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Vol.04,Issue.18,

November-2016,

Pages:3522-3525

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A Methodology Used to Reduce Papr in LTE Systems M. RESHMA BAI

1, A. KARUNAKAR

2

1PG Scholar, Dept of ECE, Sree Rama College of Engineering, Tirupati, A.P, India, E-Mail: jhansireshma@gmail.com.

2Asst. Professor, Dept of ECE, Sree Rama College of Engineering, Tirupati, A.P, India, E-Mail: ayyalakarunakar@gmail.com.

Abstract: Multiple-Input Multiple-Output orthogonal

frequency division multiplexing (MIMO-OFDM) technology

is one of the most attractive candidates for fourth generation

(4G) mobile radio communication. It effectively combats the

multipath fading channel and improves the bandwidth

efficiency. At the same time, it also increases system capacity

so as to provide a reliable transmission. However, the main

drawback of MIMO-OFDM system is high peak-to-average

power ratio (PAPR) for large number of sub-carriers, which

result in many restrictions for practical applications. Coding,

phase rotation and clipping are among many PAPR reduction

schemes that have been proposed to overcome this problem.

In this paper, we mainly investigate the PAPR reduction

performance with two different PAPR reduction methods:

partial transmit sequence (PTS) and selective mapping (SLM).

As an extension to this paper we are proposing other novel

method that is modified PTS in LTE systems and compare

these results with the other two methods. The results shows

better PAPR reduction compared to the existing algorithms.

Results are verified using MATLAB software.

Keywords: Orthogonal Frequency Division Multiplexing

(OFDM), Peak-To-Average Power Ratio (PAPR), Selected

Mapping (SLM), Partial Transmit Sequence (PTS), Sub-

Optimal Algorithm, Complementary Cumulative Distribution

Function (CCDF).

I. INTRODUCTION

Third generation (3G) mobile communication Technologies

implemented almost around the world, are not able to meet

the requirements of the day viz., high data rates nor

throughputs. Besides, voice communication in 3G relies on

circuit switching technology, the same as in second-

generation (2G) communication systems, rather than pure

Internet Protocol (IP) approach. Researchers have been trying

for the next evolutionary fourth generation (4G)

communication systems to provide a comprehensive and

secure IP solution where voice, data, and multimedia can be

offered to users at "anytime, anywhere" with higher data rates

than previous generations [1]. Since bandwidth resource for

4G mobile communications is still scarce. To improve

spectrum efficiency and achieve higher rates at more than

100Mbps more advanced techniques need to be implemented.

The limitation of modulation schemes in existing

communication systems has become an obstruction in further

increasing the data rate. Hence, next generation mobile

communication systems need more sophisticated modulation

scheme and information transmission structure. Multiple input

multiple outputs (MIMO) and orthogonal frequency division

multiplexing (OFDM) have therefore been adopted due to

their superior performance. Long Term Evolution (LTE) is

standardized by the 3rd Generation Partnership Project

(3GPP) as an evolution of the 3G systems to meet the

requirements of increasing the data rates, high mobility and

low latency over a bandwidth of up to 20 MH. Researchers

have been trying for the next evolutionary fourth generation

(4G) communication systems to provide a comprehensive and

secure IP solution where voice, data and multimedia can be

offered to users at "anytime, anywhere" with higher data rates

than previous generations.

Multiple input multiple outputs (MIMO) and (OFDM)

modulation have therefore been adopted due to their superior

performance. These developing modulation used in LTE

which promise to become the key for ” high-speed wireless

communication technologies” and combining them can

provide wireless industry evolution from 3G to 4G systems. In

OFDM systems which are use MIMO state that, the output is

the superposition of Multiple sub-carriers in this case,

instantaneous power outputs increases and may demand

higher powers than the mean power of the system since the

phases of these carriers are the same. OFDM is multicarrier

multiplexing access Technique for Transmitting Large data

over Radio waves. One of the major drawbacks of OFDM

signals is its large PAPR, which causes poor power efficiency

to transmit amplifier`s power [4].And to reduce the PAPR,

many techniques have been proposed, such as clipping,

coding, PTS, selected mapping (SLM), interleaving[5][6],

nonlinear transforms[20]. These schemes can mainly be

categorized into signal scrambling techniques, such as PTS,

and signal distortion techniques such as clipping, companding

techniques; in this Paper all techniques which can be used to

reduce PAPR in OFDM system are listed. The rest of this

paper is organized as follows. Section II first reviews existing

method. Our proposed method is described in Section III.

Then experimental results are reported in Section IV to

demonstrate the superior performance of our framework.

Finally, conclusions are presented in Section V.

II.PROBABILITY DISTRIBUTION FUNCTION OF

PAPR According to central limit theorem, for a large number of

sub-carriers in multi-carrier signal, the real and imaginary part

M. RESHMA BAI, A. KARUNAKAR

International Journal of Innovative Technologies

Volume.04, Issue No.18, November-2016, Pages: 3522-3525

of sample values in time-domain will obey Gaussian

distribution with mean value of 0 and variance of 0.5.

Therefore, the amplitude of multi-carrier signals follows

Rayleigh distribution with zero mean and a variance of N

times the variance of one complex sinusoid [15]. Its power

value obeys a 12 distribution with zero mean and 2 degrees of

freedom. Cumulative Distribution Function (CDF) is

expressed as follows

(1)

Assuming that the sampling values of different sub-channels

are mutually independent, and free of oversampling operation,

the probability distribution function for PAPR less than a

certain threshold value, is therefore expressed as

(2)

In practice, it is preferred to take the probability of PAPR

exceeding a threshold as measurement index to represent the

distribution of PAPR. This can be described as

“Complementary Cumulative Distribution Function” (CCDF),

and its mathematical expression as

(3)

III.SIGNAL SCRAMBLING TECHNIQUES

The emergence of high peak power signal in OFDM system

is due to the superposition (IFFT operation) of multiple sub-

carrier signals. If multiple sequences which carry the same

information are used to represent one transmission process,

then the best one can be chosen among those candidates for a

given PAPR threshold condition. In this way the occurrence

Probability of peak power signal can significantly be reduced.

The phase rotation method used for reducing the PAPR of

OFDM signal is a special case of Multiple Signal

Representation (MSR) technology. Its fundamental principle

is: Generating multiple signal waveforms which carry the

same information and then choose the waveform from those

candidates with the smallest PAPR for transmission. This

approach reduces the occurrence probability of high peak

power signal effectively and optimizes the statistical

characteristics of PAPR. This method is one of the non-

distortion methods used for reducing PAPR. However, until

now, there exist two most effective and meritorious proposals

which are called SLM and PTS. In this paper, investigation

into these two potent probabilistic approaches is conducted.

The two have received so much attention since they provide a

low reduction in throughput.

IV. PROPOSED MITIGATION SCHEME

A) Selection Mapping Technique (SLM)

The CCDF of the original signal sequence PAPR above

threshold PAPR0 is written as 𝑝𝑟(𝑃𝐴𝑃𝑅 > 𝑃𝐴𝑃𝑅0).Thus for

K statistical independent signal waveforms, CCDF can be

written as 𝑷𝒓 𝑷𝑨𝑷𝑹 > 𝑷𝑨𝑷𝑹𝟎 𝑹, so the probability of

PAPR exceed the same threshold. The probability of PAPR

larger than a threshold Z can be written as

𝑷 𝑷𝑨𝑷𝑹 < 𝒁 = 𝑭 𝒁 𝑵= 𝟏 − 𝐞𝐱𝐩(−𝒛) 𝑵 (4)

Assuming that M-OFDM symbols carry the same

information and that they are statistically independent of each

other. In this case, the probability of PAPR greater than Z is

equals to the product of each independent probability. This

process can be written as

𝑷 𝑷𝑨𝑷𝑹𝑳𝑶𝑾 > 𝒁 = (𝑷 𝑷𝑨𝑷𝑹 > 𝒁 )𝑴=((𝟏 − 𝐞𝐱𝐩(−𝒛))𝑵)𝑴 (5)

In selection mapping method, firstly M statistically

independent sequences which represent the same information

are generated, and next, the resulting M statistically

independent data blocks𝑆𝑚 = 𝑆𝑚 ,0, 𝑆𝑚 ,1, 𝑆𝑚 ,𝑁−1 𝑇, for

m=1,2,...,M are then forwarded into IFFT operation

simultaneously. 𝑥𝑚 = 𝑥1, 𝑥2 , 𝑥𝑁 𝑇 in discrete time-domain

are acquired and then the PAPR of these M vectors are

calculated separately. Eventually, the sequences xd with the

smallest PAPR is selected for final serial transmission. Figure

1 shows the basic block diagram of selection mapping

technique for suppressing the high PAPR.

Fig. 1. The Block Diagram of Selected Mapping Technique

B) Partial Transmit Sequence (PTS)

Partial Transmit Sequence (PTS) algorithm is a technique

for improving the statistics of a multicarrier signal. The basic

idea of partial transmit sequences algorithm is to divide the

original OFDM sequence [9] into several sub-sequences and

for each sub-sequences multiplied by different weights until

an optimum value is chosen.

Fig.2. The Block diagram of PTS Technique

Figure 2 [10] is the block diagram of PTS technique. From

the left side of diagram, the data information in frequency

domain X is separated into V non overlapping sub-blocks and

each sub block vectors has the same size N. So for each and

every sub-block it contains N/V nonzero elements and set

the rest part to zero. Assume that these sub-blocks have the

A Methodology Used to Reduce Papr in LTE Systems

International Journal of Innovative Technologies

Volume.04, Issue No.18, November-2016, Pages: 3522-3525

same size and no gap between each other. The sub-block

vector is given by

𝑿 = 𝒃𝒗𝑽𝒗=𝟏 𝒙𝒗 (6)

𝑤ℎ𝑒𝑟𝑒 𝑏𝑣 = 𝑒𝑗𝜑𝑣(𝜑𝑣𝜖 0,2𝜋 ){v=1,2,……,𝑋𝑣}is a weighting

factor been used for phase rotation.The signal in time domain

is obtained by applying IFFT operation [11] on, that is

𝒙 = 𝑰𝑭𝑭𝑻 𝑿 = 𝒃𝒗𝑰𝑭𝑭𝑻 𝑿𝒗 = 𝒃𝒗𝑿𝒗𝑽𝒗=𝟏

𝑽𝒗=𝟏 (7)

For the optimum result one of the suitable factor from

combination b = [b1, b2 , .., bv] is selected and the

combination is given by

𝒃 = 𝒃𝟏, 𝒃𝟐, … , 𝒃𝒗 = 𝒓𝒈𝒎𝒊𝒏(𝒃𝟏,𝒃𝟐,…,𝒃𝒗)(𝒎𝒂𝒙𝟏≤𝒏≤𝑵 𝒃𝒗𝑽𝒗 𝑿𝒗

𝟐)

(8) Where arg min [(·)] is the condition that minimize the output

value of function.

C. Modified PTS

The step by step procedure of the modified PTS is given

below:

Step-1: In PTS, data symbols in X are partitioned in to V disjoint sub

blocksX(i)

,where 1≤i≤V, such that X = sum (X(i)

) ; 1 ≤ i ≤V

Step-2: The sub blocks X

(i) are transformed in to V time-domain

partial transmit sequences. x(i)

= IFFT (X(i)

) ; 1 ≤ i≤ V

Step-3: These sequences are independently rotated by some phase

factors, bi equal to exp(j*φi) ; 1≤ i ≤ V.

Step-4: These are then combined to produce the time domain OFDM

signal packet back x = sum(bi * x(i)

; 1≤ i ≤ V

PTS algorithm selects a vector bi such that the PAPR of the

corresponding transmit sequence x(t) will be the lowest. IQ

vector of different wireless standards will have different

PAPR time domain pattern based on complex modulation

schemes used and OFDM/OFDMA structure. Hence during

simulation vivid iterations need to be carried out. The

comparison between these three methods can be done by the

simulation results.

V. RESULTS

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

M. RESHMA BAI, A. KARUNAKAR

International Journal of Innovative Technologies

Volume.04, Issue No.18, November-2016, Pages: 3522-3525

V. CONCLUSION

OFDM is a very attractive technique for wireless

communications due to its spectrum efficiency and channel

robustness. One of the serious drawbacks of OFDM systems

is that the composite transmit signal can exhibit a very high

PAPR when the input sequences are highly correlated. In this

paper, several important aspects are described as well as

mathematical analysis is provided, including the distribution

of the PAPR used in OFDM systems. As an extension we

proposed other method called modified PTS method and we

compared the results. The results show that the modified PTS

method is the best method to reduce the PAPR.

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