Spreading Sequence Design and Theoretical Limits for Quasisynchronous CDMA Systems

Abstract

<p>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.</p

New quasi‐orthogonal BCH‐derived sequences for CDMA applications

“…The numerical results we obtained confirmed that the new family of even BCH-derived sequences contains a high number of (quasi-orthogonal) sequences with low correlation levels, better than those presented by the even sequences of the recent UMTS system [16,17] and higher PGs, even when compared with the LS sequences [19,25] of the LAS-CDMA system. These characteristics can make the even BCH-derived sequences very attractive in fading dispersive channels, either in synchronous or quasi-synchronous channels.…”

“…This can be a disadvantage when compared, for instance, with the optimised even BCH-derived sequences which do not need of a new construction of sequences for variations of Z. Figures 2 and 3 show the results we obtained for the PG as a function of the period, for odd sequences (LS [19,25], Gold, large set of Kasami, BCH, Gold-like and msequences [24]) and even sequences (even BCH-derived, Walsh/Hadamard [6,31] and m-sequences with Manchester pulses), respectively.…”

“…We also analysed the cross-correlation results (they are not indicated in the paper), and we concluded that the even BCH-derived sequences can have lower crosscorrelation values than the traditional odd PN sequences referred in Section 1. Table 2 compares the quasi-orthogonal (binary) families of (optimised) even BCH-derived (with GRC) and GMWderived sequences [19,27] and the orthogonal (ternary) families of LA and LS y sequences [19,25] relative to the parameters: (a) sequence length N, (b) family size M and (c) (out-of-phase auto and cross) correlation peak value j"j inside the zone of length Z. The sequences were compared taking in account a fixed correlation zone Z and an identical sequence length N (whenever possible).…”

“…We find applications of these results in (quasi-synchronous) large area synchronised CDMA (LAS-CDMA) [19] and multicarrier CDMA (MC-CDMA) [17,19] systems, which are both strong candidates for the fourth generation of mobile communication systems.…”

“…Recently, quasi-synchronous CDMA (QS-CDMA) systems using sets of orthogonal [18,19] and quasi-orthogonal [19] spreading sequences have been actively investigated, due to their superior immunity against synchronisation errors [18,19], higher bandwidth efficiency [20], and consequently, better performance, when compared with those obtained with the traditional spreading sequences [18,19,21]-the quasi-orthogonal spreading sequences are characterised by a correlation zone almost zero around the zero-phase shift. We find applications of these results in (quasi-synchronous) large area synchronised CDMA (LAS-CDMA) [19] and multicarrier CDMA (MC-CDMA) [17,19] systems, which are both strong candidates for the fourth generation of mobile communication systems.…”