Thales Research & Technology (UK) Limited
UWB interference to airborne receiver
UK TG1/8 CP(03)05
ESTIMATES OF AGGREGATE UWB INTERFERENCE TO AN
AIRBORNE RECEIVER – ISSUE 2
1
Introduction
Concern has rightly been expressed about the possible effects on existing systems of the
aggregate interference from a large number of UWB devices present in an urban area. Particular
concern has been expressed about the effect on aircraft systems. This paper gives some
simplified estimates of the interference that would be experienced by an aircraft radio receiver
flying at different heights over a large city like London.
A simple calculation is presented with (hopefully) realistic and fully visible assumptions. The
value of simple calculations is that that they can be checked easily, and that there are no hidden
assumptions buried in the depths of the modelling. Comments on the assumptions and method
of calculation are welcome.
2
Assumptions
The following assumptions are made:
1)
There is a uniform density of D “active” UWB emitters per sq km over the urban area
(i.e. D gives the density of UWB devices actually transmitting at one moment in time.)
2)
The urban area has a radius of 25 km (approximately the radius of the M25) for the
case of London.
3)
The aircraft is flying over the centre of the urban area
4)
The UWB devices and the aircraft receiver all have an antenna with polar diagram like
that of a vertical half wave dipole, i.e. with constant azimuth gain and vertical gain as
shown below
Antenna vertical polar diagram
Figure 1 Antenna vertical polar diagram (radial scale in volts)
5)
Free space propagation applies out to the radio horizon (4/3 effective earth radius)
6)
All active UWB emitters are transmitting at the FCC limits on EIRP (i.e. –41.3dBm/MHz
in the band 3.1 to 10.6 GHz, 10/20 dB lower in 1.9 to 3.1 GHz, and 34 dB lower in 0.96
to 1.61 GHz). Note that this is a pessimistic assumption in that it is very unlikely that a
UWB device would be at the FCC limit across the whole of each frequency band.
7)
Building penetration loss is 12 dB (from ITU P1411-1)
Page 1 of 8
UWB interference to airborne receiver
Calculations
The path loss between UWB emitter and aircraft receiver, including the effect of the antenna
polar diagrams at the two ends, is shown below. (Here the antenna gain is in dBd, i.e. has a
value of 1 in the horizontal direction).
Path loss to elevated receiver
60
80
100
Path gain dB
3
Thales Research & Technology (UK) Limited
120
140
160
180
200
0.01
0.1
1
10
100
Horizontal range km
10m aircraft height
100m aircraft height
1000 m aircraft height
1000m aircraft height
Figure 2 Path loss between UWB emitter and aircraft receiver including antennas
The total UWB interference power experienced by the aircraft receiver can be expressed as:
D  p  g  2r  Gain(r )  dr
(watts), where:
D is the active emitter density per sq km
p is the EIRP of each active UWB emitter in watts (in a horizontal direction)
g is the power gain of the receiver antenna in a horizontal direction with respect to an
isotropic antenna
r is the horizontal range
Gain(r) is the path gain as a power ratio (i.e. isotropic path loss times antenna gain at
the two ends, the antenna gains being scaled to 1 in the horizontal direction). The dB
equivalent is given in Figure 2.
The integration upper limit used here is the radio horizon or city limit, as shown in the
Figure below:
Page 2 of 8
Thales Research & Technology (UK) Limited
UWB interference to airborne receiver
Radio horizon or city limit
30
Upper limit of integral km
28
26
24
22
20
18
16
14
12
10
10
100
1 10
Aicraft height m
3
1 10
4
Figure 3 Upper limit of path gain-area integral
The value of the integral above is given (in dB) below:
Value of integral in dB
Integral of path gain x area
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
10
3
1 10
100
1 10
4
Aircraft height m
Figure 4 Values of path gain – area integral
The maximum value of the integral is at 10m, i.e. this is the height at which maximum
interference will be experienced by the aircraft receiver.
The actual interference experienced is given below for different levels of UWB active emitter
density. The interference will be noise like and is expressed as a noise figure (i.e. the level of the
aggregate UWB interference above thermal noise.) If for example an aircraft receiver had a 5 dB
noise figure, the effect of a UWB environmental “noise figure” of 5 dB would be to double the
noise experienced, i.e. reduce the overall sensitivity by 3dB.
Page 3 of 8
UWB interference to airborne receiver
Thales Research & Technology (UK) Limited
Aggregate interference - 3.1 to 10.6 GHz
Aggregate noise figure at receiver (dB)
30
25
20
15
10
5
0
5
10
15
20
10
3
100
1 10
Active UWB emitter density (per sq km)
4
1 10
All UWB outdoor
All UWB indoor
20% UWB outdoor
Figure 5 Aggregate UWB interference to airborne receiver at 100m in 3.1 to 10.6 GHz band
Aggregate noise figure at receiver (dB)
Aggregate interference - 1.99 to 3.1 GHz
10
5
0
5
10
15
20
10
3
100
1 10
Active UWB emitter density (per sq km)
4
1 10
All UWB outdoor
All UWB indoor
Figure 6 Aggregate UWB interference to airborne receiver at 100m in 1.99 to 3.1 GHz band
Page 4 of 8
Thales Research & Technology (UK) Limited
UWB interference to airborne receiver
Aggregate noise figure at receiver (dB)
Aggregate interference 0.96 - 1.61 GHz
0
5
10
15
20
25
30
10
3
100
1 10
Active UWB emitter density (per sq km)
4
1 10
All UWB outdoor
All UWB indoor
20% UWB outdoor
Figure 7 Aggregate UWB interference to airborne receiver at 100m in 0.96 to 1.61 GHz
band
4
Discussion
The crucial parameter in the above calculations is of course the UWB active emitter density, and
this is the greatest unknown. The density will depend on the success of UWB technology.
For example, we may assume that:

The city has a population of 10,000,000

There is 1 UWB emitter per head of population

The duty cycle of each UWB emitter is 10%

20% of UWB active emitters are outdoor
For this case, D = 0.1 x 10,000,000/(π x 25 x 25) = 510, i.e. an average spacing between active
UWB emitters of 44m. This would give the following UWB interference “noise figures”:

In 3.1 – 10.6 GHz:
7 dB

In 1.99 – 3.1 GHz:
-8 dB

In 0.96 to 1.61 GHz
-27 dB
Increasing D by a factor of 10 (i.e. an average spacing between active UWB emitters of 14m)
would increase these figures by 10 dB, i.e. giving figures of 17, 2 and −17 dB respectively.
5
Comments on Assumptions
1)
For convenience, a uniform density of UWB emitters has been assumed. In practise,
the density will probably be greater in the middle of the city. However, from Figure 3, it
can be seen that above 40m aircraft height, all the city is within the radio horizon and
so the aircraft is receiving interference from all over the city (except from directly below
where the antenna gain is zero). A non uniform distribution will still have some effect
on the calculations, but results will vary according to where the aircraft is in relation to
the high UWB density areas. For simplicity, the uniform assumption is maintained.
Page 5 of 8
UWB interference to airborne receiver
The use of free space propagation may be considered too pessimistic at low elevation
angles where there will be significant building clutter between the UWB emitter and the
aircraft. An alternative integration has been done using inverse fourth law path gain at
elevation angles under about 50. The integration used the following formula for path
gain (as a power ratio):
Path gain = (Free space gain) for r < 10z
Path gain = (Free space gain) x {11z/(r + z)}2 for r > 10z
where r = horizontal range and z = aircraft height
The path loss curves and path gain area integrals are now as given below:
Path loss to elevated receiver
60
80
100
Path gain dB
2)
Thales Research & Technology (UK) Limited
120
140
160
180
200
0.01
1
0.1
10
100
Horizontal range km
10m aircraft height
100m aircraft height
1000 m aircraft height
1000m aircraft height
Free space at 10m aircraft height for comparison
Figure 8 Path loss using inverse 4th law for elevation angles below 50
Page 6 of 8
Thales Research & Technology (UK) Limited
UWB interference to airborne receiver
Value of integral in dB
Integral of path gain x area - in air
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
10
100
1 10
3
4
1 10
Aircraft height m
Free space plus inverse 4th below 5 degrees
Free space alone for comparison
Figure 9 Path gain-area integral using inverse fourth law for elevation angles below 5 0
It can be seen that the worst case aircraft height is now below 600m, and the worst
case value of the integral is only 5dB lower than in the free space case. The free
space figures can be regarded as an upper bound on interference, and the results
based on Figure 9 (i.e. 5 dB lower) as a lower bound.
6
Conclusions
The aggregate UWB interference experienced by an airborne receiver flying over an area of
dense UWB activity causes an effective rise in environmental noise at the receiver. The noise
power is directly proportional to the active UWB emitter density. Some example values of UWB
environmental “noise figure” are given below:
Active UWB emitter
density
Noise figure in
3.1 to 10.6 GHz
Noise figure in
1.99 to 3.1 GHz
Noise figure in
0.96 to 1.61 GHz
500 / sq km
2 to 7 dB
-13 to –8 dB
-32 to -27 dB
5000 / sq km
12 to 17 dB
-3 to +2 dB
-22 to -17 dB
Table 1 Example UWB environmental noise figures at worst case aircraft heights
The impact on aircraft systems depends on which frequency bands are used.
If any of the above figures are unacceptable, reductions in EIRP limits within the relevant parts
of the frequency bands should be considered. However, EIRP limits should not be reduced
across a whole band to deal with potential problems in just one part of a band. Various notching
techniques exist within UWB to reduce EIRP at selected frequencies.
Page 7 of 8
UWB interference to airborne receiver
Thales Research & Technology (UK) Limited
P.J.Munday
Thales Research and Technology (UK)
5.2.03
Page 8 of 8