Digital Signal Processing Reference
In-Depth Information
Figure 17.13. 16-QAM recovered
constellation.
The receiver also has a number of other problems to contend
with. The sampling is assumed to be at the correct instant in time
when one symbol has a non-zero value in the signal. The receiver
must determine this correct sampling time, usually by a combi-
nation of trial and error during the initial part of the reception,
and sometimes by having the transmitter send a predetermined
(or training) sequence known by both transmitter and receiver.
This process is known as acquisition, where the receiver tries to
fine-tune the sampling time, the symbol rate, the exact frequency
and phase of the carrier and other parameters which may be
needed to demodulate the received signal with a minimum of
errors. And once all this information is determined, it must still be
tracked, to account for differences in transmit and receive clocks,
or other impairments.
Figures 17.13 and 17.14
are plots from both a 16-QAM and
64-QAM constellation after being sampled by an actual digital
receiver
a WiMax wireless system operating in the presence of
noise. Each receiver signal has the same average energy. The
receiver does manage to do a sufficiently good job at detection so
that each of the constellation points is clearly visible. As the
e
