Intelligent reflecting surface (IRS) is amenable to assisting cell-free wireless networks because of its low cost and high spectrum efficiency. However, solving the IRS optimization problem usually requires high computational complexity. Therefore, we propose a novel multi-agent reinforcement learning (MARL) based joint optimization scheme to improve the system performance. First, we model the MARL framework to portray the dynamic wireless communication environment and interactive agent entities, which interprets the relationship between reinforcement learning and communication strategies. Moreover, in an IRS-aided cell-free massive MIMO system, entries are collaborated by the CPU for joint optimization, and the actions are continuous. Thus we propose a multi-agent deep deterministic policy gradient scheme for jointly optimizing the user's power control and IRS's passive beamforming (MADDPG-JPA-PB). Via centralized training and decentralized execution, the system gradually converges into an optimized policy associated with the maximization of the success transmission rate. Simulation results verify that the performance of the proposed scheme outperforms various independent deep deterministic policy gradient (DDPG) schemes and traditional optimization methods.
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