Beam selection in multiuser millimeter-wave systems with sub-array user terminal architectures

Hybrid preceding, which refers to the combined use of phased arrays and digital (baseband) beamforming, is the main transceiver architecture under consideration for millimeter-wave (mmWave) wireless systems. However, this configuration is not suitable for receivers with a single RF chain, that must rely on fully-analog beamforming. In this paper, we consider the problem of finding the best analog beam combination when user terminals have independent analog sub-arrays. We present a novel algorithm for beam selection that separates analog and digital beamforming components. First, we use hierarchical analog beamforming codebooks to obtain a combination of beamformers that maximize SNR at the receivers. We leverage multiple RF chains at the AP and sub-arrays at the users to design such hierarchical codebooks. Second, we use digital zero-forcing beamforming at the transmitter to eliminate inter-user interference. Preliminary numerical results show the effectiveness of our algorithm in simple scenarios.