# A Review of Pedestrian Indoor Positioning Systems for Mass Market Applications
## 2 Network Based Systems
### Range Free
#### 2.2.1 Proximity  
The proximity algorithms are based on the following simple idea: if a user is receiving a signal from an anchor node, the position of the user must be near the position of the anchor node. The operation mode is as follows: first, the user scans the channel looking for the radio signals from the anchors nodes. Once an anchor node is detected, the position of the user is estimated as the position of the anchor node. In the case of detecting more than one radio signal, the anchor node with the strongest received signal is selected.  
Figure 5 describes the method, where the circles represent the coverage area of the anchor nodes. Any user located in the circle of node \(s_{1}\) will estimate its own position as the position of the anchor node \(s_{1}\) whereas if the object is located in the circle of anchor \(s_{2}\) it will estimate the position as the position of the anchor node \(s_{2}\). In the intersection of both circles the selection of the anchor node will be done in terms of the RSS.  
One of the first systems to employ the proximity method was the Active Badge system [24]. This system uses a network of infrared sensors that detects the signals transmitted by the active badge and provides a localization estimation with room accuracy. More recently, some authors decided to employ Bluetooth Low Energy (BLE) beacons. For example in [27] a set of BLE beacons are deployed in a hospital for the tracking of the patients. Other works employing BLE technology can be found in [28; 29]. It is expected that BLE beacons will be deployed in millions of buildings around the world in a near future. Thus, there will be no need for investment in infrastructure reducing the cost of the system at a minimum as modern smartphones are equipped with BLE transceivers and can be used as positioning devices.  
Figure 5: Proximity method concept.  
The error committed by the proximity methods is directly related to the size of the coverage areas. Furthermore, if the coverage area of the anchor nodes is reduced, the number of anchor nodes needed for a total coverage of an indoor area increases. For this reason, the Radio-Frequency Identification (RFID) technology is particularly interesting for proximity methods because the deployment of a large number of tags in a building does not escalates the cost of the system [25]. Moreover, the use of passive RFID tags reduces the maintenance cost of the network as the battery of the anchor nodes must not be regularly replaced.  
A more general way of using connectivity information is employed in the centroid algorithm, where the estimation of position is computed as the centroid of the position of the anchor nodes received [26], that is,  
\[\hat{\mathbf{m}}=\left[\frac{1}{N}\sum_{i=1}^{N}x_{i},\frac{1}{N}\sum_{i=1}^{ N}y_{i}\right], \tag{4}\]  
where \(\hat{\mathbf{m}}\) is the estimated position of the mobile node and \(x_{i},y_{i}\) are respectively the \(x\) and \(y\) coordinates of the \(i\)-th anchor node. In the centroid method the accuracy of the position estimation is also dependent on the number of nodes. In general, proximity based methods cannot obtain highly accurate position estimations, since the obtained accuracy is in the order of the average distance between the anchors deployed in the building. However, the simplicity of these methods offers a good solution for room accuracy systems based on low complex wireless networks.  
Examples of proximity methods based on RFID can be found in [73; 74; 75] where the authors review the RFID based localization methods available in the literature.  
Proximity methods provide position estimation with room accuracy using low complex algorithms. The scalability of the system can be achieved by adding more nodes to the network in order to increase the area of coverage. Finally, the cost of the system is low if passive RFID tags are employed as a large number of tags can be deployed at a low price.