Presentation on theme: "6: Opportunistic Communication and Multiuser Diversity"— Presentation transcript:
16: Opportunistic Communication and Multiuser Diversity
2Traditional Approach to Wireless System Design Compensates for deep fades via diversity techniques over time, frequency and space.(Glass is half empty.)
3Example: CDMA frequency diversity via Rake combining time diversity via interleaving and codingmacro-diversity via soft handofftransmit/receive antenna diversityinterference diversity: averaging of interference from many users.
4Multipath 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.
6Multiuser DiversityIn 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)
7Application 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.
8Symmetric UsersServing the best user at each time is also fair in terms oflong term throughputs.
9Asymmetric 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 tcof the application.
10Proportional Fair Scheduler Schedule the user with the highest ratioRk = current requested rate of user kTk = average throughput of user k in the past tc time slots.(Tse 99)De-facto scheduler in Ev-DO and similar algorithmsused in HSDPA.
18Beamforming Interpretation Antenna array:BeamformingOmni-directional antennaBeamforming direction is controlled by the relativephase (t).Dumb antennas sweeps a beam over all directions.
19Dumb Antennas in Action: One User Most of the time, the beam is nowhere near the user.
20Many 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.
21Performance Improvement Mobile environment: 3 km/hr, Rayleigh fadingFixed environment: 2Hz Rician fading with Efixed/Escattered =5.
22Smart vs Dumb AntennasSpace-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.
23Cellular 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.