ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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email: [email protected]
Z. Ghassemlooy, W.O. Popoola, and E. Leitgeb*
Optical Communications Research Group, NCRLabNorthumbria University, Newcastle upon, UK.
*Institute of Broadband Communications, TU, Graz, Austria
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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S PROBLEM DEFINITON / MOTIVATION
FREE-SPACE OPTICS
ATMOSPHERIC TURBULENCE
SUBCARRIER MODULATION
SPATIAL DIVERSITY
RESULTS
CONCLUSIONS
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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STHE NEED TO COMMUNICATE, EXCHANGE AND SHARE
INFORMATION PROMPTED THE DEPLOYMENT OF DEVICES AND
APPLICATIONS SUCH AS:
THESE APPLICATIONS AND THE EMERGING ONES ALL HAVE
ONE “COMMON EFFECT” ON THE END USERS/BUSINESSES…….
MOBILE PHONES, HOME
PHONES, BROADBAND, OFFICE
PHONES, DIGITAL TV, VIDEO
CONFERENCING,
TELEMEDICINE, TELE-
TEACHING AND MANY OTHERS
ARE EMERGING BY THE DAY
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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…..INCREASED BANDWIDTH REQUIREMENT !
OPTICAL FIBRE, of course!
HOW CAN USERS HAVE ACCESS TO THE REQUIRED HIGH BANDWIDTH ?
AND THAT IS ?
DEPLOYMENT OF OPTICAL FIBRE IN THE BACKBONE AND METRO
NETWORKS HAVE MADE HUGE BANDWIDTH AVAILABLE TO WITHIN
ONE MILE OF BUSINESSES/HOMES IN MOST PLACES.
But, HUGE BANDWIDTH IS NOT AVAILABLE TO END USERS.
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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OPTICAL FIBRECOPPER CABLE
B BusinessC O Central officeH HomeU University BuildingN Network node
FIBRE BASED RING NETWORK
REGIONALFIBRE RING
METRO FIBRE RING
N
N
N
N
N
N
RF DOMINATEDACCESS NETWORK
C O
HU
B
H
H
BH
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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S FIBRE TO THE HOME
(FTTH)- NOT CHEAP HYBRID FIBRE COAXIAL-
STILL HAS COPPER
FIBRE TO THE HOME (FTTH)- NOT CHEAP
HYBRID FIBRE COAXIAL-
STILL HAS COPPER
FREE SPACE OPTICS (FSO)
(From NTT website)
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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STERRESTRIAL FSO BLOCK DIAGRAM
1010 1010
DA
TA
IN
LED
/LDD
RIV
ER
PH
OT
OD
ET
EC
TO
R
SIG
NA
LP
RO
CE
SS
OR
DA
TA
OU
TATMOSPHERIC CHANNEL
TRANSMITTER RECEIVER
A PRACTICAL LINK HEAD IS CAPABLE OF FULL
DUPLEX TRANSMISSION
FEATURES:
HAS SAME BANDWIDTH AS FIBRE OPTICS
NO SUNK COST
CHEAP AND FAST TO DEPLOY
NO LICENSE
NO EM INTERFERENCE
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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REGIONALFIBRE RING
METRO FIBRE
RING
N
N
N
N
N
N
C O
U
B
B
ACCESS NETWORK
WI-FI CAN BE USED WITHIN BUILDINGS FOR CONNECTIVITY
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SIN ADDITION TO BRINGING HUGE BANDWIDTH TO BUSINESSES
/HOMES FSO ALSO FINDS APPLICATIONS IN :
Multi-campus universityHospitals
Others Disaster recovery Fibre communication
back-up Video conferencing Links in difficult terrains Temporary links
e.g. conferencesCellular communication back-haul
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SMAJORLY ATMOSPHERIC FACTORS
(OPPORTUNITIES FOR RESEARCH !!!)
OTHERS:
BUILDING SWAY
BACKGROUND RADIATION
LINE OF SIGHT (LOS)
REQUIREMENT
LASER SAFETY
AEROSOLS SMOKEGASES
FOGRAIN TURBULENCE
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SCHALLENGE EFFECTS OPTIONS REMARK
RAIN Photon absorption
Increase transmit optical power Effect not significant
AEROSOLS
GASES & SMOKE
Mie scattering Photon absorption Rayleigh scattering
Increase transmit
power Diversity techniques
Effect not severe
TURBULENCE
Irradiance fluctuation
(scintillation) Image dancing Phase fluctuation Beam spreading Polarisation
fluctuation
Diversity techniques Coding Robust modulation
techniques Adaptive optics
Significant for long
link range (>1km) Turbulence and thick fog
do not occur together
FOG
Mie scattering Photon absorption 270 dB/km
Increase transmit
power Hybrid FSO/RF
Thick fog limits link range
to ~500m Safety requirements limit
maximum optical power
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SCAUSE:
ATMOSPHERIC INHOMOGENEITY / RANDOM TEMPERATURE VARIATION (0.1 degree) ON THE SPATILA SCALE OF 0.1 cm to 10 m.
DEPENDS ON:• Altitude, Pressure, Wind speed• Temperature and relative beam size
Eddies of different sizesand refractive indices
The atmosphere behaves like prismof different sizes and refractive indices
Phase and irradiance fluctuation
Result in deep signal fades that
lasts for ~1-100 μs
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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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
Based on the modulation process where the fluctuation of light radiation traversing turbulent atmosphere is assume to consist of small scale (scattering) and large scale (refraction) effects..
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 the received irradiance is
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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Serial to Parallel
Converter
.
.
.
.
.
coswc2t
Σ Σ Laserdriver
)(tdInput data
g(t)A1
AM
A2
m(t)
DC bias
b0
Photodetector
ir
x g(-t) Sampler
BPSK Demodulator
at coswc2t
BPSK Demodulator
at coswcMt
Parallel to Serial
Converter
BPSK Demodulator
coswc1t
)(ˆ td Output data
.
.
.
Atmopsheric channel
x
coswc1t
BPSK modulator
BPSK modulator
coswcMt
BPSK modulator
g(t) x
coswc1t
PSK modulator
+ Laserdriver
m(t)
DC bias
b0
Input data
Atmopsheric channel x g(-t) Sampler
PSK Demodulator
coswc1t
Photodetector
ir
Output data
)(ˆ td)(td
SINGLE SUBCARRIER
MULTIPLE SUBCARRIER
))(1( tmRPir
R = ResponsivityP = Average power = Modulation indexm(t) = Subcarrier signal
b0 Drive current
Outputpower
m(t)2maxP
P
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SWhy Subcarrier Modulation ?
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
Performs optimally without adaptive threshold as is the case with optimal OOK
Use of efficient coherent modulation (PSK, QAM etc. ) where signal processing is carried out in RF domain where devices such as stable, low phase noise oscillators and selective filters are readily available.
System capacity/throughput can be increased using multiple subcarrier
It outperforms OOK in atmospheric turbulence (data is hidden in subcarrier phase as against laser irradiance in OOK).
But..
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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Selection Combining (SELC)
))()...(),(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 TechniquesDiversity Combining Techniques
The spacing between detectors > the transverse correlation size ρo of the laserradiation, since ρo = a few cm in atmospheric turbulence
ai is the scalingfactor
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SAverage Bit-Error-Rate (BER) √
Without channels state information (CSI),
SNRe = BPSK subcarrier signal-to-noise ratiop(I) = Irradiance PDF
Outage Probability (Po)Measures the probability that the instantaneous BER is less that a pre-determined threshold level
0
1)2
(2/)(
0
)2()()(
)(2)(
)()(
dIIISNRQ
dIIpSNRQP
e
ee
Has no closed form solution. Have adopted a numerical evaluation.
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SBER performance against the normalized SNR with No Diversity
25 30 35 40 45 50 55 60 65
10-6
10-5
SNR (dB)
BE
R
weakmoderatestrong
Turbulence Regime
σl2=0.2
σl2=1.6
σl2=3.5
~35 dB
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SBER performance against the normalized SNR when using two photodetectors for EGC and MRC spatial diversities
20 25 30 35 40 45 50
10-6
10-5
10-4
SNR (dB)
BE
R
WeakRegime
StrongRegime
ModerateRegime
2 Photodetectors
MRC
EGC
5 dB improvement compared with no diversity case
20 dB improvement compared with no diversity case
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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SSNR at BER =10-6 against the number of photodetector with MRC diversity
1 2 3 4 5 6 7 8 9 1010
20
30
40
50
60
70
No of Photodetectors
SN
R a
t B
ER
= 1
0-6
WeakModerateStrong
Turbulence Regime
The decrease in the diversity gain in
strong turbulence can be attributed to
the increasing loss of spatial coherence of the laser radiation resulting in deep
fades.
ICTON 2007, Rome, ICTON 2007, Rome, ItalyItaly
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S FSO IS PROMISING IN SOLVING THE ACCESS NETWORK BOTTLENECK BY MAKING HUGE BANDWDITH AVAILABLE TO BUSINESSES/END USERS
FSO BASED ON SUBCARRIER MODULATION AND ITS ERROR PERFORMANCE HAVE BEEN PRESENTED
RESULTS SHOWING THE GAINS OF SPATIAL DIVERSITY IN MITIGATING ATMOSPHERIC TURBULENCE EFFECT ALSO PRESENTED
FSO SHOULD NOT BE VIEWED AS A SUBSTITUTE TECHNOLOGY TO RF BUT CAN ADEQUATELY COMPLIMENT IT.
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Acknowledgment
My PhD student Popoola Colleague at Erich Leitgeb at Graz University
Austria Northunmbria University for the research funding
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