1、 Rec. ITU-R S.1595 1 RECOMMENDATION ITU-R S.1595 Interference mitigation techniques to facilitate coordination between non-geostationary fixed-satellite service systems in highly elliptical orbit and non-geostationary fixed-satellite service systems in low and medium Earth orbit (Question ITU-R 231/
2、4) (2002) The ITU Radiocommunication Assembly, considering a) that in some frequency bands non-geostationary orbit (non-GSO) satellite systems are required to coordinate with other non-GSO satellite systems if there is frequency overlap; b) that systems using highly elliptical orbits (HEO) have uniq
3、ue features, such as transmitting and receiving only during a long dwell time (normally around the apogee) of a single satellite over specific range of latitude and longitude, stable and predictable service arcs, and few satellites required to provide coverage over an entire hemisphere; c) that stud
4、ies have shown, without the use of interference mitigation techniques, it will be impracticable for non-GSO systems to share the same frequencies and meet performance objectives; d) that studies have shown there are several different interference mitigation techniques that may allow non-GSO systems
5、to share; e) that the effectiveness of a mitigation technique to decrease interference may be determined by an improvement in several different performance parameters, i.e. C/I, I/N or C/(I + N); f) that if the clear sky C/N is large, of the order of 25 to 35 dB, typical ranges for allowable I/N val
6、ues can have a negligible effect on clear sky-link performance, i.e. C/(N + I) obtained with and without mitigation techniques; g) that, in the situation described in considering f), C/I may be a more useful parameter than I/N in evaluating the effectiveness of interference mitigation techniques; h)
7、 that, in situations other than those described in considering f), the use of I/N to evaluate the effectiveness of interference mitigation techniques may be more appropriate; 2 Rec. ITU-R S.1595 j) that there are some non-GSO systems in operation in the fixed-satellite service (FSS) that have large
8、clear-sky C/N values on the order of 25 to 35 dB; k) that Recommendation ITU-R S.1323 provides methodologies for determining maximum permissible levels of interference between non-GSO FSS systems; l) that the maximum permissible levels of interference discussed in Recommendation ITU-R S.1323 are bas
9、ed on an increase in the unavailability of the victim system/network, recommends 1 that the mitigation techniques described in Annex 1 of this Recommendation should be considered for use to facilitate coordination between non-GSO FSS systems in HEO and non-GSO FSS systems in low Earth orbit (LEO) an
10、d medium Earth orbit (MEO) when coordination is required by No. 9.12 of the Radio Regulations (RR) (see Notes 1 and 2); 2 that non-GSO FSS systems with large clear-sky C/N requirements, on the order of 25 dB to 35 dB, may use as a starting point to interference assessment, C/I as a criterion to dete
11、rmine the effectiveness of interference mitigation techniques (see Note 3); 3 that a more thorough assessment of the effectiveness of mitigation techniques should also take into account the methodologies of Recommendation ITU-R S.1323. NOTE 1 Annex 2 shows that the mitigation techniques described in
12、 Annex 1 can have adverse operational impacts on the systems implementing them (e.g. coverage, capacity, etc.). The severity of these impacts vary with system characteristics. NOTE 2 The mitigation techniques of Annex 1 may also be applicable to sharing between non-homogeneous HEO systems. Further s
13、tudy is required to determine this applicability. NOTE 3 Other interference criteria may be more appropriate for non-GSO FSS systems with typical clear-sky C/N requirements. ANNEX 1 Mitigation techniques This Annex provides a summary of mitigation techniques that may be used by non-GSO systems to fa
14、cilitate sharing between non-GSO systems in HEO and non-GSO systems in LEO and MEO. Four techniques for mitigating interference have been identified and studied. This Annex does not represent an exhaustive list of mitigation techniques, others may be identified in the future. These interference miti
15、gation techniques and combinations thereof are useful to different degrees in facilitating sharing between non-GSO systems in HEO and non-GSO systems in LEO and MEO. Finding the optimum mitigation technique(s) to be applied between any two non-GSO FSS systems, one of which is a HEO, may, in some cas
16、es, best be able to be accomplished during inter-system coordination. Annex 2 provides an example of the results of simulations using these mitigation techniques. Rec. ITU-R S.1595 3 1 Satellite diversity An in-line event occurs when one interfering non-GSO satellite is directly between the wanted n
17、on-GSO earth station and the wanted non-GSO satellite. Satellite diversity means that in-line events are avoided by the interfering non-GSO system selecting another visible satellite (with available beams) whenever the current satellite approaches an in-line event with a satellite operating in anoth
18、er non-GSO FSS system. To accomplish this technique, an avoidance angle of X in reference to the victim earth station is utilized. A cone of X is placed around the link from the victim earth station and the victim satellite assigned to downlink to that earth station. As a satellite from the interfer
19、ing system enters that cone, the interfering satellite is not allowed to transmit within the cell radius of the victim earth station. Satellite diversity implies performing a handover (switching) process due to reselecting the satellite for interference avoidance. Satellite diversity may require a c
20、omplex process involving cooperation among the systems involved. This mitigation technique requires extensive knowledge of the location of both non-GSO systems satellites and inherently reduces the service capacity of the mitigating satellite system. To use satellite diversity as an interference mit
21、igation technique, it is necessary for the interfering non-GSO FSS system to be designed to have a sufficient number of satellites with enough beams per satellite capable of serving a given earth station location simultaneously. Not all non-GSO systems meet these criteria. 2 Satellite selection stra
22、tegies The algorithm chosen for satellite selection by a given non-GSO FSS system may enhance the ability of that system to share with other non-GSO FSS systems. In general, earth stations will communicate with the satellite that is at the highest elevation. If a system chooses to use a different tr
23、acking technique, such as selecting the satellite that has the largest angular discrimination with respect to the satellites of other non-GSO FSS systems, the sharing situation may improve with the expense of added complexity and/or reduced capacity in system operation. This mitigation technique req
24、uires extensive knowledge of the location of both satellite systems. 3 Earth station site diversity In some cases, it may be possible to use earth station site diversity as a mitigation technique. This technique involves separating the earth stations so that when an in-line event occurs the interfer
25、ing satellite is not pointed toward the victim earth station but is pointed toward another earth station further away. This ensures that there is no main beam-to-main beam interference, rather it is side lobe to main beam interference thus reducing the amount of interference. The fewer earth station
26、s involved the more practical this becomes. This mitigation technique reduces the number of earth stations a non-GSO system could use and restricts the location of those earth stations. Therefore the capacity on the ground and the ability to serve certain areas are reduced but not necessarily the ca
27、pacity on the satellite. 4 Rec. ITU-R S.1595 4 HEO apogee avoidance HEO apogee avoidance uses a concept similar to the GSO arc avoidance technique proposed by several non-GSO systems which intend to operate in the bands 17.8-18.6 GHz and 19.7-20.2 GHz where protection of the GSO networks by non-GSO
28、systems is required by RR Article 22. This concept uses the fact that an HEO apogee service arc can be defined based on the fact that many HEO systems use an inclination of approximately 63 and an argument of perigee of 270 or 90. Many HEO systems employ this common inclination because for inclined
29、elliptical orbits the argument of perigee changes due to the non-uniform gravitational pull of the Earth. Lunar and solar gravitation are secondary causes for movement in the argument of perigee. For an HEO type orbit it is necessary to keep the argument of perigee stable so that the operational por
30、tion of the orbit (or where the satellite dwells the longest) is consistently in the same place above the Earth. The formula for rate of change of the argument of perigee is: ydegrees/da)1cos5()(14.98223.522iaRee=& where: & : rate of change of the argument of perigee e : eccentricity of the orbit Re
31、: radius of the Earth a : semi-major axis of the orbit i : inclination of the orbit. When the inclination of the orbit is equal to 63.4 or 116.6 the argument of perigee remains constant since at these angles cos2i = 1/5 and & is zero. Therefore, for any satellite orbit semi-major axis and eccentrici
32、ty, if the inclination is 63.4, then the argument of perigee (and therefore the apogee service arc) is constant and well defined. A northern HEO apogee service arc is defined when the argument of perigee for the HEO orbit is 270. A southern HEO apogee service arc is created when the argument of peri
33、gee for the HEO orbits is 90. If the HEO system has known earth station locations then the beams on the LEO or MEO satellite could be directed away from those earth stations when the LEO or MEO satellite is within the HEO apogee service arc. If the HEO systems has ubiquitous earth stations then the
34、LEO or MEO satellite would have to direct the beams away from all earth station locations for which there might be Rec. ITU-R S.1595 5 an in-line event when the LEO or MEO satellite is in the HEO apogee service arc. This redirection of beams would then avoid in-line conjunction, and thus reduce the
35、interference to acceptable levels between the two non-GSO satellites without having to have extensive knowledge of the location of the HEO satellites. This mitigation technique inherently reduces the service capacity for the mitigating satellite system. Redirecting the beams away from several known
36、earth station locations would have less impact on the interfering systems satellite coverage capability than redirecting the beams away from all of the possible earth station locations with the ubiquitous case. ANNEX 2 Results from simulations using mitigation techniques 1 System characteristics The
37、 dynamic simulations of this study were performed using data provided in RR Appendix 4 coordination information, Recommendation ITU-R S.1328 and through other contributions to ITU-R. Five systems are characterized by the information found in Table 1: two LEO systems, two MEO systems and one HEO syst
38、em. For all systems, the satellite selection strategy for nominal operations is assumed to be highest elevation angle, unless otherwise stated for a mitigation technique. The USCSID-P system is modelled such that the satellites do not transmit to any earth station when the subsatellite latitude of t
39、he HEO satellite is below 35 in order to protect the GSO arc. This GSO avoidance technique is a typical HEO operational characteristic and is not intended to reflect the actual characteristics of USCSID-P. The earth station locations of each system were modelled as concentric circles around a co-loc
40、ated earth station with the victim non-GSO system. The distance between earth stations was based on the beamwidth of the satellite downlink such that the non-GSO system would not cause self interference. For the HEO system only one earth station was modelled because the earth stations for this syste
41、m are very far apart and multiple earth stations would not have any cumulative effect on the results of the simulations. 6 Rec.ITU-R S.1595TABLE 1 System characteristics Characteristic USCSID-P LEOSAT-1 LEOSAT-2 USAMEO-2 USAMEO-3 Number of satellites 8 288 63 15 20Number of planes 8 12 7 3 4 Number
42、of satellites per plane 1 24 9 5 5 Plane spacing (degrees) 45 15.36 51.43 120 90 Inclination (degrees) 63 84.7 48 50 55 Orbit altitude (km) Apogee = 39 400 Perigee = 1 000 1 375 1 400 10 355 10 352 Inter-plane phasing (degrees) 45*( j 1) j = 1, 3, 5, 745*( j + 1) j = 2, 4, 6, 8 Random 28,57 24 0 Min
43、imum earth station elevation angle (degrees) 3 40 16 25 30 Downlink transmission parameters Carrier bandwidth (MHz) 3 200 500 35 222 133.47 Power control No No Yes No No Earth station receive peak gain (dB) 70 and 59.5 34.1 34.2 51.7 35.9 Earth station receive antenna pattern Rec. ITU-R S.1428 Rec.
44、ITU-R S.1428 Rec. ITU-R S.1428 Rec. ITU-R S.1428 Rec. ITU-R S.1428 Earth station receive antenna diameter (m) 20 and 6 0.3 0.35 2.2 0.36 Earth station receive noise temperature (K) 255 288 678.4 259.6 192 Satellite transmit peak gain (dB) 51 34.7 to 35.7 34.3 41.28 44.6 Satellite transmit antenna pa
45、ttern Rec. ITU-R S.672 0.5 edge of coverage, 25 near side lobe, 30 far side lobe Rec. ITU-R S.672 LN= 25 dB Rec. ITU-R S.672 LN= 25 dB Rec. ITU-R S.672 LN= 25 dB Satellite transmit e.i.r.p. (dBW) 70 53.9 37.22 42.5 60.34 Number of co-frequency co-polarized transmit beams 1 8 260 24 20 Number of eart
46、h stations modelled 1 91 91 91 91 Rec. ITU-R S.1595 7 2 Analysis Simulations were run to determine the effectiveness of the different mitigation techniques and to understand the impact of these mitigation techniques on the individual systems analysed. Interference, in the form of C/I, is compared to
47、 a given threshold. For these simulations the threshold was set arbitrarily at C/I = 20 dB. For a large clear sky C/N such as 35 dB, rain could lower the C/N to approximately 20 dB. The effect of rain includes signal attenuation, an increase in the earth station receiver noise temperature, and (if d
48、ual polarization) an increase in cross-polarization interference. A C/N of 20 dB combined with C/I of 20 dB gives a C/(N + I) of 17 dB, which is considered sufficient for link performance in this study. The C/I threshold chosen does not reflect the actual C/I threshold of any of the systems modelled
49、 and could be refined using the criterion specified in Recommendation ITU-R S.1323 (10% aggregate reduction in unavailability). Additionally, the joint probability of fading and interference should be considered. However, for this analysis, interference is considered acceptable if the resulting C/I value equals or exceeds 20 dB. The impact of a given mitigation technique is measured by the decrease in the number of satellites in the mitigating system available to service a given area. Another type of impact, which is not included in this study, is the increa
