1. ICASSP 2006 Plenary Modern Wireless Communication 1 Modern Wireless Communication: When Shannon Meets Marconi David Tse Wireless Foundations, University of California, Berkeley ICASSP 2006 Toulouse, France May 16, 2006

2. ICASSP 2006 Plenary Modern Wireless Communication 2 Drive Towards Spectral Efficiency Huge surge in air interface research in the past decade, driven by: explosive demand for tetherless connectivity dramatic progress in implementation technology success of second-generation cellular standards, esp. CDMA (IS-95) Significant impact has already been made. Eg. IS-95 (mid 90’s)  CDMA 2000 1x EV-DO (mid 00’s) 4 to 8 fold increase in spectral efficiency …….and more to come.

3. ICASSP 2006 Plenary Modern Wireless Communication 3 Historical Perspective Wireless communication has been around since 1900’s. Ingenious system design techniques……. but somewhat adhoc Claude Shannon Gugliemo Marconi Information theory says every channel has a capacity. Many recent advances based on understanding wireless channel capacity. New points of views arise. 1901 1948

4. ICASSP 2006 Plenary Modern Wireless Communication 4 Multipath Fading Classical view: fading channels are unreliable Modern view: multipath fading can be exploited to increase spectral efficiency. 16dB

5. ICASSP 2006 Plenary Modern Wireless Communication 5 Talk Outline Two stories: I: opportunistic communication (implemented in most 3G standards) II multiple antenna (MIMO) communication (emerging standards) Real advances based on integration of theory and system considerations.

6. ICASSP 2006 Plenary Modern Wireless Communication 6 I: Opportunistic Communication

7. ICASSP 2006 Plenary Modern Wireless Communication 7 Traditional Approach to Wireless System Design Compensates for deep fades via diversity techniques over time, frequency and space. (Glass is half empty.)

8. ICASSP 2006 Plenary Modern Wireless Communication 8 Example: CDMA frequency diversity via Rake combining time diversity via interleaving and coding macro-diversity via soft handoff transmit/receive antenna diversity interference diversity: averaging of interference from many users.

9. ICASSP 2006 Plenary Modern Wireless Communication 9 Multipath Fading: Another Look Multipath fading provides high peaks to exploit. Channel capacity is achieved by such an opportunistic strategy. (Goldsmith & Varaiya 93) Point-to-point performance benefits mainly in the energy- limited rather than the bandwidth-limited regime.

10. ICASSP 2006 Plenary Modern Wireless Communication 10 Multiuser Opportunistic Communication

11. ICASSP 2006 Plenary Modern Wireless Communication 11 Multiuser Diversity In a large system with users fading independently, there is likely to be a user with a very good channel at any time. Long term total throughput can be maximized by always serving the user with the strongest channel (Knopp&Humblet 95)

12. ICASSP 2006 Plenary Modern Wireless Communication 12 Application to CDMA 2000 1x EV-DO Multiuser diversity provides a system-wide benefit. Challenge is to share the benefit among the users in a fair way.

13. ICASSP 2006 Plenary Modern Wireless Communication 13 Symmetric Users Serving the best user at each time is also fair in terms of long term throughputs.

14. ICASSP 2006 Plenary Modern Wireless Communication 14 Asymmetric Users: Hitting the Peaks Want to serve each user when it is at its peak. A peak should be defined with respect to the latency time-scale t c of the application.

15. ICASSP 2006 Plenary Modern Wireless Communication 15 Proportional Fair Scheduler Schedule the user with the highest ratio R k = current requested rate of user k T k = average throughput of user k in the past t c time slots. (Tse 99) De-facto scheduler in Ev-DO and similar algorithms used in HSDPA.

16. ICASSP 2006 Plenary Modern Wireless Communication 16 Performance

17. ICASSP 2006 Plenary Modern Wireless Communication 17 Channel Dynamics Channel varies faster and has more dynamic range in mobile environments.

18. ICASSP 2006 Plenary Modern Wireless Communication 18 Inducing Randomness Scheduling algorithm exploits the nature-given channel fluctuations by hitting the peaks. If there are not enough fluctuations, why not purposely induce them?

19. ICASSP 2006 Plenary Modern Wireless Communication 19 Dumb Antennas The information bearing signal at each of the transmit antenna is multiplied by a time-varying phase. (Viswanath,Tse & Laroia 02)

20. ICASSP 2006 Plenary Modern Wireless Communication 20 Slow Fading Environment: Before

21. ICASSP 2006 Plenary Modern Wireless Communication 21 After

22. ICASSP 2006 Plenary Modern Wireless Communication 22 Beamforming Interpretation Omni-directional antenna Antenna array: Beamforming Beamforming direction is controlled by the relative phase  (t). Dumb antennas sweeps a beam over all directions.

23. ICASSP 2006 Plenary Modern Wireless Communication 23 Dumb Antennas in Action: One User Most of the time, the beam is nowhere near the user.

24. ICASSP 2006 Plenary Modern Wireless Communication 24 Many users: Opportunistic Beamforming In a large system, there is likely to be a user near the beam at any one time. By transmitting to that user, close to true beamforming performance is achieved, without knowing the locations of the users.

25. ICASSP 2006 Plenary Modern Wireless Communication 25 Performance Improvement Mobile environment: 3 km/hr, Rayleigh fading Fixed environment: 2Hz Rician fading with E fixed /E scattered =5.

26. ICASSP 2006 Plenary Modern Wireless Communication 26 Smart vs Dumb Antennas Space-time codes improve reliability of point-to-point links but reduce multiuser diversity gain. Dumb antennas add fluctuations to point-to-point links but increases multiuser diversity gains.

27. ICASSP 2006 Plenary Modern Wireless Communication 27 Cellular Systems: Opportunistic Nulling In a cellular systems, users are scheduled when their channel is strong and the interference from adjacent base-stations is weak. Dumb antennas provides opportunistic nulling for users in other cells. In practice, performance may be limited by interference averaging: a user may be within range of several base-stations.

28. ICASSP 2006 Plenary Modern Wireless Communication 28 II: MIMO Communication

29. ICASSP 2006 Plenary Modern Wireless Communication 29 Channel Resources Two fundamental resources: –Power (SNR) –Bandwidth (W) Shannon’s famous capacity formula: In high SNR (high spectral efficiency) regime, degree of freedom gain has a much larger impact than power gain. MIMO is an approach to yield such gain.

30. ICASSP 2006 Plenary Modern Wireless Communication 30 Line-of-Sight Environment Energy is focused along a narrow beam. Power gain but no degree-of-freedom gain.

31. ICASSP 2006 Plenary Modern Wireless Communication 31 Multipaths Comes to the Rescue A scattering environment provides multiple degrees of freedom a line-of-sight environment doesn’t have. (Foschini 96)

32. ICASSP 2006 Plenary Modern Wireless Communication 32 Performance Limits Given a scattering environment and antenna arrays, what is maximal degrees of freedom achievable? Does not grow unbounded with number of multipaths due to limit in angular resolution.

33. ICASSP 2006 Plenary Modern Wireless Communication 33 Angular Resolution Antenna array of length L lambdas provides angular resolution of 1/ L : paths that arrive at angles closer are not very distinguishable. (Sayeed 02, Poon,Broderson & Tse 05)

34. ICASSP 2006 Plenary Modern Wireless Communication 34 Clustered Model How many degrees of freedom are there in this channel?

35. ICASSP 2006 Plenary Modern Wireless Communication 35 Dependency on Antenna Size

36. ICASSP 2006 Plenary Modern Wireless Communication 36 Clustered Model For L t,L r large, number of d.o.f.: where  t,  r are the total angular spreads of the scatterers at the transmitter and the receiver. (Poon,Brodersen,Tse 05) device environment

37. ICASSP 2006 Plenary Modern Wireless Communication 37 Spatial Channel Resource Single-antenna: T seconds of transmission over a channel of bandwidth W yields WT degrees of freedom (Nyquist). MIMO: Antenna array of size L over a channel with angular spread  yields L  spatial degrees of freedom per second per Hz.

38. ICASSP 2006 Plenary Modern Wireless Communication 38 Diversity and Multiplexing: Old Meets New MIMO allows spatial multiplexing But MIMO provides diversity as well. In a richly scattered environment, there are resolvable angular paths. This is the maximum amount of diversity available. Increasing the amount of spatial multiplexing reduces the amount of diversity.

39. ICASSP 2006 Plenary Modern Wireless Communication 39 Diversity-Multiplexing Tradeoff (Zheng & Tse 03) Richly scattered environment: L t  t = n t, L r  r = n r

40. ICASSP 2006 Plenary Modern Wireless Communication 40 System Considerations MIMO makes sense in indoor environments with high SNR and rich scattering. MIMO-based products have started to appear in the WiFi space. (emerging 802.11n standard) In wide-area cellular networks, users have wide ranges of SNR’s and angular spreads, so system design becomes more challenging. How to get spatial degrees of freedom gain even when there is limited angular spread?

41. ICASSP 2006 Plenary Modern Wireless Communication 41 Space-Division Multiple Access SDMA exploits the geographical separation of users. Increase system throughput. But how to get high per-user peak rate when there is limited angular spread? Idea: cooperation.

42. ICASSP 2006 Plenary Modern Wireless Communication 42 Infrastructure Cooperation Base-stations cooperate to form a macro-array with large angular spread at each mobile. MIMO BS

43. ICASSP 2006 Plenary Modern Wireless Communication 43 User Cooperation Users relay information for each other and act as virtual scatterers to increase the effective angular spread.

44. ICASSP 2006 Plenary Modern Wireless Communication 44 Distributed MIMO Node cooperation can increase effective angular spread. Can it also be used to overcome device limitation? Each single-antenna source node wants to talk to a specific destination node. Without cooperation, total capacity is bounded irrespective of n. (interference-limited) With joint processing, capacity grows linearly with n. (MIMO gain) Interestingly, cooperation can achieve a capacity scaling of at least n 2/3. (Aeron & Saligrama 06) n source nodes n destination nodes

45. ICASSP 2006 Plenary Modern Wireless Communication 45 Conclusions Modern wireless communication theory exploits fading to increase spectral efficiency. Real advances require marriage of theory with understanding of system issues. The new point of view even suggests that fading can be induced by appropriate system design.