Method for carrying out a blind handover in an interfrequency handover in mobile communication systems

The present invention relates to a method for performing an inter-frequency handover and an inter-system blind handover of radio channel switching in cellular telephone systems, without preliminary handover checking, so-called Blind Handover, especially in heterogeneous network structures created by participating cellular systems.

Channel switching, i.e. Blind Handover does not guarantee that this handover will be successfully performed with heterogeneity of different network structures interacting in different frequency layers or in different supply areas.

The current specification, eg published in 3GPP TS 23.009 V5.1.0 (2002-06) "3rd Generation Partnership Project"; Technical Specification Group Core Network; Handover Procedures (Release 5) envisages that, for example, in the case of Handover switching between the network layer of the UMTS telephony system and the network layer of the GSM telephony system, it is possible to configure one candidate for Blind Handover for each field designated by the base station of the UMTS system. In this case, it is assumed that the cell field feeding area of ​​the target base station coincides with the cell field feeding area of ​​the source base station, i.e. the cell feeding area of ​​the UMTS base station and the GSM base station cells at the Handover site coincide. Otherwise, an ambiguous situation arises, which may result in a system error in Handover, and thus the risk of losing the connection (Call Drop).

In order to avoid such problems, so-called dual cell phones can also be used for handover switching, i.e. cell phones with two separate transceivers that can operate simultaneously on two different frequencies or on two different cellular networks, but with the disadvantage of such a solution there is a significant increase in the production cost of such a mobile phone.

The subject of the international publication WO2000 / 28774 is a method of interconnection and interfrequency handover within a CDMA system network, in which a mobile mobile station is supplied with radio signals from several base stations and at least one is selected by means of a database based on the location of the mobile station. base station suitable for intersystem or inter-frequency handover. The selected base station data required for Handover is communicated to the mobile station and the mobile station may then perform Handover to the selected base station. The mobile station determines its location by means of a satellite-assisted GPS positioning system and / or by means of timing measurements of the radio signals and transmits the determined location data to the cellular telephone system. The disadvantage of such a method of performing a handover is the necessity to equip the mobile stations with an integrated and coupled GPS location system, which entails additional costs and increases the size of the devices.

The subject of EP 1058473A1 is a radio telecommunications system and its operation. According to the method, the transmission characteristics are monitored between the base station and each mobile station, and those mobile stations that meet a predetermined first criterion (e.g., a signal strength threshold) are grouped, then a first communication link is established between the base station. and each of the mobile stations in the group, and a cell parameter (e.g. network load) is monitored and checked, whether it satisfies a predetermined second criterion, whereafter a handover signal (handover signal) is sent as long as the cell parameter satisfies the second criterion, and finally a second communication link is established and operated in response to the handover signal. This second communication link may involve changing the frequency of the same base station or switching to a different base station. The method and the system relate to both the UMTS system network and the GSM system, for example it is possible to switch an ongoing communication connection from a base station in the UMTS system network to a base station in the GSM system network.

The object of the described method according to the invention is not, however, the signaling exchange between the subscriber's mobile telephone and the network nodes involved in the communication, such as base stations, RNCs (Radio Network Controllers) and exchanges (UMSC) participating in the mobile telephone network during Handover .

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The object of the invention is to provide such a method that will overcome the disadvantages of the methods known so far and make it possible to perform Blind Handover also between different layers of cellular telephone systems, even when they do not form a common network structure.

This task is accomplished by a method for performing an inter-frequency Handover and an inter-system Blind Handover radio channel switching in cellular systems, which is characterized in that the base station automatically generates a signal for switching the communication channel of this UMTS base station to the base station channel of another GSM communication system. sending it to the mobile end station, wherein the channel switching and signaling performed by the individual other base stations are synchronized by a central clock radio linked to the stations, and the synchronization frame deviation signals determined between the base stations as a difference of the clocks of these stations and the central clock are transferred to a memory in these base stations to the precise location of the mobile end station.

Upon receipt of the radio channel handover signal generated at the selected base station, the end station preferably automatically tunes the communication frequency to another GSM system.

The switching of radio channels of the mobile station preferably occurs when the feed area by signals from one base station does not coincide with the feed area by signals from another base station.

The switching of the radio communication channels of the mobile station preferably occurs when the feed area by signals from one base station coincides at least partially with the feed area by signals from another base station.

According to the invention, the mobile end station, before carrying out the handover, is powered from several base stations, i.e. in addition to the base station currently used, it can also receive radio signals from several other base stations.

Thanks to the solution according to the invention, the mobile end stations do not need to be provided with additional equipment, since the determination of the location of the mobile station is based solely on the timing of the transmission of radio signals.

According to the invention, the mobile end station performs a transit time measurement of the signals received over the radio link from the base stations. The measurement of the signal transmission time must be forced on the terminal apparatus depending on the signal level conditions. The measured transmission times are transmitted to one of the base stations. In the cellular network, the location of the mobile end station is determined based on the measurement data. Then, using the database, at least one suitable base station for an intersystem or inter-frequency handover is selected based on the determined location. The selected station data necessary for this handover is sent to the mobile end station. Based on this, it is possible to handover from the mobile end station to the selected base station.

The switching method according to the invention can also be used to reliably perform a Handover with a more advanced Blind Handover, hereinafter referred to as Blind Handover Advanced.

The method according to the invention offers several important advantages, namely:

- no time is lost to make the Blind Handover according to the invention, while for Handover switching in the UMTS system in the compressed mode (UMTS Compressed Mode), appropriate measurements must be carried out, which, depending on the situation and the number of candidates for radio communication, require several general GAP (General Access Profile) power profiles.

- in the method according to the invention, an increase in the efficiency of the system is achieved, since the Compressed Mode is not required at all, while the mechanisms triggered in the Compressed Mode cause additional interference, thereby reducing the efficiency of the system.

- the method according to the invention enables the implementation of other services in a cellular network which require information about the subscriber's location without great expenditure.

- the method according to the invention also works both inside and outside buildings without the use of additional GPS devices.

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- end stations do not have to be equipped with GPS modules or with dual transmitting and receiving systems, which significantly reduces their production costs.

To determine location information, the subscriber's end station must measure the signaling conditions in its own cellular field and at least in one or two further cellular fields. In addition to the level of signals, the times of signal transmission over the radio link are also measured. If, under the current conditions, the measurement of the signal level in the available cell is not generally necessary, then the end station is forced to perform such a measurement. This takes place e.g. in such a way that other thresholds for the power level will be consciously introduced to the end station, which will force the measurement or the network parameters are set in advance in such a way that the measurement is obligatory.

The information on the signal transmission times generated in this way is sent to the network. To use this information when switching Blind Handover, you first need to analyze the layer where the potential cell is located in terms of the best available base station and therefore the best server. This can be done in different ways. Firstly, using the appropriate method, it is possible to determine the area of ​​power from the best server, and secondly, it can be based on the available measured values. The resulting Best Servers can be allocated to each polygon point.

The endstation coordinates are then compared to the Best Server databank, which enables the best target cell to be selected. Such a target cell is then carried by a handover command to the end station, and thus advanced radio channel handover, the so-called Blind Handover Advanced.

The method according to the invention is explained in an exemplary solution in the drawing, in which Fig. 1 shows an exemplary section of the half-cell structures of two overlapping and different mobile networks, e.g. UMTS and GSM networks.

The UMTS system network comprises a large number of radio cell fields 10, 11, 12, 13, 14, which are supplied with radio signals by a large number of permanently installed base stations 20, 23, 24. Similarly, the GSM system network comprises a large number of radio cell fields 1, 2, 3, 4, 5, 6 and 7, which are supplied with radio signals by a large number of permanently installed base stations 20, 21 and 22.

UMTS and GSM networks have, for example, a common location for the base station 20.

Mobile endstation 30 is located in a field within a cell of UMTS system 10 and is e.g. powered by radio signals from base station 24. Mobile endstation 30 is about to perform a system Blind Handover to a suitable other radio cell in the GSM system network.

Therefore, the location of this mobile end station 30 is first determined and by means of a suitable application in this end station it is forced to measure the level and quality of the signal originating from base station 24 and from neighboring base stations of another UMTS system 20, 23. The end station 30 must for this purpose firstly, unambiguously identify all three base stations 20, 23, 24 and determine their transmission times over the radio link. Information from neighboring cells and from the own cell is sent as an information packet to one of the base stations, e.g. 24. From the two measured neighbor cells and the own cell, the location of the end station 30 in the UMTS network can be calculated. This method does not depend on whether the end station is located inside or outside the building.

In order to locate a subscriber's end station without knowing the direction information, at least three base stations 20, 23, 24 are required, the exact location of which is known. Based on the timing of the signal transmission between the end station and each of the base stations, circles may be drawn to define the distance range of that station from the base station. There is a base station in the center of each circle, and the three circles' intersection together designates the end station's location. In this case, the locations of the base stations constitute reference points, the location coordinates being available in the network user's location database.

Theoretically, these three circles can intersect at one point. In real conditions, however, it is impossible, because the principle of measuring the time of signal transmission depends on the conditions of the propagation of radio waves and the speed of signal processing in the end station microcircuit (microcircuit clock frequency). The distance segments for each measuring interval must therefore not be arbitrarily small.

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In practice, this means that the location of the end station is determined by a certain area of ​​intersection of the circles, and the accuracy of the location determination increases with the number of measured base stations.

For example, for the chip frequency of 3.84 MHz, the calculated minimum measuring interval is a = Speed ​​of light C / Chip frequency fBlT = 300E6 / 3.84E6 = 78 m

The more modern the end stations (end devices), the faster and more accurate the measurement.

This accuracy also depends on the end station receiver. Such a receiver must be capable, to provide results with an accuracy of 10 m, of reading at intervals that are within the frequency of the microcircuit. The common area of ​​intersecting cell field circles gives the likely location of the end station.

Since the end station does not have any information about the frame synchronization of the cooperating base stations, the determined location of the end station is burdened with additional measurement errors.

In order to avoid this situation, two options are usually used:

- Base stations are synchronized by a central clock or GPS clock.

Based on the measurements performed by the base stations, the size of asynchronization with the other base stations is determined and the results are entered into the matrix.

Based on the determined location of mobile end station 30 and on a database existing in the cellular network, the cell or base station of the GSM system network is determined best suited to perform a Handover. In Fig. 1, this is e.g. a base station of the GSM system 20, which includes, inter alia, the cell of the GSM system 1.

In order for the mobile end station 30 to be able to perform a handover also to the corresponding target cell of the base station of the GSM system 1, after analyzing its location information (measurement values), it must receive information about the selected target cell or the corresponding base station 20. This is done directly as a result of the command. Handover handover sent to the mobile end station.

It follows that the functionality of the appropriate network node, e.g. RNC, must be extended enough to obtain location information from the analysis of measurement data, from which, using the Best Server database, the best available base station for the target cell is determined and transferred it is available to the end station and the base stations involved in a Handover.