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Abstract Due to the limitation of mobility and cost, it is inappropriate to utilize ground base station (GBS) to provide service in emergency scenarios or harsh environments. Unmanned aerial vehicle (UAV) is considered as a feasible way to solve the problem. Millimetre wave communication and UAV can be used together to offer wireless communications flexibly and rapidly for target area. However, fuselage jittering and energy supply would affect the quality and reliability of UAV communication seriously. To overcome the weakness of UAV, the authors prefer to use multiple UAVs to provide wireless communication service simultaneously. This paper investigates the design of hybrid beamforming in the scenarios including multiple UAVs and one end user. Cooperation strategy and alternating optimization method are applied in this design. The distributed UAV base stations are regarded as an entirety and the reference beamforming scheme obtained. Then, the beamforming problem for each UAV is formulated. Moreover, residual error minimization (REM) algorithm is proposed to find the solution of hybrid beamformers at UAV side. Finally, the beamformer of the receiver is optimized with the assist of effective channel, which includes the gain of both channel and beamformers of the transmitters. Simulation results show that the proposed algorithm can achieve near‐optimal spectral efficiency in different scenarios.

Abstract In massive multiple‐input multiple‐output systems, hybrid beamforming (HBF) is an attractive technique due to its excellent tradeoff between system performance and hardware implementation cost. In this paper, the signal‐to‐leakage‐plus‐noise ratio is considered as the optimization criterion and investigate the HBF design for multi‐user millimetre‐wave massive multiple‐input multiple‐output systems. To overcome the difficulty of solving the multi‐variable design problem, a novel two‐stage HBF scheme to optimize the analogue and digital beamformers are proposed. In particular, an orthogonal matching pursuit‐based method and a joint design method are introduced respectively to find the solution in the analogue stage. Then, in the digital stage, the digital precoder and combiner are designed to suppress the inter‐user interference plus noise, aiming at maximizing the sum‐signal‐to‐leakage‐plus‐noise ratio of multi‐user systems. Simulation results show that the proposed HBF scheme with lower complexity can achieve superior performance over the existing HBF schemes. Moreover, it is also indicated that the performance of the HBF scheme remains strong even with the imperfect channel state information.