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Effect of Signal Wavelength and Aperture Area of
Detector on Performance of Free Space Optical
Link
Anil J. KshatriyaDepartment of Electronics and Communication
Government Engineering College
Chandkheda,Gujarat,India
Email:anil.kshatriya@yahoo.com
Pravin R. Prajapati
Department of Electronics and Communication
A.D.Patel Institute of Technology
New V.V. Nagar,Gujarat,India
Email:pravinprajapati05@yahoo.com
AbstractBit error rate performance of a FSO link and visi-bility range are adversely affected by, harsh weather conditions,and atmospheric turbulence like fog, rain, clouds and dry snow.Wavelength of signal and aperture area of optical detector affectsquality factor of receiver. This paper presents performance ofFSO link with different wavelengths and different aperture areaof optical detector. Effects of different wavelengths on visibilityrange and quality factor of optical receiver have been simulated.For simulation, license versions of OPTSIM 5.2 and MATLAB6.00 have been used.
Index TermsFSO, PRBS, BER, aperture area
I. INTRODUCTION
Free space optical communication has very large informa-
tion handling capacity. Free space optical (FSO) is a promising
technology capable of full duplex transmission of data, voice
and video in certain applications [1]. The link can be installed
within a day. Compared to fiber communication, FSO does not
require digging to lay the fiber and it does not require permis-
sion from the landowners. W. Popoola et al and Muhammad
Saleem Awan et al [2], [3] reported performance of FSO in
different weather conditions like thick fog, moderate fog, light
fog, heavy rain, moderate rain and light rain etc. Fang Xu et
al reported efficiency of different channel coding techniques
for different time diversity orders and turbulence conditions[4]. Performance of FSO link using MIMO transmitter and
receiver investigated by Zeinab Hajjarian et al [5]. W. O.
Popoola et al presented error performance of FSO using a
subcarrier intensity modulation based on a binary phase shift
keying scheme in turbulent atmosphere [6]. A large effective
aperture can be achieved by combining the output signals from
an array of smaller receivers.Chen [7] reported the non coher-
ently combined heterodyne receiver combined system, which
can offer superior performance for given a fixed collecting
area.M.A. Khalighi et al [8] investigated the impact of aperture
averaging on the performance of FSO systems under different
atmospheric turbulence regimes and performance evaluation
is made in terms of the average BER. Nazmi A. Mohammed
et al [9] reported that the BER performance of FSO can be
improved by using forward error correcting codes. Ivan B.
Djordjevic et al [10] reported that the LDPC coded MIMO
schemes can operate under a strong atmospheric turbulence
and at the same time provide excellent coding gains compared
with the transmission of uncoded data. Sachin M. Kale et al
[11] reported that for a 2 km FSO link, relative improvements
in SNR and BER due to combined effect of partially coherent
optical beam and the aperture averaging of the received optical
beams in different atmospheric turbulence conditions. AhmedA. Farid et al[12] derived a statistical model for the optical
intensity fluctuation at the receiver due to the combined effects
of atmospheric turbulence and pointing errors. Colin Reinhardt
et al [13] described an improved method for estimation of the
atmospheric channel impulse response function, for adverse
visibility conditions of fog to improve the performance of
terrestrial FSO system. Thomas Plank et al [14] reported some
major performance improvement obtained by employing some
specific modulation and coding schemes
II. SIMULATION
The block diagram for the simulations link of free spacecommunication is shown in Fig. 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. 1. This model generates a binary sequence of several
different types like alternating one and zero sequence, PRBS,
only sequence of one and only sequence of zero. NRZ driver
converts an input binary signal into an output electrical signal.
The output signal may be specified as either voltage or current.
Here NRZ modulation is considered. The input data source
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modulates at the LED beam by using an LED driver.LED
source generates the LED beam at 1550 nm. 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
have been considered The OptSim photo receiver model is
composed of several individual building blocks: the photo
detector, the preamplifier, and the post amplifier/filter complex.
Here PIN photo detector with quantum efficiency of 80%, dark
current of 10 A and ionization coefficient of 1 considered.BJT based preamplifier with noise like shot noise and thermal
noise etc considered. Bessel filter with 4th order and 1 GHz
bandwidth has been considered.
III. RESULTS AND DISCUSSION
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 proportionalto wavelength and visibility () = 3.91
V
550
q
, where V is
visibility in km and q is the size distribution of the scattering
particles [2],[6].According to the Kim model, q is taken as
1.6 for V > 50 , 1.3 for 6 < V < 50, 1.6V + 0.34 for1 < V < 6, V 0.5 for 0.5 < V < 1 and 0 for V < 0.5.According to the Kruse model, q is taken as 1.6 for V > 50,1.3 for 6 < V < 500 and 0.585V
1
3 for V < 6 [2], [6]. So,attenuation decreases with the increment of wavelength,which
is shown in Fig. 2. From Fig. 3, it is observed that the
quality factor of optical receiver increases as an increase
in wavelength of signal. The lower wavelength is scattered
much more than the higher wavelength. The attenuation is
less at higher wavelength compared to the lower wavelength.
So under the environment of scattering losses, higher signal
wavelength gives improvement. So the selection of wavelength
is important in order to reduce scattering coefficient and at-
mospheric attenuation. The higher wavelengths are also better
for eye-safety. The wavelength radiation above 1.5 m is lessharmful and safe for eyes. It also allowed for carrying higher
power than the shorter wavelengths. 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. As receiver aperture area increases,
sensitivity of receiver increases due to increment in received
optical power, which leads to increment in quality factor ofreceiver. The effective aperture area of the receiver improves
the quality factor of FSO link. The increase in the aperture area
of the receiver increases the sensitivity of the receiver.Figs.
4 and 5 reported 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.
IV. CONCLUSION
It is concluded that due to reduction in scattering loss at
higher wavelength; as wavelength increases, quality factor of
Fig. 1. Optsim 5.2 Simulation link of free space communication
Fig. 2. Relationship between visibility range (km) for different wavelengths(nm)
Fig. 3. Quality factor of receiver as function of input signal wavelength fordifferent receiver aperture area
Fig. 4. Quality factor of optical receiver as function of attenuation fordifferent receiver aperture area
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.
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Fig. 5. Quality factor of optical receiver as function of receiver aperture areafor different atmospheric attenuation conditions
ACKNOWLEDGMENT
The authors would like to thank the management of Charu-
tar Vidya Mandal , Vallabh Vidyanagar and management of A
D Patel Institute of Technology for their continuous support
and encouragement. The authors also like to thank the Rsoftdesign group for the OPTSIM 5.2 simulation software and
Fiber Optics Services, Bombay.
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