# Experiencing LoRa Network Establishment on a Smart Energy Campus Testbed
## 5 Experiments for LoRa MAC
### Impact of LBT Sensing Threshold on LoRa  
As discussed briefly in Sections 2.4 and 3, Korean regulations state that the LBT sensing threshold must be more than \(-65\) dBm. According to the experimental results in this paper and those from previous research, with a sensing threshold of \(-65\) dBm, the detection range becomes less than 50 m. This setting is highly likely to cause severe collisions, as a radio signal in LoRa reaches several kilometers in general. Low threshold values such as \(-180\) dBm, on the other hand, can cause a node to misinterpret noise as a normal signal, and thus, no data transmission is performed. This can result in long delays and/or low network use. To understand how the LBT sensing threshold affects LoRa MAC performance in detail, we configure the parameters as shown in Table 6 (Exp.C) and ran experiments. In the experiment, we fix the data period at 180 s and 60 s. The threshold value is increased from \(-130\) dBm to \(-65\) dBm with a 5 dBm interval. The other parameters remain the same as in the previous experiment. The experimental results with a data period of 180 s are shown in Figure 16(a). PDR performance does not worsen much as the threshold value increases. With 200 nodes, the PDR stays above 90%  
Figure 16: We split the results of PDR values for variable data periods based on SF values for each number of nodes. for all SF nodes over all threshold values. Even for SF 12, the PDR is 94% for a \(-\)65 dBm threshold, which is analyzed in the following way. Recall that \(-\)65 dBm indicates a detection range of around 50 m. Nodes with SF 12 are usually far from the gateway (1344 m at most), which implies that two nodes are likely to be more than 50 m away when there are few nodes. That is, nodes are hidden from each other, meaning that a node senses a channel is empty all the time and starts transmitting data immediately. However, there are only 22 nodes with SF 12 that send data packets every 180 s. This minimizes the possibility that two nodes send data simultaneously. Moreover, a signal at SF 12 is orthogonal to them at other SFs, avoiding another possibility for collision. These conditions result in high PDR performance. With 3000 nodes, on the other hand, the performance for SF 12 decreases from 62% to 45% as the threshold value increases from \(-\)130 dBm to \(-\)65 dBm. Recall that \(-\)130 dBm at SF 12 indicates a detection range of 1344 m and that nodes with SF 12 are deployed at a distance between 1197 m and 1344 m from the gateway. Even with \(-\)130 dBm, more than 50% of the nodes are still hidden from a given node. This leads to low performance. When the threshold value decreases to \(-\)65 dBm, neighboring nodes that were within the detection range are now out of range. This increases the possibility for collision, resulting in a 45% performance. Figure 17: _Cont._  
As the number of nodes increases, performance degrades. The higher the SF value, the lower the PDR. An interesting observation is made for the SF 7 nodes. With 3000 nodes, PDR performance for SF 7 nodes goes below that for SF 8 nodes when the threshold becomes higher than \(-115\) dBm. It performs even worse than that for SF 9 nodes after a threshold of \(-100\) dBm. This is mainly attributed to the huge number of SF 7 nodes; around 1,380 nodes at SF 7 are deployed while there are 250 nodes and 300 nodes at SF 8 and 9, respectively. As the threshold increases, the detection range decreases, which prevents an SF 7 node from detecting that neighboring nodes are transmitting. This results in packet collision because there are many SF 7 nodes nearby. With 1500 nodes, on the other hand, fewer nodes are deployed, and thus, this hidden terminal problem barely affects PDR performance. To highlight the impact of threshold values on MAC performance, the experiment presented in this subsection stresses the network more by reducing the data period from 180 to 60 s. The experimental results are shown in Figure 17b.