Single-user sparse channel acquisition in multiuser DS-CDMA systems

Single-user channel estimation in multiuser DS-CDMA systems for the case of sparse channels with large delay spreads is addressed. In addition, practical pulse shapes are considered. In sparse channels, the efficient way to estimate the parameters is to estimate the continuous delays of each path, instead of using the typical discrete tapped delay-line model. Due to the facts that the desired delays are not drawn from a simple finite set and that band-limited pulse shapes are employed, the resulting methods require numerical optimization techniques. To facilitate estimation, it is proposed to optimize the spreading code employed during the training, or estimation, phase. The optimal single-path spreading code is derived and extended for estimation in the multipath scenario. Both single-path and multipath channel estimation are considered. The proposed algorithms are evaluated through simulation and via the determination of the Cramér-Rao lower bound on the estimation variance. Analytical approximations of key performance measures are also derived and are seen to be tight for a variety of scenarios.