using collision-free scheduling: dream or reality?
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Using collision-free scheduling: dream or reality?. Eytan Modiano Massachusetts Institute of Technology. Some thoughts…. 1) A bit of history 2) What do we mean by “collision free”? 3) Impact of new physical layer models 4) Issues with “collision free” scheduling - PowerPoint PPT PresentationTRANSCRIPT
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Communications and Networking Research GroupEytan Modiano
Slide 1
Using collision-free scheduling:
dream or reality?
Eytan Modiano
Massachusetts Institute of Technology
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Communications and Networking Research GroupEytan Modiano
Slide 2
Some thoughts…
1) A bit of history
2) What do we mean by “collision free”?
3) Impact of new physical layer models
4) Issues with “collision free” scheduling
5) Issues with random access approaches
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Communications and Networking Research GroupEytan Modiano
Slide 3
History (from a practical perspective)
• Early work on wireless networks focused on random access and its variants – Packet radio - 1970’s, aloha, CSMA, etc.
• Most systems in practice did not use random access– Military wireless system: EPLRS, SINCGARS, JTRS are TDMA
based
• Most commercial wireless data systems are “collision free”– CDMA or TDMA systems
– Random access often used for control channel signaling
… and then of course there is 802.11
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Communications and Networking Research GroupEytan Modiano
Slide 4
What is collision free?
• Simple model: simultaneous transmissions interfere and fail– Provides the basis for Aloha, CSMA, etc.
– Much of the work on “collision free” scheduling assumes simple interference models • Primary interference, secondary interference, interference sets,
etc.• Goal to activate a set of links that do not “interfere”
• In reality interference can be a “soft” notion
– Power and data rate can be controller to deal with interference
– No such thing as a collision
– No such thing as “collision free”
– Need to consider SINR based models• Communicate in the presence of interference
• Simple “collision” and “collision-free” models are useful for developing guiding principles, intuition, algorithms, etc.
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Communications and Networking Research GroupEytan Modiano
Slide 5
Impact of new (more sophisticated) physical layer models
• SINR-based models– “soft” notion of a link– Power and rate control
• Multi-user decoding– Reduce or eliminate interference
• Multi-channel and multi-radio capabilities– How would a random access protocol work in this context?
• MIMO systems
• Sophisticated physical layer techniques can significantly increase system capacity - but will also be more complex to implement– Added cost of “scheduling” may be relatively small
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Communications and Networking Research GroupEytan Modiano
Slide 6
Collision-free scheduling
• E.g., TDMA-based systems– “non-interfering” links are activated according to some
scheduling algorithm– Max-weight scheduling (Tassiulas/Ephremides)– Rate-based scheduling (e.g., Hajek and Sasaki)
• Issues:– What does it mean for two links to be non-interfering?
– How do we determine which links do not interfere?
– How do we deal with time-varying channels? mobility?
– Complexity of scheduling algorithms?
– Distributed implementations?
• Finding an optimal scheduling scheme may be impractical - but very simple suboptimal schemes exist that may outperform random access– Especially for multi-hop networks
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Communications and Networking Research GroupEytan Modiano
Slide 7
Random Access
• Random access is relatively simple
• 802.11 is out there
• Issues:– Performance in multi-hop setting– How to provide QoS guarantees
• Hybrid Schemes– Schedule at high loads– Random access at Low loads– Random access for control signaling