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Traditional Approach to Wireless System Design

Traditional Approach to Wireless System Design. Compensates for deep fades via diversity techniques over time and frequency. Multipath Fading: Another Look. Multipath fading provides high peaks to exploit. Channel capacity is achieved by such an opportunistic strategy. .

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Traditional Approach to Wireless System Design

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

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

  3. Multiuser Opportunistic Communication

  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 strongestchannel

  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.

  6. Symmetric Users 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.

  8. Proportional Fair Scheduler Schedule the user with the highest ratio Rk = current requested rate of user k Tk = average throughput of user k in the past tc time slots. De-facto scheduler used in HSDPA.

  9. Performance

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

  11. 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?

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

  13. Slow Fading Environment: Before

  14. After

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

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

  17. 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.

  18. Performance Improvement

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