In this dissertation, we analyze the performance of code tracking loops in code division multiple access (CDMA) environments, and bit error rate (BER) performance of the CDMA system with PN code tracking error. We consider both the analog and digital delay-locked loops (DLL's) and propose the all-digital non-coherent DLL (NC-DLL) which is suitable for tracking the orthogonally-coded CDMA signal. We investigate the effect of the multiple access interference on the commonly used code tracking scheme, NC-DLL in asynchronous CDMA systems. Also according to this analysis, the BER of the binary phase shift keying (BPSK) data modulation in CDMA systems is obtained. The analysis enables the calculation of BER when imperfect PN code synchronization is assumed. In order to compute BER of the overall CDMA system, it is first necessary to compute the probability density function (pdf) of the tracking error process. By combining these results, an average BER result can be obtained.
First, the performance of the analog NC-DLL in CDMA environments is analyzed. Most of previous analyses of DLL have only assumed the point to point communication systems without multiple access interference. But in a multiple access communication system, we must analyze the effect of the multiple access interference in order to evaluate the performance of the communication system more accurately. In CDMA environments, the multiple access interference is generally approximated by Gaussian. In this dissertation, we exactly obtain the effect of multiple access interference. The theoretical analysis is done by using the renewal Markov theory. We obtain performance curves in terms of steady-state jitter and mean time to lose lock as a function of signal-to-noise ratio and number of users. And the effect of code tracking error on BER in CDMA systems are considered in this dissertation.
Second, we focus onto the performance of all digital code tracking loop for band-limited CDMA signals using a periodic extension approach. The continued progress in increased performance, speed, reliability, and the reduction in size and cost of very large scale integrated circuits (VLSI) has resulted in the spread spectrum system implementation in the digital domain. The key design issues of the digital implementation of the spread spectrum receiver are the PN code acquisition as well as the PN code tracking. Hence, we briefly review some basic concepts about the band-limited CDMA system and the scheme of the non-coherent all-digital code tracking loop. And we drive the root mean square timing jitter and mean time to lose lock in CDMA environments. In addition, we calculate the pdf of the tracking error in CDMA environments. Using this pdf, we obtain the performance of the band-limited CDMA system with the code tracking error.
Finally, we propose an all-digital code tracking scheme for a band-limited Direct Sequence Code Division Multiple Access (BL DS-CDMA) cellular system. The system employs the band-limited Nyquist chip pulse shaping and orthogonal coding. The proposed code tracking loop has a decision-directed noncoherent structure to handle the orthogonally coded DS-CDMA signals. The performance of the proposed code tacking loop is derived analytically and simulation of the proposed structure based on the linearized analysis.
