Performance of ODS-CDMA QPSK in a soft-limiting AM/AM nonlinearity channel

The performance of orthogonal direct sequence code division multiple access (ODS-CDMA) signaling in a soft-limiting, memoryless AM/AM nonlinearity channel is analyzed. Depending upon the backoff of the composite ODS-CDMA signal from the input saturation level, the chip-to-chip fluctuating amplitude of the composite signal may be clipped, distorting the composite signal, reducing the correlation of the received codeword with its local replica, and producing both self- and multiple access interferences at the output of a conventional, inner product demodulator. Using the central limit theorem, an expression is derived for the signal-to-distortion ratio at the demodulator output as a function of the channel loading, backoff from saturation input, and system parameters. This expression is then used to determine: (1) the backoff required to produce an allocated degradation at both full and partial channel loading, and (2) the effect on the uncoded bit error probability. The results show that approximately 6 dB backoff is required to produce less than 0.5 dB degradation.