Very minimum-shift keying
Encyclopedia : V : VE : VER : Very minimum-shift keying
| Disputed Science: Very Minimum Shift Keying modulation | |
|---|---|
| Disciplines: | Radio technology; Radio modulation modes |
| Core Tenets: | |
| VMSK achieves high speed data transfer while maintaining a very narrow bandwith in direct violation of the mathematical principles of digital communications discovered by Harry Nyquist and Claude Shannon. | |
| Year Proposed: | 1995 |
| Original Proponents: | Harold (Hal) R. Walker |
| Current Proponents: | Harold (Hal) R. Walker |
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.
Personalities
Today, most VMSK or related modulation schemes seem to regularly studied, developed and documented by a certain H.R. Walker, owner of the ["Welcome to the Ultra Narrowband Club"] website, and author of several papers and publications on the arguments. Some of Mr. Walker's works have even gained the attention of telecommunications sector publications like [EDN], [Microwaves & RF] and [CommsDesign] , as well as several cooperations and paper co-authorships with academic environments [link].
Interestingly, Mr. Walker often refers to the works of a certain "Professor Howe" in many of his VMSK related papers, who allegedly had proposed and proven that a VMSK-like modulation scheme could work since 1939 (Professor Howe, as published in K.R. Sturley, Frequency Modulation, Chemical Publishing Co., Brooklyn, NY (from "Wireless Engineer," November 1939, p. 547.). However, other than in Mr. Walker's works, references to "Professor Howe", his academical career and publications or even his actual existence are obscure at best.
xG
Early, sketchy descriptions of xG technology made it appear as if it was another "ultra narrowband" scheme similar to VMSK. However, it has been more recently described as a hybrid of a narrow band, low speed channel used to coordinate the actual data transmission on an ultra wideband channel. This would remove it from the "ultra narrowband" category.
External links
- [Welcome to the Ultra Narrowband Club] VMSK developer's website
- [AeroTelesis] another company claiming an ultra narrowband scheme
- [xG Technology]
- [The VMSK Delusion] Phil Karn's analysis of ultra narrowband claims
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