Hybrid Precoding Design Achieving Fully Digital Performance for Millimeter Wave Communications

As a promising candidate for millimeter wave (mmWave) multiple-input and multiple-output (MIMO) communications, hybrid precoding techniques can reap the benefit of large antenna arrays, yet with only limited number of radio frequency (RF) chains. In this paper, we investigate the problem of achieving the same performance of the fully digital system with hybrid precoding. Specifically, for the single user MIMO system, we propose a closed form hybrid precoding design that can achieve the optimal fully digital performance for both frequency-flat and frequency-selective channels, and only requires the number of RF chains to equal the number of paths of the channel. The design for the case with even less RF chains is also given. Furthermore, for the multiuser (MU) system with single antenna at each mobile terminal (MT), two MU beamforming schemes are considered, which are the directional beamforming and zero-forcing. We show that for both schemes, the fully digital performance can be achieved with our proposed hybrid precoding designs with the number of RF chains no less than the sum number of channel paths from the base station to all the selected MTs. Numerical results are provided to validate our analytical results and show the performance gain of the proposed hybrid precoding designs compared to other benchmark schemes.