Very minimum-shift keying

VMSK, for Very Minimum Shift Keying modulation, is one of a growing class of digital modulation methods claimed to send high speed digital data through very low bandwidth (or narrowband) channels. A typical claim is a data rate of 6 Mbit/s in a bandwidth of 1 KHz or less using the same (or even less) transmitter power than conventional schemes.

Such claims are to communications what Perpetual Motion is to energy production, as they run afoul of firmly established mathematical principles such as the Shannon-Hartley theorem and the Nyquist-Shannon sampling theorem. Proponents claim to have built working hardware. However, a careful analysis shows that all these schemes actually produce various forms of ultra wideband modulation to which a strong narrowband component is added as a distraction. Only the strong narrowband component is easily seen on a spectrum analyzer, so it is incorrectly alleged that it must convey the information being transmitted. This claim is readily dismissed with the Nyquist-Shannon sampling theorem which states that for a receiver to be able to distinguish among data-dependent changes made to a signal at a given rate, the channel bandwidth must be at least one half that signaling rate.

The information in all these allegedly ultra-narrowband signals is actually conveyed in the very weak, very wideband spectral components that generally fall well below the noise floor of a spectrum analyzer. The conspicuous narrowband component conveys no information; it merely wastes most of the transmitter power.

In other words, these schemes represent viable -- though extremely inefficient and useless -- means of sending digital data. Only the bandwidth claims made for them are impossible.

Part of the misconception rests on the fact that there is no single, universally applicable definition of the word "bandwidth". Different definitions exist for different applications, and using the wrong one can lead to meaningless or absurd results such as the claims made for ultra narrowband modulation.

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See also: Very minimum-shift keying, Bandwidth, Modulation, Narrowband, Nyquist-Shannon sampling theorem, Perpetual Motion, Shannon-Hartley theorem, Ultra wideband