2 Phase modulations: Differential Phase-Shift Keying, Quadrature Phase-Shift Keying
2.1 Differential Phase-Shift Keying

The principle

Similarly as for OOK, DPSK can be implemented as RZ and NRZ.

It is a type of phase modulation that conveys data by changing the phase of the carrier wave. All subsequent information is coded as 180° phase reversals of the carrier frequency.

  • A 180-degree carrier phase reversal preceding a chip shall characterize that chip as a binary 1.
  • The absence of a preceding phase reversal shall denote a binary 0.

Main benefits

The main advantage of DPSK is the 3-dB improvement it offers in the receiver sensitivity compared to OOK.

The difference in results between NRZ-DPSK and RZ-DPSK is primarily related to the wider spectrum of RZ-DPSK format.

Transceivers for DPSK

Transceiver’s construction:

  • NRZ-DPSK transmitter's design is similar to that of NRZ, but instead of the MZM, a phase modulator with 180° phase shift is deployed.
  • A low-pass filter is used between the driver and modulator to include the impact of a non-ideal binary to electrical signal conversion.
  • The RZ-DPSK transmitter includes additionally among others a second modulator to generate RZ output pulses.
  • A DPSK receiver consists of a delay interferometer for decoding purposes and a balanced receiver modelled using two regular compound receivers, where the electrical output of the first one is added to the second's inverted output.
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The principle of BPSK modulation – binary zeros and ones are represented by phase states shifted by π from each other, e.g. binary zero is assigned phase π and binary one, phase 0 radians.
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The principle of DPSK modulation – a binary one shifts the symbol’s phase by π, binary zero does not shift the phases.