# Polling Mechanisms for Industrial IoT Applications in Long-Range Wide-Area Networks
## 3 Materials and Methods
### Technology Overview and Operation in LoRaWAN  
At the physical layer, the radio transceivers of the end devices use Semtech's long-range (LoRa) technology [13]. With this, wireless low-power transmitters can forward small packets of data to a receiver over long distances up to several kilometers, depending on the environment, in a point-to-point link. LoRa modulation is based on CSS (Chirp Spread Spectrum), using linear frequency modulation chirp pulses with high bandwidth to encode information. These chirp pulses are sinusoidal signals varying in frequency over time, which determines the symbols that represent the information. The spreading factor (SF) determines the number of bits encoded in each symbol, with a 2 SF relationship. Valid SF values range from 7 to 12. Equation (1) shows how to calculate the symbol duration (Ts) in seconds based on SF and bandwidth (BW). \[\mathrm{T_{s}}=\frac{2^{\mathrm{SF}}}{\mathrm{BW}} \tag{1}\]  
According to Equation (1), increasing SF reduces the bit rate and extends the packet's time on air (ToA). This also increases power consumption because the radio interface stays active longer for data transmission. However, higher SF values enhance coverage range due to the increased ToA, making them more resistant to noise [14]. LoRaWAN, an open protocol defined by LoRa Alliance, operates on top of LoRa technology. A central network server coordinates all network devices (end nodes and gateways), including selecting the optimal gateway for each node. As illustrated in Figure 1, the LoRaWAN architecture utilizes application servers to forward information to other systems and networks using various application protocols. Figure 1: LoRaWAN architecture. The latest version of the protocol released by LoRa Alliance is version 1.1 [15]. The gateways are connected to the network server through a conventional TCP/IP SSL network, while the end devices use LoRa to communicate with one or more gateways. Another matter to keep in mind when sending and receiving messages in a LoRaWAN is to comply with spectrum regulations. The duty cycle (DC), which is the percentage of time a device is using or occupying the channel, is regulated in Europe, as seen in Table 1 or in detail in [16]. In the described work, the LoRaWAN Regional Parameters 1.0.2 Rev B is used to define duty-cycled limited transmissions to comply with the European Telecommunications Standards Institute (ETSI) regulations. In ETSI, most bands use a maximum duty cycle of 1%, but some use 0.1% and 10%. For any network, SF11 and SF12 are only allowed when the ADR (Adaptive Data Rate) mechanism is enabled. It is important to highlight that increasing the spreading factor (SF) by one almost doubles the ToA (for the same BW), which also directly impacts the duty cycle waiting time, as it is proportional to ToA. For example, a single transmission on SF10 takes more time than six transmissions on SF7. This behavior also influences the maximum payload size for each SF. The default transmission configuration recommended for the downlink is 869.525 MHz (G3 sub-band offering the highest DC) with SF9 and 125 KHz bandwidth [16]. Nevertheless, the default channel can be modified by gateways using MAC commands in accordance with the LoRaWAN Regional Parameters. Regarding the MAC access layer, there are three classes of LoRaWAN nodes (Figure 2). 1. Class A: This is the most basic class of the LoRaWAN node. It can only receive downlinks when it is actively sending an uplink, so it is not suitable for polling \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **Band (MHz)** & **Limitations** \\ \hline G & 8630–8680 MHz & EIRP \(<\) 25 mW—duty cycle \(<\) 1\% \\ G1 & 8680–8686 MHz & EIRP \(<\) 25 mW—duty cycle \(<\) 1\% \\ G2 & 8687–8692 MHz & EIRP \(<\) 25 mW—duty cycle \(<\) 0.1\% \\ G3 & 8694–86965 MHz & EIRP \(<\) 500 mW—duty cycle \(<\) 10\% \\ G4 & 8697–8700 MHz & EIRP \(<\) 25 mW—duty cycle \(<\) 1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Duty cycle restrictions on EU bands.