Digital Signal Processing Reference
In-Depth Information
antenna. There is a good reason why this matter is brought to the attention of
the reader, though. Wideband antennas are very difficult to design. Matching
problems over a wide spectral range (500 MHz+ for the uwb specifications
by the fcc) and the physical dimensions of the antenna causes dispersion of
the signal. Even for a perfectly matched antenna, any conducting element in the
neighborhood changes its impedance. The result is that the rf-energy of the
pulses is spread over a longer time. The time-limited pulse that is transmitted,
collapses during transport and different frequency components will arrive with
different time delays at the output of the receive antenna.
For the receiver, it makes no sense to make the window length shorter than
the total dispersion time caused by the transmit or receive antenna: dispersion
invalidates the assumption of independent fading between the distinct paral-
lel receive slots, which is necessary precondition to increase reliability. The
reader should be aware that is not an isi-related problem, since it only affects
the multipath resolvability of the pulse-based radio receiver. The best way to
compensate for the reduced number of slots per receive antenna is to increase
the number of physical antennas.
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Architecture of the interleaved ISSR decoder
The purpose of the interleaved issr decoder described here is to merge mul-
tiple independently fading symbol streams which are captured by parallel re-
ceive units into a single output stream with a more reliable signal quality (i.e.
a lower standard deviation on the received signal power). The entire process
of signal recombination occurs before symbol-to-bit demapping, which means
that no permanent or irreversible decisions about the received symbol are taken
by the interleaved issr decoder itself. This task is entrusted to the error cor-
rection mechanism which is located further on in the signal processing chain.
For more information on the topic of issr, the reader is referred to the issr
decoder introduction in Section 5.3. Remember that the internal workings be-
hind the scenes of issr are based on the reconstruction of frequency domain
information which has gone missing as a result of frequency-selective fading.
As a result of intersymbol interference, each of the parallel receive modules
of the pulse-based radio receiver offers a frequency-selective fading symbol
stream. It would be an interesting approach if the available frequency bands
from every symbol stream could be joined (Figure 5.12), in the attempt to fill
in missing holes in the frequency spectrum as much as possible. Any parts of
the spectrum that remain missing, or are unavailable in all streams due to nar-
rowband interference are subsequently reconstructed using issr. It is evident
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Note that the physical distance between the antennas must be large enough to ensure independent fading
characteristics ([Fuj01] p. 183).
