Reduced-complexity downlink cell-free mmWave Massive MIMO systems with fronthaul constraints

Cell-free architectures have recently emerged as a promising architecture with the potential to offer equal user-rates throughout the coverage area. Given the spectral congestion at sub-6 GHz bands, there is a pressing interest in evaluating the cell-free performance in the mmWave regime. This paper addresses the design and performance evaluation of the downlink segment of cell-free mmWave Massive MIMO system using hybrid precoders under the realistic assumption of capacity-constrained fronthaul links. Towards this end, a hybrid digital-analog beamforming is proposed where the high-dimensional analog part only depends on second-order large scale information. The low-dimensional digital part can then be implemented using standard precoding techniques that rely on instantaneous CSI. Numerical results demonstrate that this reduced-complexity architecture, when combined with an adequate user selection (scheduling), attains excellent Max-Min performance when operating under limited-fronthaul constraints.