Performance and User Association Optimization for UAV Relay-Assisted mm-Wave Massive MIMO Systems

Unmanned aerial vehicle (UAV) relaying is deemed as a promising solution to enhance the achievable rate and widespread connectivity in millimeter-wave (mm-wave) systems for tomorrow's 6G wireless networks. In this paper, we study both the performance and user association optimization for the UAV relay-assisted mm-wave massive multiple-input multiple-output (MIMO) communication system, where multiple base stations (BSs) serve their respective users with the help of one beamforming UAV relay. Both the beamforming and the UAV relay have essential impact on the achievable sum-rate of the system. Thus, a multi-user hybrid beamforming scheme is designed to mitigate the inter-user interference issues and achieve better trade-off between performance and complexity in UAV-enabled communications. Also, to exploit UAV relay-based architecture in serving different ground BS-user pairs, we propose a UAV relay-assisted multi-BS mm-wave massive MIMO system with hybrid beamforming architecture, which prevent sudden link disconnections caused by high path loss and line-of-sight (LoS) blockage in the mm-wave frequency band. Then, we formulate a user association problem with multiple constraints such that the sum-rate of the overall UAV relay-assisted mm-wave massive MIMO system is maximized. Simulation results are provided to demonstrate the effectiveness of the proposed UAV relay-enabled architecture.