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Digital communications
Offset or staggered
modulation
This type of modulation addresses transmitter problems in the communication
design. RF ampli
fi
ers can be made to operate more ef
fi
ciently if the signals they
are amplifying are nearly constant in amplitude. This is especially important for
satellites deployed in space. The dif
fi
culty is that the ampli
fi
ers have a nonlinear
response in this regime. The nonlinearities are often not problematic as long as
the amplitude variations are contained within a small band. Unfortunately, normal
QPSK modulation does not meet this criterion. Remember that even though the
constellation points are de
fi
ned with constant amplitude, the RF amplitude varies
as it transitions from one point to the next. For transitions of 180°, the signal
power will momentarily go all the way down to zero. Nonlinear ampli
fi
ers forced
to make such a transition will create out-of-band interference, thus, defeating the
whole purpose of the pulse shaping
fi
lters.
Offset modulation addresses this problem by modifying the modulation to prevent
a transition through the origin. (See Figure 9.)
Figure 9: Offset modulation prevents transitions through the origin
In normal QPSK modulation, I and Q data are shifted into the pulse shaping
fi
lters
simultaneously. With offset or staggered modulation, the shifting of data for the
two channels is offset by half a symbol period. First I is shifted in. One half a
symbol period later, Q is shifted in. One half a symbol period later, the next I is
shifted in, and so on. On the IQ plane, I transitions are strictly horizontal, and Q
transitions are strictly vertical. However, since both transitions cannot happen
simultaneously, the trajectory must follow the outside edges between constellation
points. It can never go through the origin, thus, solving the problem.
Rotating constellations
Offset modulation is not the only method of preventing transitions through the
origin. The second commonly employed technique is to rotate the constellation
after each symbol. This strategy is exempli
fi
ed by the
π
/4 DQPSK and the 3
π
/8 8
PSK constellations. (See Figure 10.)
TSG4100A Series RF Signal Generators User Manual
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