[1]

CHAPTER 1 MULTICARRIER COMMUNICATION

SYSTEMS

uring last two decades, the demand for multimedia wireless communication services has

grown tremendously and this trend are expected to continue in the near future. This high

demand of multimedia wireless communication services force the development of advance digital

wireless communication system which can provide high data rate transmission at low cost to as many as

users with high reliability, large bandwidth, and with great flexibility for varying traffic condition [102

and 113]. Today’s wireless communication system provides services like high quality music, video,

Internet and high quality games on mobile etc. Music, video, and games on demand while on move are

the trend of present generation and youngsters. Nowadays, people generally accessed their E-mails,

Facebook and Twitter account on Mobile. The target of next generation communication systems is to

create a “global information village”, which consists of various components at different scales ranging

from global to picocellular size. To support these types of services for large number of users and with

limited spectrum, current wireless communication system adopted technologies which are bandwidth

efficient and robust to multipath channel condition (i.e., multi-carrier communication). Orthogonal

frequency division multiplexing (OFDM) is one of such multi-carrier techniques which have attracted

vast research attention from academician, researchers and industries since last two decades. It has

become part of new emerging standards for broadband wireless access [11 and 69]. This chapter

contains an introduction to current scenario of wireless communication system and also includes an

insight about the evolution of different generation of wireless communication (form 1G to 4G). Besides

this, historical development of OFDM system, principle of OFDM, orthogonality condition is also

discussed in this chapter.

1.1 EVOLUTION OF MOBILE COMMUNICATION SYSTEMS

The wireless (mobile) communication system took time to evolve to where it is today, changed

drastically since Guglielmo Marconi first demonstrated radio’s ability to provide continuous contact

with ships sailing the English channel. Historically, growth in the mobile communications field has

come slowly, and has been coupled closely to technological improvements. The ability to provide

wireless communications to an entire population was not even conceived until Bell Laboratories

developed the cellular concept in the 1960s and 1970s.With the development of highly reliable,

miniature, solid-state radio frequency hardware in the 1970s, the wireless communication systems era

D

[2]

was born. The recent exponential growth in cellular radio and personal communication systems

throughout the world is directly attributable to the new technologies of the 1970s, which are mature

today [127, 131]. The development in the field of digital signal processing (DSP) also plays an

important role in the progress of modern digital communication system.

The history of mobile communication can be categorized into three periods (a) the pioneer era,

(b) the pre-cellular era, and (c) the cellular era [38, 45, and 131]. The period of 1800 and early 1900 is

comes under pioneer era. In this period, the fundamental research in the field of wireless communication

took place. The postulates of electromagnetic (EM) waves by James Clark Maxwell during the 1860s in

England, the demonstration of the existence of these waves by Heinrich Rudolf Hertz in 1880s in

Germany, and the invention and first demonstration of wireless telegraphy by Guglielmo Marconi

during the 1890s in Italy are the some examples of this pioneer era.

On the basis of these fundamental researches, many applications of wireless telegraphy started

from the year 1920s.The pre-cellular era began with the first land-based mobile wireless telephone

system installed in 1921 by the Detroit Police department to dispatch patrol cars, followed in 1932 by

the New York City Police [130]. The end of the Second World War saw the expansion of mobile

telephone services to the commercial arena. In 1946, the first commercial mobile telephone system was

set up by Bell Telephone Laboratories in St. Louis [38]. This system used three channels at 150 MHz

band. These channels were based on frequency modulation (FM) and utilized wide area architecture.

Initially a manually operated telephone exchange was installed; and in 1948 first fully automatic mobile

telephone system was put into operation in Richmond, Indiana.

Later on, many other wireless systems were also developed during this era, collectively called as

Mobile radio telephone. Since they were the predecessors of the first generation of cellular telephones,

these systems are sometimes retroactively referred to as Zero Generation (0G) systems. Few popular

systems of 0G are Push to Talk (PTT) or manual, Mobile Telephone System (MTS) in 1946, Improved

Mobile Telephone Service (IMTS) in 1969, and Advanced Mobile Telephone System (AMTS) [131].

The primary users of these systems were loggers, construction foremen, realtors, and celebrities. These

mobile telephones were usually mounted in cars or trucks.

However, because of their large coverage area, these systems could not manage a large number

of users. The cellular concept was basically developed to overcome this problem so that large number of

users can accommodate with better bandwidth efficiency. The essential features of the cellular

architecture are low power transmitter with smaller coverage zone, frequency reutilization (Frequency

[3]

Reuse), cell splitting to increase capacity and hand-off control [131]. This use of cellular concept

launched the third era, known as the cellular era. This cellular era is continued till date and updated

accordingly depending upon the demands of different types of services.

In the cellular era, many mobile radio standards have been developed for wireless systems

throughout the world, and more standards are likely to emerge. These systems are different to each other

in terms of services provided by them (voice, video and data), technology used (digital or analog),

period of existence, and many other communication parameters like multiple access technique, channel

bandwidth, frequency band, and modulation technique. So collectively, these wireless communication

standards or systems are classified on the basis of generation wise development [127]. They are called

First Generation (1G), Second Generation (2G), Third Generation (3G) and Fourth Generation (4G).

1.1.1 First Generation (1G) Mobile Standards

These are the analog cell phone standards that were designed in 1970s, deployed in early 1980s

and continued until being replaced by 2G digital cell phones. First generation system provided only

voice and other telephone services and used frequency division multiple access (FDMA) techniques for

resources sharing. Several analog cellular radio systems have been developed in Europe and Japan in

parallel with the Advanced Mobile Phone System (AMPS) in the US. The world’s first commercial

cellular system was implemented by the Nippon Telephone and Telegraph Company (NTT) in Japan.

The system, deployed in 1979, uses 600 FM duplex channels in the 800 MHz band [127, 131]. Several

other analog standards were also developed in 1980, shown in Table-1.1. The first generation systems

were incompatible with one another because of the different frequencies and communication protocol

used. This incompatibility between various systems of analog 1G standards precluded roaming; users

had to change their mobile terminals when they moved to another country. Another problem of all 1G

systems is the capacity; the limited number of users can be supported by these standards because of the

use of FDMA. To overcome these problems of roaming and capacity, a new generation of Mobile

standards was evolve during 1900 called as second generation of Mobile standards.

1.1.2 Second Generation (2G) Mobile Standards

The technologies which are using by most of the today’s ubiquitous cellular networks are come

under the 2G-(Second Generation) cellular standards. Unlike 1G cellular system that relied exclusively

[4]

on FDMA and analog FM, 2G standards use digital modulation formats with Time Division Multiple

Access (TDMA) and Code Division Multiple Access (CDMA) techniques. 2G services are frequently

referred as Personal Communications Service (PCS) in the United States. The most popular 2G

standards include three TDMA standards (GSM, IS-136, and PDC) and one CDMA standard (IS-95)

[127 and 131]. The Global System for Mobile (GSM) is most successful and popular 2G standard based

on TDMA technique and was first deployed in Germany in 1981. It supports eight time slotted users for

each 200 KHz radio channel and has been deployed widely in Europe, Asia, Australia, South America

and some parts of the US.

The Interim standard 136 (IS-136) is another 2G standard based on TDMA. It supports three

time slotted users for each 30 KHz radio channel and is widely used in North America, South America,

and Australia. The third 2G standard which is based on TDMA is Pacific Digital Cellular (PDC). It is a

Japanese TDMA standard that is similar to IS-136 with more than 50 million. The Interim standard 95

(IS-95) is another popular 2G standard which is based on CDMA. It is also known as cdmaOne, which

supports up to 64 users that are orthogonally coded and simultaneously transmitted on each 1.25 MHz

channel. CDMA is widely deployed by carriers in North America, as well as in Korea, Japan, China,

South America, and Australia. The summary of 1G and 2G mobile standards are given in Table-1.1.

[127]

Table - 1.1: The most common standards of 1G and 2G used in different parts of world.

Standard Type Region Year of Introduction

Multiple Access

Frequency Band

(MHz) Modulation

Channel Bandwidth

(KHz)

AMPS 1G US 1983 FDMA 824-894 FM 30

NMT-900 1G Europe 1986 FDMA 890-960 FM 12.5

NTT 1G Japan 1979 FDMA 400-800 FM 25

GSM 2G Europe 1990 TDMA 890-960 GMSK 200

DCS-1900 (GSM) 2G US 1994 TDMA 1850- 1910 GMSK 200

IS-95 2G US 1993 CDMA 824-894, 1800-2000

QPSK/ BPSK 1.25

PDC 2G Japan 1993 TDMA 810-1501 π/4 DQPSK 25

[5]

1.1.3 Enhance Second Generation (2.5G) Mobile Standards

The 2G technologies use circuit-switched data modems that limit data users to a single circuit-

switched voice channel. Due to this circuit switching, the data rates available with 2G networks are very

low. All 2G networks can support single user data rates on the order of 10 Kbps. With this low data rate,

2G standards are able to support limited Internet browsing and sophisticated short messaging

capabilities. Short Messaging Service (SMS) is a popular feature of GSM.

In an effort to retrofit the 2G standards for compatibility with increased throughput data rates

that are required to support modern Internet applications, new data-centric standards have been

developed that can be overlaid upon existing 2G technologies. These new standards represent 2.5G

technology which allow existing 2G equipment to be modified and supplemented with new base station

add-ons and subscriber unit software upgrades to support higher data rate transmissions for Internet

browsing, e-mail accessing, mobile commerce, and location based mobile services [102]. The 2.5G

technologies also support a popular new web browsing format language, called Wireless Application

Protocol (WAP), which allows standard web pages to be viewed in a compressed format. The three

popular 2.5G standards which are based on TDMA includes: (a) High Speed Circuit Switched Data

(HSCSD) for 2.5G GSM (b) General Packet Radio Service (GPRS) for 2.5G GSM and IS-136 (c)

Enhanced Data Rates for GSM Evolution (EDGE). For CDMA based 2G standard (IS-95), only one

interim data solution is available called as IS-95B.

HSCSD is a circuit switched technique that allows a single mobile user to use consecutive user

time slots in the GSM standard. HSCSD relaxes the error control coding algorithms of original GSM

and increase the available application data rate to 14.4 Kbps, as compared to the original 9.6 Kbps in the

GSM specification. GPRS is a packet based data network, which is well suited for non-real time Internet

usage, (like e-mail access) where the user downloads much more data than it uploads on the Internet.

Unlike HSCSD, which dedicates circuit switched channels to specific users, GPRS supports multi-user

network sharing of individual radio channels and time slots [127]. Thus GPRS can support many more

users than HSCSD. When all eight time slots of a GSM radio channel are dedicated to GPRS, an

individual user is able to achieve as much as 171.2 Kbps.

EDGE is a more advanced upgrade to the GSM standard which requires the addition of new

hardware and software at existing base stations. EDGE introduces a new digital modulation format, 8-

PSK, which is used in addition to GSM’s standards GMSK modulation. EDGE allows for nine different

[6]

air interface formats, known as multiple modulation and coding schemes, with varying degrees of error

control protection. When EDGE uses 8-PSK modulation without any error protection, and all eight

times slots of a GSM radio channel are dedicated to a single user, a raw peak throughput data rate of

547.2 Kbps can be provided. The IS-95B supports medium data rate service by allowing a dedicated

user to command up to eight different users Walsh codes simultaneously and in parallel for an

instantaneous throughput of 115.2 Kbps per user.

1.1.4 Third Generation (3G) Mobile Standards

The 3G mobile standards are the successor of 2G and enhance 2G (2.5G) mobile standards.

Although, the 2G mobile standards can accommodate as much user with lot of data services and

international roaming, but it still unable to provide high speed Internet and multimedia services on

mobile. An interim solution in terms of 2.5G standards have been developed to provide such services

with high data rate. However, all these solution were not able to provide services like high quality

music, video, games, high speed web browsing, e-commerce, mobile banking, e-reservation etc. on

mobile with high data rate.

This demand of high data rate mobile communication initiated the research activity in 1995 with

an outcome of new mobile standards called as Third Generation (3G) mobile standards [131]. It is based

on the International Telecommunication Union (ITU) family of standards under the International Mobile

Telecommunications programme, "IMT-2000". 3G technologies enable network operators to offer the

users a wider range of more advanced services while achieving greater network capacity through

improved spectral efficiency.

The ITU IMT-2000 standards organizations worked under two separate 3G camps: 3G

Partnership Project for Wideband CDMA (3GPP) standards based on backward compatibility with GSM

and IS-136/PDC) and 3G Partnership Project for cdma2000 (3GPP2) standards based on backward

compatibility with IS-95 [131]. IMT-2000 aims to realize 144 Kbps, 384 Kbps, and 2 Mbps under high-

mobility, low-mobility, and stationary environments, respectively. In IMT-2000, on the basis of CDMA,

three radio-access schemes have been standardized: (a) Direct-Sequence CDMA (DS-CDMA)-

frequency division duplex (FDD) (b) Multi-Carrier CDMA (MC-CDMA) - FDD, and (c) DS-CDMA-

time division duplex (TDD).

[7]

The first commercial 3G network was launched by NTT DoCoMo in Japan in October 2001 and

branded as FOMA (Freedom of Mobile Multimedia Access). In Europe, the first commercial network

based on Wideband CDMA (WCDMA) was opened for business by Telenor in December 2001 with no

commercial handsets and thus no paying customers. Two major 3G standards of IMT-2000 are UMTS

(WCDMA) and CDMA2000. The summary of both 3G standards are given in Table 1.2.

Table - 1.2: Different parameters of 3G standards

Parameters WCDMA (UMTS) CDMA2000

Frequency Band 2 GHz Band 2 GHz Band

Bandwidth 1.25/5/10/20- MHz (DS-CDMA) 1.25/5/10/20- MHz (DS-CDMA)

3.75/5- MHz (MC-CDMA)

Chip Rate 3.84 Mcps (DS-CDMA-FDD,

DS-CDMA-TDD)

3.84 Mcps (DS-CDMA-FDD)

3.6864 Mcps (MC-CDMA-TDD)

Data Rate

144 Kbps (High Mobility)

384 Kbps (Low Mobility)

2 Mbps (Stationary)

144 Kbps (High Mobility)

384 Kbps (Low Mobility)

2 Mbps (Stationary)

Universal Mobile Telecommunications System (UMTS) is an air interface standard developed

under the flagship of European Telecommunications Standards Institute (ETSI) in 1998 [131]. UMTS,

or WCDMA, assures backward compatibility with the 2G GSM, IS-136, and PDC TDMA technologies,

as well as 2.5G TDMA technologies. WCDMA require a minimum spectrum allocation of 5 MHz. The

cdma2000 standard has been developed under the auspices of working group 45 of the

Telecommunications Industry Association (TIA) of the US and involves the participation of the 3GPP2

working group.

1.1.5 Fourth Generation (4G) Mobile Standards

Future technologies for wireless communication come under 4G standards. A 4G system will

provide an end-to-end IP solution where voice, data and streamed multimedia can be served to users on

[8]

an "Anytime, Anywhere" basis at higher data rates than previous generations. No formal definition is set

about 4G, but the objectives that are predicted for 4G can be summarized as follows [113]:

4G will be a fully IP-based integrated system of systems and network of networks.

4G will be achieved after the convergence of wired and wireless networks as well as computer,

consumer electronics, communication technology, and several other convergences.

It will be capable of providing 100 Mbps and 1 Gbps, respectively, in outdoor and indoor

environments with end-to-end quality of service and high security

It will provide any kind of services at anytime and at anywhere, at affordable cost and one

billing.

As opposed to earlier generations, 4G systems are fully depends on multi-carrier communication

and packet switching. The key parameters of 3G and 4G are compared in Table 1.3.

Table - 1.3: Comparison between 3G and 4G systems

Parameters 3G 4G

Frequency Band 1.8 – 2.5 GHz 2- 8 GHz

Bandwidth 5 to 20 MHz 5 to 20 MHz

Max data rate 2 Mb/s 20 Mb/s (or 100 Mb/s)

Multiple access WCDMA, TDMA MC-CDMA, OFDM

Switching Circuit/packet Packet

Coding Convolution Codes,

rates 1/2 & 1/3

Concatenated codes,

LDPC codes

In March 2008, the International Telecommunications Union-Radio communications sector

(ITU-R) specified a set of requirements for 4G standards called as International Mobile

telecommunications Advanced (IMT-A) specification [51]. In IMT-A, the speed requirements for 4G

service at 100Mbps for high mobility communication (such as from trains and cars) and at 1 Gbps for

low mobility communication (such as pedestrians and stationary users) are specified [51].

[9]

1.2 NEED OF MULTI-CARRIER COMMUNICATION SYSTEM

Various kind of multiple access techniques (FDMA for 1G, TDMA and CDMA for 2G, and

WCDMA for 3G) have been developed for mobile communication system for achieving demands of

high data rate, high capacity, and for the integration of different services in a cellular network. The main

limitation of FDMA and TDMA are the capacity (the number of users, which these can accommodate).

The CDMA techniques have been developed mainly for capacity reasons. CDMA techniques can

potentially accommodate more users than either TDMA or FDMA. WCDMA is a 3G wireless standard,

evolved from CDMA to support the multimedia wireless services at data rate as high as 2 Mbps [102].

DS-CDMA used in 2G systems and WCDMA used in 3G systems have a common problem of

frequency selective multipath fading. Frequency selective multipath fading is common in urban and

indoor environment and is significant source of performance degradation. The performance degrades

more if the number of users increases rapidly. The rapid fluctuation of the amplitude of a radio signal

over a short period of time or travel distance is known as fading which arises due to combination of

multipath waves at the receiver antenna to give a resultant signal, which can vary widely in amplitude

and phase, depending on the distribution of the intensity and relative propagation time of the waves and

the bandwidth of the transmitted signal [55, 127]. Depending on the relation between the signal

parameters (such as bandwidth, symbol period etc.) and the channel parameters (like rms delay spread,

coherence bandwidth, Doppler spread, and coherence time), the fading is classified as:

Based on multipath time delay spread

Flat Fading Frequency Selective Fading

BW of signal < BW of Channel

Delay Spread < Symbol Period

BW of signal > BW of Channel

Delay Spread > Symbol Period

Based on Doppler spread

Fast Fading Slow Fading

[10]

High Doppler spread

Coherence time < Symbol Period

Channel variations faster than

baseband signal variation

Low Doppler spread

Coherence time > Symbol Period

Channel variations slower than

baseband signal variations

If the signal bandwidth is larger than the channel bandwidth than the signal undergo frequency

selective fading otherwise flat fading. It is clear from the above discussion that the solution of frequency

selective multipath fading is to use narrowband signals because narrowband signals are less sensitive to

Inter Symbol Interference (ISI) and frequency selective multipath fading. This idea of using narrow

band signals evolves the concept of multi-carriers transmission. Hence, multi-carrier communication

techniques like MC-CDMA and OFDM have been proposed for current broadband wireless access like

IEEE 802.11, 16 and for 4G mobile communication systems [45].

In MC-CDMA, a block of serial data or chip (pseudo random code) is converted into a parallel

bit stream and each stream is then modulated by a sub-carrier, which is orthogonal to other sub-carriers.

By parallel conversion, the duration of bit is increased thus frequency spectrum is reduced. Due to this

parallel conversion, the signals which are transmitted through channel are narrow band signals which are

less sensitive to frequency selective fading. The orthogonal nature of sub-carriers allowed the

accommodation of large numbers of users efficiently because orthogonal signal do not interfere with

each other with in a time period.

1.3 ORIGIN AND HISTORICAL DEVELOPMENT OF OFDM

The initial development of multi-carrier communication system was basically done by military

systems in the late 1950s and mid 1960s, such as KINEPLEX [98] and KATHRYN [31, 82, and 90].

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 1960s were dedicated for

MCM employing overlapped band-limited orthogonal signals [18, 103, and 104]. In the year 1967, B.

R. Saltzberg suggested a MCM system employing Orthogonal time-staggered Quadrature Amplitude

Modulation (O-QAM) on the carriers [18]. The concept of MCM scheme employing time-limited

orthogonal signals, which is similar to OFDM, was first given by H. F. Marmuth [37] in 1960.

[11]

A U.S. patent in which the name of “OFDM” was first appeared was issued in January 1970

[89]. But it gained more popularity and attentions only after the use of the discrete Fourier transform

(DFT) to generate orthogonal sub-carriers at the transmitter as — suggested by Weinstein and Ebert

[106] in 1971. The use of IDFT/DFT totally eliminates bank of subcarrier oscillators at the

transmitter/receiver. The availability of fast DFT algorithm (i.e., FFT) significantly reduces the

implementation complexity of OFDM modems. The further development and advances in integrated

circuit technology have made the implementation of OFDM cost effective. Later on, many other works

in 1980s [10, 14, and 132] suggested the use of DFT based OFDM for high-speed modems and digital

mobile communication system. One of such systems was suggested by B. Hirosaki in the year 1981 for

Saltzberg’s O-QAM OFDM system [14]. A US patent was obtained in June 1980 for high speed modem

suggesting the use of OFDM [132]. A few years later in 1985, Cimini provided analytical and

simulation results on the performance of OFDM modems in mobile communications channels [70].

In the late 1980’s the work began on the development of OFDM for commercial use, with the

introduction of the Digital Audio Broadcasting (DAB) system. The DAB standard was in fact the first

OFDM-based standard (project started in 1988 by ETSI and completed in 1995). 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 (VDSL; 100 Mbps), DAB and High Definition Television (HDTV)

terrestrial broadcasting [44, 59, 62, and 93].

In 1990s, parallel to commercial development, the studies regarding the performance of OFDM

system in different environment or channels with different parameters were also made [29, 30, 52, 80,

and 112]. Casas et.al [29 and 30] gives the analytical and experimental results of OFDM system over

mobile radio channel. Later on, Hara et.al [112] reported the bit error rate performance of OFDM

system over Rayleigh frequency selective fading channel. The performance of OFDM system in

different channel conditions (flat fading, frequency selective fading, etc.) with frequency offset has been

analyzed by different authors [60, 61, and 91]. More recent advances in OFDM transmission are

presented in the impressive state-of-art collection of works edited by S Hara & R Prasad [113], Taha &

Salleh [39], and Taewon Hwang et.al in 2009 [120].

1.4 PRINCIPLES OF OFDM

[12]

The basic principle of OFDM is to split a 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 decreases the relative

amount of dispersion in time caused by multipath delay spread. OFDM can be seen as either a

modulation technique or a multiplexing technique [102].

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.

Figure-1.4.1: Comparison of FDM and OFDM [102]

In conventional broadcasting, 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. With an OFDM transmission such as DAB, the information signals from

multiple stations are combined into a single multiplexed stream of data. This data is then transmitted

using an OFDM symbol that is made up from a dense packing of many subcarriers. These subcarriers

are orthogonal to each other, thus required no band gap which improves the spectral efficiency. The

principle of orthogonality is discussed in next sub-section. The difference between FDM and OFDM is

shown in Figure-1.4.1.

[13]

1.4.1 Orthogonality Principle

Two functions or signals are said to be orthogonal if they are mutually independent of each

other. Orthogonality is a property that allows multiple information signals to be transmitted perfectly

over a common channel with the successful detection [16 and 21]. Many common multiplexing schemes

are inherently orthogonal. Time Division Multiplexing (TDM) allows transmission of multiple

information signals over a single channel by assigning unique time slots to each separate information

signal. During each time slot only the signal from a single source is transmitted preventing any

interference between the multiple information sources. Because of this, TDM is orthogonal in nature.

The principle of orthogonality state that if the time-averaged integral of product of any two

functions from a set of functions {푔 (푡),푔 (푡),푔 (푡), … ,푔 (푡)}, over a joint existence time interval [푡 ,

푡 + 푇] is equal to zero, irrespective of the limit of existence of the functions, then the functions are said

to be orthogonal to each other within this time-interval [16]. Mathematically, it can be expressed as –

1푇

푔 (푡) 푔 (푡)

푑푡 = 0 ∀ 푖 ≠ 푘

(1.4.1.1)

Similarly, if the set of functions are to be an ortho-normal set then they have to be orthogonal as

well as being normalized, i.e.,

1푇

푔 (푡) 푔 (푡)

푑푡 =0 ∀ 푖 ≠ 푘

1 ∀ 푖 = 푘

(1.4.1.2)

In the context of OFDM, the set of complex exponential function 푒 , defined over

the period [0, T], represents different sub-carriers at 푓 = , 푓표푟 푘 = 0,1,2⋯ ,푁 − 1. Using the

expression (1.4.1.2) for OFDM system –

1푇

푒 푒 푑푡 = 1푇

푒 푒

푑푡 = 1푇

푒 ( ) 푑푡

(1.4.1.3)

Further simplification gives –

[14]

1푇

푒 푒 푑푡 = 0 ∀ 푖 ≠ 푘

1 ∀ 푖 = 푘

(1.4.1.4)

Taking the discrete samples with the sampling instances at 푡 = 푛푇 = 푛푇/푁,푛 = 0,1,2, … ,푁 −

1, (1.4.1.4) can be written in the discrete time domain as –

1푁

푒 푒 = 1푁

푒 ( )=

0 ∀ 푖 ≠ 푘

1 ∀ 푖 = 푘

(1.4.1.5)

Figure-1.4.1.1: Frequency response of the sub-carriers in a 5 tone OFDM signal

The orthogonality property of OFDM signals can be shown with the help of its spectrum. In the

frequency domain each OFDM subcarrier has a 푠푖푛푐 = sin(푥) /푥 frequency response, as shown in

[15]

Figure-1.4.1.1. Five sub-carriers have been taken in this simulation exercise. It is clearly visible from the

plots that each carrier has a peak at the centre frequency and nulls evenly spaced with a frequency gap

equal to the carrier spacing. It is also observable from the plots that the peak of each sub-carrier

coincides with the nulls of all other sub-carriers. This verifies the orthogonal nature of the OFDM

signal.

1.5 APPLICATION OF OFDM SYSTEMS

During the past decade, OFDM has been adopted in many wireless communication standards,

including European digital audio broadcasting [33], terrestrial digital video broadcasting [34] and

satellite–terrestrial interactive multiservice infrastructure in China. In addition, OFDM has been

considered and approved by many IEEE standard working groups, such as IEEE 802.11a/g/n, IEEE

802.15.3a, and IEEE 802.16d/e [46 - 50]. The applications of OFDM include wireless personal area

networks, wireless local area networks, and wireless metropolitan networks. Currently, Orthogonal

Frequency Division Multiple Access (OFDMA) is being investigated as one of the most promising radio

transmission techniques for LTE of the 3rd Generation Partnership Project (3GPP), International

Mobile Telecommunications—Advanced Systems [51].

(A) Digital Audio Broadcasting (DAB) [33]

In 1995, the Digital Audio Broadcasting (DAB) was standardized by the ETSI as the first

standard which uses OFDM. DAB has three transmission modes using different sets of OFDM

parameters, which are listed in Table–1.4. One important reason to use OFDM for DAB is the

possibility to use a single frequency network, which greatly enhances the spectrum efficiency. The DAB

transmitted data consists of a number of audio signals each sampled at 48 KHz with an input resolution

up to 22 bits. The digital audio signal is compressed to a rate in the range of 32 to 384 Kbps, depending

on the desired quality. The signal is divided into frames of 24 ms.

Table - 1.4: Key parameters of DAB standard

Parameters Mode 1 Mode 2 Mode 3

Number of Sub-carriers (N) 1546 768 384

Modulation DQPSK

Useful Symbol length (푇 ) 1 ms 250 µs 125 µs

[16]

Sub-carrier Separation ∆푓 3968 KHz 1.984 KHz 0.992 KHz

Length of Guard Interval (푇 ) 푇 /4 (250 µs) 푇 /4 (62.5 µs) 푇 /4 (31.25 µs)

Forward Error Correction Code Convolution Code

Information Transmission Rate 2.4 Mbps

Bandwidth 1.536 MHz (B) Terrestrial Digital Video Broadcasting (DVB-T) [34]

In 1993, a pan-broadcasting-industry group started the Digital Video Broadcasting (DVB)

project. Within this project, a set of specifications was developed for the delivery of digital television

over satellites, cable, and through terrestrial transmitters. This section describes the terrestrial DVB

system, which was standardized in 1997. Terrestrial DVB uses OFDM with two possible modes, using

1705 and 6817 sub-carriers, respectively. These modes are referred to as 2K and 8K modes,

respectively, as these are the sizes of the FFT/IFFT needed to generate and demodulate all subcarriers.

All other parameters of DVB are listed in Table-1.5.

Table - 1.5: Key parameters of DVB-T standard

Parameters Mode 2K Mode 8K

Number of Sub-carriers (N) 1705 6817

Modulation QPSK 16 and 64 QAM

Useful Symbol length (푇 ) 224 µs 896 µs

Sub-carrier Separation ∆푓 4.464 KHz 1.116 KHz

Length of Guard

Interval (푇 )

푇 /4 (56 µs), 푇 /8 (28 µs),

푇 /16 (14 µs), 푇 /32 (7 µs)

푇 /4 (224 µs), 푇 /8 (112 µs),

푇 /16 (56 µs), 푇 /32 (28 µs)

Forward Error Correction Code

Inner Code – Convolution Code (R = 1/2, 2/3, 3/4 5/6, 7/8)

Outer Code – Reed-Solomon Code (204, 188)

[17]

Information Transmission Rate 4.98 – 31.67 Mbps

Bandwidth 7.61 MHz

[18]

(C) IEEE 802.11 and HIPERLAN/2 [32, 46, 48 and 49]

In July 1998, the IEEE 802.11 standardization group decided to select OFDM as the basis for

their new 5-GHz standard, targeting a range of data rates from 6 Mbps to 54 Mbps. This new standard

becomes the first one to use OFDM in packet-based communications, while the use of OFDM until now

was limited to continuous transmission systems like DAB and DVB. Following the IEEE 802.11, ETSI

BRAN (Broadband Radio Access Networks) also adopted OFDM for their physical layer standards.

IEEE802.11a has the same physical layer as HIPERLAN/2 with the main difference between the

standard corresponding to the higher-level network protocols used. Table-1.6 lists the main parameters

of the OFDM given by IEEE 802.11 standard.

Table - 1.6: Main parameters of IEEE 802.11 standards Parameters Specification

Data rate 6, 9, 12, 18, 24, 36, 48, and 54 Mbps

Modulation BPSK, QPSK, 16-QAM, 64-QAM

Coding rate 1/2, 2/3, and 3/4

Total number of sub-carriers (Pilot + Data) 4 + 48 = 52

Number of Pilot sub-carriers 4

OFDM Symbol Duration 4 µs

Guard Interval 800 ns

Sub-carrier spacing 312.5 KHz

Channel Spacing 20 MHz

(D) IEEE 802.16 for Mobile and Fixed Wireless Systems [47 and 50]

In the year 1999, IEEE standards board established a working group to develop standards for

broadband for Wireless Metropolitan Area Networks (WMAN). Although, the first document of IEEE

802.16 came in the year 2001, this project concluded in 2004 with the release of 802.16-2004 which

superseded the earlier 802.16 documents, including the “a” and “c” amendments. IEEE 802.16 defines

the Wireless MAN air interface specification for metropolitan area networks (MANs), which attempts to

replace “ the last mile wired access” with cable modem and Digital Subscriber Lines (DSL) by

Broadband Wireless Access (BWA).

[19]

In this standard, the OFDM and OFDMA has been adopted to support peak data rate up to 75

Mbps at the frequency bands under 11 GHz. In IEEE 802.16-2004 for OFDMA, the size of FFT is fixed

at 256 and the sub-channel space is varied according to the bandwidth of the system. Whereas, in IEEE

802.16e-2005, OFDMA maintains the same sub-channel space and varies the size of FFT according to

bandwidth of the system. The IEEE 802.16e-2005 is an amendment to 802.16-2004. It basically deals

with the mobility along with better support for quality of service and the use of scalable OFDMA, and is

sometimes called Mobile Wi-MAX (Worldwide Interoperability for Microwave Access). The main

parameters of OFDM adopted in IEEE 802.16 are shown in Table-1.7 and Table- 1.8.

Table - 1.7: Main parameters of IEEE 802.11-2004 standard Parameters Specification

Modulation BPSK, QPSK, 16 QAM, and 64 QAM

Coding rate 1/2 , 2/3, 3/4, and 5/6

FFT Size 256

Sampling Frequency (MHz) 2 4 6.32 8

Symbol Duration (푇 휇푠푒푐) 128 64 40 32

Duration of Guard Interval 푇 /4 , 푇 /8 , 푇 /16 , 푇 /32

Sub-carrier Spacing (KHz) 7.81 16.6 25.0 31.3

Bandwidth (MHz) 1.75 3.5 5.5 7

Table - 1.8: Main parameters of IEEE 802.11e-2005 standard Parameters Specification

Modulation BPSK, QPSK, 16 QAM, and 64 QAM

Coding rate 1/2 , 2/3, 3/4, and 5/6

FFT Size 128 512 1024 2048

Sampling Frequency (MHz) 1.4 5.6 11.2 22.4

Symbol Duration (푇 휇푠푒푐) 91.4

Guard Interval 푇 /4 , 푇 /8 , 푇 /16 , 푇 /32

Sub-carrier Spacing (KHz) 10.9

Bandwidth (MHz) 1.75 3.5 5.5 7

[20]

(E) Multi-Band OFDM for UWB Systems [28]

The research activities in the Ultra-Wideband (UWB) technology have grown extensively after

the declaration of UWB as an unlicensed band (3.1 GHz – 10.6 GHz) by Federal Communications

Commission (FCC) in 2002. The available UWB spectrum of 7.5 GHz is divided into several sub-bands,

each with bandwidth of at least 500 MHz. For the efficient use of this considerable bandwidth (7.5

GHz), the Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) has been accepted as

commercially more feasible than ‘impulse-radio-based’ and CDMA based’ UWB transmission because

of its flexible spectrum configuration and robustness against frequency selective fading channel. It has

been proposed for IEEE802.15.3a ultra-wideband standard and for standard ECMA-368 (PHY and

MAC specification) [28]. In 2006, Bluetooth-SIG has adopted MB-OFDM as their on-air interface for

high speed wireless.

MB-OFDM is a combination of OFDM modulation with data transmission by Frequency

Hopping (FH) over different frequency bands. The basic idea of MB-OFDM is to divide the spectrum

into several sub-bands, and a data stream is transmitted over each band by OFDM. The MB-OFDM-

based UWB system achieves data rates ranging from 55 to 480 Mbps over distances up to 10 m. The

entire available UWB spectrum between 3.1-10.6 GHz is divided into 14 sub-bands, each with 528 MHz

bandwidth. The specifications of UWB-OFDM are listed in Table-1.9.

Table - 1.9: Specifications of UBW MB-OFDM Parameters Specification

Data Rate 55, 80, 110, 160, 200,320 and 480 Mbps.

FFT Size 128

Symbol duration 312.5 nsec

Length of CP 60.6 nsec

Length of Guard Interval 9.5 nsec

Sub-carrier Spacing 3.2 MHz

Bandwidth 528 MHz

[21]

1.6 ADVANTAGES AND DRAWBACKS OF OFDM SYSTEM

As explored in previous section that the OFDM system has been adopted by many current

wireless standards and also proposed for future wireless communication systems due to its several

advantageous features. The key advantages of OFDM system are as follows [69, 102 and 113] :

Less Implementation Complexity: For a given delay spread of multipath fading channel, the

implementation compexity is significantly lower than that of single carrier system with an

equalizer (single tap equalizer is required in OFDM system).

Robustness Against Narrowband Interference: OFDM is more robust against narrowband

interference because in a single carrier system, a single fade or interferer can cause the entire

link to fail, but in a multicarrier system (like in OFDM), only a small percentage of subcarriers

will be affected.

Immune to Frequency Selective Fading: OFDM is highly immune to frequency selective

fading because of parallel transmission (each sub-carrier has narrow bandwidth in comparison

to overall bandwidth of signal). It converts a frequency selective fading channel into several

nearly flat fading channels.

High Spectral Efficiency: Due to orthogonal nature of each sub-carrier, large number of sub-

carriers can be accomadate in a very narrow spectral region thus increasing the spectral

efficiency.

Efficient Modulation and demodulation of OFDM symbol are possible with the help of IFFT

and FFT.

Besides these advantageous features, the OFDM systems also have some major problems like:

Symbol Timing Offset (STO): OFMD is highly sensitive to STO. Due to the use of IFFT and

FFT for modulation and demodulation at transmitter and receiver respectively, correct timing

(start of FFT window) is required at the receiver otherwise one FFT window will take sample

from two transmitted OFDM symbol. This deteriorates the performance of OFDM system [78].

Carrier Frequency Offset (CFO): OFMD is highly sensitive to carrier frequency offset. Most

of the advantages of OFDM are due to the orthogonal nature of sub-carriers and this

orthogonality between sub carriers will be destroyed if frequency offset arises between them.

[22]

The major cause of CFO is Doppler shifts (due to relative motion between transmitter and

channel) [78, 83, 119 and 102].

High Peak-to-Average Power Ratio (PAPR): 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 [122].

Inter-Carrier Interference (ICI): The result of all above mention problem will appear in the

form of Inter-Carrier Interference (ICI) at the receiver. The ICI decreases the SNR and degrades

the BER performance of OFDM system [86 and 87].

1.7 ORGANIZATION OF THESIS

From the literature survey, it is found that besides so many advantageous features of OFDM

system, some major issues are still present (as mention above in previous section) which must be

resolved to get full advantages. Lots of work has been reported in the literature to solve these problems.

This thesis presents the analysis of all these major issues of OFDM system with their comparative

study, followed by proposed solution to each and every problem. The work reported in the thesis is

organized in six Chapters, as given below –

Chapter-2 presents an in-depth literature survey on OFDM and its major issues. This chapter

starts with basic system model of baseband OFDM including transmitter, channel and receiver model

with the expression of transmitted and received OFDM signal. The impulse response of multipath

fading channel has also been given. After that, the major issues of OFDM system like high PAPR,

timing synchronization, frequency synchronization and ICI reduction has been discussed. This survey

also summarizes the solutions of these problems available in the literature. Different PAPR reduction

techniques, timing offset estimation methods, frequency offset estimation methods and different

window functions used for ICI reduction have been analyzed.

Based on the conclusions drawn in Chapter-2, the problem of timing offset estimation has been

considered in Chapter-3. A new timing offset estimation method is proposed in this Chapter. Here, first

the effect of timing offset has been analyzed for different scenario. The expression of recovered signal

(after FFT) in the presence of timing offset has been derived mathematically. Thereafter, a brief review

of different timing offset estimation methods are given with their timing metric and preamble structure.

Subsequently, the fractional Fourier transform (FRFT), the chirp signal, the FRFT of chirp signal and

the correlation theorem for FRFT domain has been presented. After that, the FRFT based proposed

[23]

timing offset estimation method, proposed training structure and timing metric is given. Finally, the

performance of proposed timing offset estimation method is compared with other existing methods in

terms of mean and mean square error of timing offset and implementation complexity.

In Chapter-4, a new frequency offset estimation method has been introduced. Here, first of all

the effect of frequency offset and expression of ICI coefficient has been derived. After that, a brief

review of different frequency offset estimators is discussed. Subsequently, the modulation property of

FRFT, and characteristic of chirp signal and its FRFT has been analyzed. It has also been shown that

FRFT is better tool than FT for estimating frequency offset when chirp signal is used as a training

sequence. After that proposed method of fractional frequency offset estimation and proposed algorithm

for integer frequency offset estimation has been presented. Finally, a performance comparison is made

between proposed and existing methods in terms of mean square error of estimation and

implementation complexity.

The issue of high PAPR in OFDM system has been considered in Chapter-5. Here, first the

definition and distribution of PAPR is defined followed by the discussion on some major PAPR

reduction techniques like non linear companding, selective mapping (SLM) and partial transmit

sequence (PTS). Then after, the proposed method of PAPR reduction (combination of pre-coding and

clipping) has been explored. For this method, the system model with pre-coding and clipping is also

given which includes transmitted and received signal expression. For the generation of pre-coding

matrix, a new window is proposed and for clipping process a new clipping algorithm has been

introduced. After that, the performance of proposed hybrid method and proposed clipping algorithm is

evaluated and compared with exponential companding and conventional clipping method.

In Chapter-6, the ICI reduction method using receiver windowing has been described. For this,

an OFDM system model with receiver windowing is chosen. The expression of ICI coefficient, ICI

power, and Signal to Interference (SIR) ratio while using receiver windowing has been derived.

Subsequently, it has been shown that the ICI power depends on the Fourier transform of pulse shaping

function being used. After that, a MBH window family is proposed for reducing the ICI. The

expression of window family (modified for the use in OFDM system) both in time and frequency

domain has also been included in this chapter. Further, it has been shown that many existing pulse

shape are the member of this window family at different pulse shape parameter. Lastly, the proposed

MBH window family is compared with other existing pulse shapes in terms of both ICI power and bit

error rate.

Finally, the conclusion of the thesis along with the possible future scope is included in Chapter-7.

[24]

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