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Iran 2008
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Professor Z GHASSEMLOOY
Associate Dean for Research Optical Communications Research Group,
School of Computing, Engineering and Information SciencesThe University of Northumbria
Newcastle, U.K.http://soe.unn.ac.uk/ocr/
Free Space Optical Communications
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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
Photonic Switching
Indoor
Wired Wireless
Free-Space Optics(FSO)
Free-Space Optics(FSO)
OCRG - Research Areas
• Chromatic dispersion compensation 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/Wavelet based receivers
4HK Poly-Univ. 2007
Staff• Prof. Z Ghassemlooy• J Allen• R Binns• K Busawon• Wai Pang Ng
Visiting Academics• Prof. Jean Pierre, Barbot France • Prof. I. Darwazeh UCL• Prof. Heinz Döring Hochschule 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-Chip
• Photonics in communications: expanding and scaling
Health(“bio-photonics”)
Environmentsensing
Securityimaging
• Photonics: diffusing into other application sectors
RF & Optical Communications - Integration
TraditionalRadio
TraditionalRadio
TraditionalOptics
TraditionalOptics
Radio onFibre
Radio onFibre
OpticalWireless
FibreFibre Free SpaceFree Space
Lig
htw
ave
Lig
htw
ave
RFRF
Transmission ChannelTransmission Channel
Sou
rce
Sou
rce
Free Space Optical (FSO)
Communications
….. 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 700-1500 nm rangeAbundance of unregulated bandwidth - 200 THz in the 700-1500 nm range
What does
It Offer
?
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 out doors)High data rate – In particular line of sight (in and out doors)
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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Drawbacks
Multipath induced dispersion (non-line of sight, indoor) - Limiting data rate Multipath 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 - Outdoor High transmitted power - Outdoor
Receiver sensitivity Receiver sensitivity
Large area photo-detectors - Limits the bandwidthLarge area photo-detectors - Limits the bandwidth
May be high cost - Compared with RF May 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 factorsLimited 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 service provider’s C.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 ~1km
Cable Shared network resulting in quality and security issues. Low data rate during peak times
FTTx Expensive Right of way required - time consuming Might contain copper still etc
Access Network Technology
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Optical Wireless Communications
Using optical radiation to communicate between two points through unguided channels
Types- Indoor
- Outdoor (Free Space Optics)
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DR
IVE
R
CIR
CU
IT
SIG
NA
LP
RO
CE
SS
ING
PH
OT
OD
ET
EC
TO
R
Link Range L
FSO - Basics
Cloud Rain Smoke Gases Temperature variations Fog and aerosol
Transmission of optical radiation through the atmosphere obeys the Beer-Lamberts’s law:
α : Attenuation coefficient dB/km – Not controllable and is roughly independent of wavelength 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
22 10
)(L
tr LDd
dPP
Dominant term at 99.9% availability
Dominant term at 99.9% availability
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
Operating Wavelength
(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 to 40Gbps50–65 times as much power compared with 780-850 nm
~10,000
Quantum cascade laser (QCL)
Expensive, very fast and highly sensitiveIdeal for indoor (no penetration through window)
For indoor applications LEDs are also used17Eye 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 and Quatum-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)
No
rma
lise
d p
ow
er/u
nit
wa
vele
ng
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 signal
gain 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
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 3B
Laser at 100 mW) Microcell and cell-site backbone – GSM, GPRS, 3G and EDGE traffic
o No Frequency licenseo No Link Engineeringo Management via SNMP, RS232o or GSM connection
Last mileo 155 Mbps STM-1 linkso 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 finds applications 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 challenges…FSO challenges…
FSO - Applications
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 by rain
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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DR
IVE
R
CIR
CU
IT
POINT APOINT APOINT BPOINT B
SIG
NA
LP
RO
CE
SS
ING
PH
OT
OD
ET
EC
TO
RMajor challenges are due to the effects of:
CLOUD,
RAIN, SMOKE, GASES,
TEMPERATURE VARIATIONS FOG & AEROSOL
FSO - Challenges
To achieve optimal link performance, system design involves
tradeoffs of the different parameters.
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Effects Options Remarks
Photon absorption
Increase transmit
optical power
Effect not significant
FSO Challenges - Rain
= 0.5 – 3 mm
FSO Challenges - Physical ObstructionsPointing Stability and Swaying Buildings
Effects Solutions Remarks Loss of signal Multipath induced Distortions Low power due to beam divergence and spreading Short term loss of
signal
Spatial diversity Mesh architectures: using diverse routes Ring topology: User’s n/w become nodes at least one hop 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 transmit
power 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 mm
In heavy fog conditions, attenuation is almost constant with wavelength over the
780–1600 nm region.In fact, there are no benefits until one gets
to millimeter-wave wavelengths.
In heavy fog conditions, attenuation is almost constant with wavelength over the
780–1600 nm region.In fact, there are no benefits until one gets
to millimeter-wave wavelengths.
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Weather condition
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
FSO Challenges - Beam Divergence
Beam width Typically, for FSO transceiver is relatively wide: 2–10-mrad
divergence, (equivalent to a beam spread of 2–10 m at 1 km), as is generally 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
transceiver’s anticipated platform motion.
For automatic pointing and tracking, Beam width can be narrowed significantly (typically, 0.05–1.0 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
Free Space Optics Characteristics Challenges Turbulence
- Subcarrier intensity multiplexing- Diversity schemes
Results and discussions
Wavelet ANN Receiver
Final remarks
Effects Options Remarks
Irradiance fluctuation (scintillation) Image dancing Phase fluctuation Beam spreading Polarisation
fluctuation
Diversity techniques Forward error control control Robust modulation techniques Adaptive optics Coherent detection not used due to Phase fluctuation
Significant for long link range (>1km)Turbulence and thick fog do not occur together In IM/DD, it results in deep irradiance fades that could last up to ~1-100 μs
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 irradiance fluctuation
• Zones of differing density act as lenses, scattering light away from its intended path. • Thus, multipath.
Result in deep signal fades that
lasts for ~1-100 μs
FSO Challenges - Turbulence
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Gamma-Gamma All regimes
Model Comments
Log Normal Simple; tractable
Weak regime only
I-K Weak to strong turbulence 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 distributionIy: due to small scale effects; obeys Gamma distributionKn(.): modified Bessel function of the 2nd kind of order n σl
2 : Log irradiance variance (turbulence strength indicator)
yx III Based on the modulation process the received
irradiance is
Irradiance PDF:
02
220
2
)2/)/(ln(exp
1
2
1)(
I
l
l
lI
II
IIp
To mitigate turbulence effect we, employ subcarrier modulation with spatial diversity
Turbulence – Channel Models
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40
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 which results in sub-optimal performance in turbulence regimes
Turbulence Effect on OOK
dI
II
I
iRIi
l
l
l
rr
2
220
20
2
22
2
2/)/ln(exp
.1
2
1
2
))((exp
))(/()(ˆ maxarg tdiPtd rd
Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable 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
Log Intensity Standard Deviation
Th
resh
old
lev
el,
ith
0.5*10-2
10-2
3*10-2
5*10-2
Noise variance
OOK based FSO requires adaptive threshold to performoptimally….
….but subcarrier intensity modulated FSO does not
41
Photo-detector
array
Atmosphericchannel
Serial/parallelconverter
Subcarrier modulator
.
.Data in
d(t)
Summing circuit
.
.
DC bias
m(t) m(t)+bo
Optical transmitter
Spatial diversity combiner
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. KIm et 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 a
coding gain >20 dB compared with similarly coded OOK (I. B. Djordjevic, et al 2007)
SIM with space-time block code with coherent and differential detection (H. Yamamoto, et al 2003)
However, error control coding introduces huge processing delays 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 active tracking
provides independent aperture averaging with multiple separate aperture system, than in a single aperture where the aperture size has to be far greater than the irradiance spatial coherence distance (few centimetres)
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 alternative configuration 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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45
M
jjcjj twtgAtm
1
)cos()()(
Serial to Parallel
Converter
.
.
.
.
.
.
PSK modulator
at coswc1t
PSK modulator
at coswcMt
PSK modulator
at coswc2t
Σ Σ Laserdriver
)(tdInput data
g(t)
g(t)
g(t)
A1
AM
A2
m(t)
DC bias
b0
Atmopsheric channel
Subcarrier Modulation - Transmitter
1'00,0 ][ ct NPRh
Modulation 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)2maxP
P
5-subcarriers
M
jjcjj twtgAtm
1
)cos()()(
Subcarrier Modulation - Transmitter
1'00,0 ][ ct NPRh
Photodetector
ir
x g(-t) Sampler
PSK Demodulator
at coswc2t
PSK Demodulator
at coswcMt
Parallel to Serial
Converter
PSK Demodulator
coswc1t
)(ˆ td Output data
.
.
.
SIM - Receiver
)())(1()( tntmIRtir
Photo-current
R = Responsivity, I = Average power, = Modulation index, m(t) = Subcarrier signaldi(t) = Data
2
2
2
)(
IRASNRele
47
cN
iiiitir tntftdPhP
1, )(2cos()(1
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48
Performs optimally without adaptive threshold as in OOK Use of efficient coherent modulation techniques (PSK, QAM etc.)
- bulk of the signal processing is done in RF where matured devices like stable, low phase noise oscillators and selective filters are readily available.
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 number of subcarrier increases or suffers from signal clipping. Intermodulation distortion due to multiple subcarrier impairs its performance
But..
Subcarrier Modulation
SIM - Spatial Diversity
Single-input-multiple-output Multiple-input-multiple-output (MIMO)
49
Selection Combining (SELC). No need for phase information
))()...(),(max()( 21 titititi NT ii ia
Maximum RatioCombining (MRC)[Complex but optimum]
Naaa ...21
Equal Gain Combining (EGC)
FSO CHANNEL
PSK Subcarrier
Demodulator....
)(ˆ td
)(1 ti
)(2 ti
)(tiN
a2
a1
aN
Combiner
)(tiT
Diversity Combining Techniques
ai is the scalingfactor
)()cos()(1)( tntwtgAIN
Rti i
M
jjcjjiri
SIM - Spatial Diversity
Assuming identical PIN photodetector on each links, the photocurrent on each link is:
50
SIM Spatial Diversity – Assumptions Made
Spacing between detectors > the transverse correlation size ρo of the laser radiation, because ρo = a few cm in atmospheric turbulence
Beamwidth at the receiver end is sufficiently broad to cover the entire field of view of all N detectors.
Scintillation being a random phenomenon that changes with time makes the received signal intensity time variant with coherence time o of the order of milliseconds.
Symbol duration T << o , thus received irradiance is time invariant over one symbol duration.
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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
Free Space Optics Characteristics Challenges Turbulence
- Subcarrier intensity multiplexing- Diversity schemes
Results and discussions
Wavelet ANN Receiver Final remarks
1 2 3 4 5 6 7 8 9 10-10
-5
0
5
10
15
20
Number of subcarrier
No
rmal
ised
SN
R @
BE
R =
10
-6
(dB
)
0.10.20.50.7
Log intensityvariance
Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels for BPSK
Increasing the number of subcarrier/users, resultsIn increased SNR
SNR gain compared with OOK
Error Performance – No Spatial Diversity
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20 25 30 35 4010
-10
10-8
10-6
10-4
10-2
SNR (dB)
BE
R
DPSK
BPSK
16-PSK
8-PSK
Log intensity
variance = 0.52
0
22
)()/sin(loglog
2dIIpMMSNRQ
MBER e
BPSK based subcarrier modulation is the most power efficient
BPSK BER against SNR for M-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
Div
eris
ty G
ain
(d
B)
Weak
Saturation
Moderate
2 Photodetectors3 Photodetectors
Spatial Diversity Gain
Spatial diversity gain with EGC against Turbulence regime
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
Lin
k m
arg
in (
dB
)
0.22
0.52
0.72
1
Log IntensityVariance
EGCSel.C
BER = 10-6
].)(1[2
))22exp((
1
1)(
220 llixK
n
i
NiiNSelCe exerfw
NP
ni i
x1= Zeros of the nth order Hermite polynomial
ni i
w1
= Weight factor of the nth order Hermite polynomial
NARIK 200 2
Dominated by received irradiance,reduced by factor N on each link.
Link margin for SelC is lower
than EGC by ~1 to ~6 dB
1 2 3 4 5 6 7 8 9 100
5
10
15
20
25
30
No of Receivers
Sp
atia
l D
iver
sity
Gai
n
(dB
)
MRCEGC
Log Intensity variance
1
0.52
0.22
Most diversity gain region
The optimal but complex MRC diversity is marginally superior to the practical EGC
Spatial Diversity Gain for EGC and MRC
BER = 10-6
mx
i
ZEGCe
uuieKQw
dZdZPZK
P
1
)2(1
0
2/
0
22
21
)(
)(1
)()(sin2
exp1
2/
0
0
)(
,)(1
)(/
dS
IdIPIQP
N
IMRCMRCe
58
Multiple-Input-Multiple-Output
BPSK Modu-Lator
and
Laser driver
d(t) ...
It1
It2
ItH
FSO CHANNEL
BPSK Subcarrier
Demodulator....
)(ˆ 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
jijiele I
HN
RASNR
59
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)
BE
R
1X5MIMO
1X8MIMO
4X4MIMO2X2MIMO
1X4MIMO
2/
0
,)(1
dSP Ne
m
juujj x
KwS
12
22 )]2(2exp[
sin2exp
1)(
HN
ARIK
2
02
2
log intensity variance= 0.52
At BER of 10-6:
2 x 2-MIMO requires additional ~0.5 dB of SNR compared with 4-photodetector single transmitter-multiple photodetector system.
4 x 4-MIMO requires ~3 dB and ~0.8 dB lower SNR compared with single transmitter with 4 and 8-photodetectors , respectively.
60
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 - Receiver Models
TX Channel
Noise
…
+
Slicer
MF Equaliser Slicer Data out
CWT NN Slicer Data out
Data in
MMSE
Wavelet - NN
Data out
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PPM System – NN Equalization
PPMEncoder
h(t)∑
NeuralNetwork
DecisionDevice
OpticalTransmitter
Optical
Receiver
n(t)
PPMDecoder
X(t)
MatchedFilter
ZjZj
Zj-1
.
Zj-n
.
Yj
Z(t)
M
0 0 1 0 Ts = M/LRb
Xj
M0 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 delta function which is equivalent to a impulse response of the ideal channel
Impulse response of unequalized channel
impulse response of equalized channel
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Results (1)
Adaptive linear equalizer with least mean square (LMS) algorithm is used.
The performance of ANN equalizer is almost identical to the linear equalizer.
Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 50 Mbps
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Results (2)
Unequalized performance at higher data rate is unacceptable at all SNR range
Linear and neural equalization give almost identical performance.
Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 100 Mbps
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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 and Disadvantages
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 pulse
modulation techniques.
Downlink
Uplink
Visible-light communication system
01
23
45
0
1
2
3
4
5200
400
600
800
1000
1200
1400
x[m]
Distribution of horizantal illuminance [lx]
y[m]
Illum
inan
ce[lx
]
Number of LEDs60 x 60 (4 set)
Distribution of illuminance
Visible Light Optical Wireless System with OFDM
FSO Network – Two Universities in Newcastle
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Agilent Photonic Research Lab
Research Collaboration
Free space optical Du-plex communication
link (Northumbria
and Newcastle Universities)at a data rate of 155 MbpsOptical Fibre
A-Block
Agilent PhotonicResearch Lab
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 based
deviced Integration with existing systems Lover receiver sensitivity Of course more research and development at all
levels
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74
Summary
Access bottleneck has been discussed
FSO introduced as a complementary technology
Atmospheric challenges of FSO highlighted
Subcarrier intensity modulated FSO (with and without 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 research fellows
