WO1997050274A1

WO1997050274A1 - Location management and paging method in a cellular radio system

Info

Publication number: WO1997050274A1
Application number: PCT/FI1997/000391
Authority: WO
Inventors: Markku Verkama
Original assignee: Nokia Telecommunications Oy
Priority date: 1996-06-26
Filing date: 1997-06-18
Publication date: 1997-12-31
Also published as: EP0976280A1; FI103556B; FI103556B1; FI962645A; AU3178197A; FI962645A0; JP2000512822A

Abstract

The invention relates to a location management and paging method in a cellular radio system comprising base stations (BTS1-BTS8) transmitting their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies, and mobile stations (MS) receiving said information for monitoring base stations. In the location management method according to the invention, the contact area of the mobile station (MS) is stored in connection with each network activity and updated when the mobile station (MS) does not receive any identity belonging to the contact area on the broadcast frequency. The paging method according to the invention utilizes the stored contact area and its neighbouring cell information when paging the mobile station (MS). The invention also relates to a cellular radio system and a mobile station implementing the above-described functionality.

Description

LOCATION MANAGEMENT AND PAGING METHOD IN A CELLULAR RADIO SYSTEM

FIELD OF THE INVENTION

The invention relates to a location management and paging method in a cellular radio system comprising base stations that transmit their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies. In the location management method, the system also comprises mobile stations that receive said base station information for the purpose of monitoring base stations. The invention also relates to a cellular radio system and a mobile station. The cellular radio system comprises base stations that transmit their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies, and mobile stations that receive said base station information for the purpose of monitoring base stations.

BACKGROUND OF THE INVENTION

In mobile systems, mobile station location data is required for the routing of incoming calls and for other network services. The location of each mobile station is stored in the system registers usually with the accuracy of a location area. The size of a location area is determined by the network operator, but it should not be too small since in such a case the location updating of a mobile station causes a great signalling load, thus consuming the limited radio resources of the network, for example. On the other hand, a large location area does decrease the amount of location updating signalling, but paging a mobile station within a large area causes a great signalling load both over the radio path and between the network exchange and the base stations, when each base station of the location area in question is guided to transmit a call to the mobile station. Therefore, the problem with designing location areas is the unnecessary location updating signalling caused by small location areas, on the one hand, and the great call signalling load caused by large location areas when a mobile station is paged, on the other hand.

Figure 1 of the accompanying drawing is a block diagram of a simplified example of the structure of the GSM mobile system. The units shown in the figure can also be found in other mobile networks, but they may be called with a different name. A mobile station MS that is either in an idle or active mode is at all times connected to a base transceiver station BTS, in the case shown in Figure 1 to base station BTS1. A base station system BSS consists of a base station controller BSC and BTSs under its control. A mobile services switching centre MSC usually controls several BSCs. An MSC communicates with other MSCs and via a gateway mobile services switching centre GMSC with the public switched telephone network. The mobile station location data and other subscriber data are permanently stored in a home location register HLR of the system and temporarily in the visitor location register VLR in the area of which the MS happens to be located. The VLR stores the location data of the MS with the accuracy of a location area LA.

The geographical area controlled by the VLR is divided into one or several location areas and the MS may roam freely within each area without notifying the VLR. One or several BTSs may operate within each location area. The BTSs continuously transmit information on themselves and their environment, such as a base station identity code BSIC, neighbouring cell information and a location area identifier LAI. On the basis of the last- mentioned, an MS that is locked onto the BTS knows the LA where it is situated currently. When changing the BTS, if the MS finds out that the LAI of the base station has changed, it transmits a location updating request to the network. The location area of the MS is updated to the VLR in the area of which the MS is currently located. Information concerning the VLR in the area of which the MS is located is forwarded to the HLR.

An MS continuously measures the signals of the base stations situated close to the cell where it is located, for example, for a possible cell change. On the basis of the neighbouring cell information transmitted by each base station on a broadcast control channel, the mobile station identifies the neighbouring cells it should monitor. For example in the GSM mobile system, the MS may simultaneously measure the signal level and/or quality of at most 32 other base stations in addition to the serving base station. The mobile station location data stored in the registers of the mobile system is used, for example, in the routing of an incoming call. On the basis of the mobile station location data obtained from the HLR and the VLR, an incoming call is routed to the MSC in the area of which the MS of the called subscriber is currently located according to the location data. The MSC asks for the data of the called subscriber from its own VLR for the purpose of call set-up. The VLR transmits the required data of the MS, including for example the LA of the MS stored during the latest location updating, i.e. LA1 in the case shown in Figure 1. On the basis of the location area data, the MSC transmits a paging message to the MS via all the BTSs of the location area, in the case shown in Figure 1 via BTS1 , BTS2 and BTS3. When the paged MS has received the paging message, it transmits an answer to the paging message via the most suitable base station, e.g. BTS1 in Figure 1 , whereafter the network allocates a traffic channel for the call and sets up a speech connection to the MS.

GB 2 243 976 discloses a mobile system where dynamically changeable location areas partly overlap, so that location updating back and forth would be avoided in the vicinity of the border between two location areas. A mobile station according to the reference comprises a route memory where it stores the identity codes of the base stations the cells of which it has passed through after the latest location updating procedure. In connection with location updating, the mobile station transmits the stored route list to the base station, simultaneously emptying its route memory. The base station calculates a cumulative value of the route lists transmitted by all the mobile stations for each neighbouring base station and remodels its location area to include the neighbouring base stations via which a majority of mobile stations have passed, i.e. which have the highest cumulative value. The number of base stations belonging to a location area is changed on the basis of the location updating and paging load of the base stations. Continuous location area planning is thus carried out in the system. In the mobile system disclosed in the reference, a base station transmits location area data comprising a list of the base stations of the location area either in the form of continuous broadcast or a message addressed to a certain mobile station. In an embodiment of the reference, a mobile station compares the base station identity code it has received to the base station identity codes it has stored in its route memory. If a corresponding base station identity code cannot be found from the route list, the mobile station compares the base station identity codes on the location area data list received from the base station to the identity code of the base station in the area of which the mobile station carried out the latest location updating procedure. The mobile station transmits a location updating request if the aforementioned base station identity code is not included in the location area list. Otherwise, the mobile station stores the new base station identity code in its route memory. When a mobile station is paged, the paging message is transmitted via all the base stations that currently belong to the location area. The problem with the arrangement disclosed in the reference is particularly the complicated structure of the base stations and the mobile stations, including the changing tables of neighbouring base stations and route memories. The calculation and storage of the dynamic location areas and the transmission of the continuously changing location area list require special arrangements at the base station. The long base station identity code list alone, transmitted for the purpose of location management, increases the size of the signalling messages. In order to provide an optimum location area structure, location area planning is to be implemented on the basis of rather complicated criteria. The above-described decision-making concerning mobile station location updating requires processing capacity of the mobile station for comparing and storing different long lists. Also, in some cases changing location areas may create a need for extra location updating if the cell where the mobile station is currently located is removed from the location area of the mobile station.

EP 454 648 discloses a mobile system where a mobile station is paged in an extended paging area so that the call could be routed despite erroneous location data. Location areas where the mobile station could possibly be located are also stored in the "uncertainty" list of each mobile station in addition to the normal location updating of the mobile station. In an embodiment disclosed in the reference, the uncertainty list comprises the current location area of the mobile station, at least one previous location area where the mobile station has been registered, and the areas from where the mobile station has made a call or responded to a transmitted paging message. A paging message is transmitted to a mobile station in an extended paging area according to the uncertainty list stored in advance, if the mobile station does not respond to a paging message transmitted in the primary MSC area indicated in the location data. A paging message may also be transmitted in one location area at a time. The problem with this arrangement is the heavy paging that creates a great deal of signalling when a paging message is transmitted simultaneously in all the location areas of the extended paging area. The extended paging area may comprise several location areas that are situated in different parts of the country if the mobile subscriber travels a great deal, which results in unnecessary paging load on the network. When a mobile station is paged in one location area at a time, the waiting period for the call set-up may become too long for the calling subscriber. Also, forming and storing an uncertainty list for all the mobile stations of the system loads the fixed network and requires special arrangements of the network.

BRIEF DESCRIPTION OF THE INVENTION The purpose of the present invention is to provide location management and paging in a cellular radio system in a simple manner that loads the system only a little, thus also decreasing unnecessary signalling in the network.

This new type of location management is achieved with a method according to the invention which is characterized by storing the contact area of the mobile station in connection with each network activity, and updating the contact area data when the mobile station does not receive any identity data belonging to the contact area on the broadcast frequency of the base station.

The new type of paging of a mobile station is achieved with a method according to the invention which is characterized by paging the mobile station via the base stations of the stored contact area, which is stored in connection with the latest network activity, and paging the mobile station via the base stations indicated in the neighbouring cell information of the stored contact area, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the contact area base stations.

The location management and paging according to the invention can be implemented with a cellular radio system of the type described in the preamble, characterized according to the invention in that the system is arranged to store the contact area of the mobile station in connection with each network activity, to update the contact area data when the mobile station does not receive any identity code belonging to the contact area on the broadcast frequency of the base station, to transmit a paging message to the mobile station via the base stations to the contact area, and to transmit a paging message to the mobile station via the base stations of the cells indicated in the neighbouring cell information of the contact area, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the base stations of the contact area.

The invention also relates to a mobile station that is arranged to receive a base station identity code and neighbouring cell information transmitted by a base station on its broadcast frequency for the purpose of monitoring base stations, and that is characterized according to the invention in that the mobile station is arranged to store the contact area in connection with each network activity and to update the contact area data when it does not receive any identity code belonging to the contact area on the broadcast frequency of the base station.

The invention is based on the idea that the location management and paging are arranged on the basis of the cellular structure of the network, by utilizing the neighbouring cell information of the base stations without separate location areas. Location management according to the invention is implemented by storing, both in the fixed network and in the mobile station, the identity of the contact area, e.g. the base station identity code of the contact cell or both the aforementioned base station identity code and the neighbouring base station identity codes. When moving from one cell to another in the network, a mobile station compares the base station identity code it has received from the cell base station and the identity codes of the neighbouring base stations to be monitored with the identity codes of the contact area. When none of the identity codes of the contact area matches the base station identity code received by the mobile station or the neighbouring base station identity codes to be monitored, the mobile station notifies the cell base station, whereupon the fixed network and the mobile station update the identity of the cell area in question, e.g. the cell base station identity code or this identity code and the neighbouring base station identity codes, as the identity of the contact area. If the mobile station communicates with the network for some other reason, e.g. when it makes or receives a call, uses some other teleservice or when it transmits an additional service request to the network, the new contact area identity is stored as the identity of the contact area after the service.

Paging according to the invention is carried out by first transmitting a paging message to the mobile station via the base stations of the contact area stored in connection with location management. If the mobile station does not respond to the page within a predetermined period of time, the paging message is transmitted to the mobile station via the base stations indicated in the neighbouring cell information of the base stations in the stored contact area. The paging message to be transmitted in the second stage can be limited to be transmitted via only the neighbouring base stations that do not belong to the contact area. The first stage of the paging can be carried out, if required, in the form of two pagings, whereupon the mobile station is first paged via the base station of the stored contact cell and if the mobile station does not respond to the transmitted paging message within a predetermined period of time, the paging message is transmitted to the mobile station via the base stations indicated in the stored neighbouring cell information of the contact cell.

Such a location management and paging method has the advantage that there is no need to design separate location areas in the network. Further, the paging method according to the invention has the advantage that it decreases the signalling load on the radio network. The paging of a mobile station according the invention can be directed at only one cell, for example when the mobile subscriber is an office user that remains nearly stationary, in which case the paging signalling is rather minimal. Another advantage of the location management method according to the invention is that it is not necessary to transmit location area identities or codes on the broadcast frequencies of the base stations when the network does not comprise separate location areas. A base station in a cellular radio system according to the invention does not transmit any data that would be intended particularly for location management only, and therefore signalling from the base station to a mobile station can be minimized with the present arrangement.

The location management method also has the advantage that the location updating load is distributed evenly to different base stations of the radio network when the network is not divided into location areas that trigger the location updating.

The advantage of the cellular radio system and the mobile station according to the invention is their simple structure since the implementation of the invention utilizes structures and functionalities intended for other network facilities.

LIST OF FIGURES

The invention will be described in greater detail below with reference to the accompanying drawings, in which

Figure 1 is a block diagram of a simplified structure of a cellular radio system, Figure 2 shows a first example of the cellular structure of a radio network,

Figure 3 is a flow chart illustrating location management according to the primary embodiment of the invention, Figure 4 shows another example of the cellular structure of a radio network,

Figure 5 is a flow chart illustrating a paging method according to the primary embodiment of the invention, and

Figure 6 is a flow chart illustrating a paging method according to the secondary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be applied in any cellular mobile system. The invention will be described below by way of example in connection with the pan-European digital mobile system GSM. Figure 1 shows a simplified structure of the GSM network described above. For a more detailed description of the GSM system, reference is made to the GSM recommendations and to The GSM System for Mobile Communications by M. Mouly and M. Pautet, Palaiseau, France, 1992, ISBN:2-9507190-0-7.

Fiugre 2 shows a simplified example of a cellular structure where a geographical area covered by the network is divided into cells C1 to C8 such that when an MS is situated in each cell it communicates with the network via a base station BTS1 to BTS8 situated in the cell. According to the present invention, the cellular structure of the network is not divided into location areas. The network base stations BTS1 to BTS8 transmit, for example, their own base station identity code and neighbouring cell information on their broadcast frequencies according to the GSM recommendations. The neighbouring cell information transmitted may comprise data about cells other than those conventionally regarded as neighbouring cells. The term "neighbouring cell" does not necessarily refer in the present application to cells that are situated next to each other geographically, but to all foreign cells indicated by the base station on the broadcast frequency. All the mobile stations situated in the cell area receive the broadcast transmitted by the base station and measure the signal of the indicated base stations in a manner determined in the GSM recommendations. The term "cell identity" refers in the present application to any identity data with which a network cell can be identified and the geographical scope of the contact area can thus be determined. Even though the invention will be described below mostly in connection with processing a base station identity code, contact area identities may also include other unambiguous identities that a base station broadcasts. A base station identity code should therefore be considered to generally refer in the present application to any identity data defining a network cell.

In a cellular radio system according to the invention, contact area data is used as the location data of a mobile station instead of location areas. In the primary embodiment of the invention, a contact area is a contact cell, i.e. a cell via which the mobile station has last communicated with the network. Mobile station activities that set up a connection to the network include for example making or receiving a call, requesting for additional services and other teleservices provided by the network, and updating a contact area described below. In the following, the invention will be described in greater detail by means of the primary embodiment with reference to Figure 2. The identity of the contact cell, for example the base station identity code, is stored both in the MS and in the fixed network, for instance in the MSC or the BSC, when the connection from the mobile station to the network is terminated. In Figure 2, it is assumed that the last network activity of the MS terminated in the area of cell C1 , and therefore the base station identity code BSIC1 of the base station BTS1 of the contact cell C1 is stored for example in the BSC and in the MS. For the purpose of base station measurements, the MS continuously receives from the broadcast frequency of BTS1 base station information including the identity code BSIC1 of the transmitting base station and the identity codes BSIC2, BSIC3, BSIC4, BSIC5, BSIC6 and BSIC7 of the neighbouring base stations BTS2, BTS3, BTS4, BTS5, BTS6 and BTS7. The MS according to the invention also uses this base station information for location management by comparing the base station information to the identity code BSIC1 of the contact cell C1 it stored last. As a result of the comparison, a common identity code BSIC1 is found in both pieces of the compared data, whereupon the location management does not call for any actions. When the MS moves to C7 according to the route 20 shown in the figure, the MS receives base station information that is transmitted by BTS7 and that contains base station identity codes BSIC7, BSIC2, BSIC1 , BSIC6 and the identity codes of the neighbouring base stations of BTS7 that are not shown in Figure 2. The mobile station compares the received base station information to the stored identity code BSIC1 of the contact cell that is still the common identity code for both pieces of data to be compared, and therefore the location management does not still require any action to be taken. When the MS travels further along the route 20, it correspondingly receives base station information that is transmitted by BTS6 of C6 and that it compares again to the stored identity code BSIC1 of the contact cell, as described above. Further, the MS may find that BSIC1 is among the neighbouring cell identity codes of BTS6, in a similar manner as previously in the area of C7. The MS next moves along the route 20 to the area of C8, where it receives base station information that is transmitted by BTS8 and that contains the base station identity codes BSIC8, BSIC6, BSIC5 and the identity codes of the neighbouring base stations of BTS8 that are not shown in the figure. The mobile station compares the identity code BSIC1 of the contact cell to the received base station information. The comparison does not reveal identical base station identity codes, wherefore the mobile station transmits a contact cell updating message to BTS8 and updates at the same time BSIC8 as its own contact cell identity code. Correspondingly, in the fixed network for example the BSC stores the identity code BSIC8 of BTS8 as the new contact cell identity of the MS. Figure 3 shows the location management of a mobile station according to the primary embodiment in the form of method steps. The process moves from step 30 to step 31 if the MS is engaged in a network activity. When the connection to the network is terminated between BTS1 and the MS, in step 31 the identity code of the last serving base station BTS1 , e.g. BSIC1 , is stored in the MS and in the fixed network, e.g. in the MSC or the BSC, as the contact cell identity of the mobile station. If the MS is not engaged in a network activity, in step 33 the base station information received from the base station is compared to the stored contact cell identity in the MS. When the MS is located in C7 of Figure 2, it compares the base station information BSIC7, BSIC2, BSIC1 , BSIC6 etc. to the contact cell identity BSIC1. Since both pieces of data to be compared include the same identity code BSIC1 , no contact cell updating is carried out. The mobile station may move on without contacting the network. When the MS arrives in the area of C8 of Figure 2, no common identity code is found for both pieces of data to be compared in the comparison carried out in step 33. The MS then transmits a contact cell updating message via BTS8. The fixed network and the MS store BSIC8 as the new contact cell identity (step 34).

In the secondary embodiment of the cellular radio system according to the invention, a contact area includes the contact cell and the cells indicated in the neighbouring cell information of the contact cell base station. In the following, the invention will be described in greater detail by means of the secondary embodiment with reference to Figure 4, where the geographical area covered by the network is divided into cells C1 to C13. In connection with a network activity of the MS, when the network connection is terminated in the area of the contact cell C1 , for example the base station identity code BSIC1 of the contact cell and the neighbouring base station identity codes BSIC2, BSIC3, BSIC4, BSIC5, BSIC6 and BSIC7 contained in the neighbouring cell information of BTS1 are stored in the fixed network and in the MS. When the MS moves according to a route 40 shown in Figure 4 to C7, it receives base station information from BTS7, as described above in connection with Figure 2. The MS compares the received base station information BSIC7, BSIC2, BSIC1 , BSIC6 etc. to the base station identity codes BSIC1 , BSIC2, BSIC3, BSIC4, BSIC5, BSIC6 and BSIC7 of the stored contact area. The comparison results in the discovery of four common base station identity codes BSIC1, BSIC2, BSIC6 and BSIC7. The location management does not require any measures to be taken. In a corresponding manner, in the area of C6, the MS discovers the common base station identity codes BSIC1 , BSIC5, BSIC6 and BSIC7 as a result of the comparison. Even when it moves to the area of C8, the MS finds the common base station identity codes BSIC5 and BSIC6 as a result of the comparison, and therefore no measures for updating the contact area are required. When the MS moves to the area of C11 according to the route 40, it receives from BTS11 base station identity codes BSIC11 , BSIC12, BSIC8, BSIC10 etc. as base station information. When the MS compares these identity codes to the identities of the stored contact area, the comparison does not result in any identical base station identity codes. In such a case, the MS transmits a contact area updating message to the network via BTS11. The fixed network and the MS update the base station identity code BSIC11 and the neighbouring base station identity codes BSIC12, BSIC8, BSIC10 etc. as the new contact area. In the primary embodiment of the cellular radio system according to the invention, when an MS is paged a paging message is transmitted in two stages, if required. At first the paging to the MS is transmitted via the base station of the contact cell stored in the fixed network, e.g. the MSC or the BSC. When the stored contact cell is C1 in the case shown in Figure 2, the paging message to the MS is transmitted via BTS1. If the MS does not respond to the page within a predetermined period of time, the paging message is transmitted to the MS via BTS2, BTS3, BTS4, BTS5, BTS6 and BTS7 in C2, C3, C4, C5, C6 and C7 indicated in the neighbouring cell information of the contact cell C1. The second stage of the paging can also be carried out by transmitting the paging message both in the contact cell and in its neighbouring cells. Figure 5 shows the paging of the MS in the form of method steps in the primary embodiment. In step 51 , a paging message is transmitted to the MS in the contact cell stored in the network. In step 53, it is monitored whether the MS responds to the transmitted paging message. If the MS does not respond within a predetermined period of time, the paging message is transmitted to the MS in the cells indicated in the neighbouring cell information of the contact cell (step 55).

In the secondary embodiment of the invention, the paging of the MS is carried out in three steps, if required. Figure 6 shows the paging of the MS in the form of method steps in the secondary embodiment. At first in step 61 , a paging message is transmitted to the MS via the base station of the contact cell stored in the network, i.e. via BTS1 in the contact cell C1 shown in Figure 4. In step 62 of Figure 6, it is monitored whether the MS responds to the page. If the MS does not respond within a predetermined period of time, the paging message is transmitted to the MS in the cells C2, C3, C4, C5, C6 and C7 indicated in the neighbouring cell information of the contact cell C1 stored in the network (step 63 of Figure 6). The paging message in the second stage can also be transmitted in the contact cell C1 in addition to the aforementioned cells. In step 65, it is monitored whether the MS responds to the page transmitted in the second stage. If the MS still does not respond, the paging message is transmitted to the MS, as shown in step 67, in all the cells indicated in the neighbouring cell information of the contact area base stations BTS1 , BTS2, BTS3, BTS4, BTS5, BTS6 and BTS7 that are situated outside the contact area, in Figure 4 in C13, C9, C8 and C12 and in other neighbouring cells of the contact area that are not shown in Figure 4. The last stage of the paging can be alternatively carried out by transmitting the paging message in all the cells indicated in the neighbouring cell information, i.e. also in the cells that belong to the contact area.

The mobile station according to the invention requires rather minor changes in the structure of the prior art mobile station. The storing, comparing and message-forming means of the mobile station used in the conventional location management can be modified to carry out the functionality according to the present invention based on the idea of the contact area. The mobile station according to the invention obtains all the data it requires in the location management from the system base station, and therefore the mobile station only has to generate contact area updating messages, The memory of the mobile station only stores the identities of the latest contact area, and therefore the mobile station does not have to maintain data concerning the cellular structure of the network.

The above-described location management method can be used in a cellular radio system either alone or combined suitably with the prior art location updating method. When the location management according to the invention is combined with the method using conventional location areas, it is possible to optimise the location management for users moving in different manners. The drawings and the related description are only intended to illustrate the inventive idea. The details of the location management and paging method, the cellular radio system and the mobile station according to the invention may vary within the scope of the claims. In the cellular radio system according to the invention, the contact data can also be stored in a separate auxiliary register.

Claims

1. A location management method in a cellular radio system comprising base stations (BTS1-BTS8) that transmit their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies, and mobile stations (MS) that receive said base station information for the purpose of monitoring base stations, characterized in that the method comprises the steps of storing the contact area of the mobile station (MS) in connection with each network activity, and updating the contact area data when the mobile station (MS) does not receive any identity data belonging to the contact area on the broadcast frequency of the base station (BTS).

2. A location management method according to claim 1, cha¬ racterized in that in order to update the contact area data, the method comprises the steps of comparing the base station data of the base station information received in the mobile station (MS) to the base station data of the stored contact area, and storing a new contact area when the comparison does not result in any identical base station data.

3. A location management method according to claim 1 or 2, characterized in that in order to store a new contact area, the method comprises the steps of transmitting a contact area updating message from the mobile station (MS), and storing the contact area both in the fixed network and in the mobile station (MS).

4. A location management method according to claim 1, 2 or 3, characterized by storing the identity of the contact cell of the mobile station (MS) as the contact area.

5. A location management method according to claim 1, 2 or 3, characterized by storing the contact cell identity of the mobile station (MS) and the identities of the cells indicated in the neighbouring cell information of the contact cell as the contact area.

6. A location management method according to claim 4 or 5, characterized by storing the base station identity code as the cell identity.

7. A method for paging a mobile station (MS) in a cellular radio system comprising base stations (BTS1-BTS8) that transmit their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies, characterized in that the method comprises the steps of paging the mobile station (MS) via the base stations of the stored contact area, which is stored in connection with the latest network activity, and paging the mobile station (MS) via the base stations indicated in the neighbouring cell information of the stored contact area, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the contact area base stations.

8. A method according to claim 7 for paging a mobile station (MS), characterized in that in order to page the mobile station via the base stations of the stored contact area, the mobile station (MS) is paged via the base station of the stored contact cell.

9. A method according to claim 7 for paging a mobile station (MS), characterized in that the paging of the mobile station via the base stations of the stored contact area comprises the following steps of paging the mobile station (MS) via the base station of the stored contact cell, and paging the mobile station (MS) via the base stations indicated in the stored neighbouring cell information of the contact cell, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the contact cell base station.

10. A method according to claim 7 for paging a mobile station (MS), characterized by paging the mobile station (MS) via the base stations indicated in the neighbouring cell information of the stored contact area, excluding the base stations of the contact area, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the base stations of the contact area.

11. A cellular radio system comprising base stations (BTS1-BTS8) that transmit their own base station identity code and neighbouring cell information as base station information on their own broadcast frequencies, and mobile stations (MS) that receive said base station information for the purpose of monitoring base stations, characterized in that the system is arranged to store the contact area of the mobile station (MS) in connection with each network activity, the system is arranged to update the contact area data when the mobile station (MS) does not receive any identity code belonging to the contact area on the broadcast frequency of the base station (BTS), the system is arranged to transmit a paging message to the mobile station (MS) via the base stations of the contact area, and the system is arranged to transmit a paging message to the mobile station (MS) via the base stations of the cells indicated in the neighbouring cell information of the contact area, if the mobile station does not respond within a predetermined period of time to the paging message transmitted via the base stations of the contact area.

12. A cellular radio system according to claim 11, char¬ acterized in that the contact area comprises the contact cell of the mobile station (MS).

13. A cellular radio system according to claim 11, char¬ acterized in that the contact area comprises the contact cell of the mobile station (MS) and the cells indicated in the neighbouring cell information of the contact cell.

14. A cellular radio system according to claim 11, char¬ acterized in that the system is arranged to transmit a paging message to the mobile station (MS) via the base stations of the cells indicated in the neighbouring cell information of the contact area that are situated outside the contact area.

15. A mobile station arranged to receive a base station identity code and neighbouring cell information transmitted by a base station (BTS) on its broadcast frequency for the purpose of monitoring base stations, charac- t e r i z e d in that the mobile station (MS) is arranged to store the contact area in connection with each network activity, and the mobile station (MS) is arranged to update the contact area data when it does not receive any identity code belonging to the contact area on the broadcast frequency of the base station (BTS).

16. A mobile station according to claim 15, characterized in that in order to update the contact area data, the mobile station (MS) is arranged to compare the base station data of the received base station identity code and the neighbouring cell information to the base station data of the stored contact area, and to register a new contact area when the comparison does not result in any identical base station data.

17. A mobile station according to claim 16, characterized in that in order to register a new contact area, the mobile station (MS) is arranged to transmit a contact area updating message to the base station (BTS), and the mobile station (MS) is arranged to store the new contact area in its memory.