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Multiple frequency-shift keying
From Wikipedia, the free encyclopedia
Multiple frequency-shift keying (MFSK) is a variation of frequency-shift keying (FSK) that uses more than two frequencies. MFSK is a form of M-ary orthogonal modulation, where each symbol consists of one element from an alphabet of orthogonal waveforms. M, the size of the alphabet, is usually a power of two so that each symbol represents log2M bits.
Like other M-ary orthogonal schemes, the required Eb/N0 ratio for a given probability of error decreases as M increases without the need for multisymbol coherent detection. In fact, as M approaches infinity the required Eb/N0 ratio decreases asymptotically to the Shannon limit of -1.6 dB. However this decrease is slow with increasing M, and large values are impractical because of the exponential increase in required bandwidth. Typical values in practice range from 4 to 64, and MFSK is combined with another forward error correction scheme to provide additional coding gain.
Defined examples of a multiple frequency-shift keying system include dual-tone multi-frequency (DTMF), which is used in touch tone phones and the Multi-frequency trunk signals used in Twentieth Century telephone exchanges.
MFSK or "polytone" modes used for shortwave communications:
- Olivia MFSK
- MT63
- COQUELET
- PICCOLO
MFSK modes used for VHF, UHF communications
- DTMF, mainly for telemetry control
- Radioteletype
- frequency-hopping spread spectrum also uses many different frequencies, where each symbol uses only one frequency.