Hemant Kumar Rath, Abhijeet Bhorkar, Vishal Sharma Dept. of Electrical Engg., IIT-Bombay

1. An Opportunistic Uplink Scheduling Scheme to Achieve Bandwidth Fairness and Delay forMulticlass Traffic in Wi-Max (IEEE 802.16)Broadband Wireless Networks Hemant Kumar Rath, Abhijeet Bhorkar, Vishal Sharma Dept. of Electrical Engg., IIT-Bombay {hemantr,bhorkar,vsharma@ee.iitb.ac.in} IEEE Globecom – 2006 San Francisco, CA

2. Motivation • Request-grant mechanisms, service types defined in std. • Request is either in Contention mode or Contention free (Polling) mode • Service types need QoS in terms of delay guarantees • Scheduling mechanisms are not defined • Scheduling in both uplink and downlink is open • Providers/vendors can have their own scheduling algos. • Scheduling mechanism must balance.... • Fairness in bandwidth alloc. with delay guarantees

3. Motivation (cont’d) • Polling mode • Poll each SS in every frame or in every k frames • Polling interval k is a function of • Delay tolerance Td • UGS: 10ms, rtPS: 50ms, nrtPS: 200ms, BE: 500ms • Fairness measure • System efficiency • Provider selects k to balance efficiency & fairness • k may depend upon class of traffic

4. SS1 BS SS3 SS4 SS2 Uplink Scheduling Scheme Requests Requests Grants Grants Requests Requests Grants Grants

5. Optimum Polling Interval k • Polling mode • BS polls each SS every k frames • Worst case fairness is better if polled in every frame • Normalized delay is better if polled in some k frames • Design problem is to find an optimum k • Approach: Minimize weighted sum of • Normalized delay • Worst case fairness in bandwidth allocation

6. SINR1 d1(t) SINR2 d2(t) SINRk BS dk(t) Opportunistic Scheduling q1(t) SS1 q2(t) SS2 Scheduler qk(t) SSk BS takes into account • Channel characteristics • Queue lengths • Delay counters at scheduling instant, based on COS

7. Opportunistic Deficit Round Robin(O-DRR) • Channel is static in a frame interval • Slot assignment is opportunistic • Assign slots only if channel is good and flow is active • DRR variant for slot assignment • Use queue state, delay requirements and lag/lead info. • Works for single- and multi-class traffic • SS with large Td relinquishes resources to SS with small Td

8. O-DRR Uplink Scheduling SS1 SS6 SS2 SS5 SS3 Scheduling Instant Scheduling Instant Scheduling Instant SS4 Tf kTf Scheduling Epoch Scheduling Epoch Scheduling Epoch

9. Scheduling Multiclass Traffic • Number of slots assigned to an SS depends upon • Delay counter • How close a HOL packet is to its delay bound • Weight is more if it closer to the delay limit • Deficit counter • Weight is more if the deficit counter is high • Weights w •  1/delay counter •  deficit counter

10. SS1 SS2 SS6 SS5 SS5 SS3 SS4 Tf SS3 kTf O-DRR Uplink Scheduling d1=10, d2=30, d3=25, d4=20 SS1 SS1=28, SS2=6, SS4=15, SS6=11 Schedule: weights (wi) and lag/lead counter Schedulable Set {SS1,SS2 ,SS4, SS6} Scheduling Epoch Eligible Set {SS1, SS2, SS4, SS6} SINRi > SINRth & Backlogged

11. SS1 SS2 SS6 SS6 d1=10, d2=30, d3=25, d4=20 SS1=23, SS2=5, SS4=13, SS6=9 SS5 SS5 Sch Set {SS1,SS2, SS4,SS6} SS3 SS4 Tf SS3 kTf Scheduling Epoch Eligible Set {SS1, SS2, SS4, SS6} SINRi > SINRth & Backlogged O-DRR Uplink Scheduling SS1 d1=5, d2=25 SS1=46, SS2=14 Sch Set {SS1,SS2} SS4

12. SS1 SS1 SS2 SS6 Sch Set {SS1,SS2, SS4, SS6} SS5 SS5 Sch Set {SS1,SS2} SS3 SS4 Tf SS3 kTf Scheduling Epoch Eligible Set {SS1, SS2, SS4 , SS6} SINRi > SINRth & Backlogged O-DRR Uplink Scheduling Sch Set {SS2,SS6} SS4

13. SS1 SS1 SS2 SS6 SS5 SS5 SS3 SS4 Tf kTf Scheduling Epoch Eligible Set {SS1, SS2, SS4, SS6} SINRi > SINRth & Backlogged O-DRR Uplink Scheduling Scheduling Epoch Eligible Set {SS2, SS3, SS4, SS6}

14. Simulation Setup • No. of users = 100 • No. of classes = 2 • k = 75, 100 • All flows backlogged (heavy traffic assumption) • Delay requirements • Class1 = 200ms • Class2 = 500ms • Total no. of frames scheduled = 2000 • Uplink slots per frame = 100 • Drop packets only if delay is violated

15. Fairness and Throughput • O-DRR is fair • Fair among users • Max. difference in allocated bandwidth < 10 % of average • Fair among traffic classes • Both class1 and class2 traffic get almost equal number of slots • As k increases, fairness decreases (intuitively expected)

16. Delay Performance • Meets delay guarantees of different classes of traffic • Packets are dropped only if delay is violated • Packet drop is less than 8.5% for both classes of traffic • For larger k, the dropping percentage is higher • For worst case k=100, 91.5% of traffic meets its delay

17. Choosing Polling Interval k • Jain’s fairness index is more than 95% • A series of k are tested for fairness • Possible to trade off fairness for delay • Appropriate k to satisfy • Fairness & bandwidth requirements

18. Discussion • Low complexity scheduling algorithm • The scheduling is done in the MAC layer • It is a cross layer scheduling scheme involving PHY and MAC layer • Jain's fairness index remains above 90% • It is possible to tradeoff fairness for delay • O-DRR ensures delay requirements of users

19. Future Work • Multi-rate users (SSs) based on channel condition • Adaptive to channel condition where SS can select a particular modulation scheme and data rate • Effect of location-dependent channel variations • Stability analysis of the individual queues

20. Thank You

21. Example of O-DRR Scheme • Assumptions • Total no of slots = 60 • Number of users = 6 • Per user (quantum) = 10 • Tf= 5, K = 3 Scheduling Epoch1, Scheduling Instant1

22. Example of O-DRR Scheme Scheduling Epoch1, Scheduling Instant2