©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
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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
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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
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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
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SS5
SS1 SS2
SS4
O-DRR Uplink Scheduling
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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O-DRR Uplink Scheduling
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
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O-DRR Uplink Scheduling
SS1
Tf
kTfScheduling EpochEligible Set
{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
O-DRR Uplink Scheduling
Tf
kTfScheduling EpochEligible Set
{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
©Copyright 2005-2006All Rights Reserved
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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