Modifed PTS Scheme Based PAPR Reduction In OFDMA System

IRJMST Vol 5 Issue 6 [Year 2014] ISSN 2250 – 1959 (0nline) 2348 – 9367 (Print)

International Research Journal of Management Science & Technology http://www.irjmst.com Page 114

Modifed PTS Scheme Based PAPR Reduction In OFDMA System

Jitender1 Amit Deroliya2 Pradeep3

Assisstant professor M.tech Research Scholar Assisstant professor

Department of Electronics Department of Electronics Department of Electronics

& Communication Engineering & Communication Engineering & Communication Engineering

B.I.T.S Bhiwani, India B.I.T.S Bhiwani, India B.I.T.S Bhiwani, Indi

Abstract: — Orthogonal Frequency- Division Multiplexing (OFDM) is a popular technique for

achieving high-bit-rate wireless data transmission. A very major disadvantage of orthogonal frequency-

division multiplexing (OFDM) is the high peak-to-average power ratio (PAPR) of the transmitted signal.

Partial Transmit Sequence (PTS). In PTS technique, the data block to be transmitted is divided into

disjoint subblocks. Each subblock is multiplied by the phase sequence and calculates the combined

PAPR for each phase sequence and selects the lowest PAPR sequence for transmission.There are various

techniques through which we can reduce PAPR.but here we use modified PTS (partial transmit

sequence)

Keywords— OFDM, PAPR, PTS,CCDF

I. Introduction

Orthogonal frequency division multiplexing (OFDM) is a promising solution for high data rate

transmission in frequency-selective fading channel. But it also have some good advantage

Like High spectral efficiency, Simple digital realization by using the FFT operation.etc

Besides a lot of advantages, OFDM has some disadvantages also. High peak - to – average power ratio

(PAPR) is one of the major disadvantages of OFDM.

OFDM signal is the sum of many independent signals modulated onto sub channels of equal bandwidth.

Let us denote the data block of length N as a vector S = [S0, S1, . .., SN-1]T where N is the number of sub

carriers. The complex baseband representation of an OFDM signal consisting of N subcarriers is given

by [1]



s t =1

N Xn . ej2πn△ftN−1

n=0 , 0 ≤ t < 𝑁𝑇 ... (1)

Where 𝑗 = −1 , ∆𝑓 is the subcarrier spacing and NT denotes the useful data block period. PAPR of

the transmitted signal in (1) is defined as [1]

𝑃𝐴𝑃𝑅 = 𝑠 𝑡 0≤𝑡<𝑁𝑇

𝑚𝑎𝑥 2

1𝑁𝑇 𝑠 𝑡 2 𝑑𝑡

𝑁𝑇0

... (2)

PAPR reduction techniques are mainly concerned with the reduction of 𝑚𝑎𝑥 𝑠 𝑡 . The PAPR problem

is more important in the uplink since the efficiency of power amplifier is critical due to the limited

battery power in the mobile terminal.

The PAPR of an OFDM signal is defined as the ratio of the maximum instantaneous power to its

average power (avP ):

...(3)

where (avP ) = average power of x(t).

In PTS,PAPR reduction is based on the sampled, discrete-time signal which can be written as

...(4)

where L is the over sampling factor

2.(| ( ) | )[ ( )]

av

Max x tPAPR x t

P 0 t NT .......3.3

21

0

1x[n] ,

j nkN

LNk

k

X eN

0 1n LN .......3.4



The PAPR computed from the L-times over sampled OFDM signal can be defined as

...(5)

where E{.} denotes the expectation operator

The cumulative distribution function (CDF) of PAPR is one of the most important parameter to measure

the performance of PAPR reduction technique. The complementary cumulative distribution function

(CCDF) is used generally instead of CDF for performance analysis of the PAPR reduction technique.

The CCDF shows the probability that the PAPR of data block exceeds a given threshold. The CDF of

the amplitude of a signal sample is given by [1]

𝐹 𝑥 = 1 − 𝑒𝑥 ... (6)

The CCDF of the PAPR of a data block is given by [1]

𝑃 𝑃𝐴𝑃𝑅 > 𝑥 = 1 − 𝑃 𝑃𝐴𝑃𝑅 ≤ 𝑥

= 1 − 𝐹 𝑥 𝑁

= 1 − 1 − 𝑒−𝑥 𝑁 ... (7)

The expression in (7) assumes that the N time domain signals samples are mutually independent. A

large number of solutions have been proposed to solve the PAPR problem in

OFDM. One of the important technique is partial transmit sequence (PTS), in which input data blocks

are divided into disjoint subblocks and multiplied by different phase sequences to generate modified

data blocks. Modified data block with the minimum PAPR is selected for transmission.

In this paper, we introduce PAPR reduction techniques such as Partial Transmit Sequence

2

2

.(| (n) | )[X(n)]

( | [ ] | )

Max xPAPR

E x n 0 1n LN .......3.5



(PTS) with different number of subblocks. The two PAPR reduction techniques are implemented and

evaluated with respect to the decrement in CCDF of PAPR with respect to the different number of

subblocks and different number of phase sequences in PTS. The simulation results are reported and

analysed at the end of this paper. The PTS techniques considered in this paper are explained below.

Input

Data

Source

N-point

IFFT

N-point

IFFT

Phase optimization

N-point

IFFT

+

Serial to

Parallel

And

Partition

Into

subblocks

X

X1

X2

XV

x1

x2

XV

𝑏 1

𝑏 2

𝑏 𝑉

𝑥 .

.

.

.

.

.

Figure 1 Block Diagram of Partial Transmit Sequence (PTS) Technique



II. Partial Transmit Sequence Technique

In the PTS approach, an input data blocks of N symbols is partitioned into M disjoint

subblocks. The subcarriers in each subblock are weighted by a phase factor for that

subblock. The phase factors are selected such that the PAPR of the combined signal are

minimized [1]-[4]. Figure 1 shows the block diagram of PTS technique. In this technique,

the input data block X is partitioned into M disjoint subblocks as follows [5]:

𝑋𝑚 = 𝑋𝑚 ,0, 𝑋𝑚 ,1, … , 𝑋𝑚 ,𝑁−1 𝑇

... (8)

Here 𝑚 = 1,2, … , 𝑀 and Xm are disjoint subblocks. These M subblocks are combined to

minimize the PAPR in the time domain. The L-timed oversampled time domain signal of

𝑋𝑚 is denoted by [5]:

𝑥𝑚 = 𝑥𝑚 ,0, 𝑥𝑚 ,1, … , 𝑥𝑚 ,𝑁𝐿−1 𝑇

. 𝑥𝑚 ... (9)

Here 𝑚 = 1,2, … , 𝑀 and 𝑥𝑚 is obtained by taking IFFT of length NL on 𝑋𝑚 . These are

called the partial transmit sequence. Then each partitioned subblock is multiplied by a

corresponding complex

phase factor 𝑏𝑚 = 𝑒𝑗∅𝑚 m = 1, 2, M, to combine the PTSs. The time domain signal after

combining is given by [5]

𝑥 ′ 𝑏 = 𝐼𝐹𝐹𝑇 𝑏𝑚 𝑋𝑚

𝑀

𝑚=1

= 𝑏𝑚 . 𝐼𝐹𝐹𝑇 𝑋𝑚

𝑀

𝑚=1

= 𝑏𝑚 𝑥𝑚

𝑀

𝑚=1

… (10)

Where 𝑥 ′ 𝑏 = 𝑥0′ 𝑏 , 𝑥1

′ 𝑏 … 𝑥𝑁𝐿−1′ 𝑏

𝑇.The PTS technique suffers from the

complexity of searching for the optimum set of phase factors, especially when the

number of subblocks increases [10]. Minimization of PAPR is related to the minimization

of the

max0≤𝑘≤𝑁𝐿−1 𝑥𝑘′ (𝑏)



The set of allowed phase factor is written as:

𝑏 = 𝑒𝑗2𝜋𝑙/𝑊 |𝑙 = 0, 1, … , 𝑊 − 1

where W is the number of allowed phase factors, WM-1

sets of phase factors should be searched to find the optimum set of phase vectors. So, the complexity is increases exponentially with number of subblocks increases. PTS needs M IFFT operations for each data block. Main idea of PTS is data blocks are divided into non overlapping sub-block with independent rotation factor. This rotation factor generates time domain data with lowest amplitude

modified data blocks [9]. The decrease in PAPR of the transmitted signal is dependent

upon the number of the phase sequences U and the design of the phase sequences [10].

Main idea of PTS is data blocks are divided into non overlapping sub-block with

independent rotation factor. This rotation factor generates time domain data with lowest

amplitude.

CCDF- Cumulative distributive function is to describe the probability of the random

variable with the given probability distribution function Complementary cumulative

distributive function is also called as tail distribution which is used to illustrate the PAPR

of OFDM signal The output curve is used to conclude the design parameters of the

modulation system The CCDF of an OFDM system to measure the PAPR can be given as

IV. Simulation Results

A. Partial Transmit Sequence Technique

The Partial Transmit Sequence technique is implemented and CCDF of PAPR is measure

with different number of subcarriers. The simulation results are plotted in the various

figures 2.1 , 2.2 , 2.3 with different number of subcarriers.

The number of sucarriers (N) is taken as 64,128,256 . It has been observed from the

various figures that, as the number of subcarriers N are increases the PAPR increases

From figures 2.1, 2.2 and 2.3 we observed that as the numbers of subcarriers increases

( )r oCCDF P PAPR PAPR .......3.6



the PAPR of OFDM system increases this is because of larger the numbers of subcarriers

larger will be peak values and hence PAPR.

The proposed scheme reduces PAPR upto 8.3 dB for N=256 subcarriers at CCDF 10-4.

.

Figure 2.1: Comparison of Standard OFDM and PTS scheme based OFDM for N=64


3 4 5 6 7 8 9 10 1110

-4

10-3

10-2

10-1

100

PAPR0 [dB]

CC

DF

(P

r[P

AP

R>

PA

PR

0])

Standard OFDM versus PTS Scheme based OFDM using QPSK modulation

Orignal

PTS

PTS Scheme Based PAPR

Original PAPR

4 5 6 7 8 9 10 1110

-3

10-2

10-1

100

PAPR0 [dB]

CC

DF

(P

r[P

AP

R>

PA

PR

0])


Orignal

PTS

PTS Scheme Based PAPR Original PAPR




V. Conclusion

In this paper, we investigate the effect of PAPR performance of an OFDM system with

modified PTS technique when number of phase carrier increases then PAPR is also

increases.this is the main disadvantage of standard OFDMA.

In fig 2.1 we cleary see that the original PAPR of standard OFDMA is 10.2 db and when

we use the modified PTS technique

We obtain the PTS Scheme based PAPR is 7.8db.

So from PTS technique we improve the PAPR is

2.4 db.Note- when we use the number of subcarriers =64.

Similarly in fig 2.2 & fig 2.3 we improve the PAPR is 2.2 db & 1.6 db

When we use the number of subcarrier is 128 & 256.

FUTURE SCOPE- . A very major disadvantage of standard orthogonal frequency-

division multiplexing (OFDM) is that we use lager number of subcarriers which leads

Higher PAPR to overcome this disadvantage we use SC-FDMA (Single Carrier

Frequency Division Multiple Access) system to improve PAPR.

5 6 7 8 9 10 1110

-4

10-3

10-2

10-1

100

PAPR0 [dB]

CC

DF

(P

r[P

AP

R>

PA

PR

0])


Orignal

PTS

PTS Scheme Based PAPR Original PAPR



VI. References

[1]. Han, S.H. and Lee, J.H. “An overview of peak-to-average power ratio reduction techniques for multicarrier transmission”. IEEE Wireless Commun., April 2005, pp. 56–65.

[2]. L. J. Cimini, Jr. and N. R. Sollenberger, “Peak-to-Average Power Ratio Reduction of an OFDM Signal Using Partial Transmit Sequences,” IEEE Commun. Lett., vol. 4, no. 3, Mar. 2000, pp. 86–88.

[3]. D. S. Jayalath and C. Tellambura, “Adaptive PTS Approach for Reduction of Peak-to-Average Power Ratio of OFDM Signal,” IEEE Elect. Lett., vol. 36, no. 14, July 2000, pp. 1226–28.

[4]. S. H. Han and J. H. Lee, “PAPR Reduction of OFDM Signals Using a Reduced Complexity PTS Technique,” IEEE Sig. Proc. Lett., vol. 11, no. 11, Nov. 2004, pp. 887–90.

[5]. Yong Soo Cho and Jaekwon Kim and Won Young Yang and Chug- Gu Kang,” MIMO – OFDM Wireless Communication”, John Wiley & Sons (Asia) Pvt. Ltd, Singapore, IEEE 2010.

[6]. Di- xiao Wu, “Selected Mapping and Partial Transmit Sequence Schemes to Reduce PAPR in OFDM Systems”, IEEE IASP 2011, pp. 1-5.

[7]. S.H .Muller and J.B. Huber , “ A Novel Peak Power Reduction Scheme for OFDM, ” Proc . IEEE PIMRC 97, Helsinki, Finland, Sept. 1997, pp. 1090–94.

[8]. L. J. Cimini, Jr. and N.R.Sollenberger, “Peak-to-Average Power Ratio Reduction of an OFDM Signal Using Partial Transmit Sequences”, IEEE Commun. Lett., vol.4, no. 3, Mar. 2000, pp. 86–88.

[9]. S.H. Müller and J. B. Huber, “A Comparison of Peak Power Reduction Schemes for OFDM,” Proc. IEEE. GLOBECOM '97, Phoenix, AZ, Nov. 1997, pp. 1–5.

[10]. R. W. Bäuml, R. F. H. Fisher, and J. B. Huber, “Reducing the Peak-to-Average Power Ratio of Multicarrier Modulation by Selected Mapping,” IEEE Elect. Lett., vol. 32, no. 22, Oct. 1996, pp. 2056–57.



Authors

Teaching and research. His research interests include OFDM system in Wireless

Communication.

.

Jitender received his B.E. degree in

ECE from B.I.T.S Bhiwani In

2008, AND M.Tech degree also in

ECE From PDM Bhadurgarh.He

has an experience of 4 year in

Teaching and research.His research

interests include Pseudo-noise

sequence generator in C.D.M.A.

Amit Deroliya received his

B.Tech degree in Electronics

and communication in 2011

from BPRCE,Gohana (India),

M.Tech. (ECE) in 2014 from

B.I.T.S, Bhiwani (India). He

has an experience of 1 year in

teaching

Modifed PTS Scheme Based PAPR Reduction In OFDMA System

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