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Scheduling Techniques for Improving Call Capacity for VoIP Traffic in MIMO-OFDMA Networks

Scheduling Techniques for Improving Call Capacity for VoIP Traffic in MIMO-OFDMA Networks. M. Nicolaou, S. Armour, A. Doufexi, Y. Sun (Toshiba Research Ltd.). Vehicular Technology Conference, VTC-2009 Fall, 20-23 Sept. 2009. Introduction. Emergence of new applications for wireless systems

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Scheduling Techniques for Improving Call Capacity for VoIP Traffic in MIMO-OFDMA Networks

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  1. Scheduling Techniques for Improving Call Capacity for VoIPTraffic in MIMO-OFDMA Networks M. Nicolaou, S. Armour, A. Doufexi, Y. Sun (Toshiba Research Ltd.) Vehicular Technology Conference, VTC-2009 Fall, 20-23 Sept. 2009

  2. Introduction • Emergence of new applications for wireless systems • Quality of Service (QoS) provision essential • MIMO and OFDMA provide a good solution towards increasing throughput, enhancing coverage and improving resource allocation fairness • WiMAX and LTE downlink of the air interface OFDM/OFDMA based, with MIMO support • Consideration of real-time, delay-sensitive VoIP traffic

  3. QoS Support in WiMAX • QoS control maintained by connection-oriented MAC architecture, • Downlink and uplink connections are controlled by the serving Base Station (BS) • Five scheduling services defined • Unsolicited grant services (UGS), • Real-time polling services (rtPS), • Non-real time polling service • (nrtPS), Best-effort (BE) service • Extended real-time variable rate (ERT-VR) service

  4. System and Channel Model • OFDMA system with BW=10MHz • Dedicated band of 175 KHz, split into 16 non-contiguous PRBs, allocated exclusively for VoIP traffic. • 3GPP-Spatial Channel Model Extended (SCME) Urban Micro Environment • Single User MIMO (SU-MIMO) precoding scheme

  5. Parameters

  6. VoIP Traffic Modelling • VoIP transmission with Voice Activity Detection (VAD) modelled by a two-state Markov process • Alternating periods of activity and silence exponentially distributed • Constant packet arrival of 20ms during active state • Fixed sized packets of 32 bytes

  7. VoIP QoS Requirements • Maximum packet latency set at D=30ms • Packets exceeding maximum latency are assumed to timeout • Data transmitted on packets that timeout is lost • Maximum tolerable packet timeout ratio set at 4% • Users exceeding timeout threshold are in QoS outage • Wireless systems should aim to ensure a target QoS outage probability

  8. Packet Scheduler Structure • The packet scheduling structure at the BS consists of three blocks: • Packet Classifier (PC) • Buffer Management Block (BMB) • Packet Scheduler (PS)

  9. Packet Scheduling Algorithms • Max. Rate Scheduling: • Proportional Fair (PF) Scheduling: being the previous throughput utilisation , updated over an exponentially weighted window of length tc. • Time or Frequency domain PF scheduling possible in multicarrier systems

  10. Packet Scheduling Algorithms • PF scheduling designed for continuous traffic • Unequal ON states and packet arrivals not considered • Packet arrivals considered as independent events • Utilisation is reset for each new event, and set to a non-zero value

  11. Packet Scheduling Algorithms • Relative Strength Scheduling: α: tuning fairness parameter • Resource allocation fairness in the short term • May not converge to throughput allocation fairness in the long term

  12. Packet Scheduling Algorithms • Urgency Based Scheduling: • Time utility Function (TUF): γ:slope parameter, c the location parameter of the inflection point • Joint consideration of Head of Line (HOL) packet delay and channel strength

  13. Packet Scheduling Algorithms-Urgency Based Scheduling • Increased priority to VoIP over the marginal scheduling time interval (MSTI) • Packets with same urgency scheduled based on max. Rate • Peak Urgency before extreme timeout point • No scheduling possible outside (MSTI) Zero urgency

  14. QoS Performance Analysis-Average Packet Timeout • Max. Rate achieves lowest packet timeouts for a given load • Urgency Based scheduling attains highest packet timeouts • Throughput fairness oriented algorithms (PF) achieve intermediate packet timeouts

  15. QoS Performance Analysis-User Satisfaction • Customer satisfaction defined by the packet timeout ratio • Used to define max. call admittance capacity • Max. Rate manages highest call admittance capacity for a given tolerable QoS outage

  16. QoS Performance Analysis-Packet Delay Distribution • Lowest average packet delays for max. Rate • Highest delays for Urgency based scheduling due to idle state outside MSTI • Fairness oriented algorithms do not guarantee lower packet delays

  17. Conclusions • Classical notion of fairness fails to accommodate QoS requirements for real-time traffic • Significant differences of the real-time traffic scenario and full buffer with no delay constraints • Fairness should be considered with regards to the aggregate QoS performance and not in terms of resource scheduling • Max. Rate ensures highest QoS, improving call admittance capacity per Hz, without additional metrics (HOL packet delay, timeout ratios etc) • Max. Rate serves stronger users faster, removing them from the BMB, freeing up more resources for weaker users • Urgency based scheduling unsuitable for dedicated VoIP as it results in idle scheduling instants

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