Underwater Optical Wireless
Communications:
Mid-Ranged Line-of-Sight Channel Modelling
2nd School of Engineering Postgraduate Symposium
Laura Johnson, Roger Green, Mark Leeson
Overview
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Introduction
Underwater link configurations
Problem identification
Channel variation
Variability characterisation
Significant results
Summary
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Introduction
• Acoustic: long range (km),
low bandwidth (kHz).
low efficiency (100 bits/Joule)
• Optical:
short to mid range (<200m),
high bandwidth (GHz),
energy efficient (30k bits/Joule)
Applications: environmental monitoring, defence
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Underwater Link Configurations
LOS
point-to-point
LOS
retroreflector
LOS
diffuse
non-LOS
diffuse
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Problem Identification
Performance figure PF = Bandwidth*Length*Attenuation
Hanson and Radic (2008)
1 Gb/s for 2 meters in moderately clear seawater
PF = 550 x106
Pontbriant et al (2008)
5 Mb/s for 200 meters in clear sea water
PF = 40 x106
Cochenour et al (2007)
5 Mb/s for 3.6 meters in turbid sea water
PF = 4.14 x106
Problem: PFs in real ocean tests are much lower
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Channel Variation
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2
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Image: Google Earth (accessed 03/03/13)
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Channel Variation
𝐼 = 𝐼0 𝑒 −
𝑎+𝑏 𝑟
Attenuation
- wavelength
- in-organic particles
-- chlorophyll
dissolved organic matter
- particulate organic matter
Scattering
- density changes
Link geometry
- orientation
- water type
- distance to shore
- length
- depth
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Channel Variation
𝐼 = 𝐼0 𝑒 −
Attenuation
𝑎+𝑏 𝑟
Link geometry
- orientation
Scattering
- water type
- density changes - distance to shore
- salinity
- length
- temperature
- depth
- -refractive
Index
pressure
- turbulence
- wavelength
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Variability Characterisation
Attenuation changes with
depth
- modelled theoretically
- derived from chlorophylldepth profiles
- dependant on surface
chlorophyll level
- Gaussian with maximum
at 20-200m
attenuation (m-1)
0
0.1
0.2
0
-50
depth
-100
(m)
-150
-200
-250
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Variability Characterisation
refractive index
Refractive changes with
depth
- modelled using
measured data
- salinity, temperature
and pressure changes
> 300m
- only pressure changes
< 300m
1.335
0
1.34
1.345
500
depth
(m)
1000
1500
2000
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Variability Characterisation
Turbulence
- refractive index changes derived from first principles
- typical ocean turbulence strength considered
- resultant scattering negligible
Intermittency
- calculated likelihood of disruption from marine
different locations
- investigated ways to reduce it
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Significant Results
Ideal transmission wavelengths
4 mg/m3
0 mg/m3
surface
chlorophyll
0m
530 nm
250 m
490 nm
430 nm
500 m
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Significant Results
Estimated receiver displacements
expected
location
true
location
displacement
- found through refractive gradient and ray tracing
- varies by angle of transmission
- up to 0.3m for 200m link
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Summary
• What: investigated realistic channels aspects for
LOS underwater optical links.
• Why: improve performance predictions for real
channels.
• How: modelling of attenuation and refractive
changes with depth, turbulence and intermittency.
• Significant results: ideal transmission wavelengths,
estimated receiver displacement.
• Future: Monte Carlo simulations, real channel
experiments
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Any Questions?
E-mail: laura.j.johnson@warwick.ac.uk
laura.j.johnson@warwick.ac.uk
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