CN112074010A

CN112074010A - Wireless multi-hop time slot synchronization method of Internet of things

Info

Publication number: CN112074010A
Application number: CN202010895473.8A
Authority: CN
Inventors: 刘进志; 张阳; 李鹏志; 林凯恩; 王峰; 王莉; 陈爽; 孙绍华
Original assignee: Qingdao University of Science and Technology
Current assignee: Qingdao University of Science and Technology
Priority date: 2020-08-31
Filing date: 2020-08-31
Publication date: 2020-12-11
Anticipated expiration: 2040-08-31
Also published as: CN112074010B

Abstract

The invention relates to a wireless multi-hop time slot synchronization method of the Internet of things, belonging to the technical field of global time slot synchronization of networks. The invention comprises the following steps: s1, operating a heuristic algorithm to distribute the flooding time slot based on the topological structure; s2: entering a synchronous time slot triggering mode after the timer is configured; s3: the synchronous timer is continuously triggered and enters an interrupt program; s4: and the nodes in the multi-channel period broadcast the synchronization messages according to the allocated time slots: in a multi-channel period, recording local time and global time after receiving a synchronous message; the synchronization message is transmitted in the allocated time slot. The invention realizes higher synchronization precision by coupling the synchronous timer with the linear regression algorithm; a multi-channel TDMA flooding time slot allocation algorithm is provided, so that stable synchronous operation is provided while the synchronous message overhead is reduced; the technology is flexible, high in portability and low in development difficulty, and is suitable for various multi-hop Internet of things synchronization applications.

Description

Wireless multi-hop time slot synchronization method of Internet of things

Technical Field

The invention relates to a wireless multi-hop time slot synchronization method of the Internet of things, belonging to the technical field of global time slot synchronization of networks.

Background

IOT (Internet of Things), Internet +, and other concepts have become one of the most concerned hot spots for the national people. The wireless sensor network is one of the important technologies of the internet of things. An allocation-based TDMA (Time Division Multiple Access) MAC is more efficient in a wireless sensor than a contention-based csma (carrier Sense Multiple Access) MAC. But implementing TDMA requires global slot synchronization of the network. Global slot synchronization of a network is a difficulty.

There exist several techniques for global time synchronization based on multiple hops. For example, the time synchronization protocol FTSP, based on linear regression, provides a global synchronization time. Unlike time synchronization, slot synchronization requires that synchronized slots be provided for TDMA use. There is also a gap between time synchronization techniques and the time slot synchronization required to implement TDMA, and time slot synchronization is more difficult to implement than time synchronization.

The existing technology for realizing time slot synchronization is less. The technology based on constructive interference can realize accurate and stable message flooding. However, the disadvantage (1) is that slot synchronization is not provided and the slots used for data communication need to be developed later. (2) The method is too dependent on the coupling with the bottom layer, the flexibility is lacked during later application development, the time slot synchronization is easy to be unstable and unreliable, and the development difficulty is high.

Wireless-based synchronization relies on the successful receipt and transmission of synchronization messages. The communication of synchronization messages is an overhead for TDMA data communication. Therefore, it is critical how to reduce the overhead of the synchronization messages and provide stable synchronization.

Disclosure of Invention

Aiming at the defects in the prior art, the invention provides a wireless multi-hop time slot synchronization method of the Internet of things, which realizes higher synchronization precision by coupling a synchronization timer with a linear regression algorithm; a multi-channel TDMA flooding time slot allocation algorithm is provided, so that stable synchronous operation is provided while the synchronous message overhead is reduced; the technology is flexible, high in portability and low in development difficulty, and is suitable for various multi-hop Internet of things synchronization applications.

The invention relates to a wireless multi-hop time slot synchronization method of the Internet of things, which comprises the following steps:

and S1, operating a heuristic algorithm based on the topological structure to allocate the flooding time slot: the system initializes the configuration of a synchronous timer, operates a multi-channel TDMA flooding time slot distribution algorithm based on the topological structure of the wireless network, distributes time slots and waits for receiving synchronous messages;

s2: entering a synchronous time slot triggering mode after the timer is configured: judging whether the synchronous information is received for the first time or not, aligning time slots, and recording local time and global time;

s3: the synchronous timer is triggered continuously and enters an interrupt program: a linear regression algorithm is operated in an interrupt program to correct the interrupt time of the synchronous timer so as to ensure the synchronization of the time slots, and a superframe structure of the time slots is constructed based on the counting value of the time slots;

s4: and the nodes in the multi-channel period broadcast the synchronization messages according to the allocated time slots: in a multi-channel period, recording local time and global time after receiving a synchronous message; the synchronization message is transmitted in the allocated time slot.

Preferably, the step S1 is to run a multi-channel TDMA flooding timeslot allocation algorithm based on the topology of the wireless network, and includes the following steps:

s11: hop-based channel allocation: the communication equipment allocates channels based on the hop count, and each hop selects one channel as a receiving channel; a sender needs to switch to a channel of a receiver when sending a message; only one device in the devices belonging to the same hop in a certain time slot can initiate broadcasting; the use of the channel adopts a circulating mode, and the equipment selects the channel again after a certain hop number according to the total number of the used channels;

s12: a time slot allocation heuristic algorithm based on a tree topology structure:

step 1: device.coverage and device.edge of all devices are set to zero;

step 2: clearing a time slot counter: the value of the time slot counter represents the current time slot, and flooding is initiated by the device 0;

and step 3: device 0 initiates flooding, meaning that the first hop device receives the synchronization broadcast message, is covered and becomes a covered edge device, and sets device.

And 4, step 4: searching and judging whether the whole network has uncovered equipment;

and 5: judging, if the devices which are not flooded are still available, sequentially executing 6, 7, 8, 9 and 10;

step 6: adding 1 to a time slot counter;

and 7: device with the deepest hop count and uncovered device is searched in the devices with the device number of 1 in the whole network, and the device is allocated to the time slot; the uncovered device with the deepest hop count means that the number of hops required for flooding to reach the network edge is the largest;

and 8: searching equipment meeting the conditions in each hop according to the hop number from large to small and allocating time slots;

and step 9: correspondingly operating the equipment which is newly allocated with the time slot in the step 7 and the step 8;

step 10: and returning to the step 4 to perform the loop judgment until the judgment of the step 4 is false, and then jumping to the step 11.

Preferably, the synchronization method of step S2 includes the following steps:

s21: time association based on synchronization messages: carrying time information through wireless broadcast messages to realize time association between sending equipment and receiving equipment;

s22: linear regression based linear fitting: the wireless communication equipment carries out mapping from local time to global time through the held local time so as to obtain the global time, each piece of synchronous message corresponds to the mapping from the local time to the global time, and the wireless communication equipment deletes old mapping by adding new mapping and finally keeps the mapping of the latest N pieces of synchronous message records.

Preferably, the synchronization timer in step S3 is configured to perform steps in a form that a hardware timer is coupled to a time synchronization algorithm, and includes the following sub-steps:

s31: firstly, selecting a timer of a microcontroller as a time slot synchronization special timer, wherein the timer is connected with the external interrupt of an initial frame delimiter SFD of a wireless communication module;

s32: configuring a timer for synchronization in initialization as a continuous working mode;

s33: coupling of the timer to the time synchronization algorithm: the wireless communication equipment carries out time slot synchronization based on global time, carries out mapping from the global time to local time based on a synchronization algorithm in the interruption of a synchronization timer, deduces a counting value of next interruption from the timer and completes the setting of a register TBCCR 0;

s34: the wireless communication SFD interruption competes with the interruption of the timer interruption, a global time timing mark is set in the synchronous timer interruption processing function, and the time is corrected when the SFD time is sent to a linear regression module for global time fitting.

Preferably, the time slot superframe in the step S4 is composed of a plurality of time slots, the length of the superframe is selectively set within 5-6 seconds, and the length of the superframe affects the accuracy of synchronization;

the multi-channel time interval is composed of a plurality of time slots and is used for broadcasting the multi-channel TDMA synchronous message;

the length setting of the multi-channel period is obtained by a heuristic algorithm according to a topological structure, and the synchronization based on linear regression is updated through a periodic superframe structure.

Preferably, the step S21 of associating time based on a synchronization message specifically includes the following steps:

s211: the synchronous protocol writes the local time and the global time of the sending equipment into the synchronous message, and the synchronous protocol of the sending equipment updates the global time according to the accumulated local time in the interruption of the start frame delimiter SFD;

s212: the receiving device records the local time of the receiving device in an interrupt also according to the start frame delimiter SFD;

s213: the difference value between the SDF interruption time of the sending equipment and the SFD interruption time of the receiving equipment is theoretically equal to the air propagation delay of radio waves from the sending equipment to the receiving equipment, the time obtained when the receiving equipment receives the SFD interruption is accurate global time, and meanwhile, the local time of the receiving equipment and the mapping between the global time and the global time are obtained through the local time of the receiving equipment recorded in the SFD interruption of the receiving equipment, so that the time correlation between the wireless sending equipment and the receiving equipment is completed;

s214: and (3) finishing accurate mapping between the local time and the global time, namely fitting from the local time to the global time based on the latest N pieces of synchronous information by utilizing a linear regression algorithm.

The invention has the beneficial effects that:

(1) the invention can realize stable time slot synchronization through a synchronization mechanism of coupling the timer and the linear regression algorithm, is suitable for general wireless communication equipment, and is easily transplanted to the wireless communication equipment with a microprocessor and a communication module frame;

(2) the invention provides the use of multi-channel TDMA to flood the synchronous messages, and provides a heuristic algorithm for time slot distribution, wireless synchronization depends on the broadcast of the synchronous messages, compared with the traditional broadcast based on CSMA, the invention can ensure that the whole network is flooded more quickly in a limited time, the broadcast of the synchronous messages is carried out as much as possible, the synchronous cost is reduced, and the synchronous accuracy is ensured;

(3) the present invention analyzes and improves the potential contention between designing wireless communication interrupts and timer interrupts;

(4) the time slot synchronization mechanism based on the coupling of the timer and the linear regression algorithm not only has the function of realizing stable time slot synchronization, but also can realize higher synchronization accuracy.

Drawings

FIG. 1 is a flow diagram of the present invention.

Fig. 2 is a topological structure diagram of wireless communication.

Fig. 3 is a schematic diagram of the transmission and reception of synchronization messages.

Fig. 4 is a diagram of slot change caused by contention interruption.

Fig. 5 is a diagram of synchronized slot data that runs stably after contention resolution.

Fig. 6 is a diagram of a channel allocation scheme based on hop count.

Fig. 7 is a channel cycling diagram.

FIG. 8 is one of the topology structural diagrams of embodiment 3.

FIG. 9 is the second topology structure of embodiment 3.

FIG. 10 is a third topological structure diagram in accordance with embodiment 3.

FIG. 11 is the fourth of the topology structure of embodiment 3.

Fig. 12 is a schematic diagram of a time-slot superframe structure.

Fig. 13 is a schematic diagram of a multi-channel broadcast time slot.

FIG. 14 is a graph comparing the average synchronization error of the implementation of the present invention with the prior art work.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1:

as shown in fig. 1, the method for synchronizing wireless multi-hop timeslots of the internet of things according to the present invention includes the following steps:

Wherein, the step S1 is to run a multi-channel TDMA flooding timeslot allocation algorithm based on the topology of the wireless network, and includes the following steps:

step 1: device.coverage and device.edge of all devices are set to zero;

step 6: adding 1 to a time slot counter;

The synchronization method of step S2 includes the following steps:

In step S3, the synchronization timer performs steps in a form of coupling a hardware timer with a time synchronization algorithm, including the following steps:

In the step S4, the time slot superframe is composed of a plurality of time slots, the length of the superframe is selectively set within 5-6 seconds, and the length of the superframe affects the accuracy of synchronization;

Wherein, the time association based on the synchronization message in step S21 specifically includes the following steps:

The wireless multi-hop time slot synchronization method of the Internet of things realizes higher synchronization precision by coupling the synchronization timer with the linear regression algorithm; a multi-channel TDMA flooding time slot allocation algorithm is provided, so that stable synchronous operation is provided while the synchronous message overhead is reduced; the technology is flexible, high in portability and low in development difficulty, and is suitable for various multi-hop Internet of things synchronization applications.

Example 2:

in this case, the wireless communication device composed of the TI MSP430F1611 low-power microcontroller and the CC2420 wireless communication chip is taken as an example for design and implementation. CC2420 has a half-duplex wireless transceiver. The time slot synchronization technology designed by the invention can be easily transplanted to other wireless communication devices. The technical framework of the present invention is applicable to general wireless communication devices.

The invention carries out global time slot synchronization of the multi-hop wireless network based on wireless communication. Since the clock of each wireless communication device, i.e. the local clock, has unavoidable physical deviation and the wireless communication has uncertain delay and fluctuation, it is necessary to estimate the global-time (global-time) from the local-time (local-time) by using a statistical idea. The invention adopts a linear regression method to perform linear fitting of global time so as to complete the mapping from the global time to the local time and from the local time to the global time. For this reason, the wireless transmitting device and the receiving device need to perform time association through a synchronization message to acquire association data between the local time and the global time. In the invention, the time of the base station with the equipment number of 0 is taken as a reference, and other communication is synchronized with the base station through a synchronization message. As shown in fig. 2 as an example, the

devices

1, 2 synchronize with respect to the device 0. After

devices

1, 2 are synchronized with device 0, device 3 synchronizes according to

devices

1, 2 and device 4 synchronizes according to device 2. From a topological point of view, device 1 is a parent node of device 3, and device 3 is a child node of device 1.

It should be noted that: based on the time association of the synchronization messages. The invention carries time information through wireless broadcast messages. Fig. 3 shows a time association between a transmitting device and a receiving device based on a synchronization message. The sequence numbers in fig. 3 are steps performed in time. The synchronization protocol writes the local time and global time of the sending device to the synchronization message in 1 and the wireless transmission is performed in 3. The time from 1 to 3 is uncertain, and the synchronization protocol updates the global time according to the accumulated local time of 1-3 in the interruption of the initial frame delimiter SFD in 3; the receiving device records the local time of the receiving device in 4 also in response to the start frame delimiter SFD interrupt. The SDF interruption time in 3 and the SFD interruption time in 4 are theoretically different by the air propagation delay of the radio wave from the transmitting device to the receiving device, and can be ignored. Therefore, the time obtained in the SFD interrupt reception in 4 is an accurate global time, and meanwhile, the local time of the receiving device and the mapping between the global time and the global time can be obtained through the local time of the receiving device recorded in the SFD interrupt reception in 4, so as to complete the time association between the wireless transmitting device and the receiving device. The exact mapping between the local time and the global time, i.e. the fitting from the local time to the global time, is done in 6 using a linear regression algorithm based on the latest N pieces of synchronization information.

It should be noted that: linear fitting based on linear regression. The wireless communication device maps the local time to the global time through the held local time, so as to obtain the global time. Each synchronization message corresponds to a mapping from local time to global time. To make this mapping accurate, the wireless synchronization device performs a linear fit based on the N synchronization messages. The larger N is, the higher the precision is, but the synchronization message has timeliness, and the value is generally 8. That is, the wireless communication device adds a new mapping, deletes an old mapping, and finally keeps the mapping of the latest N synchronization message records to perform the following calculation:

the Local _ to _ global mapping of Local time to global time is:

gt＝lt+oa+(sk*(lt-la))；

global _ to _ Local, the global time to Local time map is:

lt＝gt-oa-(sk*((gt-oa)-la))；

wherein: lt is the local time; gt is the global time; la is the average of N local times; global time minus local time is the offset time, and oa is the average of the offset times; sk is a clock drift compensation coefficient and is estimated by a least square method according to N records.

It should be noted that: a method for realizing time slot synchronization. The slot synchronization contains time synchronization information, but cannot be simply obtained based on the time synchronization. Practical experience has shown that a simple timer setting based on the synchronization time does not achieve accurate and reliable time slot synchronization. The invention adopts the mode that the hardware timer is coupled with the time synchronization algorithm to realize the time slot synchronization. The proposed framework of timer and time synchronization algorithm coupling is adapted to general wireless communication devices.

Firstly, a Timer (Timer) of the microcontroller is selected as a time slot synchronization dedicated Timer. One of the acquisition timers of this timer should be connected to a Start Frame Delimiter (SFD) external interrupt of the wireless communication module. The MSP430F1611 used in the present invention has two 16-bit timers. In this case, the TimerB of MSP430F1611 is selected as the timeslot synchronization specific timer.

Secondly, the timer for synchronization is configured to be in a continuous operation mode in initialization. The configuration is as follows:

TBCTL＝0；

TBR＝0；

TBCCR0＝(uint16_t)Slot_Size+1；

TBCCTL0|＝CCIE；

TBCTL&＝～(MC1|MC0)；

TBCTL|＝MC0；

wherein: TBCTL is the TimerB control register, TBR is the TimerB counter, tbcc 0 is the TimerB capture comparator 0, CCIE is the capture compare interrupt enable flag, MC1, MC0 two flags represent the operating mode, and when set to 10, the timer is in the continuous operating mode. The Slot _ Size is the width of the Slot, and the timer is interrupted once every Slot _ Size timer clock (tick) after configuration. The length of each Slot may be set by the Slot Size parameter according to the specific application.

Again, the coupling of the timer to the time synchronization algorithm: the wireless communication device performs slot synchronization based on the global time, and therefore, during the synchronization timer interrupt, the mapping from the global time to the local time is performed based on the synchronization algorithm, so as to calculate the count value from the timer interrupt next time, and complete the setting of the register tbcc 0.

The handling of timer interrupts is as follows:

(1) adding 1 to the time slot count;

(2) accumulating the local time to obtain a value (TBCCR 0-1);

(3) and correcting the response time of the next synchronous timer. The specific operation is as follows: 1) setting a next interrupt corresponding global time value nextfireglobalsime + ═ Slot _ Size; 2) the mapping obtains a Local time nextfirelocatimei at the next timer interruption, which is global _ to _ Local (& nextfireglobal time), where global _ to _ Local is a mapping from the global time to the Local time. 3) Setting the synchronization timer tbcc 0 to nextFireLocalTime-LastlocalTime +1 according to the local time value obtained in 2), where LastlocalTime is the local time when the last timer was interrupted.

It is noted that there is a problem of interrupting contention during the implementation. Contention exists between the wireless communication SFD interrupt and the timer interrupt. Such contention occurs with a very low probability and is therefore not easy to find, but because both the SFD and the timer interrupt are closely linked to the synchronization time, once contention occurs, it will cause a steep increase in synchronization error. The specific reason is as follows: the SFD interrupt is an interrupt acquired from the outside of a timer, whether the interrupt process is carried out or not when the interrupt occurs, and the SFD time is the real time when the interrupt occurs. If the interrupt processing of the synchronous timer is in progress when the SFD interrupt occurs, a large error occurs when the SFD time is sent to the linear regression module for fitting the global time, which causes the length of the time slot to increase suddenly. This is because the synchronization timer itself is the engine of time. Fig. 3 shows a phenomenon in which the interruption of contention causes a sudden increase in the slot width. 33593 and 33594, the error of the time slot suddenly increases, and the size of the sudden increase is about half the time slot length, which causes the synchronization of the time slot to be broken, and the time slot edges are not aligned any more.

As a countermeasure, a global time count flag is set in the synchronous timer interrupt processing function. With this flag, the time is corrected when the SFD time is then sent to the linear regression module for global time fitting. Fig. 5 shows the running of stable synchronized slot data after resolving the interrupt contention. The synchronous errors are randomly distributed, and the situation that the errors are not increased steeply occurs.

It should be noted that: a flooding mechanism based on multi-channel tdma (time Division Multiple access). The operation of wireless synchronization relies on the flooding of synchronization messages from device 0 to other devices. Successful transmission and reception of synchronization messages directly affects the performance of synchronization. Existing CSMA broadcast-based flooding mechanisms not only lack efficiency, but also have problems such as hidden terminals. The time slot synchronization technology realized by the invention can be used by TDMA, so the invention provides a flooding mechanism based on multi-channel TDMA based on self realization.

First, a heuristic time slot allocation algorithm. In TDMA communication, the synchronization message belongs to the overhead, so that the efficient flooding mechanism can shorten the occupied time proportion, thereby reducing the synchronization overhead. In addition, the global time estimation method based on linear regression used in the present invention operates based on the latest N synchronization messages, and thus it is desirable that a device be able to broadcast more useful synchronization messages in a limited time. The invention provides a heuristic algorithm aiming at the tree-based topological structure to realize time slot allocation of TDMA flooding. Experiments show that the realized multi-channel TDMA flooding mechanism presents high efficiency and reliability.

Second, channel assignment based on hop count. CC2420 has 16 channels, channel 11 through channel 25. The communication device allocates channels based on the number of hops, and as shown in fig. 6, selects one channel per hop as a reception channel. The sender needs to switch to the receiver's channel when sending the message. Only one of the devices belonging to the same hop in a certain time slot can initiate a broadcast. The channels are used in a cyclic manner, as shown in fig. 7, the device 0 selects the channel 11, and the device reselects the channel 11 after a certain number of hops according to the total number of channels used. For example: if 4

channels

11, 12, 13, 14 are used, the fourth hop reuses channel 11, where device 0 is hop 0.

A time slot allocation heuristic algorithm based on a tree topology structure:

1)device.coverage＝0，device.edge＝0；

2)slotNo＝0；

3) device 0 broadcasts a wireless synchronization message, wherein device.coverage of the first hop device is 1, and device.edge is 1;

4) checking whether equipment with device.coverage of 0 exists;

5) if (true) the

n execution

6, 7, 8, 9, 10;

6)slotNo++；

7) device with edge of 1, which has the deepest hop count and is not covered, is searched and allocated to the slot.

8) Except the hop number corresponding to the device in 7, the other devices search the devices meeting the following conditions in each hop for time slot allocation according to the hop number from large to small: (1) edge of the device is 1, the device in the hop has the largest subtree of uncovered equipment, the value of the time slot counter allocated last time plus 1 is not equal to the current slot No, and the device has a flooding object; if no equipment meeting the condition (1) exists, searching equipment meeting the condition (2); (2) coverage is 1 for this device, and the number of allocated slots is the smallest among the devices of this hop, and the last allocated slot counter value plus 1 is not equal to the current slotNo and owns the flooding object.

9) The devices newly allocated time slots in 7 and 8 operate accordingly: (1) reset to 0, (2) device.coverage of the next hop device is 1, and device.edge is 1, and (3) record the assigned time slot value.

10) Return 4 execution

11) end of else slot allocation

It should be noted that:

all devices are provided with flood coverage flag bits device. Device. coverage and device. edge of all devices are set to zero in step 1.

And (3) clearing the time slot counter in the step 2. The value of the slot counter represents the current slot. Flooding is initiated by device 0.

In step 3, device 0 initiates flooding, meaning that the first hop device receives the synchronization broadcast message, is covered, and becomes an covered edge device, thus setting their device.

And 4, searching and judging whether the whole network has uncovered equipment.

And step 5, judging, and if the devices which are not covered by the flooding exist, sequentially executing 6, 7, 8, 9 and 10.

In step 6 the slot counter is incremented by 1.

In step 7, the device having the uncovered device with the deepest hop count is searched for from the devices whose device.edge is 1 in the entire network, and is allocated to this time slot. The uncovered device with the deepest hop count means that the flooding takes the most hops to reach the network edge.

Except the hop count corresponding to the device searched in step 7, in step 8, the devices meeting the conditions (1.1) - (1.4) at the same time are searched in each hop according to the hop count from large to small, and the time slot is allocated. (1.1) edge flag bit device. edge of this device is 1. I.e. it belongs to the flooded edge. (1.2) among the devices of the hop, the device has the largest uncovered device sub-tree. The sub-tree refers to all child nodes taking the device as a parent node in the tree topology and child nodes of the child nodes. The size of a sub-tree refers to the number of nodes that this sub-tree contains in the topology. (1.3) the allocated time slots are not adjacent. I.e., the last allocated slot counter value plus 1 is not equal to the current slotNo. (1.4) the device has at least one child node as a broadcast object in the slot. I.e. flooding, is significant. If there are no devices satisfying the conditions (1.1) - (1.4) at the same time, a device satisfying the conditions (2.1) - (2.4) is searched for. (2.1) the coverage flag bit of this device is 1. I.e. it belongs to a covered device. (2.2) the minimum number of allocated slots among the devices of this hop. (2.3) the allocated time slots are not adjacent. I.e., the last assigned slot counter number plus 1 is not equal to the current slotNo. (2.4) the device has at least one child node as a broadcast object in the slot. I.e. flooding, is significant. Conditions (2.1) to (2.4) correspond to covered devices.

The device to which the time slot is allocated, of course, broadcasts the synchronization message in this time slot, so step 9 performs corresponding operations for the devices to which the time slot is newly allocated in step 7 and step 8: (1) edge flag bit is reset to 0. The assigned slot value is recorded. (2) The coverage flag device of the next-hop device is set to 1, and the edge flag bit device is set to 1, that is, the coverage is enlarged after flooding, and the edge node is extended outward.

Step 10 returns to step 4 to perform a loop judgment until the judgment of step 4 is false, and then jumps to step 11.

Example 3:

the following examples are given for heuristic algorithms:

all devices except device 0 run a heuristic algorithm according to the topology of the network to obtain the broadcast time slot. Device 0, due to its particularity, can simply set up to broadcast the synchronization message every slot.

Example 1: a heuristic algorithm is used to assign the multiple channel TDMA time slots to the network topology shown in fig. 8.

As shown in FIG. 8, the detailed process of the heuristic operation in the topology of example 1 is as follows:

1) device.

2) The slot counter is cleared. Device 0 initiates flooding.

3) Node 1 and node 7 set device. The slot counter is incremented by 1.

4) The hop depth of the uncovered device of node 1 is 1 and the hop depth of the uncovered device of node 7 is 7, while node 7 satisfies other conditions, so the current time slot, i.e. time slot 1, is allocated to select node 7.

5) Edge flag bit device of node 7 is reset to 0. The assigned slot value is recorded. The coverage flag device. coverage of the device node 8 of the next hop is set to 1, and the edge flag bit device. edge is set to 1, that is, the coverage after flooding is enlarged, and the edge node is extended to the node 8.

6) And adding 1 to the time slot counter, and entering the next round of time slot allocation.

7) The maximum hop depth for node 8 to have uncovered devices is found to be 6, while node 8 satisfies other conditions, so the current time slot, i.e., slot 2, is assigned to select node 8. Node 8 belongs to hop 2, except for this hop, node 1 has a subtree of largest uncovered devices. Node 1 is the parent node of the subtree, the size of which is 5. Thus allocating a time slot to node 1 in hop 1. The current time slot, i.e. slot 2, is allocated to the selection node 1. Edge flag bit device of

nodes

1 and 8 is reset to 0. Meanwhile, the device.coverage of the

nodes

2, 3, 4, 5, 6, and 9 is set to 1, and the edge flag bit device.edge is set to 1, that is, after flooding, the coverage is enlarged, and the edge nodes are extended outwards.

8) And continuing to enter the next round of time slot allocation process. Until all nodes are covered. The device information allocated to each timeslot after the algorithm is applied is shown in table 1.

Table 1 table of correspondence between time slot and device in example 1

Time slot counting	Obtaining settings of time slotsStandby number
		0	0
1	0，7
		2	0，1，8
3	0，2，7，9
		4	0，1，3，10
5	0，4，7，9，11
		6	0，1，5，10，12
7	0，6，7，9，11，13

The time slots allocated by each device are shown in table 2.

Table 2 corresponding relation table between devices and time slots in example 1

Blank indicates that the device has no transmission time slot and is only in a receiving state.

Example 2: a heuristic algorithm is used to assign the multiple channel TDMA time slots to the network topology shown in fig. 9. The device information allocated to each timeslot after the algorithm is applied is shown in table 3.

Table 3 table of correspondence between time slot and device in example 2

Time slot counting	Obtaining a device number for a time slot
		0	0
1	0，1
		2	0，2，12
3	0，3，7，15
		4	0，4，8，18
5	0，5，9，20
		6	0，6，10，22
7	0，11，13，24
		8	0，7，14，26
9	0，8，16，28

The time slots allocated by each device are shown in table 4.

Table 4 device to time slot correspondence table in example 2

Example 3: a heuristic algorithm is used to assign the multiple channel TDMA time slots to the network topology shown in fig. 10. The device information allocated to each timeslot after the algorithm is applied is shown in table 5.

Table 5 table of correspondence between time slot and device in example 3

Time slot counting	Obtaining a device number for a time slot
		0	0
1	0，1
		2	0，2，10
3	0，1，3，11
		4	0，2，4，10，12
5	0，1，3，5，11，13
		6	0，2，4，6，10，12，14
7	0，1，3，5，7，11，13，15
		8	0，2，4，6，8，10，12，14，16
9	0，1，3，5，7，9，11，13，15，17

The time slots allocated by each device are shown in table 6.

Table 6 device to time slot correspondence table in example 3

Example 4: a heuristic algorithm is used to assign the multiple channel TDMA time slots to the network topology shown in fig. 11. The device information allocated to each timeslot after the algorithm is applied is shown in table 7.

Table 7 table of correspondence between time slot and device in example 4

Time slot counting	Obtaining a device number for a time slot
		0	0
1	0，1
		2	0，8，13
3	0，4，9，18
		4	0，7，14，16
5	0，6，19，20
		6	0，1，10，26
7	0，11，13，27

The time slots allocated by each device are shown in table 8.

Table 8 corresponding relation table between devices and time slots in example 4

It should be noted that: a superframe structure. The counting value of the time slot can be transmitted through the synchronization message, and the equipment can simply realize the synchronization of the time slot, thereby realizing a time slot superframe structure. As shown in fig. 12, a time slot superframe is composed of a plurality of time slots, and experiments show that the length of the superframe can be selectively set within 5-6 seconds, and the length of the superframe affects the accuracy of synchronization. The M period is a multi-channel period and is composed of a plurality of time slots, as shown in fig. 13. The M period is used for the broadcast of the multi-channel TDMA synchronization message. The length setting of M is obtained by a heuristic algorithm based on the topology. The synchronization based on linear regression is updated by a periodic superframe structure.

Device 0 acts as the initiator of the synchronization and its workflow is relatively simple. The equipment starts and enters a synchronous time slot triggering mode after timer configuration, and the synchronous timer is continuously triggered and enters an interrupt program. In the interrupt routine, the device 0 is also equipped with a linear regression algorithm, which makes it possible to correct the time. In practice, however, the linear regression algorithm of device 0 does not work in the present invention, and the global time used is the local time of device 0, since device 0 does not receive a time slot in the proposed multi-channel TDMA algorithm. Experiments show that this has no significant effect on the synchronization accuracy. The superframe structure of the slots is constructed based on the count value of the slots in the synchronization timer interrupt routine. In the time slot of the M segments, a synchronization message is transmitted.

The other devices except device 0 start, configure a timer, run a multi-channel TDMA flooding time slot allocation algorithm based on the topology of the wireless network, allocate time slots, and then wait for receiving the synchronization message initiated by device 0. At this point, although the synchronization timer has started to trigger, no excessive processing is performed until a synchronization message is received. And after receiving the synchronous message, entering the correction of a synchronous timer, aligning the edge of the time slot and synchronizing the counting value of the time slot. After that, the synchronous timer is continuously triggered and enters into an interrupt routine. And a linear regression algorithm is operated in the interrupt program to correct the interrupt time of the synchronous timer so as to ensure the synchronization of the time slots, and a superframe structure of the time slots is constructed based on the counting value of the time slots. In the M time period, recording local time and global time after receiving the synchronous message; the synchronization message is transmitted in the allocated time slot.

The invention provides and realizes a time slot synchronization technology which can be used for time synchronization or TDMA in multi-hop wireless communication. The beneficial effects are mainly shown in the following four aspects:

(1) the invention designs a synchronization mechanism for coupling the timer of the microprocessor and the linear regression algorithm, and can realize stable time slot synchronization. The frame of the time slot synchronization mechanism designed and realized by the invention is suitable for general wireless communication equipment and can be easily transplanted to the wireless communication equipment with a microprocessor and communication module frame. The slot synchronization contains time information, and not vice versa. Many time synchronization mechanisms exist, but there are fewer stable slot synchronization mechanisms. The existing time slot synchronization mechanism based on constructive interference is over dependent on coupling with the bottom layer, and has the defects of lack of flexibility and higher portability and application development difficulty.

(2) The invention proposes to use multi-channel TDMA for the flooding of synchronization messages and a heuristic algorithm for the allocation of time slots. Wireless synchronization relies on the broadcast of synchronization messages. Compared with the traditional CSMA-based broadcasting, the algorithm can ensure that the whole network is flooded more quickly in a limited time, and the broadcasting of the synchronous messages is carried out as much as possible. The accuracy of synchronization is ensured while the synchronization overhead is reduced.

(3) In detail, potential contention between wireless communication interruption and timer interruption was discovered over long experimental tests. The time slots in which contention may occur become large and the present invention analyzes and improves the design to eliminate it.

(4) Experiments show that the time slot synchronization mechanism based on the coupling of the timer and the linear regression algorithm not only has the function of realizing stable time slot synchronization, but also can realize higher synchronization accuracy. The invention is realized and tested on a platform consisting of an MSP430F149 microcontroller and a CC2420 half-duplex wireless transceiver. A large number of experiments are carried out, and experimental data statistics shows that under the condition of ideal wireless signal quality, the synchronization technology can realize high precision with the average error smaller than single step counting (1tick) in the 8-hop range. The timer counts the master frequency using 32Khz, and the single step counting duration is equal to about 30.5 us. The detailed data are shown in table 9.

TABLE 9 statistical table of average synchronous error of time slot synchronization of the present invention

The data obtained by running the existing linear regression-based time synchronization protocol under the same platform, the same environment, and the same parameter settings are shown in table 10.

TABLE 10 statistical table of mean synchronization errors for existing time synchronization

Compared with the method shown in fig. 14, in the case of 1 hop, the accuracy of the existing time synchronization is slightly better than that achieved by the present invention, but when the number of hops becomes large, the accuracy achieved by the time slot synchronization achieved by the present invention is better than that of the existing time synchronization, which benefits from the mechanism of the present invention in which the timer is coupled to the linear regression algorithm. It should be noted that (1) the hardware platform used in this experiment has better consistency. When the crystal oscillator precision error on the hardware platform becomes large, the measured synchronous error value may slightly differ from the value obtained in the experiment. (2) The existing time synchronization protocol does not realize the time slot synchronization function.

The invention can be widely applied to the global time slot synchronization occasion of the network.

It is noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A wireless multi-hop time slot synchronization method of the Internet of things is characterized by comprising the following steps:

2. The method for wireless multi-hop time slot synchronization of the internet of things according to claim 1, wherein the step S1 is executed by a multi-channel TDMA flooding time slot allocation algorithm based on the topology of the wireless network, and comprises the following steps:

step 1: the flag positions of device. coverage and device. edge of all the devices are set to be zero;

step 6: adding 1 to a time slot counter;

3. The method for wireless multi-hop timeslot synchronization of the internet of things as claimed in claim 1, wherein the synchronization method of step S2 includes the following steps:

4. The method for wireless multi-hop timeslot synchronization of the internet of things as claimed in claim 1, wherein the synchronization timer in step S3 is configured to perform steps in a form that a hardware timer is coupled to a time synchronization algorithm, and the steps include the following steps:

5. The method for wireless multi-hop time slot synchronization of the internet of things according to claim 1, wherein the time slot superframe in the step S4 is composed of a plurality of time slots, the length of the superframe is selectively set within 5-6 seconds, and the length of the superframe affects the accuracy of synchronization;

6. The method for wireless multi-hop time slot synchronization of the internet of things according to claim 1, wherein the step S21 of associating time based on the synchronization message specifically includes the steps of: