Performance Analysis of Optical Wireless Communication Systems for Security Enhancement

Abstract

The internet requirement is ever increasing demand and to combat this demand of different internet services such as high-quality multimedia, smart cell phones, online gaming and shopping, cloud computing etc. The optical communication is the heart of networking system which supports high capacity and data rate. In order to meet the exponential growth in internet services and to cope up with the present users, there is immense need of high-capacity wireless networks. There are certain limitations that exist in both short distance and long haul when condition of large capacity needs to be fulfilled. In the past, multiplexing with electrical techniques was used to meet the capacity requirement, but in that case the data rate achieved was lower. Although with incorporation of WDM technique the capacity increased and rate escalated little. Optical code division multiple access (OCDMA) technique has become the prominent technology to support high capacity and multiple transmission of data with complete coding and synchronization. This coded feature of OCDMA technology provides high security level in comparison to the existing techniques. The studies revealed that in case of simple transmission of data with OCDMA technique leads to traffic jam and eavesdropping. Hence, security leakage has become a major concern in these networks. The data is easily interpreted by the eavesdropper and either jammed or modified. Thus, the main motive of this thesis is to increase the security level in IS-OWC networks by making availability of linearly polarised modes and multiplexing methods to avoid eavesdropping. Initially, the issue of cross correlation in IS-OWC networks is addressed. For eliminating cross correlation in IS-OWC system, different existing one-dimensional codes (MD code, Double diagonal weight, random diagonal code and EDW code) are discussed. Further, an enhanced code than the existing ones is presented that also improved bandwidth efficiency. In most of the suggested codes, cross correlation is present either due to variable weight or fixed code length. Therefore, code comprising of features like minimum multiple access interference and bit error rate with zero cross correlation is evaluated firstly. A novel code called zero cross correlation resultant weight is proposed to eliminate cross correlation in the IS-OWC networks. It is depicted that the data is transmitted without compromising the performance of the system. Further, to increase the capacity of OWC system mode division multiplexing is taken into consideration. The effect of mode division multiplexing with various types of spatial modes (LG11-15/HG11-15) for improving the capacity of the system is analysed. Also, the security

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leakage issue due to lack of dual polarization has been discussed mathematically. It can be determined that overlapping among the users caused cross correlation that eventually degraded the system performance. Hence, it is observed that to achieve minimum MAI and BER, users are sent at different modes. The absence of modes can easily result in low performance of the system as capacity will lower and even the data rate will be impacted. Hence, to novel mode

division multiplexed strategy is opted to increase the data rate of correct information. An IS- OWC network is simulated with desired frequency range and it is revealed that the data can be

transmitted securely without any sort of eavesdropping. The less development in the security area poses many challenges as there is either overlapping or breaching of the data during transmission over a single mode. The existing MDM systems require improved MUX and DEMUX which support multiple modes, fibres which support spatial modes, optical amplifiers capable of operating with minimum components on multiple modes. The optical wireless communication is dominating the field of communication and also in the sensing region. The OWC is capable to perform even for long distances and in case of non-line of sight region. The positive features of OWC comprise of low weight, compact size, immune to EMI, low power requirement and low price. These days security is of major concern in all areas, hence to attain enhanced level different type of techniques comprising of efficient codes, modulation schemes and combination of spatial modes are essential. As per the analysis, if modes are combined in the simple coding, then the security is increases to a next level because eavesdropper needs a greater number of combinations to decode the information. Even if more users are employed, it also increases security as probability of detecting correct codeword decreases. A new expression for P (E) has been derived in which a new parameter ‘r’ has been presented which denotes the number of Hermite Gaussian and Laguerre-Gaussian modes. Further it is noted that presence of multiple users at eavesdropper decreases the chance of P (E). In addition to it, in codes like ZCCRW the greater code length leads to more combinations at the eavesdropper thus leading to higher security level. Finally, the multilevel dual polarisation-based modulation in OWC system is taken into consideration. These days the normal encrypted messages are easily decoded by the eavesdroppers. Hence, to achieve a high level of security, a next level comprising of different polarised modes is employed in the system. An IS-OWC network is simulated with proposed mode selectors and it is analysed that data can be easily transmitted without being lost or misused. The probability of correct data detection by the spy at the receiver is minimized.

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Introducing the distinctive features with simple set up and reduced error rate, the proposed ZCCRW code design is capable to cope up with the interference during the transmission by utilizing minimum bandwidth. This investigation can be further enhanced by using mode division multiplexing to increase the system security and speed during transmission of the information. Hence, the work shown in this thesis relates the concept of zero cross correlation in IS-OWC to lower the demand of bandwidth and variable weight as different modes are combined during transmission to avoid eavesdropping. The analytical study of polarization inscribed in modulation techniques has been used for better system performance like high quality factor and eye diagrams. The research provides wide scope in inter satellite communication, military areas and research areas where security is of major concern. The study includes the establishment of new design, simulation and optimization of a newly developed Is-OWC code named as zero cross correlation resultant weight (ZCCRW) code. Most of the research content has been published in different SCI/SCIE journals as listed below.