Achieving multiple degrees of freedom in long-range mm-wave MIMO channels using randomly distributed relays

Multi-Gbps, long-range wireless communication at millimeter wave frequencies is characterized by channels with strong line-of-sight signal components, with link budgets relying on highly directional and dense transmit and receive antenna arrays with sub-wavelength inter-element spacing. A natural method to further increase data rates over such channels is to spatially multiplex several data streams by providing additional antenna arrays at both ends of the communication system. However, at the link ranges of interest, the resulting MIMO channel rank, largely governed by the Rayleigh criterion, is deficient for inter-array spacings that can be realized with reasonable node size. As exact relay placement is out of the question, one scalable approach to obtaining the maximum available degrees of freedom is to introduce relay nodes randomly distributed over a sufficiently large region that the effective inter-relay spacing satisfies a probabilistic version of the Rayleigh criterion. In this paper, we present analysis and simulation results which provide design guidelines regarding the required size of the relay region, and quantify the dependence of performance on the number of relays.