Decoder-Assisted Timing Synchronization in Multiuser CDMA Systems

The problem of synchronizing weak user signals in multiuser code-division multiple access systems is addressed. We integrate timing synchronization into an iterative multiuser detection (MUD) framework for the improved synchronization performance of weak users. First, the effect of correctly detected users on the remaining undetected users is suppressed via soft parallel interference cancellation. Then, the time offset (TO) of each undetected user is estimated. To speed up the MUD, interference cancellation is performed after only a few decoder iterations of the detected users, and the TO estimator is designed taking into account the residual multiple access interference. A low-complexity non-data-aided TO estimator that estimates the TO of each undetected user separately is proposed. The proposed technique significantly improves the synchronization performance of weak users under high near-far conditions compared to the full interference case. Assuming Gaussian distributed log-likelihood ratios at the decoder outputs, we directly relate the missed detection probability of a desired user to the root-mean-squared error of the detected user´s data sequences. The extrinsic information transfer chart of the decoder is used to analyze the acquisition performance of the desired user with the number of decoder iterations of the detected users. An upper bound for the missed detection probability of the proposed estimator is derived.