International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 9- Sep 2013
Multiple Carrier Modulation Technique OFDM for
High Data Rate Next Generation Wireless Systems
1
Vikas#1, Satnam Singh*2
M. Tech Scholar, Department of ECE, SSCET, Badhani, Pathankot, Punjab, India
2
AP, Department of ECE, SSCET, Badhani, Pathankot, Punjab, India
Abstract— Orthogonal frequency division multiplexing (OFDM)
has been an effective technique to combat multipath fading in
wireless communications. It has been successfully used for HF
radio applications and has been chosen as the standard for
digital audio broadcasting and digital terrestrial TV
broadcasting in Europe and high-speed wireless local areas
networks. In this paper an overview of a multiple carrier
modulation technique known as OFDM (Orthogonal Frequency
Division Multiplex) is given and discusses the problems, and
some of the potential solutions, in implementing an OFDM
system.
Keywords— OFDM, BER, WLAN, DAB.
I. INTRODUCTION
Orthogonal frequency division multiplexing (OFDM) is a
multi-carrier transmission technique that has been recently
recognized as an excellent method for high speed bidirectional wired/wireless data communication. High capacity
and variable bit rate information transmission with high
bandwidth efficiency are just some of the requirements that
the modern transceivers have to meet in order for a variety of
new high quality services to be delivered to the customers.
Because in the wireless environment signals are usually
impaired by fading and multipath delay spread phenomenon,
traditional single carrier mobile communication systems do
not perform well. In such channels, extreme fading of the
signal amplitude occurs and Inter Symbol Interference (ISI)
due to the frequency selectivity of the channel appears at the
receiver side. This leads to a high probability of errors and the
system’s overall performance becomes very poor.
frequencies of the carriers in the system. In a normal
frequency-division multiplex system, many carriers are spaced
apart in such a way that the signals can be received using
conventional filters and demodulators. In such receivers,
guard bands are introduced between the different carriers and
in the frequency domain, which results in a lowering of
spectrum efficiency.
It is possible, however, to arrange the carriers in an
OFDM signal so that the sidebands of the individual carriers
overlap and the signals are still received without adjacent
carrier interference. To do this, the carriers must be
mathematically orthogonal. The receiver acts as a bank of
demodulators, translating each carrier down to DC, with the
resulting signal integrated over a symbol period to recover the
raw data. If the other carriers all beat down the frequencies
that, in the time domain, have a whole number of cycles in the
symbol period T, then the integration process results in zero
contribution from all these other carriers. Thus, the carriers
are linearly independent (i.e., orthogonal) if the carrier
spacing is a multiple of 1/T. 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, although their spectra overlap. The separation
between carriers is theoretically minimal so there would be a
very compact spectral utilization.
Techniques like channel coding and adaptive
equalization have been widely used as a solution to these
problems. However, due to the inherent delay in the coding
and equalization process and high cost of the hardware, it is
quite difficult to use these techniques in systems operating at
high bit rates, for example, up to several M bps. An alternative
solution is to use a multi carrier system. Orthogonal
Frequency Division Multiplexing (OFDM) is an example of it
[1].
OFDM is a technique for transmitting data in parallel
by using a large number of modulated sub-carriers. These subcarriers (or sub-channels) divide the available bandwidth and
are sufficiently separated in frequency (frequency spacing) so
that they are orthogonal. The word orthogonal indicates that
there is a precise mathematical relationship between the
ISSN: 2231-5381
Fig.1: Spectra of (a) an OFDM sub-channel and (b) and OFDM signal.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 9- Sep 2013
OFDM systems are attractive for the way they handle
ISI, which is usually introduced by frequency selective multipath fading in a wireless environment. Each sub-carrier is
modulated at a very low symbol rate, making the symbols
much longer than the channel impulse response. In this way,
ISI is diminished. Moreover, if a guard interval between
consecutive OFDM symbols is inserted, the effects of ISI can
completely vanish. This guard interval must be longer than the
multipath delay. Although each sub-carrier operates at a low
data rate, a total high data rate can be achieved by using a
large number of sub-carriers. ISI has very small or no affect
on the OFDM systems hence an equalizer is not needed at the
receiver side.
III. DRAWBACKS OF OFDM
There are two main drawbacks:
(1) The large dynamic range of the signal, also known as
the peak-to-average-power ratio (PAPR). Solutions
to deal with this problem have been (and still are)
developed and one of the most used ones is clipping.
(2) Sensitivity to frequency errors.
Most research centres throughout the world are mainly
focusing their work on these two topics in their attempt to
optimize OFDM.
IV. OFDM GENERATION
II. HISTORICAL BACKGROUND
From an historical point of view, the theoretical bases for
the development of OFDM systems were laid out by
R.W.Chang,1966 [2], where the orthogonality conditions for
the perfect recovery of the transmitted signals were derived,
while the possibility of efficiently realizing the multicarrier
modulators through DFT processors was shown as early as
1970, in [3,4] . Nevertheless it was not until the late 1980s
that, thanks to the advances in Digital Signal Processing
technology that the implementation of OFDM systems began
to appear as feasible [5, 6]. In the last few years, the filter less
OFDM system with cyclic prefix is chosen, as the modulation
format for several digital communication systems throughout
the world.
In USA, it was adopted in the ADSL and HDSL standards
for high bit rate data transmission over the twisted pair [7,8],
whereas in Europe it is used by the standards for the digital
terrestrial radio broadcasting of sounds (DAB) [9, 10] and
television, first with the dTTb project [11,12,13] and later
with the DVB-T standard [14].
In the OFDM system, Inverse Fast Fourier Transform/Fast
Fourier Transform (IFFT /FFT) algorithms are used in the
modulation and demodulation of the signal. The length of the
IFFT/FFT vector determines the resistance of the system to
errors caused by the multipath channel. The time span of this
vector is chosen so that it is much larger than the maximum
delay time of echoes in the received multipath signal.
OFDM is generated by firstly choosing the spectrum required,
based on the input data, and modulation scheme used. Each
carrier to be produced is assigned some data to transmit. The
required amplitude and phase of the carrier is then calculated
based on the modulation scheme (typically differential BPSK,
QPSK, or QAM). Then, the IFFT converts this spectrum into
a time domain signal. The FFT transforms a cyclic time
domain signal into its equivalent frequency spectrum. The
amplitude and phase of the sinusoidal components represent
the frequency spectrum of the time domain signal.
ISSN: 2231-5381
To generate OFDM successfully the relationship between
all the carriers must be carefully controlled to maintain the
orthogonality of the carriers. For this reason, OFDM is
generated by firstly choosing the spectrum required, based on
the input data, and modulation scheme used. Each carrier to be
produced is assigned some data to transmit. The required
amplitude and phase of the carrier is then calculated based on
the modulation scheme (typically differential BPSK, QPSK,
or QAM). The required spectrum is then converted back to its
time domain signal using an Inverse Fourier Transform. The
IFFT performs the transformation very efficiently, and
provides a simple way to ensuring that the carrier signals
produced are orthogonal.
The Fast Fourier Transform (FFT) transforms a cyclic time
domain signal into its equivalent frequency spectrum. This is
done by finding the equivalent waveform, generated by a sum
of orthogonal sinusoidal components. The amplitude and
phase of the sinusoidal components represent the frequency
spectrum of the time domain signal.
The IFFT performs the reverse process, transforming a
spectrum (amplitude and phase of each component) into a
time domain signal. An IFFT converts a number of complex
data points, of length which is a power of 2, into the time
domain signal of the same number of points. Each data point
in frequency spectrum used for an FFT or IFFT is called a bin
[15].
The orthogonal carriers required for the OFDM signal can
be easily generated by setting the amplitude and phase of each
bin, then performing the IFFT. Since each bin of an IFFT
corresponds to the amplitude and phase of a set of orthogonal
sinusoids, the reverse process guarantees that the carriers
generated are orthogonal.
.A block diagram showing a simplified configuration for an
OFDM transmitter and receiver is given in Figure 2.2.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 4 Issue 9- Sep 2013
V. CONCLUSIONS
OFDM allows for a high spectral efficiency as the carrier
power and modulation scheme can be individually controlled
for each carrier. Therefore OFDM can be considered as
promising technique to reduce the effects of multipath fading
in wireless communications. There is a wide scope of OFDM
in data transmission because it is one of the best way of
transmitting Multipath signal by controlling the effect of
channel fading.
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Fig. 2 Basic OFDM system (a) Transmitter (b) receiver
The OFDM signal generated by the system in Figure
2 is at base-band; in order to generate a radio frequency (RF)
signal at the desired transmit frequency filtering and mixing is
required.
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