SMART TRANSMITTER AND RECIEVER FOR UNDERWATER FREE SPACE OPTICAL COMMUNICATION

INTRODUCTION:

• Underwater vehicles, sensors, and observatories require a communication interface.• Underwater Free space optical communication-Promising

alternative for Short range links.• Point to point.• A new optical front-end for underwater free-space optical

communication- smart transmitter & receiver.

Benefits of smart optical systems

• Non-mechanical pointing and tracking on a moving underwater vehicle.• Maintaining link with a stationary node as an underwater vehicle

does a drive-by.• Providing sensory information to underwater vehicles• Duplex multi-user system.• Optical backscatter estimation to assess water quality.• Electronic Switched Pointing & Tracking.

Properties of under water channel:• Beam attenuation coefficient(c(λ)): ratio of energy absorbed or

scattered from an incident power per unit distance.• It denotes total energy lost• Single-scattering albedo(ωo): ratio of scatteringcoefficient to

beam attenuation coefficient .• It denotes the probability that a photon will bescattered rather

than absorbed.• Volume scattering function: fraction of scattered power to incident power as a function of direction ψscattered into

a solid angle ΔΩ.

• A  3.66m long,1.22m wide,1.22 m tall indoor water tankconstructed .• Maalox-commercial antacid-scattering agent –controls attenuation coefficient of water.• Nigrosin dye - control the albedo.

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

Smart Receiver:• Goal quasi- Omni directional system to reduce pointing and tracking requirement characteristics• Increased field of view.•  Angle of arrival estimation.

Design:

• Consists of a 3-D spherical array of lenses all focusing to a 2-D planar array of photodiodes.• A prototype has been constructed using seven lenses and

seven photodiode.• Lens at the receiver :

• Existing terrestrial optical front-end arrays.•  Photodiode arrays with no lenses.• A single lens with multiple photodiodes.• Multiple lenses focusing on separate photodiodes.

• Proposed design– array of lenses & photodiodes with multiple combination of optical path.

• Quasi- Omni directionality : Photodiode array under middle layer increases the FOV from 5-40degree.Addition of outer lenses at 130 with respect to middle layer increase the FOV to 120degree.

• Angle of Arrival Estimation: The intensity of light received by each photodiode can be used to roughly estimate the angle of arrival of light.

•Photodiode output combining:• Connect the array of photodiodes in parallel.• An ideal combining technique:

• maintain bandwidth• minimize noise• maximize SNR

• Linear diversity combining techniques:• Equal Gain Combining(EGC)• Maximum Selection Combining(SEL)

Smart Transmitter:• The smart transmitter has the following 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 FOV of the LED to a narrower beam in a particular direction.

• Each LED is uniquely addressed and driven, which allows the modulator to select an output direction.

• This forms the mechanism for a simple switched beam steering at the transmitter CDMA Coding .

•The on-off-keying modulation methods in optical communication are uni polar as opposed to the bipolar modulation required by Gold codes

• Instead optical CDMA typically uses prime codes.

• A large code sequence will decrease overall BW but allow for more users with a corresponding increase in SNR.

EXPERIMENTAL RESULTS:• Prototype lens-photodiode arrays and LED-arrays were constructed for the

receiver and transmitter front -ends to collect data.• Receiver - Designed to pre amplify and digitize the receiver outputs.• Transmitter - Designed to receive up to seven different streams of data and

drive up to seven different LEDs on the LED-array.• Under water channel used was 3.66m long laboratory water tank.• Channel condition are adjusted by controlled addition of liquid Maalox.• The resulting attenuation coefficient was measured.• Experiments are conducted for characterizing the receivers and transmitters

& to demonstrate their capabilities.

Characterization of the Receiver Lens-photodiode Array:• Experiments were conducted for the receiver pointed in all

directions and intensities were observed at all photodiode outputs stored as a function of the spherical co-ordinates.• A pan and rotate system constructed using digital servos.• Seven amplified photodiode outputs digitized using a

simultaneous 8 channel digitizer.

Angel of arrival estimation:• Estimating the direction of arrival of the incident light based on

the relative output powers at each photodiode.• Outputs of receiver at each instant are sorted and compared with

receiver output patterns.• Angle of best match is taken as the estimated angle.

Backscatter Estimation:Transmitter• A co-located receiver to estimate the attenuation coefficient.• Can estimate the quality of water.• adaptively change its transmit power, data rate ,code rate

or other parameters.• challenge is that the return beam from backscatter depends on

the attenuation coefficient of the channel.• Remedy-sending a higher power training sequence it increasing

the receiver gain.

Figure : Results of the backscatter estimation experiment

CONCLUSION:• Results show that design is capable of acting as a smart system.• Backscatter estimation experiment demonstrates linear relationship

between return beam intensity and channel attenuation coefficient.• It helps to estimate water quality.• Smart Receivers• increased field of view.• estimate angle of arrival.

• Transmitters :• quasi Omni directional.• allow electronic switched beam steering.