Underwater communication systems are in high demanded for subaquatic environment activities as the sea is an enormous and mostly unexplored place. The ten-meter long and few giga-bit per second range optical communication technique is feasible and has bright future compared to the mature but low data rate (few kilobits per second) acoustic technology and short distance (several meters) radio-frequency signaling schemes. The underwater wireless optical communication (UWOC) technique takes advantage of wide bandwidth, low attenuation effect in the visible range for multiple applications such as seafloor and offshore exploration, oil pipe control and maintenance, and pipeline leak detection. Nowadays, visible light-emitting diode (LED)-based and laser diode (LD)-based UWOC system are attractive and much related research is being conducted in the field. However, the major challenges of developing UWOC systems are the attenuation, scattering and turbulence effects of the underwater environment. The temperature gradient, salinity gradient, and bubbles make underwater optical channel predictable challenging and degrade the optical beam propagating distance and quality. Most studies focus on the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations as well as the presence of air bubbles. In this thesis, we experimentally investigate the reciprocity nature of underwater turbulence caused by the turbidity, air bubbles, temperature variations, and salinity. Bit error rate measurement and statistical data analysis reveal the high reciprocal nature of turbulence that can be induced by the presence of bubbles, temperature, and salinity. The mitigation strategies for the different turbulence scenarios are discussed.

The Internet of Underwater Things (IoUT) paradigm is expected to enable various practical applications such as environmental monitoring, underwater exploration, and disaster prevention. Supporting the concept of IoUT requires robust underwater wireless communication infrastructure. Optical wireless communication has the superiority of wide bandwidth, low latency, and high data capacity over its counterparts, namely, acoustic and radio-frequency. However, the transmission of the optical beam has inherent drawbacks in a harsh environment. Obstructions such as geometrical underwater terrains and underwater turbulence can pose a serious challenge to the alignment of the transmitter and the receiver. Non-line-of-sight (NLOS) configuration is a generalized alignment scheme between the transmitter and the receiver such that the strict requirement of precise alignment (point-to-point) is no longer needed. In this dissertation, the effectiveness of NLOS to withstand challenging underwater turbulence is examined. Thermal gradients with a maximum temperature difference of 10 ◦C had a negligible effect on the received power. The presence of air bubble clouds caused an increase of 38% of the received power when the bubble area increased from 5.2 to 80 mm$^2$. Additionally, various salinity concentrations ranging between 30-40‰ are emulated. A gain of 32.5% in the signal-to-noise ratio is observed when the salinity gradient increased from 0.08 to 0.4‰·cm$^{−1}$. Moreover, a reduction of 2.35 dB/m of the pathloss is noticed. The bit-error ratio is used to examine the communication quality using on-off-keying modulation scheme. In addition, this dissertation shows a practical wavelength-division multiplexing method based on large-area detection and wide field-of-view (FoV) photonic receiver. The dual-antenna is made of scintillating fibers with distinctive characteristics. An aggregated data rate of 1 Gbps is achieved. Two methods of wavelengths separation are demonstrated. Additionally a field deployment verification in an outdoor water pool is conducted at a maximum separation distance of 10m. The presented promising results pave the way for a robust underwater wireless optical sensor network that serves as a building block for achieving the goal of establishing IoUT.

Eelume AS is a norwegian company that develops autonomous underwater vehicles. Their flagship model is an underwater snake robot that performs inspection, maintenance and repair operations. For the time being, Eelume has been using acoustic communications between their AUV's and the docking station, but it has a big drawback: bandwidth. Eelume is interested in streaming live high-quality video from their AUV's to the docking station, which requires several megabits per second. As underwater radio frequency communications are not possible, wireless optical communications seem to be the best available alternative.   The focus of this Thesis is to design an underwater wireless optical communications system that could be implemented on the Eelume AUV, although it is designed as a standalone embedded system that could be integrated into any other platform. Two prototypes were designed and tested through-air: a low-cost system featuring a PIN photodiode that can stream a 1.5 Mbps video signal over 0.5 meters and a high-sensitivity system featuring an avalanche photodiode that can stream a 2.5 Mbps video signal over 10.5 meters.   Even if further underwater testing is needed and some inherent limitations in the design like the precise calibration or the ambient light noise effects could be mitigated. The results achieved by this high-sensitivity system demonstrates that a high-bandwidth mid-range underwater wireless optical communication system featuring a blue/green LED array as the light source and an avalanche photodiode as the photodetector is a viable solution for streaming live high-quality video over several meters even in very turbid seawaters.

In Underwater Acoustic Sensor Networks (UWASNs), cameras have recently been deployed for enhanced monitoring. However, their use has faced several obstacles. Since video capturing and processing consume significant amounts of camera battery power, they are kept in sleep mode and activated only when ultrasonic sensors detect a target. The present study proposes a camera relocation structure in UWASNs to maximize the coverage of detected targets with the least possible vertical camera movement. This approach determines the coverage of each acoustic sensor in advance by getting the most applicable cameras in terms of orientation and frustum of camera in 3-D that are covered by such sensors. Whenever a target is exposed, this information is then used and shared with other sensors that detected the same target. Compared to a flooding-based approach, experiment results indicate that this proposed solution can quickly capture the detected targets with the least camera movement.

The means of communication in oceanic environments is currently dominated by sonar. Although it is reliable for long-distance transmission, the vision of internet of underwater things (IoUT) requires an alternate means for high-data-rate transmission. It is also envisaged that a networked underwater and above-water objects, such as sensor nodes, and autonomous underwater vehicles will benefit seafloor exploration. The use of laser-based optical communication is poised to realize this dream while working hand-in-hand with acoustic and radio-frequency technologies from the littoral zone to deep blue sea. While blue and green lasers are typically utilized depending on the optical properties of the water, laser-based white light is attractive in a number of aspects. In this thesis, we proposed and realized the use of white light to model the channel and to provide the immediate decision for the preferred system configuration, which is critical for developing reliable communication links, particularly, in the presence of turbulence, which makes the alignment of underwater wireless optical communication (UWOC) links challenging. Temperature and salinity changes are among factors that change the refraction index, giving rise to beam wander. This thesis explores the dependence of underwater turbulence on the wavelength. After comparing the performance of red, green, and blue lasers, an ultra-fast comprehensive method that utilizes a white-light source that can produce a wide range of wavelengths is implemented. Experimental results show an 80%-decrease in the scintillation index as the wavelength is increased from 480 to 680 nm in weak turbulence caused by a 0.02-℃/cm temperature gradient with a 40-ppt salt concentration, which emulates conditions found in the Red Sea. The effect of turbulence on the bit error ratio (BER) is also investigated experimentally. Temperature gradients increased the BER especially for shorter wavelengths. The results along long-transmission distances were verified using Monte Carlo simulations. The correlation matrix between wavelengths was studied, which is important for designing multiple-input multiple-output systems. The results obtained show that as the difference in the wavelengths increases, the correlation decreases. Based on the interplay among scintillations, scattering, absorption, and the correlation between different wavelengths, it is possible to design a more reliable UWOC link.

Yes / Underwater communication applications extensively use localization services for object identification. Because of their significant impact on ocean exploration and monitoring, underwater wireless sensor networks (UWSN) are becoming increasingly popular, and acoustic communications have largely overtaken radio frequency (RF) broadcasts as the dominant means of communication. The two localization methods that are most frequently employed are those that estimate the angle of arrival (AOA) and the time difference of arrival (TDoA). The military and civilian sectors rely heavily on UWSN for object identification in the underwater environment. As a result, there is a need in UWSN for an accurate localization technique that accounts for dynamic nature of the underwater environment. Time and position data are the two key parameters to accurately define the position of an object. Moreover, due to climate change there is now a need to constrain energy consumption by UWSN to limit carbon emission to meet net-zero target by 2050. To meet these challenges, we have developed an efficient localization algorithm for determining an object position based on the angle and distance of arrival of beacon signals. We have considered the factors like sensor nodes not being in time sync with each other and the fact that the speed of sound varies in water. Our simulation results show that the proposed approach can achieve great localization accuracy while accounting for temporal synchronization inaccuracies. When compared to existing localization approaches, the mean estimation error (MEE) and energy consumption figures, the proposed approach outperforms them. The MEEs is shown to vary between 84.2154m and 93.8275m for four trials, 61.2256m and 92.7956m for eight trials, and 42.6584m and 119.5228m for twelve trials. Comparatively, the distance-based measurements show higher accuracy than the angle-based measurements.

Approximately, two-thirds of earth's surface is covered by water. There is a growing interest from the military and commercial communities in having, an efficient, secure and high bandwidth underwater wireless communication (UWC) system for tactical underwater applications such as oceanography studies and offshore oil exploration. The existing acoustic and radio frequency (RF) technologies are severely limited in bandwidth because of the strong frequency dependent attenuation of sound in seawater and the high conductivity of seawater at radio frequencies, respectively. Recently, underwater wireless optical communication (UWOC) has been proposed as the best alternative or complementary solution to meet this challenge. Taking advantage of the low absorption window of seawater in blue-green (400-550 nm) regime of the electromagnetic spectrum, UWOC is expected to establish secure, efficient and high data rate communication links over short and moderate distances (< 100 m) for versatile applications such as underwater oil pipe inspection, remotely operated vehicle (ROV) and sensor networks. UWOC uses the latest gallium nitrite (GaN) visible light-emitting diode (LED) and laser diode (LD) transmitters. Although some research on LED lased UWOC is being conducted, both the military and academic 5 research communities are favoring the use of laser beams, which potentially could enhance the available bandwidth by up to three orders of magnitude. However, the underwater wireless channel is optically very challenging and difficult to predict. The propagation of laser beams in seawater is significantly affected by the harsh marine environments and suffers from severe attenuation which is a combined effect of absorption and scattering, optical turbulence, and multipath effects at high transmission rates. These limitations distort the intensity and phase structure of the optical beam leading to a decrease in signal-to-noise ratio (SNR) which ultimately degrades the performance of UWOC links by increasing the probability of error. In this dissertation, we seek to experimentally demonstrate the feasibility of short range (≤ 20 m) UWOC systems over various underwater channel water types using different modulation schemes as well as to model and describe the statistical properties of turbulence-induced fading in underwater wireless optical channels using laser beam intensity fluctuations measurements.

Nowadays, Opportunistic Routing (OR) is widely considered to be the most important paradigm for Multi-hop wireless networks (MWNs). It exploits the broadcast nature of wireless medium to propagate information from one point to another within the network. In OR scheme, when a node has new information to share, it rst needs to set its forwarding list which include the IDs and/or any relevant information to its best suited neighboring nodes. This operation is supported by the use of appropriate metrics. Then, it executes a coordination algorithm allowing transmission reliability and high throughput among the next-hop forwarders. In this paper, we provide a comprehensive guide to understand the characteristics and challenges faced in the area of opportunistic routing protocols in MWNs. Moreover, since the planet we live on is largely covered by water, OR protocols have gained much attention during the last decade in real-time aquatic applications, such as oil/chemical spill monitoring, ocean resource management, anti-submarine missions and so on. One of the major problems in Underwater Wireless Sensor Network (UWSNs) is determining an e cient and reliable routing methodology between the source node and the destination node. Therefore, designing e cient and robust routing protocols for UWSNs became an attractive topic for researchers. This paper seeks to address in detail the key factors of underwater sensor network. Furthermore, it calls into question 5 state-of-the-art routing protocols proposed for UWSN: The Depth-Based Routing protocol (DBR), the Energy-E cient Depth-Based Routing protocol (EEDBR), the Hydraulic-pressure-based anycast routing protocol (Hydrocast), the Geographic and opportunistic routing protocol with Depth Adjustment for mobile underwater sensor networks (GEDAR), and the Void- Aware Pressure Routing for underwater sensor networks (VAPR). Finally, it covers the performance of those protocol through the use of the R programming language.

Το αντικείμενο της παρούσας διπλωματικής εργασίας είναι η δημιουργία ενός υποβρύχιου ασύρματου δικτύου αισθητήρων για την πραγματοποίηση της μέτρησης της στάθμης μίας δεξαμενής γεμισμένης με νερό και λύματα. Πραγματοποιήθηκε μία πλήρη βιβλιογραϕική αναζήτηση πάνω στο θέμα των των υποβρύχιων ασύρματων δικτύων αισθητήρων και στην συνέχεια αγοράστηκε ο κατάλληλος εξοπλισμός για την πραγματοποίηση των πειραμάτων. Με την χρήση του ολοκληρωμένου εξοπλισμού evaluation kit EK010-JN5148, δημιουργήσαμε ένα δίκτυο μεταξύ ενός συντονιστή, ενός δρομολογητή και διαϕόρων τερματικών συσκευών. Ο δρομολογητής και οι τερματικές συσκευές πραγματοποιούν μετρήσεις της θερμοκρασίας στο υδάτινο περιβάλλον και ο δρομολογητής είναι υπεύθυνος για την μεταϕορά των πληροϕοριών εκτός του υποβρύχιου περιβάλλοντος, στον συντονιστή, ο οποίος απεικονίζει τα πακέτα δεδομένων στην LCD οθόνη. Με αυτό τον τρόπο, ο χρήστης βλέπει ανά πάσα στιγμή τις μετρήσεις που τον ενδιαϕέρουν, αλλά και ταυτόγχρονα παρακολουθεί την ισχύ του δικτύου στα διάϕορα βάθη,στα οποία εμβυθίζονται οι μικροεπεξεργαστές που ϕέρουν τους αισθητήρες. Απώτερος στόχος της εργασίας είναι η εξοικείωση του αναγνώστη με το αντικείμενο των ασύρματων δικτύων υποβρύχιων αισθητήρων και η ανάδειξη της χρησιμότητάς τους μέσω των πολυάριθμων εϕαρμογών τους. Τα πειράματα που πραγματοποιήθηκαν έδωσαν ένα αριθμό κριτηρίων για την διαπίστωση της ισχύος των ηλεκτρομαγνητικών κυμάτων στο νερό και τελευταίο και μη αμελητέο η εϕαρμογή που δημιουργήθηκε αποτελεί μία σημαντική λύση στο πρόβλημα ανίχνευσης της στάθμης των λυμάτων σε μία δεξαμενή γεμισμένη με νερό και λύματα, ουσίες οι οποίες πρέπει να διαχωριστούν στην συνέχεια. Στα πλαίσια της διπλωματικής εργασίας, έγινε λεπτομερής μελέτη της λειτουργίας του Υδροηλεκτρικού Σταθμού του Γλαύκου στην περιοχή της Αχαίας και των αναγκών του σταθμού, όπου και στο τέλος προτάθηκαν λύσεις για την βελτίωση και διευκόλυνση της ετήσιας πραγματοποίησης μετρήσεων πάνω στην ποιότητα του αρδεύσιμου νερού, χρησιμοποιώντας τον αγορασθέντα εξοπλισμό. / The object of this thesis is to create an underwater wireless sensor network for the embodiment of the level measurement of a tank filled with water and wastewater. A search in literature was conducted on the topic of underwater wireless sensor network, in order to further purchase the appropriate equipment to perform the experiments. Using the integrated equipment kit EK010-JN5148, a network was created between a coordinator device, a router device and various terminals. The router and terminal devices operate temperature measurements in the aquatic environment, while the router has the additional role to transfer the gathered information to the coordinator, who is placed outside the aquatic environment. Afterwards, the coordinator illustrates the data packets on the LCD screen for the user to see. The advantage of the network utilization is that the user can benefit from the update of the information and choose the way to depict them and concurrently monitor the power of the network in various depths. The ultimate goal of this paper is to familiarize the reader with the object of underwater wireless sensor networks highlighting their usefulness through numerous applications. The experiments that were carried out provide criteria to determine the effect of the electromagnetic waves in water. Finally, through coding in language C, an application was created to serve as a solution to the problem of detecting the level of water waste in an industrial tank and give necessary information to facilitate its separation later in the process. During the thesis, a detailed study was made on the operation of a hydroelectric power plant in Glavkos in the region of Achaea, Greece. Solutions including the use of the kit EK010-JN5148 were proposed in order to enhance and facilitate the annual measurements on the quality of the irrigable water.

Recreational scuba diving is a highly social activity where divers are encouraged to work in groups of two or more people. Though collaborative, divers are unable to freely and naturally communicate. Additionally, the distortion of sensory information (e.g. distances and sounds cannot be judged as accurately underwater) affects the ability to keep track of critical information which impairs their ability to engage in this underwater world. We have studied and designed a fault tolerant system, including the software, the device, and the network, to foster underwater communication. We studied the technology required, the software design for both single user and multiple users, as well as, the network design in order to support such a system. In the thesis, we have set up and analyzed the result of three user studies and a simulation to investigate the viability of the proposed design.