Complex Signal Demodulation Research Articles

Radar-Enabled non-contact speed estimation for rotating electrical machinery

Electric motors play a pivotal role in contemporary industrial processes, prompting the necessity for their continuous monitoring through an array of sensors in various applications. Among the crucial parameters under scrutiny, the operational speed of electric motors holds a prominent position. Current methods for speed detection conventionally entail the utilization of physically connected sensors or the measurement of some electrical variables. Nonetheless, a non-contact speed measurement approach with a mobile device promises low cost, efficiency, and even safety in some specific cases. This research delves into the remote speed detection of electric motors employing Continuous Wave radar technology. To this end, phase extraction techniques, which have demonstrated robust performance in diverse application domains, have been assessed for their efficacy in motor speed detection. First, five distinct methods have been utilized to extract the phase of baseband signals obtained from a 24 GHz IQ-demodulated radar system directed toward an asynchronous motor. These methodologies have undergone comprehensive evaluation in experimental measurements and simulation encompassing ideal and noisy conditions. Complex Signal Demodulation, one of the phase extraction methods, exhibits a noteworthy superiority over alternative approaches in motor speed measurement experiments. These experiments have entailed three different load scenarios and encompassed ten distinct speeds, equally spaced between 500 and 1400 rpm. The Complex Signal Demodulation method has delivered exceptional results, successfully detecting motor speeds across thirty experiments with a mean absolute percentage error of merely 0.08. Radar-based non-contact speed detection holds immense promise, particularly in fault diagnosis and predictive maintenance.

Ultra-Wideband Impulse Radar Through-Wall Detection of Vital Signs

This paper presents a new system for the detection of human respiration behind obstacles using impulse ultra-wideband (UWB) radar. In complex environments, low signal-to-noise ratios (SNRs) as they can result in significant errors in the respiration, heartbeat frequency, and range estimates. To improve the performance, the complex signal demodulation (CSD) technique is extended by employing the signal logarithm and derivative. A frequency accumulation (FA) method is proposed to suppress mixed products of the heartbeat and respiration signals and spurious respiration signal harmonics. The respiration frequency is estimated using the phase variations in the received signal, and a discrete short-time Fourier transform (DSFT) is used to estimate the range. The performance of the proposed system is evaluated along with that of several well-known techniques in the literature.

Vector Modulation by Laser and Electroabsorption Modulator With Digital Pre-Coding and Pre-Compensation

We present a method to achieve vector modulation of lightwave using an electroabsorption modulator intregrated laser. The laser section is used for direct phase modulation followed by an amplitude modulation of the electroabsorption modulator section in a cascade manner, which is an alternative method to conventional IQ modulator. To allow an improved phase demodulation, we propose a digital pre-compensation technique for a transmitter in optical coherent system for the first time, which includes a phase-balanced differential phase shift keying encoding and a digital domain pre-equalization filtering. The phase-balanced encoding technique applies parallel DC-balance coding with a phase-balanced symbol mapping, which results in a bounded cumulative phase and a constant statistical characteristic of the modulating signal in differential M-ary phase shift keying and M-ary quadrature amplitude modulation formats. The digital pre-equalization filter also compensates for the phase response of the laser section. Experimental results show that the pre-digital compensation technique allows the improved complex signal demodulation compared to conventional DQPSK modulation in a back-to-back setup.

Polyphase-Basis Discrete Cosine Transform for Real-Time Measurement of Heart Rate With CW Doppler Radar

This paper presents a novel polyphase-basis discrete cosine transform that can be used for challenges to observe heart rate (HR) variability and sudden changes of HR in short time. To succeed in the challenges, real-time measurement which requires a short window length is needed. As the window length decreases, however, increased main-lobe width (MLW) and sidelobe width (SLW) result errors in measuring HR. Unlike commonly used discrete Fourier transform (DFT), the proposed method is based on DCT, which improves the accuracy of measured HR even though the short window length is used for the real-time measurement of HR. We demonstrate that the DCT can give 2 times shorter MLW and SLW than the DFT can. To verify the proposed method, it is compared with well-known existing methods, arctangent demodulation and complex signal demodulation, which use the DFT. We measured HR using a 10.225-GHz continuous-wave Doppler radar. In each experiment, the window lengths of 3, 2, and 1.5 s were used to measure HR in real time. Experimental results show that the proposed method has lower mean and standard deviation of errors than the existing methods have.

Phase-Based Methods for Heart Rate Detection Using UWB Impulse Doppler Radar

Ultra-wideband (UWB) pulse Doppler radars can be used for noncontact vital signs monitoring of more than one subject. However, their detected signals typically have low signal-to-noise ratio (SNR) causing significant heart rate (HR) detection errors, as the spurious harmonics of respiration signals and mixed products of respiration and heartbeat signals (that can be relatively higher than heartbeat signals) corrupt conventional fast Fourier transform spectrograms. In this paper, we extend the complex signal demodulation (CSD) and arctangent demodulation (AD) techniques previously used for accurately detecting the phase variations of reflected signals of continuous wave radars to UWB pulse radars as well. These detection techniques reduce the impact of the interfering harmonic signals, thus improving the SNR of the detected vital sign signals. To further enhance the accuracy of the HR estimation, a recently developed state-space method has been successfully combined with CSD and AD techniques and over 10 dB improvements in SNR is demonstrated. The implementation of these various detection techniques has been experimentally investigated and full error and SNR analysis of the HR detection are presented.

Fast Acquisition of Heart Rate in Noncontact Vital Sign Radar Measurement Using Time-Window-Variation Technique

The fast acquisition of heart rate (HR) is a challenge in noncontact vital sign detection using a Doppler radar system. Most of the previous studies use long-period time windows to guarantee a sufficient frequency spectrum resolution for HR measurement using the peak searching method on the frequency spectrum. For fast acquisition of HR, the length of the time window is less than 5 s and the accuracy is significantly degraded due to insufficient spectrum resolution. For vital sign detection using complex signal demodulation (CSD), measuring HR variation becomes a difficult job due to respiration harmonic interference and insufficient frequency spectrum resolution. In this paper, a time-window-variation technique is developed for fast acquisition of HR from short-period time windows and measuring HR variation using CSD. Experiments are performed on four human subjects under controlled laboratory conditions by a 5.8-GHz continuous-wave Doppler radar vital sign detection system. The HR measurement results within 2–5-s time windows are compared by applying a simple fast Fourier transform (FFT) and peak searching method, a 10-s sliding-time-window FFT method, and the proposed method. The proposed method achieves the smallest average error among the three techniques. The proposed method has also proved to be able to measure HR variation using CSD.

Random Body Movement Cancellation in Doppler Radar Vital Sign Detection

The complex signal demodulation and the arctangent demodulation are studied for random body movement cancellation in quadrature Doppler radar noncontact vital sign detection. This technique can be used in sleep apnea monitor, lie detector, and baby monitor to eliminate the false alarm caused by random body movement. It is shown that if the dc offset of the baseband signal is accurately calibrated, both demodulation techniques can be used for random body movement cancellation. While the complex signal demodulation is less likely to be affected by a dc offset, the arctangent demodulation has the advantage of eliminating harmonic and intermodulation interference at high carrier frequencies. When the dc offset cannot be accurately calibrated, the complex signal demodulation is more favorable. Ray-tracing model is used to show the effects of constellation deformation and optimum/null detection ambiguity caused by the phase offset due to finite antenna directivity. Experiments have been performed using 4-7 GHz radar to verify the theory.