3.4.2 Coherence Bandwidth (CBW)
The frequency selective nature of the propagation channel arises when there exists dominant multipath
at significant time of arrival offsets. This next parameter is an attempt to statistically measure the BW
of the wireless channel. The CBW is defined as the separation of frequencies over which the propagation
channel has the same fading statistics (in other words range of frequencies where the channel is
considered to be frequency flat). Here the frequency separation between two carriers is increased until
the fading observed by each carrier begins to vary differently. This measured similarity between the
two carriers is defined where the correlation function is above 0.5 and is given below [1].
(3.42)
Note other definitions exist depending on the choice of the correlation function and relative importance
to the system designer. This shows the CBW is inversely proportional to the RMS delay spread
of the channel.
Let??™s consider the following two-ray multipath PDP example (see Fig. 1.17).
Given the PDP below, the mean excess delay is given as
t
(1) # (0) (0.1) # (2)
1 0.1
0.2
1.1
0.18sec
CBW
1
2p # tRMS
t2
ak
P(tk) # t2k ak
P(tk)
ak
a2k # t2k
ak
a2k
tRMS 2t2 (t)2
t
ak
a2k
# tk
ak
a2k
t
ak
P(tk) # tk
ak
P(tk)
WIRELESS MULTIPATH CHANNEL 131
And the RMS delay spread is calculated as
We can now write down the CBWfor this type of channel as
If the transmit occupied BW is larger than or similar to 277 kHz, then the transmitted signal will
encounter an FSF channel.
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