Boston CommSoc 1 Dec 2011

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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1 Dec 2011

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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1 Dec 2011

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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1 Dec 2011

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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Free space optical comm 33J Kaufmann - Boston IEEE Comm

1 Dec 2011

Atmospheric Effects: Intensity Scintillation


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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


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


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


Probability

-50 -40 -30 -20 -10 0 1010

-5

10-4

10-3

10-2

10-1

100


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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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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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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1 Dec 2011

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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1 Dec 2011

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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1 Dec 2011

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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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

010101

~3Km

Slide courtesy of Prof. V. Chan, EECS Dept., MIT


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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

Documents

Boston CommSoc 1 Dec 2011