How not to leave any money on the table:interference, feedback and side information

David TseWireless Foundations

U.C. Berkeley

June 7, 2012

TCE ConferenceTechnion

2

Drive for Spectral Efficiency

• Lots of advances in the physical layer in the past 15 years.

• 4 to 8 fold increase in spectral efficiency from 2G to 3G and several more folds from 3G to 4G,

• Gains pertain mainly to point-to-point and single-cell performance.

• Interference is the current research frontier.

The Interference BarrierInterference gets worse as:• density of wireless nodes increases.• wireless architectures become more decentralized

Heterogeneous networks

Macro-cell

pico-cell

Tx1

Rx1

Peer-to-peer networks

Tx2

Rx2

Tx3

Rx3

Interference MitigationSome emerging techniques:

• Multiuser MIMO

• Network MIMO

• Interference alignment

Multiuser MIMOmitigates intra-cell interference

Network MIMOmitigates inter-cell interference

Central Unit

Downlink Interference Alignment(Suh, Ho & T. 10)

Downlink Interference Alignment

Interference alignment between out-of-cell and intra-cell interferenceWill see how each cell can use

2 out of 3 dimensions.

21

1

1

2

2

transmission over 3 sub-carriers

(Suh, Ho & T. 10)

For K users/cell, can use K out of K+1 dimensions.

Interference-free performance is approached as K grows.

Key Enabler For All Techniques

Timely channel knowledge at the transmitters:

• Multiuser MIMO

• Network MIMO

• Interference alignment.

Central Unit

21

1

1

2

2

Theory to Practice

• In a FDD system, channel has to be measured at the receivers and fed back to the transmitter.

• The fed-back channel may be outdated.

• Current approach: channel prediction

Prediction is very difficult, especially if it’s about the future.

Niels Bohr

(or was it Yogi Berra?)

Outdated FeedbackWe ask:

What if current channel is completely independent of the fed back information?

Conventional wisdom:

Feedback is totally useless.

Downlink MIMOPerfect channel knowledge: 2 symbols per time slot

A

B

Downlink MIMOPerfect channel knowledge: 2 symbols per time slot

No channel knowledge: 1 symbol per time slot

Downlink MIMO• Outdated channel knowledge? 4/3 symbol per time slot

reconstruct

Scalability of Technique

......

symbol/time slot

K antennas

K users (Maddah-Ali & T. 11)

Role of feedback in CommunicationOld:

• ARQ

• channel prediction.

New:

• exploiting side information

Exploiting Side Information

reconstruct

side info.

side info.

Interference, Side Info. And Feedback

• Broadcast nature of wireless creates interference.

• Interference can be viewed as side information.

• Side information can be exploited by feedback of previous channel.

Optimal scheme leaves no money on the table.

Side Information in Other Settings(Katti et al 06)(Ahlswede et al 00)

+

+ +X X

Another related work: index coding problem (Bar-Yossef et al)

Conclusions• Interference is a central barrier and new approaches to

mitigate interference are emerging.

• Innovative use of feedback is a key enabler for these approaches.

• New role of feedback emerges: exploiting side information.

• Optimal use of feedback leaves no money on the table.