## 1 Introduction  
The rapid diffusion of Internet of Things (IoT)-based facilities is leading to the definition of day-by-day more complex technological architectures, encompassing a large number of devices ranging from simple sensor nodes to complex edge computing equipment within single heterogeneous infrastructures. The different requirements of these elements have paved the way to the adoption of a plethora of different data transmission technologies, of which the technical features are designed to satisfy specific system requisites. In particular, they have driven the technological development of IoT data transmission techniques that implement crucial characteristics for a myriad of application scenarios: reduced power consumption, long transmission range and low cost. In the last five years, a number of technologies satisfying these requirements emerged, leading to the definition of the Low Power Wide Area Network (LPWAN) concept. Under the LPWAN umbrella, well-known technologies such as SigFox, Long Term Evolution for Machines (LTE-M), Long Range Wide Area Network (LoRaWAN) and Narrow Band IoT (NB-IoT) lie at the moment, for which the specific features can lead to the choice of any of them according to the needs of the application scenario. Nevertheless, the different features of these technologies may also be simultaneously exploited to create more complex network architectures, with the aim of covering specific operational gaps that may emerge when shaping network infrastructures to specific application contexts. One example concerns the usability of LoRaWAN networks in remote places,where some kind of network connection is required for packet forwarding from LoRaWAN Gateways to the Network Server. This is in general provided by means of standard cellular connectivity, relying on power-consuming radio modules, thus requiring some kind of continuous power source (e.g., energy harvesting systems or mains). Other examples include coexistence within the same network of standalone nodes and sub-clusters: in this case, different technologies may be combined according to the specific needs. For example, standalone SigFox nodes may be enabled to exchange data with LoRaWAN Nodes that are integrated into LoRaWAN sub-networks. With the aim of demonstrating the feasibility of this typology of architectures, this paper focuses on two specific technologies, belonging to the two macro-areas in which LPWANs are usually distinguished (i.e., the cellular and the non-cellular ones). Among cellular LPWANs, NB-IoT [1,2] is the one that has seen the largest diffusion and has been widely employed in several application scenarios [3, 4, 5]. The non-cellular chosen one is LoRaWAN, which can be actually considered as de facto standard among non-licensed LPWANs operating in the sub-GHz band (i.e., at frequencies below 1 GHz). Since LoRaWAN demonstrated excellent features in terms of transmission distance and low power consumption [6, 7, 8], it was applied in a plethora of different use-cases [9, 10, 11]. Despite their diffusion, these two technologies were integrated within the same infrastructure, though only in very limited cases. This paper attempts to fill this gap by demonstrating the feasibility of a hybrid LoRaWAN-NB-IoT network, pointing out the achievable improvements and the different implementable services. In particular, the usage of NB-IoT as a link between LoRaWAN Gateways and the Cloud is experienced: this solution is relevant in all applications concerning a limited number of LoRaWAN End Devices, each of them transmitting a reduced amount of data. In this case, the throughput of the NB-IoT link can be considered adequate to ensure packet forwarding, with a notable reduction in power consumption with respect to other standard cellular connections.