Field Guide · technology

Also known as: pi/4-DQPSK, π/4 DQPSK, differential QPSK

π/4-DQPSK (π/4-shifted differential quadrature phase-shift keying) is a four-symbol phase modulation in which the constellation is rotated by 45° (π/4) every symbol and information is carried in the change of phase rather than its absolute value.1 It is the air-interface modulation of TETRA.

IQ
π/4-DQPSK alternates between two QPSK constellations offset by 45°, so symbols never pass through the origin.

Overview

By alternating between two QPSK constellations offset by 45°, π/4-DQPSK guarantees a phase transition at every symbol (helping clock recovery) while avoiding transitions through the origin, which keeps the signal’s amplitude envelope more constant and easier to amplify efficiently.

Relevance

Differential encoding means the receiver only needs to measure phase changes, so it tolerates a constant carrier-phase offset without an absolute phase reference. This robustness is why TETRA and several other professional systems adopted it.

Sources

  1. Phase-shift keying — π/4–QPSK — Wikipedia, for the differential π/4-shifted QPSK definition. ↩

See also

Learn it step by step

From the blog