Scheduling with QoS Support in IEEE 802'16 Networks

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Scheduling with QoS Support in IEEE 802.16
Networks

• Hsi-Lu Chao
• Department of Computer Science
• National Chiao Tung University
• hlchao_at_cs.nctu.edu.tw

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Outline

• Introduction
• IEEE 802.16 WiMAX features
• Adaptive modulation and coding (AMC)
• Scheduling
• Inter-class channel-aware scheduling
• References

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WiMAX Features

• Theoretical coverage range
• Up to 50km for fixed stations.
• 5-15km for mobile stations.
• Maximum data rate 70 Mbps
• Medium access control OFDMA
• Advanced antenna techniques
• Beamforming
• MIMO

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WiMAX QoS Service Classes

• Unsolicited Grant Scheme (UGS)
• Real Time Polling Service (rtPS)
• Non Real Time Polling Service (nrtPS)
• Best Effort (BE)
• Extended Real Time Polling Service (ertPS)

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WiMAX QoS Parameters

• Maximum delay
• Minimum reserved traffic rate
• Maximum sustained traffic rate
• Traffic priority

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AMC (1/3)
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AMC (2/3)
Mode 5
Mode 3
Mode 4
Mode 1
Mode 2
Mode 6
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AMC (3/3)

• Rayleigh channel error model
• Delivery rate0.99

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Scheduling (1/3)

• A main component of MAC layer that helps assure
  QoS to various service classes.
• Three distinct scheduling processes
• BS downlink scheduling
• BS uplink scheduling
• MS uplink scheduling

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Scheduling (2/3)

• Design factors
• QoS parameters
• Throughput optimization
• Fairness
• Energy consumption and power control
• Implementation complexity
• Scalability

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Scheduling (3/3)

• Classification of schedulers
• Timestamp-based vs. credit-based schedulers
• Channel-unaware vs. channel aware schedulers
• Intra-class vs. inter-class schedulers

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Fair Queuing (1/2)

• Multiple queues for each port
• One for each source or flow.
• Queues serviced round robin.
• Each busy queue (flow) gets exactly one packet
  per cycle.
• Load balancing among flows.
• No advantage to being greedy.
• Your queue gets longer, increasing your delay
• Short packets penalized as each queue sends one
  packet per cycle.
• Comparison with FIFO

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Fair Queuing (2/2)
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Generalized Processor Sharing (GPS)

• Multiple queues as in FQ
• Send one bit from each queue per round
• Longer packets no longer get an advantage.
• However, we wish to send packets, not bits.

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Fair Queuing

• Packet approximation of GPS
• With a link data-rate of R, at any given time the
  N active data are serviced each with an average
  data rate of R / N.
• Compute virtual start and finish time as before.
• When a packet finished, the next packet sent is
  the one with the earliest virtual finish time.

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Examples of Fair Queueing
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Examples of PS and BRFQ
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Weighted Fair Queue

• FQ can not provide different capacities to
  different flows.
• Enhancement called Weighted fair queue (WFQ).
• From GPS, allocate weighting to each flow that
  determines how many bits are sent during each
  round.
• If weighted 5, then 5 bits are sent per round.
• Give means of responding to different service
  requests.

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Comparisonof FIFO, WFQ
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Inter-Class Channel-Aware Scheduling
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Approach 1 (1/4)

• BS downlink scheduling only
• Utilizing Adaptive Modulation and Coding (AMC)
  scheme
• AWGN channel model
• PER?SNR thresholds
• (N1) modes
• Mode 0 cannot transmit data.
• Scheduler design
• Scheduling UGS connections
• No AMC scheme.
• NrNd-NUGS

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Approach 1 (2/4)

• Scheduling rtPS, nrtPS and BE connections
  priority-based.
• rtPS connections
• Fi(t) delay satisfaction indicator

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Approach 1 (3/4)

• nrtPS connections
• Fi(t) rate satisfaction indicator

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Approach 1 (4/4)

• BE connections no QoS guarantee
• For different service classes, ßrtPS, ßnrtPS,
  ßBE0.8, 0.6, 0.4 provides comparable
  priorities.
• For different connections in the same service
  class, priority depends on normalized channel
  quality and satisfaction index.

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Approach 2 (1/3)

• QoS scheduling architecture

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Approach 2 (2/3)

• Packet classifier
• Only consider rtPS, nrtPS, and BE.
• Timestamp each arriving packet according to its
  arrival time.
• Bandwidth requests with CINR reports
• Each SS monitors its channel state and sends this
  information to the scheduler in the BS.
• AMC scheme activated.

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Approach 2 (3/3)

• Packet scheduler
• Start and finish tags are updated per queue
  basis.
• The queue with the smallest virtual finish time
  is selected and its HOL packet is transmitted.
• Channel error compensator
• Credit/debit
• Substitution
• Same queue
• Update virtual start and finish tags.
• Update credit and debit.
• Different queue
• No virtual start tag and finish tag updates for
  the bypassed queue.
• No credit/debit updates.

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Approach 3 (1/4)

• AMC scheme activated.
• Scheduling is divided into two sub-problems.
• Allocate time slots among the service classes.
• Allocate time slots to active connections of the
  same class.
• Total slots per frame K
• Allocated slots of class i Ki
• Blocking probability of class i is

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Approach 3 (2/4)

• Optimization of Inter-class scheduling the
  blocking probability of each class is minimized.

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Approach 3 (3/4)

• Intra-class scheduling priority assignment
• UGS connections
• rtPS connections

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Approach 3 (4/4)

• nrtPS connections
• BE connections

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Approach 4 (1/5)

• Transmission opportunity vs. channel quality

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Approach 4 (2/5)

• AMC networking coding
• Sending side
• n packets are grouped as a batch.
• m linear-independent coefficient sets are
  generated.
• Each set contains n linear-independent
  coefficients.
• mgtn
• m coded packets are formed by executing linear
  combination on n original packets.
• Receiving side
• The destination can correctly decode n original
  packets through at least n receipts among m coded
  packets.

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Approach 4 (3/5)

• Improvement of operating SNR ranges

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Approach 4 (4/5)

• Design challenges
• How to select the coding factor (n and m)?
• credit?mmax
• n
• Minimize redundancy degree (m/n)

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Approach 4 (5/5)

• Latency performance

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References

• Approach 1
• Q. Liu, X. Wang, and G. B. Giannakis, A
  Cross-Layer Scheduling Algorithm with QoS Support
  in Wireless Networks, IEEE Transactions on
  Vehicular Technology, Vol. 55, No. 3, May 2006.
• Approach 2
• A. Iera, A. Molinaro, abd S. Pizzi,
  Channel-Aware Scheduling for QoS and Fairness
  Provisioning in IEEE 802.16/WiMAX Broadband
  Wireless Access Systems, IEEE Network,
  Sept./Oct. 2007.
• Approach 3
• N. A. Ali, M. Hayajneh, and H. Hassanein, Cross
  Layer Scheduling Algorithm for IEEE 802.16
  Broadband Wireless Networks, IEEE ICC 2008.