Hybrid digital and analogue beamforming design for millimeter wave relaying systems

Millimeter-wave (mm-wave) communication has been considered as a potential technology for next-generation mobile cellular networks because of the vast amount of underutilised spectrum in these frequency bands. The very short wavelength enables the use of a massive number of antenna arrays to obtain sufficient beamforming gains and received power. In practice, mm-wave communications adopt a hybrid radio frequency (RF) and baseband (BB) beamforming design to avoid allocating a dedicated RF chain for each antenna. In this paper, we propose hybrid beamforming schemes for multiuser mm-wave relaying systems. In particular, the RF and BB beamforming designs are separated to avoid the intractable searching problem in a joint optimization. Simulation results show that the proposed hybrid beamforming can use a small number of RF chains to approach the performance of conventional digital beamforming, in which each antenna is assigned to an individual RF chain. Results also demonstrate that the proposed design outperforms the conventional joint RF/BB design. Finally, the proposed hybrid beamforming method is further validated by introducing channel estimation error, to which it is shown to be robust.