Performance analysis of an optical CDMA transmission system impaired by four wave mixing in a single mode fiber

Abstract

Optical CDMA is one of the most important research topics to the experts on optical communication for its several attractive features such as asynchronous access, privacy and security. Four wave mixing (FWM) is one of the most dominating nonlinear effects in optical communications. But all of the research works on optical COMA system, performance analyses are carried out without considering the effect of FWM. In this thesis, an asynchronous direct-sequence optical CDMA (DSOCDMA) system, with intensity modulation and direct detection (IM/OD) transmission link using sequence inversion keyed (SIK) switched correlator receiver, is considered. Firstly, the system is theoretically analyzed in present of FWM in single mode dispersion shifted fiber (DSF) and mathematical expressions of FWM noise, multiple access interference (MAl) and bit error rate (BER) are obtained using necessary assumptions. The BER performances of 100 Mb/s optical COMA system using OSF are evaluated using analytical expressions. Further, to verify the analytical expressions, FWM noise, MAl and BER"performance are evaluated by computer simulation with random data sequences and different m-sequences as well as Gold sequence of each user. It is investigated that simulation results and analytical results are similar in all cases for optical CDMA system using OSF. The BER performance is found to be degraded due to FWM effect if the transmitted power, transmission distance and number of simultaneous users are increased. It is seen that the BER performance can be improved by increasing the number of chips per bit. It is investigated that transmitted power per user, transmission distance and number of simultaneous users are limited due to FWM effect. Maximum allowable transmitted power per user, transmission distance and number of simultaneous users are evaluated at a BER of 10-9 to design an effective optical CDMA system using OSF. Also, the value of transmitted power per user is evaluated to obtain minimum BER performance for different values of transmission distance at different chips per bit encoding for different number of simultaneous users. Further, the analytical approach is modified to evaluate the BER performance of the optical COMA system using normal dispersive fiber. The BER performance is evaluated for the OS-OCOMA system using normal fiber with 63 chip m-sequence as well as Gold sequence and compared with the results obtained using OSF. It is found that BER performance using OSF is better than the BER performance using normal dispersive fiber. The BER performance of the optical COMA system using normal dispersive fiber found to be degraded significantly if the transmission distance is increased.