241 Ofdm for Wirless Communication

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OFDM FOR WIRLESS COMMUNICATION


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ABSTRACT

This thesis investigates the effectiv eness of Orthogonal Frequency Division

Multiplexing (OFDM) as a m odulation technique for wireless radio

applications. The main aim was to assess the suitability of OFDM as amodulation technique for a fixed wire less phone system for rural areas of

Australia. However, its suita bility for m ore general wi reless applications is

also assessed. Most third generation mobile phone systems are proposing to

use Code Division Multiple Acce ss (CDMA) as their m odulation

technique.. It was found t hat OFDM perform s extremel y well compared

with CDMA, providing a very high tolerance to multipath delay spread, peak

power clipping, and channel noise. In addition to this it provi des a high

spectral efficiency. Orthogonal FDM' s (OFDM) spread spectrum technique

distributes the dat a over a large number of carrier s that are spaced apart atprecise frequencies. This spacing prov ides the "orthogonality" in this

technique, which prevents the dem odulators from seeing frequencies other

than their own. The benefits of OFDM are high spectral effici ency,

resiliency to RF interferen ce, and lower m ulti-path distortion. This is useful

because i n a typical terrestrial broa dcasting scenario there are multipath-

channels (i.e. the trans mitted signal a rrives at the receiver using various

paths of different length). Since multiple versions of the signal interfere with

each other (inter sym bol interference (ISI)) it becomes very hard to extract

the original information. Orthogonal FDM deals with this multipath problem

by splitting carriers into sm aller sub ca rriers, and then broadcasting thosesimultaneously. This reduces m ultipath distortion and reduces RF

interference (a mathematical form ula is used to ensure the sub carrier s'

specific frequencies are "orthogonal," or non-interfering, to each other),

allowing for greater throughput. The only main weak point t hat was found

with using OFDM, was that it is ve ry sensitive to fre quency, and phase

errors between the transmitter and receiver. The main sources of these errors

are frequency stability problem s; phase noise of the transm itter; an d any

frequency offset errors be tween the transmitter an d receiver. This problem

can be mostly overcome by synchronizing the clocks between the transmitterand receiver, by designing the system appropriately


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S e m i n a r R e o r t -1

CHAPTER 1

INTRODUCTION


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INTRODUCTION

OFDM stands for Orthogonal Frequency Division Multiplexing and is an up

and coming modulation technique for transmitting large amounts of digitaldata over a radio wave. W-OFDM stands for Wideband OFDM.

OFDM is conceptu ally sim ple, but the devil is in the details! The

implementation relies on very high speed digital signal processing. OFDM is

conceptually sim ple, but the devil is in the details! The im plementation

relies on very high speed digital signal processing and this has only recently

become available at a pr ice that m akes OFDM a competitive technology in

the marketplace. OK, so what is th e simple concept behind OFDM ? Take

one carrier and m odulate it using Quad rature Phase Shift Keying (QPSK)where each sym bol encodes 2 bits. M odulation theory tells us that the

spectrum of such a m odulated signal w ill have a sin (x)/x shape with nulls

spaced by the bit rate. In OFDM, the ca rriers are spaced at the bit rate, so

that the carriers fit in the fit in the nulls of the other carriers. Another view of

Orthogonal Another view of Orthogonal is that each carrier has an integer

number of sine wave cycles in one bit period

The problem with the sim ple-minded approach is that it ta kes lots of local

oscillators each locked to the others so that the frequencies are the exact

multiples that they should be. This is difficult and expensive. DSP to therescue! Each of the oscillators can be a digital representation of the sine

carrier wave that can be m odulated in the numerical dom ain. This can

happen simultaneously for all of the ca rriers. The resulting output of each

channel is added and then bl ocked. Since we have a representation of the

signal in the frequency domain but need to modulate an actual carrier in the

time domain, we just perform an Inverse Fast Fourier Transform (IFFT) to

convert the block of frequenc y data to a block of ti me data that m odulates

the carrier . The receiver acquires the signal, digit izes it, and perform s an

FFT on it to get back to t he frequency domain. From there, it is relativelyeasy to recover the modulation on each of the carriers.


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CHAPTER 2

SUBJECT DETALING


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2.1 WHAT IS OFDM

OFDM (Orthogonal Frequency Division Multiplexing) is a method of using

many carrier waves instead of only one, and using each carrier wave for onlypart of the message. OFDM is also called multicarriermodulation (MCM) or

DiscreteMulti-Tone (DMT). It is important to stress that OFDM is not really

a m odulation scheme sinc e it does not confli ct with other m odulation

schemes. It is m ore a coding sche me or a transportscheme. Orthogonal

Frequency Division is where t he spaci ng between carriers is equal to the

speed (bit rate) of the message.

A m ultiplex was prim arily used to allow many users to share a

communications medium like a phone trunk between two telephone centraloffices. In OFDM, i t typical to assign all carriers to a single user; hence

multiplexing is not used with its generic meaning.

Orthogonal frequency division m ultiplexing is then the concept of typically

establishing a co mmunications link using a multitude of carriers each

carrying an amount of i nformation iden tical to the separation between the

carriers.

2.2 QUALITATIVE DESCRIPTION OF OFDM

Figure 0.4shows structure of a multicarrier system.

th e general

Figure 0.4: Basic structure of a multicarrier system


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The original data stream of rate R is multiplexed into N parallel data streams

of rate each of the data streams is modulated with a

different frequency and the resulting si gnals are transm itted together in the

same ban d. Correspondingly the recei ver consists of N parallel receiver

paths. Due to the prolonged distance in between transmitted symbols the ISI

for each sub system reduces to

In the case of DVB-T we have N=8192 leading to an ISI of

Such little ISI can often be tolerated and no extra counter measure such as an

equalizer is needed. Alas as far as the co mplexity of a receiver is concerned

a system with 8192 paralle l paths still isn' t feasible. This asks for a slight

modification of the approach which leads us to the concept of OFDM.

In OFDM, each carrier is ort hogonal to all other carriers. However, thi s

condition is not always maintained in MCM. OFDM is an optimal version of

multicarrier transmission. In OFDM, each carrier is orthogonal to all other

carriers. However, this condit ion is not always maintained in MCM. OFDM

is an optimal version of multicarrier transmission Schemes.


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Fig. 3 The effect of adopting a multicarrier system. For a given overall data

rate, increasing the num ber of carriers reduces the data rate that each

individual carrier must conve y, and hence (for a given m odulation system)

lengthens the sym bol period. This m eans that the intersym bol interference

affects a smaller percentage of each symbol as the


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In OFDM, the data is divided am ong large number of cl osely spa ced

carriers. This accounts for the frequenc y division m ultiplex part of the

name. This is not a m ultiple access technique, since there is no com mon

medium to be shared

Instead of transm itting in serial way, da ta is transferred in a parallel way .

Only a small am ount of the data is carried on each carrier, and by this

lowering of the bit rate per carrier (not the total bit rate), the influence o f

intersymbol interfe rence is significan tly reduced. In principle, many

modulation schemes could be used to modulate the data at a low bit rate onto

each carrier.

Orthogonal Frequency Division Multiplexing: A method for multiplexing

signals, which divides the available bandwidth into a series of frequencies

known as tones. Modulation on each tone is usually quardature amplitudemodulation. As shown in figure

Orthogonal tones do not interfere with each other because the bandwidth of

a modulated carrier sinc shape (sinx/x) with nulls spaced by the bit rate. In

OFDM, the carriers fit in the nulls of the other carriers..


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All frequencies fade but the rapi d switching, frequency-hopping technique

is intended to all ow more robust data se rvice. Because of the orthogonal of

the signals, establish overlap in frequency without interfe ring with each

other, thus reducing the system bandwidth

Ofdm frequency domaine shown in figure

.SO OFDM can be simply defined as a form of multicarrier modulation

where its carrier spacing is carefully selected so that each subcarrier is

orthogonal to the other sub carriers


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9

OFDM can be simply defined as a form of multicarrier modulation where its

carrier spacing is carefully selected so that each subcarrier is orthogonal to

the other sub carriers

2.3 The importance of orthogonality

The orthogonal part of the OFDM name indicates that there is a precise

mathematical relatio nship between th e frequencies of the carriers in the

system. In a normal FDM system, the many carriers are spaced apart in such

way that the signals can be recei ved using conventio nal filters and

demodulators. In such receivers, guard bands have to be introduced between

the different carrier s and the lowering of the spectrum E fficiency. It is

possible, however, to arrange the carri ers in an OFDM signal so that the

sidebands of the individual carriers overlap and the signals can still be

received without adjacent carrier interference. In order to do this the carriers

must be mathemati cally orthogonal. T he receiv er acts as a bank of

demodulators, translating each carrier down to DC, the resulting signal then

being integrated over sym bol period to recover the raw data. If the other

carriers all beat down to frequencies wh ich, in thtime domain, have a wholenumber of cycles in the symbol period (t), then the integration process

results i n zero contributi on from all th ese carriers. Thus the carriers ar e

linearly independent (i.e. orthogonal) if the carrier spacing is a m ultiple of

1/t. Mathematically, suppose we have a set of signa ls y , where y p is the p-

th element in the set


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1

2.4 Mathematical description of OFDM

After the qualitative description of the sy stem, it is valuable to discuss the

mathematical definition of the modulation system. This allows us to see how

the signal is generated and how rece iver m ust operate, and transm ission

channel. As noted above, OFDM tran smits a large num ber of narrowband

carriers, closely spaced in the frequency domain.

Mathematically, each carrier can be described as a complex wave:

The real signal is the real part of sc(t ). Both Ac (t) and sc(t), the amplitude

and phase of the carrier, can vary on a symbol by symbol basis. The values

of the parameters are constant over the sym bol duration peri od t. OFDM

consists of many carriers. Thus the complex signals s (t)) is represented

This is of course a con tinuous signal. If we consider the waveforms of each

component of the si gnal overcome sym bol period, then the variables Ac (t)

and fc(t) take on fixed values, wh ich depend on the frequency of that

particular carrier, and so can be rewritten:


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If the signal is sampled using a sampling frequency of 1/T, then the resulting

signal is represented by:

At this point, we have restricted the time over which we analyse the signal to

N samples. It is convenient to sam ple over the period of one data sym bol.

Thus we have a relationship=NT

If we now simplify eqn. 3, without a loss of generality by letting w0=0, then

the signal becomes:

Now Eq. 4 can be co mpared with the general form of the inverse Fourier

transform:

In eq. 4, the

is no more than a definition of the si gnal in the sampled frequency domain,

and s (kT) is the time domain representation. Eqns. 4 and 5 are equivalent if:

This is the same condition that was required for orthogonality


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2.5 Implementation of ofdmIf ofdm is implemented through multicarrier system

Then the receiver and transmitter is as shown in figure


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Above figures shows that generation of a number of carriers using separate

local oscillators. This was inefficient and costly (though increased the data

rate). . DSP to the rescue

2.6 The use of the FFT in OFDM

The main reason that the OFDM techni que has taken a long time to become

a prominence has been practical. It has been difficult to generate such signal,and even harder to receive and demodulate the demodulators, was somewhat

impractical for use in the civil systems. The ability to define the signal in the

frequency domain, in soft ware onVLSI processors, and to generate the

signal using the inverse Fourier transform is the key to its current popularity.

The use o f the reverse process in the receiver is essential if cheap and

reliable. Although the origi nal proposals were made a long time ago [5], it

has taken at the transmitter; the signal is defined in the frequency domain. It

is spectrum exists only at discrete frequencies. Each OFDM carrier

corresponds to oneelement of this discrete Fourier spectrum. The amplitudes

and phases of t he carriers depend on t he data to be transm itted. Th e datatransitions are synchronized at the carri ers, and can be processed together,

symbol by symbol

The definition of the (N-point) discrete Fourier transform (DFT) is:


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and the (N-point) inverse discrete Fourier transform (IDFT):

A natural consequence of this method is that it allows us to generate carriers

that are orthogonal. The members of an orthogonal set are linearly

independent. Consider a data sequence (d0, d1, d2, , dN-1), where each

dn is a complex number dn=an+jbn. (an, bn=1 for QPSK, an, bn=1, 3 for

16QAM,)

Where fn=n/(NDT), tk=kDt and Dt is an arbitrarily chosen symbol duration

of the serial data sequence dn. The real part of the vector D has components

If these components are applied to a low-pass filter at time intervals Dt, a

signal is obtained that closely approximates the frequency division

multiplexed signal

The inco ming seri al data is first converted form serial t o parallel and

grouped into x bits each to form a co mplex number. The number determines

the signal constellation of the corresponding subcarrier, such as 16 QAM or


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32QAM. The com plex num bers are modul ated i n the base band by the

inverse FFT (IFFT) and converted back to serial data for transm ission. A

guard interval is inserted between symbols to avoid intersymbol interference

(ISI) caused by m ultipath distortion. The discrete symbols are converted toanalog and low-pass filtered for RF up conversion. The receiver perform s

the inverse process of the transm itter. One-tap equalizer issued to correct

channel distortion. The tap-coefficients of the filter are calculated based on

the channel information.

Fig 4a shows the spe ctrum of an OFDM sub channel and Fig. 4b and Fig. 6

present compositeOFDM s pectrum. By car efully selecting the carrier

spacing, the OFDM signal spectrum can be made flat and the orthogonality

among the sub channels can be guaranteed.

If the signal is passed through a time-dispersive channel, by appending a

cyclic prefix at the front of every OFDM symbol. The cyclic prefix is a copy

of the last part of the OFDM symbol of length equal to or greater than the

maximum delay spread of the channel


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2.7 Coded Orthogonal Frequency Division Multiplexing

In practice, some of the carriers are used for channel estimation and there are

extra bits added for error detection and correction. Doing this is called

Coded Orthogonal Frequency Division Multiplexing (COFDM). Coding is

now so common that many people drop the "C", as unnecessary, assuming

that coding is used.


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2 8

When the radio signals travel fro m one location t o another, they

maybounceoffsurrounding objects (Figure 1), resulting in m ultiple paths

between transmitter and receiver. This is analogous to echoes or reflections

causing multiple copies of the message to arrive at the receiv er at different

times. The com bination of all Modulated message signal t o be distorted. A

simple example is where there are only two paths, the line of sight path andreflected path from the ground. If message is sent at the right speed, then the

second (reflected) copy of the Message may arrive exactly one bit t ime later

than the first (direct) copy. The Receiver will then receive two different bits

mixed together, thus distorting the Original message bit (Figure 1). Wireless


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communication syst ems have to be designed t o cope with this so-

calledmultipath distortion.

Figure 1

The main idea of usi ng OFDM is to avoid problems caused by

multipathreflections by sending the m essage bits slowly enough so that any

delayed copies (reflections) are late by only a small fraction of a bit time. To

maintain high bit rate, m ultiple carriers are used to send many low speed

messages at the same time which can be combined at thereceiver to makeup one high speed message. In this way, we avoid the distortion

caused by reflections.

2.8.2 ofdm act as a antinode for intersymbol interference


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2.9 BLOCK DIAGRAM OF OFDM

3.1 Advantages of OFDM

Spectral efficiency

{The orthogonal sub channels are spaced 1/T Hz apartandoverlap in

frequency)

Simple implementation

{IFFT/FFT pair

ADC/DAC pair)

Mitigation of ISI

{Cyclic prefix/suffix guardinterval)

3.2 The disadvantages of the OFDM

OFDM signal is contaminated by non-linear distortion of transmitter power

amplifier, because it is a combined amplitude-frequency modulation (it is

necessary to maintain linearity)

OFDM is very sensitive to carrier frequency offset caused by the jitter of

carrier wave and Doppler effect caused by moving of the mobile terminal.

At the receiver, it is very difficult to decide the starting time of the FFT symbolOFDM

stands for Orthogonal Frequency Division Multiplexing and is an up and


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coming modulation technique for transmitting large amounts of digital data

over a radio wave

OFDM is currently a very popular choice for future wireless applications,

including wireless LANs, cellular and PCS data, and possibly 4G systems.

Hopefully, inexpensive products that provide high-speed communications to

individualsand appliances around the globe.


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Conclusions

OFDM/COFDM has long been studie d and im plemented to com bat

transmission channel impairments. Its applications have been extended from

high frequency radio comm unications to telephone networks, digital audio

broadcasting and terrestrial broadc asting of digital television. The

advantages of COFDM, especially in the multipath propagation, interference

and fading environment, m ake the t echnology a prom ising alternative in

digital communications including mobile multimedia.


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6.REFERENCE

[1] R. Prasad, An overview of millimetre waves for future personal wireless

communication systems,

Proc. IEEE First symposium. on communications and vehicular technology

in the Benelux, K3, Delft,

Netherlands, Oct. 27-28. 1993.

[2] Ministerie van Verkeer and Waterstaat, Hoofddirectie Telecommunicatie

en Post,Frequency

allocations in the Netherlands, 2nd edition, Groningen, 1993.

[3] R.W. Chang, Synthesis of Band-Limited Orthogonal Signals for

Multichannel Data Transmission,

Bell Syst. Tech. J., vol.45, pp. 1775-1796, Dec. 1966.

[4]B.R. Salzberg, Performance of an efficient parallel data transmission

system,IEEE Trans.

Commun. Technol., vol. COM-15, pp. 805-813, Dec. 1967.

[5]S.B. Weinstein and P.M. Ebert, Data transmission by frequency-division

multiplexing using the

discrete Fourier transform,IEEE Trans. Commun. Technol., vol. COM-19,

pp. 628-634, Oct. 1971.

[6]A.W.M. van den Enden and N.A.M. Verhoeckx,Discrete-time signal

processing: an introduction.

London: Prentice Hall Int., 1989., ISBN 0-13-216763-8

[7]A.V. Oppenheim and R.W. Schaffer,Discrete -time signal processing,

Prentice-Hall International,

1989., ISBN 0-13-216771-9

241 Ofdm for Wirless Communication

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