## 3 Results & Observations
### Size of Payload & Transmission Interval  
Duty cycle limitations are not as important as we thought prior to our experiments. We perceived these regulations as a great obstacle for the network scalability, but on the other hand a lot of preceding factors affect this aspect of LPWAN deployments. Some experiments with 1000 EDs, distributed in every DR spectrum based on distance, reveal to us the spectrum limits in terms of data acquisition frequency. We have noticed that while 100% of 10 byte payloads are sent with a 200 s transmission interval, 89% were sent with a 100 s interval and 71% with a 60 s interval due to reaching the duty cycle limitation. The fact is that such transmission interval settings are, in one way or another, not viable for large scale deployments as we will argue with the following observations. For 5000 EDs we can achieve 0.96 PDR when we have small payload (10 bytes), medium transmission interval (1 h), in a range of 2 km with all the devices set to DR5 to avoid long ToA. With the same settings, but using DR3 devices instead, the PDR reduced to 0.88 and the higher chance of successful packet decoding due to the higher SF is not real. We can therefore determine that the importance of the ToA factor is higher than SF ability to decode in noisy environments, when we analyze collisions. It is worth mentioning that on a series of simulations where we have set a very short transmission interval (1 min), the PDR cannot climb higher than 0.11 when more than 2000 EDs exist on the network, and is zero for lower DRs, with even two GWs in some cases. Such low PDR in that case was caused due to GW channel unavailability. Because the distance is some km, the transmission gets wider in time so a huge number of messages arrive at the same time to the GW. Even a random transmission time within the range of 60 s, which is the time between two consecutive payloads, is not enough. The only way to have channel availability in such a small transmission interval is to use DR5 and deploy less than 2000 EDs, but again the collision problem due to ToA is huge (0.54 PDR). From the gateway perspective, based on our experiments, for less than 100 EDs, GW channels are enough in any case, but the GW may be unavailable for receiving packets when the transmission interval is less than 10 minutes or when the transmission retries are getting increased. When we reduce the EDs to 3000, with the previous settings, PDR jumps to 0.98, but reduces if we differentiate the payload size (0.97 for 30 bytes, 0.95 for 50 bytes). When we have a lot of EDs (3000) and high coverage is required, the only way to reduce the collisions with increased ToA (only  
Figure 5: DR allocation can further downgrade the capacity by decreasing transmission interval and increasing payload size. DR0 and 50 byte payload size) is to increase the transmission interval. In such a way we can achieve 0.95 and 0.97 PDR for 50 bytes and 10 bytes respectively. It needs to be noted that, for such rare in time transmission interval settings (>10 h), the PDR is 1.0 when the DR is 4 or 5--independently of the payload size--and drops to 0.99 when DR3 is used. Another way to decrease the collisions, when you have a lot of EDs in a great distance that need to be allocated in DR0, is to deploy more gateways. Our observation here is that for a network of 400 EDs with a 10 min interval between the transmissions of 20 byte payloads, the PDR is 0.74 when four GWs are deployed, while with one GW it is 0.56, but the deployment of more EDs along with more GWs is not a choice at all to scale the network. This observation reveals that if we want to deploy a medium-to-small capacity network (<500 EDs), with great coverage and kind of frequent data acquisition the only way is to deploy two GWs instead of one. This situation is not analogous for more EDs. In another experiment with 1000 EDs in DR3 we have noticed that if we send the same amount of data in one payload in a more rare interval it is better than sending every time data are available.