Dissertations / Theses on the topic 'Underwater wireless communications'

In this thesis the possibility of building an underwater communication system usingelectromagnetic waves has been explored. The focus became designing and testingan antenna even if the entire system has been outlined as well. The conclusion isthat using magnetically linked antennas in the near field it is a very real possibilitybut for long EM waves in the far field more testing needs to be done. This isbecause a lack of equipment and facilitates which made it hard to do the realworld testing for this implementation even if it should work in theory.

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.

Underwater communication is necessary for a variety of applications, including transmission of diver speech, communication between manned and/or unmanned underwater vehicles, and data harvesting for environmental monitoring, to name a few. Examples of communication between underwater vehicles include unmanned or autonomous underwater vehicles (UUV or AUV) for deep water construction, military UUVs such as submarine drones, repair vehicles for deep water oil wells, scientific or resource exploration, etc. Examples of underwater communication between fixed submerged devices are sensor networks deployed on the ocean floor for seismic monitoring and tsunami prediction, pollution monitoring, tactical surveillance, analysis of resource deposits, oceanographic studies, etc. The underwater communication environment is a challenging one. Radio signals experience drastic attenuation, while optical signals suffer from dispersion. Because of these issues, acoustic (sound) signals are usually used for underwater communication. Unfortunately, acoustics has its own problems, including limited bandwidth, slow propagation, and signal distortion. Some of these limitations can be overcome with advanced modulation and coding, but to do so requires better understanding of the underwater acoustic propagation environment, which is significantly different than air- or space-based radio propagation. The underwater environment must be studied and characterized to exploit these advanced modulation and coding techniques. This thesis addresses some of these concerns by proposing a derivation of the envelope level crossing rate of the underwater channel, as well as a simulation model for the channel, both of which agree well with the measured results. A velocity estimator is also proposed, but suffers from a high degree of root mean square error

Existen muchos trabajos relacionados con el diseño y desarrollo de nodos sensores, donde se presentan gran variedad de aplicaciones. Las redes de sensores inalámbricos pueden facilitarnos y mejorar algunos aspectos de nuestra vida diaria. Es fácil, pensar que si este tipo de dispositivo es tan beneficioso para nosotros y para el entorno donde vivimos, su precio debería ser relativamente barato. Pero podemos comprobar que esto no es así. ¿Por qué estos dispositivos son tan caros? ¿Sería posible desarrollar dispositivos con las mismas capacidades y precios más económicos? ¿Cómo puedo fabricar mis nodos sensores de bajo coste? Esta tesis responde a estas preguntas y muestra algunas de las muchas aplicaciones que los nodos sensores pueden tener. En esta tesis hemos propuesto (e implementado en algunos casos) el desarrollo de nodos sensores para la monitorización del medio, a partir de dispositivos de bajo coste. Para la implementación de un nodo sensor, y en definitiva la red que une a todos estos nodos, es importante conocer el medio donde trabajarán. A lo largo de este documento se presentan las investigaciones llevadas a cabo para el desarrollo de sensores en tres ámbitos de aplicación. En el primero de ellos, se desarrollan dispositivos multisenores para la monitorización del medio. La aplicación de las redes de sensores inalámbricas al medio natural, precisa un estudio de cómo se ven afectadas las señales, en función de la distancia, vegetación, humedad del ambiente, etc. Focalizamos nuestros desarrollos en la verificación de incendios en zonas rurales y en el control de plagas en viñedos donde la detección precoz de estos eventos genera elevados ahorros económicos. También proponemos el desarrollo de una red de collares sensores para ganado domestico, que nos ayudará a reducir y prevenir en muchos casos, los ataques de lobos y hurtos de crías. Por último, dentro de este grupo, presentamos una red permite detectar anomalía de los materiales en edificios y red de sensores que nos permite monitorizar las personas mayores o deficientes, que se mueven junto con un grupo, en una excursión o actividad. El segundo grupo de aplicaciones, hace referencia a la monitorización de espacios en entornos de interior. Para ello hemos analizado el comportamiento de las señales inalámbricas en diferentes escenarios. Los resultados, nos han permitido extraer un nuevo método de diseño de las redes inalámbricas en interiores. Nuestro método, permite definir la mejor ubicación de los dispositivos de red y nodos sensores en interiores con un ahorro en el número de sensores del 15%. Por último, se presenta el estudio sobre las comunicaciones subacuáticas basadas en las ondas electromagnéticas donde analizamos la dependencia de las comunicaciones subacuáticas en agua dulce en función de la frecuencia, temperatura, tasas de transferencia de datos y modulación. Relacionado con el medio subacuático, presentamos 2 propuestas. La primera de ellas hace referencia a la implementación de una red de sensores para granjas marinas que nos permite reducir la cantidad de residuos depositados en el lecho marino y reducir el porcentaje de comida desperdiciada. La segunda propuesta es el desarrollo de dos sensores oceanográficos que nos permitirían controlar la cantidad de comida y heces depositadas en el suelo y controlar la turbidez del agua de manera muy simple y económica Todos estos desarrollos y propuestas, han estado precedidos por un exhaustivo estudio sobre los problemas energéticos que las redes de sensores inalámbricas presentan y las técnicas que pueden emplearse, para prolongar la vida útil de la red y mejorar su estabilidad.
There are many works related to the design and development of sensor nodes which present several applications. Wireless sensor networks can facilitate and improve some aspects of our daily lives. It is easy to think that if this type of device is so beneficial to us and to our environment, its price should be relatively cheap. But we can see that this is not true. Why these devices are so expensive? Would it be possible to develop devices with the same capabilities and lower prices? How can I make my low-cost sensor nodes? This dissertation answers these questions and shows some of the many applications that sensor nodes may have. In this dissertation, we propose (and implement in some cases) the development of sensor nodes for environmental monitoring, from low-cost devices. For the implementation of a sensor node and network which joins all these nodes, it is important to know the environment where they will work. Throughout this dissertation, we present the research carried out for the development of sensors in three main application areas. In the first of these areas, we present multisensor devices developed for environmental monitoring. The application of wireless sensor networks to the environment requires a study of how signals are affected depending on the distance, vegetation, ambient humidity, etc. We focus our developments on the fire detection in rural areas and on the control of pests in vineyards where the early detection of these events generates high economic savings. We also propose the development of a sensor network which will help us to reduce and prevent wolves¿ attacks and theft in livestock. Finally, within this group, we present a network to detect material anomalies in building and a sensor network which allows us to monitor the elderly or disabled people who move along with a group on a tour or activity. The second group of applications is related to the monitoring of spaces in indoor environments. For this, we analyze the behavior of wireless signals in different scenarios. These results allowed us to extract a new method for designing wireless networks in indoor environments. Our method allows defining the best location of network devices and sensor nodes indoors saving 15% of the sensors needed. Finally, we present a study on underwater freshwater communications based on electromagnetic waves, where we analyze the dependency of underwater communications as a function of working frequency, temperature, data transfer rates and modulation. Related to underwater environment, we present two proposals. First one refers to the implementation of a sensor network for marine farms which allows us to reduce the amount of waste deposited on the seabed and reduce the percentage of wasted food. The second proposal is the development of two oceanographic sensors which allow us to control the amount of food and feces deposited in seabed and the water turbidity control in a very simple and inexpensive way. All these developments and proposals have been preceded by a comprehensive study on the energy problems in wireless sensor networks. We have also presented several techniques which can be used to prolong the network lifetime and improve its stability.
Sendra Compte, S. (2013). Deployment of Efficient Wireless Sensor Nodes for Monitoring in Rural, Indoor and Underwater Environments [Tesis doctoral]. Editorial Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/32279
Alfresco

On assiste au développement des activités humaines liées au monde océanique, mais aucune norme n'a encore émergé pour l'Internet des objets appliqué aux objets autonomes marins. Bien qu'elle possède une bande passante limitée, l'onde acoustique est le seul moyen de communiquer sur des distances importantes et elle est donc utilisée par de nombreux systèmes sous-marins pour communiquer, naviguer ou déduire des informations sur l'environnement. Cela a conduit à une forte demande de réseaux sans fil qui nécessitent à la fois une bonne efficacité spectrale et énergétique avec la faible complexité des algorithmes associés. Par conséquent, au cours de ce doctorat, nous avons proposé plusieurs solutions originales pour relever le défi de développer des techniques numériques, capables de faire face au canal acoustique.En raison d’une diversité inhérente d'espace du signal (SSD), les constellations tournées permettent de meilleures performances théoriques que les constellations conventionnelles et ce, sans détérioration spectrale. Nous passons en revue les propriétés structurelles des constellations tournées M-QAM uniformément projetées, afin de proposer une technique de demapping souple à faible complexité pour les canaux à fading. Puis, nous proposons une technique originale de réduction du PAPR pour les systèmes OFDM utilisant les constellations tournées. Afin de réduire la complexité du décodage aveugle, nous nous appuyons sur les propriétés des constellations tournées M-QAM uniformément projetées, pour concevoir un estimateur de faible complexité. De plus, pour faire face à la sélectivité du canal acoustique, nous avons proposé un turbo-détecteur parcimonieux adaptatif avec seulement quelques coefficients à mettre à jour afin de réduire la complexité. Enfin, nous avons proposé un algorithme original auto-optimisé pour lequel les tailles de pas de l'égaliseur sont mises à jour de manière adaptative et assistées par des informations souples de manière itérative, afin de répondre à l'exigence de convergence rapide et de faible erreur quadratique sur des canaux variant rapidement dans le temps
There has been recently a large development of human activities associated to the ocean world, where no standard has emerged for the Internet of Things (IoT) linked to marine autonomous objects. Though it has a limited bandwidth, the acoustic wave is the only way to communicate over average to large distances and it is thus used by many underwater systems to communicate, navigate, or infer information about the environment. This led to a high demand for wireless networks that require both spectral efficiency and energy efficiency with the associated low-complexity algorithms. Therefore, in this Ph.D. thesis, we proposed several original solutions to face this challenge.Indeed, due to the inherent Signal Space Diversity (SSD), rotated constellations allow better theoretical performance than conventional constellations with no spectral spoilage. We review the structural properties of uniformly projected rotated M-QAM constellations, so as to propose a low complexity soft demapping technique for fading channels. Then, we present an original blind technique for the reduction of the PAPR for OFDM systems using the rotated constellations with SSD. In order to reduce the complexity of blind decoding for this technique, we again rely on the properties of uniformly projected M-QAM rotated constellations to design a low-complexity estimator. Moreover, to face the selectivity of the acoustic channel, we suggest a sparse adaptive turbo detector with only a few taps to be updated in order to lower down the complexity burden. Finally, we have proposed an original self-optimized algorithm for which the step-sizes of both the equalizer and the phase estimator are updated adaptively and assisted by soft-information in an iterative manner, so as to meet the requirement of fast convergence and low MSE over time-varying channels

Underwater wireless communication refers to transmitting data in unguided water environment through the use of wireless carriers, i.e., radio-frequency wave, acoustic wave, and optical wave. We focus, in this thesis, on the underwater wireless optical communication (UWOC) that employs optical wave as the transmission carriers. In comparison to RF and acoustic counterparts, UWOC has a much higher transmission bandwidth, thus providing much higher data rate. Due to this high-speed transmission advantage, UWOC has attracted considerable attention in recent years. Many potential applications of UWOC systems have been proposed for environmental monitoring, offshore exploration, disaster precaution, and military operations. However, UWOC systems also suffer from severe absorption and scattering introduced by underwater channel. In order to overcome these technical challenges, several new system design approaches, which are different from the conventional terrestrial free-space optical communication, have been explored in recent years. In this thesis, we provide a comprehensive survey of the state-of-the-art of UWOC research in three aspects: channel characterization, channel modulation and coding techniques, and practical implementations of UWOC. Based on the comprehensive understanding of UWOC, we also investigate the outage performance for vertical buoy-based UWOC with pointing errors. Closed-form outage probability with zero boresight pointing errors and outage probability bounds with nonzero boresight pointing errors have been derived.
Applied Science, Faculty of
Engineering, School of (Okanagan)
Graduate

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 61-63).
Though acoustic modems have long been the default wireless communication method for underwater applications due to their long range, the need for high speed communication has prompted the exploration of non-acoustic methods that have previously been overlooked due to their distance limitations. One scenario that drives this need is the monitoring of deep sea oil wells by AUVs that could be stationed at the well and communicate surveillance data wirelessly to a base station. In this thesis, optical communication using LEDs is presented as an improvement over acoustic modems for scenarios where high speed, but only moderate distances, is required and lower power, less complex communication systems are desired. A super bright blue LED based transmitter system and a blue enhanced photodiode based receiver system were developed and tested with the goal of transmitting data at rates of 1 Mbps over distances of at least 10 meters. Test results in a fresh water tow tank showed the successful transmission of large data files over a distance of 13 meters and at transmission rates of at least 3 Mbps. With an improved test environment, even better performance may be possible.
by Heather Brundage.
S.M.

This thesis studies the modelling and characterisation of underwater optical wireless communication links, particularly short-range diffuse links, by using numerical Monte Carlo (MC) simulation. MC simulation provides a flexible, intuitive and accurate modelling of the underwater channel, which is severely affected by absorption and scattering processes. In diffuse Underwater Optical Wireless Communication (UOWC) links, scattering is expected to have a larger impact on communication link performance due to the wider beam divergence compared to collimated beams. Thus, this thesis will investigate the characterisation of path loss, spatial, temporal and angular dispersions of diffuse links in various types of water. Firstly, a detailed investigation on the path loss performance of diffuse beam in three types of water is presented. This includes the study on the contribution of unscattered and scattered components of light to the total received power and how they are attenuated. From the percentage of unscattered light that contributed to the total power reception, the distance at which the unscattered component drops to zero can be estimated. This distance is used to predict the transition point from minimal scattering to multiple scattering regime for diffuse beams in coastal and turbid water. In addition to this, the spatial dispersion effect is also studied at off-axis locations. To further understand the behaviour of scattering in diffuse links, the scattering order probability is evaluated for various beam sizes in various types of water. Currently, this kind of information cannot be obtained either analytically or experimentally. The information on the scattering order is used as the parameter to classify the links into three scattering regimes, namely minimal, intermediate and multiple scattering regimes. Further investigations into the transition regimes are conducted by investigating the impulse response and frequency response performance for temporal dispersion effects. From the impulse response and frequency response analysis, the bandwidth that can be supported by the channel can be predicted, which provides some insight into the potential and limits of the links. In addition to temporal dispersion, the angular dispersion performance is also evaluated. It is shown through the angle of arrival (AOA) distribution that diffuse beams exhibit significant angular dispersions, implying that a large receiver field of view (FOV) is needed for optimum power performance. The information on the AOA distribution is then used to study the impact of receiver FOV on the bandwidth. Finally, the effect of aperture on the power received and scattering order histogram is evaluated. As a conclusion, the numerical results presented in this thesis will provide an improved understanding of the effect of scattering on path loss, spatial, temporal and angular dispersions along with their relationships with each other.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 187-197).
Underwater exploration and surveillance currently relies on subsea cables and tethers to relay data back to the user. The cause for this is that water heavily absorbs most electromagnetic signals, preventing effective radio communication over large distances, and that underwater communication with acoustic signals affords only bit rates on the the order of Kilobits per second. In this thesis we present a novel design and implementation for an underwater data muling system. This system allows for automatic collection of underwater datasets without the need to physically connect to or move the sensors by using mobile robots to travel to the sensors and download the data using wireless optical communication to bring it back to the base station. The system consists of two parts. The first part is a modular and adaptive robot for underwater locomotion in six degrees of freedom. We present a hardware design as well as control algorithms to allow for in-situ deployment without the need for manual configuration of the parameter space. To achieve this we designed a highly parameterizable controller and methods and algorithms for automatically estimating all parameters of this controller. The second part of the data mulling system is a novel high-bandwidth optical underwater communication device. This device allows for transfer of high-fidelity data, such as high-definition video and audio, images, and sensor logs. Finally we present algorithms to control the robots path in order to maximize data rates as it communicates with a sensor while using only the signal strength as a measurement. All components and algorithms of the system have been implemented and tested in the real world to demonstrate the validity of our claims.
by Marek Wojciech Doniec.
Ph.D.

Gateways play a major role in communications over two-hop wireless or hybrid systems. For instance, indoor-outdoor communication networks, satellite communications, and hybrid underwater-wireless communications. It can extend the coverage of cellular systems, and connect the isolated users in rural areas. This thesis considers mainly four different themes. Firstly, the design and performance analysis of a single gateway equipped with a single antenna was investigated over different wireless channel models. The gateway uses random access protocol to connect a random number of independent non-collaborative users. New mathematical expressions were derived for the overall spectral efficiency. Afterwards, the results were extended to include collaborative users in a space division multiple access (SDMA) scenario, where the linear zero-forcing (ZF) beamformer was employed at the gateway. New analytical expressions were derived for the overall spectral efficiency (SE) in both collaborative and non-collaborative SDMA scenarios. In addition to that, different types of fading and shadowing models were considered. Thereafter, a new model for cooperative multiple gateways was introduced and analysed in details for two different scenarios, namely, single and multiple antenna gateway. In this aspect, it was assumed that the getaways are arbitrarily distributed within the service area. Finally, a comprehensive analytical framework for hybrid underwater-wireless communication where the gateway acts as a convert-and-forward relay was developed. New analytical results were derived for the spectral efficiency over different wireless/underwater channel models. The accuracy of the new mathematical results was confirmed by Monte Carlo simulation. The results in our thesis showed that the gateway can greatly enhance the communication systems performance. Moreover, enhancement in spectral efficiency and coverage area extension can be achieved by integrating linear precoder with gateway systems.

CAPES
Nesta Tese de Doutorado propõe-se um modelo de consumo de energia para redes acústicas subaquáticas que leva em consideração as especificidades do ambiente subaquático, como a comunicação utilizando ondas acústicas, a dependência da largura de banda do canal com a perda de percurso, que varia tanto com a distância quanto com a frequência, e o ruído. O desvanecimento, resultado do multipercurso, normalmente modelado pelas distribuições Rayleigh e Rice nas transmissões terrestres, é modelado pela distribuição K, que melhor representa a severidade do ambiente subaquático. O modelo considera uma rede subaquática linear de múltiplos saltos e a possibilidade de retransmissões para calcular a energia total consumida para cada bit de informação transmitido com sucesso entre a fonte e o destino. A fim de obter o menor consumo de energia, a SNR e a frequência de operação também são otimizadas, sendo considerado o uso de códigos convolucionais, cuja taxa ótima que leva ao menor consumo é determinada. Uma análise teórica é desenvolvida para cenários com e sem limitação de atraso. No primeiro caso avalia-se o consumo de energia quando retransmissões não são permitidas ou devem ser limitadas e, portanto, uma FER residual deve ser tolerada. No segundo caso infinitas retransmissões são permitidas até que um pacote seja recebido sem erros. Para ambos cenários o número ótimo de saltos que minimiza o consumo de energia é determinado, e na sequência o impacto do número de tentativas de transmissão é considerado. Resultados numéricos são apresentados, mostrando que o esquema de múltiplos saltos é mais eficiente em termos de consumo de energia que a transmissão direta. Além disso, os resultados mostram que um número pequeno de tentativas de transmissão é suficiente para alcançar uma redução considerável no consumo de energia em redes de múltiplos saltos, limitando o atraso médio por pacote transmitido, o que é muito interessante em aplicações reais.
In this Thesis, an energy consumption model for underwater acoustic networks is proposed. The model takes into account the specificities of the underwater environment, such as the use of acoustic waves for communication, dependence of the underwater acoustic channel bandwidth with the path loss, which varies with both the distance and frequency, and noise. The fading, usually modeled by Rayleigh and Rice distributions on terrestrial communications, in this work, is modeled by the K distribution, which best represents the severity of the underwater environment. The model considers a linear multi-hop underwater network and the possibility of retransmissions to calculate the total energy consumed for each bit of information successfully transmitted between the source and the destination. In order to obtain the minimum energy, the SNR and the operating frequency are also optimized. The use of convolutional codes is considered and the optimal code rate, which leads to the minimum energy consumption, is determined. A theoretical analysis was developed for two scenarios: delay constrained and delay unconstrained networks, indicating the optimal number of hops which minimizes energy consumption. Next, the impact of the number of transmission trials was considered. A numerical analysis was also performed for both the scenarios. The numerical results validate the theoretical analysis, showing that the multi-hop scheme is more efficient in terms of energy consumption when compared to direct transmission. Furthermore, the results show that a small number of transmission trials is sufficient to achieve a considerable reduction in energy consumption in multi-hop networks, limiting the average delay per packet transmitted, which is very interesting in real applications.

Underwater optical wireless communications (UOWC) is an exciting new field that has grasped attention from several research groups worldwide .This this aimed to conduct a profound study of thew most relevant aspects of a UOWC system. A complete simulation framework for various underwater conditions was developed, a study of different modulation formats for free space optical communications was conducted and a functional prototype using state of the art optical components was developed.

Multi-Input Multi-Output (MIMO) principle is based on transmitting digital data from Nt transmitters to Nr receivers within a frequency band. In the last decade, theoretical works and practical experiments in wireless and cellular networks have convincingly proved that MIMO has been a real find in digital communications. Nowadays, MIMO principle is being applied to underwater acoustic communications (UAC) and it is showing encouraging results. But very few research has been done on the relation between different parameters of MIMO and its gain. Our primary purpose in this project is to analyse the MIMO gain theoretically using Shannon capacity analysis and suggest ways to maximize capacity with limited bandwidth by varying other parameters . As we will show in the later pages of this project that underwater acoustic channel (UAC) is highly selective in frequency, data transmission cannot be increased by simply increasing the transmission bandwidth. This difficult situation can be found in wireless communication where there is an increasingly large requirement in high speed data transfer but available bandwidth is constrained by the frequency allocation law. In those communication fields where radio frequency is used , this bandwidth limitation problem has been overcome by introducing MIMO techniques, which provide significant gain in data transmission rate while keeping the transmission Bandwidth constant.In the previous years, the contribution of MIMO to UAC systems has mostly been thoroughly analyzed via spatial modulation or multi-carrier modulation. Initial simulation and experimental results have showcased a larger gain over conventional single input single output (SISO) but the results strongly vary depending on the modulation scheme chosen by us and the receiver algorithm as well as the underwater channel environment.

Now a days, underwater wireless communication is getting lot of popularity. Few years before it was related only to military and defense applications. But now it is used in commercial field also. Design of Underwater wireless communication is very difficult as behavior of water is completely different than air. We have to take lot of parameters into consideration at the time of design. Also, the losses and noise are comparatively very high in underwater than that of in air. Hence long distance communication is practically very difficult to design. The Electromagnetic waves (EM), Magnetic Induction (MI), Optical waves or acoustic waves are the choices for underwater wireless communication systems. Out of these, acoustics are proven to be best underwater although they are limited with bandwidth due to thermal noise. But still acoustics provide communication at large distance comparing to other due to its special property of low attenuation of sound underwater. The currently favorable technology for underwater communication is upon acoustics. In this thesis we present the parameters of acoustic channel, characteristics of acoustics, its mathematics and analysis. Also, we take a look at modulation technique that can be used in transmitter and receiver.

In past few decades, researchers have delivered about great advances in the underwater wireless communication. Underwater communication has found an increasing use in an wide range of applications, such as environmental research, navigation technology via autonomous underwater vehicle (AUV), and civil missions in ocean. Since, electromagnetic waves propagates extremely short distance because of high attenuation, we use acoustic waves. The underwater acoustic channel has unique characteristics due to which there are no good synthetic channel models. Some of these characteristics are low speed of acoustic waves, Doppler distortion in the signal, frequency dependent path attenuation. Due time variability and multipath, estimation of channel is very difficult. In this thesis we consider frequency selective channel modelled by Nakagami-m distribution. There are many noise sources are present in underwater. Hence, standard Gaussian noise model is not applicable for underwater environment conditions. Depending upon the frequency of operation, noise will also change. There are several noise models are proposed, we consider Generalized Gaussian distribution model. Because of the frequency selective nature of the channel, intersymbol interference (ISI) is very high. In order to mitigate the ISI, we use the ZP-OFDM. The speed of acoustic wave is very slow, hence it induces extreme Doppler effect. So, before detecting the received data we need to mitigate the Doppler distortion. In this thesis, we first model the channel by considering Nakagami-m distribution along with GG distribution noise model. Then, we estimated the Doppler scaling factor by appending the preamble and postamble to transmitting signal. Further, we design receiver algorithm when each path having common Doppler scaling factor as well as different Doppler scaling factor. Finally, we incorporate the receiver diversity by employing the maximal ratio combining where we observed improvement in the BER performance.

The underwater communications are essential for the operation and collect large amounts of data (video and images) obtained by Autonomous Underwater Vehicles (AUVs) and remotely controlled underwater vehicles (ROVs) in inspection and monitoring missions at sea. Acoustic waves, despite the high range, allow only narrowband communications, which prevents quick and effective transfer of data. On the other hand, aquatic environment, in particular salt water, severely limits the scope of networks based on electromagnetic waves, having such a range of radius of a few meters. This thesis aims to study and evaluate the use of small vehicles (underwater drones - date mules) capable of transporting data across networks with tolerance to delay (Delay-Tolerant Networks - DTN) between a transmitter and an underwater receiver, taking advantage of high transfer rates at close range. The student must implement a file transfer application that can tolerate high delays in the delivery of information packages. The application will be tested in the underwater environment using the large tank available at INESC TEC and tight cylinder, and compared with simulation results for this scenario. 163/5000 This thesis also presupposes the elaboration of a scientific article for publication in a conference or magazine to disseminate the relevant results of the work.
As comunicações subaquáticas são essenciais para a operação e recolha de grandes quantidades de dados (vídeo e imagens) obtidas por Veículos Autónomos Subaquáticos (AUVs) e por veículos subaquáticos controlados remotamente (ROVs) em missões de inspeção e monitorização no mar. As ondas acústicas, apesar do elevado alcance, permitem apenas comunicações de banda estreita, o que inviabiliza a transferência desses dados de forma rápida e eficiente. Por outro lado, o meio aquático, em especial a água salgada, limita severamente o alcance das redes baseadas em ondas eletromagnéticas, tendo estas um raio de alcance de apenas alguns metros. Nesta tese pretende-se estudar e avaliar a utilização de pequenos veículos (drones subaquáticos - data mules) capazes de transportar dados através de redes tolerantes ao atraso (Delay-Tolerant Networks - DTN) entre um emissor e um recetor subaquático, tirando partido das elevadas taxas de transferência a curto alcance. O estudante deverá implementar uma aplicação de transferência de ficheiros capaz de tolerar elevados atrasos na entrega de pacotes de informação. A aplicação será testada em ambiente subaquático recorrendo ao tanque de grandes dimensões disponível no INESC TEC e a cilindros estanques, e comparada com resultados de simulação para esse cenário. Esta tese pressupõe também a elaboração de um artigo científico para publicação em conferência ou revista para disseminação dos resultados relevantes do trabalho.

The underwater communications are essential for the operation and collect large amounts of data (video and images) obtained by Autonomous Underwater Vehicles (AUVs) and remotely controlled underwater vehicles (ROVs) in inspection and monitoring missions at sea. Acoustic waves, despite the high range, allow only narrowband communications, which prevents quick and effective transfer of data. On the other hand, aquatic environment, in particular salt water, severely limits the scope of networks based on electromagnetic waves, having such a range of radius of a few meters. This thesis aims to study and evaluate the use of small vehicles (underwater drones - date mules) capable of transporting data across networks with tolerance to delay (Delay-Tolerant Networks - DTN) between a transmitter and an underwater receiver, taking advantage of high transfer rates at close range. The student must implement a file transfer application that can tolerate high delays in the delivery of information packages. The application will be tested in the underwater environment using the large tank available at INESC TEC and tight cylinder, and compared with simulation results for this scenario. 163/5000 This thesis also presupposes the elaboration of a scientific article for publication in a conference or magazine to disseminate the relevant results of the work.
As comunicações subaquáticas são essenciais para a operação e recolha de grandes quantidades de dados (vídeo e imagens) obtidas por Veículos Autónomos Subaquáticos (AUVs) e por veículos subaquáticos controlados remotamente (ROVs) em missões de inspeção e monitorização no mar. As ondas acústicas, apesar do elevado alcance, permitem apenas comunicações de banda estreita, o que inviabiliza a transferência desses dados de forma rápida e eficiente. Por outro lado, o meio aquático, em especial a água salgada, limita severamente o alcance das redes baseadas em ondas eletromagnéticas, tendo estas um raio de alcance de apenas alguns metros. Nesta tese pretende-se estudar e avaliar a utilização de pequenos veículos (drones subaquáticos - data mules) capazes de transportar dados através de redes tolerantes ao atraso (Delay-Tolerant Networks - DTN) entre um emissor e um recetor subaquático, tirando partido das elevadas taxas de transferência a curto alcance. O estudante deverá implementar uma aplicação de transferência de ficheiros capaz de tolerar elevados atrasos na entrega de pacotes de informação. A aplicação será testada em ambiente subaquático recorrendo ao tanque de grandes dimensões disponível no INESC TEC e a cilindros estanques, e comparada com resultados de simulação para esse cenário. Esta tese pressupõe também a elaboração de um artigo científico para publicação em conferência ou revista para disseminação dos resultados relevantes do trabalho.

During the past years, there has been an increasing interest in the exploration of underwater wireless communications. This interest has been related mainly to the need for establishing a reliable way of transferring large amounts of data gathered on remote locations in the ocean. This data comes from environmental exploration, oil and gas industries, or marine data from Autonomous Underwater Vehicles (AUVs). These activities require innovative solutions that can provide high bitrates at low costs. With this in mind, and given the current solutions - Optical, Acoustic, and Radio Frequency -, there is the need to create a solution that takes advantage of each technology and overcomes their limitations. In the case of optical communications, they can provide high bitrates, but requires line of sight, and depend significantly on water turbidity. Although acoustic solutions can provide a large range of operation, they have a low bandwidth due to the frequency of operation, and so they are not suitable for transferring high amounts of data. Finally, current radio frequency (RF) solutions allow high bit rates but are limited by the operation range due to the substantial attenuation of electromagnetic waves underwater. With this in mind, it is possible to say that currently, there is no solution for broadband long-range underwater communications. This dissertation aims to develop a solution that allows the increase of throughput and range of underwater wireless communications. To achieve this, a set of underwater data mules will be used. They will take advantage of the high bitrates of RF wireless communications and the long-range associated with acoustic solutions. With this dissertation, communication protocols designed for delay and disruption tolerant networks (DTNs) will be explored, and a protocol that will enable the scheduling of mules will be proposed and implemented, taking advantage of an out-of-band acoustic channel for controlling the mules, and the DTN for data transfer. The solution will be evaluated in a freshwater testbed.

During the past years, there has been an increasing interest in the exploration of underwater wireless communications. This interest has been related mainly to the need for establishing a reliable way of transferring large amounts of data gathered on remote locations in the ocean. This data comes from environmental exploration, oil and gas industries, or marine data from Autonomous Underwater Vehicles (AUVs). These activities require innovative solutions that can provide high bitrates at low costs. With this in mind, and given the current solutions - Optical, Acoustic, and Radio Frequency -, there is the need to create a solution that takes advantage of each technology and overcomes their limitations. In the case of optical communications, they can provide high bitrates, but requires line of sight, and depend significantly on water turbidity. Although acoustic solutions can provide a large range of operation, they have a low bandwidth due to the frequency of operation, and so they are not suitable for transferring high amounts of data. Finally, current radio frequency (RF) solutions allow high bit rates but are limited by the operation range due to the substantial attenuation of electromagnetic waves underwater. With this in mind, it is possible to say that currently, there is no solution for broadband long-range underwater communications. This dissertation aims to develop a solution that allows the increase of throughput and range of underwater wireless communications. To achieve this, a set of underwater data mules will be used. They will take advantage of the high bitrates of RF wireless communications and the long-range associated with acoustic solutions. With this dissertation, communication protocols designed for delay and disruption tolerant networks (DTNs) will be explored, and a protocol that will enable the scheduling of mules will be proposed and implemented, taking advantage of an out-of-band acoustic channel for controlling the mules, and the DTN for data transfer. The solution will be evaluated in a freshwater testbed.

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.

Oceans form about three quarters of our planet Earth, and house immense resources that are critical for future generations. Exploring and monitoring such resources is becoming essential to protect the effected ones by the irresponsible human behavior, and to discover new ones. The limitations of depths mandate the search for alternatives and where human divers become endangered. Using remotely operated vehicles is commonly used for marine explorations, while tethered to ships. To be fully autonomous and to avoid damaging the fragile marine environment, they must be equipped with wireless communication solutions that enable real-time control and feedback on their maneuvering and mobility. Also, the ultimate tool to monitor, inspect and repair underwater structures is to use video streaming to mimic the reality of those unseen parts of the world. Existing underwater communications do not provide the necessary features to transmit video from the deep. Acoustic waves as well as the radio frequency waves are either limited in bandwidths or strongly attenuated by the water medium. On the other hand, wireless optical communication is an emerging technology that provides high transmission speeds and can enable video streaming underwater. This motivates bringing wireless optical technologies for real-time video streaming underwater to a practical implementation by undertaking theoretical and experimental studies of systems and techniques that can provide optimized solutions within our proposed framework. We present our video transmission architecture that facilitates programmable system configurations. Software defined platforms provide us with the means of configuring several setups to test our approach. In order to fully utilize the available optical spectrum, we have additionally implemented several modulation techniques in various laboratory scenarios. Real-time and ultra-high definition video has been successfully demonstrated. The overall system performance and throughput analysis have been provided. A thorough investigation of the system performance under various underwater channel conditions was undertaken. Also, as the delay resulting from queuing, when video packets are waiting for service, is key in time critical applications, we derive the mathematical model and investigate the delay effects and the packet dropping probability on the overall system performance when our setup is extended to a multi-channel configuration.

In underwater wireless optical communications (UWOC), the obstruction of light propagation by air bubbles is one of the main factors which causes light power to fluctuate at the receiver. In this thesis, we construct a statistical model for the received power in the presence of air bubbles. First, we postulate some random variables based on some real experiments, such as the size of a bubble, the generation of each bubble, and the horizontal and vertical movements of a bubble. Second, we mathematically express the amount of obstructed power which the shade of each bubble causes over the beam area and sum them all up to get the total obstructed power. In order to use the method of moments, we find the expectation, the second and/or the third moments of the total obstructed power. Lastly, we use these two or three moments of it to find suitable distributions that match the simulation data, which are the Weibull distribution and Generalized gamma distribution respectively. With these distributions, we construct the statistical model of the received power. Furthermore, we show that those distributions fit well to the simulation data.

Traditionally, noise in communication systems has been modeled as an additive, white Gaussian noise process with independent, identically distributed samples. Although this model accurately reflects thermal noise present in communication system electronics, it fails to capture the statistics of interference and other sources of noise, e.g. in unlicensed communication bands. Modern communication system designers must take into account interference and non-Gaussian noise to maximize efficiencies and capacities of current and future communication networks. In this work, I develop new multi-dimensional signal processing methods to improve performance of communication systems in three applications areas: (i) underwater acoustic, (ii) powerline, and (iii) multi-antenna cellular. In underwater acoustic communications, I address impairments caused by strong, time-varying and Doppler-spread reverberations (self-interference) using adaptive space-time signal processing methods. I apply these methods to array receivers with a large number of elements. In powerline communications, I address impairments caused by non-Gaussian noise arising from devices sharing the powerline. I develop and apply a cyclic adaptive modulation and coding scheme and a factor-graph-based impulsive noise mitigation method to improve signal quality and boost link throughput and robustness. In cellular communications, I develop a low-latency, high-throughput space-time-frequency processing framework used for large scale (up to 128 antenna) MIMO. This framework is used in the world's first 100-antenna MIMO system and processes up to 492 Gbps raw baseband samples in the uplink and downlink directions. My methods prove that multi-dimensional processing methods can be applied to increase communication system performance without sacrificing real-time requirements.
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Underwater wireless optical communication (UWOC) has attracted increasing interest for data transfer in various underwater activities. However, the complexity of the water environment poses considerable challenges to establish aligned and reliable UWOC links. Therefore, solutions that are capable of relieving the requirements on positioning, acquisition and tracking (PAT) are highly demanded. Different from the conventional blue-green light band utilized in UWOC, ultraviolet (UV) light is featured with low solar background noise, non-line-of-sight (NLOS) and good secrecy. The proposed work is directed towards the demonstration and evaluating the feasibility of high- speed NLOS UWOC for easing the strict requirement on alignment, and thus circumvent the issues of scintillation, deep-fading, and complete signal blockage presented in conventional LOS UWOC. This work was first started with the investigation of proper NLOS configurations. Path loss (PL) was chosen as a figure-of-merit for link performance. With the understanding of favorable NLOS UWOC configurations, we established a 377-nm laser-based, the first-of-its-kind NLOS UWOC link. The practicality of such NLOS UWOC links has been further tested in a field trial. Besides the underwater communication links, UV-based NLOS is also appealing to be the link for direct communication across the wavy water-air interface. Investigations for such a direct communication link have been carried out to study data rate, coverage and robustness to the dynamic wave movement, based on the performance of different modulation schemes, including non-return-to-zero (NRZ)-OOK and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM). Further this study, an in-Red Sea canal field in-situ test has been conducted, showing strong robustness of the system. In addition, an in-diving pool drone-aided real-application deployment has been carried on. The trial results indicate link stability, which alleviates the issues brought about by the misalignment and mobility in harsh environments, paving the way towards real applications. Our studies pave the way foreventual applications of UWOC byrelieving the strict requirements on PAT using UV-based NLOS. Such modality is much sought-after for implementing robust, secure, and high-speed UWOC links in harsh oceanic environments.

Tese de doutoramento do Programa Doutoral em Engenharia Eletrónica e de Computadores
Underwater wireless communication systems are becoming a priority in terms of research and technological development due to the increasing demand for exploring the oceans’ potential in areas such as pharmaceutical, oil, minerals, environmental and biodiversity. This demand is increasing exponentially with the need for high data rate and near-real-time communications between submerged mobile and static agents. The existing wireless communication technologies using electromagnetic waves or lasers are not very efficient due to the large attenuation in aquatic environment. Ultrasound reveals a lower attenuation, and thus has been used in underwater long-distance communications. But the underwater acoustic medium is one of the less reliable communication channels which represent major challenges for communications. With relatively slow sound speed propagation (~1500 m/s) the delay may represent a problem for communications with real-time applications. A theoretical model of an underwater communication system was also developed. The model allows to emulate the emitter, the hydrophone and the underwater acoustic channel, which includes attenuation, environmental noise, Doppler Effect, multipath and propagation delay. This model supported the study of wireless communications by emulating the transmission of acoustic signals using different types of digital modulations. The acoustic signal attenuation, multipath, ambient noise in several environments theoretical results were compared to those obtained experimentally. Allowing to conclude that the model represents a suitable approximation to the real subaquatic communication channel for the evaluation of digital acoustic communications. An optimization study of ultrasound transducers for underwater communications was addressed, focusing on a piston type emitter operating in the thickness mode (d33). It was discussed how the acoustic impedance, thickness, resonance frequency and structure affect the transducer performance. This work allowed a better understanding of the emitter transducer characteristics allowing reaching the optimum point of operation for specific applications. Focusing on underwater communication, the transducer was optimized by finite element computer simulations. The results were compared with experimental tests and show that four-layer structures increase up to 16 dB in performance when compared to single-layer transducer disks. For high data-rates and real-time applications it was necessary to develop ultrasound transducers able to work at high frequencies and wideband, with suitable responses to digital modulations. It was thus also included a comparison study that shows how the acoustic impedance influences the performance of an ultrasonic emitter when using different digital modulations and operating at frequencies between 100 kHz and 1 MHz and some tens of meters of distance. It is presented a Finite Element Method (FEM) and a MATLAB/Simulink simulation with an experimental validation to evaluate two types of piezoelectric materials: one based in ceramics (high acoustic impedance) with a resonance design and a polymer based (low acoustic impedance) system, designed to optimize the performance when using digital modulations. The transducers performance for Binary Amplitude Shift Keying (BASK), On-Off Keying (OOK), Binary Phase Shift Keying (BPSK) and Binary Frequency Shift Keying (BFSK) modulations with a 1 MHz carrier at 125 kbps baud rate were compared. The transducers materials used were the ceramics PZT-5H and the polymer PVDF. The results show that PVDF transducer has a better performance to digital modulations than PZT-5H transducer, providing the signal full demodulation for all digital modulations tested. On the other hand, the PZT-5H transducer showed a higher output, but fails to perform accurate modulated signals. Finally, the system was validated by the implementation of a full duplex point-to-point communication at 1 Mbps using OOK modulation with a 1 MHz single carrier. The system was successfully tested in a swimming pool at a distance of 6 meters with a 1 Mbps rate, achieving a 3x10-3 Bit Error Rate (BER) using just 1.4 W of power consumption. These results represent an advance in underwater acoustic communications, being the first practical system to achieve data rates up to 1 Mbps.
O desenvolvimento de sistemas de comunicação subaquáticos sem fios está a tornar-se uma prioridade na comunidade científica no sentido de aumentar o desenvolvimento tecnológico. Este facto deve-se à crescente necessidade de exploração do potencial dos oceanos em áreas científicas diversas como farmacêutica, petrolífera, mineral, ambiental e até do próprio estudo da biodiversidade. Essa necessidade aumenta exponencialmente com a necessidade de comunicações de alto débito e em tempo real entre agentes submersos móveis e estáticos. As tecnologias de comunicações sem fios existentes, nomeadamente as que utilizam ondas eletromagnéticas ou lasers não são muito eficientes, devido, em grande parte, à atenuação no ambiente subaquático. Os ultrassons revelam uma menor atenuação tendo sido, por isso, utilizados em comunicações subaquáticas em longas distâncias. Contudo o canal acústico subaquático definisse como um dos mais difíceis, devido em parte as suas características únicas, o que apresenta ser um enorme desafio. Como a velocidade de propagação do som é relativamente lenta (~1500 m/s), o atraso pode representar um problema para as aplicações em tempo real. Foi desenvolvido um modelo teórico do sistema de comunicações subaquáticos que permite emular o emissor, o hidrofone e o canal acústico subaquático. No canal acústico subaquático foi simulado o efeito da atenuação, ruído ambiente, efeito de Doppler, multipath e atraso de propagação. Este modelo é indicado para o estudo das comunicações subaquáticas, emulando a transmissão de sinais acústicos utilizando diferentes tipos de modulações digitais. Neste estudo foram testados, a atenuação do sinal acústico, multipath, ruído em diversos ambientes e os resultados teóricos foram comparados com os obtidos experimentalmente. Permitindo concluir que o modelo representa uma aproximação adequada do canal de comunicação, permitindo a avaliação das comunicações digitais acústicas. Inclui ainda um estudo de otimização de transdutores de ultrassons para comunicações subaquáticos, tendo como base o emissor do tipo pistão, operando ao longo da espessura (d33). Foi analisada ainda a forma como a impedância, espessura, frequência de ressonância acústica e estrutura afetam o desempenho do transdutor. Este trabalho permitiu uma melhor compreensão das características do transdutor emissor que permitem atingir o ponto ótimo de operação para aplicações específicas. Tendo como base a comunicação subaquática, o transdutor foi otimizado usando os resultados de simulações pelo Método dos Elementos Finitos. Os resultados foram comparados com os testes experimentais, onde se mostra que as estruturas de quatro camadas podem aumentar até 16dB no desempenho quando comparados com discos de transdutor de única camada. Para aplicações em tempo real e de elevado debito, foi necessário desenvolver transdutores de ultrassons capazes de operar em banda larga a altas frequências, com resposta adequada às modulações digitais. Foi, portanto, incluído também um estudo comparativo que mostra como a impedância acústica influencia o desempenho do emissor de ultrassons quando se utilizam modulações digitais a operar com frequências entre 100 kHz e 1 MHz abrangendo distâncias de algumas dezenas de metros. São apresentadas simulações por Método de Elementos Finitos (MEF) e MATLAB/Simulink com validação experimental de modo a avaliar dois tipos de materiais piezoelétricos: um com base cerâmica PZT-5H (alta impedância acústica) com um design de ressonância e outro de base de polimérica PVDF (baixa impedância acústica), otimizado para modulações digitais. O desempenho dos transdutores foi comparado para as modulações: Binary Amplitude Shift Keying (BASK), On-Off Keying (OOK), Binary Phase Shift Keying (BPSK) e Binary Frequency Shift Keying (BFSK) com uma portadora de 1 MHz a 125 kbps. Os resultados mostram que o transdutor de PVDF tem um melhor desempenho do que transdutor PZT-5H, proporcionando a desmodulação completa do sinal para todas as modulações digitais testadas. Por outro lado, o transdutor de PZT-5H mostrou uma potência acústica mais elevada, embora não consiga produzir sinais modulados precisos. Finalmente, o sistema foi validado através da implementação de uma comunicação ponto-aponto bidirecional de 1 Mbps utilizando uma modulação OOK com uma portadora de 1 MHz. O sistema foi testado com sucesso numa piscina a uma distância de 6 metros com uma taxa de 1 Mbps, com um BER (Bit Error Rate) de 3x10-3, utilizando apenas 1,4 W de consumo de potência. Estes resultados representam um avanço nas comunicações acústicas subaquáticas, sendo o primeiro sistema prático de atingir velocidades até 1 Mbps.

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.

碩士
國立臺北科技大學
光電工程系
106
Underwater wireless optical communication (UWOC) has elicited significant interest over the past few years as a provider of high-speed underwater links. Many applications of UWOC systems have been proposed for environmental monitoring, disaster precaution, and underwater oil pipe investigation. To satisfy the requirements of different applications, a high-speed UWOC system with limited underwater link is needed. In this paper, two-stage injection-locked and laser beam expander techniques had been adopted to improve the transmission performance. Nevertheless, these techniques have not been utilized to improve the transmission performance of the UWOC system. Two-stage injection-locked technique exhibits an enhancement in frequency response, and the laser beam expander technique can effectively improve the transmission performance of UWOC systems with highly turbid harbor water link due to the higher amounts of light received by optical receiver. Therefore, we successfully show that a 25-Gbps data stream (NRZ-OOK data format) can be delivered to a maximum of 5-m highly turbid harbor water link. To our understanding, it is the first link test to employ a two-stage injection-locked technique and a laser beam expander in a high-speed 25-Gbps UWOC system. Satisfactory BER performance and clear eye diagram are acquired over a 5-m highly turbid harbor water link.

Now a day though wireless communication technology has turned out to be a part of regular human life, the idea of wireless UW communication system still seems to be fantastical. However, researchers have spent over ten years on structuring the mechanisms for UW wireless information transmission. In the past decades, the most widely technique is used acoustic wireless communication for underwater wireless communication. Acoustic communication is use physical layer technology for UWCNs. It is use for long distance communication [1], [2], [3]. But it has many disadvantages, such as air bubbles in water and temperature gradients are effect on speed of transmission [4]. Acoustic wireless communication has poor performance in shallow water. Underwater wireless communication is useful in pollution monitoring, disaster prevention, offshore exploration and other applications. Wireless communication is a part of our life but under water wireless communication is still a lucrative field. The environment of subsea is challenging for wireless communication because the medium in which waves are propagating is not air, it is propagating through different fractions of water having different densities. Magnetic induction (MI) is a technique which is not affected by multipath propagation, large propagation delays and fading. MI communication can be accomplished with small size coil. MI technique creates constant channel condition and This (MI) technique is very useful in reducing path loss. In this paper, we analysis path loss theoretically and numerically. We also analysis, how to change path loss with electrical conductivity in a day and a year.

The thesis addresses the modelling and control problem of an Autonomous Underwater Vehicle (AUV) that is attached with a 2-link manipulator. Firstly, an INFANTE AUV and 2-link manipulator are modeled. A system comprising of both, an AUV and a manipulator is formed and the effects of different forces acting on them are studied in detail. Further a Model Reference Adaptive Control (MRAC) algorithm is proposed for the Autonomous Underwater Vehicle Manipulator System (AUVMS) for trajectory tracking. Since the number of states of the system increases due to addition of manipulator, a resolution method is applied to remove the redundant states from the system. It may be noted that due to the hydrodynamic effects, the AUVMS experiences a dissipative natured drag force that has no contribution to the motion but consumes much energy thereby reducing the efficiency of the system. Therefore, Steepest Descent method have been employed to reduce the drag on the system. The positive gains for position and velocity are varied by developing an adjustment using MIT Rule. It optimizes the error and subsequently generates the required thruster force to traverse the desired reference path. The manipulator begins to function when the AUV gets stable. The links of the manipulator moves from an initial angular position to a desired angular position based on the control input torque generated by the respective joints of the manipulator. The effectiveness of the simulated result is described in MATLAB and SIMULINK environment. Subsequently, a Model Predictive Controller (MPC) is designed for the position control of the AUV and the manipulator. Force generated by a thruster for an AUV has a practical limit and cannot exceed beyond its defined range.This aspect of imposing constraint over the control input has not been considered in the MRAC algorithm which can be resolved by designing an MPC. A linear model is derived for both the AUV and the 2-link manipulator and further an MPC is developed for controlling the AUVMS. The constraints are set over the thruster force and torque to control the AUV and the manipulator respectively, to maintain a desired fixed position. The performance of the controller is analyzed from the MATLAB and SIMULINK simulation results.

碩士
國立臺灣大學
工程科學及海洋工程學研究所
103
An underwater wireless optical communication system that adopts laser and LED as the transmission medium is demonstrated. It serves as a short-distance and full-duplex wireless communication interface between underwater vehicles and humans in a shallow or narrow water environment where acoustic waves show unsatisfied communication performance. This system has two parts: static node and autonomous underwater vehicle node. They all adopt PIN photodiodes as optical receiver. The former uses the high power LED as its transmitter, and the latter was equipped a structured light system which includes a camera for structured light detection and the auxiliary navigation and control for the connection between the two nodes. These nodes connect each other with optical waves using On-Off Keying modulation method to carry data packets. The packets are the result of improved x-modem protocol that is constructed by Reed-Solomon code and Manchester code to reduce the probability of error bit happening during transmission. Finally, experiments were conducted in a water tank to prove the packet error rates (PER) can be satisfactory and the feasibility of a guidance system for communication.

Autonomous Underwater Vehicles (AUVs) are robots which autonomously carry out desired underwater operations. With the advancements in efficient sensors, actuators and processors, AUVs can be deployed for various applications such as underwater mineral detection, studying life patterns of aquatic plants and animals so that measures can be taken to protect endangered species, scanning deep sea beds for installing underwater pipelines, overhauling and repair of installed pipelines and defence applications which includes carrying as well as detecting warheads. In order to accomplish the aforementioned applications, various sensors are necessary to be installed with the AUV. Thus, deploying a group of AUVs with a distributed sensor configuration, helps in reducing the payloads on a single AUV. Moreover, when AUVs are deployed in a group, larger areas can be scanned in less time and it also makes the mission redundant in case a single AUV fails. There are various approaches of formation control such as behaviour based, synchronization based, virtual structure based and leader-follower based approach. The leader-follower approach is one of the most widely used formation control approaches owing to its simple structure. In this approach, the leader AUV states are sent to the follower AUVs for maintaining the formation. These states are transferred in underwater via acoustic modem. However, the low data rate of acoustic medium introduces communication delay into the AUV motion control system. This delay varies as the distance between the leader and follower AUVs varies with the ocean currents. In the literature [1, 2], most often, this delay is minimized by either transferring least number of AUV states or accurately estimating these states. However, the communication delay can still increase as well as vary significantly due to change in distance between the AUVs with the ocean current. Hence, in this thesis, a leader-follower approach is considered for studying the effects of this variable communication delay on the performance of follower AUVs. Optimal control facilitates path following by optimizing the error between the desired and actual AUV positions [3, 4]. Moreover, when this control is employed for formation control while considering the communication delay among multiple AUVs, it also provides an optimal control law which can handle the effects of delay along with ensuring smooth path following during formation control. Thus, in the thesis, an optimal kinematic control is developed to deal with the communication delay during formation control. In this approach, a linear augmented system comprising the kinematics of both leader as well as the follower AUVs is constructed. This co-operative AUV system also takes into consideration the leader AUV states received by the follower AUVs. Further, an optimal control law is derived for both the leader and follower AUVs. By using these optimal gains, the leader AUV converge to the desired path and also the follower AUVs maintain the formation under the effects of communication delay without yielding oscillations. The performance of the optimal control scheme is compared with a Linear Quadratic Regulator (LQR) based formation controller. The results clearly show that the follower AUVs in case of proposed controller follows the path without oscillations, whereas, in case of LQR, for a chosen set of Qlqr and Rlqr matrices, the response is oscillatory. The simulation results also show that though the steady state error occurs in case of the proposed controller, it is significantly less than LQR. To minimize the aforesaid steady-state error, a convex optimization based modified constrained adaptive control is then proposed in the thesis considering non-linear kinematics and dynamics of AUV. In this approach, the cost function comprising communication delay information based position and velocity error equations is minimized in every iteration. Thus, the effect of communication delay is considered in every sampling time and adaptive control is developed to deal with these effects. This proposed controller also resolves the issue of actuator saturation. However, the results in this case are compared to a backstepping control based formation controller. The backstepping controller is one of the most widely adopted control paradigms for non-linear systems as it considers all the system non-linearities while calculating the control input. Moreover, as backstepping is a Lyapunov function based approach, it also guarantees asymptotic stability of the system. Nevertheless, from the results, it is clearly evident that, under the effects of communication delay, in case of the constrained adaptive control, the AUVs follow the desired path within an accuracy of 5cm, thereby reducing the steady-state error. But in case of backtepping control, with stamping based technique is used in this approach to determine the communication delay. In this technique, the packet of leader AUV states also contains time information at which it is sent. When this packet is received by the follower AUV, the time at which it is received is also noted. The time-difference between these two time-stamps gives the actual delay which is used for controller design. This technique not only increases the packet-size, which subsequently increases the communication delay, it also necessitates that all the clocks pertaining to every AUV to be synchronized. To address increased communication delay and clock synchronization issues, next, a gradient descent method based delay estimator is designed to handle the effects of the time stamping of every sent packet of the leader AUV states. It also does not necessitate time synchronization of AUV system clocks. This estimator is employed along with the developed constrained adaptive controller to reduce the delay and achieve the desired formation control performance. The results show that the gradient-descent method based estimator accurately estimates the communication delay. Moreover, employing the proposed estimator also helps in significantly reducing the communication delay occurring in each sampling-time. The formation control of AUVs considering small or negligible communication delay has been widely studied in the available literature. However, in this thesis, a leader-follower approach based formation control of AUVs under the effects of significantly large variable communication delay has been investigated. Simulations were conducted by implementing a group of a leader AUV and two follower AUVs using MATLAB/Simulink. In case of all the aforementioned proposed algorithms, it is observed that, the AUVs follow the desired formation with accuracy and without oscillations. To verify the efficiency of all the developed control schemes for formation of AUVs, in real-time, an experimental setup is developed. A torpedo-shaped prototype AUV is designed for implementation of proposed control algorithms. Moreover, the hydrodynamic parameters of the developed prototype AUV are also determined. Since, in the proposed constrained adaptive technique which is employed to handle varying communication delay between the AUVs, the determination of the hydrodynamic parameters a priori helps in minimizing the computational burden. Moreover, the determined parameters can also be used for controller design in the future applications. Furthermore, a virtual AUV is considered as the leader AUV and the prototype AUV with Robot Operating System which supports python based programming is employed as the follower AUV. The experimental results also support the observations from the simulation studies. In case of all the aforesaid proposed algorithms such as optimal and constrained adaptive control, the follower AUV follows the desired orientation, under communication delay without oscillations. Moreover, the designed estimator helps in estimating the communication delay accurately and also helps in reducing the delay which occurs due to time-stamping. Thus, from the results it is evident that for small linearization errors or slowly changing paths (as the linearization errors depend on angular velocity of the desired path), the proposed optimal kinematic control law can be employed to deal with communication delay during formation control. Since, the optimal control gains in this case are calculated off-line, it helps in reducing the computation burden, while achieving smooth formation between the AUVs. As the linearization error increases, the constrained adaptive control can be employed for the aforementioned objective. This control adapts on-line to the variable communication delay occurring during formation control and also helps in avoiding actuator saturation. This proposed control being adaptive in nature, still computes the control law within the considered sampling time, thereby adhering to the limit of computation time. Moreover, this adaptive control law when employed along with the gradient descent method based estimator helps in accurately estimating and reducing the communication delay by avoiding time-stamping. However, based on these aforesaid advantages, it is evident that, with the advancements in the processor speed, the adaptive control algorithm offers better performance and the delay estimator can appropriately replace time-stamping technique.

In scuba diving any problem that can be solved underwater will be solved underwater. This helps to prevent a dive from being disrupted. If a diver is separated from the group and is unable to find the group within a short time both the diver and dive group must surface to find each other and rejoin. To prevent the separation of divers a Stray Diver Alert System has been devised involving wireless communication to track the diver's position relative to the dive masters. Underwater communication holds many challenges that are not found in above water networks. Through simulation, it has been shown that the communication requirements for the Stray Diver Alert can be met with existing technology and protocols. This has been done by evaluating the resolution, power consumption and physical size of the device for three different communication protocols. This has shown that current technology is capable of meeting the requirements of the stray diver alert system.

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.

碩士
國立成功大學
系統及船舶機電工程學系碩博士班
97
The RF (radio frequency) technique utilizes radio-frequency waves to transmit messages. Radio frequency usually means the radio-wave frequency is less than 300GHz. In this study, RF wireless technologies are applied to network communications and the control of underwater mini-vehicles. The RFID (radio frequency identification) system is a typical integration of RF technologies and network communications. In the first part of this study, the data are collected by the RFID system and are then stored in the Visual Basic software based database. A WAP server is constructed so that the cell phone can access the content of the database through the GPRS and internet. In the second part of this study, the RF technology is applied to the control of underwater mini-vehicles. The TG-11 wireless module is installed in an underwater mini-vehicle to serve as the transmitter and receiver. This wireless module will control the motor to drive the underwater mini-vehicle toward the forward, backward, left and right directions.

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.

Harvesting energy became one of the most prominent research topics around the world, not only for research institutes and universities but also for technology companies as well. Mainly focused on internet of things (IoT) applications, harvesting energy is a crucial factor for reducing costs that come with the use of batteries and increasing the devices’ working time. Simultaneous lightwave information and power transfer is a technique that seeks to use wireless optical communication to achieve both fundamental objectives in modern communication systems. This work presents the main techniques that are used to achieve SLIPT, a novel circuit that improves the standard methods and applications employing this circuit.