Communications and Networking Research GroupEytan Modiano

Slide 1

Using collision-free scheduling:

dream or reality?

Eytan Modiano

Massachusetts Institute of Technology

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

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

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.

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

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

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