Cumulative Effect Models

-1-

ACP/WGF12 - WP/14

Agenda Item 4:Ultra Wide Band (UWB)

Cumulative Effect Models

(Presented by John Mettrop)

SUMMARY

This paper compares the two models proposed for assessing the cumulative effects of multiple Ultra-Wideband on aeronautical receivers and concludes that they are equivalent for the purpose proposed.

1.0Introduction

ITU –R Task Group 1/8 was established to investigate whether Ultra-Wideband technology can be introduced without causing harmful interference to existing services and if so what spectral mask Ultra- Wideband must conform to.

At the last meeting of TG 1/8 in Boston two methods for assessing the compatibility of Ultra-Wideband with Aeronautical services were proposed. Both methods essentially were the same with respect as to how to assess the compatibility of a single Ultra-Wideband transmitter and an aeronautical service. Where the two methods differed is in the model proposed for assessing the cumulative effect of multiple Ultra-Wideband transmitters on an aeronautical receiver. This paper compares the two models.

2.0Description of The Models

Both Models considered make the following assumptions:-

a)that all emitters are uniformly distributed in a circular area below the victim receiver;

b)that all emitters radiating on the same frequency, the same power levels and using isotropic antennas with omni-directional radiation pattern;

c)that free space propagation are assumed; and

d)that all single emitters are statistically independent and the power contribution of all single emitters can be added arithmetically.

Based on these assumptions the aggregate interference can be calculated by summing the effects of a number of concentric rings of thickness δr, centred on the ground directly below an aircraft from radius Rmin to Rmax as shown below.

Figure 1

The cumulative methodology

Where the two models differ is in the assumption about the earth. For the model proposed by ICAO the earth is assumed to be flat out to the radio horizon whilst for the NTIA model earth curvature is taken into account.

Assuming that Rmax is set to the effective radio horizon and Rmin equals 0 then the two models produce the following two equations for the evaluation of aggregate interference level.

2.1ICAO Model

2.2NTIA Model

Where:-

A=Average aggregate interference in (watts per unit bandwidth)

Weirp=Average UWB device EIRP (watts per unit bandwidth)

=Wavelength (metres)

Gr=Victim receiver antenna gain (Assumed to equal unity)

=Average density of UWB emitters (emitters per metre2)

Re=Effective earth radius (metres)

Rmax=Radio horizon

h=Height of the receive antenna above the ground (metres)

H=Re(1-cos(Rmax/Re)

3.0Comparison of the Models

Both models described above are intended, based on similar assumptions, to assess the aggregate interference from a uniformly distributed set of interference transmitters. Therefore the models in theory should produce the same results. By varying the four main variables given below in turn whilst keeping the other two constant the two models can be compared and assessed:-

Weirp=Average UWB device EIRP (watts per unit bandwidth)

=Average density of UWB emitters (emitters per metre2)

=Wavelength (metres)

h=Height of the receive antenna above the ground (metres)

3.1Variation of Weirp

UWB EIRP (dBm/MHz) / NTIA Model / ICAO Model
0 / -43.6 / -47.2
-10 / -53.6 / -57.2
-20 / -63.6 / -67.2
-30 / -73.6 / -77.2
-40 / -83.6 / -87.2
-50 / -93.6 / -97.2
-60 / -103.6 / -107.2
-70 / -113.6 / -117.2
-80 / -123.6 / -127.2
-90 / -133.6 / -137.2
Effective Earth Radius / 8494667 / m
Density of interferers / 10000 / per km2
Frequency / 2000 / MHz
Height above Ground / 1000 / m

3.2Variation of 

Density of Interferers (/km2) / NTIA Model / ICAO Model
10 / -113.6 / -117.2
20 / -110.6 / -114.2
50 / -106.6 / -110.2
100 / -103.6 / -107.2
200 / -100.6 / -104.2
500 / -96.6 / -100.2
1000 / -93.6 / -97.2
2000 / -90.6 / -94.2
5000 / -86.6 / -90.2
10000 / -83.6 / -87.2
Effective Earth Radius / 8494667 / m
Height Above Ground / 1000 / m
Frequency / 2000 / MHz
Interferer EIRP / -40 / dBm/MHz

3.3Variation of 

Frequency (MHz) / NTIA Model / ICAO Model
10 / -37.6 / -41.2
20 / -43.6 / -47.2
50 / -51.6 / -55.2
100 / -57.6 / -61.2
200 / -63.6 / -67.2
500 / -71.6 / -75.2
1000 / -77.6 / -81.2
2000 / -83.6 / -87.2
5000 / -91.6 / -95.2
10000 / -97.6 / -101.2
Effective Earth Radius / 8494667 / m
Density of interfers / 10000 / per km2
Height above Ground / 1000 / m
Interferer EIRP / -40 / dBm/MHz

3.4Variation of h

Height Above Ground
(m) / NTIA Model / ICAO Model
10 / -81.9 / -85.5
20 / -82.1 / -85.8
50 / -82.4 / -86.1
100 / -82.7 / -86.3
200 / -82.9 / -86.6
500 / -83.3 / -86.9
1000 / -83.6 / -87.2
2000 / -83.9 / -87.5
5000 / -84.4 / -88.0
10000 / -84.8 / -88.4
Effective Earth Radius / 8494667 / m
Density of interfers / 10000 / per km2
Frequency / 2000 / MHz
Interferer EIRP / -40 / dBm/MHz

4.0Conclusions

The two models track each other exactly with only a 3.6dB differential between the results with the NTIA model and that proposed by ICAO.

Since the NTIA model is the more conservative model and has already been adopted within TG1/8 as a suitable model for assessing aggregate interference it is recommended that this model be adopted for the aeronautical section of the TG1/8 compatibility report.

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