WiMAX OFDM PHY Overview Chen-Nien Tsai Institute of Computer Science and Information Engineering National Taipei University of Technology 2006.10.24.

WiMAX OFDM PHY OverWiMAX OFDM PHY Overviewview

Chen-Nien TsaiChen-Nien TsaiInstitute of Computer Science and InforInstitute of Computer Science and Infor

mation Engineeringmation EngineeringNational Taipei University of TechnologyNational Taipei University of Technology

2006.10.242006.10.24

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OutlineOutline

► IntroductionIntroduction►Review of the OFDM SystemReview of the OFDM System►OFDM PHYOFDM PHY►SummarySummary

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IntroductionIntroduction

► WiMAXWiMAX Worldwide Interoperability for Microwave AccWorldwide Interoperability for Microwave Acc

essess Replace last mileReplace last mile Cost savingCost saving Easy to deployEasy to deploy

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Basic WiMAX Network ArchitectureBasic WiMAX Network Architecture

Subscribe Station (SS)

Subscribe Station

Radio tow er

Wireless link

Radio tow er

Radio tow er

Radio tow er

Base Station (BS)

Subscribe Station

Core network

Wired/wireless links

Users

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Reference ModelReference Model

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Physical LayerPhysical Layer

► WirelessMAN-SC PHYWirelessMAN-SC PHY► WirelessMAN-SCa PHYWirelessMAN-SCa PHY► WirelessMAN-OFDM PHYWirelessMAN-OFDM PHY► WirelessMAN-OFDMA PHYWirelessMAN-OFDMA PHY

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OFDM PHYOFDM PHY

► Based on OFDM modulation.Based on OFDM modulation. 256 subcarriers256 subcarriers

► Designed for NLOS operation in the frequeDesigned for NLOS operation in the frequency band below 11 GHz. ncy band below 11 GHz.

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OutlineOutline

► IntroductionIntroduction►Review of the OFDM SystemReview of the OFDM System►OFDM PHYOFDM PHY►SummarySummary

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Review of the OFDM SystemReview of the OFDM System

► OFDM stands for OFDM stands for Orthogonal Frequency DiOrthogonal Frequency Division Multiplexingvision Multiplexing..

► It was proposed in mid-1960s and used in It was proposed in mid-1960s and used in several high-frequency military system.several high-frequency military system.

► It is a It is a multicarrier transmissionmulticarrier transmission technique. technique. Divides the available spectrum into many subDivides the available spectrum into many sub

carriers, each one being modulated by a low dcarriers, each one being modulated by a low data rate stream.ata rate stream.

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The Applications of OFDMThe Applications of OFDM

► High-definition TelevisionHigh-definition Television► Wireless LANsWireless LANs

IEEE 802.11a/gIEEE 802.11a/g HIPERLAN2HIPERLAN2

► IEEE 802.16 (WiMAX)IEEE 802.16 (WiMAX)► IEEE 802.20IEEE 802.20

Mobile Broadband Wireless Access (MBWA)Mobile Broadband Wireless Access (MBWA) Group’s activities were temporarily suspendeGroup’s activities were temporarily suspende

d. d.

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Single carrier and Multicarrier TranSingle carrier and Multicarrier Transmissionsmission

► Single carrier transmissionSingle carrier transmission Each user transmits and receives data stream Each user transmits and receives data stream

with only one carrier at any time.with only one carrier at any time.

► Multicarrier transmissionMulticarrier transmission A user can employ a number of carriers to traA user can employ a number of carriers to tra

nsmit data simultaneously.nsmit data simultaneously.

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Single carrier and Multicarrier TranSingle carrier and Multicarrier Transmissionsmission

cos(2 )cf t

ib ( )s t

S/Pib

1cos(2 )f t

2cos(2 )f t

cos(2 )Nf t

∑( )s t

Single carrier transmission

Multicarrier transmission

NN oscillators are require oscillators are requiredd

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The Basic Principles of OFDMThe Basic Principles of OFDM

► FFT-based OFDM systemFFT-based OFDM system

► Modulation and mappingModulation and mapping

► OrthogonalityOrthogonality

► Guard interval and Cyclic ExtensionGuard interval and Cyclic Extension

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FFT-based OFDM systemFFT-based OFDM system

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FFT-based OFDM systemFFT-based OFDM system

► Generation of OFDM signalGeneration of OFDM signal Discrete/Fast Fourier Transform implementatiDiscrete/Fast Fourier Transform implementati

on.on. No need for N oscillators to transmit N subcarNo need for N oscillators to transmit N subcar

riers.riers.

S/Pib

1cos(2 )f t

2cos(2 )f t

cos(2 )Nf t

∑( )s t

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Why FFT-based (1/3)Why FFT-based (1/3)

► A OFDM subcarrier signal can be expresseA OFDM subcarrier signal can be expressed asd as

► Suppose there are N subcarrier signalsSuppose there are N subcarrier signals

[2 ( )]( ) ( ) c ci f t tc cS t A t e

1[2 ( )]

0

1( ) ( ) k

Ni k ft t

s kk

S t A t eN

amplitude

phase

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Why FFT-based (2/3)Why FFT-based (2/3)

►After samplingAfter sampling

► If If

12

0

1( ) k

Ni i k fn t

s kk

S n t A e eN

1(sample rate )

t

1f

N t

21

0

1( ) k

knN ii Ns k

k

S n t A e eN

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Why FFT-based (3/3)Why FFT-based (3/3)

►The definition of IDFTThe definition of IDFT21

0

1( ) ( )

nkN iN

k

x n t X k f eN

21

0

1( ) k

knN ii Ns k

k

S n t A e eN

Identical

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Modulation and MappingModulation and Mapping

► Modulation types over OFDM systemsModulation types over OFDM systems Phase Shift Keying (PSK)Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM)Quadrature Amplitude Modulation (QAM)

► WiMAX OFDM PHYWiMAX OFDM PHY BPSKBPSK QPSKQPSK 16-QAM16-QAM 64-QAM64-QAM

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BPSK

QPSK

16-QAM

64-QAM

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An ExampleAn Example

► Input streamInput stream 11 01 10 1111 01 10 11

►Output stream (I, Q)Output stream (I, Q) 1, 11, 1 -1, 1-1, 1 1, -11, -1 1, 11, 1

QPSK

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Orthogonality (1/5)Orthogonality (1/5)►Time domainTime domain

►Frequency domainFrequency domain

*1 2( ) ( ) 0x t x t dt

*1 2( ) ( ) 0X f X f df

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Orthogonality (2/5)Orthogonality (2/5)►Two signalsTwo signals

121( ) i f ts t e 22

2 ( ) i f ts t e

1 2 1 22 2 2 ( )*12 0 0

( )

sin( )

T Ti f t i f t i f f t

i ft

e e dt e dt

fTe

f

12If , is non-zero integer, then 0fT n n

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Orthogonality (3/5)Orthogonality (3/5)

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Orthogonality (4/5)Orthogonality (4/5)

1Subcarrier f

2Subcarrier f

3Subcarrier f

4Subcarrier f

Time Domain

Frequency Domain

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Orthogonality (5/5)Orthogonality (5/5)

Time Domain

Frequency Domain

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Guard interval and Cyclic Guard interval and Cyclic ExtensionExtension

► Inter-symbol interference (ISI)Inter-symbol interference (ISI) The crosstalk between signals within the samThe crosstalk between signals within the sam

e subcarrier of consecutive OFDM symbols.e subcarrier of consecutive OFDM symbols. Caused by multipath fading.Caused by multipath fading.

► Inter-carrier interference (ICI)Inter-carrier interference (ICI) The crosstalk between adjacent subcarrier of The crosstalk between adjacent subcarrier of

frequency bands of the same OFDM symbols.frequency bands of the same OFDM symbols.

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Guard IntervalGuard Interval

►To eliminate the effect of ISITo eliminate the effect of ISI Guard interval is used in OFDM systemsGuard interval is used in OFDM systems

DATAGuard Interval

gT T

OFDM symbol duration total gT T T

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Guard IntervalGuard Interval

► The guard interval could consist of no signThe guard interval could consist of no signals at all.als at all. Orthogonality would be violated.Orthogonality would be violated. The problem of ICI would arise.The problem of ICI would arise.

► Call for cyclic extension (or cyclic prefix).Call for cyclic extension (or cyclic prefix).

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Cyclic ExtensionCyclic Extension

Guard Interval(Cyclic Extension)

COPY

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OFDM symbol time OFDM symbol time

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OutlineOutline

► IntroductionIntroduction►Review of OFDM SystemReview of OFDM System►OFDM PHYOFDM PHY►SummarySummary

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OFDM SymbolOFDM Symbol

►Time domainTime domain

g

b

TG

T

1 1 1 1Possible values: , , ,

4 8 16 32

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OFDM Frequency DescriptionOFDM Frequency Description

► Frequency domainFrequency domain Data subscarriers: For data transmissionData subscarriers: For data transmission Pilot subscarriers: For various estimation purPilot subscarriers: For various estimation pur

posesposes Null subscarriers: For guard bands, non-active Null subscarriers: For guard bands, non-active

subcarriers, and the DC subcarriersubcarriers, and the DC subcarrier

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OFDM Frequency DescriptionOFDM Frequency Description

► Subchannel is a combination of data subcSubchannel is a combination of data subcarriers.arriers. Subcarriers in a subchannel can be adjacent oSubcarriers in a subchannel can be adjacent o

r spread out.r spread out.► 256 subcarriers per carrier256 subcarriers per carrier

1 DC subcarrier (index 0)1 DC subcarrier (index 0) 55 Guard subcarriers 55 Guard subcarriers data subcarriers + pilot subcarriers = 200 subcdata subcarriers + pilot subcarriers = 200 subc

arriersarriers

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16 subchannels

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Channel CodingChannel Coding

►Channel coding is composed of three Channel coding is composed of three stepssteps RandomizationRandomization FECFEC InterleavingInterleaving

Data to transmit

Randomizer FEC Bit Interleaver

Modulation

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RandomizationRandomization

►Purpose: additional privacyPurpose: additional privacy

►For each allocation of data block, the For each allocation of data block, the randomizer shall be used randomizer shall be used independently.independently.

►Each data byte shall enter sequentially Each data byte shall enter sequentially into the randomizer, MSB first.into the randomizer, MSB first.

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►PBRS (Pseudo-Random Binary PBRS (Pseudo-Random Binary Sequence) of randomization with Sequence) of randomization with generator 1+Xgenerator 1+X1414+X+X1515

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Initialization vectorInitialization vector

►UplinkUplink

►For burst #1, the initialization vector isFor burst #1, the initialization vector is

1 0 1 1 000 0 1 0 0 0 000

DIUC: Downlink Interval Usage Code

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Initialization vectorInitialization vector

►DownlinkDownlinkUIUC: Uplink Interval Usage Code

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FECFEC

► Forward Error CorrectionForward Error Correction Concatenated Reed-Solomon-convolutional cConcatenated Reed-Solomon-convolutional c

ode (RS-CC) – Mandatoryode (RS-CC) – Mandatory Block Turbo Coding (BTC) – optionalBlock Turbo Coding (BTC) – optional Convolutional Turbo Codes – optionalConvolutional Turbo Codes – optional

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Binary Convolutional EncoderBinary Convolutional Encoder► Each Each mm-bit information to be encoded is tr-bit information to be encoded is tr

ansformed into an ansformed into an nn-bit symbol-bit symbol► Code rate = Code rate = mm//nn► To convolutionally encode data:To convolutionally encode data:

kk memory registers ( memory registers (kk = 6 in OFDM PHY) = 6 in OFDM PHY) Input bits are fed into the leftmost registerInput bits are fed into the leftmost register Output bits are generated by the generator poOutput bits are generated by the generator po

lynomials and the existing values in the remailynomials and the existing values in the remaining registersning registers

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Binary Convolutional EncoderBinary Convolutional Encoder

2 3 61

2 3 5 62

1 For X

1 For Y

G x x x x

G x x x x

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Puncturing PatternPuncturing Pattern

►““1” means a transmitted bit and “0” 1” means a transmitted bit and “0” denotes a removed bitdenotes a removed bit

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An ExampleAn Example

►Code rate = 5/6Code rate = 5/6► Input data = 0100100100Input data = 0100100100►Output data will be 12 bits.Output data will be 12 bits.

►All memory registers start with a value All memory registers start with a value of 0.of 0.

510 126

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1 00000000000 1100

1 1111 0 0

111 00 1 1

Input

0 1 1 1 1 1 10 1 1

0 0 01 1 10 0 0 0

0 0 00 0 01 1 1 1 1 1

Initial values of registers

G1

G2

Y

X

Output

1 1 2 3 4 5X YY X Y XPuncturing PatternPuncturing Pattern

1. Bitwise multiplication

2. Summation

011

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Interleaveing (1/3)► Why bother?Why bother?

FEC codes are effective when transmission errFEC codes are effective when transmission errors occur randomly in time.ors occur randomly in time.

In most cases, errors occur burstly.In most cases, errors occur burstly. Without interleavingWithout interleaving

With interleavingWith interleaving

aaaabbbbccccddddeeeeffffgggg

aaaabbbbccc____deeeeffffgggg

abcdefgabcdefgabcdefgabcdefg

abcdefgabcd bcdefgabcdefg

aa_abbbbccccdddde_eef_ffg_gg

De-interleaving

Error-free transmission transmission with a burst error

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Interleaveing (2/3)► LetLet

kk be the index of the coded bit before the first be the index of the coded bit before the first permutation.permutation.

mmkk be the index of the coded bit after the first a be the index of the coded bit after the first and before the second permutation.nd before the second permutation.

jjkk be the index after the second permutation. be the index after the second permutation. NNcpccpc be the number of coded bits per subcarrier. be the number of coded bits per subcarrier.

►BPSK BPSK 1 1 16-QAM 16-QAM 4 4►QPSK QPSK 2 2 64-QAM 64-QAM 6 6

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Interleaveing (3/3)►The first permutationThe first permutation

►The second permutationThe second permutation

( ) mod 12 ( )12 12cbps

k

N km k floor

12( ) ( ( )) mod k kk k cbps

cbps

m mj s floor m N floor ss N

( 2)cpcs ceil N

0, 1, , 1cbpsk N

0, 1, , 1cbpsk N

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De-interleaveing► LetLet

jj be the index of a received bit before the first be the index of a received bit before the first permutation.permutation.

mmjj be the index of that bit after the first and b be the index of that bit after the first and before the second permutation.efore the second permutation.

kkjj be the index of that bit after the second per be the index of that bit after the second permutation.mutation.

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De-interleavingDe-interleaving

►First permutationFirst permutation

►Second permutationSecond permutation

12( ) ( ( )) mod jcbps

j jm s floor j floor ss N

1212 ( 1) ( )j

j j cbpscbps

mk m N floor N

0, 1, , 1cbpsj N

0, 1, , 1cbpsj N

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Block Sizes of the Bit InterleaveBlock Sizes of the Bit Interleaverr

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OutlineOutline

► IntroductionIntroduction►Review of OFDM SystemReview of OFDM System►OFDM PHYOFDM PHY►SummarySummary

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Summary (1/3)Summary (1/3)

► Advantages of the OFDM systemAdvantages of the OFDM system Better bandwidth usage than traditional FDMBetter bandwidth usage than traditional FDM

►The subcarrier is keep orthogonality with overlapThe subcarrier is keep orthogonality with overlap►No guard band among subcarriersNo guard band among subcarriers

Low complexityLow complexity►Using off-the-shelf DFT/FFT DSP technologiesUsing off-the-shelf DFT/FFT DSP technologies

Tolerate ISI and ICITolerate ISI and ICI►Guard intervalGuard interval►Cyclic extensionCyclic extension

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Summary (2/3)Summary (2/3)

►Disadvantages of the OFDM systemDisadvantages of the OFDM system Cyclic prefix overheadCyclic prefix overhead Frequency synchronizationFrequency synchronization

►Sampling frequency synchronizationSampling frequency synchronization►Carrier frequency synchronizationCarrier frequency synchronization

Symbol synchronizationSymbol synchronization►Timing errorsTiming errors►Carrier phase noiseCarrier phase noise

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Summary (3/3)Summary (3/3)

MAC Layer

PHY Layer

MAC PDU

Randomizer

FEC

Bit Interleaver ModulatorIFFT

Backup MaterialsBackup Materials

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Modulation and MappingModulation and Mapping

QPSK 16-QAM

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Example OFDM Uplink RS-CC EncoExample OFDM Uplink RS-CC Encoding (1/3)ding (1/3)

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Example OFDM Uplink RS-CC EncoExample OFDM Uplink RS-CC Encoding (2/3)ding (2/3)

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Example OFDM Uplink RS-CC EncoExample OFDM Uplink RS-CC Encoding (3/3)ding (3/3)

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