1、 ITU-R ITU-6 PN-452-6 94 4855232 O522382 607 375 Rec. ITU-R PN.452-6 SECTION 5G: PROPAGATION DATA REQUIRED FOR THE EVALUATION OF INTERFERENCE: SPACE AND TERRESTRIAL SYSTEMS RECOMMENDATION ITU-R PN.452-6 PREDICTION PROCEDURE FOR THE EVALUATION OF MICROWAVE INTERFERENCE BETWEEN STATIONS ON THE SURFACE
2、 OF THE EARTH AT FREQUENCIES ABOVE ABOUT 0.7 GHz (Question IT-R 208/3) (1970- 1974- 1978-1982- 1986-1992- 1994) The ITU Radiocommunication Assembly, considering - a) services, between systems in the same service and between systems in the terrestrial and Earth-space services; that due to congestion
3、of the radio spectrum, frequency bands must be shared between different terrestrial b) prediction procedures are needed that are accurate and reliable in operation and acceptable to all parties concerned; that for the satisfactory coexistence of systems sharing the same frequency bands, interference
4、 propagation c objectives; that interference propagation predictions are required to meet “worst-month” performance and availability d) that prediction methods are required for application to all types of path in all areas of the world, recommends 1. that, for frequencies above about 0.7 GHz, the mi
5、crowave interference prediction procedure given in Annex 1 be used for the evaluation of the available propagation loss in interference calculations between stations on the surface of the Earth. ANNEX i 1. Introduction Congestion of the radio-frequency spectrum has made necessary the sharing of many
6、 of the microwave frequency bands between different radio services, and between the different operators of similar radio services. In order to ensure the satisfactory coexistence of the terrestrial and Earth-space systems involved, it is important to be able to predict with reasonable accuracy the l
7、evels of interference that might exist between them, using prediction procedures and models which are acceptable to all parties concerned, and which have demonstrated accuracy and reliability. Many types and combinations of interference path may exist between stations on the surface of the Earth and
8、 between these stations and stations in space, and prediction methods are required for each scenario. This Annex addresses one of the more important sets of interference problems, Le., those situations where there is a potential for interference between microwave radio stations located on the surfac
9、e of the Earth. I l The prediction procedure detailed below is appropriate to terrestrial microwave link stations and satellite earth stations operating in the frequency range of about 0.7 GHz to 30 GHz. The method includes a comprehensive set of propagation models which ensure that the predictions
10、embrace all the significant propagation mechanisms that can arise. Techniques for analysing the radio-meteorological and topogaphical features of the path are provided so that it is possible to generate a prediction for any practical type of interference path. 376 ITU-R ITU-R PN.452-6 94 m 4855212 0
11、522383 543 Rec. ITU-R PN.452-6 2. Interference propagation mechanisms Microwave interference propagation may arise through a range of propagation mechanisms whose individual dominance depends on many factors, including climate, radio frequency, time percentage of interest, distance and path topograp
12、hy. Ar any one time a single mechanism or more than one may occur. The eight principal interference propagation mechanisms are outlined here, of which the first four may be regarded as long-term, or continuous, interference mechanisms (Fig. i), and the remaining four as short-term, or anomalous, mec
13、hanisms (Fig. 2) usually generating much higher interfering signal levels. FIGURE 1 Long-term interference propagation mechanisms i . . . . . . i . . . il . , . _. . . . *DOS 2.1 Line-of-sight The most straightforward interference propagation mechanism is line-of-sight under well-mixed atmospheric c
14、onditions. An additional complexity can, however, come into play when sub-path diffraction causes an increase in signal level. 2.2 Diffraction The accuracy to which this mechanism can be modelled often determines the density of microwave systems that can be achieved in a given area. The diffraction
15、prediction capability must have sufficient utility to cover smooth- earth, discrete obstacle and irregular terrain situations. 2.3 Tropospheric scatter This mechanism defines the “background” interference level for longer paths (e.g., 100-150 km). The prediction requirement for this mode in the inte
16、rference context is somewhat different from that needed for troposcatter link design as it is time percentages below 50% that are of interest, often on paths with a highly asymmetrical geometry and with the common volumes being generated by antenna side lobes. 1TLJ-R ITU-R PN.452-6 74 W 485.5212 052
17、2384 48T = Rec. ITU-R PN.452-6 377 FIGURE 2 Short-term (enhanced) propagation mechanisms 2.4 Scatterfrom terrain and buildings (not illustrated in Fig. i) This has not hitherto been a problem, but may become important in the future if crossing paths and high- density networks of paths become more co
18、mmonplace. This mechanism is not currently covered by this prediction procedure. 2.5 Enhanced line-of-sight The interference propagation mechanism of unobstructed transmission on line-of-sight paths may sometimes have levels raised by multipath and focusing effects. 2.6 Surface super-refraction and
19、ducting This is the most important short-term interference mechanism over water and in flat coastal land areas. 2.7 Elevated layer refection and refraction The treatment of reflection and/or refraction from layers at heights up to a few hundred metres is of major importance as these mechanisms enabl
20、e signals to bypass the diffraction loss of the terrain very effectively under favourable path geometry situations. 2.8 Hydrometeor scatter Hydrometeor scatter is particularly important as a possible source of interference because it may act virtually omni-directionally . A basic problem in interfer
21、ence prediction (which is indeed common to all tropospheric prediction procedures) is the difficulty of providing a unified consistent set of practical methods covering a wide range of distances and time percentages; i.e., for the real atmosphere in which the statistics of one mechanism merge gradua
22、lly into another as meteorological andor path conditions change. Especially in these transitional regions, a given level of signal may occur 378 ITU-R ITU-IR PN.452-6 94 4855232 0522385 33b Rec. ITU-R PN.452-6 for a time percentage which is the sum of those in different mechanisms. The approach in t
23、his procedure has been deliberately to keep separate the prediction of interference levels from the different propagation mechanisms up to the point where they can be combined into an overall prediction for the path. This is especially the case for the two somewhat different circumstances of clear-a
24、ir and hydrometeor scatter conditions. 3. Clear-air interference prediction 3.1 Global and European procedures The text below provides two complementary versions of a comprehensive clear-air interference prediction procedure which takes into account the clear-air propagation mechanisms outlined in $
25、 2 above. The first version, the “global” procedure ($ 3.2), can be applied (for all practical purposes) on a world-wide basis, and uses radio- meteorological data to describe the location variability of interference propagation conditions. The second version, the “European” procedure ($ 3.3) offers
26、 improved precision for North-West Europe, from where a significant amount of measured data has allowed more accurate determination of propagation characteristics based on land, sea and coastal zone concepts. The two procedures are essentially the same apart from the method of defining the radio-met
27、eorological influences affecting the anomalous propagation conditions on the path. - The procedures below are intended for computer implementation, although it is possible to hand-work the important clear-air calculations for individual paths if required. Maps are used to provide essential input dat
28、a, but the intention is that these be realized as simple databases for use by computer software. Appendix 1, which contains the maps, also suggests one convenient method of creating suitable databases. 3.2 Deriving a prediction using the global procedure 3.2.1 Outline of the procedure The global pro
29、cedure allows average year or (quasi) worst-month predictions to be derived for any path bounded by 80” North and 70” South. The North-South limits are imposed by the current availability of radio- meteorological data. The procedure embraces all the clear-air propagation mechanisms mentioned in $ 2
30、above (with the exception of terrain and building scatter). The variations in propagation conditions around the world are described by means of maps of long and short term radio refractive index conditions. (The resolution and accuracy of the data in these maps is currently being improved for areas
31、where better data have become available since the original work.) There are seven stages to deriving a prediction using the global method: Step 1: Decide whether an average year or worst-month prediction is required. Step 2: Assemble the basic input data. Step 3: Derive the annual or worst-month rad
32、io-meteorological data from the maps provided. Step 4: Analyse the path profile, and classify the path according to the path geometry. Step 5: Identify which individual propagation models need to be invoked. Step 6: Calculate the individual propagation predictions using each of the models identified
33、 in Step 5. Step 7: Combine the individual predictions to give the overall statistics. 3.2.2 Choice of average year or worst-month prediction The decision as to whether annual or “worst-month” predictions are required is dictated by the quality (Le., performance and availability) objectives of the i
34、nterfered-with radio system at the receiving end of the interference path. As interference is often a bi-directional problem, two such sets of quality objectives may need to be evaluated in order to determine the worst-case situation upon which the prediction needs to be based. In the majority of ca
35、ses the quality objectives will be couched in terms of a percentage “of any month”, and hence worst-month data will be needed. True worst-month data can only be provided when there exists an extensive database of long-term radio measurements (ideally 10 years or more). No such database exists for in
36、terference propagation data from around the world, and thus the global method has to rely on quasi worst-month data derived from long-term meteorological measurements, on the assumption that reliable relationships exist between the observed meteorological conditions and the actual propagation condit
37、ions. However, this approximation of the worst-month situation has proved realistic in tests against such measured data as are available, and provides prediction results far closer to operational requirements than have previously been available. Parameter Preferred resolution 3.2.3 Assembling the in
38、put data Description The basic input data required for the global procedure is given in Table 1: f P TABLE 1 0.01 GHz Frequency (GHz) 0.001% Required time percentage(s) for which the calculated basic transmission loss is not exceeded Basic input data Wrv Wr hg7 hrg 0.ool0 Longitude of station (degre
39、es) Irn Antenna centre height above ground level (m) Gi, Gr 1- (Pt. Vr I o.oolo I Latitude of station (degrees) 0.1 dBi Antenna gain in the direction of the horizon along the great-circle interference path (dBi) It should be noted that, for the interfering and interfered-with stations: t: interferer
40、 r : interfered-with. In addition, where the path is partly over the sea or other large areas of water, certain additional information about the great-circle interference path needs to be determined, as indicated in Table 2. All other information required to complete the prediction is derived from t
41、hese basic data during the execution of the procedure. 3.2.4 Radio-meteorological data for the path The global model uses two radio-meteorological parameters to describe the variability of background and anomalous propagation conditions at the different locations around the world. - AN, the average
42、radio-refractive index lapse-rate through the first 1 km of the atmosphere, provides the data upon which the appropriate effective earth radius can be based for path profile and diffraction obstacle analysis. Figures 5 and 6, respectively, provide world maps of average annual values of AN and maximu
43、m monthly mean values of M (for worst month prediction). - The parameter o% is the time percentage for which super-refractive lapse-rates with gradients of modulus exceeding 100 N-unitskm can be expected in the lower atmosphere, and is used as a measure of the incidence of fully developed anomalous
44、propagation in the area under consideration. Figures 7 and 8, respectively, provide the annual and maximum monthly mean time percentages, o% relevant to these anomalous propagation conditions. 380 Parameter Preferred resolution TABLE 2 Description Additional input data required for paths with one or
45、 more sea sections db 1 km Aggregate length of the path sections over water (km) d,) d,I) W equation (29) in Appendix 2. For totally overland paths w = O Distance from the first terminal (the interference source) to the coast along the great-circle interference path (km) Corresponding distance for t
46、he second (interfered-with) station (km) 0.1 km 0.1 km Fraction of the total path over water: where dis the great-circle distance (km) calculated using w = dbld (1) The correct input data for the prediction models is given by the average of the path-end and path centre values of AN and o% appropriat
47、e to the path in question from Figs. 5 and 7 for any (average year) or Figs. 6 and 8 (for the worst month) as decided in the procedural step in 9 3.2.2. However, in areas where the values are reasonably constant the path centre value alone can safely be used. Using the radio-meteorological determine
48、d: (Note that AN is considered to be data, the median effective earth radius factor k50 for the path can be a positive quantity in this procedure.) When diffraction calculations need to be undertaken (see Table 4 for conditions), the basic transmission loss variability with time percentage due to th
49、e diffraction propagation mechanism is assumed to be the result of changes in bulk atmospheric radio refractivity lapse rate. As the time percentage, p, reduces, the effective earth radius factor k(p) is assumed to increase according to the function: Note that this expression is only valid for p% I 50%. Using the value of k50 found from equation (2), and assuming a true earth radius of 6 375 km, the median value of effective earth radius a, can be determined: a, = 6375 k50 km (3) Once again, under the specific conditions which make necessary the diffraction calculations, a time-v
