On Mon, 27 Jun 2016, Jason Abele wrote:
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.
Thanks for the more detailed information.
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.
Cellular mostly works because they have hundreds/thousands of channels rather
than tens.
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.
compatibility and the fact that stations with different spreading algorithms
still interfere with each other. Also, coordinating the 'right' spreading
algorithm for each station with each AP (including ones with hidden SSIDs)
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.
Almost all the mesh work I see is using a mesh of APs, anything beyond that is
wishful thinking.
Even mu-mimo requires some client support.
David Lang
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