AMCP WG-F/8 WP 05
(REV)
Agenda Item 9 : 2700 - 2900 MHz band issues
RADAR SHARING WITH MOBILE SERVICES IN BAND 2700-2900 MHz
( Presented by Secretariat )
SUMMARY
This paper presents developments in CEPT Europe on proposals made to share radar frequencies in the band 2700-2900 MHz with mobile services, in particular with ENG/OB 1 and with Third Generation mobile services. The paper describes the results of studies by aviation (Eurocontrol) and by ITU-R SG8B, and the publication of a CEPT ECC Report.
The paper recommends policy and actions for use by national aviation authorities involved in discussions with regulatory and other bodies. on this subject.
The band 2700-2900 MHz is allocated to the Aeronautical Radionavigation Service, and is utilized by Airfield Surveillance Radar ( ASR )systems, with which ATC services can monitor the Terminal
Area (TMA) of an airport, and apply separation between aircraft in the TMA. The service is of high criticality in operational terms, and particularly at busy airports and in conditions of adverse weather and low visibility. In some world areas, for example Europe, the band is in intensive use and becoming more intensive as airport congestion increases.
The bands below 2700 MHz are planned for intensive use by the 3 rd generation UMTS mobile services in the future. To complement this planning, 2 separate proposals have been made for the 2700-2900 MHz band above. One, to move ENG/OB services, in Europe currently occupying the UMTS band to the radar band. Secondly to examine the possibility of an overflow band for
3G in the longer term. The WRC-2003 is expected to propose an Agenda Item for WRC-2006 addressing this sharing. In Europe the CEPT 2 organization has studied the first of these. The
Report of this work will shortly be placed in the public domain as an ECC Report (Ref 1)
3.1 ITU-R Recs
1
ENG/OB : Electronic News Gathering / Outside Broadcast : A mobile service used by broadcasting services to relay news and other outside events from mobile units back to broadcast studies. The link may use terrestrial or satellite means
2 CEPT : Conference of European Postal and Telecommunications Administrations - An indepentant (i.e non-Governmental) organization with membership from 52 European countries. CEPT is the coordinator of European preparations for ITU WRC. The ECC covering all telecommunications matters has only recently replaced the earlier ERC which concentrated on radio only.
1

ITU-R has only recently made studies in the field of radar.
Most relate to general purpose PPI type radars, usually marine radars. More recently - last WG 8B meeting - new material based on civil aviation plot extracted radars with more stringent performance requirements (shown by underlining), typical of those employed in civil aviation, has been added. The 2 Recommendations of relevance to this paper are :
Rec. M 1461 : . Procedures for determining the potential for interference between radars operating in the radiodetermination service and systems in other services.
Rec. M.1464 : Characteristics of and protection criteria for radionavigation and meteorological radars operating in the band 2700-2900 MHz
The latest modifications, still to be fully approved , derive from an aviation input based on
Eurocontrol sponsored studies ( the TU Graz study ).
These Preliminary Draft Modified versions of ITU-R Recs incorporating the new material are considered by aviation to be the definitive ones for radar system protection purposes.
3.2 Aviation Standards
There are no ICAO SARPS for the detailed performance requirements for radar systems.
However, ICAO Doc 8071- Manual of Flight Testing , details specific test procedure for flight testing. In Europe, Eurocontrol radar system performance standards have been agreed which form the basis of assessing acceptable performance. The main parameters of interest are P d the probability of detection, P fa
the probability of false alarm, and azimuth accuracy,
CEPT SE PT34 held 9 meetings in which 3 European CAAs participated together with ICAO and
Eurocontrol . Of the many aviation inputs the Eurocontrol sponsored study (Ref 2) in collaboration with Austrocontrol (and Graz University) and NATS UK formed the aviation basis for the material in the ECC Report, as well as the amendments to ITU-R Recs. 1461 and 1464. The study included both theoretical analysis of system performance and propagation factors, in addition to measurements on an operational radar under live traffic conditions. Interfering signals used were white noise, and a simulated standard COFDM signal regarded to be used by ENG/OB. Tests were made on both magnetron type, and the more modern solid state type radars. Several propagation models, as specified in ITU-R Rec,P 452 were examined in detail as to their applicability. An important outcome of the study was the derivation of a relationship between the performance degradation parameters and the input interference to noise ratio (I/N) at the input to the receiver actually measured at the IF i.e required interference acceptibility for assessing protection and mimimum separation distances. .
A summary of the main findings, summarized from the recommendation section of the study (page
21, see also attachment B to this paper) are:
(i)
(ii)
The level of -6 dB below noise power spectral density for an interferer (as specified for
I/N in the unmodified ITU-R Recs ) is insufficient to protect civil aviation radars in this frequency band. The level required is at least - 10 dB and desirably -12 dB. :
For the front end saturation mode ( these radars have front end LNB of 200 MHz band
(iii)
(iv) width ) a maximum level of 15 dB below radar signal level at the radars 1 dB compression point is required. The maximum acceptable level depends on the acceptable reduction of dynamic range of the received echo signal. Since this effects mainly strong echos, 15 dB may not be necessary, but the interference level should be below 1 dB compression.
The propagation model should be based on line of sight, together with enhancements and other short term effects as described in ITU-R Rec. P.452 but regarding gas attenuation which reducesthe interference level slightly. The Hata model used in terrestrial mobile work is not considered appropriate
Calculation of separation distances based on above
2

ECC Report No.6 is a jointly prepared document between experts on aviation radar and in aviation frequency management, and radio regulatory and mobile radio experts from CEPT member countries. It is intended to be used by CEPT radio administrations and others, in the spectrum management work of integrating mobile services in to this ARNS band.
On the basis of its findings frequency management decisions will be taken by other CEPT groups, notably that on Frequency Management (WGFM) , on allocations and deployment of assignments.
The report is exploratory in character, and does not presume as its initial assumption that integration of the new services is either possible or practicable. Of necessity the findings must be accurate, meaningful and sufficient. It does not address in any way the regulatory and control aspects of operating two services, each highly individual in character, in a real life environment.
The Report very adequately describes the two services, their technical parameters, and the interference mechanisms of importance. Nevertheless, attention has to be paid to the conflict of interest inherent in the aspirations of the two communiities, the mobile service with its desire to find new spectrum, and the aeronautical service to protect important safety of life services. Hence the reporting is a faithful record in regard to the material included.
From an aeronautical viewpoint, a criticism may be made that it does not sufficiently stress important elements in the findings of the Eurocontrol studies. For example the decision by the parent body ( CEPT WGSE) to only employ ITU-R recommendations as the technical basis has meant that the very comprehensive aviation study is not fully accepted as valid material. The report must hence be regarded by aviation as a transitional document towards final conclusions.
One of the other shortcomings is that no positive recommendations are made on radar protection requirements, and for example both I/N figures - existing ITU-R recommendations and the recently modified figure - are both quoted as equal alternatives. There are other equally important examples. Aviation services must hence decide and promote only those elements which accord with the best and most expert input. .In the case of radar, this is the findings of the aviation study which extended and categorized the knowledge of radar protection requirements in an expert and professional manner. There is no better material available anywhere at the moment.
The Report includes extensive calculations of separation distances to protect radars. These can be regarded as typical rather than actual, and can only be used for broad strategic planning purposes, and only in those cases where aviation accepted criteria have been employed. An important conclusion is that the possibility of co channel operation is not considered to be practical. The high values of separation, and the cross border coordination aspects - both for planning and for clearance of interference purposes - combine to create a situation of extreme difficulty in European conditions.
The frequency management implications considering that ENG/OB a mobile system is, which has the potential of interfering with a radionavigation service, are expected to be very difficult to handle and, any constraint to be placed on ENG/OB is also expected to be very difficult to enforce.
A further difficulty is the consideration of future needs in the aviation service. It can not be assumed that no more 10 cm radars will be installed, particurlarly as congestion at major airports in Europe is increasing creating the need for more ASR radars. The involvement of military radar is a further factor needing consideration in this regard. The time scale of the Mobile requirement is many years away and in the same time frame as possible radar expansions. Attention must therefore be given to this factor.
The allocation aspect is a further important consideration. Presently the band is not allocated to the Mobile Service and a WRC agreement is necessary to change the situation. WRC-2003 has no directly appropriate Agenda Item , although there is one proposed for WRC-2006
In the belief that radars make an inefficient use of spectrum some European administrations are proceeding with plans to apply financial pressures through higher licensing charges. This opinion is based on the use of solid state systems being more spectral efficient than magnetron systems.
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Evidence for this view presented in the CEPT studies have not confirmed this assumption.
Further work is necessary to evaluate what the real situation is.
Whilst no change can be made to allocations until 2006, the progress of this subject within CEPT is regarded as urgent. ENG/OB use of the band below 2900 MHz is confined to a few countries only. The need for decisions is hence not immediately apparent. The need to take time to get the decisions right is equally important. From an aviation standpoint rapid progressing is not essential, or desirable..
From an international standpoint, it is desirable to formulate guidelines for future action by aviation authorities. The points of importance in this process are :
ITU-R Recs M.1461 and M.1464

In respect of civil aviation radar systems, support and agree those amendments to ITU-R
Recs M. 1461 and 1464 which have been derived from studies undertaken and / or endorsed by experts in those systems. These are the underlined parts of the Preliminary Draft Modified
Versions agreed at the WG 8 B meeting held in ] Sept 2001 ]

For other new material agree to its inclusion only on the basis that it does not degrade or invalidate existing material on this subject. ;
ECC Report No 6

Support deletion of the text indicated in Attachment A ( Example from one participating aviation authority )

Support the incorporation of a Recommendation that the conclusions and recommendations of the aviation study (Ref.4 ) be adopted as the definitive criteria for the protection of ASR radars in the band 2700-2900 MHz . ( see Attachment B )
National Discussions

In regard to proposals made by radio regulatory authorities to accept sharing with mobile services, insist on the application of the more stringent of the conditions contained in ECC
Report No. 6.as identified in Ref.4

Insist on the inclusion of future aviation requirements for ASR at airports, resisting also planning in CEPT or at national level which does not take in to account these projected needs of aviation up to at least the year [ 2015 ] ;

Resist any suggestions that radars are inefficient in spectrum use terms until a full examination is made.and discussed within the aviation community.
References
1 Technical Impact on Existing Primary Services in the band 2.7-2.9 GHzdue to the Proposed
Iintroduction of New Systems. CEPT ECC Report No. 6
2 Study on 2700-2900 MHz frequency band sharing between existing aeronautical radar equipment and planned digital ENG/OB and digital aeronautical telemetry services : . By AustroControl and
Technical University, Graz, Austria sponsored by Eurocontrol.
4

Attachments
A : Suggested amendment to Draft ECC Report No, 6
B : Extract from Ref. 2
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ATTACHMENT A
Modification to Draft ECC Report No.6
DELETE:
Proposal for deletion of the the following text in the "Executive Summary" (3 rd paragraph)
and in the "Conclusions" (page 30, 2 nd paragraph):
“In general, the radiohorizon is the limiting factor with regard to required separation distance. Assuming
LOS-propagation and co-channel operation, separation distances in the order of 400 km requires significant altitudes in the order of 10 km. For typical ENG/OB scenarios considered in this report, these altitudes, and therefor the separation distances, are significantly lower.”
REASON:
This text was added to the ECC-Report without approval by SE34 and changes one of the main arguments for protection of
ARNS in a way not complying with relevant ITU-R recommendations.
In particular, in the definition of the term "Line-of-sight" in Recommendation ITU-R P.452 it is clearly stated that propagation situations are more complex than assumed in the above piece of text. As a conclusion from this statement in ITU-R P.452, the requirement for separation distances in the order of 400 km can not only occur in cases where the interfering transmitter is operated at a significant altitude above ground in the order of 10 km.
From the excerpt from Recommendation ITU-R P.452-10 as reproduced below (in particular the highlighted sections) it can be seen, that also trans-horizon (non-LOS) propagation can produce signal strengths at, or even above, the LOS-level. This can particularly happen due to surface ducting and elevated layer reflection and refraction.
It is therefor necessary to take into consideration the longer separation distances as agreed by SE34 in the original version of
ECC-Report 6 when calculating/predicting interference between Digital ENG/OB transmitters and radio navigation services
(radar) receivers.
Excerpt from Recommendation ITU-R P.452-10:
– Line-of-sight (Fig. 1): The most straightforward interference propagation situation is when a line-of-sight transmission path exists under normal (i.e. well-mixed) atmospheric conditions. However, an additional complexity can come into play when sub-path diffraction causes a slight increase in signal level above that normally expected. Also, on all but the shortest paths (i.e. paths longer than about 5 km) signal levels can often be significantly enhanced for short periods of time by multipath and focusing effects resulting from atmospheric stratification (see Fig .
2).
– Diffraction (Fig. 1): Beyond line-of-sight and under normal conditions, diffraction effects generally dominate wherever significant signal levels are to be found. For services where anomalous short-term problems are not important, the accuracy to which diffraction can be modelled generally determines the density of systems that can be achieved. The diffraction prediction capability must have sufficient utility to cover smooth-Earth, discrete obstacle and irregular
(unstructured) terrain situations.
– Tropospheric scatter (Fig. 1): This mechanism defines the “background” interference level for longer paths (e.g. more than 100-150 km) where the diffraction field becomes very weak. However, except for a few special cases involving sensitive Earth stations or very high power interferers (e.g. radar systems), interference via tropo-scatter will be at too low a level to be significant.
–
Surface ducting (Fig. 2): This is the most important short-term interference mechanism over water and in flat coastal land areas, and can give rise to high signal levels over long distances (more than 500 km over the sea).
Such signals can exceed the equivalent “free-space” level under certain conditions.
–
Elevated layer reflection and refraction (Fig. 2): The treatment of reflection and/or refraction from layers at heights up to a few hundred metres is of major importance as these mechanisms enable signals to overcome the diffraction loss of the terrain very effectively under favourable path geometry situations. Again the impact can be significant over quite long distances (up to 250-300 km).
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Acceptable interference power spectral density -6 dB below noise power spectral density, as stated by Recs. ITU-R
M.1461 and M.1464 is insufficient. It causes 1 dB S/N reduction, which further can lead to about 15 % (percentage points) reduction in probability of detection (P d
) for interference by OFDM signals. Even at I/N = -10 dB, some 10 % reduction have been measured with a conventional operational ASR.
It is recommended here to reduce acceptable interference power spectral density to less than –
10 dB relative to system noise level for interference from OFDM transmitters. Additional measurements on advanced ASR systems need to be carried out in order to determine a new acceptable I/N.
rd
Received interference power, aggregated over the full RF-bandwidth, must not be higher than the level which causes output power of that particular element in the receiver chain, which goes first into saturation, to remain a sufficient separation (in case of the Graz ASR this is 15 dB) below the 1-dB-compression point, so not to take away too much dynamic range and not to produce 3 rd -order intermodulation products exceeding the acceptable I/N in the radar receiver's
IF-band .
Any bandwidth restriction in a later stage of the receiver is of no help against this negative effect. For details on this topic refer to Appendix J.
Apply the line-of-sight propagation model for site isolation calculation, with direct beam coupling , as applied also in Doc. SE34 (00) 35rev2, but including gas attenuation (here applied: 0.0075 dB/km), as well as some allowance for short-term fluctuation term E s
(p)
(as given in ITU-R P.452), unless a detailed path analysis is made , according to the procedures given in Rec. ITU-R
P.452 ,
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