Downlink Multiuser Beamforming With Compensation of Channel Reciprocity From RF Impairments

In this paper, we consider the uncertainty of channel reciprocity due to radio frequency (RF) frontend impairments for downlink multiuser beamforming in time division duplexing (TDD) based systems. In particular, we focus on conversion gain error (CGE) in analog RF frontends that results in independent in-phase and quadrature-phase (IQ) imbalances for each RF chain. A parameter of conversion gain factors for each RF chain is considered for the compensation of downlink beamforming as it can be easily estimated by a base station (BS) itself. With the set of these parameters, we study downlink multiuser beamforming for given uplink channel vectors that are available through uplink training phases. Based on performance analysis, we observe that the CGE in receive RF frontend can result in more performance degradation than those in transmit RF frontend.