[e2e] Looking for SNR traces.

Detlef Bosau detlef.bosau at web.de
Tue Jul 1 08:39:53 PDT 2008

David P. Reed wrote:
> SNR is a derived number that you probably don't want to use for analyses.

Why not? At least, it is used for theorems like Shannon-Hartley ;-)

Although you might object, that the Shannon rate is a theoretical upper 
limit for a bit rate, that there are quite a few reasons why this is 
hardly achieved in reality and that the simple "signal to noise ratio" 
is not the only factor which determines whether a Transbort Block / 
Burst / however it is called is successfully deflifered or not.

At least, it is a point to start. And, as you know from many of my posts 
on the list, it took me several wrong tracks to _stop_ trying to 
understand all the gory details of RLC etc., and now I think it is at 
least a reasonable way to model some aspects (not all, but some) of 
mobile networks to simply _assume_ that we can send packtes / blocks / 
- with Shannon rate
- error free.

Of course, this model is wrong. Like any other model. Otherwise, it 
wouldn't be a model :-)

I simply got bogged down in too much details of simulation and 
simulators of mobile networs during the last few months, and I'm 
(frankly spoken) somewhat fed up of looking for explanations why one 
simulator yields other results than another.  Although it may be 
worthwile to compare simulator artifacts, you do not learn very much 
about your problem that way.

So, when Lachlan suggested, to use Shannon rates some weeks ago, I first 
was rather upset and thought, he were pulling my leg.
But some days later I understood: That's indeed a reasonable way to go.
> You might want to just ask a simpler question: has anyone got a trace 
> of N for realistic activity patterns of a cellphone that is held up to 
> the ear or mounted in a vehicle, for the bands (and receiver 
> front-end) in question.

This is a simpler question. Yes. And admittedly, those patterns are hard 
to find as well.

> I think you will find the answer is no.
> Regarding signal, this is so highly dependent on "tower" placement 
> that it's best dealt with by declaring a minimum acceptable signal 
> above the "standard noise floor in dBm", then moving the "towers" 
> around to achieve the desired signal.

I know.

However, what I'm interested in is the question: Is the PF algorithm 
(Jalali, Tse, Hosein, Holtzmann, ...) ressoure fair (wrt to STTI 
considred as ressources) or not?

 From Holtzmann's papers, I understand that PF is ressource fair if the 
Raleigh Fading part in all competing channles is i.i.d.

And although I did not read each simulator code line by line, I think in 
many models based upon Jakes Model the simulated Rayleigh fading _is_ i.i.d.

Personally, I strongly doubt that the PF algorithm is ressource fair. 
(Maybe, someone can convince me that I'm wrong?) but from my 
simulations, it _is_.
And now, I'm looking for the problem:

Some possibilities:

1.: Me. (Always a valid guess :-))
2.: The simulated scenraios: These might be scenarios, where PF works 
pretty fine.

> Perhaps you are interested in the PDF describing the derivative of 
> "signal level".   The PDF would allow you to fit a single-parameter 
> gaussian to it, and then verify the Rayleigh-fading parameter.

No, I'm certainly not interested in the PDF but in the time series 
itself. And because some authors claim that the PF scheduling algorithm 
_is_ ressource fair for all scenarious, it would be sufficient to find 
one (or some very few) counterexampels to justify more investigation in 
this field.
> In general, this fails the sensible scientific approach of measuring 
> given controllable.  

Absolutely, however for a _counterexample_, this should be sufficient as 
long as the traces are can be reproduced.

At least, the other way round, providing some examples where an approach 
holds and use them as a proof is not really better...

> Thus, despite all the math, the models are witchcraft, slightly above 
> Astrology in validity.
You are sooooooooooo right ;-)

(However, I somewhat lost the belief, everything in computer networks 
can be "proven" in a reliable manner. I think, some degree of wichcraft 
or Stonehenge or Nazca or crystall skull will always remain.)


Detlef Bosau                          Mail:  detlef.bosau at web.de
Galileistrasse 30                     Web:   http://www.detlef-bosau.de
70565 Stuttgart                       Skype: detlef.bosau
Mobile: +49 172 681 9937

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