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ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING(OFDM)
byB.KesavaRao, Asst.Prof.
MGIT
UNIT-8
Index
• Introduction
• Basic Principles of Orthogonality
• Single Vs Multi channel systems
• OFDM block diagram
• OFDM signal Mathematical representation
Objective of OFDM
Traditional vs. OFDM Communication
Effect of ISI
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.
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.
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
Ofdm time and frequency response
0 2 4 6 8 10 12-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
FREQUENCY DOMAIN: OFDM Subcarriers 2 through 10
Frequency (Normalized by 1/Tofdm)
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-1.5
-1
-0.5
0
0.5
1
1.5TIME DOMAIN: 2 OFDM subcarriers (BPSK)
Time (Normalized by Tofdm)
Ofdm versus fdm
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
Input Signal
OFDM System
Output Signal
S/P
frequency frequency
freq
uenc
y re
spon
se
freq
uenc
y re
spon
se
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
OFDM is a considerable option when the channel introduces ISI
Applications: ADSL, DAB, DVB, Hiperlan/2, ...
Why OFDM?
OFDM – Orthogonal Frequency Division Multiplexing
• Uses the entire bandwidth • Splits bandwidth into subchannels
• Short symbol times
• This causes ISI
• Sends information in parallel
• OFDM: orthogonal subcarriers
Single Carrier Multicarrier
Modulation techniques: monocarrier vs. multicarrier
To improve the spectral efficiency:
To use orthogonal carriers (allowing overlapping)Eliminate band guards between carriers
– 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 exploitFrequency 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 deploy2D coding techniques
– Dynamic signalling
N carriers
B
Pulse length ~ N/B
Similar toFDM technique
– Data are shared among several carriers and simultaneously transmitted
B
Pulse length ~1/B
– Data are transmited over only one carrier
Channel
Guard bands
Channelization
OFDM and Multicarrier Transmission
• Single and multicarrier transmissionSingle carrier transmission
1cos(2 )f t
ib ( )s tS/P
1cos(2 )f t
2cos(2 )f t
cos(2 )Nf t
ib ( )s t
Multicarrier carrier transmission
2/8
Orthogonality in OFDM
• Orthogonality Time domain frequency domain
Band pass signal
Where is the equivalent low pass signal of
if , n is an non-integer i.e. Then
*1 2( ) ( ) 0x t x t dt
*1 2( ) ( ) 0X f X f df
2 ( ) 2( ) cos(2 ( ) ) Re Re ( )c m ci f f t i f tm c m lmx t f f t e x t e
2( ) mi f tlmx t e ( )mx t
1 2 1 22 2 2 ( )*12
0 0
sin( )( )
T Ti f t i f t i f f t i fTfTe e dt e dt e
f
f T n nfT
12 0
OFDM and Multicarrier Transmission
(A)
(E)
(D)
(C)
(B)
1cf f 2cf f 3cf f 4cf f 5cf f
1
T
1
T
Orthogonal
Non-orthogonal
Orthogonal, n=3
Orthogonal, n=2
Orthogonal, n=1(OFDM)
4/8
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
OFDM and Multicarrier Transmission
1subcarrier f
2subcarrier f
3subcarrier f
4subcarrier f
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1.5
-1
-0.5
0
0.5
1
1.5
2
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
3
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-2
-1
0
1
2
3
4
-10 -5 0 5 10-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
5/8
Time domain Frequency domain
Orthogonal Frequency Division Modulation
Data coded in frequency domain
N carriers
B
Transformation to time domain:each frequency is a sine wavein time, all added up.
f
Transmit
Symbol: 8 periods of f0
Symbol: 4 periods of f0
Symbol: 2 periods of f0
+
Receivetime
B
Decode each frequencybin separately
Channel frequencyresponse
f
f
Time-domain signal Frequency-domain signal
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 byadding a cyclic prefix to the OFDM frame to be sent. The cyclic prefix consists ofthe L last samples of the frame, which are copied and placed in the beginning ofthe 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 trainingsequence or sending known so-called pilot symbols at predefined sub-carriers. Decoding and de-interleaving
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
Disadvantages of OFDM
Synchronization
Need FFT units at transmitter, receiver
Sensitive to carrier frequency offset
High peak to average power ratio
Application of OFDM
• Worldwide Interoperability for Microwave Access (WiMAX).
• Terrestrial Digital Audio Broadcasting (DVB-T).
• Wireless Metropolitan Area Network (WMAN).(IEEE
802.16d).
• Wireless Local Area Network (WLAN). ETSI HiperLAN.
• Digital Audio Broadcasting (DAB).
• Digital Video Broadcasting (DVB).
• High Definition Television (HDTV).
• Broadband Internet Access