View source for Underwater acoustic communication

{{Short description|Wireless technique of sending and receiving messages through water}}
[[File:Vixar3.jpg|bottom|thumb|300px|Example of multi-path [[radio propagation|propagation]]]]
'''Underwater acoustic communication''' is a technique of sending and receiving messages in water.<ref>I. F. Akyildiz, D. Pompili, and T. Melodia, "Underwater Acoustic Sensor Networks: Research Challenges," Ad Hoc Networks (Elsevier), vol. 3, no. 3, pp. 257-279, March 2005.</ref> There are several ways of employing such communication but the most common is by using [[hydrophone]]s. Underwater communication is difficult due to factors such as multi-path [[radio propagation|propagation]], time variations of the channel, small available bandwidth and strong [[attenuation|signal attenuation]], especially over long ranges. Compared to terrestrial communication, underwater communication has low data rates because it uses [[acoustic wave]]s instead of [[electromagnetic waves]].

At the beginning of the 20th century some ships communicated by underwater bells as well as using the system for navigation. [[Submarine signals]] were at the time competitive with the primitive maritime [[radionavigation]].<ref>{{Cite web|title = Submarine Signaling on Steamships|url = https://www.ggarchives.com/OceanTravel/InterestingFacts/1905-10-SubmarineSignaling.html|website = www.ggarchives.com|access-date = 2016-01-18}}</ref> The later [[Fessenden oscillator]] allowed communication with submarines.

== Types of modulation used for underwater acoustic communications ==
{{Generalize|section|date=January 2015}}
{{anchor|modulation}}In general the modulation methods developed for radio communications can be adapted for underwater acoustic communications (UAC). However some of the modulation schemes are more suited to the unique underwater acoustic communication channel than others. Some of the modulation methods used for UAC are as follows:

* [[Frequency-shift keying]] (FSK)
* [[Phase-shift keying]] (PSK)
* [[Frequency-hopping spread spectrum]] (FHSS)
* [[Direct-sequence spread spectrum]] (DSSS)
* [[Frequency modulation|Frequency]] and [[pulse-position modulation]] (FPPM and PPM)
* [[Multiple frequency-shift keying]] (MFSK)
* [[Orthogonal frequency-division multiplexing]] (OFDM)

The following is a discussion on the different types of modulation and their utility to UAC.

=== Frequency-shift keying ===
FSK is the earliest form of modulation used for acoustic modems. FSK usually employs two distinct frequencies to modulate data; for example, frequency F1 to indicate bit 0 and frequency F2 to indicate bit 1. Hence a binary string can be transmitted by alternating these two frequencies depending on whether it is a 0 or 1. The receiver can be as simple as having analogue matched filters to the two frequencies and a level detector to decide if a 1 or 0 was received. This is a relatively easy form of modulation and therefore used in the earliest acoustic modems. However more sophisticated demodulator using [[digital signal processor]]s (DSP) can be used in the present day.

The biggest challenge FSK faces in the UAC is multi-path reflections. With multi-path (particularly in UAC) several strong reflections can be present at the receiving hydrophone and the threshold detectors become confused, thus severely limiting the use of this type of UAC to vertical channels. Adaptive equalization methods have been tried with limited success. Adaptive equalization tries to model the highly reflective UAC channel and subtract the effects from the received signal. The success has been limited due to the rapidly varying conditions and the difficulty to adapt in time.

=== Phase-shift keying ===
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave). The signal is impressed into the magnetic field x,y area by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs , RFID and Bluetooth communication.

=== Orthogonal frequency-division multiplexing ===
Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channels by incorporating closely spaced orthogonal sub-carrier signals.

OFDM is a favorable communication scheme in underwater acoustic communications thanks to its resilience against frequency selective channels with long delay spreads.<ref>E. Demirors, G. Sklivanitis, T. Melodia, S. N. Batalama, and D. A. Pados, "Software-defined Underwater Acoustic Networks: Toward a High-rate Real-time Reconfigurable Modem," IEEE Communications Magazine, vol. 53, no. 11, pp. 64 – 71, November 2015.</ref><ref>S. Zhou and Z.-H. Wang, OFDM for Underwater Acoustic Communications. John Wiley and Sons, Inc., 2014.</ref><ref>E. Demirors, G. Sklivanitis, G.E. Santagati, T. Melodia, and S. N. Batalama, "Design of A Software-defined Underwater Acoustic Modem with Real-time Physical Layer Adaptation Capabilities," in Proc. of ACM Intl. Conf. on Underwater Networks & Systems (WUWNet), Rome, Italy, November 2014.</ref>

== Use of vector sensors ==
{{anchor|vector sensor}}
Compared to a scalar pressure sensor, such as a hydrophone, which measures the scalar acoustic field component, a vector sensor measures the vector field components such as acoustic particle velocities. Vector sensors can be categorized into inertial and gradient sensors.<ref>{{cite conference|first =T. B.|last = Gabrielson|title =Design problems and limitations in vector sensors|conference= Workshop on Directional Acoustic Sensors |location = Newport, RI|date = 2001|url = https://apps.dtic.mil/sti/pdfs/ADA633245.pdf|page = 29}}</ref>

Vector sensors have been widely researched over the past few decades.<ref>Proc. AIP Conf. Acoustic Particle Velocity Sensors: Design, Performance, and Applications, Mystic, CT, 1995.</ref><ref name="Nehorai">A. Nehorai and E. Paldi, “Acoustic vector-sensor array processing,” IEEE Trans. Signal Process., vol. 42, pp. 2481–2491, 1994.</ref> Many vector sensor signal processing algorithms have been designed.<ref>K. T. Wong & H. Chi, "Beam Patterns of an Underwater Acoustic Vector Hydrophone Located Away from any Reflecting Boundary," IEEE Journal of Oceanic Engineering, vol. 27, no. 3, pp. 628-637, July 2002.</ref>

Underwater vector sensor applications have been focused on sonar and target detection.<ref name="Nehorai" /> They have also been proposed to be used as underwater multi‐channel communication receivers and equalizers.<ref>A. Abdi and H. Guo, “A new compact multichannel receiver for underwater wireless communication networks,” IEEE Trans. Wireless Commun., vol. 8, pp. 3326‐3329, 2009.</ref> Other researchers have used arrays of scalar sensors as multi‐channel equalizers and receivers.<ref>T. C. Yang, “Temporal resolutions of time-reversal and passive phase conjugation for underwater acoustic communications,” IEEE J. Oceanic Eng., vol. 28, pp. 229–245, 2003.</ref><ref>M. Stojanovic, J. A. Catipovic, and J. G. Proakis, “Reduced-complexity spatial and temporal processing of underwater acoustic communication signals,” J. Acoust. Soc. Am., vol. 98, pp. 961–972, 1995.</ref>

== Applications ==

=== Underwater telephone ===
The underwater telephone, also known as UQC, AN/WQC-2, or Gertrude,  was used by the [[U.S. Navy]] in 1945 <ref>{{cite news|author1=Quazi, A.|author2=Konrad, W.|date=March 1982|title=Underwater Acoustic Communications |pages=24–29|work=IEEE Comm Magazine}}</ref> after in Kiel, Germany, in 1935 different realizations at sea were demonstrated.<ref>{{cite news|author1=Nissen, I.|date=March 2017|title=GERTRUDE - 80 years underwater telephony|pages=1–13|work=DAGA 2017| url=https://www.researchgate.net/publication/361801155}}</ref> The terms UQC and AN/WQC-2 follow the nomenclature of the [[Joint Electronics Type Designation System]].<ref>{{Cite web |title=Acoustic influence of underwater mobile survey vehicles on the soundscape of Pacific rockfish habitat |url=https://pubs.aip.org/asa/jasa/article/146/1/EL45/997200/Acoustic-influence-of-underwater-mobile-survey |access-date=2023-07-03 |website=pubs.aip.org}}</ref> The type designation "UQC" stands for <code>General Utility (multi use)</code>, <code>Sonar and Underwater Sound</code> and <code>Communications (Receiving/Transmitting, two way)</code>. The "W" in WQC stands for <code>Water Surface and Underwater combined</code>. The underwater telephone is used on all crewed [[submersible]]s and many Naval surface ships in operation. Voice or an audio tone (morse code) communicated through the UQC are [[heterodyne]]d to a high pitch for acoustic transmission through water.<ref>{{cite web|website=Discovery of Sound in the Sea |url=https://dosits.org/people-and-sound/communication/how-is-sound-used-to-communicate-underwater/|title = How is sound used to communicate underwater?|publisher = University of Rhode Island|date = 2021}}</ref>
{{expand section|Scuba diver wireless communications, closed diving bell emergency communications |date=May 2022}}

=== JANUS ===
In April 2017, NATO's [[Centre for Maritime Research and Experimentation]] announced<ref>{{cite web | url = https://www.nato.int/cps/bu/natohq/news_143247.htm | title = A new era of digital underwater communications | date = 2017-04-27 | publisher = NATO }}</ref> the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound (like [[modem]]s and [[fax]] machines do over telephone lines).<ref>{{cite web | url = http://www.januswiki.com/tiki-index.php | title = JANUS Community Wiki }}</ref> Documented in [[Standardization Agreement|STANAG]] [https://nso.nato.int/nso/nsdd/main/standards?search=ANEP-87 4748], it uses 900&nbsp;Hz to 60&nbsp;kHz frequencies at distances of up to {{convert|28|km|mi}}.<ref>{{cite web | url = https://www.linux.com/blog/event/elce/2017/internet-underwater-things-open-source-janus-standard-undersea-communications | title = The Internet of Underwater Things: Open Source JANUS Standard for Undersea Communications | date = 2017-08-15 | first = Eric | last = Brown | work = Linux.com | publisher = The Linux Foundation }}</ref><ref>{{cite web | url = https://robohub.org/janus-creates-a-new-era-for-digital-underwater-communications/ | title = JANUS creates a new era for digital underwater communications | date = 2017-05-04 | first =  Francesca | last = Nacini | work = Robohub}}</ref> It is available for use with military and civilian, NATO and non-NATO devices; it was named after the [[Janus|Roman god]] of gateways, openings, etc.

The JANUS specification ([https://nso.nato.int/nso/nsdd/main/standards?search=ANEP-87 ANEP-87]) provides for a flexible plug-in-based payload scheme. A baseline JANUS packet consists of 64 bits to which further arbitrary data (Cargo) can be appended.<ref>{{Cite web |date=March 2017 |title=ANEP-87 Ed: A, Ver. 1, DIGITAL UNDERWATER SIGNALLING STANDARD FOR NETWORK NODE DISCOVERY & INTEROPERABILITY |url=https://nso.nato.int/nso/nsdd/main/standards?search=ANEP-87 |website=NATO Standardization Office}}</ref> This enables multiple different applications such as Emergency location, Underwater AIS (Automatic Identification System), and Chat. An example of an Emergency Position and Status message is the following [[JSON]] representation:<ref name=":1">{{cite web |title=Popoto User's Guide PMM5021 |url=https://www.popotomodem.com/static/fc5c5c6e4a70daa0d4de18fd16fe0d1a/PMM5021UsersGuide.pdf |page=44}}</ref><ref>{{Cite web |title=JANUS Community Wiki {{!}} Class User Id: 002 Underwater AIS |url=http://www.januswiki.com/tiki-index.php?page=Class+User+Id%3A+002+Underwater+AIS |access-date=2023-07-15 |website=JANUS Community Wiki |language=en}}</ref><syntaxhighlight lang="json">
{
   "ClassUserID":0,
   "ApplicationType":3,
   "Nationality":"PT",
   "Latitude":"38.386547",
   "Longitude":"-9.055858",
   "Depth":"16",
   "Speed":"1.400000",
   "Heading":"0.000000",
   "O2":"17.799999",
   "CO2":"5.000000",
   "CO":"76.000000",
   "H2":"3.500000",
   "Pressure":"45.000000",
   "Temperature":"21.000000",
   "Survivors":"43",
   "MobilityFlag":"1",
   "ForwardingCapability":"1",
   "TxRxFlag":"0",
   "ScheduleFlag":"0"
}
</syntaxhighlight>This Emergency Position and Status Message (Class ID 0 Application 3 Plug-in) message shows a Portuguese submarine at 38.386547 latitude -9.055858 longitude at a depth of 16 meters. It is moving north at 1.4 meters per second, and has 43 survivors on board and shows the environmental conditions.

=== Underwater messaging ===
Commercial hardware products have been designed to enable two-way underwater messaging between scuba divers.<ref>{{Cite web |date=2008-02-04 |title=Israel's UTC brings text-messaging underwater [VIDEO] |url=https://www.israel21c.org/israels-utc-brings-text-messaging-underwater-video/ |access-date=2022-08-30 |website=ISRAEL21c |language=en-US}}</ref><ref>{{Cite web |date=2019-02-08 |title=Ultrasonic dive computer lets divers communicate through "pings" |url=https://newatlas.com/oceans-s1-ultrasonic-dive-computer/58384/ |access-date=2022-08-30 |website=New Atlas |language=en-US}}</ref> These support sending from a list of pre-defined messages from a dive computer using acoustic communication.

Research efforts have also explored the use of smartphones in water-proof cases for underwater communication, using acoustic [[modem]] hardware as phone attachments<ref>{{Cite book |last1=Restuccia |first1=Francesco |last2=Demirors |first2=Emrecan |last3=Melodia |first3=Tommaso |title=Proceedings of the International Conference on Underwater Networks & Systems |chapter=ISonar |date=2017-11-06 |chapter-url=https://dl.acm.org/doi/10.1145/3148675.3148710 |language=en |location=Halifax NS Canada |publisher=ACM |pages=1–9 |doi=10.1145/3148675.3148710 |isbn=978-1-4503-5561-2|s2cid=11584770 }}</ref> as well as using a software app without any additional hardware.<ref name=":0">{{Cite book |last1=Chen |first1=Tuochao |last2=Chan |first2=Justin |last3=Gollakota |first3=Shyamnath |title=Proceedings of the ACM SIGCOMM 2022 Conference |chapter=Underwater messaging using mobile devices |date=2022-08-22 |language=en |location=Amsterdam Netherlands |publisher=ACM |pages=545–559 |doi=10.1145/3544216.3544258 |isbn=978-1-4503-9420-8|s2cid=251496040 |doi-access=free }}</ref> The [[Android (operating system)|Android]] software app, AquaApp, from University of Washington uses the microphones and speakers on existing smartphones and smart watches to enable underwater acoustic communication.<ref>{{Cite web |title=Finally, an underwater messaging app |url=https://social.techcrunch.com/2022/08/29/finally-an-underwater-messaging-app/ |access-date=2022-08-30 |website=TechCrunch |date=29 August 2022 |language=en-US}}</ref> It had been tested to send digital messages using smartphones between divers at distances of up to 100 m.<ref name=":0" /> 

== See also ==
*[[Acoustic communication in aquatic animals]]
*[[Acoustic communication in fish]]
* [[Telecommunications]]
* {{annotated link|Acoustic release}}
* {{annotated link|Underwater acoustics}}

== References ==
<references />

== External links ==
*{{Citation | vauthors=((Jarrot, A.)), ((Ioana, C.)), ((Quinquis, A.)) | title=Europe Oceans 2005 | year=2005 | chapter=Denoising underwater signals propagating through multi-path channels | pages=501–506 Vol. 1 | publisher=IEEE | doi=10.1109/OCEANSE.2005.1511765 | isbn=0-7803-9103-9 | s2cid=32055087 | chapter-url=http://dx.doi.org/10.1109/OCEANSE.2005.1511765}} 
* [http://www.dspcomm.com/ DSPComm – underwater acoustic modem manufacturer]
* [https://docs.unavlab.com/documentation/EN/uWAVE/uWAVE_Specification_en.html uWAVE - the smallest underwater acoustic modem]
* [https://www.tetcos.com/underwater_networks.html NetSim UWAN - underwater acoustic network simulation]
{{hydroacoustics}}
{{Underwater diving|divequ}}
[[Category:Telecommunications techniques]]
[[Category:Acoustics]]