Timing and frequency offset estimation in asynchronous multi-user OFDM systems

A symbol timing and carrier frequency offset estimation scheme for asynchronous multi-user orthogonal frequency division multiplexing (OFDM) systems with antenna diversity is proposed. Compared to a single-user OFDM system, an asynchronous multi-user OFDM system based on spatial multiplexing contains many parameters to estimate so that joint maximum likelihood (ML) based techniques are difficult to implement. In this paper, the estimation problem is simplified by assigning a unique set of pilot symbols to each user. Employing a fractional frequency offset estimation technique that exploits virtual carriers, a subspace-based projection scheme is proposed for tracking the timing offsets. The simulation results demonstrate that the timing and frequency offset information of each user is acquired with sufficient accuracy at moderate SNR's.