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.
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.
Like any other form of angle modulation that transmits a single RF tone that varies only in phase or frequency, MFSK produces a constant envelope.
DTMF and MF use different tone frequencies largely to keep end users from interfering with inter-office signaling.
These signals are distinctive when received aurally as a rapid succession of tone pairs with almost musical quality.
When several separate paths from transmitter to receiver exist, a condition known as multipath, they almost never have exactly the same length so they almost never exhibit the same propagation delay.
With appropriate parameter selection, MFSK can tolerate significant Doppler or delay spreads, especially when augmented with forward error correction.
Because a long symbol contains more energy than a short one for a given transmitter power, the detector can more easily attain a sufficiently high signal-to-noise ratio (SNR).
Some of them are: Piccolo was the original MFSK mode, developed for British government communications by Harold Robin, Donald Bailey and Denis Ralphs of the Diplomatic Wireless Service (DWS), a branch of the Foreign and Commonwealth Office.
The current specification "Piccolo Mark IV" was still in limited use by the UK government, mainly for point-to-point military radio communications, up to the late 1990s.
Greenman has also developed DominoF and DominoEX for NVIS radio communications on the upper MF and lower HF frequencies (1.8–7.3 MHz).
"CIS-36 MFSK" or "CROWD-36" (Russian: Сердолик) is the western designation of a system similar to Piccolo developed in the former Soviet Union for military communications.
[8] [9] [10] "XPA" and "XPA2" are ENIGMA-2000 [11] designations for polytonic transmissions, reportedly originating from Russian Intelligence and Foreign Ministry stations.
MFSK modes used for VHF, UHF communications: 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.