Near-Field XL-MIMO Systems with Sparse UPAs

This paper investigates a downlink near-field extremely large-scale multiple-input multiple-output (XL-MIMO) communication system with sparse uniform planar arrays (UPAs). Based on the Green's function-based channel model, the paper focuses on the power distribution of the arrived signal near the focused point of the transmit sparse UPA. In the vicinity of the focused point, along the x-axis and z-axis directions, closed-form expressions for the power distributions are derived. Based on that, expressions for the width and length of the main lobe are obtained in closed form, both of which decrease as the antenna spacing increases. Furthermore, the paper introduces a crucial constraint on system parameters, under which effective degrees-of-freedom (EDoF) of XL-MIMO systems with sparse UPAs can be precisely estimated. Then, the paper proposes an algorithm to obtain a closed-form expression, which can estimate EDoF with high accuracy and low computational complexity. The numerical results verifies the correctness of the main results of this paper. Furthermore, the numerical results reveals the improvement in the performance of XL-MIMO systems with the use of sparse UPAs.