Multiuser Diversity Gain Enhancement by Guard Time

Reduction

Hend Koubaa, Vegard Hassel, Geir E. Øien

Norwegian University of Science and Technology (NTNU) Dept. of Electronics and Telecommunications

Trondheim, Norway

Outline

Multi-user diversity scheduling Performance criteria Contention-less feedback algorithm Contention feedback algorithm Results Conclusion

Multiuser diversity scheduling Multiple users with different channel fluctuation

characteristics Exploiting this diversity among users by selecting the

best user to transmit with the best rate Increasing the Maximum Average System Spectral

Efficiency (MASSE) Basic mechanism

The sender requests the CNRs of users The users send back their CNRs: feedback

mechanism The sender chooses the best CNR user and transmits

with the rate corresponding to this CNR

Performance criteria Maximising network throughput

Maximising the potential rate to be used Reducing the cost of the feedback

mechanism Minimising the number of users’ feedbacks

(saving users’ powers) Reducing the guard time (overhead time between

BS queries) Trade off between 1 and 2! Realising fairness among users Fairness: Deeper study in a future work

Using multiple feedback thresholds Opportunistic scheduling algorithm to be presented at VTC

Spring 2005 (Hassel, Alouini, Gesbert, and Øien). Employing multiple (“nested”) optimized feedback CNR

thresholds to minimize average feedback load. Denote the thresholds by th,L>th,L−1> · · ·>th,0 (For

convenience th,L = ∞ and th,0 = 0) The base station initially requests feedback from those

users whose CNRs are above th,L−1. If there are none, the threshold is successively lowered, to th,L−2, th,L−3, · · ·, th,0.

The best user is thus always selected, but the average feedback load is significantly reduced compared to the conventional rate-optimal MCS (Max Channel SNR) scheduling algorithm.

Contention-less feedback algorithm (1) Assume N users. 1. Check if any CNRs are above th,L−1. All users wait for a

mini-slot TMS. If there are none, the threshold is successively lowered to th,L−2 - and users again wait for their mini-slot TMS. And so on...

2. In the interval [th,l,th,l+1], all users having a CNR above th,l will send their feedback - all others stay silent.

If only one user is above the threshold, there is no collision problem.

If multiple users have a CNR above th,l: collision problem. Solution: ranking the users

The base station sends a ranked user list The time is initially slotted to N ranked mini-slots TMS The user of rank j will start sending his feedback at the start of

the jth minislot, if his CNR is above th,l. Those users having a CNR lower than th,l will simply be silent

during their assigned mini-slots.

Contention-less feedback algorithm (2) Assume TFB is the total time needed to send one user’s

feedback (and assume TFB ≥TMS). [th,l, th,l+1] denotes the “best user” interval. Assume that all users can detect feedback initiated from

others, and stay silent if feedback from another user is detected.

After each feedback transmission from one user (of rank j), the time is again slotted, now to N-j mini-slots.

This mechanism is repeated until the user of rank N sends his feedback - or is silent - during the Nth (last) mini-slot.

The base station finally schedules the best user out of those who gave feedback within the current interval.

Contention-less feedback algorithm (3) Example 4 thresholds (th,3>th,2> th,1>th,0 ) 3 users The CNRs of the best users, u1 and u3, are assumed

to reside in the interval [th,1,th,2]First step [th,2,th,3] Tms

u1 sends his feedback

u1

The remaining time is slotted to 2 mini-slots

u3u2

u2 is silent and u3 sends his feedback

u3u2

th,0=0 th,1 th,2 th,3= ∞

Second step [th,1,th,2]: collision

TmsTms

u1 & u3

Third step slotting

u1 u3u2TFBTms

Contention feedback algorithm N users If only one user exists in the best user interval, then

this user will send his feedback just after the base station's query, and data transmission can start.

However if multiple users have a CNR above th,l: all send simultaneously, and a collision takes place.

Solution: exponential backoff scheme Each user having a CNR above th,l and detecting a

collision (by not receiving data) will retransmit his feedback with probability q < 1.

After a collision have occurred i times , each user will send his feedback with probability qi

This mechanism will last until one successful feedback transmission takes place.

Comparison between both proposed algorithms

Contention-less The best user is

selected (highest rate but also highest guard time)

Contention A random user in

the “best user interval” is selected (suboptimal rate but also lower guard time)

Results: (general and Rayleigh fading model) Expressions for the guard time (GT) are derived for

both approaches (see the TD). The MASSE in the contentionless approach (Rayleigh):

The MASSE in the contention approach (Rayleigh):

NB: The above MASSEs both have to be multiplied by the fraction

where TTS is total time between base station queries.

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MASSE for both approaches (5 users)

Masse with 5 usersM

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Number of thresholds

MASSE for both approaches (10 users)

Masse with 10 usersM

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Number of thresholds

MASSE for both approaches (long data packet)

Masse with 10 usersM

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Number of thresholds

MASSE for both approaches (short data packet)

Masse with 10 usersM

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Number of thresholds

Guard time in both approaches

Contention-less feedback

Contention channel for feedback

Number of thresholds

Guard

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Guard time with 5 users

MASSE for both approaches (short data packet case: TTS = 20TMS)

Contention-less feedback

Contention channel for feedback

Number of thresholds

Mass

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bit

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Conclusion (1) Study of guard time and MASSE in a certain multi-user

diversity scheduling algorithm with two different feedback protocols.

Use of multiple thresholds is better than the full feedback mechanism (=no thresholds) w.r.t. overall MASSE.

The impact of the feedback mechanism has to be studied in detail.

Preliminary conclusion: To maximize overall MASSE, choose the number of feedback thresholds such that the overall guard time is: not too long if the data packet size is short not too short if the date packet size is long

Conclusion (2) Two proposed feedback mechanisms

Contention-less: ranked user list Contention: exponential backoff scheme

The contention-less feedback protocol is more suitable for long data packet applications (optimal rate counts for more than short guard time)

The contention mechanism is more suitable for short data packet applications (shorter guard time compensates for suboptimal rate)