In mobile communication, ARQ (Automatic Repeat reQuest) is applied to downlink data from a radio communication base station apparatus (hereinafter abbreviated to a “base station”) to radio communication mobile station apparatuses (hereinafter abbreviated to “mobile stations”).  That is, mobile stations feed back response signals representing error detection results of downlink data, to the base station.  Mobile stations perform a CRC (Cyclic Redundancy Check) of downlink data, and, if CRC=OK (no error), feed back an ACK (ACKnowledgement), and, if CRC=NG (error present), feed back a NACK (Negative ACKnowledgement), as a response signal to the base station.  These response signals are transmitted to the base station using uplink control channels such as PUCCH's (Physical Uplink Control CHannels). 
Also, the base station transmits control information for carrying resource allocation results of downlink data, to the mobile stations.  This control information is transmitted to the mobile stations using downlink control channels such as L1/L2 CCH's (L1/L2 Control CHannels).  Each L1/L2 CCH occupies one or a plurality of CCE's (Control Channel Element), depending on the coding rate of control information.  For example, when an L1/L2 CCH for carrying control information of a coding rate of ⅔ occupies one CCE, an L1/L2 CCH for carrying control information of a coding rate of ⅓ occupies two CCE's, an L1/L2 CCH for carrying control information of a coding rate of ⅙ occupies four CCE's, and an L1/L2 CCH for carrying control information of a coding rate of 1/12 occupies eight CCE's.  If one L1/L2 CCH occupies a plurality of CCE's, the plurality of CCE's occupied by the L1/L2 CCH are consecutive.  The base station generates an L1/L2 CCH per mobile station, allocates a CCE that should be occupied by the L1/L2 CCH depending on the number of CCE's required by control information, maps the control information on physical resources associated with the allocated CCE's and transmits the results. 
Also, to use downlink communication resources efficiently without signaling to carry PUCCH's from the base station to the mobile stations for transmitting response signals, studies are underway to associate CCE's and PUCCH's on a one-to-one basis (see Non-Patent Document 1).  According to this association, each mobile station can decide the PUCCH to use to transmit a response signal from that mobile station, from the CCE associated with a physical resource on which control information for that mobile station is mapped.  Therefore, each mobile station maps a response signal from that mobile station on a physical resource, based on the CCE associated with the physical resource on which control information for that mobile station is mapped.  For example, when a CCE associated with a physical resource on which control information for a mobile station is mapped is CCE #0, the mobile station decides that PUCCH #0 associated with CCE #0 is the PUCCH for that mobile station.  Also, for example, when CCE's associated with physical resources on which control information for that mobile station is mapped are CCE #0 to CCE #3, the mobile station decides that PUCCH #0 associated with CCE #0 of the minimum number among CCE #0 to CCE #3 is the PUCCH for that mobile station, or, when CCE's associated with physical resources on which control information for that mobile station is mapped are CCE #4 to CCE #7, the mobile station decides that PUCCH #4 associated with CCE #4 of the minimum number among CCE #4 to CCE #7 is the PUCCH for that mobile station. 
Also, as shown in FIG.  1, studies are underway to perform code-multiplexing by spreading a plurality of response signals from a plurality of mobile stations using ZAC (Zero Auto Correlation) sequences and Walsh sequences (see Non-Patent Document 2).  In FIG.  1, (W0, W1, W2, W3) represent Walsh sequences having a sequence length of 4.  As shown in FIG.  1, in a mobile station, first, an ACK or NACK response signal is subject to the first spreading in the frequency domain by a sequence having a characteristic of a ZAC sequence (having a sequence length of 12) in the time domain.  Next, the response signal subjected to the first spreading is subject to an IFFT (Inverse Fast Fourier Transform) in association with W0 to W3.  The response signal spread in the frequency domain is transformed to a ZAC sequence having a sequence length of 12 in the time domain by this IFFT.  Further, the signal subjected to the IFFT is subject to second spreading using Walsh sequences (having a sequence length of 4).  That is, one response signal is allocated to each of four SC-FDMA (Single Carrier-Frequency Division Multiple Access) symbols S0 to S3.  Similarly, response signals of other mobile stations are spread using ZAC sequences and Walsh sequences.  Here, different mobile stations use ZAC sequences of different cyclic shift values in the time domain or different Walsh sequences.  In this case, the sequence length of a ZAC sequence in the time domain is 12, so that it is possible to use twelve ZAC sequences of cyclic shift values “0” to “11,” generated from the same ZAC sequence.  Also, the sequence length of a Walsh sequence is 4, so that it is possible to use four different Walsh sequences.  Therefore, in an ideal communication environment, it is possible to code-multiplex maximum 48 (12×4) response signals from mobile stations. 
Also, as shown in FIG.  1, studies are underway to code-multiplex a plurality of reference signals (e.g. pilot signals) from a plurality of mobile stations (see Non-Patent Document 2).  As shown in FIG.  1, when three reference signal symbols R0, R1 and R2, are generated from a ZAC sequence (having a sequence length of 12), first, the ZAC sequence is subjected to an IFFT in association with orthogonal sequences [F0, F1, F2] having a sequence length of 3 such as a Fourier sequence.  By this IFFT, a ZAC sequence having a sequence length of 12 in the time domain is provided.  Further, the signal subjected to the IFFT is spread using the orthogonal sequences [F0, F1, F2].  That is, one reference signal (i.e. ZAC sequence) is allocated to each of three symbols R0, R1 and R2.  Similarly, other mobile stations allocate one reference signal (i.e. ZAC sequence) to each of three symbols R0, R1 and R2.  Here, different mobile stations use ZAC sequences of different cyclic shift values in the time domain or different orthogonal sequences.  In this case, the sequence length of a ZAC sequence in the time domain is 12, so that it is possible to use 12 ZAC sequences of cyclic shift values “0” to “11” generated from the same ZAC sequence.  Also, the sequence length of an orthogonal sequence is 3, so that it is possible to use three different orthogonal sequences.  Therefore, in an ideal communication environment, it is possible to code-multiplex maximum 36 (12×3) response signals from mobile stations. 
As a result, as shown in FIG.  1, seven symbols of S0, S1, R0, R1, R2, S2, S3 form one slot. 
Here, cross-correlation between ZAC sequences of different cyclic shift values generated from the same ZAC sequence, is virtually zero.  Therefore, in an ideal communication environment, a plurality of response signals subjected to spreading and code-multiplexing by ZAC sequences of different cyclic shift values (0 to 11), can be separated in the time domain by correlation processing in the base station, virtually without inter-code interference. 
However, due to the influence of, for example, transmission timing difference in mobile stations and multipath delayed waves, a plurality of response signals from a plurality of mobile stations do not always arrive at a base station at the same time.  For example, if the transmission timing of a response signal spread by a ZAC sequence of the cyclic shift value “0” is delayed from the correct transmission timing, the correlation peak of the ZAC sequence of the cyclic shift value “0” may appear in the detection window for the ZAC sequence of the cyclic shift value “1.”  Further, if a response signal spread by the ZAC sequence of the cyclic shift value “0” has a delayed wave, interference leakage due to the delayed wave may appear in the detection window for the ZAC sequence of the cyclic shift value “1.”  That is, in these cases, the ZAC sequence of the cyclic shift value “1” is interfered by the ZAC sequence of the cyclic shift value “0.”  Therefore, in these cases, the separation performance degrades between a response signal spread by the ZAC sequence of the cyclic shift value “0” and a response signal spread by the ZAC sequence of the cyclic shift value “1.”  That is, if ZAC sequences of adjacent cyclic shift values are used, the separation performance of response signals may degrade. 
Therefore, up till now, if a plurality of response signals are code-multiplexed by spreading using ZAC sequences, a cyclic shift interval (i.e. a difference of cyclic shift values) is provided between the ZAC sequences such that inter-code interference does not occur between the ZAC sequences.  For example, when the cyclic shift interval between ZAC sequences is 2, only six ZAC sequences of cyclic shift values “0,” “2” “4,” “6,” “8” and “10” are used in the first spreading of response signals, among twelve ZAC sequences of cyclic shift values “0” to “11” having a sequence length of 12.  Therefore, if Walsh sequences having a sequence length of 4 are used in second spreading of response signals, it is possible to code-multiplex maximum 24 (6×4) response signals from mobile stations. 
However, as shown in FIG.  1, the sequence length of orthogonal sequences used to spread reference signals is 3, and therefore only three different orthogonal sequences can be used to spread reference signals.  Therefore, when a plurality of response signals are separated using the reference signals shown in FIG.  1, only maximum 18 (6×3) response signals from mobile stations can be code-multiplexed.  Therefore, three Walsh sequences among four Walsh sequences having a sequence length of 4 are enough, and therefore one Walsh sequence is not used. 
Also, one SC-FDMA symbol shown in FIG.  1 may be referred to as one “LB (Long Block).”  Therefore, a spreading code sequence used for spreading in symbol units (i.e. in LB units) is referred to as a “block-wise spreading code sequence.” 
Also, studies are underway to define 18 PUCCH's shown in FIG.  2.  Normally, between mobile stations using different block-wise spreading code sequences, the orthogonality of response signals do not collapse unless those mobile stations move fast.  However, between mobile stations using the same block-wise spreading code sequence, especially when there is a large difference of received power between response signals from those mobile stations in a base station, one response signal may be interfered with from another response signal.  For example, in FIG.  2, a response signal using PUCCH #3 (cyclic shift value=2) may be interfered with from a response signal using PUCCH #0 (cyclic shift value=0). 
To reduce such interference, a technique of cyclic shift hopping is studied (see Non-Patent Document 3).  Cyclic shift hopping is the technique of changing the cyclic shift values to allocate to the symbols in FIG.  1, over time, in a random manner.  By this means, it is possible to randomize the combinations of response signals to cause interference, and prevent only part of mobile stations from having strong interference continuously.  That is, by cyclic shift hopping, it is possible to randomize interference. 
Here, interference between response signals can be classified broadly into inter-cell interference which refers to the interference caused between cells and intra-cell interference which refers to the interference caused between mobile stations in one cell.  Therefore, interference randomization is classified broadly into inter-cell interference randomization and intra-cell interference randomization.     Non-Patent Document 1: Implicit Resource Allocation of ACK/NACK Signal in E-UTRA Uplink (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-072439.zip)    Non-Patent Document 2: Multiplexing capability of CQIs and ACK/NACKs form different UEs (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—49/Docs/R1-072315.zip)    Non-Patent Document 3: Randomization of intra-cell interference in PUCCH (ftp://ftp.3gpp.org/TSG_RAN/WG1_RL1/TSGR1—50/Docs/R1-073412.zip)