WHAT IS FADING ?
Due to various
shadowing and terrain effects the signal level measured on a circle around base
station shows some random fluctuations around the mean value of received signal
strength. The long-term fades in signal strength, r, caused by the terrain
configuration and man made environments form a log-normal distribution, i.e
the mean received signal strength, r, varies log-normally in dB if the signal
strength is measured over a distance of at least 40l.
Experimentally
it has been determined that the standard deviation,s, of the mean
received signal strength, r, lies between 8 to 12 dB with the higher s generally
found in large urban areas.
RAYLEIGH FADING
This
phenomenon is due to multipath propagation of the signal.
The Rayleigh
fading is applicable to obstructed propagation paths.
All the
signals are NLOS signals and there is no dominant direct path.
Signals from
all paths have comparable signal strengths.
The
instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna
RICEAN FADING
This
phenomenon is due to multipath propagation of the signal.
In this case
there is a partially scattered field.
One dominant
signal.
Others are
weaker.
DOPPLERS SHIFT
Dopplers shift
is the shift in frequency due to the motion of mobile from the actual carrier
frequency.
Consider a
mobile moving at a constant velocity v along a path segment having a length d
between points X and Y while it receives signal from a remote source S.
The Change in frequency due to dopplers shift is given by
fd
= (v/l) * cos(f)
It can be seen
from the above equation that if the mobile is moving towards the direction of
arrival of wave the dopplers shift is positive I.e. the apparent received
frequency is increased. .
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