Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-11-09
2001-12-04
Nguyen, Lee (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S422100, C342S357490, C342S450000
Reexamination Certificate
active
06327474
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method defined in the preamble of claim 1 for locating a mobile station (MS) in a distributed manner.
Locating methods are known which are based on the transfer of radio signals between a mobile station and mobile network. The location is determined on the basis of signals transmitted from a mobile station or from base stations and on the measurement of those signals as well as on processing the results in a substantially centralized manner.
One such method is a locating method used in the GSM system, based on the propagation time of a radio transmission and utilizing the time difference of arrival (TDOA), wherein a mobile station transmits a signal to at least three base transceiver stations (BTS) which measure the arrival times of the signals so that their time differences can be calculated. The time differences are obtained using the impulse response which is a result of correlation between a known bit pattern and a received burst signal. The bit pattern is a so-called training sequence or a corresponding known sequence. The time difference can be determined on the basis of the impulse response e.g. by selecting the point corresponding to the highest correlation or the point corresponding to the first arrived component. First arrived component refers to the signal that arrived via the shortest route in multipath propagation and the impulse response peak caused by the signal at the point corresponding to the signal. The time differences are used in a location service center (LSC) to produce at least two hyperbolas the intersection of which indicates the position of the mobile station. Because of inaccuracies in the time differences the intersection of the hyperbolas define an area, not a singular point. Positions of the hyperbolas are determined with respect to the positions of the base stations.
In a second such method a mobile station receives signals from base stations. The location is calculated in the mobile station or the measurement data, i.e. observed time difference (OTD), are sent to the mobile network where the location is then calculated.
People belonging to special groups needing emergency services, say, those suffering from heart troubles or epilepsy, nowadays have alarm devices with which they can call for immediate help. These alarm devices require special arrangements and indicate the location of the person needing help and the need for help to an emergency center. In one particular case, these devices are signalling apparatus connected to the public telephone network at the patient's home, preferably beside the bed and in the living room but also elsewhere, e.g. in the garden. As a switch in such a device is pressed, the device transmits an alarm message via the public telephone network to an emergency center. The location of the device is known in the emergency center.
A problem with the known methods is the need for signalling capacity between base stations and the location service center and the mobile station. Another problem is the computing capacity of the processor in the mobile station when the location is calculated by the mobile station. A further problem related to the indication of an emergency is brought about by the necessary special arrangements and the fact that the person using them has to keep in the vicinity of them and, when calling to an emergency center by means of a mobile station, by the fact that the caller has to know his exact location and has to communicate it verbally.
Furthermore, a problem with the known mobile-based positioning methods is that the mobile station carries out constant measurements without taking into account the surroundings and other such parameters.
SUMMARY OF THE INVENTION
An object of the invention is to eliminate the aforementioned disadvantages of the prior art.
The method according to the invention is characterized by what is expressed in claim 1. Preferred embodiments of the invention are described in the subclaims.
The invention concerns a method for determining the location of a mobile station by means of radio signals between the mobile station and mobile network. In accordance with the invention, location computing is distributed between a mobile network element and the mobile station in such a manner that location computing is divided into operation sequences, and those operation sequences are carried out in the mobile network element and in the mobile station. In the mobile network, computing is preferably performed by a location service center. The distributed units exchange information via the mobile network and air interface.
In an application of the method, the first operation sequences are carried out in the mobile station. Base stations send to the mobile station a positioning signal which may be a normal base station signal or a special signal optimized for positioning, preferably controlled from a location service center. At least the arrival of the signal is measured in the mobile station. Alternatively, a preliminary analysis is carried out on the measurement data in the manner according to distributed positioning. The type of analysis is chosen according to the locating method and used to determine the information necessary for the method. For example, in the OTD method, correlation between a signal received from a neighboring base station and a bit pattern in a known signal is used to produce an impulse response the shape of which is checked. As the mobile station has adjusted its timing to be in accordance with the signals of the serving base station, the impulse response is used to determine an observed time difference (OTD) between the serving base station and the neighbour base station, preferably by finding the center of mass or the first rising edge of the impulse response. If necessary, the location service center sends to the mobile station additional data needed in the analysis. Those additional data are e.g. data based on the serving cell of the cellular mobile network, such as correction co-efficients or other environmental parameters. For example, in the OTD method, additional data may indicate parameters related to the shape of the impulse response expected in that particular environment. The relative height of the highest peak in the impulse response with respect to the surrounding lower peaks should preferably exceed a certain threshold level, otherwise it is found that the shape of the impulse response is wrong and the measurement has failed.
In an application of the method the first operation sequences for positioning are selected from among a set of operation sequences on the basis of a command from the mobile network. Such a command is issued on the basis of environmental or measurement data, for example. Environmental data can be obtained on the basis of the cell identity, for example. Measurement data comprise at least data on signals received at the mobile station or at base stations. The command is determined in the mobile network preferably by a location service center LSC. In practice, when using the OTD method, for example, a mobile network may find that the mobile station is located in a mountainous area where there is high probability of delayed components, such as reflections from mountain sides, arriving after the signal component that arrives via the direct route. In such a case, intensity in the impulse response is systematically shifted later and, therefore, choosing the first rising edge is a better alternative than the center of mass. So, the mobile network can drive the mobile station to carry out the impulse response analysis with the first rising edge, i.e. to select, for instance, the first impulse response tap the height of which exceeds 40% of the height of the highest tap. In the mobile network it is advantageously calculated the dispersion of the OTD values reported by the mobile station, and if the dispersion is too high, it is deduced that it is impossible in the mobile station to determine the correct OTD value by means of the impulse response and the mobile station is instructed
Alanen Marko
Gunnarsson Gudni
Hyvarinen Olli
Rantalainen Timo
Ruutu Ville
Nguyen Lee
Nguyen Simon
Nokia Mobile Phones Limited
Perman & Green LLP
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