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.
