Opportunistic beamforming is a powerful technique increasing system throughput in a static channel environment. Since the opportunistic beamforming technique requires quite large user population to exploit sufficient multiuser diversity, a Codebook-based Opportunistic BeamForming (COBF) technique is proposed. The COBF technique uses a unitary matrix, which changes with time, to induce larger and faster channel fluctuations in the static channel. Also, the COBF technique uses a codebook to provide further selection diversity to the conventional opportunistic beamforming technique to overcome the problem of limited multiuser diversity in a small population. Compared to the recently proposed multiple-pilot based opportunistic beamforming technique in , the COBF technique reduces required number of pilots. Moreover, since the size of codebook, not the number of pilots, determines the amount of supplementary selection diversity, the system throughput can be increased without limitation from the number of pilots. Some computer simulation results show that system throughput of the COBF technique is higher than that of other conventional techniques and required number of unitary matrices, which needs to be kept as a memory at mobile stations, is only ten in a simulation environment.