# Overcoming Limitations of LoRa Physical Layer in Image Transmission
## 3 Background
### The LoRa Physical Layer  
LoRa can commonly refer to two distinct layers: a physical layer using the CSS and a link layer. The LoRa physical layer consists of many parameters which can be configured into 6720 different settings [15] to offer a wide range of choices for ensuring a good quality link or consuming less energy. The most important parameters of the LoRa physical layer are the carrier frequency (CF), spreading factor (\(SF\)), bandwidth (\(BW\)), transmission power (TP), and coding rate (CR). These are discussed further in the proceeding paragraphs, and Table 1 below.  
The following are definitions for these parameters:  
* Carrier Frequency: CF represents the central transmission frequency used in a band. This can be programmed between 137 MHz to 1020 MHz, in steps of 61 Hz.
* Spreading factor: \(SF\) is the ratio between the symbol rate and the chip rate. It can be set between 6 and 12 in LoRa [16]. For each \(SF\), there are \(2^{SF}\) chips per symbol. \(SF\) has a more significant influence on energy consumption than increasing the transmission power, which increases the communication range. Hence, modifying the \(SF\) is more effective in reducing the energy consumption while maintaining the communication range. A larger \(SF\) increases the communication sensitivity and data bit rate and reduces the time on air (TOA).
* Bandwidth: \(BW\) is the range of frequencies available for transmission. A larger \(BW\) allows transmission at higher data rates but with a shorter TOA and lower sensitivity. In contrast, a lower \(BW\) enables a higher sensitivity but a lower data bit rate.
* Coding rate: FEC offers protection against interference and can be configured by setting the CR to 4/5, 4/6, 4/7, or 4/8. In LoRa, more robust protection from interference requires a higher CR, which increases the TOA.
* Transmission power: In LoRa, the power required to transmit a specific data packet can be adjusted as appropriate. The relationship between \(SF\) and the data bit rate in LoRa is defined as follows \[R_{b}=SF\times\frac{1}{[2^{SF}/BW]}\text{bits}/\text{s}\] (1)  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Setting** & **Values** & **Effects** \\ \hline Bandwidth & 125, \(\dots\), 500 kHz & A higher bandwidth is required for transmitting data at high rates (1 kHz = 1 kcps). However, increasing this parameter decreases the communication range and sensitivity. \\ \hline Spreading Factor & \(2^{6}\), \(\dots\), \(2^{12}\)\(\frac{\text{chips}}{\text{symbol}}\) & A higher spreading factor (\(SF\)) increases the communication range, radio sensitivity, and the signal-to-noise ratio (SNR). However, energy consumption consequently increases. \\ \hline Coding Rate & \(4/5\), \(\dots\), \(4/8\) & Bigger coding rates increase the protection against decoding errors and interference bursts at the expense of longer packets and higher power consumption. \\ \hline Transmission Power & \(-4\), \(\dots\), 20 dBm & The signal-to-noise ratio is increased by increasing the transmission power at the cost of energy expenditure. \\ \hline \hline \end{tabular}
\end{table}
Table 1: LoRa parameter settings and their effects on communication performance [2].  
where \(BW\) is the modulation bandwidth in Hz. From Equation (1), we note that spreading factor is directly proportional to the data rate [17].