Multiple frequency-shift keying
MFSK uses multiple tones to transmit data with constant envelope.
Multiple frequency-shift keying (MFSK) is a variant of frequency-shift keying that employs more than two distinct frequencies. It belongs to a class of M-ary orthogonal modulation, where each transmitted symbol is one waveform selected from a set of mutually orthogonal waveforms. The alphabet size, denoted M, is typically a power of two, so each symbol conveys log₂ M bits of data.
In an M-ary signaling system such as MFSK, an alphabet of M tones is defined, and the transmitter sends one tone at a time from this set. Because M is often a power of two, each transmitted tone represents log₂ M bits. The scheme is considered orthogonal because each of the M detection filters at the receiver responds only to its assigned tone and ignores the others, ensuring independence between tones.
Like other M-ary orthogonal schemes, the required Eb/N0 ratio for a given error probability decreases as M increases, without needing multisymbol coherent detection. As M approaches infinity, the required Eb/N0 asymptotically approaches the Shannon limit of −1.6 dB. However, this improvement is slow with increasing M, and large values become impractical due to an exponential increase in bandwidth. Practical M values typically range from 4 to 64, and MFSK is often combined with forward error correction to gain additional coding gain.
The spectral efficiency of MFSK decreases as the modulation order M increases. Because MFSK is a form of angle modulation that transmits a single RF tone varying only in phase or frequency, it produces a constant envelope. This property greatly simplifies RF power amplifier design, allowing higher conversion efficiency than linear amplifiers.
Two MFSK systems can be combined to increase link throughput. The most widely used two-tone MFSK system is dual-tone multi-frequency (DTMF), known by its AT&T trademark "Touch Tone." Another is the multi-frequency (MF) scheme used in the 20th century for in-band signaling on trunk lines between telephone exchanges. Both are in-band signaling schemes, meaning they share the user's communication channel. In DTMF and MF, symbols are sent as tone pairs: DTMF selects one tone from a high group and one from a low group, while MF selects both tones from a common set. Their tone frequencies differ largely to prevent end users from interfering with inter-office signaling.
- Modulation type
- M-ary orthogonal
- Typical m range
- 4 to 64
- Asymptotic eb/n0 limit
- −1.6 dB
- First developed mode
- Piccolo (1962)
- Standardized protocol
- MIL-STD-188-141B (ALE)
Lore & Background
MFSK is classed as an M-ary orthogonal signaling scheme because each of the M tone detection filters at the receiver responds only to its tone and not at all to the others; this independence provides the orthogonality. 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. As M approaches infinity the required Eb/N0 ratio decreases asymptotically to the Shannon limit of −1.6 dB, though 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.
A 2-tone MFSK system combines two MFSK systems to increase throughput. The most widely used is dual-tone multi-frequency (DTMF), better known by its AT&T trademark of 'Touch Tone'. Another is the Multi-frequency (MF) scheme used during the 20th century for in-band signalling on trunks between telephone exchanges. Symbols in DTMF and MF alphabets are sent as tone pairs; DTMF selects one tone from a 'high' group and one from a 'low' group, while MF selects its two tones from a common set. The simultaneous transmission of two tones directly at RF loses the constant-envelope property of the single tone system.
Reader's Guide
MFSK is notable for its ability to tolerate significant Doppler or delay spreads on HF channels, especially when augmented with forward error correction. A long delay spread with little Doppler spreading can be mitigated with a relatively long MFSK symbol period to allow the channel to 'settle down' quickly at the start of each new symbol. 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. The resultant throughput reduction can be partly compensated with a large tone set so that each symbol represents several data bits; a long symbol interval allows these tones to be packed more closely in frequency while maintaining orthogonality. Conversely, if the Doppler spread is large while the delay spread is small, a shorter symbol period may permit coherent tone detection and the tones must be spaced more widely. The most challenging case is when delay and Doppler spreads are both large, which is more common on auroral and EME channels than on HF. A wide variety of MFSK schemes have been developed for HF, including Piccolo (the original MFSK mode, developed for British government communications and first used in 1962), Coquelet, MFSK8, MFSK16, Olivia MFSK, DominoF, DominoEX, THROB, CIS-36 MFSK (or CROWD-36), XPA, XPA2, and the ALE protocol standardized as MIL-STD-188-141B.
Did You Know?
- MFSK produces a constant envelope, which relaxes RF power amplifier design.
- The required Eb/N0 ratio for MFSK decreases asymptotically to the Shannon limit of −1.6 dB as M approaches infinity.
- Piccolo was the original MFSK mode, first used in 1962 for British government communications.
- DTMF, better known as 'Touch Tone', is a widely used 2-tone MFSK system.
More in Radio Modulation Modes, Part 2 1-24
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