Large-scale unmanned cluster distributed TDMA time self-synchronization method

Abstract

The invention belongs to the technical field of wireless ad hoc networks, and particularly relates to a large-scale unmanned cluster distributed TDMA time self-synchronization method. According to the distributed multi-hop time self-synchronization method for the large-scale unmanned cluster, in the networking process of the hundreds of nodes using the TDMA protocol, time fine synchronization is completed for the nodes of the whole network, the convergence rate of the time fine synchronization is improved, and the method does not depend on an external time service source (such as GPS) and the like); and the method is suitable for the conditions of network topology dynamic change and node failure.

Description

Technical Field

[0001] This invention belongs to the field of wireless ad hoc network technology, specifically relating to a large-scale unmanned cluster distributed TDMA time self-synchronization method. Background Technology

[0002] In unmanned swarm collaborative operations, the accuracy of time synchronization directly affects the reliability and efficiency of task allocation, data transmission, path planning, and collaborative control.

[0003] Currently, time synchronization technologies for small-scale networks are relatively mature, such as those based on GPS (Global Positioning System), NTP (Network Time Protocol), or PTP (Precision Time Protocol). However, when the number of nodes reaches hundreds and operates in a distributed wireless TDMA (Time Division Multiple Access) communication protocol environment, traditional synchronization methods have the following shortcomings:

[0004] 1) GPS timing is limited by indoor / obstructed or interference-prone environments and cannot be guaranteed to be available at all times;

[0005] 2) In wireless multi-hop networks, the delay of NTP / PTP is uncontrollable, making it difficult to achieve sub-microsecond synchronization accuracy, which is insufficient to meet the time synchronization accuracy requirements of TDMA.

[0006] 3) In large-scale TDMA networks, the network topology changes dynamically, and there are factors such as propagation delay and hardware clock drift between nodes. Therefore, high-precision clock alignment of the entire network cannot be achieved through a single synchronization.

[0007] Therefore, a precise time synchronization solution is needed for large-scale distributed unmanned clusters with hundreds of nodes and TDMA protocol. Summary of the Invention

[0008] To address the aforementioned issues, this invention provides a distributed multi-hop time self-synchronization method for large-scale unmanned clusters. In the process of networking hundreds of nodes in a large-scale unmanned cluster using the TDMA protocol, it achieves precise time synchronization for all nodes in the network, improves the convergence rate of precise time synchronization, does not rely on external time sources (such as GPS), and is applicable to situations with dynamic changes in network topology and node failures.

[0009] The technical solution adopted in this invention is:

[0010] A large-scale unmanned cluster distributed TDMA time self-synchronization method includes the following steps:

[0011] S1. Assign ID numbers to the nodes deployed in the network. Specifically, define the maximum node capacity of the network as N, and assign a unique ID number to each node within the range of 1 to N. After completing the ID number assignment, the node with the smallest ID number is the time reference node by default. Divide the nodes in the network into time-synchronized nodes and nodes waiting to be time-synchronized. At this time, the time reference node is the only time-synchronized node in the network, and the remaining nodes are nodes waiting to be time-synchronized.

[0012] The preset TDMA time frame structure includes six time slots: service time slot, data time slot, primary synchronization time slot, secondary synchronization time slot, primary flight control time slot, and secondary flight control time slot. The service time slot and data time slot are adjacent. The service time slot is used to send coarse synchronization information in fine synchronization mode and to send fine synchronization requests in coarse synchronization mode. The data time slot is used for data exchange when no fine synchronization request is received in the service time slot, and to reply to a fine synchronization response when a fine synchronization request is received in the service time slot. The primary synchronization time slot and secondary synchronization time slot are adjacent. The primary synchronization time slot is used to send fine synchronization requests; the secondary synchronization time slot is used to reply to fine synchronization responses. The primary flight control time slot is used for nodes to send flight control commands in fine synchronization mode and for nodes to send synchronization requests in coarse synchronization mode. The secondary flight control time slot is used for data exchange when no fine synchronization request is received in the primary flight control time slot; and to reply to a fine synchronization response when a fine synchronization request is received in the primary flight control time slot.

[0013] S2. According to the preset time frame structure, the time synchronization node broadcasts coarse synchronization information in the duty time slot. The waiting time synchronization node receives the coarse synchronization information, selects the best upstream time reference, and completes the coarse time synchronization. The specific method is as follows:

[0014] The time-synchronized nodes broadcast coarse synchronization information, which includes the sending node's ID, sending time T1, the time reference node's ID, the time reference node's sequence number, and the distance to the time reference node (i.e., the hop count). The time-synchronized nodes select a parent node in the following priority order: smallest time reference node ID, latest time reference node sequence number, smallest distance to the time reference node, and smallest parent node ID. The time-synchronized nodes then synchronize their time with the parent node, completing the coarse time synchronization.

[0015] S3. The node awaiting time synchronization, having completed coarse time synchronization, initiates a fine time synchronization query to its parent node in the service time slot, main synchronization time slot, and main flight control time slot, according to the preset time frame structure. After completing fine time synchronization, the node awaiting time synchronization is converted to a synchronized node. The specific method is as follows:

[0016] A node awaiting time synchronization, having completed coarse time synchronization, sends a fine time synchronization request to its parent node using a preset time frame structure. Based on the request response, it obtains the time synchronization deviation with the parent node, corrects its own time, and completes fine time synchronization with the parent node, becoming a time-synchronized node. This ultimately achieves self-synchronization of network nodes. The fine synchronization query process includes:

[0017] Within the service time slot, main synchronization time slot, and main flight control time slot, the node to be added to the network sends a time fine synchronization frame to the parent node. The time fine synchronization request frame carries the transmission time T1. After receiving the time fine synchronization frame, the parent node records the arrival time T2 of the time fine synchronization request frame. In the adjacent time slot, it replies with a time fine synchronization response frame. The time fine synchronization response frame is sent at time T3 and carries the synchronization deviation Δ = T3 + T2 - T1. When the node to be added to the network receives the time fine synchronization response frame, it records the arrival time T4 of the time fine synchronization response frame and corrects its own time to (Δ + T4) / 2.

[0018] S4. All time-synchronized nodes, according to the preset time frame structure, initiate a fine-synchronization query to the parent node in the primary synchronization time slot to maintain fine-synchronization. The specific method is as follows:

[0019] A time-synchronized node sends a time fine synchronization request to its parent node in the primary synchronization time slot. Based on the response to the request, it obtains the time synchronization deviation with the parent node, corrects its own time, and periodically maintains time fine synchronization.

[0020] S5. The time-synchronized nodes monitor the existence of the time reference node. When the time reference node fails, the node with the smallest ID among the remaining nodes in the network is elected as the new time reference node. The specific method is as follows:

[0021] The time reference node periodically sends coarse synchronization information, incrementing the time reference sequence number by 1 with each transmission. The new time reference sequence number is forwarded to the entire network through other synchronized nodes. The synchronized nodes periodically receive the updated time reference sequence number and determine that the time reference node still exists. When the time reference node fails, if the time reference sequence number is not updated within a set time, the synchronized nodes determine that the time reference node has failed and elect the node with the smallest ID among the remaining nodes as the new time reference node.

[0022] The beneficial effects of this invention are as follows:

[0023] 1) At a scale of hundreds of nodes, the time synchronization accuracy can reach the sub-microsecond level;

[0024] 2) It does not rely on external time sources and can still achieve distributed synchronization through synchronization information exchange to maintain high-precision clock alignment even when GPS is unavailable;

[0025] 3) Supports mobile unmanned cluster networks, where nodes can dynamically join and leave the network;

[0026] 4) Allows nodes to send synchronization request messages using service time slots and primary synchronization time slots in coarse synchronization state. The synchronization convergence time is fast and it is suitable for unmanned clusters with dynamic topology changes. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a time frame structure with a capacity of 128 nodes.

[0028] Figure 2 This is a diagram illustrating the relationship between each node and its parent node.

[0029] Figure 3 This is a schematic diagram of the time synchronization process.

[0030] Figure 4 This is a flowchart illustrating the time self-synchronization method.

[0031] Figure 5 This is a schematic diagram of the frame structure during time self-synchronization.

[0032] Figure 6 This is a schematic diagram of a finite state machine for a node during time self-synchronization. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this application clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0034] like Figure 3 As shown, the present invention includes the following steps:

[0035] S1. Assign ID numbers to the nodes deployed in the network. Specifically, define the maximum node capacity of the network as N, and assign a unique ID number to each node within the range of 1 to N. After completing the ID number assignment, the node with the smallest ID number is designated as the time reference node by default. Divide the nodes in the network into synchronized nodes and nodes awaiting time synchronization. At this point, the time reference node is the only synchronized node in the network, and the remaining nodes are nodes awaiting time synchronization.

[0036] S2. According to the preset time frame structure, the time synchronization node broadcasts coarse synchronization information in the duty time slot. When the time synchronization node receives the coarse synchronization information, it selects the best upstream time reference and completes the coarse time synchronization.

[0037] S3. The node that has completed coarse time synchronization initiates a fine time synchronization query to the parent node in the service time slot, main synchronization time slot and main flight control time slot according to the preset time frame structure, and completes fine time synchronization, becoming a time-synchronized node.

[0038] S4. The time-synchronized node initiates a fine-time synchronization query to the parent node in the main synchronization time slot according to the preset time frame structure to complete the fine-time synchronization maintenance.

[0039] S5. The time synchronization node monitors the existence of the time reference node. When the time reference node fails, the node with the smallest ID among the remaining nodes in the network is elected as the new time reference node. In S2, the time frame structure has six preset time slots: service time slot, data time slot, primary synchronization time slot, secondary synchronization time slot, primary flight control time slot, and secondary flight control time slot. A specific time frame structure with a capacity of 128 nodes is as follows: Figure 1 As shown.

[0040] Service time slots and data time slots are adjacent time slots. Service time slots are used to send coarse synchronization information in fine synchronization state and to send fine synchronization requests in coarse synchronization state (in coarse synchronization state, nodes cannot use time slots to transmit services and can only request synchronization from their parent node. Here, to speed up fine synchronization network access, nodes can send fine synchronization requests not only in their own main synchronization time slot but also in their own service time slots and main flight control time slots. In fine synchronization state, nodes broadcast recruitment in service time slots, telling other nodes that they can obtain fine synchronization through them because they have already obtained fine synchronization, and others can first coarsely synchronize with them and then finely synchronize with them). The number of service time slots is consistent with the network capacity, and service time slot numbers are allocated sequentially from 1 to N. Each node occupies a service time slot with the same number according to its device number. Data time slots are used for data exchange when no fine synchronization request is received in a service time slot, and are used to reply to a fine synchronization response when a fine synchronization request is received in a service time slot.

[0041] Coarse synchronization message frames, such as Figure 5 The coarse synchronization message frame (a) contains the frame type, sending node ID, sending time T1, time reference node ID, time reference node sequence number, and distance to the time reference node. The type of the coarse synchronization message frame is 0. The sending node ID represents the ID of the node that sent the frame. The sending time T1 is set by the sending node when sending. The time reference node ID is the ID of the time reference node elected by the sending node. The time reference sequence number is the sequence number broadcast by the time reference node, which increments by 1 each time the time reference node broadcasts it. Other time-synchronized nodes only forward it without modifying it. The distance to the time reference node represents the number of hops between the sending node and the network time reference node.

[0042] The purpose of the time reference sequence number in this scheme is that, since this scheme allows the time reference node in the network to be destroyed, the node can reselect a time reference node; each time the time reference node sends, the sequence number of the time reference node is incremented by 1, and other nodes will carry the new time reference node sequence number when forwarding coarse synchronization information; after the time reference node is destroyed, the time reference node sequence number recorded by other nodes will not be updated for a long time, so the time reference node sequence number can be used to detect whether the time reference node still exists.

[0043] Fine synchronization request frame such as Figure 5 The b-precise synchronization request frame contains frame type, sending node ID, sending time T1, and receiving node ID; the type of the precision synchronization request frame is 1; the sending node ID represents the node ID that sends the frame; the sending time T1 is set by the sending node when sending; the receiving node ID is the ID of the parent node elected by the sending node.

[0044] Precise synchronization response frame such as Figure 5 The fine synchronization response frame in the middle contains frame type, sending node ID, synchronization deviation Δ, and receiving node ID; the type of fine synchronization response frame is 2; the sending node ID represents the node ID that sent the frame; the synchronization deviation Δ is calculated by the sending node and set at the time of transmission; the receiving node ID is the node ID that sent the fine synchronization request frame.

[0045] The primary synchronization time slot and the secondary synchronization time slot are adjacent time slots; the primary synchronization time slot is used to send fine synchronization requests; the number of primary synchronization time slots is consistent with the network capacity, and the primary synchronization time slot numbers are allocated sequentially from 1 to N. Each node occupies a primary synchronization time slot with the same number according to its device number; the secondary synchronization time slot is used to reply to fine synchronization responses; the number of secondary synchronization time slots is consistent with the network capacity, and the secondary synchronization time slot numbers are allocated sequentially from 1 to N. Each node occupies a secondary synchronization time slot with the same number according to its device number.

[0046] The primary and secondary flight control time slots are adjacent time slots. In fine synchronization mode, the primary flight control time slot is used by nodes to send flight control commands; in coarse synchronization mode, it is used by nodes to send fine synchronization requests. The number of primary flight control time slots is consistent with the network capacity, and the primary flight control time slots are numbered sequentially from 1 to N. Each node occupies a primary flight control time slot with the same number according to its device number. The secondary flight control time slot is used to send flight control commands when no fine synchronization request is received in the primary flight control time slot; and it is used to reply to a fine synchronization request when a fine synchronization request is received in the primary flight control time slot. The number of secondary flight control time slots is consistent with the network capacity, and the secondary flight control time slots are numbered sequentially from 1 to N. Each node occupies a secondary flight control time slot with the same number according to its device number.

[0047] The time-synchronized node broadcasts coarse synchronization information. This information includes the sending node's ID, transmission time T1, the time reference node's ID, the time reference node's sequence number, and the distance to the time reference node (i.e., the hop count). After receiving the coarse synchronization information, the time-synchronized node selects a parent node based on the following priority order: smallest time reference node ID, latest time reference node sequence number, smallest hop count to the time reference node, and smallest parent node ID. Figure 2 As shown, the time synchronization node synchronizes its time with the parent node to complete coarse time synchronization. The node records the received coarse synchronization information in a synchronization table, with each sending node ID corresponding to one synchronization table entry; combined with... Figure 2 This section describes the process of nodes using a synchronization table to elect a parent node. It assumes that node 1 is the time base node, nodes 2 and 3 are time-synchronized nodes, and the remaining nodes are nodes waiting to be time-synchronized. Node 1 carries the sequence number of the time base node in the coarse synchronization information, which has been incremented to 3. After receiving the information, node 2 sends the coarse synchronization information in the duty time slot, where the sequence number of the time base node is 3.

[0048] When node 4 receives the coarse synchronization information sent by node 2, it records a synchronization entry for node 2, as shown in Table 1. The distance from the time reference node represents the distance from this node to the time reference node via the sending node, i.e., the number of hops.

[0049] Table 1. Synchronization table for node 4

[0050]

[0051] At this time, in the synchronization entry recorded by node 4 for node 3, the sequence number of the time reference node is still the sequence number 2 carried by node 3 when it last sent coarse synchronization information; according to the priority order of parent node election, although both node 2 and node 3 elect node 1 as the time reference node ID, the sequence number of the time reference node announced by node 2 is the latest, so node 4 elects node 2 as the parent node.

[0052] After node 3 receives the coarse synchronization information sent by node 2, the sequence number of the time reference node in the coarse synchronization information sent by node 3 is also 3. When node 4 receives the coarse synchronization information sent by node 3, it updates the synchronization table entry for node 3, as shown in Table 2. According to the priority order of parent node election, although both node 2 and node 3 elect node 1 as the time reference node ID and the sequence number of the time reference node is the same, node 2 has the smallest hop count from the time reference node, so node 4 still elects node 2 as the parent node.

[0053] Table 2. Synchronization table after node 4 update

[0054]

[0055] In S3, a node that has completed coarse time synchronization sends a fine time synchronization request to its parent node using a preset time frame structure. Based on the response to the request, it obtains the time synchronization deviation with the parent node, corrects its own time, and completes fine time synchronization with the parent node, becoming a time-synchronized node. This ultimately achieves self-synchronization of network node time.

[0056] Specifically, such as Figure 4 As shown, the node to be added to the network sends a time fine synchronization frame to the parent node during the service time slot, the main synchronization time slot, and the main flight control time slot. The time fine synchronization request frame carries the transmission time T1. After receiving the time fine synchronization frame, the parent node records the arrival time T2 of the time fine synchronization request frame. It replies with a time fine synchronization response frame in the adjacent time slot. The time fine synchronization response frame is sent at time T3 and carries the synchronization deviation Δ = T3 + T2 - T1. When the node to be added to the network receives the time fine synchronization response frame, it records the arrival time T4 of the time fine synchronization response frame and corrects its own time to (Δ + T4) / 2.

[0057] like Figure 6 As shown, the node uses a finite state machine to perform the time self-synchronization process. The finite state machine contains three states: listening, coarse synchronization, and fine synchronization.

[0058] When a node is in the listening state, it only receives coarse synchronization information and does not send any frames. The listening state lasts for L time frames, which is usually the same as the maximum number of hops in the network. For example, for a 4-hop network, it can be set to 4. When a node receives synchronization information, if the sending node is selected as the parent node, it synchronizes with the parent node's time to complete coarse time synchronization and transitions to the coarse synchronization state.

[0059] When a node is in coarse synchronization state, it sends a fine synchronization request to its parent node in the service time slot, main synchronization time slot, and main flight control time slot. The coarse synchronization state lasts for C time frames, which can usually be set to 1. After receiving the fine synchronization request, the parent node replies with a fine synchronization response in the next time slot. When the node receives the fine synchronization response, it completes the fine synchronization and transitions to the fine synchronization state.

[0060] When a node is in fine synchronization state, it sends coarse synchronization information in the service time slot, sends fine synchronization requests in the main synchronization time slot (except for the time reference node), and sends flight control commands in the main flight control time slot.

[0061] The node uses a synchronization counter to determine whether the listening state and coarse synchronization state have timed out; when the node's service time slot begins, the synchronization counter decrements by 1; when it decrements to 0, it is determined that a timeout has occurred.

[0062] In S4, a time-synchronized node sends a time fine synchronization request to its parent node in the primary synchronization time slot. Based on the response to the request, it obtains the time synchronization deviation with the parent node, corrects its own time, and periodically maintains time fine synchronization.

[0063] In S5, nodes determine the existence of a time reference node by periodically updating the sequence number of the time reference node. Taking the failure of node 1 as an example, node 1 no longer sends coarse synchronization information. The sequence number of time reference node 1 recorded by the remaining nodes will not be updated for a long time, and the remaining nodes determine that node 1 has failed. Among the remaining nodes, the node with the smallest ID is node 2, so node 2 is elected as the new time reference node.

[0064] The method of the present invention was verified by simulation. In the simulation scenario, the time slot length was 550 μs, the number of nodes was 128, and they were randomly distributed in a range of 2.5 km × 2.5 km. The transmission distance was 1 km, and the maximum network distance was 4 hops. According to the simulation results, when all nodes start up at the same time, all nodes can complete precise time synchronization within 400 ms.

Claims

1. A large-scale unmanned cluster distributed TDMA time self-synchronization method, characterized in that, Includes the following steps: S1. Assign ID numbers to the nodes deployed in the network. Specifically, define the maximum node capacity of the network as N, and assign a unique ID number to each node within the range of 1 to N. After completing the ID number assignment, the node with the smallest ID number is the time reference node by default. Divide the nodes in the network into time-synchronized nodes and nodes waiting to be time-synchronized. At this time, the time reference node is the only time-synchronized node in the network, and the remaining nodes are nodes waiting to be time-synchronized. The preset TDMA time frame structure includes six time slots: service time slot, data time slot, primary synchronization time slot, secondary synchronization time slot, primary flight control time slot, and secondary flight control time slot. The service time slot and data time slot are adjacent. The service time slot is used to send coarse synchronization information in fine synchronization mode and to send fine synchronization requests in coarse synchronization mode. The data time slot is used for data exchange when no fine synchronization request is received in the service time slot, and to reply to a fine synchronization response when a fine synchronization request is received in the service time slot. The primary synchronization time slot and secondary synchronization time slot are adjacent. The primary synchronization time slot is used to send fine synchronization requests; the secondary synchronization time slot is used to reply to fine synchronization responses. The primary flight control time slot is used for nodes to send flight control commands in fine synchronization mode and for nodes to send synchronization requests in coarse synchronization mode. The secondary flight control time slot is used for data exchange when no fine synchronization request is received in the primary flight control time slot; and to reply to a fine synchronization response when a fine synchronization request is received in the primary flight control time slot. S2. According to the preset time frame structure, the time synchronization node broadcasts coarse synchronization information in the duty time slot. The waiting time synchronization node receives the coarse synchronization information, selects the best upstream time reference, and completes the coarse time synchronization. The specific method is as follows: The time-synchronized nodes broadcast coarse synchronization information, which includes the sending node's ID, sending time T1, the time reference node's ID, the time reference node's sequence number, and the distance to the time reference node (i.e., the hop count). The time-synchronized nodes select a parent node in the following priority order: smallest time reference node ID, latest time reference node sequence number, smallest distance to the time reference node, and smallest parent node ID. The time-synchronized nodes then synchronize their time with the parent node, completing the coarse time synchronization. S3. The node awaiting time synchronization, having completed coarse time synchronization, initiates a fine time synchronization query to its parent node in the service time slot, main synchronization time slot, and main flight control time slot, according to the preset time frame structure. After completing fine time synchronization, the node awaiting time synchronization is converted to a synchronized node. The specific method is as follows: A node awaiting time synchronization, having completed coarse time synchronization, sends a fine time synchronization request to its parent node using a preset time frame structure. Based on the request response, it obtains the time synchronization deviation with the parent node, corrects its own time, and completes fine time synchronization with the parent node, becoming a time-synchronized node. This ultimately achieves self-synchronization of network nodes. The fine synchronization query process includes: Within the service time slot, main synchronization time slot, and main flight control time slot, the node to be added to the network sends a time fine synchronization frame to the parent node. The time fine synchronization request frame carries the transmission time T1. After receiving the time fine synchronization frame, the parent node records the arrival time T2 of the time fine synchronization request frame. In the adjacent time slot, it replies with a time fine synchronization response frame. The time fine synchronization response frame is sent at time T3 and carries the synchronization deviation Δ = T3 + T2 - T1. When the node to be added to the network receives the time fine synchronization response frame, it records the arrival time T4 of the time fine synchronization response frame and corrects its own time to (Δ + T4) / 2. S4. All time-synchronized nodes, according to the preset time frame structure, initiate a fine-synchronization query to the parent node in the primary synchronization time slot to maintain fine-synchronization. The specific method is as follows: A time-synchronized node sends a time fine synchronization request to its parent node in the primary synchronization time slot. Based on the response to the request, it obtains the time synchronization deviation with the parent node, corrects its own time, and periodically maintains time fine synchronization. S5. The time-synchronized nodes monitor the existence of the time reference node. When the time reference node fails, the node with the smallest ID among the remaining nodes in the network is elected as the new time reference node. The specific method is as follows: The time reference node periodically sends coarse synchronization information, incrementing the time reference sequence number by 1 with each transmission. The new time reference sequence number is forwarded to the entire network through other synchronized nodes. The synchronized nodes periodically receive the updated time reference sequence number and determine that the time reference node still exists. When the time reference node fails, if the time reference sequence number is not updated within a set time, the synchronized nodes determine that the time reference node has failed and elect the node with the smallest ID among the remaining nodes as the new time reference node.

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