A frequency domain approach to timing estimation in multiuser communications

Early work on code division multiple access multiuser receiver assumed perfect timing recovery of all users. Chang and Chen (see Proc. IEEE Vehicular Technology Conference, 1997) confirmed two stage timing recovery for multiuser synchronization, acquisition and tracking, and further derived optimal timing estimator based on the maximum a posteriori probability (MAP) rule, in spite of its non-polynomial (NP) complexity. Optimum timing estimation transforming acquisition into serial search can reduce its complexity but with NP acquisition time. Analytical investigation on optimum timing estimator motivates study over frequency domain in this paper. By examining frequency domain characteristics, we not only propose a novel filtering method to improve the acquisition time by more than 2 dB in the low signal-to-noise ratio (SNR) region, but also suggest the possible polynomial complexity timing estimator by using properly designed signature sequences. Under the restriction of computation, the existence of two users´ case has been verified to demonstrate the effectiveness of our proposed methodology