1. How not to leave any money on the table: interference, feedback and side information David Tse Wireless Foundations U.C. Berkeley June 7, 2012 TCE Conference Technion TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAAAAAA

2. 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.

3. The Interference Barrier Interference 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

4. Interference Mitigation Some emerging techniques: Multiuser MIMO Network MIMO Interference alignment

5. Multiuser MIMO mitigates intra-cell interference

6. Network MIMO mitigates inter-cell interference Central Unit

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

8. Downlink Interference Alignment Interference alignment between out-of-cell and intra-cell interference Will see how each cell can use 2 out of 3 dimensions. 2 1 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.

9. Key Enabler For All Techniques Timely channel knowledge at the transmitters: Multiuser MIMO Network MIMO Interference alignment. Central Unit 2 1 1 1 2 2

10. 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 The fed-back channel may be outdated.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

11. Prediction is very difficult, especially if its about the future. Niels Bohr ( or was it Yogi Berra? )

12. Outdated Feedback We ask: What if current channel is completely independent of the fed back information? Conventional wisdom: Feedback is totally useless.

13. Downlink MIMO Perfect channel knowledge: 2 symbols per time slot A B

14. Downlink MIMO Perfect channel knowledge: 2 symbols per time slot No channel knowledge: 1 symbol per time slot

15. Downlink MIMO Outdated channel knowledge?4/3 symbol per time slot reconstruct

16. Scalability of Technique symbol /time slot K antennas K users (Maddah-Ali & T. 11)

17. Role of feedback in Communication Old: ARQ channel prediction. New: exploiting side information

18. Exploiting Side Information reconstruct side info.

19. 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.

20. 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)

21. 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.