Frequency-Shift Keying (FSK)
Frequency-shift keying (FSK) refers to a type of frequency modulation that assigns bit values to discrete frequency levels. In noncoherent forms of FSK, the instantaneous frequency is shifted between two discrete values termed the mark and space frequencies. Coherent forms of FSK exist that have no phase discontinuity in the output signal. FSK modulation formats generate modulated waveforms that are strictly real-values, and thus tend not to share common features with quadrature modulation (QM) schemes.