ABSTRACT

Wi-MAX technology based communication system has evolved with key

advantages such as capacity improvement per station, increased data rate,

standardization. Multi-carrier OFDM system has increased the capability of

WiMAX system because of its high spectral efficiency, robustness against multi-

path fading and simple receiver structure. In the OFDM system, orthogonally

placed sub – carriers are used to carry the data from the transmitter end to the

receiver end.

Presence of guard band in this system deals with the problem of ISI and noise is

minimized by larger number of sub – carriers. But the large Peak – to – Average

Power Ratio of these signal have some undesirable effects on the system.

In this work, we have focused on learning the basics of an OFDM system and have

used two different methods to reduce the PAPR in the system with special

emphasis on “Windowing Technique”.

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ACKNOWLEDGEMENT

In the name of Almighty Allah who gave us courage and support to complete the

Research work presented in this dissertation.

Day by day computer networking becomes very important part of present IT world. Computer networking is now more developed and 4G technology is introduced as Wi-Max technology. WiMax(Worldwide Interoperability for Microwave Access) is a telecommunications protocol that provides fixed and mobile Internet access. Because of the importance of computer networking this thesis paper has been done by a group of CSE students on the subject of WiMax technology.

The work presented in this dissertation was carried out under the supervision of Mahmuda Begum, lecturer, Dept. of ETE, The Peoples’ University of Bangladesh, whose kind attention and advice helped us to successfully complete the research work. We are grateful to all our teachers’ especially to Md. Masud Reza, Assistant Professor, Department of CSE, PUB for his guidance and advice to complete this research work.We would also like to thank Syed Ahsanul Kabir, Head, Dept. of CSE, PUB for his cooperation to complete the work. Finally, we express our deepest gratitude and sincerity to Prof. Mustafizur Rahman, Vice-Chancellor, PUB.

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DECLARATION

This is certified that the accomplished thesis paper was done by us under CSE-400 “Project and Thesis” and it has not been submitted elsewhere for the requirement of any degree or diploma or any other purposes except for other relevant purposes.

Signature of the students-

(Jams Chiran)

(Al-Rajhi Talukdar)

(Amirul Islam)

(Md Ebrahim Khalil)

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ACCEPTANCE

This thesis “A Simulation Study of WiMAX based communication system using Windowing Technique to improve Peak to Average Power Ratio (PAPR) of OFDM signal" is accepted as to style and content.

MAHMUDA BEGUMSUPERVISORLECTURERDEPARTMENT OF ELECTRONICS & TELECOMMUNICATION ENGINEERINGTHE PEOPLES UNIVERSITY OF BANGLADESH

SYED AHSANUL KABIRCHAIRMANASSOCIATE PROFESSORDEPARTMENT OF COMPUTER SCIENCE & ENGINEERINGTHE PEOPLE’S UNIVERSITY OF BANGLADESH

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Contents

Title …………………………………………………………………………….. IAbstract ………………………………………………………………………… IIAcknowledgements …………………………………………………………….. IIIDeclaration ……………………………………………………………………… IVAcceptance ……………………………………………………………………… VContents …………………………………………………………………………. VI

Chapter 1: Introduction1.1 : Wireless Communication

1.1.1: Introduction ……………………………………………………… 021.1.2: Background ……………………………………………………… 021.1.3: Spectrum Allocation …………………………………………….. 031.1.4: Introduction of cellular concept based Mobile Communication System ………………………………… 051.1.5: Standards ………………………………………………………… 05

1.2 : WiMAX Communication System 1.2.1: History …………………………………………………………... 091.2.2: Presently Available Technology ………………………………... 091.2.3: Motivation for Research & Development in WiMAX

Technology based communication system …………………….. 11

1.3 : Objective of the thesis …………………………………………………….. 12

1.4 : Organization of the thesis ………………………………………………… 12

Chapter 2: PAPR reduction Technique in OFDM signal 2.1: OFDM ……………………………………………………………… 142.2: PAPR ………………………………………………………………. 152.3: PAPR Reduction Techniques ……………………………………... 162.4: Comparative Analysis of Different Techniques ……………………. 232.5 Characteristic Equations of Some PAPR Reduction Techniques …… 24

Chapter 3: Results & Discussion 3.1: Windowing technology for PAPR reduction ……………………………… 28

3.2: Simulation Results …………………………………………………………. 30

3.4: Graphs for number of QAM……………………………………………… ... 31

3.5: Flow Chart …………………………………………………………………... 70

Chapter 4: Overall Discussion4.1: Simulation Output Analysis…………………………………………………. 734.2: Conclusion ………………………………………………………………….. 754.3: Future Work ………………………………………………………………… 75

References ………………………………………………………………………... 76

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Chapter 1

Introduction

1.1: Wireless Communication

1.1.1: Introduction

In 1895, Guglielmo Marconi opened the way for modern wireless communications by transmitting the three-dot Morse code for the letter ‘S’ over a distance of three kilometers using electromagnetic waves [1]. From this beginning, wireless communications has been developed into a key element of modern society. From satellite transmission, radio and television broadcasting to the now ubiquitous mobile telephone, wireless communications has revolutionized the way societies function.

1.1.2: Background

Marconi’s pioneering work quickly led to variety of commercial and government (particularly military) developments and innovations. In the early 1900s, voice and then music was transmitted and modern radio was born [2]. By 1920, commercial radio had been established with Detroit station WWJ and KDKA in Pittsburgh [2]. Wireless telegraphy was first used by the British military in South Africa in 1900 during the Anglo-Boer war [3]. The British navy used equipment supplied by Marconi to communicate between ships in Delagoa Bay. Shipping was a major early client for wireless telegraphy and wireless was standard for shipping by the time the Titanic issued its radio distress calls in 1912.

Effective international coordination involving two features, local coordination & coordination between countries was required at the beginning for solid foundation of worldwide accepted communication system. First, the potential for interference in radio transmissions meant that at least local coordination was needed to avoid the transmission of conflicting signals. Secondly, with spectrum to be used for international communications and areas such as maritime safety and navigation, coordination was necessary between countries to guarantee consistency in approach to these services. This drove government intervention to ensure the coordinated allocation of radio spectrum.

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1.1.3: Spectrum Allocation

Radio transmission involves the use of part of the electromagnetic spectrum. Electromagnetic energy is transmitted in different frequencies and the properties of the energy depend on the frequency. For example, visible light has a frequency between 4×1014 and 7.5×1014 Hz. Ultra violet radiation, X-rays and gamma rays have higher frequencies (or equivalently a shorter wave length) while infrared radiation, microwaves and radio waves have lower frequencies (longer wavelengths). The radio frequency spectrum involves electromagnetic radiation with frequencies between 3000 Hz and 300 GHz. The spectrum allocation is shown in Figure 1.1 [25] and the ranges of wireless services are shown in Table 1.1.

Figure 1.1: Frequencies and Wave lengths for electromagnetic signal

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Table 1.1: The range of wireless service [26]

Band Name ITU band

Frequencyand

wavelength in air

Example uses

subHz (sub-hertz)

0 < 3 Hz> 100,000 km

Natural and man-made electromagnetic waves (millihertz, microhertz, nanohertz) from earth,

ionosphere, sun, planets, etc[citation needed]

ELF (Extremely low frequency)

1 3–30 Hz100,000 km –

10,000 km

Communication with submarines

SLF (Super low frequency)

2 30–300 Hz10,000 km –

1000 km

Communication with submarines

ULF (Ultra low frequency)

3 300–3000 Hz1000 km – 100 km

Communication within mines

VLF (Very low frequency)

4 3–30 kHz100 km – 10 km

Submarine communication, avalanche beacons, wireless heart rate monitors, geophysics

LF (Low frequency)

5 30–300 kHz10 km – 1 km

Navigation, time signals, AM longwave broadcasting, RFID

MF (Medium frequency)

6 300–3000 kHz1 km – 100 m

AM (medium-wave) broadcasts

HF (High frequency)

7 3–30 MHz100 m – 10 m

Shortwave broadcasts, amateur radio and over-the-horizon aviation communications, RFID

VHF (Very high frequency)

8 30–300 MHz10 m – 1 m

FM, television broadcasts and line-of-sight ground-to-aircraft and aircraft-to-aircraft communications. Land

Mobile and Maritime Mobile communicationsUHF (Ultra high frequency)

9 300–3000 MHz1 m – 100 mm

Television broadcasts, microwave ovens, mobile phones, wireless LAN, Bluetooth, GPS and two-way radios such as Land Mobile, FRS and GMRS radios

SHF (Super high frequency)

10 3–30 GHz100 mm – 10 mm

Microwave devices, wireless LAN, most modern radars, communications satellites

EHF (Extremely high frequency)

11 30–300 GHz10 mm – 1 mm

Radio astronomy, high-frequency microwave radio relay, microwave remote sensing

THz (Terahertz) 12 300–3,000 GHz1 mm – 100 μm

Terahertz imaging – a potential replacement for X-rays in some medical applications, ultrafast molecular

dynamics, condensed-matter physics, terahertz time-domain spectroscopy, terahertz

computing/communications, sub-mm remote sensing

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1.1.4: Introduction of cellular concept based Mobile Communication System

The history of mobile telephones can be broken into four periods. [4] The first (pre-cellular) period involved mobile telephones that exclusively used a frequency band in a particular area. These telephones had severe problems with congestion and call completion. If one customer was using a particular frequency in a geographic area, no other customer could make a call on that same frequency. Further, the number of frequencies allocated by the FCC in the U.S. to mobile telephone services was small, limiting the number of simultaneous calls. Similar systems, known as A-Netz and B-Netz were developed in Germany.

The introduction of cellular technology greatly expanded the efficiency of frequency use of mobile phones. Rather than exclusively allocating a band of frequency to one telephone call in a large geographic area, a cell telephone breaks down a geographic area into small areas or cells. Different users in different (non-adjacent) cells are able to use the same frequency for a call without interference.

1.1.5: Standards

There are different organizations or authorities around the globe for standardization of wireless communication like ITU, ATIS, TIA, ETSI, TTC and IEEE. Among them IEEE is the most acceptable and reliable authority for standardization of wireless communication. Here different standards of IEEE are mentioned bellow [5].

Table 1.2: IEEE 802 Wireless Standards

802 Overview Basics of physical and logical networking concepts.

802.1 Bridging

LAN/MAN bridging and management. Covers management and the lower sub-layers of OSI Layer 2, including MAC-based bridging (Media Access Control), virtual LANs and port-based access control.

802.2 Logical LinkCommonly referred to as the LLC or Logical Link Control specification. The LLC is the top sub-layer in the data-link layer, OSI Layer 2. Interfaces with the network Layer 3.

802.3 Ethernet

"Grandaddy" of the 802 specifications. Provides asynchronous networking using "carrier sense, multiple access with collision detect" (CSMA/CD) over coax, twisted-pair copper, and fiber media. Current speeds range from 10 Mbps to 10 Gbps. Click for a list of the "hot" 802.3 technologies.

802.4 Token Bus Disbanded

802.5 Token RingThe original token-passing standard for twisted-pair, shielded copper cables. Supports copper and fiber cabling from 4 Mbps to 100 Mbps. Often called "IBM Token-Ring."

802.6 Distributed "Superseded **Revision of 802.1D-1990 edition (ISO/IEC

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queue dual bus (DQDB)

10038). 802.1D incorporates P802.1p and P802.12e. It also incorporates and supersedes published standards 802.1j and 802.6k. Superseded by 802.1D-2004." (See IEEE status page.)

802.7Broadband LAN Practices

Withdrawn Standard. Withdrawn Date: Feb 07, 2003. No longer endorsed by the IEEE. (See IEEE status page.)

802.8Fiber Optic Practices

Withdrawn PAR. Standards project no longer endorsed by the IEEE. (See IEEE status page.)

802.9Integrated Services LAN

Withdrawn PAR. Standards project no longer endorsed by the IEEE. (See IEEE status page.)

802.10Interoperable LAN security

Superseded **Contains: IEEE Std 802.10b-1992. (See IEEE status page.)

802.11 Wi-Fi

Wireless LAN Media Access Control and Physical Layer specification. 802.11a, b, g etc. are amendments to the original 802.11 standard. Products that implement 802.11 standards must pass tests and are referred to as "Wi-Fi certified."

802.11a  

Specifies a PHY that operates in the 5 GHz U-NII band in the US - initially 5.15-5.35 AND 5.725-5.85 - since expanded to additional frequencies

Uses Orthogonal Frequency-Division Multiplexing Enhanced data speed to 54 Mbps

Ratified after 802.11b

802.11b  

Enhancement to 802.11 that added higher data rate modes to the DSSS (Direct Sequence Spread Spectrum) already defined in the original 802.11 standard

Boosted data speed to 11 Mbps 22 MHz Bandwidth yields 3 non-overlapping channels

in the frequency range of 2.400 GHz to 2.4835 GHz

Beacons at 1 Mbps, falls back to 5.5, 2, or 1 Mbps from 11 Mbps max.

802.11d  

Enhancement to 802.11a and 802.11b that allows for global roaming

Particulars can be set at Media Access Control (MAC) layer

802.11e  

Enhancement to 802.11 that includes Quality of Service (QoS) features

Facilitates prioritization of data, voice, and video transmissions

802.11g   Extends the maximum data rate of WLAN devices that operate in the 2.4 GHz band, in a fashion that permits interoperation with 802.11b devices

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Uses OFDM Modulation (Orthogonal FDM)

Operates at up to 54 megabits per second (Mbps), with fall-back speeds that include the "b" speeds

802.11h  

Enhancement to 802.11a that resolves interference issues

Dynamic frequency selection (DFS)

Transmit power control (TPC)

802.11i  

Enhancement to 802.11 that offers additional security for WLAN applications

Defines more robust encryption, authentication, and key exchange, as well as options for key caching and pre-authentication

802.11j   Japanese regulatory extensions to 802.11a specification

Frequency range 4.9 GHz to 5.0 GHz

802.11k   Radio resource measurements for networks using

802.11 family specifications

802.11m   Maintenance of 802.11 family specifications

Corrections and amendments to existing documentation

802.11n  

Higher-speed standards -- under development Several competing and non-compatible technologies;

often called "pre-n" Top speeds claimed of 108, 240, and 350+ MHz

Competing proposals come from the groups, EWC, TGn Sync, and WWiSE and are all variations based on MIMO (multiple input, multiple output)

802.11x   Mis-used "generic" term for 802.11 family

specifications

802.12 Demand PriorityIncreases Ethernet data rate to 100 Mbps by controlling media utilization.

802.13 Not used Not used

802.14 Cable modemsWithdrawn PAR. Standards project no longer endorsed by the IEEE.

802.15Wireless Personal Area Networks

Communications specification that was approved in early 2002 by the IEEE for wireless personal area networks (WPANs).

802.15.1 BluetoothShort range (10m) wireless technology for cordless mouse, keyboard, and hands-free headset at 2.4 GHz.

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802.15.3a UWB Short range, high-bandwidth "ultra wideband" link

802.15.4 ZigBee Short range wireless sensor networks

802.15.5 Mesh Network

Extension of network coverage without increasing the transmit power or the receiver sensitivity

Enhanced reliability via route redundancy

Easier network configuration - Better device battery life

802.16Wireless Metropolitan Area Networks

This family of standards covers Fixed and Mobile Broadband Wireless Access methods used to create Wireless Metropolitan Area Networks (WMANs.) Connects Base Stations to the Internet using OFDM in unlicensed (900 MHz, 2.4, 5.8 GHz) or licensed (700 MHz, 2.5 – 3.6 GHz) frequency bands. Products that implement 802.16 standards can undergo WiMAX certification testing.

802.17Resilient Packet Ring

IEEE working group description

802.18Radio Regulatory TAG

IEEE 802.18 standards committee

802.19 Coexistence IEEE 802.19 Coexistence Technical Advisory Group

802.20Mobile Broadband Wireless Access

IEEE 802.20 mission and project scope

802.21Media Independent Handoff

IEEE 802.21 mission and project scope

802.22Wireless Regional Area Network

IEEE 802.22 mission and project scope

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1.2: WiMAX Communication System

1.2.1: History

WiMAX Technology is an IP based [21], wireless broadband access technology that provides performance similar to 802.11/Wi-Fi networks with the coverage and QoS (Quality of Service) of cellular networks. WiMAX is also an acronym meaning "Worldwide Interoperability for Microwave Access (WiMAX). The idea of WiMAX technology based communication was introduced by WiMAX forum in June 2001 [6].The ultimate goal of the WiMAX Forum is to promote and accelerate the introduction of cost-effective broadband wireless access services into the marketplace. Standards-based, interoperable solutions enable economies of scale that, in turn, drive price and performance levels unachievable by proprietary approaches, making WiMAX Forum Certified products the most competitive at delivering broadband services on a wide scale.

1.2.2: Presently Available Technology

WiMAX is based upon the IEEE 802.16 WMAN technology family [7], which provides specifications of the media access control (MAC) layer and the physical (PHY) layer. The 802.16 specification further subdivides the MAC sub-layer into three sub layers: the convergence sub-layer (CS), the common part sub-layer (CPS), and the security sub-layer. The CS aims to enable 802.16 to better accommodate the higher layer protocols placed above the MAC layer. The 802.16 specification assumes there will be two predominant types of traffic transported across the 802.16 network: ATM and IEEE 802.3 (Ethernet). Thus, there are two CS specifications: ATM and packet. The CS receives data frames from a higher layer and classifies the frame. On the basis of this classification, the CS can perform additional processing, such as payload header compression, before passing the frame to the Medium Access Control Common Part Sub-layer (MAC CPS). The CS also accepts data frames from the MAC CPS. If the peer CS has performed any type of processing, the receiving CS will restore the data frame before passing it to a higher layer. The CS is separate from the remainder of the 802.16 MAC such that vendors who wish to support other protocols can develop Specialized CSs.

The CPS is the central piece of the 802.16 MAC, defining the medium access method (Figure 1.2). The CPS provides functions related to duplexing, network entry and initialization, framing, quality of service (QoS), and channel access. The security sub-layer, also referred to as the privacy sub-layer,

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Figure 1.2: The logical architecture of IEEE 802.16

has been designed to meet two primary goals: providing subscribers with privacy across the wireless network and providing operators with strong protection from theft of service. The PHY layer then converts MAC layer frames into signals to be transmitted across the air interface. Consequently, the security sub layer has two component protocols: an encapsulation protocol and a privacy key management protocol.

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1.2.3: Motivation for Research & Development in WiMAX technology based communication System

WiMAX has already evolved as a key technology for wireless communication with the advantages such as:

It is the large range wireless system covering many KMs and single station can serve several users.

This enables the user to place the WiMAX subscriber unit in the best recipient & able to use the WiMAX network from any place within their residence.

Much faster deployment of new users comparing to wired networks 802.16e provides full mobility (soft & hard handover between base stations) support

which has become most important aspects of ‘Mobile WiMAX’. Provides advanced ‘antenna diversity’ scheme, ‘Hybrid Automatic Repeat-request

(HARQ) scheme’, ‘Adaptive Antenna System (AAS)’ & ‘MIMO’ technology. Significant development of WiMAX technology is its ‘spectral efficiency’ (3.7Hz & 3.5

Hz for 4G wireless system)

These notable advantages & development of WiMAX comes from combining SOFDMA (Scaling OFDM Access) with ‘smart antenna’ technologies. This multiplies the effective spectral efficiency through multiple reuse and smart network deployment topologies. But there are several areas where R & D is required to improve the performance of the system. Such as improving of PAPR, increasing coverage area, speed, self-installation, power consumption, frequency re-use and bandwidth efficiency.

Any successful technology must fit a key to determine the current status of the market and investigates requirements for a mass-market success. Evaluations are made based on the current performance of various components of a particular technology, including need, market potential, technology evolution, maturation, price points and the companies and resources.

WiMAX is an important, highly visible part of the evolving fixed/portable field and is gaining momentum with the recent deployment. The broadband wireless technologies and WiMAX in particular will have great success in markets because they are well suited to meet the growing needs of residential, business, and government users. WiMAX and other applications of MIMO-OFDM based systems marks a demarcation between it and prior fields of wireless development.

Therefore the WiMAX technology must have a compelling title like ‘killer technology’ as it has brought evolutionary change in the field of wireless communication and it has also satisfied a long-term success in the marketplace.

Although WiMAX and similar fixed wireless technologies are already delivering broadband access for niche applications like rural deployments and backhaul, timing is of the essence for both the early real-world success of the future potential of WiMAX.

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1.3: Objective of the Thesis

Our research on WiMAX based communication system has mainly five objectives

1. To be familiarized with the WiMAX based communication system.2. To be familiarized with Orthogonal Frequency Division Multiplexing (OFDM)

technology3. To analyzing the Bit-Error Rate (BER) probability of equalized OFDM signal.4. To observe the effect of using ‘Windowing’ technique to improve the Peak to Average

Power Ratio (PAPR) of OFDM signal.5. To compare the performance of Windowing technique with the Clipping.

1.4: Organization of the Thesis:

The thesis is divided into three chapters. In chapter one, introductory remarks about the wireless communication system along with its background, spectrum allocation, range, cellular concept based mobile communication system and different standards are explained. Here we also had discussed about WiMAX communication system, available technologies and motivation for R& D. In chapter two, we discussed details about OFDM, PAPR and different techniques for reduction of PAPR in OFDM signal. In the final chapter, an analytical discussion on windowing technique to reduce PAPR has been made. The results of our simulation study are reported and concluding summarized remarks are also made.

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Chapter 2

PAPR Reduction Techniques

2.1: Orthogonal Frequency Division Multiplexing (OFDM)

OFDM is a modulation scheme that allows digital data to be efficiently and reliably transmitted over a radio channel and it performs well even in multipath environments with reduced receiver complexity. Using OFDM, it is possible to exploit the time domain, the space domain, the frequency domain and even the code domain to optimize radio channel usage [22].

OFDM transmits data by using a large number of narrow-band subcarriers. These subcarriers are regularly spaced in frequency, forming a block of spectrum. The frequency spacing and time synchronization of the subcarriers are chosen in such a way that the subcarriers are orthogonal, meaning that they do not cause interference to one another, despite the fact that subcarriers overlap with one another in the frequency domain. The name ‘OFDM’ is derived from the fact that the digital data is sent using many subcarriers, each of a different frequency (Frequency Division Multiplexing), which are orthogonal to each other, hence Orthogonal Frequency Division Multiplexing.

Figure 2.1(a) shows the construction of such an OFDM signal (real part only) with 5 subcarriers. Note that each subcarrier has an integer number of cycles per symbol, making them cyclic. Adding a copy of the symbol to the end would result in a smooth join between symbols (Figure 2.1c). These subcarriers have sinc (sinx/x) response in frequency domain (Figure 2.1b). This is a result of the symbol time corresponding to the inverse of the subcarrier spacing, Δf = 1/T where T is OFDM symbol period. The sinc shape has a narrow main lobe, with many side-lobes that decay slowly with the magnitude of the frequency difference away from the centre. Each subcarrier has a peak at the centre frequency and nulls evenly spaced with a frequency gap equal to the subcarrier spacing. The orthogonal nature of the transmission is a result of the peak of each subcarrier corresponding to the nulls of all other subcarriers. Therefore, there is no intercarrier interference (ICI). Figure 2.1(d) shows the overall combined frequency response of OFDM. Since, the entire channel bandwidth is divided into many closely spaced sub-bands (or subcarriers); the frequency response over each subcarrier becomes relatively flat, making equalization potentially simpler than single-carrier system.

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Figure 2.1: Construction of OFDM with 5 subcarrier (a) Time Domain Representation (b) Frequency Domain Representation (c) Overall sum of subcarriers (Time Domain) (d) Overall combined frequency response of

subcarriers

2.2: Peak to Average Power Ratio (PAPR)

Presence of large number of independently modulated sub-carriers in an OFDM system the peak value of the system can be very high as compared to the average of the whole system. This ratio of the peak to average power value is termed as Peak-to-Average Power Ratio. Coherent addition of N signals of same phase produces a peak which is N times the average signal.

Let the data block of length N is represented by a vector . Duration of

any symbol in the set X is T and represents one of the sub – carriers set. As the N sub – carriers chosen to transmit the signal are orthogonal to each other, so we can have , where and NT is the duration of the OFDM data block X. The complex data block for the OFDM signal to be transmitted is given by [23]

, Where ………………… (2.1)

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The PAPR of the transmitted signal is defined as

………………………. (2.2)

Reducing the max |x(t)| is the principle goal of PARP reduction techniques. Since, discrete- time signals are dealt with in most systems, many PAPR techniques are implemented to deal with amplitudes of various samples of x(t). Due to symbol spaced output in the first equation we find some of the peaks missing which can be compensated by oversampling the equation by some factor to give the true PAPR value.

The most severe disadvantage of using several subcarriers in parallel using IFFT is the highly non-constant envelope of the transmit signal, making OFDM very sensitive to nonlinear components in the transmission path. A key component is the high power amplifier (HPA). Due to cost, design and most importantly power efficiency considerations, the HPA cannot resolve the dynamics of the transmit signal and inevitably cuts off the signal at some point causing additional in-band distortions and adjacent channel interference. The power efficiency penalty is certainly the major obstacle to implement OFDM in low-cost applications.

2.3: Different PAPR Reduction Techniques

There have been many new approaches developed during the last few years. Several PAPR reduction techniques have been proposed in the literature. These techniques are divided into two groups. These are signal scrambling techniques and signal distortion techniques [24].

The signal scrambling techniques are: Block coding Selective Level Mapping (SLM) Partial Transmit Sequences (PTS)

Signal scrambling techniques work with side information which minimized the effective throughput since they commence redundancy. Signal distortion techniques introduce band interference and system complexity also. Signal distortion techniques minimize high peak dramatically by distorting signal before amplification.

The signal distortion techniques are: Clipping Peak windowing Peak cancellation Peak power suppression Weighted multicarrier transmission

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Table 2.1: Signal Scrambling Techniques

Sl Technique Main Feature Technology Advantage Disadvantage

1 Block Coding

Techniques

This Block coding technique has been proposed by Wilkinson and Jones in 1965 for the minimization of the peak to mean envelope power ratio of multicarrier communication system [8].

Can be applied for signal scrambling, M sequences, Golay complementary sequences, Shapiro-Rudin sequences codes can be used to reduce the PAPR efficiently.

The block coding techniques have three stages for the development. The first stage works with the collection of appropriate sets of code words for any number of carriers, any M-ary phase modulation method, and any coding rate. The second stage works with the collection of the sets of code words which enable proficient implementation of the encoding/decoding. The third stage offers error deduction and correction potential.

1. This technique is simple and accurate for short codes because it needs extreme computation.

2. Natural algorithms are mainly sophisticated searching techniques.

3. It can be used for the collection of longer code words.

1. The receiver needs to know the phase shifts to decode correctly.

2. The set of phase shifts needs to be found and can become computationally intensive.

2 Block Coding Scheme

with Error Correction

It has been proposed by Ahn and et.al in [9] to introduce a new block coding proposal for minimization of peak to average power ratio (PAPR) of an Orthogonal Frequency Division Multiplexing (OFDM) system

The key object

A k bit data block (e.g. 4-bit data) is encoded by a (n, k) block code with a generator matrix ‘G’ in the transmitter of the system. Followed by the phase rotator vector b to produce the encoded output x=a.G+b(mod 2).

1. Block coding has error correction capability.

2. In block coding method, the OFDM symbol can be reduced by selecting only those code words with lower PAPR

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of the method is proposed that properly designed block codes can not only minimize the PAPR, but also give error correction capability

3. This method can improve the overall system performance and provides error correction capability.

3 Selected Mapping (SLM)

Selective Mapping (SLM) approaches have been proposed by Bauml in 1965 [10].

This method is used for minimization of peak to average transmit power of multicarrier transmission system with selected mapping.

A complete set of candidate signal is generated signifying the same information in selected mapping, and then concerning the most favorable signal is selected as consider to PAPR and transmitted. In the SLM, the input data structure is multiplied by random series and resultant series with the lowest PAPR is chosen for transmission. To allow the receiver to recover the original data to the multiplying sequence can be sent as ‘side information’.

1. One of the preliminary probabilistic methods is SLM method for reducing the PAPR problem.

2. It doesn’t eliminate the peaks, and can handle any number of subcarriers.

The drawback of this method is the overhead of side information that requires to be transmitted to the receiver of the system in order to recover information.

4 Partial Transmit Sequence

(PTS)

Partial Transmit Sequence (PTS) technique has been proposed by Muller and Hubber in 1997 [11].

This proposed method is based on the phase shifting of sub-

The main purpose behind this method is that the input data frame is divided into non-overlapping sub blocks and each sub block is phase shifted by a constant factor to reduce PAPR.

This method is flexible and effective for OFDM system.

PTS method works better than SLM method

There is no

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blocks of data and multiplication of data structure by random vectors.

need to send any side information to the receiver of the system, when differential modulation is applied in all sub blocks.

5 Interleaving Technique

Interleaving technique has been proposed by Jayalath and Tellambura [12], for reduction peak to average power ratio of an OFDM transmission.

The notion that highly correlated data structures have large PAPR can be reduced, if long correlation pattern is broken down.

The basic idea in adaptive interleaving is to set up an initial terminating threshold. PAPR value goes below the threshold rather than seeking each interleaved sequences. The minimal threshold will compel the adaptive interleaving (AL) to look for all the interleaved sequences.

The main important of the scheme is that it is less complex than the PTS technique but obtains comparable result.

1. This method does not give the assurance result for PAPR reduction.

2. In this circumstance, higher order error correction method has to use in addition to this method.

6 Tone Reservation

(TR)

Tone Reservation (TR) method is proposed for PAPR reduction [13].

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

This method explains an additive scheme for minimizing PAPR in the multicarrier communication system.

It shows that reserving a small fraction of tones leads to large minimization in PAPR ever using with simple algorithm at the transmitter of the system without any

1. It is less complex, no side information and also no additional operation is required at the receiver of the system.

2. Tone reservation method is based on adding a data block and

Tone reservation method is based on adding a data block and time domain signal. A data block is dependent time domain signal to the original multicarrier signal to minimize the high peak.

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solved accurately. The amount of PAPR reduction depends on some factors such as number of reserved tones, location of the reserved tones, amount of complexity and allowed power on reserved tones.

additional complexity at the receiver end. Here, N is the small number of tones, reserving tones for PAPR reduction may present a non–negligible fraction of the available bandwidth and resulting in a reduction in data rate.

time domain signal. A data block is dependent time domain signal to the original multicarrier signal to minimize the high peak.

3. This time domain signal can be calculated simply at the transmitter of system and stripped off at the receiver.

7 Tone Injection

(TI)

Tone Injection (TI) method has been recommended by Muller, S.H., and Huber, J.B.[14].

This technique is based on general additive method for PAR reduction. Using an additive method achieves PAPR reduction of multicarrier signal without any data rate loss.

Tone injection (TI) uses a set of equivalent constellation points for an original constellation points to reduce PAPR.

The main idea behind this method is to increase the constellation size. Then, each point in the original basic constellation can be mapped into several equivalent points in the extended constellation, since all information elements can be mapped into several equivalent constellation points. These additional amounts of freedom can be utilized for PAPR reduction.

In this method replacing the points in the basic constellation for the new points in the larger constellation which corresponds to injecting a tone of the proper phase and frequency in the multi-carrier symbol.

The drawbacks of this method are; need to side information for decoding signal at the receiver side, and cause extra IFFT operation which is more complex.

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Table 2.2: Signal Distortion Techniques

Sl Technique Main Feature Technology Advantage Disadvantage

1 Peak Windowing

The peak windowing method has been suggested by Van Nee and Wild [15]. This method, proposes that it is possible to remove large peaks at the cost of a slight amount of self-interference when large peaks arise infrequently. Peak windowing reduces PAPRs at the cost of increasing the BER and out-of-band radiation.

In peak windowing method we multiply large signal peak with a specific window, for example; Gaussian shaped window, cosine, Kaiser and Hanning window. In view of the fact that the OFDM signal is multiplied with several of these windows, consequential spectrum is a convolution of the original OFDM spectrum with the spectrum of the applied window.

The technique of peak windowing offers better PAPR reduction with better spectral properties.

Windowing method, PAPRs can be obtained to 4 dB which from the number of independent subcarriers.

The loss in signal-to-noise ratio (SNR) due to the signal distortion is limited to about 0.3dB.

A back off relative to maximum output power of about 5.5dB is needed in spectra distortion at least 30 dB below the in-band spectral density.

The window should be as narrow band as possible, conversely the window should not be too long in the time domain because various signal samples are affected, which results an increase in bit error rate (BER).

2 Envelope Scaling

The Envelope Scaling technique has been proposed by Foomooljareon and Fernando in [16].

They proposed a new algorithm to reduce PAPR by scaling the input envelope for some subcarriers before they are sent to IFFT.

The key idea of this scheme is that the input envelope in some sub carrier is scaled to achieve the smallest amount of PAPR at the output of the IFFT. Thus, the receiver of the system doesn’t need any side information for decoding the receiver sequence.

This scheme is appropriate for QPSK modulation; the envelopes of all subcarriers are equal. Results show that PAPR can be reduced significantly at around 4 dB

The system of single scaling factor and number of clusters equal to number of sub carriers is recommended.

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3 Peak Reduction

Carrier

Peak Reduction Carrier has been proposed by Tan and Wassell to use of the data bearing peak reduction carriers (PRCs) to reduce the effective PAPR in the OFDM system [17]. This scheme includes the use of a higher order modulation scheme to represent a lower order modulation symbol. This permits the amplitude and phase of the PRC to be positioned within the constellation region symbolizing the data symbol to be transmitted.

For example, to use a PRC that employs a 16-PSK constellation to carry QPSK data symbol, the 16-phases of the 16-PSK constellations are divided into four regions to represent the four different values of the QPSK symbol.

This scheme is appropriate for PSK modulation; where the envelopes of all subcarriers are equal. When the QAM modulation scheme will be implemented in the OFDM system, the carrier envelope scaling will result in the serious BER degradation.

To limit the bit error rate (BER) degradation, amount of the side information would also be excessive when the number of subcarriers is large.

4 Clipping and Filtering

One of the simple and effective PAPR reduction techniques is clipping, which cancels the signal components that exceed some unchanging amplitude called clip level. However, clipping yields distortion power, which called clipping noise, and expands the

Then clipping operation is performed to cut high peak amplitudes and frequency domain filtering is used to reduce the out of band signal, but caused peak re-growth [20]. This consists of two FFT operations. Forward FFT transforms the clipped signal back to discrete

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

Clipping is nonlinear process and causes in-band noise distortion, which causes degradation in the performance of bit error rate (BER) and out-of-band noise, which decreases the spectral efficiency [19].

The iterative signal takes long time and it will

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transmitted signal spectrum, which causes interfering [18] [20].

frequency domain. The in-band discrete components are passed unchanged to inputs of second IFFT while out of band components are null. The clipping and filtering process is performed iteratively until the amplitude is set to the threshold value level to avoid the peak out-of band and peak re-growth

increase the computational complexity of an OFDM transmitter [18].

2.4: Comparative Analysis of Different Techniques

There are several techniques has been proposed in different literature. Thus, it is possible to reduce the large PAPR by using the different techniques. Note that the PAPR reduction technique should be chosen with awareness according to various system requirements [24].

Table 2.3: Comparison of PAPR Reduction Techniques

Name of SchemesName of parameters

Distortion less Power increases

Data rate loss

Clipping and Filtering No No NoCoding Yes No YesPartial Transmit Sequence(PTS)

Yes No Yes

Selective Mapping (SLM) Yes No YesInterleaving Yes No YesTone Reservation (TR) Yes Yes YesTone Injection(TI) Yes Yes No

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There are many issues to be considered before using the PAPR reduction techniques in a digital communication system. These issues include PAPR reduction capacity, power increase in transmit signal, BER increase at the receiver, loss in data rate, computational complexity increase and so on. Simultaneously most of the techniques are not proficient to obtain a large reduction in PAPR with low coding overhead, with low complexity, without performance degradation and without transmitter and receiver symbol handshake.

2.5: Characteristic Equations of Some PAPR Reduction Techniques

Amplitude Clipping and Filtering (Distortion Technique)

Amplitude clipping is considered as the simplest technique which may be under taken for PAPR reduction in an OFDM system. A threshold value of the amplitude is set in this case to limit the peak envelope of the input signal. Signal having values higher than this pre-determined value are clipped and the rest are allowed to pass through un-disturbed [27].

…………… (2.3)

Where,

B(x) = the amplitude value after clipping. x = the initial signal value. A = the threshold set by the user for clipping the signal.

The problem in this case is that due to amplitude clipping distortion is observed in the system which can be viewed as another source of noise. This distortion falls in both in – band and out – of – band. Filtering cannot be implemented to reduce the in – band distortion and an error performance degradation is observed here. On the other hand spectral efficiency is hampered by out – of – band radiation. Out – of – band radiation can be reduced by filtering after clipping but this may result in some peak re – growth. A repeated filtering and clipping operation can be implemented to solve this problem. The desired amplitude level is only achieved after several iteration of this process.

Selected Mapping (Scrambling Technique)

The main objective of this technique is to generate a set of data blocks at the transmitter end which represent the original information and then to choose the most favorable block among them for transmission. Let us consider an OFDM system with N orthogonal sub – carriers. A data block is a vector composed of N complex symbols , each of them representing modulation symbol transmitted over a sub – carrier. X is multiplied element by element with U

vector composed of N complex numbers , , defined so that

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, where |.| denotes the modulus operator. Each resulting vector , where

, produces after IDFT, a corresponding OFDM signal given by

, ………………. (2.4)

Where, T is the OFDM signal duration and is the sub – carrier spacing [28, 29].

Among the modified data blocks, the one with the lowest PAPR is selected for transmission. The amount of PAPR reduction for SLM depends on the number of phase sequences U and the design of the phase sequences.

Partial Transmit Sequence (Scrambling Technique)

In the PTS technique, input data block X is partitioned in M disjoint sub – blocks,

, m=1,2, …, M, such that and the sub – blocks

are combined to minimize the PAPR in the time domain. The L times oversampled time domain signal of , , is obtained by taking the IDFT of length NL on concatenated with (L–1)N zeros. These are called the partial transmit sequences. Complex phase

factors, , are introduced to combine the PTSs. The set of phase factors is

denoted a vector .

The time domain signal after combining is given by

…………………..……… (2.5)

where . The objective is to find the set of phase factors that

minimizes the PAPR. Minimization of PAPR is related to the minimization of

.

Tone Reservation (Scrambling Techniques)

Proposed by Tellado and Cioffi (1998), the PAPR is reduced by adding time domain signals to the data sequences that lay in disjoint frequency space to the multicarrier data symbols. With this formation, optimizing the time domain signal leads to a convex optimization problem that can be transformed into a linear program (LP). Solving the LP exactly leads to PAPR reduction of 6-10dB, but a simple gradient algorithm could achieve most of this reduction after a few iterations. These additive signals can be easily removed from the received signal without transmitter receiver symbol handshake.

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If we do not transmit data on all tone, i.e. some values of data vector are zero; we can use these values for reducing PAPR. If data vector Xj=0 for j {j1,… jL}, then the transmitter can add any vector c that satisfies, cj =0 for j {j1… jL} to the data vector and remove it at the receiver.

IDFT (X+C) = Q(X+C) =x+QC =x+c where Q denotes IDFT matrix.

………………………….….. (2.6)

To minimize the PAPR of (x+c), we must compute the vector c* that minimizes the maximum peak value.

………………………… (2.7)

Using gradient algorithm iteratively, PAPR can be reduced. This algorithm has complexity, O(N) and can be applied to any number of carriers. But, there is little increases in transmit power and little data rate loss.

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CHAPTER- 3

Results and Discussion

3.1: Windowing Technique for PAPR reduction

The basic idea of peak windowing is to multiply the envelop of OFDM signal with a window function [30]. Therefore,

………………… (3.1)

Where,

w(t): is the window function.

: denotes the position of a local maximum of the envelop, . : attenuation constant.

When the amplitude of envelop amplitude of the OFDM signal exceeds a threshold, a window function is applied to the envelop of the OFDM signal to eliminate the peak amplitude. Windowing results in a smooth signal. The peak window is given by:

w (t) = 0.5 − 0.5 cos(2πt/T) 0≤ t ≤ T ………………(3.2)

Interleaving

In this approach k interleavers are used at the transmitter. These interleavers produce K permuted frames of the input data sequence. These permutations can be done either before or after the modulation (mapping). The minimum PAPR frame of all the K frames is selected for transmission. The identity of the corresponding interleaver is also sent to the receiver as side information.

We have carried out simulation study based on the technique proposed by Van Nee and Wild [15] as shown in Figure 3.1.

First, the interleaving approach is used and the signal with lowest PAPR is then passed through windowing method. The intention to combine these two methods is to obtain signal with lower PAPR than in the case of interleaving method and with lower bit error rate in the case of windowing method.

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Figure 3.1: OFDM system with proposed technique for PAPR reduction

Computer simulations can be used to clarify the peak power reduction capability and the BER performance with the proposed technique. This simulated system employs an OFDM signal with N = 256, 512, and 1024 subcarriers using QPSK, 16 QAM, and 64 QAM. The high power amplifier (HPA) is Rapp’s solid state power amplifier model (SSPA) with the characteristic

………… (3.3)

Where vin, vout are respectively the complex input and complex output signals and vsat is the output saturation level. The parameter p, often called “knee factor”, controls the smoothness of the characteristic.

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3.2: Simulation Results

The BER performance is evaluated for Additive White Gaussian Noise (AWGN) channels. In the peak windowing method, a window is applied to a region where exist large peaks. The window function is multiplied with the signal in such a way that the signal peaks fall in the center of the window while the signal samples with lower amplitudes align themselves with the large amplitude segment of the window function. In this paper, the hanning window will be used as a window function.

Here from Figure: 3.1 to 3.36, we have given simulation graphs of our work output.

From Figure: 3.1 to 3.12, we have put graphs for the constellation points of 16 (m=16) where Figure: 3.1 to 3.3 for fft size of 4, Figure: 3.4 to 3.6 for fft size of 16, Figure: 3.7 to 3.9 for fft size of 32 and Figure: 3.10 to 3.12 for fft size of 64. For all fft size we have taken three different combinations with a cyclic prefix coefficient of value Gi=0.25, 0.5 and 0.75 whose are the first, second and third values of the mention Figures respectively.

From Figure: 3.13 to 3.24, we have put graphs for the constellation points of 32 (m=32) where Figure: 3.13 to 3.15 for fft size of 4, Figure: 3.16 to 3.18 for fft size of 16, Figure: 3.19 to 3.21 for fft size of 32 and Figure: 3.22 to 3.24 for fft size of 64. For all fft size we have taken three different combinations with a cyclic prefix coefficient of value Gi=0.25, 0.5 and 0.75 whose are the first, second and third values of the mention Figures respectively.

From Figure: 3.25 to 3.36, we have put graphs for the constellation points of 64 (m=64) where Figure: 3.25 to 3.27 for fft size of 4, Figure: 3.28 to 3.30 for fft size of 16, Figure: 3.31 to 3.33 for fft size of 32 and Figure: 3.34 to 3.36 for fft size of 64. For all fft size we have taken three different combinations with a cyclic prefix coefficient of value Gi=0.25, 0.5 and 0.75 whose are the first, second and third values of the mention Figures respectively.

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