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
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
Figure 5 cyclic extension of sinusoid
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Seminar Report ’03 Wideband - OFDM
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
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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.
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Seminar Report ’03 Wideband - OFDM
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
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Seminar Report ’03 Wideband - OFDM
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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