Study On QS-CDMA Systems Based On Zero Correlation Zone Sequence

Even in quasi synchronize code division multiple access (QS-CDMA) communication systems, the relative delay among received signals should be under the maximum. We have researched the CDMA systems from the sixties of the twenty century, but our comparative research are the synchronized CDMA (S-CDMA) and the asynchronized CDMA (A-CDMA). The QS-CDMA system is presented resently and has attracted a great deal of attention because it not only possesses the advantages of the S-CDMA system but also reduces the complexity of the A-CDMA system. In QS-CDMA systems, the spreading sequences should have perfect correlation property around the origin. ZCZ sequences are used as spreading sequences in order to realize the advantage. In this thesis, the ZCZ sequences are applied to QS-CDMA systems, and the corresponding system performance are analysed. Topics on the correlation property analysis of 2-phase, 4-phase and 6-phase ZCZ sequences are performed, and the applications of ZCZ sequences in QS-CDMA systems are elaborated.First of all, the correlation definitions of ZCZ sequences are provided, including the periodic correlation property, aperiodic correlation property and the odd periodic correlation property. Meanwhile, the concept of ZCZ sequences and the theoretical bounds are investigated based on the correlation definitions of ZCZ sequences. Then based on the analysis of the constructions of 2-phase ZCZ sequences, the corresponding correlation properties are derived and the 4-phase ZCZ sequences are also analysed. And the method of designing 4-phase ZCZ sequences is extended, furthermore, the 4-phase and 6-phase ZCZ sequences are constructed, the corresponding correlation properties are also derived. It is shown that the 4-phase and the 6-phase ZCZ sequences can approach to the tighter theoretical bounds. Afterwards, the concept of pseudoperiodic sequences is associated with the ZCZ sequence, both the length and the family size of ZCZ sequences are extended. These ideas, in fact, open a new direction in the extention of fixed length of the ZCZ sequence.Next, the applications of ZCZ sequences to the QS-CDMA systems in AWGN channel are considered in this thesis. The 2-phase and 4-phase ZCZ sequences are applied to the BPSK and QPSK QS-CDMA systems respectively, the SNR expressions and the bit-error-rate(BER) results are derived. Both the theoretical analysis and the computer simulation show that the BER and the correlation property are closely-related. The results of simulation also show that the performance of 4-phase ZCZ sequences is better than that of 2-phase ZCZ sequences. Meanwhile, the ZCZ sequences are applied to QPSK so as to augment the system capacity effectively.In the end, the QS-CDMA system performance with ZCZ sequences as spreading sequences is analysed, the computer simulation shows that the ZCZ's ability to restrain multiple access interference is close to the S-CDMA system, and is much better than that of A-CDMA system.