FREE-SPACE OPTICAL COMMUNICATION USING SUBCARRIER INTENSITY MODULATION
description
Transcript of FREE-SPACE OPTICAL COMMUNICATION USING SUBCARRIER INTENSITY MODULATION
1
FREE-SPACE OPTICAL COMMUNICATION FREE-SPACE OPTICAL COMMUNICATION USING SUBCARRIER INTENSITY USING SUBCARRIER INTENSITY
MODULATIONMODULATION
POPOOLA, Wasiu O.(2nd Year PhD student)
Optical Communication Research Lab., CEIS[email: [email protected] ]
Supervision Team:Supervision Team:Fary Ghassemlooy – Director of studiesJoseph AllenErich Leitgeb ( University of Technology, Graz, Austria.)Steven Gao (now at University of Surrey)
Research Plan (1)
TASKS YR ONE
YR TWO YR THREE
Literature search/review Background readings Random process Optical Detection Modulation Techniques
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
IPP
Performance analysis of FSO based on: o OOK o Subcarrier Modulation
Turbulence induced fading effect on performance: o Weak (Lognormal model) o Moderate(Gamma-
Gamma) o Strong (Gamma-Gamma) o Saturation (Negative exp.)
MPP
YR ONE
YR TWO YR THREE
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Research Plan (2)
Subcarrier modulated FSO with spatial diversity: o MRC o EGC o Sel.C
Experimental work/Hardware realisation: o System design o System Installation o Data acquisition and
analysis
Laser non-linearity effects of performance
Subcarrier modulated FSO with forward error control: o Turbo code o LDPC
Reduction of average power requirement for multiple SIM
Thesis : o Writing down o Writing up
Submission
VIVA
4
Problem definition
FSO Introduction
FSO challenges
Subcarrier Intensity Modulation (with and without diversity)
Results and Discussions
Summary and Future work
Outline
5
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
HBH
75% of businesses are within one mile of fibre backbone, yet only 5% have access to it - RHK
Problem Definition
6
(Source: NTT)
Access Network bottleneck
7
xDSL: 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
Access network tech.
8
THE USE OF OPTICAL RADIATIONS TO COMMUNICATE
BETWEEN TWO POINTS THROUGH
UNGUIDED CHANNELS
What is it?
Free- space optical communication
9
SPACE
WATER
ATMOSPHERE
Unguided channels
(H. Hemmati, NASA)
10
DR
IVE
R
CIR
CU
IT
POINT A POINT B
CLOUD, RAIN, SMOKE, GASES, TEMPERATURE VARIATIONS FOG & AEROSOL
SIG
NA
LP
RO
CE
SS
ING
PH
OTO
DE
TEC
TOR
Link Range
FSO basics
11
Selected FSO DetectorsSelected FSO 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.
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
FSO Optical (Laser diode) SourcesFSO Optical (Laser diode) Sources
FSO components
12
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
FSO basics
13
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)
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
FSO Features
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 ?
17
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. conferencesCellular communication back-haul
FSO challenges…FSO challenges…
Some areas of use
18
DR
IVE
R
CIR
CU
IT
POINT A POINT B
SIG
NA
LP
RO
CE
SS
ING
PH
OTO
DE
TEC
TOR
Link Range
Major challenges are due to the effects of:
CLOUD,CLOUD,
RAIN,RAIN, SMOKE, GASES,SMOKE, GASES,
TEMPERATURE VARIATIONSTEMPERATURE VARIATIONS FOG & AEROSOLFOG & AEROSOL
FSO challenges
19
Aerosols SmokeGases
AEROSOLSGASES & SMOKE
Mie scattering Photon absorption Rayleigh scattering
Increase transmit power Diversity techniques
Effect not severe
CHALLENGE EFFECTS OPTIONS REMARK
FSO challenges
20
Rain
CHALLENGE EFFECTS OPTIONS REMARK
RAIN Photon absorption
Increase transmit optical power Effect not significant
FSO challenges
21
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…Fog effect on performance…
FSO challenges
22
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
Fog effect
23
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
Turbulence
Others:
Building sway
Background radiation
LOS requirement
Laser safety
FSO challenges
24
CAUSE:CAUSE: Atmospheric random temperature variation along beam path.
Depends on:Depends on:Altitude/Pressure, Wind speed,Temperature and relative beam size.
Eddies of different sizesand refractive indices
The atmosphere behaves like prismsof different sizes and refractive indices
Phase and irradiance fluctuation (fading)
Incoming optical radiation
Atmospheric turbulence
25
Model Remarks
Log Normal Simple; tractable but for weak regime only
Gamma-Gamma All regimes
Negative Exponential Saturation regime only
I-K Weak to strong turbulence regime
K Strong regime only
Turbulence models
26
Log Normal Simple; tractable but for weak regime only
Irradiance PDF :I : Received irradiance
Io: mean irradiance without turbulence σl
2 : Log irradiance variance (turbulence strength indicator)02
220
2
)2/)/(ln(exp1
21)(
I
l
l
lI
III
Ip
Gamma-Gamma All regimes
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
Irradiance PDF by Andrews et al (2001):
I : Received irradianceIx: 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 modulation concept i.e.
Turbulence models
27
OOK threshold level at various turbulence levels
Increasing turbulence effect
OOK based FSO requires adaptive threshold to perform optimally….
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Log Intensity Standard Deviation
Thre
shol
d le
vel,
ith
Noise variance = 10-2
dI
II
IiRIi
l
l
l
rr
2
220
20
2
22
22/)/ln(
exp
.1
2
12
))((exp
))(/()(ˆ maxarg tdiPtd rd
Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data:
Turbulence effect on OOK
28
A
No Pulse Bit “0” Pulse Bit “1”
No Intensity Fading
With Intensity Fading
A
Threshold level
A/2
Turbulence effect on OOK
All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes
29
The need for adaptive threshold is circumvented through, subcarrier modulation
Subcarrier modulation
Standard RF BPSK modulator
+
-
+
-
30
TRANSMITTER
Subcarrier modulation
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
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
32
))(1( tmRPir R = ResponsivityP = Average power = Modulation indexm(t) = Subcarrier signal
RECEIVER
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
.
.
.
Subcarrier modulation
33
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..
Subcarrier modulation
34
Selection Combining (Sel.C)
ii ia Naaa ...21 ))()...(),(max()( 21 titititi NT
Maximum RatioCombining (MRC)
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
Spatial diversity
Spatial diversity
Eric Korevaar et. al
A typical reduction in intensity fluctuation with spatial diversity
One detector
Two detectors
Three detectors
36
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
dIIpSNRQBER e )()(0
SNRe = BPSK subcarrier signal-to-noise ratiop(I) = Irradiance PDF
2. Outage Probability (Po)Measures the probability that the instantaneous BER is greater than a pre-determined/specified threshold level
*)( BERBERPPo BER*: Threshold BER
Performance metrics
1 2 3 4 5 6 7 8 9 10-10
-5
0
5
10
15
20
Number of subcarrier
Norm
alis
ed S
NR @
BER
= 1
0-6 (
dB)
0.12
0.52
0.72
Log intensityvariance
37
Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels (No diversity)
Increasing the number of Subcarrier/users, resultsIn increasing SNR
Gained SNR Compared with OOK
Some results
38
20 25 30 35 4010
-10
10-8
10-6
10-4
10-2
SNR (dB)
BE
RDPSKBPSK16-PSK8-PSK
Log intensityvariance = 0.52
0
22
)()/sin(loglog
2 dIIpMMSNRQM
BER e
BPSK based subcarrier modulation is the most power efficient
BER against SNR for M-ary-PSK for log intensity variance = 0.52. (No diversity)
Some results
39
Outage probability against power margin for various fading strength (No diversity)
30 35 40 45 50 55 60 6510
-10
10-8
10-6
10-4
10-2
100
Power Margin (dBm)
Out
age
Pro
babi
lity,
P
o0.22
0.52
0.71
Log intensityVariance
2/2ln2exp 22
lloPm
Power (dBm) needed to achieve outage probability, Po
m (dBm)
Some results
40
Spatial diversity gain with EGC against Turbulence regime
10
20
30
40
50
60
70
Turbulence Regime
Dive
risty
Gai
n (d
B)
Weak
Saturation
Moderate
2 Photodetectors3 Photodetectors
Some results
1 2 3 4 5 6 7 8 9 100
5
10
15
20
25
30
No of Receivers
Spat
ial D
iver
sity
Gai
n (d
B)
MRCEGC
Log Intensity variance
1
0.52
0.22
41
Spatial diversity gain (EGC and MRC) against the number of receivers.
The most diversity gain is obtained with up to 4 photodetectors
Most diversity gain region
The optimal but complex MRC diversity is marginally superior to the practical EGC
Some results
42
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
CompletedOn going
Progress chat
Task Remark FSO with OOK
FSO employing subcarrier intensity modulation
(Turbulence induced fading strength)
No diversity MRC S. diversity
EGC S. diversity
Sel.C S. diversity
Weak (Log normal model)
Medium (Gamma-gamma model)
Strong (Gamma-gamma model)
Saturation (Negative Expo.)
Laser non-linearity effects
MPP Practical design, installation and data acquisition
March 2008
44
Publications Journal Papers
1. W.O. Popoola, Z. Ghassemlooy,: “MIMO Free-Space Optical Communication Employing Subcarrier Modulation in Clear Atmospheric Turbulence” IEEE Transaction on communications (Under review)
2. 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, (In print).
3. 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, 2006.pp. 56-61.
Conference Papers
1. W.O.Popoola and Z. Ghassemlooy,: “Performance of Subcarrier Modulated Free-Space Optical Communication Link in Negative Exponential Atmospheric Turbulence Environment”, IEEE-ICC 2008 (Under review)
2. 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, pp.
3. Z. Ghassemlooy, W.O. Popoola, and E. Leitgeb. “Free-Space optical communication using subcarrier modulation in Gamma-Gamma atmospheric turbulence” Invited paper. 9th International Conference on Transparent Optical Networks, July 1-5, 2007 - Rome, Italy, pp.
4. 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), 28th & 29th June 2007, Liverpool, UK.
5. 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.
6. 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.
7. Popoola, W. O., Ghassemlooy, Z., and Aldibbiat, N. M.: "DH-PIM employing LMSE equalisation for indoor infrared diffuse systems", 14th ICEE 2006, Tehran, Iran.
8. 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
1. 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.
45
Practical FSO implementation and data analysis (Joint project with Newcastle University)
FSO with forward error control
SIM average power reduction
Future work
46
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
Appreciation
47
I will like to acknowledge Northumbria University for the following awards:
ORSA and
Research studentship
Acknowledgement
48
Questions and Comments
Thank you.
49
ServiceAvailability (%)
Cities Range(m)
99.5
Phoenix – atmospherically excellent Denver – atmospherically good Seattle – atmospherically fair London – atmospherically poor
10,000+24001200630
99.9
Phoenix – atmospherically excellent Denver – atmospherically good Seattle – atmospherically fair London – atmospherically poor
5200850420335
99.99
Phoenix – atmospherically excellent Denver – atmospherically good Seattle – atmospherically fair London – atmospherically poor
460290255185
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)
FSO availability