| In modern information age, we have increasingly high demand for information transmission. Code Division Multiple Access(CDMA) has become the mainstream technology of 3G mobile communication. The system capacity needs to be increased with number of mobile communication customers and the diverse demand for mobile communication services increasing. The main reason for restricting the system capacity to improve is the multiple access interference of system, which is mainly caused by the non-orthogonality of code sequences. So sequence design is the key technology to the development of wireless communications. This paper focuses on the spread sequence design for quasi-synchronous CDMA communication system. The main contributions of this work are as follows:1. Five constructions of the biphase sequence which have the good autocorrelation property are proposed. And if 2N×2 interleaving sequences are used in the constructions, the values of period autocorrelation function of the new sequences achieve optimal except one shift point. The first construction is based on an arbitrary ideal 2-level autocorrelation sequence. The other four constructions are based on a GMW sequence of period N=22k-1 and its modified version, a twin prime sequence of length p(p+2) and its modified version, and two types of Legendre sequences mentioned in Tang and Gong.2. A new general construction of zero correlation zone (ZCZ) sequence set, which should pre-select two given ZCZ sequence sets, is introduced. Compared with the two given sequence sets, the resultant sequence set not only has larger family size and longer period, but also provides more flexible choices of pre-selected sequences, ZCZ length and family size. 3. Two new constructions of ZCZ sequence sets, which are based on interleaving technique and affine transformations, are proposed. The resultant sequence sets not only have larger family size and longer period, but also are more flexible when choosing the length of ZCZ, compared with the known constructions. The second construction is optimal or almost optimal with respect to Tang-Fan-Matsufuji bound.4. Four general constructions of low correlation zone (LCZ)/ZCZ sequence sets based on interleaving technique and affine transformations are proposed. A larger family of LCZ/ZCZ sequence sets with longer period is generated by these constructions, which are more flexible among the selection of the alphabet size, the period of the sequences and the length of LCZ/ZCZ, compared with those generated by the known constructions. A comparison is carried on about the advantages and disadvantages of the four constructions. Especially, two families of the newly constructed sequences may achieve or almost achieve the theoretic bound.5. A new symbol cjl called a carrier to study the structures and properties of single cycle T-functions, which has been lately proposed by Klimov and Shamir. And only 2j successive values of the word x are needed to recover all the data including the least significant bit on jth position. Under some conditions, less data are needed to recover the word x on jth position. So secure parameters should be adopted to avoid hidden hazards when a cryptography system based on single cycle T-functions is constructed. |
