The invention relates to a method for transferring special cell information in a communications system and to a cellular system and mobile station that realise the method. The invention can be advantageously applied in cellular systems in which users are assigned cell-specific services such as cell-dependent charging. Such cellular systems may be digital cellular systems such as GSM (Global System for Mobile Communications).
First it will be discussed the prior art by disclosing the call setup and handover procedures in GSM. Then it will be described a known arrangement for implementing a cell-specific service as well as problems involved in the solutions described.
In cellular mobile communication systems a mobile station can move freely in the area of the mobile communication network and choose the best base station signal available at a particular moment. Usually all base stations offer substantially the same services to the mobile stations in the network. Some base stations, however, may be specified so as to offer a special service to all mobile stations in the network, e.g. a call charge rate cheaper than the normal rate. The base station informs on its broadcast control channel (BCCH) about such a special service so that the mobile subscribers in the area will know that they are in a special service area of the network and can utilise the service.
FIG. 1 of the attached drawing shows a simplified block diagram of the pan-European GSM. A mobile station MS is connected by radio with a base transceiver station BTS, in the case depicted by FIG. 1 to base transceiver station BTS1. The mobile station MS receives in the standby mode the transmission of a base station it has selected. A base station system BSS comprises a base station controller BSC which controls base transceiver stations BTS. A mobile services switching centre is connected with other mobile services switching centres, a gateway mobile services switching centre GMSC and possibly with an intelligent network IN. Through the gateway mobile services switching centre GMSC the GSM network is connected with other networks, such as a public switched telephone network PSTN, public land mobile network PLMN, integrated services digital network ISDN or with an intelligent network IN. The operation of the whole system is supervised by an operation and maintenance centre OMC.
A mobile station""s subscriber data are permanently stored in the system""s home location register HLR and temporarily in the visitor location register VLR in the area of which the mobile station is located at a given moment. The visitor location register VLR contains location information for a mobile station MS with the accuracy of a location area LA. The geographical area controlled by the visitor location register is divided into one or more location areas LA within which a mobile station may move freely without registering in the visitor location register. Each location area may have one or more base transceiver stations BTS operating in it.
Base transceiver stations BTS continuously transmit on their broadcast control channels information about them and their surroundings, such as cell identity CI, information about neighbour cells and the location area identity LAI. The location area identity tells a mobile station, which receives a base transceiver station""s transmission on the broadcast control channel, the location area in which it is located at that moment. If a mobile station detects in conjunction with a handover or cell reselection that the base station""s location area identity has changed, it sends a location update request to the network.
While moving in the area of a mobile network a mobile station normally listens to the base station having the strongest signal. Thus, as a mobile-terminated or mobile-originated connection is to be set up, it is first attempted to use that base station. If that base station cannot provide a traffic channel for the connection, the mobile station tries to set up the connection via the base station offering the next strongest signal. If, during the connection established, the mobile station moves within the operating areas of more than one base station, handovers are carried out during the connection according to the principle described above.
FIG. 2a in the drawing attached hereto shows signalling in GSM between a mobile network and mobile station in mobile-originated call setup. When a mobile station wants to set up a connection, it sends to the network a Channel_Request (message 21) to which the network responds with an Immediate_assignment message 22 by means of which it assigns a standalone dedicated control channel SDCCH to the mobile station MS for signalling purposes. The mobile station MS sends on the signalling channel assigned a CM_service_request message 23, indicating the type of the service requested, in this case a call setup on a traffic channel. Subscriber authentication is carried out using an Authentication_request message 24 and Authentication_response message 25. In the authentication, information stored in the network is compared with information stored in the mobile station MS with the aim of controlling access to the network, among other things, in order to prevent different violations.
After successful authentication a Cipher_mode_command message 26 and Cipher_mode_complete message 27 carry information about the protection algorithm used on the connection. A Setup message and Call_proceeding message 29 carry more detailed information about the connection to be established. Such information includes e.g. the called number transmitted from the mobile station to the network. Then the network assigns a traffic channel to the mobile station using an Assignment_command message 30, acknowledged by the mobile station MS with an Assignment_complete message 31. In step 32 ringing tone is transmitted to the mobile station MS and when subscriber B has answered, the call is established using the Connect 33 and Connect_acknowledge 34 messages. The call then proceeds on the traffic channel as usual.
In a like manner FIG. 2b in the drawing attached hereto shows the call setup signalling in GSM in the case of a mobile-terminated connection. Like messages in FIGS. 2a and 2b have like reference designators. The network pages a mobile station MS using a Paging_request message 35 to which the mobile station responds by sending to the network a Channel_request (message 21) for a signalling channel. The network assigns to the mobile station MS a signalling channel using an Immediate_assignment message 22 as described above. The mobile station MS responds to the paging message on the signalling channel assigned to it using a Paging_response message 36. Messages 24 to 27 are used for subscriber authentication and for transmitting the protection algorithm data in the manner described above. Call setup is initialised in the Setup message 37 and Call_confirmed message 38, and a traffic channel for the call is assigned to the mobile station MS in the Assignment_command 30 and Assignment_complete 31 messages. In step 39 the mobile station MS indicates that it is alerting and when the mobile subscriber has answered, the call is established using the Connect 40 and Connect_acknowledge 41 messages. The call then proceeds on the traffic channel as usual.
In cellular-type mobile communication systems radio coverage is achieved using several slightly overlapping radio cells. As a mobile station moves from a cell to another, a handover is carried out in accordance with predetermined handover criteria. The handover should as little as possible disturb the existing connection. Normally, a handover is based on radio path criteria but it may be carried out for other reasons, too; in order to distribute load or reduce transmission power, for instance. A handover may also be carried out within a cell, between traffic channels.
A neighbour cell priority list may also be specified for a base transceiver station BTS so that one or some of the neighbour cells of the base transceiver station are defined as primary handover target cells. In a handover situation, the connection is then handed over to one of these priority cells always when possible. The neighbour cell priority list is base transceiver station specific, and all mobile stations perform the handover in the same manner because of the list.
A mobile station MS continuously measures the signals of the base transceiver stations BTS closest to the cell in which it is located e.g. to determine the base transceiver station that offers the best signal, and in order to prepare for a possible handover. On the basis of the neighbour cell information transmitted by each base transceiver station on its broadcast control channel the mobile station recognises the neighbour cells that it should monitor. For example, in GSM a mobile station MS may measure the signal level and quality of the serving base transceiver station and simultaneously the signal levels of 32 other base transceiver stations at the most. The mobile station MS regularly sends the measurement results in the form of a report message via the serving base transceiver station BTS1 to a base station controller BSC. The report message contains the measurement results for the serving base transceiver station and six best neighbour base transceiver stations at the most. A handover between the serving cell and a neighbour cell or between channels of the serving cell may occur e.g. when the measurement results of the mobile station and/or base transceiver station indicate a low signal level and/or quality for the channel of the current serving cell and a neighbour cell offers a better signal level or another channel offers a better signal quality or when a neighbour cell/another channel offers a chance to use lower transmission power. The selection of the handover target cell is based e.g. on the signal level and/or load of the target cell. So, commonly used handover criteria include, among others, the signal level and quality of the radio path, signal levels of the source cell and target cell, signal quality of the source cell, and the transmission power which is on the one hand required of and on the other hand allowed for the mobile station in the target cell. A handover between traffic channels or signalling channels is usually carried out once the handover criteria set by the operator are met. A handover may also occur because of an overload. A so-called directed retry is also considered a handover when in the setup phase the traffic channel is chosen from a cell different from that in which signalling was carried out.
FIG. 3 in the drawing attached hereto shows signalling in a handover between two base station systems BSS in a GSM network. In FIG. 3, a radio connection has been established to a mobile station MS, e.g. in order to transmit speech, data or signalling, via a base transceiver station BTS1 and base station controller BSC1 (not shown) in a base station system BSS1. The base station controller BSC1 receives measurement results from measurements by the base transceiver station BTS1 and mobile station MS and, if necessary, hands the connection over from the base transceiver station BTS1 to another base transceiver station under its management, for example. When the base station system BSS1 detects that the connection should be handed over to a base transceiver station in another base station system it sends to the mobile services switching centre MSC a handover request in a Handover_required message 1, indicating also the reason for the handover, e.g. the quality of the radio link or the signal level, as well as the identity/identities of the handover target cell. The mobile services switching centre MSC sends to the base station system of the target cell, in the case depicted by FIG. 3, the base station system BSS2, a Handover_request message 2, indicating that the radio connection of the mobile station MS is handed over to the base station system BSS2. The base station system BSS2 responds to the mobile services switching centre MSC using a Handover_request_acknowledge message 3, indicating the new radio channel assigned to the mobile station MS. In a Handover_command message 4 the mobile services switching centre MSC sends to the base station system BSS1 radio channel information which the base station system BSS1 further sends to the mobile station MS in a Handover_command (message 5). The mobile station MS performs the handover by adopting the new assigned radio channel and acknowledges the base station system BSS2 of a successful handover with a Handover_complete message 6. The base station system BSS2 further informs the mobile services switching centre MSC about the successful handover using a Handover_complete message 7. From then on the traffic to and from the mobile station MS flows via a base transceiver station and base station controller in the base station system BSS2.
For a mobile subscriber to be able to use cell-specific services in an area specified for the subscriber the setup and handover methods have to allow for the direction of the mobile station to the special cells in mobile networks offering tailored special services. Then the base station controller must know the special cells of the mobile subscriber. This information is advantageously stored in the home location register HLR or in a database of an intelligent network wherefrom it could be transferred to the base station controller during call setup and in conjunction with a handover when moving into the coverage area of a new base station controller. This, however, involves the problem that the transfer of the special cell information from the home location register would result in an unreasonably long delay in the call setup/handover.
A solution is applicable to this problem which according to the information available to the applicant has not been disclosed in public at the moment of filing the present application. The solution is based on the idea that base station selection is carried out on the basis of a special cell list created for the mobile station and stored on the mobile station. When a base station controller controls the base station selection the mobile subscriber""s special cell list is sent from the mobile station to the base station controller in connection with a call setup and, if necessary, in connection with the execution of a handover. An attempt is made to direct the mobile station to its special service area in call setup and handover. By means of the special cell list the cells in the network can be divided, as far as a single mobile station is concerned, into special cells, such as primary cells, for example, and normal cells. In call setup the mobile station is assigned a traffic channel of a base station of a primary cell specified for the mobile station when the mobile station is located in the area of such a primary cell. A base station of a primary cell specified for the mobile station is selected as the handover target cell when the signal of the primary cell base station is suitable for handover. This is realised e.g. by setting a handover criterion that the base station signal of a cell defined as primary in the special cell list must meet in order for the handover to be carried out from a normal cell of the network to a primary cell of the mobile station. By means of such base station selection carried out in handover and call setup the mobile station is directed to the special service and kept in the special service area when it is reasonable from a radio standpoint.
The problem with the solution described above is that special cell information, especially the special cell identities, contains rather a big amount of data so that the transfer of special cell information from a mobile station to a base station system uses an unreasonably big portion of the radio path""s signalling capacity.
An object of the invention is to provide a solution for the transfer of special cell information from a mobile station to a base station system in such a manner that mobile subscriber specific special cell services can be realised with short delays in the transfer of special cell information and with a light network load.
An idea of the invention is that special cell identities are transferred from a mobile station to a base station system encoded in such a manner that the transfer of special cell identity information requires fewer bits than what would be needed for the transfer of special cell identities as such. Cell identities are advantageously chosen such that the cell identities of a single area can be coded with a low number of bits.
In accordance with the principle according to the invention the transfer of special cell identities can be advantageously performed in such a manner that the first special cell identity is transferred as such and after that it is transferred transfer codes that are differences of two consecutive special cell identities in the special cell list. Then the special cell identities are advantageously arranged in the order of magnitude in the special cell list.
In most cases the solution according to the invention makes it possible to transfer the special cell information on the radio path using a single L2-frame-size message so that the transfer of special cell information can be carried out quickly without using a lot of the radio path""s capacity in connection with call setup and handover.
The method according to the invention for transferring special cell information in a cellular system from a mobile station to a base station system, wherein said special cell information comprises a list of special cell identities (CI), is characterised in that
a transfer code is determined on the basis of at least two special cell identities,
said transfer code is transferred from the mobile station to the base station system and
the special cell is identified on the basis of the transfer code transferred.
The cellular system according to the invention which comprises means for selecting the serving base station on the basis of a special cell list is characterised in that it comprises means for transferring special cell information from a mobile station to a base station system using a transfer code which is created on the basis of at least two special cell identities and the bit number of which is smaller than the bit number of a single special cell identity.
The mobile station according to the invention is characterised in that it comprises means for storing special cell information, means for creating a transfer code on the basis of at least two special cell identities the bit number of which is smaller than the bit number of a single special cell identity, and means for sending the transfer code to a base station system.
Preferred embodiments of the invention are described in the sub-claims.