## 6 Results  
### Power Consumption of End User Devices  
Figures 7-9 show power consumption results with the defined power consumption models from the end user perspective. All used WiFi and LTE models are applicable to out-of-band D2D communication scenarios. In addition, the LTE models are applicable to in-band overlay D2D where D2D links use dedicated resources. The power consumption of the total D2D network, as well as the power consumption of the cluster head of a network in each case, is given in Figure 7 for a cluster size of \(N=4\) nodes using the Huang model [19] for the LTE and WiFi interfaces. It is seen that with the low throughput values it is best that only the cluster head actively receives the data from the LTE base station. Then it uses WiFi for relaying the data to requesting users. However, it can be seen that from the cluster head perspective this is the second most power consuming option and thus there might be a need to change the cluster head from time to time in order to prevent it draining the battery completely. When the higher throughput \(>\)6 Mbps is required, the most power efficient option from the end user perspective is to receive all the data directly from the base station.  
Figure 6: Resource allocation from the BS perspective assuming two mobile devices. (**A**) Case 1, (**B**) Cases 2 and 3, and (**C**) Cases 4 and 5.  
Figure 8: Results with the Lauridsen LTE and the 802.11n WiFi; Power consumption of a whole network (**left**) and the cluster leader (**right**).  
Figure 7: Power consumption with the Huang LTE and WiFi models, 4 nodes; whole network (**left**) and the cluster leader (**right**).  
Figure 9: Results with the Lauridsen LTE and the 802.11 ac (**left**) and 802.11 ad (**right**) WiFi, whole network considered.  
When the Lauridsen model is adopted for LTE and 802.11n for WiFi, the observations are a bit different, as seen in Figure 8. We have assumed \(S_{\rm rx}=-50\) dBm and \(S_{\rm rx}=10\) dBm for a D2D LTE link. The total power consumption in case 2 with a higher number of nodes is even more advantageous due to the lower power consumption of the WiFi. Case 4 demands the active operation of both LTE and WiFi interfaces. This is not good from the power consumption point of view due to the static part of the power consumption that comes from keeping the air interface active, i.e., \(\beta\) in Equations (5) and (6). Thus, the latest power consumption models propose that dividing the data into smaller streams and changing the missing packets over the air is not efficient due to the simultaneous use of several active interfaces. WiFi relaying is a good option up to 20 Mbps data rate. However, also in this case, one has to take care that the cluster head is changed from time to time in a mobile network to keep all the nodes alive for longer periods of time.  
The situation is quite similar when the 802.11 ac and 802.11 ad WiFi models are adopted as seen in Figure 9. The results cover the whole network and show that with the latest radios, where the power consumption is static regardless of the data rate, the best option is to use LTE alone. Either the conventional cellular operation or relaying with LTE are the best choices. This is due to the high power consumption of WiFi models with any data rate. An active WiFi interface consumes a lot of power. WiFi could be used to enhance the data rate of the devices if very high data rate services were needed.  
The used models are applicable both to LTE and LTE-advanced systems. Only some parameter updates are needed e.g., regarding the power model given in the Equation (1). e.g., release 12 equipment in our lab uses the old HW and only the SW is updated in the base station compared to the older releases. Power consumption is affected by the software as well, but the LTE-A base station power consumption can be described with the same model due to the slow evolution of the devices.