Also known as: phase-shift keying, PSK, QPSK

Phase-shift keying (PSK) is digital modulation that switches a carrier’s phase between fixed angles while amplitude stays constant.¹ Two phases is BPSK; four is QPSK (2 bits per symbol).

PSK encodes bits in the carrier's phase; QPSK uses four phase points (2 bits each). P25 Phase 2 uses a PSK variant.

How it works

Each symbol is a phase angle, read as a point on the IQ plane. A Costas loop recovers the carrier phase so the constellation lines up. PSK is spectrally efficient.

Relevance to SDR

P25 Phase 2 uses a PSK variant, and many satellite and broadcast links are PSK; tracking phase accurately is key to decoding them.

Sources

1. Phase-shift keying — Wikipedia, for the definition and the BPSK/QPSK variants. ↩

See also

• Frequency-shift keying (FSK)
• Quadrature amplitude modulation (QAM)
• CQPSK
• Phase
• Costas loop
• Constellation diagram

Learn it step by step

• Digital modulation & constellations

From the blog

• SDR Internals, Part 6: Demodulation