1、CCIR RECMNa44b-4 93 m 4855232 O520933 856 m 32 Rec. ITU-R S.446-4 RECOMMENDATION ITU-R S.446-4* CARRIER ENERGY DISPERSAL FOR SYSTEMS EMPLOYING ANGLE MODULATION BY ANALOGUE SIGNALS OR DIGITAL MODULATION IN THE FIXED-SATELLITE SERVICE (1966- 1974- 1978- 1992- 1993) The ITU Radiocommunication Assembly,
2、 considering a) that use of carrier energy dispersai techniques in systems in the fixed-satellite service can result in a substantial reduction of interference to stations of a terrestrial service operating in the same frequency bands; b) that in many cases the use of such techniques can result in a
3、 moderate to substantial reduction in the level of interference between systems in the fixed-sateiite service operating in the same frequency bands, although in other cases the use of such techniques may not reduce the level of interference between such systems; cl that such techniques are being reg
4、ularly and successfully employed in systems in the fixed-satellite service without noticeable deterioration of the quality of operation; 4 that Recommendation No. 103, relating to canier energy dispersal in systems in the fixed-satellite service, was adopted by the World Administrative Radio Confere
5、nce for Space Telecommunications (Geneva, 1979); e) that performance evaluation of various techniques of FM-TV signal dispersion is given in Annex 1 to Recommendation ITU-R S.671*, recommends 1. that systems in the fixed-satellite service should use carrier energy dispersai techniques, as far as is
6、practicable and in a manner consistent with satisfactory operation of the systems, with a view to spreading energy such that the interference to stations of a terrestrial service operating in the same frequency bands is maintained within specified tolerable limits at all times; 2. that the capabilit
7、y for carrier energy dispersai up to the maximum degree practicable should be included in the design of satellite systems to be available for implementation when necessary to maintain a reduced level of interference between systems in the fixed-satellite service operating in the same frequency bands
8、; 3. Note I - Annex 1 describes various dispersion techniques for use with FM analogue and PSK digital signals which can be recommended for practical utilization. that the following Note should be regarded as part of this Recommendation. ANNEX 1 Energy dispersal in the fixed-satellite service 1. Int
9、roduction It is clear from studies of frequency sharing between the fixed-satellite service and terrestrial radio-relay systems and between different fixed-satellite networks that, to ensure that mutual interference between the systems is kept to a tolerable level, it will be essential in most cases
10、 to use energy dispersai techniques to reduce the spectral * New version of CCIR Recommendation 446. This Recommendation was developed rom work carried out under ex-CCIR Study Programme 27N4 which was suppressed according to Resolution 109 (Dsseldorf, 1990). * Former CCIR Recommendation 671. CCIR RE
11、CMN*YYb-4 93 4855232 8520732 772 W Rec. ITU-R S.446-4 33 energy density of the transmissions of the fixed-satellite service during periods of light loading. The reduction of the maximum energy density will also facilitate: efficient use of the geostationary-satellite orbit by minimizing the orbital
12、separation needed between satellites using the same frequency band; and multiple-carrier operation of broadband transponders. - - The amount of energy dispersal required obviously depends on the characteristics of the systems in each particular case. It is clear, however, that it is desirable that t
13、he maximum energy density under light loading conditions should be kept as close as possible to the value corresponding to the conditions of busy hour loading. In this Annex, the results of some theoretical and experimental studies of energy dispersal techniques, separately applicable to analogue fr
14、equency-modulation and to digital radiocommunication-satellite systems, are reported. It is concluded that substantial energy dispersal can be obtained in most circumstances. However, there are some possible limitations on the efficiency of the dispersai and these are mentioned in the Annex. 2. Ener
15、gy dispersal for analogue multi-channel telephony FM systems In studying ways of achieving high degrees of carrier energy dispersal, it is useful to know what is the dispersing effect of the fully-loaded baseband signai, in order to have some reference value with which to compare what can be obtaine
16、d artificially. It is legitimate, for the general class of wide-deviation frequency-modulation systems under consideration, i.e. those in which the multi-channel r.m.s. deviation (6F) exceeds the highest baseband frequency, and greatly exceeds the lowest baseband frequency, to assume that the mean p
17、ower spectrum under the conventional busy-hour loading conditions is of Gaussian form. Hence, the dispersing effect obtained under these conditions is: 6F is expressed in MHz) The dispersing effect when 6F is less than the highest baseband frequency can be calculated using the information contained
18、in Annex 1 of Recommendation ITU-R SF.675* . There are a number of methods of maintaining a high degree of carrier energy dispersal in telephony systems, where the obtained dispersal is a function of the complexity of the means of dispersal and the increase in occupied radio-frequency bandwidth resu
19、lting from distortion. The methods fall into one or other of two general cases; one which adds a dispersal waveform not necessarily of constant magnitude to the input signal and the second which, in addition, effectively controls the deviation sensitivity of the frequency modulator. Various arrangem
20、ents of these two me.thods are illustrated in Fig. 1. 2.1 Dispersal by added waveforms 2.1.2 Method la) The simplest way of bringing about some degree of carrier energy dispersal is to add to the baseband signal, a suitable low-frequency dispersing waveform of fixed magnitude, as in Method la) of Fi
21、g. 1. Of a variety of dispersal waveforms that have been proposed, the following are examined in this Annex: - - a sinusoidal signai (Curve A of Fig. 2), a sinusoidal signal plus 30% third harmonic added in suitable phase (Curve B of Fig. 2), * Former CCIR Recommendation 675 CCIR RECMN*446-4 93 W 48
22、55212 0520933 629 34 Rec. RU-R S.446-4 - a band of low-frequency noise (Curve C of Fig. 2), a low-frequency triangular waveform (Curve D of Fig. 2). - FIGURE 1 Simplified block diagram 4- ? 1 I F Method A = E-i- U 5 tG A: baseband signal input B: r.m.s. detector C: amplifier 1 D: amplifier 2 E: r.m.
23、s. detector F: output to frequency-modulator G: dispersal waveform Full load No control No control Gain O v, Gain 1 1 O 5 Full load (Possible filters, buffer-amplifiers and gain-regulating pilots omitted) To provide some basis for comparing the efficiencies of these waveforms, the maximum energy spe
24、ctral density, which they produce when applied to an unmodulated carrier, has been calculated for an assumed 10% increase in occupied radio-frequency bandwidth. The results are plotted in Fig. 2, relative to that which would occur under the conditions of busy-hour loading; the curves of Fig. 2 have
25、been designated A-D as described above. However some approximations bave been made, as the problem of the relation between signai distortion and radio-frequency bandwidth limitation has been avoided by assuming: - Carson bandwidth occupancy (with peak-to-r.m.s. ratio of 12 dB) throughout; - that thi
26、s bandwidth formula may also be applied to the sum of the signal and dispersed r.m.s. deviation when the dispersal is by noise band; - in other cases, that the occupied radio-frequency bandwidth is increased by the peak-to-peak dispersal deviation. The errors so incurred are not thought to be large,
27、 and in any case should be in the same sense for all waveforms. As a further approximation, each type of dispersing waveform is represented in Fig. 2 by a single curve. CCIR RECMN*446-4 93 D 4855212 0520934 565 Rec. ITU-R S.446-4 FIGURE 2 Energy dispersal for multi-channel telephony systems Typical
28、number of channels 20 40 80 100 200 400 800 1200 “ 0.1 0.2 0.5 1 2 Multi-channel r.m.s. deviation (MHz) 5 10 35 d08 2.1.1.1 Sinusoidal dkpersal It is evident from Curve A of Fig. 2 that carrier energy dispersal by a sinusoidal signal is rather inefficient, while Curve B shows that a sinusoidal signa
29、l with 30% of third harmonic is only about 2 dB better. For a typical 20-channel transmission, the maximum power density in either case, exceeds that which occurs under full-loading conditions by about 10 dB. It is a feature of both these types of dispersal that the amount by which the dispersed pow
30、er-density exceeds that at full loading, increases with the r.m.s. multi-channel deviation and hence with channel capacity. For example, the excess for 1200 channels is about 18 dB. 2.1.1.2 Triangular dkpersal The most effective way, for a given increase in occupied bandwidth, of dispersing the ener
31、gy present in a single spectral line is, at least theoretically, by the application of a iriangular waveform. The dispersed power-density is inversely proportionai to the permitted percentage increase in radio-frequency bandwidth and Curve D of Fig. 2 shows that, if a 10% increase in occupied bandwi
32、dth is permitted, the dispersed power per 4 kHz exceeds that under full-loading conditions by only about 4.5 dB for most numbers of channels. When a low-frequency (I 1 kHz) triangular dispersai waveform is used on multi-channel systems, a wanted single-channel-per-carrier system can be exposed to ne
33、arly the full power of an interfering carrier for significant periods of time. CCIR RECMNa446-4 93 W 4855212 0520935 4Tl Frequency of triangular waveform (Hz) 36 Rec. ITU-R S.446-4 Increase in energy density The triangular waveform evidently offers a simple and efficient means of dispersing the ener
34、gy present in isolated spectral lines of telephony transmissions. It must be remembered, however, that its effectiveness depends upon faithful preservation of the shape of the wave until it appears as frequency-modulation, particularly when a high degree of dispersai is required. If 32 dB of dispers
35、ai were required for a 1200-channel transmission, for example, flattening of the extremities of the wave by only 0.25% might lead to a local doubling of spectral energy density. 20 80 150 The triangular signal may have to be filtered before being applied, to prevent the harmonics of the fundamental
36、from disturbing the lower channels of the telephone multiplex. For triangular waveform frequencies of up to 150 Hz and for a low frequency multi-channel baseband of 4 kHz, filtering causes deformation at the angles of the signai waveform and thus at the energy density peaks at the extremities of the
37、 modulation spectrum under light- loading conditions. 3 5 7 Table 1 shows measured values for the increase in energy density at the extremities of the spectrum in relation to the density at the centre frequency of the spectrum, for a 132-channel multiplex, as a function of the frequency of the trian
38、gular waveform. Discontinuous single step regulation was used for this system. TABLE 1 The low-pass filter used was a 7-pole Chebyshev-type filter with a cut-off frequency of 2.7 kHz and an attenuation at 4 kHz equal to 34 dB. It is, however, possible to take account of the presence of the continuit
39、y pilot at the modulator input provided it is generated independently of the telephone multiplex. Under the same measurement conditions as indicated earlier, the application of a pilot at a level of -20 dBm0 makes it possible to reduce the energy density peaks at the extremities of the band from 7 t
40、o 3 dB. 2.1.1.3 Dispersal by a band of low-frequency noise A form of carrier energy dispersal that is not criticai in its application and shares with triangular dispersal the property of yielding a maximum energy spectral-density, inversely proportional to the amplitude of the waveform, may be accom
41、plished by adding a band of low-frequency noise to the multi-channel baseband. Curve C of Fig. 2 shows that, for a 10% increase in occupied bandwidth, the maximum dispersed power per 4 ICHZ exceeds that under full-loading conditions by about 9.5 dB for all numbers of channels. Note 1 - When the leve
42、l of the dispersal signal is not fixed, the required amount of dispersion can be attained by other methods. 2.1.2 Method Zb) An obvious variant of Method la) would incorporate an automatic means for adjusting the degree of artificial energy dispersal which would be applied according to the state of
43、loading of the system, as shown in Method lb) of Fig, 1. It might, in fact, be possible by using noise band dispersal in this way, to maintain the maximum energy spectral-density of a transmission quite close to its full-loading value without any increase in occupied radio-frequency bandwidth. The p
44、erformance that could be achieved in practice, would depend on the distortion produced by the interaction (via radio-frequency bandwidth limitation and other transmission characteristics) of the dispersal waveform CCIR RECMNm44b-4 93 4855232 0520936 338 Rec. ITU-R S.446-4 37 and isolated tones and a
45、ctive telephone channels under light-loading conditions. It is probable that the matter can only be settled experimentally, since there is as yet no generally accepted way of calculating the distortion that frequency- modulation signals undergo during transmission, even for the simplest case of whit
46、e-noise loading. A particular method which has been proposed for applying the variable degree of dispersal to which the present sub-section relates, relies on filling a suitable proportion of unoccupied telephone channels with simulated speech (i.e. band-limited noise). Although full dispersal could
47、 in this way be maintained without increase of bandwidth, the complexity of the apparatus likely to be required for the method is a serious disadvantage, as is the probable necessity for applying it at the audio switchboards from which the baseband originates. 2.2 Dispersal by automatic deviation co
48、ntrol 2.2.1 General It would clearly be possible to adjust the signal level entering the system frequency modulator so as to maintain the r.m.s. (or peak) frequency deviation at some constant value. The desired level could be obtained merely by subjecting whatever the baseband content happens to be,
49、 to sufficient amplification, or by so amplifying after the addition of some fixed or variable amount of artificial dispersal. The overall baseband transmission loss of the system would be kept sensibly constant by a compensating adjustment of the post-demodulation gain through the medium of a system pilot tone. The possibilities are discussed in the following paragraphs. 2.2.2 Method 2a) The most general method of caniei energy dispersal of which the others are in a sense degenerate forms, is Method 2a) in Fig. 1. This consists in adding to the baseband, before the application of autom
