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Scheduling in WiMAX

Scheduling in WiMAX. Hemant Kumar Rath Dept. of Electrical Engg., IIT-Bombay {hemantr@ee.iitb.ac.in} Guide: Prof. Abhay Karandikar Comnet 2007. Motivation. Request-grant mechanisms, service types defined in std. Request is either in Contention mode or Polling mode

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Scheduling in WiMAX

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  1. Scheduling in WiMAX Hemant Kumar Rath Dept. of Electrical Engg., IIT-Bombay {hemantr@ee.iitb.ac.in} Guide: Prof. Abhay Karandikar Comnet 2007

  2. Motivation • Request-grant mechanisms, service types defined in std. • Request is either in Contention mode or 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 • RTT or RTO in case of TCP (BE services) • Provider selects k • To balance efficiency & fairness • k may depend upon class of traffic • Should be such that the TCP timeouts are avoided

  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 • Demand of users

  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. 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

  10. 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

  11. 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

  12. 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}

  13. Scheduling for Best Effort Services • No guarantee on delay or bandwidth for BE services • BE services are mainly of TCP based applications • TCP is a greedy protocol • Congestion window increases as long as there is no congestion in the network • Results in unfairness among users • Assigning slots equally among the users… • More un-used slots in every frame • Congestion windows (cwnd) are fixed for one RTT • i.e., requirements are fixed for one RTT • Slot assignment should be based on cwnd • TCP time outs due to scheduling should be avoided

  14. TCP Window Aware Scheduling • Need: Separate scheduling algo. for BE services • Polling should be based on the RTT of the applications • Users should be scheduled before timeout occurs • Achieved by introducing deadlines • Assign slots based on the requirement • More the congestion window, more the slots assigned • Ensure fairness by introducing deficit counter • Users having higher congestion window will have to reduce their congestion window • Fixed modulation schemes and coding is considered • Airlink is not fully utilized

  15. TCP Window Aware Scheduling - With Adaptive Multi Rate Coding - • Modulations Schemes • WirelessMan-SC • QPSK, 16QAM, 64QAM • WirelessMAN-OFDM • BPSK, QPSK, 16QAM, 64QAM • WirelessMAN-OFDMA • BPSK, QPSK, 16QAM, 64QAM • Modulation schemes are decided based on the channel condition • Maximize the use of airlink • Rates between BS and SS are different • Fairness measure should be on the amount of data transferred • Not on the amount of slots assigned • Is fair in short- and long-term

  16. Thank You

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