Background of the Invention The most significant factor reducing the capacity of radio systems is the limited frequency spectrum available. The capacity of a radio system is thus dependent on how efficiently the radio frequencies allocated to the sys- tem can be utilized. In cellular radio networks, enhanced utilization of radio fre- quencies is based on frequency reuse: the same frequency is reused at sev- eral locations that are sufficiently spaced apart, which affords a vast increase in system capacity. This is counteracted by increased complexity in the net- work as well as in the mobile units that must be capable of selecting a base station from among several possible base stations. For example, if the same frequency is reused in every ninth cell, the spectral allocation of N frequencies permits the use of N/9 carriers simultaneously in any cell. With certain limita- tions the same frequency bands can be used in all cells simultaneously. There are various configurations of cellular networks, such as micro cellular net- works, multi-layered networks, underlay-overlay networks, umbrella cells, etc.

In cellular mobile communication systems a radio terminal can move freely in the area of the cellular network and camp on the most suitable cell at each time. Cell re-selection (also called camping) is the act of changing the serving cell from an old cell to a new cell. When re-selection is made and possible registration is performed, the radio terminal is said to be attached to the cell. Cell re-selection during a call is also known as a handover. For that purpose, the cell site (the base station) of each cell broadcasts system infor- mation on the broadcast channel. The broadcast information includes informa- tion on the serving cell and information about neighbour cells, e. g. a neighbour cell list. The radio terminal measures the signal level/quality of the serving and neighbor cells. On the basis of the measurements, the radio terminal perform ranking of the neighbour cells according to suitable criteria and/or algorithm, so that selection of the best (most suitable) cell is attempted first.

Professional Mobile Radio or Private Mobile Radio (PMR) systems are dedicated radio systems developed primarily for professional and govern- mental users, such as the police, military forces, oil plants, etc. PMR services have been offered via dedicated PMR networks built with dedicated PMR

technologies. This market is divided between several technologies-analog, digital, conventional and trunked-none of which has a dominating role.

TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital PMR systems.

One special feature offered by the PMR systems is group commu- nication. Group communication is applicable to other kind of users too, such as firms or other groups using public cellular networks. The term"group", as used herein, refers to any logical group of three or more users intended to par- ticipate in the same group communication, e. g. call. The groups are created logically, i. e. special group communication information maintained on the net- work side associates specific user with a particular group communication group. This association can be readily created, modified or canceled. The same user may be a member in more than one group communication group.

Typically, the members of the group communication group belong to the same organization, such as the police, the fire brigade, a private company, etc. Also, typically, the same organization has several separate groups, i. e. a set of groups.

In cellular systems using group communication, the operational, area of a group is usually restricted to pre-defined geographical area. One reason is the system capacity; if group area covers the whole network, setting up a call to such a group uses a lot of system resources (traffic channels and speech lines between switches). Furthermore the communication need is usu- ally only local, i. e. if you are located outside the normal operational area of the group, the need to participate in the communication vanishes.

The area of the group communication is usually defined to contain one or several base stations (radio network cells). To have better coverage and better service for the radio users, the radio network is usually defined such that the coverage areas of neighbour cells overlap. Thus several base stations may provide service for a radio terminal in a certain geographical area. When roaming near the border of the operational area of the group, the radio terminal can select such a cell from the several alternatives which is out- side the group area.

EP 0 555 441 discloses an approach for overcoming this problem.

The approach described in the patent is based on sending group call paging messages also in nearby cells not belonging to the group area directing the radio terminals to join the call on a traffic channel of a cell belonging to the group area. Thus, the radio terminal changes the cell only after receiving the

group call paging message indicating call establishment in a nearby cell (be- longing to the group area). This means that the radio terminal may have to register on the new cell (where the call is set up) and gain air interface encryp- tion information from the system before being able to join the call. This may cause delay in being able to join the call.

TETRA standard provides support for group communication only for the currently transmitting party (the talking party) who is performing announced type 2 or type 1 cell re-selection ; the transmitting party informs the new cell to the system via the old cell before the cell change. The system can respond to this cell change request negatively indicating"restoration cannot be done on the cell". For the several receiving parties or for the case that no group call is going on, there is no cell change related group communication support defined in the TETRA standard.

Summary of the Invention An object of the invention is to provide a new way for a radio termi- nal to select a new cell when the radio terminal belongs to at least one com- munication group.

This object of the invention is achieved by a method, a radio sys- tem, and a radio terminal as disclosed in the attached independent claims 1,9 and 18, respectively. Preferred embodiments of the invention are described in the dependent claims.

A cell site broadcasts neighbour cell broadcast messages contain- ing information of all neighbour cells available to all radio terminals within a cell. In addition to this conventional broadcast, the cell site sends at least one further message indicating how the communication group or groups is or are available in the neighbour cells. Upon receiving this further information, a radio terminal belonging to at least one communication group makes a decision on selection of a new cell based on the availability of said at least one communi- cation group if there are two or more candidates for the new cell. By the merits of the present invention, when performing cell re-selection the radio terminal knows, before selecting the other cell, whether the nominated cell supports the desired group (s) or not, and can thus select a cell where the desired group communication is possible.

The present invention provides various advantages in comparison with the prior art approaches. Firstly, the present invention provides solution also for receiving parties in a group call and also for a case in which there is

no group call currently going on when the cell re-selection is performed. This is a significant improvement in comparison with the present TETRA standard.

Secondly, no information of the operational area of the groups needs to be pre-programmed to the radio terminals. This is an advantage over a hypothetical alternative approach to define"preferred cells"to the radio ter- minals reflecting the normal operational area of the terminal user, so that the radio terminal prefers these cells to other cells. Contrary to such a hypothetical approach, in the present invention the changing of the operational area of the groups does not cause re-programming the radio terminals. Further, the area definitions of the groups¢ (listened to by the same radio terminal user) do not have to be identical.

Thirdly, advantages as compared with EP0555441 include that a radio terminal knows as early as before camping on the cell and without wait- ing for the next call establishment whether the group is available on the cell, i. e. whether the nominated cell belongs to the group area. The present inven- tion decreases the number of cell selections and registrations as compared to EP055544. Further in EP055544, a radio terminal roaming while in a group call does not know which cell to select so that the call can be continued on the new cell. In the present invention, the radio terminal can be sure that the group is available within the whole coverage area of the cell.

Another alternative approach which could be employed instead of the present invention is making the decision to stay on the new cell after camping on the new cell. The radio terminal checks the availability of the group on the new cell by the"group attachment procedure"defined in the TETRA standard. If the response to this group attachment request is negative (group identity is not available within the current area), the radio terminal does not stay on the cell but tries to select another cell where the group is available.

However, the present invention gains advantages as compared with this hypo- thetical approach. The radio terminal knows as early as before camping on the cell and without sending an attachment request whether the group is available on the cell, i. e. whether the nominated cell belongs to the group area. The present invention decreases the number of cell selections, registrations and uplink signalling. In addition, the"service break" (group not available) is mini- mised by the present invention.

Brief Description of the Drawings In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying draw- ings, in which Figure 1 illustrates an exemplary network architecture, Figure 2 illustrates a cell configuration and group areas in a cellular network, Figure 3 illustrates an example of SDS messages sent by the cell sites in Fig. 2, and Figure 4 illustrates an example of neighbour cell broadcast mes- sages sent by the cell sites in Fig. 2.

Preferred Embodiments of the Invention The present invention can be applied to any cellular mobile com- munication network supporting group communication.

Fig. 1 illustrates an exemplary network architecture. The cellular network shown in this example comprises mobile exchange (s) MX, to which the base stations BS are connected via a suitable transmission system. A cel- lular network also comprises some kind of subscriber database system that may be distributed or centralized. Typically there are two types of logical data- base functions, a home location register (HLR) for permanent storage of data of the network's own subscribers and visitor location register (VLR) for storing the subscriber data temporarily while the subscriber is located in that VLR area. The VLR is often associated with or integrated into the mobile exchange MX, each MX having a dedicated VLR. The HLR may be associated with or in- tegrated into the MX, each having a dedicated HLR. More typically, there is one HLR which is shared by two or more mobile exchanges MX. In profes- sional systems there is typically also a dispatcher system (s) 10 for providing a variety of dispatching and management functions to the users. The dispatcher may lead, control and supervise the communication in the field as well as per- form management functions, such as group management and subscriber management. For example, the group management can include group con- figuration, i. e. creation, modification, deletion of groups or temporary groups, and control of the group area. Also a network management system NMS may be able to perform the group management. When a group is created, the group information, such as a group identity which uniquely identifies the group in the network and the group area is stored in the system databases. If the

group data, for example the group area, is later modified, the change is up- dated to the system databases. The members of the group may be defined when the group is created, or the information on the memberships may be achieved later on. This group membership information may also be stored to a system database, together with the group data, or to subscriber HLRNLR.

Now the dispatcher 10 or any of the radio terminals belonging to the group are able to initiate and receive a group call within the group area. An example of such a cellular system is the TETRA system defined by ETSI. The TETRA network is commercially available from Nokia Corporation.

The specific architecture of a cellular network employed is not rele- vant to the present invention, and the present invention is not intended to be restricted to any specific network configuration or architecture. For the basic invention it is only relevant that at least some of the cells of the network send, in addition to the conventional neighbour cell broadcast messages, at least one further message indicating how the communication group or groups is or are available in the neighbour cells. In the following, the preferred embodi- ments will be described.

According to a first aspect of the invention, the system, e. g. the cell site or the base station BS, sends data messages having a broadcast address and indicating the groups available by each neighbour cell. The information in- cluded into these data messages contains the cell identity of the neighbour cell and group identities of all the groups available on the new cell. In addition the normal neighbour cell broadcast messages having a broadcast address and indicating all the neighbour cells are also broadcasted. All radio terminals RT receive the information on all available neighbour cells (neighbour cell broad- cast messages are sent with a broadcast address), and therefore, the radio terminals RT that do not support a cell re-selection algorithm according to the present invention are also supported by the network (backward compatibility).

In other words, these radio terminals RT perform the cell re-selection in a con- ventional manner, e. g. as defined in the present TETRA standard. All radio terminals also receive the availability information of all groups in the data mes- sages according to the invention. The radio terminals RT incompatible with the present invention ignore these additional messages. However, a radio terminal RT supporting the cell re-selection algorithm according to the invention makes the final decision based on the availability of the desired group (s) if there are several candidates for the new cell.

The additional data messages may be short data messages (SDS) defined in the TETRA standard, short messages (such as in the GSM), USSD messages, or any similar messaging messages.

The data messages containing the group availability information can be generated and/or sent by any suitable network entity. This network entity can be a base station or a mobile exchange, for example. Preferably this net- work entity is the same network entity that generates the normal neighbour cell broadcast messages, since this network entity has the neighbour cell informa- tion readily available. Further, when a new group is created, the group area in- formation is stored in a memory or database of the network entity. If the group is modified, also the stored information is modified. If the group is deleted, the corresponding information in the network entity is deleted.

The information can be stored in a search table having a group identity as an index and listing the neighbour cell identifiers of each group. A sending routine in the network entity goes sequentially through the table and sends neighbour cell broadcast messages having a broadcast address and containing a group identity and a respective list of neighbour cell identifiers.

One data message may contain information on one or several groups. It is also possible that similar messages having a group address and listing the neighbour cell IDs for the group are sent.

As another example, the information can be stored in a search table having a neighbour cell ID as an index and listing the identities of the groups available in the indexed neighbour cell. A sending routine in the network entity goes sequentially through the table and sends neighbour cell broadcast mes- sages each having a broadcast address and containing a neighbour cell IDs and a list of group identities of the group available in that neighbour cell. One data message may contain information of one or several neighbour cells.

For example in the TETRA system, in addition to the normal neighbour cell D-NWRK BROADCAST messages, the system sends SDS messages containing the groups available by each neighbour cell in accor- dance with the present invention. The information included into these SDS messages contains the cell ID of the neighbour cell and group identities (GSSI/GTSI) of all the groups available on the new cell. These SDS messages are sent with broadcast address (all bits"ones"), and thus all radio terminals located on the cell will receive them. A short data message SDS with a User defined data 4 format defined in the TETRA standard can be used, and one of the protocol identifiers in the SDS messages can be reserved for the purpose

"group availability in neighbour cells". The SDS message is received and processed as any SDS message by the conventional means in the radio ter- minal. On the basis of the new protocol identifier, a radio terminal is able to recognise that the received SDS message relates to the cell re-selection algo- rithm according to the invention, and forwards the information to the cell re- selection routine performing the cell re-selection algorithm. In an embodiment of the invention, this routine is implemented by means of a program run in a controller of the radio terminal.

The broadcast SDS message may contain the following information elements: cell identifier of the neighbour cell and a list of group identi- ties/numbers (GSSls/GTSls) of the groups available in the cell corresponding to the cell identifier. One SDS message can contain information on one or sev- eral neighbour cells.

In another example, the SDS message may contain the following in- formation elements : Group identity/number (GSSI/GTSI) and a list of cell iden- tifiers of the neighbour cells in which the group corresponding to the group identity is available. One SDS message can contain information on one or several groups.

Let us illustrate the above embodiments by means of a practical example. In Figure 2, five cell sites or cells Q, X, Y, W and Z are shown. Also shown are group areas for groups A, B, and C. Cells X, Y, Z, W and Q are all each other's neighbours. Cells X and Y support groups A and B. Cells Z and W support group C. Cell Q supports groups B and C.

The contents of the SDS messages sent by the cell sites Q, X, Y, W and Z are shown in Figure 3 in accordance with one of the embodiments de- scribed above. For example, the cell site X will send SDS messages contain- ing the following information: Cell Y: groups A and B Cell Z : group C Cell W: group C Cell Q: groups B and C Alternatively, in accordance with the other embodiments described above, the cell site X will send SDS messages containing the following infor- mation: Group A: Cell Y Group B: Cells Y and Q Group C: Cells Z, W and Q

The contents of the SDS messages sent by the other cell sites Q, Y, W and Z are arranged similarly.

According to a second aspect of the invention system, e. g. the cell site or the base station BS sends modified neighbour cell broadcast messages with a group address. Each group addressed broadcast message contains only those neighbour cells where the group is available (the cells in the mes- sage belong to a group area). The same neighbour cell may be included in several broadcast messages sent with different group address. In addition to these messages, conventional neighbour cell broadcast messages having a broadcast address and listing all the neighbour cells are also sent. All radio terminals RT receive the information of all available neighbour cells, and there- fore, also the radio terminals that do not support a cell re-selection algorithm according to the present invention are supported by the cellular network (back- ward compatibility). Those radio terminals RT that are interested in a group (i. e. willing to receive communication addressed to the group) receive also the availability information on the group in the group addressed broadcast mes- sages, in other words, information on neighbour cells belonging to the group area. If there are several candidates for the new cell, the radio terminal sup- porting the cell re-selection algorithm according to the present invention makes the final decision based on the availability of the desired group (s).

The modified neighbour cell broadcast messages having a group address can be generated by the same network entity which generates the normal neighbour cell broadcast messages. This network entity can be a base station or a mobile exchange, for example. For that purpose, when a new group is created, the group area information is stored in a memory or data- base of the network entity. If the group is modified, also the stored information is modified. If the group is deleted, the corresponding information in the net- work entity is deleted. The information can be stored in a search table having a group address as an index and listing the neighbour cell IDs of each group.

A sending routine in the network entity goes sequentially through the table and sends sequentially neighbour cell broadcast messages, each having the in- dexed group address and the respective list of neighbour cell IDs. These modified neighbour cell broadcast messages can be sent interleaved with the normal neighbour cell broadcast messages, for example.

When this approach is applied in a TETRA system, a network broadcast message D-NWRK BROADCAST as defined in the TETRA stan- dard may contain information on one or several neighbour cells. The D-NWRK

BROADCAST message is sent via TMA-SAP containing an address field in the MAC header. In a normal case the address used in the MAC header is the broadcast address (all ones). According to the present invention, the system also sends D-NWRK BROADCAST messages with a group address, i. e. the address used in the MAC header is a GSSI (group short subscriber identity).

Each group addressed D-NWRK BROADCAST message contains only those neighbour cells where the group is available (i. e. the cell belongs to a group area). Those radio terminals that are interested in receiving communication (voice and data) addressed to the group have the GSSI of the group attached (as valid layer 2 address) and thus are able to receive and identify also the group addressed broadcast messages. The information content of these mes- sages is forwarded to a cell re-selection routine performing the cell re-selection algorithm according to the present invention. In an embodiment of the inven- tion, this routine is implemented by means of a program run in a controller of the radio terminal.

The second approach is illustrated by means of a practical example referring again to the configuration shown in Figure 2. Cells X, Y, Z, W and Q are all each other's neighbours. Cells X and Y support groups A and B. Cells Z and W support group C. Cell Q supports groups B and C. The D-NWRK BROADCAST messages sent by different cell sites X, Y, Z, W and Q are shown in Fig. 4. More specifically, the system sends D-NWRK BROADCAST messages so that in a message addressed to group A, there are cells X and Y. In a D-NWRK BROADCAST message addressed to group B, there are cells X, Y and Q included. In a D-NWRK BROADCAST message addressed to group C, cells Z, W and Q are included. However, the current serving cell in which the message is sent is not included in the D-NWRK BROADCAST mes- sage. If a radio terminal RT1 has only group A attached, it will receive D- NWRK BROADCAST messages addressed to group A only. If a radio terminal RT2 has groups A and B attached, it will receive D-NWRK BROADCAST messages addressed to groups A and B. Let us assume that the radio termi- nals RT1 and RT2 are both currently located in cell X. The RT1 receives D- NWRK BROADCAST message telling that group A is supported in cell Y. The RT2 receives D-NWRK BROADCAST messages informing that group A is supported in cell Y and group B is supported in cells Y and Q.

The cell re-selection algorithm can be based on any suitable criteria employed in a specific radio system to which the present invention is applied to. For the purposes of the present invention, the selected cell re-selection al-

gorithm is modified such that it takes into account the group availability infor- mation sent by the radio system in accordance with the present invention. De- tails of such modification depend on the requirements of each specific imple- mentation.

One suitable cell re-selection algorithm is based on measurements of signal power received from serving and neighbour cells and path loss pa- rameters calculated on the basis of these measurements. Such algorithm is used in the TETRA system and defined in the TETRA standard. Let us con- sider the general aspects of such an algorithm. When a radio terminal is syn- chronised to a cell, it directly measures the received power of that cell and di- rectly obtains the broadcast and synchronisation information for that cell by decoding the Broadcast Network CHannel BNCH and Broadcast Synchroniza- tion CHannel BSCH. In that way, the radio terminal obtains a C1 path loss pa- rameter for a neighbour cell. The radio terminal also calculates another path loss parameter C2 of neighbour cells, using information about the neighbour cells broadcast by the serving cell. The path loss measurements, C1 and C2, can be used to maintain a ranked list of neighbour cells. The ranking algorithm will rank the monitored neighbour cells in strict order of downlink radio connec- tion quality. The results of this algorithm can be used to determine when a cell is deemed to be radio usable, radio relinquishable or radio improvable. For ex- ample, a cell may be required to meet the following minimum criteria in order to be included in the ranking list of neighbour cells : C1 > 0 or C2 > 0. If these criteria are not satisfied, a radio terminal does not include that cell in the rank- ing list and so does not consider that cell for cell re-selection. Alternatively, the ranking may be based on C1 or C2 only in certain situations. Also other broad- cast information, such as the supported services for the neighbour cells, can be used in the ranking and cell re-selection. Finally, the radio terminal chooses the cell which has the highest ranking. It should be appreciated that this is only one example of re-selection algorithms. Any other suitable algorithm can be used instead.

The present invention improves any such conventional algorithm in that the broadcast group availability information provides additional information for the radio terminal selecting the new cell : the radio terminal knows before the cell selection attempt which groups are supported on each of the neighbour cells. Thus, the radio terminal can rank higher than normally those neighbour cells that support the desired group (s) so that one of them is most likely or exclusively selected, if otherwise satisfactory.

For example, a radio terminal may rank higher those cells that sup- port one of the following groups: 1) group where there is an ongoing call ; or 2) group that is currently actively used (so-called selected group); or 3) all of the attached groups of the radio terminal ; or 4) most of the attached groups of the radio terminal; or 5) most of the important groups attached in the radio termi- nal.

The invention can be used also if the network is planned so that there is double coverage. For example, if one"umbrella cell"covers the areas of several normal/small size cells, load of the umbrella cell can be decreased by means of directing the group communication of desired groups to the other cells. This is achieved so that the broadcast messages of the other cells do not indicate that the groups are available via the umbrella cell. So the radio terminals that are interested in the group communication tend to favour the normal/small size cells.

The description only illustrates preferred embodiments of the inven- tion. The invention is not, however, limited to these examples, but it may vary within the scope and spirit of the appended claims.