Codebook-based opportunistic beamforming for downlink multiple input multiple output systems

Opportunistic beamforming (OBF) is an efficient technique to exploit multiuser diversity in static channel environments. Since the OBF experiences lack of diversity when user population is low, a codebook-based opportunistic beamforming (COBF) is proposed. The COBF allows users to choose the best beam among multiple random beams. The COBF uses fixed pilots for channel estimation and generates multiple random beams by using a codebook composed of multiple fixed beams and a random matrix altering the beams with time slots. Compared with the conventional multi-beam opportunistic beamforming (MOBF), which also provides the beam selection diversity to the OBF, the number of beams in the COBF is related to the size of codebook, not the number of pilots. Accordingly, the COBF can increase the beam selection diversity without a restriction of pilot overhead. When the COBF is extended to a system that transmits multiple data streams in a time slot, the system throughput improvement over the OBF and the MOBF grows as the number of streams is increased because the pilots can be saved further.