WO2008113211A1

WO2008113211A1 - Method and system for determining a set of target cells suitable for handover form a source cell in a cellular communication system

Info

Publication number: WO2008113211A1
Application number: PCT/CN2007/000910
Authority: WO
Inventors: Henrik Olofsson; Michael Roberts; Johan Johansson
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2007-03-20
Filing date: 2007-03-20
Publication date: 2008-09-25
Also published as: CN101637043A

Abstract

The present invention relates to a method in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein each of said cells transmits identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other. The method comprises the steps of, from a serving cell radio transmitter transmitting, to a user entity in the coverage area of said serving cell, a representation of a subset of said cells, wherein only cells of said subset constitute allowed cells for a subsequent handover or cell reselection from said serving cell to a target cell. The present invention also relates to a system, a user entity, a computer program and a computer program product.

Description

Method and system for determining a set of target cells suitable for handover from a source cell in a cellular communication system

Field of the Technology

The present invention relates to a method for determining a set of target cells suitable for handover from a source cell in a cellular communication system according to the preamble of claim 1. The present invention further relates to a communication system according to the preamble of claim 27, a user entity according to claim 53, a computer program according to claim 55 and a computer program product according to claim 56.

Background of the Invention

Wireless mobile communication systems are usually cellular, i.e., the total coverage area of such systems is divided into smaller areas wherein each of these smaller areas are associated with a radio base station having one or more radio transmitters for providing communication resources via a radio interface for communication with user entities (UEs). The areas are usually called cells, and the UEs can, e.g., consist of mobile phones, smartphones or handheld computers having communication capabilities, located in said area. Two or more cells may share a base station, e.g., the base station may be provided with radio transmitters that transmit in different directions, the coverage area of each transmitter being defined as a cell. Alternatively, a single cell may comprise two or more radio transmitters for providing radio coverage in a specific area. In the simplest form of a cellular communications network, the cells are located adjacent to each other to jointly provide coverage in the geographical area that the communication system is intended to cover. The network supports mobility by allowing the interface between the UE and the network (i.e., connection to a radio transmitter) to move by connecting the UE to different radio transmitters as the UE moves around in the network. When the UE is switched on and is communicating with the network, it can be in an active or an idle state. When the UE is in an idle state, the activity of the UE is reduced to a minimum to reduce power consumption. However, the UE will regularly and/or continuously and/or at certain occasions listen to radio transmitters surrounding the radio transmitter to which the UE is presently connected in order to identify suitable surrounding (adjacent or neighbour) cells to connect to (camp on to) as it moves around in the network coverage area. In idle mode, this is called cell reselection. The UE will reselect to a new neighbour cell if it is determined that the new cell is better for service than the camped cell.

. When in active mode, the UE also regularly and/or continuously and/or at certain occasions measures suitable cells. If the UE moves from one cell to another in active mode, however, a simple cell reselection is not sufficient, since there is an ongoing communication and the communication channel(s) need to be relocated from the current cell (serving cell) to a suitable cell (target cell). This relocation of communication channels is called handover. The network uses measurement reports from the UE to decide whether to do a handover or not.

In order to reduce the amount of signalling from the UE to the network regarding measurement reports of surrounding cells, a neighbour cell list (NBL) is transmitted from the network to the UE. The NBL is cell specific, and one purpose of the list is to define suitable target cells surrounding the cell transmitting the list. The lists are necessary since the radio environment may create situations wherein a cell which, based on radio quality measurements alone, appears to be the most preferred cell, may not be suitable for handover at all.

The neighbour cell lists are transmitted, e.g., on a Broadcast Control Channel

(BCCH), which is designed to cover the whole cell area. The BCCH is a common downlink channel which does not have any uplink acknowledgement channel associated with it. This means that the data must be robustly encoded and therefore consume relatively large amount of radio resources.

In fact, there is a risk that in some cases the transmission of the neighbour list may consume too many of the available resources for one cell. One example of this is the current 3rd Generation Partnership Program (3 GPP) work on defining a future cellular communication system, presently called the Evolved Universal Terrestrial Radio Access (E-UTRA) or Long Term Evolution (LTE), in which a rather small bandwidth may be used in a cell, for example 1.25 MHz. Apart from the small bandwidth, the coverage areas of these cells are usually small, resulting in a lot of surrounding cells being potential target cells. In such situations, the neighbour cell list may be extensive, which further increases bandwidth usage for the list.

A solution to the above problem is disclosed in 3GPP document R2-070493, "Neighbour cell list reduction", which describes a method wherein the UE implicitly generates a neighbour cell list based on information transmitted in each cell. In this solution, a number of standard cellular configurations (patterns) are predefined and preconfigured. Each cell is allocated a specific identity (logical cell value, LCV) in a pattern. Since the neighbour cells are numbered using a specific pattern, the UE can read the logical cell identity of the serving cell and implicitly deduce the numbers allocated to each real neighbouring cell.

During both idle and active mode mobility, however, the cells must be identified during communication between the network and the UE. For example, in active mode, the UE report measurements on neighbouring cells, wherein the UE needs to signal which cell was detected and measured. Although the LCV is already a compression since each pattern only covers a subset of all the cells in the network, there exists a need for a method to further reduce signalling in the system.

Further, in reality, cell plans rarely contains cells of equal size ordered in a strict pattern. Consequently, there is also a need for an improved system having improved capabilities of handling such situations.

Summary of the Invention

It is an object of the present invention to provide a method in a cellular communication system that solves the above mentioned problems. This object is achieved by a method according to the characterizing portion of claim 1.

It is a further object of the present invention to provide a communication system that solves the above mentioned problems. This object is achieved by a communication system according to the characterizing portion of claim 27.

According to the present invention, it is provided a method in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein each of said cells transmits identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other. The method comprises the step of, from a serving cell radio transmitter, transmitting, to a user entity in the coverage area of said serving cell, a representation of a subset of said cells, wherein only cells of said subset constitute allowed cells for a subsequent handover or cell reselection from said serving cell to a target cell.

This has the advantage that even though, in principle, neighbour cell lists are transmitted by cells in the communication system, these lists can be represented by referring to portions of a cell pattern, which, consequently, can allow a substantial reduction of data transmitted from cells in the system. This is particularly true in areas wherein the possible number of neighbour cells, and thereby the extent of the lists, is large, since, instead of transmitting the identities of all cells in the list, a single common reference, e.g., a maximum distance from the serving cell, can include all cells that are to be considered as allowed cells.

Further characteristics of the present invention, and advantages thereof, will be evident from the following detailed description of preferred embodiments and appended drawings, which are given by way of example only, and are not to be construed as limiting in any way.

Brief Description of the Drawings

Fig. 1 discloses an example of a Long Term Evolution (LTE) system in which the present invention may advantageously be utilised.

Fig. 2 discloses an exemplary cell pattern according to the present invention. Fig. 3 a discloses a further example of a cell pattern according to the present invention.

Fig. 3b discloses an example of cell renumbering according to the present invention.

Fig. 4 discloses the set of cells of fig. 3 a, with the second tier renumbered according to the invention.

Fig. 5 discloses a plurality of smaller cells being arranged to cover the coverage area of a single cell position in a cell pattern. Fig. 6 discloses usage of the cell pattern in different frequencies having different cell sizes. Detailed Description of the Invention

In the description and claims of the present invention, the term "handover" is intended to include not only a handover in active mode, i.e., handover of one or more communication channels, but also the situation when a user entity (UE) in idle mode performs a cell reselection. Further, the term "serving cell" is intended to comprise not only the cell a UE in active mode has an ongoing communication with, but also the cell a UE is camped on to in idle mode.

The present invention will now be described with reference to the 3rd Generation Partnership Program (3 GPP) work on defining a future cellular communication system, which presently is called the Evolved Universal Terrestrial Radio Access (E- UTRA) or Long Term Evolution (LTE).

An example of the architecture of an E-UTRA system 100 is shown in fig. 1, and consists of two kinds of nodes, radio base stations (stationary radio transceivers), eNBs (enhanced Node B) 101-103 and aGWs (access GateWay) 104-105, where the eNBs 101-103 belongs to the evolved UTRAN (E-UTRAN) 106 and the aGW belongs to the evolved packet core (EPC) 107. A user entity (UE) 108 connects to the network (E-UTRAN and EPC) by means of a radio interface, Uu interface 109. The eNB 101-103 handles communication over the radio interface in a certain coverage area, i.e., cell, which is the area wherein the radio signal is strong enough to allow a satisfactory communication with UEs within said area.

When the UE 108 moves around in the coverage area provided by the communication system, the UE 108 will move from one cell to another, and thereby an ongoing communication will be transferred from the eNB to which the UE presently belongs, i.e., the serving cell, to the cell into which the UE is entering, i.e., the target cell. This is accomplished by establishing a communication channel on the Uu interface of the target cell, and terminating the communication channel on the Uu interface of the source cell.

The target cell is selected based on measurements of potential target cells performed by the UE, which measurements are transmitted to the eNB for processing by the system. The target cells that are classified as "potential" are defined in a neighbour cell list, which is transmitted to UEs within range of the eNB on a radio channel. As was stated above, there exits a desire to reduce the transmission of information associated with the transmission of neighbour cell lists, in particular when the cells are small and, consequently, the number of cells are high, e.g., the neighbour cell list may comprise up to 96 neighbour cells.

Further, in idle mode operation the UE does not transmit any measurement reports to the eNB, and no explicit signalling is required when the UE is changing cells, at least not as long as it is moving around within a tracking area. However, the need of neighbour cell lists in idle mode can still be similar to when in active mode, with similar problems regarding excessive use of bandwidth.

According to the present invention, a method is provided that allows the neighbour cell list information to be determined in the UE, in active and/or idle mode, without the need of transmitting extensive lists of surrounding, or neighbouring, cells from each cell in the system. This is accomplished by a system wherein the cells, instead of transmitting neighbour cell lists, transmit cell-specific information that includes data from which a representation of its location in a cell pattern can be derived. The cell pattern describes the relative location of cells in the system with respect to each other, and which can be seen as representation of the cells of a geographical area. Further, according to the present invention, the serving base station (eNB) (i.e., the base station providing one or more communication channels or to which an idle UE is camped on to) transmits information to the UE regarding which of the cells in the pattern that are to be considered when establishing suitable neighbour cells, i.e., which cells that are to be considered as allowed cells. Such information can, e.g., include information (cell pattern positions, herein denoted logical cell values LCV) identifying allowed cells, or identities (logical cell values) of the restricted cells, i.e., all cells but the allowed cells (i.e., the cells that should not be considered). This has the advantage that a set of suitable target cells can be easily derived by the UE in a system wherein the cells only need to transmit cell-specific data, apart from the information regarding restricted and/or allowed cells, which information, however, due to the pattern, can be kept to a much smaller amount as compared to prior art cell lists. The transmission of which further has the advantage that the network can ensure that the UE only considers cells that are considered appropriate by the network. Consequently, advantages of the prior art method of transmitting a controlled neighbour cell list containing appropriate cells can be accomplished by the present invention as well, although with the advantages of the reduced signalling, and thereby reduced consumption of radio resources as well,

In fig 2 is shown an exemplary cell pattern according to the present invention. In the disclosed example, the parameters "Row offset" and "Maximum LCV" is used to define a cell pattern, having the meaning that the cell pattern consists of a linear row of 16 cells, and the cell pattern of the total coverage area is created, in the x-direction, by immediately beginning with a new pattern where the previous ends, i.e., a cell numbered "15" is followed by cell number "0". In order to obtain a two-dimensional pattern, rows are put together also in the y-direction, wherein each row is followed by a row having a Row offset of 4.5, i.e., the start of a consecutive rows in the y- direction are offset by 4.5 cells in the x-direction. This can, e.g., be seen by comparing cell 201 with pattern number, or, as the pattern numbers are called in this description and claims, logical cell value LCV, "0", while cell 202 has LCV "5" (as can be noted in the figure, no row is shown in its entirety). Consequently, if, as in the figure, the number of cells in a system is large, the cell pattern can be reused, i.e., duplicated, a number of times. Naturally, patterns containing a larger number of cells can be used instead, or a combination thereof.

Each cell transmits data, which the UE can use to determine the relative location and identity of the cell with respect to the source (serving) cell and/or other cells in the system. This data may, for example, consist of the LCV, i.e., the value that represents the specific location or position of the cell within the cell pattern. This LCV can, e.g., be transmitted on a downlink common channel or of any other suitable channel, as long as it is possible to decode data transmitted on the channel by UEs currently camping in neighbouring cells. For example, the LCV could be sent on BCH or any broadcasted channel (for example SCH).

The LCV is assigned to the cell from the set of LCVs defined in the specific cell pattern based on the location of the cell in relation to its surrounding cells.

In order to reduce the number of cells a UE in, e.g., cell 203 is required to consider for a handover in idle and/or active mode, the network signals a representation of an allowed set of cells for this purpose. For example, by appropriate signalling from the base station of cell 203, the network can tell the UE how many surrounding "cell circles" surrounding the serving cell, herein called tiers, the UE should consider as potential handover candidates (neighbours). In the example shown in fig. 2, two tiers are to be considered as neighbours, i.e., allowed cells. The first tier consists of the immediate neighbours, i.e., the cells directly bordering to the cell 203. The second tier consists of the immediate neighbours to the cells of the first tier. Consequently, this is one way for the network to limit which LCVs that should be considered by the UE. By signalling the tier value, the network can in a simple manner create a subset of LCVs, i.e., allowed cells, in a much simpler, and thereby bandwidth efficient, manner as compared to conventional neighbour cell lists. The UE can then perform a cell search to find all detectable cells and for each detected cell determine whether the LCV of the detected cell belongs to the allowed LCV set, and if so, add it to the UE determined list of suitable cells for handover.

As an alternative to signalling the number of tiers to be considered, tiers not to be considered as neighbouring cells can be signalled to the UE instead. For example, tiers >3 can be considered as restricted cells. As a further alternative to signalling the tier value, a distance value could be signalled instead, indicating the maximum relative distance to a cell that should be considered. In such a solution, the cell pattern can be used to determine a distance between the currently camped cell and a measured cell. Since the pattern is a schematic representation of actual locations of cells, the determined distances can constitute distances in the pattern and need not constitute actual distances between the actual cells in the system.

So far, only complete tiers have been considered as belonging to an allowed set of LCVs. However, there are circumstances when one or more cells in the allowed set of cells are unsuitable, and therefore should be excluded from the allowed set of cells. One way of accomplishing this is to signal the explicit LCV of the one or more cells to be excluded. Such signalling, however, can require a large number of bits, for example if the pattern is large and comprise a high number of cells, or if, as in fig. 2, there are ambiguities regarding an LCV, e.g., LCVs 10, 12, 14 occurs twice in the allowed set, which means that these have to be distinguished if only one of these cells are to be excluded. According to the present invention, if an allowed LCV set has been defined, such as in fig. 2, cells of the allowed LCV set can be provided with a second identity to reduce such signalling. This will be explained with reference to figs. 3a-b. Fig 3a shows a further example of a cell pattern, wherein the cell pattern is given by the parameters "Geometrical shape" and "size", with the values: Geometrical shape: rhombic, and size: 4*4, i.e., the cell pattern consists of 16 positions arranged as a rhomb. In this example, the allowed set of LCVs are restricted to the immediate neighbours, i.e., closest tier of the serving cell 5. If it is desirable to restrict a cell in the allowed set, i.e., restrict any of the cells 0, 1, 4, 6, 9, 10, four bits are required to identify the cell, i.e., to address any of the cells 0 to 15. According to the present invention, however, the number of bits required can be reduced by renumbering cells in the allowed set. This is exemplified in fig. 3b, wherein the number of bits required to identify a cell is reduced to three. As can be seen in the figure, the cells in the allowed set, including the serving cell, is renumbered from left to right, top to bottom. In this way, only numbers zero to six has to be represented, with the result that only three bits are required. Naturally, the gain is dependent on number of LCVs in a pattern. Should more than 16 LCVs be used in a pattern, more than four bits would be required and, consequently, the potential gain in using renumbering according to the present invention is even greater. The described method requires that the addressing within the allowed LCV is always unambiguously defined, which, e.g. can be accomplished by the above way of numbering from top left and move to the right and then one step down. Another exemplary method is to start numbering from the centre (serving) cell and then successively extend into the outer tiers, if any, starting the numbering at a predefined position in each tier.

In some scenarios, there may be a need to add one or more additional LCVs, outside an allowed LCV set, to the allowed LCV set. The above renumbering method is applicable in such situations as well. The LCVs surrounding the allowed LCV set can be numbered in a predefined way, thereby reducing the signalling of adding additional cells. This is illustrated in Fig. 4, which discloses the set of cells of fig. 3a, however with the second tier renumbered according to the invention. Note that in this particular example, the actual gain of this compression is limited, since four bits still are required to uniquely identify a cell in the second tier. As was stated above, this solution is most efficient if the LCV range is much larger than the allowed LCV set.

So far, only patterns containing cells being substantially homogeneous in cell size have been discussed. However, due to geographical, user density or other reasons it can be necessary that a plurality of smaller cells are arranged to cover the coverage area of a single LCV in a pattern. Such a situation is shown in fig. 5, in which , e.g., cells 4, 5, 9, 10 of a pattern as disclosed in fig. 3a is shown. Cell 9, however, is, in this example, replaced by a plurality of smaller cells having identities 1-7. Assuming, for simplicity, that cell 5 is a serving cell and only cells 4, 10 and the plurality of cells representing cell 9 belongs to the allowed set, it is advantageous if these cells can be distinguished from each other. This can, for example, be accomplished by adding an additional address valid inside the LCV. These cells (1-7) can, for example, broadcast the LCV value 9 and, in addition, an additional field, hereinafter called intra LCV address tag (ILAT), which is the identity of the cell within the LCV.

If the range of the ILAT is quite small, e.g., 0-7, i.e., requiring three bits, the same ILAT bit length can, for example, be used for all LCV consisting of a plurality of cells. However, if the range is quite large and is varying throughout the system, it can be beneficial to vary the signalling range of the ILAT. For example, one possibility is to signal the value range of the ILAT from the serving cell to the UE. The serving cell can be arranged to broadcast the ILAT range of all neighbouring LCVs. Alternatively, the serving cell can be arranged to only broadcasting the ILAT range for LCVs having an ILAT range different from a default value.

Continuing with a solution such as the one in fig. 5, the situation may arise, wherein only part of the cells with ILAT 1-7 constituting LCV 9 is to be included in an allowed set of cells for establishing suitable handover candidates. In such a situation, it can be desirable to address a set of cells, e.g., part of the cells 9:1-7 in fig.

5 in order to define a set of cells which should not be considered by the UE during mobility. For example, cells 3, 6, 7 could be excluded. Such exclusion is often called black listing. If this it to be done within an LCV, as in the figure, the ILAT can be used. One possibility is then to list the LCV+ILAT for each cell that is to be black listed.

Another solution is to carefully plan the usage of ILAT and address many cells at a time. One possibility is to disregard the Least Significant Bits of the ILAT, and only transmit the Most Significant Bits. By doing so, multiple cells can be addressed with one signalling value. Yet another possibility is to transmit the starting ILAT and the ending ILAT with the result that these and all ILAT in-between are black listed.

Finally, it is also possible for each cell to signal its membership in black list groups (BLG). This signalling would be done by each cell broadcasting a bit sequence containing as many entries as there are BLG. This is exemplified in table 1 below.

Table 1

As can be seen in table 1 , there are four BLGs, and the membership of a specific cell in such a group is indicated by a "1". For example, cell 5 is a member of BLGs 0 and 2, and if any of these groups are blacklisted, cell 5 will become a restricted cell. In the example of fig. 5, the serving cell, i.e., cell 5 can, e.g., transmit 9:1, which means that cells within LCV 9 and belonging to BLG 1 are restricted, i.e., cells 3, 6, 7 of LCV 9 are restricted. Naturally, BLG setting can be different for different cells surrounding an LCV containing BLGs. For example, if LCV 10 is the serving cell, 9:3, i.e., cells 1, 3, 6 of LCV 9 are restricted. Further, different LCVs in a pattern may be represented different number of cells, and thereby different number of ILATs.

In such a solution, the ILAT cells can, e.g., either be arranged to transmit LCV (9) and an ILAT number, in which case the UE, if provided with information regarding which ILAT that belong to which BLG, can determine whether the cell should be included in the allowed cell or not. Alternatively, the ILAT cell can transmit LCV and

BLG, e.g., cell 9:1 in fig. 5 could transmit 9: [1 0 0 1], defining that it belongs to BLGs 0 and 3. Consequently, in such a situation, no ILAT has to be transmitted from an ILAT cell.

Further, the serving cell can be arranged to transmit a similar signal, i.e. 9: [1 1 0 0], meaning that cells with LCV 9 and belonging to blacklist groups 0 and 1 should be excluded from the allowed set.

In one embodiment, the LCV can be implicitly signalled by, for example, mapping another broadcasted value to a certain LCV. Assuming that the cellular system is using some form of cell identifier, cell-ID, one example of implicit signalling is to group these cell identifiers into groups where each group points to a specific LCV. The UE then only needs to decode the cell identity to be able to derive the LCV of a particular cell. This mapping of the cell-ID:s to LCVs can, for example, be static and stored in the UE. Alternatively, the system can signal the relationship at the entering of a new cell pattern area, i.e., an area in which a certain cell pattern is valid. If such an implicit method is used for broadcasting the LCV, it can be further extended to also signal the ILAT. If, for example, 512 cell identities are used for broadcast in the system, i.e., 9 bits are required, these 9 bits can be used to signal 64 different LCV (6 bits) and still leave room to signal 8 ILAT (3 bits).

In the prior art, the cell-ID is often coded in such a manner that it is immediately retrievable for a UE that has detected a cell in order to allow a fast cell identification. For example, in LTE, the cell-ID can be coded so that the UE only needs to decode the synch channel (SCH) to retrieve cell-ID, wherein the cell-ID is implicitly coded as one of a plurality of reference sequences on the SCH. Consequently, using the above mapping of LCV onto cell-ID, the UE will immediately know the LCV of the cell. This has the advantage that a very fast determination of the LCV can be obtained. Hitherto, methods for determining the allowed set have been described. However, even though these methods allow a substantial reduction in number of cells that the UE has to consider, there is still a limited measurement capacity in the UE, both due to processing capability and due to battery consumption. There is also a limited reporting capability, due to battery consumption and radio resource capacity shortage. Therefore, when selecting which cells the UE shall measure (idle and active mode) and report (active mode), the UE can prioritize the cells based on some criteria, e.g., the relative distance. This means that the UE can measure and/or report relatively closer cells more often. For example, if the allowed set consists of two tiers, cells of the inner tier can be reported more often than the cells of the outer tier. The network can also choose to limit how many cells the UE is allowed to report.

In some cases, different LCV within the allowed LCV set should be prioritized differently. One example of this is during inter-frequency mobility, when the UE is moving between two frequencies. In this case, the cells on the different frequencies may be of different size. For example, one frequency can be used for macro-cells and one frequency can be used for micro cells. In such situations, there is an uncertainty when moving from larger to smaller cells since the actual location of the UE within the source cell is unknown. An example of such a scenario is depicted in fig. 6.

In this scenario, a group of cells should be considered to be of equal priority in the target frequency. For example, if a UE is in the larger cell LCV of frequency 1 , 1 :5, cells 4, 5, 8, 9, 10, 12, 13, 14 of the second frequency are to be considered as having equal priority. As an alternative to signalling specific LCVs of the second frequency, this relaxation of priority handling in the target frequency can be solved by including a parameter describing the relative difference in cell size between the different frequencies. For example, the number of tiers of smaller cells that is typically covered by a larger cell can be signalled. Of course, such a solution can also be applied for inter-Radio Access Technology (RAT) mobility. Although the present invention has been described in connection with an LTE system, the principles of the invention applies to cellular access systems in general, and are therefore applicable in any cellular system, such as, but not limited to, CDMA2000, UMTS & GSM.

Claims

1. Method in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein each of said cells transmits identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other, characterised in that the method comprises the steps of: from a serving cell radio transmitter,

- transmitting, to a user entity in the coverage area of said serving cell, a representation of a subset of said cells, wherein only cells of said subset constitute allowed cells for a subsequent handover or cell reselection from said serving cell to a target cell.

2. Method according to claim 1, characterised in that said identity data represents the position of said cells in said cell pattern.

3. Method according to claim 1, characterised in that said cells in said pattern are distinguishable by a logical cell value (LCV) representing the position of a cell in said pattern, and wherein said identity transmitted from said cells at least partly consist of a representation of said logical cell value.

4. Method according to claim 1, characterised in that said method further includes the step of, in said user entity,

- generating a set of target cells based on said signals transmitted from said cells and said representation of allowed cells transmitted from said radio transmitter.

5. Method according to claim 1, characterised in that said representation of a subset of said cells consists of one or more from the group: a maximum distance to a cell from said serving cell, one or more tiers of cells surrounding said serving cell, a range of identities.

6. Method according to claim 1, characterised in that a plurality of said allowed cells are represented by a common representation, and that said transmitter further transmits a representation of one or more additional cells to be included in said allowed set of cells.

7. Method according to claim 1, characterised in that said transmitter further transmits a representation of one or more cells to be excluded from said allowed set of cells.

8. Method according to claim 6 or 7, characterised in that said representation of one or more cells consists of any from the group: one or more tiers of cells surrounding said serving cell, one or more cell identities.

9. Method according to claim 6, characterised in that said representation of an additional cell consists of a second identity, said second identity being an identity derived from simplified identity valid in an area surrounding said allowed set.

10. Method according to claim 7, characterised in that said representation of a cell to be excluded consists of a second identity, said second identity being an identity derived from a simplified identity valid within said allowed set.

11. Method according to claim 1, characterised in that a plurality of cells share an identity of a single position in said cell pattern.

12. Method according to claim 11, characterised in that said plurality of cells sharing an identity of a position of said cell pattern are distinguishable from each other by means of a second identity.

13. Method according to claim 12, characterised in that said plurality of cells are arranged according to a second pattern within said cell identity.

14. Method according to claim 1, characterised in that at least some of said plurality of cells belong to at least one blacklist group, wherein transmission of a representation of a blacklist group excludes members of said group from said allowed set.

15. Method according to claim 14, characterised in that a cell belonging to a blacklist group transmits an identity of the cell and an indication of the one or more blacklist groups to which it belongs.

16. Method according to claim 1, characterised in that said user entity perform and report measurements of characteristics of cells of said allowed set.

17. Method according to claim 16, characterised in that when sending measurements, cells are identified according to a second identity, said second identity being a simplified identity valid within said allowed set.

18. Method according to claim 16, characterised in that said measurements are reported to said radio transmitter.

19. Method according to claim 16, characterised in that said cells of said allowed set are provided with a priority, wherein cells of higher priority are measured more often than cells of lower priority.

20. Method according to claim 19, characterised in that said priority is based on a quality of a radio transmission from said cell, and/or the distance to said cell.

21. Method according to claim 1, characterised in that said identity is mapped onto a cell-ID, so as to allow said identity to be determined by determining said cell- ID.

22. Method according to claim 21, characterised in that said cell-ID is coded such that it is immediately decodable when said cell is detected by said user entity.

23. Method according to claim 1, characterised in that said pattern is reused so that that the pattern is repeated a plurality of times in said communication system.

24. Method according to claim 1, characterised in that said radio transmitter is a radio transceiver.

25. Method according to claim 1, characterised in that said radio transmitter is a radio base station.

26. Method according to claim 25, characterised in that said radio transceiver is an eNB (enhanced Node B) in a Evolved UTRA (E-UTRA) system.

27. A cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein each of said cells is arranged to transmit identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other, characterised in that the system comprises means for: from a serving cell radio transmitter,

28. System according to claim 27, characterised in that said identity data is arranged to represent the position of said cells in said cell pattern.

29. System according to claim 27, characterised in that said cells in said pattern are distinguishable by a logical cell value (LCV) representing the position of a cell in said pattern, and wherein said identity transmitted from said cells at least partly consist of a representation of said logical cell value.

30. System according to claim 27, characterised in that said system further includes means for, in said user entity,

31. System according to claim 27, characterised in that said representation of a subset of said cells consists of one or more from the group: a maximum distance to a cell from said serving cell, one or more tiers of cells surrounding said serving cell, a range of identities.

32. System according to claim 27, characterised in that a plurality of said allowed cells are represented by a common representation, and that said transmitter further is arranged to transmit a representation of one or more additional cells to be included in said allowed set of cells.

33. System according to claim 27, characterised in that said transmitter further is arranged to transmit a representation of one or more cells to be excluded from said allowed set of cells.

34. System according to claim 32 or 33, characterised in that said representation of one or more cells consists of any from the group: one or more tiers of cells surrounding said serving cell, one or more cell identities.

35. System according to claim 32, characterised in that said representation of an additional cell is arranged to consist of a second identity, said second identity being an identity derived from simplified identity valid in an area surrounding said allowed set.

36. System according to claim 33, characterised in that said representation of a cell to be excluded is arranged to consist of a second identity, said second identity being an identity derived from a simplified identity valid within said allowed set.

37. System according to claim 33, characterised in that a plurality of cells are arranged to share an identity of a single position in said cell pattern.

38. System according to claim 37, characterised in that said plurality of cells being arranged to share an identity of a position of said cell pattern are distinguishable from each other by means of a second identity.

39. System according to claim 38, characterised in that said plurality of cells are arranged according to a second pattern within said cell identity.

40. System according to claim 27, characterised in that at least some of said plurality of cells belong to at least one blacklist group, wherein transmission of a blacklist group excludes members of said group from said allowed set.

41. System according to claim 40, characterised in that a cell belonging to a blacklist group is arranged to transmit an identity of the cell and an indication of the one or more blacklist groups to which it belongs.

42. System according to claim 27, characterised in that said user entity is arranged to perform and report measurements of characteristics of cells of said allowed set.

43. System according to claim 42, characterised in that when sending measurements, cells are identified according to a second identity, said second identity being a simplified identity valid within said allowed set.

44. System according to claim 42, characterised in that said measurements are reported to said radio transmitter.

45. System according to claim 42, characterised in that said cells of said allowed set are provided with a priority, wherein cells of higher priority are measured more often than cells of lower priority.

46. System according to claim 45, characterised in that said priority is based on a quality of a radio transmission from said cell, and/or the distance to said cell.

47. System according to claim 27, characterised in that said identity is mapped onto a cell-ID, so as to allow said identity to be determined by determining said cell- ID.

48. System according to claim 47, characterised in that said cell-ID is coded such that it is immediately decodable when said cell is detected by said user entity.

49. System according to claim 27, characterised in that said pattern is arranged to be reused so that that the pattern is repeated a plurality of times in said communication system.

50. System according to claim 27, characterised in that said radio transmitter is a radio transceiver.

51. System according to claim 27, characterised in that said radio transmitter is a radio base station.

52. System according to claim 51, characterised in that said radio transceiver is an eNB (enhanced Node B) in a Evolved UTRA (E-UTRA) system.

53. User entity for use in a cellular communication system, said communication system including a plurality of cells, each of said cells including at least one radio transmitter for providing communication with at least one user entity in a coverage area of said cell, wherein each of said cells is arranged to transmit identity data from which a position of said cell in a cell pattern can be derived, said cell pattern describing the relative location of said cells with respect to each other, characterised in that the user entity comprises means for:

- receiving, from a serving cell radio transmitter, a representation of a subset of said cells, wherein only cells of said subset constitute allowed cells for a subsequent handover or cell reselection from said serving cell to a target cell.

54. User entity according to claim 53, characterised in that it further includes means for:

55. Computer program, characterised in code means, which when run in a computer causes the computer to execute the method according to any of the claims 1- 26.

56. Computer program product including a computer readable medium and a computer program according to claim 55, wherein said computer program is included in the computer readable medium.

57. Computer program product according to claim 56, characterised in that said computer readable medium consists of one or more from the group: ROM (Read-Only Memory), PROM (Programmable Read-Only Memory), EPROM (Erasable PROM), Flash memory, EEPROM (Electrically Erasable PROM), hard disk drive.