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Propagation Measurements and Models for Wireless Communications Channels
Jorgen Bach Anderson, Theodore S. Rappaport, and Susumu Yoshida
Complex interactions between geometry, radio propagation effects, and
mobility make prediction of coverage and path loss a practically
intractable problem; rudimentary models have been developed, but
empirical or heuristic methods nearly inevitably fare better.
- Propagation mechanisms: wave propagation is physically explained
by three phenomena - reflection (reflective object >> wavelength),
diffraction (obstructive object >= wavelength), and scattering (obstructive
object < wavelength).
- Raleigh fading: the field detected at the receiver is
the sum of many random contributions of different phase and directions,
due to multipath effects deriving from the 3 propagation mechanisms.
Raleigh fading implies rapid fluctuation - "small scale fades".
Raleigh fading occurs typically for non-line-of-sight (NLOS) environments.
In LOS environments, smaller delay spreads (Ricean distribution) dominates.
- Path loss: path loss at some distance d from a transmitter,
PL(d), is a local average measurement of received signal strength.
The empirical path loss relationship is
PL(d) = PL(do) +
10nlog(d/do) + Xsigma, where PL(do) is some reference
distance, n is a power law parameter known as the "power-delay
index", and Xsigma is a Gaussian random fudge factor. n=3-4 is
typical of NLOS Raleigh fading, and n=2 is typical of Ricean fading or
free space. A more complex indoor propagation formula is in Randy's
- Delay spread: multipath effects create time dispersion, a
smearing or spreading out of the signal. Urban delay spreads of around
3 microseconds are commonplace, but much higher values such as 10-100 usec
have been observed. Important characteristics are the maximum excess
delay (MED), the MED at X db monotonically less than the maximum,
the mean excess delay (first moment of power delay profile), and RMS
- Path loss models: All models of path loss are empirical in
nature, and quite inaccurate. Macrocells in particular suffer from
extreme variations in path loss, while the lower power and coverage
of microcells makes the propagation characteristics "milder",
enabling broadband techniques.
- Indoor propagation: Signal characteristics are much less
predictable indoors. Building materials, physical layout, and the
number and position of receivers all greatly affect coverage. The
motion of people within the building causes Ricean fading in LOS paths,
while Raleigh fading still dominates OBS paths. CAD-like tools can
be used to aid prediction, although their accuracy is low.
The physics of radio propagation dictate wireless networks' physical
layer characteristics, which in turn affect the design and performance
limitations of all higher level network layers. Knowledge of radio
propagation characteristics may give insight into wireless systems design.
- Radio propagation characteristic models are inaccurate and highly
heuristic. This is a fault of the science in general, not this paper.
- The paper is a bombardment of factoids; it is hard to weed through
them to find the truly valuable insights.
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