1、CCIR RECMNn795 92 4855232 0520030 781 W Rec. 795 RECOMMENDATiON 795* TECHNIQUES FOR ALLEVIATING MUTUAL INTERFERENCE BETWEEN FEEDER LINKS TO THE BSS (Question 86/1 I) 161 (1992) The CCIR, considering a) that interference on the feeder link will impact on the overail broadcasting-satellite service (BS
2、S) system performance; b) that the number of feeder link stations is limited compared to the number of BSS receiving earth stations; c) that special measures are possible and feasible to alleviate the impact of mutual interference between feeder links; d) that the most critical cases of feeder link
3、interference are for the case of cross-polar channels transmitted to Co-located satellites; e) that for the case of Co-located satellites advantage can be taken of the difference in directivity between the BSS receive station antenna and the feeder link transmit antenna in order to minimize the inte
4、rference between cross- polar channels, recommends that one or more of the following techniques be considered to alleviate mutual interference between feeder the use of a homogeneous set of feeder link technical parameters between feeder links serving BSS satellites that are closely spaced in orbit;
5、 adjustments of the maximum level of e.i.r.p. of potential interfering feeder links or feeder links subject to excessive interference, provided that adequate carrier-to-noise and carrier-to-interference ratios on the adjusted feeder links are maintained; when studies indicate that harmful interferen
6、ce could be experienced between closely spaced satellites the off-axis CO- and cross-polar side-lobe reference patterns of the earth station transmitting antenna should meet the 29 - 25 log 0 (dBi) side-lobe discrimination pattern down to -10 dBi; where insufficient cross-polar isolation is achieved
7、, the off-axis cross-polar side-lobe reference pattern of the earth station transmitting antenna be limited to 24 - 25 log 8 down to -10 dBi for the cross-polar pattern; modifying the satellite receiving antenna beam pattern shape, size andor side-lobe response e.g., a multiple beam or shaped beam a
8、ntenna); off-setting the beam-pointing direction of the satellite receiving antenna subject to maintaining the target carrier-to-noise ratio; setting an upper limit on the feeder link margin allocated to rain attenuation; improving the beam pointing accuracy for the satellite receiving antenna; sepa
9、rating satellite orbital positions by, for example ? 0.2“, from the nominal position (see Annex 1). * This Recommendation should be brought to the attention of Study Group 4. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCCLR RECMN*7
10、95 92 Li855212 0520031 b1i8 M 162 Rec, 795 ANNEX 1 interference between cs-located satellites The most critical cases of feeder-link interference are for cross-polar channels transmitted to co-located satellites. For the case where co-located s?tellites use a common cross-polarized channel, a protec
11、tion ratio of 40 dB is needed. Discrimination of more than about 30 dB from the siitellite receiving antenna pattern requires geographical separation of feeder-link service arens. The discrimination is the difference in eo-polar gain towards points within the wanted service area and the cross-polar
12、gain towards tlie closest point in the interfering service area. Satellite antenna patterns are typically given as functions of cp/w where cp is the exocentrie angle between the on-axis direction and the direction of interest, and cpo is the 3 dB betamwidth of the satellite ,antenna. The discriminat
13、ion between wanted and interfering signals is then the difference between the gain towards the wanted feeder-link station nnd the gain at angle cp. If the maximum discrimination is taken to be the opposite of the on-axis gain, 40 dB discrimination at the edge of service area would require an on-axis
14、 gain of 43 dB and viilues of cplcp greater than 2. Satellite antenna gcains of 43 dB are not consistent with country-wide feeder-link service areas for many countries. Provisions for inhomogeneities in received signais due to rain attenuation and unequal transmit power levels would require even hig
15、her antenna gains. An on-axis gain of 49 dB (8.6 becamwidth) would provide, at best, a 6 dB margin for min attenuation. In considering the case where co-located satellites owrating on croas-poliinzed adjacent channels have common feeder-link service area, it is assumed that the discrimination capabi
16、lities are 25 dB for the satellite receiving antenna and 30 dB for the earth-station transmitting antenna. Since tlie two interferencc components may be in phase, voltage addition must be used to determine tlie interference level. In clear skies, the feeder-link C/l for an adjacent channel would be
17、21.1 dB. When the wanted feeder-link path is subjected to 10 dB rain attenuation, the feeder-link C/l drops to 11.1 dB. The protection ratio of 24 dB implied by the WARC-BS-77 cannot be achieved for this example, even under clear-sky conditions. One possible solution to the problem of adjacent chann
18、el interference ia to provide a slight separation between Co-located satellites. A study performed in Ganada showed tliat i111 improvement in isolation can be obtained in the case of two satellites tr,ansmitting cross-polarized adjacent chiuinels by separating these satellites by a fraction of a deg
19、ree such that they are seen as two distinct orbikd locations by the feeder-link transmitting antennas but as co- located by the smaller receiving antennas. This removes aimost completely the susceptibility of overall link adjacent channel C/l to rain fades on the feeder links at the cost of a small
20、gain loss at the receiving terminal. Figure 1 shows the results of a pariunctric study giving the overall adjacent channel C/I as a function of orbital separation and for different tr,ansmitting antenna sizes. The technical parameters adopted at rhe RARC SAT-83 including the transmit and receive ant
21、enna mispointings were used in this analysis. The figure also gives the variation in receiving antenna gain as a function of the orbital separation. It should bc notcd that 1 dB receiving gain loss due to mispointing is already taken into account in the ecuth-stntion G/Tcnlculation. The optimum orbi
22、tai separation is the point of bcst polarization discrimination for faded condition on the feeder link. This represents the best trade-off between feeder-link polcuization discrimination and down-link loss in gain. This optimum is found to be 0.4 for 5 m feeder-link transmitting antennas. This separ
23、ation was used in the development of the pkan for Region 2 at the RAEC SAT-83. The use of larger lmnsmitting antennas will shift this optimum to smaller orbitid separation (e.g. 8.3 for 8 m antennas and 0.27 for 11 m antennas). The WARC ORB-88 decided that administrations could place the satellites
24、of a same “group“ of satellites (Le, sharing the same nominal position in the Pican) at any position no further thnn 8.2 away from the nominal position, provided that the agreement of the otlier xlministrations on that orbital position is obtained. The advantage of this arrangement is that it permit
25、s additional discrimination between fecder links (large transmitting antennas) whilst for the purposes of reception of the down link (small iuitennas) these mellitca ctan still be considered as being at the same position. COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicens
26、ed by Information Handling Services- CCIR RECMN*795 92 4855232 0520032 554 = Rec. 795 FIGURE 1 Cross-polar CII improvement through orbital separation OTO1 0.02 0.05 0.1 0.2 0.5 1 .o 2.0 5.0 Orbit separation (degrees) CurvesA : B: c: A, : A, : B, : B, : A, : clear-air conditions on feeder links and d
27、ovn link 10 dB rainfall attenuation on the wanted feeder link degradation of the earth-station receive Co-polar gain overall link cross-polar C/I for 5 m antennas at the feeder-link sites (clear-air situationi overall link cross-polar C/I for 8 m antennas at the feeder-link sites (clear-air Situatio
28、n) overall link cross-polar C/I for 11 m antennas at the feeder-link sites (clear-air situation) overall link cross-polar C/I for 5 m antennas at the feeder-link sites (10 dB fade situation) overall link cross-polar C/Z for 8 m antennas at the feeder-link sites (10 dB fade situation) 163 B, : overall link cross-polar C/Z for 11 m antennas at the feeder-link sites (10 dB fade situation) COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
