Capacity bounds for multi-user MIMO systems in terrestrial line-of-sight environments

We derive capacity upper and lower bounds for a multi-user multiple-input multiple-output (MU-MIMO) system in a two dimensional line-of-sight environment where the central node is equipped with a uniform circular array and the remote nodes are distributed from the central node at an equal distance and equal angular separation. We first show that this channel can be represented by a circulant matrix that facilitates the derivation of the bounds. We then show that those bounds are robust to non-equal distances and non-equal angular separation, and closely indicate the performance of three dimensional MU-MIMO channel taking into account the antenna height, antenna patterns, and ground reflection in the terrestrial environment. We also show the MU-MIMO channel capacity for the central node using a uniform linear, triangular, square, and circular array as a function of remote node locations and central node antenna spacing, and conclude that the circular array provides the best performance.