# Feasibility Analysis of a LoRa-Based WSN Using Public Transport
## 4 Network Architecture
Similarly, the end-nodes-gateways communication can be analyzed for fault detection. Apart from leading to a higher level of redundancy without consistently improving the performance and robustness of the network, this implementation may solve another major problem. If, by chance, no gateway happens to be in proximity of the end-node at the time of the transmission, the information would not be lost; in fact, in the current implementation, the end-node repeatedly sends its request-for-transmission packet until a gateway replies. Figure 3: A representation of the proposed multilevel architecture in its correct operation. A direct consequence of the network’s modus operandi is the redundancy highlighted at the supergateway level. Concerning the power consumption, since the gateways need to be always listening for incoming messages, they must always be on. According to [33], their mean supply current in receiver mode is about 11 mA. To save power, different solutions may be adopted, according to different installation scenarios. Among them, it is possible to highlight the following two strategies:  
* To save power during transmission, it is possible to send a lower number of packets by adding two or more messages coming from nodes in a single packet. It depends on the size of the received packets. In this case, the time-on-air of a single packet will be higher but the number of transmissions will be lower, hence saving power. * If gateways are located at bus stops, the power system could be improved by including small photovoltaic solar panels to generate electricity and charge the battery powering the gateway. Another alternative could be activating the gateways only during specific periods of time: however, this strategy is not recommended since there will be a high probability of not hearing the incoming packets (it depends on the network deployment, the geographic distribution of the gateways, the number of end-nodes transmitting, and how often the end-nodes are transmitting). It may happen that if any of the gateways receive the broadcast message, the end-nodes will keep sending messages looking for another gateway, which means that the power required at the first mode will be higher. It is also to be noted that the network proposed in this feasibility analysis is based on LoRa technology. The applicable restrictions regarding the time-on-air depend only on the frequency sub-band in use. For the frequency in use (868-868.6 MHz), the regulatory regulations for Europe are given by the European Telecommunications Standards Institute ETSI under the ETSI EN 300 220-2 Standard [41] and ERC Recommendation 70-03 as follows [42] as reported in Table 2. Based on that information, time-on-air restriction policies will be applied to comply with the regulations. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Frequency** & **Max Radiated** & **Spectrum Access and Mitigation** & **Notes** \\
**Band (MHz)** & **Power (mW)** & **Requirements** & **Notes** \\ \hline \hline \multirow{2}{*}{868–868.6} & \multirow{2}{*}{25} & \(\leq\)1\% duty cycle or Lister Before Talk (LBT) & Narrowband/Wideband \\  & & + Automatic Frequency Agility (AFA) & modulation \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Duty-cycle restrictions according to international regulations. Figure 4: A hypothetical case of failure of a gateway, which results nonetheless in correct delivery of the packet and fault detection by the supergateway.