DQPSK

DQPSK, or differential quadrature phase shift keying, is a differential modulation scheme in which data are encoded as phase differences between successive symbols rather than as absolute phase. By using the previous symbol as a reference, the receiver can detect information without requiring an exact carrier phase estimate, improving robustness to phase noise and frequency offset.

In DQPSK, each symbol carries two bits, selecting one of four phase increments separated by 90 degrees. A common mapping assigns successive phase changes as 00→0, 01→π/2, 11→π, and 10→3π/2. At the receiver, a differential detector compares the phase of adjacent symbols to recover the two transmitted bits. This approach reduces the need for precise carrier phase recovery while maintaining relatively high spectral efficiency.

DQPSK is spectrally efficient, transmitting 2 bits per symbol, and is well suited for high‑speed links where absolute phase recovery is difficult. It is used in various communication systems, including optical communications, and can be implemented with polarization multiplexing in optical networks (often referred to in the context of DP‑DQPSK).

Advantages of DQPSK include improved tolerance to carrier phase noise and reduced receiver complexity since coherent phase estimation is not strictly required. Disadvantages include a potential performance penalty relative to ideal coherent QPSK in favorable conditions and vulnerability to cycle slips or large, rapid phase changes that can cause bit errors.

Applications include radio frequency links, satellite communications, and fiber‑optic networks where differential detection offers robustness to phase instability and simplifies receiver design.