Exploring the Advantages of Sparse Arrays in XL-MIMO Systems: Do Half-Wavelength Arrays Still Offer an Edge in the Near Field?

원문 초록

Extremely large-scale multiple-input multiple-output (XL-MIMO) has attracted extensive research attention due to its potential to meet the increasingly demanding requirements of future communication systems. Meanwhile, recent researches have indicated that sparse arrays may offer promising advantages for XL-MIMO systems in the near-field. Motivated by these, this paper investigates a downlink near-field 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.