Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling :

Orthogonal Frequency Division Multiplexing (OFDM): Concept and System-Modeling Klaus Witrisal
Signal Processing and Speech Communication Lab
Technical University Graz, Austria
VL: Mobile Radio Systems, Ch. 5: “Wideband Systems”
24-Nov-05

Outline :

Outline Introduction
What is OFDM?
Multipath fading radio-channel
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM in Practice
Summary

What is OFDM? :

What is OFDM? Modulation technique
Requires channel coding
Solves multipath problems OFDM modulation Source coding I/Q-mod., up-
converter I/Q RF Channel coding / interleaving Transmitter: Info
Source

Multipath Propagation :

Multipath Radio Channel Multipath Propagation Reflections from walls, etc. Time dispersive channel
Impulse response:
Problem with high rate data transmission:
inter-symbol-interference

Inter-Symbol-Interference :

Multipath Radio Channel Inter-Symbol-Interference Transmitted signal:
Received Signals:
Line-of-sight:
Reflected:
The symbols add up on the channel
Distortion! Delays

Outline :

Outline Introduction
What is OFDM?
Multipath fading radio-channel
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM in Practice
Summary

Concept of parallel transmission (1) :

OFDM Technology Concept of parallel transmission (1) Channel impulse
response

The Frequency-Selective Radio Channel :

Multipath Radio Channel The Frequency-Selective Radio Channel Interference of reflected (and LOS) radio waves
Frequency-dependent fading

Concept of parallel transmission (2) :

OFDM Technology Concept of parallel transmission (2) Channel impulse
response 1 Channel (serial) Channel
transfer function Frequency Frequency Time

Concept of an OFDM signal :

Implementation and System Model Concept of an OFDM signal Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Saving of bandwidth Ch.3 Ch.5 Ch.7 Ch.9 Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Conventional multicarrier techniques Orthogonal multicarrier techniques 50% bandwidth saving frequency frequency

Outline :

Outline Introduction
What is OFDM?
Multipath fading radio-channel
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM in Practice
Summary

Generating the OFDM signal (1) :

Generating the OFDM signal (1) Symbol (QPSK) of sub-carrier i at time k
Other symbol-alphabets can be used as well (BPSK, m-QAM)
Baseband signal is generated by DSP

Spectrum of the modulated data symbols :

Spectrum of the modulated data symbols Rectangular Window of duration T0
Has a sinc-spectrum with zeros at 1/ T0
Other carriers are put in these zeros
sub-carriers are orthogonal Frequency Magnitude

Generating the OFDM signal (2) :

Generating the OFDM signal (2) serial-to-parallel xn IDFT
(IFFT) x0,k x1,k xN,k parallel-to-serial s0,k s1,k sN,k sn N data symbols:
(in frequency-
domain) Base-band
signal
(time-domain)

Idea of Guard Interval (GI) :

Introduction Idea of Guard Interval (GI) FFT-part time Insertion of guard interval (cyclic prefix): Channel impulse response (shorter than GI): t 1 OFDM symbol Cyclic convolution of transmitted signal
with channel impulse response
multiplication in frequency-domain

Slide 16:

Guard interval (2) - Cyclic extension

OFDM Symbol Configuration (1) :

System Proposal OFDM Symbol Configuration (1)

OFDM System Model :

Introduction OFDM System Model Multiplication of data symbols with (complex-valued) channel transfer-function:

OFDM Block Diagram :

Introduction OFDM Block Diagram Transmitter

Outline :

Outline Introduction
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM System Design
Parameter selection
Implementation Issues
Summary and Applications

Design of an OFDM System :

Introduction Design of an OFDM System

OFDM Symbol Configuration (1) :

OFDM System Design OFDM Symbol Configuration (1)

Spectral Shaping by Windowing :

OFDM System Design Spectral Shaping by Windowing

OFDM Symbol Configuration (2) :

Design of an OFDM System OFDM Symbol Configuration (2) Not all FFT-points can be used for data carriers
Lowpass filters for AD- and DA-conversion
oversampling required
DC offsets; carrier feedtrough; etc.

Outline :

Outline Introduction
What is OFDM?
Multipath fading radio-channel
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM in Practice
Summary

Advantages of OFDM :

Advantages of OFDM Solves the multipath-propagation problem
Simple equalization at receiver
Computationally efficient
For broadband systems more efficient than SC
Supports several multiple access schemes
TDMA, FDMA, MC-CDMA, etc.
Supports various modulation schemes
Adaptability to SNR of sub-carriers is possible
Elegant framework for MIMO-systems
All interference among symbols is removed

Problems of OFDM (Research Topics) :

OFDM Technology Problems of OFDM (Research Topics) Synchronization issues:
Time synchronization
Find start of symbols
Frequency synchr.
Find sub-carrier positions
Non-constant power envelope
Linear amplifiers needed
Channel estimation:
To retrieve data
Channel is time-variant

Correlation-based Frequency-sync. :

Introduction Correlation-based Frequency-sync. Correlation of duplicated parts of OFDM signal
e.g.: Cyclic prefix (Guard interval - GI): Guard interval
(M samples) FFT-part
(L samples) si: Peak at optimum position
Phase frequency-offset Received signal with f-offset:
Constant phase offset between samples spaced by L

Outline :

Outline Introduction
What is OFDM?
Multipath fading radio-channel
Principle of OFDM
OFDM Implementation and System Model
Advantages and Disadvantages
OFDM in Practice
Summary

Applications of OFDM :

Summary and Applications Applications of OFDM Wireless LAN
IEEE802.11a/g
HYPERLAN
DAB, DVB, etc.
Digital Audio/Video Broadcasting
xDSL (Digital Subscriber Line)
uses Discrete Multitone (DMT)

Summary – Essential “Ingredients” :

Summary – Essential “Ingredients” IFFT & FFT
For efficient implementation
Guard interval insertion
Obtaining simple equalization
Removing all IS- & IC-interferences
Error correction coding
To restore bits that are lost on weak sub-carriers

OFDM Block Diagram :

OFDM Technology OFDM Block Diagram Transmitter

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