In case of emergencies such as fires in highway/railway tunnels, using firefighting drones to enter the tunnel for fire reconnaissance and firefighting operations is a very effective method. During the operation of the firefighting drone, it is necessary to transmit the filmed video in real time to the monitoring station outside the tunnel, and the operator at the monitoring station controls the flight posture and route of the drone, and issues control commands at the appropriate time to control the drone to complete the operation task. Therefore, establishing an efficient and stable wireless communication link from the drone to the monitoring station outside the tunnel is a necessary condition for the completion of the firefighting drone tunnel operation.
The UAV networking communication system developed by our company is installed on a certain type of firefighting drone. In early December 2023, it completed the operation task in the tunnel at a certain test site. During the operation, the video of the drone's pod and the data of the airborne payload were transmitted to the monitoring station at the tunnel entrance through the communication system, and the operator controlled the monitoring station to send instructions to the drone through the communication system to complete the operation task. During the operation task, the UAV networking communication system served as the only means of communication throughout the process, providing reliable and stable link guarantee conditions for the overall operation task.
Characteristics and Difficulties of Tunnel Communication
The characteristics and difficulties of wireless communication in tunnels mainly include the following aspects:
1.Signal attenuation and obstruction
The space in the tunnel is narrow and is a closed space. The concrete, rock and other materials in the tunnel will obstruct, absorb and scatter the transmission of electromagnetic wave signals, causing signal attenuation. The tunnel cannot be a straight line, and there must be multiple turns in the middle, each of which will seriously affect the transmission of the signal.
2.Multipath effect
There are a large number of reflections and refractions in the tunnel, which will cause multipath propagation of the signal, causing the receiver to receive multiple signals from different paths, causing multipath interference of the signal.
3.Various interference sources
There may be a large amount of electromagnetic interference caused by metal structures, cable lines, etc. in the tunnel, such as electromagnetic wave reflection, refraction, etc., which further interfere with signal transmission.
4.High reliability requirements
When a fire occurs in a tunnel, the wireless communication link is the only communication link between the firefighting drone and the monitoring station. If the communication link is interrupted, it will cause the drone to lose control and cause accidents. Therefore, the reliability of wireless communication must withstand the test.
System Working Principle
The UAV networking communication system includes three parts: the remote control station outside the tunnel, the fixed node equipment in the tunnel, and the UAV onboard communication equipment. Among them, there is one onboard communication equipment installed on the drone. There are several fixed nodes in the tunnel. The number of fixed nodes in the tunnel is reasonably arranged according to the length, height, turning angle, etc. of the tunnel. Considering the special scene in the tunnel, the spacing between nodes is about 1km~1.5km. There is one remote control station outside the tunnel. If conditions permit, a remote control station can be arranged on both sides of the tunnel to increase the reliability of the system through redundant backup.
Main Features of the System
1.Fixed node chain networking, with self-healing function
The chain networking technology is used between the nodes in the tunnel. Chain networking technology can specify the transmission order of nodes, so that data can flow along the chain structure, thereby achieving efficient data transmission.
There are two forms of chain networking, one is a one-way chain structure, and the other is a two-way chain structure. In a one-way chain structure, nodes can only transmit data to the back of the chain, while nodes in a two-way chain structure can transmit data to the back of the chain as well as to the front of the chain.
This system adopts a two-way chain structure. In the chain structure, each node only communicates directly with several neighboring nodes before and after it. In this way, the entire network is divided into orderly transmission units. When a node needs to transmit data to another node, it only needs to initiate the data from the front end of the chain, pass through several middle nodes of the chain in turn, and finally transmit to the target node. Moreover, when one of the nodes cannot work, the previous node can still communicate with the next node:
There are 9 nodes in the figure, and we set it to a two-way chain networking mode. Node 3 can communicate with adjacent nodes 1, 2, 4, and 5. Normally, the signal from node 1 is transmitted to node 5 through nodes 2, 3, and 4. If node 3 cannot work properly, the signal from node 1 can also be transmitted to node 5 through nodes 2 and 4.
This transmission method can also greatly reduce data conflicts in the network. For example, nodes 1 and 6 in the figure above are not in the same chain communication network, and the frequency and bandwidth resources of nodes 1 and 6 can be reused, improving the transmission efficiency of data.
2.Automatic routing of airborne communication equipment
The automatic routing technology in ad hoc network communication is adopted between the airborne communication equipment and the ground fixed nodes. Each fixed node discovers its surrounding neighbor nodes through broadcasting or neighbor query, and establishes a neighbor relationship. Nodes exchange routing information with each other, including node status, link quality, network topology, etc., so that all nodes understand the situation of the entire network.
The system adopts an automatic routing algorithm that combines the shortest path and load balancing. The airborne communication equipment automatically selects a route path to access a certain ground fixed node based on the current network topology information, link quality, and the status of each fixed node, based on the shortest path between it and the ground node, and the communication bandwidth load of the node, ensuring the accessibility, reliability, and delay performance of the communication link and the quality of network communication.
3.High Communication Speed, Low Latency
The data transfer rate from the drone communication equipment to the monitoring station outside the tunnel can reach more than 6Mbps, and the communication latency between nodes is less than 35ms, which fully guarantees the requirements of firefighting drone tunnel operations.
Basic Technical Parameters of the System
(1)Possess the communication capability of video transmission and data transmission;
(2)Real-time communication rate of the drone: ≥6Mbps;
(3)Uplink remote control data of the remote control station: ≥115200bps;
(4)Communication latency between nodes: ≤35ms;
(5)Video interface: support 2 network interfaces, 1 visible light, and 1 infrared;
(6)Data interface: 1 RS422 interface;
(7)Node equipment in the tunnel: usually powered by 220V, when emergency conditions shut down the 220V power supply, it is powered by its own battery;
(8)Self-healing ability of nodes in the tunnel: when individual node equipment in the tunnel is damaged, the system can still work normally and realize the communication between the drone and the remote control station.
System Testing and Verification
In early December 2023, in cooperation with a certain host, it completed the unmanned operation task in the tunnel at a certain test site, providing a two-way data communication link between the drone in the tunnel and the monitoring station at the tunnel entrance. The overall system schematic diagram is as follows:
An airborne communication device is installed on the drone, 2 ground nodes are arranged in the tunnel, and 1 ground node is arranged at the tunnel entrance. One airborne device and three ground nodes form a communication network. The drone takes off from the tunnel entrance and flies into the tunnel under the control of the operator at the monitoring station. When the drone arrives near the operation area, the operator issues instructions to complete the operation task.
The UAV networking communication system works stably and reliably, and has received unanimous praise from all members of the host's test team.