Scheduling Schemes for Multimedia Service in Wireless OFDM Systems

1. Scheduling Schemes for Multimedia Service in Wireless OFDM Systems Paper by Haiying Julie Zhu and Roshdy H.M. Hafez Presentation by Deepa Jandhyala

2. Introduction Scheduling schemes play a key role in the system performance of broadband wireless systems. Scheduling scheme is defined as the method of user selection and how to allocate available resources to selected user. Multimedia service scheduling schemes take into account the different data rate requirements for services such as video and internet surfing.

3. What is OFDM? OFDM stands for Orthogonal Frequency Division Multiplexing and is a modulation technique for transmitting large amounts of digital data over a radio wave. The implementation relies on very high speed digital signal processing Allows for transmission of different data rate requirements in multimedia servicing while combating signal fading. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions

4. Use of Radio Resources Problems: Scarcity of radio resources Diverse quality of service (QOS) requirements Wireless channel conditions complicate scheduling and radio resource management. Solution: Use a Scheduler to allocate resources Key component across physical (PHY) and medium access control (MAC) layers.

5. Radio Resources Scheduling scheme intelligently allocates radio resources to achieve high system performance in terms of efficiency and fairness. Efficiency measured by system throughput Unfair to those far away or with bad channel conditions Absolute fairness may lead to low bandwidth efficiency Need effective TRADE-OFF between Efficiency and Fairness

6. Main topics of Paper Conventional Scheduling Schemes Round Robin (RR) and Maximum Signal-to-Noise Ratio (MaxSNR) Proportional Fair (PF) scheme provides trade-off between efficiency and fairness in TDMA (time division multiple access) and CDMA (code division multiple access) systems Paper extends PF scheme to include Orthogonal frequency-division multiplexing (OFDM)-based systems called as (OPF) Proposes 3 variations on OPF for multirate multimedia services: Adaptive OPF (AOPF) Multimedia AOPF (MAOPF Normalized MAOPF (NMAOPF) Comparison of all scheduling schemes using efficiency and fairness criteria

7. RR and MaxSNR Round Robin (RR) Runs scheduling in a fixed cycle Each user is allocated time slots in a ring fashion Transmission will not be regenerated to same user before all other users have been serviced Scheduling scheme is �fair��all users get equal time slots Maximum Signal-to-Noise Ratio (MaxSNR) All active users are ranked periodically by SNR values Highest SNR user gets allocated time slots Possible for same users to rescheduled before other users Scheduling scheme is �efficient��high system throughput

8. Proportional Fair (PF) Selects user with maximal priority metric defined by the formula: j = selected user, i = user index, N = total number of users, Di(t)= current supportable data rate Ri(t-1) = average experienced rate by this user User is selected when it has a good channel (high Di(t)) to keep system throughput high Users with bad channels are also considered since their they have low average rate Ri(t � 1)

9. OFDM Version of PF (OPF) Implemented in 802.16a OFDM-based system User selection Criterion: I = selected user index, i = user index, N = total number of users, j = symbol index (OFDM value) Di,j = supportable rate of symbol j of user i Ri,j = avg experience rate until symbol j of user i

10. Adaptive OPF (AOPF) Assumption that all users are active at all times Criterion: Same as OPF except for adaptive component ni,j to equalize the value of Di,j/Ri,j, and M is the total number of symbols in each frame (OFDM)

11. Multimedia Adaptive OPF (MAOPF) Based on AOPF with added parameter for required data rate of user Ri_r, j Criterion:

12. Normalized MAOPF (NMAOPF) Normalization is done to provide better user satisfaction and average user rates Criterion:

13. Comparison of AOPF, MAOPF, NMAOPF AOPF considers supportable and experienced data rates (fairness for location of user and changes in wireless channel rate) MAOPF takes into account also the data rate requirement for each user (fairness for users with different data requirement for multimedia services) NMAOPF adds normalization factor to get better user satisfaction rates

14. Simulations of Scheduling Schemes Joint PHY and MAC layer simulations Implemented in C++ and MatLab Users are uniformly distributed around the base station within 1.5km 15 users, 1 base station

15. Efficiency: System Throughput Comparison

16. System Throughput Comparison Results RR has lowest throughput OPF improves system throughput over RR by 10 Mb/s AOPF and MAOPF slightly lower than MaxSNR NMAOPF is slightly higher than MaxSNR over 50 Mb/s traffic load

17. Efficiency: Mean Delay Comparison

18. Mean Delay Comparison Results Mean packet Delay � mean value of packet completion time Instant of packet arrives until the user receives the packet OPF and all proposed schemes are better than MaxSNR in throughput range between 38 to 48 Mb/s�but worse above 48 Mb/s, except NMAOPF NMAOPF and MaxSNR are almost the same

19. Fairness: User Satisfaction Rate Comparison

20. User Satisfaction rate Comparison Results User Satisfaction rate � ratio of satisfied (no blocking on arrival and no dropped packets) users to total users OPF outperforms RR and MaxSNR, and gives the highest user satisfaction rate when throughput is larger than 35 Mb/s AOPF and MAOPF have worst fairness rate NMAOPF is better than RR and MaxSNR but worse than OPF

21. Fairness:Average User Rate Comparison

22. Average user rate comparison Results Average user rate � sum of user rate divided by number of users. NMAOPF performs a little better than OPF Other trends are similar to User satisfaction rate comparison graph Results show that OPF and other schemes support not only users with high rates but also high experienced (average) rates. Supports throughput for user with bad channel connection so user will not �starve� if they do not get first time around.

23. Conclusion OPF increases system throughput by 10Mb/s over RR, and gives highest user satisfaction rate as well as average user rate. Compared to RR and MaxSNR, OPF provides better trade-off in terms of mediated throughput and best fairness. Results for the 3 new proposed schemes: AOPF and MAOPF improve system throughput and delay statistics at the cost of fairness. NMAOPF breaks the supposed upper bound of throughput without sacrificing fairness. NAMOPF is the best overall proposed scheme in terms of efficiency and fairness.

24. References H. Zhu, R. Hafez, �Scheduling Schemes for Multimedia Service in Wireless OFDM Systems�, IEEE Wireless Communications, October 2007, pp. 99-105. E. Crozier, A. Klein, �WiMAX�s technology for LOS and NLOS environments�, WiMax Forum, www.wimaxforum.org/technology/downloads/WiMAXNLOSgeneral-versionaug04.pdf http://www.sss-mag.com/ofdm.html http://en.wikipedia.org/wiki/COFDM