[e2e] I got lost in opportunistic scheduling.

Detlef Bosau detlef.bosau at web.de
Mon May 14 08:17:41 PDT 2007

Excuse me, when I return to this somewhat annoying subject.

Admittedly, I get somewhat lost here. My leading question is:
- Are there any peculiarities in mobile networks, which can affect upper 
Then I got lost in the assumption, that PF scheduling may lead to delay 
spikes which can cause trouble to upper layers.
My next question was. How large are these?

So, I read a lot of papers, put them into water and converted them into 
paper-mache and built a new house from it - and totally got lost.

So, my first question is, and if there is anybody who can help me there 
I would greatly appreciate this:
Why are we doing opportunistic scheduling at all?

The question seems to be very "local" at a first glance, but in fact 
opportunistic scheduling basically may turn store´n forward components 
into stop´n pray for restart components, so at least they are not 
"completely transparent" from an end to end perspective.

To my understanding, the key idea in opportunistic scheduling is to have 
a user (in a multiuser mobile network) send preferably in periods of 
"good network condition" (whatever this may be).

To my understanding, one reason for doing so is to increase the spectral 
efficiency in a network. Instead of mitigating errors by averaging, as 
it is done as a side effect from spreading by the use of long spreading 
sequences, the user avoids sending in periods with "bad network 
condtion" (w.t.m.b.) and therefore errors are avoided. Side effect: 
Spreading sequences can be shortened and throughput can be increased.

At least, I think that is one lesson I have learned from some slides by 
David Tse.

So, we have three issues here.

1. A user shall send in periods with "good network condtion" (...).

However, there are two ancillary conditions:

2. There shall be fairness between users. (There exists some work on 
this, I did not yet read this all, otherwise our whole street would have 
got new houses from paper-mache. We could rename it then from 
"Galileistrasse" into "Potemkin Boulevard".)
3. From an end to end perspective, a user wating for a time slot 
allocation might want to get it allocated some day. ("When I get older, 
losing my hear...")

In David Tse´s talks about "Multiuser Diversity", the first issue 
corresponds to "Hitting the Peaks".

The other ones correspond to the scheduling metric which is actually 
chosen and the time constants of smoothing components being actually in use.

Perhaps, the fist problem is of particularly interest for people living 
in San Franciso: I´m told, that there are small and tiny earth quakes 
almost every few days. So it´s not the problem to escape from an average 
earthquake but it´s interesting to know to "hit the peak"  - and hit the 
road then, because it´s eventually quite a good idea to leave the city ;-)

However, I totally got lost here and even for the very simple question 
of finding appropriate EWMA filters or to decide which kind of filter 
makes sense, I don´t see where to start.

Perhas, some better understanding of some communication engineering 
details would be helpful here - but at the moment I simply got lost.

So, I would appreciate any hint here.



Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
Mobile: +49 172 681 9937

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