## 2 Research Background
### Sensor-Based Site Hazard Alarm Systems and Knowledge Gaps  
In general, many of the safety injuries at construction sites would be associated with whether the workers have a possibility to be close to or make contact with any existing safety risks such as floor holes or open spaces without proper safety protection, which can lead to \"falls from height\", or working areas that are exposed to \"struck-by hazards\" (e.g., flying, falling, swinging, and rolling objects). As a result, monitoring the locations of workers or any sources of forcible contact (e.g., equipment) is important to provide a timely alarm to workers and avoid potential hazards. The primary function of such sensor-based hazard alarm systems would be to obtain accurate worker location information by using various positioning and localization sensors such as Global Positioning Systems (GPSs), Radio Frequency Identification (RFID), Ultra-Wide Band (UWB), Wireless Local Area Networks (WLANs), or BLE. For example, [PERSON] [47] designed an alarm system that obtained distance data through a GPS to remind railway construction workers of a coming train at the sites. [PERSON] et al. [48] used RFID to localize workers to provide a timely warning if a worker was near predefined hazardous areas. [PERSON] et al. [49] applied UWB technologiesto develop a real-time monitoring system for workers and equipment. [PERSON] et al. [50] developed a WLAN-based indoor positioning system to track laborers at construction sites and conducted a case study at the Guangzhou MTR tunnel construction site. [PERSON], [PERSON], [PERSON], and [PERSON] [8] proposed a BLE beacon-based safety alert system for indoor worker safety monitoring [7].  
[PERSON], [PERSON], [PERSON], and [PERSON] [48] pointed out several features of sensors when selecting localization technologies, such as (1) accuracy, (2) affordability (e.g., required capital hardware investment), (3) need for line of sight, (4) wireless communication, (5) need for a power supply, and (6) scalability. Considering these features, BLE beacons have comparative advantages compared with others, especially for sensor deployment at dynamically changing construction environments. These days, various cheap, off-the-self BLE beacons with long-lasting batteries (more than 2-3 years without changing batteries) are available on the market. They can communicate with mobile phones to serve as signal receivers and hazard alarming devices by using mobile applications. Even though they may suffer from low accuracy in long-distance and line-of-sight issues (blocked signals can be degraded), deploying multiple BLE beacons somehow can address these issues. In particular, they can be quickly installed and relocated only by attaching and detaching them at the site, allowing flexible deployment when considering dynamic hazardous site conditions.  
However, from a practical point of view, how to deploy the beacon sensors is still challenging. In general, two different strategies for beacon deployment have been widely used: (1) point-based (deploying at specific places) and (2) grid-based (deploying to cover the entire space) beacon deployment. For hazard alarm systems, point-based deployment would be preferred, as the alarm system aims to give an alarm to workers who are exposed to hazardous areas or objects, and it can also save the costs and efforts of buying and managing multiple beacons. However, point-based deployment may need to compromise the reliability of safety alarms due to the limited coverage and vulnerability to blockage of BLE beacons. In this regard, understanding how beacon-based alarms can be affected by various site conditions would be essential for more cost-effective beacon deployment for hazard alarm systems.