Joint Frequency Offset and Channel Estimation in Distributed MIMO-OFDM Systems

In recent times, cooperative communications or distributed multi-input multi-output (DMIMO) systems are being considered as key enabler for increasing spectral efficiency and coverage area in 4G and beyond wireless communication system. DMIMO system employs multiple relays each with single or multiple antennas in between the source and destination to form a virtual antenna array. However, a few of the fundamental difficulties in deployment of DMIMO networks with coherent communications are synchronization, tracking of independent oscillators, and channel estimation. The signal received at the destination is characterized by multiple carrier frequency offsets (MCFOs) and MIMO channel effects due to sparsely distributed relay nodes. In this paper, we address the issue of joint estimation of frequency offset and channel gains in a DMIMO orthogonal frequency division multiplexing (OFDM) systems deploying multiple antennas at the relays. We consider estimate-and-forward (EF) protocol for relaying where the coarse estimation process at the low cost relays generates inter carrier interference (ICI) in a multi-carrier system resulting in severe performance degradation and decrease in reliability of the network. The two iterative estimators, expectation conditional maximization (ECM) and space-alternating generalized expectation maximization (SAGE), are proposed to jointly estimate MCFOs and channel in the presence of ICI. The robustness of the estimators are evaluated through mathematical analysis and validated using simulations. The performance of the estimators is measured in terms of mean square error (MSE) and bit error rate (BER). Simulation results show that the proposed estimators provide a significant performance gain in DMIMO-OFDM systems with MIMO configuration at the relays compared to single-input single-output (SISO) system.