e., Eb/No).
Earlier we discussed modulation schemes whose phase trajectories cross the origin had tremendous
spectral regrowth issues. That??™s why schemes that avoided them had less spectral regrowth.
In this subsection, we will provide a short overview of some of the digital cellular standard??™s choices
in the modulation format.
GSM system uses GMSK, which we saw earlier as a spectrally efficient modulation scheme with
the benefit of having an approximate constant envelope waveform. It can be viewed as OQPSK with
sinusoidal pulse shaping and can be detected either coherently or noncoherently with a differential
detector.
JDC and NADC use /4-DQPSK, which is a spectrally efficient modulation scheme, provided the
PA is operating in quasi-linear region. It avoids the origin and can be detected noncoherently through
the use of a differential detector or limiter discriminator.
In the above-mentioned TDMA schemes, the UL transmission is a major consumer of battery current
and thus will play a significant role in determining the MS or UE talk time. It behooves the communication
system designer to choose a modulation scheme that not only avoids the origin of the
signal constellation diagram, but also resembles a circle as much as possible.
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