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COLLEGE OF MATHEMATICAL SCIENCES &
INFORMATION TECHNOLOGY
AHLIA UNIVERSITY
Orthogonal Frequency Division
Multiplexing (OFDM)
By
Eng. Husein A.A.Alenzi
Submitted in partial fulfillment of the requirements for
M.Sc Degree in Information Technology at the AHLIA
UNIVERSITY
Advisor:Dr. Ahmed J. Jameel
April 25, 2009
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Acknowledgements
This project would not have been possible without the support of many people.
Many thanks to my advisor Dr. Ahmed J. Jameel, who has been with me in every
step of the project and helped me to finally come up with this completed project.
Also thanks to the entire member of AHLIA UNIVERSITY faculty , staff and
Especially to Dr. Abdulla Al Hawajfor his great effort and support in maintaining
The quality of the whole learning process at the university.
Also thanks to my friendMuneer Aljufiri
for his support and my friendAbdullah
AlenzChairmanof subbiya TX radio station for his support & thanks for my friend
Faleh Almutterifor his support also.
And thanks a lot to my AuntTariyah Aldahok, which have always encourage me to
complete the Master's degree. And thanks also to my Father and my brotherAbudulhameed to fully support.
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ABSTRACT
Orthogonal Frequency Division Multiplexing (OFDM) is a communications technique
that divides a communications channel into a number of equally spaced frequency
bands. A subcarrier carrying a portion of the user information is transmitted in each
band. Each subcarrier is orthogonal (independent of each other) with every other
subcarrier.
OFDM is a multi-carrier modulation technique that is unlike other modulation
techniques. In OFDM, the carriers have substantial overlap. For each single high
frequency carrier used, OFDM transmits multiple high data rates signals concurrently
using sub carriers. The sub-carriers are orthogonal with each other and hence do notinterfere with each other.
In recent years Orthogonal Frequency Division Multiplexing (OFDM) has gained a
lot of interest in digital communication application. This has been due to its properties
like high spectral efficiency and robustness to channel fading. Today OFDM is
mainly used in digital audio broadcasting (DAB), digital video broadcasting (DVB),
Wireless Local Area Networks (WLAN), and other high speed data application for
both wireless and wired communications.
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Table Of Contents
CHAPTER .1. Introduction----------------------------------------------------------------1
History of OFDM---------------------------------------------------------2Multiple Access Techniques--------------------------------------------3
CHAPTER . 2. OFDM (Orthogonal Frequency Division Multiplexing-------------5
2.1Introduction----------- - -------------------------------------------------62.2OFDM (Orthogonal Frequency Division Multiplexing)-----------72.3The principle of OFDM--------------------------------------------------72.4OFDM Transmitter-------------------------------------------------------8
2.4.1 series and parallel converter-----------------------82.4.2 Quadrature phase shift keying (QPSK)- -------92.4.3 Fast Fourier Transform in OFDM --------------102.4.4 Guard Interval and Cyclic Extension-----------11
CHAPTER .3. Modulation & Coding in OFDM---------------------------------------14
3.1 Introduction---------------------------------------------------------------153.2 Modulation----------------------------------------------------------------15
3.2.1 Amplitude Shift Key Modulation------------------153.2.2 Phase Shift Key Modulation------------------------163.2.3 Quadrature Amplitude Modulation--------------17
3.3 Coding in OFDM---------------------------------------------------------203.4 Convolutional Encoding------------------------------------------------213.5 Concatenated coding----------------------------------------------------21
CHAPTER .4. OFDM Applications-------------------------------------------------------22
4.1 Digital Audio Broadcasting (DAB)-----------------------------------234.2 Digital Video Broadcasting (DVB)-----------------------------------244.3 OFDM for Wireless LAN-----------------------------------------------27
4.3.1 MAGIC WAND---------------------------------------284.3.2 MAGIC WAND Physical layer--------------------28
4.4 ADSL System-------------------------------------------------------------29ltiplexing)Design OFDM(Orthogonal Frequency Division MuCHAPTER .5.
using SIMULINK-----------------------------------------------------------------------------30
5.1 Design OFDM 4QAM using SIMULINK .- -----------------------------34
5.2 IQ MAPPER---------------------------------------------------------------345.3 OFDM Modulation-------------------------------------------------------36
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5.4 The AWGN Channel-----------------------------------------------------385.5 OFDM Demodulator-----------------------------------------------------455.6 IQ Demapper--------------------------------------------------------------47
CHAPTR .6. CONCLUSION---------------------------------------------------------------58
6.1 Conclusion-----------------------------------------------------------------59
6.2.Future work --------------------------------------------------------------60
References -------------------------------------------------------------------------------------62
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Table of Figure
Figure 2.1. Concept of OFDM signal (a) Conventional Multi-carrier technique
(b) orthogonal Multi-carrier modulation technique.[1]---------------------------------3
Figure 2.2 The principle of OFDM[5]------------------------------------------------------6
Figure 2.3. A modulation scheme-----------------------------------------------------------7
Figure 2.4. OFDM Transmitter[7]---------------------------------------------------------7
Figure 2.5 QPSK Transmitter--------------------------------------------------------------8
Figure 2.7 OFDM symbol duration.[5]--------------------------------------------------9
Figure 2.8 Guard Interval and Cyclic Extension[6]----------------------------------11
Figure 2.9 Guard Interval.[5]-------------------------------------------------------------12
Figure 2.10 Effect of multipath with zero signals in the guard interval[7]--------12
Figure 2.11 . Time and frequency representation of OFDM with guard
intervals.[7]-------------------------------------------------------------------------------------13
Figure 3.1: Amplitude Shift Key Modulation[14]--------------------------------------13
Figure 3.2: Phase Shift Key Modulation[14]--------------------------------------------15
Figure 3.3. QAM transmitter[16].---------------------------------------------------------16
Figure 3.4 QAM - Quadrature Amplitude Modulation[17].-------------------------17
Fig. 3.5. 4-QAM constellation[18]--------------------------------------------------------17
Fig. 3.5. Example OFDM waveform produced by [0 0 0 1 1 0 1 1].[18].----------18
Figure (3.6 ) Two dimensional coding for OFDM.[1]----------------------------------19
Fig ( 3.7 )Concatenated coding with interleaving.--------------------------------------20
Figure 4.1.spectrum of a Digital Radio Signal.[19].------------------------------------21
Figure 4.2. spectrum of a DVB Signal.[20].---------------------------------------------24
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Figure5.1 . OFDM BLOCK DIGRAM MAIN SYSTEM -----------------------------26
Figure 5.2 (function block parameter) -------------------------------------------------34
Figure 5.3 (function block parameter)-------------------------------------------------34
Figure 5.4 (function block parameter)-------------------------------------------------35
Figure 5.5 (IQ MAPPER)-----------------------------------------------------------------35
Figure 5.6 (function block parameter)-------------------------------------------------36
Figure 5.7 (function block parameter)-------------------------------------------------36
Figure 5.8 (function block parameter)------------------------------------------------37
Figure 5.9 OFDM Modulation--------------------------------------------------------------38
Figure 5.10 (function block parameter)------------------------------------------------38
Figure 5.11 (function block parameter)------------------------------------------------39
Figure 5.12 (function block parameter)-----------------------------------------------40
Figure 5.13 (function block parameter)------------------------------------------------41
Figure 5.14 Matrix Concatenation---------------------------------------------------------42
Figure 5.15 (function block parameter)------------------------------------------------42
Figure 5.16 (function block parameter)------------------------------------------------43
Figure. 5.17- Add Cyclic Prefix------------------------------------------------------------44
Figure 5.18 (function block parameter)------------------------------------------------44
Figure.5.19- The AWGN Channel--------------------------------------------------------45
Figure 5.20 (function block parameter)------------------------------------------------45
Figure. 5.21- OFDM Demodulator--------------------------------------------------------46
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Figure 5.22 (function block parameter)------------------------------------------------47
Figure 5.23 (function block parameter)------------------------------------------------47
Figure 5.24 (function block parameter)------------------------------------------------48
Figure 5.25 (function block parameter)------------------------------------------------48
Figure 5.26 (function block parameter)------------------------------------------------49
Figure. 5.27- IQ Demapper-----------------------------------------------------------------49
Figure.5.28 - Data Sink-----------------------------------------------------------------------50
Figure. 5.29-OFDM (Orthogonal Frequency Division Multiplexing) 4QAMusingSIMULINK-------------------------------------------------------------------------------------50
Figure. 5.30.Resulats ------------------------------------------------------------------------52
Figure. 5.31. system performance test ---------------------------------------------------53
Figure. 5.32.Resulats ------------------------------------------------------------------------54
Figure. 5.33 Resulats ------------------------------------------------------------------------55
Figure 5.34 .4QAM with BER.-------------------------------------------------------------56
Figure 5.35. Symbol error probability curve for QPSK(4-QAM)-------------------57
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CHAPTER .1.
Introduction
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1.1History of OFDM
Orthogonal frequency-division multiplexing, or OFDM, is a process of digital
modulation that is used in computer technology today. Essentially, OFDM is
configured to split a communication signal in several different channels. Each of these
channels is formatted into a narrow bandwidth modulation, with each channel
operating at a different frequency. The process of OFDM makes it possible for
multiple channels to operate within close frequency levels without impacting the
integrity of any of the data transmitted in any one channel .
The history of OFDM goes back to the 1960s. At the time, there was a need to make
more efficient use of bandwidth transmissions without creating situations where
signals would be subject to a phenomenon referred to as crosstalk. Essentially,
crosstalk occurs when two audio sources are broadcasting at the same time. The end
result is that the message of each broadcast is partially obscured for anyone
attempting to listen to either of the messages. Crosstalk can be compared to two
people choosing to speak while another individual is already speaking .[1]
Generally, the process of OFDM is focused on preventing the occurrence of crosstalk,
or any other type of outside interference with the quality of the transmission.
However, the method does have some limited capability to attempt to enhance the
quality of the transmission proper. For example, it is sometimes possible to make use
of OFDM in order to minimize background noise that is resident in the transmission,
or to boost the volume level if the transmission has weak sound clarity .
The use of OFDM is common worldwide. Many radio networks around the globe
make use of OFDM to service their broadcast ranges. Some amateur radio systems
also employ elements of OFDM for sending out signals as well. There are some
applications of OFDM that lend well to the audio component of digital television, and
it is also possible to make use of OFDM to boost the speed of an Internet connection
over a standard telephone line. With the emergence of more wireless methods of
communication, OFDM is also finding a place in local wireless networks. [2]
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1.2 Multiple Access Techniques
A limited amount of bandwidth is allocated for wireless services. A wireless system is
required to accommodate as many users as possible by effectively sharing the limited
bandwidth. Therefore, in the field of communications, the term multiple access could
be defined as a means of allowing multiple users to simultaneously share the finite
bandwidth with least possible degradation in the performance of the system.[3]
Figure 1. A schematic comparison of FDMA, TDMA, and CDMA multiple-
access techniques.[4]
In frequency-division multiplexing (FDM) and frequency-division multiple access
(FDMA), the passband of a channel is shared among multiple users by assigning
distinct and nonoverlapping sections of the electromagnetic spectrum within the
passband to individual users. The information stream from a particular user is
encoded into a signal whose energy is confined to the part of the passband assigned to
that user.[4]
Time-division multiplexing (TDM) and time-division multiple access (TDMA) permit
a user access to the full passband of the channel, but only for a limited time, after
which the access right is assigned to another user. Normally the access rights are
assigned in a cyclical order to the competing users. However, statistical time-division
multiplexing assigns time on the channel on a demand basis, which typically increases
the number of users who may be accommodated on the same channel, but may result
in delays in accessing the channel during periods when the demand exceeds the
supply.[4]
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In code-division multiple access (CDMA), all users are assigned the entire passband
of the channel and are permitted to transmit their information streams simultaneously.
To maintain the ability to recover the individual signals at the receiver, at the
transmitter each signal has impressed on it a characteristic signature.[3]
OFDMA is a multi-user OFDM that allows multiple access on the same channel uses.
OFDMA distributes subcarriers among users so all users can transmit and receive at
the same time within a single channel on what are called subchannels.
OFDM overcomes most of the problems with both FDMA and TDMA. OFDM
divides the available bandwidth into many narrow band channels . The carriers for
each channel are made orthogonal to each other, allowing them to be spaced very
close together. The orthogonality of the carriers means that each carrier has an integer
number of cycles over a symbol period. Due to this, the spectrum of each carrier has a
null at the centre frequency of each of the other carriers in the system. This results in
no interference between the carriers, allowing then to be spaced as close as
theoretically possible. This overcomes the problem of overhead carrier spacing
required in FDMA. Each carrier in an OFDM signal has a very narrow bandwidth (i.e.
1 kHz), thus the resulting symbol rate is low. This will give the signal a high tolerance
to Multipath delay spread, because the delay spread must be very long to cause
significant inter-symbol interference.[2]
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CHAPTER . 2.
OFDM (Orthogonal Frequency DivisionMultiplexing)
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2.1 Introduction
Orthogonal Frequency Division Multiplexing(OFDM), beginning with short
description of OFDM technology . The multiplexing is a technique that allows the
simultaneous transmission of multiple signal across a single data link. The Orthogonal
Frequency Division Multiplexing is a communication technique thatdivides a channel
into a number of equally spaced frequency band.
The OFDM is used mainly for transmission of digital data is currently used in digital
audio broad casting (DAB) .
The idea is to used large number of parallel narrow band subcarriers instead of a
single wide band carrier to transport information.
OFDM is multi carrier modulation technique that is unlike other modulation
technique .In OFDM the carrier have substantial overlap .For each single high
frequency carrier used, OFDM transmits multiple high data rates signals concurrently
using sub carriers.[5,7,8]
Figure 2.1. Concept of OFDM signal (a) Conventional Multi-carrier technique
(b) orthogonal Multi-carrier modulation technique.[1]
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OFDM (Orthogonal Frequency Division Multiplexing)
Orthogonal frequency-division multiplexing, or OFDM, is a process of digital
modulation that is used in computer technology today. Essentially, OFDM is
configured to split a communication signal in several different channels. Each of these
channels is formatted into a narrow bandwidth modulation, with each channel
operating at a different frequency. The process of OFDM makes it possible for
multiple channels to operate within close frequency levels without impacting the
integrity of any of the data transmitted in any one channel.
2.3 The principle of OFDM:
Figure 2.2 The principle of OFDM[5]
Suppose that this transmission takes four seconds. Then, each piece of data in the left
picture has a duration of one second.On the other hand, OFDM would send the fourpieces simultaneously as shown on the right. In this case, each piece of data has a
duration of four seconds. [5.6.7]
A modulation scheme is a mapping of data words to a real (In phase) and imaginary
(Quadrature) constellation, also known as an IQ constellation. Each data word is
mapped to one unique IQ location in the constellation.[5,6]
Figure 2.3. A modulation scheme
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2.4 OFDM Transmitter
Figure 2.4. OFDM Transmitter[7]
2.4.1 series and parallel converter
In OFDM system design, the series and parallel converter is considered to realize the
concept of parallel data transmission.
Example the if input :x=[0,0,0,1,1,0,1,1,.]
The output will be a parallel :x1=[0,0]x2=[0,1]x3=[1,0]x4=[1,1] ..
Series:
In a conventional serial data system, the symbols are transmitted sequentially, with
the frequency spectrum of each data symbol allowed to occupy the entire available
bandwidth.
When the data rate is sufficient high, several adjacent symbols may be completely
distorted over frequency selective fading or multipath delay spread channel. [5,6.11]
Parallel:
The spectrum of an individual data element normally occupies only a small part of
available bandwidth.
Because of dividing an entire channel bandwidth into many narrow sub bands, the
frequency response over each individual sub channel is relatively flat.
A parallel data transmission system offers possibilities for alleviating this problem
encountered with serial systems. [5,6,11]
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2.4.2 Quadrature phase shift keying (QPSK)
QPSK is a method for transmitting digital information across an analog channel. Data
bits are grouped into pairs, and each pair is represented by a particular waveform,
called a symbol, to be sent across the channel after modulating the carrier. The
receiver will demodulate the signal and look at the recovered symbol to determine
which pair of bits was sent. This requires having a unique symbol for each possible
combination of data bits in a pair. Because there are four possible combinations of
data bits in a pair, QPSK creates four different symbols, one for each pair, by
changing the I gain and Q gain for the cosine and sine modulators .
The QPSK transmitter system uses both the sine and cosine at the carrier frequency to
transmit two separate message signals, sI[n] and sQ[n], referred to as the in-phase and
quadrature signals. Provided that a coherent receiver system is employed, both the in-
phase and quadrature signals can be recovered exactly, allowing us to transmit twice
the amount of signal information at the same carrier frequency as we could with a
single oscillator.
Figure 2.5 QPSK Transmitter[8]
2.4.3 Fast Fourier Transform in OFDM
Why do we use FFT in OFDM system?.To spread the data in time. And because its
faster than a DFT .
The fast Fourier transform (FFT) is merely a rapid mathematical method for computer
applications of DFT. It is the availability of this technique, and the technology that
allows it to be implemented on integrated circuits at a reasonable price, that has
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permitted OFDM to be developed as far as it has. The process of transforming from
the time domain representation to the frequency domain representation uses the
Fourier transform itself, whereas the reverse process uses the inverse Fourier
transform. [8]
The use of the Fast Fourier Transform in OFDM
OFDM systems are implemented using a combination of fast Fourier Transform
(FFT) and inverse fast Fourier Transform (IFFT) blocks that are mathematically
equivalent versions of the DFT and IDFT, respectively, but more efficient to
implement. An OFDM system treats the source symbols (e.g., the QPSK or QAM
symbols that would be present in a single carrier system) at the transmitter as though
they are in the frequency-domain.
These symbols are used as the inputs to an IFFT block that brings the signal into the
time-domain. The IFFT takes in N symbols at a time where N is the number of
subcarriers in the system. Each of these N input symbols has a symbol period of T
seconds. Recall that the basis functions for an IFFT are N orthogonal sinusoids. These
sinusoids each have a different frequency and the lowest frequency is DC. Each input
symbol acts like a complex weight for the corresponding sinusoidal basis function.
Since the input symbols are complex, the value of the symbol determines both the
amplitude and phase of the sinusoid for that subcarrier. The IFFT output is the
summation of all N sinusoids. Thus, the IFFT block provides a simple way to
modulate data onto N orthogonal subcarriers. The block of N output samples from the
IFFT make up a single OFDM symbol. The length of the OFDM symbol is NT where
T is the IFFT input symbol period mentioned above.
After some additional processing, the time-domain signal that results from the IFFT is
transmitted across the channel. At the receiver, an FFT block is used to process the
received signal and bring it into the frequency-domain. Ideally, the FFT output will be
the original symbols that were sent to the IFFT at the transmitter. When plotted in the
complex plane, the FFT output samples will form a constellation, such as 16-QAM.
However, there is no notion of a constellation for the time-domain signal. When
plotted on the complex plane, the time-domain signal forms a scatter plot with no
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regular shape. Thus, any receiver processing that uses the concept of a constellation
(such as symbol slicing) must occur in the frequency-domain. [8]
Following are equations of Discrete Fourier Transform (DFT) and Inverse Discrete
Fourier Transform (UDFT). N points x(n) signal is transformed to N points X(k) by
DFT. Fast computation algorithm of DFT is Fast Fourier Transform (FFT). But, FFT
needs the restriction N=2l(l=integer). IFFT is Fast computation algorithm of IDFT.
The IFFT & FFT equations can be written as follows:
IFFT X(k) = )
FFT X(n) = )
2.4.4 Guard Interval and Cyclic Extension:
Figure 2.6 OFDM symbol duration.[5]
Two different sources of interference can be identified in the OFDM system.
Inter symbol interference (ISI) is defined as the crosstalk between signals within the
same sub-channel of consecutive FFT frames, which are separated in time by the
signaling interval T.
gT T
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Inter-carrier interference (ICI) is the crosstalk between adjacent sub channels or
frequency bands of the same FFT frame.[7.11]
Figure 2.7 Guard Interval and Cyclic Extension[6]
To eliminate ICI, the OFDM symbol is cyclically extended in the guard interval. This
ensures that delayed replicas of the OFDM symbol always have an integer number of
cycles within the FFT interval, as long as the delay is smaller than the guard interval.
[6,10,11]
Figure 2.9 Guard Interval.[5]
I f T g < T dely-spread
T g S y m b o l 1 T g S y m b o l 2 T g S y m b o l 3 T g S y m b o l 4
T d e l y - s p r e a d
I f T g > T d e l y - s p r e a d
T g S y m b o l 1 T g S y m b o l 2 T g S y m b o l 3
T g S y m b o l 1 T g S y m b o l 2 T g S y m b o l 3 T g S y m b o l 4
T g S y m b o l 1 T g S y m b o l 2 T g S y m b o l 3
T d e l y - s p r e a d
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Effect of multipath with zero signals in the guard interval, the delayed subcarrier 2
causes ICI on subcarrier 1 and vice versa.[7,9,11]
Figure 2.8 Effect of multipath with zero signals in the guard interval[7]
Figure 2.9 . Time and frequency representation of OFDM with guardintervals.[7]
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CHAPTER .3.
Modulation &
Coding in OFDM
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3.1 Introduction
Modulation and channel coding are very important in a digital communication system.
Modulation is the process of mapping the digital information to analog form, so it can
be transmitted over the channel. The inverse process called demodulation, done by the
receiver to recover the transmitted digital information. An OFDM system performs
modulation and demodulation for each subcarrier separately, and usually in serial
form to reduce complexity.
3.2 Modulation
Modulation can be done by changing the amplitude, phase, or frequency of
transmitted radio channel signal. In the case of OFDM system the first two methods
can be used, but frequency modulation can not be used because subcarriers are
orthogonal in frequency and carry independent information. Modulating the carrier
frequency will destroy the orthogonality between the subcarriers; this makes
frequency modulation unusable for OFDM systems[ 2].
3.2.1 Amplitude Shift Key Modulation
In this method the amplitude of the carrier assumes one of the two amplitudes
dependent on the logic states of the input bit stream. A typical output waveform of an
ASK modulator is shown in the figure below. The frequency components are the USB
and LSB with a residual carrier frequency. The low amplitude carrier is allowed to be
transmitted to ensure that at the receiver the logic 1 and logic 0 conditions can be
recognised uniquely.[14]
Figure 3.1: Amplitude Shift Key Modulation[14]
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3.2.2 Phase Shift Key Modulation
With this method the phase of the carrier changes between different phases
determined by the logic states of the input bit stream.
There are several different types ofphase shift key(PSK) modulators.
Two-phase (2 PSK) Four-phase (4 PSK) Eight-phase (8 PSK) Sixteen-phase (16 PSK) Sixteen-quadrature amplitude (16 QAM)
The 16 QAM is a composite modulator consisting of amplitude modulation and phase
modulation. The 2 PSK, 4 PSK, 8 PSK and 16 PSK modulators are generally referred
to as binary phase shift key (BPSK) modulators and the QAM modulators are referred
to as quadrature phase shift key(QPSK) modulators.
Two-Phase Shift Key Modulation
In this modulator the carrier assumes one of two phases. A logic 1 produces no phase
change and a logic 0 produces a 180 phase change. The output waveform for this
modulator is shown below.
Figure 3.2: Phase Shift Key Modulation[14]
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3.2.3 Quadrature Amplitude Modulation
QAM (quadrature amplitude modulation) is a method of combining two amplitude-
modulated (AM) signals into a single channel, thereby doubling the effective
bandwidth. QAMis used with pulse amplitude modulation (PAM)in digital systems,
especially in wirelessapplications.
In a QAM signal, there are two carriers, each having the same frequency but differing
in phase by 90 degrees (one quarter of a cycle, from which the term quadrature
arises). One signal is called the I signal, and the other is called the Q signal.
Mathematically, oneof the signals can be represented by a sine wave, and the other
by a cosine wave. The two modulated carriers are combined at the source for
transmission. At the destination, the carriers are separated, the data is extracted from
each, and then the data is combined into the original modulating information. [15].
Figure 3.3. QAM transmitter[16].
Figure 3.4 QAM - Quadrature Amplitude Modulation[17].
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This is the most complicated step in the OFDM system. The binary stream must be
converted to an actual OFDM waveform. The technique used in this simulation is
known as QAM or quadrature amplitude modulation. Before this technique can be
implemented, the binary stream created in the previous step must be separated into
blocks of 8-bits. Then this block of 8-bits must be further broken down into sets of 2-
bits. These 2-bit sets are converted into a waveform using Equation and Fig.3.5
.[18]
( ) ( ) ( )tBtAts 00 sincos +=
Fig. 3.5. 4-QAM constellation[18]
The 2-bits sets will be 1 of 4 combinations, [0 0], [0 1], [1 0], or [1 1]. Depending on
which combination it is, A and B will either be a 1 or a 1 as seen in Fig. 3.5. The
values of A and B will then make up the waveform whose equation is given by
Equation (10). Once this has been done, only one 2-bit set of the 8-bit block has been
converted into a waveform. This must be done for all 4 2-bit sets within the 8-bit
block. Each resulting waveform created using Equation (10) is given a different
frequency (0) depending on which 2-bit set is currently being manipulated. The first
2-bit set is given a low frequency and the next 2-bit set is given a higher frequency
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and so on. The 4 resulting waveforms at 4 orthogonal frequencies will then be added
together to produce the actual OFDM waveform.
There is one unique feature of OFDM that makes this whole process different from
any other technique. The waveform construction is done entirely in the frequency
domain on the real and imaginary axes. Taking the IFFT of the frequency domain
information then produces the waveforms. The following table summarizes how each
2-bit set is transformed into a waveform. The frequency domain representation can be
found in most digital communication textbook.[18]
TABLE 3.1. Representation of waveforms in time and frequency domains.[18]
Binary
Word
Time Domain
Representation
Frequency Domain Representation
00 ( ) ( )tt 00 sincos + )()( 021
21
021
21 ffjffj +++
01 ( ) ( )tt 00 sincos + )()( 021
21
021
21 ffjffj ++
10 ( ) ( )tt 00 sincos )()( 0212102121 ffjffj ++
11 ( ) ( )tt 00 sincos )()( 021
21
021
21 ffjffj ++
Once the waveforms are constructed in the frequency domain, an IFFT operation is
performed producing the actual time domain waveforms. The time domain
representation is shown in Fig. 3.5 below.
Fig. 3.5. Example OFDM waveform produced by [0 0 0 1 1 0 1 1].[18].
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Fig. 3.5 shows one possible OFDM waveform. Each waveform will be different
depending on what the 8-bit word is. Recall this process needs to be done for each 8-
bit block until the entire binary stream has been covered.
3.3 Coding in OFDM
To achieve satisfactory performance in application of OFDM, the addition of some
form of coding is needed. High signal to noise ratio are required to achieve reasonable
bit error rate in the presence of fading channel. Wireline systems, usually use large
constellation size to achieve high bit rates. Coding in this case is essential for
achieving the highest possible rates in the presence of noise and interference.
Proper coding is very important for OFDM. There are several factors should be taken
into account, such as the required coding gain, channel characteristics, source coding
requirement, modulation.[1]
In OFDM system, coding can be implemented in time and frequency domain.
Interleaving play key role to achieve the above goal as shown in figure (3.6 )
Impulse response in each ineach time/frequency bin
Figure (3.6 ) Two dimensional coding for OFDM.[1]
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3.4 Convolutional Encoding
The purpose of a convolutional encoder is to take a single or multi-bit input and
generate a matrix of encoded outputs. One reason why this is important is that in
digital modulation communications systems (such as wireless communication
systems, etc.) noise and other external factors can alter bit sequences. By adding
additional bits we make bit error checking more successful and allow for more
accurate transfers. By transmitting a greater number of bits than the original signal
we introduce a certain redundancy that can be used to determine the original signal in
the presence of an error. For our illustration we will assume a 5-bit input and rate-1/2
code (two output bits for every input bit). This will yield a 2x5 output matrix, with
the extra bits allowing for the correction.
3.5. Concatenated coding
Combining convolutional and block codes in a concatenated code is a particularly
powerful technique. The block code is the outer code, that is applied first at the
transmitterand last at the receiver. The inner convolutional code is very effective at
reducing the error probability, particularly when soft decision decoding is employed.
Figure (3.7 )show concatenated coding with interleaving.
Fig ( 3.7 )Concatenated coding with interleaving.
However when a convolutional code make an error, it apears as a large burst. This
occurs when the Viterbi algorithm chooses a wrong sequence. The outer block code ,
especially an interleaved Reed-Solomon code, is then very effective in correcting that
burst error. For a maximum effectiveness the two codes should be interleaved, with
different interleaving patterns[30].
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CHAPTER .4.
OFDM Applications
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4.1 Digital Audio Broadcasting (DAB)
Current analog FM radio broadcasting system cannot satisfy
the demands of the future, which are
- Excellent sound quality
- Large number of stations
- Small portable receivers
- No quality impairment due to multipath propagation or signal fading.
Current analog FM radio broadcasting systems have reached the limits of technical
improvement.
- DAB is a digital technology offering considerable advantages over today'sFM radio.
Digital audio broadcasting (DAB), also known as digital radio and high-definition
radio, is audiobroadcasting in which analogaudio is converted into a digital signal
and transmitted on an assigned channel in the AM or (more usually) FM frequency
range. DAB issaid to offer compact disc (CD)- quality audio on the FM (frequency
modulation) broadcast band and to offer FM-quality audio on the AM (amplitude
modulation) broadcast band. The technology was first deployed in the United
Kingdomin 1995, and has become commonthroughout Europe. [15]
Digital audio broadcast signals are transmittedin-band, on-channel (IBOC). Several
stations can be carried within the same frequency spectrum. Listeners must have a
receiver equipped to handle DAB signals. At the transmitting site, the signal is
compressed using MPEG algorithms and modulated using coded orthogonal
frequency division multiplexing (COFDM).A digital signal offers severaladvantages
over conventional analog transmission, including improved sound quality, reduced
fading and multipath effects, enhanced immunity to weather, noise, and other
interference, and expansionof the listenerbaseby increasing the numberof stations
that can broadcast within a given frequency band. [15]
A DAB receiver includes a small display that provides information about the audio
content in much the same way that the menuscreen provides an overview of programs
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in digital television (DTV). Some DAB stations provide up-to-the-minute news,
sports, and weather headlines or bulletins in a scrolled text format on the display.
Using the DAB information, it may also be possible to see what song is coming up
next.
Figure 4.1.spectrum of a Digital Radio Signal.[19].
4.2. Digital Video Broadcasting (DVB)
Digital Video Broadcasting (DVB) is a set of standards that define digital
broadcasting using existing satellite, cable, and terrestrial infrastructures. In the early
1990s, European broadcasters, consumer equipment manufacturers, and regulatory
bodies formed the European Launching Group (ELG) to discuss introducing digital
television(DTV) throughout Europe. The ELG realized that mutual respect and trust
had to beestablished between members later became the DVB Project. Today, the
DVB Project consists of over 220 organizations in more than 29 countries worldwide.
DVB-compliant digital broadcasting and equipment is widely available and is
distinguished by the DVB logo. Numerous DVB broadcast services are available in
Europe, North and South America, Africa, Asia, and Australia. The term digital
television is sometimes used as a synonym for DVB. However, the Advanced
Television Systems Committee (ATSC) standard is the digital broadcasting standard
used in the U.S. [15]
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A fundamental decision of the DVB Project was the selection of MPEG-2, one of a
series of MPEG standards for compression of audio and video signals. MPEG-2
reduces a single signal from166Mbits to 5 Mbits allowing broadcasters to transmit
digital signals using existingcable, satellite, and terrestrial systems. MPEG-2 uses the
lossy compressionmethod, whichmeans that the digital signal sent to the television is
compressed and some data is lost. This lost data does not affect how the human eye
perceives the picture. Two digital television formats that use MPEG-2 compression
are standard definition television (SDTV) and high definition television (HDTV).
SDTV's picture and sound quality is similar to digital versatile disk (DVD). HDTV
programming presents five times as much information to the eye than SDTV,
resulting in cinema-quality programming.
DVB uses conditional access (CA) systems to prevent external piracy. There are
numerous CA systems available to content providers allowing them to choosethe CA
system that they feel is adequate for the services they provide. Each CA system
provides a security module that scrambles and encrypts data. This security module is
embedded within the receiver or is detachable in the form of a PC Card. Inside the
receiver, there is a smart card that contains the user's access information. The
following describes the conditional access process:
- The receiver receives the digital data stream.- The data flowsinto the conditional access module, which contains thecontent
provider's unscrambling algorithms.
- The conditional access module verifies the existence of a smart card thatcontains the subscriber's authorization code.
- If the authorization code is accepted, the conditional access moduleunscrambles the data and returns the data to the receiver. If the code is not
accepted, the data remainsscrambled restricting access.
- The receiver then decodesthedata andoutputs it for viewing.For years, smart cards have been used for pay TV programming. Smart cards are
inexpensiveallowing the content provider to issue updated smart cards periodically to
prevent piracy. Detachable PC cardsallow subscribers to use DVB services anywhere
DVB technology is supported. [15]
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DVB is an opensystem as opposed to a closed system. Closed systems are content
provider-specific, not expandable, and optimized only for television. Open systems
such as DVB allows the subscriber to choose different content providers and allows
integration of PCs and televisions. DVB systems are optimized for not only television
but also for home shopping and banking, private network broadcasting, and
interactive viewing. DVB offers the future possibilities of providing high-quality
television display in buses, cars, trains, and hand-held devices. DVB allows content
providers tooffer their services anywhere DVB is supported regardless of geographic
location, expand their services easily and inexpensively, and ensure restricted access
to subscribers, thus reducing lost revenue due to unauthorized viewing.[15].
Figure 4.2. spectrum of a DVB Signal.[20].
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4.3 OFDM for Wireless LAN
Multicarrier modulation is a strong candidate for packet switched wireless
applications and offers several advantages over single carrier approaches. For higher
data rate applications ranging from 10Mb/s up to 50Mb/s, an OFDM system is viable
for the following reasons:
- Robustness against delay spread: Data transmission in wireless environmentexperience delay spread up to 800ns which cover several symbols at baud
rates of 10Mb/s and higher. In a single carrier system an equalizer handle
detrimental effects of delay spread. Where delay spread is more than 4
symbols, use of maximum likelihood sequence estimator structure is not
practical due to its exponentially increasing complexity [23]. Linear equalizer
is not suitable for this application either since in a frequency selective channel
it amounts to significant noise enhancement [22,24]. Hence other equalizer
structure such as decision feedback equalizer are used. Number of taps of the
equalizer should be enough to cancel the effect of inter-symbol interference
and perform as a matched filter too. In addition , equalizer coefficient should
be trained for every packet, as the channel characteristics are different for each
packet. A large header is usually needed to guarantee the convergence of a
adaptive training techniques [23]. A multicarrier system is robust against delay
spread and does not need a training sequence. Channel estimation is required
however.
- Fall-back mode: Depending on the delay spread of different applications adifferent number of carriers is required to null the effect of delay spread.
- Computational efficiency: Use of FFT structure at the receiver reduce thecomplexity toNlog2N . As the number of carrier grows the higher efficiency
can be achieved.
- Fast synchronisation : OFDM receivers are less sensitive to timing jittercompared to spread spectrum techniques.
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4.3.1. MAGIC WAND
The Magic WAND( Wireless ATM Network Demonstrator) project was part of the
European ACTS ( Advanced communications technology and Server) program. The
Magic WAND consortium members implemented a prototype wireless ATM network
based on OFDM modulation. This prototype had a large impact on standardization
activities in the 5GHz band. First by employing OFDM based modems, Magic
WAND helped to gain acceptance for OFDM as viable modulation type for high rate
wireless communications[21]. Second , the wireless ATM based approach of Magic
WAND forms the basis for the standardization of the HIPERLAN type 2 Data Link
Layer.
4.3.2 MAGIC WAND Physical layer
The main parameter of the WAND physical layer are listed in Table (4.1 ). OFDM
with 16 subcarrier is used, the number of which was chosen to facilitate
implementation. The 400ns guard time provide a delay spread tolerance of about
50ns. Because of a 240ns rolloff time, the effective guard times is only 160ns. While
this is sufficient for most office building and the WAND trial site, a realistic product
would require more delay spread robustness to also cover large office building and
factory halls[22 ].
The OFDM subcarriers are 8-PSK modulated. At a symbol rate of 13.3 MS/s, this
give a raw bit rate of 40Mb/s. The rate complementary coding reduces the data rate
to 20Mb/s. The subcarrier spacing is 1.25MHz, which gives a total (3-dB) bandwidth
of 20MHz. The packet preamble is 8.4/zs in duration and consist of one OFDM
symbol, repeated seven
times. This preamble is used for packet detection, automatic gain control, frequency
offset estimation, symbol timing, and channel estimation.
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Table (4.1 ) Main parameter of the WAND OFDM modem
Number of subcarriers 16
Modulation 8-PSK
Coding Two interleaved length 8
Complementary codes, rate
Bit rate (after decoding) 20Mb/s (24 bits per symbol)
Guard time 0.4jus
Symbol time 1.2jus
Widowing Raised cosine, rolloff factor =0.2
Subcarrier spacing 1.25MHz
Training length 7 symbols
Carrier frequency 5.2GHz
Peak output power 1w
The PHY payload holds an odd number of half-slots. Each half slots consist of 9
symbols or 27 bytes. This number was chosen so that a full slot of 54 bytes can hold
an ATM cell(which is 52 bytes long), and is also a multiple of 3 bytes, which is
imposed by the PHYs modulation scheme[22].
4.4 ADSL System
A ubiquitous communication channel is the subcarrier line, or loop consisting of an
unshielded twisted pair of wires, connecting any home or office to a telephone
companys central office. The overwhelming majority of the channels are used to caryanalogue voice conversation, which require a bandwidth of less than 4KHz. It has
been recognized that most subcarrier lines can support much wider bandwidth
[21,28]. In particular, to cary high rate digital signals. The first such widespread use is
for access to a basic rate ISDN, in which the subcarrier line carries 160Kb/s
simultaneously in both directions over a single pair.[25].
More recently, higher rates have been introduced into numerous systems. Of
particular interest here in ADSL which is primarily intended to provide access for
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residential applications. Most of such applications require a high data rate in the
downstream direction ( to the customer). This primary application of ADSL are the
delivery of digitally encoded video, and access to digital services , particularly the
Internet. ADSL meets these needs by providing a high rate digital downstream signal
over 1Mb/s, a moderate rate upstream signal , and a normal analog voice channel, all
over a single wire-pair. Because virtually all customers have a wire-pair channel
providing voice service, no additional channel need to be installed to provide this new
service. It only require the installation of terminating equipment at the customers
premises and at the central offices. OFDM, typically referred to as DMT ( Discrete
Multi-tone) in this application, has been adopted as the standard for transmission of
the digital information.[27]
Two classes of ADSL have been standardized recently[23], with many options in each.
Full rate ADSL can carry up to approximately 8Mb/s downstream and 800Kb/s
upstream. A simpler class, commonly called ADSL Lite carries up to approximately
1.5Mb/s downstream and 500Kb/s upstream. In both classes, data rates can be
adjusted to any value in steps of 32Kb/s. An analog voice channel is provided on the
same pair. The target error probability is 10"7per bit, with some required margin.
The two classes are somewhat compatible with each other. In both cases subcarriers
are spaced 4312.5Hz apart in both directions. After every 68 frames of data, a
synchronization frame is inserted. Because of this and the cyclic prefixes, the net
useful number of data frames is 4000 per second in all cases. One of the subcarriers of
the frame is devoted to synchronization. Adaptive bit allocation over the subcarriers is
performed in all cases. This process is critical to ensure system performance. In the
full rate downstream direction, a block of 255 complex data symbols. Including
several of value zero, are assembled. These will correspond to sub-channels 1 to 255.
The lower ones can not be used because of the analog voice channels, nor can the 255th
.
Therefor , the highest frequency allowed subcarrier is centered at 1.095MHz.
subcarrier which can not support at least a 4 point constellation at the desired error
probability will also be unused. Conjugate appending is performed on the block
followed by a 512 point DFT. This result in frame of 512 real values. A cyclic prefix
of 32 samples is added, and the resultant 2.208M samples per second transmitted over
the line.
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Upstream, 31 sub-channels are processed, although(gain) the lower few and the 31st
can not be used. The same processing is performed with a cyclic prefix of 4 samples.
The digitally encoded video, and access to digital services , particularly the Internet.
ADSL meets these needs by providing a high rate digital downstream signal over
1Mb/s, a moderate rate upstream signal , and a normal analog voice channel, all over a
single wire-pair. Because virtually all customers have a wire-pair channel providing
voice service, no additional channel need to be installed to provide this new service. It
only require the installation of terminating equipment at the customers premises and at
the central offices. OFDM, typically referred to as DMT ( Discrete Multi-tone) in this
application, has been adopted as the standard for transmission of the digital
information.[26]
Two classes of ADSL have been standardized recently[23], with many options in each.
Full rate ADSL can carry up to approximately 8Mb/s downstream and 800Kb/s
upstream. A simpler class, commonly called ADSL Lite carries up to approximately
1.5Mb/s downstream and 500Kb/s upstream. In both classes, data rates can be
adjusted to any value in steps of 32Kb/s. An analog voice channel is provided on the
same pair. The target error probability is 10"7per bit, with some required margin.
The two classes are somewhat compatible with each other. In both cases subcarriers
are spaced 4312.5Hz apart in both directions. After every 68 frames of data, a
synchronization frame is inserted. Because of this and the cyclic prefixes, the net
useful number of data frames is 4000 per second in all cases. One of the subcarriers of
the frame is devoted to synchronization. Adaptive bit allocation over the subcarriers is
performed in all cases. This process is critical to ensure system performance. In the
full rate downstream direction, a block of 255 complex data symbols. Including
several of value zero, are assembled. These will correspond to sub-channels 1 to 255.
The lower ones can not be used because of the analog voice channels, nor can the 255th
.
Therefor , the highest frequency allowed subcarrier is centered at 1.095MHz.
subcarrier which can not support at least a 4 point constellation at the desired error
probability will also be unused. Conjugate appending is performed on the block
followed by a 512 point DFT. This result in frame of 512 real values. A cyclic prefix
of 32 samples is added, and the resultant 2.208M samples per second transmitted over
the line.
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Upstream, 31 sub-channels are processed, although(gain) the lower few and the 31st
can not be used. The same processing is performed with a cyclic prefix of 4 samples.
The upstream and downstream sub-channels may overlap. This provides a larger data
rate, but require the use of echo cancellation. The bit streams may be treated as several
multiplexed data channels. Each such channel may be optionally Reed-Solomon
coded, with a choice of code and interleaving depth. Other optional codes include a
CRC error check, and a 16 state 4-dimensional trellis code. The trellis code, when
present, operates over the non-zero subcarriers of a block, and is forced to terminate at
the end of each block [23].
ADSL Lite is intended as a simpler lower cost system, with greater range of
coverage because of the lower rate. One important difference is the elimination of
filters at the customers premises to separate the voice and the data channels. The
upstream channel is created identically to that of the full rate system, except that the
first 6 sub-carriers must be zero. The downstream transmitted sampled rate is reduced
by a factor of two, to 1.104M samples per second. The IDFT is performed over an
initial block of 127 complex numbers, of which the first 32 must be zero. The highest
subcarrier is now at 543KHz. In this case, the upstream and downstream sub-carriers
do not overlap. The signal is treated as a single bit stream. Reed-Solomon and CRC
coding are again optional, but there is no trellis coding.
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CHAPTER .5.
Design OFDM (Orthogonal
Frequency Division Multiplexing)
using SIMULINK .
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5.1. Design OFDM (Orthogonal Frequency Division Multiplexing) 4QAMusing
SIMULINK .
From chapter 2 we have a clear idea
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