PULSE-BASEDWIDEBAND RADIO - Ultra-Wideband Pulse-based Radio - page 91

the receiver. 9 The same holds for accidental in-band narrowband interferers:
they are suppressed by the ratio between the on- and off-state (duty-cycle)
of the receive window. This implies that if the gating switch is located nearby
the antenna terminal, the share of interferer power in the total received power
figure is effectively reduced. One might think about this receive window as
the time-domain equivalent of a channel-select filter in the frequency-domain.
The result is the same, though, as unwanted interferer power is blocked early
in the receive chain, and the signal-of-interest is passed unattenuated to the
subsequent input stages.
For example, if the total on-time of the window adds up to 10% of the aver-
age interval between two pulses, an unwanted narrowband interferer will be
suppressed by 10 dB. For an ofdm-based radio approach, the unattenuated in-
terferer will have to pass through the entire receive and ad conversion chain
before it can be suppressed in the digital back-end of the receiver. As noted
earlier, the receiver puts a lot of effort (i.e. power) in the pursuit of an accurate
representation of this interferer in the digital domain, only to get rid of it at the
first available opportunity. Of course, an interferer suppression of only around
10 dB is far from sufficient for any practical application. However, the pulse-
based receiver architecture which will be introduced later on in this section
employs a two-level interference suppression scheme. Most of the interferer
energy is suppressed by the coarse window switch early in the receive chain.
This greatly relaxes the linearity requirements of the successive analog front-
end. In a second step, the remaining unwanted spurious energy is removed by
a more flexible, fast-adaptive filter in the digital domain.
In the discussion above, the outlines of a pulse-based radio system have
been described, but only very superficially. As the attentive reader may have
guessed, the benefits of using pulses instead of continuous-time modulated
symbols do not come for free. Two major problems have to be solved before
such an unconventional radio system can be deployed.
Window alignment
First, there is the problem of getting the receive window in line with the ar-
rival time of the pulses at the antenna. Failing to do this results in a complete
signal loss, since a misaligned window must be regarded as the time-domain
equivalent of a channel-select filter that is tuned into the wrong frequency
band. The shorter the duration of the pulses, the higher the gain in terms of
signal-to-noise ratio, but the more difficult it becomes to 'tune' into the correct
window position. As in many situations, there is always a trade-off between
9 In a narrowband subsampling receiver , exactly the opposite effect is experienced. The reason for this is
that a subsampling receiver rejects some valuable signal power, as a result of which the snr is decreased.