CELL SELECTION IN A RADIO NETWORK

Abstract: A method for selecting a cell for a mobile station (MS) in a cellular telecommunications network, wherein the mobile station (MS) sends a radio station controller (BSC) measurement reports (6-4) comprising information (61, 631 ... 64N) about the mobile station's serving cell and its neighbour cells. The mobile station or its subscriber is associated with at least one set of special cells called LSA cells. Information on the set of special cells is transferred to the radio station controller (BSC), and the mobile station (MS) at least participates in the transferring of information, preferably by sending an explicit list of its LSA cells or by including in at least one measurement report (6-4) a flag bit (62, 641 ... 64N) indicating whether or not the cell in question is a special cell.

Description: CELL SELECTION IN A RADIO NETWORK BACKGROUND OF THE INVENTION The invention relates to cell selection in a mobile telecommunications network which comprises a packet radio network. An example of such networks is a combined GSM/GPRS network. A mobile station (MS) in a cellular system, when switched on, selects the most suitable cell. Later, when the MS is in idle mode (i.e. does not have a call in progress) and it is camped normally on a cell of the selected PLMN, it monitors its neighbouring cells for a possible handover in case one of the neighbouring cells provides better service. This process is called cell re- selection. The GSM/GPRS cell reselection chapters are based on ETSI recommendations GSM 03.22, GSM 03.60, GSM 03.64 and GSM 05.08. These recommendations are incorporated herein by reference, but for convenience, the parts of those documents which are essential for understanding the inven- tion are repeated in this application. Cell reselection in GSM A cell reselection can be triggered if a path loss criterion parameter C1 indicates that the path loss to the cell has become too high; or the current serving cell has become barred; or there is a better cell in terms of the cell re- selection parameter C2 in the same location area (LA), or a much better cell in another LA of the selected PLMN. There are other reasons for a cell reselection (such as a downlink signalling failure, or a random access attempt fails after a certain number of repetitions) but they are not essential for understanding the invention. The MS will reselect a new cell in order to achieve the following conditions which are commonly referred to as a "normal service state": a) The registered PLMN is the selected PLMN; b) The MS is camped on a suitable cell of the LA on which the last successful LA update was performed, and that cell has the highest value of C2 for any unbarred cell in that LA; c) The value of C2 for the MS is not lower than the value of C2 for any suitable cell in another LA of the selected PLMN by more than the hysteresis value CRH (Cell Reselect Hysteresis). There may be temporary conditions under which not all these are fully satisfied, e.g., during cell selection in a PLMN or while in the process of reselecting another cell. However, if the above cannot be satisfied for a certain period of time, the MS will normally enter a limited service state in which it will try to camp on an acceptable cell. In this state, only emergency calls are possible. Parameters C1 and C2 are used to ensure that the MS is camped on the cell with which it has the highest probability of successful communication on uplink and downlink. The MS uses C1 to determine whether a cell is suitable to camp on. C1 depends on 4 parameters: i) RLA (Received Level Average) is the received signal level suitably averaged; ii) RXLev_Access_Min which is broadcast on the BCCH, and is related to the minimum signal that the operator wants the network to receive when being initially accessed by an MS; iii) MS_TXPWR_MAX_CCH which is also broadcast on the BCCH, and is the maximum power that an MS may use when initially accessing the network; and iv) P which is the maximum power of the MS. Use of the parameter C1 enables the MS to determine whether communication is possible with the network in the absence of interference. However, an additional safeguard against interference is used. This is to monitor the success rate of the MS in decoding signalling blocks of the paging subchannel. If there is a downlink signalling failure, i.e. the success rate drops too low, this indicates probable interference on the downlink, and the MS at- tempts to find another suitable cell. The path loss criterion parameter C1 used for cell selection and reselection is defined by: C1 = (A - Max(B.O)), where A = RLA - RXLev_Access_Min B = MS_TXPWR_MAX_CCH - P All values are expressed in dBm. The path loss criterion is satisfied if C1 > 0. In order to optimize cell reselection, additional cell reselection parameters can be broadcast on the BCCH of each cell. The cell reselection process employs a parameter C2 that depends on these parameters. The pa- rameters used to calculate C2 are as follows: Cell_Reselect_Offset which applies an offset to the C2 reselection criterion for that cell. Penalty Time: When the MS places the cell on the list of the strongest carriers, it starts a timer, which expires after the Penalty_Time. This timer will be reset when the cell is taken off the list. For the duration of this timer, C2 is given a negative offset (Temporary_Offset). This will tend to pre- vent fast moving MSs from selecting the cell. The all-ones bit pattern on the Penalty_Time parameter is reserved to change the sign of Cell_Reselect__Offset and the value of Temporary_Offset is ignored as indicated by the equation defining C2. Temporary_Offset which applies a negative offset to C2 for the duration of Penalty_Time after the timer T has started for that cell. An infinite value can be applied, but a number of finite values are also possible. Timer T is implemented for each cell in the list of strongest carriers. T is started from zero at the time the cell is placed by the MS on the list of strongest carriers, except when the previous serving cell is placed on the list of strongest carriers at cell reselection. In this case, T is set to the value of Penalty " Time (i.e. expired). The reselection criterion C2 is used for cell reselection only and is defined by: C2=C1+CelLReselect_Offset-Temporary_Offset*H(Penalty_Time-T) for Penalty_Time<>11111 C2=C1-Cell_Reselect_OffeetforPenalty_Time=11111 for non-serving cells: H(x) = 0 for x < 0; and H(x) = 1 for x >= 0; and for serving cells: H(x) = 0 Cell reselection in GPRS A major difference between GSM and GPRS is that, in GSM, the network makes the decisions concerning handovers. In GPRS, the concept of handover is not used, and usually, the mobile station makes the decisions concerning cell reselection. In order to receive GPRS service, a cell reselection may be required. When the MS is in GPRS Idle state and it wishes to initiate a GPRS attach procedure, the following applies. If the currently camped-on cell supports GPRS then no cell reselection is required. If the currently camped-on cell does not support GPRS, then reselection of a cell supporting GPRS is required before execution of the attach procedure. If, however, the MS is in GPRS Standby or Ready state, cell selection and reselection procedures spe- cific to GPRS are used (GPRS mode). Instead of the parameters C1 and C2, a GPRS MS applies the corresponding GPRS parameters C31 and C32, if provided. If the MS is in active mode, then the change from one cell to another is performed according to the GSM handover procedures. In GPRS Standby and Ready states, the MS performs cell reselec- tion, except for a class A MS while in a circuit switched connection, in which case the cell is determined by the network according to the GSM handover procedures. The GPRS mode cell reselection procedures apply to the MSs attached to GPRS if a PBCCH (Packet Broadcast Control Channel) exists in the serving cell. Otherwise the MS shall perform cell reselection according to the GSM idle mode procedures. Optionally, the network may control the cell selection. The cell reselection criteria C31 and C32 are provided as a complement to the current GSM cell reselection criteria. This provides a more general tool to make cell planning in GPRS as similar as possible to existing cell planning in GSM. The GPRS-specific cell reselection algorithm is only applica- ble if the PBCCH is allocated. If the PBCCH is not allocated, the MS shall perform cell reselection according to the C2 criteria. The following cell reselection steps shall be followed, wherein (s) and (n) denote a serving cell and a neighbour cell respectively: 1) Path loss criterion (C1). The path loss criterion C1 >= 0 is used as a minimum signal strength criterion for cell selection for GPRS in the same way as for GSM in Idle mode. 2) Signal strength threshold criterion (C31) for hierarchical cell structures (HCS). 3) Cell ranking (C32). 4) Cell reselection rules. The MS selects the cell having the highest C32 value among those having the highest priority class and fulfilling the criterion C31 >= 0. The prior- ity classes may correspond to different HCS layers. They may also be used for other purposes. If no cells fulfil the criterion C31 >= 0, the MS selects the cell having the highest C32 value among all cells. The HCS signal strength threshold criterion (C31) is used to decide whether the cell is qualified for prioritized hierarchical cell reselection. C31 (s) = RLA(s) - HCS JTHR(s) >= 0, (serving cell) C31(n) = RLA(n) - HCSJΗR(n) >= 0, (neighbour cell) where HCS_THR is the signal threshold for applying HCS reselection. The cell ranking criterion (C32) is used to select cells among those with the same priority: C32(s) = C1 (s) + GPRS_Reselect_Offset(s), (serving cell) C32(n) = C1(n) + GPRS_Reselect_Offset(n) - Temporary_Offset(n) * H(Penalty_Time(n) - T(n)), (neighbour cell) where GPRS_Reselect_Offset applies an offset and hysteresis value to each cell; H(x) = 0 for x < 0; and H(x) = 1 for x >= 0 Network Controlled Cell Reselection It is possible to order an MS in Ready state to send a measurement report to the network. The measurement report is a regular packet transmission addressed to the proper network entity. It is also possible for the network to order an individual MS in Ready state to perform cell reselection to a cell appointed by the network, possibly in combination with a penalty parameter to keep the MS from returning to the original cell immediately. A network induced cell reselection temporarily overrides the MS originated cell reselection. The degree to which the mobile station shall resign its radio network control is controllable, and is specified in detail by the parameter Net- work_Control__Order. The following actions are possible to order to the mobile stations. NCO: normal MS control (the MS performs autonomous cell reselection); NC1 : MS control with measurement reports (the MS sends measurement reports to the network according to additional information in the message NC1. It continues its normal cell reselection. NC2: Network control (the MS sends measurement reports to the network according to additional information in the message NC2. It performs no cell reselection on its own, and can only make a cell reselection according to a cell reselection command received from the network. Two sets of parameters are broadcast on PBCCH and are valid in Transfer and Wait states respectively for all mobile stations in the cell. Net- work_Control_Order can also be sent individually to an MS on PACCH, in which case it overrides the broadcast parameter. The measurement reports are sent individually from each MS as RLC transmissions, addressed to the RR management in a BSS. The measurement reporting is maintained in the MS RR states Wait with Cell Update and Transfer and in the BSS RR states Measurement Report Reception and Transfer. The cell reselection commands include: 1) target cell channel de- scription (GSM 04.08); 2) penalty to prevent immediate return (3 bits); and 3) penalty timer (5 bits). There may appear exceptional situations where the network controlled cell reselection procedures (NC1 or NC2 modes of operation) should not be used: 1. When a class A mobile station is simultaneously involved in a circuit switched service and in a GPRS transfer. In this case, a change to circuit switched service has precedence over GPRS network controlled cell reselection. In order to avoid reporting measurements twice by the MS (once for GPRS, once for the circuit switched service), thus leading to inefficient usage of the radio bandwidth, network controlled cell reselection should not be acti- vated in this case. 2. When an MS is performing a process called Anonymous Access, cell reselection implying a change of routing area results in the MS returning to the GPRS MM Idle state. Therefore, there might be cases where the network controlled cell reselection would result in a failure of the Anonymous Access. Since the BSS is not able to detect such situations, the MS must inform the BSS when such a situation is encountered, so that the BSS can select a more appropriate mode of operation. This coordination procedure is as follows. When an MS in Transfer or Wait state has been asked by the network to behave according to the NC1 or NC2 modes of operation and the MS encounters one of the above-mentioned situations, it continues reporting measurements and it will not obey the network controlled cell reselection commands, if received. As soon as an uplink transmission opportunity occurs, the MS informs the network that the requested mode of operation should be suspended. It is up to the network to decide whether to maintain or to change the mode of operation. The network replies with the value to be used. The MS adapts its behaviour accordingly. If no answer is received from the network, the MS assumes no change and continues to inform the network, as described above, until an answer is received. When the situation which triggered the above-mentioned actions ends, and provided that the MS is still in Transfer or Wait state, as soon as an uplink transmission opportunity occurs, the MS informs the network that the initial mode of operation (NC1 or NC2) may resume. It is up to the network to decide whether to maintain or to change the mode of operation. The network replies with the value to be used. The MS adapts its behaviour accordingly. If no answer is received from the network, the MS assumes no change (i.e. it continues to apply the last value received from the network) and it continues to inform the network, as described above, until an answer is received. Localized service areas (LSA) In cellular mobile communications systems, a mobile station may roam freely within the area of the mobile communications network and connect to the base transceiver station signal received best at a given time. Usually, all base transceiver stations provide substantially similar services for the mobile stations in a network. Some base transceiver stations can, however, be defined to provide a certain special service for all mobile stations of the network, e.g. call charges below the normal tariff. The base transceiver station broad- casts a message on such a special service on its broadcast control channel (BCCH), whereby mobile subscribers in the area note that they are within a special service area of the network and may take advantage of this service. Within the context of this application, such special service areas are referred to as localized service areas (LSA) and the support of LSA is called SoLSA. A subscriber having SoLSA service is called a SoLSA subscriber. A mobile station currently having support for SoLSA is the to be in LSA mode. This can mean e.g. that the mobile station indicates to its user that certain special features (like lower rates or extra services) are available, and it uses these features when applicable. SoLSA for a GSM environment is described in a proposal to ETSI (European Telecommunication Standard Institute), document Tdoc 98S147. For convenience, the subject matter of the '147 document is repeated here in a somewhat rearranged form. Within the GSM standards there are features and capabilities defined which allow groups of subscribers to be managed according to a number of operator specific criteria, e.g. CAMEL, USSD, SIM Application Toolkit (SAT) and "Multiple Subscriber Profile". Future trends in GSM system platform development are towards customised system solutions where, for example, radio access solutions will be enhanced to take into account indoors environment (micro and picocells) such as office buildings. New means to provide radio ac- cess solutions together with service and tariff packages (e.g. flat rate charging) can be introduced. The concept of LSA extends the operator's capability to offer different service features to subscribers or subscribers groups, different tariffs and different access rights depending on the location of the subscriber. Examples of LSAs are indoors office cells (those provided by indoors base stations), a home or an office and its neighbourhood, an industry area (covering all company buildings and the space between), or a part of a city or several locations. Support of LSA (SoLSA) can be specified as a set of service features which are used in conjunction with existing toolkits, e.g. CAMEL and SAT. For example CAMEL phase 2 functionality already provides the means to offer cell based applications, as a CAMEL Service Environment (CSE) can be aware of the cell from which a call was originated. CSE capabilities include e.g. modified charging and barring of calls based on cell information. However, operators typically have several service platforms. It should be noted that CAMEL is but one example of a platform for the SoLSA concept. SoLSA can enhance radio specific support for an LSA by standardising one or more of the following: definition of an LSA, LSA indication, LSA support (in idle mode (camping), or in active mode (cell reselection)), LSA only access, exclusive access, and preferential access. Figure 1 shows the parts of the mobile network system that are es- sential for understanding the invention and figure 2 illustrates the concept of SoLSA. Subscriber-specific LSA information can be stored and managed either in the CSE or in the Home Location Register HLR. Two types of LSAs can be foreseen, either a truly customised LSA for a small office area and a residential area ("office/home zone"), or a predefined LSA covering large cam- puses, sections of cities and districts ("city zones") etc. These two LSAs fulfil different needs. They complement each other and they can be used simultaneously. A SoLSA subscriber can have a home LSA consisting of 1 or 2 cells, an office LSA consisting of 1 to 5 cells and a city LSA with dozens or hundreds of cells. The first two LSAs are preferably tailored on a per-subscriber basis. A truly subscriber-specific LSA can be defined on a cell identity basis and it is possible to define it as small as possible, consisting of even only one cell, if necessary. The third LSA could rely on a certain predefined LSA definitions. LSA information can consist of: • List of cell identities belonging to an LSA (Cell identity can be defined as specified in GSM 08.08, e.g.: Cell Global Identity (MCC+MNC+LAC+CI), MNC+LAC+CI LAC+CI or Cl). • LSAJD (The LSA ID is a number uniquely assigned to an area in one network. LSA ID can be associated with a Cell Identity list.) • LSA name (a text string describing area and/or service, e.g. "Etsi Building"). • LSA indication (a flag to activate/deactivate LSA indication) • LSA priority (a number value needed to support overlapping LSAs, for example city zone and office cells in same area). • LSA-only access (Service outside LSAs is restricted). • LSA-only access indication text (Text to be displayed when out of LSA area). • Exclusive access (User has exclusive access to an LSA). • Relevant MSP profile (an LSA may be associated with some MSP profiles). • Idle/active mode support (preferred cell selection in idle/active mode may be utilised in an LSA). Examples of LSA information: LAC+CI, "Home", Priority=3, MSP Profile = 1 LAC+CI, LAC+CI, LAC+CI, LAC+CI, "Corporate", Priority=1 , MSP Profile = 2 (a user with two LSAs) Excl_access = yes, Active mode support = mandatory MCC+MNC+LAC+CI, MCC+MNC+LAC+CI, "MoU at Fiji" Priority=2 (a user with exclusive access and LSA in VPLMN ) LSA Only Access = yes OutOfLSA="No calls allowed" LAC+CI, LAC+CI, LAC+CI, LAC+CI, "Corporate", (a user with limited mobility) The subscriber's LSA information can be stored within the subscriber's SIM. This information can be initially stored by the operator for new subscribers. The operator can manage LSA information in a SIM by using SIM data download of SAT (SIM Application Toolkit), USSD (Unstructured Supplementary Service Data) or SMS (Short Message Service) by concatenating multiple short messages if necessary. LSA information transfer from the CSE can take place any time. It takes place at least when the subscriber's LSA parameters are changed, e.g. due to possible changes in the radio network's configuration. The MS can receive the LSA information for cell reselection procedures, etc. Idle mode support makes the behaviour of the MS more consistent and reliable in an LSA. A preferential cell reselection mechanism can be defined so that when camping on a cell in idle mode (MS in SERV state), the subscriber's MS favours the cells belonging to his/her LSA. The MS shall always attempt to reselect a cell in a higher priority LSA. This requires new functionality in the MS. The MS can recognise the LSA by the LSAJD or a cell id list stored in the SIM. Figures 3 to 4 illustrate cell reselection in SoLSA. GSM phase 2 de- fines the above parameters C1 (for normal cell selection) and C2 (for cell re- selection). C2 is evaluated for the six strongest neighbouring cells, and the cell with the highest priority and C2 is reselected. If there are more than one LSA cell available which meet the reselection criteria, the highest LSA priority cell is selected. For normal cells the calculation of C2 involves a parameter RXLev_Access_Min, but for LSA cells the LSA_RXLev_Access_Min is used instead. The value for the parameter is broadcast to the MS for each cell in the system information messages. For SoLSA, a new parameter LSA_Camping_Margin is also broad- cast to the MS for each cell in the system information messages. If the C2 of a lower priority LSA cell or a normal cell exceeds the C2 of an LSA cell by this margin, the LSA MS will no longer camp on the highest priority LSA cell even if its C1 is positive. This can be realised by adding LSA_CampingJvlargin and (LSA__RXLev_Access_Min - RXLev_Access_Min) to the C2 of the LSA cells. This causes the MS to favour the LSA cells and minimises the negative effect of LSA mobiles on the network's spectral efficiency. New parameters can be added to appropriate system information messages to enable broadcasting of the LSA cell reselection parameters. These new parameters are transferred to the BSS from the O&M with other radio network parameters. Existing cell reselection parameters will affect the behaviour of all GSM subscribers in the same way. For SoLSA, there is a need for customer-specific idle mode supporting parameters. This enables SoLSA to handle LSA customers in e.g. traffic hot spots and in office environments without affecting regular GSM customers. A Base Station Controller BSC needs subscriber-specific LSA information to favour LSA cells during active mode. This information can be delivered to the BSC from an MSC (Mobile services Switching Centre) during call setup and inter-BSC handover by using existing BSSAP messages or proprietary messages. The LSA information should identify the LSA cells, LSA priorities and the LSA-only access indicator. The MSC can retrieve the LSA information from the CSE or from the Visitor Location Register VLR, if available. During inter-BSC handover, the LSA information can be delivered from the source BSC to the target BSC by including the LSA cell list to the HANDOVER_REQUEST and HANDOVER_REQUIRED messages (each of which will require a new information element). The transfer of the LSA information between the BSSs is transparent to the MSC. A subscriber-specific identifier can be assigned to each LSA (e.g. an alphanumeric text or an icon). This identifier can be provided to a user in idle or active mode. It is also possible to indicate a change of the LSA during idle and active mode. The indication is a network option (activated/deactivated by the network). When the subscriber moves into and away from his/her LSA, the LSA indication may be given by the MS, as guided by the LSA indication bit. In the idle mode the MS can indicate the LSA e.g. by displaying the LSA name string (e.g. "Etsi Building"), an icon, or another signal to the subscriber, as a result of camping on an LSA cell. An exclusive access (EA) feature makes it possible to guarantee that the members of a user group are the only users of the radio channels (physical resources) within a cell. In order to support EA in idle mode, other users' mobile stations must be prevented from camping on that cell. This can be done by using a Cell_Bar_Access parameter. Users whose LSA the cell belongs to shall ignore the Cell_Bar_Access parameter and camp on an LSA cell normally. In order to bar the cell also for LSA users, a new broadcast parameter LSACell_Bar_Access can be used. In order to support EA in active mode, handovers of non-EA LSA users to EA LSA cells should be prevented. This feature can be supported e.g. by flagging EA cells in the BSC and permitting handovers to EA cells only for users for whom a matching LSA id information is found in the LSA information delivered to the BSC. An LSA-only access (LO) feature makes it possible to restrict the users' access to a defined area. Originating and/or receiving calls outside that area can be prevented. It is also possible to prevent handovers to cells not belonging to allowed LSAs. In idle mode an MS with LO is only allowed to access cells that are defined in the subscriber's LSA information. All cells with no match with the LSA information stored in the SIM are regarded as not suitable. An indication can be given to the user (e.g. a text string like "No Service Allowed"). If LO is set and a non-LSA cell is selected, only emergency calls are allowed. To support LO during active mode, the BSC can compare each handover candidate cell with LSA information delivered to the BSC and permit handover only if a match is found. CAMEL CSE (or NSS) may be used to pre- vent call attempts (MO and/or MT) outside the LSA area as required by SoLSA application. Preferential access (PA) means the ability to favour the allocation of radio resources at call setup and during the active mode to LSA users. Alloca- tion of resources could be achieved by defining a specific load alarm for PA cells in the BSC. Upon triggering the alarm threshold, access to PA cells would be granted only to LSA subscribers. In call setup phase and in handovers, PA can be supported by using the LSA information delivered to BSC (see section Localised service area support in active mode). For Camel Service Environment (CSE), the following functionality is required from a SoLSA server: (1) storage and management of subscriber- specific LSA information, (2) interface to service provision and management system, (3) LSA information transfer to the MS, (4) conformance with SIM Application Toolkit (classl) data download procedure, and (5) LSA information transfer to the MSC. Using the CAMEL platform provides a number of benefits. The CAMEL Application Part (CAP) protocol is able to transfer the cell id to the CSE for LSA information processing during call setup. That would make it easy for any CAMEL application (e.g. tariff applications) to use SoLSA capa- bility. The CAMEL platform would also enable SoLSA based services to roaming subscribers. To support SoLSA, the MS and the SIM must support SIM Application Toolkit (class 1). The MS must also have: support for subscriber-specific LSA indication; LSA support in idle mode (decoding and processing of new SoLSA-specific system information parameters and LSA specific cell reselection algorithm); LSA support for EA in Idle state (ignore Cell_Bar_Access bit while in LSA and support for decoding and processing of LSACell_Bar_Access bit while in LSA); and LSA support for LSA only access in Idle state (indication to subscriber when not in an LO cell). The BSC must support: LSA in active mode (reception and processing of LSA information during call setup, storage and execution of an LSA favouring algorithm, and transfer of LSA information between BSCs during external handovers); EA; LO; and PA. A general problem associated with the above-mentioned embodi- ment for providing special services in a network is that it is not clear how SoLSA and GPRS should interwork while using the network-controlled cell reselection feature described above. Normally (i.e. without SoLSA) the BSC should tell the MS which cell to use because the network does not know the SoLSA cells for the MS. A circuit-switched call is normally a SoLSA call if it is initiated within an LSA cell. Packet-switched operation is somewhat more complicated because every packet is charged separately. In other words, there are no 'calls'. Packet-switched network use the concept of 'session' but the duration of a session can be days or weeks. Consequently, it is not feasible to treat a session as an LSA session if it is initiated within an LSA cell. Moreover, a conventional BSC has no way of knowing which subscribers are SoLSA subscribers and which are not. BRIEF DESCRIPTION OF THE INVENTION An object of this invention is to develop methods and network elements for overcoming the above problem. More specifically, the invention should provide optimal cell reselection in a GPRS SoLSA environment. The methods and network elements according to the invention should help the MS to remain in its LSA area especially when it is transmitting. The object is achieved by means of methods and equipment which are characterized by what is disclosed in the characterizing part of the appended independent claims. Preferred embodiments of the invention are specified in the dependent claims. The invention is based on a vision that it is not sufficient for a BSC (or any other network element making decisions oncell reselection) to know which subscribers are SoLSA subscribers. Such knowledge would help the BSC to decide which subscribers can be handed over to different cells without affecting their SoLSA benefits. However, in network-controlled cell reselection a BSC has to be able to change an MS to a different cell. In such situations the BSC should know, for each SoLSA subscriber, the respective LSAIist. For attaining this object, the invention is also based on the idea that the mobile station should indicate to the BSC that the MS's subscriber is a SoLSA sub- scriber. According to a first alternative procedure, the MS's measurement reports to the BSS can be complemented to include an indication that the MS's subscriber is a SoLSA subscriber. The BSS will then download from the network (e.g. SGSN, HLR...) the list of LSA cells, or some other equivalent information. The BSS will make use of this indication during network-controlled cell reselection. In particular, the BSS should try to keep the MS in its LSA area as long as acceptable radio conditions can be maintained. According to a second alternative procedure, after a BSC change (attach, RA update, etc.) the MS sends the BSC a list of its LSA cells. According to a third alternative procedure, the MS can indicate for each cell whether or not the cell in question is an LSA cell. The methods and network elements according to the invention extend the concept of SoLSA to a packet radio network by solving the inter- working problems of prior art systems. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which Figure 1 shows the parts of the mobile network system that are essential for understanding the invention; Figure 2 illustrates the concept of SoLSA; Figures 3 to 4 illustrate cell reselection in SoLSA; Figure 5A is a signalling diagram illustrating the invention; Figure 5B illustrates a possible structure for the list of LSA cells; Figure 6A illustrates an alternative signalling diagram; Figure 6B illustrates a preferred embodiment of a measurement report; and Figure 7 illustrates yet another signalling diagram. DETAILED DESCRIPTION OF THE INVENTION The invention can be applied to many kinds of cellular mobile communications systems. In the following, the invention will be described in greater detail by way of example mainly with reference to the pan-European digital GSM (Global System for Mobile Communication) system in connection with a GPRS system (General Packet Radio Service). The following table 1 shows a possible set of additional criteria for network controlled cell reselection for all combinations of serving and neighbour cells. signal level signal level than serving cell Table 1 In other words, when the cell's LSA status does not change, it is sufficient that the neighbour cell is better in some respect (e.g. it provides better signal level than the serving cell). Alternatively, cell reselection can be performed if the MS's serving cell is heavily loaded while its neighbour cell is not. When changing from a normal cell to an LSA cell, the latter should provide at least an adequate signal level. When changing from an LSA cell to a normal cell, the latter should provide a clearly better signal level than the serving cell. This means that the user of the MS is willing to give up the LSA advantages only if the signal level in the serving LSA cell is marginal and the signal level in the neighbour cell is clearly (at least a few dB) better. Figure 5A illustrates a first embodiment of the invention, according to which the MS will, after a BSC change, send a list of its LSA cells to the BSC. In step 5-0 the MS and the SGSN perform an attach or a routing area update procedure, the details of which have been omitted for clarity. In step 5-3 the MS sends the BSC a list of its LSA cells. In step 5-4 the MS provides the BSS with measurement reports concerning the MS's serving cell and the neighbouring cells (candidate cells for cell reselection). In this embodiment, the measurement reports can be conventional. Some time later, in step 5-5 the BSS, using the measurement reports sent by the MS, sends the MS a cell reselection command. Figure 5B illustrates a possible structure for the list of LSA cells (for the message in step 5-3). In essence, this list consists of a header with a length parameter and pairs of LAC and Cl for each cell in the MS's LSA list. Figure 6A illustrates a second embodiment of the invention. Step 6-0 corresponds to the attach or a routing area update procedure. In step 6-4 the MS provides the BSS with measurement reports concerning the MS's serving cell and the neighbouring cells. In this embodiment, the measurement reports indicate which cells are LSA cells. There are several possibilities for such measurement reports. One possibility is shown in Figure 6B, wherein the measurement report for the MS's serving cell comprises part 61 which is known from the prior art, e.g. the current GSM/GPRS specifications. Additionally the measurement report comprises an indication 62 (e.g. one bit) as to whether the MS's current serving cell is an LSA cell or not. Similarly, a meas- urement report for the MS's neighbour cells comprises, for each of the N neighbour cells, a known part 63, to 63 6 and an indication 64 N to 64 N indicating whether or not the respective neighbour cell is an LSA cell. Currently, an MS measures up to six neighbour cells, i.e. N=6. According to current GSM/GPRS specifications, every bit of the measurement report is used for some purpose. Thus the prior art measurement report would have to be lengthened by one bit for each measured cell. Such lengthening of the measurement reports can be avoided by noticing and making use of the fact that the LSA information (i.e. which cells are LSA cells) is rather static. Consequently, the MS could, in one measurement report (e.g. the first time the MS reports a certain cell to a new BSC) report only five neighbour cells and use the space of the sixth cell (field 63 N in figure 6B) for sending all the flag bits 62 and 64, ... 64 N . Alternatively, some other bits could be used in a novel manner. For example, such a special measurement report could report some parameter with a resolution which has one bit less than in normal measurement reports. Of course, the BSC must be informed of such a different interpretation of bits. The MS could do so by indicating in the attach or RA update procedure 6-0 that the first measurement of a cell should be interpreted differently. Figure 7 illustrates an alternative embodiment of the invention, ac- cording to which the BSS has a more active role. In this case the SGSN downloads the list of the MS's LSA cells to the BSS which independently makes all the decisions concerning cell reselection. In step 7-0 the MS performs an attach or an RA update. In step 7-1 it sends the BSS an indication (e.g. in a Setup message) that its subscriber is a SoLSA subscriber. In step 7- 2 the BSS requests the MS's LSA cells from the SGSN. In step 7-3 the SGSN provides the BSS with a list of the MS's LSA cells. The remaining steps 8-4 and 8-5 correspond to similarly numbered steps in figure 5A. It will be apparent to one skilled in the art that the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above but may be modified within the scope of the appended claims. Abbreviations: BSC = Base Station Controller CAMEL = Common Applications for Mobile Enhanced Logic Cl = Cell identifier CSE = CAMEL Service Environment EA = Exclusive Access HCS = hierarchical cell structures LA = Location Area LAC = Location Area Code LO = LSA Only access LSA = Localized Service Area MCC = Mobile Country Code MNC = Mobile Network Code MO/MT = Mobile Originating/ Mobile Terminating MSC = Mobile services Switching Centre PA = Preferential Access RA = Routing Area SAT = SIM Application Toolkit SoLSA = Support of LSA

Claims: CLAIMS 1. A method for cell reselection for a mobile station (MS) in a cellular packet radio network comprising a radio station controller (BSC), the mobile station and the radio station controller having a capability to make decisions concerning cell reselection, the method comprising the steps of: sending from the mobile station (MS) to the radio station controller (BSC) measurement reports (5-4, 6-4, 7-4) comprising information (61, 63, ... 63 N ) about the mobile station's serving cell and its neighbour cells; and in response to fulfilment of a predetermined set of conditions, ordering the mobile station to at least partially surrender its capability to make decisions concerning cell reselection; characterized by: associating the mobile station and/or its subscriber with at least one set of special cells (LSA); transferring information (5-3, 64, ...64 N , 7-3) on said at least one set of special cells to the radio station controller (BSC) wherein the mobile station (MS) at least participates in the transferring of information; and using the information on the special cells at the radio station con- troller for cell reselection when the mobile station has been ordered to at least partially surrender its capability to make decisions concerning cell reselection. 2. A method according to claim ^ characterized in that the mobile station (MS) sends the radio station controller (BSC) at least one explicit message (5-3, 51) which indicates the set of special cells. 3. A method according to claim ^ characterized in that the mobile station (MS) indicates to the radio station controller (BSC) in at least one measurement report (6-4) indicating whether or not the cell in question is a special cell. 4. A method according to claim 3, characterized in that the mobile station (MS) performs the indication only seldom, preferably once for each reported cell. 5. A method according to claim ^ characterized in that the mobile station (MS) sends the radio station controller (BSC) an indication (7-1) that the set of special cells exists; and the radio station controller (BSC) re- quests (7-2) and receives (7-3) the set of special cells from another network element, preferably an SGSN node. 6. A mobile station (MS), adapted to: send to a radio station controller (BSC) measurement reports (5-4, 6-4, 7-4) about the mobile station's serving cell and its neighbour cells in a cellular telecommunications network providing packet radio service; make decisions concerning cell reselection and in response to a command from the radio station controller, at least partially surrender its ability to make decisions concerning cell reselection; the mobile station being associated with at least one set of special cells (LSA); characterized in that the mobile station is adapted to at least participate in transferring of information on the at least one set of special cells to the radio station controller (BSC). 7. A measurement report (6-4) from a mobile station (MS) to a radio station controller (BSC) in a cellular telecommunications network providing packet radio service, the measurement report comprising information (61, 63, ...63 N ) about the mobile station's serving cell and its neighbour cells; characterized in that the measurement report (6-4) at least indirectly indi- cates, for each reported cell, whether or not the cell in question is a special cell. 8. A measurement report (6-4) according to claim 7, characterized by including, for each reported cell, a flag bit (62, 64, ...64 N ) indicating whether or not the cell in question is a special cell.

Inventors: HAUMONT SERGE, MUHONEN AHTI, VAARA TOMI, HAUMONT, SERGE, MUHONEN, AHTI, VAARA, TOMI
Applicants: HAUMONT SERGE, MUHONEN AHTI, NOKIA NETWORKS OY, VAARA TOMI, HAUMONT, SERGE, MUHONEN, AHTI, VAARA, TOMI
Application year: 2000
Main classification: H04Q
IPC codes: H04M3, H04W4, H04W8, H04W36