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Synchronous Fiber-Optic PPM-CDMA Systems with Interference Reduction

In this paper, we propose a multi-user interference estimation and cancellation technique for direct-detection fiber-optic code division multiple-access (FO-CDMA) communication systems employing pulse position modulation (PPM). Interference estimation is based on the unique grouping property of modified prime code sequences. By having a code unused in each group of the code sequence, the estimated interference can be canceled out from the received signal after the photo-detection process. We have used APD photo-detector in our proposed system. An upper bound on the bit-error rate (BER) for the proposed system is derived and compared with a lower bound on the BER for the system without cancellation. In the presence of multiple users interference (MUI), shot noise, thermal noise, dark current noise and surface leakage current, our results clearly indicate that the performance, in terms of the bit error probability, of the proposed system is significantly improved compared with that of the system without cancellation. In our proposed system, we have also used Manchester codes in signaling the transmitted data to further improve the bit error rate (BER).