The Impact of Link Orientation on
Underwater Optical Wireless Communication
Systems
Laura Johnson, Roger Green and Mark Leeson
project sponsors
IC1101, special interest group UW-OWC
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Underwater optical communications
Atlantic Ocean
Thames, UK
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Underwater optical communications
Atlantic Ocean
 Laser likely
 Longer range
 Tracking issues
Thames, UK
 LED likely
 Shorter range
 Multipath issues
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Link orientation: why it matters
dissolved and
particulate
substances
attenuation coefficient
temperature
salinity
pressure
refractive index
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Previous studies
All studies acknowledge that the attenuation varies by
location…
… but none have considered it varying in a single
optical link!
Some consider local changes in refractive index…
... but not global changes, like with depth.
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Aims of this study
1. Estimate the attenuation change in communication
links at various angles,
this has implications on wavelength choice and
link length.
2. Estimate the refractive change on links at different
angles and determine the beam’s deviation from
the non-refracted path
this has implications on pointing accuracy and
transmitter and receiver size FOVs.
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Attenuation variation
attenuation coefficient (m-1)
Find attenuation with depth
using
- bio-optical models of
phytoplankton with depth
- relation between constituent
concentrations
0
0.1
0.2
0
-50
-100
depth
Limited scope to
- Case 1 water, 0-4 mg/m3
surface chlorophyll
L. J. Johnson et al., App. Opt. 52(33), 2013
-150
-200
-250
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Attenuation variation
Transmitted 200m links from a fixed stating position,
record average attenuation for each angle.
attenuation coefficient (m-1)
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Attenuation variation
Links with high attenuation are found where the
geometry has caused them to stay near the peak.
What we found (low - high turbidity):
Surface to peak ratio
Min to peak ratio
Min to average* ratio
1:1.9 - 1:1.3
1:3.7 - 1:7.4
1:2.4 - 1:5.8
*over first 250m
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Attenuation variation
𝐼𝑟 = 𝐼0 𝑒 −𝑐𝑟
Are the findings significant?
Yes! Big implications on link distance.
For example, for low turbidity…
100m, minimum attenuation
52m, surface attenuation
42m, average attenuation
27m, peak attenuation
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Refractive variation
Found refractive gradients
using
- CTD data available for
research
- An algorithm which
calculates refractive
indicies, based on the
values of temperature,
wavelength, salinity,
pressure
refractive index
1.334
0
1.339
1.344
500
1000
depth
1500
2000
2500
3000
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Refractive variation
Used ray tracing to plot 200m link paths, which had
different starting angles and depths, and measured
size of the deviation created by refraction
magnitude of deviation (m)
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Refractive variation
Are the findings significant?
depends on beam angle, transmitter FOV, the
amount of scattering, where in the link scattering
occurred…
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Future work
- Numerical simulations with variable refractive index
and attenuation coefficients
- Field measurements of link performance with depth
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Summary
- Introduced underwater optical wireless
communications
- Emphasized the importance of link orientation in
communication link power and directional properties
- Seen how attenuation depended on the transmitted
angle relative to the peak location
- Looked at how refractive index gradients causes the
link path to deviate from a straight line
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Link orientation: why it matters
T
T
T
R
T
R
R
R
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Link orientation: why it matters
T
T
T
non-refracted
path
refracted
path
T
attenuation coefficient
R
refractive index
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