说明：  Differential PSK (DPSK) 􀂄 Inexpensive 􀂄 Quadrature Phase Shift Keying (QPSK) 􀂄 Twice as spectrally efficient than BPSK 􀂄 Offset QPSK (OQPSK) 􀂄 reduce envelope variation 􀂄 π/4 QPSK 􀂄 reduces envelope variations to a lesser extent than OQPSK can be noncoherently detected