The Doppler spread is the spectral widening of the carrier signal caused by the time variations of
the channel. Recall the max Doppler frequency is dependent on the carrier frequency, vehicle speed,
and angle of arrival with respect to the velocity vector (direction of movement).
The CT is inversely proportional to the Doppler Spread [1, 7].
(3.47)
It is defined as the time duration in which two samples have a strong correlation. If the symbol time
duration is much less than the CT then the channel will not change over its transmission. If the CT is
defined as the time where the autocorrelation function is greater than 0.5, then we have the following
relationship (with the maximum Doppler spread defined as ).
(3.48)
Given the normalized time correlation function of
(3.49)
The corresponding spectrum is equal to
(3.50)
(3.51)
This is equivalent to the Doppler spectrum given earlier. The Fourier transform of the temporal correlation
produces the Doppler spectrum [18].
3.4.4 Indoor Delay Spread Measurements
In this section a brief overview of some of the delay spread measurements for an indoor environment
will be provided. [19, 20] First we present a measured PDP taken at 2.4 GHz band in Australia (see
Fig.
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