Improvement of BER Performance of DS-OCDMA in Dispersive Fiber Medium

Abstract

Among the dispersion mechanisms fiber group velocity dispersion (GVD) and polarization mode dispersion (PMD) are the key limiting factors for high-speed, long length and highperformance fiber optic communications, especially, for optical code division multiple access (OCDMA). In this study, an analytical model is proposed to evaluate the bit error rate (BER) performance of direct sequence (DS) OCDMA with cascaded in-line optical amplifiers in presence of GVD, PMD, and their compensations using fiber Bragg grating (FBG) and frequency advanced higher-order PMD vectors. Optical orthogonal codes (OOC) are used as address sequence, and Intensity modulation direct detection technique is employed in a single mode fiber optic system operating at 1550nm. Optoelectronic conversion is performed by an avalanche photodiode (APD) in an optical correlator receiver. The system BER performance is determined on account of receiver, optical amplifier, and multiple access interference (MAI) noises. The power penalty suffered by the system is determined at BER of 10 as a function of system parameters. The BER performance of the proposed OCDMA system is investigated in two steps. The impact of GVD and its compensation is studied in the first step taking into account of super Gaussian and Hyperbolic-Secant-shaped OOC's as address sequence. In the second step the influence of PMD and its compensation is analyzed under GVD-induced penalty compensated condition. The results obtained in the first step indicate that the performance of proposed system severely degrades due to interchip interference caused by GVD-induced spreading and overlapping of short duration chips which limits the fiber length, chip rate, and number of simultaneous user. The numerical results also indicate that the system BER performance is highly dependent on the chipshape, and suffers minimum penalty when Hyperbolic-Secantshaped chip is used instead of super-Gaussian-shaped chip. The system performance is further determined in presence of FBG-based compensator. It is found that about 90% of the GVD induced penalty can be compensated depending on the compensator parameters. In the second step, the BER performance of proposed DS-OCDMA system is evaluated in presence of PMD and PMD compensation using Gaussian shaped OOC's. The system BER performance is found to degrade more at higher chip rate, and longer fiber length due to the effect of PMD. However, employing PMD compensation significant improvement of the system performance is found by third-order PMD compensation with respect to first-, and second-order compensations.