Towards Optimal Cooperative Beamforming in Millimeter Wave Systems via Directional Transmissions

Deployments of millimeter wave systems suffer from increased propagation and blockage losses that lead to a focus on small cell coverage. With an increased densification of such systems, cooperative transmissions are possible at a user equipment (UE) from multiple transmission reception points (TRPs) and with repeater nodes (e.g., relay UEs, intelligent reflecting surfaces, reflectarrays) in the vicinity of a certain UE. In this context, this work proposes and studies the performance of a class of directional beam steering and co-phasing schemes that leverage the existing set of beam steering solutions for point-to-point transmissions. To benchmark the performance of the proposed schemes, an unrealizable upper bound to the SNR is first established. We then quantify the SNR gap between the proposed directional schemes and the unrealizable upper bound via theory and numerical studies and show that the proposed solutions are of low complexity and comparable performance to the unrealizable upper bound.