International Journal of Engineering Trends and Technology (IJETT) – Volume 15 Number 2 – Sep 2014
Split FFT Technique: A New Scheme for ICI
Reduction in OFDM System
Anurag Pandey, Sandeep Sharma
Department of Electronics and Communication Engineering, DIT University
Abstract: OFDM is being widely used in many
communication systems for its ability to enhance the
data rate and reduce the bandwidth. When the system is
placed in a multipath environment is undergoes
intercarrier interference. A new idea is proposed in the
paper to reduce the intercarrier interference produced
due to the multipath effects. The results are based on
the bit error rate performance of the system.
Simulations of the OFDM system is carried out in
rician fading channel to understand the effect of
channel fading and to obtain bit error rate for the
improved system.
Keywords: OFDM, BER, Rician fading channel, Split
FFT
I. Introduction
The ever remaining demand of enhanced data rate
along with increased reliability and reduced bandwidth
in a communication system requires robustness against
fast frequency selective multipath fading. Wireless
communication systems widely adopted OFDM
(Orthogonal Frequency Division Multiplexing) scheme
for its high data rate transmission along with high
bandwidth efficiency [1].
In real the transmission of data is much more complex
than it seems in the block diagrams. Various factors
influence the efficiency of the system like the density
of users, line of sight between the transmitter and
receiver, mutual movement between the transmitter and
receiver etc. To consider these factors different channel
models are developed like AWGN (Additive White
Gaussian Noise) channel, Rayleigh fading channel,
Rician fading channel etc [2]. Rician fading channel is
selected here to study the behavior of new proposed
scheme for ICI reduction in OFDM system.
The paper focuses upon the performance comparison of
simple OFDM system and OFDM system with new
FFT scheme both under rician fading channel. The rest
of the paper is organized as follows. Section II presents
a brief introduction of OFDM system. Section III gives
knowledge of rician fading channel in brief. Section IV
presents the specifications of IEEE 802.11a.
Simulation results and proposed scheme is presented in
section V. Section VI concludes the paper.
ISSN: 2231-5381
II. OFDM
OFDM [3] is a parallel transmission scheme, where a
high-rate serial data stream is split up into a set of lowrate sub-streams, each of which is modulated on a
separate sub-carrier. The bandwidth of the sub-carriers
becomes small compared with the coherence
bandwidth of the channel. The symbol period of the
sub-streams is made long compared to the delay spread
of the time-dispersive radio channel.
Selecting a special set of (orthogonal) carrier
frequencies high spectral efficiency can be obtained
because the spectra of the sub-carrier overlaps, while
mutual influence among the sub-carriers can be
avoided. By introducing a guard interval (Fig. 1), the
orthogonality can be maintained over a dispersive
channel. OFDM scheme is advantageous because of
high spectral efficiency, easy adaptability to severe
channel conditions without complex time domain
equalization, robustness against inter symbol
interference (ISI) and fading caused by multipath
propagation, efficient implementation using Fast
Fourier Transform (FFT), less sensitive to time
synchronization errors etc.
Fig.1 Effect of guard interval insertion
If T denotes the duration of OFDM symbol, fo is the
central frequency, Tg is the guard interval and Δf is the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 15 Number 2 – Sep 2014
subcarrier spacing then transmitted signal can be
expressed as
( )=
√
∑
where ∈ [−
OFDM systems as per IEEE 802.11a standard here are
considered for rician fading channel and given in Table
1.
…(1)
Parameter
Value
FFT Size
64
No. of Subcarriers
52
FFT Sampling
frequency
20 MHz
Subcarrier Spacing
312.5 KHz
Subcarrier Index
-26 to -1 and 1 to 26
Data Symbol Duration
3.2 µsec
Cyclic Prefix Duration
0.8 µsec
Total Symbol
Duration
4 µsec
Modulation Schemes
BPSK, QPSK, 16-QAM, 64QAM
, ]
is the yth subcarrier frequency and is given by
= + ( − 1)∆
…(2)
where fo is the first carrier frequency
if
The signal at the receiver can be modeled as
(
∑
( )=
∑
ℎ ()
( ))
+
( )
…(3)
where hx(t) and τx(t) are the time varying gain and
delay of xth path.
√
III. Rician Fading Channel
In a multipath channel the transmitted signal reaches
the receiver as a train of impulses. In rician fading
channel there is a direct line of sight (LOS)
propagation path [2]. The multipath signals arriving at
different angles are added to the stationary (LOS)
signal which is similar to the addition of a DC
component in the multipath signals. Rayleigh fading is
the specialized model for stochastic fading when there
is no line of sight signal, and is sometimes considered
as a special case of the more generalized concept of
Rician fading.
AWGN channel model is widely used in studying
OFDM. In this model there is only linear addition of
white noise with a constant spectral density and
Gaussian distribution of amplitude but the model does
not consider fading, frequency selectivity, interference
etc.
A comparative study of the BER responses of OFDM
systems under the influence of AWGN and Rician
faded systems can be made using the graphs presented
in [4].
Since a multipath channel reflects signals at multiple
places, a transmitted signal travels to the receiver
through several paths that may have different lengths
and hence different associated time delays. Fading
occurs when these signals travelling through different
paths interfere with each other.
IV. IEEE 802.11a System Specifications
The IEEE 802.11a standard specifies an OFDM
physical layer that splits an information signal across
52 separate subcarriers. 4 subcarriers are pilot
subcarriers and remaining 48 subcarriers provide
separate wireless pathways for sending the information
in a parallel manner. The resulting subcarrier spacing is
0.3125 MHz (for a 20 MHz bandwidth with 64
possible frequency slots). The basic parameters for
ISSN: 2231-5381
Table 1. OFDM System Specification
V. Proposed Scheme and Simulation Results
The OFDM system is developed, simulated and
analyzed in MATLAB version 7. Bit Error Rate is
calculated for different modulation schemes under
Rician Fading channel model.
A variety of ICI cancellation algorithms and techniques
has been presented so far [5-8]. The proposed idea is to
first split the received signal that is going for FFT, into
two parts. After this each part is Fourier transformed
and then the outputs are combined together. Consider a
system with ‘n’ number of signals waiting for Fourier
transform at the receiver. These ‘n’ signals are split
into two equal parts of length ‘n/2’. Each signal is
passed through Fourier transform. It will result in two
sequences each of length ‘n’. These signals will be then
added to produce the final transformed signal. After the
demodulation of the added signal the received bits are
compared with the transmitted bits. The error ratios are
presented in the following figures.
The rician channel used is a static rician channel with
default Jake’s spectrum.
The symbol to noise ratio can be calculated using
. =
+ 10
+
. 10
(
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)
…(4)
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International Journal of Engineering Trends and Technology (IJETT) – Volume 15 Number 2 – Sep 2014
The BER performance of BPSK, QPSK and 16 & 64
QAM for OFDM system using rician channel is shown
in Fig.3
From fig.4 to fig.7 it can be observed that the split FFT
technique improves the BER of the system
Fig.8 shows a comparative evaluation of the new
technique for all four modulation techniques in normal
and in the faded environment..
Fig.5 Performance improvement in QPSK system
Fig.3 BER comparison of different modulation
techniques under rician channel
Fig.4 Performance improvement in BPSK system
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Fig.6 Performance improvement in 16-QAM system
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International Journal of Engineering Trends and Technology (IJETT) – Volume 15 Number 2 – Sep 2014
VI. Conclusion
The extent of BER improvement is different for
different modulated systems yet the efficiency is good
for BPSK modulated system. The result presented
clearly shows that the method is quite efficient in
reducing the error in the received bits. The improved
BER curves show that the carrier interference is
reduced along with the reduction in complexity in the
presented modulated OFDM systems.
References:
[1] Theory and Applications of OFDM and CDMA by Henrik
Schulze and Christian Luders, John Wiley & Sons Ltd
[2] Wireless Communications: Principles and Practices By T S
Rappaport, Printice Hall
[3] OFDM for Wireless Communications Systems by Ramjee Prasad,
Artech House Publication
[4] Anurag Pandey and Sandeep Sharma, “ICI Reduction in OFDM
Systems: A Review”, Advances in Wireless and Mobile
Communications, Vol. 7, No. 1, 2014
Fig.7 Performance improvement in 64-QAM system
[5] Anurag Pandey and Sandeep Sharma,” BER Performance of
OFDM System in AWGN and Rayleigh Fading Channel”,
International Journal of Engineering Trends and Technology
(IJETT), Vol. 36, No. 15, 2000
[6] Srinivas Yerramalli, Milica Stojanovic, and Urbashi Mitra,
“Partial FFT Demodulation: A Detection Method for Highly Doppler
Distorted OFDM Systems,” IEEE transactions On Signal
Processing, Vol. 60, No. 11, November 2012
[7] A. Ghassemi, T.A. Gulliver, “Decimation-in-Time FFT
Subblocking for Partial Transmit Sequence OFDM”, Vehicular
Technology Conference, 2007.
[8] Chao-Ming Chen, Yuan-Hao Huang, “Partial cached-FFT
algorithm for OFDMA communications”, TENCON 2007 IEEE
Region 10th Conference, 2007.
Fig.8 Performance comparison of all systems
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