DESIGN AND IMPLEMENTATION OF OFDM SYSTEM AND REDUCTION OF INTER-CARRIER INTERFERENCE

International Journal of Technical Research and Applications e-ISSN: 2320-8163,
www.ijtra.com Volume 2, Issue 4 (July-Aug 2014), PP. 173-175
173 | P a g e
DESIGN AND IMPLEMENTATION OF OFDM
SYSTEM AND REDUCTION OF INTER-CARRIER
INTERFERENCE
Gaurav Verma1, Navneet Singh2
1Research Scholar. JCDMCOE, Sirsa
2Assistance Professor, JCDMCOE, Sirsa
gaurav.vermamec@gmail.com
Abstract- Orthogonal frequency-division multiplexing (OFDM)
[1] is a method of encoding digital data on multiple carrier
frequencies. OFDM[1] has developed into a popular scheme
for wideband digital communication, whether wireless or
over copper wires, used in applications such as digital television
and audio broadcasting, DSL Internet access, wireless networks,
powerline networks, and 4G mobile communications. In the
Several wireless standards such as IEEE 802.11a[2] and
HiperLAN2[3].The orthogonality of the subcarriers is no longer
maintained which results in ICI (Inter carrier Interference)[4]
.ICI reduction techniques achieve a better SNR and BER in
OFDM at zero phase noise variance . This technique will use a
large number of closely spaced orthogonal subcarriers to avoid
phase noise. It provides high data rates with sufficient robustness
to radio channel damages. A major problem in OFDM is carrier
frequency offset error between the transmitted and received
signals. Due to this the orthogonality of the subcarriers is no
longer maintained which results in ICI (Inter carrier
Interference). In this paper, we used the ICI self-cancellation
technique and reduced the ICI and improved the BER and SNR
we are also calculate the SNR=15db and 20db at different phase
noise variance.
Index Terms- Inerter Carrier Interference, multicarrier
frequency, self-cancellation, Phase noise.
I. INTRODUCTION
OFDM[1] is a frequency-division multiplexing (FDM) scheme
used as a digital multi-carrier modulation method. A large
number of closely spaced orthogonal sub-carrier signals are
used to carry data on several parallel data streams or channels.
Each sub-carrier is modulated with a conventional modulation
scheme (such as quadrature amplitude modulation or phase-
shift keying) at a low symbol rate, maintaining total data rates
similar to conventional single-carrier modulation schemes in
the same bandwidth.
The primary advantage of OFDM over single-carrier schemes
is its ability to cope with severe channel conditions (for
example,attenuation of high frequencies in a long copper wire,
narrowband interference and frequency-selective fading due
to multipath) without complex equalization filters.
Channel equalization is simplified because OFDM may be
viewed as using many slowly modulated narrow band signals
rather than one rapidly modulated wideband signal. The low
symbol rate makes the use of a guard interval between
symbols affordable, making it possible to eliminate inter
symbol interference (ISI) and utilize echoes and time-
spreading (on analogue TV these are visible as ghosting and
blurring, respectively) to achieve a diversity gain, i.e. a signal-
to-noise ratio improvement. This mechanism also facilitates
the design of single frequency networks (SFNs), where several
adjacent transmitters send the same signal simultaneously at
the same frequency, as the signals from multiple distant
transmitters may be combined constructively, rather than
interfering as would typically occur in a traditional single-
carrier system.OFDM is a special form of multicarrier
modulation technique which is used to generate waveforms
that are mutually orthogonal and then distributes the data over
a large number of carriers that are spaced apart at precise
frequencies. This spacing provides the "orthogonality" in this
technique which prevents the demodulators from seeing
frequencies other than their own. In an OFDM scheme, a large
number of orthogonal, overlapping, narrow band subcarriers
are transmitted in parallel. These carriers divide thea vailable
transmission bandwidth. The separation of the subcarriers is
such that there is a very compact spectral utilization. With
OFDM, it is possible to have overlapping sub channels in the
frequency domain (Figure 1), thus increasing the transmission
rate.
Fig.1:- power spectrum of the transmitted signal
In order to avoid a large number of modulators and filters at
the transmitter and complementary filters and demodulators at
the receiver, it is desirable to be able to use modern digital
signal processing techniques, such as fast Fourier transform
(FFT). OFDM is a promising candidate for achieving high
data rates in mobile environment because of its multicarrier
modulation technique and ability to convert a frequency
selective fading channel into several nearly flat fading
channels. This technology has been chosen as the transmission
method of many standards, such as Digital Subscriber Line
(DSL), European Digital Audio and Video Broadcasting
terrestrial (DAB/DVB-T), European HIPERLAN/2 and IEEE
802.11 a/g for wireless local area networks (WLAN),
Worldwide Interoperability for Microwave Access (WiMAX),
etc. However, OFDM systems exhibit a sensitivity to phase
noise higher than single carrier modulations due to its long
symbol period. Because carriers are kept very close to each

174 | P a g e
other, OFDM is very sensitive to distortion that may remove
the orthogonality between carriers.
Phase noise can cause several types of signal degradation that
are usually very difficult to quantify analytically. When the
modulation experiences phase noise, it encounters two
problems:
a) A common phase rotation over all the carrier
frequencies which rotate the entire signal space for a
given OFDM symbol
b) Inter-carrier interference due to the loss of
orthogonality between subcarriers.
Especially, the ICI seriously degrades system predominance
because it may break down the orthogonality between
subcarriers.
II. ORTHOGONAL FREQUENCY DIVISION
MULTIPLEXING
Orthogonal Frequency Division Multiplexing [5] is
a technology related to Frequency Division Multiplexing.
With it, many different signalscan be sent over the same
medium, at the same time. Each signal uses a different basis
function. By using the basis function given, the sender and
recipient will then see their signal better, the other signals will
be clearly separated.
Fig.2:-An example o OFDM with 4 different signal shown
in different colour
I. Orthogonality
If two signals are said to be orthogonal then their dot product
is zero. As the subcarriers are orthogonal then the spectrum of
each subcarrier has a null at the center frequency of the other
subcarriers in the system. It is as shown in the figure.2
II. OFDM Generation And Reception
Fig 3 TRANSMITTER
An OFDM carrier signal is the sum of a number of orthogonal
sub-carriers, with baseband data on each sub-carrier being
independently modulated commonly using some type
ofquadrature amplitude modulation (QAM) or phase-shift
keying (PSK). This composite baseband signal is typically
used to modulate a main RF carrier. is a serial stream of
binary digits. By inverse multiplexing, these are first
demultiplexed into parallel streams, and each one mapped
to a (possibly complex) symbol stream using some modulation
constellation (QAM, PSK, etc.). Note that the constellations
may be different, so some streams may carry a higher bit-rate
than others.
An inverse FFT is computed on each set of symbols, giving a
set of complex time-domain samples. These samples are
then quadrature-mixed to passband in the standard way. The
real and imaginary components are first converted to the
analogue domain using digital-to-analogue
converters (DACs); the analogue signals are then used to
modulatecosine and sine waves at the carrier frequency, ,
respectively. These signals are then summed to give the
transmission signal, .
Fig: 4 RECIVER
The receiver picks up the signal , which is then quadrature-
mixed down to baseband using cosine and sine waves at the
carrier frequency. This also creates signals centered on , so
low-pass filters are used to reject these. The baseband signals
are then sampled and digitised using analog-to-digital
converters (ADCs), and a forward FFT is used to convert back
to the frequency domain.

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This returns parallel streams, each of which is converted to
a binary stream using an appropriate symbol detector. These
streams are then re-combined into a serial stream, , which
is an estimate of the original binary stream at the transmitter.
III. ICI SELF CANCELLATION METHODS
It is seen that the difference between the ICI co-efficient of the
two consecutive sub-carriers is very small. This makes the
basis of ICI self cancellation. Here one data symbol is not
modulated in to one sub-carrier, rather at least in to two
consecutive sub-carriers. If the data symbol =a‘ is modulated
in to the 1st sub-carrier then =-a‘ is modulated in to the 2nd
sub-carrier. Hence the ICI generated between the two sub-
carriers almost mutually cancels each other. This method is
suitable for multipath fading channels as here no channel
estimation is required.
IV. CONCLUSION
In this paper we have calculated the BER and SNR at
different phase noise variance from zero to 10 and we have
seen that when the phase noise variance is increased BER and
SNR performance is decreased as well as we calculated the
BER at SNR=15 db and 20 db. The improved graph as bellow.
Fig 5:- Best BER performance with ICI cancellation at
zero phase noise variance
Fig 6: BER at different phase noise variance at SNR=15db
and 20db
REFERENCES
[1] M.M. Islam, D. Zhang, G. Lu. A geometric method to compute
directionality features for texture images. In proceeding of:
Proceedings of the 2008 IEEE International Conference on
Multimedia and Expo (ICME), 2008.
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DESIGN AND IMPLEMENTATION OF OFDM SYSTEM AND REDUCTION OF INTER-CARRIER INTERFERENCE