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Traditional Approach to Wireless System Design
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Traditional Approach to Wireless System Design Compensates for deep fades via diversity techniques over time and frequency 1
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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. . - PowerPoint PPT Presentation
Transcript of Traditional Approach to Wireless System Design
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Traditional Approach to Wireless System Design
Compensates for deep fades via diversity techniques over time and frequency
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Multipath Fading: Another Look
Multipath fading provides high peaks to exploit.
Channel capacity is achieved by such an opportunistic strategy.
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Multiuser Opportunistic Communication
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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
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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.
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Symmetric Users
Serving the best user at each time is also fair in terms of long term throughputs.
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Asymmetric Users: Hitting the Peaks
Want to serve each user when it is at its peak.
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Proportional Fair SchedulerSchedule the user with the highest ratio
Rk = current requested rate of user kTk = average throughput of user k in the past tc time slots.De-facto scheduler used in HSDPA.
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Performance
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Channel Dynamics
Channel varies faster and has more dynamic range in mobile environments.
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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?
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Dumb Antennas
The information bearing signal at each of the transmit antennais multiplied by a time-varying phase.
(Viswanath,Tse & Laroia 02)
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Slow Fading Environment: Before
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After
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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.
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Dumb Antennas in Action: One User
Most of the time, the beam is nowhere near the user.
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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.
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Performance Improvement
