Rec. ITUR P.1321
RECOMMENDATION ITUR P.1321^{*}
PROPAGATION FACTORS AFFECTING SYSTEMS USING DIGITAL
MODULATION TECHNIQUES AT LF AND MF
(Question ITUR 225/3)
(1997)
Rec. ITUR P.1321
The ITU Radiocommunication Assembly,
considering
a) that digital modulation methods for sound broadcasting purposes at LF and MF are currently being studied under Question ITUR 217/10;
b) that information on the propagation characteristics at these frequencies is necessary for use in the design of modulation methods,
recommends
1 that the information given in Annex 1 should be taken into account in the design of digital 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 ITUR P.368). The limiting coverage range, during daytime and in the absence of interference, is determined by the intensity of radio noise due to lightning and to manmade sources (see Recommendation ITU R P.372) and by the required signaltonoise ratio. During the hours of darkness, ionospheric sky wave modes become important (see Recommendation ITUR P.1147). For analogue amplitude modulation, these modes limit the coverage, since the waveinterference between the ground wave and the varying and phase delayed sky wave modes results in unsatisfactory signal quality. Skywave signals from other distant transmissions may also add significant nighttime interference, 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.
Digital modulation methods may also be affected by the presence of delayed signal 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 methods may be required for service planning purposes.
2.1 Ground wave mode
The ground wave is essentially constant and does not exhibit significant changes with time or with atmospheric variability. As shown in Recommendation ITUR P.368, the signal amplitude depends on range and on the electrical characteristics of the ground.
2.2 Skywave modes
During daylight hours, signal attenuation in the lower Dregion part of the ionosphere effectively prevents sky wave propagation. This Annex concentrates on the conditions at night when skywave propagation may be significant.
The Elayer 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 Elayer, and multihop reflections will also be supported. Signals at higher frequencies may still be reflected from the Elayer, particularly to longer ranges, but signals will also penetrate the Elayer to be reflected from the higher F region. Using a simple model for the Elayer, Fig. 1 illustrates the 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 ITUR P.1147, provides predictions for the composite signal power for the available skywave 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 the ionospheric reflection coefficient for sunspot minimum conditions, based on experimental results, and on some assumptions, as stated in the Recommendation.
Using the above simple propagation models, Fig. 2 shows the expected median field strengths and relative time delays 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 skywave signals at the shorter ranges.
The mode shown at 0 ms is for the ground wave, and field strengths are shown for three values of ground conductivity; 5 S/m (sea water), 10^{–2} (good ground), and 10^{–3} (poor ground).
The skywave components are marked with the relevant mode and the levels approximately represent the median field strengths four hours after sunset at sunspot minimum.
Figure 3 indicates the delay of the one hop E and Fregion skywave modes relative to the ground wave for ranges out to 1 000 km, while Fig. 4 gives the relative delays between single and multihop skywave modes.
The range of distances for which the ground and skywave 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 quality MF broadcasting.
The ground wave mode may be assumed to be unfading. Skywave modes will be subject to long term night to night variability where the hourly median values have a log normal distribution with a semiinterdecile range of between 3.5 and 9 dB. Within 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 per hour.
For cases when the composite amplitude of the ground wave and skywave modes 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.
The frequency shift of skywave modes, due to the Doppler effect on reflection from moving ionospheric layers, will be small.
FIGURE 1/M.1321...[D01] = 3 CM
FIGURE 2/M.1321...[D02] = 3 CM
FIGURE 3/M.1321...[D03] = 3 CM
FIGURE 4/M.1321...[D04] = 3 CM
5 Conclusions
Recommendation ITUR 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.
APPENDIX 1
TO ANNEX 1
The composite signal amplitude, e, for the combination of a steady ground wave signal and a log normally distributed sky wave signal is obtained by a power summation:
where e_{e}_{ }and e_{i} are the levels of the ground wave and skywave components, usually expressed in V/m.
The skywave component e_{i} has a log normal distribution (see Recommendation ITUR P.1057, equation (6)). For convenience it is supposed ostensibly that the ground wave component is also log normally distributed, and the final result is obtained by setting its standard deviation to 0 dB.
The combination of two lognormal distributions is also lognormally distributed where the mean level is the sum of the individual mean levels (i.e. in amplitude, not in decibels) and the variance is the sum of the two variances.
For a lognormal distribution (see Recommendation ITUR P.1057) the mean and the standard deviation of the signal levels (V/m) are given by:
where m is the mean and is the standard deviation of the lognormal distribution.
Using these considerations it is possible to evaluate the parameters for the combined distribution. Table 1 gives example results where the standard deviation of the lognormal skywave component is 3 dB.
TABLE 1
e_{i} / e_{e}

Mean level relative to the mean of the ground
wave component

Standard deviation

0.5 (– 6 dB)

1.3 dB

0.72 dB

1 (0 dB)

4.4

1.35

2 ( 6 dB)

5.7

2.0

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