In UNITS (Universal Mobile Telecommunications System) networks, for the purpose of improving spectral efficiency and further improving data rates, by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), it is performed exploiting maximum features of the system based on W-CDMA (Wideband Code Division Multiple Access). For the UMTS network, for the purpose of further increasing high-speed data rates, providing low delay and the like, Long Term Evolution (LTE) has been studied.
In the 3G system, a fixed band of 5 MHz is substantially used, and it is possible to achieve transmission rates of approximately maximum 2 Mbps in downlink. Meanwhile, in the LTE system, using variable bands ranging from 1.4 MHz to 20 MHz, it is possible to achieve transmission rates of maximum 300 Mbps in downlink and about 75 Mbps in uplink. Further, in the UMTS network, for the purpose of further increasing the wide-band and high speed, successor systems to LTE have been studied (for example, LTE Advanced (LTE-A)). For example, in LTE-A, it is scheduled to increase 20 MHz that is the maximum system band in LTE specifications to about 100 MHz.
Meanwhile, in LTE-scheme systems, MIMO (Multi Input Multi Output) systems are proposed as radio communication techniques for transmitting and receiving data using a plurality of antennas and improving a data rate (spectral efficiency) (for example, see Non-patent Document 1). In the MIMO systems, the transmitter/receiver is provided with a plurality of transmission/reception antennas, and simultaneously transmits different transmission information sequences from different transmission antennas. Meanwhile, the receiver side exploits the fact that different fading variations occur in between transmission and reception antennas, and divides the simultaneously-transmitted information sequences to detect, and it is possible to increase the data rate (spectral efficiency).
In the LTE-scheme systems, specified are Single User MIMO (SU-MIMO) in which transmission information sequences simultaneously transmitted from different transmission antennas are all for the same user and Multiple User MIMO (MU-MIMO) in which the transmission information sequences are for different users. In the SU-MIMO and MU-MIMO, the receiver side selects a phase/amplitude control amount (PMI: Precoding Matrix Indicator) to set on an antenna of the transmitter, and notifies the transmitter as feedback. The transmitter side performs precoding on each transmission antenna based on the PMI that is fed back from the receiver, and transmits transmission information sequences.
Meanwhile, in LTE-A scheme systems, to reserve flexibility of precoding on each transmission antenna in the transmitter, proposed is Zero Forcing MU-MIMO (ZF MU-MIMO) in which the receiver side transmits a CDI (Channel Direction Indicator) indicative of a channel state as feedback, and the transmitter side performs precoding on each transmission antenna based on the CDI, and transmits transmission information sequences (see Non-patent Document 2). In this ZU MU-MIMO, it is possible to transmit transmission information sequences to respective different users from eight transmission antennas scheduled in LTE-A, and to significantly increase the data rate when the spatial correlation between channels is high.