Joint Estimation of Time, Frequency Offsets, and Channel Gains With ICIs in EF Multi-relay DMIMO-OFDM System

The cooperative communication or distributed multiple-input multiple-output (DMIMO) system, combined with orthogonal frequency division multiplexing (OFDM), is considered as an emerging paradigm for link reliability, high data rate, and coverage extension in fifth-generation (5G) wireless communication systems. The DMIMO system employs multiple relays with single or multiple antennas that opportunistically form virtual antenna array (VAA) in between the source and destination. Moving to the DMIMO-OFDM system from the conventional network with uncoordinated nodes requires robust synchronization and tracking. The signal received at the destination is characterized by multiple timing offsets (MTOs), multiple carrier frequency offsets (MCFOs), and frequency-selective channel gains. In this paper, we address the issue of joint time frequency and channel gain estimation for the estimate-and-forward (EF) relaying protocol. EF is a cost-effective solution but provides coarse estimation at the relays introducing intercarrier interferences (ICIs). OFDM system is very sensitive to the ICIs. We propose two iterative estimators: expectation conditional maximization (ECM) and space-alternating generalized expectation maximization (SAGE) to jointly estimate MTOs, MCFOs, and channel gains in the presence of ICIs. The robustness of the estimators is analyzed through mathematical formulation and simulations. The performances of the estimators are measured in terms of mean square error (MSE) and bit error rate (BER). The MSE performance is further verified with the theoretical Cramer-Rao lower bound (CRLB) derived. Simulation results show that the proposed estimators provide a significant performance gain in the DMIMO-OFDM system with MIMO configuration at the relays compared to the single-input single-output (SISO) system.