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6: Opportunistic Communication and Multiuser Diversity. Traditional Approach to Wireless System Design. Compensates for deep fades via diversity techniques over time, frequency and space. (Glass is half empty.). Example: CDMA. frequency diversity via Rake combining - PowerPoint PPT Presentation
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Fundamentals of Wireless Communication: Tse&Viswanath1
6: Opportunistic Communication and Multiuser Diversity
6: Opportunistic Communication and Multiuser Diversity
Fundamentals of Wireless Communication: Tse&Viswanath2
6: Opportunistic Communication and Multiuser Diversity
Traditional Approach to Wireless System Design
Compensates for deep fades via diversity techniques over time, frequency and space.(Glass is half empty.)
Fundamentals of Wireless Communication: Tse&Viswanath3
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath4
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath5
6: Opportunistic Communication and Multiuser Diversity
Multiuser Opportunistic Communication
Fundamentals of Wireless Communication: Tse&Viswanath6
6: Opportunistic Communication and Multiuser Diversity
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)
Fundamentals of Wireless Communication: Tse&Viswanath7
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath8
6: Opportunistic Communication and Multiuser Diversity
Symmetric Users
Serving the best user at each time is also fair in terms of long term throughputs.
Fundamentals of Wireless Communication: Tse&Viswanath9
6: Opportunistic Communication and Multiuser Diversity
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 tc of the application.
Fundamentals of Wireless Communication: Tse&Viswanath10
6: Opportunistic Communication and Multiuser Diversity
Proportional Fair SchedulerSchedule 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.
(Tse 99)
De-facto scheduler in Ev-DO and similar algorithms used in HSDPA.
Fundamentals of Wireless Communication: Tse&Viswanath11
6: Opportunistic Communication and Multiuser Diversity
Performance
Fundamentals of Wireless Communication: Tse&Viswanath12
6: Opportunistic Communication and Multiuser Diversity
Channel Dynamics
Channel varies faster and has more dynamic range in mobile environments.
Fundamentals of Wireless Communication: Tse&Viswanath13
6: Opportunistic Communication and Multiuser Diversity
Why No Gain with High Mobility?
3 km/hr 30 km/hr 120 km/hr
Can only predict the average of the channel fluctuations,not the instantaneous values.
Fundamentals of Wireless Communication: Tse&Viswanath14
6: Opportunistic Communication and Multiuser Diversity
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?
Fundamentals of Wireless Communication: Tse&Viswanath15
6: Opportunistic Communication and Multiuser Diversity
Dumb Antennas
The information bearing signal at each of the transmit antennais multiplied by a time-varying phase.
(Viswanath,Tse & Laroia 02)
Fundamentals of Wireless Communication: Tse&Viswanath16
6: Opportunistic Communication and Multiuser Diversity
Slow Fading Environment: Before
Fundamentals of Wireless Communication: Tse&Viswanath17
6: Opportunistic Communication and Multiuser Diversity
After
Fundamentals of Wireless Communication: Tse&Viswanath18
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath19
6: Opportunistic Communication and Multiuser Diversity
Dumb Antennas in Action: One User
Most of the time, the beam is nowhere near the user.
Fundamentals of Wireless Communication: Tse&Viswanath20
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath21
6: Opportunistic Communication and Multiuser Diversity
Performance Improvement
Mobile environment: 3 km/hr, Rayleigh fading
Fixed environment: 2Hz Rician fading with Efixed/Escattered =5.
Fundamentals of Wireless Communication: Tse&Viswanath22
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath23
6: Opportunistic Communication and Multiuser Diversity
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.
Fundamentals of Wireless Communication: Tse&Viswanath24
6: Opportunistic Communication and Multiuser Diversity
Conventional vs Opportunistic Communication