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: wasiu.popoola@unn.ac.uk ]

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

14

Source:

Cost comparison

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

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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

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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

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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

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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

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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.

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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

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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