Digital Signal Processing Reference
In-Depth Information
of the delay profile that are caused by the clock offset between transmitter and
receiver. This causes a slow shift between the mutual alignment of transmit
and receive window, but should not be accounted for in the window-generating
subsection of the receiver. It means that for the receiver, no fixed relationship
is required between the clock of the window-generating circuit and the digi-
tal back-end. Clock-offset will indeed cause a rotating constellation in of the
received qpsk signal as seen by the signal processor, but this issue can be
solved without the need for a feedback path all the way up to the reference
local oscillator of the analog section.
Power efficiency and bandwidth compression
There is still one aspect of the pulse-based
radio system that has not been touched on in
this overview. It was already mentioned at
the beginning of this section that by adding
a gated transmit layer to the system, the
symbol rate could be decoupled from the
multipath resolvability of the system. How-
ever, the large bandwidth which is needed
to represent such short pulses consumes a
lot of power in the analog signal chain of the receiver. For reasons of power ef-
ficiency, the receiver must convert the pulse stream to the original continuous-
time qpsk modulated signal as soon as possible. From then on, the well-known
coherent receiver architecture can be mobilized for all further processing of the
baseband qpsk signal. Fortunately, the description of the transition process
from rf-pulses to a continuous-time qpsk modulated signal is more complex
than the actual implementation. Just like any other narrowband architecture,
the rf-pulses are first converted from passband to baseband by i/q-mixing
the received pulse stream with the center frequency of the transmit carrier. At
the output of the mixer, of course, nothing has been gained yet with respect to
the signal bandwidth. The actual bandwidth compression is obtained by low-
pass filtering the i/q mixer outputs beyond the symbol rate of the transmission.
Due to this bandwidth limitation, the signal at the output of the low-pass filter
is unable to track the fast changes of the down-converted pulses, and only the
envelope of the signal is retained. The i/q outputs of the envelope detector
23
yield the original continuous-time qpsk modulated symbol stream. From this
frequency
BANDWIDTH
COMPRESSION
23
Remark that the coherent envelope detector does not suffer from the dead-zone sensitivity problems as the
non-coherent diode-detector. In the am broadcasting service, the dead-zone of diode-detector based radios
is circumvented by limiting the modulation depth to about 85% of the peak carrier amplitude. At receiver
side, the strength of the residual carrier in the transmitted signal is commonly used to drive the agc circuit.
However, the rf carrier contains no information and is for the transmitter a pure waste of energy.
