icton 2007, rome, italy 1 email: [email protected]@unn.ac.uk z....

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


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S PROBLEM DEFINITON / MOTIVATION

FREE-SPACE OPTICS

ATMOSPHERIC TURBULENCE

SUBCARRIER MODULATION

SPATIAL DIVERSITY

RESULTS

CONCLUSIONS


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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


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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.


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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


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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)


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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


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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


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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


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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


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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


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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


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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


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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


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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..


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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


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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.


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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


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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


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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.


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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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icton 2007, rome, italy 1 email: [email protected]@unn.ac.uk z....

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