Cell selection method in a cellular radio mobile network system

The present invention relates generally to cellular mobile radio communication, that is to say, the cellular networks of mobile radio and the mobile stations for use in such networks.

The present invention relates more particularly to cell selection when the access by a mobile station in such a network, for communication.

It is known to carry out a cell selection for access to the network, by the mobile station itself, the corresponding algorithm used in the mobile station being also called cell selection algorithm.

Specifically, the network broadcasts in the serving cell on a particular channel called beacon channel, the information needed to implement such an algorithm, these particular information containing a list of neighboring cells to listen for the mobile station to determine which of these neighboring cells or the serving cell, the signal level received on the control channel is the most favorable. The cell is thus determined, provided that certain other criteria are satisfied, the selected cell for network access. In addition, the commissioning of the mobile station, no serving cell having been selected, the mobile station looks at the collection of the beacon channels, and determines the serving cell according to the same criteria.

For a more complete description of the cellular mobile radio such as the GSM system (for "Global System for Mobile communications" in English), reference may be made for example to the book of M. Mouly and MB PAUTHET, entitled "The GSM System for Mobile Communications "(Palaiseau, France, 1992, ISBN: 2-9507190-07-7).

The present invention relates more particularly to cell selection when the access to the network for communication in a cellular mobile radio network including different types of cells in which different types of services are available.

An example of such a network consists of a network in which are gradually being introduced into an existing infrastructure, corresponding to a system such as a second generation system (in particular the aforementioned GSM system), new radio access means this infrastructure, corresponding to a system such as a third generation system (in particular the UMTS, for "Universal Mobile Telecommunication system") for the progressive introduction of new services, especially transmission services high-speed data.

Such a system thus comprises cells or resources in which said new services are available, in this case of UMTS cells, or GSM / UMTS cells (ie cells with a common control channel to means 'GSM radio access and the means of UMTS radio access) and cells in which said new services are not available in this case GSM cells.

In such a system, the possibility exists that a selected cell according to the cell selection algorithm mentioned above is a GSM cell, when the required service, for a given call for which the mobile station accesses the network, require UMTS or GSM / UMTS cell, or conversely the selected cell is a UMTS cell or GSM / UMTS while the service required to satisfy a GSM cell, so that the allocation of radio resources in such a network is not in any case optimized depending on the services required by users.

Another example of such a network is a network in which are introduced into an existing infrastructure for a given system, for example the GSM system, and in a given geographical area, additional radio access means, a group dedicated to specific users, and allowing these users to have additional services in this geographical area.

In such a system, is also getting a cell selected by the cell selection algorithm mentioned above is a cell in which said additional services are not available, particularly in the case where a user authorized to receive such additional services is limited in this geographical area, in which case neither the user nor the provider of these services are not satisfied.

So the problem is usually made in a cellular mobile radio system with different types of cells in which different types of services are available, the selected cell for network access does not necessarily correspond to a better cell in terms of services .

It is also known, in a cellular mobile radio network such as the GSM network, perform, during communication, cell selection to determine the best cell to which to transfer this communication, the corresponding procedure is also known in English as the "handover". It is also known to produce such a selection cell by the network, based on measurement results that it receives from the mobile station. Specifically, the selected cell is the first of a list of candidate cells called cells (classified in a following order said measurement results) to which a transfer is possible, that is to say in particular in which resources are available for this communication.

Also known in a cellular mobile radio network such as the GSM network, the concept says in English "directed retry", in which a communication transfer (or "handover" in English), a cell initially selected for access to the network to better subsequently selected cell, may be necessary to reflect the fact that the cell selection is made differently in both cases, as has just been recalled, and this can lead to selecting different cells.

Can further be mentioned WO97 / 44922, describes a cellular mobile radio system having different types of cells, which are available in various multimedia type services, and wherein the selection of a best cell to which a communication transfer course is performed on the measurement results as a basis for candidate cells, the availability of resources in these candidate cells, and the type of service required. This document does not address the cell selection when accessing the network for communication. In addition, this document describes a mobile station equipped with means to implement such a selection of best cell to which to transfer an ongoing call. If such cell selection was implemented in the mobile station for the cell selection when accessing the network for communication, it would include the drawback of requiring numerous exchanges of signaling between mobile station and network for its implementation used, of which a substantial consumption of radio resources for such trade, and therefore an increase of the overall interference level in the network. The present invention also aims to avoid these drawbacks.

One can also mention the EP-A-0785696 for a multi-layered architecture system in which, to allow good quality communications for the type of call (voice or data or image), cell selection is performed not only on the basis of the speed of mobile, but also based on the type of call.

One can also mention the EP-A-0808073 addresses the problem of the selection visited system (or "visited system") to provide the best rates available to users.

The present invention is thus intended in particular to allow access by a mobile station in a cellular mobile radio network including different types of cells in which different types of services are available, avoiding various drawbacks mentioned in the foregoing.

The present invention thus relates to a cell selection method in the access by a mobile station in a cellular mobile radio network for communication, said method being essentially characterized in that, said network including different types of cells in which different types of services are available, comprises the following steps:<ul><li>cell selection by said mobile station to access the network,</li><li>network access in the cell so selected, corresponding to a given cell type,</li><li>depending on the type of service required for the communication, decision to transfer or not this communication to a cell of another type constituting a better cell in terms of services.</li></ul>

In other words, and by reference to the concept known in English as "directed retry", the invention can be regarded as introducing a new application of such a concept, that is made a transfer of communication (or "handover" in English) of a selected cell initially for access to the network, to another cell, constituting a better cell in terms of service.

Another object of the present invention consists in a cellular mobile radio network, a switching center and a base station controller for implementing such a method (the present invention advantageously requires no modification of the mobile stations to allow its implementation).

Other objects and features of the present invention will become apparent on reading the following description of an embodiment taken in conjunction with the accompanying drawings, in which:<ul><li>Figure 1 is a diagram illustrating an exemplary system to which the present invention is applicable,</li><li>Figure 2 is a diagram illustrating the various steps of an exemplary method according to the invention,</li><li>Figures 3 and 4 are diagrams to illustrate an example of means to provide a network in which the present invention is applicable, to allow use of the process according to the invention.</li></ul>

A cellular mobile radio system essentially comprises, as shown in Figure 1:<ul><li>a set of base stations (BTS for "Base Transceiver Station" in English), such as those noted BTS0, BTS1, BTS2, BTS3 BTS4 and each of these base stations being in the example shown, assigned to a cell (denoted respectively C0, C1, C2, C3, and C4), and these base stations being connected via a radio interface with mobile stations, such as MS0, MS1, MS2, MS3, MS4,</li><li>a set of base station controllers (BSC for "Base Station Controller" in English), as noted BSC 1 and BSC 2, each BSC controlling the point of view of the radio resource management, a sub- set of cells, the base station controller BSC1 controlling in the example shown the cells C0, C1 and C2, and the base station controller BSC 2 controls in this example the cells C3 and C4,</li><li>a plurality of mobile switching centers, also called MSC (for "Mobile Switching Center" in English), only one of which, denoted by MSC1, is shown in the figure, each MSC being, a non-illustrated specifically manner, relationship with external networks such as including PSTN (for "Public Switched Telephone Network") or ISDN (for "Integrated Services Digital Network"), and controlling the point of view of communications management, a subset of base station controllers, mobile switching center MSC1 controlling in the illustrated example the base station controllers BSC1 and BSC2.</li></ul>

The assembly formed by the BTS and BSC, also called BSS (for "base station subsystem" in English), is what has been called above radio access network resources.

As an example to which this invention is applicable, the radio access means formed BTS3 elements BTS4 and BSC2 constitute new radio access means corresponding to the UMTS system, introduced into an existing infrastructure corresponding to the GSM system already having its own radio access means formed BTS0 elements BTS1, BTS2 and BSC1. Cells such as C0, C1 and C2 then constitute a first type of cells, corresponding to GSM cells, and cells such as C3 and C4 constitute a second type of cells, corresponding to the UMTS or GSM / UMTS cells. The mobile stations are also considered dual mode mobile stations adapted to communicate on one or the other radio interfaces corresponding to the different radio access means.

Figure 2 illustrates the various steps of an exemplary method according to the present invention.

The following steps are illustrated in Figure 2:<ul><li>cell selection by a mobile station for access to the network, for example according to the cell selection algorithm mentioned above (step 1, denoted "cell selection")</li><li>network access, for a given communication in the cell so selected, corresponding to a given cell type (step 2, denoted by "Access") </li><li>phase of establishing communication (step 21, denoted "Establishing communication"), in turn comprising, besides signaling exchange steps in this establishment of communication (such as steps 22 and 23 marked "Display") , a transfer decision step or not this communication to a cell of another type constituting a better cell in terms of service (step 3 rated "transfer?")</li><li>communication phase in the cell thus selected (step 24, noted "Communication"), or at least the beginning of this communication phase, one or more transfers can indeed be made later, during this communication.</li></ul>

In case the transfer is not determined in Step 3, the cell thus selected remains that selected initially for access to the network.

In cases where a transfer is decided in step 3, the following steps are performed to determine to which cells of said other type communication is to be transferred, these steps may be similar to those commonly employed to select a better cell to which a transfer in progress, except that they do not apply here to cells of any type, but the cells of said other type:<ul><li>a step in which it is determined whether the candidate cells said other type can be determined (step 4, denoted "cell (s) candidate (s)?")</li><li>a step in which it is determined whether a transfer to either of these candidate cells, taken in a certain order of preference, is possible, that is to say if it does not act to a rejected cell (5 step denoted "cell (s) candidate (s) rejected (s)?").</li></ul>

In the case where such a non-rejected candidate cell is determined, a transfer to the cell is controlled (step 6 denoted "Transfer").

In the case where such a candidate cell, or such a non-rejected candidate cell, can not be found, a transfer can be attempted later, preferably within limits of time (steps 7 and 8 marked "Time? "). If no transfer to a better cell in terms of service is definitely possible, communication is left in the cell selected initially for access to the network. The network can then provide, for example in the case considered here of a mixed GSM / UMTS, a lower quality of service, including a data transmission service at lower speed, or any replacement service up is a next transfer to a better cell in terms of service is possible, according to the movements of the user and the radio conditions change and traffic within the network during this communication.

It should be noted that according to another example cells determined as the candidate cells could be cells of any type, and the candidate cells to which transfer is attempted, only said other type of cells.

In the example illustrated in Figure 1, if the C0, C1 and C2 cells are GSM cells, access to the network by a mobile station MS0 such as for example, corresponding to a communication, for example to a transmission service high-speed data available in a UMTS cell or GSM / UMTS such that the cells C3 and C4 for example, can be performed as follows:<ul><li>selecting a GSM cell, for example cell C0, by the mobile station MS0, </li><li>access to network in cell C0 so selected,</li><li>based on signaling data transmitted by the mobile station during the establishment phase of the call, decision of transfer to a UMTS or GSM / UMTS cell; in the illustrated example, it may be for example of cell C3.</li></ul>

Figures 3 and 4 are diagrams to illustrate an example of means to provide in the network, respectively, in the MSC and BSC entities, to use a method according to the invention.

As illustrated in Figure 3, an MSC receives a mobile station MS when accessing the network and the establishment of a communication phase<ul><li>INF1 of information to identify the user,</li><li>INF2 of information to identify the type of service required for the call concerned.</li></ul>

MSC receives in addition a database as including the HLR database (for "Home Location Register" in English), in response to INF1 information, INF3 information identifying the services that the user is authorized to use, and possibly the geographical restrictions for such use.

MSC illustrated in Figure 3 further includes means rated M for determining, based on information INF 2 and INF 3, the INF4 information indicating the type of cell constituting a better cell in terms of service for this user and this communication.

For example, in the case considered here of a mixed GSM / UMTS, and as already indicated above, such a best cell in terms of services may, in the case where the requested service is a transmission service high-speed data available only in a UMTS or GSM / UMTS cell, be such a UMTS or GSM / UMTS cell, or, otherwise, be a GSM cell.

As illustrated in Figure 4 means rated M 'are provided in the BSC entity, to:<ul><li>firstly decide whether or not a transfer must be made to a better cell in terms of service, this decision being taken on the basis:<ul><li>- Cell type selected to access the network, represented by a INF5 information that can be provided by the BSC considered (that is to say, the BSC controlling the current serving cell, that is to say, in the selected cell case for access to the network)</li><li>- The type of cell constituting a better cell in terms of service, represented by INF4 information</li></ul></li><li>on the other hand, in the case where it is decided that such a transfer should actually be performed, select one of these cells to which the transfer is to be made, this selection being made on the basis of:<ul><li>- Information such as INF6 corresponding to the measurement results provided by the mobile station MS,</li><li>- Information on the availability of resources in the cells considered candidates, such as INF7 information provided by the radio resource management means within the BSC considered (in the case of transfer said internal, or intra-BSC ), or such as INF8 information provided by the MSC that controls the BSC (in the case of external transfer is, or inter-BSC)</li></ul></li><li>information about the type of cell constituting a better cell in terms of service, such as INF4 information.</li></ul>

The means M 'operate essentially by the method described above, for, from information such as INF4 INF8 to determine whether a transfer to a better cell in terms of service is to be performed, and if so select one of these cells to which this transfer is to be performed, the cell thus selected being indicated by information such as INF9 transmitted to the mobile station MS.

The particular embodiment of the means M and M 'in order to operate the set according to the process described, presenting no particular difficulty for the skilled, requires no specific description. Furthermore, the distribution of different functions between MSC and BSC entities can be other than that contemplated above as an example.

In addition, the transmission of information such as INF1 to INF9, within the network, can be carried out according to known communication protocols within such a network, that do not require to be described again here.

The example describes particular allows for optimization of the allocation of radio resources within the network, based on the services required by the users.

The invention is not limited in this application example, if the cell selected initially for access to the network is a GSM cell and wherein a transfer is then performed to a UMTS cell or GSM / UMTS . It can also be used in the case where the selected cell initially for access to the network is a UMTS cell or GSM / UMTS and where a transfer is then performed to a GSM cell (if permitted for the requested service) , thereby further optimizing radio resource allocation within the network.

Furthermore, as already indicated, the invention is not limited to the application example described corresponding to a mixed GSM / UMTS.