Re: [Make-wifi-fast] [Cerowrt-devel] more well funded attempts showing market demandfor better wifi

[moeller0 <moeller0@gmx.de>]

Hi David,

> On Jun 27, 2016, at 09:44 , David Lang <david@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