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International Journal of Electronics,
Communication & Instrumentation Engineering
Research and Development (IJECIERD)
ISSN 2249-684X
Vol. 2 Issue 4 Dec 2012 71-80
© TJPRC Pvt. Ltd.,
PAPR REDUCTION IN OFDM BY CLIPPING TECHNIQUE
1PRASHANT MARUTI JADHAV,
2L.S.ADMUTHE &
3A.P.BHADVANKAR
1Department of Electronics TEI, Rajwada Ichalkaranji, Maharashtra, India
2Department of Electronics TEI, Rajwada Ichalkaranji, Maharashtra, India
3Department of Electronics & TC Sharad Institute of technology, Yadrav, Maharastra, India
ABSTRACT
Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication
applications. . Long term evolution (LTE) is the last step toward the 4th generation (4G) of radio technologies designed to
increase the capacity and speed of mobile telephone networks. LTE has adopted orthogonal frequency division
multiplexing (OFDM) for the downlink transmission. OFDM meets the LTE requirement for spectrum flexibility and
enables cost-efficient solutions for very wide carriers with high peak rates. Potentially large peak-to-average power ratio
(PAPR) of the transmitting signals has limited its application. This high PAPR causes interference when the OFDM signals
are passed through an amplifier which does not have enough linear range. OFDM signal with a large peak-to-mean
envelope power ratio, result in significant distortion when passed through a nonlinear device such as a transmitter power
amplifier. We investigate, through extensive computer simulations, the effects of clipping and filtering on the performance
of OFDM, including the power spectral density, the crest factor, and the bit-error rate. Our results show that clipping and
filtering is a promising technique for the transmission of OFDM signals using realistic linear amplifiers. One such
algorithm is the iterative clipping and filtering (ICF). Also technique of interleaving is discussed and simulated.
Simulation results confirm that the using QPSK modulation we get maximum PAPR reduction while IFFT size is 32 for
512 data bits. It’s up to: 17.384791 dB. Simulation results show that the proposed scheme may obtain significant PAPR
reduction while the BER of performance of system is improved at the same clipping parameter. ICF is a widely used
technique to reduce the PAPR of OFDM signals. In this paper different modulation techniques like 16-QAM, QPSK and
BPSK are simulated for clipping techniques in two ways.
KEYWORDS: Orthogonal Frequency Division Multiplexing (OFDM), Peak-to-Average power ratio (PAPR), Peak
Envelope Power (PEP), Selective Level Mapping (SLM)
INTRODUCTION
ORTHOGONAL frequency division multiplexing (OFDM) is a very attractive technique for the transmission of
high-bit-rate data in a radio environment [4]. The further increasing demand on high data rates in wireless communications
systems has arisen in order to support broadband services. Long term evolution (LTE) is standardized by the third
generation partnership project (3GPP) and is an evolution to existing 3G technologies in order to meet projected customer
needs over the next decades. Current working assumptions in 3GPP LTE are to use orthogonal frequency division
multiplexing access (OFDMA) for downlink and single carrier-frequency division multiple access (SC-FDMA) for uplink.
[1], [2], [3]. However, any multicarrier signal with a large number of sub channels is burdened with a large crest factor
(CF peak voltage/rout mean square (rms) voltage). When passed through a nonlinear device, such as a transmitter power
amplifier, the signal may suffer significant spectral spreading and in-band distortion. The conventional solutions to this
problem are to use a linear amplifier or to back off the operating point of a nonlinear amplifier; both approaches resulting
in a significant power efficiency penalty. Moreover, the low data rate on the subcarriers mitigates the effect of the
multipath problem in wireless environment. Advantage of OFDM system is that it can be implemented via (inverse) fast
Fourier transform ((I)FFT) [10] which makes it fast and efficient. Unfortunately, due to the superposition of a large number
72 Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar
of individual sub channels, the amplitude of the transmitted OFDM signal generally suffers from high peak-to-average
power ratio (PAPR). This fact complicates implementation of the analog radio frequency (RF) frontend. When the PAPR is
high, the digital-to-analog converter (DAC) and power amplifier (PA) of the transmitter require high dynamic ranges to
avoid amplitude clipping. Such high dynamic range increases complexity, reduces efficiency, and increases cost of the
components. On the other hand, if the dynamic range is too low, there would be substantial amount of signal distortion
which in turn will raise the amount of bit error rate (BER). Furthermore, the distortion would cause unwanted out-of-band
radiation. Various kinds of methods to combat PAPR problem have been proposed [11]. To reduce the PAPR, many
techniques have been proposed. Such as clipping, coding, partial transmit sequence (PTS), selected mapping (SLM),
interleaving [20][21], nonlinear companding transforms[22] [23], hadamard transforms[24] and other techniques etc. these
schemes can mainly be categorized into signal scrambling techniques, such as PTS, and signal distortion techniques such
as clipping, companding techniques. Among those PAPR reduction methods, the simplest scheme is to use the clipping
process. However, using clipping processing causes both in-band distortion and out-of-band distortion and further causes
an increasing of error bit rate of system. In this paper focus is given on the clipping technique to reduce PAPR of OFDM
system. Using simulation results the effect of clipping technique is compared for the various modulation systems as BPSK,
QPSK and QAM.
The remainder of this paper is organized as follows: in section II, we describe the wireless communication
systems model. In section III, amplitude clipping PAPR reduction techniques is analyzed. In section IV, we simulated and
compare the clipping method with the different level of clip and filtering level for various modulation techniques. Finally,
conclusions are made in section V according to simulation results.
OFDM Basics
ORTHOGONAL frequency-division multiplexing (OFDM) is a technique widely used for wireless applications
[12].Due to its multicarrier feature, OFDM systems are more sensitive than single-carrier systems to frequency
synchronization errors [13].
Fig. 1: Block Diagram of OFDM System
OFDM is a special case of FDM. As we know in single carrier modulation channels are frequency selective so
they add ISI at the receiver side because channels not having flat response in frequency domain. Even though if we do
amplification at the receiver side noise also get amplified so multicarrier modulation is so much popular. OFDM is a
multicarrier modulation in which orthogonality allows lot of subcarriers in tight frequency space without interference from
each other. Due to limitation of bandwidth in communication we need to divide data stream in to many small bands and
also carriers. Then we multiply carriers with data stream then we modulate each carrier at lower data rate and adding them
together for transmission. OFDM is a form of multicarrier transmission that sends information simultaneously over N
Serial
to paralle
l
IFFT Parallel
to serial
DAC
and LPF
Channel
Parallel
to serial
FFT Serial
to
Parallel
ADC
Serial data in
Serial data out
PAPR Reduction in OFDM by Clipping Technique 73
orthogonal carriers. It introduces frequency diversity by making the bandwidth of each carrier smaller than the coherence
bandwidth of the channel. Each carrier may still suffer from flat-fading, however. OFDM is considered a good candidate
for high data rate wireless systems and is currently used for the Hyper LAN II standard [14]. The transmitted signal over a
symbol duration T is: [15]
)...(
0)(2exp(Re),(
110
1
0
0
−
−
=
=
≤≤
+= ∑
N
N
i
si
cccc
Tttiffjctcs π
The code word c consists of N symbols chosen from a many modulation method. All of the code words form the set C. For
M PSK.
Mi
aM
j
i Zaeci
ε
π)(
2
=
The duration of an OFDM symbol T is N times the duration of the symbols ci plus the duration of the cyclic prefix or guard
band. The complex envelope of the transmitted signal, sampled at 1/T, is:
)/2(),(~1
0
NnijxpecncsN
i
i π∑−
=
=
This equation can be recognized as the IDFT of the sequence co …cN-1. FFT is the efficient algorithm to find the
DFT, so in block diagram of OFDM at the transmitter side we are using IFFT block first and at the receiver side we are
using FFT block. FFT converts time domain signal in to frequency domain and IFFT is vice versa. In transmitter taking
FFT means only multiplying by D-1 block after multiplication we serially convert the parallel signal and transmit it. This
whole process is nothing but linear convolution of two signals so there is found overlapping of last L-1 bits to avoid this
we can pad zeros or we can cyclic prefix to the original data blocks. An important limitation of OFDM is that it suffers
from a high Peak-to-Average Power Ratio (PAPR). OFDM is the time domain signal which is a sum of several sinusoids
leads to High PAPR; resulting from the coherent sum of several carriers. This forces the power amplifier to have a large
input back off and operate inefficiently in its linear region to avoid inter modulation products. High PAPR also affects D/A
converters negatively and may lower the range of transmission.
Basics of PAPR
PAPR is defined as:
[ ]2
2
)(
)(max
tsE
tsPAPR =
Theoretically, the PAPR can be as high as N, but the occurrence of such peaks is rare. The summation of a large number of
carriers assumes a Gaussian distribution. The numerator, max|s(t)|2, is also known as the PEP (Peak Envelope Power).
It is also equal to:
)(~)(~ *tstsPEP = .
Therefore, it is desirable to reduce the PAPR.
74 Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar
CLIPPING TECHNIQUE Clipping and Filtering
A high PAPR brings disadvantages like increased complexity of the ADC and DAC and also reduced efficiency
of radio frequency (RF) power amplifier. One of the simple and effective PAPR reduction techniques is clipping, which
cancels the signal components that exceed some unchanging amplitude called clip level. In Clipping, the amplitudes of the
input signal are clipped to a predetermined value. However, clipping yields distortion power, which called clipping noise,
and expands the transmitted signal spectrum, which causes interfering [16].
Clipping and filtering technique is effective in removing components of the expanded spectrum. Although
filtering can decrease the spectrum growth, filtering after clipping can reduce the out-of-band radiation, but may also cause
some peak re-growth, which the peak signal exceeds in the clip level [17].
Fig. 2: Block Diagram of OFDM System in Clipping and Filtering Approach
The technique of iterative clipping and filtering reduces the PAPR without spectrum expansion. However, the
iterative signal takes long time and it will increase the computational complexity of an OFDM transmitter [16]. But without
performing interpolation before clipping causes it out-of-band. To avoid out-of-band, signal should be clipped after
interpolation. However, this causes significant peak re-growth. So, it can use iterative clipping and frequency domain
filtering to avoid peak re-growth. In the system used, serial to parallel converter converts serial input data having different
frequency component which are base band modulated symbols and apply interpolation to these symbols by zero padding in
the middle of input data. Then clipping operation is performed to cut high peak amplitudes and frequency domain filtering
is used to reduce the out of band signal, but caused peak re-growth [17]. This consists of two FFT operations. Forward FFT
transforms the clipped signal back to discrete frequency domain. The in-band discrete components are passed unchanged to
inputs of second IFFT while out of band components are null. But heavy clipping causes about 1 dB lower average EVM.
Clipping introduces in band distortion and out-of-band signals, which can be controlled by proper filtering.
REPEATED CLIPPING AND FREQUENCY DOMAIN FILTERING
A clipping method in its basic form is based on simple time domain signal limitation. Clipped signal ��c(t) can be
expressed by following relationship:
��(t)=� �. ����, |���| � �
���, |���| � � �
Where A is the clipping level and �� is the phase of original signal ���. By this limitation, the peak values of
signal are removed that results in PAPR reduction. However, the clipping introduces signal distortion resulting in adjacent
channel emissions. This undesirable effect can be suppressed by low pass filtering of clipped signal that unfortunately
further increases the PAPR.
Clipping
and
frequency
domain
filtering
Output
OFDM
OFDM
Clipping
and
frequency
domain
filtering
input
PAPR Reduction in OFDM by Clipping Technique 75
Armstrong [18] developed a method based on K-times repetition of the clipping and filtering process. Therefore
both PAPR and adjacent spectral emissions are reduced, although the PAPR reduction is far from simple clipping case. In
this paper results for repeated clipping are discussed .
COMBINATION OF INTERLEAVING WITH REPEATED CLIPPING AND FILTERING
In paper [20], authors used a combination of interleaving (adaptive symbol selection) with simple clipping
followed by a filter increasing the PAPR. We have chosen a concatenation of interleaving and repeated clipping and
frequency domain filtering or its simplified non iterative alternative. First, the interleaving approach is used and the signal
with lowest PAPR is then passed through clipping and filtering method. The intention to combine these two methods is to
obtain signal with lower PAPR than in the case of interleaving method and with lower distortion (and thus lower bit error
rate) than in the case of standalone Repeated clipping and filtering.
Fig.3: Actual Simulation approach of OFDM System
As both methods used in the combination suffer from high complexity, the main disadvantage of the combined
method is above all the complexity. Moreover, side information (SI) to identify the interleaver with lowest PAPR has to be
sent to receiver for each OFDM symbol. Without this side information, it is not possible to decode the data. As the correct
decoding of side information is fundamental for the performance of OFDM modem, the SI can thus be either mapped using
modulation with lower number of states or encoded by FEC.
The complexity of the presented combined method can be dramatically reduced using the recently proposed
method Simplified clipping and filtering instead of the repeated clipping and frequency domain filtering method. This case
has been also considered in our paper and this method is recommended for practical use.
CONCLUSIONS
In this paper some PAPR reduction carried out by clipping technique in two ways for modulation techniques like
BPSK, QPSK, 16-QAM. Results are compared as per the tabular data shown over here.
Here we can conclude that in case of BPSK modulation we get maximum PAPR reduction for IFFT size of 32 while data
points are 512. It’s up to : 13.525361 dB. Here also we can conclude that In case of QPSK modulation we get maximum
PAPR reduction while IFFT size is 32 for 512 data bits. It’s up to: 17.384791 dB
Sk Sn QAM/PS
K mapping
S/
P IFFT
P/S and
PAPR
Reduction
D/A
and
HPA
A/D
S/P and
Inverse
PAPR
Reduction
QAM/PSK DE
mapping P/
S FFT
Wn Sn
’
Sk
’
76 Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar
Table 1: Simulation Results with HPA Effect for BPSK and QPSK
PSK
Type
IFFT
Size
Data
Points
PAPR without
Clipping after
HPA
PAPR with
Clipping
and HPA
BPSK 8 32 14.060386 5.856270
BPSK 16 64 15.511691 6.557389
BPSK 16 128 17.990750 4.897381
BPSK 32 512 21.304113 7.778752
QPSK 8 256 14.792368 7.081363
QPSK 8 512 16.733045 7.685571
QPSK 16 256 19.738100 7.694189
QPSK 16 512 18.211730 6.630577
QPSK 32 512 25.225245 7.840454
Simulation results with phase rotation and without phase rotation:
BER curve for 8 frames for 16-QAM:
BER Curve for 16 Frames for 16-QAM:
PAPR Reduction in OFDM by Clipping Technique 77
BER Curve for 32 Frames for 16-QAM:
BER curve for 100 frames for 16-QAM:
Table 2: Simulation Results with Filtering for QAM
No of
frames
PSD PAPR
before
Clipping
PAPR after
Clipping
BER
8 0.025020 6.719467 1.790023 0.171250
16 0.023034 5.641196 1.838601 0.167656
32 0.023787 8.496442 2.094995 0.168177
100 0.027488 4.490738 1.689178 0.165967
Results for repeated clipping and filtering:
Table 3: Simulation Results with Repeated Clipping and Filtering for QAM
NO OF
DATA
POINTS
ORIGINAL
PAPR
PAPR 1ST
CLIP
PAPR 2ND
CLIP
PPAR 3RD
CLIP
PAPR 4TH
CLIP
1024 9.074727 7.533764 6.842138 6.459559 6.253227
2048 9.417887 7.698206 6.932880 6.507286 6.277734
4096 9.737573 7.861174 7.023105 6.554821 6.302306
8192 10.08860 8.013226 7.106953 6.599251 6.325240
78 Prashant Maruti Jadhav, L.S.Admuthe & A.P.Bhadvankar
PAPR for 1024 Bits:
PAPR for 2048 Bits:
PAPR for 4096 Bits:
PAPR for 8192 Bits:
For different variations in number of frame for QAM modulation we get the almost maximum reduction in PAPR
of up to 6dB. Where BER remains almost constant for varying frames. Also PSD remains constant and within range of up
to 0.02 to 0.03. For 40 frames I get the maximum PAPR reduction but PSD is also MAX.
No specific PAPR reduction technique has been the best solution for all multicarrier transmission system. It has
been suggested that the PAPR reduction technique should be carefully chosen according to various system requirements.
Clipping and Partial Transmit Sequence are more practical than other techniques.
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