Digital communications
where T is the symbol period and N denotes the number of bits per symbol.
Phase trellis diagram.
As mentioned previously, CPM modulation is a form of
continuous phase FSK. However, it can also be viewed as a special form of offset
phase shift keying, OPSK, with sinusoidal symbol weighting. Ultimately, this
means that CPM transmissions may be decoded by demodulating the frequency
or, alternatively, the phase. For binary CPM the phase will traverse h
π
for every
symbol transition. Thus, one can map out a trellis diagram of allowed transitions
and phases over time. (See Figure 5.)
Figure 5: Phase trellis diagram for binary CPM with a rectangular
fi
lter
Note that if h is a simple rational fraction, the allowed phases will map onto
a
fi
nite number of allowed phases. For h = 1/2, for instance, there are only 4
allowed phases: 0,
π
/2,
π
, and 3
π
/2. Only 2 of the 4 phases are allowed at each
transition, however.
QAM (quadrature
amplitude modulation)
In quadrature amplitude modulation (QAM), both the amplitude and phase of the
constellation points are varied, usually in a rectangular array. In all other respects,
it is identical to phase shift keying.
QAM constellations.
The generators provide default constellations for QAM 4,
QAM 16, QAM 32, QAM 64, and QAM 256. The constellations are all arranged
as rectangular arrays with a simple right to left and top to bottom naming pattern.
The front panel displayed power corresponds to the constellation points in the
corners of the array. For QAM 32, it indicates the power of the “missing” point in
each corner. (See Figure 6.)
TSG4100A Series RF Signal Generators User Manual
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