rate-optimal multiuser scheduling with reduced feedback vegard hassel, mohamed-slim alouini, geir...
Post on 19-Dec-2015
218 views
TRANSCRIPT
Rate-Optimal Multiuser Scheduling with Reduced
Feedback
Vegard Hassel, Mohamed-Slim Alouini, Geir Øien and David
Gesbert
Gotland Workshop
24. - 26. August 2004
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• ORRF:– Feedback load– MASSE– Impact of scheduling delay– Impact of outdated channel estimates
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
Scheduling and Adaptive Modulation
To have efficient multiuser scheduling in wireless networks, we have to combine scheduling with adaptive modulation
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
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.
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
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!
Feedback Load
The normalized feedback load for the ORRF algorithm can be minimized using an optimal threshold value:
€
γth* = −γ ln 1−
1
N +1
⎛
⎝ ⎜
⎞
⎠ ⎟
1N ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟, N = 2,3,4,...
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
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+
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