Abstract:
A terminal device is equipped with: a sequence determination unit that determines a sequence number with a different pattern than the assignment pattern for the sequence number assigned to the cell in which the host device resides; a reference signal generation unit that generates a reference signal for a sequence group corresponding to the sequence number that has been determined; and a wireless transmission unit that transmits the reference signal that has been generated to a base station. The sequence determination unit employs a configuration whereby, with the sequence number assigned to the cell in which the host device resides as a cell-specific sequence number, a dynamic offset which periodically changes over time and for which the same value is not consecutive is added to the cell-specific sequence number, thereby determining a sequence number which never overlaps with the cell-specific sequence number.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a terminal apparatus, a base station apparatus, and a communication method each used for transmitting a reference signal. 
       BACKGROUND ART 
       [0002]    LTE (Long Term Evolution) Rel. 8 (Release 8) developed in 3GPP (3rd Generation Partnership Project Radio Access Network) employs SC-FDMA (single-carrier frequency-division multiple-access) involving a low PAPR (peak-to-average power ratio) and high electric-power use efficiency of a terminal apparatus (also referred to as UE) as an uplink communication scheme (refer to Non-Patent Literatures (hereinafter, abbreviated as NPLs) 1, 2, and 3). 
         [0003]    In the uplink of LTE, both a data signal of PUSCH (Physical Uplink Shared Channel) and a control signal of a PUCCH (Physical Uplink Control Channel) are transmitted on a per-subframe basis (refer to NPL 1). 
         [0004]    Each subframe includes two time slots. In each time slot, a plurality of SC-FDMA symbols and a DMRS (demodulation reference signal, also abbreviated as DRS) as one of the reference signals are time-multiplexed.  FIG. 1  illustrates an example of a subframe configuration of a PUSCH to which normal cyclic prefix is applied. 
         [0005]    Upon reception of a data signal on the PUSCH, the base station performs channel estimation using a DMRS. The result of channel estimation result is used to demodulate and decode an SC-FDMA symbol. In addition, in LTE-A (LTE-Advanced) Rel. 10 (Release 10), DFTS-OFDM (discrete-Fourier-transform spread orthogonal frequency division multiplexing), which is an extended version of SC-FDMA, has also become available. This is a method of dividing into two spectra the data signals of the PUSCH obtained as illustrated in  FIG. 1 , and mapping the divided signals to different frequencies, and thereby expanding the flexibility of scheduling without any change in the configuration of the PUSCH from that of SC-FDMA. 
         [0006]    On the other hand, the control signal of PUCCH has a plurality of formats each having a subframe configuration different from that of PUSCH (refer to NPL 1). Any one of the formats involves a subframe consisting of two slots, and each slot includes a plurality of SC-FDMA symbols and a plurality of DMRSs. The control signal of PUCCH is demodulated on the basis of a channel estimation result using the DMRS similarly to the data signal of PUSCH. 
         [0007]    In order to correctly decode information transmitted on the PUSCH and PUCCH, highly precise channel estimation by the DMRS is indispensable. In Rel. 10 of LTE-A, the DMRSs of different UEs connected to the single cell are transmitted while different cyclic shifts (CSs, equivalent to phase rotations) or orthogonal codes (also referred to as orthogonal cover codes (OCCs)) are allocated to the DMRSs. Accordingly, these DMRSs can be orthogonally multiplexed and orthogonally demultiplexed. 
         [0008]    On the other hand, the DMRSs transmitted by different UEs connected to different cells cannot be demultiplexed. Therefore, in order to suppress inter-cell interference and to perform highly precise channel estimation, it is necessary to use different signal sequences having a small correlation between different cells. 
         [0009]    Consequently, in Rel. 8-10 of LTE-A, 30 different base sequence groups mutually having low correlations are defined and are respectively allocated to cells. Moreover, in order to avoid causing uneven magnitude of interference, sequence group hopping that changes the sequence group used for every time slot is introduced. 
         [0010]    The base sequence group refers to a group including a plurality of base sequences having a plurality of sequence lengths. Moreover, the sequence group refers to a group obtained by gathering a plurality of signal sequences obtained by cyclically shifting all the respective base sequences in one base sequence group. Therefore, allocating a base sequence group to a cell means the same as allocating a sequence group corresponding to the base sequence group to the cell. 
         [0011]      FIG. 2  illustrates the feature of a sequence group allocated to each cell. 
         [0012]    For example, when N cell ID  represents a cell-specific identifier ID (cell ID number), a sequence group corresponding to a number given by the following expression is allocated in an n s -th slot in the cell. 
         [0000]      [1] 
         [0000]        u =( f   gh ( n   s )+ f   ss )mod 30 , n   s =0˜19  (Expression 1)
 
         [0013]    In expression 1, f gh (n s ) is a hopping pattern represented with a sequence group determined by a PN (pseudo-random) sequence having an initial value equal to an integer value obtained by truncating N cell ID /30 after the decimal point, and N cell ID  is given by an integer of 0 to 503. 
         [0014]    Offset f ss  is an integer determined by the following expression, and is provided to each cell. 
         [0000]    
       
         
           
             
               
                 
                   ( 
                   
                     Expression 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     f 
                     ss 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               N 
                               
                                 cell 
                                  
                                 
                                     
                                 
                                  
                                 ID 
                               
                             
                              
                             mod 
                              
                             
                                 
                             
                              
                             30 
                           
                         
                         
                           
                             for 
                              
                             
                                 
                             
                              
                             PUCCH 
                           
                         
                       
                       
                         
                           
                             
                               N 
                               
                                 cell 
                                  
                                 
                                     
                                 
                                  
                                 ID 
                               
                             
                             + 
                             
                               
                                 Δ 
                                 ss 
                               
                                
                               mod 
                                
                               
                                   
                               
                                
                               30 
                             
                           
                         
                         
                           
                             for 
                              
                             
                                 
                             
                              
                             PUSCH 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   2 
                   ] 
                 
               
             
           
         
       
     
         [0015]    In expression 2, Δ ss  is one of the integers of 0 to 29 and is indicated to a UE as a cell-specific parameter. The sequence group hopping can also be disabled (that is, setting f gh (n s )=0) on a per-cell basis or on a per-UE basis. 
         [0016]    The following is found from the above. 
         [0017]    1. In many cells included in a system, according to 504 pieces of different N cell ID , 30 sequence groups are repeatedly used in different cells. 
         [0018]    2. A sequence hopping pattern (f gh (n s )) is the same among 30 cells having continuous N cell ID , and the hopping patterns of sequence groups are different between the cells having an interval of N cell ID  equal to or more than 30. A cell group with the same hopping pattern (f gh (n s )) constitutes a cell cluster. 
         [0019]    3. In one cell cluster, offset f ss  varies from cell to cell in principle. In other words, this is equal to using patterns obtained by applying cell-specific sequence offset f ss  to a common sequence group hopping pattern. 
         [0020]      FIG. 3  illustrates an example of sequence group hopping.  FIG. 3  illustrates hopping patterns of a sequence group used in a cell cluster including N cell ID =0 to 29. For the sake of simplicity, it is assumed that Δ ss =0 in all cells. 
         [0021]    As illustrated in  FIG. 3 , a hopping pattern is common in one cell cluster, but a different sequence offset is given according to the value of N cell ID . Therefore, in a cell cluster, a different sequence group is always used in a different cell. Moreover, hopping patterns themselves differ in different cell clusters. Therefore, in cells in different cell clusters, even if sequence groups overlap, causing an increase in interference, the interference does not continue over a plurality of slots. 
         [0022]    The base sequence group allocated to each cell in this way is used, for example, for an uplink SRS (sounding reference signal) that is one of the reference signals and modulation of a PUCCH in addition to the DMRS. 
       CITATION LIST 
     Non-Patent Literatures 
     NPL 1 
       [0000]    
       
         3GPP TS 36.211 V10.4.0, “Physical Channels and Modulation (Release 10),” December 2011 
       
     
       NPL 2 
       [0000]    
       
         3GPP TS 36.212 V10.4.0, “Multiplexing and channel coding (Release 10),” December 2011 
       
     
       NPL 3 
       [0000]    
       
         3GPP TS 36.213 V10.4.0, “Physical layer procedures (Release 10),” December 2011 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0026]    CoMP reception (coordinated multipoint reception) has been discussed in Rel. 11 (Release 11) of LTE-A. The CoMP reception is a scheme that causes a plurality of base stations (also referred to as reception points (RPs)) to receive a PUSCH data signal and a PUCCH control signal from a UE and to combine these signals, thereby improving the throughput. In particular, the CoMP reception is effective in a heterogeneous network (HetNet) environment where a pico cell or RRH (remote radio head) having a smaller coverage is placed in a macro cell. This is because large interference caused between a macro cell and a pico cell and considered as a problem in HetNet can be combined as a desired signal when the CoMP reception is performed. 
         [0027]    As uplink CoMP in HetNet, an operation giving different N cell ID  to macro cells and pico cells (CoMP scenario 3) and an operation giving the same N cell ID  (CoMP scenario 4) are discussed. 
         [0028]    In the case of CoMP scenario 3, as illustrated in  FIG. 4 , UEs use different sequence groups according to the connected macro cell and pico cell. At this time, if cells connected to a plurality of UEs to be simultaneously subjected to CoMP reception are different, and different sequence groups are used for DMRSs of the UEs, the DMRSs cannot be orthogonally demultiplexed, which results in deterioration of channel estimation accuracy due to interference. 
         [0029]    On the other hand, in the case of CoMP scenario 4, as illustrated in  FIG. 5 , all UEs in a macro area use the same sequence group. Therefore, unlike CoMP scenario 3, different UEs use signal sequences obtained by cyclically shifting one base sequence using different shift amounts, thereby enabling orthogonal demultiplexing of DMRSs of the different UEs. However, only one sequence group can be used in a macro area, and therefore the number of usable signal sequences may be insufficient if the number of UEs increases. 
         [0030]    In order to solve such problems described above, enabling allocation of another base sequence to the UEs is discussed.  FIG. 6  explains an example in which some of the UEs use a base sequence not included in a base sequence group allocated to the cell under the HetNet environment.  FIG. 6  does not illustrate UEs using a base sequence group allocated to the cell but illustrates only UEs using a base sequence not allocated thereto. However, such a favorable method of allocating a base sequence has not been found yet. 
         [0031]    It is an object of the present invention to provide a terminal apparatus, a base station apparatus, and a communication method that can secure a large number of signal sequences that can be used by UEs in one cell without increasing deterioration of the process accuracy (deterioration of channel estimation accuracy in case of DMRSs and deterioration of estimation accuracy of channel quality in case of SRSs) due to interference to the reference signals. 
       Solution to Problem 
       [0032]    A terminal apparatus according to an aspect of the present invention is configured to communicate with a base station in a communication system in which a plurality of reference signals of a plurality of sequence groups are beforehand determined in association with a plurality of sequence numbers, the sequence numbers each having an offset added thereto are allocated to cells, the offset temporally varies depending on a hopping pattern that is common in one cell cluster and that is different among a plurality of cell clusters, and the offset is different from those of other cells belonging to the same cell cluster, the apparatus comprising: a sequence determination section that determines the sequence number using a pattern that is different from an allocation pattern of the sequence number allocated to the cell which contains the terminal apparatus; a reference signal generation section that generates a reference signal of the sequence group corresponding to the determined sequence number; and a radio transmission section that transmits the generated reference signal to the base station, wherein the sequence determination section determines the sequence number that does not overlap with a cell-specific sequence number always, by adding a variable offset that regularly varies depending on time and that does not have the same value continuously, wherein the cell-specific sequence number is the sequence number allocated to the cell which contains the terminal apparatus. 
         [0033]    A base station apparatus according to an aspect of the present invention is an apparatus in a communication system in which a plurality of reference signals of a plurality of sequence groups are beforehand determined in association with a plurality of sequence numbers, the sequence numbers each having an offset added thereto are allocated to cells, the offset temporally varies depending on a hopping pattern that is common in one cell cluster and that is different among a plurality of cell clusters, and the offset is different from those of other cells belonging to the same cell cluster, the apparatus comprising: a reception section that receives a radio signal from a terminal apparatus; and a signal extraction section that extracts the reference signal from the received radio signal, wherein the reference signal extracted by the signal extraction section includes the reference signal transmitted from the terminal apparatus in association with the sequence number which is determined by adding a cell-specific sequence number to a variable offset that regularly varies depending on time and that does not have the same value continuously, the determined sequence number not overlapping with the cell-specific sequence number always, wherein the cell-specific sequence number is the sequence number allocated to the cell which contains the terminal apparatus. 
         [0034]    A communication method according to an aspect of the present invention is a method used in a communication system in which a plurality of reference signal of a plurality of sequence groups are beforehand determined in association with a plurality of sequence numbers, the sequence numbers each having an offset added thereto are allocated to cells, the offset temporally varies depending on a hopping pattern that is common in one cell cluster and that is different among a plurality of cell clusters, and the offset is different from those of other cells belonging to the same cell cluster, the method comprising: determining the sequence number that does not overlap with a cell-specific sequence number always, by adding a variable offset that regularly varies depending on time and that does not have the same value continuously to the cell-specific sequence number by a terminal apparatus, wherein the cell-specific sequence number is the sequence number allocated to the cell which contains the terminal apparatus; generating the reference signal of the sequence group corresponding to the determined sequence number by the terminal apparatus; transmitting the generated reference signal to a base station by the terminal apparatus; and receiving the transmitted reference signal by the base station. 
       Advantageous Effects of Invention 
       [0035]    The present invention can secure a large number of signal sequences that can be used by UEs in one cell without increasing deterioration of channel estimation accuracy or estimation accuracy of channel quality due to interference in reference signals. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0036]      FIG. 1  illustrates a subframe configuration of a PUSCH to which a normal cyclic prefix is applied; 
           [0037]      FIG. 2  illustrates the feature of a sequence group allocated to each cell; 
           [0038]      FIG. 3  illustrates an example of sequence group hopping; 
           [0039]      FIG. 4  is an explanatory diagram illustrating an example operation of CoMP scenario 3; 
           [0040]      FIG. 5  is an explanatory diagram illustrating an example operation of CoMP scenario 4; 
           [0041]      FIG. 6  explains an example in which some UEs use a base sequence not included in a base sequence group allocated to the cell under the HetNet environment; 
           [0042]      FIG. 7  is a functional block diagram illustrating a main part of an RP according to an embodiment of the present invention; 
           [0043]      FIG. 8  is a block diagram illustrating the details of the RP according to the embodiment of the present invention; 
           [0044]      FIG. 9  is a functional block diagram illustrating a main part of a terminal apparatus according to an embodiment of the present invention; 
           [0045]      FIG. 10  is a block diagram illustrating the details of the terminal apparatus according to the embodiment of the present invention; 
           [0046]      FIG. 11  illustrates the situation of interference between a sequence group allocated by signaling for an individual terminal and a sequence group allocated to the cell in Embodiment 1; 
           [0047]      FIG. 12  explains the relationship between a first example of a sequence group allocated by signaling for an individual terminal and a sequence group allocated to the cell in Embodiment 2; 
           [0048]      FIG. 13  explains a second example of a sequence group allocated by signaling for an individual terminal in Embodiment 2; 
           [0049]      FIG. 14  explains a third example of a sequence group allocated by signaling for an individual terminal in Embodiment 2; 
           [0050]      FIG. 15  illustrates a sequence group allocated by signaling for an individual terminal in Embodiment 3; 
           [0051]      FIG. 16  illustrates a sequence group allocated by signaling for an individual terminal in Embodiment 6; and 
           [0052]      FIG. 17  illustrates a sequence group allocated by signaling for an individual terminal in Embodiment 7. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0053]    Each embodiment of the present invention will be explained below in detail with reference to the accompanying drawings. 
       Embodiment 1 
     Outline of Communication System 
       [0054]    A communication system according to the present embodiment includes a plurality of RPs  10  distributed at many points and a plurality of terminal apparatuses  20  moving freely around the respective points. The plurality of RPs  10  are placed at many points so that each macro cell can cover the almost entire moving ranges of the UEs. Each macro cell is supplied with 504 kinds of cell IDs (N cell ID =0 to 503), and a plurality of cell clusters are configured by grouping the macro cells into 30 continuous macro cells. The present embodiment can also be applied to the case of CoMP scenario 3 allocating different cell IDs respectively to placed RPs  10  in addition to macro cells, in the completely same way. 
         [0055]    If focusing on only a region around one terminal apparatus  20 , the communication system according to the present embodiment includes one or more RPs  10  and terminal apparatus  20  in the macro cell. Here, when the plurality of RPs  10  are provided, the plurality of RPs  10  are connected through interfaces, such as optical fibers, having a low delay and a large capacity. RP  10  may be a macro base station covering the range of a macro cell and may be a pico base station covering the range of a pico cell. Alternatively, RP  10  may be a receiver that is connected to a network and used for uplink reception. Terminal apparatus  20  and RP  10  transmitting and receiving the PUSCH are mutually determined by a prior hand-over process and a prior control process. 
         [0056]    [Configuration of RP  10  (Base Station)] 
         [0057]      FIG. 7  is a functional block diagram illustrating a main part of RP  10  according to the embodiment of the present invention. 
         [0058]    As illustrated in  FIG. 7 , RP  10  according to the present embodiment includes antenna  101 , radio reception section  104  receiving a radio signal through antenna  101 , reference signal extraction section  12  extracting a DMRS from the received signal, and reception signal processing section  13  removing received data from a data signal using the extracted DMRS. RP  10  includes transmission processing section  11  converting transmission data into a signal for radio transmission and radio-transmitting the signal through antenna  101 . 
         [0059]    When terminal apparatus  20  transmits the DMRS of a sequence group allocated to the cell, reference signal extraction section  12  extracts the DMRS of this signal sequence from the radio signal. Moreover, when terminal apparatus  20  transmits the DMRS of a sequence group different from that allocated to the cell, reference signal extraction section  12  extracts the DMRS according to this other sequence group from the radio signal. How to allocate the other sequence group will be explained below in detail. 
         [0060]    Reception signal processing section  13  performs channel estimation from the extracted DMRS and removes received data from the signal received from terminal apparatus  20  using this channel estimation result. 
         [0061]      FIG. 8  is a block diagram illustrating the details of RP  10  according to the embodiment of the present invention. 
         [0062]    As illustrated in  FIG. 8 , RP  10  specifically includes antenna  101 , radio transmission section  102 , scheduling information generation section  103 , radio reception section  104 , CP removal section  105 , FFT section  106 , demapping section  107 , demultiplexing section  108 , equalization section  109 , inter-RP IF (inter-RP interface)  110 , channel estimation section  111 , control section  112 , IDFT section  113 , demodulation section  114 , descrambling section  115 , decoding section  116 , determination section  117 , and retransmission control section  118 . RP  10  receives a PUSCH data signal transmitted from the terminal and a PUCCH control signal. 
         [0063]    In these sections, CP removal section  105 , FFT section  106 , demapping section  107 , and demultiplexing section  108  function as reference signal extraction section  12  in  FIG. 7 . CP removal section  105 , FFT section  106 , demapping section  107 , demultiplexing section  108 , equalization section  109 , inter-RP IF  110 , channel estimation section  111 , control section  112 , IDFT section  113 , demodulation section  114 , descrambling section  115 , decoding section  116 , determination section  117 , and retransmission control section  118  function as reception signal processing section  13  in  FIG. 7 . Scheduling information generation section  103  and radio transmission section  102  function as transmission processing section  11  in  FIG. 7 . 
         [0064]    Hereinafter, the details of each section will be explained. 
         [0065]    Control section  112  sends information on a base sequence used for a DMRS and information on a scrambling sequence used for scrambling data to channel estimation section  111  and descrambling section  115 , respectively. Specifically, control section  112  performs the following processes. 
         [0066]    Control section  112  generates a base sequence used for a DMRS multiplexed into the PUSCH data signal transmitted by terminal apparatus  20 , and sends the base sequence to channel estimation section  111 . 
         [0067]    Whether a base sequence to be used by terminal apparatus  20  is allocated to a cell or allocated by signaling for an individual terminal is beforehand determined in the network and is indicated to terminal apparatus  20 . The network refers to a whole network in which all RPs  10  and a plurality of control apparatuses controlling a communication system are wired-connected. This indication may be sent by correspondent RP  10  or may be sent by another RP  10  connected to the network. Moreover, this indication may be sent as RRC (Radio Resource Control) control information for an individual terminal, and may be indicated by being included in a MAC (Media Access Control) header. Alternatively, this indication may be sent by a control signal transmitted through a downlink (PDCCH). In addition, this indication is sent to terminal apparatus  20  only when a base sequence is allocated individually, and the base sequence allocated to the cell may be used when this indication is not sent. 
         [0068]    When the base sequence allocated by signaling for an individual terminal is a sequence determined and indicated by correspondent RP  10 , correspondent RP  10  indicates this information to another RP  10  through inter-RP IF  110 . On the other hand, when the sequence is determined and indicated by another RP  10 , correspondent RP  10  receives this information from the other RP  10  through inter-RP IF  110 . Therefore, control section  112  can know the base sequence used by terminal apparatus  20 . 
         [0069]    Control section  112  also generates a scrambling sequence used for scrambling data and sends the scrambling sequence to the descrambling section. Whether a scrambling sequence to be used is allocated to a cell or allocated by signaling for an individual terminal is beforehand determined in the network and is indicated to terminal apparatus  20 . These determination and indication may be performed by correspondent RP  10  and may be performed by another RP  10  connected to the network. Moreover, this indication may be sent as RRC (Radio Resource Control) control information for an individual terminal, and may be included in a MAC header. Alternatively, this indication may be sent by a control signal transmitted through a downlink (PDCCH). When this indication is not sent particularly, terminal apparatus  20  uses the scrambling sequence allocated to the cell. 
         [0070]    When the scrambling sequence subject to signaling for an individual terminal is a sequence determined and indicated by correspondent RP  10  for terminal apparatus  20 , correspondent RP  10  indicates the information to another RP  10  through inter-RP IF  110 . On the other hand, when the scrambling sequences is determined and indicated by another RP  10  for terminal apparatus  20 , correspondent RP  10  receives the information from the other RP  10  through inter-RP IF  110 . Therefore, control section  112  can know the scrambling sequence used for scrambling on data. 
         [0071]    Scheduling information generation section  103  indicates the bandwidth and the frequency position of a PUSCH included in a control signal (PDCCH) to terminal apparatus  20 , through radio transmission section  102  and antenna  101 . The determination of the bandwidth and the frequency position is called frequency mapping. An instruction on frequency mapping may be issued by correspondent RP  10  or may be issued by another RP  10  connected to the network. 
         [0072]    When information on frequency mapping is information determined and indicated by correspondent RP  10  for terminal apparatus  20 , scheduling information generation section  103  indicates the information to another RP  10  through inter-RP IF  110 . On the other hand, when the information is determined and indicated by another RP  10  for terminal apparatus  20 , scheduling information generation section  103  receives the information from the other RP  10  through inter-RP IF  110 . 
         [0073]    Scheduling information generation section  103  sends the above-described information on frequency mapping to demapping section  107 . 
         [0074]    Radio reception section  104  receives a PUSCH data signal transmitted from the terminal, through the antenna, and performs a reception process such as down conversion and A/D conversion. 
         [0075]    CP removal section  105  removes a CP (Cyclic Prefix) added to the DMRS and the data signal of the PUSCH and sends the signal after the elimination to FFT section  106 . 
         [0076]    FFT section  106  applies FFT (Fast Fourier Transform) to the DMRS and the data signal of the PUSCH to convert the signals into frequency domain signals, and then sends the converted signals to demapping section  107 . 
         [0077]    Demapping section  107  demaps the inputted frequency domain PUSCH according to scheduling information. 
         [0078]    Demultiplexing section  108  demultiplexes the DMRS and the data signal, which are time-multiplexed, from the PUSCH signal inputted from demapping section  107 , and sends these signals to channel estimation section  111  and equalization section  109 , respectively. 
         [0079]    Channel estimation section  111  performs channel estimation using base sequence information inputted from control section  112  and a received DMRS inputted from demultiplexing section  108 . Channel estimation section  111  sends a channel estimation result to equalization section  109 . 
         [0080]    Equalization section  109  performs the following processes. 
         [0081]    Specifically, equalization section  109  equalizes the data signal inputted from demultiplexing section  108  using the channel estimation result inputted from channel estimation section  111 . This process may be performed independently by each RP  10  participating in the reception. Alternatively, this process may be performed by single RP  10  among RPs  10  participating in the reception, by transmitting and receiving the channel estimation results and the data signals, which are acquired by the plurality of RPs  10 , through inter-RP IF  110 . 
         [0082]    When each RP  10  participating in the reception performs the equalization independently, equalization section  109  transmits and receives the output of demultiplexing section  108  through inter-RP IF  110 , and aggregates the results in one of RPs  10  participating in the reception. On the other hand, when single RP  10  among RPs  10  participating in the reception performs the equalization by transmitting and receiving the channel estimation results and the data signals, which are acquired by the plurality of RPs  10 , through inter-RP IF  110 , equalization section  109  performs the equalization using the plurality of aggregated channel estimation results and the plurality of received data signals. 
         [0083]    In both of the above cases, the equalization output is obtained by equalization section  109  of single RP  10  among RPs  10  participating in the reception. Equalization section  109  outputs the signal that has been equalized and outputted, to IDFT section  113 . 
         [0084]    IDFT section  113  converts the equalized frequency domain data signal into a data symbol sequence in a time domain and sends this data symbol sequence to demodulation section  114 . 
         [0085]    Demodulation section  114  performs data demodulation on the input signal, acquires a bit sequence, and outputs the bit sequence to descrambling section  115 . 
         [0086]    Descrambling section  115  descrambles the bit sequence received from demodulation section  114 , on the basis of information on a scrambling sequence received from control section  112 . The bit sequence after the descrambling process is then outputted to decoding section  116 . 
         [0087]    Decoding section  116  performs error correction decoding on the input bit sequence and outputs the bit sequence after the decoding process to determination section  117 . 
         [0088]    Determination section  117  performs error detection on the inputted bit sequence. The error detection is performed using CRC bits added to the bit sequence. If the determination result based on the CRC bits has no error, determination section  117  removes received data and indicates ACK (Acknowledgement) to retransmission control section  118 . On the other hand, if the determination result based on the CRC bits has an error, determination section  117  indicates NACK (Negative Acknowledgement) to retransmission control section  118 . 
         [0089]    Retransmission control section  118  performs retransmission control on the data signal of the bit sequence including the detected error. 
         [0090]    [Configuration of Terminal Apparatus  20 ] 
         [0091]      FIG. 9  is a functional block diagram illustrating a main part of terminal apparatus  20  according to the embodiment of the present invention. 
         [0092]    As illustrated in  FIG. 9 , terminal apparatus  20  according to the present embodiment includes antenna  201 , sequence determination section  22  determining a sequence group used for generation of a DMRS, DMRS generation section  203  generating the DMRS using the determined sequence group, and transmission processing section  23  performing radio transmission by generating a signal for radio transmission from transmission data and the generated DMRS. Terminal apparatus  20  also includes reception processing section  24  receiving radio signal sent from RP  10  and extracting received data. 
         [0093]    When an instruction to use the sequence group allocated to the cell is issued, sequence determination section  22  determines this sequence group as a sequence group for generating the DMRS. On the other hand, when an instruction to use a sequence group which is different from that allocated to the cell is issued, sequence determination section  22  determines this different sequence group as a sequence group for generating the DMRS. Hereinafter, a determination method for this different sequence group and an instruction indicating which sequence group to use will be explained in detail. 
         [0094]    DMRS generation section  203  generates a DMRS of a sequence group determined in sequence determination section  22 , and sends the DMRS to transmission processing section  23 . 
         [0095]    Transmission processing section  23  maps transmission data and the generated DMRS into a resource block to generate a transmission signal, and transmits this transmission signal by radio through antenna  201 . 
         [0096]      FIG. 10  is a block diagram illustrating the details of terminal apparatus  20  according to the embodiment of the present invention. 
         [0097]    Specifically, as illustrated in  FIG. 10 , terminal apparatus  20  includes antenna  201 , control section  202 , DMRS generation section  203 , encoding section  204 , scrambling section  205 , modulation section  206 , multiplexing section  207 , DFT section  208 , mapping section  209 , IFFT section  210 , CP addition section  211 , radio transmission section  212 , radio reception section  213 , and scheduling information extraction section  214 . Terminal apparatus  20  transmits a PUSCH data signal and a PUCCH control signal to RP  10 . 
         [0098]    In these sections, control section  202  functions as sequence determination section  22  in  FIG. 9 . Control section  202  also includes a functional section (corresponding to a standard sequence determination section) determining a signal sequence to be used for a DMRS from the sequence group allocated to the cell. Encoding section  204 , scrambling section  205 , modulation section  206 , multiplexing section  207 , DFT section  208 , mapping section  209 , IFFT section  210 , CP addition section  211 , and radio transmission section  212  function as transmission processing section  23  in  FIG. 9 . Radio reception section  213  and scheduling information extraction section  214  function as reception processing section  24  in  FIG. 9 . 
         [0099]    Next, the details of each section will be explained. 
         [0100]    Control section  202  outputs information on a base sequence used for a DMRS and information on a scrambling sequence used for scrambling on data, to DMRS generation section  203  and scrambling section  205 , respectively. Specifically, control section  202  performs the following processes. 
         [0101]    Control section  202  outputs a base sequence used for a DMRS to be multiplexed on a PUSCH, to DMRS generation section  203 . Whether the base sequence to be used is allocated to the cell or allocated by signaling for an individual terminal is beforehand indicated from the network to terminal apparatus  20 . When this indication is not especially sent, terminal apparatus  20  may use the base sequence allocated to the cell. 
         [0102]    Control section  202  generates a scrambling sequence to be used for scrambling on data and outputs the generated scrambling sequence to scrambling section  205 . Whether the base sequence to be used is allocated to the cell or allocates by signaling for an individual terminal is beforehand indicated from the network to terminal apparatus  20 . In the case of no indication, control section  202  generates the scrambling sequence allocated to the cell. 
         [0103]    Encoding section  204  adds CRC bits to the bit sequence of transmission data inputted from a high layer configuration, performs error correcting coding, and outputs a codeword bit sequence to scrambling section  205 . 
         [0104]    Scrambling section  205  scrambles the code word bit sequence using the scrambling sequence inputted from control section  202 . Scrambling section  205  then sends the result from the scrambling to modulation section  206 . 
         [0105]    Modulation section  206  performs data modulation on the inputted bit sequence and sends a data signal after the modulation to multiplexing section  207 . 
         [0106]    Scheduling information extraction section  214  first receives a downlink control signal (PDCCH) through antenna  201  and radio reception section  213 , and obtains the bandwidth and the frequency position of a PUSCH indicated from the network. The bandwidth and the frequency position are called frequency mapping information. Scheduling information extraction section  214  then sends the acquired frequency mapping information to mapping section  209  and DMRS generation section  203 . 
         [0107]    DMRS generation section  203  generates a DMRS having a sequence length corresponding to the bandwidth of the PUSCH inputted from scheduling information extraction section  214  using the base sequence inputted from control section  202 , and inputs the generated DMRS to multiplexing section  207 . 
         [0108]    Multiplexing section  207  configures a subframe while time-multiplexing the data signal inputted from modulation section  206  and the DMRS inputted from the DMRS generation section and sends the signal of this subframe to DFT section  208 . 
         [0109]    DFT section  208  applies DFT (Discrete Fourier Transform) to the input signal and acquires a frequency domain signal. DFT section  208  sends this frequency domain signal to mapping section  209 . 
         [0110]    Mapping section  209  maps the inputted frequency domain signal according to the frequency mapping information inputted from scheduling information extraction section  214 , and sends the mapped signal to IFFT section  210 . 
         [0111]    IFFT section  210  applies IFFT (Inverse Fast Fourier Transform) to the input signal and acquires a time-domain signal, and outputs the acquired time-domain signal to CP addition section  211 . 
         [0112]    CP addition section  211  adds a CP (Cyclic Prefix) to the inputted data signal and DMRS and outputs the resultant signal to radio transmission section  212 . 
         [0113]    Radio transmission section  212  performs transmitting processes such as D/A conversion and up-conversion, and transmits a PUSCH signal to RP  10  through antenna  201 . 
         [0114]    [DMRS Transmission-Related Operations] 
         [0115]    Next, the process flow relating to transmission of a DMRS will be explained with reference to Steps 1 to 5. 
         [0116]    (Step 1) Before transmission and reception of a PUSCH, the network indicates whether a base sequence to be used for a DMRS is allocated to the cell or allocated by signaling for an individual terminal, to terminal apparatus  20 . This indication is delivered to control section  202 . 
         [0117]    (Step 2) Upon receipt of an instruction to use the base sequence group allocated, control section  202  determines employment of a base sequence group given by expression 1. DMRS generation section  203  then generates the DMRS of this sequence group in response to an instruction from control section  202 . When no instruction is particularly issued from the network, a DMRS is generated using the base sequence group given by expression 1. 
         [0118]    On the other hand, when use of the base sequence allocated by signaling for an individual terminal is indicated, control section  202  determines employment of the base sequence group given by expression 3 described below. DMRS generation section  203  then generates the DMRS of this sequence group in response to an instruction from control section  202 . 
         [0000]      [3] 
         [0000]        ũ= ( f   gh ( n   s )+ {tilde over (f)}   ss ( n   s ))mod 30  (Expression 3)
 
         [0119]    In expression 3, f ss ˜(n s ) is a sequence offset amount varying depending on the value of n s  (0 to 19) and has the following features. 
         [0000]    
       
         
           
             
               
                 
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         [0120]    X is the total number of elements of a set of the values that can be taken by variable x, and Pr represents the probability to satisfy an expression in the brackets. In expression 4, n s  represents each time slot in one cycle period (n s =0 to 19) in which sequence groups are allocated repeatedly. Hereinafter, the variables and functions with a tilde symbol in expressions are indicated by attaching “˜” to the side of the alphabet symbols. 
         [0121]    Here, F(n s ) represents the time variable component of a sequence offset (corresponding to the variable offset). Sequence number u˜ to be allocated by signaling for an individual terminal is a value obtained by adding time variable component F(n s ) of the variable offset to sequence number u to be allocated to the cell modulo 30 (refer to expression (1) and (3)). Therefore, a cell in which sequence number u allocated to the cell overlaps with sequence number u˜ allocated by signaling for an individual terminal is determined by the value of F(n s ). 
         [0122]    The first expression of the simultaneous expressions of expression 4 represents a condition on which F(n s ) is not equal to zero modulo 30. In other words, this expression represents a condition on which sequence number u˜ allocated by signaling for an individual terminal always does not overlap with sequence number u allocated to the cell including terminal apparatus  20 . 
         [0123]    The second expression of the simultaneous expressions of expression 4 represents that if time variable component F(n s ) of the sequence offset takes X different values x (=x 1 , x 2 , x 3  . . . ) during one cycle period (n s =0 to 19), a probability that F(n s ) is equal to each value is equal to 1/X. In other words, this expression represents a condition that each value (x=x 1 , x 2 , x 3  . . . ) appears equally as the value of F(n s ). 
         [0124]    If value x of F(n s ) does not take the same value continuously, the same value may be taken around two or three times. In other words, in one cell, sequence number u allocated to the cell may discontinuously overlap with sequence number u˜ allocated to terminal apparatus  20  around two or three times during one cycle period. This condition can be represented as Pr[F(n s )=x]≦3/X. 
         [0125]    Moreover, it is significantly favorable that time variable component F(n s ) of the sequence offset takes values different from one another in one cycle period (n s =0 to 19). In other words, it is significantly favorable that sequence number u allocated to each cell in the same cell cluster overlaps with sequence number u˜ allocated by signaling for an individual terminal, and that the cell differs in all time slots n s . In this case, the element number of a set of values which x can take is equal to X=20. 
         [0126]    In addition, it is sufficient that cells corresponding to sequence numbers u and u˜ that overlap with each other are distributed to such a degree that the cells differ at about 50 or more percent in one cycle period (n s =0 to 19). This condition can be represented as X≧0.5×20. 
         [0127]      FIG. 11  illustrates the situation of interference between the base sequence group given by expressions 3 and 4 and the sequence group allocated to the cell. 
         [0128]    In  FIG. 11 , six macro cells having N cell ID =0 to 5 exist, and a base sequence group given by expressions 3 and 4 is used in a macro cell having N cell ID= 0. In the example of  FIG. 11 , the value of Δ ss  in expression 2 is set to 0 in any cell, but may be set to any value. These macro cells belong to the same cell cluster and use a common pattern of sequence group hopping. 
         [0129]    In  FIG. 11 , an arrow extending from point Pu using a base sequence group given by expressions 3 and 4 indicates that this base sequence group overlaps a base sequence group allocated to a cell (referred to as a cell-specific base sequence group) indicated by the arrow. For example, in time slot n s =0, the base sequence group given by expressions 3 and 4 overlaps with the cell-specific base sequence group of the macro cell having N cell ID =3. Therefore, the influence of interference with a DMRS transmitted and received in this macro cell increases. However, in time slot n s =1, the value of sequence offset f ss ˜(n s ) varies, and the base sequence group given by expressions 3 and 4 overlaps with the base sequence group allocated to the macro cell having N cell ID =1. This base sequence group is different from that of the macro cell having N cell ID =3 involving the previous overlapping. This reduces interference with a DMRS transmitted and received in the macro cell having N cell ID =3. Similarly, each time slot n s  varies as 2, 3, . . . , a cell involving the overlapping changes as the macro cell having N cell ID =4, the macro cell of N cell ID =2, . . . . 
         [0130]    (Step 3) Terminal apparatus  20  configures a subframe while time-multiplexing the generated DMRS and data signal generated independently, and maps a PUSCH signal according to frequency mapping information indicated from RP  10 . Then, this signal is transmitted to RP  10 . 
         [0131]    (Step 4) RP  10  demultiplexes the DMRS and the data signal from the received PUSCH signal. Then, RP  10  performs channel estimation using the DMRS. 
         [0132]    (Step 5) RP  10  equalizes a data signal on the basis of the channel estimation result. Methods for this equalization include a method of independently performing equalization in the plurality of RPs  10  participating in the reception, and a method of collectively performing equalization by aggregating received signals from the plurality of RPs  10  participating in the reception. When each RP  10  independently performing equalization, the equalization result is aggregated in one of RPs  10 , and RP  10  involving the aggregation performs demodulation and decode processes on the received data. On the other hand, when equalization is performed in an aggregated manner in the plurality of RPs  10  participating in the reception, the channel estimation result and the reception data signal are aggregated in one of RPs  10 , and RP  10  involving the aggregation performs equalization and synthesis. Then, this RP  10  performs demodulation and decode on the received data. 
         [0133]    As described above, according to the communication system, terminal apparatus  20 , and RP  10  in Embodiment 1, terminal apparatus  20  is enabled to use a sequence group different from a sequence group allocated to the cell. This can secure many sequence groups that can be used by terminal apparatus  20  in one macro cell. 
         [0134]    Furthermore, a sequence group allocated by signaling for an individual terminal always differs from a sequence group allocated to the macro cell including terminal apparatus  20 . Therefore, a DMRS does not interfere with another terminal apparatus  20  using the cell-specific sequence group in the same macro cell. 
         [0135]    Furthermore, a sequence group allocated by signaling for an individual terminal overlaps a sequence group allocated to another macro cell, but this overlapping does not continuously occur in the same macro cell. Furthermore, macro cells involving the overlapping are distributed in each time slot in the same cell cluster. Therefore, in relatively nearby cells, interference in DMRSs does not occur continuously. Therefore, in RPs  10  of each cell, the accuracy of channel estimation does not significantly deteriorate due to interference in DMRSs. 
         [0136]    Furthermore, Embodiment 1 adds offset component F(n s ) varying with time to the expression of a sequence group allocated specifically to a cell, and determines a sequence group allocated by individual signaling for terminal apparatus  20 . Therefore, if this F(n s ) is regular, the hopping pattern of a sequence group used by terminal apparatus  20  is not the same as or similar to the hopping pattern of another cell cluster. Therefore, in respective cells of another relatively distant cell clusters, any one cell does not tend to have overlapping sequence groups. That is, in cells of the other cell cluster, even if a cell has sequence groups overlapping continuously, such overlapping does not occur for a long period. Therefore, also in each RP  10  of the other cell cluster, the accuracy of channel estimation does not significantly deteriorate due to interference in DMRSs. 
         [0137]    Moreover, existing base sequence group can be reused in order to secure many sequence groups that can be used by terminal apparatus  20  in one macro cell in Embodiment 1, and it is not necessary to define a new base sequence group or a new hopping pattern. Moreover, a sequence group allocated by signaling for an individual terminal can be determined only by adding one parameter F(n s ) to the determination expression of a sequence group allocated specifically to a cell. Therefore, the circuit size for determining these sequence groups does not increase. 
       Embodiment 2 
       [0138]    In a communication system according to Embodiment 2, only the allocation pattern of the base sequence group performed by signaling for an individual terminal differs from Embodiment 1. Therefore, only different points will be explained in detail. 
         [0139]    A base sequence group allocated by signaling for an individual terminal in Embodiment 2 is characterized by sequence offset f ss ˜(n s ) of expression 5 described below. Number u˜ of the base sequence group allocated by signaling for an individual terminal is determined by expression 3. 
         [0000]      [5] 
         [0000]        {tilde over (f)}   ss ( n   s )=( f   ss   +λ·n   s   +z )mod 30  (Expression 5)
 
         [0140]    In expression 5, λ and z are integers that are not equal to 0. 
         [0141]      FIG. 12  illustrates allocation patterns of base sequence groups in λ=1 and z=1. 
         [0142]    In this example, similarly to the case of  FIG. 11 , assuming that some terminal apparatuses  20  in the macro cell having N cell ID =0 use a base sequence group determined by expression 5. In this case, a base sequence group allocated to these terminal apparatuses  20  overlaps with a base sequence group allocated to the macro cell having N cell ID =1 in time slot n s =0. The base sequence group overlaps with a base sequence group allocated to the macro cell having N cell ID =2 in time slot n s =1. Similarly, base sequence groups allocated to the terminal apparatus  20  in time slots n s =2 to 19 sequentially overlap with each of the base sequence groups allocated to the macro cells having N cell ID =3 to 20 once. On the other hand, base sequence groups allocated to terminal apparatus  20  do not overlap with base sequence groups allocated to the cells having N cell ID =0 and N cell ID =21 to 29 once. 
         [0143]    Attention is focused on the case where a base sequence group is allocated to terminal apparatus  20  in the macro cell having N cell ID =1 on the basis of expression 5. In this case, the sequence group overlaps with a base sequence group allocated to the cell having N cell ID =2 in time slot n s =0, and overlaps with a base sequence group allocated to the cell having N cell ID =3 in time slot n s =1. Subsequent overlapping of the sequence group similarly occurs in each of the plurality of cells until n s =19 once. 
         [0144]    Here, sequence groups allocated by signaling for an individual terminal are compared between the macro cell having N cell ID =0 and the macro cell having N cell ID =1. As a result, these sequence groups are always different from each other in each time slot n s . As described above, it is found that overlapping always does not occur between base sequence groups individually allocated to respective terminal apparatuses  20  connected to different cells belonging to the same cell cluster. 
         [0145]      FIG. 13  illustrates allocation patterns of base sequence groups in the case of varying parameter z.  FIG. 14  illustrates allocation patterns of base sequence groups in the case of varying parameter λ.  FIGS. 13 and 14  illustrate that a base sequence group of each cell overlapping with an arrow line overlaps with a base sequence group allocated by signaling for an individual terminal in each time slot. 
         [0146]    As illustrated in  FIGS. 13 and 14 , variations in parameters λ and z cause variations in allocation patterns of base sequence groups by signaling for an individual terminal. As illustrated in  FIG. 13 , a variation in parameter z cause variations in the allocation patterns so that a cell and time slot n s  which involve overlapping of base sequence groups may be shifted by a number proportional to parameter z. As illustrated in  FIG. 14 , a variation in parameter λ may also cause, for example, such a variation that base sequence groups overlap twice in a cell. 
         [0147]    As described above, according to the communication system, terminal apparatus  20 , and RP  10  in Embodiment 2, the same advantageous effects as Embodiment 1 can be acquired only by adding a time variable component of a simple linear function to a sequence offset. 
         [0148]    Furthermore, according to Embodiment 2, parameter z and parameter λ are suitably selected to thereby enable constant prevention of overlapping between respective sequence groups allocated by signaling for an individual terminal in a plurality of cells belonging to the same cell cluster. Therefore, interference in DMRSs does not occur between the plurality of terminal apparatuses  20  included in each of a plurality of cells subject to this allocation. Moreover, suitable selection of parameter z and parameter λ for each cell also enables such configuration that the above-described sequence groups overlap only at a predetermined ratio. 
         [0149]    Embodiment 2 has been described using an example configuration based on adding a linear function of time slot n s  to sequence offset f ss . However, as long as the conditions of expression 4 is satisfied, Embodiment 2 may be provided by applying a configuration based on adding a polynomial, such as a quadratic function of time slot n s , or various functions to sequence offset f ss . 
         [0150]    Moreover, Embodiment 2 has been described using an example configuration based on finding sequence offset f ss ˜(n s ) including a time variable component through a calculation process. However, an alternative configuration may also be employed in which such sequence offset f ss ˜(n s ) is stored in a storing section, for example, as a data table, and then control section  202  reads and finds the sequence offset from the storing section. 
         [0151]    An example of a data table storing sequence offset f ss ˜(n s ) will be illustrated below. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 n s   
                 {tilde over (f)} ss (n s ) 
               
               
                   
               
             
             
               
                 0 
                 f ss  + 10 
               
               
                 1 
                 f ss  + 5 
               
               
                 2 
                 f ss  + 8 
               
               
                 3 
                 f ss  + 2 
               
               
                 4 
                 f ss  + 9 
               
               
                 5 
                 f ss  + 1 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                 19  
                 f ss  + 6 
               
               
                   
               
             
          
         
       
     
         [0152]    The configuration in which to store sequence offset f ss ˜(n s ) including a time variable component can be employed as described above to thereby represent even a sequence offset including a time variable component that cannot be represented by a simple function easily. Moreover, sequence offsets do not need to be found through calculation, and therefore a necessary calculation circuit can be reduced. 
       Embodiment 3 
       [0153]    In a communication system according to Embodiment 3, the allocation pattern of a base sequence group by signaling for an individual terminal is applicable to an OCC multiplex transmission scheme for a DMRS, and the configuration other than that is the same as that of Embodiment 1. Therefore, only different points will be explained in detail. 
         [0154]    In a PUSCH, two DMRSs located in a subframe are multiplied by [+1, +1] or [+1, −1] as an orthogonal cover code (OCC) to thereby enable two different terminal apparatuses  20  to code-multiplex and transmit the DMRSs of the same sequence. However, in this transmission scheme, each terminal apparatus  20  needs to use a DMRS of the same sequence in two time slots of a subframe. 
         [0155]    Incidentally, Rel. 10 of LTE-A includes a mechanism for stopping the hopping of a base sequence group by a terminal-specific RRC control signal, and can stop the hopping to thereby employ DMRS transmission using OCC multiplex. 
         [0156]      FIG. 15  is an explanatory diagram illustrating sequence groups allocated by signaling for an individual terminal in Embodiment 3. 
         [0157]    As illustrated in  FIG. 15 , sequence groups allocated by signaling for an individual terminal in Embodiment 3 employ configurations in which a base sequence group is allocated by adding sequence offsets of expressions different between an odd time slot (n s  mod 2=1) and an even time slot (n s  mod 2=0). Consequently, Embodiment 3 allocates the same sequence number to an odd time slot (n s  mod 2=1) and an even time slot (n s  mod 2=0). 
         [0158]    Specifically, such allocation can be implemented as follows. That is, a configuration adding sequence offsets of expression 6 described below in an odd time slot and an even time slot and obtaining sequence numbers can be employed. 
         [0000]      [6] 
         [0159]    In a time slot satisfying (n s  mod 2)=0, 
         [0000]        {tilde over (f)}   ss ( n   s )=( f   ss   +λ·n   s /2 +z )mod 30 
         [0160]    In a time slot satisfying (n s  mod 2)=1, 
         [0000]        {tilde over (f)}   ss ( n   s )= {tilde over (f)}   ss ( n   s −1)  (Expression 6)
 
         [0161]    As a specific example of this,  FIG. 15  illustrates base sequence group hopping patterns in λ=1 and z=1. As illustrated in  FIG. 15 , it is found that base sequence groups are the same in two time slots in a subframe. 
         [0162]    According to the communication system of Embodiment 3, base sequence groups allocated by signaling for an individual terminal can be used for transmission of DMRSs using an OCC multiplex in two terminal apparatuses  20 . 
         [0163]    Moreover, a base sequence group allocated to the cell has a sequence varying in units of slots. Therefore, a sequence group allocated by signaling for an individual terminal does not continuously overlap with a sequence group allocated to the cell in the same macro cell. Therefore, a method for allocating a base sequence group in Embodiment 3 can also reduce interference from a DMRS of terminal apparatus  20  having a sequence group allocated by signaling for an individual terminal with a DMRS of terminal apparatus  20  to which a cell-specific sequence group is applied, similarly to Embodiment 1 and Embodiment 2. 
         [0164]    In Embodiment 3, an alternative configuration can also be employed in which sequence offset f ss ˜(n s ) including a time variable component is stored in a storing section as a data table and then is read and found from the storing section. 
         [0165]    An example of a data table storing sequence offset f ss —(n s ) in Embodiment 3 will be illustrated below. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 n s   
                 {tilde over (f)} ss (n s ) 
               
               
                   
               
             
             
               
                 0 
                 f ss  + 10 
               
               
                 1 
                 f ss  + 10 
               
               
                 2 
                 f ss  + 2 
               
               
                 3 
                 f ss  + 2 
               
               
                 4 
                 f ss  + 4 
               
               
                 5 
                 f ss  + 4 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                 . 
                 . 
               
               
                 19  
                 f ss  + 6 
               
               
                   
               
             
          
         
       
     
         [0166]    The configuration in which to store sequence offset f ss ˜(n s ) including a time variable component can be employed as described above to thereby represent the same sequence offsets in two continuous time slots easily. Furthermore, the configuration can easily represent even a sequence offset including a time variable component that cannot be represented by a simple function. Moreover, sequence offsets do not need to be found through calculation, and therefore a necessary calculation circuit can be reduced. 
         [0167]    In addition, Embodiment 3 may switch whether hopping of a base sequence group allocated for an individual terminal has a pattern varying for each time slot or a pattern varying for each subframe, according to a next control parameter. 
         [0168]    In Rel. 10 of LTE-A, signaling of the high-order layer aiming at performing DMRS multiplex by an OCC has already been regulated. This signaling is performed using control parameters of “dmrs-WithOCC-Activated” or “groupHoppingDisabled” indicated for an individual terminal by RRC control information. 
         [0169]    Therefore, when a base sequence group is allocated by terminal-specific signaling in Embodiment 3, one of the control parameters, i.e., “dmrs-WithOCC-Activated” and “groupHoppingDisabled” can be used. When one of the control parameters, i.e., “dmrs-WithOCC-Activated” and “groupHoppingDisabled” is “True”, terminal apparatus  20  employs a hopping pattern varying for each subframe using a sequence offset from expression 6 or Table 1, and generates a DMRS subject to OCC multiplex. On the other hand, when one of the control parameters, i.e., “dmrs-WithOCC-Activated” and “groupHoppingDisabled” is “False”, terminal apparatus  20  employs a hopping pattern varying for each time slot using a sequence offset described in Embodiment 2, and generates a DMRS. 
         [0170]    Such a configuration can transmit a DMRS subject to OCC multiplex between terminals using base sequence groups allocated by terminal-specific signaling without adding new signaling. 
       Embodiment 4 
       [0171]    In configurations of a communication system, terminal apparatus  20 , and RP  10  according to Embodiment 4, a network determines the allocation pattern of a base sequence group performed by signaling for an individual terminal, and indicates the allocation pattern to terminal apparatus  20 . The configurations other than that are the same as those of Embodiment 2. Hereinafter, different points will be explained in detail. 
         [0172]    [Outline of Communication System] 
         [0173]    The communication system in Embodiment 4 includes including one or more RPs  10  in a macro cell and terminal apparatus  20  similarly to Embodiment 1. In the communication system of Embodiment 4, the network beforehand determines parameters “λ, z” in Expression 5 or 6 representing a base sequence group allocated by signaling for an individual terminal. 
         [0174]    [Configuration of RP  10 ] 
         [0175]    Control section  112  of RP  10  outputs parameters “λ, z” to channel estimation section  111 . Parameters “λ, z” are beforehand determined in the network and are indicated to RP  10  through inter-RP IF  110 . Correspondent RP  10  may determine parameters “λ, z.” When correspondent RP  10  determines the parameters, the information is indicated to another RP  10  through inter-RP IF  110 . On the other hand, when another RP  10  determines the parameters, correspondent RP  10  receives the information from the other RP  10  through inter-RP IF  110 . 
         [0176]    [Configuration of Terminal Apparatus  20 ] 
         [0177]    Control section  202  of terminal apparatus  20  inputs parameters “λ, z” into DMRS generation section  203 . Parameters “λ, z” are beforehand determined in the network and are indicated to terminal apparatus  20 . This indication may be sent as RRC control information for an individual terminal and may be included in a MAC header. Alternatively, this indication may be sent by a control signal transmitted through a downlink (PDCCH). When this indication is not especially sent, both parameters “λ, z” are set to “0” (equivalent to using the base sequence group allocated to the cell). 
         [0178]    [DMRS Transmission-Related Operations] 
         [0179]    In the communication system of Embodiment 4, when new parameters “λ, z” are indicated from the network to terminal apparatus  20 , control section  202  of terminal apparatus  20  overwrites parameter “λ, z” in expression 5 (or expression 6). Moreover, when indicating new parameters “λ, z” to terminal apparatus  20 , the network indicates parameters “λ, z” to control section  112  of each RP  10  participating in the reception, and control section  112  of RP  10  overwrites parameters “λ, z” in expression 5 (or expression 6). 
         [0180]    Other process flows relating to transmission of DMRSs performed by RP  10  and terminal apparatus  20  are the same as those in Embodiment 1. 
         [0181]    According to the communication system, RP  10 , and terminal apparatus  20  in Embodiment 4, the network can appropriately select parameters “λ, z” according to the placement of the cells to thereby set sequence groups suitable for the placement of the cells. Specifically, the network enable a control so as to determine a sequence group allocated by signaling for an individual terminal in terminal apparatus  20  so that overlapping of sequence groups does not occur in other nearby cells tending to involve large influence of interference. 
         [0182]    In stead of a configuration indication parameter information that can represent all the values and combination that can be taken as parameters “λ, z,” the network or RP  10  may also have a configuration indication parameter information that has a small bit length and can represent only some of the combinations of parameters “λ, z.” This configuration can reduce the number of transmission bits required for the indication of parameters “λ, z”. 
         [0183]    Alternatively, a configuration may be used in which a region of nine or more bits of established control data is defined as an indication region for terminal-specific signaling, and the information on this indication region is used to determine a base sequence group used for generation of a DMRS by terminal apparatus  20 . Specifically, when a value (for example, 0 to 503 in the case of nine bits) representing a cell ID is indicated in the indication region, terminal apparatus  20  generates a DMRS using a base sequence group obtained by rewriting N cell ID  in expressions 1 and 2 with a value corresponding to the information in the indication region. Moreover, when a value not representing a cell ID is indicated in the indication region, a DMRS is generated using a base sequence group given by expression 3 (for example, expressions 5 and 6). This configuration may also be used. Also in such a configuration, when information in the indication region is not indicated, terminal apparatus  20  can generate a DMRS using a base sequence group allocated to the cell. 
         [0184]    This configuration can select a base sequence group used for generation of a DMRS by terminal apparatus  20 , according to the value of the indication information. That is, a base sequence group to be used can be selected from a base sequence group allocated a connected cell, a base sequence group allocated to an unconnected cell, and a base sequence group allocated for an individual terminal. 
         [0185]    Furthermore, a configuration may be employed in which parameters “λ, z” are associated with values representing information other than a cell ID in the indication region, and are indicated to terminal apparatus  20  using the value representing information other than a cell ID. Specifically, values representing information other than a cell ID (for example, 504 to 511 in the case of nine bits) in the indication region are associated with eight combinations of parameters “λ, z,” and when this value is indicated, control section  202  of terminal apparatus  20  overwrites parameters “λ, z” in expression 5 (or Expression 6). Terminal apparatus  20  then generates and transmits a DMRS using a sequence group determined by expression 5 (or expression 6) after the overwriting. 
         [0186]    This configuration enable the network to select parameters “λ, z” suitable for the cell placement and to indicate the parameters to terminal apparatus  20 , without adding a new indication bit. 
         [0187]    The indication region has nine or more bits in the above explanation of the configuration, but can also have less than nine bits. When a cell-specific base sequence group of another cell is allocated as a base sequence group allocated by signaling for an individual terminal, allocating the same base sequence group as that of a nearby cell should be avoided in order to suppress influence of the interference. Especially, influence of the interference may be increased by allocating a base sequence group of another cell in the same cell cluster for an individual terminal. Therefore, the number of base sequence groups allocated to other cells that can be used as terminal-specific base sequence groups in reality is smaller than the number of all the cell IDs of “504.” Therefore, the cell ID of a cell-specific base sequence group that can be allocated can be indicated with the number of indication bits smaller than nine. For example, the offset for the cell ID of a connected cell can be indicated with the number of indication bits smaller than nine, and the cell ID of another cell can be indicated. 
         [0188]    This configuration can reduce the number of bits of indication information. 
       Embodiment 5 
       [0189]    Hereinafter, other examples will be explained on a determination method for a base sequence group allocated by signaling for an individual terminal. 
         [0190]    In Embodiments 1 to 4, sequence offset f ss ˜ is represented by a function of time slot n s . In contrast to this, in Embodiment 5, sequence offset f ss  is a fixed value, and PN hopping pattern f gh ˜ is represented as a function of time slot n s . Specifically, Embodiment 5 determines sequence number u˜ using expression 7. 
         [0000]      [7] 
         [0000]        ũ= ( {tilde over (f)}   gh ( n   s )+ f   ss )mod 30  (Expression 7)
 
         [0191]    In expression 7, {tilde over (f)} gh (n s ) is a sequence group hopping pattern determined by a PN sequence having an initial value equal to └N cell ID /30┘+G(n s ), and G(n s ) is a function satisfying a condition of Expression 8 described below. A symbol └ ┘ refers to truncation after the decimal point. 
         [0000]    
       
         
           
             
               
                 
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         [0192]    Here, Y is the total number of values that can be taken by variable y. When sequence number u˜ overlaps with a base sequence group allocated to the connected cell in expression 7, a different value is substituted into sequence number u˜. Alternatively, f gh ˜(n s ) may be given as values stored as a data table in a storing section instead of a function. 
         [0193]    According to this determination method for a base sequence group, hopping of the sequence group is enabled using new hopping pattern different from all cell clusters, without significantly changing the determination expression for a base sequence group given by expressions 1 and 2. 
         [0194]    Alternatively, sequence offset f ss ˜(n s ) including a time variable component described in Embodiments 1 to 4 may be added to hopping pattern f gh ˜(n s ) given by the above-described function of time slot n s  to thereby determine sequence number u˜. Moreover, both the PN hopping pattern and the sequence offset represented by functions of time slot n s  may also be provided as a data table. 
       Embodiment 6 
       [0195]    Embodiment 6 solves a problem of securing a large number of base sequence groups that can be used by terminal apparatus  20  in a connected cell when the influence of interference with another cell is not significantly large. 
         [0196]      FIG. 16  illustrates a sequence group allocated by signaling for an individual terminal in Embodiment 6. 
         [0197]    In Embodiment 6, expressions 1 and 2 determining a cell-specific base sequence group are used to change the value of N cell ID  in the expressions into the value (constant value) of new parameter K and to determine a sequence group allocated by signaling for an individual terminal. The value of new parameter K is set as N cell ID  of a cell other than the connected cell in the same cell cluster. 
         [0198]      FIG. 16  illustrates an example case where new parameter K is set as the value “K=3” of the cell ID of another cell (N cell ID =3) in the same cell cluster when terminal apparatus  20  is located in the connected cell (N cell ID =0). 
         [0199]    In this case, a base sequence group allocated by signaling for an individual terminal is the same as a cell-specific base sequence group of other cell (N cell ID =3), but does not overlap with a cell-specific base sequence group of any other cell in the same cell cluster once. 
         [0200]    According to the communication system of Embodiment 6, a large number of base sequence groups that can be used in one cell can be secured by allocating a base sequence group by signaling for an individual terminal using new parameter K. This configuration is useful when the influence of interference with another cell is not large, for example, when a few other terminal apparatuses  20  are connected to another cell (N cell ID =3). 
         [0201]    In addition, new parameter K may also be determined in a network and may be indicated from RP  10  to terminal apparatus  20 . 
       Embodiment 7 
       [0202]    Embodiment 7 solves a problem of securing a large number of base sequence groups that can be used by terminal apparatus  20  in a connected cell when the influence of interference with a cell in another cell cluster is not significantly large. 
         [0203]      FIG. 17  illustrates a sequence group allocated by signaling for an individual terminal in Embodiment 7. 
         [0204]    In Embodiment 7, expressions 1 and 2 determining a cell-specific base sequence group are used to change the value of N cell ID  in the expressions into the value (constant value) of new parameter K and to determine a sequence group allocated by signaling for an individual terminal. The value of new parameter K is set as N cell ID  of a cell in another cell cluster. 
         [0205]      FIG. 17  illustrates an example case where new parameter K is set as the value “K=30” of the cell ID of a cell (N cell ID =30) in another cell cluster when terminal apparatus  20  is located in the connected cell (N cell ID =0). 
         [0206]    In this case, a base sequence group allocated by signaling for an individual terminal always overlaps with a cell-specific base sequence group of one cell in the other cell cluster, but randomly overlaps with a cell-specific base sequence group of each cell in the same cell cluster. 
         [0207]    According to the communication system of Embodiment 7, a large number of base sequence groups that can be used in one cell can be secured by allocating a base sequence group by signaling for an individual terminal using new parameter K. This configuration is useful when it is known that the influence of interference with a specific cell in another cell cluster hardly occurs, for example, when a specific cell in another cell cluster is significantly distant. 
         [0208]    In addition, new parameter K may also be determined in a network and may be indicated from RP  10  to terminal apparatus  20 . 
         [0209]    In the above, each embodiment of the present invention has been explained. 
         [0210]    In each above-described embodiment, a specific example of a base sequence group allocated by signaling for an individual terminal has been explained. However, the present invention is not limited to the expressions or the data tables illustrated in each embodiment. The present invention includes all allocation patterns of sequence groups satisfying the following conditions 1 to 3. 
         [0211]    1. The base sequence group is always different from a base sequence group allocated to a connected cell of a terminal apparatus. 
         [0212]    2. The sequence group has a hopping pattern different from that of a base sequence group allocated to a cell belonging to another cell cluster. 
         [0213]    3. Overlapping with a base sequence group allocated to another cell in the same cell cluster does not concentrate in a single cell. 
         [0214]    Although the above-described embodiments have been explained using a DMRS as a target reference signal, an SRS or other reference signals can also be used as a target reference signal. Moreover, the antenna explained in the above-described embodiments can be replaced with an antenna port. 
         [0215]    Although each above-described embodiment has been explained using a case where the present invention is implemented with hardware, as an example, the present invention can be implemented with software in cooperation with hardware. 
         [0216]    Furthermore, each function block employed in the explanation of each above-described embodiment may typically be implemented as an LSI constituted by an integrated circuit. These function blocks may be individual chips or partially or totally contained on a single chip. The term “LSI” is adopted herein but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI,” depending on the differing extents of integration. 
         [0217]    The method of implementing integrated circuit is not limited to LSI, and implementation by means of dedicated circuitry or a general-purpose processor may also be possible. After LSI manufacture, utilization of a field programmable gate array (FPGA) or a reconfigurable processor where connections and settings of circuit cells in an LSI can be reconfigured is also possible. 
         [0218]    If a new integrated circuit implementation technology replacing LSI is introduced because of advancement in semiconductor technology or a different technology derived therefrom, the function blocks may of course be integrated using that technology. For example, application of biotechnology is possible. 
         [0219]    The disclosure of Japanese Patent Application No. 2012-016414, filed on Jan. 30, 2012, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0220]    The present invention is applicable to a cellular portable telephone, a base station therefor, and a communication method therefor. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  RP (Base station) 
           11  Transmission processing section 
           12  Reference signal extraction section 
           13  Reception signal processing section 
           101  Antenna 
           102  Radio transmission section 
           104  Radio reception section 
           111  Channel estimation section 
           112  Control section 
           20  Terminal apparatus 
           22  Sequence determination section 
           23  Transmission processing section 
           24  Reception processing section 
           201  Antenna 
           202  Control section 
           203  DMRS generation section 
           212  Radio transmission section 
           213  Radio reception section