In recent years, for example, multimedia data is being increasingly distributed and broadband for downlinks in particular is being studied frequently (e.g., see Non-Patent Document 1). Non-Patent Document 1 discusses downlink high-speed packet transmission adopting an OFDM (Orthogonal Frequency Division Multiplexing) scheme which is considered as a promising communication scheme to be used for a next-generation radio communication system. The OFDM scheme is one of multicarrier communication schemes and a technology for transmitting data mapped on a plurality of subcarriers and has advantages such as strong resistance to frequency selective fading.
Furthermore, as a high-speed packet transmission on a downlink, an HSDPA (High Speed Downlink Packet Access) standard is being developed by the 3GPP (3rd Generation Partnership Project). Adaptive modulation, scheduling and HARQ (Hybrid Automatic Repeat reQuest) are indispensable technologies for the HSDPA standard.
Adaptive modulation in HSDPA is a technology for a base station apparatus to transmit data to a mobile station apparatus by changing, for example, a modulation scheme and coding rate (MCS: Modulation and Coding Scheme) according to channel quality and thereby making a transmission rate variable. When the base station apparatus changes an MCS, an optimum MCS is selected based on an index of channel quality (CQI: Channel Quality Indicator) reported from the mobile station apparatus (e.g., see Non-Patent Document 2).
Furthermore, HARQ is a technology for a mobile station apparatus to send an ACK/NACK indicating whether or not data has been received from a base station apparatus normally and for the base station apparatus to control retransmission by receiving the ACK/NACK. When sending feedback information such as CQI or ACK/NACK described above, the mobile station apparatus sends the feedback information with transmit power with a predetermined offset set, for example, in a DPCCH (Dedicated Physical Control Channel) (e.g., see Non-Patent Document 3).
The feedback information includes important information which constitutes an element to control downlink transmission at a base station apparatus and needs to be received by the base station apparatus accurately. Therefore, this feedback information may be transmitted with relatively high transmit power. As for ACK/NACK in particular, in order to improve the efficiency of data retransmission, a required BER (bit error rate) required from the base station apparatus is high and a high offset is set for a DPCCH.    Non-Patent Document 1: “Experiment Result of Packet Combination Type Hybrid ARQ in Downlink VSF-OFCDM Broadband Radio Access” Miki, Abeta, Higuchi, Atarashi, Sawabashi, pp. 15-pp. 22, TECHNICAL REPORT OF IEICE RCS2003-26, 2003-May.    Non-Patent Document 2: 3GPP TR25.858 V5.0.0 “HSDPA physical layer aspects” (2002-March).    Non-Patent Document 3: 3GPP TS25.213 V5.4.0 “Spreading and Modulation (FDD).”