For DQPSK/PAM2 [14], the theoretical receiver sensitivity penalty as compared
to BPSK with the same bit rate) is 4.7 dB, about 1.7 dB smaller than 8-ary DPSK
which is also a 3-bit/symbol format. At 4-bit/symbol and 5-bit/symbol, the hybrid
DPSK/PAM offers higher optimal receiver sensitivities than m-ary DPSK, as shown
in Figure 4.11. However, DPSK/PAM usually suffers more transmitter/receiver band-
width limitation induced ISI and nonlinear penalty than m-ary DPSK which may be
regarded as a constant-amplitude format. Note that in principle, one can improve the
receiver sensitivity of hybrid formats based on DPSK and PAM (or ASK) by
arranging the signal constellation such that more DPSK phase states are assigned
at high power regime than at low power ...