Analysis of High-Speed Optical Wavelength/Time CDMA Networks Using Pulse-Position Modulation and Forward Error Correction Techniques

This paper presents a comprehensive analysis of an optical code-division multiple access (OCDMA) network based on two distinct modulation formats, namely ON-OFF keying (OOK) and pulse-position modulation (PPM). We also investigate how each of these modulation formats performs under two distinct 2-D coding schemes, i.e., single-pulse per row (SPR) and multiple-pulse per row (MPR). For both cases, we have accounted for the simultaneous effect of many different dispersion and noise mechanisms (including multiple access interference (MAI) that impair the overall system performance. We have included the laser relative intensity noise at the transmitter side, the fiber dispersive effects (group velocity dispersion (GVD), and first-order polarization-mode dispersion (PMD), and beat, avalanche photodiode (APD), and thermal noises at the receiver side. The effect of GVD and PMD, as well as the influence of noises, on the performance of SPR and MPR codes is also investigated. Another effect studied in this paper is the influence of the APD photodetector on the beat noise of an incoherent OCDMA network. To mitigate systems noises and bit error rate (BER), we have adopted a forward error correction (FEC) RS(255, 239) algorithm in both networks investigated here. New expressions for the BER with all noises and dispersion mechanisms were also derived for the SPR and MPR code schemes. Results indicated that OOK and PPM modulation schemes without additional mechanisms to mitigate MAI and other noise effects are not sufficient to accommodate 32 simultaneous users in an error-free environment (BER < 10-12). This occurs due to the already high BER at the FEC input, which severely affects FECs at the receiver side.