# Robust Downlink Mechanism for Industrial Internet of Things Using LoRaWAN Networks
## 3 Materials and Methods
### Interference in Industrial Scenarios  
Interference occurs due to adverse EM interactions that may disturb connectivity between devices and their respective GW. In LoRa, two types of interference sources can be found: LoRa signals, if devices use exactly the same set of transmission parameters (DR, BW, channel); and other signals. While in the ns3-simulator the former is already considered as a possible event during normal operation, external sources sharing the same frequency band and coexisting in the scenario were not implemented directly, but only as background channel noise. Figure 3: ADR mechanism overview for uplink as recommended by the standard. Channel noise is often modelled as additive white Gaussian noise (AWGN), a basic noise model used to reproduce the effect of random processes that occur in nature. The term white derives from the idea that it has uniform power across the frequency band for the information system (as the colour white has uniform emissions at all frequencies in the visible spectrum), and Gaussian because the noise signal PDF (probability density function) follows a Gaussian distribution. However, in real life, and especially in industrial scenarios, interference sources can appear due to the characteristics of some machinery or electronic devices present in the floor-plant, adding even more disturbing components to the received signal (as seen in Figure 5). Motors, conveyor belts, electricity generators, and other elements can produce high power RF signals that hinder communications and can saturate the channel. Therefore, for a LoRaWAN receiver listening for significant data (checking if the bytes received represent a preamble to keep listening for the full packet), this interference signal can mask partially or completely the real network packet transmitted by a LoRaWAN source (either GW or node). Even with the mechanisms provided by LoRaWAN to protect against this, if the interference signal is too strong the expected signal cannot be recovered, leading to packet losses. Figure 4: Proposed DROB mechanism overview for downlink. Figure 5: Signal received also contains noise and interference added in the channel. For this work, these interference sources have been added to the ns3 simulator modules by representing them as devices that can transmit \"packets\" of configurable length, power, and periodicity, but without information that the network devices can interpret as LoRaWAN data. This means that these sources are not limited by duty cycles, maximum power allowed or packet size limitations. Furthermore, interference sources can be located physically at specific points of the scenario, as they would be in real life, so the ubication and power of their signal varies its repercussion on the operation of the network deployed. Figure 6 shows the temporal evolution of the interference sources' output signal, with a fixed power of 28 dBm and 120 s of duration (t\({}_{\text{on}}\)), but the periodicity is randomized between 220 and 600 s (t\({}_{\text{off}}\)). This signal represents a strong and repetitive interference source meant to test saturation in the network simulated, in order to validate the implementation of the interference sources. With this signal configuration, the network is as expected unable to operate because the power and duration of interference makes receivers unable to recover valid packets. Once validated, the interferers are configured with signals that, according to consulted sources, can be expected to appear, with t\({}_{\text{on}}\) in the order of hundred milliseconds and with lower power levels. The choice of interference signals that can represent real sources is made assuming that these sources appear during shorter periods of time, often matching floor plant operation events, such as personnel shifts. This may occur for example two or three times per day.