4-phase DS-CDMA performance over LEOS/MEOS channels

In this paper, we investigate the downlink performance of DS-CDMA using QPSK modulation and 4-phase spreading sequence in a fading satellite channel. Two different types of spreading sequences will be evaluated. One type of sequence construction will be to use two binary sequences; one for the in-phase channel and the other for the quadrature channel. The binary sequences will be binary Gold sequences. The other type of sequence will utilize sequences from GF(4) which have near optimum correlation properties. The 4-phase sequences have been shown to increase the number of users of a multiple access system by 50% in a Gaussian environment. We investigate the performance of the above sequences in a non-geostationary satellite channel. This channel model for non-geostationary satellites is characterized by Rician fading and lognormal shadowing where the fading/shadowing characteristics depend on the elevation angle of the satellite. The upper bound on the symbol error rate (SER) is derived and is computed for both a LEO and MEO system.