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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 1960â„¢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.
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 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
usersâ„¢ 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 consumerâ„¢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 usersâ„¢ 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. wca.org
6. magisnetworksofdm.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
