This paper presents a suboptimal modified code tracking loop (SO-MCTL) that mitigates the effect of multiple- access interference (MAI) for synchronous direct-sequence code division multiple access (DS-CDMA) systems. Weighted linear combinations of the interfering users' signals are introduced into the local signals to facilitate the optimization of the tracking performance. Instead of search for the optimal weighting vectors that yields the minimum mean square tracking error, the mean square tracking error is minimized with respect to the weighting parameters given the constraints that ensure a stable bias-free S-curve. Simulations based on Gold codes compare the S-curve and root-mean-square (rms) tracking error of the SO-MCTL and traditional modified code tracking loop (T-MCTL). It is shown that SO-MCTL outperforms T-MCTL greatly in that it provides unbiased code tracking with reduced mean square tracking error and near-far resistance. PN code's autocorrelation function in addition to the imperfect compensation of MAI. In view of the performance impairment caused by MAI on the tracking loop, we propose a suboptimal modified code tracking loop (SO-MCTL), which is designed for stable code tracking under multiuser environment. In SO-MCTL, weighted assistant despreading signals are introduced into the local reference signals. Instead of searching for the optimum weighting vectors to achieve the global minimum tracking variance, the constrained optimization based on the minimum mean square tracking error criterion is performed, subject to the constraints that ensure a stable bias-free S-curve. SO- MCTL is shown to be able to provide unbiased stable tracking with reduced mean square error in a multiuser environment. The paper is organized as follows. In Section II, the system and signal model is introduced and the bias problem with T-MCTL is briefly discussed. Section III describes the pro- posed SO-MCTL and presents the constrained optimization to minimize the mean square tracking error. Simulation results based on Gold code sequences are shown in Section IV to compare the performance of SO-MCTL with T-MCTL. Finally, conclusions are given in Section V.
['Yun T. Wu', 'Shu Hung Leung', 'Wai Keung Wong', 'Yajun Zhu']
A Suboptimal Modified Code Tracking Loop for Synchronous DS-CDMA Systems