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ACP-WGW01/WP-01
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International Civil Aviation Organization
INFORMATION PAPER / ACP-WGF20/IP-01
11/03/09

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

20TH MEETING OF WORKING GROUP F

Montreal, Canada 24 March – 3 April 2009

Agenda Item 7: / Interference from non-aeronautical sources

S-Band Radar Receiver Selectivity Issues

(Presented by John Mettrop)

SUMMARY
This paper provides information on the activities within the UK with respect to the introduction of WiMAX/3G services below 2690 MHz and it’s impact on radars operating above 2700 MHz.
ACTION
To note the paper and for members to take it into account when considering possible deployment of WiMAX/3G services below 2690 MHz

1.INTRODUCTION

1.1At the 2000 World Radiocommunications Conference the frequency band 2500 – 2690 MHz was allocated to the mobile service for use by administrations wishing to implement International Mobile Telecommunications (IMT). This allocation was then confirmed and the relevant Resolution amended at the 2007 World Radiocommunications Conference. As a result of this allocation a number of administrations have been progressing towards the implementation of IMT in their States.

1.2In the UK Ofcom released a “Notice of Ofcom’s proposal to make regulations in connection with the award of 2500-2690 MHz and 2010-2025 MHz”[1] on the 4th May 2008. In the document they detailed the spectrum that was to be released to the market and the technical constraints applicable to any system deployed as a result of the proposed future auction. These constraints included maximum radiated power, bandwidth, out of band spurious emissions and constraints imposed by out of band spurious emissions from radars operating above 2700 MHz

1.3At the time neither the CAA, nor NATS officially responded to the consultation. The rationale for not responding was that there would be at least 10 MHz separation from the lower end of the 2700 – 2900 GHz Aeronautical Radionavigation band used for airport primary surveillance radar and hence any deployment of a mobile system below 2690 MHz was unlikely to affect the operation of Radars deployed above 2700 MHz.

2.discussion

2.1As part of their due diligence process, and being aware of the sensitivity of radars, Ofcom decided to undertake some practical measurements to assure themselves that the out of band spurious limits they had set in the notice would indeed protect radars operating above 2700 MHz. A small contract was let to undertake this work with ERA (Ofcom’s approved test house).

2.2The trials were undertaken on a representative test radar and carried out by injecting signals directly into the radar receiver. The results of these trials showed that the spurious level quoted in the Ofcom notice would not cause any interference to the radar receiver. As a final step it was decided as all the equipment was in place to just confirm that the radar receiver would not see the fundamental signal of any transmitter deployed below 2 690 MHz in accordance with the Radio Regulations.

2.3The results of this final test were surprising and showed that the radar receiver was sensitive to signals generated below 2690 MHz and that they would cause interference. The diagram produced by ERA to demonstrate the issue is given below:-


note: The X axis scale should be offset to the left by 50 MHz such that the right hand end =2.75 GHz

2.4 Calculations based on these results indicated that if a transmitter, operating in accordance with the Ofcom notice at the maximum permitted power, were placed 1 km from the radar under test then the 1dB compression point would be exceeded by approximately 40 dB. This equated to a required separation distance, assuming free space path loss, of 80 km. It was realised that this was a serious issue that had significant implications for the proposed deployment of systems below 2690 MHz.

2.5Ofcom, realising the significance of these results approached the CAA and MoD for their views and assistance to resolve the issue who confirmed the gravity of the situation and the potential impact on aviation services if deployment went ahead without a solution having implemented. It was agreed that the first step would to ascertain whether the results achieved by ERA were a peculiarity to that test radar, that particular model of radar or was more general.

2.6The design authority for the radar was commissioned to undertake a study to:-

  • confirm whether the measured results obtained by ERA were representative of that radar model,
  • model the radar receiver chain in order to explain the apparent difference in measured and modelled results
  • investigate the feasibility of modifying the radar such that it would ideally not lose performance but be immune to interference from the proposed systems due to operate below 2690 MHz.
  • The results to date from the design authority have shown that:-
  • the apparent anomaly between the measured and theoretical results achieved by ERA can be explained and result from the loses in the system (see below) not originally taken into account by ERA.
  • the results achieved by ERA were representative of that radar model.
  • the determining component of the 1dB compression point is not as originally thought the low noise amplifier but the 1st IF but the effects on the Low Noise Amplifier, E2V TR cell and the Intermediate Frequency stages needs careful consideration.
  • a modification could be implemented that would resolve the issue.
  • when considering the 1dB compression for other radars the effects on the entire receiver chain needs to be considered

2.8Whilst this work was being undertaken a questionnaire was sent to all known manufacturers/design authorities of radars operated in the UK. As a result of this questionnaire it has been confirmed that the issue is not confined to a single radar model but to a greater or lesser extent affects all aeronautical radars (including military) operating in the UK and that the limiting factor is not necessarily the LNA and that the whole receiver chain including the E2V TR cell and the various IF stages needs to be considered. The table below illustrates the required separation distance under a given set of assumptionsand the variation in values dependent on the radar model.

2.9

Level at the LNA for 1dB Compression / -27 dBm to -46 dBm
Transmission Line Loss Between LNA and the Antenna / 1.5 to 4.5 dB
Antenna Gain towards the Horizon / 27.5 to 34.5 dB
Receiver Discrimination at 2690 MHz / 0 to 1 dB
Maximum Interference Power at the Antenna / -57 – -72 dBm
EIRP of the Transmitter operating below 2690 MHz / 31dBW
Required Separation Assuming Free Space Path Loss / 6.5 to 36 km
Assumed Additional Factors[2] / Cross Polarisation Loss / 3 dB
Antenna Gain Variation / 2 dB
Multipath Propagation Enhancement / 8 dB
Multiple Interference Sources / 6 dB
Required Separation taking into Account the Assumed Additional Factors / 29 to 160 km

2.10The UK are continuing their work to investigate this issue and potential solutions and will keep ACP WG F informed of our progress.

3.ACTION BY THE MEETING

3.1The ACP WGF is invited to note the information provided and, where appropriate, for members to take it into account in their own State.

[1] The consultation document can be found at

[2] These values are under discussion in the UK