1、 Rep. ITU-R M.2124 1 REPORT ITU-R M.2124*Interference calculations to assess sharing between the mobile-satellite service and space research (passive) service in the band 1 668-1 668.4 MHz (2007) 1 Introduction Under WRC-07 Agenda item 1.7, Resolution 744 (WRC-03) requests studies into sharing betwe
2、en the mobile satellite service and space research (passive) service in the band 1 668-1 668.4 MHz. Two relevant space-VLBI (Very Long Baseline Interferometry) systems have been identified. The HALCA system began operation in 1997 and ceased operation in 2005. A proposed system, Radioastron, is plan
3、ned for this band and characteristics have been made available. The band 1 668-1 668.4 MHz is allocated for MSS (Earth-to-space) but is unlikely to be useable in the North America, where alternative fixed and mobile uses are planned. This and other restrictions which are likely in other parts of the
4、 world mean that this band is unlikely to be used by non-GSO MSS systems. Therefore this Report assesses the interference received by the HALCA and Radioastron systems from GSO MSS networks only. 2 Calculations of interference from MESs to space VLBI receivers 2.1 Characteristics of space VLBI satel
5、lites The Radioastron satellite is planned to operate in an elliptical orbit, which is allowed to drift in inclination. There is also some variation in apogee and perigee height, however the representative values are: apogee height = 364 000 km, perigee height = 10 000 km, eccentricity = 0.9153, inc
6、lination 0 to 80. The HALCA system operated in an elliptical orbit at a fixed inclination of 30. Other key parameters are: height of apogee = 21 000 km, height of perigee = 560 km, eccentricity = 0.5956. The HALCA receiver consisted of two channels of 16 MHz which could be tuned within the range 1 6
7、00-1 730 MHz. The receiver therefore had some ability to avoid frequencies where high interference could be anticipated. The Radioastron system on the other hand is planned to operate with a fixed receiver bandwidth. A space VLBI satellite employs a large reflector antenna. The gain characteristics
8、of such antennas are discussed in 2.3. 2.2 Interference criterion The threshold pfd levels for VLBI observations. These pfd levels are based on a criterion of interference equal to 1% of the receiver system noise. Both the proposed Radioastron system and the HALCA system have receiver temperatures o
9、f about 70 K. In a bandwidth of 400 kHz, this equates to an interference threshold of 174 dBW. *This Report should be brought to the attention of Radiocommunication Working Parties 7B and 7D. 2 Rep. ITU-R M.2124 Current technology for spaceborne receivers would allow a system noise temperature of ab
10、out 30-40 K. Furthermore, it is also envisaged that in the future lower system noise temperatures may be achievable. By about 2010, values of about 20 K can be expected. In a bandwidth of 400 kHz, this equates to an interference threshold of 179.6 dBW. However, as can be seen from the results, the o
11、rbital characteristics of the satellite may have a more significant impact on the S-VLBI protection requirements. Recommendation ITU-R RA.1513 recommends that interference from a network may cause a radio astronomy data loss of up to 2%. Therefore the criterion used is based on 1% of the receiver sy
12、stem noise (I/N = 20 dB) which may be exceeded by a single network for no more than 2% of the time. Recommendation ITU-R RA.1513 also recommends that data loss of up to 5% may be permitted from all networks and hence a second criterion is used based on 1% of the receiver system noise (I/N = 20 dB) w
13、hich may be exceeded by all networks for no more than 5% of the time. 2.3 Characteristics of space radio telescope on-board antennas The HALCA system used a large reflector antenna (8 m) and the Radioastron system also proposes to use a large reflector antenna (about 10 m diameter). The difference b
14、etween the gain of the main antenna beam and far side- and back-lobes is about 40 dB and more for these antennas. But at the same time the D/ value of these antennas is less than 100, taking into account the operating frequency range (1 668 MHz). Additionally, it should be noted that because of the
15、design features of the Radioastron spaceborne antenna (deployable umbrella-type antenna) there is limited scope to significantly suppress the side- and back-lobes. The angle between the direction to the observed space radio source and nadir (direction to the Earth centre) depends on the Sun position
16、, the orbital location of the spacecraft and the direction to the earth control station (radio channel of delivering information). It can be assumed that this angle always exceeds 90. Unfortunately, the antenna patterns of the HALCA and Radioastron satellites, that characterizes the sensitivity to t
17、he radiation from the far side- or back-lobes, was not measured. Moreover it should be mentioned that measurement of these levels is very difficult problem from the technical point of view, first of all because the level and arrangement of the antenna side- and back-lobes very largely depend on loca
18、tions of other spacecraft components. It should also be noted that, due to distribution of interference sources and the satellite movement, the gain representative of the average gain in the direction of earth is required. The subsections below provide two alternative approaches by which it is propo
19、sed that the appropriate value of the antenna gain be determined. The Radioastron system will operate with both LHC and RHC polarization and hence no polarisation discrimination between the space-VLBI system and MSS systems can be assumed. 2.3.1 Approach 1 Examination of ITU-R Recommendations The fi
20、eld test measurements of the onboard Radioastron antenna were made for verification of the main beam and determination of the gain value and location of the first side-lobe of it. Therefore the real measured Radioastron antenna far side- and back-lobes gain are not currently available. The existing
21、Recommendation ITU-R SA.509-2, showing analytical expressions for antenna radiation patterns of receiving stations of the radioastronomy service cannot be applied for side- and back-lobe Radioastron spaceborne antenna gain because the D/ value of this antenna is less than 100. Recommendation ITU-R R
22、A.1631 may not be applicable for antennas with such a small value of D/. The analysis of existing Recommendations (ITU-R S.465, ITU-R S.580, ITU-R S.731, ITU-R F.699, ITU-R F.1245, ITU-R S.1428, ITU-R S.672, ITU-R S.1528) and Appendixes 7 and 8 of the Radio Regulations (RR) with analytical expressio
23、ns for antenna radiation patterns (with D/ Rep. ITU-R M.2124 3 value less than 100) of different radio services shows that the level of the antenna back- and side-lobes is between 0 dBi and 11.7 dBi. At this time, Recommendations ITU-R S.672-4 and ITU-R S.1528 which describe the satellite antenna ra
24、diation pattern in the fixed satellite service (GSO and non-GSO systems) can be used for preliminary calculations to assess sharing between the MSS and the space research service (passive) in the band 1 668-1 668.4 MHz. Based on the analytical expressions shown in these Recommendations, the best cas
25、e back-lobe gain ( more than 90) of the Radioastron spaceborne receive antenna toward the Earth will be no less than 0 dBi. Recommendation ITU-R S.672-4 is intended as a design objective for the antennas of GSO FSS satellites. For angles in the range 90-180, the sidelobe level is given by: LB: 15 +
26、LN+ 0.25 Gm+ 5 log z dBi or 0 dBi whichever is higher. Taking the example of the Radioastron antenna parameter values, LN= 18 dB, Gm= 42 dBi and z = 1. This leads to a value of LB= 7.5 dBi. Recommendation ITU-R S.1528 is intended for non-GSO FSS systems below 30 GHz. It recommends that, where possib
27、le, a measured pattern be used and alternatively, that patterns given by one of a number of equations are used. Taking the equations given in recommends 1.1 as an example, the gain for angles in the range 90-180 is given by the same equation as for Recommendation ITU-R S.672: LB: 15 + LN+ 0.25 Gm+ 5
28、 log z dBi or 0 dBi whichever is higher. For the Radioastron antenna parameter values, this naturally leads to the same value: 7.5 dBi. However, neither of these recommendations are appropriate to the case in hand, for several reasons: Both recommendations apply to FSS satellites, where large unfurl
29、able antennas of about 10 m diameter are unheard of. Recommendation ITU-R S.672 applies to GSO satellites and Recommendation ITU-R S.1528 applies to non-GSO satellites with multiple-beam antennas. Neither is appropriate for non-GSO systems with single beam antennas which would be appropriate in this
30、 case. Both recommendations are for peak envelope rather than average gain patterns. Furthermore, for any directional antenna, the average gain of the side lobes must be less than 0 dBi and this is clearly not the case for an antenna for which the minimum gain value is 7.5 dBi. Hence the values give
31、n by these recommendations cannot be appropriate for this case. While there are no ITU-R Recommendations explicitly related to the case in hand, one can identify other recommendations which point towards the possibility of much lower values. It is particularly relevant to examine recommendations whi
32、ch address the average antenna pattern. For example: in Recommendation ITU-R S.1428, in the case of D/ = 55, the average sidelobe gain value for angles in the range 80-120 is 4 dBi and for angles in the range 120-180 is 9 dBi. in Recommendation ITU-R RA.1631, the average antenna gain for angles in t
33、he range 80-120 is 7 dBi and for angles in the range 120-180 is 12 dBi. in Recommendation ITU-R F.1245, in the case of D/ = 55, the sidelobe value beyond 48 is 11.7 dBi. Hence examples of Recommendations aimed at average antenna gain values suggest a value in the range 4 to 11.7 dBi. It should be ta
34、ken into consideration that the Radioastron system suggests observations of space radio sources in wide ranges of angles relative at the Earth. 4 Rep. ITU-R M.2124 Based on the aggregate above-mentioned information it has been proposed to use the value 5 dBi for the average gain of the side- and bac
35、k-lobes of the space-VLBI on-board antenna pattern in the sharing studies between MSS (E-s) and SRS (passive), in the absence of other information. 2.3.2 Approach 2 Spherical integration While it would be useful to have actual measured antenna patterns of the satellite antenna, it is also possible t
36、o take a more theoretical approach. Taking the Radioastron system as an example, the antenna diameter is about 10 m. The peak antenna gain is 42 dBi and the antenna beamwidth is about 1.4. Reflector antenna gain patterns generally follow a common shape, illustrated in Fig. 1: the main lobe follows a
37、 parabolic equation; the near side lobes follow a constant 25 log pattern; the far side lobes are relatively constant. FIGURE 1 Typical antenna pattern envelope The curve in Fig. 1 is based on the equations given in Recommendation ITU-R F.699. Similar equations are used to represent reflector antenn
38、as in other recommendations for space and terrestrial applications. For the studies in question, it is necessary to consider the interference received from a distribution of MESs. Hence it is appropriate to determine the average, rather than peak, gain in the direction of Earth. For any antenna, the
39、 average gain found by integrating throughout the sphere is 0 dBi. (If fact it is the directivity rather than the gain, but in practice the two values are very close). Hence, the desired antenna pattern, representative of the average side lobe level, should integrate to 0 dBi. An integration of the
40、pattern in Fig. 1 has been carried out and this has been found to result in the value 1.3 dB. The level of the near side lobe and far side lobe curve may therefore be lowered until the integral is equal to 0 dBi. By an iterative process, it was found that curves should be lowered by 3 dB to lead to
41、an integral result of 0 dBi. The resulting pattern is shown in the dotted line in Fig. 2. Rep. ITU-R M.2124 5 FIGURE 2 Typical and adjusted antenna patterns The horizontal part of the curve, representative of the average far side lobe level is 10.5 dBi. Based on this approach it has been proposed to
42、 use the value 10 dBi for the average gain of the side- and back-lobes of the space-VLBI on-board antenna pattern in the sharing studies between MSS (E-s) and SRS (passive), in the absence of other information. 2.3.3 Conclusions regarding the satellite antenna Actual measured antenna patterns for sp
43、ace-VLBI satellites would be required to accurately assess the average side lobes of the space-VLBI antenna. Until such patterns are available, it is necessary to assume a range of possible values. Based on the above analysis, values of 5 dBi and 10 dBi are considered. 2.4 Interference simulations C
44、omputer simulations have been performed to model the interference received by a space-VLBI satellite receiver from an MSS network. Four representative MES types have been considered (Table 1). The Type A and Type B MESs are portable terminals, using directional antennas. The Type C and Type D MESs a
45、re hand-held terminals, using an omnidirectional antenna. The MES transmitter parameters are shown below. For MES Types A and B the MES antenna patterns are based on those given in Recommendation ITU-R M.1091 and shown in Fig. 3. 6 Rep. ITU-R M.2124 TABLE 1 Mobile earth station parameters Type A Typ
46、e B Type C Type D Peak antenna gain (dBi) 7.5 16.5 0 3 Polarization Circular Circular Circular Circular e.i.r.p. (dBW) 9 21 3.5 5 Channel bandwidth (kHz) 100 200 31.25 31.25 Maximum e.i.r.p. spectral density (dBW/4 kHz) 4.0 5.0 5.4 3.9 Power spectral density delivered to the antenna (dBW/4 kHz) 11.5
47、 11.5 5.4 6.9 Total power delivered to the antenna (dBW) 1.5 4.5 3.5 2 FIGURE 3 MES antenna patterns It is necessary to model a distribution of MESs. Using the Inmarsat-4 characteristics, the satellite provides coverage to almost the entire visible earth surface with 228 spot beams. Frequencies are
48、re-used on a 7-cell repeat basis, meaning each frequency may theoretically be used up to 32 times. It should be noted that, in practice, frequencies are not re-used to this extent due to the uneven geographical distribution of traffic. However, for the purpose of this analysis, the more conservative
49、 assumption has been retained. A fully loaded MSS satellite has been assumed (i.e. each cell is fully loaded with traffic). Rep. ITU-R M.2124 7 For the purpose of simulation the following assumptions have been made: The variant 1: the Type A MES only will operate in the frequency band 1 668-1 668.4 MHz. Hence there are 128 Type A MESs. The variant 2: Half of the bandwidth is dedicated to the Type A MES and the other half is dedicated to the Type B MES. Hence, within the 400 kHz bandwidth, there are 64 Type A MESs a
