 Open Access
 Total Downloads : 163
 Authors : Sohangjot Kaur Randhawa, Prof. Daljeet Singh Bajwa
 Paper ID : IJERTV6IS070250
 Volume & Issue : Volume 06, Issue 07 (July 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS070250
 Published (First Online): 26072017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
PAPR Reduction in OFDM using PTS Technique
Sohangjot Kaur Randhawa
Department of ECE BBSBEC Fatehgarh Sahib,India
Prof. Daljeet Singh Bajwa
Department of ECE BBSBEC Fatehgarh Sahib,India
Abstract OFDM system is widely used in the field of trending communication. With the increasing use of the OFDM system many issues related to OFDM comes into existence. PAPR is one of the issues of OFDM. It refers to the increment in the peaktoaveragepower ratio of signals that are transmitted to the receiver. It occurs due to the superposition of many sub carriers. The quality of the signals is tainted and the complexity is increased while performing conversion process. Hence it is mandatory to develop such an approach which can help to reduce the PAPR. There are many techniques have been suggested for PAPR reduction, with different levels of success and complexity. Techniques like clipping, filtration, PTS etc were proposed but these techniques achieve PAPR reduction at the bit error rate increase, data rate loss, computational complexity increase, and so on.
This study provides a review over the techniques meant for reducing the PAPR in OFDM systems. It represents various techniques along with their equations.
Keywords: PAPR,SLM,PTS,Clipping and Filtering.

INTRODUCTION
Since the very genesis of man, communication has been one of the fundamental parts of the human life .Previously different techniques like gesture based communications were executed for this reason. At present there are many ways that through which we communicate, email, internet, social networking, letters, cell phones, signs, newspapers, radio, magazines, the list goes on and on.
With the advancement in the technique of wireless communication channel the need of high rate data transmission comes into existence. To transfer the data on a high rate OFDM is used. OFDM stands for Orthogonal Frequency Division Multiplexing. It is an effective method or technique adopted in wireless channels.

PAPR
PAPR stands for PeaktoAverage Power Ratio. PAPR is a exigent issue in OFDM systems. If the PAPR goes high then it leaves direct effects on the performance of the system by degrading it. The PAPR degrades the efficiency of the transmitted signals. Low PAPR increases the efficiency of the power amplifier but high value of PAPR is totally opposite to this.
Figure1. Peak value issue in OFDM system
PAPR of a signal is measured or represented in the form of decibels as shown below. It will also express the need of resolving the problem of PAPR.
Equation for PAPR as:
PAPRdb = 10 log (max[x (t) x * (t)] / E[x (t) x * (t)])

Thus, PAPR defined as the ratio of maximum peak power which will be dividing by average power of OFDM signal.
In the above equation, E shows expected value. Now, PAPR for a single complex tone
X (t) = e2ft

Where t shows period and peak value of the signal: Max[x (t) x * (t)] = max [e2ft e 2ft] = max [e0] =1
As a result, calculate mean square value of the signal:
E[x (t) x * (t)] = E [e2ft e 2ft] =1

Generated output from the above equation is 0db i.e. shows the value of PAPR. Now, consider OFDM time signal which comprises of more than k complex tones (sub Carrier tones).These tones are known as subcarriers. Therefore, the above signal representation will be like:
X (t) = ke j2 k t / T
Consequently, PAPR reduction can be possible by increasing
the probability of getting low PAPR values. PAPR should be reduced in order to enhance the lifetime of the network. In comparison with single carrier systems PAPR is quite high in multi carrier system. Highest value of PAPR reduces the efficiency of the Power amplifier (Transmitter) [3]. PAPR affects the transmitted signal. PAPR is the problem exists in OFDM system. The input symbol stream in IFFT should have a constant value of power spectrum. But the output of IFFT can result in a variable value and fluctuated wave or spikes. Only few of the sub carriers are allotted with energy to transmit the data. This problem gives rise to other problems in OFDM system. So OFDM signal has a very large PAPR, which is very sensitive to non linear high power comprised amplifier. In OFDM, a block of N symbols{X , k = 0,1,….,N 1} k , is formed with each symbol modulating one of a set of subcarriers,
{f , k = 0,1,…..,N 1} k .

The N subcarriers are chosen to be orthogonal, that is, f k f k = D , where Df =1 NT and T is the original time period.

PAPR of single tone: Consider a sin signal as having the period t. The peak value of the signal is: . The Mean square value of the signal is 5.
Given so, the PAPR of a single sine tone is, PAPR OF A MULTICARRIER SIGNAL
Let the data block of length N be
represented by a Duration of vector any system in the set X is T and represents one of the sub carriers set. As the N sub carriers selected for transmission of the signal are orthogonal to each other, so we can have NT is the duration of the OFDM data block X. Reducing the maxx(t) is the principle goal of PARP reduction techniques. In these cases Discrete Time signals are used. There are many PAPR reducing techniques which can be used to handle amplitude value of x (t).
TECHNIQUES FOR PAPR REDUCTION
There are many techniques used for reducing the PAPR in OFDM. But all of these techniques are not suitable in every case. The use of PAPR reduction techniques based on the needs of the system and various elements.

Signal Scrambling Techniques

Block Coding Techniques

Block Coding Scheme with Error Correction

Selected Mapping (SLM)

Partial Transmit Sequence (PTS)

Interleaving Technique

Tone Reservation (TR)

Tone Injection (TI)


Signal Distortion Techniques

Peak Windowing

Envelope Scaling

Peak Reduction Carrier

Clipping and Filtering


SELECTED MAPPING TECHNIQUE (SLM)
SLM stands Selected Mapping Technique. In this technique input data is divided into various sub blocks on the basis of given N length. It uses serial to parallel converter for converting the data stream. When the conversion is applied to the signals then the blocks of the OFDM system are arranged in a sequence as follows:
Where u= [0, 1, 2.U], to make OFDM data blocks to be phase rotated. Therefore X (u) expressed as,
After data blocks are phase rotated, the rotated OFDM data blocks represents similar information which are unmodified OFDM data blocks, provided with known phase sequence.
PARTIAL TRANSMITS SEQUENCE (PTS)
According to PTS technique an N symbol input block is taken, and is divided into V disjoint subblocks. After that the divided subbocks are weighted by the phase vector sequence. The selection of phase factor such that the PAPR of the resultant signal is minimum.
Where v = [1, 2, 3., V], Then, the subblocks X are transformed into timedomain partial transmit sequence x, by using IFFT which can be represented as
V
xm IFFT ( Xm)
v1

INTERLEAVING TECHNIQUE
It is a PAPR reduction technique which collects the interleavers for reducing the value of PAPR. It is quite different from PTS and SLM as it does not uses the set of phase sequence for PAPR reduction.

CLIPPING AND FILTERING
This is easiest and simplest way or technique to redue the PAPR. It is the combination of two processes i.e. clipping and filtering. Clipping is a technique in which a user defined threshold level or clip level is defined. This threshold level is predefined and then the signals are compared by these predefined levels and the signal which crosses the threshold levels clipped or cancelled. The process of clipping causes inband or outofband noise in the signals and clipping is a nonlinear process. This process may reduce the spectral efficiency of the signals and also increases the BER. After applying clipping, filtering is applied to the signals which are received after clipping. Filtering is applied to remove the noise from the signals. It is applied to remove the outofband distortion and spectrum growth efficiency of the signals. After applying filtering signal may rises above the clip level which is considered as the regrowth f the signals. This is the lacking point of applying filtering after clipping. To overcome the disadvantage of filtering the process clipping and filtering should be applied alternatively.
CONCLUSION
From above literature it is concluded that the OFDM suffers from the problem of highest PAPR. In order to reduce the problem of PAPR large number of techniques has been developed. But these techniques are not work sufficiently to reduce the effect of PAPR on OFDM. Hence still the research is under process to reduce the problem of PAPR.
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