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

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

Multiuser Opportunistic Communication 3

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

Application to CDMA x EV-DO  Multiuser diversity provides a system-wide benefit.  Challenge is to share the benefit among the users in a fair way. 5

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

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

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.

Performance 9

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

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?

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

Slow Fading Environment: Before 13

After 14

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.

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

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.

Performance Improvement 18