 Open Access
 Total Downloads : 439
 Authors : Gagandeep Singh, Ranjit Kaur
 Paper ID : IJERTV3IS090223
 Volume & Issue : Volume 03, Issue 09 (September 2014)
 Published (First Online): 12092014
 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 Reduced Complexity Selective Mapping Technique
Gagandeep Singh
PG Student,ECE Department Punjabi University
Patiala, Punjab,INDIA
Dr. Ranjit Kaur
Associate Professor,ECE Department Punjabi University
Patiala, Punjab,INDIA
Abstract Orthogonal Frequency Division Multiplexing (OFDM) is the most promising technique available and is most commonly used due to its advantages like very high data rate, noise immunity and efficient bandwidth utilization. One of the major drawback of OFDM is its high Peak to Average Power Ratio (PAPR). In this paper Selective mapping is used to perform PAPR reduction and same time reducing its complexity so that its implementation uses less floor area and on chip requirements. Results shows that PAPR reduction is similar to Conventional SLM but new technique is less complex than original one in terms of IFFT blocks used and Multipliers used in the process.
This paper is organized as follows: Section I describes OFDM system. Section II describes PAPR in OFDM system. Section III describes Improved SLM as PAPR reduction techniques. Section IV describes simulation results and Section V describes conclusion.
II. PAPR IN OFDM SIGNAL
Let the input data block of length N be represented by block X S [ X1 , X 2, X 3 …X N 1 ] , Thus OFDM symbol can be written as:
e
N 1
Keywords – Orthogonal frequency division multiplexing (OFDM), Peak to average power ratio (PAPR), Selective Mapping (SLM), Bit Error Rate (BER), Digital Subscriber Line
(DSL), Partial Transmit Sequence (PTS).
Z (s) = X
k 1
j 2kf0t
S
(1)
Where Z(s) is the OFDM symbol,
X S is the input data

INTRODUCTION
Orthogonal Frequency Division Multiplexing (OFDM) is a digital transmission Technique developed to meet the increasing demand of higher data rates in wireless and
block and N is the number of symbols in input data block.
PAPR is defined as the ratio of Peak to Average Power. Mathematically PAPR can be defined as following:
wired communications. Due to its high data rate handling capacity, it is most often used in high speed networks like Digital Video Broadcasting (DVB), Digital Subscriber Line
(DSL) and in wireless communication[3][12]. The main
PAPR Z
s
maxZ s 2
EZ s2
(2)
disadvantage in using OFDM is its high Peak to Average Power Ratio (PAPR). Such a high PAPR causes the linear amplifier to have large dynamic range which is difficult to accommodate[8].
Where Z (s) is the OFDM signal,
signal power, E Z s2 power[3].
maxZ s2 is the peak is the average signal
Thus it becomes necessary to reduce the PAPR to keep the High Power Amplifier (HPA) in linear range. There are numerous techniques available for PAPR reduction. These are categorized as Signal Distortion Techniques and Signal Scrambling Techniques. Distortion Techniques causes signal distortion and Bit Error Rate (BER) to grow after certain point, whereas Signal Scrambling techniques causes no harm to the OFDM signal. The only disadvantage in using signal scrambling techniques is their increased system complexity[3][6].

SELECTIVE MAPPING
Selective Mapping is a Signal Scrambling technique to reduce PAPR in OFDM system. The advantage in using this technique is that it does not effects the system performance in terms of Bit Error Rate. The only disadvantage in using this technique is its high complexity. The basic concept behind this technique is the phase rotation of the modulated data before performing IFFT operation. After performing phase rotations, the signal with lowest PAPR is selected [2][8][4].
Fig. 1. Block diagram of SLM technique[2]
Let the input data block be represented as
Where is the oversampling factor, N is no. of subcarrier, U is total no. of independent phase sequences.
A. IMPROVED SELECTIVE MAPPING
The drawback in using Conventional Selective Mapping is its high Implementation complexity. The complexity of this technique increases exponentially upon increasing the number of phase sequence and number of subcarriers in OFDM system.
So, here a new and improved SLM technique is presented which is less complex than conventional SLM but the original concept remains intact and PAPR reduction is similar to Conventional SLM. This technique is modified
X S [ X 0
, X1,
X 2 …X
T
]
N 1
(3)
to incorporate only one IFFT block and multiplier in place
And independent phase sequences are given by
of multiple ones and replace the parallel processing with serial processing. Figure below illustrates the Single IFFT
BU [BU , BU …..BU
]T ,U (1,2….U )
(4)
block SLM.
0 1 N 1
Where BU
is the phase sequence, U is the total no. of
If the number of subblocks is M, then the data block can
Phase sequences and T is the length of input data block.
After applying Phase rotation, IFFT is applied to obtain data block with different PAPR value and phase
be expressed as:
(7)
sequences[2].
After serial to parallel conversion one data block can be
represented as:
X U [ X U , X U ,…..X U ]T
(5)
0 1 N 1
(8)
Where X U is the OFDM symbol generated after IFFT
operation. After this the stream with lowest PAPR is selected for transmission. CCDF is used for PAPR representation. CCDF of PAPR in SLM can be represented as
Each of the subblocks is then multiplied with independent phase sequences and passed through an Npoint IFFT block to obtain the corresponding SLM. Which is given by:
P(PAPR PAPR0) (1 (1 e PAPR 0 ).N )U
(6)
Fig. 2. Block diagram of proposed SLM
Let the Phase Sequences is represented by:
Where M is the total no of input data blocks and u is the no. of phase sequences of length equal to n. Considering
Pu [ pu , pu …. pu ]
(9)
the number of phase sequences in SLM be 4 and input data
0 1 n1
Where u is the no. of total phase sequences each of lenth n.The above Phase sequence is multiplied with data block to perform phase rotation.
block be 64, then quantitative analysis can be summarized as in following table:
Table II. Shows quantative analysis with example
u
Zn Xn * P
(10)
Process
Original SLM
Single IFFT Block
SLM
N point IFFT
blocks required
256
64
Phase Multipliers
required
256
64
PAPR calculations
performed
256
256
SLM output for first block in first phase sequence will be:
Yn IFFT [Zn ]
(11)

SIMULATION RESULTS
It is observed that the results obtained from both the
PAPR of this SLM rotated OFDM symbol can be calculated as:
techniques are similar in PAPR reduction values but variation in time consumption of the two algorithms.
2
2
PAPR [Yn ] =
max[Yn ]
(12)
CCDF Plot for PAPR using SLM with 64 subcarriers
0
primary OFDM 

SLM U=2 SLM U=4 

10
E[Yn ]
Pr(PAPR>PAPR0)
10
Similarly, the input data block is rotated U number of times 1
and is stored in Memory Buffer temporarily. The PAPR is calculated for each sequence and lowest one is selected for transmission. Similarly next data block enters the system and same steps are repeated for total number of data
2
blocks. 10
Simple mathematical calculations have been done for both the techniques to find out the number of times these operations have to be performed. The values are enlisted in the following Table:
Process 
Original SLM 
Single IFFT Block SLM 
N point IFFT blocks required 

Phase Multipliers required 

PAPR calculations performed 
Table I. Shows quantative analysis
3
10
2 4 6 8 10 12 14
PAPR0/dB
Fig 3. CCDF Plot of PAPR using proposed SLM for 64 subcarriers
with U=2,4.
In the above graph of proposed SLM for 64 subcarriers , PAPR levels are reduced with the increase phase sequences(U).
CCDF Plot for PAPR using SLM with 64 subcarriers
0
10
primary OFDM
SLM U=8
SLM U=16
SLM U=32
1
Pr(PAPR>PAPR0)
10
2
10
3
10
2 4 6 8 10 12 14
PAPR0/dB
Fig. 4. CCDF Plot of PAPR using proposed SLM for 64 subcarriers with U=8,16,32.
From the above graph of proposed SLM it is observed that PAPR levels keep on decreasing as Phase sequences are increased. but the margin of decrease in value of PAPR keeps on decreasing with the increase in value of U.
IV. CONCLUSION
In this paper, OFDM is studied and new SLM technique is implemented. This technique performs PAPR reduction similar to original SLM technique as shown in the graphs but implementation complexity is reduced to great extent. This technique does not cause out of band radiations or degrades BER performance. As in this technique hardware is reduced to great extent in terms of IFFT blocks and Phase multipliers used. But this technique reduces throughput of data as it takes large time for processing of data. So we can use this technique for less no. of phase sequences.
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