Performance of constrained and unconstrained adaptive multiuser detectors for quasi-synchronous CDMA

It has previously been shown that multiuser interference in a quasi-synchronous code-division multiple-access (QS-CDMA) system can be effectively rejected using a nonadaptive decorrelator. However, the QS decorrelator is orthogonal to the inflated subspace spanned by the undesired users with arbitrary delays in the QS uncertainty interval and, hence, does not perform as well as a decorrelator with a priori knowledge of the delays. Here, we consider a constrained adaptive linear suppression filter which implicitly estimates the delays of the QS undesired signals, and thus approaches the ideal decorrelator of Lupas and Verdu (1990). For QS-CDMA, it is shown through analysis and simulations that the constrained recursive least-squares (C-RLS) algorithm converges faster than an unconstrained RLS adaptive filter, and generally outperforms the nonadaptive QS decorrelator. A cellular quasi-synchronous system is discussed in the present paper