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

©Copyright 2005-2006All Rights Reserved

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,[email protected]}

IEEE Globecom – 2006San Francisco, CA

IEEE Globecom-2006, NXG-02: Broadband Access©Copyright 2005-2006All Rights Reserved

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


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Uplink Scheduling Scheme

Requests

GrantsGrants

Grants

Requests

Requests

Grants

Requests

SS2

SS1

SS3

SS4

BS


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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


6

BS takes into account Channel characteristics Queue lengths Delay counters at scheduling instant, based on COS

SSk

SS1

SS2

SINR1

SINR2

SINRkScheduler

Opportunistic Scheduling

1 2[ , , ]kSINR SINR SINR1 2[ ( ), ( ), ( )]kq t q t q t

1 2[ ( ), ( ), ( )]kd t d t d t

q1(t)

q2(t)

qk(t)

d1(t)

d2(t)

dk(t)

BS


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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


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O-DRR Uplink Scheduling

Tf

kTfScheduling Epoch

Scheduling Epoch

Scheduling Epoch

SS1 SS2

SS3

SS4

SS5

SS6

Scheduling Instant

Scheduling Instant

Scheduling Instant


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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

( )i d i fd T nT


10

SS5

SS1 SS2

SS4


Tf

kTf

SS1SS6

SS5

SS3SS3

Schedulable Set

{SS1,SS2 ,SS4, SS6}

Schedule: weights (wi) and lag/lead counter

SS1=28, SS2=6, SS4=15, SS6=11

d1=10, d2=30, d3=25, d4=20

Scheduling Epoch

Eligible Set

{SS1, SS2, SS4, SS6}SINRi > SINRth & Backlogged


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Tf

kTfScheduling EpochEligible Set

{SS1, SS2, SS4, SS6}SINRi > SINRth

& Backlogged

Sch Set

{SS1,SS2}

SS1=46, SS2=14Sch Set

{SS1,SS2,

SS4,SS6}

SS1=23, SS2=5,

SS4=13, SS6=9

d1=10, d2=30,

d3=25, d4=20

d1=5, d2=25

SS5

SS1 SS2SS1SS6

SS5

SS3SS3

SS6

SS4SS4


12


SS1

Tf


{SS1, SS2, SS4 , SS6}SINRi > SINRth

& Backlogged

Sch Set

{SS2,SS6}

Sch Set

{SS1,SS2}

Sch Set

{SS1,SS2,

SS4, SS6}

SS5

SS2SS6

SS5

SS3SS3

SS1

SS4SS4


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SS1


Tf


{SS1, SS2, SS4, SS6}SINRi > SINRth

& Backlogged

Scheduling Epoch

Eligible Set

{SS2, SS3, SS4, SS6}

SS5

SS2

SS4

SS6

SS5

SS3

SS1


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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


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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)


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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


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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


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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

Discussion


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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

Future Work


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Example of O-DRR Scheme

Assumptions Total no of slots = 60 Number of users = 6 Per user (quantum) = 10 Tf= 5, K = 3

SS Cl SNR Qstate DRR Flag

Lag/Lead (before)

di Wi Slots assigned

Lag/Lead (after)

1 1 31 1 1 30 10 0.46 28 12

2 2 30 1 1 20 30 0.10 6 24

3 1 20 1 0 -35 20 0.0 0 -25

4 2 35 1 1 40 25 0.25 15 35

5 1 23 1 0 15 18 0.0 0 25

6 2 32 1 1 23 20 0.18 11 22

Scheduling Epoch1, Scheduling Instant1


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Example of O-DRR Scheme

SS Cl SNR Qstate DRR Flag

Lag/Lead (before)

di wi Slots assigned

Lag/Lead (after)

1 1 31 32 1 1 112

10 5 0.77 46 -24

2 2 30 34 1 1 124

10 25 0.23 14 20

3 1 20 22 1 0 0-25

10 15 0 0 -15

4 2 35 25 1 1 035

10 20 0 0 45

5 1 23 24 1 0 025

10 13 0 0 35

6 2 32 21 1 1 022

20 15 0 0 32

Scheduling Epoch1, Scheduling Instant2

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

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