On DS-CDMA Systems Employing Generalized Orthogonal Sequences And Their Performance Evaluation

In Code-Division Multiple-Access (CDMA) systems, the auto- and cross-correlation properties of the spreading sequences employed play an important role on the level of the self-interference (SI), multiple access interference (MAI) and adjacent cell interference (ACI), and therefore directly influence the performance and capacity of the systems. In this thesis, the zero correlation zone (ZCZ) or generalized orthogonal (GO) sequences are applied to direct-sequence CDMA (DS-CDMA) systems, and the corresponding system performance are analysed. Topics on the correlation property analysis of Hadamard Walsh (WH) sequences, GO sequences, and Loosely Synchronous (LS) sequences are performed, and the system model and performance evaluation of single-cell and multi-cell synchronous CDMA(S-CDMA) systems employing GO sequences, and the applications of GO sequences in quasi-synchronous CDMA (QS-CDMA) systems are elaborated.First of all, the correlation definitions of spreading sequences, which are used widely in the system performance evaluation, are provided, including the cross-conrrelation property at a specific shift, the periodic (cross-) correlation mean square property and the aperiodic (cross-) mean square correlation property. Then based on the analysis of the constructions of WH, GO and LS sequences, the corresponding correlation properties are derived and methods of selecting subsets with good periodic mean-square cross-correlation property or aperiodic mean-square correlation property from the WH set and GO sequence set whose GO zone Z=l are investigated. Furthermore, the methods of truncating the GO sequences to obtain the quasi-orthogonal (QO) set whose family size M is larger than the length L and the quasi-generalized-orthogonal (QGO) set whose size M is larger than LI2Z are discussed, together with the numerical results of the correlation properties.Thereafter, the applications of GO sequences in single-cell S-CDMA systems are discussed in this thesis. In additive white Gaussion noise (AWGN) channel, interference components in S-CDMA systems accommodating more users than the spreading factor is analyzed based on the theoretical bound of the periodic correlation mean square property. Then two interference-suppressed S-CDMAsystem models of augmented capacity are proposed, along with the analysis and simulation results. Moreover, the performance of the single-cell S-CDMA system employing GO sequences under the multi-path propagation channel is analyzed, resulting analytical SIR expressions, the numerical and simulation error results, for Rayleigh and Rice fading channels respectively.Next, a two-level spreading-despreading scheme is proposed, and the GO sequences are applied to the forward link of a multi-cell CDMA system in this thesis. Two multi-cell S-CDMA system models based on LS sequences and two-level spreading scheme respectively are presented here, as well as the numerical and simulation results which reveal that, due to the restricted intra-cell interference, the system performance is improved distinctly.In the end, the applications of GO sequences to the QS-CDMA systems in AWGN channel are considered in this thesis. The GO sequences are applied to the high-order modulation QS-CDMA systems, and for the MPSK and M-QAM modulation scheme, the maximum allowable timing delays, the SIR expressions and the bit-error-rate (BER) results are derived. Meanwhile, truncated GO sequences are applied to QS-CDMA system so as to augment the system capacity effectively. Then three QS-CDMA system models based on two-level spreading scheme are proposed and both the theoretical analysis and the computer simulation show that the improved error performance and augmented capacity can be achieved in the QS-CDMA systems proposed.