KR20100102010A - Location tracing system and tag driving method therein - Google Patents

Abstract

PURPOSE: A location tracing system and a tag driving method thereof are provided to construct a location tracking system economically by only using general APs(Access points) and RTLS(Real-Time Locating System) tags. CONSTITUTION: A location measurement server judges the location of a tag, and plural reference APs broadcasts a reference signal for the reference signal which is measured by a tag. Communication APs supports a roaming function for the tag, and the location measurement server judges the distance between the AP and the tag from the reception strength information for the reference signal of each AP.

Description

Location Tracing System and Tag Driving Method therein}

The present invention relates to a position measuring system for determining a position of a tag and a method of driving the tag, and more particularly, to a method of roaming an RTLS tag using Wi-Fi.

Looking at the configuration of a location measurement system using Wi-Fi, it consists of three tags, an access point (AP) and a location measurement engine.

In the method of increasing the accuracy of tag tracking in the Wi-Fi positioning system, there may be a difference in data processing method and tag performance of a signal sent from a tag, but above all, proper placement of an AP is important.

Once APs are placed as tightly as possible, they will increase the accuracy even more than expected, but in this case, the cost of AP growth is expected to rise. In addition, if the site is large, a roaming function is required for the tag.To support this, a network must be established between APs so that the tag can be connected to the location engine in the Real-Time Locating System (RTLS) server. Since data can be sent, the cost of building a network increases.

An object of the present invention devised to solve the above problems is to provide a location measurement system that can be economically constructed with only general-purpose APs and Real-Time Locating System (RTLS) tags.

Position measuring system according to an aspect of the present invention for achieving the above object, the position measuring server for determining the position of the tag; A plurality of reference APs on which the tag broadcasts a reference signal for measuring reception strength; And a plurality of communication APs for receiving the received strength information on the reference signals received by the tag from the tag and transmitting the received strength information to the location measurement server.

According to another aspect of the present invention for achieving the above object, the tag driving method of the position measurement system for measuring the position by using the reference signals transmitted from a plurality of AP, selecting a reference signal having the highest reception strength ; Interpreting the reference signal; Determining whether the AP which sent the reference signal is an AP for communication; And if the determination result is a reference AP, next selecting a reference signal having a high reception strength, repeating the analysis of the reference signal, and if the determination result is a communication AP, forming a communication channel with the corresponding communication AP. .

By implementing the position measuring system and the tag driving method of the present invention according to the above configuration, there is an advantage that can be used economically the existing AP network.

On the other hand, in the position measurement system of the present invention, since the reference AP and the communication AP are separated, an encrypted and inaccessible AP can be utilized as the reference AP. This is because basically any AP supports the function of broadcasting the SSID. Accordingly, the present invention has the advantage of improving not only the efficiency of network construction but also communication compatibility.

1 shows the reception strength in a tag of an AP's reference signal according to the distance between the tag and the AP.

1 is a value of RSSI reception strength of a tag when the height of the AP is 1.5m and the height of the tag is 1.2m. The vertical axis represents the RSSI value of the received strength of the tag received by the AP in dBm units, and the horizontal axis represents the distance between the AP and the tag.

In FIG. 1, the distance that can communicate data between the AP and the tag is about 120m (when -64dB is set as the communicable threshold (limit), it is about 120m, but when the communication limit is lowered, it can be longer than 120m), but after 60m RSSI Looking at the change in intensity, it is no longer changing at the beginning of -60dBm.

That is, in case of location tracking using RSSI in RTLS using WiFI, RSSI measurement value of 60m or more cannot be used to estimate the distance between the AP and the tag using RSSI strength. That is, -61 dB in the figure is the threshold of the linear attenuation region for position measurement.

Eventually, the AP placement should be shorter than 60m to conclude that accurate positioning is possible. Of course, the 60m interval may vary depending on the height between the AP and the tag, but according to a simulation experiment, it is possible to implement the RTLS with the expected position accuracy by placing the AP in the interval of 30m ~ 70m.

In the following description of the present invention, the maximum distance in which the contour of the reception intensity changes linearly, such as the area from FIG. 1 to 60 m, is called a 'linear attenuation distance' and is received over a communication standard such as 120 m. The longest distance with strength will be called 'communicable distance'. In addition, a circular area within the linear decay distance will be referred to as a "linear decay area", and a circular area within the communicable distance will be referred to as a "communicable area".

FIG. 2 is a diagram illustrating APs disposed at 60m intervals reflecting the above characteristics. In the drawing, 9 APs are connected at 60m intervals, but if the RTLS site is large, there may be 9 or more. Note that APs installed at 60m intervals are connected to the network via wired or wireless.

On the other hand, since the tag itself cannot communicate over a distance of 100m or more, it is necessary to find a nearby AP and send data. This is called roaming. 3 illustrates roaming of tags in RTLS using a Wi-Fi network. Roaming of RTLS is similar to roaming in general wireless networks. For example, as shown in the figure, when the tag 2 connected to the BSS (Basic Service Set) composed of AP-A in the Wi-Fi network configuration moves to AP B, the tag 2 releases the BSS of the AP-A and automatically connects to the BSS of the AP-B. Means a series of procedures.

However, the communication distance between the tag and the AP is about 120m, and the distance may be increased depending on the antenna performance of the AP. Obviously a waste of administrative costs.

In addition, when a plurality of APs are installed at intervals shorter than a communication distance of 120m, the tag roams more frequently than otherwise, which causes a problem of power waste of the tag due to frequent roaming.

The present invention proposes to install APs for RTLS construction into two types, AP for communication and reference AP, as a method of reducing the network construction cost. Here, the communication AP serves as a kind of base station used to form a communication channel for data communication with a tag. On the other hand, the reference AP only serves as a kind of beacon signal generator. That is, the reference AP is intended to increase the accuracy of the position measurement. Preferably, the communication AP also plays a role of a beacon signal generator such as a reference AP.

4 shows the structure of an RTLS implemented according to the above scheme.

The illustrated RTLS includes a RTLS server for determining a location of a tag; Reference APs LS_1 to LS_5 for broadcasting the reference signal to which the tag measures reception strength; And communication APs AP_1 to AP_4 that receive reception strength information on reference signals received by the tag from the tag and transmit the received strength information to the location measurement server.

In this case, the distance between the communication AP or the reference AP adjacent to each reference AP is equal to or shorter than the linear attenuation distance of the reception strength of the reference signal. More preferably, they are spaced apart by the linear attenuation distance.

That is, in order to increase the accuracy of the tag location tracking, reference APs which serve only as a generator of location measurement beacon signals and do not perform a communication function are arranged as shown. At this time, the arrangement interval of the reference AP, it is preferable to adjust so that the distance to the adjacent communication AP is 30m ~ 70m.

In addition, the height of the AP and the height of the tag is well adjusted to arrange the radio wave map, because the use of the fingerprinting method is well distinguished because the radio map is the most important factor to determine the accuracy.

For example, as shown, the reference AP LS_2 is installed such that two communication APs AP_1 and AP_3 and one reference AP LS_3 exist in a linear attenuation distance.

Meanwhile, the distance between each communication AP and adjacent communication APs is longer than the distances of the communication APs or reference APs adjacent to each reference AP, and is equal to or shorter than twice the communication distance between the communication AP and the tag. . That is, the distance between each communication AP and the adjacent communication APs is longer than the linear attenuation distance shown and is set equal to or shorter than twice the communication possible distance.

In order to ensure the quality of the Wi-Fi communication, it is desirable to calculate the size of the RTLS service site and to properly arrange the APs for communication so that there is no propagation shadow area and communication with the tag and the RTLS server is desired.

For example, as illustrated, AP_1, which is a communication AP, is installed to be located at a distance shorter than twice the communication distance with all of AP_2 to AP_4 which are adjacent communication APs.

As a result, the tag is located within the coverage area of the at least one communication AP, even if the tag is present in the service area, and the tag transmits reception strength information about received reference signals to the RTLS server via the communication AP. Can be.

For example, when shown, the tag selects one of the communication APs to form a Wi-Fi wireless Internet communication channel as a communication channel for data communication.

Meanwhile, each communication AP and the reference AP installed in the service target area broadcast a reference signal for location measurement as a kind of beacon signal. The reference signal may be a broadcast signal for a Wi-Fi connection. In this case, the reference AP broadcasts a reference signal that mimics a broadcast signal for a Wi-Fi connection. Of course, the Wi-Fi connection is not achieved by the reference signal of the reference AP.

The tag existing in the service target area receives the reference signals broadcast by the communication APs and the reference APs and detects the reception strength. Of these, only the reference signals higher than the linear attenuation threshold strength (-61 dB) of FIG. 1 generate reception strength information and transmit it to the RTLS server.

The RTLS server determines the distance between the AP and the tag from the reception strength information for each AP, and determines the location of the tag from the distance information of the plurality of APs and the tag. Detailed description of the RTLS server operation will be described later.

Tag according to the spirit of the present invention, it is preferable to embed an algorithm for roaming and communicating the two APs.

5A illustrates an embodiment of a driving method performed in a tag of RTLS according to the spirit of the present invention. The driving method is a concept including a roaming method of a tag and a data communication method for position measurement.

The illustrated tag driving method includes: initializing a tag (S110); Selecting a reference signal having the highest reception strength (S120); Interpreting the reference signal (S120); Determining whether the AP which has transmitted the reference signal is an AP for communication (S140); And if the determination result is a reference AP, next, a reference signal having a high reception strength is selected (S190), and the analysis of the reference signal is repeated (that is, returns to step S110), and if the determination result is a communication AP, The method may include forming a communication channel with a communication AP.

The tag that forms a communication channel with a specific communication AP according to the above process comprises: receiving reference signals from at least three communication or referral APs; Calculating reception strengths of the reference signals; And transmitting information on the reception strengths to an external location measurement server through a communication AP that forms the communication channel (S160).

Then, the location measurement server determines the location of the tag, and transmits the location information of the tag to the tag. Then, the tag that has received the location information of the tag from the location measurement server may notify the user of this.

The tag, in step S110, performs internal initialization for UDP communication.

In step S120, the tag scans the SSID of the AP around the tag and reads a configuration factor that determines the tag operation upon initialization. The setting factor can be broadly classified into a wireless network setting item and a tag function setting item. If you look at the wireless network setting items, IP address setting part, gateway, tag IP of RTLS server. There is a communication channel and SSID setting portion of the AP to communicate with. If you look at the tag function setting items, you can set the LED and button control panel, and set the motion sensor.

For example, when setting the scan method, there are two methods of active scanning and passive scanning. The difference is whether to request or receive the SSID of the AP or just to SCAN. This is related to the power consumption of the tag and the SSID reception rate. This is related to the setting of neighboring APs. Once the SSID is set to Broadcast and the period is set to 100mSec, the passive scan method may be used.

When the initialization of step S110 is completed, the tag scans the SSID and MAC address of the AP around (S120). When scanning, a channel may be selected and received, which may also be adjusted with the configuration factor. The scanned SSID, MAC address, and RSSI value are converted into a predetermined protocol with the RTLS server, and the result is transmitted through a UDP communication method (S160).

The process illustrated in FIG. 5A may also be performed in a roaming process for a tag. Referring to the process shown in the roaming process in detail as follows.

In order to distinguish between the communication AP and the reference AP, the tag according to the spirit of the present invention, the tag algorithm has a SSID filtering function when resetting the communication AP. In detail, if the communication AP does not exist in the priority SSID, the tag roaming is performed by selecting one of the SSIDs among the superior SSIDs among the SSID reception rate and the RSSI strength in the scan result. Do this.

Here, the SSID reception rate determination is determined by the reception rate within a predetermined time and by the number of times missing from the total number of receptions. If the reception rate is 20% and the RSSI is less than the reference value within 3 seconds, roaming is applied to the reception rate. Can be implemented.

When roaming is performed in the above process, control of the communication channel is transferred from the previously formed BSS to another BSS. At this time, the important thing is to set the BSS by finding the AP for communication with the string set in the tag when setting the AP to communicate when moving to another BSS.

We propose SSID filtering function to distinguish between reference AP and communication AP. For example, SSIDs of communication APs may be represented by AP_1, AP_2, AP_2. … If the tag is set to have a certain string of “AP_” as shown above, the tag is roamed by filtering with a string of “AP_“ when searching for a new BSS when roaming, and the APs of LS_1 to LS_5 which are reference APs of FIG. Rather than performing UDP communication, a basic service set (BSS) is newly configured with AP_1 to AP_4, which are communication APs.

In addition, the filtering value may be adjusted by adjusting the tag configuration factor. For example, if the SSID of the reference AP is null and the communication AP has a name, the tag setting factor may be changed to implement filtering and roaming functions.

5B illustrates an embodiment in which a step of sleeping a tag is added to reduce power consumption of the tag.

That is, after the wireless communication channel is formed, the illustrated tag transmits the information on the reception strengths to an external positioning server, and enters a sleeping mode for power saving (S170), and the wakeup condition of the tag is satisfied. When the tag wakes up (S180), the process returns to the initialization process (S110). The wakeup condition of the tag may be a user's operation, a lapse of a predetermined time after the slipping point, and the like.

If there is a stable power source other than a battery in the tag, the sleeping function and the wake up function may be disabled.

The RTLS server of FIG. 4 calculates the position of the tag from received strength values of beacon signals received from the tag according to the spirit of the present invention.

The position calculation may include a process of estimating a distance based on a received strength value and a position calculating process by triangulation.

Triangulation is a geometric method that is most commonly used in RTLS to estimate the real-time position of a moving object on a two-dimensional plane. At least three reference points are required to estimate the real-time position of a moving object on a two-dimensional plane.

Therefore, if the triangulation method is used in the position recognition method according to the spirit of the present invention, reference signals broadcast from at least three APs should be received.

4 is a distance for measuring the position from the three reference points expressed as d1, d2, d3. The distance from each reference point to the target point for position measurement may be calculated by Pythagorean theorem of Equation 1 below.

d ₁ ² = (xx ₁ ) ² + (yy ₁ ) ²

d ₂ ² = (xx ₂ ) ² + (yy ₂ ) ²

d ₃ ² = (xx ₃ ) ² + (yy ₃ ) ²

When triangulation and radio signal strength information (RSSI) are used, the distance d between the moving object (RTLS tag) and the reference point AP may be obtained through the Friis formula of Equation 2 below.

λ is the wavelength of radio waves, c is the speed of radio waves, f is the frequency, and L is the transmission loss (ie, received signal strength) of the signal transmitted by the mobile entity. The transmission loss of the transmitted signal is obtained by the difference between the signal transmission strength of the mobile object and the strength of the received signal at the reference point.

In order to calculate the position of the RTLS tag, in addition to the triangulation method mentioned in the above equations, radios such as Time of Arrival (TOA), Time of Flight (TOF), Time Difference of Arrival (TDOA), and Angle of Arrival (AOA) Positioning techniques can also be applied.

The reception strength value decreases as the distance increases, and the reception strength value may be weakly measured even where the distance is not large due to an obstacle such as a wall or a building. This phenomenon may cause an error in the position calculation of the RTLS server according to the spirit of the present invention.

There are two ways to prevent the error.

In the first method, the RTLS server holds data on a region where an error with respect to a reception strength value occurs in a database, and searches the database every time the position calculation is performed based on the reception strength value to determine whether or not an error area exists. Check and correct. For correction, the database may have a data structure that stores identifiers of the AP detected in the area where an error occurs and its reception strength values, and maps the location information measured in each case.

The second method is to predict the occurrence of an error according to an algorithm applying a predetermined path loss model. Path loss models include simple path loss model, wall attenuation model, and layer attenuation coefficient model. The most known outdoor path loss model is the Okumura-Hata model, which is an empirical formula that measures the actual urban environment.

Although the technical spirit of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

For example, in the above description, the location measurement system is described in detail by using RTLS implemented using a Wi-Fi wireless Internet communication network. However, the idea of the present invention can be applied to other location measurement systems, which is also within the scope of the present invention. Of course it belongs.

1 is a graph showing reception strength in a tag of an AP reference signal according to a distance between a tag and an AP.

2 is a structural diagram showing a position measuring system according to the prior art.

3 is a conceptual diagram illustrating roaming of tags in RTLS using a Wi-Fi network;

4 is a structural diagram showing a position measuring system according to an embodiment of the present invention.

5A and 5B are flowcharts illustrating embodiments of a driving method performed in a tag of RTLS according to the spirit of the present invention.

Claims (10)

A location measurement server for determining a location of a tag; A plurality of reference APs on which the tag broadcasts a reference signal for measuring reception strength; And A plurality of communication APs receiving reception strength information on reference signals received by the tag from the tag and transmitting the received strength information to the location measurement server Position measuring system comprising a. The method of claim 1, The distance between the communication AP or the reference AP adjacent to each of the reference AP, And a position equal to or shorter than a linear attenuation distance of the received intensity of the reference signal. The method of claim 2, The distance between the communication AP and the adjacent communication AP, And longer than the distances of the respective reference APs, adjacent communication APs or reference APs, and equal to or shorter than twice the receiving distance of the communication AP and the tag. The method of claim 1, The communication APs support a roaming function for the tag, And the reference APs block roaming for the tag. The method of claim 1, The tag is, And receiving strength information on the reference signal of the received communication AP, and reception strength information on the reference signal of the received reference AP, to the location measurement server. The method of claim 1, The tag is, Selecting one of the communication APs to form a communication channel for data communication. The method of claim 1, The location measurement server, The distance between the AP and the tag is determined from the reception strength information on the reference signal of each AP, And determining the position of the tag from distance information of a plurality of APs and tags. In the tag driving method of the position measurement system for measuring the position using reference signals transmitted from a plurality of APs, Selecting a reference signal having the highest reception strength; Interpreting the reference signal; Determining whether the AP which sent the reference signal is an AP for communication; And If the determination result is a reference AP, the next step is to select a reference signal having a high reception strength and repeat the analysis of the reference signal. If the determination result is a communication AP, establishing a communication channel with a corresponding communication AP; Tag driving method of a positioning system comprising a. The method of claim 8, After forming the communication channel, Receiving reference signals from at least three communication or referral APs; Calculating reception strengths of the reference signals; And Transmitting the information on the reception strengths to an external server through the communication AP which formed the communication channel. Tag driving method of a positioning system further comprising. The method of claim 9, After transmitting the information on the reception strengths to an external server, Receiving location information of a tag from the external server Tag driving method of a positioning system further comprising.

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