Joint Estimation of RF Impairments, Channel, and Low Complexity Iterative Equalization Technique for High Mobility SC-FDMA/OFDMA Uplink Systems

In this paper, we consider the problem of joint estimation and equalization of doubly selective channels and all radio frequency impairments, namely transmitter IQ imbalance (TIQI), receiver IQI (RIQI) and carrier frequency offsets (CFO) in the uplink of single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) systems with direct conversion transceiver architecture and high mobility users. We combine the effect of TIQI, RIQI and DSC to a new effective channel and propose the use of a Hermite polynomial based basis expansion model for approximating it to avoid the identifiability issue associated with this complete parameter estimation problem. As the maximum likelihood solution to this joint estimation problem requires complex multi-dimensional searches, we propose a novel subspace nulling based iterative estimation technique that effectively reduces the multi-user interference and replaces the multi-dimensional search with single-dimensional searches. Further, without much compromise on the performance, we also derive a low complexity version of the proposed technique. Additionally, we derive the Cramer-Rao bound for the joint estimation of CFO and effective channel. Moreover, we propose two equalization techniques for iteratively equalizing the mirror subcarrier pairs. The proposed techniques do not put any restriction on the carrier assignment scheme and provide good performance even at high mobile speeds.