Download Terrestrial Free-Space Optical Communications

Terrestrial Free-Space Optical
Communications
Popoola, W.O.
Optical Communication Research Group, NCRLab.
Supervision Team:
Fary Ghassemlooy – Director of studies
Joseph Allen
Erich Leitgeb ( University of Technology, Graz, Austria)
Popoola, W.O. 2009
1
Outline
Part 1
Introduction to FSO
Part 2
My Research
Discussions and Remarks
Popoola, W.O. 2009
2
Part 1: Introduction
Free-space optics (FSO) started around 18781 and was made popular by the photo
phone experiment of Alexander G. Bell in 18802 involving the modulation of sun
radiation with voice signal.
Radiation
from the sun
The photophone schematic
Alexandra Graham Bell’s Photophone experiment
1http://modulatedlight.org/Modulated_Light_DX/ModLightBiblio.html
2Alexander
Graham Bell, "On the production and reproduction of sound by light," American Journal of Sciences, Series 3, pp. 305 - 324, Oct. 1880.
Popoola, W.O. 2009
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FSO witnessed a tremendous growth in the 1960s fuelled by the discovery of
lasers.
Variants of FSO
Indoor Applications
Including TV and Sound system
remote controls – very low data rate
(Source: Boston University)
Outdoor Applications
(Deep space and terrestrial)
Emphasis on outdoor FSO….
Popoola, W.O. 2009
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In
the
early
days,
terrestrial
FSO
(civil)
The early terrestrial applications were mainly for analogue signals (mostly sound)
applications
stalled
because
In the 1970s, the main attention was for deep space application .
existing
communications
were adequate to handle
Notable
space FSO flagship systems
projects :
• NASA’s Mars Laser Communication Demonstration (MLCD) and
ESA’s Semiconductor-laser
Inter-satellite Link Experiment (SILEX).
the• demands
of the time.
Popoola, W.O. 2009
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But….
….the increasing demand for more bandwidth in the face of new
and emerging applications implies that the old practice of
relying on just one access technology to connect with the end
users has to give way.
This among others, led to the resurgence of terrestrial FSO for
various applications including sound, data and video transmission.
Popoola, W.O. 2009
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Terrestrial FSO block diagram
Transmitter
Transmit
Telescope
Atmospheric Channel
Absorption
Receiver
Receive
Telescope
Turbulence
Scattering
Optical Source
(LED/LASER)
Message
Background
radiation noise
Optical
Filter
Driver Circuit
Photodetector
Modulator
Post detection
Processor
Popoola, W.O. 2009
Estimated
Message
7
Features of FSO
No
trenches
No license
fee
No electromagnetic
interference
Complements other
access network
technologies
Huge bandwidth
similar to fibre
Quick to install; only
takes few hours
Requires no right of way
Achievable range limited by thick fog to ~500m
Over 3 km in clear atmosphere
Popoola, W.O. 2009
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Terrestrial FSO Challenges
The main challenges are due to the nature of the atmospheric channel
Rain
snow
The resulting effects: Signal attenuation/power loss
Received optical power fluctuation (fading)
Popoola, W.O. 2009
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Rain and snow
A heavy rainfall of 15 cm/hour causes 20 - 30
dB/km loss in optical power
light snow about 3 dB/km power loss
Blizzard could cause over 60 dB/km power loss
Popoola, W.O. 2009
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Aerosols and Gases
Effects:
Scattering - Change in direction of photons
Absorption - Photons extinguished and
energy lost into the aerosol in
form of heat.
Both contribute to signal attenuation,  ( )
()   m ()   a ()  m ()  a ().
Absorption coefficient
Scattering coefficient
(molecular and aerosol)
(molecular and aerosol)
Popoola, W.O. 2009
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Within the wavelength band used in FSO, (0.5 μm – 2 μm) scattering dominates the
attenuation process.
That is:  ()   a ()
 

a ()  3.91 
V 550
Scattering coefficient in terms of visibility, V,
and wavelength, λ.
Scattering process at λ = 850 nm
Type
Radius(µm)
Size parameter,
Scattering process
xo
Air Molecules
0.0001
0.00074
Rayleigh
Haze particle
0.01 – 1
0.074 – 7.4
Rayleigh – Mie
Fog droplet
1 – 20
7.4 – 147.8
Mie - Geometrical
Rain
100 – 10000
740 – 74000
Geometrical
Snow
1000 – 5000
7400 –37000
Geometrical
Hail
5000 –50000
37000 – 370000
Geometrical
Popoola, W.O. 2009
Kim model,
1.6
1.3

  0.16V  0.34
V  0.5

0
V  50
6  V  50
1V  6
0.5  V  1
V  0.5
Kruse model,

1.6

  1.3

1
0.585V 3
V  50
6  V  50
V 6
12
Signal Attenuation
Attenuation coefficient at λ = 850 nm.
Thick fog
Thick fog causes the
most attenuation of over
100 dB/km.
Limits link range to
~500 km.
Clear air
In clear atmosphere, the
attenuation is as low as
0.4 dB/km. Longer
ranges of over 3 km
therefore practical.
Weather conditions and their visibility range values3
Weather condition
Thick fog
Moderate fog
Light fog
Thin fog/heavy rain
(25mm/hr)
Haze/medium rain
(12.5mm/hr)
Clear/drizzle (0.25mm/hr)
Very clear
Popoola, W.O. 2009
3Free-space
Visibility range (m)
200
500
770 – 1000
1900 – 2000
2800 – 40000
18000 – 20000
23000 – 50000
optics by Willebrand and Ghuman, 2002
13
Experimental attenuation measurement in Milan (11th January 2005)5
1000
900
Visibility Range ( m )
800
700
600
500
400
300
Low visibility,
high attenuation
200
100
250
160
300
350
400
450
500
550
600
650
700
750
Tim e ( m in )
Sp. Attenuation (dB/km )
140
120
100
80
Measured laser attenuation (red
curve) and the estimate from
the visibility values(blue
curve).
60
40
20
0
250
300
350
400
450
500
550
600
650
700
750
Tim e ( m in )
Popoola, W.O. 2009
5Courtesy:
TU Graz Collaborator – Prof. Erich Leitgeb
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Scintillation Effect
Channel temperature fluctuation = changes in refractive index of the channel
P:
Te:
n:
λ:
Channel pressure
Channel temperature
Channel refractive index
Beam wavelength
Effects:
Signal fluctuation (fading)
Complete loss of signal
Loss of signal coherence
Wave front deformation
Scintillation
Refraction and diffraction of beam
Popoola, W.O. 2009
Image dancing
15
Other Challenges
Pointing
Alignment
Line of sight
Building sway
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Part 2: My Research
Research Theme
Combating FSO channel (fading) effect using:
1. Robust modulation technique
2. Spatial diversity and
3. Temporal diversity
Popoola, W.O. 2009
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Research Objectives:
 To investigate, model and analyse:
1. On-Off keying (OOK) modulated FSO (existing technique)
2. Subcarrier intensity modulated (SIM) FSO
3. Compare 1 and 2 above
4. Spatial and Temporal diversity schemes on FSO
5. Laser non-linearity effects
 Link budget analysis
 To design an FSO test bed
 Recommendations
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Laboratory investigation
Simplified diagram of the experimental setup
Laser
Turbulence simulation chamber
Popoola, W.O. 2009
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Rm E105
BPSK modulator
•Carrier
1.5 MHz
•Data rate
200 kbps
Turbulence chamber
•Dimension
140 x 30 x 30 cm
•Temp. range 24oC – 60oC
Laser Module
(Direct Modulation)
Power = 3mW
λ = 785nm
Reflecting
mirror
OOK & BPSK
Mod. +Dem.
Turbulence chamber
Reflecting mirror
Thermometers
PIN Detector
+ Amplifier
Optical power
meter head
Heaters + Fans
Popoola, W.O. 2009
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Turbulence Simulation
Received mean signal + Noise + Scintillation
Input signal :
Pure sinusoid
with zero mean
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Turbulence Simulation - Signal distribution
With signal
scintillation
Histogram of mean
- no scintillation
Lognormal
Gaussian
fit fit
Mean
=1
Mean
=
-0.0012
16
Variance
= 9e-3
= 5e-5
2.5Variance
Gaussian fit
14
2
Lognormal fit
10
Bin Size
Bin Size
12
1.5
6
1
8
4
0.5
Observations
2
Total fluctuation variance = 9.10-3 (V2)
0 0
0.85
0.9
1 0
-0.05
-0.04 Lognormal
-0.03
-0.020.95 -0.01
• Weak Scintillation obeys
Signal
level
Signal level
distribution (variance < 1)
• Scintillation dominates over noise
• Simulated turbulence is very weak.
Popoola, W.O. 2009
1.05
0.01
0.021.1
0.03 1.150.04
22
OOK:
Data bit 1: Transmission of an optical pulse + Noise
Data bit 0:
No pulse
+ Noise
In turbulent channels
ith depends on:
No atmospheric turbulence
Noise level and
•
Turbulence strength
Bit “1”
Threshold position. ith
Signal Distribution
Bit “0”
•
0
2
2

  ((ir  RI )  ir ) 

   exp 

2 2


0




  ln( I / I )   2 / 2

0
l
exp 
2
2 l


Eg

2
1
2 l 2
1
.
I


dI


Bit separation
ith = Eg/2
Popoola, W.O. 2009
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OOK threshold level against the turbulence strength
Noise level
0.5
 2 = 5*10-3
 2 = 10-2
0.45
 2 = 3*10-2
 2 = 5*10-2
Threshold level, i
th
0.4
For optimum performance,
threshold level must adapt to the
channel noise and fading.
0.35
0.3
0.25
That, is a tough call.
0.2
0.15
0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Log Intensity Standard Deviation,  l
0.8
0.9
1
Fading strength
All commercially available FSO systems use OOK
with fixed threshold which results in suboptimal performance in turbulence regimes
Popoola, W.O. 2009
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OOK with Scintillation effect
Received Signal
8
7.5
7
No Scintillation
Threshold position. ith
Bin Percentage
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
1.5
Received
Transmitted
1
0.5
0
-0.2492 -0.1992 -0.1492 -0.0992 -0.0492 0.0008 0.0508
Signal Level
1.4
0.1008
0.1508
0.2008 0.2492
Received Signal ≈ 400mV p-p
With Scintillation
1.3
1.2
Threshold
range
1.1
Bin Percentage
1
0.9
0.8
Observation:
The optimum symbol decision position in
OOK depends on scintillation level and
noise
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.3488
-0.2488
-0.1487
-0.0487
0.0513
Signal Level
0.1513
0.2513
0.35130.3987
Popoola, W.O. 2009
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Subcarrier Intensity Modulation (SIM)
+
+
-
-
Data
Standard RF BPSK modulator
Popoola, W.O. 2009
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Subcarrier Intensity Modulation (SIM)
a1c
g(t)
X
m1c(t)
cos(ωc1t+φ1)
g(t)
X
m1s(t)
.
.
.
.
.
aNc
.
.
.
.
.
g(t)
sin(ωc1t+φ1)
.
.
.
X
mNc(t)
m(t)
Σ
Σ
Driver
circuit
Serial-to-parallel converter
and encoder
d(t)
a1s
bo
DC bias
Atmospheric channel
cos(ωcNt+φN)
aNs
g(t)
X
mNs(t)
sin(ωcNt+φN)
Subcarrier multiplexing for increased throughout or capacity
Popoola, W.O. 2009
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BPSK-SIM signal distribution
In turbulent channel
Bit “0”
Signal Distribution
Threshold position. ith
Bit “1”
-Eg
ith = 0
Threshold level does not
depend on turbulence
Eg
Popoola, W.O. 2009
Bit separation
28
BPSK-SIM with Scintillation Effect
Received Signal
Demodulated
(No low
Pass filtering)
Before
demodulation
Subcarrier
phase preserved
Demodulated Signal ≈ 400mV p-p
Popoola, W.O. 2009
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BPSK-SIM with Scintillation Effect
3.5
3.4
No Scintillation
3.2
3
2.8
Threshold position. ith
2.6
BIn Percentage
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.2
-0.15
-0.1
-0.05
0
0.05
Singal Level
0.1
0.15
0.2
3.5
3.4
With Scintillation
3.2
3
2.8
2.6
Threshold position. ith
Observation:
Scintillation does not affect the
symbol decision position in
BPSK-SIM
Bin Percentage
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-0.25
-0.2
-0.15
Popoola, W.O. 2009
-0.1
-0.05
0
0.05
Signal Level
0.1
0.15
0.2
0.25
30
Error performance of OOK Vs BPSK-SIM
-1
10
Fixed threshold
OOK
-2
10
-3
Performance based on
similar electrical SNR
10
BER
-4
10
-5
10
-6
10
Log irradiance Variance = 0.2
OOK Threshold Level
-7
10
-8
10
0.2
0.5
Adaptive
BPSK-SIM
10
15
20
25
Electrical SNR (dB)
30
35
Atmospheric turbulence, σl2 = 0.2.
Popoola, W.O. 2009
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BPSK-SIM BER against the normalised SNR in atmospheric turbulence.
-2
10
-4
10
-6
BER
10
-8
10
Fading strength
-10
10
 2l = 0.1
 2l = 0.3
 2l = 0.5
-12
10
Fading penalty
 2l = 0.7
No fading
15
20
25
30
Normalised SNR (dB)
Popoola, W.O. 2009
35
40
45
32
BPSK-SIM BER against turbulence strength for SNR (dB) = [5, 20, 25, 30].
0
10
-5
10
-10
10
Increasing power helps
to combat fading but
only during very weak
fading.
-15
BER
10
-20
10
-25
10
-30
10
Normalised SNR
5 dB
20 dB
25 dB
30 dB
-35
10
-40
10
-2
10
-1
0
10
10
Fading
strengthvariance,  2l
Log irradiance
Popoola, W.O. 2009
33
Diversity scheme:
Exploiting the spatial variations in the channel characteristics
Combiner
It1
It2
d(t)
BPSK
ModuLator
and
.
.
.
Laser
driver
ItH
F
S
O
i1 (t )
C
H
A
N
N
E
L
i2 (t )
iN (t )
a1
a2
.
.
.
.
a

iT
BPSK
Subcarrier
Demodulator
dˆ (t )
N
ai is the scaling
factor
Diversity Combining Techniques
Maximum Ratio
Combining (MRC)
[Complex but optimum]
ai
 ii
Equal Gain Combining
(EGC)
Selection Combining
(SELC). No need for phase
information
a1  a2  ...  a N
Popoola, W.O. 2009
iT (t )  max( i1 (t ), i2 (t )...i N (t ))
34
Spatial Diversity Gain:
30
Log
Intensity
Turbulence
variance
strength
Spatial Diversity Gain (dB)
25
1
20
MRC
EGC
15
0.52
10
5
0
0.22
1
2
3
4
5
6
No of Receivers
7
8
9
10
Four detectors: Good compromise between performance and complexity
Popoola, W.O. 2009
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Diversity scheme:
Exploiting the temporal variations in the channel characteristics
Retransmission on different subcarriers
Other possibilities: different wavelengths
different polarisations
Delay ≥ Channel coherence time
This process is reversed at the receiver side to recover the data
Popoola, W.O. 2009
36
Temporal Diversity Gain:
No fading
No TDD
1-TDD
3-TDD
5-TDD
2-TDD
-2
10
-4
BER
10
-6
10
-8
10
BER =10-9
-10
10
Rb = 155Mbps
Log irradiance
var =0.3
-32
-30
-28
-26
-24
-22
-20
Receiver sensitivity, Io (dBm)
-18
-16
No TDD
-17.17
Single delay path is
the optimum
Io (dBm)
(no fading: -27.05)
Fading penalty (dB) 9.88
Diversity gain (dB) 0
(gain / path)
(0)
Popoola, W.O. 2009
1-TDD 2-TDD
-19.17 -19.85
3-TDD
-20.13
5-TDD
-20.3
7.88
2
(2)
6.92
2.96
(0.99)
6.75
3.13
(0.63)
7.2
2.68
(1.34)
37
Research Output
Refereed Journal Articles
1. W. O. Popoola, Z. Ghassemlooy,: “BER and Outage Probability of DPSK Subcarrier Intensity Modulated Free Space Optics in Fully
Developed Speckle” Journal of Communications (In print)
2. W. O. Popoola, and Z. Ghassemlooy,: “BPSK Subcarrier Modulated Free-Space Optical Communications in Atmospheric Turbulence” IEEEJournal of Lightwave Technology, Vol. 27. No 8, pp. 967-973, April 15 2009.
3. W. O. Popoola, Z. Ghassemlooy, and V. Ahmadi,: “Performance of sub-carrier modulated Free-Space optical communication link in negative
exponential atmospheric turbulence environment,” International Journal of Autonomous and Adaptive Communications Systems, Vol. 1, No.
3, pp.342–355, 2008.
4. *W. O. Popoola, Z Ghassemlooy, J I H Allen, E Leitgeb, S Gao: “ Free-Space Optical Communication employing Subcarrier Modulation and
Spatial Diversity in Atmospheric Turbulence Channel” IET Optoelectronics, Vol. 2, Issue 1, Feb. 2008, pp.16 – 23.
5. Ghassemlooy, Z., Popoola, W. O., and Aldibbiat, N. M.: “Equalised Dual Header Pulse Interval modulation for diffuse optical wireless
communication system”, Mediterranean J. of Electronics and Communications, Vol. 2, No. 1, pp. 56-61, 2006.
6. W. Popoola, Z. Ghassemlooy, C. G. Lee, and A. C. Boucouvalas “Scintillation Effect on Intensity Modulated Laser Communication Systems A Laboratory Demonstration”, Elsevier’s Optics and Laser Technology Journal (Under review)
*According to IET, this paper ranked No. 2 in terms of the number of full text downloads within IEEE Xplore in 2008, from the hundreds of papers
published by IET Optoelectronics since 1980.
Conference Articles
7. Z. Ghassemlooy, W. O. Popoola, V. Ahmadi, and E Leitgeb: “MIMO Free-Space Optical Communication Employing Subcarrier Intensity
Modulation in Atmospheric Turbulence Channels” Invited paper. The First International ICST Conference on Communications Infrastructure,
Systems and Applications in Europe (EuropeComm 2009) 11 -13 August, 2009, London, UK
8. W. Popoola, Z. Ghassemlooy, M. S. Awan, and E. Leitgeb: “Atmospheric Channel Effects on Terrestrial Free Space Optical Communication
Links” Invited paper. 3rd International Conference on Computers and Artificial Intelligence (ECAI 2009), 3-5 July, 2009, Piteşti, Romania (To
appear).
9. W. O. Popoola, Z. Ghassemlooy and E. Leitgeb “BER Performance of DPSK Subcarrier Modulated Free Space Optics in fully Developed
Speckle”, IEEE - CSNDSP, 23-25 July 2008, Graz, Austria, pp. 273-277.
Popoola, W.O. 2009
38
Research Output
10. Z. Ghassemlooy, W.O. Popoola, S. Rajbhandari, M. Amiri, S. Hashemi,: “A synopsis of modulation techniques for wireless infrared
communication”, Invited paper. IEEE - International Conference on Transparent Optical Networks, Mediterranean Winter (ICTON-MW)
Dec, 6-8, 2007 - Sousse, Tunisia, pp. 1-6.
11. W.O. Popoola and Z. Ghassemlooy.: “Free-Space optical communication in atmospheric turbulence using DPSK subcarrier modulation”,
Ninth International Symposium on Communication Theory and Applications, ISCTA'07, 16th - 20th July, 2007, Ambleside, Lake District,
UK.
12. Z. Ghassemlooy, W.O. Popoola, and E. Leitgeb. “Free-Space optical communication using subcarrier modulation in Gamma-Gamma
atmospheric turbulence” Invited paper. IEEE - 9th International Conference on Transparent Optical Networks, July 1-5, 2007 - Rome, Italy.
13. W. O. Popoola, Z. Ghassemlooy and J. I. H. Allen. “Performance of subcarrier modulated Free-Space optical communications”, 8th Annual
Post Graduate Symposium on the Convergence of Telecommunications, Networking and Broadcasting (PGNET), 28 th & 29th June 2007,
Liverpool, UK.
14. S. Rajbhandari, Z. Ghassemlooy, N. M. Aldibbiat, M. Amiri, and W. O. Popoola: “Convolutional coded DPIM for indoor non-diffuse optical
wireless link”, 7th IASTED International Conferences on Wireless and Optical Communications (WOC 2007), Montreal, Canada, May-Jun.
2007, pp. 286-290.
15. Popoola, W. O., Ghassemlooy, Z., and Amiri, M.: "Coded-DPIM for non-diffuse indoor optical wireless communications", PG Net 2006,
ISBN: 1-9025-6013-9, Liverpool, UK, 26-27 June 2006. pp. 209-212.
16. W. O. Popoola, Z Ghassemlooy, and N. M. Aldibbiat,: "DH-PIM employing LMSE equalisation for indoor infrared diffuse systems", 14th
ICEE 2006, Tehran, Iran.
17. W. O. Popoola, Z. Ghassemlooy and N. M. Aldibbiat: "Performance of DH-PIM employing equalisation for diffused infrared
communications", LCS 2005, London, Sept. 2005, pp. 207-210.
Posters
18. Popoola, W. O., and Ghassemlooy, Z.: “Free space optical communication”, UK GRAD Programme Yorkshire & North East Hub, Poster
Competition & Network Event, Leeds, 9 May 2007, Poster No. 52.
19. Burton, A, Ghassemlooy, Z, Popoola, W.O., Simulation software for the evaluation of free space optical communications links, 19th
International Scientific Conference, Mittweida, Germany, July 2008.
Popoola, W.O. 2009
39
Summary
FSO introduced and its challenges highlighted
OOK and its pros and cons discussed
Subcarrier intensity modulation and
Diversity techniques discussed
Popoola, W.O. 2009
40
Acknowledgement
Supervision team members
Fabulous
Jolly
Fary Ghassemlooy
Joe Allen
Exquisite
Erich Leitgeb (TU Graz)
– Director of study
– 1st Supervisor
– 2nd Supervisor
My fellow research students apprentices
Northumbria Univ. for funding my research
Popoola, W.O. 2009
41
With plentiful:
Patience
Hard work
&
Due Diligence
I look forward to (successfully) completing my
Popoola, W.O. 2009
PhD Degree
42
Thank you.
Questions/Remarks/Contributions
Popoola, W.O. 2009
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