Wireless Sensor Networks : Technology, Protocols, and Applications

RADIO TECHNOLOGY PRIMER 95
Ionosphere
Sky Wave
Surface Wave
Direct (Free-Space) Wave
Ground-Reflected Wave
Ground
Figure 4.1 Radio propagation modes. [Note: For WSNs the direct (free-space) wave is the
most common.]
in a straight line, filling the entire spherical volume of space with radio energy that
varies in strength with a 1/(distance) 2 rule (or 20 dB per tenfold increase in
distance) [4.1]; attenuation in environments that are not free space (e.g., waters,
coaxial cable, heavily wooded areas, confined rooms or structures) is considerably
more severe.
Three basic physical mechanisms affect radio propagation [4.2]:
1. Reflection. A propagating wave impinges on an object that is large compared
to the wavelength. (e.g., the surface of the Earth, buildings, walls).
2. Diffraction. A radio path between the transmitter and receiver is obstructed
by a surface with sharp irregular edges; waves bend around the obstacle, even
when line of sight (LOS) does not exist.
3. Scattering. Objects smaller than the wavelength of the propagating wave are
encountered along the way (e.g., foliage, street signs, lampposts).
These phenomena cause radio signal distortions and signal fading as described in
Table 4.1. Signal strength fluctuations caused by the fact that the composite signal
received comprises a number of components from the various sources of reflections
from different directions as well as scattered and/or diffracted signal components
affect both mobile and stationary receivers, whether the receivers are indoors or
outdoors. In this phenomenon, called multipath, signal fluctuations can be as
much as 30 to 40 dB. The intrinsic electromagnetic (radio) signal strength attenuation
caused by these phenomena is called a large-scale effect; signal-strength
fluctuations related with the motion of the broadcasting or receiving antenna are
called small-scale effects.
Reflection, diffraction, and scattering all give rise to additional radio propagation
paths beyond the direct line-of-sight path between the radio transmitter and receiver;