Mobile network cross-channel seamless switching method and system for indoor and outdoor AGV communication

Abstract

The invention provides a mobile network cross-channel seamless switching method and system for indoor and outdoor AGV communication, and the method comprises the steps: enabling an AGV to obtain a wireless data frame through a wireless network card, extracting the intensity of a received signal, and carrying out the scanning of surrounding wireless access points; acquiring a data queue length and a detection time delay; obtaining the weight of each AP based on the time slot occupancy rate, the detection time delay and the received signal strength of the APs meeting the communication requirements; obtaining an optimal switching target AP based on the weight; and calculating the load of the optimal switching target AP after switching, and obtaining a communication time slot allocation scheme between the AGV and the AP. According to the method, the time slot number required by AGV communication in different environments and the change of the network load state are considered, and the problems that the real-time and reliable data transmission cannot be ensured when the AGV is switched between the APs and the time slot resource utilization rate is low under the division multiple access technology are solved, so that the real-time performance and the overall throughput of the AGV communication network are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication and channel switching, and in particular relates to a method and system for seamless cross-channel switching of a mobile network for indoor and outdoor AGV communications. Background Art

[0002] With the advent of the intelligent era, new mobile services are constantly developing in industrial scenarios, and automated guided vehicles (AGVs) have put forward new requirements for the performance of wireless transmission networks. One of the most important requirements in industrial applications is reliable real-time communication. WLAN networks based on Time Division Multiple Access (TDMA) have the characteristics of wide coverage and support for mobility. Compared with existing protocols such as WirelessHART and ISA100.11a, WLAN networks based on TDMA can provide higher throughput and larger coverage, so they have been widely used in AGVs.

[0003] However, WLAN networks based on TDMA also face the problem of switching delay. A wireless local area network is a large-scale wireless network composed of multiple wireless access points (APs). When a mobile terminal switches between different APs, it will cause a switching delay problem, which is mainly caused by two processes: one is that the mobile terminal disconnects from the current AP and establishes a new connection with the target AP; the other is that after the mobile terminal establishes a connection with the new AP, it waits for the new AP to allocate the time slot required for communication.

[0004] The main methods to solve the switching delay are fixed and dynamic time slot allocation. Fixed time slot allocation allocates a fixed and unique number of time slots to each mobile terminal, but it cannot be adjusted dynamically, which easily leads to time slot resource waste or network congestion. Although dynamic time slot allocation can dynamically allocate time slots according to the time slot requirements of mobile terminals, it cannot make corresponding adjustments according to the changing network load status in mobile scenarios, resulting in the delay jitter performance of the network cannot be guaranteed.

[0005] Although some studies have been devoted to reducing the switching delay, these studies still have some limitations, such as poor effect on outdoor scenarios. Therefore, a mobile network cross-channel seamless switching method for indoor and outdoor AGV communication is proposed. This scheme considers the number of time slots required for AGV communication and the changes in network load status, thereby improving the real-time performance and overall throughput of the AGV communication network. Summary of the invention

[0006] In view of the deficiencies in the prior art, the present invention provides a method and system for seamless cross-channel switching of a mobile network for indoor and outdoor AGV communications, so as to solve the problem that the mobile device cannot ensure real-time and reliable data transmission when switching between APs and the TDMA time slot resource utilization is low.

[0007] To achieve the above object, the present invention provides the following solutions:

[0008] A mobile network cross-channel seamless switching method for indoor and outdoor AGV communication includes the following steps:

[0009] The AGV obtains the wireless data frame through the wireless network card and extracts the received signal strength. When the received signal strength is less than the preset scanning threshold within the preset detection times, the AGV scans the surrounding APs;

[0010] Obtain the data queue length and detection delay of data transmitted between the AGV and all scanned APs;

[0011] Based on the data queue length, the number of time slots required by the AGV and the number of idle time slots of the AP are obtained and compared, and the AP that meets the communication requirements of the AGV is screened out;

[0012] Based on the time slot occupancy rate of the AP that meets the AGV communication requirements, the detection delay and the received signal strength, combined with indoor and outdoor environmental factors, the weight of each AP is obtained; based on the weight of each AP, the best switching target AP is obtained;

[0013] Calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between the APs meets the preset threshold, and the AGV is connected to the best switching target AP;

[0014] The load of the optimal switching target AP after switching is calculated, and based on the size relationship between the load and the preset load threshold, the communication time slot allocation scheme between the AGV and the AP is obtained to complete the seamless cross-channel switching of the mobile network for indoor and outdoor AGV communication.

[0015] Preferably, the method for extracting the received signal strength of the wireless data frame obtained by the AGV through the wireless network card is:

[0016] The AGV obtains wireless data frames through the wireless network card installed on it;

[0017] The wireless data frame is parsed to obtain the received signal strength and the received signal strength is sent to the current AP, and the SDN controller is used to periodically obtain and store the received signal strength.

[0018] Preferably, the method for obtaining the detection delay is:

[0019] When the wireless network card sends data, add a sending timing statement to the beginning of the sending function to obtain the accurate time when the wireless network card sends the data packet;

[0020] When the wireless network card receives data, add a receiving timing statement to the beginning of the receiving function to obtain the accurate time when the wireless network card receives the data packet;

[0021] The detection delay is obtained based on the accurate time when the wireless network card sends the data packet and the accurate time when the data packet is received.

[0022] Preferably, the method for screening out APs that meet the AGV communication requirements is:

[0023] Based on the length of the data queue sent by the AGV, the current data transmission rate, and the length of a single time slot, the number of time slots required by the AGV is obtained. The calculation formula is as follows:

[0024]

[0025] Among them, Slot need is the number of time slots required by AGV, L queue is the length of the data queue sent by AGV, V is the current data transmission rate, L slot is the length of a single time slot;

[0026] Compare the number of time slots required by the AGV and the number of idle time slots of the AP to obtain the AP that meets the communication needs of the AGV.

[0027] Preferably, the method for obtaining the weight of each AP is:

[0028] Based on the number of AP idle time slots and the total number of time slots, obtain the AP time slot occupancy rate;

[0029] Processing the detection delay based on the detection response delay and the number of detection responses;

[0030] Smoothing the received signal strength values ​​received within a preset period to obtain a smoothed received signal strength;

[0031] Based on the adaptive weight adjustment mechanism, under the influence of indoor and outdoor environmental factors, the AP time slot occupancy weight, the processed detection delay weight and the smoothed received signal strength weight are calculated respectively;

[0032] The AP time slot occupancy weight, the processed detection delay weight and the smoothed received signal strength weight are normalized respectively to obtain normalized parameters; based on the normalized parameters, the AP weight is calculated, and the AP with the largest weight is used as the best switching target AP.

[0033] Preferably, the method for calculating the change trend of the received signal strength of the current AP and the best switching target AP is:

[0034] A linear regression received signal strength trend change model is established, and the formula is as follows:

[0035] r(t)=k×t+b,

[0036] Wherein, k represents the received signal strength change rate, t represents the measurement time, r(t) represents the received signal strength value at time t, and b is the regression coefficient;

[0037] The least square method is used to solve the received signal strength trend change model to obtain the received signal strength change rate within the measurement time. The formula is as follows:

[0038]

[0039] in, represents the average measurement time, Indicates the mean value of the received signal strength;

[0040] Based on the received signal strength change rate, a received signal strength change trend is obtained.

[0041] Preferably, the conditions for triggering the switching are:

[0042] When the signal receiving strength change rate of the current AP is less than 0, the receiving signal strength change rate of the best switching target AP is greater than 0, and the difference between the receiving signal strengths of the best switching target AP and the current AP is greater than a preset threshold, switching is triggered.

[0043] The present invention also provides a mobile network cross-channel seamless switching system for indoor and outdoor AGV communication, which is used to implement the method, including:

[0044] A scanning module is used for the AGV to obtain wireless data frames through the wireless network card, extract the received signal strength, and when the received signal strength is less than a preset scanning threshold within a preset number of detections, the AGV scans the surrounding APs;

[0045] The detection delay acquisition module is used to obtain the data queue length and detection delay of the data transmitted between the AGV and all scanned APs;

[0046] An AP screening module is used to obtain and compare the number of time slots required by the AGV and the number of idle time slots of the AP based on the length of the data queue, and screen out APs that meet the communication requirements of the AGV;

[0047] An AP weight calculation module is used to obtain the weight of each AP based on the time slot occupancy rate of the AP that meets the AGV communication requirements, the detection delay, and the received signal strength, combined with indoor and outdoor environmental factors; based on the weight of each AP, obtain the best switching target AP;

[0048] An AP switching module is used to calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger the switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between the APs meets the preset threshold, and the AGV is connected to the best switching target AP;

[0049] The load calculation module is used to calculate the load of the best switching target AP after switching, and based on the size relationship between the load and the preset load threshold, obtain the communication time slot allocation plan between the AGV and the AP, and complete the seamless switching of the mobile network across channels for indoor and outdoor AGV communication.

[0050] Compared with the prior art, the present invention has the following beneficial effects: by using a switching triggering scheme based on RSS change trend, the AP switching accuracy is improved. By using an AP selection scheme based on time slot occupancy and detection delay in the AP selection stage, the communication quality of the AGV after switching is guaranteed. By applying a dynamic time slot allocation scheme based on queue length in the time slot allocation stage, the time slot utilization is improved and the network delay jitter performance is guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0052] Figure 1 is a flow chart of a method according to an embodiment of the present invention;

[0053] Figure 2 A flowchart of determining scanning and switching timing according to an embodiment of the present invention;

[0054] Figure 3 A schematic diagram of a switching trigger according to an embodiment of the present invention;

[0055] Figure 4 A flowchart of screening access points that can meet time slot requirements according to an embodiment of the present invention;

[0056] Figure 5 A flowchart of calculating the weight of a wireless access point according to an embodiment of the present invention;

[0057] Figure 6A flowchart of a time slot allocation scheme for selecting an embodiment of the present invention;

[0058] Figure 7 Schematic diagram of common time slot allocation methods; among them, (a) is a schematic diagram of continuous allocation; (b) is a schematic diagram of uniform allocation; (c) is a schematic diagram of random allocation. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0060] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0061] Embodiment 1

[0062] like Figure 1 As shown, a mobile network cross-channel seamless switching method for indoor and outdoor AGV communication includes the following steps:

[0063] S1: The AGV obtains the wireless data frame through the wireless network card and extracts the received signal strength. When the received signal strength is less than the preset scanning threshold within the preset detection times, the AGV scans the surrounding APs. In this embodiment, the AGV monitors the RSS in real time during the movement. When the RSS of the current AP is detected to be less than the scanning threshold for four consecutive times, the AGV starts to scan the surrounding APs.

[0064] A further implementation method is that the method for extracting the received signal strength from the wireless data frame obtained by the AGV through the wireless network card is:

[0065] The AGV obtains wireless data frames through the wireless network card installed on it;

[0066] The wireless data frame is parsed to obtain the received signal strength and send it to the current AP. The SDN controller is used to periodically obtain and store the received signal strength.

[0067] Specifically, in this embodiment, before calculating and analyzing the signal strength, it is necessary to first obtain the signal strength information. According to the IEEE802.11 wireless protocol standard, the header of a wireless data frame includes two parts, one is Radiotap and the other is a control frame. Radiotap contains information such as channel, signal strength, timestamp, etc., so the signal strength value can be extracted from the wireless data frame obtained by the wireless network card. It is only necessary to parse and extract the information received in the wireless data frame to obtain the signal strength value. In the present invention, a wireless network card driver ath9k is used. When the network card receives a wireless data frame, the status information of the data frame will be stored in the structure ath_rx_status, and the received signal strength value can be directly obtained from this structure.

[0068] The SDN controller periodically obtains the signal strength value by sending subscription messages, and stores the signal strength value in the time period to obtain the RSS set sequence R, which is expressed as R = {r(t0), r(t1), ..., r(t m )}, where R is the RSS set obtained by the SDN controller during the measurement time, t m represents the mth measurement moment, r(t m ) indicates t m The RSS value measured at the moment.

[0069] During data transmission, AGV will put the data to be sent into the corresponding cache queue. The amount of data in the cache queue is the required queue length. Latency refers to the total time it takes for a data packet to be sent from the source address to the destination address. Latency is very important for analyzing the quality of service and congestion of the network. Before calculating the AP weight, you need to obtain the AGV queue length and detection delay information.

[0070] S2: Obtain the data queue length and detection delay of the data transmitted between the AGV and all scanned APs.

[0071] A further implementation method is that, S21: obtaining the length of the data queue: the present invention uses the wireless network card driver ath9k, and the relevant status information of the data queue is stored in the structure struct ath_txq. The variable axq_depth in the structure records the current queue length. When data is added to or sent out of the network card, the queue length axq_depth will be adjusted in real time. Therefore, you only need to obtain axq_depth from the structure to get the current data queue length.

[0072] A further implementation method is that the detection delay refers to the time period between the AGV successfully sending a Probe Request on a certain channel and successfully receiving a Probe Response from the AP. The detection delay is calculated using the formula T Delay =T recv -T send Calculate, where T Delay represents the detection delay, T recv Indicates the time when AGV receives Probe Response, T send Indicates the time when the AGV sends the Probe Request.

[0073] In this embodiment, S2: the method for obtaining the detection delay is:

[0074] When the wireless network card sends data, add a sending timing statement to the beginning of the sending function hard_start_xmit() to obtain the accurate time when the wireless network card sends the data packet;

[0075] When the wireless network card receives data, add a receiving timing statement to the beginning of the receiving function netif_rx() to obtain the accurate time when the wireless network card receives the data packet;

[0076] The detection delay is obtained based on the accurate time when the wireless network card sends a data packet and the accurate time when the data packet is received. The present invention estimates the state of the AP by measuring the delay generated during the AGV detection of the AP, and modifies the wireless network card driver ath9k so that it can directly obtain the time difference from the sending and receiving of the network card, thereby eliminating the error and obtaining an accurate detection delay.

[0077] S3: Based on the data queue length, obtain and compare the number of time slots required by the AGV and the number of idle time slots of the AP, and filter out the AP that meets the communication needs of the AGV; the time slot requirement of the AGV changes dynamically with the queue length. To ensure the communication quality after switching the AP, the number of idle time slots of the selected AP must be greater than the number of time slots required by the AGV. The present invention compares the number of time slots required by the AGV and the number of idle time slots of the AP, and filters out the AP that can meet the time slot requirement of the AGV from the accessible AP list.

[0078] A further embodiment is that Figure 4 As shown, the method for screening out APs that meet the AGV communication requirements is:

[0079] S31: Based on the length of the data queue sent by the AGV, the current data transmission rate, and the length of a single time slot, the number of time slots required by the AGV is obtained. The calculation formula is as follows:

[0080]

[0081] Among them, Slot need is the number of time slots required by AGV, L queue is the length of the data queue sent by AGV, V is the current data transmission rate, L slot is the length of a single time slot;

[0082] S32: Get the number of AP idle time slots:

[0083] The time slot table is stored in an array. When the AGV connects to the AP, the AP will assign an Association Identifier (AID) to the AGV. The AID is bound to the MAC address of the AGV and is unique. When the AP assigns a time slot to the AVG, the value of the time slot at the corresponding position in the array will be changed to the AID of the AGV.

[0084] Specifically, the present invention uses the following code to obtain the number of idle time slots of the AP in the wireless network card driver ath9: free :

[0085]

[0086] S33: Compare the number of time slots required by AGV need And the number of AP idle time slots free , obtain the AP that meets the AGV communication requirements. When the number of AP idle time slots is greater than the number of time slots required by the AGV, the AP is marked as available, otherwise it is marked as unavailable.

[0087] After the scan is triggered, it is necessary to select an AP with better performance in the network to ensure the communication quality of the AGV. In the traditional switching process, RSS is used as the basis for AP selection, and the AP with the largest RSS is selected as the target AP. This selection method is not optimal and has many problems because there are many factors in the network that affect AP performance, such as RSS, terminal association number, network load, etc.

[0088] The present invention comprehensively considers the two influencing factors of time slot occupancy and detection delay under the TDMA mechanism, and invents an AP selection scheme based on time slot occupancy and detection delay. The main idea is: first, through step S3, select the AP that can meet the communication needs of AGV according to the number of time slots required by AGV and the number of AP idle time slots, and then comprehensively consider the AP time slot occupancy, detection delay and RSS, under the influence of indoor and outdoor environmental factors, through the adaptive weight adjustment method and normalization idea, calculate a weight for each AP, and use this weight as the basis for AP selection.

[0089] S4: Based on the time slot occupancy rate, detection delay and received signal strength of the AP that meets the AGV communication requirements, combined with indoor and outdoor environmental factors, the weight of each AP is obtained; based on the weight of each AP, the optimal switching target AP is obtained; Figure 5 As shown, a further implementation method is that the method for obtaining the weight of each AP is:

[0090] S41: Based on the number of AP idle time slots and the total number of time slots, obtain the AP time slot occupancy rate Slot used_rate :

[0091] Specifically, use the formula Calculate AP timeslot occupancy, where Slot free is the number of idle time slots, Slot total is the total number of time slots.

[0092] S42: Based on the response delay of the detection and the number of detection responses, the detection delay is processed; the IEEE802.11 protocol standard stipulates the detection residence time of the AGV on each channel, namely the maximum channel time (MaxChanTime) and the minimum channel time (MinChanTime). After sending the Probe Request, the AGV first waits for MinChanTime. If no response is received, it switches to other channels for detection. If a response is received, it continues to wait on the channel until the MaxChanTime times out. During the detection time, the AGV may receive multiple responses from the same AP, so the detection delay needs to be processed before it can be utilized.

[0093] Specifically, the detection delay calculation is smoothed and optimized through the idea of ​​sequence delay difference, using the formula Processing, where T Probe is the processed detection delay, T i is the response delay of the ith probe, and N is the number of probe responses.

[0094] S43: Smoothing the received signal strength values ​​received within a preset period to obtain a smoothed received signal strength. Specifically, in order to reduce the impact of the change in RSS on AP selection, the RSS received within the period is calculated using the formula RSS=rss t-1 *α+rss t *(1-α) is smoothed, where RSS is the signal strength value after smoothing, rss is the average signal strength value after processing within the period, and α is an influence factor, which is set to α=0.25 in this embodiment.

[0095] S44: Based on the adaptive weight adjustment mechanism, under the influence of indoor and outdoor environmental factors, the AP time slot occupancy weight, the processed detection delay weight, and the smoothed received signal strength weight are calculated respectively;

[0096] There are significant differences in the communication characteristics of indoor and outdoor environments, so the impact of environmental factors must be fully considered when designing a decision-making mechanism. The AGV described in the present invention uses laser radar SLAM positioning technology. The laser radar generates point cloud data of the surrounding environment in real time by emitting a laser beam and receiving a reflected signal. Based on the SLAM algorithm, the AGV can simultaneously perform positioning and environmental mapping, and has real-time detection and classification functions, which can determine whether the AGV is in an indoor or outdoor environment. Based on this indoor and outdoor environment judgment capability, the present invention introduces the environmental factor E env , and stipulates the environmental factor E env =1 indicates indoor environment, E env =0 indicates an outdoor environment, and this factor is used to adjust the impact of time slot occupancy, detection delay, and received signal strength on the decision.

[0097] Analysis of environmental characteristics:

[0098] (1) Indoor environment:

[0099] In indoor environments, due to the high density of APs and fierce resource competition, AGVs rely more on time slot occupancy when switching APs to ensure efficient resource utilization. Therefore, setting a higher time slot occupancy weight can optimize AP selection and reduce resource waste.

[0100] Since the signal is stronger, the weight of signal strength can be relatively reduced. Processing delay is usually lower indoors, so a moderate weight is sufficient.

[0101] (2) Outdoor environment:

[0102] In outdoor environments, signal strength is a key factor because the AP is far away and signal attenuation will significantly affect communication quality. Increasing the weight of signal strength can help AGV select APs with better signal quality.

[0103] Since there are fewer APs, the influence of time slot occupancy is relatively weakened, so its weight can be reduced to focus on other important indicators (such as signal strength and detection delay).

[0104] The detection delay may increase in outdoor environments, so its weight is appropriately increased to ensure the real-time data transmission.

[0105] The specific calculation formula of the adaptive weight adjustment mechanism is:

[0106] (1) Calculation formula for time slot occupancy weight

[0107] In a real environment, time slot occupancy may change at any time, and sometimes network congestion is only a temporary phenomenon. Therefore, the fluctuation range and historical time slot occupancy can be introduced so that the weight adjustment depends not only on the current occupancy, but also on past historical data and fluctuations in time slot occupancy. The system can avoid selecting APs with large load fluctuations, ensuring the stability of the network connection and avoiding short-term misjudgments.

[0108] According to the characteristics of indoor and outdoor environments, if AGV is in an indoor environment (E env =1), the time slot occupancy rate should be more important, and the influence of the time slot occupancy rate is amplified in indoor environments because the competition for AP resources is more intense.

[0109]

[0110] Where: W slot is the time slot occupancy adaptive weight; α slot is the time slot occupancy basic weight; β hist is the historical occupancy rate impact coefficient; Slot hist is the historical occupancy rate, Calculation, Slot t is the time slot occupancy rate at time t, and n is the number of calculations. var is the occupancy rate fluctuation influence coefficient.

[0111] Var(Slot used_rate ) is the fluctuation of time slot occupancy rate, calculate;

[0112] Slot th It is the threshold of the time slot occupancy rate, which is used to normalize the impact of the current time slot occupancy rate and the historical and fluctuation information. It is set to a reasonable upper limit value according to actual needs.

[0113] (2) Detection delay weight calculation formula

[0114] The single detection delay may be affected by temporary network fluctuations or the environment. By adding the average value of the most recent detections, the change of the detection delay can be smoothed to ensure a more stable switching decision. In addition, by introducing the success rate of the most recent detections, the overall quality of the current channel can be reflected. If the success rate is low, it means that even if the delay is short, the network quality is still not ideal. The weight should increase the attention to the detection success rate to avoid switching to an unstable AP and reduce the occasional impact of the detection delay.

[0115] According to the characteristics of indoor and outdoor environment, in outdoor environment (E env =0), the weight of the detection delay will be amplified so that APs with shorter delays can be preferentially selected during handover between distant APs.

[0116] In indoor environment (E env =1), the weight of the detection delay remains unchanged, because the APs in indoor environments are usually relatively close and the delay has little impact.

[0117]

[0118] Where: W T is the detection delay adaptive weight; α T is the basic weight of the detection delay; β avg is the recent detection delay mean impact coefficient;

[0119] Avg(T) is the average of recent detection delay, given by Calculate, T t is the detection delay at time t. suc is the detection success rate, Calculate, n suc is the number of successful detections, n total is the total number of detections. th It is the threshold of the detection delay, which is used to standardize the impact of the mean detection delay and the detection success rate. It should be set as a reasonable upper limit of the delay based on actual needs.

[0120] (3) RSS weight calculation formula

[0121] Although the instantaneous value of signal strength is important, a signal with large fluctuations may not be suitable for long-term communication. By introducing the fluctuation range of signal strength, the system can avoid APs with large signal fluctuations, thereby improving the stability and reliability of communication.

[0122] According to the characteristics of indoor and outdoor environment, env =0), the weight of signal strength should be amplified to ensure that in outdoor environments, signal strength is the key factor when AGV selects AP.

[0123] Indoors (E env =1), the influence of signal strength is moderate because indoor signals are usually more stable.

[0124]

[0125] Where: W RSS is the RSS adaptive weight, α RSS is the RSS basic weight, RSS is the signal strength value after smoothing in S43, δ hist is the influence coefficient of historical signal strength;

[0126] Var(RSS) is the fluctuation of signal strength, which is given by and Computing, RSS t is the RSS at time t.

[0127] RSS th Signal strength threshold, used to standardize the impact of signal strength fluctuations, should be set as a reasonable upper limit of the delay according to actual needs

[0128] S45: Normalize the AP time slot occupancy weight, the processed detection delay weight, and the smoothed received signal strength weight respectively to obtain normalized parameters; calculate the AP weight based on the normalized parameters, and use the AP with the largest weight as the best switching target AP.

[0129] In this embodiment, the SDN controller uses the normalization idea based on the parameters such as the slot occupancy rate, detection delay and RSS calculated in steps S41, S42 and S43 to first convert the weight W obtained in S44 into slot , W T , W RSS Separately through Perform normalization processing to obtain the normalized parameter W sl ' ot , W T ′、W R ' SS , and then use the formula for AP Calculate the weights, where is the weight of the ith AP. slot , α T , α RSS , the present invention sets them to 0.4, 0.4, and 0.3 respectively. By calculating the weight of each AP Choose from The largest AP is used as the best switching target AP.

[0130] S5: Calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between APs meets the preset threshold, and the AGV is connected to the best switching target AP.

[0131] In an industrial environment, RSS is prone to large fluctuations. Therefore, it is difficult to make accurate switching trigger decisions based on the RSS size at a single moment, and the "ping-pong effect" is prone to occur. How to process and fully utilize RSS information is a major problem. The present invention proposes a new RSS information processing and utilization method, which summarizes the RSS within a time period, calculates and analyzes the rate of change of RSS within the time period, and then uses the rate of change of RSS to indicate whether the AGV is moving away from or approaching a certain AP, thereby improving the accuracy of switching:

[0132] A further implementation method is that the method for calculating the change trend of the received signal strength of the current AP and the best switching target AP is:

[0133] A linear regression received signal strength trend change model is established, and the formula is as follows:

[0134] r(t)=k×t+b,

[0135] Wherein, k represents the received signal strength change rate, t represents the measurement time, r(t) represents the received signal strength value at time t, and b is the regression coefficient;

[0136] The least squares method is used to solve the received signal strength trend change model to obtain the received signal strength change rate during the measurement time. The formula is as follows:

[0137]

[0138] in, represents the average measurement time, Indicates the mean value of the received signal strength;

[0139] Based on the received signal strength change rate, the received signal strength change trend is obtained. The calculated k value can be used to determine the current AGV movement state. When k is less than 0, it means that the RSS is decreasing, and the AGV is moving away from the AP. When k is greater than 0, it means that the RSS is increasing, and the AGV is approaching the AP. The larger the k value, the faster the AGV approaches the AP.

[0140] It is very important to know when to switch. If you only rely on an RSS value to determine whether to switch, it will inevitably lead to many unnecessary switching actions. The present invention proposes a switch trigger judgment mechanism based on the RSS change trend. The current motion state of the AGV is judged by the RSS change trend, thereby improving the accuracy of the switch trigger and using the switch threshold to determine the scanning and switching timing. Figure 2 shown.

[0141] A further implementation method is that the condition for triggering the switching is:

[0142] When the change rate of the signal receiving strength of the current AP is less than 0, the change rate of the received signal strength of the best switching target AP is greater than 0, and the difference between the received signal strength of the best switching target AP and the current AP is greater than a preset threshold, the switching is triggered. Figure 3 As shown, the switching condition can be expressed as Combining multiple conditions to make a handover trigger decision can improve the accuracy of handover triggering and reduce frequent handovers caused by RSS fluctuations.

[0143] S6: Calculate the load of the best switching target AP after switching, and obtain the communication time slot allocation plan between AGV and AP based on the size relationship between the load and the preset load threshold, so as to complete the seamless switching of mobile network cross-channel for indoor and outdoor AGV communication.

[0144] As the AP load increases, the probability of TDMA time slot conflict will also increase. When the AP load exceeds a certain threshold, the time slot conflict will cause the overall performance of the network to show a downward trend. For AGV, since it is constantly moving, the load state of the AP changes greatly. Therefore, it is necessary to allocate time slots according to the changing AP load to reduce time slot conflicts. The present invention takes the AP load state into consideration in the time slot allocation scheme, and selects the corresponding time slot allocation scheme according to the relationship between the current AP load and the threshold by setting a suitable load threshold:

[0145] S61: Obtain the number S of AGVs connected to the best switching target AP:

[0146] STA

[0147] In the TDMA mechanism, the occurrence of time slot conflicts is mainly related to the number of AGVs connected to the AP and the time slot allocation method. Therefore, the present invention estimates the AP load according to the number of AGVs connected to the AP.

[0148] Specifically, when an AGV connects to an AP, the AP will assign an ID to the AGV, referred to as Aid. The Aid is bound to the MAC address of the AGV and is unique. By counting the number of Aids, the number of AGVs connected to the AP can be obtained. STA , the code is as follows:

[0149]

[0150]

[0151] S62: Calculate AP load using the formula:

[0152] Through step S61, the number S of AGVs connected to the AP can be obtained. In the TDMA mechanism, the maximum

[0153] STA

[0154] The maximum number of connectable AGVs is the total number of slots in the superframe. total , according to S STA and Slot total It can be done by formula Calculate the AP load.

[0155] AP

[0156] S63: Obtain the communication time slot allocation scheme between the AGV and the AP, such as Figure 6 As shown:

[0157] In a TDMA-based WLAN, the TDMA time slot allocation method has a great impact on the overall network performance, and there are different ways to select the position of the allocated time slots in the superframe, such as random allocation, continuous allocation, uniform allocation, etc. Each allocation method has its own advantages and disadvantages. Continuous allocation means that the time slots belonging to a certain AGV are arranged continuously so that the position of the AGV's time slots in the superframe is continuous, such as Figure 7 (a); Uniform allocation means that the time slots belonging to a certain AGV are evenly arranged so that they are evenly placed in the superframe, such as Figure 7 (b); Random allocation means randomly placing the time slots belonging to a certain AGV in the superframe, such as Figure 7 (c) as shown.

[0158] When time slots are continuously allocated, a higher network throughput can be obtained, and when time slots are evenly allocated, the delay jitter performance in the network can be guaranteed. When the AP load is large, the uniformity of time slot allocation decreases, resulting in the inability to guarantee the delay jitter performance in the network. Therefore, the present invention selects a suitable time slot allocation method according to the AGV queue length and the AP load status, thereby improving the time slot utilization and ensuring the network delay jitter performance.

[0159] Specifically, after calculating the number of time slots required by the AGV and the load size of the AP,

[0160] total AP will be Load AP and the set load threshold Load THR For comparison, when Load AP <Load THR When Load>Load, select the continuous allocation mode. AP allocates a corresponding number of time slots to AGV and places these time slots in the superframe so that the position of the AGV's time slot in the superframe is continuous. When Load>Load, select the uniform allocation mode.

[0161] AP THR

[0162] The allocation method is first determined by the number of time slots required. need and the total number of time slots in the superframe total Use the formula Calculate the ideal time slot interval for evenly distributed time slots interval , according to the calculated ideal time slot interval Slot interval , place the time slots allocated to AGV at equal intervals in the superframe. When a time slot is occupied, continue to search backward until the next idle time slot is found.

[0163] Embodiment 2

[0164] The present invention also provides a mobile network cross-channel seamless switching system for indoor and outdoor AGV communication, which is used to implement the method of embodiment 1, including:

[0165] A scanning module is used for the AGV to obtain wireless data frames through the wireless network card, extract the received signal strength, and when the received signal strength is less than a preset scanning threshold within a preset number of detections, the AGV scans the surrounding APs;

[0166] The detection delay acquisition module is used to obtain the data queue length and detection delay of the data transmitted between the AGV and all scanned APs;

[0167] The AP screening module is used to obtain and compare the number of time slots required by the AGV and the number of idle time slots of the AP based on the length of the data queue, and screen out the APs that meet the communication requirements of the AGV;

[0168] The AP weight calculation module is used to obtain the weight of each AP based on the time slot occupancy, detection delay and received signal strength of the AP that meets the AGV communication requirements, combined with indoor and outdoor environmental factors; based on the weight of each AP, the best switching target AP is obtained;

[0169] An AP switching module is used to calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger the switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between the APs meets the preset threshold, and the AGV is connected to the best switching target AP;

[0170] The load calculation module is used to calculate the load of the optimal switching target AP after switching, and based on the size relationship between the load and the preset load threshold, obtain the communication time slot allocation plan between AGV and AP, and complete the seamless switching of mobile network cross-channel for indoor and outdoor AGV communication.

[0171] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A mobile network cross-channel seamless switching method for indoor and outdoor AGV communication, characterized in that: The following steps are involved: The AGV obtains the wireless data frame through the wireless network card and extracts the received signal strength. When the received signal strength is less than the preset scanning threshold within the preset detection times, the AGV scans the surrounding APs; Obtain the data queue length and detection delay of data transmitted between the AGV and all scanned APs; Based on the data queue length, the number of time slots required by the AGV and the number of idle time slots of the AP are obtained and compared, and the AP that meets the communication requirements of the AGV is screened out; Based on the time slot occupancy rate of the AP that meets the AGV communication requirements, the detection delay, and the received signal strength, combined with indoor and outdoor environmental factors, the weight of each AP is obtained; Based on the weight of each AP, the best switching target AP is obtained; Calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between the APs meets the preset threshold, and the AGV is connected to the best switching target AP; The load of the optimal switching target AP after switching is calculated, and based on the size relationship between the load and the preset load threshold, the communication time slot allocation scheme between the AGV and the AP is obtained to complete the seamless cross-channel switching of the mobile network for indoor and outdoor AGV communication.

2. The method for seamless cross-channel switching of mobile networks for indoor and outdoor AGV communication according to claim 1 is characterized in that: The method for extracting the received signal strength of the wireless data frame obtained by the AGV through the wireless network card is as follows: The AGV obtains wireless data frames through the wireless network card installed on it; The wireless data frame is parsed to obtain the received signal strength and the received signal strength is sent to the current AP, and the SDN controller is used to periodically obtain and store the received signal strength.

3. The method for seamless switching of mobile networks across channels for indoor and outdoor AGV communications according to claim 1 is characterized in that: The method for obtaining the detection delay is: When the wireless network card sends data, add a sending timing statement to the beginning of the sending function to obtain the accurate time when the wireless network card sends the data packet; When the wireless network card receives data, add a receiving timing statement to the beginning of the receiving function to obtain the accurate time when the wireless network card receives the data packet; The detection delay is obtained based on the accurate time when the wireless network card sends the data packet and the accurate time when the data packet is received.

4. The method for seamless cross-channel switching of mobile networks for indoor and outdoor AGV communication according to claim 1 is characterized in that: The method to select APs that meet the AGV communication requirements is: Based on the length of the data queue sent by the AGV, the current data transmission rate, and the length of a single time slot, the number of time slots required by the AGV is obtained. The calculation formula is as follows: Among them, Slot need is the number of time slots required by AGV, L queue is the length of the data queue sent by AGV, V is the current data transmission rate, L slot is the length of a single time slot; Compare the number of time slots required by the AGV and the number of idle time slots of the AP to obtain the AP that meets the communication needs of the AGV.

5. The method for seamless switching of mobile networks across channels for indoor and outdoor AGV communications according to claim 1, characterized in that: The method to obtain the weight of each AP is: Based on the number of AP idle time slots and the total number of time slots, obtain the AP time slot occupancy rate; Processing the detection delay based on the detection response delay and the number of detection responses; Smoothing the received signal strength values ​​received within a preset period to obtain a smoothed received signal strength; Based on the adaptive weight adjustment mechanism, under the influence of indoor and outdoor environmental factors, the AP time slot occupancy weight, the processed detection delay weight and the smoothed received signal strength weight are calculated respectively; The AP time slot occupancy weight, the processed detection delay weight, and the smoothed received signal strength weight are normalized to obtain normalized parameters; Based on the normalized parameters, the weights of the APs are calculated, and the AP with the largest weight is used as the best switching target AP.

6. The method for seamless switching of mobile networks across channels for indoor and outdoor AGV communications according to claim 1, characterized in that: The method for calculating the received signal strength change trend of the current AP and the best switching target AP is: A linear regression received signal strength trend change model is established, and the formula is as follows: r(t)=k×t+b, Wherein, k represents the received signal strength change rate, t represents the measurement time, r(t) represents the received signal strength value at time t, and b is the regression coefficient; The least square method is used to solve the received signal strength trend change model to obtain the received signal strength change rate within the measurement time. The formula is as follows: in, represents the average measurement time, Indicates the mean value of the received signal strength; Based on the received signal strength change rate, a received signal strength change trend is obtained.

7. The method for seamless switching of mobile networks across channels for indoor and outdoor AGV communications according to claim 6, characterized in that: The conditions that trigger the switch are: When the signal receiving strength change rate of the current AP is less than 0, the receiving signal strength change rate of the best switching target AP is greater than 0, and the difference between the receiving signal strengths of the best switching target AP and the current AP is greater than a preset threshold, switching is triggered.

8. A mobile network cross-channel seamless switching system for indoor and outdoor AGV communication, used to implement the method described in any one of claims 1 to 7, characterized in that: include: A scanning module is used for the AGV to obtain wireless data frames through the wireless network card, extract the received signal strength, and when the received signal strength is less than a preset scanning threshold within a preset number of detections, the AGV scans the surrounding APs; The detection delay acquisition module is used to obtain the data queue length and detection delay of the data transmitted between the AGV and all scanned APs; An AP screening module is used to obtain and compare the number of time slots required by the AGV and the number of idle time slots of the AP based on the length of the data queue, and screen out APs that meet the communication requirements of the AGV; An AP weight calculation module is used to obtain the weight of each AP based on the time slot occupancy rate of the AP that meets the AGV communication requirements, the detection delay, and the received signal strength, combined with indoor and outdoor environmental factors; Based on the weight of each AP, the best switching target AP is obtained; An AP switching module is used to calculate the received signal strength change rate of the current AP and the best switching target AP, and trigger the switching when the received signal strength change rate meets the preset requirement and the received signal strength difference between the APs meets the preset threshold, and the AGV is connected to the best switching target AP; The load calculation module is used to calculate the load of the best switching target AP after switching, and based on the size relationship between the load and the preset load threshold, obtain the communication time slot allocation plan between the AGV and the AP, and complete the seamless switching of the mobile network across channels for indoor and outdoor AGV communication.

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