©Copyright All Rights Reserved Scheduling in WiMAX Hemant Kumar Rath Dept. of Electrical Engg., IIT-Bombay Guide: Prof. Abhay Karandikar Comnet 2007

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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 T d 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

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 4 Uplink Scheduling Scheme Requests Grants Requests Grants Requests SS 2 SS 1 SS 3 SS 4 BS

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 5  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 Optimum Polling Interval k

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 6 BS takes into account  Channel characteristics  Queue lengths  Delay counters at scheduling instant, based on COS  Demand of users SS k SS 1 SS 2 SINR 1 SINR 2 SINR k Scheduler Opportunistic Scheduling q 1 (t) q 2 (t) q k (t) d 1 (t) d 2 (t) d k (t) BS

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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 T d relinquishes resources to SS with small T d

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 8 O-DRR Uplink Scheduling TfTf kT f Scheduling Epoch SS 1 SS 2 SS 3 SS 4 SS 5 SS 6 Scheduling Instant

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 9 SS 5 SS 1 SS 2 SS 4 O-DRR Uplink Scheduling TfTf kT f SS 1 SS 6 SS 5 SS 3 Schedulable Set {SS 1,SS 2,SS 4, SS 6 } Schedule: weights (w i ) and lag/lead counter SS 1 =28, SS 2 =6, SS 4 =15, SS 6 =11 d 1 =10, d 2 =30, d 3 =25, d 4 =20 Scheduling Epoch Eligible Set {SS 1, SS 2, SS 4, SS 6 } SINR i > SINR th & Backlogged

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 10 O-DRR Uplink Scheduling TfTf kT f Scheduling Epoch Eligible Set {SS 1, SS 2, SS 4, SS 6 } SINR i > SINR th & Backlogged Sch Set {SS 1,SS 2 } SS 1 =46, SS 2 =14 Sch Set {SS 1,SS 2, SS 4,SS 6 } SS 1 =23, SS 2 =5, SS 4 =13, SS 6 =9 d 1 =10, d 2 =30, d 3 =25, d 4 =20 d 1 =5, d 2 =25 SS 5 SS 1 SS 2 SS 1 SS 6 SS 5 SS 3 SS 6 SS 4

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 11 O-DRR Uplink Scheduling SS 1 TfTf kT f Scheduling Epoch Eligible Set {SS 1, SS 2, SS 4, SS 6 } SINR i > SINR th & Backlogged Sch Set {SS 2,SS 6 } Sch Set {SS 1,SS 2 } Sch Set {SS 1,SS 2, SS 4, SS 6 } SS 5 SS 2 SS 6 SS 5 SS 3 SS 1 SS 4

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 12 SS 1 O-DRR Uplink Scheduling TfTf kT f Scheduling Epoch Eligible Set {SS 1, SS 2, SS 4, SS 6 } SINR i > SINR th & Backlogged Scheduling Epoch Eligible Set {SS 2, SS 3, SS 4, SS 6 } SS 5 SS 2 SS 4 SS 6 SS 5 SS 3 SS 1

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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

Comnet 2007, IIT Bombay ©Copyright All Rights Reserved 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

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