Walker still doesn't get it!
10 April 2001

On April 10, 2001 I noticed an update to Walker's VMSK Delusion page with another failed attempt to rebut my debunking of VMSK.

Walker's near-total ignorance of the fundamental principles of modulation and signal theory is exceeded only by his stubborn refusal to admit his mistakes and learn from them even when the rest of the world can see he's wrong. Perhaps the latter explains the former.

In any event, Walker's document now mentions my recent simulation program:

Phil has just added a simulation program that misses the mark. We have repeatedly emphasized that there is no known narrow band filter that functions at baseband. The full Nyquist BW must be used. This is seen in the Nonsense file and some others. If one uses a narrow band filter at baseband, only a sine wave at bit rate comes out. This is also why we cannot use conventional filters at RF and why simulation programs such as LabView, Matlab, SysView and EEsoft fail.
Of course there is no known narrow band filter that will allow VMSK to function at baseband. Showing that was the entire purpose of my simulation! What Walker still can't understand is that the exact same analysis applies at both baseband and RF. There is no reason why they should be different.

When a baseband signal is applied to a balanced modulator, the baseband frequency spectrum is shifted by the carrier frequency. The positive frequency components in the baseband signal become the upper sideband and the negative frequency components become the lower sideband. Because the negative baseband components are equal to the positive components, the lower modulation sideband is a mirror image of the upper sideband.

At baseband, the negative and positive frequencies overlap. But at RF, they are separated into two sidebands. Therefore the occupied RF bandwidth is fully twice that of the baseband signal. Single sideband techniques can remove one of the redundant sidebands, but they cannot reduce the occupied RF bandwidth to less than that of the original baseband signal.

When he successfully comes up with an RF simulation that shows the single frequency AND THE DETECTED OUTPUT shown in the Grass is Gone" paper, the world will accept the gospel according to Karn.
What a strange statement. Of course I will never be able to simulate a pure single-frequency system that produces detected output because I've been saying all along that this is impossible!

On the other hand, I could simulate a working VMSK system where the spectral density of the data-carrying grass is low enough to fall below the noise floor of a real spectrum analyzer, making it appear that the narrowband clock is the only relevant signal component. For this I need the complete phase-frequency response curves of Walker's "magic filters" out to at least ten or twenty times the data rate, depending on his "code".

Walker has repeatedly ignored my requests for this information. What could he be hiding?

I'd also need an accurate model of any stray RF coupling that may exist between modulator and receiver on the lab bench that might bypass his filters. I note that such stray coupling is far more likely at RF than at baseband, as it can take many paths: free space between unshielded equipment, along common power supply leads, etc. At Qualcomm we often have to go to great lengths to preclude stray RF coupling in equipment tests, e.g., through the use of copper "screen rooms".

If stray RF coupling is bypassing Walker's filters, this could help explain Walker's delusion that VMSK can somehow tolerate tighter filtering at RF than at baseband.

We say it without tongue in cheek or double meaning. This work by Karn is darn good. The coding is excellent, even praiseworthy. His FFT is excellent. Unfortunately, he tried what about 6 or 7 others have tried unsuccessfully. VMSK cannot be analyzed at baseband unless you use the full Nyquist BW.
Thank you, Harold. Once again you've conceded my point: VMSK requires the full Nyquist bandwidth.

Walker's irrelevant questions

Walker goes on to ask several irrelevant questions. I might as well answer them anyway.
Here are a few pointed questions to Karn: VMSK is coded BPSK with end to end pulse width modulation transmitted single sideband.

1) Does BPSK work? If you say NO, then you had better argue that with some people at Qualcomm who use it for Chipping in CDMA.

Of course BPSK works; I never said it didn't. However, BPSK requires a minimum RF bandwidth equal to the data rate. Real systems generally use somewhat more. Qualcomm IS-95 CDMA chips at 1.2288 million chips per second, and the occupied RF bandwidth is about 1.25 MHz. A carefully designed digital filter (described in the IS-95 spec) is required to confine the CDMA signal to that bandwidth.

So if VMSK is coded BPSK, then it too requires an RF bandwidth equal to the Nyquist rate.

2) Can you filter off the upper harmonics and keep only the inner fundamentals? Ditto-ask Qualcomm and all the text book writers. ( See Feher and Rappaport).
If you mean the harmonics of your square wave clock, yes. You can remove these without harm because they are well outside the Nyquist limit. This can be shown quite easily with my simulation. But you must still pass (at some amplitude) all of the components within the Nyquist limit, or the resulting intersymbol interference will destroy your modulation.
3) Does pulse width and/or pulse position modulation work? If you say NO, then argue that Federal Radio, who made the gear and AT&T who used it extensively in the 50's. Also tell the text and reference book authors to please remove it from the old books.
Sure, PWM and PPM work just fine -- as long as you don't try to filter them below the Nyquist limit.
4) When using PPM, can you throw away the upper harmonics and keep the fundamentals only? If you say NO, then argue that with Federal Radio and AT&T, plus the texts. Notably Schwartz.
Sure, once again you can throw away anything beyond the Nyquist limit. But not within the limit.
5) Is the VMSK pulse width and rise time any different from that of a BPSK signal transmitting a 101010101 pattern? We don't think so. It is a sinx/x pulse with a pulse width = 2X the bit period and a rise time equal to the bit period.
As I have repeatedly explained, the modulation spectra produced by a short repeating bit pattern like 101010... consists of discrete spectral lines. But a modem that only lets the user send such a sequence is rather useless, wouldn't you say? The only meaningful tests are with random data. And random data always produces a broad, diffuse, noise-like spectrum. Just like the "grass" that has proven to be such an embarassment to you.
6) Does Single Sideband work? A great many people ( including Hams who have been using it ) seem to think so.
Sure, SSB works fine. But it still requires an RF bandwidth equal to or greater than the baseband audio bandwidth.
7) Is the definition of Shannon's Channel Capacity - that it is the "sampling rate times the energy per sample correct"? If not, argue it with Schwartz and Proakis, who say it is in their texts. We accept Schwartz and Proakis. We sample at the data rate and change the filter BW to keep the proper value of C/N.
As worded, this question doesn't make sense. The "sampling rate times the energy per sample" gives the total received power. Exactly how does this relate to Shannon?

Shannon and bandwidth, again

Shannon's channel capacity theorem gives the capacity, in bits per second, for a bandlimited, noisy channel. No system can ever exceed channel capacity. The fact that VMSK appears to do so does not disprove Shannon or show that he is "misunderstood" as Walker so erroneously claims.

Indeed, it quickly became obvious to everyone but Walker just how he has deluded himself into thinking he has built an ultra-narrowband modulation scheme. Walker defines "bandwidth" to mean the range of frequencies that contain most -- but not all -- of his signal power. While this definition is in common use, it is not the correct one when Nyquist and Shannon are involved.

I am familiar with the descriptions of the Shannon capacity theorem in Schwartz and Proakis, which are correct. Perhaps Walker should re-read them himself until he understands them.

Walker questions my motives

This one is just too outrageously funny to ignore:
Qualcomm does have a financial interest in discrediting VMSK. VMSK R&D is supported by the GSM Consortium, which does not like Qualcomm's CDMA.
Actually, my manager and colleagues think I'm just wasting my (personal) time debunking VMSK, because it is so obviously flawed that no competent professional could ever be fooled for very long. While Qualcomm (and I) may consider GSM to be inferior to CDMA, I have to assume that the GSM Consortium is comprised of largely competent individuals. So I find it rather hard to accept Walker's claim that they are funding him.

This fits one of Walker's well-established patterns: in an effort to garner respect, he'll claim support from numerous "scientists", "universities", "commercial labs" and "cellular companies". But when pressed for the identities of those supporters, he hotly refuses and says that it's his own business. This is certainly his right, but then it's our right to wonder if any of those backers actually exist.

I have to give Walker some credit, though. A recent addition to his main page says the following:

Companies contemplating investing in VMSK technology are urged to conduct a very thorough due diligence study, including the technology, plus FCC, ITU and CTIA matters, test site results,
Indeed! And anyone who does conduct a thorough due diligence study will quickly realize that VMSK is in the same category as perpetual motion and "free" energy.