# Eco-Efficient Mobility in Smart City Scenarios
## 4 Proposed Solution
### Evaluation Tests  
Once we validated the functionality of our OBU prototype, we conducted a series of experimental tests in which we evaluated the performance of the two LPWAN technologies integrated into the unit. Concretely, we made some rides around the university campus ring to measure the Signal-to-Noise Ratio (SNR) obtained for both communication alternatives as well as additional network performance metrics such as Packet Delivery Ratio (PDR) and Round Trip Time (RTT). Figure 6 presents two heat-maps indicating the real-time position of the device and the quality of the connection in each point in terms of Signal-to-Noise Ratio (SNR) for the LoRaWAN and NB-IoT links. Regarding the LoRaWAN experiment, a decay in the link quality in the southwest area of the campus can be noticed, due to some buildings that blocked the signal from the base station. This is proven by low SNR values and some noticeable packet losses (long gaps between consecutive successful transmissions). As mentioned above, our unit just connected to the unique LoRaWAN gateway installed by us in our facilities. On the other hand, in the case of NB-IoT, we could not control the base-station that the device was connected to, as this is an internal and automatic process managed by the communication interface. At the time of performing these tests, our NB-IoT service provider (Vodafone) had some base stations active surrounding the university campus. Even so, some lacks of coverage are also evidenced in the north and south areas. We expect this situation to improve by deploying additional gateways/base stations in the surrounding areas. From a network performance perspective, Table 1 shows a comparison between both LPWAN communication technologies, attending to SNR, PDR, RTT, and power consumption measurements. It can be observed that, although the average SNR attained for LoRaWAN is better than that  
Figure 6: SNR obtained in one test ride around the campus ring. obtained for NB-IoT, the evaluated network metrics demonstrate a superior performance of the latter. Focusing on the PDR, the packet loss recovery mechanisms implemented by NB-IoT permits to retransmit lost packets, allowing it to reach almost 100% of PDR. On the other hand, LoRaWAN does not implement any similar loss recovery scheme; hence, lower PDR values are obtained, although it is close to a notable figure of 80%. The greatest difference between both technologies is evidenced in the measured RTTs. The shorter RTT achieved by NB-IoT is justified by its faster transmission data rate on the order of kilo bits per second (kbps), which is not heavily impacted by a longer network path through the operator' network. In turn, the conservative configuration selected in our experiments for LoRaWAN (Spreading Factor (SF) = 12) permits to establish more robust links at the expense of dramatically reducing the transmission rate down to just a few bits per second (bps). For this reason, the time-on-air taken by the transmitted packets (data payload of 50 bytes) is more than two seconds in each trip. Therefore, it can be concluded that, from a networking perspective, NB-IoT offers superior performance to LoRaWAN for this type of mobile setting. Observing the energy consumption of both interfaces, it can be seen that the instant current requirements of NB-IoT are higher than the ones of LoRaWAN (data extracted from the specification sheet of each modem). This fact is even more exacerbated given the operational mechanisms of each technology. While NB-IoT requires a regular synchronization of the end-device with the infrastructure by means of the exchange of multiple control messages, LoRaWAN does not need such mechanism, so the number of transmissions and receptions are notably lower in this case. Therefore, it is evidenced that the superior communication capabilities provided by NB-IoT are achieved at the expense of greater power consumption.