Adaptive PN code synchronisation in DS-CDMA systems.

Spread Spectrum (SS) communication, initially designed for military applications, isnow the basis for many of today's advanced communications systems such as CodeDivision Multiple Access (CDMA), Global Positioning System (GPS), Wireless LocalLoop (WLL) , etc. For effective communication to take place in systems using SSmodulation, the Pseudo-random Noise (PN) code used at the receiver to despread thereceived signal must be identical and be synchronised with the PN code that was used tospread the signal at the transmitter. Synchronisation is done in two steps: coarsesynchronisation or acquisition, and fine synchronisation or tracking. Acquisitioninvolves obtaining a coarse estimate of the phase shift between the transmitted PN codeand that at the receiver so that the received PN code will be aligned or synchronisedwith the locally generated PN code. After acquisition, tracldng is now done whichinvolves maintaining the alignment of the two PN codes.This thesis presents results of the research calTied out on a proposed adaptive PN codeacquisition circuit designed to improve the synchronisation process in Direct SequenceCDMA (DS-CDMA) systems. The acquisition circuit is implemented using a MatchedFilter (MF) for the correlation operation and the threshold setting device is an adaptiveprocessor known as the Cell Averaging Constant False Alarm Rate (CA-CFAR)processor. It is a double dwell acquisition circuit where the second dwell isimplemented by Post Detection Integration (PDI). Depending on the application, PDIcan be used to mitigate the effect of frequency offset in non-coherent detectors and/or inthe implementation of multiple dwell acquisition systems. Equations relating theperformance measures - the probability of false alarm (Pra ), the probability of detection (P d) and the mean acquisition time (E {Tacq}) - of the circuit are deri ved. Monte Carlosimulation was used for the independent validation of the theoretical results obtained,and the strong agreement between these results shows the accuracy of the derivedequations for the proposed circuit. Due to the combination of PDI and CA-CFARprocessor in the implementation of the circuit, results obtained show that it can providea good measure of robustness to frequency offset and noise power variations in mobileenvironment, consequently leading to improved acquisition time performance. Thecomplete synchronisation circuit is realised by using this circuit in conjunction with aconventional code tracking circuit. Therefore, a study of a Non-coherent Delay-LockedLoop (NDLL) code tracking circuit is also calTied out