Cooperative MIMO in Wireless Networks: Where are we?

Srikanth Krishnamurthy

What is cooperation?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 ?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

AssumptionsAssumptions

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 nutshellOur 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.

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.

The story in picturesThe story in pictures

• Paper in INFOCOM 2006

A closer lookA 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 802.11n

Experiments with 802.11n

Use of STBC Use of STBC

MIMO: STBC SISO

• Not much to be gained at lower rates !!!

To summarize …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!

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 ?

Where did we go wrong ?

Long story -- but in short -- PHY layer issues need to be well understood.

Long story -- but in short -- PHY layer issues need to be well understood.

A list of issuesA 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…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.

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Need to rethink designNeed 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.

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.

The Road AheadThe 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 youThank you

ModelModel 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.