Design and Analysis of Reliable Wireless Optical Communication System for the Underwater Channels

Abstract

Internet of Things (IoT) is a technology that deals with devices and protocols employed to interconnect smart devices that are deployed in benign and hazardous environments to collect or exchange information. In recent years, attempts have been made to interconnect devices deployed in underwater environments. These devices have been employed in underwater environments to monitor physical phenomena such as climate and ocean monitoring and enable activities such as underwater surveillance and ocean exploration. This technology is referred to as the Internet of Underwater Things (IoUT). IoUT is a technology, which requires a high degree of information integrity, high data transfer rates and energy e ciency for e ective deployment. Optical signaling gives the exibility of providing high data-rates than that of acoustic and RF signaling for medium link-ranges. Hence, optical signaling is an excellent candidate to enable high speed IoUT communication link between the underwater devices. The devices that employ optical signaling to enable communication between underwater vehicles and the underwater channel together constitute the Underwater Wireless Optical Communication (UWOC) system. The transmitted optical data experiences beam attenuation, turbulence and pointing errors, all of which can contribute to introduce errors in the received data stream. We initiated the work in this thesis by conducting experimental work to determine various parameters a ecting the propagation of light in an underwater channel and determining the optimum wavelength for UWOC communication. This is followed by a discussion of channel models that are appropriate descriptors of weak and strong turbulence in the underwater medium. Analytic models describing strong and weak turbulence have been derived, and simulation studies (Monte- Carlo simulations) that determine the accuracy of these analytic models have been carried out. The performance of UWOC system is mainly dependent on the underwater turbulence, beam attenuation and pointing errors, to mitigate these e ects we have introduced multiple input multiple output (MIMO), forward error control codes and Space-Time Block Codes (STBCs) to the proposed UWOC system. In many instances, the UWOC link operating under the surface of the water has v to be linked with a RF system operating over the water surface. Such a link is referred to as a cooperative RF-UWOC system. Channel models for the combined RF-UWOC system have been drawn up. It has been recognized that in addition to channel induced impediments, a major cause of link outage is the introduction of pointing errors due to the physical displacement of the transmitter-receiver pair, which causes the Line of Sight (LOS) requirement to be disturbed. Loss of LOS has the potential to severely compromise the working of the UWOC/ RFUWOC system. The performance of the RF-UWOC (co-operative IoT and IoUT) system in the presence of pointing errors has been studied and various outage probabilities have been determined. We have concluded the technical contributions of the thesis by studying underwater image transmission through the turbulent oceanic medium and suggesting various remedial techniques for proper image reception and enhancement. The thesis has been concluded by drawing conclusions from the research work conducted in the thesis and suggesting avenues for further research.