Radio Wave Propagation
The initial understanding of radio wave propagation goes back to the pioneering work of James ClerkMaxwell, who in 1864 formulated the electromagnetic theory of light and predicted the existence of radio
waves. In 1887, the physical existence of these waves was demonstrated by Heinrich Hertz. However, Hertz
saw no practical use for radio waves, reasoning that since audio frequencies were low, where propagation
was poor, radio waves could never carry voice. The work of Maxwell and Hertz initiated the field of radio
communications: in 1894 Oliver Lodge used these principles to build the first wireless communication
system, however its transmission distance was limited to 150 meters. By 1897 the entrepreneur Guglielmo
Marconi had managed to send a radio signal from the Isle of Wight to a tugboat 18 miles away, and in
1901 Marconi’s wireless system could traverse the Atlantic ocean. These early systems used telegraph
signals for communicating information. The first transmission of voice and music was done by Reginald
Fessenden in 1906 using a form of amplitude modulation, which got around the propagation limitations
at low frequencies observed by Hertz by translating signals to a higher frequency, as is done in all wireless
systems today.
Electromagnetic waves propagate through environments where they are reflected, scattered, and
diffracted by walls, terrain, buildings, and other objects. The ultimate details of this propagation can
be obtained by solving Maxwell’s equations with boundary conditions that express the physical charac-
teristics of these obstructing objects. This requires the calculation of the Radar Cross Section (RCS) of
large and complex structures. Since these calculations are difficult, and many times the necessary param-
eters are not available, approximations have been developed to characterize signal propagation without
resorting to Maxwell’s equations.
The most common approximations use ray-tracing techniques. These techniques approximate the
propagation of electromagnetic waves by representing the wavefronts as simple particles: the model
determines the reflection and refraction effects on the wavefront but ignores the more complex scattering
phenomenon predicted by Maxwell’s coupled differential equations. The simplest ray-tracing model is the
two-ray model, which accurately describes signal propagation when there is one direct path between the
transmitter and receiver and one reflected path. The reflected path typically bounces off the ground, and the two-ray model is a good approximation for propagation along highways, rural roads, and over water.
We next consider more complex models with additional reflected, scattered, or diffracted components.
Many propagation environments are not accurately reflected with ray tracing models. In these cases it
is common to develop analytical models based on empirical measurements, and we will discuss several of
the most common of these empirical models.
Often the complexity and variability of the radio channel makes it difficult to obtain an accurate
deterministic channel model. For these cases statistical models are often used. The attenuation caused
by signal path obstructions such as buildings or other objects is typically characterized statistically, as
described in Section 2.7. Statistical models are also used to characterize the constructive and destructive
interference for a large number of multipath components, as described in Chapter 3. Statistical models
are most accurate in environments with fairly regular geometries and uniform dielectric properties. In-
door environments tend to be less regular than outdoor environments, since the geometric and dielectric
characteristics change dramatically depending on whether the indoor environment is an open factory, cu-
bicled office, or metal machine shop. For these environments computer-aided modeling tools are available
to predict signal propagation characteristics.
