# Collision Avoidance Adaptive Data Rate Algorithm for LoRaWAN
## 2 Technological Background
### Interference Susceptibility of LoRaWAN  
Asymmetric interference occurs between wireless sensor networks, including LoRaWAN networks and other wireless protocols. LoRaWAN networks, in particular, may encounter different kinds of interference that could drastically affect network performance.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Spreading Factor** & **Required SNR [dB]** & **Sensitivity [dBm]** \\ \hline
7 & \(-\)7.5 & \(-\)123 \\
8 & \(-\)10 & \(-\)126 \\
9 & \(-\)12.5 & \(-\)129 \\
10 & \(-\)15 & \(-\)132 \\
11 & \(-\)17.5 & \(-\)134.5 \\
12 & \(-\)20 & \(-\)137 \\ \hline \hline \end{tabular}
\end{table}
Table 1: SNR and sensitivity thresholds per spreading factor.  
Wideband interference can be caused by other wireless devices operating on similar frequencies, such as microwave ovens, cordless phones, electrical noise from power lines, and even natural sources such as lightning strikes. Narrowband interference is generally the result of the presence of signals that occupy a small frequency band, typically a few kilohertz or less, within the frequency band like Bluetooth.  
Within the LoRaWAN network itself, there exist forms of interference known as co-spreading factor (co-SF) as well as inter-spreading factor (inter-SF) interference [19; 24]. Co-SF interference develops when two LoRaWAN end nodes use identical spreading factors and transmission occurs concurrently and on the same frequency. This can result in collisions and lost packets, leading to reduced network performance unless one of the signals is 6 dB stronger than the other, called the capture effect expressed in (2). Given two colliding EDs, ED\({}_{i}\) and ED\({}_{j}\), using the same SF, with average received power \(P_{rx_{i}}\) and \(P_{rx_{j}}\), respectively,  
\[C_{ij}^{*}=\begin{cases}1\,,&\text{if}\quad P_{rx_{i}}-P_{rx_{j}}\leq 6,\\ 0\,,&\text{otherwise}\,.\end{cases} \tag{2}\]  
Inter-SF interference, on the other hand, occurs when two LoRaWAN devices transmit simultaneously using different SFs on the same frequency. This can result in the deterioration of the signal-to-noise ratio (SNR) and a rise in the bit error rate (BER) of the received signal, resulting in increased energy consumption. Demodulation of the transmitted packets occurs if the received power differential exceeds their signal-to-interference-plus-noise ratio (SINR). According to the findings outlined in [28], the demodulation of signals characterised by co-SF interference presents a greater challenge compared to inter-SF interference. Due to the rapid growth in the IoTs, especially in urban areas, interference becomes a major concern that requires immediate solutions as the sensors are battery operated and their lifetime is limited and, therefore, needs to be extended. The quasi-orthogonality of the spreading factors means that they have minimal interference with each other. This is because LoRaWAN operates in unlicensed frequency bands, where multiple devices can transmit data simultaneously, and interference can occur between devices using the same or different spreading factors. Table 2 shows the interference thresholds per spreading factor. Req is the required SF and Int is the interfering SF. These SIR thresholds are empirical values.