Digital Signal Processing Reference
In-Depth Information
Figure 17.14. 64-QAM recovered
constellation.
receiver noise level increases, the constellation samples would
quickly start to drift together on the 64-QAM constellation, and it
would not be possible to accurately determine which constella-
tion point a given symbol should map to. The 16-QAM system is
more robust in the presence of additive noise and other impair-
ments than the 64-QAM.
The
mo
dulation and demodulation (modem) ideas presented
in this chapter are used in most digital communication systems,
including satellite, cable and fiber systems used to transmit video
signals. In QPSK all four symbols have the same amplitude. The
phase in the complex plane is what distinguishes the different
symbols, each of which is located in a different quadrant. For
QAM, the amplitude and phase of the symbol
is needed to
distinguish a particular symbol.
In general, communication systems are full of trade-offs. The
most important comes from a famous theorem developed by
Claude Shannon which gives the maximum theoretical data bit-
rate which can be communicated over a channel, known as the
Shannon limit. This depends upon bandwidth, transmit power
