On the feasibility of beamforming in millimeter wave communication systems with multiple antenna arrays

The use of the millimeter (mm) wave spectrum for next generation (5G) mobile communication has gained significant attention recently. The small carrier wavelengths at mmwave frequencies enable synthesis of compact antenna arrays, providing beamforming gains that compensate the increased propagation losses. In this work, we investigate the feasibility of employing multiple antenna arrays (at the transmitter and/or receiver) to obtain diversity/multiplexing gains, where each of the arrays is capable of beamforming independently. Considering a codebook based beamforming system (the set of possible beamforming directions is fixed a priori, e.g., to facilitate limited feedback), we observe that the complexity of jointly optimizing the beamforming directions across the multiple arrays is highly prohibitive, even for very reasonable system parameters. To overcome this bottleneck, we develop two complementary approaches, premised on the sparse multipath structure of the mmwave channel. Specifically, we reduce the complexity of the joint beamforming optimization problem by restricting attention to a small set of candidate directions, based on: (a) computation of a novel spatial effective power metric, and (b) estimation of the channel's angles of arrival. Our methods enable a drastic reduction of the optimization search space (a factor of 100 reduction), while delivering close to optimal performance, thereby indicating the potential feasibility of achieving diversity and multiplexing gains in mmwave systems.