# LoRa Architecture for V2X Communication: An Experimental Evaluation with Vehicles on the Move
## 6 Outdoor Testing and Analysis of the Prototypes  
### Preliminary Handoff Testing with and without Kalman Filter  
The spreading factor (SF) of LoRa plays an important role in any network deployment as the data rate, the transmission distance, time-on-air (latency) and energy consumption depend on it [72]. The RFMHCW module is used in both the OBU and RSU and they have the spreading factor from SF7-SF12. Increasing the SF would increase the transmission range significantly but that would also affect the data rate and latency. V2X has time-critical applications where higher latency can hamper the performances drastically. Moreover, higher data rates would also support the architecture to transmit more data which would be vital for applications like traffic flow optimization and optimized traffic light control [73]. Table 3 shows the SFs and the corresponding parameters of the LoRa communication for a data packet of 15 bytes over 125 kHz bandwidth. SF7 has the highest throughput of 5.47 kbps whereas with SF12 the data rate is 0.29 kbps. The time-on-air is only 36 ms (25-byte packet) with SF7 in comparison to the 682 ms by SF12, but at SF7 the signal-to-noise ratio (SNR) is limited to \(-6\) compared to \(-20\) at SF12. Considering these parameters, SF7 is chosen for the proposed V2X architecture to facilitate the least time-on-air and highest possible data rate with LoRa. All the field tests, experiments, and evaluations are done with SF7 at 125 kHz. Outdoor testing of the prototypes is conducted with actual cars on the road, by integrating the OBU at the car roof and connecting RSUs with computers for data analysis. Figure 8 presents the part of the testing set up with OBU on the car roof and testing the car on the track. One of the important challenges of the proposed architecture is the handoff from one RSU to another in V2I. To evaluate the handoff, the architecture is tested with multiple RSUs in a university environment. As the earlier tests suggest, RFM69 HCW can communicate consistently up to 160 m in a mobile environment with regular traffic on the road. Distances between two consecutive RSUs are  
Figure 8: Testing setup for outdoor experiments. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**SF** & **Data Rate (kbps)** & **Time-on-Air (ms)** & **Receiver Sensitivity (dBm)** & **SNR (dB)** \\ \hline
7 & 5.47 & 36 & \(-\)123 & \(-\)6 \\
8 & 3.13 & 64 & \(-\)126 & \(-\)9 \\
9 & 1.76 & 113 & \(-\)129 & \(-\)12 \\
10 & 0.98 & 204 & \(-\)132 & \(-\)15 \\
11 & 0.54 & 365 & \(-\)134.5 & \(-\)17.5 \\
12 & 0.29 & 682 maintained to 320 m allowing each RSU to have a radius of 160 m. Two RSUs are placed at a distance of 320 m from each other as in Figure 9. The architecture is primarily tested with simple 'Hello' message, thus the data packet is consisting of only 16 bytes in comparison to generated vehicle data of 40 bytes. Moreover, there will be no data generation time as the OBU is sending only the 'Hello' message to OBU which means the OBU is no needed to collect and parse the data from the sensors to send each packet data. It reduces the burden on the processing unit and sends each data packet with an interval of only (1.67 s). According to Algorithm 1, when a car goes along the road it should communicate with the RSU depending on the best link quality and should switch from that RSU to another when the link quality degrades and reaches a defined threshold value. Thus, if two RSUs are placed at an equal interval and a vehicle with OBU goes along the road, the RSUs should receive an equal number of data packets. This preliminary test has been carried out first without the Kalman filter in the algorithm. The average of three trials of the test is taken into consideration in this work. Figure 10 shows the number of data packets received by each of the RSUs in this preliminary test. It shows a considerable percentage of discrepancies in the number of data packets received among the RSUs whereas according to the V2I algorithm, they should have received an equal number of data packets from the OBU.