UNCLASSIFIED

Free Space Optical Communications using

Software Defined Radio

Pranav Mistry

1 Feb, 2018

C4ISR Systems Group

Defence Technology Agency

1 Feb 2018

UNCLASSIFIED

Where did my bandwidth go ?

• Proliferation of emitters means increasing congestion

• Potential for intercept and jamming

Image source : https://www.rsm.govt.nz/

2 Feb 2018

UNCLASSIFIED

What is free space optical communication ?

• Optical Fibres

In use since 1970s

Long ranges (> 50 km)

Requires underground cable

• Free space optical systems

Transmit light through air

Low probability of detection

No licensing issues

Image source : https://www.scienceabc.com/innovation/

3 Feb 2018

UNCLASSIFIED

History

200 BC Semaphore 1880 Photophone 1910 Heliograph

Image source : en.wikipedia.org/wiki/ 4 Feb 2018

UNCLASSIFIED

When RF spectrum is unavailable Navy needs…

• Communications during underway replenishment

• Interoperability – communicate with allies

• Bandwidth of kilobit per second

• Out to horizon range (15Km)

• Eye safe

Image source : https://en.wikipedia.org/wiki/Signal_lamp#

5 Feb 2018

UNCLASSIFIED

Current challenges and solutions

Challenge Solution Features Shortfall

“Last Mile” problem

• High data rate • Low cost

Range <5 km

Underway Replenishment

• Led based • Wide angle

Range <1 km

Military jamming (Land)

• Long range (>50 km) • Uses lasers

Not eye safe* *At close range

Source: www.fsona.com

Source: tplogic.com

Source://www.doncio.navy.mil/chips/

6 Feb 2018

UNCLASSIFIED

DTA optical communications project

• Problem

Provide low bandwidth communications

Use low cost COTS hardware

Long ranges (> 15 km)

Must be eye safe

• Can Software defined radio platforms be used

with optics ?

Universal software defined radio peripheral (USRP)

Designed to drive radio frequency (RF) antennas

Image source : amazon.com

7 Feb 2018

Image source : ettus.com

UNCLASSIFIED

DTA optical comms technology demonstrator

• Hardware

Software defined radio & PC

630nm / 3mW Laser diode

Amplified photo diode detector

Bias Tee Circuit ( Shifts AC output of USRP to DC for laser )

• Implemented in the lab with up to 8 m optical path length

8 Feb 2018

UNCLASSIFIED

Hardware - architecture

User-defined

Gnu-Radio

Code

Optical

detector

Sender

USRP (mother board) PC

USB FPGA

Receiver

FPGA

Bias Tee

Laser diode

Digital to

Aanalog

Converter

User-defined

Gnu-Radio

Code

Aanalog

to Digital

Converter

USRP (mother board)

9 Feb 2018

UNCLASSIFIED

Software - Gnu Radio flow graph example

10 Feb 2018

UNCLASSIFIED

Progression of our Gnu-radio/USRP work

• Initial testing (Jan 2017)

Implemented basic binary phase shift keying tests

15 kHz bandwidth (text messages)

• Video streaming (Oct 2017)

FFMpeg streaming using UDP packets (Jan 2018)

Motion Jpeg vs H.264 (1Mbps)

Gaussian minimum shift keying blocks with packet encoders

11 Feb 2018

UNCLASSIFIED

Live video streaming at 1Mbps

12 Feb 2018

UNCLASSIFIED

Future work

• Atmospheric effects

Absorption / Scattering - wavelength diversity

Turbulence - Multiple transmitters and receivers

o 20-30 dB losses, time scale milliseconds

o Beam wander

o Beam divergence

• Alternative waveforms

Pulse width modulation

Pulse position modulation

Novel waveforms

Image source : dst.defence.gov.au

13 Feb 2018

Image source : en.wikipedia.org/wiki/

UNCLASSIFIED

Future work

Challenge Solution Features Shortfall

Channel Emulation

• Deflect partial beam • Use with beam shaping

optics

Response time

Platform stability

• Wide angle of acceptance

• No stabilisation required

Asymmetric Small diameter Range <4 km

Platform stability (Fast Morse code)

• Aldis lamp replacement

• Wide angle • Long range • Fast cameras required

High probability of detection

Source: thorlabs.com

Source: bostonmicromachines.com

Source: schoolofsurvival.com

14 Feb 2018

UNCLASSIFIED

Questions

15 Feb 2018

Image source : https://en.wikipedia.org/wiki/Optical_communication