1、 Rec. ITU-R SM.443-4 1 RECOMMENDATION ITU-R SM.443-4 Bandwidth measurement at monitoring stations (Question ITU-R 26/1) (1966-1978-1995-2005-2007) Scope This Recommendation recommends methods for determining occupied bandwidth at monitoring stations for different classes of emission. The ITU Radioco
2、mmunication Assembly, considering a) the need for the measurement of bandwidths of emissions at monitoring stations to promote efficient use of the radio-frequency spectrum; b) the need for uniform, easy-to-perform and reliable results of bandwidth measurement at monitoring stations, to enable a com
3、parison of the results obtained by different monitoring stations; c) the definitions of different bandwidths in the Radio Regulations (RR) and Recommendation ITU-R SM.328, especially the definitions of occupied bandwidth and x dB bandwidth; d) the increasing availability of equipment able to directl
4、y measure the occupied bandwidth, including equipment employing digital signal processing and fast Fourier transform (FFT) techniques; e) Chapter 4.5, on bandwidth measurement, of the ITU-R Spectrum Monitoring Handbook, (Edition 2002), recommends 1 that the direct “% method” specified in Annex 1 sho
5、uld be used at monitoring stations when measuring the occupied bandwidth; 2 that the “x dB method” specified in Annex 2 should be used at monitoring stations when measuring the x dB bandwidth; 3 that the occupied bandwidth can be estimated from the x dB bandwidth using the procedure described in Ann
6、ex 3 when the conditions for accurate measurement of occupied bandwidth are not met, or in the absence of equipments capable of performing % measurement. 2 Rec. ITU-R SM.443-4 Annex 1 Measurement method of occupied bandwidth (% method) 1 Introduction RR No. 1.153 and Recommendation ITU-R SM.328 defi
7、ne the term occupied bandwidth as follows: “Occupied bandwidth: The width of a frequency band such that, below the lower and above the upper frequency limits, the mean powers emitted are each equal to a specified percentage /2 of the total mean power of a given emission. Unless otherwise specified i
8、n an ITU-R Recommendation for the appropriate class of emission, the value of /2 should be taken as 0.5%.” In line with 2 of Recommendation ITU-R SM.328 on an emission of a transmitter, the optimum from the standpoint of spectrum efficiency is reached when the occupied bandwidth is equal to the nece
9、ssary bandwidth of the relevant class of emission, given by Recommendation ITU-R SM.1138, which is incorporated into the RR by reference. According to the definition above, the occupied bandwidth can be measured using either digital swept spectrum analysers or digital monitoring receiver allowing to
10、 store the recorded traces in memory for later graphical processing, or with analysers employing FFT techniques. 2 General conditions for bandwidth measurement General conditions for bandwidth measurements are the following: The line-of-sight (LoS) with Fresnel curve between the transmitting and rec
11、eiving antenna must be secured to ensure the high degree of discrimination of source of emission. A directional antenna with high directivity and high front-back ratio should be used to minimize the influence on multipath fading effects. Any suitable spectrum analyser or digital monitoring receiver
12、can be used. Impulse interferences should not occur (for example, interference from an ignition source). 3 Measurement procedure The spectrum analyser or digital monitoring receiver is adjusted with the following settings: Frequency: estimated centre frequency of the emission Span: 1.5 to 2 times th
13、e estimated bandwidth of the emission Resolution bandwidth (RBW): less than 3% of the span Video bandwidth (VBW): 3 times RBW or more Level/Attenuation: adjusted so that the S/N ratio is more than 30 dB Detector: peak or sample Sweep time or acquisition time: auto (for pulsed emissions long enough s
14、o that one pulse is recorded for every pixel on the screen) Trace: MaxHold (for analogue modulation), ClearWrite (for digital modulation). In most digital systems, the occupied bandwidth is constant over time, because normally a data stream is transmitted with a constant symbol rate. In these cases,
15、 the momentary value of the calculated bandwidth will be relatively constant for each recorded trace. To smooth the results of Rec. ITU-R SM.443-4 3 different subsequent measurements, a longer sweep time can be set. This will make the reading of the result easier. In analogue systems, especially whe
16、n audio signals are transmitted (F3E, A3E, J3E), the momentary occupied bandwidth changes rapidly with the modulation. In these cases, monitoring stations are only interested in the maximum occupied bandwidth within a certain observation time (e.g. one hour). To get this result, the “MaxHold” functi
17、on has to be applied. After the trace has been recorded, the displayed spectrum is mathematically analysed to calculate the occupied bandwidth as follows: The spectral power (or level) of each frequency line of the stored trace is added throughout the adjusted span to give the 100% reference power.
18、In a second calculation, starting from the lowest frequency recorded, the spectral power of each frequency line is again added up until the sum reaches 0.5% of the predetermined total power. At this point, a marker is set. The same calculation is then performed starting from the highest frequency re
19、corded (the right edge of the display) until again 0.5% of the total power is reached and a second marker is set. The occupied bandwidth is the frequency difference between the two markers. 4 Measurement conditions and accuracy The relative accuracy depends on: The spectral shape of the signal When
20、the signal raises and falls steeply towards the edges of the used channel, the accuracy is higher. The resolution bandwidth Smaller RBW result in higher accuracy, because the bandwidth calculation is based on the graphical shape of the displayed trace which is always widened by the measurement filte
21、r. The frequency span If the frequency span is too wide, more and more noise will be included in the calculation process which results in less accuracy. However, the span has to be wide enough to include at least some spectral components below the 0.5% (or 26 dB) points. The noise and interference l
22、evel Since noise and interference outside the used channel is included in the calculation process, a high difference between useful signal and interference will increase the accuracy. Therefore a minimum adjacent channel power ratio (ACPR) or minimum difference of the peak level and level of outmost
23、 frequencies of 30 dB is recommended to ensure a measurement error of less than 10% (see Fig. 1 and Fig. 2). Measurement trials The fluctuation of digital signal due to the non-constant modulation signal can lead uncertainty on the measurement result. Therefore the measurement trials of at least 400
24、 times are recommended to obtain the average occupied bandwidth. Care should be taken that no interfering signal can be seen inside the recorded span because this would be treated as part of the wanted signal which may result in a high measurement error. 4 Rec. ITU-R SM.443-4 Annex 2 Measurement met
25、hod of x dB bandwidth (x dB method) 1 Introduction The “x dB bandwidth” (Recommendation ITU-R SM.328 ( 1.14) is defined as the width of a frequency band such that beyond its lower and upper limits any discrete spectrum component or continuous spectral power density is at least x dB lower than a pred
26、etermined 0 dB reference level. In cases where it is specifically needed, for example to determine the border to the out-of-band domain of radar emissions, the x dB bandwidth can be measured using any spectrum analyser or digital monitoring receiver. Rec. ITU-R SM.443-4 5 Moreover in line with the r
27、equirements of item 2 of Recommendation ITU-R SM.328 on an emission of a transmitter, optimum from the standpoint of spectrum efficiency, x dB bandwidth can be also related to the necessary bandwidth of the relevant class of emission given by Recommendation ITU-R SM.1138, which is incorporated into
28、the RR by reference. 2 Measurement procedure First, the 0 dB reference level has to be determined. This is usually the level of the highest spectral line when the spectrum is recorded with a narrow RBW on the analyser. Under the assumption that, during the recording time, the transmitter power retur
29、ns to the carrier at least once, this level is equal to the total emitted power. However, in digitally modulated signals, this assumption can not be made. Although the 0 dB reference is still set to the level of the highest spectral line, it is not the total emitted power, which is the reason that t
30、he x values, to gain comparable results, are different for analogue and digital emissions. The spectrum analyser/digital receiver should be adjusted as follows: Centre frequency: f0(carrier frequency or estimated centre frequency of the emission) Span: 1.5 times the estimated bandwidth of the emissi
31、on RBW: less than 3% of the span VBW: 3 times RBW or more Detector: peak Trace: MaxHold. When the trace has built up, the peak level is searched. The value x dB bandwidth is read on the equipment screen as the width of a frequency band such that beyond its lower and upper limits any discrete spectru
32、m component is at least x dB lower than a predetermined 0 dB reference level. If more than two spectral lines have this level, the outmost frequencies are taken. The frequency difference between the two markers is the x dB bandwidth. 6 Rec. ITU-R SM.443-4 3 Measurement conditions and accuracy The ac
33、curacy of the x dB method depends on: The spectral shape of the signal When the signal raises and falls steeply towards the edges of the used channel, the accuracy is higher. The resolution bandwidth Smaller RBW result in higher accuracy, because the displayed shape of the signal is always widened b
34、y the measurement filter. The frequency span If the frequency span is too wide, less display lines are available to show the signal and set the markers. Less resolution increases the measurement uncertainty. The noise and interference level The signal-to-noise ratio must be sufficient according to t
35、he analysed emission. A minimum of x + 5 dB is recommended to ensure a measurement error of less than 10%. One of the best ways, when the dynamic of the signal is not sufficient in the monitoring station, is to make the measurement with a mobile station which can go near the transmitter. Interfering
36、 signals inside the displayed span can be tolerated and will not influence the result of a manual x dB measurement, if it is narrow compared to the bandwidth of the wanted signal, and if the interference frequency does not fall on one of the x dB points. 4 Measurement of dB bandwidths in conditions
37、of influence of interferences In some cases values of dB bandwidth can be measured or, at least, estimated in the presence of interferences whose levels exceed value of a measuring level dB. As it is shown in Fig. 4, in the case when borders of dB bandwidth from each edge of a spectrum of an emissio
38、n under consideration are not masked by interferences (spectra 1 and 2), the bandwidth of this particular spectrum is measured, not taking into account spectra of interferences. In other words, in the case shown in Fig. 4, the dB bandwidth is equal to mand not B. In case of doubts whether spectra 1
39、and 2 belong to interferences, the interfering transmitters can be determined using a two-channel correlometer by a small value of mutual correlation factor between the signal corresponding to a spectrum of the emission under consideration and the signal of potential interference ( 4.9.5.1 of the IT
40、U Spectrum Monitoring Handbook (Edition 2002). Even in case of masking of one dB bandwidth border by an interfering spectrum as shown in Fig. 5 (spectrum 3), if the spectrum of wanted signal is symmetrical, like most cases, the estimation of the bandwidth value can be carried out on the basis of hal
41、f of spectrum width, i.e. = 2 k. It is natural that measurements in conditions of influence of interferences are burdened by greater errors, than measurements in the absence of interferences. However, estimations obtained in some cases are quite suitable for practical applications. Rec. ITU-R SM.443
42、-4 7 8 Rec. ITU-R SM.443-4 Annex 3 Estimation of the occupied bandwidth using the x dB method 1 Introduction In the following situations, the % method can not be applied to directly measure the occupied bandwidth: in-band interference at levels that are higher than the level of the wanted signal; su
43、itable equipment capable of employing the % method is not available. In these cases, the x dB method described in Annex 2 can be used to estimate the occupied bandwidth. To give the occupied bandwidth as a result of the x dB method, the values for the 0 dB reference level and for x have to be select
44、ed properly. Generally, there are two different approaches to compare the measured bandwidth with the necessary or occupied bandwidth: Always measure the 26 dB bandwidth and apply a conversion factor. Measure the signal at specific values of x dB that are different for each class of emission. 2 Esti
45、mation of occupied bandwidth from the 26 dB bandwidth With this method, the bandwidth is always measured at the 26 dB points following the procedure in Annex 2. The conversion factors between 26 dB bandwidth B 26and necessary bandwidth Bnin Table 1 should be used for estimation of occupied or necess
46、ary bandwidth. TABLE 1 Class of emission Relationship between B26and BnA1A, A1B, A2A, A2B B26= 0.9 BnF1B B26= BnF3C B26= BnF7BDX B26= 0.9 Bn3 Direct estimation of occupied bandwidth from x dB bandwidth measurement With this method, the bandwidth is measured using the x dB method described in Annex 2
47、. The 0 dB reference level is always set to the peak level of the resulting curve. The values for x should be taken from Table 2 according to the modulation of the signal. Rec. ITU-R SM.443-4 9 TABLE 2 Class of emission (See RR Appendix 1) Values of “x dB” to be used when measuring x dB bandwidth fo
48、r estimation of occupied bandwidth Remarks A1A A1B 30 A2A A2B 32 A3E 35 B8E 26 F1B 25 F3C 25 F3E G3E 26 F7B 28 H2B 26 H3E 26 J2B 26 J3E 26 R3E 26 C7W (8-VSB) 12(1)Average of more than 300 sweeps G7W (T-DAB) 8(1), (2)Average of more than 100 sweeps (1)According to Recommendation ITU-R SM.328, the uni
49、t of these values is dBsd because the reference level was chosen to the maximum value of power spectral density (psd) within the necessary bandwidth. (2)This value is derived from experiments with T-DMB using a T-DAB network taken from Report ITU-R BT.2049. The resulting value of the measurement is the estimation of the occupied bandwidth.
