## 3 Cellular Network Modeling
### The Overall Schema  
We have now completed the illustration of all the components of the overall ER schema of the cellular network shown in Figure 6. The schema introduces several (total and disjoint) specializations that allow, for each entity, the modeling of the properties (attributes and relationships) that are specific to each type of network. As for the cell specialization, since we consider administrative identifiers for the cells, the partial key can be either _ci_ (in the case of GSM or UMTS technologies) or _eci_ (for LTE). In the schema, they are represented using the single attribute _eci/ci_.  
The schema also includes the \"POSITIONING SYSTEM\" subschema, which represents the module in charge of dealing with the location of a device in space. In this work, we consider a fingerprint-based positioning system that makes use of cellular networks.  
In the schema, a device (identified by its _id_) sends to the system several observations at different time instants. These observations can be associated with a GNSS position (_GNSSfix_) or not. If the GNSS position is not available, an estimation (_GNSSestimation_) may be derived using suitable positioning algorithms (dependently on the specific system and  
Figure 5: Conceptual schema for LTE networks.  
cellular technology; see [3] for fingerprinting techniques, especially signal-based ones). Observations sent to the system consist of data about one or more observed cell. One is the serving cell, to which the device is directly connected. The others are the remaining neighboring cells detected by the device. Each cell is received with a _signal_strength_ that can have a different meaning according to the type of network under consideration: receiving level (RXLEV) for GSM, received signal code power (RSCP) for UMTS, and reference signal received power (RSRP) for LTE. Furthermore, for GSM and LTE serving cells, the timing advance (TA) could be detected, which measures the time a signal takes to reach the base station of the serving cell from the device (attribute _tu_). Of course, observations may in principle be extended with additional attributes encoding information such as the speed or the direction of a device, and the object that is being tracked (e.g., a person or an animal).  
As mentioned in Section 3.1, it still remains to be explained how the _coverage_ of a cell is calculated. This is done by aggregating the positions (attribute _GNSS_fix_) of the observations that have perceived the cell, either as serving or neighboring ones (e.g., calculating the convex hull).  
The derived attribute _position_, which is part of the network controller and base station, can be inferred using the observations as well (there exist ad hoc algorithms; see, for  
Figure 6: Complete cellular network schema, integrated with the part related to the positioning system and equipped with temporal support. T denotes entities that feature transaction time, whereas LS denotes instances that feature valid time.  
instance, [35]). Moreover, from the position of the BASE STATIONS, one can then derive that of the linked network controller. Finally, the calculation of the remaining derived spatial attributes is trivial, since they can be obtained by aggregation of other spatial attributes.