ES2345024A1 - System and localization procedure based on the passive measures of temporary differences taken from listening of time of arrival of other nodes. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Location system and procedure based on passive measurements of temporal differences taken from listening for arrival time of other nodes. Procedure to obtain the location of a node (called passive node) from the location requests made by a subset of nodes of the same network (called active nodes). The network in which the nodes operate must be a dissemination network and therefore operate through a shared access mechanism. The active nodes, that is, those that initiate an exchange of messages with the network to know their position, use a localization technique based on the estimate of the round trip time between the node and a network element (typically access point). Or base station). The presented procedure allows the location of nodes in a passive way, transforming the time measures with which the active nodes operate, in temporary differences in the passive nodes. These temporary differences will be calculated by the passive node when listening to the messages exchanged between the active swimmers and the network. Therefore, the passive node will not generate any signaling or traffic in the network to obtain its position. (Machine-translation by Google Translate, not legally binding)

Description

Location based system and procedure in passive measures of temporary differences taken from You hear the arrival time of other nodes.

Technical sector

Mobile communications. Geolocation Telematic engineering

State of the art

Currently there are many techniques of location that can be used to determine the position of a node in cellular networks. All of them have very disparate in terms of accuracy, coverage, integrity, consistency and scalability. The various agencies responsible for regulating the operation of wireless networks have endeavored to standardize the most promising, ensuring adequate levels of quality of service.

Location techniques can be classified According to multiple parameters. One of them, perhaps the most discriminant, is the type of environment in which they will be positioned the nodes In this sense, it differs between conceived techniques for outdoor location and those developed for interiors Outdoor location has been widely treated and there are currently multiple regulated solutions for its achievement, among which the family of solutions stands out based on GPS technology. However, the location in Interior presents a set of differentiating features. The first is that signals from cellular networks public are clogged by multiple elements, making that multiple viable outdoor techniques present large problems for indoor location. A clear example is can appreciate in GPS technology, whose use in spaces of interior is inconvenient due to low signal level received in this type of environment. In addition, indoor environments they present another quality that the techniques of location intended to position in this type of environment: multi-path losses. It is a fact that indoor spaces they make the diffraction and reflection of the signal phenomena power with respect to their homonyms abroad. Another problem at what indoor positioning techniques should face It is the need for great precision. For example, an error of 10 meters, normally accepted outside value, can mean that a node is located in another building, in an environment of inside.

In this way, multiple have been developed location techniques specific to the indoor environment, whose task is to deal with the aforementioned problems. There are multiple solutions in this field, although the most employees are based on three families:

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Solutions based on cell identification They are based on identifying the network element to which the node connects and estimate its position from the position of said network element.

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Solutions based on fingerprints signal. These types of techniques establish two processes. A first called training stage, in which a set of representative points of the area to position. Measures performed compete at the level of signal received at one point per part the different network elements that are within reach of the receiver employee. These data will form a database, used in the later stage Under operating conditions, a node performs the same measures as those performed in the training stage and These measures are sent to a network entity, whose task will be compare the measurements sent with the data stored in the database data. Depending on a certain algorithm it will be estimated which is the most likely position for the sent measurements.

?

Solutions based on radio channel modeling. These types of solutions calculate the distance between two network elements from the power received in one of them. This requires a precise model of radio channel, which allows estimating the propagation loss to from the distance that separates both network elements. Should Note that, although the power received is a value normally accessible in all communication systems, it is very dependent on the environment and therefore the establishment of a Precise channel model is a difficult task to perform.

?

Time-based solutions of arrival. They are based on measuring the distance between the node and an element network (typically an access point) from the time that It takes a certain signal to cover that distance. To do this Requires a synchronized network. The need for synchronism is you can ignore calculating the times in the node using a round-trip strategy, that is to measure twice the time of signal transit [US20070002813].

The procedure presented is especially suitable for indoor location but also applicable to exterior and is based on the family of arrival time techniques. These types of techniques measure the distance from a node to a set network elements and then estimate the position of the node using triangulation techniques Normally, for a positioning in 2 dimensions the node is required to receive signal of at least 3 network elements This need translates into two problems. He First is that every node that wants to know its location generates an exchange of signaling (traffic) with multiple elements in the net. The number of items with which it is exchanged Information is variable. All this signaling generated by the node reduces the capacity of the network, being able to limit the scalability of the system. The second problem is that if the number is not reached minimum of 3 network elements to communicate with, the positioning cannot be carried out. In this way, it is possible dark areas are generated where there is communication with the network but the location of nodes is not possible. It should be noted that the dark areas in positioning may arise from a bad planning (number of visible network elements less than required by the location technique) or problems with the System integrity: failure of any of the network elements.

The present invention proposes a procedure with which to maximize the coverage, integrity and scalability of location techniques based on time of arrival in networks not synchronous

References

[US20070002813] Nathan Edward Tenny and Farrokh Khatibi , "Apparatus and method for determining WLAN Access point position". Patent published on January 4, 2007.

[EURASIP2008] Israel Martin-Escalona , Francisco Barcelo-Arroyo , "A new time-based algorithm for positioning mobile terminates in wireless networks", Cooperative Localization in Wireless Ad Hoc and Sensor Networks, special edition within the EURASIP Journal on Advances in Signal Processing , vol. 2008, March 2008.

[SAYED03] Ali H. Sayed , "Fundamentaláis of Adaptive Filtering", John Wiley & Sons , 2003 .

Description

Most of the networks deployed in interiors (based mostly on 802.11 technology), operate asynchronously This fact conditions the calculation of the time of transit of a signal, since all the calculations made they must have the same time base to apply an algorithm of triangulation To address this problem, the techniques based on the calculation of transit time operate by round trip procedures [US20070002813]. This The sender sends a message to the receiver and when he receives it resends to the sender again. Being the issuer that generates the message and the one who finally receives it, can compute the measure of transit time regardless of the clock used in the receiving element in the network, using the equation

one

where d indicates the distance between the two elements,? the time taken by the signal to go from the sender to the receiver and from the receiver to the sender and c represents the propagation speed of the emitted signal.

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The presented system is intended to obtain the location of a node, which will be called a passive node, from of the current location requests generated by subset of nodes of the same network, called active nodes. Active nodes they use a technique based on the calculation of distances by means of transit time estimation, calculated using a mechanism based on equation (1). The network nodes will access it through shared access, in which all nodes are capable of receiving the rest emissions. To facilitate the explanation and without prejudice to the generalization of the procedure described here, the proposed nomenclature for IEEE 802.11 networks will be used.

The procedure for locating the proposed system consists of taking advantage of the messages that circulate in the network for the positioning of the active nodes to, without introducing additional signaling, position the passive nodes. Fig. 1 shows an example of the procedure to follow. In said figure, the node ND 1 is considered as an active node, while the node ND 2 will be the passive node. Both nodes receive a signal from three access points: AP 1, AP 2 and AP 3. At a given time, the active node ND1 starts a localization process. To do this, it proceeds to estimate the distances that separate it from the access points it has in sight. In this way, ND 1 sends a message M 1 to AP 1 at time t 1. This message is received at t {2} by AP _ {1}. In response to the message M _ {1}, {1} _ AP sends at t {4} a message A _ {1} to {1 ND _}, reaching the message at time t _ {1}. Thus, using t 5 and t 1 and equation (1) an estimate of the distance separating the ND 1 from the AP 1 is obtained. Since the network is broadcast, the message sent M 1 by ND 1 at time t 1 is also heard in t 3 by ND 2. The same applies to the message A 1 generated in t 4 by AP 1 in response to the message M 1 from ND 1. Thus, ND _ {2} receives a copy of the messages sent by and to ND _ {1} in the times t {3} and {6} t _ respectively. By computing the difference between both times, that is t 6 -t 3, a time difference is obtained that results in a hyperbola of possible positions for the node ND 2. ND1 repeats the procedure described with AP2 and AP3 . In this way, ND 1 sends the message M 2 on t 7, which is received on t 8 by AP 2 and on t 9 by ND 2}. The message A 2 generated in t 10 by AP 2 in response to the message M 2 is later received by ND 1 and ND 2 at times t _ {11} and {12} t _. Finally, ND _ {1} sends at t {13} the message M _ {3}, which is received at t {14} by ND _ {2} and t _ {15} by AP _ {3} . The message A 3 generated in t 16 by AP 3 in response to the message M 3 is later received by ND 2 and ND 1 at moments t _ {17} and {18} t _. In this way, by applying equation (1) the node ND 1 is able to obtain its distance to AP 2 and AP 3, while the node ND 2 obtains two more temporary differences (t {12} - t {9} and {17} _ t - t {14}), which will result in two possible hyperbolas over positions that can be found ND _ {2}. The intersection between the hyperbolas found allows the positioning of the passive node as long as the position of the access points ( AP1 , AP2 and AP3 ) is known.

It should be noted that the example proposed in Fig. 1 involves only one active node and three access points. Without However, it is logical to think that in a network there will be multiple nodes assets and access points and therefore a passive node can Receive multiple measures of this type. In general it is necessary Know the position of the active nodes involved. Thus, a protocol must be established that allows obtaining said data in the passive node. The protocol could be centralized, where a network entity would receive the measures of the passive nodes and the position of the assets and calculate the position of the nodes liabilities, or distributed, in which the active nodes would remit its position to the rest of the network nodes. The knowledge of the Position of active nodes is not strictly necessary if the active nodes execute the distance estimation process with a high number of access points and passive nodes are in willingness to listen to the messages generated in said procedures

In this way, every passive station will conform with the information mentioned above the following matrices:

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2

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where P will contain the positions of the different access points (AP) and Q ( t ) will keep the positions of the known active nodes. The matrices P and Q ( t ) are used to produce the distance matrix

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3

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Together with the distance matrix R , the distance difference matrix T is generated, based on the measures of temporal differences ( DT ).

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4

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The method used to obtain the position of the passive node can be based on any of those proposed for location techniques based on the difference in arrival times (TDOA). The most conventional method, although not the only one possible, is to minimize the quadratic error associated with the matrix T by means of a least squared algorithm [SAYED03]. In the case described, the application of said algorithm produces the system of equations

5

wherein X {ij}, Y {ij} and Z {ij} are X {j} - X {i}, Y {j} - Y {i} and Z {j} - Z i respectively. X j, Y j and Z j indicate the position of the access point j , X i, Y i and Z i indicate the position of the active node i and X k, Y k and Z k indicate the position of the passive node. W \ langle i , k \ rangle indicates the distance between the active node and the passive node. Finally the different coefficients B ij_ {u} are calculated as

6

The described procedure is susceptible to incorporate refinement stages. Those stages are based on reducing positioning error discarding those measurements from the calculation of temporary differences of lower quality. The quality of the Measures can be assessed using multiple metrics and criteria, such as the following:

?

Standard deviation. Consists in discard those measures that foreseeably incur a greater error, that is, that are clearly far from the value expected for the measured time difference.

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Study of geometry. To the get the position of the passive node you can generate a diagram of the geometry that represents the network elements involved in The position calculation. In this way, they could be eliminated those measurements from inadequate geometries in location terms (for example situations in which items were found online).

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Error in the position of the active nodes This way the differences would be weighted temporary measurements based on the positioning error of the active nodes, using any of the techniques available in the literature.

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The proposed procedure allows 3 advantages important on procedures based on calculation of distances by estimating transit times:

one.

Scalability improvement . The procedure improves the scalability of this type of techniques, since it allows the positioning of nodes in the network without introducing any signaling, from a reduced subset of active nodes. In this way, and by combining both techniques in an optimal way, the set of network nodes could be located minimizing the impact that location traffic has on it.

2.

Increase coverage of the location system . The techniques based on triangulation of distances estimated from transit times require at least 3 distances to perform a 2-dimensional positioning. This implies receiving a signal of at least 3 access points. This requirement is not easy to fulfill, since in most networks the objective is to deploy the network with minimal cost and therefore the overlapping of access points is limited. However, the proposed procedure is capable of positioning a node with only 2 access points, covering areas where positioning with conventional techniques would not be possible or would not be with acceptable accuracy. In extreme cases, the proposed technique is capable of obtaining the positioning of the passive node with only one access point, provided there are at least 3 active nodes from which it receives a signal. In this way, the coverage of the location system is maximized without requiring the deployment of new infrastructure.

3.

Improvement in the integrity of the location system . Even if the location system has been carefully designed and the necessary coverage is ensured at all points to be located, it can be conditioned by failures at the access points. These failures may be due to multiple causes. As an example, one might think of a fire that would disable one of them. In this type of situation, the described procedure allows positioning even if any of the access points has ceased to function.

[EURASIP2008] provides data detailed and metric about the expected improvements when using the technique described in this document.

Claims (6)

1. Procedure for calculating the position of a wireless node in broadcast networks with shared access to the medium, characterized by comprising the steps of:

TO)

Play shared waiting cycles back paging messages (M _ {1}) and back (A {1}) emitted by active nodes half.

B)

Calculating times when messages back (t {3}) and back (t {6}) according to the extracted information received from the messages M _ {1} and A {1} captured by the passive node.

C)

Generate temporary differences with the times extracted in step B ( t 6 -t 3).

D)

Calculate node location liabilities from the temporary differences generated, using for this, any algorithm based on the time difference of Arrival (TDOA).

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2. Method according to claim 1, characterized in that it comprises in the passive node a step (E) of obtaining the positions of the neighboring active nodes. These positions are transmitted to the passive node by any other element of the network. 3. Method according to claim 1, characterized in that it comprises three stages (F, G, H) oriented to position refinement:

F)

Assessment of the quality of the measures of temporal differences according to available metrics in the node passive.

G)

Selection of temporary differences most suitable for the calculation of the position of the passive node, in function of its quality

H)

Execution of the procedure according to claim 1 using only the measures resulting from the stage (G).

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4. Method according to claim 3, characterized by the fact of using in step (F) the estimated position error of the network entities involved in the positioning of the passive node. 5. Method according to claim 3, characterized in that the step geometry derived from the position of the network entities involved in the positioning of the passive node is used in step (F). 6. System for calculating the position of a wireless node passively, characterized in that it comprises at least one active and another passive wireless node to be positioned and configured to implement the method proposed in any of the preceding claims.