Digital Signal Processing Reference
In-Depth Information
to arrive.
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Doing this way blocks a lot of interferer power immediately from
the input of the receiver, which is a good thing from a linearity perspective.
The input section of the pulse-based receiver also employs a bandwidth com-
pression technique, which converts the wideband character of the received
pulses back into a regular baseband qpsk signal. Since this pulse-to-baseband
conversion process occurs early in the signal chain of the receiver, it is suc-
cessfully prevented that the entire receive chain must deal with the wideband
character of the pulses. The remaining part of the receiver deals with the base-
band signal as any other qpsk modulated signal. Of course, at this point there
is still no benefit of using pulses for the reliability of the radio link. For this
purpose (Section 5.2), the receiver employs a set of parallel pulse-based re-
ceive units, which are dynamically allocated to a receive slot within the time
frame between to consecutive pulses.
Diversity combining: interleaved ISSR
Thanks to the resolvability of the short pulses, each receive unit will expe-
rience an independent fading (virtual) channel. It is evident that each of the
receive units still suffers from a fading channel, since closely separated multi-
path components still arrive within the duration of the receive slot. However,
the combined forces of multiple parallel receive units will boost the reliability
of the link (Section 4.4). For this purpose, the output of the individual pulse-
based qpsk receive units is first processed by the
interleaved
issr decoder
(Section 5.3). In the interleaved version of issr, the decoder employs the un-
affected bands supplied by different receive units in order to reconstruct the
original signal. The result is a much more reliable radio link because - except
for the case of interference - chances are very slim that the same frequency
band is unavailable at the same moment for different receive units operating in
(virtual) channels that have independent fading characteristics.
Finally, the whole concept of pulse-based radio with parallel receive units has
an additional advantage, related to the synchronization of the receiver on the
received pulse stream (Section 5.2): instead of chasing after every individual
'pulse' that may arrive at the antenna, the receiver deliberately ignores the fact
that pulses shift in and out of the receive slot allocated to a certain pulse-based
unit. This is caused by changes in the arrival time of the pulses, either due to
a varying propagation length of a certain multipath component or due to clock
offset between transmitter and receiver. Of course, this will result in a rotating
constellation diagram in the baseband output of the qpsk receiver, but this
issue must be handled at the level of the signal processor in the back-end of the
receiver.
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The length of a receive slot is longer than the pulse duration to simplify the synchronization process.
