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VMSK/2 Promises 5 Times More Bandwidth
Posted by
Hemos
on Tue Aug 22, 2000 10:04 AM
from the that's-a-lot-of-howard-stern dept.
from the that's-a-lot-of-howard-stern dept.
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.
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VMSK/2 Promisses 5 Times More Bandwidth
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And then something magic happens... (Score:3)
We can write the modulated signal as
v(t) = A * exp(j * m * (d(t) - 0.5))
where d(t) is a binary data signal. If m=Pi/2 then we have a BPSK style modulation. Make m small and we get a VMSK signal. This small value of m is what gives them a proportionally smaller occupation of the spectrum for the bit rate.
NoW consider the constellation of this modulation in the complex plane, we have two points representing the bits 0 and 1. For BPSK they are seperated by Pi radians, while with VMSK they are much closer together. VMSK is therefore clearly more vunerable to noise than a BPSK scheme. At the above website they clearly state that the noise resistance (C/I) is significantly better with VMSK!!!
It seems clear that there is a basic fault in their reasoning, and they can't beat Shannon in this manner. D.
Compressed more than John Candy's seat cushion. (Score:4)
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.
Shannon worse than Gates (Score:3)
We need more scientists like Moore, who ensured that chips would continue to get faster and cheaper, and they have. That's progress. That's a good guy.
All Shannon, Nyquist, and Einstein have done is limit the rates of communication we can attain and bloat our harddrives with 2F-sampled signals (Mp3z, pr0n). What jerks. Much worse than Gates.
jeb
Problems with mobile applications? (Score:3)
VMSK/2 seems to be mostly an academic modulation right now- they need to send it through much more rigorous testing in the real world, with real world components.
I'm skeptical when people talk about overturning Shannon's limit- people would love to disprove it, but it holds up time and time again.
Re:Compressed more than John Candy's seat cushion. (Score:3)
Transmission rate is still dependant on the information theory(bandwidth and the more power you put out, the better transmission rates you can expect, square power and rate doubles, double bandwidth and rate doubles). Could still improve the transmission rate of telephone line by improving the s/n-ratio(putting out more power) but benefits would be small compared to the cost..
FM-broadcasts are much more powerful than your average telephone conversation and s/n-ratio could be made better thus improving transmission rates. Increased bandwidth does have a bigger impact, however. Would probably want to use frequency bands over 1ghz where a bandwidth of 500khz-1mhz is obtainable..
You can stuff as many carriers you want into a bandwidth but limiting factor is going to be the modulating frequency which is going to appear on the both sides of the carrier(could suppress one). you don't want interference..
I See a Bad Trend Forming (Score:3)
With this technology, boasting that they could deliver 100 128Kbps MP3 channels probably means that they are probably planning on doing such a thing, maybe even squeezing the compression a little more to get a few more channels out of it.
While this is all great from a technological standpoint (and probably a business one at that), I see a trend of lower quality broadcasting coming about. While I understand that radio transmission is typically less than perfect (static, power wires, etc), digital artifacts of compression come through even on the clearest of signals. And on a side note, with digital cable, sometimes it almost looks like I'm watching a video screen with an 8-bit color depth.
I guess my rant is that the people on the delivery side of television and radio are letting their quality standards decrease. They can pack more content into the wires/airwaves by using compression, and this leads to lower quality decoded signals. For the average person, this is not an issue (and you're probably smirking right now). For the audiophile, this is quite a disturbing trend.
My two cents; no refunds.
--
Alphacom Communication is a multilevel marketing (Score:3)
Typically ML organisations are more interested
in building a network of "sales" agents and
taking their money than in delivering anything
like they promise.
Be skeptical.
See Alphacom Corporate page [networkalpha.com]
for their spin.
Impossible to implement (Score:4)
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
MS VMS? (Score:4)
Doubtful (Score:4)
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.
Shadiness in the spectral bandwidth claims. (Score:3)
You can think of VMSK/2 as a form of duty-cycle modulation (Figure 1). Think of a "square" wave whose total period does not vary but which, depending on whether a given bit interval contains a 1 or a 0, spends slightly more or slightly less than half the period in the high state.
Problem - this kind of nudging of the duty cycle causes spreading in the frequency domain. In fact, it is these additional frequencies that encode the change in the duty cycle.
If you try to transmit a signal modulated using this technique through a very narrow channel centered about the carrier frequency, you will lose a lot of the duty cycle information, and your data signal will degrade a *lot*.
I am skeptical of this getting much more effective use of bandwidth than conventional encoding schemes. The best I can see them getting is a modest gain if this technique is less sensitive to common types of noise (which has yet to be demonstrated).
Re:how they do that? (Score:5)
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.
Re:Translation (Score:3)
No, they are actually doing the opposite from that: instead of multiple bits/symbol, they are using one bit/symbol. The bit is initially encoded by a change in duty cycle of a square(-ish) wave. They then modulate a carrier with this signal, remove the carrier and a sideband, finally filtering the resulting sideband with extremely sharp, patented-technology filters.
I'm real suspicious... the initial square wave would have to be several MHz, and it seems that the subsequent processing would either wind up stripping out the information or result in a multi-MHz bandwidth in the resulting signal. I could be all wrong, but until I saw the math that shows exactly how the bits are encoded and extracted, I'd be skeptical.