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

The invention relates to a method for carrying out a so-called "blind handover" in an inter-system and inter-frequency handover in mobile communication systems, in particular in inhomogeneous network structures of the participating mobile communication systems.

With a so-called Bind Handover (HO), in the case of inhomogeneity of the different network structures, ie different frequency locations or service areas of the participating networks, no guarantee can be given for the successful performance of the HO.

The currently valid specification, eg disclosed in 3GPP TS 23.009 V5.1.0 (2002-06) "3rd Gerneration Partnership Project"; Technical Specification Group Core Network; Handover Procedures (Release 5), provides that eg in a handover between a UMTS layer and a GSM layer per UMTS cell an HO candidate can be configured for a blind handover. In this case, it is assumed that the coverage area of ​​the target cell coincides with the source cell, ie the coverage areas of the respective cell of the UMTS and GSM networks are congruent at the location of the handover. Otherwise the situation is not clear. The result would be a HO error and thus the risk of losing the connection (call drop).

To avoid these problems in a blind handover, so-called dual-mobile terminals, ie mobile phones with two separate transceiver devices can be used, which can work simultaneously on two frequencies or in two mobile networks.

In <patcit id="pcit0001" dnum="WO0028774A"><text>WO 00 28774 A</text></patcit> A method is proposed for performing inter-system and inter-frequency handover within CDMA networks, in which a mobile station is supplied with radio signals by a plurality of base stations, and by means of a database based on the location of the mobile station at least one suitable base station for the inter-system or inter-frequency Handover is selected. The data of the selected base station required for a handover is transmitted to the mobile station so that the mobile station can perform the handover to the selected base station. The mobile station determines its location by means of a satellite-based GPS positioning system and / or by measuring the transit time of the radio signals and transmits the determined position data to the mobile radio system. The disadvantage is that the mobile stations for carrying out this handover method preferably have to be equipped with a GPS positioning system, which causes additional costs and increases the size of the devices.

The subject matter of the method described here is not the signaling that emerges during a handover between the subscriber terminal and the participating network nodes, such as base stations, RNCs (Radio Network Controllers) and exchanges (UMSCs) of the mobile communication networks involved. Therefore, it will not be discussed further below.

The object of the invention is to provide a method with which blind handover without great risks between different layers of mobile communication systems are feasible, even if they have no common network structure. The method should be based purely on a transit time measurement of the radio signals for determining the location of the mobile station, so that no additional hardware is required by the mobile stations.

This object is achieved by the features of claim 1.

The invention is based on the assumption that a mobile station is supplied by several base stations before the handover, ie that it can also receive signals from several other base stations in addition to the serving base station.

According to the invention, the mobile station carries out a transit time measurement of the signals received by the base stations on the air interface. Depending on the level conditions of the signals, the terminal must be forced to perform this runtime measurement. The measured transit times are transmitted to one of the base stations. Subsequently, the location of the mobile station is determined by the mobile communication network on the basis of the transmitted transit time measurement data. With the aid of a database, at least one suitable base station for an intersystem or inter-frequency handover is then selected on the basis of the determined location, and the data required for a handover of the selected base station is transmitted to the mobile station. Based on this information, the mobile station can handover to the selected base station.

With this method it is possible, with a further developed blind handover, in the following also referred to as blind handover advanced, to carry out such HO unerringly.

The described method yields several important advantages:<ul id="ul0001" list-style="dash"><li>In the so-called UMTS Compressed Mode, required HO measurements must be triggered, which require several GAPs (General Access Profile) depending on the situation and the number of HO candidates. In a blind HO according to the invention no time is lost.</li><li>The mechanism of Compressed Mode (CM) creates additional interference in the network. This in turn means a reduction in capacity. In contrast, the described method achieves an increase in capacity since no CM is necessary.</li><li>The method can be implemented without much additional effort other mobile services that require location information of the subscriber.</li><li>The procedure works both inside and outside buildings without additional GPS.</li><li>The terminals do not have to be equipped with GPS or dual terminals (with two transceivers) and can therefore be produced more cheaply.</li></ul>

Advantageous embodiments and modifications of the invention will become apparent from the features of the dependent claims.

To determine the location information, the subscriber terminal has to measure the level relationships in the own cell and at least in one or two further cells. In addition to the signal levels, the signal propagation times on the air interface are also measured. If this is actually not necessary due to the current level conditions of the supplying cell, then the terminal must be forced to perform this measurement. This can eg This can be achieved by specifically notifying the terminal of other supply level thresholds which force a measurement, or by setting the parameters of the network from the outset in such a way that these measurements are obligatorily carried out.

The thus generated information about the signal propagation times is transmitted to the network. In order to use this information for a blind HO, the layer in which the potential target cell is located must first be analyzed for the best serving base station, ie the best server. This can be done in different ways. On the one hand, the coverage area of ​​the Best Server can be determined using appropriate procedures and, on the other hand, this can be done from available measurement data. The resulting Best Servers can then be assigned via the polygon at each point.

The coordinates of the terminal are then compared to the Best Server database, thus selecting the appropriate target cell. This target cell is then transferred via HO ​​command to the terminal and thus specifically executed the Blind Handover Advanced.

An embodiment of the invention will be explained in more detail with reference to the drawing figure.

Figure 1 shows an example of a section of the cell structures of two overlapping mobile communication networks, such as a UMTS network and a GSM network.

The UMTS network comprises a plurality of radio cells 10-14, which are supplied with radio signals from a plurality of fixed base stations 20, 23, 24. Likewise, the GSM network includes a plurality of radio cells 1-7, which are supplied with radio signals from a plurality of fixed base stations 20-22.

For example, the UMTS and the GSM network have the location for the base station 20 in common.

A mobile station 30 is located within the UMTS cell 10 and is supplied by the base station 24 with radio signals, for example. The mobile station 30 wants to perform a blind handover to a suitable radio cell of the GSM network.

According to the invention, the location of the mobile station 30 must first be determined for this purpose.

By a suitable application, the terminal is requested to measure the supply level and the quality of the base station 24 and the neighboring UMTS base stations 20, 23. In this case, the terminals 30 must identify the corresponding base stations 20, 23, 24 unambiguously and determine the associated transit times of the signals on the air interface. This information of the neighboring cells and the own cell are sent as an information packet to a base station, eg 24, sent.

The location of the terminal 30 can therefore be calculated in the UMTS network from only two measured neighbor cells and one's own cell. This method is not dependent on whether the location of the terminal is inside or outside a building.

In order to determine the location of a subscriber terminal without knowing the direction information so at least three base stations 20, 23, 24 are necessary, whose location is known exactly. On the basis of the transit time measurement of the signals between the terminal and each of the base stations, it is possible to calculate circular rings which define the distance range of the terminal to the respective base station. At the center of each circle is a base station. The common intersection of the three annuli is the location of the terminal. The locations of the base stations here are the reference points, with the location coordinates from the location database of the network operator are available.

In theory, three circles intersect in one point. This is impossible under real conditions, since the transit time measurement principle depends on the propagation conditions and the processing speed of the signals in the microchip of the terminal (chip frequency). The sections per measurement interval can not be arbitrarily small.

This means in practice that the location of the terminal is described by a sectional area. The accuracy of the location determination increases with the number of base stations measured.

By means of a chip frequency of, for example, 3.84 MHz, the smallest measurement interval a per chip is calculated <maths id="math0001"><math display="block"><mi mathvariant="normal">a</mi><mo>=</mo><mi>Speed ​​of light C</mi><mo>/</mo><msub><mi>Chip frequency f</mi><mi>bit</mi></msub><mo>=</mo><mn>300</mn><mo>⁢</mo><mi mathvariant="normal">e</mi><mo>⁢</mo><mn>6</mn><mo>/</mo><mn>3</mn><mo>.</mo><mn>84</mn><mo>⁢</mo><mi mathvariant="normal">e</mi><mo>⁢</mo><mn>6</mn><mo>=</mo><mn>78</mn><mspace width="1em" /><mi mathvariant="normal">m</mi></math><img file="EP1516507B1_D0001.tif" /></maths>

With modern terminal devices, much better measuring accuracies can be achieved in practice.

Accuracy also depends on the receiver of the terminal. The receiver must be able to resolve time intervals within the chip frequency to deliver results in the 10m range. The common intersection of the circles indicates the probable location of the terminal.

Since the terminal has no information about the frame synchronicity of the participating base stations, the determined location of the terminal will be subject to additional measurement errors.

To circumvent this, there are two main possibilities:<ul id="ul0002" list-style="dash" compact="compact"><li>The base stations are synchronized via a central clock or via GPS time.</li><li>Measurements by the base stations determine the degree of asynchrony with other base stations and store them in a matrix.</li></ul>

Based on the determined location of the mobile station 30 is determined by means of a data base existing in the mobile communication network for a handover most suitable radio cell or base station of the GSM network. In FIG. 1, this is, for example, the GSM base station 20, which, among other things, supplies the GSM radio cell 1.

So that the mobile station 30 also executes a HO to the corresponding GSM target cell 1, it must be notified of the target cell or the corresponding base station 20 after the evaluation of the location information (measured values) of the terminal. This can be done directly in the form of an HO command to the mobile station.

The result of this is that the functionality of the corresponding network node, eg RNC, must be extended such that the evaluation of the measurement data results in location information, determines the best-serving base station of the target cell from a Best Server database and the terminal and base stations involved in the handover Is made available.