A constrained minimum mean square error code tracking loop for synchronous DS-CDMA systems

A constrained minimum mean square error code tracking loop (CMMSE-CTL) is proposed in this paper for synchronous direct sequence code division multiple access (DS-CDMA) systems in a multiuser environment. By introducing assistant weighted despreading signals into the local reference signals of the loop, we exploit the resultant extra degrees of freedom to improve the tracking performance under multiuser environment. The derived mean square tracking error is taken as the cost function and minimized with respect to the weight parameters, under the constraints which ensure an odd-symmetric bias-free SS-curve with enhanced tracking range. Simulations based on Gold codes compare the SS-curve, root-mean-square (rms) tracking error, mean-time-to-lose-lock (MTLL) performance of CMMSE-CTL and the traditional modified code tracking loop (T-MCTL). It is shown that CMMSE-CTL outperforms T-MCTL greatly in that it provides unbiased stable code tracking with reduced mean square tracking error, an enhanced tracking range, and near–far resistance.