Introduction. Orthogonal frequency division multiplexing (OFDM) is the dominant modulation scheme in mobile communications. OFDM systems should be capable of operating across a wide range of multipath fading channel conditions. The recent research focus in this field has been on the design of OFDM receivers based on machine learning, including artificial neural networks. Neural networks in such receivers are typically trained for one specific communication system configuration. This complicates the use of neural network-based receivers in real-world systems, thus rendering development of more flexible schemes highly relevant.
Aim. To obtain and optimize the structure of an OFDM receiver based on an artificial neural network and consisting of separate modules that can be combined depending on the configuration of the pilot signals and the modulation used.
Materials and methods. Computer simulation in the MATLAB environment.
Results. The proposed architecture of a neural network-based OFDM receiver uses a combination of two multilayer perceptrons, one of which implicitly implements channel state information estimation and equalization, and the other performs demodulation. The first perceptron forms intermediate representations of data symbols, for which there were no specific references during network training, while the instances of the second perceptron work with these representations for individual data symbols. Variants of the second perceptron were trained for three quadrature modulation (QAM) constellations: 4QAM, 16QAM, and 64QAM.
Conclusion. The proposed OFDM receiver for all considered modulation types provided error rates comparable to those of the baseline algorithms under favorable channel conditions (moderate delay spread with low Doppler spread) and outperformed baseline algorithms in severe conditions (channel with a large delay spread and high Doppler spread). Further research directions involve neural network-based generation of soft decisions of the demodulator and development of specialized layers of the neural network that would facilitate approximation of the necessary operations.

Introduction. Periodic pulse signals are used in various fields, including radar systems. The parameters of periodic pulse signals, such as period, pulse duration and shape, and radio frequency content, can vary significantly and, as a rule, are unknown a priori. Under the conditions of a priori uncertainty and low signal-to-noise ratios, the detection of periodic pulse signals, estimation of their parameters, and direction finding of the source is a non-trivial task.
Aim. To develop algorithms for detecting, estimating parameters, and direction finding of periodic pulse signals in the presence of low signal-to-noise ratios and the absence of a priori information about the parameters of a periodic pulse signal.
Materials and methods. The methods of statistical radio engineering, mathematical statistics, estimation of signal parameters against interference, and computer simulation were used.
Results. Simple-to-implement algorithms for detecting periodic pulse signals, evaluating their parameters, and direction finding of the source have been developed. The noise immunity characteristics of the algorithms obtained by computer simulation were successfully tested when receiving actual signals. The noise immunity of the algorithms was shown to increase with a decrease in the duty ratio of the signal. The developed algorithms allow periodic pulse signals to be distinguished from signals of wireless communication systems, such as GSM, UMTS, LTE, Wi-Fi, 5G. The latter signals, although having a periodic component, are not pulsed signals.
Conclusion. The developed algorithms function successfully in the absence of a priori information about the parameters of a periodic pulse signal and at low signal-to-noise ratios (up to –15 dB). A significant gain in the noise immunity of direction finding was achieved in comparison with the standard phase difference algorithm. The parameters of a periodic pulse signal evaluated using the developed algorithms can be used for source identification.

Introduction. The advent of wide-gamut color reproduction devices (LED, laser, or OLED) has exacerbated the problem of color rendering related to individual differences in color perception and referred to as observer metameric failure. Currently, designers of smartphone and TV displays, as well as other playback devices, are aiming to reach a wider gamut; however, there is a lack of device-specific methodologies for assessing the degree of observer metameric failure.
Aim. To develop a method for estimating the degree of observer metameric failure. This method can be used to assess the need for a color correction of a particular device in order to compensate for the color rendering problem.
Materials and methods. Categorical observers, formed by clustering of color matching functions of individual users, were used. The procedure of searching for pairs of colors that would be indistinguishable for one categorical observer, but would look different for another, i.e., would cause an observer metameric failure, was developed based on the solution of an optimization problem. The decision on the need to implement a color correction for the designed device is made based on the degree of errors on the found pairs of colors for a set of categorical observers.
Results. It was shown that modern displays are already associated with the effect of observer metameric failure. Further development of display technologies and an extension of color coverage due to narrowing of spectral characteristics of basic colors will make the problem of observer metameric failure more pronounced, thus requiring special measures of color correction.
Conclusion. A method for estimating the degree of observer metameric failure for a particular color reproduction device is proposed. This method can be used to assess the need for implementing color correction measures to compensate for the problem of color rendering by individual users.

Introduction. The development of broadband reflectarray antennas for the microwave band remains a key challenge in the context of stricter requirements imposed on telecommunications systems, including 5G/6G networks and satellite communications. Despite a significant number of studies devoted to methods of extending the operating frequency band, it is of interest to analyze data from experimental studies of the developed reflectarrays, confirming the effectiveness of the considered approaches.
Aim. Generalization of the design approaches used to extend the operating frequency band of reflectarrays. The main attention is paid to the experimental verification of the considered approaches, i.e., the use of multilayer structures and spatial diversity and geometric optimization of elements, with the purpose of clarifying their practical applicability.
Materials and methods. An analysis of existing techniques (numerical modeling of FI, FM, electrodynamic calculation based on a Floquet cell) and the results of original experimental research in this field was conducted. Measurements were carried out on printed, all-metal, and conformal reflectarrays using an Antast B3-1 near-field scanner and an Agilent N5230A PNA-L vector circuit analyzer. The phase error minimization algorithms were adapted to work in the extended frequency range.
Results. The study experimentally confirmed the extension of the operating frequency band in terms of the 3 dB criterion from the maximum value of the gain to 40 % for multilayer printed circuit boards and 19.6 % for corner structures. Optimization of the geometry of the elements based on dumbbell cross-shaped structures provides a relative band of 28 % with a decrease in gain by 0.5 dB. All-metal slit tubes demonstrate resistance to extreme conditions, although requiring consideration of the possibility of excitation of plane-parallel waveguide modes at the design stage, which have a significant impact on their characteristics.
Conclusion. Recommendations on the choice of geometry, design, and manufacturing technology of various reflectarrays based on the experience of theoretical and experimental research conducted at the Department of Theoretical Foundations of Radio Engineering of Saint Petersburg Electrotechnical University in 2010–2025 are presented. These data form the basis for designing antenna arrays that meet the requirements of high-speed telecommunications systems and indicate areas for further research, including miniaturization and increased structural stability.

Aim. To determine the dimensions of the high correlation region by studying the shape of a volumetric image of the module of the spatial correlation function, depending on the direction of arrival of the signal of interest.

Materials and methods. The uncertainty function of the space–time signal was investigated using statistical simulation methods. Calculations were performed in the Mathcad 15 software package.

Results. A volumetric image of the ambiguity function module of a space–time signal is constructed. The minimum and maximum values of the width of the high correlation region are determined by angular coordinates, which directly affect the accuracy of direction finding of the repeater satellite using a graphical method. At an elevation angle equal to zero, the minimum value of the width of the correlation function is obtained, equal to θ_кор = θ_{кор min} = 7° and the maximum uncertainty in relation to the true value of the azimuth. At the boundary of the scanning area of the radiation pattern θ₀ = 60°, we obtain θ_{кор max} = 12°, in this case, the parameter A_{кор min} = 7°. The analytical method allowed us to obtain: A_{кор min} » 6° at θ₀ = 60°; A₀ = 90, 270° and θ_{кор min} » 5° at θ₀ = 0°; A₀ = 0,180, 360°.

Conclusion. The results obtained can be used when developing mobile space communication systems with phased antenna arrays. Further research directions include the development of conformal phased antenna arrays with a controllable directional diagram.

Introduction. In modern communication systems, the requirements imposed on the weight, size, and frequency characteristics of filters in the input and output paths of antenna-feeder devices (AFD) are becoming increasingly stringent. Taking this into account, we investigate a transverse waveguide ridge resonator of a quarter-wave structure. The use of a quarterwave resonator, rather than a half-wave structure, allows the width and length of the filter to be reduced. The influence of various resonator parameters on its resonant frequency is demonstrated. The dependence of the loaded Q-factor on the height of the transverse ridge quarter-wave resonator is calculated. The calculated waveguide filters on resonators of this type offer the possibility of forming attenuation poles both above and below the passband.
Aim. Investigation of various types of filters on ridge quarter-wave resonators, as well as evaluation of the rejection band by level and by width.
Materials and methods. Numerical studies were carried out using the methods of finite elements (FEM) and finite difference in the time domain (FDTD).
Results. Simulation of various types of fiveand ten-order filters on ridge quarter-wave resonators was performed. Five-order filters, depending on the arrangement of the resonators, are capable of forming attenuation poles both above and below the passband. A ten-order filter on transverse ridge quarter-wave resonators at receiving frequencies of the X-band (7.25…7.75 GHz) provides insertion losses of no more than 1.2 dB, while the attenuation level in the transmission frequency range (7.9…8.4 GHz) is at least 80 dB.
Conclusion. The use of filters on ridge quarter-wave resonators ensures a significant reduction in length and improvement in weight and size characteristics, while maintaining a high level of attenuation in the stop band.

Introduction. Modern complex systems with a network structure are characterized by spatial and temporal long-term dependence of flows. The existing models of mass service theory based on the assumptions of stationarity and mutual statistical independence of fluctuations in the intensity of incoming flows significantly underestimate real delays.
Aim. Development of an improved model for estimation of aggregated traffic delays in highly loaded networks taking into account statistical characteristics of interrelations between activity fluctuations in nodes and channels of the network.
Materials and methods. A superstatistical approach is applied to analytically correct the Kingman formula for estimating the waiting time based on the calculation of the coefficients of variation of arrival intensities and mutual correlations between the intensities of the traffic generated by different nodes. Analytically obtained approximations of probability densities of delay distribution by q-exponential distributions are used to estimate the characteristics of aggregated traffic, the results of which are confirmed by the data of simulation modeling of aggregated traffic. In addition, the validation of the proposed estimations is performed on the example of analyzing empirical traffic data of the MAWI academic backbone network. The duration of the analyzed time segments of the traffic was adapted to adequately compare the results for model and empirical data, with integral statistics constructed based on the results of the analysis of several full-day records.
Results. An analytical model for estimating delays in aggregated traffic was developed, taking into account the coefficients of variation of arrival intensities and mutual correlations of traffic intensities originating from different nodes in the network. The analytical estimation of delay distribution was shown to give an intermediate result between the estimations obtained by using two modeling schemes, which is caused by the prevalence of errors of discreteness or finiteness of data sampling depending on the modeling scheme.
Conclusion. The application of the superstatistical approach to account for statistical interrelationships allows the estimates of delay times in highly loaded networks to be clarified on the basis of substituting the adjusted characteristics of aggregated traffic into the Kingman formula, thus providing more detailed estimates of delays in complex engineering systems with a network structure.

Introduction. An autocorrelation method can be used for calibration of phased antenna arrays (PAA) in the presence of interference. In scenarios where the PAA size is substantial, the initial elements of post-calibration are designated as a reference element for subsequent comparison with the following antenna elements. However, this method becomes time-consuming when the PAA size increases, also affecting the adaptive calibration proposed in this work. In practical applications, the calibration of PAA may be affected by various factors, such as intentional interference, passive interference, weather conditions, equipment aging, etc. Therefore, the impact of different interference levels on the calibration accuracy of PAA should be investigated. In addition, using a calibration antenna instead of a reference antenna may decrease the calibration accuracy of the received signal.
Aim. To design and investigate a method for calibrating a PAA with high accuracy and low complexity based on an autocorrelation algorithm.
Materials and methods. The efficiency of the developed algorithm was estimated using MATLAB/Simulink-based simulation and experimental validation.
Results. To verify the feasibility of the proposed method for a large-scale antenna array, a 2 × 8 phased array antenna is implemented at 3 GHz. The proposed autocorrelation method for PAA exhibited superior performance over the conventional autocorrelation method. In comparison with the conventional autocorrelation technique, the developed method enhances the peak value of the combined beam in the E-plane by 3.2 and 3.7 dB, respectively. Furthermore, the beams at a distance between two antennas equal 0.625λ were tilted by 1.5 and 8° for the proposed and conventional autocorrelation methods, respectively.
Conclusion. The validation through actual measurement data confirmed that the proposed autocorrelation method is more accurate than conventional methods in determining amplitude and phase offsets. The paper points out that the proposed autocorrelation calibration method performs well in large-scale on-site and factory-level calibration, being also effective in scenarios under the presence of external interference. 

Introduction. In most technological processes, the parameters of transistors may exhibit variations in values. As a result, integrated circuit (IC) parameters may spread beyond the nominal values stated in the technological specification. Parametric reliability of the designed devices is an important goal of parametric analysis based on simulation. This paper presents a numerical analysis of a pseudomorphic GaAs/AlGaAs/InGaAs high electron mobility transistor conducted in the TCAD environment. Particular attention is paid to the analysis of the drain and transfer characteristics taking into account 10% deviations from the pHEMT parameters specified by the manufacturer. High-frequency properties of the simulated pHEMT are evaluated. The effect of the spacer thickness on the drain and drain-gate characteristics is analyzed. The work is based on a large amount of experimental data.
Aim. Numerical analysis of a pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor in the TCAD environment.
Materials and methods. The simulation approach involved solving the fundamental equations of semiconductor electronics using numerical analysis methods. A hydrodynamic two-dimensional numerical pHEMT model was used, which takes into account the influence of quantum wells, the effects of non-stationary dynamics, and the phenomena of charge carrier transport. The experimental data of pHEMT were obtained at the production facility of JSC Svetlana-Rost.
Results. The conducted parametric analysis revealed the concentration of the AlGaAs donor layer to be a critical parameter having a significant impact on the characteristics of pHEMT transistors. Changes in the channel length, gate length, and gate depth in the GaAs layer have a less pronounced effect on the electrical characteristics of pHEMT. The drain and drain-gate characteristics of the numerical model of pHEMT demonstrated a high degree of agreement with the experimental data. The experimental and calculated I–V characteristics obtained by varying the thickness of the spacer layer made it possible to clarify the value of the spacer thickness implemented in production conditions. As part of this analysis, the dependence of the cutoff frequency on the gate voltage was obtained.
Conclusion. The conducted analysis revealed the parameters affecting the characteristics of the numerical model of GaAs/AlGaAs/InGaAs pHEMT. Critical deviations of the studied characteristics were detected as a result of 10 % variation in the concentration of the AlGaAs donor layer. The analysis of experimental and calculated I–V characteristics, under varied spacer values, established the spacer thickness which showed agreement with the experimental structures. Parametric stability is a critical aspect in the production of microelectronic devices, affecting reliability, durability, performance, and compliance with standards. Improved parametric stability reduces the level of defects and optimizes production processes.