Underwater Optical Wireless Communications:
Mid-Ranged LOS Channel Modelling
Laura J. Johnson, Roger J. Green and Mark S. Leeson

I. INTRODUCTION
II. CHANNEL FUNDAMENTALS
Seawater exhibits a window of low attenuation for blue-green
visible wavelengths. This, combined with recent advances in
visible-light technology, has caused a surge of interest in
underwater optical wireless communications (UWOC).
Recent experimental research into UWOC has shown this
technology to be capable of high bandwidths, up to 1Gb/s [1],
and at lengths of up to 200m [2]. It is considered a
complementary technology to conventional acoustic links.
Thus far, studies in UWOC have been confined to ideal
or controlled laboratory settings which overestimate the
performance of a real system [3]. By advancing the
understanding of the ocean as a transmission medium, this
research aims to create a realistic underwater channel model.
Seawater is described optically by two properties: the
attenuation coefficient, c, used to determine the optical
power loss over a link, and the refractive index, which
determines directional properties. Attenuation occurs in
UWOC links because photons are absorbed by seawater or
scattered away from the receiver. Beer’s law is used to
determine the overall power loss I, due to bulk absorption a
(m-1) and scattering b (m-1) [5]:
A. Link types
The scope of UWOC has been limited to mid-ranged line-ofsight (LOS) links for this study. Mid-ranged links arise when
laser diodes are used as the transmitter source as they are high
power with a low field-of-view, leading to direct point-topoint links (fig. 1a), as opposed to diffuse LED links (fig. 1b).
Line-of-sight (LOS) refers to the ability to see the transmitter
from the receiver; non-LOS systems can occur either when
using a reflection from the ocean surface or a link that has
been diffused past an object [4].
B. Applications
Applications for UWOC include environmental monitoring
and mapping, oil/gas monitoring and security. Receiver
location in these applications can be in or above the water:
e.g. AUV (autonomous underwater vehicle) to AUV, AUV to
buoy or submarine to satellite.
a)
b)
(
)
( )
where r is the length of the optical link and I0 is the power at
the source and
. The imaginary refractive index, m,
is comprised of the real refractive index n and a loss term,
which is the normalised absorption coefficient [5]:
( )
where is the transmission wavelength.
A. Optically significant constituents
The optical properties of seawater vary with its composition.
Towards the shoreline and ocean surface, the availability of
light and nutrients supports the growth of phytoplankton –
microscopic organisms comprised of chlorophyll –
increasing the amount of dissolved and particulate matter.
Absorption subsequently ranges between 0.11-0.27 m-1 and
scattering between 0.27-1.82 m-1 [6].
Temperature, salinity, pressure and wavelength are
known to impact on the real part of the index of refraction.
B. Existing models
Models exist to determine how light will propagate between
a source and receiver. With the exception of Beer’s law in
(1), all existing channel models are based on solving the
radiative transfer equation (full description available in [7]).
Probabilistic Monte Carlo solutions are found to be the most
versatile with comparatively low computational times, at the
expense of statistical errors [7]. However, this numerical
solution, as other models, is limited by the accuracy of the
underlying channel description.
III. REAL CHANNEL MODELLING
Fig. 1. Geometry of a a) line-of-sight point-to-point link and b) line-of-sight
diffuse link.
A. Attenuation with depth
Depth variations in the attenuation coefficient have been
calculated using a one-parameter model based on the
chlorophyll concentration (which is closely linked to the
phytoplankton population) and experimentally-determined
C. Turbulence
Turbulence occurs when a medium experiences rapid, local
changes in refractive index. In optical wireless
communications through air, turbulence is significant
because it creates large refractive index differentials based on
changes in pressure and temperature. If this idea is extended
underwater, considering pressure and temperature difference
caused by ocean currents, the low compressibility and higher
specific heat capacity of seawater means the refractive index
should remain fairly invariant. This is supported by
calculations using [10]. Combining this result with
turbulence compensation techniques (e.g. aperture
averaging), means that UWOC should be resilient to
turbulence created by regular ocean movement.
D. Marine Life
Marine life is known to be attracted to light; fish that live in
open ocean prefer blue-green wavelengths whilst those in
freshwater prefer yellow-green [11]. These wavelengths
represent the optimal wavelengths of transmission in the
respective locations. There is a possibility to ward them away
with erratic duration of illumination (flashing) or significant
brightness, but both of these reduce link security. In addition,
UWOC systems which are stationary long-term have issues
with algae growth on and around the transmitter. Both these
issues need to be addressed in a real communication system.
absorption coefficient,
a (m-1)
0
0.1
b)
0.2
-50
-100
refractive index, n
1.334
0
0
1.336
1.338
50
depth (m)
B. Refractive change with depth
In a graded-index optical fibre, and satellite laser
communications, a source is transmitted through a gradedrefractive index at an angle. This causes the light beam to go
through multiple refractive interfaces, where the beam is
refracted. The accumulated effect of this is a beam which
appears to bend, missing the direct receiver location.
The ocean also has refractive gradients, found in [9] from
ocean measurements, and shown in fig. 2b. This profile
shows that the refractive index increases non-linearly with
depth. This is mainly due to pressure changes, although
salinity and temperature also contribute near the surface.
Using a link of 200m, the study in [9] calculated the
maximum displacement from expected receiver location was
up to 0.3m, depending on wavelength, and angle sent at;
near-horizontal angles and larger visible wavelengths (up to
700nm) caused greater displacements. However, careful
selection of the laser FOV and pointing angle correction can
overcome this, with a power cost.
a)
depth (m)
chlorophyll-depth profiles [8]. Attenuation has a Gaussian
relation with depth, as in fig. 2b, which depends on the
surface chlorophyll concentration; a property which is found
from satellite images. The maximum attenuation occurs
higher up for locations with higher surface chlorophyll level,
typically 20-200m [8].
Understanding the attenuation-depth profile is useful for
vertical links or horizontal links with variable depth. When
the amount of chlorophyll becomes negligible, the ideal
transmission wavelength is 430nm, increasing to 530nm at
the maxima.
100
-150
150
-200
200
-250
250
Fig. 2. Optical properties changing over depth for a 500nm link where
a) absorption and b) refractive index.
IV. CONCLUSION AND FUTURE WORK
The four UWOC channel aspects which have been described
in this research – attenuation with depth, refractive index
with depth, turbulence and marine life – assist the basic
attenuation and refractive models to give a better
understanding of real world underwater channels. The next
step is to include these aspects into Monte Carlo simulations
and, where appropriate, do laboratory and real-channel
experiments to support theoretical models.
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