[e2e] I got lost in opportunistic scheduling.

Detlef Bosau detlef.bosau at web.de
Mon May 14 13:28:22 PDT 2007

marbukh at antd.nist.gov wrote:
> It appears that problems you touched are pretty much open.

Hopefully :-)

At the moment, I see two possible problems:
- Either I´m too stupid to find the solutions in literature,
- or I´m too stupid to find the solutions myself ;-)

And the more I try to understand these issues, the more I see, that this 
seems to be really difficult.

> Opportunistic scheduling allows one to achieve
> the maximum theoretical end-to-end throughput region
> without knowing the pattern of link capacity variability,
> e.g., due to fading, mobility, etc.
O.k., so my guess is correct: The reason for op. sch. is to achieve 
maximum spectral efficiency ( = throughput / bandwidth), correct?

If so, I finally have understood, why we´re doing this :-)

> The problem is that it can be done at the expense of
> the end-to-end delays, while throughput/delay trade-offs
> in variable connectivity networks with opportunistic scheduling
> is to a large degree an open problem.
Fine :-)

> Some initial ideas include delay-limited bandwidth,
> but as far as I know these ideas have been developed
> only for cellular networks.
To my understanding, opportunistic scheduling mainly exploits multi user 
diversity, sometimes it´s metioned in conjuncition with multi path 
diversity and thus with the MIMO- and beamforming stuff.

To my understanding, both approaches / ideas primarily attempt to 
mitigate the effects of fast faiding (typically modeled as Rayleigh 
fading) in wireless networks, hcne things become interesting for _fast_ 
moving users. (whatever may be the meaning of "fast (tm)").
(A rough definition of fast is "neither still standing nor pedestrian".)

What makes things extremely difficult is that I have absolutely no idea 
how to model the wireless channel. (I´m computer scientist and no 
communication engineer, so I´m in the need of advice here.) Ideally, I 
would appreciate a model which yields a BLER with respect to time. 
However, I don´t know whether we have those models. And I dare not to 
think about HARQ there, because a simple BLER model perhaps will not be 
sufficient when it comes to adaptive puncturing which is as well a 
technique to adapt to fast fading and wich to my understanding also may 
change delay because it changes the "payload length" of a radio block. 
O.k., I learned that CE people do not understand me here, so it changes 
the code rate with respect to time. %-)

I got in touch with a colleague here in Germany about this topic, but at 
the moment we both got mad about the differences in terminology.


Detlef Bosau
Galileistrasse 30
70565 Stuttgart
Mail: detlef.bosau at web.de
Web: http://www.detlef-bosau.de
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