This paper studies the optimal bit error rate (BER) performance of chip-level detection for optical code division multiple-access (OCDMA) systems in the presence of shot noise. To attain the optimal BER of chip-level detection, a chip-level detector that uses a limiter on the photon counts at each weighted chip is first proposed. Then, the monotonicity of the likelihood ratio used in the maximum a posteriori (MAP) probability criterion is established. With this monotonicity, a sufficient condition characterized by the cap levels of the limiters and the average photon counts per pulse is obtained, under which the detector is simply an AND detector to attain the optimal BER of chip-level detection. The detector' performance and its comparison to those of other detectors are also studied by numerical studies.

In this paper, the monotonicity of the likelihood ratio used in the optimal correlation detection is established for optical code division multiple-access (OCDMA). The monotonicity property not only guarantees the optimality of the detection, it also enables us to calculate the bit-error-rate bounds for OCDMA in the presence of uncertain a priori probabilities.