PAPR REDUCTION OF OFDM SIGNAL BY USING COMBINED HADAMARD AND MODIFIED MEU-LAW COMPANDING TECHNIQUES

International Journal of Computer Networks & Communications (IJCNC) Vol.6, No.5, September 2014
PAPR REDUCTION OF OFDM SIGNAL BY
USING COMBINED HADAMARD AND
MODIFIED
MEU-LAW COMPANDING TECHNIQUES
Mohamed M. El-Nabawy1, Mohamed A. Aboul-Dahab2
and Khairy El-Barbary3
1Electronic and Communication Dept., Modern Academy for Eng. & Tech in Maadi
(M.A.M), Cairo, Egypt
2Electronic and Communication Dept., Arab Academy for Science and Technology and
Maritime Transport (AAST), Cairo, Egypt
3 Electronic and Communication Dept., Canal University, Cairo, Egypt
ABSTRACT
Orthogonal frequency division multiplexing (OFDM) is a technique which gives high quality of service
(QOS) to the users by mitigating the fading signals as well as high data rates in multimedia services.
However, the peak-to-average power ratio (PAPR) is a technical challenge that reduces the efficiency of
RF power amplifiers. In this paper, we propose the combined Hadamard transform and modified meu-law
companding transform method in order to lessen the effects of the peak-to-average power ratio of the
OFDM signal. Simulation results show that the proposed scheme reduces PAPR compared to other
companding techniques as well as the Hadamard transform technique when used on its own.
INDEX TERMS
Orthogonal Frequency Division Multiplexing (OFDM), Cumulative Complementary Distribution Function
(CCDF), Peak to Average Power Ratio (PAPR), Bit Error Rate (BER).
1.INTRODUCTION
Orthogonal frequency division multiplexing (OFDM) is attractive multicarrier modulation
schemes for high bandwidth efficiency and highly strong immunity to multipath fading. Its basic
concept is similar to frequency division multiplexing but there is much dissimilarity; under the
assumption that the gap between subcarriers is fixed to the inverse of symbol duration, which is
divided into N subcarriers, spectrum overlap but their orthogonal property is remained. Thus high
bandwidth efficiency is achieved. By parallel transmitted with N subcarriers, the symbol duration
becomes N times as long as compared with a single carrier transmission. These allow strong
immunity to multipath fading and reduce the complexity of equalizers [1]. Nowadays OFDM
have been contained in the IEEE 802.11, IEEE 802.16 wireless broadband access systems, and in
digital audio/video broadcasting (DAB/DVB) standard in Europe, etc. On the other hand, the
main disadvantage of OFDM signals is their large peak-to-average power ratio (PAPR), which
leads to poor power efficiency, and subsequent performance degradation, of the signal transmitted
through the power amplifier [2].
DOI : 10.5121/ijcnc.2014.6505 71

International Journal of Computer Networks & Communications (IJCNC) Vol.6, No.5, September 2014
For signals with a high PAPR, the average input power must be reduced so that the peak power of
the input signal is maintained at most at the saturation input level of the high power amplifier
(HPA). If the input power is not backed off, signal distortion takes place resulting in out-of-band
spectral regrowth [3]. The amount of back-off must be at least equal to the PAPR (unless some
kind of pre-distortion technique is applied). However, HPAs are most efficient when they are
driven into saturation. Therefore, input power back-off reduces the efficiency of the HPA and this
is detrimental to battery low powered devices. As a consequence, many studies have focused on
PAPR reduction leading to the development of many schemes to address it such as coding and
tone reservation [4][5][6], pre-distortion schemes, clipping [7][8][9], partial transmit sequence
(PTS), selected mapping (SLM), interleaving [10][11][12], nonlinear companding transforms
[13][14], Hadamard transforms [15][16] and other techniques etc. Of all those PAPR reduction
systems, the easiest system is to use the clipping process. Nevertheless, using clipping processing
leads to both in-band and out-of-band distortions and further causes an increase in the BER of the
system. Another way of approach is the use of companding techniques which give better
performance than clipping techniques because the inverse companding transform (expanding) is
applied on the receiver end to reduce the distortion of signals. The use of Hadamard transform
may reduce PAPR of OFDM signal while the error possibility of system does not increase [17].
In this paper, an efficient technique for reducing PAPR is introduced. It is based upon joint
modified μ-Law companding and Hadamard transform method. This paper is organized as
follows: The PAPR problem in OFDM systems is presented in section 2. The modified μ-law
companding transform and Hadamard transform are introduced in sections 3 and 4. In section 5, a
description of a PAPR reduction scheme that combines the modified μ-law companding
transform and Hadamard transform is described. Simulation results are reported in section 6 and
section 7 presents the conclusion.
72
2.SYSTEM MODEL
In this section, the basics of OFDM transmitters and the PAPR definition are reviewed. Consider
an OFDM consisting of N subcarriers. Let a block of N symbol X={Xk ,k=0,1,2,...,N-1} is formed with
each symbol modulating one of a set of subcarriers {fk ,k=0,1,2,...,N-1} .the N subcarriers are chosen to
be orthogonal , that is , fk =kf , where f= 1 (NT) and T is the original symbol period. Therefore, the
complex baseband OFDM signal can be written as
(1)
In general, the PAPR of an OFDM signal, x(t), is defined as the ratio period between the
maximum instantaneous power and its average power during an OFDM symbol
(2)
Reducing themax x(t) is the principle goal of PAPR reduction techniques. In practice, most
systems deal with a discrete-time signal, therefore, we have to sample the continuous time signal
x(t) .

International Journal of Computer Networks Communications (IJCNC) Vol.6, No.5, September 2014
To better approximate the PAPR of continuous-time OFDM signals, the OFDM signals samples
are obtained by L times oversampling. By sampling x(t) defined in Eq. (1), at frequency fs = L T ,
where L is the oversampling factor, the discrete-time OFDM symbol can be written as
73
(3)
Equation (2) can be implemented by using a length (NL) IFFT operation. The new input vector X
is extended from original X by using the so-called zero-padding scheme, i.e. by inserting （L
−1（N zeros in the middle of X. The PAPR computed form the L-times oversampled time domain
OFDM signal samples can be defined as
(4)
However, the PAPR does not increase significantly after L = 4. So as to be away from aliasing the
out-of-band distortion into the data bearing tones and in so as to accurately have the description of
the PAPR an oversampling factor L 4 is necessarily required.
3.MODIFIED μ-LAW COMPANDING TRANSFORM
In this section, we simply review the companding techniques [7][8][15][18] of OFDM signal
PAPR reduction. This transform reduces the PAPR of OFDM signals by amplifying the small
signals and shortening the big signals. In companding, the OFDM signal is compressed at the end
of the transmitter after the IFFT process and expanded at the receiver end prior to the FFT
process. In this companding method, the compressor characteristic is piecewise, and it consists of
a linear segment for low level inputs and a logarithmic segment for high level inputs. The A-law
compressor characteristics for different values of A are shown in Figure 1. Corresponding to A=1,
we notice that the characteristic is linear (no compression) which corresponds to a uniform
quantization. A-law has mid riser at the origin. Hence it contains non-zero value. The practically
used value of “A” is 87.6. The A-law companding is used for PCM telephone systems [19].
Figure 1 :Characteristics of A-law Compressor
The linear segment of the characteristic is for low level inputs while the logarithmic segment is
concerned with high level inputs. It is mathematically expressed as,

74
(5)
Where x=input signal, y= output signal, Sgn(x) =sign of the input (+ or -), |x|=absolute value
(magnitude of x).
In the μ-law companding, the compressor characteristic is piecewise, and is made up of a linear
segment for low level inputs and a logarithmic segment for high level inputs. The μ-law
compressor characteristics for different values of μ are shown in figure 2. The higher the value of
“μ”, the greater the compression. Corresponding to μ=0, we observe that the characteristic is
linear (no compression) which corresponds to a uniform quantization. μ-law has mid tread at the
origin. Hence it contains zero value. The practically used value of “μ” is 255. The μ-law
companding is used for speech music signals [19].
Figure 2 :Characteristics of μ-law Compressor
It is mathematically expressed as,
(6)
Where x= amplitude of the input signal at a particular instant of time, = maximum
uncompressed analog input amplitude, C|x|=compressed output amplitude, and |x| absolute value
(magnitude of x). Note that μ is the parameter used to define the amount of compression or
compression factor.
The first difference between A and μ-law is the dynamic range of the output. μ-law has a larger
dynamic range than A-law. Dynamic range is fundamentally the ratio between the quietest and
loudest sound that can be represented in the signal. The downside of having a higher dynamic
range is greater distortion of small signals; in other words, for soft sound input A-law sounds
better than μ -law. The Compression is performed according to the well-known modified μ-Law
viz. After companding, the signal now become

75
{ }

log 1
x
PR X
peak
( ) ( ) ( ) sgn( )
T n C x n PR X x
log(1 )
peak
μ
μ
+
= =
+
(7)
And,
V
PR
= (8)
x peak
Where PR is peak ratio, V is the peak amplitude of the input and output signals specified for the
μ-Law compander, x
peak is the peak amplitude of the actual signal to be companded x is the
instantaneous amplitude of the input signal. The companding transform has to satisfy the
following two conditions in particular:
2 2
(1) E( T ( n ) ) E( x ( n
) )
»
(2) T ( n ) x ( n ) when x ( n ) V
;
³ £
( ) ( ) when ( ) .
T n x n x n V
³ £
(9)
Decompression is simply the inverse of equation (5), the expanding equation is
μ
Peak peak y n C r n x
{ }
(1 )
-1
x
PR X PR X
( ) ( ) 10 - 1 sgn( )
μ
+
= =

(10)
4.HADAMARD TRANSFORM
Park et.al, have suggestions of in [16] a system for PAPR reduction in OFDM transmission using
Hadamard transform. The Hadamard transform scheme may reduce the occurrence of the high
peaks compared to the original OFDM system. The idea of using the Hadamard transform is to
reduce the autocorrelation of the input sequence to reduce the PAPR problem without needing the
transmission of any side information to the receiver. In this section, we shorty review Hadamard
transform. We assume H is the Hadamard transform matrix of N orders, and that the Hadamard
matrix is a standard orthogonal matrix. Every element of the Hadamard matrix is either 1 or -1.
The Hadamard matrix of 2 orders is stated by
(11)
Hadamard matrix of 2N order may be constructed by
(12)
Where is the complementary of . Hadamard matrix satisfy the relation
T T
= = (13)
H H H H I
2 2 2 2 2
N N N N N
Where
HT
2
N
is the transport matrix,
2
I
N
is the unity matrix of 2N order. Note that the
Hadamard can be implemented by using a butterfly structure as in FFT. This means that applying
Hadamard transform does not require the extensive increase of system complexity.

After the sequence is transformed by a Hadamard matrix of N order, the
new sequence is
= . The expander signal will pass through the S/P converter after which
76
Y = HX (14)
5.PROPOSED SCHEME
The proposed system block diagram is shown in Figure (3). The modification in this system from
the classical OFDM system is the combination of the companding transform and the Hadamard
transform. These two modifications are added to the OFDM system as shown in Figure (3).
Figure 3: Block diagram of OFDM system with proposed PAPR reduction technique
The signal output of the Hadamard transform unit is given by (14). After the IFFT module the
T
signal will be y=IFFT(Y) , where [ (1) (2) ....... ( )]
y = y y y N . The signal will pass through the P/S
converter after which it is applied to the modified μ-law compander. The output of the compander
is given by T (n ) = C {y (n )} . The compander signal passes through the AWGN channel. At the
receiver, the received signal is transform by modified μ-law expander. The output of expander is
Ù given by y −
1
(n ) C { r (n )
}
Ù Ù
it is applied to FFT module. The output signal of FFT module is given by Y FFT (y )
= . The
signal output from the inverse Hadamard transform is given by
Ù Ù
T
= then the signal X
X H Y
Ù
is
de-mapped to bit stream.
6.SIMULATION RESULTS
In this section, the simulation results of the proposed scheme are presented. These results are the
complementary CDF (CCDF) performance and the bit error rate (BER) achieved. The CCDF
performance of the proposed scheme is evaluated by two steps of simulations. Simulations are
done on the Matlab package. Simulations are made using of QPSK as a modulation scheme with
64 subcarriers and an additive white Gaussian noise (AWGN) channel.
The cumulative distribution function (CDF), a widely used parameter, is employed to measure
how efficient PAPR techniques are. The CDF of the amplitude of a signal sample is given by
F (z ) = 1- exp(z ) (15)
However, the complementary CDF (CCDF) is used instead of the CDF, which helps us to
measure the probability that the PAPR of a certain data block exceeds the given threshold. The

CCDF of the PAPR of the data block is desired in our case to compare outputs of various
reduction techniques. It is given by
77
N
P ( PAPR z ) = 1- P ( PAPR z ) = 1- (1- exp(- z ))
(16)
In the first simulation, the value of PR=1 while the companding factor μ has four different values.
First, when μ=1 the performance of the proposed scheme provides better performance than that of
the simple companding, Hadamard or classical OFDM. at CCDF= 10−4 , Figure (4-a) illustrates this
achievement where the PAPR of the proposed scheme is almost 2.8 dB smaller than that of
classical OFDM scheme. The Hadamard matrix transform will reduce 1.2 dB PAPR compared to
that of classical OFDM signal.
Second when μ=2 the PAPR is reduced by 3.4 dB compared to classical OFDM as shown in
figure (4-b).Third when μ=5 Fourth when μ=10 this gets a reduction value of 5.8 dB as shown in
Figure (4-c) and Figure (4-d) respectively.
(a)
(b)

78
(c)
(d)
Figure 4:Comparison of PAPR for OFDM, Hadamard transform techniques, companding techniques, and
proposed scheme at PR=1 and different value μ (a) μ=1, (b) μ=2, (c) μ=5, (d) μ=10.
In the second simulation the value of peak ratio PR=2 with the same four values of used in the
first simulation., Figure 3 show that the proposed scheme is lower than classical OFDM system
and Hadamard and modified μ-law companding techniques . Although the reduction of this
simulation is low but it is not as low as the first simulation. This is obvious in Figure (5-a) the
PAPR of proposed scheme is almost 2.2 dB smaller than that of classical OFDM signal. This
value of reduction is less than the previous by 0.6 dB.

79
(a)
(b)
(c)

80
(d)
Figure 5:Comparisons of PAPR for OFDM, Hadamard transform techniques, companding techniques, and
proposed scheme at PR=2 and different value μ (a) μ=1, (b) μ=2, (c) μ=5, (d) μ=10.
Table I summarize the values of PAPR for the classical OFDM system, Hadamard transform,
companding technique and the proposed scheme.
As an example for PR=1 and μ=5, the PAPR of the proposed scheme, companding technique,
Hadamard transform, and OFDM system is equal 6.1, 7.1, 9.8, 11 dB respectively. We can see
the amount of reduction of OFDM system, Hadamard transform, and companding technique with
respect to proposed scheme is equal 4.9, 3.7, 1 dB respectively. From table-1 we can see that at
different values of μ, the PAPR of proposed scheme is less than that of other techniques, But the
value of reduction at PR=1 is greater than of the value at PR=2.
TABLE I. VALUE OF PAPR0 IN dB AT CCDF = 4
10
− FOR DIFFERENT PAPR REDUCTION TECHNIQUES.
Tech
μ
Proposed Companding
Hadamard OFDM
PR=1 PR=2 PR=1 PR=2
1 8.2 8.8 9.5 9.6 9.8 11
2 7.2 8.4 8.4 9.3 9.8 11
5 6.1 7.2 7.1 8.1 9.8 11
10 5.2 6.2 6 7.1 9.8 11
Figure (6) presents the BER performance over AWGN channel for OFDM system, Hadamard
transform, companding technique, and proposed scheme. The companding and proposed scheme
uses two different values of PR. In Figure (6-a) and Figure (6-b) we can see that the performance
of proposed system did not degrade compared to other systems. While in Figure (6-c) and Figure
(6-d) we can see at the large value of companding factor μ, the performance of BER is degraded.
But at the large value of PR this degraded can be minimized.

81
(a)
(b)
(c)

82
(d)
Comparisons of BER for OFDM, Hadamard transform techniques, companding techniques, and proposed
scheme at PR=1 and 2 and different value μ (a) μ=1, (b) μ=2, (c) μ=5, (d) μ=10.
Table II summarize the value of Eb / N in dB for different value of μ and PR for different
0 techniques of PAPR reduction at the BER= − 4
10
. The table shows the BER performance at low
values of μ did not degraded that mean higher value of PR will not increase the performance of
the BER. But at the large value of μ you must use large value of PR to upgrade value of BER. As
example at μ=2 the value of Eb / N approximately don’t changed for different techniques and
0 PR. But at μ=10 the value of Eb / N for OFDM system, Hadamard is equal. But for proposed
0 system the value of Eb / N is changed by 2.1 dB at PR=1, this value is large, we can upgrade
0 this by increase value of PR. At PR=2 the changed of Eb / N w.r.t OFDM system at BER= −
4
0 10
equal 1dB. By comparing change in PR modify in degraded of BER.
TABLE II. THE VALUE OF Eb /N 0 IN dB FOR DIFFERENT VALUE OF PR DIFFERENT TECHNIQUES OF PAPR
REDUCTION AT BER= −
4
10
Tech
μ
Proposed Companding
Hadamard OFDM
PR=1 PR=2 PR=1 PR=2
1 8.4 8.4 8.4 8.4 8.4 8.4
2 8.6 8.4 8.6 9.3 8.4 8.4
5 9.5 8.8 9.5 8.8 8.4 8.4
10 10.5 9.4 10.5 9.4 8.4 8.4
7.CONCLUSIONS
In this paper, a PAPR reduction scheme which is basically based on combination of Hadamard
transform and modified μ-law companding techniques is proposed. This modified μ-law profile is
based upon the peak ratio (PR). The PAPR reduction performance and BER performance are
evaluated through computer simulation. Two different simulations have been carried out for the
proposed scheme. The first simulation is made for the case PR=1 and μ have four different values

which are the second simulation uses the same values of μ but PR=2. Simulation results show
that the PAPR reduction performance is improved compared to only using companding
transforms. On the other hand, the BER of the system using the proposed PAPR reduction scheme
does not degrade at small values of the companding factor, μ. At large values of μ, however, the
BER performance degrades. This issue can be solved by increasing the value of the peak ratio,
PR. As the PR increases, the PAPR reduction decreases; regardless, the proposed scheme still
remains better than the companding techniques, Hadamard transform, and OFDM system.
83
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84
Authors
Mohamed M. El-Nabawy received the B.S degree in Electronics Engineering and
communications Technology (with average grade excellent with honor degree) from Modern
Academy for Eng. Tech. (M.A.M), Cairo, Egypt, in 2007. Currently pursuing the Master
degree working as Teaching Assistant in Electronics and communications engineering in
M.A.M., His Research area includes Wireless Communication, Mobile Communication,
Information Coding Theory, etc.
Mohamed A. Aboul-Dahab received the B.S.degrees in communication engineering from
Faculty of Engineering, Cairo Univ., in 1973. He received the M.Sc. and Ph.D. degrees in
Communications Engineering from Alexandria Univ., Egypt in 1980 and 1986 respectively.
He is a professor in the Electronics and Communications Engineering department since
1999. He has been the Head of the Electronics and Computer Engineering Department,
College of Engineering and Technology (Alexandria Campus), Arab Academy for Science
and Technology and Maritime Transport, AASTMT (1995- 2002), and the Dean of College
of Engineering and Technology (Cairo Campus), AASTMT (2002-2008) .He has 37 years of teaching
experience for the undergraduate students and postgraduate. He has supervised many undergraduate
projects and many master and Ph.D. theses in AASTMT and other universities. His research activities are in
the field of antennas, digital communication and satellite communication.
K.A. ElBarbary received the B. S. degree (with Average Grade Very Good) in Electric
Engineering from Military Technical College, Egypt , the M.S. degree in Electric
Engineering from Military Technical College, Egypt, the Ph.D. degree in Communications
from George Washington University, Washington DC, USA, in 1981, 1986 and 1993,
respectively. He was faculty member at Military Technical College, Egypt in the Electric
Engineering Department (Chair of communications and Electronic warfare) (1994-2010).
there he became a professor of Electric Engineering(2009), Head of Electronic Warfare Engineering
Department (2000), and Head of Electrical Engineering Department (2009). He has been a professor of
signal and systems and Communications at Suez Canal University, Egypt in Electric Engineering
Department. Currently he is the Head of Electric Engineering Department, Suez Canal University, Egypt.
His research interests include Statistical signal and array processing and related applications in
communications, radar, Electronic Intelligence, Multi user Detectors and wide band communication
systems.

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PAPR REDUCTION OF OFDM SIGNAL BY USING COMBINED HADAMARD AND MODIFIED MEU-LAW COMPANDING TECHNIQUES