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• ORRF:– Feedback load– MASSE– Impact of scheduling delay– Impact of outdated channel estimates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