Two novel iterative joint frequency-offset and channel estimation methods for OFDMA uplink

We address joint estimation of frequency offsets and channel responses in OFDMA uplink. A cyclically equal-spaced, equal-energy interleaved pilot preamble is proposed by which two iterative estimators are developed. In the first estimator, we develop a modified SAGE (space alternating generalized expectation-maximization) method by incorporating multiuser interference cancellation in both time and frequency domains into the SAGE method. The proposed modified SAGE method yields a faster convergence rate, a better estimation performance, and a lower complexity than the existing conventional SAGE method from [10]. In the second method, we propose a very low complexity ad hoc method to replace the high complexity frequency offset estimator of the first method. The proposed ad hoc method is developed based on the time-domain characteristics of the pilot preamble and the frequency-domain power concentration property of OFDMA systems. It achieves almost the same estimation performance as the first proposed method but requires significantly lower complexity. Both proposed methods are not only better in estimation performance, convergence rate, and complexity, but also more robust to the number of active uplink users than the existing method while requiring only one OFDM training symbol and no restriction of the data subcarrier assignment scheme.