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1. ABSTRACT
Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region. The evolution to location-dependent services and applications in wireless systems continues to require the development of more accurate and reliable mobile positioning technologies. The major challenge to accurate location estimation is in creating techniques that yield acceptable performance when the direct path from the transmitter to the receiver is intermittently blocked. This is the Non-Line-Of-Sight (NLOS) problem, and it is known to be a major source of error since it systematically causes mobile to appear farther away from the base station (BS) than it actually is, thereby increasing the positioning error.
In this paper, we present a simple method for mobile telephone tracking and positioning with high accuracy. Our paper presents the location of a mobile telephone by drawing a plurality of circles with the radii being the distances between a mobile telephone and a several base stations (it will be found using Time Of Arrival (TOA)) and the base stations at their centers, and using location tracking curves connecting the intersection points between each circle pair instead of the common chords defined by the circles. We use location tracking curves connecting the intersection points of the two circles which will be drawn by ordinary TOA method, instead of the common chord as in TDOA.
2. NEED FOR MOBILE TRACKING
Recent demands from new applications require positioning capabilities of mobile telephones or other devices. The ability to obtain the geo-location of the Mobile Telephone (MT) in the cellular system allows the network operators to facilitate new services to the mobile users. The most immediate motivation for the cellular system to provide MT position is enhanced in accident emergency services. The positioning of the mobile user could provide services like
Emergency service for subscriber safety.
Location sensitive billing.
Cellular Fraud detection.
Intelligent transport system services.
Efficient and effective network performance and management.
3. EXISTING TECHNOLOGIES & CONSTRAINTS
3.1. NETWORK ASSISTED GLOBAL POSITIONING SYSTEM (GPS)
A mobile telephone can be located by a mobile telephone itself or through a mobile telecommunication network. To locate the mobile telephone by itself, the mobile telephone is provided with a GPS receiver to calculate its location in latitude and longitude coordinates based on the location information received from a satellite through the GPS receiver.
Increases the price and the size of the mobile telephone.
The load on the mobile telephone is increased.
Power consumption is high.
3.2. NETWORK BASED MOBILE POSITIONING
In the case that the mobile telephone network locates the mobile telephone, at least three base stations (BSs) receive a signal from the mobile telephone; calculate the distances between the BSs and the mobile telephone using the arrival time of the signals at the BSs, then determine the location of the mobile telephone using the trigonometry. This location service is provided generally by a location data processor included in a base station controller (BSC). Upon a request for service about the location of a specific mobile subscriber, the BSC selects the three adjacent BSs surrounding the mobile telephone for use in the location service, and these selected BSs are ready for communication with the mobile telephone.
3.3. TIME OF ARRIVAL (TOA)
The TOA method calculates the distance of a mobile telephone and a BS based on the TOA of a signal transmitted from the mobile telephone at the BS. It is assumed that the mobile telephone is located at the intersection point of three circles having the radius of the distances between the BSs and the mobile telephone. The distance is calculated by the following equation,
Ri = C i = sqrt ( (xi “ X ) 2 + (yi “ Y) 2 ) where,
C “ propagation speed of electromagnetic wave,
i “ propagation of time from the mobile telephone to ith base station,
xi, yi -- location of ith base station,
X, Y “ mobile position.
3.4. TIME DIFFERENCE OF ARRIVAL (TDOA)
The TDOA method assumes that the TDOAs of a signal transmitted from the mobile telephone at the three BSs define a set of points on a hyperbola, and the mobile telephone is located at the intersection point of at least three hyperbolas.
The implementation requires accurate synchronization of each BS.
The signal of the mobile telephone often travels a longer path to a BS due to the multi-path fading characteristic and the Non- Line Of Sight (NLOS) effects.
In this method, three circles or hyperbolas do not meet at one point but overlap each other over an area.
Figure 1, illustrates a typical TOA method for locating a mobile telephone.
As shown in Figure 1, three circles C1, C2, and C3, whose radii are the distance between the mobile telephone M1 and at least three BSs T1, T2, and T3, are overlapped across an area. The mobile telephone M1 is located in the overlap area. One approach to locating the mobile telephone M1 in the overlap area 1 is to use a common chord, as shown in Figure. 2. When at least three circles C1, C2, and C3 are overlapped over an area without meeting at one point, the mobile telephone M1 is considered to exist at the intersection point of three common chords L1, L2, and L3. The above method using the common chord is not very accurate in locating the mobile telephone except in the case where the mobile telephone is at an approximate equal distance from the selected BSs and in a similar propagation environment to each respective BS.
Figure 2, illustrates the TDOA method of locating a mobile telephone.
In the case that a first mobile telephone M1 is nearer to the first BS T1, as shown in Figure 2, the procedure will be described by a way of example. In Figure 2, two circles C11 and C21 are drawn based on the TOAs of a signal transmitted from the first mobile telephone M1 at the first and the second BSs T1 and T2. A first common chord L1 is defined by the intersection between the circles C11 and C21. But if the path between the first mobile telephone M1 and the second BS T2 is in an NLOS condition and the path between the first mobile telephone M1 and the first BS T1 is in a line-of-sight (LOS) condition, the common chord L1 is positioned far left from the actual location of the mobile telephone M1. The effect is the same in the opposite case. If the path between the first mobile telephone M1 and the second BS T2 is in the LOS condition and the path between the first mobile telephone M1 and the first BS T1 is in the NLOS condition, the common chord L1 is also far right from the actual location of the mobile telephone M1. In this method using a common chord involves a huge location error unless the paths between the mobile telephone and each BS have the same propagation environment.
4. LOCATION TRACKING CURVE METHOD
4.1. PROPOSAL
Figure 3, illustrates the configuration of a typical mobile telecommunication network.
As shown in Figure 3, the mobile telecommunication network includes a several base stations (BSs) T 1 to T N for providing mobile telecommunication service to a mobile subscriber through a mobile telephone M1, a base station controller (BSC) for controlling the BSs T 1 to T N, and a mobile switching center (MSC) for connecting the BSC to another BTS or a PSTN (Public Switched Telephone Network). In a cellular mobile telecommunication network, the whole service area is divided into a several coverage areas having respective base stations (BS). Each BS coverage area is called a "cell." An MSC controls these BSs so that a subscriber can continue his call without interruption while moving between different cells. The MSC can reduce the time required for calling a subscriber by locating the cell of the subscriber. In case of an emergency like a fire, or a patient needing first aid treatment, the mobile subscriber should be accurately located. Tracking the location of a mobile subscriber within the boundary of a cell in a mobile telecommunication network is known as "location service."
The method proposed by us for tracking the location of a mobile telephone using curves connecting the points where circles intersect one another, the circles radii being the distances between BSs and the mobile telephone. The steps involved are:
a. Each base station nearer to a mobile telephone receives a predetermined signal from the mobile telephone and calculates the distance between the mobile telephone and the base station and the variances of time arrival of the signal at the base station;
b. A circle is drawn to have a radius being the distance and the coordinates of the base station being the center of the circle;
c. A pair of the first and the second base stations is selected among the base stations. A several location tracking curves connecting two intersection points between the selected circles corresponding to the first and the second base stations are drawn. One of the location tracking curves is selected using the variances of the first and the second base stations;
d. The steps c. and d. are repeated for the other pairs of the base stations;
e. The intersection points are obtained among the location tracking curves selected in step d. and e. and,
f. The location of the mobile telephone is determined using the coordinates of the intersection points obtained in step e.
Figure 4, depicts a flowchart showing the steps involved in locating a mobile telephone.
The several location tracking curves are parts of circles with centers near to the base station with smaller variances between the first and the second base stations. The circles formed by the location tracking curves have the centers on a line connecting the coordinates of the first and the second base stations. The larger variances between the variances of the first and the second base stations are compared to the variances of the several location tracking curves, and one of the location tracking curves is selected according to the comparison result. The location coordinates of the mobile telephone are determined by averaging the coordinates of the intersection points obtained in step (f).
4.2. DESCRIPTION
When a location service is requested about a specific mobile telephone by a user or a network, the location data processor draws two circles C1 and C2 with their respective centers set at BSs T1 and T2 based on the TOAs of a signal transmitted from the corresponding mobile telephone M1 or M2 to the two BSs T1 and T2 located near the mobile telephone M1 or M2. The two circles C1 and C2 define a common chord L1.
Figure 5, illustrates a proposed method for mobile telephone location.
However, if each mobile telephone M1 or M2 is placed in a different propagation environment with respect to the BSs T1 and T2, the location of the mobile telephone M1 or M2 can not be determined by the common chord L1. Therefore, we use location tracking curves TR1 and TR2 connecting the same two intersection points P1 and P2 of the two circles C1 and C2, instead of the common chord L1. The process of determining the location tracking curves will be explained later. The two curves TR1 and TR2 have their middle points intersecting the line ST, which connects the positions of the two BSs T1 and T2 and the parts of two circles C1 and C2 drawn to connect the two intersection points P1 and P2.
Instead of the common chord L1, the location data processor uses the curve TR1 for the mobile telephone M1 and the curve TR2 for the mobile telephone M2. It prevents the location error caused by the multi-path fading or the NLOS path characteristics. If the radio propagation environment between the mobile telephone and the BS is poor due to the multi-path fading or the NLOS effects, the TOA of the received signal has error. The TOA error can be compensated by appropriately selecting a desired curve (reference circle).
4.3. DETERMINATION OF LOCATION TRACKING CURVE
The NLOS environment has been compared with the LOS environment and we see that the variances of the TOAs of a signal transmitted from a mobile telephone are higher in the NLOS environment. By knowing this, appropriate curves can be selected by comparison between the variances of TOAs of an input signal. That is, the mobile telephone is nearer from the common chord L1 to the one with the larger variances out of the two BSs in Figure 5. Therefore, the BS with the smaller variances should be selected to draw reference circles based on the variances.
For example, since the first mobile telephone M1 is near the first BS T1, the variances of the TOAs of a signal transmitted from the mobile telephone M1 at the first BS T1 will be higher than those of the signal at the second BS T2. Hence, the reference circle C1 is obtained around the second BS T2 with smaller variances.
Figure 6, illustrates the determination of location tracking curve.
From Figure 6, assuming that the first and the second BSs T1 and T2 selected for use in the location tracking are present at positions (x1, y1) and (x2, y2), respectively, in the second-dimensional coordinates, the location data processor draws the two circles C1 and C2 with the coordinates (x1, y1) and (x2, y2) of the two BSs T1 and T2 at their centers. The curve connects the two points P1 and P2 at which the two circles C1 and C2 intersect each other. The coordinates of the intersection points P1 and P2 are (xA, yA) and (xB, yB), respectively.
Since the mobile telephone is near the first BS T1 with respect to the common chord L1, the variances of the TOAs of a signal transmitted from the mobile telephone at the first BS T1 will be larger than those of the signal at the second BS. Therefore, reference circles TR1 to TR4 are drawn with respect to the second BS T2 with smaller variances, as shown in Figure 6.
The coordinates of the reference circle can be obtained (using minimum variance) which has its center on the line ST passing through (x1, y1) and (x2, y2) and passes through (xA, yA) and (xB, yB). Selecting the center of the reference circle is significant as the mobile telephone is located on the reference circle. The location data processor selects the desired curves (reference circles) with respect to the several BSs selected for location tracking. In Figure 6, as the real location of the mobile telephone deviates farther from the circle C2 with the second BS T2 at its center, the center of a reference circle is farther from the location of the second BS T2. That is, the center of a desired reference circle is farther from the second BS T2 in the case of a third mobile telephone M3 (curve C3) than in the case of a fourth mobile telephone M4.
4.4. REFERENCE CIRCLE SELECTION
The variances of the TOAs of a signal which arrives at the two BSs T1 and T2 from different paths are used to find the curve on which the actual location of the mobile telephone is determined.
If the TOAs of the signal at the first BS T1 from N propagation paths are t1, t2, . . . , tN, the first BS T1 calculates the variances of t1, t2, . . . , tN. The location data processor compares the variances calculated by the first BS T1 with the variances calculated by the second BS T2 and considers that the mobile telephone is near to that BS with the larger variances (the first BS T2 in Figure 6).
Hence, the reference circle has its center near to the BS with the smaller variances (the second BS T2 in Figure 6) on the line ST. With the larger variances, the center of a reference circle gets farther to the right from the center of the second BS T2. In order to select the desired curve, the location data processor initializes the reference circles with predetermined radii and the variances of TOAs of a signal transmitted from the mobile telephone located on the reference circles, and compare the preset variances with real variance measurements.
The location data processor sets a several reference circles based on the distances between the mobile telephone and the BS with the smaller variances(the second BS T2) In Figure 6, as an example, the first to the fourth reference circles TR1 to TR4 have radii twice, three times, four times, and five times, respectively, of that of BS T2, where all these points of reference circles TR1 and TR4 are located along the line ST. The variances of the second BS T2 smaller than those of the first BS T1 are used as a criterion for selecting an optimal reference circle. Therefore, the location data processor predetermines the reference variances for the first to the fourth reference circles TR1 to TR4 to be compared with respect to the second BS T1. It is assumed in the following description that 1, 2, and 3 are reference variances and
1< 2< 3
The location data processor compares the variances calculated by the two BSs T1 and T2 and selects the base station with smaller variances as a reference point to draw the reference circle. If the selected variances (those of the second BS T2) are , the location data processor compares the selected variances , with the preset reference variances 1, 2, and 3.
If <= 1, the curve of the first reference circles TR1 is selected.
If 1 < <= 2, the curve of the second reference circles TR2 is selected.
If 2 < <= 3, the curve of the third reference circles TR3 is selected.
If 3 < , the curve of the fourth reference circles TR4 is selected.
As we have seen, the location data processor selects the optimal curve (reference circle) for the two BSs among the several BSs, and selects another optimal circle for another BS pair, and so on. When curves are selected for all selected BS pairs, the location data processor obtains the intersection points among the selected curves as shown in Figure 7. However, as the selected curves do not intersect at one point due to the multi-path fading or the NLOS effects, the midpoint of these intersection points is determined as the location of the mobile telephone.
Figure 7, illustrates the positioning of mobile telephone by the proposed method.
Tracking the location of a mobile telephone requires at least three BSs. As shown in Figure 7, the first to the third BSs T1 to T3 form the first to the third circles C1 to C3, respectively. The location data processor selects a first optimal curve TR1 for the first and the second BSs T1 and T2, a second optimal curve TR2 for the second and the third circles C2 and C3, and a third optimal curve TR3 for the first and third circles C1 and C3. As the three intersection points M1 (xA, yA), M2 (xB, yB), and M3 (xC, yC) are defined by the three curves TR1 to TR3, the location data processor considers the mobile telephone to be located at (x, y). While the three BSs are selected for the location service using the TOAs of a signal arrived at each BS from a mobile telephone has been described in the embodiment of the present invention, more BSs can be used to increase the accuracy in locating the exact position of the mobile station. If Nth intersection points are defined by location tracking curves obtained according to the present invention and an ith intersection point is at (xi, yi), coordinates (x, y) indicate the location of the mobile telephone.
After the location of the mobile telephone, that is, the intersection points among the curves are obtained, the location data processor represents the intersection points in the latitude and the longitude coordinates and transmits the position coordinates to the network (BS/BSC/MSC) and the mobile telephone.
5. CONCLUSION
Our proposal is advantageous in that the location of a mobile telephone can be accurately tracked even in the multi-path fading and the NLOS environment, by using more accurate tracking curves connecting the intersection points among circles with the radii being the distances between corresponding BSs and the mobile telephone in a cellular mobile communication system. We have described about accurate positioning of mobile telephones, which can be used for several applications. The important considerations to be undertaken while selecting a location based technology are location accuracy, implementation cost, reliability, increasing functionality.