Tài liệu Complex Modulation Techniques

Implementations of complex modulations may have constellations with as many as
256 or 512 signal points. They correspond to operating at 8 bits/baud and 16
bits/baud. Great care is taken to arrange the signal points so that they are equidistant
from one another. This is necessary to provide an equal area around each point in
which errored signals may fall. An example of a 16-point constellation (4 bits/baud)
174 Connections, Codes, Signals, and Error Control
Figure A.6 Example of QAM to create a signal in which each symbol represents 2 bits.
is given in Figure A.7. In the upper diagram, the signal points are formed from a
minimum combination of two amplitudes and eight phase angles. The 16 signal
points are not uniformly distributed over signal space and the inner ring of eight
points has less signal space per point to cope with errors than the outer ring. To correct
this, a practical 16-point constellation is formed out of the combination of three
amplitudes and 12 phase angles shown in the lower diagram. The signal points are
distributed uniformly, and each has the same signal space as its neighbors.
The successful deployment of various flavors of digital subscriber lines depends
on the use of complex passband signal processing algorithms. Some of them are:
ã Pulse amplitude modulation: A popular modulation format uses trellis-coded
PAM with 3 bits per symbol and a 16-level constellation. The coding employs
twice as many signal points in the constellation as are needed to represent the
signal points. This redundancy is a form of forward error correction coding
and is used to reduce errors