an overview : peak to average power ratio (papr) in ofdm system using some new papr techniques (with...

7/23/2019 An Overview : Peak to Average Power Ratio (PAPR) in OFDM system using some new PAPR techniques (with matl

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An Overview : Peak to AveragePower Ratio (PAPR) in OFDMsystem using some new PAPRtechniques (with matlab code)

Zainab S. H. AL-Hashmi


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An Overview : Peak to AveragePower Ratio (PAPR) in OFDM

system using some new PAPRtechniques (with matlab code)

ByZainab Saad Hadi AL-Hashmi

A graduate of University of

Baghdad, College of EngineeringElectronic & CommunicationsEngineering Department


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Acknowledgments

praise belongs to God who showed favour to us through His

religion, singled us out for His creed, and directed us onto the

roads of His beneficence, in order that through His kindness we

might travel upon them to His good pleasure, a praise which He

will accept from us and through which He will be pleased with

us. !Allah send peace and blessings upon Mohammed and his

progeny (S.A.W.)

Finally I would like to thank my family,

Especially my grandfather Mr. Hassan Ali Zwain,

my mother, Mr. Qasim Hassan Zwain and Mr. Maythem Hassan

Zainab saad hadi

2015


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Abstract

The Orthogonal frequency division multiplexing (OFDM) is multicarrier

modulation scheme which has recently become comparatively popular in

both wireless and wired communication systems for transfer the

multimedia data. OFDM could be used at the core of well-known systems

like Asymmetric digital subscriber line (ADSL) internet, digital

television/radio broadcasting, wireless local area network (LANs), and

Long Term Evolution (LTE).

High PAPR is the major drawback of OFDM, which results in lower

power efficiency hence impedes in implementing OFDM. The PAPR

problem is more significant in the uplink because the efficiency of power

amplifier is critical because a mobile terminal has a limited battery

power.

High peak-to-average power ratio (PAPR) occurs due to large envelope

fluctuations in OFDM signal this requires a highly linear high power

amplifier (HPA). Power amplifiers with large linear range are expensive,

bulky 50% of the size of a transmitter lies and difficult to manufacture.

In order to reduce the PAPR, several techniques have been proposed in

this thesis, primarily the repeated frequency domain filtering and clipping

(RFC) has been proposed and compared with the existing method

repeated clipping and frequency domain filtering (RCF). The RFC is

better than RCF in performance especially when I 2, although they havethe same complexity and cost.

The proposed method is not only improving PAPR but also improving

BER. Best case for the bit error rate (BER) is at I =4 and CR =4, where

Signal to Noise Ratio (SNR) at BER improved by (5.7601 dB)and Complementary Cumulative Distribution Function (CCDF) of PAPR

was improved by (4.775 dB) and PAPR was improved by (11.4177 dB).

The best one improvement in PAPR and CCDF of PAPR So as not to

BER deteriorate is at I =4 and CR =1.75. The improvement in PAPR by =

(18.2789 dB), CCDF of PAPR = (8.0187 dB), and the SNR at

BERby = (0.6101 dB).In addition to (RFC) six new types of companding have been proposed

and compared with the -law and A-law compandings. all these proposedmethods have better performance than the -law and A-law compandings,and the best one is Absolute Exponential (AEXP) companding and the


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best one improvement in PAPR and CCDF of PAPR is at d= 1.1. The

improvement in PAPR by = (17.6492 dB), and CCDF of PAPR = (7.2405

dB), while the SNR at BER

deteriorated by = (-3.4186 dB).

Five types of pre-coding are used in this work and then compared them

with each other. The best type of precoding in term of reduced PAPR and

BER is the Discrete Fourier Transform (DFT) pre-coder, while the least is

the Walsh Hadamard Transform (WHT) pre-coding.

Also four new types of hybrids PAPR reduction techniques have been

proposed. These methods are:

1. RCF with precodings (WHT, Discrete Cosine Transform (DCT),

Discrete Sine Transform (DST),and Discrete Hartley Transform (DHT)).

2. RCF with compandings (the all proposed compandings, -law and A-law compandings).

3. RFC with compandings (the all proposed compandings, -law and A-law compandings).

4.and finally precodings (WHT, DCT, DST,and DHT), with compandings

(the all proposed compandings, -law and A-law compandings).

The best one improvement is at (RFC with AEXP) because the PAPR,

CCDF of PAPR, and BER. This improvement in PAPR and CCDF of

PAPR is at d = 0.6 and CR =4. The improvement in PAPR by

(21.0509dB), CCDF of PAPR = (8.7178 dB), and the SNR at

BER by (0.0116 dB).The DHT with tangent Rooting (tanhR) have acceptable results where the

PAPR and CCDF of PAPR were improved while BER was degarded.

The best one improvement for this case is at k=15, y=.8 and DHT. The

improvement in PAPR by = (22.7711 dB), and CCDF of PAPR = (8.9691

dB), while the SNR at BERdeteriorated by = (-1.1828 dB).All methods are simulated using matlab.


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Contents

Chapter One: Introduction 1

1.1 Introduction 1

1.2 Literature survey 1

Chapter Two: LTE and OFAM 7

2.1. Introduction 7

2.2. LTE Requirements 7

2.3. LTE Architecture 8

2.4. Air interface in LTE 9

2.5 History of OFDM 10

2.6 OFDM 12

2.6.1 Orthogonality of the subcarriers and OFDM 15

2.6.2 Guard Interval 17

2.6.3 One-tap equalizer 18

2.7 OFDM based Multiple Access 19

2.8 Orthogonal Frequency Division Multiple Access 20

2.9 SC-FDMA 24

Chapter Three: Peak To Average Power Ratio Reduction 27

3.1Definitions of PAPR 27

3.2 PAPR of OFDM signal 28

3.3 Oversampling discrete OFDM symbols to find true (continuous) peaks 29

3.4 Distribution of PAPR 29

3.5 Identification of the Problem 32

3.5.1 Nonlinear HPA and DAC 32

3.5.2 Power Saving 35

3.6 Factors influencing the PAPR 35

3.6.1 The number of sub carriers 35

3.6.2 The order of Modulation 35

3.6.3 Constellation shape 36

3.6.4 Pulse Shaping 36


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3.7 The gauge for judgment of the PAPR reduction in OFDM systems 36

3.8 Fitness function-based approach for determining an appropriate Algorithm 37

Chapter Four: PAPR Reduction Techniques 39

4.1There are three different way to divide the PAPR 39

4.1.1The first way is 39

4.1.2 The second way 43

4.1.3The third way 45

4.1.4 And finally there is Hybrid techniques 45

4.2 Clipping and Filtering 46

4.3 Peak Windowing Method 47

4.4 Envelope Scaling 48

4.5 Peak Reduction Carrier 48

4.6 Companding Technique 49

4.6.1 Square-Rooting Companding Technique ( SQRT) for PAPR Reduction in

OFDM Systems

50

4.6.2 Exponential Companding Algorithm 51

4.6.3 Trapezoidal power companding 53

4.6.4 Hyperbolic tangent () companding 534.6.5 Error Function ( ) Companding 544.6.6 Logarithm Function (log) Companding 54

4.7 Coding techniques 54

4.8 Selective Mapping (SLM) 56

4.9 Partial Transmit Sequence (PTS) 57

4.10 Tone Reservation 59

4.11 Tone Injection 60

4.12 Interleaving 61

4.13 Active Constellation Extension (ACE) 61

4.14 Dummy Sequence Insertion (DSI) 62

Chapter Five: Simulation Results and Analysis 63

5.1 OFDM System model 63

5.2 PAPR techniques used 65


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5.2.1 Repeated clipping and frequency domain filtering (RCF) 65

5.2.2 Repeated frequency domain filtering and clipping RFC 72

5.2.3 The OFDM System with discrete time companding 77

5.2.3.1 A companding 77

5.2.3.2 Companding 815.2.3.3 Rooting Companding Technique (RCT) 85

5.2.3.4 New error function Companding (NERF) 87

5.2.3.5 Absolute Exponential companding (AEXP) 89

5.2.3.6 Cos companding 91

5.2.3.7 tangent Rooting Companding (tanhR) 95

5.2.3.8 Logarithmic Rooting Companding (logR) 101

5.2.4 OFDM System with pre-coding 104

5.2.4.1 Pulse Shaping or Pre-coding 104

5.2.4.2 Discrete Hartley transform (DHT) 105

5.2.4.3 Walsh-Hadamard Transform (WHT) 105

5.2.4.4 Discrete Cosine Transform (DCT): 106

5.2.4.5 Discrete Sine Transform (DST) Precoding Technique 107

5.2.4.6 The Discrete Fourier Transform (DFT) Precoding 107

5.2.4.7 Simulation results and analysis of OFDM system with pre-coding 108

Chapter six: Simulation Results and Analysis of Hybrid PAPR techniques 110

6.1 Hybrid pre-coding with RCF 110

6.2 Hybrids RCF with companding 119

6.2.1 RCF + A companding 119

6.2.2 RCF +

121

6.2.3 RCF + RCT 123

6.2.4 RCF + AEXP 126

6.2.5 RCF + cos 128

6.2.6 RCF + NERF 130

6.2.7 RCF + tanhR 131

6.2.8 RCF +logR 132

6.3 Hybrid RFC with companding 134

6.3.1 RFC + A companding 134


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6.3.2 RFC + companding 1376.3.3 RFC + RCT 139

6.3.4 RFC + AEXP 141

6.3.5 RFC + cos 143

6.3.6 RFC + NERF 145

6.3.7 RFC + tanhR 146

6.3.8 RFC +logR 147

6.4 Pre-coding + companding 148

6.4.1 Pre-coding + A companding 149

6.4.2 Pre-coding + 1526.4.3 Pre-coding + RCT 154

6.4.4 Pre-coding + AEXP 156

6.4.5 Pre-coding + cos 159

6.4.6 Pre-coding + tanhR 161

6.4.7 Pre-coding + logR 162

6.4.8 Pre-coding + NERF 163

Chapter seven : Conclusions and future work 165

7.1Conclusions 165

7.2Future work 167

References 168

Appendices

Appendix A : Table of Results A.1

Appendix B : MATLAB Code B.1


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Chapter One Introduction

1

Chapter One

Introduction

1.1 Introduction:

During the last two decades, the demand for multimedia wireless communication

services have grown tremendously and this trend are expected to continue in the near

future. Orthogonal frequency division multiplexing (OFDM) is one of such multi-

carrier techniques which have attracted vast research attention from academics,

researchers and industries since last two decades. It has become part of new emerging

standards for broadband wireless access [1].

Energy efficiency, particularly matters in future mobile communications networks. A

key driving factor is the growing energy cost of network operation which can make up

as much as 50% of the total operational cost nowadays [2].

The transmitted signal of OFDM exhibits a high Peak-To-Average Power Ratio(PAPR). This high PAPR reduces the efficiency of high power amplifier and degrades

the performance of the system [3].

A major source for reducing energy costs is to increase the efficiency of the high

power amplifier (HPA) in the radio frequency (RF) front end of the base stations [4].

However, the efficiency of the HPA is directly related to the PAPR of the input signal.

The problem, especially, becomes serious in OFDM multicarrier transmission, which

is applied in many important wireless standards such as the third Generation

Partnership Project (3GPP) Long Term Evolution Advanced (LTE-A). The PAPR

problem still prevents OFDM from being adopted in the uplink of mobile

communication standards, and, besides from power efficiency, it can also place severe

constraints on output power and therefore coverage in the downlink. In the past, there

have been many efforts to deal with the PAPR problem resulting in numerous papers

and several overview articles, e.g., [5], [6], [7].

PAPR has a deleterious effect on battery lifetime in mobile applications. As handy

devices have a finite battery life, it is significant to find ways of reducing the PAPR

allowing for a smaller, more efficient HPA, which in turn will mean a longer lasting

battery life.

In many low-cost applications, the problem of high PAPR may outweigh all the

potential benefits of multicarrier transmission systems [6]. A number of promisingapproaches or techniques have been proposed & implemented to reduce PAPR with

the expense of increase transmit signal Power, bit error rate (BER) & computational

complexity and loss of data rate, etc. So, a system trade-off is required [8].

1.2 Literature survey:

In 1996 Robert [9]. The selected mapping was used for the reduction of PAR. The

selected mapping can be used for arbitrary numbers of carriers and any signal

constellation. The selected mapping provides significant gains at moderate additional

complexity. Actually, it is appropriate for all kinds of multiplex techniques, whichtransform data symbols to the transmit signal. Even in single carrier systems where


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PAR grows as the roll of factor of the pulse shaping filter decreases, selected mapping

can be applied advantageously.

The first nonlinear companding transform (NCT) for PAPR reduction was given by

Wang et.al in 1999 [10]. It was based on the speech processing algorithm -law and ithas found better performance than that of clipping technique. The -law companding

transform mainly focuses on enlarging small amplitude signals while keeping peak

signals unchanged, and thus it increases the average power of the transmitted signals

and may lead to overcome the saturation region of the HPA to make the performance

of the system worse. In order to overcome the problem of -law companding

(increasing average power) and to have an efficient PAPR reduction. [10]

In 2000 Myonghee et.al [11] Hadamard transform is an effective technique to reduce

the PAPR of an OFDM system. The PAPR can be reduced in OFDM system without

any power increase and side information. The increase of system complexity is not

much. As further study, the equalization problem combining with Hadamard

transform, which is induced to reduce PAPR, over multipath fading channel, is

considered.

In 2001 J. Armstrong [12] the clipping and frequency domain filtering PAPR

reduction technique has been described in which an interpolated version of the

baseband signal is clipped and then filtered with a new form of filter. The filter

consists of a forward and an inverse fast Fourier transform (IFFT). It is designed to

remove the out-of-band (OOB) noise without distorting the in-band discrete signal. Itis shown that significant PAPR reduction can be achieved without any increase in

OOB power. Some in-band distortion results, but this will have negligible effect on

the overall BER in most systems.

In 2002 J. Armstrong [13] the repeated clipping and frequency domain filtering of an

OFDM signal can significantly reduce the PAPR of the transmitted signal. This

method causes any increase in OOB power. Considerable PAPR reduction can be

obtained with only moderate levels of clipping noise.

In 2004 Ryu, et al. [14] The Dummy Sequence Insertion (DSI) technique reduces

PAPR through increased the average power of the signal. Herein, after switchting the

input data stream into parallel through the serial to parallel converter a, dummy

sequence is inserted in the input signal. Thus, the average value is raised and the

PAPR is reduced later.

In 2005 Tao Jiang et.al [15] exponential companding. It can adjust the amplitudesof both large and small input signals, while maintaining the average power unchanged

by properly choosing transform parameters, so as to make the output signals have a

uniform distribution (with a specific degree). The exponential companding schemes

can efficiently reduce PAPR for various modulation formats and sub-carrier sizes.
http://www.mathworks.com/help/matlab/ref/ifft.htmlhttp://www.mathworks.com/help/matlab/ref/ifft.html


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The exponential companding schemes make less spectrum side-lobes than -law

companding. Simulation results have shown that exponential companding schemes

could provide better system performance in terms of PAPR reduction, power

spectrum, BER, and phase error than the -law companding scheme.

In 2007 Wisam et.al [16] square rooting companding (SQRT) companding a simple

method of reducing the PAPR value of OFDM symbol by changing the statistical

characteristics of the output signals . This was achieved by applying a non-linear

square rooting operation of the OFDM signals. The process changed also the

describing parameters of power signals: average and peak power values, and as a

result the PAPR value is reduced. This companding more suitable for OFDM

applications that do not have sophisticated processor, since it allows significant

reduction in PAPR value with very low cost of computational complexity, and only

slight performance degradation.

In 2008 Pisit et.al [17] the simple PAPR reduction method by using the dummy sub-

carriers. The features of proposed method is to decide the frequency data for dummy

subcarriers theoretically by using the certain number of larger amplitude levels

detected in the time domain signal and to achieve the better PAPR performance with

less computational complexity.

In 2008 Carole et.al [18] they present an incipient PAPR reduction technique which

exploits the utilization of used carriers in addition to the phase information of pilot

signals in OFDM systems to limit the PAPR while not degrading channel estimationor frequency offset. Compared to conventional techniques like clipping and

windowing, this technique introduces significantly lower OOB distortions and

provides a lower BER since there is no interference to the original data signals. There

is additionally no requisite for side information to be transmitted to the receiver.

In 2009 Kazuki and Fumiyuki [19] A tone injection (TI) has been suggested which

exploits the property of a nonlinear modulo function. The TI is identically equivalent

to the one that superimposes a quadrature amplitude modulation (QAM) signal on the

data symbol to reduce the PAPR. Without the transmission of the side information,

the TI dramatically reduces the PAPR level. Albeit the TI-OFDM reduces the 1%

PAPR level by about 3~4.5dB, the BER performance remarkably degrades. However,

the utilization of antenna diversity reception can reduce the BER performance

degradation.

In 2010 Zhongpeng et.al [20] a combined companding transform and hadamardtransform technique is suggested to reduce PAPR of OFDM signal .Simulation results

shows that the PAPR reduction performance is improved compared with companding

transform used only. On the other hand, the BER of system using proposed PAPR

reduction scheme is not degraded.


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In 2010 Imran and Varun [21] the PAPR of discrete hartley transform (DHT)-

Precoded OFDM system for M-ary Quadrature Amplitude Modulation(M-QAM)

(where M=16, 32, 64, 256). The Matlab simulation shows that DHT-Precoded OFDM

System shows better PAPR gain as compared to OFDM-Original system, Walsh

Hadamard transformation (WHT)-Precoder Based OFDM system and selectivemapping (SLM) OFDM (with V=2) system respectively. Thus, it is concluded that

DHT Precoder Based OFDM System shows better PAPR reduction then WHT-

Precoder Based OFDM System, SLM-OFDM System and OFDM-Original system for

MQAM. Additionally, the DHT-Precoded OFDM system does not require any power

increase, complex optimization and side information to be sent for the receiver.

In 2011 Zhongpeng [22] a combined reduction in PAPR of the

OFDM signals based on the combination of the discrete cosine transform (DCT) with

companding. While taking both BER performance and PAPR performance intoaccount, a united DCT and companding scheme to reduce the PAPR of OFDM

signals.

In 2011 Hem [23] a combinational method of pre-coding and clipping is proposed to

reduce the PAPR of an OFDM system. The proposed technique is better than

conventional because it does not require any increase in roll-off factor to reduce

PAPR. Thus, it reduces the overhead in comparison to conventional pre-coding

method. Increasing the roll-off factor degrades the BER as given in [24]. The clipping

after pre-coding reduces PAPR but degrades BER. However, this degradation in not

significant in comparison to degradation obtained by increasing roll off factor.

In 2012 Malhar and Prof.Abhishek [25] tone reservation includes no of set of

reservation of tones. By using this technique reserved tones can be utilized to

minimize the PAPR. This method is used for multicarrier transmission and also

demonstrated the reserving tones to limit the PAPR. Advantage of this tone

reservation is very positive that no process is needed at receiver end. Furthermore

there is no need to transmit the side information combined with the transmitted signal.

In 2012 Eugen [26] The PAPR reduction technique based on combination of a WHT

and a new signal companding function. The numerical results show that the hybrid

scheme realizes an improved PAPR reduction than the component methods. The

computation complexity increases linearly with the number of considered signal

variants because of several signal variants are applied to the precoding block. This

problem can be solved, by using few subcarriers as markers.

In 2012 Chau, and Hsuan [27] presents a combination scheme, which using a

combination of precoding by utilizing least null subcarriers in the frequency domain

and nonlinear companding technique by applying proper

-Law characteristic in time

domain, for reducing PAPR. Simulation results indicate that the proposed scheme


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achieves a advantageous trade-off between OOB power emission in OFDM systems

and the reduction of PAPR.

In 2013 Sroy et.al [28] an Iterative Clipping and Filtering (ICF) Technique for PAPR

Reduction of OFDM Signals: System Using DCT/inverse discrete cosine transform(IDCT) Transform. The OFDM symbol enters the ICF block with DCT/IDCT

technique, then clipping and filtering is iteratively performed. Although we

demonstrate that significant PAPR reduction is obtained through Iterative clipping

and filtering using fast Fourier transform (FFT)/IFFT transform, but better results are

observed applying DCT/IDCT in the classical ICF technique.

In 2013 Zihao et.al [29] a trapezoidal power companding method which could

significantly reduce the PAPR for a complex OFDM or Filterbank Based Multicarrier

Transmission (FBMC) system. The proposed approach provides a convenient way for

designing a compander where the trade-offs among several system performances

(such as PAPR, power spectral density (PSD) and BER) can be made.

In 2013 Mohit et.al [30] the performance of tanh and erf companding is

approximately. Log companding is better than the hyperbolic tangent and error

function companding . -law and A-law companding give the same performance andthe -law and A-law companding is better than the tanh, log and erf companding.Some more non-linear transform have been suggested in the paper [31, 32, 33, 34, and

35]

In 2013 Jaspreet et.al [36] the performance analyzed in terms of PAPR in Orthogonal

Frequency Division Multiple Access (OFDMA) by utilizing some pre-coding

techniques, called Zadoff-Chu Transform (ZCT) and WHT with the -law

companding to limit the PAPR of the OFDM signals .These pre-coding techniques

produced the lower PAPR as compared to the conventional OFDM system.

Furthermore ZCT is better than WHT because it produced the lowest PAPR than

WHT. -law companding further reduces PAPR of OFDM signal and as withincreasing the value the PAPR reduces. The obtained results approved that the

proposed method have gotten better results than conventional OFDM.

In 2013 Navneet and Lavish [37] The PAPR reduction method is based on combining

clipping with WHT. Combined technique is simple to implement and has no

limitations on the system parameters such as number of subcarriers modulation order,

and constellation type. This system produces the lowest PAPR and is efficient, signal

independent, distortion less and do not require any complex optimizations

representing better PAPR reduction methods than others existing techniques because

it does not require any power increment, complex optimization and side information

to be sent to the receiver.


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In 2013 Mohit et.al [38] To reduce the PAPR of OFDM has been proposed Hybrid

Clipping-Companding techniques for PAPR Reduction. the performance of hybrid

PAPR reduction scheme with either tanh or erf as companding function is

approximately same .Hybrid PAPR reduction scheme with log companding function

is better than the last two. Hybrid PAPR reduction scheme with either -law or A-lawcompanding gives the same performance and the Hybrid PAPR reduction schemewith either -law or A-law companding is best.

In 2013 K. muralibabu et.al [39] In the proposed scheme, a combined reduction in

PAPR of the OFDM system by grouping the sub carrier on the basis of the

combination of joining the Discrete Cosine Transform (DCT) with companding

technique. The simulation results indicat that the proposed scheme can yield good

tradeoff between computational complexity and PAPR reduction performance

In 2014 Jijina et.al [40] a comparative study is made on the three typical linear

precoding techniques: Hadamard transform precoding, Discrete Sine Transform

(DST) precoding and Square root raised cosine function precoding used in OFDMA

system. The performance of these different schemes in terms of PAPR reduction is

analyzed with the conventional Random Interleaved OFDMA system. Linear

precoding schemes are efficient, signal independent, distortion less and do not require

complex optimization when compared to the other reduction schemes.


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Chapter Two LTE and OFAM

7

Chapter Two

LTE and OFAM

2.1. Introduction:

The growth in data intensive mobile services and applications like Web browsing,social networking, video streaming and music has become a driving force for

development of the next generation of wireless standards. Thus, new standards are

being developed to provide the data rates and network capacity needful to support

worldwide delivery of these kinds of rich multimedia application. LTE have been

developed to respond to the requirements of this generation and to achieve the aim of

realizing global broadband mobile communications [41].

2.2. LTE Requirements:The demand for high speed and widespread network access in mobile

communications increases every day as the number of users increases andapplications are constantly developed with greater demand for network resources. Asa result of this trend, mobile communications has experienced significant

developments within the last two decades, which is the result of tremendous research

that has been carried out. [42]

Requirements and objectives for the LTE Discuss the main requirements for the new

LTE system Resulted in a the creation of a formal

Study Item in 3GPP with the specific aim of evolving the 3GPP radio accesstechnology to guarantee competitiveness over a ten-year time-frame. Depending on

the study of this Study Item, the requirements for LTE Release 8 were revised and

crystallized. They can be summed up as follows [41,43, and 44]:

High peak data rates and diminished delays, in both connection establishmentand transmission latency. These improvements are to be realized through the

simplification of the overall system, the decrease of complexity and the

automated process of system management (i.e. optimization).

greater flexibility of spectrum usage, in each of the new and pre-existing bands;

Seamless integration with existing systems (Universal Mobile

Telecommunications System (UMTS), Wireless Fidelity (Wi-Fi), etc.).

Infrastructure-building economy. Although the implementation of every new

system brings construction and building costs, LTE should be realized through

minimal investment and use as much of the existing mobile communication

infrastructure as possible.

Multi-antenna support. Improved system capacity and coverage

Reasonable power consumption for the mobile terminal. The mobile terminal isbeing associated with mobile phones and similar devices which have limited

battery capacities. Therefore a flexible bandwidth system (with lower

frequencies used for uplink transmission) and automated signal power-level

optimization have to be included into LTE [45].

Seamless mobility, including between different radio-access technologies;

Simplified network architecture;

Increased cell-edge bit-rate, for unification of service provision;

Increased user data rates;

Reduced cost per bit, implying an enhanced spectral efficiency;


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Packet switched domain utilization. To eliminate additional system complexity,

introduced through the support of both the circuit switched and packet switched

domain, the circuit switched domain will not be included into the LTE system.

The traditional voice and text messaging services must be replaced with system-

external subsystems (e.g. Information Management System (IMS)).

High-level security and mobility. As the mobile communication system is nowsimilar to a data network (e.g. internet), additional emphasis will be set on new

security measures in combination with IP (Internet Protocol)-security functions.

Mobility efficiency is provided through the use of evolved base stations, i.e.

eNodeBs (E-UTRAN Node-B or Evolved Node-B).

These main targets resulted in the creation of additional requirements and spin-off

functionalities, whose realizations were researched, developed and evolved by 3GPP

and hence introduced in LTEs specifications and standardization upgrades.These improvements were further evolved and enhanced in Release 9, which

contained additional techniques, functionalities and technology approaches to enable a

quick, efficient and low-cost implementation of the LTE system. The followingtechniques are included:

introduction to Self-Organizing Networks (SON),

improved approach to emergency calls, as they oppose the systems securitypolicy,

multiple-eNodeB broadcast signal combination (LTE MBMS),

further improvement of Frequency Division Duplex (LTE-FDD) and Time

Division Duplex (LTE-TDD),

improvement of SON technologies and mechanisms, and

Minimization of system drive-tests (MDT).

The LTE system and its standardization are 3GPPs most significant milestoneachieved so far, triggering an increase of participation in their further projects and

worldwide acknowledgement of their existing work. Takahiro Nakamura, the 3GPP

RAN Chairman, states: Operators need to work on issues that have been created insignaling and the volume of data being carried. Therefore, further improvements to

the 3GPP system are being driven by that data explosion. A continued evolution ofthe system is given in Releases 10, 11 and 12, introducing an improved mobile

communication standard named LTE-Advanced [45].

2.3. LTE Architecture:The LTE architecture was highly simplified and flattened, as shown in Figure 2.1. Thesystem contains only two types of nodes namedMobility Management Entity/System

Architecture Evolution Gateway (MME/SAE GW) and evolved Node-B (eNB) [46,

47].

All LTE network interfaces are based on IP protocols and therefore two major

changes were made compared to previous cellular radio architectures. The first

significant modify is that the Radio Network Controller (RNC) is removed from the

data path and its functions are now situated in eNB [46]. The main benefits of this

type of single node access network are the diminished latency and the distribution of

the RNC processing overhead into multiple eNBs. The second major change is that

there are no nodes for CircuitSwitched (CS) domain, such as the Mobile Switching


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9

Centre (MSC). Therefore speech services are handled as Voice over IP (VoIP) calls in

the LTE network [47, 48].

The eNBs are connected to each other via X2 interface and to Evolved Packet Core

(EPC) through S1 interface, as also shown in Figure 2.1. The S1 interface supports in

addition many-to-many relations between MMEs / SAE Gateways and eNBs [46].

SAE Gateway contains two logical gateway entities named as the Serving Gateway(SGW) and the Packet Data Network Gateway (P-GW). The S-GW is responsible for

receiving and forwarding IP packets. Therefore, it can be seen as a local mobility

anchor to the eNBs [48]. The P-GW, on the other hand, is responsible for handling the

internet protocol functions, like routing, packet filtering, policy enforcement and

address allocation [47].

The new system architecture was designed so that it will reduce the overhead from

increased traffic. This is achieved because only the MME is responsible for signaling

and therefore the user IP packets do not go over MME. This way the network capacity

stays on a good level as the signaling and the traffic can grow separately [49]. The

main responsibilities of MME are idle-mode User Equipment (UE) reachability

including the control and execution of paging retransmission, different type ofauthentication procedures with Non-Access Stratum (NAS) signaling, roaming, P-

GW/S-GW selection, tracking area list management and bearer management

including dedicated bearer establishment [47,48].

2.4. Air interface in LTE:The air interface and communication environment used in LTE mobile

communication systems is called the LTE Radio Access Network. [45]

The LTE air interface is based on OFDMA for the downlink. OFDMA is an extension

of OFDM for the implementation of a multi-user communication system. For the

uplink, a single-carrier frequency-division multiple access (SC-FDMA) technique hasbeen selected. Advantages of this method include the relatively low adjacent channel

power, even if the power amplifier is not 100% linear. With SC-FDMA, no exacting

requirements are imposed on the linearity of the power amplifier in the mobile

handset. As a result, power consumption can be kept within limits. [50]

The utilization of OFDM provides considerable advantages over alternative multiple-

access techniques and signals severe departure from the past. Among the benefits are

adaptability for broadband data transmission and high spectral efficiency, impedance

to Inter Symbol Interference (ISI) resulting from the multipath fading, naturally

provide MIMO (Multiple Input Multiple Output) schemes, and provide frequency-

domain techniques like frequency-selective scheduling [51].

The design of the time-frequency representation of OFDM to provide high levels offlexibility in allocation of each of the time frames for transmission and the spectra.

The spectrum flexibility in LTE supports not only a scalable set of bandwidths, but

also a variety of frequency bands. LTE also supplies a small frame size of 10 ms in

order to reduce latency. By designate short frame sizes, LTE allows better channel

estimation to be carried out the mobile, allowing timely feedbacks needful for link

adaptations to be supplied to the base station.[41]


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10

Figure 2.1: System architecture for LTE Rel-8 network [47].

2.5 History of OFDM:The initial development of multi-carrier communication system was basically done by

military systems in the late 1950s and mid-1960s. KINEPLEX, ANDEFT andKATHRYN etc. are the few OFDM based systems utilized by US military systems for

high frequency applications [10].

In 1966, the concept of multicarrier communication was first introduced by Chang

[60] .He suggested a multicarrier scheme utilizing the parallel data transmission by

means of 10 frequency division multiplexing (FDM) with overlapping subcarriers. It

was found to be an efficient scheme for bandwidth utilization and to mitigate the

effect of multipath propagation. It also eliminated the requirement of high-speed

equalization technique. He gave the basic concept of OFDM and outlined a theoretical

way to transmit simultaneous data stream trough linear band limited channel without

Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI) [61] [62].


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11

These systems are called classical Multicarrier Modulation (MCM) system and

transmitted data on non-overlapped band-limited orthogonal signals. These systems

require analog oscillator and filter of much wider bandwidth and sharp cut-off.

Therefore, the concept of OFDM was not gained so much attention or popularity

because of the difficulty in subcarrier recovery without inter-subcarrier interference

by means of analog filters. Due to this reason only, a number of studies in the 1960swere dedicated for MCM employing overlapped band-limited orthogonal signals [63,

64, and 65]. In the year 1967, B. R. Saltzberg suggested a MCM system employing

Orthogonal time-staggered Quadrature Amplitude Modulation (O-QAM) on the

carriers [63]. The concept of MCM scheme employing time-limited orthogonal

signals, which is similar to OFDM, was first given by H. F. Marmuth [66] in 1960.

[10]

The KINEPLEX system was developed by Collins Radio Company for data

transmission at high frequency over multipath fading channel, in this system, 20 tones

are modulated by DQPSK without filtering, which resulted in overlapping channels.

Initially the implementation of an OFDM system with large number of subcarriers

was very complex and expensive because it requires the array of sinusoidal generatorsand coherent demodulators for parallel operations. In order to avoid the crosstalk

between the subcarriers, the system should be free from frequency and timing offsets

[62].

A major breakthrough in the history of OFDM came in 1971 when Weinstein and

Ebert used Discrete Fourier Transform (DFT) to perform baseband modulation and

demodulation which eliminated the need of bank of subcarrier oscillators thus making

the operation efficient and simpler [1,67].

In 1979, after evolutionary growth and development in signal processing and VLSI

technologies, high speed chips can be built around special-purpose hardware

performing the large size Fast Fourier Transform (FFT) (efficient algorithm for DFT)

at affordable price [68], [69].

All the proposals of OFDM systems used guard spaces in frequency domain and

a raised cosine windowing in time domain to combat ISI and ICI. Another milestone

for OFDM history was when Peled and Ruiz introduced Cyclic Prefix (CP) or cyclic

extension in 1980 [67,70] .This solved the problem of maintaining orthogonal

characteristics of the transmitted signals at severe transmission conditions. The

generic idea that they placed was to use cyclic extension of OFDM symbols instead of

using empty guard spaces in frequency domain. This effectively turns the channel as

performing cyclic convolution, which provides orthogonality over dispersive channels

when CP is longer than the channel impulse response [56,70].

Since 1990s, OFDM has been utilized for many broadband communication systems, itincludes high-bit-rate digital subscriber lines (HDSL) [71], asymmetric digital

subscriber lines (ADSL) [72], very high-speed digital subscriber lines (VHDSL) [72],

high definition television (HDTV) terrestrial broadcasting etc. It has also been utilized

by many wireless standards like Digital Audio Broadcasting (DAB) [73] The DAB

standard was in fact the first OFDM-based standard (project started in 1988 by ETSI

and completed in 1995), Digital Video Broadcasting (DVB) [74].

Many standards have been proposed for wireless local area networks (WLANs)

operating in ISM band, which are based on spread-spectrum technology. A number of

studies regarding the commercial applications of OFDM were made during 1990s like

High Bit rate Digital Subscriber Lines (HDSL; 1.6 Mbps), Asymmetric Digital

Subscriber Lines (ADSL; 6 Mbps), Very High Speed Digital Subscriber Lines


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12

(VDSL; 100 Mbps), DAB and High Definition Television (HDTV) terrestrial

broadcasting [75].

In 1997, first OFDM-based WLAN standard, IEEE 802.11, was released. IEEE

802.11 can support a data rate up to 2 Mbps. Later on, in 1999, IEEE approved an

OFDM based standard 802.11a for supporting a data rate up to 54 Mbps. During this

period ETSI has also standardized the HiperLAN/2 standard, which has adoptedOFDM for their PHY standards [1].

In 2001, the FCC came with some new rules for modulations scheme operating in the

2.4 GHz range, which allow IEEE to extend 802.11b to 802.11g standard. Now days,

it has also been used in WiMAX (IEEE 802.16), and mobile broadband wireless

access (MBWA) IEEE 802.10. It is 11 also utilized by 4G wireless communication

systems, such as IMT-A. It is also been considered for 3GPP Long Term Evolution,

which is under deployment [62].

2.6 OFDM:With the ever growing require of this generation, the necessity for high speed

communication has become a top priority. Different multicarrier modulation

techniques have developed to meet these demands, a few prominent among them

being OFDM and Code Division Multiple Access (CDMA) [52].

The fundamental principle of OFDM is a division of high data rate streams into a

number of lower data rate streams and then transmitted these streams in parallel using

several orthogonal sub-carriers (parallel transmission). Due to this parallel

transmission, the symbol duration increases, thus decrease the prorated amount of

dispersion in time resulting from the multipath delay spread. OFDM can be seen as

either a modulation technique or a multiplexing technique [10].

OFDM communication systems do not depend on increased symbol rates for

achieving higher data rates. That makes the task of managing ISI much easier.Because data is transmitted in parallel instead of serially, OFDM symbols are

basically much longer than symbols on single carrier systems of equivalent data rate

[53].

The concept of OFDM is very much similar to the well-known and extensively used

technique of Frequency Division Multiplexing (FDM). OFDM uses the principles of

FDM to allow multiple messages to be sent over a single radio channel. It is however

in a much more controlled manner, allowing an improved spectral efficiency [54].

In conventional broadcast, each radio station transmits on a different frequency,

effectively using FDM to maintain a separation between the stations. Due to non-

orthogonal nature of carrier frequencies in FDM, a large band gap is required to avoid

inter-channel interference, which reduces the overall spectral efficiency. Thedifference between FDM and OFDM is shown in Figure 2.2 [55].


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13

Figure 2.2: Comparison of FDM and OFDM [55]

The sub-carriers are mutually orthogonal (The principle of orthogonality is discussed

in next sub-section.) in the frequency domain which alleviates the effects of ISI as

indicated in the Figure 2.3. All of these sub-carriers experiences flat fading becausethey have a bandwidth less than the Mobile channel coherence bandwidth [56].

Figure 2.4 shows a baseband transceiver structure for OFDM utilizing the Fourier

transform for modulation and demodulation. Here the serial data stream is mapped tocomplex data symbols (Phase Shift Keying (PSK), QAM, etc.) with a symbol rate

of . The data is then demultiplexed by a serial to parallel converter resulting in ablock of N complex symbols, .The parallel samples are then passedthrough an N point IFFT (in this case no oversampling is assumed) with a rectangular

window of length N.Ts, resulting in complex samples .Assuming the incoming complex data is random it follows that the IFFT

is a set of independent random complex sinusoids summed together. Thesamples,

are then converted back into a serial data stream producing a

baseband OFDM transmit symbol of length T=N.Ts [57].

A Cyclic Prefix (CP), which is a copy of the final part of the samples, is appended to

the front of the serial data stream before RF up conversion and transmission. The CP

combats the disrupting effects of the channel which introduce ISI.

In the receiver the whole process is reversed to recover the transmitted data, the CP is

removed prior to the FFT which reverses the effect of the IFFT [58]. The complex

symbols at the output of the FFT, are then decoded and the original bitsteam recovered.

Thus, the IFFT and FFT blocks at the transmitter and at the receiver, respectively, are

important components in an OFDM system. A lot of work has gone into the

optimization of the FFT implementations and the design community has leveraged this

trend to advantage leading to the popularity of OFDM based systems. The time-


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14

frequency view of an OFDM signal is shown in Figure 2.5, where the important

parameters like subcarrier spacing and OFDM symbol period are shown [59].

Figure 2.3 OFDM subcarrier spacing [56].

Figure 2.4 a block diagram of a basic OFDM system.

Signal

Mapper

Signal

demapper

EqualizerAnd

P/S

DFTOR

FFT

S/P

D/AAdd

CP

IDFT

OR

IFFT

P/SS/P

Multipath

Fading Ch.

&

noise

A/DRemove

CP

Input

output


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15

Figure 2.5 Time-Frequency view of OFDM signal [59]

2.6.1. Orthogonality of the subcarriers and OFDM:Two functions or signals are said to be orthogonal if they are mutually independent of

each other. Orthogonality is a feature that lets multiple information signals to be

transmitted skillfully over a common channel with the successful detection [24 and

76].

The subcarrier spacing is chosen so that the waveforms transmitted on different sub

carriers are orthogonal in time, but overlap in frequency. The orthogonality is

achieved by making the peak of each subcarrier signal coincide with the null of the

other subcarrier signals resulting in a perfectly aligned and spaced subcarrier signal

[77].The principle of orthogonality state that if the time-averaged integral of the product of

any two functions from a set of functions { }, over ajoint existence time interval [ ] is equal to zero, irrespective of the limit ofexistence of the functions, then the functions are told to be orthogonal to each other

within this time-interval [16]. Mathematically, it can be expressed as

(2.1)The orthogonality property of OFDM signals can be shown with the help of its

spectrum. In the frequency domain every OFDM subcarrier has a frequency response, as shown in Figure 2.6 [10].One of the key advantages of OFDM is its efficient use of the frequency band as thesubcarriers are allowed to overlap each other in the frequency domain. The N equally

spaced subcarriers will be orthogonal if the frequency separation between subcarriers

is f = , where N.Ts is symbol duration, and rectangular windowing of theIFFT is performed. Under these conditions the subcarriers will have a waveformfrequency response [78].

Simple rectangular pulse of the length is used and rectangular shape in timedomain corresponds to a

-square shaped spectrum in frequency domain as

illustrated in Figure 2.6. The LTE sub-carrier spacing is set to f= 15 KHz [62].


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16

Figure 2.6 Per-subcarrier pulse shape and spectrum of basic OFDM transmission [48]

Figure 2.7 shows the frequency response of a 5 carrier system where it is seen that

because of the orthogonal relationship the maximum of a particular samplecorresponds to a null in all other carriers, therefore eliminating the effects of

interference.

Figure 2.7: Frequency spectrum of 5 orthogonal subcarriers of an OFDM transmit

signal [78].

The orthogonality among sub carriers can be viewed in time domain as shown in

Figure 2.8. Each curve represents the time domain view of the wave for a subcarrier.

As seen from Figure 2.3, in a single OFDM symbol duration, there are integer

numbers of cycles of each of the subcarriers [62]


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17

Figure 2.8: Time domain representation of the signal waveforms to show

orthogonality among the subcarriers [62]

2.6.2. Guard Interval:Individual sub channels can be perfectly separated by the FFT at the receiver when

there are no ISI and Inter-channel Interference (ICI) introduced by channel distortion.

Practically these conditions cannot be acquired. Since the spectra of an OFDM signal

is not precisely band limited, linear distortion like multipath fading caused sub

channel to spread energy in the adjacent channels [79, 80].

Figure 2.9 illustrates the CP insertion technicality, the Guard Interval or CP is a

periodic addition of the final part of an OFDM symbol that is added to the front of the

symbol in the transmitter, and at the receiver the CP is removed before demodulation[81].

It serves as a recurrence of the end of the symbol, so allowing the linear convolution

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

which in turn might be transformed to the frequency domain utilizing a discrete

Fourier transform (DFT). This process allows for simple frequency domain processing

like channel estimation and equalization [82].

CP insertion, therefore, increases the size of the data symbol from to , being the duration of the guard-period containing the CP. The standard length ofthe guard-period in LTE is defined to be 4.69 s, allowing the system to tolerate pathvariations up to 1.4 km (considering the standard LTE symbol length of 66.7 s).

When a cyclic extension longer than a channel impulse response is added, thenegative effect of the previous symbol can be avoided by simply removing that

extension. CP insertion implies the copying of the last part of the OFDM data symbol

and attaching it to the timing at the beginning of the symbol, creating a break between

signals (hence: guarding-period). The receiver can then sample the incoming

waveform at optimum time, as time-dispersion problems (i.e. delays caused by

reflections of the signal) up to the length of the guarding-period are ignored [45].


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18

Figure 2.9 the CP insertion mechanism [83]

2.6.3 One-tap equalizer [10]:The tap-delay line model with path is considered for multipath fading channel.After Considering the effect of the multipath fading channel, the samples of Thereceived signal can be expressed as:

(2.2)

where, is the impulse response of multipath fading channel with path gains{ }, is the path delay of path, and is a zero-mean, unit variance complex Gaussian noise.

After discarding first G sample of the received signal and taking Z-point FFT, the

output of FFT block is given as : (2.3)Where, the term

is the channel response to the

subcarrier frequency and

is

theAdditive white Gaussian noise(AWGN) term in the frequency domain. To

compensate the fading effect of the channel, one-tap equalizer is used and each

element of the vector is multiplied by an equalized gain factor the output of

equalizer may be written as

(2.4)Where, is defined as

||

. (2.5)


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19

2.7 OFDM based Multiple Access:Various multiple access schemes can be combined with OFDM transmission and they

include orthogonal frequency division multiplexing-time division multiple access

(OFDM-TDMA), OFDMA, and multicarrier code division multiple access (MC-

CDMA). In OFDM-TDMA, time-slots in multiples of OFDM symbols are used to

separate the transmissions of multiple users as shown in figure. 2.10. This means thatall the used subcarriers are allocated to one of the users for a finite number of OFDM

symbol periods.

The only difference from OFDM-TDMA is that the users capture the channel and use

it for certain duration, i.e., the time dimension is used to separate the user signals [84]

Figure 2.10: TimeFrequency view of an OFDM-TDMA Signal

In OFDMA systems, both time and/or frequency resources are used to separate the

multiple user signals. Groups of OFDM symbols and/or groups of subcarriers are the

units used to separate the transmissions to/from multiple users. In figure 2.11, the

time, frequency view of a typical OFDMA signal is shown in a case where there are 3users. It can be seen from figure 2.11 that users signals are separated either in thetime-domain by using different OFDM symbols and/or in the subcarrier domain.

Thus, both the time and frequency resources are used to support multiuser

transmissions. We shall discuss this technique in more detail in the subsequent

sections and also compare it with OFDM-TDMA [85].


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20

Figure 2.11: TimeFrequency view of an OFDMA Signal [85]

2.8 Orthogonal Frequency Division Multiple Access:The approach used in LTEs access techniques consists of using OFDMA for thedownlink (DL) and SC-FDMA for the uplink (UL).

The main reason that justifies different access techniques for the UL and DL is the

fact that SC-FDMA optimizes range and power consumption at the UE, while

OFDMA minimizes receiver complexity and enables frequency domain scheduling

with flexibility in resource allocation. OFDMA is a multi-carrier transmission scheme

in opposition to SC-FDMA. Both allow multiple user access, depending on the

available bandwidth, by dynamically allocating each user to a specific time-frequency

resource, depending on which duplexing is deployed. OFDM requires a large dynamic

range due to PAPR [86 and 87].

The main difference between an OFDM system and an OFDMA one is represented in

Figure 2.12. The different colors represent different users using resources. In OFDM,

users are assigned to resources in the time domain only, while in OFDMA, users can

be assigned also in the frequency domain, optimizing resource usage.

In OFDMA systems, the multiple user signals are separated in the time and/or

frequency domains. OFDMA has been developed with multi-user operation as itspurpose, allowing a flexible assignment of bandwidth to users according to their

needs.

Typically, a burst in an OFDMA system will consists of several OFDM symbols. The

subcarriers and the OFDM symbol period are the finest allocation units in the

frequency and time domain, respectively. Hence, multiple users are allocated different

slots in the time and frequency domain, i.e., different groups of subcarriers and/ or

OFDM symbols are used for transmitting the signals to/from multiple users. For

instance, we illustrate an example in figure 2.13 wherein the subcarriers in an OFDM

symbol are represented by arrows and the lines shown at different times represent the

different OFDM symbols. We have considered 3 users and we have shown how

resources can be allocated by using the different subcarriers and OFDM symbols [88and 89].


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21

Figure 2.12 Difference between OFDM and OFDMA resource by user allocation [86].

Figure 2.13: Example allocation of resources to users in an OFDMA system [85].

Figure 2.14 is a detailed block diagram of OFDMA. The LTE PHY (Physical Layer)

specification has been designed to adapt bandwidths from 1.25 MHz to 20 MHz

OFDM was selected as the main modulation scheme due to its robustness with asevere multipath fading. Downlink multiplexing is achieved through the OFDMA.

OFDM is the modulation scheme for the DL. The primary subcarrier spacing is 15

kHz, with lower subcarrier spacing of 7.5 kHz available for some MB-SFN

(Multicast-broadcast single-frequency network) scenarios. OFDM modulation

parameters summarizes in Table 2-1 [90]


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22

Table 2-1 Downlink OFDM Modulation Parameters [90]

Transmission

BW

1.25 MHz 2.5

MHz

5 MHz 10 MHz 15 MHz 20 MHz

Sub-frame

duration

0.5 ms

Sub-carrier

spacing

15 kHz

Sampling

frequency

192 MHz

(1/2 x 3.84

MHz)

3.84

MHz

7.68

MHz

(2

x 3.84

MHz)

15.36

MHz

(4 x

3.84

MHz)

23.04

MHz (6

x 3.84

MHz)

30.72 MHz

(8 x 3.84

MHz)

FFT size 128 256 512 1024 1536 2048

No. of

occupied

subcarrier

76 151 301 601 901 1201


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Figure 2.14 Complete block diagram of an OFDMA transmitter and receiver [91]


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24

2.9 SC-FDMA:In cellular systems, the wireless communication service in a certain geographical area

is supplied by multiple base stations. The downlink transmissions in cellular systems

are one-to-many, whilst the uplink transmissions are many-to-one. A one-to-many

service means that a base station transmits concurrent signals to multiple users

equipments in its coverage area. This demands that the base station has very hightransmission power ability; as a result of the transmission power is involved for

transmissions to multiple users equipments [92]. On the other hand, in the uplink, asingle users equipment has all its transmission power available for its uplinktransmissions to the base station. In the uplink, the design of an effective multiple

access and multiplexing scheme is more challenging than on the downlink because of

the many-to-one nature of the uplink transmissions. Another consequential requisite

for uplink transmissions is the low signal peakiness by means of the limited

transmission power at the users equipment [92].

One of the main parameters that affect all mobile UE devices is their battery life. It is

therefore necessary to ensure an economic and efficient power use in the transmission

and reception of signals. With the RF power amplifier (i.e enhancer of mixed signals)

and the transmitter being the parts with the highest energy consumption within the

mobile UE; it is essential to establish a transmission model with near constant

operating power level [45].

The downlink physical layer of LTE is depending on OFDMA. Thus, in spite of its

many advantages, OFDMA has specific drawbacks like high sensitivity to frequency

offset (Doppler spread by cause of mobility and Arising from the instability of

electronics) and PAPR. PAPR occurs due to the random constructive addition of sub-

carriers and results in spectral spreading of the signal which leads to adjacent channelinterference. It is a problem that could be insurmountable with high compression point

power amplifiers and amplifier linearization techniques. While these approaches may

be utilized on the base station, they become costly on the UE [93 and 94].

In LTE, a new concept is used for the access technique of the uplink, called SC-

FDMA. Its characteristics combine lower PAPR of single-carrier systems because

there is only a single carrier unlike N carriers. (Which allows maintaining a lower

operating power level than OFDMA) with immunity to multipath interference, as well

as flexible subcarrier frequency allocation (as a crucial part of OFDM) [45]. Figure

2.15 shows the concepts of OFDMA and SC-FDMA.


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25

Figure 2.15 frequency domain description of downlink and uplink LTE access

technologies

SC-FDMA differs from OFDMA in one additional transmission step, caused by the

single-path transmission of single-carrier systems. That transmission step, called

resource element mapping (and its counterpart, resource element selection), shifts all

symbols obtained through the FFT to the desired center frequency and passes them on

to the IFFT for further conversion Figure 2.16.

Since the power of the modulation signals used in this process is constant (QPSK

(Quadrature Phase Shift Keying), 16QAM and 64QAM) and the result of the resource

element mapping step is a waveform similar to the original, on another center

frequency; the required result of a constant-power signal is achieved [45].For practicality, SC-FDMA is implemented in LTE utilizing a Discrete Fourier

Transform Spread OFDM transmission (DFTS-OFDM) which is repeatedly referred

to as a frequency-domain generalization of SC-FDMA. The DFT is used to multiplex

uplink transmissions in definite frequency allocation blocks within the general system

bandwidth in accordance with eNodeB scheduler instructions. The bandwidth of the

single carrier is specified based on the desired data rate by the user. Data remains

serial and not parallelized as done on the downlink with OFDMA (i.e. one

information bit is being transmitted at a time). This results in similar link performance

parameters for the uplink and downlink. Nevertheless, there would be comparatively

high ISI for the uplink because of the single carrier modulation. Thus, the eNodeB

receiver requires a low-complexity equalizer to rectify for the distorting impacts ofthe radio channel. SC-FDMA is not as sensitive to Doppler Effect and frequency

instability the as OFDM by cause of its single carrier nature [93].


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Figure 2.16 Block diagram of an SC-FDMA transmitter and receiver [37]


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Chapter Three Peak-to-Average Power Ratio Reduction

27

Chapter Three

Peak-to-Average Power Ratio Reduction:

High PAPR of transmitted signals is one of the major issues of the OFDM system. A

large dynamic range of input data symbols is the main cause of getting high PAPR.

An OFDM signal consists of independent data symbols modulated on N orthogonalsubcarriers, and when these signals are added to the same phase, higher peakamplitude is observed. The value of this peak may be times of the averageamplitude [10].

3.1 Definitions of PAPR:For a continuous time baseband OFDM signal, the PAPR of any signal is defined as

the proportion of the maximum instantaneous power of the signal and its averagepower. If x (t) is a transmitted baseband OFDM signal, then PAPR is defined as:

,- , - (3.1)Where, is the average power of x (t) and can be computed in frequency domain

because IFFT is a unitary transformation is useful duration of an OFDM symbol[95].

For a discrete OFDM signal, the PAPR is computed from

time oversampled

OFDM signal as:

,- (3.2)The ,- at (dB) = (3.3)Where,, - denotes the expectation operator and is the total number of sub-carriers. The PAPR of pass band OFDM signal is approximately twice that of

baseband PAPR [95].

The above power characteristics can also be described in terms of their magnitudes

(not power) by defining the crest factor (CF), which is defined as the ratio between

maximum amplitude of OFDM signal and root-mean-square (RMS) of thewaveform. The CF is defined as:

||,||||- (3.4)In most cases, the peak value of signal

is equals to a maximum value of its

envelope

||However, it can be seen from Figure 3.1 that the appearance of peak

amplitude is very rare, thus it does not make sense to use max ||to represent the


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28

peak value in real application. Therefore, the PAPR performance of OFDM signals is

commonly measured by certain characterization constants which relate to probability

[96].

Figure 3.1: High PAPR when sub-carriers are modulated by same symbols [96]

3.2 PAPR of OFDM signal [62]:The discrete time baseband OFDM signal is defined in (6). The PAPR of the discrete

time OFDM signal determines the complexity of the digital circuitry in terms of the

number of bits necessary to achieve the desired signal to quantization noise ratio

during signal digitization and recovery. To better approximate the PAPR of a

continuous time OFDM signal, the discrete time OFDM signal is to be obtained by L

times oversampling. The oversampled discrete time OFDM signal can be obtained by

performingLNpoint IFFT on the data block with (L-1) N zero padding as follows:

,- , 0 n NL-1 (3.5)PAPR of the oversampled OFDM signal of becoming

,- , -, - (3.6)where, E[. ] denotes the expectation operator and N is total number of sub-carriers.

The PAPR of passband OFDM signal is approximately twice that of baseband PAPR.

Complementary Cumulative Distribution Function (CCDF) for an OFDM signal can

be written as:

P (PAPR > PAP= (3.7)Where PAP is the clipping level.This equation can be read as the probability that the PAPR of a symbol block exceeds

some clip level PAP.


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3.3 Oversampling discrete OFDM symbols to find true (continuous)

peaks:The PAPR for the discrete-time baseband signal x [n] may not be the same as that of

the continuous-time baseband signal

In fact, the PAPR for

,- is lower than

that for

, simply because

,-may not have all the peaks of

In practice, the

PAPR for the continuous-time baseband signal can be measured only after

implementing the actual hardware, including digital-to-analog convertor (DAC). In

other words, measurement of the PAPR of the continuous-time baseband signal is not

straightforward. Therefore, there must be some means of estimating the PAPR from

the discrete-time signal,-. Fortunately, it is known that ,-can show almost thesame PAPR as if it is L-times interpolated (oversampled) as shown in Figure 3.2where L 4 [97 and 98].

Figure 3.2 Block diagram of L times interpolator [83]

3.4 Distribution of PAPR:To design and develop an effective PAPR reduction technique, it is very important to

accurately identify the distribution of PAPR in OFDM systems. The distribution of

PAPR plays an important role in the design of the whole OFDM system. Thedistribution of PAPR can be used in determining the proper output back-off of the

HPA to minimize the total degradation. It can be used directly to calculate the BER

and to estimate the achievable information rates [10].

For the OFDM system, if we assume that the input data stream is statistically

independent and identically distributed (i.e.) then the real and imaginary parts ofx[n]

are uncorrelated and orthogonal. From central limit theorem, it follows that, for large

values of N, the real and imaginary parts of x[n] are independent and identically

distributed (i.e.) Gaussian random variables, each with zero mean and variance

,| ,- | - . (3.8)

The probability distribution of complex OFDM signals with large N is a complex

Gaussian distribution given by following relation:

*,-+ . ,- / (3.9)Where Pr{.} denotes the probability distribution function. Where, is the varianceof , -.The amplitude of OFDM signal has a Rayleigh distribution and its

probability density function(PDF) is given by:

*,-+ |,- | . |,- | / (3.10)


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The histogram plots for the real part, imaginary part and the absolute value of a time

domain OFDM signal are shown in Figure 3.3(a), (b) and (c) respectively. The plots

shown in Figures 3.3(a) and (b) are obtained after performing the computer

simulations of an OFDM system having N=256 QPSK modulated subcarriers as

shown in Fig. 2.4. The signal obtained from IFFT block of Figure 2.4 is complex

OFDM signal. After that real, imaginary and absolute values of OFDM signal (x[n])are calculated and their histograms are plotted [62].

The power of OFDM signal has chi-square distribution. The distribution of PAPR is

often expressed on the one hand Complementary Cumulative Distribution Function

(CCDF). In probability theory and statistics, the CCDF describes the probability that a

real-valued random variable X with a given probability distribution will be found at a

value greater than or equal to x [99 and 10].

The Cumulative Distribution Function (CDF) of the PAPR of the amplitude of a

signal sample is given by

(3.11)The CCDF of the PAPR of the data block is desired in our case is to compare outputs

of different reduction techniques. This is given by:

(3.12) (3.13)

(3.14)

Where, is the given reference level.

Figure 3.3 (a)


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Figure 3.3 (b)

Figure 3.3 (c)

Figure 3.3: Histogram of (a) Real part of OFDM signal amplitude (b) Imaginary part

of OFDM signal amplitude (c) OFDM signal magnitude [63].


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3.5 Identification of the Problem:Multi-carrier phenomena is considered to be one of the major development in wireless

communication and among them OFDM is becoming the important standard.

However, high PAPR is the major drawback of OFDM, which results in lower power

efficiency hence impedes in implementing OFDM. To overcome the low power

efficiency requires not only large back off and large dynamic range DAC but alsohighly efficient HPA and linear converters. These demands result in costly hardware

and complex systems. Therefore to lessen the difficulty of complex hardware design it

has become imperative to employ efficient PAPR reduction techniques [100 and 101].

The drawback of a large dynamic range is that it places pressure on the design of

components such as the word length of the IFFT/FFT pair, mixer stages, and most

importantly the HPA, which must be designed to handle irregularly occurring large

peaks, decreases the SQNR (Signal-to-Quantization Noise Ratio) of ADC (Analog-to-

Digital Converter) and DAC, The PAPR problem is more important in the uplink

since the efficiency of power amplifier is critical due to the limited battery power in a

mobile terminal. Failure to design components with a sufficiently large linear range

result in saturation of the HPA [98, 78]. Saturation creates both in band distortion,

increasing the BER and out of band distortion, or spectral splatter, which causes

Adjacent Channel Interference (ACI). One obvious solution is to design the

components to operate within large linear regions, however this is impractical as the

components will be operating inefficiently and the cost becomes prohibitively high.

This is especially apparent in the HPA where much of the cost and ~50% of the size

of a transmitter lies which will be explained in next sections [98, 78].

3.5.1 Nonlinear HPA and DAC:

HPA are used in the transmitter of communication systems for sufficient transmissionpower. To achieve maximum output power efficiency they have to be operated at or

near the saturation region. [100]

If the data on the subcarriers add up in a constructive manner at the transmitter, the

resulting signal could exhibit large PAPR. As a result, the composite transmit signal

could be severely clipped by the DAC and power amplifiers for their bounded

dynamic range as described in Figure 3.4. In this case, the reconstructed output can possess a significant amount of distortion. It can be reduce the PAPR of an

OFDM signal by modifying the signal characteristics in time-domain or frequency

domain clipping of the composite OFDM signal causes several undesirable outcomes,

such as signal distortion and spectral regrowth. For instance, clipping causes in band

noise that results in a degradation of the BER performance .Moreover, higher-orderharmonics that spill over into OOB spectrum can also result from signal clipping.

Although filtering after the HPA can be employed to remove this spectral leakage, it

is very power-inefficient, so it is an undesirable solution. Therefore, the dynamic

range of DAC should be large enough to accommodate the largest peaks of signals or

high PAPR values [102].

A high-precision DAC support high PAPR with acceptable amount of quantization

noise, but could be very costly to a certain sampling rate of the system. On the other

hand, a low-precision DAC would be cheaper, but the quantization noise will be

significant, which reduces the signal SNR (Signal to Noise Ratio) when the dynamic

range of DAC is increased to support high PAPR. Otherwise, the DAC will saturateand clipping will occur [48, and 103].


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Figure 3.4 An example illustrating effect of clipping.

The dynamic range of the power amplifiers should also be large enough to

accommodate large PAPR values. Otherwise, the power amplifiers may saturate and

clipping might occur. The component cost of the DAC and power amplifiers increase

with the increase in the dynamic range.


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It is worth mentioning that the clipping of high signal peaks rarely happens, resulting

in a comparatively low incidence clipping noise. In this manner, the impact of

clipping at the transmitter on the error performance of the OFDM system liable to be

subjected frequency selective fading is minimal [102].

If an HPA with limited linear range is utilized for amplification, it may operate near

saturation and can cause OOB radiations and in-band distortion. The OOBdistortion/noise is a major concern, especially in wireless communications, where

large differences in signal strength from a mobile transmitter impose stringent

requirements on ACI [104]

Figure 3.5 demonstrates a classic input-output characteristic of a power amplifier. For

prevent or limit signal distortion input signals must be preserved below the Non-linear

area. The result is that the amplifier is not completely used [105]

IBO = 10 (3.15)OBO = 10 (3.16)IBO (Input Back-Off) or OBO (Output Back-Off)

High PAPR results in a wide variety of OFDM signal amplitudes which due to

nonlinear characteristics of HPA findings in inter-modulation among the various sub

carriers and leading to an increment in BER. To realize a low BER and minimal

signal distortion, HPA must be a large dynamic range and work in the linear amplifier

region. But, these types of HPA are expensive and smaller power efficient. The power

efficiency in wireless communication is very important for achieving efficient area

coverage and small size terminals. Thus, the power efficient process of non-linearHPA is so important. Accordingly, it is best to target the reduction of PAPR the

OFDM signals before transmitting the signal into nonlinear DAC and HPA [100].

Figure 3.5 Typical input-output characteristics of a power amplifier showing the

Relation between Output Back-Off (OBO) and Input Back-Off (IBO) [98].


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3.5.2 Power Saving [100]:A high dynamic range HPA has low power efficiency. The power could save by

reducing PAPR. This power saving that is implemented in such a way to provide a

direct correlation with the desired average output power.

On the assumption a linear model of HPA, the power efficiency is:

(3.17) (3.18)The = HPA efficiency .= the average of the output power. .= A fixed amount of power regardless of their input power.For example: an OFDM signal with 256 sub carriers that demand an IBO equal to the

PAPR at the probability level lower than 0.01%, i.e. (25.235).This makes

= 0.5/25.2351.98%

The PAPR of OFDM systems has to reduce for avoiding this level of power

inefficiency.

3.6 Factors influencing the PAPR:

3.6.1 The number of sub carriers:In Multi-Carrier Systems the complex base band signal for one symbol in an OFDM

system can be expressed as follows:

(3.19)Where N is the modulating symbol and is the number of sub carriers. For moderately

large numbers of m-PSK (multiple phase-shift keying) sub carriers the quadrature

components of x (t) each tends towards a Gaussian distribution (giving the sum of

their power amplitude a Rayleigh distribution). Consequently, whilst the peak value

possible is N times the individual sub carrier peak, the probability of any value close

to that peak occurring is very low. For example, with only 24 sub carriers, the

probability of the PAPR exceeding 4dB is

and of exceeding 8dB is only

[99].3.6.2 The order of Modulation:High data bandwidth efficiency (in terms of b/s/Hz) this can be achieved by utilizing

higher order modulations based, for instance, on QAM. When using a higher-order

modulation such as QAM type, the PAPR of the summed OFDM signal is increased

by the PAPR of the QAM constellation utilized. Nevertheless, the probability of these

higher peaks happening is accordingly less. Furthermore, since among benefits of

OFDM is one that sub carriers could have their modulation independently varied to

adapt to channel conditions, the joined PAPR in any system utilizing this technique

might are hard to predict and control. PAPR for an unfiltered base band signal is listedin the following Table 3.1. [100].


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Table 3.1 PAPR for picked modulation formats

3.6.3 Constellation shape:The last entry in Table 3.1 is for a constellation obtained by modifying 256- QAM to

reduce PAPR. This modified constellation shape is shown in figure 3.6. However,

there is an additional processor load associated with encoding and decoding this

constellation.

Figure 3.6 256-QAM constellations: (a) regular and (b) modified mapping to reduce

PAPR

3.6.4 Pulse Shaping:In terrestrial communications, it is popular to use pulse shaping to the base band

signal, to decrease the bandwidth of the transmitted spectrum, but this causes

overshoot and can increase the PAPR of the modulating signal by 4-5 dB [100].

3.7 The gauge for judgment of the PAPR reduction in OFDM systems

[106, 107, 108]:Every method used to reduce the PAPR has some drawbacks and merits. There is

always a trade-off between PAPR reduction and some other factors like bandwidth,

computational complexity, average power etc. An ideal PAPR reduction technique

should have following characteristics:

1) High potential to limit the PAPR: It is a key factor to consider in the selection of

technology to reduce the PAPR with few adverse side effects like in-band distortion

and OOB radiation.

2) Low average power: even though it can reduce PAPR through the average power of

the original signals increase, it needs a bigger linear operation region in HPA and

which led in the deterioration of BER performance.

Modulation PAPR

256-QAM 4.23dB

64-QAM 3.68dB256-QAM (modified) 2.85dB

16-QAM 2.55dB

m-PSK (reference) 0 dB


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3) Low implementation complexity: mainly, complexity techniques viewing better

capability of PAPR reduction. Nevertheless, practically, both time and hardware

requisites for the PAPR reduction must be minimal.

4) No bandwidth expansion: The bandwidth is an infrequent resource in systems. The

bandwidt

an overview : peak to average power ratio (papr) in ofdm system using some new papr techniques (with...

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