1、 Recommendation ITU-R F.1096-1(04/2011)Methods of calculating line-of-sight interference into fixed wireless systemsto account for terrain scatteringF SeriesFixed serviceii Rec. ITU-R F.1096-1 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and econ
2、omical 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
3、 World 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 us
4、ed for 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. Se
5、ries of 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, radiodete
6、rmination, 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 Sa
7、tellite 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, 2011 ITU 2011 All rights reserved. No part of this pub
8、lication may be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R F.1096-1 1 RECOMMENDATION ITU-R F.1096-1 Methods of calculating line-of-sight interference into fixed wireless systems to account for terrain scattering1(1994-2011) Scope This Recommendation is intende
9、d for the calculation of a terrestrial surface impact on formation of interference signal to the fixed wireless systems. The ITU Radiocommunication Assembly, considering a) that interference from other fixed wireless systems and other services can affect the performance of a line-of-sight fixed wire
10、less system; b) that the signal power from the transmitting antenna in one system may propagate as interference to the receiving antenna of another system by a line-of-sight (LoS) great-circle path; c) that the signal power from the transmitting antenna in one system may propagate as interference to
11、 the receiving antenna of another system by the mechanism of scattering from natural or man-made features on the surface of the Earth; d) that terrain regions that produce the coupling of this interference may not be close to the great-circle path, but must be visible to both the interfering transmi
12、t antenna and the receive antenna of the interfered system; e) that the component of interference power that results from terrain scattering can significantly exceed the interference power that arrives by the great-circle path between the antennas; f) that efficient techniques have been developed fo
13、r calculating the power of the interference scattered from terrain, recommends 1 that the effects of terrain scatter, when relevant, should be included in calculations of interference power when the interference is due to signals from the transmitting antenna of one system into the receiving antenna
14、 of another and when either or both of the following conditions apply (see Note 1): 1.1 there is a LoS propagation path between the transmitting antenna of the interfering system and the receiving antenna of the interfered system; 1.2 there are natural, or man-made, features on the surface of the Ea
15、rth that are visible from both the interfering transmit antenna and the interfered receive antenna; 2 that the methods given in Annex 1 be used to calculate the contribution of interference due to terrain scatter. 1Recommendation ITU-R P.452 Prediction procedure for the evaluation of interference be
16、tween stations on the surface of the Earth at frequencies above about 0.1 GHz, in Radiocommunication Study Group 3 addresses other propagation mechanisms. 2 Rec. ITU-R F.1096-1 NOTE 1 Specular reflections or propagation by diffraction are not treated by the calculation methods described in this Reco
17、mmendation. Annex 1 Interference to fixed wireless systems caused by terrain scattering 1 Introduction Terrain scattering has been found to be a particularly strong mechanism for coupling interference between radio-relay systems in cases where two paths cross each other and the terrain at the inters
18、ection is visible from both the transmitting antenna of one hop and the receiving antenna of the other. In this case, the main lobes of the two antennas couple through a common area of terrain and the interference is like ground clutter received in a bi-static radar system. In the past, interference
19、 between fixed wireless systems was determined by calculations based on the mechanism of near great-circle propagation which includes LoS paths, the effects of atmospheric refraction, diffraction by the surface of the Earth and tropospheric forward scatter. Such great-circle techniques combined with
20、 antenna side-lobe coupling have also been used for many years by some administrations to determine intra- and inter-system interferences in terrestrial fixed wireless systems. Field measurements in recent years have indicated that great-circle propagation is often a minor contributor compared to te
21、rrain scattering. In contrast to the case of inter-system interference, the discrepancies between great-circle predictions and measurements become smaller for intra-system interference, where the major cases have usually been between two adjacent hops on the same route. In the case of interference b
22、etween an earth station and a terrestrial station, coupling may also occur through the main beam of the terrestrial antenna, the area around the earth station, and the side lobes of the earth station antenna. Depending on geometric circumstances, either ground scattering or great-circle coupling may
23、 be the predominant interference mechanism. 2 Mathematical model of terrain scatter The interference power, Pr, received through the mechanism of ground scatter from a transmitting antenna radiating power, Pt, may be determined from the bi-static radar equation: ()2trrt e322trGGP=P dA4RR, (1) where
24、t and r denote transmitter and receiver respectively, G(,) is the respective antenna power gains in the direction of the scatterers in the elemental scattering area d Ae, represents the azimuth and the elevation from the centerline of the antenna (see Fig. 1), Rtand Rrare the respective slant distan
25、ces from the antennas to the scattering element, is the wave length, and is the modified scattering coefficient describing the incoherent energy scattered by the elemental area. The elemental area d Aeis defined to be the minimum of the areas of the scatterer normal to the slant vectors from the tra
26、nsmitter and the receiver. Rec. ITU-R F.1096-1 3 Equation (1) assumes that the scattered fields from different areas or objects are incoherent, and the region contains all elemental regions that contribute to the received scattered energy. In evaluating the integral for a portion of the surface of t
27、he Earth, it is necessary to account for the shadowing of individual elemental areas. Only unshadowed areas, which are visible to both the transmitting and receiving antennas, will contribute. Comparison of measured and calculated interference values has shown that the modified scattering coefficien
28、t can be assumed to be constant over fairly large areas of terrain. Characteristic values of , which were determined by one administration for several different land covers, are given in Table 1. FIGURE 1 Scattering from terrain Portion of terrainTxRtRrRxPortion of elemental scatterervisible from bo
29、th T and RxxTABLE 1 Scatterer type (dB) Land cover types (from United States of America database) urban residential commercial and services deciduous forest mixed forest 8 7 16 20 Man-made structures from Federal Aviation Administration database 10.4 The integral for determining the interference pow
30、er, (1), may be expressed as a finite summation: () ()() ()tirirt ie,i22tiriGpGpP=C ARpRp(2) 4 Rec. ITU-R F.1096-1 23(4 )ttCP=(3) where: i: coefficient of the i-th cell pi: cell midpoint Ae,i: effective area of the i-th cell contained in the universe of cells, . Interference measurements made at a f
31、ixed frequency typically show time fluctuations around the mean value computed by equation (2). This is due to the movement of scattering objects like trees and vegetation or to temporal variations in atmospheric conditions, which may induce amplitude and phase variations between scatter returns fro
32、m different scattering areas. Similarly, fluctuations about the same mean value are observed when the carrier frequency in the experiment is changed. Fortunately, for the important case of interference between digital radio systems, only the mean value given by equation (2) is of importance. On the
33、other hand, if the strong carrier of a low-index FM signal is the source of interference then fluctuations of this carrier above the mean (up fades) have to be taken into account. 2.1 Application of the model Practical considerations of computer resources impose a limit on the physical size of the p
34、ortion of the region over which the integral will be evaluated. One approach is to take the region of integration as a quadrilateral region on the surface of the Earth defined by the intersection of an azimuthal sector centered on the transmit interfering antenna with an azimuthal sector centered on
35、 the receiving interfered antenna. Figure 2 shows such a region for the case where the path from station 1 to station 4 intersects with that from station 3 to station 2, resulting in terrain scatter interference from station 1 into station 2. The sectors would be centered on the main beam azimuth of
36、 the respective antennas and could include the azimuths where the directive gain in the azimuthal direction is no more than, say, 30 dB below the maximum gain. Evaluation of the interference integral over such a region is described in 3.1. Bounding techniques offer a less arbitrary, more accurate, a
37、nd more efficient means of evaluating the scatter interference power. Since most of the received energy, which has been scattered by the terrain, is usually contributed by scattering from the regions close to the intersection of the main beams of the interfering and interfered antennas, an accurate
38、integration is only required in the vicinity of this intersection. The contribution from the remainder of can be determined from an upper bound. The application of bounding techniques is described in 3.2. An elemental terrain area can have a non-zero contribution to the interference integral only if
39、 it is not shadowed. That is, it must be visible to both the transmitting and receiving antennas. In evaluating the shadowing of an elemental area it is necessary to consider both macro- and micro-shadowing. In macro-shadowing, an element is not visible because of obstruction by higher terrain that
40、is closer to one of the antennas; in micro-shadowing, the element presents no effective area, Ae,ito one of the antennas because of its orientation. The conditions necessary to determine whether an elemental region is macro- or micro-shadowed are described in 4.1 and 4.2, respectively. Rec. ITU-R F.
41、1096-1 5 FIGURE 2 Interference geometry 3214RL XT 22 11In the following developments, it is assumed that reliable digital elevation maps are available for the terrain of integration. These data take the form of elevations for a set of points defined on geodetic coordinates of latitude and longitude.
42、 While high resolution map data based on 3 arc-second intervals are available and can be used, adequate accuracy can be obtained using 15 arc-second data. In computations, the antenna patterns are often based on measurements that are stored in computer look-up tables, or they can be analytic express
43、ions of measured patterns. For simplicity, the readily available azimuth pattern may be rotated about the boresight axis. 3 Integration procedures 3.1 Direct evaluation The integration over a selected region S0can be evaluated as: () ()() ()i0tirir,o t i e,i22pStiriGpGpP=C ARpRp(4) 6 Rec. ITU-R F.10
44、96-1 where points pibelong to regions Ae,iwhich constitute a regular partition of the region S0: ifor0e,i e,ie,jSA AA ij= (5) Although the integrands are defined on a rectangular grid, it is convenient to use triads of points to define planar elemental regions. These determine triangular regions tha
45、t are used to develop Ae,ithe minimal area that is visible by the transmitter and receiver: Ae,i= minAt,i, Ar,i (6) If a region is shadowed, the corresponding term is excluded from integration. The elemental area Ae,iis determined by the visible part of the projection of the surface Sionto the plane
46、 which is perpendicular to the ray direction (see Fig. 3): Ae,i= minBt,i, Br,i (7) FIGURE 3 Elemental scatterer BedSLandcoverAntennaThe maximal visible elemental area can be expressed as: abcbezxvzyuvuhyxhB+=+=),()2sin()2cos()cos,(sin5.0(8) Here zbaand zbcare the elevation increments of the points o
47、f a grid triangle with respect to the elevation of the right angle point (see Fig. 4); x and y are the grid cell dimensions, and are the elevation and azimuth of the triangle mid-point; is the angle between the chord and the tangent of the scattered ray (see Fig. 5). Note that Ae,i is the projected
48、area onto the unit sphere of the terrain area Siwhich does not depend on propagation conditions. Rec. ITU-R F.1096-1 7 FIGURE 4 Scatterer effective area acCABSzyxbr0Flat earthFIGURE 5 Elevation angle OAntennaRmvHorizontal8 Rec. ITU-R F.1096-1 This equation does not require the calculation of any ang
49、les since it is possible to explicitly calculate all the trigonometric functions in (8): () ()122122;z2 1 ; 42d21421/ 2 1/ 222 22/2ee/2eexysin , cosx+y x+ydsin( )RRzcos( ) ,RR= =+ = + = (9) where = (x2+ y2+ z2)is the distance between the antenna and scatterer, d = (x2+ y2)is its projection onto the horizontal plane, Re= a k, a is the radius of the Earth, k is the Earth radius factor that depends on the refractivity gradient in t
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