Othoginal frequency Division Multiplexing

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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]

    http://www.answers.com/topic/frequency-division-multiplexinghttp://www.answers.com/topic/tdmahttp://www.answers.com/topic/tdmahttp://www.answers.com/topic/frequency-division-multiplexing
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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]

    http://www.answers.com/topic/cdma-abbreviationhttp://www.answers.com/topic/cdma-abbreviation
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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