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Rate-Optimal Multiuser Scheduling with Reduced Feedback Vegard Hassel, Mohamed-Slim Alouini, Geir Øien and David Gesbert Gotland Workshop 24. - 26. August 2004
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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modulation MCS, SMUD & ORRF scheduling algorithms ORRF: –Feedback load –MASSE –Impact of scheduling delay –Impact of outdated channel estimates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 SCHEDULER 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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SMUD CDF Expression for the CDF of the CNR of N users:
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SMUD PDF Expression for the PDF of the highest CNR between all users using a Rayleigh channel:
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ORRF PDF Expression for the PDF of the highest CNR between all users using a Rayleigh channel: Same as for the MCS algorithm!
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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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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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Closed-Form Expressions for ORRF and SMUD (II) Average BER: 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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