Wireless Network Design: Optimization Models and Solution ...

48 K. V. S. Hari
terrain of operation, atmospheric conditions, relative mobility of transmitters and
receivers, types of antennas used, heights of antennas, and other practical parameters.
3.3 Propagation Model for Wireless Channel
The propagation model for the wireless channel is characterized by path loss, multipath
fading and Doppler spread (or Doppler spectrum). All these characteristics are
affected by the physical environment between the transmitter and receiver as well
as system dependent parameters such as antenna heights, antenna beam-widths, antenna
polarization, and mutual coupling between multiple antennas.
Path Loss characterizes the attenuation between the transmitted and received signals.
This is typically based on physical phenomena like absorption, diffraction,
scattering, reflection and refraction. Absorption of RF (microwave frequency band)
signals in the atmosphere is due to the phenomenon of molecular resonance due to
the presence of water and oxygen molecules. Absorption, therefore, is higher during
rain and snowfall. Absorption also occurs when the propagation is through solids.
Buildings with metal attenuate between 10-25 dB depending on the thickness of
the walls, trees attenuate about 10-12 dB and metals attenuate the most. Diffraction
around objects occurs when the wavelength of the signal is comparable to the size of
the object. In the microwave region, for example, a 2.5 GHz signal has a wavelength
of 12 cm. Diffraction occurs when the signal encounters objects of these sizes and if
there are sharp edges, the signal ‘diffracts’ across the edges. Diffraction is also observed
in a city with buildings. Lower frequency signals can diffract over hilltops.
Scattering is a phenomenon which occurs when the signal encounters a cluster of
objects and multiple reflections from these objects causes the random scattering effect.
An example of a scattering environment is foliage. Trees with lots of branches
and moving leaves (due to wind) scatter the signal significantly.
Path loss is a function of the frequency of operation, the distance between the
transmitter and receiver, terrain of operation, and system parameters like antenna
heights and antenna characteristics.
Fading is a phenomenon which occurs when the scattering environment between
the transmitter and the receiver varies with time. This change in the scatterers alters
the signals being added constructively or destructively at the receiver location,
as a function of time. This change in the signal level is called fading. It is obvious
that the terrain of operation (open spaces, dense foliage, tall buildings, hilly areas)
will determine the level of fading. Microscopic fading occurs when the receiver
receives multiple copies of signals due to scatterers close to the receiver. This type
of fading is also called flat fading. Macroscopic Fading occurs when the receiver
receives multiple delayed copies of the transmitted signal from a collection of scatterers,
which are distributed in space over a large region. This type of fading is also
called frequency selective fading. Frequency selective fading is characterized by a
parameter called delay spread of a channel [40]. It indicates the statistical nature