1. Traditional Approach to Wireless System Design Compensates for deep fades via diversity techniques over time and frequency 1

2. Multipath Fading: Another Look 2  Multipath fading provides high peaks to exploit.  Channel capacity is achieved by such an opportunistic strategy.

3. Multiuser Opportunistic Communication 3

4. 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 4

5. 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. 5

6. Symmetric Users 6 Serving the best user at each time is also fair in terms of long term throughputs.

7. Asymmetric Users: Hitting the Peaks Want to serve each user when it is at its peak. 7

8. Proportional Fair Scheduler 8 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. De-facto scheduler used in HSDPA.

9. Performance 9

10. Channel Dynamics Channel varies faster and has more dynamic range in mobile environments. 10

11. Inducing Randomness 11  Scheduling algorithm exploits the nature-given channel fluctuations by hitting the peaks.  If there are not enough fluctuations, why not purposely induce them?

12. Dumb Antennas 12 The information bearing signal at each of the transmit antenna is multiplied by a time-varying phase. (Viswanath,Tse & Laroia 02)

13. Slow Fading Environment: Before 13

14. After 14

15. Beamforming Interpretation 15 Omni-directional antenna Antenna array: Beamforming Beamforming direction is controlled by the relative phase  (t). Dumb antennas sweeps a beam over all directions.

16. Dumb Antennas in Action: One User 16 Most of the time, the beam is nowhere near the user.

17. Many users: Opportunistic Beamforming 17 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.

18. Performance Improvement 18