Performance Analysis of One-Dimensional and Two-Dimensional Codes in Optical Code Division Multiple Access System

Abstract

Code Division Multiple Access (CDMA) is a "spread spectrum" technology, allowing many users to occupy the same frequency band at the same time. As its name implies, CDMA assigns unique codes to each user to differentiate it from others in the same spectrum. In a world of finite spectrum resources, CDMA enables many more people to share the airwaves at the same time. The optical fiber communications have the high bandwidth, large capacity and high reliability by the use of the broadband of the optical fiber. To take the full advantage of both of the technologies, CDMA and optical fiber communication, one of the basic concepts is the idea of allowing several users to transmit data simultaneously over the optical fiber communication channel by simultaneously allocating the available bandwidth to each user, which is called multiple access. There are several techniques to provide multiple access and one of them is optical code division multiple access (OCDMA). In OCDMA, the type of codes used is a major factor influencing its performance. The objective of the thesis is to analyze the performance of one and two dimensional codes for an OCDMA system. Firstly, the design, implementation and performance analysis of various one dimensional codes in an OCDMA system for different data formats is presented. A number of different codes are used with Optical CDMA to improve its error performance. Here, three such codes, Optical orthogonal codes (OOC), Walsh Hadamard codes and Zero cross correlation (ZCC) codes have been compared using different data formats, NRZ raised cosine, NRZ rectangular, RZ raised cosine and RZ rectangular. It is found that NRZ raised cosine has the best system performance for all the codes used. After that, these three codes have been compared in terms of the BER, eye diagrams and received optical power using NRZ raised cosine modulation format. It is analyzed that ZCC codes have zero cross correlation property. The simulation results revealed that ZCC codes can provide a better BER compared to the OOC and Walsh Hadamard codes and it is most suitable to be employed in the OCDMA systems. Secondly, the design, implementation and performance analysis of two dimensional wavelength/time codes in OCDMA system are also presented. The 2-D codes are constructed by a technique based on folding of Golomb rulers. The performance evaluation of OCDMA system based on wavelength/time code has been analyzed by measuring the values of bit error rates and eye diagrams for different number of active users. It is shown that eye opening decreases and BER increases with increase in number of active users. It is also shown that BER further increases with increase in number of active users when number of decoders increases on receiver side. Hence, it is concluded that MAI is the dominant source of BER and there is graceful degradation in system performance when number of simultaneously active users increases. The received optical power is also measured at different transmission distances. It has been observed that received optical power decreases with increase in length of fiber due to attenuation. Lastly, the security enhanced OCDMA system based on spectral encoding using code switching scheme is analyzed. The security issues are investigated by measuring eye diagrams and received signals for various cases. It has been observed; an eavesdropper based on a simple energy detector can easily read the information being transmitted by a single user using On-Off keying. In order to increase the security a code switching scheme is implemented on OCDMA. It is shown that the eye diagram at the eavesdropper becomes true noise waveform due to code switching scheme and at the receiver end a clear eye diagram is observed. The code switching scheme shows an immunity of the OCDMA system against eavesdropping and like conventional OCDMA schemes an authorized user clearly decodes an original data when a single user is active in the network. Thus, the thesis presents the performance analysis of one and two dimensional codes for an OCDMA system.