GMSK

Frequency-shift keying (FSK) is a form of frequency modulation in which the modulating signal shifts the output frequency between predetermined values.

Usually, the instantaneous frequency is shifted between two discrete values termed the mark frequency and the space frequency.

Continuous phase forms of FSK exist in which there is no phase discontinuity in the modulated signal.

Other names for FSK are frequency-shift modulation and frequency-shift signaling.

Minimum frequency-shift keying or minimum-shift keying (MSK) is a particularly spectrally efficient form of coherent frequency-shift keying. In MSK the difference between the higher and lower frequency is identical to the bit rate. As a result, the number of carrier periods used to represent a 0 and a 1 bit differs by exactly one. This is the smallest FSK modulation index that can be chosen such that the waveforms for 0 and 1 are orthogonal. A variant of MSK called GMSK is used in the GSM mobile phone standard.

http://en.wikipedia.org/wiki/Frequency-shift_keying

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