Design Of Spreading Spectrum Sequences And Their Applications In Wireless Communications

In Code Division Multiple Access (CDMA) systems, the correlation properties of the spreading sequences play an important role and directly influence the system interference-resist ability, and the size of code set influences the system capacity. In the 3^rd generation mobile communication systems employing single codes as spreading codes, the system capacity is reduced by multipath interference (MI) and multiple access interference (MAI) due to the non-ideal correlation properties of the codes. Generalized Orthogonal (GO) sequences possesse ideal correlation properties in their zero correlation zone (ZCZ). Therefore, the systems employing GO sequences as spreading codes can eliminate MI and MAI effectually.The main contents of this work are listed as follows:The correlation functions definitions of spreading sequences are presented, including the (even) periodic correlation function, odd periodic correlation function, aperiodic correlation function and spectral correlation function. The correlation properties of several existion spreading sequences are discussed and their applications in spreading spectrum systems are analyzed. The parameters of the code set are investigated, and their limitations are point out. Then the relationship between the code set size and anti-interference ability of the system has been discussed, and their mutual restriction relation are obtained.Based on the knowledge of m-sequences, a novel binary generalized orthogonal code, called m-ZCZ code, is constructed, and its correlation properties is presented. Besides having the same characteristics as conventional GO sequences, the value of ZCZ length breaks through the restriction, and can be flexibly chosen to match the practical channel in a high degree. So that the number of codes is increased and the system capacity is improved. Then, the applications of m-ZCZ code in CDMA systems are discussed, and the theory analysis of system BER performance is presented.The MC-CDMA systems' performance impacted by MAI is analyzed, and the limitations of spreading codes employed in MC-CDMA systems in some literatures are pointed out. The criteria of spreading codes design to achieve an MAI-free system are analyzed. According to the criteria, a multiphase MAI-free spreading code, called GW code, is proposed, and its ideal spectral correlation properties are proved. Then, the performances of the system employing GW codes and the systems employing conventional spreading codes are compared, and the simulation results of BER are given. It is shown that the system performance is improved by using GW codes.In the end, several existing problems in the research of sequence design and their applilcations are pointed out and they outline the direction of future work.