In radio links, three kinds of scattering are utilized: tropospheric, ionospheric, and meteor scattering. The tropospheric scattering is due to turbulence in the troposphere and is the most widely utilized scattering mechanism in communication. This may provide a 500-km hop in the microwave region. The ionospheric scattering is caused by the cloudlike structures of the lowest layers of the ionosphere, and the meteor scattering is caused by the ionized trails of meteors. (The ionized trail is due to the high temperature produced by friction as the meteor enters the atmosphere at high velocity.) These scattering mechanisms may provide radio hops of 2,000 km at frequencies from 30 to 80 MHz. Furthermore, raindrops and snowflakes cause scattering, which may introduce interference between different radio links.

Fading or a random variation of the signal power level is typical for radio systems based on scattering. This phenomenon may be divided into a slow fading, which is due to large changes in the propagation conditions, and into a fast fading, which is due to multipath propagation. Fading can be partially compensated by a feedback, which is used to stabilize the signal power level, or by using diversity, as described in Section 10.6.

10.8 Propagation via Ionosphere

The highest layers of the Earth’s atmosphere are called the ionosphere, because they contain plasma, which is ionized gas (free electrons and ions). The ionosphere extends from 60 to 1,000 km. Below 60 km the ionization is insignificant because the solar ionizing radiation is getting weaker due to absorption in the higher layers, and because recombination of plasma is fast due to high density of molecules. Above 1,000 km the density of molecules is too low for a significant phenomenon. It is possible to distinguish different layers in the ionosphere, as shown in Figure 10.15; they are called D, E, F1 , and F2 layers. The electron density and the height of these layers depend on the solar activity, on the time of day and season, and on the geographical location. During night the D layer nearly disappears, and the F1 and F2 layers merge together. The highest electron density is about 1012 electrons/m3, and it can be found at daytime at the altitude of about 250 to 400 km in the F2 layer.

Let us consider radio wave propagation in plasma. The electric field having a strength E affects (accelerates) a charge q by force qE. If there are N charges in a unit volume, the current density in case of a sinusoidal field is (F = ma = mdv /dt = mjvv )

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Figure 10.15 Electron density in the ionosphere versus altitude.

where v is the velocity of the charge and m is its mass. Maxwell’s IV equation, (2.21), can now be written as

The charges cause a decrease of the permittivity of medium. Because the electron rest mass is only 1/1,836 of that of a hydrogen ion, the electrons determine the permittivity. The relative permittivity of plasma is

where e is the electron charge and m e is the electron mass, and

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Figure 10.16 Wave propagation via the ionosphere.

surface wave. The electric field strength of the wave decreases rapidly as the distance from the surface increases. At low frequencies the attenuation of such a ground wave is small, and the wave can propagate beyond the horizon thousands of kilometers, especially over seawater. However, the attenuation of a ground wave increases rapidly with frequency. Therefore this propagation mechanism is useful only below 10 MHz.

Attenuation of the wave depends on conductivity and permittivity of the surface (land, lake, or sea). Table 10.1 shows some typical characteristics at frequencies below 30 MHz. At microwave frequencies these characteristics change strongly as a function of frequency. ITU-R publishes field strength graphs for different surface types. Figure 10.17 presents the electric field strength versus distance when the wave propagates along the surface of a medium dry land and sea [14]. The transmitting antenna is a vertical monopole, and the power transmitted is 1 kW. The graphs show that at 10 kHz the electric field strength decreases as 1/r (as in free space) up to a distance

Table 10.1

Electrical Properties of Different Ground Surfaces (f < 30 MHz)

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of 1,000 km. At higher frequencies the attenuation is much higher. Over the sea the wave attenuates much slower than over medium dry land. In practice the electrical characteristics of the surface may vary a lot between the transmitting and receiving stations, and therefore a prediction of the power to be received is difficult.

References

[1]Salonen, E., et al., ‘‘Modeling and Calculation of Atmospheric Attenuation for Low- Fade-Margin Satellite Communications,’’ ESA Journal, Vol. 16, 1992, pp. 299–317.

[2]Zhang, W., S. I. Karhu, and E. T. Salonen, ‘‘Predictions of Radiowave Attenuations Due to a Melting Layer of Precipitation,’’ IEEE Trans. on Antennas and Propagation, Vol. 42, No. 4, 1994, pp. 492–500.

[3]Salonen, E. T., J. K. Tervonen, and W. J. Vogel, ‘‘Scintillation Effect on Total Fade Distributions for Earth-Satellite Links,’’ IEEE Trans. on Antennas and Propagation, Vol. 44, No. 1, 1996, pp. 23–27.

[4]van de Kamp, M. M. J. L., et al., ‘‘Frequency Dependence of Amplitude Scintillation,’’

IEEE Trans. on Antennas and Propagation, Vol. 47, No. 1, 1999, pp. 77–85.

[5]Skolnik, M. I., Introduction to Radar Systems, 2nd ed., New York: McGraw-Hill, 1981.

[6]Rappaport, T. S., Wireless Communications, Principles, and Practice, Upper Saddle River, NJ: Prentice Hall, 1996.

[7]Parsons, J. D., The Mobile Radio Propagation Channel, 2nd ed., Chichester, England: John Wiley & Sons, 2000.

[8]Foschini, G. J., and M. J. Gans, ‘‘On Limits of Wireless Communications in a Fading Environment When Using Multiple Antennas,’’ Wireless Personal Communications, Vol. 6, No. 3, 1998, pp. 311–335.

[9]Bach Andersen, J., T. S. Rappaport, and S. Yoshida, ‘‘Propagation Measurements and Models for Wireless Communications Channels,’’ IEEE Communications Magazine, Vol. 33, No. 1, 1995, pp. 42–49.

[10]Bertoni, H. L., Radio Propagation for Modern Wireless Systems, Upper Saddle River, NJ: Prentice Hall, 2000.

[11]Hata, M., ‘‘Empirical Formulae for Propagation Loss in Land Mobile Radio Services,’’

IEEE Trans. on Vehicular Technology, Vol. VT-29, No. 3, 1980, pp. 317–325.

[12]Walfisch, J., and H. L. Bertoni, ‘‘A Theoretical Model of UHF Propagation in Urban Environments,’’ IEEE Trans. on Antennas and Propagation, Vol. 36, No. 12, 1988, pp. 1788–1796.

[13]Keenan, J. M., and A. J. Motley, ‘‘Radio Coverage in Buildings,’’ British Telecom Technology Journal, Vol. 8, No. 1, 1990, pp. 19–24.

[14]International Telecommunication Union, ‘‘Ground-Wave Propagation Curves for Frequencies Between 10 kHz and 30 MHz,’’ Recommendation ITU-R P.368-7.