Digital Signal Processing Reference
In-Depth Information
of the rf-signal is effectively
gated
to the antenna. The linearity of the power
amplifier is of little importance, because the spectral footprint of the transmis-
sion is determined by the duration and the smoothness of the transmission win-
dow. The opening and closing times of the transmit window are synchronized
with the underlying qpsk transmitter, in the sense that a transition between
two constellation points always occurs during the off-state of the transmit gate.
The way in which the transition occurs is not of importance, as long as the
phase of the carrier has been settled before the start of the next
transmission
gate
. Since only a small portion of the modulated carrier is injected into the
channel, synchronization of the window with the phase modulator ensures that
all necessary phase information is included in the transmitted signal.
The time-domain output of the transmitter has the appearance of a stream of
very short pulses, hence the name
pulse-based radio
that is used throughout
this text. The period between two consecutive pulses shows small time varia-
tions, caused by the phase modulation of the underlying qpsk engine. How-
ever, it is strongly advised not to drift away from the original concept and
become obsessed by the pulse-thing from this moment on. It is also an unfor-
givable mistake to try to build a receiver which is based on a 'pulse detector'
or - even worse - try to measure the spacing between pulses and call this ap-
proach 'pulse position (de)modulation' [Mag01]. Instead of this, the reader is
strongly encouraged to consider the transmitted signal of the pulse-based radio
system as any other single-carrier phase-modulated system. The only differ-
ence being that some non-crucial portion of the time-domain signal is cut away
by the transmitter in order to increase the multipath resolvability of the trans-
mission (Figure 5.15). When the stream of uniformly spaced pulses arrives at
Multiple delayed versions of the
same signal arrive at the receiver.
The resolvability of a narrowband system is too
low to differentiate between multipath streams.
A pulse-based radio system has a better multi-
path resolvability for the same symbol rate.
The receiver can freely choose the resolution
bandwidth, determined by the rx window length.
Figure 5.15.
Using the same baseband symbol rate, the pulse-based system offers
a better multipath resolvability than a continuous-time modulated sig-
nal. This approach eliminates the need for an impractically high-speed
and power-hungry baseband section.
