## 2 An Overview of the Cellular Networks
### Common Issues Related to the Management of the Networks  
Irrespective of the kind of network (GSM, UMTS, LTE) considered, there are some underlying issues that have to be taken into account for successful modeling.  
First, as previously remarked, reaching comprehensive knowledge of the network arrangement is of utmost importance to improve the localization performance of any cellular-network-based positioning system. Note how, in turn, this affects the effectiveness of any other task that exploits information regarding the positions of users, such as behavioral analysis, emergency responses, contact tracing, and logistics. Nevertheless, determining the state of the network arrangement is not trivial when relying on just crowdsourced measurements: often, the information sensed by the devices provides only a partial view of the environment--for instance, because they just aim to quickly discriminate between local cells. It is therefore, essential to make full use of the existing information, enhancing it through the already available network data and possibly exploiting related spatial knowledge.  
Another major challenge is the inherent temporal evolution of the cellular network layout. Indeed, a number of reconfigurations can happen over time. For instance, in [15], the authors considered a cellular fingerprint dataset composed of 785,000 GSM observations collected in 2 years over a wide area of 1,800,000 km\({}^{2}\), finding 198 instances of cells that had changed one or more of their identifiers. The phenomenon is named cell renaming--that is, a change, even a partial one, of the cell identifier (e.g., the LAC parameter for a group of cells). Observe that such an alteration has a temporal connotation, as it must hold that the first observation of a new cell takes place when the old one is not visible anymore. Moreover, the base station physically remains at the same place, having the same coverage, through the parameters of a cell change. Aside from cell renaming, base station relocations may be performed, where a cell gets assigned the same logical identifiers that were previously ascribed to a different one placed at another location. Such events are complex to model and deal with, as they are characterized by both a spatial and a temporal change. Moreover, renaming events concerning administrative areas (i.e., registration or routing areas) or the physical portions of the network (e.g., network controllers and base stations) are also to be hypothesized.  
It is clear why these phenomena are a problem for localization if not properly handled. In the case of renaming, it would be impossible to exploit the information of the old cell, as it would not be associated with the currently visible one. This can naturally lead to a substantial degradation of the positioning performance in the affected area, as a considerable amount of data would suddenly no longer be available. The phenomenon would be accentuated in areas with a low cell density, such as rural regions. In [15], it is shown how accounting for such network changes allows one to increase the average number of candidates available for each position estimation by 12.1%. In the case of relocation, the problem would be an inconsistency of the data, which would again result in a positioning error, potentially of high magnitude, being closely related to the spatial displacement affecting the cell. Thus, a system that is able to effectively and efficiently manage the information required to identify these phenomena has great potential in the domain. Finally, observe that the actual identification of the previously mentioned phenomena (which can be delegated to appropriate algorithms) is only one side of the matter; the system needs also to be capable of harmonizing and keeping track of the old and the new data, to guarantee the overall consistency of the information before and after the network modification events.