# A Comprehensive Review of Indoor Localization Techniques and Applications in Various Sectors
## 2 Technology of Indoor Localization
### Wi-Fi Module  
The ESP8266 Wi-Fi module (Figure 2) is a compact, affordable microchip designed to provide Wi-Fi connectivity for various applications. Operating at a 2.4 GHz frequency and compliant with 802.11 b/g/n Wi-Fi standards, this module connects to most modern wireless networks. It supports both client mode (connecting to existing networks) and access point mode (creating its own Wi-Fi network), offering versatility for a range of use cases. With a transmission power of +25 dBm, the ESP8266 ensures reliable communication over moderate distances [47]. Its multiple GPIO pins (general purpose input/output) enable easy connection with sensors, actuators, and other hardware, increasing its flexibility.  
The ESP8266 is widely used in Wi-Fi-based RTLSs, especially in healthcare environments, for tracking assets, staff, and patients. Hospitals can integrate the ESP8266 into wearable devices or Wi-Fi-enabled tags to efficiently track medical equipment like wheelchairs, infusion pumps, and defibrillators. This approach leverages the existing hospital Wi-Fi infrastructure, making real-time tracking cost-effective and scalable. Additionally, Wi-Fi-based positioning systems support the simultaneous tracking of multiple devices over large hospital areas without the need for additional specialized hardware. This is particularly beneficial in high-density environments where traditional tracking systems like BLE (Bluetooth low energy) may have insufficient range. Wi-Fi-based systems can also gather data on hospital traffic patterns, helping optimize facility management and workflow efficiency [48].  
The ESP8266 plays a crucial role in Internet of Things (IoT) applications, connecting medical devices to cloud platforms for real-time data collection and analysis. This capability supports predictive maintenance, automated patient flow management, and improved decision-making in hospital operations. For example, a Wi-Fi-based localization system using received signal strength indicator (RSSI) values and a backpropagation-based artificial neural network (BP-ANN) can achieve high localization accuracy, particularly for tracking Alzheimer's patients. The neural network processes RSSI data from multiple anchor nodes placed in key locations to estimate patient positions with a low localization error of around 0.964 m. This system helps caregivers monitor patient movements in real time, ensuring safety by promptly responding to wandering or disorientation incidents [4; 49; 50].  
Furthermore, the ESP8266 can be paired with sensors to monitor environmental conditions such as temperature, humidity, and air quality. These real-time data help maintain safety standards in hospitals and industrial settings. The module also supports hospital automation systems, controlling functions like lighting, HVAC systems, and security systems to improve operational efficiency. With its low cost, low power consumption, and versatility, the ESP8266 Wi-Fi module is a valuable tool for enhancing real-time tracking, asset management, patient safety, and overall healthcare quality [51].  
Figure 2: Wi-Fi module of an ESP8266 device.