ISSN: 2278 – 7798
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 2, Issue 5, May 2013
1119 All Rights Reserved © 2013 IJSETR
Abstract— In wireless communication systems, the design and
analysis of an orthogonal frequency division multiplexing
(OFDM) system signal is highly affected by the nonlinear effects
of high power amplifier (HPA). The nonlinear effects of HPA on
the OFDM system are the most critical because it is not only
reducing the system performance but also creates interference
on adjacent channels. This paper explores the analysis of
nonlinear distortion effects in an OFDM system when the signal
is passed through a nonlinear high power amplifier (HPA).
Throughout this paper we also present the AM/AM analysis of
HPA on OFDM. Finally we present the simulation results to
show the nonlinear effects of HPA on OFDM.
Index Terms—AM/AM, HPA, OFDM and Nonlinear effect.
I. INTRODUCTION
In recent years, there has been a tremendous demand for
reliable, high speed digital wireless communications. Besides
the cellular phone, there are wireless modems, high definition
television (HDTV) and digital radios. Orthogonal Frequency
Division Multiplexing (OFDM) is a multicarrier modulation
technique that’s considered as one of the high spectral
efficient modulation techniques. Although OFDM plays an
important role in wide band transmission schemes, but its
performance is degraded by various factor that is very
common in any wireless communication system [1].
However, one of the major problems associated with OFDM
is the nonlinear distortion in the transmitted OFDM signal
when it is passed through a nonlinear HPA. This nonlinear
distortion causes serious in-band distortion as well as adjacent
channel interference due to spectrum re-growth in the
transmitted signal. The performance of wireless
communication is highly related with the power amplifier.
Orthogonal frequency division multiplexing (OFDM)
signalling exhibits high peak-to-average power ratio (PAR)
which results in large sensitivity to nonlinear distortion
created by the use of a high power amplifier (HPA) at the end
of a wireless transmitter [2]-[5].
In [6] the authors addressed the effects of HPA phase
distortion on system performance, in which they discussed
mainly on the influence of AM/PM distortion of HPA by
evaluating BER performance. Modern power amplifier
Manuscript received April, 2013.
Tushar Kanti Roy, Department of Electronics and Telecommunication
Engineering, RUET, Rajshahi, Bangladesh.
Monir Morshed, Department of Information and Communication
Technology, Mawlana Bhashani Science and Technology University,
Tangail, Bangladesh.
designer face much tougher environment [7]. Nonlinearity in
power amplifier response leads to nonlinear amplification of
OFDM signal. The nonlinearity at the output of a PA was
modeled as AM/AM (amplitude distortion which depends on
the amplitude of the input). It presumed that the nonlinear
noise at the receiver can be modeled as a complex Gaussian
process uncorrelated with the input process and hence
evaluated the OFDM system performance with nonlinear
distortions in Additive White Gaussian Noise (AWGN)
channels. To achieve maximum efficiency the power
amplifier should be driven near the saturation region, but
since the OFDM signal has high PAPR these power amplifiers
will cross over to the nonlinear region causing serious in-band
distortions as well as adjacent channel interference with
spectrum re-growth in the transmitted signal. To minimize the
nonlinear effects it is desirable to operate HPA in the linear
region i.e. with huge back off, but in this case, efficiency has
to be compromised. So this degrades the performance of an
OFDM system. A measure of the degradation can be very
helpful in evaluating the performance of a given system and in
designing a signaling set that avoids degradation. Throughout
this paper, we analyze the effects of nonlinearity of HPA in
the OFDM system which degrades the system performance.
The rest of this paper is organized as follows. Section II,
briefly describes the basic concepts of OFDM transceiver.
Section III, introduces the power amplifier model. Section IV,
shows the non linearity analysis and simulations of OFDM
system for non linear power amplifier. Finally, the paper is
concluded in Section V.
II. SYSTEM DESCRIPTION
In this section, we describe the OFDM transceiver system.
Orthogonal Frequency Division Multiplexing (OFDM) is a
broadband multicarrier modulation method that has superior
performance and benefits over older, more traditional
single-carrier modulation methods (Laakso et. al., 1971).
Before transmitting information bit in AWGN or Rayleigh
fading channel through OFDM transmitter, the incoming
modulated serial bits are converted into parallel streams by
using a serial to parallel converter. Its then transforms this
spectral representation of the data into the time domain using
an Inverse Fast Fourier Transform for baseband OFDM
modulation. To prevent overlapping of the data at the receiver
cyclic prefix is inserted whose duration is one fourth of the
total OFDM symbol duration. At the receiver side, firstly the
data is received through linear receivers followed by a linear
combiner. This linear combiner is designed in such a way that
the output SNR is maximized at each instant of time. Then this
data is converted again to the digital domain by passing it
through an analog to digital converter. After removing the
High Power Amplifier Effects Analysis for
OFDM System
Tushar Kanti Roy and Monir Morshed
ISSN: 2278 – 7798
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 2, Issue 5, May 2013
1120
All Rights Reserved © 2013 IJSETR
cyclic prefix, data is again converted into serial to parallel by
a serial-to-parallel converter. These parallel bit streams are
demodulated using Fast Fourier Transform (FFT) to get back
the original data by converting parallel bit streams into serial
bit streams.
In Orthogonal Frequency Division Multiplexing, multiple
sinusoidal with frequency separation T/1 is used where, T is
the active symbol period. The information kg to be send on
each subcarrier k is multiplied by the corresponding
carrier Tktj
k etg2
)( and the sum of such modulated
sinusoidal form the transmit signal. Therefore, the sinusoidal
used in OFDM can be defined as (Roy et. al., 2012):
)()(2
twetg Tktj
k
(1)
where, k =0, 1, ...N-1 correspond to the frequency of the
sinusoidal and w(t) =u(t)-u(t-T) is a regular window over [0,
T]. Since OFDM uses multiple sinusoidal having frequency
separation 1/T, therefore each sinusoidal is modulated by
independent information. Mathematically we can write the
transmit signal through the channel is,
)(1
)(
)()()()(
1
0
2
1
0
111100
tweN
tg
tgtgtgtS
N
k
Tktj
k
N
k
kk
NN
(2)
where, k is the kth
symbol in the message symbol sequence
for k in [0, N-1], N is the number of carriers. In an OFDM receiver, we will multiply the received signal
with a bank of correlator and integrate over the period T.
Therefore, the information sends on subcarrier
km
kmgetS
T
kT
m tj
T,0
,)(
12
(3)
where, m takes values from 0 to k-1.
III. POWER AMPLIFIER MODEL
In this section we describe the model of the high power
amplifier for OFDM systems. High power amplifier
nonlinearities significantly affect the performance of OFDM
systems. In this section, the effects of amplifier nonlinearities
are modelled in an OFDM system. Consider an input signal in
polar coordinates as (Hardeep et. Al., 2012)
jex (4)
where, 𝜌 is the amplitude and is the phase of input signal.
The output of the power amplifier can be written as
))(()()( pjeMxg
(5)
where, g(𝑥) represents the output of the power amplifier,
)(M represents the AM/AM conversion and 𝑝(𝜌)
represents the AM/PM conversion characteristics of the
power amplifier.
IV. SIMULATION RESULT
In this section, numerical simulation results are presented to
investigate the nonlinear effects of HPA on an OFDM system.
HPA nonlinearity may have bad influence on OFDM signals
mainly on two aspects: (a). Out-of-band distortion, which will
cause the OFDM power spectrum distortion, i.e. the spectral
spreading of the amplified signal and introduce adjacent
channel interference (ACI). Requirements on ACI for RF
systems are very strict especially for large number of
subscribers; therefore, it is of great importance to decrease the
out-of-band distortion. (b). In-band distortion, which may
disturb the OFDM constellations. PSD is utilized to evaluate
the effects of AM/AM distortions on OFDM signals. Fig.1 (a)
shows the time domain waveform of a typical OFDM signal
before passing through the HPA.
Fig.1(a): OFDM waveform before HPA
Fig.1 (b): OFDM waveform after HPA
0 10 20 30 40 50 60 70-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
Time
Am
plitu
de
OFDM Signal before HPA
0 10 20 30 40 50 60 70-2.5
-2
-1.5
-1
-0.5
0
0.5
1
Time
Am
plitu
de
OFDM Signal after HPA
ISSN: 2278 – 7798
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 2, Issue 5, May 2013
1121 All Rights Reserved © 2013 IJSETR
Fig. 2: Power spectrum of OFDM signal before and after amplifier
Fig.3: AM/AM response of OFDM signal
As is mentioned earlier, HPA nonlinearity may introduce the
spectral spreading of OFDM signals; for that we make a
simulation that shown in Fig.2 to illustrate the out-of-band
distortion caused by AM/AM distortions. From Fig.2, it is
clear that the AM/AM distortion has great influence on HPA
power spectral density performance.
Fig.3 demonstrates the comparison of idealized and practical
AM/AM response of power amplifier at the RF transmitter
side of OFDM system. From the input =1, non linearity starts
for the high power amplifier. AM/AM distortions have
distinct effects on OFDM signal constellations that shown in
Fig.4. It is clear to see from simulation Fig.4 that the AM/AM
distortion causes not only the dispersion but also the rotation
of signal constellations which greatly degrade the
performance of the OFDM system.
V. CONCLUSION
In this paper, we have highlighted the effects of
nonlinearities in the power amplifier over OFDM system. So
it is obvious from the simulation results that the performance
of the OFDM system will be greatly degraded if we will not
minimize the nonlinear effects of HPA. It is also clear from
the simulation results that the distortion in OFDM system due
to high power amplifier, either limiting or nonlinearity effects
are highly related to the distribution parameter of signal which
controls the dynamic range itself rather than the clipping level
or the saturation level of the amplifier. It is noticed that the
effects of nonlinearity of the high power amplifier depends
upon the type of modulation used in OFDM system which is
our next research task.
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[3] A. A. Saleh (1981), ―Frequency independent and frequency dependent
nonlinear models of TWT amplifiers,” IEEE Trans. Commun., vol.
COM29, pp. 1715–1720.
[4] D. Dardari, V. Tralli, and A. Vaccari(2000), ―A theoretical
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[5] P. Banelli (2003), ―Theoretical analysis and performance of OFDM
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[6] AI Bo, ZHANG Tao-tao, PAN Chang-yong, YANG Zhi-xing, WANG
Yong, ZHAO Huai-xun (2007), ―Effects of HPA Phase Distortion on
System Performance,‖ Journal of System Simulation, Vol. 19, No. 2.
[7] Kenington (2000), ―High-Linearity RF amplifier Design,‖ Artech
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[8] F. H. Gregorio and T. I. Laakso (2005), ―The Performance of
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[9] Roy T. K (2012), ―Comparative BER Performance Analysis of OFDM
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[10] Amanjot Singh and Hardeep Kaur (2012) ―Non Linearity Analysis of
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Computer Applications (0975 – 8887), Vol. 37, No. 2.
-10 -8 -6 -4 -2 0 2 4 6 8 10-5
0
5
10
15
20
25
30
Frequency
sp
ectru
m sig
na
ls
Spectrum Effects
Befor Amplifier
After Amplifier
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
1
2
3
4
5
Input Amplitude
Ou
tpu
t A
mp
litu
de
AM/AM response of power amplifier
Linear Response
Amplifier Response
Mean of OFDM Amplitude
-1.5 -1 -0.5 0 0.5 1 1.5-1.5
-1
-0.5
0
0.5
1
1.5
I channel
Q c
ha
nn
el
signal Constelleations
After Amplifier
Before Amplifier