11-12-2012, 06:49 PM
Mobile positioning technolog
tracking and positioning of mobile phones (Autosaved).doc (Size: 143 KB / Downloads: 26)
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. Through this we will discuss some technology used for mobile positioning and tracking.
INTRODUCTION TO MOBILE TECHNOLOGY
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 telephone switching office (MTSO) 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." Each BS is provided with a frequency of a range between 450 to 900 MHz . More than one cells can use same frequency . Only condition is that no two adjacent cells must have same frequencies. An MTSO controls these BSs so that a subscriber can continue his call without interruption while moving between different cells. The MTSO 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 based services. Mobile technology includes mainly two functions. They are call fixing and hands-off process. All the BSs are sending a signal of power 25 to 30w to the mobile unit. When a user switches ON his mobile, it will search for the strongest signal and got connected to that BS. Then the mobile unit sends an identification signal to the BS. When he fixes a call, the BS accepts the request and sends the request to the BSC and MTSO. Then the MTSO will searches, where the subscriber is and connects the call.
When a user moves to another cell the MTSO will change the frequency allotted to it and allots the frequency of the new BS. For both these processes GEOLOCATION of the mobile unit is essential.
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.
EXISTING TECHNOLOGIES & CONSTRAINTS
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 of the mobile and telephone .The load on the mobile telephone is increased. Power consumption is high.
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, BSC the 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.
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,
R i = C ,i = s q r t ( (xi â €œ X ) 2 + (y i â €œ Y) 2 ) where, C â €œ propagation speed of electro magnetic wave, i â €œ propagation of time from the mobile telephone to the base station, xi, y i -- location of the base station, X, Y â €œ mobile position.
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.