Wireless communications has had its share of “incredible claims”. Some are relatively mild and are due to the unfortunately very common practice of highly exaggerated marketing claims regarding the performance of well established technologies. These claims are usually very misleading but are not necessarily technically impossible. For example, performance results are often quoted without specifying the conditions under which they would hold. An analysis of the numbers may reveal that the results would hold under ideal conditions which are rarely, if ever, met in practice. We will provide some examples in a later posting.
Some claims are more extreme in that they are simply not possible based on known and well established principles of communications theory. A fascinating example is a class of modulation techniques which attempt to transmit high data rates through very narrow channels.
VMSK , for Very Minimum Shift Keying modulation, is one of several digital modulation methods claimed to send high speed digital data through very low bandwidth 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. For a detailed description of this “technology” see The Ultra Narrowband Club . Reading this material is quite painful to anyone who has a reasonable understanding of the Fourier transform and the concept of bandwidth. It is very clear that the author has a fundamental lack of understanding of frequency analysis and its principal mathematical tool, the Fourier transform. Starting with this fundamental misunderstanding he then builds a very elaborate and seemingly consistent story, which has a “fatal flaw” which the author is apparently unable to see. For a detailed critical discussion of this technique see The VMSK Delusion and the links therein.
A related “technology” called Ultra Spectral Modulation (USM) has beed used by several companies including Photron Technologies Ltd. and aeroTelesis Inc. For a discussion of this “technology” we refer the reader to a document with the unassuming name of “USM: Defining the Breakthrough Technology Which Will Redefine the Next Generation In Telecommunications – A Simple Primer” . The document talks about a communication system with a spectral efficiency of 100 bps/Hz! It is completely silent about the fact that such a system would require an SNR of at least 301dB (!) to operate. Yes, that is unfortunately what the pesky Shannon theorem requires …