Design of Non-Uniform 3-D Arrays for Near-Field XL-MIMO Communications

In this letter, a general design criterion is proposed for optimizing non-uniform three-dimensional (3D) arrays to enhance near-field capacity in extremely large-scale MIMO (XL-MIMO) communications. Specifically, an analytical characterization for the asymptotic channel eigenvalues with deployed non-uniform 3D arrays is first presented. Subsequently, with the objective of maximizing system capacity, the array optimization problem is mathematically formulated based on the derived expressions. It is shown that the optimal non-uniform 3D antenna array exhibits a centrifugal arrangement, with elements sparsely arranged at the center and densely packed in the surrounding areas. Numerical results demonstrate a significant capacity gain of the optimized non-uniform 3D arrays over regular arrays.