Re: [RFC/T][PATCH][V3] mac80211: Exponential moving average estimate for rc80211_simple

[Mattias Nissler <mattias.nissler@gmx.de>]

On Tue, 2007-11-27 at 16:35 +0100, Stefano Brivio wrote:
> On Mon, 26 Nov 2007 22:30:05 +0100
> Mattias Nissler <mattias.nissler@gmx.de> wrote:
> 
> > This changes rc80211_simple failed frame percentage estimation to use an
> > exponential moving average method. Failed frames percentage is sampled
> > over time and a smoothed average is computed. This average is examined
> > periodically and the rate adjusted eventually.
> 
> This can be seen as a particular example of a PID controller [1]. It's
> actually a PI controller, with no derivative terms in it. It could be
> interesting to implement a regular PID controller.
> 
> This is clearly a MIMO model, with the setpoints being a reasonable value
> of TX failures and the highest achievable rate, the process input being the
> bitrate and the process output being TX failures and successes; and
> obviously, the big issue being the implementation without floating-point.
> Thus, with some tuning, you could probably get a very good rate control
> algorithm.

Good point, I'm not too familiar with control theory, but I'll do some
homework :-)

However, maybe you can answer some questions:

Is it sensible to preprocess (i.e. calculate a smoothed average) the
input data for a PID controller? If so, should we compute a time average
or just average over the last N frames?

Note that the rates (or actually modulations) are not a continuous
entity, but we only have a limited set of choices. Normally, the PID
would output a value from some continuous range. What are good ways to
map the PID controller output to a rate?

Mattias