Code sequence analysis in asynchronous coherent time-spreading optical code-division multiple access

An analysis of the code parameters that are important to asynchronous coherent time-spreading optical code-division multiple-access (OCDMA) systems employing Gold codes is presented. Multiple-access interference (MAI) and beat noise (BN) are analyzed according to the aperiodic cross-correlation function. The relationship between the mean intensity of aperiodic cross-correlation and MAI and BN is deduced. Considering the mean system performance, MAI and BN can be calculated directly from the aperiodic autocorrelation function, and the aperiodic cross-correlation function is not needed. Hence, a computational saving can be achieved. Considering the worst system performance, the upper bound of the bit-error-rate performance is evaluated by the maximum value of the mean intensity of aperiodic cross correlation. For a 511 length Gold sequence, a coherent time-spreading OCDMA system can support 12 interfering users for the mean performance. However, an OCDMA system can support fewer than five interfering users in the worst performance.