U.S. patent application number 14/365831 was filed with the patent office on 2014-12-04 for method for transmitting data in a communications network.
The applicant listed for this patent is SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Rudolf Sollacher.
Application Number | 20140355575 14/365831 |
Document ID | / |
Family ID | 47469882 |
Filed Date | 2014-12-04 |
United States Patent
Application |
20140355575 |
Kind Code |
A1 |
Sollacher; Rudolf |
December 4, 2014 |
METHOD FOR TRANSMITTING DATA IN A COMMUNICATIONS NETWORK
Abstract
A method transmits data in a communications network containing a
plurality of nodes. The transmission of suitable configuration data
between a node and its neighboring node ensures that the time slots
used for the data transmission are used only by one node, thus
preventing collision. The method is preferably used in wireless
sensor networks, in which the individual sensor nodes exchange data
between one another. The method guarantees reliable data
transmission with low energy consumption by the individual sensor
nodes. The method can be combined with a decentralized pattern
detection, for which mean values are decentrally determined in a
suitable manner in the individual nodes via protocols known per se,
particularly via a consensus protocol or via a tree aggregation
protocol. The method is used particularly in a communications
network for an automation system or a power network or a transport
network.
Inventors: |
Sollacher; Rudolf; (Eching,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SIEMENS AKTIENGESELLSCHAFT |
MUENCHEN |
|
DE |
|
|
Family ID: |
47469882 |
Appl. No.: |
14/365831 |
Filed: |
December 3, 2012 |
PCT Filed: |
December 3, 2012 |
PCT NO: |
PCT/EP2012/074193 |
371 Date: |
June 16, 2014 |
Current U.S.
Class: |
370/336 |
Current CPC
Class: |
H04W 72/0446 20130101;
H04L 67/12 20130101; H04W 72/0406 20130101; H04L 67/325 20130101;
H04W 56/001 20130101; Y04S 40/18 20180501; H04W 56/00 20130101;
H04W 84/18 20130101 |
Class at
Publication: |
370/336 |
International
Class: |
H04L 29/08 20060101
H04L029/08; H04W 56/00 20060101 H04W056/00; H04W 72/04 20060101
H04W072/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 2011 |
DE |
10 2011 088 884.5 |
Claims
1-15. (canceled)
16. A method for transmitting data in a communications network
containing a plurality of nodes, which comprises the steps of:
specifying at least one interval on a basis of a synchronized time
synchronized for all of the nodes, the interval containing a group
of first time slots and a group of second time slots, wherein the
first time slots being utilized for data transmission by every node
and the second time slots being reserved by respective ones of the
nodes in order to be utilized for data transmission by the
respective nodes; determining, via a respective node, whether
and/or which neighboring nodes within communication range have
reserved the second time slots and generates from this coordination
data according to which a second time slot is reserved by the
respective node which is not reserved by neighboring nodes, and the
coordination data furthermore containing information as to whether
and/or which of the second time slots are reserved several times by
the neighboring nodes; sending out, via the respective node, the
coordination data generated by the respective node to the
neighboring nodes of the respective node within a first time slot,
wherein each neighboring node with the second time slots reserved
several times according to the coordination data reserves a new
second time slot, no reservation of which by another node is known
thereto; and sending out the data by the respective nodes which
have reserved corresponding second time slots within the second
time slots.
17. The method according to claim 16, wherein the nodes perform a
decentralized time synchronization for determining the synchronized
time.
18. The method according to claim 16, which further comprises
reserving one of the second time slots for a broadcast transmission
by the respective node in accordance with the coordination data
generated by the respective node.
19. The method according to claim 16, which further comprises
reserving one of the second time slots for a predetermined link
between the respective node and a predetermined neighboring node
according to the coordination data generated by the respective
node, first data being transmitted by the respective node to the
predetermined neighboring node and second data being transmitted by
the predetermined neighboring node to the respective node within
the second time slot, wherein, in a case where the transmission of
the first data, the second data or the first and second data is not
successful, the first and second data are discarded.
20. The method according to claim 16, which further comprises
determining at least one parameter value in the respective nodes
which is specific for the respective node.
21. The method according to claim 20, which further comprises
determining the data transmitted in the second time slots and
processing the data determined on a basis of a protocol in such a
manner that a mean value of parameter values of all the nodes is
estimated in each of the nodes.
22. The method according to claim 21, which further comprises
selecting the protocol from the group consisting of a consensus
protocol, a tree aggregation protocol, and both the consensus
protocol and the tree aggregation protocol.
23. The method according to claim 21, wherein based in a
decentralized manner on status values which are present locally in
the respective nodes and are acquired in the respective nodes, a
pattern represented by all the status values of the nodes is
recognized from a plurality of patterns in each of the nodes on a
basis of the mean value of the parameter values.
24. The method according to claim 23, wherein, in each of the
nodes, depositing the multiplicity of patterns with in each case a
probability which specifies how probable a status variable present
locally in the respective node is in dependence on a respective one
of the patterns.
25. The method according to claim 24, which further comprises
determining logarithms of probabilities as parameter values in the
respective node for the status variable present locally in the
respective node with the presence of the respective patterns, and
the probability with which each pattern is represented by the
status variables present locally in all of the nodes is determined
in each of the nodes via the mean value of the logarithms for a
respective pattern, wherein the respective pattern having a highest
probability represents a detected pattern.
26. The method according to claim 16, wherein the first time slots
are carrier sense multiple access time slots and the second time
slots are time division multiple access time slots.
27. The method according to claim 16, wherein the communications
network is a wireless communications network in which the nodes
contain at least partially sensors which communicate with one
another wirelessly.
28. The method according to claim 16, wherein the method is used in
a communications network for an automation plant, a power system, a
traffic network or a combination of the automation plant, the power
plant and the traffic network.
29. The method according to claim 16, wherein the communications
network is a wireless sensor network in which the nodes contain at
least partially sensors which communicate with one another
wirelessly.
30. A communications network, comprising: a plurality of nodes
configured such that during an operation of the communications
network: at least one interval is specified on a basis of a
synchronized time synchronized for all of said nodes, said interval
contains a group of first time slots and a group of second time
slots, wherein the first time slots can be utilized for data
transmission by every one of said nodes and the second time slots
can be reserved by said nodes to be utilized for data transmission
by said nodes; each of said nodes determining whether and/or which
neighboring nodes within communication range have reserved the
second time slots and generates from this coordination data
according to which a second time slot is reserved by a respective
node of said nodes which is not reserved by said neighboring nodes,
and the coordination data furthermore contain information as to
whether and/or which of the second time slots are reserved several
times by said neighboring nodes; said respective node sending out
the coordination data generated by said respective node to said
neighboring nodes within said first time slot, wherein each of said
neighboring nodes with said second time slots reserved several
times according to the coordination data reserves a new second time
slot, no reservation of which by another node is known thereto; and
data are sent out by said nodes which have reserved corresponding
second time slots within the second time slots.
31. The communications network according to claim 30, wherein said
nodes perform a decentralized time synchronization for determining
the synchronized time.
32. The communications network according to claim 30, wherein the
communications network further configured to reserve one of the
second time slots for a broadcast transmission by said respective
node in accordance with the coordination data generated by said
respective node.
33. The communications network according to claim 30, wherein the
communications network further configured to reserve one of the
second time slots for a predetermined link between said respective
node and a predetermined neighboring node according to the
coordination data generated by said respective node, first data
being transmitted by said respective node to said predetermined
neighboring node and second data being transmitted by said
predetermined neighboring node to said respective node within the
second time slot, wherein, in a case where the transmission of the
first data, the second data or the first and second data is not
successful, the first and second data are discarded.
34. The communications network according to claim 30, wherein the
communications network further configured to determine at least one
parameter value in said respective node which is specific for said
respective node.
35. The communications network according to claim 34, wherein the
communications network further configured to determine the data
transmitted in the second time slots and processing the data
determined on a basis of a protocol in such a manner that a mean
value of parameter values of all said nodes is estimated in each of
said nodes.
36. The communications network according to claim 35, wherein the
communications network further configured to select the protocol
from the group consisting of a consensus protocol, a tree
aggregation protocol, and both the consensus protocol and the tree
aggregation protocol.
37. The communications network according to claim 35, wherein based
in a decentralized manner on status values which are present
locally in said respective nodes and are acquired in said
respective nodes, a pattern represented by all the status values of
said nodes is recognized from a plurality of patterns in each of
said nodes on a basis of the mean value of the parameter
values.
38. The communications network according to claim 37, wherein, in
each of said nodes, depositing the multiplicity of patterns with in
each case a probability which specifies how probable a status
variable present locally in said respective node is in dependence
on a respective one of the patterns.
39. The communications network according to claim 38, wherein the
communications network further configured to determine logarithms
of probabilities as parameter values in said respective node for
the status variable present locally in said respective node with
the presence of the respective patterns, and the probability with
which each pattern is represented by the status variables present
locally in all said nodes is determined in each of said nodes via
the mean value of the logarithms for a respective pattern, wherein
the respective pattern having a highest probability represents a
detected pattern.
40. The communications network according to claim 30, wherein the
first time slots are carrier sense multiple access time slots and
the second time slots are time division multiple access time
slots.
41. The communications network according to claim 30, wherein the
communications network is a wireless communications network in
which the nodes contain at least partially sensors which
communicate with one another wirelessly.
42. The communications network according to claim 30, wherein the
communications network is configured to be disposed in an
automation plant, a power system, a traffic network or a
combination of the automation plant, the power plant and the
traffic network.
43. The communications network according to claim 30, wherein the
communications network is a wireless sensor network in which said
nodes contain at least partially sensors which communicate with one
another wirelessly.
Description
[0001] The invention relates to a method for transmitting data in a
communications network comprising a plurality of nodes and to a
corresponding communications network.
[0002] In communications networks comprising a plurality of nodes
such as, e.g., in wireless sensor networks, the need often exists
that the data acquired by the individual nodes are reliably
conveyed to neighboring nodes within communication range of the
respective node. Since each node only knows some of the nodes of
the network, conflicts may arise in this context resulting in two
nodes which are not within communication range with respect to one
another transmitting data at the same time to the same node, which
leads to collisions and to the loss of these data.
[0003] In communications networks, a central entity in the form of
a gateway or a central controller is frequently used in which the
data from all nodes are collected. In this arrangement, however, it
is disadvantageous that the data transmission collapses when the
central entity fails and, furthermore, the communication load of
the individual nodes towards the central entity increases so that
nodes within the spatial or topological vicinity of the central
entity impair the life and the performance of the network.
[0004] It is the object of the invention, therefore, to create a
method by means of which data can be transmitted in a simple and
reliable manner in a communications network between nodes.
[0005] This object is achieved by the method as claimed in claim 1
and the communications network as claimed in claim 14,
respectively. Further developments of the invention are defined in
the dependent claims.
[0006] In the method according to the invention, one or more
successive intervals are specified on the basis of a time
synchronized for all nodes of the communications network, which
intervals in each case comprise a group of first time slots and a
group of second time slots, wherein the first time slots can be
utilized for data transmission by every node and the second time
slots can be reserved by respective nodes in order to be utilized
for data transmission by the respective node. In a preferred
embodiment, the first time slots are CSMA time slots known per se
(CSMA=Carrier Sense Multiple Access) which can be utilized by any
node if the time slot is not yet occupied by another node. By
comparison, the second time slots are preferably TDMA time slots
known per se (TDMA=Time Division Multiple Access) which are
suitably reserved exclusively for particular nodes or data
transmissions.
[0007] Within the context of the method according to the invention,
a respective node in the communications network determines whether
and/or which neighboring nodes within its communication range have
reserved second time slots. From this information, the respective
node generates coordination data (e.g. coordination packets)
according to which a second time slot is reserved by the respective
node which is not reserved by neighboring nodes. Furthermore, the
coordination data contain the information as to whether and/or
which second time slots are reserved several times by neighboring
nodes. This multiple occupancy can occur when, although two
neighboring nodes are within communication range from the node
currently considered, they are not within communication range with
one another.
[0008] According to the invention, a respective node sends the
coordination data generated by it to its neighboring nodes within a
first time slot, wherein the respective neighboring nodes which
have reserved the same second time slot according to the
coordination data reserve a new second time slot, no reservation of
which by another node is known thereto. Subsequently, data are sent
out by the respective nodes which have reserved the corresponding
second time slots within the second time slots.
[0009] The method according to the invention provides for a
decentralized time slot allocation without collisions in a simple
manner by means of self organization of the nodes, so that data can
be transmitted reliably between a node and its neighboring node. In
this context, the method can be used in arbitrary communications
networks and especially in wireless communications networks. The
method is used preferably in wireless sensor networks in which at
least some of the nodes comprise sensors which communicate
wirelessly with one another in order to exchange, e.g., sensed
measurement values by this means. The method according to the
invention can be used in arbitrary technical fields of application.
As an example, the method can be used in a communications network
for an automation plant, e.g. for production automation or process
automation, and/or for a power system and/or for a traffic network.
In such fields of application, it is often necessary to exchange
data between the nodes via a decentralized organization of the
network.
[0010] In a particularly preferred embodiment of the method
according to the invention, the nodes perform a decentralized time
synchronization for determining the synchronized time, on the basis
of which the time slots are specified. In this context, methods,
known per se, for decentralized time synchronization can be used,
such as, e.g., the method described in German patent application 10
2010 042 256.8. The entire content of disclosure of this
application is incorporated in the content of the present
application by means of reference.
[0011] In a further, particularly preferred embodiment, a second
time slot is reserved for a broadcast transmission by a respective
node in accordance with the coordination data generated by the
respective node.
[0012] In particular applications, particularly in the case of a
data transmission according to a consensus protocol, the data
transmission between a node and a neighboring node should be
symmetric, i.e. if a node sends data to a neighboring node, these
neighboring nodes should also send data back to the node. If this
is not the case, the corresponding data should not be processed
further. To achieve this, in a preferred embodiment a second time
slot is reserved for a predetermined link between a respective node
and a predetermined neighboring node according to the coordination
data generated by the respective node, both first data being
transmitted by the respective node to the predetermined neighboring
node and second data by the predetermined neighboring node to the
respective node within this second time slot, wherein, in the case
where the transmission of the first and/or second data is not
successful, the first and second data are discarded. According to
this embodiment, it is extensively ensured that data are always
transmitted symmetrically between nodes and neighboring nodes on
the corresponding links. The establishment of whether the data
transmissions were successful can be achieved, e.g., via a
corresponding confirmation which is sent out in response to the
reception of the first and second data, respectively, by the
respective node.
[0013] In a further preferred embodiment of the invention, at least
one parameter value is determined in the respective nodes which is
specific for the respective node. One such parameter value can be,
e.g., a measurement value or be based on a measurement value which
is detected by the node or a sensor in the node, respectively. As
part of the data transmission between the nodes, these parameter
values or, respectively, data based on these parameter values can
be transmitted. In this context, in particular, parameter values
updated in each new interval are determined and transmitted.
[0014] In a particularly preferred embodiment, the data transmitted
in the second time slots are determined and processed on the basis
of a protocol in such a manner that the mean value of the parameter
values of all nodes is estimated in each node. Such protocols are
sufficiently well known from the prior art and provide for an
estimation of the mean value in each node without the parameter
values of all other nodes having to be known in the respective
nodes. Instead, it is sufficient that the respective node can
exchange data directly only with some of the nodes of the network.
In one variant of the invention, a consensus protocol, known per
se, or possibly also a tree aggregation protocol which is also
previously known is used as protocol for averaging the parameter
values.
[0015] In a further variant, the data transmission method according
to the invention is utilized for the purpose that, based in a
decentralized manner on status values which are in each case
present locally in a node and are preferably acquired in the
respective nodes, a pattern represented by all status values of the
nodes is recognized from a plurality of patterns in each node on
the basis of the mean value of the parameter values which is made
known by means of a suitable protocol. This decentralized pattern
recognition is preferably implemented in such a manner that, in
each node, the multiplicity of patterns is deposited with in each
case a probability which specifies how probable a status variable
present locally in the respective node is in dependence on the
respective pattern. In particular, in this context, the logarithms
of the probabilities are determined as parameter values in the
respective node for the status variable present locally in the
respective node with the presence of the respective patterns. In
this context, the probability with which each pattern is
represented by the status variables present locally in all nodes is
determined in each node via the mean value of the logarithms for a
respective pattern. The pattern having the highest probability then
represents the detected pattern.
[0016] Apart from the method described above, the invention also
relates to a communications network comprising a plurality of nodes
which are designed in such a manner that the method according to
the invention or, respectively, one or more variants of the method
according to the invention can be carried out in the operation of
the communications network.
[0017] In the text which follows, exemplary embodiments of the
invention are described in detail with reference to the attached
figures, in which:
[0018] FIG. 1 shows a diagrammatic representation of a
communications network in the form of a wireless sensor network in
which an embodiment of the method according to the invention is
carried out;
[0019] FIG. 2 shows the representation of an interval of first and
second time slots in which data are transmitted on the basis of a
variant of the method according to the invention; and
[0020] FIG. 3 shows a diagram which illustrates the accuracy of a
mean-value estimation based on one embodiment of the invention.
[0021] In the text which follows, the invention will be described
on the basis of a communication in a wireless sensor network, FIG.
1 showing by way of example such a sensor network. The sensor
network comprises seven sensor nodes S1, S2, . . . , S7, which can
exchange data with one another via a suitable wireless protocol. In
this context, a respective sensor node only knows particular number
of neighboring nodes in its environment due to the limited
communication range of the wireless transmission. In the scenario
of FIG. 1, the node S1 only knows, e.g., nodes S2 and S3 and not
the remaining nodes. Similarly, certain other nodes may know
particular nodes in their neighborhood but not the node S1. The
wireless sensor network operates in a completely decentralized
manner, i.e. there is no central entity to which corresponding data
which are detected by the individual sensor nodes can be
transmitted. The aim of the embodiment, described here, of the
method according to the invention is then to detect in each
individual node a pattern of a system status of the entire network,
although a respective node only knows some of its neighboring
nodes. To achieve this, a consensus protocol is used which is
described below. In this context, however, it must be ensured that
each individual sensor node transmits its data reliably to its
neighboring nodes.
[0022] In the scenario of FIG. 1, each sensor node detects at
regular time intervals a measurement value, e.g. a temperature
value or a brightness value, these measurement values being
designated by z1, z2, . . . , z6 for the individual sensor nodes.
In the embodiment described here, the measurement value represents
a brightness value which can be divided into the "bright" class or
into the "dark" class. Thus, a pattern in the form of the
corresponding states "bright" or "dark" of the individual sensors
is represented by all sensor measurement values. This pattern
represents the abovementioned system status which is designated in
FIG. 1 by m for illustration. In this context, a multiplicity of
patterns exists for each possible combination of bright or dark
values of the individual sensors. For each pattern in this case, a
value p1, p2, . . . , p7 is deposited in the respective nodes
which, in dependence on the respective pattern m, specifies the
probability with which a corresponding brightness value z1, z2, . .
. , z6 is measured in the respective sensor nodes S1, S2, . . . ,
S6. As will be described below, estimations are calculated for the
probability of a pattern m in dependence on the measured brightness
values of all nodes via a consensus protocol. The pattern having
the highest probability value then represents the pattern detected
in a decentralized manner.
[0023] In order to guarantee a reliable transmission of data
between neighboring sensor nodes, the timing arrangement
represented in FIG. 2 is used in the embodiment described here.
FIG. 2 shows a time interval I which is passed successively as part
of the method according to the invention, updated data of the
respective nodes being sent out in each time interval. The time
interval comprises first time slots t1 which are the first five
slots from 0 to 4 in FIG. 2. Furthermore, the interval I comprises
second time slots t2 which are the slots 5 to 24 in FIG. 2. In one
implementation of the method according to the invention, the time
slots have a length of 20 ms in each case. In this context, the
time slots t1 are so-called CSMA time slots (CSMA=Carrier Sense
Multiple Access), according to which each node can listen to the
radio channels and send data via a free radio channel. By
comparison, the time slots t2 are TDMA time slots (TDMA=Time
Division Multiple Access) which are reserved suitably for data
transmission by the respective nodes. In a preferred variant here,
the data transmission takes place at the physical layer based on
the IEEE 802.15.4 Standard known per se.
[0024] In order to implement the data transmission according to the
intervals I, the times of the individual sensor nodes are
synchronized, wherein a method known in the prior art can be used
for synchronization, such as, e.g., the method described in German
patent application 10 2010 042 256.8. As part of this method, a
protocol is used by means of which the estimated global network
time is exchanged in packet headers. The protocol reduces the
adaptation rate of the sensor nodes which are already synchronized
with their neighbors, as a result of which the effects of errors of
sensor nodes newly added are reduced. Furthermore drifts in the
clocks of the sensors are compensated for. When a timer with a 32
kHz clock frequency is used, the synchronization error for the
protocol used lies within a range of about 30 .mu.s and is
significantly smaller than the length of one time slot in the
intervals I. The starting times and the sequence numbers for the
time slots are established on the basis of the synchronized
time.
[0025] In order to allocate time slots for the consensus protocol
described below, the individual sensor nodes transmit special
coordination data in the form of coordination packets within the
first five time slots t1 of the interval I. With these packets, the
nodes load a second time slot not reserved by other nodes and, in
doing so, at the same time transmit a list of the second time slots
which are occupied by more than one neighboring node in their
environment. Such multiple occupancies can occur when a sensor node
is added to the sensor network which sees two neighboring nodes in
its environment which are not within range of one another. On the
basis of the coordination packets transmitted within the first time
slots, the corresponding neighboring nodes can select time slots
which are not reserved by direct neighbors and for which no
allocation conflict is known in the case of multiple occupancies.
To save energy, the individual sensor nodes switch to an energy
saving mode in all time slots apart from the first five time slots
of the interval I and the time slots in which they send out, or
receive from their neighbors, data by broadcast.
[0026] To achieve reliable estimation of the system status on the
basis of the consensus protocol described below, it should be
ensured that each sensor node which sends data to a neighboring
node also receives data from this neighboring node. That is to say
the links between the sensor nodes should be symmetric. In a
development of the method according to the invention, the data are
therefore not transmitted by a broadcast between the sensor nodes,
but a unicast is used with a three-way communication. In this
context, a sensor node sends in a corresponding second time slot an
enquiry to a predetermined neighboring node in which, apart from a
specification of the interval I, it reports its estimated value
determined as part of the consensus protocol. When the neighboring
node receives this enquiry, it responds analogously with the
estimated value determined thereby. If this response is received by
the original node, the latter responds with a confirmation. If this
confirmation is then received by the neighboring node, the link is
symmetric. If the neighboring node does not receive the enquiry, it
will also not send a return response so that the link remains
symmetric. If the neighboring node receives the enquiry but its
response is lost, the neighboring node will also not receive a
confirmation from the original node with the consequence that it
discards the enquiry of the original node. It is only when the
confirmation is not received by the neighboring node as part of the
three-way communication that the link can be asymmetric, since in
this case only the neighboring node discards the data transmitted
thereto. The corresponding node repeats the procedure just
described in the current time slot with all its neighboring nodes
apart from those which have already previously completed the
three-way communication successfully in the current interval I.
[0027] In the text which follows, the consensus protocol already
mentioned above will now be described. By means of this protocol, a
mean-value estimation is carried out via the local data exchange of
one node with its neighboring node, and via this means a pattern m
is detected in a decentralized manner. It is assumed that each
sensor node n, which corresponds to one of the nodes S1 to S7 in
FIG. 1, measures a value {circumflex over (z)}.sub.n which
corresponds to the corresponding parameters z1, z2 etc. of FIG. 1.
During a classification, the binary value 1 is assigned to each
measurement value with a probability of .sigma.({circumflex over
(z)}.sub.n|w) and the binary value 0 with a complementary
probability of 1-.sigma.({circumflex over (z)}.sub.n|w). In the
abovementioned scenario of brightness values, the binary value 1
then corresponds, e.g., to the status of "bright" and the binary
value 0, e.g., to the status of "dark". A parameter vector
w=(w.sub.1,w.sub.2) specifies the corresponding probability
function which is deposited in each of the nodes and is designated
by p1, p2, etc. in FIG. 1. For example, this probability function
can be given for the sensor node n by the logistic function
.sigma.({circumflex over (z)}.sub.n|w)=1/(1+exp(-({circumflex over
(z)}.sub.n-w.sub.1)/w.sub.2)).
[0028] The probability for particular bit pattern m (e.g. 1101 in
the case of four sensors) is now to be determined via a pattern
recognition with the assumption of the sensor values {circumflex
over (z)}.sub.n. This probability is given by the following
equation (see also printed document [1]):
p ( m z ^ 1 , , z ^ N ) = p ( z ^ 1 , , z ^ N m ) p m m ' p ( z ^ 1
, , z ^ N m ' ) p m ' = i = 1 N ( p i ( z ^ i m ) ) p m m ' i = 1 N
( p i ( z ^ i m ' ) ) p m ' p i ( z ^ i m ) = { .sigma. ( z ^ n w )
, bit i ( m ) = 1 1 - .sigma. ( z ^ n w ) , bit 1 ( m ) = 0 ( 1 )
##EQU00001##
[0029] In this context, it is assumed implicitly that the
measurements of the different sensors are statistically independent
for the given pattern. N designates the total number of all sensors
in the network. p.sub.m' corresponds to an a priori probability
distribution, which may be present, for the pattern m'. Without
prior knowledge, this probability is set to 1/M, as a rule, M
representing the total number of possible patterns.
[0030] The product in the above equation (1) can be modified by
forming the logarithms of the probabilities as follows:
i = 1 N p i ( z ^ i m ) = exp ( N 1 N i = 1 N 1 n ( p i ( z ^ i m )
) ) ( 2 ) ##EQU00002##
[0031] This mean value in the exponent can now be determined on the
basis of the consensus protocol which only needs the local
information exchange with neighboring sensor nodes. If it is
assumed that each sensor node knows a number of neighboring nodes K
in the network and furthermore the possible patterns in each node
are known, each sensor node can determine the probability for each
pattern without the sensed measurement values {circumflex over
(z)}.sub.i having to be distributed in the entire network or a
central calculation having to be carried out. As a result, the
pattern which has the highest probability is finally detected in
each node.
[0032] In the variant of the method according to the invention,
described here, a typical consensus protocol known from the prior
art for data transmission and local mean-value estimation of the
logarithms is used. According to this protocol, a local estimation
of the mean value is initialized in each node with a local
calculation based on the measured sensor value, i.e. with the
logarithm of the probabilities p.sub.i({circumflex over
(z)}.sub.i|m) of the respective patterns. The local estimations are
exchanged iteratively with the neighboring nodes until a
convergence criterion is reached. The algorithmic implementation
used for this purpose is based on the following equation:
x i ( t + 1 ) = x i ( t ) + k w ik ( t ) ( x k ( t ) - x i ( t ) )
w ik ( t ) = { a ik ( t ) > 0 , if i has a link with k 0 else (
3 ) ##EQU00003##
[0033] In this context, x.sub.i(t) designates the estimation of the
mean value of the sensor node i. The couplings .alpha..sub.ik(t)
can be time-dependent weights for each existing link to a neighbor.
In this context, a suitable specification of the couplings is
described in printed document [2]. If necessary, other concensus
protocols can also be used for forming the mean value, e.g. the
protocol described in printed document [3].
[0034] Instead of a consensus protocol, a tree aggregation protocol
can also be used for the decentralized determination of the mean
values, if necessary. In this context, a node in the network acts
as root node in which the data of all other nodes, which lastly
arrive in aggregated form, are summed and then the mean value is
formed. In this context, a tree structure having the root node as
root and corresponding parent and child nodes is specified by means
of methods known per se. All other nodes apart from the root node
collect the aggregated measurement value sums and measurement value
quantities as part of the tree aggregation protocol from their
child nodes, add these measurement value sums and their measurement
value or the measurement value quantities and one and forward the
new values to their respective parent nodes. In this manner, the
mean value of the measurement values which subsequently can be
distributed again in the reverse direction to the nodes in the tree
is then obtained in the root node. The data transmission then takes
place analogously to the above consensus method based on the TDMA
time slots t2 which are allocated suitably within the CSMA time
slots by the nodes.
[0035] During the initialization of the method according to the
invention, configuration data, particularly the plurality of the
patterns m described above, must be distributed initially to all
nodes in the network. This is achieved by means of a dissemination
protocol, known per se, in a preferred embodiment of the
invention.
[0036] The method according to the invention has been tested on the
basis of a network of four sensor nodes. For this purpose, data
were considered from 129 pattern detections. On the basis of these
data, the correct probabilities were determined for three
predetermined patterns on the basis of the above equation (1).
These probabilities were compared with probabilities which had been
estimated with an implementation of the method according to the
invention for the four sensor nodes. In FIG. 3, the error
statistics for these probabilities are shown. In this context, the
corresponding number NT of the intervals I already passed is
reproduced along the abscissa. The difference .DELTA.p between the
probability estimated according to the invention and the actual
probability is represented with corresponding standard deviation
along the ordinate. It can be seen that the protocol used converges
very rapidly to a very low mean error (approximately
-7.3.times.10.sup.-6) after passing through a few intervals I. The
standard deviation converges at 7.3.times.10.sup.-3.
[0037] The embodiment of the invention described in the preceding
text has a number of advantages. The decentralized allocation of
time slots enables the corresponding communications network itself
to organize the media access without using a central entity. By
using a consensus or tree aggregation protocol, a decentralized
determination of mean values is achieved, wherein a node only needs
to know the nodes within its vicinity for this purpose. On the
basis of a decentralized mean-value formation, a pattern
recognition can be carried out during this process. The
communication effort is distributed relatively uniformly to all
network nodes. When using battery-operated sensor nodes, the
demands on energy storage in the individual nodes are thus
lowered.
LIST OF LITERATURE REFERENCES
[0038] [1] R. Olfati-Saber, E. Franco, E. Frazzoli and J. S.
Shamma: Belief Consensus and Distributed Hypothesis Testing in
Sensor Networks, In: Networked Embedded Sensing And Control:
Workshop NESC'05, University of Notre Dame, USA, October 2005
Proceedings, Springer-Verlag New York Inc, 2006. [0039] [2] Lin
Xiao: Decomposition and fast distributed iterations for
optimization of networked systems, Stanford University,
Dissertation, 2004. [0040] [3] S. Barbarossa, G. Scutari:
Decentralized Maximum Likelihood Estimation for Sensor Networks
Composed of Nonlinearly Coupled Dynamical Systems, In: IEEE
Transactions on Signal Processing 55 (2007), July, No. 7, Part 1,
3456-3470, http://dx.doi.org/10.1109/TSP.2007.89392-DOI
10.1109/TSP.2007.893921.
* * * * *
References