Dept. of Electronics and communicationSeminar Presentation

SMART TRANSMITTERS AND RECEIVERS

FOR UNDERWATER COMMUNICATION

February 6, 2013

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Contents

Introduction

Advantages

Properties

Smart Receivers

Smart Transmitters

Study

Summary

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Introduction

Underwater Freespace optical communication-Promisingalternative for Short range links.

Considered to be point to point.

New optical front-end proposed-the concept of smarttransmitters and receivers .

Smart Receivers-capable of detecting angle of arrival of signals.

Smart tranmitters

Electronically steers output beam towards particular direction.Estimates water quality from back scattered light

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Advantages

Non-mechanical pointing and tracking on a moving underwatervehicle.

Providing sensory information to underwater vehicles.

Duplex multi-user system

Spatial diversity allows for simultaneous reception from two nonco-located transmitters.

Monitors optical backscattering while transmitter is active.

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Figure : Multi-user reception system scenario with three nodes. A and Care transmitting. B is receiving

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Properties

Beam attenuation coefficient:ratio of energy absorbed or scatteredfrom an incident power per unit distance.

Single- Scattering albedo: ratio of scattering coefficient to beamattenuation coefficient.

Study

A 3.66m long,1.22m wide,1.22 m tall indoor water tank constructed .

Maalox -controls attenuation coefficient of water.

Nigrosin dyein-controls albedo.

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Figure : 1000 gallon water tank built and used for underwater free-spaceoptical communication experiments done in lab at NCSU.

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Figure : Relationship between attenuation coefficient and SNR forexperiments in laboratory test tank.

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Smart Receivers

Goal-to develop a quasi omnidirectional system that reduces pointingand tracking requirements.

Characteristics

increased FOV.

angle of arrival estimation

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Design

3-D spherical array of lenses all focusing to a 2 D planar array ofphotodiodes.

A prototype constructed using seven lenses and seven photodiodes.

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Lens at the receiver

research in the domain of indoor optical wireless in use of sphericalphotodiode arrays for increasing FOV.

Existing optical front-end arrays use:

Photodiode arrays with no lenses.Single lens with multiple photodiodes.Multiple lenses focusing on separate photodiodes.

Angle Of Arrival Estimation

Intensity of light received can be used to estimate the angle of arrivalof light.

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Photodiode output combining

Connect the array of photodiodes in parallel.

An ideal combining technique

maintain bandwidthminimize noisemaximize SNR

Linear diversity combinig techniques

Equal Gain Combining(EGC)

Maximum Selection Combining(SEL)

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Smart Transmitters

Characteristics

increased directionality.

electronic switched beamsteering.

Design

Consists of a truncated hexagonal pyramid with seven LEDs.

Each LED is coupled with its own lens that converges the wide FOVof the LED to a narrower beam in a particular direction.

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Study

Characterization of the Receiver Lens-photodiode Array

Experiments were conducted for the receiver pointed in all directionsand intensities were observed at all photodiode outputs stored as afunction of the spherical co-ordinates.

A pan and rotate system. constructed using digital servos.

Seven amplified photodiode outputs digitized using 8 channeldigitizer.

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Angle of Arrival Estimation

Involves estimating the direction of arrival of the incident light basedon relative output powers observed at each photodiode.

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Backscatter estimation

Experiments were conducted with amplied detectors as well as theprototype transmitter and receiver to collect and observe a linearrelationship between the known attenuation coeffcient of the waterand the amount of backscattered light collected.

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Figure : Results of the backscatter estimation experiment

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Summary

Results show that design also capable of acting as smart system.

Backscatter estimation experiment demonstrates linear relationshipbetween return beam intensity and channel attenuation coefficient.

smart receivers

increased field of view

ability to estimate angle of arrival.

Smart transmitters allows electronic switched beamsteering.

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