Two-timescale hybrid analog-digital beamforming for mmWave full-duplex MIMO multiple-relay aided systems

Due to the severe pathloss experienced by electromagnetic wave transmission in the mmWave band, one challenge for the design of millimeter wave (mmWave) communication systems is coverage extension. Aiming to improve the coverage and sum rate performance of mmWave communications, we investigate new schemes for the design of full-duplex (FD) mmWave multiple-input multiple-output (MIMO) multiple-relay systems. Specifically, we propose a novel two-timescale analog-digital hybrid beamforming scheme to maximize the sum rate, while reducing the system complexity and channel state information (CSI) signalling overhead, as well as mitigating the effects of self interference and CSI errors caused by the delays. In the proposed scheme, the long-timescale analog beamforming matrices are designed based on the available channel statistics and updated in a frame-based manner, where a frame contains a fixed number of time slots, while for each time slot, the short-timescale digital beamforming matrices are optimized based on low-dimensional effective CSI matrices available in real-time. We develop an effective analog beamforming algorithm based on the cut-set bound and stochastic successive convex approximation (SSCA) and an innovative digital beamforming algorithm that relies on the theory of penalty dual decomposition (PDD) to maximize the system sum rate. The convergence properties and computational complexity of the proposed algorithms are also examined. Our simulation results show that the proposed two-timescale hybrid beamforming design significantly outperforms the conventional beamforming algorithms both in terms of the CSI-signalling overhead and the achievable sum rate in the presence of CSI delays