U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page i

A PROJECT REPORT

ON

“ICI SELF CANCELLATION TECHNIQUES IN

OFDM”

B.TECH- IV (ELECTRONICS & COMMUNICATION)

SUBMITTED BY:

SUNNY GANGISETTI (U11EC033)

BARANWAL SWEETY BINDUKUMAR (U11EC051)

IPPALA P BHAVANI SHANKAR REDDY (U11EC121)

BHASKARANI SAI KRISHNA PRADEEP (U10EC133)

ARIYARATNAM ARCHCHUNAH (U11EC144)

GUIDED BY:

PROF. SHILPI GUPTA

ECED, SVNIT

DEPARTMENT OF ELECTRONICS ENGINEERING

Year: 2014-15

SARDAR VALLABHBHAI NATIONAL INSTITUTE OF

TECHNOLOGY (SVNIT)

SURAT-395007

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page ii

Acknowledgements

It gives us immense pleasure to present our project report on “ICI Self Cancellation

Techniques in OFDM”. No work, big or small, has ever been done without contributions of

others.

We would like to express deep gratitude towards Prof. Shilpi Gupta (Assistant professor at

Electronics Engineering Department, SVNIT) who gave us valuable suggestions,

motivation and the direction to proceed at every stage. She extended towards a kind and

valuable guidance, indispensible help and inspiration at times. In appreciation we offer her our

sincere gratitude.

In addition, we would like to thank Dr. Upena. D. Dalal (Head of Electronics Engineering

Department, SVNIT) and the entire Department for providing all the required resources for

our project. Finally, yet importantly, we would like to express our heartfelt thanks to our

beloved families for their blessings and my friends/classmates for their help and wishes for the

successful completion of this project.

SUNNY GANGISETTI (U11EC033)

BARANWAL SWEETY BINDUKUMAR (U11EC051)

IPPALA P BHAVANI SHANKAR REDDY (U11EC121)

BHASKARANI SAI KRISHNA PRADEEP (U10EC133)

ARIYARATNAM ARCHCHUNAH (U11EC144)

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page iii

Abstract

OFDM is a promising Technique for achieving high data rates in mobile environment because

of its multicarrier modulation technique and ability to convert a frequency selective fading

channel into several nearly flat fading channels. As the sub carriers are orthogonal, the

spectrum of each carrier has a null at the center frequency of each of the other carriers in the

system. Rapid varying channel between mobile User Equipment (UE) and base station in high

vehicular speed environment have led to significant degradation in Signal to Noise ratio due to

destruction of orthogonality.

A well known problem of orthogonal frequency division multiplexing (OFDM), however, is

its sensitivity to frequency offset between the transmitted and received signals, which may be

caused by Doppler shift in the channel, or by the difference between the transmitter and receiver

local oscillator frequencies. This carrier frequency offset causes loss of orthogonality between

sub-carriers and the signals, transmitted on each carrier are not independent of each other. The

orthogonality between subcarriers in orthogonal frequency division Multiplexing (OFDM)

which leads to inter-carrier interference (ICI). In the literature, various studies have been

proposed to cancel the effects of ICI in high speed scenario like mobile, railway and

aeronautical communication. Two different approaches like estimation and compensation of

the frequency offset/phase noise at the receiver and another approach is to use signal processing

techniques in the transmitter for reducing carrier/phase offsets by using frequency domain

coding called as ICI self-cancellation at transmitter have been studied.

In ICI self cancellation various techniques are used to map one data symbol on two sub carrier

with careful selection of weighting coefficient. There are Data conversion scheme, Data

conjugate scheme, Real constant weighted scheme, Plural weighted scheme and Symmetric

data conversion scheme. These schemes improve the system performance or mitigate the effect

of ICI and also simulate their BER and CIR performances.

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page iv

Sardar Vallabhbhai National Institute of Technology, Surat-07

Electronics Engineering Department

CERTIFICATE

This is to certify that candidates Mr..Bhaskarani.Sai.Krishna.Pradeep (U10EC133),

Mr..Sunny.Gangisetti (U11EC033), Ms..Baranwal.Sweety.Bindukumar (U11EC051),

Mr..Ippala.P.Bhavani.Shankar.Reddy (U11EC121), Mr..Ariyaratnam..Archchunah

(U11EC144) of B.TECH IV, 8TH Semester have successfully and satisfactorily presented

Project Report on the topic entitled “ICI Self Cancellation Techniques in OFDM” for the

partial fulfillment of the degree of Bachelor of Technology (B.Tech) in May. 2015.

Guide: Name: Prof. Shilpi Gupta Sign: ______________

Examiner 1 Name: ______________ Sign: ______________

Examiner 2 Name: ______________ Sign: ______________

Examiner 3 Name: ______________ Sign: ______________

Head,

ECED, SVNIT.

(Seal of the Department)

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page v

CONTENTS

TOPICS Page No.

LIST OF FIGURES vii

CHAPTERS

1. Introduction 1

1.1 Objective of Thesis 3

1.2 Planning of the Thesis 3

1.3 Outline of the Thesis 3

1.4 Literature Survey 4

2. Basics Of OFDM 6

2.1 Orthogonality 6

2.2 OFDM Generation and Reception 7

2.2.1 Signal Mapping 8

2.2.2 Serial to Parallel & Parallel to Serial Conversion 9

2.2.3 Frequency to Time Domain Conversion 9

2.3 Inter-symbol and Inter-carrier interference 9

2.4 Guard Period 10

2.5 Cyclic Prefix 11

2.6 Different channels which are employed in the project 12

2.6.1 AWGN Channel 12

2.6.2 Rayleigh Channel 13

2.7 OFDM Advantages 14

2.8 Limitations of OFDM 14

2.9 Methods of ICI Reduction 15

2.9.1 Frequency Domain Equalization 15

2.9.2 Time Domain Windowing 16

2.9.3 Pulse Shaping 17

2.9.4 ICI Self Cancellation 17

2.10 OFDM variants and applications 18

2.10.1 OFDM variants 18

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2.10.2 Applications of OFDM 19

3. ICI Self Cancellation Techniques 20

3.1 ICI Self Cancellation 20

3.2 System Model 20

3.3 Analysis Of Inter Carrier Interference 21

3.4 ICI Self Cancellation Scheme 23

3.4.1 ICI Cancelling Modulation 24

3.4.2 ICI Cancelling Demodulation 25

3.5 Various ICI Self Cancellation Techniques 26

3.5.1 Data Conversion ICI Self Cancellation Technique 26

3.5.2 Data Conjugate ICI Self Cancellation Technique 27

3.5.3 Symmetric Data Conversion ICI Self Cancellation Technique 27

3.5.4 Real Constant Weighted Conversion ICI Self Cancellation Technique 28

3.5.5 Plural Weighted Conversion ICI Self Cancellation Technique 28

3.5.6 Plural Conjugate Data Conversion ICI Self Cancellation Technique 28

4. Simulation Results 29

4.1 OFDM Model Used For Simulation 29

4.2 Graphs 29

5. Conclusion 35

5.1 Scope of Future Work 36

REFERENCES 37

ACRONYMS 39

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LIST OF FIGURES

Figure 1: Orthogonally placed OFDM sub-carriers 7

Figure 2: Block diagram of a typical OFDM Transceiver 7

Figure 3: Guard period insertion in OFDM 11

Figure 4: Pilot sub carrier arrangement 16

Figure 5: Spectrum of a 64 sub carrier OFDM 16

Figure 6: N-sub carrier OFDM system model 21

Figure7: Frequency offset model 21

Figure 8: ICI Coefficients for N=16 Carriers 23

Figure 9: Comparison of ICI coefficients 24

Figure 10: CIR versus epsilon for standard and self-cancellation applied OFDM 26

Figure 11: OFDM Signal at transmitter end 29

Figure 12: OFDM Signal after passing through channel 30

Figure 13: SNR Vs BER Graph 30

Figure 14: SNR Vs BER Curve for Rayleigh and AWGN Channels 31

Figure 15: Bit error probability curve for different offsets 31

Figure 16: OFFSET Vs CIR 32

Figure 17: CIR curve for symmetric data conversion self cancellation scheme 32

Figure 18: CIR curve for conjugate data self cancellation scheme 33

Figure 19: CIR curve for plural weighted self cancellation scheme 33

Figure 20: CIR curve for different self cancellation schemes together 34

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Chapter 1: Introduction

The ever increasing demand for very high rate wireless data transmission calls for technologies

which make use of the available electromagnetic resource in the most intelligent way. Key

objectives are spectrum efficiency, robustness against multipath propagation, power

consumption and implementation complexity. These objectives are often conflicting, so

techniques and implementations are sought which offer the best possible trade off between

them. The Internet revolution has created the need for wireless technologies that can deliver

data at high speeds in a spectrally efficient manner. However, supporting such high data rates

with sufficient robustness to radio channel impairments requires careful selection of

modulation techniques.

Currently, the most suitable choice appears to be OFDM (Orthogonal Frequency Division

Multiplexing).Orthogonal frequency division multiplexing (OFDM) is becoming the chosen

modulation technique for wireless communications. OFDM can provide large data rates with

sufficient robustness to radio channel impairments.

Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multi carrier

modulation technique which is used to generate waveforms that are mutually orthogonal. In an

OFDM scheme, a large number of orthogonal, overlapping, narrow band sub-carriers are

transmitted in parallel. These carriers divide the available transmission bandwidth. The

separation of the sub-carriers is such that there is a very compact spectral utilization. With

OFDM, it is possible to have overlapping sub channels in the frequency domain, thus

increasing the transmission rate.[1]

OFDM has been accepted as standard in several wire line and wireless applications. Due to the

recent advancements in digital signal processing (DSP) and very large-scale integrated circuits

(VLSI) technologies, the initial obstacles of OFDM implementations do not exist anymore. In

a basic communication system, the data are modulated onto a single carrier frequency. The

available bandwidth is then totally occupied by each symbol. This kind of system can lead to

inter-symbol-interference (ISI) in case of frequency selective channel.

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The attraction of OFDM is mainly because of its way of handling the multipath interference at

the receiver. Multipath phenomenon generates two effects (a) Frequency selective fading and

(b) Inter symbol interference (ISI). The "flatness" perceived by a narrowband channel

overcomes the frequency selective fading. On the other hand, modulating symbols at a very

low rate makes the symbols much longer than channel impulse response and hence reduces the

ISI.

The focus of future fourth-generation (4G) mobile systems is on supporting high data rate

services such as deployment of multi-media applications which involve voice, data, pictures,

and video over the wireless networks. At this moment, the data rate envisioned for 4G networks

is 1 GB/s for indoor and 100Mb/s for outdoor environments. Orthogonal frequency division

multiplexing (OFDM) is a promising candidate for 4G systems because of its robustness to the

multipath environment.

The main disadvantages of OFDM are the inter carrier interference (ICI) and high peak to

average power ratio (PAPR), which is caused by the sensitivity of the OFDM system due to

carrier frequency offset and Doppler shift and a large variation in envelope of OFDM signal

The undesired ICI degrades the performance of the system. To reduce the ISI, a guard interval

larger than that of the estimated delay spread is added. If the guard interval is left empty, the

orthogonality of the sub carriers no longer holds, i.e., ICI (inter carrier interference) still exists.

To prevent both the ISI as well as the ICI, OFDM symbol is cyclically extended into the guard

interval. Thus, an accurate and efficient Intercarrier Interference (ICI) reduction procedure is

necessary to demodulate the received data.

To mitigate the effect of ICI in OFDM system, ICI cancellation techniques such as frequency

domain equalization, time domain windowing method, pulse shaping and ICI self cancellation

are used. These different ICI cancellation techniques are used for different applications and

different causes of ICI. ICI self cancellation techniques is very simple and easy way to

minimize the effect of ICI in OFDM system. So, we have focused on ICI self cancellation

technique which eliminates the inter carrier interference in sub-carriers of OFDM symbols in

a OFDM signal. The various ICI self cancellation techniques are data conversion schemes, data

conjugate scheme, plural weighted scheme, real constant weighted scheme and symmetric data

conversion scheme. But the cost of the ICI self cancellation techniques are reduction in the

bandwidth efficiency.

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1.1 Objective of Thesis

The main objective of this thesis is to reduce the effect of ICI in OFDM system using ICI self

cancellation techniques to improve the performance of OFDM system in terms of inter carrier

interference (ICI), bit error rate (BER), carrier to interference ratio (CIR).

Proper ICI self cancellation techniques makes a system possible to transmit data with minimum

interference. Several scheme of ICI self cancellations have been presented to reduce ICI

including Data conversion scheme, Data conjugate scheme, Plural weighted scheme, real

constant weighted scheme. Different modulation techniques (BPSK, QPSK, QAM) are

considered for ICI self cancellation and compared their BER performances over AWGN

(Additive White Gaussian Channel) and Rayleigh channel.

1.2 Planning of the Thesis

In this particular thesis following steps have been followed:

First the concept of OFDM system was studied and understood their features to

implement in MATLAB.

The concept of ICI self cancellation was studied in detail to mitigate the effect of ICI

in OFDM system.

A number of ICI self cancellation schemes are studied, these ICI self cancellation

schemes are simulated and compared in terms of BER and CIR performance.

1.3 Outline of the Thesis

The organization of this thesis follows in this mannerism

Chapter 1: Introduction to the OFDM system and ICI schemes is done. Literature

survey related to Orthogonal Frequency Division Multiplexing (OFDM) system,

various ICI self cancellation schemes has been discussed.

Chapter 2: Introduces the brief description of basic Orthogonal Frequency Division

Multiplexing system. The concept of orthogonality is discussed in detail. OFDM

system including its transmission and reception, advantages, applications and its major

limitations such as peak-to-average power ratio (PAPR) and inter-carrier interference

(ICI) are also briefly discussed in this chapter.

Chapter 3: This chapter introduces the mechanism of ICI in OFDM system and

analyzes the effect of ICI on the received signal with different normalized frequency

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offset values. The concept of ICI self cancellation such as ICI self cancellation

modulation and ICI self cancellation demodulation are also discussed. A comparison

has been made for different Self Cancellation techniques CIR (carrier to interference

ratio) of conventional OFDM system with the CIR of ICI self cancellation OFDM

system.

Chapter 4: Simulation results using MATLAB software have been shown. This

chapter contain all results such as, carrier to interference ratio, bit error rate

performance comparison of OFDM system. These CIR performance comparisons have

been made by taking into account the combination of various ICI self cancellation

schemes.

Chapter 5: Conclusion drawn from the simulation and future scope has been

incorporated in this thesis.

1.4 Literature Survey

It is well known that Chang proposed the original OFDM principles in 1966, and successfully

achieved a patent in January of 1970. OFDM is a technique for transmitting data in parallel by

using a large number of modulated sub-carriers. These sub-carriers divide the available

bandwidth and are sufficiently separated in frequency so that they are orthogonal. The

orthogonality of the carriers means that each carrier has an integer number of cycles over a

symbol period.

In 1971, Weinstein and Ebert proposed a modified OFDM system [7] in which the discrete

Fourier Transform (DFT) was applied to generate the orthogonal subcarriers waveforms

instead of the banks of sinusoidal generators. Their scheme reduced the implementation

complexity significantly, by making use of the inverse DFT (IDFT) modules and the digital-

to-analog converters. In their proposed model, baseband signals were modulated by the IDFT

in the transmitter and then demodulated by DFT in the receiver. Therefore, all the subcarriers

were overlapped with others in the frequency domain, while the DFT modulation still assures

their orthogonality.

Cyclic prefix (CP) or cyclic extension was first introduced by Peled and Ruiz in 1980 [8] for

OFDM systems. In their scheme, conventional null guard interval is substituted by cyclic

extension for fully-loaded OFDM modulation. As a result, the orthogonality among the

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subcarriers was guaranteed. With the trade-off of the transmitting energy efficiency, this new

scheme can result in a phenomenal ISI (Inter Symbol Interference) reduction. Hence it has been

adopted by the current IEEE standards. In 1980,Hirosaki introduced an equalization algorithm

to suppress both inter symbol interference (ISI) and ICI [9], which may have resulted from a

channel distortion, synchronization error, or phase error. In the meantime, Hirosaki also applied

QAM modulation, pilot tone, and trellis coding techniques in his high-speed OFDM system,

which operated in voice-band spectrum.

In 1985, Cimini introduced a pilot-based method to reduce the interference emanating from the

multipath and co-channels [10]. In the 1990s, OFDM systems have been exploited for high

data rate communications. In the IEEE 802.11 standard, the carrier frequency can go up as high

as 2.4 GHz or 5 GHz. Researchers tend to pursue OFDM operating at even much higher

frequencies nowadays. For example, the IEEE 802.16 standard proposes yet higher carrier

frequencies ranging from 10 GHz to 60 GHz. However, one of the main disadvantages of

OFDM is its sensitivity against carrier frequency offset which causes inter carrier interference

(ICI). The undesired ICI degrades the performance of the system. Number of authors has

suggested different methods for ICI reduction. These methods are investigated in this thesis

and their performances are evaluated

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Chapter 2: Basics of OFDM

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission

technique, which divides the bandwidth into many carriers, each one is modulated by a low

rate data stream In term of multiple access technique, 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 spaced carriers.

2.1 Orthogonality

Two signals are orthogonal if their dot product is zero. That is, if you take two signals multiply

them together and if their integral over an interval is zero, then two signals are orthogonal in

that interval.The reason why the information transmitted over the carriers in OFDM can still

be separated is the so called orthogonality relation giving the method its name. By using an

IFFT for modulation we implicitly chose the spacing of the subcarriers in such a way that at

the frequency where we evaluate the received signal (indicated as peaks) all other signals are

zero. In order for this orthogonality to be preserved the following must be true: [2]

1. The receiver and the transmitter must be perfectly synchronized. This means they both

must assume exactly the same modulation frequency and the same time-scale for

transmission (which usually is not the case).

2. The analog components, part of transmitter and receiver, must be of very high quality.

3. There should be no multipath channel.

In particular the last point is quite a pity, since we have chosen this approach to combat

the multipath channel. Fortunately there's an easy solution for this problem: The

OFDM symbols are artificially prolonged by periodically repeating the 'tail' of the

symbol and precede the symbol with it. At the receiver this so called guard interval is

removed again. As long as the length of this interval 𝚫 is longer than the maximum

channel delay 𝑻𝒎𝒂𝒙 all reflections of previous symbols are removed and the

Orthogonality is preserved.

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Figure.1: Orthogonally placed OFDM Subcarriers [13]

2.2 OFDM Generation And Reception

Figure.2: Block diagram of a typical OFDM Transceiver

The high data rate serial input bit stream is fed into serial to parallel converter to get low data

rate output parallel bit stream. Input bit stream is taken as binary data. The low data rate parallel

bit stream is modulated in Signal Mapper.

Modulation can be BPSK, QPSK, QAM etc. The modulated data are served as input to inverse

fast Fourier transform so that each subcarrier is assigned with a specific frequency. The

frequencies selected are orthogonal frequencies. In this block, orthogonality in subcarriers is

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page 8

introduced. In IFFT, the frequency domain OFDM symbols are converted into time domain

OFDM symbols. Guard interval is introduced in each OFDM symbol to eliminate inter symbol

interference (ISI).

All the OFDM symbols are taken as input to parallel to serial data. These OFDM symbols

constitute a frame. A number of frames can be regarded as one OFDM signal. This OFDM

signal is allowed to pass through digital to analog converter (DAC). In DAC the OFDM signal

is fed to RF power amplifier for transmission. Then the signal is allowed to pass through

additive white Gaussian noise channel (AWGN channel).

At the receiver part, the received OFDM signal is fed to analog to digital converter (ADC) and

is taken as input to serial to parallel converter. In these parallel OFDM symbols, Guard interval

is removed and it is allowed to pass through Fast Fourier transform. Here the time domain

OFDM symbols are converted into frequency domain. After this it is fed into Signal Demapper

for demodulation purpose. And finally the low data rate parallel bit stream is converted into

high data rate serial bit stream which is in form of binary.

2.2.1 Signal Mapping

A large number of modulation schemes are available allowing the number of bits transmitted

per carrier per symbol to be varied. Digital data is transferred in an OFDM link by using a

modulation scheme on each subcarrier. A modulation scheme is a mapping of data words to a

real (In phase) and imaginary (Quadrature) constellation, also known as an IQ constellation.

For example 256-QAM (Quadrature Amplitude Modulation) has 256 IQ points in the

constellation constructed in a square with 16 evenly spaced columns in the real axis and 16

rows in the imaginary axis.

The number of bits that can be transferred using a single symbol corresponds to log2𝑀, where

M is the number of points in the constellation, thus 256-QAM transfers 8 bits per symbol.

Increasing the number of points in the constellation does not change the bandwidth of the

transmission, thus using a modulation scheme with a large number of constellation points,

allows for improved spectral efficiency. However, the greater the number of points in the

modulation constellation, the harder they are to resolve at the receiver.

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2.2.2 Serial to Parallel and Parallel to Serial Conversion

Data to be transmitted is typically in the form of a serial data stream. In OFDM, each symbol

transmits a number of bits and so a serial to parallel conversion stage is needed to convert the

input serial bit stream to the data to be transmitted in each OFDM symbol. The data allocated

to each symbol depends on the modulation scheme used and the number of subcarriers. At the

receiver the reverse process takes place, with the data from the subcarriers being converted

back to the original serial data stream. [3]

2.2.3 Frequency to Time Domain Conversion

The OFDM message is generated in the complex baseband. The frequency spacing between

adjacent subcarriers is Nπ/2, where N is the number of subcarriers. This can be achieved by

using the inverse discrete Fourier transform (IDFT), easily implemented as the inverse fast

Fourier transform (IFFT) operation. As a result, the OFDM symbol generated for an N-

subcarrier system translates into N samples, with the ith sample being, [3, 4]

𝑥𝑘=∑ 𝑐𝑛𝑁−1𝑛=0 𝑒−𝑗2𝜋𝑘𝑛/𝑁 , 0 ≤ k ≤ N-1. (2.1)

At the receiver, the OFDM message goes through the exact opposite operation in the Fast

Fourier transform (FFT) to take the corrupted symbols from a time domain form into the

frequency domain. In practice, the baseband OFDM receiver performs the Fast Fourier

transform (FFT) of the receive message to recover the information that was originally sent.

2.3 Inter-Symbol and Inter-Carrier Interference

In a multipath environment, a transmitted symbol takes different times to reach the receiver

through different propagation paths. From the receiver‘s point of view, the channel introduces

time dispersion in which the duration of the received symbol is stretched. Extending the symbol

duration causes the current received symbol to overlap previous received symbols and results

in inter symbol interference (ISI).

In OFDM, ISI usually refers to interference of an OFDM symbol by previous OFDM symbols.

For a given system bandwidth the symbol rate for an OFDM signal is much lower than a single

carrier transmission scheme. However for OFDM the system bandwidth is broken up into N

subcarriers, resulting in a symbol rate that is N times lower than the single carrier transmission.

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page 10

This low symbol rate makes OFDM naturally resistant to effects of Inter-Symbol Interference

(ISI) caused by multipath propagation. Multipath propagation is caused by the radio

transmission signal reflecting off objects in the propagation environment, such as walls,

buildings, mountains, etc. These multiple signals arrive at the receiver at different times due to

the transmission distances being different. This spreads the symbol boundaries causing energy

leakage between them.

In OFDM, the spectra of subcarriers overlap but remain orthogonal to each other. This means

that at the maximum of each sub-carrier spectrum, all the spectra of other subcarriers are zero.

The receiver samples data symbols on individual sub-carriers at the maximum points and

demodulates them free from any interference from the other subcarriers. Interference caused

by data symbols on adjacent sub-carriers is referred to Inter-carrier interference (ICI).

ICI occurs when the multipath channel varies over one OFDM symbol time. When this

happens, the Doppler shift on each multipath component causes a frequency offset on the

subcarriers, resulting in the loss of orthogonality among them. This situation can be viewed

from the time domain perspective, in which the integer number of cycles for each subcarrier

within the FFT interval of the current symbol is no longer maintained due to the phase transition

introduced by the previous symbol. Finally, any offset between the subcarrier frequencies of

the transmitter and receiver also introduces ICI to an OFDM symbol.

2.4 Guard Period

The effect of ISI on an OFDM signal can be further reduced by the addition of a guard period

to the start of each symbol. This guard period is a cyclic copy that extends the length of the

symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol

with no guard period added, which is equal to the length of the IFFT size used to generate the

signal) has an integer number of cycles. Because of this placing copies of the symbol end-to-

end results in a continuous signal, with no discontinuities at the joins. In addition to

protecting the OFDM from ISI, the guard period also provides protection against time-offset

errors in the receiver.

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Figure.3: Guard Period Insertion in OFDM [14]

2.5 Cyclic Prefix

The term cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Although

the receiver is typically configured to discard the cyclic prefix samples, the cyclic prefix serves

two purposes.

As a guard interval, it eliminates the inter-symbol interference from the previous

symbol.

As a repetition of the end of the symbol, it allows the linear convolution of a frequency-

selective multipath channel to be modeled as circular convolution, which in turn may

be transformed to the frequency domain using a discrete Fourier transform. This

approach allows for simple frequency-domain processing, such as channel estimation

and equalization.

In order for the cyclic prefix to be effective, the length of the cyclic prefix must be at least

equal to the length of the multipath channel. Although the concept of cyclic prefix has been

traditionally associated with OFDM systems, the cyclic prefix is now also used in single carrier

systems to improve the robustness to multipath propagation.

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2.6 Channels Employed in The Project

2.6.1 Additive White Gaussian Noise (AWGN) Channel

Noise exists in all communications systems operating over an analog physical channel, such as

radio. The main sources are thermal background noise, and electrical noise in the receiver

amplifiers, and inter-cellular interference. In addition to this noise can also be generated

internally to the communications system as a result of Inter-Symbol Interference (ISI), Inter-

Carrier Interference (ICI), and Inter- Modulation Distortion (IMD).

These sources of noise decrease the Signal to Noise Ratio (SNR), ultimately limiting the

spectral efficiency of the system. Noise, in all its forms, is the main detrimental effect in most

radio communication systems. It is therefore important to study the effects of noise on the

communications error rate and some of the trade-offs that exists between the level of noise and

system spectral Efficiency. [4]

Most types of noise present in radio communication systems can be modelled accurately using

Additive White Gaussian Noise (AWGN). This noise has a uniform spectral density (making

it white), and a Gaussian distribution in amplitude (this is also referred to as a normal

distribution). OFDM signals have a flat spectral density and a Gaussian amplitude distribution

provided that the number of carriers is large (greater than about 20 subcarriers), because of this

the inter-cellular interference from other OFDM systems have AWGN properties.

For the same reason ICI, ISI, and IMD also have AWGN properties for OFDM signals. In the

study of communication systems, the classical (ideal) additive white Gaussian noise (AWGN)

channel, with statistically independent Gaussian noise samples corrupting data samples free of

Inter symbol interference (ISI), is the usual starting point for understanding basic performance

relationships. An AWGN channel adds white Gaussian noise to the signal that passes through

it.

In the receiver side, the entire process at the transmitter side is reversed like the analog signal

received is converted to digital form using DAC and then the after removing the cyclic prefix,

the serial data is converted to parallel form after which the time domain signal is converted to

frequency domain by FFT algorithm. After passing this frequency domain signal through the

detector, it is converted to serial from which is our required output.

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2.6.2 Rayleigh Channel

Rayleigh fading is a statistical model for the effect of a propagation environment on

a radio signal, such as that used by wireless devices. Rayleigh fading models assume that the

magnitude of a signal that has passed through such a transmission medium (also called

a communications channel) will vary randomly, or fade, according to a Rayleigh distribution —

the radial component of the sum of two uncorrelated Gaussian random variables.

Rayleigh fading is viewed as a reasonable model for tropospheric and ionospheric signal

propagation as well as the effect of heavily built-up urban environments on radio

signals. Rayleigh fading is most applicable when there is no dominant propagation along a line

of sight between the transmitter and receiver. If there is a dominant line of sight, Rician

fading may be more applicable.[5]

Rayleigh fading is a reasonable model when there are many objects in the environment

that scatter the radio signal before it arrives at the receiver. The central limit theoremholds that,

if there is sufficiently much scatter, the channel impulse response will be well-modelled as

a Gaussian process irrespective of the distribution of the individual components. If there is no

dominant component to the scatter, then such a process will have zero mean and phase evenly

distributed between 0 and 2π radians. The envelope of the channel response will therefore

be Rayleigh distributed.

The requirement that there be many scatterers present means that Rayleigh fading can be a

useful model in heavily built-up city centres where there is no line of sight between the

transmitter and receiver and many buildings and other objects attenuate, reflect, refract, and

diffract the signal. Experimental work in Manhattan has found near-Rayleigh fading

there. Intropospheric and ionospheric signal propagation the many particles in the atmospheric

layers act as scatterers and this kind of environment may also approximate Rayleigh fading. If

the environment is such that, in addition to the scattering, there is a strongly dominant signal

seen at the receiver, usually caused by a line of sight, then the mean of the random process will

no longer be zero, varying instead around the power-level of the dominant path. Such a

situation may be better modelled as Rician fading. [6]

Note that Rayleigh fading is a small-scale effect. There will be bulk properties of the

environment such as path loss and shadowing upon which the fading is superimposed. How

rapidly the channel fades will be affected by how fast the receiver and/or transmitter are

moving. Motion causes Doppler shift in the received signal components.

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2.7 OFDM Advantages

OFDM has been used in many high data rate wireless systems because of the many advantages

it provides.

Immunity to selective fading: One of the main advantages of OFDM is that is more

resistant to frequency selective fading than single carrier systems because it divides the

overall channel into multiple narrowband signals that are affected individually as flat

fading sub-channels.

Resilience to interference: Interference appearing on a channel may be bandwidth

limited and in this way will not affect all the sub-channels. This means that not all the

data is lost.

Spectrum efficiency: Using close-spaced overlapping sub-carriers, a significant

OFDM advantage is that it makes efficient use of the available spectrum.

Resilient to ISI: Another advantage of OFDM is that it is very resilient to inter-symbol

and inter-frame interference. This results from the low data rate on each of the sub-

channels.

Resilient to narrow-band effects: Using adequate channel coding and interleaving it

is possible to recover symbols lost due to the frequency selectivity of the channel and

narrow band interference. Not all the data is lost.

Simpler channel equalization: One of the issues with CDMA systems was the

complexity of the channel equalization which had to be applied across the whole

channel. An advantage of OFDM is that using multiple sub-channels, the channel

equalization becomes much simpler.[7]

2.8Limitations of OFDM

(a) High Peak to Average Power Ratio (PAPR)

Time domain OFDM signal is a summation of several orthogonal sub-carriers, so OFDM

signal has high variation in its envelope. High power transmitter amplifiers need linearization.

OFDM signal has a noise like amplitude with a very large dynamic range when passes through

RF power amplifiers produces high PAPR. It causes signal distortion. So to reduce PAPR we

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need linear amplifiers at the transmitter. But linear amplifiers are less efficient and costly

compared to non-linear amplifiers.

(b) Sensitive To Carrier Offset And Drift (ICI)

Because of the orthogonality of the sub-carriers, we are able to extract the symbols at the

receiver as they do not interfere with each other. Orthogonality is preserved as long as sub

carriers are harmonics to each other. But at the receiver end, if there is a change of frequency

of the sub-carriers due to any reason then the orthogonality among them is lost & ICI occurs.

As a result the signal degrades heavily. This change in frequency is called frequency offset.

There are two main reasons for frequency offset. (a) Frequency mismatch between transmitter

& receiver (b) Doppler effect. So ICI has to be reduced for effective performance of the

system and some methods are discussed. [7]

2.9 Methods of ICI Reduction

1) Frequency Domain Equalization

2) Time Domain Windowing

3) Pulse Shaping

4) ICI Self Cancellation

From the above four methods the first two methods are the initial approach, whereas the last

two methods are very effective.

2.9.1 Frequency Domain Equalization

The fading distortion in the channel causes ICI in the OFDM demodulator. The pattern of ICI

varies from frame to frame for the demodulated data but remains invariant for all symbols

within a demodulated data frame. The equalizer co-efficient for eliminating ICI in the

frequency domain can be derived from the pattern of the pilot symbol & hence a suitable

equalizer can be constructed [8,9]

Drawbacks:

It can only reduce the ICI caused by fading distortion which is not the major source of ICI.

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The major source of ICI is due to the frequency mismatch between the transmitter and

Figure.4: Pilot subcarrier arrangement

receiver, and the Doppler shift. The above method cannot address to it.Again it is only suitable

for flat fading channels, but in mobile communication the channels are frequency selective

fading in nature because of multipath components. Here also the channel needs to be estimated

for every frame. Estimation of channel is complex, expensive & time consuming. Hence the

method is not effective one.

2.9.2 Time Domain Windowing

We know that OFDM signal has widely spread power spectrum. So if this signal is transmitted

in a band limited channel, certain portion of the signal spectrum will be cut off, which will lead

to inter carrier interference.

Figure.5: Spectrum of a 64 subcarrier OFDM [15]

To diminish the interference the spectrum of the signal wave form need to be more

concentrated. This is achieved by windowing the signal. Basically windowing is the process of

multiplying a suitable function to the transmitted signal wave form. The same window is used

in the receiver side to get back the original signal. The IC1 will be eliminated if the product of

the window functions satisfies the Nyquist’s vestigial symmetry criterion.

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Drawbacks:

It can only reduce the ICI caused by band limited channel which is not the major source of ICI.

The major source of ICI is due to the frequency mismatch between the transmitter and receiver,

and the Doppler shift. The above method cannot address to it. Windowing is done frame by

frame & hence it reduces the spectral efficiency to a large extent. Hence the method is not

effective one.

2.9.3 Pulse Shaping

As we have seen in the OFDM spectrum that each carrier consist of a main lobe followed by a

number of side lobes with reducing amplitude. As long as orthogonality is maintained there is

no interference among the carriers because at the peak of the every carrier, there exist a spectral

null. That is at that point the component of all other carriers is zero. Hence the individual carrier

is easily separated.[8]

When there is a frequency offset the orthogonality is lost because now the spectral null does

not coincide to the peak of the individual carriers. So some power of the side lobes exists at

the centre of the individual carriers which is called ICI power. The ICI power will go on

increasing as the frequency offset increases. The purpose of pulse shaping is to reduce the side

lobes. If we can reduce the side lobe significantly then the ICI power will also be reduced

significantly.

Drawback:

Complex in implementation

2.9.4 ICI Self Cancellation

It is seen that the difference between the ICI co-efficient of two consecutive sub-carriers are

very small. Here one data symbol is not modulated in to one sub-carrier, rather at least in to

two consecutive sub-carriers. If the data symbol ‘a’ is modulated in to the 1st sub-carrier then

‘-a’ is modulated in to the 2nd sub-carrier. Hence the ICI generated between the two sub-

carriers almost mutually cancels each other. [8]

Drawbacks:

The major drawback of this method is the reduction in band width efficiency as same symbol

occupies two sub-carriers.

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2.10 OFDM Variants and Applications

2.10.1 OFDM Variants

There are several other variants of OFDM for which the initials are seen in the technical

literature. These follow the basic format for OFDM, but have additional attributes or variations:

COFDM: Coded orthogonal frequency division multiplexing. A form of OFDM

where error correction coding is incorporated into the signal.

Flash OFDM: This is a variant of OFDM that was developed by Flarion and it is a

fast hopped form of OFDM. It uses multiple tones and fast hopping to spread signals

over a given spectrum band.

OFDMA: Orthogonal frequency division multiple access. A scheme used to provide

a multiple access capability for applications such as cellular telecommunications when

using OFDM technologies.

VOFDM: Vector OFDM. This form of OFDM uses the concept of MIMO technology.

It is being developed by CISCO Systems. MIMO stands for Multiple Input Multiple

output and it uses multiple antennas to transmit and receive the signals so that multi-

path effects can be utilised to enhance the signal reception and improve the transmission

speeds that can be supported.

WOFDM: Wideband OFDM. The concept of this form of OFDM is that it uses a

degree of spacing between the channels that is large enough that any frequency errors

between transmitter and receiver do not affect the performance. It is particularly

applicable to Wi-Fi systems.

Each of these forms of OFDM utilize the same basic concept of using close spaced orthogonal

carriers each carrying low data rate signals. During the demodulation phase the data is then

combined to provide the complete signal. [7]

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2.10.2 Applications of OFDM

Cable

ADSL and VDSL broadband access via POTS copper wiring,

DVB-C2, an enhanced version of the DVB-C digital cable TV standard,

Power line communication (PLC),

ITU-T G.hn, a standard which provides high-speed local area networking of existing home

wiring (power lines, phone lines and coaxial cables).

TrailBlazer telephone line modems,

Multimedia over Coax Alliance (MoCA) home networking.

Wireless

The wireless LAN (WLAN) radio interfaces IEEE 802.11a, g, n, ac and HIPERLAN/2.

The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD

Radio, T-DMB and ISDB-TSB.

The terrestrial digital TV systems DVB-T and ISDB-T.

The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and Media FLO forward link.

The.wireless personal.area.network (PAN) ultra-wideband (UWB) IEEE

802.15.3a implementation suggested by Wi-Media Alliance.

The OFDM based multiple access technology OFDMA is also used in several 4G and pre-

4G cellular networks and mobile broadband standards:

The mobility mode of the wireless MAN/broadband wireless access (BWA) standard IEEE

802.16e (or Mobile-WiMAX).

The mobile broadband wireless access (MBWA) standard IEEE 802.20.

The downlink of the 3GPP Long Term Evolution (LTE) fourth generation mobile

broadband standard. The radio interface was formerly named High Speed OFDM Packet

Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA) [7]

OFDM, orthogonal frequency division multiplexing has gained a significant presence in the

wireless market place. The combination of high data capacity, high spectral efficiency, and its

resilience to interference as a result of multi-path effects means that it is ideal for the high data

applications that have become a major factor in today's communications scene.

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Chapter 3: ICI Self Cancellation Techniques

3.1 ICI Self Cancellation

It is seen that the difference between the ICI co-efficient of two consecutive sub-carriers are

very small. This makes the basis of ICI self cancellation. Here one data symbol is not

modulated in to one sub-carrier, rather at least in to two consecutive sub-carriers. If the data

symbol ‘a’ is modulated in to the 1st sub-carrier then ‘-a’ is modulated in to the 2nd sub-carrier.

Hence the ICI generated between the two sub-carriers almost mutually cancels each other. This

method is suitable for multipath fading channels as here no channel estimation is required

.Because in multipath case channel estimation fails as the channel changes randomly. Thus, the

ICI signals become smaller when applying ICI cancelling modulation. On the other hand, the

ICI cancelling demodulation can further reduce the residual ICI in the received signals. The

combined ICI cancelling modulation and demodulation method is called the ICI self-

cancellation scheme.

Merits of self cancellation technique

It is suitable for multipath fading channels

It is also suitable for flat channels

Channel estimation is not required

Channel equalization is not required

It is simple in implementation

It is less complex and effective

3.2 System Model

The modulated data are served as input to ICI cancelling modulation. ICI coefficients can

be found in this. If there is frequency mismatch between transmitter and receiver local

oscillators frequency offset occurs. Doppler shift also introduces frequency offset. This

frequency offset (ε) occurs in OFDM signal due to these reasons.

ICI canceling demodulation is performed on the received OFDM symbols. ICI cancelling

modulation and ICI cancelling demodulation together known as ICI Self Cancellation. After

this it is fed into Signal De-Mapper for demodulation purpose. And finally the low data rate

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parallel bit stream is converted into high data rate serial bit stream.

Figure. 6: N–subcarrier OFDM system model [16]

3.3 Analysis of Inter-Carrier Interference

The main disadvantage of OFDM, however, is its susceptibility to small differences in

frequency at the transmitter and receiver, normally referred to as frequency offset. This

frequency offset can be caused by Doppler shift due to relative motion between the transmitter

and receiver, or by differences between the frequencies of the local oscillators at the transmitter

and receiver. In this project, the frequency offset is modeled as a multiplicative factor

introduced in the channel, as shown in Figure.

Figure.7: Frequency offset model

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The received signal is given by,

𝑦(𝑛) = 𝑥(𝑛)𝑒𝑗2𝜋𝑛𝜀

𝑁 + 𝑤(𝑛) (3.1)

Where ε is the normalized frequency offset, and is given by ΔfNTs. Δf is the frequency

difference between the transmitted and received carrier frequencies and Ts

is the subcarrier

symbol period. w(n) is the AWGN introduced in the channel. The effect of this frequency offset

on the received symbol stream can be understood by considering the received symbol Y(k) on

the kth

sub-carrier.

𝑌(𝑘) = 𝑋(𝑘)𝑆(0) + ∑ 𝑋(𝑙)𝑆(𝑙 − 𝑘)𝑁−1𝑙=0,𝑙≠𝑘 + 𝑛𝑘 (3.2)

𝑘 = 0,1,2, …………… ,𝑁 − 1

Where N is the total number of subcarriers, X(k) is the transmitted symbol (M-ary phase-shift

keying (M-PSK), for example) for the kth

subcarrier, nk is the FFT of w(n), and S(l-k) are the

complex coefficients for the ICI components in the received signal. The ICI components are

the interfering signals transmitted on sub-carriers other than the kth

sub-carrier. The complex

coefficients are given by

𝑆(𝑙 − 𝑘) =sin(𝜋(𝑙+𝜀−𝑘))

𝑁𝑠𝑖𝑛(𝜋(𝑙+𝜀−𝑘)/𝑁)× 𝑒

(𝑗𝜋(1−1

𝑁)(𝑙+𝜀−𝑘))

(3.3)

To analyze the effect of ICI on the received signal, we consider a system with N=16 carriers.

The frequency offset values used are 0.2 and 0.4, and l is taken as 0, that is, we are analyzing

the signal received at the sub-carrier with index 0. The complex ICI coefficients S(l-k) are

plotted for all sub-carrier indices in Figure 3.2.

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Figure 8: ICI Coefficients for N=16 Carriers

This figure shows that for a larger ε, the weight of the desired signal component, S(0),

decreases, while the weights of the ICI components increases. The authors also notice that the

adjacent carrier has the maximum contribution to the ICI. This fact is used in the ICI self-

cancellation technique described in Section 4.

The carrier-to-interference ratio (CIR) is the ratio of the signal power to the power in the

interference components. It serves as a good indication of signal quality. It has been derived

from (3.2) in [10] and is given below. The derivation assumes that the standard transmitted

data has zero mean and the symbols transmitted on the different sub-carriers are statistically

independent.

𝐶𝐼𝑅 = |𝑆(𝑘)|2

∑ |𝑆(𝑙−𝑘)|2𝑁−1𝑙=0,𝑙≠𝑘

=|𝑆(0)|2

∑ |𝑆(𝑙)|2𝑁−1𝑙=0

(3.4)

3.4 ICI Self-Cancellation Scheme

ICI self-cancellation is a scheme that was introduced by Yuping Zhao and Sven-Gustav

Häggman in 2001 in [10] to combat and suppress ICI in OFDM. Succinctly, the main idea is

to modulate the input data symbol onto a group of subcarriers with predefined coefficients such

that the generated ICI signals within that group cancel each other, hence the name self-

cancellation.

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3.4.1 ICI Cancellation Modulation

The ICI self-cancellation scheme requires that the transmitted signals be constrained such that,

X(1)= -X(0), X(3)= -X(2), ……………, X(N-1)= -X(N-2). Using (3.3), this assignment of

transmitted symbols allows the received signal on subcarriers k and k + 1 to be written as

𝑌′(𝑘) = ∑ 𝑋(𝑙)[𝑆(𝑙 − 𝑘) − 𝑆(𝑙 + 1 − 𝑘)] + 𝑛𝑘

𝑁−2

𝑙=0,𝑙=𝑒𝑣𝑒𝑛

𝑌′(𝑘 + 1) = ∑ 𝑋(𝑙)[𝑆(𝑙 − 𝑘 − 1) − 𝑆(𝑙 − 𝑘)] + 𝑛𝑘+1𝑁−2𝑙=0,𝑙=𝑒𝑣𝑒𝑛 (3.5)

and the ICI coefficient S’(l-k) is denoted as

𝑆 ′(𝑙 − 𝑘) = 𝑆(𝑙 − 𝑘) − 𝑆(𝑙 + 1 − 𝑘) (3.6)

Figure 4.1 shows a comparison between |S’(l-k)| and |S(l-k)| on a logarithmic scale. It is seen

that |S’(l-k)| << |S(l-k)| for most of the l-k values. Hence, the ICI components are much smaller

in (3.6) than they are in (3.3). Also, the total number of interference signals is halved in (3.6)

as opposed to (3.3) since only the even subcarriers are involved in the summation.

Figure.9: Comparison of ICI coefficients

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3.4.2 ICI Cancellation Demodulation

ICI modulation introduces redundancy in the received signal since each pair of subcarriers

transmit only one data symbol. This redundancy can be exploited to improve the system power

performance, while it surely decreases the bandwidth efficiency. To take advantage of this

redundancy, the received signal at the (k + 1)th

subcarrier, where k is even, is subtracted from

the kth

subcarrier. This is expressed mathematically as

𝑌′(𝑘) = 𝑌′(𝑘) − 𝑌′(𝑘 + 1)

= ∑ 𝑋(𝑙)[−𝑆(𝑙 − 𝑘 − 1) + 2𝑆(𝑙 − 𝑘) − 𝑆(𝑙 − 𝑘 + 1)] + 𝑛𝑘 − 𝑛𝑘+1𝑁−2𝑙=0 (3.7)

Subsequently, the ICI coefficients for this received signal becomes

𝑆 ′′(𝑙 − 𝑘) = −𝑆(𝑙 − 𝑘 − 1) + 2𝑆(𝑙 − 𝑘) − 𝑆(𝑙 − 𝑘 + 1) (3.8)

When compared to the two previous ICI coefficients |S(l-k)| for the standard OFDM system

and |S’(l-k)| for the ICI canceling modulation, |S’’(l-k)| has the smallest ICI coefficients, for

the majority of l-k values, followed by |S’(l-k)| and |S(l-k)|. This is shown in Figure.8 for N =

64 and ε = 0.4. The combined modulation and demodulation method is called the ICI self-

cancellation scheme.

The reduction of the ICI signal levels in the ICI self-cancellation scheme leads to a higher

CIR. From (4.4), the theoretical CIR can be derived as

𝐶𝐼𝑅 =|−𝑆(−1)+2𝑆(0)−𝑆(1)|2

∑ |−𝑆(𝑙−1)+2𝑆(𝑙)−𝑆(𝑙+1)|2𝑁−1𝑙=2,4,6…

(3.9)

Figure (9) below shows the comparison of the theoretical CIR curve of the ICI self-cancellation

scheme, calculated by (3.9), and the CIR of a standard OFDM system calculated by (3.4). As

expected, the CIR is greatly improved using the ICI self-cancellation scheme. The

improvement can be greater than 15 dB for 0 < ε < 0.5.

As mentioned above, the redundancy in this scheme reduces the bandwidth efficiency by half.

This could be compensated by transmitting signals of larger alphabet size. Using the theoretical

results for the improvement of the CIR should increase the power efficiency in the system and

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gives better results for the BER. Hence, there is a tradeoff between bandwidth and power

tradeoff in the ICI self-cancellation scheme.

Figure.10: CIR versus epsilon for standard and self-cancellation applied OFDM

3.5 Various ICI Self Cancellation Techniques

It is seen that the difference of ICI coefficient between two consecutive subcarrier S(l-k) and

S(l+1-k) is very small. Hence the idea of self-cancellation is generated. The main idea is to

modulate one data symbol onto a group of subcarriers with predefined weighting coefficients.

By doing so, the ICI signals generated within a group can be self-cancelled each other . Thus

it is called self-cancellation method. There are various self cancellation methods which have

been employed in present. Let’s have an overview of them.

3.5.1 Data Conversion ICI Self Cancellation Technique

The data-conversion self-cancellation scheme for ICI mitigation based on a data symbol

allocation of X’ (k) = X (k), X’(k +1) = -X (k), for k = 0,2,…. N - 2 in consecutive subcarriers to

deal with the ICI. The received signal on subcarrier k will be

𝑌′(𝑘) = ∑ 𝑋(𝑙)[𝑆(𝑙 − 𝑘) − 𝑆(𝑙 + 1 − 𝑘)] + 𝑛𝑘𝑁−2𝑙=0

𝑙=𝑒𝑣𝑒𝑛 (3.10)

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And on the subcarrier k+1 the received signal will be. To further reduce ICI, demodulation is

done. The resultant signal Y (k) is determined by the difference between the adjacent subcarrier.

𝑌′′(𝑘) =1

2(𝑌′(𝑘) − 𝑌′(𝑘 + 1)) (3.11)

CIR of data conversion method is given as

𝐶𝐼𝑅 =|−𝑆(−1)+2𝑆(0)−𝑆(1)|2

∑ |−𝑆(𝑙−1)+2𝑆(𝑙)−𝑆(𝑙+1)|2𝑁−2𝑙=2,4,6,..

(3.12)

3.5.2 Data Conjugate ICI Self Cancellation Technique

In the data-conjugate scheme, subcarrier signals are remapped in the form of

X’ (k) = X (k), X’(k +1) = - X*(k), for k= 0,2 ….. N-2

The final recovered signal is as follows

𝑌′′(𝑘) =1

2(𝑌′(𝑘) − 𝑌′∗(𝑘 + 1)) (3.13)

CIR of data conjugate scheme is given by

𝐶𝐼𝑅 =|𝑆(0)+𝑆∗(0)|2+|𝑆(1)+𝑆∗(−1)|2

∑ |𝑆(𝑙)+𝑆∗(𝑙)|2𝑁−2𝑙=2,4,6,.. +|𝑆(𝑙+1)+𝑆∗(𝑙−1)|2

(3.14)

3.5.3 Symmetric Data Conversion ICI Self Cancellation Technique

In the symmetric data-conversion scheme, subcarrier signals are remapped in the form of X’

(k) = X (k), X’(N-k -1) = - X(k) for k= 0,2 ….. N-2

The final recovered signal is as follows

𝑌′′(𝑘) =1

2(𝑌′(𝑘) − 𝑌′∗(𝑁 − 𝑘 − 1)) (3.15)

CIR of data conjugate scheme is given by

𝐶𝐼𝑅 =|2𝑆(0)−𝑆(𝑁−1)−𝑆(1−𝑁)|2

∑ |𝑆(𝑙)+𝑆(−𝑙)−𝑆(𝑁−𝑙−1)−𝑆(𝑙−𝑁+1)|2𝑁−2𝑙=2,4,6,..

(3.16)

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3.5.4 Real Constant Weighted Conversion ICI Self Cancellation Technique

In the constant weighted data-conversion scheme, subcarrier signals are remapped in the form

of X’ (k) = X (k), X’(k +1) = - μX(k) for k= 0,2 ….. N-2, where μ is a real constant.

The final recovered signal is as follows

𝑌′′(𝑘) =1

1+𝜇(𝑌′(𝑘) − 𝑌′(𝑘 + 1)) (3.17)

CIR of data conjugate scheme is given by

𝐶𝐼𝑅 =|(1+𝜇)𝑆(0)−𝜇𝑆(1)−𝑆(−1)|2

∑ |(1+𝜇)𝑆(𝑙)−𝜇𝑆(𝑙+1)−𝑆(𝑙−1)|2𝑁−2𝑙=2,4,6,..

(3.18)

3.5.5 Plural Weighted Data Conversion ICI Self Cancellation Technique

In the plural weighted data-conversion scheme, subcarrier signals are remapped in the form of

X’ (k) = X (k), X’(k+1) = 𝑒−𝑗𝜋/2 X(k), for k= 0,2 ….. N-2

The final recovered signal is as follows

𝑌′′(𝑘) =1

2(𝑌′(𝑘) − 𝑌′(𝑘 + 1)𝑒−𝑗𝜋/2) (3.19)

CIR of data conjugate scheme is given by

𝐶𝐼𝑅 =|(2𝑆(0)−𝑒

−𝑗𝜋2 [𝑆(1)−𝑆(−1)]|

2

∑ |2𝑆(𝑙)−𝑒−𝑗𝜋2 [𝑆(𝑙+1)−𝑆(𝑙−1)]|

2𝑁−2𝑙=2,4,6,..

(3.20)

3.5.6 Plural Conjugate Data Conversion ICI Self Cancellation Technique

In this scheme, subcarrier signals are remapped in the form of X’ (k) = X (k), X’(k +1) = 𝑒𝑗𝜋/2

X*(k) for k= 0,2 ….. N-2

The final recovered signal is as follows

𝑌′′(𝑘) =1

2(𝑌′(𝑘) − 𝑌′∗(𝑘 + 1)𝑒−𝑗𝜋/2) (3.21)

CIR of data conjugate scheme is given by

𝐶𝐼𝑅 =|𝑆(0)+𝑆∗(0)|2+|𝑒𝑗𝜋/2𝑆(1)+𝑒−𝑗𝜋/2𝑆∗(−1)|

2

∑ |𝑆(𝑙)+𝑆∗(𝑙)|2𝑁−2𝑙=2,4,6,.. +|𝑒𝑗𝜋/2𝑆(𝑙+1)+𝑒−𝑗𝜋/2𝑆∗(𝑙−1)|

2 (3.22)

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Chapter 4: Simulation Results

4.1 OFDM Model Used For Simulation

Figure.6 shows the Fast Fourier transform (FFT) based N-subcarrier OFDM system model

used for simulation. The simulation parameters used for the above model is as given below.

Simulation Parameters:

Parameter Specifications

IFFT Size 1024

Number of Sub Carriers 64

Channel AWGN, Rayleigh

Modulation BPSK

4.2 Graphs

Figure.11: OFDM signal at transmitter end

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Figure.12: OFDM Signal after passing through channel

Figure.13: SNR vs BER Graph

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Figure.14: SNR vs BER curve for Rayleigh and AWGN channels

Figure.15: Bit error probability curve for different offsets

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Figure.16: Offset Vs CIR

Figure.17: CIR curve for symmetric data conversion self cancellation scheme

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Figure.18 : CIR curve for conjugate data self cancellation scheme

Figure.19: CIR curve for plural weighted self cancellation scheme

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Figure.20: CIR curve for different self cancellation schemes together.

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Chapter 5: Conclusions

OFDM is a present day Modulation Technique with wide range of applications. orthogonality

of the sub-carriers in OFDM helps to extract the symbols at the receiver without interference

with each other. Orthogonality is preserved as long as sub carriers are harmonics to each other.

But if there are frequency offsets in the sub-carriers due to any reason then the orthogonality

among them is lost & ICI occurs.

One of the main limitations of OFDM is its sensitivity against carrier frequency offset which

causes attenuation and rotation of subcarriers, and inter carrier interference (ICI). The

frequency offset is due to frequency mismatch between the transmitter and receiver local

oscillators, and Doppler shift. The undesired ICI degrades the signal heavily and hence

degrades the performance of the system.

So, ICI mitigation techniques are essential in improving the performance of an OFDM system

in an environment which induces frequency offset error in the transmitted signal. This project

investigates an ICI self-cancellation schemes for combating the impact of ICI on OFDM

systems for different frequency offset values. It is also suitable for multipath fading channels.

We have worked on several ICI mitigation techniques that have been introduced in the chapters

above to achieve an optimum result. With the help of suitable MATLAB codes these have been

verified and from the results given above we can conclude that among all the ICI self

cancellation schemes that consider “grouping of two method”, Symmetric Data Conversion

gives the best result for the communication.

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5.1 Scope Of Future Work

Following are the areas of future study which can be considered for further research work.

1. In this work the BER performance of the OFDM system is evaluated considering BPSK and

QPSK Modulation system. It can be tested with other modulation systems such as QAM and

GMSK. ICI reduction using self cancellation technique can be used for COFDM (Coded

OFDM) Systems.

2. In this work, the group size in ICI techniques is considered as 2 thereby making it less

complicated to perform. In future the group size can be increased to three or four.

3. The sequential Monte Carlo (SMC) method called sequential importance sampling (SIS) can

be implemented which requires very lower computational complexity and estimates accurately

high value frequency offsets. However, the SIS performs slightly better, which is expected due

to the nonlinearity of the state–space and it is bandwidth efficiency scheme.

4. In this dissertation the polarization effects have not been taken into account. Simulation

studies can be done for same architectures while taking into account the polarization effects.

5. We have used channel spacing of 100 GHz in this dissertation. This can be further reduced

for more bandwidth utilization and some other techniques can be introduced for further

network sharing

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REFERENCES

[1] V.N. Richard and R. Prasad, “OFDM for Wireless Multimedia

Communication”, Artech house Publisher, London, 2000.

[2] T. S. Rappport, “Wireless Communications, principles and practice”,

2nd Edition, prentice- Hall publications, 2002.

[3] S .Weinstein and P. Ebert, “Data Transmission by Frequency Division

Multiplexing Using the Discrete Fourier Transform”, IEEE Trans. On

Commun., vol.19, Issue: 5, pp. 628–634, Oct.1971

[4] L. J. Cimini, “Analysis and simulation of a digital mobile channel using

orthogonal Frequency division multiplexing”, IEEE Trans. Communications.,

vol. COM-33, pp. 665-675. July 1985 .

[5] John G. Proakis (1995). Digital Communications (3rd ed.). Singapore:

McGraw–Hill Book Co. pp. 767–768.ISBN 0-07-113814-5.

[6] Dmitry Chizhik, Jonathan Ling, Peter W. Wolniansky, Reinaldo A.

Valenzuela, Nelson Costa, and Kris Huber (April 2003). "Multiple-Input–

Multiple-Output Measurements and Modeling in Manhattan". IEEE Journal on

Selected Areas in Communications 21 (3): 321–331.

doi:10.1109/JSAC.2003.809457

[7] Radio Electronics,”OFDM Tutorial” http://www.radio-

electronics.com/info/rf-technology-design/ofdm/ofdm-

[8] A Survey of ICI Reduction Techniques in OFDM System

SonikaChouhan, Deepak Sharma (IJCTT) – volume 4 Issue 8–August 2013

[9] N.A. Dhahi., “Optimum finite-length equalization for multicarrier

transceivers,” IEEE Trans. Commun., vol. 44, pp. 56–64, Jan. 1996.

[10] J. Armstrong, “Analysis of new and existing methods of reducing

intercarrier interference due to carrier frequency offset in OFDM,”IEEE Trans.

Commun., vol. 47, no. 3, pp. 365–369, Mar. 1999.

[11] P. H. Moose, “A technique for orthogonal frequency division

multiplexing ……frequency offset correction”, IEEE Trans. Commun., vol. 42,

no.10, pp. ……2908–2914, 1994

U11EC033, U11EC051, U11EC121, U11EC144, U10EC133 PROJECT 2014-15 Page 38

[12] Rfmw.em.keysight.com, ”Concepts of OFDM”

.http://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems

./wlan-ofdm/Content/ofdm_basicprinciplesoverview.htm

[13] http://ecee.colorado.edu/~ecen4242/WiMax/WiMAX_802_16e.htm

[14] www.dsplog.com,”OFDM basics

http://www.dsplog.com/category/ofdm/

[15] ni.com,” OFDM and Multi-Channel Communication Systems

http://www.ni.com/white-paper/3740/en/

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ACRONYMS

ADC Analog to Digital Converter

ADSL Asymmetric Digital Subscriber Line

AWGN Additive White Gaussian Noise

ADSL Asymmetric Digital Subscriber Lines

BER Bit Error Rate

BPSK Binary Phase Shift Keying

BWA Broadband Wireless Access

CIR Carrier to Interference Ratio

COFDM Coded Orthogonal Frequency Division Multiplexing

DAB Digital Audio Broadcasting

DFT Discrete Fourier Transform

DSL Digital Subscriber Line

DSP Digital Signal Processing

DVB Digital Video Broadcasting

DVB-C Digital Video Broadcasting - Cable

DVB-T Digital Video Broadcasting - Terrestrial

FFT Fast Fourier Transform

FOFDM Flash Orthogonal Frequency Division Multiplexing

GMSK Gaussian Minimum Shift Keying

HIPERLAN High Performance Radio LAN

HSPOA High Speed OFDM Packet Access

ICI Inter Carrier Interference

IDFT Inverse Discrete Fourier Transform

IEEE Institute of Electrical and Electronics Engineers

IFFT Inverse Fourier Transform

ISDB Integrated Service Digital Broadcasting

ISI Inter symbol Interference

IMD Inter Modulation Distortion

LAN Local Area Network

LTE Long Term Evolution

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MAN Metropolitan Area Network

MBWA Mobile Broadband Wireless Access

MIMO Multiple Input – Multiple Output

OFDM Orthogonal Frequency Division Multiplexing

OFDMA Orthogonal Frequency Division Multiplexing Access

PAN Personal Area Network

PAPR Peak to Average Power Ratio

PLC Power Line Communication

PSK Phase Shift Keying

QAM Quadrature Amplitude Moduation

QPSK Quadrature Phase Shift Keying

SNR Signal to Noise Ratio

UMTS Universal Mobile Telecommunication System

UWB Ultra Wide Band

VDSL Very High Bit Rate Digital Subscriber Line

VLSI Very Large Scale Integration

VOFDM Vector Orthogonal Frequency Division Multiplexing

WiMAX Worldwide Interoperability for Microwave Access

WOFDM Wideband Orthogonal Frequency Division Multiplexing