International Civil Aviation Organization

INFORMATION PAPER

ACP-WGF24/IP-08

AERONAUTICAL COMMUNICATIONS PANEL (ACP)

TWENTY FOURTH MEETING OF WORKING GROUP F

Paris, France 17 – 21 March 2011

Agenda Item 8: Any other business

Update on Compatibility Issues Around 2.7 GHz between WiMAX/LTE and Radar

(Presented by John Mettrop, UK)

SUMMARY

This paper recaps the history of the issues between WiMAX/LTE operating in the frequency band below 2 690 MHz and Radar operating above 2.7 GHz as well as provides the current status from a UK perspective.

ACTION

To note the content of the paper and if appropriate use it as background information in national processes

1. HISTORY

1.1 At the 2000 World Radiocommunication Conference the frequency band 2 500-2690 MHz was allocated to the mobile service for the implementation of IMT-2000. Since then two technologies, WiMAX and LTE have been developed to deliver what was envisaged by IMT-2000 and more.

1.2 Aviation paid little attention to these developments since there was a guard band of 10 MHz in which the Radio Astronomers operate between the upper frequency available to the mobile service and the lower edge of the radar band. The belief at the time was that there would be no issues for the radar but that the mobile service would receive some degradation to it’s services in the upper part of the frequency band allocated but since radar was an incumbent service article 3.31 of the Radio Regulations would apply.

1 Transmitting and receiving equipment intended to be used in a given part of the frequency spectrum should be designed to take into account the technical characteristics of transmitting and receiving equipment likely to be employed in neighbouring and other parts of the spectrum, provided that all technically and economically justifiable measures have been taken to reduce the level of unwanted emissions from the latter transmitting equipment and to reduce the susceptibility to interference of the latter receiving equipment.

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1.3 Ofcom in preparation for auctioning the frequency band 2 500-2 690 MHz carried out a measurement campaign to assess the level of out of band emissions from radar in the UK that fell into the frequency band. This information was published in the consultation document in 2007 which also contained the transmission masks for both the WiMAX/LTE mobile and base stations. The base station being specified to have a 15dB lower out-of-band emission mask than required by either ITU or CEPT regulations.

1.4 Ofcom following the publication of their consultation document decided to do some final injected measurements with a test radar. During these trials they also decided as part of due diligence to check the susceptibility of radar to the WiMAX/LTE signals. The results of these measurements were a shock to both Ofcom and the CAA. They showed that the radar under test had no pre-LNA filtering and hence was open to interference from the fundamental signals of the WiMAX/LTE signal and that interference occurred well below the maximum level of WiMAX/LTE signal permitted. This interference being so severe that the radars would not be fit for purpose and hence procedural approaches would have to be used limiting movement rates and having a major economic impact.

1.5 Since then Ofcom and the CAA have been working together to understand the various interference mechanisms and the action that needs to be taken to remedy the situation. In 2009 one company trialled a revised radar receiver front end but this whilst improving the situation did not solve the problem and hence further work has been undertaken.

1.6 The auction of the frequency band 2 500-2 690 MHz has been delayed until a workable solution can be found with a provision date now being set for the auction consultation document to be released in the autumn of this year.

1.7 Since the UK raised the issue a number of other countries have either experienced interference to their radars (e.g. NOAA weather radars in the USA) or have carried out test to assess the potential for interference the results of which appear to be in line with the UK findings.

1.8 I have briefed WG-F at previous meetings on the outcomes of these studies and also provided similar information to the ITU.

2. CURRENT SITUATION

2.1 As a result of the work carried out by Ofcom and the CAA on assessing the potential impact of WiMAX/LTE signals on radar performance it became clear that there were 4 issues to be resolved in order to ensure compatibility:-

1) Radar rejection of the fundamental signal of the WiMAX/LTE signal

2) WiMAX/LTE spurious emissions into the Radar band

3) WiMAX/LTE rejection of the fundamental signal of Radar

4) Radar spurious emissions into the WiMAX/LTE band

2.2 In order to ensure the continued availability of radar to provide a service the first two issues need to be resolved. However in order for both services to operate in a compatible manner all four issues will eventually have to be addressed.

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2.3 Radar Rejection of WiMAX/LTE Signals

2.3.1 Having established that the interference threat was real through field trials the UK is now looking at what a realistic worst case scenario would be against which a pre-LNA filter can be designed by the various manufacturers for fitting to the radar receiver. The following table shows a calculation of the WiMAX/LTE base station signal level that the radar receiver will have to capable of rejecting and the interference margin against it’s current 1dB compression point:-

Radar 1(Magnetron)

Radar 2(TWT)

Radar 3(Solid State)

BS average radiated power (dBm/5 MHz) 61Peak to average signal ratio (dB) 14Multipath gain (dB) 6Aggregation for 23 base stations (dB) 14Free space path loss for 1km (dB) 101Radar antenna Gain (dBi) 34 33.5 35Receiver filter rejection (RF) (dB) 0 0 0Power at the receiver front-end (dBm) 24 27.5 29Receiver 1dB compression point (dBm) -27 -43 -27Indicative interference margin (dB) 51 70.5 56

2.3.2 Designing a filter to provide 60dB of rejection whilst maintaining good phase linearity and keeping the insertion loss to a minimum is a difficult task. Advice is currently being sought from manufacturers as to the feasibility of designing such a filter for their radar design(s). Early indications are that in order to achieve the 60dB rejection requirement and maintain the phase linearity of the received signal the filter will have to roll-off over a number of 10’s of MHz and have an insertion loss of around 0.5dB. The diagram below shows an assessment carried out by the CAA on the impact an additional 0.5dB of insertion loss would have on a typical radar’s coverage at 1,000ft

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2.3.3 Based on the replies from the various radar manufacturers a decision will be made as to the way forward and whether the radar manufacturers/operators are going to have to cover the costs or whether at least the development work will be either part or fully funded by government.

2.3.4 Both Ofcom and the CAA are keen to ensure that the radar manufacturers do not profit out of the current situation which is largely of their own making. To this end we are working closely with other CAA’s where they are doing similar work and ensuring that the manufacturers know that we are working together.

2.3.5 What the final solution is, who will pay for it and whether it will be fit for purpose has yet to be determined.

2.4 WiMAX/LTE Out-of-Band Emissions

2.4.1 The ITU and CEPT out-of-band emission limit for mobile services is -30dBm/MHz. The UK did initially specify an out-of-band emission limit of -45dBm/MHz but as the table below demonstrates neither of these limits are sufficient to protect radar receivers from interference assuming main-beam coupling.

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Base Station, 1km Separation User Equipment 500m Separation

Spurious emission level (ITU) (dBm/MHz) -45 -30 -30Antenna Gain (dBi) 17 0Free space path loss for 1km (dB) 101 98Multipath gain (dB) 6Radar antenna gain (dBi) 34Power received (dBm/MHz) -89 -74 -88Typical minimum discernable signal (dBm/MHz) -120 Interference margin (dB) -31 -46 -32

2.4.2 From these figures it can be concluded that a tighter out-of-band emission mask will be required to protect radar.

2.4.3 Measurements made on a couple of pre-production WiMAX/LTE base stations show that the out-of-band emissions are significantly lower than the ITU limit and are for a significant portion of the frequency band 2 700-2 900 MHz below the level required to protect radar.

2.4.4 For the user terminals which are unlikely to be in the main-beam gain the following graph shows the interference potential of real equipment taking into account the beam pattern of the radar antenna.

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2.4.5 For both the mobile base station and the user equipment it would appear that the manufactured equipment is likely to have out-of-band emissions that are well below those required by either the ITU or CEPT. Therefore were the masks to be tightened this should not be a significant problem for WiMAX/LTE.

2.5 Work in CEPT SE 21

2.5.1 Project Team SE21 have responsibility within CEPT for work on Unwanted emissions and following a meeting of their parent group WG SE in September 2010 they were asked to study the issue of compatibility between WiMAX/LTE and Radar systems operating around 2 700 MHz. The timeline for the work being to produce initial findings by summer 2011 with a final report being completed by the end of 2011.

2.5.2 Since being tasked with undertaking the studies two meetings have been held. The last meeting being held 14th-16th March 2011. At that meeting two major contributions were received, one from France and the other from the UK detailing some of the work they had done with respect to this issue. As a result of these contributions and others from various countries that focused more on the potential impact on the WiMAX/LTE equipment a document has been produced listing the scenarios and parameters with the intention of using this information to produce the final report. Copies of the outline report and the French and UK contributions are attached.

2.5.3 The studies within SE21 are on-going with the next meeting due to be held on the 18 th/19th April 2011.

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3. ACTION BY THE MEETING

3.1 The ACP WG-F is invited to:

To note the content of the paper and if appropriate use it as background information in national processes

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Attachment 1Relevant documents From SE21

French contribution

UK Contribution

List of Scenarios and parameters

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