Wireless communication systems are developing to support
more users and provide higher data rates in a limited radio-
frequency (RF) spectrum. Protocols that support modern
wireless communication systems allow users to access a
variety of multimedia services, potentially providing it with a
speed of 100 Mb/s. In the transmitter chain, modulator, PA
and filter are the most challenging blocks. As nonlinear
elements, these components cause distortion to the transmitted
signals which significantly degrade the quality of the signal.
The Quadrature and In-phase carriers in the analog modulator
do not have exactly the same amplitudes and phase
differences. These discrepancies are called gain/phase
imbalance and can cause crosstalk between the I and Q
channels, which degrade quality of the signal [1]-[4].
This paper is an evaluation of the nonlinearity effects with
and without I/Q imbalance in reconfigurable RF circuits for
wireless transmitters. It is applied for analyzing 16 QAM
OFDM/64 QAM OFDM signal in wireless transmitter. It will
be shown that by introducing I/Q imbalance an additional
distortion appears in wireless transmitter. Experimental
analysis for LTE R9 16 QAM (16 QAM OFDM) and LTE R9
64 QAM (64 QAM OFDM) signals of a wireless transmitter
shows that out-of-band distortion increase correspondingly
with addition of the mentioned undesired effect. The paper is
organized as follows. At first, the effects of I/Q imbalance is
explained using baseband methodology. Experimental setup
and results are presented in section III. Finally, the conclusion
is given in section IV.
Nonlinearity of the components in transmitter chain
produces compression of signal amplitude and phase, degrades
the quality of the transmitted signal. Modulator is nonlinear
element which up-converts the baseband signal to RF.
Problem with modulator is that it has phase and gain
imbalances that affect transmitter’s performances. This
disturbs the ideal 90˚ degree phase relationship between I and
Q signals along with gain imbalance [5]-[7]. The output of
modulator can be represented as:
󰇟
 
󰇠
󰇟 

󰇠 (1)
where is the imbalanced signal, is gain imbalance, is
phase imbalance and is 16/64 QAM OFDM input signal.
1. Introduction
2. I/Q Imbalance
Joint Evaluation of I /Q Imbalance and Reconfigurable RF Filter
Nonlinearity in LTE Transmitters
M. BOZIC, A. ANASTASIJEVIC, K. RABBI, N. MOHOTTIGE, D. BUDIMIR
Wireless Communications Research Group, Faculty of Science and Technology
University of Westminster, London, W1W 6UW, UK
Abstract: In this paper, a joint evaluation of I/Q imbalance and reconfigurable bandpass filter
nonlinearity in wireless transmitters is described. An experimental analysis of complete Orthogonal
Frequency Division Multiplex (OFDM) transmitter is presented for the purpose of quantifying these
nonlinearities using LTE R9 3 MHz 16 QAM and LTE R9 3 MHz 64 QAM signals.
Keywords: LTE, Nonlinear distortion, reconfigurate filters; I/Q imbalance; wireless transmitters.
Received: June 28, 2021. Revised: December 2, 2021. Accepted: December 17, 2021. Published: January 5, 2022.
WSEAS TRANSACTIONS on COMMUNICATIONS
DOI: 10.37394/23204.2022.21.2
M. Bozic, Α. Anastasijevic, K. Rabbi, N. Mohottige, D. Budimir
E-ISSN: 2224-2864
10
Volume 21, 2022
I PATH
Q PATH
90°- θI + θQ
TO RECEIVER
LPF SWITCH
AMPLIFIER
AMPLIFIER
LPF MODULATOR
RECONFIGURABLE
FILTER
Fig. 1. Wireless transmitter with undesired effects
Measurement setup consists of signal generator Agilent
MXG N5182A, reconfigurable microstrip bandpass filter as
DUT, which is illustrated in Fig. 2, used to emulate wireless
transmitters. The layout of the reconfigurable circuit
(bandpass filter) is based on the combination of a bent single
λg/2 resonator and a pair of bent λg/4 short circuited resonators.
The filter is inductively coupled to the source. Compactness of
this filter is attained through the reduction of filter length and
width. In addition, two bent short stubs are shorted to common
ground in order to miniaturize the filter. This filter is designed
to have a 3 dB passband from 925 MHz – 960 MHz (LTE
band 8) with a mid-band frequency of 942.50 MHz. The
proposed filter layout comprises of lines of width 1.2 mm and
the open circuited stub leading to the gap is 2.4 mm wide.
Fig. 2. Reconfigurable bandpass filter as DUT.
General-purpose interface bus (GPIB) was used to connect
this generator with PC. The signals were created in Matlab and
download to MXG using Agilent Signal Studio Toolkit. The
signal named RF output was passed through DUT
(reconfigurable pin switch based bandpass filter). Finally, the
signal was captured with VSA 4406A for signal analysis. The
measurement setup is shown in Fig. 3.
MXG-N5182A
DUT
GPIB
PC
Agilent Signal Studio Toolkit
VSA 4406A
RF OUT
Matlab
Advanced Design System
Fig. 3. Measurement setup of wireless transmitter.
Experiment was conducted for two different cases of input
signals, LTE R9 3 MHz 16 QAM and LTE R9 3 MHz 64
QAM signals with and without I/Q imbalance. Spectrum of
the LTE R9 3 MHz 16 QAM signals with and without I/Q
imbalance are shown in Figs. 4a, 4b, 4c and 4d respectively.
The measured power spectrum of the LTE R9 3 MHz 64
QAM signals with/without I/Q imbalance are shown in Figs.
5a, 5b, 5c and 5d respectively.
3. Results
3.1 Measurement Setup
3.2 Measurement Results
WSEAS TRANSACTIONS on COMMUNICATIONS
DOI: 10.37394/23204.2022.21.2
M. Bozic, Α. Anastasijevic, K. Rabbi, N. Mohottige, D. Budimir
E-ISSN: 2224-2864
11
Volume 21, 2022
Fig. 4a. The measured power spectra of the LTE R9 3 MHz
16 QAM signal at the input of the reconfigurable filter
without I/Q imbalance.
Fig. 4b. The measured power spectra of the LTE R9 3 MHz
16 QAM signal at the output of the reconfigurable filter
without I/Q imbalance.
Fig. 4c. The measured power spectra of the LTE R9 3 MHz
16 QAM signal at the input of the reconfigurable filter with
I/Q imbalance.
Fig. 4d. The measured power spectra of the LTE R9 3 MHz
16 QAM signal at the output of the reconfigurable filter
with I/Q imbalance.
Fig. 5a. Spectrum of the LTE R9 3 MHz 64 QAM signal at
the input of the reconfigurable filter without I/Q imbalance.
Fig. 5b. Spectrum of the LTE R9 3 MHz 64 QAM signal at
the output of the reconfigurable filter without I/Q
imbalance.
WSEAS TRANSACTIONS on COMMUNICATIONS
DOI: 10.37394/23204.2022.21.2
M. Bozic, Α. Anastasijevic, K. Rabbi, N. Mohottige, D. Budimir
E-ISSN: 2224-2864
12
Volume 21, 2022
Fig. 5c. Spectrum of the LTE R9 3 MHz 64 QAM signal at
the input of the reconfigurable filter with I/Q imbalance.
Fig. 5d. Spectrum of the LTE R9 3 MHz 64 QAM signal at
the output of the reconfigurable filter with I/Q imbalance.
Quantitative measures of distortion is defined as:
Δ   (2)
where  represents output power of the transmitted signal
and  represents distortion power. Measurement results
with and without I/Q imbalance are presented in Tables 1 and
2, respectively.
TABLE 1: DISTORTION OF RECONFIGURABLE BANDPASS FILTER IN
WIRELESS TRANSMITTER WITHOUT I/Q IMBALANCE
M QAM
modulation
Δ [dB]
without I/Q
imbalance
16
0.46
64
1.93
TABLE 2: DISTORTION OF RECONFIGURABLE BANDPASS FILTER IN
WIRELESS TRANSMITTER WITH I/Q IMBALANCE
M QAM
modulation
Δ [dB] with
I/Q imbalance
16
3.82
64
6.7
Financial support was provided by the EU-Erasmus Mundus
Action 2 project EUROWEB.
[1] D. Saffar, N. Boulejfen, F. Ghannouchi , M. Helaoui and A. Gharssalah,
“Behavioral Modeling of MIMO Transmitters Exhibiting Nonlinear
Distortion and Hardware Impairements,” Proceedings of the 6th
European Microwave Integrated Circuits Conference
[2] Y.-D. Kim, E.-R. Jeong and Y. H. Lee, "Adaptive Compensation for
Power Amplifier Nonlinearity in the Presence of
QuadraturModulation/Demodulation Errors," IEEE Transactions Signal
Process., vol. 55, no. 9, pp. 4717-4721, Sep. 2007
[3] F. H. Raab, P. Asbeck, S. Cripps, P. B. Kenington, Z. B. Popovic, N.
Pothecary, J. F. Sevic, and N. O. Sokal, “Power Amplifiers and
Transmitters for RF and Microwave“, IEEE Transactions on Microwave
Theory and Techniques, vol. 50, no. 3, pp. 814-826, March 2002.
[4] P. B. Kenington, "High Linearity RF Amplifier Design", Artech House,
2000.
[5] S.Bhattacharjee, Rawat, K.; Rawat, M.; Donglin Wang; Helaoui, M.;
Leung, H.; Ghannouchi, F.; , "Joint evaluation and mitigation of RF
impairments and nonlinear distortion in WiMAX Transceiver under
Nakagami-m fading channel," Electrical and Computer Engineering
(CCECE), 2011 24th Canadian Conference on , vol., no., pp.000926-
000929, 8-11 May 2011
[6] L.Ding, Z. Ma, D. Morgan, M. Zierdt, and G. Tong Zhou,
“Compensation of frequency-dependent gain/phase imbalance in
predistortion linearization systems,” IEEE Transactions Circuits and
Systems I, vol 55, no. 1, pp. 390-397, Feb. 2008.
[7] L. Anttila, P. Handel, and M. Valkama, “Joint mitigation of power
amplifier and IQ modulator impairments in broadband direct-conversion
transmitters,” IEEE Transactions Microwave Theory and Techniques,
vol. 58, no. 4, pp. 730-739, Apr. 2010.
Acknowledgement
References
Creative Commons Attribution License 4.0
(Attribution 4.0 International, CC BY 4.0)
This article is published under the terms of the Creative
Commons Attribution License 4.0
https://creativecommons.org/licenses/by/4.0/deed.en_US
WSEAS TRANSACTIONS on COMMUNICATIONS
DOI: 10.37394/23204.2022.21.2
M. Bozic, Α. Anastasijevic, K. Rabbi, N. Mohottige, D. Budimir
E-ISSN: 2224-2864
13
Volume 21, 2022