For example, in DSCDMA
systems the PDP is continually estimated and updated in order to find multipaths that can be
used in the RAKE receiver.
For sake of discussion we will assume the wireless multipath channel consists of a two-ray
model. Where h1(t) and h2(t) represent the time-varying channel response of the first arriving and
second arriving multipaths, respectively. In the case of ???K??? multipath rays we have provided earlier
an equation to determine the rms delay spread of the channel that can also be used. Continuing along
with the simplified example, we can take the ratio of the relative channel powers to determine if
the channel is very frequency selective (i.e., both have large average power) or somewhat frequency
selective (i.e., large difference in average powers) or nonfrequency selective (i.e., single
large average power).
As an example, we may want to enable another mode (say mode #2) in the receiver if the delay
spread is significant otherwise continue with mode #1. This can be based on the following
comparison:
or (6.105)
The benefit of this technique is that time slot synchronization may not be needed depending on the
method used to derive the channel estimates. Also we can make use of the information that is already
available in the receiver so additional signal processing may not be required.
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