Re: research problem


Steve Underwood skrev:
> porterboy76@xxxxxxxxx wrote:
> >>
> > Yes, thats one of the reasons I like digital comms so much. It is very
> > easy to tell a good algorithm from a bad algorithm objectively, unlike
> > for example in audio or imaging, where there is a lot of subjectivity
> > (I've stared blankly at a lot of similar Lena jpgs and not really been
> > able to tell the difference). I think that's why ended up in digital
> > comms, I originally started a PhD in imaging and switched almost
> > immediately. Are you saying that seismic (seismological?) signal
> > processing is a more objectively defined field than image and audio
> > processing? What kind of algorithms are involved... wait, OK, time for
> > Google ;-)
>
> Interesting view.
>
> Those Lena images, they always miss the best parts of the original :-) .

Does anybody know the history of those images? Is it as simple as
some geek wanted an excuse to read playboy at work? Or was caught
reading it and had to come up with an explanation...?

> You just get used to picking out these differences. I found similar
> issues with colour work, and speech coding. Differences that were very
> hard to detect on the first day stood out like sore thumbs pretty soon.

Would those differences be perceptible by a layman end user?

> I don't agree with your notion about the ease of recognising good comms
> alorithms. The behaviour or radio in complex environments like dense
> city areas is still poorly understood.

Imagine to provide a radio com link in a densely populated area
where

- the speed of light is unknown and varies between blocks
- the reflection coefficient from the ground and walls of
buildings are unknown
- signals propagate through buildings and experience
internal reflections inside rooms before they propagate
to the reciever, all transmission and reflection coefficients
being unknown, as well as the speed of light inside each room
- you have only nominal control of the source signal,
i.e. you control the time at witch it will send pulse and
the general frequency band, but that's it
- the width of the roads vary from block to block
- the length of the blocks vary

and you start approaching the world of the seismic signal analyst.

> To take a pretty simple radio
> system as an example - 1200bps POCSAG paging generally worked better in
> dense cities than the original 512bps. That surprised most radio systems
> researchers, so they clearly had a weak idea of what works well. :-)

Was the bandwidth proportinal to the bit rate? Were the two radios
working in different frequency bands? Did they use the same
modulation scheme?

If the 1200bps system has a higher bandwidth, everything else being
equal, it might be expected to work better than the 512bps system,
since the relative bandwidth of nulls to signal bandwidth in the
Lloyd mirror equivalent (the superposition of direct and reflected
signal) is lower in the 1200bps system. neither would work perfectly,
but the higher bandwidth one would work better.

If the 1200bps system works at a higher carrier frequency, the
transmision band will be more diffuse than the lower-carrier 512bps
system. Again, it has to do with the effects of reflected and
direct signals adding up.

Modulation schemes are difficult to analyse, but there could be
different robustness to reflectied signals that arrive at a
given delay. If so, what works well in test scenario A needs not
work in test scenario B.

I once used the Lloyd mirror to explain why a radio link at sea
had a very short range, only 1/4th of what the same radio achieved
over land. It made the day for the guy whos neck was on the line
for not getting that link to work.

It doesn't take much complexity for a propagation channel to become
seriously messed up.

Rune

.

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