Enhancing the BER and ACLR for the HPA Using Pre-Distortion Technique

International Journal of Electrical and Computer Engineering (IJECE)
Vol. 9, No. 4, August 2019, pp. 2725~2731
ISSN: 2088-8708, DOI: 10.11591/ijece.v9i4.pp2725-2731  2725
Journal homepage: http://iaescore.com/journals/index.php/IJECE
Enhancing the BER and ACLR for the HPA
using pre-distortion technique
Ehab Abdulrazzaq Hussein
Department of Electrical Engineering, Faculty of Engineering, University of Babylon, Iraq
Article Info ABSTRACT
Article history:
Received Mar 9, 2018
Revised Jan 26, 2019
Accepted Mar 11, 2019
Power amplifiers are key components in wireless transceivers. Their function
is to amplify signal and generate the required Radio Frequency (RF) power
that allows to transmit the signal over an appropriate range. The Orthogonal
Frequency Division Multiplexing (OFDM) systems are highly sensitive to
nonlinear distortion introduced by High Power Amplifier (HPA). The HPA
nonlinearity causes in-band and out-of-band distortions. The linearization
techniques are used to compensate the nonlinear effects of the high power
amplifier. These techniques correct the distortion effects resulting from
nonlinearities in the transmitted signal. Many linearization techniques have
been developed to improve power amplifier linearity and to decrease both Bit
Error Rate (BER) and Adjacent Channel Leakage Ratio (ACLR). This work
is set to run the high power amplifier in the nonlinear region. It is also
attempting to analyze the resulting signal in terms of the BER and ACLR,
next employs pre-distortion linearization techniques to reduce the distortion
introduced in this region. According to Digital Video Broadcasting-
Terrestrial (DVB-T) standard the linearization techniques, circuit and the
OFDM transmitter and receiver is designed and implemented through using
computer simulation of AWR Design Environment.
Keywords:
ACLR
BER
High power amplifier
Linearization technique
OFDM
Predistortion
Copyright © 2019 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Ehab Abdulrazzaq Hussein,
Department of Electrical Engineering,
University of Babylon,
Hillah-Najaf Road, Elect. Dept., Faculty of Eng., University of Babylon, Babylon, Iraq.
Email: dr.ehab@itnet.uobabylon.edu.iq
1. INTRODUCTION
OFDM, the multi-carrier transmission technique, is an excellent method for high speed bi-
directional wireless data communication [1]. It is very similar to the well-known technique of Frequency
Division Multiplexing (FDM). It uses the same principles of FDM to allow multiple messages to be sent over
a single radio channel. It offers a much more controlled manner, which is allowing an improved spectral
efficiency [2]. An OFDM signal consists of a sum of subcarriers that is modulated by using Phase Shift
Keying (PSK) or Quadrature Amplitude Modulation (QAM). Although OFDM provides good spectral
efficiency, it produces a signal with a fluctuating envelope that generates Intermodulation Distortion (IMD)
at the system’s power amplifiers. Most of the IMD power appears as an interference between adjacent
channels, which requires the use of linear high power amplifiers. Considering back-off power is the simplest
way to improve the linearity of the power amplifier which corresponds to moving the operating point of the
amplifier to the linear region. Increasing the back-off of the power amplifier means that the signal is covered
better in the linear range, and thus the effects of nonlinearities are reduced. However, power efficiency is
reduced as well. So a tradeoff between efficiency and linearity is needed. Linearization techniques prove to
be the best solution in order to improve power amplifier linearity without having a negative impact on
efficiency [3]. Several kinds of linearization techniques for power amplifiers have been proposed such as

 ISSN: 2088-8708
Int J Elec & Comp Eng, Vol. 9, No. 4, August 2019 : 2725 - 2731
2726
Feedforward, Feedback, Predistortion, Linear Amplification using the Nonlinear Component [LINC] and
Envelope Elimination and Restoration [EER]. This work aims to study the impact of amplifier nonlinearity
for different modulation techniques and the effect of the pre-distortion linearization technique on it in OFDM
system. Pre-distortion linearization technique is used widely because of its stability and its high efficiency.
2. BASIC PRINCIPLE OF OFDM
The OFDM idea utilizes information spreading which is transmitted over an extended number of
carriers, each being balanced at a low rate. The transporters are generated to be orthogonal to each other by
suitably picking the recurrence separating between them [5,6]. With a specific end goal to avoid countless
channels on the transmitter and reciprocal channels and demodulators at the beneficiary, it is attractive to
have the capacity to utilize the current computerized signal handling procedures, for example, Fast Fourier
Transform (FFT) [7]. In a run of the mill OFDM transmission framework an information bit stream is
injected into a channel encoder that partitions the information into N distinctive sub channels.
The information is mapped onto M-PSK or M-QAM body. At that point, a N-point is connected onto OFDM
mediator where IFFT is added to change the images from a recurrence area in the time space. Therefore, the
time domain signals pass from parallel to serial transformation, and prepared to send. Today, the OFDM is
commonly used for the most part in the transmission of computerized information where right now utilized
on DAB, DVB, HDTV, ADSL, VDSL, Wireless LAN Networks, HIPERLAN/2, IEEE 802.11a, IEEE
802.11g, and IEEE 802.16 Broadband Wireless Access System.
Numerous advantages are gained by using OFDM, including: high range productivity as in Figure 1,
strength to channel blurring, insensitivity to test timing balances than single bearer frameworks,
invulnerability to motivation impedance, and simplicity of sifting through the clamor. The fundamental
burden of OFDM framework can be described as follows: (a) its RF signal experiences a high crest to the
normal force proportion (PAPR) because of the transmission of numerous sub-transporters and (b) it is very
delicate to Inter Carrier Interference (ICI) because of recurrence counterbalance. The PAPR of a sign is
given as:
𝑃𝐴𝑃𝑅 =
𝑆𝑖𝑔𝑛𝑎𝑙 𝑃𝑒𝑎𝑘 𝑉𝑎𝑙𝑢𝑒
𝑆𝑖𝑔𝑛𝑎𝑙 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑣𝑎𝑙𝑢𝑒
(1)
Figure 1. The concept of OFDM: (a) Multi-Carrier technique.
(b) Orthogonal Multi-Carrier technique.
3. RESULTS AND ANALYSIS
The force enhancer is the key component in the transmitter. In light of the vital issue, high power,
productivity is a fundamental necessity for a powerful speaker. In the meantime, high linearity is increasingly
alluring today, to minimize the recurrence obstruction and permit higher transmission limit in wideband
correspondence frameworks. More client channels can be fitted into the accessible range if the range of
linearity is extended in transmitter [8]. Nonlinear intensification yields IMD items and results in unsuitable
ghastly regrowth in the contiguous channels. Direct intensification is required when the signal contains both

Int J Elec & Comp Eng ISSN: 2088-8708 
Enhancing the BER and ACLR for the HPA using pre-distortion technique (Ehab Abdulrazzaq Hussein)
2727
sufficiency and state regulation. It can be proficient either by a chain of direct PAs or by a blend of nonlinear
PAs [10]. In a perfect Power enhancer the unpredictable exchange capacity is given as:
𝐺 = 𝐴𝑒 𝑗𝛷 (2)
Where G and Φ is the linear gain and phase constant respectively. When a real amplifier is considered,
the gain A(s(t)) and phase-shift Φ(s(t)) are represented as functions of the input signal s(t) while the complex
transfer function depends on input power of the amplifier and is described mathematically as follows [5]:
𝐺 (𝑠(𝑡)) = 𝐴(𝑠(𝑡)) 𝑒 𝑗𝛷(𝑠(𝑡)) (3)
Two types of distortion are introduced due to the nonlinear characteristics of power amplifiers AM/AM
(amplitude modulation/amplitude modulation) and AM/PM (amplitude modulation/phase modulation)
effects. The characteristics of amplitude and phase of the output signal are known as AM-AM and AM-PM
characteristics respectively. They result in output signal amplitude and phase modulation when the input
signal envelope fluctuates [11]. The AM-AM and AM-PM characteristics can be obtained in digital
communication systems as a function of the complex form of the input signal sample. For an nth
complex
input sample zn with a magnitude | zn | and a phase are (zn), the complex transfer function of the nonlinear
PA is represented by [5]:
𝐴(𝑧 𝑛) = 𝐴𝑀(|𝑧 𝑛|2
)𝑒 𝑗(𝑎𝑟𝑔(𝑧 𝑛)+𝑃𝑀(|𝑧 𝑛|2)) (4)
Where 𝐴𝑀(|𝑧 𝑛|2
) and 𝑃𝑀(|𝑧 𝑛|2
) are mathematically modeled by polynomial functions, both of them are
derived from the AM-AM and AM-PM characteristics, respectively. |zn|2
is the power of the nth
complex
input sample. Different factors can measure nonlinearity of the power amplifier such as third order intercept
point (IP3), harmonics 1dB compression point, and Adjacent Channel Power Ratio (ACPR). The amount of
signal energy which is leaking from the main an adjacent channel can be determined by the help of ACPR.
The overall system response of non-constant modulated signals are expected to be enhanced using RF power
amplifier linearization techniques. This can be achieved by reducing ACPR in the PA output power spectral
density [3] as presented in the following equation:
𝐴𝐶𝑃𝑅 =
𝑇𝑜𝑡𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛 𝑎𝑑𝑗𝑎𝑐𝑒𝑛𝑡 𝑐ℎ𝑎𝑛𝑛𝑒𝑙
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛 𝑡ℎ𝑒 𝑚𝑎𝑖𝑛 𝑐ℎ𝑎𝑛𝑛𝑒𝑙
(5)
4. PROPOSED MODEL OF DISTORTION TECHNIQUE
Proficiency is an essential sympathy toward the power amplifier creators. In any case, the tradeoff
for high effectiveness is regularly results in reducing in linearity, and the diminishing linearity is added to
ghastly obstruction outside the proposed transfer speed. Pre-distortion gives better effectiveness, lower
expense and appropriateness of linearity from the other linearization systems. This is achieved based on the
grounds that a nonlinear circuit is embedded between the information signal and the PA. This nonlinear
circuit creates the IMD items backwards to those delivered by the PA and cross out the impact of PA
nonlinearity accordingly. Figure 2 demonstrates an outline of a proposed pre-distortion linearizer.
The pre-distortion linearizer circuit is composed of two circles and three ways. Where the
information signal is separated into three parallel ways in the circuit and afterward recombined toward the
end of pre-distortion linearizer. Two ways are direct and can essentially be a length of transmission line.
The other way is nonlinear trademark that can be acquired from a speaker crashed into immersion area.
In the first loop, the power amplifier amplifies the input signal in the upper branch with a voltage
gain of Av which leads to amplify the input signal as well as the appearance of distortion voltage vd by a
certain amount.
𝑣𝑠 = 𝑣𝑖𝑛 𝐴 𝑣 + 𝑣 𝑑. (6)
The attenuator circuit with the transfer function
1
Av
that located after the amplifier is used to
decrease the amplified signal to the level of linear signal.
𝑣 𝐴 =
𝑣𝑠
𝐴 𝑣
= 𝑣𝑖𝑛 +
𝑣 𝑑
𝐴 𝑣
(7)

 ISSN: 2088-8708
2728
Figure 2. Block diagram of power amplifier system with predistortion linearizer.
The signal of the middle branch is a phase shifted version of the original RF signal. While the signal
in the upper branch will be 180° out of phase. This signal is contaminated with distortion that generated by
the power amplifier. Thus, when the two signals in this loop are recombined at the output, will get only over
the distortion signal.
𝑣 𝑐1 = 𝑣 𝐴 − 𝑣𝑖𝑛 = 𝑣𝑖𝑛 +
𝑣 𝑑
𝐴 𝑣
− 𝑣𝑖𝑛 =
𝑣 𝑑
𝐴 𝑣
. (8)
In the second loop, the signal results from the first combiner is phase shifted by 180° and
recombined with the signal that coming from lower path. The result is the original signal minus it the output
distortion that generated by the pre-distorter Amplifier.
𝑣 𝑐2 = 𝑣𝑖𝑛 − 𝑣 𝑐1 = 𝑣𝑖𝑛 −
𝑣 𝑑
𝐴 𝑣
. (9)
The output signal that is coming from a circuit of pre-distortion linearize contains distortion which
is the opposite of the distortion that results from amplification. They will cancel each other and the output
signal will be almost free of distortion.
𝑣 𝑜𝑢𝑡 = 𝑣𝑖𝑛 𝐴 𝑣 + 𝑣 𝑑 (10)
𝑣 𝑜𝑢𝑡 = (𝑣𝑖𝑛 −
𝑣 𝑑
𝐴 𝑣
)𝐴 𝑣 + 𝑣 𝑑 = 𝑣𝑖𝑛 𝐴 𝑣 − 𝑣 𝑑 + 𝑣 𝑑 (11)
𝑣 𝑜𝑢𝑡 = 𝑣𝑖𝑛 𝐴 𝑣 (12)
The key to good predistortion filtering is to match the AM/AM and AM/PM responses of the
distortion generating element with the AM/AM and AM/PM responses of the amplifier being pre-distorted.
5. EXPERIMENTAL RESULTS
Experimental work was carried out in this study using AWR Design Environment. The pre-
distortion linearization technique is simulated for power amplifier in OFDM system.
5.1. System parameters
The OFDM signal for simulation parameters is selected based on the DVB-T standard as in Table 1.
Table 1. OFDM Model System Parameters
Source of Data Random
Type of Modulations PSK, QAM
M 16, 64, 256
Number of Subcarriers 1705
Power Amplifier G = 26, P1dB = 45
Coding Convolution with rate 1/2
Mode 2k
Subcarrier Spacing 4.464 (kHz)
Guard Interval 0.03125
Bandwidth 7611.12 (MHz)

2729
5.2. A Comparison of BER performances of 16PSK with 16QAM
Figure 3 (a, b, and c) shows BER vs. SNR for 16PSK and 16QAM OFDM system with and without
PDLT (Pre-Distortion Linearization Technique) for different values (15, 20 and 25) dBm of input power for
OFDM system.
Figure 4 (a, b, and c) shows the BER vs. SNR as in Figure 3 but the value of M was changed to the
value of 64.
BER performance results comparison with SNR for the OFDM system with 256PSK and 256QAM
modulations with and without PDLT as shown in Figure 5 (a, b, and c).
Figures (3, 4 and 5) show BER vs. SNR for (16, 64, 256) PSK and QAM, OFDM system with and
without PDLT (Pre-Distortion Linearization Technique) for different values (15, 20 and 25) dBm of input
power for OFDM system.
From the above shapes and tables, it can be observed that as the value of input power increased, the
BER increases, where the input power impacts significantly on BER. Also, as SNR increased the PSK or
QAM BER curve leans downward this refers to the reduction in bit error rate. Also, it is noticed that the
value of the BER suffers significantly from reduction if used PDLT circuit.
5.5. Power amplifier spectra
Figure 6 (a, b, c and d) presents the ACLR (Adjacent Channel Leakage Ratio) level of the power
amplifier with and without the presence of pre-distortion linearization technique and different values of the
input power level.
From Figure 6 (a, b, c and d) it is noticed that the spectral regrowth of amplifier reduced when used
predistortion technique, which will lead to the reduction of the ACLR of the power amplifier.
5.6. AM to AM of power amplifier
Figure 7 displays AM to AM of predistortion linearizer and PA with and without using of
predistortion linearization technique. From Figure 7, it can be observed that the linear region of the power
amplifier, is increased significantly when we use PDLT because the transfer function of the predistortion
linearizer is the opposite of the transfer function of PA in the nonlinear region.
(a) (b) (c)
Figure 3 (a, b and c). BER performance of 16PSK & 16QAM in OFDM system with and without PDLT
(a) (b) (c)

 ISSN: 2088-8708
2730
(a) (b) (c)
(a) (b)
(c) (d)
Figure 6 (a, b, c and d). Power amplifier spectra
Figure 7. AM to AM of Power Amplifier
6. CONCLUSIONS
This work has developed a technique to run the high power amplifier in the nonlinear region. The
results depict following conclusions: It is noticed that when using the pre-distortion method can be reduced
the nonlinearity of the HPA. This can be enhanced BER and ACLR which will increase the efficiency of the
HPA, and that will lead to an increase in the efficiency of OFDM system. It is also noticed that a bit error rate
of the power amplifier depends on the types of modulation methods and Input Power (IP). Thus, it is less for
the signal modulated with QAM as compared to PSK modulation and whenever the value of the IP has been
reduced the results will be better. So the signal which is modulated with QAM has minimum distortion in
OFDM system with the least possible value of IP.

2731
REFERENCES
[1] Paloma García-Dúcar, Jesús de Mingo and Antonio Valdovinos, “Misalignments Feedforward Transmitter
Correction Design for Nonlinear Distortion Cancellation in OFDM Systems,” 1-4244-0063-i2006 IEEE, 2006.
[2] A. Singh, H. Kaur, “Nonlinearity Analysis of High Power Amplifier in OFDM System,” International Journal of
Computer Applications, Vol. 37– No.2, January 2012.
[3] Ziad El-Khatib, Leonard MacEachern and Samy A. Mahmoud, “Distributed CMOS Bidirectional Amplifiers,”
Carleton University, 2012.
[4] Suranjana Julius, “Design and Implementation of An ETSI-SDR OFDM Transmitter with Power Amplifier
Linearization,” M.Sc. Thesis, University of Saskatchewan, 2010.
[5] Ehab A. Hussein, Mohammed A. Abdulkadhim, “Performance Improvement of BER in OFDM System using Feed
Forward Technique on Power Amplifier,” International Journal of Computer Applications (IJCA), Vol. 75, No.4,
August 2013.
[6] Brahmaji T.A.R.K, “An efficient ICI Cancellation Technique for OFDM Communication Systems,” M.Sc. Thesis.
National Institute of Technology, Rourkela, 2009.
[7] Ming Xiao, “Novel Predistortion Techniques for RF Power Amplifiers”, The University of Birmingham, 2009.
[8] Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington, Zoya B. Popovic, Nick Pothecary, John F.
Sevic and Nathan O. Sokal, “RF and Microwave Power Amplifier and Transmitter Technologies — Part 1,” High
Frequency Electronics, May 2003.
[9] Tzi-Dar Chiueh and Pei-Yun Tsai, “OFDM Baseband Receiver Design for Wireless Communications,” National
Taiwan University, Taiwan, 2007.
BIOGRAPHY OF AUTHOR
Ehab Abdulrazzaq Hussein, PhD. MSc. Electrical Engineering was born in Babylon on January 1,
1976. He obtained his BSc degree (1997) in Electrical Engineering at the Faculty of Engineering,
University of Babylon and MSc degree (2000), in electrical engineering at the Department of
Electrical Engineering, University of Technology and his PhD. Degree from the Department of
Electrical Engineering at the Faculty of Engineering, University of Basrah, Currently he works as
assistant professor at the Electrical Department at the Faculty of Engineering, University of Babylon.
His main interest is signal processing, analysis, information transition, sensors and control system
analysis.

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