International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 8-April 2015
PAPR Reduction for OFDM system using iterative
clipping and filtering and performance improvement
using different FEC code
Nidhi Jain #1, Vinod Sonkar *2
#
Department of Electronics & Communication Engg. Indore(M.P.)
Sushila Devi Bansal College of Technology, Indore
Abstract— Wireless communication has been one of the
several wireless standards such as digital audio
broadcasting (DAB), digital video broadcasting
(DVB-T), the IEEE 802.11a LAN standard and the
IEEE 802.16a [2] MAN standard.
OFDM has become a popular technique for
transmission of signals over wireless channels [3].
The OFDM converts a frequency-selective channel
into a parallel collection of frequency flat subchannels. If knowledge of the channel is available
at the transmitter, then the orthogonal frequency
division multiplexer transmitter can adapt its
signaling strategy to match the channel. The
available bandwidth is used very efficiently.
OFDM
(orthogonal
frequency
division
multiplexer) is a block modulation scheme where a
block of N information symbols is transmitted in
parallel on N sub-carriers. The time duration of an
Keywords—
OFDM
(Orthogonal
Frequency
Division
Multiplexing), PAPR, iterative Clipping and Filtering (ICF), orthogonal frequency division multiplexer symbol
Forward Error Control (FEC)
is N times larger than that of a single-carrier
system.. The OFDM systems are attractive for the
way they handle ISI, It is usually introduced by
INTRODUCTION
frequency selective multipath fading in a wireless
1. Wireless communication
environment [4]. ISI has very small or no effect on
Wireless communication is one of the dynamic the OFDM systems hence an equalizer is not
areas in the communication field today. Wireless needed at the receiver side.
communication is
the transfer
of
information without any electrical conductor 2. Peak to Average Power Ratio
between two or more points. There are various In OFDM system, because the transmitted signal is
generation in the wireless system existing now a the sum of a set of modulated signal, the peak
day such as 2G, 3G and 4G etc[1][2].Currently 4G power of the transmitted signal can be very high
wireless systems are preferred for high speed data compared to its average power. Although occurring
communication. Orthogonal Frequency division only with low probability, such large peaks have
multiplexing is a type of multicarrier modulation negative ramifications for the overall system. High
scheme which are used in wireless communication. Peak-to-Average Power Ratio has been recognized
OFDM systems have lot of advantages in providing as one of the major practical problem involving
high data rates with sufficient robust against radio OFDM modulation [6]. The PAPR of OFDM
channel impairments. OFDM has been adopted in signals x(t) is defined as the ratio between the
fastest growing segments in the telecommunication
industry. Orthogonal Frequency Division Multiplexing
(OFDM) is considered to be a promising technique against
the multipath fading channel for wireless communications.
However, OFDM faces the Peak-to-Average Power Ratio
(PAPR) problem that is a major drawback of multicarrier
transmission system which leads to power inefficiency in
RF section of the transmitter. large Peak-to-Average
Power Ratio of these signal have some undesirable effects
on the system. Tough, many schemes presented to reduce
the PAPR, most of them are at the cost of increasing the
systems’ complexity. Iterative clipping and Filtering (ICF)
in combination with powerful forward error correction
(FEC) can provide significant performance improvement
in OFDM. By the method of PAPR reduced for OFDM is
presented here, the PAPR, can be lowered to some degree
without excessively increasing the complexity of the system
and degradation the quality of communication.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 8-April 2015
maximum instantaneous power and its average
power during an OFDM symbol.
The PAPR can be defined as
2.1
Where Ppeak and Pav can be compute as:
2.2
Pav=
Hence, the PAPR is expressed as:
2.3
When N sinusoids add, the peak magnitude would
have a value of N, in average might be quite low
due to the destructive interference between the
sinusoids.
In this probabilistic approach certain schemes have
been proposed by researchers. These include
clipping, coding and signal scrambling techniques.
Under the heading of signal scrambling techniques
there are two schemes included. Which are Partial
transmit sequence (PTS) and Selected Mapping
(SLM). Although some techniques of PAPR
reduction have been summarized in [7], it is still
indeed needed to give a comprehensive review
including some motivations of PAPR reductions.
An effective PAPR reduction technique should be
given the best trade-off between the capacity of
data rate loss, PAPR reduction and transmission
power, implementation complexity and Bit-ErrorRatio (BER) performance etc. In this paper, firstly
the distribution of PAPR based on the
characteristics of the OFDM signals are
investigated then typical PAPR reduction
techniques are analyzed.
PAPR can be described by its
complementary cumulative distribution function
(CCDF). The CCDF of PAPR is defined as the
probability that the PAPR of the OFDM symbols
exceeds a given threshold A. The complementary
cumulative distribution function (CCDF) is the
distribution of PAPR and has stochastic
characteristics.
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2.4
2.1 PAPR Reduction Techniques
PAPR=10 log10
PAPR=10log10
CCDF=1-Pr (PAPR
Several PAPR reduction techniques have been
proposed they are mainly divided in two types
which are as follows.
I.
Signal scrambling techniques.
II.
Signal distortion techniques.
Clipping & Filtering method:
The paper, by Wang and Luo, [8] proposes the
method of PAPR reduction by manipulating the
OFDM signal with a suitable additive correcting
function. In this approach, the amplitude peaks are
corrected (or signal is modified) in such a way that
a given amplitude threshold of the signal is not
exceeded after the correction.
The clipping is the easiest technique to reduce the
power by setting a maximum level for the
transmitted signal , we can use filtering operation to
decrease the appearance of the out-of-band
radiation but the signal may exceed the maximum
level of the clipping operation [9]. The outcome of
the filtering stage is a less degraded BER
performance and a reduced out-of-band radiation.
Though, the PAPR reductions improvements are
gained at the cost of regrowth the peak where the
signal could go beyond the clipping level after
applying the filtering operation.
3. Error Correcting codes:
There is different error correcting codes. In this
paper we used Reed Solomon (RS) code and
Convolution (CC) codes.
Reed Solomon codes are systematic linear block
codes and are an important sub class of non-binary
BCH codes. RS codes operate on the information
by dividing the message stream into blocks of data,
in adding redundancy per block depending only on
the current inputs. The symbols in RS coding are
elements of a finite field or Galois Field (GF). GF
arithmetic is used for encoding and decoding of
reed Solomon codes.
As any binary code, convolution codes protect
information by adding redundant bits. A rate-k/n
convolution encoder processes the input sequence
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 8-April 2015
of k-bit information symbols through one or more 5. Result:
binary shift registers (possibly employing feedback).
The MATLAB code has been developed for OFDM
based data communication. The problem of PAPR
4. System Description
has been addressed using the clipping technique.
To resolve the problem regarding BER degradation
These techniques generate distortion which is
we are using Convolution coding process that not
simulated by the different clipping ratio and recover
only reduce PAPR but also abate the BER.
the performance by filtering and FEC code.
TABLE 5.1 LIST OF PARAMETERS
Serial no.
1.
Parameter
Modulation
Technique
FFT size
Data carrier
Cyclic Prefix
FEC code
No. of OFDM
symbol
PAPR reduction
technique
Crest Ratio
2.
3.
4.
5.
6.
7.
8.
Value
QPSK,QAM16,QAM-32
64
26,52
¼
RS ,CC
50
Clipping and
filtering
1.1/1.2/1.3
Figure 3. Iterative Clipping & Convolution coded
OFDM system model
The following simulation has been performed.
1. QAM-16 based OFDM simulation with
Crest Ratio(Cr) =1.1
The block diagram of OFDM model using iterative
2.
QAM-16 based OFDM simulation with
clipping & Filtering method is shown in Fig 3.
Crest Ratio(Cr) =1.2
Firstly, the serial to parallel converter converts the
3.
Figure 5.1 PAPR vs. CCDF curve for QAMbit stream into a number of parallel bit streams.
16 & Crest Ratio-1.1.
Then, the bit streams are modulated according to
modulation techniques for 4-QAM, 8-QAM, and
16-QAM etc. Then, each block of bits is forwarded
to the Inverse fast Fourier transform (IFFT) and
transformed into an OFDM signal.
The length of the cyclic prefix is chosen to be larger
than the maximum path delay of the channel. Then,
the converted parallel to serial signals are sent out.
At the receiver, assuming a perfect timing and
carrier frequency
synchronization, first the
received signals are converted to parallel signals
and then the cyclic prefix removed. After going
SNR Vs. SER curve with and without ICF
through the demodulation process, with the
10
estimated channel information the data symbols are
detected. Then the output we get decoding process
in serial data. This procedure shown in the above
10
fig.3 gives not only reduced PAPR value but also
improves BER performance of the system. The
graphs shown in simulation & result section of the
paper clarify the process shown in the system
10
model.
Without code
PAPR Vs. CCDF Curve with and without ICF
1
0.9
0.8
0.7
CCDF
0.6
0.5
0.4
0.3
Without code
With RS code
With CC code
With RS CC code
0.2
0.1
0
1
2
3
4
5
6
7
8
9
14
16
10
PAPR
0
SER
-1
-2
With RS code
With CC code
With RS CC code
-3
10
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2
4
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6
8
10
12
SNR
18
20
22
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International Journal of Engineering Trends and Technology (IJETT) – Volume 22 Number 8-April 2015
Figure 5.2 SNR vs. SER curve for QAM-16 & code will improve the significant performance as
compare to RS and without FEC code.
Crest Ratio-1.1
PAPR Vs. CCDF curve with and without ICF
1
REFERENCES
0.9
[1]
0.8
[2]
0.7
CCDF
0.6
[3]
0.5
0.4
[4]
0.3
Without code
With RS code
With CC code
With RS CC code
0.2
0.1
0
1
2
3
4
[5]
5
6
7
8
9
10
[6]
PAPR
Figure 5.3 PAPR vs. CCDF curve for QAM-16 &
[7]
Crest Ratio-1.2
[8]
SNR Vs. SER curve with and without ICF
0
10
[9]
SER
[10]
-1
10
[11]
Without code
With RS code
With CC Code
With RS CC Code
-2
10
2
4
6
8
[12]
10
12
SNR
14
16
18
20
22
[13]
Figure 5.4 SNR vs. SER curve for QAM-16 &
Crest Ratio-1.2
6. Conclusion:
[14]
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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. These aspects of
the OFDM have been addressed in this project and
simulate the system for clipping technique with
filter and FCE code. The various modulations has
been tried to simulate the effect of clipping for
different transmission speed. The level of clipping
is also studied for its performance study. It was
found that higher clipping can’t control effectively
by FEC code or filtering effect. However the CC
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