Optical pulse detection photoplethysmography (PPG) provides a means of low cost and unobtrusive physiological monitoring that is popular in many wearable devices. However, the accuracy, robustness and generalizability of single-wavelength PPG sensing are sensitive to biological characteristics as well as sensor configuration and placement; this is significant given the increasing adoption of single-wavelength wrist-worn PPG devices in clinical studies and healthcare. Since different wavelengths interact with the skin to varying degrees, researchers have explored the use of multi-wavelength PPG to improve sensing accuracy, robustness and generalizability. This paper contributes a novel and comprehensive state-of-the-art review of wearable multi-wavelength PPG sensing, encompassing motion artifact reduction and estimation of physiological parameters. The paper also encompasses theoretical details about multi-wavelength PPG sensing and the effects of biological characteristics. The review findings highlight the promising developments in motion artifact reduction using multi-wavelength approaches, the effects of skin temperature on PPG sensing, the need for improved diversity in PPG sensing studies and the lack of studies that investigate the combined effects of factors. Recommendations are made for the standardization and completeness of reporting in terms of study design, sensing technology and participant characteristics.

Range sidelobe suppression in pulse-compression radars and sonars is conventionally performed using amplitude windowing in either the time or frequency domains. Using peak power limited transmitters, this inevitably causes a degradation in the signal-to-noise ratio available at the receiver output owing to shading or mismatch losses. Nonlinear frequency modulation (NLFM) chirps have been suggested as a solution to this problem and have been successfully implemented in a number of radar systems. Using appropriate NLFM functions it is possible to shape the energy spectrum of a chirp, achieving low-range sidelobes, without the need for any amplitude windowing and hence maintaining maximum efficiency. In the field of active sonar, NLFM chirps have been largely ignored because of their poor Doppler tolerance compared with conventional linear period modulation (LPM) chirps. It is shown that a combination of amplitude windowing and NLFM can be used to achieve a compromise between sidelobe suppression, Doppler tolerance and shading loss. Using a novel family of NLFM functions a significant performance advantage over LPM chirps can be attained over a useful range of target velocities. Experimental results support this argument.

The performance of active sonars operating in shallow water is often limited by the reverberation level (Waite, 1998). If a target is moving relative to the reverberating scatterers, it may be possible to isolate its Doppler-scaled echo from the zero-Doppler reverberation provided that the Doppler shift is greater than the bandwidth of the transmitted pulse. In practice, this generally implies the use of narrowband long-duration continuous-wave (CW) pulses. The major disadvantages of this method are the poor range resolution of such pulses leading to poor reverberation processing with low Doppler targets. To detect slowly moving or stationary targets, linear period modulation (LPM) chirps are often used in preference. Several new classes of pulse design have been proposed which theoretically provide superior reverberation processing to CW pulses by virtue of their comb-like spectra. The paper reviews the theory behind Newhall trains, sinusoidal frequency modulated pulses and geometric comb waveforms, comparing their theoretical reverberation processing gains against CW and LPM pulses using the Q-function. Experimental results from low-frequency active sonar sea trials are also presented to verify theoretical predictions.

The majority of active sonar systems detect and classify a target based on the amplitude of the received echo strength or the induced Doppler shift. However, additional classification information is available from the phase shift introduced by some targets as a result of the acoustic boundary conditions. For example, reverberation returns from the sea surface and from the swimbladders of various fish introduce an additional phase shift that is not usually present in returns from the seabed or man-made targets. Techniques based on the use of sub-band correlators are presented for measuring the phase shifts associated with certain stationary and moving targets when insonified by broadband transmissions. The performance of the target-phase measurement technique is derived for transmit signals used in existing sonar systems such as continuous wave (CW), linear frequency-modulated (LFM), and hyperbolic frequency-modulated (HFM) pulses in the presence of target or platform motion. The use of HFM signal processing with sub-band correlators to measure the Doppler shift of very slowly moving targets is presented. Modifications to sinusoidal frequency-modulated (SFM) and HFM transmission types are proposed in order to maintain a good compromise between overall detection and target-phase classification performance, i.e., between range resolution and phase measurement. Field trial results are presented for a plankton classification sonar system and a collision-avoidance system designed for operation near the sea surface

This paper addresses the significant need for improvements in device version reporting and practice across the academic and technical activity monitoring literature, and it recommends assessments for new and updated consumer sensing devices. Reproducibility and data veracity are central to good scholarship, and particularly significant in clinical and health applications. Across the literature there is an absence of device version reporting and a failure to recognize that device validity is not maintained when firmware and software updates can, and do, change device performance and parameter estimation. In this paper, we propose the use of tractable methods to assess devices at their current version and provide an example empirical approach. Experimental results for heart rate and step count acquisitions during walking and everyday living activities from Garmin Vivosmart 3 (v4.10) wristband monitors are presented and analyzed, and the reliability issues of optically-acquired heart rates, especially during periods of activity, are demonstrated and discussed. In conclusion, the paper recommends the empirical assessment of new and updated activity monitors and improvements in device version reporting across the academic and technical literature.

Epilepsy is a neurological disorder that affects 50 million people worldwide. It is characterised by seizures that can vary in presentation, from short absences to protracted convulsions. Wearable electronic devices that detect seizures have the potential to hail timely assistance for individuals, inform their treatment, and assist care and self-management. This systematic review encompasses the literature relevant to the evaluation of wearable electronics for epilepsy. Devices and performance metrics are identified, and the evaluations, both quantitative and qualitative, are presented. Twelve primary studies comprising quantitative evaluations from 510 patients and participants were collated according to preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Two studies (with 104 patients/participants) comprised both qualitative and quantitative evaluation components. Despite many works in the literature proposing and evaluating novel and incremental approaches to seizure detection, there is a lack of studies evaluating the devices available to consumers and researchers, and there is much scope for more complete evaluation data in quantitative studies. There is also scope for further qualitative evaluations amongst individuals, carers, and healthcare professionals regarding their use, experiences, and opinions of these devices.