Digital Signal Processing Reference
In-Depth Information
filter, located at the output of the downconversion mixer. From this moment on,
it will appear as if a regular qpsk modulated signal is being received: because
the bandwidth of low-pass filter and the consecutive analog signal processing
chain is too small to represent the short pulses, the gap between two pulse sym-
bols is automatically filled just as if a regularly continuous modulated qpsk
signal is being received.
The bandwidth of the filter cannot be chosen arbitrarily, though: the infor-
mation gets corrupted when the minimum bandwidth constraint necessary to
represent the original baseband signal is not met. It is also clear that an analog-
domain implementation of the low-pass filter will cause considerable inter-
symbol interference (isi) when the 3 dB-bandwidth of the filter approaches the
Nyquist bandwidth of the baseband qpsk stream. This is due to the infinite
impulse response of analog (continuous-time) filters. However, complex mod-
ifications to the analog front-end should be avoided as this is only a minor issue
that can be easily solved by the equalization filter and the issr section in the
digital back-end of the receiver.
Looking back to the start of this section, the bandwidth compression technique
described here is in fact a very basic implementation of a matched filter re-
ceiver. By looking at the problem from a unusual point of view, it was found
that the classic heterodyne topology can be transformed into a pulse-based ra-
dio system with only minor conceptual changes. The only important thing to
remember from this section is that a pulse-based radio can be seen as a mod-
ified version of a single-carrier radio system. A considerable portion of the
carrier is cut away just before the rf-signal is applied to the antenna. How-
ever, the internal workings of the system are no different than for any other
qpsk transmitter-receiver configuration.
As a closing note, it is stressed again that the main focus of pulse-based wide-
band radio should be on the problem that is being addressed. In this case
the goal is to improve the multipath resolvability of the system in an indoor
channel with considerable multipath scattering but a limited delay spread. An
approach that focuses too early on
pulse-shaping
,
pulse modulation
or the
Ultra
in Ultra-Wideband is in danger of getting caught up in low-level details
(Figure 5.8) and may end up in a deadlock situation.
5.2
Synchronization
The previous section has introduced the basic ideas behind pulse-based wide-
band radio. It was shown that the names 'pulse-based radio' or uwb may be
a proper description of the characteristics of a pulse-based radio signal in re-
spectively the time and the frequency domain, but are in fact very misleading
names when it comes to understanding the larger picture. After all, it is quite
