Peak to Average Power Ratio Reduction in Ofdm

7/30/2019 Peak to Average Power Ratio Reduction in Ofdm

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PEAK TO AVERAGE POWER RATIO

REDUCTION IN OFDM

A Seminar Report

Submitted by

SAMHITA HISWANKAR

I n partial fu lf ilment for the award of the degree

Of

BACHELOR OF TECHNOLOGY

IN

ELECTRONICS AND TELECOMMUNICATION

Guided by

Prof. V. M. Kulkarni

MAHARASHTRA INSTITUTE OF TECHNOLOGY

AURANGABAD2012- 2013


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CERTIFICATE

This is to certify that, the report Peak to Average Power Ratio Reduction in OFDM

submitted by Samhita Hiswankar is a bonafide work completed under my supervision and

guidance in partial fulfilment for award of Bachelor of Technology (Electronics and

Telecommunication) Degree of Maharashtra Institute of Technology Aurangabad.

Place: Aurangabad

Date :

Dr. S. P. Bhosle

Principal

Maharashtra Institute of Technology

Aurangabad

Prof. V.M. Kulkarni

Guide

Prof. V. M. Kulkarni

Head of the Department


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ABSTRACT

Orthogonal Frequency Division Multiplexing is a multicarrier modulation technique

being used in state of the art communication systems today. It has been known to have many

advantages such as minimal inter symbol interference, minimal frequency selective fading

due to multipath, avoidance of complex equalization filters and many more. However its

biggest disadvantage is high Peak to Average Power Ratio (PAPR). The high PAPR

necessitates the use of high resolution analog to digital convertor and digital to analog

convertor. That unnecessarily increases the cost of equipment radically

There are many techniques used for reduction of PAPR. This seminar focuses on one of them

namely Iterative Clipping and filtering. It involves clipping the rare but present peaks inOFDM signal.

However clipping of the original signal introduces noise in the system. That has to be

removed. This removal is done by the use of filters that are inserted in the system

This seminar also includes various simulation results and deep theoretical study of the topic

done from various well known journals and papers.


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List of Figures

Fig No. Title Page no.

2.1 FDM Multiplexing process 9

2.2 Block Diagram of Multicarrier Modulation 10

2.3 Spectrum of OFDM pulse 10

Error! No

text of

specified

style in

document.2.4

Symbol Structure of OFDM word with Cyclic Prefix 13

2.5 Block Diagram of Modulation and Demodulation of

OFDM

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2.6 Block Diagram : Block Coding Technique 20

2.7 Block Diagram : Selective Mapping Technique 21

2.8 Block Diagram: Partial Transmit Sequence Technique 21

2.9 Process of Peak Windowing 22

2.10 Block Diagram of Envelop Scaling 22

2.11 Block Diagram of iterative clipping-filtering. 25

List of Tables

Table No Title Page no.

Table 2.1 Comparison of various PAPR reduction schemes 21

Table 3.1 PAPR without and with clipping for PSK 24

Table 3.2 PAPR before and after clippingfiltering for QAM 25

Table 3.3 PAPR for QAM: Original and repetitive clipping -

filtering.

25


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List of Acronyms

OFDM Orthogonal Frequency Division Multiplexing

PAPR Peak To Average Power Ratio

CDMA Code Division Multiple Access

FDM Frequency Division Multiplexing

ISI Inter Symbol Interference

ICI Inter Carrier Interference.

PAP Peak Average Power

PA Power Amplifier

QPSK Quadrature Phase Shift Keying

DFT Discrete Fourier Transform

IDFT Inverse Discrete Fourier Transform

PTS Partial Transmit Sequence.

RF Radio Frequency

BER Bit Error Rate

CDF Cumulative Distribution Function

FEC Forward Error Correction

QAM Quadrature Amplitude Modulation


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Contents

Page No.

Abstract 3

List of Figures

List of Tables

List of Acronyms

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1. INTRODUCTION 8

2. LITERATURE SURVEY 9

2.1 Introduction to Orthogonal Frequency Division Multiplexing

2.2DEVELOPMENT OF OFDM SYSTEMS

2.2.1 Frequency Division Multiplexing

2.2.2 Multicarrier modulation.

2.3OFDM Theory

2.3.1 Orthogonality

2.3.2 Sub carriers.

2.3.3 Inter-symbol Interference

2.3.4 Inter-carrier Interference

2.3.5 Cyclic Prefix

2.3.6 Inverse Discrete Fourier Transform

2.4Modulation and Demodulation in OFDM system

2.4.1 Modulation :QAM

2.4.2 IFFT

2.4.3 Parallel to Series Convertor

2.4.4 Guard Interval Insertion2.4.5 Transmit Filter

2.4.6 Communication channel

2.4.7 Demodulation Blocks

2.5Peak To Average Power Ratio in OFDM: An Overview

2.5.1 Introduction

2.5.2 Peak To Average Power Concern

2.5.3 PAPR of Multicarrier Signal

2.6PAPR Reduction Techniques.

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2.7Amplitude Clipping and Filtering

2.7.1 Introduction

2.7.2 Clipping Filtering

2.7.3 Repetitive Clipping and Frequency Domain Filtering

2.7.4 Combination of Interleaving with Repetitive Clipping and

Frequency Domain Filtering

3 System Performance

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4 Conclusion and Future Scope 29References 33

Acknowledgement 35


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1. IntroductionOrthogonal frequency division multiplexing (OFDM) technology is one of the most

attractive candidates for fourth generation (4G) wireless communication. It effectively

combats the multipath fading channel and improves the bandwidth efficiency. At the sametime, it also increases system capacity so as to provide a reliable transmission. OFDM uses

the principles of Frequency Division Multiplexing (FDM) but in much more controlled

manner, allowing an improved spectral efficiency. The basic principle of OFDM is to split a

high-rate data stream into a number of lower rate streams that are transmitted simultaneously

over a number of subcarriers. These subcarriers are overlapped with each other. Because the

symbol duration increases for lower rate parallel subcarriers, the relative amount of

dispersion in time caused by multipath delay spread is decreased. Inter-symbol interference

(ISI) is eliminated almost completely by introducing a guard time in every OFDM symbol.

OFDM faces several challenges. The key challenges are ISI due to multipath-use guard

interval, large peak to average ratio due to non linearity of amplifier; phase noise problems of

oscillator, need frequency offset correction in the receiver. Large peak-to-average power

(PAP) ratio distorts the signal if the transmitter contains nonlinear components such as power

amplifiers (PAs). The nonlinear effects on the transmitted OFDM symbols are spectral

spreading, inter modulation and changing the signal constellation. In other words, the

nonlinear distortion causes both in-band and out-of-band interference to signals. Therefore

the PAs requires a back off which is approximately equal to the PAPR for distortion-less

transmission. This decreases the efficiency for amplifiers. Therefore, reducing the PAPR is of

practical interest.

Many PAPR reduction methods have been proposed. Some methods are designed based

on employing redundancy, such as coding, selective mapping with explicit or implicit side

information, or tone reservation. An apparent effect of using redundancy for PAPR reduction

is the reduced transmission rate. PAPR reduction may also be achieved by using extended

signal constellation, such as tone injection, or multi-amplitude CPM. The associated

drawback is the increased power and implementation complexity. A simple PAPR reduction

method can be achieved by clipping the time-domain OFDM signal. In this paper, I focus on

Iterative clipping and filtering technique to reduce the PAPR.


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2. Literature Survey2.1Introduction to Orthogonal Frequency Division Multiplexing

With the ever growing demand of this generation, need for high speed communication has

become an utmost priority. Various multicarrier modulation techniques have evolved in orderto meet these demands, few notable among them being Code Division Multiple Access

(CDMA) and Orthogonal Frequency Division Multiplexing (OFDM). Orthogonal Frequency

Division Multiplexing is a frequency division multiplexing (FDM) scheme utilized as a

digital multicarrier modulation method. A large number of closely spaced orthogonal sub

carriers is used to carry data. The data is divided into several parallel streams of channels, one

for each sub carriers. Each sub carrier is modulated with a conventional modulation

scheme (such as QPSK) at a low symbol rate, maintaining total data rates similar to the

conventional single carrier modulation schemes in the same bandwidth.

2.2Development Of OFDM SystemsThe development of OFDM systems can be divided into three parts. This comprises of

Frequency Division Multiplexing, Multicarrier Communication and Orthogonal Frequency

Division Multiplexing.

2.2.1 Frequency Division Multiplexing

Frequency Division Multiplexing is a form of signal multiplexing which involves

assigning non overlapping frequency ranges or channels to different signals or to each

user of a medium. A gap or guard band is left between each of these channels to ensure that

the signal of one channel does not overlap with the signal from an adjacent one. Due to lack

of digital filters it was difficult to filter closely packed adjacent channels.

Figure 2.1 FDM Multiplexing Process


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2.2.2 Multicarrier Communication

As it is ineffective to transfer a high rate data stream through a channel, the signal is

split to give a number of signals over that frequency range. Each of these signals are

individually modulated and transmitted over the channel. At the receiver end, these signals

are fed to a demultiplexer where it is demodulated and recombined to obtain the original

signal.

Figure 2.2 Block Diagram of Multicarrier Modulation

2.3OFDM THEORYOrthogonal Frequency Division Multiplexing is a special form of multicarrier modulation

which is particularly suited for transmission over a dispersive channel. Here the different

carriers are orthogonal to each other, that is, they are totally independent of one another. This

is achieved by placing the carrier exactly at the nulls in the modulation spectra of each other.


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Figure 2.3 Spectrum of OFDM pulse [2]

2.3.1 Orthogonality

Two periodic signals are orthogonal when the integral of their product over one

period is equal to zero.

For the case of continuous time:

For the case of discrete time:

Where mn in both cases.

2.3.2 SubCarriers

Each sub carrier in an OFDM system is a sinusoid with a frequency that is an

integer multiple of a fundamental frequency. Each sub carrier is like a Fourier series

component of the composite signal, an OFDM symbol.

The subcarriers waveform can be expressed as


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Where,

The sum of the subcarriers is then the baseband OFDM signal:

2.3.3 InterSymbol Interference

Inter symbol interference (ISI) is a form of distortion of a signal in which one

symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous

symbols have similar effect as noise, thus making the communication less reliable. ISI is

usually caused by multipath propagation or the inherent non linear frequency response of a

channel causing successive symbols to blur together. The presence of ISI in the system

introduces error in the decision device at the receiver output. Therefore, in the design of the

transmitting and receiving filters, the objective is to minimize the effects of ISI and thereby

deliver the digital data to its destination with the smallest error rate possible.

2.3.4 InterCarrier Interference

Presence of Doppler shifts and frequency and phase offsets in an OFDM system

causes loss in orthogonality of the sub carriers. As a result, interference is observed

between subcarriers. This phenomenon is known as intercarrier interference (ICI).


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2.3.5 Cyclic Prefix

The Cyclic Prefix or Guard Interval is a periodic extension of the last part of an

OFDM symbol that is added to the front of the symbol in the transmitter, and is removed at

the receiver before demodulation.

The cyclic prefix has to two important benefits

The cyclic prefix acts as a guard interval. It eliminates the intersymbol interference

from the previous symbol.

It acts as a repetition of the end of the symbol thus allowing the linear convolution of

a frequency selective multipath channel to be modelled as circular convolution

which in turn maybe transformed to the frequency domain using a discrete Fourier

transform. This approach allows for simple frequency domain processing such as

channel estimation and equalization.

Figure 2.4 Symbol Structure of OFDM word with Cyclic Prefix [2]

2.3.6 Inverse Discrete Fourier Transform

By working with OFDM in frequency domain the modulated QPSK data symbols are

fed onto the orthogonal sub-carriers. But transfer of signal over a channel is only possible in

its time-domain. For which we implement IDFT which converts the OFDM signal in from

frequency domain to time domain.


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IDFT being a linear transformation can be easily applied to the system and DFT can

be applied at the receiver end to regain the original data in frequency domain at the receiver

end. Since the basis of Fourier transform is orthogonal in nature we can implement to get the

time domain equivalent of the OFDM signal from its frequency components.

Usually, in practice instead of DFT and IDFT we implement Fast Fourier Transformation for

an N-input signal system because of the lower hardware complexity of the system.


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2.4Modulation & Demodulation In OFDM Systems

Figure 2.5 Block Diagram of Modulation and Demodulation of OFDM [2]

2.4.1 Modulation: QAM

Modulation is the technique by which the signal wave is transformed in order to send

it over the communication channel in order to minimize the effect of noise. This is done in

order to ensure that the received data can be demodulated to give back the original data. In an

OFDM system, the high data rate information is divided into small packets of data which are

placed orthogonal to each other. This is achieved by modulating the data by a desirable

modulation technique (QAM).

Like all modulation schemes, QAM conveys databy changing some aspect of a

carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case

of QAM, the amplitude of two waves, 90 out-of-phase with each other (in Quadrature) are

changed (modulated or keyed) to represent the data signal. Amplitude modulating two

carriers in Quadrature can be equivalently viewed as both amplitude modulating and phase

modulating a single carrier.
http://en.wikipedia.org/wiki/Modulationhttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Sine_wavehttp://en.wikipedia.org/wiki/Sine_wavehttp://en.wikipedia.org/wiki/Carrier_wavehttp://en.wikipedia.org/wiki/Datahttp://en.wikipedia.org/wiki/Modulation


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2.4.2 IFFT:

After this, IFFT is performed on the modulated signal to convert the signal from

frequency domain to time domain as only a time domain signal can be transmitted over a

carrier. The size of IFFT is chosen with care as the no. of data points used for IFFT affect the

systems performance on PAPR front.

2.4.3 Parallel to Series Convertor

IFFT provides parallel inputs. That needs to be converted into serial one for

transmission over channel. Parallel to series convertor gives a serial output to GII

2.4.4 Guard Interval Insertion

Guard interval or cyclic prefix is added to OFDM symbol to avoid inter symbol

interference. It acts as a repetition of the end of the symbol thus allowing the linear

convolution of a frequency selective multipath channel to be modelled as circular

convolution

2.4.5 Transmit Filter

Transmit filter (BPF) centred around the subcarrier frequencies are used to filter out

the individual subcarrier components of OFDM for noise removal

2.4.6 Communication Channel

This is the channel through which the data is transferred. Presence of noise in this

medium affects the signal and causes distortion in its data content. It can be coaxial cable,

copper wire or wireless RF.

2.4.7 Demodulation Blocks

Demodulation is the technique by which the original data (or a part of it) is recovered

from the modulated signal which is received at the receiver end. In this case, the received

data is first made to pass through a low pass filter to remove any noise inserted while

transmission through channel. The GIR removes the cyclic prefix. FFT of the signal is done

to convert the time domain signal into the original frequency domain. Then it is made to pass

through a serial to parallel converter. A demodulator is used, to get back the original

signal.


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2.5Peak To Average Power Ratio In OFDM: An Overview2.5.1 Introduction

OFDM is one of the many multicarrier modulation techniques, which provides high

spectral efficiency, low implementation complexity, less vulnerability to echoes and non

linear distortion. Due to these advantages of the OFDM system, it is vastly used in various

communication systems. But the major problem one faces while implementing this system is

the high peak to average power ratio of this system. A large PAPR increases the

complexity of the analog to digital and digital to analog converter and reduces the

efficiency of the radio frequency (RF) power amplifier. Regulatory and application

constraints can be implemented to reduce the peak transmitted power which in turn reduces

the range of multi carrier transmission. This leads to the prevention of spectral growth and the

transmitter power amplifier is no longer confined to linear region in which it should operate.

This has a harmful effect on the battery lifetime. Thus in communication system, it is

observed that all the potential benefits of multi carrier transmission can be out - weighed by a

high PAPR value.

There are a number of techniques to deal with the problem of PAPR. Some of them

are amplitude clipping, clipping and filtering, coding, partial transmit sequence (PTS),

selected mapping (SLM) and interleaving. These techniques achieve PAPR reduction at the

expense of transmit signal power increase, bit error rate (BER) increase, data rate loss,

computational complexity increase, and so on .

2.5.2 PeakToAverage Power Ratio

Presence of large number of independently modulated sub-carriers in an OFDM

system the peak value of the system can be very high as compared to the average of the

whole system. This ratio of the peak to average power value is termed as Peak-to-Average

Power Ratio. Coherent addition of N signals of same phase produces a peak which is N times

the average signal.

The major disadvantages of a high PAPR are-

1. Increased complexity in the analog to digital and digital to analog converter.

2. Reduction is efficiency of RF amplifiers.


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2.5.3 PAPR Of A Multicarrier Signal

Let the data block of length Nis represented by a vector .

Duration of any symbol in the set Xis T and represents one of the sub carriers

set. As the N sub carriers chosen to transmit the signal are

orthogonal to each other, so we can have where and NT is the

duration of the OFDM data block X. The complex data block for the OFDM signal to be

transmitted is given by

The PAPR of the transmitted signal is defined as

Reducing the max|x (t)| is the principle goal of PARP reduction techniques. Since,

discrete- time signals are dealt with in most systems, many PAPR techniques are

implemented to deal with amplitudes of various samples ofx (t).

The Cumulative Distribution Function (CDF) is one of the most regularly used

parameters, which is used to measure the efficiency of any PAPR technique. Normally, the

Complementary CDF (CCDF) is used instead of CDF, which helps us to measure the

probability that the PAPR of a certain data block exceeds the given threshold.

By implementing the Central Limit Theorem for a multi carrier signal with a large

number of sub-carriers, the real and imaginary part of the time domain signals have a mean

of zero and a variance of 0.5 and follow a Gaussian distribution. So Rayleigh distribution is

followed for the amplitude of the multi carrier signal, where as a central chi-square

distribution with two degrees of freedom is followed for the power distribution of the system.

The CDF of the amplitude of a signal sample is given by


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The CCDF of the PAPR of the data block is desired is our case to compare outputs of various

reduction techniques. This is given by


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2.6PAPR Reduction Techniques:2.6.1 Introduction

PAPR reduction techniques vary according to the needs of the system and are

dependent on various factors. PAPR reduction capacity, increase in power in transmit signal,

loss in data rate, complexity of computation and increase in the bit-error rate at the receiver

end are various factors which are taken into account before adopting a PAPR reduction

technique of the system.

The PAPR reduction techniques are basically of two types. They are Signal Scrambling

and Signal Distortion. Their respective types are as follows.

Signal Scrambling Techniques

Block Coding

Figure 2.6 Block Diagram : Block Coding Technique[6]

The fundamental idea is that of all probable message symbols, only those which have lawpeak power will be chosen by coding as valid code words for transmission. No introduction

of distortion to the signals [4].


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Selected Mapping

Figure 2.7 Block Diagram : Selective Mapping Technique[7]

In this a set of sufficiently different data blocks representing the information same as the

original data blocks are selected. Selection of data blocks with low PAPR value makes it

suitable for transmission [4].

Partial Transmit Sequence

Figure 2.8 : Block Diagram: Partial Transmit Sequence Technique[8]

Transmitting only part of data of varying sub-carrier which covers all the information to be

sent in the signal as a whole is called Partial Transmit Sequence Technique [4].

Tone Reservation

The main idea of this method is to keep a small set of tones for PAPR reduction. This can be

originated as a convex problem and this problem can be solved accurately. Tone reservation

method is based on adding a data block and time domain signal [4].


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Tone Injection

This technique is based on general additive method for PAPR reduction. Using an additive

method achieves PAPR reduction of multicarrier signal without any data rate loss. TI uses a

set of equivalent constellation points for an original constellation points to reduce PAPR [4].

Interleaving

The notion that highly correlated data structures have large PAPR can be reduced, if long

correlation pattern is broken down. The basic idea in adaptive interleaving is to set up an

initial terminating threshold. PAPR value goes below the threshold rather than seeking each

interleaved sequences [4].

Signal Distortion Techniques

Peak Windowing

Figure 2.9 Process of Peak Windowing[ 9]

This method, proposes that it is possible to remove large peaks at the cost of a slight amount

of self interference when large peaks arise infrequently. Peak windowing reduces PAPRs at

the cost of increasing the BER and out-of-band radiation [4].

Envelope Scaling

Figure 2.10 Block Diagram of Envelop Scaling [10]


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The key idea of this scheme is that the input envelope in some sub carrier is scaled to achieve

the smallest amount of PAPR at the output of the IFFT. Thus, the receiver of the system

doesnt need any side information for decoding the receiver sequence. [4]

Peak Reduction Carrier

It includes the use of a higher order modulation scheme to represent a lower order modulation

symbol. The amplitude and phase of the PRC is positioned within the constellation region

symbolizing the data symbol to be transmitted. This method is suitable for PSK modulation

[4].

Amplitude Clipping And Filtering

A threshold value of the amplitude is set in this process and any sub-carrier having amplitude

more than that value is clipped or that sub-carrier is filtered to bring out a lower PAPR value.

One of the simple and effective PAPR reduction techniques is clipping, which cancels the

signal components that exceed some unchanging amplitude called clip level. However,

clipping yields distortion power, which called clipping noise, and expands the transmitted

signal spectrum, which causes interfering. Clipping is nonlinear process and causes in-band

noise distortion, which causes degradation in the performance of bit BER and out-of-band

noise, which decreases the spectral efficiency [4].

The following table shows a basic comparison of various PAPR reduction schemes according

to multiple criteria.

Table 2.1 Comparison of Various PAPR Reduction Techniques [4].


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2.7Amplitude Clipping And Filtering2.7.1 Introduction

Amplitude clipping is considered as the simplest technique which may be under taken for

PAPR reduction in an OFDM system. A threshold value of the amplitude is set in this case to

limit the peak envelope of the input signal. Signal having values higher than this pre-

determined value are clipped and the rest are allowed to pass through un-disturbed.

Where,

B(x) = the amplitude value after clipping.

x = the initial signal value.

A = the threshold set by the userfor clipping the signal.

The problem in this case is that due to amplitude clipping distortion is observed in the

system which can be viewed as another source of noise. This distortion falls in both in band

and outofband. Filtering cannot be implemented to reduce the in band distortion and

an error performance degradation is observed here. On the other hand spectral efficiency is

hampered by outofband radiation. Outofband radiation can be reduced by filtering

after clipping but this may result in some peak re growth. A repeated filtering and clipping

operation can be implemented to solve this problem. The desired amplitude level is only

achieved after several iteration of this process.

2.7.2 ClippingFiltering

Clipping and filtering technique is effective in removing components of the expanded

spectrum. Although filtering can decrease the spectrum growth, filtering after clipping can

reduce the out-of-band radiation, but may also cause some peak re-growth, which the peak

signal exceeds in the clip level.


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Figure2.11 Block Diagram of iterative clipping-filtering. [3]

The technique of iterative clipping and filtering reduces the PAPR without spectrum

expansion. However, the iterative signal takes long time and it will increase the

computational complexity of an OFDM transmitter. But without performing interpolation

before clipping causes it out-of-band. To avoid out-of-band, signal should be clipped after

interpolation. However, this causes significant peak re-growth. So, it can use iterative

clipping and frequency domain filtering to avoid peak re-growth. In the system used, serial to

parallel converter converts serial input data having different frequency component which are

base band modulated symbols and apply interpolation to these symbols by zero padding in

the middle of input data. Then clipping operation is performed to cut high peak amplitudes

and frequency domain filtering is used to reduce the out of band signal, but caused peak re-

growth. This consists of two FFT operations. Forward FFT transforms the clipped signal back

to discrete frequency domain. The in-band discrete components are passed unchanged to

inputs of second IFFT while out of band components are null. But heavy clipping causes

about 1 dB lower average EVM.

Clipping introduces in band distortion and out-of-band signals, which can be

controlled by proper filtering.

2.7.3 Repeated Clipping and Frequency Domain Filtering

A clipping method in its basic form is based on simple time domain signal limitation.

Clipped signal can be Expressed by following relationship:


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Where A is the clipping level and is the phase of original signal S (t) by this

limitation, the peak values of signal are removed that results in PAPR reduction. However,

the clipping introduces signal distortion resulting in adjacent channel emissions. This

undesirable effect can be suppressed by low pass filtering of clipped signal that unfortunatelyfurther increases the PAPR.

Armstrong developed a method based on K-times repetition of the clipping and

filtering process. [5]. Therefore both PAPR and adjacent spectral emissions are reduced,

although the PAPR reduction is far from simple clipping case. In this paper results for

repeated clipping are discussed.

2.7.4 Combination of Interleaving With Repeated Clipping and Filtering

In paper, authors used a combination of interleaving (adaptive sym1bol selection)

with simple clipping followed by a filter increasing the PAPR. We have chosen a

concatenation of interleaving and repeated clipping and frequency domain filtering or its

simplified non iterative alternative. First, the interleaving approach is used and the signal

with lowest PAPR is then passed through clipping and filtering method. The intention to

combine these two methods is to obtain signal with lower PAPR than in the case of

interleaving method and with lower distortion (and thus lower bit error rate) than in the case

of standalone Repeated clipping and filtering.

As both methods used in the combination suffer from high complexity, the main

disadvantage of the combined method is above all the complexity. Moreover, side

information (SI) to identify the interleaver with lowest PAPR has to be sent to receiver for

each OFDM symbol. Without this side information, it is not possible to decode the data. As

the correct decoding of side information is fundamental for the performance of OFDM

modem, the SI can thus be either mapped using modulation with lower number of states or

encoded by FEC.The complexity of the presented combined method can be dramatically reduced using the

recently proposed method Simplified clipping and filtering instead of the repeated clipping

and frequency domain filtering method. This case has been also considered in our paper and

this method is recommended for practical use.


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3. System Performance.As discussed in earlier chapters, OFDM is an advantageous scheme used in almost all

modern and up to date communication systems. The main disadvantage it has is the high

PAPR. The report suggests the method of repetitive (iterative) clipping and filtering to

overcome this problem. In the system overview some simulation results are shown to

illustrate the point that PAPR can be effectively reduced using the suggested technique.

The first simulation result shows that PAPR is reduced by the use of suggested technique

for phase shift keying modulation. The table gives the near accurate PAPR readings before

and after clipping-filtering and high power amplification for QPSK and BPSK. It also shows

that PAPR values also depend upon IFFT size and number of data points.

Table3.1 PAPR without and with clipping for PSK [1]

The next table is the obtained simulation result for single iteration of clipping

filtering for QAM. The table considers different numbers of frame and different power

spectral densities of QAM. It shows significant decline in PAPR and a small value of Bit

Error Rate (BER)


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Table3.1 PAPR before and after clippingfiltering for QAM [1]

The third table shows the results of repetitive clipping and filtering for various numbers of

data points. A significant decline is observed for every iteration of the clippingfiltering

operation.

Table3.2 PAPR for QAM: Original and repetitive clipping - filtering. [2]


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4. Conclusions and Future Scope

During the literature survey for this seminar, I have covered the basics of FDM, OFDM

and PAPR reduction. I have also taken introduction of various schemes for PAPR reduction

and elaborated clipping filtering.

This report also contains detailed description of OFDM basics, Modulation and

demodulation process and entire theoretical explanation for the execution of Clipping-

Filtering method of reduction in PAPR. The system Performance overview also shows the

results of simulations that are referenced bellow.

In this paper some PAPR reduction carried out by clipping technique in two ways for

modulation techniques like BPSK, QPSK, 16-QAM. Results are compared as per the tabular

data shown over here. Here we can conclude that in case of BPSK modulation we get

maximum PAPR reduction for IFFT size of 32 while data points are 512. Its up to:

13.525361 dB. Here also we can conclude that In case of QPSK modulation we get maximum

PAPR reduction while IFFT size is 32 for 512 data bits. Its up to: 17.384791 dB.

The robust high-bandwidth capabilities of orthogonal frequency division multiplexing

(OFDM) confer immediate advantages on wireless products that can take advantage of it--and

many types of networking systems are doing so. OFDM underlies the existing IEEE 802.11a

wireless LAN (WLAN) standard and the proposed IEEE 802.11g WLAN standard, as well as

digital cable, DSL, digital TV, and power-line networking products. OFDM is also being

considered for use in 4G cellular systems.

The reasons for this widespread interest become clear from a glance at OFDM

characteristics. In 802.11a, OFDM provides raw data rates up to 54 Mbits/s in a 20-MHz

channel. In addition to supporting high data capacity and resisting degradation from various

types of radio effects, OFDM makes highly efficient use of the available spectrum. The latter

characteristic will become crucial in coming years as wireless networks are built out,

especially in enterprise environments.

4.1Managing Imperfect Airwaves

All wireless systems have to deal with the many unruly ways in which radio signalsbehave in the real world. Along with the general challenges of signal-to-noise ratio, the main


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types of problems are self-interference (intersymbol interference or ISI) and fading owing to

multipath effects, which occur when the same signal arrives at a receiver via different paths.

The main way to prevent multipath errors is to transmit a short block of data (a symbol)

then wait until all the multipath echoes fade before sending another symbol. This waiting

time is often referred to as the guard interval.

The longer the guard interval, the more robust the system is in the presence of multipath

effects. But during the guard interval, the system gets no use from the available spectrum. So

the longer the wait, the lower the effective channel capacity.

Some guard interval is necessary for any wireless system, but the goal is to minimize that

interval and maximize the symbol transmission time. OFDM meets this challenge by dividing

transmissions among multiple subcarriers. The same guard interval can then be applied to

each subcarrier, while the symbol transmission time is multiplied by the number of

subcarriers.

Since 802.11a OFDM uses 52 subcarriers, for example, an 802.11a WLAN can afford 52

times the guard interval than a single-carrier system could. The 802.11a subcarriers are

spaced 312.5-kHz apart. The symbol period is 3.2 s plus an 800-ns guard interval.

The system thus tolerates peak multipath delays of nearly 800 ns. Compared with the 65

ns of multipath tolerance provided by many 802.11b direct sequence spread spectrum (DSSS)

based products, OFDM represents a 12-plus times improvement in multipath tolerance.

Using multiple subcarriers also makes OFDM systems more robust in the presence of

fading. Because fading typically decreases the received signal strength at particular

frequencies, the problem affects only a few of the subcarriers at any given time. Error-

correcting codes provide redundant information that enables OFDM receivers to restore the

information lost in these few erroneous subcarriers.

Each of the subcarriers in an OFDM system can be modulated individually using

whatever technique suits the application. In 802.11a, the choices include BPSK, QPSK, 16-

QAM, and 64-QAM.


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After modulation, the data from all the subcarriers are converted to a single stream of

symbols for transmission. At the receiver, the stream is converted to the frequency domain

via fast Fourier transform (FFT), then each "frequency bin" (subcarrier) is decoded

separately.

4.2Need of OrthogonalityTraditionally, frequency division multiplexing (FDM) has used conventional filtering to

separate subcarriers at the receiver. This approach required the insertion of significant guard

bands between the subcarriers (different from the guard intervals that prevent ISI).

Making the subcarriers mathematically orthogonal was a breakthrough for OFDM

because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the

guard bands. As Fig. 1 shows, OFDM subcarriers can overlap to make full use of the

spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is

zero.

OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler

system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse

FFT.

To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are

not misaligned), the system must keep the transmitter and receiver clocks closely

synchronized--within 2 parts per million in 802.11a systems. The 802.11a standard therefore

dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to

track the phase and frequency of the incoming signal. This method also eliminates low-

frequency phase noise.

Separating the subcarriers via an FFT requires about an order of magnitude fewer

multiply-accumulate operations than individually filtering each carrier. In general, an FFT

implementation is much simpler than the RAKE receivers used for CDMA and the decision-

feedback equalizers for TDMA.

The complexity advantage of OFDM grows dramatically as the data rate increases. The

complexity of the transceivers is hidden inside the chipsets that implement a particular

standard, but reducing device complexity and signal-processing requirements lead to benefits


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customers can see. A simpler chip is more reliable and can reduce costs. Perhaps more

important, the simpler circuitry helps reduce the system's power demands--a crucial

advantage for mobile devices.

4.3The Wireless FutureSince inexpensive and high-performance CMOS 802.11a chipsets entered the market in

September 2001, the relative merits of 802.11a and 802.11b have increasingly been debated.

However, 802.11a's underlying OFDM technology is easily superior to 802.11b's DSSS

approach in terms of both bandwidth and robustness, so for technologists the debate has been

a nonstarter.

The supporters of 802.11b have even adopted OFDM as the technology of choice for

eventual successor products to 802.11b in the 2.4-GHz band. The proposed standard for these

products is 802.11g.

The introduction of chipsets that support both 802.11a and 802.11b as well as the

802.11g draft standard has resolved the debate about WLAN standards. With the ability to

choose any of these standards, increasing numbers of users will see the advantages of OFDM

first hand. 802.11a's combination of OFDM and the interference-free 300-MHz-wide 5-GHz

band are proving that the future of WLANs lies in this direction. [11]


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References

1. Papr Reduction In Ofdm By Clipping Technique, By Prashant Maruti Jadhav,

L.S.Admuthe & A.P.Bhadvankar In International Journal Of Electronics,

Communication & Instrumentation Engineering Research And Development

(Ijecierd) Vol. 2 Issue 4 Dec 2012 71-80

2. Papr Reduction In Ofdm Using Clipping And Filtering By W. Aziz, E. Ahmed, G.

Abbas, S. Saleem And Q. Islam In World Applied Sciences Journal 18 (11): 1495-

1500, 2012

3. Ofdm Systems And Papr Reduction Techniques In Ofdm Systems: A Thesis

Submitted In Partial Fulfillment Of The Requirements For The Degree Of Bachelor

Of Technology In Electronics And Communication Engineering By Abhishek Arun

Dash And Vishal Gagrai Department Of Electronics And Communication

Enginnering National Institute Of Technology, Rourkela

4. Comparative Study Of Papr Reduction Techniques In Ofdm By Md. Ibrahim

Abdullah, Md. Zulfiker Mahmud, Md. Shamim Hossain, Md. Nurul Islam In Arpn

Journal Of Systems And Software Vol. 1, No. 8, November 2011

5. Peak To Average Power Ratio Reduction For Ofdm By Repeated Clipping And

Frequency Domain Filtering By J. Armstrong In Electronics Letters, 28 th February

2002, Vol. 38, No. 5

6. Peak-To-Average Power Ratio Reduction In Ofdm System Using Block Coding

Technique By Ms Snehal B. Meshram In International Journal Of Research In

Computer And Communication Technology, Ijrcct, Issn 2278-5841, Vol 1, Issue 7,

December 2012.


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7. Papr Reduction Using Modified Selective Mapping Technique And Turbo Coding By

Shweta Jain*, Vikas Gupta And Divya Jain In Ijaet/Vol.Ii/ Issue Iv/October-

December, 2011

8. An Efficient Method For Papr Reduction In Ofdmsystems With Reduced Complexity

By 1pritanjali Kumari & 2us Triar In International Journal Of Electrical And

Electronics Engineering (Ijeee), Issn (Print): 22315284 Vol-1 Iss-4, 2012

9. An Enhancement Of Peak To Average Power Ratio Reduction In Ofdm Using Cap-Pt

Method By C. Raja Rajeshwari1, K. Manojkumar2 In International Journal Of

Modern Engineering Research (Ijmer) Www.Ijmer.Com Vol.2, Issue.5, Oct-Oct. 2012

Pp-3699-3704

10.Papr Reduction In Ofdm System By P. Foomooljareon And W.A.C. Fernando In

Thammasat Int. J. Sc. Tech., Vol.7, No.3, September-December 2002

11.Http://Www.Electronicproducts.Com/Analog_Mixed_Signal_Ics/Ofdm_Carries_The_

Future_Of_Wireless_Networking.Aspx
http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspxhttp://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspxhttp://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspxhttp://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspxhttp://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspxhttp://www.electronicproducts.com/Analog_Mixed_Signal_ICs/OFDM_carries_the_future_of_wireless_networking.aspx


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ACKNOWLEDGEMENT

The potion of success is brewed by the efforts put in by many individuals. It isconstant support provided by people who give you the initiative, who inspire you at each step

of your endeavour that eventually helps you in your goal.

I wish to express my deep gratitude and heartily appreciation for the invaluable

guidance of our professors throughout the span of preparing this seminar. We are indebted to

our college Principal Dr. S. P. Bhosle.

I am also thankful to ourHOD and my Seminar Guide Prof. Mrs. V. M. Kulkarni

for her invaluable and elaborate suggestions. Her excellent guidance made me to complete

this task successfully within a short duration.

The inspiration behind the every aspect of life constructs a way to get success, which I

have got from all the professors of the department.

No thanks giving would be complete without mentioning our parents and friends,

without their constant support and encouragement, this assignment would have not been

successful.

SAMHITA HISWANKAR

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Peak to Average Power Ratio Reduction in Ofdm

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