Analysis of a CDMA downlink with non-orthogonal spreading sequences for fading channels

Since the transmitted signals in the downlink of a CDMA system can be time synchronized, orthogonal spreading codes are often used to separate the users' signals. In systems supporting different data-rates, sets of orthogonal codes of different periods can be applied. An important example are the Orthogonal Variable Spreading Factor (OVSF) codes in the 3GPP-WCDMA (UTRA) standard. Since the number of orthogonal codes is limited and a capacity restriction in a CDMA downlink due to code shortage should be avoided, additional (non-orthogonal) spreading codes must be available. In recent years, several construction methods for these codes have been discussed in connection with the development of CDMA-based mobile communication systems. In this paper, the resulting performance when nonorthogonal spreading codes are used is discussed for an AWGN and a fading scenario.