Communication systems, particularly, mobile-phone-related wireless communication systems, are continuously developing as high-speed and high-capacity communication systems. LTE (Long Term Evolution, the 3.9G wireless access technology), which is a wireless communication standard by 3GPP (3rd Generation Partnership Project), and LTE-A (LTE-Advanced, the evolved version of LTE), which is the advanced form of LTE, adopt OFDMA (Orthogonal Frequency Division Multiple Access) as a transmission scheme for the downlink (the wireless communication link from a base station to a terminal). OFDMA is highly robust against frequency selective fading and has a high affinity with MIMO (Multiple Input Multiple Output) transmission. As for a transmission scheme for the uplink (the wireless communication link from a terminal to a base station), on the other hand, the cost and scale of the terminal are important.
However, multi-carrier transmission, such as OFDMA or MC-CDMA (Multi-Carrier Code Division Multiple Access), requires a terminal to include a power amplifier having a wide linear region because of a high PAPR (Peak to Average Power Ratio) of the transmit signal, which thus makes multi-carrier transmission unsuitable for the uplink transmission. That is, to maintain a wide coverage (which is a communication coverage range, for example, the distance to a base station) in the uplink, single-carrier transmission is desirable in which the PAPR is low. For the uplink, LTE adopts SC-FDMA (Single Carrier Frequency Division Multiple Access, also referred to as DFT-S-OFDM), which is single-carrier transmission.
Also, methods for achieving a wide coverage include transmit antenna diversity (also referred to as “transmit diversity”). In transmit diversity, in the case of the uplink for example, signals that have undergone different kinds of signal processing are transmitted from multiple antennas of a transmission apparatus (which indicates a transmission unit of a terminal in this case) and are received by a receive antenna of a reception apparatus (which indicates a reception unit of a base station in this case). In this manner, transmit antenna diversity gain can be obtained. Transmit diversity is roughly categorized into open-loop transmit diversity, in which transmission is performed without using channel state information regarding channels between the transmission apparatus and the reception apparatus; and closed-loop transmit diversity, in which transmission processing is performed based on channel state information regarding channels between the transmission apparatus and the reception apparatus.
Open-loop transmit diversity includes STBC (Space Time Block Coding), SFBC (Space Frequency Block Coding), and CDD (Cyclic Delay Diversity). Closed-loop transmit diversity includes antenna selection transmit diversity and maximum ratio transmit receive antenna diversity. It has been decided that the uplink of LTE-A that uses this closed-loop transmit diversity adopts precoding based on codebooks (code tables) described in NPL 1. In precoding, phases of transmit signals of individual transmit antennas are rotated before transmission so that the signals transmitted from the multiple transmit antennas of a transmission apparatus are received to be combined in phase by a reception apparatus. In this manner, the reception power can be increased at the reception apparatus.
Furthermore, multiple antennas of a transmission apparatus in a wireless communication system are used not only to improve the communication quality through transmit diversity but also to achieve spatial multiplexing transmission, which can improve the transmission rate by transmitting independent signals from the individual antennas at the same time on the same frequency. In spatial multiplexing transmission, the number of simultaneously transmit signals is called the number of streams, the number of ranks, or the number of layers. The signals transmitted from individual antennas are demultiplexed through signal demultiplexing processing, such as spatial filtering or MLD (Maximum Likelihood Detection). Also, PTL 1 and PTL 2 disclose methods for performing transmission by making different frequency allocation (also referred to as “assignment” or “mapping”) for each transmit antenna because the frequency with a good channel characteristic differs for each transmit antenna. By allowing the use of different frequency allocation for each transmit antenna, frequencies with high gain can be selected for each transmit antenna to perform communication. Thus, spatial multiplexing transmission with a high reception quality can be performed.
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