[Battlemesh] Why does .11ax increase the guard interval?

[Eoin]

Non line of sight propagation creates longer signal paths - that
introduce delays. Though almost insignificant amounts at these
distances - usually only noticeable in satellite communications.

Possibly related is non line of sight produces multi paths - that is
the signal propagates in different routes bouncing around the
obstacles. These multiple signals arrive at the receiver in different
phases because of the varying distances each has taken. You can then
experience constructive and destructive interference where the wave
form peaks either combine or cancel one another. A longer sample time
will increase the likelihood of receiving an in phase signal.


regards

Eoin




On 2 February 2017 at 00:47, Juliusz Chroboczek
<jch at pps.univ-paris-diderot.fr> wrote:
>> Does anyone understand why .11ax increases the guard interval?
>
> I found the answer here:
>
>   https://mentor.ieee.org/802.11/dcn/15/11-15-0099-04-00ax-payload-symbol-size-for-11ax.pptx
>
> It turns out that .11ax aims to improve behaviour in non-line-of-sight
> outdoors situations, where multipath issues are more severe than in the
> usual cases (indoors nLOS, outdoors LOS).  To that, they:
>
>   - increase the symbol time (and therefore the size of the FFT);
>   - increase the guard interval.
>
> I'm not sure what problems outdoors nLOS is supposed to solve, but if the
> problem does exist, it makes sense to relax the timing parameters.
>
> -- Juliusz
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