2 Phase modulations: Differential Phase-Shift Keying, Quadrature Phase-Shift Keying
2.2 Quadrature Phase-Shift Keying and Differential Quadrature Phase-Shift Keying

The principle

Differential Quadrature Phase-Shift Keying (DQPSK) is a multilevel format.

QPSK is effectively two independent BPSK systems, and therefore exhibits the same performance but twice the bandwidth efficiency.

  • Pairs of bits are assigned a specific phase, as for example:
    • 00 → 45°
    • 01 → 135°
    • 10 → 315°
    • 11 → 225°

There are many options of QPSK – the pairs of bits can be assigned different phase and different pairs can be the neighbour pairs, which can be observed in the following constellation diagrams.

  • In DQPSK, the pairs of bits correspond to a given phase shift from a reference (initial) phase, or, in other words, by 90° between the neighbour symbols.
  • The initial phase can be 0° or non-zero.
    • 00 → shift by 0° from the initial phase.
    • 01 → shift by 90° from the initial phase.
    • 10 → shift by 180° from the initial phase.
    • 11 → shift by 270° from the initial phase.

Main benefits

  • Symbol rate is 2x slower than the bit rate
  • DQPSK benefits in good optical signal to noise ratio
  • robustness against polarization mode dispersion due to its longer symbol duration
  • compression in frequency
  • increased tolerance to chromatic dispersion
  • narrow optical spectrum     
  • NRZ-DQPSK is promising even for terabit transmission
  • DQPSK can also perform well at 40 Gbps
  • RZ-DQPSK enables the longest optical reach
  • RZ-DPSK offers the largest nonlinearity tolerance for a single 160 Gbps channel
  • Among DQPSK formats, the highest values of Q-factors for each optical channel can be achieved for RZ-DQPSK.

Transceivers

Transceiver’s construction:

  • In a NRZ-DQPSK transmitter, the two input encoded in-phase and quadrature binary signals are converted into electrical waveforms which drive two MZMs.
  • A continuous laser source is used for both MZMs. The output of one of the modulators travels through a phase modulator used to obtain an additional phase shift of 90°, which is required for the quadrature component.
  • Signals are then combined together to generate a single DQPSK modulated signal.
  • Additional components include two low pass filters between electrical signal generators and MZMs to consider also the non-ideal binary to electrical signal conversion.
  • For RZ-DQPSK, an additional MZM is used to create the RZ output pulses.
  • DQPSK receiver is modelled using two balanced 2DPSK receivers for the in-phase and quadrature signals, each composed of a tunable Mach Zehnder interferometer and two PIN photodetectors.
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The principle of QPSK modulation. Sample constellation diagram. Pairs of bits are assigned particular symbols; since there are four combinations of the pairs, the modulation has four states (levels).
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Sample constellation diagrams for OOK and BPSK for comparison purposes. Phase and amplitude expressed by a constellation diagram.