On the deorthogonalization of ODS-CDMA 4-PSK in frequency-nonselective shadowed Rician fading

Using a conventional demodulator, a sufficient condition to maintain orthogonal direct sequence-code-division multiple access (ODS-CDMA) codeword orthogonality is constant relative channel amplitude over the codeword duration. Fading produces a fluctuating amplitude that deorthogonalizes the codewords, resulting in an additional noise similar to the multiaccess interference (MAI) found in asynchronous DS-CDMA. For ODS-CDMA 4-PSK in a frequency-nonselective, shadowed Rician fading channel, we derive and evaluate: (i) an exact expression for the uncoded bit error probability, and (ii) an upper bound for the convolutionally-coded bit error probability. We find that the degradation to BER depends on both the fading parameters and the ODS-CDMA channel loading.