Page 27 - Integrated Wireless Propagation Models

I n t r o d u c t i o n t o M o d e l i n g M o bi l e S i g n a l s i n W i r e l e s s C o m m u n i c a t i o n s 5

height of the mobile antenna is close to the ground level, and usually the mobile signal
cannot be a line-of-sight signal received from the base station. Two or more wave paths
of a signal would be created by reflection, refraction, or diffraction from the surround­
ings and arrive at the mobile receiver antenna at different times.
The combination of these wave paths generates a resultant signal that can be very
different from the original transmitted signal in both amplitude and phase while arriv­
ing at the mobile. Its received signal is also different from each location every multiple
of a half wavelength apart on a road.

1.6.1.2 In a Moving Condition
When a mobile unit is traveling along a street, climbing a hill, or moving inside a build­
ing, the mobile antenna receives a signal, strong and weak, along the path. This is called
signal fading. Mobile signal fading is hard to handle in mobile communication recep­
tion. If we can predict the received mobile statistically (i.e., long-term fading) before the
system is deployed, then we can plan a highly efficient system in the field. The time
consumed, the labor, and the cost for deploying a system will be reduced. Otherwise,
the cut-and-try method in the field has to be used. In addition, the prediction tool also
helps engineers find reasons that signal strength is unexpectedly high or low.

1.6. 1.3 Obstruction Condition
There are two obstruction conditions that the received mobile signal may experience:
1. Obstructed or nonobstructed by human-made structures or a forest:
• Nonobstructed by a human-made structure or forest-it is in a LOS
condition.
• Obstructed by a human-made structure or forest-it is in a non-line-of-sight
(NLOS) condition.
2. Obstructed or nonobstructed by the terrain:
• Nonobstructed by terrain-it is in a nonshadow region.
• Obstructed by terrain-it is in a shadow region.

1.6.1.4 Direct Path and Diffraction Path
1. Direct path-A radio path is not obstructed by the terrain. There are LOS path
and NLOS paths.
2. Diffraction path-A radio path is obstructed by the terrain in a shadow region.

1.6.2 T y pes of Signal Fading
1
Different types of fading 7 depend on the natural properties of the original transmitted
signal and related parameters, such as path loss, bandwidth, symbol period, and frequency.
Various types of scattering and multipath phenomena, which can cause severe sig­
nal fading, are attributable to the mobile-radio communications medium and also affect
mobile-radio fading signals. Two types of fading, long term and short term, compound
the mobile-radio signal fading as mentioned earlier.
In short-term fading, the types of fading can be further classified as follows:

1. Based on the time-delay spread affected in the received signal:
Flat fading and frequency-selective fading.