1、ITU-R RECMNUP. SERIES 95 m 4855212 052789 BT m Rw. ITU-R P.53.0-6 229 RECOMMENDATION UU-R P.530-6 PROPAGATION DATA AND PREDICTION METHODS REQUIRED FOR THE DESIGN OF TERRESTRIAL LINE-OF-SIGHT SYSTEMS (Question ITU-R 204/3) ( 1978- 1982-1986- 1990-1992- 1994- 1995) The 112T Radiocommunication Assembly
2、, considering a) that for the proper planning of terrestrial line-of-sight systems it is necessary to have appropriate propagation prediction methods and data; b) that methods have been developed that allow the prediction of some of the most important propagation parameters affecting the planning of
3、 terrestrial line-of-sight systems; c) that as far as possible these methods have been tested against available measured data and have been shown to yield an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for most present applications in system pl
4、anning, recommends 1 sight systems in the respective ranges of parameters indicated. that the prediction methods and other techniques set out in Annex 1 be adopted for planning terrestrial line-of- ANNEX 1 1 Introduction Several propagation effects must be considered in the design of line-of-sight r
5、adio-relay systems. These include: - diffraction fading due to obstruction of the path by terrain obstacles under adverse propagation conditions; - attenuation due to atmospheric gases; - fading due to atmospheric multipath or beam spreading (commonly referred to as defocusing in the English technic
6、al literature) associated with abnormal refractive layers; fading due to multipath arising from surface reflection; attenuation due to precipitation or solid particles in the atmosphere; variation of the angle-of-arrival at the receiver terminal and angle-of-launch at the transmitter terminal due to
7、 refraction; reduction in cross-polarization discrimination in multipath or precipitation conditions; signal distortion due to frequency selective fading and delay during multipath propagation. - - - - - One purpose of this Annex is to present in concise step-by-step form simple prediction methods f
8、or the propagation effects that must be taken into account in the majority of fixed line-of-sight links, together with information on their ranges of validity. Another purpose of this Annex is to present other information and techniques that can be recommended in the planning of terrestrial line-of-
9、sight systems. Prediction methods based on specific climate and topographical conditions within an administrations temtory may be found to have advantages over those contained in this Annex. ITU-R RECMN*P* SERIES 95 W 4855232 0527590 OTL D 230 Rec. ITU-R P.530-6 With the exception of the interferenc
10、e resulting from reduction in cross-polarization discrimination, the Annex deals only with effects on the wanted signal. Other interference aspects are treated in separate Recommendations, namely: - - To optimize the usability of this Annex in system planning and design, the information is arranged
11、according to the propagation effects that must be considered, rather than to the physical mechanisms causing the different effects. It should be noted that the term “worst month” used in this Recommendation is equivalent to the term “any month” (see Recommendation ITU-R P.581). intederence involving
12、 other terrestrial links and earth stations in Recommendation ITU-R P.452, interference involving space stations in Recommendation ITU-R P.619. 2 Propagation loss The propagation loss on a terrestrial line-of-sight path relative to the free-space loss (see Recommendation IT-R P.525) is the sum of di
13、fferent contributions as follows: - attenuation due to atmospheric gases, - diffraction fading due to obstruction or partial obstruction of the path, - fading due to multipath, beam spreading and scintillation, - attenuation due to variation of the angle-of-arrivaMaunch, - attenuation due to precipi
14、tation, - attenuation due to sand and dust storms. Each of these contributions has its own characteristics as a function of frequency, path length and geographic location. These are described in the subsections that follow. Sometimes propagation enhancement is of interest. In such cases it is consid
15、ered following the associated propagation loss. 2.1 Attenuation due to atmospheric gases Some attenuation due to absorption by oxygen and water vapour is always present, and should be included in the calculation of total propagation loss at frequencies above about 10 GHz. The attenuation on a path o
16、f length d km is given by: The specific attenuation ya (dB/km) should be obtained using Recommendation ITU-R P.676. NOTE 1 - On long paths at frequencies above about 20 GHz, it may be desirable to take into account known statistics of water vapour density and temperature in the vicinity of the path.
17、 Some information on water vapour density is given in Recommendation ITU-R P.836. 2.2 Diffraction fading Variations in atmospheric refractive conditions cause changes in the effective Earths radius or k-factor from its median value of approximately 4/3 for a standard atmosphere (see Recommendation I
18、TU-R P.310). When the atmosphere is sufficiently sub-refractive (large positive values of the gradient of refractive index, low k-factor values), the ray paths will be bent in such a way that the Earth appears to obstruct the direct path between transmitter and receiver, giving rise to the kind of f
19、ading called diffraction fading. This fading is the factor that determines the antenna heights. k-factor statistics for a single point can be determined from measurements or predictions of the refractive index gradient in the first 100 m of the atmosphere (see Recommendation ITU-R P.453 on effects o
20、f refraction). These gradients need to be averaged in order to obtain the effective value of k for the path length in question, k,. Values of ke exceeded for 99.9% of the time are discussed in terms of path clearance criteria in the following section. ITU-R RECMN*P* SERIES 75 4855232 0527573 T38 Rec
21、. ITU-R P.530-6 231 2.2.1 Diffraction loss dependence on path clearance Diffraction loss will depend on the type of terrain and the vegetation. For a given path ray clearance, the diffraction loss will vary from a minimum value for a single knife-edge obstruction to a maximum for smooth spherical Ea
22、rth. Methods for calculating diffraction loss for these two cases and also for paths with irregular terrain are discussed in Recommendation -R P.526. These upper and lower limits for the diffraction loss are shown in Fig. 1. FIGURE 1 Diffraction loss for obstructed line-of-sight microwave radio path
23、s -10 O - 3 8 o 10 determine the antenna heights required for the appropriate median value of the point k-factor (see 8 2.2; in the b) c) clearance radii: obtain the value of k, (99.9%) from Fig. 2 for the path length in question; calculate the antenna heights required for the value of k, obtained f
24、rom step b) and the following Fresnel zone 0.0 FI (Le. grazing) if there is a single isolated path obstruction 0.6 FI for path lengths greater than about 30 km I I 0.3 Fi if the path obstruction is extended along a portion of the path d) In cases of uncertainty as to the type of climate, the more co
25、nservative clearance rule for tropical climates may be followed or at least a rule based on an average of the clearances for temperate and tropical climates. Smaller fractions of F1 may be necessary in steps a) and c) above for frequencies less than about 2 GHz in order to avoid unacceptably large a
26、ntenna heights. use the larger of the antenna heights obtained by steps a) and c). Higher fractions of F1 may be necessary in step c) for frequencies greater than about 10 GHz in order to reduce the risk of diffraction in sub-refractive conditions. ITU-R RECMN*P. SERIES 95 m 4855212 0527593 800 m Re
27、c. I“-R P.530-6 1.1 1 0.9 O. 8 eu 0.7 0.6 0.5 0.4 0.3 10 FIGURE 2 Value of R, exceeded for approximateiy W.9% of the worst month (Continental temperate climate) 10 Path length beam spreading (commonly referred to as defocusing in the English technical literature), antenna decouping, surface multipat
28、h, and atmospheric multipath. Most of these ITU-R RECMNaP. SERIES 95 4855212 0527594 747 W 234 Rec. ITU-R P.530-6 mechanisms can occur by themselves or in combination with each other (see Note 1). A particularly severe form of frequency selective fading occurs when beam spreading of the direct signa
29、l combines with a surface reflected signal to produce multipath fading. Scintillation fading due to smaller scale turbulent irregularities in the atmosphere is always present with these mechanisms but at frequencies below about 40 GHz its effect on the overall fading distribution is not significant.
30、 NOTE 1 - Antenna decoupling governs the minimum beamwidth of the antennas that should be chosen. Two methods for predicting the single-frequency (or narrow-band) fading distribution at large fade depths in any pari of the world are given below. Method 1 in 8 2.3.1 does not make use of the path prof
31、ile and can be used for initial planning, licensing, or even design purposes. Method 2 in 8 2.3.2, which does employ the path profile, should be used for critical link designs, particularly if geoclimatic factors Kare available from measured data. Appropriate equations are given in these methods for
32、 overland and overwater paths. A third method in 8 2.3.3 that is suitable for all fade depths employs Methods 1 or 2 for large fade depths as appropriate and an interpolation procedure for small fade depths. All methods take into account an average mixture of the clear-air fading mechanisms noted ab
33、ove. A method for predicting enhancement is given in 8 2.3.4. The method uses the fade depth predicted by the method in 8 2.3.1 or $ 2.3.2 as the only input parameter. 2.3.1 Method for initial planning purposes at small percentages of time (Method 1) a) For the path location in question, estimate th
34、e geoclimatic factor K for the average worst month from fading data for the area if these are available (see Annex 2). If such data are not available, K can be estimated from the contour maps of Figs. 7-10 of Recommendation -R P.453 for the percentage of time pr, that the average refractivity gradie
35、nt in the lowest 100 m of the atmosphere is less than -100 N uniMun, and the following empirical relations: K = 1046.5 - C, - Ch) pL1.5 overland links for which the lower of the transmitting and (4) receiving antennas is less than 700 m above mean sea level (see Note 2) K = 1047.1 - C, - Ch,) pL1.5
36、overland links for which the lower of the transmitting and receiving antennas is higher than 700 m above mean sea level (see Notes 1 and 2) K = 1045.9-C,-C,)1.5 links over medium-sized bodies of water (see Note 3), coastal areas beside such bodies of water (see Note 4), or regions of many lakes (see
37、 Note 5) K = 10-(5.5-C,-C,)1.5 links over large bodies of water (see Note 3), or coastal areas beside such bodies of water (see Note 4) where the coefficient C, of latitude 5 is given by: c, = o for 53“s 25 I 53“N c, = -5.3 + 5/10 for 53“N or S O, repeat steps a) to c) for A = 25 dB to obtain the de
38、finitive value of qt. e) For A 25 dB or A 35 dB, as appropriate, calculate the percentage of time pw that the fade depth A is exceeded using Method 1 or Method 2 as desired. For A e 25 dB, or A 1OdB (34) where E is the enhancement (dB) not exceeded for p% of the time and Ao.01 is the predicted deep
39、fade depth using equations (15) or (28) exceeded for pw = 0.01% of the time. ITU-R RECMNIP- SERIES 95 m 4855232 0527598 392 m 238 Rec ITU-R P.530-6 For the enhancement between 10 and O dB use the following step-by-step procedure: a b) Calculate q using: Calculate the percentage of time pw with enhan
40、cement less or equal to 10 dB (E = 10) using equation (34). cl calculate the parameter qs from: 4s = 2.05 4; - 20.3 d) Calculate qe for the desired E using: qe = 8 + i + 0.3 x 10-E20 104*7E120 qs + 12 ( + E/8W) e) The percentage of time that the enhancement E (dB) is not exceeded is found from: O 10
41、 m 20 z = T Tz al U e, U 30 40 50 pw = 100 - 58.21 1 - exp -10-4e ( FIGURE 3 Percentage of the time fade depth exceeded in an average worst month, with q, (equation (33) ranging from -2 to 7 (37) 10-1 10-2 10-3 10-4 10-5 :xz 102 10 1 0-9 * *si Percentage of time, p, ITU-R RECMNwP- SERIES 35 M 985523
42、2 0527539 229 = Rc. ITU-R P.530-6 239 The set of curves in Fig. 4 with qs as a parameter gives a graphical representation of the method. FIGURE 4 Prediction of enhancement for various percentages of time, with qs (in equation (37) ranging from -14 to 2 20 18 16 14 i 12 rn w- f lo s 8 =8 6 4 2 O 1 O“
43、 10-3 10-2 10-1 1 10 lo2 Percentage of time (100 - pw) *I* I * S.,: For prediction of exceedance percentages for the average year instead of the average worst month, see 0 2.3.5. 2.3.5 The fading and enhancement distributions for the average worst month obtained from the methods of 0 2.3.1-2.3.4 can
44、 be converted to distributions for the average year by employing the following procedure: Conversion from average worst month to average annual distributions 4 worst month from equations (15) or (28). calculate the percentage of time pw fade depth A is exceeded in the large tail of the distribution
45、for the average b) Calculate the logarithmic geoclimatic conversion factor AG from (39) AG = 10.3 - 5.0log f Ico21) - 2.8 logd + 1.8 log where AG I 10.8 dB and the positive sign in equation (39) is employed for 6 545“ and the negative sign for 5 45“. and where: 6 : latitude (ON or “S) d: path length
46、 (km) I cP I : magnitude of path inclination (obtained from equation (14). ITU-R RECMNxP. SERIES 95 4855232 0527600 870 240 RW ITU-R P.530-6 C) the average year from: Calculate the percentage of time p fade depth A is exceeded in the large fade depth tail of the distribution for d) the method of 0 2
47、.3.3, replacing pw by p. If the shallow fading range of the distribution is required (i.e. A 100 mmh. use the value 100 mmfh in place of R0.01. Step 4: An estimate of the path attenuation exceeded for 0.01% of the time is given by: Step 5: following power law: Attenuation exceeded for other percenta
48、ges of time p in the range 0.001% to 1% may be deduced from the This formula has been determined to give factors of 0.12, 0.39, 1 and 2.14 for 1%, 0.1%, 0.01% and 0.001% respectively, and must be used only within this range. 1TU-R RECMN*P- SERIES 95 = LIB55212 0527603 707 W Rs ITTJ-R P.530-6 241 Ste
49、p 6: If worst-month statistics are desired, calculate the annual time percentages p corresponding to the worst-month time percentages pw using climate information specified in Recommendation -R P.841. The values of A exceeded for percentages of the time p on an annual basis will be exceeded for the corresponding percentages of time pw on a worst- month basis. The prediction procedure outlined above is considered to be valid in all parts of the world at least for frequencies up to 40 GHz and path lengths up to 60 km. 2.4.2 Frequency
