PERFORMANCE ANALYSIS OF OPTICAL CDMA USING W/T/ CODES

Abstract

The OCDMA system plays a vital role in long haul and high speed communication networks like LAN and MAN. These systems have advantage of high speed, large capacity and huge bandwidth. The TDMA, WDMA and CDMA are available major multiple access techniques. The throughput of the TDMA system is limited by product of number of user and their respective transmission rate. In WDMA system each channel is transmitted on a single wavelength of light. This allows each user to transmit the peak speed of network hardware, but the complexity of the system increases due to dynamic system of multiple users because of the significant amount of the coordination among the nodes required for successful operation. If a dynamic user base has to built with WDMA system the control channel and collision detection schemes would need to be implemented but that would waste significant bandwidth. Both TDMA and WDMA system require time or frequency management system however the CDMA system does not require any centralized control and thereby reduce the complexity of the system. Optical CDMA combines the large bandwidth of the fiber medium with the flexibility of the CDMA technique to achieve the high-speed connectivity. The establishment of OCDMA needs to overcome the code orthogonality problem and also in 1-D optical orthogonal codes, the ratio of code length to the code weight grows rapidly as the number of users increases. If we consider 2-D optical orthogonal code then, it reduce the length of the code as a result of which BER performance of the system improves. Wavelength-Time (W/T) encoding of the two dimensional code is practical in FOCDMA networks. They can be classified mainly into two types: 1. Hybrid sequences, where one type of sequence is crossed with another to improve the cardinality and the correction properties and 2. Matrix codes 1-D sequence converted to 2-D codes or 2-D codes by construction to reduce the time spread of the sequences /codes.In the project I have discussed the performance analysis of an incoherent optical code division multiple access scheme based on wavelength/time codes. The OCDMA system can support only 25 users for permissible BER rate of 10-9 with -15dB received power. The performance of the system is also analyzed on BER and EYE diagram under the influence of number of simultaneous users with different received power.