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.  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.  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.

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

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

Our work in a nutshell  Design of MAC layer extensions to the 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.  Design of MAC layer extensions to the 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.

The story in pictures Paper in INFOCOM 2006

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

Experiments with n  Use of STBC MIMO: STBCSISO Not much to be gained at lower rates !!!

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!

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

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.  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.

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.  MIMO seems to help in coping with far away interferers.  Increased connectivity true to some extent  higher rates.  Can help in power savings.

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

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

Thank you

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.  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.