 Open Access
 Total Downloads : 2
 Authors : Pankaj Shrotriya, Mahesh Kumar Porwal
 Paper ID : IJERTCONV2IS10055
 Volume & Issue : NCETECE – 2014 (Volume 2 – Issue 10)
 Published (First Online): 30072018
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
PAPR Reduction of OFDM Using Companding of Selective Mapping and Partial Transmit Sequence
Pankaj Shrotriya
M.Tech Digital Communication Shrinathji Institute of technology and Engineering,
Nathdwara, India pankaj.shrotriya1@gmail.com
Mahesh Kumar Porwal
Department of Electronics and communication Shrinathji Institute of technology and Engineering,
Nathdwara, India porwal5@yahoo.com
Abstract In recent years orthogonal frequency division multiplexing (OFDM) has gained a lot of involvement in diverse digital communication applications. It is a new ensuring transmission scheme for broadband communications over a wireless channel. In OFDM data is transmitted simultaneously through multiple frequency bands. It offers many advantages over single frequency transmission such as high spectral efficiency, unrefined to channel fading and the capability to handle frequencyselective fading without resorting to complex
scheme. In SLM we can get better PAPR by modifying the OFDM without distortion.

BACKGROUND OF PAPR
The PAPR of OFDM is defined as the ratio between the maximum power and the average power, The PAPR of the OFDM signal X(t) is defined as
= n
channel equalization schemes. Despite the fact that OFDM has a number of advantages, one of the major drawbacks of OFDM signal is its large envelope fluctuation, likely resulting in large
PAPR =
Ppeak
Paverage
max [ x 2] E[ xn 2]
(1)
peaktoaverage power ratio (PAPR). These degradations would seriously affect the performance of OFDM systems. The PAPR reduction scheme changes the formation of the OFDM signals with high PAPR before multicarrier modulation, e.g. coding, partial transmit sequence (PTS) and selective mapping (SLM). With PTS and SLM companding we can get better PAPR by modifying the OFDM without distortion
Key Words OFDM; RSSLM; PTS; PAPR; IFFT
Where xn = An OFDM signal after IFFT (Inverse Fast Fourier transform)
E[.] = Expectation operator, it is an average power. The complex baseband OFDM signal for N subcarriers represented as
X(t) = 1 1 2 ,(0 ) (2)

INTRODUCTION
=0
Orthogonal Frequency Division Multiplexing is a special form of multicarrier modulation which is particularly suited for transmission over a dispersive channel. Here the different carriers are orthogonal to each other so that they all are totally independent of one another. This is done by placing the carrier exactly at the nulls in the modulation spectra of each other. In other words, when the OFDM signal with high PAPR passes through nonlinear device, (for example power amplifier working in the saturation region); the signal will suffer significant nonlinear distortion.
This nonlinear distortion will result in inband distortion and outofband radiation. Because of inband distortion system performance degradation results and due to outofband radiation adjacent channel interference (ACI) results that affects working of the neighbour bands systems. To lessen the signal distortion, it requires a linear power amplifier with large dynamic range. However, this linear power amplifier has poor efficiency and is so expensive.
The PAPR reduction scheme changes the formation of the OFDM signals with high PAPR before multicarrier modulation, e.g. coding, and selective mapping (SLM). The researches reduce the computational complexity for the SLM


PAPR PROBLEM &TECHNIQUE FOR
REDUCTION
There have been many new approaches developed during the last few years. Several PAPR reduction techniques have been proposed in the literature. These techniques are divided into two groups, which are signal scrambling techniques and signal distortion techniques. The signal scrambling techniques are:

Amplitude Clipping And Filtering
In this approach, we can perform timedomain based clipping or frequencydomain based coding. The simplest approach for PAPR reduction is to deliberately clip the amplitude of the signal to a predefined value before amplification. However, there are several drawbacks of this approach, such as signal distortion and spectral regrowth. Hence simple clipping is not enough, we have to use coding techniques that are applied to OFDM signals in order to find the optimum threshold for every specific signal\. However, this technique works well only when the number of subcarriers is small, because at higher subcarriers, the clipping ratio is to be very low which will lead to more distortion.

SELECTIVE MAPPING
This method is used for minimization of peak to average transmit power of multicarrier transmission system with selected mapping. A complete set of candidate signal is generated signifying the same information in selected mapping, and then concerning the most favourable signal is selected as consider to PAPR and transmitted. In the SLM, the input data structure is multiplied by random series and resultant series with the lowest PAPR is chosen for transmission. To allow the receiver to recover the original data to the multiplying sequence can be sent as side information.
IFFT
Let us define data stream after serial to parallel conversion as X=[X0, X1………, XN1]T. Initially each input X(u)can be
Minimization of PAPR has the relation to the minimization of 01 .


PROPOSED PTSSLM ALGORITHM
Data Input
Partitioning into Blocks
Serial to parallel conversion
defined as equation:
n
n
x(u) = x
n
Partial Transmit Sequence
n
. b(u) (3)
B(u) can be written as x(u) = [x(u), x(u),……, x(u) ]T
Selective Mapping
n 0 1 N2
Where n = 0, 1, 2 N1 and (u=0, 1, 2……U) to make the U phase rotated OFDM data blocks. All U phase rotated OFDM data blocks represented the same information as the unmodified OFDM data block provided that the phase sequence is known.
When we applying the SLM technique, then the complex envelope of the transmitted OFDM signal becomes
Minimum No
Value?
Yes
1 1
Companding
2
() =
=0
, (0 ) (4)
Wheref = 1
Final Output Data
NT
, and NT is the OFDM data block duration.
Lowest PAPR output data is selected for transmission. Effect of PAPR reduction will be better when the copy block number U is increased. SLM method reduces PAPR effectively without any signal distortion.

PARTIAL TRANSMIT SEQUENCE
Figure 1: Flow diagram for proposed work


SIMULATION AND RESULT
0 N=4
10
Orignal PTS
In the PTS technique, input data block X is partitioned in M SLM
disjoint subblocks = [ ,0, ,1, . . ,1], =
1
C
10 SLM OMPANDED
CCDF (Pr[PAPR>PAPR0])
=1
1,2, , , , , , , , such that = and the sub blocks are PTS OMPANDED
C
combined to minimize the PAPR in the time domain. PTS OMPANDED LM
C S
2
The times oversampled time domain signalof , = 10
1,2, , , , , , , , is obtained by taking the IDFT of
length on concatenated with ( 1) zeros. These are 3
called the partial transmit sequences. Complex phase 10
4
factors, = , = 1,2, , , , , , , are introduced to combine the PTSs. The set of phase factors is denoted a
vector [1, 2 , . ]. The time domain signal after 10
combining is given by
=1
=
.
(5)
5
10
2 3 4 5 6 7 8 9 10 11 12
Where = [ , , . ()]. The objective
PAPR0 [dB]
0 1 1
is to find the set of phase factors that minimizes the PAPR.
Figure 2: Comparison of different schemes (showing PAPR v/s CCDF)

CONCLUSION
This article proposes an improved algorithmPTSSLM Companded which is based on SLM and PTS. Both the numerical analysis and the simulation results predict that the new algorithm is effective in reducing the PAPR of OFDM system. Though its PAPR performance is a little worse than the SLM scheme, this new algorithm can reduce complexity of OFDM system greatly.
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