# On the Design of a Network Digital Twin for the Radio Access Network in 5G and Beyond
## 2 NDT Architecture for the RAN
### Service Mapping Models
#### 2.2.2 Functional Models  
The functional models cover network analysis, emulation, diagnosis, prediction and assurance. In Figure 1, three subgroups of functional models are proposed.  
The first subgroup is the _emulation dynamics_, which refers to those models that allow controlling the RAN environment dynamics in the NDT. The proposed models in this subgroup are the following:  
* _Traffic generation_ model: allows emulating a realistic behaviour of the traffic demand in the real RAN at different levels (e.g., packet, user, service, network slice or system levels), accounting for the temporal and spatial distributions in the considered RAN area. The traffic generation can be either based on available models in the literature (e.g., session generations according to a Poisson distribution and session durations modelled by an exponential distribution) or on geo-localized traces and measurement reports generated by UEs during their connection to the network. The traffic generation model interacts with the UEs model in Figure 1 to generate new UE sessions.
* _Mobility_ model: emulates how the users move in the RAN area, characterizing their trajectories and speed. Different models can be defined for the existing user types in the RAN area (e.g., pedestrian, vehicular, static, drones, etc.). This model interacts with the UEs model to modify the UE's positions and takes into account the scenario topology model for users' trajectories (e.g., pedestrian users usually walk on the pavement while vehicles drive on the road).
* _Propagation_ model: allows generating different radio propagation conditions, capturing effects such as path loss, attenuations due to diffraction and rain, shadowing or fast fading. The outcomes of the propagation model are provided to the channel model in Figure 1.  
The second subgroup is the _RAN management & optimization_. The models belonging to this subgroup are management and optimization models that are applied in the real RAN and, thus, impact the network behaviour. This includes _RRM policies_ (e.g., admission control, resource allocation, packet scheduling, etc.), _optimization functions at network level_ (e.g., interference coordination, handovers) and _SON functions_ (e.g., capacity and coverage optimization, mobility load balancing, cross-slice capacity optimization, etc.). These functions tune and control the different basic models.  
The last group of functional models is the _exploitation_, embracing models for exploiting the outputs of the rest of the _service mapping models_. The proposed models in this group are the _monitoring_ model, which allows obtaining detailed information of the functional and basic models in the RAN NDT, the _Key Performance Indicator (KPI) reporting_ model, which generates reports with KPI metrics of interest (e.g., resources utilization, throughput, QoS satisfaction, etc.), the _analysis_ model, for processing the obtained KPIs to assess the performance of the different NDT models, and the _visualization_ model, which can provide different kind of graphs such as the evolution of different KPIs, coverage maps of the different gNBs included in the RAN NDT, etc.