OFDM_Introduction.pdf

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2.5G or even 3G networks. Along with this, interoperability of the networks has to be

considered [3].

1.1.1 FIRST GENERATION (1G)1G, which stands for "first generation," refers to the first generation of wireless

telecommunication technology, more popularly known as cell phones. A set of wireless

standards developed in the 1980's, 1G technology replaced 0G technology, which featured

the mobile radio telephones and such technologies as Mobile Telephone System (MTS),

Advanced Mobile Telephone System (AMTS), Improved Mobile Telephone Service (IMTS),

and Push to Talk (PTT).

Unlike its successor, 2G, which made use of digital signals, 1G wireless networks used

analog radio signals. Through 1G, a voice call gets modulated to a higher frequency of about

150MHz and up as it is transmitted between radio towers. This is done using a technique

called Frequency-Division Multiple Access (FDMA) [4]. In terms of overall connection

quality, 1G compares unfavorably to its successors. It has a low capacity, unreliable handoff,

poor voice links, and no security at all since voice calls were played back in radio towers,

making these calls susceptible to unwanted eavesdropping by third parties. Different 1G

standard was used in various countries.

Advanced Mobile Phone System (AMPS) was a 1G standard used in the United

States.

Nordic Mobile Telephone was a 1G standard used in Nordic countries (Denmark,

Finland, Iceland, Norway and Sweden), as well as in its neighboring countries

Switzerland and Netherlands, Eastern Europe, and Russia.

Italy used a telecommunications system called RTMI.

In the United Kingdom, Total Access Communication System was used.

France used Radiocom 2000.

1.1.2 SECOND GENERATION (2G)

Second-generation (2G) mobile systems were introduced at the end of 1980s. Three primary

benefits of 2G networks over their predecessors were that phone conversations were digitally


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encrypted; 2G systems were significantly more efficient on the spectrum allowing for far

greater mobile phone penetration levels; and 2G introduced data services for mobile, starting

with SMS text messages. This 2G technology increase in the user capacity of around three

times. Consequently, and compared with first-generation systems, higher spectrum

efficiency, better data services, and more advanced roaming were offered by 2G systems [5].

2G technologies can be divided into TDMA-based and CDMA-based standards depending

on the type of multiplexing used. The main 2G standards are:

GSM (TDMA-based), originally from Europe but used in almost all countries on

all six inhabited continents. Today accounts for over 80% of all subscribers

around the world. Over 60 GSM operators are also using CDMA2000 in the 450

MHz frequency band (CDMA450).

IS-95 (CDMA-based, commonly referred as simply CDMA in the US), used in the

Americas and parts of Asia. Today accounts for about 17% of all subscribers

globally. Over a dozen CDMA operators have migrated to GSM including

operators in Mexico, India, Australia and South Korea.

PDC (TDMA-based), used exclusively in Japan

iDEN (TDMA-based), proprietary network used by Nextel in the United States

and Telus Mobility in Canada

IS-136 aka D-AMPS (TDMA-based, commonly referred as simply 'TDMA' in the

US), was once prevalent in the Americas but most have migrated to GSM.

2G services are frequently referred as Personal Communications Service (PCS), in the

United States. 2G networks were built mainly for voice services and slow data transmission.

Some protocols, such as EDGE for GSM and 1x-RTT for CDMA2000, are defined as "3G"

services (because they are defined in IMT-2000 specification documents), but are considered

by the general public to be 2.5G services (or 2.75G which sounds even more sophisticated)because they are several times slower than present-day 3G services.


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2.5G (GPRS)

2.5G is a stepping stone between 2G and 3G cellular wireless technologies. The term

"second and a half generation" is used to describe 2G-systems that have implemented a

packet switched domain in addition to the circuit switched domain. It does not necessarily

provide faster services because bundling of timeslots is used for high speed circuit switched

data services (HSCSD) as well.

The first major step in the evolution of GSM networks to 3G occurred with the introduction

of General Packet Radio Service (GPRS). CDMA2000 networks similarly evolved through

the introduction of 1xRTT. The combination of these capabilities came to be known as 2.5G.

GPRS could provide data rates from 56 kbit/s up to 115 kbit/s. It can be used for services

such as Wireless Application Protocol (WAP) access, Multimedia Messaging Service

(MMS), and for Internet communication services such as email and World Wide Web

access. GPRS data transfer is typically charged per megabyte of traffic transferred, while

data communication via traditional circuit switching is billed per minute of connection time,

independent of whether the user actually is utilizing the capacity or is in an idle state.

1xRTT supports bi-directional (up and downlink) peak data rates up to 153.6 bit/s,

delivering an average user data throughput of 80-100 kbit/s in commercial networks [5,6]. Itcan also be used for WAP, SMS & MMS services, as well as Internet access.

2.75G (EDGE)

GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding.

Enhanced Data rates for GSM Evolution (EDGE), Enhanced GPRS (EGPRS), or IMT Single

Carrier is a backward-compatible digital mobile phone technology that allows improved

data transmission rates, as an extension on top of standard GSM. EDGE was deployed onGSM networks beginning in 2003initially by Cingular (now AT&T) in the United States

[3].

EDGE is standardized by 3GPP as part of the GSM family and it is an upgrade that provides

a potential threefold increase in capacity of GSM/GPRS networks. The specification


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achieves higher data-rates ( up to 236.8 kbit /s) by switching to more sophisticated methods

of coding (8PSK), within existing GSM timeslots.

1.1.3 THIRD GENERATION (3G)

3G systems are an extension on the complexity of second generation systems and are already

introduced. The system capacity is expected to be increased to over ten times original first

generation systems. This is going to be achieved by using complex multiple access

techniques such as Code Division Multiple Access (CDMA), or an extension of TDMA, and

by improving flexibility of services available [6]. The main feature includes support for

multimedia services, data transmission rate of at least 384kbit/Sec, and global roaming.

Compared with the 1G and 2G, high-speed data transmission and broadband multimedia

service can be achieved in the 3G communication system. However, because of differentstandards of regional communication systems, 3G is still unable to meet the future

requirements of higher data transfer rates. The 3G cellular services know as a universal

mobile telecommunication system (UMTS) or IMT-2000 will sustain high higher data rates

and open the door to many Internet style applications. The main characteristics of IMT-2000

3G systems are:

A single family of compatible standards that can be used worldwide for all mobile

applications.

Support for both packet-switched and circuit-switched data transmission.

Data rates up to 2 Mbps (depending on mobility).

High spectrum efficiency.

IMT-2000 is a set of requirements de-fined by the international telecommunication union

(ITU). IMT stands for the international mobile telecommunication, and 2000 represent both

the scheduled year for initial trial systems and the frequency range of 2000 MHz.


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1.1.4 FOURTH GENERATION (4G)

Therefore, 4G mobile communication system, with the OFDM modulation technique, started

to enter the horizon and became a research hotspot. For high-volume and high-speed

wireless mobile communication systems, OFDM is a promising modulation scheme, and will

play an increasingly important role in the future development of wireless mobile

communication networks.

4G mobile communications will have transmission rates up to 20 Mbpshigher than of 3G.

The technology is expected to be available by the year 2010. Presently, NTT DOCOMO and

Hewlett-Packard are on their agenda to make it available by the year 2006. 4G is being

developed with the following objectives:

Speeds up to 50 times higher than 3G. However, the actual available bandwidth of 4G

is expected to be about 10 Mbps.

Three-dimensional virtual realityimagines personal video avatars and realistic

holograms, and the ability to feel as if you are present at an event even if you are not.

People, places, and products will be able to interact as the cyber and real worlds

merge.

Increased interaction between corroborating technologies; the smart card in yourphone will automatically pay for goods as you pass a linked payment kiosk, or will

tell your car to warm up in the morning as your phone has noted you leaving the

house.

1.2 BRIEF HISTORY OF OFDM SYSTEMAlthough OFDM has only recently been gaining interest from the telecommunications

industry, it has a long history of existence. It is reported that OFDM-based systems were in

existence during the Second World War. OFDM had been used by the US military in several

high-frequency military systems such as KINEPLEX, ANDEFT, and KATHRYN [6].

KATHRYN used AN/GSC-10 variable rate data modem built for high-frequency radio.


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In December 1966, Robert W. Chang1 outlined a theoretical way to transmit simultaneous

data stream through linear band-limited channel without inter-symbol interference (ISI) and

inter-carrier interference (ICI) [7]. Subsequently, he obtained the first US patent on OFDM

in 1970. Around the same time, Saltzberg performed an analysis of the performance of the

OFDM system [8]. Until this time, we needed a large number of subcarrier oscillators to

perform parallel modulations and demodulations.

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

discrete Fourier transform (DFT) to perform baseband modulation and demodulation

focusing on efficient processing [8]. This eliminated the need for a bank of subcarrier

oscillators, thus paving the way for easier, more useful and efficient implementation of the

system.

All the proposals until this time used guard spaces in the frequency domain and a raised

cosine windowing in the time domain to combat ISI and ICI. Another milestone for OFDM

history was when Peled and Ruiz introduced cyclic prefix (CP) or cyclic extension in 1980

[10]. This solved the problem of maintaining orthogonal characteristics of the transmitted

signal at severe transmission conditions. The generic idea that they placed was to use cyclic

extension of OFDM symbols instead of using empty guard spaces in the frequency domain.

This effectively turns the channel as performing cyclic convolution, which provides

orthogonality over dispersive channels when CP is longer than the channel impulse response

[6]. It is obvious that introducing a CP causes loss of signal energy proportional to length of

CP compared to symbol length, but, on the other hand, it facilitates a zero ICI advantage

which pays off.

By this time, inclusion of FFT and CP in OFDM system and substantial advancements in

digital signal processing (DSP) technology made it an important part of telecommunicationslandscape. In the 1990s, OFDM was exploited for wideband data communications over

mobile radio FM channels, high-bit-rate digital subscriber lines (HDSL at 1.6 Mbps),

asymmetric digital subscriber lines (ADSL up to 6Mbps), and very-high-speed digital

subscriber lines (VDSL at 100 Mbps). Digital audio broadcasting (DAB) was the first


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commercial use of OFDM technology. Development of DAB started in 1987. By 1992, DAB

was proposed and the standard was formulated in 1994. DAB services came to reality in

1995 in the United Kingdom and Sweden [1]. The development of digital video broadcasting

(DVB) started in 1993. DVB along with high-definition television (HDTV) terrestrial

broadcasting standard was published in 1995. At the dawn of the 20th

century, several

wireless local area networks (WLAN) standards adopted OFDM on their physical layers.

Development of European WLAN standard Hyper-LAN started in 1995. HIPERLAN/2 was

defined in June 1999 which adopts OFDM in physical layer. IEEE 802.11a has also adopted

OFDM in its PHY layer. Perhaps of even greater importance is the emergence of this

technology as an enabler for future 4th generations (4G) wireless systems, such as IMT-A.

These systems, expected to emerge by the year 2015, promise to at least deliver on the

wireless Nirvana of anywhere, anytime, anything communications. OFDM promises to gain

prominence in this arena; therefore, it is expected to become the technology of choice in

most wireless links.

1.3 THE PRINCIPLE OF OFDMOFDM is a multicarrier transmission technique, which divides the bandwidth into many

carriers; each one is modulated by a low rate data stream. In terms of multiple access

technique, OFDM is similar to FDMA in that the multiple user access is achieved by

subdividing the available bandwidth into multiple channels that are then allocated to users

[6]. However, OFDM uses the spectrum much more efficiently by spacing the channels

much closer together. This is achieved by making all the carriers orthogonal to one another,

preventing interference between the closely spaced carriers. The figure shows the difference

between the conventional non-overlapping multicarrier technique and overlapping

multicarrier modulation technique. As shown in figure 1.1, by using the overlapping

multicarrier modulation technique, we save almost 50% of the bandwidth. To realize the

overlapping multicarrier technique, however we need to reduce crosstalk between

subcarriers, which means that we want orthogonality between the different modulated

carriers.


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Figure 1.1 Concept of OFDM Signal: Conventional Multicarrier Technique versus

Orthogonal Multicarrier Technique

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

over a symbol period. Due to this, the spectrum of each carrier has a null at the center

frequency of each of the other carriers in the system. This results in no interference between

the carriers, allowing them to be spaced as close as theoretically possible. This overcomes

the problem of overhead carrier spacing required in FDMA. Each carrier in an OFDM signal

has a very narrow bandwidth (i.e.1 kHz), thus the resulting symbol rate is low. This results

in the signal having a high tolerance to multipath delay spread, as the delay spread must be

very long to cause significant inter-symbol interference (e.g. > 500 sec).

1.3.1 ORTHOGONALITY PRINCIPLE

Two functions or signal is said to be orthogonal if they are mutually independent of eachother. Orthogonality is a property that allows multiple information signals to be transmitted

perfectly over a common channel with the successful detection [11].


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

two factions from the set of functionalities {g1 (t), g2 (t), gN(t)} over a joint existence

time interval [t1 .. t1+T] is equal to zero. Irrespective of the limit of existence of the

function then the functions are said to be orthogonal to each other within this time interval

[6].

Similarly if the set of functions is to be an orthogonal set then they have to be orthonormal

set then they have to be orthogonal as well as normalized.

In the context of OFDM, the set of complex exponential function Defined overthe period [0, T] represent different subcarrier at for k=0 to k=N-1 using theexpression (1.1).

1.4 APPLICATION OF OFDM

OFDM is widely applied from theory to practice. In the past decade, the rapid development

of large scale integrated circuit technology and the realization of a large numberpoints

high-speed FFT promoted the wide application of OFDM theory. The first OFDM based

standard was digital audio broadcasting (DAB) [12].

1. OFDM is used in broadcast audio and video and civilian communications, includingasymmetric digital subscriber line (ADSL).


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2. Digital audio broadcasting (DAB) [1,6]3. High definition television (HDTV)4. IEEE 802.11 [6]5. HIPERLAN/2 [6]6. Digital video broadcasting (DVB) [1]7. 4G LTE (Long Term Evolution)

1.5 ADVANTAGES AND PROBLEMS OF OFDM1.5.1 Advantages

OFDM has several advantages over single carrier modulation systems and these make it a

viable alternative for CDMA in future wireless networks. Some of the advantages arediscussed below.

OFDM is highly immune to multipath delay spread that causes inter-symbol

interference in wireless channels. Since the symbol duration is made larger (by

converting a higher data rate signal into a low rate signals), the effect of delay spread

is reduced by the same factor. Also by introducing the concepts of guard time and

cyclic extension, the effects of inter-symbol interference (ISI) and inter-carrier

interference (ICI) can be removed completely.

In the conventional FDM method, the frequency band is divided into several disjoint

sub-frequency band to transmit parallel data streams, and separated to the various

sub-channels at the receiver with a set of filters. Compared with this, the

orthogonality of sub-carriers of OFDM system allows overlapping spectrum of sub-

channels hence OFDM system can maximize the using of spectrum resources.

In each sub-channel, IDFT and DFT can be used to achieve orthogonal modulation

and demodulation. When N is a large number, we can use FFT to achieve. IFFT and

FFT are very easy to implement using DSP technology.

If the channel undergoes a frequency selective fading, then complex equalization

techniques are required at the receiver for single carrier modulation techniques. But


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in the case of OFDM the available bandwidth is split among many orthogonal

narrowly spaced sub-carriers. Thus the available channel bandwidth is converted into

many narrow flat- fading sub-channels. Hence it can be assumed that the subcarriers

experience flat fading only, though the channel gain/phase associated with the sub-

carriers may vary. In the receiver, each sub-carrier just needs to be weighted

according to the channel gain/phase encountered with it. Even if some sub-carriers

are completely lost due to fading, proper coding and interleaving at the transmitter

can recover the user data.

OFDM system can be easily combined with a variety of other access methods to form

multi-carrier code division multiple access (MC-CDMA), frequency hopping OFDM.

It allows multiple users to transmit information from OFDM technology

simultaneously.

Because the narrow-band interference can only affect a small part of the sub -carriers,

the OFDM system can resist this narrow-band interference to some extent.

1.5.2 MAJORPROBLEMS OF OFDM

It is vulnerable to the impact of frequency deviation. Because of the time variability

of the wireless channel, wireless signal frequency offset always occurs during

transmission, such as Doppler frequency shift. Then the orthogonality between

subcarriers of OFDM system will be destroyed, resulting in ICI between sub-

channels. Hence sensitivity to the frequency deviation is the main drawback of

OFDM system.

There exists a problem of higher PAPR. Compared with the single-carrier system,

because the output of multi-carrier modulation system is a superposition of multiple

sub-channel signals, so if the phases of some signals are same, instantaneous signal

power of the superposition of signals received will be far greater than the average

power, which lead to larger PAPR. It needs higher level linear requirement for an

amplifier of the transmitter. If the dynamic range of amplifier cannot meet the signal

changes, it will bring the signal distortion, which could change the spectrum of the


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signal. So, the orthogonality between different sub-channels will be destroyed,

making the system performance worse.

1.6 ORGANIZATION OF THESIS

This thesis is organized as follows: In chapter 2, Literature survey which includes the

characteristics of radio channel, basic OFDM system and channel estimation is discussed, In

Chapter 3, an overview of different approaches of channel estimation in OFDM systems is

presented and different channel estimation and interpolation techniques are discussed.

Chapter 4 demonstrates Simulation. Chapter 5 concludes the thesis and areas for future work

are also suggested.

Documents

OFDM_Introduction.pdf