A study of smart code in LAS-CDMA system and performance comparison with spreading codes for CDMA systems

Abstract

In DS(Direct Sequence)-CDMA systems, the choice of the spreading codes predetermines the properties of CDMA systems. Traditional CDMA systems have mainly used Walsh-Hadamard code, Maximal-length sequences, Gold sequences or Kasami sequence, etc. These codes exhibit good correlation properties in Synchronous-CDMA systems. But because of the difficulty in timing synchronization and the large cross-correlation values around the origin, there exists a lot of tasks to be settled. In recent years, the smart codes, which have the almost ideal correlation properties, have been studied by many researchers. The smart codes have the ZCZ(zero correlation zone) or IFW(Interference Free Window) outperforms traditional spreading sequences. Due to the ZCZ or IFW, the multi-path interference and multi user interference can be tremendously reduced. With this concept, LAS(Large Area Synchronous)-CDMA system was proposed to the Third Generation Partnership Project 2 (3GPP2) in 2000. LAS-CDMA system combine LA codes with LS codes. LA code are mainly used to reduce the adjacent cell interference. The pulses of an LA code are formed by passing through an LS code to its matched filter at the required pulse positions. LS codes are used for spreading. LS codes are based on Golay complementary pairs(GCP). Since 1969, when Golay first proposed the GCP, lots of studies have been taken for GCP. There are many GCPs, but the main problem is how many orthogonal sets do exist and how to find the orthogonal sets among them. Until now, several generating algorithms for orthogonal sets from GCP have been proposed, but they are still expressed as a word, “exhaustive”. In this thesis, we introduce new expansion method. With the proposed algorithm, one can easily generates GCP orthogonal sets.