Performance Evaluation of Optical Code Division Multiple Access System

Abstract

This thesis deals with Performance evaluation of Optical Code Division Multiple Access System. The major noise source in Optical CDMA is co-channel interference from other users known as Multiple Access Interference (MAI). The system performance in terms of bit error rate (BER) degrades as a result of increased MAI. It is perceived that number of users and type of code used for optical system directly decides the performance of system. MAI can be restricted by efficient designing of optical codes and implementing it with unique architecture to accommodate more number of users. Hence it is a necessity to design 2-D wavelength-time and 3-D spectral-phase-time codes, which can provide better cardinality and good correlation property. The designing and investigation of efficient 2-D and 3-D coding techniques for different number of users in terms of Bit Error Rate (BER) performance aiming to counter the ill effects of MAI has been presented. Firstly, a WDM compatible optical CDMA system incorporating 3-D spectral-phase-time encoding/decoding is demonstrated. Coding and decoding using binary [0, π] phase chips is demonstrated for six users at 10 Gb/s, and a single coded signal is separated with acceptable bit-error rate ≤ 10-9. The coding and decoding method is based on 3-D coding of tightly spaced phase-locked laser lines that is compatible with conventional WDM networking. In our proposed optical CDMA system, encoding and decoding is done by converting Hadamard codes (used for conventional CDMA system) to phase codes. Duo-binary modulation format is reported to be the best with adequate bandwidth compression. Simulation results too confirm better results in terms of BER and MAI. The obtained simulation results have been further verified that achieved bit error rate (BER) by the use of the bipolar coding method is much improved as compared to the unipolar scheme, especially when the received power is large. Hence it is summarized that whenever the system needs good performance to transmit multimedia data, we can use bipolar scheme in the network and if the users only transmit voice data the unipolar method can be employed. Further, a new optimized class of optical codes known as Message Priority & Fast Routing (MPFR) is presented. We have presented two dimensional code constructions based on message priority and routing at faster rate. Tridiagonal matrix is used for code construction and message priority is used which enables the priority of messages by simply assigning the priority value in packet header. We have proposed an algorithm based on multiple user environments used in- accordance with the packet header. The message size is increased and priority bit is added which leads to faster and effective data transfer in communication. MPFR code development is based on minimizing code length. Variation in weight due to priority provides fixed data rates but can support various different qualities of service processes and in other case; length variation can provide data variation to cater the user specific needs. An OCDMA environment is created and an interference sensing algorithm is introduced. The results revealed that proposed MPFR codes provide better performance as compared to Flexible Cross Correlation (FCC) codes, Zero Cross Correlation (ZCC) codes and Prime Hop codes (PHC) in term of BER, packet delivery ratio and MAI. Also we have studied and analyzed the spectrally encoding/decoding OCDMA system for different lengths of fiber in terms of quality factor (Q) and BER performance. The performance characteristics like bit error rate, eye diagrams and eye closure penalty at the output are studied using simulation methods for different lengths of fiber. An upper bound on the bit error probability for phase encoded OCDMA system is maintained under the above considerations. The advantages of this system over the conventional time-encoded system include the availability of larger number of low cross-correlation sequences and the implementation of efficient decoders for low error probability detection. In addition, we have presented the optical simulation technique of Encoding/Decoding for different lengths & gain in terms of Quality factor and BER performance. The system supports up to sixty four asynchronous users, while maintaining BER < 10−11, for the correctly decoded signal. We have designed and simulated a Tree Network Topology Optical Code Division Multiple Access System, for large number of users using wavelength–time code and then analyzed the performance of the system based on BER and Eye Diagram under the influence of number of simultaneous users with different received powers. The thesis also highlights the investigative study of proposed Optical CDMA network, by implementing spectral-coding of incoherent broadband optical sources. We have utilized the transmissive spectrum characteristics of FBG (Fiber Bragg Grating) to design encoder/decoder. In this design, we have not used any circulator; thereby reducing the cost as well as power loss. Signal at the receiver is extracted with acceptable bit-error rate ≤ 10-9. The coding and decoding method is based on spectral-amplitude coding of FBG. Here FBG are used to control and modify the amplitude and phase spectrum of broadband incoherent optical signal. The effect of FWM (Four Wave Mixing) is avoided to a larger extent by using this optical CDMA coding technique. Simulation results too confirm better results in terms of Q factor, SNR and bit error rate in favour of NRZ modulation format. Therefore, this study establishes the design and optimization of Optical CDMA system resulting in the revolutionary growth of data traffic with enhanced supported data rate with acceptable BER.