Conventionally, in wireless communication systems, a technology such as Space Multiplexing Multi-Input and Multi-Output (hereinafter, abbreviated as “SM-MIMO”) that allows to simultaneously transmit mutually different data from a plurality of transmission antennas has been studied as a transmission method for improving transmission efficiency. In SM-MIMO, because mutually different data are simultaneously transmitted from multiple antennas, when the reception quality of a receiving apparatus is good, the data transmission efficiency can be improved. However, when the reception quality is bad, data re-transmission is frequently performed, so that the transmission efficiency may be reduced. One measure against this is to combine transmission diversity (e.g., STC (Space-Time Coding) that allows to simultaneously transmit mutually identical data from a plurality of transmission antennas with SM-MIMO, and to transmit data by switching between transmission methods for the transmission diversity and for SM-MIMO depending on the reception quality (see, for example, Japanese Laid-open Patent Publication No. 2005-39807).
In this case, to reduce a processing load for switching the transmission methods, it has been studied that a modulation scheme and a code rate are also changed depending on the reception quality of the receiving apparatus during the data transmission in the same manner as normal AMC (Adaptive Modulation and Coding). In AMC, a transmitting apparatus receives a report on the reception quality from the receiving apparatus, and adaptively changes the code rate and the modulation scheme (MCS: Modulation and Coding Scheme) for transmission data depending on the reception quality. That is, the transmitting apparatus increases the code rate and a modulation level (e.g., 64QAM) for data to be transmitted to a receiving apparatus having good reception quality, and decreases the code rate and the modulation level (e.g., QPSK) for data to be transmitted to a receiving apparatus having bad reception quality.
A hierarchical modulation scheme has also been known as the transmission method for improving the transmission efficiency. The hierarchical modulation scheme is frequently used in the field of digital broadcasting or the like, and in which a plurality of bits transmitted by one symbol is hierarchized and different data is mapped to each level. For example, important data is mapped to a bit in a high level for which an error is less likely to occur, and less important data is mapped to a bit in a low level for which an error is more likely to occur. Consequently, frame data or the like indispensable to video reproduction can infallibly be transmitted without issuance of a retransmission request therefor, so that the transmission efficiency can be improved.
However, when the above-mentioned transmission method is implemented, there is a problem in that the transmission efficiency is not improved to more than a predetermined level especially in a wireless communication system in which data is transmitted to a plurality of user terminal devices. That is, in the wireless communication system, as illustrated in FIG. 1 for example, user data #0 and user data #1 to be transmitted to different user terminal devices are mapped to a channel #0 and a channel #1 corresponding to the user terminal devices, respectively. Furthermore, in the wireless communication system employing MCS for example, when the state of the channel #0 is degraded, the reception quality of the user data #0 is degraded, so that the code rate and the modulation scheme for the channel #0 are changed. However, because practically changeable code rate and modulation scheme are limited, it is not always possible to select optimal MCS for the state of the channel #0.
Furthermore, when the reception quality of the user data #1 is good, a code rate and a modulation scheme that are excessive for the actual reception quality may be selected when selecting MCS in a practical range, so that resources may be wasted. As described above, the user data #0 and #1 are respectively mapped to the channels #0 and #1 that are independent of each other, so that when there is a large difference in the reception quality between the respective user terminal devices corresponding to the channels #0 and #1, there may be a case in which resources are wasted in one channel while resources run short in the other channel.