Analog beamsteering for flexible hybrid beamforming design in mmwave communications

In this paper, we propose an analog beamsteering approach for enabling flexible hybrid beamforming design that can achieve performance close to singular value decomposition (SVD) based digital beamforming with single user case. As a starting point, assuming the use of a codebook with infinite precision, we apply transposes of antenna array response matrices as analog beamformers at the BS and the UE sides. The resulting effective channel matrix including both propagation channel and analog beamformers is shown to be able to provide a maximal achievable rate very close to SVD based digital beamforming at low to medium SNR. Next, the impact of a finite size codebook is analyzed for practical analog beamformer design. A closed-form derivation is obtained for mapping rate loss to the number of antennas, code-book sizes and the received SNR. The analysis shows that in order to achieve performance comparable to analog beamforming with infinite precision, the codebook size should be at least larger than the number of implemented antennas. Therefore, the proposed analog beamforming approach can be designed independently without taking digital beamforming into account. Such an approach can lead to the development of flexible hybrid beamforming architectures that could adapt to a changing propagation environment via agile analog beam selection and maximize spatial multiplexing gain via proper digital beamformer design.