Method for positioning a mobile station

#The invention relates to mobile communication technology and is especially directed to the positioning of mobile stations in the manner specified in the characterizing part of claim 1.

#One of the cornerstones of mobile communication technology is the determination of the position of a mobile station for call maintenance and billing purposes. Moreover, there are situations in which the user of a mobile station needs to determine his position coordinates either when orientating himself, having lost his way, having had an accident or because of an attack of illness or some other much more general reason, like in order to inquire the location of a service in a certain area, for example.

#There are a plurality of ways to determine the position of a mobile station. A widely used method called the Global Positioning System (GPS) is based on signals received from satellites orbiting Earth. This method requires a GPS receiver which should be integrated in the mobile station and thus would result in additional costs. Since positioning is an essential part in the already existing functionality of a mobile station it is advantageous to use the cellular system's own radio signals transferred between the mobile station and base stations to determine the position of the mobile station, even though some mobile station models may include a GPS receiver, too.

#The invention is below described more closely, referring to the preferred embodiments presented as examples and to the accompanying drawings where

#FIG. 1 shows a mobile station, base stations and repeaters in a terrain,

#FIG. 2 shows a block diagram of a method according to an embodiment of the invention,

#Like elements in the figures are denoted by like reference numerals.

#FIG. 1 illustrates through the use of an example the problems related to OTD positioning, for instance, caused by a delay which results from signal processing in repeaters 106 and 107 and, on the other hand, signal propagation along its path. In the example of FIG. 1 a mobile station 117 communicates with a base station 101. Initially the mobile station was at point 115 where the signal from base station 101 was the strongest and, therefore, the mobile station 117 selected base station 101 as the serving base station. The mobile station started moving in order to circle an obstacle 120, whereby it still communicates with base station 101 but the signal between the mobile station 117 and base station 101 now passes through repeaters 106 and 107. As this signal is the strongest on the route 116 traveled by the mobile station 117, the latter will not attach itself to base station 103, 104 or 105. When the mobile station has arrived at the end of the route 116, the signal from base station 101 is still the strongest but that is because of the repeater 106. As the radio signal will not penetrate through the obstacle 120, a direct connection between the mobile station 117 and base station 101 is not possible. When the mobile station 117 receives a positioning request, the time differences from the OTD measurement for the signal propagation time from base station to mobile station will be different because of the geometric position of the different base stations and repeaters with respect to one another. Because of the processing related to the reception and transmission of the signal, the repeaters 106 and 107 cause an extra delay Tin the propagation time of the signal for each repeater station, which delay is summed into the propagation time t+t′+t″ proper of the radio signal between the mobile station 117 and base station 101, which propagation time proper is the time used by the radio signal to travel the routes 119, 113 and 112. So, the total propagation time with base station 101 in OTD measurement would be t+t′+t″+T+T when T is the average repeater station delay of the individual repeater stations. The delays are not necessarily equal, but they may be of the same order of magnitude. Depending on the implementation and technology of the electronic circuitry these speeds may be clearly different compared to the propagation time of the radio signal through a medium. Base stations 103, 104 and 105 are situated clearly a shorter distance away from the mobile station 117 than base station 101, measured along routes 112, 113 and 119, which is the distance traveled by the radio signal. Thus signaling in OTD measurement will produce values that differ considerably from each other, adding to the inaccuracy of positioning.

#In a method according to an embodiment of the invention for positioning a mobile station 117 a serving base station 101 is not selected as reference base station which means that one of those base stations which are in the coverage area of the mobile station in a neighboring cell, is selected as reference. In the example of FIG. 1 such base stations are 103, 104 and 105. The number of suitable base stations may vary a great deal from case to case, depending on the location of the mobile station and type of the location area. According to an embodiment of the invention the mobile communication network has information about base stations (101) which communicate via a repeater in some part of their coverage area. In that case, the mobile station receives information about the base stations (103, 104 and 105) which are available for positioning without repeaters involved. By measuring the time differences of such stations with respect to the base station selected as reference the positioning parameters can be determined with an accuracy clearly better than in the prior art, also in the example case illustrated in FIG. 1.