The paper investigates an unmanned aerial vehicle (UAV)-assistant air-to-ground communication system, where multiple UAVs form a UAV-enabled virtual antenna array (UVAA) to communicate with remote base stations by utilizing collaborative beamforming.
The key highlights are:
The authors formulate a UAV-enabled collaborative beamforming multi-objective optimization problem (UCBMOP) to simultaneously maximize the transmission rate of the UVAA and minimize the energy consumption of all UAVs by optimizing the positions and excitation current weights of the UAVs.
The UCBMOP is challenging as the two optimization objectives conflict with each other and are non-concave with respect to the optimization variables. Traditional optimization methods are inefficient in solving this problem.
The authors propose a multi-agent deep reinforcement learning (MADRL) approach, specifically an improved heterogeneous-agent trust region policy optimization (HATRPO) algorithm called HATRPO-UCB, to address the UCBMOP.
HATRPO-UCB introduces three techniques to enhance the performance: observation enhancement, agent-specific global state, and Beta distribution for policy. These techniques help the agents learn better strategies for the UVAA collaborative beamforming.
Simulation results demonstrate that the proposed HATRPO-UCB algorithm outperforms other classic antenna array solutions and baseline MADRL algorithms in learning an effective strategy for the UVAA communication system.
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