Low-energy-consumption clustering time synchronization method in air-ground cluster network

Abstract

The invention discloses a low-energy-consumption clustering time synchronization method in an air-ground cluster network, and relates to the technical field of network time synchronization. Establishing an air-ground cluster network model; performing hierarchical level distribution and clustering on common nodes in the air-ground cluster network model by using a CTPSN hierarchical discovery stage algorithm and an improved LEACH algorithm; performing time synchronization of the cluster head nodes; and carrying out time synchronization on the common nodes in the cluster. An improved clustering protocol is used for selecting a cluster head, then cluster division is completed, time synchronization is completed on the cluster head through a time synchronization algorithm, when time synchronization is carried out on a common node in a cluster, a local clock is updated through calculation and prediction by monitoring a data packet sent by the cluster head, and then time synchronization is completed. According to the method, the aim of achieving dual optimization of the synchronization precision and the energy efficiency in a complex air-ground cluster network environment is achieved, so that the problem that the energy consumption is too large generally when the high synchronization precision is kept in an existing time synchronization algorithm is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of network time synchronization, and in particular to a method for low-energy clustering time synchronization in an air-ground cluster network. Background Art

[0002] With the rapid development of communication technology, the application scope of air-ground cluster systems with drones and unmanned vehicles as the core is gradually expanding. Time synchronization is the basis and premise of many applications of this system. However, due to different production processes of the crystal oscillators of the nodes, the time of each node in the system is not synchronized. Therefore, the time synchronization algorithm of the air-ground cluster network has universal significance and existence value, and has always been a hot topic of concern in the industry at home and abroad. However, the traditional time synchronization algorithm completes time synchronization through two-way data packet exchange, and finally realizes time synchronization of the whole network. A large number of data packet exchanges will consume huge energy. In order to better balance the time synchronization accuracy and energy loss, it is necessary to propose a new low-energy clustering time synchronization method. On the basis of ensuring the time synchronization accuracy, the energy consumption of the algorithm is reduced from the hierarchical discovery stage and the time synchronization stage, thereby balancing the energy load of the air-ground cluster network and extending the life cycle of the network.

[0003] The Chinese patent "CN202211144309.9 A method, system and device for micro-power time synchronization in a wireless sensor network" provides a method, system and device for micro-power time synchronization in a wireless sensor network. The invention is that the child node monitors the synchronization message of a pair of parent nodes, and establishes a loss function of the relative clock frequency deviation and relative clock offset of the child node and the parent node it seeks to synchronize based on the timestamp of the node's clock information exchange, and solves to obtain the corresponding relative clock frequency deviation estimate and relative clock offset estimate to compensate the clock of the corresponding child node. According to the successful clock synchronization data of the child node, a linear regression model of the relative clock frequency deviation and relative clock offset with respect to the communication distance and the remaining energy of the node is constructed respectively. When the child node clock synchronization fails, the linear regression model is used to calculate the relative clock frequency deviation and offset, and the clock compensation is performed again according to the calculated results.

[0004] However, the regression model constructed in the design of the Chinese patent "CN202211144309.9 A method, system and device for micro-power time synchronization in a wireless sensor network" and the algorithm for solving the optimization problem are highly complex, which is a major challenge for wireless sensor nodes with limited computing resources, and may affect real-time performance or occupy too many computing resources. From the perspective of energy consumption, there are deficiencies in terms of increasing network energy consumption, high requirements for node resources, and environmental adaptability. At the same time, when the network topology changes frequently or there is a lot of interference, the pre-built model may not be able to accurately predict, resulting in reduced synchronization efficiency and accuracy. Summary of the invention

[0005] In view of the deficiencies in the prior art, the purpose of the present invention is to propose a method for low-energy clustering time synchronization in an air-ground cluster network, which uses an improved clustering protocol to select cluster heads and then complete the cluster division, and uses a time synchronization algorithm to complete time synchronization for the cluster heads. When ordinary nodes in the cluster perform time synchronization, they use the data packets sent by the cluster heads to update their own local clocks through calculation and prediction, and then complete time synchronization, so as to achieve the goal of dual optimization of synchronization accuracy and energy efficiency in a complex air-ground cluster network environment, so as to solve the problem of excessive energy consumption that is common in existing time synchronization algorithms while maintaining high synchronization accuracy.

[0006] A method for low energy consumption clustering time synchronization in an air-ground cluster network comprises the following steps:

[0007] Step 1: Establish an air-ground cluster network model;

[0008] The method for establishing the air-ground cluster network model is specifically as follows: in a rectangular monitoring area with a side length of L and an area of ​​L×L, N ordinary nodes are randomly deployed, each ordinary node represents a drone or an unmanned vehicle, and then a time reference node Sink is deployed at the center of the rectangular monitoring area;

[0009] The time reference node Sink has unlimited energy and a fixed position that does not change. The time reference node Sink has a GPS module to provide it with absolute time. The energy of the remaining N common nodes except the time reference node Sink is provided by batteries and has the same and limited initial energy.

[0010] Each of the N common nodes is identified by a unique ID number and can calculate its own residual energy;

[0011] The communication radius of each of the N common nodes is not completely consistent. The communication radius is changed by adjusting its own transmission power. The common node determines the distance between itself and other common nodes according to the strength of the received signal. Any common node has at least one common node with which it can communicate within its communication radius.

[0012] Step 2: Use the CTPSN hierarchical discovery phase algorithm and the improved LEACH algorithm to perform hierarchical level allocation and clustering of common nodes in the air-ground cluster network model;

[0013] Step 2.1: Set the time base node Sink as the root node, set its level to 0, and set the current round to 1;

[0014] Step 2.2: The root node broadcasts a hierarchical data packet. The ordinary nodes that receive the hierarchical data packet join the cluster where the root node is located and are divided into two categories according to their own location information, one in the left area of ​​the root node and the other in the right area of ​​the root node; the hierarchical data packet includes the identity ID and hierarchical level of the root node or cluster head node that sends the hierarchical data packet;

[0015] Step 2.3: Execute the improved LEACH algorithm for the common nodes in the left area and the common nodes in the right area respectively, and select the cluster head nodes in the left area and the right area respectively;

[0016] The improved LEACH algorithm is specifically:

[0017] For each common node, when it is running to become a cluster head node, a random number is generated between 0 and 1 and compared with the expected probability T(n). If the expected probability T(n) is greater than the generated random number, the node becomes the cluster head node, otherwise it does not become the cluster head node.

[0018] The expected probability calculation formula is:

[0019]

[0020] Where T(n) is the expected probability of a node becoming a cluster head in the rth round, n is the node number, p is a set ratio, G is the set of ordinary nodes that have not been selected as cluster head nodes in 1 / p rounds, and E re (n) is the remaining energy of the nth common node, E ave is the average energy of the current air-ground cluster network model;

[0021] Step 2.4: The cluster head node and common nodes in the current cluster perform hierarchical level update;

[0022] The method for updating the hierarchical level is specifically as follows: taking the hierarchical level in the received hierarchical data packet plus 1 as the updated hierarchical level;

[0023] Step 2.5: The cluster head node after the hierarchy level is updated broadcasts a hierarchical data packet;

[0024] Step 2.6: When an ordinary node that has not joined any cluster receives a hierarchical data packet broadcast by a cluster head node after the hierarchical level is updated, it joins the cluster where the cluster head node is located, updates its own hierarchical level, and executes the improved LEACH algorithm to select the cluster head node of the current hierarchical level; when an ordinary node or cluster head node that has joined a cluster receives a hierarchical data packet broadcast by the updated cluster head node, it discards the hierarchical data packet;

[0025] Step 2.7: Repeat steps 2.5 to 2.6 until no new cluster head nodes are generated, and then execute step 2.9;

[0026] Step 2.8: Determine whether there are common nodes that have not yet joined the cluster in the current air-ground cluster network model. If so, treat these common nodes as isolated nodes and execute step 2.9. Otherwise, directly execute step 3.

[0027] Step 2.9: The isolated node broadcasts a cluster entry request data packet. After receiving the cluster entry data packet, all the ordinary nodes that have already joined the cluster execute the improved LEACH algorithm to select the cluster head node. The cluster head node broadcasts a cluster entry confirmation data packet. The cluster entry confirmation data packet contains the identity ID and hierarchical level of the cluster head node. The isolated node updates its own hierarchical level to the hierarchical level of the cluster head node plus 1, and joins the cluster where the cluster head node is located.

[0028] Step 3: Use the cluster head time synchronization algorithm to synchronize the time of the cluster head node;

[0029] Step 3.1: Each cluster head node sends a time synchronization data packet to the parent node at the next level at time T1;

[0030] Step 3.2: The parent node receives the time synchronization data packet at time T2 and prepares to return the confirmation data packet;

[0031] Step 3.3: The parent node sends a confirmation data packet to the child node at time T3, and the child node receives the confirmation data packet at time T4;

[0032] Step 3.4: Calculate the local clock deviation between the child node and the parent node, and the child node corrects the local clock according to the calculated time deviation;

[0033] The local clock deviation between the child node and the parent node is:

[0034] [(T2-T1)-(T4-T3)] / 2

[0035] Step 4: Time synchronization of common nodes within the cluster using the intra-cluster time synchronization algorithm;

[0036] Step 4.1: The cluster head node acts as a sender. When the sender detects that the channel is idle, it prints a timestamp before the start character of the synchronization data packet to be sent, and then immediately sends the synchronization data packet with the timestamp;

[0037] The timestamp is the local clock of the sender at the absolute time t0, marked as T0;

[0038] Step 4.2: Calculate the transmission delay by using the transmission rate and the number of bits of the transmission synchronization data packet;

[0039] Transmission delay:

[0040]

[0041] Among them, n is the size of the transmitted data packet, p0 is the transmission rate, and λ0 is the transmission delay;

[0042] Step 4.3: As a receiver, the ordinary node in the cluster prints the arrival time timestamp of the synchronization data packet at the MAC layer. After the synchronization data packet is received and before submitting it to the upper layer service, it prints another timestamp. The receiving delay is calculated by the difference between the two timestamps. The two timestamps are the local clock of the receiver at the absolute time t1 and t2, marked as T1 and T2.

[0043] Step 4.4: Ignore the propagation delay and synchronize the local clocks of common nodes in the cluster according to the transmission delay and reception delay;

[0044] T=T0+λ0+(T2-T1)

[0045] Wherein, T is the local clock of the common node after time synchronization.

[0046] Compared with the prior art, the beneficial effects of the present invention are:

[0047] A low-energy clustering time synchronization method in air-ground cluster networks can significantly extend the network life by reducing unnecessary energy consumption in critical stages, ensuring the long-term stable operation of the air-ground cluster network. At the same time, by optimizing clustering and time synchronization strategies, the adaptability and flexibility of the network are enhanced. The method combines the advantages of multiple classic algorithms and can cope with complex and changeable environmental conditions in air-ground cluster networks and meet the needs of different scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a flow chart of a method for low energy consumption clustering time synchronization in an air-ground cluster network in an embodiment of the present invention;

[0049] Figure 2 This is a network model diagram of an air-ground cluster in an embodiment of the present invention;

[0050] Figure 3 Flow chart of the CTPSN hierarchical discovery phase algorithm in an embodiment of the present invention;

[0051] Figure 4 A network topology diagram formed by completing the CTPSN hierarchical discovery phase algorithm in an embodiment of the present invention;

[0052] Figure 5 is a flow chart of a cluster head time synchronization algorithm according to an embodiment of the present invention;

[0053] Figure 6 It is a schematic diagram of the CTPSN intra-cluster time synchronization algorithm in an embodiment of the present invention;

[0054] Figure 7 This is a schematic diagram of energy consumption of TPSN and CTPSN in the hierarchical division stage in an embodiment of the present invention;

[0055] Figure 8 Schematic diagram of energy consumption of TPSN and CTPSN in the time synchronization stage in an embodiment of the present invention;

[0056] Fig. 9 Schematic diagram of TPSN and CTPSN error analysis in an embodiment of the present invention. DETAILED DESCRIPTION

[0057] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0058] The core of the present invention is to propose an innovative clustering time synchronization protocol, CTPSN (ClusteringTiming-sync Protocol for Sensor Networks), which aims to achieve efficient and low-energy time synchronization in air-ground cluster networks, and specifically solves the challenge of high precision and low energy consumption that are common in current time synchronization algorithms. CTPSN cleverly combines the time synchronization accuracy advantages of the TPSN protocol and the low energy consumption characteristics of the RBS protocol, while introducing the optimization strategy of the LEACH clustering algorithm, realizing the innovation of time synchronization technology in complex network environments.

[0059] The key innovations of CTPSN are reflected in two aspects: First, in the hierarchical discovery stage, through the improved LEACH algorithm strategy, only the root node and the cluster head node broadcast hierarchical information, which significantly reduces the number of broadcast data packets and effectively reduces the energy burden of the network in the early stage. Secondly, in the time synchronization stage, CTPSN innovatively adopts a dual-track synchronization strategy. The cluster head node continues to use the TPSN algorithm to ensure high-precision time synchronization, while the ordinary nodes in the cluster monitor the broadcast data of the cluster head and use efficient calculation prediction methods to autonomously update the local clock, which greatly reduces the amount of data exchange during the synchronization process and further saves energy resources.

[0060] A low-energy clustering time synchronization method in air-ground cluster networks, such as Figure 1 As shown, the following steps are included:

[0061] Step 1: Establish an air-ground cluster network model;

[0062] The method for establishing the air-ground cluster network model is as follows: in a rectangular monitoring area with a side length of L and an area of ​​L×L, N ordinary nodes are randomly deployed, each ordinary node represents a drone or an unmanned vehicle, and then a time reference node Sink is deployed in the center of the rectangular monitoring area, such as Figure 2 As shown;

[0063] The time reference node Sink has unlimited energy and a fixed position that cannot be changed. The time reference node Sink has a GPS module to provide it with accurate absolute time. The energy of the remaining N common nodes except the time reference node Sink is provided by batteries and has the same and limited initial energy.

[0064] Each of the N common nodes is identified by a unique ID number and can calculate its own residual energy;

[0065] The communication radius of each of the N common nodes is not completely consistent. The communication radius is changed by adjusting its own transmission power. The common node determines the distance between itself and other common nodes according to the strength of the received signal. Any common node has at least one common node with which it can communicate within its communication radius.

[0066] Step 2: Use the CTPSN hierarchical discovery phase algorithm and the improved LEACH algorithm to perform hierarchical level allocation and clustering of common nodes in the air-ground cluster network model;

[0067] In the hierarchical discovery phase of the CTPSN time synchronization algorithm, the root node at layer 0 broadcasts a hierarchical data packet, and the nodes that receive the hierarchical data packet update their hierarchical level to level 1. Then all nodes at level 1 continue to broadcast a hierarchical data packet, and the nodes that receive the hierarchical data packet set their hierarchical level number to the level number of the node in the previous layer plus 1, and then continue to broadcast. Repeat the above operations until all nodes in the air-ground cluster network model are assigned hierarchical level numbers, and the hierarchical discovery phase is completed. Therefore, all nodes in the air-ground cluster network model need to broadcast hierarchical data packets in the hierarchical discovery phase, and sending too many data packets will consume a lot of network energy. In order to reduce the amount of data packets sent and thus reduce the energy consumption of the network, the idea of ​​clustering is used to establish a hierarchical structure of the network.

[0068] The specific process of the CTPSN hierarchical discovery phase algorithm is as follows: Figure 3 As shown:

[0069] Step 2.1: Set the time base node Sink as the root node, set its level to 0, and set the current round to 1;

[0070] Step 2.2: The root node broadcasts a hierarchical data packet. The ordinary nodes that receive the hierarchical data packet join the cluster where the root node is located and are divided into two categories according to their own location information, one in the left area of ​​the root node and the other in the right area of ​​the root node; the hierarchical data packet includes the identity ID and hierarchical level of the root node or cluster head node that sends the hierarchical data packet;

[0071] Step 2.3: Execute the improved LEACH algorithm for the common nodes in the left area and the common nodes in the right area respectively, and select the cluster head nodes in the left area and the right area respectively;

[0072] The cluster head node manages the energy information, location information, time information, communication status information and configuration parameter information of the common nodes in the cluster;

[0073] The LEACH algorithm can divide the network into several clusters. Compared with ordinary nodes, cluster head nodes tend to consume more energy in each cycle because they have to manage various information of ordinary nodes in the cluster. However, when each node in the LEACH algorithm is running for cluster head, it just randomly generates a number between 0 and 1, and then compares the number with the expected probability T(n) to determine whether it can become a cluster head. The current energy of the node is not taken into account. If a node with less remaining energy is selected as a cluster head, it is easy to cause the death of the node due to the high energy consumption, which reduces the life cycle of the network. Therefore, in the improved LEACH algorithm, the current energy state of the node is added to the expected probability formula of cluster head selection.

[0074] The improved LEACH algorithm is specifically:

[0075] For each common node, when it is running to become a cluster head node, a random number is generated between 0 and 1 and compared with the expected probability T(n). If the expected probability T(n) is greater than the generated random number, the node becomes the cluster head node, otherwise it does not become the cluster head node.

[0076] The expected probability calculation formula is:

[0077]

[0078] Where T(n) is the expected probability of a node becoming a cluster head in the rth round, n is the node number, p is a set ratio used to determine how many nodes should become cluster heads in any given round. This ratio helps balance the load in the network and ensures that not all nodes try to become cluster heads at the same time, thereby reducing energy consumption and extending the life cycle of the network. G is the set of ordinary nodes that have not been selected as cluster head nodes in round 1 / p, and E re (n) is the remaining energy of the nth common node, E aveis the average energy of the current air-ground cluster network model, mod is the "modulo" operation, which is the remainder of the division of two numbers;

[0079] In the expected probability calculation, in order to match the classification of nodes by broadcasting messages in the hierarchical discovery phase of the TPSN time synchronization algorithm, in the cluster head selection of each large cycle, the restrictions on nodes that have become cluster heads within the cycle should be reduced to prevent a node from not joining any cluster in a certain round, but being unable to become a cluster head again because all of its broadcast neighbor nodes have become cluster heads, resulting in the node not joining any cluster in the round and being unable to achieve time synchronization with the parent node, making the CTPSN time synchronization algorithm unable to converge.

[0080] Step 2.4: The cluster head node and common nodes in the current cluster perform hierarchical level update;

[0081] The method for updating the hierarchical level is specifically as follows: taking the hierarchical level in the received hierarchical data packet plus 1 as the updated hierarchical level;

[0082] Step 2.5: The cluster head node after the hierarchy level is updated broadcasts a hierarchical data packet;

[0083] Step 2.6: When an ordinary node that has not joined any cluster receives the hierarchical data packet broadcast by the cluster head node after the hierarchical level is updated, it joins the cluster where the cluster head node is located, updates its own hierarchical level and executes the improved LEACH algorithm to select the cluster head node of the current hierarchical level;

[0084] When a common node or cluster head node that has joined a cluster receives a hierarchical data packet broadcast by the updated cluster head node, it discards the hierarchical data packet;

[0085] After the root node receives the hierarchical data packet sent by the first-level cluster head node for the first time, it will determine which side of itself the cluster head node is on, and then broadcast a completion message to inform other nodes on that side that they do not need to execute the LEACH algorithm anymore;

[0086] Step 2.7: Repeat steps 2.5 to 2.6 until no new cluster head nodes are generated, and then execute step 2.9;

[0087] Step 2.8: Determine whether there are common nodes that have not yet joined the cluster in the current air-ground cluster network model. If so, treat these common nodes as isolated nodes and execute step 2.9. Otherwise, directly execute step 3.

[0088] Step 2.9: The isolated node broadcasts a cluster entry request data packet. After receiving the cluster entry data packet, all the ordinary nodes that have already joined the cluster execute the improved LEACH algorithm to select the cluster head node. The cluster head node broadcasts a cluster entry confirmation data packet. The cluster entry confirmation data packet contains the identity ID and hierarchical level of the cluster head node. The isolated node updates its own hierarchical level to the hierarchical level of the cluster head node plus 1, and joins the cluster where the cluster head node is located.

[0089] After completing the hierarchical discovery phase algorithm of CTPSN, the network topology diagram is as follows: Figure 4 shown.

[0090] In the hierarchical discovery phase of CTPSN, the TPSN hierarchical discovery phase algorithm is optimized to avoid the need for all nodes in the network to broadcast hierarchical data packets. Instead, only the time reference node and cluster head node broadcast hierarchical data packets, and ordinary nodes only need to receive data packets, reducing the amount of data packets sent and thus reducing the energy consumption of the network.

[0091] Step 3: Use the cluster head time synchronization algorithm to synchronize the time of the cluster head node;

[0092] In the process of time synchronization, the cluster head node is not only a reference node for time synchronization of ordinary nodes in the cluster, but also needs to synchronize time with the lower-level cluster head nodes, so it is necessary to ensure the time accuracy of the cluster head node. Considering that the number of child nodes that can become cluster head nodes in each cluster head node is small, in the process of using the TPSN algorithm for time synchronization, the cumulative error caused by the need for serial execution when different child nodes synchronize time with the same parent node is small, and the time accuracy of the child nodes can be ensured. Therefore, the time synchronization between cluster heads adopts the principle of the TPSN time synchronization algorithm. The algorithm execution process is as follows: Figure 5 shown.

[0093] Step 3.1: Each cluster head node sends a time synchronization data packet to the parent node at the next level at time T1;

[0094] The parent node is the cluster head node of the previous level, and this parent node is responsible for collecting data from its child nodes (i.e., the cluster head nodes of the next level) and forwarding it further upward;

[0095] The data packet contains the node ID, timestamp, hierarchical level, data packet type, and time synchronization status to help the parent node identify the source and destination of the data packet;

[0096] Step 3.2: The parent node receives the time synchronization data packet at time T2 and prepares to return the confirmation data packet;

[0097] Step 3.3: The parent node sends a confirmation data packet to the child node at time T3, and the child node receives the confirmation data packet at time T4;

[0098] Step 3.4: Calculate the local clock deviation between the child node and the parent node. The child node corrects the local clock according to the calculated time deviation. When all cluster head nodes successfully complete the time synchronization process, the time consistency of the entire network is guaranteed, and the cluster head time synchronization algorithm ends.

[0099] The local clock deviation between the child node and the parent node is:

[0100] [(T2-T1)-(T4-T3)] / 2

[0101] This is because there are two transmission delays from the child node sending a data packet to the parent node receiving the data packet, then the parent node sending a confirmation data packet, and finally the child node receiving the confirmation data packet. During the execution of one round of the time synchronization algorithm, the position of the node will not change;

[0102] Step 4: Time synchronization of common nodes within the cluster using the intra-cluster time synchronization algorithm;

[0103] After the hierarchical discovery phase of the CTPSN time synchronization algorithm is completed, the network is divided into clusters one by one, and the cluster head node uses a single-hop method to communicate directly with the ordinary nodes in the cluster. The number of nodes in the cluster is unevenly distributed, with some clusters having more nodes and some clusters having fewer nodes. If the ordinary nodes in the cluster still execute the TPSN algorithm for time synchronization, the time synchronization accuracy of some clusters with more nodes will decrease as the number of nodes increases because the ordinary nodes need to serially exchange time synchronization packets with the cluster head node to complete time synchronization. In addition, in the time synchronization phase of the classic TPSN time synchronization algorithm, all nodes except the root node need to complete two-way data packet exchange with their parent node. Frequent data packet exchange requires the entire air-ground cluster network to consume a lot of energy.

[0104] The CTPSN time synchronization algorithm refers to the principle of time synchronization in a single broadcast domain of the RBS time synchronization algorithm. It uses the cluster head to broadcast time synchronization packets in a unidirectional manner, and the nodes in the cluster monitor the wireless channel to complete time synchronization. By reducing the number of times the cluster head node communicates with the nodes in the cluster during the time synchronization process, the number of packets sent and received during time synchronization is optimized, thereby reducing the energy consumption of the algorithm and extending the network life cycle.

[0105] The principle diagram of the CTPSN cluster time synchronization algorithm is as follows Figure 6 shown.

[0106] Step 4.1: The cluster head node acts as a sender. When the sender detects that the channel is idle, it prints a timestamp before the start character of the synchronization data packet to be sent, and then immediately sends the synchronization data packet with the timestamp;

[0107] The timestamp is the local clock of the sender at the absolute time t0, marked as T0. This process is selected at the stage when the sender sends synchronization data packets to the physical layer in bit sequence, avoiding the influence of transmission delay and access delay on time synchronization calculation.

[0108] Step 4.2: Calculate the transmission delay by using the transmission rate and the number of bits of the transmission synchronization data packet;

[0109] Transmission delay:

[0110]

[0111] Assume the size of the transmitted data packet is nbit and the transmission rate is p0 (bit / s);

[0112] Step 4.3: As a receiver, the ordinary node in the cluster prints the arrival time timestamp of the synchronization data packet at the MAC layer. After the synchronization data packet is received and before submitting it to the upper layer service, it prints another timestamp. The receiving delay is calculated by the difference between the two timestamps. The two timestamps are the local clock of the receiver at the absolute time t1 and t2, marked as T1 and T2.

[0113] Step 4.4: Ignore the propagation delay and synchronize the local clocks of common nodes in the cluster according to the transmission delay and reception delay;

[0114] It can be found that there is a strict correspondence between the local clocks of the receiver and the sender, as shown in the formula:

[0115] T=T0+λ0+(T2-T1)

[0116] Among them, T is the local clock of the common node after time synchronization;

[0117] In theory, the receiver can use the above calculation results to update its local clock and complete the time synchronization with the cluster head node.

[0118] The CTPSN cluster time synchronization algorithm has a simple structure, low complexity, low communication overhead, and low energy consumption. It is a lightweight and efficient time synchronization mechanism that can be widely used in small-scale networks but in broadcast domains. When performing time synchronization, since the distance between nodes is much larger than the speed of electromagnetic waves, the propagation delay has little effect on the calculation of node time synchronization. Therefore, although the receiving node cannot accurately estimate the propagation delay, the CTPSN cluster time synchronization algorithm also has good time synchronization accuracy. In addition, the cluster heads of most clusters have both ordinary nodes as child nodes and cluster head nodes of other clusters as child nodes. Therefore, ordinary nodes can calculate clock offsets by listening to the data packets sent by cluster head nodes in the TPSN time synchronization algorithm, which greatly reduces the number of data packets exchanged when executing time synchronization. At the same time, in order to ensure that some clusters with only ordinary nodes can also successfully complete time synchronization, the cluster head node needs to broadcast a time synchronization data packet after completing time synchronization, so that the nodes in the cluster can calculate their own clock offsets through the timestamps in the data packet and update their local clocks.

[0119] The clustered time synchronization protocol CTPSN of the present invention has demonstrated significant advantages and effects in the application of air-ground cluster networks, especially in ensuring time synchronization accuracy and reducing energy consumption. Experimental results show that the energy consumption of CTPSN in the synchronization stage is significantly lower than that of RBS, and its synchronization error is significantly reduced compared with RBS, which means that CTPSN has successfully reduced the overall energy consumption of the algorithm while maintaining high synchronization accuracy similar to TPSN. This improvement directly extends the service life of the air-ground cluster network and improves the energy efficiency of the network. The clustering design of CTPSN enhances the network's adaptability to dynamic changes in topology, and can maintain good time synchronization performance even in the case of changes in network scale or node failures, ensuring the stability and reliability of network operation.

[0120] The CTPSN time synchronization algorithm introduces the improved LEACH protocol into the hierarchical division stage, which almost only requires the cluster head to broadcast hierarchical messages, while the TPSN time synchronization algorithm requires each node to broadcast hierarchical messages. Therefore, the exchange volume of algorithm data packets in the hierarchical division stage of CTPSN is significantly reduced, which greatly reduces the energy consumption of the air-ground cluster system and improves the network life cycle. Figure 7 The energy consumption of TPSN and CTPSN in each of the 12 synchronization rounds without changing the node deployment positions during the hierarchical division phase is described.

[0121] Since the node position does not change and the node broadcast radius does not change in each round of TPSN and CTPSN algorithms, the energy consumed in each round of TPSN algorithm is the same. Six rounds of CTPSN algorithm constitute a large cycle. In a large cycle, due to the increase of synchronization rounds, the cluster head threshold becomes larger, the number of nodes that may become cluster heads increases, and the number of data packets broadcast by nodes increases, so the energy consumption will increase.

[0122] The CTPSN time synchronization algorithm uses different methods to synchronize time for different node types during the time synchronization phase. TPSN is used for time synchronization for cluster head nodes. For the other ordinary nodes with cluster head nodes in the cluster, the standard time is obtained by channel monitoring of the data packets exchanged between cluster head nodes for time synchronization, and the local clock deviation is calculated using the formula. For ordinary nodes without other cluster heads in the cluster, a data packet will be broadcast after the cluster head of the cluster completes time synchronization, so that these ordinary nodes can complete time synchronization. TPSN requires all nodes to perform two-way data packet exchange with the upper parent node to complete time synchronization. Therefore, CTPSN also greatly reduces the exchanged data packets during the time synchronization phase, reducing the energy consumption of the algorithm. Figure 8 The energy consumption of TPSN and CTPSN in each of the 12 synchronization rounds without changing the node deployment locations during the time synchronization phase is described.

[0123] Since the position of the node does not change and the broadcast radius of the node does not change in each round of the TPSN algorithm, the network topology tree does not change. Therefore, the energy consumed in each round of the TPSN algorithm is the same. In a large cycle of the CTPSN algorithm, due to the increase in synchronization rounds, the cluster head threshold becomes larger, the number of nodes that may become cluster heads increases, and the cluster head nodes perform TPSN two-way message exchange when performing time synchronization, so the energy consumption may increase; but on the other hand, in a large cycle, the threshold of the node that has become a cluster head will be smaller than the threshold of the node that has not become a cluster head, which may lead to a decrease in the number of nodes that become cluster heads in this round, resulting in reduced energy consumption.

[0124] In order to compare the synchronization accuracy of CTPSN algorithm and classic TPSN algorithm in time synchronization, simulation experiments were carried out on TPSN and CTPSN algorithms, and the experimental results were compared. Fig. 9 The mean error of each of the 12 rounds of synchronization between TPSN and CTPSN without changing the node deployment locations is described.

[0125] The CTPSN time synchronization algorithm uses the idea of ​​clustering to complete time synchronization. Although the tree network topology height formed in the hierarchical division stage may be higher than the tree network formed by TPSN, since each node in the cluster completes time synchronization by monitoring the channel, a single broadcast message can achieve time synchronization of multiple nodes, avoiding the accumulation of errors caused by the cluster head node having to perform point-to-point time synchronization with its multiple child nodes, thereby improving the synchronization accuracy of the algorithm.

Claims

1. A low-energy clustering time synchronization method in air-ground cluster networks. It is characterized in that The steps include: Step 1: Establish an air-ground cluster network model; Step 2: Use the CTPSN hierarchical discovery phase algorithm and the improved LEACH algorithm to perform hierarchical level allocation and clustering of common nodes in the air-ground cluster network model; Step 3: Use the cluster head time synchronization algorithm to synchronize the time of the cluster head node; Step 4: Use the intra-cluster time synchronization algorithm to synchronize the time of common nodes in the cluster.

2. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 1, It is characterized in that The method for establishing the air-ground cluster network model described in step 1 is specifically as follows: in a rectangular monitoring area with a side length of L and an area of ​​L×L, N ordinary nodes are randomly deployed, each ordinary node represents a drone or an unmanned vehicle, and then a time reference node Sink is deployed in the center of the rectangular monitoring area; The time reference node Sink has unlimited energy and a fixed position that does not change. The time reference node Sink has a GPS module to provide it with absolute time. The energy of the remaining N common nodes except the time reference node Sink is provided by batteries and has the same and limited initial energy. Each of the N common nodes is identified by a unique ID number and can calculate its own residual energy; The communication radius of each of the N ordinary nodes is not completely consistent. The communication radius is changed by adjusting its own transmission power. The ordinary node determines the distance between itself and other ordinary nodes according to the strength of the received signal. Any ordinary node has at least one communicative ordinary node within its communication radius.

3. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 2, It is characterized in that Step 2 specifically includes: Step 2.1: Set the time base node Sink as the root node, set its level to 0, and set the current round to 1; Step 2.2: The root node broadcasts a hierarchical data packet. The ordinary nodes that receive the hierarchical data packet join the cluster where the root node is located and are divided into two categories according to their own location information, one in the left area of ​​the root node and the other in the right area of ​​the root node; the hierarchical data packet includes the identity ID and hierarchical level of the root node or cluster head node that sends the hierarchical data packet; Step 2.3: Execute the improved LEACH algorithm for the common nodes in the left area and the common nodes in the right area respectively, and select the cluster head nodes in the left area and the right area respectively; Step 2.4: The cluster head node and common nodes in the current cluster perform hierarchical level update; Step 2.5: The cluster head node after the hierarchy level is updated broadcasts a hierarchical data packet; Step 2.6: When an ordinary node that has not joined any cluster receives a hierarchical data packet broadcast by a cluster head node after the hierarchical level is updated, it joins the cluster where the cluster head node is located, updates its own hierarchical level, and executes the improved LEACH algorithm to select the cluster head node of the current hierarchical level; when an ordinary node or cluster head node that has joined a cluster receives a hierarchical data packet broadcast by the updated cluster head node, it discards the hierarchical data packet; Step 2.7: Repeat steps 2.5 to 2.6 until no new cluster head nodes are generated, and then execute step 2.9; Step 2.8: Determine whether there are common nodes that have not yet joined the cluster in the current air-ground cluster network model. If so, treat these common nodes as isolated nodes and execute step 2.

9. Otherwise, directly execute step 3. Step 2.9: The isolated node broadcasts a cluster-joining request packet. After all ordinary nodes that have already joined the cluster receive the cluster-joining packet and execute the improved LEACH algorithm to select the cluster head node, the cluster head node broadcasts a cluster-joining confirmation packet. The cluster-joining confirmation packet contains the identity ID and hierarchical level of the cluster head node. The isolated node updates its own hierarchical level to the hierarchical level of the cluster head node plus 1, and joins the cluster where the cluster head node is located.

4. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 3, It is characterized in that The improved LEACH algorithm described in step 2.3 is specifically: For each common node, when it is running to become a cluster head node, a random number is generated between 0 and 1 and compared with the expected probability T(n). If the expected probability T(n) is greater than the generated random number, the node becomes the cluster head node, otherwise it does not become the cluster head node. The expected probability calculation formula is: Where T(n) is the expected probability of a node becoming a cluster head in the rth round, n is the node number, p is a set ratio, G is the set of ordinary nodes that have not been selected as cluster head nodes in 1 / p rounds, and E re (n) is the remaining energy of the nth common node, E ave is the average energy of the current air-ground cluster network model.

5. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 3, It is characterized in that The method for updating the hierarchical level in step 2.4 is specifically: taking the hierarchical level in the received hierarchical data packet + 1 as the updated hierarchical level.

6. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 1, It is characterized in that Step 3 specifically includes: Step 3.1: Each cluster head node sends a time synchronization data packet to the parent node at the next level at time T1; Step 3.2: The parent node receives the time synchronization data packet at time T2 and prepares to return the confirmation data packet; Step 3.3: The parent node sends a confirmation data packet to the child node at time T3, and the child node receives the confirmation data packet at time T4; Step 3.4: Calculate the local clock deviation between the child node and the parent node, and the child node corrects the local clock according to the calculated time deviation; The local clock deviation between the child node and the parent node is: [(T2-T1)-(T4-T3)] / 2.

7. A method for low energy consumption clustering time synchronization in an air-ground cluster network according to claim 1, It is characterized in that Step 4 specifically includes: Step 4.1: The cluster head node acts as a sender. When the sender detects that the channel is idle, it prints a timestamp before the start character of the synchronization data packet to be sent, and then immediately sends the synchronization data packet with the timestamp; The timestamp is the sender's absolute time t 0 The local clock at time T 0 ; Step 4.2: Calculate the transmission delay by using the transmission rate and the number of bits of the transmission synchronization data packet; Transmission delay: Where n is the size of the transmitted data packet, p 0 is the transmission rate, λ 0 is the transmission delay; Step 4.3: As a receiver, the ordinary node in the cluster prints the arrival time stamp of the synchronization data packet at the MAC layer. After the synchronization data packet is received, another time stamp is printed before submitting it to the upper layer service. The difference between these two timestamps is used to calculate the reception delay. The two timestamps are the absolute time t of the receiver and the time delay t of the receiver. 1 and t 2 The local clock at time T 1 and T 2 ; Step 4.4: Ignore the propagation delay and synchronize the local clocks of common nodes in the cluster according to the transmission delay and reception delay; T=T 0 +λ 0 +(T 2 -T 1 ) Wherein, T is the local clock of the common node after time synchronization.

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