Multiuser diversity is one of diversities caused by independently time-varying a channel environment between users in a wireless communication system. Multiuser diversity gain is obtained by allocating a resource when a channel of each user reaches the best condition through scheduling during observation of a change of a channel of each user. As the change of the channel is increased, the diversity gain is increased. Accordingly, the diversity gain is limited under a channel environment where the change of the channel is small or slow. Under the above-mentioned environment, the diversity gain is generated by inducing a large and quick change of the channel using multiple transmission antennas. This method is called opportunistic beamforming (hereinafter referred to as “OBF”) or random beamforming.
OBF technology is limited to the diversity thereof under an environment corresponding to a small number of users, since it uses only one beam to search an optimal user at each time slot. Therefore, technologies that generate multibeams and select an optimal beam of each user to increase multiuser diversity gain, have been proposed in order to overcome the above-mentioned limitation. As these technologies in the related art, there are codebook-based opportunistic beamforming (hereinafter referred to as “COBF”) and codebook-based beamforming with code rotation (hereinafter referred to as “CBF-CR”), where the codebook contains multiple vectors used for multibeams.
The COBF uses a pilot only for channel estimation, and allows each user to estimate a channel. A base station receives from each user the maximum beam index and channel quality information (hereinafter referred to as “CQI”) corresponding to the beam, as feedback. Then, the base station performs scheduling by allocating a user having the maximum gain to each beam thereof, and transmits data. In the COBF method, a unitary matrix varying with time is multiplied by one of beam vectors in codebook, so that a beam varying with time is obtained. The COBF has a merit in that a pilot is fixed to a pilot used for channel estimation and a selective diversity effect is thus increased to infinity.
The CBF-CR generates multiple codebooks by grouping large-sized codebooks and then obtains multiple beams that vary with time by diversely using the codebooks with change of time. Therefore, the same effect as the COBF method is obtained in the CBF-CR.
In the COBF, multibeams have been obtained by multiplying unitary matrices, which vary with time, by beam vectors of one codebook. In contrast, in the CBF-CR, multibeams are obtained by generating multiple codebooks, and then using the codebook while exchanging the codebook with time.
However, the method of designing a beam and the method of designing a beam parameter in the related art have a problem in that performance deteriorates.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.