1、CCIR RECNN*793 92 4855232 0520002 B37 Rec. 791 RECOMMENDATION 791* CHOICE OF POLARIZATION FOR THE BROADCASTINGSATELLITE SERVICE* (Questions 83/11 and 85/11 133 (1 992) The CCIR, considering a) that the choice of polarization used in the broadcasting-satellite service (BSS) impacts on system performa
2、nce, spectrumlorbit utilization, and the level of interference within the BSS and between services sharing the same frequency band; b) that misalignment between linearly polarized signals of interfering systems and the misalignment between transmitters and receivers has: - - - - c) that linear polar
3、ization can provide significant advantages over circular polarization with respect to atmospheric effects such as rain attenuation and depolarization, particularly at the higher frequency bands (see Annex 1); d) that, with linear polarization, accurate alignment between transmitter and receiver woul
4、d be extremely difficult to achieve for reception by portable receivers and receivers in vehicles; e that for certain frequency bands allocated to the BSS and the associated feeder links there are Plans in the Radio Regulations where circular polarization was adopted and that there are satellite net
5、works, operating or planned. in these bands, no effect on C/N of linearly and circularly polarized systems, no effect on C/I of circularly polarized systems, little effect on Co-polar C/l of linearly polarized networks, but has a strong effect on the cross-polar C/I of linearly polarized networks; r
6、ecommends 1. that circular polarization be used where reception is required by portable receivers and receivers in vehicles and for fixed reception except for the situation referred to in 0 2; 2. that, for the frequency bands where atmospheric effects are predominant and when maximum frequency re-us
7、e from the same orbital location is needed, the advantages of linear polarization should be taken into account. Should linear polarization be employed, it should be fully specified and any additional information required to ailow the IFRB to determine the affected administrations should be given; No
8、te I - A summary of the factors affecting the choice of polarization is given in Annex 1. 3. polarization adopted in the Plans. that for bands where Plans exist, the choice of polarization considered in 4 2 should take into account the * Note from the Director, CCIR - Reports 814-2, 564-4 and 122-3
9、were used in preparing this Recommendation This Recommendation should be brought to the attention of Study Groups 4,5 and 9. * CCIR RECMN*79b 92 H ItS552S2 052D003 753 H 134 Rec. 791 ANNEX 1 1. Introduction For purposes of planning the BSS in the bnnd 11.7-12.5 GHz in Region 1 and 11.7-12.2 GHz in R
10、egion 3, right- and left-hand circular polarization was adopted. Similarly, in Region 2, right- anci left-hand circular polarization was selected for the Plan for the broadcasting-siitellite service in the band 12.2-12.7 GHz as well as for the associated feeder-link Plan in the bniid 17.3-17.8 GHz.
11、Furtllcrmore, at the WARC ORB45 the frequency bands 14.5-14.8 GHz (for countries outside Europe and for Mdi) and 17.3-18.1 GHz were selected for the planning of fceder links for the broadcasting-satellite service in Regions 1 and 3. It was assumed that circular polarization would be used for plannin
12、g. Alternatively linear polarization could bc used, subject to the agreement of ail administrations sharing the given orbitai position, This Annex presents a summary of the factors that were consiered in making a choice, both for the record and for the design of future systems in other bands that ar
13、e or may be allocated to the broadcasting-satellite service. 2. Comparison between linear and circular polarizatlon The polarization type has an influence on system and service design. Although the WARC-BS-77, RARC SAT-83 Plians and WARC ORB-88 feeder-link Plan used circular polarization, linear pol
14、arization has some distinct advantages with respect to circular polarization. One advantage is that there is a cross-polarization improvement factor which varies with poliwization tilt angle and amounts to 15 dB fer O“ and 90 local polnrization tilt angles. A further advantage of linear polnrization
15、 is that it is easier to achieve adequate CO-polar side-lobe suppression and cross-polar discrimination in the recciving antenna, The comparative advnntages and disadv,antiiges of linear and circular polarization for use in the broadcasting- satellite service are summiarized in Table 1. The symbols
16、in the last two columns of the table indicate for each factor which type of polarization, linear (L) or circular (C), is considered to have the advantage. In evaluating these comparative advantages and disridvantages, it must of course be recognized that the different factors arc not all of equal pr
17、actical importance and that their relative importance is also amatter of engineering judgement. To aid in evaluating the importance of siitellite antenna orientation on the choice of polarization (item 3 in Table i), a short quantitative discussion of the effects of system geometry on linear polcari
18、zation is given in Appendix 1. The choice between linear and circular polarization for planning the BSS is governed by two major factors: - the effect of rain attenuation - the effect on interference of the misalignment between the reference linear polarization vectors and of the misalignment of ear
19、th station and satellite polarizers. Rain affects both the C/N and C/I of linearly and circularly polarized waves for small percentages of time. On the other hand, the misalignment between linearly polarized signals of interfering networks and the misalignment between transmitters and receivers has:
20、 - no effect on C/N of linearly ,and circularly polarized networks; - - no effect on C/Z of circularly po1,uized networks; little effect on Co-polar C/I of linearly polarized networks; but - has a strong effect on the cross-polar C/Z of linearly polarized networks. This is significant since linear p
21、okwization is normally used to improve C/Z during rain but, as a consequence of the misalignment, it may decrease C/I far below the maximum possible discrimination capability of the satellite and earth-station antennas. CCIR RECflN*7ql 72 rn 4855232 0520004 b7T Rec. 791 TABLE 1 Some aspects of hear
22、as compared with circular polarization Factor 1. Alignment of receiving antenna 2. Effect of misalignment on cross-polarization 3. Onentation of satellite antenna 4. Sharing with other services 5. Propagation effects Remarks Alignment of the polarkation direction is not necessary for circular polari
23、zation Misalignment of polarization direction of both transmitting and receiving antennas required with linear polarization, 2 to 4 dB extra cross-polar protection margins in comparison with circular polarization With linear polarization, the plane of polarization will not in general correspond to t
24、he major or minor axes of a beam with elliptical cross-section; therefore: a) polarization (in particular for elliptical beams); it may be difficult to produce a good cross-polar response with linear b) transfer to a spare satellite at a different orbital position would probably be more difficult wi
25、th linear polarization because of the need to realign the polarization plane a) If circular polarization is chosen for the broadcasting-satellite service and other services use linear polarization, up to 3 dB protection between these services and the broadcasting-satellite service is assured b) If b
26、oth the broadcasting-satellite and other services, e.g. futed- satellite and terrestrial services, use hear polarization, then in isolated cases, where the dominant interference arrives near the main beam of a receiving antenna, it may be possible to increase the isolation by the use of orthogonal P
27、olarization Circular polarization is more affected by atmospheric conditions than hear polarization for high rainfall rates (greater than 12.5 mm/h) and low angles of arrival For example, the cross-polar attenuation may be 20 dB for 1% of the time with circular polarization according to some measure
28、ments in Switzerland at 12 GHz. This disadvantage of circular polarization may not be significant if compared with linear polarization transmission on or near a 45“ plane 135 Advantage i! C C C C C C L L (1) C: circular L linear It was found that even for high rainfall zones, the effect of rain on C
29、o-polar and cross-polar CIZ is small for ail but 1% of the worst month. For smaller percentages of time, the effect of rain becomes more important and its effect on cross-polar C/Z depends on the type of polarization and on the reference vectors for linear polarization. There are two main reference
30、systems that can be used to define linear polarization: - canted linear polarization: Vertical polarization is defined so that the polarization vector is perpendicular to the satellite antenna beam axis and lies in the plane defined by the satellite antenna beam axis and the local vertical. Horizonr
31、d polarization is defined so that the polarization vector is perpendicular to the satellite antenna beam axis and is contained in the local honzontal plane. These vectors will be in the direction closest to the local horizontai or local vertical at the boresight of the satellite antenna; CCIR RECMN*
32、?1 92 W Li855212 52005 526 M 136 Rec. 791 - equatorial linear pokarizatiun: Polar polarization is defined so that the polarization vector is perpendicular to the satellite antenna beam axis and lies in the piane defined by the satellite antenna beam axis and a line parallel to the Earths polar axis,
33、 Equatorial polarization is defined so that the polarization vector is perpendicular to the satellite antenna beam axis and parallel to the equatorial plane. in general, the better performance that would be available from linear polarization requires two conditions which are, in most cases, incompat
34、ible. On the one hand, the best performance of linear polarization is obtained when the signal is received vertically polarized. On the other hand, the orthogonally-polarized interfering signals must be received exactly at 90 from the wanted signal. Because of the geometry of the problem these two c
35、onditions cannot be met simultaneously. A slightly better performance can be obtained with linear polarization when the reference vector is defined perpendicular to the equatorial plane, and if the receiver misalignment does not exceed 3“ for all CO-polar systems. This implies that the polarization
36、is not received vertically, and therefore does not meet the first condition. Since this improvement is considered marginal considering the additional constraint imposed on the receiver polarizer alignment, circular polarization is therefore suggested for planning of the broadcasting-satellite servic
37、e. 3, Experimental results A study has been performed on the choice between linear and circular polarization for the BSS down links. The cross-polar discrimination capability of the earlh-station antenna XPZB was assumed to be in the range of 20 to 25 dB and the XPIT of the satellite antenna in the
38、range of 27 to 33 dB for both linear and circular polarization. The effect of rain attenuation and depolarization on the down link CII has been studied for rain climatic zones E, K and N using the rain model of Recommendation 618. The results have shown that for all but 1% of the worst month, the ef
39、fect of rain attenuation and depolarization on CO-polar and cross-polar CIZ is very small. For mailer percentages of time, the effect of rain attenuation and depolarization on cross-polar CIZ depends on the type of polarization and on the reference vectors for linear polarization. Figure 1 gives the
40、 results of the availability of cross- polar CII between homogeneous satellite down-link beams (2 diameter) using canted linear or equatorial linear or circular polarization. The elevation angle to the satellite is 25 and the earth-station antenna is assumed to be perfectly pointed to the satellite
41、and aligned to the wanted polarization. However, when the polarization misalignment, BR is 0.1 for equatorial linear polarization and 10.3“ for canted linear polarization, the corresponding tilt angles for the wanted and interfering beams are around 5 and 10“ respectively for canted linear polarizat
42、ion and for equatorial linear polarization, Figure 1 shows that for 1 dB attenuation, the cross-polar CIZ is 19.5 dB for circular polarizXion and for equatorial linear polarization. However, for canted linear polarization, the 10.3 misalignment, BR, decreases the cross-polar CII to 13.5 dB for 1 dB
43、rain attenuation. For larger values of rain attenuation, the CIZ is governed by the differential attenuation between linear horizontal and vertical polarization and by depolarization. Canted vertical polarization is the least attenuated and therefore can surpass the CIZ performance of horizontal equ
44、atorial, circular and even vertical equatorial polarization. However, these changeovers in cross-polar CII performance generally occur for very mull percentages of time when the signal attenuation is excessive (greater than 18 dB). If the effect of rain at 12 GHz on CII ia ignored, the effect of the
45、 total misalignment between linearly polarized networks, BT, on the clear-sky CIZ is illustrated in Fig. 2 and is compared to the performance of circular polarization. The figure shows the rapid decrease of the clear sky cross-polar CIZ with the misalignment between linear polarizations. Circularly
46、polarized receive antennas with 20 and 25 dB XPZ give higher cross-polar CII than linearly polarized antennas for any misalignment greater than 5 and 2 respectively. The figure also shows some typical values of total misalignment, Bn assuming a satellite antenna rotation error Es = f 1 and an earth-
47、station alignment CCIR RECMN*793 92 4855232 052000b Yb2 m I Rec. 791 137 I The maximum 5“ total misalignment may give a higher cross-polar CIZ with linear polarization than with circular polarization depending on the discrimination capabilities of the antenna. A maximum of 5“ misalignment can only b
48、e achieved with equatorial hear polarization when it is possible to align and maintain the polarizer of the receiver to within about 3“ of the wanted polarization. The use of canted hear polarization with typical minimum value of misalignment, BR, of 4“ would, in the majority of cases, give worse CI
49、Z than circular polarization. error BES = f5“ for both equatorial and canted linear polarization. It would appear difficult to align and maintain the polarizers of millions of low-cost receive antennas to better than +5“ of the wanted polarization. I (The antennas are assumed to be perfectly aligned) FIGURE 1 Availability of cross-polar CI1 at edge of coverage area between co-located satellites serving adjacent beam areas Attenuation for circular polarization (dB) 12 GHz down Links Co-located satellites Satellite antenna beam size: 2 Elevation angle: 25“ XPI,
