A Novel Energy-Efficient Contention-Based MAC Protocol Used For OA-UWSN
sensorsArticleA Novel Energy-Efficient Contention-Based MACProtocol Used for OA-UWSNJingjing Wang 1 , Jie Shen 1 , Wei Shi 1, *, Gang Qiao 2 , Shaoen Wu 3 and Xinjie Wang 4, *1234*School of Information Science and Technology, Qingdao University of Science & Technology,Qingdao 266061, China; wangjingjing@qust.edu.cn (J.W.); 4016110010@mails.qust.edu.cn (J.S.)College of Underwater Acoustic Engineering, Harbin Engineering University, Harbin 150001, China;qiaogang@hrbeu.edu.cnDepartment of Computer Science, Ball State University, Muncie, IN 47304, USA; Swu@bsu.eduSchool of Information and Control Engineering, Qingdao University of Technology, Qingdao 266000, ChinaCorrespondence: 03244@qust.edu.cn (W.S.); wangxinjie@qut.edu.cn (X.W.); Tel.: 86-152-7521-8040 (W.S.);Fax: 86-532-8895-9036 (W.S.)Received: 28 October 2018; Accepted: 26 December 2018; Published: 7 January 2019 Abstract: A hybrid optical-acoustic underwater wireless sensor network (OA-UWSN) was proposedto solve the problem of high-speed transmission of real-time video and images in marine informationdetection. This paper proposes a novel energy-efficient contention-based media access control (MAC)protocol (OA-CMAC) for the OA-UWSN. Based on optical-acoustic fusion technology, our proposedOA-CMAC combines the postponed access mechanism in carrier sense multiple access with collisionavoidance (CSMA/CA) and multiplexing-based spatial division multiple access (SDMA) technologyto achieve high-speed and real-time data transmission. The protocol first performs an acoustichandshake to obtain the location information of a transceiver node, ensuring that the channel is idle.Otherwise, it performs postponed access and waits for the next time slot to contend for the channelagain. Then, an optical handshake is performed to detect whether the channel condition satisfiesthe optical transmission, and beam alignment is performed at the same time. Finally, the nodestransmit data using optical communication. If the channel conditions do not meet the requirementsfor optical communication, a small amount of data with high priority is transmitted through acousticcommunication. An evaluation of the proposed MAC protocol was performed with OMNeT simulations. The results showed that when the optical handshaking success ratio was greater than50%, compared to the O-A handshake protocol in the literature, our protocol could result in doubledthroughput. Due to the low energy consumption of optical communication, the node’s lifetime is30% longer than that of pure acoustic communication, greatly reducing the network operation cost.Therefore, it is suitable for large-scale underwater sensor networks with high loads.Keywords: MAC protocol; OA-UWSN; high throughput; low energy; contention-based1. IntroductionUnderwater wireless sensor networks (UWSN) are typically used to monitor underwaterenvironments and collect underwater data, such as marine data collection, pollution monitoring,marine exploration, assisted navigation, and tactical monitoring [1–4]. The vast areas to be detectedin marine environments result in sparse deployment of the network and widespread use of mobilesensors. In order to cover the entire target area, the size of a network is generally large, and thetopology is usually based on multi-hop wireless [5].Three ways are normally employed to transfer data between UWSN nodes, namely, underwaterelectromagnetic communication, underwater acoustic communication, and underwater opticalSensors 2019, 19, 183; doi:10.3390/s19010183www.mdpi.com/journal/sensors
Sensors 2019, 19, 1832 of 14communication. Electromagnetic waves attenuate very quickly in water, so they are rarely used [6].Underwater acoustic communication is the most mature technique, because acoustic signal attenuationunderwater is small and thus can achieve a relatively long-distance transmission (usually usedfor UWSN data transmission), but the bandwidth of acoustic communication is narrow andthe delay is long, which is not suitable for large-capacity data transmission such as images andvideo [7]. Underwater optical communication has the advantages of high speed and low powerconsumption [8], and the attenuation of underwater blue-green light is relatively low, enablingunderwater short-distance and large-capacity data transmission [9]. In recent years, combiningthe advantages of acoustic communication and optical communication, a new underwater datatransmission mode has been proposed [10,11]. The routing algorithm of the design in Reference [12]improved the lifetime of underwater acoustic communication nodes. Compared to acoustic communication,optical communication has inherent advantages in node lifetime. Reference [13] proposed a solutionfor the combined use of acoustic communication and optical communication that addresses thebandwidth limitation of the acoustic channel while achieving optical alignment through acousticposition, thereby performing optical communication. Reference [14] proposed that autonomousunderwater vehicles (AUVs) use long-distance acoustic communications, nodes using high-bandwidthoptical communications, and a data transmission mode combining two communication modes.Compared with references [13,14], we previously proposed and designed a new underwaternetworking method: A hybrid optical-acoustic underwater wireless sensor network (OA-UWSN) [15].Using optical communication for high-speed and short-distance data transmission, and using acousticcommunication for control commands and node positioning, the OA-UWSN enables solving theproblem of high-speed and low-cost wireless transmission of real-time or quasi-real-time video andimages in marine information detection, while keeping system power consumption small.In a wireless sensor network, a media access control (MAC) protocol is critical, and is used toprevent collisions when more than two transmissions occur [16]. At present, the network layer routingprotocol has only been tested in simulations, such as the multi-level routing protocol for acoustic-opticalhybrid underwater wireless sensor networks (MURAO) routing protocol [17]. The existing underwaterwireless sensor network MAC protocols are only for a single transmission medium network, andthere is no mature MAC protocol applicable to an underwater optical-acoustic hybrid wirelesssensor network.At present, MAC protocols adopted by UWSNs based on underwater acoustic communicationhave been studied for years, generally falling into two types: A MAC protocol based on time slotallocation and a MAC protocol based on contention. References [18,19] proposed that one nodebe responsible for scheduling the allocation of time slots across the entire network. Meanwhile,Reference [18] proposed a scheme based on time division multiple access (TDMA), where the sink nodedetects its distance from all neighboring nodes and notifies all nodes of the scheduling transmissionsequence through superframes. Reference [19] proposed a slotted floor acquisition multiple access(S-FAMA) protocol, which divides time into slots. All packets must be transmitted at the beginning ofeach time slot, so that collisions can be avoided. In a contention-based MAC protocol, all nodes equallyaccess the channel through competition. Reference [20] proposed that in an ordered carrier sensemultiple access (CSMA), the scheduler notifies all nodes of the transmission schedule so that eachnode knows when to transmit information. Reference [21] proposed a low-energy MAC protocol(T-Lohi), which contends for the channel by sending a short frame. If there is only one node competing,then the data can be transmitted. Otherwise the competing node waits and then competes again.At present, underwater optical communication generally uses blue-green light in the visible light bandfor data transmission. However, UWSN research based on underwater optical communication is full ofchallenges. In particular, there is great need for an efficient MAC protocol.This paper proposes a novel optical-acoustic competitive MAC protocol (OA-CMAC) based onthe underwater optical-acoustic hybrid wireless sensor network in our previous work. The protocolcombines the advantages of fast transmission, low loss, high bandwidth of underwater optical
Sensors 2019, 19, 183Sensors 2018, 18, x FOR PEER REVIEW3 of 143 of 14communication, and long-distance propagation of underwater acoustic communication [11]. Basedcommunication, and long-distance propagation of underwater acoustic communication [11]. Based onon optical-acoustic fusion technology, it uses the postponed access of carrier sense multiple accessoptical-acoustic fusion technology, it uses the postponed access of carrier sense multiple accesswith collision avoidance (CSMA/CA) technology to achieve high-speed and reliable transmission ofwith collision avoidance (CSMA/CA) technology to achieve high-speed and reliable transmission ofunderwater data while ensuring channel utilization and energy efficiency.underwater data while ensuring channel utilization and energy efficiency.2. UnderwaterUnderwater Optical-AcousticOptical-Acoustic HybridHybrid WirelessWireless taData2.TransmissionProcessProcessTransmissionAs shownshown inin FigureFigure 11 [15],[15], thethe networknetwork topologytopology of the OA-UWSN is composed of fixed nodes,Asmobile nodes,nodes, sinksink nodes,nodes, andand controlcontrol centers.centers. TheThe fixedfixed nodesnodes areare deployeddeployed onon thethe seasea floorfloor toto collectcollectmobilemonitoring datadata (such(such asas temperature,temperature, salinity,salinity, andand depth),depth), images,images, oror videos.videos. TheThe mobilemobile nodesnodes arearemonitoringsuspended inin thethe sea,sea, completingcompleting thethe datadata collectioncollection andand passingpassing thethe datadata toto thethe upperupper mobilemobile nodesnodessuspendedor thethe sinksink nodesnodes andand thenthen toto thethe controlcontrol centers.centers. SinkSink nodesnodes andand controlcontrol centerscenters areare deployeddeployed inin thetheorsea andand receivereceive thethe collectedcollected remoteterminals.terminals.seaFigure 1.1. NetworkNetwork relesssensornetwork,data communicationis mainlyIn relesssensornetwork,data ionmainly divided into three phases. In the first phase, an acoustic handshake is performed to obtain theinformationof a transceivernode to ensurethe thatchannelis idle. Otherwise,a postponedaccessposition informationof a transceivernode tothatensurethe channelis idle. Otherwise,a postponedisperformedto waittoforthefornextcompeteagain.again.In theInsecondphase,accordingto theaccessis performedwaitthetimenextslottimetoslotto competethe secondphase,accordingtopositioninformationof thetransmittingandreceivingnodesobtainedthe positioninformationof firstfirststage,stage,thethe nodenodeperformsalignment (that(that orms beam uiredfor opticalcommunicationis satisfied.the thirdthethebit biterrorrateratethresholdrequiredfor opticalcommunicationis satisfied.In theInthirdphase,phase,if theifopticalthe opticalcommunicationrequirementis met,datatransmittedbetweenbetweennodesnodes throughthrough opticalcommunicationrequirementis unication. If optical communication requirements are not met,met, datadata areare transmittedtransmitted betweenbetweennodesnodes throughthrough acousticacoustic communication.communication.3.3. OA-UWSNOA-UWSN MACMAC ProtocolProtocol DesignDesign3.1.3.1. DataData LinkLink LayerLayer ChannelChannel AccessAccessInIn thisthis CSMA/CACSMA/CA datadata linklink laccess method, a spatial multiplexing-based spatial division multiple access is introduced, csof lightofto lightincreasethe data transmissiondistance.full ofusethe directionaltransmissioncharacteristicsto increasethe data nce. Figure 2 shows a superframe structure designed for the OA-UWSN in this divides a superframe into different periods: An acoustic handshake period, an opticalahandshakedata transmissionand a postponedaccessperiod, aperiod,data transmissionperiod,and period.a postponed access period.
Sensors 2019, 19, 1834 of 14Sensors 2018, 18, x FOR PEER REVIEW4 of 14Superframe cousticCTSSuperframe #2OpticalhandshakeOpticalRTSSuperframe #.Data TransmissionSuperframe #nPostponed accessOpticalCTSFigure 2.2. SuperframeSuperframe structure.structure.Figure3.2.3.2. DataData Transmission,Transmission, Reception,Reception, andand ConfirmationConfirmationThisThis paperpaper combinescombines thethe advantagesadvantages ofof bothboth opticaloptical andand acousticacoustic communicationcommunication modesmodes andandproposesHow nodenode AA andand nodenode Bproposes anan OA-CMACOA-CMAC communicationcommunication mechanism.mechanism. HowB ofof thethe lowerlower layerslayerstransmittransmit datadata toto thethe upperupper nodenode isis illustratedillustrated asas anan exampleexample toto describedescribe thethe datadata sending,sending, ninFigure3,thespecificstepsareasfollows:and confirming mechanisms. As shown in Figure 3, the specific steps are as follows:1.1.The mobilemobile nodenode isis boundbound toto aa pressurepressure sensor,sensor, which can measure its depth and send an acousticTheinterrogation signalsignal atat a certain frequency. This signal can reach the water surface directly atinterrogationatTime 0, or it can be forwarded by the fixed node and reach the water surface at TimeTime 1.1. The timedifference and the measured depth are used to locate the mobile node [22];differenceperform anan2. TheThe lowerlower nodesnodes AA andand B need to send acoustic RTS1 and RTS2, respectively, to performacoustichandshake. TheThe d)includesincludesinformationsuchlocationacoustic handshake.informationsuchas esmoothly;thatthethenextopticalhandshakecancanbe becompletedmoresmoothly;3. Afterthe firstfirst iatelyrepliesreplieswithacousticCTS(Clear3.After receivingreceiving thewithananacousticCTS(CleartotoSend).If theall thenodesreceivethe CTS,theyknowcan whichknow whichnodesare competingto theSend).If allnodesreceivethe CTS,they cannodes arecompetingto the channel,channel,and the uncommittednodes postponedperform postponedandthewaitfor thetochanneland the uncommittednodes performaccess andaccesswait forchannelbe idle; to beidle;4.After successful competition, the lower node A sends an optical RTS to perform an optical4. Aftersuccessfullowernode Anodesendsan opticalto receiving thetheRTS,the upperrespondswithRTSan opticalCTS ndswithanopticalCTStoconfirmwhether the communication mode is optical or acoustic;whetherthe communicationis optical withinor acoustic;5.If the opticalhandshaking ismodenot completedthe time slot, the node that competes in the5. Iftheopticalhandshakingisnotcompletedwithinthe timeslot, the node withthat competesthechannel through the acoustic handshake performs acousticcommunicationthe upper cousticcommunicationwiththeuppernode.If the two instances of handshaking are successful, optical data communication is performed.Iftwoinstancesof handshakingare istime,the uppernode broadcastsa busy-tonesignaldatathroughthe acousticissignalto nalthroughtheacousticsignaltonotifyallall lower nodes that it is busy, and the node that succeeded in competition performs dedincompetitionperformsopticalcommunication according to the communication mode, modulation, and coding informationcommunicationaccordingto the communication mode, modulation, and coding informationdetermined by theoptical handshaking;determined by the optical handshaking;6.After the end of one frame of data transmission, the upper node first sends an ACK6.After the end of one frame of data transmission, the upper node first sends an ACK(Acknowledgement) confirmation signal to the lower node. Then the upper node broadcasts(Acknowledgement)confirmation signal to the lower node. Then the upper node broadcasts aa free signal through the acoustic signal, and all lower nodes recompete for the next frame offree signal through the acoustic signal, and all lower nodes recompete for the next frame ofinformation transmission.information transmission.Each frame is subjected to an acoustic and optical handshake, and the position information of theEach frame is subjected to an acoustic and optical handshake, and the position information of themutual communication node is transmitted during the acoustic handshake. If the node moves and themutual communication node is transmitted during the acoustic handshake. If the node moves and theposition information changes, the optical alignment is realigned in the optical handshake phase.position information changes, the optical alignment is realigned in the optical handshake phase.
Sensors2018,2019,18,19,x183SensorsFOR PEER A-FreeDataA-BusyA-CTSA-FreePostponed OpticalHandshakeHandshakeTimeTime3.3.In aa framethetheacoustichandshaketimetimeis fixed.In contrast,the opticalInframe acoustichandshakeis fixed.In tical handshaking needs to perform optical alignment through beam training. Therefore, the timefor the opticalis uncertain.If it laststoolong,throughputof the ofOA-UWSNis notrequiredfor thehandshakingoptical handshakingis uncertain.If itlaststoothelong,the throughputthe usticcommunication.Therefore,itisnecessarytois not necessarily higher than that of a UWSN with pure acoustic communication. Therefore, it atissuperiortopureacousticcommunication.necessary to limit the optical handshake time to obtain throughput that is superior to pure acousticThis section presentsthe relationshipthebetweenoptical thehandshakingsuccess rateandratethecommunication.This sectionpresents the betweenrelationshipoptical d the upper limit of the optical handshake time when the optical communication rate and thecommunicationrate are fixed.acousticcommunicationrate are fixed.Networkthroughputrefers toto meduringduringNetwork throughput refersthesimulationtime,asshowninEquation(1):the simulation time, as shown in Equation (1):received packet bytesreceived packet bytes𝑇hroughput(1)(1)Throughput simulation time .simulation timeIn the OA-UWSN, with the denotation that the frame length is n, the acoustic handshake time isIn the OA-UWSN, with the denotation that the frame length is n, the acoustic handshake time is c,c, the optical handshake time is d, the acoustic bit rate is x, the optical bit rate is y, the success rate ofthe optical handshake time is d, the acoustic bit rate is x, the optical bit rate is y, the success rate of thethe acoustic handshake is 𝛽, the optical handshake success rate is 𝛼, the simulation time is t, andacoustic handshake is β, the optical handshake success rate is α, the simulation time is t, and then thethen the throughput expression of the OA-UWSN is shown in Equation (2):throughput expression of the OA-UWSN is shown in Equation (2):() ()().𝑇 (2)(1 α) β(n c d) x αβ(n c d)yT .(2)The network throughput1when purely using acousticcommunication ist(). communication is𝑇 acousticThe network throughput when purely using(3)If the OA-UWSN throughput is higher thanβthepurec) x acoustic network throughput, namely𝑇 (n .(3)T2 𝑇 , thent()()If the OA-UWSN throughput is higheracoustic network throughput, namely.𝑑 than the pure(4)()T1 T2 , thenThe upper limit of the optical handshakingcanbexobtainedby deriving Equation (4). Table 1α(ntime c)(y ).(4)d lists the parameter settings for the simulation(4). Figure 4 shows the relationshipx inα(Equationy x)betweenupperlimitof s rate.Thetheupperlimitof theopticalhandshakingtimecanbe theobtainedderiving Equation(4). Table 1lists the parameter settings for the simulation in Equation (4). Figure 4 shows the relationship betweenthe upper limit of the optical handshake time and the optical handshake success rate.
Sensors 2019, 19, 183Sensors 2018, 18, x FOR PEER REVIEW6 of 146 of 14Table1. ChannelChannel simulationsimulation parameters.parameters.Table 1.ParametersParametersFrame lengthFrame lengthAcoustichandshakingtimeAcoustic handshakingtimeOpticalhandshakingtimeOptical handshakingtimeAcoustic bitAcousticbitraterateOpticalbitrateOptical bit rateValueValue2s2s0.4 s 0.4 sdd10 kbps10 kbps1 Mbps1 Mbps1.81.6Optical training tical handshake success rate0.80.91Figure 4.4. RelationshipRelationship betweenbetween opticaloptical handshakehandshake successsuccess rateFigurerate andand opticaloptical handshakehandshake time.time.FigureFigure 44 showsshows thatthat asas thethe successsuccess raterate ofof thethe opticaloptical handshakinghandshaking increases,increases, thethe upperupper limitlimit ofoftheincreases. IfIf thethe opticaloptical handshakinghandshaking successsuccess raterate isis constant,constant,the opticaloptical handshakehandshake timetime graduallygradually increases.thethroughput ofthe OA-UWSNOA-UWSN isis higherhigher thanthan thatthat ofof aa purepure acousticacoustic communicationcommunication networknetwork onlyonlythe throughputof thewhenFor example,example, ifif thethe opticaloptical handshakehandshakewhen thethe opticaloptical handshakehandshake timetime isis lowerlower thanthan thethe upperupper limit.limit. herthanthatofapureacousticsuccess rate is 20%, the throughput of the OA-UWSN is higher than that of a pure acoustic UWSNUWSNonlyonly whenwhen thethe opticaloptical handshakehandshake timetime isis lessless thanthan 1.41.4 s.s.4. OA-UWSN MAC Protocol Simulation Environment Settings4. OA-UWSN MAC Protocol Simulation Environment Settings4.1. Optical Properties of Underwater Channels4.1. Optical Properties of Underwater ChannelsWater is a complex physical, chemical, and biological system. The optical properties of water areWateris athreecomplexchemical,biologicalsystem. Thepropertiesof waterrelatedto themainphysical,factors: Purewater,anddissolvedsubstances,and opticalsuspension[23]. tsvarious components on the beam in a water medium lie in their absorption and scattering effects onof variouscomponents on the beam in a water medium lie in their absorption and scattering effectsthebeam [24].on the beam [24].4.1.1. Attenuation Effect of Water4.1.1. Attenuation Effect of WaterThe incident light can be absorbed or scattered in water. The total effect of these two mechanismsThe attenuation,incident lightresultingcan be absorbedor scatteredin water.The totaleffect andof thesetwo mechanismsis calledfrom watermoleculesand atoms,biomass,chemicalsdissolvedis calledattenuation,resultingfrom watermoleculesand atoms,biomass, tattenuationat a wavelengthof chemicals450–530 nmis atawavelengthof450–530nmismuchsmallersmaller than other light wavebands, so blue-green light [25,26] is generally assumed for underwaterthan otherlight wavebands, so blue-green light [25,26] is generally assumed for underwater opticalopticalcommunication.communication.The total absorption coefficient of water can be expressed as the sum of the absorptioncoefficients of various substances in water, as shown in Equation (5):𝑎(𝜆) 𝑎 (𝜆) 𝑎 ℎ (𝜆) 𝑎 (𝜆) 𝑎 (𝜆).(5)
Sensors 2019, 19, 1837 of 14The total absorption coefficient of water can be expressed as the sum of the absorptioncoefficients of various substances in water, as shown in Equation (5):a(λ) aw (λ) a ph (λ) ad (λ) ay (λ).(5)In Equation (5), aw is the absorption coefficient of pure water, a ph is the absorption coefficientof chlorophyll or phytoplankton, ad is the absorption coefficient of nonpigment suspended particles,and ay is the absorption coefficient of yellow substances.When light travels in water, the scattering effect causes the light to deviate from the originalpropagation direction. The scattering of light by water is caused by water itself, chlorophyll,and suspended particles [27]. The scattering coefficient is shown in Equation (6):b(λ) bw (λ) b ph (λ) bd (λ).(6)In Equation (6), bw is the scattering coefficient of pure water, b ph is the scattering coefficient ofchlorophyll or phytoplankton in water, and bd is the scattering coefficient of nonpigmented suspendedparticles. The parameters in Equations (5) and (6) are all relative to the wavelength m 1 in units.4.1.2. The Model of Energy AttenuationThe loss of light waves during their propagation is shown in Equation (7) [28]:c ( λ ) a ( λ ) b ( λ ).(7)In Equation (7), a(λ) is the total absorption coefficient, b(λ) is the total scattering coefficient,and λ is the wavelength.The attenuation law of energy transmitted by light in water is exponentially distributed [29],and the average power received by the receiver is shown in Equation (8):Pr Pt exp[ c(λ)r ].(8)In Equation (8), Pt is the transmitted optical power, r is the underwater communication distance ofthe optical signal, and c(λ) is the total attenuation coefficient of the light beam propagating in seawater.Considering the loss of the system device and optical path expansion, the received power ofunderwater wireless optical communication is shown in Equation (9) [30,31]:Pr Pt ηηt ηr [1 exp( 2 2r)]exp[ c(λ)r ].θ 2 R2(9)In Equation (9), ηt is the light-emitting power generated by the emitting device represented,ηr is the light source representing the received power generated by the receiving device of the lightsource, η represents the noise power, and r is the aperture radius of the optical receiving antenna.The parameter value settings are shown in Table 2.Table 2. Optical energy consumption parameters.ParametersValueηtηrCaliber D(mm)DivergenceAngle θ (mrad)Sensitivity(µW)AttenuationCoefficient (m 1 )0.910.9161.3511.53714.2. Acoustic Properties of Underwater ChannelsMarine media are very complex sound propagation channels that are subject to various naturalconditions, geographical conditions, and random factors, which makes their physical properties very
Sensors 2019, 19, 1838 of 14complex and unstable and causes delays, distortions, loss, and other changes in the transmission ofunderwater acoustic signals, loss, and other changes, which become the key factors affecting thetransmission of sound waves underwater [32].4.2.1. Propagation LossA seawater medium is a non-ideal loss medium. During the propagation of acoustic waves,the intensity of the seawater is attenuated in the propagation direction. Propagation loss is often usedto represent the loss of acoustic signal energy in the ocean [32]. It can be assumed that the propagationloss consists of two parts: Expansion loss and attenuation loss. The extended loss is the geometric effectwhere the sound intensity is regularly weakened when the acoustic signal propagates outward fromthe sound source. It is also called geometric loss. Attenuation losses include absorption and scattering,where absorption refers to the effect where part of the acoustic energy is lost due to conversion tothermal energy.The overall acoustic path loss can be estimated using the semi-empirical formula shown inEquation (10) [33]:A(l, f ) (l/lr )k a( f )l lr .(10)In Equation (10), (l/lr )k is the expansion loss, which is related to the propagation mode anddistance of sound waves; l indicates the distance traveled; lr indicates the reference distance; k is theexpansion loss index, according to different acoustic propagation conditions, whose value variesusually between 1 and 2, corresponding to cylinder expansion and spherical expansion; and a( f )l lr isattenuation loss, where f is the signal frequency and a( f ) is the absorption coefficient, which increasessignificantly with an increasing frequency of sound waves, in
consumption [8], and the attenuation of underwater blue-green light is relatively low, enabling underwater short-distance and large-capacity data transmission [9]. In recent years, combining the advantages of acoustic communication and optical communication, a new underwater data transmission mode has been proposed [10,11].
The Static Single-Phase Contention-Free FF (S2CFF) [9] (Fig. 2a) is based on the dynamic True Single-Phase Clock (TSPC) FF [18], with an additional conventional slave latch, and uses 24 transistors (equal to the conventional TGFF). It is a fully-static circuit without contention issues, which suggests suitability for NTV operation.
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