1
PAPR Reduction through Lossy Coding in BPSK
OFDM
M.Vasantha Lakshmi, B.Kanmani
M.Vasantha Lakshmi, assistant Professor, BMS College of Engineering, Bangalore -560019 phone: 9741557906; e-mail: (vasantha.tce@bmsce.ac.in).
B.Kanmani, Professor and Dean Academic, BMS College of Engineering, Bangalore-560019. (e-mail: bkanmani.tce@bmsce.ac.in).
ABSTRACT
OFDM is a multicarrier modulation technique efficiently used for high speed data communication. OFDM provides high spectral
efficiency and high tolerance to multipath fading hence it is considered as one of the popular technique for digital
communication. In OFDM output is the superposition of multiple subcarriers hence, some instantaneous power output may
become very high greater than the mean power of the system resulting in the high Peak to Average Power of the system. But the
major disadvantage of OFDM system is the high Peak to Average Power Ratio (PAPR) which results in the degradation of the
signal quality. Such high PAPR cannot be used for any practical applications. To reduce the PAPR in the OFDM signal, a Lossy
Coding method was proposed and implemented in Lab VIEW. By generating the OFDM symbols by considering different carrier
frequencies from 10 to 60 KHz for symbol length M=6, PAPR has been calculated for all possible combinations of the inputs. It
was observed that out of the 2M different combinations of the inputs only four combinations has very high PAPR compared to
the other possible combinations of the inputs with the introduced Bit Error Rate (BER). Though we introduce the BER, typical
OFDM is of larger symbol length and by increasing the value of M the Bit error rate will reduce. The experiment has been
repeated for symbol length ranging from 6 to 16 and for different sets of carrier frequencies. It was observed that for all the
symbol length from 6 to 16, only four combinations have very high PAPR. By toggling the LSB of the four combinations by
using Combinational Logic, the PAPR can be reduced to 40%..
Keywords:
Bit Error rate, OFDM, Peak to Average Power, Symbol Length
1. INTRODUCTION
In a 4G Wireless communication systems one of the popular
multicarrier techniques used is the Orthogonal Frequency
Division Multiplexing (OFDM). In OFDM bandwidth is
divided into data streams and then modulated using different
carrier frequencies. This increases the spectral efficiency and
hence finds the wide application in the field of wireless
communication. OFDM is the efficient method for high speed
data transmission because of the spectral efficiency.
With the growth of telecommunication and the increasing
technology, there is a greater demand for the higher
bandwidth. OFDM efficiently utilizes the available bandwidth
of the spectrum but still there is a greater scarcity of the
bandwidth allocation with the increasing demand. The
limitation in the modulation schemes of the existing
communication systems does not support the higher data rate.
OFDM output is the superposition of the multiple sub
carriers. While increasing the bandwidth some power may
become very high instantaneously greater than the mean power
of the system which results in the high peak power compared
with the average power of the system resulting in high PAPR.
This is the major disadvantage of the OFDM systems. High
PAPR will result in the degradation of the quality of the signal
and hence reduces the efficiency of the system to a greater
extent. Hence it becomes a major goal to reduce this high peak
power in any of the communication systems. Several methods
like Signal Scrambling Techniques and signal Distortion
Techniques has been proposed to reduce the PAPR in OFDM
systems and has been implemented and a significant reduction
in PAPR has been achieved. In the Section –II, we propose a
new method to reduce the PAPR in BPSK OFDM through
Lossy Coding. This has been implemented in LabVIEW and
the result of the implementation is presented in Section-III.
Finally we conclude in Section-IV.
2. PROPOSED METHOD
To reduce the PAPR in OFDM system, we propose a new
Lossy Coding method with the introduced Bit Error Rate. The
methodology used is a follows:
Let M be the length of the OFDM symbol to be transmitted
where M = 1, 2,…..16. The possible combinations are 2M.
OFDM signals have been generated for all the possible
combinations of M bits by considering different carrier
frequencies for each bit. The peak power has been calculated
for all the possible combinations and it has been observed that
2
out of the 2M combinations, only 4 combinations have very
high peak value compared with the other possible
combinations of M bits. This high peak value results in the
increase in the PAPR. The proposed method is to avoid these
four combinations with the high PAPR and represent these
four combinations in terms of other possible combinations
with the reduced PAPR. This can be done by toggling the
LSB of the bit pattern based on the Combinational Logic as
shown in Figure 1. The proposed method reduces the PAPR
with the introduced BER.
Though we introduce the BER, Typical OFDM is of larger
symbol length and by increasing the value of M, the BER will
be reduced.
For M=6, the possible combinations are 000000 to 111111.
For all the 2M combinations, OFDM symbols are generated
with the carrier frequencies as 10, 20, 30, 40, 50 and 60Hz and
the peak values are calculated. The peak values lie between 9.5
and 36. For the four combinations 000000, 010101, 101010
and 111111, the peak values are 36 and the remaining
combinations has a peak value ranging from 16.3 to 9.5. Based
on the observations, if we toggle the LSB of these four bit
patterns say instead of 000000 if we transmit 000001 the BER
will be reduced from 36 to 16 with the 50% reduction in the
PAPR with the introduced BER of 0.01 which is not
acceptable. By increasing the value of M, BER will be
reduced.
The experiment has been performed using LabVIEW 2014
as hardware implementation to simulate the Lossy encoding
method to compute PAPR. Any circuit simulation tool like
Multisim/ Pspice or others can be used to perform the same.
The results can be reproduced with the suitable hardware.
Figure 2 shows the OFDM Signal generation for the bit pattern
000000 for M=6 along with the peak value of 36 generated
and Figure 3 shows the OFDM signal generated for the
toggled bit pattern of 000000 to 000001 with the reduced peak
value of 16.
Figure 4 shows the OFDM signal generated for the Bit pattern
111111 for M = 6 with the peak value of 36 and Figure 5
shows the OFDM signal generated for the toggled bit pattern
111111 to 111110 with the reduced value of 16.
Figure1 Combinational Logic of toggling the LSB
3
a)
Figure 2 OFDM signal for 000000 bit pattern for M=6
Figure 3 OFDM signal with the toggled bit pattern 000001for M=6
Figure 4 OFDM signal for 111111 bit pattern for M =6
Figure 5 OFDM signal with the toggled bit pattern 111110 for M =6
4
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8.
3. CONCLUSION
The proposed Lossy encoding method has been implemented
in LABVIEW for the value of M ranging from 6 to 16. It has
been observed that only four combinations out of 2M have
very high peak value compared with the other combinations. If
we change the bit patterns of the four combinations by
toggling the LSB of the bit patterns, significant reduction in
the peak value has been achieved with the introduced BER. By
increasing the value of M the BER will be reduced.
ACKNOWLEDGMENT
The research work presented in this paper has been supported
by the College through TEQIP-II of MHRD, Government of
India and the authors acknowledge the technical support by the
National Instruments.
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BIOGRAPHY
M.Vasantha Lakshmi, obtained her Bachelors in Electronics
and Communication Engineering form Visveswaraiah
Technological University in 2003, M.Tech Degree in Digital
communication and Networking from Visveswaraiah
Technological University in 2008 and pursuing PhD in the
domain of Wireless Communication. She has been with BMS
College of Engineering, Bangalore, since 2005, and has to her
credit over 8 International publications.
As Assistant
Professor in the department, she teaches under-graduate
courses related to Communication Domain. Her prior
employment as a teaching faculty was with Kuppam College of
Engineering (AP). She is Member IEEE, and ARSI.
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B Kanmani, obtained her Bachelors in Electronics and
Communication Engineering form Nagarujuna University in
1987, M.Tech. degree in Digital communication from Indian
Institute of Technology, Kanpur in 1990, and PhD from the
Indian Institute of Science Bangalore (IISc) in the year 2006.
She has been with BMS College of Engineering, Bangalore,
since 1995, and has to her credit over 45 International
publications. She has delivered lectures on ‘Signals &
Systems’, during the VTU-EDUSAT Program -10, and
contributed modules on CNX.ORG. Having held the position
of Head of the department of Telecommunication Engineering,
for more than a decade, she has steered the department to
effective implementation of Outcomes Based Education. As
Professor in the department, she teaches under-graduate
courses related to Signal Processing and Communication.
Presently, she holds the position of Dean Academics. Her prior
employment as a teaching faculty was with Thadomal Shahani
College of Engineering (Mumbai) and K L College of
Engineering (Guntur). She is Senior Member IEEE, and Life
Member ISTE.