# A Low Power IoT Sensor Node Architecture for Waste Management Within Smart Cities Context
## 4 Node Architecture
### LoRa Transmission Module
Following these simple requirements, it is possible to determine the advantages and drawbacks of the three network categories listed before for the proposed scenario:  
* **Local Area Networks:** since the area to be covered is notably larger than the transmission range of these technologies, ad-hoc architectures should be studied. Using WiFi or BLE would require the set up of a large number of gateway nodes or the presence of a capillary WiFi infrastructure. This would notably affect the overall cost of the deployed infrastructure. Another option could be the deployment of a mesh ZigBee (or BLE mesh) network: mesh networks are not energy efficient since they allow a considerable reduction in power consumption of End Device nodes but not for Router nodes that have to be always switched on. This means that, even though their overall current absorption is on average quite low (\(\sim\)40 mA [63]), their lifetime when powered by batteries is however in the order of some days. * **Wide Area Networks:** LPWAN solutions are intrinsically characterized by low power consumption [64]: their larger transmission range allows to set up star or multi-star networks with few access points, permitting the sensor node to exploit duty-cycling policies that allow to turn off the node for prolonged periods. In the case of medium-sized cities a single gateway node could be sufficient for the whole network. Finally, the cost of LPWAN radio modules is extremely low consenting their use in large quantities without the additional costs as it would be in the case of SIM-based services. * **Global Area Networks:** this solution is the easiest one to be deployed since it does not require access points or the realization of an ad-hoc infrastructure. Unfortunately, it has many other critical aspects, the first of which is the cost: GPRS/UMTS modules, even the cheapest ones among these technologies, are still more expensive than other solutions. Moreover, they are SIM-based thus they require the payment of a subscription that, when planning to deploy thousands of nodes, becomes not negligible. In addiction to it, also the question of power consumption has to be taken into account: the average current absorption of UMTS is ten times greater than the one of ZigBee or LPWAN modules [65], making it not suitable for low power applications. All the previous considerations suggest that the best choice for the proposed scenario is represented by Wide Area Networking solutions, since they provide the best ratio in terms of transmission range (and then complexity of the network to be deployed), costs and power consumption (and then lifetime). Thence, the choice for the transmission module to employ fell on LoRa technology and LoRaWAN protocol: this solution allows the development of a fully customized network (unlike the SigFox technology which is based on a proprietary cloud platform), that is widely employed and then well-known in terms of performances and reliability. Moreover, a wide range of devices are already available on the market having low prices, under few euros, hovering around \(15\div 25\) euros. The radio module used for the development of the node prototype is a Libelium SX1272 LoRa module [66]: this device was chosen due to its ease in the interfacing with an Arduino UNO board (used for the first tests) through an ad-hoc Multiprotocol Radio Shield. Moreover, following a set of tests among similar platforms (e.g., [67]) performed in the historic centre of Siena, Italy, it proved to provide the best results in terms of transmission ranges, reaching 3 km in urban areas. Table 1 shows the main features of the radio module. Since the LoRa module is controlled by the microcontroller, then the Serial Peripheral Interface (SPI) bus is exploited in order to manage the communication between these two components. Therefore, these devices communicate synchronously in full-duplex in a master-slave fashion where the microcontroller acts as the master.