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The feasibility of implementing digital predistortion for next-generation wireless communication is faced with a dilemma due to the ever-increasing demand for faster data rates. This causes the utilized bandwidth to increase significantly, as seen in the 5G NR standard in which bandwidths as high as 400 MHz are utilized. Hence, the development of new predistortion techniques in which the forward and feedback paths operate at lower sampling rates is of utmost importance to realize efficient and practical predistortion solutions. In this work, a novel predistortion technique is presented by which the predistortion is divided between the digital and analog domains. The predistorter is composed of a memoryless AM/AM gain function that is implementable in the analog domain, and a nonlinear model with memory effects in the digital domain to relax the sampling rate requirements on both the forward and feedback paths. Experimental validation was carried out with a 20 MHz and a 40 MHz 5G signal, and the results indicate minimal linearization degradation with a sampling rate reduction of 50% and 30%, respectively. This sampling rate reduction is concurrently applied in the digital-to-analog converter of the forward path and the analog-to-digital converter of the feedback path. [ABSTRACT FROM AUTHOR]

Wireless communication infrastructure is a key enabling technology for smart cities. This paper investigates a novel technique to enhance the performance of 5G base stations by addressing the compensation of nonlinear distortions caused by radiofrequency power amplifiers. For this purpose, a sequential digital predistortion approach that uses twin nonlinear two-box structure along with reduced sampling rates in the feedback path is proposed to implement a linearization system. Such a system is shown to have a correction bandwidth that exceeds the bandwidth of the feedback path. This is achieved by synthesizing the predistortion function in two successive characterization iterations. Both characterizations use the same hardware, which has a reduced sampling rate in the feedback path. Hence, the proposed predistorter scheme does not require any additional hardware compared to standard schemes. Moreover, coarse delay alignment is performed while identifying the memory polynomial function in order to further reduce the computational complexity of the proposed system. Experimental results using an inverse Class-F power amplifier demonstrate the ability of the proposed predistorter to achieve a correction bandwidth of 100 MHz with a feedback sampling rate as low as 25 MSa/s.

Background. he dependences of the light intensity on time and the photocurrent flowing in the resistor are generally described by various functions. Methods have been studied for obtaining the time dependence of the intensity of light pulses of arbitrary shape, following with a frequency ω, from the time dependence of the photocurrent in the presence of recombination of charge carriers on the surface of the semiconductor. Materials and methods. The results were obtained based on a study of the kinetics of photoconductivity of a resistor for linear and quadratic recombination laws in the bulk of a semiconductor. The diffusion of nonequilibrium charge carriers to the surfaces of the photoresistor with their subsequent surface recombination is taken into account. The first method is based on the use of the photocurrent differentiation operation. The second method allows you to restore the intensity of an optical pulse of an arbitrary shape from the amplitudes of the harmonics of the expansion of the function that specifies the dependence of the current on time in a Fourier series. Results and conclusions. Nonlinear, frequency and phase distortions in the region of high frequencies are small. The proposed methods are valid when the inequality ωτ≥1 (τ is the effective lifetime of the main charge carriers) is fulfilled.

In this paper, a hybrid predistortion architecture that allies the benefits of digital and analog predistortion techniques is investigated. This Hybrid Digital/Analog Predistorter (H-DAPD) uses a low order memory polynomial as a baseband digital predistortion function. To alleviate the complexity of the analog predistorter implementation, an amplitude only predistortion function is adopted. The analog predistortion function is used to perform a coarse linearization of the amplifier. The digital predistortion function is then applied to linearize the system made of the analog predistorter and the power amplifier. The H-DAPD predistortion system is found to be superior to purely analog and to purely digital predistortion systems by combining key advantages present in each of these systems. The robustness of the proposed H-DAPD with respect to the hardware impairments that are typically associated with the implementation of analog predistortion is thoroughly discussed, and shown to be superior to that of conventional hybrid and analog predistortion systems. Experimental validation performed on a commercial power amplifier prototype using 5G new radio (NR) test signals demonstrates the enhanced robustness of the proposed H-DAPD to hardware impairments. It was found that standard compliant adjacent channel leakage ratio is obtained with up to 0.3dB resolution in the analog predistorter’s AM/AM function, and up to 3ns delay misalignment between the input and control signals of the analog predistorter. The proposed system is expected to pave the road for future hybrid predistortion systems.

Introduction. The general system of parameters, including compression points and intersection points of various harmonics, is unsuitable for devices operating under the influence of baseband pulses (for example, before the modulator in the transmitter, after the demodulator in the receiver). Previously, the authors have developed simple models in the form of nonlinear recursive filters, which give a satisfactory error in describing the response of a wide class of baseband pulse circuits. The system of nonlinear functions of such models can be considered as a new system of characteristics of nonlinear-dynamical baseband impulse circuits, for which it is necessary to develop a method that ensures their measurement with an acceptable error. In the general formulation, this problem is rather challenging; therefore, this article considers only a first-order nonlinear recursive filter. However, the obtained result gives satisfactory results for devices without flat top transient overshoot.Aim. To consider a method for measuring the characteristics of nonlinear-dynamical baseband impulse devices without overshoot on the flat top of the transient response.Materials and methods. The considered recursive filter is represented by an equivalent circuit consist of nonlinear resistive and capacitive elements. Therefore, the task is reduced to measuring their current-voltage characteristics (IV) and charge-voltage characteristics (CVC). IV characteristics are measured at the flat tops of the transient characteristics of the device. Having a certain IV, we obtain the opportunity to calculate the current (and then the charge) of the capacitive element. The experimental study was carried out on the example of a three-stage amplifier with an aperiodic transient response.Results. The transient characteristics of the filter are approximated by the model with an accuracy of no worse than 3.2 %.Conclusion. The considered system of functions can be obtained with a definable error that satisfactory for practice, which allows it to be considered as a new system of parameters for nonlinear baseband pulse devices.

The feasibility of implementing digital predistortion for next-generation wireless communication is faced with a dilemma due to the ever-increasing demand for faster data rates. This causes the utilized bandwidth to increase significantly, as seen in the 5G NR standard in which bandwidths as high as 400 MHz are utilized. Hence, the development of new predistortion techniques in which the forward and feedback paths operate at lower sampling rates is of utmost importance to realize efficient and practical predistortion solutions. In this work, a novel predistortion technique is presented by which the predistortion is divided between the digital and analog domains. The predistorter is composed of a memoryless AM/AM gain function that is implementable in the analog domain, and a nonlinear model with memory effects in the digital domain to relax the sampling rate requirements on both the forward and feedback paths. Experimental validation was carried out with a 20 MHz and a 40 MHz 5G signal, and the results indicate minimal linearization degradation with a sampling rate reduction of 50% and 30%, respectively. This sampling rate reduction is concurrently applied in the digital-to-analog converter of the forward path and the analog-to-digital converter of the feedback path.

The characterization of analog optical links was examined, and it is shown that the study, accounting, and reduction of nonlinearities introduced by the elements of an optical link into the output signal during photodetection are of particular practical interest. Methods for the analysis of nonlinear distortions of analog optical paths with external modulation are presented, and it is proposed to use high-resolution optical spectrum analyzers for their characterization. The analysis methods use the results of measurements of the optical spectrum, which directly characterize the optical signal of the link without taking into account the effect of nonlinearities introduced during photodetection. An analytical model of the process of modulation of monochromatic light radiation by a two-tone radio frequency signal using an electro-optical Mach–Zehnder modulator is presented. The mutual interaction between the nonlinear spectral components of the optical modulated signal and the radio frequency signal at the photodetector output was examined. The optical and radio frequency spectra of AOL signals were experimentally studied on a specially assembled measurement stand. The results of experiments on measuring the spurious free dynamic range (SFDR) of analog optical paths using high-resolution radio frequency and optical spectrum analyzers are presented. Possible reasons for the degradation of the SFDR during photodetection are discussed. Using an analytical model of the nonlinearities of the Mach–Zehnder modulator, the contribution of the nonlinearity of photodetection to the distortions of the output signal of the optical link was experimentally estimated, and the differences in the power of cross-modulation products for high frequency photodiodes of two types were demonstrated. [ABSTRACT FROM AUTHOR]

Traditional multi-channel active noise control (MCANC) is based on adaptive filtering and usually uses a separate control unit for each channel. This paper introduces a deep learning based approach for multi-channel active noise control (ANC). The proposed approach, called deep MCANC, encodes optimal control parameters corresponding to different noises and environments, and jointly computes the multiple canceling signals to cancel or attenuate the primary noises captured at error microphones. A convolutional recurrent network (CRN) is employed for complex spectral mapping where the summated power of error signals is used as the loss function for CRN training. Deep MCANC is a fixed-parameter ANC approach and large-scale multi-condition training is employed to achieve robustness against a variety of noises. We explore the performance of deep MCANC with different setups and investigate the impact of factors such as the number of loudspeakers and microphones, and the position of a secondary source, on ANC performance. Experimental results show that deep MCANC is effective for wideband noise reduction and generalizes well to untrained noises. Moreover, the proposed approach is robust against variations in reference signals and works well in the presence of nonlinear distortions. [ABSTRACT FROM AUTHOR]

Structural analysis of signals is an important part of smart energy. It includes adaptive control of the aperiodic, low-frequency, and signal-harmonics levels for relay protection and automation tasks, fault location, and monitoring and diagnostics of electrical systems. For real-time signal processing, adaptive filtering is widely used and elements of artificial intelligence are increasingly being employed. This paper considers feed-forward artificial neural networks (multilayer perceptron) for signal processing. The simplest artificial neural networks (ANNs) can replace neural networks with a more complex structure (convolutional, recurrent), but within the framework of a sequential recurrent algorithm. This makes it possible to control the quality of parameter identification and signal processing at each stage of calculations. The proposed algorithm is tested using the example of sliding window processing of a signal with nonlinear distortions and at CT saturation. It is shown that the amplitude, frequency, and phase of a utility frequency signal with a high level of harmonics and aperiodic component can be identified with an accuracy of a few percent in a time not exceeding a few milliseconds. To improve the accuracy at each calculation step, in addition to ANNs, traditional methods can be used, such as averaging and median smoothing. The presented results demonstrate the possibility of real-time structural analysis of signals. [ABSTRACT FROM AUTHOR]

Echo cancellation has been effectively achieved through the successful application of subband adaptive filtering (SAF) algorithms. However, the performance of these algorithms may significantly deteriorate when the system exhibits nonlinear distortion. The Van der Pol–Duffing oscillator (VDPDO) system is widely recognized as a complex nonlinear system that displays intricate dynamical behaviors, emphasizing the importance of identifying this nonlinear model. This paper proposes a novel SAF, termed nonlinear SAF (NLSAF) for VDPDO system identification. The NLSAF is based on the functional link artificial neural network (FLANN) to achieve superior nonlinear modeling capability. In particular, the Andrew's sine estimator is integrated into the NLSAF, generating NLSAF-ASE algorithm for performance improvement. The simulation results provide evidence of the accurate identification of the Van der Pol system through the effective utilization of the NLSAF-ASE algorithm. [ABSTRACT FROM AUTHOR]

The paper presents the results of modeling the electrophysical parameters of AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (pHEMT) structures via self-consistent solution of the Schrödinger and Poisson equations. On the basis of numerical calculations, a method for analyzing nonlinear distortions of the transfer current–voltage characteristics of such transistors is proposed. The effect of the spacer layers and the doping level of the δ layer on the nonlinearity of the current–voltage characteristics is estimated. [ABSTRACT FROM AUTHOR]

Such tasks as the determination of the level of harmonics upon nonlinear distortions and separation of the amplitude, phase, and signal frequency of fundamental harmonics in many cases require signal processing with a small time delay or in real time. At standard frequencies of signal digitization of 600–2400 Hz, it is difficult to provide accurate determination of signal parameters during an interval amounting to fractions of an industrial frequency period. This paper demonstrates that this can be aided by neural-network algorithms and artificial feedforward neural networks of direct propagation with simple structure. In the case of adequate training and testing of such a neural network, it allows signal parameters to be recovered in fractions of the main frequency period of 50 Hz. It is demonstrated that trained neural network can be used in a regular microcontroller. The algorithm is analyzed using the example of recovery of a fundamental frequency signal upon nonlinear distortions, when the level of the second harmonics exceeds half of the amplitude of the fundamental frequency signal. Estimation errors of recovery of signal amplitude, phase, and frequency are given. It is demonstrated that recovery can be carried out in a sliding window with the duration from units of milliseconds. Herewith, the errors of recovery of signal parameters do not exceed units of percent. The possibility of using the proposed algorithm for real-time signal processing is discussed. The rates of computations are verified and estimated for microprocessors with standard parameters for digital substations. [ABSTRACT FROM AUTHOR]

A new look-up table based behavioral model for dynamic nonlinear power amplifiers is proposed. This model labelled as hybrid look-up tables model is based on the combination of a memoryless look-up table sub-model and a nested look-up tables one. It is demonstrated that the proposed model circumvents the computational complexity associated with the parameters identification in analytically defined behavioral models. Moreover, the proposed model reduces the size of the standalone nested look-up tables model by approximately 80% while maintaining its accuracy. Furthermore, a novel slew-rate based trimming and indexing technique to reduce the nested look-up tables model size is developed and corroborated experimentally. Additionally, the two-box structure of the hybrid look-up tables model makes it suitable for bandwidth scalability. Experimental validation using LTE-advanced test signals with up to 120MHz bandwidth demonstrates the ability of the proposed hybrid look-up tables model to be bandwidth scalable with less than 0.5dB degradation in the normalized mean-squared error.

In this paper, we propose a novel nonlinear resilient learning post equalizer named TFDNet in UVLC system. Unlike the traditional deep neural network (DNN) based post equalizers which merely consider the time domain, the proposed TFDNet exploits time-frequency image analysis which considers the time and frequency domains simultaneously and transforms the signal into 2D time-frequency image, which is further learned by neural network. Experimental results demonstrate that TFDNet outperforms Volterra and DNN based methods for compensating nonlinear distortions through a 1.2 m underwater channel using 64 quadrature amplitude modulation-carrierless amplitude modulation (64QAM-CAP). Even under severe nonlinear distortions where Volterra and DNN cannot work, TFDNet retains valid bit error rate (BER) below the 7% forward error correction (FEC) limit of 3.8 × 10-3. The performance of TFDNet verifies the effectiveness of time-frequency image analysis which has been applied to tackle nonlinear distortions in UVLC system for the first time.

Nonlinearities in optical fibers deteriorate system performances and become a major performancelimiting issue. This article aims to investigate the compensation of nonlinear distortions in optical communication systems based on different wavelength propagations over few-mode fiber (FMF). The study adopted Space Division Multiplexing (SDM) based on decision feedback equalizer (DFE). Various transmission wavelength of the FMF system is applied to mitigate the attenuation effect on the system. In this paper, different wavelengths (780, 850 and 1550 nm) are used in SDM. Extensive simulation is performed to assess the attenuation and Bit Error Rate (BER) in each case. The results show that the wavelength of 1550 nm produces higher power and less attenuation in the transmission. Furthermore, this wavelength produces the best distance with less BER compared to 780 nm and 850 nm wavelengths. Moreover, the validations show improvement in BER and eye diagram.

Channel independent precoding technique has been widely used in optical discrete multi-tone (DMT) transmission systems to compensate unbalanced impairments induced by bandwidth limitations and imperfect frequency responses of electrical/optical devices and various interferences. However, the comparison of different precoding techniques in terms of peak-to-average power ratio (PAPR) reduction, nonlinear distortion tolerance, implementation complexity, and bit error rate (BER) improvements has not been fully studied. In this article, we comparatively investigate seven most commonly used precoding techniques, i.e., discrete Fourier transform (DFT), orthogonal circulant matrix transform (OCT), constant amplitude zero autocorrelation sequence-based matrix transform (CAZACT), Zadoff-Chu matrix transform (ZCT), discrete cosine transform (DCT), discrete Hartley transform (DHT), and Walsh-Hadamard transform (WHT), through both numerical simulations and offline experiments. Simulations show that the ZCT can achieve the best PAPR reduction, and the OCT cannot reduce the PAPR. Besides, DFT, CAZACT, ZCT, DCT, and DHT precoded DMT signals have superior error vector magnitude performance after passing through nonlinear models. And the corresponding precoded QPSK-DMT signals have better BER performance than both OCT/WHT precoded and conventional ones in the distortion-limited scenarios. However, the precoded 16/64QAM-DMT signals, excluding OCT precoded one, are more sensitive to nonlinear distortions and provide minor BER improvement or even may degrade the BER performance. Complexity analysis exhibits the WHT precoding does not require multiplications and therefore has the lowest implementation complexity. In the inter-symbol interference-limited case, OCT precoding can still achieve a good signal-to-noise ratio (SNR) balance and provide the best BER performance. A simple timing synchronization point adjustment (TSPA) method is employed to enhance SNR balance. Simulated and experimental results exhibit that similar BER performance can be achieved by using these precoding techniques together with TSPA in noise-limited scenarios. Considering the implementation complexity, WHT precoding may be a good option to compensate unbalanced impairments in the short-reach DMT transmission system. [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel nonlinear resilient learning post equalizer named TFDNet in UVLC system. Unlike the traditional deep neural network (DNN) based post equalizers which merely consider the time domain, the proposed TFDNet exploits time-frequency image analysis which considers the time and frequency domains simultaneously and transforms the signal into 2D time-frequency image, which is further learned by neural network. Experimental results demonstrate that TFDNet outperforms Volterra and DNN based methods for compensating nonlinear distortions through a 1.2 m underwater channel using 64 quadrature amplitude modulation-carrierless amplitude modulation (64QAM-CAP). Even under severe nonlinear distortions where Volterra and DNN cannot work, TFDNet retains valid bit error rate (BER) below the 7% forward error correction (FEC) limit of 3.8 × 10−3. The performance of TFDNet verifies the effectiveness of time-frequency image analysis which has been applied to tackle nonlinear distortions in UVLC system for the first time. [ABSTRACT FROM AUTHOR]

The article presents the results of a study of a 220 V/50 Hz AC power source consisting of a power factor corrector and a DC/DC converter. The proposed model of the source, designed to work in the LTspice environment, allows you to investigate the power factors, nonlinear distortions of the network current, efficiency, as well as the spectra of the conductive interference generated during the operation of the source. The efficiency of the proposed model is confirmed by the fact that it provides efficient operation of the source in the range of the input voltage from 128 to 400 V. The studies of the power factor correction source model were conducted using independent automatic power factor correction key control systems and a DC/DC converter, which made it possible to study the level of conductive interference generated by the impulse power source from the power switching frequency of each key from the efficiency of the power keys the use of blurring of the spectrum of the signal of correction of the power factor and the DC/DC converter. Efficiency comparisons were made on the level of spectral components in the current spectrum of the network from which the source is supplied. Studies have shown the feasibility of using spectrum blurring technology in DC/DC converters. The efficiency of using this technology to reduce conductive interference is from 8 to 10 dB, depending on the power source parameters. It is shown in the paper that the selection of the switching frequency of the power factor correction key switches and the DC/DC converter should be made by simulation with the blur mode in the converter on. It has been shown that incorrect frequency selection of the power factor correction key and DC/DC converter key leads to a 10 dB increase in conductive interference. The analysis of the models of the power source and its individual units makes it possible reasonably make both the choice of the switching frequency of the power switches of the power source from the AC power source and the use of effective methods of reducing the electromagnetic interference created during the operation of the pulse power sources. The results obtained allow us to evaluate not only the potential of different methods of reducing the level of electromagnetic interference in the variation of the parameters of the control signals of the power keys of the converter nodes, but also to formulate recommendations on the combination of these methods for the best result. [ABSTRACT FROM AUTHOR]

The method for compensation of 2-nd and 3-rd order nonlinear distortions in multistage amplifiers is considered. It is shown, that equalization of nonlinear components in multistage structures is determined by schemes of one-from-last and last cascades. Realization of multistage amplifier for reducing 2-nd and 3-rd order nonlinear distortions is proposed.

The described method refers to compensation of nonlinear distortions, originating in laser diodes, used in wideband SCM-communication systems. Its essence is presenting in forming of a function that is opposite of the function describing the W-A characteristic of the emitting diode. Main element is the predistorter, which is connected in series before the emitting diode. This way, an improving of the optical transmitter’s parameters and characteristics is being realized, by using a purposive entering predistortion.

ABSTRACT This paper addresses the problem of model size estimation in power amplifiers behavioural modeling and predistortion applications using multi-basis functions memory polynomials. A low complexity sequential approach is proposed along with a suitable model. The proposed model and size estimation approach are experimentally assessed for the modeling and predistortion of an LTE envelope-tracking power amplifier. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:726-732, 2016 [ABSTRACT FROM AUTHOR]

In quite some applications, the linear dynamics of the system under test evolve over time in a smooth nonperiodic manner. The dynamic behavior of such systems is uniquely described by the time-variant frequency response function (TV-FRF). However, since most real-life systems behave nonlinearly to some extent, it is important to detect and quantify the deviation between the ideal linear time-variant (LTV) framework and the true non-LTV dynamics. Therefore, in this paper, a nonparametric estimation procedure is developed for detecting and quantifying the noise level and the level of the nonlinear distortions in TV-FRF measurements using random-phase multisine excitations. As a result, the users can decide whether or not the LTV framework is accurate enough to describe the true dynamics in their particular application. The proposed measurement procedure is illustrated on a time-variant electronic circuit. [ABSTRACT FROM PUBLISHER]

Using laser vibrometry and a stimulation and signal analysis method based on multisines, we have measured the response and the nonlinearities in the vibration of the rabbit middle ear at the level of the umbo and the stapes. With our method, we were able to detect and quantify nonlinearities starting at sound pressure levels of 93-dB SPL. The current results show that no significant additional nonlinearity is generated as the vibration signal is passed through the middle ear chain. Nonlinearities are most prominent in the lower frequencies (125 Hz to 1 kHz), where their level is about 40 dB below the vibration response. The level of nonlinearities rises with a factor of nearly 2 as a function of sound pressure level, indicating that they may become important at very high sound pressure levels such as those used in high-power hearing aids. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of crest factor reduction and digital predistortion on the performances of communication and broadcasting systems. The overdrive of digitally predistorted power amplifiers is proposed for power efficient linear amplification of amplitude modulated signals having high peak to average power ratio. This is achieved by adding, at the input of the digital predistortion system, a clipping function to allow for an overdrive of the linearized power amplifier while meeting spectrum emission mask linearity requirements. The proposed technique is experimentally validated using a GaN Doherty power amplifier driven by long-term evolution (LTE) signals. Measurement results show that the overdriven power amplifier can deliver up to 2.5 dB extra output power, up to 80% enhancement in the power utilization factor, and up to 20% relative increase in the power efficiency while being compliant with the linearity requirements of the LTE standard. [ABSTRACT FROM PUBLISHER]

Satellite communication is facing the urgent need of improving data rate and efficiency to compete with the quality of service offered by terrestrial communication systems. An imminent gain, achievable without the need of upgrading current satellite technology, can be obtained by exploiting multicarrier operation at the transponder and using highly efficient modulation schemes. However, on-board multicarrier joint amplification of high order modulation schemes is a critical operation as it brings severe non-linear distortion effects. These distortions increase as the on-board High Power Amplifier (HPA) is operated to yield higher power efficiencies. In this work, we propose novel techniques to implement on ground predistortion that enable multicarrier transmission of highly efficient modulation schemes over satellite channels without impacting infrastructure on the downlink. [ABSTRACT FROM PUBLISHER]

This paper concerns the theoretical modeling of OFDM coded signal's degradation caused by pseudo-random nonlinear distortions introduced by an analog-to-digital or digital-to-analog converter. A new quantity, the effective number of samples, is defined and used for the derivation of accurate expressions for the autocorrelation function and the total power of the distortions. The derivation is based on a probabilistic model of the signal and its transition probability. It is shown that for digital (discrete and quantized) signals the effective number of samples replaces the total number of samples and is the proper quantity defining their properties. [ABSTRACT FROM AUTHOR]

Frequency response function (FRF) measurements are very often used to get a quick insight into the dynamic behavior of complex systems; even if it is known that these systems are only approximately linear and time-invariant. Therefore, it is important to detect and quantify the deviation from the ideal linear time-invariant framework that is inherent to the concept of an FRF. This paper presents a method to detect and quantify the nonlinear and time-variant effects in FRF measurements using periodic excitations. The proposed method can handle noisy input, noisy output data, and nonlinear time-variant systems operating in feedback. [ABSTRACT FROM PUBLISHER]

The present state of the metrological facilities for measuring the total harmonic distortion factor, based on digital methods of measuring and synthesizing signals, is considered. The principal metrological characteristics of the SK6-20 calibrator-meter, the SK6-122 generator-calibrator and the SK6-20A metercalibrator are presented. [ABSTRACT FROM AUTHOR]

A method of estimating nonlinear distortions, which differs from the well-known ones in that it enables one to determine the nonlinear distortions of random signals, which carry useful information, is described. A new criterion is proposed for estimating these distortions, the basis of which is a calculation of the mean value of the relative change in the gain of the circuit which introduces the distortions. A block diagram of the measuring circuit is presented. [ABSTRACT FROM AUTHOR]

The paper proposes a nonlinear acoustic echo cancellation setup based on a decoupled scheme of the second-order adaptive Volterra filter. The acoustic distortions that take place in the Loudspeaker-Enclosure-Microphone setup are modeled using measured linear and quadratic Volterra kernels. The adaptation of the filter weights is achieved by exploiting the Normalized Least-Mean-Fourth algorithm. The proposed method is compared with the usual direct adaptation procedure of the second-order Volterra filter using the same algorithm and the same step size values. Comparisons between these techniques regard the evolution of the Echo Return Loss Enhancement in terms of convergence rate and steady-state error. Experimental results for Laplacian distributed excitation show that the new approach increases the speed of convergence in comparison to the one given by the coupled version without enhancing the steady-state error. [ABSTRACT FROM AUTHOR]

In this paper, a nonparametric estimation procedure is presented in order to track the evolution of the dynamics of continuous (discrete)-time (non)-linear periodically time-varying (PTV) systems. Multisine excitations are applied to a PTV system since this kind of excitation signals allows us to discriminate between the noise and the nonlinear distortion from a single experiment. The key idea is that a linear PTV system can be decomposed into an (in)finite series of transfer functions, the so-called harmonic transfer functions (HTFs). Moreover, a systematic methodology to determine the number of significant branches is provided in this paper as well. Making use of the local polynomial approximation, a method that was recently developed for multivariable (non)-linear time invariant systems, the HTFs, together with their uncertainties embedded in an output-error framework, are then obtained from only one single experiment. From these nonparametric estimates, the evolution of the dynamics, described by the instantaneous transfer function (ITF), can then be achieved in a simple way. The effectiveness of the identification scheme will be first illustrated through simulations before a real system will be identified. Eventually, the methodology is applied to a weakly nonlinear PTV electronic circuit. [ABSTRACT FROM AUTHOR]

The frequency response function (FRF) is a common nonparametric modeling tool in many practical engineering problems used for obtaining insight in the device under test. However, the device often behaves nonlinearly. When nonlinearities are detected, the user wants to find out how large these are with respect to the measurement noise. In this paper, we describe a method, based on an overlap technique and periodic excitations, that accurately estimates the FRF, the level of nonlinear distortion, and the measurement noise using only two periods and two random phase realizations of the input signal. [ABSTRACT FROM AUTHOR]

In this paper, we present a new method for characterization of radio frequency Power Amplifier (PA) in the presence of nonlinear distortions which affect the modulated signal in Radiocommunication transmission system. The proposed procedure uses a gray box model where PA dynamics are modeled with a MIMO continuous filter and the nonlinear characteristics are described as general polynomial functions, approximated by means of Taylor series. Using the baseband input and output data, model parameters are obtained by an iterative identification algorithm based on Output Error method. Initialization and excitation problems are resolved by an association of a new technique using initial values extraction with a multi-level binary sequence input exciting all PA dynamics. Finally, the proposed estimation method is tested and validated on experimental data. [ABSTRACT FROM AUTHOR]

Abstract: It is generally accepted that the middle ear acts mainly as a linear system for sound pressures up to 130dB SPL in the auditory frequency range. However, at quasi-static pressure loads a strong nonlinear response has been demonstrated. Consequently, small nonlinear distortions may also be present in the middle ear response in the auditory frequency range. A new measurement method was developed to quickly determine vibration response, nonlinear distortions and noise level of acoustically driven biomechanical systems. Specially designed multisines are used for the excitation of the test system. The method is applied on a gerbil eardrum for sound pressures ranging from 90 to 120dB SPL and for frequencies ranging from 125Hz to 16kHz. The experiments show that nonlinear distortions rise above noise level at a sound pressure of 96dB SPL, and they grow as sound pressure increases. Post-mortem changes in the middle ear influence the nonlinear distortions rapidly until a stabilization occurs after approximately 3h. [Copyright &y& Elsevier]

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This work proposes a novel two-box dual-input single-output digital predistorter (labeled as the LUT MISO predistorter) suitable for linearizing dynamic nonlinear envelope tracking power amplifiers. The LUT MISO predistorter is built by cascading a single-input memoryless nonlinear function with a two-input polynomial model with memory. The predistorted signal is synthesized as a function of the baseband complex waveform as well as the corresponding envelope-shaped signal. The LUT MISO predistorter is experimentally benchmarked against its single-box dual-input counterpart. It is shown that for the same number of coefficients, reduced spectrum regrowth and better adjacent channel power ratio were obtained with the LUT MISO predistorter. Moreover, the proposed predistorter was found to pass the ACPR requirements of the LTE standard while its conventional counterpart fails. [ABSTRACT FROM PUBLISHER]

A method for the detection and estimation of nonlinear distortions when identifying multivariable frequency response functions (FRF) is considered. The method is successfully applied to experimental data, which were collected in closed loop, from an industrial robot. The results show that nonlinear distortions are indeed present and cause larger variability in the FRF than the measurement-noise contributions. [ABSTRACT FROM AUTHOR]

We report on a fiber-optic transport system for next-generation wireless-communication systems utilizing 4 times 4 multiple-input multiple-output (MIMO) orthogonal-frequency-division-multiple-access (OFDMA) technology. Our system supports time-division-duplex (TDD)-based wireless signals operating at 3.775 GHz. To accommodate the TDD-based MIMO signals over a single strand of optical fiber, we utilize nine-channel coarse wavelength-division-multiplexed optical channels: one for link delay measurement and TDD control signal transmission, four for downlink, and the others for uplink. The system first measures the propagation delay between the central base station (CBS) and the remote antenna (RA) and sends the result to delay modules to compensate for the delay added by the transmission link. This procedure makes the CBS and RA emit the downstream signals simultaneously into the air and, consequently, helps avoid the performance degradation caused by the propagation delay of the radio-over-fiber system. The system then sends the MIMO signals together with TDD control signals to the RA. Our experimental demonstration is carried out with 1-Gb/s OFDMA signals having pilot, control, and data channels. For downstream, the error vector magnitudes (EVMs) are measured to be < -30 dB after a 3.9-km transmission over conventional single-mode fiber when the antenna output power is set to be 24 dBm. For upstream, the best EVMs are measured to be < -35 dB. Both the downstream and upstream performance is limited by the shot and thermal noise of the receiver when the signal power is low, whereas nonlinear distortions of electrical amplifiers start to degrade the system performance as the signal power increases. We also measure the crosstalk between channels. It is measured to be less than -42 dB for all channels, which is found to be caused by board-to-board interference at the RA. [ABSTRACT FROM PUBLISHER]

This paper presents a new method of digital adaptive predistortion for linearization of power amplifiers (PAs) exhibiting memory effects. The predistorter (PD) device consists of a lookup table (LUT) gain followed by a codebook of filters addressed by the index of the LUT. The adaptation is derived from direct learning for the LUT gains and indirect learning for the filter coefficients. We compared our results with those of two reference methods: a simple LUT system (with direct learning) and a memory polynomial system (with indirect learning). The performances of the new approach lie between those of the two reference methods in terms of adjacent channel power regrowth and error vector magnitude. The LUT is the less complex of the three methods, but it is a memoryless system, and it cannot correct the memory effects in the PA. The memory polynomial PD is more powerful, but its complexity is very high. The new technique, due to the addition of filters to the LUT, has possibilities to compensate not only for the nonlinearity but also for the memory effects in the PA, and it is one order of magnitude less complex than the memory polynomial system. [ABSTRACT FROM AUTHOR]

Using specifically designed broadband periodic random excitation signals, the best linear approximation of RF amplifiers is measured. The proposed technique: 1) takes into account the measurement uncertainty and the nonlinear distortions and 2) detects, quantifies, and classifies the nonlinear distortions with confidence bounds. The approach is suitable for the experimental characterization of existing amplifiers. [ABSTRACT FROM AUTHOR]

Coping with nonlinear distortions in fingerprint matching is a challenging task. This paper proposes a novel method, a fuzzy feature match (FFM) based on a local triangle feature set to match the deformed fingerprints. The fingerprint is represented by the fuzzy feature set: the local triangle feature set. The similarity between the fuzzy feature set is used to characterize the similarity between fingerprints. A fuzzy similarity measure for two triangles is introduced and extended to construct a similarity vector including the triangle-level similarities for all triangles in two fingerprints. Accordingly, a similarity vector pair is defined to illustrate the similarities between two fingerprints. The FFM method maps the similarity vector pair to a normalized value which quantifies the overall image to image similarity. The proposed algorithm has been evaluated with NIST 24 and FVC2004 fingerprint databases. Experimental results confirm that the proposed FFM based on the local triangle feature set is a reliable and effective algorithm for fingerprint matching with nonlinear distortions. [ABSTRACT FROM PUBLISHER]