A New Codebook Structure for Enhanced Multi-User MIMO Transmission in mmWave Hybrid-Beamforming System

A multi-user hybrid-beamforming (H-BF) system consisting of both analog beamforming and digital beamforming is considered for millimeter-wave (mmWave) channels. In the hybrid-beamforming system, analog beams are first allocated to scheduled users and then each user reports channel state information of the effective channel, i.e. channel between transmit and receive digital paths. Since amplitudes of the effective channel elements can largely be different from each other due to the analog beamforming which has a certain directivity, the conventional feedback scheme, e.g. precoder matrix indicator (PMI) in long-term evolution (LTE), that does not reflect this channel characteristic may not work well. In this paper, we propose a new feedback scheme for multi-user H-BF systems in mmWave channels. The proposed feedback scheme jointly utilizes two types of codebooks: 1) a direction of the H-BF channel; and 2) amplitudes of the channel elements. In numerical results, it is shown that the proposed feedback scheme largely improves the average throughput performance, especially when the analog beamwidth is narrow.