DIGITAL UNDERWATER ACOUSTIC COMMUNICATIONS Tianzeng Xu Lufen Xu AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON NEWYORK(cid:129)OXFORD(cid:129)PARIS(cid:129)SANDIEGO SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO AcademicPressisanimprintofElsevier AcademicPressisanimprintofElsevier 125LondonWall,LondonEC2Y5AS,UnitedKingdom 525BStreet,Suite1800,SanDiego,CA92101-4495,UnitedStates 50HampshireStreet,5thFloor,Cambridge,MA02139,UnitedStates TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UnitedKingdom ©2017ChinaOceanPress.PublishedbyElsevierInc.Allrightsreserved. Nopartofthispublicationmaybereproducedortransmittedinanyformorbyanymeans,electronicor mechanical,includingphotocopying,recording,oranyinformationstorageandretrievalsystem,without permissioninwritingfromthepublisher.Detailsonhowtoseekpermission,furtherinformationaboutthe Publisher’spermissionspoliciesandourarrangementswithorganizationssuchastheCopyrightClearance CenterandtheCopyrightLicensingAgency,canbefoundatourwebsite:www.elsevier.com/permissions. ThisbookandtheindividualcontributionscontainedinitareprotectedundercopyrightbythePublisher (otherthanasmaybenotedherein). Notices Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchandexperiencebroaden ourunderstanding,changesinresearchmethods,professionalpractices,ormedicaltreatmentmaybecome necessary. Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgeinevaluatingand usinganyinformation,methods,compounds,orexperimentsdescribedherein.Inusingsuchinformationor methodstheyshouldbemindfuloftheirownsafetyandthesafetyofothers,includingpartiesforwhomthey haveaprofessionalresponsibility. Tothefullestextentofthelaw,neitherthePublishernortheauthors,contributors,oreditors,assumeany liabilityforanyinjuryand/ordamagetopersonsorpropertyasamatterofproductsliability,negligenceor otherwise,orfromanyuseoroperationofanymethods,products,instructions,orideascontainedinthe materialherein. LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary ISBN:978-0-12-803009-7 ForinformationonallAcademicPresspublications visitourwebsiteathttps://www.elsevier.com/ Publisher:GlynJones AcquisitionEditor:SimonTian EditorialProjectManager:ViviLi ProductionProjectManager:JasonMitchell Designer:MarkRogers TypesetbyTNQBooksandJournals About the Authors TianzengXuisaprofessorandthesupervisor Ministry of Science and Technology of the of a Ph.D. program at the College of Ocean and People’sRepublic of China. Earth Sciences of Xiamen University and Key Lufen Xu is a senior engineer and master Laboratory of Underwater Acoustic Communi- instructorattheCollegeofOceanandEarthSci- cation and Marine Information Technology, encesofXiamenUniversityandKeyLaboratory Ministry of Education. He had been appointed of Underwater Acoustic Communication and as director of the Subtropical Marine Institute, MarineInformationTechnology,MinistryofEd- Head of Oceanography Department, Xiamen ucation. He is vice dean of teaching and the University and vice chairman of the China researchsectionofmarinephysicsofthecollege. Marine Physical Society. The advanced worker of national high-new technical developing (863) program vested by vii Foreword Underwater acoustic communications have basic premise for establishing innovative digital covered the areas of national defense and civil underwater communication equipment that can ocean development and exploration. Therefore, adapt topeculiar underwateracoustic channels. they have been highly valued by maritime na- The contents discussed in this book will tions in the past several decades. mainly focus on digital underwater acoustic Analogunderwateracousticcommunications communications forcivil applications. have played an important role for many years. Military underwater acoustic communication However,theyhavesomedrawbackslikeunsta- has its own special requirements and corre- ble communication quality. To adapt peculiar sponding difficulties. For example, it requires underwater acoustic communication channels, long-rangeandstrictlyconfidentialcommunica- digital underwater acoustic communications tion,whichsometimesappliestohigh-speedmo- have raised vast concerns and achieved crucial bilecommunication.However,theappliedfields developments and massive technological break- and operating specifications between military throughs. With regard to many underwater and civil underwater acoustic communications acoustic communication fields, like underwater crosseachother.Inparticular,thetheoreticalba- data transmission in an underwater acoustic sis is generally identical. Therefore,the contents network,digitalcommunicationshavegradually discussed in this book have a common replacedanalog ones. significance. In principle, digital underwater acoustic Thisbookhasfourchaptersandanappendix. communications are an extension and develop- Afterreviewingthedevelopmentofunderwater ment from digital radio communications. How- acoustic communications, Chapter 1 is focused ever, there are many essential differences ondescribingthedifferencesbetweenunderwa- betweenunderwateracousticandradiocommu- ter acoustic communication channels and radio nication channels, including randomly variant ones. The peculiarities existing in the former spatial-temporal-frequency parameters, large indicatethenecessitytoexploresomeinnovative attenuation, severe multipath effects, a strict signal processing systems employed effectively band-limiting property, high noise level, and in digital underwater acoustic communications. low sound velocity. In particular, underwater After that, a communication sonar equation acoustic communication channels are generally will be derived, and an active sonar equation not linear nor time-invariant. Therefore, some against noise background will also be included. present advanced signal processing systems, A specific example to show how to use the suchasmatchedfilter,orevensomebasicprinci- communication sonar equation to select the ples,likeShannontheorem,inradiocommunica- relative parameters in designing an improved tions cannot perfectly be employed, and they FH-SS (Frequency-Hopped Spread-Spectrum) especially cannot be copied mechanically in system communication sonar will be provided. underwater acoustic communications. So, Lawsoftransmittedsoundinunderwateracous- exploring innovative signal processing systems tic channels will be discussed in Chapter 2, to suitably employ in peculiar underwater includingsoundtransmissionloss,multipathef- acoustic communication channels is a valuable fects, sound scattering and fluctuations, and and challenging research topic, which is also a noise in the sea, especially their impacts on ix x FOREWORD digital underwater acoustic communications and (3) a rapid, adaptive channel-modifying and the possible countermeasures to adapt to net that can adapt to underwater acoustic them. They are the physical basis for designing communication channels having a random innovative digital underwater acoustic commu- spatial-temporal-frequency variability. nication signal processing systems. Digital Many notable books have been published in underwater acoustic communication signal thefieldofunderwateracoustics;however,there processing will be described in Chapter 3. After are not any monographs or systematic reports discussing some signal processing schemes about digital underwater acoustic communica- employed in digital underwater acoustic com- tion. Despite the fact that the first author is munications at present, the explorations estab- over 70years old, he invited his daughter to lishing innovative digital underwater acoustic writethisbookbasedonyearsofteachingexpe- communication systems are emphatically dis- riences, including two doctor’s degree courses: cussed. According to the peculiarities of under- “Applied Underwater Acoustics” and “Under- water acoustic communication channels, by water Acoustic Data Transmissions.” Especially combiningsomekeytechniques,whichurgently relevant was the experience of research and need to be solved in civil underwater acoustic development, including undertaking and communications, it is possible to establish an participating in five topics with respect to a na- innovative, adaptive pseudo-random frequency tional high, new technical developing program modulation (abbreviated as APNFM) system to (since to make some contributions to this pro- beemployedindigitalunderwateracousticcom- gram, one has to be chosen as an advanced munications. It may be expected to adapt to worker by National Science and Technology peculiar underwater acoustic communication Committee and win the third prize), and it is channelsandobtainanapproximatelyoptimum our wish to use that to add a brick and a tile detection result at the criterion of maximum for building the edifice of Underwater Acoustic output SNR against a random multipath inter- Communications. At the same time, the authors ferenceandnoisebackground.Digitalunderwa- hopethecommunicationsdevelopwithoutinter- ter acoustic communication equipment will be ruption, and become more and more mature in discussedinChapter4,including three types of servicetohumanitytoexploreandusetheocean civil digital underwater acoustic communica- in a peacefulway. tion equipment developed by the authors: (1) Duringtheprocessofwritingthisbook,theau- underwater acoustic telecontrol communication thors had consulted many brilliant works and equipment in which a digital time correlation theses, and they received some inspirations accumulation decision system has been from them. At the same time, the authors want employed; (2) underwater acoustic multimedia togiveheartfeltthankstocolleaguesfortheirsup- communication equipment in which an portandhelpduringthelengthyworkprocess. improved FH-SS system has been used; and This book is designed to serve as a reference (3) a digital underwater acoustic communica- book for postgraduate students and practicing tion prototype in which an innovative APNFM engineers involved in the design and analysis system has been employed. There are three of underwater acoustic communications, as innovative key parts in APNFM: (1) adaptive wellasrelativeunderwateracousticengineering. total time sampling processing for PN As a background, we presume that the readers (Pseudo-random)sequencesinsteadoftheusual have a prior knowledge of underwater acoustic synchronical schemes; (2) an adaptive Rake physics and digital communications. receiver to adapt complicated and rapid vary- As we all know, the Monkey King in Journey ing multipath effects and utilize its energy; to the West can freely travel in the ocean to visit xi FOREWORD hisfriendDragonKingintheCrystalPalace,but basis for designing ultrasonic sensing systems itisalsoaregrettablethingthat hecannotcarry employedintheairmedium.Next,threenewul- out underwatercommunicationwithhis master trasonicsensingsystemsdevelopedwillemphat- and apprentices. ically be discussed in this appendix, which are The authors believe that with unremitting ef- ultrasonic ranging and bearing sensing system fortswecanalsotravelthroughtheoceansome- employedinconcretejettingmanipulators,ultra- day and live in the man-made“Crystal Palace.” sonicterrainobstaclessensingsystememployed Moreover, people can be guided by tortoises in mobile robots, and ultrasonic navigating and dance with whales. Furthermore, like the sensing system employed in automatic guided people who live on the earth, everyone can vehicles.Thisappendixissuitableforpracticing haveanunderwatermobilephonetocommuni- engineers and postgraduate students as a refer- cate with their relatives and friends in multi- ence in ultrasonic sensing systems and their media ways at anytime and anywhere. If the applications. Monkey King knew that, he wouldbe jealous. Itshouldbepointedoutthattheauthorspub- The appendix includes relative ultrasonic lishedDigitalUnderwaterAcousticCommunication sensing systems in the air medium. The rules in Chinese in 2010. Now, we have carried out with respect to ultrasonic radiation, transmis- some supplements and expect a wider range sion,scattering,andreceivingintheairmedium of interchanges, if it is published in English, will first be described briefly. It is a physical becauseit has beentranslated by the authors. C H A P T E R 1 Introduction Digital underwater acoustic communications 1.1 OVERVIEW OF UNDERWATER for civil applications will be discussed in this ACOUSTIC COMMUNICATION book. An appendix on ultrasonic sensing sys- DEVELOPMENT tems in the air medium willalso be included. Afterreviewingthedevelopmentofunderwa- Formation and development of a discipline ter acoustic communications, the emphasis in come from practical needs. The urgent needs thischapterwillbefocusedondescribingthedif- from both navy military activities and ocean ferences between underwater acoustic commu- resource exploitations have become a strong nication channels and radio channels. The drivingforceforthedevelopmentofunderwater peculiarities existing in the former mean that it acoustic communications. is necessity to explore some innovative signal Modernunderseawarfarebelongstothewar processing systems employed in digital under- ofinformationtechnology.Underwateracoustic water acoustic communications, which is also a communications are the important methods of basic premise to establish new digital underwa- information acquisition, transmission, and ter acoustic communication equipment. Also, a control. communication sonar equation will be derived, The main military applications for underwa- which provides the references for predicting ter acoustic communications are as follows theperformancesofpresentandnewcommuni- [1,2]: the communications between submarines, cation sonars being designed. An active sonar submarines and surface ships, submarines and equation against noise background will also be shore base stations, and submarines and under- included. A specific example to show how to water combating platforms, as well as military usethisequationtoselecttherelativeparameters divers; in addition, there are military applica- indesigninganFHSSsystemcommunicationso- tions for communication among nodes in mili- nar is also introduced. tary underwater acoustic networks, submarines In this chapter, the general idea of the book (now as mobile nodes) and nodes, and so forth. willbesummarized,andtheroleofeachchapter Tomeettheurgentneedsofthepeacefuluses in overall will becoherently defined. of the ocean, the applications of underwater DigitalUnderwaterAcousticCommunications http://dx.doi.org/10.1016/B978-0-12-803009-7.00001-5 1 ©2017ChinaOceanPress.PublishedbyElsevierInc.Allrightsreserved. 2 1. INTRODUCTION acoustic communications have been rapidly parameters.Inthistypeofmarineobservation extended from current military to civil fields. station,thevarioussensorsofoceanographic The typical applications are as follows: parametersandadatacommunicationsonar arearrangedinanunderwaterplatform. 1. Underwateracousticcommunicationsamong Samplingvesselsregularlyvisitthesestations surface command ships and divers andsendcommandsinthevicinityofthe performing underwater exploration, rescue, underwaterplatformtoacquiretherelative and salvage [3]. Generally, short-range (such parametersbyusingunderwateracousticdata as a few kilometers) voice and image communications.Comparedwiththe communications are required. Moreover, traditionalfieldmeasurementsbyusing servicesmust bedesigned to besmallin size surveyingships,thevesselshavethe and lightin weight tofacilitate the divers to outstandingadvantagesofsavingtime,effort, carry and operate them. Therefore,higher andcost. operating frequencies,such as 20kHz or 5. Formation of civil underwater acoustic more, will beselected. networks [6]. The oceanographic parameters 2. Multimediacommunicationsbetweensurface acquiredby asinglebuoy-based ocean ships and the various types of underwater observatory station mentioned earlier can robots, AUVs (autonomous underwater only get a single pointdata; thus in-depth vehicle), deep submergences, etc. [4]. These analyses of marine environmental variations communication equipments generally work andnumericalpredictionsareverydifficult.If in the deepsea, and where there are good anunderwater networkis formed, the conditionsofverticalsoundtransmission,the multilevelparameters will bemeasured in coherent detection system canbe used (such every node. Then by applying underwater as DPSK (differential phase shift keying)). acousticdatacommunication amongthe Communication distances are thus greater nodes, the parameters willbe transmitted by (eg, more than several kilometers), and the antenna on the surface buoyof the transmission rates are also higher, so real- gateway node into radiochannels and timevoice or even color image collectedby a shorestation datacenter. communication can beachieved [5]. Therefore,multiparameter, large area, 3. Applicationsof underwater acoustic simultaneous, continuous, and long-term communications tothe explorationand valuabledata can beobtained. utilization of marine mineralresources [4], suchasunderwatermonitoringofoffshoreoil People are ready to capture the formation of drilling platforms, underwater acoustic data an autonomous oceanographic sampling telemetry of oil wells,underwater network[7,8].Thisnetworkwillprovidetheex- positioning, and monitoring for laying change of data, such as control, telemetry, and pipelines.Thesetypesofunderwateracoustic video signals, among network nodes. The communications belong to high-frequency, networknodes,stationaryandmobileunderwa- short-range, and point-to-point types; ter vehicles or robots, will be equipped with operating conditions are better,and thus various surveying instruments, such as current it is easier todesign corresponding meters, seismometers, sonars, and video cam- communication sonars. eras. A remote user can gather various oceano- 4. Underwateracousticdata(includingremote graphic data using direct computer access via a controlcommands)communicationsforthe radio link to a central network node based on a automaticmonitoringofmarineenvironmental surface buoy. 3 1.1 OVERVIEWOFUNDERWATERACOUSTICCOMMUNICATIONDEVELOPMENT Inaddition,underwateracousticcommunica- occur anytime and anywhere. Adapting tionscanalsobeusedinsomeotherfields,such communication functionsto underwater asmarinedisasterforecasting,thepositioningof acousticcommunication environments that ablackboxfromaplanecrash,orthecommuni- randomly vary in spatial,temporal, and cation between underwater tour boats and the frequency aspects isvery difficult. shore station. The application fields of civil un- 4. Military underwater acoustic derwater acoustic communications can be ex- communicationsmay beapplied to high- pected to be rapidly extended in the future. speed mobileones. In these cases, there exist Theappliedfieldsandperformancespecifica- dramaticchangesin the communication tions between military and civil underwater environments,largeDopplerfrequencyshifts, acoustic communications cross each other, and the high noise levels of theships [10].In whereas more are essentially different. particular, since the usualunderwater Military underwater acoustic communica- acousticsignal processing schemes,such as tions have their own special requirements and the matchedfilter and OFDM(orthogonal corresponding difficultiesrealizing robust ones. frequencydivisionmultiplexing),donothave frequency shift adaptability, realizing high- 1. Military underwateracoustic quality mobile underwater acoustic communicationsgenerallyrequirelong-range communicationsis quite difficult. propagation.Iftheinformationtobeobtained 5. Military underwater acoustic is more distant, there is a wider range for communicationshave a requirement to be acting opportunity. strictly confidential. Ifthis difficult problem 2. Military underwateracoustic cannot beovercome, perhaps transmitted communications generally require high data sound power and the corresponding rates.Oncetheinformationisreceivedfaster, communication distanceshave tobereduced actions can beobtained in anadvanced to exchange forthe improvement of amount of time. confidential communication performance [1]. Compatibly satisfyingthe long range,r, 6. Military underwater acoustic and the high rate,R, underwater acoustic communicationsgenerallyneed multimedia communications canbe quite difficult. Based information to adapt to differentapplied on the experimental results acquired by fields. Installing a lot of different American scholars, anupperlimit product communication equipment(including between R (kbps) and r (km) [9] isgivenby different transducer-amplifier of power modules) in the shipswith limitedspacesis R (cid:1)40 (1.1) r generally not allowed.Thus,the compatibility problemsof different Generally,itisdifficult toreachthisupper communication media, communication limit. For example,whenR¼1kbps is sonar, and activesonar transducers, etc., required, communication distance would be would cautiously besolved [1]. less than 40km. 3. Military underwateracoustic In contrast, the problems existing in military communications demand high robustness to underwater acoustic communications are not as avoidaccidental incidents. However,marine outstandingasinmilitaryones.Civilcommunica- communication environments are tionequipmentisusuallyoperatedatshorterdis- complicatedandvaried;whereasunderwater tances and communication time intervals also warfare and their battlefields are likely to have a great flexibility; thus the compatibility 4 1. INTRODUCTION problem of either communication distances or underwateracousticcommunicationsonarshave data rates is not present. Moreover, they usually someparticularrequirementsinsize,weight,po- belong to fixed points or, at low-speed mobile wer consumption, and cost. In particular, it is communications, communication environments difficulttoacquirethepriorknowledgeofunder- that are more stable, so there is a lighter burden water acoustic communication channels for the on the Doppler frequency shift correction, and civil users. Therefore, such equipment must be thenoiselevelsarealsolower.Thereisnorequest able to accomplish the communication without forconfidentialityforsuchcommunicationequip- the prior knowledge of the channels. We see ment.Aslongasthepermissionsaremetforen- that there are some special difficulties for ergy consumption, size, and weight, we can designingsuchcivilcommunicationsonars. maketheinformationdetectionunderthecondi- Thus, even if for civil underwater acoustic tion of high signal-to-noise ratio (SNR). In addi- communications,achievingmultimediacommu- tion, we can produce a variety of underwater nications with longer distances, higher data acoustic communication equipment having rates, and high robustness are still of very diffi- different specifications and applied fields for cult. Many core techniques critically wait to be different users. Generally, the complex compati- solved, which will be discussed in this book. bilityproblemsmentionedearlierdo nothaveto Some particularly difficult problems existing in beconsidered. military underwater acoustic communications, Ofcourse,thedegreeofdifficultybetweenmil- such as secure, high-speed mobile communica- itary and civil underwater acoustic communica- tions, will not be discussed in depth in this tions is relative. Realizing civil underwater bookbecausethecontentswillfocusonthedig- acousticcommunicationswithhighperformances ital underwater acoustic communications is still a difficult task. If the communication dis- against for civil applications. tances can be extended to be farther, this will As noted earlier, because of the complexity widely adapt to practical requirements. Some andvariabilityofunderwateracousticchannels, communicationmediathathavelargeinformation there are many practical difficulties to achieve contents, such as in image communication, still high-quality underwater acoustic communica- have high required data rates. For example, we tions. However, driven by actual military and haddevelopedashallowwaterimagecommuni- civilneeds,theunderwateracousticcommunica- cation prototype [11]. Although the data rates tion discipline has advanced greatly in the pro- havereached8kbpsforasimpleblackandwhite cess to overcomethe varied difficulties. image consisting of 320(cid:3)200(pixels)(cid:3)16(gray From the layout of the communications, sim- level)withoutusingdatacompression,thetrans- ple, static point-to-point communications have mission time is still 32s. According to Eq. (1.1), spreadtomobileones;nowunderwateracoustic the communication distances are below 5km. In networks have been formed, and we are pre- someseaareas,adversecommunicationenviron- pared to establish land/sea/air three- ments will be encountered. A signal processing dimensional mobile communication networks. system with excellent channel adaptability is Fromthecommunicationsystems,theanalog also needed for ocean developments and utiliza- underwater acoustic communications have tion.Someappliedfields,suchasthecommunica- gradually been transited to digital ones. More- tionbetweenasurfacecommandshipandAUVs, over, some advanced radio communication sys- are still considered mobile. Moreover, the multi- tems, such as spread spectrum [12,13] and media communications and corresponding OFDM systems have been applied to underwa- compatibility problems will also be encountered ter acoustic communications. We can achieve insomeintegralapplyingfields.Inaddition,civil distances above 100km in low data rate digital