Abstract:
The invention pertains to an improved method and arrangement for cell selection performed by a mobile station in a cellular radio system. Particularly the invention pertains to cell selection in a situation where the mobile station is connected to a special service such as the GPRS (General Packet Radio Service) offered by the current cell. An essential idea of the invention is that a base station sends to the mobile station information about whether the base station of a neighbouring cell offers the same service. This information is sent preferably in a signalling message of a certain service, such as the PSI 3  message of the GPRS system ( 70 ) or in a system information SI message of a lower-level service, e.g. in a SI 3  message. The mobile station changes serving base stations directly, without having to use lower-level system signalling ( 72 ) in between. The invention helps adjust cell reselection such that the load on the mobile station and radio signalling, caused by measurement of possible new cells and by message traffic, is as small as possible.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an improved method and arrangement for cell reselection by a mobile station in a cellular radio system. In particular the invention relates to cell reselection in a situation where the mobile station is attached to a special service such as the General Packet Radio Service (GPRS), for example, offered by the current cell. 
     To provide background for the invention it will be next described by way of example prior-art arrangements for changing the active cell in the GSM (Global System for Mobile telecommunications) system and in a GPRS service used in the GSM. 
     FIG. 1 shows the basic structure of digital cellular systems. Typically, a mobile communications network comprises at least one core network CN and one or more radio access networks RAN. The core networks CN consist of various central systems which may offer various intelligent network services in addition to versatile communications possibilities. A core network comprises mobile services switching centers MSC, other network elements (which in the GSM include the serving GPRS support node, SGSN, and gateway GPRS support node, GGSN, for example), and the associated transmission systems. The radio access networks are located between the core network and mobile stations. A radio access network comprises base transceiver stations BTS and a radio network controller RNC. Each base station BS has a fixed connection to the radio network controller RNC. The radio network controllers in turn have fixed connections to at least one core network node. Between the mobile stations MS and core networks CN there may operate one or more radio access networks, and through a given radio access network a mobile station MS can be connected with several core networks CN. 
     FIG. 2 shows the coverage areas of services offered by a cellular system. Area  20  shows the coverage of basic GSM services, and areas  21 ,  28  and  29  are GPRS coverage areas within area  20 . Typically, area  21  may be the center of a city, area  28  may be e.g. an airport, and area  29  may be a research and development or other office complex where GPRS service is needed. 
     FIG. 3 shows in more detail the GPRS base stations  23 ,  24 ,  25  and  26  and their coverage areas  23   a ,  24   a ,  25   a  and  26   a  in a city center  21 . FIG. 3 does not show the basic-service GSM base stations the coverage areas of which are within area  21 . FIG. 3 shows that even an area that offers GPRS services may have locations, here location  27 , which are not within the coverage area of any GPRS base station. In such a location a mobile station has to connect to a base station which only offers the basic GSM service. In addition, it can be seen from FIG. 3 that when a mobile station is moving in an area where the base station coverage areas are small, the serving base station has to be changed at short intervals. 
     A mobile station in a cellular radio system always tries to choose a base station coverage area to camp on such that the quality of the radio connection is adequate. Traditionally, the cell selection has been based on the measurement of the received radio signal level either at the mobile station or at the base station For example, in the GSM system each base station transmits a signal on a so-called broadcast control channel BCCH which has different frequencies at adjacent base stations. Mobile stations measure the strengths of the BCCH signals they receive and decide on the basis of the measurements which cell is the most advantageous from the radio connection quality perspective. Base stations also inform mobile stations about the BCCH frequencies used in the neighbouring cells so that the mobile stations will know what frequencies they have to listen to in order to find the BCCH transmissions of the neighbouring cells. In each cell the BCCH channel transmission also includes information about how the mobile stations can make so-called random access requests in that particular cell in order to set up a connection. 
     FIG. 4 shows the GSM broadcast frequency channels in a so-called  51  multiframe. In addition to the BCCH, there are included the frequency correction channel FCCH, synchronization channel SCH, and the common control channel CCCH. 
     FIGS. 5 a ,  5   b  and  5   c  depict the GPRS  51  and  52  multiframe structures. In the GPRS broadcast channels the GSM BCCH and CCCH channels are replaced by their GPRS counterparts, namely the PBCCH and PCCCH channels. In the  51  multiframe structure, FIG. 5 a , the FCCH and SCH channels are illustrated, even though they are not specified in the current GPRS specification. In the GPRS  52  multiframe structure no FCCH or SCH channels not shown, due to the fact that they are not specified in the  52  multiframe structure of the current specification. In the  52  multiframe structure shown in FIG. 5 b  there is PBCCH (PSI) only in one radio block per multiframe, while in the  52  multiframe structure shown in FIG. 5 c  there is PBCCH (PSI) information in four radio blocks per multiframe. Even though  51  multiframe structure is used as an example in this text, the ideas illustrated here are also valid for the  52  multiframe structures. This is because of the fact that there are no essential differences between the two frame structures In the GPRS system, the list of the base stations&#39; neighbour cells is sent in a so-called Packet Information type  3  (PSI 3 ) and Packet Information type 3bis (PSI3bis) messages transmitted on the PBCCH channel. 
     In the GSM system, each base station includes in its BCCH signal the data required for connection requests. In addition, each base station may regularly transmit at the ECCH frequency so-called system information (SI) messages. These messages contain information about the ability of the base station to provide advanced services, especially GPRS network service. 
     If a mobile station camped on a cell that offers basic GSM service needs to change to another cell, it listens to the neighbouring cells&#39; BCCH channels and receives SI messages from these. On behalf of this information the mobile station can determine whether a cell supports GPRS service or not, and if it does, the mobile station can start operating on the GPRS channel immediately. For example, an idling mobile station can at regular intervals update its information about the neighbouring cells&#39; characteristics by receiving SI messages so that the information can be quickly retrieved from the mobile station&#39;s memory at the moment of possible cell reselection. 
     FIG. 6 shows the prior-art process that takes place when a mobile station already in the GPRS service needs to change serving cell. The mobile station has already received a neighbour cell list from the serving base station. This list is used in order to continuously examine whether or not there is a need to change the serving cell, phase  60 . If the mobile station finds that cell reselection is necessary, it tunes to the neighbour cell and starts receiving the system information of the BCCH of the chosen cell, phase  61 . On the basis of these data the mobile station makes a preliminary selection of the cell and examines that the cell can offer a minimum of necessary service, phase  62 . When the mobile station has chosen a suitable cell to get service from, the mobile station starts receiving the necessary SI or PSI from this cell in order to obtain GPRS service, phase  63 . The chosen cell may or may not offer GPRS service. 
     A disadvantage of the method described above is that it is not sensible, as regards power consumption and communications resources, for the mobile station, while camped on a GPRS cell, to receive basic-GSM SI messages, but in practice the mobile station starts receiving SI messages only after the cell reselection procedure has started. Thus the mobile station does not keep an up-to-date list of the neighbour cell characteristics. The arrangement described above puts a load on the mobile station connected with the GPRS service because the mobile station camped on a cell first has to read from the signal sent by the base station of the cell a list of the other BCCH frequencies and then receive, demodulate and decode a short period of the BCCH transmission arriving at each of the frequencies mentioned in the list in order to find out whether the neighbour cells can provide GPRS network service. It may be that a major part of the neighbour cells do not support the GPRS, in which case the mobile station only wastes time and electric power in receiving, demodulating and decoding the BCCH channels of such cells. The reception of said GSM SI messages may take nearly 10 seconds, which usually is too long a delay in cell reselection, especially if the mobile station is moving fast. Consequently, the mobile station uses more power, data communications slows down and the connection with the base station may be temporarily cut off. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a method and arrangement in which the use of communications resources to find a new suitable cell is more efficient than in prior-art methods and arrangements. 
     An essential idea in the invention is that the serving base station sends to the mobile station the information about whether the base station in a neighbour cell provides a given service. This information is preferably sent in a signalling message on a channel of a given service, such as GPRS, to which the mobile station is connected. The cell reselection is then preferably performed directly at the level of the determined service. 
     The “determined service” means here most preferably a service other than the lowest, basic service level of the communication system. 
     For example, when the mobile station is operating on GPRS packet channels, the information about whether a neighbour cell&#39;s base station is offering GPRS or not, is advantageously sent in a PSI 3  message on a GPRS packet channel. At its simplest, the invention can be realized in the GPRS system by adding to the packet channel&#39;s PSI 3  message a one-bit information element for each neighbour cell which indicates whether the neighbour cell in question supports the GPRS service. If the mobile station is not connected to a special service such as the GPRS, the information can be sent in the basic system&#39;s information message, such as SI 3 . 
     The invention makes it possible for a mobile station connected to a given service to perform a direct change between the serving base station and another base station that offers the same service, without having to resort to lower-level system signalling. So, a mobile station connected to GPRS, for example, need not receive system information messages transmitted by the neighbour cells on the basic GSM channels in conjunction with cell reselection but it can select the new GPRS-supporting cell directly at GPRS level. 
     Cell reselection in mobile stations can be made more efficient than in prior-art systems if the mobile stations have prior knowledge of the service levels offered by the base stations. In the arrangement according to the invention a base station sends information about the service levels of the neighbouring base stations in signalling messages of the current service so that in order to find a new cell the mobile station receives, demodulates and decodes only the transmissions of those neighbouring base stations which transmit a strong enough signal and which according to the information sent by the current base station can offer the current service level. The mobile station may maintain various lists of such neighbouring base stations into whose cells it may move in the near future. 
     So, the invention helps adjust cell reselection such that the load on the mobile station and radio signalling, caused by measurement of possible new cells and reception of messages, is as small as possible. 
     In accordance with the invention a mobile station can as quickly as possible select from among available cells a new cell which best meets the communications needs of the mobile station. Cell reselection may be performed while the mobile station is in the so-called idle state, in which there is no active data connection between the mobile station and a base station, or it may be performed during an active data connection. 
     The method according to the invention for selecting a new serving cell in a cellular radio system where a determined service level is required in the new cell, the method including a phase of transmitting information about neighbouring cells from a base station of a cellular radio system to a mobile station, wherein the base station generates a message containing information about the neighbouring cells and sends the message to the mobile station, is characterized in that the base station includes in said message information about whether the neighbour cell mentioned in the message offers the determined service to mobile stations and the cell reselection is performed directly at the level of the determined service. The method according to the invention for realizing cell reselection in a cellular radio system which comprises base stations with associated cells as well as mobile stations, is characterized in that it comprises phases in which 
     a) a mobile station links up with a service offered by the system, 
     b) a base station sends to the mobile station a signalling message in accordance with a determined service, containing information about neighbour cells located near to the base station as well as information about whether said neighbour cells provide a determined service, 
     c) on the basis of said message the mobile station preliminarily selects a group of new cells offering a determined service, 
     d) the mobile station performs measurements among the preliminarily selected cells in order to find a suitable new cell, whereby 
     if the measurements indicate that among the preliminarily selected cells there is at least one suitable new cell, the mobile station selects a new cell from among the preliminarily selected cells, and 
     if the measurements indicate that among the preliminarily selected cells there is no suitable new cell, the mobile station selects a new cell from among cells other than the preliminarily selected cells, and 
     e) the cell reselection is performed directly at the level of the determined service. 
     The invention is also directed to a method for transmitting information about neighbouring cells from a base station of a cellular radio system to a mobile station, wherein the base station generates a message containing information about the neighbouring cells and sends the message to the mobile station, and which is characterized in that the base station includes in said message information about the service level offered to mobile stations by the neighbour cell mentioned in the message and that said signalling message includes at least one of the following pieces of information: 
     whether or not the neighbour cell is in blocking state, 
     length of the multiframe used by the neighbour cell, 
     location of control channel synchronization information. 
     The invention is also directed to a base station and mobile station of a cellular radio system. 
     The base station according to the invention, comprising means for offering a determined service and means for generating signalling messages and means for transmitting them to mobile stations, is characterized in that 
     it contains information about a group of neighbour cells located in the vicinity of the base station, including information about whether a neighbour cell provides said determined service, 
     it is equipped so as to include said information in a signalling message, and 
     it is equipped so as to the cell selection is perform the cell reselection directly at the level of the determined service. 
     The cellular mobile station according to the invention, equipped with means for connecting to a determined service, and comprising means for receiving signalling messages from base stations as well as means for performing cell-specific measurements to find a suitable cell for the purpose of cell reselection, is characterized in that it is equipped so as to indicate information about the service level of a neighbour cell based on signalling messages transmitted by the current serving base station, and to perform cell reselection on the basis of said information directly at the level of the determined service. 
     The invention is further directed to a cellular radio system, which comprises base stations and, in association with them, cells, as well as mobile stations and in which 
     the base stations are equipped so as to generate messages and send them to mobile stations in the form of signalling messages, and 
     the mobile stations are equipped so as to operate at a determined service level and receive signalling messages from base stations, and which is characterized in that it contains information accessible by a base station about a group of neighbour cells in the vicinity of the cell of the base station, including information about the service level offered in and by said neighbour cells to mobile stations, the system in question being arranged so as to transmit said information from the base station to a mobile station in a signalling message for the purpose of selecting a new serving cell to perform the cell reselection directly at the level of the determined service. 
     Preferred embodiments of the invention are described in the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described in more detail with reference to the preferred embodiments presented by way of example and to the accompanying drawings wherein 
     FIG. 1 shows a cellular radio system according to the prior art, 
     FIG. 2 shows coverage areas in a cellular radio system, 
     FIG. 3 shows cells in an area offering a certain service, 
     FIG. 4 shows a GSM  51  multiframe, 
     FIG. 5 a  shows a GPRS  51  multiframe, 
     FIG. 5 b  shows a GPRS  52  multiframe with PBCCH in one radio block, 
     FIG. 5 c  shows a GPRS  52  multiframe with PBCCH in four radio blocks, 
     FIG. 6 shows a flowchart of a prior-art method for changing serving cells, 
     FIG. 7 shows a flowchart of a method according to the invention for chancing serving cells, and 
     FIG. 8 shows a cellular mobile station according to the invention and its connection to a cellular radio system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Above in conjunction with the description of the prior art reference was made to FIGS. 1 to  6 , so below in the description of the invention and its preferred embodiments reference will be made mainly to FIGS. 7 and 8. 
     FIG. 7 shows a flowchart of a method according to the invention for changing the serving cell. A mobile station finds out if there is need to change the serving cell and receives PSI 3  messages from the packet broadcast control channel PBCCH or packet associated control channel PACCH of the current base station in phase  70 . A PSI 3  message contains a list of neighbour cells and information about whether each particular cell supports the current service level, such as the GPRS network service. The information about whether a neighbouring base station supports the current GPRS service is transmitted in the PSI 3  message preferably in the form of a one-bit field per each neighbouring base station. In that case, value 0 for the bit could mean that the base station does not support the GPRS service and value 1 could mean that the base station supports the GPRS service. The information may also be transmitted in a lower service level system information (SI) message, say, in a SI 3  message, especially if the mobile station is not connected to the GPRS service. 
     Next, the mobile station may preliminarily select the cells that support the current service, phase  71 . The selection of the group of cells in phase  71  may also be based on that the mobile station tries to find a neighbour cell which, according to a PSI message sent by the current base station, belongs to the same location area with the current base station or to the home location area of the mobile station or to a private picocell network, in which the operation is more advantageous to the user than on average. The invention does not limit on what grounds the cells are discriminated in phase  71 , in addition to the information about whether a particular neighbour base station supports the current service level, in this case the GPRS service. 
     Then the mobile station selects a new cell from among the preliminarily selected cells on the basis of channel measurement data, among other things, in a known manner by measuring the power level of the signal received from the current base station and/or the error rate of the demodulated and decoded signal, and attempts to make the selected cell the new serving cell, phase  72 . 
     So, in phase  72  the mobile station tries to find a new cell from among the neighbour cells which in phase  71  were found suitable on the basis of their service level and possible other criteria. As a prerequisite for the mobile station to change cells, the new cell has to meet known suitability criteria, which e.g. in the GSM system mean high enough values of known cell-specific C 1  and C 2  parameters. The suitability criteria and other known cell selection functions in the GSM and DCS 1800  systems are specified in European Broadcasting Union&#39;s (EBU) and European Telecommunications Standards Institute&#39;s (ETSI) standards ETS 300 535 (GSM 03.22) and ETS 300 578 (GSM 05.08). On a general level, the same approach can be applied to other digital cellular radio systems as well. 
     In the above-mentioned phases the mobile station need not start receiving lower service level signaling channels. If, however, it is not possible to select a cell offering the current service level, the mobile station starts in phase  73  looking for a suitable cell from among the cells discriminated in phase  71 . If the mobile station has time, it can look for a suitable cell from among the discriminated cells, in accordance with phase  73 , even if a suitable cell with a “better level” had been found in phase  72 . By looping continuously or regularly the loop formed by states  70 ,  71 ,  72  and  73  the mobile station can continuously maintain a list that indicates the best possible new cells. 
     A base station can advantageously include in PSI 3  messages information primarily about those neighbour cells that support the same service as the base station itself. If it is not necessary to include in every message the information of all neighbour cells, the length of the messages can be made shorter if desired. 
     Below it is described in more detail a possible PSI 3  message structure as information elements: 
     &lt;PSI 3  Message Content&gt;::= 
     &lt;MESSAGE_TYPE: bit ( 6 )&gt; 
     &lt;PSI 3 _BIS_COUNT: bit ( 3 )&gt; 
     {L|H&lt;Global TFI: Global TFI IE&gt;} 
     &lt;Serving Cell parameters: Serving Cell parameters struct&gt; 
     &lt;General Cell Selection parameters: Gen Cell Sel Struct&gt; 
     &lt;Neighbour Cell parameters: Neighbour Cell parameters struct&gt; 
     &lt;spare padding&gt; 
     &lt;Serving Cell Params struct&gt;::= 
     &lt;RA_CODE: bit ( 8 )&gt; 
     &lt;GPRS_RXLEV_ACCESS_MIN: bit ( 6 )&gt; 
     &lt;GPRS_MS_TXPWR_MAX_CCH: bit ( 5 )&gt; 
     { 0 | 1 &lt;HCS Serving Cell parameters: HCS struct&gt;} 
     &lt;HCS struct&gt;::= 
     &lt;GPRS_PRIORITY_CLASS: bit ( 3 )&gt; 
     &lt;GPRS_HCS THR_: bit ( 5 )&gt; 
     &lt;Gen Cell Sel struct&gt;::= 
     &lt;GPRS_CELL_RESELECT_HYSTERESIS: bit ( 3 )&gt; 
     &lt;C 31 _HYST: bit ( 1 )&gt; 
     { 0 | 1 &lt;RA_RESELECT_HYSTERESIS: bit ( 3 )&gt;} 
     &lt;Neighbour Cell params struct&gt;::= 
     { 1 &lt;START_FREQUENCY: bit ( 0 )&gt; 
     &lt;Cell selection params: Cell selection struct&gt; 
     &lt;NR OF_REMAINING_CELLS: bit ( 4 )&gt; 
     &lt;FREQ_DIFF_LENGTH: bit ( 3 )&gt; 
     {&lt;FREQUENCY_DIFF: bit (n)&gt; 
     &lt;Cell selection params: Cell selection struct&gt;}*}* 0 ; 
     &lt;Cell selection struct&gt;::= 
     &lt;BSIC: bit ( 6 )&gt; 
     &lt;SAME_RA_AS_SERVING_CELL: bit ( 1 )&gt; 
     &lt;Cell Reselection parameters: Cell reselection params struct&gt; 
     { 0 | 1 &lt;GPRS_RXLEV_ACCESS_MIN: bit ( 6 )&gt;&lt;GPRS_MS_TXPWR_MAX_CCH: bit ( 5 )&gt;} 
     { 0 | 1 &lt;GPRS_TEMPORARY_OFFSET: bit ( 3 )&gt;&lt;GPRS_PENALTY_TIME: bit ( 5 )&gt;} 
     { 0 | 1 &lt;GPRS_RXLEV_ACCESS_MIN: bit ( 6 )&gt;} 
     { 0 | 1 &lt;HCS params: HCS struct&gt;} 
     The information according to the invention about the ability of a neighbour cell to support the GPRS network service is advantageously coded in the &lt;Cell Reselection parameters: Cell reselection params struct&gt;field which according to the invention is added to the PSI 3  message in this case. The contents of this field are e.g. as follows: 
     &lt;Cell reselection params struct&gt;::= 
     &lt;GPRS_SUPPORTING_CELL: bit ( 1 )&gt; 
     &lt;CELL_BARR_INDICATION: bit ( 1 )&gt; 
     &lt; 51 _MF_FRAME_STRUCTURE: bit ( 1 )&gt; 
     { 0 | 1 &lt;ARFCN: bit ( 10 )&gt;} 
     The first of these bits, i.e. &lt;GPRS_SUPPORTING_CELL: bit ( 1 )&gt;indicates whether the cell supports the GPRS service (value=1) or not (value=0). The second bit, i.e. &lt;CELL_BARR_INDICATION: bit ( 1 )&gt;indicates whether the cell is in the blocking state ( 1 ) or not ( 0 ). Blocking state means that the cell is blocked for use i.e. it is barred. The last bit, i.e. &lt; 51 _MF_FRAME_STRUCTURE: bit ( 1 )&gt;indicates whether the GPRS service uses the  51  multiframe ( 0 ) or the  52  multiframe 
     The optional 10-bit-long { 0 | 1 &lt;ARFCN: bit ( 10 )&gt;} element may indicate where the control channel synchronization data can be found in case the FCCH and SCH channels are not in use. 
     Apart from the &lt;Cell Reselection parameters: Cell reselection params struct&gt; field the PSI 3  message described above is in accordance with the prior art. The described cell selection parameters can naturally be included in other System Information messages than described, such as SI 3 , SI 4 , SI 7 , SI 8  or SI 13 . Also the transmission of the concerned System Infonnation message may depend on whether GPRS is supported or not. Also the names of the bit fields may naturally be different from the ones described. 
     Let us next examine in more detail, how the cell reselection can be accomplished directly at the level of the GPRS service. 
     As described above, the information on whether a neighbouring cell supports a GPRS can be included in PSI 3  or PSI 3 bis message of the serving cell. This way the mobile station receives information—already before finishing the the actual cell reselection—whether the possible new cell supports supports GPRS or not. The mobile station utilize this information in the cell reselection process. In case 
     PBCCH is mapped on a non-hopping carrier, an ARFNC field (as described above) can be added. in the message. 
     In a case (I), for example, the new cell supports GPRS and the PBCCH is mapped on a non-hopping carrier, and the mobile station can tune directly to the PBCCH of the new cell without a need to read the BCCH information of the new cell. 
     Also in the case (II) where the new cell supports GPRS, and the PBCCH is mapped in a on a non-hopping carrier, and the mobile station does not receive the ARFCN information on the serving carrier, the mobile station can find this information from the SI 13  message of the new cell. 
     On the other hand, in a case (III) where the new cell supports GPRS and the PBCCH is mapped on a hopping carrier (PBCCH), the MS needs additional information from the new cell in order to describe the hopping PBCCH. This information can be received from the SI 13  information element of the new cell. 
     In cases where the mobile station already has information that the new cell supports GPRS (eg. GPRS_SUPPORTING_CELL bit) the mobile station may, if necessary, tune to the new cell and read the SI 13  information element which includes information on the location of the PBCCH of the new cell. This can be performed already before the cell reselection is finished. This reading of SI 13  can be performed while the mobile station is still operating on the current serving cell. By doing this the mobile station can receive information on PBCCH of the new cells and act according to this information before actually tuning to the new cell (cases II and III above). This procedure will remove the necessity for the mobile station to read the whole set of SI messages on the BCCH of the new cell during the cell reselection, thereby reducing the actual cell reselection time. 
     FIG. 8 shows a simplified block diagram of a mobile station  800  according to the invention and its connection to a cellular system. The mobile station comprises an antenna  801  for receiving radio-frequency (RF) signals transmitted by base stations. A received RF signal is directed by a switch  802  to a RF receiver  811  where the signal is amplified and converted digital. The signal is then detected and demodulated in block  812 . Block  813  performs decryption and deinterleaving. Then follows signal processing in block  830 . Received data may be stored as such in the mobile station&#39;s memory  804  or, alternatively, the processed packet data are taken after the signal processing to a possibly external device such as a computer. A control unit controls the above-mentioned reception blocks in accordance with a program stored in the unit. 
     Transmission from the mobile station is performed e.g. as follows. Controlled by the control block  803 , block  833  performs possible signal processing on the data and block  821  performs interleaving and encryption on the processed signal to be transmitted. The encoded data are arranged in bursts, block  822 , which are modulated and amplified into a RF signal to be transmitted, block  823 . The RF signal to be transmitted is lead to an antenna  801  by means of a switch  802 . The processing and transmission functions described above are controlled by the control unit  803 . 
     In the mobile station of FIG. 8, the components that are essential from the invention&#39;s perspective include the known reception blocks  811 - 813 , by means of which the mobile station receives, demodulates and decodes the messages transmitted by base stations and measures the levels of the received signals, as well as the control block  803  which processes the information contained in the messages and controls the operation of the mobile station. Part of the memory  804  of the mobile station has to be allocated to information regarding neighbour cell selection, said information including data received from base stations about the neighbour cells&#39; PBCCH frequencies and service levels. Naturally, a detachable memory medium, such as the known SIM card, may also be used for said purpose. The mobile station uses the transmission blocks  821 - 823  to transmit the messages related to cell selection to base stations. 
     Compared to the prior art the invention&#39;s demands on the base stations&#39; and mobile stations&#39; hardware are quite low. A base station  851  and/or base station controller  852  has access to a database (not shown) which includes information about the neighbour cells&#39; PBCCH frequencies and service levels, such as the ability to offer GPRS service. An operator usually has operation and maintenance (OM) capabilities for altering the contents of these databases statically (e.g. when a new base station starts operation in the vicinity of existing base stations) or dynamically (e.g. when a nearby base station is driven down for service). Prior-art base stations generate and transmit many kinds of messages, so the creation and transmission of messages concerning the information in the database can be realized by means  851 ,  852  according to the prior art. 
     The embodiments described above are naturally exemplary only and do not limit the application of the invention. Especially it should be noted that even though the above examples pertain to GSM and GPRS systems, the invention can be applied to any other digital cellular system as well. Such systems include especially the Digital Communications System at 1800 MHz (DCS1800), Interim Standard 54 (IS-54), and the Personal Digital Cellular (PDC) system. Particularly in forthcoming so-called third-generation digital cellular systems the service levels offered by the cells to the mobile stations may considerably differ from a cell to another. Proposed third-generation systems include the Universal Mobile Telecommunications System (UMTS) and the Future Public Land Mobile Telecommunications System/International Mobile Telecommunications at 2000 MHz (FPLMTS/IMT-2000). In these proposed systems the cells are divided according to their size and characteristics into different-sized cells, and the transmission rate can be used as an example of the service level. Picocells have the highest transmission rate and macrocells the lowest. The cells may partly or totally overlap and there may be different mobile stations, so that all mobile stations may not be able to use the service levels of all base stations. Thus the invention can be applied to transmit various data about different services supported by neighbour base stations in the form of signalling messages of the current service.