Performance Analysis Of Quasi-Synchronous MC-DS-CDMA System Employing GO Sequence

In the uplink of the cellular communication systems, for the reason that all users are not synchronized, and the fading among different users are independent from each other, thus bring severe multiple access interference. MC-DS-CDMA is a multiple access scheme based on DSSS and OFDM technologies. It can reduce transmission speed in sub-channels and receive signals at low power spectrum in order to combat ISI and frequency-selective fading. Multiple access interference (MAI) is the main factor to decide the performance of MC-DS-CDMA system. It can be reduced effectively by using orthogonal spreading sequences when the system is synchronized. Based on common orthogonal sequence, the generalized orthogonal spreading sequences are applied in this thesis. The spreading sequences remain orthogonal when the relative shifts among spreading sequences lie within the zero correlation zone, which highly improves system bit error rate performance.The objective of this thesis is to conduct the research on the quasi-synchronous (QS) MC-DS-CDMA system, where signals from different users are assumed to arrive at the receiver quasi-synchronously and the relative timing errors among them are far less than the symbol duration. Here, Generalized Orthogonal (GO) sequences are applied to the QS MC-DS-CDMA system and the generalized orthogonal zone property of GO sequence are utilized to suppress the multiple access interference. Both the simulation and theoretical results reveal that the system employing GO sequences outperforms that employing traditional orthogonal or random sequence. Besides, considering the fact that in traditional MC-DS-CDMA system, no frequency diversity can be obtained for the reason that different data bits are transmitted on different subcarriers, a novel frequency diversity scheme is proposed in this thesis, where the same data bit stream is transmitted on several parallel subcarriers and the combining is performed at the receiver. The core idea of this scheme is to obtain the improvement of error performance at the cost of lower spectrum efficiency.In this thesis, the QS MC-DS-CDMA system employing GO sequence is evaluated by both computer simulation and theoretical analysis. First of all, the history and the research background about multi-carrier communication technologies are presented. Meanwhile, the principles of DS-CDMA are introduced, with the emphasis on the structure of MC-DS-CDMA system and comparison with MC-CDMA system. The construction methods and the correlation properties of GO sequence are presented. Thereafter, the multi-user QS uplink MC-DS-CDMA system model in the context of AWGN channel is discussed in this thesis. System BER expressions are derived from Gaussian approximation together with analysis of system multiple access performance. In addition, the multi-user asynchronous uplink MC-DS-CDMA system model in the context of multi-path fading channel is discussed. The BER expressions based on MRC combining method is derived. Both numerical computation and computer simulation results are provided. The conclusions obtained reveal that GO sequences provides better error performance than orthogonal sequences and random sequences, and the frequency diversity scheme proposed in this thesis can provide diversity gain.