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PERFORMANCE EVALUATION OVER FREESPACE OPTICAL (FSO) COMMUNICATION,
USING APD DETECTOR
Hamid Aslani
Department of Telecommunication
Multimedia (MMU) university
Cyberjaya, Malaysia
[email protected]
Abstract
Over last decade, free-space optical communication has become one of the more interesting
schemes in long distance outdoor communication process. FSO improves solutions for RF
communication in some special fields such as aerospace and military applications. This article gives an
overview of the communication process, challenges of FSO application and also some compressions
between RF and FSO concepts in the distance of 105 Km.
Keywords
Free-space optical communication, Radio frequency (RF) and free-space optical (FSO) compression, APD detector.
I.
INTRODUCTION
Optical wireless system and FSO links provide
higher performance to the communication
system.FSO links improve data rate transmission and
overall SNR of the system for deep-space
communication applications. The most important
parameter for an optical system is low weight and
coast of system are improving fast. For a system by
improving signal qualities some parameters such as
coast, weight, and size will be reduced a lot.
Transmitter part has some challenges in operation,
size and the quality of the transmitter parts. . Based
on these statements the (FSO) system becomes more
and more interesting concept for these
communication solutions. [1]
In FSO concept, some parameters such as
signal attenuation in the beam that affects by the
atmospheric parameters and also the process of
system adjustment and primary installation of the
FSO unites have most significant effects on the
system performance. [2] Also light propagation
process which in some situation can be block by
birds and limitation for light power or eye-safe range,
and also the wavelength and f-number should be
taking in to account. Improving these factors will
increase the ability of transmitter to tolerate
minimum divergence. In receiver side, aperture size
and the f-number effect light collecting and also
increase the quality of the system. F-number
determines the field of view in receiver detectors. By
considering these parameters system performance
can be improved more and more. [3]
Optical links are more preferable comparing
with radio frequency (RF) for some specific
applications with long distance communication.
Some applications such as Unmanned Aerial
Vehicles (UAVs), aircraft and satellite
communications have wide range of usage in both
civil and military aspects. Moreover, there is high
amount of data which should be transferred between
both sides of transmission at all moments.
This article is including an overview about
two models of RF and optical concepts. It also gives
some compression between two sample of FSO and
RF system to find out some of these application
performances in data rate transmission, accuracy, and
quality of received signals.
II.
THEORY OF THE OPERATION
At the transmitter part data is converted to
the light source to be for transmission through the
FSO link. The drive circuit convert the electrical
signal to the optical by changing the currents
between its optical light sources. There are two types
of light source which depends on the application can
be used, they can be a light emitting diode (LED) or
a Laser diode (LD). [4]
At receiver side, the optical signals will convert to
the electrical form again. Receivers are a
combination of two devices; first part is the optical
detector, and second part is the signal conditioning.
At the receiver detectors has the most significant
responsibility to detect the light signal. In this part
signal is received by the detector in the light form
and then a circuit which calls signal-conditioning will
adjust the output of the detector and extract the
original data from arrival signal with minimum
distortion in electric form. Figure 1 introducing a
basic of optical communication. [5]
manner will be explained in following parts. There
are so many types of photonic detector and also APD
detectors in deferent size and shape and each one for
specific purpose. Basically at the transmitter part the
light is generated by the optical source which it can
be generated by LED or LD.
An exemplary link budget for a 105 Km for RF and
FSO link are shown in Table 1 and 2.
RF Link
Transmitter power, Pt1
Pt_RF = 5;
Transmitter power, Pt2
Pt_RF2 = 15; Watt
Watt
Transmitter power, Pt2
Pt_RF3 = 25; Watt
Figur1. Block diagram of an optical wireless link showing the
front end of an optical transmitter and receiver.
Transmitter antenna diameter , Dt
Receiver antenna diameter, Dr
In some applications like UAVs APD detectors
have higher performance; the reason of this manner
will be explained in fallowing parts. There are so
many types of photonic detector and also APD
detectors in deferent size and operation for different
proposes. Basically at the transmitter part the light is
generated by an optical source which normally is
LED or LD. The best choice from the two types of
light generator also should be considered. The light
will propagate from the light generator trough the
FSO link and then will detect by the photo
detectors.[6]
Dt_RF = 1; m
Dr_RF = 1.6; m
0.85 *10 -6 m
Wavelength
Transmitter transmission loss Lt
Receiver transmission loss
Alpha_t = 0.3
Alpha_r = 0.5
Noise figure
Boltzmann constant
3 dB
K
1.38 * 10 ^ (-23)
System temperature (Kelvin)
290 K
Table 1: an exemplary parameters for RF link systems.
III.
OPTICAL TRANSMISSION PROCEDURE
Basically in transmitter part data is created in
electric form, which will be converted to the light
form. When signal received by the detector signal will
detected and then converted to the electric form again
for more processing. A sample block diagram of the
transmission procedure is shown in figure 2.[7]
Optical link
Transmitter power, Pt1
1 mW
Transmitter power, Pt2
5 mW
Transmitter power, Pt2
10 mW
Transmitter antenna diameter , Dt
Receiver antenna diameter, Dr
0.305
0 .605
Quantum efficiency
0.3
Surface leakage current
Equivalent resistance, Req
0
50 K
Noise figure
3 dB
Dark current noise
Boltzmann constant
0.05
K
Electron charge, q
nA
1.38 * 10 ^ (-23)
APD gain
Planck’s constant, h
m
m
2
6.626068*10 -34
1.6*10 6
Figure 2: sample block diagram of optical transmission process.
Table 2: exemplary parameters for FSO link systems.
IV.
LIGHT
TRANSMISSION
AND
DETECTION
EVALUATION
In some applications such as UAV, APD
detectors are more preferable, the reason of this
A.
RF link performance evaluation
An exemplary system is evaluated it this
part. RF system with project cost in table 2 are shown
in figure 3,4, and 5. SNR, BER and also data rate are
introduced as a function of distance. There are also
compressions for different rates of transmission
power.
8
B. Optical link performance over communication
parameter variations.
In this part the performance of optical system will
be evaluated when APD detector and EDFA
preamplifier are used at receiving process. Figure
shows the SNR performance as a function of
distance. An exemplary link cost also introduced in
table 2.The system SNR will be introduce as equation
1,3 and 5. Signal and noise against distance with
using APD detectors and EDFA amplifier and all
equations for the overall received signal power SNR
and received signal quality will be achieved by
following equations:
RF Link
x 10
14
12
Data Rate
10
8
6
SNREDFA=
4
2
0
1
10
2
10
3
10
4
10
(1)
5
10
10
Distance (m)
S Spontaneous emission (ASE) and self-mixing noise
(ASE ASE) are depending on the system
equipments,
Figure3: RF link data rate as a function of distance.
RF link with different transmit power
0
10
5 Watt
15 Watt
-1
SNREDFA|back=
10
(2)
-2
10
When S×ASE noise is not dominants, as it would be
for ideal reception:
-3
10
-4
10
BER
SNREDFA|S×ASE=K3R-2
-5
(3)
10
Where
-6
10
-7
10
(4)
-8
10
-9
10
0
10
20
30
40
SNR (dB)
50
60
70
80
When ASE×ASE noise is dominant,
SNREDFA|ASE×ASE=K4R-4,
Figure 4: BER versus SNR with different
transmit power (5 and 15 Watt).
(5)
Where,
RF Link
5 Watt
15 Watt
25 Watt
160
(6)
Primary parameters are introduced in Table 1 and 2.
140
120
Optical Link
M=2
M=5
M=10
160
80
140
60
120
40
SNR (dB)
SNR (dB)
100
20
0
1
2
3
4
5
Distance (m)
6
7
8
9
10
100
80
4
x 10
60
Figure 5 : SNR versus Distance for different RF
transit power (5, 15, and25 Watt).
40
0
your table title. Run-in heads, such as “Abstract”, will
require you to apply a style (in this case, italic) in
addition to the style provided by the drop down menu
to differentiate the head from the text.
1
2
3
4
5
Distance (m)
6
7
8
Figure6: SNR variation versus Distance.
9
10
4
x 10
system, which indicates that data rate transmission, is
more than RF system.
Optical Link
Pr=0.001
Pr=0.005
Pr=0.010
160
RF Link
Optical Link
160
140
140
120
SNR (dB)
120
100
SNR (dB)
100
80
80
60
60
40
40
0
1
2
3
4
5
Distance (m)
6
7
8
9
20
10
4
x 10
0
0
1
2
3
4
Figure7: optical link data rate with APD gain as a function of
5
Distance (m)
6
7
8
9
10
4
x 10
Figure 10: RF and optical SNR as a function of distance.
distance.
8
x 10
8
15
RF Link
Optical Link
14
Optical Link
x 10
12
14
13
10
Data Rate
12
Data Rate
11
8
10
6
9
4
8
2
7
6
0
1
10
2
10
3
10
4
10
5
10
10
Distance (m)
5
4
0
10
1
2
10
3
10
4
10
Figure 11: RF and optical data rate compression.
5
10
10
Distance (m)
0
10
Figure 8: optical link data rate with APD gain
as a function of distance.
RF Link
Optical Link
-1
10
-2
10
Optical Link
0
10
Pr=0.001
Pr=0.005
Pr=0.010
-1
10
-3
10
-4
10
BER
-2
10
-5
10
-3
10
-6
10
-4
BER
10
-7
10
-5
10
-8
10
-6
10
-9
10
-7
10
-8
0
10
20
30
40
SNR (dB)
50
60
70
80
Figure 12: SNR and bit error rate comparison between RF and
FSO.
10
-9
10
0
10
20
30
40
SNR (dB)
50
60
70
80
Figure 9: SNR versus BER for an optical link equipped
with APD gain
All graphs from 1 to 9 showed that the system
performance will be improved by using APD detector
and EDFA amplifier.
C. Optical and RF link performance compression.
Conclusion
Compression between RF and Optical system are
shown in figure 7, 8 and 9.Figure 5.8 shows that the
optical link has higher SNR compare with RF system.
Figure 9 also shows the higher performance in optical
Comparing with RF technology, FSO link has better
performance by using APD detector and EDFA
preamplifier. Optical system becomes able to transfer
higher amount of data with higher SNR with 10 7 Km
distance. This range of support can increasing
performance in some applications such as UAV,
Aircraft communications and also improves the
accuracy of data transmission comparing with RF
system. In FSO concept SNR can be significantly
improved by applying APD detector and also the
performance of the optical system can be improved
rather than before by using an EDFA preamplifier at
receiver side.
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