Presentation is loading. Please wait.

Presentation is loading. Please wait.

Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 1.

Similar presentations


Presentation on theme: "Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 1."— Presentation transcript:

1 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 1

2 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 2 Traditional Approach to Wireless System Design Compensates for deep fades via diversity techniques over time, frequency and space. (Glass is half empty.)

3 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 3 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.

4 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 4 Multipath Fading: Another Look Multipath fading provides high peaks to exploit. Channel capacity is achieved by such an opportunistic strategy. Point-to-point performance benefits mainly in the energy- limited rather than the bandwidth-limited regime.

5 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 5 Multiuser Opportunistic Communication

6 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 6 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)

7 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 7 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.

8 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 8 Symmetric Users Serving the best user at each time is also fair in terms of long term throughputs.

9 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 9 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.

10 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 10 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.

11 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 11 Performance

12 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 12 Channel Dynamics Channel varies faster and has more dynamic range in mobile environments.

13 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 13 Why No Gain with High Mobility? 3 km/hr30 km/hr120 km/hr Can only predict the average of the channel fluctuations, not the instantaneous values.

14 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 14 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?

15 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 15 Dumb Antennas The information bearing signal at each of the transmit antenna is multiplied by a time-varying phase. (Viswanath,Tse & Laroia 02)

16 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 16 Slow Fading Environment: Before

17 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 17 After

18 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 18 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.

19 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 19 Dumb Antennas in Action: One User Most of the time, the beam is nowhere near the user.

20 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 20 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.

21 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 21 Performance Improvement Mobile environment: 3 km/hr, Rayleigh fading Fixed environment: 2Hz Rician fading with E fixed /E scattered =5.

22 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 22 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.

23 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 23 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.

24 Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 24 Conventional vs Opportunistic Communication


Download ppt "Fundamentals of Wireless Communication: Tse&Viswanath 6: Opportunistic Communication and Multiuser Diversity 1."

Similar presentations


Ads by Google