25-08-2017, 09:32 PM
DESIGN AND PERFORMANCE ANALYSIS OF OPTICAL CDMA SYSTEM USING NEWLY DESIGNED MULTIWAVELENGTH PRIME SEQUENCE CODES AT 1GBPS BIT RATE
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ABSTRACT
This research work focuses to design OCDMA system with improved coding techniques
which will help reduce hardware requirement and still offer better performance for higher
number of users. These codes have been designed by remodeling and improvising the
existing concept of Prime Sequence Codes. A three way correlation is derived between
various input parameters e.g. bit rate, received power with respect to number of users and
performance indicators i.e. BER and Quality factor. The codes designed result in improved
performance compared to previous researches in terms of quality factor and bit error rate
with minimum input requirements of power, bit rate etc for even in case of larger number
of users. This system can support 21 users with -15db received power for 1Gbps bit rate
for permissible bit error rate of e-9. Moreover these results are more realistic as practical
impairments have also been considered.
INTRODUCTION
OCDMA is an essential part of the digital
communication system now days for long haul, high
speed LAN and MAN networks [1]-[3]. The biggest
challenge with Optical CDMA system is to maintain
the performance of the system and offer high
bandwidth in case of higher number of users at
minimum cost. As the number of users increase, the
input requirements i.e. transmitted power, bit rate etc
start increasing sharply which contribute to the
additional cost [3]-[5]. In this research the available 2D
codes under the OCDMA system were studied and
with a view to overcome the limitations in terms of
expanding the user base (number of users, speed of the
system, bit error rate, quality factor, required received
power etc.), a new code set have been introduced based
on rigorous analysis.
CODING ARCHITECTURE
The code design is based on the prime sequence
code pattern. One code from prime sequence family
becomes the master code in new code design set which
also fixes the time slot pattern while wavelength as the
other dimension is varied with various combinations
according to prime sequence code family. The
specialty of this coding technique is the generation of a
large number of code set by using a low code weight
value. i.e. three. This technique uses delay lines which
are similar to each other for implementation of encoder
which reduces the complexity and makes the system
more cost effective. In this coding structure, seven
mode locked lasers along with three time delay lines
are used to design forty nine codes.
OCDMA SYSTEM DESIGN
To implement both the coding techniques, a
realistic simulation model for OCDMA system is
designed which considers all practical impairments e.g.
transmitter, channel and receiver noise etc. while
analyzing the performance of designed codes as shown
in Figure 1. Two different encoders are designed after
confirming the viability of the system.
The performance of the codes is evaluated using
this simulation model by varying number of users at
different received power i.e. -15db to -30db for various
data rates. As per the requirement of codes constructed
seven lasers have been used at the transmitter end as
represented in Figure 1. The NRZ pulses are used for
designing the OCDMA system as shown in Figure 2.
CONCLUSION
The OCDMA system designed for this new
coding architecture, results in improved performance
compared to previous researches in terms of quality
factor and bit error rate with minimum input
requirements of power, bit rate even in case of larger
number of users. Moreover these results are more
realistic as practical impairments have also been
considered. With -15db received power and for
permissible BER of 10-9 this OCDMA system can
accommodate up to 21 users at 1Gbps bit rate. If
received power is kept lower i.e.-22db, the same
system can accommodate 16 users for 1Gbps bit rate.