Binary Pulse Position Modulation Simulation System in Free Space Optical Communication Systems

7/27/2019 Binary Pulse Position Modulation Simulation System in Free Space Optical Communication Systems

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ICIAS 2010: International Conference on Intelligent and Advanced Systems

Binary Pulse Position Modulation Simulation

System in Free Space Optical Communication

Systems

AbstractFree space optical communication system

(FSO) is a potential solution for increasing bandwidth

demands. The FSO has a capability to provide high speed

data communication, economic and quick deployable.

Although FSO has several advantages, but at the same time

FSO faces a major challenge from scintillation introduced

by atmospheric turbulence. In this paper, the simulation

performance for the binary pulse position modulation

(BPPM) system is done under weak and strong atmospheric

turbulence. The performances are analyze in terms of bit

error rate (BER) and eye diagrams. The system use

avalanched photodiode (APD) receiver. The simulation was

verified with ideal calculated performance for BPPM. The

results show the simulation system is capable to give theBER similar like mathematical model and give the optimum

gain of APD to achieve the best BER in atmospheric

turbulence environment. The optimum gain for APD result

from the simulation for weak turbulence is 150-165 and 160

for strong turbulence.

Index TermsAtmospheric Turbulence, Avalanche

Photodiode, Binary Pulse Position Modulation, Free Space

Optical Communication Systems.

I. INTRODUCTION

FSO typically currently used intensity modulation with

direct detection (IM/DD) since this system associated

with complexity of phase and frequency modulation [1].

However, in practice, the performance of FSO can be

degraded and scintillation induced by atmospheric

turbulence as a major impairment. Atmosphericturbulence happens due to variations in the index or

refraction caused by temperature fluctuation. The impact

of atmospheric turbulence is it can cause random

variations in signal intensity. The turbulence is classified

as weak when scintillation index is less than 0.75 while

scintillation index of 1 represents a strong turbulence [8].

Generally, scintillation index is a complicated function of

the beam parameters, propagation distance, height of the

transmitter and receiver, and the fluctuations in the index

of refraction [2].

A system using PPM is better than On Off Keying

(OOK) because PPM is power efficient compared toOOK since satellite communication links required the

large peak laser power level to survive huge losses during

transmission [4]. The presence of a pulse in the symbol

frame regardless of the transmitted symbol benefits the

clock recovery subsystem, whereas an On-Off Keying(OOK) system may suffer synchronization loss if a

sequence of zeros is encountered [6]. However the

current technology, Q-switched laser cannot be toggled

between on and off states at a very high rate. This

scenario automatically is limiting the data rate that can be

supported using OOK transmission scheme.Avalanche photodiodes (APD) are used to boost the

signal level over additive noise level present at thereceiver [4]. This is because the signal received by the

detector is attenuated due to large distances and effect

from the atmosphere. APD will magnify each incident

photon to a high number of randomly distributed

postdetection electrons. An APD can provide mean gain

values in the range 50 to 200 [1]. In BPPM systems, they

will increase the bit error rate (BER) since APD will

produce excess noise factor. This excess noise factor was

affected by APD gain and ionization factor of APD. Inother word, there is very important to use suitable gain of

APD in FSO systems since APD gain leads to an increase

of excess noise factor and ultimately reducing the BER.

This paper will discuss the details about BPPM insection II. It is consists the format of BPPM system, the

N. Tahir1, N. Mohamad Saad

1, B. B. Samir

1, V. K. Jain

1, S. A. Aljunid

2

1Electrical & Electronic Engineering Department, Universiti Teknologi PETRONAS,

Bandar Seri Iskandar, 31750 Tronoh, Perak, MALAYSIA

Tel: +605-368-8000 Fax: +605-365-74432Research and Development Unit, Universiti Malaysia Perlis

01000 Kangar, Perlis, MALAYSIA

Tel: +04-9798784 Fax: +04-9798790

[email protected], [email protected],[email protected],

[email protected], [email protected]

Free space optical communication systems (FSO) is an

optical communication that uses laser light to transmit

data between two points. The laser light propagates infree space so that this technology becomes the best

solution to overcome the problems occurred by using

optical fiber. Systems of FSO can function over distances

of several kilometers as long as there is a clear line of

sight between the transmitter and the receiver,

communication is theoretically possible. Even if there is

no direct line of sight, strategically positioned mirrors can

be used to reflect the energy. The beams can pass through

clean glass windows with little or no attenuation.


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photodetection process and the probability of word error.

The simulation system of PPM using APD withatmospheric turbulence is briefly described in section III.

In section IV, this paper shows results of simulation

systems including the simulation signals; the clock signal,

the information signal, the BPPM modulated signal and

the detected signal at the receiver.

II. BINARY PULSE POSITION MODULATIONIn BPPM, two bits are transmitted in block instead of

one at a time. The block is called a BPPM frame. Optical

pulse is placed in one of two adjacent time slots to

represent the data block. The optical block encoding isachieved by converting each block of two bits into one of

optical fields for transmission. At the receiver, decoding

of each block is achieved by determining which of the

fields is being received during each block time.

The optical PPM direct detection receiver block

diagram is shown in Fig. 1[1]. At receiver, PPM decoder

must decide which one of the slots occurring during aframe time contains the optical pulse. The incoming field

is photodetected, and slot integration is made for each

slot time by a synchronized slot clock. The sequence of

slot integrations, (v1,v2,,vM) collected over a frame time

are then compared for the maximum, with the largest oneidentifying the signal slot for that frame. This maximum

comparison among the slot values is in fact the decoding

test producing the minimum probability of a decoding

error. Decoding word error happens when incorrect slot

produces a higher integration value than the correct slot.

The integrators densities depend on photodetection

model.

(a)

(b)

Fig. 1. Binary Pulse Position Modulation format. (a) Encoder. (b)

Receiver and decoder

The probability word error (PWE) represents the

probability of decoding the incorrect PPM pulse position.

But, the incorrect decoded word may still produce some

correct bits. The bit error probability (PE) is different

from the PWE. This relation can be obtained by

determining the probability that a given bit of the word

will be incorrect after incorrect decoding. If incorrectly

decoded word is equally likely to be any of the remaining

word, then a given bit will be decoded as any of the bits

in the same position of each word. In two equally bit

patterns, a given bit position will be a one or zero one

times. In summary, the probability of a given bit being inerror is this probability times the probability that the word

was in error.

III. SIMULATION SYSTEM MODEL

The modulator is built to perform the random datasignals into PPM format. A clock is added to make the

random data coded. Pulse will coded as zero when the

clock strikes and it does not receive signal until one cycle

later. While when the clock strikes and received a signal,

the pulse will be coded as one. Then the demodulator will

recover the modulated data. Electrical rescale is used to

scale the maximum and minimum values of the inputsignals. The oscilloscope is used to monitor the signal so

that the signal is modulated in the PPM format.

The modulated signal is transmitted using laser with

wavelength of 1550 nanometer. The medium used FSOlink with affected atmospheric turbulence. Since the

atmospheric turbulence usually expressed in terms of

normalized intensity variance or scintillation index [7],

the Rytov approximation is used to get the turbulence

strength in m-2/3 unit. By using the Rytov relationship,

the turbulence strength will be performed in optical signalattenuation in dB/km.

Fig. 2. BPPM system simulation model

In the receiver part, APD is used as a photodetector.

The gain of APD varies from 50 to 200 to get the bestBER in turbulence FSO channel. The ionization factor

is fixed to 0.028 for all simulation done. The dark

current of APD in this simulation system is 10 nano

Ampere while the responsivity is 70 A/W. The main

reason of using APD is because this photodetector has

more sensitivity than PIN receiver, making so that it

suitable to use for long distance.


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Eye diagram for

scintillation index 0.35

Eye height2.8097X10-6

Minimum BER6.8197X10-6

Eye diagram forscintillation index 0.55

Eye height1.1860X10-6

Minimum BER

1.6245X10-4

IV. RESULTSAND DISCUSSION

The oscilloscopes are used to monitor the signal sothat the simulation system model is in BPPM format.

The bit rate for this system is 2.4 Giga bit per second

(Gbps) when the noise temperature is in room

temperature. The figures below show the simulation

signals. They included the clock signal, theinformation signal, the BPPM modulated signal and

the detected signal at the receiver.

(a)

(b)

(c)

(d)

Fig. 3. BPPM simulation signals. (a) The clock signal. (b) Theinformation signal. (c) The modulated BPPM signal. (d) The detected

signal at receiver

The simulation is performed for several of APD gain

from the range 50 to 200. All the simulation is divided by

two scenarios, which are for weak turbulence and strong

turbulence. For weak turbulence, the scintillation index is

in the range of 0 to 0.75 while for strong turbulence, thescintillation index is 1. In this paper, the simulations for

weak turbulence are done for scintillation index 0.2, 0.35

and 0.55. The simulation system performance is

monitored using eye diagram analyzer. From this

analyzer, we also get the value for eye height and

minimum BER for the simulation system.

Figure 4 (a) and (b) shows the eye diagram for

scintillation index 0.35 and 0.55. The other parameters

are remaining fixed. For scintillation index 0.35, the eyeheight of eye diagram is bigger than eye height for

scintillation index 0.55 for the same APD gain. While for

BER, the scintillation index of 0.35 shows smaller BER

than scintillation index 0.55. As expected, the increasingatmospheric turbulence results in an increase in the

required signal level to achieve the same performance.

To determine the best APD gain for weak turbulence,

the simulation system model is done for three scintillation

indexes, which are scintillation index 0.20, 0.35 and 0.55.Figure 5 shows the results of BER versus APD gain forvarious scintillation indexes. The graph shows the

improvement of BER with increasing APD gain. But, at

APD gain more than 170, the BER starts to increase. This

is because the large APD gain leads to an increase in

excess noise factor, and resulting the increase of BER.

In Figure 6, the result for strong turbulence is done for

various APD gain to find the best one. As expected, the

trend of the graph follows the trends of weak turbulence

but with bigger BER. The excess noise factor caused by

large gain of APD is verified in this simulation. For

strong atmospheric turbulence, the best of APD gain is in

the area of 160. The major effect that affects the BER forBPPM system is excess noise factor of photodetector,

while other parameters are constant. It is very important

to get the best value of APD gain so that the FSO systems

will perform the best BER for atmospheric turbulence

scenario.

(a)

(b)

Fig. 4. (a) The eye diagram for scintillation index 0.35. (b) The eyediagram for scintillation index 0.55


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Fig. 5. BER vs APD gain for various scintillation index of weak

turbulence for BPPM simulation system

Fig. 6. BER vs APD gain for strong turbulence BPPM simulation

system

IV. CONCLUSION

The simulation system of BPPM for weak and strongturbulence channel are successfully done using Optic

Software (OptiSys). The results show perfect simulation

signals included the clock signal, the information signal,

the BPPM modulated signal and the signal detected at the

receiver. The results also show the graph for BER in

different gain of APD and various scintillation index ofatmospheric turbulence.

Therefore, the major contribution from this paper is in

designing a simulation system model for M-ary PPM inweak and strong turbulence channel using the same

software. The simulation and numerical result will be

compared to see the performance of BPPM in FSO

channel. The simulation was verified with ideal

calculated performance for BPPM.

ACKNOWLEDGEMENT

This research has been supported by the Postgraduate

Office of Universiti Teknologi Petronas.

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Documents

Binary Pulse Position Modulation Simulation System in Free Space Optical Communication Systems