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

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DESCRIPTION

Procedure for performing blind handover in case of intersystem and interfrequency handover in mobile communication systems.

The invention relates to a method for carrying out a so-called blind handover ("Blind Handover or HO") in the event of an intersystem and interfrequency handover in mobile communication systems, especially in non-homogeneous network structures of communication systems. mobile involved.

In a so-called blind handover (HO), in case of inhomogeneity of the different network structures, i.e. different frequency positions or coverage areas of the networks involved, no guarantee of the satisfactory realization of the HO can be provided. .

The specification in force so far, for example, published in 3GPP TS 23.009 V5.1.0 (2002-06) “3rd Generation Partnership Project”, Technical Specification Group Core Network; Handover Procedures (edition 5), foresees that, for example, in case of a handover between a UMTS layer (Universal Mobile Telecommunication System) and a GSM layer (Group Special Mobile, special group of mobile communications) by UMTS cell, an HO candidate can be configured for a blind handover. In this case, it is assumed that the coverage area of ​​the destination cell coincides with the source cell, that is, the coverage area of ​​the corresponding cell of the UMTS and GSM network at the handover location coincide. Otherwise, the situation that occurs is not unequivocal. The consequence would then be a HO failure and, with it, the risk of losing the connection (Call Drop).

To avoid this problem during a blind handover, so-called dual mobile devices can be used, that is, mobile phones with two independent transmitting / receiving devices that can operate simultaneously on two frequencies or on two mobile radio transmission networks.

Document WO0028774A proposes a method for carrying out an intersystem or interfrequency handover within AMCD networks in which several base stations supply radio transmission signals to a mobile station and with the help of a database it is selected, based on the location of the mobile station, at least one base station suitable for intersystem or inter-frequency handover. The data necessary for a handover of the selected base station 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-assisted GPS location system and / or by measuring the transit time of radio transmission signals, and transmits the determined position data to the mobile radio transmission system. It is disadvantageous that mobile stations for carrying out this handover procedure preferably have to be equipped with a GPS location system, which causes additional costs and increases the construction size of the apparatus.

The signal emissions exchanged during a handover between the subscriber terminal and the network nodes involved, such as, for example, base stations, RNC (Radio Network Controller) and central stations are not the object of the procedure described here. (UMSC) of the mobile communication networks involved. Therefore, it does not go into more detail.

The object of the invention is to indicate a method with which a blind handover can also be carried out without great risks between different layers of mobile communication systems, even if they do not have a common network structure. The procedure should be based purely on a measurement of the transit time of the radio transmission signals to determine the location of the mobile station, so that no additional hardware is required by the mobile stations.

This objective is achieved according to the invention thanks to the features of claim 1.

The invention starts from the fact that a mobile station is supplied before handover by several base stations, that is to say, in addition to the supplying base station, it can also receive signals from several other base stations.

According to the invention, the mobile station performs a measurement of the transit time of the signals received by the base stations at the air interface. Depending on the noise level conditions of the signals, the terminal must be asked to carry out this transit time measurement. The measured transit times are determined at one of the base stations. Furthermore, the mobile communication network determines the location of the mobile station based on 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 of the selected base station necessary for the handover is transmitted to the mobile station. Based on this information, the mobile station can handoff to the selected base station.

With this method, it is possible to safely carry out an improved blind handover, also referred to hereinafter as advanced blind handover (Blind Handover).

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The procedure described produces some important advantages:

- In the so-called "UMTS Compressed Mode", the necessary HO measurements must be initiated which require, depending on the situation and the number of HO candidates, several GAPs (General Access Profile). In the case of a blind handover according to the invention, no time is wasted.

- Thanks to the “Compressed Mode (CM)” mechanism, additional interference is generated on the network. This in turn means a reduction in capacity. In contrast to this, thanks to the described procedure, an increase in capacity is achieved since a CM is not required.

Thanks to the method, other mobile radio transmission services that require subscriber location information can be carried out without great additional cost.

- The procedure works both inside and outside buildings without additional GPS.

- The terminals do not have to be equipped with GPS or as dual terminals (with two transmitting and receiving units) and can therefore be produced more economically.

Advantageous configurations and variants of the invention appear from the characteristics of the dependent claims.

To determine the location information, the subscriber terminal must measure the noise conditions in the cell itself and at least one or two other cells. In addition to signal noises, signal transit times at the air interface are also measured. If this is not really necessary due to the current noise conditions in the supply cell, then the terminal must be required to do this measurement. This can happen, for example, because other coverage level threshold values ​​that require a measurement are piped to the terminal, or because the network parameters are set in advance so that these measurements are performed on a mandatory basis.

The information generated in this way on signal transit times is transmitted to the network. To be able to use this information for a blind handover, the layer in which the potential destination cell is located must be previously analyzed according to the best provider base station, that is, the best server. This can be done in different ways. On the one hand, the coverage area of ​​the best server can be determined with the corresponding procedure and, on the other hand, this can be done from available measurement data. The best servers thus obtained can then be assigned to any point using the polygons.

The terminal coordinates are then compared with the database of best servers and the corresponding destination cell is thus selected. The destination cell is then transmitted by HO instruction to the terminal and thereby the forward blind handover is performed in a piped manner.

An embodiment of the invention is explained in more detail by means of the figure in the drawing.

Figure 1 shows by way of example a detail of the cell structures of two overlapping mobile communication networks, for example a UMTS network and a GSM network.

The UMTS network comprises a plurality of radio transmission cells 10-14 which are supplied with radio transmission signals by a plurality of permanently installed base stations 20, 23, 24. Similarly, the GSM network comprises a plurality of radio transmission cells 1-7 which are supplied with radio transmission signals by a plurality of permanently installed base stations 20-22.

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

A mobile station 30 is located within the UMTS cell 10, and is supplied with broadcast signals by the base station 24. Mobile station 30 would like to perform a blind handover in a suitable radio transmission cell of the GSM network.

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

By means of 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. For this, the terminal 30 must unequivocally identify the corresponding base stations 20, 23, 24 and the corresponding transit times of the signals at the air interface. This information from neighboring cells and the cell itself is sent as an information packet to a base station, for example 24.

With this, from only two contiguous cells measured and one's own cell, the location of the terminal 30 can be calculated in the UMTS network. This procedure does not depend on whether the location of the terminal is inside or outside a building.

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Therefore, to determine the location of a subscriber terminal without knowing the address information, at least three base stations 20, 23, 24 are necessary whose location is precisely known. By measuring the transit time of the signals between the terminal and each of the base stations, circular rings can be calculated that define the area of ​​separation of the terminal with respect to the corresponding base station. At the center point of each circular ring a base station is arranged. The common cut-off point of the three circular rings is the location of the terminal. The base station locations are in this case the reference points, the location coordinates being available from the network operator's location database.

In theory, three circles converge at one point. This is impossible in real conditions since the transit time measurement principle is indicated in the propagation conditions and the speed of signal processing in the terminal microchip (chip frequency). Span sections per measurement interval cannot be the desired size.

This means in practice that the location of the terminal is described by a cutting surface. In this case, the accuracy of the location determination increases with the number of measured base stations.

Using a chip frequency of, for example, 3.84 MHz, the minimum measurement interval a per chip is calculated for a = speed of light C / chip frequency f_Bit = 300E6 / 3.84E6 = 78 m

With modern terminals, much better measuring accuracies must be achieved in practice.

The precision also depends on the receiver of the terminal. The receiver must be able to resolve time intervals within the chip frequency to be able to deliver results on the order of 10 m. The common cut surface of the circle indicates the actual location area of ​​the terminal.

Since the terminal does not have information about the frame synchronization of the base stations involved, the calculated location of the terminal will be linked to additional measurement failures.

To solve this, two possibilities are mainly indicated:

- the base stations are synchronized by a central clock or via GPS time.

- by means of measurements carried out by the base stations, the measure of asynchronicity with respect to other base stations is determined and stored in a matrix.

By means of the determined location of the mobile station 30, the most suitable radio transmission cell or base station of the GSM network for handover is determined with the aid of a database present in the mobile communication network. In FIG. 1 this is, for example, the GSM base station 20, which supplies, inter alia, the GSM radio transmission cell 1.

In order for the mobile station 30 to also carry out an HO to the corresponding GSM destination cell 1, after evaluating the status information (measurement values) of the terminal, the corresponding destination cell or base station 20 must be communicated. This can be done directly in the form of an HO instruction to the mobile station.

It follows that the functionality of the corresponding network node, for example RNC, must be extended in the sense that the evaluation of the measurement data provides status information from which it is determined, on the basis of data from better servers, the base station that best supplies the destination cell and is provided to the terminal involved in the handover and the base stations.