1. INTRODUCTION
World Health Organization (WHO) has pointed out that falls are the second leading cause of unintentional injury deaths worldwide [1]. Fall can also cause post-fall syndrome (e.g., the fear of falling again) which may reduce the independence of their daily life [2]. Currently, many researchers developed "pre-impact fall detection systems" using wearable sensors [3] to support fall protection systems for preventing severe injuries [4]. The purpose of pre-impact fall detection systems is to detect the occurrence of falls during falling with a lead time which is determined as the period between the fall detection of the system and the ground impact. A longer lead time indicates that the protection system has more response time to activate the fall protection device as the duration of falling is less than 800ms in most fall instances [5]. It shows the requirement to develop reliable pre-impact fall detection systems to quickly and accurately detect early falls.