1、ITU-R RECMN*M* 1179 95 4855232 0525973 8bl Rec. ITU-R M.1179 41 RECOMMENDATION ITU-R M.1179* PROCEDURES FOR DETERMINING THE INTERFERENCE COUPLING MECHANISMS AND MITIGATION OPTIONS FOR SYSTEMS OPERATING IN BANDS ADJACENT TO AND IN HARMONIC RELATIONSHJP WITH RADAR STATIONS IN THE RADIODETERMINATION SE
2、RVICE (Question ITU-R 207/8) ( 1995) Summary This Recommendation provides administrations with measurement procedures to identify the interference coupling mechanism(s), receiver front-end overload or radar spurious emissions, and methods to mitigate the interference from radar stations in the radio
3、determination service to fixed radio-relay stations and fixed-satellite earth stations. The ITLJ Radiocommunication Assembly, considering a that both fixed and mobile radar stations in the radiodetermination service are widely implemented in bands adjacent to and in harmonic relationship with the 4,
4、5 and 6 GHz bands used by the fixed and fixed-satellite services; b) that fixed radio-relay stations and fixed-satellite earth stations are vulnerable to interference from radar stations that have fundamental (necessary) and spurious emissions with high peak power levels; C) that fixed radio-relay s
5、tations have largely adopted digital modulation and fixed-satellite earth stations are rapidly moving towards the use of digital modulations which may be more susceptible to interference from radar pulsed type emissions; d) that fixed radio-relay stations and fixed-satellite earth stations use low-n
6、oise amplifiers which have inherent wide bandwidths and gains of 10-20 dB for fixed radio-relay stations and 50-65 dF3 for earth stations; e that under the conditions stated in a) through d), interference to stations in the fixed and fixed-satellite services may be caused by radar station necessary
7、emissions causing receiver front-end overload; f, that high levels of radar spurious emissions may cause interference to fixed radio-relay stations in the 4,5 and 6 GHz bands and fixed-satellite earth stations in the 4 GHz band; g) that Radiocommunication Study Group 8 is studying the question of ef
8、ficient use of the radio spectrum by radar systems including the study of inherent spurious emission characteristics of various types of output devices (see Question ITU-R 20U8); h) that Radiocommunication Study Group 9 has been studying the effects of unwanted emissions from radar systems on system
9、s in the fixed service (see Question i“lJ-R 159/9); j) that Radiocommunication Study Group 9 has proposed interference mitigation options to enhance compatibility between radar stations in the radiodetermination service and fixed radio-relay stations (Recommen- dation IT-R F. 1097); k) that measurem
10、ent procedures to positively identify the interference coupling mechanisms and mitigate the interference are not widely known, * This Recommendation should be brought to the attention of the International Maritime Organization (IMO), the International Civil Aviation Organization (ICAO), the Intemati
11、onal Maritime Radio Association (CIRM), the World Meteorological Organization WO) and Radiocommunication Study Groups 4 and 9. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services42 Rec. ITU-R M.1179 recommends 1 operating in the 4, 5 and
12、6 GHz bands, the following action(s) should be taken: - that in cases of radar interference to earth stations operating in the 4 GHz band and fixed radio-relay stations the procedures described in Annex 1 should be used to identify the interference coupling mechanism (see Notes 1 and 2); after the i
13、nterference coupling mechanism has been identified, one of the interference mitigation options described in Annex 2 for the interference coupling mechanism identified should be applied. - NOTE 1 - If the receiving system does not have a bandpass (preselector) filter ahead of the low-noise amplifier,
14、 it may be more cost-effective to install a bandpass (preselector) filter ahead of the low-noise amplifier prior to performing the tests in Annex 1. If the interference continues after the installation of the filter, the Annex 1 measurements to identify the interference coupling mechanism must be pe
15、rformed. NOTE 2 - If the interference coupling mechanism is identified as radar transmitter spurious emissions, the radar will need to be identified if it is desirable to filter the radar transmitter output. Installation of filters in the radar transmitter may not be achievable if the radiodetermina
16、tion station is mobile and there are numerous radars such as the case of radionavigation radars operated under licences of many administrations. Also, the installation of a filter in a radar may not be achievable due to the type of output device, size, weight and performance trade-offs. ANNEX 1 Dete
17、rmination of interference coupling mechanism 1 Introduction Interference mitigation measures will be ineffective unless the correct interference coupling mechanism (receiver front-end overload or radar transmitter spurious emissions) is identified. This Annex describes methods by which the interfere
18、nce mechanism may be determined, CO that appropriate mitigation measures may be implemented as reliably as possible. It should be noted that the tests and measurements required to determine the interference mechanism are not necessarily easy to perform, even if personnel at the facility has access t
19、o the necessary test equipment (a spectrum analyser and digital oscilloscope are recommended, at a minimum). (In carrying out measurements on radar emissions, care should be taken to ensure that overload of the measurement equipment does not occur as damage may be caused to measuring equipment.) Com
20、mercially available RF front-end bandpass filters are relatively inexpensive and can be installed relatively quickly. Installation of such a filter ahead of the first LNA/LNB/LNC (low noise amplifier/mixer/downconverter) in the receiver RF front-end is recommended as a first step when the interferin
21、g radar signal is outside the normal allocated receive band for the fixed radio-relay or earth station. If the only interference mechanism is receiver front-end overload and the radar signal is occurring outside the receive band, then the filter should mitigate the problem. It is also possible for r
22、eceiver front-end overload interference and radar spurious emission interference to occur simultaneously. In that case, installation of a bandpass filter on the receive station will only eliminate the front-end overload interference component; the station will still experience interference effects d
23、ue to the radar spurious emissions. Because of this possibility, the installation of a bandpass filter on the receive station ahead of the LNA/LNB/LNC is recommended before tests for radar spurious emission interference are attempted. If no such filter is installed, then the absence of receiver fron
24、t-end overload must be verified through tests described below. Finally, it should be noted that a receiver front-end amplifier which also incorporates a mixer/downconverter (LNB or LNC) may generate undesired products in the desired receive band when it is in an overload condition, as shown in Annex
25、 2. These products may be easily mistaken for radar spurious emissions in the earth station band. Thus, it is critical that the possibility of receiver front-end overload be eliminated by installation of a front-end bandpass filter before tests for spurious emissions are performed. COPYRIGHT Interna
26、tional Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R M.1179 2 Measurement procedures 43 2.1 Determination of front-end overload There are several steps in the process of determining whether or not front-end overload is occurring in a receiver. Th
27、e first and most obvious step is to physically examine the RF front-end of the system, usually at the antenna, and determine if any preselection already exists. It is important not to be misled by schematic diagrams, which may indicate the presence of filters which may not have actually been install
28、ed, or by narrow frequency ranges which are specified on an amplifier case (e.g. “3.7-4.2 GHz”); the actual amplifier response may be much wider than the label indicates. If RF bandpass filtering ahead of the first preamplifier is verified as being present, then it is very unlikely that the coupling
29、 mechanism is receiver front-end overload. If no such filtering is present, then such a filter should be installed. If the installation results in elimination of the problem, then the mechanism is probably front-end overload. If a bandpass filter for the earth station is unavailable, or for any othe
30、r reason the presence of front-end overload must be independently verified, then the following measurement procedure can be performed through the front-end of the receive station during an interference event. The goal of this measurement is to determine the extent, if any, to which the amplifier is
31、gain-compressing when energy from the radar is received. In order to document this effect clearly, it is necessary to simultaneously monitor the radar energy at the radar fundamental frequency, as well as the response of the earth station to that energy. A block diagram for the hardware arrangement
32、to be used in this test is shown in Fig. 1. FIGURE 1 Block diagram fur deteniinalion of interference coupling mechanisa Characteristics of either front-end overload or radar spurious ettissions are observed in coincidence with radar pulses Antenna feed , i LNA or l LNB or i LNC I Recording device (e
33、.g., computer or camera) Video (trigger source) Digital scope ChannelA LNA (output 3 7004 200 MHz) or LNB (output 950-1 450 MHz) or I LNC (OUtDUt 270-770 MHz) Spectrum analyser: tuned to equivalent preamplifier output of radar fundamental frequency (frequency span = O Hz) 33- Splitter I receiver J 1
34、TU-R RECNN*N- 3179 95 4855232 0525975 634 COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services44 Rec. ITU-R M.1179 With reference to Fig. 1, this test is performed with the antenna feed horn connected directly into the earth stations front
35、-end amplifier (LNALNBLNC). THERE SHOULD NOT BE ANY BANDPASS FILTER AHEAD OF THE LNALNEVLNC DURING THIS TEST UNLESS IT IS NORMALLY A PART OF THE SYSTEM. The signal out of the amplifier is then split into two paths. One side of the split is sent to a spectrum analyser, and the analyser video output i
36、s in turn routed to one channel of an oscilloscope (see Note 1). The analyser should be tuned to the equivalent preamplifier output of the radar fundamental frequency and the analyser frequency span should be set to O Hz. (If two or more radar fundamentals are produced, any one of them will suffice.
37、) The analyser IF bandwidth should be set to 1 MHz, and analyser trace sweeping should be suspended. NOTE 1 - Documentation of these measurements is important. Either a digital oscilloscope that can transfer data to a magnetic medium or an analogue oscilloscope with a camera can be used for this pur
38、pose. The other side of the split is routed to the earth station receiver, and the receivers IF output is routed to a second channel of the oscilloscope. The oscilloscope should be triggered from the radar pulse train coming out of the spectrum analyser. Thus, both the radar pulse train and the rece
39、iver response to that pulse train may be simultaneously observed on the oscilloscope. If the radar is overloading the earth station front-end, then gain compression should be observed on the IF trace when pulses from the radar are observed on the other oscilloscope trace. Examples of such responses
40、are shown for an LNA and an LNB in Figs. 2-5. A vanation on this technique can be implemented on an antenna that incorporates two cross-polarized feeds: install a bandpass filter ahead of the preamplifier on one feed. If interference subsequently occurs on the unfiltered feed but not on the filtered
41、 feed, then the problem is receiver front-end overload. As a caveat, however, it should be noted that interference may be polarization-dependent, and the cross polarization between feeds can unintentionally produce a filtering effect of its own. So it is critical, if this technique is attempted, tha
42、t both feeds are known to have previously been affected by the interference simultaneously. 2.2 Simultaneous occurrence of front-end overload and spurious emission interference As stated at the beginning of this section, it is entirely possible for both receiver front-end overload and radar spurious
43、 emission interference to occur simultaneously. This would be the case if a radar produced strong spurious emissions at the receive stations centre frequency, while the receive station was being operated with an unpreselected front-end. However, before the spurious emission interference problem can
44、be addressed, the possibility of front-end overload must first be eliminated by installing an RF filter on the earth station. 2.3 Determination of radar spurious emissions If tests for receiver front-end overload are negative or interference persists when a bandpass filter has been installed ahead o
45、f the first RF amplifier in the receive station, then the interference is probably occumng as a result of radar transmitter spurious emissions in the receive band of the station. It must be emphasized that, if the tests for radar spurious emission interference are to utilize unpreselected earth stat
46、ion RF front-ends, as shown in Fig. 1, then the possibility that interference is caused by receiver front-end overload must be eliminated before these tests are conducted. While the use of a bandpass filter on the earth station front-end is not absolutely required under these circumstances, the pres
47、ence of such a filter during the tests increases the confidence that no receiver front-end overload is occurring. ITU-R RECflNaM- 3179 95 4855232 052597b 570 9 COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R M.1179 o. 1 0.05
48、 45 I l l - : TI Osciiioscope channel B: spectrum analyser video output l - I u_- I FIGURE 2 ChaMels A and B of osdoscope when test arrangemnt of Fig. 1 is used and LNB front-end is not overloaded. Osciiioscope channel A receiver IF output I l , Note that time axis is much faster, due to faster expe
49、cted recovery time of LNB I I o. 1 m anal the time-domain spacing will be the same as the nominal pulse width. It is also possible for spurious emissions to appear as noise-like pulses. A disadvantage to this method is that the presence of the desired earth station signal may have the effect of masking the radar spurious emissions. FIGURE 6 Channels A and B of oscilloscope when test arrangement of Fig. 1 is used and spurious emssions occur at received frequency Desired signai must be eliminated to perform this test , 0.05 I ! 1 Oscilloscope channel B: spectnim analyser video output I
