Gaussian minimum shift keying or GMSK is a kind of continuous phase modulation. The basebandmodulation is generated by starting with a bitstream 0/1 and a bit-clock giving a timeslice for each bit. 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 decoder), high density codings can be decoded efficiently. Currently the densest coding being used is 5-GMSK.
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.
GMSK was developed specifically for GSM by the COST (Council on Science and Technology, a scientific advisory group funded by CCITT and later ETSI).
GMSK uses a special waveform to achieve a gradual change in frequency when the two consecutive binary bits to be transmitted are 01 or 10.
The important thing about use of GMSK for GSM is that the power produced in an adjacent modulated carrier bandwidth (the 200 kHz section of spectrum next to the desired channel) is less than 1/2 percent of the desired signalpower.
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