c (m-1)
Underwater Optical Wireless Communication:
Challenges of the Environment
108
106
104
102
100
10-2
10-14
Introduction
visible band
10-10
10-6
10-2
𝜆 (m)
102
106
Figure 1: electromagnetic attenuation spectrum [1]
Optical wireless communications are a high-bandwidth alternative to conventional acoustic systems for communication in vast open oceans. Visible light (380 –
700 nm) is chosen for transmission as the electromagnetic attenuation spectrum of water exhibits a window of low attenuation around the visible region (figure 1).
Channel losses in these communication links are typically characterised by Beer’s Law, where received power decreases exponentially with respect to link
distance and a constant attenuation coefficient c (m-1); the latter is dictated by both environmental and design factors. However, such a model provides only an
outline of the true channel behaviour, and real underwater environments encompass many other challenges, investigated theoretically in this study.
Aim: to fully characterise the underwater channel in an open-ocean environment in order to determine ideal parameters of transmission such as wavelength,
power, field of view and modulation scheme and to be able to accurately predict the performance of any underwater optical wireless communication system.
Variability
Turbulence
Divergence
Marine Life
The ocean is made up of four optically unique
constituents; pure water, chlorophyll, fulvic
acids and humic acids. Each of the properties
varies by location (latitude and depth) and with
time (seasonally and during the day), leading to
changes in the attenuation coefficient.
The refractive index of sea water, n, varies
naturally throughout the ocean by up to 3%.
The exact value depends on several factors:
When refractive index changes are gradual,
such as with increasing depth, it is possible for
a communication link to be diverted away from
its expected path. For a vertical link of fixed
wavelength, there is a linear rise in refractive
index driven by increasing water pressure, such
that:
𝑛 ≈ 𝐶𝑧
Light is a fundamental part of life, even for
creatures living in deep oceans, and interactions
between marine life and a visible light
communication
system
are
potentially
destructive for both parties.
0.0
0.2
0.0
0.4
-50
-20
-100
-40
-150
4.0
z (m)
0
z (m)
0
2.0
-60
-200
-80
-250
-100
Figure 2: chlorophyll distribution Cchl with depth z, for
varying surface chlorophyll concentration [2]
Figure 3: the chaotic nature of turbulent flow [3]
Abrupt local changes in these factors, above
that of typical ocean values, is known as
turbulence. It can lead to temporal and spatial
dispersion in communication systems due to
the consecutive refractive index boundaries, in
addition to secondary attenuation effects from
bubbles. This type of dispersion has a direct
impact on the modulation scheme and bit rate
employed in the communication system.
The causes of turbulence include ocean
currents, animal/vehicular movement and
interfaces between fresh water and seawater.
The resultant beam displacement from the
receiver (figure 4) is estimated based on
models from multimode graded-index fibres,
where there is also a gradual change in
refractive index.
receiver
Bioluminescence is also a significant factor
underwater as it can lead to additional noise
across the receiver. Fish typically use this for
finding prey or food and for camouflage.
n
transmitter
expected path
z
diverged path
Figure 4: refractive index profile causing beam diversion
Initial Findings and Future Plans
Variability: the attenuation coefficient was quantified over varying wavelength and depth for
surface levels of chlorophyll between 0 - 4 mg/m3 (figure 5). There is a peak in attenuation where
the chlorophyll level is at its highest, significantly, this occurs at a greater depth for areas with low
surface chlorophyll concentration. The ideal wavelength of transmission for a general link in the
first 100 m was found to be 490 nm (blue-green) and 430 nm (blue-violet) below this depth. The
applications of this are low-noise, secure underwater communications and vertical links from the
ocean surface or above. When communicating from above water, the air/water interface becomes
important - the behaviour of light at the interface is to be modelled and validated experimentally.
There is also scope to look into the temporal variability of attenuation.
Turbulence: calculations deduced that turbulence caused by ocean current is negligible due to
the relative incompressibility of water (compared to that of air). However, turbulence is likely to be
significant around ships and estuaries, the effects of which shall be determined experimentally.
Divergence: theoretical beam bending equations were found; it is expected to have up to a 1 m
displacement for a 200 m length link. There is scope to extend this model to include attenuation
variability for the new path shape and within alignment-correction methods.
Marine life: it was found that fish are typically attracted to the optimum transmission wavelength
of a given location (blue-violet for deep ocean, up to green for near-surface). However, visible
intermittency can be introduced to ward away shoals of fish.
Low Cchl at surface:
c (m-1)
Cchl
Cchl (mg/m3)
(mg/m3)
where z is depth from the surface and C is a
positive constant. Salinity and temperature also
change with depth but their effect on the
refractive index is negligible.
Firstly, fish and microalgae are known to be
attracted to certain wavelengths and intensities
of light; high intensity blue lights are often
present on fishing boats. A shoal of fish
attracted to a transmitter would lead to a highly
unreliable, intermittent link, so communication
systems should be designed to avoid this.
1
0.5
0
0
z (m) -100
-200
High Cchl at surface:
700
600
500
400
𝜆 (nm)
1
c (m-1)
To determine the variability of attenuation, first
the four-parameter model must be simplified.
This is done using a one-parameter
approximation based around the concentration
of chlorophyll. This new model can be
combined with surface chlorophyll data and
empirical depth-chlorophyll profiles (figure 2) to
estimate the variable attenuation coefficient.
𝑛 = 𝑓(𝑃, 𝑆, 𝑇, 𝜆)
here P represents pressure, S is salinity, T is
ocean temperature and 𝜆 is the wavelength of
transmission.
0.5
0
0
z (m) -40
-80
700
600
500
400
𝜆 (nm)
Figure 5: attenuation
variation
Figures/Acknowledgements
References
ref:
[1]: Mobley, C. D., and C. D. Mobley. Light and water. San Diego, CA: Academic press,1:1994.
2: phytoplankton
daoijdwan djkdkwejdkew
dewkd
ewkocean:
dekw an
d
[2]:Morel, A. et al. “Vertical distribution of
communities
in open
another.
assessment based on surface chlorophyll,” J. of Geophys. Research, vol. 111,
2006.
[3] Smith, R., Symscale computation fluid mechanics, 2007
Laura
Johnson
Laura
Johnson
Laura.J.Johnson@warwick.ac.uk
Laura.J.Johnson@warwick.ac.uk
Supervisors: Prof.
R. Green and Dr. M. Leeson
Supervisors:
Prof. R.bodies:
GreenEPSRC,
and Dr.Thales
M. Leeson
Funding
UK
Funding bodies: EPSRC, Thales UK