1、CCIR VOLUME*IV-3 90 W 4855232 - 0509885 b = Rec. 481-2 29 RECOMMENDATION 481-2 MEASUREMENT OF NOISE IN ACTUAL TRAFFIC FOR *SYSTEMS IN THE FIXED-SATELLITE SERVICE FOR TELEPHONY USING FREQUENCY-DIVISION MULTIPLEX (Question 20/4) (1974-1978-1986) The CCIR, CONSIDERING (a) that measurements by means of
2、a generator producing white noise (Recommendation 482) are only possible when the radio-frequency channel is not carrying traffic; (6) that systems carrying multi-channel telephony cannot be withdrawn from service for measurement at will; (c) that protection channels, similar to those used in terres
3、trial radio-relay systems, are not available for maintenance purposes; (d) that maintenance measurements of the total noise (thermal and intermodulation noise) are useful for determining the quality of a system and must be made while the system is carrying traffic; (e) that it is convenient to place
4、 the channels used for this kind of measurement outside the total bandwidth of the multiplex signal; (f) that, when these measuring channels are located outside the total multiplex signal band, they should be positioned as near the limits of the total signal band as possible, to measure the intermod
5、ulation products due to the non-linearity of the system; (g) that, on the other hand, to facilitate and to minimize the cost of filter construction, the measuring channels should not be positioned too near these limits; (h) that measurements in channels about 10% above the upper limit of the totai m
6、ultiplex signal band are generally sensitive to changes of thermal and intermodulation noise in the radio-frequency and intermediate- frequency circuits of the equipment; (j) that it is usually necessary to use band-stop filters at the input of the system to minimize noise on the incoming circuit in
7、 the bands occupied by the measuring channels, and that it will be necessary to specify the minimum performance of these filters, both in the stopband of these filters and at the edges of the total multiplex signal band, UNANIMOUSLY RECOMMENDS 1. measured at the output of the system in relatively na
8、rrow bands situated above the total multiplex signal band; that noise occurring in links in the fixed-satellite service while actual traffic is being carried should be 2. that the centre frequencies of these measuring bands should be those listed in Table I; 3. that the attenuation of the .band-stop
9、 filters at the input of the system should exceed 50 dB over a minimum frequency band of f (0.005 f + 2) kHz (f being the centre frequency in kHz of the measuring channel). The additional attenuation caused by the insertion of the band-stop filters at the upper edge of the total multiplex signal ban
10、d shall not exceed 0.3 dB referred to the additional attenuation caused in the centre of the multiplex signal band; 4. use with the input band-stop filter mentioned above; that the effective bandwidth of the bandpass filters in the receiving equipment should be small enough for 5. that, in all cases
11、 where different frequency bands are used, or where there are differences between the measurement techniques, special agreements should be made between the administrations which use the same space systems of the fixed-satellite service. - - lb CCIR VOLUME*IV-L 90 4855212 0509886 Rw. 481-2 . TABLE I
12、System -capacity (number of channels) 12 24 36 48 60 72 96 132 192 252 312 372 432 492 552 612 792 972. 1092 1200 1332 1 872 .Limits of band occupied by telephone channels (kHz) 12- 60 1% 108 12- 156 12- 204 12- 252 12- 300 12- 408 12- 552 12- 804 12-1 052 12-1 300 12-1 548 12-1 796 12-2 o44 12-2 292 12-2 540 12-3 284 12-4 028 12-4 892 12-5 340 12-5 884 12-8 120 Centre frequencies, f of noise-measuring channels (kHz) 66 116 172 224 277 33 1 448 607 884 1157 1499 1730 1 976 2 248 2 438 2 794 3 612 4 430 5 381 5 874 6 300 8 932
