Professor Z GHASSEMLOOY
Associate Dean for Research
Optical Communications Research Group,
School of Computing, Engineering and Information Sciences
The University of Northumbria
Newcastle, U.K.
http://soe.unn.ac.uk/ocr/
Iran 2008
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OCRG Research Areas
Optical Communications
Wired Wireless
Optical Fibre
Communications
•
Chromatic dispersion compensation using optical signal processing
•
Pulse Modulations
• Optical buffers
•
Optical CDMA
Photonic
Switching
• Fast switches
•
All optical routers
Indoor
•
Pulse Modulations
• Equalisation
•
Error control coding
• Artificial neural network &
Wavelet based receivers
Free-Space
Optics
(FSO)

Subcarrier modulation
 Spatial diversity
 Artificial neural network/Wavelet based receivers
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HK 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)
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)

• Photonics in communications: expanding and scaling
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Long-Haul Metropolitan Home access
Board -> Inter-Chip -> Intra-Chip
• Photonics: diffusing into other application sectors
Health
(“bio-photonics”)
Environment sensing
Security imaging
Iran 2008

Radio on
Fibre
Traditional
Radio
Traditional
Optics
Optical
Wireless
Fibre Free Space
Transmission Channel

AND THAT IS ?
….. BANDWIDTH when and where required.
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.
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10
Abundance of unregulated bandwidth 200 THz in the 700-1500 nm range
No multipath fading Intensity modulation and direct detection
What does
It
Offer
?
High data rate – In particular line of sight (in and out doors)
Improved wavelength reuse capability
Flexibility in installation
Secure transmission
Flexibility Deployment in a wide variety of network architectures.
Installation on roof to roof, window to window, window to roof or wall to wall.
Iran 2008
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11
c k b a s
D r a w
Multipath induced dispersion (non-line of sight, indoor) Limiting data rate
SNR can vary significantly with the distance and the ambient noise
(Note SNR

P r
2 )
Limited transmitted power Eye safety (indoor)
High transmitted power Outdoor
Receiver sensitivity
May be high cost Compared with RF
Large area photo-detectors Limits the bandwidth
Limited range: Indoor: ambient noise is the dominant (20-30 dB larger than the signal level . Outdoor: Fog and other factors
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12
12
(Source: NTT)
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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
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 Using optical radiation to communicate between two points through unguided channels
 Types
- Indoor
- Outdoor (Free Space Optics)
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 Cloud
 Rain
 Smoke
 Gases
 Temperature variations
 Fog and aerosol
Transmission of optical radiation through the atmosphere obeys the Beer-
Lamberts’s law:
2
P r

P t
 d
1
2
( d
D
2

L )
2

10
 
L / 10
Dominant term at
99.9% availability
α : Attenuation coefficient dB/km – Not controllable and is roughly independent of wavelength in heavy attenuation conditions.
d
1 and d
2
: Transmit and receive aperture diameters (m)
D: B eam divergence (mrad)(1/ e for Gaussian beams; FWHA for flat top beams),
This equation fundamentally ties FSO to the atmospheric weather conditions
Link Range L
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 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

Operating
Wavelength
(nm)
~850
~1300/~1550
Laser type Remark
VCSEL Cheap, very available, no active cooling, reliable up to ~10Gbps,
Fabry-Perot/DFB Long life, compatible with EDFA, up to
40Gbps
50 –65 times as much power compared with 780-850 nm
~10,000 Quantum cascade laser (QCL)
Expensive, very fast and highly sensitive
Ideal for indoor (no penetration through window)
For indoor applications LEDs are also used
 Eye safety 17 Class 1M
Iran 2008
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Material/Structure
Silicon PIN
InGaAs PIN
Silicon APD
Wavelength
(nm)
300
1000
400
– 1100
– 1700
– 1000
Responsivity
(A/W)
Typical sensitivity
0.5
0.9
-34dBm@
155Mbps
-46dBm@
155Mbps
77 -52dBm@
155Mbps
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Gain
1
1
150
InGaAs APD
Quantum –well and
Quatum-dot
(QWIP&QWIP)
1000 – 1700
~10,000
10
Germanium only detectors are generally not used in FSO because of their high dark current.
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Iran 2008
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 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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1.2
1
0.8
0.6
0.4
0.2
0
P ave)amb-light
>> P ave)signal
(Typically 30 dB with no optical filtering)
Sun Incandescent
1 st window IR
Fluorescent x 10
2 nd window IR
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20
Wavelength (
 m)
Iran 2008

 Narrow low power transmit beaminherent 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)
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Source:
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Iran 2008

 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 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

800BC - Fire beacons (ancient Greeks and Romans)
150BC - Smoke signals (American Indians)
1791/92 - Semaphore (French)
1880 Alexander Graham Bell demonstrated the photophone – 1 st
FSO ( THE GENESIS)
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(www.scienceclarified.com)
1960s - Invention of laser and optical fibre
1970s - FSO mainly used in secure military applications
1990s to date - Increased research & commercial use due to successful trials
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Iran 2008

In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in :
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Hospitals
Cellular communication back-haul
Others:
 Inter-satellite communication
 Disaster recovery
 Fibre communication back-up
 Video conferencing
 Links in difficult terrains
 Temporary links
Multi-campus university e.g. conferences
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FSO challenges…
Iran 2008


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

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Video-conference for Tele-medicine CIMIC-purpose and disaster recovery
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Iran 2008

Major challenges are due to the effects of:
CLOUD,
RAIN,
SMOKE, GASES,
TEMPERATURE VARIATIONS
FOG & AEROSOL
POINT A
To achieve optimal link performance, system design involves tradeoffs of the different parameters.
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POINT B
Iran 2008
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
= 0.5 – 3 mm
Effects Options Remarks
Photon absorption  Increase transmit optical power
Effect not significant
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Iran 2008
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FSO Challenges Physical Obstructions
Pointing Stability and Swaying Buildings
Effects Solutions
 Loss of signal  Spatial diversity
 Multipath induced  Mesh architectures: using

Distortions
Low power due to spreading
 beam divergence and diverse routes
Ring topology: User ’ s n/w become nodes at least one hop away from the ring
 Fixed tracking (short
 Short term loss of buildings) signal
 Active tracking (tall buildings)
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Remarks
 May be used for urban areas, campus etc.
 Low data rate
 Uses feedback

Effects
 Mie scattering
 Photon absorption
 Rayleigh scattering
Solutions
 Increase transmit power
 Diversity techniques
Remarks
 Effect not severe
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
= 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.
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Effects
 Mie scattering
 Photon absorption
Options
 Increase transmit optical power
 Hybrid FSO/RF
Remarks
 Thick fog limits link range to ~500m
 Safety requirements limit maximum optical power
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Iran 2008

Weather condition
Dense fog
Thick fog
Moderate fog
Light fog
Thin fog
Haze
Light haze
Clear
Very clear
Precipitation
Snow
Snow Cloudburs t
Snow Heavy rain
Snow
Snow
Snow
Medium rain
Light rain
Drizzle
Amount
(mm/hr)
Visibility dB
Loss/km
Typical Deployment Range
(Laser link ~20dB margin)
100
25
12.5
2.5
0.25
0 m
50 m
200 m
500 m
770 m
1 km
1.9 km
2 km
2.8 km
4 km
5.9 km
10 km
18.1 km
20 km
23 km
50 km
-271.65
-59.57
-20.99
-12.65
-9.26
-4.22
-3.96
-2.58
-1.62
-0.96
-0.44
-0.24
-0.22
-0.19
-0.06
122 m
490 m
1087 m
1565 m
1493 m
3238 m
3369 m
4331 m
5566 m
7146 m
9670 m
11468 m
11743 m
12112 m
13771 m
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(H.Willebrand & B.S. Ghuman, 2002.)
Iran 2008
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 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.

 Background radiation
 LOS requirement
 Laser safety
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Iran 2008

 Free Space Optics
 Characteristics
 Challenges
 Turbulence
- Subcarrier intensity multiplexing
- Diversity schemes
 Results and discussions
 Wavelet ANN Receiver
 Final remarks

Effects Options
 Irradiance fluctuation
 Diversity techniques
(scintillation)
 Forward error control
 Image dancing
 Phase fluctuation control
 Robust modulation
 Beam spreading techniques
 Polarisation fluctuation
 Adaptive optics
 Coherent detection not used due to Phase fluctuation
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Remarks
 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 ~1100 μs

Cause: Atmospheric inhomogeneity / random temperature variation along beam path.
The atmosphere behaves like prism of different sizes and refractive indices
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Phase and irradiance fluctuation
Result in deep
• Zones of differing density act as lenses, signal fades that lasts for ~1100 μs scattering light away from its intended path.
• Thus, multipath .
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.
Iran 2008

39
Irradiance PDF: p
I
( I )

1
2
  l
1 exp
I



(ln( I / I
0
)

2
 l
2
 l
2
/ 2 )
2
I

0
Model
Log Normal
I-K
Comments
Simple; tractable
Weak regime only
Weak to strong turbulence regime
Strong regime only
Saturation regime only
K
Rayleigh/Negative
Exponential
Gamma-Gamma All regimes
Irradiance PDF by Andrews et al (2001):
Based on the modulation process the received irradiance is I

I x
I y p ( I )

2 (

)
(
  
) / 2

(

)

(

)
I
(
  
2
)

1 
  
( 2

I ) I

0 I x
: due to large scale effects; obeys Gamma distribution
  


 exp


( 1

0 .
49
 l
2
1 .
11
 l
12 / 5
)
7 / 6
  


 exp


( 1

0 .
0 .
51
 l
69
 l
12
2
/ 5
)
5 / 6




1




1
I y
: due to small scale effects; obeys Gamma distribution
K n (.): modified Bessel function of the 2nd kind of order n




1




1 with spatial diversity
σ l
2 : Log irradiance variance
(turbulence strength indicator)
To mitigate turbulence effect we, employ subcarrier modulation
Iran 2008

No Pulse Bit “0”
No Intensity Fading
Threshold level
A/2
With Intensity Fading
A
Pulse Bit “1”
A
All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes
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Iran 2008
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Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data: d
ˆ
( t )
 arg max d
P ( i r
/ d ( t ))
 


0 exp exp





(( i r ln( I

RI )
2  i r
2
)
2

2
/ I
0
)
2
 l

2
 l
2
/ 2

2
 dI
1
2
 l
2
1
.
I
0.5
0.45
0.4
Noise variance
0.5*10
-2
10
-2
3*10
-2
5*10
-2
0.35
0.3
OOK based FSO requires adaptive threshold to perform optimally….
0.25
0.2
….but subcarrier intensity modulated FSO does not
0.15
0.1
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
Log Intensity Standard Deviation
0.8
0.9
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1

d ( t )
Data in
Serial/parallel converter .
.
Subcarrier modulator
DC bias m ( t )
.
.
Summing circuit m ( t )+ b o
Optical transmitter
Atmospheric channel d ’( t )
Data out
Parallel/serial converter
.
.
Subcarrier demodulator
Spatial diversity combiner i r
Photodetector array
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 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)
43

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

45
A
1 g ( t )
PSK modulator
at cosw c1 t
Input data d ( t )
Serial to
Parallel
Converter
.
.
.
.
A
2
.
.
A
M g ( t )
PSK modulator
at cosw c2 t m ( t )

M
 j

1
A j g ( t ) cos( w cj t
  j
)
Σ m ( t ) Σ Laser driver
Atmopsheric channel
DC bias b
0 g ( t )
PSK modulator
at cosw cM t
Modulation index is constrained to avoid over modulation
45
 
[ Rh
0
P t , 0

0
N c
'
]

1
Iran 2008

46
1
2
0
-1
-2 m ( t )
 j
M


1
A j g ( t ) cos( w cj t
  j
)
5-subcarriers
-3
-4
-5
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Output power
P

P max
2
 
[ Rh
0
P t , 0

0
N c
'
]

1
m(t) b
0 Drive current
Iran 2008

SNR ele

( IRA )
2
2
 2 x g(-t)
PSK Demodulator
Sampler
P r
 i
N  c

1 h i
P t , i

1
  d i
( t ) cos( 2
 f i t

 cosw c1 t n ( t )
Photodetector PSK Demodulator
at cosw c2 t
i r
.
.
.
PSK Demodulator
Photo-current
at cosw cM t i r
( t )

R I ( 1
  m ( t ))
 n ( t )
R = Responsivity, I = Average power,

=
Modulation index, m ( t ) = Subcarrier signal di( t ) = Data
47
Parallel to Serial
Converter d
ˆ
( t )
Output data

 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
But..
 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
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Iran 2008
48

 Single-input-multiple-output
 Multiple-input-multiple-output (MIMO)
49

N
N
E
L
C
H
A
F
S
O
i
1
( t ) a
1 i
2
( t ) i
N
( t ) a
2
.
.
.
a
N
.
Combiner

Assuming identical PIN photodetector on each links, the photocurrent on each link is : i
T
( t ) i ri
( t )

PSK
Subcarrier
Demodulator d
ˆ
( t )
R
N
I i

1

M
 j
A j g ( t ) cos( w cj t
  j
)
 n i
( t ) a i is the scaling factor
Maximum Ratio
Combining (MRC) a i i i
Diversity Combining Techniques a
1
Equal Gain
Combining (EGC)
 a
2

...
 a
N
Selection Combining
(SELC). No need for phase information i
T
( t )
 max( i
1
( t ), i
2
( t )...
i
N
( t ))
50

 Spacing between detectors > the transverse correlation size ρ o of the laser radiation, because ρ atmospheric turbulence o
= a few cm in
 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.
51

52
One detector
Two detectors
Three detectors
A typical reduction in intensity fluctuation with spatial diversity Eric Korevaar et. al
Iran 2008

 Free Space Optics
 Characteristics
 Challenges
 Turbulence
- Subcarrier intensity multiplexing
- Diversity schemes
 Results and discussions
 Wavelet ANN Receiver
 Final remarks

Normalised SNR at BER of 10 -6 against the number of subcarriers for various turbulence levels for BPSK
20
15
10
5
Increasing the number of subcarrier/users, results
In increased SNR
0
-5
-10
1 2 3 4 5 6
Number of subcarrier
7 8
Log intensity variance
0.1
0.2
0.5
0.7
9 10
SNR gain compared with OOK

55
BPSK BER against SNR for M-ary-PSK for log intensity variance = 0.5
2
10
-2
10
-4
DPSK
BPSK
16-PSK
8-PSK
Log intensity variance = 0.5
2
BPSK based subcarrier modulation is the most power efficient
10
-6
10
-8
BER

2 log
2
M


0
Q

SNR e log
2
M sin(

/ M )
 p ( I ) dI
10
-10
20 25
SNR (dB)
30 35 40
Iran 2008

Spatial diversity gain with EGC against Turbulence regime
Saturation
70
60
50
40
30
20
10 Weak
2 Photodetectors
3 Photodetectors
Moderate
Turbulence Regime
Iran 2008
56

Spatial Diversity Gain for EGC and SeLC
25
20
15
Log Intensity
Variance
0.2
2
0.5
2
0.7
2
1
Link margin for SelC is lower than EGC by ~1 to ~6 dB
10
5
0
Dominated by received irradiance, reduced by factor N on each link.
-5
-10
1
EGC
Sel.C
BER = 10 -6
2 3
P e ( SelC )

2 N
N

4 5 6 7 8 9 i n 
[


1 w i

No of Receivers
1 erf ( x i
)

N

1
.
e
(

K
0
2 exp( 2 x i
  i i n
= Zeros of the n th

1 order
Hermite polynomial
10
2
 l
  l
2
))
]
  n
= Weight factor of the n th order i

1
Hermite polynomial
K
0

RI
0
A 2
 2
N

Spatial Diversity Gain for EGC and MRC
30
BER = 10 -6
25
Log Intensity
variance
1
P e ( EGC )

0



 
/
0
2 exp


2
K
1
2 sin
2
(

)
Z
2
1
 m

1 w i
Q ( K
1 e
( x i
2
 u
  u
)
)
P
Z
( Z ) d
 dZ
20
MRC
EGC
15
10
5
0.5
2
0.2
2
P e ( MRC )


1



0
Q


MRC
/ I
 
P
I
 ( I

) d I


/

0
2 
S (

)

N d

,
0
1 2 3
Most diversity gain region
4 5 6
No of Receivers
7 8 9 10
The optimal but complex MRC diversity is marginally superior to the practical EGC
58

I t1 d ( t )
I t2
BPSK
Modu-
Lator and
Laser
driver
I tH
.
.
.
H
A
N
N
E
L
F
S
O
C i
1
( t ) a
1
Combiner
 i
T
BPSK
Subcarrier
Demodulator d
ˆ
( t ) i
2
( t ) i
N
( t ) a
2
.
.
a
N
.
.
By linearly combining the photocurrents using MRC, the individual SNRe on each link
SNR ele
 i



RA
2 N
 2
59
H j
H 

1
I ij


2

10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9 log intensity variance= 0.5
2
1X5MIMO
1X8MIMO
4X4MIMO
2X2MIMO
1X4MIMO
At BER of 10 -6 :

2 x 2-MIMO requires additional ~0.5 dB of SNR compared with 4photodetector single transmittermultiple photodetector system.

4 x 4-MIMO requires ~3 dB and ~0.8 dB lower SNR compared with single transmitter with 4 and 8photodetectors , respectively.
26
P e
12

1

14 16 18 20 22
SNR (R*E[I])
2
/ No (dB)


/
0
2 
S (

)

N d

,
24
S (

)

1
 j m 

1 w j
K exp





K
2
2
2 sin 2
 exp[ 2 ( x j
RI
0
A
2

2 N
 2
H
60
2
 u
  u
)]





 Free Space Optics
 Characteristics
 Challenges
 Turbulence
- Subcarrier intensity multiplexing
- Diversity schemes

Results and discussions
 Wavelet ANN Receiver
 Final remarks

Data in TX Channel +
Noise
Data out
… Slicer
MMSE
Data out Slicer Equaliser MF
CWT Data out Slicer
Wavelet - NN
NN
Iran 2008
62

n ( t )
M
0 1 0 0
PPM
Encoder
X j Optical
Transmitter
X ( t ) h ( t )
∑
Z(t )
Optical
Receiver
M
0 0 1 0
PPM
Decoder
Decision
Device
Y j
Neural
Network
.
Z j
Z j -1
.
Z j n
Z j
T s
= M / LR b
Matched
Filter
 A feedforward back propagation neural network .
 ANN is trained using a training sequence at the operating SNR.
 Trained AAN is used for equalization
63
Iran 2008

64
Impulse response of unequalized channel 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
Iran 2008

Slot error rate performance of 8- PPM in diffuse channel with D rms
Mbps of 5ns at 50
65

Adaptive linear equalizer with least mean square (LMS) algorithm is used.

The performance of ANN equalizer is almost identical to the linear equalizer.
Iran 2008

Slot error rate performance of 8- PPM in diffuse channel with D rms
Mbps of 5ns at 100
66
 Unequalized performance at higher data rate is unacceptable at all SNR range
 Linear and neural equalization give almost identical performance.
Iran 2008

67
Wavelet
SNR Vs. the RMS delay spread/bit duration
Iran 2008

 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.
68
Iran 2008

Visible Light Optical Wireless System with
OFDM
Visible-light communication system
Down link
Up link
Distribution of illuminance
Distribution of horizantal illuminance [lx]
Number of LEDs
60 x 60 (4 set)
1400
1200
1000
800
600
400
200
5
4
3 y[m]
2
1
0 0
1
2 x[m]
3
4
5

Agilent Photonic
Research Lab
Research Collaboration
A-Block
71
Optical Fibre
Free space optical
Du-plex communication link
(Northumbria and Newcastle Universities) at a data rate of 155 Mbps
Iran 2008

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

 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
Iran 2008
73

 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

74
Iran 2008
74

 To many colleagues (national and international) and in particular to all my MSc and PhD students
(past and present) and post-doctoral research fellows
75
Iran 2008