SYSTEM AND METHOD FOR EVALUATION OF A DIRECTION TO A MOBILE UNIT IN A CELLULAR RADIO NET

Abstract: A method and a system for evaluating a direction between a first station and a mobile station in a cellular radio net, for instance the GSM system, are based on the first station having an antenna arrangement which includes at least two antennas that have different directional characteristics and directions and each transmit an identifiable signal. A characteristic is established with respect to the difference between the signals from the two antennas in different directions form the first station. The two signals are sensed in the mobile station and the difference between the signals evaluated. The aforesaid direction is evaluated by the characteristic on the basis of the signal difference in the mobile station.

Description: SYSTEM AND METHOD FOR EVALUATION OF A DIRECTION TO A MOBILE UNIT IN A CELLULAR RADIO NET The present invention relates to a system and to a method of evaluating the direction between a base station and a mobile unit in a cellular radio net in accordance with the preamble of respective independent Claims. It is known to use radio data according to the specification for a cellular radio network, such as the GSM system, for evaluating the position of a mobile radio unit (a mobile telephone) . However, available present-day systems/methods have various drawbacks. For instance, they do not produce a satisfactorily reliable evaluation result and/or require large investment. The object of the present invention is to provide a technique which will enable a direction between, e.g., a base station and a mobile station in a cellular radio network, such as the GSM system, to be evaluated with the aid of simple means. A further object is to enable said direction to be evaluated with the aid of existing base station antennas. These objects are achieved with the invention defined in the accompanying independent Claims. Further embodiments of the invention are set forth in the accompanying dependent Claims. According to the invention, the base station shall have two or more antennas that are spaced negligibly apart. The antennas may, for instance, be situated on one and the same mast or on one and the same building. An essential feature is that the antenna signals from the antennas to the mobile station travel essentially along the same path and are thereby attenuated to essentially the same extent. The antennas shall also have mutually different directional characteristics and send signals that have an identity, i.e. signals that can be identified with respective antennas. This enables a characteristic to be established with respect to the difference between two antenna signals that are comprehended by a mobile station in different directions from the base station. The antennas that are bought so as to be fitted to a base station are accompanied with highly comprehensive documentation which, among other things, gives an accurate account of the directional amplification characteristic of the antenna, this documentation being, of course, well preserved by the purchaser. Because existing base stations will normally have at least two, and often three, antennas, the inventive technique can be implemented in existing base stations without needing to reconstruct their antenna arrangements. For instance, the difference characteristic of respective pairs of antennas can be readily evaluated with the aid of the documentation relating to the directional characteristics and directions of the antennas, and so on. Alternatively, the difference characteristic of respective antenna pairs may, for instance, be determined by registering the signal difference at a receiver which moves around the station. Subsequent to having evaluated or calculated such a characteristic, the signal difference can be determined in respect of a mobile station whose position in the area covered by the base station is unknown. The direction sought for can then be derived from the characteristic with the aid of an input value consisting of the signal difference measured at the mobile station. Any false directions obtained from the characteristic are discarded on the basis of other information obtained with regard to said direction. The possibilities of discarding a false direction are normally good even when the base station has only two antennas, since the mobile telephone will normally be served by the antenna that delivers the stronger signal to the mobile unit. The base station may have two antennas with the same directional amplification, although it is not necessary for the antennas to be pointed in one and the same direction. It is natural to measure the signals in dBm in the first instance and to thus calculate said differences in dB. The inventive technique can be considered as a virtual direction finding technique. Because the position of the base station is assumed to be known to the operator, the position of the mobile station can be determined in a known manner, with the aid of simple means. For instance, the position of the mobile station can be determined with the aid of the direction from some other base station to the mobile station where the position of the base station is known. The inventive position determining method can be considered as being a relative position determining method. The base station may, of course, have three or more antennas mounted on one and the same place, these antennas having different direction-dependent amplifications. This enables the direction between the base station and the mobile station to be evaluated still more positively. For instance, if the base station has three antennas which each delivers its respective identifiable signal, characteristics associated with the different signal differences can be established and the mobile station is able to detect the difference between the different pairs of signals and evaluate the direction desired on the basis of these characteristics. For instance, if each characteristic should generate one false and one true direction for each input value of the measured signal difference, there would be obtained six different directions, of which three would be identical. The remaining directions can then be safely discarded as being false. Communication between the net and the mobile unit for directional/positional determination, can be carried out beneficially in the data channels of the system (the SMS channel) . The distance between the mobile terminal concerned and the base station concerned (between which the direction is determined in accordance with the invention) can also be evaluated. Although the timing advance method can be used in this respect, it is preferable to use predicted signal attenuation from the base station to the mobile terminal as a measurement of the distance in the direction concerned. Naturally, other, nearby base stations will conveniently be used in the position determining process, for the sake of safety. The inventive method, which can be considered to be a relative method, provides directional information and data with a relatively high degree of safety and reliability. When seen against the background that different position determining methods have different degrees of reliability in different parts of the GSM system, due to the topography of the area, the presence of buildings, signal disturbances, the number of base stations in the area of the mobile station, etc., it is suitable to work simultaneously with several different methods of determining the position of the mobile station. In this regard, the various position determining methods can be allocated different degrees of safety or "tolerance" in accordance with well-tested practice, wherewith the results of these methods can be weighed against each other on the basis of the known uncertainties. If one of the position determining methods should fail locally, or temporarily, the position of the mobile station can still be determined with the aid of the remaining methods/systems . The inventive technique will now be described in more detail by way of example with reference to the accompanying drawings . Fig. 1 is a horizontal view of a base station and a mobile station. Fig. 2 is a curve which show a characteristic of the variation between two different signals transmitted from the base station, depending on the common direction of both signals from the station 1. Fig. 3 illustrates schematically the directions that can be read from Fig. 2 on the basis of a signal difference measured at the mobile station 3. Fig. 4 illustrates schematically the determination of the position of a mobile station on the basis of the direction obtained in accordance with the invention, in combination with other information obtained from other stations. Shown in Fig. 1 is an antenna arrangement 1 which may comprise a base station that has three antennas 11, 12, 13. For the sake of simplicity, these antennas are shown at a relative angular spacing of 120° and are also shown to have generally the same lobe-wise directional amplification in relation to one another. The signals transmitted via the antennas 11, 12, 13 are indicated at A, B and C respectively. It can be assumed that the geographical position of the base station 1 is known and that a reference direction is also known, for instance the direction north N. An antenna arrangement characteristic can now be established, for instance by registering the difference between the signals A and B in a receiver 3, said receiver 3 being moved one revolution around the station 1 in the process. There is then obtained a characteristic of the general nature shown in Fig. 2. The difference A minus B is independent of the distance between the station 1 and the receiver 3. A further characteristic corresponding to Fig. 3 can then be established in respect of the difference between the signals BC (B minus C) and CA (C minus A). When a mobile station 3 is then placed in an unknown position in the vicinity of the station 1, the signal differences A minus B, B minus C, C minus A can be read on the mobile station 3. Let us assume that the measured signal difference A minus B in respect of the station 3 in the position shown in Fig. 1 is 0 dB. There can then be inserted in the curve characteristic shown in Fig. 2 a straight line A minus B = 0 dB, in other words a line that coincides with the abscissa in the illustrated example. The intersection between the curve characteristic and the line 0 dB gives two directions αl and α2 respectively. One of these directions is false. A corresponding direction evaluation can then be carried out on characteristics for B minus C and C minus A respectively. The directions thus obtained may, for example, be inserted in a coordinate system in which the ordinate corresponds to the direction north. As will be understood, the result will be three parallel directions αl corresponding to the direction from the base station 1 to the mobile station 3 in Fig. 2. The directions α2 are false directions and are mutually different and can be discarded, since the three results αl confirm that the true direction is αl . The station 3 may be a mobile unit, i.e. a mobile telephone in a cellular radio network, for instance a GSM telephone network. The mobile unit may send the detected or sensed signal differences to the station 1 concerned for instance, where the direction to the mobile unit can be evaluated on the basis of these signal differences and on the earlier measured or calculated signal difference characteristics of the station antennas. Fig. 4 illustrates another base station 1' of the same design as the base station 1. The direction a x ' between the base station 1' and the mobile station 3 can be determined in the same way as that described above with reference to Figs. 1-3. The intersection between the two directions concerned corresponds to the position of the mobile station 3. Instead of information relating to the direction a x ' from the station 1' to the station 1, there can be used a distance x, x', x" to the station from one of the base stations 1, 1', 1" located in the area, said distance being obtained in a known manner. Since the positions of the stations is known, the position of the mobile station 3 can be determined easily. Although Figs. 1-3 illustrate an antenna arrangement that includes three antennas 11-13, it will be obvious that the inventive technique can be practised with an antenna arrangement that includes only two antennas which have different directional characteristics, so as to enable a characteristic to be calculated or evaluated that shows the difference between the antenna signals as it varies with the direction from the antenna arrangement. Thus, in the simplest case, there can be used two antennas that have the same directional amplification, although the antennas may not then be trained in the same direction. The inventive virtual direction finding technique can be combined with other techniques for determining the position of the mobile station, for instance by combining the direction finding process with a process for determining the distance between the same base station, or another base station, and the mobile station, or determining a direction from another station. As an alternative or supplement to the aforedescribed relative position determining technique, there can be used an absolute position determining method that fundamentally involves considering a common coverage area of at least two base stations, and preferably three or more base stations. Each base station delivers at least one station signal with its own identity and with a known characteristic. If it is assumed that signal attenuation from the station to a mobile terminal is distance dependent, it is possible to establish in the coverage area a prediction with which each part-area in said coverage area is given an estimation of the strength of the signal that will be received by the mobile station from the base station. This prediction can be registered and a comparison that is made between the prediction and signals that are measured at the mobile station will provide information relating to the part-area in which the mobile unit is situated. A prediction expressed in dB can be established in each part- area also in this case, when making a comparison with the corresponding signals actually measured in the mobile unit. A so-called interpolation method may also be used to provide from time to time a prognosis over the position of the mobile station on the basis of these positions at earlier points in time. When it is possible to establish that a mobile unit is travelling at a given speed along a given path or route with the aid of one or more position determining methods, it will be possible to extrapolate said path and, on the basis of said speed, produce a prognosis of the position in which the mobile unit can be expected to be situated at a later point in time. Each of these position determining methods, i.e. the relative, the absolute and the interpolating method, have their individual advantages and drawbacks. According to one embodiment of the invention, these three methods are used side-by-side for determining the position of a mobile station. The position determined with each method can be allocated a "tolerance", i.e. a measurement of the reliability of the determining process in the area concerned. Thus, the positional determinations obtained with the three different methods can be weighed against each other on each position determining occasion on the basis of the "tolerance" of respective determinations. For instance, if two position determining methods give roughly the same result, whereas the third method differs radically from the other two, this third, radically differing determination can be discarded and the final position determination based solely on the results of the values of the two remaining methods. The inventive direction determining technique provides a relative position determining method which is both simple and reliable, and therefore advantageous. By using such a relative method together with an absolute method and an interpolation method in the aforedescribed manner, a highly reliable position determination can be obtained in the most varying environments and situations. One advantage afforded by the invention is that the existing antennas of the majority of base stations can be utilised, i.e. those antennas that are normally required for communication in the network. If it is necessary to replace antennas with new antennas for instance, it is a simple matter to evaluate or calculate the new signal difference characteristic of the base station.

Claims: CLAIMS 1. A method of evaluating a direction between a first station and a mobile station in a cellular radio net, wherein the first station has an antenna arrangement that includes two antennas which have different directional characteristics and which each transmit an identifiable signal, characterised by establishing a characteristic with respect to the difference between the signals from the two antennas in mutually different directions from the first station; sensing the two signals in the mobile station and evaluating the difference between said signals; and evaluating said direction with the aid of said characteristic on the basis of the signal difference in the mobile station. 2. A method according to Claim 1, characterised by discarding a false direction possibly obtained with said evaluation, on the basis of other directional information. 3. A method according to Claim 1 or 2 , characterised in that the antenna arrangement comprises at least three antennas that have different directional characteristics; in that at least three different signal difference characteristics are formed on the basis of the available pairs of signals; in that the direction between a mobile station and the first station is determined with the aid of each of said available characteristics and an associated difference that has been calculated for the two associated signals that have been measured at the mobile station; and in that any false directions that have been generated in the evaluation are discarded by choosing as the true direction those directions that are generally the same in said evaluation. 4. A method according to any one of Claims 1-3, characterised by determining the relative position of the mobile station on the basis of information relating to the distance from the mobile station to said first station or to a second, fixed station whose position is known to the mobile station, or on the basis of information relating to the direction to the mobile station from another, fixed station whose position is known. 5. A method according to any one of Claims 1-4, characterised by also determining the position of the mobile station by an absolute method and by an interpolation method; and estimating the true position of the mobile station by weighing together the results of the relative, the absolute and the interpolating methods. 6. A method according to Claim 5, characterised by allotting to the positional results obtained with the relative, the absolute and the interpolating methods respective tolerance values corresponding to the estimated reliability of the results; and by taking these tolerances into account when weighing together the results. 7. A method according to any one of Claims 1-6, characterised by sensing the signals in dBm and by determining the signal differences in dB. 8. A system for evaluating a direction between a first station and a mobile station in a cellular radio net, wherewith the first station has an antenna arrangement which includes at least two antennas that have mutually different directional characteristics and that each transmit an identifiable signal, characterised by means which includes a characteristic established with respect to the difference between the signals from the two antennas in different directions from said first station, and by means for deriving a direction from said characteristic on the basis of a corresponding difference between two signals that can be sensed in the mobile station. 9. A system according to Claim 8, characterised in that the characteristic is established by measuring and registering the difference between the two signals at a sensing point that is moved one revolution around said station, or by weighing the two corresponding antenna characteristics and antenna directions against one another. 10. A system according to Claim 8 or 9 , characterised in that the system is used to generate a relative position determination of the mobile station on the basis of the direction determined from the first station whose position is assumed to be known; and in that the position of the mobile station is also determined, generally in parallel, with an absolute method and with an interpolation method, wherewith the probable position of the mobile station is determined with the aid of means for weighing together the results of the relative, the absolute and the interpolating position determining method.

Inventors: SICKING MAGNUS, SICKING, MAGNUS
Applicants: SICKING MAGNUS, UNWIRE AB, SICKING, MAGNUS
Application year: 1999
Main classification: H01Q
IPC codes: H01Q1