## 3 Indoor Positioning Systems
### Systems Based on Radio Frequency
#### 3.1.2 Range-Free  
Range-free methods are based on information obtained from the connectivity of a wireless network [25]. The values obtained can be used to estimate the position without calculating any measure of reach at an anchor node [58; 59]. There are mainly two types of flawless algorithms: proximity and digital printing methods. Proximity methods: These methods use the connectivity information to directly infer the position of the user based on the number of anchors in the neighborhood. Proximity algorithms are based on a simple idea: if a user is receiving a signal from an anchor node, the user's position must be close to the anchor node's position [59]. The operation takes place according to the following steps: first, the user searches the channel looking for radio signals from the anchor nodes. After detecting an anchor node, the user's position is estimated to the position of this anchor node. When there are several nodes available, the model will select the one with the strongest signal. Figure 4 shows how the method works: the circles represent the coverage area of the anchor nodes, and it is necessary to create an intersection between them. When a user is around the circle maintained by node s1, the estimation of their position will use anchor node S1 as a reference. When a user is in the circle of anchor S2, the position will be estimated as the position of anchor node S2. The selection of the anchor node will be made in terms of the RSSI at the intersection of the two circles. One example of using the proximity method is the Active Badge system [60]. This system uses a network of infrared sensors that detect signals transmitted by active tags and provides an accurate location estimate of a room. More recently, many authors have tried using Bluetooth Low-Energy (BLE) beacons to identify and locate targets. Bluetooth is a wireless communication technology that uses information that is digitally incorporated into radio frequency signals [58; 61]. The standard was initially intended for the exchange of data over short distances, with operating policies and protocol defined by the IEEE 802.15.1 standard [58]. The main objectives of the technology are to communicate between fixed and/or mobile  
Figure 4: Proximity method concept. devices to reduce the necessary infrastructure, eliminating cables and physical connectors, and to facilitate the synchronization of data and people [61]. Bluetooth technology has been considered as a resource for indoor position systems and a competitor to Wi-Fi. The widespread adoption of BLE occurred due to its availability (most modern smartphones support it), low cost, and extremely low power consumption. BLE devices allow fixed emitters to run on batteries for several months or even years [58]. [PERSON] was among the authors who studied the use of BLE beacons in an indoor environment to track patients in a hospital [62]. Many authors have indicated that BLE technologies could be deployed in millions of buildings to provide the locations of places, objects, and people, without the need to invest in infrastructure, as indicated by [PERSON] et al. and [PERSON] et al. [63; 64]. The level of error presented by proximity methods is causally related to the size of the coverage area. That is, if the coverage area is large, the error is increased, and if the coverage area is smaller, the number of anchor nodes required for the total coverage of an internal area increases. For this reason, several authors have experimented with applying complementary technologies, such as radio frequency identification (RFID), which allows reducing the level of error, with a low impact on the cost of the system [65]. Also, the use of passive RFID tags reduces the cost of maintaining the network, as the battery of the anchor nodes does not need to be replaced regularly.