9.PAPR reduction

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.


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


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

’


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:


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


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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9.PAPR reduction