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LINK BUDGET ANALYSIS OF FREE-SPACE OPTICAL (FSO) COMMUNICATION NETWORKS
Serhat Aslandoğdu- Özer Yıldız
Electronic and Communication Engineering Department, Cankaya University
Abstract
The link budget calculation, which determines if an FSO link is
able to deliver sufficient optical power to the opposite end of the
link for a given worst case condition, is the most important
consideration in the design of an FSO system. This paper
examines a sample calculation for the fade margin.
Introduction
Fade margin takes into consideration the characteristics of the
FSO system (total transmitted power, total receiver aperture
area, receiver sensitivity, coupling losses) and the enviroment
where the link is to be deployed (geometric path loss, loss
through windowpanes, beam misalignment loss). A sample
calculation for the fade margin, which only takes into account
the power emitted at the transmitter end, the power collected at
the reciever end, the geometric loss and fixed system losses
inherent to the FSO system.
IDA Technology Group Team analyzed the link budget
calculation in the experiments where they worked on FSO
system in Singapore [1]. The company of Lightpointe prepared a
link budget calculation table for their products and gave system
variables [2].
Path Length: The physical distance between the two
transceivers of a single FSO link. This is the distance the laser
beam travels through the atmosphere.
Fixed System Loss: Losses caused by transmitter and
receiver. In transmitter, it caused by converting electrical signal
to optical signal. In receiver, it caused by converting optical
signal to electrical signal.
function pushbutton1_Callback(hObject, eventdata, handles)
Geometrical Path Loss: The loss in optical power directly
resulting from the spreading of the laser beam as it propagates
from one end of the link to the other.
 4 AN R 
10log10 
2 
  (d ) 
Tp=str2double(get(handles.edit1,'String'));
Nt=str2double(get(handles.edit2,'String'));
Beamdiv=str2double(get(handles.edit3,' String '));
NRx=str2double(get(handles.edit4,' String '));
Rs=str2double(get(handles.edit5,' String '));
DRx=str2double(get(handles.edit6,' String '));
L=str2double(get(handles.edit7,' String '));
Vis=str2double(get(handles.edit8,' String '));
Fix=str2double(get(handles.edit9,' String '));
Sci=str2double(get(handles.edit10,' String '));
Atotal= NRx * pi*((DRx/100)^2);
Lg=-10 * log10(4*Atotal/(pi*((L*Beamdiv/1000)^2)));
Link Availability: The percentage of time that the link is operating
satisfactorily when the fade margin is not exceed is known as link
availability.
Diameter of Receiver Aperture: The area on the
transceiver through which optical signals arriving from the
opposite end of the link is able to enter according to the
diameter of receiver.
Algorithm of Link Budget
Calculation
Scintillation: The shimmering of the laser beam caused by
random refraction of the laser beam in the air.
is the receiver aperture area according to receiver diameter, N R
is the number of receivers, d is the path length,  is the beam
divergence.
Receiver Sensitivity: The minimum optical power the
receiver needs to detect in order to sustain a specified
level of performance (e.g. bir-error rate of 108).
According to this algorihm, the interface of link budget calculator is
done. Changing parameters in figure 1, the figure 2 is found. It shows
us FSO fade mardgin graph will determine us at which path length
with which availability. According to availability we can choose our
link distance or we can change our parameters to obtain availability.
Most of the codes are gave automatically by the program. The
algorithm of link budget calculation is given below:
A
Beam divergence: The angle of the cone of light emitted
from the transmitter. This angle directly determines how
fast the width of the beam grows along the path of the
laser beam.
the fixed system loss.
Weather Visibility Losses: Losses caused by weather
conditions (e.g. rain, haze, fog, etc.).
The Link Budget Calculation
Figure 1: The Link Budget Calculation
T is the transmission power, NT is the number of transmitter, R
is the receiver sensitivity, LG is the geometrical path loss, L F is
Table 1: Attenuations at varius path length according to reference
[1]
Fade Margin: The amount of allowance (in decibels) an installed
wireless link has against time-varying conditions (e.g. rain,
scintillation, fog) that may cause the signal to attenuate.
The fade margin of an FSO system is given by:
T  10log10 NT  R  LG  L F
Total= 10*log10(Tp) + 30 + 10*log10(Nt) - (Rs+30) - Lg - Fix Sci - (Vis*L/1000);
set(handles.text01,' String ',Lg);
set(handles.result,' String ',Total);
ser=[]
res=[]
avb=[]
avaib=[]
for L1=0:100:L+1000;
avb1 = 8 + L1*0.016;
avb2 = 11 + L1*0.024;
Lg1 = -10 * log10(4*Atotal/(pi*((L1*Beamdiv/1000)^2)));
Total1 = 10*log10(Tp) + 30 + 10*log10(Nt) - (Rs+30) - Lg1 Fix - Sci - (Vis*L/1000);
ser=[ser Total1];
avb=[avb avb1];
avaib=[avaib avb2];
res=[res L1];
end
axes(handles.axes2);
plot(res,ser,res,avb,'r--',res,avaib,'r--+');
grid on;
xlabel('Path Length in m','FontSize',10.4,'FontWeight','bold');
ylabel('Fade Margin in dB','FontSize',10.4,'FontWeight','bold');
set(handles.axes2);
Figure 2: Link Availability Fade Margin Graph
Conclusion
The link budget calculation, which determines if an FSO link is able
to deliver sufficient optical power to the opposite end of the link for
a given worst case condition, is the most important consideration in
the design of an FSO system. This paper examines a sample
calculation for the fade margin.
Literature cited
[1] IDA Technology Group Team, A Trial-Based Study Of Free
Optics Systems in Singapore (October 2002)
[2] internet// www.tera-wave.com/lightpointe.xsl
[3] L.C. Andrews and R. L. Philips, Laser Beam Propagation
through Random Media(SPIE, 1998)
[4] D. Hanselman and B. Littlefield, The Student Edition of
Matlab(Prentice Hall, Upper Saddle River, NJ 07458)
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