KR100736419B1 - Estimation of a signal delay - Google Patents

Abstract

The present invention relates to a method for estimating a delay of a signal received at a mobile station (MS) from specific network elements (BS1, BS2) of a network to determine the location of a mobile station. In order to reduce the false alarm rate and increase the probability of acquisition, it is proposed that the method comprises estimating a delay within a search window. The search window is determined based on location information available at specific network elements BS1, BS2 and known distances from the mobile station MS to at least one other network element BS0, BS1. The invention also relates to a communication system comprising a corresponding mobile station (MS), a corresponding network element (BS0), and at least one mobile station and at least two network elements.

Description

Estimation of a signal delay

The present invention relates to a method for estimating a delay of signals received at a mobile station from a particular network element of a network to determine the location of the mobile station. The invention also relates to a mobile station, a network element of a network, and a communication system.

The location of the mobile station may be determined by a positioning procedure, for example a positioning procedure that is part of the location service.

Location services in cellular systems are based on determining the delay of signals transmitted by different network elements of the cellular network to the mobile station whose location is determined. In the case of line-of-sight transmission, the delay of the signals is directly proportional to the distance between the mobile station and each transmission network element.

Location services have a requirement for acquisition of signals that are different from normal communication services. In particular, there is a requirement for a finger searching process for delay estimation using information of an impulse response.

In ordinary communication services, the strongest signals are the most important. Thus, the search can be performed by selecting peaks in the impulse responses. In contrast, for location services, the object is to find the first arrived signal, preferably a straight line signal, if available.

For ordinary communication services, the covering range is more different because only signals from the serving network element have to be obtained. The serving network element is a network element that provides a service to the cell where the mobile terminal is currently located. Thus, for normal communication services, the delay profile is mainly determined by the cell size of the network. In contrast, for location services, the coverage is determined by the size of the server cell and neighboring cells since signals from a plurality of network elements are needed to determine the exact location of the mobile station.

For example, FIG. 1 shows a cellular network having a server base station BS _s as a serving network element in a server cell 10 having a hexagonal shape. The mobile station MS whose current position is determined is located in the server cell 10. The server cell 10 is surrounded by six additional hexagonal cells 11 serviced by immediate neighboring base stations BS _n constituting additional network elements. Further base stations farther away from the mobile station MS may be considered further.

A search procedure for detecting delayed signals for location services is generally performed in the impulse response of signals received from different network elements. The search may be performed using a correlation procedure, for example using a matched filter starting from zero delay. The delay is increased until a match is found. The delay can be estimated, for example, by edge detection in the impulse response profile of the received signals. The length of the impulse response profile is much longer than the width of the signal shape. Thus, the search for the edge begins from any position of the signal by comparing the size of the sampling data with a predefined threshold. On the other hand, this threshold should be set high enough to avoid noise peaks being detected as signal edges, resulting in false alarms. On the other hand, the threshold should be set low enough to ensure that signal edges are detected even if the signal strength is rather weak.

In general, the length of the impulse response should be long enough to include the maximum delay possible. However, it is known from the search procedures for delayed signals that the length of the search range generally affects the false-alarm rate upon acquisition of the signal. The longer the search range, the higher the false alarm rate. The problem with location services is even more serious because the signals are partially much weaker and the noise is greater with respect to neighboring network elements. Most noise when picking up neighboring network elements is interference caused by server network elements.

The same kind of problems may arise in other cellular network based location procedures.

It is an object of the present invention to obtain a high acquisition probability and to reduce false alarm rates when determining the position of a mobile station.

This object is achieved according to the invention using a method of estimating the delay of a signal received at a mobile station from a particular network element of the network to determine the position of the mobile station. It is proposed that the method includes estimating a delay within a search window. The search window is determined based on location information available at a particular network element and a known distance from the mobile station to at least one other network element. The estimated delay of signals from a particular network element corresponds to the distance to that particular network element.

It is also an object of the present invention to provide a corresponding mobile station comprising means for receiving signals from a plurality of network elements of the network to determine the location of the mobile station, and means for determining a delay of the received signals using each search window. Is achieved using. The mobile station also comprises means for receiving an indication of a search window for each of the network elements, or means for determining the search window itself according to the proposed method.

The object of the invention is also achieved using a corresponding network element of a network comprising means for transmitting signals to the mobile station for determining the location of the mobile station. The network element further comprises means for determining a search window for at least one additional network element of the network according to the proposed method, and means for transmitting information about the determined search window to the mobile station. Or the network element further comprises means for retrieving location information for at least one additional network element and transmitting it to the mobile station. The network unit may for example be a base station of a radio access network.

Finally, it is an object of the present invention to provide at least two network elements for transmitting signals for determining the position of a mobile station, at least one mobile station having means for determining a delay of received signals based on a search window, and A communication system comprising means for determining a search window according to the proposed method is achieved. The latter means may be included in the mobile station or in the network element.

The present invention proceeds from the idea that the search range can be limited by placing a search window in the impulse response profile in order to reduce false alarm rates in acquisition. If the distance to at least one network element is known and additional geometry information for the particular network element for which the distance is to be determined is available, the search window may be limited based on the information. Geometry information may be information about the location relative to the exact location or known location of a particular network element. The information available allows in particular to determine the minimum and maximum distance that a mobile station can have to a particular network unit, and thus to determine the possible minimum and maximum delay of signals transmitted by that particular network unit. Thus, a search window may be selected to enable searching of the signal arriving with a delay between the minimum and maximum delays.

An advantage of the present invention is that it increases the probability of acquisition and reduces false alarm rates, for example when determining the position of a mobile station in location services. In addition, the amount of noise component is cut in half, for example.

The invention further enables the capture of weaker signals, and thus the acquisition of more network elements. This also allows for more accurate positioning.

In addition, an advantage of the present invention is that the search range is reduced, so that the search operation and the search operation are faster than known solutions. Compared to the search range used in the conventional signal acquisition procedure for location services, the search range can be reduced by 50%, for example by using the proposed search window.

Preferred embodiments of the invention become apparent from the dependent claims.

In a particularly simple embodiment of the present invention, only one known distance from the mobile station to one other network element may be used to determine search windows for one or more specific network elements. For example, the known distance may be given by the maximum distance to the network element serving the cell at the boundary of the cell where the mobile station is currently located. The search window for all particular network elements can thus have a length corresponding to twice the maximum distance. At this time, the search window should be shifted for each particular network element only by relative position.

Advantageously, however, more detailed knowledge of the available distance is used. If a serving network element is captured, the known distance may be the determined distance from the mobile station to the serving network element. If the distance to at least two other network elements has been estimated beforehand, the distance to all of them can be considered in determining the search window. The search window may in particular be selected to include the intersections of all the circles around the captured network elements with a radius that is each determined distance. In particular to enable a small search window, the search window can be subdivided into at least two sub-windows, each sub-window comprising a respective intersection.

In order to ensure reliability in the second approach, the delay of the signals from the closer network elements or from the network elements that provide stronger signals is preferably first determined. If the estimation of the parameter value of the second object is based on the estimation of the parameter value obtained for the first object, and if the parameter value for the first object is not reliable, the method does not converge. In the proposed method, the most reliable object is the serving network element, which should be the nearest network element or network element that provides the strongest signals.

In a further preferred embodiment of the invention, the power level used by the particular network element for transmitting signals and the covering range of the particular network element are further considered in determining the search window.

Advantageously, the search window is used not only to limit the search range for the delay of the received signal, but also to set the threshold for the signal strength of the received signals. The smaller the search window, the lower the threshold for obtaining the same false alarm rate and the same capture probability.

The invention may be used with any kind of network for transmitting signals over an air interface, for example a cellular network, a radio station network and the like. Correspondingly, the present invention is directed to certain cellular systems, such as global system for mobile communications (GSM), code division multiple access (CDMA) and / or general packet radio systems (GPRS); It can be used in general packet radio systems, but it can also be used in any system that uses the same non-cellular network. In addition, the present invention may be used with some kind of location service, for example, a service using Observed Time Difference (OTD).

Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings.

1 illustrates a mobile station in a cellular communication system.

2 illustrates various distances in a cellular communication system.

3a to 3c show the principle of the method according to the invention in connection with FIG. 2.

4 is a flow diagram illustrating one embodiment of a method according to the present invention.

FIG. 5 shows the first steps in the flowchart of FIG. 4.

6 shows false alarm rate simulation results for different search window sizes.

7 is a diagram illustrating capture probability simulation results for different search window sizes.

1 has already been described above.

2 and 3A-3C illustrate by way of example the principle of a method according to the invention for use in a cellular communication system. The communication system includes a mobile network and a cellular network having a plurality of cells. Each cell is served by another base station. The communication system enables certain location services. In general, the location of the mobile station is determined using these location services.

2 shows various distances in a communication system. Several cells 20, 21, 22 of the cellular network are arranged in a row. The straight lines shown in parallel in this line associate different characters O, A to E, ..., X for different positions in the communication system. The positions A to D are at the same distance from each neighboring position.

The mobile station MS is in the server cell 20. Server cell 20 extends from position (O) to position (B). The server base station BS ₀ providing service to the server cell 20 is located at the center of this cell 20 at position A. The first neighboring cell 21 extends from position B to position D. The server base station BS ₁ , which services the cell 21, is located at the center of the cell 21 at location C. The second neighboring cell 22 starts at position D and extends beyond position E. FIG. The server base station BS ₂ serving the cell 22 is located at the center of this cell 22 at location E. Additional base stations can follow accordingly and extend to the maximum position X. The total length of the impulse response profile of the signals transmitted by the base stations BS ₀ , BS ₁ , BS ₂ includes the distance from position O to position X.

The mobile station MS now determines the distance to each base station BS ₀ , BS ₁ , BS ₂ to determine the current position of the mobile station MS.

Before the measurement for the location service starts, the server base station BS ₀ is known. The server base station BS ₀ knows its own location and cell size and also knows the location and coverage of the other base stations BS ₁ , BS ₂ .

Assume that communication is being performed between the mobile station MS and the server base station BS ₀ . During this communication, the server base station BS ₀ transmits information and geometry information about the cell size and coverage of the base stations to the mobile station MS. For cell size and coverage, an indication of each maximum distance from the coverage area or base station to the border of the cell may be provided, for example.

Based on the received information, the mobile station MS determines a dedicated search window for each of the base stations BS ₀ , BS ₁ , BS ₂ capable of receiving signals.

The resulting search windows are shown in FIGS. 3A-3C. Each drawing has 0 and | OX | A time line is shown with a delay of the impulse response between. | OX | is the delay that the signal receives in the transmission path from position O to position X.

In the server base station BS ₀ itself, since the mobile station MS is known to be located in the server cell 20, the search range is a circular area having a radius that is the distance between the position O and the position A. FIG. Must include. Thus, mobile station MS determines a search window that includes a delay from zero to delay | OA |, where delay | OA | is the propagation time required for a straight path in the distance from O to A. The search window for the server base station BS _{0 is} shown in FIG. 3A. All components outside this range in the impulse response can be excluded by the mobile station MS in the acquisition for the server base station BS ₀ .

Since the mobile station MS is located in the server cell 20, the minimum distance between the base station BS ₁ and the mobile station MS is known to be the distance between the positions B and C, and the base station BS ₁ and the mobile station ( The maximum distance between MS) is known to be the distance between positions O and C. Therefore, for the base station BS ₁ , the corresponding delay | OA | To | OC |, a search window is determined, where delay | OA | is equal to delay | BC |. This search window is shown in FIG. 3B.

Since the mobile station MS is known to be located in the server cell 20, the minimum distance between the base station BS ₂ and the mobile station MS is known to be the distance between the positions B and E, and the base station BS ₂ And the maximum distance between mobile stations MS is known to be the distance between positions O and E. Therefore, for the base station BS ₂ , the corresponding delay | OC | To | OE |, a search window is determined, where delay | OC | is equal to delay | BE |. This search window is shown in FIG. 3C.

The search window may also be limited by the available information about the coverage of neighboring base stations BS ₁ , BS ₂ . For example, if the coverage of the base station BS ₂ is known to the mobile station MS as having a radius of only AE, the search range may be further limited from A to B, and the search window correspondingly | OC It can be limited by the delay from | to | OD |.

In the example described, the mobile station MS determines the search windows based on the information received from the server base station BS ₀ . Alternatively, the server base station BS ₀ may determine a search window for each of the base stations BS ₀ , BS ₁ , BS ₂ and provide only information about the search windows to the mobile station MS.

In other cases, the mobile station MS arrives from different base stations BS ₀ , BS ₁ , BS ₂ by putting the determined search window in the impulse response profile of the received signals and by performing edge detection within this search window. Estimate the delay of the signals. From the estimated delays, the mobile station MS calculates each distance to the base stations BS ₀ , BS ₁ , BS ₂ . It can be assumed that the current position of the mobile station MS is at the intersection of the circles around the base stations BS ₀ , BS ₁ , BS ₂ with each determined distance.

In the proposed method, the search range is reduced and thus the false alarm rate in acquisition is reduced. As in the example shown in Figures 2 and 3, assuming the cells of the same size, the search range is approximately 1/3 for the base station BS ₁ and for the base station BS ₂ compared to the conventional approach It is reduced by about 3/5.

Of course, in practice, the cell coverages overlap each other and the cell sizes and shapes are not the same. Nevertheless, a search window may be considered by the mobile station to include all possible delays for capturing a particular base station, similar to those mentioned above.

4 is a flow diagram illustrating one embodiment of a method according to the present invention. The method further shortens the search range for capturing neighboring base stations for location services in a cellular communication system.

In a first step, the server base station (BS ₀₎ is the maximum cell radius (R ₀₎ of the information, the server base station (BS ₀₎ for the position of the cellular network server base station (BS _0), and the neighbor base stations (BS _i), which And an indication of the coverages of neighboring base stations BS _i to the mobile station MS.

In the next step, the mobile station MS determines a search window for the server base station BS ₀ as mentioned above. The server base station BS ₀ serves a cell with a maximum radius of approximately R ₀ . Thus, the determined search window has a size corresponding to a search range of R ₀ . After acquiring the server base station BS ₀ , the mobile station MS uses the search window to determine the delay of the signals received from the server base station BS ₀ . The mobile station MS can calculate a distance D ₀ from this delay to the server base station BS _{0 in} which the mobile station MS is currently located. 5 shows a cellular environment with a server base station BS ₀ , a radius R ₀ and a determined distance D ₀ as an example.

In order to estimate the distance to further base stations BS _i , the mobile station MS combines the received geometry information and the information for the previously acquired base stations.

The mobile station MS _first selects the nearest neighboring base station BS ₁ . The neighboring base station BS ₁ serves a cell with a maximum radius of approximately R ₁ . The cell partially overlaps with the cell provided by the server base station BS ₀ .

In the example provided with reference to FIGS. 2 and 3, the search range for the base station BS ₁ will now be approximately 2R ₀ . However, it is already known that the mobile station MS is on a limited ring around the server base station BS ₀ , indicated with a dashed line in FIG. 5. The ring has a distance D ₀ to the server base station BS ₀ . Therefore, the search range for the base station BS ₁ may be located between D ₁₁ and D ₁₂ . D ₁₁ corresponds to a distance obtained by subtracting D ₀ from the distance between the base station BS ₁ and the server base station BS ₀ . D ₁₂ corresponds to a distance obtained by adding D ₀ to a distance between a neighboring base station BS ₁ and a server base station BS ₀ .

Thus, the length of the search range required is 2D ₀ , which may be much smaller than the range of 2R ₀ used in the examples of FIGS. 2 and 3A-3C.

The mobile station (MS) is the capture and determining the delay of the signal received by using the received search window on the base station (BS ₁₎ to a base station (BS ₁₎ to a neighbor, and to the base station (BS ₁₎ from the delay Determine the distance D ₁ .

If a distance to an additional base station BS _i is required, the mobile station MS continues to select each next nearest neighboring base station BS _i and advances the location information for that base station and the mobile station MS. The search window for the base station BS _i is determined based on the obtained location information.

For example, in the following cycle, the two base stations (BS _0, BS ₁₎ the distance (D _0, D ₁₎ to the already been captured, the base stations in the mobile station _{(MS) (BS 0, BS} 1) Was determined. Thus, the mobile station MS must be located at the intersection of the two rings. One of the two rings has a radius D ₀ surrounding the base station BS ₀ , and the other of the two rings has a radius D ₁ surrounding the base station BS ₁ . When searching for other base stations, this information can be used to limit the size of the search window to include a very small search range. The search window can be divided into two sub-windows, in particular comprising two small search areas around two intersection areas of the two rings. Show the coverage of the neighboring base stations (BS _i) may be previously used to check if it can reach a mobile station (MS) both to the signal from the base station (BS _i) for each neighbor with sufficient signal strength.

As more base stations are acquired, more accurate information about the location of the mobile station is obtained. The location information can in turn be used to capture additional base stations. Therefore, the false alarm rate can be reduced to a minimum.

6 and 7 present simulation results showing improvements that can be achieved using the method according to the invention.

6 shows false alarm rates for set thresholds for search ranges of 150m, 500m, 1000m and 5000m. The search range corresponds directly to the size of the search window used. The curve provided roughly links the simulation results for each search range. The threshold indicates the strength that the received signal must have in order to be detected by the mobile station and is expressed as a product of the noise mean with values from 0.5 to 3.5. False alarm rate is the ratio of noise peaks that are falsely detected as being signal edges because the threshold is set too low.

For certain noise environments, the threshold is set according to the desired false alarm rate. As can be seen in FIG. 6, the smaller the search range, the lower the threshold required to achieve the same false alarm rate. For example, false alarm rates may be required to be less than 0.2. If the search range can be limited to 150 meters, the threshold setting should be set greater than approximately 1.8. In contrast, if the search range is 2000 meters, the threshold should be set greater than 2.6. This means that signal power should be greater for larger search ranges. Thus, more neighboring base stations with smaller search windows obtained in accordance with the present invention can be considered in location services.

FIG. 7 shows acquisition probability with a percentage unit for set thresholds for search ranges of 150m, 500m, 1000m and 5000m. In addition, the curve provided roughly links the simulation results for each search range. A signal-to-noise ratio of 9 dB is assumed. The acquisition probability considers on the one hand the probability that the correct signal is detected and on the other hand considers the probability that the detected signal is not a noise signal. Since both probabilities depend on the threshold selected in a contrasting manner, the resulting curves have a maximum and thus represent an optimal threshold.

As can be seen in FIG. 7, the signal acquisition probability increases when the search range is reduced. For example, if the search range is 150 meters, the maximum capture probability may reach approximately 85%, but if the search range is 2000 meters, the maximum capture probability will only be approximately 67%. Thus, the gain obtained by reducing the search range is clear.

It should be noted that the described embodiments constitute only selected ones of the various possible embodiments of the present invention.

Claims (23)

A method of estimating a delay of a signal received at a mobile station from a particular network element of a network to determine a location of the mobile station, Estimating the delay within a search window, The search window is determined based on location information available at the particular network element and a known distance from the mobile station to at least one other network element, wherein the search window increases an acquisition probability for the signal. Characterized in that the method. The method of claim 1, The at least one other network element includes a serving network element that provides service to a server cell in which the mobile station is currently located, and the maximum distance from the boundary of the server cell to the serving network element is from the mobile station to the serving network element. Defining a known distance. The method of claim 1, The at least one other network element includes a serving network element that provides service to the server cell in which the mobile station is currently located, and the known distance is based on delay measurements for signals from the serving network element at the mobile station. And a distance to the serving network element determined. The method according to any one of claims 1 to 3, Said at least one other network element comprising at least two network elements, each distance being predetermined based on delay measurements for signals from said at least two network elements. The method of claim 4, wherein And the search window is selected to include the intersections of all the circles around the at least two network elements with a radius that is each determined distance. The method of claim 5, The search window is subdivided into at least two sub-windows, each sub-window comprising a respective intersection. The method according to any one of claims 1 to 3, Wherein each search window is determined for at least two specific network elements starting from the network element closest to the mobile station, in order of distance to the mobile station. The method according to any one of claims 1 to 3, A search window is determined for at least two specific network elements starting from the network element providing the strongest signal, in order of signal strength at the mobile station of signals transmitted by the network elements. The method according to any one of claims 1 to 3, The covering range of the particular network element is further considered to limit the search window. The method according to any one of claims 1 to 3, Determining a threshold based on the determined size of the search window, wherein the threshold defines a minimum signal strength of signals received at the mobile station for which delay is estimated. In the mobile station, Means for receiving signals from a plurality of network elements of a network to determine a location of the mobile station; Means for determining a search window based on location information available for a particular network element and a known distance from the mobile station to at least one other network element, wherein the search window is an acquisition probability for a signal received from the particular network element. search window determining means for increasing (acquisition probability); And Means for determining a delay of received signals using the determined search window, respectively. In the mobile station, Means for receiving signals from a plurality of network elements of a network to determine the location of the mobile station and receiving an indication of a search window for each of the network elements; And Means for determining a delay of received signals using each search window, And the search window increases the acquisition probability for the received signals. In the network element for a network, Means for transmitting signals to the mobile station to determine a location of the mobile station; Means for determining a search window based on location information available for an additional network element and a known distance from the mobile station to at least one other network element that is not the additional network element, the search window from the additional network element. Search window determining means for increasing an acquisition probability for a signal received at the mobile station; And Means for transmitting information about the determined search window to the mobile station. delete In a communication system, At least two network elements for transmitting signals for determining the position of the mobile station; At least one mobile station having means for determining a delay of received signals based on a search window; And Means for determining a search window based on location information available for a particular network element and a known distance from the mobile station to at least one other network element, wherein the search window is a signal received from the particular network element to the mobile station. Means for determining a search window for increasing an acquisition probability for the < RTI ID = 0.0 > The method of claim 15, Means for determining the search window is included in at least one of the at least two network elements. The method of claim 15, Means for determining the search window is included in the at least one mobile station. The method of claim 11, And the means for determining a search window is configured to select the search window comprising intersections of all circles around the at least two network elements each having a radius that is a determined distance. The method of claim 18, And said means for determining a search window is subdivided into at least two sub-windows, each sub-window configured to include a respective intersection. The method of claim 13, Said means for determining a search window is configured to select said search window comprising intersections of all circles around said at least two network elements each having a radius that is a determined distance. The method of claim 20, Said means for determining a search window is subdivided into at least two sub-windows, each sub-window being configured to comprise a respective intersection. The method of claim 15, The means for determining a search window is configured to select the search window comprising intersections of all circles around the at least two network elements each having a radius that is a determined distance. The method of claim 22, Said means for determining a search window is subdivided into at least two sub-windows, each sub-window being configured to include a respective intersection.

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