GMSK: <from wikipedia>

Gaussian minimum shift keying or GMSK is a kind of continuous phase http://en.wikipedia.org/wiki//wiki/Frequency-shift_keyingfrequency-shift keying. The baseband http://en.wikipedia.org/wiki//wiki/Modulationmodulation is generated by starting with a http://en.wikipedia.org/wiki//wiki/Bitstreambitstream 0/1 and a bit-clock giving a http://en.wikipedia.org/wiki//w/index.php?title=Timeslice&action=edittimeslice for each http://en.wikipedia.org/wiki//wiki/Bitbit. This is the type of modulation used in http://en.wikipedia.org/wiki//wiki/Global_System_for_Mobile_CommunicationsGlobal System for Mobile Communications (GSM) - although that system includes http://en.wikipedia.org/wiki//wiki/Differential_encodingdifferential encoding of the bitstream, which makes the overall modulation behave like a kind of continuous phase http://en.wikipedia.org/wiki//wiki/Binary_Phase_Shift_KeyingBinary Phase Shift Keying with constant http://en.wikipedia.org/wiki//wiki/Envelope_detectorenvelope.

The baseband http://en.wikipedia.org/wiki//wiki/Signal_%28information_theory%29signal is generated by first transforming the zero/one encoded bits into -1/+1 encoded bits. This -1/+1 signal is then http://en.wikipedia.org/wiki//wiki/Electronic_filterfiltered in such a way that the "boxcar" shaped +1/-1 http://en.wikipedia.org/wiki//wiki/Pulsepulses are transformed into http://en.wikipedia.org/wiki//wiki/Normal_distributionGaussian-shaped signals. The baseband signal is then modulated using http://en.wikipedia.org/wiki//wiki/Frequency_modulationfrequency 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 http://en.wikipedia.org/wiki//wiki/Intersymbol_interferenceintersymbol 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 http://en.wikipedia.org/wiki//wiki/Viterbi_algorithmViterbi algorithm decoders), high density codings can be decoded efficiently. Currently the highest density coding being used is 5-GMSK.

GMSK has high http://en.wikipedia.org/wiki//wiki/Spectral_efficiencyspectral efficiency, but it needs higher http://en.wikipedia.org/wiki//wiki/Electrical_powerpower level than for instance http://en.wikipedia.org/wiki//wiki/Quadrature_phase-shift_keyingQuadrature phase-shift keying (http://en.wikipedia.org/wiki//wiki/QPSKQPSK) to reliably http://en.wikipedia.org/wiki//wiki/Communicatecommunicate the same amount of http://en.wikipedia.org/wiki//wiki/Datadata.

<Don't ask me what it means>

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