1. 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

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

3. What is OFDM? Modulation technique Requires channel coding
Solves multipath problems
Transmitter:
I/Q RF
Source coding
Channel coding / interleaving
OFDM modulation
I/Q-mod., up-
converter
Info
Source
e.g. Audio
0110
OFDM de-modulation
Source decoding
Down-converter, I/Q-demod.
I/Q RF
Decoding / deinter-leaving
Receiver:
Radio-
channel
Info
Sink f
PSD * f
PSD fc
-fc

4. Multipath Propagation
Reflections from walls, etc.
Time dispersive channel
Impulse response:
Problem with high rate data transmission:
inter-symbol-interference t
[ns] p (
) (PDP)
Multipath Radio Channel

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

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

7. Concept of parallel transmission (1)
Channel impulse
response
Time
1 Channel (serial)
2 Channels
Channels are transmitted
at different frequencies
(sub-carriers)
8 Channels
In practice: 50 … 8000
Channels (sub-carriers)
OFDM Technology

8. The Frequency-Selective Radio Channel
-10 -5 5 10 15 20
Frequency
Power response [dB]
Interference of reflected (and LOS) radio waves
Frequency-dependent fading
Multipath Radio Channel

9. Concept of parallel transmission (2)
Channel impulse
response
Channel
transfer function
Frequency
Time
Signal is
“broadband”
1 Channel (serial)
Frequency
2 Channels
Frequency
8 Channels
Frequency
Channels are
“narrowband”
OFDM Technology

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

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

12. 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
Window function Re Im
xi,k
Sub-carrier

13. 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
Magnitude
Frequency
N sub-carriers:
resembles
IDFT!

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

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

16. Guard interval (2) - Cyclic extension

17. OFDM Symbol Configuration (1)
System Proposal

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

19. OFDM Block Diagram Transmitter Receiver time 010101001 0110
OFDM modulation (IFFT)
Channel coding / interleaving
Guard interval
I/Q
Symbol mapping (modulation)
0110
FFT-part
time
1 OFDM symbol
Channel
impulse response:
N symbols
OFDM demod.
(FFT)
Decoding / deinter-leaving
Guard interval removal
Time sync.
I/Q
symbol de-mapping (detection)
Channel est.
Receiver
Introduction

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

21. Design of an OFDM System
Nr. of
carriers
Data rate;
modulation
order
Channel
impulse
response
Guard
interval
length
x(4 … 10)
FFT
symbol
length
Other constraints:
Nr. of carriers should match FFT size
and data packet length
considering coding and modulation schemes
Channel
Parameters
are needed
Introduction

22. OFDM Symbol Configuration (1)
OFDM System Design

23. Spectral Shaping by Windowing
OFDM System Design

24. 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.
Design of an OFDM System

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

26. 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

27. Problems of OFDM (Research Topics)20 40 60 80
100
120
140
160
180
200
-0.2
-0.1
0.1
0.2
time domain signal (baseband)
sample nr.
imaginary
real
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
OFDM Technology

28. 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:
conj. 
...
… (M times)
Peak at optimum position
Phase  frequency-offset
Received signal with f-offset:
Constant phase offset between samples spaced by L
Introduction

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

30. 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 and Applications

31. 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