1、 Rec. ITU-R S.1061-1 1 RECOMMENDATION ITU-R S.1061-1*Utilization of fade countermeasure strategies and techniques in the fixed-satellite service (1994-2007) Scope Satellite systems are one of the most effective ways to provide high-speed multimedia services. Use of higher frequency bands due to limi
2、ted RF spectrum and the demand for high-speed services necessitates efficient countermeasure strategies for rain attenuation in the satellite link. This Recommendation presents an overview of these techniques, briefly describing guidance for planning the utilization of fade countermeasure techniques
3、 in the fixed-satellite service (FSS). The ITU Radiocommunication Assembly, considering a) that pressure on the limited RF spectrum from the increased demand for satellite services is leading to the use of higher frequency bands; b) that one of the major drawbacks associated with higher frequency sa
4、tellite systems is the large signal attenuation caused by rain; c) that satellite channel performance as set forth in Recommendations ITU-R S.353, ITU-R S.522, ITU-R S.614, ITU-R S.1062, ITU-R S.1420, ITU-R S.1711, ITU-R S.579 and ITU-R S.1424 may be difficult to be achieved, in an economic way, by
5、resorting only to power margin; d) that several systems have been developed to cope with rain attenuation whose performance and complexity are such that their applicability depends on the particular type of network involved, recommends 1 that the material contained in Annex 1 should give guidance fo
6、r planning the utilization of fade countermeasure techniques in the fixed-satellite service (FSS). NOTE 1 It should be noted that the subject techniques may even be used in combination provided that no basic incompatibility exists among them. *Radiocommunication Study Group 4 made editorial amendmen
7、ts to this Recommendation in 2001 in accordance with Resolution ITU-R 44 (RA-2000). 2 Rec. ITU-R S.1061-1 Annex 1 Fade countermeasures in satellite communication systems 1 Site diversity operation 1.1 General design considerations The performance required for diversity earth stations is decided not
8、only by the rain climate but also by the diversity configuration. The first kind of configuration is balanced diversity (diversity by two earth stations with equal performance). The other configuration is unbalanced diversity. In this configuration, the performance of one earth station (main station
9、) is made sufficiently high, so that the performance requirements of the other station (sub-station) may be considerably reduced. Such an unbalanced diversity configuration is expected when the main station antenna is equipped with a multiple-frequency band feeds such as 6/4 GHz and 14/11 GHz, and/o
10、r when the sub-station has to be simplified for technical and operational reasons. Table 1 summarizes the results of sample calculations of the antenna diameter and the maximum transmit power required for balanced diversity links with low elevation angles. Estimates are given for two assumed diversi
11、ty links: (A) Yamaguchi-Hofu (diversity distance = 20 km) and (B) Yamaguchi-Hamada (100 km); both in Japan. It is seen from this Table that the antenna diameters required for the 14/11 GHz FM link (14 GHz for uplink and 11 GHz for downlink) are about 28 m and 19 m for cases (A) and (B), respectively
12、. When the diameter of the main station can be made larger than those values, the required diameter for the sub-station becomes smaller. Values shown in this Table are derived using many of the link parameters established for Intelsat-V satellites, so they are subject to change when the link paramet
13、ers are different from those used here. Rec. ITU-R S.1061-1 3 TABLE 1 Sample calculations of the required performances for balanced diversity links with low elevation angles (14/11 GHz) Location (A) Yamaguchi-Hofu (B) Yamaguchi-Hamada Elevation angle (degrees) 9.1 9.1 9.1 8.4 Diversity distance (km)
14、 20 100 FM Required antenna diameter (m) Required transmit power(1)(W) (maximum value) 28/32 730 19/22 510 TDMA(2)Required antenna diameter (m) Required transmit power (W) (maximum value) 17/19 530 11/12 400 (1)Values for 792 channel FDM-FM carrier (25 MHz). (2)Values for 4-phase CPSK at 120 Mbit/s
15、with forward error-correction. Assumptions : Frequency: 14.5 (uplink)/11.7 (downlink) GHz Orbital position of satellite: 63 E, 0 N Satellite e.i.r.p.: 41.1 dBW Antenna diameters are estimated for two cases, namely: Ts= 50 K and Ts= 150 K Ts: system noise temperature of the earth-station antenna Effi
16、ciency of the earth-station antenna: 65% Estimates are based on the rain-rate statistics obtained for those locations. The calculation methods of the required performances (antenna diameter and e.i.r.p.) for the diversity earth stations are different depending on the diversity configurations. In the
17、 design of a balanced diversity link, calculations have to be based upon the joint probability distribution of the rain attenuation at both locations, while in the case of an unbalanced diversity configuration, the cumulative time distribution of the rain attenuation and the conditional probability
18、of the attenuation are required. The conditional probability P(L“/L ) is the probability with which the rain attenuation at the site of the sub-station exceeds L“ under the condition that the rain attenuation at the main site exceeds L. In order to perform reliable estimates of the earth station req
19、uirements, reliable statistics on the basis of the long-term propagation measurements are needed. 1.2 Site diversity switch-over operation To implement diversity earth stations, care should be exercised on the switch-over operation, because, in the event of switch-over, short duration signal loss or
20、 overlap may occur due to the difference in path length of diversity routes or carrier phase discontinuity. In analogue transmissions such as FM-FDMA, switch-over in transmitting will necessarily cause discontinuity of carrier phase which will result in a signal transient at the demodulator output i
21、n the receiving earth stations. Signal transients due to switch-over at the receive earth station may be 4 Rec. ITU-R S.1061-1 avoided by carefully adjusting the electrical path length of each diversity link measured from the switch-over equipment to the satellite. In digital transmissions it is pos
22、sible to avoid signal transients even in the event of switch-over at the transmit earth station by providing dummy intervals in the transmitting signal sequence and making the switch-over during the dummy interval. In the receiving earth stations the dummy intervals should be discarded whether or no
23、t switch-over took place. Transient switch-over both in the transmitting and receiving of the diversity system may most conveniently be achieved in TDMA transmission. The dummy intervals are built-in because TDMA transmission occupies only a part of the TDMA frame. Furthermore, TDMA demodulators are
24、 capable of receiving burst mode carriers of incoherent phase. Therefore, phase incoherency of TDMA carriers does not cause any difficulty. The only possible problem of site diversity operation of TDMA transmission would be the necessity of very precise transmit timing control even for the initial t
25、ransmission from the stand-by station. This may be solved either by continuously transmitting a dummy burst from the stand-by station or by obtaining sufficiently accurate ranging data of the satellite which is possible when the TDMA system employs open loop synchronization. In TDMA transmission, th
26、e path lengths of diversity routes can be equalized using the reception timing of frame synchronization signals. The reception timing of signals from both diversity routes can be automatically equalized by controlling the variable delay line inserted in one of the diversity routes. An experimental s
27、ystem using the dummy burst technique has been tested. For route selection in diversity operation, it is necessary to measure the transmission quality of diversity routes. Because the diversity effect may degrade depending on the choice of the measuring method of link quality, care should be taken o
28、n selecting the measuring time duration and achievable accuracy. 1.3 Diversity interconnect link A factor which must be considered is that the ITU-R hypothetical reference circuit contained in Recommendation ITU-R S.352 and the ITU-R hypothetical reference digital path contained in Recommendation IT
29、U-R S.521 include the diversity interconnection links (DIL) to the diversity switching point and any additional modulation/demodulation equipment required. This would mean that system noise budgets must include all the effects of the DIL. 1.3.1 Basic configuration 1.3.1.1 Physical aspects There are
30、a number of different specific configurations which can be considered and there could be reasons for preferring one of these. Two of these are identified and described in this Annex as (see Fig. 1): a main site which contains the diversity switch and the terrestrial interface. The diversity site is
31、connected by a two-hop microwave DIL using either an active, or passive, repeater. (A repeater site is assumed, since the likelihood of mutual visibility of the diversity sites is small); dual diversity sites and a separate control site with the interface and diversity switch; single microwave hops
32、for each site to the control site. Rec. ITU-R S.1061-1 5 It may also be possible to employ cable or waveguide links for the DIL. When both FDM-FM and TDM (FDMA or TDMA) are used at an earth station, two parallel links would usually be required. 1.3.1.2 Modulation requirements When FDM-FM is used, re
33、modulation will be required because the satellite link modulation and baseband configurations are usually different from those conventionally used for terrestrial systems. The main difference is associated with the channel packaging. The terrestrial system will usually combine the channels in one or
34、 more basebands in each direction and will use a relatively low modulation index. The earth station will break these basebands down into multiple, multi-destination, transmit basebands; different from those on the terrestrial system and using a different modulation index. The receive basebands are e
35、ven more numerous and may consist of only a few channels and these must be re-combined into the terrestrial basebands. This process requires modulation/demodulation equipment at the main earth station site and at the diversity site where conventional design of the DIL is used. All configurations can
36、 be implemented using the remodulation technique at the expense of providing duplicate equipment at the diversity site. An alternative technique is to use the same modulation arrangements on the terrestrial system as used on the satellite system. Such a technique would appear to be technically feasi
37、ble although not conventional. The incentive is to save the cost of remodulation equipment at perhaps some added cost to the terrestrial system, although savings may also result for this element as well. The use of such a technique is only applicable to the second configuration of Fig. 1. When TDM i
38、s used (FDMA or TDMA), either technique could be employed. In the case of TDMA, diversity switching is performed between bursts (see 1.2). The same modulation could be used on the DIL as used on 6 Rec. ITU-R S.1061-1 the satellite system although the data rates would not normally be those of a conve
39、ntional terrestrial digital radio system. 1.3.2 Technical factors 1.3.2.1 Frequency selection Frequency selection for a microwave DIL requires careful study to ensure that the required overall performance is obtained. Information on terrestrial microwave propagation is shown in the relevant texts of
40、 Radiocommunication Study Group 3. 1.3.2.2 Bandwidth requirements The bandwidth required to implement the DIL can be related to the earth station bandwidth by a factor which may be unity or less, depending upon whether re-modulation is used or not. If only frequency translation is used then bandwidt
41、h requirements must be MHz for MHz. By re-modulating, a greater channel density can be achieved by using smaller FM modulation indices at the expense of a substantial multiplex interface. 1.3.2.3 Rain attenuation Further factors are rain attenuation and site diversity characteristics which are both
42、related to rainfall phenomenon. A dry climate is preferred. Diversity action is a function of the site spacing. It is expected that the nominal spacing required is about 16 km. The best orientation of a line connecting the sites may be assumed to be perpendicular to the direction of predominant weat
43、her patterns since the most severe attenuation condition would not be expected to affect both sites simultaneously and maximum diversity action would be obtained. The weather effects on the microwave DIL must be accounted for if the higher frequencies are used for these links, although this should b
44、e a secondary consideration. 1.3.2.4 Variations in transmission delay due to diversity switching Another element of importance is associated with the differential transmission delay between the diversity signals as they arrive at the switching point. 1.3.3 General considerations Two particular aspec
45、ts of the diversity interconnection links (DIL) are important: the contribution to the overall system noise budgets, and the contribution to system outage. These subjects are studied here to develop the effects of the important parameters and the interrelationship with the satellite link parts of th
46、e system. The diversity link design can be made on two bases. If a re-modulation system is selected, then conventional radio-relay designs can be used. If a translation system is selected, then the design can follow a different pattern and will be very similar to the satellite system transmission de
47、sign. Fading margins and noise contributions must be accounted for in overall performance. In the special case where the same frequencies are used for the DIL as for the satellite system, then interference noise allowances must also be made. 1.3.3.1 Noise budgets for FDM-FM The contributions of the
48、DIL to the overall noise of the hypothetical reference circuit have to be made reasonably small in order to maintain the system performance in accordance with Recommendation ITU-R S.353. Rec. ITU-R S.1061-1 7 It seems reasonable to assume that the DIL noise contribution would be considered as part o
49、f the earth station budget (usually 1 500 pW0p), as the DIL actually provides part of the normal earth station function. It only needs to be determined that such a contribution can be kept sufficiently small so that the total of 1 500 pW0p is not exceeded. The fading of the DIL will contribute to the overall short-term noise budget of the link. The noise contribution from the DIL would have a number of components depending upon the implementation configuration and the frequency bands used. The components are: a) Thermal noise Conventional ITU-R designs for radio-relay are 1 to
