NDA chip synchronizers for bandlimited DS/SS signals using CDMA on frequency nonselective fading channels

The authors investigate the chip synchronization for a bandlimited direct-sequence spread spectrum (DS/SS) M-PSK signal transmitted over a slowly time-varying nonselective fading channel. Two maximum-likelihood (ML) nondecision-aided (NDA) chip synchronization algorithms are derived. The first algorithm is nonchannel-aided (NCA): timing is acquired independently of the channel gain estimation. The second algorithm is channel-aided (CA): it makes use of (an estimate of) the instantaneous channel gain; as the channel gain estimator requires timing information, use of the CA algorithm implies that the timing and the channel gain must be acquired jointly. They also examine the influence of multiuser interference on the tracking performance for synchronous CDMA, where the M-PSK symbols from the various users are (almost) perfectly aligned at the receiver input, and asynchronous CDMA, where the misalignment is uniformly distributed over the symbol interval