Abstract
A new estimator is presented which enables a baseband digital compensation of carrier and clock frequency offsets in OFDM systems. The estimator is obtained by applying the joint maximum likelihood criterion to one or several frequency domain symbols, with a model that explicitly takes into account the channel frequency selectivity. Under certain assumptions a linearization can be applied, leading to an analytical expression of the estimator. Otherwise, two-dimensional optimization is required. Several versions of the estimator are presented according to whether the modulation is PSR or QAM, differential or coherent. The performance improvement is evaluated in two contexts: digital audio broadcasting and wireless local area networks (HIPERLAN/2-like systems). In the context of vehicular DAB, the influence of Doppler is emphasized. In HIPERLAN/2, simulations show that a significant gain (up to 1 dB) can be achieved with the new method compared to basic linear least squares estimators, enabling an all-digital compensation of frequency offsets with negligible degradation of the BER performance.
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