Abstract
For various quasisynchronous (QS) CDMA systems such as LAS-CDMA system which emerged recently, in order to reduce or eliminate the multiple access interference and multipath interference, it is required to design a set of spreading sequences which are mutually orthogonal within a designed shift zone, called orthogonal zone. For traditional orthogonal sequences, such as Walsh sequences and orthogonal Gold sequences, the orthogonality can only be achieved at the inphase point; in other words, the orthogonality is destroyed whenever there is a relative shift between the sequences, that is, their orthogonal zone is 0. In this paper, new concepts of generalized orthogonality (GO) and generalized quasiorthogonality (GQO) for spreading sequence design in both direct sequence (DS) QS-CDMA systems and time/frequency hopping (TH/FH) QS-CDMA systems are presented. Besides, selected GO/GQO sequence designs and general theoretical periodic and aperiodic limits, together with several applications in QS-CDMA systems, are also reviewed and analyzed.
Highlights
In a typical direct sequence (DS) code division multiple access (CDMA) system, all users use the same bandwidth, but each transmitter is assigned a distinct spreading sequence [1]
In order to eliminate the multiple access interference, it is required to design a set of spreading sequences with impulsive autocorrelation functions (ACFs) and zero crosscorrelation functions (CCFs)
The new concepts, generalized orthogonality (GO) and generalized quasiorthogonality (GQO) [4], are introduced, which can be employed in quasisynchronous CDMA (QS-CDMA) to eliminate the multiple access interference and multipath interference
Summary
In a typical direct sequence (DS) code division multiple access (CDMA) system, all users use the same bandwidth, but each transmitter is assigned a distinct spreading sequence [1]. In A-CDMA system, the spreading sequences are normally designed to have low autocorrelation sidelobes and low crosscorrelations, such as Gold sequences, Kasami sequences, and so forth [2, 3, 18] To overcome these difficulties, the new concepts, generalized orthogonality (GO) and generalized quasiorthogonality (GQO) [4], are introduced, which can be employed in quasisynchronous CDMA (QS-CDMA) to eliminate the multiple access interference and multipath interference. As for TH/FH sequence design, five parameters are normally involved, the size q of the time/frequency slot set F, the sequence length N, the family size M, the maximum Hamming autocorrelation sidelobe Ha, and the maximum Hamming crosscorrelation Hc, where Hm = max{Ha, Hc}. Due to the limited space, only basic ideas and selected constructions are given without proofs
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