AU2062000A

AU2062000A - A method of selecting cells in a cellular mobile radio system

Info

Publication number: AU2062000A
Application number: AU20620/00A
Authority: AU
Inventors: Francois Courau; Michael Roberts
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: Alcatel Lucent SAS
Priority date: 1999-03-05
Filing date: 2000-03-02
Publication date: 2000-09-07
Also published as: CN1267176A; DE60031735D1; ES2275476T3; FR2790632A1; EP1037490B1; ATE345023T1; EP1037490A1; DE60031735T2; FR2790632B1; CN1157970C

Abstract

The cell selection technique has a process in which mobile units are informed of nearby cells and which measure the effect and report towards the network. The network has different types of cells for different services and the mobile station (1) is informed of preferred services (2) and reports (3) to the network measurement of the results.

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT oooe oe Invention Title: .e 'A method of selecting cells in a cellular mobile radio system' The following statement is a full description of this invention, including the best method of performing it known to us: FH4PSvFYrXNATP2fNi N~f6R4Q9 A METHOD OF SELECTING CELLS IN A CELLULAR MOBILE RADIO

SYSTEM

The present invention relates generally to cellular mobile radio systems, i.e. cellular mobile radio networks and mobile stations for use in such networks.

BACKGROUND OF THE INVENTION In prior art cellular mobile radio systems a distinction is drawn between two mobile station operating modes: dedicated mode, in which a mobile station has one or more dedicated channels available to it (either traffic channels used to transmit traffic information or signaling channels used to transmit signaling information either during a call or between calls), and standby mode, in which a mobile station has no So. dedicated channels available to it but must nevertheless remain in contact with the network, for which purpose it uses channels referred to as common channels.

In the prior art, each cell is associated with a particular frequency, referred to as the broadcast control channel frequency, on which said common channels are broadcast and, in outline, the process of server cell selection (or reselection) by a mobile station is as follows.

In standby mode, the mobile station receives on the broadcast control channel frequency of its server cell first system information consisting of a list of broadcast control channel frequencies associated with cells adjoining its server cell. The mobile station then scans each of those broadcast control channel frequencies in succession to perform radio measurements on them and, after synchronization, to acquire second system information consisting of parameters which are used in conjunction with the results of the radio measurements to determine if the current broadcast control channel frequency meets a selection criterion and then, by comparing results obtained in the same fashion for the other broadcast control channel frequencies, to determine which cell to select. The mobile station then uses third system information broadcast on the broadcast control channel of the cell to be selected to determine whether that cell is in the same location area as the current server cell. If it is, the cell to be reselected becomes the new server cell. If it is not, the mobile station advises the network of this beforehand, by means of a location updating procedure, in order to have the network verify the user's right to access the cell determined in this manner. If the response from the network is in the affirmative, that cell becomes the new server cell.

no When a mobile station enters service, at which time .no server cell has yet been selected, it selects a server S 15 cell according to the same principle but scans all the broadcast control channel frequencies and uses a 00.:procedure referred to as registration to verify the right to access the selected cell.

If a call cannot be set up in a cell selected in this way in standby mode (in particular for traffic reasons, because of a traffic overload in that cell, or for radio reasons, because of radio propagation problems in that cell), the network commands the mobile station to try to access the network again in cells which are indicated to it in the call set-up phase. This is referred to as "directed retry".

The procedure by which a mobile station changes from standby mode to dedicated mode is referred to as the access procedure.

A similar cell selection process is used in dedicated mode except that the mobile station reports measurement results to the network, which determines the target cell to be selected, i.e. the cell to which handover is to be effected, on the basis of the measurement results and other parameters. To be more precise, the cell selected is the first of the reported cells (classified in a particular order, on the basis of the measurement results) to which the network determines that handover is possible, i.e. in particular in which resources are available for a handover. This process is referred to as "mobile-assisted handover". The directed retry procedure can likewise be used if it is not possible to handover to the cells tried in this way (in particular for traffic reasons, because of a traffic overload in those cells, or for radio reasons, because of radio propagation problems in those cells).

For a more comprehensive description of cellular mobile radio systems such as the Global System for Mobile communications (GSM) see "The GSM System for Mobile Communications" by M. Mouly and M.B. Pautet (Palaiseau, France, ISBN 2-9507190-07-7) The present invention relates more particularly to cell selection problems caused by changes associated with the introduction of new services.

~The present invention therefore relates more particularly to the problem of cell selection in a cellular mobile radio network including different types of cells in which different types of service are available.

One example of a network of the above kind is a network in which a new radio interface is progressively introduced into an existing infrastructure corresponding to a system such as a second generation system (in particular the aforementioned GSM), the new interface corresponding, for example, to circuit-switched data services, such as the high-speed circuit-switched data (HSCSD) service, or packet-switched data services, such as the general packet radio service (GPRS), or to a third generation system (in particular the universal mobile telecommunications system (UMTS)).

A system of the above kind therefore includes cells in which new services are available, for example UMTS or GSM/UMTS cells cells having a broadcast control channel frequency common to the GSM radio interface and the UMTS radio interface), and cells in which new services are not available, for example GSM cells.

In a system of the above kind, the cell selection process outlined above can select a GSM cell when the required service necessitates a UJMTS or GSM/UMTS cell or a UMTS or GSM/UMTS cell when a GSM cell would be satisfactory for the required service, so that the allocation of radio resources in a network of the above kind is in no case optimized as a function of the services required by users.

Another example of a network of the above kind is a network in which an additional radio interface dedicated •go• :to a particular group of users and enabling those users to access additional services in a given geographical area is introduced into an existing infrastructure corresponding to that geographical area and to a given system, for example the GSM.

In a system of the above kind, the cell selection o process outlined above can select a cell for a user •ooo authorized to use additional services in which the additional services are not available, in which case **neither the user nor the provider of those services is satisfied. This can occur in particular if the power at which broadcast control channel frequencies associated with cells in which the additional services are available are transmitted is made lower than the power at which broadcast control channel frequencies associated with cells in which the additional services are not available are transmitted, in order to avoid interfering with the frequency plan of the system into which the additional radio interface is introduced (in particular the GSM) Thus the problem of the cell selected not necessarily being the best possible cell in terms of services generally arises in a cellular mobile radio system including different types of cells in which different- types of services are available.

Document W097/44922 describes a cellular mobile radio system including different types of cells in which different multimedia type services are available and a best cell is selected in dedicated mode on the basis of a number of parameters including measurement results for candidate cells, the availability of resources in those candidate cells, the type of service required, etc. The solution described in W097/44922 has the drawback of a relatively great increase in the complexity of the mobile stations and/or of necessitating many exchanges of signaling between mobile stations and the network (over and above those already provided and outlined above), leading to a high consumption of radio resources for such exchanges, and therefore to non-optimum use of the 9s15 available radio resources and an increase in the overall 9999 level of interference in the network.

Document W098/56207 describes a cellular mobile radio system including different types of cells in which different types of services are available and in which, in contrast to the system described in the previous document, when the network selects an appropriate cell in 9. terms of services in dedicated mode it takes account of .999 subscription data relating to the user concerned and itself containing service information. One drawback of a solution of this kind is that the selected cell is not necessarily the optimum, in particular because no account is taken of the radio environment as perceived by the mobile stations.

W096/34499 describes a system in which fixed subscribers have limited rights of access to particular cells forming what is referred to as a fixed subscription area. It describes a "redirection" function whereby, if a cell other than those of a fixed subscription area is selected in standby mode, and if at least one cell of the fixed subscription area is a cell adjoining the selected cell, then the network indicates the adjoining cells to the mobile station so that the mobile station can select CA00367458.2 6 one of them to set up a call. W096/34499 does not address the problem of cell selection in dedicated mode in a cellular mobile ratio system including different types of cells in which different types of services are available, in particular the problem of mobile-assisted handover.

Document W098/38827 addresses this problem and describes a system including cells in which special services are available to some users. According to W098/38827, a special list of cells is drawn up for a mobile station and transmitted to a base station controller during call set-up, and if necessary during handover, in order to redirect a mobile station to a special cell, if necessary. In W098/38827, the unit which controls handover (for example the base station controller) monitors the requirement for handover in accordance with °.measurements performed as in the prior art. When handover is required the base station controller selects appropriate target cells on the basis of the measurement results. According to W098/38827, if one of the selected cells is in said special oo 15 list, it is selected as the target cell for the handover. If none of the target cells is in the special list, the base station controller selects as the target cell a cell which satisfies the prior art handover criterion. According to W098/38827, it is therefore still possible for the cell selected not to be the optimum in terms of service.

a. a Thus the prior art does not provide an optimum solution to the problem of cell selection in dedicated mode in a cellular mobile radio system including different types of cells in which different types of services are available.

Summary of the invention The present invention provides a method of selecting a cell in dedicated mode in a cellular mobile radio system, in which method a mobile station is informed of cells adjoining its current server station and on which measurements are to be performed and reports measurement results obtained in this way to the network, which includes different types of cells in which different types of services are available, wherein the mobile station is informed of which adjoining cells on which measurements are to be performed are preferred in terms of services and reports to the network the results of measurements on those cells.

The measurement results reported to the network therefore relate to cells which are preferred in terms of service, so guaranteeing selection by the network of an optimum cell from the service point of view (it being 15 understood that from a radio point of view the cells which are preferred in terms of service do not necessarily constitute the best cells, but that the cell selected from the cells which are preferred in terms of services can be the best from the radio point of view, depending on the measurement results).

In other words, the present invention adapts the standard dedicated mode cell selection procedure (or mobile-assisted handover) to the situation of networks including different types of cells in which different types of services are available, in a manner which is relatively simple and does not require major modification of the network or the mobile stations.

According to another feature of the invention, the mobile station is supplied with the information referred to above via a dedicated signaling channel assigned to the mobile station.

This addresses the following problems. In the prior art methods referred to above, the system information required for cell selection is broadcast on a common channel which in the GSM is referred to as the broadcast control channel CBCCH) Clearly, in the methods described above the acquisition and use of such 0A00367458.2 8 information by mobile stations are complex operations which are costly in terms of processing time. This problem is accentuated in systems like those described above in which new services are introduced and which therefore require a large amount of information in order to be able to describe them to mobile stations in sufficient detail to enable them to perform the cell selection process efficiently. Also, such broadcasting would give rise to many practical implementation problems in a network of the above kind because of the relatively small capacity of the broadcast channels, initially determined for systems including only one type of cell in all of which the same services are available.

These problems are addressed by a method of the above kind of transmitting cells which are preferred in terms of services on a dedicated signalling channel assigned to the mobile station.

This mode of transmission also enables the information transmitted to be S °•personalized to suite the profile of the user, which is not possible with the broadcast 15 transmission used.

According to another feature of the invention the cells adjoining the current server cell and on which measurements are to be performed and which are preferred in terms of services are determined within the network on the basis of the type of .service required by the mobile station.

According to another feature of the invention the type of service required can be indicated to the network by the mobile station itself.

According to another feature of the invention the type of service required can be determined within the network by interrogating a mobile user database in the network and containing service information.

According to another feature of the invention the cell selection in dedicated mode can be effected during handover to a best cell in terms of services from a cell which was initially selected in standby mode for a mobile station to access the network and which did not constitute a best cell in terms of services.

This enables an optimum cell to be selected to access the network without the various problems referred to previously.

The present invention also provides a mobile station for implementing a method of the above kind.

The present invention further provides a cellular mobile radio network entity for implementing a method of the above kind.

X ~BRIEF DESCRIPTION OF THE DRAWINGS Other objects and features of the present invention will become apparent on reading the following description 15 of one embodiment of the invention, which is given with oooe reference to the accompanying drawings, in which: Figure 1 is a diagram showing one example of a too.. system to which the present invention can be applied, Figure 2 is a diagram illustrating one example of the method according to the invention, and Figures 3 and 4 are diagrams illustrating one '*top. example of the respective resources to be provided in a .000 mobile station and in a network to which the present invention can be applied to enable use of the method according to the invention.

MORE DETAILED DESCRIPTION As shown in Figure 1, a cellular mobile radio system essentially includes: a set of base transceiver stations (BTS), such as stations BTSO, BTS1, BTS2, BTS3 and BTS4, connected via a radio interface to mobile stations such as stations MSO, MS1, MS2, MS3, MS4, the base transceiver stations being assigned to respective cells CO, Cl, C2, C3 and C4 in the example shown, a set of base station controllers (BSC) such as controllers BSCl and BSC2, each BSC controlling, from the radio resource management point of view, a subset of cells, the base station controller BSC1 controlling the cells CO, C1 and C2 and the base station controller BSC2 controlling the cells C3 and C4 in this example, a set of mobile switching centers (MSC), only one of which, denoted MSC1, is shown in the figure, each MSC being connected in a manner that is not specifically illustrated to external networks such as the public switched telephone network (PSTN) or the integrated services digital network (ISDN) and controlling, from the call management point of view, a subset of base station controllers, the mobile switching center MSC1 controlling the base station controllers BSC1 and BSC2 in the example shown, and databases such as the home location register (HLR) or the visitors location register (VLR) to which the MSC are connected and which are used in particular to manage subscription data and roaming.

The base station subsystem (BSS) formed by the BTS and the BSC constitutes what is referred to above as the network radio interface.

The combination of the MSC, HLR and VLR is referred 9 to as the network and switching subsystem (NSS) In an example of a system to which the present invention can be applied, the radio interface formed by 25 the units BTS3, BTS4 and BSC2 constitutes a new radio interface introduced into an existing infrastructure corresponding to the GSM, for example, and already having its own radio interface formed of the units BTSO, BTS1, BTS2 and BSC1. Cells such as cells CO, C1 and C2 then constitute a first type of cell, corresponding to GSM cells, and cells such as cells C3 and C4 constitute a second type of cell, in which additional services are available.

Figure 2 is a diagram illustrating a dedicated mode cell selection method according to the invention.

As shown diagrammatically in Figure 2,"a mobile station: receives information informing it of the cells adjoining its server cell which are preferred in terms of services and for which measurements are to be performed (step i), performs measurements on those cells (step and reports the measurement results obtained to the network (step 3) As already indicated, the measurement results reported to the network relate to cells which are preferred in terms of service, guaranteeing selection by the network of an optimum cell to which to handover the ooe call (step 4) The mobile station can perform the measurements and report the measurement results obtained to the network in 15 a conventional way which does not need to be described here. In particular, the mobile station could report only a subset of the cells on which the measurements are oo• performed, in particular the ones which are best from the radio point of view.

20 Similarly, the network could select a cell on the *basis of the reported measurement results in a ~conventional manner that does need to be described here.

The information advising the mobile station of the e cells adjoining its server cell which are preferred in terms of services and for which measurements are to be performed is advantageously transmitted to the mobile station by the network on a dedicated signaling channel assigned to that mobile station.

For example, in a cellular system such as the GSM, the dedicated signaling channel can be the standalone dedicated control channel (SDCCH) used to transmit signaling information between calls, for example during roaming management procedures or call set-up and clearing down phases, or the slow associated control channels (SACCH) or fast associated control channels (FACCH) used to transmit signaling information during a call.

The information transmitted to the mobile station by the network in this way can be produced in the network on the basis of the type of service required by the mobile station.

The type of service required can be indicated to the network by the mobile station itself, in particular during call set-up.

The type of service required can be determined within the network by interrogating a mobile user database in the network (in particular the HLR/VLR in a GSM network) and containing service information.

A cell can be selected in dedicated mode during handover to the best cell in terms of services from a cell which was initially selected in standby mode for a 15 mobile station to access the network and which did not o• constitute the best cell in terms of services. An application of this kind is particularly advantageous .o because it avoids the problems referred to above in relation to prior art cell selection principles in systems of the above kind.

"Thus in the simplified example shown in Figure 1, it may be assumed, for example, that new services are not available in cells CO, C1 and C2, that new services are available in cells C3 and C4 and that cell CO was initially selected for a mobile station, for example station MSO, to access the network. In this example the adjoining cells which are preferred in terms of services and on which the mobile station has to perform measurements could include cells C3 and C4. On this basis the mobile station could report to the network measurement results relating to cells C3 and C4 and the network could select cell C3, for example.

As shown in Figure 3, and in a manner that is well known in the art, a mobile station includes air interface means 10 which are used: to format information from the man-machine interface 11 in a form suitable for transmission via the air interface, and to format information received via the air interface for transmission to the man-machine interface ii.

In a manner that is well known in the art, the air interface 10 includes a transceiver 12 and a control unit 13 and the transceiver 12 includes baseband processing functions and radio functions.

The control unit 13 enables transmission and coo reception of signaling information exchanged between the S"mobile stations and the network using existing procedures or protocols of the network.

oo In particular, in the dedicated mode cell selection S"procedure considered here, the control unit 13 can include: means for enabling the mobile station to receive information on which cells adjoining its current server 20 cell are preferred in terms of services, and *oco S" means for reporting to the network measurement e*o.

results obtained in this way on those cells.

These information reception and measurement result reporting operations can themselves use an existing S 25 procedure or protocol of the system.

Procedures like these are well-known to the skilled person and their particular implementation will not present any particular difficulty to the skilled person, so they do not need to be described here in any more detail than by stating their function.

Figure 4 is a diagram illustrating the type of resources to be provided in a cellular mobile radio network entity (in particular a base station subsystem (BSS)) in a system according to the invention.

As shown in Figure 4, and in a manner that is wellknown in the art, an entity such as the BSS includes an air interface 20 which: formats information from a higher level entity 21 of the system concerned (such as a network and switching subsystem (NSS)) in a form suitable for transmission via the air interface, and formats information received via the air interface for transmission to the higher level entity 21.

As previously described for the mobile station, the air interface 20 conventionally includes a transceiver 22 and a control unit 23 and the transceiver 22 includes baseband processing functions and radio functions.

The control unit 23 sends and receives signaling messages exchanged between the mobile stations and the network using existing procedures or protocols of the system.

In particular, in the dedicated mode cell selection S"procedure considered here, the control unit 23 can include: means for transmitting to the mobile station information on the cells adjoining its current server cell which are preferred in terms of services, and means for receiving the reported measurement results for those cells.

These information transmission or reported measurement result reception operations can themselves use an existing procedure or protocol of the system.

Similarly, procedures like these are well-known to the skilled person and their particular implementation will not present any particular difficulty to the skilled person, so they do not need to be described here in any more detail than by stating their function either.

Claims

1. A method of selecting a cell in dedicated mode in a cellular mobile radio system, in which method a mobile station is informed of cells adjoining its current server station and on which measurements are to be performed and reports measurement results obtained in this way to the network, which includes different types of cells in which different types of services are available, wherein the mobile station is informed of which adjoining cells on which measurements are to be performed are preferred in terms of services and reports to the network the results oooo of measurements on those cells.

2. A method according to claim 1, wherein the mobile station is supplied with said information via a dedicated 15 signaling channel allocated to that mobile station.

3. A method according to either claim 1 or claim 2, wherein said cells adjoining said current server cell and on which measurements are to be performed and which are preferred in terms of services are determined within the eeoc network on the basis of the type of service required by the mobile station.

4. A method according to claim 3, wherein the type of service required is indicated to the network by the mobile station itself.

5. A method according to claim 3, wherein the type of service required is determined within the network by interrogating a mobile user database in the network and containing service information.

6. A method according to any of claims 1 to 5, wherein said cell selection in dedicated mode is effected during handover to a best cell in terms of services from a cell which was initially selected in standby mode for a mobile CA00367458.2 16 station to access the network and which did not constitute a best cell in terms of services.

7. A mobile station for implementing the method according to any one of claims 1 to 6, the station including: means enabling the mobile station to receive information on which cells adjoining its current server cell are preferred in terms of services, and means for reporting to the network the results of measurements obtained in this way on those cells.

8. A cellular mobile ratio network entity for implementing a method according to any one of claims 1 to 6, the entity including: means for transmitting to the mobile station information on which cells adjoining its current server cell are preferred in terms of services, and oo means for receiving said reported measurement results on those cells.

9. A method of selecting a cell in a cellular mobile radio system substantially as hereinbefore described with reference to the accompanying drawings.

10. A mobile station substantially as hereinbefore described with reference to the accompanying drawings. S"

11. A cellular mobile ratio network entity substantially as hereinbefore described with reference to the accompanying drawings. Dated this 2nd day of March 2000 ALCATEL by its attorneys Freehills Patent Attorneys