1、50 Rec. ITU-R S.353-8 RECOMMENDATION ITU-R S.353-8 ALLOWABLE NOISE POWER IN THE HYPOTHETICAL REFERENCE CIRCUIT FOR FREQUENCY-DIVISION MULTIPLEX TELEPHONY IN THE FIXED-SATELLITE SERVICE (Question ITU-R 27/4) (1 963-1966-1970-1978-1982-1986-1990-1992-1994) The ITU Radiocommunication Assembly, consider
2、ing a) that the hypothetical reference circuit is intended as a guide to the design and construction of actual systems; b) overall signal-to-noise performance requirements; that the cost of establishing and maintaining systems in the fixed-satellite service is critically dependent on the c) conversa
3、tion in most telephone calls or the transmission of telephone signalling; that the total noise power in the hypothetical reference circuit should not be such as would affect appreciably d) that it may be necessary to take account of fading due to meteorological events, particularly rain; e) that spe
4、cifications of circuit availability are contained in Recommendation ITU-R S.579; f) that there may be other sources of noise of short duration; s) the information presented in Annexes 1 and 2, recommends 1. circuit as defined in Recommendation ITU-R S.352 should not exceed the provisional values giv
5、en below: that the noise power, at a point of zero relative level in any telephone channel in the hypothetical reference 1. I 10 O00 pW0p psophometrically-weighted one-minute mean power for more than 20% of any month; 1.2 50000 pWOp psophometrically-weighted one-minute mean power for more than 0.3%
6、of any month; 1.3 1 O00000 pW0 unweighted (with an integrating time of 5 ms), for more than 0.01% of any year; 2. that the following Notes should be regarded as part of the Recommendation. Note I - Noise in the multiplex equipment is excluded from the above. For frequency-division multiplex telephon
7、y, noise additional to the above values is introduced by the equipment necessary to translate the satellite baseband to and from the multiplex level required for interconnection to a terrestrial link. In considering the performance of an overall connection, the noise introduced by such equipment sho
8、uld be added to the values given in 9 1. The noise allowed by the ITU-T for multiplex equipment is given in ITU-T Recommendation G.222, 8 4. Note 2 - It is assumed that noise surges and clicks from power supply systems and from switching apparatus (including switching from satellite-to-satellite) ar
9、e reduced to negligible proportions and therefore will not be taken into account when calculating the noise power. Rec. ITU-R S.353-8 51 Note 3 - In applying the hypothetical reference circuit and the allowable circuit noise to the design of satellite and earth-station equipment for a given overall
10、signai-to-noise performance, the system characteristics preferred by the ITU-R, as found in its Recommendations, should be used where appropriate; where more than one value is recommended, the designer should indicate the value chosen; in the absence of preferred values, the designer should indicate
11、 the assumptions used. Note 4 - For frequency-division multiplex telephony, it will be assumed that, during the busy hour, the baseband signal can be represented by a uniform-spectrum signal, the mean absolute power-level of which, at a point of zero relative level is equal to -15 + 10 log N (dBm) f
12、or 240 channels or more, and -1 3. 4 log N (dBm)* for number of channels between 12 and 240, N being the number of channels. These formulae apply only to baseband signals without pre-emphasis and using independent amplifiers or repeaters for the two directions of transmission. Further information on
13、 the conventional load, in particular in the case of a repeater which is common to both directions of transmission, is given in ITU-T Recommendation G.223. * It is considered that these formulae give a good approximation in calculating intermodulation noise when N 2 60. For small numbers of channels
14、, however, tests with uniform-spectrum random noise are less realistic, due to the wide difference in the nature of actual and test signals. Note 5 - Compandors are sometimes used in achieving a performance considered equivalent to that of 9 1.1 with typical compandor gain values of approximately 10
15、 dB for speech-like signais. Note 6 -The noise power indicated in 5 1 above should include interference noise (see Recommendations ITU-R SF.356 andITU-R S.466) and noise resulting from atmospheric absorption and increased noise temperature due to rain. In certain cases, such as extremely long links
16、and low elevation angles where margins must be higher, additional noise may cause limits of the general objectives to be slightly exceeded. This should not cause serious concern, provided that the provisions of ITU-T Recommendation G.222, 9 2.6, are met. Note 7 - The value given in 0 1.3 on occasion
17、 may be exceeded due to solar interference in the beam of the antenna but this noise is assumed to result in unavailability of the circuit. Detailed information is given in Appendix 2 of Recommendation ITU-R 5.733. Note 8 - The objectives set out in this Recommendation are performance objectives, as
18、 distinct from availability objectives. Note 9 - It may be necessary to make special provision regarding the performance of inter-satellite links. The extent of this provision is a matter requiring further study. Note 10 - Short interruptions (less than 10 s) shall be treated as equivalent to the ca
19、se where noise power of a circuit is more than lo6 pW0 unweighted. Note 11 - It is desirable that systems be planned on the basis of propagation data covering a period of at least four years. The performance recommended to be met in “any year” should be based on the cumulative propagation statistics
20、 for all the complete years for which reliable data are available. The performance recommended to be met for “any month” should be based on the propagation data corresponding to the “worst month of the year taken from the monthly statistics for all the years for which reliable data are available”. T
21、he “worst month” should be calculated in accordance with Recommendation ITU-R PN.58 1. Note 12 - This Recommendation applies only when the system is considered available in accordance with Recommendation ITU-R S.579. Note 23 - In order to comply with the values given in $ 1, for systems operating ab
22、ove 10 GHz, it may be advantageous to make use of fade countermeasures strategies and techniques for which basic guidance is provided in Recommenda- tion ITU-R S.1061. 52 Rec. ITU-R S.353-8 ANNEX 1 Form of the hypothetical reference circuit and allowable noise standards for frequency-division multip
23、lex telephony 1. Form of the hypothetical reference circuit The concept of the hypothetical reference circuit (HRC) has been used by the ITU-T in developing the requirements for making international connections for telephony and television and has been typically associated with terrestrial systems.
24、This topic is discussed in ITU-T Recommendation G.222 along with the associated standards of noise performance. The HRC applicable to the fixed-satellite service (FSS) is given in Recommendation ITU-R S.352 which includes provisions for the possibility of the use of diversity at the higher frequency
25、 bands where rain attenuation is a factor in the performance. The HRC is applicable to all types of analogue transmissions and generally comprises a single, geostationary- orbit satellite link although a satellite-to-satellite link can be used. With respect to television, the definition and characte
26、ristics of reference chains comprising one or more hypothetical reference circuits and corresponding to different services, have been studied by the ex-CMTT and are reflected in Recommendations ITU-R S.354 and ITU-R CMTT.567. 2. Allowable noise standards 2. z General considerations The noise standar
27、ds for international connections for analogue telephony services are established by the ITU-T taking into account the need to provide a minimum quality for the longest connections considered reasonable and, from this minimum quality for the end-to-end connection, an allowance for each portion of the
28、 circuit can be determined. The current status of this work is that specifications exist for the basic HRC which apply to terrestrial systems, the length being 2500 km and the total noise allowance being loo00 pWOp including the multiplex. This specification was made in 1963 and it seemed appropriat
29、e at the time to consider the satellite link as the equivalent of a single HRC with a proviso that multiplex noise was excluded. This latter provision accounts for the fact that only a single multiplexing operation is a normal part of a satellite link. Four such international connections were deemed
30、 to make up the longest connections. Hence it was implicit that this part of a connection could have a total noise of 40000 pWOp. Additionally, the local part of the circuit could be considered to add another 10000 pWOp for an end-to-end total of 50000 pWOp or -43 dBmOp. This level of noise has gene
31、rally been accepted as the level where user difficulties begin to be of concern even though noise of twice this level is found to be acceptable by as many as half the users. 2.2 Allowable noise in the HRC: frequency-division multiplex telephony 2.2.1 Factors influencing the allowable noise The total
32、 noise of a typical HRC of 10000 pWOp for terrestrial links is translated into a distance dependant factor of 3 pWOp/km, after making an allowance of 2 500 pWOp for the multiplex and the specified length of 2 500 km. The total of 4 such HRCs providing an international connection of 10000 km was prob
33、ably reasonable in 1963, however, such a length is not consistent with present practice. For example, the ISDN hypothetical reference connection is 27500 km and ISDN performance is based on that concept. At the same time, the practice of system designers of terrestrial systems has long been to use a
34、n objective of 1 pWOp/km instead of 3 pWOp/km. This objective allows for twice as many HRCs in a connection without exceeding the 40000 pWOp limit for the long haul portion of the connection. This assumes that the multiplex contribution is maintained at 2500 pWOp for each HRC. The effect is to incre
35、ase the length of a connection, which meets the overall requirement, to over 20000 km, which is more representative of modern communications. ITU-R RECMN*S- 353-8 74 = 4855232 0522174 779 Rec. ITU-R S.353-8 53 The use of satellites, with their distance independent performance, and their ability to p
36、rovide connections of as much as 17 O00 km with a single link, makes it clear that such links having the noise level corresponding to a single HRC will provide for improved end-to-end performance in many cases. The satellite HRC has a short-term noise limit that would also have to be adjusted so tha
37、t margins would be still reasonable. This is quite feasible to do for the 6/4 GHz bands where the actual margin requirements are small (of the order of 3 dB or less). For this case then, the current short term allowance of 50000 pWOp for 0.3% of the month is easily met as would be the O. 1 % of the
38、month specified in ITU-T Recommendation G.222. For the 14/11 GHz bands, this is not the case and the short-term value will be affected depending upon the margin used. For example, on a link designed to a long term value of IOOOO pWOp with 10 dB of margin, the short term noise could reach a level of
39、1OO pWOp, which, while still a usable circuit for many users, exceeds the current limits. The ITU-T itself would prefer this value for the short term allowance but it would be too difficult to measure, and therefore adopted the 50000 pWOp number. The subject of increased noise is also discussed in A
40、nnex 2 from a slightly different point of view; however, the same general conclusions are reached. Finally, still in the context of the 14/11 GHz bands, the results of an analysis are provided in the following, which highlights the impact of introducing the concept of availability as far as performa
41、nce compliance with ITU-T Recommendation G.222 is concerned. A standard link concept is used for the analysis to correspond to the current practice of other terrestrial systems of allowing 1 pWOp/km for design, or a link of 10000 km. The operational locations for such links are typically at 40“ lati
42、tude and 25“ elevation angle. The climates for these latitudes exhibit rain rates, for 0.01% of the time, between 30 and 60 mdh. A value of 50 mm/h is chosen for the analysis. Calculations of the rain attenuation are made in accordante with the methods of Radiocommunication Study Group 3 (ex SG 5).
43、The loo00 pWOp requirement for 20% of any month is interpreted as applying to the worst month .e. for the poorest propagation month. The definition of the worst month is provided in Recommendation ITU-R PN.581. The same interpretation is applied to the 50000 pWOp clause. Propagation availability fac
44、tor (as defined in Annex 1 to Recommendation ITU-R S.579) is taken as 10% of the duration of fade which results in reaching the system threshold. Two cases are shown in Fig. 1, one at 50 O00 pWOp and one at 100000 pWOp. The margin in the first case is 7 dB and is 10 dB for the second. The performanc
45、e for the path expressed in terms of the available time will meet all of the ITU-T Recommenda- tion G.222 performance objectives for the climate and latitudes assumed in this study. For low antenna elevation angles and higher rain rates, it may be more difficult to meet ITU-T Recommendation G.222. 2
46、.2.2 The concept of satellite equivalent distance For the purposes of ISDN, the distance independence of satellite links was recognized by the use of a satellite equivalent distance to permit the use of the same performance degradationkm applied to other transmission systems. In the long distance, h
47、igh quality portion of the HRX for ISDN, half of the noise allocation was assigned to satellite HRDPs which can be translated to an equivalent distance of 12500 km. Although application of this concept to the analogue case could result in an increased allocation of noise for a satellite HRC without
48、violating the overall end-to-end performance of connections made using a satellite link, there are many situations using a satellite link where better performance is desirable. 2.2.3 Propagation availability Recommendation ITU-R S.579 covers the subject of availability of the HRC and includes all ou
49、tages which persist for longer than IO s. In particular, propagation fades which are the major source of degradation to satellite circuits, have been studied to establish a relationship between fading level and the duration. For the fade levels which are typical for satellite system designs, a value of 0.1 has been determined to be appropriate for the ratio of available to unavailable time for propagation fades. This factor has more impact at the higher frequencies. Rec. ITU-R S.353-8 FIGURE 1 14/11 GHz noise performance as a function of available time I I , . 2 I= 7dB I I= 10dB I I I /Il
