Date:
Meeting:
8th May 2002
PPWG
Document Filename:
Paper Number:
RADIOCOMMUNICATIONS
AGENCY
Project title
System Signature Prediction Model
Author: M RELMY
Date: April 2002
533544781
PPWG (02-02)/024
System Signature Prediction Model
CONTENTS
Acknowledgements…………………………………………………………...1
ABSTRACT…………………………………………………….…………….2
1.
INTRODUCTION……………………………………………………………3
1.1 Fresnel Zone……………………………………………………………..3
1.2 Dispersive fading………………………………………………………..3
1.3 Digital radio signature………………………………………………….4
1.3.1 Despersive fade model…………………………………………………………....4
1.3.2 Signature curves…………………………………………………………………..5
1.3.3 Selective outage probability………………………………………………………6
2.
Methodology…………………………………………………………………..6
3.
Results…………………….…………………………………………………...7
3.1 Selective outage probability improvement using adaptive equaliser…8
3.2 Selective outage probability for different modulation schemes………8
3.3 Selective outage probability for 10-3 and 10-6 BER…………………...10
3.4 Selective outage probability for systems with adaptive equalisation..11
3.4.1 Selective outage probability for the 4 GHz band………………………………………11
3.4.2 Selective outage probability for the 6 GHz lower band……………………………….12
3.4.3 Selective outage probability for the 6 GHz upper band ………………………………12
3.4.4 Selective outage probability for the 7.5 GHz band…………………………………….14
3.4.5 Selective outage probability for the 13 GHz band……………………………………..15
3.5 Selective outage probability for the 38 and 52 GHz bands…………...15
4.
Conclusion and recommendation…………………………………………..17
4.1 Conclusion……………………………………………………………...17
4.2 Outage probability investigation……………………………………...17
4.3 Recommendation………………………………………………………19
5.
Appendix…………………………………………………………………….21
6.
Bibliography ………………………………………………………………...24
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Acknowledgements
The author would like to thank first the manufactures for providing theirs radio relay
systems signature parameters, which were necessary, for this project.
The author was very grateful for the help and the support during this project from the
following persons:

Dr David Bacon

Mr Steve Lynch

Mr Nicholas Woollard

Mr Ian Flood

Mr James Richardson

Mr Duncan Gallon

Mr Stan Murphy
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ABSTRACT
A major concern for microwave system users is how often and how long a system
might be out of service. Various statistical models and analysis methods have been
developed in order to predict and measure the outage and availability over a period of
time.
This report deals with the calculation of the selective outage probability using radio
relay systems signatures. The system signature is the measurement of the tolerance of
a digital radio system to multipath fades and was raised for consideration during the
review of the link length policy within fixed links consultative committee (FLCC) and
Point to Point Working Group formerly RSSPWG. The selective outage probability is
defined as the probability that the Bit Error Ratio (BER) is larger than a given
threshold which is based on the system signatures. The methodology used in the
calculation is based on the ITU-R 530-9 Recommendation. The calculations were
performed on all the UK fixed terrestrial (point-to-point) links in the 4, lower 6, upper
6, 7.5 and 13 GHz bands. The selective outage probability was performed then plotted
against path length using developed software for all the UK fixed links (FL) in the
bands under study.
The objective of the study was achieved and it was clear from the results that longer
links have a selective outage probability greater than the acceptable value (10-4). Also
in this report it was concluded that to improve the selective outage probability for
longer links it is necessary to
1) increase the height of both antennas, and/or
2) increase the inclination angle
Option 2 gives greatest benefit for the reduction of selective outage probability.
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1. Introduction
1.1 Fresnel Zone
In microwave line of sight systems it is necessary to provide clearance of around 0.6
Fresnel zone to ensure that attenuation due to obstacles, in or near the ray beam path,
is negligible. The first Fresnel zone is the zone within which the reflected wave has an
indirect path of a half wavelength longer than the direct path.
Fresnel zone
Direct path
Indirect path
rn
Receiver
Transmitter
d1
d2
Figure 1: Fresnel zone
The radius of nth Fresnel zone (rn) is derived as follow:
(d1 + rn)1/2 + (d2 + rn)1/2 = (d1 +d2) +n(/2)
For rn << d1, d2
we can use (1 + x )n  1 + nx for x very small
Then
1
  n   d1 d2 
rn = 

 d1  d2 
2
1.2 Dispersive fading
Multi-path propagation causes dispersive fading when microwave signals reach a
receiver having travelled over more than a single path with significant relative delays
and comparable signal amplitudes. This may happen due to the presence of reflective
layers in the atmosphere or reflective areas on the ground. Signals arriving via the
reflective layers or areas are delayed relative to the direct line-of-sight signal. All
signals combine at the receiver antenna can cause amplitude and phase distortion of
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the direct signal. Normally this distortion is minimal and easily handled by the
receiver because the direct signal is much stronger than all the reflected signals.
However, under certain conditions, the direct signal may be attenuated and the
distortion increased to the point where frequency selective notches result and
dispersive fading is said to be present.
1.3 Digital radio signatures
1.3.1 Dispersive fading model
W.D. Rummler of Bell laboratories has developed a 3-path model of multi-path
propagation, see figure 2. The transfer function for this model is given by the
following expression:
H() = a[1 – b.e j( - 0)]
Where:
 a is the attenuation due to flat fading (randomly varying flat fade across the
channel)
  = 2f, centre frequency of the notch in radians
 0 = 2f0 , centre frequency of the RF channel in radians
 b is the ratio of the amplitude of the main signal (direct) to the delayed (indirect)
signal, b (0 b  1) determines the notch depth.
  = 6.3ns [1] which the standard delay time between the two signals
 The  signs in the exponent correspond to non-minimum phase and minimum
phase fades respectively, which are explained in the next section.
Direct path
1
2
Tx
3
Indirect path
Rx
Figure 2: Diagram of the 3-path model showing the direct (1) and indirect (2&3) paths
[1] Rummler has shown that 6.3 ns is approximately the delay time measured on real microwave links in the USA .
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1.3.2 Signature curves
It is easier to understand and measure the performance of a microwave radio under
multi-path receptions when considering just one direct signal and one indirect signal
having a different amplitude and relative time delay (as shown in figure 3). When the
amplitude of the direct signal is higher than that of the indirect signal, the notch is
called a minimum-phase notch. Conversely, when the delayed signal has higher
amplitude than the direct signal, the notch is a non-minimum phase notch. Because a
receiver can respond differently to these types of notches, it is important to test the
radio under both minimum and non-minimum phase conditions. In general, nonminimum is more severe than minimum phase dispersive fading, but under most
conditions, typically the direct signal is stronger (minimum phase notch).
Spectrum
Analyzer
Variable
attenuation
Tx
3dB
coupler
3dB
coupler
MOD
Rx
DEM
Fixed
delay
Variable
attenuation
Pattern
Generator
(2n-1)
Phase
shifter
Error
Detector
(2n-1)
Figure 3: Block diagram of a test bench for the experimental measurement of signature
The measurement is performed by setting the centre frequency of the notch
somewhere in the RF channel of the particular radio system under test. The depth is
adjusted until BER degrades to 10-3 or 10-6. The phase shifter is then adjusted so that
the notch appears at a different frequency (in the RF channel) and again the notch
depth is adjusted to a reference BER (T). This process is repeated at a number of
frequencies resulting in a signature curve by plotting of non-minimum and minimum
phase notch versus notch depth. The signature curve shows the depth of the notch
required, at a particular frequency relative to the centre of the RF channel to cause the
BER to degrade to T.
The signature of a radio relay system (Figure 4) for a given BER can be idealised by a
rectangular block. The system signature parameters are:
Wx: the width of the system signature in MHz.
Ndx: The signature depth in dB.
whith x denoting either minimum phase (m) or non-minimum phase (nm) fades.
 : Reference delay used to obtained the signature.
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The system will suffer outage if the multipath notch intrudes in to the signature.
Freq in MHz
Ndx
Selective fade
attenuation in dB
(Notch)
Wx
Idealised system signature
Figure 4: The selective fading attenuation characteristic and the idealised system
signature
1.3.3 Selective outage probability
A major concern for microwave system users is how often and how long a system
might be out of service. Various statistical models and analysis methods have been
developed in order to predict and measure the outage and availability over a period of
time.
This report is about the calculation of the outage probability / outage time based on
ITU-R P.530-9 for radio relay systems in the 4, lower 6, upper 6, 7.5 and 13 GHz FS
bands. The selective outage probability is defined as the probability that the BER is
larger than a given threshold (T) (Note: This probability does not translate directly to
the unavailable time). The calculation of selective outage probability is based on
system signatures.
The next section will describe the steps that were followed to calculate the selective
outage probability based on ITU-R P.530-9 Recommendation.
2.
Methodology
ITU-R P.530-9 Recommendation was used to calculate the selective outage
probability that the system performs more poorly than a specified BER threshold (T).
In this study the threshold for the BER is 10-6 (T = 10-6).
The outage probability is calculated using the following steps and is in appendix A of
this report.
Step 1: Calculation of the mean time delay from:
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d
m  0.7   
 50 
1.3
Where d is the path length (km)
Step 2: Calculation of the multipath activity parameter  from:
0.75
  1  e  0.2 po
Where po = pw/100 is the multipath accurrence factor corresponding to the
percentage of time pw (%) of exceeding A = 0 dB in the average worst month.
pw  K ( d)
3.0
  1  p
  1.2  10 0.033 f 0.001 hl
Where f is the frequency (GHz), hl is the altitude of the lowest antenna K is the
geoclimatic factor (in UK a typical value is 8.241 10-5) and p = (height of the
transmitter – height of the receiver) / d : is the inclination angle in mrad (1 mrad 
0.03)
Step 3: Calculation of the selective outage probability from:
 Ndm
 Ndnm 

2

3
20
3
20   m 
Ps  2.15      Wm 10  10
 Wnm 10  10


 

Where:
Wx: signature width (GHz)
Ndx: signature depth (dB)
with x denoting either minimum phase (m) or non-minimum phase (nm) fades.
: the reference delay (ns) used to obtained the signature
3. Results
Mathcad tool was used to calculate and plot the selective outage probability for all the
FS links in UK in the following bands:





4 GHz band
Lower 6 GHz band
Upper 6 GHz band
7.5 GHz band
13 GHz band
Results are shown in the following section. The algorithm for the calculation for the
selective outage probability for 4 GHz band is in the appendix B. For other bands the
procedure remains the same but with changed parameters.
In the calculation of the selective outage probability the fade margin depth is set for
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0 dB. The signature parameters of the radio relay system with or without adaptive
equaliser used in the calculation are obtained from manufacture’s data. Manufactures
are not identified for the sake of confidentiality.
3.1 Selective outage probability improvement using adaptive equaliser
Selective outage probabilities were calculated for all UK FL in the band under study
using system signature with and without adaptive equaliser. The calculation
performed shows that selective outage probability for system without adaptive
equaliser is improved by approximately 94.3 % using adaptive equaliser as illustrated
in figure 5.
Figure 5: Selective outage probability for 4 GHz band
Blue crosses for radio relay system without adaptive equaliser
Red crosses for radio relay system with adaptive equaliser
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3.2 Selective outage probability for different modulation schemes
Calculations of selective outage probability were performed for all UK FL in the band
under study for systems with 64 and 128 TCM. The results as in figures 6 and 7 (7.5
GHz band) show that the selective outage probability is the same using different
modulation schemes.
Figure 6: Selective outage probability against path length for a 7.5 GHz band
Red crosses for 10-6 BER for a 128 TCM system
Blue crosses for 10-6 BER for a 64 TCM system
Figure 7: Selective outage probability against path length for a 7.5 GHz band
Red crosses for 10-3 BER for a 128 TCM system
Blue crosses for 10-3 BER for a 64 TCM system
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3.3 Selective outage probability for 10-3 and 10-6 BER
The calculations of the selective outage probability were performed on the UK FL
bands under study using system signatures for a 10-3 and 10-6 BER for different
modulation schemes. The results as illustrated in figures 8 and 9 shows that the
selective outage probability for a 10-3 BER is improved by approximately 40 % than a
10-6 BER.
Figure 8: Selective Outage probability for 7.5 GHz band using 64 TCM system
Red crosses for a 10-3 BER
Blue crosses for a 10-6 BER
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Figure 9: Selective Outage probability for 7.5 GHz band using 128 TCM system
Red crosses for a 10-3 BER
Blue crosses for a 10-6 BER
3.4 Selective outage probability for systems with adaptive equalisation
3.4.1 Selective outage probability for the 4 GHz band
Figure 10: Selective outage probability that BER > 10-6 for all UK FL
in the 4 GHz band
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3.4.2 Selective outage probability for the 6 GHz lower band
Figure 11: Selective outage probability that BER > 10-6 for all UK FL
in the lower 6 GHz band
3.4.3 Selective outage probability for the 6 GHz upper band
Figure 12: Selective outage probability that BER > 10-6 for all UK FL
in the upper 6 GHz band
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Figure 13: Selective outage probability that BER > 10-6 for all UK FL in the upper
6 GHz band (16x2 Mb/s system signature parameters)
3.4.4
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Selective outage probability for the 7.5 GHz upper band
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System Signature Prediction Model
Figure 15: Selective outage probability that BER > 10-6 for all UK FL
in the 7.5 GHz band
Figure 16: Selective outage probability that BER > 10-6 for all UK FL
in the 7.5 GHz band
3.4.5 Selective outage probability for the 13 GHz band
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Figure 17: Selective outage probability that BER > 10-6 for all UK FL
in the 13 GHz band
3.5 Selective outage probability for the 38 and 52 GHz bands
The next figures show the selective outage probability function (E) against the height
of antenna (transmitter) for 38 and 52 GHz bands. The parameters from left to right of
the E function are the maximum link path length in km obtained from Second MPL
Report v5, the height of transmitter ASL in metres and the height of the receiver ASL
in meters. The signature parameters of the radio relay systems in the frequency range
of 4-14 GHz were used to estimate the selective outage probability for the 38 and 52
GHz bands.
Figures 18 and 19 shows that the selective outage probability is below 10-10. The peak
represents the worst case (inclination angle is 0 i.e. antennas height are equal). There
is a small sensitivity to increasing the height of the receiver.
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Figure 18: Selective outage probability against height of transmitter ASL
for 38 GHz band
Figure 19: Selective outage probability against height of transmitter ASL
for a 52 GHz band
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4 Conclusion and recommendation:
4.1 Conclusion
From the results, most of the selective outage probabilities for UK fixed links in the
bands under study are below 10-4 which maybe, acceptable. However there are two
links in the lower 6 GHz and the 7.5 GHz bands where the selective outage
probability is greater than the acceptable value (10-4).
Also the results shows that the selective outage probability for the fixed links is:
1) Nearly the same for signatures obtained from different manufactures.
2) The same for different modulation schemes (64 and 128 TCM).
3) Improved by 94.3 % when adaptive equaliser is used.
4) Difference of about 40 % between 10-3 and 10-6 BER.
The improvement in the selective outage probability is due to the increase in the notch
depth of the signature which reduces the possibility of the multipath notch intruding in
to the signature. This will reduce outage in the system.
In summary the results show that the selective outage probability is higher for longer
path links.
4.2 Outage probability investigation
The parameters from left to right of the Ps function in figures 20 and 21 are path
length, centre frequency, the height of the lowest antenna, the inclination angle and
the point refractivity gradient.
3
Selective outage probability
4.55210
Ps  di  3.9  0  0   40
Outage probability for BER > 10^-6
0.006
0.0054
0.0048
Ps  di  6.175 0  0   40 0.0042
0.0036
Ps  di  6.775 0  0   40
0.003
Ps  di  7.65  0  0   40
0.0024
Ps  di  13  0  0   40
0.0018
0.0012
 106 10
3.72710
4
0
0
0
15
30
45
60
75
90
di
Path length in km
105
120
135
150
150
Figure 20: Outage probability for the worst case ( hl = 0 and  = 0) for the 5 bands under
study against path length
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7.107  10
Outage probability for BER > 10^-6
 4 8 10 4
4

Ps  d i  3.9  10  10   40 
7.2 10
4
6.4 10
Ps  d i  6.175  10  10   40 5.6 10
4
4.8 10
Ps  d i  6.775  10  10   40 
4
4 10
Ps  d i  7.65  10  10   40 
4

3.2 10
4
Ps  d i  13  10  10   40 
4.234  10
4
2.4 10
4

1.6 10
5
 11 8 10
0
0
15
30
45
0
60
75
90
105
120
135
di
Path length in km
150
150
Figure 21: Outage probability (hl = 10 and  = 10) for the 5 bands under study against
path length
Figure 21 and 22 shows grate sensitivity to increasing the inclination. Also increasing
the height of the lowest antenna can reduce the outage probability.
Figure 22 shows the variation of the outage probability set at 60 km path length, the
lowest antenna height set at 100 m for varying inclination angle against the frequency.
Ps
Ps
Ps
O u t a g e p ro b a b i l i t y
Ps
Ps
Ps
Ps
Ps
Ps
Ps
Ps
Ps
Ps
Ps
Ps
2  10
5
1.9  10
5
1.8  10
5
1.7  10
5
1.6  10
5
1.5  10
5
1.4  10
5
1.3  10
5
1.2  10
1.1  10
5
1  10
5
9  10
6
8  10
6
7  10
6
6
 5
1.941  10
 60
 60
 60
 60
 60
 60
 60
 60
 60
 60
 60
 60
 f i  10 0  5   40

 60
 60
 60
 f i  10 0  13 0   40
6  10
5  10
 f i  10 0  14 0  
4  10
6
3  10
6
2  10
6
1  10
6

40 
40 
40 
40 
40 
40 
40 
40 
 f i  10 0  10   40
 f i  10 0  20  
 f i  10 0  30  
 f i  10 0  40  
 f i  10 0  50  
 f i  10 0  60  
 f i  10 0  70  
 f i  10 0  80  
 f i  10 0  90  

40 
 f i  10 0  10 0   40
 f i  10 0  12 0  
 f i  10 0  15 0  
4.843  10

40 
40 
 7
Outage probability for BER > 10^-6
5
6
0
0
0
1.6
3.2
4.8
6.4
8
fi
Frequ ency in GHz
9.6
11 .2
12 .8
14 .4
16
16
Figure 22: Outage probability for a 60 km path length, 100 m height for the lowest
antenna and varying inclination angles against the frequency
Figure 23 shows the outage probability against the path length (red crosses) and the
inclination angle in mrad (blue crosses) for the UK fixed links in the 4 GHz band.
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Figure 23: Selective outage probability against path length in km (Red crosses) and
inclination angle in mrad (Blue crosses) for the UK fixed links in the 4 GHz band
4.3 Recommendation
Based on the investigation in the above section, the selective outage probability of the
links can be improved by increasing the inclination angle or increasing the altitude of
the two antennas keeping the same inclination angle. This is shown in the graphs (24
and 25) for the longest fixed link in the lower 6 GHz band.
The other recommendation is the limitation on the path length.
The solution is dependent on the location of the fixed link and if the recommendations
are feasible.
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Figure 24: Selective outage probability for a BER > 10-6 for a 140.3 km link in the 7.5
GHz band (company B signature) against the inclination angle
Figure 25: Selective outage probability for a BER > 10-6 for a 140.3 km link in the 7.5
GHz band (company B signature) against the height of the lowest antenna
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5. Appendix:
5.A
Methodology used to calculate the selective outage probability
1. Centre frequency
f1: the lower frequency in GHz
f2: the upper frequency in GHz
f: the center frequency in GHz
 f1  f2 

 2 
f  
2. System Signature parameters obtained from manufactures
Nd: Notch depth in dB
Ndnm: notch depth for non -minimum phase fade
Ndm: notch depth for minimum phase fade
W: Signature width in MHz
Wnm: signature width for non -minimum phase fade
Wm: signature width for minimum phase fade
Reference delay used to obtain the signature in ns :
3. Point to Point Links data
ht is the transmitter hight above sea level in m
hr is the receiver hight above sea level in m
d is the path length in km
h is the path heigth antennas difference
hl is the altitude of the lower antenna in m
hl  min ( ht  hr)
4. Calculation of the selective outage probability
4.1 The mean time delay in ns
d
m  0.7   
 50 
1.3
4.2 Point refractivity gradient in the lowest 65 m of the atmosphere not exceeded for 1% of
an average year
dN1  40
This is a typical value for UK
4.3 The geoclimatic factor for the average worst month
K  10
 4.20.0029 dN1
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4.4 Magnitude of the path inclination in mrad
p 
h
d
4.5 The percentage of time pw
pw  K ( d)
3.0
  1  p
  1.2  10 0.033 f 0.001 hl
4.6 The multipath occurrence factor
po 
pw
100
4.7 Multipath activity parameter
  1  e
0.75
 0.2 po
4.8 The selective outage probability
 Ndm
 Ndnm 

2 




m
Ps  2.15      Wm 10  3  10 20  Wnm 10  3  10 20  

 

4.9 The outage time in s / year
Ot = Ps x second in a year ( s / year )
Ot = Ps x 31.536 x 10^6
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System Signature Prediction Model
5.B Calculation and plotting the selective outage probability against path length
for the FS UK links in the 4 GHz band
f  3.9
5
  6.3
K  8.24 10
E( d  ht  hr) 
h  ht  hr
Wm  28
Wnm  28
Ndnm  22 Ndm  27
hl  min( ht  hr)
 d

 50 
1.3
m  0.7 
p 
h
d
3.0
pw  K d
po 
 1  p
 1.2
0.033f 0.001hl
10
pw
100
0.75
 0.2 po
 1e
 Ndm
 Ndnm 

2

3
20
3
20  m
Ps  2.15   Wm10 10
 Wnm10 10


i  0  rows( A)  1
Psi  E Ai  0  Ai  1  Ai  2
1 10
5
1 10
6
1 10
7
1 10
8
1 10
9
1 10
10
1 10
11
1 10
Psi 1 10 12
13
1 10
14
1 10
15
1 10
16
1 10
17
1 10
18
1 10
19
1 10
20
1 10
Selective outage probability
4
0
10
20
30
40
50
60
70
Ai  0
Path length in km
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System Signature Prediction Model
6. Bibliography

ITU- R P.530-9

ITU-R F.1093-1

Western Multiplex: Dispersive fade margin
(WesternMultiplex@http://wmux.com)

IEEE 802.16sc-99/13: Dispersive fade margin

Telettra: Introduction to high capacity digital transmission systems

Radio System Design For Telecommunications (ROGER L. FREEMAN)

Principles of radio communication ( Fraidoom Mazda)
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