Closed-Form Beamforming Aided Joint Optimization for Spectrum- and Energy-Efficient UAV-BS Networks

In this paper, we investigate a spectrum- and energy-efficient emergency wireless communication system with an unmanned aerial vehicle (UAV) mounted base station (BS). To the best of our knowledge, this is the first work to investigate joint optimization of three-dimensional (3D) beamforming (BF), power allocation (PA), user scheduling and UAV trajectory, to maximize the average spectrum efficiency (SE) while maintaining high energy efficiency (EE). Furthermore, a closed-form BF design in both angle- and power-domains is proposed for the first time for UAV-BS assisted wireless systems. Thanks to the closed-form solutions, a low-complexity iterative algorithm is proposed for joint optimization, which converges within only one or two iterations. Simulation results show that 3D BF plays a dominant role in the overall performance, and the proposed closed-form 3D BF design achieves near-optimal performance in terms of both EE and SE, while requiring much lower complexity than exhaustive search. The maximum UAV speed with respect to the optimal EE is also obtained.