## 3 System Model and D2D Use Cases for Combined LTE/5G and WiFi  
Figure 4 presents our high-level system model for D2D communications in a 5G network. There are many types of users that are connected to the base stations using cellular interfaces. Nodes can also communicate directly using D2D communication links between nodes that are in proximity to each other. Direct links between user devices such as phones and laptops may use several RATs, including 3 GPP evolution, as described in Section 2, Bluetooth, or WiFi standards. Cars also use a dedicated 802.11p standard in the intelligent transport system (ITS) band in 5.9 GHz for V2X communications. In the future, autonomous and remote-controlled ships will also use more and more ship-to-ship communications, possibly also radios specifically developed for these purposes. Both in the V2X communications among cars and in maritime communications, integrated 5G satellite-terrestrial systems will be needed [8; 9]. The system has a connection to the Internet and the connectivity provider to make all the required services available to the end users. The 5G core supports seamless cooperation between different RATs and the terrestrial and satellite segments. It also enables QoS management of data transmission e.g., by dedicating part of the resources to applications with higher priority. There could even be end-to-end network slices dedicated to autonomous driving and other use cases so that QoS requirements can be met in any circumstances via proper resource allocation and isolation mechanisms. Network  
Figure 4: High-level system model for D2D communications in 5G; BS: Base station. virtualization and slicing techniques enable different operators to share network resources with other (virtual) operators and to provide end-to-end connectivity across operator boundaries. In addition to network management with the core network, the 5G networks will also use spectrum sharing technologies to utilize available radio resources as efficiently as possible. We assume a licensed spectrum access (LSA) approach, where the incumbent operators are required to provide a priori information about their spectrum use over the area of interest to the database. They tell explicitly where, when, and which parts of the frequency bands are available for the secondary use. This most probably requires a third party to operate the LSA system, since operators are often not willing to share the information about their spectrum use with other spectrum users. Let us now look at the simplified model for the analysis that is presented in Figure 5. The model is based on the high-level system model described above. Wireless mobile users are connected to the base station using the LTE interface. There are \(N\) nodes in the network. We assume that links L\({}_{12}\) (between Node 1 and Node 2), L\({}_{13}\), L\({}_{23}\), L\({}_{3n}\) can be either LTE or WiFi links. Only user equipment such as phones, tablets, and laptops are used as nodes in the network. Link attenuations between the base station and the user equipment are assumed to be equal, as well as the direct links between nodes. All the links between the user equipment and the base station are using 3 GPP interfaces. D2D communication is controlled by the base station, which enables interference management and assures QoS to the end users. Nodes can form a cluster around the cluster head which may be the only node discussing with the base station. In order to estimate the power consumption in the depicted system model both from the user device and the base station perspectives, we need to define practical use cases for analysis. Based on Figure 5, we can define several different use cases for delivering Internet data or some other data from the content provider that certain node(s) want to access through the base station. Five different cases are described in the following as [30; 31]:  
1. **Case 1**: The base station sends the data directly to the requesting node(s). 2. **Case 2**: Nodes with social ties form a cluster.