Abstract:
Multi carrier direct sequence code division multiple access (MC DS CDMA) technique, which is a combination of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA) has been considered as an important technique for the future generation wireless communication systems due to its bandwidth efficiency, frequency diversity and immunity to channel dispersion. OFDM has already been employed in many areas, such as digital audio and video broadcasting, wireless local/metropolitan area networks and asynchronous digital subscriber lines (ADSL). Leveraging the multiple access capability of CDMA, the MC DS CDMA technique is an important enhancement to OFDM.
Nevertheless, a major drawback of the MC DS CDMA system is the high sensitivity to timing errors between transmitter and receiver due to the use of a large number of carriers and the superposition of signals and multiple users. In this thesis, the expression of the signal power, multiple access interference (MAI) power in presence of timing jitter over a Rayleigh fading channel for MC DS CDMA wireless system is formulated which is used to determine the signal to interference noise ratio (SINR), average bit error rate (BER) and power penalty at the output with and without Rake Receiver. The average bit error rate (BER) is determined in terms of different system parameters and optimum system design parameters are evaluated for a given timing jitter variance. The analytical approach is further carried out for a MC OFDM DS CDMA system considering a Rake Receiver. The analysis is developed to find the probability density function (pdf) at the output of maximal ratio combining (MRC) receiver combiner considering combined influence of fading and timing jitter. The performance results are evaluated numerically in terms of BER and required power penalty considering system parameters like number of users, number of sub-carriers. The result shows significant deterioration in performance parameters due to fading. For a given power penalty at a fixed BER, how the allowable jitter variance is influenced by the number of users has also been represented. Moreover, numerical results show that the inclusion of Rake Receiver combats the limitations imposed by timing jitter by reducing the BER and power penalty.
