Multiuser Diversity Gain Enhancement by Guard Time Reduction

1. 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

2. Outline • Multi-user diversity scheduling • Performance criteria • Contention-less feedback algorithm • Contention feedback algorithm • Results • Conclusion

3. 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

4. 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

5. 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.

6. 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.

7. 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.

8. First step [th,2,th,3] Tms Second step [th,1,th,2]: collision Tms Tms u1 & u3 Third step slotting u1 u2 u3 Tms TFB u1 u1 sends his feedback u2 u3 The remaining time is slotted to 2 mini-slots u2 u3 u2 is silent and u3 sends his feedback 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] th,3= ∞ th,0=0 th,1 th,2

9. 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.

10. 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) Comparison between both proposed algorithms

11. 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.

12. MASSE for both approaches (5 users) Masse with 5 users Masse (bits/sec/Hz) Number of thresholds

13. MASSE for both approaches (10 users) Masse with 10 users Masse (bits/sec/Hz) Number of thresholds

14. MASSE for both approaches (long data packet) Masse with 10 users Masse (bits/sec/Hz) Number of thresholds

15. MASSE for both approaches (short data packet) Masse with 10 users Masse (bits/sec/Hz) Number of thresholds

16. Guard time in both approaches Guard time with 5 users Contention-less feedback Guard time (TMS) Contention channel for feedback Number of thresholds

17. MASSE for both approaches (short data packet case: TTS = 20TMS) Masse with 5 users Contention channel for feedback Contention-less feedback Masse (bits/sec/Hz) Number of thresholds

18. 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

19. 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)