1、 Recommendation ITU-R P.1321-5 (07/2015) Propagation factors affecting systems using digital modulation techniques at LF and MF P Series Radiowave propagation ii Rec. ITU-R P.1321-5 Foreword The role of the Radiocommunication Sector is to ensure the rational, equitable, efficient and economical use
2、of the radio-frequency spectrum by all radiocommunication services, including satellite services, and carry out studies without limit of frequency range on the basis of which Recommendations are adopted. The regulatory and policy functions of the Radiocommunication Sector are performed by World and
3、Regional Radiocommunication Conferences and Radiocommunication Assemblies supported by Study Groups. Policy on Intellectual Property Right (IPR) ITU-R policy on IPR is described in the Common Patent Policy for ITU-T/ITU-R/ISO/IEC referenced in Annex 1 of Resolution ITU-R 1. Forms to be used for the
4、submission of patent statements and licensing declarations by patent holders are available from http:/www.itu.int/ITU-R/go/patents/en where the Guidelines for Implementation of the Common Patent Policy for ITU-T/ITU-R/ISO/IEC and the ITU-R patent information database can also be found. Series of ITU
5、-R Recommendations (Also available online at http:/www.itu.int/publ/R-REC/en) Series Title BO Satellite delivery BR Recording for production, archival and play-out; film for television BS Broadcasting service (sound) BT Broadcasting service (television) F Fixed service M Mobile, radiodetermination,
6、amateur and related satellite services P Radiowave propagation RA Radio astronomy RS Remote sensing systems S Fixed-satellite service SA Space applications and meteorology SF Frequency sharing and coordination between fixed-satellite and fixed service systems SM Spectrum management SNG Satellite new
7、s gathering TF Time signals and frequency standards emissions V Vocabulary and related subjects Note: This ITU-R Recommendation was approved in English under the procedure detailed in Resolution ITU-R 1. Electronic Publication Geneva, 2013 ITU 2013 All rights reserved. No part of this publication ma
8、y be reproduced, by any means whatsoever, without written permission of ITU. Rec. ITU-R P.1321-5 1 RECOMMENDATION ITU-R P.1321-5 Propagation factors affecting systems using digital modulation techniques at LF and MF (Question ITU-R 225/3) (1997-2005-2007-2009-2013-2015) Scope This Recommendation pro
9、vides information on the characteristics of LF and MF ground-wave and sky-wave propagation which may affect the use of digital modulation methods in those bands. Keywords MF propagation; seasonal variation The ITU Radiocommunication Assembly, considering a) that digital modulation methods for sound
10、broadcasting purposes at LF and MF are currently being studied; b) that information on the propagation characteristics at these frequencies is necessary for use in the design of modulation methods, recommends that the information given in Annex 1 should be taken into account in the design of digital
11、 modulation methods for broadcasting at MF and LF. Annex 1 1 Introduction The majority of broadcasting services in the MF and LF bands are based on the characteristics of the ground-wave propagation mode (see Recommendation ITU-R P.368). The limiting coverage range, during daytime and in the absence
12、 of interference, is determined by the intensity of radio noise due to lightning and to man-made sources (see Recommendation ITU-R P.372) and by the required signal-to-noise ratio. During the hours of darkness, ionospheric sky-wave modes become important (see Recommendation ITU-R P.1147). For analog
13、ue amplitude modulation, these modes limit the coverage, since the wave-interference between the ground wave and the varying and phase delayed sky-wave modes results in unsatisfactory signal quality. Sky-wave signals from other distant transmissions may also add significant night-time interference,
14、which may also restrict the service coverage to ranges where the ground wave provides a sufficiently strong signal; aspects of interference from other signals are not further considered in this annex. 2 Rec. ITU-R P.1321-5 Digital modulation methods may also be affected by the presence of delayed si
15、gnal modes, but suitable modulation design may counter or exploit this effect. This annex presents some very simple models for this multipath environment, which are expected to be suitable for the design of modulation methods. Dependent on the modulation method chosen, more detailed prediction metho
16、ds may be required for service planning purposes. 2 Propagation modes 2.1 Ground wave mode The ground wave may often not be constant (see 4). Also as shown in Recommendation ITU-R P.368, the signal amplitude depends on range and the electrical characteristics of the ground. Also the signal amplitude
17、 does not remain constant for small changes in location (from several hundred metres). 2.2 Sky-wave modes During daylight hours, signal attenuation in the lower D-region part of the ionosphere effectively prevents sky-wave propagation. This annex concentrates on the conditions at night when sky-wave
18、 propagation may be significant. The E-layer of the ionosphere decays after sunset, but the critical frequency, foE, will be within the MF broadcast band, at least for times in the first part of the night. Signals at frequencies less than the critical frequency will always be reflected by the E-laye
19、r, and multihop reflections will also be supported. Signals at higher frequencies may still be reflected from the E-layer, particularly to longer ranges, but signals will also penetrate the E-layer to be reflected from the higher F-region. Using a simple model for the E-layer, Fig. 1 illustrates the
20、 available signal modes for three frequencies in the MF band, showing the way in which mode availability varies with ground range and with time after sunset. These modes will be time delayed compared with the ground wave mode. Recommendation ITU-R P.1147 provides predictions for the composite signal
21、 power for the available sky-wave modes, and thus does not give the necessary information for the relative amplitudes for individual modes. However, Recommendation ITU-R P.684 does provide information, although primarily intended for frequencies less than 500 kHz. In particular, it gives values for
22、the ionospheric reflection coefficient for sunspot minimum conditions, based on experimental results, and on some assumptions, as stated in the Recommendation. 3 Multipath time spread Using the above simple propagation models, Fig. 2 shows the expected median field strengths and relative time delays
23、 for three ranges, 100, 200 and 500 km, and two frequencies, 700 kHz and 1 MHz. The field strengths are for 1 kW e.m.r.p. and do not include the effect of the vertical radiation pattern of the transmitting antenna this may reduce the levels of the sky-wave signals at the shorter ranges. The mode sho
24、wn at 0 ms is for the ground wave, and field strengths are shown for three values of ground conductivity; 5 S/m (sea water), 102 (good ground), and 103 (poor ground). The sky-wave components are marked with the relevant mode and the levels approximately represent the median field strengths four hour
25、s after sunset at sunspot minimum. Rec. ITU-R P.1321-5 3 FIGURE 1 Available propagation modes P . 1 3 2 1 - 0 1T i me aft er s u n s et (h )T i me aft er s u n s et (h )T i me aft er s u n s et (h )Pathlength(km)0 . 7 MH z 1 . 0 MH z 1 . 5 MH z E an d F E an d F E an d F Pathlength(km)Pathlength(km)
26、0 2 4 6 8 10 124000EF0 2 4 6 8 10 125004000EF0 2 4 6 8 10 121 6 0 01 2 0 08004000EF4 Rec. ITU-R P.1321-5 FIGURE 2 Examples of time delay spread P . 1 3 2 1 - 0 26040200(dB(V/m)m1 0 0 k m(ms )0 1 2 35103G1E2E(ms )0 1 2 3 4 55103G1F2F3F1023E10260402002 0 0 k m(ms )0 1 2 35103G1E2E(ms )0 1 2 3 4 55103G
27、1F2F3F1023E102(dB(V/m)m5 0 0 k m(ms )0 1 2 360402005102G1E2E7 0 0 k H z(ms )0 1 2 3 4 55102G1E2E1 MH z2F3F(dB(V/m)mRec. ITU-R P.1321-5 5 Figure 3 indicates the delay of the one hop E- and F-region sky-wave modes relative to the ground wave for ranges out to 1 000 km, while Fig. 4 gives the relative
28、delays between single and multihop sky-wave modes. FIGURE 3 Relative delay of sky-wave signal relative to ground-wave signal P . 1 3 2 1 - 0 31 . 20 . 80 . 4100 300 1 0 0 00Td(ms)d (k m)FEFIGURE 4 Mutual delay of sky-wave signals for different hop numbers P . 1 3 2 1 - 0 4100 300 1 0 0 03210Td(ms)d
29、(k m )1 -31 -2FF1 -21 -3EThe range of distances for which the ground and sky-wave signal amplitudes are similar have particular interest since the fading exhibited in this zone is particularly severe. This has been called the “night fading zone” and has often set the limit for the range of good qual
30、ity MF broadcasting. 6 Rec. ITU-R P.1321-5 4 Variability 4.1 Time-variations signal in daytime 4.1.1 Seasonal variations The field strength of terrestrial waves can vary with seasonal temperature. For MF at mid latitudes with a continental climate and with a significant density of wooded areas, the
31、range of seasonal changes of the field strength of ground waves on links of up to approximately 100 km is on average within the limits of 10-18 dB. The smaller ranges are related to links beginning inside a large city (10 dB) or crossing a city (up to 15 dB). The greatest range has links which are i
32、n rural areas (15-18 dB). Similar results may be expected in other regions with similar climatic and natural conditions. The above paragraph refers to the East-European zone where the average January temperature is 10C. For other geographic zones, the average range of seasonal changes depends on the
33、 average January temperature, as shown in Table 1, since for links with similar soil/vegetative conditions the variation is only distinguished by the average January temperature. It is expedient to make an approximate estimate of the seasonal change in field strength proportionate to temperature ran
34、ge, taking account of differences in climatic conditions in various geographic zones. For example, for a link beginning in a city where the January temperature is +4C, the field strength range would be approximately 10 x (4/13) 3 dB, and for rural links would be approximately (15 18) x (4/13) (4.6.5
35、.5)dB, using data from the previous paragraph and from Table 1. TABLE 1 Average northern hemisphere January temperature (C) 4 0 10 16 Winter-summer field strength range, u (dB) 4 8 13 15 For LF the range of variation of field strength at mid latitudes with a continental climate (as measured in conti
36、nental Europe and the Siberian region) depends on distance and frequency, dependent on the parameter q = d f1/2, where d is the distance (km) and f is the frequency (MHz). Values of q 500, concern ionospheric sky waves. The corresponding formulas for the range of the variation are: for paths with a
37、small proportion of woodland: dB005.01023 25/ qqU ws for paths with a large proportion of woodland: dB124.21)(n1409.6/ qU wL Here the indexes s/w and L/w indicate a small proportion of woodland (approximately up to 30%) and a large proportion of woodland (more than 50%), respectively. Rec. ITU-R P.1
38、321-5 7 4.1.2 Day-by-day variations of the hour median For the value of the root-mean-square (RMS) deviation (L for LF and M for MF) the hourly median field strength from monthly median at LF depends on the path length, while at MF it depends on frequency. At LF, in medium latitudes with a middle pr
39、oportion of woodland, this dependence is: L = 0.073 d0.5 + 0.00122 d dB At MF, the RMS deviation with frequency for paths 20 km to 120 km without division into seasons, is: M = 0.0018f + 0.6 dB In these equations, L, M are RMS in dB, d is the distance in km, f is the frequency in kHz. 4.2 Variations
40、 of a signal in daytime from place to place At MF, the changes of level of a signal between locations separated by distances of the order of 1 km have similar values of standard deviation in different parts of the world. The probability distribution practically coincides with a log-normal law with a
41、 root-mean-square deviation = 3.7 dB, as shown in Fig. 5. In urban conditions in streets and areas, the standard deviation also is close to 4 dB. In densely built-up parts of a city, especially at small distances from the transmitter (up to 1 km), the standard deviation rises, reaching 7-8 dB. Insid
42、e buildings in rare cases additional absorption can reach 20 dB. 4.3 Variations of a signal in night-time Sky-wave modes will be subject to long-term night-to-night variability where the hourly median values have a log normal distribution with a semi-interdecile range of between 3.5 and 9 dB. Within
43、 the hour fading of individual modes also has a log normal distribution; there are few measurement data, but a typical value for the standard deviation of about 3 dB may be assumed. The fading rate is between 10 and 30 fades/h. For cases when the composite amplitude of the ground wave and sky-wave m
44、odes needs to be considered, i.e. in cases where the modes cannot be separated in the receiving system, the fading distribution of the signal is discussed in Appendix 1 to Annex 1. The frequency shift of sky-wave modes, due to the Doppler effect on reflection from moving ionospheric layers, will be
45、small. 8 Rec. ITU-R P.1321-5 FIGURE 5 The law of distribution of deviations P . 1 3 2 1 - 0 5150 . 10 . 0ProbabilitydeviationV al u e o f d ev i at i o n s (d B )10 5 0 10 1550 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 0Me as u re m en t re s u l t sC al cu l at i o n w i t h = 3 . 7 2 d B4.4 The ch
46、aracteristics of excesses and fades in LF and MF ionosphere channels For analysis and planning of digitally modulated radiocommunication systems in the LF and MF bands, the characteristics of the average values and signal dispersion appear to be insufficiently described. It is necessary to take into
47、 account more detailed properties of excesses and fades, in particular the probability distribution of excesses and fade durations need to be understood, at various signal-to-interference levels. The statistical characteristics of excesses and fades were obtained for a two-year period on two links,
48、one LF (1 550 km at 155 kHz) and one MF (860 km at 539 kHz), and are given below in Appendix 2. The results concern the middle geographical latitudes of the eastern hemisphere and moderate sun-spot activity (SSN 40). In Appendix 2, the number of excesses and fades per hour for each link are given in
49、 Tables 3 and 4. Figures 6 and 7 show scatter diagrams of the number (%) of median threshold excess durations for each link. 5 Conclusions Recommendation ITU-R P.1407 identifies a set of parameters for use in describing multipath effects. The “delay window”, containing more than say 98% of the total energy, may be determined from inspection of Fig. 2 as less than 3 ms. It may be noted that in some circumstances the initial multipath component will not be that with the greatest amplitude. Rec. ITU-R P.1321-5 9
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