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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.
This is a emf signal, not a sound wave. How fast do you drive?
Doppler happens with any motion. How do you think the nice traffic officer figures out how fast you're moving? With the precision of this modulation scheme, the little bit of doppler that happens will become significant.
I work at Alphacom and have talked with several folks working on VMSK/2 and yes it is true it is an improvement. But it's definately not as great of an increase as this article claims.
After reading this article my impression is the author decided to exaggerate the technology to make his article more exciting. From what I've heard so far, the maximum transfer rates VMSK/2 will achieve is around 256kbit/s in optimal conditions. Most users can expect better then dual channel ISDN, which I think is pretty good for wireless.
Note: These are my personal views and do not reflect the views of Alphacom Communications.
> No one really "owns" the airwaves
Do you really believe that? If you live in the US, then the airwaves are owned (licensed? bought? stolen?) by a few large media corporations. Infinity, Clear Channel, and AMFM come to mind. Learn more about the situation at http://www.mediademocracynow.org.
If you really want the airwaves to be free, join us in protest of the NAB (National Association of Broadcasters) on September 20-23 in San Francisco during their convention.
Ryan
I understand that nudging the duty-cycle would cause spreading in the frequency domain (at least, I think I do) but didn't the article say that the signal is then run through some narrow bandpass filters or something? So then wouldn't the nudging sort of degrade to a phase shift or something?
My point was that this filtering would lose information. Your available bandwidth influences how fine and how fast a phase shift you can detect.
Good question, though.
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.
The information-carrying capacity (C) of a channel is a function of its bandwidth B and signal (S) to noise (N) ratio. The formula (this is elementary information theory) is
C=B log2(1+S/N)
We can plug in the given values for B (200 KHz) and C (12.8M bit/sec) and determine the required signal to noise ratio:
12.8e6=2e5 log2(1+S/N)
log2(1+S/N)=64
S/N=2^64 - 1
Each bit of signal-to-noise ratio corresponds to 3 decibels (6 dB if you're talking about power, which I won't, just to be charitable), so the required S/N ratio is around 190 dB.
Is this achievable? In a word, no. It's not even close. They're off by more than 100 dB.
When these guys claim that `when properly implemented and used under the right conditions, the digital modulation scheme reportedly delivers 90 bps/Hz-over the air' they're blowing smoke. They're saying they need a 270 dB S/N ratio. (That's larger than my number because I gave them some extra slack.) If they could get it, their scheme would work, but they can't, so it won't.
-Tom Duff
Yeah, I didn't count that, since it's not actually carrying data. In fact, the FCC doesn't like to license stations less than 500kHz apart. 'Course, New York and L.A. are big markets...
Stereo pilot is at 19khz if i'm not terribly mistaken.Yeah... I've been thinking more of MTS stereo on TV, which is basically just the same thing as FM stereo with a different pilot frequency.
All frequencies higher than 18khz or so are filtered out(so that they don't mess with stereo carrier). 40khz bandwidth is a good estimate..Well, remember, they're not filtered out, they're modulated into the additive and difference signals. The (L+R) signal that you hear on a mono FM radio, then the pilot which turns on the FM stereo circuits, then the (L-R) difference signal which is used to create the stereo image. All that space is available under such a scheme.
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..Why, when you can basically run a dozen 56k modems slingshotted through the same given bandwidth? Use the bandwidth efficiently, and you don't actually need the 500-1,000kHz bandwidth.
Besides, UHF/microwave sucks. It's way too position dependant. I don't want a Walkman that has to be moved three feet to the left before I'm able to receive the Howard Stern show again. Since UHF/microwave tend to bounce off lots of things, reflections and self-cancelling signals are going to be a constant pain in the ass.
Fire and Meat. Yummy.
MTV stopped playing music years ago because there simply isn't enough good music to fill the airwaves. Britney Spears, N'Sync, and everyone else that will be on the FOX Teen Choice Awards tonight are proof that there is something wrong. Regardless of what percentage they make up in the general population, teenage girls seem to have a HUGE influence on the music that is chosen to be "popular". Popular music hasn't appealed to the masses in decades.
You can have all the bandwidth in the world -- if nobody's interested in what's on, it's all a big waste of time.
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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.
For the benefit of us poor slobs who had the unutterably bad taste to attend college 20 years ago, when the best modulation methods going were things like Huffman codes and trellis codes, the notion of sending 19 bps/Hz (much less 90 bps/Hz) is a bit mysterious. Can one of you folks with a modern education post a few URLs where I could see how they are managing to accomplish this? Thanks, from the annoying old fart in the next cube.
WWJD for a Klondike Bar?
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
From my reading of this, this modulation scheme relies heavily on timing- which would really get screwed up if you happened to be moving, because of doppler shifts. Keeping accurate track of timing is very, very hard when moving. Also, they don't really address interference (intentional or incidental) which will make it that much harder to reach those bandwidths.
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.
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.
--
Never hit your grandmother with a shovel, for it leaves a bad impression on her mind...
I guess that's how the whole dang human comedy keeps perpetuating itself. ok bye.
Axel
Axel
mhm23x3, alt.fan.karl-malden.nose
Multilevel Marketing Organisation.
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
for their spin.
Ethics II Axiom 2. "Man thinks." B. Spinoza
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
I like! Now, how stable can we make an FM signal to a computer? This could damn well be another method of providing high-bandwidth internet access, and a good one at that.
Because No one really "owns" the airwaves, like one does a cable, you could get a lot of competition amongst providers for customers.
I can see the Laptop/Handheld owners screaming now. I have a palm, and the thought of cheap high bandwidth wireless access for it amuses me.
-i
Nice to see this "confession" from someone in the know. It's still not enough, though. He says that the increase is not as big as the article claims when in fact there *cannot* be any increase in performance compared to state-of-the-art techniques like QAM.
The implementation of VMSK/2, however, is significantly simpler. VMSK/2 can be implemented with an extremely low gate count without quadrature A/D converters, complex digital signal processing and equalization. So if its performance is not too suboptimal it might still be interesting. It would have been much more interesting 10 years ago, though. Today you can put half a million transistors on a chip without blinking.
----
Stop worrying about the risks of nuclear power and start worrying about the risks of not using nuclear power.
Am I the only one that read that as "VMS 2K?" Now there's a scary thought...
I Think the operative word here is *may*
I bet this is vapor ware and nothing will ever come of it.
___________________
___________________
He who laughs last... Thinks slowest
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.
From the article:
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).
I agree, although I felt the journal article was just about right for my two years rusted EMC and Communications stuff.
;-)
The main thing that I picked up on was the sheer lack of spacing between the transitions of the final data and the original clock. As I read it, this means that the clock recovery circuitry is going to have to be spot on every single cycle.
Admittedly things may well have changed since I last saw them, but PLLs were notoriously unstable when I played with them. I wonder how they're going to make the receivers sufficiently portable to make this worthwhile in a mobile situation.
What we probably need is the original StrongARM design crew from DEC and give them something like a DSP32 to play with
Alpha Communications is such a scam!1 0710
1 0710
Notice how on the page they don't even mention what VMSK means? They do on their business scam site: http://www.alphacomopportunity.com/cgi-bin/d.cgi/
If you go to their page you see they avoid discussing their technology and talk all about their business plans.
I'm surprised these guys aren't advertising snake oil or $4000 a month work at home jobs in the newspapers. Don't be fooled they are scam artists.
I also saw on www.indexdirect.com/classifieds this advertisement:
Business Opportunities
FOR SALE Increase your current internet connection
Posted on: 2000-08-11 .
Details:
Increase your current internet connection by thirty times it's current speed! FREE for 5 days Become a distributor (under 10 bucks) Check out the VMSK technology!! Have a GREAT day
Email : carters@polbox.com
Contact: Shelby Carter
Website: http://www.alphacomopportunity.com/cgi-bin/d.cgi/
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.
Correct in respect to Doppler but wrong in its application. A radar gun actually does not measure anything but Delta D over Delta T (change of distance over change of time). Granted it does this quickly but it does not use the doppler effect to measure the speed. For doppler to really become an issue I would think that you would really need to be traveling around 1/10000th the speed of light or even 1/100000th for something like this.
Sorry, *absolutely* correct in its application. The only sort of police "radar" that uses timing to determine distance is a laser radar gun. The traditional RF police speed radar (known as Doppler radar) mixes a sample of the outgoing signal with a received signal and makes a beat between the two. Generally this beat is in the audio frequency range. By beating the outgoing with the incoming it doesn't really matter how much your main oscillator drifts, because over the microsecond or so that the signal takes to make a round trip it won't have drifted far. This sort of radar (as I have described it) can't distinguish between incoming and outgoing targets, but there are DSP based police doppler radars that have this ability, with a different downconversion technique.
Time to go back to basic physics- for a speed much less than c the doppler shift of a signal will be a factor of 1/(1 +/- (v/c)) where v is the radial velocity to or from the observer (- when approaching the observer, + when going away).
For example, with a radar operating at 18GHz, the doppler shift of a car moving at 100 ft/s is about 1.85 kHz. Check out this page for more information on doppler. Things do change as you approach the speed of light, but doppler still does matter, and I think it matters in this situation because of the extreme timing accuracy that this modulation requires. If the timing accuracy goes down, so will the bit-rate.
This whole topic is some combination of genuinely good technology, hype, exaggerations by imperfect journalists, and fraud. From where I'm sitting, I don't know which of the above causes is the true explanation. But I doubt Shannon's law has been repealed. Alas...
Anybody have a translation of this?
It's 10 PM. Do you know if you're un-American?
Correct me if I'm wrong but doesn't Hz mean "per second". Does this mean that we now have 90 bits per second per second?
The author is using a shorthand reference to Hz of bandwidth.
Um, maybe it's just me, but I thought that Shannon's law had been prove n within the bounds of mathematics.
Secondly, the article gives _no_ information that Shannons law has been broken.
In fact, it hasn't. All Shannons law says is the relationship between the signal to noise ration, and the size of the channel, to how much data you can put through it.
In this case, Shannons law allows this to happen. But requires powers of something like (quick mental sums) 280dB.
That's, like, stupidly high.
It's possible that EDN got this all wrong, and someone has actually found an end-run around a whole lot of difficult stuff in conventional signalling theory. But I wouldn't bet on it, and if you're smart, you won't either. This may be some scheme to get investment money out of a bunch of suckers, and it behooves you to not be one of them.
--
Time is Nature's way of keeping everything from happening at once... the bitch.
I wonder what kind of range this system would have? Like current radio? Could it be deployed in a cellular-like fashion, so when I drive cross-country, I could access my mp3s on my machine back home? This could be really cool - Not needing to burn MP3s to CD so I can have car mixes, just fire up a radio connection and network over it. What about using multiple channels for faster throughput? e.g. recieve on 101.3, 101.5, and transmit back on 101.7? Could this be deployed in a car-to-car fashion? Like people who have their HAM callsign on their vanity license plate....
Don't pick up the pho*(@)$*@&@!@ NO CARRIER
No, it is not okay to point out spelling mistakes.
Your post should be moderated as
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or, to save karma from meta-moderation, -1, undderrrattedd
Have a nice day. ^_^
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