However, when the environment varies the threshold needs to be reset or new activities need to be identified, which limits the adaptability and instantaneity of this system. Traditional HAR systems distinguish the difference of CSI between action phase (the case that someone is interfering with the link) and stationary phase (the case that no one is interfering with the link) and artificially set the threshold to detect the start and the end of the activity. Therefore, CSI-based HAR is popular with researchers and has been extensively studied. Channel statement information (CSI) is a kind of fine-grained physical layer information with high resolution. Human activity recognition (HAR) is the vital technology nowadays, and it enables to use for realizing applications such as intelligent sensory games, smart homes, and human body posture monitoring and some other individual applications. CDHAR ensures high recognition accuracy while improving the adaptability and instantaneity. According to the extracted data, the recognition accuracy in outdoor and indoor environments can reach 91.2% and 90.2%, respectively. The experiment results tell that even if experimental scenario varies, the accuracy of activity durations extraction can reach 98% and 99.60% whether in outdoor or indoor environments. Finally, we prototype CDHAR on commercial WiFi devices and evaluate its performance in both indoor environment and outdoor environments. Second, we proposed a random subspace classifier ensemble method for classification, which applies the frequency domain feature instead of the time domain feature, and we choose each kind of feature in the same amount. Firstly, by using machine learning, CDHAR applies kernel density estimation (KDE) to obtain adaptive detection thresholds to complete the extraction of activity duration. In this paper, we propose a CSI-based device-free HAR (CDHAR) system with WiFi-sensing radar integrated on UAVs to recognize everyday human activities.
(2) A sole classifier is used to complete the recognition, resulting in poor robustness and relatively low recognition accuracy. However, in the existing CSI-based HAR system, there are two disadvantages: (1) The detection threshold is manually set, which limits its adaptability and immediacy in different wireless environments. In recent years, the HAR system based on CSI based on WiFi radar has received widespread attention due to its low cost and privacy protection property. Nowadays, with the extensive use of unmanned aerial vehicles (UAVs) in the civil field, integrating wireless signal receivers on UAVs could be a better choice to receive hearable signals more conveniently. Features extraction and analysis for human activity recognition (HAR) have been studied for decades in the 5th generation (5G) and beyond the 5th generation (B5G) era.
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