Underwater Optical Wireless Communications in Swarm Robotics: A Tutorial

Peter A. Hoeher, Jan Sticklus, Andrej Harlakin
2021 IEEE Communications Surveys and Tutorials  
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information : DOI 10.1109/COMST.2021 this problem is easier to tackle. Since our focus is on AUV swarms and therefore on mobility, however, in the remainder only LED-based solutions will
more » ... e explored. Regarding underwater laser communications, the interested reader is referred to [27]-[29] and related papers. Besides ultrasonic and optical communication, electromagnetic waves sometimes are proposed for wireless underwater communication [30] , [31] . Electromagnetic (EM) waves are radiated by antennas, i.e., the far-field regime d λ is of interest. With ultra long-wavelength radio (< 3 kHz) significant distances can be covered, at the expense of huge antenna sizes, low data rates, and high transmit powers, which is beyond the scope of AUV swarms. The low frequency (30 − 300 kHz), medium frequency (0.3 − 3 MHz), and high frequency (3 − 30 MHz) ranges are easier handable, but the attenuation of the medium increases with frequency in this regime. At radio frequencies used for cellular radio (> 800 MHz) and Wi-Fi (2.4/5 GHz), the penetration of electromagnetic waves in seawater is a few centimeters only [30] . For this reason, in shallow waters sometimes the air wave is exploited for range extension [31] . For the sake of completeness, it should be mentioned that although GHz radio frequency waves are not suitable as a communication medium for submerged AUVs, radio communication with an escort ship or satellites is frequently used when surfaced. An alternative to electromagnetic waves is magneto-inductive (MI) communication, which is traditionally based on a modulated magnetic field emitted by a transmitter coil and picked-up by a receiver coil [32], [33] . Magneto-inductive communication operates in the near-field d < λ, i.e., in the non-radiating regime around the radiator. Hence, by definition MI communication is (like UOWC) a near-range technique. In the non-radiating regime, signal fading does not occur because waves cannot superimpose destructively. This is a distinctive difference compared to sound and EM waves. A decisive advantage magnetic fields have over acoustic waves is the high underwater propagation speed, which is similar to that of UOWC. Compared to UOWC, key advantages are the independence of visibility and the low sensitivity to water turbidity. MI communication is the only technology which is capable to simultaneously operate below and above the water surface or above and below the ground. The main disadvantage is the strong signal attenuation in salty water. Recently, in [34]-[36] a novel approach based on magnetic fields was proposed, where the receiver coil is replaced by a high-sensitivity wideband low-noise magnetic field detector, for instance an anisotropic magneto-resistive (AMR) sensor. Compared to conventional MI communication systems employing two coils, an additional advantage is that this family of sensors is small, lightweight, and offers a high bandwidth. This allows for a streamlined integration of both transmitter and receiver into the AUV, since the transmitter coil can also be integrated into non-metal hulls [35] . Electric currents have been proposed as an alternative to the use of EM or magnetic fields for digital underwater communications in [37] . Instead of antennas or coils, electrodes are used. This technology is bio-inspired by fish using weak electric fields for electrocommunication and sensing [38] . Since all mentioned wireless communication media have pros and cons, heterogeneous networking employing hybrid communication is a favourable strategy. For example, acoustic communication may serve as a wide-area umbrella cell, complemented by optical and/or MI communication in nearrange links. Focus, however, will subsequently be on UOWC. Among all media suitable for wireless underwater communications, UOWC systems offer the largest bandwidth and channel capacity needed for high-speed communication in swarm networks [39] , [40] . Besides UOWC between AUVs and between AUVs and underwater sensors, wireless light communication may also replace some of the cabling inside the AUV hull in order to save weight and cost. C. Related Overview Articles
doi:10.1109/comst.2021.3111984 fatcat:dbu6v37agnaa7o46emq4qjjc4q