Rate-Optimal Multiuser Scheduling with Reduced Feedback Vegard Hassel, Mohamed-Slim Alouini, Geir...
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![Page 1: Rate-Optimal Multiuser Scheduling with Reduced Feedback Vegard Hassel, Mohamed-Slim Alouini, Geir Øien and David Gesbert Gotland Workshop 24. - 26. August.](https://reader035.fdocuments.net/reader035/viewer/2022062320/56649d2b5503460f94a00a8d/html5/thumbnails/1.jpg)
Rate-Optimal Multiuser Scheduling with Reduced
Feedback
Vegard Hassel, Mohamed-Slim Alouini, Geir Øien and David
Gesbert
Gotland Workshop
24. - 26. August 2004
![Page 2: Rate-Optimal Multiuser Scheduling with Reduced Feedback Vegard Hassel, Mohamed-Slim Alouini, Geir Øien and David Gesbert Gotland Workshop 24. - 26. August.](https://reader035.fdocuments.net/reader035/viewer/2022062320/56649d2b5503460f94a00a8d/html5/thumbnails/2.jpg)
Agenda• What is scheduling?
• Scheduling and adaptive modulation
• MCS, SMUD & ORRF scheduling algorithms
• ORRF:– Feedback load– MASSE– Impact of scheduling delay– Impact of outdated channel estimates
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What Is Scheduling?
How capacity is shared between users in a network
TDM:
A set of rules that specify which user is allowed to transmit and which user is allowed to receive at each time-slot
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Scheduling and Adaptive Modulation
To have efficient multiuser scheduling in wireless networks, we have to combine scheduling with adaptive modulation
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BASE STATION
EXTERNAL NETWORK
USERS
Feedback
TRANSCEIVER
TRANSCEIVERSCHEDULER
CHANNEL
ESTIMATOR
TRANSCEIVER
CHANNEL
ESTIMATOR
TRANSCEIVER
CHANNEL
ESTIMATOR
Data User 1
Data User 3
Data User 2
Data User 3
Data User 2
Data User 1
System Model
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MCS Algorithm
• MCS=Maximum CNR Scheduling• The time-slot is given to the user with the best
CNR • Rate-optimal algorithm (?)• All users feed back channel state information
for each time-slot• This algorithm is only fair if the channels of
the users are i. i. d.
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SMUD Algorithm
• SMUD=Selective Multi-User Diversity
• The users feed back channel state information only if their CNRs are above a threshold
• The user with the highest CNR is chosen
• If none of the users succeed to exceed the threshold value, a random user is selected
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ORRF Algorithm
• ORRF=Optimal Rate, Reduced Feedback• The users feed back channel state information
only if their CNRs are above a threshold• If none of the users succeed to meet the
threshold value, the scheduler requests full feedback
• The user with the highest CNR is always chosen!
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Feedback Load
The normalized feedback load for the ORRF algorithm can be minimized using an optimal threshold value:
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γth* = −γ ln 1−
1
N +1
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⎠ ⎟
1N ⎛
⎝ ⎜ ⎜
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⎠ ⎟ ⎟, N = 2,3,4,...
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Closed-Form Expressions for ORRF
Maximum average system spectral efficiency:
• Shannon: AWGN & Rayleigh
• Adaptive M-QAM:– Constant-Power, Continuous-Rate – Constant-Power, Discrete-Rate– Continuous-Power, Continuous-Rate– Continuous-Power, Discrete-Rate
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Scheduling Delay
In real-life systems we have to assume a delay because the scheduler:
• Collects channel estimates
• Takes a scheduling decision
• Notifies the selected user
Assumes that a perfect channel estimate is available at time t+
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Outdated Channel Estimates
• A perfect channel estimate is NOT available at time t+
• Both the scheduling decision and the decision of the constellation size have to be taken based on an estimate at time t
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