1. FREE-SPACE OPTICAL COMMUNICATION USING SUBCARRIER INTENSITY MODULATION
POPOOLA, Wasiu O.
(2nd Year PhD student)
Optical Communication Research Lab., CEIS
[ ]
Supervision Team:
Fary Ghassemlooy – Director of studies
Joseph Allen
Erich Leitgeb ( University of Technology, Graz, Austria.)
Steven Gao (now at University of Surrey)

2. Research Plan (1)

3. Research Plan (2)

4. Subcarrier Intensity Modulation (with and without diversity)
Outline
Problem definition
FSO Introduction
FSO challenges
Subcarrier Intensity Modulation
(with and without diversity)
Results and Discussions
Summary and Future work

5. FIBRE BASED RING NETWORK
Problem Definition
C O H U B
REGIONAL
FIBRE RING
METRO FIBRE
RING N
FIBRE BASED RING NETWORK
RF DOMINATED
ACCESS NETWORK
75% of businesses are within one mile of fibre backbone, yet only 5% have access to it - RHK
B Business
C O Central office
H Home
U University Building
N Network node
OPTICAL FIBRE
COPPER CABLE

6. Access Network bottleneck
(Source: NTT)

7. Access network tech. xDSL: Radio link: Cable: FTTx:
Copper based (limited bandwidth)- Phone and data combined
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

8. Free- space optical communication
What is it?
THE USE OF OPTICAL RADIATIONS TO COMMUNICATE
BETWEEN TWO POINTS THROUGH
UNGUIDED CHANNELS

9. Unguided channels
SPACE
WATER
(H. Hemmati, NASA)
ATMOSPHERE

10. FSO basics Link Range DRIVER CIRCUIT CLOUD, RAIN, SMOKE, GASES,
PROCESSING
SIGNAL
PHOTO
DETECTOR
DRIVER CIRCUIT
CLOUD,
RAIN,
SMOKE,
GASES,
TEMPERATURE VARIATIONS
FOG & AEROSOL
POINT A
POINT B
Link Range

11. Operating Wavelength (nm)
FSO components
FSO Optical (Laser diode) Sources
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 40Gbps
~10,000
Quantum cascade laser (QCL)
Expensive, very fast and highly sensitive
Selected FSO Detectors
Material/Structure
Wavelength
(nm)
Responsivity
(A/W)
Typical sensitivity
Gain
Silicon PIN
300 – 1100
0.5
155Mbps 1
InGaAs PIN
1000 – 1700
0.9
155Mbps
Silicon APD
400 – 1000 77
150
InGaAs APD 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.

12. Preceive = Ptransmit * exp(-αR)
FSO basics
The transmission of optical radiation through the atmosphere obeys the Beer-Lamberts’s law which says:
α -- Attenuation coefficient that results from absorption and scattering from
the constituents of the atmosphere
R – Link Range
Preceive = Ptransmit * exp(-αR)
This equation fundamentally ties FSO to the atmospheric weather conditions

13. FSO Features
Similar bandwidth/data rate as optical fibre
Very narrow beam – inherent security
No EM interference
No license issues
Cheap (cost about $4/Mbps/Month according to fSONA)

14. Cost comparison
Source:

15. FSO Features
Fast to deploy (few hours)
Transferable (no sunk cost)
Suffers atmospheric effects most deleterious being thick fog
Strictly line of sight - Pointing, Tracking and Alignment issues
FSO can therefore complement/co-exist with all existing access network tech. to guarantee end users improved quality and more services

16. When did it all start ?
800BC Fire beacons (ancient Greeks and Romans)
150BC Smoke signals (American Indians)
1791/ Semaphore (French)
Alexander Graham Bell demonstrated the photophone – 1st FSO (THE GENESIS) (
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

17. Some areas of use
In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in :
Multi-campus university
Hospitals
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…

18. FSO challenges Link Range Major challenges are due to the effects of:
CLOUD,
RAIN,
SMOKE, GASES,
TEMPERATURE VARIATIONS
FOG & AEROSOL
PROCESSING
SIGNAL
PHOTO
DETECTOR
DRIVER CIRCUIT
POINT A
POINT B
Link Range

19. FSO challenges Aerosols Smoke Gases CHALLENGE EFFECTS OPTIONS REMARK
GASES & SMOKE
Mie scattering
Photon absorption
Rayleigh scattering
Increase transmit
power
Diversity techniques
Effect not severe

20. FSO challenges Rain CHALLENGE EFFECTS OPTIONS REMARK RAIN
Photon absorption
Increase transmit
optical power
Effect not significant

21. FSO challenges Fog CHALLENGE EFFECTS OPTIONS REMARK FOG Mie scattering
Photon absorption
Increase transmit
power
Hybrid FSO/RF
Thick fog limits link range
to ~500m
Safety requirements limit
maximum optical power
Fog effect on performance…

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

23. FSO challenges Turbulence CHALLENGE EFFECTS OPTIONS REMARK TURBULENCE
Irradiance fluctuation
(scintillation)
Image dancing
Phase fluctuation
Beam spreading
Polarisation
fluctuation
Diversity techniques
Forward error control
Robust modulation
techniques
Adaptive optics
Significant for long
link range (>1km)
Turbulence and thick fog
do not occur together
Others:
Building sway
Background radiation
LOS requirement
Laser safety

24. Atmospheric turbulence
CAUSE:
Atmospheric random temperature variation along beam path.
The atmosphere behaves like prisms
of different sizes and refractive indices
Phase and irradiance fluctuation (fading)
Incoming optical radiation
Eddies of different sizes
and refractive indices
Depends on:
Altitude/Pressure, Wind speed,
Temperature and relative beam size.

25. Turbulence models Model Remarks Log Normal
Simple; tractable but for weak regime only K
Strong regime only
I-K
Weak to strong turbulence regime
Gamma-Gamma
All regimes
Negative Exponential
Saturation regime only

26. Turbulence models Log Normal
Simple; tractable but for weak regime only
I : Received irradiance
Io: mean irradiance without turbulence
σl2 : Log irradiance variance
(turbulence strength indicator)
Irradiance PDF :
Gamma-Gamma
All regimes
Based on modulation concept i.e.
Irradiance PDF by Andrews et al (2001):
I : Received irradiance
Ix: due to large scale effects;
obeys Gamma distribution
Iy: due to small scale effects;
Kn(.): modified Bessel function
of the 2nd kind of order n
σl2 : Log irradiance variance
(turbulence strength indicator)

27. Turbulence effect on OOK
Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data:
OOK threshold level at various turbulence levels
Increasing turbulence effect
OOK based FSO requires adaptive threshold to perform optimally….

28. Turbulence effect on OOK
No Intensity Fading
No Pulse Bit “0”
Threshold level
A/2
Pulse Bit “1” A
With Intensity Fading A
All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes

29. Subcarrier modulation
The need for adaptive threshold is circumvented through, subcarrier modulation + -
Standard RF BPSK modulator

30. Subcarrier modulation
TRANSMITTER

31. Subcarrier modulation
5-subcarriers b0
Drive current
Output
power
m(t)

32. Subcarrier modulation
RECEIVER
R = Responsivity
P = Average power
= Modulation index
m(t) = Subcarrier signal

33. Subcarrier modulation
Performs optimally without adaptive threshold as is the case with optimal
OOK
Efficient coherent modulation techniques such as PSK, QAM can be easily
used because the 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
It 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..

34. Selection Combining (Sel.C)
Spatial diversity
Diversity Combining Techniques
Maximum Ratio
Combining (MRC)
Equal Gain
Combining (EGC)
Selection Combining (Sel.C)

35. Spatial diversity
One detector
Two detectors
Three detectors
A typical reduction in intensity fluctuation with spatial diversity
Eric Korevaar et. al

36. Performance metrics
System performance analysis is carried out considering the following metrics:
1. Average Bit-Error-Rate (BER)
Models the number of bits received in error as a fraction of total transmitted bits
SNRe = BPSK subcarrier
signal-to-noise ratio
p(I) = Irradiance PDF
2. Outage Probability (Po)
Measures the probability that the instantaneous BER is greater than a pre-determined/specified threshold level
BER*: Threshold BER

37. Some results
Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels (No diversity)
Increasing the number of
Subcarrier/users, results
In increasing SNR
Gained SNR Compared
with OOK

38. Some results BPSK based subcarrier modulation is the most
BER against SNR for M-ary-PSK for log intensity variance = (No diversity)
BPSK based subcarrier
modulation is the most
power efficient

39. Some results
Outage probability against power margin for various fading strength (No diversity)
Power (dBm) needed to achieve outage probability, Po
m (dBm)

40. Some results
Spatial diversity gain with EGC against Turbulence regime

41. Some results
Spatial diversity gain (EGC and MRC) against the number of receivers.
The most diversity gain is obtained
with up to 4 photodetectors
The optimal but complex MRC diversity is marginally superior to the practical EGC
Most diversity gain region

42. Access bottleneck has been discussed
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

43. Progress chat
March 2008
On going
Completed

44. Publications

45. Practical FSO implementation and data analysis
Future work
Practical FSO implementation and data analysis
(Joint project with Newcastle University)
FSO with forward error control
SIM average power reduction

46. Appreciation
Academic Staff:
Prof. Fary Ghassemlooy ; J.I.H. Joe Allen; Dr. Erich Leitgeb; Dr.
Steven Gao; Dr. Krishna Busawon and Dr. Wai Pang.
My Colleagues:
Wisit, Ming-Feng, Maryam, Sujan , Kamal, Rupak and every
member of NCRLab

47. Research studentship Acknowledgement
I will like to acknowledge Northumbria University for the following awards:
ORSA and
Research studentship

48. Questions and Comments
Thank you.
Questions and Comments

49. FSO availability Service Availability (%) Cities Range (m) 99.5
Phoenix – atmospherically excellent
Denver – atmospherically good
Seattle – atmospherically fair
London – atmospherically poor
10,000+
2400
1200
630
99.9
5200
850
420
335
99.99
460
290
255
185
Availability ranges are based upon two 125/155 Mbit/s FSO transceivers that are located outdoors and transmitting through clear air under normal operating conditions. (Bloom, S. et al. 2003)