1、 Rec. ITU-R F.1097-1 1 RECOMMENDATION ITU-R F.1097-1 INTERFERENCE MITIGATION OPTIONS TO ENHANCE COMPATIBILITY BETWEEN RADAR SYSTEMS AND DIGITAL RADIO-RELAY SYSTEMS (Question ITU-R 159/9) (1994-2000) Rec. ITU-R F.1097-1 The ITU Radiocommunication Assembly, considering a) that radar systems can produc
2、e interference to digital radio-relay systems (DRRSs) in some situations; b) that there are two coupling mechanisms by which radiated energy from radar stations may be coupled into radio-relay systems: radar spurious emission in the radio-relay bands; radio-relay system front-end overload (receiver
3、desensitization) caused by the radar fundamental frequency; c) that the most desirable method of mitigating the interference may be to reduce the spurious emissions at the radar transmitter to a sufficiently low level; d) that some of the techniques employed by radio-relay system designers to enhanc
4、e system performance are expected to reduce the susceptibility of these systems to interference from radar transmitters, recommends 1 that the interference mitigation options for radar systems listed below should be taken into consideration in order to enhance compatibility with DRRSs: operational m
5、easures, according to agreement with the agency responsible for the radar system; selection or adjustment of transmitter frequency; replacement of transmitter device; RF filter installation in the radar transmitter; 2 that the interference mitigation options listed below should be taken into conside
6、ration in the design and implementation of DRRSs in order to enhance compatibility with radar systems: microwave RF filters before the front-end of the receiver; antenna selection (side-lobe characteristics); antenna diversity (space or angle); forward error correction (FEC) coding; additional bit i
7、nterleaving technique (BIT); alternate channel use, in same band; alternate band deployment; path re-routing; other possible techniques; 3 that Annex 1 should be referred to for additional guidance relating to this Recommendation. 2 Rec. ITU-R F.1097-1 ANNEX 1 Options to enhance compatibility betwee
8、n radar systems and DRRSs 1 Radar system options The options listed below all depend on the condition that a particular radar installation has unequivocally been identified as the one causing the interference. So far, stationary air surveillance radars (ASR) operating near 1.3 GHz and near 3 GHz, an
9、d meteorological radars operating near 5.6 GHz have been encountered by DRRS operators. Furthermore merchant marine (mobile) navigation radars operating near 3 GHz have been encountered by operators with DRRSs in coastal areas. 1.1 Operational measures, sector blanking When the radar installation an
10、d the agency responsible for its operation are known, an agreement with the agency may be made, that the radar is momentarily switched off, when its main beam is pointing in the direction of the DRRS location. This is commonly known as sector blanking. If sector blanking is agreeable to the radar op
11、erating agency, it is simple to implement, either by hardware measures in older radars, or by control software commands in modern installations. Also, minimal or no expenses are incurred. This kind of mitigation option has already been implemented in some countries resulting in a successful co-exist
12、ence of installations of the radiodetermination service and the fixed service (see also Appendix 1). 1.2 Operational measures, selection or adjustment of transmitter frequency In some types of fixed radar systems it may be possible to select or adjust the fundamental frequency of the radar transmitt
13、er within the frequency range allowed for the radar system, so that the second or third harmonic spurious emissions will not be received by the DRRS. In particular, it may be possible to place the radar harmonic in the guardband, between upper and lower radio-relay half band of the frequency plan, o
14、r outside the radio-relay band all together. If this retuning is agreeable to the radar operating agency, for this measure too, minimal or no expenses are incurred. This kind of mitigation option has already been implemented in some countries resulting in a successful co-existence of installations o
15、f the radiodetermination service and the fixed service. 1.3 Replacement of transmitter device Variations in ground-based radar spurious emission levels have been observed in radars using either conventional or coaxial magnetron power tubes. These variations may be attributed to aging phenomena, resu
16、lting in: changes in the pulse shaping networks of the modulator; changes in anode voltage and current of the power tube; or arcing in the tube. The ground-based radar operators, on a routine basis, may need to perform periodic checks of the radar transmitter to determine whether these transmitters
17、have, because of aging, developed spurious components that were of low level or not present when the transmitter was new. In some reported cases, interference problems have been corrected by replacing the radar transmitter output device. Rec. ITU-R F.1097-1 3 1.4 RF filter installation in the radar
18、transmitter Radio frequency (RF) waveguide filters have been used in several types of radar to reduce interference to radio-relay systems to acceptable, low levels. Thus RF low pass, absorptive filters have been used in fixed 1.3 GHz ground-based radars to mitigate interference by the third harmonic
19、 into the 4 GHz band allocated to the fixed service. Similarly, 5 GHz ground-based radars had band pass filter (BPF)/low pass filter installed, to suppress spurious components interfering in the lower and upper 6 GHz fixed service bands (see Fig. 1). Such filters have been known in the radar industr
20、y for over 30 years. They will suppress radar spurious emissions by approximately 40 to 50 dB, while having an insertion loss of a few tenths of a dB at the fundamental operating radar frequency. The radar performance (detection range) is reduced by a small amount only by such filters. When interfer
21、ence into DRRSs is caused by spurious emissions from radars, the installation of an RF filter in the radar transmitter is considered to be the preferred solution, provided that it is technically possible. The expense incurred by installing filters in radar transmitters should be related to the cost
22、of the entire radar installation. The measures discussed in 1.3 and 1.4, in principle also apply to maritime mobile radar systems. 2 Radio-relay system options When interference from a radar system is observed in a radio-relay system, the first step in attempting to reduce the interference is to det
23、ermine if the coupling mechanism is: front-end overload of the radio-relay receiver caused by the radar fundamental frequency; or a radar spurious component occuring at the receiver channel frequency. In the case of large stationary ASRs with MW peak power output at their fundamental, design frequen
24、cy, the level of unintentionally generated and inadvertently emitted spurii which may be intercepted by a radio-relay receiver is often higher than the level of the desired radio-relay signal. In the case of mobile merchant marine navigation radars, the transmitter parameters are significantly diffe
25、rent from those of the ASR transmitters. The output levels at the fundamental and thus the spurious levels as well, are lower and the pulse durations are much shorter. Only if it turns out to be difficult or impossible to suppress at the source (i.e. the radar) the spurious components occurring at t
26、he receiver channel frequency, should measures at the victim radio-relay receiver be attempted. 2.1 Microwave RF filters If front-end overload by the radar signal, of a low-noise preamplifier (LNA) (which is common to all radio-relay channels) is the coupling mechanism, an RF BPF ahead of this pre-a
27、mplifier may be used to protect it from radar interference. Front-end LNA overload from 5 GHz ground-based radars has been encountered by some operators with DRRSs operating in the lower 6 GHz or the upper 6 GHz band. Installation of a BPF solved the problems. Receiver RF filters will not be effecti
28、ve against interference on, or near to, the radio-relay receiver channel frequencies. 2.2 Antenna selection After a radio-relay site survey, where appropriate electronic intelligence has been collected, the approximate location of a radar and the levels of spurii which may cause interference will be
29、 known. Thus the path geometry from the radio-relay receiver to the desired radio-relay transmitter and toward the radar may be established. 4 Rec.ITU-RF.1097-1 1097-017 6003 600 5 6004 400 5 000 5 925 6 4253 700 4 200 MHz+50010050+1007 6003 600 5 6004 400 5 000 5 925 6 4253 700 4 200 MHz+100+500100
30、50FIGURE 1Radiated spectrum measurements of a 5 GHz radar without and with an RF filterWithout filterRelative scale (dB)With filterFIGURE1/M.1097-1.D01= 3CMRec. ITU-R F.1097-1 5 If it appears that the radar main beam will intercept the radio-relay receiver antenna through its side lobes, then select
31、ion of a radio-relay antenna type with sufficiently low side-lobe levels may contribute toward protection of the receiver from spurious interference. The level of the interference ingressing through the side lobes into the receiver may exceed the thermal background noise by a small amount at most, i
32、n order not to compromise the transmission performance of the DRRS. Well designed, shrouded parabolic antennas are a good choice. If the interference arrives from an angle in the forward field of view of the antenna, between about 20 and 60 from main beam, then an antenna with an offset feed system
33、(shell-type) may yield better suppression than a shrouded parabola; however, it is significantly more expensive. 2.3 DRRS antenna diversity DRRSs susceptible to radar interference are wideband, high capacity systems for trunk transmission. They are normally employing channels in the 4 GHz, 5 GHz, lo
34、wer 6 GHz or upper 6 GHz bands allocated to the fixed service. These bands are chosen in order to permit long spans or hops to be bridged. Because of the propagation phenomena encountered in these bands, on most hops receiver antenna diversity is used, either space diversity or sometimes angle diver
35、sity. Thereby the probability of a marginal received signal level is minimized. Only in this manner may the stringent requirements to signal transmission quality (see Recommen-dation ITU-R F.1189 and ITU-T Recommendation G.826) be met in normal operation. Thus diversity will also protect against rad
36、ar spurious interference, provided this is incident on the radio-relay receiving site at a modest level. In such cases, the spurious levels may be well above the receiver background noise; however, the signal-to-total-noise ratio is still sufficient for proper detection/demodulation of the desired s
37、ignal. 2.4 Antenna site shielding This mitigation possibility has been suggested; it is sometimes used for satellite ground station antennas, located at ground level. Here the use of radar fences or soil dikes has been useful. However considering the normal placement of radio-relay antennas, 50 to 1
38、00 m above ground level on towers or masts, the shielding of antenna side lobes by nearby devices or means is unrealistic. 2.5 Forward error correction (FEC) FEC coding is a method used in most digital microwave systems to improve the BER performance. The utilization of FEC coding techniques permits
39、 a limited number of random errors to be corrected at the receiver by means of special coding software implemented at both ends of the hop. Measurements have shown that for a double-error correcting code, the threshold for interference breaking through and causing errors in the desired signal is imp
40、roved by approximately 10 dB when the interfering pulse duration is 1 Bd interval (i.e. the interference produces a receiver impulse response). Trunk transmission DRRSs operate at rates of about 30 MBd corresponding to baud intervals of the order of 30 ns. However, spurious pulses from ASRs (being g
41、enerated mainly during pulse rise and fall time of the intentionally generated radar pulse) have a considerably longer duration, of the order of 100 ns, and thus traditional FEC will not be effective against such interference. 2.6 Additional bit interleaving The situation is different when the inter
42、ference originates from a navigation radar, because the intentional pulse duration is much shorter ( 100 ns) than in ASRs. Therefore the spurious pulses generated during rise and fall times are exceedingly short ( 15 ns), but may still be picked up by the wideband radio-relay receivers. 6 Rec. ITU-R
43、 F.1097-1 A useful mitigation method has been found for this situation. It consists of additional bit interleaving. Upon reception of a signal which is corrupted by short bursts of errors due to the interference, the errors are spread out in time due to the de-interleaving. Thereafter the individual
44、 bit errors may be corrected by the FEC process. The disadvantage of the BIT is the additional processing time required at the receiver. Therefore the BIT should be available as an add-on option to normal receivers, to be used only on radio-relay hops where interference from navigation radars is enc
45、ountered. The BIT mitigation method has been in successful operational use in Japan for several years (see also Appendix 1). 2.7 Radio-relay transmitter power increase This mitigation possibility has been suggested; however, normally it is not likely to improve the situation for the following reason
46、s: a few dB of additional power from the radio-relay transmitter is not likely to improve the signal-to-total-noise ratio significantly, so the transmission performance will remain compromised; a significantly more powerful radio-relay transmitter would definitely be uneconomical; additional power i
47、ncreases the likelihood of interference with other distant DRRSs which are reusing the particular channel. 2.8 Alternate radio-relay RF channel selection This mitigation measure might be applied in certain situations only, since it is definitely not viewed as efficient spectrum utilization. The cond
48、itions for this measure are: the interference is caused by a reasonably well defined harmonic of the radar, so only a particular radio-relay channel is affected; for some reason it is not possible to retune the radar, as above, in 1.2; an undisturbed RF channel may be made available by the administr
49、ation, on the hop under consideration. 2.9 Alternate band employment This mitigation measure could be considered when planning a new DRRS, and when a site survey has indicated the likelihood of interference problems in bands allocated to the fixed service, which would otherwise be considered first choice. Also, this would depend on the availability of channels in a suitable alternate band. However, it is not an economical alternative for an established fixed service operation. It will usually mean a major expense to re-equip t
