final doc of ofdm

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A

Project reportOn

Implementation

Of FFT/IFFT Blocks for

Orthogonal Frequency Division

Multiplexing(OFDM)


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ORTHOGONALORTHOGONAL FREQUENCYFREQUENCY DIVISION MULTIPLEXINGDIVISION MULTIPLEXING

PROJECT REPORTPROJECT REPORTSUBMITTEDSUBMITTED

IN PARTIAL FULFILMENT OF THE REQUREMENTSIN PARTIAL FULFILMENT OF THE REQUREMENTS

FOR THE AWARD OF THE DEGREE OFFOR THE AWARD OF THE DEGREE OF

BACHELOR OF TECHNOLOGYBACHELOR OF TECHNOLOGY

ININ

ELECTRONICS AND COMMUNICATION ENGINEERINGELECTRONICS AND COMMUNICATION ENGINEERING

BYBY

DEPARTMENT OFDEPARTMENT OF

ELECTRONICS AND COMMUNICATION ENGINEERINGELECTRONICS AND COMMUNICATION ENGINEERING

------- UNIVERSITY COLLEGE OF ENGINEERING------- UNIVERSITY COLLEGE OF ENGINEERING


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----- UNIVERSITY COLLEGE OF ENGINEERING, HYDERABAD----- UNIVERSITY COLLEGE OF ENGINEERING, HYDERABAD

CERTIFCATECERTIFCATE

This is to certify that the project work entitledThis is to certify that the project work entitled

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXINGORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

Is a bonafide work done byIs a bonafide work done by

The students of B.Tech in Electronics and Communication Engineering during the

year 2010-2011 as a partial fulfillment of the requirement for the award of B.Tech

degree by -------- University College of Engineering, Hyderabad.

(Internal Guide)(Internal Guide) (Head, Dept of ECE)(Head, Dept of ECE)


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ACKNOWLEDGEMENTACKNOWLEDGEMENT

We are grateful to Department of Electronics and Communication Engineering,University college of Engineering, Hyderabad, Which gives us the opportunity to have

profound technical knowledge. Theyre by enabling us to complete the project.

We express our sincere and heartful thanks to ------- (PRINCIPAL university

college of engineering hyderabad) for his kind permission to undertake this project work.

We are extremely grateful to ----- (HOD of ECE, university college of

Engineering ,Hyderabad) for her valuable suggestions and timely help in the endeavor

and which paved the way for the successful completion of this project.

We specially surrender humble thanks and record our deep sense of gratitude to

our guide, who helped us a lot, guided us in excellent way by keeping us always in

positive mood and our wills alive. He is none other than ------------.

Last but not least, we express our heartfelt thanks to all this staff members and

friends for all help and co-operation extended in bringing out this project successfully in

time.


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SYNOPSYSSYNOPSYS

Orthogonal Frequency Division Multiplexing (OFDM) is a scheme used in the area

of high-data-rate mobile wireless communications such as cellular phones, satellite

communications and digital audio broadcasting. This technique is mainly utilized to

combat inter-symbol interference.

The OFDM technology was first conceived in the 1960s and 1970s during research

into minimizing Inter-Symbol Interference (ISI) due to multipath. OFDM is a special

form of M ulti Carrier Modulation (MCM) with densely spaced sub carriers withoverlapping spectra, thus allowing for multiple-access. MCM is the principle of

transmitting data by dividing the stream into several bit streams, each of which has a

much lower bit rate and by using these sub-streams to modulate several carriers. This

technique is being investigated as the next generation transmission scheme for mobile

wireless communications networks.

A multiple-access is a transmission scheme where several simultaneous users can

use the same fixed bandwidth. Some other multiple access schemes are TDMA (Time

Division Multiple Access), FDMA (Frequency Division Multiple Access and CDMA

(Code Division Multiple Access).

The purpose of this project was to implement an OFDM system within the

Matlab software package. We are transmitting text, image and speech at a time and de-

multiplexing at the receiver side. The simulation explored the advantages and

disadvantages of the OFDM communication technique.


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ABSTRACT

Orthogonal Frequency Division Multiplexing is a scheme used in the area of high-

data-rate mobile wireless communications such as cellular phones, satellite

communications and digital audio broadcasting. The Fourier transform, in essence,

decomposes or separates a waveform or function into sinusoids of different frequencies

which sum to the original waveform. It identifies or distinguishes the different frequency

sinusoids and their respective amplitudes.

In many applications high-speed performance is required. For this purpose,

conventional multi-carrier techniques are usually chosen, but this result in the lowering of

spectrum efficiency. So, the principles of Orthogonal Frequency Division Multiplexing

are used in such applications. This paper gives the details of the development of IFFT &

FFT algorithms to be used in OFDM systems based on the IEEE 802.11a standard for

WLAN. This system consists of separate OFDM transmitter & receiver. Actually, in the

entire architecture of OFDM system, all the mathematical manipulations take place in

these two blocks only, i.e. IFFT & FFT blocks while rest of the blocks convert the data

from one format to another format. In this paper we have implemented FFT and IFFT

blocks. The speed enhancement is the key contribution of the main processing blocks inOFDM system.

However, the advent of the Discrete Fourier Transform (DFT) made this

transmission scheme more plausible. The Fast Fourier Transform (FFT) and the Inverse

Fast Fourier Transform (IFFT) are the more efficient implementations of the DFT, are

utilized for the base band OFDM modulation and demodulation process.


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Contents:Contents:S.NoS.No TopicTopic Page No.Page No.

1. Chapter 1

Introduction .

Objective

Back ground.

Multiple Access Techniques

FDMA (Frequency Division Multiple Access).

TDMA (Time Division Multiple Access).

CDMA (Code Division Multiple Access)..

CDMA Generation

2. Chapter 2

Theory & Research Introduction

OFDM Principles

Fourier Transform.

Orthogonality..OFDM Carriers

OFDM

(Orthogonal Frequency Division Multiplexing)

OFDM Generation.

Modulation Techniques..

QAM.

QPSK..

FFT & IFFT..

Adding a Guard Period of OFDM

Transmitter & Receiver Structures..


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3. Chapter 3

Propagation Characteristics of mobile radio channel..

Attenuation

Multipath Effects.

Doppler Shift.

ISI (Inter Symbol Interference).

Chapter 4

Implementation of OFDM System.

OFDM Model Used

System Flowchart

Phase Shift Correction..

High Data Rates

Advantages of OFDM..

Disadvantages of OFDM..

OFDM Applications.

Chapter-5

Result Analysis

Future Enhancements..

Conclusion

Bibliography.


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CHAPTER 1CHAPTER 1

INTRODUCTIONINTRODUCTION

&&

BACKGROUNNDBACKGROUNND


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1.1 INTRODUCTION1.1 INTRODUCTION

Initial proposals for OFDM were made in the 60s and the 70s. It has taken more

than a quarter of a century for this technology to move from the research domain to theindustry.The concept of OFDM is quite simple but the practicality of implementing it has

many complexities.So, it is a fully software project.

OFDM depends on Orthogonality principle. Orthogonality means, it allows the

sub carriers, which are orthogonal to each other, meaning that cross talk between co-

channels is eliminated and inter-carrier guard bands are not required. This greatly

simplifies the design of both the transmitter and receiver, unlike conventional FDM; a

separate filter for each sub channel is not required.

Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi carrier

modulation scheme, which uses a large number of closely spaced orthogonal sub-carriers.

A single stream of data is split into parallel streams each of which is coded and

modulated on to a subcarrier, a term commonly used in OFDM systems.

Each sub-carrier is modulated with a conventional modulation scheme (such as

quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to

conventional single carrier modulation schemes in the same bandwidth. Thus the high bit

rates seen before on a single carrier is reduced to lower bit rates on the subcarrier.


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In practice, OFDM signals are generated and detected using the Fast Fourier

Transform algorithm. OFDM has developed into a popular scheme for wideband digital

communication, wireless as well as copper wires.

Actually, FDM systems have been common for many decades. However, in FDM,

the carriers are all independent of each other. There is a guard period in between them

and no overlap whatsoever. This works well because in FDM system each carrier carries

data meant for a different user or application. FM radio is an FDM system. FDM systems

are not ideal for what we want for wideband systems. Using FDM would waste too much bandwidth. This is where OFDM makes sense.

In OFDM, subcarriers overlap. They are orthogonal because the peak of one

subcarrier occurs when other subcarriers are at zero. This is achieved by realizing all the

subcarriers together using Inverse Fast Fourier Transform (IFFT). The demodulator at the

receiver parallel channels from an FFT block. Note that each subcarrier can still be

modulated independently.

Since orthogonality is important for OFDM systems, synchronization in

frequency and time must be extremely good. Once orthogonality is lost we experience

inter-carrier interference (ICI). This is the interference from one subcarrier to another.

There is another reason for ICI. Adding the guard time with no transmission causes

problems for IFFT and FFT, which results in ICI. A delayed version of one subcarrier can

interfere with another subcarrier in the next symbol period. This is avoided by extending

the symbol into the guard period that precedes it. This is known as a cyclic prefix . Itensures that delayed symbols will have integer number of cycles within the FFT

integration interval. This removes ICI so long as the delay spread is less than the guard

period.


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1.2 Objective:1.2 Objective:The aim of this project is to investigate the OFDM scheme, and realize a fully

functional system in software and analyzing how it is reducing the inter-symbol

interference caused by the multipath fading channels and different effects and estimating,evaluating the performance of it.

1.3 Background:1.3 Background:

Most first generations systems were introduced in the mid 1980s, and can be

characterized by the use of analog transmission techniques and the use of simple multipleaccess techniques such as Frequency Division Multiple Access (FDMA). First generation

telecommunications systems such as Advanced Mobile Phone Service (AMPS) only

provided voice communications. They also suffered from a low user capacity, and

security problems due to the simple radio interface used. Second generation systems were

introduced in the early 1990s, and all use digital technology. This provided an increase

in the user capacity of around three times. This was achieved by compressing the voice

waveforms before transmission.

Third generation systems are an extension on the complexity of second-generation

systems and are expected to be introduced after the year 2000. The system capacity is

expected to be increased to over ten times original first generation systems. This is going

to be achieved by using complex multiple access techniques such as Code Division

Multiple Access (CDMA), or an extension of TDMA, and by improving flexibility of

services available.

The telecommunications industry faces the problem of providing telephone

services to rural areas, where the customer base is small, but the cost of installing a wired

phone network is very high. One method of reducing the high infrastructure cost of a

wired system is to use a fixed wireless radio network. The problem with this is that for

rural and urban areas, large cell sizes are required to get sufficient coverage.


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Fig.1.1 shows the evolution of current services and networks to the aim of

combining them into a unified third generation network. Many currently separate systems

and services such as radio paging, cordless telephony, satellite phones and private radio

systems for companies etc, will be combined so that all these services will be provided by

third generation telecommunications systems.

Fig: 1.1 Evolution of current networks to the next generation of wireless networks.

Currently Global System for Mobile telecommunications (GSM) technology is

being applied to fixed wireless phone systems in rural areas. However, GSM uses time

division multiple access (TDMA), which has a high symbol rate leading to problems with

multipath causing inter-symbol interference. Several techniques are under consideration

for the next generation of digital phone systems, with the aim of improving cell capacity,

multipath immunity, and flexibility. These include CDMA and OFDM. Both these

techniques could be applied to providing a fixed wireless system for rural areas.

However, each technique as different properties, making it more suited for specific

applications.

OFDM is currently being used in several new radio broadcast systems including

the proposal for high definition digital television (HDTV) and digital audio broadcasting

(DAB). However, little research has been done into the use of OFDM as a transmission

method for mobile telecommunications systems. In CDMA, all users transmit in the same


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broad frequency band using specialized codes as a basis of channelization. Both the base

station and the mobile station know these codes, which are used to modulate the data

sent. OFDM/COFDM allows many users to transmit in an allocated band, by subdividing

the available bandwidth into many narrow bandwidth carriers. Each user is allocated

several carriers in which to transmit their data.

The transmission is generated in such a way that the carriers used are orthogonal

to one another, thus allowing them to be packed together much closer than standard

frequency division multiplexing (FDM). This leads to OFDM/COFDM providing a high

spectral efficiency.

Orthogonal Frequency Division Multiplexing is a scheme used in the area of high-

data-rate mobile wireless communications such as cellular phones, satellite

communications and digital audio broadcasting. This technique is mainly utilized to

combat inter-symbol interference.

1.4 Multiple Access Techniques:1.4 Multiple Access Techniques:

Multiple access schemes are used to allow many simultaneous users to use the

same fixed bandwidth radio spectrum. In any radio system, the bandwidth, which is

allocated to it, is always limited. For mobile phone systems the total bandwidth is

typically 50 MHz, which is split in half to provide the forward and reverse links of the

system.

Sharing of the spectrum is required in order increase the user capacity of any

wireless network. FDMA, TDMA and CDMA are the three major methods of sharing the

available bandwidth to multiple users in wireless system. There are many extensions, and

hybrid techniques for these methods, such as OFDM, and hybrid TDMA and FDMA

systems. However, an understanding of the three major methods is required for

understanding of any extensions to these methods.


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1.4.1 Frequency Division Multiple Access (FDMA):1.4.1 Frequency Division Multiple Access (FDMA):

In Frequency Division Multiple Access (FDMA), the available bandwidth is

subdivided into a number of narrower band channels. Each user is allocated a unique

frequency band in which to transmit and receive on. During a call, no other user can use

the same frequency band.

Each user is allocated a forward link channel (from the base station to the mobile

phone) and a reverse channel (back to the base station), each being a single way link. The

transmitted signal on each of the channels is continuous allowing analog transmissions.

The bandwidths of FDMA channels are generally low (30 kHz) as each channel only

supports one user. FDMA is used as the primary breakup of large allocated frequency

bands and is used as part of most multi-channel systems.

Fig. 1.2 & Fig. 1.3 show the allocation of the available bandwidth into several channels.

1.4.2. Time Division Multiple Access:1.4.2. Time Division Multiple Access:

Time Division Multiple Access (TDMA) divides the available spectrum into multiple

time slots, by giving each user a time slot in which they can transmit or receive. Fig. 1.4shows how the time slots are provided to users in a round robin fashion, with each user

being allotted one time slot per frame. TDMA systems transmit data in a buffer and burst

method, thus the transmission of each channel is non-continuous.


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Fig 1.4 TDMA scheme, where each user is allocated a small time slot

The input data to be transmitted is buffered over the previous frame and burst

transmitted at a higher rate during the time slot for the channel. TDMA can not send

analog signals directly due to the buffering required, thus is only used for transmitting

digital data. TDMA can suffer from multipath effects, as the transmission rate isgenerally very high. This leads the multipath signals causing inter-symbol interference.

TDMA is normally used in conjunction with FDMA to subdivide the total available

bandwidth into several channels. This is done to reduce the number of users per

channel allowing a lower data rate to be used. This helps reduce the effect of delay spread

on the transmission. Fig. 1.5 shows the use of TDMA with FDMA. Each channel based

on FDMA, is further subdivided using TDMA, so that several users can transmit of the

one channel. This type of transmission technique is used by most digital second

generation mobile phone systems. For GSM, the total allocated bandwidth of 25MHz is

divided into 125, 200 kHz channels using FDMA. These channels are then subdivided

further by using TDMA so that each 200 kHz channel allows 8-16 users.

Fig. 1.5 TDMA/FDMA hybrid, showing that the bandwidth is split into frequency channels and time slots.


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1.4.3. Code Division Multiple Access:1.4.3. Code Division Multiple Access:

Code Division Multiple Access (CDMA) is a spread spectrum technique that uses

neither frequency channels nor time slots. In CDMA, the narrow band message (typically

digitized voice data) is multiplied by a large bandwidth signal, which is a pseudo random

noise code (PN code). All users in a CDMA system use the same frequency band and

transmit simultaneously. The transmitted signal is recovered by correlating the received

signal with the PN code used by the transmitter. Fig. 1.6 shows the general use of the

spectrum using CDMA.

Some of the properties that have made CDMA useful are: Signal hiding and non-

interference with existing systems, Anti-jam and interference rejection, Information

security, Accurate Ranging, Multiple User Access, Multipath tolerance.

Fig. 1.6 Code Division Multiple Access (CDMA)

Fig.1.7 shows the process of a CDMA transmission. The data to be transmitted (a)

is spread before transmission by modulating the data using a PN code. This broadens the

spectrum as shown in (b). In this example the process gain is 125 as the spread spectrum

bandwidth is 125 times greater the data bandwidth. Part (c) shows the received signal.

This consists of the required signal, plus background noise, and any interference from

other CDMA users or radio sources.

The received signal is recovered by multiplying the signal by the originalspreading code. This process causes the wanted received signal to be dispread back to the

original transmitted data. However, all other signals, which are uncorrelated to the PN

spreading code used, become more spread. The wanted signal in (d) is then filtered

removing the wide spread interference and noise signals.


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Fig. 1.7 Basic CDMA Generation.

1.4.4 CDMA Generation:1.4.4 CDMA Generation:

CDMA is achieved by modulating the data signal by a pseudo random noise

sequence (PN code), which has a chip rate higher then the bit rate of the data. The PN

code sequence is a sequence of ones and zeros (called chips), which alternate in a random

fashion. The data is modulated by modular-2 adding the data with the PN code sequence.

This can also be done by multiplying the signals, provided the data and PN code is

represented by 1 and -1 instead of 1 and 0. Fig. 1.8 shows a basic CDMA transmitter.

Fig. 1.8 Simple direct sequence modulator

The PN code used to spread the data can be of two main types. A short PN code

(Typically 10-128 chips in length), can be used to modulate each data bit. The short PNcode is then repeated for every data bit allowing for quick and simple synchronization of

the receiver. Fig.1.9 shows the generation of a CDMA signal using a 10-chip length short

code. Alternatively a long PN code can be used. Long codes are generally thousands to

millions of chips in length, thus are only repeated infrequently. Because of this they are

useful for added security as they are more difficult to decode.


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Fig.1.9 Direct sequence signals


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CHAPTER 2CHAPTER 2

THEORY THEORY &&

REASEARCHREASEARCH


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2.1 Theory & Research Introduction:2.1 Theory & Research Introduction:

The OFDM technology was first conceived in the 1960s and 1970s during

research into minimizing ISI, due to multipath. The expression digital communications in

its basic form is the mapping of digital information into a waveform called a carrier

signal, which is a transmitted electromagnetic pulse or wave at a steady base frequency of

alternation on which information can be imposed by increasing signal strength, varying

the base frequency, varying the wave phase, or other means. In this instance,

orthogonality is an implication of a definite and fixed relationship between all carriers in

the collection. Multiplexing is the process of sending multiple signals or streams of

information on a carrier at the same time in the form of a single, complex signal and then

recovering the separate signals at the receiving end.

Modulation is the addition of information to an electronic or optical signal carrier.

Modulation can be applied to direct current (mainly by turning it on and off), to

alternating current, and to optical signals. One can think of blanket waving as a form of

modulation used in smoke signal transmission (the carrier being a steady stream of

smoke). In telecommunications in general, a channel is a separate path through which

signals can flow. In optical fiber transmission using dense wavelength-division

multiplexing, a channel is a separate wavelength of light within a combined, multiplexed

light stream. This project focuses on the telecommunications definition of a channel.

2.2 OFDM Principles:2.2 OFDM Principles:

OFDM is a special form of M ulti Carrier Modulation (MCM) with densely spaced sub

carriers with overlapping spectra, thus allowing for multiple-access. MCM) is the

principle of transmitting data by dividing the stream into several bit streams, each of

which has a much lower bit rate, and by using these sub-streams to modulate several


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carriers. This technique is being investigated as the next generation transmission scheme

for mobile wireless communications networks.

2.3 Fourier Transform:2.3 Fourier Transform:

Back in the 1960s, the application of OFDM was not very practical. This was

because at that point, several banks of oscillators were needed to generate the carrier

frequencies necessary for sub-channel transmission. Since this proved to be difficult to

accomplish during that time period, the scheme was deemed as not feasible.

However, the advent of the Fourier Transform eliminated the initial complexity of

the OFDM scheme where the harmonically related frequencies generated by Fourier and

Inverse Fourier transforms are used to implement OFDM systems. The Fourier transform

is used in linear systems analysis, antenna studies, etc., The Fourier transform, in essence,

decomposes or separates a waveform or function into sinusoids of different frequencies

which sum to the original waveform. It identifies or distinguishes the different frequency

sinusoids and their respective amplitudes.

The Fourier transform of f(x) is defined as:

dxe x f F x j

= )()(

and its inverse is denoted by:

=

d e F x f x j)(21

)(

However, the digital age forced a change upon the traditional form of the Fourier

transform to encompass the discrete values that exist is all digital systems. The modified

series was called the Discrete Fourier Transform (DFT). The DFT of a discrete-time

system, x(n) is defined as:

(1)

(2)


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=

=

1

0

2

)()( N

n

kn N

jen xk

1 k N

and its associated inverse is denoted by:

= =

1

0

2

)(1

)( N

n

kn N

jek

N n x

1 n N

However, in OFDM, another form of the DFT is used, called the Fast Fourier Transform

(FFT), which is a DFT algorithm developed in 1965. This new transform reduced the

number of computations from something on the order of

2 N to .log2

2 N N

2.4 Orthogonality:2.4 Orthogonality:

In geometry, orthogonal means, "involving right angles" (from Greek ortho ,

meaning right , and gon meaning angled ). The term has been extended to general use,

meaning the characteristic of being independent (relative to something else). It also can

mean: non-redundant, non-overlapping, or irrelevant. Orthogonality is defined for both

real and complex valued functions. The functions m(t) and n(t) are said to be

orthogonal with respect to each other over the interval a < t < b if they satisfy the

condition:

=b

amm

dt t t ,0)()(*

Where n m

2.5 OFDM Carriers:2.5 OFDM Carriers:

As fore mentioned, OFDM is a special form of MCM and the OFDM time

domain waveforms are chosen such that mutual orthogonality is ensured even though

(3)

(4)

(5)

(6)


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sub-carrier spectra may over-lap. With respect to OFDM, it can be stated that

orthogonality is an implication of a definite and fixed relationship between all carriers in

the collection.

It means that each carrier is positioned such that it occurs at the zero energy frequency

point of all other carriers. The sinc function, illustrated in Fig. 2.1 exhibits this property

and it is used as a carrier in an OFDM system.

f u is the sub-carrier spacingFig .2.1. OFDM sub carriers in the frequency domain

2.6 Orthogonal Frequency Division Multiplexing:2.6 Orthogonal Frequency Division Multiplexing:Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier

transmission technique, which divides the available spectrum into many carriers, each

one being modulated by a low rate data stream. OFDM is similar to FDMA in that the

multiple user access is achieved by subdividing the available bandwidth into multiple

channels that are then allocated to users. However, OFDM uses the spectrum much more

efficiently by spacing the channels much closer together. This is achieved by making all

the carriers orthogonal to one another, preventing interference between the closely spacedcarriers.

Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as

OFDM except that forward error correction is applied to the signal before transmission.


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This is to overcome errors in the transmission due to lost carriers from frequency

selective fading, channel noise and other propagation effects. For this discussion the

terms OFDM and COFDM are used interchangeably, as the main focus of this thesis is on

OFDM, but it is assumed that any practical system will use forward error correction, thus

would be COFDM.

In FDMA each user is typically allocated a single channel, which is used to

transmit all the user information. The bandwidth of each channel is typically 10 kHz-30

kHz for voice communications. However, the minimum required bandwidth for speech is

only 3 kHz. The allocated bandwidth is made wider then the minimum amount required

preventing channels from interfering with one another. This extra bandwidth is to allow

for signals from neighboring channels to be filtered out, and to allow for any drift in the

center frequency of the transmitter or receiver. In a typical system up to 50% of the total

spectrum is wasted due to the extra spacing between channels.

This problem becomes worse as the channel bandwidth becomes narrower, and

the frequency band increases. Most digital phone systems use vocoders to compress the

digitized speech. This allows for an increased system capacity due to a reduction in the

bandwidth required for each user. Current vocoders require a data rate somewhere

between 4- 13kbps, with depending on the quality of the sound and the type used. Thus

each user only requires a minimum bandwidth of somewhere between 2-7 kHz, using

QPSK modulation. However, simple FDMA does not handle such narrow bandwidths

very efficiently. TDMA partly overcomes this problem by using wider bandwidth

channels, which are used by several users. Multiple users access the same channel by

transmitting in their data in time slots. Thus, many low data rate users can be combined

together to transmit in a single channel, which has a bandwidth sufficient so that the

spectrum can be used efficiently.

There are however, two main problems with TDMA. There is an overhead

associated with the change over between users due to time slotting on the channel. A

change over time must be allocated to allow for any tolerance in the start time of each


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user, due to propagation delay variations and synchronization errors. This limits the

number of users that can be sent efficiently in each channel. In addition, the symbol rate

of each channel is high (as the channel handles the information from multiple users)

resulting in problems with multipath delay spread.

OFDM overcomes most of the problems with both FDMA and TDMA. OFDM

splits the available bandwidth into many narrow band channels (typically 100-8000). The

carriers for each channel are made orthogonal to one another, allowing them to be spaced

very close together, with no overhead as in the FDMA example. Because of this there is

no great need for users to be time multiplex as in TDMA, thus there is no overheadassociated with switching between users.

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

center 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 results in the signal having a high tolerance to multipath delay spread, as the

delay spread must be very long to cause significant ISI (e.g > 500usec).

2.7 OFDM generation:2.7 OFDM generation:

To generate OFDM successfully the relationship between all the carriers must be

carefully controlled to maintain the orthogonality of the carriers. For this reason, OFDM

is generated by firstly choosing the spectrum required, based on the input data, and

modulation scheme used. Each carrier to be produced is assigned some data to transmit.

The required amplitude and phase of the carrier is then calculated based on the

modulation scheme (typically differential BPSK, QPSK, or QAM).


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The required spectrum is then converted back to its time domain signal using an

Inverse Fourier Transform. In most applications, an Inverse Fast Fourier Transform

(IFFT) is used. The IFFT performs the transformation very efficiently, and provides a

simple way of ensuring the carrier signals produced are orthogonal.

The Fast Fourier Transform (FFT) transforms a cyclic time domain signal into its

equivalent frequency spectrum. This is done by finding the equivalent waveform,

generated by a sum of orthogonal sinusoidal components. The amplitude and phase of the

sinusoidal components represent the frequency spectrum of the time domain signal.

. The IFFT performs the reverse process, transforming a spectrum (amplitude and

phase of each component) into a time domain signal. An IFFT converts a number of complex data points, of length, which is a power of 2, into the time domain signal of the

same number of points. Each data point in frequency spectrum used for an FFT or IFFT

is called a bin. The orthogonal carriers required for the OFDM signal can be easily

generated by setting the amplitude and phase of each bin, then performing the IFFT.

Since each bin of an IFFT corresponds to the amplitude and phase of a set of orthogonal

sinusoids, the reverse process guarantees that the carriers generated are orthogonal.

Fig. 2.2 OFDM Block Diagram


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Fig. 2.2 shows the setup for a basic OFDM transmitter and receiver. The signal

generated is a base band, thus the signal is filtered, then stepped up in frequency before

transmitting the signal. OFDM time domain waveforms are chosen such that mutual

orthogonality is ensured even though sub-carrier spectra may overlap. Typically QAM or

Differential Quadrature Phase Shift Keying (DQPSK) modulation schemes are applied to

the individual sub carriers. To prevent ISI, the individual blocks are separated by guard

intervals wherein the blocks are periodically extended.

2.8 Modulation Techniques:2.8 Modulation Techniques:

2.8.1 Quadrature Amplitude Modulation (QAM):2.8.1 Quadrature Amplitude Modulation (QAM):

This modulation scheme is also called quadrature carrier multiplexing. Infact, this

modulation scheme enables to DSB-SC modulated signals to occupy the same

transmission BW at the receiver o/p. it is, therefore, known as a bandwidth-conservation

scheme . The QAM Tx consists of two separate balanced modulators, which are supplied,

with two carrier waves of the same freq but differing in phase by 90 . The o/p of the two

balanced modulators are added in the adder and transmitted.

Fig. 2.3 QAM System

The transmitted signal is thus given by


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S (t) = X1 (t) A cos (2 Fc t) + X2 (t) A sin (2 Fc t)

Hence, the multiplexed signal consists of the in-phase component A X1 (t) and

the quadrature phase component A X2 (t).

Balanced Modulator:Balanced Modulator:

A DSB-SC signal is basically the product of the modulating or base band signal

and the carrier signal. Unfortunately, a single electronic device cannot generate a DSB-

SC signal. A circuit is needed to achieve the generation of a DSB-SC signal is called

product modulator i.e., Balanced Modulator.

We know that a non-linear resistance or a non-linear device may be used to

produce AM i.e., one carrier and two sidebands. However, a DSB-SC signal contains

only 2 sidebands. Thus, if 2 non-linear devices such as diodes, transistors etc., are

connected in balanced mode so as to suppress the carriers of each other, then only

sidebands are left, i.e., a DSB-SC signal is generated. Therefore, a balanced modulator

may be defined as a circuit in which two non-linear devices are connected in a balanced

mode to produce a DSB-SC signal.

2.8.2 Quadrature Phase Shift Keying (QPSK) :2.8.2 Quadrature Phase Shift Keying (QPSK) :

In communication systems, we have two main resources. These are:

1. Transmission Power

2. Channel bandwidth

If two or more bits are combined in some symbols, then the signaling rate will be

reduced. Thus, the frequency of the carrier needed is also reduced. This reduces thetransmission channel B.W. Hence, because of grouping of bits in symbols; the

transmission channel B.W can be reduced. In QPSK two successive bits in the data

sequence are grouped together. This reduces the bits rate or signaling rate and thus

reduces the B.W of the channel. In case of BPSK, we know that when sym. Changes the


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level, the phase of the carrier is changed by 180 . Because, there were only two syms in

BPSK, the phase shift occurs in 2 levels only. However, in QPSK, 2 successive bits are

combined. Infact, this combination of two bits forms 4 distinct syms. When the sym is

changed to next sym, then the phase of the carrier is changed by 45 degrees.

S.No I/p successive bits symbol phase shift in carrier

I=1 1(1v) 0(-1v) S1 /4I=2 0(-1v) 0(-1v) S2 3/4I=3 0(-1v) 1(1v) S3 5/4I=4 1(1v) 1(1v) S4 7/4

0.

Generation of QPSK:Generation of QPSK:

Here the i/p binary seq. is first converted into a bipolar NRZ type of signal. This

signal is denoted by b (t). It represents binary 1 by +1V and binary 0 by -1V. The

demultiplexer divides b (t) into 2 separate bit streams of the odd numbered and even

numbered bits. Here Be (t) represents even numbered sequence and Bo (t) represents odd

numbered sequence. The symbol duration of both of these odd numbered sequences is

2Tb. Hence, each symbol consists of 2 bits.

Fig.2.4 Generation of QPSK

It may be observed that the first even bit occurs after the first odd bit. Hence, even

numbered bit sequence Be (t) starts with the delay of one bit period due to first odd bit.

Thus, first symbol of Be (t) is delayed by one bit period due to first odd bit. Thus, first

symbol of Be (t) is delayed by on bit period Tb with respect to first symbol of Bo (t).

This delay of Tb is known as offset . This shows that the change in the levels of Be (t) and


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Bo (t) cant occur at the same time due to offset or staggering. The bit stream Be (t)

modulates carrier cosine carrier and B0(t) modulates sinusoidal carrier. These modulators

are the balanced modulators. The 2 carriers are Ps.cos (2 Fc.t) and Ps.sin (2 Fc.t)

have been shown in fig. Their carriers are known as quadrature carriers. Due to the

offset, the phase shift in QPSK signal is /2.

2.8.3 FFT & IFFT:2.8.3 FFT & IFFT:

In practice, OFDM systems are implemented using a combination of FFT and

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 Tx as though they are in the freq-

domain. These syms are used as the i/ps to an IFFT block that brings the sig into the

time domain. The IFFT takes in N syms at a time where N is the num of sub carriers in

the system. Each of these N i/p syms has a symbol period of T secs. Recall that the basis

functions for an IFFT are N orthogonal sinusoids. These sinusoids each have a different

freq and the lowest freq is DC. Each i/p symbol acts like a complex weight for thecorresponding sinusoidal basis fun. Since the i/p syms are complex, the value of the sym

determines both the amplitude and phase of the sinusoid for that sub carrier.

The IFFT o/p is the summation of all N sinusoids. Thus, the IFFT block provides

a simple way to modulate data onto N orthogonal sub carriers. The block of N o/p

samples from the IFFT make up a single OFDM sym. The length of the OFDM symbol is

NT where T is the IFFT i/p symbol period mentioned above.

Fig. 2.5 FFT & IFFT diagram


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After some additional processing, the time-domain sig that results from the IFFT

is transmitted across the channel. At the Rx, an FFT block is used to process the received

signal and bring it into the freq domain. Ideally, the FFT o/p will be the original syms

that were sent to the IFFT at the Tx. When plotted in the complex plane, the FFT o/p

samples will form a constellation, such as 16-QAM. However, there is no notion of a

constellation for the time-domain sig. When plotted on the complex plane, the time-

domain sig forms a scatter plot with no regular shape. Thus, any Rx processing that uses

the concept of a constellation (such as symbol slicing) must occur in the frequency-

domain.

2.9 Adding a Guard Period to OFDM:2.9 Adding a Guard Period to OFDM:

One of the most important properties of OFDM transmissions is the robustness

against multipath delay spread. This is achieved by having a long symbol period, which

minimizes the ISI. The level of robustness, can infact is increased even more by the

addition of a guard period b/w transmitted syms. The guard period allows time for

multipath sigs from the pervious symbol to die away before the information from the

current symbol is gathered.

The most effective guard period to use is a cyclic extension of the symbol. If a

mirror in time, of the end of the symbol waveform is put at the start of the symbol as the

guard period, this effectively extends the length of the symbol, while maintaining the

orthogonally of the waveform. Using this cyclic extended symbol the samples required

for performing the FFT (to decode the sym), can be taken anywhere over the length of the

sym. This provides multipath immunity as well as sym time synchronization tolerance.

As long as the multipath delay echos stay within the guard period duration, there

is strictly no limitation regarding the signal level of the echos: they may even exceed the

signal level of the shorter path! The signal energy from all paths just adds at the input to


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the receiver, and since the FFT is energy conservative, the whole available power feeds

the decoder.

If the delay spread is longer then the guard interval then they begins to cause ISI.

However, provided the echos are sufficiently small they do not cause significant

problems. This is true most of the time as multipath echos delayed longer than the guard

period will have been reflected of very distant objects. Other variations of guard periods

are possible. One possible variation is to have half the guard period a cyclic extension of

the symbol, as above, and the other half a zero amplitude signal. This will result in a

signal as shown in Fig.2.6.

Using this method the symbols can be easily identified. This possibly allows for

symbol timing to be recovered from the signal, simply by applying envelop detection.

The disadvantage of using this guard period method is that the zero period does not give

any multipath tolerance, thus the effective active guard period is halved in length. It is

interesting to note that this guard period method has not been mentioned in any of the

research papers read, and it is still not clear whether symbol timing needs to be recovered

using this method.

Fig. 2.6 Section of an OFDM signal showing 5 symbols, using a guard period which

is half a cyclic extension of the symbol, and half a zero amplitude signal.


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CHAPTER 3CHAPTER 3

PROPAGATIONPROPAGATION

OFOF

CHANNELCHANNEL


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CHARACTERISTICCHARACTERISTIC

SS

3.1 Propagation Characteristics of mobile radio channels:3.1 Propagation Characteristics of mobile radio channels:

In an ideal radio channel, the received signal would consist of only a single direct

path signal, which would be a perfect reconstruction of the transmitted signal. However

in a real channel, the signal is modified during transmission in the channel.

It is known that the performance of any wireless systems performance is affected by the medium of propagation, namely the characteristics of the channel . In

telecommunications in general, a channel is a separate path through which signals can

flow. In the ideal situation, a direct line of sight between the transmitter and receiver is

desired. But alas, it is not a perfect world; hence it is imperative to understand what goes


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on in the channel so that the original signal can be reconstructed with the least number of

errors.

The received signal consists of a combination of attenuated, reflected, refracted,

and diffracted replicas of the transmitted signal. On top of all this, the channel adds noise

to the signal and can cause a shift in the carrier frequency if the transmitter, or receiver is

moving (Doppler effect). Understanding of these effects on the signal is important

because the performance of a radio system is dependent on the radio channel

characteristics.

3.1.1. Attenuation:3.1.1. Attenuation:

Attenuation is the drop in the signal power when transmitting from one point to

another. It can be caused by the transmission path length, obstructions in the signal path,

and multipath effects. Fig.3.1 shows some of the radio propagation effects that cause

attenuation. Any objects, which obstruct the line of sight signal from the transmitter to

the receiver, can cause attenuation.

Fig. 3.1. Some channel characteristics

Shadowing of the signal can occur whenever there is an obstruction between the


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transmitter and receiver. It is generally caused by buildings and hills, and is the most

important environmental attenuation factor. Shadowing is most severe in heavily built up

areas, due to the shadowing from buildings. However, hills can cause a large problem due

to the large shadow they produce.

Radio signals diffract off the boundaries of obstructions, thus preventing total

shadowing of the signals behind hills and buildings. However, the amount of diffraction

is dependent on the radio frequency used, with low frequencies diffracting more then

high frequency signals. Thus high frequency signals, especially, Ultra High Frequencies

(UHF), and microwave signals require line of sight for adequate signal strength. To over

come the problem of shadowing, transmitters are usually elevated as high as possible to

minimize the number of obstructions. Typical amounts of variation in attenuation due toshadowing are shown in Table 3.1.

Table.3.1 Typical attenuation in a radio channel.

Shadowed areas tend to be large, resulting in the rate of change of the signal

power being slow. For this reason, it is termed slow-fading, or lognormal shadowing.

3.1.2 Multipath Effects:3.1.2 Multipath Effects:

3.1.2.1. Rayleigh fading:3.1.2.1. Rayleigh fading:

In a radio link, the RF signal from the transmitter may be reflected from objects

such as hills, buildings, or vehicles. This gives rise to multiple transmission paths at the

receiver. Fig. 3.2 show some of the possible ways in which multipath signals can occur.


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Fig.3.2 Multipath Signals

The relative phase of multiple reflected sigs can cause constructive or destructive

interference at the Rx. This is experienced over very short distances (typically at half wavelength distances), thus is given the term fast fading. These variations can vary from

10-30dB over a short distance.

Fig. 3.3 Typical Rayleigh fading while the mobile unit is moving.

The Rayleigh distribution is commonly used to describe the statistical time

varying nature of the received signal power. It describes the probability of the signal

level. being received due to fading. Table 3.2 shows the probability of the signal level for

the Rayleigh distribution.


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Table 3.2 Cumulative distributions for Rayleigh distribution

3.1.2.2. Frequency Selective Fading:3.1.2.2. Frequency Selective Fading:

In any radio transmission, the channel spectral response is not flat. It has dips or

fades in the response due to reflections causing cancellation of certain frequencies at the

receiver. Reflections off near-by objects (e.g. ground, buildings, trees, etc) can lead to

multipath signals of similar signal power as the direct signal. This can result in deep nulls

in the received signal power due to destructive interference. For narrow bandwidth

transmissions if the null in the frequency response occurs at the transmission frequency

then the entire signal can be lost. This can be partly overcome in two ways.

By transmitting a wide bandwidth signal or spread spectrum as CDMA, any dips

in the spectrum only result in a small loss of signal power, rather than a complete loss.

Another method is to split the transmission up into many small bandwidth carriers, as is

done in a COFDM/OFDM transmission. The original signal is spread over a wide

bandwidth thus; any nulls in the spectrum are unlikely to occur at all of the carrier

frequencies. This will result in only some of the carriers being lost, rather then the entire

signal. The information in the lost carriers can be recovered provided enough forward

error corrections are sent.

3.1.2.3. Delay Spread:3.1.2.3. Delay Spread:

The received radio signal from a transmitter consists of typically a direct signal,

plus reflections of object such as buildings, mountings, and other structures. The reflected

signals arrive at a later time than the direct signal because of the extra path length, giving


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rise to a slightly different arrival time of the transmitted pulse, thus spreading the

received energy. Delay spread is the time spread between the arrival of the first and last

multipath signal seen by the receiver.

In a digital system, the delay spread can lead to inter-symbol interference. This is

due to the delayed multipath signal overlapping with the following symbols. This can

cause significant errors in high bit rate systems, especially when using time division

multiplexing (TDMA). Fig.3.4 shows the effect of inter-symbol interference due to delay

spread on the received signal. As the transmitted bit rate is increased the amount of inter-

symbol interference also increases. The effect starts to become very significant when the

delay spread is greater then ~50% of the bit time.

Fig.3.4 Multi delay spread

shows the typical delay spread that can occur in various environments. The maximum

delay spread in an outdoor environment is approximately 20usec, thus significantintersymbol interference can occur at bit rates as low as 25kbps.


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Table. 3.3 Typical Delay Spread

Inter-symbol interference can be minimized in several ways. One method is to

reduce the symbol rate by reducing the data rate for each channel (i.e. split the bandwidth

into more channels using frequency division multiplexing). Another is to use a codingscheme which is tolerant to inter-symbol interference such as CDMA.

3.1.3 Doppler Shift:3.1.3 Doppler Shift:

When a wave source and a receiver are moving relative to one another the

frequency of the received signal will not be the same as the source. When they are

moving toward each other the frequency of the received signal is higher then the source,

and when they are approaching each other the frequency decreases. This is called the

Doppler Effect. An example of this is the change of pitch in a cars horn as it approaches

then passes by. This effect becomes important when developing mobile radio systems.

The amount the frequency changes due to the Doppler effect depends on the relative

motion between the source and receiver and on the speed of propagation of the wave. The

Doppler shift in frequency can be written:

Where f is the change in frequency of the source seen at the receiver, fo is the frequency

of the source, v is the speed difference between the source and transmitter, and c is the

speed of light.

For example: Let fo = 1GHz, and v = 60km/hr (16.7m/s) then the Doppler shift will

be:

This shift of 55Hz in the carrier will generally not effect the transmission. However,


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Doppler shift can cause significant problems if the transmission technique is sensitive to

carrier frequency offsets (for example COFDM) or the relative speed is higher (for

example in low earth orbiting satellites).

3.2 Inter Symbol Interference:3.2 Inter Symbol Interference:

As communication systems evolve, the need for high symbol rates becomes more

apparent. However, current multiple access with high symbol rates encounter several

multi path problems, which leads to ISI. An echo is a copy of the original signal delayed

in time. ISI takes place when echoes on different-length propagation paths result in

overlapping received symbols. Problems can occur when one OFDM symbol overlaps

with the next one. There is no correlation between two consecutive OFDM symbols andtherefore interference from one symbol with the other will result in a disturbed signal

In addition, the symbol rate of communications systems is practically limited by

the channels bandwidth. For the higher symbol rates, the effects of ISI must be dealt

with seriously. Several channel equalization techniques can be used to suppress the ISIs

caused by the channel. However, to do this, the CIR channel impulse response, must be

estimated.

Recently, OFDM has been used to transmit data over a multi-path channel.

Instead of trying to cancel the effects of the channels ISIs, a set of sub-carriers can be

used to transmit information symbols in parallel sub-channels over the channel, where the

systems output will be the sum of all the parallel channels throughputs.

This is the basis of how OFDM works. By transmitting in parallel over a set of

sub-carriers, the data rate per sub-channel is only a fraction of the data rate of a

conventional single carrier system having the same output. Hence, a system can be

designed to support high data rates while deferring the need for channel equalizations.


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In addition, once the incoming signal is split into the respective transmission sub-

carriers, a guard interval is added between each symbol. Each symbol consists of useful

symbol duration, T s and a guard interval, t , in which, part of the time, a signal of T s is

cyclically repeated. This is shown in Fig.3.5.

Fig. 3.5 Combating ISI using a guard interval

As long as the multi path propagation delays do not exceed the duration of the

interval, no inter-symbol interference occurs and no channel equalization is required.

CHANNELS We Used:

The transmission signal models of the electromagnetic wave which travels form

transmitter to receiver. Along the way the wave encounters a wide range of different

environments. Channel models represent the attempt to model these different

environments. Their aim is to introduce well defined disturbances to the transmission

signal. In this lecture we discuss channel models which are typical for DAB transmission.

We consider the effects of noise, movement, and signal reflection. The general strategy is

to have a pictorial representation of the channel environment before we introduce the

mathematical model.

Overview Diagram

The following figure shows again the block diagram of communication system. Such a

system consists of Sender, Channel and Receiver. In this lecture we focus on the


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channel aspect of the communication system. In the block diagram, s(t) is the

transmission signal and s(t) is the received transmission signal.

Frequency offset channel

The frequency offset channel introduces a static frequency offset. One possible cause for

such a frequency offset is a slow drifting time base, normally a crystal oscillator, in either

transmitter or receiver. The frequency offset channel tests the frequency correction circuit

in the receiver. The following figure shows the block diagram of the Frequency shift

channel.

The mathematical model follows as:

.AWGN channel

For the Additional White Gaussian Noise (AWGN) channel the received signal is equal

to the transmitted signal with some portion of white Gaussian white noise added. Thischannel is particularly important for discrete models operating on a restricted number

space, because this allows one to optimise the circuits in terms of their noise

performance. The block diagram of the AWGN channel is given in the next figure.

s(t) = s(t) + n(t)

where n(t) is a sample function of a Gaussian random process. This represents whiteGaussian noise.

Multi path channel

The multipath channel is the last of the static channels. It reflects the fact that

electromagnetic waves can travel over various paths from the transmission antenna to the


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receiver antenna. The receiver antenna sums up all the different signals. Therefore, the

mathematical model of the multipath environment creates the received transmission

signal by summing up scaled and delayed versions of the original transmission signal.

This superposition of signals causes ISI.

The following figure shows a multipath environment.

The block diagram, shown in the next figure, details a DSP model for the multipathenvironment.

The mathematical model follows as:

Fading channels

Fading channels represent a mathematical model for wireless data exchange in a physical

environment which changes over time. These changes arise for two reasons:

1. The environment is changing even though the transmitter and receiver are

fixed; examples are changes in the ionosphere, movement of foliage andmovement of reflectors and scatterers.

2. Transmitter and receiver are mobile even though the environment might be static.

3. The next figure shows a multipath fading environment. The fading is modeled bythe fact that the environment is changing.


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The block diagram, shown in the next figure, details a DSP model for the multipathenvironment

Mathematically the DSP model can be formulated as follows:

DSP model and mathematical description are close to the underlying physical

phenomena. This makes them unsuitable for practical channel models. To establish

practical channel models we employ statistical methods to abstract and generalize thefading channel models. In the following two subsections we discuss Rayleigh and Rician

fading channels. Both represent statistical channel modes, the difference between them is

that the Rayleigh model does not assume a direct or prominent path and the Ricien model

assumes a direct path. The last channel model extends the ideas of Rayleigh and Rician


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fading channels with mobility aspects. The resulting mobile fading channels model the

degrading effects in the frequency domain of wireless multipath channels.

Rayleigh fading:

Rayleigh fading is caused by multipath reception. The mobile antenna receives a large

number, say N , reflected and scattered waves. Because of wave cancellation effects, the

instantaneous received power seen by a moving antenna becomes a random variable,

dependent on the location of the antenna.

To simplify the derivation of the fading models an un-modulated carrier of

the form s(t) = Acos(2pifct) as transmission signal is used. Based on the block diagram

the complex envelope of the received signal is:

where ai (t) is the gain factor and Ti (t) is the delay for a specific path i at a specific time

t.

where rRa (t) is a sample function of a Rayleigh distributed random process:

and the is uniformly distributed in the interval [0, 2pi).

The general form of this channel model is:

again, and are amplitude and phase from a particular measurement of a

rayleigh distributed

random process. This channel is called rayleigh fading channel.

Rician fading channel

Rician fading


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The model behind Rician fading is similar to that for Rayleigh fading, except that in

Rician fading a strong dominant component is present. This dominant component can for

instance be the line-of-sight wave. Refined Rician models also consider

1. that the dominant wave can be a phasor sum of two or more dominant signals, e.g.

the line-of- sight, plus a ground reflection. This combined signal is then mostly

treated as a deterministic (fully predictable) process

2. that the dominant wave can also be subject to shadow attenuation. This is a

popular

Assumption in the modeling of satellite channels. Besides the dominant component, the

mobile antenna receives a large number of reflected and

Scattered waves.

A Rician fading channel indicates that there is a prominent or direct path over which theelectromagnetic wave can travel. Compared to the Rayleigh channel model, Equation 1,

the Rician fading channel model has an additional Acos(2pifct) component to reflect the

prominent path:

Above Equation can be written as:

where rRi (t) is a sample function of a random process with a Rician distributed

probability density function (pdf):

Where I0 is the zero order modified Bessel function of the first kind given by:

and the distribution of is:


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where is the error function defined as:

The ratio , referred as the K-factor, relates the power in un faded and faded

components. Values of K >> 1 indicate less severe fading, whereas K


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CHAPTER 4CHAPTER 4

IMPLEMENTATIOIMPLEMENTATIO

NN

OFOF

OFDM SYSTEMOFDM SYSTEM

4.1 Implementation of OFDM System:4.1 Implementation of OFDM System:


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An OFDM system was modeled using Matlab to allow various parameters of the

system to be varied and tested. The aim of doing the simulations was to measure the

performance of OFDM under different channel conditions, and to allow for different

OFDM configurations to be tested.

4.2 OFDM Model Used:4.2 OFDM Model Used:

The OFDM system was modeled using Matlab and is shown in Fig.5.1 . A brief

description of the model is provided below.

Fig. 4.1 OFDM Model used for simulations

4.2.1 Serial to Parallel Conversion:4.2.1 Serial to Parallel Conversion:

The input serial data stream is formatted into the word size required for

transmission, e.g. 2bit/word for QPSK, and shifted into a parallel format. The data is then

transmitted in parallel by assigning each data word to one carrier in the transmission.

Modulation of Data:Modulation of Data:


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The data to be transmitted on each carrier is then differential encoded with

previous symbols, then mapped into a phase shift-keying format. Since differential

encoding requires an initial phase reference an extra symbol is added at the start for this

purpose. The data on each symbol is then mapped to a phase angle based on the

modulation method. For example QPSK the phase angles used are 0, 90, 180, and 270

degrees. The use of phase shift keying produces a constant amplitude signal and was

chosen for its simplicity and to reduce problems with amplitude fluctuations due to

fading.

4.2.3 Inverse Fourier Transform:4.2.3 Inverse Fourier Transform:

After the required spectrum is worked out, an inverse Fourier transform is used to

find the corresponding time waveform (IFFT Convert frequency domain signal to time

domain signal). The guard period is then added to the start of each symbol.

4.2.4 Guard Period:4.2.4 Guard Period:

The guard period used was made up of two sections. Half of the guard period time

is a zero amplitude transmission. The other half of the guard period is a cyclic extension

of the symbol to be transmitted. This was to allow for symbol timing to be easily

recovered by envelope detection.

However it was found that it was not required in any of the simulations as the

timing could be accurately determined position of the samples. COFDM as a modulation

technique for wireless telecommunications, with a CDMA. After the guard has been

added, the symbols are then converted back to a serial time waveform. This is then the

base band signal for the OFDM transmission.

.

4.2.5 Channel:4.2.5 Channel:

A channel model is then applied to the transmitted signal. The model allows for

the signal to noise ratio, multipath, and peak power clipping to be controlled. The signal


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to noise ratio is set by adding a known amount of white noise to the transmitted signal.

Multipath delay spread then added by simulating the delay spread using an FIR filter. The

length of the FIR filter represents the maximum delay spread, while the coefficient

amplitude represents the reflected signal magnitude.

4.2.6 Receiver:4.2.6 Receiver:

The receiver basically does the reverse operation to the transmitter. The guard

period is removed. The FFT of each symbol is then taken to find the original transmitted

spectrum. The phase angle of each transmission carrier is then evaluated and converted

back to the data word by demodulating the received phase. The data words are then

combined back to the same word size as the original data.

4.3 High Data Rates:4.3 High Data Rates:

An advantage of the OFDM scheme is that it naturally combats inter-symbol

interference while still allowing a high data rate. In order to fully understand this, we will

first take a look at a traditional wireless transmission as seen in Fig. 5.17.

Fig. 4.2. Traditional symbol transmission

In a traditional transmission, if the system was to have a high data rate the symbol

length would have to be short thus making the rate high as seen in the Fig. 5.17. The

problem with this technique is that in wireless transmission, inter symbol interference

presents a huge problem when short symbols are used. As seen in Fig. 5.18, even smallISI consumes much of the actual symbol. In this example, ISI consumes roughly one

third of each symbol. Therefore, a large portion of the actual information is corrupted.


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Fig. 4.3. Traditional symbol transmission with ISI

Now lets take a look at an OFDM transmission.

Fig. 4.4. OFDM symbol transmission

In an OFDM transmission, the symbols do not need to be short to produce a high

data rate. This is illustrated in Fig 5.19. Recall that in an OFDM transmission, the

information is represented in the frequencies of the symbol and not the symbol itself.Therefore, the symbol may be very lengthy but can still can a large amount of

information in its component frequencies. A large symbol length is a natural way to

combat ISI as seen in Fig.. The ISI in Fig. is the same length as in Fig. but only consumes

roughly one eighth of each symbol.

Therefore, a large portion of the actual information is preserved.

Fig. 4.5. OFDM transmission with ISI


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4.4 Advantages of OFDM signaling:4.4 Advantages of OFDM signaling: Makes efficient use of the spectrum by allowing overlap.

By dividing the channel into narrowband flat fading sub channels, OFDM is

more resistant to frequency selective fadings than single carrier systems are.

Eliminates ISI and IFI through use of a cyclic prefix.

Using adequate channel coding and interleaving one can recover symbols lost due

to the frequency selectivity of the channel.

Channel equalization becomes simpler than by using adaptive equalization

techniques with single carrier systems.

It is possible to use maximum likelihood decoding with reasonable complexity.

OFDM is computationally efficient by using FFT techniques to implement the

modulation and demodulation functions. Also, for multiple communication

channels, as is the case in digital audio broadcasting (DAB) systems, partial FFT

algorithms can be used in order to implement program selection and decimation.

In conjunction with differential modulation there is no need to implement a

channel estimator.

Is less sensitive to sample timing offsets than single carrier systems are.

Provides good protection against co channel interference and impulsive parasitic

noise.

Preservation of orthogonality in severe multipath.

Used for highest speed applications.

Supports dynamic packet access.

Support for TX and RX diversity.

Support for

o adaptive antenna arrays

o MINO/space time coding

o adaptive modulation and tone/power allocation


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4.5 Disadvantages of OFDM signaling:4.5 Disadvantages of OFDM signaling:

The OFDM signal has a noise like amplitude with a very large dynamic range,

therefore it requires RF power amplifiers with a high peak to average power ratio.

It is more sensitive to carrier frequency offset and drift than single carrier systems

are due to leakage of the DFT.

4.6. Problems with OFDM:4.6. Problems with OFDM:

44 .6.1.1 Peak-to-Mean Power Ratio:.6.1.1 Peak-to-Mean Power Ratio:

An OFDM signal may exhibit a high instantaneous peak power with respect to the

average signal level, seeing that the OFDM signal is the superposition of a large number

of modulated sub channel signals. In addition, when a time-domain signal moves from a

low to a high instantaneous power waveform, large amplitude swings are encountered. In

the context, the peak-to-mean power envelope fluctuates considerably, when traversing

the origin upon switching from one phasor to another.

In order to solve these problems, two things can be done:

Reduce the peak-to-mean power ratio . Using a different encoding or mapping

scheme before modulation can achieve this.

Improve the amplification stage of the transmitter , such as post processing the

time-domain OFDM signal or employ some sort of adaptive sub carrier allocation in

order to reduce the Crest factor (peak-to-mean signal ratio).

4.6.1.2 Synchronization:4.6.1.2 Synchronization:

To optimize the performance of an OFDM link, time and frequency

synchronization between the transmitter and receiver is of absolute importance. This is

achieved by using known pilot tones embedded in the OFDM signal or attach fine


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frequency timing tracking algorithms within the OFDM signals cyclic extension (guard

interval).

4.6.1.3 Co channel Interference:4.6.1.3 Co channel Interference:

Co channel interference in cellular communications systems are combated by

combining adaptive antenna techniques with OFDM transmissions. With the aid of beam

steering, it is possible to focus the receivers antenna bean on the served user, while

attenuating the co channel interferers. This is significant since OFDM is sensitive to co

channel interferences.


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4.7 OFDM Applications:4.7 OFDM Applications: DAB

HDTV

ADSL & HDSL

WLANs (IEEE 802.11 & Hiper LAN II


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CHAPTER 5CHAPTER 5

RESULT RESULT

ANALYSIS ANALYSIS


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SIMULATION RESULTS:

FIG:5.1 BUTTERFLY RESULTS


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FIG: 5.2 FAST FOURIER TRANSFORM(8 POINT FFT)


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FIGURE:5.3 INVERSE FAST FOURIER TRANSFORM(8 POINT IFFT)


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SYNTHESIS RESULTS

XILINX GENERATED ARCHITECTURE FIG 5.4


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Internal Architecture FIG 5.6

Timing Summary:

---------------

Speed Grade: -5

Minimum period: 2.289ns (Maximum Frequency: 436.862MHz)

Minimum input arrival time before clock: 4.501ns

Maximum output required time after clock: 48.491ns

Maximum combinational path delay: 2.540ns


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Conclusion:Conclusion:

The purpose of this document was to give some insight into the power of theOFDM transmission scheme. It has discussed not only the transmission scheme itself, but

also some of the problems that are presented in mobile communications as well as the

techniques to correct them.

Digital Communications is a rapidly growing industry and Orthogonal Frequency

Division Multiplexing is on the forefront of this technology. OFDM will prove to

revolutionize mobile communications by allowing it to be more reliable and robust while

maintaining the high data rate that digital communications demands.

The number of clock cycles required is reduced and both blocks gives

the final outputs as desired. Here, the real value inputs are given to

the FFT blocks while all the imaginary input values are zero.

We have successfully implemented the 8-point IFFT & FFT algorithms

using VHDL to be used in the 802.11a architecture of OFDM transmitter

& receiver. The performance of the main processing block of OFDM

transceiver is upgraded by reducing the clock cycles in the above work


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Bibliography:Bibliography:

Bahai, A., and B. Saltzberg. Multicarrier Digital Communications: Theory and Applications of OFDM . New York: Kluwer Academic/Plenum Publishers, 1999

Van Nee, R., and R. Prasad. OFDM Wireless Multimedia Communications .

Boston: Artech House, 2000

Couch II, L. W. Digital and Analog Communication Systems . New Jersey:

Prentice-Hall, 1997

Keller, T., and L. Hanzo. Adaptive Multicarrier Modulation: A Convenient

Framework for Time-Frequency Processing in Wireless Communications. Proceedings of the IEEE 88.5 (2000) 609 - 639

OFDM Wireless Technology, Eric Lawrey and Craig Blackburn. 2000. James

Cook University. < http://www.eng.jcu.edu.au/eric/thesis/Thesis.htm >.

Spread Spectrum Scene , SSS Online, Inc. 2001 < http://sss-mag.com/index.html

>

Wireless Resource Center , PaloWireless.Com. 2001 .

OFDM Receiver for Broadband Receivers , Michael Speth. Institute for Integrated

Signal Processing Systems. 2001. < http://www.ert.rwth-aachen.de/index_e.htm >.
http://www.eng.jcu.edu.au/eric/thesis/Thesis.htmhttp://sss-mag.com/index.htmlhttp://www.palowireless.com/ofdm/tutorials.asphttp://www.ert.rwth-aachen.de/index_e.htmhttp://www.eng.jcu.edu.au/eric/thesis/Thesis.htmhttp://sss-mag.com/index.htmlhttp://www.palowireless.com/ofdm/tutorials.asphttp://www.ert.rwth-aachen.de/index_e.htm

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