Presentation on theme: "6: Opportunistic Communication and Multiuser Diversity"— Presentation transcript:
1 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 Example: 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.
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.
6 Multiuser 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)
7 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.
8 Symmetric UsersServing the best user at each time is also fair in terms oflong term throughputs.
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 tcof the application.
10 Proportional 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.
18 Beamforming Interpretation Antenna array:BeamformingOmni-directional antennaBeamforming direction is controlled by the relativephase (t).Dumb antennas sweeps a beam over all directions.
19 Dumb Antennas in Action: One User Most of the time, the beam is nowhere near the user.
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 Performance Improvement Mobile environment: 3 km/hr, Rayleigh fadingFixed environment: 2Hz Rician fading with Efixed/Escattered =5.
22 Smart 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.
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.