Of late, Optical Code Division Multiple Access
(OCDMA) has attracted much attention because of its vast bandwidth availability and for allowing simultaneous users
to access the same optical channel with less delay. The construction of codes with
good auto and cross correlation properties with low Multiple Access Interference (MAI)
is a big challenge in OCDMA system. Hence, there is increasing research in this area.
In this paper, we propose a reliable simulated model for OCDMA system with
improved performance as compared to previous designs. The system has been designed for
a Metropolitan Area Network (MAN) with optical fiber length of 60 km. In this
paper, the optical CDMA system has been designed and simulated by using redesigned W/T matrix code and WDM-type components for high data rates, i.e., 1 Gbps, 1.25
Gbps, 2.5 Gbps and 5 Gbps. A comparative Bit Error Rate (BER) and eye diagram
analysis of high speed OCDMA system for asynchronous concurrent communication of
multiple users at the above-mentioned data rate has been carried out. Results show that
this proposed system can accommodate 16 simultaneous users for 2.5 Gbps for
BER e-10. Previous designs have been on a bit rate of 1.25 Gbps for 16 users with
BER e-9 (Mendez et al., 2004). With -19 db received power, 16 simultaneous
users can be accommodated at the BER of
e-16, e-13, e-10 and
e-4 for 1 Gbps, 1.25 Gbps,
2.5 Gbps and 5 Gbps respectively. Hence, the system designed in the present work
is well-suited for 16 simultaneous users for the data rates, i.e., 1 Gbps, 1.25 Gbps
and 2.5 Gbps, and can accommodate up to eight numbers of simultaneous users for
data rate 5 Gbps.
Optical Code Division Multiple Access (OCDMA) is considered as an important area
in multiple access techniques because of its advantages, such as utilization of
terahertz bandwidth offered by optical fiber with secure transmission.
The block diagram of OCDMA system is represented in Figure
1. The choice of the codes with good auto and cross-correlation properties is the challenge in transmission and access network.
In OCDMA system, each bit of data is modulated by optical modulator. The bit `1'
is transmitted by sending an optical pulse and `0' is transmitted by sending no pulse.
The transmitted bits are further encoded by the signature sequence encoder where each
user has been assigned a unique signature sequence. So the resultant signal is a unique
optical pulse sequence where pulses are put onto their respective positions according to
the signature sequence (Fan et al., 1989; Jawad 1989; and Jawad et al., 1989).
One dimensional codes are very long to reduce the multiple access interference
and for increasing the code cardinality. However, in two dimensional codes, the number
of simultaneous users can be increased cleverly by adding one more dimension. In
this proposed design, we have used two parameters, i.e., wavelength and time. The
Wavelength/Time (W/T) matrix codes are specifically used to increase the number of subscribers
with low Bit Error Rate (BER) (Kwon, 1994a and 1994b; Wing et al., 1996; Tung-Wah and Edward, 2001; and Stok and Sargent, 2002), where W is the number of wavelength and T is the number of time slots. Here the incoming pulse is split into distinct chips on the
basis of signature sequence on the time axis, each of specific colors. At the receiver end,
the matched decoder realigns the delays between the different pulses and gives
an autocorrelation peak. |
