ITU-R S 446-3-1992 Carrier Energy Dispersal for Systems Employing Angle Modulation by Analogue Signals or Digital Modulation in the Fixed-Satellite Service - Section 4C - Earth Station.pdf

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1、146 CCIR RECMN*446-3 92 4855232 0538768 406 Rec. 446-3 RECOMMENDATION 446-3 CARRIER ENERGY DISPERSAL FOR SYSTEMS EMPLOYING ANGLE MODULATION BY ANALOGUE SIGNALS OR DIGITAL MODULATION IN THE FEEDSATELLITE SERVICE (Question 27/4) (1966- 1974-1978-1992) The CCIR, considering a) substantial reduction of

2、interference to stations of a terrestrial service operating in the same frequency bands; that use of carrier energy dispersal techniques in systems in the fixed-satellite service can result in a b) that in many cases the use of such techniques can result in a moderate to substantial reduction in the

3、 level of interference between systems in the fixed-satellite 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; c) without noticeable deterioration of the quality of operation; that such te

4、chniques are being regularly and successfully employed in systems in the fixed-satellite service d) was adopted by the World Administrative Radio Conference for Space Telecommunications (Geneva, 1979); that Recommendation No. 103, relating to carrier energy dispersai in systems in the fixed-satellit

5、e service, e) Kecommetidation 67 I, that performance evaluation of various techniques of FM-TV signal dispersion is given in Annex 1 to recommends 1. that systems in the fixed-satellite service should use carrier energy dispersal techniques, as far as is practicable and in a manner consistent with s

6、atisfactory 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 hits at all times; 2. that the capability for carrier energy dispersal up to the maximu

7、m 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; 3. that the following Note should be regarded

8、 as part of this Recommendation. Note 1 - Annex 1 describes various dispersion techniques for use with FM analogue and PSK digital signals which can be recommended for practical utilization. CCIR RECMN*446-3 92 4855212 0518767 342 Rec. 446-3 147 ANNEX 1 Energy dispersai in the fEed-satellite service

9、 1. Introduction 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 mos

10、t cases to use energy dispersal techniques to reduce the spectral 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 t

11、he orbital 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 desirab

12、le that the 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 an

13、alogue frequency-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 dispersal and these are mentioned in the Annex

14、. 2. Energy dispersai 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 signal, to have some reference value with which to compare what can be obtaine

15、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 (SF) exceeds the highest baseband frequency, which in turn greatly exceeds the lowest baseband frequency, to assume that

16、the mean power spectrum under the conventional busy-hour loading conditions is of Gaussian form. Hence, the dispersing effect obtained under these conditions is: maximum ener =lolog= 0.004 =-(28+10logSF) 10 log total enZ:y dB (SF is expressed in ME) The dispersing effect when SF is less than the hig

17、hest baseband frequency can be calculated using the information contained in Annex 1 of Recommendation 675. There are a number of methods of maintaining a high degree of carrier energy dispersal in telephony systems, with particular reference to the dependence of the obtainable dispersal on the comp

18、lexity of the means of dispersal and the attendant increase in occupied radio-frequency bandwidth as a function of 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,

19、in addition, effectively controls the deviation sensitivity of the frequency modulator, Various arrangements of these two methods are illustrated in Fig. 1. CCIR RECVN*446-3 92 m 4855212 0518770 Ob4 m 148 Rec. 446-3 FIGURE 1 Simplified block diagram (Possible filters, buffer-amplifiers and gain-regu

20、lating pilots omitted) A- ,F Method A: baseband signal input B: r.m.s. detector C: amplifier 1 D: amplifier 2 E r.m.s. detector F: output to frequency-modulator G: dispersal waveform Gain O Gain O hl Gain h I “ Gain 1 1 1 O O Zero gain O Full load No control No control Gain I 1 Gain I 1 1 O O Gain O

21、 Lii- v2 Full load 2.1 Dispersal by added waveforms 2.1.1 Method l(a) 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 l(a) of Fig. 1. Of a variety of dispersal w

22、aveforms that have been proposed, the following are examined in this Annex: - a sinusoidal signal (Curve A of Fig. 2), - a sinusoidal signal plus 30% third harmonic added in suitable phase (Curve B of Fig. 2), - a band of low-frequency noise (Curve C of Fig. 2), - a low-frequency triangular waveform

23、 (Curve D of Fig. 2). CCIR RECMN*446-3 92 m 485523i- 0538773 TTO m on Rec. 446-3 FIGURE 2 Energy dispersal for multi-channel telephony systems Typlcal number of channels 800 1200 4n Rn IW 200 400 0.1 0.2 0.5 1 2 Multi-channel r.m.s. deviation (MHz) 5 149 10 To provide some basis for comparing the ef

24、ficiencies of these waveforms, the maximum energy spectral 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 condit

25、ions of busy-hour loading; the curves of Fig. 2 have been designated A-D as described above. Some approximation occurs here, because difficult questions of the relation between signal distortion and radio-frequency bandwidth limitation have been avoided by assuming: - Carson bandwidth occupancy (wit

26、h peak-to-r.m.s. ratio of 12 dB) throughout; - that this 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 deviati

27、on. The errors so incurred are not thought to be large, 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. 150 CCIR RECNN*446-3 92 4855232 O538772 937 Rec. 446-3 2.1.1.1 Sinusoidal di

28、spersal 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 signal with 30% of third harmonic is only about 2 dB better. For a typical 20-channel transmission, the maximum power density in either case,

29、 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 power-density exceeds that at full loading increases, with the r.m.s. multi-channel deviation, and hence, with channel capacity. For examp

30、le, the excess for 1200 channels is about 18 dB. 2.1.1.2 Triangular dispersal The most effective way, for a given increase in occupied bandwidth, of dispersing the energy present in a single spectral line is, at least theoretically, by the application of a triangular waveform. The dispersed power-de

31、nsity is inversely proportional to the permitted percentage increase in radio-frequency bandwidth and Curve D of Fig. 2 shows that, if a 10% increase in occupied bandwidth is permitted, the dispersed power per 4 ICHZ exceeds that under full-loading conditions by only about 4.5 dB for most numbers of

32、 channels. With the use of low-frequency (I 1 kHi) triangular dispersai waveform on multi-channel systems a wanted single-channel-per-carrier system can be exposed to nearly the full power of an interfering carrier for significant periods of time. The triangular waveform evidently offers a simple an

33、d efficient means of dispersing the energy 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 di

34、spersal is required. If 32 dB of dispersal 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. The triangular signal may have to be filtered before being applied, to prevent the

35、 harmonics of the fundamental 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 signal waveform and thus at the energy density p

36、eaks at the extremities of the modulation spectrum under light- loading conditions. 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 th

37、e frequency of the triangular waveform. Discontinuous single step regulation was used for this system. TABLE 1 Frequency of triangular waveform Increase in energy densiy (Hz) (a) 20 3 80 5 150 7 The low-pass filter used was a 7-pole Chebyshev-type filter with a cut-off frequency of 2.7 kHz and an at

38、tenuation at 4 kHz equal to 34 dB. It is, however, possible to take account of the presence of the continuity 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 le

39、vel of -20 dBmO makes it possible to reduce the energy density peaks at the extremities of the band from 7 to 3 dB. Rec. 446-3 151 2.1.1.3 Dispersal by a band of low-frequency noise A form of carrier energy dispersal that is not critical in its application and shares with triangular dispersal the pr

40、operty of yielding a maximum energy spectral-density, inversely proportional to the amplitude of the waveform, may be accomplished 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 powe

41、r per 4 kHz exceeds that under full-loading conditions by about 9.5 dB for all numbers of channels, Nute I - When the level of the dispersal signal is not futed, the required amount of dispersion can be attained by other methods. 2.1.2 Method l(b) An obvious variant of Method l(a), would incorporate

42、 automatic means for adjusting the degree of artifical energy dispersal, applied according to the state of loading of the system, as shown in Method l(b) of Fig. 1. It might, in fact, be possible in this way, using say, noiseband dispersal, to maintain the maximum energy spectral-density of a transm

43、ission quite close to its full-loading value without any increase in occupied radio-frequency bandwidth. The performance 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

44、 the dispersal waveform and isolated tones and active 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

45、transmission, even for the simplest case of white-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. ba

46、nd-limited noise). Although full dispersal could 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 ori

47、ginates. 2.2 Dispersal by automatic deviation control 2.2.1 General It would clearly be possible to adjust the signai 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 subject

48、ing whatever the baseband content happens to be, 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

49、-demodulation gain through the medium of a system pilot tone. The possibilities are discussed in the following paragraphs. 2.2.2 Method 2(a) The most general method of carrier energy dispersal of which the others are in a sense degenerate forms, is Method 2(a) in Fig. 1. This consists in adding to the baseband, before the application of automatic deviation control, a source of artificial energy dispersal whose amplitude is made to depend upon the loading conditions. The use of this method might add little or nothing to the occupied radio-frequency bandwidth. Furthermore, if the application