Adaptive PN code synchronisation in DS-CDMA systems.
Job Obiebi · Edinburgh Napier Research Repository (Edinburgh Napier University) · 2026
Spread spectrum (SS) communication, initially designed for military applications, is now the basis for many of today’s advanced communication systems such as Code Division Multiple Access (CDMA), Global Positioning System (GPS), Wireless Local Loop (WLL), etc. For effective communication to take place in systems using SS modulation, the pseudo-random noise (PN) code used at the receiver to despread the received signal must be identical to and synchronised with the PN code used to spread the signal at the transmitter. Synchronisation is performed in two steps: coarse synchronisation (acquisition) and fine synchronisation (tracking). Acquisition involves obtaining a coarse estimate of the phase shift between the transmitted PN code and that at the receiver so that the received PN code can be aligned with the locally generated PN code. After acquisition, tracking is performed to maintain this alignment. This thesis presents results of research carried out on a proposed adaptive PN code acquisition circuit designed to improve the synchronisation process in Direct Sequence CDMA (DS-CDMA) systems. The acquisition circuit is implemented using a matched filter (MF) for correlation, while the threshold-setting device is an adaptive processor known as the Cell Averaging Constant False Alarm Rate (CA-CFAR) processor. It is a double dwell acquisition circuit in which the second dwell is implemented by post-detection integration (PDI). Depending on the application, PDI can be used to mitigate the effect of frequency offset in non-coherent detectors and/or in the implementation of multiple dwell acquisition systems. Equations relating key performance measures—the probability of false alarm (Pfa), the probability of detection (Pd), and the mean acquisition time (E{Tacq})—of the circuit are derived. Monte Carlo simulation was used for independent validation of the theoretical results, and strong agreement between the results demonstrates the accuracy of the derived equations for the proposed circuit. Owing to the combination of PDI and the CA-CFAR processor, the results show that the circuit provides robustness to frequency offset and noise power variations in mobile environments, thereby improving acquisition time performance. The complete synchronisation circuit is realised by combining this acquisition circuit with a conventional code tracking circuit. Consequently, a study of a non-coherent delay-locked loop (NDLL) code tracking circuit is also carried out.