Smart transmitters & receivers for underwater freespace optical communication

1. SMART TRANSMITTERS & RECEIVERS
FOR UNDERWATER FREE-SPACE
OPTICAL COMMUNICATION

2. CONTENTS
Introduction
Underwater optical channel
Smart Receivers
Smart Transmitters
Experimental Results
Benefits
Disadvantages
Conclusion
References

3. INTRODUCTION
Underwater communication is of great interest for
military , industry and scientific communities.
Fiber optic and copper cabling can be used for
stationary devices , a wireless link is usually
desirable.
Radio frequencies are heavily attenuated in sea
water.
Acoustic communication suffers from low data
rates.
For short range links free space optical
communication is promising alternative.

4. Underwater free space optical communication
takes advantage of “blue green optical window”
of relatively low attenuation of blue green
wavelength of electromagnetic spectrum
underwater.
Laser based systems can be used for extended
ranges and high data rates.
Led based systems have been demonstrated for
low cost , low power and compact systems.

5. The smart receivers have wide FOV and are
capable of detecting angle of arrival of signal in
order to adjust FOV towards desired signal.
Smart transmitters are capable of using this
information to electronically steer its o/p beam
towards a particular direction.
Also capable of estimating water quality from
its backscattered light collected by its co-located
receiver.

6. UNDERWATER OPTICAL CHANNEL
Underwater free space optical channel is very
different from atmospheric channel.
From an optical comm. perspective the 3
important properties are : beam attenuation
coefficient , volume scattering function and
albedo.
Light interacts with water and materials
dissolved in it in 2 different ways: Absorption
& scattering.

7. Beam attenuation coefficient :
Amount of energy absorbed or scattered from an
incident power per unit distance.
Denotes the total energy lost.
Sum of absorption and scattering coefficient.
Albedo:
Ratio of scattering coefficient to beam attenuation
coefficient .
Denotes probability that a photon will be
scattered rather than absorbed.

8. Volume scattering function:
Defined as the fraction of scattered power to
incident power
Water type Attenuation coeff.(1/m)
Clear ocean 0.152
Coastal ocean 0.399
Harbor water 2.195

9. SMART TRANSMITTERS
Should have following characteristics:
1] Increased directionality
2] Electronic switched beam steering
Design:
 Unlike receiver each LED is coupled with its own
lens that converges wide FOV of LED to narrower
beam in a particular direction.

10. Mechanism of beam steering:
 Each LED is uniquely addressed and driven.
 This allows the modulator to select an output
direction.
 This forms the mechanism of simple switched
beam steering at the transmitter.

11. SMART RECEIVERS
Goal is to develop a system to reduce pointing and
tracking requirements associated with free space
optical systems.
Allows to estimate angle of arrival.
Smart receivers have following characteristics :
 Increased FOV
 Angle of arrival estimation

12. Design considerations
1] Lens at the receiver:
Existing optical receivers uses only photodiodes.
Proposed design uses an array of lenses as well as
an array of photodiodes.
2] Angle of arrival estimation:
The intensity of light received by each
photodiode can be used to roughly estimate angle
of arrival of light.

13. 3] Photodiode o/p combining:
 Important consideration for the performance of
the system.
 Simplest interface to the array of photodiodes is
to connect in parallel.
 But results in low bandwidth.
2 combining techniques :
 Equal gain combining (EGC)
 Selection combining(SEL)

14. Figure1 Figure 2
Figure1:Sketch showing light entering through top lens
Figure2:Sketch showing light entering through side lens and
falling on photodiode.

15. EXPERIMENTAL RESULTS
0 1 2 3 4 5 6 7 8 9
Attenuation coefficient , c (m-1)
Power (μW)
0
2
4
This graph shows a plot of attenuation co-eff v/s power.It shows a
linear relationship between the power of backscattered light and
attenuation co-efficient.

16. BENEFITS
Non mechanical pointing & tracking on a moving
underwater vehicle
Maintaining link with stationary node as an
underwater vehicle does a drive by
Providing sensory information to underwater
vehicles
Duplex multiuser system
Optical backscatter estimation to assess water
quality

17. A B
C
Figure1: Multi-user reception
1.
B
C
Figure2: Electronic switched
pointing and tracking
2.
3.
B
Figure3:Optical backscatter estimation

18. DISADVANTAGES
 Point to point communication
 Point to multipoint communication is not
possible because of the use of led or laser.
 System may receive from two different
system but it is not possible to transmit to
two different system.

19. CONCLUSION
Results show that design is capable of
acting as a smart system.An increased
field of view and the capability to evaluate
the angle of arrival by the smart receiver
along with the estimation of water quality
by measuring the optical backscatter from
transmitted light by the transmitter is
depicted.

20. REFERENCES
[1]Simpson, Hughes, John F “Smart transmitters
and receivers for underwater free-space optical
communication”.
[2]M.A. Chancey, “Short range underwater optical
communication links”, Masters Thesis, North
Carolina State University, 2005, p. 277, Jan.2008.