# LoRa Architecture for V2X Communication: An Experimental Evaluation with Vehicles on the Move
## 4 Communication Algorithms of Proposed System  
The performance of any low power wireless communication depends vastly on its link quality and data overheads. Link quality is measured with received signal strength and error rates. Received signal strength indicator (RSSI) of any received data depicts the quality of any wireless link. It estimates the power level of a received data from any sender or access point. Transmission throughput is another important metric that can provide the estimation of link quality for ensuring higher reliability. Some of the research works [67] have also considered throughput along with metrics like RSSI for reliable communication in vehicular networks. As throughput is beyond the scope of this work, the proposed architecture utilizes RSSI values to choose a better communication link, and for obtaining a reliable communication link with lower overheads. \begin{table}
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As V2X presents a dynamic environment with moving OBUs along the roads, data handover from one RSU to another plays an important role. With the increased speed of the vehicles, the perfect handover becomes more challenging. Other vehicles including roadside infrastructures might also cause to prevent direct line of sight communication which makes the data handover more challenging. Thus, the proposed architecture emphasizes the smooth handover of the data from one RSU to another. LoRa communication supports WAN through the LoRaWAN server. In LoRaWAN, the data packet from any end device is sent to the server through gateways and after that, from the server, it reaches the intended destination. Routing through the server introduces higher latency which might hamper any V2X applications badly. Thus, the proposed system is based on D2D communication which reduces the communication latency by direct communication rather than unnecessary routing through the server. Moreover, the architecture is designed to facilitate modularity which can offer the legacy vehicles V2X capability without adapting any modification of the vehicles' circuitry. A RFM69 HCW LoRa module which operates on 915 MHz and has a maximum throughput of 300 kbps with up to 100 mW of transmitting power is used as the transceiver [68]. Though it has a transmission range of 500 m in the direct line of sight, a simple quarter-wave monopole wire antenna is used in the transceiver module which might affect the range. Moreover, RSUs and OBUs will surely not be at an ideal location with a direct line of sight all the time. The research group has tested that, on a regular road with obstacles such as moving cars and pedestrians, a RFM69 HCW placed at 1 m height can communicate consistently with almost no packet drop up to 160 m with an RSSI value of approximately -60 dBm. At a 40 m distance, the RSSI value improved to a value of approximately -35 dBm. The proposed V2X communication architecture is presented in Figure 2. The green links denote the exchange of vehicle data whereas the red links denote that vehicles or RSUs are within the transmission range of each other but do not exchange the vehicle data according to the defined algorithms 1 & 2. While going through a road, the OBU in the car communicates with RSUs in its vicinity. CAR1 finds RSU1 and RSU2 within its transmission range whereas CAR2 finds RSU2 and RSU3 within its range. Depending on the higher RSSI value from the RSUs, CAR1 starts sending sensor data to the RSU2 and maintain sending the data to it until it reaches the threshold value of -60 dBm. At the same time, it looks for any other vehicle within its radius of 40 m. CAR1 finds CAR2 and vice-versa within the designated range and sends vehicle data to each other. Though RSU1 is within the transmission range of CAR1, it keeps sending data to the only RSU2 to reduce the data overheads. Similarly, CAR2 sends data to RSU2 only following the V2I algorithm even after having RSU3 in the transmission range. Moreover, CAR2 does not send sensor data to CAR3 as it is out of the designated perimeter.