ofdm

International Journal of Advanced Computer Research (ISSN (print):2249-7277 ISSN (online):2277-7970)
Volume-3 Number-2 Issue-10 June-2013
67
Behaviour of OFDM System using MATLAB Simulation
S.S.Ghorpade1
, S.V.Sankpal2
Abstract
Orthogonal Frequency Division Multiplexing
(OFDM) is mainly designed to combat the
effect of multipath reception, by dividing the
wideband frequency selective fading channel into
many narrow flat sub-channels. OFDM offers
flexibility in adaptation to time varying channel
condition by adopting the parameters at each
subcarrier accurately. To avoid ISI due to
multipath, successive OFDM symbols are separated
by guard band. This makes the OFDM
system resistant to multi-path effects [1].The idea of
using parallel data transmission by FDM was
published in mid 60s [2].However, recently the
attention toward OFDM has grown rapidly in the
field of wireless and wired communication systems.
This is reflected by the adoption of this technique
in applications such as digital audio/video
broadcast, wireless LAN (802.11a and
HiperLAN2), broadband wireless (802.16) and
xDSL [3]. In this paper design of OFDM system
transmitter and receiver is introduced and
Simulation is done using MATLAB.
Keywords
FFT, IFFT, OFDM, QAM
1. Introduction
Orthogonal Frequency Division Multiplexing is a
special form of multicarrier modulation which is
particularly suited for transmission over a dispersive
channel.. Here the different carriers are orthogonal to
each other, that is, they are totally independent of one
another. Orthogonal Frequency Division
Multiplexing (OFDM) is a wideband modulation
scheme that is designed to cope with the problems of
the multipath reception. Essentially, the wideband
frequency selective fading channel is divided into
many narrow-band sub channels. If the number of
sub channels is high enough, each sub channel could
be considered as flat.
S.S.Ghorpade, E&TC, DYP College, Kolhapur, India.
S.V.Sankpal, E&TC, DYP College, Kolhapur, India.
This is because we transmit many narrowband
overlapping digital signals in parallel, inside one
wide band.
Figure 1: A) Spectrum of FDM showing guard
Bands
B) Spectrum of OFDM showing overlapping
subcarrier
Increasing the number of parallel transmission
channels reduces the data rate that each individual
carrier must convey, and that lengthens the symbol
period. Therefore the delay time of reflected waves is
suppressed to within 1 symbol time. Fig.1 compares
the utilization of FDM and OFDM.
The development of OFDM systems can be divided
into three parts. They are Frequency Division
Multiplexing, Multicarrier Communication and
Orthogonal Frequency Division Multiplexing [4].
Frequency Division Multiplexing is a form of signal
multiplexing which involves assigning non
overlapping frequency ranges or channels to different
signals or to each user of a medium. A gap or guard
band is left between each of these channels to ensure
that the signal of one channel does not overlap with
the signal from an adjacent one. Multicarrier
Communication involves splitting of the signal to
give a number of signals over that frequency range.
Each of these signals are individually modulated and
transmitted over the channel. At the receiver end,

68
these signals are fed to a demultiplexer where it is
demodulated and recombined to obtain the original
signal.
2. Implementation
As shown in Fig.2, we have to implement the OFDM
System below.
Figure 2: OFDM Block Diagram
Modulation
Modulation is the technique by which the signal
wave is transformed in order to send it over the
communication channel in order to minimize the
effect of noise. This is done in order to ensure that
the received data can be demodulated to give back
the original data. In an OFDM system, the high data
rate information is divided into small packets of data
which are placed orthogonal to each other. This is
achieved by modulating the data by a desirable
modulation technique like Quadrature Amplitude
Modulation [7]. After this, IFFT is performed on the
modulated signal which is further processed by
passing through a parallel to serial converter. Guard
Interval Insertion (GII) is done in order to avoid ISI.
Communication Channel
This is the channel through which the data is
transferred. Presence of noise in this medium affects
the signal and causes distortion in its data content.
Demodulation
Demodulation is the technique by which the original
data is recovered from the modulated signal which is
received at the receiver end. In this case, the received
data is first made to pass through a low pass filter and
the Guard Interval Removal (GIR ) is done. FFT of
the signal is done after it is made to pass through a
serial to parallel converter. A demodulator is used, to
get back the original signal. The bit error rate and the
signal to noise ratio is calculated by taking into
consideration the unmodulated signal data and the
data at the receiving end.
3. Simulation
Figure 3: OFDM Simulation Flowchart
OFDM Simulation Process
Simulation flowchart is shown in fig.3. MATLAB
code for 256 bits processing is given below.
The transmitter first converts the input data from a
serial stream to parallel sets. Each set of data contains
one symbol, Si, for each subcarrier. For example, a set
of four data would be [S0 S1 S2 S3].Before performing
the Inverse Fast Fourier Transform (IFFT), this
example data set is arranged on the horizontal axis in
the frequency domain as shown in Fig:4. This
symmetrical arrangement about the vertical axis is
necessary for using the IFFT to manipulate this data.
Figure 4: Frequency Domain Distribution of
Symbol

69
An inverse Fourier transform converts the frequency
domain data set into samples of the corresponding
time domain representation of this data. Specifically,
the IFFT is useful for OFDM because it generates
samples of a waveform with orthogonal frequency
components. Then, the parallel to serial block creates
the OFDM signal by sequentially outputting the time
domain samples.
The channel simulation will allow examination of the
effects of noise, multipath, and clipping. By adding
random data to the transmitted signal, simple noise
can be simulated. Multipath simulation involves
adding attenuated and delayed copies of the
transmitted signal to the original. This simulates the
problem in wireless communication when the signal
propagates on many paths. For example, a receiver
may see a signal via a direct path as well as a path
that bounces off a building. Finally, clipping
simulates the problem of amplifier saturation. This
addresses a practical implementation problem in
OFDM where the peak to average power ratio is
high. The receiver performs the inverse of the
transmitter. First, the OFDM data are split from a
serial stream into parallel sets. The Fast Fourier
Transform (FFT) converts the time domain samples
back into a frequency domain representation. The
magnitudes of the frequency components correspond
to the original data. Finally, the parallel to serial
block converts this parallel data into a serial stream to
recover the original input data.
MATLAB Code
%code for OFDM signal transmission and reception
in AWGN channel
% code
n = 256; % Number of bits to process
x = randint(n,1); % Random binary data stream
M = 16; % Size of signal constellation
k = log2(M); % Number of bits per symbol
xsym = bi2de(reshape(x,k,length(x)/k).','left-msb');
% Convert the bits in x into k-bit symbols.
y = modulate(modem.qammod(M),xsym);
% Modulate using QAM
tu=3.2e-6;%useful symbol period
tg=0.8e-6;%guard interval length
ts=tu+tg;%total symbol duration
nmin=0;
nmax=64;%total number of subcarriers
scb=312.5e3;%sub carrier spacing
fc=3.6e9;%carrier frequency
Rs=fc;
tt=0: 6.2500e-008:ts-6.2500e-008;
c=ifft(y,nmax);%IFFT
s=real(c'.*(exp(1j*2*pi*fc*tt)));%bandpass
modulation
figure;
plot(real(s),'b');title('OFDM signal transmitted');
figure;
plot(10*log10(abs(fft(s,nmax))));title('OFDM
spectrum');
xlabel('frequency')
ylabel('power spectral density')
title('Transmit spectrum OFDM');
snr=10;%signal to noise ratio
ynoisy = awgn(s,snr,'measured');%awgn channel
figure;
plot(real(ynoisy),'b');title('received OFDM signal
with noise');
z=ynoisy.*exp(j*2*pi*fc*tt);%Bandpass
demodulation
z=fft(z,nmax);%FFT
zsym=demodulate(modem.qamdemod(M),z);%demo
dulation of bandpass data.
z = de2bi(zsym,'left-msb'); %Convert integers to bits.
z = reshape(z.',prod(size(z)),1);%matrix to vector
conversion
[noe,ber] = biterr(x,z) ;%BER calculation figure;
subplot(211);stem(x(1:256));title('Original
Message');
subplot(212);stem(z(1:256));title('recovered
Message');
4. Results
Simulation results for MATLAB code are shown in
fig.5, 6, 7, 8.
Figure 5: Transmit spectrum of OFDM

70
Fig 6: OFDM Signal Transmitted
Fig 7: Received OFDM Signal with Noise
Figure 8: Original message and Recovered
Message
5. Conclusion
The main aim of the project is to implement the core
signal processing blocks of OFDM system. These
blocks are simulated using MATLAB. Design Suite
is tested for data patterns and the results are as
shown. The results are matching with expected
results. In this project OFDM system is simulated
using 64 subcarriers. This is very basic
implementation and has advantage of less processing
time requirement and complexity. The spectral
efficiency can be increased by increasing the number
of subcarriers. The problem of Peak-to-average ratio
can be reduced by using power amplifier with wide
linear range at the front end of transmitter. Some
other methods like Clipping, Peak Cancellation can
be used. Synchronisation can be achieved by using
Cyclic extension and Training Sequences.
Acknowledgment
I wish to thank the referee for the careful reading of
the paper and giving the valuable suggestions.
References
[1] Masoud Olfat, “Spatial Processing ,Power
Control, and Channel Allocation for OFDM
Wireless Communications”.-Ph,d Report , 2003.
[2] B. Salzberg,“Performance of an efficient parallel
data transmission system”. IEEE Trans.
Commun Technol., vol. COM-15, pp.805-813,
Dec 1967.
[3] Chide, Nilesh, Shreyas Deshmukh, and P. B.
Borole. "Implementation of OFDM System using
IFFT and FFT." International Journal of
Engineering Research and Applications (IJERA)
3, no. 1 (2013): 2009-2014.
[4] H. Taub, D. L. Schilling, G. Saha,
“Taub’sPrinciples of Communication Systems”.
Tata McGraw Hill, 2008.
[5] S. Weinstein and P. Ebert, “Data transmission by
frequency-division multiplexing using the fourier
transform” ,IEEE Trans. Commun. Technol.,vol.
COM-19, pp. 628-634, Oct 1971.
[6] T.Pollet, M. Bladel, and
M.Moeneclaey,“Sensitivity of OFDM systems to
carrier frequency offset and wiener phase noise”,
IEEETransaction on Communications, vol. 43,
pp. 191-193, Apr 1995.
[7] Horosaki B., “An orthogonally multiplexed QAM
system using the DFT,” IEEE Trans. Comm.
Vol.COM-29, pp. 982-989, July 1981.

71
[8] P. Banelli and S. Cacopardi, “Theoretical
analysis and performance of OFDM signals
in channels,” IEEE Trans. Communication.,vol.
48, pp. 430–441, Mar. 2000.
S.S.Ghorpade, received her B.E.(I.E.)
degree from Shivaji University.She is
student of M.E.(E&TC) in DYP
College, Kolhapur.Her research interest
include OFDM.She published 1
international conference paper, 1
international journal paper.
Swati V.Sankpal is currently working
as associate professor at D. Y. Patil
college of Engineering and
Techonology, Kolhapur. She has total
28 years of experience. She has
completed her B.E .and M.E.in
Electronics from Walchand College of Engineering Sangli
and earned her Ph.D. from Shivaji University, Kolhapur.
She has published totally 28 national conference papers, 29
international conference papers, 25 international journal
papers.Under her guidance three students have completed
their P.G.(E.&.TC) and ,three students have submitted their
thesis and ten are pursuing.

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