Digital Signal Processing Reference
In-Depth Information
between the received pulses and the window position controlled by the re-
ceiver: a small frequency offset between the clocks of the transmitter and the
receiver will cause an increasing error between their time references. If the re-
ceiver is unable to monitor these offsets, this eventually will result in signal loss
from the moment the pulses shift outside the scope of the receive window.
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For example, if the clock offset between transmitter and receiver is 25 ppm and
the duration of the receive window is set to 1 ns, it will take around 40
s for the
pulses to go completely out of scope. For a symbol rate of 100 Msymb/s, the
tracking algorithm has a time frame of 4,000 pulses to track and compensate
for this error. Despite the fact that in a pulse-based radio system the symbol rate
is decoupled from the pulse duration, this finding shows that there is a physical
constraint on the ratio between the pulse duration and the symbol interval: for
a very low bit rate, the number of symbols before the pulses become out-of-
sync with the receive window is too limited for the receiver to be able to obtain
a reliable estimate on the clock offset. A possible solution for these low data
rate communication systems could be to use a sufficiently high symbol rate
and to transmit data in short bursts. But the reader should be aware that if the
idea behind using a low data rate is to save power, the use of a pulse-based
wideband radio system may not be a good design choice.
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After all, a pulse-
based receiver must keep the entire wideband front-end running in order to
monitor the channel for incoming messages.
μ
Bandwidth compression: the basis of power efficient pulse-based radio
The second issue that has to be addressed by the designer of a pulse-based radio
receiver is the considerable bandwidth of the received pulses. The advantage
of using short pulses (typically 2 ns or less) instead of a continuous-modulated
signal is that the symbol rate is independent from the spectral characteristics
of the transmitted signal. The use of short pulses provides an improved multi-
path resolvability in indoor channels with a low delay-spread, without the need
to increase the symbol rate to impractical high levels. In terms of power con-
sumption, the big bonus of using pulses is that a significant amount of interferer
power can be eliminated early in the receive chain.
The benefit in terms of bandwidth depends on where the demodulation of the
pulses is done. If demodulation and symbol demapping is deferred to the digital
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The frequency offset of an xtal-based oscillator is typically a few tens of ppm.
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One of the applications where the use of a low-throughput wideband system could be justified is the
use in rfid asset tracking tags [Fon04]. The tags themselves do not have their own power supply (under
a best-case scenario a small battery) and the tag contains only a small pulse-based transmitter. The use of
short pulses allows a network of receivers to collect ranging data - much like a radar system - in order to
determine the approximate location of a tag.
