# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture
### LoRa Transmission Module  
As anticipated in the previous section, the data transmission module plays a significant role in power consumption optimization, therefore it has to be carefully chosen. Since replicas of the proposed IoT node are expected to be deployed outdoor, in large quantities and across large areas, only wireless solutions will be taken into account: indeed, it is obviously unthinkable to have a wired connection (e.g., Ethernet) at each trash bin. First of all, wireless connections can be subdivided according to their data transmission ranges. In particular, for IoT communication infrastructures, they can be summarized in three categories:  
* **Local Area Networks:** based on different technologies and protocols, these networks have limited transmission ranges, usually lower than 100 m: this means that the data collection centre has to be located in the same area of the nodes. Alternatively, a sort of gateway node is required, which is in charge of receiving data from the nodes and forwarding them to a remote data collection centre through the Internet. The most common technologies belonging to this category are Bluetooth Low Energy (BLE), ZigBee and WiFi. * **Wide Area Networks:** this category encompasses the emerging LPWAN Sub-GHz technologies like LoRa and SigFox, that provide transmission ranges that can reach 20 km in line-of-sight. In urban areas these technologies are able to transmit data even at a 3 km distance, representing the ideal solution for Smart City scenarios. Indeed, no middle layer is required between the nodes and the data collection centre: a single gateway can be able to collect data from large quantities of nodes distributed in wide areas: tests on transmission ranges proved that a single gateway could be able to cover a medium-sized city centre [62]. However, for larger areas more gateway nodes could be set up with a limited impact on costs and complexity of the network. * **Global Area Networks:** they allow the transmission of data anytime and anywhere on the Internet as long as there is network coverage. This category encompasses all the cellular networks as well as more complex technologies like satellite communications. 2G (GSM, GPRS) and 3G (UMTS) radio modules are widely used for IoT nodes as well as 4G (LTE)-based solutions. Among the latter, the most promising one is the Narrow Band IoT (NB-IoT). All the modules available on the market are currently quite expensive and relatively power-hungry. However, with the emergence of the upcoming 5G technologies, new scenarios are expected to open for IoT architectures, favored by the promise of low costs and reduced energy consumption. Nonetheless, being SIM-based systems, all these technologies are characterized by fixed service costs that may be significant in case of IoT infrastructures constituted by large quantities of nodes. Figure 2: Circuit of the IoT sensor node. Since all these technologies represent good alternatives for the realization of IoT nodes, the choice for the best one has to be made according to the specifications of the deployment scenario that has to be carefully studied. Regarding the one proposed in this paper, i.e., a Smart City solution for the real time monitoring of filling level of trash bins, the following assumptions can be made:  
* The nodes are expected to be deployed in large quantities since even in a medium-sized city the number of trash bins amounts to some hundreds. Therefore, their cost has to be kept as low as possible and, at the same time, the maintenance operations have to be minimized;
* The area to be covered is large, some tens of km\({}^{2}\);
* The sampling rate can be kept very low. One sample per hour may be sufficient for the purposes of the whole monitoring infrastructure.