UNCLASSIFIED Free Space Optical Communications using Software Defined Radio Pranav Mistry 1 Feb, 2018 C4ISR Systems Group Defence Technology Agency 1 Feb 2018
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
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
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