# December 2021, Volume 30, Number 4 [DOI: 10.13164/re.2021-4]

**P. Kadera, J. Lacik, H. Arthaber**
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[DOI: 10.13164/re.2021.0595]
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Effective Relative Permittivity Determination of 3D Printed Artificial Dielectric Substrates Based on a Cross Unit Cell

This paper proposes closed-form analytical models for the determination of the effective relative permittivity of 3D printed artificial dielectric substrates based on a cross unit cell. The parallel plate capacitor approach is used to describe the real physical shape of the unit cell allowing to include anisotropic properties as well. A detailed comparison of the analytical models and effective medium approximations is carried out for air host material and inclusion materials with relative permittivities in the range from 2.5 to 1000 and the inclusion volume fraction from 0.1 to 1. It is observed that the proposed models predict the effective relative permittivity with much better accuracy than frequently used effective medium theory-based formulas and due to their closed-form expressions provide faster calculations than numerical methods. The proposed models were verified experimentally, achieving a very good agreement with simulations.

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Keywords: Dielectric substrates, permittivity, analytical models, material characterization, three-dimensional printing

**M. H. Ahmad**
[references] [full-text]
[DOI: 10.13164/re.2021.0611]
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Efficient Method for Solving TM-Polarized Plane Wave Scattering from Two-Dimensional Perfect Conductor Surfaces Using Fourier Series Approximation of the Green’s Function

The method of moments generates a matrix which is usually solved using iterative methods due to the high computational complexity of a direct inversion. The cost of matrix-vector multiplications and memory requirement at each iteration step is proportional to O(N2), where N is the number of unknowns in the problem. To reduce the computational complexity, the Green’s function is approximated using Fourier series. This will allow to separate the source points from the observation points. Hence, aggregate all source points and then multiply it with each observation point with a small adjustment in the aggregation term. The proposed method is tested by solving electromagnetic wave scattering from perfect conductor two-dimensional basic canonical shape, i.e., circular cylinder. The results showed that the proposed method is accurate and for large N it has a computational complexity less than the direct matrix-vector multiplication.

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Keywords: Electromagnetic wave scattering, Fourier series, method of moments, perfect conductor surfaces, two-dimensional

**S. Karamzadeh, V. Rafiei, H. Saygin**
[references] [full-text]
[DOI: 10.13164/re.2021.0617]
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Metasurface Interlaced SR CP Patch with the Capability to Change Polarization Diversity

In this work, an affordable solution for the improved performance of circular polarization diversity array antenna by helping metasurface structure (MTS) is presented. The basic structure includes a multi-input feed network which is ended to a 2×5 sequentially rotated subarray. A layer of MTS has been used to modify basic antenna characteristics of inspiring ref. [5]. This innovation is aimed to increase the bandwidth of basic antenna from 14.7% (5.05-5.85 GHz) to 37.8% (4.5-6.6 GHz), and 3-dB AR about 4%. Employing MTS layer leads to an increase in the gain of the antenna to 15 dBic. More details of the antenna are reported in the text.

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Keywords: Circular polarization (CP), metasurface, polarization diversity

**J. Lu, X. Cao, J. Gao, H. H. Yang, L. Jidi, K. Gao**
[references] [full-text]
[DOI: 10.13164/re.2021.0622]
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High-gain and Low-RCS Linear Polarization F-P Resonant Cavity Antenna Based on Metasurface

Fabry-Perot (F-P) resonant cavity antenna is a high-gain antenna, which can increase the gain of microstrip antenna significantly, without a complicated feed network. It has a simple structure and it is easy to process. In this paper, the polarization grid structure and the zigzag structure are combined to design a linear-circular polarization conversion metasurface unit. This unit can convert the linearly polarized incident wave that is perpendicular to the polarization grid into circularly polarized transmitted wave in the range of 9.26~10.84 GHz. The unit and its mirror image unit are arranged in a checkerboard shape as a metasurface and placed above the microstrip antenna. Thus a linear polarized F-P resonant cavity antenna with high gain and low radar cross section (RCS) is proposed. In order to verify the performance of the antenna, the F-P resonant cavity antenna was processed and measured. The measured results and simulated results have shown good consistency. Compared with the original microstrip antenna, the polarization purity of the F-P resonator cavity antenna is enhanced in the range of 9.55~9.85 GHz, and the gain is improved in the working frequency range of 9.50~10.04 GHz with a maximum increase of 5.55 dB, and the RCS reduction was achieved in the range of 9.47~11.97 GHz with a maximum reduction of 17.47 dB.

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- ZHANG, L., WAN, X., LIU, S., et al. Realization of low scattering for a high-gain Fabry-Perot antenna using coding metasurface. IEEE Transactions on Antennas and Propagation, 2017, vol. 65, no. 7, p. 3374–3383. DOI: 10.1109/TAP.2017.2700874
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Keywords: F-P resonant cavity antenna, metasurface, high gain, low RCS

**S. C. Yadav, V. Sivavenkateswara Rao, S. P. Duttagupta**
[references] [full-text]
[DOI: 10.13164/re.2021.0631]
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A Novel Unidirectional High-Gain Cascaded Square Ring Antenna for WLAN Base Station Applications

This paper designs a unidirectional high gain, low-cost cascading ring antenna with coaxial feeding and metal without the dielectric. The designed antenna suits for high power transfer applications such as Radar communications, wireless local area network (WLAN), and base stations. The use of gap-coupled cascading rings in the design enhances the gain to 13.4 dBi at the resonance frequency of 2.45 GHz, improving the side lobes level and front-to-back ratios. The proposed antenna has symmetrical half power beam width (HPBW) in H-plane and E-plane of 37 degree and 36.5 degree, respectively. The cross-polarization field component of the antenna is below -25 dB in H-plane, and below -45 dB in E-plane is obtained from the measurements. The measured antenna has 10 dB bandwidth of 95 MHz i.e., 2.405-2.50 GHz that covers the ISM 2.45 GHz band. The designed antenna is planar in structure with compact radiating rings of size 1.16λ×0.4λ×0.1λ. The measured and HFFS simulated results are found in good agreement.

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Keywords: Cascaded square ring, high gain, metallic structure, symmetrical radiation pattern, wireless local area network (WLAN).

**M. Boozari, M. Khalaj-Amirhosseini**
[references] [full-text]
[DOI: 10.13164/re.2021.0639]
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Development of an Analytical Method for Pattern Synthesis of Arbitrary Shaped Planar Arrays

In practice, it is often necessary to design an array that will yield desired radiation pattern. For this purpose, several time-consuming algorithms are introduced in the literature. In this paper, an analytical method is presented to synthesize the radiation pattern of planar and ring arrays. In this method, two new parameters are defined to reconstruct the array factor and simplify the calculation complexity. To accomplish this, we use the double integral to generate two distinct Sinc functions from a bivariate function utilizing the sampling theory notion. This stage generates a set of linear equations that, when solved, yields the complex excitation coefficients. The proposed method is verified by presenting several practical examples. Also, the performance of the method is compared with that of other approaches. The results show that the proposed method is a good candidate for synthesizing a prescribed pattern of planar arrays.

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Keywords: Array factor, integration, least square method, pattern synthesizing

**S. Patra, S. K. Mandal, G. K. Mahanti, N. N. Pathak**
[references] [full-text]
[DOI: 10.13164/re.2021.0646]
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Synthesis of Dual-Beam Patterns by Exploiting Time-Modulation in Unequally Spaced Linear Arrays

In this paper, a novel approach for synthesizing multiple radiation patterns with reduced hardware complexity in the feed network by exploiting the additional degree of freedom ‘time’ in time modulated unequally spaced linear array (TMUSLA) is presented. In the proposed approach, with a suitable common set of element position of TMUSLA, the desired dual-beam pattern with low sidelobe level (SLL) is obtained by simply controlling the ON-OFF time sequence of the RF switches connected to the array elements. To show the effectiveness of the proposed array synthesis method, two dual beam patterns - first one as pencil (sum) beam (PB) and flat-topped beam (FTB) pattern, and the second one as sum and difference pattern with different constraints have been synthesized. For the successful generation of the desired power patterns, differential evolution (DE) algorithm is employed to obtain the optimum possible solution in terms of common element position, time-modulation, switching sequences and applicable excitation phase for the desired shape beam patterns. The superiority of the proposed approach with the favourable improved performance have been demonstrated by comparing the realized patterns with the state-of-the-art relevant reported works.

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Keywords: Pattern synthesis, on-time duration, side lobe level, sideband level, differential evolution

**D. Krutilek, Z. Raida, J. Drinovsky**
[references] [full-text]
[DOI: 10.13164/re.2021.0654]
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Experimental Characterization of Aircraft Electromagnetic Protections

In the paper, an original construction of a coaxial flange for measurements of shielding efficiency of composite materials is presented. The measurement procedure is conceived as a differential method to suppress influence of a flange. Attention is turned to measurements of carbon composites used in aerospace industry. The studied materials exhibit a significant ability to shield electromagnetic radiation. The shielding efficiency is rising with material thickness and with the number of fiber-to-fiber contacts. The optimal composite structure consists of 4 layers of carbon composite; more layers do not influence the shielding efficiency significantly.

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Keywords: Carbon composite, carbon-fiber-reinforced polymer (CFRP), glass-fiber composite (GFC), shielding efficiency, measurements, aerospace applications, coaxial flange

**A. Hoseinabadi, M. B. Tavakoli, M. J. Rastegar Fatemi, F. Setoudeh**
[references] [full-text]
[DOI: 10.13164/re.2021.0662]
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A New Method for Designing Modified Compact Microstrip LPF with Sharp Roll-Off and Wide Stopband

A new method for designing a compact microstrip lowpass filter (LPF) with wide stopband width (SBW) and sharp roll off (ROF) is presented. In proposed designing procedure, high impedance microstrip lines are bent to achieve an LPF with compact size. Then, to compensate for the effect of bending microstrip lines, the lengths of the lines are mathematically modified. Moreover, adding a suppressing cell composed of Radial stub resonator and a Butterfly stub resonator increases the SBW. Also, an elliptic filter structure is used to obtain sharp ROF. In this work, an LPF with 1.12 GHz cutoff frequency, 0.147 λg — 0.133 λg filter size; where λg is the guided wavelength at cutoff frequency, the SBW equal to 13.4 GHz, and the ROF more than 201 dB / GHz, is designed, simulated and fabricated to demonstrate efficiency of the proposed method. Also, the other conventional characteristics for the fabricated LPF such as 0.3 dB insertion loss, 14.4 dB return loss, and suppression factor equal to 2.2, are in the appropriate range of their amounts.

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Keywords: Lowpass filter, microstrip, elliptic filter, stopband width, roll off, compact size

**K. R. Komatla, S. R. Patri**
[references] [full-text]
[DOI: 10.13164/re.2021.0670]
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A Self-Start-Up Sub-Threshold DC/DC Boost Converter Using Bootstrap Driver for Self-Powered Sensor Nodes

In this paper, a fully autonomous and integrated sub-threshold DC-DC converter is presented for energy harvesting from ambient sources to self-powered IoT nodes. The proposed converter and its clock generator are designed by exploiting body biasing technique for low power operation and operate in sub-threshold regime. This bulk driven technique can dynamically enhance the on-current during conducting state and decrease reverse current during the non-conducting state. A bootstrap driver with dynamic body bias is employed to drive the phase generator at the output of the ring oscillator to decrease the settling time of the charge pump. This further improves the driving capacity of the clock along with extended rail to rail output voltage swing. Also, a novel cross clock scheme is proposed to improve the output voltage's transient response and conversion efficiency of a converter by reducing reverse current loss. The proposed circuit is implemented in CMOS 0.18 μm process. The proposed design requires very low start-up voltage of 400 mV and exhibits output voltage of 1.98 V, settling time of 33 μs, and pumping efficiency of 99% with a total power dissipation limited to just 1.5 μW.

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Keywords: Bootstrap driver, body biasing, charge transfer switches (CTS), DC-DC converter, internet of things (IOT)

**W. Jin , Y. Z. Guo , W. M. Jia , J. W. Zhao**
[references] [full-text]
[DOI: 10.13164/re.2021.0680]
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Null Broadening Robust Beamforming Based on Decomposition and Iterative Second-order Cone Programming

To solve the problem that the performance of adaptive beamformer degrades severely in the presence of steering vector mismatch or non-stationary interference, a null broadening robust beamforming based on decomposition and iterative second-order cone programming (SOCP) is proposed. The width and depth of the nulls is controlled. The magnitude response constraints are applied to control the beamwidth and ripple of mainlobe, so the SV mismatch can be overcame. Due to the decomposition of the non-convex magnitude response constraints, the proposed approach can be solved by decomposition and iterative SOCP. Simulation results show that the proposed approach can effectively broaden the null width and enhance the null depth, and it is also robust against SV mismatch, especially large SV mismatch. The proposed approach is jointly robust against the SV mismatch and non-stationary interference, and is still effective in the case of low snapshot, which enhances the robustness of adaptive beamformer in complex environments.

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Keywords: Robust adaptive beamforming, null broadening, magnitude response constraints, second-order cone programming, joint robustness

**X. Huo, W. Guo, H. Zhao, Y. Liu, Y. Tang**
[references] [full-text]
[DOI: 10.13164/re.2021.0688]
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Low-Complexity Suppression of Adjacent Channel Interference in FDD Transceiver

The transmitter-induced adjacent channel interference (ACI) due to power amplifier nonlinearity poses severe desensitization to the receiver in frequency-division duplexing transceivers. To tackle this issue, this paper proposed a digital suppression method with a low-complexity circuit structure to eliminate the interference. The transmitter baseband signal and leakage were employed to estimate the system nonlinear parameters in the digital baseband domain, and the interference was regenerated and then subtracted from the received signal. The proposed method can simplify the circuit structure and facilitate engineering implementation in practice. The simulation and experimental results show that the proposed method can suppress about 25 dB ACI, which can effectively improve the signal-to-interference-plus-noise ratio of the desired signal.

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Keywords: ACI suppression, parameters estimation, signal regeneration, nonlinear distortion, low-complexity

**H. He, S. Kojima, T. Omura, K. Maruta, C. J. Ahn**
[references] [full-text]
[DOI: 10.13164/re.2021.0695]
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Generalized Regression Neural Network Based Channel Identification and Compensation Using Scattered Pilot

In the high-speed mobile environment, channel state information (CSI) estimated at the beginning of the packet is quite different at the last part because the actual channel state changes with time. To overcome this problem, a neural network (NN) based channel compensation method was previously developed. Due to inaccurate channel estimation of decision feedback channel estimation (DFCE), the pilot-aided CSI of the first symbol and DFCE-aided CSIs in the intermediate data part will cause inexact channel state transition even though the application of NN. Accordingly, the channel compensation performance is still degraded, especially in the last part of the packet. This paper proposes a new version of GRNN based channel identification and compensation method by introducing scattered pilot. It can improve the tracking capability of GRNN thanks to densely arranged pilot in the time-domain while it cannot reduce the transmission efficiency. Simulation results show that the proposed method is more effective than the conventional ones in terms of RMSE and BER performance, even in the fast fading environment.

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Keywords: OFDM, fast fading, channel estimation, generalized regression neural network, DFCE, scattered pilot

**Y. Feng, G. Wang, Z. Liu, B. Cui, Y. Yang, X. Xu, H. Han**
[references] [full-text]
[DOI: 10.13164/re.2021.0704]
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Recognition of Radar Emitters with Agile Waveform Based on Hybrid Deep Neural Network and Attention Mechanism

With the increasing complexity of the electromagnetic environment and the continuous development of radar technology, more and more modern digital programmable radars using agile waveform will appear in the future battlefield. It is difficult to effectively identify these radar emitters with complex system only by relying on traditional recognition models. In response to the above problem, this paper proposes a recognition method of radar emitters with agile waveform based on hybrid deep neural network and attention mechanism to deal with the problem of variable conventional characteristic parameters of radar emitter signals with agile waveform. First, we perform a distributed representation of the pulse signal data to generate high-dimensional sparse signal features. Then we design to use a dynamic Convolutional Neural Network to extract features of structural details of radar emitter signals with agile waveform at different levels, and use a Long Short-Term Memory to extract its timing features. In order to obtain the deep features that can characterize the agility of the waveform, the attention mechanism-based method is used to fuse the extracted structural features and timing features, and at the same time it can reduce the influence of noise in complex electromagnetic environment on the characteristic data of radar emitter. Finally, the deep feature is input into the Softmax layer to complete the recognition of radar emitters with agile waveform. The experimental results show that the method proposed in this paper can effectively solve the problem of the recognition of radar emitters with agile waveform, and the recognition accuracy is improved by 1.26% compared with the traditional models and other deep models.

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Keywords: Agility waveform, radar emitter, hybrid deep neural network, attention mechanism

**L. Li, Z.Y. Dong, X. R. Yu, Z. Y. Ren, Z. G. Zhu, L. Jiang**
[references] [full-text]
[DOI: 10.13164/re.2021.0713]
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UAV Communication Signal Recognition: A New Feature Representation and Deep-Learning Method

As the threats from unmanned aerial vehicles (UAVs) increases gradually, to recognize and classify unknown UAVs have became more and more important in both civil and military security fields. Classification of signal modulation types is one of the basic techniques for specific UAV recognition. In this paper, to represent the hidden features involved in the transmitted signals from UAVs, we propose a two-dimensional squeezing transform (TDST) to characterize the UAV communication signals in a compressed time-frequency plane. The new time-frequency representation, TDST, retains the instantaneous characteristics of the UAV signal, and is with excellent data reduction and noise suppression capabilities. The TDST plane of different modulation types are then considered as input features, and we propose a convolutional neural network (CNN) based on deep-learning to recognize the UAV signals. We design an interception system and consider 10 types of UAV signals with random initial phase, bandwidth and frequency offset. Experimental results demonstrate the effectiveness and superiority of the proposed algorithm.

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Keywords: Automatic modulation classification, unmanned aerial vehicles, squeezing transform, convolutional neural network

**L. Zhang, Y. Chen, S. Liu, B. Zou**
[references] [full-text]
[DOI: 10.13164/re.2021.0719]
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A Matching Pursuit-Based Vehicle Wheel Parameter Extraction Method from Micro-Doppler Radar Signal

Micro-Doppler effects of moving vehicles in a radar system are mainly induced by the rotation of wheels, whose features are closely related to the numbers, positions and radiuses of wheels. These parameters of wheels are critical for the vehicle classification and recognition. However, most micro-Doppler features extraction works of vehicles are unable to explicitly extract parameters of wheels. In this paper, a parameter extraction method of vehicle wheels using micro-Doppler features based on the matching pursuit (MP) is proposed. The micro-Doppler signals of wheels are generally weak comparing to redundant echo signals induced by other irrelevant parts of the vehicle, which makes the micro-Doppler features difficult to extract. In this case, several signal atom sets are created according to the motion states of irrelevant parts of vehicle and MP is performed to suppress the redundant signals. After the suppression, micro-Doppler signals induced by wheels have become the major part of the echo signal. Another atom set is generated according to the rotational motion of wheels to perform MP again. Then the wheel parameters, such as the estimated numbers, positions and radiuses, are extracted. Simulation results demonstrate that the proposed method is feasible in feature extraction of moving vehicle. Besides, the accuracy can be guaranteed when the signal-to-noise ratio is greater than –5 dB.

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Keywords: Micro-Doppler, moving vehicle, feature extraction, matching pursuit (MP), radar echo analysis

**S. Batool, M. Imran, M. Imran, E. Elahi, A. Maqbool, S. A. A. Gillani**
[references] [full-text]
[DOI: 10.13164/re.2021.0729]
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Development of an Improved Frequency Limited Model Order Reduction Technique and Error Bound for Discrete-Time Systems

Frequency limited model order reduction algorithm presented by Wang & Zilouchian for discrete-time systems provide unstable reduced-order models and also do not provide a priori error bound formula. Many stability-preserving model order reduction algorithms were presented; however, these methods produce significant approximation errors in the desired frequency interval. An improved algorithm of model order reduction for the discrete-time systems is presented. The proposed technique gives the stable reduced-order model and also provides less approximation error as compared with other algorithms and also provides the formula for the frequency response a priori error bound. Numerical examples provided at the end of the section show the efficacy of the proposed technique.

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Keywords: Model order reduction, controllability Gramians, observability Gramians, error bound, balanced truncation

**A. M. P. de Lucena, F. de A. T. F. da Silva, A. S. da Silva**
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[DOI: 10.13164/re.2021.0739]
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Scintillation Effects in S-Band Telemetry Link of INPE’s Earth Station in Cuiaba-Brazil

One of the main earth stations that INPE uses to track and control its satellites is located in the city of Cuiaba (15.33°S, 56.46°W, dip latitude, 6.1°S), between the magnetic equator and the peak of the equatorial anomaly. Based on the GISM model, it is determined that the ionospheric scintillation index (S4) for the telemetry link in the S band (2208 MHz), between Cuiaba station and the SCD2 satellite, depending on the date and time, can reach values greater than 0.8. This is the first study conducted on ionospheric S-band scintillation in this region of the earth. In this article, the channel model for the link and the telemetry receiver architecture are presented in order to subsequently evaluate some effects of ionospheric scintillation on the functioning of the communication system. The modulation used is OQPSK and a fully-digital demodulator recovers the carrier phase using a Costas loop and synchronizes the symbols using a Gardner synchronizer. The design of OQPSK demodulator is detailed and the impact of ionospheric scintillation on general demodulator performance and on the functioning of its modules is discussed. The system bit error rate, the error variances of the carrier phase and symbol delay in different conditions of severity of ionospheric scintillation were figured out through computer simulation. From the presented results, it is evident that, for the adopted receiver architecture, which was designed for a space channel without scintillation, there is a substancial degradation on performance of the system even for S4=0.5 and, for the scenario where S4=0.8, the link becames practically inoperative.

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Keywords: Equatorial scintillation, S-band space link, OQPSK demodulator, telemetry, satellite communications