Wideband - OFDM

Seminar Report ’03 Wideband - OFDM

INTRODUCTION

Orthogonal frequency division multiplexing (OFDM) is a multicarrier

transmission technique that has been successfully applied to wide variety of

digital communication applications. Although the concept of OFDM has been

around for a long time, it has been recently recognized as an excellent method

for high speed bi-directional wireless data communication. This technology is

used in broad cast systems such as Asymmetric Digital Subscriber Line

(ADSL), European Telecommunications standard Institute (ETSI), radio (DAB:

Digital Audio broadcasting) and TV (DVB: Digital Video broadcasting-

Terrestrial) as well as being proposed for wireless LAN standards.

OFDM efficiently squeezes multiple modulated carriers tightly together

reducing the required bandwidth but keeping the modulated singles orthogonal

so that they do not interface with each other. Any digital modulation technique

can be used on separate carriers. The output of the modulated carriers is added

together before transmission. At the receiver, the modulated carriers are

separated before demodulation.

W- OFDM will allow the deployment of 4 G wireless networks that

enable phones to transmit data at rates of up to megabits per second.OFDM

segment are according to frequency. It is a technique that divides the spectrum

in to a number of equally spaced tones and carriers a portion of a users

information on each tone. A tone can be thought of frequency. Each tone is

orthogonal to the other. OFDM is also called multi tone modulation.

OFDM can be considered as a multiple access technique, because an

individual tone or groups tones can be assigned to different users. Multiple

users share a given bandwidth in this manner, yielding the system called

Dept. of AEI MESCE Kuttippuram1


OFDMA. Each user can be assigned a predetermined number of tones when

they have information to send, or alternatively a user can be assigned a variable

number of tones based on the information that they have to send.W-OFDM can

overcome problems of high peak-to-average signal amplitude and fading due to

multipath affects. W-OFDM enables the implementation of low power

multipath RF networks that minimize interference with adjacent networks.



OFDM FOR MOBILE COMMUNICATION

OFDM represents a different system design approach it can be though of

as combination of modulation and multiple across schemes that segment a

communications channel in such a way that many users share it. Where as

TDMA segments are according to time and CDMA segments are according to

spreading codes ,OFDM segments are according to frequency. It is a technique

that divides the spectrum into a number of equally spaced tones and carries a

portion of a users information on each tone. A tone can be thought of a

frequency, much in the same way that each key on a pain represents unique

frequency. OFDM has a special property that each tone is orthogonal with each

other. There will be frequency guard bands b/w frequencies so that they do not

interfere with each other. OFDM allows the spectrum of each tone to overlap

and because they are orthogonal they donot interfere with each other. This

reduces the required spectrum.

Figure 1Spectrum of an OFDM Signal With Three sub-carriers



OFDM is a modulation technique that enables user data to be modulated

onto the tones. The information is a modulated into a tone by adjusting the

tones phase amplitude or both. In the most basic form, a tone may be present or

disabled to indicate a one or zero bit of information; however, either phase shift

keying (PSK) or quadrate amplitude modulation (QAM) is typically employed.

An OFDM system takes a data stream and splits it into N parallel data streams

each at a rate 1/N of the original rate. Each stream then mapped to a tone at a

unique freq and combined together using the inverse fast Fourier transform

(IFFT) to yield the time-domain waveform to be transmitted.

For example, if a 100-tone system were used, a single data stream with a

rate of 1 mega bit per second (Mpbs) would be converted into 100 streams of

10 kilobits per second (Kpbs). By creating parallel data streams, the bandwidth

of modulation symbol is effectively decreased by a factor of 100. OFDM can

also be considered a multiple access technique because an individual tone or

groups of tone can be assigned to different users. Multiple users share a given

band with in this manner, yielding the system called OFDMA. Each user can be

assigned a predetermined number of tones when they have information to send,

or alternatively, a user can be assigned a variable number of tones on the

amount of information that they have to send.

OFDM can be combined with frequency hopping to create a spread

spectrum system, realizing the benefits of frequency diversity and interference

averaging property. In frequency hopping spread spectrum system, each users’

set of tones is changed after each time period. By switching frequencies after

each symbol time, the losses due to frequency selective fading are

minimized.OFDM therefore provides the best of the benefits of TDMA in that

users are orthogonal to one another and CDMA-while avoiding the limitations

of each including the need for TDMA frequency planning and multiple access

interference in the case of CDMA.



W-OFDM SYSTEM ARCHITECTURE

Fig2 shows the processing blocks of the W-OFDM.

Encoder

The encoder prepares the bits so that the decoder can correct the errors

that may occur during transmission. The bits entering the encoder are grouped

into bocks.

Modulator

The modulator transforms the encoded block of bits into a vector of

complex values which is the W-OFDM symbol in the frequency domain.

Groups of bits are mapped onto a modulation constellation producing a

complex value representing a modulated carrier. The carrier representing DC is

not modulated to eliminate complications with DC levels. Some carriers called

pilot carriers are modulated with known values to allow the demodulator to

adjust amplitude and phase. There are multiple pilot carriers to improve SNR.



Signal whitener

The signal whitener reduces the peak to average power level ratio that

must pass through the radio amplifiers and A/D converters; it can also provide

a level of security. The W-OFDM symbol is multiplied by a vector of complex

value, R that is known to the transmitter and receiver. There are many vectors

that can be used for, R and different R can be used for each W-OFDM symbol;

thus this stage can be used as a level of security.

IFFT

The IFFT processing blocks transforms the W-OFDM symbol from the

frequency to the time domain. It prepares the time domain W-OFDM symbol

for transmission. The vector is cyclically intended to reduce the effects of

intersymbol interference at the receiver as shown in Fig.3

Figure 3 W-OFDM solves the problem of intersymbol interference due to

multipath delays by incorporating a cyclic prefix.



FFT

The FFT block transforms the W-OFDM symbol from the time to the

frequency domain.

SYNCHONIZATION

For synchronization, a direct sequence (DS) spread spectrum signal is

used. The OFDM receiver recovers the gain and frequency error information

from the synchronization message.

CHANNEL ESTIMATION

The amplitude and phase distortion caused during transmission is

determined by comparison of the original known signal with the OFDM signal.

EQUALIZER

Equalizer removes the channel distortion and the prewhitening. The W-

OFDM vectors is multiplied by the pre-computed channel estimation.

DEMODULATOR

The W-OFDM symbol is converted back into a block of bits. Each

carrier is converted back to bits based on the modulation technique.

DECODER

The decoder detects and corrects bits in error producing the original

block of bits. It ignores bits that were on carriers with low SNR.



THEORY OF OFDM OPERATION

The sinusoidal waveforms making up the tones in OFDM have the very

special property of being functions of a linear channel. This property prevents

adjacent tones in OFDM systems from interfacing with one another, in the

same manner that human ear can clearly distinguish between each of the tones

created by the adjacent keys of a piano. This property and incorporation of a

small amount of guard time to each symbol, enables the orthoganolity between

tones to be preserved of multipath. This is what enables OFDM to avoid the

multiple access interference that is present in CDMA systems.

The frequency domain representation of a number of tones, shown in

figure 4. Highlights the orthogonal nature of the tones used on OFDM system.

Notice that the peak of each tone corresponds to a zero level or null of every

other tone. The result of this is that there is no interference between tones.

When the receiver samples at the center frequency of each tone, the only

energy present is that of the desired signal, plus whatever other noise happens

to be in the channel. To maintain orthogonality between tones, it is necessary to

ensure that symbol time contains one or multiple cycles of each sinusoidal tone

waveform. This is normally the case, because the system numerology is

constructed such that tone frequencies are integer multiples of the symbol

period, as it is subsequently highlighted, where the tone spacing is 1/T. figure.

Shows three tones over a single symbol period, where each tone has an integer

number or cycles during the symbol.



Figure4 Time and Frequency domain representation of OFDM

The absolute terms, to generate a pure sinusoidal tone require the signal

start at time minus infinity. This is because tones are the only waveform that

can ensure orthogonally. Fortunately, the channel response can be treated as

finite, because multipath components decay overtime and channel is effectively

band limited.

By adding a guard time called a cyclic prefix, the channel can be made

to behave as if the transmitted waveforms were from time minus infinity and

thus ensure orthogonality, which essentially prevents one sub carrier from

interfacing with another ( called intercarrier interference ) . The cyclic prefix is

actually a copy of the last portion of the data symbol appended to the front of

the symbol during guard interval as shown . multipath causes tones and delayed

replicas of tones to arrive at the receiver with some delay spread. This leads to

misalignment between sinusoids which needs to aligned as in figure5, to be

orthogonal. The cyclic prefix allows the tones to be realigned at the receiver,

thus regaining orthogonality.



Figure 5 cyclic extension of sinusoid



KEYBENEFITS OF OFDM

Bandwidth efficiency

A key aspect of all high speed communications lies in the bandwidth

efficiency. This is especially important for wireless communications where all

current future devices are expected to share an already crowded range of carrier

frequencies, where the consumer’s appetite for wireless internet services has

steadily been growing. For wireless networks to remain profitable, it is

necessary to achieve maximum bandwidth efficiency.

In OFDM, the frequency band containing the message is divided up into

parallel bit streams of lower frequency carriers or sub carriers. These

subcarriers are designed to be orthogonal to another, so that they can be

separated out at the receiver without interference from neighbouring carriers. In

this manner, OFDM is able to space the channels much closer together, which

allows for more efficient use of spectrum than through simple FDM.

Multipath fading

When radio signal travel from point to point they may bounce off

surrounding objects, resulting in multiple paths between transmitter and

receiver. This leads to several copies of the message arriving at the receiver.

This is called multipath fading. The combination of all paths at the receiver

causes the modulated message to be distorted. Thus the individual pulses

overlap one another, and this is called intersymbol interference. Each subcarrier

in an OFDM signal has a very narrow bandwidth, thus the symbol rate is very



low. This results in the signal having high tolerance to multi path delay spread,

reducing any significant intersymbol interference.

RF interference

To combat the effects of random signal noise, which can prevent the

receiver from fully recovering the signal, a spreading forward error correcting

code is applied to the signal before transmission this has the effect of spreading

the symbols over many frequencies, white maintaining the ability to recover the

symbols even if some carriers are subjected to noise.



LEADING – EDGE MOBILE OFDM TECHNOLOGIES

Unlike most existing forms of wireless access, including 3G

technologies, conventional wireless systems have been designed primarily at

the physical layer. To address the unique demands posed by mobile users of

high speed data applications, new air interface must be designed and optimized

across all the layers of the protocol stack, including networking layers. A prime

example of this is flash-OFDM. It is a system level technology that exploits the

unique physical properties of OFDM, enabling significant higher layer

advantages that contribute to very efficient packet transmission in a cellular

network.

Packet switched Air interface

The telephone network, designed basically for voice is an example of

circuit switched systems. Circuit switched systems exist only at the physical

layer that uses the channel resource to create a bit pipe. circuit switched

systems are very inefficient for burst data traffic. Packet switched systems on

the other hand, are very efficient for data traffic but require control layers in

addition to the physical layer that creates the bit pipe. The internet is the best

example for packet switched interface network.

Because all conventional cellular wireless systems are designed for

circuit switched voice, they are designed and optimized at the physical layer

flash OFDM is a packet-switched designed for data and is optimized across the

physical MAC,Link and network layers.



CONCLUSION

W-OFDM for mobile data communication can be thought of as

combination of modulation and multiple access schemes. Like CDMA multiple

users share a given bandwidth in OFDM. OFDM provides the best of the

benefits of TDMA. OFDM divides the spectrum into a number of equally

spaced tones and carries a portion of the users’ information on each tone. A

tone can be thought of as a frequency. OFDM has an important property that

each tone is orthogonal to the other.

OFDM is a modulation technique that it enables user data to be

modulated onto the tones. The information is modulated into tone by adjusting

the tones phase, amplitude or both. In addition to high speed wireless

application wired systems such as asynchronous digital subscriber line (ADSL)

and cable modem utilized. OFDM as its underlying technology to provide a

method of delivering high speed data. OFDM has also been adopted into

several European wireless communications such as digital video broadcast and

terrestrial digital video broadcast.



BIBLIOGRAPHY

1. Justin chuang ,Leonard j.cimini. IEEE Communications Magazine.

November 2001.

2. Des Brisay, Greg. “Basics of orthogonal FDM”

3. Welling, Keith . “Coded orthoragothonal FDM”

4. “Wireless Data communications” September 2001

5. www.wca.org

6. www.magisnetworks.com/ofdm.pdf.



CONTENTS

1. INTRODUCTION

2. OFDM FOR MOBILE COMMUNICATION

3. W- OFDM SYSTEM ARCHITECTURE

4. THEORY OF OFDM- OPERATION

5. KEY BENEFITS OF OFDM

6. LEADING EDGE MOBILE OFDM TECHNOLOGIES

7. CONCLUSION



ABSTRACT

Orthogonal frequency Division Multiplexing (OFDM) is multicarrier

transmission technique. OFDM is a communication technique that divides the

communication channel into a number of equally spaced frequency bands. A

sub carriers a portion of the users information in each band. Each sub carriers is

orthogonal (Independent of each other) with every other sub carrier. OFDM

efficiently squeezes multiple modulated carriers tightly together reducing the

required bandwidth.

OFDM was invested in 1960’s, only recently it has recognized as an

excellent method for bi-directional wireless data communication. It is

extremely efficient in mitigating common problems in high-speed

communication such as multipath fading and RF noise interference. It can be

considered as multiple access technique OFDMA.


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Wideband - OFDM