smart transmitters and receivers forunderwater

964 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 30, NO. 5, JUNE 2012

Smart Transmitters and Receivers forUnderwater Free-Space Optical Communication

Jim A. Simpson, Student Member, IEEE, Brian L. Hughes, Member, IEEE, and John F. Muth, Member, IEEE

Abstract—The number of unmanned vehicles and devicesdeployed underwater is increasing. New communication systemsand networking protocols are required to handle this growth. Un-derwater free-space optical communication is poised to augmentacoustic communication underwater, especially for short-range,mobile, multi-user environments in future underwater systems.Existing systems are typically point-to-point links with strictpointing and tracking requirements.In this paper we demonstrate compact smart transmitters and

receivers for underwater free-space optical communications. Thereceivers have segmented wide field of view and are capable ofestimating angle of arrival of signals. The transmitters are highlydirectional with individually addressable LEDs for electronicswitched beamsteering, and are capable of estimating waterquality from its backscattered light collected by its co-locatedreceiver. Together they form enabling technologies for non-traditional networking schemes in swarms of unmanned vehiclesunderwater.

Index Terms—Freespace optical, Underwater communication,Autonomous underwater vehicle, Unmanned underwater vehicle,Angle diversity, Imaging diversity, Multiuser, Adaptive, Swarm

I. INTRODUCTION

UNDERWATER communication is of great interest to mil-itary, industry, and scientific communities. Underwater

vehicles, sensors, and observatories require a communicationsinterface with data rates in the few to tens of Mbps. Whilefiber optic or copper cabling can be used for sufficientlylarge or stationary devices, a wireless link is desirable inmany situations. Radio frequencies are heavily attenuated inseawater. Acoustic communication is the existing prevalentmethod but suffers from low data rates, high latencies, andmultipath issues. For short-range links, free-space opticalcommunications is a promising alternative.Recently, underwater free-space optical communication has

witnessed a surge in interest from developments in blue-greensources and detectors [1],[2],[3],[4]. These take advantage ofthe “blue-green optical window” of relatively low attenuationof blue-green wavelengths of the electromagnetic spectrumunderwater. Laser-based systems have been demonstrated forextended ranges, high data rates and low latencies [5]. LED-based systems have been demonstrated for low-cost, low-power, and compact systems [6].Underwater free-space optical communication is typically

considered point-to-point and, thus far, most researchers have

Manuscript received 13 August 2011; revised 30 April 2012. This workwas supported by the Office of Naval Research via NRL grant N00173-07-1-G904 and STTR N00014-07-M-0308 and by the National Science Foundationunder grants CCF-0515164 and ECCS-0636603.The authors are with the Department of Electrical and Computer Engineer-

ing, NC State University, Raleigh, NC, 27695-7911 (e-mail: [email protected]).Digital Object Identifier 10.1109/JSAC.2012.120611.

treated it as such. Point-to-point links require strict pointingand tracking, especially on mobile platforms. This is reason-able in systems that use collimated laser links and are largeenough to afford dedicated gimbal systems. Some systemsalso use very large aperture (∼20 inch) photomultiplier tubes(PMTs) that increase the receiver field of view (FOV) [1].In some situations, however, compact systems are desired.

Smaller platforms with compact systems do not have the vol-ume or energy budget for sophisticated pointing and tracking.Large-area PMTs can be expensive and bulky. In the presenceof multiple users or strong background ambient light a single-output wide FOV can also be a disadvantage.In traditional RF wireless, smart antennas are an integral

part of most mobile communications standards, such as 4GLTE Advanced. Smart antennas are capable of signal pro-cessing to provide angle of arrival information and transmitbeamforming. In indoor optical wireless, multiple antennaswith spatial and angular diversity are used for non-line-of-sightcommunications, ambient light rejection, electronic pointingand tracking, relative localization, and multi-hop networking.It is natural to consider the benefits of such techniquesextended to the underwater environment.In this paper, we propose a new optical front-end for

underwater free-space optical communication. The new front-end introduces the concept of smart receivers and transmitters.The smart receivers have segmented wide FOV and are capableof detecting angle of arrival of signals in order to adjust andorient FOV towards the desired signal. The smart transmittersare capable of using this information to electronically steerits output beam towards a particular direction. The smarttransmitters are also capable of estimating water quality fromits backscattered light collected by its co-located receiver.This can be used to adapt to changing water conditions.The transmitter outputs are also code division multiple access(CDMA) coded for operation in a multi-user environment.

II. BENEFITS OF SMART OPTICAL SYSTEMS FOR UUVS

Smart receivers and transmitters are the physical layerenabling technology for coordinated sensing and communi-cating, and the focus of this paper. As an example, considersmart optical transmitters and receivers that can estimate theapparent optical properties of water, send a beam of light in adesired direction, and determine the direction and identity oflight that is being received.This information can then be used in many different phases

of an underwater vehicle’s operation. Determining the wa-ter quality can be used to adaptively change the power oftransmission or gain of the receiver during detection and

0733-8716/12/$31.00 c© 2012 IEEE

SIMPSON et al.: SMART TRANSMITTERS AND RECEIVERS FOR UNDERWATER FREESPACE OPTICAL COMMUNICATION 965

Fig. 1. Multi-user reception system scenario with three nodes. A & C aretransmitting. B is receiving. Note: Only three field of views (channels) peruser are shown, for clarity.

Fig. 2. Optical backscatter estimation at B from its co-located transmitter.

acquisition of another platform. Knowledge of vehicle orien-tation, identity, and relative angle can be used to localize anddetermine the relative positions of vehicles. Brief descriptionsof potential benefits are listed in sections A-F below.

A. Non-mechanical pointing and tracking on a moving under-water vehicle

Depending on the sea state and the controls of an under-water vehicle, an optical transmitter or receiver mounted onit can go in and out of alignment with another stationaryor mobile platform as the vehicle moves. An optical front-end capable of changing its effective FOV, detecting angle ofarrival at its receiver, as well as electronically steer its outputbeam, can potentially maintain a communications link in suchan environment. Furthermore, one can utilize signal diversitytechniques to improve signal reliability.

B. Maintaining link with a stationary node as an underwatervehicle does a drive-by

Station keeping and maintaining a precise relative positioncan be energy intensive and difficult for underwater vehicles.The ability to interrogate and collect data from a station-ary sensor node as a vehicle drives by can add significantoperational capability. Thus, a quasi-omnidirectional receivercapable of continually adapting its FOV and optical power isof value.

C. Providing sensory information to underwater vehicles

In a swarm environment, localization information can begathered from angle of arrival information as various nodestalk to each other. This information can be passed to thevehicle to augment its other sensory information for navigationand collision avoidance purposes. A smart optical front-endcan also provide other sensory information such as waterquality measurements obtained from the communications link.

Fig. 3. Electronic switched pointing & tracking. B can sense the directionof C and point.

D. Duplex multi-user system

Each transceiver consists of a smart receiver and a smarttransmitter. Spatial diversity allows for simultaneous receptionfrom two non-co-located transmitters. Since each transmitteris CDMA coded, the smart receiver at B is also capable ofassociating A and C data streams with their correspondingdirections. In the event that A and C are co-linear, the CDMAcode still allows for separating the two transmit streams at thereceiver on B.In a mesh network scenario, as illustrated in Fig. 1, users

A and C are not within range of each other. Assuminglocalization information from angle of arrival is kept at eachnode, B can relay messages between A and C through ahop network. If B is a mobile node, it can be positioned toeffectively extend the optical communication range betweenA and C when needed.

E. Optical backscatter estimation to assess water quality

The duplex system provides a way for a receiver to monitoroptical backscattering while its co-located transmitter is active,as illustrated in Fig. 2. Based on the modulation schemeused, steps would need to be taken to isolate background,interference, and unmodulated light. Using volume scatteringinformation, an estimate of the attenuation coefficient can bemade based on the measured amount of backscatter. Thiscomplements SNR measurements that can be obtained fromthe tx/rx signals.

F. Electronic Switched Pointing & Tracking

Angle of arrival information at a receiver is passed along toits co-located transmitter. The transmitter can then switch toa beam that points its output in the direction of the incomingbeam to optimize the link, as illustrated in Fig. 3.

III. BACKGROUND

A. Underwater Optical Channel

The underwater free-space optical channel is very differentfrom the atmospheric channel. While there have been exten-sive studies on the optical properties of water, the focus ofmost of these studies has been on the geo-physical, bio-optical,and remote sensing applications. Thus, the underwater channelfrom an optical communication perspective is still very muchunknown. Nonetheless, a good overview of the properties canbe obtained from literature [7], [8], [9].


TABLE IEXAMPLE ATTENUATION COEFFICIENTS

Water Type c (1/m)Clear ocean 0.152Coastal ocean 0.399Harbor water 2.195

From an optical communications perspective, the threeimportant properties are beam attenuation coefficient, volumescattering function, and albedo. Light interacts with water andthe materials suspended and dissolved in it in two differentways: absorption and scattering. Absorption is the change ofelectromagnetic radiation into other forms of energy such asheat. Scattering is the redirection of electromagnetic radiation.Photons essentially change their direction by means of reflec-tion, refraction, and diffraction. In small particles, Mie andRayleigh scattering determine the magnitude and direction ofthe scattered photon [7]. This dependence can be describedby a phase function. In water, the phase function is usuallystrongly forward peaked. There can also be a significantbackscattered component [10].1) Beam attenuation coefficient: The ratio of energy ab-

sorbed or scattered from an incident power per unit distanceis defined as beam attenuation coefficient. It denotes the totalenergy lost and can be shown to be the sum of the absorptionand scattering coefficients, a(λ) and b(λ). It has units of m−1

and is denoted by c(λ).

c(λ) = a(λ) + b(λ) (1)

Beer’s law defines the attenuation of an optical signal as afunction of attenuation coefficient and distance d as:

I = I0 e−c(λ)d (2)

where I is the intensity at the receiver and I0 is the intensityat the transmitter. The exponential term, c(λ)d, is calledattenuation length and is a unitless term that expresses theexponential loss of light in water.2) Single-scattering albedo: Single-scattering albedo is de-

fined as the ratio of scattering coefficient to beam attenuationcoefficient and denotes the probability that a photon will bescattered rather than absorbed. It is a unitless term and isdenoted by ωo. It is defined as

ωo ≡ b(λ)c(λ)

(3)

As expected, highly (primarily) scattering environmentsresult in an albedo close to 1, and highly (primarily) absorbingenvironments result in an albedo close to 0. Since scatteredphotons are not converted to other forms of energy, single-scattering albedo is also known as the probability of photonsurvival.3) Volume scattering function: The volume scattering func-

tion (VSF) is defined as the fraction of scattered power (Φs)to incident power (Φi) as a function of direction ψ scatteredinto a solid angle ΔΩ. It has units of m−1sr−1 and is denotedby β(ψ, λ).

Fig. 4. 1000 gallon water tank built and used for underwater free-spaceoptical communication experiments done in lab at NCSU.

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c(53

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datafitted curveprediction bounds

Fig. 5. Relationship between attenuation coefficient and SNR for experimentsin laboratory test tank. Attenuation and albedo are controlled by introducingmeasured amounts of scattering particles such as liquid Maalox and Nigrosindye to mimic ocean water.

β(ψ, λ) =d(Φs(ψ,λ)

Φi(λ) )

dr dΩ(4)

B. Underwater Freespace Optical Communication

We have previously developed systems for underwaterfree-space optical communication that use laser sources andphotomultiplier tubes as detectors [5] as well as LEDs andphotodiodes [6], both capable of implementing digital signalprocessing techniques.For laboratory testing, a 3.66 m long, 1.22 m wide, 1.22 m

tall indoor water tank was constructed. Maalox, a commercialantacid was added as a scattering agent. The addition ofMaalox controls the attenuation coefficient of water, whileabsorption can be independently controlled by adding Nigrosindye to control the albedo [11]. During the progression of anexperiment, Maalox was added in a controlled manner usinga programmable syringe pump. The resulting sweep resultsin a reproducible relationship between attenuation coefficientand the electrical signal-to-noise ratio (SNR) of the receivedsignal as shown in Fig. 5.


Fig. 6. Experimental performance of various error correction codes im-plemented for underwater free-space optical communication at NCSU as afunction of code rate vs. performance in Eb/N0 at a BER of 10−4.

On compact platforms that we have built, to improve theperformance of the links, forward error correction was addedand theoretical and experimental performance was compared[12],[13]. Reed-Solomon, turbo, and low-density parity-check(LDPC) codes were implemented and their performance com-pared. The (255,129) Reed-Solomon code provides a codinggain of 2.5 dB over uncoded data at a bit-error-rate of 10−4.UMTS and CCSDS turbo codes provide gains ranging from6.8 dB to 9.5 dB for code rates ranging from 1/2 to 1/6. TheDVB-S2 LDPC code provides a gain of 7.7 dB to 9.2 dB forrates from 1/2 to 1/4.

C. Geometrical Loss in Link Budget

The biggest losses in the link budget comes from theexponential path loss of light in water and from geometricallosses. In the simplest scenario of a well aligned and pointedsystem, consider the transmitter beam to have a divergenceangle of θdiv . Let d be the perpendicular distance between thetransmitter and the receiver. At that distance, the transmittedbeam would have an area (approximated using the small angleapproximation):

ATX =π

4(d · θdiv)2 (5)

A receiver with diameter DRX has an area:

ARX =π

4D2RX (6)

Geometric loss is then calculated as the ratio of receiverarea to transmitter beam spot area at the receiver as:

LossG(d) =D2RX

(d · θdiv)2(7)

Thus, it is desirable to have a large receiver aperture size.However, practical size, weight, and cost requirements limitaperture sizes.

D. Field of View and Pointing

One method of increasing aperture size is to use a lightcollecting lens in front of the smaller area optical detector.

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(a) FOV of a bare photodiode (ac-tive area diameter of 2.65 mm).

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(b) FOV of a 1′′ lens-photodiodepair. Photodiode is at focus.

Fig. 7. Comparison of field of view and light collecting ability of a barephotodiode (a) vs. a lens-photodiode pair (b) shown to illustrate the gain inlight collection by using a lens, but at the expense of FOV. FOV decreasesfrom ∼120◦ to ∼10◦ .

However, this limits the FOV of the receiver, adding additionalpointing requirements.The FOV of a typical photodiode can be between 60 to

120 degrees based on the type of packaging used. Adding alens restricts the FOV severely, depending on the radius of thephotodiode active area rphotodiode and the focal length of thelens flens as:

φ = 2 · tan−1

(rphotodiodeflens

)(8)

The FOV is mostly limited by the imaging property of thelens: as the incoming beam enters the lens off-axis, the focalpoint moves laterally as a function of the tangent of the angle.Beyond a particular off-axis angle, the focal point starts to“walk off” the photodiode active area.Even without the restrictions of a limited FOV at the

receiver, free-space optical communications are inherentlypoint-to-point. Hence the performance of a practical systemdepends on how well aligned the transmitter and receiver are.While bigger platforms can afford dedicated gimbal systems,smaller platforms do not have the volume or energy budgetfor sophisticated pointing and tracking.

E. Existing Systems and Methods

1) In underwater optical communication: Wide FOV istypically achieved using very large aperture devices such asphotomultiplier tubes (PMT). They have very short rise timesand wide spectral response, including the blue-green windowof interest in underwater optical communication. PMTs alsohave a wide range of aperture sizes ranging from 10 mm toas high as 500 mm (20 inches) in diameter. These large-areaPMTs have been used in underwater optical communicationsystems to avoid active pointing and tracking [1].

2) Modulating retroreflector: A modulating retroreflectorcan be used to address power, size, and pointing requirementsat the receiver [14]. A modulating retroreflector eliminatesthe need for a transmitting laser on the data-bearing platformand reduces the pointing requirements by retroreflecting themodulated light back to the interrogating source. For theunderwater environment, we have built a MEMS Fabry-Perot modulating retroreflector operating in the blue-greenwavelengths and at data rates of 500 kbps and 1 Mbps


[15]. This technology can also complement the diversityapproaches described in Section III.

3) In indoor optical wireless: There has been some re-search in the domain of indoor optical wireless in the use ofspherical photodiode arrays for increasing FOV [16]. Initialprototypes have been built and shown to be sufficient inlow attenuation channels such as the indoor optical wirelesschannel [17].The imaging property of a lens has been leveraged in

the low attenuation indoor wireless channel by replacingthe traditional single-element photodiode with a largerarea segmented-photodiode [18]. Additionally, the use ofcompound parabolic concentrators instead of traditionalspherical lenses have been shown to improve the performanceof such systems. An improvement in range by a reductionin path loss, multipath distortion, and background noisehas been shown by using such a system and by optimallycombining the photodiode outputs [19].

4) In RF communications: Terrestrial RF communicationshave benefited from recent advancements in spatial diversityand smart antennas. A smart antenna is an antenna array withsignal processing capable of 1. estimating direction of arrivaland 2. beamforming. Advanced adaptive array systems arecapable of focusing towards a desired signal while simul-taneously nulling interferences. Mobile communications alsoprovide insight towards some of the applications possible withan antenna with such capabilities. However, in optical systems,we do not have the RF advantage of being able to use coherentbeamforming or phased arrays.

IV. SMART RECEIVER

The goal of the smart receiver research is to develop aquasi-omnidirectional system to reduce pointing and track-ing requirements typically associated with free-space opticalsystems. The compound eye type lens array that has beendeveloped has segmented wide FOV, which allows for otherbenefits in addition to the original goals.In addition to potentially reducing pointing and tracking

requirements, this design also potentially allows one to es-timate angle of arrival. This can be used in combinationwith a CDMA type multiple access system. Thus, potentially,signals from different platforms can be distinguished fromtheir coded signals and have an indication of their position.This opens the door for a large number of applications suchas localization, navigation assistance, and mesh networking.Using multiple input multiple output (MIMO) techniques, thisoptical approach potentially also provides angle and spatialdiversity for improving the performance of point-to-pointlinks. Previous work in this area shows ideal methods tocombine the output of such an array of photodiodes suchas to maximize signal and minimize noise, without loss inbandwidth [20].As such, the smart receiver has the following characteristics:

• Increased field of view• Angle of arrival estimation

A. Design and Conceptual Operation

The proposed design consists of a 3-D spherical array oflenses all focusing to a 2-D planar array of photodiodes.Close packing is achieved by using a hexagonal structure forboth arrays. A prototype has been constructed using sevenlenses and seven photodiodes. Figs. 8c and 8d shows a solidrendering of the prototype lens head.Figs. 8a and 8b shows a sketch of light entering through

a single lens and falling on multiple photodiodes. For theprototype lens head, with 7 lenses and 7 photodiodes, thereare 49 such lens-photodiode pairs.

B. Design Considerations

The following section discusses the different aspects of thedesign and their importance to underwater free-space opticalcommunication.

1) Lens at the Receiver: Existing optical front-end arraysin terrestrial free-space optics and indoor optical wireless useeither photodiode arrays with no lenses, a single lens withmultiple photodiodes, or multiple lenses focusing on separatephotodiodes. The proposed design is novel in that it usesan array of lenses as well as an array of photodiodes, withmultiple combinations of optical paths in between.

2) Quasi-omnidirectionality: Increased FOV has beenthe primary design consideration for the optical front-end.Simulations and early experimental results show that thephotodiode array under the middle lens increases the FOVfrom ∼5◦ to ∼40◦. The addition of the outer lenses at 130◦

with respect to the middle lens increases the FOV to ∼120◦.The envelope of the combined FOV can be adjusted byadditionally intentionally defocusing the photodiode array.

3) Angle of Arrival Estimation: The intensity of lightreceived by each photodiode can be used to roughly estimatethe angle of arrival of light. The hexagonal structure allowsthe receiver to estimate this angle with good resolution inthe 0.5π steradian (corresponding to ±20◦) and a lesserresolution in the 3π steradian (corresponding to ±60◦).

4) Photodiode Output Combining: Output combining is animportant consideration for the performance of the system.The simplest interface to the array of photodiodes would beto connect them in parallel. This would allow a single outputto be digitized for processing. However, the total capacitancewould be the sum of the individual capacitances resulting in amuch lower bandwidth. An ideal combining technique wouldmaintain bandwidth, minimize noise, and maximize SNR.Two simple linear diversity combining techniques are equal

gain combining (EGC) and maximum selection combining(SEL). In an equal gain combiner, the receiver output signalsare summed with equal weights to generate the combinedsignal. This can be represented as

y = y1 + y2 + · · · + yn (9)


(a) Sketch showinglight entering throughtop lens.

(b) Sketch showing light entering through side lensand falling on three photodiodes.

(c) Isometric view.

(d) Top view of solidrendering.

1

2 4

7

6

53

(e) Lens array namingconvention.

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25.4

X (mm)

Y (m

m)

(f) Photodiode arraynaming convention.

Fig. 8. Design of prototype lens-photodiode array. Lens-array is a truncated hexagonal pyramid structure with 7 lenses, underneath which is a planarphotodiode-array consisting of 7 photodiodes. Solid renderings of design in (c) and (d). Sketches of side view in (a) and (b) show light entering through eachlens and falling on multiple photodiodes. 49 such directions (“channels”) exist for the prototype lens-photodiode array.

−50 −25 0 25 50−50

−25

0

25

50

X (mm)

Y (

mm

)

Fig. 9. Example simulation for a normal incident light source, visualizingcombined receiver plane with summation of focal points (x-y plane) and theirintensities. x-y plane is a 50 mm x 50 mm receiver plane underneath lens arraywith (0,0) centered underneath, and 40 mm below the middle lens. Overlaidon the plot is the photodiode array placement. This type of simulation canbe used to optimize photo diode placement to maximize light collected whilealso maximizing separation between photodiodes.

In selection combining, the receiver with the largest signalpower at each instant of time is chosen as the combined signal.The combined signal is therefore

y = max (y1, y2, · · · , yn) , (10)

where y1 is the signal at the first receiver, y2 is the signal atthe second receiver, and yn is the signal at the nth receiver.

C. Modeling and Simulation

A model of the lens-photodiode array system was built inMATLAB and used to simulate light incident on the system toobserve: the power at the face of each lens on the lens-array,

300 400 500 6000

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utoc

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Fig. 10. Simulation correlator outputs for the data sequence “111001010”spread using a 32-chip prime code sequence of P = 5 with 7 padding zeros.

the power at the receiver plane underneath the lens-array, andfinally, the power at each photodiode on the photodiode-array.The goal of the model is to simulate different scenarios toselect ideal parameters for the prototype to be constructed.

V. SMART TRANSMITTER

The smart transmitter has the following characteristics:

• Increased directionality• Electronic switched beamsteering

A. Design and Conceptual Operation

Similar to the smart receiver, the smart transmitter consistsof a truncated hexagonal pyramid with seven LEDs. Unlike thereceiver, each LED is coupled with its own lens that convergesthe wide FOV of the LED to a narrower beam in a particulardirection. Each LED is uniquely addressed and driven, whichallows the modulator to select an output direction. This formsthe mechanism for a simple switched beamsteering at thetransmitter.For a multi-user environment it is important to provide

multiple access to the medium. LEDs at different wavelengthscan be used, but receivers would need multiple filters. TimeDivision Multiple Access would require synchronous clocks.Among asynchronous methods, Code Division Multiple Ac-cess (CDMA) was chosen as the multiple access scheme.


Fig. 11. Image of top-view of prototype receiver lens-array constructed.

B. CDMA Coding

CDMA techniques have been extensively studied and usedin RF communications. Incoherent optical detection and pro-cessing make typical RF maximum length and Gold codesequences not ideal. The on-off-keying modulation methods inoptical communication are unipolar as opposed to the bipolarmodulation required by Gold codes. Instead optical CDMAtypically uses prime codes [21].Prime codes of length N = P 2 are obtained from prime

sequences of prime number length P generated from a GaloisField GF (P ). Here we use P = 5 to generate a code oflength 25, which is then zero padded to N = 32. The codewas simulated for K = 5 users. The theoretical SNR can becalculated using the below equation [21] as 14.4 dB.

SNRprime,th ≈ 10.29

[N

K − 1

](11)

Fig. 10 shows the autocorrelation and cross correlationfunction for the data sequence 111001010 coded with theprime code. The empirical SNR is obtained as the ratio ofthe square of the maximum of the autocorrelation function tothe sum of the variances of the cross correlations with theinterferers, and found to match the theoretical SNR.The particular length prime code chosen allows for 5

simultaneous users. This is sufficient for the initial prototype.A longer code sequence will decrease overall bandwidth butallow for more users with a corresponding increase in SNR.

VI. EXPERIMENTAL RESULTS

Prototype lens-photodiode arrays and LED-arrays were con-structed for the receiver and transmitter front-ends and usedto collect experimental data. At the receiver, a 7-channelelectronics front-end was designed and built to pre-amplifyand digitize the receiver outputs. At the transmitter, a 7-channel electronics front-end was designed and built to receiveup to seven different streams of data from an FPGA and driveup to seven different LEDs on the LED-array. The underwaterchannel used was the 3.66 m long laboratory water tankpreviously described. The channel conditions were adjustedby the controlled addition of liquid Maalox as a scatteringagent, and the resulting attenuation coefficient was measuredusing a WET Labs C-Star transmissometer.The different experiments conducted are for the primary

purpose of characterizing the receivers and transmitters, aswell as to serve as a demonstration of their capabilities. First,

Fig. 12. Image of top-view of prototype transmitter LED-array constructed.

a characterization of the lens-photodiode array is conducted,then an algorithm to extract angle of arrival information isdiscussed and its results presented. The lens-photodiode arrayis then used to measure the amount of backscatter from aco-located transmitter and its relationship to the changingattenuation coefficient of the water is shown. For diversitycombining, two different techniques are implemented and theirresults given. Finally, a experiment using two transmitterssimultaneously transmitting to a single receiver is conducted.The different experiments are detailed in Sections A-E below.

A. Characterization of the Receiver Lens-photodiode Array

Experiments were first conducted to characterize the re-sponsivity of the prototype receiver’s lens-photodiode arrayas a function of the incident light, and in turn derive its FOV.Experiments were conducted for the receiver pointed in alldirections in a 2π steradian and intensities were observed atall photodiode outputs stored as a function of the sphericalco-ordinates (θ, φ) with 0 ≤ θ ≤ π/2 and 0 ≤ φ ≤ 2π. Apan and rotate system was constructed using digital servosand was used to point the array towards a constant, expanded,white light source. The seven amplified photodiode outputswere digitized using a simultaneous 8-channel digitizer. Fourseparate experiments were then conducted for four differentlens-photodiode separations, 40, 50, 60, and 70 mm.Graphs showing the two-dimensional (single-axis) and

three-dimensional FOV for each of the four separation dis-tances between the lens-array and photodiode-array are in-cluded in Fig. 13. As this separation determines the distancebetween the focal points of each lens and the receiver plane,increasing separations defocus the focal point to a conicsection projection. The 2D FOV shows the individual lens-photodiode pair FOVs along one axis of the lens-photodiodearray, while the 3D FOV shows all possible lens-photodiodepair FOVs formed. The plots also help visualize the segmentedwide-FOV, which has been the primary design goal. Amongmany parameters, the separation between the lens-array andthe photodiode-array can be used to control the width andoverlap between the single FOVs, as well as the total FOV.As anticipated, increasing the separation between the lensesand photodiodes defocuses the focal points and effectivelywidens each individual lens-photodiode FOV. The smallerseparation distance of 40 mm gives the maximum total FOV at∼120◦, and the larger separation distance of 70 mm gives theleast total FOV at ∼90◦. However, the increased total FOVcomes at the expense of larger gaps between the individual


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Fig. 13. Single-axis two-dimensional FOV (left) and three-dimensional FOV(right) plots for four different separations between lens-array and photodiode-array. 3D FOV is a top-view of responsivities plotted on a spherical surfacecorresponding to (θ, φ) of incident light. θ = 0◦ lies at the center andincreases outwardly till θ = 90◦ at the edge of the hemisphere. φ = 0◦points south and increases clockwise. φ = 180◦ points north.

FOVs. By the same mechanism, smaller gaps between theindividual FOVs and more overlap between the individualFOVs is achieved at the expense of decreased total FOV.

B. Angle of Arrival Estimation

Angle of arrival estimation involves estimating the directionof arrival of the incident light based on the relative outputpowers observed at each photodiode. The preliminary algo-rithm explored here uses the uniqueness of the relationshipbetween the photodiode outputs to potentially extract the angleof arrival.The approach taken is based on a look-up table traversal

generated using characterization data previously collected.Effectively, the seven receiver outputs at each instant are sortedand compared against all possible receiver output patterns.The angle corresponding to the best match is chosen as theestimated angle. The best match is defined as, and obtained

−90 −45 0 45 90−90

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Fig. 14. Results for the single-axis angle of arrival estimation algorithminvestigated show low bias within the combined field of view of the lens-photodiode array. Results shown are for the experimental data collected using60 mm separation between lens array and photodiode array.

by, the look-up table location that is the closest match for thechannel with the most power, followed by the next most powerand so on.The result of this algorithm is shown in Fig. 14. The

algorithm matches angle of arrivals with low error within thecombined FOV of the lens-photodiode array. Simulated noisewas added to observe the degradation of the performance andit was noted that the algorithm continued to perform well inregions of transition between the individual lens-photodiodeFOVs.While the design allows for a look-up table-based algorithm

to be used in estimating the single-axis angle of arrival withreasonable accuracy, estimating the three-dimensional spheri-cal angle (θ, φ) is more complicated. One possible solution isto use an algorithm similar to Markov localization. Such anestimator would incorporate prior and posterior probabilitiesfor the estimate, including a convolution between the measure-ments. Such a convolution can be intentionally introduced byrotating the receiver by a known φ′′ between measurements.The convolution can also arise from movement of the platformthe receiver is mounted to.

C. Backscatter Estimation

A smart transmitter can perform estimation of the waterquality by using its backscattered return light and a co-located receiver to estimate the attenuation coefficient (channelstate) of the channel at the transmitter. This technique hasthe advantage of knowing the water quality without relyingon a back-channel for back-telemetry or even a separateinstrumentation sensor. Knowing this information allows thetransmitter to, for example, adaptively change its transmitpower, data rate, code rate, or other parameters. The challengeto this technique is that the return beam from backscatter,depending on the attenuation coefficient of the channel, canbe as low as approximately six orders of magnitude below theoutput power of the transmitter. To some extent, this can besolved by a few methods including, but not limited to: sendinga higher power training sequence for the purpose of increasingthe amount of backscattered light used for estimation, thereceiver correlating the captured light to the actual data beingtransmitted, or even temporarily increasing the receiver gain.Techniques such as the use of a lock-in amplifier can also beused and are aided by the fact that the transmitter and thebackscatter-receiver are co-located.


0 1 2 3 4 5 6 7 8 90

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2

3

4

5

Attenuation Coefficient, c (m−1)

Pow

er (µ

W)

Fig. 15. Results of the backscatter estimation experiment showing linearrelationship between power of the backscattering from transmitted light asmeasured by its adjacent receiver, and the true attenuation coefficient mea-sured using a transmissometer suspended in the water during the experiment.

Experiments were conducted with commercial off-the-shelfamplified detectors as well as the prototype transmitter andreceiver to collect and observe a linear relationship betweenthe known attenuation coefficient of the water and the amountof backscattered light collected. For this purpose, duringexperimentation, a transmissometer was introduced in thewater to collect true attenuation coefficient measurements. Theattenuation coefficient was changed as previously mentionedby the addition of Maalox. Results are shown in Fig. 15.The results provide a measure of the integral of the full

range of backscattering angles. However, no attempt is madeto extract an actual backscattering coefficient from thesemeasurements. Such measurements are technically difficult[22]. Exact measurements, although required for an instru-mentation device, are not necessary for some schemes ofadaptive communication. An instrumentation device wouldalso have a small or no separation of distance between thelight source and the detector. However, the relationship toattenuation coefficient is linear and, after calibration, can beused to set the output power of the transmitter in order tocompensate for water conditions.

D. Diversity Combining

Diversity combining is the process of combining the mul-tiple outputs from the receiver front-end in order to provide asingle improved signal for processing. The two types of com-bining techniques investigated here are equal gain combining(EGC) and selection combining (SEL). In a typical commu-nications system over a fading channel, the multiple receiverstypically observe copies of the same signal with time-varyingpower levels. However, in the case of this particular receiverfront-end introduced in this paper, the receiver front-end isanalogous to an angular diversity system. As such, it is notalways desirable to sum all receiver outputs together withequal gain as one would with EGC.For the purposes of quantifying the detrimental effects

of EGC, an experiment was conducted using a single 7-channel transmitter and a single 7-channel receiver, both ofwhich were stationary during the duration of the experiment.First, the receiver channel pointed towards the transmitterwas considered. From that point, other receiver channels, notoriented towards the transmitter, were added incrementally,until all seven channels were considered.The results are plotted in Fig. 16. As expected, SEL combin-

ing always switches to the channel with the most power, while

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EGC (PSD Noise Floor)EGC (SNR)SEL (SNR)

Fig. 16. SNR and noise floor of the power spectral density (PSD), for equalgain combining (EGC) and selection combining (SEL), as the number ofchannels are added incrementally. Only one receiver is pointed towards andactively receiving data from the transmitter in this experimental scenario.

EGC gets incrementally worse as more channels are added.The term diversity is used loosely to imply that in a mobilesituation, the channel with the most power changes as userschange position. The combining technique is implemented ona packet basis and will select the ideal receiver channels aspositions change. The SNR estimator used saturates below10 dB and therefore an additional metric, the noise floor of thepower spectral density of the combined signal, is also includedin the figure for reference.It is possible that in waters with very high attenuation

coefficients, the multiply scattered photons typically carrymore power than ballistic photons and as result will resultin power at multiple receiver channels simultaneous. In thiscase, a more efficient combining technique such as maximumratio combining will potentially be advantageous. More workis needed to study this scenario.

E. Multi-user: CDMA and SDMA

One of the advantages of the proposed design is the spatialdivision multiple access (SDMA) inherent to the design.However, this is not applicable in two scenarios: first, whentwo or more transmitters are co-located, and second, when twoor more different signals arrive at the same photodiode throughtwo different lenses. In either case, SDMA is not applicable,and for this reason, code division multiple access (CDMA)spreading is employed at the transmitter. The codes used andtheir properties have been detailed in a previous section. Here,we experimentally validate simultaneous channel access usingCDMA.An experiment was conducted through water, with the

receiver (User B) at the end of the tank, and two co-locatedtransmitters (Users A and C) at the other end. Data wascollected for the instances where only user A is transmitting,only user C is transmitting, and when both users A and Care simultaneously transmitting. The signal received at thereceiver was digitized and processed with their results shownin Fig. 17. The results in Figs. 17a and 17b show strongcorrelation in the absence of interferers. Fig. 17c is consistentwith theory as well, and shows multiple access interference


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(c) User A (Code 1) on, User C (Code 3) on

Fig. 17. Correlation of the received signal at User B from two differenttransmitters, User A and User C, with their outputs spread using the respectiveCDMA codes 1 and 3. Graph shown is for a bit “1” spread using two different32-chip prime codes. The bits are intentionally synchronized between the twotransmitters for the sake of comparison, although synchronization between thetransmitters is not required.

(MAI) inherent to asynchronous multi-user CDMA. Althoughthis degrades performance, a code-matched filter and multi-user detection techniques such as a decorrelating detector canbe used to decouple users.

VII. DISCUSSION

Freespace optical communication is inherently line-of-sightand hence highly dependent on the field of views of, andpointing and tracking between, transmitters and receivers.The goal of this paper has been to present one method ofrelaxing these requirements. The results show that in additionto satisfying this goal, the design is also capable of acting as a“smart” system with the additional sensing capabilities at thereceiver and the control capabilities at the transmitter.The characterization experiments reveal that it is possible

to extend the directed FOV of a single lens-photodiode pairin multiple directions using the design described in this paper.It is also shown that, while the combined FOV has “nulls” insensitivity, this can be mitigated by intentionally defocusingthe receiver plane. The angle of arrival estimation algorithmpresented, although preliminary, serves as a proof-of-conceptthat a simple pattern matching algorithm can obtain a single-axis angle of arrival. It is proposed that a more advanced

R1

R7

Diversity Combiner

Angle of Arrival Estimator

Backscatter Estimator

Direction Control

Power and rate Control

T1

T7

Output Switching

RX

TX

Fig. 18. Closed-loop system architecture of smart-transceiver. The transmitteris capable of direction, output power, and rate control, based on feedback frombackscatter and angle of arrival estimator at the receiver.

precoding-based or Markov localization-based algorithm canprovide a three-dimensional estimate. The backscatter estima-tion experiment demonstrates a linear relationship between thereturn beam intensity and channel attenuation coefficient. Itis anticipated that higher level controls are required to co-ordinate the gain changes at the receiver required during thisprocess, as well as that some calibrations are necessary toextend the laboratory tank measurements to the “real world.”Through diversity combining experiments, it can be seen

that selection combining is a better alternative to equal gaincombining in an angular diversity scenario. However, it isexpected that in highly scattering environments, an optimalmaximum ratio combiner is desirable. Preliminary multi-userexperiments conducted show that although CDMA is requiredin certain alignments of users, the design inherently acts asa spatial division multiple access system. Further experimentsare necessary to verify whether, in a highly scattering envi-ronment in coordination with the CDMA coded transmitteroutputs, a diversity combiner similar to techniques used in aRAKE receiver is beneficial.The block diagram shown in Fig. 18 takes advantage of the

design and its capabilities to illustrate a closed-loop scenario.The output from the angle of arrival estimator is used by thetransmitter to switch LEDs, and thus electronically beamsteerits outputs. The output from the backscatter estimator is usedto adjust the output power of the LEDs and, if necessary, otherparameters such as the data rate and code rate. Currently, allprocessing takes place offline on a PC running MATLAB.However, the hardware constructed is capable of implementingthis proposed closed-loop scheme on a field programmablegate array or other embedded digital signal processor.

VIII. CONCLUSION

This paper demonstrates the feasibility of implementingsmart transmitters and receivers for free-space underwateroptical communication systems and presents data obtainedin a tank designed to simulate ocean water conditions. Thesmart receivers have increased field of view and the abil-ity to estimate angle of arrival. The transmitters are quasi-omnidirectional, allow electronic switched beamsteering, andwith an adjacent receiver, enable estimation of water quality by


measuring the optical backscatter from transmitted light. Thissmart transceiver approach mitigates pointing and trackingrequirements, which can be difficult for underwater platformsand enable adaptive communication techniques, which areexpected to be useful due to the variable dynamic range of thecommunications signal as platforms change relative range andpose. Thus, we anticipate that implementations such as thosepresented combined with CDMA and other coding techniqueswill be useful in enabling a physical layer for networkingschemes in swarms of unmanned underwater vehicles in thefuture.

ACKNOWLEDGMENT

The authors are grateful for helpful discussions and assis-tance from William Cox and Kevin Wolf.

REFERENCES

[1] C. Pontbriand, N. Farr, J. Ware, J. Preisig, and H. Popenoe, “Diffusehigh-bandwidth optical communications,” in Proc. OCEANS Conf. 2008,Quebec, Canada, Sept. 15-18 2008.

[2] B. Cochenour, L. Mullen, and A. Laux, “Phase Coherent DigitalCommunications for Wireless Optical Links in Turbid UnderwaterEnvironments,” in Proc. OCEANS Conf. 2007, Vancouver, BC, Canada,2007.

[3] M. Doniec, I. Vasilescu, M. Chitre, C. Detweiler, M. Hoffmann-Kuhnt,and D. Rus, “AquaOptical: A lightweight device for high-rate long-range underwater point-to-point communication,” in Proc. OCEANSConf. 2009, Biloxi, MS, Oct 26-29 2009.

[4] F. Hanson and S. Radic, “High bandwidth underwater opticalcommunication,” Applied Optics, vol. 47, no. 2, p. 277, Jan. 2008.

[5] W. C. Cox, “A 1 Mbps Underwater Communication System Using a 405nm Laser Diode and Photomultiplier Tube,” M.S. thesis, North CarolinaState University, Raleigh, 2007.

[6] J. A. Simpson, “A 1 Mbps Underwater Communications System usingLEDs and Photodiodes with Signal Processing Capability,” M.S. thesis,North Carolina State University, Raleigh, 2007.

[7] C. D. Mobley, Light and water : radiative transfer in natural waters.San Diego: Academic Press, 1994.

[8] C. Mobley, “Ocean Optics Web Book,” 2011. [Online]. Available:http://www.oceanopticsbook.info/

[9] W. C. Cox, “Simulation, Modeling, and Design of Underwater OpticalCommunication Systems,” Ph.D. dissertation, North Carolina StateUniversity, Raleigh, 2012.

[10] B. M. Cochenour, L. J. Mullen, and A. E. Laux, “Characterization ofthe Beam-Spread Function for Underwater Wireless Optical Communi-cations Links,” IEEE J. Ocean. Eng., vol. 33, no. 4, pp. 513–521, Oct.2008.

[11] L. Mullen, D. Alley, and B. Cochenour, “Investigation of the effect ofscattering agent and scattering albedo on modulated light propagationin water,” Applied Optics, vol. 50, no. 10, p. 1396, Mar. 2011.

[12] J. Everett, “Forward-Error Correction Coding for Underwater Free-spaceOptical Communication,” M.S. thesis, North Carolina State University,Raleigh, 2009.

[13] J. A. Simpson, W. C. Cox, J. R. Krier, and B. Cochenour, “5 Mbpsoptical wireless communication with error correction coding for under-water sensor nodes,” in Proc. OCEANS Conf. 2010, Seattle, WA, 2010.

[14] W. Rabinovich, R. Mahon, P. Goetz, E. Waluschka, D. Katzer, S. Binari,and G. Gilbreath, “A cat’s eye multiple quantum-well modulating retro-reflector,” IEEE Photonics Techn. Lett., vol. 15, no. 3, pp. 461–463,Mar. 2003.

[15] W. C. Cox, K. F. Gray, J. A. Simpson, B. Cochenour, B. L. Hughes,and J. F. Muth, “A MEMS Blue / Green Retroreflecting Modulator forUnderwater Optical Communications,” in Proc. OCEANS Conf. 2010,Seattle, WA, 2010.

[16] J. Akella, C. Liu, D. Partyka, M. Yuksel, S. Kalyanaraman, and P. Dutta,“Building blocks for mobile free-space-optical networks,” in WOCN2005, 2005.

[17] A. Sevincer, M. Bilgi, M. Yuksel, and N. Pala, “Prototyping Multi-Transceiver Free-Space Optical Communication Structures,” in Int. Conf.on Communications 2010, 2010.

[18] A. Tang, J. Kahn, and K. Ho, “Wireless infrared communication linksusing multi-beam transmitters and imaging receivers,” in Int. Conf. onCommunications 1996, vol. 1., 1996.

[19] J. Carruther and J. Kahn, “Angle diversity for nondirected wireless in-frared communication,” IEEE Transactions on Communications, vol. 48,no. 6, pp. 960–969, Jun. 2000.

[20] J. A. Simpson, B. L. Hughes, and J. F. Muth, “A spatial diversity systemto measure optical fading in an underwater communications channel,”in Proc. OCEANS Conf. 2009. Biloxi, MS: IEEE, 2009.

[21] P. Prucnal and M. Santoro, “Spread spectrum fiber-optic local areanetwork using optical processing,” J. Lightwave Technology, vol. 4,no. 5, pp. 547–554, 1986.

[22] D. Haubrich, J. Musser, and E. S. Fry, “Instrumentation to measure thebackscattering coefficient b(b) for arbitrary phase functions.” Appliedoptics, vol. 50, no. 21, pp. 4134–47, Jul. 2011.

Jim Simpson (S’06) received the B.S. degree andM.S. degree in electrical engineering from NorthCarolina State University, Raleigh, NC, in 2006and 2007, respectively. He is currently a Ph.D.candidate in electrical engineering at North CarolinaState University. His research interests include dig-ital communication systems and underwater opticalcommunications.

Brian Hughes (S’84-M’85) was born in Baltimore,MD, on July 16, 1958. In 1980, he received theB.A. degree in mathematics from the Universityof Maryland, Baltimore County. He received theM.A. degree in applied mathematics as well asthe Ph.D. degree in electrical engineering from theUniversity of Maryland, College Park, in 1983 and1985, respectively.From 1980 to 1983, he worked as a mathemati-

cian at the NASA Goddard Space Flight Center inGreenbelt, MD. From 1983 to 1985, he was a Fellow

with the Information Technology Division of the Naval Research Laboratoryin Washington, DC. From 1985 to 1997, he served as Assistant and then Asso-ciate Professor of Electrical and Computer Engineering at The Johns HopkinsUniversity in Baltimore, MD. In 1997, he joined the faculty of North CarolinaState University in Raleigh, where he is currently Professor of Electrical andComputer Engineering. His research interests include communication theory,information theory, and communication networks.Dr. Hughes has served as Associate Editor for Detection of the IEEE

Transactions on Information Theory, Editor for Theory and Systems of IEEETransactions on Communications, and as Guest Editor for two special issuesof IEEE Transactions on Signal Processing. He has also co-chaired the2008 Globecom Wireless Communications Symposium, the 2004 GlobecomCommunication Theory Symposium, as well as the 1987 and 1995 Con-ferences on Information Sciences and Systems. He has also served on theprogram committees of numerous international conferences, including theIEEE Global Communications Conference, IEEE International Communica-tions Conference, IEEE International Symposium on Information Theory, andthe IEEE Wireless Communications and Networking Conference.

John Muth (M’07) received the B.Sc. degree inapplied engineering physics from Cornell University,Ithaca, NY, in 1988, after serving as submarineofficer in the U.S. Navy he received a Ph.D. in solidstate physics from North Carolina State University,Raleigh, in 1998.He is a professor of Electrical and Computer

Engineering at North Carolina State University,Raleigh, NC. His research interests growth andfabrication of novel photonic materials and devicesas well as underwater optical communications. He

has over 90 peer-reviewed publications and 6 awarded patents.Dr. Muth has received several awards including the Office of Naval Re-

search Young Investigator Award (2003), the National Academy of EngineersFrontiers of Science Award (2004) and a Bronze Star for meritorious servicewhile on active duty in Iraq (2008).

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