On Mon, 27 Jun 2016, moeller0 wrote:
Hi David,
On Jun 27, 2016, at 09:44 , David Lang <da...@lang.hm> wrote:
On Mon, 27 Jun 2016, Sebastian Moeller wrote:
On a wireless network, with 'normal' omnidirctional antennas, the signal drops
off with the square of the distance. So if you want to service clients from 1
ft to 100 ft away, your signal strength varies by 1000 (4 orders of magnatude),
this is before you include effects of shielding, bounces, bad antenna
alignment, etc (which can add several more orders of magnatude of variation)
The receiver first normalized the strongest part of the signal to a constant
value, and then digitizes the result, (usually with a 12-14 bit AD converter).
Since 1000x is ~10 bits, the result of overlapping tranmissions can be one signal
at 14 bits, and another at <4 bits. This is why digital processing isn't able
to receive multiple stations at the same time.
But, I you add 10 Bits to your AD converter you basically solved this. Now,
most likely this also needs to be of higher quality and of low internal noise,
so probably expensive... Add to this the wide-band requirement of the sample
the full band approach and we are looking at a price ad converter. On the
bright side, mass-producing that might lower the price for nice oscilloscopes...
well, TI only manufactures AD converters up to 16 bit at these speeds, so 24 bit
converters are hardly something to just buy. They do make 24 and 32 bit ADCs, but
only ones that could be used for signals <5MHz wide (and we are pushing to 160
MHz wide channels on wifi)
But David’s idea was to sample the full 5GHz band simultaneously, so we
would need something like a down-mixer and an ADC system with around 2GHz
bandwidth (due to Nyquist), I believe multiplexing multiple slower ADC’s as
done in better oscilloscopes might work, but that will not help reduce the
price not solve the bit resolution question.
loosing track of the Davids here :-)
it's not just the super high-speed, high precision ADCs needed, it's also the
filters to block out the other stuff that you don't want.
If you want to filter a 1 GHz chunk of bandwidth, you need to try and filer out
signals outside of that 1GHz range. The wider the range that you receive, the
harder it is to end up with filters that block the stuff outside of it. A strong
signal outside of the band that you are trying to receive, but that partially
makes it through the filter is as harmful to your range as a strong signal in
band.
also note my comment about walls/etc providing shielding that can add a few
more orders of magnatude on the signals.
Well, yes, but in the end the normalizing amplifier really can be
considered a range adjustor that makes up for the ADC’s lack of dynamik
resolution. I would venture the guess not having to normalize might allow
speed up the “wifi pre-amble” since one amplifier less to stabilize…
not really, you are still going to have to amplify the signal a LOT before you
can process it at all, and legacy compatibility wouldn't let you trim the
beginning of the signal anyway.
And then when you start being able to detect signals at that level, the first
ones you are going to hit are bounces from your strongest signal off of all
sorts of things.
But that is independent of whether you sample to whole 5GHz range in one
go or not? I would guess as long as the ADC/amplifier does not go into
saturation both should perform similarly.
if you currently require 8 bits of clean data to handle the data rate (out of 14
bits sampled) and you move to needing 16 bits of clean data to handle the
improved data rate out of 24 bits sampled, you haven't gained much ability to
handle secondary, weak signals
You will also find that noise and distortion in the legitimate strong signal
is going to be at strengths close to the strength of the weak signal you are
trying to hear.
But if that noise and distortion appear in the weak signals frequency
band we have issues already today?
no, because we aren't trying to decode the weak signal at the same time the
strong signal is there. We only try to decode the weak signal in the absense of
the strong signal.
As I said, I see things getting better, but it’s going to be a very hard
thing to do, and I'd expect to see reverse mu-mimo (similarly strong signals
from several directions) long before the ability to detect wildly weaker
signals.
You are probably right.
I also expect that as the ability to more accurately digitize the signal
grows, we will first take advantage of it for higher speeds.
Yes, but higher speed currently means mostly wider bands, and the full
4-5GHz range is sort of the logical end-point ;).
not at all. There is nothing magical about round decimal numbers :-)
And there are other users nearby. As systems get able to handle faster signals,
we will move up in frequency (say the 10GHz band where police radar guns
operate) or higher.
David Lang
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