In recent years, with the increased interest in smart home technology and the increased need to remotely monitor patients due to the pandemic, demand for contactless systems for vital sign measurements has also been on the rise. One of these kinds of systems are Doppler radar systems. Their design is composed of several choices that could possibly have a significant impact on their overall performance, more specifically those focused on the measurement of cardiac activity. This review, conducted using works obtained from relevant scientific databases, aims to understand the impact of these design choices on the performance of systems measuring either heart rate (HR) or heart rate variability (HRV). To that end, an analysis of the performance based on hardware architecture, carrier frequency, and measurement distance was conducted for works focusing on both of the aforementioned cardiac parameters, and signal processing trends were discussed. What was found was that the system architecture and signal processing algorithms had the most impact on the performance, with FMCW being the best performing architecture, whereas factors like carrier frequency did not have an impact.This means that newer systems can focus on cheaper, lower-frequency systems without any performance degradation, which will make research easier.

Human identification using gait as a biometric feature has gained significant attention in recent years, showing notable advancements in medical fields and security. A review of recent developments in remote radar-based gait identification is presented in this article, focusing on the methods used, the classifiers employed, trends and gaps in the literature. Particularly, recent trends highlight the increasing use of Artificial Intelligence (AI) to enhance the extraction and classification of features, while key gaps remain in the area of multi-subject detection. In this paper, we provide a comprehensive review of the techniques used to implement such systems over the past 7 years, including a summary of the scientific publications reviewed. Several key factors are compared to determine the most suitable radar for remote gait-based identification, including accuracy, operating frequency, bandwidth, dataset, range, detection, feature extraction, size and number of features extracted, multiple subject detection, radar modules used, AI used and their properties, and the testing environment. Based on the study, it was determined that Frequency-Modulated Continuous-Wave (FMCW) radars were more accurate than Continuous-Wave (CW) radars and Ultra-Wideband (UWB) radars in this field. Despite the fact that FMCW is the most closely related radar to real-world scenarios, it still has some limitations in terms of multi-subject identification and open-set scenarios. In addition, the study indicates that simpler AI techniques, such as Convolutional Neural Network (CNN), are more effective at improving results.

Bio-Radar (BR) systems measure physiological activity by using the micro-Doppler effect to detect signal reflections from the human body. The quality of the received signal is influenced by several factors, most notably the Radar Cross Section (RCS) of the subject, which becomes especially complex in close-proximity conditions. To optimize BR system performance, this paper investigates the radio channel propagation in short-range conditions, focusing on RCS variations at close distances. The study was conducted using a Continuous-Wave (CW) radar system operating at 5.8 GHz, with measurements taken in an anechoic chamber. Ten human subjects with varying characteristics were analyzed. The findings demonstrate that RCS fluctuates with distance, exhibiting oscillatory behavior at short-range distances, with values ranging between −45 dBsm and −20 dBsm. They also reveal a correlation between RCS measurements and the extraction of vital signs, identifying specific conditions under which vital sign detection is significantly compromised. Additionally, the data indicate that male subjects tend to have higher RCS values than female subjects.