Overloading Performance of OCDMA/OCDMA Scheme for Cellular DS-CDMA System on Rayleigh Fading Channel

Overloading is a technique to accommodate more number of users than the spreading factor N. This is a bandwidth efficient scheme to increase the number users in a fixed bandwidth. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O). The first set is assigned to the N users and the second set is assigned to the additional M users. An iterative multistage detection (IMSD) technique is used to cancel interference between the two sets of users. The interference cancellation receiver uses hard decisions (HDIC) or soft decisions (SDIC) to estimate the interference. In this paper, the BER performance of s-O/O overloading scheme using Walsh-Hadamard (WH) orthogonal codes is evaluated with SDIC receiver on a Rayleigh fading channel. It is observed that this scheme provides the channel overloading of 33% at a BER of 0.0005, with an SNR degradation of about 1 dB as compared to single user performance on a Rayleigh fading channel with real scrambling. With complex scrambling, overloading performance increases significantly to 75%. This is a significant amount of overloading on a Rayleigh fading channel.