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Attached Fig.3 8PSK Diagram
Quadrature amplitude modulation QAM
QAM performs suppressed carrier double-sideband amplitude modulation for two quadrature
carriers with the same frequency by using two independent baseband signals and realizes
transmission of two parallel digital information by using the orthogonality of the frequency spectrum of
such modulated signals in the same bandwidth. The modulation methods include 4QAM (namely
QPSK), l6QAM, 32QAM and 64QAM... and the constellations have 4, 16, 32 and 64... vector
endpoints.
For 4QAM, its generation, demodulation, performance, and phase vector are the same as QPSK
when the amplitudes of the two signals are equal. Obviously, when the symbol point is greater than
or equal to 16, the distance between QAM symbol points is greater than that of PSK if the number of
symbols is the same. Since the bandwidth utilization of the QAM system is higher than that of the
QPSK, it is a very promising modulation method in a frequency band-limited system and has already
been used in communication systems, but its performance is not as good as that of the QPSK system.
Now, 128QAM and 256QAM are used in digital television systems. The 16QAM and 32QAM
constellations are shown in the figures below (Attached Figures 5 and 6).
Attached Figure 5 16QAM Constellation
Attached Figure 6 32QAM Constellation
Differential modulation method π/4DQPSK
The QPSK signal discussed earlier has a high frequency band utilization. However, a 180°
carrier phase jump occurs when the code group jumps from 00 to 11 or from 01 to 10. This phase
jump will cross the original point, causing large envelope fluctuations. When passing through
non-linear components, the distortion of the time-domain waveform will cause distortion in the
frequency domain so that the out-of-band components that have been filtered out will be recovered
again, and the extended spectrum will cause interference with adjacent channels. In order to
eliminate such 180° phase jump, a modulation method called π/4-DQPSK is obtained after the
correction is made on the basis of QPSK. π/4-DQPSK is a differential modulation and it is a
synthesized constellation in the Attached Figure 7. Differential modulation uses relative changes of
phases, not absolute phase symbols, to represent “0” or “1” bit data. The even symbol point here is
on the I/Q axis and the odd symbol point (with *) is at the position 45° away from the I/Q axis. It can
be seen that the maximum phase jump is limited to 135 degrees. There are four phase changes in
π/4-DQPSK, including 45°, 135°, -45° and -135° respectively. There is always a phase change for
each symbol due to the use of two QPSK constellations with 45° offsets. This avoids a 180° phase
change and the trace of its vector diagram does not pass zero point.
