Beam Selection And Precoding In User-Centric Cell-Free Mm Wave MIMO Systems

With the continuous development of today’s wireless communication technology,the emergence of millimeter wave technology provides a feasible technical idea to satisfy the user’s demand for higher rate transmission,and the introduction of a user-centric deployment of a cell-free large-scale multiple-input multiple-output antenna(MIMO)system can further alleviate the problems of path loss and channel blockage that exist in millimeter wave technology.In this paper,we focus on the optimization of a distributed precoding scheme design for user-centric decentralized large-scale MIMO millimeter-wave systems.The main tasks are as follows:(1)We convert the channel matrix from the original physical space domain to the beamspace domain,while optimizing the beam selection algorithm to suppress inter-beam interference intra-cluster to maximize the sum-rate.(2)We propose a new beam selection algorithm and precoding scheme design for user-centric decoupled large-scale MIMO millimeter-wave based systems,in which a weighted system sum-rate maximization problem is formulated.Since this maximization problem is a nonconvex optimization problem with binary beam selection constraints and coupled variables,the computational complexity associated with solving this joint optimized beam selection matrix and precoding matrix problem according to the general nonconvex optimization algorithm is impractical.Therefore,we propose a practical solution that will design the beam selection algorithm and precoding scheme separately.In particular,our proposed beam selection algorithm aims to reducing the intercluster inter-beam interference,while we propose a precoding scheme based on the weighted sum mean-square error(WSMSE)framework,where the precoding matrix can be updated in a distributed manner.(3)We further employ low-rank decomposition and Neumann series expansion to reduce the computational complexity of precoding.At the same time,we perform a computational complexity analysis of the proposed beam selection algorithm and precoding design algorithm,and demonstrate that our proposed algorithm outperforms the existing beam selection algorithm based on the minimization of inter-cluster interference and the existing zero-forcing(ZF)precoding scheme in the centralized implementation of conventional cell-free MIMO systems in terms of computational complexity.