Centre for Communications Research

Scheduling Techniques for Improving Call Capacity for VoIPTraffic in MIMO-OFDMA NetworksM. 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• 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

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

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

ParametersParameter Value

FFT size 1024

Useful Subcarriers 768

Guard Interval Length 176

Subcarrier Frequency Spacing 10.94 KHz

Symbol Duration 102.9 μS

MAC Frame Duration 5 ms

No. Rx Antennas 2

No. Tx Antennas 2

Exponentially Weighted Window (tc) value 1000

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

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

Packet Scheduler Structure• The packet scheduling structure at the BS consists of three blocks:

Packet Classifier (PC) Buffer Management Block (BMB) Packet Scheduler (PS)

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

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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

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

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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

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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

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

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

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

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