WO2008086648A1

WO2008086648A1 - Cell search in a cellular communications system

Info

Publication number: WO2008086648A1
Application number: PCT/CN2007/000037
Authority: WO
Inventors: Martin Bakhuizen; Henrik Olofsson; Michael Roberts
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2007-01-08
Filing date: 2007-01-08
Publication date: 2008-07-24
Also published as: WO2008086648A8; CN101507335A

Abstract

The invention concerns a method for cell search of the type performed in conjunction with selecting a Public Land Mobile Network (PLMN) in a cellular communications system, wherein the cellular communications system includes at least one terminal and at least two cells, the method including:- said at least one terminal obtaining, in any of said at least two cells, data on at least one candidate camping cell, - using said data for cell search. The method is distinguished by:- in said step of obtaining data, obtaining data including information on preference for at least one of said at least one candidate camping cell,- in said step of cell search, searching a cell based on said information on preference. The invention also concerns a terminal, a base station and a cellular communications system.

Description

Cell search in a cellular communications system

Field of the invention

The present invention concerns a method for cell search in a cellular communications system. Further, it concerns a terminal for a cellular communications system, a base station for a cellular communications system and a cellular communications system.

Background of the invention

5.1 Public Land Mobile Network background A public land mobile network (PLMN) is a cellular system comprising a number of cells of which the user equipment (UE), for instance a cellular phone or terminal, has to select one cell for accessing service. The PLMN can use one or more radio access technologies (RAT), e.g. 2G GSM (Global System for Mobile Communications), 3 G UTRAN (Universal Terrestrial Radio Access Network) or 3 G LTE (Long Term Evolution). From one location the UE can normally discover several cells belonging to different PLMNs, and normally several cells belonging to the same PLMN. One PLMN may have organized its cells in a so called Hierarchical Cell Structure (HCS) as depicted in Figure 1. When in idle mode, the UE has to select one of the cells that the UE can detect, where it can camp for later access of a service. The idle mode procedure is described in section 5.2 and the cell selection procedure is further described in section 5.3.

5.2 Idle mode procedure Overview

The basic procedures and rules for how to make the cell selection in a 3 G network is outlined in the respective 3GPP specifications, of which some are found in references [1], [2], [3] and [4], specified at the end of the description. These procedures are described in the context of UMTS (Universal Mobile Telecommunications System). However, many of these are applicable to other digital cellular radio systems. An example of such a system is GSM.

When a UE is switched on, a public land mobile network (PLMN) is selected. In order to select a PLMN, a search for cells has to be effected such that at least the PLMN identity of the cells can be read and the selection be made. The UE then searches for a suitable cell of the selected PLMN and selects that cell to provide available services, and tunes to its control channel. This selecting is known as "camping on the cell". The UE will then, if necessary, register its presence, by means of a Non- Access Stratum (NAS) registration procedure, in the registration area of the chosen cell and as outcome of a successful Location Registration the selected PLMN becomes the registered PLMN [5].

If the UE finds a more suitable cell, it reselects onto that cell and camps on it. If the new cell is in a different registration area, location registration is performed.

If necessary, the UE may search for higher priority PLMNs at regular time intervals and search for a suitable cell if another PLMN has been selected by NAS.

If the UE loses coverage of the registered PLMN, either a new PLMN is selected automatically (automatic mode), or an indication of which PLMNs are available is given to the user, so that a manual selection can be made (manual mode). The purpose of camping on a cell in idle mode is fourfold: a) It enables the UE to receive system information from the PLMN. b) When registered and if the UE wishes to establish an RRC (Radio Resource Control) connection, it can do this by initially accessing the network on the control channel of the cell on which it is camped. c) If the PLMN receives a call for the registered UE, it knows (in most cases) the registration area of the cell in which the UE is camped. It can then send a "paging" message for the UE on control channels of all the cells in the registration area. The UE will then receive the paging message because it is tuned to the control channel of a cell in that registration area and the UE can respond on that control channel. d) It enables the UE to receive cell broadcast services.

If the UE is unable to find a suitable cell to camp on, or the User Subscriber Identity Module (USIM) is not inserted, or if the location registration failed, it attempts to camp on a cell irrespective of the PLMN identity, and enters a "limited service" state in which it can only attempt to make emergency calls. The idle mode tasks can be subdivided into three processes:

PLMN selection; Cell selection and reselection; Location registration. The relationship between these processes is illustrated in Figure 2.

5.3 Cell Selection procedure Overview

The NAS (Non- Access Stratum) can control the RAT(s) (Radio Access Technology(ies)) in which the cell selection should be performed, for instance by indicating RAT(s) associated with the selected PLMN (public land mobile network), and by maintaining a list of forbidden registration area(s) and a list of equivalent PLMNs. The UE selects a suitable cell and the radio access mode based on idle mode measurements and cell selection criteria.

In order to speed up the cell selection process, stored information for several RATs may be available in the UE.

When camped on a cell, the UE shall regularly search for a better cell according to the cell reselection criteria. If a better cell is found, that cell is selected. The change of cell may imply a change of RAT.

For normal service, the UE has to camp on a suitable cell, tune to that cell's control channel(s) so that the UE can:

Receive system information from the PLMN;

• Receive registration area information from the PLMN, e.g., location area and routing area; and

• Receive other AS and NAS Information; - If registered:

• Receive paging and notification messages from the PLMN; and

• Initiate call setup for outgoing calls or other actions from the UE.

With reference to figure 3, the reference letters denote as follows: a): cell information stored for the PLMN b): no cell information stored for the PLMN c): suitable cell found d): no suitable cell found e): leave idle mode f): return to idle mode g): NAS indicates that registration on selected PLMN is rejected h): trigger As seen in Figure 3, the UE will use one of the following two search procedures: a) Initial Cell Selection

This procedure requires no prior knowledge of which Radio Frequency (RF) channels are Universal Terrestrial Radio Access (UTRA) carriers. The UE scans all RF channels in the UTRA bands according to its capabilities to find a suitable cell. On each carrier, the UE need only search for the strongest cell. Once a suitable cell is found this cell is selected. b) Stored Information Cell Selection

This procedure requires stored information of carrier frequencies and optionally also information on cell parameters, e.g. scrambling codes, from previously received measurement control information elements. Once the UE has found a suitable cell the UE shall select it. If no suitable cell is found the Initial cell selection procedure is started.

The intention with the Stored Information Cell Selection procedure is that it will be quicker than the Initial Cell Selection. The UE will spend less time searching for a suitable cell, and will more quickly come to the camped state so that it is ready for the initiation of a service.

5.4 Stored Information Cell Selection procedure The main source of information is the information broadcasted by each cell. In UTRAN it consists of a Master Information Block (MIB) and a number of System Information Blocks (SIBs). Example of information available in the MIB and SIBs is:

• PLMN identity(ies) of the cell,

• access restrictions like Cell Barred and Access Class Barring, • "Intra-frequency cell info list"

• "Inter-frequency cell info list" "Inter-RAT cell info list",

"Use of HCS (Hierarchical Cell Structure)",

• "HCS neighbouring cell information"

5.5 Problem

With the addition of more RATs (Radio Access Technologies)(e.g. LTE (Long Term Evolution)) and the addition of more spectrum for cellular systems and the capability of a UE (User Equipment, for instance a cellular phone or terminal) to connect to these different RATs on different frequencies, the information that the UE can collect from the PLMN about the cell and its neighbours is expanding. The UE will also normally be able to detect an increasing number of cells. This results in that the UE will have an increasing number of cells to choose from and on which to perform a search, in order to do a cell selection. Thus, it is a problem that the Cell Selection procedure in many cases will take an unacceptable long time to complete, a time while initiation of service is not possible. The same is true for the selection of a PLMN, due to the necessity to search for cells in order to at least discover the PLMN identity of the cells so that a selection can be made. In addition to these problems the industry is entering an era where the cell sizes may be very small with low power amplifiers (the so called femto cells, for home cell deployment and indoor applications). With this type of deployment there is no guarantee that when a mobile is switched on that such cellular coverage is available. Given that an operator will still try to accomplish indoors coverage from the macro/umbrella cell, a mobile may find itself searching for many small cells rather than trying to get immediate service from umbrella or macro cellular layer.

The Stored Information Cell Selection procedure provides some mitigation of the problem of slow cell search by using stored information on cells in the search. However, due to the anticipated boost in available cells described above, cell search times will still be problematic in many situations. As has been said, this unacceptably affects both PLMN selection and cell selection times.

Summary of the invention

It is an object of the present invention to propose a solution for or a reduction of the problems of prior art. A main object is consequently to provide a method that allows for a more swift PLMN and cell selection in cellular radio networks.

The main idea of the invention is that the network will signal some additional information about the cells that will help the UE (User Equipment, for instance a cellular phone or terminal) in finding and selecting a suitable cell in conjunction with selecting a Public Land Mobile Network, i.e. after switch on and after loss of coverage when the selected PLMN cannot be found. This will decrease the time for finding a suitable cell, both for PLMN selection and for cell selection, and thus shorten the time while the UE is unavailable for setting up a service. It is assumed that after the UE has been switched on for a very first time it has collected information about the network including different cell information by either receiving specific cell information, both for the currently selected cell and it's neighbours, from the network or by the UE measuring and building it's own neighbour cell information list.

This knowledge may contain very many candidates. In order to speed up the cell search and decrease the time until a suitable cell is found, it may be necessary to reduce the number of candidates, or indicate which cells are of higher probability to be a suitable cell, for the cell search procedure. According to the invention this is accomplished by sending some sort of information on preference for cells to the UE. This can be done in many ways, e.g. by broadcasting information or by dedicated signalling when the UE is detaching from the network.

Furthermore the current information that the UE can collect contains very limited information about the HCS, if and when implemented by the PLMN. E.g. the UE cannot determine which of the neighbour cells is a larger cell covering the last accessed cell. This larger cell is the most probable to be available, but as said cannot be accurately distinguished by the currently available information. According to the invention this can be avoided by sending said information on preference, whereby preferred cells can be searched first.

According to the invention, the problems of prior art are also solved or reduced according to the invention in a terminal, a base station and a cellular communications system respectively having the features of claims 16, 19 and 20 respectively.

The dependent claims disclose further advantageous aspects of the invention.

Brief description of the drawings

Embodiments exemplifying the invention will now be described, on hand of the appended drawings, wherein Fig. 1 illustrates a Cellular system with HCS (Hierarchical Cell Structure)

Fig. 2 illustrates an Overall Idle Mode process in 3GPP UTRAN (Universal Terrestrial

Radio Access Network)

Fig. 3 illustrates Idle Mode Cell Selection and Reselection in 3GPP UTRAN, Fig. 4 illustrates how the invention would modify, as an example, the existing Overall

Idle Mode process in 3GPP UTRAN,

Fig. 5 illustrates HCS cell layers and neighbour cell pointers, and

Fig. 6 illustrates an example of a Cellular communications system with two base stations, forming each a cell, and one terminal.

Detailed description of preferred embodiments

7.1 Overview

The very first time a UE is switched on it will have to perform the Initial Cell Selection procedure. It will then acquire information about the network, either by explicit signalling from the network or complemented by the UE' s own measurements. The subsequent times when the UE is switched on it may use this information to make a Stored Information Cell Selection, which is normally faster than the Initial Cell Selection. The Stored Information Cell Selection procedure is also used after loss of coverage from the selected PLMN, where the UE is registered. While the currently provided information from the network is aimed at helping the UE at both cell selection and cell reselection, the solution in the invention is targeted at assisting the

UE in the cell selection procedure and in PLMN selection. The existing solution signals a complete list of neighbour cells via channels in the selected cell where the UE is camping.

According to a method of the invention, cell search of the type performed in conjunction with a terminal (also known as a UE (User Equipment)) selecting a Public Land Mobile Network (PLMN), i.e. after switch on of the terminal and after loss of coverage when the selected PLMN cannot be found, in a cellular communications system, is performed as follows. The cellular communications system includes at least one terminal and at least two cells. The method includes the steps: - said at least one terminal obtaining, in any of said at least two cells, data on at least one candidate camping cell and - said data would be used for cell search.

Further, in said step of obtaining data, data would be obtained including information on preference for at least one of said at least two candidate camping cells, and - in said step of cell search, searching a cell based on said information on preference.

Thus, according to the invention, the network points out the cells that are preferred or more likely to be available at PLMN/cell selection and then the terminal can use the information on preference to search for a cell, whereby the search would be quicker compared to prior art.

A conceptual view of an example of the invention could be found in Figure 4, where a) denotes data, including information on preference, signalled in order to aid the terminal in performing the PLMN selection and Cell Selection respectively.

It is also possible for the network to give more information about the HCS configuration, the size of the current and neigbouring cells and the probability to reconnect to the current and neigbouring cells and/or layer.

The cell search of the method of the invention could be performed in order to facilitate a selection of a Public Land Mobile Network (PLMN) or a selection of a cell.

Thus, for both PLMN selection and Stored Information Cell Selection, the procedure is enhanced, so that the UE starts to search for cells based on preference, e.g cells with the highest preference or priority according to the new information. This will increase the probability to quickly find a suitable cell.

The assumption made is that the UE is normally switched on in the same geographical area as where it was switched off. E.g. a user switches his UE off before going to bed and switches it on in the morning.

7.2 Signalling priority of the cells

Currently in UTRAN the network can signal up to 32 neighbouring UTRAN FDD (Frequency Division Duplex) and additionally up to 32 GERAN (GSM/EDGE Radio Access Network) neighbouring cells. Also the UE can get information about up to 32 UTRAN TDD (Time Division Duplex) cells. This is already up to 96 cells. With the addition of LTE the total is expected to increase, possibly with another 32 neighbours.

According to the invention the network will by signalling means indicate which cells are preferred or the most probable to be a suitable cell. This can be done in several ways.

When identifying suitable cells, the following steps of the method of the invention may be possible to identify each member:

• The information on preference is obtained as a ranked order of cells. Thus, the order of neighbour cells is used (if transmitted to the UE) to express the preference of cells. • The information on preference is obtained as an individual preference factor for a cell.

Thus, using a preference factor, information on the cells may be sent in any order to the terminal (or UE).

• Signal a group ID in which the cell(s) belongs

• Signal a value other than the Cell ID in each cell, which uniquely identifies the cell among the cells that the UE can concurrently receive information from, and use that as a reference in the list

Further, in order to organise data on one or more candidate camping cells, according to a variant step of a method of the invention, it is possible to obtain said data on at least one candidate camping cell contained in at least one list. Having received such a list, a method of a variant of the invention could include a step wherein in cell selection, said at least one list would be traversed in accordance to cell preference when searching a candidate camping cell.

Lists containing candidate camping cells may be formatted in different ways. According to a variant step of a method of the invention, it is possible to obtain a said at least one list containing all neighbour cells of a cell. Alternatively, a list containing a subset of all neighbour cells of a cell could be used. Thus, the amount of content in lists can vary according to the invention.

Alternatively, a separate list independent of the existing lists can be used.

The content of lists obtained according to a step of the method of the invention may vary.

For instance a list may contain cells of the same radio access technology. Alternatively a list may contain cells of different radio access technologies. The list may use the full cell ID for the cells in the list. It is possible to compress lists of candidate camping cells. According to a preferred step of the method of the invention, it is possible to obtain at least one list that is at least partly compressed by declaring only a difference of at least one member of the list to a fully defined member of the list. According to a further preferred step, the difference may be given in the form of the least significant bits of a full cell id. Thus, compressed cell IDs would be signalled, e.g. by giving one full cell ID, then for the other cells just the least significant bits of the full cell ID, or a difference to the first cell ID.

In the case of a plurality of lists, according to a variant step of the method of the invention, it is possible to obtain at least one list of said plurality of lists containing an indication, such as an endpoint marker, anywhere in said at least one list where to switch to another list when performing cell search. In this way, the most preferred cells of each list would be searched first. If none of the cells were found, the search could continue after the indication in a list. Possibly, also an indication of in which priority order the different lists should be used (e.g. first UTRAN FDD list, then GERAN list, then UTRAN TDD list and so on) could be signalled to the terminal.

The signalling of the information on preference to at least one terminal could according to a variant step of a method of the invention be accomplished in accordance with any of the ways from the group consisting of: broadcasted in the current cell, signalled dedicated upon detach and signalled when going back to idle mode from connected mode. Basically, any suitable way and any suitable occasion signalling the information on preference could be employed.

7.3 Signalling the preferred HCS layer and cell

In the current UTRAN network the network signals only a HCS priority value (0-7) as an indication of the HCS layer. The neighbour cell list may contain one or more neighbours with the same HCS priority value, and with a value with higher priority than the currently selected cell. The UE will not know which of these higher layer cells is the one that overlaps the currently selected cell. The invention proposes that the network signals which cell(s) of each higher HCS layer that overlaps with the currently selected cell. In Figure 5, the network is configured with two HCS layers (for simplicity, could be more layers). The currently UE selected cell is in HCS layer 2 and has a small square pattern. With the current UTRAN network the network would signal a list of neighbour cells containing all the cells with a pattern (dotted, dashed, waved, squared, total 25 cells). In the solution proposed by the invention, the network can point out the dashed cell in the higher HCS layer 1 as the most probable to be a suitable cell at switch on. The network could also indicate the closest neighbours in the HCS layer 2, the waveform patterned ones, as being highly probable to be suitable.

In addition the network may indicate to the UE which HCS layer is the preferred one for the cell selection procedure.

7.4 Signalling the size of the current cell and/or layer

The network can indicate the HCS layer or an explicit cell size for all or a subset of the cells in the different lists. A larger cell gives higher probability to be found to be a suitable cell. If this cell is not in the most suitable cell layer, cell reselection procedure will take care of the necessary reselection. The main importance is to quickly gain access to the system.

7.5 Indicating the feasibility to reconnect to a cell and/or layer Add a probability factor to each cell in the respective cell lists.

7.6 Signalling methods for the added information ^•

This new information about the cells can be signalled by the network in different ways: It may be broadcasted in the current cell, signalled dedicated upon detach or signalled when going back to idle mode from connected mode.

7.7 On the different steps of the method of the invention

The different steps of the method of the invention described above can be combined or performed in any suitable order. A condition for this of course, is that the requirements of a step, to be used in conjunction with another step of the method of the invention, in terms of available measures such as terminals, list of candidate camping cells having a certain format etc., must be fulfilled.

8 Other embodiments of the invention Other embodiments of the invention will now be described, with reference to figure 6. In figure 6, 1 denotes a Cellular communications system, 2 denotes a base station forming each, in this example, a cell 4 and 3 denotes a terminal.

Thus, the invention comprises a terminal 3 for a cellular communications system 1 , the terminal 3 being adapted to perform cell search of the type performed in conjunction with selecting a Public Land Mobile Network (PLMN) in said cellular communications system 1 , wherein the cellular communications system includes at least two cells 4,

- said terminal 3 being adapted to obtain, in any of said at least two cells 4, data on at least one candidate camping cell 4, - said terminal 3 being adapted to use said data for cell search, wherein

- said terminal 3 being adapted to, when obtaining said data, obtaining data including information on preference for at least one of said at least two candidate camping cells 4,

- said terminal 3 being adapted to, when using said data for cell search, search a cell 4 based on said information on preference. In this way, cell search in a terminal could be performed more swiftly.

In a variant embodiment of a terminal of the invention, said terminal could be adapted to perform said cell search in order to select a Public Land Mobile Network (PLMN). Alternatively, said terminal could be adapted to perform said cell search in order to select a cell. In this way, a terminal could decrease any of PLMN and selection times.

The terminal of the invention can be adapted to include means for performing any of the steps of the method of the invention. A trivial requirement is of course that such a step do involve a terminal.

The invention also comprises a base station 2 for a cellular communications system, the base station being adapted to assist in cell search of the type performed in conjunction with selecting a Public Land Mobile Network (PLMN) in said cellular communications system 1, wherein the cellular communications system includes at least two cells 4, - said base station 2 being adapted to transmit, to a terminal 3, data on at least one candidate camping cell 4, wherein - said base station 2 being adapted to, when transmitting said data, transmit data including information on preference for at least one of said at least two candidate camping cells 4.

The base station of the invention can be adapted to perform any of the steps of the method of the invention. A trivial requirement is of course that such a step do involve a base station.

Further, the invention also comprises a cellular communications system 1, the cellular communications system 1 being adapted to perform cell search of the type performed in conjunction with selecting a Public Land Mobile Network (PLMN) in said cellular communications system, wherein the cellular communications system 1 includes at least one terminal 3 and at least two cells 4,

- said at least one terminal 3 being adapted to obtain, in any of said at least two cells 4, data on at least one candidate camping cell 4,

- said terminal 3 being adapted to use said data for cell search, wherein

- said terminal 3 being adapted to, when using said data for cell search, search a cell 4 based on said information on preference.

The cellular communications system of the invention can be adapted to perform any of the steps of the method of the invention.

9 APPLICATIONS AND ALTERNATIVES

The invention can be used in any communication network where there exist mobility and cell selection. 10 REFERENCES

[1] 3GPP TS 25.304, v6.9.0, "User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode"

[2] 3GPP TS 23.122, "NAS functions related to Mobile Station (MS) in idle mode" [3] 3GPP TS 43.022, "Functions related to Mobile Station in idle mode and group receive mode"

[4] 3GPP TS 25.214, "Physical layer procedures (FDD)"

[5] 3GPP TS 25.331 , v6.11.0, "Radio Resource Control (RRC); Protocol

Specification" [6] 3GPP TDoc R2-063305, "Reduction of neighbour cell list information sent to

UE"

[7] 3GPP TS 36.300, vθ.2.0, "Evolved Universal Terrestrial Radio Access (E-

UTRAN) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (Release 8)",

2006-11 [8] 3GPP TR 25.913, V7.0.0, "Requirements for Evolved UTRA (E-UTRA) and

Evolved UTRAN (E-UTRAN) (Release 7)", 2005-06

[9] UK Patent Application GB 2 420 937 "A mobile radio communications device connecting to a network of a radio access technology (RAT)"of the type performed when selecting a new Public Land Mobile Network (PLMN) in a cellular communications system

Claims

1. A method for cell search of the type performed in conjunction with a terminal selecting a Public Land Mobile Network (PLMN) in a cellular communications system I₅ wherein the cellular communications system 1 includes at least one terminal 3 and at least two cells 4, the method including:

- said at least one terminal 3 obtaining, in any of said at least two cells 4, data on at least one candidate camping cell 4,

- using said data for cell search, characterised by:

- in said step of obtaining data, obtaining data including information on preference for at least one of said at least two candidate camping cells 4,

- in said step of cell search, searching a cell 4 based on said information on preference.

2. A method for cell search according to claim I₅ wherein:

- performing said cell search in order to select a Public Land Mobile Network (PLMN).

3. A method for cell search according to claim 1, wherein: - performing said cell search in order to select a cell.

4. A method for cell search according to any of claims 1, 2 or 3, wherein:

- the information on preference is obtained as a ranked order of cells.

5. A method for cell search according to any of claims 1, 2 or 3, wherein:

- the information on preference is obtained as an individual preference factor for a cell.

6. A method for cell search according to claim I₅ wherein: - obtaining said data on at least one candidate camping cell contained in at least one list.

7. A method for cell search according to claim 6, wherein:

- in cell selection, traversing said at least one list according to cell preference when searching a candidate camping cell.

8. A method for cell search according to claim 6, wherein:

- obtaining a said at least one list containing all neighbour cells of a cell.

9. A method for cell search according to claim 6, wherein:

- obtaining a said at least one list containing a subset of all neighbour cells of a cell.

10. A method for cell search according to any of claims 6-9, wherein:

- obtaining a said at least one list containing cells of a same radio access technology.

11. A method for cell search according to any of claims 6-9, wherein: - obtaining a said at least one list containing cells of different radio access technologies.

12. A method for cell search according to any of claim 6-9, wherein:

- obtaining a said at least one list that is at least partly compressed by declaring only a difference of at least one member of the list to a fully defined member of the list.

13. A method for cell search according to claim 12, wherein:

- obtaining a said at least one list that is at least partly compressed by the difference being given in the form of the least significant bits of a full cell id.

14. A method for cell search according to any of claims 6-9, wherein:

- in the case of a plurality of lists, obtaining a said at least one list of said plurality of lists containing an indication anywhere in said at least one list where to switch to another list when performing cell search.

15. A method for cell search according to any of claims 1, 2 or 3, wherein:

- the information on preference is signalled to said at least one terminal in accordance with any of the ways from the group consisting of: broadcasted in the current cell, signalled dedicated upon detach and signalled when going back to idle mode from connected mode.

16. A terminal 3 for a cellular communications system I₅ the terminal 3 being adapted to perform cell search of the type performed in conjunction with selecting a Public Land

Mobile Network (PLMN) in said cellular communications system 1, wherein the cellular communications system includes at least two cells 4,

- said terminal 3 being adapted to obtain, in any of said at least two cells 4, data on at least one candidate camping cell 4, - said terminal 3 being adapted to use said data for cell search, characterised in that:

- said terminal 3 being adapted to, when obtaining said data, obtaining data including information on preference for at least one of said at least two candidate camping cells 4, - said terminal 3 being adapted to, when using said data for cell search, search a cell 4 based on said information on preference.

17. A terminal for a cellular communications system according to claim 16, wherein:

- said terminal being adapted to perform said cell search in order to select a Public Land Mobile Network (PLMN).

18. A terminal for a cellular communications system according to claim 16, wherein:

- said terminal being adapted to perform said cell search in order to select a cell.

19. A base station 2 for a cellular communications system I₅ the base station 2 being adapted to assist in cell search of the type performed in conjunction with selecting a Public Land Mobile Network (PLMN) in said cellular communications system 1 , wherein the cellular communications system 1 includes at least two cells 4,

- said base station 2 being adapted to transmit, to a terminal 3, data on at least one candidate camping cell 4, characterised in that:

- said base station 2 being adapted to, when transmitting said data, transmit data including information on preference for at least one of said at least two candidate camping cells 4.

20. A cellular communications system 1, the cellular communications system being adapted to perform cell search of the type performed in conjunction with selecting a

Public Land Mobile Network (PLMN) in said cellular communications system 1 , wherein the cellular communications system includes at least one terminal 3 and at least two cells 4,

- said terminal 3 being adapted to use said data for cell search, characterised in that:

- said terminal 3 being adapted to, when using said data for cell search, search a cell based on said information on preference.