Optical Engineering 49共1兲, 015001 共January 2010兲
Underwater optical wireless communication network Shlomi Arnon, MEMBER SPIE Ben-Gurion University of the Negev Electrical and Computer Engineering Department Satellite and Wireless Communications Laboratory P.O. Box 653 Beer-Sheva, IL-84105, Israel E-mail:
[email protected]
Abstract. The growing need for underwater observation and subsea monitoring systems has stimulated considerable interest in advancing the enabling technologies of underwater wireless communication and underwater sensor networks. This communication technology is expected to play an important role in investigating climate change, in monitoring biological, biogeochemical, evolutionary, and ecological changes in the sea, ocean, and lake environments, and in helping to control and maintain oil production facilities and harbors using unmanned underwater vehicles 共UUVs兲, submarines, ships, buoys, and divers. However, the present technology of underwater acoustic communication cannot provide the high data rate required to investigate and monitor these environments and facilities. Optical wireless communication has been proposed as the best alternative to meet this challenge. Models are presented for three kinds of optical wireless communication links: 共a兲 a line-of-sight link, 共b兲 a modulating retroreflector link, and 共c兲 a reflective link, all of which can provide the required data rate. We analyze the link performance based on these models. From the analysis, it is clear that as the water absorption increases, the communication performance decreases dramatically for the three link types. However, by using the scattered light it was possible to mitigate this decrease in some cases. It is concluded from the analysis that a high-data-rate underwater optical wireless network is a feasible solution for emerging applications such as UUV-to-UUV links and networks of sensors, and extended ranges in these applications could be achieved by applying a multi-hop concept.
© 2010 Society of Photo-Optical Instrumentation Engineers. 关DOI: 10.1117/1.3280288兴
Subject terms: optical communication; underwater; subsea; FSO; ocean. Paper 090580PR received Jul. 30, 2009; revised manuscript received Nov. 8, 2009; accepted for publication Nov. 11, 2009; published online Jan. 15, 2010. This paper is a revision of a paper presented at the SPIE conference on Free-Space Laser Communications IX, August 2009, San Diego, California. The paper presented there appears 共unrefereed兲 in SPIE Proceedings Vol. 7464.
1
Introduction
The present technology of acoustic underwater communication is a legacy technology that provides low-data-rate transmissions for medium-range communication. Data rates of acoustic communication are restricted to around tens of thousands of kilobits per second for ranges of a kilometer, and less than a thousand kilobits per second for ranges up to 100 km, due to severe, frequency-dependent attenuation and surface-induced pulse spread.1–4 In addition, the speed of acoustic waves in the ocean is approximately 1500 m / s, so that long-range communication involves high latency, which poses a problem for real-time response, synchronization, and multiple-access protocols. As a result, the network topology is simple and goodput is low. In addition, acoustic waves could distress marine mammals such as dolphins and whales. As a result, acoustic technology cannot satisfy emerging applications that require around the clock, high-data-rate communication networks in real time. Examples of such applications are networks of sensors for the investigation of climate change; monitoring biological, biogeochemical, evolutionary, and ecological processes in sea,
ocean, and lake environments; and unmanned underwater vehicles 共UUVs兲 used to control and maintain oil production facilities and harbors 共Fig. 1兲. An alternative means of underwater communication is based on optics, wherein
Fig. 1 The line-of-sight communication scenario.
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Optical Engineering
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January 2010/Vol. 49共1兲
Arnon: Underwater optical wireless communication network
high data rates are possible. However, the distance between the transmitter and the receiver must be short, due to the extremely challenging underwater environment, which is characterized by high multiscattering and absorption. Multiscattering causes the optical pulse to widen in the spatial, temporal, angular, and polarization domains. Although high data rates are threatened by extremely high absorption and scattering, there is evidence that broadband links can be achieved over moderate ranges. Hanson and Radic5 demonstrated 1-Gbit/ s transmissions in a laboratory experiment with a simulated aquatic medium with scattering characteristics similar to oceanic waters. Cochenour, Mullen, and Laux6 measure both the spatial and temporal effects of scattering on a laser link in turbid underwater environments. Using Monte Carlo simulations and measurement results, they predict longer-range underwater free-space optical performance with bandwidths greater than 5 GHz for a range of 64 m in clear ocean water, dropping to 1 GHz for a range of 8 m in turbid harbor water. The authors of Refs. 7 and 8 examine the fundamental physics and natural variability of underwater optical attenuation and discuss the design issues of underwater optical communications associated with oceanic physics and parameter variability. In Ref. 9 the authors examine the potential of subsea free-space optics for sensor network applications, leveraging the emerging technologies of highly sensitive photon-counting detectors and semiconductor LED and laser light sources in the solar blind UV. The authors of Ref. 10 propose to use retroreflecting free-space optical links in water, which allow much of the weight and power payload of the system to be located at one end. Arnon and Kedar11 propose a novel non-line-of-sight network concept in which the optical link is implemented by means of back reflection of the propagating optical signal at the ocean-air interface, which could help to overcome obstructions. In Ref. 12 the possibility of a wireless sensor network concept dubbed “optical plankton” is described and evaluated. The paper by Jaruwatanadilok13 presents the modeling of an underwater wireless optical communication channel using the vector radiative transfer theory. The vector radiative transfer equation captures the multiple scattering in natural water, and also includes the polarization of light. I present models of three kinds of optical wireless communication: 共a兲 a line-of-sight link, 共b兲 a modulating retroreflector link, and 共c兲 a reflective link, all of which can provide the required data rate. I also present performance analyses based on these models. From the analyses it is clear that as the water absorption increases due to changes in water turbidity, the communication performance decreases dramatically for all three link types, but the modulated retroreflector link is the most affected. However, the absorption coefficient increases more moderately than does the water turbidity. We conclude from the analysis that a high-data-rate underwater optical wireless network is a feasible solution for emerging applications such as UUV-toUUV links and networks of sensors. Extended ranges in these applications could be achieved by applying a multihop concept. The remainder of the paper is organized as follows. Section 2 describes the properties of the underwater optical wireless communication channel. Section 3 presents the Optical Engineering
Extinction coefficient (m-1) Clean Ocean Coastal Ocean Turbid Harbor
0.15 0.30 2.19
Fig. 2 Absorption, scattering, and extinction coefficients for four types of water—pure sea water, clean ocean water, coastal ocean water, and turbid harbor water—at 520-nm wavelength.
communication link models. Section 4 contains a discussion and a numerical example. Finally, Sec. 6 summarizes our results. 2
The Properties of the Underwater Optical Wireless Communication Channel Light pulses propagating in aquatic medium suffer from attenuation and broadening in the spatial, angular, temporal, and polarization domains. The attenuation and broadening are wavelength dependent and result from absorption and multiscattering of light by water molecules and by marine hydrosols 共mineral and organic matter兲. The extinction coefficient c共兲 of the aquatic medium is governed by the absorption and scattering coefficients ␣共兲 and 共兲, respectively, and we have9 c共兲 = ␣共兲 + 共兲.
共1兲
Figure 2 depicts the absorption, scattering, and extinction coefficients for four types of water—pure sea water, clean ocean water, coastal ocean water, and turbid harbor water—at 520-nm wavelength.6,10,14 It is clear that an increase in the turbidity dramatically increases the extinction coefficient, from less than 0.1 m−1 for pure water up to more than 2 m−1 for turbid harbor water. However, the absorption coefficient increases more moderately than does the turbidity. The propagation loss factor as a function of wavelength and distance z is given by Lpr共,z兲 = exp关− c共兲z兴.
共2兲
3 Communication Link Models We now consider three types of communication links: the line of sight, the modulating retroreflector, and the reflective. In addition, we perform a bit error rate 共BER兲 calculation.
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January 2010/Vol. 49共1兲
Arnon: Underwater optical wireless communication network
冉
PRគlos = PTTRLpr ,
冊
ARec cos d . cos 共d tan 0兲2
共4兲
3.2 Modulating Retroreflector Communication Link The modulating retroreflector link10 is used when one party 共for example, a submarine兲 has more resources another one 共for example, a diver兲, as in Fig. 3共b兲. In this case, the submarine has more energy, payload, and lifting capacity than the diver. Therefore it would be wise to put most of the complexity and power requirement of the communication system into the submarine. In a modulating retroreflector link, the interrogator sits at one end 共in our case, in the submarine兲, and a small modulating optical retroreflector sits at the remote end. In operation, the interrogator illuminates the retroreflecting end of the link with a continuouswave beam. The retroreflector inactively reflects this beam back to the interrogator while modulating the information on it. The received power in this scenario is given by
冉
PRគRetro = PTTRecRetroLpr , ⫻
冋
册
2d cos
冊冋
ARetro cos 2d2共1 − cos 0兲
ARec cos , 共d tan 0retro兲2
册
共5兲
where Retro is the optical efficiency of the retroreflector, is the angle between the perpendicular to the receiver plane and the transmitter-receiver trajectory, ARetro is the retroreflector’s aperture area, and 0retro is the retroreflector’s beam divergence angle. Fig. 3 共a兲 The line-of-sight communication scenario. 共b兲 The modulating retroreflector communication scenario. 共c兲 The reflection communication scenario.
3.1 Line-of-Sight Communication Link The most common link between two points in optical wireless communication systems is a line-of-sight 共LOS兲 link as illustrated in Fig. 3共a兲. In this scenario, the transmitter directs the light beam in the direction of the receiver. The optical signal reaching the receiver is obtained by multiplying the transmitter power, telescope gain, and losses and is given by Ref. 11 as
冉
PRគlos = PTTRLpr ,
冊
ARec cos d , cos 2d2共1 − cos 0兲
共3兲
where PT is the average transmitter optical power, T is the optical efficiency of the transmitter, R is the optical efficiency of the receiver, d is the perpendicular distance between the transmitter and the receiver plane, is the angle between the perpendicular to the receiver plane and the transmitter-receiver trajectory, ARec is the receiver aperture area, and 0 is the laser beam divergence angle. When the transmitter beam divergence angle is very narrow 共0 / 20兲, Eq. 共3兲 can be approximated as Optical Engineering
3.3 Reflective Communication Link In some communication scenarios the line of sight is not available due to obstructions, misalignment, or random orientation of the transceivers.11 To address this problem a reflective communication link could be used. In this case, the laser transmitter emits a cone of light, defined by inner and outer angles min and max, in the upward direction 关Fig. 3共c兲兴. Here i and t are the angles of incidence and of transmission, respectively. 共The latter is derived from the former using Snell’s law.兲 The light reaching the ocean-air surface illuminates an annular area and is partially bounced back in accordance with the reflectivity. Since the refractive index of air is lower than that of water, total internal reflection 共TIR兲 can be achieved above a critical incidence angle. When the transmitter is at depth h, the illuminated annular surface with equal power density at depth x is given by Aann = 2共h + x兲2共1 − cos max − 1 + cos min兲 = 2共h + x兲2共cos min − cos max兲.
共6兲
Equation 共6兲 describes an annular area taken from a sphere of radius h + x, which would have uniform power density in free space. If we model the ocean-air surface as smooth, then = i, and we can derive the link budget by using the variables defined in Eq. 共3兲. Then we can define the auxiliary function and calculate the received power as
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Arnon: Underwater optical wireless communication network
f Rគref共兲 =
PT cos Aann
冦
冉 冉
TRLpr , TRLpr
冊 再冋 冊
h+x 1 cos 2
tan共t − 兲 tan共t + 兲
册 冋 2
+
h+x , , cos
PRគref共兲 ⬇ ARec f Rគref共兲.
共8兲
3.4 Bit Error Rate Calculation The simplest and most widespread modulation technique in optical wireless communication is intensity-modulation, direct-detection on-off keying 共OOK兲. In this technique, the receiver is based on the emerging technology of silicon photomultipliers 共SiPMs兲.15 These photodetector devices are fabricated in the form of arrays of photodiodes that are operated in Geiger mode to create a photon-counting device. If we assume that a large number of photons are received, then according to the central limit theorem, the Poisson distribution can be approximated by a Gaussian distribution and the BER is given by11
再
1 erfc 2
r 1T − r 0T
冑2关共r1T兲1/2 + 共r2T兲1/2兴
冎
.
共9兲
Here r1 = rd + rbg + rs and r0 = rd + rbg, where rd and rbg represent the sources of additive noise due to dark counts and background illumination, respectively, and erfc共兲 =
2
冑
冕
⬁
exp共− ␥2兲d␥ .
2
, min 艋 ⬍ c ,
冧
共7兲
the receiver results in a considerably higher photon count for a given sensor node separation than a reflective or retroreflector link. For instance, for a node separation of 30 m, 8,000 photons would be received from a signal in a LOS link, 2 photons would be received from a retroreflector link, and only 10 would be received in a reflective link where the transmitter depth is 20 m and the receiving nodes are also at a depth of 20 m. However, if a single point-to-point link were to fail, the transmitted signal would be lost, while in the reflective underwater network solution a number of nodes would be expected to receive the signal. Even in the severe case where several nodes fail, with sufficient node redundancy there would still be additional nodes that could relay the signal further. In Fig. 5 we can see that BER values of 10−4 are obtained for a reflective link when the node separation is 40 m, while a BER of 10−4 could be achieved in a LOS link and a retroreflector link when the node separation is 60 m and 50 m, respectively. From this result it is easy to understand that acceptable BER performance could be achieved for short ranges on the order of tens of meters for all three models. In Fig. 6 we compare the numbers of photons received for a link operated in turbid harbor water for two cases: 共a兲 when only absorption is considered and 共b兲 when absorption and scattering are considered. From this figure it is easy to see that in the absorption case the number of received photons reduces from 105 to 1 for increases in distance separation from 1 to 65 m, while in the case of ab-
共10兲
4 Discussion and Numerical Example The three types of link models could be used to design sophisticated networks. It is clear that line of sight using narrow beam divergence provides the maximum range; however, in this case the precise locations of the two platforms are required. On the other hand, when it is required to simultaneously broadcast 共for example兲 from a submarine to several platforms 共UUVs or divers, for example兲, the best option is to use LOS with a wide beam divergence. However, if obstructions between the two platforms block the line of sight, a reflective communication link is preferred. When one party has more resources than the other one in the link, the modulated retroreflector is the best option. In this section we simulate the performance of the three links, using practical values for clean ocean water with an extinction coefficient of 0.15 m−1. The values of the simulation parameters are given in Table 1. It is evident from Fig. 4 that a single LOS underwater link using a pulsemodulated laser transmitter and a SiPM detector array in Optical Engineering
册冎
c 艋 ⬍ max .
At the plane of the receiving sensor, node coverage is provided within an annular area bounded by radii 共h + x兲 tan min and 共h + x兲tan max. Equation 共7兲 can be simplified on the assumption that the receiver aperture is small relative to h + x, yielding the approximate received power as
BER =
sin共 − t兲 sin共 + t兲
Fig. 4 Graph showing number of received photons as a function of transmitter-receiver separation for clean ocean water with extinction coefficient equal to 0.15 m−1.
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Arnon: Underwater optical wireless communication network Table 1 Parameters used in numerical calculations. We assume ⌰retro is much greater than the diffraction-limited divergence angle. Parameter
Typical value
Extinction coefficient, clear ocean 共m−1兲
0.1514
Refractive index
1.33643
Critical angle 共deg兲
48.44
Transmission wavelength 共nm兲
532
Optical efficiency of retroreflector
0.9
Optical efficiency of transmitter
0.9
Optical efficiency of receiver
0.9
Average transmitter power 共W兲
10
Pulse duration 共ns兲
1
Data rate 共Mbit/s兲
0.5
Receiver aperture area 共m2兲
0.01
Retroreflector aperture area 共m 兲 2
Fig. 5 Graph showing BER as a function of transmitter-receiver separation for clean ocean water with extinction coefficient equal to 0.15 m−1.
transceiver and an acoustical transceiver. A hybrid communication system can provide high-data-rate transmission by using the optical transceiver. When the water turbidity is high or the distance between the terminals is large, the system can switch to a low data rate using the acoustic transceiver, thereby increasing the average data rate and availability. However, the complexity and cost of the system are increased. In this kind of system, smart buffering and prioritization could help to mitigate short-term data rate reduction. Many aspects of the proposed system remain to be investigated; for example, rigorous modeling of the reflective nature of the ocean-air surface, including ocean surface roughness, as well as solar radiance penetration. Extensive studies should be made of the nature of multiple scattering in different oceanic channels and the limitation of the
0.01
Retroreflector beam divergence ⌰retro 共deg兲
10
Laser beam divergence angle 0 共deg兲
68
Transmitter inclination angles min, max 共deg兲
0, 68
Dark counting rate 共MHz兲
1
Background counting rate 共MHz兲
1
Counting efficiency 共%兲
16
Transmitter depth h 共m兲
20
Receiver depth x 共m兲
20
sorption and scattering the number of received photons reduces from 105 to 1 for increases in distance separation from 1 to 8 m This result indicates that receiving more scattered light and performing the required signal processing in the time domain could dramatically improve the performance of an optical wireless system in turbid water. 5 Summary and Conclusions The results presented indicate that networks based on underwater optical wireless links are feasible at high data rates for medium distances, up to a hundred meters. Such networks could serve subsea wireless mobile users. In addition, by placing multiple relay nodes between the chief network nodes, messages could traverse very long distances despite severe medium-induced limitations on the transmission ranges of individual links. Additional improvements to the availability of the network could be achieved by a hybrid communication system that would include an optical Optical Engineering
Fig. 6 Graph showing number of received photons for line-of-sight scenario as a function of transmitter-receiver separation for two cases: absorption and extinction.
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Arnon: Underwater optical wireless communication network
modulating retroreflector range due to light backscattered into the receiver before reaching the retroreflector. Some seminal theory necessary to describe spatial spreading of an optical beam in the presence of scattering agents under water was presented in Ref. 16. Future work on these subjects should refine the analysis and yield more accurate numerical results. Additional open issues to be addressed at higher layers of the network design include multiple access, such as wavelength division multiplexing 共WDM兲 at blue-green wavelengths, and code division multiple access 共CDMA兲 or clustering. However, the fundamental concept has been shown to be feasible and practical. References 1. I. F. Akyildiz, D. Pompili, and T. Melodia, “Underwater acoustic sensor networks: research challenges,” Ad Hoc Networks 3共3兲, 255– 256 共2005兲. 2. J. Heidemann, W. Ye, J. Wills, A. Syed, and Y. Li, “Research challenges and applications for underwater sensor networking,” in Proc. IEEE Wireless Communications and Networking Conf., pp. 228–235 共2006兲. 3. T. Dickey, M. Lewis, and G. Chang, “Optical oceanography; recent advances and future directions using global remote sensing and in situ observations,” Rev. Geophys. 44共1兲, RG1001 共2006兲. 4. C. Detweiller, I. Vasilescu, and D. Rus, “AquaNodes: an underwater sensor network,” in Proc. Second Int. Workshop on Underwater Networks, pp. 85–88, IEEE 共2007兲. 5. F. Hanson and S. Radic, “High bandwidth underwater optical communication,” Appl. Opt. 47共2兲, 277—283 共2008兲. 6. B. Cochenour, L. Mullen, and A. Laux, “Spatial and temporal dispersion in high bandwidth underwater laser communication links,” in Proc. IEEE Military Communications Conf., pp. 1–7 共2008兲. 7. J. H. Smart, “Underwater optical communication systems part 1: variability of water optical parameters,” in Proc. IEEE Military Communications Conf., pp. 1140–1146 共2005兲. 8. J. W. Giles and I. N. Bankman, “Underwater optical communications systems part 2: basic design considerations,” in Proc. IEEE Military Communications Conf., pp. 1140–1146 共2005兲. 9. D. Kedar and S. Arnon, “Subsea ultraviolet solar-blind broadband free-space optics communication,” Opt. Eng. 48共4兲, 046001 共2009兲. 10. L. Mullen, B. Cochenour, W. Rabinovich, R. Mahon, and J. Muth, “Backscatter suppression for underwater modulating retroreflector links using polarization discrimination,” Appl. Opt. 48共2兲, 328–337 共2009兲. 11. S. Arnon and D. Kedar, “Non-line-of-sight underwater optical wireless communication network,” J. Opt. Soc. Am. A 26共3兲, 530–539 共2009兲. 12. D. Kedar and S. Arnon, “Optical plankton: an optical oceanic probing
Optical Engineering
scheme,” J. Appl. Remote Sensing 1, 013541 共2007兲. 13. S. Jaruwatanadilok, “Underwater wireless optical communication channel modeling and performance evaluation using vector radiative transfer theory,” IEEE J. Sel. Areas Commun. 26共9兲, 1620–1627 共2008兲. 14. R. P. Bukata, J. H. Jerome, K. Y. Kondratyev, and D. V. Pozdnyakov, Optical Properties and Remote Sensing of Inland and Coastal Waters, CRC Press, Boca Raton, FL 共1995兲. 15. P. Eraerds, M. Legre, A. Rochas, H. Zbinden, and N. Gisin, “SiPM for fast photon-counting and multiphoton detection,” Opt. Express 15共22兲, 14539—14549 共2007兲. 16. B. M. Cochenour, L. J. Mullen, and A. E. Laux, “Characterization of the beam-spread function for underwater wireless optical communications links,” IEEE J. Ocean. Eng. 33共4兲, 513–521 共2008兲. Shlomi Arnon is a faculty member in the Department of Electrical and Computer Engineering at Ben-Gurion University, Israel. There, in 2000, he established the Satellite and Wireless Communication Laboratory, which has been under his directorship since then. During 1998–1999 Professor Arnon was a postdoctoral associate 共Fulbright Fellow兲 at LIDS, Massachusetts Institute of Technology 共MIT兲, Cambridge, USA. His research has produced more than fifty journal papers in the area of satellite, optical, and wireless communication. During part of the summer of 2007, he worked at TU/e and Philips Lab, Eindhoven, Nederland, on a novel concept of a dual communication and illumination system. He was a visiting professor during the summer of 2008 at TU Delft, Nederland. Professor Arnon is a frequent invited speaker and program committee member at major IEEE and SPIE conferences in the USA and Europe. He was an associate editor for the Optical Society of America’s Journal of Optical Networks for a special issue on optical wireless communication that appeared in 2006, and is now on the editorial board for the IEEE Journal on Selected Areas in Communications for a special issue on optical wireless communication. Professor Arnon continuously takes part in many national and international projects in the areas of satellite communication, remote sensing, and cellular and mobile wireless communication. He consults regularly with start-up and well-established companies in optical, wireless, and satellite communication. In addition to research, Professor Arnon and his students work on many challenging engineering projects with especial emphasis on the humanitarian dimension. For instance, a longstanding project has dealt with developing a system to detect human survival after earthquakes, with an infant respiration monitoring system to prevent cardiac arrest and apnea, and with detection of falls in the case of epilepsy sufferers and elderly people.
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