MFSK

Multiple frequency-shift keying (MFSK) is a variation of frequency-shift keying (FSK) that uses more than two frequencies. [1] 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.

• M is usually between 2 and 64
• Error Correction is generally also used

How it works

Like other M-ary orthogonal schemes, the required E_b/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 E_b/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 (systematic) coding gain.

Types

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.

These signals are distinctive when received auraly. Their main feature is a rapid succession of tones with almost musical quality.

HF communications

MFSK or "polytone" modes used for shortwave communications:

• MFSK8
• MFSK16
• Olivia MFSK
• Coquelet
• Piccolo
• ALE (MIL-STD 188-141)
• DominoF
• DominoEX
• THROB
• CIS-36 MFSK or CROWD-36
• XPA, XPA2

Piccolo was the original MFSK mode, developed for British government communications by Harold Robins, Donald Bailey and Denis Ralphs of Diplomatic Wireless Service (DWS), a branch of the Foreign and Commonwealth Office. It was first used in 1962.

Coquelet is a similar modulation system developed by the French Government for similar applications.

MFSK8 and MFSK16 were developed by Murray Greenman, ZL1BPU for amateur radio communications on HF. Olivia MFSK is also an amateur radio mode. Greenman has also developed DominoF and DominoEX for NVIS radio communications on the lower HF frequencies (1.8-7.3 MHz). [2]

ALE (automatic link establishment) is a protocol developed by the USA military and used mainly as an automatic signalling system between radios. It is used extensively for military and government communications worldwide and by radio amateurs.

"CIS-36 MFSK" or "CROWD-36" is the western designation of a system similar to Piccolo developed in the former Soviet Union for military communications.

"XPA" and "XPA2" are ENIGMA-2000 designations for polytonic tranismissions, reportedly originating from Russian Intelligence and Foreign Ministry stations. Recently the system was also described as "MFSK-20".

VHF & UHF communications

MFSK modes used for VHF, UHF communications:

• DTMF
• FSK441
• JT6M
• JT65

FSK441, JT6M and JT65 are parts of the WSJT family or radio modulation systems, developed by Joe Taylor, K1JT, for long distance amateur radio VHF communications under marginal propagation conditions. These specialized MFSK modulation systems are used over troposcattering, EME (earth-moon-earth) and meteoscattering radio paths.

DTMF was initially developed for telephone line signaling. It is frequently used for telecommand (remote control) applications over VHF and UHF voice channels.

Spectrum efficiency versus other HF modes

Source: Greenman 2005 [3]

See also

• Radioteletype
• frequency-hopping spread spectrum also uses many different frequencies, where each symbol uses only one frequency.
• DTMF
• Olivia MFSK
• ALE (MIL-STD 188-141)
• WSJT