Lately, Multiple Input Multiple Output (MIMO) data transmission using plural antennas attracts attention in order to enhance the data rate and/or quality of radio communication. In MIMO data transmission, it is possible to accomplish parallel transmission of plural pieces of data, such that a transmitting station transmits plural pieces of data at a same frequency and concurrently, using plural transmit antennas, and a receiving station receives data with plural receive antennas and demultiplexes the data.
Moreover, Multi-User (MU) MIMO, i.e., MIMO data transmission between a transmitting station having plural antennas and multiple receiving stations is under consideration. In MU-MIMO data transmission, paths between the plural antennas of the transmitting station and all antennas of the multiple receiving stations are regarded as a MIMO channel. The transmitting station concurrently transmits individual data signals to each of the receiving stations and the multiple receiving stations concurrently access the MIMO channel, so that system throughput can be improved.
With MIMO or MU-MIMO data transmission, plural pieces of data concurrently transmitted in parallel (called data transmission of plural layers) are first mixed in a spatial channel. Accordingly, it is necessary to implement demultiplexing the plural pieces of data at the receiving stations. Diverse methods for this implementation are proposed. MIMO data transmission includes, for example, an eigen beam space division multiplexing (ESDM: Eigen Beam SDM) scheme in which, on a MIMO channel, plural orthogonal beams between transmitting and receiving stations are formed by MIMO transmission processing and MIMO reception processing, and plural pieces of data are transmitted using these orthogonal beams (see patent literature 1). Meanwhile, in MU-MIMO data transmission, since receiving stations cannot cooperate with each other, a transmitting station needs to perform transmission processing so that signals destined for each receiving station do not arrive at another receiving station. For this purpose, MU-MIMO transmission processing initiative by the transmitting station, i.e., so-called precoding is used (see non-patent literature 1 and non-patent literature 2). Furthermore, in a case where each receiving station has plural antennas, combined usage of MU-MIMO transmission processing at a transmitting station and MU-MIMO reception processing at a receiving station is possible (see non-patent literature 3 and non-patent literature 4).
Then, in order to properly demodulate data demultiplexed for each receiving station, each receiving station needs to know an effective channel for each transmission data (each layer) in MIMO or MU-MIMO data transmission. This effective channel is defined as a virtual channel between each received data and transmitted data corresponding to the received data, after data transmitted in parallel by MIMO processing or MU-MIMO transmission and/or reception processing has been demultiplexed for each receiving station. That is, it represents a change (gain) obtained for each layer, resulting from MIMO/MU-MIMO transmission and reception processing. After having known this effective channel, i.e., this change (gain), each receiving station demodulates and decodes data of each layer. Because all receive antennas are located on one receiving station in MIMO transmission and the receiving station can estimate an entire spatial channel, it can calculate effective channels that are formed by the MIMO transmission and reception processing, if arrangements for MIMO transmission and reception processing for data are established between the transmitting station and receiving station. Meanwhile, each receiving station in MU-MIMO can estimate only a part of a spatial channel and, therefore, it is for the receiving station impossible to calculate effective channels.