Abstract:
Disclosed are a wireless transmitter and a reference signal transmission method that improve channel estimation accuracy. In a terminal ( 100 ), which transmits a reference signal using n (n is a non-negative integer 2 or greater) band blocks (which correspond to clusters here), which are disposed with spaces therebetween in a frequency direction, a reference signal controller ( 106 ) switches the reference signal formation method of a reference signal generator ( 107 ) between a first formation method and a second formation method based on the number (n) of band blocks. In addition, a threshold value setting unit ( 105 ) adjusts a switching threshold value based on the frequency spacing between band blocks. Thus, the reference signal formation method can be selected with good accuracy and, as a result, channel estimation accuracy is further improved.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a radio transmission apparatus and a reference signal transmission method. 
       BACKGROUND ART 
       [0002]    For an uplink channel of LTE-Advanced, which is an evolved version of 3rd generation partnership project long-term evolution (3GPP LTE), using both contiguous frequency transmission and non-contiguous frequency transmission is under consideration (see Non-Patent Literature 1). That is, in communication from each radio communication terminal apparatus (hereinafter referred to as “terminal”) to a radio communication base station apparatus (hereinafter referred to as “base station”), contiguous frequency transmission and non-contiguous frequency transmission are switched. 
         [0003]    Contiguous frequency transmission is a method of transmitting a data signal and a reference signal (RS) by allocating such signals to contiguous frequency bands. For example, as shown in  FIG. 1 , in contiguous frequency transmission, a data signal and a reference signal are allocated to contiguous transmission bands. In contiguous frequency transmission, a base station allocates contiguous frequency bands to each terminal based on the reception quality per frequency band for each terminal, so that it is possible to obtain frequency scheduling effects. 
         [0004]    On the other hand, non-contiguous frequency transmission is a method of transmitting a data signal and a reference signal by allocating such signals to non-contiguous frequency bands, which are dispersed in a wide range of band. For example, as shown in  FIG. 2 , in non-contiguous frequency transmission, it is possible to allocate a data signal and a reference signal to transmission bands which are dispersed all over the frequency band. In non-contiguous frequency transmission, compared to contiguous frequency transmission, the flexibility of assignment of a data signal and a reference signal to frequency bands is improved, so that it is possible to gain greater frequency scheduling effects. Further, in non-contiguous frequency transmission, it is possible to decrease the probability that all of a data signal or a reference signal of a terminal will get in a valley in fading. That is, according to non-contiguous transmission, it is possible to obtain frequency diversity effects and suppress deterioration of reception characteristics. Further, in LTE, as shown in  FIGS. 1 and 2 , a terminal transmits a data signal and a reference signal in the same transmission band (see Non-Patent Literature 2). Then, a base station estimates a channel estimation value of the transmission band to which a data signal of each terminal is allocated, using a reference signal, and demodulates the data signal using the channel estimation value. 
         [0005]    Further, in LTE, as a reference signal to use for propagation path estimation of an uplink channel, an orthogonal code called a cyclic shift sequence, which has high interference suppression effects, is employed (see Non-Patent Literature 3). Because one code sequence (ZC sequence) allocated to each base station (cell) is cyclically shifted by a different amount of cyclic shift, it is possible to obtain a plurality of cyclic shift sequences which are orthogonal to each other. An amount of shifting between cyclic shift sequences is set greater than delay time in a multipath channel. As shown in  FIG. 3 , a terminal transmits a cyclic shift sequence generated using a different amount of cyclic shift per terminal or antenna. A base station obtains a correlation value corresponding to each cyclic shift sequence by receiving a plurality of cyclic shift sequences that are multiplexed in a channel and performing a correlation calculation on a received signal and a base code sequence. That is, as shown in  FIG. 4 , the correlation value corresponding to cyclic shift sequence (CS # 2 ) appears at the position which is shifted by cyclic shift width Δ from the position at which the correlation value corresponding to cyclic shift sequence (CS # 1 ) appears. By setting cyclic shift width Δ greater than delay time in a multipath channel, it is possible to extract a correlation value in the period (detection window) in which an incoming wave of the desired wave exists. 
         [0006]    Here, as a method of transmitting a reference signal in non-contiguous frequency transmission, two methods are possible. First, in transmission method (a) in  FIG. 5 , reference signals are generated from one code sequence. That is, transmission is performed by dividing one code sequence by a width corresponding to the bandwidth of each contiguous frequency band (hereinafter referred to as “cluster”), and allocating the obtained subsequence to each cluster. 
         [0007]    On the other hand, in transmission method (b) in  FIG. 6 , reference signals are generated from a plurality of code sequences. That is, transmission is performed by generating a plurality of code sequences corresponding to the bandwidth of each cluster, and allocating each code sequence to clusters. 
       CITATION LIST 
     Non Patent Literature 
     NPL 1 
       [0008]    RI-090257, Panasonic, “System performance of uplink non-contiguous resource allocation” 
       NPL 2 
       [0009]    3GPP TS 36.212 V8.3.0, “E-UTRA Multiplexing and channel coding (Release 8),” 2008-05 
       NPL 3 
     3GPP TS 36.211 V8.3.0, “Physical Channels and Modulation (Release 8),” 2008-05 
     SUMMARY OF INVENTION 
     Technical Problem 
       [0010]    However, the above-described method of transmitting a reference signal in non-contiguous frequency transmission has the following problem. 
         [0011]    In transmission method (a), compared to transmission method (b), a coding sequence (a correlation length) can be made longer. That is, transmission method (a) has an advantage of reducing interference. Specifically, in the case where a ZC sequence is used as a code sequence, when a sequence length is N, the cross-correlation value between ZC sequences will be constant at 1/√N. When sequence length N doubles, the cross-correlation value will be 1/√2 times, making it possible to suppress inter-cell interference power value lower by 3 dB. 
         [0012]    However, transmission method (a) has a problem that accuracy of channel estimation deteriorates when the number of clusters is large or channel variation in the frequency band between clusters is significant. As shown in  FIG. 7 , when transmission method (a) is adopted, a base station obtains a correlation value (that is, a delay profile) by performing complex division on a received reference signal that is obtained by connecting a reference signal received as a plurality of clusters back to one code sequence, and a reference signal replica, in the frequency domain, and by performing IDFT processing on the result of division to convert into the time domain. At the point where reference signals are connected, channel variation becomes non continuous, and interference occurs resulting from this noncontinuity. This interference increases as the number of clusters is greater, because the number of noncontinuous points increases as the number of clusters is greater. Further, when the number of clusters is greater, a bandwidth per cluster becomes narrower and a correlation length becomes smaller, decreasing interference suppression effects and further increasing interference effects. As described above, when interference increases, the accuracy of detecting a desired wave deteriorates and separation of a plurality of cyclic shift sequences becomes difficult, drastically deteriorating the accuracy of channel estimation as well. 
         [0013]    On the other hand, transmission method (b) has an advantage that deterioration of the accuracy of channel estimation can be prevented even when channel variation between clusters is significant. As shown in  FIG. 8 , when transmission method (b) is adopted, a base station obtains a correlation value (delay profile) by performing complex division on a received reference signal of each cluster and a reference signal replica, in the frequency domain, and by performing IDFT processing on the result of division to convert into the time domain. In transmission method (b), because there is no noncontinuous point of channel variation as is in transmission method (a), it is possible to prevent interference from occurring. 
         [0014]    However, transmission method (b) has a problem that, because a sequence length per cluster (a correlation length) is shorter, compared with transmission method (a), interference suppression effects decrease and the accuracy of channel estimation deteriorates. For example, when the number of clusters is 2 and the bandwidths of the two clusters are equal, the interference level in transmission method (b) increases 3 dB greater than the interference level in transmission method (a). It is therefore an object of the present invention to provide a radio transmission apparatus and a reference signal transmission method for improving the accuracy of channel estimation. 
       Solution to Problem 
       [0015]    One aspect of a radio transmission apparatus according to the present invention employs a configuration to have a radio transmission apparatus that transmits a reference signal using n (n is a natural number of 2 or greater) bandwidth blocks that are positioned at intervals from each other in a direction of frequency, the apparatus comprising: a formation section that forms the reference signal based on one of a first formation method, in which n subsequences are formed as the reference signal by dividing one base code sequence into a length to match each bandwidth block, and a second formation method, in which n code sequences are formed as the reference signal by adjusting lengths of n base code sequences to match each bandwidth block; and a switch section that switches reference signal formation methods in the formation section between the first formation method and the second formation method based on a switch threshold value and the number of the bandwidth blocks n. 
         [0016]    One aspect of a reference signal transmission method according to the present invention employs a configuration to have a reference signal transmission method of transmitting a reference signal using n (n is a natural number of 2 or greater) bandwidth blocks that are positioned at intervals from each other in a direction of frequency, the method comprising steps of: forming the reference signal based on one of a first formation method, in which n subsequences are formed as the reference signal by dividing one base code sequence into a length to match each bandwidth block, and a second formation method, in which n code sequences are formed as the reference signal by adjusting lengths of n base code sequences to match each bandwidth block; and switching reference signal formation methods in a formation section between the first formation method and the second formation method based on a switch threshold value and the number of the bandwidth blocks n. 
       Advantageous Effects of Invention 
       [0017]    According to the present invention, it is possible to provide a radio transmission apparatus and a reference signal transmission method for improving the accuracy of channel estimation. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  shows contiguous frequency transmission; 
           [0019]      FIG. 2  shows non-contiguous frequency transmission; 
           [0020]      FIG. 3  shows cyclic shift sequences; 
           [0021]      FIG. 4  shows correlation values corresponding to cyclic shift sequences; 
           [0022]      FIG. 5  shows reference signal transmission method (a) in non-contiguous frequency transmission; 
           [0023]      FIG. 6  shows reference signal transmission method (b) in non-contiguous frequency transmission; 
           [0024]      FIG. 7  shows a problem in reference signal transmission method (a); 
           [0025]      FIG. 8  shows a problem in reference signal transmission method (b); 
           [0026]      FIG. 9  is a block diagram showing a configuration of a terminal according to Embodiment 1 of the present invention; 
           [0027]      FIG. 10  is a block diagram showing a configuration of a base station according to Embodiment 1 of the present invention; 
           [0028]      FIG. 11  is a block diagram showing a configuration of the channel estimation section in  FIG. 10 ; 
           [0029]      FIG. 12  shows a relationship between the first reference signal formation method and the second reference signal formation method according to Embodiment 1; 
           [0030]      FIG. 13  shows switch control between the first reference signal formation method and the second reference signal formation method according to Embodiment 1; 
           [0031]      FIG. 14  shows adjustment of threshold values used to switch reference signal formation methods; 
           [0032]      FIG. 15  shows switch control between the first reference signal formation method and the second reference signal formation method according to Embodiment 1; 
           [0033]      FIG. 16  shows an embodiment when applied to LTE-Advanced; 
           [0034]      FIG. 17  shows a relationship between the first reference signal formation method and the second reference signal formation method according to Embodiment 2; 
           [0035]      FIG. 18  shows switch control between the first reference signal formation method and the second reference signal formation method according to Embodiment 2; and 
           [0036]      FIG. 19  shows adjustment of threshold values used to switch reference signal formation methods. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
     Configuration of Terminal 
       [0038]      FIG. 9  is a block diagram showing a configuration of terminal  100  according to Embodiment 1 of the present invention. In  FIG. 9 , terminal  100  is provided with RF reception section  101 , demodulation section  102 , decoding section  103 , resource assignment information setting section  104 , threshold value setting section  105 , reference signal control section  106 , reference signal generation section  107 , encoding section  108 , modulation section  109 , fast Fourier transform (FFT) section  110 , mapping section  111 , inverse fast Fourier transform (IFFT) section  112 , and RF transmission section  113 . 
         [0039]    RF reception section  101  performs reception processing such as down-conversion and A/D conversion on a signal received via an antenna, and outputs the signal on which reception processing is performed to demodulation section  102 . 
         [0040]    Demodulation section  102  performs equalization processing and demodulation processing on the signal received from RF reception section  101 , and outputs the processed signal to decoding section  103 . 
         [0041]    Decoding section  103  performs decoding processing on the signal received from demodulation section  102  and extracts reception data and control information. 
         [0042]    Encoding section  108  encodes transmission data and outputs the obtained encoded data to modulation section  109 . 
         [0043]    Modulation section  109  modulates the encoded data received from encoding section  108  and outputs the modulated signal to FFT section  110 . 
         [0044]    FFT section  110  performs FFT processing on the modulated signal received from modulation section  109  and outputs the obtained signal to mapping section  111 . 
         [0045]    Mapping section  111  maps a data signal received from FFT section  110  and a reference signal received from reference signal generation section  107  to a frequency domain resource according to a frequency assignment information received from resource assignment information setting section  104 , and outputs the obtained signal to IFFT section  112 . 
         [0046]    Threshold value setting section  105  adjusts a switch threshold value in reference signal control section  106 . Threshold value setting section  105  receives information about clusters from resource assignment information setting section  104  and adjusts a switch threshold value in reference signal control section  106  based on a frequency interval between clusters. 
         [0047]    Reference signal control section  106  receives information about clusters from resource assignment information setting section  104 , compares which one of the number of clusters and the switch threshold value is smaller or greater, and, based on the result of the comparison, determines a method of forming a reference signal in reference signal generation section  107 . Reference signal control section  106  switches reference signal formation methods in reference signal generation section  107  by outputting identification information of the determined reference signal formation method to reference signal generation section  107 . 
         [0048]    Resource assignment information setting section  104  outputs frequency assignment information about a reference signal and a data signal, including the number of clusters, a frequency position and a bandwidth of each cluster, to threshold value setting section  105 , reference signal control section  106 , and mapping section  111 . Contents of resource assignment information are reported from base station  200  (described later) to terminal  100 , and are input to resource assignment information setting section  104  via RF reception section  101 , demodulation section  102 , and decoding section  103 . 
         [0049]    Reference signal generation section  107  generates a reference signal based on a reference signal formation method indicated by identification information received from reference signal control section  106 , and outputs the reference signal to mapping section  111 . As methods of forming a reference signal, as described above, there are a first formation method (transmission method, (a)), in which subsequences corresponding to the number of clusters are formed as a reference signal by dividing one base code sequence into the length to match each cluster, and a second formation method (transmission method (b)), in which base code sequences corresponding to the number of clusters are formed as a reference signal by adjusting lengths of base code sequences corresponding to the number of clusters to match each cluster. 
         [0050]    IFFT section  112  performs IFFT processing on the signal received from mapping section  111  and outputs the obtained signal to RF transmission section  113 . 
         [0051]    RF transmission section  113  performs transmission processing such as D/A conversion, up-conversion, and amplification on the signal received from IFFT section  112 , and trans&#39; its the obtained signal by air to base station  200  via an antenna. 
         [0052]    [Configuration of Base Station] 
         [0053]      FIG. 10  is a block diagram showing a configuration of base station  200  according to Embodiment 1 of the present invention. In  FIG. 10 , base station  200  is provided with RF reception section  201 , discrete Fourier transform (DFT) section  202 , demapping section  203 , resource assignment information setting section  204 , threshold value setting section  205 , channel estimation control section  206 , channel estimation section  207 , frequency domain equalization section  208 , IFFT section  209 , demodulation section  210 , and decoding section  211 . 
         [0054]    RF reception section  201  performs reception processing such as down-conversion and A/D conversion on a signal received via an antenna, and outputs the obtained signal to DFT section  202 . 
         [0055]    DFT section  202  performs DFT processing on the signal received from RF reception section  201  to convert a time domain signal into a frequency domain signal. Then, DFT section  202  outputs the frequency domain signal to demapping section  203 . 
         [0056]    Demapping section  203  extracts a data signal and a reference signal from the frequency domain signal received from DFT section  202  according to the frequency assignment information received from resource assignment information setting section  204 . Then, demapping section  203  outputs the extracted data signal to frequency domain equalization section  208  and outputs the reference signal to channel estimation section  207 . 
         [0057]    Resource assignment information setting section  204  outputs a frequency assignment information that is allocated to terminal  100 , including the number of clusters, a frequency position and a bandwidth of each cluster, to threshold value setting section  205 , channel estimation control section  206 , and demapping section  203 . In this regard, contents of resource assignment information are reported from base station  200  to terminal  100  in advance. 
         [0058]    Threshold value setting section  205  adjusts a switch threshold value in channel estimation control section  206 . Threshold value setting section  205  receives information about clusters from resource assignment information setting section  204 , and adjusts the switch threshold value in channel estimation control section  206  based on the frequency interval between clusters. 
         [0059]    Channel estimation control section  206  switches a channel estimation method in channel estimation section  207  to a channel estimation method corresponding to the reference signal transmission method in terminal  100 . That is, channel estimation control section  206  receives information about clusters from resource assignment information setting section  204 , compares which one of the number of clusters and a switch threshold value is smaller or greater, and, based on the result of the comparison, determines a channel estimation method in channel estimation section  207 . Channel estimation control section  206  switches channel estimation methods in channel estimation section  207  by outputting identification information of the determined channel estimation method to channel estimation section  207 . 
         [0060]    Channel estimation section  207  performs channel estimation using the channel estimation method indicated by identification information received from channel estimation control section  206 , and outputs the result of channel estimation to frequency domain equalization section  208 . A configuration of channel estimation section  207  will be described in detail later. 
         [0061]    Frequency domain equalization section  208  performs equalization processing on the data signal received from demapping section  203  using the channel estimation result (i.e. a channel frequency response) received from channel estimation section  207 . Then, frequency domain equalization section  208  outputs the result of equalization processing to IFFT section  209 . 
         [0062]    IFFT section  209  performs IFFT processing on the data signal received from frequency domain equalization section  208  and outputs the obtained signal to demodulation section  210 . 
         [0063]    Demodulation section  210  performs demodulation processing on the signal received from IFFT section  209  and outputs the obtained signal to decoding section  211 . 
         [0064]    Decoding section  211  performs decoding processing on the signal received from demodulation section  210 , and outputs the obtained reception data. 
         [0065]      FIG. 11  is a block diagram showing a configuration of channel estimation section  207 . In  FIG. 11 , channel estimation section  207  is provided with switching switch  220 , estimation processing section  230 , and estimation processing section  240 . 
         [0066]    Switching switch  220  redirects the output of the reference signal received from demapping section  203  to estimation processing section  230  or estimation processing section  240  based on identification information received from channel estimation control section  206 . 
         [0067]    Estimation processing section  230  performs a first channel estimation method corresponding to the first reference signal formation method. Estimation processing section  230  is provided with cluster combination section  231 , division section  232 , IFFT section  233 , mask processing section  234 , and DFT section  235 . 
         [0068]    Cluster combination section  231  connects, in a frequency domain, a plurality of clusters used to transmit a reference signal in terminal  100 , and outputs the received reference signal thus obtained to division section  232 . 
         [0069]    Division section  232  performs complex division on the received reference signal received from cluster combination section  231  using the reference signal replica (i.e. the reference signal transmitted from terminal  100 ). Then, division section  232  outputs the result of division (i.e. a correlation value) to IFFT section  233 . 
         [0070]    IFFT section  233  performs IFFT processing on the signal received from division section  232 , and outputs the obtained signal to mask processing section  234 . 
         [0071]    Mask processing section  234 , as an extraction means of a requested desired wave, extracts a correlation value at a period (a detection window) in which a correlation value of the desired cyclic shift sequence by performing mask processing on the signal received from IFFT section  233 , which is equivalent to a delay profile, based on an amount of cyclic shift used in terminal  100 . Then, mask processing section  234  outputs the extracted correlation value to DFT section  235 . 
         [0072]    DFT section  235  performs DFT processing on the correlation value input from mask processing section  234  and outputs the obtained signal to frequency domain equalization section  208 . This signal output from DFT section  235  is a channel estimation value in which channel variation (i.e. a channel frequency response) is estimated. 
         [0073]    Estimation processing section  240  performs a second channel estimation method corresponding to the second reference signal formation method. Estimation processing section  240  is provided with cluster extraction section  241  and estimation value calculation sections  242 - 1  to n which correspond to each cluster. Estimation value calculation section  242  is provided with division section  243 , IFFT section  244 , mask processing section  245 , and DFT section  246 . 
         [0074]    Cluster extraction section  241  outputs each of the number of clusters n used to transmit a reference signal in terminal  100  to estimation value calculation sections  242 - 1  to n. Estimation value calculation section  242  performs the same processing as performed in division section  232 , IFFT section  233 , mask processing section  234 , and DFT section  235 . 
         [0075]    [Operation of Terminal] 
         [0076]    Terminal  100  having the above configuration will be described below. 
         [0077]    As described above, in terminal  100 , reference signal control section  106  switches reference signal formation methods by controlling reference signal generation section  107 . 
         [0078]    The above-described first reference signal formation method (transmission method (a)) and second reference signal formation method (transmission method (b)) have the relationship shown in  FIG. 12 . That is, the accuracy of channel estimation is constant regardless of the number of clusters when the second reference signal formation method is used. On the other hand, the accuracy of channel estimation tends to lower as the number of clusters increases when the first reference signal formation method is used. Therefore, with a certain number of clusters N being a threshold, the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed. That is, when the number of clusters is N or smaller, the channel estimation value of the first reference signal formation method exceeds the channel estimation value of the second reference signal formation method, while, inversely, when the number of clusters is greater than N, the channel estimation value of the second reference signal formation method exceeds the channel estimation value of the first reference signal formation method. 
         [0079]    Therefore, by using the number of clusters of a point at which the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed as a switch threshold value, it is possible to select a more advantageous reference signal formation method with respect to the accuracy of channel estimation according to the number of clusters. By performing this kind of switch control of reference signal formation methods, base station  200  can obtain the accuracy of channel estimation shown with the solid line in  FIG. 13 . 
         [0080]    Further, as shown in  FIG. 14 , the accuracy of channel estimation of the first reference signal formation method depends on the frequency interval between clusters. That is, the accuracy curve of channel estimation shifts upward as the frequency interval between clusters is narrower. Therefore, when the frequency interval between clusters changes, the point at which the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed also shifts. 
         [0081]    Therefore, because threshold value setting section  105  adjusts a switch threshold value in reference signal control section  106  based on the frequency interval between clusters, it is possible to accurately select a reference signal formation method. 
         [0082]    The above-described switch control of a transmission method in terminal  100  is summarized in  FIG. 15 . That is, when the frequency interval is Y or greater, N 1  is used as a switch threshold value, and transmission method (a) and transmission method (b) are switched based on which one of this threshold value and the number of clusters is smaller or greater. On the other hand, when the frequency interval is smaller than Y, N 2  is used as a switch threshold value. 
         [0083]    As described above, according to the present embodiment, in terminal  100  that transmits a reference signal using n (n is a natural number of 2 or greater) band blocks (here, equivalent to clusters) which are positioned at intervals from each other in a direction of frequency, reference signal control section  106  switches the reference signal formation methods in reference signal generation section  107  between the first formation method and the second formation method, based on the number of band blocks n. 
         [0084]    By this means, it is possible to select a more advantageous reference signal formation method with respect to the accuracy of channel estimation, and, as a result of this, it is possible to improve the accuracy of channel estimation. 
         [0085]    Further, in terminal  100 , threshold value setting section  105  adjusts the switch threshold value based on the frequency interval between band blocks. 
         [0086]    By this means, it is possible to accurately select a reference signal formation method, and, as a result of this, it is possible to further improve the accuracy of channel estimation. 
         [0087]    Further, a case has been described with the above description where each cluster is treated as a band block. However, the present invention is by no means limited to this, and it is equally possible to use a band block formed with a plurality of clusters as an equivalent of the cluster described in Embodiment 1. That is, when there are a plurality of band blocks formed with a plurality of clusters, it is possible to employ a first formation method in which subsequences corresponding to the number of band blocks are formed as a reference signal by dividing one base code sequence into the length to match each band block, and a second formation method in which code sequences corresponding to the number of band blocks are formed as a reference signal by adjusting the lengths of base code sequences corresponding to the number of band blocks to match each band block. 
         [0088]    For example, a component carrier, which is a predetermined system bandwidth in LTE-Advanced, is equivalent to this band block. For a component carrier, a maximum value of the number of clusters that can be contained is defined by, for example, restricting the signaling format. Therefore, in such a case, it is possible to switch reference signal transmission methods according to the number of component carriers. For example, in the case where the maximum value of the number of clusters in component carrier is 2, as shown in  FIG. 16 , it is possible to obtain the same effects as in above Embodiment 1 even by selecting transmission method (a) when the number of component carriers is 1, while selecting transmission method (h) when the number of component carriers is 2 or greater. 
       Embodiment 2 
       [0089]    A case will be described here with Embodiment 2 where reference signal formation methods are switched based on a “cluster bandwidth.” That is, reference signal formation methods are switched based on a total bandwidth of n clusters in addition to a switch threshold value and the number of clusters n. Further, basic configurations of a terminal and a base station according to the present embodiment are the same as the configurations of the terminal and the base station explained in Embodiment 1. Therefore, the terminal and the base station according to the present Embodiment will also be explained using  FIGS. 9 and 10 . 
         [0090]    [Configuration of Terminal] 
         [0091]    Reference signal control section  106  in terminal  100  according to Embodiment 2 receives information about clusters from resource assignment information setting section  104 , and first calculates a “cluster bandwidth.” This “cluster bandwidth” means an average bandwidth per cluster and can be obtained by dividing a total bandwidth of n clusters by the number of clusters n. 
         [0092]    Then, reference signal control section  106  compares which one of the cluster bandwidth and a switch threshold value is smaller or greater, and, based on the result of the comparison, determines a reference signal formation method in reference signal generation section  107 . Reference signal control section  106  switches reference signal formation methods in reference signal generation section  107  by outputting identification information of the determined reference signal formation method to reference signal generation section  107 . 
         [0093]    [Configuration of Base Station] 
         [0094]    Further, channel estimation control section  206  in base station  200  according to Embodiment 2 switches the channel estimation method in channel estimation section  207  to the channel estimation method corresponding to the reference signal transmission method in terminal  100 . That is, channel estimation control section  206  receives information about clusters from resource assignment information setting section  204 , and first calculates a “cluster bandwidth,” as is the case with reference signal control section  106 . 
         [0095]    Then, channel estimation control section  206  compares which one of the cluster bandwidth and a switch threshold value is smaller or greater, and, based on the result of the comparison, determines a reference signal formation method in reference signal generation section  107 . Channel estimation control section  206  switches channel estimation methods in channel estimation section  207  by outputting identification information of the determined channel estimation method to channel estimation section  207 . 
         [0096]    [Operation of Terminal] 
         [0097]    As described above, in terminal  100 , reference signal control section  106  switches reference signal formation methods by controlling reference signal generation section  107 . 
         [0098]    The above-described first reference signal formation method (transmission method (a)) and second reference signal formation method (transmission method (b)) have a relationship shown in  FIG. 17 , when the horizontal axis of the graph shows a cluster bandwidth. 
         [0099]    Especially, as shown in  FIG. 17 , the performance of transmission method (a) depends on the cluster bandwidth and deteriorates as the cluster bandwidth is narrower. When the cluster bandwidth is narrower, the number of clusters tends to increase. Therefore, when the number of non-continuous points in channel variation in calculating channel estimation increases, interference increases. Further, the performance of transmission method (b) also depends on the cluster bandwidth and deteriorates as the cluster bandwidth is narrower. Because the correlation length becomes smaller according to the cluster bandwidth, the interference suppression effects are lowered. This performance deterioration in transmission method (b) is greater than the performance deterioration in transmission method (a). 
         [0100]    On the other hand, the performance in transmission method (b) is greater than the performance in transmission method (a) when the cluster bandwidth is wider. In transmission method (b), when the cluster bandwidth is greater, sufficient interference suppression effects can be obtained and interference can be suppressed to a noise level. Further, while, in transmission method (b), the performance does not deteriorate even the number of clusters is large, in transmission method (a), great interference due to non-continuous of channel variation occurs even when the cluster bandwidth is wide. 
         [0101]    That is, here again, with a certain cluster bandwidth M being a threshold, the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed. That is, when the cluster bandwidth is M or narrower, the channel estimation value of the first reference signal formation method exceeds the channel estimation value of the second reference signal formation method, while, inversely, when the cluster bandwidth is wider than M, the channel estimation value of the second reference signal formation method exceeds the channel estimation value of the first reference signal formation method. 
         [0102]    Therefore, by using a cluster bandwidth of the point at which the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed as a switch threshold value, it is possible to select a more advantageous reference signal formation method with respect to the accuracy of channel estimation according to the cluster bandwidth. By performing this kind of switch control of reference signal formation methods, base station  200  can obtain the accuracy of channel estimation shown with the solid line in  FIG. 18 . 
         [0103]    Further, as shown in  FIG. 19 , the accuracy of channel estimation of the first reference signal formation method depends on the frequency interval between clusters even when the horizontal axis of the graph shows a cluster bandwidth. That is, the accuracy curve of channel estimation shifts upward as the frequency interval between clusters is narrower. Therefore, when the frequency interval between clusters changes, the point at which the accuracy of channel estimation of the first reference signal formation method and the accuracy of channel estimation of the second reference signal formation method are reversed also shifts. 
         [0104]    Therefore, because threshold value setting section  105  adjusts a switch threshold value in reference signal control section  106  based on the frequency interval between clusters, it is possible to accurately select a reference signal formation method. 
         [0105]    As described above, according to the present embodiment, in terminal  100 , reference signal control section  106  switches reference signal formation methods based on the “cluster bandwidth.” That is, reference signal formation methods are switched based on a total bandwidth of n clusters in addition to a switch threshold value and the number of clusters n. 
         [0106]    By this means, it is possible select a more advantageous reference signal formation method with respect to the accuracy of channel estimation, and, as a result of this, it is possible to improve the accuracy of channel estimation. 
         [0107]    Although cases have been described with the above embodiments where reference signal formation methods are switched based on the cluster bandwidth, it is possible to use the narrowest bandwidth out of the bandwidths of n clusters instead of the cluster bandwidth. 
       Other Embodiment 
       [0108]    Cases have been described with above Embodiment 1 and Embodiment 2 where both of reference signal transmission methods in terminal  100  and channel estimation methods in base station  200  switch according to the number of clusters or a cluster bandwidth. However, it is possible to switch only channel estimation methods in base station  200 . That is, it is possible to fix the reference signal transmission method in terminal  100  to transmission method (a) or transmission method (b), and switch channel estimation methods in base station  200  according to the number of clusters or a cluster bandwidth. By this means, it is also possible to obtain effects similar to the effects of Embodiment 1 and Embodiment 2. 
         [0109]    Also, although cases have been described with the above embodiments as examples where the present invention is configured by hardware, the present invention can also be realized by software. 
         [0110]    Each function block employed in the description of each of the aforementioned embodiments may typically be implemented as an LSI constituted by an integrated circuit. These may be individual chips or partially or totally contained on a single chip. “LSI” is adopted here but this may also be referred to as “IC,” “system LSI,” “super LSI,” or “ultra LSI” depending on differing extents of integration. 
         [0111]    Further, the method of circuit integration is not limited to LSI&#39;s, and implementation using dedicated circuitry or general purpose processors is also possible. After LSI manufacture, utilization of a programmable FPGA (Field. Programmable Gate Array) or a reconfigurable processor where connections and settings of circuit cells within an LSI can be reconfigured is also possible. 
         [0112]    Further, if integrated circuit technology comes out to replace LSI&#39;s as a result of the advancement of semiconductor technology or a derivative other technology, it is naturally also possible to carry out function block integration using this technology. Application of biotechnology is also possible. 
         [0113]    The disclosure of Japanese Patent Application No. 2009-018632, filed on Jan. 29, 2009, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0114]    A radio transmission apparatus and a reference signal transmission method according to the present invention are useful for improving the accuracy of channel estimation.