We
began with BPSK modulation scheme, which required operating the PA in the linear region and
coherent detection for bit extraction. We then doubled our spectral efficiency to 2 bps/Hz, by using
QPSK. Here the constellation diagram has phase trajectories that crossed the origin and were problematic
in that they contribute to the spectral regrowth phenomenon when the modulated signal passes
through a nonlinearity. In an effort to avoid the origin, OQPSK was selected. Here the troublesome
trajectories were split up into two /2 trajectories, thus avoiding crossing the origin. However, we
didn??™t give many options to the receiver.
Next we mentioned that if spectral regrowth could have been tolerated or less-efficient PAs could
have been used, then DQPSK may be another option. Here the phase changes were inserted into the
transmitted signal by phase differences. The origin was crossed, but we gave the communication system
designer options for the receiver.
In an attempt to recollect the PA high-efficiency benefit while maintaining options at the receiver,
/4-DQPSK was presented. Here the origin was completely avoided; however, depending on the
choice of the LPF used, the phase trajectories came dangerously close.
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