Digital Signal Processing Reference
In-Depth Information
frequency
functionality of correlator
taken over by low-pass
filter and time-window.
I/Q
time-window
DC
continuous-time LO signal
Ruling out the matched filter receiver, the reader may question if there is still
any difference with a standard heterodyne receiver architecture. Indeed, the
only remaining part making the described architecture suited for pulse-based
radio reception is the window switch in front of the receive chain. In combina-
tion with the mixer and the low-pass filter, the switch forms a basic correlator
structure and is the essential component to resolve the closely spaced multi-
path components. Based on the previous reflections, the reader should start to
see the idea of pulse-based radio in a new light. Instead of focussing on the
generation and demodulation of pulses, the pulse-based radio system that will
be elaborated in the next sections should be considered as an
extension on the
narrowband radio architecture
.
This cryptic statement should be understood as follows. Suppose a single-
carrier qpsk modulated radio system. Without any change to its working
principles, the transceiver hardware is expanded with an extra layer between
the transmitter (receiver) and the channel, with the purpose to increase the
multipath resolvability of the system: instead of dropping the rf-carrier of
the transmitter directly into the transmission antenna, only a small fraction
of the rf-signal is allowed to pass (Figure 5.7). The signal that is applied to
the channel has the appearance of short pulses, but is in fact a well-controlled
qpsk-modulated signal. Remark that cutting away some of the time-domain
information does not alter the phase-information contained in the residual sig-
nal that is applied to the antenna. The duration of the time-window determines
the spectral mask of the transmission, but apart from this, the window is not im-
portant for the qpsk symbol as such. Also important is that jitter on the edges
of the window does not corrupt the phase angle of the transmitted constellation
point. The window switch in the analog front-end has to be synchronized with
the i/q-modulator in the digital part of the transmitter. Transitions between
two constellation points in the qpsk constellation diagram must occur during
the off-state of the transmit window. The way the transition is done is of little
importance, as long as the carrier phase has been settled when the transmit
