Performance Evaluation of WiMAX / IEEE 802.16 OFDM Physical Layer

Mohammad Azizul Hasan

Master’s thesis presentation, 5th June, Espoo

Supervisor: Prof. Riku JänttiInstructor: Lic. Tech. Boris Makarevitch HELSINKI UNIVERSITY OF TECHNOLOGY

Communications Laboratory

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Agenda

Introduction

IEEE 802.16 and Wireless Broandband Access

IEEE 802.16 Physical Layer

Simulation Model

Simulation Results

Conlusion and Futurework

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Introduction Background and Motivation

Broadband Wireless Access Promising solution for last mile access High speed internet access in residential as well as small and medium sized enterprise sector

Advantages of BWA– Ease of deployment and installation– Much higher data rates can be supported– Capacity can be increased by installing more base stations

Challenges for BWA

– Price

– Performance– Interoperability issues

Broadband access is currently dominated by DSL and cable modem technologiesLimitations:

• dsl can reach only three miles from central office switch

• Lack of return channel in older cable network

• Commercial areas are often not covered by cable networks

IEEE 802.16 is the first industry based standard for BWA Objective

Evaluate the effect of various modulation and coding schemes as well as interleving on PHY layer performance Methodology

PHY layer simulation is used to investigate the performance

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IEEE 802.16 and Broadband Wireless Access (BWA) (1/5)

• Evolution of IEEE family of standard for BWA

-EEE 802.16 Working group on BWA is responsible for development of the standards-The standard provides secification for PHY and MAC layer

IEEE 802.16-2001-First issue of the family intend to provide fixed BWA access in a point-to-point (PTP) topology.-Single carrier modulation-10-66 GHz frequency range-QPSK, 16-QAM (optional in UL) and 64-QAM (optional) modulation scheme

IEEE 802.16a-Added physical layer support for 2-11 GHz-Non Line of Sight (NLOS) operation becomes possible-Advanced power management technique and adaptive antenna arrays were included -OFDM was included as an alternative to single carrier modulation-BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM (optional)

IEEE 802.16-2004-2-11 GHZ frequency range-256 subcarriers OFDM Technique-BPSK, QPSK, 16-QAM, 64-QAM-Fixed and Nomadic access

IEEE 802.16e-Scalable OFDMA-Mobile BWA

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IEEE 802.16 and BWA (2/5)

IEEE 802.16 Protocol Stack

MAC Layer

Service specific convergence Sublayer(CS)-MAC CS receives higher level data

-provides transformation and mapping into MAC SDU

-ATM CS and packet CS

MAC Common Part Sublayer (CPS)

- System access, bandwidth allocation, connection

management

-QoS provisioning

Privacy Sublayer

-Authentication, secure key exchange, encryption

PHY Layer-Four different physical layer specifications

-SC, SCa, OFDM, OFDMA

Service-Specific Convergence Sublayer

(CS)

MAC Common Part Sublayer (MAC CPS)

Security Sublayer

Physical Layer (PHY)

CS SAP

PHY SAP

MAC SAP

Data /Control Plane

PHY

MAC

Scope of standard

Management Entity

Service Specific CS

Management Entity

MAC CPS

Security Sublayer

Management EntityPHY

Management Plane

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IEEE 802.16 and BWA (3/5)

Network Architecture and Deployment Topology

Architecture Resembled to cellular networks Each cell consists of a BS and one or

more SS BS provides connectivity to core network

Topology Point to point (PTP) Point to multi point (PTM) Mesh

BS

SSsBS

SSs

BS

SSs

Core Network

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IEEE 802.16 and BWA (4/5)

Application-Supports ATM, IPv4, IPv6, Ethernet and VLAN

Cellular Backhaul

- hotspots, PTP back haul

Residential Broadband

-fill the gaps in cable and dsl coverage

Underserved Areas

-rural areas

Always Best Connected

- roaming

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IEEE 802.16 and BWA (5/5)

WiMAX Forum and IEEE 802.16

Worldwide Interoperability for Microwave Access (WiMAX) An allince of telecommunication equipment and component manufacturers and service

providers Promotes and certify the compatibility and interoperability of BWA products Adopted two version of the IEEE 802.16 standard

Fixed/nomadic access: IEEE 802.16-2004 OFDM PHY layer

Portable/Mobile access: IEEE 802.16e

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IEEE 802.16 Physical Layer (1/4)

PHY Layer attributes:

Defines duplexing techniques (TDD, FDD)

Supports multiple RF bands 10-66 GHz for LOS below 11GHz for NLOS

Flexible bandwidths Up to 134 MHz in 10-66 GHz band Up to 20 MHz in < 11GHz band

Defines multiple PHYs for different Applications SC for point-to-point long range application OFDM for efficient Point-to-Multi-Point high data rate applications OFDMA more optimized for mobility, using sub-channelizationon on Downlink and Uplink

Specifies Modulation and channel coding schemes

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IEEE 802.16 Physical Layer (2/4)

Desgnation Band of operation Duplexing Technique Notes

WirelessMAN-SC™ 10-66 GHz TDD,

FDD

Single Carrier

WirelessMAN-SCa™ 2-11 GHz

Licensed band

TDD,

FDD

Single Carrier technique for NLOS

WirelessMAN-OFDM™ 2-11 GHz

Licensed band

TDD,

FDD

OFDM for NLOS operation

WirelessMAN-OFDMA™ 2-11 GHz

Licensed band

TDD,

FDD

OFDM Broken into subgroups to provide multiple access in a

single frequency band

WirelessHUMAN™ 2-11 GHz

Licensed Exempt Band

TDD May be SC, OFDM, OFDMA. Must include Dynamic Frequency Selection to mitigate interfarence

IEEE 802.16 Airinterface nomenclature and description

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IEEE 802.16 Physical Layer (3/4)

WirelessMANTM OFDM PHY Layer

Flexible Channel Bandwidth integer multiple of (1.25 1.5, 1.75, 2 or 2.75) MHz with a maximum of 20 MHz

Robust Error Control Mechanism outer Reed-Solomon (RS) code and inner Convolutional code (CC). Turbo Coding (optional)

Adaptive Modulation and Coding 8 different scheme

Adaptive Antenna System Transmission of DL and UL burst using

directed beams

Transmit Diversity

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IEEE 802.16 Physical Layer (4/4)

OFDM Special form of MCM technique Dividing the total bandwidth into a number of sub-carriers

Densely spaced and orthogonal sub-carriers Orthogonality is acheived by FFT ISI is mitigated

Comparison between conventional FDM and OFDM

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Simulation Model (1/5)

PHY Layer Setup

Random data generation

Channel Encoding

Mapping

Cyclic Prefix removal

FFT

IFFT Cyclic Prefix insertion

De-mappingChannel decoding

Output Data

Transmitter

Receiver

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Simulation Model (2/5)

Channel coding

Mandatory channel coding per modulation

Modulation Uncoded Block Size

(bytes)

Coded Block Size

(bytes)

Overall coding rate

RS code CC code rate

BPSK 12 24 1/2 (12,12,0) 1/2

QPSK 24 48 1/2 (32,24,4) 2/3

QPSK 36 48 3/4 (40,36,2) 5/6

16-QAM 48 96 1/2 (64,48,8) 2/3

16-QAM 72 96 3/4 (80,72,4) 5/6

64-QAM 96 144 2/3 (108,96,6) 3/4

64-QAM 108 144 3/4 (120,108,6) 5/6

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Simulation Model (3/5)

Channel Coding (contd.)

Data randomization• Implemented with PRBS generator• 15-stage shift register• XOR gates in feedback

RS-encoding• Derived from RS(N=255, K=239, T=8)

• Shortend and punctured

CC Encoder• Native code rate ½• Supports punctureing to acheive variable code rate

Interleaver• Two step permutation• First step:adjacent coded bits are mapped onto non-adjacent subcarriers • Second step: adjacent coded bits are mapped alternately onto less or more significant bits of the constellation

Data Randomization

Reed-SolomonEncoding

ConvolutionalEncoding

Interleaving

FEC

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Simulation Model (4/5)

Simulator Description

Each block of the transmitter, receiver and channel is written in separate ’m’ file

The main procedure call each of the block in the manner a communication system works

initialization parameters: number of simulated OFDM symbols, CP length, modulation and coding rate, range of SNR values and SUI channel model for simulation.

The input data stream is randomly generated

Output variables are available in Matlab™ workspace

BER and BLER values for different SNR are stored in text files

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Simulation Model (5/5)

Channel model wireless channel is characterized by: Path loss Multipath delay spread Fading characteristics Doppler spread Co-channel and adjacent channel interference

Stanford University Interim (SUI) channel models -empirical model -six channel model to address three different terrain types -3 taps used to model multipath -tap delay: 0-20 µs

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Simulation results (1/10)

Scatter plots• '+' transmitted data

• '*' received data.

Sppead reduction is taking place with

the increaseing values of SNR

Validates the implementation

of channel model

Scatter Plots for 16-QAM modulation (RS-CC 1/2) in SUI-1 channel model

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Simulation results (2/10)

BER Performance

BER vs. SNR plot for different coding profiles on SUI-2 channel

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Simulation results (3/10)

BPSK ½ QPSK ½ QPSK ¾ 16-QAM ½ 16-QAM ¾ 64-QAM 2/3 64-QAM 3/4

Channel SNR (dB) at BER level 10-3

SUI-1 4.3 6.6 10 12.3 15.7 19.4 21.3

SUI-2 7.5 10.4 14.1 16.25 19.5 23.3 25.4

SUI-3 12.7 17.2 22.7 22.7 28.3 30 32.7

SNR required at BER level 10-3 for different modulation and coding profile

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Simulation results (4/10)

BER performance:variations with the change in channel conditions

Severity of corruption is highest on SUI-3 Lowest in SUI-1

Tap power dominates in determining

the order of severity of corruption

BER vs. SNR plot for 16-QAM 1/2 on different SUI channel

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Simulation results (5/10)

BLER performance

BLER vs. SNR plot for different modulation and coding profile on SUI-1

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Simulation results (6/10)

BLER Performance

BPSK ½ QPSK ½ QPSK ¾ 16-QAM ½ 16-QAM ¾ 64-QAM 2/3

64-QAM 3/4

Channel SNR (dB) at BLER level 10-2

SUI-1 7.3 7 11 12.6 15.6 19.6 21.3

SUI-2 10.7 12.7 15.4 16.5 20.8 23.8 26.1

SUI-3 15 17.7 22.7 24.4 28.8 31.2 33.8

SNR required at BLER level 10-2 for different modulation and coding profile

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Simulation results (7/10)

BLER performance:variations with the change in channel conditions

• Results are consistant with

the BER performance

BLER vs. SNR plot for 64-QAM 2/3 modulation and coding profile on different SUI channel

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Simulation results (8/10)

Effect of Forward Error Correction

FEC gains 4.5 dB improvement

at BER level of 10-3

Effect of FEC in 64-QAM 2/3 on SUI-3 channel model

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Simulation results (9/10)

Effect of Reed-Solomon Encoding

QPSK ½ 16-QAM ½ 64-QAM 2/3

SNR(dB) at BER 10-3

1 1.2 1.4

SNR(dB) at BLER 10-2

3 4.5 5

Performance improvement due to RS Coding

Effect of Reed Solomon encoding in QPSK ½ on SUI-3 channel model

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Simulation results (10/10)

Effect of Bit interleaver

Effect of Block interleaver in 64-QAM 2/3 on SUI-2 channel model

BPSK 1/2 QPSK ½ 16-QAM ½ 64-QAM 2/3

SNR(dB) at BER 10-

3

2.2 0.8 1.4 2.2

SNR(dB) at BLER 10-2

1 1.2 1.7 2.5

Performance improvement due to bit interleaving

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Conclusion and Future Work

Conclusion

• Lower modulation and coding scheme provides better performance with less SNR• The results are ovious from constallation mapping point of view• Results obtain from the simulation can be used to set threshold SNR to implement adaptive modulation scheme

to attatin highest transmission speed with a target BER• FEC improves the BER performance by 6 dB to 4.5 dB at BER level 10 -3

• RS encoding improves the BER performance by 1dB to 1.4 dB at BER level 10 -3 • RS encoder provides tremendous performance when it is concatenated with CC

Future Works

The implemented PHY layer model still needs some improvement. The channel estimator can be implemented to obtain a depiction of the channel state to combat the effects of the channel using an equalizer.

The IEEE 802.16 standard comes with many optional PHY layer features, which can be implemented to further improve the performance. The optional Block Turbo Coding (BTC) can be implemented to enhance the performance of FEC. Space Time Block Code (STBC) can be employed in DL to provide transmit diversity.

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Thank You !