Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1997-02-14
2001-07-10
Eisenzopf, Reinhard J. (Department: 2745)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S444000, C375S341000
Reexamination Certificate
active
06259919
ABSTRACT:
This application is the national phase of international application PCT/ F196/00352 filed Jun. 13, 1996 which designated the U.S.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for identifying a fast speed mobile station in a radio system, said method comprising measuring the frequency error of radio signals received from a mobile station.
The invention further relates to a base station of a radio system, said base station comprising receiver means for receiving signals transmitted by a radio unit, and measuring means for measuring the frequency error of the received signals.
2. Description of Related Art
The invention relates particularly to the problem created in cellular radio systems when a fast speed mobile station (FSMS) roams in a microcellular network and thus changes base station frequently. Changes of base station load the network management resources significantly and, moreover, the quality of the connection deteriorates. In a situation of this kind it is preferable to hand a fast speed mobile station over to a larger radio cell which serves a larger area “above” the microcells, i.e. which covers the same geographical area as a number of microcells together. The problem that arises is that it is very difficult to identify a fast speed mobile station reliably.
Previously known methods for identifying a fast speed mobile station are based on measuring time. These methods comprise measuring the time between two handovers. If the measured time is sufficiently short, it is assumed that a fast speed mobile station is in question. Since the identification methods are based on measuring the time, a fast speed mobile station has usually reached the area of at least the second or third microcell in a microcellular network before it is identified; the identification thus takes place at a rather late stage. In addition, a mobile station moving on the periphery of a cell can be interpreted as a fast speed mobile station if it moves in an unfavourable direction through the periphery of cells. Since the above-mentioned larger radio cell has usually limited capacity, the interpretation of slow speed mobile stations as fast speed mobile stations may unnecessarily overload the larger radio cell.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the problem described above and to provide a method for identifying a fast speed mobile station more reliably and rapidly than before. This is achieved with the method of the invention, which is characterized by calculating the change in the frequency error from a time interval of predetermined length, and identifying a mobile station as a fast speed mobile station if the change in the calculated frequency error exceeds a predetermined limit value.
The invention further relates to a base station in which the method of the invention can be utilized. The base station of the invention is characterized in that it further comprises a calculator unit responsive to the measuring means and arranged to calculate the change in the frequency error in a time interval of predetermined length, and a comparator means responsive to the calculator unit and arranged to compare the change in the frequency error with a predetermined limit value and to identify a radio unit as a fast speed radio unit if the change in the frequency error exceeds the limit value.
The invention is based on the idea that a fast speed mobile station can be identified more reliably and rapidly than before by measuring the frequency error (Doppler effect) caused by the motion of the mobile station. This is because, in practice, a mobile station always passes a base station, whereby the frequency error caused by the Doppler effect changes. Passing means herein that the distance between the mobile station and the base station diminishes until the mobile station reaches a certain point after which the distance between the mobile station and the base station begins to increase. The rate of change in the frequency error, i.e. the frequency change per a unit of time, depends on the speed of the mobile station in relation to the base station. The identification of a high speed mobile station passing the base station is thus based on the fact that the change in the frequency error exceeds a certain limit. The most significant advantages of the solution of the present invention are that a fast speed mobile station can be identified at an earlier stage than before, i.e. in the first radio cell; that a “normal”, slow speed mobile station cannot be interpreted by mistake as a fast speed mobile station irrespective of its direction of motion with respect to the cells; and that the invention can be applied in existing base stations for instance by means of a computer program, since invention requires only that the base station comprises measuring devices known per se for measuring the frequency error of received signals.
According to a preferred embodiment of the invention, the change in the frequency error is found by calculating a first average frequency error during a first time interval and thereafter a second average frequency error during a second time interval, and finally the absolute value of the difference between the first and the second average. The durations of the time intervals have preferably been selected to be several seconds. The oscillation which is caused by inaccuracies and which occurs between the individual measurement results is removed by a filter, and the change in the frequency error can be calculated more accurately than before.
The base station according to a preferred embodiment of the invention comprises means for adjusting the limit value to correlate with a set value given by the operator and/or correspondingly for adjusting the length of the time interval from which the average of the frequency change on which the comparison is based is calculated. This allows the frequency change on the basis of which the base station interprets a mobile station as a fast speed mobile station to be adjusted base-station-specifically; it is therefore possible to take into account, for example, the course of a road passing the base station. It is also possible to take into account the speed limit on this section of the road.
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patent: 5396253 (1995-03-01), Chia
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Inkinen Kullervo
Suonvieri Jukka
Bhattacharya Sam
Eisenzopf Reinhard J.
Nokia Telecommunications Oy
Pillsbury Madison & Sutro Intellectual Property
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