Performance Performance Analysis Analysis of an innovative of an innovative

scheduling algorithm for OFDMA based scheduling algorithm for OFDMA based

IEEE 802.16a systemsIEEE 802.16a systems

E. Baccarelli, M.Biagi, C.Pelizzoni, N.Cordeschi

This work has been partially supported by Italian National project: Wireless8O2.16 Multi-antenna mEsh Networks (WOMEN)under

grant number 2005093248.

Outline

Introduction to the 802.16 Systems

Description of the IEEE 802.16 standard

The proposed algorithm of Scheduling for IEEE 802.16

systems

Test model and simulation results

Conclusions

Innovative Contributions

Definition of a new strategy of scheduling for IEEE 802.16 systems, able to support

multiservice traffic adaptive modulation

Implementation of the scheduling algorithm by adopting the “OPNET simulator”

Performance analysis of the proposed scheduling algorithm in terms of transfer delay, jitter and throughput

• Frequency: 2-11 GHz

• Non line of Sight

• Centralized Access Control

• Connection-Oriented MAC

• QoS implemented with a proper packet queuing and scheduling strategy

• Adaptive modulation and coding

SS 3 BS

SS n Ethernet

LAN

IP

Backbone

CS1 CS2 CS3

CS1 CS2 CS3

Ethernet LAN

IEEE 802.16 Systems

UL-MAP

downlink

uplink

Requestedband

Allocated band

BS: Base Station

SS: Subscriber Station

CS : Service Category

Stack Protocol

Convergence Sublayer (CS)

Mapping of MAC SDUs onto the 802.16 service classes

MAC Common Part Sublayer (CPS)

Radio Access Control

Traffic transport in variable length MAC PDUs (fragmentation, packing)

PHYsical layer

Adaptive modulation and coding

Supporting of the QoS

Service classes

Always on Sources

Peridically guaranteed bandwidth

Cell-based traffic

E.g.: Voice over IP

UGS

On/Off Sources Fixed bandwidth

and garanteed on demand

packet-based Traffic E.g.: Video

Conferences

Sorgenti On/Off Minimum

guaranteed bandwidth on demand

packet-based Traffic E.g.:TCP, Telnet

On/Off Source

packet-based Traffic

E.g.: E-mail

rtPS BEnrtPS

Unsolicited Grant Service real time Polling Service non real time Polling Service Best Effort

Modulation based on the current channel state

Adopted Modulations

SS3

DOWNLINK UPLINK

BS

SS1 SS

2

SS n

d 1

dn d 3

d 2 1. QPSK

2. 16 QAM

3. 64 QAM

Adaptive Modulation

Frame Structure

FCH (Frame Control Header): contains information on DL-MAP

DL–MAP: indicates the bursts position in the downlink frame and the currently adopted modulation and

coding scheme

UL-MAP: indicates the bursts position allowed to the SS into the uplink frame and the modulation and

coding scheme to be adopted

Frame length: 2 ms roll-off : 0.25 Channel bandwidth: 10 MHz

14400 simbols per frame

DL burst 2

OFDMA

symbol

uplink

DL burst 2

Pre

ambo

lo

FCH

DL

- M

AP U

L -

MA

P

sottocanale ranging

downlink

N°

logi

co s

otto

cana

le DL burst 1

DL burst 2

DL burst 3

DL burst 4

UL burst 1

UL burst 5

UL burst 3

UL burst 4

UL burst 2

DL burst 5

OFDMA (1/2)

Subchannel s

An 802.16a OFDMA symbol contains 60 sub-channels with 2 cluster each

Any cluster is composed by 14 adjacent sub-carriers (2 pilots sub-carriers e 12 data

sub-carriers)

Cluster structure for even symbols

Cluster structure for odd symbols

Subchannel 1Subchannel 1

Pilot CarriersPilot Carriers

Subchannel 2Subchannel 2

Subchannel sSubchannel s

timetime

Sub

chan

nels

Sub

chan

nels

OFDMA (2/2)

Sub-channels allocation

to different transmit users

AdvantagesResource adaptive allocation Mitigation of the intra-cell interference Mitigation of the inter-cell interferenceIncreasing of the uplink power efficiency

Counterparts High sensitivity to time

and frequency

synchronization errors

It divides the available band to the

different traffic classes by adopting

a strictly hierarchic algorithm

It updates the available band basing on current modulation

scheme

Base Station DL Scheduler(1/2)

DL Frame

Available bandwidth

Residual bandwidth

Residual bandwidth

Base Station DL Scheduler(2/2)

)(~)()(imaxarg tr

trtW

i i

iij i

i

Ti)log(

i

Modified Largest Weighted Delay First (M-LWDF)- It determinates the priority by using the following la relation

Allowed band to the current frame

Connection equivalent band

Packet delay on the top of the sub-queue

Modified Proportional Fair (M-PF)- It determinates the priority by adopting the following relation

Ti = Maximum allowable delay = probability to overcome Ti threshold

)(~)()(maxarg tr

trtW

i i

iij Allowed band to the current frame

Connection mean rate

Packet delay on the top of the sub-queue

Simulation Tool

®

Performance analisys for rtPS traffic (1/2)

Checking for the IEEE 802.16 QoS constraintsChecking for the IEEE 802.16 QoS constraints

Downlink transmission QPSK modulation Maximum system capacity : 14.4 Mb/s Total traffic 15.5 Mb/s Guaranteed traffic 14.1 Mb/s Ti=10 ms,

Downlink transmission QPSK modulation Maximum system capacity : 14.4 Mb/s Total traffic 15.5 Mb/s Guaranteed traffic 14.1 Mb/s Ti=10 ms,

Task

Operating conditions

Performance parameters

Transfer delay Jitter Throughput

Transfer delay Jitter Throughput

310

i

JitterTransfer delay

Output data rateInput data rate

Performance analysis of rtPS traffic (2/2)

Checking for the IEEE 802.16 QoS constraintsChecking for the IEEE 802.16 QoS constraints

Task

Condizioni di sistema

Performance parameters

Transfer delay of 4 service traffic classes Transfer delay of 4 service traffic classes

Downlink transmission QPSK modulation Maximum system capacity : 14.4 Mb/s Total Traffic 16.7 Mb/s Guaranteed traffic 14.2 Mb/s rtPS ( , Ti=10 ms)

nrtPS ( , Ti=10 ms)

Downlink transmission QPSK modulation Maximum system capacity : 14.4 Mb/s Total Traffic 16.7 Mb/s Guaranteed traffic 14.2 Mb/s rtPS ( , Ti=10 ms)

nrtPS ( , Ti=10 ms)2

10

i

310

i

Performance analysis of heterogeneous traffic (1/2)

Transfer delay (UGS) Transfer delay (rtPS)

Transfer delay ( nrtPS)Transfer delay (BE)

Performance analysis of heterogeneous traffic (2/2)

Task

Operating conditions

Performance parameters

Transfer delay of 4 service traffic classes Transfer delay of 4 service traffic classes

Downlink transmission Adaptive modulation (QPSK, 16 QAM, 64 QAM) Total traffic 16.7 Mb/s Guaranteed traffic 14.2 Mb/s rtPS ( , Ti=10 ms)

nrtPS ( , Ti=10 ms)

Downlink transmission Adaptive modulation (QPSK, 16 QAM, 64 QAM) Total traffic 16.7 Mb/s Guaranteed traffic 14.2 Mb/s rtPS ( , Ti=10 ms)

nrtPS ( , Ti=10 ms)2

10

i

310

i

Performance analisys with heterogeneous traffic and adaptive modulation (1/2)

Checking for the IEEE 802.16 QoS constraints when adaptive modulation is supported

Checking for the IEEE 802.16 QoS constraints when adaptive modulation is supported

Performance analisys with heterogeneous traffic and adaptive modulation (1/2)

Transfer delay rtPS

Tra

nsfe

r d

ela

y (

s)

Transfer delay nrtPS

Tra

nsfe

r d

ela

y (

s)

Transfer delay (BE)

Tra

nsfe

r d

ela

y (

s)

Conclusions

The adopted strategy of scheduling is able:

to fully meet the QoS constrains in terms of transfers delay

to efficiently manage the available bandwidth

to efficiently manage the heterogeneous traffic

to support the adaptive modulation letting the traffic transfer delay jitter be controlled basing it on the channel state conditions

The adopted strategy of scheduling is able:

to fully meet the QoS constrains in terms of transfers delay

to efficiently manage the available bandwidth

to efficiently manage the heterogeneous traffic

to support the adaptive modulation letting the traffic transfer delay jitter be controlled basing it on the channel state conditions