Robust OFDMA Uplink Synchronization by Exploiting the Variance of Carrier Frequency Offsets

In this paper, the uplink frequency offset estimation for orthogonal frequency-division multiplexing access (OFDMA) is discussed. We consider a general subcarrier allocation scheme where each user subcarrier group need not be contiguous. For an OFDMA uplink, we model the frequency offset for each user as an independent and identically distributed (i.i.d.) random variable with mean zero and variance sigma _epsiv ². An analysis of multiple access interference (MAI) is performed, and the Cramer-Rao lower bound (CRLB) for the estimation of variance of each user is derived. The signal-to-interference-plus-noise ratio (SINR) is derived as a function of the variance of the frequency offset. The variance of the frequency offset estimation error is lower bounded as a function of the signal-to-noise ratio (SNR). Successive interference cancellation (SIC) and iterative frequency offset estimation are considered. An estimate of the variance of the frequency offset is derived as a function of SINR and SNR. An estimate of the range of frequency offsets is derived using the assumption of uniformly distributed frequency offsets. Based on this estimate of the range of frequency offsets, the accuracy of any existing algorithm can be improved. Thus, new versions of the SIC-based frequency offset estimation and differential estimation algorithms are derived. Extensive simulation results are provided for a 16-user, 256-subcarrier OFDMA system over a multipath fading channel.