Fading::
· There is a large dependence of fading on distance.
o The probability of a fade of a particular depth increases with the cube of
distance. Thus, as the distance is doubled, the probability of a particular fade
depth increases by a factor of eight. Or, alternatively, the fade for a given
probability increases by 9 dB. So, doubling the distance will increase the freespace
loss by 6 dB, and increase the probability of fading by 9 dB, thus
increasing the overall link-budget loss by 15 dB.
· There is a slight dependence of fading on frequency. Increasing the frequency by
1GHz will decrease the probability of a fade by a factor of 1.08.
· There is a fairly strong dependence of fading on the height of the path above sea
level.
o There is simply less atmosphere at higher altitudes and therefore the effect of
atmospheric fading is smaller.
o For every 1000 meter increase in altitude the required fade margin reduces by
10 dB.
· Types of Fading
o Fast fading - occurs when the coherence time of the channel is small relative
to the delay constraint of the channel. Fast fading causes rapid fluctuations in
phase and amplitude of a signal if a transmitter or receiver is moving or there
are changes in the radio environment (e.g. car passing by). If a transmitter or
receiver is moving, the fluctuations occur within a few wave lengths. Because
of its short distance fast fading is considered as small-scale fading.
o Slow fading - arises when the coherence time of the channel is large relative to
the delay constraint of the channel. Slow fading occurs due to the geometry of
the path profile. This leads to the situation in which the signal gradually gets
weaker or stronger.
o Flat fading – occurs when the coherence bandwidth of the channel is larger
than the bandwidth of the signal.
o Selective fading – occurs when the coherence bandwidth of the channel is
smaller than the bandwidth of the signal.
o Rayleigh fading - assume that the magnitude of a signal that has passed
through a communications channel will vary randomly.
o Ricean fading - occurs when one of the paths, typically a line of sight signal, is
much stronger than the others.
o Nakagami fading - occurs for multipath scattering with relatively larger timedelay
spreads, with different clusters of reflected waves.
o Weibull fading - considers a signal composed of clusters of one multipath
wave, each propagating in a non-homogeneous environment.