Millimeter Wave Cell-Free Massive MIMO Systems: Joint Beamforming and AP-User Association

We consider the problem of joint beamforming and access point (AP)-user association for downlink millimeter Wave (mmWave) cell-free (CF) massive MIMO systems. Such a problem is formulated into an integer-constrained optimization problem which aims at maximizing the minimum average received signal power among users. Directly solving this problem is NP-hard due to its combinatorial nature. To address this difficulty, we first transform the problem into a feasibility checking problem within a bisection framework. Then an alternating average reflection algorithm (AARA)-based method is developed to solve the feasibility checking problem. Simulation results show that the proposed algorithm achieves a considerable performance improvement over existing methods.