Time-domain Distortion Research Articles

Distortion-Elimination Hybrid OFDM With Low Complexity for Optical Wireless Communications

In optical wireless communications (OWC), hybrid optical orthogonal frequency division multiplexing (O-OFDM) schemes, such as hybrid asymmetrically clipped O-OFDM (HACO-OFDM) and layered asymmetrically clipped O-OFDM (LACO-OFDM), enjoy both high power and spectral efficiency. However, due to the non-orthogonal transmission of signal components induced by the clipping distortion, successive interference cancellation (SIC) is required in these hybrid O-OFDM schemes, leading to notably increased complexity. In this paper, we conceive novel hybrid O-OFDM schemes to offer both high spectral and power efficiency, whilst possessing low complexity. By elaborately designing a time-domain (TD) distortion elimination methodology at the transmitter, a novel distortion-elimination hybrid O-OFDM (DEHO-OFDM) is first proposed, which combines asymmetrically clipped O-OFDM (ACO-OFDM) and pulse-amplitude-modulated discrete multitone (PAM-DMT) in an interference-free manner. In order to further improve the spectral efficiency, an enhance DEHO-OFDM (EDEHO-OFDM) is designed by activating the remaining subcarrier resources. Both DEHO-OFDM and EDEHO-OFDM can be realized through a single-IFFT transmitter and standard OFDM receiver, leading to much lower complexity than the existing hybrid O-OFDM schemes. Simulation results have demonstrated the superiorities of the proposed schemes over HACO-OFDM and LACO-OFDM in terms of peak-to-average-power ratio (PAPR) and bit error rate (BER).

Broad-bandwidth and accurate optical vibration sensing by using FDM Φ-OTDR with linear regression analysis of multi-frequency phase responses.

We demonstrate Φ-OTDR distributed acoustic sensing (DAS) that realizes both a broad bandwidth for the vibration frequency and wide dynamic range for the vibration amplitude based on optical frequency-division-multiplexing (FDM). We enhance the sampling rate of DAS by using FDM while suppressing waveform distortion in time domain (spurious components in spectral domain) caused by sensor nonlinearity inherent in Φ-OTDR, thus improving dynamic range, with linear regression analysis of multi-frequency phase responses. The proposed method compares the phase offsets and responses of each frequency to those of a common reference frequency and uses the information to calibrate each of the different responses. We clarify the physical origin of the problem and the validity of the proposed method in both simulations and experiments. Experimental results show an improvement in dynamic range by above 8 dB on average for vibration waveforms with nɛ-order amplitudes and kHz-order frequencies over 10-km single-mode fiber.

Toward Zero-Energy Devices: Waveform Design for Low-Power Receivers

The use of a low-power wake-up receiver (WUR) can significantly reduce the device’s energy consumption. To incorporate such WURs into existing communication systems, the waveform design needs to facilitate simple detection yet be compatible with the orthogonal frequency division multiplexing (OFDM) transmission. In this letter, we propose a novel phase design for the wake-up signal (WUS) so that the time-domain distortion of the WUS can be minimized. A majorization-minimization (MM) method is proposed to address the waveform design problem. Simulations show that the proposed scheme can achieve a near-optimal detection performance under different system parameters.

A spectrally efficient modified asymmetrically and symmetrically clipped optical (mASCO)-OFDM for IM/DD systems

A novel spectrally efficient modified asymmetrically and symmetrically clipped optical (mASCO)-OFDM for intensity modulated direct detection (IM/DD) systems is presented. The conventional ASCO-OFDM systems use two frames to transmit the conventional asymmetrically clipped optical (ACO)-OFDM and symmetrically clipped optical (SCO)-OFDM system. The proposed mASCO-OFDM system replaces the two frame SCO-OFDM by a single frame modified SCO (mSCO)-OFDM. The mSCO-OFDM clips the data on only one side of the symmetry and performs an absolute function on the other side of the symmetry. This allows mASCO-OFDM to be 1.333 times more spectrally efficient than the conventional ASCO-OFDM. The mASCO-OFDM reduces its receiver’s complexity by estimating and eliminating the clipping noise distortion in time domain. Overall, this system has 43% lower complexity in comparison to ASCO-OFDM system. The mASCO-OFDM shows a better BER performance and a lower Peak Average Power Ratio (PAPR) than ASCO-OFDM. The proposed system also shows better BER performance than ACO-OFDM for the same spectral efficiency.

Performance Analysis and Comparison of Clipped and Filtered OFDM Systems With Iterative Distortion Recovery Techniques

A well-known drawback of orthogonal frequency-division multiplexing (OFDM) is its signal with high peak-to-average power ratio (PAPR). Among a number of PAPR reduction techniques, clipping and filtering (CAF) is the simplest approach, which effectively reduces the PAPR of band-limited OFDM signals at the cost of increasing in-band distortion. In order to mitigate the performance degradation caused by the in-band distortion, several iterative distortion recovery techniques have been proposed in the literature, and they are largely classified into time-domain (TD) and frequency-domain (FD) compensation approaches: The former and latter are represented by the decision-aided reconstruction (DAR) and clipping noise cancellation (CNC), respectively. To date, however, their theoretical performance limits have not been studied. In this work, we revisit the performance limits of CAF and derive a closed-form signal-to-distortion power ratio (SDR) expression. Furthermore, we introduce a time-domain distortion model for characterizing the OFDM signal with CAF, based on which we make performance comparison between the two compensation approaches. Theoretical analysis and simulations in terms of their achievable symbol error rate (SER) reveal that, unlike the FD counterpart, the TD compensation may suffer from unrecoverable distortion when filtering after clipping is applied at the transmitter.

Two-stage artificial neural network-based burst-subcarrier joint equalization in nonlinear frequency division multiplexing systems.

We propose an artificial neural network (ANN)-based scheme to improve the performance of nonlinear frequency division multiplexing (NFDM) optical transmission systems in both time and frequency domain. Through two-stage ANN equalization at the receiver side, time-domain distortions between adjacent bursts and frequency-domain cross talk between neighboring subcarriers can be jointly mitigated. Burst ANN and Subcarrier ANN equalizers are characterized and validated by numerical simulations of a dual-polarization NFDM transmission system. Compared with the basic detection scheme, the proposed two-stage ANN achieves a Q-factor gain of 3.01 dB for an NFDM system with 256 Gb/s gross data rate transmitting over 960 km standard single-mode fiber (SSMF). The two-stage ANN approach offers an effective way to jointly equalize the signal in multiple dimensions.

Fatigue damage detection and growth monitoring for composite structure using coda wave interferometry

This paper presents the development of a fatigue damage detection and growth monitoring technique based on coda wave interferometry (CWI) for composite structures. The time domain distortion (TD) component of coda waves, which is typically ignored as noise in the literature, is used and experimentally validated. First, a widely used signal processing method for CWI called the stretching method is applied, and TD component is extracted in the time domain. Next, threshold values are statistically estimated using noise levels of the signal and compared with the amplitude of TD component for detecting early-stage fatigue damage. A damage index (DI) is then obtained from TD component and used to monitor fatigue damage growth. The experimental results of the damage detection and growth monitoring are validated through visual inspection during the fatigue test. Finally, the developed technique's performance is evaluated by comparing it to conventional ultrasonic and CWI-based composite fatigue damage assessment methods. The uniqueness of this study lies in (1) detection and growth monitoring of the early-stage composite fatigue damage, (2) extraction and use of TD component of CWI and investigation of the correlation with the damage existence and growth, and (3) experimental validation and performance examination by comparing with the conventional ultrasonic and CWI-based composite fatigue damage assessment methods.

Psychoacoustic evaluation of acoustic features distortion in fricative consonant of speech

The present study investigated the perceptual discrimination between typical (normal) acoustic features duration and intensity in word-initial fricative consonant /ʃ/ and atypical duration (shortened or lengthened) and intensity (weak or strong). Perceptual task distortion contrast was performed in each dimension, duration and intensity, as well as in two-dimensional interaction. Stimulus was the word /ʃuma/ with fricative /ʃ/ in the initial position. Duration and intensity of /ʃ/ were synthesized from typicaly pronounced stimuli in 16-step continuum from 135 ms to 300 ms and from −18 dB to +4.5 dB, respectively. Eight speech-language pathologies (SLPs) performed perceptual task distortion contrast in these two dimensions. Results are presented in the form of identification functions. First experiment tested two-choice decision and three-choice decision tasks in identification of distortion in time domain (shortened/normal/lengthened frictive), second experiment analyzed distortion in intensity domain (weak/normal/strong frictive), third experiment examined mutual interaction of distortion in duration and intensity, and forth experiment analyzed two-dimensional perception of simultaneous distortion in both domains, time and intensity. The results indicate regions of typical and atypical values for duration and intensity of fricative /ʃ/, in Serbian language, and also indicate that distortion in intensity domain does not influence perception of distortion in time domain, except in the case of weak friction. In two-dimensional perception of normal fricative /ʃ/, the dimension duration has higher perceptual significance over dimension intensity, whereas in perception of distorted fricative the dimension intensity has higher perceptual significance over dimension duration. In two-dimensional perception SLPs reported that they selectively recognize at first the dominant distortion and then another distortion.

Distortion Analysis for the Assessment of LTI and Non-LTI Transmitters

Bounds on the frequency domain behavior of electrically small antennas are adapted to assess the time-domain distortion or fidelity achievable by simple linear time-invariant (LTI) systems. Rigorous expressions for a TM01 spherical shell are used as a direct analog to the well-known Chu limit on an antenna’s bandwidth-efficiency product. These expressions are shown to agree with results obtained using narrowband single resonance approximations, allowing for the analysis of arbitrary small dipole-like structures through bounds on their single-frequency Q-factor and efficiency. The resulting expressions are used as a basis for assessing the performance gains of electrically small non-LTI (e.g., direct antenna modulation) transmitters for which Q-factor and impedance bandwidth are not defined but which can be analyzed directly in the time domain via distortion.

Wavelet-Based Quality-Constrained ECG Data Compression System Without Decoding Process

Retaining the reconstruction quality of electrocardiogram (ECG) data compression is crucial for diagnosis demands. However, traditional wavelet-based approaches suffer from complicated distortion control. The most fundamental way to overcome this difficulty is to avoid the use of the time-domain distortion measure that requires the complete decoding process. In this article, based on the reversible roundoff nonrecursive discrete periodized wavelet transform reversible round-off non-recursive discrete periodized wavelet transform (RRO-NRDPWT), a new transform-domain index is presented. This index accumulates the roundoff error of the RRO-NRDPWT and the quantization error as the distortion measure. The index referred to as the simplified percentage root-mean-square difference (SPRD) can completely replace the PRD for the case of ${\mathbf{PRD}} \geq 2{\mathrm{\% }}$PRD≥2%, where $\boldsymbol{SPRD}/\boldsymbol{PRD} \approx 1$SPRD/PRD≈1. By using the ECG signals in the MIT-BIH arrhythmia database, the simulation results show that for target PRD (${\mathbf{PR}}{{\mathbf{D}}_{\mathbf T}}$PRDT) equals to $3{\mathrm{\% }} \pm 0.05$3%±0.05, the SPRD can save 99.34% quantization process time compared with the traditional quality control scheme.

Lossy wavefront sensing and correction of distorted laser beams.

The art of rectifying a laser beam carrying amplitude and phase distortions has been demonstrated through several competing methods. Both wavefront sensor and wavefront sensor-less approaches show that the closed-loop correction of a laser beam can be accomplished by exploiting high-resolution sampling of the beam distortion in its spatial or time domain, respectively. Moreover, machine-learning-based wavefront sensing has emerged recently, and uses training data on an arbitrary sensing architecture to map observed data to reasonable wavefront reconstructions. This offers additional options for beam correction and optical signal decoding in atmospheric or underwater propagation. Ideally, wavefront sensing can be achieved through any resolution in spatial samples, provided that more frequent sampling in the time domain can be achieved for a reduced number of spatial samples. However, such trade-offs have not been comprehensively studied or demonstrated experimentally. We present a fundamental study of lossy wavefront sensing that reduces the number of effective spatial samples to the number of actuators in a deformable mirror for a balanced performance of dynamic wavefront corrections. As a result, we show that lossy wavefront sensing can both simplify the design of wavefront sensors and remain effective for beam correction. In application, this concept provides ultimate freedom of hardware choices from sensor to sensorless approaches in wavefront reconstruction, which is beneficial to the frontier of study in free-space optical communication, lidar, and directed energy.

Compact broadband frequency selective microstrip antenna and its application to indoor positioning systems for wireless networks

This study presents a low-profile broadband microstrip patch antenna with filtering response. The proposed antenna consists of a rectangular patch and four parasitic gap-coupled elements, two L- and two rectangular-shaped patches. A broadband quasi-elliptic boresight gain response is obtained without using any extra filtering circuits. The input impedance of each radiating element, i.e., driven patch and parasitic elements, is matched to its radiating quality factor and the couplings between patches are optimised for broadband impedance bandwidth with filtering response. Prototype hardware is designed and fabricated on Kappa 438 substrate with a relative permittivity of 4.4 and thickness of 3.2 mm. The antenna exhibits a total size of 25 × 23 × 3.2 mm3 with relative impedance bandwidth (voltage standing wave ratio<2) of 60% ranging from 4.4 to 7.8 GHz. The experimental results demonstrate good performance with nearly flat gain and good filtering response. The proposed filtering antenna exhibits low pulse distortion in time domain which makes it a good candidate for location-aware Internet-of-things applications employing the IEEE 802.15.4 ultra-wideband standard. Switchable sector base-station antenna system is studied to demonstrate the capability of this design to enhance the localisation and communication performance of the wireless network.

On the feasibility of using a bispectral measure as a nonintrusive predictor of speech intelligibility

The presence of background noise or nonlinear distortions encountered in real-world situations often reduces the intelligibility of speech signals. Several objective measurements and prediction procedures have been developed to assess speech intelligibility in noise. Most of the existing measures are, however, suitable for only a subset of specified forms of distortion. This study developed a reliable, reference-free speech intelligibility metric that uses the properties of an acoustic signal to predict the effects of a wide range of distortions that influence speech intelligibility in quiet and noisy conditions. The bispectral speech intelligibility metric (BSIM), was developed by extracting the features from the spectrogram of speech signals using the third-order statistics, which are collectively known as the bispectrum. Speech intelligibility scores predicted by the BSIM were compared to behavioral speech intelligibility scores in quiet and noise. The performance of the BSIM was also compared with that of several widely used speech intelligibility metrics. Results showed that the BSIM can successfully predict nonlinear distortions, such as peak-clipping and center-clipping, as well as time domain distortions, such as phase-jitter and reverberation. Unlike existing metrics, such as the articulation index and speech transmission index, the BSIM successfully captured the effect of fluctuating noise on speech intelligibility and predicted the effects of the degradation of noisy speech processed by the ideal time-frequency segregation method. The BSIM presents a reliable, reference-free, and objective measure of speech intelligibility that can provide real-time predictions of the effect of signal processing and acoustics distortion on speech intelligibility in quiet and noise. In addition, the BSIM could be used to analyze algorithms that process noisy speech.

Time-domain distortion of a pulse-operated high-power GaN amplifier and a reduction method

Time-domain distortion of a pulse-operated high-power GaN amplifier and a reduction method

Performance evaluation of optical beamforming-based wideband antenna array (Invited Paper)

A wideband-generalized pattern multiplication approach to evaluate the performance of an optical beamforming-based wideband antenna array is proposed and experimentally demonstrated, which enables the far-field measurement of a large wideband array with a small anechoic chamber. Because the optimum reception of a wideband microwave signal is highly related to the time-domain distortions of the beamforming system, a correlation-receiver-based radiation pattern is applied to take the fidelity of the wideband signals into account. A four-element optical beamforming system is built to verify the feasibility of the proposed method. The results achieved by the proposed method agree well with the conventional direct measurements.

Optimal digital filters for analyzing the mid-latency auditory P50 event-related potential in patients with Alzheimer’s disease

BackgroundFiltering is an effective pre-processing technique for improving the signal-to-noise ratio of ERP waveforms. Filters can, however, introduce substantial distortions into the time-domain representations of ERP waveforms. Inappropriate filter parameters may lead to the presence of statistically significant but artificial effects, whereas true effects may appear as insignificant. New methodThe present study aimed to determine the optimal digital filters for analyzing the auditory P50 component in patients with Alzheimer’s disease. To provide evidence of the optimal filter settings, different high-pass and low-pass filters were applied to ERP waveforms obtained from a conditioning-testing paradigm. The results facilitate practical recommendations for selection of filters that maximize the signal-to-noise ratio of the P50 components without introducing significant distortions. ResultsThe present study confirms that filter parameters have a significant effect on the amplitude and gating measures of the P50 component. Setting the high-pass cut-off at 0.1Hz and the low-pass cut-off at 90Hz (or above) is recommended for P50 component analyses. Comparison with existing methodsThe majority of ERP studies on sensory gating report using high-pass filters with 10-Hz cut-offs to measure P50 suppression. Such a high cut-off appeared to induce significant distortions into the ERP waveforms; thus, the authors advise against using these excessive high-pass cut-offs. ConclusionsFiltering broadband signals, such as ERP signals, necessary results in time-domain distortions. However, by adjusting the filter parameters carefully according to the components of interest, it is possible to minimize filter artifacts and obtain more easily interpretable ERP waveforms.

Application of multi‐objective fractional factorial design for ultra‐wideband antennas with uniform gain and high fidelity

An efficient design methodology is presented for developing ultra-wideband (UWB) antennas with uniform gain and high fidelity. The major challenge for designing UWB antennas is to simultaneously meet all the frequency-domain and time-domain performance criteria including good impedance matching throughout the operational frequencies, relatively uniform gain over the UWB, and high waveform fidelity on pulse-preserving capabilities. If the framework of Pareto-based evolutionary algorithms is adopted, it may require thousands of tests to find an optimum structure due to the blind-search feature. In this study, all the requirements of UWB antenna design can now be efficiently achieved by the proposed methodology, which integrates the processes of fractional factorial design of experiments and Derringer's desirability functions into a multi-objective optimisation technique. The detailed procedure and its capability are demonstrated by an UWB planar monopole antenna design which comprises nine design parameters. By performing merely 64 predefined simulations, the optimum antenna structure is determined with an excellent impedance matching by 2:1 voltage standing wave ratio and a relatively constant gain (2.48–5.04 dBi) over the frequency band of 3.1–10.6 GHz. Meanwhile, the time-domain distortion is greatly reduced in comparison with the reference design which simply optimises the characteristic of impedance matching.

사물-사람 간 개인화된 상호작용을 위한 음향신호 이벤트 감지 및 Matlab/Simulink 연동환경

Most IoT-related approaches have tried to establish the relation by connecting the network between things. The proposed research will present how the pervasive interaction of eco-system formed by touching the objects between humans and things can be recognized on purpose. By collecting and sharing the detected patterns among all kinds of things, we can construct the environment which enables individualized interactions of different objects. To perform the aforementioned, we are going to utilize technical procedures such as event-driven signal processing, pattern matching for signal recognition, and hardware in the loop simulation. We will also aim to implement the prototype of sensor processor based on Arduino MCU, which can be integrated with system using Arduino-Matlab/Simulink hybrid-interoperation environment. In the experiment, we use piezo transducer to detect the vibration or vibrates the surface using acoustic wave, which has specific frequency spectrum and individualized signal shape in terms of time axis. The signal distortion in time and frequency domain is recorded into memory tracer within sensor processor to extract the meaningful pattern by comparing the stored with lookup table(LUT). In this paper, we will contribute the initial prototypes for the acoustic touch processor by using off-the-shelf MCU and the integrated framework based on Matlab/Simulink model to provide the individualization of the touch-sensing for the user on purpose.

Dynamic Characteristics and Stability of Axially Moving Viscoelastic Plate with Piezoelectric Layer

The dynamic stability of the moving viscoelastic plate with the piezoelectric layer is studied. On the basis of the thin plate theory and the two-dimensional viscoelastic differential constitutive relation, the differential equation of the axially moving viscoelastic rectangular plate with piezoelectric layer in the Laplace domain is formulated, the equation is suitable for various viscoelastic differential models. Then, the differential equation of motion of the viscoelastic plate with elastic dilatation and Kelvin–Voigt distortion in time domain is derived, with the piezoelectric effect. The complex eigenvalue equations of axially moving viscoelastic plate are established by the differential quadrature method. The generalized eigenvalue equations are solved, and the force excited by the piezoelectric layer due to external voltage is modeled as the follower tensile force; this force is used to improve the stability of the axially moving viscoelastic plate. Via numerical calculation, the results for the instability type and the corresponding critical moving speed of viscoelastic plate are presented to show the variations in these factors with respect to the dimensionless moving speed, the dimensionless delay time and the applied voltages. The dynamic stability of the axially moving viscoelastic plates can be effectively improved by the determination of the optimal location for the piezoelectric layers and the most favorable voltage assignment.

A New Adaptive Self-Tuning Fourier Coefficients Algorithm for Periodic Torque Ripple Minimization in Permanent Magnet Synchronous Motors (PMSM)

Torque ripple occurs in Permanent Magnet Synchronous Motors (PMSMs) due to the non-sinusoidal flux density distribution around the air-gap and variable magnetic reluctance of the air-gap due to the stator slots distribution. These torque ripples change periodically with rotor position and are apparent as speed variations, which degrade the PMSM drive performance, particularly at low speeds, because of low inertial filtering. In this paper, a new self-tuning algorithm is developed for determining the Fourier Series Controller coefficients with the aim of reducing the torque ripple in a PMSM, thus allowing for a smoother operation. This algorithm adjusts the controller parameters based on the component's harmonic distortion in time domain of the compensation signal. Experimental evaluation is performed on a DSP-controlled PMSM evaluation platform. Test results obtained validate the effectiveness of the proposed self-tuning algorithm, with the Fourier series expansion scheme, in reducing the torque ripple.