# An Overview of Indoor Localization System for Human Activity Recognition (HAR) in Healthcare
## 5 Signal Technologies
### Inertial System  
An inertial system represents the positioning solution based on an inertial measurement unit (IMU) sensor. IMUs combine accelerometers (for measuring linear acceleration), gyroscopes (for measuring rotational speed), and optionally, a magnetometer (for measuring magnetic direction). This information can accurately determine orientation, velocity, and position [67]. This technique qualifies for accuracy, energy, and efficiency, provided that the inertial sensor is attached to the subject's body. However, the measurements are prone to errors and require using Kalman-type filters. In addition, its cost is relatively high and often requires implementing the network infrastructure. By processing signalsfrom these devices, it is possible to track the position and orientation of a device. The double integration of measured acceleration provides the relative position of an object concerning an initial position. Absolute positioning information may be obtained by fusing INS with complementary sensors. The signal-less localization method is based on mobile sensors such as accelerometer, gyroscope, magnetometer, and barometer, to track users by continuously estimating their displacement using dead reckoning [68]. With this technique, a sensor node uses its previous calculated position for localization at successive intervals. Although dead reckoning can provide the best available information about the current location with simple algorithms, it is subject to significant approximation errors. Another hybrid solution is to host the fake Wi-Fi printing with inertial sensors, overcoming the drawbacks of standalone solutions [69]. Figure 14 shows an architectural schema of an IPS application.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Technology** & **Measure Method** & **Cost** & **Advantages** & **Disadvantages** & **Accuracy (m)** \\ \hline RFID & Proximity, RSS & & & Does not require LOS & \begin{tabular}{c} Small coverage, \\ butreven TR and RT, \\ simultaneous and fast \\ reading of multiple tags \\ \end{tabular} & \begin{tabular}{c} Small coverage, \\ multipath effect and \\ signal fluctuation, \\ limited capabilities of \\ passive tags \\ \end{tabular} & 0.5 (passive) \\ \hline WLAN & RSS, TDOA & Medium & & \begin{tabular}{c} Does not require LOS, \\ presence in multiple \\ buildings, medium \\ scalability \\ \end{tabular} & \begin{tabular}{c} Complex methodology, \\ system redesign in case \\ of changes in the \\ environment \\ \end{tabular} & 10–50 \\ \hline Bluetooth & TDOA, RSS & Low-medium & & \begin{tabular}{c} Good accuracy, no need \\ additional infrastructure, \\ does not require LOS, \\ present in most \\ smartphones \\ \end{tabular} & RF interference, limited & \begin{tabular}{c} \\ coverage and mobility \\ \end{tabular} & 2–15 \\ \hline UWB & TDOA & High & \begin{tabular}{c} Low energy consumption, \\ high accuracy, passes \\ through walls and any \\ other obstacles \\ \end{tabular} & Needs time & \multirow{3}{*}{0.1–1} \\ \cline{4-4}  & & & & \begin{tabular}{c} Notify special \\ equipment, vulnerable \\ to interference caused by \\ types \\ \end{tabular} & \\ \hline ZigBee & RSS, AP ID & Low & Low power consumption & \begin{tabular}{c} Requires special \\ equipment, vulnerable \\ to interference caused by \\ a wide range of signal \\ types \\ \end{tabular} & 1–5 \\ \hline Ultrasound & TOA, TDOA & Medium & \begin{tabular}{c} Good accuracy, not \\ affected by multipath \\ \end{tabular} & \begin{tabular}{c} Interference by \\ high-frequency sound, \\ loss of signal for \\ obstruction \\ \end{tabular} & 0.01–0.1 \\ \hline Infrared & AOA, TOA, TDOA & Medium & \
\begin{tabular}{c} Low power, no multipath \\ effect, medium accuracy \\ \end{tabular} &
\begin{tabular}{c} Does not penetrate walls, \\ requires LOS, sunlight \\ interference, short range \\ \end{tabular} & 5–10 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of indoor localization systems.  
Figure 14: Architecture of IPS System.