Design of non-uniform antenna arrays for robust millimeter-wave LOS MIMO communications

Uniform linear array (ULA) multiple-input multiple-output (MIMO) systems with spatial multiplexing for millimeter wave in line-of-sight (LOS) links can enable the maximum capacity transmission, since the channel matrix can be made orthogonal by choosing the antenna spacing appropriately as a function of range, carrier wavelength and the number of antennas for ULAs. In this paper, we demonstrate that variations in range can cause significant degradation in performance such as large condition number and low channel capacity for a ULA optimized at a given range. Base on the condition number or capacity of the LOS MIMO channel, we design the optimized non-uniform antenna arrays that can alleviate this problem by keeping small condition number or large capacity over a significantly larger set of ranges than uniformly spaced antenna arrays. And furthermore, based on the optimized results, we find out a simple placement of antenna arrays which can avoid significant performance degradation near the optimized range.