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