how not to leave any money on the table: interference, feedback and side information david tse...
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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• The fed-back channel may be outdated.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Outdated FeedbackWe ask: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Downlink MIMO• Outdated channel knowledge? 4/3 symbol per time slot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.