[e2e] Opportunistic Scheduling.
detlef.bosau at web.de
Tue Jul 3 10:05:14 PDT 2007
ksingh at irisa.fr pointed to
> QoS control for WCDMA high speed packet data
> Hosein, P.A.
> Ericsson Wireless Commun. Inc., San Diego, CA, USA;
I think, there is a severe misconception in this paper, however this
kind of misconception is met quite freuquently.
Kelly´s objective is the assignment of shared ressources. So, when Kelly
talks about "rates", these rates are shares of shared ressources and
particularly sum up to a total which is less or equal to the maximum
amount of ressources.
Think of a 150 MBit/s link (is this OC/3? I always mix up the numbers),
than this can be shared among three flows which are assigned for instance
- 20 MBit/s
- 80 Mbit/s
- 50 MBit/s
(<)=150 MBit/s, fine. :-)
What we talk about in mobile networks are _code_ _rates_, and even this
is simplified because there might be some kind of dynamic channel
adaption by a per time slot choice of the symbol set.
So, although some flow may be scheduled "at a rate" 30 kbit/s and some
other flow may be scheduled "at a rate" 60 kbit/s, both flows will
typically (in HSDPA like systems) occupy _the_ _same_ _amount_ of
ressources because the radio blocks have the same length in symbols,
only the information words differ in lenghts depending on the actual
coding scheme / puncturing scheme.
Hence, when we use code rates, even the boundary conditions in Kelly´s
model can not be met in the original manner but would have to be adapted
to the different meaning of "rate".
Particularly, I once again emphasize that a stream scheduled "at a small
rate", i.e. code rate does _not_ occupy less ressources than one
scheduled "at a high rate".
So, simply spoken: I severely doubt Hoseins rationale.
Another point which is not convincing is, that in Hoseins paper the
"actual vector of average rates" is made to follow an "optimal vector of
rates" which is itself a moving target and may be subject to severe
Perhaps, there is a severe flaw in my way of thinking, but at the
moment, my doubts in the "opportunistic scheduling" algorithm as it is
in use today become not lesser but bigger.
Detlef Bosau Mail: detlef.bosau at web.de
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