A chip-interleaving pattern retaining orthogonality in DS-CDMA systems: application to the multicode downlink

Interleaving the chip-level sequence resulting from spreading before transmission in direct spread code division multiple access (DS-CDMA) systems have been reported to allow significant improvements over conventional (nonchip-interleaved) DS-CDMA. This transmission scheme has been shown to be of particular interest when used in conjunction with iterative receivers, allowing performance close to the multiple access (MAI) and intersymbol (ISI) interference-free case to be reached. This paper addresses the application of chip-interleaving to existing or near-future DS-CDMA systems. An interleaving pattern is introduced that allows the orthogonality property to be retained between interleaved and noninterleaved signals. In addition, a low-complexity receiver based on iterative channel equalization is designed for the multicode downlink of chip-interleaved DS-CDMA systems. Numerical simulations show that used in conjunction with the proposed interleaving scheme, the proposed receiver allows the interference-free performance of the noninterleaved case to be attained.