[Cerowrt-devel] [Make-wifi-fast] closing up my make-wifi-fast lab

[Jonathan Morton]

> On 27 Aug, 2018, at 9:00 am, Bob McMahon <bob.mcmahon at broadcom.com> wrote:
> 
> Curious to how LBT can be solved at the PHY level and if the potential solution sets preserve the end to end principle.

The usual alternatives include TDM, usually coordinated by a master device (eg. the AP); full-duplex operation via diplexers and/or orthogonal coding; and simply firing off a packet and retrying with exponential backoff if an acknowledgement is not heard.

TDM and diplexing are already used by both DOCSIS and LTE.  They are proven technology.  However, in DOCSIS the diplexing is greatly simplified by the use of a copper channel rather than airwaves, and in LTE the diplexer is fitted only at the tower, not in each client - so the tower can transmit and receive simultaneously, but an individual client cannot, but this is still useful because there are many clients per tower.  Effective diplexers for wireless are expensive.

Orthogonal coding is already used by GPS and, in a rather esoteric form, by MIMO-grade wifi.  IMHO it works rather better in GPS than in wifi.  In GPS, it allows all of the satellites in the constellation to transmit on the standard frequency simultaneously, while still being individually distinguishable.  The data rate is very low, however, since each satellite's signal inherently has a negative SNR (because there's a dozen others shouting over it) - that's why it takes a full minute for a receiver to get a fix from cold, because it simply takes that long to download the ephemeris from the first satellite whose signal is found.

A future version of wifi could reasonably use TDM, I think, but not diplexing.  The way this would work is that the AP assigns each station (including itself) a series of time windows in which to transmit as much as they like, and broadcasts this schedule along with its beacon.  Also scheduled would be windows in which the AP listens for new stations, including possibly other nearby APs with which it may mutually coordinate time.  A mesh network could thus be constructed entirely out of mutually coordinating APs if necessary.

The above paragraph is obviously a giant handwave...

 - Jonathan Morton