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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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