VHDL Modules and Circuits for Underwater Optical Wireless ... · PDF fileVHDL Modules and...

VHDL Modules and Circuits for Underwater Optical WirelessCommunication Systems

Davide Anguita, Davide Brizzolara, Giancarlo ParodiUniversity of Genova

Department of Biophysical and Electronic EngineeringVia Opera Pia 11A, Genova

ITALY{Davide.Anguita, Davide.Brizzolara, Giancarlo.Parodi}@unige.it

www.smartlab.dibe.unige.it

Abstract: Underwater wireless optical communication has been used for establish a link between mobile vehi-cles and/or fixed nodes because light, especially in the blue/green region, allows to achieve higher data-rate thanacoustical or electromagnetic waves for moderate distances. The here proposed work has the aim to pave the wayfor diffuse optical communication allowing to support optical communication in an Underwater Wireless SensorNetwork of dense-deployed fixed nodes for specific application, such as monitoring and surveillance, for shallow,coastal and inland water in the case of moderate/limited area. In particular, taking into account already developedprotocols for optical wireless communication, the focus is on the hardware implementation of different flexiblemodules to support both point-to-point and directional communication targeting the use of these technologies inthe field of Underwater Wireless Sensor Network and the integration with current terrestrial technologies for Wire-less Sensor Network. After a careful analysis of benefits and problems related to optical transmission underwater,design considerations are proposed with the description of the implemented modules. The use of light impulse forcommunication is motivated by the possibility of targeting high data-rate, low-cost and small dimension compo-nents. This paper describes an overall vision of the system: a HDL implementation of flexible modules for themanagement of optical communication (based on IEEE 802.15.4 and IEEE 802.11 standard) which target the in-terface with current terrestrial technology for Wireless Sensor Networks; the design and implementation of circuitsfor underwater optical point-to-point and planar communication. The preliminary results and design considerationare reported considering also future possible developments.

Key–Words: Underwater Wireless Communication, Optical Communication, Physical (PHY) Layer, Medium Ac-cess Control (MAC) Layer, Wireless Sensor Networks

1 Introduction

The growing need for underwater systems applied toobservation and monitoring has considerably stimu-lated the interest in advancing the technologies forunderwater wireless communication applied both forpoint-to-point communication than to support Under-water Wireless Sensor Networks (UWSNs) [1] [2].

Current technologies are mainly based on theuse of acoustic communication and makes use ofAutonomous Underwater Vehicles (AUVs), UWSNs,submarines, ships, buoys, and divers. These equip-ments are mainly developed to have a large operativearea and can achieve long distance communication (inthe range of kilometers), but still suffer from somelimitations: they are very expensive, very limited interms of data-rates transmission and they have usuallyvery large dimensions. Especially as regards the as-pects related to networking, they are often subject to

a wide range of problems due to acoustic communi-cation, especially considering that the propagation ofacoustic waves is not very fast in water (1500 m/s)[1]. The Wireless Sensor Network (WSN) paradigm,where miniaturized nodes cooperate to build a dis-tributed network for sensing the environment, hasgreatly improved terrestrial technologies in the lastfew years but it is difficult to transfer most of theknow-how developed for terrestrial devices to theirunderwater counterparts due to the particular charac-teristics of underwater world. In particular, wirelessunderwater communication still pose a wide range ofproblems [4]: the use of acoustic communication isthe most common but it has low data-rate, high powerconsumption and problems related to low propagationspeed.

The work presented in this paper explores the pos-sibility of using optical communication in Underwa-

WSEAS TRANSACTIONS on COMMUNICATIONS Davide Anguita, Davide Brizzolara, Giancarlo Parodi

ISSN: 1109-2742 525 Issue 9, Volume 9, September 2010

ter Wireless Sensor Networks (UWSNs) by designingflexible modules, devoted to manage optical commu-nication, which target the integration of existing avail-able terrestrial technologies for WSN with optical cir-cuits specifically implemented to support the genera-tion and reception of light impulses.

The use of the alternative solution of optical com-munication, despite reaching shorter distances be-cause of attenuation and scattering, reduces the prob-lem of low-speed diffusion and low data-rate of acous-tical signals. Optical underwater communication hasbeen investigated considering:

• the high data-rate, which can range from Mb/s toGb/s, if laser is employed;

• the possibility of using relatively low-powercomponents and circuits equipped with LEDsand photodiodes;

• the possibility of targeting relatively small di-mensions and low costs for the communicationsystem;

This paper is organized as follows: a briefoverview of the main aspects of underwater opticalcommunication and applications of an optical UWSN,a presentation of the current developed modules andcircuits followed by conclusion and future develop-ments.

2 ApplicationsOptical underwater communication is an effective al-ternative to current underwater technology especiallyin some particular environments such as, for instance,shallow, coastal and fresh inland waters where the useof this approach is useful to overcome all the short-comings related to the use of acoustic communicationand to allow a wide adoption of underwater monitor-ing systems [10][4]. In particular the possibility oftransferring high amount of data in a limited amountof time reducing power consumption can support theuse of cameras and thus the collection and transmis-sion of short video and pictures for a reliable moni-toring and surveillance supported by images (benthicanimals, different picture plankton and fish species.etc...).

Small dimensions and low-cost components al-low to establish a dense deployed networks perform-ing an effective fine grained sampling in the area of in-terest. The use of limited dimension imaging sensorscould be used to collect different pictures of plank-ton and fish species to compare the efficacy of re-mote monitoring against the usual approach, which

is performed by bringing samples to the laboratory.It could be possible, for instance, to perform pollu-tion monitoring and frequent data collection (watertemperature, specific conductivity, pH, turbidity, andpossibly oxygen concentration) and, by using a high-data rate optical link, periodically deliver data reduc-ing the time devoted to transmission and network con-gestion. The possibility of providing a high-data opti-cal wireless link, in fact, can support a more efficientenvironmental monitoring: an underwater device canbe equipped with the needed sensors for measuringthe fresh water environment, including water tempera-ture, specific conductivity, pH, turbidity, and possiblyoxygen concentration.

3 The State of the Art of UnderwaterOptical Communication

In this paragraph will be briefly investigated the stateof art related to underwater wireless communication.In addition researches related to Free Space Optical(FSO) communication will be presented with a briefintroduction to the most important current standardsfor optical communication which have been taken intoaccount for the development of the optical modulesdescribed the next paragraph.

Currently the use of wireless communication isvery common in a wide range of terrestrial devices.In particular, one of the most innovative applicationis related to Wireless Sensor Network (WSN), as welldetailed in [12], where a large number of small nodescommunicate and work together to perform a specifictask.

Specific applications include habitat monitoring,object tracking, nuclear reactor control, fire detection,and traffic monitoring where a large number of wire-less nodes is scattered in a region where it is meant tocollect data.

The progress and new developments in WSNsparadigm was clearly addressed by the Smart Dustproject [19], which explored whether an autonomoussensing, computing, and communication system canbe packed into a cubic-millimeter mote (a small parti-cle or speck) to form the basis of integrated, massivelydistributed sensor networks. This research led to newopportunities to observe and act on the world by usinga large number of small sensing self-powered nodes,which gather information or detect special events andcommunicate in a wireless fashion.

Also in the underwater world the application ofwireless communication can have an important role.Underwater wireless communication systems find ap-plications in oceanographic data collection, pollution



monitoring, offshore exploration, disaster prevention,assisted navigation and tactical surveillance applica-tions. Moreover, unmanned or autonomous under-water vehicles (UUVs, AUVs), equipped with sen-sors, will enable the exploration of natural undersearesources and gathering of scientific data in collabo-rative monitoring missions number of sensors and ve-hicles that are deployed to perform collaborative mon-itoring tasks over a given area.

Transferring the paradigm of WSNs to Underwa-ter Wireless Sensor Networks (UWSNs) is a challengethat can open great opportunities. In fact, at the mo-ment, current underwater wireless sensor nodes, incomparison with the terrestrial ones, are still very ex-pensive, power-consuming and their dimensions arenot reduced as for terrestrial ones.

Even if acoustic communication is the technologymore frequently used for these applications, as welldetailed in the next Paragraph 3.1, it is still an openfield of research. In addiction are also proposed sys-tems based on electromagnetic communication or op-tical waves.

Figure 1: Terrestrial [23] vs. underwater [22] nodes

3.1 UnderwaterAcoustical Communication

Since for the characteristics of the underwater channeltechnologies based on Radio Frequencies cannot beemployed, acoustic waves, which can propagate wellin seawater and can reach far distances, are widelyused in underwater communication. As discussed inthe introduction, although there are many recently de-veloped solutions for WSNs, the unique characteris-tics of the underwater acoustic communication chan-nel, such as limited bandwidth capacity and variabledelays, require very efficient and reliable new datacommunication protocols and UWSNs nodes are stillan open field of research [4][1]. The main limita-tions due to acoustic communication, which will bedescribed, can be summarized as follows:

• The available bandwidth is severely limited;

• The underwater channel is severely impaired, es-pecially due to multi-path and fading;

• Propagation delay in underwater is five orders ofmagnitude higher than in radio frequency (RF)terrestrial channels, and extremely variable;

• High bit error rates and temporary losses of con-nectivity (shadow zones) can be experienced, dueto the extreme characteristics of the underwaterchannel;

The problem of underwater acoustic communica-tion is still investigated in order to improve perfor-mances and overcome the current limitations. Someof the most interesting researches are here illustrated.

In [39] a high bit rate acoustic Link for videotransmission over an underwater channel is inves-tigated. To achieve this objective two approachesare proposed: the use of efficient data compressionalgorithms and the use of high-level bandwidthefficient modulation methods. In this paper the aimis to support a communication up to 64 Kb/s in orderto provide acoustic transmission capability neededfor near real-time video. In this case the focus is onthe use of high-level bandwidth-efficient modulationmethods. An experimental system, based on discretecosine transform, Huffman entropy coding for videocompression and and variable rate M-ary QAM hasbeen implemented. Phase coherent detection is ac-complished by decision-directed synchronization andadaptive equalization. System performance is demon-strated experimentally, using 25000 symbols/sec at acarrier frequency of 75 kHz over a short vertical path.The results were obtained using modulation methodsof 16, 32 and 64-QAM, thus achieving bit rates ashigh as 150 kbps, which are proposed as sufficient forreal-time transmission of compressed video.In [42] a preliminary study for a Low Power AcousticModem for dense underwater sensor networks isproposed. This work is interested in bringing thebenefits of terrestrial sensor networks underwater tothe underwater world targeting: an efficient wirelesscommunication, dense deployments (each sensor mayhave eight or more neighbors), self-configuration andlocal processing, and maximizing the utility of anyenergy consumed.

The datalink and PHY layers, written entirely inJava, use frequency-division multiple access (FDMA)with binary and 4-FSK (frequency shift keying) in anyfrequency band supported by the computers soundcard and can run at any bit rate supplied by the user.Besides being able to adapt to channel conditions,this software modem offers the advantage of beingfar less costly than current hardware devices, such asthe Benthos 013424 LF (9-14 kHz) omni-directionalmodem at$8800/pair as of April 2009, and of being



easily configurable.

In [42] a Reconfigurable modem (rModem) isdesigned to simplify the experimental studies of newunderwater acoustic sensor network algorithms on alllayers with the possibility of cross layer optimization.rModem provides a unified simulation and rapidprototyping environment by exploiting Simulinktools.

Also network network topology has been widelyinvestigate [1] because it is, in general, a crucial factorin determining the energy consumption, the capacityand the reliability of a network. The following sce-narios in the field of Underwater Acoustical WirelessSensor Networks (UW-ASNs) can be considered:

• Static two-dimensional UW-ASNs for ocean bot-tom monitoring. These are constituted by sen-sor nodes that are anchored to the bottom ofthe ocean. Typical applications may be environ-mental monitoring, or monitoring of underwaterplates in tectonics;

• Static three-dimensional UW-ASNs for oceancolumn monitoring. These include networks ofsensors whose depth can be controlled and maybe used for surveillance applications or monitor-ing of ocean phenomena (ocean biogeochemicalprocesses, water streams, pollution).

• Three-dimensional networks of autonomous un-derwater vehicles (AUVs). These networks in-clude fixed portions composed of anchored sen-sors and mobile portions constituted by au-tonomous vehicles.

3.2 Underwater Electromagnetic Communi-cation

The obstacle in using radio for underwater communi-cation is the severe attenuation due to the conductingnature of seawater. In particular the attenuation is veryhigh for high-frequency radio waves and, since thecurrent terrestrial technology (such as Wireless Sen-sor Network) is often based on high-frequency in therange of GHz, it’s impossible to use terrestrial devicesin underwater applications.

Extremely low frequency radio signals have beenused in military applications: Germans pioneeredelectromagnetic communication in radio frequencyfor submarines during World War II, where the an-tenna was capable of outputting up to 1 to 2 Mega-Watt (MW) of power. An extremely low frequency(ELF) signal, typically around 80 Hz at much lower

power, has been used to communicate with naval sub-marines globally today. This is possible mainly be-cause most of the transmission paths are through theatmosphere [43].

Some interesting studies and products related tothe use of electromagnetic waves underwater are herereported. In [44] a theoretical analysis and experi-ments show that radio waves within a frequency range1 to 20MHz is able to propagate over distances upto 100 m by using dipole radiation with transmissionpowers in the order of 100 W. This will yield high datarates beyond 1 Mbps which allows video images tobe propagated at standard camera frame rates (25Hz)[45]. The antenna design in such case is very differentfrom that of the antennas used for conventional ser-vice in the atmosphere: in fact, instead of having di-rect contact with seawater, the metal transmitting andreceiving aerials are surrounded by waterproof elec-trically insulating materials. This way, an EM signalcan be launched from the transmitter into a body ofseawater and picked up by a distant receiver.

Recently, in September 2006, the first commer-cial underwater radio-frequency (RF) modem in theworld, model S1510, was released by Wireless Fi-bre Systems [46]. Its data rate is 100 bps, and com-munication range is about several tens of meters. InJanuary 2007, a broadband underwater RF modem,model S5510, came into birth. It supports 1-10 Mbpswithin 1 meter range [46]. Due to the propagationproperty of EM waves, EMCOMM is an appealingchoice only for very short range applications. Oneexample is the communication between autonomousunderwater vehicles (AUVs) and base stations, wherethe AUVs can move within the communication rangeof a base station to offload data and receive furtherinstructions [43].

3.3 Underwater Optical Wireless Communi-cation

Underwater optical communication has been investi-gated both through theoretical studies [24] [4] [25]and in experimental tests [26], mainly developed inUSA, Canada and Australia.

Currently, there are not many research activitieson underwater optical communication, and few com-mercial optical modems are available specifically forunderwater communication. As well detailed in thenext paragraphs, recent interests in underwater sen-sor networks and seafloor observatories have greatlystimulated the interest in short-range high-rate op-tical underwater communication. For example, in[28][29], an optical modem prototype is designed fordeep seafloor observatories; in [30] and [31], a dualmode (acoustic and optic) transceiver is used to assist



robotic networks. Lab testings has shown that veryhigh datarates can be achieved within a short range.For example, a laboratory experiment for underwateroptical transmission achieves 1 Gbp/s rate over a 2-mpath in a water pipe [32].

In the next paragraphs different underwater sys-tems based on optical communication will be pre-sented focusing both on point-to-point than on diffu-sive optical communication.

3.3.1 Point-to-point Communication

In [33] a low power and low cost underwater op-tical communication system is proposed by usinginespensive devices that include: a light-emittingdiode (LED) source, a silicon-based photodetector re-ceiver, and off-the- shelf communication technologyutilizing the IrDA protocol. The aim is to provide asolid, robust communication link and rejects ambientexternal light noise because, considering the applica-tions for external submersible or ROV, lights could bean interference source. The proposed configurationuses an asynchronous serial communication rate of14.4 kbs, but maximum baud rate supported can behigher, according to the IrDA protocol. In this casea higher data-rate would require a lower capacitancephotodiode, which has less light-gathering capacityand would reduce the effective range of the device,which is already limited to 5 meters.Also in [34] an underwater communication systemhas been implemented for a swarm of submersibles. Itbased on an optical communication transceiver, smallin size, combining the IrDA physical layer with 3Watt high power light emitting diodes, emitting lightin the green and blue part of the visible spectrum. Asin the previous example, the approach is to use theIrDA physical layer modulation replacing the infraredlight emitting diodes (LEDs) with high power greenor blue LEDs, and also the photodiode with a typewhich is sensitive in the visible part of the spectrum.The prototype transceiver costs approximately$45per unit and, on this low hardware level, no linkmanagement or higher level error correction is done.The wide angular coverage, the uniform emissionfootprint and very high light intensity allow for eitheromnidirectional coverage up to 2 metre radius withonly five transmitters, when using simultaneouslytransmitting expensive LEDs that consume 2W or,with additional lenses, long range directional linkshaving a collimated beam (up to 5 meters).In [26] and [35] a low-cost medium-range opticalunderwater modem is proposed. The componentsused in this system are low cost costs (in the order of$10) A sophisticated detection algorithm is exploited(based on spread spectrum) to maximize the commu-

nication range. Tests have been performed up to 10meters and a data-rate of 310bps has been achieved.

In [30] and [31] the underwater wireless sensornetwork AquaNodes is described. In this applicationthe use of optical communication is shown as an effi-cient solution for data muling in the network proposedby CSIRO ICT Centre (Australia) and MIT CSIAL(USA). This research proposes a network where dataexchange is performed by connecting, both opticallyand acoustically, previously deployed static nodesand mobile (AUVs) vehicles, so that the characteris-tics of the two communication strategies can be ex-ploited. Each device is provided with an optical mo-dem, which can perform a transmission up to 300 kb/sin a range below 8 meters. This approach clearlyshows that the use of an optical communication sys-tem allows for a considerable reduction in terms of en-ergy consumption with respect to a multi-hop acous-tic transmission and a resulting increase of operationallife of the system. Moreover, considering this work,the use of optical modems instead of acoustic ones forshort range communications leads to a remarkable re-duction of costs, since the cost of an optical modem isof the order of$50 node against approximately$3000node of an acoustic modem.

In [36] is illustrated a careful analysis by model-ing the underwater channel based on underwater op-tics. Through this analysis, it is showed that a singlecolor LED is very weak in a wavelength dependentunderwater channel and, to overcome this problem,a multi-wavelength adaptive scheme combined withrate adaptive transmission is proposed taking inspi-ration from already developed algorithms. The pro-posed system can adapt to the channel by consideringthe change in power for each wavelength band, andcontrolling the data rate.In [37] a short range underwater optical communica-tion link is established by using an open source freespace modem proposed by the project RONJA. whichconsists of three modules: a transmitter, a receiver,and an interface to the source. The design was not op-timize and was less robust than expected. While suc-cessful 10 Mbps operation was demonstrated and theuniversal twisted pair boards and transmitter boardsworked according to the standards, the receivers hadproblems capturing the signal. In this work the focuswas mainly on the design and implementation of cir-cuits for transmission and reception. The design of thetransmitter is based on Cyan or Green Light and andtwo lens are used to collimate the light from the LED.The receiver too is equipped with lens to condense thephotonic area down to the size of the detector area.The used equipment is very expensive and difficult touse in real applications; as reported, the experimental



results achieved 10 Mb/s at a distance up to 5 meters.A commercial product targeting optical

underwater communication, from Ambalux(http://www.ambalux.com), includes high-bandwidthtransceivers, which allow a point-to-point transmis-sion at a data rate of 10Mbs up to 40m, dependingon environmental conditions. A set of tests based onthis commercial products has been described in[38],where a solution for omni-directional communicationis also proposed in order to improve the efficacy ofthe connection with an underwater untethered vehicle.This approach, however, is not suitable for a denseUWSN or small and low power devices, where nodesare deployed with low accuracy and concerns onsystem size and energy consumption are addressed.

3.3.2 Diffusive underwater optical communica-tion

In these paragraph are reported some researches re-lated to the use of diffusive underwater optical com-munication for different applications.

In [40] the point-to-point optical com-munication system developed by Ambalux(http://www.ambalux.com) is used to perform awide number of tests with the aim of paving the wayfor a future underwater omni-directional wirelessoptical communication systems. The LEDs usedin the test emitted light in the green and blue lightspectrum and were tested in a pool and in a tankfilled with lake water. The primary objective of thesetests was to get profiles of the behaviors of suchcommunication systems with respect to water charac-teristics such as turbidity levels, prior to building anomni-directional optical communication. The resultsof the tests indicated that turbidity level, viewingangle and separation distance plays a significant rolein the behavior of blue light in water. In particular, theaim was to define a threshold viewing angle (TVA),the minimum viewing angle at which communicationis lost when one of the communication devices isrotated with respect to a virtual axis that contains thesegment represented by the center of gravities of thetwo devices when they are aligned.

On the basis of the previous studies, two geomet-ric forms were modelled: icosahedron and sphericalhexagon. The icosahedron was retained because ofits simplicity in geometry and its ability to providecomplete free space coverage using the selected LED.In [38], the previous illustrated configuration hasbeen tested to implement a high bandwidth opticalnetworking for underwater untethered tele-roboticoperation. A rate of 4 Mb/s was achieved inhemispherical configuration up to 5 meters. Experi-mentation in the field achieved initially 115 Kb/s over

a distance of 15 metres. A second experiment, afterthe modification of the transmitter and the receiversoftware, was attempted: in this case the transmissionwas increased to 1.5 Mb/s and the first wirelessunderwater video pictures were transmitted. Despitethe originally anticipated different kinds of turbidity(fish, plankton, rocks or other objects) in the water,the experiment still received video feedback of theanchor of a floating laboratory at around 15 metresdeep in the bottom of the lake.

In [29] an optical modem technology for seafloorobservatories is illustrated and problems related to de-sign and implementation of a system for underwa-ter optical wireless communication are focused. Theidea is to implement an optical modem system whichshould provide sufficient bandwidth to allow trans-mission of compressed high resolution video (i.e., 2-10 Mbit/s for studio quality video), allowing nearlyunrestricted motion on the part of the mobile sen-sor (UUV), and working at ranges below 100 m. Inthis case, the design of the system have been devel-oped considering the following aspects: (1) selectionof a transmission light source (wavelength, power,beamwidth), (2) selection of a detector (field of view,quantum efficiency, gain), and (3) selection of an aim-ing and tracking strategy. As regards transmissionlight source, LEDs are suggested to be used for thetransmitters as they are switchable at high rates, canbe reasonably collimated, and are easily arrayed to in-crease transmit power. High intensity blue LEDs suchas those fabricated from Gallium Indium Nitride (In-GaN) on a silicon carbide (SiC) substrate can provideon the order of 10 mW each and they can be config-ured in arrays to increase the total radiant flux (op-tical power). The light from one or more LEDs canbe collimated with a lens to focus its beam. Whiletheir bandwidth is low by optical standards (30 ns risetime), a 100 ns bit duration meets the design goal of10 Mbit/s. LED transmitters provide significant flex-ibility as well: both OOK and PPM are possible withthe same hardware. As regards the receiver, in thiswork a photomultiplier tube (PMT) is chosen for thedetector because it provides higher sensitivity and lessnoise than photodiodes (including avalanche or PINtypes) although the tube size can be a limiting fac-tor in certain conditions. Considering the aiming andtracking strategies, three possible configurations foroptical communications between a fixed node and afree-swimming vehicle are suggested in this paper:

1. Pointed transmission (Tx) and reception (Rx)with acoustic or optical aiming is the most ef-ficient scenario from an optical transmissionstandpoint. The transmitter consists of several



collimated LEDs or a laser diode, and the re-ceiver is a PMT. This method requires a searchand acquisition mode by both the Rx and Txwhich consumes time and energy (regardless ofwhether the aiming is acoustic or optical);

2. Directional-Tx to omni-directional-Rx scenariocan be accomplished with a hemispherical PMTdetector and either a laser diode or a hemispheri-cal array of LEDs. In this case, the aiming prob-lem is one-sided and could be accomplished viaacoustics or with optics;

3. Omni-directional Tx and Rx which is the me-chanically simplest solution. This can be accom-plished using the large area PMT detector with amoderate output-power blue laser diode or LEDand diffusing optics. The diffusing optics can bedone either with discrete reflective and refractiveelements or with a high transmission scatteringmedium.

Taking into account the previous proposedtheoretical studies, some optical systems have beenimplemented and tested, as reported in [28]. Theomni-directional light source used was blue-green(470nm) to take advantage of the low attenuationin seawater at those wavelengths, and produced auniform light field over a2π steradian hemisphere.Six commercially available 470nm (blue) light emit-ting diodes (LED) were arranged in a hemisphericalgeometry and encapsulated in a weekly scatteringpotting material to provide some diffusion of thelight field over the full hemisphere of operation. Thereceiver consisted of a large-aperture, hemisphericalphotomultiplier tube (PMT) chosen for its highsensitivity, low noise and high speed. Apertures andmirrors have been used to obtain a 91 m path length ina 15 m deep pool and to prevent reflections from thesurrounding walls from contaminating the results. A100 meter bench test has been performed to verify therange and geometry calculation using neutral densityfilters to simulate the attenuation of water.

In the same work ([28]) different tests are per-formed to validate the concept of omni-directionalfree water optical communication in the range of 10meters. In particular, the power spectrum of the re-ceived signal during the transmission of a pulse trainat different repetition rates is reported: 1.25 MHz, 5MHz and 10 MHz.

In [47] the concept of underwater laser sensor net-work is illustrated as a new approach for broadbandcommunication in the underwater environment basedon the blue-green laser has been proposed. In this pa-per the applications of the underwater sensor network

in the undersea exploration are discussed with the dif-ficulties in the traditional the underwater acoustic sen-sor network. A basic of prototype of underwater lasersensor network is described: it includes the architec-ture of laser sensor node and protocol stack for un-derwater laser sensor network, but it does not shownimplementation or interesting practical results.

4 Optical Wireless UnderwaterCommunication Aspects

Due to the impossibility of using Radio Frequencies(RF), traditionally wireless underwater communica-tion employs acoustic waves because sound propa-gates well in water and its range can be very long(∼km). However, it has several disadvantages such asnarrow bandwidth and latency in communication dueto the slow speed of acoustic wave in water. For in-stance, at ranges of less than 100 m the data transmis-sion rates of these systems in shallow littoral watersare∼10 kb/s.

Experimental tests have shown that an alterna-tive feasible solution is optical communication espe-cially in blue/green light wavelengths, even if lim-ited to short distances (up to 100 m)[4]. Comparedto acoustic communication it offers a practical choicefor high-bandwidth communication and it propagatesfaster in the water (2.255 x108).

Nevertheless it is affected by different factors totake into account for an efficient design [8]. The at-tenuation of a light beam between two points can bedescribed as in 1 whered1 andd2 are the positions ofthe points.

A = e−k(d1−d2)

(

d1

d2

)2

(1)

In thefirst term,k is defined ask = a(λ) + b(λ) and itis dependent by the wavelength:a is the term relatedto the absorption of water whileb models the scatter-ing which depends both on light wavelength and tur-bidity. The second term, instead, models the quadraticattenuation.

4.1 Wireless optical modulation techniques

In this chapter some considerations about underwaterwireless communication are proposed. In particular,since present underwater communication systems in-volve the transmission of information in the form ofsound, electromagnetic (EM), or optical waves the ad-vantages and limitations of each of these techniquesare illustrated. The problem of modulation for wire-less optical communication has been investigated in



literature both for terrestrial [53] than for underwaterapplications [52].In this paragraph, after a brief de-scription of the most important available techniques, acomparison of the different available techniques willbe carried out taking into account the peculiarity ofthe underwater environment.

As well detailed in literature [53] basic modu-lation techniques mainly contain three formats, suchas Amplitude Modulation (ASK), Frequency Modula-tion (FSK) and Phase Modulation (PSK).

In particular is here considered the Amplitude-shift keying(ASK). It is a form of modulation that rep-resents digital data as variations in the amplitude of acarrier wave. The amplitude of an analog carrier sig-nal varies in accordance with the bit stream (modu-lating signal), keeping frequency and phase constant.The level of amplitude can be used to represent bi-nary logic 0s and 1s. For low-cost optical commu-nication system this modulation with direct detectiontechniques (IM/DD) is preferred to reduce the com-plexity of the implementation, since in this mode ofoperation, the intensity or power of the optical sourcex(t) is directly modulated by varying the drive currentwhile at the receiver, a photodetector is used to gener-ate a photocurrent y(t) which is proportional to the in-stantaneous optical power incident upon it. It is possi-ble to think the carrier signal as an ON or OFF switchwhere 0 is represented by the absence of a carrier, thusgiving OFF/ON keying operation.

4.1.1 Pulse Position Modulation

Pulse-Position Modulation (PPM) can be consideredin order to further improve the anti-disturbance ca-pacity of information transmission and it proposed indifferent systems both for terrestrial [54] than for un-derwater systems [4].

Pulse Position modulation is a well-known or-thogonal modulation technique, in which M messagebits are encoded by transmitting a single pulse in oneof 2M possible time-shift. PPM scheme, at each timeinterval is Ts and L = 2M time-shifts constitute aPPM frame. At the transmitter end, the signal willbe launched by the light pulse to form a specific timeslot, and in the receiver end, the photoelectric diodedetects the light pulse and then to judge its time slotto evaluate his position and resume the signal. In L-PPM system, a block of input bits is mapped on to oneof distinct waveforms, each including one”on” chipand M-1”off” chips. A pulse is transmitted duringthe”on” chip, it can be defined as [52]:

Pm =

{

is t ∈ [m − 1]Tf/L, mTf/L

0 else(2)

Considering the characteristics of the differentmodulation the Table 1 can be proposed for a com-parison between different techniques (RB is the base-band signal bandwidth).

4.1.2 Various improved forms of PPM modula-tion

Since the characteristics of PPM appeared to be in-teresting for a wide range of applications some al-ternative and modified PPM modulations are here de-scribed taking into account advantages and disadvan-tages. In particular the following modulation are con-sidered:

1. Differential Pulse Position Modulation (DPPM);

2. Pulse Interval Modulation (PIM);

3. Dual Header Pulse Interval Modulation (DH-PIM);

TheDPPM can be considered a simple improve-ment of PPM modulation. To get the correspond-ing DPPM signal of a PPM modulation all the ”0”times slot behind the ”1” of the PPM . In compar-ison with the PPM, the DPPM symbol has no seri-ous symbol synchronous requirements it can providea higher power utilization and bandwidth utilization.For a fixed L,L-DPPM is less average-power efficientthan LPPM because it has a higher duty cycle. How-ever, for a fixed-average bit rate and fixed availablebandwidth, it can be used a higher L with DPPM, re-sulting in a net improvement in average power effi-ciency.

An other interesting modulation is thePIM: whilethe PPM modulation indicates the information by thelocation of the light pulse, the PIM modulation indi-cates the information by the interval between two op-tical pulses. PIM modulation still divides a frame intoL time-shifts, each log L bits binary information isencoded as the number of time slot between the twoadjacent light pulses.

DH-PIM is an improved PIM modulation, andalso it is a most complicated modulation technique. Ithas been frequently used for atmospheric laser com-munications modulation in recent years. Each DH-PIM symbol consists of a header and a number ofempty information slots.

4.1.3 BER vs. SNR

Considering the previous described modulations it isinteresting to evaluate can be based on the evaluation



Modulation ImplementationComplexity

Sensitivity tomulti-path delay

Required transmittedpower

Bandwidth

OOK simple general PR−OOK 2RB

FSK most complex most anti-sensitive

12PR−OOK |f1 − f0|+2RB

(2 FSK)DPSK more complex more sensitive 1

8√

ln 2PR−OOK 2RB (2 DPSK)

PPM simple general 2PR−OOK

Llog2L

RB (L-PPM)

Table 1: Comparison of Various Modulations [52]

of the Bit Error Rate versus SNR. The following re-sults are mainly based on [52] [49]. For OOK demod-ulation format a threshold is used to compare the re-ceived voltage to decide”1” or ”0”. In AWGN chan-nel model, the received voltage is

p(t) =

{

is + nc(t), ”1”nc(t), 0”

(3)

wherenc(t) is the Gaussian process. For the data”1”,the probability density ofx(t) is:

p1(x) =1√2πσ

exp(−(x − is)

2

2σ2) (4)

for thedata bit”0” the probability density of x(t) is:

p0(x) =1√2πσ

exp(−x2

2σ2) (5)

By fixing the the judgment threshold to12 i, the biter-ror rate is defined as:

pe(ook) =1

2erfc

is

2√

2σ=

1

2erfc

√S

2√

2(6)

where erfcis the complementary error function. Ac-cording to [35], in additive Gaussian noise channel,the bit error rate of 2FSK coherent modulation and2DPSK coherent modulation are given by the follow-ing equations:

pe(FSK) =1

2erfc

√

S

4(7)

pe(DFSK) = erfc

√

S

2(1 − 1

2

S

2) (8)

For the L-PPM modulation in the Gaussian whitenoise channel, there are many performance evaluationmethods for the BER as suggested in [55]. The fol-lowing formula is used in [49]:

pe(L − PPM) =1

L[1

2erfc(

1 − k

2√

2√

LS)]+ (9)

+L − 1

2[erfc(

k

2√

2√

LS)] (10)

A simulation,carried out in [49] on the basis ofthe previous formulas, shows the Bit Error Rate withdifferent SNR condition and 4 PPM and 8 PPM is em-ployed for PPM scheme and is reported in Figure 2.

Figure 2: The BER performance of different modula-tion schemeswith SNR [49]

4.1.4 Power Consumption

Also problems related to power consumption has beeninvestigated in literature [33] [52] since they are cru-cial in underwater devices. Small and light systemsare desirable in order to improve underwater sys-tem performance and considerations about power con-sumption are important in the choice of modulation.The following considerations can be carried out forthe different modulations:



1. for the Frequency Shift Key (FSK) modulationa specific frequency carrier wave for digital”1”,and a different frequency carrier wave for digital”0” are generated. In this case the optical trans-mit power required as the transmitting durationis always on and it appears to be inefficient forapplications on power-constrained devices.

2. the Phase Shift Key (PSK) modulator generatesan in phase signal for digital”1” and an out ofphase signal for a digital”0”. In this case thePSK demodulator are complex since the differ-ent coherent demodulation are needed to com-pare the current phase with the previous phaseand it can be inefficient for power-constraineddevices.

3. the On-Off Keying (OOK) and Pulse PositionModulation (PPM) do not use completely the fre-quency or phase information, but the design ofthe receiver and the transmitter is simple. Thedata throughput of PPM modulation is smallerthan OOK modulation, but the required receivepower is just:

1√

L2 log2L

of OOK modulation at the same error rate per-formance. It means that PPM could transmitlonger distance than OOK at the same transmit-ting power condition.

If P represent the smallest pulse width, the compar-ison of different modulation techniques, taking intoaccount the Maximum rate, the Transmit power andthe Complexity of modulation is shown in Table 2

5 System Description

5.1 Design consideration

The here proposed work has the aim to implement anddesign modules for the management of optical com-munication in a Underwater Wireless Sensor Network(UWSN), targeting the interface with current terres-trial technologies (in particular those based on IEEE802.15.4).

The here proposed hardware architecture is to beconsidered as a preliminary step since it supplies ahardware structure which can be, eventually, easilyadapted to different systems or integrated with addi-tional functions for different standards. In particularthis design can be easily modified or integrated on fur-ther considerations related to the telecommunicationprotocol or future experimental tests.

The trade-off between optimization and flexibilitymotivated the following choices:

• each functional block, taking inspiration fromthe IEEE802.15.4 protocol, has been imple-mented independently, by separating the man-agement of the PHY layer, the transmission andreception of data and the interface to the MACLayer;

• each module, both for Layer management andthe transmission and reception of data, has beendivided in sub-module (combinational logic orFinite State Machine (FSM)) specifically de-voted to a task in order to simplify the adaptationor modification of the implemented functionali-ties;

• the interface between modules and sub-modulesis managed by using FIFO Buffers and controlunits supporting the coordination and the ex-change of information.

Currently the PHY Layer and a functions subsetof the MAC Layer have been described by using anHardware Description Language (HDL), in particularVHDL (Very High Speed Hardware Description Lan-guage), as suggested in [5] [6] .

P r o c e s s i n g

u P F P G A /A S I C

M e m o r y

C o m m u n i c a t i o n

S e n s o r s

P o w e r U n i t

Figure 3: WSN node overview

The systemthen has been interfaced to a software fortesting purposes. The description of each module hasbeen carried out by using an Hardware DescriptionLanguage because:

1. it could easily integrated in WSN nodes (Figure3) developed for terrestrial application equippedby a Field Programmable Gate Array (FPGA),such as the one developed by the WISE Labora-tory at DIBE [7];

2. it could be used for the implementation of anASIC/ASIP specifically devoted to manage op-tical communication. In this case the implemen-tation of a prototype on FPGA is to be consid-ered as a passage for simulation and optimization



OOK FSK DPSK 4-PPM 8-PPMMaximum rate 1

2P1

2P1

2P1

2P3

8P

Transmit power Middle Higher Highest Low LowestComplexity of modu-lation

Low Higher Highest Lower Lower

Table 2: Transfer rate versus implementation complexity [49]

whose aimis then to build a specific CHIP for themanagement of the optical communication.

The design approach has been bottom-up, startingfrom the management of the circuits for transmissionand reception to the MAC Layer, and based on a mod-ular approach. The here proposed work targets theinterface with current terrestrial technologies and it isinspired to IEEE 802.15.4 for WSN and IEEE 802.11,which supports an optical PHY Layer based on In-fraRed (IR) [9].

MAC Layer - HDL implementation

Packet Parsing

CRC

PD PLME

PHY Layer - HDL implementation

Software Interface

Receiver Transmitter

Optical circuits for Tx and Rx

CSMA/CA

Control Unit

Figure 4: HDL modules overview

It is a preliminary step since it supplies a hardwarestructure which can be, if necessary, easily adapted todifferent systems or integrated with additional func-tions for different standards and protocols. This flex-ibility is based on a modular design and well definedinterfaces between each modules.

5.2 Physical Layer StructureThe PHY Layer structure, depicted in Figure 5, iscomposed of 4 principal modules described in the fol-lowing paragraphs.

The Transmitter generates the transmission onthe physical channel managing a LED circuit. It can

Figure 5: Physical Layer Structure

generate asynchronization signal or a transmissionsignal based on PPM modulation in which bits are en-coded by the position of the light pulse in time slots.The duration of the time slot is fixed by the value ofthe input prescaler and the modulation can be a 4 or16 PPM.

TheReceivermanages the output of the receivercircuit. It has been designed to synchronize automati-cally with the transmitter baud rate: it determines howmany clock cycles is the duration of the time slot cho-sen by the transmitter by counting 32 transitions of theinput. This value is the output prescaler and it is usedto decode the following PPM transmission. The re-ceiver detects the modulation (4 or 16 PPM) and per-forms also a clock correction in order to maintain syn-chronization.

The Physical Layer Management Entity(PLME) provides the layer management serviceinterfaces through which layer management functionsmay be invoked.

The Physical Data (PD) service enables thetransmission and reception of PHY protocol data units(PPDUs) across the physical channel.

5.3 Implementation Results

As detailed in the previous paragraphs, each modulehas been simulated to test its behavioral correctness



Module Slices %PHY Layer 866 45Trasmitter 154 8Receiver 231 12PLME 173 9PD 308 16

Table 3: Implementation on Xilinx Spartan-3 FPGA

by usingXilinxISETM simulation tools since theproject has been developed on Xilinx Tools. In thisparagraph the results regarding the implementation ofthe PHY Layer module described in the previous Para-graphs are illustrated considering also ACTEL tech-nology and the use ofLiberoIDE integrated tool.In particular the implementation on the following de-vices have been targeted:

1. a Xilinx Spartan III;

2. a Actel IGLOO AGLN250;

The Xilinx Spartan III has been chosen because itcommonly integrated in low-cost student board whichoffer useful interfaces for experimental tests, illus-trated in the next chapter.

The IGLOO AGL250 has been chosen because itis a low-cost and low-power FPGA used for high vol-ume applications, such as sensor nodes, where powerand size are key decision criteria having the advan-tages of flexibility and quick time-to-market in low-power and small footprint profiles.

5.3.1 Xilinx Spartan III implementation

The Digilent Board is a powerful, self-contained de-velopment platform equipped with a Spartan-3 FPGAfrom Xilinx (Xilinx Spartan3 XC3S200-FT256) TheSpartan III has 200K System Gates, 4.320 equivalentlogic cells and a 50 MHz clock.The 4-digit seven-segment display on the board havebeen used to show the the value of some parameters(for instance the value of prescaler) with the 8 avail-able LEDs. A dedicated program inC# to managethe interface to the PHY Layer has been implemented:it allows to performs a test of transmission and setdifferent parameters of the PHY Layer. Since thePHY Layer VHDL description is developed in stan-dard VHDL by using a modular approach, when animplementation on a specific device is targeted it ispossible to perform an optimization by replacing somemodules with core-generated blocks of code. In thiscase a better result in terms of occupied area can beachieved.

In Table 3 the results of the implementation onXilinx Spartan-3 FPGA are reported in terms of slices.The Configurable Logic Blocks (CLBs) of Spartan-3FPGA Family constitute the main logic resource forimplementing synchronous as well as combinatorialcircuits. Each CLB comprises four interconnectedslices. These slices are grouped in pairs. Each pairis organized as a column with an independent carrychain. Both the left-hand and right-hand slice pairsuse these elements to provide logic, arithmetic, andROM functions. Besides these, the left-hand pair sup-ports two additional functions: storing data using Dis-tributed RAM and shifting data with 16-bit registers.The maximum frequency that can be achieved by thesystem is 40 MHz. A preliminary test has been per-formed in air at the maximum data-rate supported bythe Serial Port RS-232 of the Board (in the order of 10kb/s). by using a simple circuit composed by a LEDand a Photodetector (Figure 6).

Figure 6: Preliminary Test on Air

5.3.2 IGLOO Low Power

As detailed in the previous paragraph, the possibilityof the integration of a low-power FPGA in a WirelessSensor Node is an interesting opportunity for the man-agement of the underwater wireless optical commu-nication. Considering the current available devices,the Actel IGLOO low-power FPGA appears to be aneffective solution. By using the Actel Libero Inte-grated Design Environment, as for the implementa-tion on Xilinx, some optimizations has been carriedout. The target FPGA is a IGLOO nano Devices(AGLN250) with 250.000 system gates (6.144 equiv-alent core cells) and up to 71 I/O pins.The implementation results are reported in Table 4.The maximum clock rate achievable is up to 20 MHzwhich can easily support a communication in the or-



Module CoreCells

%

PHY LAYER 2405 40Trasmitter 347 6Receiver 735 12PLME 442 7PD 881 15

Table 4: Implementation on IGLOO Devices(AGLN250)

derof Mb/s.

5.4 Medium Access Control DesignThe MAC Layer hardware design is based on a mod-ular approach similar to that used for PHY Layer im-plementation. The MAC Layer handles the access tothe PHY Layer through the PD and the PLME, while itmanages different services for the Upper Layers, sim-ilarly to the PLME in the PHY Layer.

CSMA/CA fsm

Counter

Random Gen

Clock

Reset

maxBE

maxBC

CCAres

CCAreq

BoExp

BoNum

CCAsuc

Figure 7: CSMA-CA implementation

Currently the following functions have been imple-mented:

1. Packet parsing and addresses verification;

2. Cyclic Redundancy Check (CRC): to check theintegrity of transferred data;

3. a module to manage the transmission and recep-tion of the MAC Payload to the upper layer;

4. the CSMA/CA mechanism for channel access.

The competition for channel access is based on aCSMA/CA protocol in which a device, when it wishesto transmit data, waits for a random number of back-off periods before detecting the channel. If the chan-nel is busy, the device increases the number of at-tempts by one and checks if the maximum number ofattempts has been reached. If the limit is exceeded,the device generates a channel access error and reportsthis event to upper layers. If the number of attempts is

below the limit, the device repeats this procedure un-til it access the channel successfully or the number ofattempts exceeds the limit.

The previous described mechanism has been im-plemented in hardware by using a dedicated modulecomposed by a random number generator, for the cal-culation of the random delay, a counter and an othermodule, based on a FSM, to manage the CSMA/CAalgorithm as depicted in Figure 7.

5.4.1 Considerations on implementation

The previous reported results show that the structureand functionalities of this VHDL description of theoptical PHY Layer can be easily implemented on low-cost and low-power FPGA allowing also the integra-tion with other modules to describe the MAC Layerand the interface with upper layers. This work hastaken inspiration from different hardware implemen-tation [57] but, since the implemented protocol hasbeen adapted to optical communication, in particu-lar taking into account the characteristics of IEEE802.15.4 protocol, it is not easy to make a detailedcomparison with implementation results reported inliterature.

The examples of VHDL description and hardwareimplementation are common in literature for differentreasons. In general these implementations [56] [58]propose better performances or reduced power con-sumption by using dedicated hardware.

This design has been mainly oriented to defineand describe a flexible hardware architecture that canbe easily adapted, improved or also modified tak-ing into account deeper consideration about the pro-tocol and possible alternative choices related bothto telecommunication aspects than to implementationones.

The current implementation of this optical PHYLayer can be considered a good starting point foradding new functionalities up to the complete im-plementation of an dedicated optical module whichwould manage, in connection with the here imple-mented PHY Layer, the MAC Layer adding furtherfunctionalities as briefly described in the followingparagraph.

6 Test of the Communication SystemThe previous described modules have been synthe-sized and implemented on a Xilinx Spartan-3 FPGAand a dedicated program inC# is used to manage theinterface to the Digilent Spartan 3 Board: it allows toperform tests of transmission and set different param-eters of the PHY and MAC Layer (Figure 8).



Figure 8: User interface to optical PHY and MAC Layers

The VHDL description is developed in genericstandard VHDL and it is planned an integration ina WSN node by using an Actel IGLOO low-powerFPGA which appears to be an effective solution forits low-power and reduced costs.

Some circuits implementation and experimentaltests have been carried out. They are the basis to im-plement a diffuse optical communication which couldsupport a dense network of small nodes able to ex-change data at high data-rate (in the order of Mb/s).In general our approach has focused the possibility ofusing low-cost and low-power components.

In the next paragraph a point-to-point transceiveris described and then a 2-d structure for planar com-munication is proposed.

For point-to-point transceiver tests have been per-formed both in air than underwater by using a specif-ically designed tank. The tank (Figure 9) consists ofa 2 meters long waterproof pipe built of high-densitypolyethylene black plastic.

The transmitter generates an impulse of light ofa fixed duration (250 ns) which allows to support atransmission at 1Mb/s in the case of an 16-PPM mod-ulation or at 2Mb/s in the case of a 4-PPM. The choiceof LED wavelength has been done to maximize thepower of received signal as illustrated below (Figure17).

Particular attention has been posed on the circuitfor the reception, since the reciprocal distance of theunderwater devices cannot be fixed in advance and thereceiver has to maintain his functioning in all the cov-erage area of the transmitter.

The receiver has been designed considering dif-ferent blocks: a photodiode; a transresistance ampli-fier, to have a conversion from current to voltage; abandpass filter to eliminate noise below 10 kHz andabove 20 MHz; an Automatic Gain Control (AGC),based on a Linear Technology LT1006, used to am-plify the signal received by the first part of the circuitand to automatically increase or decrease the gain ac-cording to the signal amplitude; a comparator, to de-termine the output value by fixing a threshold.

Figure 9: Experimental set-up

7 Experimental Test Bed

The tests have been performed at the Hardware Labo-ratory of DIBE (Department of Biophysical and Elec-tronic Engineering).

The air test have been carried out by using sin-gle guides to fix the reciprocal position of a trans-mitter and a receiver and different electronic equip-ments (such as oscilloscopes) to perform the appro-priate measurements.

The underwater tests, instead, have been per-formed by using a specifically designed and imple-mented tank. The tank (Figure 10) consists of 2 meterslong waterproof pipe built of high-density polyethy-lene black plastic. Its width and depth are 20 centime-ters and it has been filled with more or less 50 litersof liquid. It has been equipped with a tap at the basisso to recycle the same water for tests on different sys-tems. The water used for the tests was clear distilledwater or tap water.

Moreover, specific mechanical components hasbeen designed to support tests with transmitter and re-ceiver. The position of the receiver is fixed at the topof the tank while the position of the transmitter can bevaried. A specific guide (Figure 11) has been designed



Figure 10: Tank for experimental tests

to allow the movement of the transmitter in the water:it can be moved step by step to reach the maximumlength of the tank so that the measurements could beperformed at different distances.

Figure 11: Mechanical guide for the transmitter

7.1 Preliminary Experimental Tests

The aim of this previous test has been to test the func-tionalities of the experimental testbed and it allow toshow, by implementing a simple circuit, the differentbehavior of underwater light propagation at differentwavelengths.The transmission of light has been performed by us-ing LEDs of different colors, each corresponding at adifferent wavelengths: blue, green and red.The experimental test bed has a transmitter for thegeneration of light of different colors and a receiver(Figure 12) composed by:

1. a transresistance amplifier: to have a conversion

Figure 12: Receiver block description

from currentto voltage (Figure 13);

2. an amplifier with a variable gain.

Figure 13: Transresistance amplifier

The transmitterand the receiver have been placedin the tank and the value V at the output has beenmeasured for different distances. For each distance,the input current to the LEDs has been set to have atthe receiver the same output voltage. This approachallows not to take into account the different spectralresponse of the photodiode for different wavelengths.In addition the output value due to the dark current ofthe photodiode has been measured in order to calcu-late the effective value of the received signal. In thiscase it is 1.52 V. The tests have been performed byusing distilled water. Considering the same config-uration of receiver and transmitter this test allows toshown the different behavior of each wavelength. Thetests are reported in tables 5 and 6.

Considering the experimental results is possibleto notice that, as expected, the value for each wave-length is different and, for the used water, the bestwavelength appears to be the green one. The use ofthis circuits could be useful to perform experimentalevaluation for different types of water. However, theproposed approach does not take into account the dif-ferent spectral response of the photodiode for differentwavelength and the different input current values usedto maintain the same output at the receiver.



Distance(cm)

Blu Green Red

100 2.01 V 2.08 V 2.02 V125 1.86 V 1.91 V 1.87 V150 1.75 V 1.81 V 1.77 V175 1.66 V 1.70 V 1.67 V

Table 5: Signal value at the receiver with distilled wa-ter

Distance(cm)

Blu Green Red

100 490 mV 560 mV 500 mV125 340 mV 390 mV 350 mV150 230 mV 290 mV 250 mV175 140 mV 180 mV 150 mV

Table 6: Effective value

7.2 Communicationtest

The PHY Layer currently has been tested performinga point-to-point optical communication. In this case,due to the limitations of the serial interface betweenthe Board and the PCs, the transmission rate has beenreduced to the maximum data-rate supported by theSerial Port RS-232 of the Board (in the order of 10kb/s) by using a preliminary version of the circuits(Appendix??).

Figure 14: Experimental Test Bed configuration

The experimental set-up (Figure??) is composedof two PCs, for running the user interface, connectedrespectively to a Digilent S3 Board where the opticalPHY layer has been implemented. Each Board is con-nected to the transmitter and receiver circuit placed inthe of clear water. The parameters of the transmissionhave been set from the user interface.

The programs allows to transmit and receive a se-quence of data which can be selected, for instance,from a file. By using the program interface is possibleto set some parameters of the PHY Layer or to send a

Figure 15: Program interface utilities

primitive in order to display its value. Some particularof the interface utilities are shown in Figure 15.

By using the interface, for instance, it is possi-ble to set the modulation (4-PPM or 16-PPM) and thevalue of prescaler which define the duration of a singleimpulse of light.

7.3 Point-to-point optical communication

7.3.1 Circuits Specification

The here described transceiver has been designed tosupport the generation and reception of light impulseson the basis of the following specifications.

The transmitter generates an impulse of light of afixed duration: in this case, the impulse duration hasbeen fixed at 250ns which allows to support a trans-mission at 1Mb/s in the case of an 16-PPM modula-tion or at 2Mb/s in the case of a 4-PPM.

Considering the application in clear water and theaspects regarding water absorption illustrated in theprevious chapters, the impulse is generated by using afixed wavelength determined as described in the nextparagraph (Figure??). As regard the distance, the tar-get is between 5 and 10 meters, since the aim is tosupport an Underwater Wireless Sensor Network ofsmall and densely deployed nodes.

Particular attention has been posed on the circuitfor the reception. In fact, since in the applicationtargeted by this work the reciprocal distance of theunderwater devices, in certain conditions, cannot befixed in advance, the receiver has to maintain his func-tioning in all the coverage area of the transmitter andthis condition has to be carefully evaluated in the de-sign.



P r e a m p l i f i e r H i g h P a s sF i l t e r

L o w P a s sF i l t e r A G C C o m p a r a t o r

O u t p u t

Figure 16: Receiver block description

7.3.2 Receiver

The receiver has been designed considering differentblocks as illustrated in Figure 16.

The block diagram consists of:

• Photodiode;

• Transresistance amplifier: to have a conversionfrom current to voltage;

• High pass filter: to eliminate noise below 10kHz;

• Low pass filter: to eliminate noise above 20MHz;

• Automatic Gain Control: it is used to amplifythe signal received by the first part of the circuitand to automatically increase the gain when thesignal amplitude is small and decrease the gainwhen the amplitude is higher;

• Comparator: to decide the value of the output byfixing a threshold.

The receiver has been implemented by using thefollowing component: Si PIN photodiode HamamatsuS5971 - high-speed photodiodes, with1mm2 surfacearea. To evaluate the better wavelength for the trans-mitter, as suggested in [30], Table 7 has been compiledconsidering the absorption coefficients for clear watercollected by Pope [?] and the photodiode sensitivityreported in the component datasheet. The output cur-rent of the photodiode is proportional to:

S × e−k(d) × P (11)

where S is sensitivity an P is the power in watts, dthe distance and k the absorption coefficient. Figure?? shows how the output current varies according todistance for different wavelengths of light, by usingthe equation 11.

Wavelength ofemitted light

PhotodiodeSensitivity

Absorption Coeffi-cient

450nm (blue) 0.37 A/W 9.2 x10−5cm−1

532nm (green) 0.39 A/W 4.4 x10−4cm−1

650nm (red) 0.42 A/W 3.4 x10−3cm−1

890nm (infrared) 0.63 A/W 6.0 x10−2cm−1

Table 7: Sensitivity for PDB-S5971 High-speed pho-todiodes andabsorption coefficients

Figure 17: Comparison between different wave-lengths



Due to the severe attenuation in water, the outputcurrent for infrared is less than for blue/green lightat 10m, even if the sensitivity of the Photodiode ishigher for infrared. It is possible to note that that redlight outperforms green light up to 1.5/2 m but blueand green are better beyond. Taking into account theprevious results, also considering that the attenuationis strongly influenced by turbidity, the better choiceappear to be blue or green light. These considera-tions are very important because the system perfor-mance is determined by the detector when signal at-tenuation along a wireless link is considered. It is im-portant for the receiver to detect low-level optical sig-nals maintaining a Signal-to-Noise Ratio (SNR) suf-ficiently large to yield an acceptable Bit Error Rate(BER).

For the AGC stage the circuit depicted in Figure18 been designed and implemented.

The circuit is used to support the automatic gaincontrol. It has been implemented by using a lowpower consumption device produced by Linear Tech-nology LT1006 and the operational amplifier AD600.By using the OrCad simulator, some tests have beenperformed on this configuration. It never reaches sat-uration for the presence of a strong impulse and, onthe other hand, amplifies low level signal. It allowsthe receiver to avoid the saturation in case of exces-sive proximity of the transmitter LED. Instead, whenthe light impulse detected by the receiver is small, itallows a good amplification.

7.3.3 Transmitter

The transmitter is a circuit that allows to drive a BlueLED in order to generate light impulse of 250 ns. Inthis case the Everlight Electronics LED, DLE-038-045 is used. It has typical luminous intensity of 1630mcd, a wavelength peak around the 428 nm and a fieldof view of 30o. The implemented transmitter havebeen used both for the tests in air than for the testsin the tank of water.

7.3.4 Tests of the receiver circuit

The received circuit has been tested in air and in wa-ter by using the transmitter, by measuring the outputvoltage at the AGC stage output, by using a waveformgenerator which allows to generate trains of impulses.The tests have been performed at different distancesboth in air than in water considering the limitation ofthe available testbed. In Table 8 and 9 the measuredvalues are reported. The reported results show thatthe use of the AGC allow to avoid saturation when thetransmitter and the receiver are practically in contactwhile the amplification at higher distances could be

Distance Output1 cm 6,20 V5 cm 6,10 V50 cm 4,90 V100 cm 3,20 V150 cm 2,60 V175 cm 2,30 V

Table 8: Measured signal values in water

Distance Output1 cm 6,40 V5 cm 6,10 V10 cm 5,30 V100 cm 4,00 V150 cm 3,20 V200 cm 2,90 V250 cm 2,40 V300 cm 2,10 V350 cm 1,70 V400 cm 1,20 V

Table 9: Measured signal values in air

improved. Nevertheless it allows to receive a signalup to 4 meters.

7.4 Design consideration for directionalcommunication

The possibility of targeting the connection betweenmore than two devices leads to the necessity ofimplementing a directional, up to omni-directional,transceiver able to send and detect optical impulse.

Figure 19: Transmitter disposed to cover a 2-D circu-lar area



Figure 18: Receiver Automatic Gain Block Design

Taking intoaccount some previous works, in thisparagraph some considerations regarding the designand implementation of a circular transceiver are re-ported with a preliminary implementation description.

Figure 20: Transmitter disposed to cover a 2-D circu-lar area(withoverlap)

In particular the collocation of components (LEDor photodiodes) on a 2D structure is considered. As-suming that n transmitter are placed at equal distancegaps on the circular node (radius r), considering thatthe diameter of a transmitter is2ρ:

τ =2πr − 2nρ

n(12)

Theangulardifference between any two neighboringtransceiver is given as:

ϕ = 3600 τ

2πr(13)

The coverage area L of a single transmitter can begiven by:

L = R2tan(ϑ) + 0.5πRtan(ϑ)2 (14)

Two cases can happen for the effective coverage areaC of a single transmitter, based on the value ofϕ, θ,R, and r:

1. if coverage area of the neighbor transmitter donot overlap 19:

Rtan(ϑ) ≤ (R + r)tan(0.5ϕ) (15)

In this case, the effective coverage area is equiv-alent to the coverage area, i.e. C=L.

2. Coverage area of the neighbor transmitter over-lap 20:

Rtan(ϑ) > (R + r)tan(0.5ϕ) (16)

In this case, the effective coverage area is equiv-alent to the coverage area excluding the area thatinterferes with the neighbor transceiver. If I is theinterference area that overlaps with the neighbortransmitter’s coverage, then C = L-I. Consider-ing that the target of our work is to have a direc-tional or at least omni-directional a good designapproach should minimize the interfere area.



If the target is to have a transmission and reception ofdata byusing all the elements placed on the circularstructure, the overlap is not a problem for the commu-nication. But, since the final idea is to add the possi-bility of supporting also a directional communicationin which a single LED could be activated, the idea isto minimize the interference between adjacent LEDsand receivers.

Taking into account the previous illustrated con-siderations, an implementation of the transmitter hasbeen tested in order to evaluate the behavior of trans-mitter signal for a 2-d structure. This implementationtargets the reduction of the overlap and the possibilityof supporting also a directional transmission, by usingonly one or a reduced set of LEDs for generating thesignal.

Figure 21: Circular LEDs configuration used for ex-perimental tests

TheLEDs disposition on a 2-d structure has beenperformed using 12 Ledman LL1503PLBL1-301 blueLED (with 30o of FOV) disposed on a circular disk of10 cm diameter. By using single receiver, equippedwith a SFH-2013P photodiode, different measureshave been considered at different distances, as re-ported in the Table 10. In particular, the profile of thereceived optical signal can be determined by consid-ering the maximum value in a line-of-sight conditionand the minimum value which is measured between aLED and another.From the measurements reported in Table 10, it isshown that the difference between the minimum andthe maximum achieved value is higher near the trans-mitter while it become less important when the dis-tance between the transmitter and the receiver in-creases. On the basis of the values of Table 10, theprevious structure can be considered acceptable as abase for the implementation of the 2-D transceiver.Currently a circular transceiver, equipped with LEDs

Distance(cm)

Max Value Min Value Difference

80 7.2 V 6 V 1.2 V100 6.5 V 5.5 V 1 V150 5.5 V 4.7 V 0.8 V200 4.9 V 4.6 V 0.3 V

Table 10: Signal values at the receiver

Component Cost (euro)LED 5.69SFH-203P 5.25AD600 12.57PDB-C139 2.91S5971 7.59OPA357 0.51LTC6244 1.61

Table 11: Cost of some components used for circuitsimplementation

and Photodiodes,is under construction.

7.5 Comparison with the state-of-the-art andconclusions

The optical communication modules illustrated in theprevious chapter and the circuits tested and describedin this chapter can be considered a first step for the im-plementation of an UWSN based on optical commu-nication. The illustrated tests and simulations, com-pleted only for a point-to-point communication, willlay down the foundation for an underwater directionalor omni-directional optical communication system foran optical UWSN.

In this paragraph, a brief comparison with currentoptical underwater communication systems is pro-posed in order to highlight the novelty of our approachand to compare the achieved results.

A trade-off between circuits specifications andlow-power and low-cost components have been tar-geted since the receiver and the transmitter can beused in a large number of devices where energy sup-ply can be difficult. Information on the cost of thecomponents is reported in Table 11.

In [37] a point-to-point underwater optical com-munication is implemented by using an off-the-shelveoptical modem (http://ronja.twibright.com/) used forterrestrial wireless optical communication which al-lows two sources to transmit and receive data usinga modulated beam of light up to 10 Mb/s. In thiscase the design of the transmitter is based on Cyanor Green Light and and two lens are used to collimate



the light from the LED. The receiver too is equippedwith lensto condense the photonic area down to thesize of the detector area. In this work, focusing onpoint-to-point communication in an experimental en-vironment, the possibility of using this equipment inreal applications is not well addressed. Complex andcostly lens and cases (up to 300 euro) and power-consuming components make it difficult to transferthis technology to small and low-cost underwater de-vices. In comparison the work illustrated in this paperproposes a low cost equipment (100 euro) and a com-munication up to 2 Mb/s managed by flexible moduleswhich can be adapted both for point-to-point and foromni-directional communication.

In [38] [40] an high bandwidth optical com-munication is proposed as a support for untetheredtelerobotic operation. In this work the researchesstart from the necessity of supporting the real-timereception and transmission of video to support un-derwater activities such mining, construction, aqua-culture. They use the previous described com-mercial optical modem from Ambalux Corporation(http://www.ambalux.com) and they adapt this systemin order to support the generation of omni-directionaloptical transmission which should easily allow the in-terface with Underwater Vehicles moving in the rangeof the transmitter. In this case the focus is mainlyon achieving an high bandwidth communication whilethe dimension and power-consumption of the compo-nents is not the main concern. The reported results[40] illustrates a communication of 1,5 Mb/s up to 15meters between a fixed node and a mobile Vehicles.

In this work the approach is different from thatproposed in this paper where the idea is to supporta network of densely deployed underwater nodes. Inthis case the basic idea is to create a higher FOV in or-der to avoid problems of tracking or pointing betweenfixed nodes and mobile underwater vehicles. Theachieved data-rate can be compared with the prelimi-nary results achieved by the underwater optical wire-less system proposed in this paper. As concern costand power-consumption, it is not possible to makecomparison because data regarding components arenot reported in these papers.

In [26] an underwater wireless optical modemis presented for point-to-point optical communicationtargeting medium-short distances, from 5 to 10 me-ters. In this work the aim is to provide a much morecost-effective alternative for applications where therequired communication range is modest, since exist-ing acoustic modems often cost on the order$1.000to $10.000, In this work the approach is similar tothat proposed in this paper, but they focus on an indi-vidual transmitter-receiver pair. In this work the sig-nal detection is based on direct sequence spread spec-

trum (DSSS), using a 32-chip Gold sequence whiledata communication beyond mere signal detection isachieved by using pulse position modulation (PPM).The cost of the proposed system is comparable withthat proposed in this system (in the order of 10 euros),even if the data-rate is approximately 310bps in com-parison with the here proposed system which allows awireless communication up to 2 Mb/s.

In [30] [31] an underwater wireless sensor net-work which consists of static and mobile underwa-ter sensor nodes is described. The nodes communi-cate point-to-point using a novel high-speed opticalcommunication system and they broadcast using anacoustic protocol integrated in the TinyOS stack. Thenodes have a variety of sensing capabilities, includingcameras, water temperature, and pressure. The mo-bile nodes can locate and hover above the static nodesfor data muling, and they can perform network main-tenance functions such as deployment, relocation, andrecovery. In this case the use of optical communica-tion is limited to point-to-point activities. The modu-lation is based on a modified VFIR optical communi-cation format the receiver gets a continuous sequenceof pulses regardless of the data (as opposed to NRZand its variants), and can be easily kept in syncroniza-tion.

As for our system they target the integration withan existing protocol and a limited range of operations(up to 5 meter). They employ an optical communi-cation based on a green LED achieving a data rate of320 kb/s. The system proposed in this paper, instead isbased on a PPM modulation and implements a differ-ent method for synchronization. aiming at achievinghigher data-rate. It targets the interface with currentavailable terrestrial technologies for WSN with opti-cal communication for connection between nodes.

In [28] diffuse optical transmission is proposedfor a connection between AUV and seafloor under-water observatories. The ideal platform for exploringand monitoring these sites, especially during episodicevents, is a remotely-piloted, unmanned, underwatervehicle (UUV) which is capable of sending back high-quality video or other high-rate sensor data. The com-bined requirement of remote-piloting and high datarates suggested the idea of a bidirectional optical wire-less communications link capable of sending and re-ceiving data at one to several Mbits per second. Theimplemented transmitter is composed by six commer-cially available 470nm (blue) light emitting diodes(LED) were arranged in a hemispherical geometryand encapsulated in a weekly scattering potting ma-terial to provide some diffusion of the light field overthe full hemisphere of operation. The selected LEDshave sufficiently fast rise and fall times to switch upto a PRF of 10 Mb/sec. The receiver consisted of



a large-aperture, hemispherical photomultiplier tube(PMT) chosenfor its high sensitivity, low noise andhigh speed. A 10 m vertical dock test was performedin slightly turbid water of 10 m depth: the powerspectrum of the received signal during the transmis-sion of a pulse train at different repetition (1.25 MHz,1OMHz, 5MHz) rates is reported.

This work focuses in particular the communica-tion between Underwater Mobile Vehicles and a fixednode targeting application in sea floor observatories.The achieved distances and data-rate are interestingbut it could be difficult to apply these results to low-cost and small devices where transmitters and re-ceivers should be placed on the same small surface.

In [32] an error free optical transmission measure-ments at 1 Gb/s over a 2 m path in a laboratory waterpipe is proposed but a high power consumption LD(laser diode) is used with the aim of showing differentbehavior in case of different water conditions. Theyemploy different kind of water (clear ocean, coastalocean, harbor water and Maalox suspension) to testthe different frequency response. In this case the pro-posed system appears to be, at the moment, useful fortheoretical consideration but further work will be re-quired to develop practical high speed sources and de-tector. Even if the achieved data-rate is of a differentorder of magnitude in comparison with that proposedin this paper, nevertheless the cost and the power in-volved in this kind of device make this solution highlyimpractical for UWSN nodes.

In conclusion, taking into account the state of artof optical underwater wireless communication, differ-ent systems and applications have been illustrated andcompared with the here proposed implemented mod-ules. This paper proposes a new approach which,starting from the management of a point-to-point op-tical underwater transmission and reception of data,aims to support the communication in a UWSN bythe design and implementation of a PHY and MACLayer targeting the interface with current terrestrialtechnologies for WSN. The high-data rate exchangeof data by using optical underwater communication(in the order of Mb/s) can support many emergingapplications which involve, for instance, the record-ing and transmission of images and short video se-quences. The use of optical communication for sup-porting an UWSN has been studied in literature andthis paper proposes a first implementation approach.As regard the preliminary tests and simulation theyappear to be a good starting point for the future devel-opments.

7.6 Planar transceiver

Taking into account the previous circuits, a 2-dtransceiver has been implemented. The LEDs dispo-

Figure 22: Circular transceiver configuration used forexperimental tests(transmitter on the top)

sition on a 2-d structure has been performed using 12Ledman LL1503PLBL1-301 blue LED (with300 ofFOV) disposed on a circular disk of 10 cm diame-ter. The reduction of the overlap between the signalgenerated by each LED has been targeted so to allowthe possibility of supporting also a directional trans-mission, by using only one or a reduced set of LEDs.The same approach has been considered for the place-ment of the photodiodes for the 2-d receiver. Tests ofthe transmitter have been carried out by using a sin-gle receiver, equipped with a SFH-2013P photodiode.Different measures have been considered at differentdistances. The profile of the received optical signal,determined by considering the maximum value in aline-of-sight condition and the minimum value whichis measured between a LED and another, show thatthe generated light impulses is uniformly distributedon the surface. Tests on the receiver have shown aperformance decrease in comparison with the point-to-point circuit due to the noise generated by the pho-todiodes which are not directly exposed to the lightimpulses. Nevertheless a reception up to 4 meters inair can be achieved.



8 Conclusion and Future Develop-ment

The heredescribed work is to be considered as a firststep targeting the implementation of an optical mod-ule interfaced with current terrestrial technology forWSN and circuits for optical communication in or-der to build an optical UWSN. The implemented cir-cuits, even if under improvement, are a good start-ing point for an effective high data-rate and low-costunderwater communication. Next developments areaimed to implements new services in the MAC Layerand to perform communication tests with more thantwo nodes starting from a planar optical connection,while currently only point-to-point tests have beenperformed. In addition, the use of a limited numberof LEDs for a directional communication, when thetopology of the network has been fixed, and the pos-sibility of changing the wavelength of the emitter, bychoosing different colors (blue, red or green), if waterturbidity varies, can be interesting to explore.

8.1 Near Future Goal

The optical wireless modules here described can beeasily modified and integrated with additional func-tionalities.

In particular, the first step, will be focused on theimplementation of the MAC Layer in order to inte-grate the here described PHY Layer, taking into ac-count the previously illustrated design considerations.

The development of the MAC Layer, while, onone hand, is inspired to IEEE 802.15.4 protocol tosupport compatibility with current terrestrial tech-nologies, on the other hand has to consider the as-pects related to the management of optical communi-cation. For instance, the following characteristics canbe taken into account:

1. the possibility of choosing between a 4 or 16PPM;

2. the possibility of managing circuits where choos-ing different wavelengths of the emitter, by usingdifferent colors (blue, red or green), in order tohave a system adaptive to water turbidity. Thisproperty, for instance, can be implemented by us-ing a mechanism to manage the correspondingparameters in the PHY Layer;

3. the possibility of an adaptive directionality of thecommunication system, by activating only someof the transducers, in order to spare energy andoptimize the communication efficiency.

The second step will be the integration witha current developed terrestrial node. A good

starting point could be the use of the wirelessnode developed by the WISI Laboratory at DIBE(http://www.wiselaboratory.it/)which is equipped byan FPGA. One of most important aspect of the in-tegration will be the interface with the upper layerstaking into account the differences of the optical lay-ers in comparison with the Radio based technologiesfor WSN. For instance, the IEEE 802.15.4 protocolcan achieve different data rate and the implementa-tion of modules on a dedicated FPGA or by using anASIP/ASIC should target the adaptation and synchro-nization with the upper layers of the node.

According to the parameters illustrated in the pre-vious paragraphs, the circuits for reception and trans-mission of data currently used for point-to-point com-munication will be disposed to perform a directionalcommunication on a surface paving the way to anomni-directional implementation. In addition, an in-terested advance will take into account the possibil-ity of adding LEDs of different colors (for instanceblue, red and green) in order to support the adapta-tion to different turbidities: this hardware, in connec-tion with MAC functionalities, will allow the nodes toexploit the shifting of the minimum attenuation win-dow toward longer wavelengths, which occurs whenthe concentration of suspended particles increases.

8.2 Future Research Directions: Au-tonomous long survival UWSN forambient monitoring

Starting from the previous described results, the cre-ation of an innovative long survival optical Underwa-ter Wireless Sensor Network (UWSN) could be tar-geted. It will able to sense, compute, communicateand cooperate in an underwater environment by us-ing long-survival, low-cost and eventually disposablenodes. Innovative approaches could be developed toaddress:

1. wireless, adaptive and low-power optical under-water communication;

2. underwater energy scavenging and harvesting;

3. autonomous and self-governing behavior of thenodes in the underwater environment, includingintelligent energy storing and utilization, to guar-antee long-term operation under a wide range ofconditions and avoid frequent and costly rescueprocedures;

The first innovation will address the communi-cation capabilities. Efficient optical communicationcould be implemented thank to a low-cost, efficient



and omni-directional communication system, basedon shortwavelength LEDs, and able to exploit theminimum absorption wavelength window, shifting to-ward longer wavelengths in turbid waters. The op-tical communication, despite reaching shorter dis-tances respect to acoustic communication, will allowto achieve high data rates with lower energy require-ments. Finally, the implementation of adaptive direc-tionality, by activating only some of the transducerswill allow to spare energy and optimize the commu-nication efficiency. Online nonlinear modeling andadaptation to the time-varying aquatic channel charac-teristics of the optical system can be used to advancethe state-of-the-art and achieve effective and efficientFree Space Omni-directional Optical (FSOO) under-water communication.

The second innovative field of research could beunderwater energy scavenging for self-supply or forincreasing the lifetime of both each node and the en-tire network. While solar power can be exploited onthe water surface and in shallow but clear waters, othertechniques must be explored for producing and storingenergy in a general underwater environment. Terres-trial energy scavenging for artificial artifacts and au-tonomous sensors relies mainly on exploiting environ-mental vibrations, converting mechanical energy inelectrical energy. In the static underwater nodes, ran-dom movements forced by the water flow, underwatercurrents and noise, can be exploited to perform un-derwater energy scavenging. The delicate equilibriumbetween energy harvesting, storage and consumptionis to addressed by developing adaptive behavior, tooptimize the survivability of both the nodes and theentire network.

The third field of research should address the de-velopment of low-power, low-cost miniaturized nodesable to sense, compute, communicate and cooper-ate in an aquatic environment. The development ofnew nodes, with volumes that are orders of magni-tude smaller than current generation equipments, willallow the development of new applications, wheretiny and, eventually, disposable nodes will be ableto perform 4D monitoring of aquatic environments.The increased density of the network, which will bepossible thank to the miniaturization and low-cost ofthe nodes, will allow to compensate for the relativelyshort range of the underwater optical communicationchannel, which is, as shown, well below 100m. Fur-thermore, in settings where environmental issues areof limited concern, the use of low-cost disposablenodes will avoid frequent and costly rescue proce-dures, by simply adding new nodes to areas not cov-ered by the network or to overcome the malfunction-ing of old nodes.

The development of the previous described fieldsof research could lead to the implementation of low-cost optically communicating nodes, able to be de-ployed with low accuracy on the area of interest, andcapable of self-configuring as a sensing network asdepicted in Figure 23. This optical UWSN (Figure23) will be able to perform data collection for ambientmonitoring and to transmit the acquired, and eventu-ally preprocessed, data to one or more Access Points(APs) or Base Stations (BSs), connected to the terres-trial data concentrator through conventional wireless(radio) channels. The optical UWSN could exploitmulti-hop communication, in order to avoid the needof a fixed infrastructure, to achieve simpler and morecost-effective deployment, and to improve the surviv-ability of the system. The BS could incorporate boththe functionalities of a node and the functionalities ofa conventional terrestrial WSN, using radio wirelesscommunications, so to integrate with existing terres-trial WSNs and also allow long distance communica-tion where needed.

Figure 24: Single Node Concept

The mainchallenge in realizing an optical UWSNis the development of an innovative node (Figure 24):an autonomous, self-powered and low-cost (eventu-ally disposable) sensing device. Each node couldmake use of FSOO communication in the upper endof the visible light spectrum, where minimum absorp-tion and scattering occurs, and will adapt to the tur-bidity of the water environment by exploiting lightsources of different wavelengths, according to colourand clarity of the water. Furthermore, FSOO couldalso allow non Line-Of-Sight (LOS) communicationbetween nodes by exploiting light scattering and re-



Figure 23: optical UWSN Concept

flection fromthe water surface. The use of an opticalcommunication channel will allow the node to achievedata rates that are orders of magnitude higher than cur-rent generation UWSNs, which exploit acoustic chan-nels. On one side, this approach will limit the totalenergy consumption by using very short communica-tion time-frames and reducing the overall activity ofthe node; on the other side it will allow the transmis-sion of moderately large quantities of data (e.g. stillimages or low-resolution video) at a sustained rate. Inacoustic based networks, the discovering of the net-work topology (e.g. using triangulation) is a difficulttask; on the contrary, the use of ad-hoc optical mod-ulations and transmission protocols will allow, afterthe deployment of an optical UWSN, to easily com-pute the position of each node. The initial mappingof the node positions will allow not only to discoverthe network topology, but will also simplify the res-cue procedure of faulty nodes and their recovery atthe end of their life, especially if disposability cannotbe exploited due to environmental reasons. Moreover,after discovering the network topology, each node willswitch to more energy-preserving modulations andprotocols for data transmission, exploiting also adap-tive directionality when targeting node-to-node trans-mission.

Even if each node could sustain part of its op-erational life through conventional batteries, it couldachieve longer operational life by harvesting energyfrom the environment mainly through mechanical en-ergy conversion. Solar energy will also be consideredas a viable option, because in some cases the nodeswill be deployed in shallow and clear waters. Themechanical energy harvesting mechanism will exploitboth advective flows, caused by river waters or wind,

and orbital movement, caused by waves, through theuse of piezoelectric materials. The mechanical energyscavenging will take advantage of both the main sup-port of the optical communication system and addi-tional whiskers mounted on the node body ( Figure24).

Environmental compatibility of the opticalUWSN node will be guaranteed by its small di-mension, respect to current-generation monitoringsystems. While buoys for sea monitoring oftensurpass10.000cm2 of water surface occupation and abuoyancy payload of several hundred kilos, this kindof nodes will not exceed a footprint of100−200cm2.Furthermore, the communication will not interferewith the water fauna thank to the almost exclusive useof return-to-zero and short-pulse optical modulationsfor data transmission. In addition, in environmentallyrich locations, the system could use a long-sleepoperation mode, stopping transmission for severalminutes so that the underwater fauna will not beattracted by lights, avoiding also heavy interferencewith data communication.

Each node should provide the flexibility to hosta large sensor payload (Figure 25), able to targetthe identified environmental monitoring scenarios, ac-cording to user needs (for instance including camerasfor image or short video recording). Both analog anddigital interfaces can be easily developed for address-ing the interfacing needs of a large number of COTSsensors for underwater applications.

In summary, future research direction could tar-get a node architecture which will inherit the structureof the terrestrial counterparts, but will introduce newarchitectural blocks, which will be developed ad-hocfor the underwater environment. The optical commu-



Figure 25: optical UWSN Node Architecture

nication subsystemwhich will consist of a reconfig-urable device, able to adapt the light emission and de-tection to the varying water conditions and a carefullydesigned 3D structure able to emit and receive lightomni-directionally. The energy harvesting subsystemwhich will be designed to extract energy from waterflows and transform it to sustain a long operationallife of the node.

Acknowledgements:We thanks Mr. Giorgio Carlinifor his support to circuits implementation.

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