Underwater wireless communications refer to data transmission in unguided water environment through wireless carriers, i.e., radio-frequency (RF) wave, acoustic wave, and optical wave. In comparison to RF and acoustic counterparts, underwater optical wireless communication (UOWC) can provide a much higher transmission bandwidth and much higher data rate. Therefore, we focus, in this paper, on the UOWC that employs optical wave as the transmission carrier. In recent years, many potential applications of UOWC systems have been proposed for environmental monitoring, offshore exploration, disaster precaution, and military operations. However, UOWC systems also suffer from severe absorption and scattering introduced by underwater channels. In order to overcome these technical barriers, several new system design approaches, which are different from the conventional terrestrial free-space optical communication, have been explored in recent years. We provide a comprehensive and exhaustive survey of the state-of-the-art UOWC research in three aspects: 1) channel characterization; 2) modulation; and 3) coding techniques, together with the practical implementations of UOWC.

A Survey of Underwater Optical Wireless Communications

Visible Light Communication (VLC) is an emerging field in Optical Wireless Communication (OWC) which utilizes the superior modulation bandwidth of Light Emitting Diodes (LEDs) to transmit data. In modern day communication systems, the most popular frequency band is Radio Frequency (RF) mainly due to little interference and good coverage. However, the rapidly dwindling RF spectrum along with increasing wireless network traffic has substantiated the need for greater bandwidth and spectral relief. By combining illumination and communication, VLC provides ubiquitous communication while addressing the shortfalls and limitations of RF communication. This paper provides a comprehensive survey on VLC with an emphasis on challenges faced in indoor applications over the period 1979–2014. VLC is compared with infrared (IR) and RF systems and the necessity for using this beneficial technology in communication systems is justified. The advantages of LEDs compared to traditional lighting technologies are discussed and comparison is done between different types of LEDs currently available. Modulation schemes and dimming techniques for indoor VLC are discussed in detail. Methods needed to improve VLC system performance such as filtering, equalization, compensation, and beamforming are also presented. The recent progress made by various research groups in this field is discussed along with the possible applications of this technology. Finally, the limitations of VLC as well as the probable future directions are presented.

LED Based Indoor Visible Light Communications: State of the Art

Recent technological developments have led to the production of inexpensive, non-invasive, miniature magneto-inertial sensors, ideal for obtaining sport performance measures during training or competition. This systematic review evaluates current evidence and the future potential of their use in sport performance evaluation. Articles published in English (April 2017) were searched in Web-of-Science, Scopus, Pubmed, and Sport-Discus databases. A keyword search of titles, abstracts and keywords which included studies using accelerometers, gyroscopes and/or magnetometers to analyse sport motor-tasks performed by athletes (excluding risk of injury, physical activity, and energy expenditure) resulted in 2040 papers. Papers and reference list screening led to the selection of 286 studies and 23 reviews. Information on sport, motor-tasks, participants, device characteristics, sensor position and fixing, experimental setting and performance indicators was extracted. The selected papers dealt with motor capacity assessment (51 papers), technique analysis (163), activity classification (19), and physical demands assessment (61). Focus was placed mainly on elite and sub-elite athletes (59%) performing their sport in-field during training (62%) and competition (7%). Measuring movement outdoors created opportunities in winter sports (8%), water sports (16%), team sports (25%), and other outdoor activities (27%). Indications on the reliability of sensor-based performance indicators are provided, together with critical considerations and future trends.

Trends Supporting the In-Field Use of Wearable Inertial Sensors for Sport Performance Evaluation: A Systematic Review

There is no escaping fact that a huge amount of unexploited resources lies underwater which covers almost 70% of the Earth. Yet, the aquatic world has mainly been unaffected by the recent advances in the area of wireless sensor networks (WSNs) and their pervasive penetration in modern day research and industrial development. The current pace of research in the area of underwater sensor networks (UWSNs) is slow due to the difficulties arising in transferring the state-of-the-art WSNs to their underwater equivalent. Maximum underwater deployments rely on acoustics for enabling communication combined with special sensors having the capacity to take on harsh environment of the oceans. However, sensing and subsequent transmission tend to vary as per different subsea environments; for example, deep sea exploration requires altogether a different approach for communication as compared to shallow water communication. This paper particularly focuses on comprehensively gathering most recent developments in UWSN applications and their deployments. We have classified the underwater applications into five main classes, namely, monitoring, disaster, military, navigation, and sports, to cover the large spectrum of UWSN. The applications are further divided into relevant subclasses. We have also shown the challenges and opportunities faced by recent deployments of UWSN.

https://journals.sagepub.com/doi/pdf/10.1155/2015/896832

Underwater sensor network applications: a comprehensive survey

Aspects of the subject disclosure may include, for example, a coupling module that includes a waveguide that guides a first electromagnetic wave conveying data from a transmitting device. A dielectric coupler receives the first electromagnetic wave from the waveguide to form a second electromagnetic wave, and that guides the second electromagnetic wave along the dielectric coupler adjacent to a transmission medium, and wherein the dielectric coupler has a length that supports a cancellation of at least one cancelled wave mode from a coupling of the second electromagnetic wave to the transmission medium.

Launcher and coupling system for guided wave mode cancellation

The velocity of a swimmer can be determined from the stroke rate and the stroke length or by integrating the forward acceleration. In competitive swimming, these parameters are very important for race planning. This paper presents a wrist mounted accelerometer and optical wireless communications to display goggles to give real time feedback to a swimmer during swimming. The system data rate is 2.4 kbps ON-OFF keying modulation for the optical wireless signal. The system uses visible light communication in the green-blue wavelength. Design challenges include interference from bubbles and strong background light. The final device is low cost with low power consumption and small size. Intra-stroke transmit times are scheduled using the acceleration sensor data. Experiments are conducted in air and under water for this system to optimize the link availability. Algorithms for finding the absolute maximum of the y-axis acceleration for each stroke cycle and the goggles display decision are implemented at the transmitter and the receiver, respectively. Hardware, software, and implementation modifications to improve the system are successfully tested.

Self Contained Adaptable Optical Wireless Communications System for Stroke Rate During Swimming

This paper reports 3 axis accelerometer data transfer over a one meter underwater path at 10 cm depth using a 2400 bps optical wireless frequency shift keying (FSK) at very low frequency (VLF). The modulation frequencies used were 10 and 12 KHz. The prototype modem was designed and implemented for real time feedback for swimmers in the pool. The optical transmitter included an accelerometer unit with a microcontroller, the modulator and a detector circuit based on an integrated detector preamplifier (IDP). The cost of the components for the optical transmitter and receiver was less than AU$25. Range experiments were performed in air and underwater, with and without bubbles. The received data was error free for 1.3 m in air and for more than 1.1 m underwater without bubbles. The underwater range decreased to 70 cm with bubbles. The availability of the link between the wrist and head of a swimmer was approximately 50% and varied with the position of the wrist. This enables stroke rate data to be presented to the swimmer via a goggle mounted display.

Low-cost short -range wireless optical FSK modem for swimmers feedback

An underwater optical link is proposed for communication between sensors on a swimmer during training.Tests in still-pool water, still-spa water, and bubbles-spa water demonstrated the attenuation of the signal. The system uses a light emitting device (LED) (520 nm) and optical detector and is operating at 520 nm. The attenuation constant was found to increase from 0.09 to 2.9 m−1 when the spa jets were turned on. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:729–732, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26664

The effect of air bubbles on an underwater optical communications system for wireless sensor network applications

Common tasks of High-Frequency Surface Wave Radars (HFSWRs) are long-range ocean state monitoring and maritime surveillance with a strong focus on the detection of ships. Due to the heterogeneous background composed of sea-clutter and external noise the application of Constant False Alarm Rate (CFAR) algorithms with a single parameter set are likely to lead to a high probability of false alarm or poor detection performance. This paper is about adaptive CFAR with presegmentation, where the presegmentation is performed globally on each range-Doppler map and divides the detection background into external noise dominated regions and sea-clutter dominated regions. With this global knowledge it is possible to individually adapt the shape of the reference window for each Cell Under Test (CUT) to obtain homogeneous reference cells and avoid clutter-edges in the reference window. To further increase detection performance, the constant scale factor is chosen with respect to the current background. This enables detection of small targets in clutter while maintaining a low false-alarm rate for targets in external noise. To prevent the saturation of the tracker, a pretracker structure is presented which distinguishes between strong and weak detections and assigns priority to strong detections.

Presegmentation-based adaptive CFAR detection for HFSWR

Effective short-distance communication of real-time data from sensors on the body of a competitive swimmer to a head-up display faces a number of environmental problems. Presented are the benefits of utilising IrDA transceivers for wireless optical underwater communications using timing jitter as a measure of the link performance looking at the impact of air bubbles and transmission through the water surface. A number of conclusions are drawn for improving the link performance without increasing the limited power budget of battery-driven devices.

Thomas Fickenscher

Papers

Self Contained Adaptable Optical Wireless Communications System for Stroke Rate During Swimming

Low-cost short -range wireless optical FSK modem for swimmers feedback

The effect of air bubbles on an underwater optical communications system for wireless sensor network applications

Presegmentation-based adaptive CFAR detection for HFSWR

Underwater wireless optical communication for swimmer feedback using IrDA transceiver