1、 Recommendation ITU-R P.681-8 (07/2015) Propagation data required for the design of Earth-space land mobile telecommunication systems P Series Radiowave propagation ii Rec. ITU-R P.681-8 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical
2、 use of the radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World
3、 and Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for
4、 the submission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series o
5、f ITU-R Recommendations (Also available online at http:/www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodeterminat
6、ion, amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management SNG Satellit
7、e news gathering TF Time signals and frequency standards emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2015 ITU 2015 All rights reserved. No part of this publicati
8、on may be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R P.681-8 1 RECOMMENDATION ITU-R P.681-8* Propagation data required for the design of Earth-space land mobile telecommunication systems (Question ITU-R 207/3) (1990-1994-1995-1997-1999-2001-2003-2009-2015) Sco
9、pe This Recommendation predicts the various propagation parameters needed in planning the Earth-space land mobile-satellite service (LMSS). The ITU Radiocommunication Assembly, considering a) that for the proper planning of Earth-space land mobile systems it is necessary to have appropriate propagat
10、ion data and prediction methods; b) that the methods of Recommendation ITU-R P.618 are recommended for the planning of Earth-space telecommunication systems; c) that further development of prediction methods for specific application to land mobile-satellite systems is required to give adequate accur
11、acy in all regions of the world and for all operational conditions; d) that, however, methods are available which yield sufficient accuracy for many applications, recommends 1 that the methods contained in Annex 1 be adopted for use in the planning of Earth-space land mobile telecommunication system
12、s, in addition to the methods recommended in Recommendation ITU-R P.618. Annex 1 1 Introduction Propagation effects in the land mobile-satellite service (LMSS) differ from those of the fixed-satellite service (FSS) primarily because of the greater importance of terrain effects. In the FSS it is gene
13、rally 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 availability percentages than the FSS. The prime availability range of interest to system designers
14、is usually from 80% to 99%. * This Recommendation should be brought to the attention of Radiocommunication Study Group 5. 2 Rec. ITU-R P.681-8 This Annex deals with data and models specifically needed for predicting propagation impairments in LMSS links, which include tropospheric effects, ionospher
15、ic effects, multipath, blockage and shadowing. It is based on measurements ranging from 870 MHz in the UHF band up to 20 GHz. 2 Tropospheric effects 2.1 Attenuation Signal losses in the troposphere are caused by atmospheric gases, rain, fog and clouds. Except at low elevation angles, tropospheric at
16、tenuation is negligible 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 ITU
17、-R P.676) and rain attenuation (Recommendation ITU-R P.618). Fog and cloud attenuation is usually negligible for frequencies up to 10 GHz. 2.2 Scintillation Irregular variations in received signal level and in angle of arrival are caused by both tropospheric turbulence and atmospheric multipath. The
18、 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. Antenna beamwidth also affects the magnitude of these scintillations. These effects are observed to be at
19、a maximum in the summer season. A prediction method is given in Recommendation ITU-R P.618. 3 Ionospheric effects Ionospheric effects on Earth-to-space paths are addressed in Recommendation ITU-R P.531. Values of ionospheric effects for frequencies in the range of 0.1 to 10 GHz are given in Tables 1
20、 and 2 of Recommendation ITU-R P.680. 4 Shadowing 4.1 Roadside tree-shadowing model Cumulative fade distribution measurements at 870 MHz, 1.6 GHz and 20 GHz have been used to develop the extended empirical roadside shadowing model. The extent of trees along the roadside is represented by the percent
21、age 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 of 55% to 75%. 4.1.1 Calculation of fading due to shadowing by roadside trees The following procedure provides estimates of
22、 roadside shadowing for frequencies between 800 MHz and 20 GHz, path elevation angles from 7 up to 60, and percentages of distance travelled from 1% to 80%. The empirical model corresponds to an average propagation condition with the vehicle driving in lanes on both sides of the roadway (lanes close
23、 to and far from the roadside trees are included). The predicted fade distributions apply for 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
24、 signal fading is tree canopy shadowing (see Recommendation ITU-R P.833). Rec. ITU-R P.681-8 3 Parameters required are the following: f : frequency (GHz) : path elevation angle to the satellite (degrees) p : percentage of distance travelled over which fade is exceeded. Step 1: Calculate the fade dis
25、tribution at 1.5 GHz, valid for percentages of distance travelled of 20% p 1%, at the desired path elevation angle, 60 20: AL( p,) M() ln ( p) N() (1) where: M() 3.44 0.0975 0.002 2 (2) N() 0.443 34.76 (3) Step 2: Convert the fade distribution at 1.5 GHz, valid for 20% p 1%, to the desired frequency
26、, f (GHz), where 0.8 GHz f 20 GHz: ffpAfpA L 115.1e x p),(),( 5.120(4) Step 3: Calculate the fade distribution for percentages of distance travelled 80% p 20% for the frequency range 0.85 GHz f 20 GHz as: pfAfpA 80ln4ln 1),%,20(),( 20for 80% p 20% (5) ),(20 fpA for 20% p 1% Step 4: For path elevatio
27、n angles in the range 20 7, the fade distribution is assumed to have the same value as at 20. Figure 1 shows fades exceeded at 1.5 GHz versus elevation angles between 10 and 60 for a family of equal percentages between 1% and 50%. 4.1.1.1 Extension to elevation angles 60 The roadside shadowing model
28、 at frequencies of 1.6 GHz and 2.6 GHz can be extended to elevation angles above 60 with the following procedure: apply equations (1) to (5) at an elevation angle of 60 at the above frequencies; linearly interpolate between the value calculated for an angle of 60 and the fade values for an elevation
29、 angle of 80 provided in Table 1; linearly interpolate between the values in Table 1 and a value of zero at 90. 4 Rec. ITU-R P.681-8 FIGURE 1 Fading at 1.5 GHz due to roadside shadowing versus path elevation angle P . 0 6 8 1 - 0 1046810121416182022242628302Fadeexceeded(dB)10 15 20 25 30 35 40 45 50
30、 55 601%2%5%10%20%30%50%Pat h el ev at i o n an g l e (d eg rees )TABLE 1 Fades exceeded (dB) at 80 elevation p (%) Tree-shadowed 1.6 GHz 2.6 GHz 1 4.1 9.0 5 2.0 5.2 10 1.5 3.8 15 1.4 3.2 20 1.3 2.8 30 1.2 2.5 4.1.1.2 Application of roadside shadowing model to non-geostationary (non-GSO) and mobile-
31、satellite systems The prediction method above was derived for, and is applied to, LMSS geometries where the elevation angle remains constant. For non-GSO systems, where the elevation angle is varying, the link availability can be calculated in the following way: a) calculate the percentage of time f
32、or each elevation angle (or elevation angle range) under which the terminal will see the spacecraft; b) for a given propagation margin (ordinate of Fig. 1), find the percentage of unavailability for each elevation angle; c) for each elevation angle, multiply the results of step a) and b) and divide
33、by 100, giving the percentage of unavailability of the system at this elevation; Rec. ITU-R P.681-8 5 d) add up all unavailability values obtained in step c) to arrive at the total system unavailability. If the antenna used at the mobile terminal does not have an isotropic pattern, the antenna gain
34、at each elevation angle has to be subtracted from the fade margin in step b) above. In the case of multi-visibility satellite constellations employing satellite path diversity (i.e. switching to the least impaired path), an approximate calculation can be made assuming that the spacecraft with the hi
35、ghest elevation angle is being used. 4.1.2 Fade duration distribution model Optimal design of LMSS receivers depends on knowledge of the statistics associated with fade durations, which can be represented in units of travelled distance (m) or (s). Fade duration measurements have given rise to the fo
36、llowing empirical model which is valid for distance fade duration dd 0.02 m. 2 )l n ()l n (e r f121)|( ddAAddFDP q(6) where )|( qAAddFDP represents the probability that the distance fade duration, FD, exceeds the distance, dd (m), under the condition that the attenuation, A, exceeds Aq. The designat
37、ion “erf” represents the error function, is the standard deviation of ln(dd), and ln() is the mean value of ln(dd). The left-hand side of equation (6) was estimated by computing the percentage number of “duration events” that exceed dd relative to the total number of events for which A Aq in data ob
38、tained from measurements in the United States of America and Australia. The best fit regression values obtained from these measurements are 0.22 and 1.215. Figure 2 contains a plot of P, expressed as a percentage, p, versus dd for a 5 dB threshold. The model given by equation (6) is based on measure
39、ments at an elevation angle of 51 and is applicable for moderate to severe shadowing (percentage of optical shadowing between 55% and 90%). Tests at 30 and 60 have demonstrated a moderate dependence on elevation angle: the smaller the elevation angle, the larger is the fade duration for a fixed perc
40、entage. For example, the 30 fade duration showed approximately twice that for the 60 fade duration at the same percentage level. 4.1.3 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 spe
41、cified fade threshold. The non-fade duration model is given by: )()|( ddAAddN F Dp q (7) where )|( qAAddN F Dp is the percentage probability that a continuous non-fade distance, NFD, exceeds the distance, dd, given that the fade is smaller than the threshold, Aq. Table 2 contains the values of and f
42、or roads that exhibit moderate and extreme shadowing i.e. the percentage of optical shadowing of between 55% and 75% and between 75% and 90% respectively. A 5 dB fade threshold is used for Aq. 6 Rec. ITU-R P.681-8 FIGURE 2 Best fit cumulative fade distribution for roadside tree shadowing with a 5 dB
43、 threshold P. 0 6 8 1 - 0 212525102102 5 2 5 2 511010 110 2PercentageoffadedurationabscissaFad e d u rat i o n (m)TABLE 2 Non-fade duration regression values for a 5 dB fade threshold at a path elevation angle of 51 Shadowing level Moderate 20.54 0.58 Extreme 11.71 0.8371 4.2 Roadside building-shado
44、wing model Shadowing by roadside buildings in an urban area can be modelled by assuming a Rayleigh distribution of building heights. Figure 3 shows the geometry. Rec. ITU-R P.681-8 7 FIGURE 3 Geometry of roadside building shadowing model P. 0 6 8 1 - 0 3dmMo b i l eh ei g h t , h mS lo pe di s ta nc
45、 et oF re s ne l c l ea r an ce po i nt , d rBu i l d i n gh ei g h t , hbH ei g h t o f ra yab o v e g ro u n dat fr o n t o fb u i l d i n g s , h1D i re ct i o n o f ro adE l ev at i o n , A zi mu t h , The percentage probability of blockage due to the buildings is given by: 212221 f o r2)(e x p1
46、00 / hhhhhp b (8) where: h1 : height of the ray above ground at the building frontage, given by: )s int a n( /1 mm dhh (8a) h2 : Fresnel clearance distance required above buildings, given by: 5.02 )( rf dCh (8b) hb : the most common (modal) building height hm : height of mobile above ground : elevat
47、ion angle of the ray to the satellite above horizontal : azimuth angle of the ray relative to street direction dm : distance of the mobile from the front of the buildings dr : slope distance from the mobile to the position along the ray vertically above building front, given by: )co ss in(/ mr dd (8
48、c) Cf : required clearance as a fraction of the first Fresnel zone : wavelength and where h1, h2, hb, hm, dm, dr and are in self-consistent units, and h1 h2. Note that equations (8a), (8b) and (8c) are valid for 0 90 and for 0 180. The actual limiting values should not be used. Figure 4 shows exampl
49、es of roadside building shadowing computed using the above expressions for: hb 15 m hm 1.5 m dm 17.5 m 8 Rec. ITU-R P.681-8 Frequency 1.6 GHz. FIGURE 4 Examples of roadside building shadowing (see text for parameter values) P. 0 6 8 1 - 0 40 10 20 30 40 50 60 70 80020406080100A zi mu t h = 9 0 A zi mu t h = 4 5 E l ev at i o n
