How to approach zero-forcing under RF chain limitations in large mmWave multiuser systems?

Abstract

High cost of millimeter wave (mmWave) hardware precludes the possibility to support each antenna element with a dedicated radio frequency (RF) chain in large-scale mmWave systems. It thus makes conventional baseband precoding, such as zero-forcing (ZF) which is deemed virtually optimal in the large array regime, impractical. In this paper, we consider the RF chain limitations present in large mmWave multiuser systems and propose to approach the desirable yet infeasible baseband ZF precoding by essentially steering the beam towards each user's strongest path, termed MUBS, through analog processing at the RF domain. Rates achieved by both baseband ZF and the proposed MUBS precoding are then derived in a closed form. We further study the rate loss of MUBS relative to the upper bound imposed by ZF precoding and find that the gap converges to a finite limit as the transmit antenna number Nt goes large. Since the achievable rate of ZF grows without limit as Nt increases, the proposed MUBS scheme thus promises to achieve most of ZF rates at large Nt. Numerical results are also provided to validate the desirable performance of the proposed MUBS scheme.