1、CCIR RECMN*794 92 4855232 0520039 010 150 Rec. 794 RECOMMENDATION 794 TECHNIQUES FOR MINIMIZING THE IMPACT ON THE OkTR4LL BSS SYSTEh PERFORMANCE DUE TO RAIN ALONG THE FEEDER-LCVK PATH* (Questions 86/11 and 98/11) 1992) The CCIR, co mide ring a) that any degradation on the feeder links will impact on
2、 the broadcasting-satellite service (BSSI performance over the whole service area; b) that the BSS satellite transponder should be maintained at nominal output level to minimize the impact of feeder-link fading on reducing the down-link e.i.r.p.; c that while excessive use of on-board automatic gain
3、 control (AGC) can result in increasing the inter- and intra-system interference it can maintain the transponder operation at or near nominal output level noting hon.ever that the feeder link carrier-to-noise will not be improved; d) that while feeder iink power control can potentially increase inte
4、r-system interference its application can maintain transponder operation at or near nominal output level without degrading the feeder link UA; e reducing feeder iink fading, particularly in high rain rate areas, that while use of site diversity can lead to complex operational considerations, it is a
5、n effective technique for recommends that the following techniques be used to the extent practicable for alleviating BSS system degradation due to rain fading on the feeder link: - - - on-board satellite AGC (see Annex 1); up-link power control (see Annex 2); site diversity (see Annex 3). ANNEX 1 On
6、-board automatic gain control (AGC) AGC on board spacecraft minimizes the effect of rain fades at the feeder-link station on the dcnvn-iink C/N ratio by keeping the TWTA at saturation. The AGC operates on individual channels and increases the transponder gain of the wanted signal and of any portion
7、of an interfering signal which falls within the filter bandwidth of the wanted channel. Therefore, during rain at the feeder-link station(s) the use of AGC permits the operation of the transponder close to saturation but the ratio of the wanted carrier eo the portion of the interfering adjacent cros
8、s-polarized carrier which falls into the filter bandwidth of the wanted channel remains constant. Therefore, the use of AGC has no effect on the cross-polar C/Z, of the two feeder links under consideration. * This Recommendation should be brought to the attention of Study Group 4. Note from the Dire
9、cor, CCIR - Report 952-2 (9 5.4.1, 5.4.2 and 7) was used in preparing this Recommendation * CCIR RECMN%794 92 m 4855232 0520020 832 m Rec. 794 151 However, the satellite using AGC radiates on the down link a constant level of the wanted signal which is attenuated on the feeder link, but re-radiates
10、on the down link a higher level of the interfering cross-polar signal on the adjacent channel, which is not attenuated when there is no rain on the interfering feeder link. This situation may cause an increase in down-link interference to other systems receiving this re-radiation as Co-channel inter
11、ference. This problem could be significant only for Co-located satellites serving common or adjacent service areas. A limit on the range of AGC, in Co-located satellites with cross-polarized channels, to less than 10 to 15 dB may beneeded to guard against this problem of re-radiation on the down lin
12、k. This problem can be reduced if satellites with cross-polarized channels serving the same service area or adjacent service areas are separated by at least 0.3“ on the geostationary orbit. Non-Co-located satellites with cross-poiatized channels need not be subject to this limit of AGC range. A 10 d
13、B limit on AGC range could be insufficient to maintain a constant TWTA output in some rain climates for certain elevation angles. The use of some other mechanism (power control, site diversity) might be required in these circumstances to maintain a constant signai level on the down link. The Region
14、2 feeder-link Plan is based on a limit of 15 dB on the dynamic range of AGC on board some cross-polarized spacecraft to guard against this problem of re-radiation on the down link. ANNEX 2 Feeder link power control Power control of feeder links is the rapid, automatic adjustment of earth-station tra
15、nsmitter power to compensate for rain-induced attenuation in the path of the desired signai to a satellite. 1. Application of power contrd In the presence of feeder-link power control (PC), the input level of the signai at the satellite transponder is maintained approximately constant and rain atten
16、uation along the feeder-link path is effectively compensated. As a consequence, during rain at the feeder-link station only, the use of feeder-link power control maintains a constant value of CINT as illustrated in Fig. 1. Experiments using the BSE of Japan have shown that power control is effective
17、 in maintaining a nearly constant level of desired carrier during periods of rain. In this experiment, at 14 GHz a variation of power received at the satellite of 6 dB (peak-to-peak) and 1.5 dB r.m.s. without power control, was reduced through the use of power control to 1.5 dB (peak-to-peak) and 0.
18、5 dB r.m.s., respectively. 2. Conditions for use of power control without increased interference I Use of power control to increase the availability of feeder links beyond the values used for planning is 1 andysed in this section. In studying feeder-link interference problems, the geographical locat
19、ions of interfering earth stations and wanted feeder-link beam areas are important factors affecting the feeder-link carrier-to-interference ratio. These factors affect the cross-polarization discrimination (XPZsat) of the wanted-satellite antenna because XPlsar is a function of the ratio of the off
20、-axis angle p to the half-power beamwidth cpo. For the satellite-receiving antenna reference pattern in Fig. 10 of Recommendation 652, the XPZSal can be graphically expressed as in Fig. 2. CCIR RECMN*774 72 W 4B55212 052i21 779 D 152 Rec. 794 FIGURE 1 The effect oPrainfall atenuatlen on ClNr in the
21、presence OP feeder-link power control (PC) 16 14 12 10 h ln 2. k0 i5 6 4 2 O at 17 GHz o24 6 8 10 12 at 12GHt Rainfail attenuation (dB) Clear-sky C/NU - 24 dB Clemsky CINd - 14.5 dB Curves A: rain at feeder-link station only B: correlated rain at feeder-link station and down-link station In order to
22、 analyse the effect of the XPZsa, on Mu, the equation of CIZU which includes the parameter XPlsar explicitly, is given in equation (1). where: P,: Pi : L,: Li : R,: Ri : Grww : Gravi : transmitter power at the wanted earth station transmitter power at the interfering earth station sprcading (?free s
23、pace?) loss on the wanted path spreading (?free space?) loss on the interfering path rain attenuation on the wanted path rain attenuation on the interfering path CO-polar gain of the wanted-mtellite receiving antenna in the direction of the wanted earth stiition CO-polar gain of the w,anted-satellit
24、e receiving antenna in the direction of the interfering earth station CCIR RECMN*794 92 4855232 0520022 605 Rec. 794 A: coefficient of depolarization due to rain as expressed in the following equation: A = 1WD/IO) where: XPD : rain polarization given by: XPD = 30 iogf - 40 log (COS e) - v log A, dB
25、where: V = 20 for below 15 GHz V = 23 for above 15 GHz Ap : Co-polar rain attenuation exceeded for 1 % of the worst month f : frequency (GHz) 8 : elevation angle (degrees). For values of 8 greater than 60, use 8 = 60“ in the above equation. 153 for 5“s 8 5 60“ XPIsat : ratio of Co-polar gain (Grmi)
26、to cross-polar gain (G-i) of the wanted-satellite receiving antenna in the direction of the interfering earth station as expressed in the following equation: XPIsat = Grcwi/G-i PIes : ratio of Co-polar (Grci) to cross-polar (G) of the interfering earth-station transmitting antenna in the direction o
27、f the wanted-satellite as expressed in the following equation: XPIes = GtCilGai FIGURE 2 Cross-polarization discrimination (XPI,) of satellite-receiving antenna (reference patterns are assumed as shown in Fig. 10 of Recommendation 652) 40 30 h 5 20 s 10 O o .1 0.2 0.5 1 2 5 10 20 Relative angle from
28、 beam axis, pipo (va : half-power beamwidth) XPI, (dB) = G,i (dB) - G-. (dB) CCIR RECMN*79LI 92 9 LIB55252 0520023 SYL 154 Rec. 794 Thus, XPZSal nnd XP b) c) it rains at the wanted and interfering sites simultaneously; and it rains at the interfering site only. Use of power control at both sites is
29、also assumed. The results are given in Figs. 5,6 and 7, respectively for scenarios a), b) and c). Although the RARC SAT-83 adopted voltage addition for C/Z calculations, these figures have been drawn on the basis of power addition. The figures indicate that the use of up-link power control increases
30、 the C/Zu when it rains at the wanted site but decreases the C/Iu when it rains at the interfering site. The use of up-link power control has no effect on cross-polar C/lu uhen it rains simultaneously at both the wanted and interfering sites. FIGURE 5 The effect of automatic gain control (ACC), or 1
31、0 dB of power control (PC), on the cross-polar CII, between circularly polarized feeder links at 17.5 GHz when it rains at the wanted site only (scenario a) 32 24 a 4 O Rainfall attenuation at 17.5 GHz (dB) XPIsat = 27 dB XPIes = 30 dB Curves A: with 10 dB power control B: with or without AGC withou
32、t power control 158 CCIR RECMN*794 92 M 4855212 0520027 597 Rec, 794 FIGURE 6 The efPect of automatlc gain control or power control on the cross-polar CIu between clrcularly polarized feeder Ilnks at 17.5 GHz when it ralns slmultaneously at the wanted and the interferlng transmltter sltes (scenarlo
33、b) Rainfall attenuation at 17.5 GMz (dB) FIGURE 7 The effect of power control (PC) on the cross-polar C/Iu between clrcularly polarized feeder links at 17.5 GHz when It rains rit the InterPering slte only (scenarlo c) Rainfall attenuation at 17.5 GHz (dB) XPImt = 27 dB XPl, =30dB Curves A: without p
34、ower control B: with 10 dB power control CCIR RECMN8794 92 4855212 0520028 023 Rec. 794 159 ANNEX 3 Diversity operation of feeder links The technique of site diversity to achieve greater availability of satellite links is well documented. Recommendation 618 indicates that the probability of attenuat
35、ion being exceeded simultaneously at two sites is less than the probability of the same attenuation being exceeded at one of the sites by a factor which decreases with increasing distance between the sites and with increasing attenuation. The relative joint probability is defined as the ratio of the
36、 former probability to the latter probability and is plotted in Fig. 8 for attenuations up to 10 dB and site separation up to 25 km on the basis of a log-normal distribution of rain cells. It is noted that for any given distance between diversity sites, the relative joint probability decreases rapid
37、ly with attenuation and remains almost constant for attenuation greater than about 10 dB. This joint probability data is used to illustrate the effect of site diversity on CINU and cross-polar Cllu. The availability of high values of UNl4 and Cil, during rain is mainly governed by rain attenuation 1
38、11 the case where it rains at the wanted feeder-link site only. This rain scenario is considered the worst case since both the C/NU and C/lu decrease on a dB-per-dB basis during rain. Figure 8 indicates that under these worst-case conditions, the use of site diversity with diversity stations separat
39、ed by at least 10 km would provide at least a factor of 10 improvement in the availability of high values of C/NU and C/llL for attenuation values greater than about 5 dB. In other words, the C/NU andlor the C/lu ratios exceeded for 99% of the worst month without site diversity (assuming an attenuat
40、ion of at least 5 dB) could be made to correspond to an availability exceeded for 99.9% of the worst month by using site diversity with diversity stations separated by at least 10 km. A further improvement in availability by an additional factor of 10 is possible with diversity stations separated by
41、 at least 20 km. Clearly, the use of site diversity is most advantageous where the combination of rain rate and elevation angle gives high values of signal attenuation because the relative joint probability decreases to a minimum with increasing attenuation for any given separation distance between
42、diversity stations. FIGURE 8 Relative joint probability of site diversity as a function of rainfall attenuation and distance between diversity sites Distance between diversity sites (km) 8 10 12 Rainfall attenuation (dB) CCIR RECMN*794 92 M YB55212 0520029 TbT M 160 Rec. 794 The use of site diversit
43、y cm only increase the availability of .high values of C/NU and C/Zu relative to the values calculated in a plan which is b,wd on a single feeder-link station. Therefore, the Region 2 Plan permits the use of site diversity in the implementation of feeder links. Even though the use of site diversity can effectively compensate for the effects of rain and depoluization, the cost and complexity of diversity stcqtions may be significant. The use of diversity for transportable stations is particularly problematic from the standpoint of cost and operational complexity.
