a comparative survey on papr reduction techniques for ofdm performance improvement€¦ · ·...

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International Journal of Electronics and Communication Engineering & Technology

(IJECET)

Volume 6, Issue 10, Oct 2015, pp. 53-65, Article ID: IJECET_06_10_006

Available online at

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ISSN Print: 0976-6464 and ISSN Online: 0976-6472

© IAEME Publication

A COMPARATIVE SURVEY ON PAPR

REDUCTION TECHNIQUES FOR OFDM

PERFORMANCE IMPROVEMENT

Rimpi Datta

Dept. of Electronics and Communication, Narula Institute of Technology, India

Anirban Bhar

Dept. of Information Technology, Narula Institute of Technology, India

Arpita Barman Santra


Sohan Ghorai


ABSTRACT

The Orthogonal Frequency Division Multiplexing is one of the modulation

techniques widely used in the broadband wireless technology. One of the main

problems of this technology is the high peak-to-average power ratio of

transmission signal due to the superposition of many subcarriers. This paper

presents a comparative studies of peak-to-average power ratio reduction

technique, which compares among the selective mapping method, clipping

method and Tone reservation techniques for PAPR reduction. The paper

presents the performance of these different PAPR reduction techniques and

compares it with unreduced PAPR.

Key words: PAPR, OFDM, Selective Mapping Algorithm, Clipping, Tone

Reservation.

Cite this Article: Rimpi Datta, Anirban Bhar, Arpita Barman Santra and

Sohan Ghorai. A Comparative Survey on PAPR Reduction Techniques for

OFDM Performance Improvement, International Journal of Electronics and

Communication Engineering & Technology, 6(10), 2015, pp. 53-65.

http://www.iaeme.com/IJECET/issues.asp?JType=IJECET&VType=6&IType=

10

1. INTRODUCTION

Orthogonal Frequency Division Multiplexing (OFDM) scheme is a popular standard

for next generation mobile radio communication system [2]. In OFDM system, serial

Rimpi Datta, Anirban Bhar, Arpita Barman Santra and Sohan Ghorai


data stream is multiplexed into a large number of orthogonal sub-channels producing

a bandwidth efficient signal spectrum. As it is robust against multipath fading, the

orthogonal frequency division multiplexing system can transmit high-speed data using

a number of orthogonal sub-carriers [4]. Various studies show that when OFDM sub-

carriers are added with the same phase [4], it produces a high peak to average power

ratio (PAPR). PAPR is the ratio between the maximum power of a sample OFDM

signal and the average power of that OFDM symbol. High PAPR introduces signal

distortion in the nonlinear region of power amplifier and the signal distortion

introduces the degradation of bit error rate [2]. Major study has been carried out on

effect of PAPR in OFDM and study shows that PAPR depends on the SNR for

Channel with signal and noise. The effect has been tested with introducing a Gaussian

noise in OFDM system.

1.1. OFDM System

OFDM has been developed as the modulation method in the new wireless

technologies. It mitigates the serious problem of multipath propagation, which causes

massive data errors, and loss of signal in the microwave and UHF spectrum. OFDM is

a broadband multicarrier modulation technique that gives very good performance and

it is very beneficial over single-carrier modulation methods as it fits better with

today’s high-speed data requirements and operation in the UHF and microwave

spectrum [1]. The main Features of OFDM systems are processing on the source data,

the symbols are modulated onto orthogonal sub-carriers, orthogonally is maintained

during channel transmission, Synchronization, Demodulation of the received signal

by using FFT, Channel equalization, Decoding and de-interleaving. In the OFDM

system, Inverse Fast Fourier Transform/Fast Fourier Transform (IFFT /FFT)

algorithms are used in the modulation and demodulation of the signal [6, 3]. The

length of the IFFT/FFT vector determines the resistance of the system to errors caused

by the multipath channel [6]. OFDM is generated by firstly choosing the spectrum

required, based on the input data and modulation scheme used. Each carrier to be

produced is assigned some data to transmit. The required amplitude and phase of the

carrier is then calculated based on the 3 modulation scheme (typically differential

BPSK, QPSK, or QAM) [6, 1]. Then, the IFFT converts this spectrum into a time

domain signal. The FFT transforms a cyclic time domain signal into its equivalent

frequency spectrum. Finding the equivalent waveform, generated by a sum of

orthogonal sinusoidal components, does this. The amplitude and phase of the

sinusoidal components represent the frequency spectrum of the time domain signal.

OFDM signal has high peak-to-average-power ratio (PAPR) value.

Figure 1 OFDM Spectrum

http://en.wikipedia.org/wiki/Crest_factor

A Comparative Survey on PAPR Reduction Techniques For OFDM Performance

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1.2. Peak to Average Power Ratio (PAPR)

Let us consider a data block of length N as a vector

where N denotes the number of sub-carriers. The duration of a symbol in X is T.

Each symbol in X modulates one of a set of subcarriers where

{ }. The modulation scheme taken either quadrature amplitude modulation (QAM) or quadrature phase shift keying (QPSK) by definition. The

subcarriers are orthogonal to each other, that is, , where and NT

is the duration of OFDM [8] data block X. The complex envelop of the OFDM signal

is expressed as

, (1)

PAPR is defined as the ratio of the maximum divided by the average power of the

signal [4]. Now the PAPR of the signal expressed in (6) can be defined as

(2)

The effects of PAPR need to be studied for a continuous time OFDM signals, as

the cost and power dissipation of the analog components dominated by this factor.

Reducing the value of PAPR dramatically increases the performance of OFDM

system and reduces the cost of system. However, most existing PAPR tuning methods

can only be implemented on discrete time OFDM signals [9]. For better calculation of

true PAPR, the OFDM signal samples are obtained by oversampling equation (6) by a

factor of L to approximate the PAPR of the true OFDM signals better [9]. The L-

times oversampled time domain samples can be obtained by an LN-point inverse

discrete fourier transform (IDFT) of the data block with zero padding [8]. It

was shown in [7] that is sufficient to get . Therefore,

I have considered for my numerical calculation in this paper [9].

One of the major drawbacks of the OFDM system. Drawback is high PAPR

(PeakTo Average Power ratio). PAPR means randomly sinusoidal leads occurred

during transmission of the OFDM signal. So in this chapter we discuss basics of

PAPR, why it is created & what the proposed solution are for reduce it. So as per the

introduction of PAPR we can judge that to reduce the PAPR is most important point

in the OFDM system. Because of when we are talking about the high speed data

communication in real life like video calling, high speed internet access, and also

main point is that high speed data access up to 2mbps while moving on the vehicle at

100km/h, digital video broadcasting (DVB), Microwave terrestrial television, Digital

audio broadcasting (DAB), 4G system, hyper LAN. So this most type of

communication systems required high data rate. But problem occurs like PAPR in

OFDM system prevent these types of facilities in the real life. So to reduce it is most

important. Various studies show that when OFDM sub-carriers are added with the

same phase [2], it produces a high peak to average power ratio (PAPR). PAPR is the

ratio between the maximum power of a sample OFDM signal and the average power

of that OFDM symbol. High PAPR introduces signal distortion in the nonlinear

region of power amplifier and the signal distortion introduces the breakdown of bit

error rate [4]. Extensive study has been carried out on effect of PAPR in OFDM

system and it shows that PAPR depends on the SNR for Channel with signal and

noise. The effects of PAPR need to be studied for a continuous time OFDM signals,

as the cost and power dissipation of the analog components dominated by this factor.

Reducing the value of PAPR dramatically increases the performance of OFDM



system and reduces the cost of system. So we want to reduce the PAPR in the OFDM

system by using different reduction technique algorithms for PAPR in the OFDM

systems and make a comparative analysis of these techniques and want to get OFDM

signal with improved higher bit error rate. MATLAB based simulation results are

included.

1.3. PAPR Reduction Techniques

Researchers have been proposed several PAPR reduction Techniques. These

techniques can be divided into two groups. There are signal scrambling techniques

and signal distortion techniques. Block coding techniques selected mapping (SLM),

partial transmit sequence (PTS) etc. are signal scrambling techniques. Signal

distortion techniques are peak windowing, envelope scaling, peak reduction carrier

and clipping and filtering. In this paper we discuss about the techniques signal

scrambling techniques and signal distortion techniques.

2. LITERATURE REVIEW

2.1. Selective Mapping

The SLM technique is developed from the idea of symbol scrambling. In this

technique, a set of candidate signals are generated to represent the same information,

the signal with lowest PAPR is selected and transmitted. The information about the

selection of these candidate signals need to be explicitly transmitted along with the

selected signal as side information. Selected mapping needs to transmit the

information to the receiver, about the selected signal, as side in formation. If there is

an error in the received side information, then it is very difficult for the receiver to

recover the information from the transmitted selected signal. That is the reason why a

strong protection against transmission errors is needed regarding side information.

Once the receiver has these side information then the decoding process is very simple.

SLM can be employed for larger number of sub carriers with moderate complexity.

The technique uses codes only for PAPR reduction and does not include error

correction capabilities of codes. The complexity is increased due to the multiple

numbers of IFFT operations. The need for transfer of side information to the receiver

without any margin for transmission errors is very crucial under the fading channels.

Under such noisy channels, the side information is distorted to result in BER

degradation in the OFDM systems. Hence, the side information affected the system

performance and its bandwidth efficiency.

Figure 2 Transmitter Block Diagram with SLM technique

The transmitter side [10] of an OFDM system with SLM technique is shown in

figure 1. In the SLM technique, a set of U different, distinct sequences-


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(0,2π), n=0,1,………N-1, u=1,2,……….U must be defined. At first, the input information is divided into OFDM data block X, which consists of N symbols, by the

serial-to parallel (S/P) conversion and then data block X is multiplied carrier wise

with each one of the U different phase sequences P(u), resulting in a set of U different

OFDM data blocks

n=0,1,……..N-1, u=1,2……..U Then all U alternative data blocks are transformed

into time domain to get transmit OFDM symbol-

The sequence with minimum PAPR is selected for transmission. The receiver

block diagram is shown in fig.3.

Figure 3 Receiver Block Diagram

To recover the data sequence X at the receiver, SLM needs to transmit the

information on the selected phase sequence as side information , which results in

some loss of transmission efficiency. If each phase sequence is –P(u)

n generated as the

cyclically shifted version of the previoussequence, the information needed to transmit

will be less to reconstruct the data. The BER performance will not be affected due to

the presence of this technique because the original information is transformed into a

different sequenceand we can rebuild the information from the selected phase

sequence itself.

2.2. Clipping

The simplest approach for reducing the PAPR of OFDM signals is clipping [11,12].

But this technique comes under the distortion based category. In this technique, the

high amplitude peaks of the signal are clipped to a predetermined threshold value

which limits the peak envelope of the input signal. Since the clipping is done on the

actual information itself, there is a possibility to loose the data. So there will be a

trade-off between the clipping and the BER performance. Let x[n] denote the pass

band signal and x˛[n] denote the clipped version of x[n],which can be expressed as

(3)



Where A is the pre-specified clipping level. This technique has some drawbacks

which causes in band signal distortion, resulting in Bit Error Rate performance

degradation. It also causes out-of-band radiation, which imposes out-of-band

interference signals to adjacent channels. This out-of-band radiation can be reduced

by filtering. This filtering of the clipped signal leads to the peakregrowth. That means

the signal after filtering operation may exceed the clipping level specified for the

clipping operation.

2.3. Tone Reservation

Tone Reservation (TR) is one of the well-known methods for PAPR reduction of

Orthogonal Frequency Division Multiplexing (OFDM) systems. In this method some

sub carriers are reserved for PAPR reduction and the symbols in these sub carriers are

selected such that the PAPR of the OFDM frame is minimized. This technique

contains some set of reservation of tones. By using this technique reserved tones can

be used to minimize the PAPR. This method is used for multicarrier transmission and

also shows the reserving tones to reduce the PAPR. This technique is depend on

amount of complexity. When there is number of tones is small reduction in PAPR

may represent non negligible samples of available bandwidth. Advantage of this tone

reservation is very positive that no process is needed at receiver end. And also do not

need to transmit the side information along with the transmitted signal. in this data

block is added to the time domain signal to reduce the peak leads.

3. SIMULATION RESULTS

Simulation has been performed using MATLAB and compared with various

algorithm.

3.1. Selective mapping algorithm

Simulation has been performed using MATLAB In selective mapping algorithm as

reduction technique for PAPR in OFDM systems, here we consider 16 subcarriers.

Here we use phase rotation by multiplying all the subcarriers in the sum by different

phases to create a new data vector, not only that but also as we don't know the

optimum phases to give the best PAPR we now create more than one vector and then

select the optimum value. Now the selector should test the PAPR for the data vectors

and select the one with the least PAPR to use in transmission. Here we can see the

Performance analysis using selective mapping.

Table 1Selective PAPR Comparison Chart for Different PAPR Selection

Selective PAPR PAPR Values

PAPR 29.2900

PAPR1 22.0610

PAPR2 31.2678

PAPR3 31.3918


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Figure 4 'PAPR against different phase shifts'

3.2. Clipping and Filtering as a PAPR reduction techniques

when OFDM signal pass through power amplifier system the signal distort. The

effects of the nonlinear distortions introduced by a High Power Amplifier (HPA) on

the system performance are evaluated in terms of the Bit Error Rate (BER). So a need

of technique which reduces PAPR and amplitude clipping is one of the techniques

used to reduce PAPR .In amplitude clipping, a threshold value of the amplitude set

and any subcarrier having the amplitude more than that value is clipped or that

subcarrier is filtered to bring out a lower PAPR values. By using this techniques we

can investigates the effects of high power amplifier and the channel noise on the

OFDM signals and then introduces clipping as a PAPR reduction method to reduce

the PAPR effect. Firstly select parameters- QPSK signal constellation having 128 data

points, size of each OFDM block-8 and assigns the length of cyclic prefix. Then

Generate 1 x 128 vector of random data points, Perform QPSK modulation then

perform IFFT for each block. Make the serial stream a matrix where each column

represents a pre-OFDM block). Firstly we have to find out the number of columns

that will exist after reshaping then secondly Create empty matrix to put the IFFT's

data next Operate column wise .After that Compute and append Cyclic Prefix.

Append the CP to the existing block to create the actual OFDM block then Convert to

serial stream for transmission. Now we get actual OFDM signal. Now by using

clipping techniques reduce the PAPR. To show the effect of the PAPR simply we add

random complex noise when the power exceeds the avg. value, otherwise it add

nothing.



Figure 5.1 x 128 vector of random data points

Figure 6 Transmitted data and phase representation

Figure 7 OFDM signal representation


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Figure 8 Clipped Signal Representation

Figure 9 OFDM Signal after High Power Amplifier (HPA)

Figure 10 clipped Signal after High Power Amplifier (HPA)



Figure 11 Received data phase representation

Figure 12 Received Data clipped representation

3.3. Tone reservation as PAPR techniques

There we use the 12 information subcarriers (BPSK modulated). Now consider the 4

reduction carriers we will map different combinations from them by changing the

reduction subcarrier case (positive or negative). Let us dedicate r1,r2,r3 and r4 as peak

reduction carriers and transmit on x1:x12 so when x1:x12 summation creates a peak, a

combination from r1,r2,r3 and r4 creates an anti-peak but not the exact anti-peak or

the output would be a flat signal from which data can be retrieved. Then the total sum

should have less peaks, but we have noted that even if it has less peaks but sometimes

the value of these peaks are twice the original peaks. So that we must take both

criteria into consideration the number of peaks and its values. Now calculate the

PAPR.


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Table 2 Tone Reservation Performance

Tone Reservation PAPR PAPR Values

PAPR 26.6393

PAPR1 29.2900

PAPR2 25.8413

PAPR3 25.4080

PAPR4 21.6240

PAPR5 21.7348

PAPR6 25.4585

PAPR7 24.3934

PAPR8 23.6455

PAPR9 24.5452

PAPR10 22.8342

PAPR11 26.3100

PAPR12 25.7989

PAPR13 26.0764

PAPR14 23.2783

PAPR15 22.1395

PAPR16 23.4401

Figure 13 Representation of sum with no peak reduction carriers (PRC)



Figure 14 Sums with no peak Figure 15 Sum with no peak Reduction carriers (PRC) Reduction carriers (PRC)

Figure 16 Sum with no Peak reduction Carriers (PRC)

4. CONCLUSION

The evaluation of PAPR performance is done by CCDF parameters. In this literature

several PAPR reduction technique has been discussed. I found that tone reservation is

a good technique for PAPR reduction but it needs side information to receiver to

recover original data block that increases algorithmic complexity. Also the above.


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Also they are less immune to dynamic noise. In future a pre-coder will be combined

with a neuro-fuzzy algorithm to reduce time complexity and increase performance.

REFERENCES

[1] Jiang Tao, Yang Yang, ong Yong-Hua,: Exponential Companding Technique for

PAPR Reduction in OFDM Systems, IEEE Transaction on Broadcasting, Vol. 51,

No. 2, June 2005, pp 244-248

[2] Yoo Seungsoo, Yoon Seokho, Kim Sun Yong, Song Iickho,: A Novel PAPR

Reduction Scheme for OFDM Systems Selective Mapping of Partial Tones

(SMOPT), IEEE Transaction on Consumer Electronics, Vol. 52, No. 1, Feb.

2006, pp 40-43

[3] Lim, Dae-Woon., No, Jong-Seon., Lim, Chi-Woo.: A New SLM OFDM Scheme

With Low Complexity for PAPR Reduction, IEEE Signal Processing Letters,

Vol. 12, No. 2, February 2005, pp 93-96

[4] Lim, Dae-Woon., Heo, Seok-Joong., No, Jong-Seno., Chung, H. : A new PTS

OFDM with Low Complexity for PAPR Reduction

[5] Kaur Parneet, Singh Ravinder,: Complementary Cumulative Distribution

Function for Performance Analysis of OFDM Signals, IOSR Journal of

Electronics and Communication Engineering, Vol. 2, No. 5, Oct. 2012, pp 851-

854

[6] Wulich, D.: Definition of Efficient PAPR in OFDM, IEEE Communication

Letters, Vol. 9, No. 9, Sept. 2005, pp 832-834

[7] Tellado, J: Peak to Average Ratio Reduction for Multicarrier Modulation, Ph.D.

Dissertation, University of Stanford, 1999.

[8] Han Seung Hee, Lee Jae Hong,: PAPR Reduction of OFDM Signals Using a

Reduced Complexity PTS Technique, IEEE Signal Processing Letters, Vol. 11,

No. 11, Nov. 2004, pp 887-890

[9] Jiang Tao, Xiang Weidong, Richardson Paul C, Guo Jinhua, Zhu Guangxi,:

PAPR Reduction of OFDM Signals Using Partial Transmit Sequences With Low

Computational Complexity, IEEE Transaction on Broadcasting, Vol. 53, No. 3,

Sept. 2007, pp 719-724

[10] Wen-Xiang Lin, Jia-Chin Lin, and Yu-Ting Sun, “Modified selective mapping

technique for papr reduction in OFDM systems,” 12th International Conference on

ITS Telecommunications (ITST), pp. 764–768, 2012

[11] Y.-C.Wang and Z.-Q. Luo, “Optimized iterative clipping and filtering for papr

reduction of ofdm signals,” IEEE Transactions on Communications, vol. 59, no.

1, 2011.

[12] Wang and C. Tellambura, “A simplified clipping and filtering technique for par

reduction in OFDM systems,” IEEE Signal Process. Lett, vol. 12, no. 6, pp. 453–

456, 2005. [11] J. Armstrong, “Peak-to average power reduction for OFDM by

repeated clipping and frequency domain filtering”, Electron. Lett, vol. 38, no. 5,

pp. 246–247, 2002.

[13] Ms. Shraddha R. Waghmare and Prof. Dr. Shripad P. Mohani. A New Approach

In Distortionless Techniques For PAPR Reduction In Multicarrier Transmission

Systems, International Journal of Electronics and Communication Engineering

& Technology, 4(5), 2013, pp. 207 – 213.

[14] Ashok Kumar Kajla, Rupesh Sharma, Yash Walia, and Sukoon Mishra. Improve

Peak To Average Power Ratio (PAPR) Reduction Techniques In OFDM

Systems, International Journal of Electronics and Communication Engineering

& Technology, 4(5), 2013, pp. 207 – 213.

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