On Mon, Jun 27, 2016 at 2:43 AM, moeller0 <moell...@gmx.de> 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. >
The reason you can not just add bits to the ADC is the thermal noise floor: https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise#Noise_power_in_decibels If you assume a maximum transmit power of ~20dBm (100mW) and a 160MHz channel bandwidth (with a consequent thermal noise floor of -92 dBm), the total possible dynamic range is ~112dB, if you receiver and transmitter a coupled with no loss. At ~6dB/bit in the ADC, anything beyond 19bits is just quantizing noise and wasting power (which is heat, which raises your local thermal noise floor, etc). If your channel bandwidth is 1GHz, the effective noise floor rises by another ~2bits, so ~17bits of dynamic range max, before accounting for path loss and distortion. Speaking of distortion, look at the intermod (IP3) or harmonic distortion figures for those wideband ADC sometime, if the signals of interest are of widely varying amplitudes in narrower bandwidths, the performance limit will usually be distortion from the strongest signal, not the thermal noise floor. This usually limits dynamic range to less than 10 effective bits. Also transmitters are usually only required to suppress their adjacent channel noise to around -50dB below the transmit power, so a little over 8bits of dynamic range before the ADC is quantizing an interferer rather than the signal of interest. I am surprised that 802.11 still uses the same spreading code for all stations. I am no expert on cellular CDMA deployments, but I think they have been using different spreading codes for each station to increase capacity and improve the ability to mathematically remove the interference of other physically close stations for decades. As complex as the 802.11 MAC is becoming, I do not understand why an approach like MU-MIMO was chosen over negotiating a separate spreading code per station. My best guess is that it keeps the complexity (and therefore power) at the AP rather than in the (increasingly mobile, power-constrained) station. Hopefully the rise of mesh / peer-to-peer networks in mobile stations will apply the right engineering pressure to re-think the idea of keeping all complexity in the AP. Cheers, Jason _______________________________________________ Cerowrt-devel mailing list Cerowrt-devel@lists.bufferbloat.net https://lists.bufferbloat.net/listinfo/cerowrt-devel
