1、1997 9 q8552L2 0530287 321 9 Rec. ITU-R S.4833 1 RECOMMENDATION ITU-R S.483-3* MAXIMUM PERMISSIBLE LEVEL OF INTERFERENCE IN A TELEVISION SATELLITE SERVICE EMPLOYING FREQUENCY MODULATION, CAUSED BY OTHER NETWORKS OF THIS SERVICE (Question IT-R 48/4) CHANNEL OF A GEOSTATIONARY-SATELLITE NETWORK IN THE
2、 FIXED- ( 1974-1 978- 1992- 1997) The ITU Radiocommunication Assembly, considering a) that TV/FM carriers are sensitive to interference from other fwed-satellite service (FSS) carriers; b) that in particular, W/FM carriers are sensitive to interference from other W/FM carriers; c) that interference
3、between networks in the FSS contributes to the noise in the system; d) that it is desirable that the interference noise in television channels of networks in the FSS caused by transmitters of different networks of this service should be such as to give a reasonable orbit utilization efficiency; e) t
4、hat the overall performance of a network should essentially be under the control of the system designer; 0 that it is necessary to protect a network in the FSS from interference by other networks of this service; g) that it is necessary to specifi the maximum permissible interference power in a tele
5、vision channel, in order to determine space station and earth station characteristics such as required protection ratios and minimum orbital spacing; h) that protection ratio masks are useful to determine minimum orbital spacing and in coordination exercises; j) that for direct to home (DTH) recepti
6、on quality grade 4, as defined in Recommendation ITU-R BT.500, is usually adequate, recommends 1 that different geostationary-satellite networks, in the FSS operating in the same frequency bands, be designed in such a manner that the interference noise power in a hypothetical reference circuit for t
7、elevision of a network in the FSS employing frequency modulation caused by the aggregate of the earth station and space station transmitters of other networks, should not exceed 15% of the permissible video noise in the hypothetical reference circuit for more than 1% of any month; 2 that the maximum
8、 level of interference noise power caused by any one satellite network into another satellite network should not exceed 4/10 of the interference noise allowance recommended in Q 1 but in some cases it may be necessary to limit the single entry value to less than 4/10 of the interference noise allowa
9、nce quoted above; 3 that the protection ratio masks in Annex 2 (Q 8) be used in cases where the interferer is another TVFM carrier (see Note 6); 4 that the maximum level of interference noise power caused to that network should be calculated on the basis of the following values for the receiving ear
10、th station antenna gain in a direction at an angle cp (degrees) referred to the main beam direction: G = 32 - 25 log cp dBi for 1“ I cp 48“ G = -10 dB i for 48 I cp I 180“ except when the actual gain is known and is less than the above value, in which case the actual value should be used; * This Rec
11、ommendation should be brought to the attention of Radiocommunication Working Party 10-1 1s. 2 STD-ITU-R RECMN S-483-3-ENGL L977 D 4855232 0530288 2b8 Rec. ITU-R S.4833 5 NOTE 1 -The above values of interference noise shall be included II the total noise allowances as defined in Recommendation ITU-R
12、S.354. NOTE 2 - In segments of the geostationary-satellite orbit not likely to be crowded, interference allowances less than those recommended above may be utilized, allowing a corresponding increase in other noise contributions within total acceptable noise limits. NOTE 3 - Particularly in cases wh
13、ere interference is caused by transmitters using code-division-multiple-access techniques, the interference from another fured-satellite network referred to in 0 2 is the composite interference hm all transmissions having overlapping spectra in that network. NOTE 4 - The methods referred to in Recom
14、mendation ITU-R S.741 and other relevant ITU-R texts can be used to calculate the carrier-to-interference ratio. NOTE 5 - Annexes 1 and 2 contain further information relevant to this Recommendation. NOTE 6 - The masks provided in Annex 2 do not take into account the TV audio requirements of certain
15、systems. When there are such requirements, the protection ratio masks may be replaced with other masks specific for such requirements, if necessary. This needs to be further studied. that the following Notes should be regarded as part of the Recommendation: ANNEX 1 1 Multiple carrier interference As
16、 TV is generally a wideband transmission, there will be cases where the interference will consist of a number of narrower bandwidth carriers. In such cases the analysis and calculation should take account of the total power of the interfering carriers that are contained within the TV bandwidth to de
17、termine the CIL 2 TV sound Television sound can be transmitted along with the TV carrier using a subcarrier frequency modulated by the programme signal and multiplexed with the composite video signal with the subcarrier placed above the TV baseband or using sound-in-sync techniques (SIS). A single,
18、high quality 15 kHz audio channel is commonly provided. For techniques of transmitting TV sound on a separate carrier, such as SCPC, there is no specific criterion in ITU-R texts and study is needed. ANNEX 2 Interference between TV/FM signals 1 Introduction Interference between TV/FM signals is of s
19、pecial importance because the impairments introduced by a TV/FM interferer have a different subjective effect on a TV picture than the effect caused by thermal noise. This Annex provides a method to evaluate the impairments caused by thermal noise and TVEM interference and the derivation of the prot
20、ection ratio masks. STD-ITU-R RECMN S-483-3-ENGL 2777 D 4855232 0530287 LT4 D Rec. ITU-R S.483-3 3 2 The impact of various types of impairments on the picture quality is usually determined using subjective assessments of a number of viewers, mostly non-experts. The methodology for the assessments sp
21、ecifjing the viewing conditions, grading scales and other details is recommended by the IT-R in Recommendation ITU-R BT.500. A five-grade scale is usually used to describe the quality and level of impairment of the TV picture. Subjective assessment of TV picture quality 3 The results of extensive as
22、sessments of impairments in a NTSC signal introduced by thermal noise lead to the following expressions, which approximate the impairments introduced by thermal noise: Impairments due to thermal noise Irh = exp 30.9 - 8.41 ln(S/Nw) In equation (i), Z, denotes the impairments caused by thermal noise
23、and is related to the impairment grade via a general equation: I = (5 - (I)/ - 1) where: Q : impairment grade on the five-grade scale SIN, : weighted (unified) signal-to-noise ratio in (dB). Expression (3) also provides a very good fit of the mean opinion scores as shown in Fig. 1 : 4 Impairments du
24、e to co-channel interference caused by another TV/FM carrier Many studies support the following conclusions in respect to interference from another TVRM carrier: - the impact of co-channel interference on the picture quality is more significant than the impact of the same amount of thermal noise; th
25、e subjective effects of the impairments caused by the co-channel interference are dependent on the desired TV picture content; the picture content of the interfering TV signal seems to play a less significant role; the deviation of the desired signal is an important factor. - - Expression (4) has be
26、en suggested earlier to relate the impairments introduced into a TV picture and the protection ratio between two co-channel NTSC TVRM signals: PRO = 16.9 - 8.7 log Iint - 20 log (0/12) where: (4) PRO : protection ratio lint : D : impairment introduced by co-channel interference peak-to-peak deviatio
27、n in (MHz) of the wanted TV signal Equation (2) relates the impairments due to interference with the corresponding impairment grade Q. Comparing the impairments introduced by thermal noise and interference, an important observation can be made; the impairments caused by co-channel interference corre
28、spond to the impairments caused by the thermal noise whose level is about 6 dB higher than the interference noise. This observation suggests the use of equation (3) adjusted for the 6 dB higher impact in modelling the impairments due to co-channel interference. Applying this approach to the example
29、in Fig. 2, the relation between the protection ratio and the impairments due to co-channel interference with an adjustment factor of 6.2 dB is given by equation (5). For this example, SIN, = C/N + 30 (dB). Then, SINinl = C/Z + 30 - 6.2 = C/Z + 23.8 =PR0 + 23.8 (dB) and equation (5) follows: lint = 1
30、(pR0 + 23W5 / 0.02712.2 (5) As shown in Fig. 3, this approach better describes the mean opinion scores for co-channel interference, especially at lower grades, than equation (4). 4 STD-ITU-R RECMN S.LI83-3-ENGL 1777 4855212 0530290 71b Rec. ITU-R S.483-3 FIGURE 1 Impairments due to thermal noise 5 4
31、 3 2 1 35 Measured: O GroupA A GroupB GroupC GroupD Mean (A-D) 40 45 50 Weighted (unified) signai-to-noise ratio (dB) 0483.101. STD-ITU-R RECMN S.483-3-ENGL 1997 4855232 0530293 Rec. ITU-R S.483-3 FIGURE 2 Impairments due to co-channel interference 10 15 20 25 30 Co-channel protection ratio (dB) SIN
32、,= CIN + 30 (dB) = 56 dB Deviation: 9.5 MHz p-p Measured: GroupA , GroupB GroupC . GroupD 852 m 5 I 1 Fit 1 _- 6 STDmITU-R RECMN S.483-3-ENGL 1997 D 4855232 0530292 - - Rec. ITU-R S.483-3 FIGURE 3 Impairments due to co-channel interference and thermal noise 10 15 20 25 Co-channel protection ratio (d
33、B) Mean scores for SINw, = CIN + 30 (dB) = 42 dB Deviation: 9.5 MHz p-p Measured: GroupA GroupB GroupC . GroupD Fit 1 _ Addition of noise with 6.2 dB weighting for interference noise Fit 2 _._._._._._._ Addition of impairments 5 Multiple Co-channel interferers 30 Studies have shown that the impact o
34、f three equal level Co-channel interferers is equivalent to the impact of a single interferer having a 3 to 5 dl3 higher power. These results suggest that the power addition law corresponds to the worst case. Rec. ITU-R S.483-3 7 6 When both thermal noise and co-channel interference are present in a
35、 TV signal, the modelling of the impairments introduced in the picture is not straightforward. It is known that a simple addition of the interference power to the thermal noise power does not adequately model the resulting impairments. Another method based on the addition of impairments caused by th
36、ermal noise and co-channel interference provides a better approach, but it is not reliable for low impairment grades. A new method based on the summation of thermal noise and weighted co-channel interference noise, proposed here, appears to provide a reasonably good approximation of the mean opinion
37、 scores over the full range of the impairment grades. The proposed method is based on the observation made in 5 4 that the impairments caused by co-channel interference are approximately equivalent to the impairments caused by the thermal noise having a power of about 6 dB higher than the co-channel
38、 interference power. This suggests that the noise power present in the TV signal and the co-channel interference power increased by about 6 dB can be summed up, an equivalent SIN, can be found and the impairments introduced can then be determined by using equation (3). The method of summation of imp
39、airments is also given for comparison. Following the proposed method a set of curves for a range of SIN, from 42 to 56 dB and a range of protection ratios fiom 10 to 40 di3 is generated and shown in Fig. 4 for the NTSC TV standard with a peak-to-peak FM deviation of 9.5 MHz. Impairments due to aggre
40、gate thermal noise and co-channel interference FIGURE 4 Impairments due to thermal noise and co-channel interference o, 0.3” p 5 4.5 4 3.5 3 2.5 2 1.5 1 10 15 20 25 30 35 X= PRO + 20 log (DB.5) dB 40 Signal-to-thermal noise ratio S/NW,* (dB) PR, = X- 20 log (Dl9.5) PR,: protection ratio D: peak-to-p
41、eak deviation (vHz) 8 Rec. ITU-R S.4833 7 Impact of desired TV/FM signal deviation The deviation of the desired TV signal has a significant effect on the impact of the co-channel interference. Many studies have shown that a “20 log” law can be applied. In other words, if the impact of a co-channel i
42、nterferer at a protection ratio PRO into a TV signal with a deviation DO is determined, then the same interferer will have the same impact into a TV signal whose deviation is D1 at a protection ratio PR1 = PR0 - 20 log (Dl/Do). Applying this, the results in Fig. 4 are generalized by denoting the X-a
43、xis as X= PR0 + 20 log (D/9.5) (dB). The effect of over deviation of the desired TV signal has not been modelled and needs to be further studied. 8 Development of protection masks All the preceding results were based on the assumption that the desired and the interfering signal are exactly co-channe
44、l. In practical applications, however, the signals are often frequency offset. Therefore, the masks providing the required protection ratios as a function of the frequency offset need to be developed. Such a mask can generally be defined by a protection ratio PRO in the flat pedestal around the zero
45、 frequency offset and the slopes for positive and negative frequency offsets. The previous sections provide a basis for determining the PRO (for NTSC TV standard). The width of the pedestal and the slopes are discussed in the following sections. 8.1 Protection ratio at zero offset (PRO) The curves i
46、n Fig. 4 can be used to determine the required protection ratio at zero frequency offset for any degradation of NTSC TV picture which is deemed permissible. For example, if a TV/FM signal with a signal-to-thermal noise ratio of 42 dB and a peak-to-peak deviation of 9.5 MHz is interfered with by anot
47、her TV/FM signal at zero frequency offset and the protection ratio is 25 dB, the combined impact of the thermal noise and interference will result in the impairment grade 3.4. If, however, the wanted signal peak-to-peak deviation is 19 MHz, the same interferer and thermal noise would result in the g
48、rade 3.5 (Fig. 4, X= 25 + 20 log (19/9.5) = 3 1 dB). 8.2 Width of the plateau and slopes of the mask Based on the available data, the width of the plateau seems to be 15 MHz. The slopes of the mask can be approximated by: S = K9.5/D - 0.295 log(D/9.5) (6) where D is peak-to-peak deviation of the des
49、ired TV signal (MHz) and K = 2.3 and 1.9 for the right and left slope, respectively. The above results have been derived from TV/FM signals using 2 MHZP- energy dispersal in the absence of the video signal and 1 MHz, in its presence. Further studies are needed to determine the impact of other energy dispersal bandwidths. The method presented above was used to generate the protection ratio masks for various frequency deviations shown in Fig. 5. 9 Permissible level of interference The permissible level of interference in a carrier
