[e2e] How many transmission attempts should be done on wireless networks?

Detlef Bosau detlef.bosau at web.de
Sat Sep 19 03:54:38 PDT 2009

Lachlan Andrew wrote:
> 2009/9/19 David P. Reed <dpreed at reed.com>:
>> Queues and retransmissions are inseparable.  You have to maintain a queue
>> while retransmitting.   Ideally (in coding theory) you never would
>> retransmit the same thing twice.   You would instead transmit a smaller and
>> different piece of information the second time, so that the combination at
>> the receiver would regenerate the lost information.
> Very true.  I believe Detlef is interested in HSPA systems which
> already do hybrid ARQ, rather than retransmitting the same thing
> twice.  His question still applies.

No, at the moment I'm definitely more generic.

The point is: When ARQ can guarantee a maximum SDU corruption ratio of, 
say, 10^-3 or 10^-4, we could treat wireless links similar to wired once 
- at a _very_ first glance.

There would be no loss differentiation problem any more - and the world 
were fine =8-)

"Always look on the bright side of life!" *whistle*

> In information-theoretic terms, Detlef's could have asked "how much
> water-filling-over-time" should there be?  

This is already a second glance ;-) And an important one too. VJCC 
attempts fair distribution of _capacity_, wrt to your terms: pipes and 
buckets. Or fair water-filling-over-time done by the individual sources. 
And actually, retransmissions appear as water-filling by an "unknown", 
yet not controlled, source.

> If a channel is bad,
> information theory tells us to delay sending information until the
> channel becomes good.

That's the approach pursued by opportunistic scheduling and a result of 
IIRC Stephen Hanly's PhD thesis.

And I think Hari has done some work in this direction as well for 802.11.

>   That is essentially what heroic hybrid ARQ
> repeated retransmissions (with increasing delays) does. 

Really? Particularly in HSDPA, I don't see e.g. an adaptation to 
changing channel conditions.

Actually, a CQI is chosen once, and then a transport block is sent in 
not more than three attempts. (Of course with HARQ, i.e. e.g. chase 
combining or, I think this is done more often, incremental redundancy.)

The limitation to a maximum of three attempts makes sense, because the 
channel estimation does not provide a forecast of the long term channel 
conditions but only for its "actual state", which may be sufficient for 
the next, say, 10 ms or so.

>  However, this
> causes obvious problems to applications, which are ignored by
> traditional information theory.
Yes. And I think, this is hard stuff! Because it's extremely difficult 
to forecast the number of sending attempts actually being needed to 
guarantee a certain SDU corruption probability.

Detlef Bosau		Galileistraße 30	70565 Stuttgart
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