On Thursday, June 23, 2016 4:52pm, "David Lang" <da...@lang.hm> said:
> On Thu, 23 Jun 2016, dpr...@reed.com wrote:
>
>> The actual issues of transmitting on multiple channels at the same time are
>> quite minor if you do the work in the digital domain (pre-DAC). You just
>> need
>> a higher sampling rate in the DAC and add the two signals together (and use a
>> wideband filter that covers all the channels). No RF problem.
>
> that works if you are using channels that are close together, and is how the
> current standard wide channels in N and AC work.
>
> If you try to use channels that aren't adjacent, this is much harder to do.
>
The whole 5 GHz U-NII band is not that wide. It's easy to find DACs that run
at 1 Gsps or better. On transmission you don't need to filter the bands where
you put no energy in the middle (or not much).
> Remember that the current adjacent channel use goes up to 160MHz wide, going
> wider than that starts getting hard.
>
>> Receiving multiple transmissions in different channels is pretty much the
>> same
>> problem - just digitize (ADC) a wider bandwidth and separate in the digital
>> domain. the only real issue on receive is equalization - if you receive two
>> different signals at different receive signal strengths, the lower strength
>> signal won't get as much dynamic range in its samples.
>>
>> But in a LAN setup, the variability in signal strength is likely small enough
>> that you can cover that with more ADC bits (or have the MAC protocol manage
>> the station transmit power so that signals received at the AP are nearly the
>> same power.
>>
>> Equalization at transmit works very well when there is a central AP (as in
>> cellular or normal WiFi systems).
>
> define 'normal WiFi system'
Ones based on access points. In general, in typical WiFi deployments one
prefers to make smaller cells so that the signal level variation between "near"
and "far" signals is modest, which makes equalization much easier or even
optional. If there is a large variation of power received at the access point
then CSMA is hard to achieve, and the far stations have to run at slow rates,
occupying more than their fair share of airtime.
(a non-normal system would be a peer-to-peer mesh over a wide enough area that
you end up with "hidden terminal" issues all over the place)
>
> It's getting very common for even moderate size houses to need more than one
> AP
> to cover the entire house.
>
Agree. No question about that.
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