VDE signal satellite access control method

Abstract

The invention relates to a VDE signal satellite access control method. The method comprises the following steps that a satellite end carries out dynamic clustering and communication time slot pre-distribution on ships in a satellite signal coverage area; the ship end in the dynamic cluster performs local negotiation distribution of communication time slots and periodically reports to the satellite end; and the satellite end dynamically optimizes the pre-allocated communication time slot in the next period according to the report of each region in each period. The method has the advantage of reducing the transmission conflict rate in a scene with heavy ship flow.

Description

Technical Field

[0001] The present application relates to the field of satellite communication, and in particular to a method for satellite access control of VDE signals. Background Art

[0002] The Vessel Data Exchange System (VDES) is an upgrade and expansion of the AIS system, capable of achieving real-time two-way data communication in any sea area of the world. Compared with the traditional AIS, the data transmission rate of VDES has increased by up to 32 times and supports satellite relay, realizing two-way communication with global coverage. This means that even when ocean-going ships leave the coverage area of coastal VHF base stations, they can still communicate with the shore through the VDES satellite link, thus expanding the scope of maritime communication.

[0003] The satellite control station acts as a centralized scheduler and manages the uplink channel through the SBB announcement and assignment mechanism. Multiple ship terminals can use TDMA to transmit in parallel within the same frame. The system differentiates the access of different satellites through orthogonalization technology. For example, when multiple VDES satellites cover an overlapping area, different frequencies / codes are used to ensure non-interference. In addition, to protect traditional VHF services such as AIS and DSC from interference, the VDES system sets duty cycle limits and priority rules for slot usage. For example, the number of slots occupied by each ship station on a single channel per minute does not exceed a certain proportion (such as 2.2%) to ensure the priority transmission of AIS messages and prevent the channel from being flooded by excessive VDES data. Through these mechanisms, the data exchange between ships and satellites can be carried out orderly and efficiently. Even in the open ocean area without shore station control, ships can rely on satellite VDES to obtain reliable data communication services.

[0004] One of the currently recommended access schemes for VDES is to follow the self-organizing TDMA protocol of AIS. However, research shows that when the ship traffic is heavy, the self-organizing TDMA protocol is prone to a significant increase in the transmission conflict rate because each station independently selects time slots and lacks global coordination. Especially in a large area covered by satellites, when a large number of ships that cannot monitor each other access simultaneously, if pure autonomous allocation is used, many conflict occupations will occur, reducing the effective throughput. Even with the introduction of a centralized assignment mechanism, random access channels may collide under high-concurrency requests, and multiple ships may compete for access in the same RACH time slot, resulting in access delay and reduced efficiency. Summary of the Invention

[0005] In order to reduce the transmission conflict rate in the scenario of heavy ship traffic, the present application provides a method for satellite access control of VDE signals.

[0006] A method for satellite access control of VDE signals provided by the present application adopts the following technical solutions: A VDE signal satellite access control method includes the following steps: S1. The satellite side dynamically clusters the ships within the satellite signal coverage area and pre - allocates communication time slots; S2. The ship sides within the dynamic cluster group conduct local negotiation and allocation of communication time slots and report to the satellite side periodically; S3. The satellite side dynamically optimizes the pre - allocated communication time slots for the next cycle according to the reports from each area in each cycle.

[0007] By adopting the above technical solution, the time slot allocation result is broadcast to all ships through the satellite announcement channel, ensuring that the ships within the cluster group know the available time slot segments corresponding to their cluster groups, so as to make requests and scheduling based on this boundary during the local negotiation process, avoiding cross - cluster occupation and conflicts. The system realizes the dynamic re - balance of resources among cluster groups, effectively alleviates the access congestion in high - density ship areas, and improves the throughput capacity and time slot utilization rate of the overall system. The whole process requires no manual intervention, and the automatic execution mechanism based on state feedback and threshold determination provides a strong guarantee for the stable operation of the VDE system in complex and dynamic maritime communication scenarios.

[0008] Optionally, the S1 includes the following steps: S11. The satellite side obtains the real - time position distribution and access density of the ships that have established communication with itself within the satellite signal coverage area; S12. Based on the real - time position distribution and access density of the ships within the satellite signal coverage area, the satellite side uses the density clustering algorithm to divide the ships into several dynamic cluster groups; S13. The satellite side initially allocates reserved TDMA time slot segments for each area according to the divided areas and the ship density and historical traffic within the areas for the ships within the areas to conduct local negotiation for access.

[0009] By adopting the above technical solution, the satellite side can real - time sense the spatial distribution trend and access load situation of the ships within the coverage area, providing a data basis for the next step of clustering and reasonable allocation of time slot resources. This comprehensive perception method based on dynamic position and access behavior not only helps to achieve a more reasonable network topology division, but also improves the pertinence and accuracy of access resource scheduling. Especially in the scenario of dense maritime traffic and high - concurrent communication requirements, it can significantly enhance the network's response ability and overall operation efficiency.

[0010] Optionally, the S12 includes the following steps: S121. According to the density clustering algorithm, set the cluster division parameters and calculate the clusters in real - time; S122. Form several logical clusters according to the calculation results of the clusters; S123. Record the cluster center positions and boundaries corresponding to each logical cluster for subsequent targeted time slot pre - allocation by satellites.

[0011] By adopting the above - mentioned technical solution, the system can flexibly adapt to the dynamic changes in the number and distribution of ships, making each cluster relatively compact in space and relatively consistent in communication requirements, facilitating the subsequent allocation of communication resources in units of partitions and reducing the complexity of global scheduling. At the same time, due to the characteristics of density clustering in automatically removing outliers and adapting to the number of clusters, it is more stable and reliable when facing a highly dynamic ship network, ensuring that the system has good real - time performance and adaptability.

[0012] Optionally, S13 includes the following steps: S131. Determine that the VDE frame period is 1 minute and there are 2,250 basic time slots in total; S132. Divide the basic time slots into to - be - allocated time slots, global elastic time slots, and global emergency time slots; among them, the global elastic time slots are used for subsequent dynamic adjustment and random access of individual ships, and the global emergency time slots are used for emergency communication between satellites and ships without negotiation; S133. According to the regions divided in the previous step and the ship density and historical traffic within the regions, divide the to - be - allocated time slots into each cluster to determine the allocation situation of each cluster in the initial state; S134. Broadcast the time slot pre - allocation information through the satellite announcement channel.

[0013] By adopting the above - mentioned technical solution, the system realizes the structured management of basic time slots.

[0014] Optionally, S2 includes the following steps: S21. Within each cluster, the ship - side executes a local negotiation process and completes local time slot allocation through the pre - allocated local channel using a three - step handshake mechanism; S22. After each ship - side within each cluster completes access negotiation within each VDE frame period, it regularly sends a regional access status message to the satellite - side, and the satellite - side completes regional load statistics based on this message.

[0015] Optionally, S21 includes the following steps: S211. Each ship - side within the cluster listens to the set of pre - allocated time slots and randomly selects an unoccupied time slot from the set of pre - allocated time slots to send an initial time slot request message at the beginning of each frame; S212. Other ships within the cluster listen and confirm the occupancy of time slots in this region. When no conflict occurs, they actively send a permission message in the subsequent confirmation time slot. If a conflict is detected, a veto message is fed back. S213. If the sender receives more than half of the permission messages, it sends a confirmation message to explicitly occupy the time slot; otherwise, it re - executes the random back - off and makes a new request. S214. After the local negotiation is completed, each ship terminal sends data within the reserved time slot after the negotiation is completed to complete the local access.

[0016] By adopting the above - mentioned technical solution, the orderly and efficient transmission of data is achieved with the minimum control load, providing a stable underlying support for communication scheduling in large - scale dynamic access scenarios.

[0017] Optionally, the S22 includes the following steps: S221. Select a central ship within the cluster. S222. The central ship terminal counts the status information of each ship terminal in the cluster, where the status information includes the negotiation success rate, the number of conflict occurrences, and the number of idle time slots. After each minute - long cycle ends, the central ship terminal sends a reporting message of the status information of the cluster during this cycle to the satellite terminal on behalf of the cluster.

[0018] By adopting the above - mentioned technical solution, the system establishes an adaptive dynamic scheduling foundation, realizes the closed - loop control of resource allocation based on network status, and greatly improves the flexibility and throughput efficiency of the network in dealing with complex navigation environments.

[0019] Optionally, the S3 includes the following steps: S31. Determine the thresholds corresponding to each status information, including the negotiation success rate threshold, the conflict rate threshold, and the proportion threshold of the number of idle time slots. S32. The satellite terminal compares the status information with the thresholds according to the access status report of each cycle area. S33. Allocate or reclaim the global elastic time slots to each area according to the comparison result to adjust the number of pre - allocated time slots in the next cycle.

[0020] In summary, the present application includes at least one of the following beneficial technical effects: 1. By using the density - based spatial clustering of applications with noise (DBSCAN) algorithm to perform real - time dynamic clustering of ships within the satellite coverage area, communication resources can be allocated to local clusters on demand, ensuring sufficient access time slots in ship - dense areas and effectively alleviating the access conflict problem of traditional SOTDMA schemes in large - scale and high - concurrency environments.

[0021] 2. By setting multi - dimensional status parameters such as the negotiation success rate, the conflict rate, and the proportion of idle time slots, and establishing a corresponding dynamic adjustment mechanism, the system can dynamically increase or decrease the time slot allocation of each cluster according to the periodic status feedback, realizing the optimal distribution of resources in the time - space dimension and improving the throughput capacity and utilization efficiency of the system.

[0022] 3. The present invention adopts a three-step handshake mechanism within the cluster to achieve distributed time slot negotiation. Ships autonomously complete time slot allocation based on local listening, and the satellite side only needs macroscopic configuration and periodic adjustment, avoiding the performance bottleneck of traditional centralized scheduling in large-scale access scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The flowchart showing the VDE signal satellite access control method in an embodiment of the present invention.

[0024] Figure 2 The flowchart showing the sub-step S1 in an embodiment of the present invention.

[0025] Figure 3 The flowchart showing the sub-step S12 in an embodiment of the present invention.

[0026] Figure 4 The flowchart showing the sub-step S13 in an embodiment of the present invention.

[0027] Figure 5 The flowchart showing the sub-step S2 in an embodiment of the present invention.

[0028] Figure 6 The flowchart showing the sub-step S21 in an embodiment of the present invention.

[0029] Figure 7 The flowchart showing the sub-step S22 in an embodiment of the present invention.

[0030] Figure 8 The flowchart showing the sub-step S3 in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following further describes the present application in detail with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0032] In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the inventive concept. As part of this specification, some of the drawings in the present disclosure represent structures and devices in block diagram form to avoid obscuring the disclosed principles. For clarity, not all features of actual specific implementations are necessarily described. In addition, the language used in the present disclosure has been primarily selected for readability and guidance purposes and may not have been chosen to delimit or circumscribe the subject matter of the invention, and thus recourse is had to the required claims to determine such inventive subject matter. References in the present disclosure to "a specific implementation" or "specific implementations" mean that the particular features, structures, or characteristics described in connection with the specific implementation are included in at least one specific implementation, and multiple references to "a specific implementation" or "specific implementations" should not necessarily be construed as all referring to the same specific implementation.

[0033] Unless explicitly defined otherwise, the terms "a", "an", and "the" are not intended to refer to a singular entity but include the general category for which a particular example may be used for illustration. Thus, the use of the term "a" or "an" may mean any number of at least one, including "one", "one or more", "at least one", and "one or more than one". The term "or" means any one of the alternatives and any combination of the alternatives, including all of the alternatives, unless the alternatives are explicitly indicated to be mutually exclusive. The phrase "at least one of" when combined with a list of items refers to a single item in the list or any combination of items in the list. The phrase does not require all of the listed items, unless explicitly so defined.

[0034] An embodiment of the present application discloses a method for satellite access control of VDE signals. Referring to Figure 1 , the method includes the following steps S1 - S3.

[0035] S1. The satellite end performs dynamic clustering and pre - allocation of communication time slots for ships within the satellite signal coverage area.

[0036] In this step, the satellite end performs dynamic clustering of ships within the satellite signal coverage area and pre - allocates communication time slots. Specifically, the satellite end refers to a satellite payload device deployed in a low - Earth orbit or a geosynchronous orbit, whose main functions include signal transmission and reception, reception of ship information and data forwarding, and unified communication resource scheduling for ships within its coverage area.

[0037] Dynamic clustering refers to the satellite side dynamically and logically partitioning the ships within a region through a specific algorithm based on the real-time obtained ship position distribution and communication requirements, forming several clusters. Different from traditional static regional partitioning, dynamic clustering adjusts in real time with the change of ship positions, and can effectively adapt to the unevenness and mobility of ship distributions. For example, assume there are 300 ships sailing in a certain sea area. The satellite side will collect the current geographical positions of these ships at regular intervals and use density clustering algorithms such as the DBSCAN algorithm to divide the ships with high spatial density and adjacent positions into independent clusters. Each cluster generally includes a dozen to dozens of ships. For example, in the Bohai Bay area, dense shipping routes may form several obvious clusters, and the satellite side separately schedules communication resources for these clusters.

[0038] Communication time slots are a kind of time resource unit defined in the TDMA communication mechanism. The communication cycle of the satellite side is generally set to 1 minute, and each minute can be divided into 2250 basic time slots. The satellite side pre-allocates a certain number of time slots for subsequent communication negotiation of the ship side according to the number of ships in each cluster, historical data communication traffic, and real-time access density. These pre-allocated time slots are broadcast to all ships within the cluster through the satellite announcement channel, enabling the ships to select idle resources for communication within the pre-allocated time slots according to their own communication requirements, without directly applying through the satellite side. Specifically, in a certain embodiment, S1 includes the following steps S11 - S13.

[0039] S11. The satellite side obtains the real-time position distribution and access density of the ships that have established communication with itself within the satellite signal coverage area.

[0040] The satellite side needs to first comprehensively sense the ships within its signal coverage area, mainly including obtaining the real-time geographical position and current access density information of each ship. The satellite signal coverage area refers to the service area formed by the satellite in its orbit in real time, generally a large-radius area centered on the satellite's sub-satellite point. In the VDE system, the coverage radius is usually about 1000 kilometers, and the coverage range can reach millions of square kilometers. The ships within this area that have completed the handshake connection with the satellite are the ships that have established communication with the satellite, and their status and information can be effectively received and processed by the satellite.

[0041] The satellite terminal regularly receives the position information sent by ships through the uplink channel. This information usually includes the longitude and latitude coordinates, altitude, speed, and equipment identification code of the ships. Each ship usually reports its own position automatically at regular time intervals (such as every 30 seconds or every 1 minute). Therefore, the satellite terminal can build a real-time position database of all active ships in the coverage area. In a certain area of the South China Sea, assuming that a VDES satellite simultaneously receives reports from 300 ships, the system calibrates these 300 position data on the geographic coordinate system to obtain a ship position distribution map in the coverage area.

[0042] In addition to position, the satellite terminal also needs to obtain the access density information, that is, how many ships are simultaneously attempting to access per unit area. This can be estimated by counting the uplink communication frequency, the number of data packets, and the random access conflict rate of ships within a certain range. For example, the entire coverage area can be divided into multiple grid areas of 50km×50km. In each grid, count the number of ships actively sending access requests per unit time, and then form a heat map to reflect the access pressure in each area. For example, in a strait channel area with a dense shipping route, there may be 5 ships simultaneously requesting access per square kilometer, while in the sea area far from the main shipping lane, there are only sporadic requests.

[0043] S12. Based on the real-time position distribution and access density of ships within the satellite signal coverage area, the satellite terminal uses the density clustering algorithm to divide the ships into several dynamic cluster groups.

[0044] Based on the obtained real-time position distribution and access density information of ships, the satellite terminal uses the density clustering algorithm to logically divide the ships to form multiple dynamic cluster groups. The core of this process is to automatically divide a large number of unevenly distributed ships in the coverage area into several partition units with local consistency according to their spatial proximity and communication load characteristics, so as to provide a logical boundary and a basis for independent scheduling for subsequent communication time slot resource allocation. The clustering method selected is the density clustering algorithm, that is, Density-Based Spatial Clustering of Applications with Noise (DBSCAN) or its improved version. It does not rely on the preset number of clusters, but dynamically generates a clustering structure based on the density relationship between data points, and is particularly suitable for target groups such as ships at sea with floating numbers and highly uneven spatial distributions.

[0045] During the implementation process, the satellite side first sets the clustering parameters, including the neighborhood radius ε and the minimum density threshold MinPts. The neighborhood radius ε defines the distance range regarded as adjacent around a certain ship, and MinPts defines how many ships need to exist within this distance range to form a valid clustering core point. For example, if ε is set to 30 kilometers and MinPts is set to 5, then when a ship detects at least 5 other ships within its 30-kilometer radius, it can be regarded as a clustering core point. Based on this rule, the algorithm starts from the core point and gradually expands into the neighborhood, incorporating ships that are density-reachable into the same cluster until no further expansion is possible.

[0046] For example, in the densely navigated area of the Yellow Sea, the satellite side receives the position data of 600 ships. After analysis using the density clustering algorithm, 8 clusters are formed. One of the clusters includes 80 ships sailing between the Shandong Peninsula and the Korean Peninsula, and another cluster covers the sea area near Lianyungang, Jiangsu, with a total of 60 ships. The distance between them exceeds the neighborhood threshold of the clustering algorithm, so they are divided into independent clusters. For those individuals sailing in remote areas with insufficient ship density around them, they are treated as noise points and are not temporarily assigned to any cluster, but are processed separately by the satellite side or have resource allocation delayed.

[0047] Specifically, in one embodiment, S12 includes the following steps S121 - S123.

[0048] S121. According to the density clustering algorithm, set the cluster division parameters and perform cluster calculations in real time.

[0049] S122. Form several logical sub-clusters according to the calculation results of the clusters; S123. Record the cluster center positions and boundaries corresponding to each logical sub-cluster for subsequent targeted time slot pre-allocation by the satellite.

[0050] The setting of the parameters needs to be combined with the actual ship distribution characteristics and communication characteristics of the sea area. For example, set ε to 25 kilometers and MinPts to 5, that is, it is required that at least 5 other ships exist within the 25-kilometer range of a ship to trigger clustering. In actual deployment, the satellite side applies this pair of parameters to the set of ship position information it receives. Whenever new ship reporting data in the VDE frame period is received, the system starts the clustering calculation and performs a complete cluster division calculation on all ships within the current satellite coverage area.

[0051] Taking the East China Sea shipping route as an example, the satellite terminal receives the position reports of approximately 450 ships within one minute and starts the clustering algorithm after parameter setting. The system checks the number of neighboring ships within a range of 25 kilometers around each ship one by one. For ships that meet MinPts = 5, they are used as the clustering core points to start expanding, and the ships that are density-reachable are successively grouped into the same cluster. During this cycle, the system identified a total of 10 effective clusters, with each cluster containing an average of 30 to 60 ships.

[0052] S13. Based on the regions divided in the previous step, as well as the ship density and historical traffic within the regions, the satellite terminal initially allocates reserved TDMA time slot segments for each region for ships within the region to negotiate local access.

[0053] The satellite terminal first counts the current number of ships in each cluster group and the ship density per unit area, and calls the historical communication traffic data collected in the past several communication cycles, including the average packet length, transmission frequency, access failure rate, etc., to comprehensively evaluate the intensity of communication resource requirements for this cluster group. In the scheduling strategy, the satellite gives priority to ensuring that high-density and high-traffic cluster groups obtain more time slot resources.

[0054] Specifically, in one embodiment, S13 includes the following steps S131 - S134.

[0055] S131. Determine that the VDE frame period is 1 minute and there are a total of 2250 basic time slots.

[0056] Based on the completed dynamic cluster group structure, as well as the ship density and historical communication traffic data within each cluster group, the satellite terminal allocates corresponding numbers of TDMA time slot segments for each cluster group for subsequent ship terminals to negotiate local access within the cluster group. In the TDMA (Time Division Multiple Access) communication mechanism, the entire communication frame is divided into several basic time slots, and each time slot can be independently occupied by a ship for data transmission. In the VDES system, a 1-minute frame period contains 2250 basic time slots, and these time slots are the key resource scheduling units in the present invention.

[0057] S132. Divide the basic time slots into time slots to be allocated, global flexible time slots, and global emergency time slots; among them, the global flexible time slots are used for subsequent dynamic adjustment and random access of individual ships, and the global emergency time slots are used for emergency communication between the satellite and ships without negotiation.

[0058] The satellite terminal structurally divides the basic communication time slot resources within the entire VDE frame period, and subdivides 2,250 basic time slots into three categories according to functional purposes: unallocated time slots, global elastic time slots, and global emergency time slots. Unallocated time slots refer to the main part for the regular communication resource allocation between clusters, with the largest proportion. The satellite terminal will segment and allocate them to different clusters based on indicators such as the access density and historical communication load of each cluster. Global elastic time slots are reserved for periodic dynamic adjustment and random access requests from individual ships not in the pre-allocated set, serving as a regulation buffer for the system to cope with traffic mutations and sudden load changes. Global emergency time slots belong to reserved channels, dedicated to transmitting high-priority information that does not require handshake negotiation, such as distress alarms and reports of maritime emergencies, ensuring a reliable transmission path for critical communications in high-congestion scenarios.

[0059] This division is uniformly completed by the satellite terminal before the start of each cycle and broadcast to all terminals through the announcement channel to ensure that each ship knows the number range and usage restrictions of various time slots during the access cycle. In implementation, the satellite terminal first determines the network load situation of the current cycle and slices the time slot resources according to the static ratio set by the system or the dynamic parameters based on feedback. For example, 75% of the time slots (i.e., 1,687) are set as unallocated time slots for each cluster to negotiate and use as needed, 15% (about 338) as global elastic time slots, which are flexibly scheduled by the satellite terminal according to the real-time state, and 10% (225) as global emergency time slots, distributed at fixed positions within the frame period to give priority to responding to emergency communications. Each type of time slot is accompanied by usage rules. For example, elastic time slots are prohibited from being continuously occupied for a long time, and emergency time slots require a high-priority identifier to be carried before transmission is allowed.

[0060] S133. Divide the unallocated time slots into each cluster according to the area divided in the previous step, the ship density and historical traffic within the area, so as to determine the allocation situation of the initial state of each cluster.

[0061] After completing the three-category division of the basic time slots, the specific time slot resources in the unallocated time slot set are segmented and allocated according to the communication requirements of each cluster divided previously, forming the time slot usage segments of each cluster in this cycle. This allocation process constructs a weighted model based on multi-dimensional parameters such as the ship density, historical traffic intensity, access success rate, and conflict rate within each cluster, and adjusts the proportion of resource allocation between clusters.

[0062] The satellite terminal calls the operation indicators of each cluster group in recent cycles through the access status historical database to form a cluster group demand weight table for the current cycle. For example, the ship density in Cluster A is high, with 680 access requests in the previous cycle and a conflict rate of 9%; Cluster B is relatively sparse, with 320 requests and a conflict rate of 2%. The system calculates the demand weights of Cluster A and Cluster B as 0.68 and 0.32 respectively through a weighting function. If the total number of time slots to be allocated is 1,687, the allocation result is that Cluster A gets 1,147 time slots and Cluster B gets 540 time slots. The specific time slot numbers are arranged by the system in sequence by segments. For example, the corresponding basic time slot numbers for Cluster A range from No. 100 to No. 1,246, and for Cluster B from No. 1,247 to No. 1,786. The segment boundaries are set to be non-crossing to ensure the physical isolation between cluster groups and avoid conflicts and interferences caused by channel overlap.

[0063] S134. Broadcast the time slot pre-allocation information through the satellite announcement channel.

[0064] After the allocation scheme is generated, it is cached in the broadcast information structure of the satellite and sent to each cluster group through the announcement channel in the next step. After each ship receives the corresponding time slot segment number of its cluster group, it can perform local negotiation operations locally. In the actual application in the Beibu Gulf area of the South China Sea, 6 cluster groups are formed in a certain frame cycle, each having different proportions of active ships and historical data traffic. Based on the feedback, the satellite terminal determines that Cluster E has an extremely high access density and conflict history, and decides to allocate 900 basic time slots to it, accounting for 53% of the entire set of time slots to be allocated, and the corresponding number range is from time slot #300 to #1,199. At the same time, only 120 time slots are allocated to the sparsely populated Cluster F for maintaining basic periodic reporting and non-emergency data communication, and the number range is from #1,640 to #1,759.

[0065] S2. The ship terminals within the dynamic cluster group perform local negotiation and allocation of communication time slots and report to the satellite terminal periodically.

[0066] The ship terminals within the cluster group execute the local negotiation process based on the time slot resources pre-allocated by the satellite terminal, complete data access, and at the end of each communication cycle, feedback the access status to the satellite terminal. The entire process coordinates resources in a distributed manner, avoiding the centralized processing burden of the satellite terminal assigning resources to each ship one by one, and realizing an efficient and low-conflict channel access control mechanism.

[0067] Specifically, in one embodiment, S2 includes the following steps S21 - S22.

[0068] S21. Within each cluster group, the ship terminals execute the local negotiation process and complete the local time slot allocation through the pre-allocated local channel with a three-way handshake mechanism.

[0069] Specifically, in one embodiment, S21 includes the following steps S211 - S214.

[0070] S211. Each ship terminal within the cluster monitors the pre - allocated time - slot set. At the beginning of each frame, it randomly selects an unoccupied time - slot from the pre - allocated time - slot set and sends an initial time - slot request message. After the ship terminal finishes monitoring the pre - allocated time - slot set, it starts the first step of the local negotiation process. That is, at the beginning of each frame period, for the TDMA time - slot segment reserved by the satellite terminal in this cluster, it randomly selects an unoccupied time - slot and sends a time - slot request message. The pre - allocated time - slot set is a continuous time - slot resource independently divided by the satellite terminal for each cluster according to factors such as the number of ships, access density, and historical traffic in S13. After being broadcast through the satellite announcement channel, all ships within the cluster can know the time - slot range corresponding to their cluster. For example, the time - slot set corresponding to cluster A is the basic time - slots numbered from 500 to 799.

[0071] In actual operation, the ship terminal selects a target time - slot from this set through a pseudo - random number generator. The generation process constructs a random factor based on information such as the system timestamp and ship ID to ensure that the selection results of each ship are different and reduce the probability of selection coincidence. To determine whether the target time - slot has been occupied by other ships, each ship locally maintains a short - term intra - cluster access status cache, recording the request, permission, and confirmation broadcast content monitored in the previous frame period. For example, if time - slot No. 520 was confirmed to be occupied by ship X in the previous frame, then at the beginning stage of the current frame, other ships will regard No. 520 as temporarily unavailable and skip this time - slot to avoid application. In addition, the ship terminal also listens to the intra - cluster broadcast channel at the beginning of the current period to monitor in real - time whether other ships have requested the same time - slot, ensuring that the selected time - slot is in an unoccupied state.

[0072] After completing the random selection, the ship terminal immediately constructs and broadcasts a time - slot request message. This message includes at least fields such as ship identification (e.g., MMSI number), target time - slot number, application priority flag, data service type identification, and application duration. Among them, the priority flag can be used to identify whether the communication is for emergency or high - timeliness data, such as course change, collision avoidance instructions, etc., while the service type field indicates the structure and expected length of this data transmission, helping the negotiation node judge whether it meets the conditions to grant permission. The broadcast of this request message serves to arouse the response mechanism of other ships within the same cluster, start the listening and permission process in S212, and also lay a signaling foundation for subsequent confirmation of the ownership of this time - slot.

[0073] Taking a cluster in the East China Sea shipping lane as an example, within the time slot set 500 to 799 of the cluster to which ship A belongs, ship A selects the 602nd time slot through a random number algorithm. After querying the cache and confirming that the time slot is not occupied, it then broadcasts a time slot request message with a request number of 602, declaring that it will soon send updated data on ship position information. After receiving this request, other ships enter the permission judgment process. If no conflict is found, they will return a permission message in the subsequent time slot. This process ensures that ships can independently complete access requests based on sufficient information perception, reducing the scheduling burden on the satellite side and significantly improving the flexibility and efficiency of resource allocation within the cluster.

[0074] S212. Other ships within the cluster listen and confirm the occupancy of time slots in this area. When no conflict occurs, they actively send a permission message in the subsequent confirmation time slot. If a conflict is detected, a veto message is fed back.

[0075] After receiving a time slot request message sent by a certain member within the cluster, other ships within the cluster immediately start the listening and judgment process to decide whether to grant permission for this request. The listening behavior is completed by continuously monitoring the cluster communication channel locally. In each VDE frame period, each ship always listens to the reserved time slot segment allocated within the scope of this cluster, especially paying attention to whether there are already occupied time slots or whether there are overlapping request behaviors occurring. The record of the occupancy situation is divided into two parts: one part comes from the confirmation message record of the previous cycle. Ships will maintain a time slot usage status table locally, marking which time slots were occupied in the previous cycle and automatically clearing it after a certain period of time; the other part comes from real-time listening. The time slot request messages, permission messages, and confirmation messages sent by other ships in the current frame period will all be recorded in this table for judging the conflict status.

[0076] The subsequent confirmation time slot refers to the dedicated period during which the receiving ship will broadcast a response message to the cluster within the next few basic time slot windows after the time slot request message is sent. The entire handshake mechanism is based on time division. In each round of negotiation, several standard response windows are set within the frame, including permission response time slots, confirmation response time slots, etc., to ensure that the negotiation process has strict timing and synchronization. During the listening process, once a ship finds that there are two or more ships broadcasting requests for the same basic time slot in this frame, or detects that a confirmation message for this time slot has been sent, it is determined that there is a conflict in this time slot. Typical situations of conflicts include: receiving request messages from multiple different ships for the same basic time slot, or another request preempting the same time slot before a request message has been confirmed.

[0077] After the judgment is completed, the monitoring party needs to send a permission message or a rejection message as a response within the specified confirmation time slot. Permission information refers to broadcasting a response message that clearly indicates that the requesting party agrees to occupy the target time slot. The message includes the ship identification of the permitting party, the target time slot number, and the permission flag field, which plays a role in supporting the requesting party to complete the negotiation. The rejection message means that the monitoring party finds that the time slot has a conflict risk or has been occupied, so it does not agree to the ship's request to use it. The structure is similar to the permission message, but the permission flag field is set to no. All ships follow a unified conflict judgment rule before responding to ensure consistent judgment criteria and synchronized feedback actions.

[0078] Taking a cluster in Bohai Bay as an example, ship A initiates a request to time slot No. 550. After monitoring, ship B and ship C respectively find that the time slot was unoccupied in the previous frame and no other requests were seen in the current frame, so they broadcast the permission message in the permitted time slot. At the same time, ship D monitors that ship E sends a request to time slot No. 550 almost at the same time, and determines that there is a conflict in the time slot, so it sends a veto message to ship E within the permitted window. This mechanism ensures that all ships respond quickly to access behaviors based on consistent perception logic, so that the system can maintain high resource utilization and scheduling fairness while avoiding conflicts.

[0079] S213. If the sender receives more than half of the permission messages, it sends a confirmation message to explicitly occupy the time slot; otherwise, it re-executes random backoff and re-requests.

[0080] The ship that initiates the time slot request enters the monitoring phase after completing the broadcast, waiting for permission information or rejection information returned by other ships in the cluster, and judging whether the right to use the time slot has been successfully negotiated based on the number of permission information received. This step is the key judgment link in the three-step handshake mechanism. Its core is to form a decentralized consensus decision through collective response to ensure the fairness and conflict-free nature of time slot occupancy. The initiating ship continuously monitors the cluster channel within the permission response window, counts all response information to its request, and decides whether to enter the confirmation phase based on whether the number of permissions exceeds the set threshold. The permission threshold is usually set to more than half of the ships in the cluster, that is, confirmation broadcasts are allowed only when the majority of ships agree, so as to avoid conflicts in multi-party occupancy caused by insufficient permissions or ambiguous judgments.

[0081] The confirmation message is a broadcast signaling, and its structure contains the unique identifier of the ship, the requested time slot number, the license statistics and the formal occupancy statement. After the broadcast, all other ships in the cluster immediately update the local time slot usage status table, marking the time slot as occupied by the designated ship to avoid subsequent repeated requests or responses. The issuance of the confirmation message indicates that the ship's exclusive right to the time slot has been established within this cycle. In the subsequent sending window, the ship will use this time slot as planned to complete data communication. If the number of licensed information does not reach the set threshold, it is considered that the negotiation has failed. The initiating ship will activate the backoff mechanism, temporarily abandon the current target time slot, delay a number of basic time slots, re-randomly select a new target time slot, and restart a new request process.

[0082] For example, in a cluster in the Zhoushan Port waterway, ship X initiated a request for basic time slot No. 720 during this cycle. In the permission window, it received permission information from 18 of the 30 ships in the cluster, exceeding the half-percent permission threshold, and immediately broadcast a confirmation message in the subsequent confirmation window. After the confirmation is completed, other ships in the cluster mark time slot No. 720 as occupied to ensure that it will not be reused. In the same period, ship Y also initiated a request for time slot No. 720, but only received 8 permission information, which did not meet the permission standard, triggering a backoff and entering the next set of available time slot application process.

[0083] S214. After the local negotiation is completed, each ship sends data in the reserved time slot where the negotiation is completed to complete the local access.

[0084] After the ship successfully completes the three-step handshake mechanism and broadcasts the confirmation message, it officially enters the data transmission phase. At this point, the ship has obtained exclusive use of the target basic time slot in this cycle, so it can send business data in the corresponding time slot window without additional conflict detection. Data transmission is strictly limited to the allocated basic time slot after the confirmation time slot, ensuring the communication order and physical isolation of the entire cluster under the time division multiple access structure.

[0085] Before sending data, the ship will call the business data ready in its local cache and perform necessary encapsulation, encoding and error checking. The content of the data packet sent may include navigation status reports, ship-shore data synchronization, meteorological measurement data, distress warning information, etc. according to the business scenario, and the format complies with the structural protocol specified by the VDE communication standard. In actual operation, the ship will send the encapsulated data frame after accurately synchronizing with the target basic time slot, which will be received by the satellite and complete the forwarding or storage operation. Since the communication time slot has been negotiated in advance and recognized by the cluster, there will be no problem of multiple ships competing for the channel in the same time slot, which fundamentally solves the packet loss and retransmission overhead caused by concurrent conflicts in the traditional ALOHA access method.

[0086] S22. After each ship terminal within each cluster completes access negotiation within each VDE frame period, it regularly sends a regional access status message to the satellite terminal, and the satellite terminal completes regional load statistics based on this message.

[0087] The ship terminal sending a message to the satellite terminal is equivalent to communicating from the MAC layer to the network layer.

[0088] Specifically, in one embodiment, S22 includes the following steps S221 - S223.

[0089] S221. Select a central ship within the cluster.

[0090] To improve the aggregation efficiency of access status within the cluster and reduce the information processing pressure on the satellite terminal, the system sets that each cluster selects a ship as the "central ship", which is specifically responsible for collecting, sorting, and reporting the communication status information of this cluster. The selection of the central ship is usually dynamically or semi - statically configured based on indicators such as stability, position centrality, and communication ability. For example, nodes that are close to the geographical center of gravity, have relatively stable speeds, and have good communication records with other ships in the current cluster can be preferentially selected to play the central role. The selection mechanism of the central ship can be sent by the satellite terminal through the announcement channel after each round of clustering is completed, and the identity information of the central node is broadcast before the start of each cycle, so that other ships within the cluster can identify its role and report local status data to it.

[0091] S222. The central ship terminal statistically calculates the status information of each ship terminal within this cluster, where the status information includes negotiation success rate, number of conflict occurrences, and number of idle time slots.

[0092] The central ship is responsible for statistically calculating the access status information of all member ships within the cluster during the access cycle and constructing an overview of the communication behavior in this cycle. The status information includes three key indicators: negotiation success rate, number of conflict occurrences, and number of idle time slots. These indicators together reflect the resource utilization efficiency, access congestion degree, and remaining channel capacity of this cluster during the current communication cycle.

[0093] The calculation of the negotiation success rate is based on the ratio of the number of valid confirmation messages received by the central ship to the total number of ships in the cluster. Whenever a ship completes the three - step handshake and broadcasts a successful confirmation message within the confirmation window, the central ship records a successful access event and associates the ship identifier with the corresponding time slot number. During the statistical process, the central ship can parse the access frame structure to extract key fields contained in the confirmation message, such as MMSI number, time slot number, confirmation flag, etc., to ensure the accuracy and non - repetition of the statistical results.

[0094] The number of conflict occurrences is obtained by listening to conflict feedback information. When any ship within the cluster broadcasts a veto message during the permission window, this behavior is recorded by the central ship as a conflict event. To avoid double counting of vetoes, the system sets up a conflict filtering logic, where multiple vetoes on the same target time slot are counted as only one valid conflict. The central ship organizes these conflict events into a conflict detail list, including the conflict time slot number, the number of ships involved, etc., to provide support for further analysis of the access efficiency.

[0095] The number of idle time slots is obtained by counting the basic time slots in the pre-allocated time slot set that are not requested or confirmed to be occupied by any ship. Before the end of each frame period, the central ship performs a reverse comparison of all occupancy confirmation and time slot request information it has monitored to identify which basic time slots have not been accessed throughout the period, thereby determining the actual number of idle time slots. This indicator directly reflects the resource redundancy within the cluster and is the basis for judging whether it is necessary to reduce the time slot allocation of this cluster.

[0096] After the end of each minute cycle, the central ship on behalf of the cluster sends a reporting message of the cluster's status information during this cycle to the satellite side.

[0097] After the central ship completes the statistics of the access status of its own cluster, at the end of each minute frame period, it sends a status reporting message to the satellite side to achieve a closed-loop status feedback from the cluster to the satellite side. This process ensures that the satellite side can timely grasp the real-time access performance of each cluster for resource optimization and scheduling decisions in the next cycle. The reporting message is encoded in a structured message format and usually contains fields such as cluster identifier, statistical cycle number, total number of ships within the cluster, number of successful accesses, number of conflict events, number of idle time slots, etc. When necessary, a summary of the conflict time slot numbers or data usage rate information can also be attached.

[0098] The sending of the message is broadcast using a dedicated reporting channel or a reserved global control time slot to ensure that it does not conflict with ordinary service data. The central ship needs to reserve a certain number of basic time slots within the frame period for system-level control signaling reporting. Such time slots are pre-allocated by the satellite side during the time slot pre-allocation stage and broadcast through the announcement channel, enabling the central ship to plan the data sending time in advance. When constructing the message, the central ship packs the statistical data sorted in the local cache in the previous stage, adds its own identifier, time stamp, and cluster number fields, and then sends it regularly within the allocated reporting window. To ensure reliable delivery of the information, after receiving the report, the satellite side can return an acknowledgment according to the ACK mechanism defined in the frame structure. The central ship judges whether the reporting task is completed based on this. If no acknowledgment is received, it can retransmit according to the set backoff strategy.

[0099] For example, in a certain cluster in the Yangtze River Estuary, the central ship Z identified a total of 50 ships in this cluster during the current cycle. Among them, 45 ships successfully accessed, there were 5 record conflicts, and 48 basic time slots were unused. The time slots numbered 2245 to 2249 at the end of the frame were reserved by the satellite announcement channel for the central ship to report dedicated channels. Ship Z sent a status report message in the 2246th time slot, and the content included cycle number #37981, cluster ID #A5F, success rate 90%, list of conflict time slot numbers [680, 681, 687, 690], etc.

[0100] S3. The satellite side dynamically optimizes the pre-allocated communication time slots for the next cycle according to the reports of each region in each cycle.

[0101] Specifically, in one embodiment, S3 includes the following steps S31 - S33.

[0102] S31. Determine the thresholds corresponding to each status information, including the negotiation success rate threshold, conflict rate threshold, and proportion threshold of idle time slots.

[0103] The satellite side determines the evaluation thresholds corresponding to various access status information according to the system control strategy and the network operation model, including the negotiation success rate threshold, conflict rate threshold, and proportion threshold of idle time slots. These thresholds constitute the standard line for judging the operation status of each cluster, and are used to judge whether a certain cluster is in an overloaded, inefficient, or resource - wasted state during the subsequent resource re - allocation process.

[0104] The negotiation success rate threshold is used to measure the normal access degree of ships in a certain cluster. The success rate refers to the ratio between the number of ships that successfully complete the handshake mechanism and complete data transmission within a communication cycle and the total number of participating ships. If the negotiation success rate of a certain cluster is lower than the set threshold, for example, 85%, it is considered that there are problems such as congestion, insufficient time slots, or failure of the negotiation mechanism in this cluster. The conflict rate threshold is used to judge the degree of time slot contention within the cluster, defined as the ratio between the number of conflict events occurring within a cycle and the total number of time slot requests. If it is higher than a set value such as 10%, it indicates that the current allocation method cannot effectively meet the coordination requirements in the scenario of dense ships. The proportion threshold of idle time slots is used to discover areas with excessive resource allocation. If the unused time slots in a certain cluster exceed 30% of its total time slots within a cycle, it is considered resource redundant, and the allocation share for the next cycle should be appropriately reduced.

[0105] The setting process of these thresholds is completed by the software control unit on the satellite side. Static fixed values can be used, or dynamic fine-tuning can be performed based on historical operation data. For example, during peak hours in the Nansha Sea area, the system analyzes the data of the past 100 cycles through a machine learning model and finds that the negotiation success rate often fluctuates around 88%. To ensure throughput, it is decided to adjust the negotiation success rate threshold of this cluster to 90% and the conflict rate threshold to 8%. For the sparse cluster in the Zhoushan Archipelago, the success rate threshold is maintained at 75%. After each type of threshold is set, it is stored in the system configuration table and participates in the subsequent automatic determination process of status comparison and resource adjustment logic, forming a complete closed-loop feedback mechanism for network situation changes. S32. The satellite side compares the status information with the thresholds based on the access status reports of each cycle area.

[0106] In S32, the satellite side retrieves the access status reports of each cluster in the previous frame cycle, mainly including the negotiation success rate, conflict rate, and the proportion of idle time slots, and compares these data item by item with the evaluation thresholds set in S31. The comparison results of each indicator are used to determine whether there is a phenomenon of insufficient resources or resource waste in this cluster. If the negotiation success rate of a certain cluster is lower than the threshold, it means that some ships fail to access successfully, and the system determines that there is a shortage of time slots in this area. If the conflict rate is higher than the threshold, it indicates that the time slot contention is serious in this area, and even with pre-allocation, the occurrence of overlapping access cannot be effectively avoided. When the proportion of idle time slots is too high, it reflects that the currently allocated resources are greater than the actual needs of this cluster, which belongs to excessive resource allocation. The satellite side generates cluster status labels through these comparison results, such as "congested", "too many conflicts", "resource surplus", etc.

[0107] S33. Allocate or reclaim the global elastic time slots to each area according to the comparison results to adjust the number of pre-allocated time slots in the next cycle.

[0108] The satellite side executes the allocation strategy of the global elastic time slots according to the above status labels. The allocation methods include "increasing allocation", "decreasing allocation", and "maintaining", and the response adjustment is made based on the current status of each cluster. For example, for a cluster in the "congested" state, the satellite allocates additional resources from the global elastic time slot pool to this cluster to improve its access capacity; for a cluster with the "resource surplus" label, the system reclaims its excess pre-allocated time slots and incorporates them into the elastic pool to release resources for other clusters to use. All adjustment results form a new time slot table to be allocated and are broadcast through the announcement channel before the start of the next cycle, so that the ship side can carry out access scheduling based on the updated time slot set.

[0109] For example, within a certain frame period in the Beibu Gulf, the negotiation success rate of cluster C is only 82%, and the conflict rate reaches 12%. The satellite determines that it is in a state of resource tension and immediately allocates an additional 150 basic time slots for it from the elastic pool to extend the available time slot segment number range in the next period. At the same time, the proportion of idle time slots of cluster F in this period reaches 45%, and the satellite end reclaims 80 time slots from it and releases them into the elastic pool. Through this increase and decrease mechanism, the system realizes the dynamic rebalancing of resources among clusters, effectively alleviates the access congestion in high-density ship areas, and improves the throughput capacity and time slot utilization rate of the overall system. The whole process does not require manual intervention. The automatic execution mechanism based on state feedback and threshold determination provides a strong guarantee for the stable operation of the VDE system in complex and dynamic maritime communication scenarios.

[0110] It should be understood that the magnitudes of the sequence numbers of the steps in the above embodiments do not mean the order of execution. The order of execution of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present invention.

[0111] Those skilled in the art can clearly understand that, for the convenience and brevity of description, only the above division of each functional unit and module is used as an example. In actual applications, the above functions can be allocated to different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above.

[0112] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included in the protection scope of the present invention.

Claims

1. A VDE signal satellite access control method, characterized in that: The following steps are involved: S1. The satellite terminal dynamically clusters and pre-allocates communication time slots for ships within the satellite signal coverage area; S2. The ship end within the dynamic cluster performs local negotiation allocation of communication time slots and periodically reports to the satellite end; S3. The satellite terminal dynamically optimizes the pre-allocated communication time slots for the next cycle based on the reports from each region in each cycle.

2. The VDE signal satellite access control method according to claim 1, characterized in that: The S1 comprises the following steps: S11. The satellite terminal obtains the real-time position distribution and access density of the ships that establish communication with itself within the satellite signal coverage area; S12. The satellite terminal divides the ships into several dynamic clusters using a density clustering algorithm based on the real-time position distribution and access density of the ships in the satellite signal coverage area; S13. The satellite terminal initially allocates reserved TDMA time slots to each area based on the divided clusters and the density and historical traffic of ships in the clusters for local negotiated access by ships in the area.

3. The VDE signal satellite access control method according to claim 2, characterized in that: The S12 comprises the following steps: S121. According to the density clustering algorithm, set the cluster partition parameters and calculate the clusters in real time; S122. Forming several logical sub-clusters according to the calculation results of the cluster; S123. Record the cluster center position and boundary corresponding to each logical cluster so that subsequent satellites can perform targeted time slot pre-allocation.

4. The VDE signal satellite access control method according to claim 3, characterized in that: The S13 comprises the following steps: S131. Determine that the VDE frame period is 1 minute, with a total of 2250 basic time slots; S132. The basic time slot is divided into a time slot to be allocated, a global flexible time slot and a global emergency time slot; wherein the global flexible time slot is used for subsequent dynamic adjustment and random access of individual ships, and the global emergency time slot is used for emergency communication between satellites and ships without negotiation; S133. According to the area divided in the previous step and the ship density and historical flow in the area, the time slots to be allocated are divided into each cluster to determine the allocation of the initial state of each cluster; S134. Broadcast the time slot pre-allocation information via the satellite announcement channel.

5. The VDE signal satellite access control method of claim 4, wherein: The S2 comprises the following steps: S21. In each cluster, the ship performs a local negotiation process and completes the local time slot allocation through the pre-allocated local channel using a three-step handshake mechanism; S22. After completing access negotiation in each VDE frame period, each ship end in each cluster periodically sends a regional access status message to the satellite end, and the satellite end completes regional load statistics based on the message.

6. The VDE signal satellite access control method of claim 5, wherein: The S21 comprises the following steps: S211. Each ship in the cluster monitors the pre-allocated time slot set, and at the beginning of each frame, randomly selects an unoccupied time slot in the pre-allocated time slot set to send an initial time slot request message; S212. Other ships in the cluster monitor and confirm the occupancy of the time slot in this area. If no conflict occurs, they actively send a permission message in the subsequent confirmation time slot. If a conflict is detected, a rejection message is fed back; S213. If the sender receives more than half of the permission messages, it sends a confirmation message to explicitly occupy the time slot; otherwise, it re-executes random backoff and re-requests; S214. After the local negotiation is completed, each ship sends data in the reserved time slot where the negotiation is completed to complete the local access.

7. The VDE signal satellite access control method of claim 6, wherein: The S22 comprises the following steps: S221. Select a central ship in the cluster; S222. The central ship terminal counts the status information of each ship terminal in the cluster, wherein the status information includes the negotiation success rate, the number of conflicts, and the number of idle time slots; S223. After each minute period ends, the central ship sends a report message on the status of the cluster within the period to the satellite on behalf of the cluster.

8. The VDE signal satellite access control method of claim 7, wherein: The S3 comprises the following steps: S31 determines the thresholds corresponding to each state information, including the negotiation success rate threshold, the conflict rate threshold, and the idle time slot ratio threshold; S32. The satellite terminal compares the status information with the threshold value according to the access status report of each periodic area; S33. Allocate or reclaim the global flexible time slots to each area according to the comparison result to adjust the number of pre-allocated time slots for the next cycle.

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