WiMAX OFDM PHY Overview

1
WiMAX OFDM PHY Overview

• Chen-Nien Tsai
• Institute of Computer Science and Information
  Engineering
• National Taipei University of Technology
• 2006.10.24

2
Outline

• Introduction
• Review of the OFDM System
• OFDM PHY
• Summary

3
Introduction

• WiMAX
• Worldwide Interoperability for Microwave Access
• Replace last mile
• Cost saving
• Easy to deploy

4
Basic WiMAX Network Architecture
5
Reference Model
6
Physical Layer

• WirelessMAN-SC PHY
• WirelessMAN-SCa PHY
• WirelessMAN-OFDM PHY
• WirelessMAN-OFDMA PHY

7
OFDM PHY

• Based on OFDM modulation.
• 256 subcarriers
• Designed for NLOS operation in the frequency band
  below 11 GHz.

8
Outline

• Introduction
• Review of the OFDM System
• OFDM PHY
• Summary

9
Review of the OFDM System

• OFDM stands for Orthogonal Frequency Division
  Multiplexing.
• It was proposed in mid-1960s and used in several
  high-frequency military system.
• It is a multicarrier transmission technique.
• Divides the available spectrum into many
  subcarriers, each one being modulated by a low
  data rate stream.

10
The Applications of OFDM

• High-definition Television
• Wireless LANs
• IEEE 802.11a/g
• HIPERLAN2
• IEEE 802.16 (WiMAX)
• IEEE 802.20
• Mobile Broadband Wireless Access (MBWA)
• Groups activities were temporarily suspended.

11
Single carrier and Multicarrier Transmission

• Single carrier transmission
• Each user transmits and receives data stream with
  only one carrier at any time.
• Multicarrier transmission
• A user can employ a number of carriers to
  transmit data simultaneously.

12
Single carrier and Multicarrier Transmission
Single carrier transmission
Multicarrier transmission
N oscillators are required
13
The Basic Principles of OFDM

• FFT-based OFDM system
• Modulation and mapping
• Orthogonality
• Guard interval and Cyclic Extension

14
FFT-based OFDM system
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FFT-based OFDM system

• Generation of OFDM signal
• Discrete/Fast Fourier Transform implementation.
• No need for N oscillators to transmit N
  subcarriers.

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Why FFT-based (1/3)

• A OFDM subcarrier signal can be expressed as
• Suppose there are N subcarrier signals

amplitude
phase
17
Why FFT-based (2/3)

• After sampling
• If

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Why FFT-based (3/3)

• The definition of IDFT

Identical
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Modulation and Mapping

• Modulation types over OFDM systems
• Phase Shift Keying (PSK)
• Quadrature Amplitude Modulation (QAM)
• WiMAX OFDM PHY
• BPSK
• QPSK
• 16-QAM
• 64-QAM

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BPSK
QPSK
64-QAM
16-QAM
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An Example
QPSK

• Input stream
• 11 01 10 11
• Output stream (I, Q)
• 1, 1
• -1, 1
• 1, -1
• 1, 1

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Orthogonality (1/5)

• Time domain
• Frequency domain

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Orthogonality (2/5)

• Two signals

24
Orthogonality (3/5)
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Orthogonality (4/5)
Time Domain
Frequency Domain
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Orthogonality (5/5)
Time Domain
Frequency Domain
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Guard interval and Cyclic Extension

• Inter-symbol interference (ISI)
• The crosstalk between signals within the same
  subcarrier of consecutive OFDM symbols.
• Caused by multipath fading.
• Inter-carrier interference (ICI)
• The crosstalk between adjacent subcarrier of
  frequency bands of the same OFDM symbols.

28
Guard Interval

• To eliminate the effect of ISI
• Guard interval is used in OFDM systems

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Guard Interval

• The guard interval could consist of no signals at
  all.
• Orthogonality would be violated.
• The problem of ICI would arise.
• Call for cyclic extension (or cyclic prefix).

30
Cyclic Extension
31
OFDM symbol time
OFDM symbol time
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Outline

• Introduction
• Review of OFDM System
• OFDM PHY
• Summary

33
OFDM Symbol

• Time domain

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OFDM Frequency Description

• Frequency domain
• Data subscarriers For data transmission
• Pilot subscarriers For various estimation
  purposes
• Null subscarriers For guard bands, non-active
  subcarriers, and the DC subcarrier

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OFDM Frequency Description

• Subchannel is a combination of data subcarriers.
• Subcarriers in a subchannel can be adjacent or
  spread out.
• 256 subcarriers per carrier
• 1 DC subcarrier (index 0)
• 55 Guard subcarriers
• data subcarriers pilot subcarriers 200
  subcarriers

36
16 subchannels
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Channel Coding

• Channel coding is composed of three steps
• Randomization
• FEC
• Interleaving

Randomizer
FEC
Bit Interleaver
Data to transmit
Modulation
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Randomization

• Purpose additional privacy
• For each allocation of data block, the randomizer
  shall be used independently.
• Each data byte shall enter sequentially into the
  randomizer, MSB first.

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• PBRS (Pseudo-Random Binary Sequence) of
  randomization with generator 1X14X15

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Initialization vector
DIUC Downlink Interval Usage Code

• Uplink
• For burst 1, the initialization vector is

41
Initialization vector
UIUC Uplink Interval Usage Code

• Downlink

42
FEC

• Forward Error Correction
• Concatenated Reed-Solomon-convolutional code
  (RS-CC) Mandatory
• Block Turbo Coding (BTC) optional
• Convolutional Turbo Codes optional

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Binary Convolutional Encoder

• Each m-bit information to be encoded is
  transformed into an n-bit symbol
• Code rate m/n
• To convolutionally encode data
• k memory registers (k 6 in OFDM PHY)
• Input bits are fed into the leftmost register
• Output bits are generated by the generator
  polynomials and the existing values in the
  remaining registers

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Binary Convolutional Encoder
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Puncturing Pattern

• 1 means a transmitted bit and 0 denotes a
  removed bit

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An Example

• Code rate 5/6
• Input data 0100100100
• Output data will be 12 bits.
• All memory registers start with a value of 0.

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Initial values of registers
Input

1. Bitwise multiplication
2. Summation

1
0
0
0
0
0
0
0
0
0
0
0
1
1
0
0
G1
G2
0
1
1
Puncturing Pattern
X
Y
Output
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Interleaveing (1/3)

• Why bother?
• FEC codes are effective when transmission errors
  occur randomly in time.
• In most cases, errors occur burstly.
• Without interleaving
• With interleaving

aaaabbbbccccddddeeeeffffgggg aaaabbbbccc____deeeef
fffgggg
Error-free transmission
transmission with a burst error
De-interleaving
abcdefgabcdefgabcdefgabcdefg abcdefgabcd
bcdefgabcdefg
aa_abbbbccccdddde_eef_ffg_gg
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Interleaveing (2/3)

• Let
• k be the index of the coded bit before the first
  permutation.
• mk be the index of the coded bit after the first
  and before the second permutation.
• jk be the index after the second permutation.
• Ncpc be the number of coded bits per subcarrier.
• BPSK ? 1 16-QAM ? 4
• QPSK ? 2 64-QAM ? 6

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Interleaveing (3/3)

• The first permutation
• The second permutation

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De-interleaveing

• Let
• j be the index of a received bit before the first
  permutation.
• mj be the index of that bit after the first and
  before the second permutation.
• kj be the index of that bit after the second
  permutation.

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De-interleaving

• First permutation
• Second permutation

53
Block Sizes of the Bit Interleaver
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Outline

• Introduction
• Review of OFDM System
• OFDM PHY
• Summary

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Summary (1/3)

• Advantages of the OFDM system
• Better bandwidth usage than traditional FDM
• The subcarrier is keep orthogonality with overlap
• No guard band among subcarriers
• Low complexity
• Using off-the-shelf DFT/FFT DSP technologies
• Tolerate ISI and ICI
• Guard interval
• Cyclic extension

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Summary (2/3)

• Disadvantages of the OFDM system
• Cyclic prefix overhead
• Frequency synchronization
• Sampling frequency synchronization
• Carrier frequency synchronization
• Symbol synchronization
• Timing errors
• Carrier phase noise

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Summary (3/3)
MAC Layer
MAC PDU
PHY Layer
Randomizer
FEC
Bit Interleaver
Modulator
IFFT
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Backup Materials
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Modulation and Mapping
QPSK
16-QAM
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Example OFDM Uplink RS-CC Encoding (1/3)
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Example OFDM Uplink RS-CC Encoding (2/3)
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Example OFDM Uplink RS-CC Encoding (3/3)
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