Optimal performance of an oversaturated OCDMA/OCDMA system based on scrambled orthogonal bases

In this paper we consider a multiple access system with oversaturation of the channel by means of a combined OCDMA/OCDMA scheme with scrambled orthogonal bases. We investigate the performance and the associated maximum channel capacity (channel load) for uncoded BPSK modulation with binary valued chip sequences and optimum multiuser detection for spreading factor N = 4, 8, 16 and 32. It is found that, for scrambling sequences that change randomly from symbol-interval to symbol-interval, the allowable channel overload is zero for N = 4, 8, 16, and is only about 3% for N = 32. Major improvements on the allowed channel capacity are achieved by selecting a ‘good’ scrambling sequence and keeping it the same for all symbol-intervals. In this way the allowed channel overload can be increased to 25 % for N = 8, to 31.25 % for N = 16 and to at least 31.25 % for N = 32.