Estimation and cancellation of multi-user interference in synchronous fiber-optic PPM-CDMA system using Manchester coding

A multi-user interference estimation and cancellation technique is proposed for direct-detection fiber-optic code division multiple-access communication systems employing pulse-position modulation. In addition, Manchester codes are used in signaling the transmitted data to further improve the bit-error rate (BER). The multi-user interference of any user is estimated with the help of properties of modified prime code sequences. The estimated interference is canceled out from the received signal after the photo-detection process. We have used PIN photo-detector in our proposed system. An upper bound on the 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) and the Poisson shot noise model, our results clearly indicate that the performance, in terms of the BER, of the proposed system is significantly improved compared with that of the system without cancellation. The effect of thermal, dark current and surface leakage noise is insignificant compared to MUI and thus will not be considered in our calculation of BER.