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[EEE/OSA/[APR International Conference on [nfonnatics, Electronics & Vision

Modeling and Performance Analysis of Free Space Optical Communication System

Md. Tawabur Rahman Department of EEE

Khulna University of Engineering & Technology, Khulna, Bangladesh

Shahid Iqbal Department of EEE

Mymensingh Engineering College Mymensingh, Bangladesh

Md. Monjurul Islam Department of CSE

Mymensingh Engineering College Mymensingh, Bangladesh

Abstract-In this paper, an attempt has been made to

investigate the impact of free space transfer function taking

into effects of atmospheric turbulence, pointing errors and

path loss factor on the performance of free space optical

communication system. The performance of the proposed free

space optical communication system is studied by developing a

MATLAB simulator using simulink in which free space is used

as a communication channel. Finally, we evaluated the Bit

Error Rate (BER) performance of the proposed system varying

with different system parameters such as distance, transmitter

power, path loss factor, atmospheric turbulence and received

signal power. The BER is highly degraded on severe

atmospheric turbulence condition ever for a short distance of

free space channel. The effect of path loss factor due to dense

fog is also severe on the BER even though the turbulence effect

and free space distance is short. The optimum transmitted

input power and receiver antenna radius at which the BER is

minimum, is strongly dependent on free space distance and

atmospheric parameters.

Keywords-free space optical communication, bit error rate

I. INTRODUCTION

Twenty first century is an era of "[nfonnation technology". There is no doubt that infonnation technology has had an exponential growth through the modem telecommunication systems. But, in existing Microwave communication bandwidth is limited as well as the number of channel. There also required radio permits and the data rate is comparatively lower. So, with highly developing information technology through the telecommunication system there must be an alternative. The only reasonable alternative is free space optical (FSO) communication [1]. It plays a vital role in the development of high quality and high-speed telecommunication systems, as it offers bandwidth of 105 times greater than the existing Microwave communication. [t also provides high security, low cost, low power and high rates due to unregulated bandwidth. FSO communication is a method of transmitting information from one place to another by sending light through the free space. It is an exciting technology that establishes point- to- point communication links through the atmosphere [2].

Optical communication has become popular nowadays in fiber optic communication. Research is going on to use Optical communication employing free space as communication channel instead of fiber optic waveguide. Because,

978-1-4673-1154-0112/$31.00 ©20 12 IEEE

• Telecommunication systems

propagation in atmosphere (free

costly than optical fiber.

using light

space) are less

• They do not require radio permits and licenses like

microwave and radio-relay systems. • Unlike optical fiber cables, FSO equipment is

recoverable and moveable and requires less than a

fifth of the capital outlay of comparable ground

based fiber optic technologies [3].

In previous studies [4], performance of free space optical communication is investigated without considering all impairments related to atmospheric conditions and related system parameters. But in this paper, the combined effect of atmospheric turbulence, path loss factor, and pointing errors are considered which fade the signal at the receiver and deteriorate the link performance. The performance of the proposed free space optical communication system is analyzed by developing a MA TLAB simulator named simulink in which free space is used as a communication channel. Finally, BER perfonnance of the proposed system is evaluated varying with different system parameters such as distance, transmitter power, path loss factor, atmospheric turbulence and received signal power. The BER has highly degraded on severe atmospheric turbulence condition ever for a short distance of free space channel. The effect of path loss factor due to dense fog is also severe on the BER even though the turbulence effect and free space distance is short. The optimum transmitted input power and receiver antenna radius at which the BER is minimum, is strongly dependent on free space distance and atmospheric parameters.

The rest of this paper is organized as follows. [n section II, Block diagram of FSO link, the atmospheric turbulence, pointing error and path loss factor are described. [n Section III, the newly proposed model is presented. The perfonnance analysis and simulation results are shown In Section IV. The paper has been concluded in Section V.

[I. SYSTEM ARCH[TECTURE

A. FSO Link

A block diagram of an FSO communication link is presented in Fig. 1. The transmitter modulates data on to the instantaneous Intensity of an optical beam . [n this paper, we consider intensity Modulated direct detection channels using

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[EEE/OSA/[APR International Conference on [nfonnatics, E[ectronics & Vision

On/Off Keying (OOK) modulation, which is widely employed in practical systems. The received photo current signal is related to the incident optical power by the detector responsivity r. [t is assumed that the receiver integrates the photo current for each bit period and removes any constant bias due to background illumination. The received signal Y suffers from a fluctuation in signal intensity due to atmospheric turbulence and misalignment, as well as additive noise, and can be well modeled as [5]

Y=HrX+N (1)

Where, X is the transmitted intensity, H is the channel state, Y is the resulting electrical signal, and N is signalindependent additive white Gaussian noise with

variance o'� . The channel state H models the random

attenuation of the propagation channel. In our model, H

arises due to three factors: path loss factor HI' geometric

spread and pointing errors H p , and atmospheric turbulence

H a [4]. The channel state can be formulated as

(2)

Here, HI is deterministic, and H p and H a are random

with distributions.

B. Atmospheric Statistical Models (Atmospheric

Turbulence)

Many statistical models for the intensity fluctuation through FSO channels have been proposed over the last two decades [6]-[8]. For weak turbulence, the intensity fluctuation probability density function (pdf) is modeled as a log-normal distribution, which has been validated through experimental measurements [1], [7], [9]. The log-amplitude of the optical Intensity has a Gaussian pdf with log-

amplitude variance o'� given by

4

O'l� is the Rytov variance defined as [6], [14]

7 11 0'; = 1.23c� K6 Z6

(3)

(4)

0'; can be measured directly from atmospheric parameters.

The intensity distribution is given by

212

(5)

The log-normal distribution can not characterize scintillation effects in strong turbulence regimes [7], [[ 0].

In a recent approach to FSO channel modeling [6]-[7] a Gamma-Gamma distribution was used to model

atmospheric fading. In this case, the pdf of Ha is given as

f (H) = 2(afJ)(a+ PJ /2 (H ) (a;fJJ_1 k (2Jaru ) 6 Ha a r(a)rCfi) a a-fJ fJ1a ()

where ka-fJ (-) is the modified Bessel function of the second

kind, and liP and 1/a are the variances of the small and large scale eddies, respectively [7].

For Gaussian beam intensity,

Where, PR = Received power

W = beam width Now, The Received power is [[ []

C. Path loss/actor

(7)

(8)

The attenuation of laser power through the atmosphere is described by the exponential Beers-Lambert Law as

P(Z) HI = -- = exp( -O'Z) P(O)

(9)

Where HI is the loss factor over a propagation path of

length Z, P(Z) is the laser power at distance Z, and (J is the attenuation coefficient [2].

Laser & Modulator

Photodiode & De-modulator (lJ--1 Receiver I

Telescope

Figure 1. Block diagram of a FSO link

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IEEE/OSA/IAPR International Conference on Infonnatics, Electronics & Vision

D. Pointing error

In line-of-sight FSO communication links, pointing accuracy is an important issue in determining link performance and reliability. However, wind loads and thermal expansions result in random building sways, which, in tum, cause pointing errors and signal fading at the receiver [4].

Where V = (J1i;,) 1(2Wz ) , and

ao = [erf(V)]2

W2 = W2 .JJierf(V) Zeq Z 2V exp( _V2)

(10)

(II)

(12)

Here, ao is the fraction of the collected power at R = 0, and

W is the equivalent beam width. We consider Zeq

independent identical Gaussian distributions for the elevation and the horizontal displacement (sway), as was done in previous work. The radial displacement r at the receiver is modeled by a Rayleigh distribution

R R2 fll(R) = -2 exp(--2)' R>O (13)

as 2as 2 Where as is the jitter variance at the receiver.

Combining (1\) and (13), the probability distribution of Hp can be expressed as

2 f (H ) -LH y2_1 O<H < p - 2 P , - P - ao Hp y ao

(14)

Where Wz I 2a" is the ratio between the equivalent beam eq l

radius at the receiver and the pointing error displacement standard deviation at the receiver.

E. Channel Statistical Model

The probability distribution of H = H,HpHa can be

expressed as

Where f(H; Wz) is a family of pdfs parameterized by

the beam width Wz, and fH / H a is the conditional

probability given a turbulence state Ha' Recall that Hi is

deterministic and acts as a scaling factor. The resulting conditional distribution can be expressed

(16)

Channel

Figure 2. Simulation model of free space optical communication system using simulink

213 ICIEV 2012


Substituting (13) into (12) gives

Simplifying and defining f1 = 2(j� (1 + 2 y2 )JrR 2 results in

Putting the value of HI' H p and H a in (18) we get our

required equation i.e. the free space transfer function.

III. PROPOSED MODEL

A. Simulation model of free space optical Communication system

The simulation model of FSO communication system is shown on Fig. 2. The simulation model is done in graphical application Simulink of Matlab program environment. The experiment is made assuming the following parameters: fixed discrete step with duration 100 [ps], the duration of one chip is defined by 10 times discretes T c= 1 [ns]; considering the chosen code length N=31 one information bit is defmed as T=31 [ns] [12].

B. Model of generating random data sequence and

assigning different code

The transmitter of simulation model is shown on Fig. 3(a), and it includes random information data 100 bits sequence for every user (Bernoulli generator). This generator switches on in arbitrary time moment which is different for every user. This represents asynchronous part of the simulation experiment. The generated data sequence for every user is coded by signature code word which is specific for different users in cases when information bit 1 is transmitted which is shown in Fig. 3(b). The information signal formed for every user is summed and send through free space channel [12].

C. Simulink model of{ree space channel

First, we have computed Fast Fourier Transform (FFT) of composite data signal to convert it into frequency domain. Then the composite FFT signal is multiplied with the coefficient of our proposed free space transfer function .Finally, Inverse Fast Fourier Transform (IFFT) is computed on the product data to get the composite data in time domain. This is illustrated in Fig. 4.

214

D. Model of Receiver

A hard-limiter for levels 0 and 1 is included in the input of receiver. This limiter provides decreasing noise influence to the common infonnation data stream and helps in correct identifying user data (Fig. 5). Existence of infonnation bit I is provided by individual correlator for every user at the moment of arriving of his specific signature code word. Information bit 0 is considered in other different cases. The properties of optical orthogonal codes OOC guarantee correct extracting of information data sequence for every user.

There is a mechanism for automatic detection activity for any user in the simulation model of the optical communication system. This is done by using automatic tracking of common detected level after all user correlators [12].

t------.c::J Bernoulli Binary

Bernoulli generator (a)

S?c-------�� I CONV j--1----I��I(O�t1 )

Hadamard Code Generator

Hadamard Code Generator

(b)

Figure 3. (a) Models of generating random data sequence (b) coding data sequence by signature code word for first user

Figure 4. Simulink model offree space channel

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'01


MinMax8

rl�rq..--s---6-----+ ma, t--+---.-----+-� Saturation COlTelation

MinMax2

Code Generator

MinMax1 Saturation1

Interval Test Dynamic

Constant

Rate Transition2

f- --I-------l.--R·at,I�,f-'"-3 --------�: k L-__ -+lr------,I---_+(2) MloMa,3 f

Code Generator1 Constant1

Figure 5. Model of Receiver

IV. PERFORMANCE ANAL YSIS AND SIMULATION RESULT

A. Bit stream transmission (without assigning code)

We have sent the bit stream to check the impact of the free space channel on this transmitted bit stream. From Fig. 9 we can see that the voltage of output bit stream

is reduced to 0.73 volt from 1 volt and it will be distorted more as the distance is increased.

As the effect of the free space channel is gradually increasing with increasing distance, so the received output is gradually distorted. But with increasing distance if we increase the power, the received output can be maintained constant(i.e. above threshold level) within a certain distance. This result is shown in Fig. 10.

Then we observed the effect of the free space channel on the output at the receiver with increasing distance for constant transmitter power. We have done this for various transmitter power (such as 100mW, 150mW, 200mW). This result is shown in Fig. I I. From this observation we can conclude that the performance will be satisfactory in this free space channel (where all the effects i.e. atmospheric turbulence, pointing error, path loss are included) between 100m to 500m.That is why, We analyze the performance of our proposed free space optical communication system model between this range.

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Interval Test Dynamic1

Figure 6. The transmitted bit stream

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Input bit pattern

Time

Mag

nitu

de

[EEE/OSA/[APR International Conference on [nfonnatics, E[ectronics & Vision

70

60

50

40

30

20

1.8 Frequency

Figure 7. The transmitted signal in frequency domain

transfer function

0.9

0.8

0.7

0.6 " '5 §, 0.5

� 0.4

0.3

0.2

0.1

O L-� __ � __ ��-L __ -L __ -L __ � __ � __ �� o 0.2 0.4 0.6 0.8

frequency 1.2 1.4 1.6 1.8

10 X 10

Figure 8. The Transfer function in frequency domain

recei\.€d bit pattern for OAkm O.s .-------�------�----�------�------.___----___.

0.7

0.6

? 0.5

� :3 0.4 ·c '" '" E 0.3

0.2

0.1

O ��----�------�----�------�----� o 200 400 600

time SOO 1000 1200

Figure 9. The received bit stream after propagating through free space

216

B. BER Characteristics

Fig. [2 illustrates the Plot of BER with respect to increasing distance. [t shows that, with the increasing length, the BER is increasing due to the increase of path loss and atmospheric turbulence with distance. Here we vary the length from 100m to 500m and the corresponding BER is plotted. Here we took the Transmitted power of 300mW. For getting lesser BER i.e. to improve the system performance we have to increase the transmitted power. Fig. 13 shows the Plot of BER with respect to increasing transmitted power. [t shows that, with the increasing transmitted power, the BER is decreasing. Here we vary the transm itted power from 200m W to I W.

1600

§" 1400 E 1200 � (II 1000 � 0 Q. BOO �

� 600 .� E 400 VI c � 200 f-

0

o

•

/ / / / ./ /' �

0.2 0.4 0.6 O.B

Distance(Km)

1.2 1.4 1.6

Figure 10. Plot of Transmitter power Vs Distance for maintaining constant output at the receiver considering the free space channel (where all the

eflects i.e. atmospheric turbulence, pointing error, path loss are included)

0.9 1----------._---------------------0.8 1--------\:---------------------O} 1-------+"<+--------------------

5" 0.6 1-------+--\-\,---------------------� 05 1--------+-++------------------, a 0.4 I--------t-t-\------------------

OJ I-------��'\----------------U.l I------------'Io:-l--'k:--------------0.1 I---------------"tc-'=....--"r--,------------

0.2 0.4 0.6 0.8

DistanceiKmj U 1.4 1.6

-+-pt·l00mW ...... pt.150mW _pt·200mW

Figure 11. Plot of Output at the receiver V s distance maintaining constant transmitter power considering the free space channel (where all the effects

i.e. atmospheric turbulence, pointing error, path loss are included)

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10.''---------'---------'-----'---------'-----'---------'----'---------' 100 150 200 250 300 350 400 450 500

distance(m)

Figure 12. Plot of BER with respect to Distance In the previous figure we have described that, if we increase the transmitted

power the BER will decrease and we can observe the improvement here. We consider the constant length of 300m.

10· 5'--------'----�-�--�---'----�-�-----.J 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Transmitted power(W)

Figure 13. Plot of Bit Error Rate (BER) with respect to Transmitted power

1 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 path loss factor(hl)

Figure 14. Plot of BER with respect to Path loss factor

217

Figure14 explains the performance of our proposed system with respect to path loss factor .As we described previously

path loss factor (HI) is a ratio of received power at distance

Z, P(Z) to the transmitted power, P(O) i.e.

H = P(Z)

I P(O) Here we can see that if the path loss factor increases the BER decrease. This can be explained as follows: if we increase the distance, the received power P(z) will decrease and path loss factor also decreases so BER will increase. Similarly, if we decrease the distance, the received power P(z) will increase and path loss factor also increases, so BER will decrease.

10-3�--�--�--�--�--�-�

10-�_'c5-----:-----:,-'c_5------:----:2:'o_5------:---73_5 Recei\A3d Signal intenSity x 107

Figure 15. Plot of BER with respect to Received signal power intensity

The above Fig. 15. shows the vanatIOn of BER with respect to received signal power intensity. Here we can see that if the received signal power intensity increases the BER decreases and vice versa. The received signal power intensity is also a measure of atmospheric turbulence. If the turbulence decreases the received signal power intensity increases, so lower BER occurs and if the turbulence effect increases the received signal power intensity decreases, hence higher BER occurs. This effect is shown in Fig. 16.

10· 5 '------�----'----�------':---�---'-------:1�0-----:'11 atmospheric turbulence x 10-8

Figure 16. Plot of BER with respect to Atmospheric turbulence effect.

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Since FSO link requires precise pomtmg, the effect of pointing errors on link performance is a vital issue. In Fig. 17. we can see the relation between the pointing errors and beam radius. From the figure we can see that if the beam radius is increased pointing error is reduced as precise pointing can be obtained easily. In our proposed model we have taken the effect of pointing error as a constant value in our free space transfer function, because we have taken the beam radius constant throughout the free space distance.

0.38,-----�--�--�--�--�--__.

0.36

0.34

0.32

� 0.3 g � 0.28 � "0 0.26 "-

0.24

0.22

0.2

0.18L---�--�--�--�--�----1 0.2 0.22 0.24 0.26

beam radius(m) 0.28 0.3 0.32

Figure 17. Plot of pointing error (hp) with respect to beam radius.

V. CONCLUSION

In this paper the combined impact of atmospheric turbulence, path loss factor and pointing error on the performance of free space optical communication system is analyzed. A simulation model of free space communication system is developed in Matlab using simulink. Bernoulli generator is used for generating random data sequence for every user and the data of individual user is coded with Hadamard code before transmission through the free space channel. At the receiving end the direct detection technique is used. Then the BER is evaluated by varying the several parameters such as distance, transmitter power, path loss factor, received signal intensity and atmospheric turbulence. At first, from observing the plot of distance Vs output magnitude curve shown in Fig. 1 I. for single bit transmission through the free space channel we have decided to implement our proposed model for the communication between 100m t0500m range, as the output magnitude has been drastically reduced beyond 500m. Then we have evaluated the BER of our proposed model with respect to distance and plotted it in a semilog scale. For 100m distance the BER was approximately 10-385 for a constant transmitter power of 0.35watt and receiver antenna radius of 1m. But if we increase the transmitter power upto 1 watt the BER decreases to approximately to 10-5. After that we have determined the BER performance of proposed system with respect to path loss factor. When the path loss factor is maximum the BER is almost approximately 10-3• This path loss factor is severe in presence of dense fog. The

218

effect of atmospheric turbulence was also observed at maximum turbulence condition the BER is also approximately 10-32. Here we have taken the effect of pointing error as a constant value in our free space transfer function, because we have taken the beam radius constant throughout the free space distance.

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