Also known as: GMSK

GMSK (Gaussian minimum-shift keying) is a continuous-phase FSK variant in which the data is passed through a Gaussian filter before modulation, smoothing phase transitions for a compact spectrum and constant envelope.¹

GMSK is continuous-phase FSK with Gaussian pulse shaping, giving a narrow spectrum; AIS uses it.

How it works

The constant envelope suits efficient non-linear amplifiers, while the Gaussian shaping limits bandwidth. These traits made GMSK the choice for GSM cellular and for AIS and D-STAR.

Relevance to SDR

A GMSK demodulator recovers the underlying frequency/phase transitions; GopherTrunk uses one in its AIS pipeline.

Sources

1. Minimum-shift keying — Wikipedia, for MSK/GMSK, Gaussian shaping, and the constant-envelope property. ↩

See also

• Frequency-shift keying (FSK)
• Automatic Identification System (AIS)
• D-STAR
• Root-raised-cosine filter

Learn it step by step

• Digital modulation & constellations

From the blog

• SDR Internals, Part 6: Demodulation