Application of adaptive filtering to direct-sequence spread-spectrum code synchronization

Direct-Sequence Spread-Spectrum (DS/SS) systems offer several attractive features such as interference rejection, multipath diversity, multiple access capability, and accurate ranging. Recently, a greater interest has been devoted to the use of DS/SS systems in many mobile radio applications including digital cellular systems and indoor wireless networks due to the expected increase in capacity and an increased demand for new services. This interest motivates growing research on code synchronization for SS systems as a result of the critical role that it plays in the operation of SS systems. This thesis introduces an application of adaptive filtering for code synchronization in DS/SS systems.; In the proposed scheme, an adaptive filter is used to carry both functions of code synchronization: Acquisition and Tracking. The filter coefficients are adjusted toward their optimal value by using the Least-Mean-Square (LMS) algorithm, and the filter tap weights are used to estimate the delay offset between the incoming SS signal and a locally generated PN sequence. Theoretical and simulation results for both acquisition and tracking modes are presented in this thesis.; The proposed acquisition system in classified as a hybrid acquisition scheme since several code phases are concurrently tested for acquisition. It is shown that the proposed acquisition scheme has similar improvements in acquisition time to that offered by conventional hybrid schemes but at a reduced hardware complexity. The proposed scheme has also a better acquisition performance than a partial matched filter system in low SNR conditions. It is also shown that the proposed scheme is robust to fading. The proposed acquisition scheme suffers only a slight degradation (about 0.5 to 3 dB) in SNR, when operating in slow frequency selective and frequency non-selective Rayleigh fading. The adaptive nature of the system is exploited to acquire and track the strongest received path.; The tracking performance is modelled and analyzed using Markov chain theory, and good tracking is demonstrated. Both the mean hold-in time and the mean false alarm penalty time are derived. It is shown that the hold-in time is robust to small fluctuations in received SNR as opposed to conventional delay-lock loops which exhibit dramatic reductions in the hold-in time with small change in SNR. It is also shown that the residual tracking error performance of the proposed tracking scheme outperforms the maximum likelihood estimator by approximately 4 dB. Tracking of fast changing delays is also investigated. The BER performance of a DS/SS system utilizing this tracking system is simulated in a relatively fast changing environment, and the degradation from the ideal perfectly synchronized system is shown to be about 1 dB.