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Professor Z GHASSEMLOOY
Associate Dean for ResearchOptical Communications Research Group,
School of Computing, Engineering and Information Sciences
The University of Northumbria
Newcastle, U.K.
http://soe.unn.ac.uk/ocr/
Free Space OpticalCommunications
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Northumbria University at Newcastle, UK
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Outline
Introduction
Why the need for optical wireless?
FSO
FSO - Issues
Some results Final remarks
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Optical Communications
Optical FibreCommunications
PhotonicSwitching
Indoor
Wired Wireless
Free-SpaceOptics(FSO)
Free-SpaceOptics(FSO)
OCRG - Research Areas
Chromatic dispersioncompensation using
optical signal processing Pulse Modulations Optical buffers Optical CDMA
Pulse Modulations Equalisation Error control coding
Artificial neural network &Wavelet based receivers
Fast switches All optical routers
Subcarrier modulation Spatial diversity Artificial neural
network/Waveletbased receivers
4HK Poly-Univ. 2007
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Staff Prof. Z Ghassemlooy J Allen R Binns K Busawon
Wai Pang Ng
Visiting AcademicsProf. Jean Pierre, Barbot
France
Prof. I. DarwazehUCL
Prof. Heinz DringHochschule Mittweida Univ.
of Applied Scie. (Germany) Dr. E. Leitgeb
Graz Univ. of Techn. (Austria)
OCRG -People
PhD M. Amiri M. F. Chiang: S. K. Hashemi R. Kharel
W. Loedhammacakra V. Nwanafio E. K. Ogah W. O. Popoola S. Rajbhandari (With IMLab)
Shalaby S. Y Lebbe
MSc and BEng A Burton D Bell G Aggarwal M Ljaz O Anozie W Leong
(BEng) S Satkunam (BEng)
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Photonics - Applications
Long-Haul Metropolitan Home access
Board -> Inter-Chip -> Intra-Chi
Photonics in communications: expanding and scaling
Health(bio-photonics)
Environmentsensing
Securityimaging
Photonics: diffusing into other application sectors
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RF & Optical Communications -
Integration
TraditionalRadio
TraditionalRadio
TraditionalOptics
TraditionalOptics
Radio onFibre
Radio onFibre
OpticalWireless
FibreFibre Free SpaceFree SpaceL
ightw
ave
L
ightwave
RF
RF
Transmission ChannelTransmission Channel
Source
Sourc
e
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Free Space Optical(FSO)Communications
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.. BANDWIDTH when and where required.
AND THAT IS ?
Over the last 20 years deployment of optical fibre cables in the backbone
and metro networks have made huge bandwidth readily available to
within one mile of businesses/home in most places.
But, HUGE BANDWIDTH IS STILL NOT AVAILABLE TO THE END
USERS.
The Problem?
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Optical Wireless Communication
Abundance of unregulated bandwidth - 200 THz in the 700-1500nm range
Abundance of unregulated bandwidth - 200 THz in the 700-1500nm range
Whatdoes
ItOffer
?
No multipath fading - Intensity modulation and direct detectionNo multipath fading - Intensity modulation and direct detection
Secure transmissionSecure transmission
High data rate In particular line of sight (in and outdoors)
High data rate In particular line of sight (in and outdoors)
Improved wavelength reuse capabilityImproved wavelength reuse capability
Flexibility in installationFlexibility in installation
Flexibility - Deployment in a wide variety of network architectures.Installation on roof to roof, window to window, window to roof or
wall to wall.
Flexibility - Deployment in a wide variety of network architectures.Installation on roof to roof, window to window, window to roof or
wall to wall.10
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Drawba
cks
Multipath induced dispersion (non-line of sight, indoor) - Limiting data rateMultipath induced dispersion (non-line of sight, indoor) - Limiting data rate
SNR can vary significantly with the distance and the ambient noise(Note SNR Pr
2)
SNR can vary significantly with the distance and the ambient noise(Note SNR Pr
2)
Limited transmitted power - Eye safety (indoor)Limited transmitted power - Eye safety (indoor)High transmitted power - OutdoorHigh transmitted power - Outdoor
Receiver sensitivityReceiver sensitivity
Large area photo-detectors - Limits the bandwidthLarge area photo-detectors - Limits the bandwidth
May be high cost - Compared with RFMay be high cost - Compared with RF
Limited range: Indoor: ambient noise is the dominant (20-30 dB larger than
the signal level. Outdoor: Fog and other factors
Limited range: Indoor: ambient noise is the dominant (20-30 dB larger than
the signal level. Outdoor: Fog and other factors
Optical Wireless Communication
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(Source:NTT)
Access Network bottleneck
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xDSL Copper based (limited bandwidth)- Phone and data combine Availability, quality and data rate depend on proximity to serviceprovidersC.O.
Radio link Spectrum congestion (license needed to reduce interference) Security worries (Encryption?) Lower bandwidth than optical bandwidth At higher frequency where very high data rate are possible,atmospheric
attenuation(rain)/absorption(Oxygen gas) limits link to ~1kmCable
Shared network resulting in quality and security issues. Low data rate during peak times
FTTx Expensive Right of way required - time consuming
Access Network Technology
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Optical Wireless Communications
Using optical radiation to communicatebetween two points through unguided
channels
Types
- Indoor
- Outdoor (Free Space Optics)
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DRIVER
CIRCUIT
SIGNAL
PROCESSING
PHOTO
DETECTOR
Link Range L
FSO - Basics
Cloud Rain Smoke Gases Temperaturevariations Fog and aerosol
Transmission of optical radiation through the atmosphere obeys the Beer-Lambertss law:
: Attenuation coefficient dB/km Not controllable and is roughly independent ofwavelength in heavy attenuation conditions.
d1 and d2: Transmit and receive aperture diameters (m)
D: Beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),
This equation fundamentally ties FSO to the atmospheric weather conditions
10/
22
1
2
2 10)(
L
trLDd
dPP
=
Dominant term at99.9% availabilityDominant term at99.9% availability
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FSO Link
Transmitter Lasers 780,850,980,1550nm, also 10 microns Beam control optics
o Multiple transmit apertures to reduce scintillation problems
o Tracking systems to allow narrow beams and reduced geometric losses
Receiver Collection lens Solar radiation filters (often several) Photodetector - Large area and low capacitance (PIN/APD) Amplifier and receiver
o Wide dynamic range requirement due to very high clear air link margin
o Automatic gain and transmitter power control
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Optical Components Light Source
OperatingWavelength
(nm)
Laser type Remark
~850 VCSEL Cheap, very available, no active cooling,reliable up to ~10Gbps,
~1300/~1550 Fabry-Perot/DFB Long life, compatible with EDFA, up to40Gbps5065 times as much power comparedwith 780-850 nm
~10,000 Quantum cascadelaser (QCL) Expensive, very fast and highly sensitiveIdeal for indoor (no penetration throughwindow)
For indoor applications LEDs are also used17
Eye safety - Class 1MEye safety - Class 1M
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Optical Components Detectors
Material/StructureWavelength
(nm)Responsivity
(A/W)Typical
sensitivityGain
Silicon PIN 300 1100 0.5 -34dBm@155Mbps
1
InGaAs PIN 1000 1700 0.9 -46dBm@155Mbps 1
Silicon APD 400 1000 77 -52dBm@155Mbps
150
InGaAs APD 1000 1700 9 10
Quantum well andQuatum-dot(QWIP&QWIP)
~10,000
Germanium only detectors are generally not used in FSO because of their high dark current.
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Existing System Specifications
Range: 1-10 km (depend on the data rates) Power consumption up to 60 W
15 W @ data rate up to 100 mbps and =780nm, short range 25 W @ date rate up to 150 Mbps and = 980nm 60 W @ data rate up to 622 Mbps and = 780nm 40 W @ data rate up to 1.5 Gbps and = 780nm
Transmitted power: 14 20 dBm Receiver: PIN (lower data rate), APD (>150 mbps) Beam width: 4-8 mRad Interface: coaxial cable, MM Fibre, SM Fibre Safety Classifications: Class 1 M (IEC) Weight: up to 10 kg
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Power Spectra of Ambient Light Sources
Wavelength ( m)
Norma
lise
dp
ower/unitw
aveleng
th
0
0.2
0.4
0.6
0.8
1
1.2
0.3
0.
4
0.5
0.6
0.7
0.8
0.9
1.0
1.
1
1.
2
1.3
1.
4
1.5
Sun Incandescent
x 10
1st window IR
Fluorescent
Pave)amb-light >> Pave)signal (Typically 30 dB with no optical filtering)
2nd window IR
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FSO - Characteristics
Narrow low power transmit beam- inherent security Narrow field-of-view receiver Similar bandwidth/data rate as optical fibre No multi-path induced distortion in LOS
Efficient optical noise rejection and a high optical signalgain Suitable to point-to-point communications only (out-door
and in-door) Can support mobile users using steering and tracking
capabilities Used in the following protocols:- Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, ATM- Optical Carriers (OC)-3, 12, 24, and 48.
Cheap (cost about $4/Mbps/Month according to fSONA)21
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22Source:
Cost Comparison
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Existing Systems
Auto tracking systems - 622 Mbps [Canobeam] TereScop - 1.5 Mbps to 1.25 Gbps (500m 5km)
Cable Free - 622 Mbps to 1.25 Gbps (High power class 3BLaser at 100 mW)
Microcell and cell-site backbone GSM, GPRS, 3G and EDGE traffico No Frequency license
o No Link Engineering
o Management via SNMP, RS232
o or GSM connection
Last mile
o 155 Mbps STM-1 links
o 622 Mbps ATM link for Banks etc
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800BC - Fire beacons (ancient Greeks and Romans)150BC - Smoke signals (American Indians)1791/92 - Semaphore (French)
1880 - Alexander Graham Bell demonstrated the photophone 1st FSO (THE GENESIS)
(www.scienceclarified.com)
1960s - Invention of laser and optical fibre1970s - FSO mainly used in secure military applications1990s to date - Increased research & commercial use due to successful trials
When Did It All Start?
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In addition to bringing huge bandwidth to businesses /homes FSO also findsapplications in :
Multi-campus universityHospitals
Others: Inter-satellite
communication
Disaster recovery
Fibre communication
back-up
Video conferencing Links in difficult terrains
Temporary links
e.g. conferences
Cellular communication
back-haul
FSO challengesFSO challenges
FSO - Applications
http://www.mobilecomms-technology.com/contractors/last_mile/pav/http://www.mobilecomms-technology.com/contractors/last_mile/pav/http://www.mobilecomms-technology.com/contractors/last_mile/pav/http://www.mobilecomms-technology.com/contractors/last_mile/pav/ -
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RF wireless networks- Broadcast RF networks are not scaleable
- RF cannot provide very high data rates
- RF is not physically secure- High probability of detection/intercept
- Not badly affected by fog and snow, affected byrain
A Hybrid FSO/RF Link- High availability (>99.99%)
- Much higher throughput than RF alone
- For greatest flexibility need unlicensed RF band
Hybrid FSO/RF Wireless Networks
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LOS - Hybrid Systems
Video-conference for Tele-medicine CIMIC-purpose and disaster recovery27
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DRIVER
CIRCUI
T
POINT APOINT B
SIGNAL
PROCESSIN
G
PHOTO
DETECTOR
Major challenges are due to the effectsof:
CLOUD,
RAIN,
SMOKE, GASES,
TEMPERATURE VARIATIONSFOG & AEROSOL
FSO - Challenges
To achieve optimal link performance,
system design involvestradeoffs of the different parameters.
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Effects Options Remarks
Photon absorption
Increase transmitoptical power Effect not significant
FSO Challenges - Rain
= 0.5 3 mm
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FSO Challenges - Physical Obstructions
Pointing Stability and Swaying Buildings
Effects Solutions Remarks Loss of signal Multipath induced
Distortions
Low power due tobeam divergence and
spreading Short term loss of
signal
Spatial diversity Mesh architectures: using
diverse routes Ring topology: Users n/w
become nodes at least onehop away from the ring
Fixed tracking (short
buildings) Active tracking (tall buildings)
May be used for
urban areas,
campus etc.
Low data rate Uses feedback
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FSO Challenges Aerosols Gases &
Smoke
Mie scattering Photon absorption
Rayleigh scattering
Increase transmitpower
Diversity techniques
Effect not severe
Effects Solutions Remarks
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Effects Options Remarks
Mie scattering Photon absorption
Increase transmit
optical power Hybrid FSO/RF
Thick fog limits link
range to ~500m Safety requirements
limit maximum optical
power
FSO Challenges - Fog
= 0.01 - 0.05 mmIn heavy fog conditions, attenuation is
almost constant with wavelength over the7801600 nm region.
In fact, there are no benefits until one gets
to millimeter-wave wavelengths.
In heavy fog conditions, attenuation isalmost constant with wavelength over the
7801600 nm region.In fact, there are no benefits until one gets
to millimeter-wave wavelengths.
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Weathercondition
Precipitation Amount(mm/hr)
Visibility dBLoss/km
Typical Deployment Range(Laser link ~20dB margin)
Dense fog 0 m50 m -271.65 122 m
(H.Willebrand & B.S. Ghuman,2002.)
Very clear 23 km50 km
-0.19-0.06
12112 m13771 m
Thick fog 200 m -59.57 490 m
Moderate fog Snow 500 m -20.99 1087 m
Light fog Snow Cloudburst 100 770 m1 km
-12.65-9.26
1565 m1493 m
Thin fog Snow Heavy rain 25 1.9 km2 km
-4.22-3.96
3238 m3369 m
Haze Snow Medium rain 12.5 2.8 km4 km
-2.58-1.62
4331 m5566 m
Light haze Snow Light rain 2.5 5.9 km10 km -0.96-0.44 7146 m9670 m
Clear Snow Drizzle 0.25 18.1 km20 km
-0.24-0.22
11468 m11743 m
FSO Challenges - Fog
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FSO Challenges - Beam Divergence
Beam width Typically, for FSO transceiver is relatively wide: 210-mrad
divergence, (equivalent to a beam spread of 210 m at 1 km), as isgenerally the case in non-tracking applications.
Compensation is required for any platform motion By having a beam width and total FOV that is larger than either
transceivers anticipated platform motion.
For automatic pointing and tracking, Beam width can be narrowed significantly (typically, 0.051.0 mrad
of divergence (equivalent to a beam spread of 5 cm to 1 m at 1 km)- further improving link margin to combat adverse weather conditions.
- However, the cost for the additional tracking feature can be significant.
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Background radiation
LOS requirement
Laser safety
FSO Challenges - Others
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Free Space Optics Characteristics Challenges Turbulence
- Subcarrier intensity multiplexing- Diversity schemes
Results and discussions
Wavelet ANN Receiver
Final remarks
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Effects Options Remarks
Irradiance fluctuation
(scintillation)
Image dancing
Phase fluctuation Beam spreading Polarisation
fluctuation
Diversity techniquesForward error control
control
Robust modulationtechniques
Adaptive opticsCoherent detection not
used due to Phasefluctuation
Significant for longlink range (>1km)
Turbulence and thickfog do not occurtogether
In IM/DD, it results in
deep irradiance
fades that could last
up to ~1-100 s
FSO Challenges - Turbulence
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Cause:Atmospheric inhomogeneity / random temperature variation along beam path.
Depends on: Altitude/Pressure, Wind speed, Temperature and relative beam size. Can change by more than an order of magnitude during the course of a day, being the
worst, or most scintillated, during midday (highest temperature). However, at ranges < 1 km, most FSO systems have enough dynamic range or margin to
compensate for scintillation effects.
The atmosphere behaves like prismof different sizes and refractive indices
Phase and irradiancefluctuation
Zones of differing density act as lenses,scattering light away from its intended path.
Thus, multipath.
Result in deepsignal fades that
lasts for ~1-100 s
FSO Challenges - Turbulence
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Gamma-Gamma All regimes
Model Comments
Log Normal Simple; tractableWeak regime only
I-K Weak to strongturbulence regime
K Strong regime only
Rayleigh/Negative
Exponential
Saturation regime only
Irradiance PDF by Andrews et al (2001):
0)2()()(
)(2
)(
1)2(
2/)(
>
=
++
IIIIp
1
6/55/12
2
1
6/75/12
2
1)69.01(
51.0exp
1)11.11(
49.0exp
+
=
+
=
l
l
l
l
Ix: due to large scale effects;obeys Gamma distribution
Iy: due to small scale effects;
obeys Gamma distributionKn(.): modified Bessel function
of the 2nd kind of ordernl
2 : Log irradiance variance
(turbulence strengthindicator)
yxIII =
Based on the modulation process the received
irradiance is
Irradiance PDF:
02
220
2
)2/)/(ln(exp
1
2
1)(
+= I
l
l
l
I
II
IIp
To mitigate turbulence effect we, employ subcarrier modulationwith s atial diversit
Turbulence Channel Models
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A
No Pulse Bit 0 Pulse Bit 1
No Intensity Fading
With Intensity Fading
A
Threshold level
A/2
All commercially available systems use OOK with fixed threshold whichresults in sub-optimal performance in turbulence regimes
Turbulence Effect on OOK
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( )dI
II
I
iRIi
l
l
l
rr
+
=
2
220
20
2
22
2
2/)/ln(exp
.1
2
1
2
))((exp
))(/()(maxarg
tdiPtd rd=
Using optimal maximum a posteriori (MAP) symbol-by-symbol detection withequiprobable OOK data:
Turbulence Effect on OOK
0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 10 .1
0.15
0 .2
0.25
0 .3
0.35
0 .4
0.45
0 .5
L o g I n te n s i ty S t a n d a r d D e v i a t i o n
Thres
holdlevel,i
th
0.5*10-2
10-2
3*10-2
5*10-2
Noise var iance
OOK based FSO requires
adaptive threshold to perform
optimally.
.but subcarrier intensitymodulated FSO does not
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Photo-detector
array
Atmosphericchannel
Serial/parallelconverter
Subcarriermodulator
.
.Data in
d(t)
Summingcircuit
.
.
DC bias
m(t) m(t)+bo
Opticaltransmitter
Spatialdiversitycombiner
Subcarrierdemodulator
Parallel/serialconverter .
.
Data out
d(t) ir
SIM System Block Diagram
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Subcarrier Intensity Modulation
No need for adaptive threshold To reduce scintillation effects on SIM
Convolutional coding with hard-decision Viterbi decoding (J. P. KImet al 1997)
Turbo code with the maximum-likelihood decoding (T. Ohtsuki, 2002) Low density parity check (for burst-error medium):
- Outperform the Turbo-product codes.
- LDPC coded SIM in atmospheric turbulence is reported to achieve acoding gain >20 dB compared with similarly coded OOK (I. B. Djordjevic, etal 2007)
SIM with space-time block code with coherent and differentialdetection (H. Yamamoto, et al 2003)
However, error control coding introduces huge processingdelays and efficiency degradation (E. J. Lee et al, 2004)
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SIM Our Contributions
Multiple-input-multiple-output (MIMO) (an array of transmitters/photodetectors) to mitigate scintillation effect in a IM/DD FSO link overcomes temporary link blockage (birds and misalignment) when
combined with a wide laser beamwidth, therefore no need for an activetracking
provides independent aperture averaging with multiple separate
aperture system, than in a single aperture where the aperture size hasto be far greater than the irradiance spatial coherence distance (fewcentimetres)
provides gain and bit-error performance Efficient coherent modulation techniques (BPSK etc.) - bulk of the
signal processing is done in RF that suffers less from scintillation
In dense fog, MIMO performance drops, therefore alternativeconfiguration such as hybrid FSO/RF should be considered
Average transmit power increases with the number of subcarriers,thus may suffers from signal clipping
Inter-modulation distortion
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=
+=M
j
jcjj twtgAtm
1
)cos()()(
Serial to
ParallelConverter
.
.
.
.
.
.
PSK m odulator
atcoswc1t
PSK m odulator
atcoswcMt
PSK m odulator
atcoswc2t
Laser
driver
)(tdInputdata
g(t)
g(t)
g(t)
A1
AM
A2
m(t)
DC b ias
b0
Atmopsher ic
channel
Subcarrier Modulation -Transmitter
1'
00,0 ][ ct NPRhModulation index is constrained to avoid over modulationModulation index is constrained to avoid over modulation
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-5
-4
-3
-2
-1
0
1
2
b0 Drive current
Outputpower
m(t)2
m a xP
P=
5-subcarriers
=
+=M
jjcjj twtgAtm
1
)cos()()(
Subcarrier Modulation -Transmitter
1'
00,0 ][ ct NPRh
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Photodetector
ir
x g(-t) Sampler
PSK D emodulator
atcoswc2t
PSK D emodulator
atcoswcMt
Parallelto Serial
Converter
PSK Demodulator
coswc1t
)( td Oud
.
.
.
SIM -Receiver
)())(1()( tntmIRtir ++=
Photo-current
R= Responsivity, I= Average power, =Modulation index, m(t) = Subcarrier signal
di(t) = Data
2
2
2)(
= IRASNR ele
47
{ }=
++=cN
i
iiitir tntftdPhP1
, )(2cos()(1
48
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Performs optimally without adaptive threshold as in OOK Use of efficient coherent modulation techniques (PSK, QAMetc.)
- bulk of the signal processing is done in RF where matureddevices like stable,
low phase noise oscillators and selective filters are readilyavailable.
System capacity/throughput can be increased Outperforms OOK in atmospheric turbulence Eliminates the use of equalisers in dispersive channels
Similar schemes already in use on existing networks The average transmit power increases as the numberof
subcarrier increases or suffers from signal clipping. Intermodulation distortion due to multiple subcarrier
impairsits performance
But..
Subcarrier Modulation
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SIM - Spatial Diversity
Single-input-multiple-output
Multiple-input-multiple-output (MIMO)
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Selection Combining(SELC). No need for phaseinformation
))()...(),(max()( 21 titititi NT =ii ia
Maximum RatioCombining (MRC)[Complex but optimum]
Naaa === ...21
Equal GainCombining (EGC)
FSO
C
HAN
NEL
PS K
SubcarrierDemodulator.
.
.
.
)( td
)(1 ti
)(2 ti
)(tiN
a2
a1
aN
Combiner
)(tiT
DiversityCombiningTechniques
ai is the scaling
factor
)()cos()(1)( tntwtgAIN
Rti i
M
jjcjjiri +
++=
SIM - Spatial Diversity
Assuming identical PIN photodetector on eachlinks, the photocurrent on each link is:
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SIM S ti l Di it A ti
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SIM Spatial Diversity Assumptions
Made
Spacing between detectors > the transverse correlationsize
oof the laser radiation, because
o= a few cm in
atmospheric turbulence
Beamwidth at the receiver end is sufficiently broad to coverthe entire field of view of all Ndetectors.
Scintillation being a random phenomenon that changeswith time makes the received signal intensity time variantwith coherence time
o
of the order of milliseconds.
Symbol duration T
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Eric Korevaar et. alA typical reduction in intensity fluctuation with spatial diversity
One detector
Two detectors
Three detectors
Subcarrier Modulation - Spatial Diversity
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Free Space Optics Characteristics Challenges Turbulence
- Subcarrier intensity multiplexing- Diversity schemes
Results and discussions
Wavelet ANN Receiver Final remarks
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1 2 3 4 5 6 7 8 9 10-1 0
-5
0
5
10
15
20
Num be r o f subca r ri e r
NormalisedSNR@B
ER=10
-6(d
B)
0.10.20.50.7
Log intensi tyvariance
Normalised SNRat BER of 10-6 against the number of subcarriers for variousturbulence levels for BPSK
Increasing the number ofsubcarrier/users, resultsIn increased SNR
SNR gain comparedwith OOK
Error Performance No Spatial Diversity
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2 0 2 5 3 0 3 5 4 01 0
-1 0
1 0-8
1 0-6
1 0-4
1 0-2
S N R (dB )
BER
D P S K
B P S K
1 6 -P S K
8 -P S K
Log i n ten s i t y
var ian c e = 0 .52
( )
0
22
)()/sin(loglog
2dIIpMMSNRQMBER
e
BPSK based subcarrier
modulation is the mostpower efficient
BPSK BER against SNR forM-ary-PSK for log intensity variance = 0.52
Error Performance No Spatial Diversity
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10
20
30
40
50
60
70
Turbulence Regime
DiveristyGain
(dB)
Weak
Saturation
Moderate
2 Photodetectors
3 Photodetectors
Spatial Diversity Gain
Spatial diversity gain with EGC against Turbulence regime
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Spatial Diversity Gain for EGC and SeLC
1 2 3 4 5 6 7 8 9 10-10
-5
0
5
10
15
20
25
No of Receivers
Linkmargin(dB)
0.22
0.52
0.72
1
Log Intensity
Variance
EGC
Sel.C
BER = 10-6
[ ] ].)(1[2
))22exp((
1
1
)(
220 llixK
n
i
N
iiNSelCeexerfw
NP
=
+=
{ }ni ix
1=
= Zeros of the nth order
Hermite polynomial{ } n
i iw
1=
= Weight factor of the nth order
Hermite polynomial
NARIK 200 2 =
Dominated by received irradiance,reduced by factorNon each link.
Link margin for SelC is lower
than EGC by ~1 to ~6 dB
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1 2 3 4 5 6 7 8 9 100
5
10
15
20
25
30
No of Receivers
SpatialDivers
ityGain
(dB)
MRC
EGC
Log Intensity
variance
1
0.52
0.22
Most diversity gainregion
The optimal but complex MRC diversity is marginally superior to thepractical EGC
Spatial Diversity Gain for EGC and MRC
BER = 10-6
+
=
=
mx
i
ZEGCe
uuieKQw
dZdZPZK
P
1
)2(1
0
2/
0
2
2
21
)(
)(1
)()(sin2
exp1
( )
[ ]
=
=
2/
0
0
)(
,)(1
)(/
dS
IdIPIQP
N
IMRCMRCe
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Multiple-Input-Multiple-Output
BPSK
Modu-
Lator
and
Laser
driver
d(t) .
.
.
It1
It2
ItH
F
S
O
C
HA
N
N
E
L
BPSK
SubcarrierDemodulator.
.
.
.
)( td
)(1 ti
)(2 ti
)(tiN
a2
a1
aN
Combiner
iT
By linearly combining the photocurrents using MRC, the individual SNRe on each
link 2
122
=
=
H
j
ijieleI
HN
RASNR
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MIMO Performance
12 14 16 18 20 22 24 26
10-9
10-8
10-7
10-6
10-5
10-4
10-3
SNR (R*E[I])2 / No (dB)
BER
1X5MIMO
1X8MIMO
4X4MIMO
2X2MIMO
1X4MIMO
[ ]
=2/
0
,)(1
dSPN
e=
+
m
j
uujj xK
wS1
2
2
2 )]2(2exp[sin2
exp1
)(
HN
ARIK
2
02
2
=
log intensity variance= 0.52
At BER of 10-6:
2 x 2-MIMO requires additional ~0.5dB ofSNRcompared with 4-photodetector single transmitter-multiple photodetector system.
4 x 4-MIMO requires ~3 dB and ~0.8dB lower SNR compared withsingle transmitter with 4 and 8-photodetectors , respectively.
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Free Space Optics Characteristics Challenges Turbulence
- Subcarrier intensity multiplexing- Diversity schemes
Results and discussions
Wavelet ANN Receiver Final remarks
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Transmission System - ReceiverModels
TX Channel
Noise
+
Slicer
MFEqualiserSlicerData out
CWTNNSlicerData out
Data in
MMSE
Wavelet - NN
Data out
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PPM System NN Equalization
PPM
Encoder
h(t)
Neural
Network
Decision
Device
Optical
Transmitter
Optical
Receiver
n(t)
PPM
Decoder
X(t)
Matched
Filter
ZjZjZj-1
.
Zj-n
.
Yj
Z(t)
M
0 0 1 0Ts = M/LRb
XjM
0 1 0 0
A feedforward back propagation neural network .
ANN is trained using a training sequence at the operating SNR.
Trained AAN is used for equalization
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Impulse Response of Equalized Channel
Pulse are spread to adjust pulse .
ISI depends on pulse spread
Equalized response in a deltafunction which is equivalent to aimpulse response of the idealchannel
Impulse response of unequalizedchannel
impulse response of equalizedchannel
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Results (1)
Adaptive linear equalizer with
least mean square (LMS)algorithm is used.
The performance of ANNequalizer is almost identical to
the linear equalizer.
Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 50Mbps
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Results (2)
Unequalized performance athigher data rate is
unacceptable at all SNR range
Linear and neural equalizationgive almost identical performance.
Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 100Mbps
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Results (3) - Wavelet-AI Receiver
SNR Vs. the RMS delay spread/bit duration
Wavelet
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Wavelet-AI Receiver - Advantages andDisadvantages
Complexity- many parameters & computations.
High sampling rates- technology limited.
Speed- long simulation times on average machines.
Similar performance to other equalisation techniques. Data rate independent
- data rate changes do not affect structure (just re-train).
Relatively easy to implement with other pulsemodulation techniques.
Vi ibl Li ht O ti l Wi l S t ith OFDM
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Downlink
Uplink
Visible-light communication system
0
1
2
3
4
5
0
1
2
3
4
5
200
400
600
800
1000
1200
1400
x[m]
Distribution of horizantal illuminance [lx]
y[m]
Illuminance[lx]
Number of LEDs
60 x 60 (4 set)
Distribution of illuminance
Visible Light Optical Wireless System with OFDM
FSO Network Two Universities in
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FSO Network Two Universities in
Newcastle
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Agilent Photonic Research Lab
Research Collaboration
Free space opticalDu-plex communication
link(Northumbria
and Newcastle Universities)at a data rate of 155 MbpsOptical Fibre
A-BlockAgilent PhotonicResearch Lab
C ll b t
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Collaborators
Graz Technical University, Austria Houston University, USA University College London, UK Hong-Kong Polytechnic University
Tarbiat Modares University, Iran Newcastle University, UK Ankara University, Turkey Agilent, UK Cable Free, UK
Technological University of Malaysia Others
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Final Remarks
Could the promise of optical wireless live up to reality?
Yes!!
But
Optical wireless must complement radio, not compete Industry must be bold in research and development
Lower component cost, and single technology baseddeviced
Integration with existing systems Lover receiver sensitivity
Of course more research and development at alllevels
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Summary
Access bottleneck has been discussed
FSO introduced as a complementary technology
Atmospheric challenges of FSO highlighted
Subcarrier intensity modulated FSO (with andwithout spatial diversity) discussed
Wavelet ANN based receivers
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Acknowledgements
To many colleagues (national and international)and in particular to all my MSc and PhD students
(past and present) and post-doctoral researchfellows