CN107395308A - A kind of Distributed Wireless Sensor Networks method for synchronizing time of low memory cost - Google Patents
A kind of Distributed Wireless Sensor Networks method for synchronizing time of low memory cost Download PDFInfo
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Abstract
The invention discloses a kind of Distributed Wireless Sensor Networks synchronous method of low memory cost, this method is to be based on distributed average algorithm, and each node memory size is controllable, and each nodal function is consistent in network, no reference mode and fixed topology；Each node periodically sends sync beacon, enters row clock adjustment using the data in the sync beacon received；Node sends the historical time stamp data being locally stored when sending sync beacon simultaneously, and receiving node can directly find available timestamp in sync beacon, data are locally stored without searching；Sending node discharges the data having been used by memory management scheme in time, reaches the purpose for reducing EMS memory occupation.The present invention can allow small, the cheap radio node of internal memory to apply in large-scale dynamic network and intensive distributed network.
Description
Technical Field
The invention relates to a time synchronization method for a wireless sensor network, in particular to a distributed wireless sensor network synchronization method with low memory overhead.
Background
The wireless sensing network is composed of a large number of sensor nodes and has data acquisition, communication and processing capabilities. With the continuous maturity of the technology, the wireless sensing network is increasingly applied to the fields of internet of things, home furnishing, industrial control, medical treatment and health and the like due to the characteristics of low cost, convenience in networking, low power consumption and the like.
Time synchronization is one of the hot problems in wireless sensor network research, and high-precision time synchronization is the basis and guarantee of data processing in network applications such as target tracking, network management, electrical monitoring and the like. Due to the difference of the processes of the node clock crystal oscillators in the sensing network and the influence of factors such as temperature, voltage and the like, the crystal oscillator is inaccurate in counting, so that the clocks of different nodes are inconsistent, and the clocks of the different nodes gradually deviate from each other.
Conventional Time Synchronization methods include RBS (Reference-Broadcast Synchronization), TPSN (Timing-Synchronization Protocol for Sensor Networks), ftsp (floating Time Synchronization Protocol), etc., and such Time Synchronization algorithms generally require a Reference node to provide Reference Time for other nodes, and are called centralized Synchronization algorithms. Centralized synchronization algorithms typically enable fast synchronization speeds and low network overhead. But since they rely on a reference node and a specific topology network (e.g. TPSN, FTSP requires a tree network), the loss of a node and the addition of a new node both easily cause the failure of the synchronization algorithm, so that the centralized time synchronization algorithm is not robust and is difficult to be applied to a dynamic network.
The distributed time synchronization algorithm is not dependent on the reference node and the network topology. The algorithm that is more prominent is a distributed averaging algorithm, such as the classical distributed time synchronization algorithm ATS (L.Schenato and F.Fiorentin, "AverageTimeSynch: A consensus based protocol for clock synchronization-in wireless sensor networks," Automatica, vol.47, No.9, pp.1878-1886,2011.). In the distributed average algorithm, each node exchanges shared information with the neighbor nodes thereof, and calculates a data average value by using the acquired information to adjust the clock of the node, thereby achieving the time synchronization of the whole network. The distributed architecture can provide high robustness and expandability, and the network stability is improved.
Compared with centralized time synchronization, distributed time synchronization can be more suitable for large-scale and dynamic wireless sensor networks. However, each node in the distributed synchronization algorithm needs to store a large amount of information of neighboring nodes, so that the problems of high network overhead and high node memory occupation exist, and the application range of the distributed synchronization algorithm is limited.
Node clock model: in the wireless sensor network, the clock reading of any node i at the physical time t can be represented by formula (1):
τi(t)＝ait+bi(1)
wherein a isiIs the drift rate of the node clock crystal oscillator, biIs the clock offset. To adjust the clock, the ATS algorithm proposes a new adjusted clock model:
whereinIn order to adjust the parameters for the drift rate,to offset the tuning parameters, the ATS synchronization algorithm is therefore aimed at adjusting the individual nodesEnabling adjusted clocks C of all nodes in the networki(t) agreement is achieved.
ATS adopts asynchronous and synchronous mode, and node i receives synchronous beacon of node j and adjusts selfSo that the adjusted clock is close to the clock of node j. To adjust forTo compensate for the drift rate, the ATS needs to know the ratio a of the drift rates between node i and node jij＝aj/aiBut due to aiAnd ajCannot be obtained directly, so the ATS calculates a in an indirect mannerij：
Wherein:
τj(t0) For node j at t0Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time;
τi,j(t0) For node i at t0Receiving a timestamp recorded when a synchronization beacon of the node j is received;
τj(t1) For node j at t1Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time;
τi,j(t1) For node i at t1And receiving the timestamp recorded when the synchronization beacon of the node j is received.
Due to the adoption of the MAC layer time stamp marking method based on SFD capture, the SFD (start frame delimiter) can be detected to record the time stamp when the node sends and receives the synchronous beacon, the output delay of the synchronous beacon can be ignored, and the receiving node and the sending node are considered to mark the time stamp (such as tau) at the same physical timej(t0) And τi,j(t0) Are all at t0Timestamp of the timestamp).
Since equation (3) requires node i to receive the synchronization beacon from node j twice, the first time the synchronization beacon is received, the timestamp data τ is generatedi,j(t0) And τj(t0) And the address of the node j needs to be stored in the memory of the node i, if a is calculatedij，aijAnd also needs to be stored in memory.
In a dense network, the number of neighbor nodes of node i is large; once node i receives a synchronization beacon from a new neighbor node, node i has to open up new memory space to store the generated timestamp data, which results in node i storing too much timestamp data. Such uncontrolled memory usage can therefore make the distributed average synchronization algorithm difficult to apply in dense or dynamic networks.
Disclosure of Invention
The invention aims to provide a distributed wireless sensor network time synchronization method with low memory overhead, which aims to overcome the problem of overlarge memory occupation of each node in the existing distributed synchronization algorithm, ensure the stability of distributed time synchronization, control the memory occupation of each node and enable wireless nodes with limited memory to be applied to large-scale dynamic networks and intensive distributed networks.
The invention provides a distributed wireless sensing network time synchronization method with low memory overhead, which comprises the following steps:
s1: initializing node configuration;
s2: the method comprises the steps that a sending node periodically broadcasts a synchronization beacon, and a historical timestamp and a first timestamp recorded when the sending node broadcasts the synchronization beacon are provided through the synchronization beacon;
s3: releasing the memory space: after the sending node broadcasts the synchronization beacon, the sent historical timestamp of the sending node is cleared, and the memory is released;
s4: clock correction: and the receiving node receives the synchronous beacon broadcasted by the sending node, acquires a first time stamp and a historical time stamp from the synchronous beacon, and records a second time stamp when the receiving node receives the synchronous beacon, so as to correct the clock.
Preferably, in step S1, the method for initializing node configuration includes: the initial drift rate parameter for each node is set to 1 and the initial offset parameter is set to 0.
Preferably, in step S2, the first node and the second node are any two adjacent nodes in the network, the second node is a sending node, and when the second synchronization beacon is periodically broadcast to the first node, the second synchronization beacon adds the history timestamp stored in the memory of the second node and the address ID of the first node corresponding to the history timestamp; the historical timestamp is a third timestamp recorded when the first node is used as a sending node to broadcast the first synchronization beacon to the second node during last communication and a fourth timestamp recorded when the second node receives the first synchronization beacon broadcast by the first node.
Preferably, the first node and the second node are any two adjacent nodes in the network, and a synchronization beacon broadcasted by the first node as a sending node during first communication does not contain a historical timestamp corresponding to the second node; the second node stores a second time stamp of the second node when receiving the synchronization beacon and a first time stamp of the first node when broadcasting the synchronization beacon, and stores the first time stamp and the second time stamp as historical time stamps corresponding to the first node; the first communication is not clock corrected.
Preferably, in step S2, the synchronization beacon includes: the method comprises the steps that a first timestamp recorded by a sending node during broadcasting of the sending node, clock parameters of the sending node and historical timestamps stored in a memory of the sending node are recorded; the clock parameters comprise a drift rate parameter and an offset parameter;
the format of the network address of the sending node is 2 bytes, the format of the clock parameter of the sending node is 4 bytes, the format of the timestamp recorded by the sending node is 6 bytes, the format of the historical timestamp stored in the sending node is K14 bytes, and K is the number of the neighbor nodes of the sending node.
Preferably, in the illustrated step S3: after the sending node broadcasts the synchronization beacon, marking the memory occupied by the sent historical timestamp as 'can be rewritten', deleting the sent historical timestamp, and releasing the memory for storing a new timestamp; when a new timestamp is stored in the freed memory, the memory removes the "rewriteable" flag.
Preferably, the step of clock correction of the receiving node in step S4 includes:
s41: calculating a ratio of drift rates of the receiving nodes;
s42: carrying out drift rate compensation and updating the drift rate parameter of the receiving node;
s43: and performing offset compensation and updating the offset parameter of the receiving node.
Preferably, the ratio a of the drift rates is calculatedijThe formula of (1) is:
wherein the node i is t1At the receiving node of time, node j is t1A sending node at a time; tau isj(t1) For node j at t1Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time; tau isij(t1) For node i at t1A timestamp recorded when a synchronization beacon broadcasted by the node j is received at the moment; tau isji(t0) For node j at t0Receiving a timestamp recorded when a synchronization beacon broadcasted by a node i is received; tau isi(t0) For node i at t0Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time;
the updated drift rate parameterThe formula of (1) is:
the update offset parameterThe formula of (1) is:
wherein,as a parameter of the drift rate of the node i,as the drift rate parameter of the node j, η∈ (0,1), η as the average parameter, v ∈ (0,1), v as the average parameter, aiIs a section ofClock crystal oscillator drift rate of point i, ajJ clock crystal oscillator drift rate of node, biIs the clock offset of node i, t1Is t1Time of day, CiAdjusted clock for node i, CjAdjusted clock for node j.
Preferably, the number of the historical timestamps stored in the memory of any node is at most 2K, which is twice the number of the timestamps generated when K neighboring nodes of the node broadcast the synchronization beacon to the node respectively.
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the distributed characteristic of the WSN, a distributed time synchronization algorithm with low memory overhead is designed, and the memory of the node is controllable, so that the node can still normally work even in a dense network and cannot be blocked due to insufficient memory;
(2) the distributed synchronization algorithm has the advantages of high robustness and expansibility, all the nodes have the same functions, no reference node is needed, no specific topological structure is needed, and the distributed synchronization algorithm can be applied to a dynamic network;
(3) the invention compensates the drift rate of the clock, so compared with the method of compensating the clock offset only, the invention can control the clock error at a lower value for a long time, thereby reducing the synchronization times and reducing the synchronization cost;
(4) the invention adopts the method of MAC layer timestamp marking based on SFD capture, thereby greatly reducing uncertain delay errors and greatly improving the synchronization precision.
Drawings
FIG. 1 is a schematic diagram of a beacon delivery process between two nodes according to the present invention;
fig. 2 is a diagram of a sync beacon packet format according to the present invention.
Detailed Description
The invention provides a distributed wireless sensor network time synchronization method with low memory overhead, and in order to make the invention more obvious and understandable, the invention is further explained below by combining specific embodiments and accompanying drawings.
The invention discloses a distributed wireless sensor network time synchronization method with low memory overhead, which comprises the following steps:
s1: initializing node configuration;
s2: the sending node broadcasts the synchronization beacon periodically and records a timestamp recorded when the sending node broadcasts the synchronization beacon;
s3: releasing the memory space: after the synchronous beacon is broadcasted, the sending node clears the sent historical timestamp of the sending node and releases the memory;
s4: clock correction: and after the receiving node receives the synchronization beacon broadcasted by the sending node, recording the timestamp recorded when the receiving node receives the synchronization beacon broadcasted by the sending node at the receiving moment, and correcting the clock.
In the step of initializing node configuration, each node's initial drift rate parameterSet to 1, initial drift adjustmentIs set to 0.
Each node in the network broadcasts a synchronization beacon with a period T, as shown in fig. 1, at T0At that time, in the network, node i broadcasts a synchronization beacon, which includes: the node i sends a time (t)0Time of day) time stamp τi(t0) Clock parameter of node iAnd the address ID of node i, where,in order to adjust the parameters for the drift rate,the parameters are adjusted for the offset. Assuming that there is no historical timestamp data in the memory of node i at this time, no additional data is added to the synchronization beacon.
As shown in FIG. 1, τi(t0) For node i at t0Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time; tau isj,i(t0) For node j at t0Receiving a timestamp recorded when a synchronization beacon of the node i is received; tau isj(t1) For node j at t1Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time; tau isi,j(t1) For node i at t1And receiving the timestamp recorded when the synchronization beacon of the node j is received.
Fig. 2 is a schematic diagram of a packet format of a synchronization beacon, wherein 2 bytes of SrcID is a network address of a transmitting node; both the 4-byte SrcDrift and the 4-byte SrcOffset are sending node clock parametersThe SrcClk with 6 bytes is a timestamp recorded by the sending node; the K14 bytes of PrevData are the historical timestamps stored at the sending node. In the memory of the PrevData, the time stamps generated by the K full neighbor nodes can be recorded.
The sending node records a timestamp when the sending node broadcasts the synchronous beacon, the receiving node records a timestamp at the receiving time of the receiving node, and the timestamp recorded by the sending node is sent to the receiving node, so that the receiving node acquires two timestamp data in one communication. Any node can store the timestamps generated by K neighbor nodes at most, and the number of the timestamp data generated by the K neighbor nodes is 2K at most.
At t0At the moment, when the receiving node is a node j, the sending node is a node i, the node j receives the synchronous beacon of the node i, and the timestamp tau is recordedji(t0) Inquiring to find the received synchronous beacon except for the node i at t0Time stamp tau recorded when broadcasting synchronization beacon at a momenti(t0) And no other timestamps are available.
Assuming node j has a memory of idle (i.e., has not recorded the timestamps generated by the full K neighbor nodes), two timestamps [ τ ] are usedji(t0),τi(t0)]And the address ID of node i are stored in the memory of node j.
At t1At the moment, when the sending node is a node j, the receiving node is a node i, and the node j broadcasts a synchronization beacon, the two historical timestamps [ tau ] in the memory of the node j are added into the synchronization beaconji(t0),τi(t0)](node j at t)0Timestamp tau recorded when a synchronization beacon broadcasted by a node i is received at that momentji(t0) And node i is at t0Time stamp tau recorded when broadcasting synchronization beacon at a momenti(t0) And the node ID of the corresponding node i.
After the sending node sends the synchronization beacon, two historical time stamps [ tau ] to be sentji(t0),τi(t0)]Marking the occupied memory block as 'rewritable', deleting the sent historical timestamp, and releasing the memory for storing the new timestamp; when the freed memory stores a new timestamp, the memory removes the "rewriteable" flag.
If the node i receives the synchronous beacon broadcasted by the node j, the node i firstly obtains the time t of the node j1Time stamp tau recorded when broadcasting synchronization beacon at a momentj(t1) And node i is at t1Timestamp tau recorded when a synchronization beacon broadcasted by a node j is received at a timeij(t1) The two time stamps [ tau ]j(t1),τij(t1)]Then two historical time stamps [ tau ] of the node j are acquired in the synchronous beaconji(t0),τi(t0)]。
Therefore, the node i can directly obtain the four time stamps [ tau ] in the communication processj(t1),τij(t1)]、[τji(t0),τi(t0)]Therefore, the ratio a of the drift rates can be calculated by the formula (4)ij：
Wherein the node i is t1At the receiving node of time, node j is t1A sending node at a time;
τj(t1) For node j at t1Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time; tau isij(t1) For node i at t1A timestamp recorded when a synchronization beacon broadcasted by the node j is received at the moment; tau isji(t0) For node j at t0Receiving a timestamp recorded when a synchronization beacon broadcasted by a node i is received; tau isi(t0) For node i at t0And broadcasting the time stamp recorded when the synchronization beacon is broadcasted at any time.
Calculating the ratio of the drift rates aijThen, the node i performs drift rate compensation, and updates the drift rate parameter by the formula (5)
After the drift rate compensation, the node i updates the offset parameter by performing offset compensation through equation (6)
Wherein,as a parameter of the drift rate of the node i,as the drift rate parameter of the node j, η∈ (0,1), η as the average parameter, v ∈ (0,1), v as the average parameter, aiClock crystal drift rate of node i, ajJ clock crystal oscillator drift rate of node, biIs the clock offset of node i, t1Is t1Time of day, CiAdjusted clock for node i, CjThe adjusted clock for node j is then used,is set to 1 as the initialization parameter of (a),is 0.
In the mutual communication between the two nodes i and j, the node j sends two time stamps [ tau ]ji(t0),τi(t0)]Is utilized by node i, thus [ tau ]ji(t0),τi(t0)]Even if deleted, it will not have much influence, so node j will send [ τ [ji(t0),τi(t0)]The flag is "rewritable". If node j receives the second synchronization beacon from node i, and two historical timestamps [ tau ] in the memoryji(t0),τi(t0)]Has not been deleted, node j can still calculate a using the timestampijNamely, formula (3) is used.
The above equations (5) and (6) are embodiments of an averaging algorithm, and the node i adjusts its own drift rate parameter and offset parameter to values closer to the node j. Without loss of generality, node i will constantly adjust the clock to its approximate average with the neighbor nodes. Therefore, for the whole network, the deviation of each node from other nodes is smaller and smaller, and after a certain period, the parameters of the clocks of the nodes are gradually converged to make the clocks of the whole network consistent.
Compared with the traditional distributed algorithm ATS, the bidirectional communication between two nodes in the invention enables one node to carry out clock correction without waiting for receiving the synchronous beacons from the same node twice. And the time for completing the two-way communication process is less than a period T, and the invention can still ensure certain synchronization efficiency under the condition of short node meeting time in the dynamic network.
The storage size of the node is artificially limited to store the timestamp data generated by K neighbor nodes at most, and K can be adjusted according to the actual condition of the memory of the node. When the node storage is full, a synchronization beacon from a new node is received, and whether a memory block is marked as 'rewritable' is inquired, if so, the data in the memory block is replaced by new data, and if not, the new data is discarded.
While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be determined from the following claims.
Claims (9)
1. A time synchronization method of a distributed wireless sensor network with low memory overhead is characterized by comprising the following steps:
s1: initializing node configuration;
s2: the method comprises the steps that a sending node periodically broadcasts a synchronization beacon, and a historical timestamp and a first timestamp recorded when the sending node broadcasts the synchronization beacon are provided through the synchronization beacon;
s3: releasing the memory space: after the sending node broadcasts the synchronization beacon, the sent historical timestamp of the sending node is cleared, and the memory is released;
s4: clock correction: and the receiving node receives the synchronous beacon broadcasted by the sending node, acquires a first time stamp and a historical time stamp from the synchronous beacon, and records a second time stamp when the receiving node receives the synchronous beacon, so as to correct the clock.
2. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
in step S1, the method for initializing node configuration includes:
the initial drift rate parameter for each node is set to 1 and the initial offset parameter is set to 0.
3. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
in the step S2, in the above step,
let the first node and the second node be any two adjacent nodes in the network,
a second node is used as a sending node, and when a second synchronous beacon is broadcast to a first node according to a period, the second synchronous beacon adds a historical timestamp stored in a memory of the second node and an address ID (identity) of the first node corresponding to the historical timestamp;
the historical timestamp is a third timestamp recorded when the first node is used as a sending node to broadcast the first synchronization beacon to the second node during last communication and a fourth timestamp recorded when the second node receives the first synchronization beacon broadcast by the first node.
4. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
let the first node and the second node be any two adjacent nodes in the network,
the synchronization beacon broadcasted by the first node as the sending node during the first communication does not contain the historical timestamp corresponding to the second node; the second node stores a second time stamp of the second node when receiving the synchronization beacon and a first time stamp of the first node when broadcasting the synchronization beacon, and stores the first time stamp and the second time stamp as historical time stamps corresponding to the first node;
the first communication is not clock corrected.
5. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
in step S2, the synchronization beacon includes: the method comprises the steps that a first timestamp recorded by a sending node during broadcasting of the sending node, clock parameters of the sending node and historical timestamps stored in a memory of the sending node are recorded;
the clock parameters comprise a drift rate parameter and an offset parameter;
the format of the network address of the sending node is 2 bytes, the format of the clock parameter of the sending node is 4 bytes, the format of the timestamp recorded by the sending node is 6 bytes, the format of the historical timestamp stored in the sending node is K14 bytes, and K is the number of the neighbor nodes of the sending node.
6. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
in the illustrated step S3:
after the sending node broadcasts the synchronization beacon, marking the memory occupied by the sent historical timestamp as 'can be rewritten', deleting the sent historical timestamp, and releasing the memory for storing a new timestamp;
when a new timestamp is stored in the freed memory, the memory removes the "rewriteable" flag.
7. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
the step of clock correction of the receiving node in step S4 includes:
s41: calculating a ratio of drift rates of the receiving nodes;
s42: carrying out drift rate compensation and updating the drift rate parameter of the receiving node;
s43: and performing offset compensation and updating the offset parameter of the receiving node.
8. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 7,
calculating the ratio a of the drift ratesijThe formula of (1) is:
<mrow> <msub> <mi>a</mi> <mrow> <mi>i</mi> <mi>j</mi> </mrow> </msub> <mo>=</mo> <mfrac> <mrow> <msub> <mi>&tau;</mi> <mi>j</mi> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>1</mn> </msub> <mo>)</mo> </mrow> <mo>-</mo> <msub> <mi>&tau;</mi> <mrow> <mi>j</mi> <mo>,</mo> <mi>i</mi> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>0</mn> </msub> <mo>)</mo> </mrow> </mrow> <mrow> <msub> <mi>&tau;</mi> <mrow> <mi>j</mi> <mo>,</mo> <mi>i</mi> </mrow> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>1</mn> </msub> <mo>)</mo> </mrow> <mo>-</mo> <msub> <mi>&tau;</mi> <mi>i</mi> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>0</mn> </msub> <mo>)</mo> </mrow> </mrow> </mfrac> <mo>;</mo> </mrow>
wherein the node i is t1At the receiving node of time, node j is t1A sending node at a time;
τj(t1) For node j at t1Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time; tau isij(t1) For node i at t1A timestamp recorded when a synchronization beacon broadcasted by the node j is received at the moment; tau isji(t0) For node j at t0Receiving a timestamp recorded when a synchronization beacon broadcasted by a node i is received; tau isi(t0) For node i at t0Broadcasting a timestamp recorded when the synchronization beacon is broadcast at any time;
the updated drift rate parameterThe formula of (1) is:
<mrow> <msub> <mover> <mi>a</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>1</mn> </msub> <mo>)</mo> </mrow> <msub> <mi>a</mi> <mi>i</mi> </msub> <mo>:</mo> <mo>=</mo> <mi>&eta;</mi> <msub> <mover> <mi>a</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>1</mn> </msub> <mo>)</mo> </mrow> <msub> <mi>a</mi> <mi>i</mi> </msub> <mo>+</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>-</mo> <mi>&eta;</mi> <mo>)</mo> </mrow> <msub> <mover> <mi>a</mi> <mo>^</mo> </mover> <mi>j</mi> </msub> <mrow> <mo>(</mo> <msub> <mi>t</mi> <mn>1</mn> </msub> <mo>)</mo> </mrow> <msub> <mi>a</mi> <mi>j</mi> </msub> <mo>;</mo> </mrow>
the update offset parameterThe formula of (1) is:
<mrow> <msub> <mover> <mi>b</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mo>:</mo> <mo>=</mo> <msub> <mover> <mi>b</mi> <mo>^</mo> </mover> <mi>i</mi> </msub> <mo>+</mo> <mrow> <mo>(</mo> <mn>1</mn> <mo>-</mo> <mi>v</mi> <mo>)</mo> </mrow> <mrow> <mo>(</mo> <msub> <mi>C</mi> <mi>j</mi> </msub> <mo>-</mo> <msub> <mi>C</mi> <mi>i</mi> </msub> <mo>)</mo> </mrow> <mo>;</mo> </mrow>
wherein,as a parameter of the drift rate of the node i,as the drift rate parameter of the node j, η∈ (0,1), η as the average parameter, v ∈ (0,1), v as the average parameter, aiClock crystal drift rate of node i, ajJ clock crystal oscillator drift rate of node, biIs the clock offset of node i, t1Is t1Time of day, CiAdjusted clock for node i, CjAdjusted clock for node j.
9. The method for time synchronization in distributed wireless sensor network with low memory overhead as claimed in claim 1,
the number of the historical timestamps stored in the memory of any node is at most 2K, which is twice the number of the timestamps generated when K neighbor nodes of the node broadcast the synchronization beacon to the node respectively.
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