## 3 Methodology
### Time on Air Constraints of Communication Based on LoRa
The duty cycle indicates the fraction of time a resource is busy. LoRaWAN enforces a per-band duty cycle limitation. In Europe, duty cycles are regulated by Section 7.2.3 of the ETSI EN300.220 standard. Since every radio device must comply with the regulated duty cycle limits, this applies to sensor nodes and gateways. For example, ETSI regulations limit the duty cycle over one-hour intervals, whereas LoRaWAN enforces compliance with such limitations over the interval between transmitting a message and the next one. Moreover, on The Things Network's public community network, a Fair Use Policy applies, which limits the uplink air time to 30 s per day per node and the downlink messages to 10 messages per day per node. If private networks are used, these limits do not apply, but the governmental and LoRaWAN limits apply. To stay within these limits, the easiest way to do this is to calculate how much air time each packet consumes to choose a good transmit interval. For instance, as presented in Table 6, the calculator [48] is used for the chosen packet payload size (the maximum of which is 51 for low data rate (SF12), up to about 222 bytes for the best conditions (SF7)), the chosen packet header size (which is at least 13 bytes for the header for regular uplinks, or a total of 23 bytes for a Join Request), the chosen coding rate (from 4/5 to 4/8, where higher values mean more overhead), the chosen number of preamble symbols (which is 8 for all regions but can be different using plain LoRa),the chosen bandwidth (which is typically 125 kHz, but can be 250 kHz or 500 kHz), the chosen spreading factor (where \"higher\" means more range and better reception, but also more air time), and the chosen total air time to send the entire packet (in ms). Therefore, as presented in Table 6, the calculated values include the TTN Fair Access Policy-based number of average messages/day and average messages/hour when sending all day, as well as the duty-cycle-based time between the start of the next packet in the same sub-band. ETSI regulations limit the duty cycle to one-hour intervals so as not to crash its servers by generating excessive traffic, and not to directly improve energy savings. As can be seen from the conducted calculations presented in Table 6, by complying with the TTN Fair Access Policy, since more messages/hour can be sent for smaller packets, for a maximum packet size of 51 bytes, a total of 10 packets/hour cannot be exceeded. The control of the air time is achieved by adapting the data rate, i.e., by changing SF values. Also, the values are presented for the three chosen values of packet application payload sizes. As the presented values show, the decrease in the spreading factor results in a lower air time. However, a higher air time allows the receiver to demodulate the message better. Instead of defining a specific data rate, LoRaWAN specifies the data rate as a combination of a spreading factor and bandwidth [47; 78]. It can be seen that DR3 uses SF9, while DR2 uses SF10. Hence, with DR3, a double amount of data can be sent simultaneously, compared to DR2 [4]. Similarly, since the symbol time for DR3 is half the symbol time for DR2, there will also be half the energy per bit if the same transmit power is used. Likewise, this is also true for SNR [4].