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Free space optical comm 1
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Free Space OpticalCommunications: An Overviewof Applications and
Technologies
John Kaufmann
Boston IEEE Communications Society Meeting
December 1, 2011
Portions of this work were sponsored by the Department of Defense, RRCO DDR&E, and by the National Aeronautics and Space Administration, under
Air Force Contract #FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily
endorsed by the United States Government.
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Free space optical comm 2
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Colleagues who contributed to this presentation:
Lincoln Laboratory
Steve Bernstein
Don Boroson
Matt Grein
Farhad Hakimi
Steven Michael
Bryan Robinson
Fred Walther
MIT
Prof. Vincent Chan
Acknowledgements
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Free space optical comm 3
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Free Space Optical CommunicationsApplications
Geostationary
(GEO) satellite
Aircraft
Optical fiber
infrastructure
Low-earth orbit
(LEO) satellite
Ground
vehicle
Terrestrial
FSO
Deep space
Last mile
Aircraft
Undersea
fiber
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Free space optical comm 4
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Why free space lasercom?
Large, unregulated spectrum High data rates over long distances (satellite, deep space) Much reduced SWaP (size, weight, and power) terminals
compared to RF Security (freedom from interference, immunity to
interception)
Why notfree space lasercom?
Requires clear line-of-sight (no clouds, physicalobstructions, etc.)
Few turn-key commercial systems available Economic considerations
Key topics for today Leveraging of COTS telecom technology for free space
Highly power-efficient receivers Pointing very narrow optical beams Challenges of atmospheric channel
Demonstrations of atmospheric and space lasercom
Motivation for Free Space OpticalCommunications (Lasercom)
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Free space optical comm 5
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Benefits of Free Space Optical Communications
Fiber telecom band has between 100 and 1000 timesmore bandwidth than all of RF
Optical spectrum is unregulated
Frequency
logarithmic scale
Wavelength
Bandwidthlinear scale
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Free space optical comm 6
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Telecom Industry Has Driven Evolution ofOptical Communications Technology for Fiber
1970s free space bulk-opticCO2 laser transceiver in
laboratory
10G telecom fiber line cardfor 1.5 mm, circa 2000
40G MSA opticaltransponder module,
circa 2010
Can we leverage telecom fiber-optic technology for free space?
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Free space optical comm 7
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Impact of EDFA Technology on Free-SpaceLasercom
EDFA = erbium-doped fiber amplifier
Prior to ~1990, most sensitive freespace lasercom systems based onheterodyne/homodyne (coherent)receiver technology
Laser frequency stability and low phasenoise were critical issues
Receiver and transmitters implemented
bulk optics Based on 0.8 mm GaAs diode, 1.06 mm
Nd:YAG solid state, or 10 mm CO2 gaslasers
Advent of commercial EDFA technologyin 1990s spurred a major shift in free
space lasercom technology to 1.5 mmtelecom fiberoptic technology
High power EDFA as transmit amplifiers
Low power EDFA as low-noise receiverpreamplifiers
Allowed size/weight reduction andmodularization of lasercom hardware
COTS highpower EDFA
COTS lowpower EDFA
Optical
modem
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Free space optical comm 8
J Kaufmann - Boston IEEE Comm
1 Dec 2011
What are the Differences Between Fiber-Optic and Free Space Channels?
Fiber Free Space
Path Loss ~e
-aL
~1/L
2
(diffraction)Chromatic dispersion yes no
Polarization mode dispersion yes no
Nonlinearities yes no
Modal dispersion yes (in MMF) no
Birefringence yes no
Absorption yes yes (in earth atmosphere)Scattering small yes (in earth atmosphere)
Clouds no yes (in earth atmosphere)
Turbulence no yes (in earth atmosphere)
Spectral efficiency important in high speed fiber telecom(ITU frequency grid channelization)
Fiber launch powers typically limited to few mW because ofnonlinearities, use inline amplification or regeneration tocompensate fiber span losses
Free space lasercom can use transmitter power, antennagain, bandwidth expansion (modulation + FEC) and sensitivereceivers to overcome channel losses
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Free space optical comm 9
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Optical Free Space Link Equation
Earth
Receiver
SpaceTransmitter
Wavelength
Range
AreaAperturer/ReceiverTransmitte
Lossr/ReceiverTransmitte
Powerd/ReceivedTransmitte
R
A
L
P
T/R
T/R
T/R
sec]/[photonsP
1sec]/[bitsRateData R
EIRP
RangelossRXantennagain
h= RX sensitivity (photons/bit) Looks just like RF link equation!
TXantennagain
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Free space optical comm 10
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1 Dec 2011
MOPA Transmitter Architecture for 1.5 mm
DFB master laser E-O modulator
Pump lasers
High speeddata driver
High power EDFA
MOPA = Master oscillator power amplifier
Fiber tofree space
interface
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Free space optical comm 11
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1 Dec 2011
Coupling Light from Fiber to Free Space
Fibertransmitter
Fiber mode fielddiameter ~10 mm
Collimatinglens
Far field is 2-D spatial Fouriertransform of transmit signal field in
telescope aperture
Antenna gain G ~
Far-field beamwidth BWD
~
2
D
Telescope is the transmitantenna Expands ~10 mm beam from fiberto much larger diameter D toproduce antenna gain
High gain narrow BW Telescope pointing accuracy iscritical!
Far-fieldreceiver
DBW
~
D
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Free space optical comm 12
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Optical vs. RF Antenna Gain Patterns
Small Terminal Examples:
Terminal
Aperture
(D)
Wavelength
()Beamwidth ~ (/D)
(degrees)
Gain ~ (D/)2(dB)
Optical 10 cm(4 inches)
1.5 mm 0.0009o 106 dBRF at 30 GHz 1.0 m
(3.3 feet)1 cm 0.7o 47 dB
Optical
Telescope
RF Antenna
D = 10 cm
D = 1 m
~60 dB gaindifference
between Opticaland RF
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Free space optical comm 13
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1 Dec 2011
A Preamplified Receiver Architecture for 1.5 mmCombiners, splitters
Isolators, filters,
circulators
Pump laser
Free spaceto fiber
interface
EDFA preamp Demodulator (DPSK) Integrated
photoreceiver
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Free space optical comm 14
J Kaufmann - Boston IEEE Comm
1 Dec 2011
Some Non-Fiber Optical Receivers for FreeSpace
PIN photodiode receiver Unity gain photodiode Post-detection electrical amplifier
provides gain Noise usually dominated by electrical
amplifier noise
Avalanche photodiode receiver Avalanche effect provides gain
Avalanche process is noisy
Sensitivity of PIN or avalanche photodiodereceivers generally inferior to EDFApreamp receiver, especially at high datarates
Photon-counting receivers Geiger mode APD Superconducting single-photon detector
(SSPD) High performance, more complexity 8x8
GM-APD ArraySSPD
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1 Dec 2011
Coherent detection making a comeback in telecom
100G Ethernet long haul being standardized around PM-QPSK forbandwidth efficiency
100G transmission impaired by polarization mode dispersion (PMD),chromatic dispersion (CD)
Telecom use motivated by coupling of coherent technology and highspeed DSP for fiber-specific requirements: compensate PMD, CD and
perform PM demuxingnot issues in free space Coherent receivers also finding application in 40G long haul telecom
What About Coherent Optical Receivers?
4 x ADC
Retiming
PM demuxing
EqualizationPhase recovery
Demuxing
ADCIx
Iy
Polsplitter
Laser
Qy
Qx
DSP/ASIC
Detectors
90O
90O
Sx
Sy
PM-QPSK receiver
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1 Dec 2011
10-1
100
101
102
103
Bandwidth Expansion (Hz/bit/s)
10
1
0.1
PhotonsperBit
Modulation and Coding for Free Space
Telecom,
High-RateLasercom
Coherent
Photon
Counting
QuantumLimit
Free space lasercom not subject tobandwidth expansion constraints of
channelized fiber telecom!
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Free space optical comm 17
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1 Dec 2011
Coupling Light from Free Space to Fiber
EDFApreamp
Light focused ontofiber core
Focused beam is 2-D spatial Fouriertransform of signal field in telescopeaperture
Fiber coupling efficiency calculation:
E = received fieldM = fiber mode
h d)M(d)E(d)(*ME(
222
Far-field
transmitter
Couplinglens
E
Goal: maximize spatialoverlap between
received signal andfiber mode
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Free space optical comm 18
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1 Dec 2011
What Happens With Pointing (Wavefront Tilt)Error?
EDFA orphotodiode
Spatial overlap (coupling efficiency) between fibermode and signal goes to zero as tilt angle approaches/D! Field of view (FOV) of fiber receiver is ~1 BW Must use spatial tracking to reduce tilt error to
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Free space optical comm 19
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1 Dec 2011
RF Beam from Geosynchronous Orbit
Footprint (~300 mi) of 0.7o RFbeam (1m at 30 GHz) from GEO
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Free space optical comm 20
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1 Dec 2011
Optical Beam from Geosynchronous Orbit
Footprint (~600 m) of15 mrad optical beam
(4 aperture at 1.55 mm)from GEO
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1 Dec 2011
Optical Beam-Pointing Challenge
The lasercom advantage
Very high antenna gain from small apertures: e.g. 106 dB fromD = 4 at = 1.55 mm wavelength
The lasercom beam-pointing challenge
Optical beamwidth qBW = /D = 15 mrad (=.0009o) 600m beam footprint at 40,000 km
Sources of pointing uncertainty
Quasi-static (1 to 10s ofmilliradians)o Satellite ephemeris error
o Terminal location uncertainty
o Local attitude uncertainty
o Mechanical misalignments
Dynamic (10s ofmicroradians to milliradians)
o Satellite reaction wheels, solar array drive, thrusters
o Platform dynamic maneuverso Engine vibrations
o Optical gimbal bearing noise
Pointing narrow beams is one of the most challenging aspects of lasercom
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1 Dec 2011
Beam-Pointing Uncertainty from GeosynchronousOrbit
1 mrad region ofpointing
uncertainty fromGEO (40 km
footprint)
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Free space optical comm 23
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1 Dec 2011
Optical Beam Pointing Subsystem Architecture
Beacon
Focal plane array
Transmitter PAM
FSM
Baseplate
Passive Isolators
ActiveBeam
Stabilization
Fiber receiver
Telescope Primary
Telescope Secondary
Pointingfeedback
Optics in Motion FSM
+
INS/IMU
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1 Dec 2011
TERMINAL 2
TERMINAL 1
TERMINAL 2 scansuplink beacon
TERMINAL 1 detects uplinkbeacon, pulls in to
coarse-track
TERMINAL 2 detectsdownlink beacon,
pulls in tocoarse-track
TERMINALS 1 and 2reduce beamwidths,
begin fine-track
Acquisition Sequence
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1 Dec 2011
Absorption
Scattering
Clouds
Turbulence
Challenges of the Atmospheric OpticalChannel
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1 Dec 2011
Atmospheric Absorption and Scattering
Absorption
zabsII exp0
Incident light beam
Clear Air Channel
Scatter Channel
Molecular, aerosol, particulate content ofatmosphere causes optical transmission losses
Incident light beam
Scattering
zscatabsII exp0
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1 Dec 2011
Atmospheric Transmission from Visible toInfrared
Reference: H. Hemmati, Deep Space Optical Communications, Chapter 3.
Calculation from MODTRAN Molecular, aerosol, particulate absorption and scattering
1.5 mm band
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1 Dec 2011
Optical Atmospheric Transmission Lossesat 1550 nm
Clear-air Propagation
High resolution transmission calculated from HITRAN
Atmospheric loss due to absorption can be very low withjudicious choice of wavelength
1.59
Wavelength (microns)
1.55 mmdownlink
1.55 1.56 1.57 1.580
0.1
0.2
0.3
0.4
0.5
0.6
Transmission
CO2
1.54 20 40 60 802.5
2
1.5
1
0.5
0
Elev (deg)
Att
en.
(dB)
Desert
Vis=23km
Vis=5km
20 deg elevation, vis 5 km
Clear transmissionwindow
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Free space optical comm 29
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1 Dec 2011
Free-Space Optical Last Mile Solutions
Alternative to fiber or RF wireless Commercial systems available
1-10 km rangeUp to a few Gbps
No licensing, etc.
Channel limits data rate, range, link availabilityLine-of-sight obstructions
Weather (rain, snow, fog, etc.)Haze, pollutionTurbulence
Lightpointe Wireless fSONA Communications
CableFree Solutions
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Conducted by group of Australian radio amateurs withhomebrew equipment
Demonstrated 288 km cloud forward scatter link
FSK intensity subcarrier modulation + FEC
Tasmanian Amateur Cloud BounceExperiment (October 2009)
See http://reast.asn.au/optical.php
Tasmania
Australia
Thin cirrusclouds (7 kmelev) over sea
lane
180W LEDarray TX
http://reast.asn.au/optical.phphttp://reast.asn.au/optical.php -
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RX
RX
RX
Terrestrial ground network
Spatial Site Diversity Increases Link Availabilityfrom Space to Ground
Cloud cover conditions decorrelate beyond ~300 miles Multiple ground sites provide diversity against clouds forspace to ground Availability >99% for 7 uncorrelated ground sites forProb(CFLOS) = 0.5 per site
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Random spatial and temporal variation of refractive indexcaused by non-uniform heating of air
Typical index variation is less than 1 ppm but enough toproduce significant spatial phase distortion at opticalwavelengths Time constants of a few ms
Characterized by Kolmogorov Theory Phase coherence length parameter ro
Turbulent Atmospheric Channel
RECEIVER
NEAR-FIELD
TURBULENCE
PHASE
DISTORTION
IN RXAPERTURE
BEAM
SPREAD
ATFIBER
INPUT
POWER
FLUCTUATIONAND STRUCTURE
IN RECEIVERAPERTURETRANSMITTER
NEAR-FIELD
TURBULENCE
Rx
Fiber
Tx
Fiber
Power
fluctuation
in
Fiber
Degradedcoupling
efficiency
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1 Dec 2011
Atmospheric Effects: Intensity Scintillation
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Free space optical comm 34J Kaufmann - Boston IEEE Comm
1 Dec 2011
Atmospheric Effects: Wavefront Phase Aberration
10 cm
-6
-4
-2
0
2
4
radians
Accumulated
Phase Aberration
Intensity
at Focus
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1 Dec 2011
Scintillation Impact on Communications
Delivered power must exceed receiver sensitivity to communicate
Scintillation induces fading that can be overpowered at short ranges
Long range case: impractical to overpower fades
Scintillation drives loss and must be mitigated
-50 -40 -30 -20 -10 0 1010
-10
10-8
10-6
10-4
10-2
100
Receiver Intensity (dB)
BitErro
rRatio
-50 -40 -30 -20 -10 0 1010
-10
10-8
10-6
10-4
10-2
100
Receiver Intensity (dB)
BitErro
rRatio
-50 -40 -30 -20 -10 0 1010
-5
10-4
10-3
10-2
10-1
100
Proba
bility
Receiver Intensity (dB)-50 -40 -30 -20 -10 0 10
10-5
10-4
10-3
10-2
10-1
100
Proba
bility
Receiver Intensity (dB)
Dynamic Loss before Mitigation Static Loss 15-km range
50-km range
F d Miti ti A h
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1 Dec 2011
Fade Mitigation Approaches
FEC and Interleaving
Temporal Diversity
Interleaver adds temporal diversity
Byte Separation >> Fade Duration
Interleaving Adds Latency But Does Not
Reduce Data Rate
Multiple Aperture Receiver
Spatial Diversity
Receiver Diameter < Scintillation Patch Size
Aperture Separation > Scintillation Patch Size
Apertures experience uncorrelated fading
Combined Optical and Electrical Mitigation
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1 Dec 2011
-50 -40 -30 -20 -10 0 1010
-5
10-4
10-3
10-2
10-1
100
Relative Receiver Power (dB)
Probability
-50 -40 -30 -20 -10 0 1010
-5
10-4
10-3
10-2
10-1
100
Relative Receiver Power (dB)
Probability
-50 -40 -30 -20 -10 0 1010
-5
10-4
10-3
10-2
10-1
100
Relative Receiver Power (dB)
Probability
Dynamic Loss before MitigationDynamic Loss before Mitigation Static LossStatic LossStatic Loss
50 km Range15 km Range
50 km Range4 ReceiversSpatial Diversity
Combined Optical and Electrical Mitigation
Techniques Enable Longer Range
Mitigation techniques make long range problem moretractable
Dynamic Loss
after Mitigation
Free-Space Optical Communication Airborne Link
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Free space optical comm 38J Kaufmann - Boston IEEE Comm
1 Dec 2011
Hemispherical beam director on Twin Otter AC
4 ground apertures on pan tilt head in roof dome
Flight paths north of MIT/LL out to 60 km
Measured tracking performance, fiber couplingefficiency, channel turbulence and commperformance by range, elevation and time of day
ee Space Opt ca Co u cat o bo e
(FOCAL) Demonstration, Sep-Oct 09
Dome onC-Bldg
Twin Otter Aircraft
MA
NH
60 km
MIT/LL
Hanscom AFB
AirborneTerminal
Ground
Terminal
25 km
FOCAL Sep-Oct 09 Demonstration
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1 Dec 2011
FOCAL Sep Oct 09 Demonstration
System Architecture
20-60km Range
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1 Dec 2011
A/C Terminal Ground Terminals
Power distribution on A/C track camera
FOCAL Tracking Performance at 35 km range
Altitude = 12 kft, Elevation Angle = 5.3o
Terminal 1 Terminal 2 Terminal 3 Terminal 4
/ D
/
D
-5 -3 -1 1 3 5-5
-3
-1
1
3
5
Power distribution on ground trackcamera #1
Aircraft Fiber Coupling Efficiency
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1 Dec 2011
Aircraft Fiber Coupling Efficiency
-15 -10 -5 0 5 1010
-6
10-4
10-2
100
Normalized Power (dB)
Prob
abilityDensity
Intensity on Focal Plane
Power in Fiber
Power in Fiber (Gimbal Jumps Removed)
Distributions of power in single mode fiber and power in aperturematch for low gimbal motion => high fiber coupling efficiency
Aircraft gimbal stiction induces occasional mirror jumps that causedeep fades in fiber power distribution
Distribution mismatch in deep fades can indicate tracking error
/ D
/
D
-4 -3 -2 -1 0 1 2 3 4
-4
-3
-2
-1
0
1
2
3
4
az
= 0.29 / D
el
= 0.25 / D
Elevation = -4.9 degAzimuth = -26.2 deg
Contours ofresidual centroid
log probability
Communications Performance in FOCAL
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1 Dec 2011
Communications Performance in FOCAL
Multiple error-free transfers demonstrated from aircraft at 12 kft altitude
to ground terminal at 25 km range (8 degree elevation)
No dropped bit in 6 minutes of data (~100 Gbyte data file)
Similar performance expected at 37 km from 18 kft altitude
Data block duration set to ensure reasonable probability of CFLOS inNew England
Data transported in standard OTU1 optical network format
2.5 Gb/s payload with 7% overhead (RS 255,239 code)
1.25 s interleaver span: byte spacing 5 ms > fade coherence time
Data rate was 2 Gb/s (used ~80% of OTU1 payload)
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1 Dec 2011
The Challenge of Deep Space Lasercom
GEO Moon
0 10 20 30 40 50 60 70 80 90 100 110
Venus
MercuryMars
Power Loss Relative to GEO (dB)
1 Gbps
100 Mbps
10 Mbps
1 Mbps
100 kbps
10 kbps
1 kbps
100 bps
DataRate
NewHorizons
Larger apertures
Higher power transmitters
More sensitive receivers
RF L S l i f S
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1 Dec 2011
RF vs. Lasercom Solutions for Space
NASA Deep Space
Network
34-m antenna
S-band (~2-2.3 GHz)
20-kW transmit power
EIRP = 8.3 GW!
EIRP = 8.3 GW!
Lunar Laser CommunicationsDemonstration
10-cm space terminal
Optical (1550 nm)
0.5-W transmit power
EIRP = 8.1 GW!
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1 Dec 2011
LCE (Laser Communication Experiment) 1994-1996
Limited demonstration of space lasercom Japanese Experimental Test Satellite VI (ETS-VI) in high
elliptical orbit (failed to achieve intended geostationaryorbit because of rocket failure)
Demonstrated optical beam acquisition and tracking
1.5 Mbps downlink @ 0.83 mm to JPL ground station atTable Mtn., CA
Recent History of Lasercom in Space (1)
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1 Dec 2011
SILEX (Semiconductor IntersatelliteLink Experiment) 2003-
Recent History of Lasercom in Space (2)
European Space Agency (ESA) program ~35,000 km operational optical uplink relay of earth imagerydata from French SPOT-4 sensor satellite in low earth polar orbitto ARTEMIS (Advanced Relay and Technology Mission) in GEO
orbit 50 Mbps uplink @0.85 mm >1000 successful link sessions Successful crosslink demonstration to Japanese OICETS(Optical Inter-orbit Communications Engineering Test Satellite) in2005
Lunar Laser Communication
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Free space optical comm 47J Kaufmann - Boston IEEE Comm
1 Dec 2011 47- DMB3
Demonstration (LLCD) Program
Space terminal developed by MIT Lincoln Laboratory to fly on NASA
Lunar Atmosphere and Dust Environment Explorer (LADEE) Launch mid-2013
3 months science
50 km orbit
3 science monitors
Neutral Mass Spectrometer UV Spectrometer
Lunar Dust Experiment
NASAs first space lasercom
Precursor to lasercom on future
NASA deep space missions
Lunar Laser Communications Demonstration
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Lunar Laser Communications Demonstration
Objectives
Demonstrate optical downlink (spaceto ground)
80-600 Mbps
16-ary pulse position modulation (PPM) Rate serially concatenated turbo code 1-1.5 dB from Shannon capacity Simple implementation
Demonstrate optical uplink (groundto space)
10-20 Mbps 4-ary PPM Serially concatenated turbo code
Demonstrate duplex lasercom between a ground
terminal and a terminal on a spacecraft in lunar orbit
Serially ConcatenatedPPM Turbo Code
16-ary PPM waveform
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Lunar Lasercom Space Terminal
10-cm aperture
2-axis gimballed coarsepointing
Inertially stabilized telescope
0.5-W Master OscillatorPower Amplifier transmitter at1.55 mm
Optically preamplified directdetection receiver
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Lunar Lasercom Ground Terminal
Downlink
4 x 40-cm telescopes Superconducting nanowire (photon-
counting) detector arrays
Uplink
4 x 15-cm
40-W transmit power
Single gimbal for coarse pointing ofarray
Each telescope equipped with a focal
plane array and fast steering mirrorfor spatial acquisition and tracking
Fiber coupled to optical transmittersand receivers Downlink Apertures
Uplink Apertures
Free Space Optical Network Research
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Free space optical comm 51J Kaufmann - Boston IEEE Comm
1 Dec 2011
Need to deal with:
Outages due to atmospheric turbulence
Multiple access and interference
Congestion
Long Round-trip times
Performance Metrics:
Throughput
Delay
Fairness
Multiple access/interference rejection
Free Space Optical Network Research
Long range, hi-capacity,stable
Long range, fading, on-offchannel
Short range fading, low-visibility channel
Bow shock
Boundary layer
010101
Bow shock
Boundary layer
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~3Km
Slide courtesy of Prof. V. Chan, EECS Dept., MIT
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7/29/2019 Boston CommSoc 1 Dec 2011
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Wrap-Up
High-performance free space lasercom leveragestelecom fiber-optic technology at 1.5 mm
But free space lasercom has some unique requirementsand challenges
Large path losses drive need for very sensitive receivers andhighly power-efficient modulation/coding techniques (butunconstrained bandwidth expansion can be employed)
Large antenna gains from small apertures reduce SWaPcompared to RF, but very accurate pointing required
Special mitigation techniques help overcome atmosphericchannel impairments
Demonstrations have validated key technologies
Pointing/tracking of very narrow beams Efficient coupling to fiber receivers Mitigation of atmospheric effects Highly power-efficient receivers High data rates over long distancesultimately to deep space