) is divided by 6. Each of the cells of the communications system  600  transmit a common reference signal by a frequency resource that corresponds to the remainder that results when the cell ID thereof is divided by 6. 
         [0095]    Thus, in the cells of the communications system  600 , by performing frequency shifting of the common reference signals, interference of common reference signals of adjacent cells can be suppressed. For example, although the cell  611  and the cell  621  are adjacent to each other, the common reference signals are transmitted at different frequencies based on the respective identification information #1, #4 and therefore, interference can be suppressed. 
         [0096]    The wireless base station  111  above, for example, is applicable to at least any one of the wireless base stations  610 ,  620 ,  630 ,  640  of the communications system  600  depicted in  FIG. 6 . The wireless terminal  131  above, for example, is applicable to a wireless terminal located in at least any one of the cells of the communications system  600  depicted in  FIG. 6 . 
         [0097]    The wireless base station  111  and the wireless terminal  131  are also applicable to the cells of a communications system in which small cells (e.g., femtocells) are present in a large cell (macrocell). In such a communications system, if a wireless terminal in the large cell is forcibly connected to a small cell to transfer the traffic load of the large cell to the small cell, the effects of the interference by the reference signal transmitted from the large cell, on the wireless terminal becomes great. Further, in such a communications system, there are multiple cell arrangements and therefore, control of intercell reference signal interference becomes difficult. In contrast, by an application of the wireless base station  111  and the wireless terminal  131 , the effects of the interference from the reference signal can be suppressed. 
         [0098]      FIG. 8  is a diagram depicting one example of a hardware structure of the wireless base station. The wireless base station  111 , for example, can be realized by a communications apparatus  800  depicted in  FIG. 8 . The communications apparatus  800  includes a CPU  801 , memory  802 , a user interface  803 , a physical line communications interface  804 , and a wireless communications interface  805 . The CPU  801 , the memory  802 , the user interface  803 , the physical line communications interface  804 , and the wireless communications interface  805  are connected by a bus  809 . 
         [0099]    The CPU  801  (central processing unit) governs overall control of the communications apparatus  800 . The communications apparatus  800  may include the CPU  801  in plural. The memory  802  includes, for example, main memory and auxiliary memory. The main memory is, for example random access memory (RAM). The main memory is used as a work area of the CPU  801 . The auxiliary memory is, for example, nonvolatile memory such as a magnetic disk, an optical disk, flash memory, etc. In the auxiliary memory, various programs that cause the communications apparatus  800  to operate are stored. The programs stored in the auxiliary memory are loaded to the main memory and executed by the CPU  801 . 
         [0100]    The user interface  803  includes, for example, an input device that receives operational input from a user, an output device that outputs information to the user, etc. The input device, for example, can be realized by keys (e.g., a keyboard), a remote controller, etc. The output device, for example, can be realized by a display, speaker, etc. Further, the input device and the output device may be realized by a touch panel and the like. The user interface  803  is controlled by the CPU  801 . 
         [0101]    The physical line communications interface  804  is a communications interface that performs communication with an external destination (e.g., a bearer network such as mobile communications network) of the communications apparatus  800 , via a physical line. The physical line communications interface  804  is controlled by the CPU  801 . The wireless communications interface  805  performs wireless communication with an external destination (e.g., the wireless terminal  131 ) of the communications apparatus  800 . The wireless communications interface  805  is controlled by the CPU  801 . 
         [0102]    The wireless transmitting unit  310 , the transmission antenna  311 , the reception antenna  312 , and the wireless receiving unit  313  depicted in  FIGS. 3A and 3B , for example, can be realized by the wireless communications interface  805 . The other processing units depicted in  FIGS. 3A and 3B  can be realized, for example, by the CPU  801 . 
         [0103]      FIG. 9  is a diagram depicting one example of a hardware structure of the wireless terminals. The wireless terminal  131 , for example, can be realized by a communications apparatus  900  depicted in  FIG. 9 . The communications apparatus  900  includes a CPU  901 , memory  902 , a user interface  903 , and a wireless communications interface  904 . The CPU  901 , the memory  902 , the user interface  903 , and the wireless communications interface  904  are connected by a bus  909 . 
         [0104]    The CPU  901 , the memory  902 , the user interface  903 , and the wireless communications interface  904  are respectively the same as the CPU  801 , the memory  802 , the user interface  803 , and the wireless communications interface  805  depicted in  FIG. 8 . However, the wireless communications interface  904 , for example, is a communications interface that performs wireless communication with an external destination (e.g., the wireless base station  111 ) of the communications apparatus  900 . 
         [0105]    The reception antenna  401 , the wireless receiving unit  402 , the wireless transmitting unit  415 , and the transmission antenna  416  depicted in  FIGS. 4A and 4B , for example, can be realized by the wireless communications interface  904 . The other processing units depicted in  FIGS. 4A and 4B , for example, can be realized by the CPU  901 . 
         [0106]    As described, according to the communications system, the wireless base station, the wireless terminal, and the communications method, common reference signals between cells are frequency shifted, and a downlink control signal can be transmitted at a time when no reference signal is transmitted and a wireless-terminal-specific reference signal is transmitted. As a result, interference can be suppressed, and the accuracy of channel estimation as well as control signal reception characteristics can be improved. 
         [0107]    Therefore, for example, even when a high-order modulation scheme or spatial multiplexing transmission scheme is applied to the transmission of a downlink control signal, deterioration of the reception characteristics of the downlink control signal can be suppressed. Consequently, deterioration of the reception characteristics of the downlink control signal can be suppressed and the utilization efficiency of radio resources for the downlink control signal can be improved. 
         [0108]    Further, even when the mapping interval of wireless-terminal-specific reference signals is small, the accuracy of channel estimation can be raised and therefore, a decrease in the radio resources to which a data signal can be mapped can be suppressed. 
         [0109]    In addition, the accuracy of channel estimation can be raised without strong transmission power of the wireless-terminal-specific reference signal. As a result, even in cases where in the time domain, the transmission power when a radio symbol is transmitted is made constant, the accuracy of channel estimation can be improved without lowering the transmission power of a data signal transmitted on the same radio symbol. Therefore, deterioration of data signal reception characteristics can be suppressed. 
         [0110]    According to one aspect, the effects of interference from a common reference signal from an adjacent wireless base station can be reduced. Further, by placing a wireless control signal in a vicinity of a reference signal used in demodulation, the reception characteristics of the wireless control signal can be improved. 
         [0111]    All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.