New Technique Combining the Tone Reservation Method with Clipping Technique to Reduce the Peak-to-Average Power Ratio

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 8, No. 6, December 2018, pp. 5215∼ 5226
ISSN: 2088-8708, DOI: 10.11591/ijece.v8i6.pp5215-5226 5215
New Technique Combining the Tone Reservation Method
with Clipping Technique to Reduce the Peak-to-Average
Power Ratio
Hajar Abdelali1
, Smail Bachir2
, and Mohamed Oumsis3
1
Lims Laboratory, Faculty of Sciences Dhar Mahraz, Sidi Mohamed Ben Abdellah University of Fez, Morocco
2
Xlim Laboratory, UMR CNRS 7252, University of Poitiers, France
3
Lrit Laboratory, Unit Associated With CNRST, URAC 29, University Mohamed V, Rabat, Maroc
Article Info
Article history:
Received Dec 26, 2017
Revised Jul 3, 2018
Accepted Jul 29, 2018
Keyword:
OFDM System
PAPR Reduction
WiMAX IEEE 802.16e
Tone Reservation
Clipping
Conjugate Gradient
ABSTRACT
Nonlinear distortions and impairments appear in multicarrier signal with high fluctu-
ations when amplified by a Radio Frequency Power Amplifier (RF PA). This article
is a contribution to improving the Tone Reservation (TR) method dedicated to reduc-
ing fluctuations in the Orthogonal Frequency Division Multiplexing (OFDM) mod-
ulation. This method exploits the null sub-carriers in communications standards in
order to generate a correction signal used to reduce the Peak-to-Average Power Ratio
(PAPR) with nevertheless limited performances due to the reduced number of these
sub-carriers. For this purpose, our contribution is to combine the TR method with the
Clipping (CL) method to improve the PAPR reduction without significantly impacting
the quality of transmissions. We also show the gain provided by this strategy compared
to the use of classical methods. Experiments using a simulated example on a complete
WiMax 802.16e transmitter have been made in order to investigate the PAPR reduc-
tion performances on presence of the non-linear Power Amplifier model based on gain
compression response and phase distortion.
Copyright c 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Hajar Abdelali,
LIMS Laboratory,
Faculty of Sciences Dhar Mahraz,
Sidi Mohamed Ben Abdellah University of Fez, Morocco.
abdelali.hajar@usmba.ac.ma
1. Introduction
OFDM technique [1] is a very popular modulation method and tends to become the most widespread
multicarrier modulation in recent radio communication systems, such as Wireless Local Area Network (WLAN),
Worldwide Interoperability for Microwave Access (WiMax) and DVB. Today, it was chosen for the fourth
Generation (4 G) mobile communication systems, where the Long Term Evolution Advanced (LTE-A) stan-
dard allows broadband services with a theoretical transmission rate up to 100 Mbps. The advantages of OFDM
modulation are: its optimal spectral congestion, its flexibility in the allocation of the sub-carriers and its sim-
plicity of implementation using Fourier transforms at the transceiver.
However, it presents some disadvantages and drawbacks such as; OFDM system generates high power
peaks in time-domain signal, termed as Peak to Average Power Ratio (PAPR) [2] results in nonlinear distortions
when amplified by the Radio Frequency Power Amplifier (RF PA) [3],[4]. These distortions lead to a in-
band spectral regrowth and out-of-band spectral regrowth that result in degraded communication metrics like
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5216 ISSN: 2088-8708
Adjacent Channel Power Ratio (ACPR), Error Vector Magnitude (EVM) and Bit Error Rate (BER).
In literature, there are several approaches for PAPR reduction like CL [5],[6], Clipping-Filtering (CF)
[7],[8],[9], Selective Mapping (SLM) [9],[10],[11], Partial Transmit Sequence (PTS) [9],[10],[12], Block Cod-
ing [13], Tone Injection [14] and TR [15],[16]. These methods differ in their principle and implementation, but
also in their drawbacks: some of them add side information decreasing data rate and/or bandwidth like PTS,
others sacrifice BER performances for the benefit of PA efficiency such as CL and CF while some others are
efficient but too difficult to implement like SLM.
In this paper, our contribution called (TRCL) is a new method based on the combination of two meth-
ods to reduce PAPR, TR [15],[16] with Conjugate Gradient Optimization algorithm [17],[18] and CL [5],[6].
These two methods present a certain complementarity: the TR method based on the addition of signal is one of
the methods having the property of downward compatibility and improving the performance of EVM and BER
due to their low degradation and their low intake of supplementary computational complexity, but its perfor-
mance remains limited because of the reduced number of free subcarriers, while the CL is very simple of those
known this day and theoretically allows an unlimited clipping of the signal but unfortunately with a degradation
of all the communication criteria and a decrease in performance caused by a loss of some information.
Despite its limitations, this is the TR method that is preferred in practice because it requires no mod-
ification the receiver. In [19], the author proposed the use of the TR method based on the conjugate gradient
algorithm in order to reduce the value of PAPR and improve the speed of convergence. In this paper, we are
going to describe and study our TRCL method and compare it with other existing methods TR and CL. We will
see below how our new method improves the performance in terms of PAPR reduction and the communication
criteria EVM and BER.
The study of the performances in terms of reduction of the PAPR is carried out on the standard WiMax
- IEEE 802.16e [20]. To study the impact of this association, simulations in the presence of a Solid State Power
Amplifiers (SSPA) modeled by a nonlinearity of the gain and phase, are carried out in order to evaluate the
quality of the transmissions.
The paper will be organized as follows: section 2 will present the Problem position, while section 3,
will discuss two methods of PAPR reduction: Clipping and Tone Reservation based on the Conjugate Gradient
Optimization Algorithm, and present the proposed technique based on the conjunction of CL and TR (TRCL).
In section 4, we present the simulation results of our new method TRCL compared to the TR and the CL.
Finally, we conclude, in section 5.
2. Problem Position
To transmit a signal, the OFDM signal model (figure 1) divides at first a frequency range into several
independent orthogonal sub-carriers which have the same width of frequency band. The representation of an
OFDM signal can then be written:
c(k) =
1
√
N
N−1
k=0
Ckej( 2πnk
N )
, 1 ≤ n ≤ N (1)
where Ck is the data symbols with k = 0, · · · , N − 1 is the index of the sub-carrier, C = [C0, C1, · · · , CN−1]
indicate the carrier vector data and N is the number of sub-carriers. Subsequently, it applies the algorithm of
the inverse Fourier Transform (IFFT) on this OFDM signal by the Following equation:
x = Q · X (2)
where Q is the matrix of transformation IFFT of size N.
From equation 1, when all the sub-carriers are weighted at high level, the constructive sum needs to
appear peaks in the temporal domain. This establishes the major inconvenience of the OFDM systems [1]. To
measure these fluctuations, the PAPR, defined as a highest peak power to average power of one OFDM symbol,
such as:
PAPR(x)dB = 10 · log10
max0≤t≤T |x(t)|2
E(|x(t)|2)
(3)
IJECE Vol. 8, No. 6, December 2018 : 5215 – 5226

IJECE ISSN: 2088-8708 5217
Channel
coding
Symbol
map
Serial to
Parallel
IFFT
to Serial
Parallel Cyclic
extension
DAC
RF tx
ADC
Timing and
synch
Remove
extension
freqCyclicSerial to
Parallel
FFT
to Serial
Parallel
Symbol
demap
Decoding
Bits
Frequency
corrected signal
Bits
OFDM Transmitter
RF rx
OFDM Receiver
Figure 1. Model of OFDM transmitter and receiver for WIMAX
In literature, we can find several techniques for PAPR reduction [2] which present some improvements
but at the cost of many inconveniences. For example, the CL method has in theoretical unlimited performances
in term of PAPR reduction but its application induces systematically errors of transmission (like increasing
BER and/or Out-Of-Band distortion, etc.). These drawbacks can be avoided using the TR method, but the use
of only nul subcarriers allows about 2 to 4 dB of improvement. Here, we propose new technique combining
TR and CL methods to improve performances by avoiding the disadvantages of each one.
As we previously noted, we will cite the advantages and the inconveniences of the two techniques TR
and CL:
• TR : this technique is one of the methods having the property of downward compatibility and improves
the performance of ”EVM” and ”BER” due to their low degradation and their low intake of supplemen-
tary computational complexity but it is limited by a restricted number of free subcarriers in the standard,
complexity of implementation, etc.
• CL : this technique is very simple and has an unlimited performances in term of PAPR reduction but
its application induces systematically degradation of all transmission criteria (BER and/or Out-of-Band
distortion)
3. Proposed Technique
Unused
subcarriers
Data
subcarriers
t
t
iFFT
PAPRReducer
x
x + c
c
Dedicated subcarriers
for corrective signal
(PRT)
+A
−A
A
t t
Figure 2. The combination of both techniques TR and CL (TRCL).
Here, we will investigate the possibilities of combining both techniques TR and CL (figure 2). These
two techniques can be complementary because the TR allows to reduce the majority of peaks in an OFDM
symbol. Still a few samples of the symbol that cannot be reduced by this method and which will be affected by
the non-linearities of the circuits such as the power amplifier. In this case, we propose to apply the CL method
to avoid any exceeding the clipping value.
New Technique Combining the Tone Reservation Method... (Hajar Abdelali)

5218 ISSN: 2088-8708
3.1. Clipping
CL [5],[6] is the simplest method of those known this day which allows reducing the value of PAPR.
The objective of this method is to clip all points whose amplitude exceeds a predetermined threshold A to
reduce the power variation. This procedure is repeated until no point exceeds this threshold. Equation 4 shows
the principle of clipping. The simplicity of this method has an impact on the signal reception performance will
be reduced because the information removed by clipping are lost.
f(x) =
A.sign(x(t)) if |x(t)| > A
x(t) otherwise
(4)
3.2. Tone Reservation
TR [15],[16] is the most recent method based on the addition of a signal to the original signal to reduce
the PAPR. The Useful sub-carriers are reserved for the broadcast of the original signal and the sub-carriers not
used are exploited to generate the signal of cancellation of peak without loss of flow of data and Reduced the
PAPR. The generation of the temporal signal for the reduction of the PAPR by process TR was formulated as
problem of convex optimization which requires a high computational cost of the order of O(NrN2
L), where
Nr is the number of reserved carriers, N the number of carriers of the OFDM and L is the over-sampling factor.
To solve this problem, there are four iterative optimization algorithms, but in this paper we are going
to concentrate on one optimization algorithm: Conjugate Gradient. It is an iterative method just like the method
of the gradient, but it uses an algorithm so that the direction of research is optimal. This direction is determined
by the gradient at the point Xn and the gradient of the previous point Xn−1 [17]. For the first iteration, the
chosen direction is the negative gradient:
d0 = −g0 (5)
At the k + 1th
iteration, the point Xn+1 is chosen by finding the minimum of the function in the
direction previously determined to reduce the number of iterations.
Xk+1 = Xk + αk.dk (6)
αk is the conjugate gradient step, and dk represents the direction of conjugate gradient. The next direction is
determined by the previous direction. This method involves combining the previous direction dk−1 with the
gradient at the point Xk to calculate the new direction:
dk = −gk + ρk.dk−1 (7)
There are several versions of the method of the combined gradient, they vary by the way the constant ρk is
calculated. Here is the method of calculation of Fletcher-Reeves [21]:
ρk =
gT
k .gk
gT
k−1.gk−1
(8)
ρk represents then the report between the standard to the square of the current gradient and the standard
to the square of the previous gradient. Here is the method of calculation of Polak-Ribire [22]:
ρk =
(gk − gk−1)T
.gk
gT
k−1.gk−1
=
∆gT
k−1.gk
gT
k−1.gk−1
(9)

IJECE ISSN: 2088-8708 5219
3.3. New TRCL Method
The number of null subcarriers in a given standard is limited, the TR method reduces the majority
of peaks in an OFDM symbol, but always remains some samples whose amplitude is high and which will
consequently be affected by the non-linearities of the circuits like the power amplifier. In this case, we propose
to apply, after the TR, the CL method in order to avoid any exceeding of the A threshold.
Therefore, the first phase is to reduce the PAPR, using the TR method which calculates the correction
signal c, from the null subcarriers, according to the Conjugate Gradient algorithm which is used to reduce the
convergence time of the optimization algorithm.
The second phase consists in applying a clipping to the same threshold A as the previous phase in
order to eliminate the remaining peaks
4. Simulation Results
In this section, we present the experimental evaluation results of our new method TRCL with the
standard IEEE 802.16e or “WiMAX” [20]. The following table 1 represents the WiMAX parameters used in
simulations to improve the PAPR reduction of OFDM signals.
Table 1. The IEEE 802.16 parameters used in simulation
Name of the parameter value
System OFDM
Modulation 64QAM
Num of Sub-carriers 512
Sub-carriers Datas 384
Sub-carriers Pilots 42
Sub-carriers GI 86
PAPR−target 2.17
This standard is a broadband wireless technology of the fourth generation allowing data transmission
on long distances (dozens of kilometers until 50km) at high speed 70 Mbits/s and with an optimal security. The
systems based on the 802.16e standard is called “Mobile Wimax”. The communication by this standard can be
realized on Line Of Sight (LOS) or Not Line Of Sight (NLOS). The number of carriers for WIMAX 802.16e
can vary between 2048, 1024, 512 and 128, and there are three types of subcarriers for OFDM: Subcarrier data,
Pilot subcarriers and Subcarriers care (or subcarriers zeros).
The distribution of these sub-carriers varies according to the mode of allocation FUSC (Fully Used
Sub Channelization) and PUSC (Partially Used Sub Channelization) and the nature of the link DL (Downlink)
or UL (Uplink): these two modes differ by the fact of total or partial use of channels and depend on the need
for the users.
In this paper, the simulation is carried out for 512 subcarriers and we consider the Mode sub-channel
FUSC in the downlink DL by using a Fast Fourier Transform (FFT) (figure 3).
4.1. PAPR reduction
To make an evaluation on the movement of a signal, there are several criteria to make it, among these
criteria we find the PAPR and the CCDF. As we have already seen previously that the PAPR is the report
between the maximum power and the averages power of a temporal signal. However, for a better movement
evaluation of a signal , we use the second parameter CCDF which is a Complementary Cumulative Distribution
Function of the movement on an interval of fixed time. Thus CCDF is the probability that the value of PAPR is
superior to a fixed value.
CCDF = Pr(PAPR ≥ PAPR0) (10)

5220 ISSN: 2088-8708
Symbol
mapping
Input
data
X1×384 X1×426
C1×86
X512×1 X512×1 x576×1 x1×576
PAPR
reduction
Serial
to
parallel
S/P
Flexible OFDM system
P/S
BER
EVM
Evaluation
block
Output
data
Q512×512
512pt. iFFT
t
PAPRReducer
c
Dedicated subcarriers
for corrective signal
(PRT)
+E
-E
Pilot
insertion
S/P ,
Remove GI ,
FFT ,
Equalizing
and
Demapping
OFDM
demodulation
Add
guard
interval
White
noise
PA
Figure 3. OFDM data symbol generation and TRCL scheme for 802.16e standard
For that purpose, for any signal, the dynamics of this signal will always be superior or equal to 0 dB
thus the value of PAPR will also have a probability superior or equal to 0 dB. The more the CCDF parameter
has a low value, the more the probability of having a high PAPR value tends to 0.
0 2 4 6 8 10 12
10
−3
10
−2
10
−1
10
0
PAPR, PR0 dB
Probabilite,PR>PR0
CCDF of PAPR with transmission IEEE 802.16e
Original
with TR
with CL
with TRCL
Figure 4. CCDF of PAPR with different method to reduce the PAPR in IEEE 802.16e
In this part, the impact of the studied methods on CCDF will be discussed. The figure 4, shows the
CCDF curves in function of PAPR0 obtained by the different methods allowing the PAPR value reduction
(TR, CL, TRCL) and the CCDF of a OFDM classic signal (i.e. without reduction technique of the PAPR
”Original”). We can notice that the combinaison TRCL reduce the PAPR from 10.6 dB to 4.8 dB with a
probability of approximately 10−2
. It represents a reduction factor of about 5.8 dB, while CL offers a reduction
factor of approximately 5.5 dB and TR offers approximately 3 dB. From that, we can conclude that the CCDF
of the TRCL and CL methods are comparable and decrease rapidly compared to the others methods (TR and
Original).
The second results shown in figure 5, represents the temporal evolution of the OFDM signal’s envelope
for the different PAPR reduction methods. These results are obtained from a unit mean power signal with a
clipping of normalized amplitude A = 1.65 to have a value of PAPR−target equal to2.17 (see equation 11).
A = 10
P AP R−target
10 .E(|x(t)|2
) (11)
It can be noted that the TR (figure 5.a) method leaves a few peaks that exceed the A, the CL method

IJECE ISSN: 2088-8708 5221
0 0.5 1 1.5 2 2.5
T ×104
0
0.5
1
1.5
2
2.5
3
3.5
|x|
Original
Avec TR
Avec CL
Avec TRCL
(a). With the different methods of the PAPR reduction.
0 0.5 1 1.5 2 2.5
T ×104
0
0.5
1
1.5
2
2.5
3
3.5
|x|
Original
Avec TR
Avec CL
Avec TRCL
(b). Without TRCL.
Figure 5. “The temporal evolution of the envelope for OFDM signal.”
(figure 5.b) clipped all the peaks that are higher than A, and for TRCL (figure 5.a) we can show that the results
obtained are identical to those of CL, but with an improvement in BER and EVM transmission errors (as we
will see later on).
In the next section, a nonlinear amplifier and a channel model will be introduced to observe the
degradation of the CL method and the performance of our TRCL method in terms of BER and EVM.
4.2. Performances in presence of nonlinear PA model
The emission information in wireless networks over a great distance such as WIMAX requires the use
of power amplifiers PA [3], to ensure a complete transmission. The role of PA is to increase the power of the
RF signal without distorting the signal at its input and without energy dissipation when issued to its output. The
following equation 12 represents the relationship non-linear between the input signal to a PA and its output:
Pout = G(Pin).Pin (12)
where Pin and Pout are the PA power of the input and the output signal, respectively, and G is the gain of the
PA depending on the amplifier’s input power. The gain value remains constant for the weak powers in the linear
zone, it will be called also linear gain, then it decreases when the power increased in the other areas.
PA is characterized by two inputs-outputs relationships also called transfer characteristics of conver-
sions AM/AM (Amplitude Modulation) and AM/PM (Phase Modulation). The AM/AM represents the varia-
tion of the output signal power depending on the input signal power (as show in figure 6), while the AM/PM
represents the phase difference between the PA output and input signals depending on the input signal power.

5222 ISSN: 2088-8708
−20 −15 −10 −5 0 5 10 15
−10
−5
0
5
10
P
in
P
out
P
out,sat
P
in,sat
1 dB compression point
Saturation point
Linear zone Compression
zone
Saturation
zone
P
in,1dBIBO (dB)
OBO
(dB)
P
out,1dB
P
out
mean
P
in
mean
Figure 6. AM/AM characteristics
The mathematical model of the amplifier PA is based on the most widespread models, namely, the
Rapp model described by the expression of conversion AM/AM (equation 13) and the Saleh model which is
described by AM/PM (as show in equation 14) [23]:
FAMAM (|x(t)|) =
|x|
1 + |x|
Asat
2p
1
2p
(13)
where |x(t)| is the instantaneous modulated envelope of the input signal. With Asat = 2.1 and p = 3
FAMP M (|x(t)|) =
αp.|x|2
1 + βp.|x|2
(14)
with αp = pi/3 and βp = 1. The parameters αp and βp are used to describe the behavior of the
amplifier.
The AM / AM curve of the PA is divided into three functioning zones (According to figure 6), the
linear zone of the amplifier where the output power is proportional to the input power according to a gain of
the amplifier which remains constant. After a certain power input, the gain starts to decrease slowly, this zone
is called compression zone. And finally, the saturation zone where the amplifier reaches its maximum output
power. The point to 1 dB compression of gain represents the power output where the difference between the
gain calculated in the linear zone and the real gain of the PA is worth than 1 dB. The saturation point is a point
where the amplifier reaches the maximum value to issue an output signal and its power appointed power of
saturation. Most used amplifiers are non-linear, since the linear amplifiers are very expensive. However, the
non-linearities of a PA has to impact the existence of distortions temporal and frequency signals to transmit
which are reflected by a distortion of the signal.
To avoid such distortions, we must take a step back in power, compared to the point of compression
to 1 dB steps. This decline allows to work in the linear zone as well as the signal is closest to the non-linear
zone of the PA. It is defined by the IBO (Input Back-off) and the OBO (Ouput Back-off) which represents the
decline in average power input and output, respectively, compared to the point of compression to 1 dB (figure
6).
In our simulations, we have added noise to simulate a Additive White Gaussian Noise channel (AWGN).
This is a fundamental characteristic of the transmission channel. This channel noise is modeled by a random
signal n(t), whose probability distribution follows the Gaussian law. The received signal:
r(t) = s(t) + n(t) (15)

IJECE ISSN: 2088-8708 5223
2 3 4 5 6 7 8 9 10 11
10
−4
10
−3
10
−2
10
−1
10
0
EbNo (dB)
BER
a. BER as a function of SNR with IBO=5
Original
with TR
with CL
with TRCL
2 3 4 5 6 7 8 9 10 11
10
15
20
25
30
35
EbNo (dB)
EVM(%)
b. EVM as a function of SNR with IBO=5
Original
with TR
with CL
with TRCL
Figure 7. EVM and BER as a function of SNR with IBO=5
2 4 6 8 10
10
−3
10
−2
10
−1
10
0
EbNo (dB)
BER
a. BER as a function of EbNo with IBO=0
Original
with TR
with CL
with TRCL
2 3 4 5 6 7 8 9 10 11
15
20
25
30
35
40
EbNo (dB)
EVM(%)
b. EVM as a function of EbNo with IBO=0
Original
with TR
with CL
with TRCL
Figure 8. EVM and BER as a function of SNR with IBO=0
where s(t)represents the transmitted signal.
After the addition of noise, we first calculated the percentage of Error Vector Magnitude (EVM) which
corresponds to the error vector’s amplitude between the origin constellation and the received constellation.
Second, we calculated the Bit Error Rate (BER) which is the ratio between the number of errors bits at reception
(Ne) and the total number of transmitted bits (N).
In the first figures (7.a and 7.b), we can show the evolution curves of the EVM and the BER according
to the SNR (Eb/No) that varies between 2 dB and 11 dB for a backward movement of IBO = 5 dB, to make a
comparison between the various methods for PAPR reduction.
For the low SNR values (SNR < 7) dB, we can see that all BER and EVM curves have almost
the same behavior. For strong SNR (SNR > 7) dB, a first remark concerns the Clipping method which
degrades all communication criteria. We also see that of TR which coincides with TRCL with a non-negligible
improvement of BER and with a slight improvement of EVM compared to the other methods.
After, the amplifier operates in the non-linear zone at the point at 1 dB of compression corresponding
to a decrease in power of zero input (IBO = 0 dB). The curves in Figure 8 show the comparison between the
BER and the EVM according to SNR (Eb/No) varying between 2 dB and 11 dB to make a comparison between
various methods which allows to reduce the value of PAPR. From the results of these figures, We can also see
that the CL method degrades all the criteria and the TRCL method maintains a good improvement when the
noise level exceeds Eb/No = 8 dB. We can notice that our TRCL method always retains the best level of
performance on both criterias.
In the following figure 9, we evaluated the transmission quality in terms of BER and EVM for each
method by varying the value of the amplifier’s input down IBO (IBO varies between −3 dB and 2 dB in a step
1 dB) with fixed SNR value = 12 dB. We noted the improvements made by applying a clipping after the TR
method.

5224 ISSN: 2088-8708
−3 −2 −1 0 1 2
10
−4
10
−3
10
−2
10
−1
IBO (dB)
BER
a. BER as a function of IBO with SNR=12
Original
with TR
with CL
with TRCL
−3 −2 −1 0 1 2
10
15
20
25
30
IBO (dB)
EVM(%)
b. EVM as a function of the IBO with SNR=12
Original
with TR
with CL
with TRCL
Figure 9. EVM and BER as a function of the IBO
−30 −20 −10 0 10 20 30
−80
−70
−60
−50
−40
−30
−20
−10
0
frequency, MHz
powerspectraldensity
Transmit spectrum OFDM (based on 802.16e)
Original
with TR
with CL
with TRCL
mask
Figure 10. Transmit spectrum OFDM based on 802.16e without amplifier
Simulation results in figure 9.a show that when the IBO increases, the BER tends toward zero with
the TRCL method. It is the proof that there is less disruption. In effect, an IBO low engenders significant
distortions in the signal.
We can also observe in figure 9.b that TRCL improves EVM by about 3% when the IBO increases,
that is to say that, when the amplification is done more and more in the linear area. However, low values of the
IBO mean that the power amplifier operates on the limit of its saturation zone and will cause distortions in the
amplified signal.
Figure 10 and 11 show the signal spectrum before and after the use of methods which reduce the
PAPR, for a bandwidth of 30 MHz. We can see that the application of TR and TRCL method lead to a spectrum
broadening of the signal, this is due to the use of the 64 free sub-carriers of the standard. It is verified on the
spectra input and output of the amplifier.
In addition, at the output of the amplifier, we can see the spectral ascents in the bands due to nonlinear
effects. However, these spectral lifts stay below the IEEE 802.16 standard mask.
−30 −20 −10 0 10 20 30
−80
−70
−60
−50
−40
−30
−20
−10
0
frequency, MHz
powerspectraldensity
Transmit spectrum OFDM (based on 802.16e)
Original
with TR
with CL
with TRCL
mask
Figure 11. Transmit spectrum OFDM based on 802.16e with amplifier

IJECE ISSN: 2088-8708 5225
5. Conclusion
OFDM is one of the most popular physical layer technologies for high data rate wireless communi-
cations due to its robustness, high spectral efficiency and low computational complexity. The transmit signals
in OFDM system can have high peak values in the time domain. Our observations about the TR method show
that it is difficult due to the specificities of current standards to eliminate all high power peaks. The study
presented in this article was used to evaluate the possibilities to associate a classic clipping, resulting in a good
compromise between the backward compatibility of the TR method and best performances in PAPR reducing
of the CL. The best performance of our TRCL technique in terms of PAPR reduction, the communication cri-
teria EVM and BER on the WiMax standard allows to envisage an improvement in energy balance sheet of the
amplification stage while maintaining a good transmission quality.
In addition, we measured the spectral of the signals at the input and at the output of the amplifier, of
different methods which reduce the PAPR. Analysis of the results shows the improved performance of ”TRCL”
while respecting the specifications of the standard used. Indeed, we also found that the spectrum of the signals
at the output of the amplifier meets to the mask imposed by the standard IEEE 802.16.
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