# Experimental Evaluation of the Packet Reception Performance of LoRa
## 5 Results and Analysis
### The Impact of the Packet Length on the Packet Reception Performance of LoRa
#### 5.4.2 The Result of the Impact of the Packet Length on the Packet Reception Performance of LoRa  
We conducted extensive experiments based on the previous experimental settings, and the packet length was 4, 32, 128, and 255 bytes. There was enough time between the two data packets for the receiver to fully receive and decode the data. The conclusions we obtained on the impact of SF and BW on the packet reception performance of LoRa are still valid. Our current focus is mainly on the impact of the packet length on the packet reception of LoRa. Figure 9 shows the results of the Packet Error Ratio for different packet lengths. We provide detailed experimental results, including the performance evaluation of different packet lengths under different SF and BW conditions. We used the experimental results obtained to verify the theoretical derivation previously proposed and the results show that the theory is correct. Therefore, we can use the obtained value to derive the Packet Error Ratio of other packet lengths and optimize the transmission scheme. For example, when SF = 7, BW = 125 kHz, CR = 4/8, and SNR = -9 dB, the Packet Error Ratio of 4 bytes is approximately equal to 0.4826, and the Packet Error Ratio of 128 bytes is approximately equal to 0.6871. We can substitute these two results into Equations (9) and (10), and the results of \(\lambda\) and c are approximately equal to 0.0041 and 0.5288, respectively. Finally, the Packet Error Ratio of 128 bytes can be substituted into Equation (10) to figure out the Packet Error Ratio of 32 bytes. The result is approximately equal to 0.5362, and our actual measurement result is approximately equal to 0.5380. The experimental results are very close to the theoretical results, with a difference of only 0.0018. The experimental results fully demonstrate the validity of the theory. We used similar methods and different experimental results to conduct further verification, and finally drew the conclusion that the derivation of the equation is correct. Therefore, based on this theory, we can calculate the Packet Error Ratio of different packet lengths and measure whether the node needs to change the packet length.  
Figure 9: _Cont._We also obtained other meaningful findings. On the one hand, longer data packets can better offset the overhead associated with the header, resulting in a higher number of correctly received and decoded data packets using long packets within the considered payload length range. On the other hand, increasing the length of data packets means a lower immunity to interference. Although long data packets have a longer time-on-air, the time is mainly consumed by the encoded data, so it does not make it easier to be received by the receiver. In addition, it is worth noting that in areas where the communication link is unreliable (PER \(>\) 0.2), the performance difference of different packet lengths is obvious, while in other areas (PER \(\approx\) 0), the Packet Error Ratio of different packet lengths is similar. As shown in Figure 9b, when SF = 8 and SNR = \(-\)12 dB, the PER of 255 bytes is 0.7652, and the PER of 4 bytes is 0.4521. However, when SNR = \(-\)8 dB, the PER of 255 and 4 bytes is 0.0047 and 0.0021, respectively. In SF10 and BW500 kHz, similar conclusions to SF10 and BW125 kHz can be obtained, as shown in Figure 9e,f. Therefore, BW does not interfere with the trend of the impact of the packet length on the reliability of LoRa. As shown in Figure 9a-h, SF does not interfere with the trend of the impact of the packet length on the reliability of LoRa.