## 3 The LOBIN Prototype: A Practical Study Case
### System Description
#### 3.1.2 Location Subsystem  
Some of the surveyed projects do not have tracking systems, either because the user location is always known or because it is not necessary. However, there are some other projects, including LOBIN, in which it is useful to be able to accurately locate patients and caregivers, since, by knowing their exact positions, the system can quickly lead the closers medical professionals to the desired locations, thus saving treatment time and avoiding major problems. Within these projects, there are some which perform outdoor location, such as MITHri LiveNet [17, 18], [PERSON] _et al._[26, 27], SmartVest [33], Personal Health Monitor [35], HeartToGo [40], and KNOWME [54]; whereas others use indoor localization, such as LOBIN [44], CodeBlue [25], and MASN [38]. In the case of outdoor localization, the use of GPS is unanimous, although there are others different methods. As for the indoor localization, much research has been carried out recently and many different solutions using different technologies, such as Assisted-GPS (AGPS) [55], 802.11 [56], Bluetooth [57, 58, 59], RFID [60, 61], Ultra Wide Band (UWB) [62], or IEEE 802.15.4/Zigbee [63, 64, 65], have been explored. The use of each technology implies some benefits and drawbacks that make it more or less suitable depending on the targeted scenario. In this case, the three aforementioned projects use IEEE 802.15.4, since it represents a good solution for sensor networks that run on limited battery power. While CodeBlue [25] and MASN [38] use an empirical localization scheme, the so-called NoteTrack, which matches the radio signature acquired by a roaming device with known locations by using a database that stores average radio signatures and know locations, LOBIN uses an indoor location algorithm based on Weighted Centroid Localization (WCL) and the LQI. The LQI measurement is a characterization of the strength and/or quality of a received packet [63, 64, 65]. The WCL algorithm [66] consists of computing a point comprised within the area covered by the BPs the target received beacons from, so that this point will be closer to those BPs from where higher LQIs are received. In order to do so, different weights (directly proportional to the received LQI) are assigned to every received beacon. This solution was chosen among the many different available ones since it represents a nice trade-off between accuracy and battery consumption. As it has already been mentioned, LOBIN Location Subsystem consists of two different devices: BPs and LWTBs. Figure 7 sketches how the Location Subsystem works. The BPs are deployed in well-known positions and are plugged into the electricity supply network. They send beacons periodically in one of the four channels scheduled for location purposes (namely, IEEE 802.15.4 channels 11, 12, 13, and 14) using a fixed and well-known transmission power. The LWTBs are carried by the users and are in sleep mode most of the time. Periodically (eventually, every 9 s, as explained in section Location Subsystem Tests), they wake up and listen to every single channel available for location purposes in order to record the LQI received from the different BPs. Note that the time that the LWTBs are listening to a given channel has to be at least twice the time between beacons from a given BP in order to avoid beacon losses. Once these data is collected, the LWTBs merge them into a single message (_i.e._, the location subsystem frame), add some necessary network layer information and send it to the Management Subsystem through the WCI Subsystem. The location subsystem frame consists of a set of pairs (BP identifier, LQI). This set is preceded by a field that identifies the number of transmitted pairs up to a maximum of 80, which is more than enough taking into account the areas to be covered by the system and the transmission range of the BPs. The location of the patients is computed at the Management Subsystem--using such pairs and the WCL algorithm--and then drawn onto a 2-D plan.