Joint Tilt Angle and User Association Optimization for Directional Antenna-Based Cell-Free Massive MIMO

With flexible antenna deployment, the Cell-Free Massive MIMO (CF-mMIMO) system can reduce wireless communication distance between access antennas and users, and provide multi-antenna access services for mobile users. In this paper, we consider introducing directional antennas in Cell-Free Massive MIMO system (CF-mMIMO-DA) to improve the network coverage and transmission capacity. In CF-mMIMO-DA systems, access control, which specifies the serving antennas, subcarriers, as well as the tilt angle and allocated transmit power, plays a crucial role for system performance. In this paper, we propose an access control optimization model in CF-mMIMO-DA systems with objective of maximizing the network throughput. Considering that the problem is NP-hard, we decompose the original problem into two subproblems, antenna tilt angle adjustment and user association. Then, to solve the two subproblems, we propose an antenna tilt angle adjustment algorithm based on the particle swarm optimization, and a user association algorithm based on the combination of concave-convex procedure and alternating direction of method of multipliers. In addition, we proposed an optimized joint access control mechanism based on alternating optimization between the two subproblems. Numerical results show that the deployment of directional antennas in CF-mMIMO can significantly improve the system throughput, and our proposed algorithm outperforms benchmarks.