## 2 Hybrid Localization Concept  
The typical received signal strength-based positioning systems, due to difficult and complex multipath propagation environments typically present indoors, do not allow for very accurate and reliable localization. Localization errors are particularly high at the edges of the area covered by the system or in closed spaces such as toilets or bathrooms. It comes from the fact that the propagation between the localized devices and most parts of the system infrastructure occurs under OLOS (Obstructed Line-of-Sight) or NLOS (Non Line-of-Sight) conditions, which introduce significant signal attenuation. The concept proposed in the paper consist in additional coverage of those and other vital places with proximity sensors. Proximity sensors are small and cheap devices, which allow to detect presence and measure distance from objects located at moderate distances up to a few meters. Although it is possible to use them for distance measurement, they should not be mistaken with much more advanced laser distance meters. Proximity sensor field of view typically has a shape of a conical beam covering an angle up to a few dozen degrees, whereas laser distance meter is much more directional. An exemplary scenario of the proposed hybrid concept use is presented in Figure 1.  
In the proposed system concept, the localization system consists of three parts:  
* a tag, which is worn by the user
* an infrastructure comprising a set of BLE anchors and proximity sensors
* a system controller  
The system operates in the following way. The tag, which is worn by the user, repeatedly sends BLE packets in the advertisement channels. The anchors receive them and measure the RSS (Received Signal Strength). Meanwhile, the proximity sensors perform independent, periodical distance measurements. Results from the infrastructure are sent to the system controller, which is an application running on a regular PC or a dedicated server and is responsible for position determination. The results can be exploited by an external system delivering services to monitored persons and their caregivers. For example, in the IONIS project [37] cloud-based solution performs these functions. The system controller manages the localization system infrastructure. In case of an anchors or tags failures (that are reported by anchors or detected if the communication with devices is lost), the alert message is sent to the cloud. For most of the time, a user's position is derived using BLE power measurement results only (Figure 1a). When the user steps into the area covered by one or more proximity sensors and their presence is detected, a hybrid algorithm combining BLE and ranging results is used. Unfortunately, since the proximity sensors have no way of discriminating between different users and the presented solution is intended for locating on individual only. That problem could be resolved by prior localization of the users based on BLE only and checking which user is closer to the sensor. However, because of low BLE accuracy, this approach would not be dependable when the users are close to each other. Since the system is intended for use by an older person living alone, it is not a major drawback. In the presented system architecture, the BLE anchors and the proximity sensors are separate devices, but it is possible to integrate them. It would simplify communications with the system  
Figure 1: Hybrid localization use scenario. (**a**) User is localized using BLE only. (**b**) User is localized using BLE and proximity sensors. controller and greatly reduce number of power supplies needed by the infrastructure. However, such an approach is not the best for localization. A proximity sensor maximum range is usually limited to a few meters and it would be best to orient its beam parallel to the ground to maximize the covered area. It would also mean placing the sensors in lower positions, so the signal could easily reflect off the localized person.