International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 10 - Apr 2014
PAPR Reduction Based on Clipping and Scaling in
SFBC OFDM Systems
Renjitha Mariam Rajan#1, Teena Rajan#2,
#
PG scholar, Assistant professor,Dep: of ECE,Kerala University
Mar Baselios College of Engineering and Technology
Trivandrum, Kerala, India
Abstract- Orthogonal frequency division multiplexing is a
common technique for higher data rate transmission over broad
band frequency selective channel with minimum intersymbol
interference due to delay spread of wireless channel. Special
diversity technique such as space frequency block codes is used to
achieve full diversity in transmission with high capacity. The
main drawback of SFBC –OFDM is high bit error rate and high
peak to average power ratio. This paper proposes a simple
technique based on clipping and differential scaling to reduce
high peak to average power ratio in OFDM-MIMO systems. The
complex OFDM signals amplitude is clipped and scaled in such a
way that the peak average power ratio is reduced without
causing much degradation in bit error rate.
Keywords— orthogonal frequency division multiplexing, multi
input multi out, peak average power ratio.
I. INTRODUCTION
OFDM is a multi carrier modulation scheme that transmits
a high rate data stream by dividing it into number of sub
carriers each carrying relatively low data rate. Multiple
antennas are employed at transmitter and receiver of a
communication system in order to exploit transmits and
receives diversity to compact fading of wireless channels [10].
Orthogonal frequency division multiplexing appears
promising technology for broad band systems due to its
immunity against delay spread. Due to the introduction of
orthogonal subcarriers, OFDM is especially suitable for
frequency-selective MIMO systems which make system
design and implementation as simple as those for flat-fading
channels[1]. In order to exploit the capacity of MIMO channel
space time coding is used, since it use both time and spatial
domain for coding data symbols. In STC diversity and spatial
multiplexing can be combined achieving robustness at the
receiver with higher data rate transmission[7]. Due to this
features STC have been incorporated in many of the last
generation and future communication link. By incorporating
STC with multi carrier modulation scheme such as orthogonal
frequency division multiplexing, space frequency block code
can be performed[8]. Code words are fed into adjacent carriers
of two consecutive OFDM symbols translated into time
domain and transmitted through several transmit antenna[10].
One of the main drawback related to OFDM is the high
peak to average power ratio (PAPR). PAPR cause distortion
and saturation in power amplifiers, resulting in inter symbol
interference. The main problem with the amplifiers is the
decrease in efficiency which is directly given by peak to
ISSN: 2231-5381
average power ratio[3]. As a result a high power amplifier
needs to be used. This paper discusses about a simple PAPR
reduction technique in SFBC OFDM based on clipping and
differential scaling. In this method the complex amplitudes are
clipped and confined in a certain amplitude range and then the
signal amplitudes are scaled so as to reduce the PAPR. The
clipping and scaling amplitudes can be determined by using
Monte Carlo method.
II. MIMO-OFDM
For communication over wireless channel orthogonal
frequency division multiplexing has become a popular
technique. Several wireless standards such as Digital video
broadcasting (DVB), DVB-T2, local area network,
metropolitan area network use OFDM technology. OFDM
converts a frequency selective channel into a parallel
collection of frequency flat sub channels. The sub carriers
have minimum frequency separation hence the various sub
carriers will overlap in frequency, thereby utilizing the
available bandwidth effectively. OFDM is a block modulation
scheme where a block of N information symbols is transmitted
in N parallel sub carriers. Time duration is N times larger than
that of single carrier system[2]. An inverse discrete Fourier
transform followed by an analog to digital converter will
constitute the OFDM modulator. Cyclic prefix is added with
each of the N IDFT coefficients to mitigate the effect of
intersymbol interface caused by channel delay spread. The
cyclic prefix is a copy of the past beginning of the current
symbol at the end, and ensures that the complete OFDM
symbol with the longer delay can also be received with the
appropriate DFT window shifting[9]. Since the OFDM output
is the superposition of multiple sub carriers some
instantaneous power output may increase greatly and become
much higher than the mean power of the system. In order to
reduce such high peak power amplifiers with high power
scope are required which are highly expensive.
Multiple antennas can be used a the transmitter and
receiver in order to exploit the channel capacity, such an
arrangement is called the multiple input multiple
output(MIMO) system. MIMO system will exploit spatial
diversity obtained by placing spatially separated antennas in
dense scattering environment. MIMO system cam be
implemented to obtain diversity gain in compact signal fading
or to obtain a capacity gain. Spatial multiplexing has been
generally used to increase the capacity by transmitting
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International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 10 - Apr 2014
multiple data streams in the same time slot and frequency
band from each transmit antenna, and differentiating each of
the signal at the receiver using the channel state information.
In spatial multiplexing, a high rate signal is split into multiple
lower rate streams and each stream is transmitted from a
different transmit antenna in the same frequency channel.
III. PEAK AVERAGE POWER RATIO IN SFBC
OFDM
One of the main drawback of the OFDM system is the peak
average power ratio. The peak value of some of the
transmitted signal is larger than the typical value. The two
possible combination of MIMO OFDM system is space time
block coded OFDM and space frequency block coded OFDM,
both suffer from high PAPR. In this paper we reduce PAPR in
SFBC OFDM based on clipping and scaling approach. In this
approach amplitude of complex OFDM signal is clipped and
scaled such that the PAPR is reduced without much
degradation in bit error rate during transmission [6]. The
amplitude of the clipped signals are scaled in different manner.
Three type of differential scaling techniques are there based
on the amplitude of the scaling signal [4]. The three technique
are scale up scale down and scale up down.
Scale up : In this method amplitude of the signal is scaled by a
factor of α. Which lead to the increase of the average value
without affecting the peak value thereby reducing the resulting
PAPR. The PAPR reduction function can be defined as:
h(x) = αxp, if x > αxp
= βx, if x < A
= x, if A ≤ x ≤ αxp.
Where xp is the amplitude peak value occurring in an OFDM
symbol block, is a factor deciding the clipping threshold in
terms of peak value and α is a scaling factor whose value is
greater than one and lies in the range [0 A].
Scale down: In this method amplitude of the signal is scaled
by a factor of γ which lead to decrease in peak value. The
average power value will fall down resulting in reduced peak
average power. The peak power reduction is higher than the
average peak power reduction. The PAPR reduction function
can be defined as:
h(x) = αxp, if x > αxp
= γx, if B ≤ x ≤ αxp
= x, if x < B
Where xp is the peak amplitude of the OFDM block signal,α
is the factor deciding the clipping threshold in terms of
percentage of peak value and γ is the scaling factor whose
value is less than one and lays in the range [β αxp]
Scale up down: This method combine advantage of both the
approaches, scale up and scale down. By this method PAPR
can be reduced considerably. The PAPR reduction function
can be defined as:
h(x) = αxp, if x > αxp
= γx, if B ≤ x ≤ αxp
= βx, if x < A = x, if A ≤ x ≤ B
Where xp is the peak value of the amplitude occurring from
the OFDM block,α is the factor deciding the clipping
ISSN: 2231-5381
threshold in terms of percentage of peak value and β is the
scaling factor whose value is greater then one and lays in the
range [0 A] and γ another scaling factor whose value is
between [β αxp] whose value is less than one.
In this paper PAPR reduction based on scale down
approach is done. PAPR can be described by its
complimentary cumulative distribution function, which
computes the power from a time domain signal. It will show
the probability that a signal is above the average power level
or the amount of time the signal is above the average time
level.
IV. RESULT
The performance of BER vs. SNR for normal OFDM is
compared with that of the SFBC-OFDM is compared using
MALTAB simulation.
From the above graph it is understood that the Bit Error Rate
(BER) value of a normal OFDM is higher than the Bit Error
rate of SFBC-OFDM. The red colour in the graph represents
the value of BER for a normal OFDM where as the blue
colour represents the value of BER in an SFBC-OFDM.
The CCDF vs PAPR plot shows that the clipping scheme
reduces the average power.
V.
CONCLUSION
The peak average power ratio is reduced in space
frequency block coded orthogonal frequency division
multiplexing system by using clipping and differential scaling
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International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 10 - Apr 2014
method. Reduced PAPR will also reduce the bit error
rate(BER) with no loss in data rate. With the rising demand
for efficient frequency spectrum utilization, OFDM proves as
a solution for next generation communication systems. Results
of simulation of clipping and scaling technique show that the
PAPR reduction of OFDM system, which further results in
high performance of wireless communication.
ACKNOWLEDGMENT
We express our heartfelt gratitude to all the staff members
of the Department of Electronics and Communication
Engineering for their profound encouragement and valuable
support for making this paper.We are also thankful to all those
who are directly or indirectly help us in making this paper
reality. Above all we praise and thank the Almighty for the
blessings He has showered throughout our work.
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