PULSE-BASEDWIDEBAND RADIO - Ultra-Wideband Pulse-based Radio - page 88

The presented wideband radio architecture is based on a single-carrier modu-
lation method, not only because of the above mentioned reasons, but to avoid
in advance all difficulties that could emerge from the high Crest factor 5 (cf)
of multicarrier modulation. Thanks to the use of the interferer suppression and
signal reconstruction method (issr) introduced in Section 5.3, the system will
be able to survive in hostile environments with in-band interference and multi-
path reflections. The performance of the presented system will be a match for
an ofdm-based system, but at only a fraction of the power consumption.

5.1 Symbol rate versus multipath resolvability

It was shown before (Section 4.4) that a system using a high symbol rate has a
high multipath resolvability if a rake-compatible 6 system architecture is used.
The ability to resolve multipath signals is an important aspect of a wireless
system since it can help to increase the reliability of the system in slow fading
channels. However, resolving multiple reflections of the same signal becomes
increasingly difficult in the indoor environment, where the received signal has
a very limited delay spread, from less than 10 ns up to a very few 100 ns. In-
creasing the symbol rate only to increase the resolvability of the system may
not be a good investment, for several reasons. First of all, increasing the sym-
bol rate will increase the bandwidth of the entire transceiver and receiver chain,
with the obvious consequences for the power consumption. In low-psd radio
applications (e.g. uwb), the extra effort does not automatically translate into
an acceptable performance in terms of throughput. The reason for this is that
the symbol rate can become higher than the theoretical capacity of the channel.
Of course, this issue is easily resolved by combining multiple data bits (also
named chips ) into a single bit of information, but there are some important
side-effects which are easily underestimated. Increasing the input bandwidth
of the receiver not only makes the signal-processing stages more vulnerable
to accidental high-power narrowband interferers, but also leaves the door wide
open for the background (thermal) noise in the channel. As a conclusion, in-
creasing the symbol rate in order to increase the reliability of the system may
cause more problems than it solves. It is this insight that leads us to the idea of
pulse-based wideband radio.
In a pulse-based radio system, the symbols which are transmitted through
the channel are represented by short pulses (Figure 5.2). The duration and
shape of the individual impulses determines the spectral footprint of the pulse-
based transmission. This in sharp contrast to the continuous-time modulation

5 Crest factor: cf = √ papr.
6 rake-compatible: any receiver which is able to disentangle the different multipath streams. This can be
in the time domain (e.g. dsss receiver) or by an ofdm-alike system which is immune to isi.