1、358 ITU-R RECMN*PN b3-3 94 4855232 052353 bb Rec. ITU-R PN.681-1 RECOMMENDATION ITU-R PN.681-1* PROPAGATION DATA REQUIRED FOR THE DESIGN OF EARTH-SPACE LAND MOBILE TELECOMMUNICATION SYSTEMS (Question ITU-R 207/3) (1990- 1993) The ITU Radiocommunication Assembly, considering that for the proper plann
2、ing of Earth-space land mobile systems it is necessary to have appropriate propagation that the methods of Recommendation ITU-R PN.618 are recommended for the planning of Earth-space that further development of prediction methods for specific application to land mobile-satellite systems is that, how
3、ever, methods are available Which yield sufficient accuracy for many applications, a) data and prediction methods; b) telecommunication systems: c) required to give adequate accuracy in all regions of the world and for ail operational conditions; d) recommends that the methods contained in Annex 1 b
4、e adopted for use in the planning of Earth-space land mobile 1. telecommunications systems, in addition to the methods recommended in Recommendation ITU-R PN.618. ANNEX 1 1. Introduction Propagation effects in the land mobile-satellite service (LMSS) differ from those of the fixed-satellite service
5、(FSS) primarily because of the greater importance of terrain effects. In the FSS it is generally possible to discriminate against multipath, shadowing and blockage through the use of highly directive antennas placed at unobstructed sites. Therefore, in general, the LMSS offers smaller link availabil
6、ity percentages than the FSS. The prime availability range of interest to system designers is usually from 80% to 99%. This Annex deals with data and models specifically needed for predicting propagation impairments in LMSS links, which include tropospheric effects, ionospheric effects, multipath, b
7、lockage and shadowing. It is based on measurements at 1.5 GHz (L-band) and 870 MHz in the UHF band. 2. Tropospheric effects 2. I Attenuation Signal losses in the troposphere are caused by atmospheric gases, rain, fog and clouds. Except at low elevation angles, tropospheric attenuation is negligible
8、at frequencies below about 1 GHz, and is generally small at frequencies up to about 10 GHz. Above 10 GHz, the attenuation can be large for significant percentages of the time on many paths. Prediction methods are available for estimating gaseous absorption (Recommendation IT-R PN.676) and rain atten
9、uation (Recommendation ITU-R PN.618). Fog and cloud attenuation is usually negligible for frequencies up to 1 O GHz. * This Recommendation should be brought to the attention of Radiocommunication Study Group 8. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Infor
10、mation Handling Services ITU-R RECMN*PN 682-2 94 4855232 052L5l19 512 m . Percentage ) a) 20 24.45 10 26.84 5 29.22 2 32.38 1 34.76 Rec. ITU-R PN.681-1 359 ) Y) -0.7351 5.991 x -0.6775 4.605 x -0.6000 3.219 x lo3 -0.5 106 1.386 x -0.4430 0.0 2.2 Scintilhtion Irregular variations in received signal l
11、evel and in angle of arrivai are caused by both tropospheric turbulence and atmospheric rnultipath. The magnitudes of these effects increase with increasing frequency and decreasing path elevation angle, except that angle-of-arrival fluctuations caused by turbulence are independent of frequency. Ant
12、enna bearnwidth also affects the magnitude of these scintillations. These effects are observed to be at a maximum in the summer season. A prediction method is given in Recommendation IT-R PN.618. 3. Ionospheric effects Ionospheric effects on Earth-to-space paths are addressed in Recommendation IT-R
13、PI.531. Values of ionospheric effects for frequencies in the range of 0.1 to 10 GHz are given in Tables 1 and 2 of Recommen- dation ITU-R PN.680. 4. Shadowing 4.1 Empirical roadside shadowing model Cumulative 1.5 GHz fade distribution measurements have given rise to the empirical roadside shadowing
14、model. The extent of trees along the roadside is represented by the percentage of optical shadowing caused by roadside trees at a path elevation angle of 45“ in the direction of the signal source. The model is valid when this percentage is in the range 55-75%. for 200 I e I 600 1% I p 5 20% where: A
15、: p: 8: fade exceeded in dB with respect to free space propagation percentage of the distance travelled over which the fade is exceeded path elevation angle to the satellite. The parameters, ol(p), (p), and y(p) are tabulated in Table 1. TABLE 1 Values a), ), and y) of the empirical roadside model C
16、OPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services360 Rec. ITU-R PN.681-1 The empirical roadside model corresponds to an average propagation condition with the vehicle driving on lanes both sides of the highway (lanes close to and far fro
17、m roadside trees are included). The model applies to highways and rural roads where the overall aspect of the propagation path is, for the most part, orthogonal to the lines of roadside trees and utility poles and it is assumed that the dominant cause of LMSS signal attenuation is canopy shadowing (
18、see Recommendation ITU-R PN.833). Figure 1 shows plots of fade exceeded versus the path elevation angle for several constant percentages, p. FIGURE 1 Fading at 1.5 GHz due to roadside shadowing versus path elevation angle 30 25 10 5 O Curves A: p=20% B: p = 10% cp= 5% Dp= 2% E:p= 1% Since the measur
19、ements that resulted in the development of the model did not discininate against multipath, it implicitly includes the effect of multipath by roadside trees. The receiving antenna employed in the measurements has an azimuthally omnidirectional radiation pattern with a peak gain of 4 dB. Fade measure
20、ments in the United Kingdom for a mixture of trees and buildings have shown that the model is also valid for mixed environments and that the model may be extended to elevation angles up to 80. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Se
21、rvices ITU-R RECMN*PN b8L-1 94 m 4855232 0523523 370 m Rec. ITU-R PN.681-1 361 4.2 Attenuation frequency scaling model Mobile fade measurements of roadside tree shadowing have shown that the ratio of fades at equal probability values is approximately consistent with the ratio of the square root of f
22、requencies,fi and f2: for 0.8 GHz I fi andf2 I 2.7 GHz 1% I p 530% - where: A: fade exceeded with respect to free space propagation (dB) 8: elevation angle. 4.3 Fade duration distribution model Optimal design of land mobile satellite receivers depends on knowledge of the statistics associated with f
23、ade durations, which can be represented in units of travelled distance, m, or in seconds. Fade duration measurements have given rise to the following empirical model which is valid for distance fade duration dd 2 0.02 m. where P(FD dd I A A,) represents the probability that the distance fade duratio
24、n, FD, exceeds the distance, dd m, under the condition that the attenuation, A, exceeds A,. The designation “eff represents the error function, d is the standard deviation of ln(dd), and in(a) is the mean value of In(dd). The left-hand side of equation (3) was estimated by computing the percentage n
25、umber of “duration events” that exceed dd relative to the total number of events for which A Aq in data obtained from measurements in the United States of America and Australia. The best fit regression values obtained from these measurements are a = 0.22 and o = 1.215. Figure 2 contains a plot of p,
26、 expressed as a percentage, versus dd for a 5 dB threshold. 4.4 Non-fade duration distribution model A “non-fade duration” event of distance duration, dd, is defined as the distance over which the fade levels are smaller than a specified fade threshold. The non-fade duration model is given by: where
27、 P(NFD dd I A c As) is the percentage probability that a continuous non-fade distance, NFD, exceeds the distance, dd, given that the fade is smaller than the threshold, A,. Table 2 contains the values of and y for roads that exhibit “moderate and extreme” shadowing as defined in 0 4.1. A 5 dB fade t
28、hreshold is used. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services362 Shadowing level ITU-R RECMN*PN 681-1 94 4855212 0521522 O07 Rec. ITU-R PN.681-1 .Y FIGURE 2 Best fit cumulative fade distribution for roadside tree shadowing with a
29、5 dB threshold Moderate Extreme TABLE 2 20.54 0.58 11.71 0.8371 Non-fade duration regression values for a 5 dB fade threshold at a path elevation angle of 51“ COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesITU-R RECMNUPN 683-3 94 4855
30、232 0523523 T43 Elevation = 30“ Rec. ITU-R PN.681-1 363 Elevation = 45“ 5. Multipath models for clear line-of-sight conditions 1.5 In many cases the mobile terminal has a clear line-of-sight (negligible shadowing) to the mobile satellite. Degradation to the signal can still occur under these circums
31、tances, due to terrain-induced multipath. The mobile terminal receives a phasor summation of the direct line-of-sight signal and several multipath signals. These multipath signals may add constructively or destructively to result in signal enhancement or fade. The multipath signal characteristics de
32、pend on the scattering cross-sections of the multipath reflectors, their number, the distances to the receiving antenna, the field polarizations, and receiving antenna gain pattern. 33.19 1.710 39.95 2.321 The multipath degradation models introduced in the following sections are based on measurement
33、s made using an antenna with the following characteristics: - omnidirectional in azimuth; - gain variation between 15 and 75 elevation less than 3 dB; - below the horizon (negative elevation angles) the antenna gain was reduced by at least 10 dB. 5.1 Multipath in a mountain environme$ The distributi
34、on of fade depths due to multipath in mountainous terrain is modelled by: p = aA-b for: 1% p 10% where: p: A: fade exceeded (dB). percentage of distance over which the fade is exceeded, and The curve fit parameters, a and b, are shown in Table 3 for 1.5 GHz and 870 MHz. Note that the above model is
35、valid when the effect of shadowing is negligible. TABLE 3 Parameters for best fit cumulative fade distribution for multipath in mountainous terrain 0.870 I 34.52 I 1.855 1 31.64 1. 2.464 I Figure 3 contains curves of the cumulative fade distributions for path elevation angles of 30“ and 45 at 1.5 GH
36、z and 870 MHz. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services ITU-R RECMN+PN 681-1 94 m 4855232 0521524 98T = 364 Rec. ITU-R PN.681-1 FIGURE 3 Best fit cumulative fade distributions for multipath fading in mountainous terrain o1 2 3
37、4 5 6 7 8 9 10 Fade depth (dB) Curves A: 870 MHz, 45“ B: lSGHz, 45“ C: 870 MHz, 30“ D: 1.5 GHz, 30“ 5.2 Multipath in a roadside tree environment Experiments conducted along tree-lined roads in the United States of America have shown that multipath fading is relatively insensitive to path elevation o
38、ver the range of 3Oo-6O0. The measured data have given rise to the following model: p = uexp(-VA) (6) for 1% c p 50% where: p: A: fade exceeded (dB). percentage of distance over which the fade is exceeded and Note that the above model assumes negligible shadowing. The curve fit parameters, u and Y,
39、are shown in Table 4. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesRec. ITU-R PN.681-1 Frequency GHz) 0.870 TABLE 4 Parameters for best exponential fit cumulative fade distributions for multipath for tree-lined roads U V Fade range (dB) 125.6 1.1 16 1-4.5 l l i l I 127.7 I 0.8573 I 1-6 365 O 1 23 45678910 Fade depth (dB) Curves A 870 MHz B: 1.5 GHz wi COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
