Joint Power Allocation, User-Subarray Pairing, and Analog Beamforming for the Spatial Non-Stationary Extremely Large-Scale MIMO

In this paper, we study the downlink precoding of spatial non-stationary (SnS) extremely large-scale multiple-input multiple-output (XL-MIMO) in the near-field channel with uniform spherical wave (USW) and mixed line-of-sight and non-line-of-sight environments. First, we present a novel precoding scheme by replacing the digital beamforming (BF) in the conventional hybrid BF with user-subarray pairing network. The presented scheme utilizes the SnS channel characteristic efficiently, and can facilitate the high-speed implementation and the deployment of low-resolution digital-to-analog converters. Next, we study the optimizations to maximize both the sum-rate and min-rate by designing the power allocation, user-subarray pairing network, and analog BF jointly. For the sum-rate maximization, we first reformulate the corresponding nonconvex problem by the quadratic transformation to facilitate the further processing. Then, the alternating optimization framework is utilized to optimize the variables alternately. Concretely, the Riemannian conjugate gradient method, projected gradient ascent method, and mathematical programming with equilibrium constraints alternating direction method are employed to optimize the analog BF, power allocation, and user-subarray pairing network, respectively. For the min-rate maximization, we extend the aforementioned solutions by proper modifications. Finally, the effectiveness of the proposed optimization algorithms is verified through computer simulations.