Direct Demodulation Research Articles

Dynamic phase measurement based on two-step phase-shifting interferometry with geometric phase grating

Abstract This paper proposes a novel dynamic 2-step phase-shifting interferometry method with a geometric phase grating and direct aberration-free object phase recovery algorithm. By exploring the amplitude-phase modulation property of the geometric phase grating, two-step phase-shifting interferograms with π / 2 phase shift can be obtained in a single shot through two kinds of spatial-multiplexing arrangements. Then the restriction on the background uniformity of the 2-step phase-shifting algorithms and the system aberration can be avoided simultaneously, based on the presented direct and fast object phase demodulation strategy with an object-free state recorded in advance, so an accurate object measurement result can be guaranteed. Accordingly, a detailed analysis of the theoretical model of error sources is conducted by taking the depolarization error of geometric phase grating and phase measurement model error into consideration, with a Least Squares Iteration optimization strategy established to further improve the measurement accuracy. Experiments on various types of phase objects, such as a photo-etched quartz substrate, the spatial light modulator, and a silicon chip, validate the effectiveness and accuracy of our method when compared with the reference phase obtained from 4-step temporal phase-shifting method. The dynamic phase measurement capability is demonstrated by exhibiting the evaporation process of alcohol droplet.

Design Optimization of a THz Receiver Based on 60 nm Complementary Metal–Oxide–Semiconductor Technology

The technology transfer of terahertz wireless communication from research laboratories to commercial applications is a global strategic achievement currently pursued to match the ever-increasing demand for high-speed communication. The use of commercial integrated electronics for the detection of THz waves is an intriguing challenge which has enticed great interest in the scientific research community. Rapid progress in this field has led to the exploitation of THz direct detection using standard CMOS technology based on the so-called self-mixing effect. Our research, stemming out of a collaboration between Sapienza University of Rome and STMicroelectronics company, is focused on the complete design process of a THz rectifier, realized using 50 nm ST B55 CMOS technology. In this paper, we report the optimization process of a case-study receiver, aimed to demonstrate the feasibility of direct demodulation of the transmitted OOK signal. A relatively limited bandwidth extension is considered since the device will be included in a system adopting a radiation source with a limited band. The design refers to a specific technology, the 60 nm MOS in B55X ST; nevertheless, the proposed optimization procedure can be applied in principle to any MOS device. Several aspects of the rectification process and of the receiver design are investigated by combining different numerical simulation methodologies. The direct representation of the rectification effect through the equivalent circuit of the detector is provided, which allows for the investigation of the detector–amplifier coupling, and the computation of output noise equivalent power. Numerical results are presented and used as the basis for the optimization of the receiver parameters.

Low-cost magnetic eddy current respiratory monitoring based on fast digital lock-in algorithm combining undersampling and oversampling

To design a low-cost non-contact respiratory monitoring system based on the magnetic eddy current sensing principle, a fast digital lock-in algorithm combining undersampling and oversampling is proposed in this paper. This algorithm enables direct digital sampling and fast demodulation of high-frequency respiratory modulated signals, decreasing the performance requirements of the analog-to-digital converter (ADC) and the microcontroller (MCU). In addition, a current source combined with the load characteristic is designed to reduce the cost of waveform generation. The experimental results show that the low-cost amplitude modulation (AM) system constructed based on the above studies can detect respiratory signals in different states with the signal-to-noise ratio (SNR) higher than 92 dB. Compared with the results of the medical monitor, the relative error range of the respiratory rate (RR) measured by the AM system is 1.15%–3.12%, and the measurement accuracy is higher than 98.9%.

Lensless complex amplitude demodulation based on deep learning in holographic data storage

To increase the storage capacity in holographic data storage (HDS), the information to be stored is encoded into a complex amplitude. Fast and accurate retrieval of amplitude and phase from the reconstructed beam is necessary during data readout in HDS. In this study, we proposed a complex amplitude demodulation method based on deep learning from a single-shot diffraction intensity image and verified it by a non-interferometric lensless experiment demodulating four-level amplitude and four-level phase. By analyzing the correlation between the diffraction intensity features and the amplitude and phase encoding data pages, the inverse problem was decomposed into two backward operators denoted by two convolutional neural networks (CNNs) to demodulate amplitude and phase respectively. The experimental system is simple, stable, and robust, and it only needs a single diffraction image to realize the direct demodulation of both amplitude and phase. To our investigation, this is the first time in HDS that multilevel complex amplitude demodulation is achieved experimentally from one diffraction intensity image without iterations.

A simple system for measuring the transmittance curve of x-ray filters at low x-ray energies.

To prevent the x-ray detectors from the interference of optical photons, x-ray filters are widely used in x-ray space missions. As the flux of low energy x-rays would be significantly attenuated by the filters, it is important to derive the transmittance of the filters below 10keV. In this paper, we present a simple experimental apparatus and data processing method to measure the transmittance curve of x-ray filters in the energy range from 1.6 to 10keV. The measured spectra were further processed with Geant4 Monte Carlo simulation results and a direct demodulation algorithm. The obtained transmittance is in good accordance with the theoretical calculation and the synchrotron radiation test result, which proves that the proposed system with the algorithm is a reasonable way to characterize the x-ray transmittance performance of filters above 1.6keV.

A novel Deep-Learning model for RDTS signal denoising based on graph neural networks

Focusing on the problem of increasing the deviation of temperature measurement for Raman-based distributed temperature sensor (RDTS) caused by random noise, a new method of applying a three-layer GraphSAGE-based graph neural network (3L-GraphSAGE) to noise reduction is proposed, where the spatial relationship between each signal is first constructed, and then the effective denoised results are obtained from the developed 3L-GraphSAGE model. First, an experimental setup is built for collecting fiber signals. Then, the datasets are input into the 3L-GraphSAGE to train the model. Finally, the test datasets are input into the well-trained 3L-GraphSAGE model to obtain effective denoised signals. To evaluate the performance of 3L-GraphSAGE, three evaluation indexes are calculated, including maximum deviation (MD), root mean square error (RMSE) and smoothness. The experimental results show that it can efficiently suppress the random noise and reduce the temperature measurement deviation in RDTS compared with direct demodulation of the raw data, and significantly improve the curve smoothness compared with wavelet transform by soft threshold function (WT-soft) and fast waveform type (FWT). Therefore, 3L-GraphSAGE model can provide an available method for improving the performance of RDTS.

A 164-$\mu$ W 915-MHz Sub-Sampling Phase-Tracking Zero-IF Receiver With 5-Mb/s Data Rate for Short-Range Applications

This article presents a 915-MHz ultra-low-power (ULP) sub-sampling phase-tracking receiver (SSPT-RX). It is targeted for the power-constrained devices that need short range but medium high-speed data receiving, such as the multi-channel neural stimulator with arbitrary waveform generation. The zero-intermediate frequency (zero-IF) phase-tracking receiver (PT-RX) topology is adopted to simplify the sub-sampling RX architecture with direct demodulation of frequency-shift keying (FSK) signal while improving the image frequency issue. The frequency of local oscillator (LO) is reduced by ten times with the proposed architecture, which leads to greatly reduced power consumption. Fabricated in 65-nm CMOS process, the RX chip occupies an active area of 0.58 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The RX consumes one of the lowest power consumptions of 164 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{W}$ </tex-math></inline-formula> from 0.5-/1-V supplies. It achieves an energy efficiency of 32.8 pJ/bit at a data rate of 5 Mb/s, which is improved by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 5.6\times $ </tex-math></inline-formula> compared to the state-of-the-art RXs. The measured sensitivity of 915-MHz FSK signal receiving is −69.5 dBm with an LO frequency of 91.5 MHz, which is 1/10 of the carrier frequency. The achieved RX sensitivity figure-of-merit (FoM) is 174.3 dB.

Development of a direct phase demodulation method for a Fabry-Pérot interferometer for temperature measurements using frequency scanning

Development of a direct phase demodulation method for a Fabry-Pérot interferometer for temperature measurements using frequency scanning

Towards fast sensing along ultralong BOTDA: flatness enhancement by utilizing injection-locked dual-bandwidth probe wave.

Brillouin optical time-domain analysis (BOTDA) using distributed Brillouin amplification (DBA) only requires a milliwatt-level pump to achieve a sensing range beyond 100 km, which provides a powerful tool for temperature/strain sensing. However, similar to the majority of other long-range BOTDAs, the state-of-the-art reports require > 1000 times average, severely restricting the sensing speed. The blind area over tens of kilometers caused by the nonuniform Brillouin response and parasitic amplitude modulation (AM) are crucial factors affecting the signal-to-noise ratio (SNR). Here, a comprehensive performance optimization and substantial enhancement for BOTDA sensors was presented by the direct demodulation of an injection-locked dual-bandwidth probe wave. Injection locking (IL) can completely eliminate the impact of AM noise; dual-bandwidth probe enables self-adaptive pulse loss compensation, thereby intensifying the SNR flatness along the ultralong fiber, and direct probe demodulation can overcome nonlocal effects and allows ∼19.7 dB enhancement of probe input power. Therefore, using only 100 times average, ∼148.3 km sensing, and ∼5 m spatial resolution were achieved with < ∼0.8 MHz standard deviation of Brillouin frequency shift (BFS) over a broad range (∼131.7 km). The reduction in averages was more than 10 times that of the reported majority of long-range BOTDAs. Such performances were achieved without using time-consuming or post-processing techniques, such as optical pulse coding and image denoising. Because this approach is compatible with optical chirp chain technique without frequency sweeping, fast acquisition (0.3 s) was also realized, which has the potential for fast sensing at 3.3 Hz along a ∼150 km fiber.

Deep learning-based M-ary spread spectrum communication system in shallow water acoustic channel

A deep learning-based M-ary spread spectrum (SS) underwater acoustic (UWA) communication system is proposed. The long short-term memory (LSTM) model is used as the receiver of the system, enabling the direct demodulation of received signals without de-carrier and de-spreading operations. LSTM architecture-based neural network models are fed with the time-domain waveform of each symbol during the training process to learn the source information carried by each symbol. Simulation results show that the deep learning-based M-ary SS UWA communication system outperforms the conventional system under the low signal-to-noise ratio and complex shallow water acoustic channel. For optimal performance, different LSTM models can be selected according to the simulation conditions. Furthermore, a water tank experiment simulating the complex shallow water channel was conducted and the results show that experimental equipment, different training sample acquisition methods, and LSTM models can affect the performance of the system.

High-frequency response current direct demodulation method for sensorless control of interior permanent magnet synchronous motor drives

High-frequency response current direct demodulation method for sensorless control of interior permanent magnet synchronous motor drives

Reliability Demodulation Algorithm Design for Phase Generated Carrier Signal

Phase-generated carrier (PGC) demodulation technology has been widely used in fiber-optic interferometric sensors system in recent years. Nonlinear errors caused by interference noise and parasitic amplitude modulation (AM) are key factors that affect the reliability of traditional PGC demodulation system based on arctangent (PGC-Arctan) algorithm. In order to enhance the reliable performance of PGC demodulation system under the conditions of low signal-to-noise ratio (SNR) and high parasitic AM interference, an ellipse fitting algorithm based on the Gauss–Newton iteration (GNI) is proposed, which is called the PGC-Arctan-GNI demodulation algorithm. The proposed algorithm uses the Euclidean distance to accurately estimate the geometric parameters of ellipse, which can correct the nonlinear distortion of the demodulated signal. In addition, the PGC demodulation system designed based on GNI algorithm has the advantages of strong antinoise ability, good antiparasitic AM ability, and widely dynamic range of the input signal amplitude. Experimental results show that, compared with the direct demodulation algorithm and the ellipse fitting algorithm based on the least squares (LS) method, the SINAD value of the proposed algorithm is always greater than 30 dB and higher than the other two algorithms under the condition of low SNR. Under the condition that the parasitic AM index <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula> changes from 0 to 0.5, the signal-to-noise and distortion ratio (SINAD) curve of the proposed algorithm is more stable than the curves of the other two algorithms, and the fluctuation range of the SINAD value does not exceed 3 dB. This algorithm also has a wider input signal amplitude range than the LS algorithm. Especially, when the amplitude of the measured signal is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pi$</tex-math></inline-formula> /2, the relative amplitude error of the GNI algorithm is less than 2% and significantly smaller than the LS algorithm. Finally, the demodulation results using real measured data in the actual system show that the frequency and amplitude of the demodulated signal are same as the original signal. At the same time, the SNIAD value of this system reaches 67.40 dB, the spurious-free dynamic range value reaches 68.82 dB, and the total harmonic distortion value reaches -68.40 dB@1 KHz. In summary, the proposed PGC-Arctan-GNI algorithm can eliminate system nonlinear errors caused by the parasitic AM, harmonics, and noise interference by estimating the ellipse parameters, thereby effectively enhancing the stability and reliability of the PGC demodulation system.

A Cascaded MEMS Amplitude Demodulator for Large Dynamic Range Application in RF Receiver.

An amplitude demodulator with a large dynamic range, based on microelectromechanical systems (MEMS), is proposed in this paper. It is implemented as a cascade of a capacitive and a thermoelectric sensor. Two types of the transducer can improve the measurement range and enhance the overload capacity. This MEMS-based demodulation is realized by utilizing the square law relationship and the low-pass characteristic during the electromechanical and thermoelectric conversion. The fabrication of this device is compatible with the GaAs monolithic microwave integrated circuit (MMIC) process. Experiments show that this MEMS demodulator can realize the direct demodulation of an amplitude modulation (AM) signal with a carrier frequency of 0.35–10 GHz, and cover the power range from 0 to 23 dBm. This MEMS demodulator has the advantages of high power handling capability and zero DC power consumption.

Optimization of the phase generated carrier demodulation algorithm based on support vector regression.

The phase generated carrier (PGC) algorithm is often used in the demodulation of interference signals in optical fiber sensors for its high sensitivity, good linearity, and large dynamic range. However, the PGC demodulation method is often distorted by the amplitude of the interference signal and the depth of modulation. In this study, the support vector regression (SVR) method is used to compensate the distortion of the PGC demodulation schemes. Simulation results showed that the SVR algorithm can effectively reduce the nonlinear error of the PGC demodulation system. The fitting accuracy of the SVR algorithm is 97.5% and greater than 90% in noiseless and noise systems, which is better than the back propagation (BP) neural network algorithm. Also, the SVR-based algorithm can better restore the amplitude with smaller mean square error and good correlation. A vibration monitoring system has been built, and experiment results confirm that the performance of the SVR-based algorithm is better than direct PGC demodulation and BP algorithm with the mean square error of 0.0005 and relevance of 0.94.

On the effect of I/Q imbalance in mixed RF/FSO cooperative relaying systems

In this paper, the impact of in-(I) phase/quadrature-(Q) phase imbalance (IQI) in mixed radio frequency/free space optical (RF/FSO) cooperative relaying systems is studied. The RF link has been assumed to undergo Nakagami-m distributed fading, while the FSO link is under the influence of atmospheric turbulence modeled by the Malaga irradiance model with pointing errors. The FSO model accounts for both kinds of optical demodulation schemes, i.e., intensity modulation direct detection (IM/DD) and coherent demodulation. For the considered system, the impact of hardware IQI has been taken into account in order to derive closed-form expressions for the outage probability and ergodic capacity of the amplify-and-forward (AF) relaying system. These expressions have been further presented in terms of elementary functions for high signal-to-noise ratio (SNR) region of operation. The presented analytical framework has been verified by Monte-Carlo simulations to illustrate the impact of various parameters on the performance of the considered system.

RDTS noise reduction: A fast method study based on signal waveform type

In order to reduce the measurement error of Raman-based distributed temperature sensor (RDTS) caused by random noise, we have proposed an effective noise reduction method, which can reduce the time consumption of data processing due to the low time complexity of the method. We classified the signal as five models based on the waveform types, thus to filter the raw data according to the waveform types. Then, we established an experimental device to test the effects of different signal noise reduction methods, including maximum deviation (MD), root mean square error (RMSE), smoothness, and time consumption. Experimental results show that it can efficiently suppress random noise and reduce temperature measurement errors in RDTS compared with direct demodulation of the raw data, and significantly improve the curve smoothness compared with D-SVD and WT-Soft. The time consumption test based on three different devices shows that the time consumption of the proposed method is approximately 10% of the D-SVD method, and about 30% of the WT-Soft method. It proves that this method is an effective and fast noise reduction method in RDTS systems.

Partial relay selection for combating the effects of co-channel interference in RF/FSO cooperative relaying

In this research work, mixed radio frequency (RF)/free space optical (FSO) communication systems have been investigated in the presence of multiple interferers at the relay node. The considered system incorporates partial relay selection (PRS) using the available outdated channel state information (CSI) at the relay node to combat the effect of interference. The fading on the PRS-aided RF link has been modeled using the Rayleigh distribution whereas the statistics of the co-channel interferers (CCIs) at the relay node has been modeled using Nakagami-m fading distribution. On the other hand, FSO link has been assumed to undergo α−μ distributed atmospheric turbulence with pointing errors. To this end, the model on the FSO link considers the impact of both kinds of optical demodulation schemes i.e., intensity modulation direct detection (IM/DD) and coherent demodulation. For the system under study, exact closed-form expressions for the outage probability, bit error rate (BER) and ergodic capacity have been presented. Further to this, asymptotic high signal to noise ratio (SNR) approximate expressions have been derived for outage probability, BER and ergodic capacity to extract quick insights into the results. Finally, numerical examples have been demonstrated to validate the findings of the research work.

On the Effect of Interference and Misalignment Error in Mixed RF/FSO Systems Over Generalized Fading Channels

In this paper, the end-to-end performance of a mixed radio frequency (RF)/free space optical (FSO) affected by co-channel interference (CCI), is studied. We consider that the RF link experiences $\eta -\mu $ fading and the FSO link is subjected to atmospheric turbulence, which is modeled by the $\alpha -\mu $ distribution. Also, the statistics of the FSO link is presented for the case of zero and non-zero boresight pointing errors. Furthermore, we assume intensity modulation with direct detection (IM/DD) and coherent demodulation. In particular, we present a closed-form expression for the probability density function of the FSO link, which is then used to obtain a closed-form and an asymptotic expression for the outage probability. In order to quantify the system performance, we utilize this asymptotic result to yield the system’s coding gain and diversity order. Moreover, we have presented analytical expressions for the average bit error rate (BER) and ergodic capacity for the system design. In order to gain more insights, high signal-to-noise ratio (SNR) approximations expressions for the BER and ergodic capacity, are also derived. Finally, the analytical results presented in the paper are validated through computer simulations.

A modified direct demodulation method applied to Insight-HXMT Galactic plane scanning survey

We present a modified direct demodulation (DD) method for the detection of fast variable objects with the Insight-HXMT Galactic plane survey, which is dedicated to hunting for new transients and monitoring known variable sources. The novelty of the improved DD method is the reconstruction of the modified response matrix, in which the variability of a source is properly handled. Monte Carlo (MC) simulation results demonstrate its power for the detection of variable sources with different time scales, resulting in more accurate location and precise flux monitoring. The application of the method to the Insight-HXMT Galactic plane survey data reveals that the type-II bursts can be well detected for the Rapid Burster MXB 1730–335, with an average rate of 2 bursts min−1 in a manner of the well-known relaxation oscillation.

Overview to the Hard X-ray Modulation Telescope (Insight-HXMT) Satellite

As China's first X-ray astronomical satellite, the Hard X-ray Modulation Telescope (HXMT), which was dubbed as Insight-HXMT after the launch on June 15, 2017, is a wide-band (1-250 keV) slat-collimator-based X-ray astronomy satellite with the capability of all-sky monitoring in 0.2-3 MeV. It was designed to perform pointing, scanning and gamma-ray burst (GRB) observations and, based on the Direct Demodulation Method (DDM), the image of the scanned sky region can be reconstructed. Here we give an overview of the mission and its progresses, including payload, core sciences, ground calibration/facility, ground segment, data archive, software, in-orbit performance, calibration, background model, observations and some preliminary results.