Line-of-Sight MIMO in Point-to-Point Millimeter Wave Links via Scattering From Rough Surfaces

We analyze a point-to-point MIMO communication system over a deterministic channel with two paths: line-of-sight (LoS) and a scattered path over a rough surface. We characterize the MIMO channel capacity as a function of the root-mean-square (RMS) surface roughness and provide three important findings: 1) capacity is monotonically decreasing with the surface roughness, 2) capacity is lower bounded by the SISO case and upper bounded by the smooth surface case, and 3) capacity has a breaking point corresponding to the maximum surface roughness that supports two spatial streams. When the roughness is below the maximum, the proposed LoS-MIMO system assigns power to two spatial streams. If surface roughness is above the maximum, then capacity is attained by assigning power to a single spatial stream. We derive a closed-form expression for the breaking point in terms of the transmitted power, antenna gains, wavelength, and path geometry parameters. We analyze the practical implications of these findings, including the potential multiplexing gains (from 68% to 86% with respect to the SISO channel) in mmWave systems using compact linear arrays. The channel capacity is evaluated numerically in three scenarios typical of point-to-point millimeter wave links to confirm our theoretical findings. We also study the sensitivity of the capacity and maximum surface roughness to the incidence angle, number of antennas, polarization, frequency, and surface material. Finally, we discuss implementation issues with fully-digital and hybrid analog/digital hardware. Our simulations show that hybrid hardware achieves 92% of the capacity of a fully-digital solution in the simulated scenarios.