WO2009079256A1

WO2009079256A1 - Cell re-selection in a tetra communication network

Info

Publication number: WO2009079256A1
Application number: PCT/US2008/085853
Authority: WO
Inventors: Jesper Evald
Original assignee: Motorola, Inc.
Priority date: 2007-12-17
Filing date: 2008-12-08
Publication date: 2009-06-25
Also published as: GB2456126A; GB2456126B; GB0724487D0

Abstract

A method for performing cell re-selection by a mobile communication unit operating within a cellular communication network, where the mobile communication unit is associated with at least one group, where the method comprises receiving a first signal from a serving cell; and receiving at least one second signal from at least one neighbouring cell. The method further comprises performing a comparison of at least one radio link parameter of the first signal with at least one radio link parameter of the second signal. The method further comprises determining whether to transition from the serving cell to at least one neighbour cell. In addition, the method comprises applying at least one group offset to at least one radio link parameter wherein the at least one group offset relates to at least one of the following: (i) the at least one group with which the mobile communication unit is associated; (ii) the serving cell; (iii) the at least one neighbouring cell. The group offset is used to provide a bias when considering the compared radio link parameters in relation to cell re-selection by the mobile communication unit.

Description

CELL RE-SELECTION IN A TETRA COMMUNICATION NETWORK

TECHNICAL FIELD The technical field relates generally to a method and apparatus for performing cell re-selection, and more particularly to a method and apparatus for performing cell re-selection by a mobile communication unit within a cellular communication network

BACKGROUND

Wireless communications systems, for example cellular telephony or private mobile radio communications systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs). The term mobile station generally includes both hand-portable and vehicular mounted radio units.

Wireless communications systems are distinguished over fixed communications systems, such as the public switched telephone networks (PSTN), principally in that mobile stations move between service providers (and/or different BTS) and in doing so encounter varying radio propagation environments. The communication link from a BTS to a MS is generally referred to as a down-link channel. Conversely, the communication link from a MS to a BTS is generally referred to as an up-link channel.

In a wireless communications system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area defines a particular range that the BTS can maintain acceptable communications with MSs operating in its serving cell. Often these cells combine to produce an expanded system coverage area. Furthermore, cells are often grouped into location areas for the purposes of tracking a MS within the coverage area whilst minimising location-updating signalling.

In wireless communication systems, such as the TErrestrial Trunked RAdio (TETRA) system, developed by the European Telecommunications Standards

Institute (ETSI), and defined by a plurality of ETSI standards, including ETSI EN 300 392 specification series, it is known for MSs to participate in group calls with multiple MSs, these being in a multicast and/or broadcast call manner. Such group calls will, hereinafter, collectively be referred to as group calls for simplicity.

In order for MS's to participate in such group calls, it is necessary to allocate specific resources for those group calls within the communication cells serving the participating MS's. If a plurality of MS's, located within a plurality of serving cells, are participating in the same group call, it is necessary to provide resources for that group call within each serving cell.

As will be appreciated by a skilled artisan, communication resources, such as those required to be allocated for group calls, are limited. Consequently, allocating resources for a particular group call within a plurality of cells is undesirable when it may be possible for resources to be allocated for the group call in fewer cells. Accordingly, it would be desirable for MS's participating in a particular group call to be located within as few serving cells as possible, in order to minimise the number of cells within which communication resources are required to be allocated for that particular group call. In particular, it would also be desirable for MS's participating in a particular group call, and geographically located proximate to one another, to be located within the same serving cell.

For wireless communication systems such as the TETRA system, cell selection and cell re-selection is generally performed by the MS's. As part of the cell (re-) selection process, the TETRA standard employs a combination of criteria to be used in determining which cell to select to provide communication, for example as a result of cell handover process:

(i) link failure; (ii) radio signal quality in a relinquishable (serving) cell;

(iii) radio signal quality in an improvable (serving) cell;

(iv) radio signal quality in an usable (neighbour) cell; and

(v) certain service levels per cell as broadcast by the network.

Link failure occurs when the quality of an uplink or downlink connection between the MS and a Base Transceiver Station (BTS) falls below a certain level. It is known in a TETRA system that a serving cell becomes radio relinquishable typically when the following conditions are met:

(i) The serving cell main carrier path loss parameter shall, for a period of five seconds, fall below a first threshold termed 'FAST RESELECT THRESHOLD'; (ii) The main carrier path loss parameter of at least one neighbour cell shall exceed, by a first factor termed 'FAST RESELECT HYSTERESIS', the path loss parameter of the current serving cell's main carrier for a period of five seconds; and

(iii) No successful cell re-selection shall have taken place within the previous fifteen seconds. It is also known in a TETRA system that a serving cell becomes radio improvable typically when the following conditions are met:

(i) The serving cell main carrier path loss parameter shall, for a period of five seconds, fall below a second threshold termed ' SLOW RESELECT THRESHOLD ' ; (ii) The main carrier path loss parameter of at least one neighbour cell shall exceed by a second factor termed 'SLOW RESELECT HYSTERESIS' the main carrier path loss parameter of the current serving cell for a period of five seconds; and

(iii) No successful cell re-selection shall have taken place within the previous fifteen seconds. It is also known in a TETRA system that a neighbour cell becomes radio usable typically when the following conditions are met:

(i) The neighbour cell shall, for a period of five seconds, have a main carrier path loss parameter which is greater than:

'FAST RESELECT THRESHOLD' + 'FAST RESELECT HYSTERESIS'; and (ii) No successful cell re-selection shall have taken place within the previous fifteen seconds.

Cell re-selection is initiated by the MS if the serving cell becomes radio improvable, and where the service provided by the serving cell and the neighbour cell, in relation to which the serving cell is radio improvable, are the same. If the service provided by the neighbour cell is lower than that provided by the serving cell, the cell re-selection process may be postponed until the serving cell is declared radio relinquishable. If the service provided by the neighbour cell is higher than that provided by the serving cell, then the cell re-selection process may be performed as soon as the neighbour cell is declared radio usable.

In this manner, the MS decides on the most appropriate cell to select/reselect, without any knowledge of what other MS's may be served by which cells, nor which cells may be allocating resources to particular group calls and the like.

The 'FAST RESELECT THRESHOLD', 'FAST_RESELECT_ HYSTERESIS', 'SLOW RESELECT THRESHOLD' and 'SLOW_ RESELECT HYSTERESIS' terms and factors (hereinafter collectively referred to as cell re-selection parameters), and radio link information used for calculating the path loss parameters (all of these hereinafter collectively referred to as radio link parameters), are provided to the MS by the BTS of the serving cell. In practice, a parameter such as rxlev access-min could also affect the re-selection parameters along with path loss parameters, in the ranking of candidate cells. These parameters are typically broadcast on a Broadcast Synchronisation CHannel (BSCH). In this manner, the Switching and Management Infrastructure (SwMI) of the TETRA network is able to influence the re-selection of cells by manipulating the cell re- selection parameters and radio link information. However, such influence is global in that it relates to all MS's, and does not distinguish between MS's of particular groups or the like.

Thus, there exists a need for a method and apparatus for performing cell re- selection, which addresses at least some of the shortcomings of past and present cell re-selection techniques.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for performing cell re-selection by a mobile communication unit, a mobile communication unit and a network element as set forth in the appended claims. BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, which together with the detailed description below are incorporated in and form part of the specification and serve to further illustrate various embodiments of concepts that include the claimed invention, and to explain various principles and advantages of those embodiments.

FIG. 1 illustrates a block diagram of a mobile communication unit in accordance with some embodiments of the invention.

FIG. 2 illustrates a flow chart of a method of performing cell re-selection in accordance with an embodiment of the invention.

FIG. 3 illustrates a flow chart of a method of performing cell re-selection in accordance with an alternative embodiment of the invention.

FIG. 4 illustrates a flow chart of a method of performing cell re-selection in accordance with a further alternative embodiment of the invention. FIG. 5 illustrates a flow chart of a method of applying cell re-selection criterion according to some embodiments of the invention.

FIG. 6 illustrates an example of part of a cellular communication system in accordance with some embodiments of the invention.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood elements that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments. DETAILED DESCRIPTION

Generally speaking, pursuant to the various embodiments, there is provided a method of performing cell re-selection by a mobile communication unit within a cellular communication network, the mobile communication unit being associated with at least one group. The method comprises the mobile communication unit receiving a first signal from a serving cell and receiving at least one second signal from at least one neighbouring cell. The method further comprises performing a comparison of at least one radio link parameter of the first signal with at least one radio link parameter of the second signal. The method further comprises determining whether to transition from the serving cell to at least one neighbour cell and choosing one. In addition, the method comprises applying at least one group offset to at least one radio link parameter wherein the at least one group offset relates to at least one of the following: (i) the at least one group with which the mobile communication unit is associated; (ii) the serving cell; (iii) the at least one neighbouring cell. The group offset is used to provide a bias when considering the compared radio link parameters in relation to cell re-selection by the mobile communication unit.

In this manner, mobile communication units associated with a particular group will tend to relocate to those communication cells having a positive bias, and away from those communication cells having a negative bias. Consequently, the number of communication cells to which mobile communication units associated with that particular group are attached will tend to be reduced, due to the tendency of mobile communication units to relocate to apparently one or more favourable communication cells. As such, the number of communication cells required to allocate resources to that group will be reduced. In this manner, for those communication cells from which all mobile communication units associated with a group have relocated, resources can be freed up.

It is envisaged that a number of individual biases may be used, with the respective individual biases relating to respective communication cells and/or respective communication groups, including a respective bias for the active group and/or communication cell that the communication unit is participating in and/or the alternative communications cells that the communication unit may consider for relocation.

It is also envisaged that a communication unit may not be prohibited from attaching to cells to which a negative bias offset is applied, if no other, more favourable cells are available. In this manner, mobility and service accessibility of a MS is not unduly restricted, and radio link quality may be maintained, since the one or more group offsets may be arranged such that they do not influence the path loss parameter below the usability criterion, improvability, relinquishability, or link failure thresholds. In one embodiment of the invention, it is envisaged that the process for obtaining the one or more group offsets, described herein, may encompass obtaining the most recent copy of the one or more group offsets. Furthermore, in one embodiment of the invention, it is envisaged that the process for obtaining the one or more group offsets may be partially or fully de-coupled from the process of obtaining one or more radio link parameters and/or measuring a suitable radio signal strength. Those skilled in the art will realize that the above recognized advantages and other advantages described herein are merely illustrative and are not meant to be a complete rendering of all of the advantages of the various embodiments of the invention. Referring now to the drawings, and in particular to FIG. 1 , a block diagram of a mobile communication unit adapted in accordance with some embodiments of the invention is shown and indicated generally at 100. The communication unit 100, in the context of the illustrated embodiment of the invention, is a mobile radio handset comprising an antenna 102. As such, the communication unit 100 contains a variety of radio frequency components or circuits 106, operably coupled to the antenna 102 that will not be described further herein. The communication unit 100 further comprises signal processing logic 108. An output from the signal processing logic 108 is provided to a suitable user interface (UI) 110 comprising, for example, a display, keypad, loudspeaker and/or microphone. The signal processing logic 108 is coupled to a memory element 116 that stores operating regimes, such as decoding/encoding functions and the like and may be realised in a variety of technologies such as random access memory (RAM) (volatile), (non-volatile) read only memory (ROM), Flash memory or any combination of these or other memory technologies. A timer 118 is typically coupled to the signal processing logic 108 to control the timing of operations within the communication unit 100.

Those skilled in the art, however, will recognize and appreciate that the specifics of this example are merely illustrative of some embodiments and that the teachings set forth herein are applicable in a variety of alternative settings. For example, the teachings described are not dependant on a mobile radio handset, and may be applied to any type of mobile communication unit, although a mobile radio handset is shown in this embodiment. As such, other alternative implementations of different types of a mobile communication unit, such as a vehicle mounted communication unit, are contemplated and are within the scope of the various teachings described. For the illustrated embodiment, the mobile communication unit 100 is arranged to operate within a cellular communication network, such as a TErrestrial Trunked RAdio (TETRA) network, and is capable of participating in group, multicast and/or broadcast calls (hereinafter collectively referred to as group calls for simplicity). More specifically, the mobile communication unit 100 is capable of being associated with one or more communication groups, and as such participating in group calls relating to the one or more communication groups with which it is associated.

Turning now to FIG. 2, there is illustrated a flow chart 200 of a method of performing cell re-selection by a mobile station (MS), such as the mobile communication unit 100 of FIG. 1 , within a cellular communication network, in accordance with an embodiment of the invention. By way of example, the signal processing logic 108 of the mobile communication unit 100 may be arranged, such as by way of instructions stored in memory element 116, to perform the method of FIG. 2. The method starts with obtaining one or more radio link parameters for a serving cell of the MS and at least one neighbouring cell, which for the illustrated embodiment may comprise path loss and cell re-selection parameters in step 210.

As will be appreciated by a skilled artisan, within a cellular communication network such as a TETRA network, the radio link strength between a MS and a Base Transceiver Station (BTS) of a cell is represented by a path loss parameter called 'Cl ' or 'C2'. 'Cl ' and 'C2' are parameters that are typically calculated from a Received Signal Strength Indicator (RSSI) measurement at the mobile, together with decoded power control parameters of a received signal. The MS may obtain power control parameters for its serving cell and for neighbouring cells by either 'scanning' or 'monitoring' those cells.

A MS is said to be 'scanning' when it is measuring the power of a cell and calculates the path loss parameter based upon information broadcast by that cell itself, for example within a Broadcast Synchronisation CHannel (BSCH) of that cell. In order to achieve this, the MS must have synchronised itself with the cell, and then decoded the system information of that cell. The path loss parameter calculated through 'scanning' is known as 'Cl '.

A MS is said to be 'monitoring' when it is measuring the received signal power of a neighbour cell and calculates the path loss parameter based upon information broadcast by the serving cell, for example within a BSCH of the serving cell, and thus not received directly from the cell being monitored. Monitoring is generally used if the MS is unable to synchronise itself with the cell. The path loss parameter calculated through 'monitoring' is known as 'C2'.

In addition to the path loss parameters, for the illustrated embodiment, cell re- selection parameters for both the serving cell and neighbouring cells are also obtained. As will also be appreciated by a skilled artisan, within the TETRA system, cell re-selection parameters are utilised in the cell re-selection process, in order to avoid a 'ping-pong' situation, for example whereby a MS at a communication cell boundary repeatedly alternates between communication cells. In particular, for the TETRA system such cell re-selection parameters comprise the following parameters:

'SLOW RESELECTION THRESHOLD'; ' SLOW RESELECTION HYSTERESIS ' ; 'FAST RESELECTION THRESHOLD'; and' 'FAST RESELECTION HYSTERESIS'.

Typically, the cell re-selection parameters are broadcast by the network, and the values used for these parameters are determined by individual network operators. Thus, the cell re-selection parameters for the serving cell and neighbouring cells may be obtained by way of being broadcast by the serving cell or the relevant neighbouring cell themselves within their respective BSCHs, in the same way as the cell information received when calculating the 'Cl ' and 'C2' parameters. Thus, referring back to FIG. 2 obtaining the radio link parameters in step 210, in one embodiment may comprise obtaining 'Cl ' and 'C2' path loss parameters and cell re-selection parameters for the serving cell and at least one neighbouring cell.

The next step, step 220, comprises obtaining one or more group offsets, the or each group offset relating to at least one group with which the MS is associated. The one or more group offsets are then applied to one or more of the radio link parameters, in step 230.

For example, in one embodiment, a group offset for each neighbour cell and the serving cell is obtained, and applied to the path loss parameter of that cell. The application of the group offset to the path loss parameter of a cell increases, decreases, or in a case of an offset of zero, maintains the path loss parameter value. In this manner, the various path loss parameters may be provided with a bias, for example an advantage or disadvantage, relative to those of other cells. In this manner, for a specific group, the group offsets for that group, when applied to the path loss parameters of neighbouring cells and/or the serving cell, will tend to make those cells for which a positive group offset is applied appear more favourable to MS's associated with that group than those cells for which a negative group offset is applied.

Having applied the one or more group offsets to the one or more path loss parameters, the method moves to step 240, where the neighbouring cells may be ranked in order of their suitability for communication, based on, say, at least a comparison of their respective path loss parameters, to one or more of which path loss parameters the one or more group offsets have been applied. As will be appreciated, by applying the one or more group offsets to one or more of the path loss parameters, the group offsets may have an affect on the ranking of the neighbouring cells. In this manner, preferred cells may be identified by way of the order in which the neighbouring cells have been ranked. For example, the five highest ranked neighbouring cells may be identified as the preferred cells for the purpose of cell re- selection.

Next, in step 250 cell re-selection criterion is applied to radio link parameters of the serving cell and the preferred cells in the same manner as described earlier. In this manner, cell re-selection may be initiated by the MS if e.g. the serving cell becomes radio improvable or radio relinquishable, or a neighbouring cell becomes usable. However, following the application of the cell re-selection criterion, in step 250, it is determined whether to initiate cell re-selection, in step 260. If it is determined that cell re-selection is to be initiated, the method moves to step 270, and cell re-selection is initiated.

As will be appreciated by a skilled artisan, the application of at least one group offset, relating to at least one group with which the MS is associated, in the comparison (ranking) of the radio link parameters, which for the embodiment of FIG. 2 comprise the path loss parameters, enables the provision of one or more of the compared radio link parameters with a bias, such as an advantage or disadvantage in relation to cell re-selection by the mobile communication unit.

In this manner, the application of one or more group offsets, relating to one or more groups, enables cells to be provided with a positive or negative bias for MS's associated with the one or more groups. In this manner, MS's associated with a particular group will tend to relocate to those cells having a positive bias, and away from those cells having a negative bias. Consequently, the number of cells to which MS's associated with that particular group are attached will tend to be reduced, due to the tendency of MS's to relocate to apparently favourable cells, and as such the number of cells required to allocate resources to that group will be reduced. In this manner, for those cells from which all MS's associated with a group have relocated, resources can be freed up. Referring now to FIG. 3, there is illustrated a flow chart 300 of a method of performing cell re-selection by a user equipment (MS), such as the mobile communication unit 100 of FIG. 1, within a cellular communication network, in accordance with an alternative embodiment of the invention. In the same way as for the method of FIG. 2, the method of FIG. 3 starts with obtaining one or more radio link parameters for a serving cell of the MS and at least one neighbouring cell, which for the illustrated embodiment comprises path loss and cell re-selection parameters in step 310. Group offsets are then obtained, as described in greater detail below, in step 320. Next, in step 330, and in contrast to the method of FIG. 2, the neighbouring cells are ranked in order of their suitability for communication, based on their respective path loss parameters, without any group offsets being applied thereto. After the neighbouring cells have been ranked, one or more group offsets, obtained in step 320, are applied to one or more path loss parameters, in step 340. For example, group offsets may be applied to the path loss parameters of the serving cell and, say the five highest ranked neighbouring cells.

Having applied the group offset(s) to the path loss parameter(s), the method moves to step 350, where cell re-selection criterion are applied to determine whether cell re-selection is to be initiated, in step 360. If it is determined that cell re-selection is to be initiated, the method moves to step 370, where cell re-selection is initiated.

As will be appreciated by a skilled artisan, by applying the group offset(s) after the ranking of neighbouring cells, the group offsets do not affect the choice of neighbouring cells to which the cell re-selection criterion are applied, for example the five highest ranked neighbouring cells. In this manner, (un-biased) signal strength and other radio link parameters will have a greater influence over the cell re-selection process, thereby helping to maintain radio link quality. Nevertheless, the application of group offsets to the path loss parameters, prior to applying the cell re-selection criterion, still provides the advantage of enabling positive and/or negative biases to particular cells for MS's associated with particular groups. In this manner, the bias provides such MS's associated with a particular group with the tendency to relocate towards or away from particular neighbouring communication cells. In this manner, the number of cells required to provide resources for that particular group will tend to be reduced, thereby freeing up resources within those communication cells away from which the MS's relocate.

Referring now to FIG. 4, there is illustrated a flow chart 400 of a method of performing cell re-selection by a user equipment (MS), such as the mobile communication unit 100 of FIG. 1, within a cellular communication network, in accordance with a further alternative embodiment of the invention.

The method starts at step 410 with the MS obtaining one or more radio link parameters for a serving cell of the MS and at least one neighbouring cell, which for the illustrated embodiment comprises path loss and cell re-selection parameters. Group offsets are then obtained, as described in greater detail below, in step 420.

Next, in step 430, the neighbouring cells are ranked in order of their suitability to provide communication, based on their respective path loss parameters, without any group offsets being applied thereto. After the neighbouring cells have been ranked, one or more group offsets, obtained in step 420, are applied to one or more cell re-selection parameters, in step 440. For example, group offsets may be applied to one or more of the cell re-selection parameters of each of the serving cell and the five highest ranked neighbouring cells.

Having applied the group offset(s) to the cell re-selection parameter(s), the method moves to step 450, where cell re-selection criterion are applied to determine whether cell re-selection is to be initiated, in step 460. If it is determined that cell re- selection is to be initiated, the method moves to step 470, where cell re-selection is initiated.

Referring now to FIG. 5, there is illustrated a flow chart 500 of an example of applying cell re-selection criterion according to some embodiments of the invention. Application of the cell re-selection criterion commences at step 510, with determining whether or not the criterion for a link failure has been met. As previously mentioned, link failure occurs when the quality of the uplink or downlink connection between the MS and a Base Transceiver Station (BTS) of the serving cell falls below a certain threshold level, typically when the value of the path loss parameter ('Cl ') of the serving cell is less than zero. If the criterion for a link failure has been met (e.g. 'Cl ' for the serving cell is less than zero), the next step is to initiate cell re-selection, in step 520, whereby the MS attempts to locate a cell, for example from the five highest ranked neighbouring cells, that is radio usable. A cell becomes radio usable when the cell has, for a period of five seconds, a main carrier path loss parameter that is greater than:

'FAST RESELECT THRESHOLD + FAST RESELECT HYSTERESIS.'

If the criterion for a link failure has not been met in step 510, the next step is to determine whether or not the criterion for the serving cell being radio relinquishable has been met, in step 530. As previously mentioned, the serving cell becomes radio relinquishable when, say, the following conditions are met:

(i) the serving cell main carrier path loss parameter shall, for a period of five seconds, fall below a 'FAST RESELECT THRESHOLD' parameter;

(ii) the main carrier path loss parameter of at least one neighbour cell shall exceed by a 'FAST RESELECT HYSTERESIS' parameter the path loss parameter of the current serving cell's main carrier for a period of, say, five seconds; and

(iii) no successful cell re-selection process shall have taken place within the previous fifteen seconds.

In this manner, a comparison is made between the path loss parameter of the serving cell and the 'FAST RESELECT THRESHOLD ' parameter for the serving cell. Furthermore, a comparison is made between the path loss parameter of the neighbouring cell and the 'FAST RESELECT HYSTERESIS' parameter of the serving cell. If the criterion for the serving cell being radio relinquishable has been met, the next step is to initiate cell re-selection, in step 520. Otherwise, the next step is to determine whether the criterion for the serving cell being radio improvable has been met, in step 540. As previously mentioned, the serving cell becomes radio improvable when the following conditions are met:

(i) the serving cell main carrier path loss parameter shall, for a period of five seconds, fall below a 'SLOW RESELECT THRESHOLD' parameter; (ii) the main carrier path loss parameter of at least one neighbour cell shall exceed by a 'SLOW RESELECT HYSTERESIS' parameter the main carrier path loss parameter of the current serving cell for a period of five seconds; and (iii) no successful cell re-selection shall have taken place within the previous fifteen seconds.

In this manner, a comparison is made between the path loss parameter of the serving cell and the 'SLOW RESELECT THRESHOLD' parameter for the serving cell. Furthermore, a comparison is made between the path loss parameter of the neighbouring cell and the 'SLOW RESELECT HYSTERESIS' parameter of the serving cell. If the criterion for the serving cell being radio improvable has been met, the next step is to initiate cell re-selection, in step 520. Otherwise, the cell re-selection process ends.

Prior to any step in the embodiment illustrated in FIG. 5 group offsets may be obtained to the extent possible, or otherwise set to zero. One or more group offsets, thus obtained, may then be applied to one or more radio link parameters, prior to performing the logic of the step. In this manner, effectively the boundaries of the serving and neighbouring cells can be adjusted in terms of the criteria for serving cell improvability and/or requishability and/or neighbouring cells ranking and/or usability an a per group call basis. As will be appreciated by a skilled artisan, for the embodiment illustrated in

FIGs. 2 and 3, wherein the group offset(s) is/are applied to the path loss parameter(s) for the serving cell and/or the neighbouring cells, the group offset(s) may affect the determination of whether the serving cell has become radio relinquishable and/or radio improvable. For the embodiment illustrated in FIG. 4, wherein the group offset(s) is/are applied to the cell re-selection parameters for the serving cell and/or the neighbouring cells, the group offset(s) will also affect the determination of whether the serving cell has become radio relinquishable and/or radio improvable, depending on to which cell re-selection parameter(s) the group offset(s) is/are applied. In accordance with one embodiment of the invention, group offsets are applied to cell re-selection parameters used for determining whether the serving cell has become radio improvable. In this manner, the criterion for a link failure, or the serving cell becoming radio relinquishable, is not affected. Thus, the radio link quality is not affected.

For example, group offsets may be applied to the ' SLOW RESELECT THRESHOLD ' parameter and/or the

'SLOW RESELECT HYSTERESIS' parameter for the serving cell and/or the neighbouring cells.

Furthermore, it is envisaged that the inventive concept is not limited to group offsets being applied to a single radio link parameter for each cell, but may comprise applying group offsets to two or more radio link parameters for each cell, and accordingly, the embodiments illustrated in FIGs. 2 to 5 may be combined in any suitable manner.

Furthermore, as mentioned above, it is envisaged that the inventive concept may be restricted to not applying group offsets to any path loss parameters below the link failure and/or FAST RESELECT THRESHOLD and/or

SLOW RESELECT THRESHOLD, for the serving or a neighbouring cell, and the embodiments illustrated in FIGs. 2 to 5 may adjusted accordingly. In this manner it can be ensured that overall service levels offered to the MS will not be sacrificed in terms of the criteria for e.g. link failure, serving cell relinquishability or neighbouring cell usability.

As mentioned above, in any embodiment of the invention it is envisaged that the process for obtaining the one or more group offsets may be decoupled from the process for applying the one or more group offsets to one or more radio link parameters. It is contemplated that group offsets may be substantially static, that is to say the offsets for each cell, in relation to a particular group, do not generally change. For example, group offsets may be set according to a network policy or plan, based on prior knowledge of the distribution of users in a group. In this manner, positive group offsets may be allocated for certain cells that are known to be frequently used by members of a group. Where group offsets are generally static in this manner, each MS associated with a group may be provisioned, or otherwise provided, with the group offsets relating to that group, which may then be stored in memory of the MS. In this manner, the MS simply obtains the required group offsets by retrieving them from memory.

In accordance with alternative embodiments of the invention, the network infrastructure, for example the Switching and Management Infrastructure (SwMI) of a TETRA network, may keep track of communication cells serving MS's associated with particular groups, and the distribution of those MS's therebetween. In this manner, the SwMI may dynamically modify group offsets in accordance with the distribution of MS's associated with groups such that the application of the group offsets to radio link parameters, as described above, will result in a tendency of the MS's to relocate towards those communication cells having the highest number of MS's associated with the relevant group, and away from those communication cells with the least number of MS's associated with the relevant group. As a consequence, cells having only a small number of MS's associated with a group will tend to be vacated by those MS's, thereby enabling the resources of those communication cells to be freed up.

Referring now to FIG. 6, an example of part of a cellular communication system, such as a Terrestrial Trunked Radio (TETRA) network in accordance with some embodiments of the invention is shown and indicated generally at 600. The network 600 comprises a Switching and Management Infrastructure (SwMI) 610. For the illustrated embodiment, the SwMI 610 comprises a Main Switching Centre (MSC) 620, operably coupled to (in one optional embodiment) one or more Local Switching Centres (LSCs) 630. FIG. 6 shows, for illustrative purposes only, two LSCs 630 being coupled to the MSC 620. The SwMI 610 further comprises a plurality of Base Transceiver Stations (BTSs) 640, operably coupled to the LSCs 630. Each BTS 640 is arranged to support communication in at least one cell. Mobile Stations (MSs), such as MS 650 are able to connect wirelessly to the SwMI 610 via the BTSs 640, and in this manner communicate with and via the SwMI 610.

As previously mentioned, and in accordance with some embodiments of the invention, one or more of the MS 650 may be associated with at least one group, and arranged to perform cell re-selection comprising at least the steps of: performing a comparison of at least one radio link parameter of a serving cell with at least one radio link parameter of at least one neighbour cell; determining whether to move from the serving cell to one of the at least one neighbour cell; and applying at least one group offset in the comparison of the radio link parameters. The at least one group offset relates to the at least one group with which the MS 650 is associated and/or one or more cells that may provide communication where the MS is located, and provides at least one of the compared radio link parameters with at least one of an advantage and a disadvantage, in relation to cell re-selection by the mobile communication unit.

In accordance with some embodiments of the invention, the, or each, MS 650 obtains the, or each, group offset from the network. By way of example, group offsets associated with a cell, and relating to a particular group, may be sent to any MS for which that cell is acting as the serving cell, in, say, a form of a Short Data Service (SDS) message.

As will be appreciated by a skilled artisan, SDS messages provide a point-to- point and point-to-multipoint capability for sending messages that comprise a limited number of data bits (up to 254 characters). Consequently, it is contemplated that SDS messaging may be utilised to provide group offsets to MS's either individually, for example when a MS, associated with a group for which offsets are available, registers with a cell. In this manner, a MS is not required to store in memory group offsets for cells through an entire network, or even a substantial part of a network, since each time the MS registers with a cell, it is provided with the required group offsets. Furthermore, where group offsets are updated or changed, whether frequently or from time to time, the MS will be provided with updated group offsets each time it registers with a new cell. Alternatively, it is contemplated that SDS messaging may be utilised to broadcast group offsets as 'in-band signalling' to all MSs of a group (for example on a per cell basis), SDS messages being broadcast in all cells in which MS's associated with that group are located. For example, a standard group SDS broadcast message may be periodically broadcast to provide MS's associated with that group with group offsets. Alternatively still, group offsets associated with a cell, and relating to a particular group, may be sent to any MS for which that cell is acting as the serving cell, in a form of an information element of Mobility Management (MM) Protocol Data Units (PDUs). In a further alternative embodiment, group offsets may be sent to MS's within a Location Update Accept message (D-LOCATION UPDATE ACCEPT), as part of a Location Update procedure. For example, when a MS attaches to a cell comprising a different Location Area Code (LAC) to that of the cell from which the MS has left, the MS initiates the Location Update procedure. In this manner, the MS may be provided with all group offsets relevant to cells within a particular location area, having the same LAC.

In a still further alternative embodiment, group offsets may be sent to MSs within a Group Attach Acknowledgement message (D-ATT ACH-GROUP- IDENTITY-ACKNOWLEDGEMENT). For example, the network may require MSs to attach to a group each time the MS changes cell, and in this manner, the MS can be sent appropriate group offsets for that group each time the MS changes cell.

It is envisaged that the inventive concept is not limited to a use of known messaging and/or broadcasting services to provide group offsets to MS's, and it is contemplated that an air interface of, for example, the TETRA system may be extended to provide a dedicated signalling scheme to relay group offset information to relevant MS's. For example, such a dedicated signalling scheme may form part of the affiliation and/or registration of a MS with a group or cell.

Referring back to FIG. 6, and in accordance with some embodiments of the invention, the SwMI 610 comprises at least one network element arranged to support communications of at least one group offset to MS's 650 connected to the network 600.

For example, the at least one network element may comprise the MSC 620. In this manner, the MSC 620 may be arranged to keep track of cells serving MS's associated with particular groups, substantially throughout the network, and the distribution of those MS's therebetween. In this manner, it is envisaged that the SwMI 610 may dynamically modify group offsets in accordance with the substantially global, that is to say substantially network wide, distribution of MS's associated with groups such that the application of the group offsets to radio link parameters, as described above, will result in a tendency of the MS's to relocate towards those cells having the highest number of MS's associated with the relevant group, and away from those cells with the least number of MS's associated with the relevant group.

Alternatively, the at least one network element may comprise an LSC 630. In this manner, the LSC 630 may be arranged to keep track of cells serving MS's associated with particular groups and the distribution of those MS's there between on a more localised basis.

A further alternative is envisaged where at least one network element comprises a BTS 640. For example, in a case where group offsets are substantially static, or are based on a network policy or plan, the BTS may be provisioned with cell offsets for itself and its neighbouring cells. In this manner, the BTS 640 itself is capable of providing cell offsets to MS's attached thereto. Alternatively, the BTS may be periodically provided with group offsets from, for example the LSC 630 and/or the MSC 620, which it stores in memory, and in turn provides to MS's as required, for example upon registration of a MS with that cell.

In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms 'comprises,' 'comprising,' 'has', 'having,' 'includes', 'including,' 'contains', 'containing' or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by 'comprises ...a', 'has ...a', 'includes ...a', 'contains ...a' does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms 'a' and 'an' are defined as one or more unless explicitly stated otherwise herein. The terms 'substantially', 'essentially', 'approximately', 'about' or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art. The term 'coupled' as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is 'configured' in a certain way is configured in at least that way, but may also be configured in ways that are not listed.

It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or 'processing devices') such as microprocessors, digital signal processors, customized processors and field programmable gate arrays (FPGAs) and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and apparatus for performing cell re-selection described herein. The non- processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform cell re-selection described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Both the state machine and ASIC are considered herein as a 'processing device' for purposes of the foregoing discussion and claim language.

Moreover, an embodiment can be implemented as a computer-readable storage element having computer readable code stored thereon for programming a computer (e.g., comprising a processing device) to perform a method as described and claimed herein. Examples of such computer-readable storage elements include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.

The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.

Claims

CLAIMSWhat is claimed is:

1. A method for performing cell re-selection by a mobile communication unit operating within a cellular communication network, the mobile communication unit being associated with at least one group of communication units; the method comprising: receiving a first signal from a serving cell; receiving at least one second signal from at least one neighbouring cell; performing a comparison of at least one radio link parameter of the first signal with at least one radio link parameter of the at least one second signal; and determining whether to transition from the serving cell to at least one neighbouring cell, based at least partly on the comparison, wherein the method is characterised by: applying at least one group offset to at least one radio link parameter; wherein the at least one group offset relates to at least one of the following: the at least one group with which the mobile communication unit is associated; the serving cell; or the at least one neighbouring cell.

2. The method of Claim 1 further characterised in that applying at least one group offset comprises applying a positive bias or a negative bias to the at least one radio link parameter prior to the comparison.

3. The method of Claim 1 or Claim 2 further characterised by the at least one radio link parameter to which the at least one group offset is applied comprises at least one path loss parameter of at least one of the serving cell and the at least one neighbour cell.

4. The method of any preceding Claim further characterised by applying the at least one group offset to the at least one path loss parameter prior to applying cell re- selection criterion to the radio link parameters.

5. The method of any preceding Claim further characterised by determining whether the serving cell is radio improvable.

6. The method of Claim 5 further characterised in that determination of whether the serving cell is radio improvable comprises a comparison being made between at least one of: the at least one path loss parameter of the serving cell and a SLOW RESELECT THRESHOLD parameter for the serving cell, and the at least one path loss parameter of at least one neighbouring cell and the at least one path loss parameter of the serving cell, and the margin between the at least one path loss parameter of the serving cell and the at least one path loss parameter of at least one neighbouring cell and a SLOW RESELECT HYSTERESIS parameter of the serving cell.

7. The method of Claim 4 further characterised by determining whether the serving cell is radio relinquishable.

8. The method of Claim 7 further characterised in that the determination of whether the serving cell is radio relinquishable comprises a comparison being made between at least one of: the at least one path loss parameter of the serving cell and a

FAST RESELECT THRESHOLD parameter for the serving cell, and the at least one path loss parameter of at least one neighbouring cell and the at least one path loss parameter of the serving cell, and the margin between the at least one path loss parameter of the serving cell and the at least one path loss parameter of at least one neighbouring cell and a FAST RESELECT HYSTERESIS parameter of the serving cell.

9. The method of Claim 4 further characterised by determining whether at least one neighbouring cell is usable.

10. The method of Claim 9 further characterised in that the determination of whether the at least one neighbouring cell is usable comprises a comparison being made between at least one of: the at least one path loss parameter of the neighbouring cell and a FAST RESELECT THRESHOLD parameter applicable for the neighbouring cell, and the at least one path loss parameter of at least one neighbouring cell and a FAST RESELECT HYSTERESIS parameter applicable for the neighbouring cell.

11. The method of any preceding Claim further characterised by not applying any group offset to a path loss parameter below zero or below a

FAST RESELECT THRESHOLD parameter.

12. The method of any preceding Claim further characterised by applying the at least one group offset to the at least one path loss parameter prior to ranking a number of neighbouring cells based on at least a comparison of their respective path loss parameters.

13. The method of any preceding Claim further characterised by the at least one radio link parameter to which the at least one group offset is applied comprises at least one cell re-selection parameter.

14. The method of Claim 13 further characterised by applying the at least one group offset to the at least one cell re-selection parameter prior to applying cell re- selection criterion to the radio link parameters.

15. The method of any preceding Claim further characterised by the cellular communication network being a TErrestrial Trunked RAdio network.

16. A network element of a cellular communication network characterised by logic for providing at least one group offset to at least one mobile communication unit connected to the cellular communication network and associated with a group and a transmitter operably coupled to the logic for transmitting the at least one group offset to the at least one mobile communication unit, wherein the at least one group offset relates to at least one of the following: the at least one group with which the mobile communication unit is associated; the transmitting cell; or the at least one neighbouring cell.

17. The network element of Claim 16 wherein the at least one group offset is arranged to, upon application of the at least one group offset by the mobile communication unit, modify at least one radio link parameter with a positive bias, or a negative bias, in relation to cell re-selection by the mobile communication unit.

18. The network element of Claim 16 or Claim 17 further characterised by the network element comprising logic for providing the at least one group offset to at least one mobile communication unit in a form of a Short Data Service message.

19. The network element of Claim 18 further characterised by the logic being arranged to provide the at least one group offset in a form of a point-to-point Short Data Service message to a mobile communication unit, associated with a group to which the at least one group offset relates, when the mobile communication unit registers with a cell of the network.

20. The network element of Claim 18 further characterised by being arranged to provide the at least one group offset in a form of a point-to-multipoint Short Data Service message to mobile communication units associated with a group to which the at least one group offset relates.

21. The network element of Claim 16 or Claim 17 further characterised by the network element comprising logic for providing the at least one group offset to at least one mobile communication unit in a form of a an information element of Mobility Management Protocol Data Units.

22. The network element of Claim 16 or Claim 17 further characterised by the network element comprising logic for providing the at least one group offset to at least one mobile communication unit within a Location Update Accept message, as part of a Location Update procedure.

23. The network element of Claim 16 or Claim 17 further characterised by the network element comprising logic for providing the at least one group offset to at least one mobile communication unit within a Group Attach Acknowledgement message.

24. The network element of any one of Claims 16 to 23 further characterised by logic arranged to track those communication cells serving mobile communication units associated with particular groups and the distribution of those mobile communication units therebetween.

25. The network element of Claim 24 further characterised by logic arranged to modify group offsets in accordance with the distribution of mobile communication units associated with groups.

26. The network element of any one of Claims 16 to 25 further characterised by the network being a TErrestrial Trunked RAdio network.

27. The network element of Claim 26 further characterised by the network element being at least one of:

(i) a Main Switching Centre, (ii) a Local Switching Centre, or (iii) a Base Transceiver Station.