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VMSK/2 Promises 5 Times More Bandwidth
ksan writes "Acording to
this article in EDN Magazine; VMSK/2, a new modulation technique may improve modem, FM, AM and other types of transmission. They say that its possible to transmit 100 channels of 128kbps MP3 over an FM channel. Anyone can say more about this?"Read below to find out the *major* problems with this article.
Well, the bandwidth of an ordinary PSTN telephone line (not DSL!) is only from about 300Hz to about 3kHz. And in that bandwidth, the practical transmission limit, the current state of the art, seems to be stuck at 56k.
You can't stream a 128kbps MP3 at 56kbps. Not in real time, as radio implies.
What's the bandwidth of a conventional FM radio station? 20kHz or so carrier deviation for mono. To light up the stereo light, the stereo pilot must be found, and that runs at about 21kHz carrier deviation, if I recall. A full FM stereo signal takes a carrier wave an modulates it about 44kHz either way, so 40kHz bandwidth is probably a practical conservative estimate and has easy enough numbers to work around.
Since a 56k modem uses quadrature amplitude modulation on ?4? simultaneous carriers in a total bandwidth of 2.7kHz (3000Hz top end - 300 Hz bottom end), then how many carriers could you stuff into a 40kHz wide data channel?
I think it's exponentially more.
This sounds very exciting.
Fire and Meat. Yummy.
Nice theory, but this is impossible to implement:
....
Do the math:
Shannon's channel capacity theorem:
C= B * log2 ( 1 + SNR )
C = capacity in bps
B = bandwidth
SNR = signal to noise ratio
Solving for SNR in dB ( = 10*log10(SNR) ) gives:
SNR_db = 10 * log10 (( 2 ^ (C/B)) - 1)
With C = 12.8 Mbit/s and B = 200 kHz you get SNR = 192 dB !
To have a SNR = 192 dB, the signal has to be 1.8 * 10^19 times stronger than the noise ! A receiver with 200kHz bandwidth will typically have a noise floor at -120 dBm, so you need more than 70 dBm received signal strength. 70 dBm is 10 W !! And that is the signal strength at the receiving antenna, so the transmitter would have to be in the gigawatt range, to reach short distances.
This means that VMSK/2 can be used, but you can't reach 12.8 Mbit/s without a nuclear powered transmitter. You can get a decent bit rate with VMSK/2 on battery powered equipment, but you have to design for a few kbit/s, not 12.8 Mbit/s. Nice theory, but
RFC1925
Am I the only one that read that as "VMS 2K?" Now there's a scary thought...
I'm HIGHLY doubtful, to say the least, and the article does nothing to give any credence to the claims. A quick search of Compendex yields this article by the research report authors. Our library doesn't carry the publication so I can't check it out. If anybody else has access, I'd really be interested in a review. What is Applied Microwave and Wireless? Is it peer-reviewed?
Author(s):
Koukourlis, C.S.
Pliatsikas, J.C.
Sahalos, J.N.
Walker, H.R.
Title:
Spectrally efficient biphase modulation FOUND IN:
Applied Microwave and Wireless v 10 n 4 May 1998. p 74, 76-81
Publ. year:
1998
Abstract:
Phase modulated biphase codes which are transmitted single sideband-suppressed carrier at RF frequencies require much less bandwidth without any significant increase in circuit complexity. These codes have an important advantage over other bandwidth efficient modulation methods in that they do not lose bit energy with increasing bandwidth efficiency (compression). Actual measurements confirm this characteristic. 10 Refs.
You can achieve very high spectral efficiencies if you use very high power. To send 2 bits/hz, just send the plain old signal single sideband AM (the bandwidth of a 1kbps baseband signal is 500hz). To send 4 bits/hz, you could (as an example), use four different voltage levels. For5 bits/Hz, use 8 levels, etc. Furthermore, you can modulate phase and amplitude independently. Doubling the data rate again.
There are a myriad of modulation schemes (and related coding schemes) for achieving spectral efficiency. Basically, beyond the simple stuff (filter off the extra sideband, use phase AND amplitude), they achieve that efficiency by encoding data in more subtle aspects of the signal (read: more noise sensitive). This VMSK/2 scheme appears to be one which generates smaller sidebands by modulating the signal less. As such, it requires higher power to achieve it's spectral efficienty (ignore the claims of lower power - that's *per carrier* in the signal, but they use more carriers).
Note also that increased spectral efficiency is only part of the issue. In the modern cellular world, you need increased efficiency in terms of bits per Hz per square kilometer (i.e. you share the frequencies over an area). A requirement for higher power (which really means a requirement for higher signal-to-noise ratio) reduces the areal sharing that you can achieve.
Ultimately, you can't beat Shannon's laws. If you can, you can also make perpetual motion machines and free energy (yeah, it's a stretch, but the connection is there).
Since this company is selling multilevel marketing, I am more than a bit suspicious of any claims. Multilevel marketing schemes are too often fraudulent and based on overblown claims. I am not saying these guys are wrong, just that their approach is suspicious.
As far as comments on here on FM signal bandwidth... FM stations use a 200kHz wide channel. A stereo signal uses a composite of simple FM for the Left+Right signal, and a subcarrier at 42kHz carrying Left-Right. There is still room left in the spectrum for an additional subcarrier (or more) - which is where you find service such as Muzak. Plain old FM mono is a *spread-spectrum* modulation scheme, in that the RF signal is occupies significantly more bandwidth than the modulating signal.
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