1. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM)
UNIT-8
ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM) by
B.KesavaRao, Asst.Prof. MGIT

2. Index Introduction Basic Principles of Orthogonality
Single Vs Multi channel systems
OFDM block diagram
OFDM signal Mathematical representation

3. Objective of OFDM

5. Traditional vs. OFDM Communication

6. Effect of ISI

7. OFDM- Introduction
The major requirement of modern wireless communication systems are high capacity and variable bit rate transmission with high BW efficiency.
But, the wireless environment signal usually impaired by fading and multipath delay spreading .
The traditional single carrier mobile communication systems do not perform well because it suffers extreme fading of the signal and Inter Symbol Interference(ISI).
This leads to a high probability of errors and the system’s overall performance becomes very poor.
Because of its high-speed data transmission and effectiveness in combating the frequency selective fading channel, OFDM technique is widely used in wireless communication nowadays.
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, which divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream.

8. Introduction to ofdm
Orthogonal Frequency Division Multiplexing (OFDM) is a frequency division multiplexing (FDM) scheme used as a digital multi carrier modulation (MCM) method.
OFDM is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies.
The main idea behind the OFDM is that since low-rate modulations are less sensitive to multipath, the better way is to send a number of low rate streams in parallel than sending one high rate waveform.
A large number of closely spaced orthogonal subcarriers are used to carry data.
OFDM is a promising technique for achieving high data rate and combating multipath fading in Wireless Communications.

9. OFDM DEFINITION OFDM = Orthogonal FDM
Carrier centers are put on orthogonal frequencies
ORTHOGONALITY - The peak of each signal coincides with other signals
Subcarriers are spaced by 1/Ts

10. Ofdm time and frequency response

11. Ofdm versus fdm

12. OFDM-Introduction Contd..
OFDM can be viewed as either a modulation technique or a multiplex technique.
Modulation technique
Viewed by the relation between input and output signals
Multiplex technique
Viewed by the output signal which is the linear sum of the modulated signals

13. Why OFDM? Single Carrier Multicarrier Uses the entire bandwidth
OFDM – Orthogonal Frequency Division Multiplexing
Single Carrier
Multicarrier
Uses the entire bandwidth
Splits bandwidth into subchannels
Short symbol times
Sends information in parallel
This causes ISI
OFDM: orthogonal subcarriers
OFDM is a considerable option when the channel introduces ISI
Applications: ADSL, DAB, DVB, Hiperlan/2, ...

14. Modulation techniques: monocarrier vs. multicarrier
Channel
Channelization
N carriers B
Pulse length ~ N/B
Similar to
FDM technique
– Data are shared among
several carriers
and simultaneously transmitted
Guard bands B
Pulse length ~1/B
– Data are transmited over
only one carrier
– Selective Fading
– Very short pulses
– ISI is compartively long
– EQs are then very long
– Poor spectral efficiency
because of band guards
Drawbacks
– It is easy to exploit
Frequency diversity
– Flat Fading per carrier
– N long pulses
– ISI is comparatively short
– N short EQs needed
– Poor spectral efficiency
because of band guards
Advantages
Furthermore
– It allows to deploy
2D coding techniques
– Dynamic signalling
To improve the spectral efficiency:
To use orthogonal carriers (allowing overlapping)
Eliminate band guards between carriers

15. OFDM and Multicarrier Transmission
2/8
Single and multicarrier transmission

16. Orthogonality Orthogonality in OFDM Time domain frequency domain
Band pass signal
Where is the equivalent low pass signal of
if , n is an non-integer i.e Then

17. OFDM and Multicarrier Transmission
4/8
Orthogonal
Orthogonal, n=3
Orthogonal, n=2
(OFDM)
Orthogonal, n=1
Non-orthogonal

18. OFDM and Multicarrier Transmission
5/8
Time domain
Frequency domain

21. Orthogonal Frequency Division Modulation
N carriers B
Symbol: 2 periods of f0
Transmit
Symbol: 4 periods of f0 + f f
Symbol: 8 periods of f0
Channel frequency
response .
Data coded in frequency domain
Transformation to time domain:
each frequency is a sine wave
in time, all added up. B
Decode each frequency
bin separately
Receive
time f
Time-domain signal
Frequency-domain signal

22. Principles OFDM
Some processing is done on the source data, such as coding for correcting errors, interleaving and mapping of bits onto symbols. An example of mapping used is QAM.
The symbols are modulated onto orthogonal sub-carriers. This is done by using IFFT.
Orthogonality is maintained during channel transmission. This is achieved by
adding a cyclic prefix to the OFDM frame to be sent. The cyclic prefix consists of
the L last samples of the frame, which are copied and placed in the beginning of
the frame. It must be longer than the channel impulse response.
Synchronization: the introduced cyclic prefix can be used to detect the start of each frame. This is done by using the fact that the L first and last samples are the same and therefore correlated. This works under the assumption that one OFDM frame can be considered to be stationary.
Demodulation of the received signal by using FFT
Channel equalization: the channel can be estimated either by using a training
sequence or sending known so-called pilot symbols at predefined sub-carriers.
Decoding and de-interleaving

23. Advantages of OFDM High spectral efficiency
Simple implementation by FFT
Low receiver complexity
Robust ability for high-data rate transmission over multipath fading channel.
High link in terms of link adaption.
OFDM eliminates Inter Symbol Interference (ISI) through the use of a cyclic prefix.
OFDM is less sensitive to sample timing offsets than the single carrier systems.
OFDM eliminates the need for equalizers

24. Disadvantages of OFDM Synchronization
Need FFT units at transmitter, receiver
Sensitive to carrier frequency offset
High peak to average power ratio

25. Application of OFDM
 Worldwide Interoperability for Microwave Access (WiMAX).
Terrestrial Digital Audio Broadcasting (DVB-T).
Wireless Metropolitan Area Network (WMAN).(IEEE d).
Wireless Local Area Network (WLAN). ETSI HiperLAN.
Digital Audio Broadcasting (DAB).
Digital Video Broadcasting (DVB).
High Definition Television (HDTV).
Broadband Internet Access