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Cooperative MIMO in Wireless Networks: Where are we?

Cooperative MIMO in Wireless Networks: Where are we?. Srikanth Krishnamurthy. What is cooperation?. Nodes cooperate to jointly transmit information. Instead of contention. Emulate an antenna array -- use of the antenna elements of different nodes jointly.

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Cooperative MIMO in Wireless Networks: Where are we?

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  1. Cooperative MIMO in Wireless Networks: Where are we? Srikanth Krishnamurthy

  2. What is cooperation? • Nodes cooperate to jointly transmit information. • Instead of contention. • Emulate an antenna array -- use of the antenna elements of different nodes jointly. • Possible use of Space Time Block Codes • PHY layer studies suggest that there is a diversity gain • Diversity gain --> reduced SNR requirement for a specific BER requirement.

  3. What did we do ? • Jumped on the bandwagon! • Assumed that PHY Layer benefits are directly applicable at higher layers

  4. Assumptions • The lowered SNR requirement can help in either: • Increased range • Increased rate • In our work, we seek to exploit the increase in range.

  5. Our work in a nutshell • Design of MAC layer extensions to the 802.11 DCF to enable cooperation. • Exchange of channel state information. • Intelligent selection of neighbors. • Design of an tightly coupled routing scheme to exploit increased range. • Compute SISO route. • Bridge this route with longer cooperative MISO hops.

  6. The story in pictures • Paper in INFOCOM 2006

  7. A closer look • Do we really get these gains ? • Or is this an artifact of the models ?

  8. Experiments with 802.11n • Use of STBC MIMO: STBC SISO • Not much to be gained at lower rates !!!

  9. To summarize … • At the very least, in commercial off the shelf hardware, the gains from MIMO (and possibly with cooperative MIMO) are not as one might expect!

  10. Where did we go wrong ? • Long story -- but in short -- PHY layer issues need to be well understood.

  11. A list of issues • Packet error rate does not linearly change with BER! • Bursty errors can make PHY layer models less accurate. • Interactions with FEC codes and OFDMA • We cannot consider one PHY layer technology in isolation if we want to determine what happens in practice.

  12. But there is hope… • MIMO seems to help in coping with far away interferers. • Increased connectivity true to some extent • higher rates. • Can help in power savings.

  13. Need to rethink design • Simultaneous transmissions due to capture are possible • However these are precluded by 802.11 DCF. • Can we somehow enable this ? • Facilitate cooperation • Tune CCA ? -- there are challenges. • Need to carefully understand when benefits are possible.

  14. The Road Ahead • Slippery but driving can be fun! • Need to understand PHY implications on higher layers.

  15. Thank you

  16. Model • Assume that nodes use cooperation to jointly transmit data. • Use of STBC codes. • We calculate the additional distance that a node can communicate given a fixed value of the “diversity gain” due to cooperative MIMO. • Diversity gain was obtained from SNR vs BER graphs from PHY layer studies on MIMO • Turns out that this gain is significant.

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