1、CCIR RECMN*30 72 W LI855212 051B679 LT9 M Rec. 730 77 RECOMMENDATION 730 COMPENSATION OF THE EFFECTS OF SWITCHING DISCONTINUITIES FOR VOICE BAND DATA AND OF DOPPLER FREQUENCY-SHIFTS IN THE FMED-SATELLlTE SERVICE (Question 7/4) (1992) The CCIR, considering a) that circular non-geostationary satellite
2、 systems are subject to Doppler frequency-shifts influencing the radio- frequency stability and to differential Doppler frequency-shifts resulting in baseband frequencies “stretch” or “shrinkage”, for analogue interconnections and in frame siips for digital interconnections, respectively; b) transmi
3、ssion path time delay of less than 3 ms; cl at the other end should not exceed 2 Hz according to CCIT Recommendation G.225; that the orbital perturbations of the geostationary satellites may result in a cyclical variation of the that the difference between an audio frequency applied to one end of th
4、e circuit and the frequency received 4 systems connected; that the maximum allowed frame slip performance of the digital interconnections depends on the network e that the hand-over operation of circular non-geostationary satellite systems will cause switching discontinuities due to the different pr
5、opagation paths via the two satellites, recommends 1, revolutions per day of the satellite relative to the Earth exceeds 0.666; that differential Doppler shift correction is necessary for analogue telephony transmission via a non- , geostationary satellite in the fixed-satellite service if the produ
6、ct(s) of the baseband (MHi) times the number of the 2, that compensation for switching discontinuities is necessary for time differences greater than 20 ms for analogue telephony transmission and that therefore suitable variable time-delay devices should be implemented for such a compensation; 3, th
7、at elastic buffer stores should be applied for the differential Doppler-shift correction of digital satellite links on either the receive or transmit side, respectively, where the buffer dimension depends on the orbit parcameters, the data rate and the maximum allowed frame siips; 4. by consideratio
8、ns which allow for the complete removal of the transmission delay variations; that for the digital interconnection of national synchronous networks the buffer store should be dimensioned that the foilowing Notes should be regarded as part of this Recommendation. I Note I - The transmission of data a
9、nd telefax in an analogue telephone channel compensated according to Q 1 and thus fulfilling the CCIT Recommendation G.225 requires no further provisions for the compensation of the differential Doppler shift. Note 2 - Data transmission requiring wider bandwidth than a single telephone channel needs
10、 correction of the differential Doppler shift at a smaiier value of s as in the case of Q 1. Note 3 - The transmission of analogue television signals in practical systems in the fixed-satellite service is not adversely effected by differential Doppler shift or switching discontinuities due to the pr
11、opagation path difference and CCIR RECMN8730 92 E 4855212 0518700 740 E 78 Rec. 730 Note 4 - For the transmission of data the occurrence of switching discontinuities may be subject to the systems error correction technique. Note 5 - For the compensation of the differentid Doppler shifts as mentioned
12、 in Q 1, variable time-delay devices as for Q 2 may be used. A common device used for both tasks may be applied. Note 6 - Recommends 1 applies for non-geostationary circular satellite orbits only. Other kinds of non-geostationary orbits such as highly inclined elliptical orbits (e.g. MOLNIYA or TUND
13、RA) need further study to provide for the future use of these systems. Note 7 - Switching discontinuities of 20 ms as considered in Q 2 are expected to be non-critical for present telefax group 3 systems, and cause no unacceptable degradations. However, future Recommendations of the CCIT on the qual
14、ity of service of telefax transmission may include dedicated information on maximum possible transmission interruptions which may then require further study. Note 8 - The analysis on which the recommends are based, is contained in Annexes 1 and 2. ANNEX 1 The effects of Doppler frequency-shifts and
15、switching discontinuities in the fixed-satellite service 1. Doppler frequency-shifts (applicable to non-geostationary satellites) The magnitude of the total Doppler frequency-shift between the terminals of a system in the fixed-satellite service depends upon the wavelengths used and the relative vel
16、ocities of the satellite with respect to the earth stations. The major component of the effect of the Doppler shift, i.e. the shift of the carrier or a reference-frequency of the transmission, can be removed in the receiver; however, it may be necessary also to compensate for the differential shift
17、across the radio-frequency spectrum of the signal that produces a frequency “stretch or “shrinkage” of the baseband signal. Depending upon the relative locations of the earth stations and the orbit, the Doppler shifts between transmitting earth station and satellite and also between satellite and re
18、ceiving earth station can either, add or subtract. If 5000 km is taken as a probable minimum orbital height for a communication satellite, then the “stretch” or “shrinkage” of the baseband signal will not exceed two parts in 105. In most practical cases, the orbital height will be greater and the Do
19、ppler shift would be considerably less than this, and in the particular case of the geostationary satellite, there would be no significant Doppler shift. The maximum value of the Doppler shift, resulting from transmission to, or from, a space station on a satellite in a circular orbit, can be estima
20、ted from the relationship: where: AF : Doppler frequency-shift F : operating frequency s : number of revolutions per day (24 hours) of the satellite with respect to a fixed point on the Earth. This relationship may also be used for calculating the maximum differential Doppler frequency-shift over a
21、frequency band. A few values of s for various circular equatorial orbit altitudes are provided below (Table 1) to facilitate the calculations for individual cases. CCIR RECMN*730 92 I 4855232 0518703 687 Period (h) Approximative altitude (km) Miniinion elevation of antenna: 5“ Maximum Doppler freque
22、ncy-shift at 6 GHz (kHz) Revolutions per day relative to the Earth, s 6 8 12 24 11 O00 14 O00 20 O00 36 O00 27.7 18.5 9.3 0.0 Rec. 730 TABLE 1 Altitude for circular equatorial orbits oan) 35 600 20 240 13 940 10 390 8 O80 6 420 5 170 4 190 79 Period (h) 24 12 8 6 4.8 4 3.4 3 In a frequency-division
23、multiple-access (FDMA) system, each participating station uses a portion of the frequency band of the satellite repeater. Since the transmissions from each station are independent in time, there is no adverse effect from any relative time-shift. There will, however, be a Doppler frequency-shift in t
24、he transmission from each station which varies with time. For satellites employed to relay signals simultaneously from a number of earth stations, special consideration of Doppler shift may be necessary. Table 2 shows the maximum possible Doppler frequency-shifts at the satellite at 6 GHz. The figur
25、es are based on equatorial orbits and assume that the satellite moves in the same direction as the surface of the Earth. Because of various perturbing forces, the position of a geostationary satellite varies. If the satellite position is maintained within f 0.1“ of longitude and latitude, the maximu
26、m relative velocity of a satellite with respect to an earth station is less than approximately 3.8 ds, and the maximum Doppler frequency-shift will not exceed 76 Hz at 6 GHz. To prevent interference between adjacent radio-frequency channels caused by Doppler frequency-shifts, guard bands can be used
27、. Depending on the location of the stations, the signal transmitted bjr one station may be shifted upward, while that from a station on an adjacent channel may be shifted downward. Alternatively, the frequency-shifts may be corrected by available techniques. For example, allowing a guard band equal
28、to the maximum possible Doppler frequency-shift shown in Table 2 for a ten-channel system, the total guard bands would then be 18 times the figures shown (at 6 GHz). TABLE 2 Maximum Doppler frequency-shift CCIR RECNNU73O 80 In the time-division multiple-access 92 4855212 0518702 513 Rec. 730 (TDMA)
29、system, all earth stations transmit on the same nominal carrier frequency. This requires that the transmitter carrier be on- only during that interval of the frame assigned to the station. During transmission, the carrier would probably be modulated by phase-shift keying or frequency-shift keying. B
30、ecause of the Doppler frequency-shift, transmissions will arrive at the satellite and be repeated at frequencies which vary above and below the nominal carrier frequency. To accommodate this shift, the earth-station receivers must be capable of adjusting to the sudden changes in carrier-frequency wh
31、ich will occur. This may impose the requirement for increased interburst guard time and for more time within the burst to be allowed for carrier recovery and burst synchronization for satellites in a non-geostationary orbit. 1.1 Telephony When frequency-division multiplex telephony is used, it is ne
32、cessaq to limit the bandwidth or the apparent geocentric angular velocity of the satellite to prevent unacceptable differential Doppler frequency-shifts (unless corrections are applied to compensate for the Doppler effects). According to CCIT Recommendation G.225, the difference between an audio-fre
33、quency applied to one end of the circuit and the frequency received at the other end should not exceed 2 Hz. The question of error in reconstituted frequency is still under study in CCITT Study Group XV. It may be noted that an error of 2 Hz is not exceeded in a single satellite link, if the product
34、(s) of the baseband (MHz) times the number of revolutions per day of the satellite relative to the Earth does not exceed 0.666; however, additional error is likely to be introduced by the multiplex equipment. Doppler effects will also shift the pilot frequencies used in FDM telephony for satellites
35、with such angular velocities. Possible methods which could be used for correction of these shifts are: - a suitable variable time-delay device; - the carrier-frequencies used in the frequency-division multiplex equipment could be automatically controlled to compensate for the effects of Doppler shif
36、t and so reduce the overall frequency errors to acceptably small values. The first of these methods has the advantage of effectively cancelling the errors resulting from the movement of the satellite, in a manner similar to that in which they are introduced (Le. by change in transmission delay durin
37、g the pass). This method would, therefore, also eliminate all the effects of Doppler shift on the baseband signais and by suitable arrangements, would avoid switching discontinuities when transferring the information flow from one satellite to the next in the orbital pattern. Control of the variable
38、 delay could be performed, either by using predicted orbit infoimation or on a servo basis employing a pilot signal transmitted from the earth station to the satellite and back to the same earth station (loop method). The loop method has the following advantages: - it would ensure that only the corr
39、ect frequencies were received at the satellite. This facility could be of particular importance for certain systems, for example, those using closely-spaced channels or blocks of channels with single-sideband modulation in the Earth-to-satellite direction; - Doppler frequency “stretch” might to some
40、 extent be obviated, e.g. by splitting the receiving bandwidth into appropriately separated portions and providing independent compensations for the blocks of circuits arriving from each of the other earth stations. CCIR RECMN*730 92 4855212 0518703 45T 9 Rec. 730 81 Alternatively, compensation for
41、the variable delays could be applied only at the receiving end and controlled by pilot signals originating at the distant stations. In this case, the Doppler frequency “stretch” or “contraction” of the baseband would need to be accommodated by adaptations of the frequency-division multiplex equipmen
42、t at each earth station. Administrations are requested to submit to the CCIT their recommendations, or findings concerning such adaptations involving control of the earth station frequency-division multiplex equipment, either on a loop basis, as is described under the fust method above, or on a rout
43、e-by-route basis. Doppler-shift correction may be necessary in any system in the fured-satellite service using single-sideband amplitude modulation. 1.2 Telegraphy and data transmissions If telephone channels comply with the requirement of CCIIT Recommendation G.225 this implies that, for telegraph
44、and data channels derived from such telephone channels, the effect of Doppler frequency-shift may be ignored or has been adequately compensated for (see 0 1.1). 1.3 Phototelegraphy If phototelegraphy channels are derived from telephone channels complying with the requirement of CCIT Recommendation G
45、.225, the effect of Doppler frequency-shift may be ignored as being adequately compensated for. 1.4 Wideband data It should be noted that Doppler correction would need to be provided for carrier-derived phototelegraphy or data channels requiring wider bandwidths than a single telephone channel (e.g.
46、 group or supergroup bandwidths). 1.5 Television The change in field frequency introduced by Doppler frequency-shift is very small. In normal monochrome television practice, the accuracy of the field frequency at the programme source is likely to be the limiting factor as far as disturbance to domes
47、tic receivers is concerned and Doppler shift wiii not be of concern. It may ultimately be desirable to correct for the effects of Doppler shift on colour television signals, but the initial tests made with the satellites have demonstrated that standard colour receivers and, in particular, those usin
48、g crystal controlled sub-carrier oscillators, will operate satisfactorily, with the order of Doppler frequency-shift likely to be encountered in a practical system in the fixed-satellite service. 2. Switching discontinuities (applicable to non-geostationary satellites) Satellites which rise and set
49、can be used by any two or more earth stations only while mutually visible. These stations must then switch or “hand-over” to another mutually visible satellite, to maintain communication with some orbit systems, or with excessively separated earth stations; relatively long interruptions may occur when mutual visibility of the first satellite is lost before another satellite has been acquired. Such interruptions can be avoided by the use of controlled, equally separated satellites of sufficient number in orbits having a recurrent earth-track. Such satellite orbit systems are often r
