Gaussian minimum shift keying or GMSK is a kind of continuous phase frequency-shift keying. The baseband modulation is generated by starting with a bitstream 0/1 and a bit-clock giving a timeslice for each bit. This is the type of modulation used in Global System for Mobile Communications (GSM).

How it works

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The baseband signal is generated by first transforming the zero/one encoded bits into -1/+1 encoded bits. This -1/+1 signal is then filtered in such a way that the "boxcar" shaped +1/-1 pulses are transformed into Gaussian-shaped signals. The baseband signal is then modulated using frequency modulation, producing a complete GMSK signal. If the Gaussian shapes do not overlap, then the modulation form is called 1-GMSK. If the slots overlap 50% (½), the modulation is called 2-GMSK, and so on.

The more the bits overlap, the more significant intersymbol interference (ISI) from adjacent bits will be, and for 4-GMSK and up, the ISI seen at any particular point in time is stronger than the signal from the bit currently being decoded. By looking at greater parts of the signal using advanced decoder techniques (including Viterbi algorithm decoders), high density codings can be decoded efficiently. Currently the highest density coding being used is 5-GMSK.

Spectral efficiency

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GMSK has high spectral efficiency, but it needs higher power level than for instance Quadrature phase-shift keying (QPSK) to reliably communicate the same amount of data.

See also

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• Phase-shift keying

External links

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• Document from the University of Hull giving a thorough description of GMSK.
  Note: Section D5 appears to state that MSK replaces a rectangular amplitude pulse shape with a sinusoidal shape and that GMSK replaces the sinusoidal shape with a Gaussian shape. But an important distinction is that the GMSK shaping is applied to the instantaneous frequency amplitude.
• Another good discussion (University of Toronto)

Radio modulation modes

GMSK | GMSK | GMSK | GMSK