CN104411008B - Event trigger consistency clock synchronization method for wireless sensor network - Google Patents

Event trigger consistency clock synchronization method for wireless sensor network Download PDF

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CN104411008B
CN104411008B CN201410578766.8A CN201410578766A CN104411008B CN 104411008 B CN104411008 B CN 104411008B CN 201410578766 A CN201410578766 A CN 201410578766A CN 104411008 B CN104411008 B CN 104411008B
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陈珍萍
黄友锐
李德权
曲立国
唐超礼
凌六一
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Anhui University of Science and Technology
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    • H04—ELECTRIC COMMUNICATION TECHNIQUE
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    • H04W56/001—Synchronization between nodes
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Abstract

The invention discloses a clock synchronization method for event triggering consistency of a wireless sensor network, which researches clock synchronization of a random topology network and considers the transmission delay and packet loss conditions among nodes. The invention broadcasts the synchronous information only when a specific event occurs, namely the difference between the current state and the last-time transmission state of the network node is greater than the trigger threshold value, and the network synchronous information amount is small; when the network node updates the state, only the local time of the neighbor node needs to be input, the synchronization information broadcasted among the nodes only contains an integer representing the local time of the node, and the length of the information packet is small; considering the influence of transmission delay and packet loss on the synchronization performance, providing a state updating method under the conditions of transmission delay and packet loss; the adjustment parameters of the consistency controller are only related to the degree of entry of the nodes, and the network topology structure does not need to be maintained. The invention also gives an upper bound of transmission delay and packet loss rate through numerical simulation.

Description

Event trigger consistency clock synchronization method for wireless sensor network
Technical Field
The invention belongs to the technical field of wireless sensor networks, and relates to a clock synchronization method for event triggering consistency of a wireless sensor network.
Background
Clock synchronization is a supporting technology of a wireless sensor network, and applications such as data fusion, speed measurement of a moving object, a time division multiple access technology and the like all require that nodes in a local range have consistent time. The existing clock synchronization method can be divided into tree/cluster-based clock synchronization and consistency clock synchronization according to whether topology needs to be maintained or not. The common point of the tree/cluster-based clock synchronization method is that a network topology structure needs to be regularly maintained and special reference nodes or cluster head nodes and gateway nodes need to be selected, the communication traffic of network topology maintenance is large, the saving of wireless node communication energy consumption is not facilitated, and the method has poor robustness on the death and the addition of the nodes; the synchronization method is based on a layered or clustered topology, so that nodes adjacent to a physical position are positioned in different layers or clusters, mutual synchronization errors are large, and the application of data fusion, a TDMA access technology and the like is not facilitated; in addition, the synchronization method synchronizes all network nodes from near to far step by step from the reference node, has the effect of synchronization error accumulation, has poor algorithm expansibility, and cannot be applied to a large-scale multi-hop wireless sensor network. The consistency synchronization can lead the states of a plurality of objects to be consistent through information exchange among the plurality of objects, and the consistency synchronization is widely applied to clock synchronization of a wireless sensor network because the network topology does not need to be maintained. The consistency synchronization method has robustness to network scale, but the synchronization information exchange of the existing method belongs to a continuous sending mode or a time triggering mode, and even if the synchronization error of the node meets the synchronization precision requirement, the node still sends a synchronization data packet; and when the time triggering algorithm is used for designing a triggering period, the time triggering algorithm needs to be designed according to the worst synchronization condition of the network, so that the waste of synchronization energy is caused, and the saving of node energy is not facilitated.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a consistent clock synchronization method for event triggering of a wireless sensor network, which researches clock synchronization of a random topology network and considers the transmission delay and packet loss conditions of synchronous information between nodes. The event triggering communication mode adopted by the invention effectively reduces the sending frequency of the synchronous information, and the invention researches the influence of different transmission delays and packet loss rates on the synchronous performance. The node of the invention only needs to broadcast the local time which is integral multiple of the synchronization period, while the prior synchronization technology needs to send phase offset and frequency offset, and the length of the synchronization data packet of the invention is correspondingly reduced. The control parameters of the consistency controller of the invention are only related to the degree of entry of the nodes, while the prior art determines the adjustment parameters according to the maximum degree of entry of the network, and needs to maintain the variable of the maximum degree of entry of the network. The specific technical scheme is as follows:
a method for synchronizing a clock with consistency triggered by an event in a wireless sensor network comprises the following steps:
step 1: let N be the number of nodes in the wireless sensor network, and c be the counter value of the ith node (i ═ 1, 2.., N) at time ti(t)，t0The time count value is ci(t0) According to
Figure BDA0000592964160000021
calculating local time of the node i and compensating the quantity alpha by the ratei(t) compensating for frequency offset of node i
Figure BDA0000592964160000022
Wherein tau isi(t0) For node i at t0The local time of the moment,
Figure BDA0000592964160000023
And fi(t) the nominal and actual frequencies of the clock oscillator, respectively, and assuming a frequency offset ri(t) is a constant number, denoted as ri；
Step 2: in that
Figure BDA0000592964160000024
When the node i (i ═ 1, 2.., N) adjusts the formula according to the state:
Figure BDA0000592964160000025
Figure BDA0000592964160000026
adjustment of
Figure BDA0000592964160000027
And
Figure BDA0000592964160000028
wherein
Figure BDA0000592964160000029
And
Figure BDA00005929641600000210
local time τ of node i respectivelyi(t) and the frequency compensation quantity αi(ti)，
Figure BDA00005929641600000211
And
Figure BDA00005929641600000212
the kth and the (k + 1) th update time of the node i;
and step 3:
Figure BDA00005929641600000213
or
Figure BDA00005929641600000214
When the local time of the node i is greater than or equal to kT for the first time and the difference between the state and the state in the last transmission meets or exceeds the preset value, the node i broadcasts the state to the output neighbor node, wherein T is the synchronization period of the network;
and 4, step 4: the node i receives the state variable input to the neighbor node broadcast, and the sending time of the node j is the time when the node i receives the state variable and assumes that no transmission delay exists, i.e. the node i
Figure BDA00005929641600000222
And 5: at reception of all neighbour nodes
Figure BDA00005929641600000223
Thereafter, node i determines the update time
Figure BDA00005929641600000224
Updating the formula according to the state:
Figure BDA0000592964160000031
Figure BDA0000592964160000032
update state and wherein c1And c2For synchronizing control parameters, kijIs the weight coefficient between nodes i and j, is the instant before the update time, where c1＝0.5，c2＝0.5/(fmax× T), (i, j) ∈ E, i ∈ not equal to j
Figure BDA0000592964160000037
Otherwise kij＝0；
Step 6: and jumping to the step 1 to circularly execute consistency synchronization.
Further, consider the transmission delay d between nodesijSetting local time when the node j sends, local time when the node i receives, and local time of the node i corresponding to the node j sends, and transmission delay between the node j and the node i is dijAccording to
Figure BDA00005929641600000311
The new state update formula obtained by calculation is as follows:
Figure BDA00005929641600000313
Figure BDA00005929641600000314
further, considering the packet loss situation of the synchronization information between the nodes, assuming that the packet loss situations from the node i to all the output neighbor nodes are the same, the packet loss coefficient of the kth data transmission of the node i is λi(k) If there is no packet loss, 1 is taken, and if there is packet loss, 0 is taken, and a new state updating formula is obtained as follows:
Figure BDA00005929641600000315
Figure BDA00005929641600000316
the new update time calculation formula is that the new weight coefficient satisfies if (i, j) ∈ E and i ≠ j
Figure BDA00005929641600000318
Otherwise k ij0, wherein
Figure BDA00005929641600000319
γ∈(0,1)。
Compared with the prior art, the invention has the beneficial effects that:
1) the node sets a triggering condition of a communication event according to the difference between the current sending state and the last sending state, realizes the high probability communication when the synchronization error is large and the low probability communication when the synchronization error is small, and compromises two contradictory indexes of the synchronization communication traffic and the synchronization precision; 2) the node only needs to broadcast the state variable of the local time, and the communication event of the node only occurs in an integral multiple of the updating period, so that the synchronous information is only an integer, and the length of the synchronous data packet is small; 3) the weight coefficient is determined by the node in-degree, and the algorithm does not need to maintain the network topology; 4) the influence of transmission delay and packet loss on a consistent clock synchronization algorithm is researched, a state updating formula under the conditions of transmission delay and packet loss is given, and upper bound values of the transmission delay and the packet loss are given in a simulation mode.
Drawings
FIG. 1 is a schematic block diagram of event triggered coherent clock synchronization;
FIG. 2 is a diagram illustrating transmission delay between nodes;
FIG. 3 shows a variant caThe network average traffic under the value;
FIG. 4 shows network synchronization errors at different transmission delays;
fig. 5 shows network synchronization errors at different packet loss rates.
Detailed Description
In order to make the technical means, the creation characteristics, the achievement purposes and the effects of the invention easy to understand, the invention is further explained in the following with the accompanying drawings and the specific examples.
Before proceeding with the detailed description, mathematical notation and node clock models used in the present invention will be described. Notation of mathematical symbols: the connection relationship of N nodes (sensors) in the WSN is represented by (V, E), where V is the set of vertices of graph G, corresponding to N sensors in the WSN,
Figure BDA0000592964160000048
for the edge set of the graph G, if the node i can receive the information of the node j, then there is { i, j } ∈ E, otherwise
Figure BDA0000592964160000049
if the { i, j } ∈ E, the node i is called as an output neighbor node of j, and j is an input neighbor node of i;
Figure BDA0000592964160000041
represents the input set of neighbor nodes for node i,
Figure BDA0000592964160000042
representing the degree of entry of the node i;
Figure BDA0000592964160000043
an output set of neighbor nodes representing node i,
Figure BDA0000592964160000044
representing the out degree of node i. Clock model of the node: let N be the number of nodes in the wireless sensor network, and c be the counter value of the ith node (i ═ 1, 2.., N) at time ti(t)，t0The time count value is ci(t0) According to
Figure BDA0000592964160000045
calculating local time of the node i and compensating the quantity alpha by the ratei(t) compensating for frequency offset of node i
Figure BDA0000592964160000046
Wherein tau isi(t0) For node i at t0The local time of the moment,
Figure BDA0000592964160000047
And fi(t) the nominal and actual frequencies of the clock oscillator, respectively, and assuming a frequency offset ri(t) is a constant number, denoted as ri。
Fig. 1 is a schematic block diagram of event triggered coherent clock synchronization for an ith node of a wireless sensor network according to the present invention. Wherein node i sends state based on event trigger mode
Figure BDA0000592964160000051
Broadcast to output neighbor nodes based on received broadcast from input neighbor nodes, taking into account transmission delay and packet loss
Figure BDA0000592964160000052
Determining state update time and state
Figure BDA0000592964160000053
And
Figure BDA0000592964160000054
and (6) updating. The method is described in detail below with reference to the accompanying drawings.
Step 1: in that
Figure BDA0000592964160000055
When the node i (i ═ 1, 2.., N) adjusts the formula according to the state:
Figure BDA0000592964160000056
Figure BDA0000592964160000057
adjustment of
Figure BDA0000592964160000058
And
Figure BDA0000592964160000059
wherein
Figure BDA00005929641600000510
And
Figure BDA00005929641600000511
local time τ of node i respectivelyi(t) and the frequency compensation quantity αi(ti)，
Figure BDA00005929641600000512
And
Figure BDA00005929641600000513
the kth and the (k + 1) th update time of the node i;
step 2:
Figure BDA00005929641600000514
or
Figure BDA00005929641600000515
When the local time of the node i is greater than or equal to kT for the first time and the difference between the state and the state in the last transmission is equal to or larger than the preset value, the node i broadcasts the state to an output neighbor node, wherein T is the synchronization period of the network;
and step 3: the node i receives the state variable input to the neighbor node broadcast, and the sending time of the node j is the time when the node i receives the state variable and assumes that no transmission delay exists, i.e. the node i
Figure BDA00005929641600000523
And 4, step 4: at reception of all neighbour nodes
Figure BDA00005929641600000524
Thereafter, node i determines the update time
Figure BDA00005929641600000525
Updating the formula according to the state:
Figure BDA00005929641600000526
Figure BDA00005929641600000527
update state and wherein c1And c2For synchronizing control parameters, kijIs the weight coefficient between nodes i and j, is the instant before the update time, where c1＝0.5，c2＝0.5/(fmax× T), (i, j) ∈ E, i ∈ not equal to j
Figure BDA00005929641600000532
Otherwise kij＝0；
And 5: and jumping to the step 1 to circularly execute consistency synchronization.
Event triggered consistency synchronization is now presented when considering transmission delays.
Fig. 2 shows a schematic diagram of the transmission delay between nodes j and i. In FIG. 2, point A
Figure BDA0000592964160000061
For local time when node j transmits, point B
Figure BDA0000592964160000062
Is the local time at which node i receives, point A
Figure BDA0000592964160000063
The local time of the corresponding node i when the node j sends is due to the existence of the transmission delay A' ≠ B. Noting the transmission delay value between the node j and the node i as dijFrom
Figure BDA0000592964160000064
Determining the local time of the node i when the node j sends according to the formula (5):
Figure BDA0000592964160000065
bringing formula (5) into formulas (3) and (4) yields an improved version of formulas (3) and (4) in the presence of transmission delays:
Figure BDA0000592964160000066
Figure BDA0000592964160000067
wherein
Figure BDA0000592964160000068
Now, consider the packet loss situation of the synchronization information between nodes.
Assuming that packet loss conditions from the node i to all output neighbor nodes are the same, and the packet loss coefficient of the kth data transmission of the node i is lambdai(k) 1 when there is no packet loss, 0 when there is packet loss, and λi(k) When 0, the output of node i is adjacent to the node
Figure BDA0000592964160000069
The synchronization information cannot be received, so the terms a in equations (6) and (7) can be redefined as:
Figure BDA00005929641600000610
since synchronization information is lost in transmission, the update time is determined according to the latest time of the synchronization message received within a certain period of time, namely:
Figure BDA00005929641600000611
wherein gamma is∈(0,1), recording the degree of entry of the node i under packet loss as
Figure BDA00005929641600000612
Weight coefficient k in equation (5)ijThe adjustment is as follows:
Figure BDA00005929641600000613
the effectiveness of the method is evaluated by defining the network synchronization error as the average network traffic index
Figure BDA00005929641600000614
Network average traffic of
Figure BDA00005929641600000615
Wherein the node is the number of times of occurrence of the communication event of the node i,
Figure BDA0000592964160000072
figure 3 shows the algorithm at different c, when transmission delay and packet loss are not consideredaMean communication probability curve of network at value, where cb＝10-6Number of iterations Iterate300; as can be seen from fig. 3, event triggering can reduce the network average traffic; c. CaAverage network traffic at 0 is 300, caAverage network traffic at 10 is ca33% of 0, the average network traffic follows caThe value is increased and decreased, and the decrease amplitude is maximum within the range of 1-8.
Assuming that the propagation delays follow an exponential distribution with a mean value γ, i.e. dij-exp (γ) ms, with γ being 0 without transmission delay, with γ > 0 representing transmission delay; FIG. 4 shows the network synchronization error curves for different transmission delays, where ca＝8、cb＝10-6Number of iterations Iterate300; it can be seen from fig. 4 that the transmission delay reduces the synchronization accuracy, the larger the transmission delay is, the larger the network synchronization error is, and when the transmission delay is greater than 5s, the network error curve is no longer converged, and the average consistency synchronization of the WSN cannot be realized; when the transmission delay exists and the algorithm is converged, the network synchronization error is not converged any more after iterating for a certain number of times, and small-range fluctuation around a certain value is maintained, and the value is smaller than the transmission delay mean value gamma by more than 0.5 times; if gamma can be controlled to be less than 1ms, after the iteration is performed for 50 times, the synchronization error is stabilized within 0.4ms, and the requirements of most applications of the WSN can be met.
assuming that the packet loss rates of all nodes of the WSN are the same, it is noted as η ═ 1-p { λi(k) 1, η > 0 represents packet loss, η 0 represents no packet loss, ca＝8、λ＝1、Iteratewhen the packet loss rate is less than 40%, after iteration is performed for 50 times, the synchronization error is stabilized within 0.5ms, and most of the application requirements of the WSN can be met.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention.

Claims (2)

1. A method for synchronizing a clock with consistency triggered by an event in a wireless sensor network is characterized by comprising the following steps:
step 1-1: let N be the number of nodes in the wireless sensor network, i ═ 1,2i(t)，t0The time count value is ci(t0) According to
Figure FDA0002604668670000011
calculating local time of the node i and compensating the quantity alpha by the ratei(t) compensating for frequency offset of node i
Figure FDA0002604668670000012
Wherein tau isi(t0) For node i at t0The local time of the moment,
Figure FDA0002604668670000013
And fi(t) the nominal and actual frequencies of the clock oscillator, respectively, and assuming a frequency offset ri(t) isA constant number ri；
Step 1-2: in that
Figure FDA0002604668670000014
When the node i, i ═ 1, 2., N adjusts the formula according to the state:
Figure FDA0002604668670000015
Figure FDA0002604668670000016
adjusting the state
Figure FDA0002604668670000017
And
Figure FDA0002604668670000018
wherein
Figure FDA0002604668670000019
And
Figure FDA00026046686700000110
local time τ of node i respectivelyi(t) and a rate compensation quantity αi(t)，
Figure FDA00026046686700000111
And
Figure FDA00026046686700000112
adjusting the time for the k-th and k + 1-th of the node i;
step 1-3:
Figure FDA00026046686700000113
or
Figure FDA00026046686700000114
I.e. local to node iIs greater than or equal to kT for the first time and the difference between the state and the state at the time of last transmission satisfies
Figure FDA00026046686700000115
Or
Figure FDA00026046686700000116
When node i is in the state
Figure FDA00026046686700000117
Broadcasting to an output neighbor node, wherein T is a synchronization period of the network;
step 1-4: node i receives the state variable broadcast by the input neighbor node
Figure FDA00026046686700000118
Wherein
Figure FDA00026046686700000119
representing an input neighbor node set of a node i, E representing a connection edge set of N nodes in the network, E representing that the node i can receive information of a node j, and recording the sending time of the node j as
Figure FDA00026046686700000120
Node i is at
Figure FDA00026046686700000121
Time of day receipt
Figure FDA00026046686700000122
Assuming no transmission delay, i.e.
Figure FDA00026046686700000123
Step 1-5: at reception of all neighbour nodes
Figure FDA00026046686700000124
After that, without considering the transmission delayAnd when packet loss occurs, the node i determines the updating time
Figure FDA00026046686700000125
Updating the formula according to the state:
Figure FDA0002604668670000021
Figure FDA0002604668670000022
updating a state
Figure FDA0002604668670000023
And
Figure FDA0002604668670000024
wherein c is1And c2For synchronizing control parameters, kijIs a weight coefficient between nodes i and j,
Figure FDA0002604668670000025
To update the time of day
Figure FDA0002604668670000026
At the moment of the previous moment, wherein c1＝0.5，c2＝0.5/(fmax× T), (i, j) ∈ E, i ∈ not equal to j
Figure FDA0002604668670000027
Otherwise kij＝0；
Step 1-6: and jumping to the step 1-1 to circularly execute consistency synchronization.
2. The method for event-triggered coherent clock synchronization in a wireless sensor network according to claim 1, wherein: the state updating of the node i is carried out according to the following steps:
step 2-1: according to
Figure FDA0002604668670000028
Computing
Figure FDA0002604668670000029
Wherein d isijFor the transmission delay between nodes j and i,
Figure FDA00026046686700000210
is the local time when node j transmits,
Figure FDA00026046686700000211
is the local time when the node i receives,
Figure FDA00026046686700000212
for the local time of the corresponding node i when the node j sends, under the condition of considering the transmission delay, the state of the node i is updated as follows:
Figure FDA00026046686700000213
Figure FDA00026046686700000214
wherein
Figure FDA00026046686700000215
Step 2-2: let the packet loss coefficient of the kth data sending of the node i be lambdai(k) Taking 1 when no packet is lost and taking 0 when packet is lost, assuming that packet loss conditions from the node i to all output neighbor nodes are the same, and considering transmission delay and packet loss conditions, redefining the item A as follows:
Figure FDA00026046686700000216
the state of node i is updated as follows:
Figure FDA00026046686700000217
Figure FDA00026046686700000218
the update time and the weight coefficient when packet loss is considered are as follows:
updating time:
Figure FDA00026046686700000219
wherein γ ∈ (0, 1);
the weight coefficient satisfies:
Figure FDA0002604668670000031
wherein
Figure FDA0002604668670000032
To account for the in-degree of node i in the case of packet loss,
Figure FDA0002604668670000033
the node i is represented as an input neighbor node set of the node i under the condition of considering packet loss, ∈ 'is represented as a connection edge set of N nodes in the network under the condition of considering packet loss, and (i, j) ∈ E' represents that the node i can receive information of the node j under the condition of considering packet loss.
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