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Tracking performance of DA and DD multiuser timing synchronizers for short code DS-CDMA systems

We consider a direct-sequence code-division multiple-access system operating over frequency-selective multipath and slowly Rayleigh fading channels. In order to implement RAKE reception, the delays of the different paths have to be estimated and the estimates have to be updated. We extend interference mitigation to this estimation problem and design timing recovery schemes accounting for the intersymbol interference and the multiple-access interference present in the signal. Starting from the maximum likelihood approach, we derive data-aided and decision-directed early-late synchronizers and study their tracking performance. The advantage of the interference mitigating synchronizer is illustrated. The sensitivity of the synchronizer to imperfect knowledge of path gains and to near-far situations is also investigated.