1、CCIR RECMN*7bb 72 W 4855232 0538579 42T Rec. 766 27 SECTION 4/9B: CO-ORDINATION AND INTERFERENCE CALCULATIONS RECOMMENDATION 766 MEXHODS FOR DETERMINING THE EFFECTS OF INTERFERENCE ON THE PERFORMANCE AND THE AVAILABILITY OF TERRESTRIAL RADIO- RELAY SYSTEMS AND SYSTEMS IN THE FIXED-SATELLITE SERVICE
2、(Questions 32/4 and 109/9) (1992) The CCIR, considering that it is necessary to evaluate the effects of interference on the performance and the availability of terrestrial that, in general, the determination of interference criteria requires suitable calculation methods; that calculation methods for
3、 determining interference to FDM-FM systems are fairly well established; that calculation methods for interference to single-channel-per-carrier (SCPC) FM telephony are to be a radio-relay systems and systems in the fixed-satellite service; b) C) i) established; e) f) 8) h need to be established; j
4、formulation, that calculation methods for interference to FM television are to be established; that calculation methods for interference to amplitude modulated (AM) telephony are to be established; that calculation methods for interference to digital transmissions are to be established; that, in fut
5、ure, calculation methods for interference to systems employing new modulation techniques may that it is desirable to provide spectra of signals for determination of interference from the general recommends that the methods described in Annex 1 be used for calculation of interference to FDM-FM system
6、s; that in the absence of more accurate information the methods described in Annex 2 be used provisionally for 1. 2. wanted signais other than FDM-FM. ANNEX I* Calculation methods for the interference of FDM-FM systems Given below is the method of calculation to determine the effects of interference
7、 to the FDM-FM systems in terrestrial radio-relay systems and systems in the fixed-satellite service. * Note from the Director, CCIR - For information, derivation of the formulae and historical development of this Annex are given by References contained in CCIR Report 388 (Dsseldorf, 1990). CCIR REC
8、flN*7bb 92 4855252 05i18580 i145 28 Rec. 766 1. Calculation methods 1.1 General formulation The relationship (this linear relationship is only valid for the lower levels of interference into FDM-FM telephony signals) between baseband interference power in a telephone channel and the carrier-to-inter
9、ference ratio involves the interference reduction factor B (in dB), defined as follows: S/Ni B= lOlogC/I where: S : test signal power in a telephone channel = 1 mW Ni : unweighted interference power in a telephone channel (bandwidth: 3.1 kHz) C : power of the wanted signal carrier (W) i : power of t
10、he interfering signal carrier (W). The weighted interference power Np (pW) is obtained as unweighted power in 1.75 kHz, which gives: 10 log Np = 87.5 - B - 10 log (C/l The interference reduction factor i3 is expressed as: with: r.m.s. test tone deviation (without pre-emphasis) of the wanted signal (
11、kHz) centre-frequency of channel concerned, within the wanted signal baseband (WZ) upper frequency of the wanted signal baseband (kHz) pre-emphasis factor for cetitre-frequency of channel concerned, within the wanted carrier baseband bandwidth of telephone channel (3.1 kJ3z) separation between carri
12、ers of the wanted and interfering signals (kHz) continuous part of the normalized power spectral density of the wanted signal with pre- emphasis (Hz-1) normalized vestigial carrier power of the wanted signal continuous part of the normalized power spectral density of the interfering signal (Hz-1) no
13、rmalized vestigial carrier power of the interfering signal amplitude-frequency response of the wanted signal receiving filter, the origin of the frequencies being the centre frequency of the interfering signai carrier. CCIR RECMN*7bb 72 4855232 0538583 088 Rec. 766 29 The power spectral densities ar
14、e normalized to unity and are assumed to be one-sided (only positive frequencies). The expression of Np in terms of the ratio C/Z is derived from expressions (2) and (3). In order to determine Np, it is necessary to determine: - - the interfering signal spectrum. The expressions of these spectra are
15、 given in 0 2 below and in D 3 of Annex 2. the wanted signal spectrum (analogue telephony), 1.2 Interference from a low-rnodulation-index FDMIFM signal to a high-modulation-index FDM-FM signal This case represents a terrestrial radio-relay system interfering into a system of the fixed-satellite serv
16、ice. The baseband channel which receives the most interference is not easily identified. However, the worst interference condition results when the wanted-to-unwanted carrier frequency separation is equal to, or less than, the top baseband frequency of the wanted signal. The factor B can be determin
17、ed from the following formula: If the modulation index of the wanted signal is greater than 3, the signai spectrum shape is near Gaussian, and formula (7) takes the following form: 7a) The definitions of the parameters in formulae (7) and (7a) have been given in 0 1.1 with the exception of the follo
18、wing: f, : r.m.s. multi-channel deviation of the wanted signal (Hz) LF : load factor, which is less than unity when not in the busy hour; y = (-15 + 10 log Nc)/20 for Ne 2 240 = (-1 + 4 log Nc)/20 for 60 5 Ne 3, the signal spectrum shape is near Gaussian. If the modulation indices of the wanted and
19、interfering signals are greater than 3, formula (7) should be applied to calculate interference, taking into account 0 1.3. In certain special cases, where the interfering signal may be characterized by its ramas. modulation index, and the upper baseband frequency is equal to the wanted signal (i.e.
20、fin, I fm2 = fm) there is the possibility of calculating the. interference function, Du, fo), very simply from the normalized curves of Fig. 1. 10 O 3 - 10 i2 - 20 E 6 2 .CI O -30 N - 40 -50 .- - i - 60 - 70 FIGURE 1 Normalized spectral density of FDM-FM signals Modulation iiidex, 111 The equivalent
21、 modulation index is determined by: m = .: + 41 !h and for this value of tti on the curves in Fig, 1 we find the values f,S(fi) atid fn,Scf2), where: and further: - CCIR RECMN*7bb 92 W 4855212 0538583 950 r- I 1.6 Interference from angle-modulated digitul signals into FDM-FM signals Digital systems
22、using PSK or FSK modulation are classes of angle-modulated systems. Consequently, the interference from these systems into analogue, angle-modulated systems is computed by the convolution integral. However, the spectral densities of digital, angle-modulated signals cannot be easily generalized; a sp
23、ecific spectrum is, however, provided in Q 3.2 of Annex 2. More generalized computation would involve the calculation of the digital spectral density (see Q 3.2 of Annex 2), the calculation of the analogue spectral density, the convolution of the two densities, and the computation of the factor B. I
24、 When a high-modulation-index FDM-FM carrier receives interference from angle-modulated digital signals that occupy a bandwidth small compared with that of the wanted signal, factor B is given roughly by formula (7). If a wanted FDM-FM signal suffers interference from an unwanted PCM-PSK or DPSK-PM
25、signal that occupies a bandwidth which is large compared with that of the wanted signal, factor B is given by the following simplified formula: Rec. 766 31 The same method may be used for the approximate determination of D (f, fo) according to the value of the “equivalent” modulation index: when: Th
26、e symbols used are defined as follows: fo : carrier frequency separation fm, fm, : mid frequency of the top baseband channel of the desired and interfering signals respectively mi, m2 : r.m.s. modulation indices of desired and interfering signal respectively. 1.5 Interference from a high-modulation-
27、index FDM-FM signal to a low-modulation-index FDM-FM signal This case is typicdiy that of a system in the fixed-satellite service causing interference in a terrestrial radio- relay system. Low-index angle modulation can be regarded as quasi-linear with respect to some types of interfering signal; th
28、e calculation of interference in these cases is performed by a simple procedure analogous to that employed for linear DSB-AM. The following approximate formula can be used: Interference power in telephone channel Thermal noise power in telephone channel - Interfering signal power in two appropriate
29、4 kHz bands at the receiver input Thermal noise power in same two 4 kHz bands at the receiver input . The normalized spectral power density of the interfering signal Pcf) used in this formula is determined by the formulae (36a - 36d) given in Q 3.2 of Annex 2. CCIR RECflN*7bb 92 H 485523Z 0518584 97
30、 H 32 Rec. 766 1.7 Intqference from AM signals into FDM-FM signals The quasi-linear properties of low-modulation-index angle-modulated signals with respect to interfering signals whose spectral densities do not exhibit excessive variations within the receiver passband, permit the use for such cases
31、of the following approximate formula: Interference power in telephone channel Thermal noise power in telephone channel * Interfering signal power in two appropriate 4 kHz bands at the receiver input Thermal noise power in same two 4 kHz bands at the receiver input Two 4 kHz bands are used in the for
32、mula since there may be asymmetry of the interfering spectrum with respect to the wanted carrier. When a high-modulation-index angle-modulated system receives interference from amplitude-modulated digital signals that occupy a bandwidth small compared with that of the wanted signal, factor B is give
33、n roughly by the formula of 0 1.2. 1.8 Interferencefrom a narrow-band system into an FDM-FM system The theoretical expression of 0 1.1 can be applied to the case of an interfering signal of arbirary modulation, but with a bandwidth small compared with that of the interfered-with signal. Interference
34、 from SCPC to FDM-FM signals is an example of such a situation. In particular, for evenly spaced SCPC carriers, the aggregate interference power in the baseband from all SCPC interference entries from one interfering network is close to thermal noise with equal power starting from five to six carrie
35、rs. 1.9 Interference from FM-TV signals into FDM-FMsignals When the FM-TV signai modulated only by the dispersai waveform is the interfering signal, the FDM-FM wanted signal with a low number of telephone channels has a spectrum with a width commensurate with that of the interfering signal spectrum,
36、 and the carrier frequencies coincide, then formula (4) takes the form: f+Afn f+Afa $ S(F) dF-YS(F) dF = 2P $ S(F) dF f-Aff.2 f+AfD f-4fn where: Af : frequency deviation of dispersal waveform (peak-to-peak) P : spectral power density of interfering signal (see Fig. 4, i = 1) = 11Af. In the condition
37、s described above, and with reference to formula (3), we may consider that: f+4f/2 $ S(F)dF=l when f =ed cf+C -ifS*s n! n=l where: cf : Dirac delta function Son * Scf) : Scf : convolution of the function Scf n times itself normalized spectral density of the signal phase: where E is the lower to uppe
38、r frequency ratio in the wanted signal baseband. (22)* * Although the series of formula (22) converges for all vaiues of system parameters. it does not always provide the most appropriate algorithm for numerical computation, particularly in cases where the normalized r.m.s. multi-channel phase and/o
39、r frequency deviation (a and rn respectively) are large. CCIR RECMN*7bb 92 W Y8552Li2 51858b bbT W 34 Rec. 766 The CCIR pre-emphasis characteristics are well approximated by the expression: 212+o.75 $i4, when EI- 1: f2 I O0 where: fs : r.m,s. multi-channel signal frequency deviation n! 232, (x) = (-
40、1)” H2ri (x) : normalized Hermite polynomial. Figures 2a to 2e contain spectral graphs plotted according to formulae (22) and (26) for modulation indices m adopted in typical radio-relay and communication-satellite systems. The curves are approximate in the region f/f, near O and 1. The exact values
41、 depend upon the particular value of E. The exact curves for several values of E are given in Figs. 2f to 2j for f/fm near zero. (The inset curves in Figs. 2d to 2e are also accurate enough for f/fnI near zero if E is equal to or greater than 0.02.) For modulation indices greater than 1.1, the follo
42、wing einpirical formula has been found to fit adequately the curves of Pcf and is a good approximation of equation (26): where: x = fllKlI 5 The avcragc probability of crrur pcrfurmaiicc of the 11-ary sclicmes versus thc carrier-tu-tlicrmal noise ratio (mcasurcd in doublc-sidcd Nyquist bandwidth) fu
43、r a whltc Gaussian channel only) 969 CCIR RECMN*7bb 92 m 4855232 O538599 2LB I Rec. 766 FIGURE 6 The probability of error performance curves of 64-QAM modulation system tersus carrier-to-thermal noise ratio and a carrier-to-interference ratio as a parameter (double-sided Nyquist bandwidth) 47 4 8 12
44、 16 20 24 28 32 36 40 44 48 52 56 CIN (dB) In Figs. 9 to 13, C/Z is defined as the ratio between carrier power at the receive filter input and interference power at the receive filter output. Carrier-to-interference power ratio at the receive filter input can be determined by subtracting the corresp
45、onding interference reduction factor which is given in the figures. Also, in these figures, CIN represents the ratio between the carrier power at the receive filter input and the noise power at the receive filter output. The carrier-to-noise ratio at the receive filter output is around 0.5 dB lower
46、because of the attenuation of the spectrum of the desired carrier by the receive filter. Figures 9 to 11 refer to Co-channel and adjacent channel interference in systems with 4-PSK, 16-QAM and 64-QAM modulations, considering several values of frequency separation between two equally modulated carrie
47、rs. Figure 12 addresses Co-channel interference between two 4-PSK carriers with different relative bandwidths. Figure i3 shows the interference effect of different modulations on the performance of a 4-PSK system. CCIR RECMN*7bb 92 W 4B55ZiI2 05iI8bO BbT 48 Rec. 766 FIGURE 7 The probability of error
48、 performance curves of 256.QAM modulation system versiis carrier-to-thermal noise ratio and a carrier.to.interference ratio as a parameter (double-sided Nyquist bandwidth) 1 on 10-1 10-2 I 0-3 8 E - O r) E b 2 10-4 i2 10-5 O .A ?2 SJ * -0 10-6 10 10-8 4 8 12 16 20 24 28 32 36 40 44 48 52 56 ClN (dB)
49、 o Results of measurements on 1.6 Mbit/s modems, G/f = 40 dB The following general conclusions can be drawn by inspection of the figures: - when the interfering signal power is equal to, or larger than, the thermai noise power, the effect of angle-modulation interference is considerably less than that of an equal amount of white Gaussian noise power; - when the interfering signal power is small compared to the themal noise power, the effect on error rate can be estimated safely by assuming that the interfering signai is equivalent to Gaussian noise of equal pow
