Efficient Hybrid Beamforming for Relay Assisted Millimeter-Wave Multi-User Massive MIMO

Applying relays in the multi-user massive Multiple Input Multiple Output (MIMO) system at Millimeter Wave (mmWave) bands can greatly help to overcome the large propagation loss at higher frequencies and boost the communication link. We propose an efficient hybrid beamforming technique for the relay assisted mmWave multi-user massive MIMO system. The design includes the analog transmit-receive coordinated beam alignment procedure and non-linear precoding based digital beamforming. The analog beam alignment can be flexibly applied to different hybrid array architectures. We propose to jointly design digital beamforming cross the base station, the relay, and the multi-antenna users to enhance the system performance. It is based on the Geometric Mean Decomposition Tomlinson Harashima Precoding (GMD-THP) scheme, and only linear processing is applied at the relay for a cost-effective reason. We evaluate the performance of the proposed technique and compare it with its linear and fully digital counterparts.