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.
>
> 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…
>
> 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.
>
> 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?
>
> 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 ;).
Best Regards
Sebastian
>
> David Lang
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