Radio Signal Propagation

The salient features of KF propagation are briefly described in this section. For a detailed treatment of this subject, the reader is referred to Kefs. 7 and 10. The three basic propaga­tive mechanisms, illustrated in Fig. 15, are reflection, diffrac­tion, and scattering. Together, these three modes enable us to estimate the signal level received by a transmitter for a given KF propagative channel.

• Reflection: occurs when a radio wave propagating in one medium is incident upon another medium that has dif­ferent electrical properties and a part of the energy is reflected back into the first medium, depending on the specific electrical properties of the second medium. If the second medium is a perfect conductor, all of the incident energy is reflected. If the second medium is a dielectric, then the energy is only partially reflected. The reflection coefficient is a function of the medium’s properties, the signal frequency, and the angle of incidence. Keflections of KF signals typically occur from objects in the propaga­tive path whose size is larger than the wavelength (A) of the KF carrier, such as buildings and walls. In the case of cellular/PCS signals at 1.9 GHz, the wavelength A =

Scattering

Radio Signal Propagation

Transmitter

Figure 15. The different modes of KF signal propagation, reflection, diffraction, and scattering.

Radio Signal Propagation

15 cm « 6 in. Hence, a variety of objects act as reflectors. Signals are also reflected from the ground. A model com­monly used to characterize KF channels is the two-ray ground reflection model (10).

• Diffraction: can be viewed as the ‘‘bending’’ of KF signals around an obstruction, as shown in Fig. 15. Diffraction occurs when the obstruction between the transmitter and receiver has sharp edges. As explained by Huygen’s prin­ciple, when a wavefront impinges on an obstruction, then secondary wavelets are produced, which give rise to bending of waves around the obstruction. The field strength of the diffracted wave in the shadowed region is the vector sum of the electric field components of the secondary wavelets. The knife-edge diffraction model (10) can be used to characterize the diffraction caused by a single object, such as building in the path of an KF sig­nal.

• Scattering: occurs when the KF signal is incident on a surface that has a certain degree of ‘‘roughness’’ (7,10). Scattering in an KF channel is commonly caused by ob­jects, such as buildings. The critical height hc = A/8 sin ei, where 0i is the angle of incidence. This implies that the maximum to minimum level of the surface must be greater than hc.

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