On the Number of RF Chains and Phase Shifters, and Scheduling Design With Hybrid Analog-Digital Beamforming

Bogale, T.E. Le, L.B. Haghighat, A. Vandendorpe, Luc [UCL]

This paper considers hybrid beamforming (HB) for 7 downlink multiuser massive multiple-input multiple-output 8 (MIMO) systems with frequency selective channels. The proposed 9 HB design employs sets of digitally controlled phase (fixed phase) paired phase shifters (PSs) and switches. For this system, first we 11 determine the required number of radio frequency (RF) chains 12 and PSs such that the proposed HB achieves the same perfor- 13 mance as that of the digital beamforming (DB) which utilizes N 14 (number of transmitter antennas) RF chains. We show that the 15 performance of the DB can be achieved with our HB just by uti- 16 lizing rt RF chains and 2rt (N − rt + 1) PSs, where rt ≤ N is the 17 rank of the combined digital precoder matrices of all subcarriers. 18 Second, we provide a simple and novel approach to reduce the 19 number of PSs with only a negligible performance degradation. 20 Numerical results reveal that only 20−40 PSs per RF chain are 21 sufficient for practically relevant parameter settings. Finally, for 22 the scenario where the deployed number of RF chains (Na) is 23 less than rt , we propose a simple user scheduling algorithm to 24 select the best set of users in each subcarrier. Simulation results 25 validate theoretical expressions, and demonstrate the superiority 26 of the proposed HB design over the existing HB designs in both 27 flat fading and frequency selective channels.