INVESTIGATION ON THE PERFORMANCE AND IMPROVEMENT … · Originality Report Certificate It is certified that PhD Thesis titled Investigation on the Performance and Improvement of Free

INVESTIGATION ON THE PERFORMANCE AND

IMPROVEMENT OF FREE SPACE OPTICAL LINK IN

ATMOSPHERIC TURBULENCE

A Thesis submitted to Gujarat Technological University

for the Award of

Doctor of Philosophy

in

[Electronics and Communication]

By

[Kshatriya Anilkumar Jagdishprasadsingh]

[Enrollment No. 119997111006]

under supervision of

[Dr. A. K. Aggarwal]

GUJARAT TECHNOLOGICAL UNIVERSITY

AHMEDABAD

[November 2016]

ii

INVESTIGATION ON THE PERFORMANCE AND

IMPROVEMENT OF FREE SPACE OPTICAL LINK IN

ATMOSPHERIC TURBULENCE

A Thesis submitted to Gujarat Technological University

for the Award of

Doctor of Philosophy

in

[Electronics and Communication]

By

[Kshatriya Anilkumar Jagdishprasadsingh] [Enrollment No. 119997111006]

under supervision of

[Dr. A. K. Aggarwal]


AHMEDABAD

[November-2016]

iii

© [Kshatriya Anilkumar Jagdishprasadsingh]

iv

DECLARATION

I declare that the thesis entitled Investigation on the Performance and Improvement of

Free Space Optical Link in Atmospheric Turbulence submitted by me for the degree of

Doctor of Philosophy is the record of research work carried out by me during the period

from November 2011 to December 2015 under the supervision of Dr. A. K. Aggarwal,

Dr. Y. B. Acharya, and Dr. A. K. Majumdar (Foreign Co-supervisor) and this has not

formed the basis for the award of any degree, diploma, associateship, fellowship, titles in

this or any other University or other institution of higher learning.

I further declare that the material obtained from other sources has been duly acknowledged

in the thesis. I shall be solely responsible for any plagiarism or other irregularities, if

noticed in the thesis.

Signature of the Research Scholar: ………………………………… Date:………...………

Name of Research Scholar: Kshatriya Anilkumar Jagdishprasadsingh

Place : Ahmedabad

v

CERTIFICATE

I certify that the work incorporated in the thesis Investigation on the Performance and

Improvement of Free Space Optical Link in Atmospheric Turbulence submitted by Shri

Kshatriya Anilkumar Jagdishprasadsingh was carried out by the candidate under my

supervision/guidance. To the best of my knowledge: (i) the candidate has not submitted

the same research work to any other institution for any degree/diploma, Associateship,

Fellowship or other similar titles (ii) the thesis submitted is a record of original research

work done by the Research Scholar during the period of study under my supervision, and

(iii) the thesis represents independent research work on the part of the Research Scholar.

Signature of Supervisor: ………………………………….… Date: ……………………….

………………………………..

Name of Supervisor: (1) Dr. A. K. Aggarwal

(2) Dr. Y. B. Acharya

Place: Ahmedabad

vi

Originality Report Certificate

It is certified that PhD Thesis titled Investigation on the Performance and Improvement of

Free Space Optical Link in Atmospheric Turbulence by Kshatriya Anilkumar

Jagdishprasadsingh has been examined by us. We undertake the following:

a. Thesis has significant new work / knowledge as compared already published or are

under consideration to be published elsewhere. No sentence, equation, diagram, table,

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found within limits as per GTU Plagiarism Policy and instructions issued from time to

time (i.e. permitted similarity index <=25%).

Signature of the Research Scholar: ……………………….…… Date: …………………….

Name of Research Scholar: Kshatriya Anilkumar J.

Place: Ahmedabad

Signature of Supervisor: ……………………………… Date: ……………………..

…………………………………



Place: Ahmedabad

vii

viii

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Name of Research Scholar: Kshatriya Anilkumar J.

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Seal:

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Thesis Approval Form

The viva-voce of the PhD Thesis submitted by Shri/Smt./Kum. Kshatriya Anilkumar

Jagdishprasadsingh (Enrollment No. 119997111006 ) entitled Investigation on the

Performance and Improvement of Free Space Optical Link in Atmospheric Turbulence

was conducted on …………………….………… (day and date) at Gujarat Technological

University.

(Please tick any one of the following option)

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xi

ABSTRACT

Free space optical (FSO) communications, is a cost-effective and high bandwidth access

technique, which is receiving growing attention with recent commercialization application.

This thesis analyses the performance of FSO communication and investigates the

techniques to improve the performance of free space optics. Spatial and wavelength

diversity techniques are studied and the effect of diversity on the performance of FSO

communication link in atmospheric turbulence conditions is analyzed. Wavelength

diversity technique is used to reduce the fading under strong atmospheric turbulence

condition. Mathematical expressions for the estimation of the outage probability under

strong atmospheric turbulence conditions are derived and considerable improvement is

found using wavelength diversity.

Performance of FSO link with different wavelengths and different aperture area of optical

detector has been analyzed. Effects of different wavelengths on visibility range and quality

factor of optical receiver is simulated to find the performance of FSO link. It is concluded

that due to reduction in scattering loss at higher wavelength; as wavelength increases,

quality factor of receiver improves. Quality factor of optical receiver is also improves with

increment in aperture area of detector due to increment in sensitivity of receiver due to

large aperture area.

The link availability calculations were made based on the power budget analysis of FSO

link and on the statistical analysis of visibility data. Four different cities were selected

across different geographical conditions across the country to compare the weather

conditions and the performance of FSO link for different cities of India is calculated. The

visibility data of the cities throughout the year is found from the website Wundermap. It is

concluded that, for a given link (i.e. Transmitted power, link range, beam divergence and

transmitter-receiver aperture area) the performance of FSO link is not similar for all the

geographical area as the visibility conditions are different. It is shown that the availability

and reliability of the FSO link can be improved by making survey of the geographical area

where the link has to be established. These data are varying seasonally and with location

of the particular area. Scattering and attenuation may be caused more in low visibility

xii

condition. The mean and variance of this visibility data should be calculated to find the

average visibility at given place in different seasons of the year. Link design of the FSO

link can be made after that and better link can be proposed so the desirable availability and

BER performance can be achieved.

xiii

Acknowledgement

Eternal gratitude go to Shri Paramhans Swami Adgadanandji Maharaj and Almighty God

for all the blessings he has showered on me, which has enabled me to write this last note in

my research work. During the period of my research, as in the rest of my life, I have been

blessed by Almighty with some extraordinary people.

At first, my deepest gratitude to my supervisors, Dr. Yashwant B. Acharya Honorary

Engineer, PRL, Ahmedabad and Dr. Akshai Aggarwal, Vice Chancellor, for their

consistent guidance support, supervision and inspiration during my doctoral programme.

His invaluable suggestions and constructive criticisms from time to time enabled me to

complete my work successfully. I would also like to thank my co-supervisor Dr. Arun K.

Majumdar, senior scientist, California, USA for his continuous guidance, supervision and

motivation.

The completion of this work would not have been possible without, the Doctorate Progress

Committee (DPC) members: Dr. Dhaval A. Pujara, Prof. Nirma University, Ahmedabad

and Dr. S.C.Bera, scientist, Space Application Center (SAC), Indian Space Research

Organization (ISRO), Ahmedabad. I am really thankful for their rigorous examinations

and precious suggestions during my research.

My gratitude goes out to the assistance and support of Dr. Rajul Gajjar, Dean, PhD

Programme, Dr. N. M. Bhatt, Dean, PhD Programme, Mr. J. C. Lilani, I/C Registrar, Ms.

Mona Chaurasiya, Mr. Dhaval Gohil, and other staff members of PhD Section, GTU for

their assistance and support.

Most importantly, none of this would have been possible without the love and patience of

my parents and my family members. This dissertation is dedicated to my parents, who

have been a constant source of love, concern, support and strength all these years. I would

like to express my heart-felt gratitude to both of them. My family members has aided and

encouraged me throughout this endeavor. I would like to thank my brothers Dr. Manoj

Kshatriya and Sushil Kshatriya for his support during the research period. I would also

like to thank my wife Anjali and my beloved son Dhruv and my dear nephew Vibhu,

Shivam and Parth for their compromise of time during this study.

xiv

Further, I want to extend my special thanks to Dr. P. R. Prajapati, associate prof., ADIT,

aanand, for his help and support. Finally, I have to give a special mention for the support

given by my senior colleagues: Dr. M.R.Patel, Prof. Usha Neelakantan, Dr. R.A Thakker

and Dr. Mihir Shah. I would also like to thank who helped me directly or indirectly to

complete my work successfully.

xv

Table of Content

Chapter 1 Introduction 1

1.1 Background 1

1.2 Research Motivation and Justification 2

1.3 Research Objectives 8

1.4 Original contributions 9

1.5 Thesis organization 10

References 12

Chapter 2

Basics of Free Space Optical Communication 21

2.1 Introduction 21

2.2 Features of FSO 21

2.2.1 Optical Sources and Detectors 23

2.3 Transmission parameters 24

2.3.1 Geometrical Attenuation 24

2.3.2 Atmospheric Attenuation 25

2.4 Atmospheric Turbulence channel 30

2.4.1 Probability Density Functions (PDF) to model

atmospheric fluctuations through turbulence

33

2.4.1.1 Lognormal Distribution 33

2.4.1.2 Gamma-Gamma Distribution 34

2.4.1.3 Negative Exponential model 35

2.5 Atmospheric Conditions and mitigation Techniques 37

2.6 Communications Systems Performance 37

2.6.1 Bit Error Rate 37

2.6.2 Link margin 39

2.6.3 Outage Probability 41

2.6.4 Probability of fade 42

2.7 Summary 42

References 44

xvi

Chapter 3

Effect of Signal Wavelength and Aperture Area of Detector

on Performance of Free Space Optical Link

48

3.1 Introduction 48

3.1.1 Quality factor of a receiver 48

3.2 Simulation for FSO link 50

3.3 Results and Discussion 51

3.4 Summary 54

References 55

Chapter 4

Communication Performance of Free Space Optical Link

Using Wavelength Diversity in Strong Atmospheric

Turbulence

56

4.1 Introduction 56

4.1.1 Diversity 57

4.1.2 Diversity combining techniques 58

4.2 Performance Analysis of FSO Link Under Strong Turbulent

Atmosphere

59

4.3 Wavelength Diversity to Mitigate the Effect of Turbulence 62

4.4 Summary 66

References 68

Chapter 5 Estimation of FSO Link Availability Using Climatic Data 71

5.1 Introduction 71

5.2 Visibility Conditions of Different Cities of India 73

5.3 Transmittance 76

5.4 Power Link Margin and outage probability 79

5.5 Link Availability of FSO Link 83

5.6 FSO Link analysis based on Atmospheric conditions 87

5.6.1 Atmospheric rainfall conditions in Ahmedabad 87

xvii

5.7 Summary 90

References 93

Chapter 6 Summary of the research work and future scope of work 95

6.1 Summary of the research work 95

6.2 Recommendations for Future Work 97

References 100

List of publications 101

xviii

List of Abbreviation

ARTEMIS Advanced Relay and Technology Mission Satellite

AWGN Additive White Gaussian Noise

BER Bit Error Rate

bps Bits per second

BPSK Binary Phase Shift Keying

CDF Cumulative Distribution Function

DPSK Differential Phase Shift Keying

EGC Equal Gain Combining

FCC Federal Communications Commission

FEC Forward Error Control

FSO Free-Space Optics

GaAs Gallium Arsenide

Ge Germanium

H-V Hufnagel-Valley model of index of refraction structure parameter

IM Intensity Modulation

IM/DD Intensity Modulation/Direct Detection

LA Link Availability

LAN Local Area Network

LED Light Emitting Diode

LEO Low Earth Orbit

M(L) Link Margin

LOS Line of Sight

LTC Laser Communication Terminal

MIMO Multi-Input Multi-Output

MLCD Mars Laser Communication Demonstration

M-PPM M-ary Pulse-Position Modulation

M-PSK M-level Phase Shift Keying

MRC Maximum Ratio Combining

NRZ Non-Return-to-Zero

OBPF Optical Band Pass Filter

xix

OOK On-Off Keying

PAPR Peak-to-Average Power Ratio

pdf Probability Density Function

PFM Pulse Frequency Modulation

PIN p-type-intrinsic-n-type photodetector

POut Outage Probability

PPM Pulse Position Modulation

PSD Power Spectral Density

PRBS pseudo-Random Binary Sequence

RF Radio Frequency

S.I. Scintillation Index

SILEX Semiconductor-Laser Inter-Satellite Link Experiment

SIM Subcarrier Intensity Modulation

SNR Signal to noise Ratio

xx

List of Symbols

A Turbulence strength at the ground level

Ageo Geometrical attenuation

Aatm Atmospheric attenuation

Asys System dependent losses

Arain Specific attenuation of free space optical link due to rainfall

Asnow Specific attenuation of free space optical link due to snowfall

α Effective number of atmospheric turbulence large scale eddies

β Effective number of atmospheric turbulence small scale eddies

�̅ Average Electrical SNR

τa Atmospheric transmittance

βabs Absorption coefficient

βscat Scattering coefficient

d1 Transmit aperture diameter

d2 Receive aperture diameter

D Beam divergence

h The Altitude in m

λ Wavelength

�� Attenuation by advection fog

� �� Attenuation by radiation fog

V Visibility

LA Link Availability

l0 Inner scale of turbulence

L0 Outer scale of turbulence

P Atmospheric pressure

Te Temperature in Kelvin

Cn2 Index of refraction structure parameter

v(r) The wind velocity perpendicular to field direction of travel

L Link length (range)

σ�� Irradiance fluctuation variance

Ψx (r, L) Complex perturbations due to large-scale

xxi

Ψy (r, L) Complex perturbations due to small-scale

Г(.) Gamma function

Kα-β modified Bessel function of the second kind and order α-β

I Irradiance

I0 Mean Irradiance

xo Particle size parameter

n0 Atmospheric channel refractive index in the absence of turbulence

p(0) Probabilities of transmitting the bit “0"

p(1) Probabilities of transmitting the bit “1"

erfc(.) Complementary error function

Η Quantum efficiency of a photodetector

Pt Total power of the emitter

Q Size distribution of the scattering particles.

Sr Sensitivity of the receiver

Pout Outage Probability

γth Threshold value of SNR

Γ Instantaneous electrical SNR

I(t) Instantaneous value of normalized irradiance

IT Threshold level of specified intensity

CPI Cumulative Probability for Irradiance

pI(I) PDF of intensity fluctuation

FI(I) Cumulative Distribution Function CDF

µ Average electrical SNR

xxii

List of Figures

Figure No. Title Page No.

2.1 Block Diagram of FSO 23

2.2 Attenuation due to rain 29

2.3 Attenuation due to snowfall 30

2.4 Atmospheric channel with turbulent eddies 31

2.5 Gamma-Gamma PDF 35

2.6 α and β parameters 36

2.7 Negative exponential model 36

3.1 Optsim 5.2 Simulation link of free space communication 51

3.2 Relationship between visibility range (km) for different

wavelengths (nm)

53

3.3 Quality factor of receiver as function of input signal wavelength

for different receiver aperture area

53

3.4 Quality factor of optical receiver as function of receiver aperture

area for different atmospheric attenuation conditions

54

4.1 Negative exponential probability density function for different

values of average irradiance, I0

60

4.2 Outage probabilities versus Average electrical SNR 62

4.3 Block schematic of wavelength diversity technique 63

4.4 Outage probabilities under diversity conditions 66

5.1 Average visibility data throughout the year for different cities (low

visibility)

75

5.2 Average visibility data throughout the year for different cities

(average visibility)

75

5.3 Visibility versus no. of days for different cities 76

5.4 Transmittance as a function of visibility (km) 77

5.5 Weekly data of transmittance at wavelength 1550 nm 78



xxiii

5.8 PDF and CDF for visibility data for Delhi 81

5.9 PDF and CDF for visibility data for Kolkata 81

5.10 PDF and CDF for visibility data for Ahmedabad 82

5.11 PDF and CDF for visibility data for Thiruvananthapuram 82

5.12 Visibility versus link availability of different cities of India 84

5.13 Link distance vs link margin at wavelength 850 nm 85



5.16 Average duration of event against the rain rate in Ahmedabad 89

5.17 Received power under different attenuation conditions due to rain 89

xxiv

List of Tables

Table No. Title Page No.

2.1 Weather conditions and their visibility range values 28

2.2 Effect of weather conditions and Mitigation Techniques 37

5.1 Average visibility in km for different cities during the year 2013

(For Average visibility case)

74

5.2 Average visibility in km for different cities during the year 2013

(For low visibility case)

74

5.3 Visibility data (percentage) for different cities (Low visibility) 80

5.4 PDF and CDF values of visibility data of different cities (Low

visibility)

80

5.5 Link availability of different cities

83

1

CHAPTER – 1

Introduction

1.1 Background

Free space optical (FSO) communication is a growing technology to handle high data rate

and it has very large information handling capacity. FSO communication systems are

presented as an available alternative to the fiber optics technology which is capable of full

duplex transmission of data, voice and video in certain applications. Even though light can

be competently inserted into fiber cables to route the light information, there are various

applications where only the free space between the transmitter and receiver is the only

available means to establish a communication link. This free space technique needs only a

clear line- of- sight (LOS) path between the transmitter and the remote receiver [1-4].

Actually, the use of light is a very old technique which was used earlier for signaling

purpose. Around 800 BC, ancients Greeks and Romans were using fire beacons for the

purpose of signaling. For the similar purpose, by 150BC the American Indians were using

smoke signals. During 1790-1794, French naval navigators were using optical telegraph

which was based on a chain of semaphores. The first wireless optical communication was

experimentally tested by Graham Bell. In 1880 Alexander Graham Bell demonstrated the

“photo-phone” communication which was modulated by sunlight. The system was

designed to transmit voice signal over a distance by modulating reflected light from the

sun on a foil diaphragm. In 1960 the invention of efficient optical sources such as laser

came into the existence and the technology of FSO has changed. Television signal was

transmitted upto about 48 km distance by researchers in the MIT Lincolns Laboratory

using GaAs LED source in 1962 [5]. In 1970s, FSO was used in secure military

Chapter 1 Introduction

2

applications. Nippon Electric Company (NEC, Japan) made the first Full duplex FSO link

of 14 km distance between Yokohama and Tamagawa using He-Ne laser of 0.6328µm in

1970 [5].

The modern period of indoor FSO communication was initiated in 1979 by F. R. Gfeller

and U. Bapst by suggesting the utilization of diffuse emissions in the infrared band for

indoor communications [3, 6]. Since that time, lot of research work has been done in

characterizing indoor channels and designing the receivers and transmitter optics. National

Aeronautics and Space Administration (NASA) demonstrated the use of FSO for deep

space applications in its Mars Laser Communication Demonstration (MLCD) program [7].

European Space Agency [ESA] also carried out FSO for space communication with

Semiconductor-laser Inter-satellite Link Experiment (SILEX) [8]. Japan also made use of

the laser communication and organized flight demonstration program [9]. From 1990s to

till date the research in this field has increased substantially and commercial use of FSO

started after successful trials. During Sydney Olympic Games in 2000, the images were

transmitted between the Waterhouse Centre and the studio [10]. In Japan, JAXA's Optical

Inter-orbit Communications Engineering Test Satellite (OICETS) was launched in 2005,

and a laser communication link with advanced relay and technology mission satellite

(ARTEMIS) was successfully established [11]. A German satellite, Terra SAR-X,

containing a Laser Communication Terminal (LCT) which allows for optical

communications at data rates of up to 5.5 Gbit/s, was launched in June 2007 [12].

1.2 Research Motivation and Justification

The demand of high bandwidth is increasing day by day. Very high data rate

communication is needed which can be given by FSO and can replace RF communication

in many applications. RF can offer data rate of upto several Mbps, but there is a limitation

of spectrum congestion, interference and issues related to license. FSO is a cost-effective

as well as high bandwidth access technique, which has received increasing consideration

with recent commercialization of the application [1-2]. New techniques to reduce the

limitation of FSO need to be explored in near future to take the maximum advantage of

wireless optical link. The FSO also gives alternative of ground to satellite link, inter-


3

satellite link along with the terrestrial link. Highly accurate tracking system is however

required to track the optical beam to reduce the geometrical attenuation in FSO links

which in turn increases the link distance. Several techniques are being looked into by

various researchers to reduce the tracking problem. The tracking issue gets more severe in

case of ground to satellite links and inter-satellite link.

Free Space Optics (FSO) technology-based wireless systems are not without challenges.

These systems are susceptible to atmospheric conditions which introduce errors and can

make the system inoperable for some time-periods. The attenuation in free space optical

link is mainly due to absorption due to water vapor, scattering due to water droplets (rain,

snow, fog etc) as well as atmospheric turbulence like scintillation and beam spreading. Bit

error rate performance of a FSO link and visibility range is adversely affected due to these.

Major destruction over FSO links is the atmospheric turbulence, which results in

fluctuations of the received signal, severely degrading the performance of FSO link.

Hence, atmosphere degrades the signal in FSO link in several ways which include

absorption, scattering, and scintillation.

The other factor that incorporates to attenuation to FSO link is loss due to geometric

attenuation. FSO systems transmit highly directional and narrow beam of light that must

impose upon the receive aperture of the telescope at the receiver side of the link. The

optical source transmits one or more beams of light that typically spreads as the distance

increases due to laser/light source divergence. Geometric attenuation is due to transmit

beam spreading with increasing range [13]. As a result, the receiver telescope would not

collect the entire light beam, and some of the signal would be lost. For a FSO link to work,

it is very much essential that transmitted beam of light should be aligned with the

receiving aperture of the transceiver at both end of the link. The alignment between

transceivers gets disturbed due to a number of reasons like wind effect, and vibration due

to transmitter and receiver platforms. Due to misalignment of transmitter and receiver the

line of sight link cannot be established and the received power is heavily attenuated or

sometimes totally lost.

Several techniques are proposed by various researchers to improve the performance of

FSO link in terms of bit error rate and link availability. Several turbulence reduction

techniques are investigated to mitigate the effect of turbulence and to improve BER and


4

outage probability of FSO [1-2]. Diversity techniques, aperture averaging, forward error

correction, different modulation and coding techniques, etc. are utilized for turbulence

mitigation.

Diversity has the capability to diminish the BER degradation which is produced by

atmospheric turbulence. In RF communication, diversity technique is used to mitigate the

effect of fading on RF signals. In principle, diversity makes use of multiple copies of the

transmitted signal in an effort to defeat a poor transmission medium and enhance the

communications systems reliability performance as well as degradation. This in turn will

improve the reliability, reduces the blockage probability, and restricts the requirement for

active tracking due to laser misalignment. Space diversity [14, 15] reception technique is

used to contest the turbulence-induced fading and to reimburse for pulse broadening

caused by scattering. Scintillation can be reduced with spatial diversity technique by

means of a number of smaller apertures that are adequately far apart that each received

signal experiences independent propagation paths and due to that the intensity variations

and phase are uncorrelated for each individual propagation paths. Nick Letzepis et al.,

investigated the use of multiple lasers and multiple apertures to mitigate the effect of

scintillation. They analyzed the outage probability of the MIMO Gaussian FSO channel

for lognormal, exponential and gamma-gamma distributed scintillation [16]. Chun-yi-chen

et al., presented the channel model of a FSO link using M transmitters and N receivers and

applied Monte Carlo approach to find out the outage probability of a FSO link with spatial

diversity under different circumstances of turbulence as well as spatial correlations [17]. S.

M. Aghajanzadeh et al., investigated the performance of receive diversity in coherent FSO

systems considering both atmospheric turbulence-induced amplitude fluctuation and phase

aberration [18]. Time diversity techniques are also discussed for performance

improvement of FSO link [19, 20]. Fang Xu et al., studied the performance of channel

coding methods for various time diversity orders as well as turbulence factors [19].

Theodoros A. Tsiftsis et al., investigated the error rate capabilities of FSO techniques for

K-distributed turbulence channels and also discussed probable benefits of spatial diversity

deployments at transceivers [21]. P. Deng et al., obtained the analytic expressions of the

scintillation index for a Gaussian beam wave propagation due to non-Kolmogorov

atmospheric strong turbulence [22]. In a recent study, Tugba Ozbilgin et al., presented

MIMO optical modulation technique over atmospheric turbulence channel and reported

better efficiency compared to conventional optical modulation methods [23]. V. Xarcha et


5

al., [24] derived mathematical expressions for the estimation of BER and outage

probability using wavelength diversity for FSO systems over log normal turbulence

channels. They considered three different wavelengths 0.83 µm, 1.31µmand 1.55µm and

used wavelength diversity technique under atmospheric turbulence conditions for link

length equal to 1Km and 1.5 Km and found significant performance improvement in terms

of outage probability [24]. A similar technique is investigated in the FSO study to alleviate

the effect of fog on optical signals. Eric Wainright et al., reported increase in received

power and as a result maximum attainable distance when wavelength diversity is applied

to design a second generation FSO [25]. They considered three separate wavelengths

(0.85µm, 1.55 µm and 10 µm) and found average power reception improvement in tens of

percent compared to the use of single link [25].

Aperture averaging technique has also been suggested as a method to alleviate turbulence

and have been found effective especially in the weak to moderate turbulence regime.

Aperture averaging is broadly used in commercial FSO systems due to its simplicity and

low cost. To quantify the performance improvement by use of aperture averaging, it is

essential to have an analytical mathematical model that can accurately describe the

probability density function (PDF) of the randomly fading irradiance at a finite-size [1]. In

aperture averaging fluctuations in intensity are averaged over the receiving aperture area.

That means the larger the receiving aperture is, the more scintillation it can combat. For

smaller aperture sizes in stronger turbulence, scintillation effects can be severe. However,

increasing the size of the receiver aperture area may not be practical, so spatial diversity is

used instead of large aperture area. The summed output from this type of array of detectors

provides aperture averaging. M .A. Khalighi et al., [26] reported the impact of aperture

averaging on the performance of FSO systems under different atmospheric turbulence.

Performance evaluation is made in terms of the average bit-error-rate. Zeinab Hajjarian et

al., [27] investigated the usefulness of aperture-averaging technique by comparing the

BER performance of single-aperture receivers of different diameters with BER

performance of MIMO system with the same total transmits power and receiving area. The

BER performance of SISO and MIMO communications systems in existence of turbulence

is compared. It is shown that transmits and receives diversity helps to defeat both

amplitude and phase fluctuations. Mohammad Ali Khalighi et al., studied the aperture

averaging effect on the performance of FSO systems under different turbulence conditions

[28].


6

The turbulence effects can be minimized by utilizing other modulation schemes like

binary phase shift keying (BPSK) subcarrier intensity modulation; though it will be at the

expenses of the bandwidth and power efficiencies. Intensity Modulation (IM) using On-

Off Keying (OOK) format is the simplest and most widely accepted signaling scheme in

FSO communication systems because of the easy implementation. Several works has been

done in the adapting and developing different modulation and coding technique in FSO

communication. W. Popoola et al., [29] discussed the challenges imposed on the design

and performance of a terrestrial laser communication system. Bobby Barua et al., obtained

the BER performance of FSO communication techniques utilizing OOK and subcarrier

BPSK modulation and observed that the BER performance of communication techniques

using subcarrier BPSK modulation is better than that of appropriate techniques using OOK

modulation [30]. Hector E. Nistazakis et al., have derived mathematical expressions in

closed form for the analysis of the normal channel capability of typical optical wireless

communication methods [31]. They also investigated the impact of turbulence on the

outage capacity of such a system, for weak to strong turbulence channels, modeled by the

I-K as well as the K-distribution [32]. Eduardo et al., described the fluctuations of the

optical signal during its propagation in the atmosphere using the lognormal and gamma-

gamma distribution models and provided closed-form mathematical expressions for the

analysis of the average channel capacity of typical FSO communication methods [33].

Alwayn J. seeds et al., studied the optical fiber transmission at increased carrier

frequencies at millimeter-wave and Tera-Hertz (THz) frequencies [34]. In a recent work,

Xuegui Song et al., studied the BER performance of different subcarrier phase-shift

keying systems with carrier phase errors (CPE) in lognormal turbulence channels [35].

Nazmi et al., [36] analyzed the BER performance of FSO Link with M-ary based on Reed

Solomon code scheme and found considerable improvement in turbulence channel. Pulse

position techniques are also proposed as an alternative to OOK for FSO communications

[37- 41]. PPM is beneficial compared to OOK because dynamic thresholding is not needed

for optimal detection when hard signal detection is performed in the receiver [37-39].

PPM is particularly used in deep space communication [40- 41]. M. Faridzadeh et al., [42]

carried out the error probability analysis for all turbulence levels using hybrid

modulation technique PPM-BPSK-SIM and shown that the performance of the proposed

technique was better than BPSK-SIM. Multipulse (PPM) technique is also used in some

literatures and it is shown that it is more advantageous compared to PPM as it is having

higher spectral efficiency as well as reduced peak-to-average power ratio (PAPR) [43-44].


7

In FSO the fog is the greatest challenge because it is composed of small water drops about

the size of near infrared wavelengths. In Fog the water particles are dense enough to

diffract the light pulse and extinct the signal. The RF signals are less affected due to fog

compared to optical beam. Similarly the Optical beam is less affected (faded) by rain

compared to RF signal [45-46]. So, a hybrid technique FSO/RF is proposed in literature

that utilizes both techniques to overcome the limitations of atmospheric loss [45-48].

Further work has been done with different modulation and coding techniques in hybrid

RF/FSO systems to get better performance [49-52].

The atmospheric conditions depend upon the geographical area, season, day-night

variation in temperature (change of refractive index of the atmosphere), humidity and

altitude etc. The availability and reliability of the FSO link can be improved by making

survey of the geographical area where the link has to be established. Statistical data of the

atmospheric conditions for a particular geographical area may be collected. One method to

approach the dependence of visibility on locality is to create a geographical contour map

presenting predictable availability for a given range, or expected range at a given

availability. These data are varying daily, seasonally and with location of the particular

area. These statistical data can be analyzed to propose better link in given area. Scattering

and attenuation are severe in low visibility conditions. Link design of the FSO system can

be made using visibility data. This value is found from the calculation of mean value of

daily visibility. The mean and variance of this visibility data should be calculated to find

the average visibility at given place in different seasons of the year. With this average

visibility data, the suitable link can be designed to perform better and reduce outage

probability. Mehdi et al., [53] analyzed FSO link availability taking the weather conditions

of Algeria as a case study. They considered the atmospheric challenges in the performance

of FSO link in Algeria weather conditions and analyzed the link availability. Jassim et

al.,[54] considered the weather conditions of Iraq for FSO link analysis. They mainly

focused on the impact of aerosols on the performance of FSO link for three wavelengths

(532 nm, 1064 nm, and 10600 nm) for a link range of 1-10 Km. They concluded that

effect of dust is high in Iraq and the higher laser wavelength of 10600 nm is more

appropriate compared to other shorter wavelengths used in the present analysis.

Indoor Free Space optical system also has been used in past few years [55-59]. A TV

remote control is a simple example of that. Indoor optical wireless communication system


8

becomes popular for more complex wireless network also. For the near future indoor

optical wireless communication may find applications in multimedia services for mobile

users as well as data networking, indoor inter-device connectivity etc. [60-62].

As the demand of high bandwidth and data rate is increasing, and also the atmospheric

turbulence has a considerable impact on the quality of the optical beam propagating

through the atmosphere, new techniques to reduce the limitation of turbulence in FSO

must be researched in near future to take the maximum advantage of wireless optical link.

The FSO also gives alternative of ground to satellite link; inter-satellite link alongwith

terrestrial link [63-65]. The future trends in satellite communications are likely to make it

essential to implement very high bandwidth, data links between different satellites and

between earth station and satellite [66-69]. The large amount of data exchange is needed

between ground stations and satellites. The tracking issue gets more severe in case of

ground to satellite links and inter-satellite link. Various mitigation techniques are required

to reduce the atmospheric attenuation due to absorption, scattering and scintillation. Other

techniques like diversity (Spatial diversity, wavelength diversity, time diversity [70-71],

Angle Diversity [72-73] etc.) can be researched in various atmospheric turbulence models

to reduce the fading.

1.3 Research Objectives

The research aim of this thesis is to develop new techniques to improve the performance

of free space optics related to optical link in presence of atmospheric turbulence. There are

various parameters/phenomena effecting FSO which can be studied and analyzed to

improve the performance of FSO under turbulent atmospheric conditions. The main

parameters of FSO system design are related to optical transmitter which includes

wavelength, power, beam width, divergence and receiver parameters like aperture area,

detector and distance between them. Performance of FSO link with different wavelengths

and different aperture area of optical detector can be analyzed. Effects of different

wavelengths on visibility range and performance of optical receiver can be simulated to

find the performance of FSO link. Different diversity techniques can be explored and

analyzed to find the performance improvement of free space optical communication in

various atmospheric turbulence models. Survey of the atmospheric conditions of the

geographical location of city/village can be made to suggest the better FSO link in that

1.4 Original Contributions of This Thesis

9

area. The FSO link can be compared for different cities having different atmospheric

conditions and a better link with suitable system design can be suggested for a particular

city. The main objectives of the thesis are summarized here.

• To develop new techniques to improve the bit-error-rate (BER) performance of

FSO link in atmospheric turbulence.

• To investigate the effect of spatial and wavelength diversity on the performance of

FSO link in order to reduce channel fading and to improve the transmission quality

by improving the BER.

• To design a suitable FSO system at a particular geographical condition. In order to

accomplish this objective, the atmospheric data (visibility, average rain, fog or

snowfall etc. during the year) will be required for a city where the FSO link

performance has to be analyzed. The atmospheric conditions are changing

seasonally as well as daily according to changes in temperature during day and

night. An attempt to survey the atmospheric conditions of the geographical

location of city/village will be made to suggest the better FSO link in that area.

These statistical data can be analyzed to propose a better link for other cities.

.

1.4 Original Contributions of This Thesis

• Performance of FSO link with different wavelengths and different aperture areas of

optical detectors have been analyzed. Effects of different wavelengths on visibility

range and quality factor of optical receiver are simulated to find the performance of

FSO link. It is concluded that due to reduction in scattering loss at higher

wavelength; the performance and thus the quality factor of receiver improves.

Quality factor of optical receiver also improves with increment in aperture area of

detector due to increase in sensitivity of the receiver due to large aperture area.

• Spatial and wavelength diversity techniques were studied in atmospheric

turbulence conditions. Wavelength diversity technique is used to reduce the fading

under strong atmospheric turbulence condition. Mathematical expressions for the

estimation of the outage probability under strong atmospheric turbulence

conditions are derived and considerable improvement is found using wavelength

diversity.


10

• The link availability calculations were made based on the power budget analysis of

FSO link and on the statistical analysis of visibility data. Four different cities were

selected across different geographical conditions across the country to compare the

weather conditions and the performance of FSO link for different cities of India is

calculated. The visibility data of the cities throughout the year is found from the

website Wundermap. It is concluded that, for a given link (i.e. transmitted power,

link range, beam divergence and transmitter-receiver aperture area) the

performance of FSO link is not same for all the geographical areas since the

visibility conditions are different. It is shown that the availability and reliability of

the FSO link can be improved by making survey of the geographical area where

the link has to be established. These data are varying seasonally and with location

of the particular area. Scattering and attenuation may be caused more in low

visibility condition. The mean and variance of this visibility data is calculated to

find the average visibility at given place in different seasons of the year. Link

design of the FSO link can be made using the data and a better link can be

proposed so that the desirable availability and BER performance can be achieved.

• Results from this thesis can be used to design a FSO communication system which

can control adaptically various design parameters based on the statistical nature of

the changes of the atmospheric conditions in various places, and thus choose the

optimum link at a given time to achieve the highest communication performance.

1.5 Thesis organization

The thesis is organized in seven chapters. Following the introduction chapter, chapter two

gives the basics of Free Space Optical communication. The transmission parameters are

presented in this chapter that includes the basic link equation of FSO technique. Both of

the main attenuation parameters i.e. the geometrical and atmospheric attenuation

parameters are discussed in detail. The empirical models of typical attenuation due to rain,

snow, fogs etc. are described. The atmospheric turbulence with possible turbulence models

which are generally used to model the fluctuation of optical beam during propagation

through the atmosphere is discussed. Communication system performance is also

1.5 Thesis Organization

11

discussed in this chapter with parameters which gives figure of merit of FSO system. The

chapter concludes with turbulence mitigation techniques.

Chapter 3 presents performance of FSO link with different wavelengths and different

aperture area of optical detector. Effects of different wavelengths on visibility range and

quality factor of optical receiver have been simulated and the effect of wavelength on

scattering loss is found. Chapter 4 presents the basic introduction of diversity techniques

alongwith different diversity combining techniques. Communication performance of FSO

link using wavelength diversity technique in strong atmospheric turbulence is discussed

with detailed mathematical analysis. The outage probability using wavelength diversity

under strong turbulence conditions is estimated.

Chapter 5 presents a feasibility study of FSO link for four different cities of India

representing different topological conditions. According to average visibility conditions of

four cities (Delhi, Kolkata, Ahmedabad and Thiruvananthapuram), feasibility of the FSO

link is analyzed in terms of link availability. Finally, Chapter 6 summarizes the research

work and future scope of work.


12

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analysis in an optical ground station-satellite uplink”, Appl. Opt., Vol. 43, No. 19, pp.

3866–3827, Jul. 2004.

[70] F. Xu, M. Ali Khalighi, P. Causse and S. Bourennane, “Performance of coded time-

diversity free-space optical links”, IEEE 24th

Biennial Symp. on Comm., pp. 146–149,

2008.

[71] L. Peng, X. Wu, K. Wakamori, T. D. Pham, M. S. Alam, and M. Matsumoto, "Bit

error rate performance analysis of optical CDMA time-diversity links over gamma-gamma

atmospheric turbulence channels”, IEEE Wireless Communications and Networking

Conference (WCNC) 2011, pp. 1932-1936, 2011.

[72] R.T. Valadas, A.R. Tavares and A.M. de Olveira Duarte, “Angle diversity to combat

the ambient noise in indoor optical wireless communication systems”, International

Journal of Wireless Information Networks, Vol. 4, No. 4, pp. 275-288, 1997.

[73] J. B. Carruthers and J. M. Kahn, “Angle diversity for nondirected wireless infrared

communication”, IEEE Trans. Comm., Vol. 48, No. 6, pp. 960–969, 2000.

21

CHAPTER – 2

Basics of Free Space Optical Communication

2.1 Introduction

In this chapter, an overview of the FSO technology is presented alongwith the advantages,

limitations and applications. Different aspects of atmospheric attenuation in FSO

technology and their impacts on the system performance are discussed. The atmospheric

turbulence models are described in terms of probability density function (PDF) which

characterizes the statistical nature of the optical fluctuations of optical propagation through

atmospheric turbulence and scattering. The communication parameters like BER, Outage

probability, probability of fade and link margin are also discussed.

2.2 Features of FSO

Free Space Optics is the transfer of signals/data or information between two locations

using optical radiation as the carrier channel. The information signal could be modulated

on the intensity/phase or frequency of the optical carrier. Line of sight (LOS) without any

obstruction is essential between the transmitter and the receiver for FSO communication to

take place. FSO has tremendous advantages compared to traditional RF communication.

Very high optical bandwidth is available in FSO link compared to Radio Frequency (RF)

which allows high data rate. FSO does not require digging to lay the fiber and it does not

Chapter 2 Basics of Free Space Optical Communication

22

require permission from the landowners. Installation can be made faster. Cost is less

compared to fiber optic communication. The FSO units are portable, compact and can be

simply replaced. The interception and detection of the laser beam is difficult that makes

FSO better for the security purpose as compared to existing RF and microwave

communication. There are several applications of FSO like temporary network connection,

LAN to LAN connections in campus, secure communication in military, connectivity

solution in the area where there is less possibility to lay the fiber optic cables, etc [1-3] .

FSO can also used for satellite communication, i.e. communication between earth station

to Low Earth Orbit (LEO) and for inter-satellite link. However with so many advantages

of FSO there is a serious limitation of atmosphere like absorption, scattering and

scintillation. There are various atmospheric transmission windows in the 0.7–10µm

wavelength range, but the majority of free-space communication systems are designed to

operate in the windows of 0.78–0.85 and 1.52–1.6 µm.

FSO basic block diagram is given in figure 2.1 [4- 6]. As shown in the figure 2.1, there are

three main functional elements in FSO that are transmitter, atmospheric channel and

receiver. At the transmitter, the modulator modulates the information signal and converts

the electrical signal to optical signal using the optical source (LED or laser). The most

commonly used modulation method is Intensity Modulation [4]. The radiation of LED or

laser is aligned by telescope to a collimated beam that is propagated towards the receiver

[6].

The transmitted signal propagates through the atmosphere where it attenuates due to

absorption, scattering and turbulence. The signal attenuate due to harsh weather conditions

like rain/snow/haze/fog/turbulence. Section 2.3 and 2.4 discusses about atmospheric

attenuation in detail.

At the receiver end, the telescope receives the incoming radiated signal and directs the

signal towards optical filter. The optical filter allows passing only the wavelength of the

signal and blocks other radiations from the atmosphere.

The detector converts the optical signal back to the electrical signal which is directed to

the amplifier to amplify the signal. The receiver processing circuits include decision

device and clock recovery circuit which recovers the information at the receiver end [5-6].

2.2 Features of FSO

23

Transmitter Receiver

Atmospheric Channel

Estimated message

FIGURE 2.1: Block Diagram of FSO

2.2.1 Optical Sources and Detectors:

Within the range of 700-10,000 nm wavelengths, several atmospheric transmission

windows are available having attenuation in the range of < 0.2 dB/km [2]. FSO

communication systems are mainly designed to operate in the windows of 780-850 nm and

1520-1600 nm [2, 8]. Vertical cavity surface emitting lasers (VCSEL) are available at 850

Modulator

Laser

Driver

LED or

Laser

Telescope

Absorption

Scattering

Turbulence

Telescope

Filter

Photodetector

Amplifier

Decision

device

Clock

Recovery

Circuit


24

nm wavelength and a highly sensitive silicon Avalanche photodiode (APD) are also

available at the same wavelength [8]. Silicon APDs are generally used at 850 nm range as

Si-APDs are more sensitive, due to an internal amplification (avalanche) process [8].

These lasers are cheap and having low power density. They are reliable upto 10 Gbps. In

the wavelength range of 1300 nm to 1550 nm, the lasers like Febry Perot (FP) lasers,

Distributed feedback (DFB) lasers are available which has higher power density upto 100

mW/cm2. The speed upto 40 Gbps can be achieved with these lasers. The 1550 nm band is

attractive because of reduced solar background/ scattering and its compatibility with

wavelength division multiplexing networks. Also about 50–65 times higher power can be

transmitted at longer wavelength (1520–1600 nm) compared to 780–850 nm as longer

wavelength are safe for human eye [8]. This is one of the advantages of using longer

wavelength. At this longer wavelength range, InGaAs is the most commonly used detector

material that is used by maximum fiber optic system [8]. At a wavelength about 10 µm,

Quantum cascade lasers and quantum well/quantum dot detectors are available but they

are comparatively new and expensive. Detector used in this range is mercury cadmium

telluride (HgCdTe). These higher wavelengths are better for transmission in fog as longer

wavelengths are less affected by fog. At near infrared range, Light Emitting diode (LEDs)

are available which are noncoherent source of light. They are cheaper and require simpler

driver circuit. The data rates available are comparatively less than 200 Mbps.

2.3 Transmission parameters

2.3.1 Geometrical Attenuation:

The performance of the FSO link is affected mainly by two types of attenuation. One is the

geometrical and second is the atmospheric attenuation [1, 7]. The optical beam should be

collected at the receiver and loss may occur because of misalignment of transmitter and

receiver that create geometrical attenuation in the link. Geometric losses arise because of

the dispersing of the transmitted optical beam between the transmitter and the receiver and

due to the pointing and tracking errors at the receiver. One amongst the significant

challenges with FSO technique is to maintain alignment between transmitter and receiver

2.3 Transmission Parameters

25

which get disturbed because of variety of factors like wind effect, vibration, etc. FSO

transmitters transmits highly directional as well as narrow beams of light that need to be

imposed on the receivers aperture area at the receiver side of the communication link. The

optical transmitter transmits beams of light that normally spreads as the distance increases.

For a FSO link to perform, it is essential that the transmitted beam of light should be

aligned with the receiving aperture of the transceiver at both end of the link. Usually, the

beam broadens to a size bigger than the aperture of the receiver, and received power

reduces. Geometric Losses can be given by the following equation as [8].

GeometricLoss�dB� = 10 log � ��∗�� (2.1)

where, d1 = aperture diameter of transmitter (m), d2 = aperture diameter of receiver (m), D

= beam divergence (mrad), L = range (km).

2.3.2 Atmospheric Attenuation:

The second type of attenuation is because of the weather which is one of the main

challenges of this technique. Due to the bad weather like rainy condition, foggy weather

condition, snowfall condition, etc., the link of communication is affected. This limitation

makes FSO suitable only for short distance communication.

Fog, clouds, snowfall, etc. plays a determinable part by attenuating light signal

propagating in free space. Due to that the link availability and reliability reduces and it

affects the performance of communication link. Absorption occurs during the interaction

between the photons propagating to the atmospheric molecules along its propagation path

[9]. Absorption is wavelength dependent and wavelengths chosen are such that they have

minimum absorption for the molecules and species at transmitting wavelength. It depends

on water vapor of the atmospheric channel, which in turn depends on humidity and

altitude. Atmospheric scattering occurs due to interaction of an element of the light with

the atoms and the molecules present in the transmission media [10]. It creates an angular

redeployment of the component of the radiance with or maybe without alteration of the


26

wavelength. The atmospheric transmission of optical signals, τa is expressed by the

following Beers law equation [10]

�� = !�"#$%�"%&#'�( (2.2)

where βabs and βscat are the absorption and scattering coefficients, respectively and R is

the atmospheric path length. The attenuation coefficient τa is the sum of the absorption and

the scattering coefficients from aerosols and molecular constituents of the atmosphere [11,

12].

The scattering impact is dependent upon the characteristic specifications parameter(x0),

such that x0 = 2πr / λ, where r is the particle size of the aerosol experienced throughout

propagation and λ is the wavelength of the optical signal [11]. If x0<< 1,the scattering

process is referred to as Rayleigh scattering, if x0 ≈ 1, then it is referred to as Mie

scattering and for x0>> 1, the scattering process is to be explained employing geometrical

scattering concept [11-14]. The Mie scattering arises if the particle size is equivalent to the

size of beam. Signal attenuation due to above factors is given by several models. The most

widely used model is Kruse and Kim [9, 11, 15] and according to that the attenuation

coefficient is approximated by the following relation:

)�*� = +,../0 1 � 2334�!5 (2.3)

“where, V= visibility in kilometers, λ= wavelength in nanometers and q= the size

distribution of the scattering particles”. According to the Kim model, q is taken as 1.6 for

V >50 km, 1.3 for 6 km< V < 50 km, 0.16V+ 0.34 for 1 km< V < 6 km, V- 0.5 for 0.5

km< V <1kmand 0 for V < 0.5 km. According to the Kruse model, q is taken as, 1.6 for V

> 50, 1.3 for 3< V < 50 and 0.585V1.3

for V<6km.

In fog the water elements are often dense enough to diffract the light pulse as well as

extinct the signal. It is difficult to describe foggy condition so it is described by physical

means. For example, dense fog or thin fog is generally used to give an explanation of the

characteristic of fog. According to that the attenuation due to this is also moderate in


27

nature and varies as dense or thin fog condition. The optical beam propagating from

through the can be absorbed and scattered depending on the atmospheric condition. The

particle size of fog is equivalent with the infrared wavelengths generally used in FSO

hence causing fog an important factor for attenuation of optical power. Fog attenuation is

inversely proportional to wavelength. There are two types of fog that are radiation fog and

advection fog [13]. Radiation fog is created while the temperature falls close to the dew

point, creating the water vapor in the environment to condense and obstruct visibility and

other is the advection fog that is created by combining pockets in the environment

containing distinct temperatures and/or densities [13]. Advection fog is created when hot,

wet air passes over a cooler surface. The air connected with the surface area is cooled

below its dew point, creating the condensation of water vapor [14]. The attenuation by

advection fog is given by the following relation [14-16].

σ6�789:;<= = 4.//>?@λ�,.@,A?B (2.4)

“where, V is the visibility in km, and λ is the wavelength in nm”. Table 2.1 shows the

weather conditions with their typical values of visibility [9]. Radiation fog appears when

the air is adequately cool and gets saturated. This fog normally arises during the night and

at the end of the day when meteorological conditions are favorable. The attenuation by

radiation fog is given by [14, 15, 16].

σC6�;6:;<= = 4./@/DAλ��4./,?4.λ�,.?34DB (2.5)

Where, V= visibility in kilometers, λ = wavelength in micrometers. However, the particles

encountered in the atmosphere have complex shapes and orientations. The visibility range

values under different weather conditions are given in table 2.1.

During rainfall, the water particles of rain cause distortion in the FSO link and the

attenuation caused by it is variable in nature i.e. the attenuation is increasing with the

increase in rainfall rate and vice versa. For example, for a rainfall rate of about 2.5

cm/hour, attenuation of approximately 6 dB/km can be observed and will increase if rate

of rain fall increases.


28

TABLE 2.1: Weather conditions and their visibility range values [9].

Sr. No. Weather condition Visibility range (m)

1 Very clear 23000 - 50000

2 Clear/drizzle 18000-20000

3 Haze/medium rain 2800 - 4000

4 Thin fog/heavy rain 1900 - 2000

5 Light Fog 770 - 1000

6 Moderate fog 500

7 Thick fog 200

Generally, the impact of rain is less in optical signal because the radius of raindrop (200

µm to 2000 µm) is much larger than the optical signal wavelength used in FSO

communication. As an example, a standard value of certain attenuations caused by rain is

between 20-30 dB/km for a rainfall rate of 150 mm/h, while specific attenuation due to

falling snowfall can attain as much as 68 dB/km [9]. Rain causes wavelength independent

scattering when the water particles of rain become comparable in size to cause refraction

or reflection of optical signal. The specific attenuation of free space optical link in dB/km

due to rainfall rate of R mm/hr is given by [15, 17, 18, 19],

Arain= 1.076 R0.67

(2.6)

The attenuation due to rainfall rate is shown in figure 2.2.

The attenuation due to snowfall falls approximately between light rains to moderate fog,

with link attenuation of approximately 3 dB/km to 30 dB/km. The relation between

snowfall rate and attenuation is given by the following equation [14-15]

Asnow [dB/km] = aSb

(2.7)

where S is the snowfall rate in mm/h, λ is wavelength in nm and a, b are constant given

by following relation [14]. For wet snow (altitude < 500 m):


29

a = 0.0001023 λ + 3.78554766 and b = 0.72 (2.8)

For dry snow (altitude > or 500 m) :

a = 0.0000542 λ + 5.4958776 and b = 1.38: (2.9)

Equations (2.7 to 2.9) are plotted in figure 2.3 for wavelength of 850 nm, which shows

snowfall rate versus attenuation. So, heavy rain and snowfall decreases the availability and

reliability of FSO link. The atmospheric conditions should be observed before designing

FIGURE 2.2: Attenuation due to rain

the link. W. Popoola et al., explained the issues imposed on the design as well as

functionality of a terrestrial laser communication system. The presence of matter (gases,

suspended particles, aerosols, fog, rain and haze) along the propagation path extinguishes

and redirects the traversing photons [9].

The FSO link performance can also be improved by reducing the effect of

fog/aerosols/rain by sending same signal at different wave length i.e., wavelength diversity

technique. Eric Wainright [13] reported increase in received power level and hence

maximum attainable transmission range using wavelength diversity. Naimullah et al.,

0 20 40 60 80 100 120 140 1600

5

10

15

20

25

30

35

Rainfall rate,mm/h

Att

enuation (

dB

)


30

presented wavelength selection criteria before installation of FSO link to attain better

system performance [20].

FIGURE 2.3: Attenuation due to snowfall.

2.4 Atmospheric Turbulence Channel:

Atmospheric turbulence is induced due to the fluctuation of refractive index of the air. In a

sunny day, the air near the earth surface area gets warmer than the air at the higher altitude

because of solar radiance. This layer of warm air gets to be much less dense and then goes

up from the earth to combine with the surrounding cooler air and due to that the air

temperature fluctuate randomly. Random variations of the atmospheric refractive index n

arise along the propagation path of the optical ray caused by atmospheric turbulence. This

fluctuation of refractive index is resulting from the random changes in atmospheric

temperature which is a function of the atmospheric air pressure, elevation, and time of the

day and also wind flow speed. The environment includes several sizes of cell as shown in

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

100

120

140

Snowfall rate, mm/h

Att

en

ua

tio

n (

dB

\km

)

dry snow

wet snow

2.4 Atmospheric Turbulence Channel

31

figure 2.4. The tiniest of the turbulence eddies are referred to as the interior size l0, while

the largest of the turbulence eddies are referred to as the outer size L0 of turbulence. l0 is

usually on the order of just a few millimeters, whereas L0 is normally on the order of a few

meters [2, 21]. The relation between the atmospheric temperature and its refractive index

is given by [2]

E = 1 + 77.6�1 + 7.52K10!,*!D� +LMN1K10!A (2.10)

where, P is the atmospheric pressure in millibars, Te is the temperature in Kelvin and λ is

the wavelength in microns. For the majority of engineering applications, the rate of change

of the refractive index with reference to channel temperature is characterized by equation

[2.2]

Input wave Velocity Output wave

Lo

lo Atmospheric channel

FIGURE 2.4 Atmospheric channel with turbulent eddies

!�=�OP = 7.8X10!3 SO8� (2.11)


32

The location and time-based index of refraction indicated by n(r; t). It could be stated as

the summation of, no (typical index value), and a turbulence-caused random fluctuation

factor n1(r; t) which is induced by spatial variation of temperature and pressure of the

atmosphere [2, 21].

n(r, t) = n0 + n1(r, t) (2.12)

Corresponding to the Taylors frozen-flow hypothesis, which suggests that the temporal

fluctuations of the refraction index of the media (atmosphere) are primarily because of the

transverse element of the wind, the randomly varying portion of equation (2.12) may then

be presented as [2, 21]

n1(r, t) = n1(r - vt) (2.13)

“where, v(r) is the wind velocity perpendicular to field direction of travel” [21, 22].

Refractive index structure parameter Cn2 is the main considerable parameter that defines

the turbulence. It depends on time, altitude and location of the day and also on the season.

The temperature gradient tends to decrease with increase in the altitude that result in

decrease in density of air and due to that the value of Cn2

becomes smaller. For a weak

turbulence at ground level the typical value for Cn2 is 10

-17 m

-2/3, whereas for strong

turbulence it could be up to 10-13

m-2/3

or larger. Several parametric models have been

established to express Cn2, among them Hufnagel- Valley (HV) model is one of the most

used model and it is given by [23, 24]

“where, h is the Altitude in m, v is the wind speed in m/s and A is the turbulence strength

at the ground level”, A= 10-14

m-2/3

. An identical parameter for temperature fluctuations is

the temperature structure parameter which is written by [2]

C=D =+ �=�OP1D COD (2.15)

)14.2(100

exp1500

exp107.21000

exp)10(27

00594.0)( 16105

2

2

−+

−×+

−

= −− h

Ahh

hhCn

ν


33

The power spectral density of the refractive index fluctuation is related to Cn2 by [2, 18,

23, 25]

∅=�K� = 0.033C=DK!///, 2π/L0<< K<< 2π/l0 (2.16)

The wind and altitude are the most important variable in the model. The scintillation is

described by a log-intensity distribution with a variance given by [2, 21, 23, 25]

.

Y(D = kC=D +Dπλ 1?/A L///A (2.17)

where, k is the constant, and it is 1.23 for the plane wave and 0.5 for the spherical wave, λ

is wavelength in nm, L is the link length in meter. Scintillation results due to atmospheric

turbulence and the attenuation due to this are unpredictable. Wind and temperature

gradients generates air pockets with quickly varying indices of optical refraction. Along

with scintillation, beam wander and widening also arise when optical signals travel in the

turbulent atmosphere and as a result of that spatial and temporal variations occur when

optical ray moving through the turbulence [8]. The scintillation is maximum during

midday when the temperature is maximum.

2.4.1 Probability Density Functions (PDF) to model atmospheric fluctuations

through turbulence

2.4.1.1 Lognormal Distribution

The log-normal models consider the log intensity I of the laser light travelling in the

turbulent atmosphere to be normally distributed with a mean value of -σI2/2. Thus the PDF

of the received irradiance is specified by [2, 21]

f�I� = /�Dπσ]� ��^ exp a−

�c=�^/^d��σ]� D�e �Dσ]� f , I ≥ 0 (2.18)

Where, I is the irradiance at the receiver, Io is the irradiance of the signal in the absence of

scintillation and σgD as given in equation (2.17). The log-normal channel is categorized as

weak turbulence that could be described by a scintillation index less than 0.75.


34

2.4.1.2 Gamma-Gamma Distribution

Al-Habash et al., suggested a statistical model that factorizes the irradiance as the product

of two independent random functions each with a Gamma PDF. Andrews et.al., suggested

the modified Rytov theory , which describe the optical field as

U(r,L)=U0(r,L) exp[Ψx(r,L)+ Ψy(r,L)] (2.19)

“where Ψx (r, L) and Ψy (r, L) are statistically independent complex perturbations which

are due only to large-scale and small-scale atmospheric effects, respectively” [26-28]. The

received irradiance is now defined as the product of two statistically independent random

processes Ix and Iy.

I = IxIy (2.20)

where Ix occur from large scale eddies and Iy arise from small scale eddies. Both the small

scale and large scale fluctuations follow gamma distribution [26-27]. Gamma pdf is used

to model small scale as well as large scale fluctuation that lead to the gamma-gamma pdf.

The PDF of the intensity fluctuation using gamma-gamma pdf model is written by [25,

29].

f�I� = D�αβ��αhβ� �⁄Г�α�Г�β� j�αhβ�� !/K�α!β�k2lαβjo,j > 0 (2.21)

“Where I is the signal intensity, α and β are parameters of the PDF, Г(.) is the gamma

function, and Kα-β is the modified Bessel function of the second kind and order α-β. Here,

α and β are the effective number of small- scale and large scale eddies of the scattering

environment” [25, 29]. These parameters can be related directly with atmospheric

turbulence as [25, 29].

r = /stuv d.wxyz��h�.��yz�� {⁄ �| }⁄ ~!/ (2.22)

) = /stuv d.{�yz��hd.}xyz�� {⁄ �{ }⁄ ~!/ (2.23)


35

Figure 2.5 and 2.6 represents gamma-gamma pdf and parameters α, β as a function of

irradiance respectively.

FIGURE 2.5: Gamma-Gamma PDF

2.4.1.3 Negative Exponential Model

In case of very strong turbulent conditions, and as the link length increases, the irradiance

fluctuation becomes strong as the scattering of independent particles becomes large. The

fluctuation of the optical beam traversing in such a condition is normally indicating a

negative exponential statistics for the irradiance. That is [29].

f(I)= 1/I0 exp(-I/I0) , I ≥ 0 (2.24)

Where, I0 indicates the mean received irradiance and I is the irradiance. Figure 2.7 shows

the probability density function (pdf) of the negative exponential channel mode for

different values of I0.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Irradiance I

Gam

ma-G

am

ma p

df,

p(I

)


36

FIGURE 2.6: α and β parameters

FIGURE 2.7: Negative exponential model

10-2

10-1

100

101

102

0

5

10

15

20

25

30

35

40

45

log intensity variance σI2

para

mete

rs :

α. β

α

β

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Irradiance,I

Ne

ga

tiv

e e

xp

on

en

tia

l p

df:

p(I

)

I 0 = 2

I 0 = 1

I 0 = 0.5

2.5 Atmospheric Conditions and Mitigation Techniques

37

2.5 Atmospheric Conditions and Mitigation Techniques

We have seen that the atmospheric conditions creates disturbance in error free

communication of high data rate FSO communication. Rain, haze, snowfall, fog,

turbulence induced scintillations, etc. creates great challenge and limits the performance.

Several turbulence mitigation techniques are found by various researchers to mitigate the

effect of turbulence and to improve BER and outage probability of FSO. Diversity

techniques, aperture averaging [30], forward error correction, different modulation and

coding techniques, etc. are utilized for turbulence mitigation. The following table gives the

summary of atmospheric effects and remedies of them [2, 5-6, 8-20, 23-25].

TABLE 2.2: Effect of Weather Conditions and Mitigation Techniques

Sr.

No.

Weather

condition

Effect Mitigation Techniques

1 Aerosols, smoke

and gases etc.

Scattering of light

(Mie/Rayleigh

Scattering)

• Increase transmitter power

• Different diversity

techniques

2 Rain, Snowfall ,

Haze, etc.

Photon absorption • Increase transmitter power

3 Fog (Thin fog,

Tick fog)

Scattering of light • Use longer Wavelength laser

• Hybrid FSO/RF

• Increase transmitter power

4 Atmospheric

Turbulence

Irradiance Fluctuation,

Phase fluctuation,

Beam spreading, Beam

Broadening

• Diversity techniques

• Different Modulation and

coding techniques

• Forward error correction

2.6 Communication System Performance

2.6.1 Bit Error Rate

The capability to predict performance of any system is important for the design of a

practical and most favorable system. The FSO must be competent to set up a

communication link with a particular data rate and error probability lower than the


38

acceptable BER. The communication system performance needs to be measured for FSO

Communication. One of the standard evaluation techniques for FSO system performance

is the available signal-to-noise ratio (SNR) available at the receiver [2]. One of the

important concerns in the design of a communication system is the system performance in

the presence of noise. In case of FSO, generally data is transmitted in digital form and the

actual performance evaluation in digital communications is not given directly by the SNR,

but it is given in terms of probability of error, also given as bit error rate (BER) [2]. The

BER is the probability that an error may occur in digital data transmission, i.e., when 1 bit

is received as 0 or vice versa. Generally, the BER is given by [2].

�� = LdD + L�D (2.25)

where P0 is the probability of receiving (by mistaken) “0” for “1”, and P1 is the probability

of receiving (by mistake) ‘1’ for ‘0’. The 1/2 multiplicative factor comes because, in a

general digital communication system, ‘0’and ‘1’ are equally likely [2]. The most

commonly used modulation technique for FSO is intensity modulation/direct detection

(IM/DD) and on-off keying (OOK) modulation. The BER of FSO systems with IM/DD

and on-off keying (OOK) modulation, in the presence of AWGN can be estimated through

the following expression [31, 32]

P = p(0)P(e\0) + p(1)P(e\1) (2.26)

with p(0) and p(1) being the probabilities of transmitting the bit ‘0’ and ‘1’, respectively

whereas P(e/0) and P(e/1) represent the conditional bit-error probabilities for the

transmitted bit [2, 25, 26]. Considering p(0) = p(1) = 0.5, and P(e/0) = P(e/1), the BER is

predicted as a function of I, as: [2, 31, 32]

P(I) = P(e/0, I) + P(e/1, I) = P(n > ηI/2) = P(n < ηI/2) =/D �� η�Dl�d� (2.27)

where erfc(.) in the above equation is the complementary error function. The performance

of the FSO system can be calculated by computing the BER of the system which depends

on the modulation format as well as on the signal-to noise ratio (SNR). Assuming


39

Gaussian distribution for signal and noise power, the BER in the absence of turbulence is

given by [2]

BER4 = /D +��gdD√D 1 (2.28)

where SNR0 is the signal to noise ratio in absence of turbulence. In the presence of

atmospheric turbulence, the BER is considered a conditional probability that required to be

averaged over PDF of the fluctuating signal at the receiver due to turbulence, to find out

the unconditional BER. For the case of OOK for pulsed modulation in binary detection,

the BER is given by [2]

�� = /D� �� + ��(�D√D��%�1∞4 (2.29)

“where < SNR > is the mean SNR in the presence of atmospheric turbulence, < is> is the

mean output signal current and pI(s) is the PDF of the received fluctuating signal at the

receiver that depends on the level of turbulence strength and the propagation path” [2].

2.6.2 Link Margin

The performance of a FSO system is also computed by the “link margin”, that is the ratio

of the received signal power to the signal power required to achieve a given data rate with

a specified acceptable BER. Generally, the power is calculated in dB. The link margin

computation is thus necessary to design a suitable FSO system. Atmospheric attenuation

can influence system performance and thus required to be calculated before designing the

link. A model for link budget should be developed which include atmospheric channel

attenuation through which the signal passes through. This model aids engineers to design

optical system main parameters so that adequate performance can be achieved by the

system [2].

The link availability of FSO system can be found by link power analysis. The link margin

depends on several parameters like the transmitter power, sensitivity of the receiver,

geometrical attenuation, atmospheric attenuation and other system dependent losses.


40

By considering a laser transmitter antenna with gain GT transmitting a total power PT at

the given wavelength, the received signal power received at the optical detector can be

expressed as [2]

Received Signal �(s� =�M�M�M��(�( (2.30)

“where τT is the transmitter optical efficiency, τATM is the value of the atmospheric

transmission at the laser transmitter wavelength, S is the free-space loss, GR is the receiver

antenna gain, and τR is the receiver optical efficiency”[2].

The transmitter gain is given by [2]

�M = /A�� (2.31)

Where θT is the transmitting divergence angle. The free space loss is given by [2]

� = + 2>��1D (2.32)

Where L is the range and λ is the wavelength. The receiver gain is given by [2]

�( = +��2 1D (2.33)

Where D is the receiver diameter. τATM can be given by [2]

� M¡ = 10�!¢�//4� (2.34)

Where α is attenuation in dB/km. The received signal can be expressed as [2]

�(s� =�M + ��1 �M10�!¢�//4��( (2.35)


41

The "required" power at the receiver PREQ (watts) to achieve a data rate, R (bits/sec), and

receiver sensitivity, Nb (photons/bit), is correlated by [2]

PREQ = NbRhv = NbRhc/λ, (2.36)

“where v is the frequency of the laser light of wavelength (h = Planck 's constant, c =

velocity of light”. [2]

Finally, we can define the link margin M as[ 2.2]

M = Received power/ required power

M = + L��$(£¤1 + ��1 �M10�!¢�//4��( (2.37)

For a laser transmitter power Pt with transmitter divergence of θt, Link range L, area of

receiver telescope A, transmit and receive optical efficiency τopt, the attainable data rate R

can be found from [2]

R = S¥¦§¨¥¦©ª«¬+®¥� 1��¯¨�°

(2.38)

“Where Ep = hc/λ is the photon energy and Nb is the receiver sensitivity in photons/bit.

τATM is the value of the atmospheric transmission at the laser transmitter wavelength”.

2.6.3 Outage Probability

Outage probability is a major performance parameter for any wireless communication link

that describes the probability of unavailability of the communication link. It represents the

probability of decreasing the instantaneous SNR at the input of the receiver below the

threshold [2, 25]. The receiver’s threshold depends on the sensitivity limit of the detector.

The outage probability is given by [25]

Pout = P(γ≤γth) (2.39)


42

Where γ is the instantaneous electrical SNR and γth is the threshold value of SNR. Outage

probability actually shows the probability of unavailability of the link. The link becomes

unavailable when the minimum required instantaneous signal not reaches to the detector.

Outage probability and BER both are important parameter to estimate the communication

performance of FSO systems.

2.6.4 Probability of fade

The aim of designing any wireless communication system is to achieve continuous

exchange of data without interruption. The randomly varying channel due to turbulent

atmosphere creates fluctuation of the received signal and there is a possibility of received

signal to fall below the sensitivity of the detector. The reliability of any wireless

communication link depends on the fading probability [2]. The fading probability of FSO

link can be found from the statistical data of intensity fluctuations and the scintillation

index [2]. If I(t) is denoted as the instantaneous value of normalized irradiance, the

fraction of time I(t) ≥ IT need to be obtained where IT is the threshold level of specified

intensity. Assuming the statistics of optical propagation as ergodic process where time

averages are equal to ensemble averages, the fraction of time I(t) ≤ IT can be given as [2].

±�²�³´µE�j ≤ jM� = ·�^�j ≤ jM� = /D� ��4 �j�¸j (2.40)

where CPI is the cumulative probability for irradiance and pI(I) is the PDF of intensity

fluctuation.

2.7 Summary

A review of the FSO technology has been described in this chapter. The basic block

diagram of FSO communications system and the transmission parameters are discussed

with detail analysis and mathematical expressions. Brief discussion about sources and

detectors used in FSO communication are presented. The atmospheric conditions which

create absorption and scattering to the photons during propagation of light are discussed in

2.7 Summary

43

detail with empirical formulas for attenuation due to rain, snowfall and fog. Visibility

ranges according to different weather conditions are listed and different mitigation

techniques are listed to improve the performance of the system. Different atmospheric

turbulence models and the communication performance parameters of FSO technology are

also discussed in this chapter. Finally the communication system parameters like BER,

probability of fade, outage probability, link margin etc., are discussed in detail.


44

References:

[1] Hennes Henniger and Otakar Wilfert, “An introduction to free-space optical

communications”, Journal of Radio Engineering, Vol. 19, No. 2, 2010.

[2] Arun K. Majumdar and Jennifier C. Ricklin, “Free space Laser Communications:

Principles and advances” , Springer, ISBN-13: 978-0-387-28652-5,2008.

[3] Steve Hranilovic, “Wireless Optical Communication Systems”, Springer, eBook ISBN:

0-387-22785-7, 2005.

[4] Z. Ghassemlooy and W. O. Popoola, Terrestrial Free-Space Optical Communications,

ch. 17, pp. 356–392., InTech, 2010.

[5] Mohammad Ali Khalighi and Murat Uysal, “Survey on Free Space Optical

Communication: A Communication Theory Perspective”, IEEE Communication Surveys

& Tutorials, Vol. 16, No. 4 , pp 2231-2258, 2014.

[6] Haim Manor and Shlomi Arnon, “Performance of an optical wireless communication

system as a function of wavelength”, Applied Optics, Vol. 42, No. 21, pp. 4285-4294, July

2003.

[7] G. Hansel and E. Kube, “Simulation in the Design Process of Free Space Optical

Transmission Systems”, Proc. 6th

Workshop, Optics in Computing Technology, Paderborn

(Germany), pp. 45-53, 2003.



pp. 178-200, June 2003.

[9] W. Popoola, Z. Ghassemlooy, M. S. Awan, and E. Leitgeb Piteti, “Atmospheric



Edition, pp. 17-23, 2009.

References

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[10] Maha Achour, S. Hwy and S. Beach, “Free-Space Optics Wavelength Selection: 10µ

Versus Shorter Wavelengths”, UlmTech, Inc., pp. 1-15.

[11] Muhammad Saleem Awan, Laszlo Csurgai Horwath, Sajid Sheikh Muhammad, Erich

Leitgeb, Farukh Nadeem and Muhammad Saeed Khan, “Characterization of Fog and

Snow Attenuations for Free-Space Optical Propagation”, Journal of Communication, Vol.

4, No. 8, pp. 533-545, 2009.

[12] Harilaos G. Sandalidis, Theodoros A. Tsiftsis, George K. Karagiannidis and Murat

Uysal,“BER Performance of FSO Links over Strong Atmospheric Turbulence Channels

with Pointing Errors”, IEEE Communication letters, Vol.12, No. 1, pp. 44-46, 2008.

[13] Eric Wainright, Hazem H. Refai and James J. Sluss, “Wavelength Diversity in Free-

Space Optics to Alleviate Fog Effects”, Free-Space Laser Communication Technologies

XVII, edited by G. Stephen Mecherle, Proceedings of SPIE, Vol. 5712, pp.110-118, 2005.

[14] Mehdi ROUISSAT, A. Riad BORSALI and Mohammad E. CHIKH-BLED, “Free

Space Optical Channel Characterization and Modeling with Focus on Algeria Weather

Conditions”, I. J. Computer Network and Information Security, Vol.4, No. 3, pp. 17-23,

April 2012.

[15] Al Naboulsi, M., Sizun H. and de Fornel F., “Propagation of optical and infrared

waves in the atmosphere.” http://www.ursi.org/proceedings/procga05/pdf/F01P.7

(01729).pdf

[16] M. Al Naboulsi, H. Sizun and F. de Fornel, “Fog Attenuation Prediction for Optical

and Infrared Waves”, Optical Engineering, Vol.43, No.2, pp.319-329, February 2004.

[17] S. Sheikh Muhammad, P. Khldorfer and E. Leitgeb, “Channel Modeling for

Terrestrial Free Space Optical Links”, ICTON, pp. 407-410, 2005.

[18] T.H. Carbonneau and D.R. Wisley, “Opportunities and Challenges for optical

wireless; the competitive advantage of free space telecommunications links in todays


46

crowded market place”, SPIE Conference on Optical Wireless Communications,

Massachusetts, 1998.

[19] Ivan B. Djordjevicr, Stojan Denic, Member, Jaime Anguita, Bane Vasic and Mark A.

Neifeld, “LDPC-Coded MIMO Optical Communication Over the Atmospheric Turbulence

Channel”, Journal of Lightwave Technology, Vol. 26, No. 5, pp. 478-487, 2008.

[20] B. S. Naimullah, M. Othman, A. K. Rahman , S. I. Sulaiman, S. Ishak ,S. Hitam and

S. A. Aljunid, “Comparison of Wavelength Propagation for Free Space Optical

Communications”, International Conference on Electronic Design, Penang, Malaysia,

December 1-3, 2008.

[21] Xiaoming Zhu and Joseph M. Kahn, “Free-space optical communication through

atmospheric turbulence channels”, IEEE Transactions on Communications, Vol. 50,

No.8, pp. 1293-1300, August 2002.

[22] Ali Abdul Hussein, Anand Oka, Trung Thanh Nguyenand and Lutz Lampe, “Rateless

Coding for Hybrid Free-Space Optical and Radio-Frequency Communication”, IEEE

Transactions on Wireless Communications, Vol. 9, No. 3, pp1-7, March 2010.

[23] Zeinab Hajjarian and Jarir Fadlullah, “MIMO Free Space Optical Communications in

Turbid and Turbulent Atmosphere”, Journal of communication, Vol.4, No.8, pp. 524-532,

September 2009.

[24] S. Mohammad Navidpour, Murat Uysal and Mohsen Kavehrad, “BER Performance

of Free-Space Optical Transmission with Spatial Diversity”, IEEE Transactions on

Wireless Communications, Vol. 6, No. 8, pp. 2813-2819, August2007.

[25] Bobby Barua, “Comparison the Performance of Free-Space Optical Communication

with OOK and BPSK Modulation under Atmospheric Turbulence”, International Journal

of Engineering Science and Technology (IJEST), Vol. 3, No. 5, pp.4391-4399, May 2011.

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[26] Murat Uysal and Jing (Tiffany) Li, “Error Rate Performance of Coded Free-Space

Optical Links over Gamma-Gamma Turbulence Channels”, IEEE Communications

Society, pp 3331-3335, 2004.

[27] M. A. Al-Habash, L. C. Andrews and R. L. Phillips, “Mathematical model for the

irradiance probability density function of a laser beam propagating through turbulent

media”, Optical Engineering, Vol. 40, No. 8, pp. 1554-1562, August 2001

[28] Bayaki, E.; Schober, R. and Mallik, R.K., “Performance analysis of MIMO free-

space optical systems in gamma-gamma fading”, Communications, IEEE Transactions,

Vol.: 57, Issue: 11, pp. 3415 – 3424, Nov. 2009

[29] Wasiu O. Popoola and Zabih Ghassemlooy, “BPSK Subcarrier Intensity Modulated

Free-Space Optical Communications in Atmospheric Turbulence”, Journal of Lightwave

Technology, Vol. 27, No. 8, pp. 967-973, April 2009.




Telecommunications - ConTEL 2009, ISBN: 978-953-184-131-3, pp. 59-66, June 2009.

[31] H. G. Sandalidis, T. A. Tsiftsis, G. K. Karagiannidis and M. Uysal, “BER

performance of FSO links over strong atmospheric turbulence channels with pointing

errors”, IEEE Commun. Lett., Vol. 12, No. 1, pp. 44-46, January 2008.

[32] H.E. Nistazakisa, V.D. Assimakopoulos and G. S. Tombras, “Performance

estimation of free space optical links over negative exponential atmospheric turbulence

channels , Optik 122, pp. 2191-2194, 2011.

[33] Fatin Hamimi Hamat, Abu Sahmah M. Supaat and Farah Diana Mahad, “Simulation

of FSO Transmission at Petaling Jaya due to Attenuations Effect”, ELEKTRIKA, Vol. 12,

No. 1, pp. 30-34, 2010.

48

CHAPTER – 3

Effect of Signal Wavelength and Aperture Area of

Detector on Performance of Free Space Optical

Link

3.1 Introduction

BER performance of a FSO link and visibility range are adversely affected by the weather

conditions which causes attenuation and outage in the FSO link. Wavelength of signal and

aperture area of optical detector affects quality factor of receiver. This chapter presents

performance of FSO link with different wavelengths and different aperture area of optical

detector. Effects of different wavelengths on visibility range and quality factor of optical

receiver have been simulated. For simulation, license versions of OPTSIM 5.2 and

MATLAB 6.00 have been used.

3.1.1 Quality Factor of a Receiver

The received SNR plays a very important role on the performance of any communication

systems. SNR is the signal power to noise power ratio. The noise in FSO system includes

the thermal noise, shot noise, background noise etc. that contribute to the total noise at the

FSO receiver system. The performance of FSO link can be calculated by the system BER

which depends on SNR value and on the modulation format used as well as on SNR [1].

Considering Gaussian distribution of noise, in the absence of atmospheric turbulence, the

SNR at the output of the photodetector is given by [1],

3.1 Introduction

49

�¹�4 = L%º+�»¼½¾ 1�L%�L½��+»¼¾N1�+w¿�À½z 1

(3.1)

where Ps is the signal power in watts, PB is the background noise in watts, η is the

quantum efficiency, e is the electronic charge in Coulombs, h is the plank’s constant, v is

the optical frequency in hertz, k is the Boltzman’s constant, B is the bandwidth, TN is the

noise temperature and R is the effective input resistance. In the presence of turbulence,

The SNR becomes a fluctuating term and the average value of it can be given as [1]

⟨�¹�⟩ = ��(dÃ+Ä%Å⟨Ä%⟩1�ÆÇ��(d�� (3.2)

where SNR0 is the SNR value and Pso is the signal power in the absence of turbulence,

<Ps> is the average input signal power and σI2

(D) is the aperture averaged scintillation

index.

The system performance can be calculated in several ways such as by analyzing the BER

and Q-factor. The performance standard for digital receivers is governed by the bit error

rate (BER). The receiver makes wrong decision due to presence of noise and bit error

occurs. Bit error rate is defined as the number of bits received by the detector to the

number of bits transmitted. In an analog system the fidelity criterion is usually specified in

terms of a peak signal to rms noise ratio. BER is used to measure the probability of error.

The analysis of BER and SNR is given in section 2.6.1.The relation between SNR and

BER in the absence and presence of atmospheric turbulence are given by the equation

(2.28) and equation (2.29)

Q-factor is a way of measurement of the signal quality. It is usually proportional to the

systems signal to noise ratio. In optical communication system, the BER is generally too

low to evaluate therefore Q-factor is much more appropriate to be used to measure the

signal quality. The relation between BER and Q-factor could be given as [2-4]

BER = /D erfc + È√D1 (3.3)

Chapter3 Effect of Signal Wavelength and Aperture Area of Detector on Performance of

Free Space Optical Link

50

If the Inter Symbol Interference (ISI) distortions do not occur and the effective amplitude

noise possesses Gaussian distribution, the signal Q-factor is given as:

É = 0�!0dÆ�!Æd (3.4)

“Where V1, V0 are the mean values for voltage v(t) amplitude high and low without ISI,

whereas σ1, σ0 are the root mean square (RMS) of the additive white noise for each

Gaussian distribution”.

3.2 Simulation for FSO link

The block diagram of the simulations link of free space communication is shown in Fig.

3.1. Transmitter section consists of the data source of pseudo-random binary sequence

(PRBS), electrical driver, LED source and optical normalizer. The data source is a non

return to zero (NRZ) format at 1.25 Gb/s bit rate and is indicated by PRBS generator, as

shown in Fig. 3.1. This system produces a binary sequence of various types like

alternating one and zero sequence, PRBS, only sequence of one and only sequence of zero.

NRZ driver transforms an input binary signal into an output electrical signal that could be

described as either voltage or current. Here NRZ modulation is considered.

The input data source modulates at the LED beam by means of an LED driver. LED

source generates the light beam at 1550nm. The output of the modulator is fed to an

optical normalize which normalizes the optical signal power by attenuating the input

optical signal(s) to the specified average output power level. Free space optical length of

500 meter is considered. For attenuation constant, different conditions of atmosphere like

thick fog, moderate fog, light fog, heavy rain, medium rain and clear conditions can be

considered. In the present analysis, different values of atmospheric attenuations are

considered and mentioned in respective results. The OptSim photo receiver model is

consists of various individual building blocks: the photo detector, the preamplifier, and the

post amplifier and filter. Here PIN photo detector with quantum efficiency of 80%, dark

current of 10 µA and ionization coefficient of1 considered. BJT based preamplifier with

3.3 Results and Discussion

51

noise like shot noise and thermal noise etc. and Bessel filter with 4th order and 1 GHz

bandwidth have been considered. The simulation results are shown in figure 3.3 and 3.4.

FIGURE 3.1: Optsim 5.2 Simulation link of free space communication


The free space optical link is simulated to find the visibility range for different

wavelengths at given attenuation. Rayleigh scattering and other scattering losses are

inversely proportional to wavelength and visibility β (λ) =3.91/V (λ/550)-q

, where, V is the

visibility in kilometers, λ is the wavelength in nanometers and q is the size distribution of

the scattering particles. The value of q is taken according to the Kim and Kruse model as

given in equation 2.3 (section 2.3.2). So, attenuation decreases with the increment of

wavelength, which is shown in Fig. 3.2. We have considered four wavelengths 0.85µm,

0.95µm, 1.33µm and 1.55µm for the simulation and observed that attenuation decreases

from about 0.22 db, 0.18db, 0.13 db and 0.1 db respectively at visibility of 10km. The

attenuation is found to be decreasing as the visibility improves. Since the scattering is

inversely proportional to wavelength, the attenuation due to scattering is less at longer



52

wavelength. Also it is possible to transmit more power at longer wavelength due to eye

safety considerations. Selection of wavelength is therefore very important in order to

reduce atmospheric attenuation.

The results of simulation of FSO communication link as shown in figure 3.1 is shown in

figure 3.3 and 3.4. From Fig. 3.3, it is observed that the quality factor of optical receiver

increases with increase in wavelength of the signal. For example at 1.3 µ wavelength, Q2

is approximately 22dB for aperture area of 170 cm2 and it is about 23 dB for 1.5 µ with

same aperture area. The higher wavelength FSO link gives opportunity to obtain better

range in bad weather conditions as compared to the currently available ones at the shorter

wavelength. The received optical power increases with the increase of the receiver

aperture area as given in equation (2.35). As receiver aperture area increases, sensitivity of

receiver increases due to increment in received optical power, which leads to increment in

quality factor of receiver. The effective aperture area of the receiver improves the quality

factor of FSO link. The attenuation also plays an important role in the received optical

power as given in equations (2.35). As the attenuation increases the received optical power

decreases, that directly affects Q-factor of the receiver. Figs. 3.3 and 3.4 shows that as

attenuation of FSO link decreases, quality factor of receiver improves, and also with

increment of receiver aperture area, due to increment in sensitivity of receiver, Q factor

improves. In figure 3.3 the quality factor is plotted as a function of wavelength for three

different values of aperture area of receiver i.e. 170 cm2, 190 cm

2 and 210 cm

2. As shown

in the figure, the quality factor is approximately 22 dB for aperture area of 170 cm2 and it

is about 23.5 dB for aperture area 210 cm2. This figure shows improvement in terms of

quality factor with both wavelength and aperture area. In figure 3.4, Quality factor of

optical receiver is shown as function of receiver aperture area for different atmospheric

attenuation conditions. The quality factor improves with decrease in the atmospheric

attenuation. As shown in the figure 3.4, for aperture area 170 cm2the quality factor is 18.5

dB for atmospheric attenuation of 8.0 dB and it is 27.8 dB for attenuation 2.0 dB.


53

FIGURE 3.2: Relationship between visibility range (km) for different wavelengths

(nm)

FIGURE 3.3: Quality factor of receiver as function of input signal wavelength for

different receiver aperture area



54

FIGURE 3.4: Quality factor of optical receiver as function of receiver aperture area

for different atmospheric attenuation conditions

3.4 Summary

The increase in the aperture area of the receiver increases the sensitivity of the receiver.

Quality factor is calculated as a function of aperture area of the detector. It is found that

quality factor of optical receive improves with increase in aperture area of detector due to

increase in sensitivity of the receiver. Quality factor is also found wavelength dependent.

It is concluded that due to reduction in scattering loss at higher wavelength; as wavelength

increases, quality factor of the receiver improves. The quality factor is shown as a function

of wavelength varying from 0.85 µ to 1.6 µ for three different values of aperture area of

receiver i.e. 170 cm2, 190 cm

2 and 210 cm

2 and the improvement of in terms of quality

factor with both wavelength and aperture area is shown. When designing the optical

receiver a large aperture area can help in improvement of the system, but at the same time

it should be considered that increase in the aperture area may also cause increase in noise

at the receiver due to increase in background radiance. Quality factor of optical receiver

also improves with decrease in atmospheric attenuation.

References

55

References:

[1] Arun K. Majumdar and Jennifier C. Ricklin, “Free space Laser Communications:

Principles and advances” , Springer, ISBN-13: 978-0-387-28652-5, 2008.

[2] Marcuse, “Calculation of Bit-Error Probability for a Lightwave System with Optical

Amplifier and Post-Detection Gaussian Noise”, Journal of Lightwave Technology, Vol. 9,

No. 4, pp. 505-513, 1991.

[3] Gerd Keiser, “Optical Fiber Communication”, Third edition, Mc Graw Hill edition,

2000

[4] John M.Senior, “Optical Fiber Communication, principles and practice”, Second

edition PHI, New Delhi, 2002.

56

CHAPTER – 4

Communication Performance of Free Space

Optical Link Using Wavelength Diversity in

Strong Atmospheric Turbulence

4.1 Introduction

Atmospheric turbulence could be considered as one of the major challenges that FSO

communication systems is facing, which degrades the performance in terms of bit error

rate. As the refractive index variation will be different for different wavelengths, the

fading is not the same for different wavelength and different paths at the same time. The

signals traveling through turbulent atmosphere undergo different amount of intensity

fluctuations. The effect of atmospheric turbulence and scintillation cause intensity

fluctuation of optical beam thus increasing the bit error rate (BER). In this chapter

wavelength diversity technique is proposed to reduce the turbulence induced fading under

strong atmospheric turbulence condition. The main purpose of this chapter is to analyze

the application of wavelength diversity in FSO to minimize the impact of turbulence on

the performance of the link. Mathematical expressions for the estimation of the outage

probability are derived and considerable improvement is found using wavelength diversity

technique.

Atmospheric turbulence is one of the greatest challenges for FSO link, especially for long

distance communication. The turbulence phenomena have an effect on the propagation of

optical beam by both the spatial as well as temporal random variations of refractive

index caused by temperature, pressure and wind fluctuations along the optical propagation

path. Continual variations in the refraction in the turbulence of the environment cause

scintillation and because of that the received optical power is fluctuating continuously.

4.1 Introduction

57

The prediction of turbulent channel is complicated because it can vary continuously with

variation in temperature, time of the day and wind velocity etc. Several techniques are

proposed by various researchers to improve the performance of FSO link in terms of bit

error rate and link availability. Ahmed A. Farid et al., obtained a statistical model for the

optical intensity fluctuation at the receiver caused by the combined effects of atmospheric

turbulence as well as pointing errors [1]. Aperture averaging technique can be employed

for mitigation of scintillation effects. In aperture averaging, fluctuations in intensity are

averaged over the receiving aperture area. For smaller aperture sizes in stronger

turbulence, scintillations can be severe and increasing the receiver size decreases the effect

of scintillation. M.A. Khalighi et al., [2] and Zeinab Hajjarian et al., [3], investigated the

impact of aperture averaging on the performance of FSO systems under atmospheric

turbulence regimes. Practically, increasing the size of the aperture area of receiver

feasible, so spatial diversity is used instead of a large aperture area. Space diversity [4, 5]

reception technique is employed to overcome the fading caused by turbulence. Time

diversity techniques are also discussed for performance improvement of FSO link [6-7].

Fang Xu et al., studied the performance of channel coding methods for various time

diversity orders as well as turbulence factors [6]. S. Mohammad et al., investigated BER

performance using space diversity technique under turbulent atmosphere having lognormal

distribution of FSO link [4]. V. Xarcha et al., studied the utilization of wavelength

diversity in FSO technology that perform in turbulence conditions that follow log normal

distribution. Mathematical expressions are also derived for the same and measurable

improvement in performance if found [8]. H. G. Sandalidis et al., evaluated the

performance of a FSO system in strong turbulence regime which follows the K

distribution and the outage probability for a single-input single-output FSO link [9]. This

chapter analyses the communications system performance in terms of outage probability

using wavelength diversity technique in mitigating strong atmospheric turbulence effects.

4.1.1 Diversity

Diversity technique is one of the most effective techniques to overcome the effect of

fading. The main idea behind the diversity technique is that the amount of fading will not

be the same for all the optical wavelength, polarization, space, and time in the atmospheric

channel which acts as a random media for propagation of signal. If one

Chapter 4 Communication Performance of Free Space Optical Link Using Wavelength

Diversity in Strong Atmospheric Turbulence

58

signal is faded deeply at some point of time then another path which is independent to the

first path may have less fade and it may receive stronger signal. As there are more than

one paths of received signal to select from, a large amount of improvement can be found

in both the average and instantaneous SNR. Various types of diversity techniques are used

to overcome the effect of fading which include wavelength diversity, time diversity,

spatial diversity, polarization diversity and angle diversity. The SNR improvement is

achieved using diversity without increasing the bandwidth and transmitted power [10].

The limitation of this technique is that the system complexity and cost increases.

4.1.2 Diversity Combining Techniques

At the receiver the signal is to be processed so that maximum efficiency of the system is

achieved. In communication receiver there are several diversity reception techniques to

employ out of them and the most common techniques are: Selection diversity, Equal gain

combining (EGC) and Maximum Ratio Combining (MRC) [10-11].

Selection Diversity: In selection diversity, the signal with highest received level is

selected and switched to the receiver. This is the least complicated method of combining

out of three as it has to process only one branch with maximum SNR value. The selection

diversity combiner selects the branch having largest SNR values. As the output of

selection diversity combiner selects largest SNR value, it is not required to find out the

coherent sum of individual branches in the present technique. [12]

Maximum Ratio Combining (MRC): MRC is the most advantageous combining

technique in the absence of interference regardless the statistics of fading. However this

most favorable combining method has the limitation of system complexity as MRC

requires information of all channel fading parameters. The MRC technique is complicated

compared to other techniques as the information of channel fading amplitude as well as

channel phases are required and therefore is not practical for noncoherent detection [9-10].

Equal Gain Combining (EGC): The EGC receiver processes all the received copies of

the signal, weights all of them uniformly and then adds them to generate the decision

statistics [12-13]. This combining technique is less complicated compared to MRC as it

4.2 Performance Analysis of FSO Link under Strong Turbulent Atmosphere

59

does not need approximation of the fading amplitude as EGC weights each branch equally

before combining. It is generally an attractive solution with coherent detection [12-13].

The performance improvement is little bit lower in EGC as there is a possibility to

combine the signals with noise and interference, with the high quality signals which are

noise free.

4.2 Performance Analysis of FSO Link under Strong Turbulent

Atmosphere

Air nearer to the earth surface becomes heater compared to that at higher elevation due to

the solar radiation during daytime. This layer of warmer air arises and

get combine turbulently with the neighboring cooler air which induces the air temperature

to vary randomly. The strength of turbulence is described by Cn2 as explained in section

2.4. It depends on time of the day, altitude and location. As the altitude increases, the

temperature gradient decreases. With the decrease in temperature gradient the air density

also decreases and so the Cn2 is also smaller at high altitude. For a weak turbulence at

ground level the typical value for Cn2

is 10-17

m-2/3

, whereas it could be upto10-13

m-2/3

or

larger for strong turbulence conditions [3]. The strong turbulence regime is also possible

when the propagation path is very large such as a few 10s of km to 100s of kilometers.

Scintillation variations in light intensity induced due to atmospheric turbulence and the

resulting attenuation due to this are random in nature and are unpredictable. Wind and

temperature gradients create air pockets with rapidly changing indices of optical

refraction. Along with scintillation, beam broadening and beam wander also arise when

optical signals propagated through the turbulent air. Due to that spatial and temporal

variations occur in the optical beam which passing through the turbulent medium [14].

The scintillation is maximum during midday when the temperature is maximum. It is also

large in summer compared to winter season. Several models are proposed for expressing

intensity fluctuations due to atmospheric turbulence such as log normal model, gamma-

gamma turbulence model, Rayleigh density model and negative exponential channel

model. As the link length increases upto few kilometers, several independent scatterers

tend to increase. The amplitude fluctuation of the field traversing the turbulent medium in

strong regime is generally follow a negative exponential statistics for the irradiance [15]

and is given by:



60

0,exp1

)(00

≥

−= I

I

I

IIf

(4.1)

where, E [I] = I0 is the average value of received irradiance and I is irradiance. Figure 4.1

shows the probability density function (pdf) of the negative exponential channel model for

different values of I0.

In digital communication systems, BER is commonly used as a figure of merit. BER is

given as the number of bits in error out of total transmitted bit at the receiver. Considering

that, the sensitivity of a receiver is defined as the minimum required received optical

power to maintain BER below a certain value. The most common modulation method for

FSO link is On OFF Keying (OOK) or Intensity modulation. The “1” and “0” bits are

transmitted by the intensity value of the optical signal. For the FSO link using Intensity

Modulation/ Direct Detection (IM/DD), optical signal propagating through Additive White

Gaussian Noise (AWGN) channel is statistically modeled as [16].

nηIxnsxy +=+= (4.2)

FIGURE 4.1.Negative exponential probability density function for different values of

average irradiance, I0

0 0.5 1 1.5 2 2.5 3 3.5 40

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Irradiance,I

Ne

ga

tiv

e e

xp

on

en

tia

l p

df:

p(I

)

I 0 = 2

I 0 = 1

I 0 = 0.5

4.2 Performance Analysis of FSO Link under Strong Turbulent Atmosphere

61

where y represents the optical signal reaching at the receiver, s = ηI is the instantaneous

intensity gain, is the photocurrent conversion ratio of the receiver, I is the normalised

irradiance arriving at the receiver, x is the binary signal that takes the value ‘0’ or ‘1’ and

n represents AWGN with zero mean and variance σ2 = N0/2, where N0 is the noise power.

As a result of the atmospheric turbulence, the normalized irradiance intesity functions as a

random variable and for the strong atmospheric turbulence conditions, it follows a

negative exponential distribution given by equation (4.1). Without loss of generality we

can assume I0 = 1 and equation (4.1) can be written as

)exp()( IIf −=

(4.3)

CDF i.e., Cumulative Distribution Function can be found as a function of I by integrating

equation(4.3) and is given as

)exp(1)( IIFI −−= (4.4)

The instantaneous SNR fluctuates at the receiver because the fluctuation of intensityof

laser beam due to atmospheric turbulence. So, the instantaneous electrical SNR is given by

[16]

Ê = �Ë��d = �Ì��d (4.5)

The average electrical SNR is given by [16]

0

2

N

Iηµ = (4.6)

where I is the mean value of the irradiance. After doing a power transformation of I from

(4.1) the following PDF is obtained for γ

−=

µ

γexp

γµ2

1 ) (f γ γ (4.7)

and by integrating, the CDF for γ can be found to be [16],

Fγ�γ� = Î1 − expÏ−ÃγµÐÑ (4.8)



62

The outage probability is given by [16]

( ) ( )

−−==≤=

µ

γexp1γFγγPP

th

thγth,out

(4.9)

In above equations γth is the threshold value of SNR and µ is the average electrical SNR.

Equation (4.9) is plotted in figure 4.2 with different values of threshold SNR. Figure 4.2

shows the relation between outage probability and electrical SNR for different threshold

values of SNR.

FIGURE 4.2: Outage probabilities versus average electrical SNR

4.3 Wavelength Diversity to Mitigate the Effect of Turbulence

Scintillation can be reduced using the wavelength diversity i.e. sending the same signal

using more than one laser with different wavelength. The time fluctuations in atmospheric

temperature and refractive index variation will be different for different wavelengths. The

effect of fading due to scintillation can be reduced because the fading is not the same for

0 5 10 15 20 25 30 35 4010

-4

10-3

10-2

10-1

100

Averge Electrical SNR(dB)

Outa

ge P

robabili

ty

Threshold SNR= -10 dB






63

different wavelength at the same time. If we put three different lasers having wavelengths

λ1, λ2 and λ3 and arranging the lasers vertically at different positions (heights) and

similarly employing three separate detectors at the receiver as shown in figure 4.3, the

signals traveling through turbulent atmosphere undergo different amount of intensity

fluctuations for the three different links. At the receiver, the signals are combined by a

selective combiner, which results in better overall combined signal intensity.

Atmospheric

Turbulence

FIGURE 4.3: Block schematic of wavelength diversity technique

Intensity fluctuation (λ1, t) ≠ intensity fluctuation (λ2, t) ≠ intensity fluctuation (λ3, t),

where λ1, λ2 and λ3 are the wavelengths of the transmitters. As the normalized irradiance

intensity at the receiver acts like a random variable, the SNR also becomes a random

variable. Due to that probability of error also becomes a random variable. If the

probability of error in link 1 with wavelength λ1, is Px1(x1), that of link 2 with wavelength

λ2, is Px2(x2), and that of link 3 with wavelength λ3 is Px3(x3), then generally Px1(x1) ≠

Transmitter

Wavelength λ1

Transmitter

Wavelength λ2

Transmitter

Wavelength λ3

Optical

receiver

Optical

receiver

Optical

receiver

Select

Largest

SNR



64

Px2(x2) ≠ Px3(x3). By using the diversity technique described here takes advantage of the

reality that the atmospheric propagation path of optical beam is statistically independent

for different operating wavelengths and the intensity fluctuations and BER performance

will be enhanced because the joint probability of error is always less than the probability

of error from individual channels.

)3()3,2,1(

,)2()3,2,1(

,)1()3,2,1(

3321

2321

1321

xpxxxp

xpxxxp

xpxxxp

xxxx

xxxx

xxxx

<

<

<

(4.10)

If Px1(x1), Px2(x2) and Px3(x3) are independent which the case is generally, then the

probability of error is

)3()2()1()3,2,1( 321321 xpxpxpxxxp xxxxxx = (4.11)

The diversity techniques that can be used under this situation are selection diversity, Equal

Gain Combining (EGC) and Maximal ratio combining (MRC). Selection diversity which

is used in here is one of the simplest diversity technique in which the signal with largest

SNR is selected by the receiver. Let γth is the threshold value of SNR that must be

achieved for proper demodulation and γk be the instantaneous SNR of the kth

branch. If

there are M links (M transmitters and M receivers), the probability that bit energy to noise

ratio of all the links are below the threshold γth is

)},.....,(max{ 21 thkout PP γγγγ <=

(4.12)

where, Pout is the outage probability which is the probability that the instantaneous SNR

falls below the threshold i.e. sensitivity of the receiver. The effect of fading due to

scintillation can be reduced because the fading is not same for different wavelengths at the

same time. Using wavelength diversity, the same communication signal is transmitted with

different wavelength. In that case the channel given in equation (4.1) can be given as [8],

MmnxInxsy mmmmmm ,..2,1, =+=+= η

(4.13)

where ym represents the optical signal arriving at each of the M (M is the number of

transrecievers (pair of transmitters and receivers of different wavelength) channels, sm =


65

ηm Im is the instantaneous intensity gain, ηm is the photocurrent conversion ratio of the

reciever, I is the normalised irradiance arriving at the receiver, x is the binary signal which

takes the value ‘0’ or ‘1’ and n represents AWGN with zero mean and variance σ2 =

N0/2.The outage probability is given by [8]

( ) ( )mthmthmout FPP ,,,

γγγ γ=≤=

(4.14)

From equation (4.9) and (4.14) the outage probability is given by

−−=

m

mthmoutP

µ

γ ,exp1

, (4.15)

where Pout is the outage probability. The intensity of the optical signal falling on the

detector is a random variable, and so as the BER . Random fluctuations of the atmospheric

refrective index n occurs along the propagation path of the optical beam due to

atmospheric turbulence. This refractive index fluctuation is due to the random variations

in atmospheric temperature which results in the fluctuation of the received optical power .

Assuming that Pout,m is independent for each of the M wavelength channels which is

generally the case, the intensity fluctuation will be different for different wavelengths. The

probability of fading due to that cannot be similar at the same time for different

wavelength. Since all the communication links are independent from each other, the total

outage probability of FSO system will correspond to the probability of outage of all the M

links with different wavelengths as shown in equation (4.11).

( ) ∏∏==

−−==

M

m m

mthM

m

mthmout FP11

,,

,exp1

µ

γγγ (4.16)

where the final product expression is valid as fading by each link is assumed independent.

Now, the outage probability for M independent link can be given as

M

thmoutP

−−=

µ

γexp1

,

(4.17)



66

Figure 4.4 shows the outage probability for diversity conditions and selection combining.

It shows a plot between threshold values of SNR (in dB) versus outage probability for

different number of channels. As shown in the figure, for the case of threshold SNR to

Average SNR value of -40 dB, the outage probability is on the order of 10-2,

10-4

and 10-6

respectively for single link, double link and three links. It is also clear that outage

probability decreases with the increase of the number of elements (M) in the

communication link. The outage probability can be reduced further by using more number

of communication links but there is a compromise between system complexity and cost.

FIGURE 4.4: Outage probabilities under diversity conditions

4.4 Summary

Intensity variation due to atmospheric turbulence is random in nature but improved

performance can be achieved in the proposed technique of wavelength diversity.

Wavelength diversity is the case where the same signal modulates laser transmitter of

-40 -35 -30 -25 -20 -15 -10 -5 0 5 1010

-6

10-5

10-4

10-3

10-2

10-1

100

Threshold SNR/Averge SNR(dB)

Ou

tag

e P

rob

ab

ility

M=1

M=2

M=3

4.4 Summary

67

different wavelengths. The probability of fading will not be similar at different

wavelengths (i.e. 850 nm, 1330 nm and 1550nm) as the atmospheric turbulence is random

in nature. The wavelength diversity can then improve the performance in terms of outage

probability. A mathematical expression has been derived for the outage probability for

number of independent links. It has been shown that the performance improves in terms of

outage probability as we increase number of channels under diverse atmospheric

conditions. Strong turbulence comes in to the picture when the link length is of several

kilometers. However it increases system complexity and cost. Further investigation can be

done to find the effect of different coding techniques other than the OOK to improve the

link performance. As atmospheric turbulence causes the variation in intensity and phase,

the FSK modulation technique as well as PPM technique can perform better than OOK in

atmospheric turbulence condition. Further research is needed in this area.



68

References:

[1] Ahmed A. Farid and Steve Hranilovic, “Outage Capacity Optimization for Free-Space

Optical Links With Pointing Errors”, Journal of Lightwave Technology, Vol. 25, Issue 7,

pp. 1702-1710, 2007.




Telecommunications - ConTEL 2009, ISBN: 978-953-184-131-3, pp. 59-66, June 2009

[3] Zeinab Hajjarian and Jarir Fadlullah, “MIMO Free Space Optical Communications in

Turbid and Turbulent Atmosphere”, Journal of communication, Vol.4, No.8, pp.524-532,

September 2009.

[4] S. Mohammad Navidpour, Murat Uysal and Mohsen Kavehrad, “BER Performance of

Free-Space Optical Transmission with Spatial Diversity”, IEEE Transactions on Wireless

Communications, Vol. 6, No. 8, pp. 2813-2819, August 2007.

[5] Z. X. Wang, W. D. Zhong, S. N. Fu, and C. Lin, “Performance comparison of different

modulation formats over free-space optical (FSO) turbulence links with space diversity

reception technique”, IEEE Photonics Journal, Vol. 1, No. 6, pp. 277-285, December

2009.

[6] Fang Xu, Ali Khalighi, Patrice Causse and Salah Bourennane, “Channel coding and

time-diversity for optical wireless links”, Optics Express, Vol. 17, No. 2, pp. 872-887,

2009.

[7] W.O. Popoola, Z. Ghassemlooy, H. Haasl, E. Leitgeb and V. Ahmadi, “Error

performance of terrestrial free space optical links with subcarrier time diversity”, IET

Communication, Vol. 6, Issue-5 pp. 1-8, 2011.

References

69

[8] V. Xarcha, A. N. Stassinakis, H. E. Nistazakis, G. P. lastas, M. P. Hanias, G. S.

Tombras and A. Tsigopoulos, “Wavelength diversity for free space optical systems:

performance evaluation over log normal turbulence channels, 19th

International

conference on Microwaves, radar and Wireless Communications, MICON-2012, Warsaw,

Poland, pp. 678-683, May 21-23, 2012.

[9] H.G. Sandalidis and T.A. Tsiftsis, “Outage probability and ergodic capacity of free-

space optical links over strong turbulence”, Electronics Letters, Vol.44, No.1, pp. 46-47,

2008.

[10] Mihajlo Stefanovic, Dragan Draca, Aleksandra Panajotovic and Nikola Sekulovic,

“Modeling and Simulation of L-branch Selection Combining Diversity Receiver in

Nakagami-m Environment using Matlab”, Proceedings of Small Systems Simulation

Symposium 2012, Nis, Serbia, 12th

-14th

, pp. 115-118, February 2012.

[11] Nikos C. Sagias, George K. Karagiannidis, Dimitris A. Zogas, and P. Takis

Mathiopoulos, “Selection Diversity for Wireless Communications with Non-Identical

Weibull Statistics”, IEEE Communications Society, Globecom , pp. 3690-3694, 2004.

[12] M. K. Simon and M.S. Alouini, “Digital Communications over Fading Channels: A

Unified Approach to Performance Analysis”, John Wiley & Sons, 2000.

[13] R. You, H. Li and Y. Bar-Ness, “Diversity combining with channel estimation”, IEEE

Trans. Commun, Vol.53, No. 10, pp.1655-1662, October 2005.

[14] Scott Bloom, Eric Korevaar, John Schuster and Heinz Willebrand, “Understanding

the performance of free-space optics [Invited]”, Journal of optical networking, Vol. 2, No.

6, pp. 178-200, June 2003.

[15] Wasiu O. Popoola and Zabih Ghassemlooy, “BPSK Subcarrier Intensity Modulated

Free-Space Optical Communications in Atmospheric Turbulence”, Journal of Lightwave

Technology, Vol. 27, No. 8, pp. 967-973, April 2009.



70

[16] H. E. Nistazakisa, V. D. Assimakopoulos and G. S. Tombras, “Performance

estimation of free space optical links over negative exponential atmospheric turbulence

channels, Optik 122, pp. 2191-2194, 2011.

71

CHAPTER – 5

Estimation of FSO Link Availability Using

Climatic Data

5.1 Introduction

FSO communication techniques systems are increasingly being accepted to offer high

speed data transmission. The communication performance of a FSO link can be severely

degraded due to atmospheric conditions that create the temporal and spatial fluctuations of

light intensity. Before establishing a FSO link, the meteorological condition of the given

geographical area should be studied so that a better link availability can be achieved. In

this paper, we present a feasibility study of FSO link for four different cities of India

representing different topological conditions as a case study.

There has been a considerable increase of interest in FSO communication research due to

its manifold advantages over other transmission techniques [1-2]. The FSO availability for

a number of cities in USA as a function of link range incorporating the local historical

weather data has been reported [3]. The important criterion for communication is bit error

rate (BER) which is adversely affected by atmospheric turbulence and worst weather

conditions. The visibility between the FSO links plays an important role for error free

communication between the transmitter and receiver. The weather like haze, fog, snowfall,

etc. decreases the visibility and these variables are moderate in nature that can take on

different values in continuous range.

The performance of the link in terms of BER degrades as the attenuation is inversely

proportional to the visibility. The atmospheric weather conditions like average visibility of

the particular geographical location should be considered before designing a FSO link for

any particular application. A contour map of average expected visibility should be created

Chapter 5 Estimation of FSO Link Availability Using Climatic Data

72

by collecting the statistical data of the weather conditions throughout the year. With this

calculated visibility data, a better link can be designed with reduced outage probability.

Mainly two types of attenuation affect the performance of FSO link that are geometrical

and atmospheric attenuation. Geometrical attenuation is due to misalignment of transmitter

and receiver as well as link range whereas atmospheric attenuation is due to scattering,

absorption and scintillation. Geometric losses occur due to the spreading of the transmitted

beam between the transmitter and the receiver. The FSO link equation can be given as [4].

�� = Pt Ó��ÔÓ��∗��Õ� ∗ 10�!�∗ Ö�d� (5.1)

where, Pt is the transmitted power and Pr is the Received power, d1is the transmit aperture

diameter (m), d2 is the receive aperture diameter (m), D is the beam divergence (mrad), L

is the range (km) and a is the atmospheric attenuation factor (dB/km). The main criteria of

atmospheric attenuation of FSO link are absorption, scattering and scintillation.

Absorption depends on the water vapor present on the propagation path of optical signal

that further depends on altitude and humidity [5]. The scattering occurs because of the

light is scattered by the atoms and molecular components present in the atmosphere.

Scattering causes redistribution of the light and reduces the incident power at the receiver

[6-7]. The atmospheric transmission of optical signals, τa, is expressed by the “Beer's law

equation” (equation 2.2) [6]

The transmittance τa given in the equation 2.2 is the sum of the absorption and the

scattering coefficients [7]. The transmitted optical beam propagating from the atmosphere

can be absorbed, scattered and dislocated depending on the atmospheric condition. Signal

attenuation due to above factors are given by several models. The most commonly used

model is Kruse and Kim [7-11] and the attenuation coefficient is approximated by the

relation given in equation 2.3. The availability calculation uses the power budget analysis

and the statistical analysis of atmospheric attenuation. The attenuation due to scattering is

calculated using visibility data of different places and subsequently power loss is

calculated.

5.2 Visibility Conditions of Different Cities of India

73


The visibility data of four different cities in India is obtained from the website

Wundermap [12] for the year 2013. The statistical data given in the website Wundermap

[12] are of three types of visibility, i.e., low, average and maximum visibility [12]. We

have considered two types only, one low for the minimum availability of the link and

other average to show the average picture of link availability. Table 5.1 gives the data of

visibility throughout the year 2013 (from 1 January 2013 to 31 December 2013). Table 2

gives the visibility data considering the low attenuation values. The average visibility data

(monthly) of four different cities are given. From the tables 5.1 and 5.2, one can see that

the average visibility throughout the year is lower in case of Delhi and maximum in

Thiruvananthapuram as compared to other cities. Figures 5.1 and 5.2 show the graph of

monthly variation of visibility for all the four cities considered in above analysis. One can

see that there is seasonal variation of visibility and the trend is different in different places.

In Ahmedabad and Thiruvananthapuram, maximum visibility is in the month of

March/April, whereas for Delhi and Kolkata, maximum visibility is in the month of

June/July. This variation is due to variation in the monsoon pattern of India. In addition,

the visibility also shows daily temporal behavior, i.e., the visibility is different in the

morning, afternoon or evening for a given city. Average monthly visibility data of four

cities are shown in figure 5.1. It is clear from figures 5.1 and 5.2 that the maximum

visibility is in the month of March for two cities, i.e., Thiruvananthapuram and

Ahmedabad, whereas it is maximum in the month of June for Delhi and it is fairly constant

for few months starting from July to September for Kolkata.

In the data average visibility throughout the year is taken. One can see from the Tables 5.1

and 5.2 that visibility of Delhi is low for more number of days compared to other cities,

and for Thiruvananthapuram the visibility is higher for more number of days compared to

other cities. The same link cannot be proposed for all the four cities. The parameters (link

range, sensitivity of the receiver, etc.) have to be changed for better performance. Figure

5.3 shows the visibility data versus the number of days which it appears in 2013 [12].


74

TABLE 5.1: Average visibility in km for different cities during the year 2013 (For

Average visibility case)

Month Visibility (km)

Delhi Kolkata Ahmedabad Thiruvananthapuram

Jan 1.48 1.74 4.03 4.55

Feb 2.25 2.32 4.25 4.96

Mar 3.00 2.61 4.32 4.84

Apr 2.80 2.83 4.43 4.9

May 3.13 3.0 4.81 4.61

Jun 2.83 2.96 3.97 3.47

Jul 3.00 3.13 3.35 3.97

Aug 2.32 3.13 3.68 4.35

Sep 2.73 3.03 3.8 4.43

Oct 2.29 2.61 3.8 4.65

Nov 1.60 2.13 3.97 4.37

Dec 1.42 1.84 3.87 4.35

TABLE 5.2: Average visibility in km for different cities during the year 2013 (For

low visibility case)

Month Visibility (km)

Delhi Kolkata Ahmedabad Thiruvananthapuram

Jan 0.35 0.84 2.29 2.45

Feb 0.75 1.36 2.29 3.14

Mar 1.03 1.48 2.71 2.87

Apr 1.60 1.73 2.5 3.67

May 1.77 1.74 2.77 3.29

Jun 2.03 1.7 1.97 1.97

Jul 1.55 1.9 1.94 2.68

Aug 1.35 1.74 2 2.94

Sep 2.03 1.9 2 3.1

Oct 1.35 1.48 1.9 3.03

Nov 0.93 1.43 1.93 2.77

Dec 0.48 0.94 2.13 2.65


75

FIGURE 5.1 Average visibility data throughout the year for different cities (low

visibility)

FIGURE 5.2 Average visibility data throughout the year for different cities (average

visibility)


76

FIGURE 5.3: Visibility versus no. of days for different cities

5.3 Transmittance

The atmospheric attenuation is mainly because of scattering due to dust and atmospheric

aerosols. The particles are of similar sizes as compared to wavelength, hence this type of

scattering is Mie scattering. This depends on the volume of the atmospheric aerosols, and

the impacts of absorption will be relatively small compared to of Mie scattering, hence,

the scattering coefficient may be calculated from the visibility distance as well

as wavelength of the incident beam. The scattering coefficient is related to concentration

of dust particles, which in turn related to visibility [13]. Transmittance can be calculated

for the FSO link if the visibility data is known as given in equation (2.2) and (2.3). Figure

5.4 shows the transmittance versus visibility for three different wavelengths (850 nm,

1330 nm and 1550 nm). As shown in the figure 5.4 the value of transmittance for the

visibility 3 km is about 0.4 for 850 nm wavelength, 0.53 for 1330nm wavelength and

5.3 Transmittance

77

about 0.57 for 1550nm wavelength. Since the visibility data are known, the transmittance

for the four cities can be estimated by substituting the visibility data. We have taken the

weekly average of the visibility data of four cities for the computation. Figure 5.5 to 5.7

shows the transmittance for the three wavelengths 1550 nm, 1330 nm and 850 nm

respectively for the whole year 2013. From these figures one can say that the transmittance

of Thiruvananthapuram is higher compared to other cities and the transmittance for Delhi

is lowest in all cities. One can also see the effect of wavelength on transmittance and it can

be shown that transmittance improves for higher wavelength.

FIGURE 5.4: Transmittance as a function of visibility (km)


78

FIGURE 5.5 Weekly data of transmittance at wavelength 1550 nm


5.4 Power Link Margin and Outage Probability

79



The performance of FSO link can be calculated from outage probability and one of the

main factors on which it depends is the visibility in this case. Table 5.3 shows the

percentage of time of the visibility data. The data is collected from the website

Wundermap [12]. The percentage visibility of the cities is found by calculating the given

visibility over 365 days. Based on these data we can calculate the Probability Density

Function (PDF) of the visibility data for all the four cities. Using the data of PDF we can

also calculate the Cumulative Distribution Function (CDF) of the visibility data for given

city. The method of PDF and CDF calculation is included in the Appendix. Table 5.4

gives the calculated PDF and CDF data. Here only low visibility case is considered as the

link availability should be considered for worst case. Figures 5.8 -5.11 shows the PDF and

CDF value of the cities. CDF is required to calculate the link availability data of the FSO

link. Link availability is considered from the CDF value of the visibility in the cities. The

power link margin of any FSO link depends on the laser power, beam divergence, receiver

sensitivity, coupling losses and receiver lens area. A correct operation of the FSO link for


80

link distance L (km) will be achieved only if the power link margin is greater or equal to

the atmospheric attenuation for the link distance. Atmospheric turbulence, scattering and

absorption will determine the power loss due to atmosphere. Absorption at FSO

wavelengths is considered to be negligible and the attenuation due to scattering will be

dominant which can be calculated using the equation (2.3). The relation also shows

dependence on the visibility.

TABLE 5.3: Visibility data (percentage) for different cities (Low visibility)

City Visibility (V in km) out of 365 days

0 (km) 1(km) 2 (km) 3 (km) 4 (km) 5 (km)

Delhi 60 181 95 23 6 0

16.44% 49.59% 26.03% 6.30% 1.64% 0%

Kolkata 19 137 209 0 0 0

5.2% 37.53% 57.26% 0% 0% 0%

Ahmedabad

1 30 224 110 0 0

0.27% 8.21% 61.37% 30.14% 0% 0%

Thiruvananthapuram 0 25 64 207 69 0

0% 6.85% 17.53% 56.71% 18.90% 0%

TABLE 5.4: PDF and CDF values of visibility data of different cities (Low visibility)

City Visibility (V in km) out of 365 days

0 (km) 1 (km) 2 (km) 3 (km) 4 (km) 5

(km)

Delhi PDF 0.164 0.496 0.26 0.063 0.016 0

CDF

[F(V)]

0.164 0.66 0.92 0.983 1 1

Kolkata PDF 0.052 0.38 0.57 0 0 0

CDF

[F(V)]

0.052 0.432 1 1 1 1

Ahmedabad PDF 0.0027 0.082 0.61 0.30 0 0

CDF

[F(V)]

0.0027 0.0847 0.695 1 1 1

Thiruvananthapu

ram

PDF 0 0.069 0.175 0.567 0.189 0

CDF

[F(V)]

0 0.069 0.244 0.811 1 1


81

For a visibility V, the link availability (LA) is defined as [6, 8].

× =�Ø [Ú ≥ Ú��Ü�×� = 1 − ±[Ú��Ü�×�] (5.2)

Where F [V=Vmin(L)] is the CDF of visibility as shown in the figures 5.8-5.11.

FIGURE 5.8 PDF and CDF for visibility data for Delhi

FIGURE 5.9 PDF and CDF for visibility data for Kolkata


82

FIGURE 5.10 PDF and CDF for visibility data for Ahmedabad

FIGURE 5.11 PDF and CDF for visibility data for Thiruvananthapuram

Table 5.5 gives the Link availability for a given city for given visibility. For a given link

(i.e. Transmitted power, link range, beam divergence and transmitter-receiver aperture

area) if the minimum visibility required is 1, than the link availability will be 34 % for

5.5 Link Availability of FSO Link

83

Delhi, 56.8% for Kolkata, 91% for Ahmedabad and 93% for Thiruvananthapuram.

Similarly if the minimum visibility 2 is required then link availability for Delhi will be

only 8%, for Kolkata it is 0%, for Ahmedabad it is 30.5% and for Thiruvananthapuram it

is still 75.6%. It can be seen that the same link cannot be proposed for all the cities. Table

5.5 and figure 5.12 show the link availability for different cities in tabular and graphical

form respectively. The X-axis shows the visibility data (km) and figure 5.12 shows the

link availability for given minimum visibility for the particular link.


The performance of a FSO system is also computed by the “link margin” as it is explained

in section 2.6.2. The link margin computation is necessary to design a suitable FSO

system that depends on various parameters including the environment where the link is to

be deployed. In the present section the link margin is explained using some typical data of

transmitted power, geometrical attenuation and sensitivity of detector for different

visibility conditions as well as at different wavelengths. The Link margin can be given

with the following equation [4, 14, 15]

Link Margin M (L) = Pt - Ageo - Sr - Asyst (5.3)

Pt - total power of the emitter (dBm), Ageo - geometrical attenuation (dB), Sr – sensitivity

of the receiver (dBm), Aatm is the atmospheric attenuation (dB), Asys - all other system

dependent losses (in dB). The system loss in a FSO system can be due to imperfect lenses

and other optical elements (such as couplers). In the present analysis all other system

dependent losses are ignored.

TABLE 5.5: Link availability of different cities

City

% Link availability

1- F[0] 1- F[1] 1- F[2] 1- F[3] 1- F[4]

Delhi 84% 34% 8% 1.7% 0%

Kolkata 94.8% 56.8% 0% 0% 0%

Ahmedabad 99.72% 91% 30.5% 0% 0%

Thiruvananthapuram 100% 93% 75.6% 18.9% 0%


84

FIGURE 5.12: Visibility versus link availability of different cities of India

Link Margin M (L) = Pt - Ageo - Sr (5.4)

As given in the equation, the link margin depends on the transmitter power, receiver

sensitivity and geometrical attenuation. In the analysis the following data are assumed: Pt -

total power of the emitter (dBm) = 13 dbm, Sr – the sensitivity of the receiver (dBm) = -46

dbm. Note that Ageo - is the geometrical attenuation (dB). Geometric losses are those

losses that occur due to the spreading of the propagated optical beam. The overfill energy

is lost when the beam spreads to a size comparatively larger than the receive aperture.

Generally, larger receive apertures or smaller transmit divergences induce lesser geometric

loss for a particular range. For a uniform transmit power distribution with a no obscured

transmitter or receiver, geometric losses may be determined with the equation (2.1). It

depends on receiver aperture diameter d2, transmitter aperture d1, range L and divergence

D. Considering d1 = d2= 0.01m, D=2.5 mrad and substituting in above equation, we can

calculate geometrical attenuation per km. For the link to survive, the atmospheric

attenuation should not exceed to the link margin. Here using the data which we considered

in present analysis, we calculated the link margin and atmospheric attenuation for different


85

visibility for three different wavelengths i.e. 850 nm, 1330nm and 1550 nm. The plot for

link margin versus link length and attenuation is given for all three wavelengths is shown

in figures 5.13, 5.14 and 5.15 for wavelengths 850 nm,1330nm, and 1550 nm respectively.

As shown in figure 5.13, the link margin is plotted at wavelength of 850 nm using the data

given above and the link length is considered to be variable from 1 to 3 km. The curve for

link margin shows that it is decreases with the increase in distance as geometrical loss

increases with the distance L. Other curves are for attenuation for visibility conditions of 1

km, 2 km and 3 km. When the atmospheric attenuation crosses the link margin, the link

fails. We can see that the link fails at about 1.9 km, about 2.3 km and about 2.7 km for and

for visibility conditions of 1km, 2km and 3 km respectively as shown in figure 5.13.

Similar analysis is done for wavelengths 1330 nm and 1550 nm and plotted in figure 5.14

and 5.15.

FIGURE 5.13: Link distance vs link margin at wavelength 850 nm


86



5.6 FSO Link Analysis Based on Atmospheric Conditions

87


In the earlier section of this chapter, a case study of four different cities of India

representing different topology using visibility data for the year 2013 has been made. In

this section, the atmospheric condition of a particular city i.e. Ahmedabad is considered

for the analysis of FSO Link.

5.6.1 Atmospheric rainfall conditions in Ahmedabad

Ahmedabad is located at an altitude of 55m above the sea level. Ahmedabad

city generally experiences extreme kind of weather conditions dominated by

three primary seasons, which are summer season, monsoon and winter. The weather

conditions of Ahmedabad vary from season to season. There is lots of variation in the day

and night temperatures. Summer season starts during the period of March and lasts by the

month of June. Monsoons generally appear in the month of July and then end in the month

of September. The winter season dominates the city for the period of November to

February. In the city like Ahmedabad there is no or very less possibility of fog and

snowfall during the year. So there is no need to worry about fog and no need to go for

longer wavelength laser and detector that can make the link costly and complicated. Rain

is the only main criteria which must be considered and analyzed to design of the FSO link

in Ahmedabad that is also for about 3-4 months during the year. There are few rainy days

in the season of monsoon that are not predictable but there is no possibility of rain

everyday during the season. The attenuation due to rain depends on the rain rate and that is

also not constant during rainfall. The empirical model for distribution of average duration

of event against the rain rate in Ahmedabad is given as [14].

Daverage = 29.39* R -0.74

min (5.5)

Attenuation due to rain can be about 20 dB per km for the rain rate of about 85 mm/h. The

rain rate above 100mm/h does not frequently occur in Ahmedabad. The heavy rainfall


88

increases the attenuation but the possibility of it is very less and also for less duration that

can be seen from figure 5.16. Before designing the FSO link, link margin should be

provided for average attenuation due to rain. The attenuation due to scintillation is also

noticeable mostly in the season of summer because of large variation in temperature

during the day. The average maximum temperature during March to June is more than

350C and it may reach above 40

0C in the month of May-June and minimum temperature is

about 200 C to 25

0 C during May - June. In the winter the maximum temperature remains

in the range of about 300 C and the minimum temperature is about 10

0 C to 15

0 C. The

signal attenuation caused by scintillation effect depends on the season and time of day as

discussed in section 2.4.

The most important method for determining the performance of the FSO link is to

calculate the link budget. Link budget or link margin considers total transmitted power,

geometric loss, total atmospheric attenuation and receiver sensitivity. Link margin is the

percentage of time that the received power at the receiver is above the sensitivity. The

expression of link margin is given in equation (5.3).

In the present link the following parameters have been taken. The transmitter and receiver

aperture diameters are taken as 0.5 cm each. The beam divergence is taken 2.5 mrad , the

transmitted power is taken 13dbm and the receiver sensitivity is taken -46 dbm. The other

system dependent losses are ignored. The simulation is based on rain attenuation only as

there is very less probability of fog or snowfall in Ahmedabad. Figure 5.17 shows received

power in different conditions of rain attenuation (Arain) versus distance between

transmitter and receiver for this link. The received power becomes less than the sensitivity

of the receiver and link fails at about 2.5 km for attenuation of 15 dB/km (rain rate

50mm/h) and at about 2 km for rain attenuation of 20dB/km (rain rate nearly 80mm/h). In

this simulation maximum attenuation due to rain is taken 20dB/km. About 110mm/h rain

rate increases the attenuation upto 25 dB/km and the distance even decreases with increase

in the rain rate but generally the probability is less of that in Ahmedabad considering its

earlier weather history. Keeping the link margin for worst conditions of rain rate can

minimize the probability of link failure. By improving the sensitivity of the receiver; the

available distance can be increased.


89

FIGURE 5.16: Average duration of event against the rain rate in Ahmedabad.

FIGURE 5.17: Received power under different attenuation conditions due to rain

0 20 40 60 80 100 120 140 160 1800.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

rainfall rate,mm/h

Avera

ge d

ura

tion,m

in

0 0.5 1 1.5 2 2.5 3-80

-70

-60

-50

-40

-30

-20

-10

0

10

20

Distance(km)

Recie

ved p

ow

er

(dB

m)

Arain = 3 dB/km

Arain = 6 dB/km

Arain = 9 dB/km

Arain = 15 dB/km

Arain = 20 dB/km


90

5.7 Summary

The result presented in this study enables to design FSO link considering visibility data. A

case study of four different cities of India representing different topology using visibility

data for the year 2013 has been made. It is observed that Thiruvananthapuram is better

suited for the link availability of FSO as compared to other three cities for the same design

parameters of FSO. We have also considered average visibility and assuming that the

average visibility data is similar for other years as well, the design parameter will remain

same. The demand for high bandwidth and secure communication is increasing in future

and FSO can provide a better alternative for that. Link design of the FSO link can be made

after making survey of the geographical area and better link can be proposed so the

desirable availability and BER performance can be achieved. In the present analysis,

feasibility of the link is analyzed in terms of link availability for four different cities of

India. In the last section the atmospheric condition of a city Ahmedabad is considered for

the analysis of FSO Link. The potential users can thus make a more knowledgeable

decision on the link availability of the FSO systems within their geographical area based

on the localized statistical weather data.

91

APPENDIX:

Computation of the PDF and CDF from the data for various cities (Figures 5.8-5.11)

Probability Density Functions (PDF):

We interpret probability density functions (PDF) as probabilities: If p(x) is a probability

density function (PDF) for some characteristic of a population, then

Þ p�x�dx = fractionofthepopulationforwhichä6 a ≤ x ≤ b

We have visibility data of 365 days for the year 2013 from Wundermap data source and

used low visibility data. The PDF p(x) are generated from dividing the number of days for

a specific visibility by the total number of days (365 days). The PDFs are thus created for

four different cities.

We also know that for any density function,

Þ p�x�dx = 1�∞!∞

Cumulative Distribution Function (CDF):

Suppose p(x) is a density function for a quantity. The cumulative distribution function

(CDF) for the quantity is defined as

CDF�x� = Þ p�t�dtç!∞

The CDF(x) gives

• The proportion of population with value less than x

• The probability of having a value less than x.


92

We have visibility data of 365 days for the year 2013 from Wundermap data source and

used low visibility data. The data available is given in six columns for the visibility 0, 1, 2,

3, 4 and 5 Km. We calculated number of days in respective columns for different

visibility and calculated PDF and then CDF.

References

93

References:

[1] Hennes Henniger and Otakar Wilfert, “An introduction to free-space

opticalcommunications”, Journal of Radio Engineering, Vol. 19, No. 2, 2010.

[2] M. A. Khalighi, N. Aitamer, N. Schwartz and S. Bourennane, “Turbulence Mitigation



Telecommunications - ConTEL 2009, ISBN: 978-953-184-131-3, pp. 59-66, June 2009

[3] Ahmed A. Farid and Steve Hranilovic, “Outage Capacity Optimization for Free-Space

Optical Links With Pointing Errors”, Journal of Lightwave Technology, Vol. 25, No. 7,

pp. 1702-1710, July 2007.



pp. 178-200, June 2003.

[5] S. Mohammad Navidpour, Murat Uysal and Mohsen Kavehrad, “BER Performance of

Free-Space Optical Transmission with Spatial Diversity”, IEEE Transactions on Wireless

Communications, Vol. 6, No. 8, pp. 2813-2819, August 2007.

[6] Maha Achour, S. Hwy and S. Beach, “Free-Space Optics Wavelength Selection: 10µ

Versus Shorter Wavelengths”, UlmTech, Inc., pp. 1-15.

[7] Muhammad Saleem Awan, Laszlo Csurgai Horwath, Sajid Sheikh Muhammad, Erich

Leitgeb, Farukh Nadeem and Muhammad Saeed Khan, “Characterization of Fog and

Snow Attenuations for Free-Space Optical Propagation”, Journal of Communication, Vol.

4, No. 8, pp. 533-545, 2009.

[8] W. Popoola, Z. Ghassemlooy, M. S. Awan, and E. Leitgeb. Piteti, “Atmospheric



Edition, pp. 17-23, 2009.


94

[9] Harilaos G. Sandalidis, Theodoros A. Tsiftsis, George K. Karagiannidis and Murat

Uysal,“ BER Performance of FSO Links over Strong Atmospheric Turbulence Channels

with Pointing Errors”, IEEE Communication letters, Vol.12, No. 1, pp. 44-46, 2008.

[10] Al Naboulsi, M., Sizun H. and de Fornel F., “Propagation of optical and infrared

waves in the atmosphere.” http://www.ursi.org/proceedings/procga05/pdf/F01P.7

(01729).pdf

[11] S. Sheikh Muhammad, P. Khldorfer and E. Leitgeb, “Channel Modeling for

Terrestrial Free Space Optical Links”, ICTON, pp. 407-410, 2005

[12]http://www.wunderground.com/history/airport//2014/8/25/MonthlyHistory.html#calen

dar.




[14] Amandeep Kaur Virk, Jagjit Singh Malhotra, Sakshi Pahuja, “Link Margin

Optimization of Free Space Optical Link under the Impact of Varying Meteorological

Conditions”, International Journal of Engineering Science and Technology (IJEST)”, Vol.

4 No.3, ISSN: 0975-5462, pp. 20-25, March 2012.

[15] Fatin Hamimi Hamat, Abu Sahmah M. Supaat and Farah Diana Mahad, “Simulation

of FSO Transmission at Petaling Jaya due to Attenuations Effect”, ELEKTRIKA, Vol. 12,

No. 1, pp. 30-34, 2010.

[16] Animesh Maitra, Saurabh Das and Ashish K. Shukla, “Joint statistics of rain rate and

event duration for a tropical location in India”, Indian Journal of radio and space physics,

Vol.38, No.6, pp.353-360, December2009.

95

CHAPTER – 6

Summary of the Research Work and

Future Scope of Work

6.1 Summary of the Research Work

The demand for high bandwidth and secure communication is increasing progressively

in future for which FSO can provide a better alternative. The research work is

contributing for the improvement of the link performance under atmospheric

turbulence conditions. The main objective of the study of FSO communication is to get

full advantages of this technology by minimizing the limitations which is caused by

adverse weather conditions. Free-space optical communication through atmosphere

turbulence is now under active research and various methods have been proposed to

improve the performance of FSO in terms of BER, outage probability etc. Also there

are a lot of studies and experiments have been done on FSO in the temperate climate

region where the attenuation due to snow and fog are the two major problems in the

deployment of FSO. However, not much study is done for the deployment of FSO in

the tropical climate region where the rain and haze are present throughout the year. In

order to address some of the current existing problems of research in FSO, we have

investigated the following which are listed below.

1. Developed new technique and demonstrated improvement in communication

performance using diversity technique by simulation. Developed new

mathematical expressions to improve communications performance of Free Space

Optical Link in presence of strong turbulence using wavelength diversity

Chapter 6 Summary of the Research Work and Future Scope of Work

96

technique. A mathematical expression has been derived for the outage probability

for three independent links. The outage probability is calculated for the FSO link

under negative exponential atmospheric turbulence conditions at three transmitter

wavelengths. The outage probability for three separate link at three different

wavelengths is compared with given electrical and threshold SNR values. The

relation between threshold values of SNR (in dB) versus outage probability for

different no. of channels is plotted. For example, for a typical value of -30 dB the

outage probability for a single link 3.1X 10-2

, for two independent links 9.69X10-4

and for three independent links 3.01X10-5

is found. The result shows that using

wavelength diversity, significant improvement is found in outage probability under

strong atmospheric turbulence conditions.

2. Simulated the effects of different wavelengths on visibility range and quality factor

of optical receiver to determine the performance of FSO link. It is concluded that

due to reduction in scattering loss at higher wavelength as wavelength increases,

quality factor of receiver improves. The quality factor is shown as a function of

wavelength varying from 0.85 µ to 1.6 µ for three different values of aperture area

of receiver i.e. 170 cm2, 190 cm

2 and 210 cm

2 and the improvement of in terms of

quality factor with both wavelength and aperture area is shown. Quality factor of

optical receiver is also calculated as function of receiver aperture area for different

atmospheric attenuation conditions.

3. Evaluated the Free Space Optical Link availability for four cities located in

different places of India to determine the feasibility of FSO link and improve

reliability. The link availability is found using the Cumulative Distribution

Function (CDF) for the visibility data. It is shown that, for a given link (i.e.

transmitted power, link range, beam divergence and transmitter-receiver aperture

area) for the year 2013, if the minimum visibility required is 1 km, than the link

availability will be 34 % for Delhi, 56.8% for Kolkata, 91% for Ahmedabad and

93% for Thiruvananthapuram. Similar results are discussed at other minimum

visibility. In another analysis, link length versus link margin and attenuation is

calculated for wavelengths 0.85µm, 1.33µm, and 1.55µm.

6.2 Recommendations for Future Work

97

Extending these basic studies to different cities in different geographical locations

can provide the Five-Nines (99.999%) reliability in FSO communications. The

fundamental diversity technique presented in this thesis to improve the FSO

communication performance using the different atmospheric dynamic conditions

for different cities will provide a very powerful tool to design a practical FSO

system which can offer uninterrupted communication connectivity with high

reliability under all-weather conditions.


• For future research, it is recommended to design a Free Space optical

communication system based on the theoretical analysis developed in this research

to verify and demonstrate the concepts developments in this thesis. This will test

the applicability of the present study to test in real-world atmospheric turbulence.

• In the present work wavelength diversity is used to improve the FSO performance

in terms of outage probability. Further continue to research different wavelength-

based the concept of wavelength-diversity and determine the optimum number of

optical wavelengths for transmitters needed under different atmospheric

conditions.

Selection combining techniques are used to select the signal with largest SNR at

the receiver end. Other diversity combining techniques such as MRC and EGC can

be used and the performance can be measured. Wavelength diversity and coding

can be used simultaneously which can reduce the probability of error further with

increase in the system complexity. Coding and diversity techniques can be

combined together and BER performance improvement can be achieved. Other

diversity technique such as time diversity can also be investigated in the presence

of turbulent atmosphere.

• The availability and reliability of the FSO link can be improved by making survey

of the geographical area where the link has to be established. Statistical data of the


98

atmospheric conditions for a particular geographical area may be collected. One of

the ways to consider the dependency of visibility and availability on locality is

usually to develop a geographical contour map presenting estimated availability at

a particular range, or even estimated range at an assigned availability. These data

are varying seasonally and with location of the particular area. These statistical

data can be analyzed to propose better link in given area. Scattering and

attenuation is usually induced much more in lower visibility conditions. The mean

and variance of this visibility data should be calculated to find the average

visibility at given place in different seasons of the year. With this average visibility

data the link can be designed to perform better and reduce outage probability. In

[1-4] the FSO link analysis is done based on the atmospheric conditions for

particular geographical area.

In the present thesis a case study of four different topological regions of India are

considered to evaluate link availability based on their visibility conditions

throughout the year is presented. The results are useful for designing a FSO link

for given performance parameters in particular region.

• The future trends in satellite communications are likely to make it essential to

implement very high bandwidth, inter-satellite links (data links between different

satellites) and links between earth station and satellite. The large amount of data

exchange is needed between ground stations and satellites that can be provided.

For future satellite-based optical communications programs which can include a

number of different cities in various geographical locations: the object is to

establish optical communications connectivity using satellite-based platforms

under all weather-conditions.

• The high bandwidth of FSO link also creates attraction for use in mobile

communications. Now a days mobile phones are used for many applications which

requires high data rate. In future, the highly efficient tracking system can be

developed to take the benefit of high bandwidth as well as high data rate of

wireless optical communication for mobile applications.


99

• Further investigation can be done to find the effect of different coding techniques

other than the OOK to improve the link performance. As atmospheric turbulence

causes the variation in intensity and phase, the FSK modulation technique as well

as PPM technique can perform better than OOK in atmospheric turbulence

condition. In most of the application of FSO intensity modulation (ON-OFF)

keying technique is used. Other modulation methods can also be used like phase-

shift keying (PSK), differential-phase-shift-keying (DPSK) etc. The orthogonal

modulation formats like M-ary pulse-position modulation (M-PPM) and

frequency-shift keying (MFSK) can also be used for FSO communications [5].

With the increase of M in Mary PPM, the power required decreases as compared to

OOK. One of the most favorable technique on which research can be done is the

polarization shift keying. The polarization state of a traversing field is not changed

(affected) by the turbulent atmosphere. This detail can be explored and the signal

can be encoded on the polarization state of the optical beam and the effect of

turbulence can be minimized.

• As longer wavelength is more efficient for FSO communication, the research and

improvement of detection module for longer wavelength will provide opportunity

to get better communication link distance in bad weather conditions. The most

degrading factor on the performance of FSO is fog. The scattering due to fog is

less at higher wavelength. Longer wavelength optical source and detectors can be

investigated and the effect of fog on the attenuation of optical beam can be

minimized. The optical beam is attenuated more in foggy weather conditions

compared to rain and RF wave is attenuated more in rainy conditions compared to

fog. In fact, a hybrid link of RF/FSO can be used as the attenuation due to rain is

less in FSO compared to RF communication and the attenuation due to fog is less

in RF communication compared to FSO. This area can be explored to meet the

communication performance requirement.


100

References:

[1] Zdenek Kolka, Otakar Wilfert and Viera Biolkova, “Reliability of Digital FSO Links

in Europe”, International Journal of Electrical and Computer Engineering, 2:12, 2007.

[2] Fatin Hamimi Hamat, Abu Sahmah M. Supaat and Farah Diana Mahad, “Simulation of

FSO Transmission at Petaling Jaya due to Attenuations Effect”, ELEKTRIKA, Vol. 12, No.

1, pp. 30-34, 2010.




[4] B.S. Naimullah, S.Hitam, N. S. M. Shah, M. Othman, S. B. A. Anas, and M. K.

Abdullah, “Analysis of the Effect of Haze on Free Space Optical Communication in the

Malaysian Environment”, International Conference on Communications, Penang,

Malaysia, May 14-17, 2007.

[5] Thomas Plank, Erich Leitgeb and Markus Loeschnigg, “Recent Developments on Free

Space Optical Links and Wavelength Analysis”, 2011 International conference on space

optical systems and applications, pp. 14-20, 2011.

List of Publications

101


International Journals

1. Anil. J. Kshatriya, Y. B. Acharya, A. K. Aggarwal and A. K. Majumdar,

“Communication Performance of Free Space Optical Link Using Wavelength Diversity in

Strong Atmospheric Turbulence, Journal of optics, ISSN:0972-8821, DOI

10.1007/s12596-015-0248-7, Vol. 44, No. 3, pp. 215-219, March-2015.

2. Anil J. Kshatriya, Y. B. Acharya, A. K. Aggarwal and A. K. Majumdar, “Estimation of

FSO Link Availability Using Climatic Data, Journal of optics, ISSN: 0972-8821, DOI

10.1007/s12596-016-0327-4, accepted for publication, February-2016.

International Conferences

1. Anil J. Kshatriya, Y. B. Acharya and A. K. Aggarwal, “Analysis of Free Space Optical

link in Ahmedabad Weather Conditions”, IEEE Conference on Information and

Communication Technologies (ICT 2013), organized by Noorul Islam Centre for Higher

Education Kumaracoil, Thuckalay, Tamilnadu, India, Print ISBN 978-1-4673-5759-3, pp.

272-276, April 11-12, 2013.

2. Anil J. Kshatriya and Pravin R.Prajapati, “ Effect of Signal Wavelength and Aperture

Area of Detector on Performance of Free Space Optical Link”, IEEE students conference

on Electrical, Electronics and Computer Sciences, organized by Maulana Azad National

Institute of Technology, Bhopal (NIT-Bhopal), Print ISBN 978-1-4673-1516-6, pp.1-3,

March 1-2, 2012.


102

National Conferences

1. Anil J. Kshatriya, Y. B. Acharya, A. K. Aggarwal and A. K. Majumdar,

“Communication Performance of Free Space Optical Link Using Wavelength and Spatial

Diversity in Atmospheric Turbulence”, National Conference on Emerging Areas of

Photonics and Electronics EAPE-2013, organized by B. P. Poddar Institute of

Management and Technology, Kolkata, pp.29-37, 30-31 August 2013.

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INVESTIGATION ON THE PERFORMANCE AND IMPROVEMENT … · Originality Report Certificate It is certified that PhD Thesis titled Investigation on the Performance and Improvement of Free