Underwater Optical Wireless Communications: An Overview
Laura Johnson
Department of Engineering
University of Warwick
January 8, 2012
Abstract
A literature review on underwater wireless optical communications is presented. The behaviour of light underwater, communication system configuration and previous experimental
performance is discussed. Existing systems are categorised by power requirement and an additional system is suggested based on the literature.
1
Introduction
Over the last five years there has been a resurgence of interest in underwater wireless optical
communications. Traditionally, acoustic communications were used in the ocean, these originally
superseded optical communications due to their ability to communicate over larger ranges [1].
However, much like the on-land phenomenon, there is now an increased need for short-range,
high-bandwidth wireless communications underwater. Acoustic systems are not suitable for this
application as they are inherently band limited; systems undergo severe, frequency dependant
dispersion, even at short ranges [2]. A promising alternative is the use of visible light, particularly
blue-green wavelengths. Light in this region propagates through water better by several orders
of magnitude than the remaining electromagnetic spectrum. Recent advances in terrestrial visible
light communications have also helped to increase the plausibility of an underwater optical wireless
system [3].
This review briefly outlines the current model of light propagation in ocean water and, in
section 3, it is shown how this model can be used to create an undersea optical communication
system. Finally, section 4 looks the at previous experimental achievements in this field.
2
Marine Optics
The performance of an underwater optical communication system relies on how well light propagates through seawater. This is an area of research which has interested oceanographers for many
years, therefore comprehensive literature exists on the topic. However, as this review will discuss,
this knowledge has been sparsely applied to models of the underwater optical channel thus far.
2.1
The Basics of Attenuation
The two main causes of electromagnetic attenuation in water are scattering and absorption. Whilst
scattering changes the path of a photon, absorption completely removes the photon from its path.
The combined rate of attenuation is described by the attenuation coefficient c, which is written in
its most basic form [4] as:
c(λ) = a(λ) + b(λ)
(1)
Where a is the absorption coefficient and b is the coefficient of scattering; both are dependent
on the illumination wavelength λ. When this wavelength is in the region 450-550 nm, there is
1
significantly less attenuation than the remaining electromagnetic spectrum. These wavelengths
represent blue-green visible light.
All reports in underwater optical communications recognise that the absorption and scattering
coefficients vary by region and typically Jerlov water types are used to classify these differences
[5]. The value of the attenuation coefficient ranges between 0.15 m−1 for the clearest open oceans
and 2.19 m−1 for turbid harbours [6]. The latter value is a conservative estimate of the maximum,
the actual uncategorised maximum is likely to be at least an order of magnitude higher. Other
types of classification include colour-matching and ternary diagrams based on the concentrations
of different ocean optical components [7] [8].
To understand the variation in coefficient values, the fundamental causes of absorption and
scattering shall be explained. Particles in the ocean are grouped by their optical properties,
higher concentrations of these particles lead to more significant attenuation. The factors which
affect absorption include; pure sea water, dissolved organic matter (CDOM), phytoplankton and
inorganic materials. Scattering, as well as being affected by pure sea water, is affected by particulate
substances and where there is a change in the ocean’s optical refractive index. A refractive index
change can happen for a number of reasons, including temperature gradients, pressure gradients
and increased salinity. It has been noted [9] that these factors have so far been omitted from
the optical communications channel model, despite causing a well documented increase on the
scattering coefficient [10]-[12]. The cause of such rapid increase is ocean currents and the resultant
turbulence, for which models have been formulated by the ocean science community [13]. The
delay in application to the underwater communications channel is likely due to the complexity of
the models.
2.2
Advanced Channel Models
In the previous section, it was shown that the current underwater channel model is likely to be
an oversimplification which systematically underestimates the scattering coefficient by omitting
scatter induced by ocean currents. Several authors show similar weaknesses in the current model
but based on different factors such as attenuation depth variation and temporal scattering, both
will be discussed in this section.
Chronologically, the first issue identified was the depth dependency of the attenuation coefficient. It arises because marine life has a tendency to cluster at specific regions, leading to a change
in optical properties as depth is varied. This behaviour was described by Smart et al, where
in-situ measurements were compared to colour satellite images [14]. There had been a previous
attempt to describe how depth affects the attenuation coefficient [15] but this was based on highly
uncertain experimental data and only indirectly considered the particulate substances causing the
attenuation. This description is also valid only in the euphoric region, the top region of the ocean
where sunlight can propagate and the majority of marine life resides. In clear open ocean water,
this region is approximately 100 m deep [16]. As many reports in underwater optical communications omit a discussion of depth variation, it must be assumed that existing link models are purely
horizontal.
Due to the attenuation depth variation, ternary diagrams are more suited for categorising the
underwater optical channel than popular Jerlov schemes. Each optical component has a unique
behaviour and ternary diagrams provide a coefficient to represent how much of a particular substance is present whereas Jerlov only provides a single coefficient for all absorption and another
for scattering. This distinction also becomes more important when the model in equation 1 is
extended to include additional effects such as temporal scattering and polarisation. An existing
example of where these diagrams could readily be applied is the work of Green et al [17].
The issue of temporal scattering was raised by a team at NC state university. It is an important factor because if temporal pulse stretching is close to the time taken for each bit of data
then inter-symbol interference will occur. This is combated by reducing the bit rate, decreasing
the performance of the communication system. The team at NC state university predicted that
temporal changes would be significant in highly turbid environments, where a large amount of
2
- R
T
(a) LOS
#
#
T #
#c
#
c
c c
c
~ R
(b) Non-LOS (reflective)
T - R
(c) Modulating retroreflector
Figure 1: Link types for underwater optical wireless communication between a transmitter T and
receiver R.
multi-scattering occurs from particulate substances [18]. However, experimental data showed turbidity to have little effect on temporal scattering, even on high information modulated signals [19].
This is quoted to be because the 3.6 m link used was short compared to the long wavelength of
3.2 m and that it may still occur in longer links. Two further reports included detailed optical
analysis and modelling of the beam spread functions in order to describe the affect of temporal
scattering [20] [21]. Despite this, the extent of temporal scattering on information bearing content
over a medium to large distance remains to be tested.
There are additional topics explored by oceanographers that have not yet been considered in the
communication channel model. There has little consideration of underwater polarisation, although
Jaruwatanadilok describes a model of the channel with use of radiative transfer theory which
inherently takes into account polarisation [22]. Moreover, the extent of refractive index change
due to vehicle induced turbulence, and subsequent bubbles [23], is unknown but could provide an
insight into how an underwater communication system is affected by its platform. A reason that
many of these factors are omitted is the lack of experimental data in true ocean environments, this
is discussed further in section 4.
3
System Configuration
This section considers options for underwater communication links and how they differ from terrestrial visible light communications. There is emphasis on the optical set up, receiver and transmitter
design and modulation schemes. Cochenour et al showed that the affect of these components on
pointing accuracy and link range, at a specified water clarity, should be considered together [19].
3.1
Optical Design
Research into terrestrial infra-red communications raised two important optical aspects for communication links [24]. First is transmitter field-of-view (FOV) which describes the sending angle and
has significant implications on required pointing accuracy. The second aspect is link configuration,
the most simple being a line-of-sight (LOS) link which is a direct link between the transmitter and
receiver, as shown in figure 1 (a). In underwater optical communications, most authors adopt a
LOS link as it is easier to implement and most energy efficient [25]. A report by Arnon suggested
a reflecting non-LOS link to overcome underwater obstacles by reflecting from the sea surface [26],
given in figure 1 (b). A retroreflective link was also suggested as it may be useful for underwater
application because it allows much of the weight and power burden of the link to remain at one
end [27], see figure 1 (c). Arnon and Vijaya et al theoretically compared these links in terms of
bit error rate performance, both concluded that close range communication (under 15 m) is viable
with all configurations [25] [28].
3
3.2
Transmitter and Receiver
Transmission is typically by either light emitting diode (LED) or laser diode. LEDs are low power,
lightweight, diffuse sources which have a wide FOV. This makes them most suited to low range,
turbid oceans or areas where tracking accuracy is low. Laser diodes, on the other hand, require high
power and have lower channel noise, making them more suitable for long distance communications
in clear ocean. Joshi et al did a cost analysis of the two technologies [29] and determined that
LEDs are the favourable technology due to versatility and portability. This analysis overestimated
the required power for laser diodes as it included a large running cost associated with keeping the
laser below critical operating temperature. As the system is submerged in seawater, which has
an average surface temperature of 3.5 ◦ C [16], it should be possible for a simple, low cost cooling
system to be designed.
There are few reports that include a discussion of optical receivers despite being more significant underwater due to increased channel attenuation. Green writes that a successful receiver in
traditional optical wireless links includes an optical concentrator, optical filter, photo receiver and
preamplifier [30]. Many of these components are omitted from experimental configurations in underwater optics. For example, Brundage detected incoming light directly using only a photodiode
in a watertight perspex box [31]. Chancey designed a unique optical concentrator which used a
series of lenses to converge light from an LED [9]. However, this design significantly reduced the
FOV and lead to alignment issues. In general, more research is needed on optical receivers for use
underwater.
3.3
Modulation and Error Correction
Existing studies of underwater wireless links use on-off shift keying (OOK) as it is theoretically and
practically easier to implement, this is suitable as many of the studies are preliminary in nature.
Karpie et al compared modulation schemes for these links and showed that M-ary pulse-position
modulation (PPM) has power and bandwidth efficiency advantages over OOK and frequency-shift
keying [32], claiming to be near optimal [33]. Whilst this is a just claim in open oceans where
not much scattering occurs, medium- to long-distance turbid links potentially cause extremely
poor performance. The reason is that PPM operates well in links where there is little temporal
dispersion, hense being good for terrestrial optical wireless [24]. However, temporal scattering is
claimed to be likely in longer turbid links [19]. In addition to this, the system requires that the
transmitter and receiver are exactly in sync which is often difficult to achieve. Sui et al noted
that it is possible to bypass this using differential PPM (DPPM) [34]. However, DPPM has an
unusually large variation in bit rate and therefore could potentially violate eye safety laws when
used in conjuction with a laser source. Cox et al were the first to implement foward error correction
coding, in the form of Reed-Solomon (RS) coding, to a 500 kb s−1 underwater connection [35].
This was later improved to 5 Mb s−1 [36].
Recent publications in terrestrial wireless optical communications have looked at alternatives to
M-ary PPM. These include orthogonal frequency division multiplexing (ODFM) which has recently
been developed for visible light [37] [38]. Another method is polarisation-division multiplexing
(PDM) [39], an M-level scheme based on polarisation angle. A more accurate model of the channel
must be made before the optimum scheme is determined.
4
Experimental Performance
Several papers have investigated the performance of underwater optical wireless communication
systems, particularly the distance and bit rate they support. A summary of these papers is given
in figure 2. This section discusses the merits and achievements of each experimental set up and
shows that systems can be categorise into high power technologies, which typically use laser diodes
and portable LED-based technologies.
4
Year
Author
Range
Data Rate
Turbidity
Notes
1992
1995
2004
2004
2005
2006
2007
2008
2010
2010
Snow et al [40]
Bales and Chryssostomidis [41]
Tivey et al [42]
Schill et al [43]
Chancey [9]
Cochenour et al [18]
Cochenour et al [19]
Hanson and Radic [44]
Brundage [31]
Simpson et al [36]
9m
20 m
2.7 m
2m
12 m
3m
3.6 m
2m
13 m
7.7 m
50 Mb s−1
10 Mb s−1
14.4 kb s−1
57.6 kb s−1
10 Mb s−1
1 Mb s−1
5 Mb s−1
1 Gb s−1
3 Mb s−1
5 Mb s−1
Low
Low
Low
Low
Low
High
High
Low
Low
Low
LD
LD
IrDA, LED
IrDA, LED
LED
LED
QAM, LED
LD
LED
RS code, LED
Figure 2: Data rates and distance of previous experiments in underwater optical communications.
Initial experimental studies in the 1990s used highly specialised and expensive equipment and
were based on light transmission from laser diodes [40] [41]. Although both these configurations
achieved high data rates at mid-length ranges, they required high operational power and a heavy
transceiver. Only recently have these data rates been surpassed; Hanson and Radic achieved a 1 Gb
s−1 connection in a 2 m water tank using a high powered laser diode and predict this bit rate could
be maintained for up to 48 m in fresh water [44]. Despite proving a strong communication ability,
these laser links cannot realistically be used in a portable communication system due to weight
and power requirements. However, based on the work of Arnon, a laser configuration may still be
possible using a modulating retroreflector link [26]. In this system, the receiver is lightweight and
portable whilst the transmission station is static. The downsides of this system include limited
communication back to the base station and low pointing accuracy due to small-angle FOV.
An alternative branch of experimental configuration considers portable, low cost systems which
were triggered by the advance of infra-red data association (IrDA) standards for infrared communications in air. Tivey at al adapted the IrDA physical layer for underwater environments and
achieved 14.4 kb s−1 over 2.7 m [42]. The transmitter consisted of 22 LEDs and used a transceiver
of dimensions 5 cm by 10 cm; a system which was considered to be too large with too poor a data
rate [43]. This system had a wide-angle FOV of 120◦ , making system alignment trivial. Schill
et al also based their system on the IrDA physical layer but achieved an increased speed of 56.7
kb s−1 which was claimed to be sufficient to control submersible vehicles. However, for current
monitoring applications it is likely that this is still too low. Despite using only four 3 W blue LEDs
for transmitters, the report admitted the transceiver had larger dimensions than that of Tivey et
al.
The bit rate of lightweight, low power systems continued to improve with the work of Brundage
and Chancey to 3 Mb s−1 and 10 Mb s−1 respectively. Chancey’s work included a unique transmitter and receiver design which created an incoherent beam with a small FOV [9]. Although this
caused an improvement in the bit rate and range, it lead to alignment issues. Brundage had a
similar problem with poor off-axis performance [31]. By improving receiver optics, as discussed in
section 3.2, it should be possible to develop a receiver which is more resistant to pointing errors.
For this, additional experimental work needs to be done to determine the off-axis performance of
generic optical receivers. Simpson et al [36] pursued portable technologies further with the inclusion of RS coding. The novel receiver design in this set up includes a photo-multiplier tube to
amplify the received signal. This reduces circuit noise but increases detected noise and also induces
shape limitations the receiver design.
As mentioned in section 2.2, Cochenour et al explored the effects of increased water turbidity.
This was done with the addition of Maalox to fresh water, a substance which has similar attenuation
behaviour to that of seawater [18]. A general criticism of existing experimental data is that there
have been no tests in true oceanic environments. In section 2.1 it was discussed how additional
5
factors, such as ocean turbulence, could mean that there is a significant difference between the
scattering observed in closed experiments and real ocean data. Cochenour et al reported a 1 Mb
s−1 bit rate in water of attenuation coefficient 3.0 m−1 , representing a turbid coastal harbour.
This result was improved in 2007 to 5 Mb s−1 by use of a quadrature amplitude modulation
(QAM) [19]. Whilst this appears a surprising choice of modulation scheme, the report assumes
that the unproven temporal scattering effects exists, making PPM unsuitable for the underwater
channel. As previously discussed, there is a need for additional data of temporal scattering at
longer distances to determine whether this is the case.
5
Conclusion
Optical wireless has shown to be a viable solution for short-range, high-bandwidth, underwater
communications. However, it is a technology which is still very much in development and the
lack of application of knowledge from the ocean science community has lead to an oversimplified
model of the underwater channel. Scattering by oceanic turbulence, temporal effects and depth
variation are some of the topics covered by various authors, not in the current model, which may
have significant effects on the system configuration and performance.
It has been illustrated by previous experimental configurations that underwater optical wireless
communication systems fall into two categories; large, high power systems, characterised by the use
of laser diodes and those where LEDs are used. LED systems are low powered and highly portable
but have reduced range capabilities and lower data rates; experimental results show laser systems to
currently be performing with a bit rate two orders of magnitude higher. Despite this, LEDs are the
prevailing technology due to superior portability. This review also described a third, semi-portable
configuration based on existing literature. The system uses a modulating retroreflector link, with
a laser transmitter at a stationary base and a light portable receiver. This configuration offers
increased range and bit rate at the expense of limited communication back to the base station.
Additional research must be carried out in order to improve receiver design and pointing accuracy, particularly in systems where the FOV is limited. Further experimental data is required to
quantify scattering in real oceanic environments, so that the true effect ocean turbulence can be
determined. This review also highlighted the need for experimental data on the extent of temporal
scattering in long-range turbid links. The significance of this is that it determines whether PPM,
and schemes derived from it, are suitable modulation schemes for the underwater channel.
References
[1] Wiener, T. F. and Karp, S., The role of blue/green laser systems in strategic submarine communications. IEEE
Transactions on Communications, 28(9), 1601-1607. (1989)
[2] Stojanovic, M., Underwater wireless communications: current achievements and research challenges. IEEE
Oceanic Engineering Society Newsletter, 41, 2-7. (2006)
[3] Haas, H. Wireless data from every light bulb. TEDGlobal 2011, 12-15 July 2011, Edinburgh, UK. Available from http://www.ted.com/talks/harald_haas_wireless_data_from_every_light_bulb.html [Accessed
02/12/11] (2011)
[4] Shifrin, K. S., Physical optics of ocean water. American Institute of Physics, pp. 1-22. (1988)
[5] Jerlov, N. G., Marine Optics. Elsevier, Amsterdam, 2nd Ed. pp. 69-72. (1976)
[6] Mobley, C. D., Light and water: radiative transfer in natural waters. Academic Press. pp. 61-142. (1994)
[7] Arnone, R.A., A.M. Wood, and R.W. Gould Jr. The evolution of optical water mass classification. Oceanography, 17(2), 14-15. (2004)
[8] Babin, M., Stramski, D., Ferrari, G. M., Claustre, H., Bricaud, A., Obolensky, G., and Hoepffner, N. Variations
in the light absorption coefficients of phytoplankton, nonalgal particles, and dissolved organic matter in coastal
waters around Europe. Journal of Geophysical Research, 108(C7), 3211. (2003)
6
[9] Chancey, M. A., Short range underwater communication links. Master thesis, North Carolina State University.
(2005)
[10] Wells, W. H., Theory of small-angle scattering. Electromagnetics of the Sea, AGARD conference proceedings,
vol. 81, pp. 3.3-13.3-19 (1973)
[11] Hodara, H., Experimental results of small-angle scattering. Electromagnetics of the Sea, AGARD conference
proceedings, vol. 81, pp. 3.4-13.4-17 (1973)
[12] Honey R. C. and Sorensen, G. P., Optical absorption and turbulence induced narrow-angle forward scatter in
the sea. Electromagnetics of the Sea, AGARD conference proceedings, vol. 77, pp. 39.139.7. (1970)
[13] Bogucki, D. J., Domaradzki, J. A., Stramski, D. and Zanevald, J. R., Optical Society of America, 37(21),
4669-4677. (1998)
[14] Smart, J. H., Underwater optical communication systems part 2: variability of water optical properties. MILCOM, 2, 1140-1146. (2005)
[15] Jerlov, N. G., Classification of sea water in terms of quanta irradiance. ICES Journal of Marine Science, 37(3),
281-287. (1977)
[16] Meadows, P. S. and Campbell, J. I., An introduction to marine science. Blackie, London, pp. 25-33. (1978)
[17] Green, R. J. and Leeson, M. S. Underwater Optical Communications: Prospects and Opportunities. Confidential report to QinetiQ. (2008).
[18] Cochenour, B., Mullen, L., Laux, A. and Curran, T., Effects of multiple scattering on the implementation of an
underwater wireless optical communications link. OCEANS 2006 Conference, 18-21 Sep 2006, pp. 1-6. (2006)
[19] Cochenour, B., Mullen, L. and Laux, A., Phase coherent digital communications for wireless optical links in
turbid underwater environments. OCEANS 2007 Conference, 29 Sep - 4 Oct 2007, pp. 1-5. (2007)
[20] Cochenour, B., Mullen, L. and Laux, A., Spatial and temporal dispersion in high bandwidth underwater
communication links. MILCOM, 16-19 Nov 2008 pp. 1-7. (2008)
[21] Cochenour, B., Mullen, L. and Laux, A., Characterization of the beam-spread function for underwater wireless
optical communications links. IEEE Journal Ocean Engineering, 33(4), 513-521. (2009)
[22] Jaruwatanadilok, S., Underwater wireless optical communication channel modelling and performance evaluation
using vector radiative transfer theory. IEEE, selected areas in communications, 26(9), 1620-1627. (2008)
[23] Zhang, X., Lewis, M. and Johnson, B., Influence of bubbles on scattering of light in the ocean Optical Society
America, 37(27), 6525-6536. (1998)
[24] Barry, J. R., Wireless infrared communications. Kulwer Academic Publishers, pp. 3-14. (1994)
[25] Arnon, S., Underwater optical wireless communication network. Optical Engineering, 49, 1-6. (2010)
[26] Arnon, S. and Kedar, D., Non-line-of-sight underwater optical wireless communication network. Optical Society
of America, 26, 530-539. (2009)
[27] Mullen, L., Cochenour, B., Rabinovich, W., Mahon, R. and Muth, J., Backscatter suppression for underwater
modulating retroreflector links using polarization discrimination. Applied Optics, 48, 328-337. (2009)
[28] Kumar, P. V., Praneeth, S. S. K. and Narendar, R. B., Analysis of optical wireless communication for underwater wireless communication. International Journal Science and Engineering Research, 2(6), 328-337. (2011)
[29] Green, R. J., Leeson, M. S., Higgins, M. and Joshi, H. The blue-green study. Confidential report to QinetiQ.
(2009).
[30] Green, R. J. and Ramirez-Iniguez, R. Optical antenna design for indoor optical wireless communication systems.
International Journal Communication Systems, 18, 229-245. (2005)
[31] Brundage, H., Designing a wireless underwater optical communication system. Master thesis, Massachusetts
Institute of Technology. (2010)
[32] Karp, S. and Gagliardi, R. M., The design of a pulse position modulated optical communications system. IEEE
Transactions on Communication Technology, 17(6), 670-676. (1969)
7
[33] Xu, T., Jiang, B., Wei, D. and Shi, Y., Design of a wireless laser communication system based on PPM
technique. ICECE, 16-18 Sep 2011, pp. 4427-4430. (2011)
[34] Sui, M., Yu, Z. and Zhou, Z., The modified PPM modulation for underwater wireless optical communication.
International conference on communication software and networks, pp. 138-142. (2009)
[35] Cox, W. C., Simpson, J. A., Domizioli, C. P., Muth, J. F. and Hughes, B. L., An underwater optical communication system implementing Reed-Solomon channel coding. OCEANS, 15-18 Sep 2008, pp. 1-6. (2008)
[36] Simpson, J. A., Cox, W. C., Krier, J. R., Cochenour, B., Huges, B. L. and Muth, J. F., 5 mbps optical wireless
communication with error correction coding for underwater sensor nodes. OCEANS, 20-23 Sep 2010, pp. 1-4.
(2010)
[37] Joshi, H., Modulation for optical wireless communication. PhD thesis, University of Warwick. (2010)
[38] Shieh, W. and Djordjevic, I. OFDM for optical communications. Academic press, pp. 32-39. (2009)
[39] Han, Y. and Li, G., Coherent optical communication using polarization multiple-input-multiple-output. Optical
Society America, 13(19), 7527-7534. (2005)
[40] Snow, J. B., Flatley, J. P., Freeman, D. E., Landry, M. A., Lindstrom, C. E., Longacre, J. R. and Schwartz, J.
A., Underwater propagation of high data rate laser communication pulses. SPIE, 1750, 419-427. (1992)
[41] Bales, J. W. and Chryssostomidis, C, High bandwidth, low-power, short-range optical communications underwater. International Symposium on Unmanned Untethered Submersible Technology, 9, 406-415. (1995)
[42] Tivey, M., Fucile, P. and Sichel, E., A Low Power, Low Cost, Underwater Optical Communication System.
Ridge 2000 Events, April 2004, pp. 27-29. (2004)
[43] Schul, F., Zimmer, U.R. and Trumpf, J., Visible Spectrum Optical Communication and Distance Sensing for
Underwater Applications. Australasian Conference on Robotics and Automation, 6-8 Dec 2004, (2004)
[44] Hanson, F. and Radic, S., High bandwidth underwater optical communication. Optical Society of America, 47,
277-283. (2008)
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