WO2013156067A1

WO2013156067A1 - Configuring a handover procedure

Info

Publication number: WO2013156067A1
Application number: PCT/EP2012/057152
Authority: WO
Inventors: Ingo Viering; Rossella De Benedittis; Marcin Wiczanowski; Nikolay Anatolievich TRUBNIKOV; Henrik MARTIKAINEN
Original assignee: Nokia Siemens Networks Oy
Priority date: 2012-04-19
Filing date: 2012-04-19
Publication date: 2013-10-24

Abstract

It is described a method for configuring a handover procedure of a user equipment from a source cell to a target cell within a cellular network system (100). The cellular network system comprises the source cell and at least one neighboring target cell. The handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset. A cell individual offset is assigned to each of the at least one neighboring target cell in relation to the source cell, wherein the cell individual offset is adaptable by a mobility robustness optimization algorithm and a mobility load balancing algorithm. The method comprises computing a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm, computing a second value lower than or equal to the first value and a third value higher than or equal to the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm, setting the cell individual offset to the first value by the mobility robustness optimization algorithm, adapting the cell individual offset between the second value and the third value by the mobility load balancing algorithm, and configuring a handover procedure for the user equipment from the source cell to a target cell based on user equipment measurements being based on the adapted cell individual offset.

Description

DESCRIPTION Title

Configuring a handover procedure

Field of invention The present invention relates to the field of cellular networks, especially to self-organizing networks, and in particular to networks being able to perform mobility robustness optimization and mobility load balancing.

Art Background

In 3GPP Rel9., Mobility Robustness Optimization (MRO) and Mobility Load Balancing have been specified as part of the self-organizing networks (SON) framework. The main task of MRO is to adapt the handover parameters such that mobility problems, in particu- lar radio link failures, handover failures and pingpongs, are minimized. A user equipment (UE) will perform measurements before a handover procedure. A so called "cell individual offset" (CIO) can be applied by the User Equipment (UE) when measuring a specific cell neighbored to the serving cell. This CIO can be modified by MRO for optimization. For instance, a default value of OdB can be used for the CIO. A positive value towards a par- ticular neighbor will make this neighbor more attractive for the UEs, thus leading to earlier handovers. This typically happens on boundaries which are dominated by fast UEs, e.g. on a street, to guarantee that handovers are initiated in time. A negative value towards a particular neighbor will make this neighbor less attractive for the UEs, thus leading to later handovers. This typically happens on boundaries which are dominated by slow UEs to avoid pingpongs. Different neighbors of the same serving cell can be configured with different CIO values.

MRO can also modify other parameters, internal or external, i.e. communicated to the connected UEs, to the serving cell, to speed up or delay a handover. The CIO adjusting part of MRO might be implemented in the base stations ("distributed MRO") or in a centralized entity such as an operation and maintenance unit ("centralized MRO"). On the other hand, the task of Mobility Load Balancing (MLB) is to move cell boundaries according to load imbalances. For instance, an overloaded cell will try to do early handovers (i.e., offload) towards a neighbor cell with spare capacity, and it would expect that this "under-loaded" neighbor cell will do late handovers in turn, in order to prevent pingpongs and to keep load away from the overloaded cell. Such a mechanism may also use CIO between the overloaded cell and the cell with spare capacity. MLB may be implemented fully in the base stations ("distributed").

MLB may counteract with MRO since they are using the same parameters. Aggressive MLB may prevent MRO from fixing mobility problems or may create additional mobility problems. Furthermore, typical time scales for MRO and MLB are different: MRO requires reliable statistics of the mobility problems and thus may work rather slowly, e.g. every 24 hours. MLB is based on load statistics which converge rather quickly, so MLB may work on a minute time scale. Finally, if MRO is centralized and MLB is distributed, a proper col- laboration will affect the interface between central entity and the base station.

There may be a need for an improved and flexible system and method being adapted to allow use of mobility robustness optimization and mobility load balancing in parallel.

Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention there is provided a method for configuring a handover procedure of a user equipment from a source cell to a target cell within a cellular network system. The cellular network system comprises the source cell and at least one neighboring target cell, wherein the handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset, wherein a cell individual offset is assigned to each of the at least one neighboring target cell in relation to the source cell, wherein the cell individual offset is adaptable by a mobility robustness optimization algorithm (MRO) and a mobility load balancing algorithm (MLB). The method comprises computing a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm, computing a second value lower than or equal to the first value and a third value higher than or equal to the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm, setting the cell individual offset to the first value by the mobility robustness optimization algorithm, adapting the cell individual offset between the second value and the third value by the mobility load balancing algorithm, and configuring a handover procedure for the user equipment from the source cell to a target cell based on user equipment measurements being based on the adapted cell individual offset.

This aspect of the invention is based on the idea to provide the possibility of using mobility robustness optimization and mobility load balancing in parallel by setting a range of values for the cell individual offset being used by both algorithms. This invention describes a method how MRO and MLB can collaborate constructively, which means that mobility problems could be fixed as much as possible, but at the same time MLB should exploit load imbalances as much as possible without enhancing mobility problems. So far MRO has been developed as a first SON mobility feature, i.e. without taking MLB into account.

As explained above current MRO implementations adjust the CIOs in every cell towards each neighbor cell, i.e. per neighbor cell relation. In particular, in a centralized implementation this adjustment may be visible on the interface between the centralized entity and the base station.

A network element, for instance the centralized entity, may estimate the most suited CIO to apply in the source cell towards a given neighbor cell relation after some evaluation. The evaluation may be indicative for the network behavior or actual network conditions. The basic idea is that MRO does not only configure the "best" or optimum CIO value per neighbor cell relation (in the following referred to as first value or CIO_defaui_t), but in addition it also estimates the range of values around this "optimum" which CIO is allowed to assume if e.g. required by MLB: an upper CIO bound CIO_upper or third value and a lower CIO bound ClOiower or second value.

According to the herein described method, the MLB algorithm is free to modify the cell individual offset towards a value being appropriate for a neighbor relation as long as it stays between CIO_!ower and CIO_upper. If MLB does not detect load imbalances, it will may ClO_de_fauit without performing any adaptation. The second and third value being computed by the MRO should be in a range still being acceptable for the MRO. That means that MRO is expected to configure CIOi_ower and CIO_Up_per such that MLB can use any value be- tween CIOi_ower and CIO_upper without creating severe mobility problems. During times or events, when the MRO needs a specific value without being able to accept deviations, MRO may inherently switch off MLB by configuring CIOi_owet=CIO_uppet=CIO_defauit.

The herein described method may be carried out in any kind of network element, like a central entity (OAM) or base station, or may be divided between different network elements. For instance, features relating to MRO may be carried out in a central entity and features relating to MLB may be carried out in a base station.

"Configuring the handover procedure" in this context may refer to the base station config- uring at the connected user equipments parameters being used for triggering the handover procedure, i.e. providing information to the user equipment on how to perform and report neighbor cell measurements which initiate a handover procedure. The cell individual offset can be applied to the results of the measurements to adapt the results per cell. "Cell individual offset" (CIO) in this context may refer to a parameter, for instance a value, which may be applied to measurements being used as a basis for handovers. The CIO may be set individually per neighboring cell and may thus be applied to the results of the measurements to adapt the measurements per cell. "Mobility Robustness Optimization" in this context may refer to any kind of algorithm being used for an optimization of mobility by adapting parameters for speeding up or delaying handovers.

"Mobility load balancing" in this context may refer to any kind of algorithm being used for changing the radio coverage of cells, in particular for load balancing by moving cell boundaries so that handovers can be carried out earlier or later, where the radio coverage or cell boarder is virtually changed providing to the connected UEs appropriate cell individual offset values towards the relevant serving cell's neighbors. According to an embodiment of the invention, the method further comprises initiating the handover procedure for the user equipment from the source cell to the target cell triggered by the user equipment measurements being based on the cell individual offset. The UE measurements, adapted by the CIO, may trigger the handover procedure. The network element, or base station, may initiate the handover in response to the trigger based on the measurements.

According to a further embodiment of the invention, computing the first value is based on an evaluation of the handover performance.

The handover performance, for instance the success rate of handovers, may be deter- mined from the HO statistics collected at the base station serving the source cell, i.e., from the handover performance counters at the source cell towards all its neighboring cells or only to one neighboring cell. Based on the result, the first CIO value may be computed. For instance, it may be determined to keep the previous CIO value, for example a default CIO, or to change the first value by increasing or decreasing.

According to a further embodiment of the invention, the method further comprises, if the handover performance changes, re-computing the first value, the second value and the third for the cell individual offset for each of the at least one neighboring target cell, resetting the cell individual offset to the re-computed first value by the mobility robustness optimization algorithm, and adapting the cell individual offset between the re-computed second value and the re-computed third value by the mobility load balancing algorithm.

This means that, if the handover performance changes, the default (first) value as well as the lower bound (second) value and the upper bound (third) value of the cell individual offset are re-computed for each of the at least one cell neighbor to the source cell, thus providing a new CIO default value and new CIO lower bound and upper bound values between which the mobility robustness optimization algorithm can derive new CIO values to configure at the connected user equipments. During changes of the network topology or conditions, the handover performance may change. Thus, it may be necessary to re-calculate the different CIO values. This may provide a dynamic method for configuring a handover procedure, as the CIO values may be adapted based on actual radio or network conditions. According to a further embodiment of the invention, the handover performance is evaluated by determining a success rate of handovers from the source cell towards each of the at least one neighboring target cell. Evaluating the handover performance may refer to a determination or evaluation of the handover success rate towards a neighbor cell. The handover may fail because not triggered in due time; this is the case of too slow handover. The handover may fail because triggered when not required; this is the case of too fast handover. The handover may fail because the wrong target cell has been selected; this is the case of handover to the wrong cell. If e.g. the handover performance towards a neighbor cell is poor because there are too many slow (/fast) handovers, MRO may configure a higher (/lower) CIO value towards that neighbor cell. Otherwise, if the handover performance is good, no change might be required and the current CIO value may be kept.

According to a further embodiment of the invention, the second value and the third value are computed as absolute cell individual offset limits. This may refer to a computation of the CIOs as values representing an absolute upper and lower limit by the MRO. If the first value changes due to actual radio or network conditions, the second and the third value may keep their absolute values.

According to a further embodiment of the invention, the second value and the third value are computed as a deviation with respect to the first value.

According to this embodiment, the second value and the third value are computed in relation to the first value, i.e., as a deviation with respect to the first value representing a CIO default value. If the first value changes, the second value and the third value changes as well as the relation changes.

According to a further embodiment of the invention, the method further comprises computing different second values and different third values for different groups of user equipments.

Based on this embodiment, different values may be computed for different groups of UEs requiring different configurations or being able to perform different radio measurements, for supporting Inter-Cell Interference Coordination (ICIC). For instance, more relaxed limits might be used for Rel10 UEs when enhanced ICIC is being used, whereas tighter limits are required for Rel9 UEs. Another example may be relaxed limits for non-real-time UEs, and tight limits for real-time UEs. According to a further embodiment of the invention, computing the second value and the third value comprises considering information with respect to actual and/or previous value of the cell individual offset applied by the mobility load balancing algorithm towards a neighbor cell of the serving cell, the information being provided by the mobility load bal- ancing algorithm.

This embodiment is based on the idea to provide a better decision or computation basis by considering values being previously used for the CIO by the MLB. When having such information, the MRO may determine the new upper and lower limits for the CIO, i.e., the new second and the third value, based on already used values and may consider determining the new values to correspond to the already used values.

According to a further embodiment of the invention, the information comprises statistic values for cell individual offsets.

For instance, MLB could provide information about the actual CIOs, i.e. the CIO values which have actually been used during a handover statistic collection period per neighbor cell relation, such as minimum and maximum CIO, a probability density function / histogram of the actually used CIOs, or any other CIO statistics.

According to a second aspect of the invention, there is provided a network element being adapted to configure a handover procedure of a user equipment from a source cell to a target cell within a cellular network system, the cellular network system comprising the source cell and at least one neighboring target cell, wherein the handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset, wherein a cell individual offset is assigned to each of the at least one neighboring target cell in relation to the source cell, wherein the cell individual offset is adaptable by a mobility robustness optimization algorithm and a mobility load balancing algorithm. The network element comprises a computation unit being adapted to compute a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm, and being adapted to compute a second value lower than the first value and a third value higher than the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm, a setting unit being adapted to set the cell individ- ual offset to the first value by the mobility robustness optimization algorithm, an adaption unit being adapted to adapt the cell individual offset between the second value and the third value by the mobility load balancing algorithm, and a configuration unit being adapted to configure a handover procedure for the user equipment from the source cell to a target cell based on measurements being based on the adapted cell individual offset.

The term "network element" in this context may denote any kind of physical entity being able to communicate with a user equipment, directly or indirectly via another network element, or any other network device. A network element in this context may be any kind of network device providing the required functionality for the method, it may also be a transceiver node in communication with a centralized entity.

The network element may comprise a receiving unit, for example a receiver as known by a skilled person. The network element may also comprise a transmitting or sending unit, for example a transmitter. The receiver and the transmitter may be implemented as one single unit, for example as a transceiver. The transceiver or the receiving unit and the sending unit may be adapted to communicate with the user equipment or any other network device via an antenna. The network element further comprises a computation unit, a setting unit, an adaptation unit and a configuration unit. The computation unit, the setting unit, the adaptation unit and the configuration unit may be implemented as single units or may be implemented for example as part of a standard control unit, like a CPU or a microcontroller. According to an embodiment of the invention, the network element is a centralized entity and/or a base station.

The term "centralized entity" in this context may denote an operation and maintenance unit or any other kind of central control unit.

The term "base station" in this context may denote any kind of physical entity being able to communicate with a user equipment or any other network device. A base station in this context may be any kind of network device providing the required functionality for the method, it may also be a transceiver node in communication with a centralized entity. The base station may be for example a NodeB or eNB. The method may be performed also partially within the centralized entity and partially within the base station.

The base station may be any type of access point or point of attachment, which is capable of providing a wireless access to a cellular network system. Thereby, the wireless access may be provided for a user equipment or for any other network element, which is capable of communicating in a wireless manner. The base station may be a NodeB, eNB, home NodeB or HeNB, or any other kind of access point. The base station may in particular be used for a B4G, LTE or 3GPP cell and communication.

In a centralized implementation of MRO, MRO and the relating features of the method may be carried out in the centralized unit and MLB and the relating features of the method may be carried out in the base station. According to a third aspect of the invention, there is provided a user equipment being adapted to communicate with a network element as described above.

The user equipment (UE) may be any type of communication end device, which is capable of connecting with the described network element. The UE may be in particular a cellular mobile phone, a Personal Digital Assistant (PDA), a notebook computer, a printer and/or any other movable communication device. In particular, in the context of this application, a UE may be any kind of communication device, for example a smart phone, being able to perform handovers based on the configuration as provided by the network element. The user equipment may comprise a receiving unit or receiver which is adapted for receiving signals from the network element or a base station communicating with the network element. The user equipment may comprise a transmitting unit for transmitting signals. The transmitting unit may be a transmitter as known by a skilled person. The receiver and the transmitting unit may be implemented as one single unit, for example as a transceiver. The transceiver or the receiver and the transmitting unit may be adapted to communicate with the network element or the base station via an antenna.

The user equipment may further comprise a control unit being adapted to control handovers. The control unit may be implemented as a single unit or may be implemented for example as part of a standard control unit, like a CPU or a microcontroller. According to a fourth aspect of the invention, there is provided a cellular network system. The cellular network system comprises a network element as described above.

Generally herein, the method and embodiments of the method according to the first aspect may include performing one or more functions described with regard to the second, third or fourth aspect or an embodiment thereof. Vice versa, the network element, user equipment or cellular network system and embodiments thereof according to the second and third aspect may include units or devices for performing one or more functions described with regard to the first aspect or an embodiment thereof.

According to a fifth aspect of the herein disclosed subject-matter, a computer program for configuring a handover procedure is provided, the computer program being adapted for, when executed by a data processor assembly, controlling the method as set forth in the first aspect or an embodiment thereof.

As used herein, reference to a computer program is intended to be equivalent to a reference to a program element and/or a computer readable medium containing instructions for controlling a computer system to coordinate the performance of the above described method.

The computer program may be implemented as computer readable instruction code by use of any suitable programming language, such as, for example, JAVA, C++, and may be stored on a computer-readable medium (removable disk, volatile or non-volatile memory, embedded memory/processor, etc.). The instruction code is operable to program a com- puter or any other programmable device to carry out the intended functions. The computer program may be available from a network, such as the World Wide Web, from which it may be downloaded.

The herein disclosed subject matter may be realized by means of a computer program respectively software. However, the herein disclosed subject matter may also be realized by means of one or more specific electronic circuits respectively hardware. Furthermore, the herein disclosed subject matter may also be realized in a hybrid form, i.e. in a combination of software modules and hardware modules. In the above there have been described and in the following there will be described exemplary embodiments of the subject matter disclosed herein with reference to a cellular network system, a network element, a user equipment and a method of for configuring a handover procedure. It has to be pointed out that of course any combination of features relating to different aspects of the herein disclosed subject matter is also possible. In particular, some embodiments have been described with reference to apparatus type embodiments whereas other embodiments have been described with reference to method type embodiments. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one aspect also any combination between features relating to different aspects or embodiments, for example even between features of the apparatus type embodiments and features of the method type embodiments is considered to be disclosed with this application.

The aspects and embodiments defined above and further aspects and embodiments of the present invention are apparent from the examples to be described hereinafter and are explained with reference to the drawings, but to which the invention is not limited.

Brief Description of the Drawing

Figure 1 shows a cellular network system according to an exemplary embodiment of the present invention.

Figure 2 shows a cellular network system according to a further exemplary embodiment of the present invention. Figure 3 shows a cellular network system according to a further exemplary embodiment of the present invention.

Figure 4 shows examples of applications for the present invention. Figure 5 shows a network element and a user equipment within a cellular network system according to an exemplary embodiment of the invention.

It is noted that in different figures, similar or identical elements are provided with the same reference signs. Detailed Description

In the following, embodiments of the herein disclosed subject matter are illustrated with reference to the drawings and reference to aspects of current standards, such as LTE, and their further developments. However, such reference to current standards is only exemplary and should not be considered as limiting the scope of the claims.

Figure 1 shows a cellular network system 100. The cellular network system comprises a source cell 103 and a target cell 104 being neighbored to the source cell. The cellular network system 100 further comprises a network element 101 , for instance a base station. A user equipment 102 is served by the source cell 103 and can be controlled by the network element 101 .

A handover procedure can be configured by the network element 101 by providing infor- mation to the user equipment 102. The handover procedure is based on measurements being performed by the user equipment 102 prior to a handover. The measurements can be based inter alia on a cell individual offset. A cell individual offset is assigned to each of the neighboring cells, i.e., also to the target cell 104, in relation to the source cell 103. The cell individual offset is adaptable by a mobility robustness optimization algorithm and a mobility load balancing algorithm.

According to the herein described method, a first value (in the following also denoted as default CIO or ClOdefault) is computed for the cell individual offset for each neighboring cell. The ClOdefault represents an optimum cell individual offset for the mobility robust- ness optimization algorithm. A second value (in the following also denoted as lower CIO or

ClOlower) lower than the first value and a third value (in the following also denoted as upper CIO or ClOupper) higher than the first value for the cell individual offset is computed for each neighboring cell. The second value represents the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm.

The cell individual offset is then set to the first value by the mobility robustness optimization algorithm and can be adapted between the second value and the third value by the mobility load balancing algorithm. The network element 101 can then configure a handover procedure for the user equipment 102 from the source cell 103 to the target cell 104 based on measurements being based on the adapted cell individual offset. As explained above, current MRO implementations adjust the CIOs in every cell towards each neighbor cell, i.e. per neighbor cell relation. In one centralized embodiment, the network element can be a centralized entity which estimates the most suited CIO to apply in the cell towards a given neighbor cell relation after evaluating the handover performance. The handover performance can be evaluated based on the handover success rate, for example, towards the neighbor cell:

Handover may fail because not triggered in due time; this is the case of too slow handover

- Handover may fail because triggered when not required; this is the case of too fast handover

Handover may fail because the wrong target cell has been selected; this is the case of handover to the wrong cell

If, for instance, the handover performance towards a neighbor cell is poor because there are too many slow (/fast) handovers, MRO may configure a higher (/lower) CIO value towards that neighbor. Otherwise, if the handover performance is good, no change might be required and the current CIO value may be kept.

The idea is that MRO will not only configure the "best" CIO value per neighbor cell relation (in the following referred to as CIO_defaui_t), but in addition it also estimates the range of values around this optimum which CIO is allowed to assume if e.g. required by MLB (an upper CIO bound CIO_Up_perand a lower CIO bound CIOi_ower.). This range can be still derived after evaluating the handover statistics per neighbor cell relation towards preconfigured reference thresholds.

The MLB algorithm is free to modify the cell individual offset towards that neighbor cell relation as long as it stays between CIOi_ower and CIO_Up_per. If MLB does not detect load imbalances, it can use CIO_defaui_t. CIOi_ower and CIO_Up_per might be configured as absolute CIO limit, or as a deviation with respect to CIOdefauit (relative).

MRO can configure CIO_iower and CIO_upper such that MLB can use any value between CIOiower and CIOupper without creating severe mobility problems. In one special case, MRO may inherently switch off MLB by configuring CIOi_OWer=CIO_Upper=CIO_defauit- In Figure 2, a further embodiment of a cellular network system 200 is shown for the case of a centralized MRO implementation. The network element 101 in this case is an operation and maintenance unit (OAM). The OAM is in communication with two base stations 200 and 201 .

In the OAM, the MRO and the CIO adjustment are carried out. The resulting values ClOdefault, ClOupper and ClOlower are sent to the base stations. The base stations may then carry out the MLB based on the received values and adapt the CIO value. The base stations may also provide some information to the OAM, like CIO statistics.

Figure 3 shows a further exemplary cellular network system 300 providing a heterogeneous network (HetNet) scenario with different cells. The small cell 301 with fast UEs may have rather tight CIO limits, since mobility is rather challenging. Thus, little or no MLB might be possible. For example, the MRO may switch off MLB.

The small cell 302 with very slow UEs may use relaxed CIO limits, mobility is not very challenging. MLB may have a lot of degrees of freedom. For example, the MRO may set a wide range for the CIO values.

The small cell 303 with pedestrian UEs may have moderate CIO limits. MLB will have some degree of freedom. Thus, the MRO may set a medium range of CIO values. Figure 4 gives an example 400 how the extended MRO algorithm may configure the thresholds ClOdefault, ClOupper and ClOlower, based on the evaluated MRO key performance indicators. "Too slow" indicates the count of handovers which were too late and have lead to a radio link failure. "Too fast" summarizes too early handovers and handovers to wrong cell both leading to a radio link failure / handover failure, and possible pingpongs (with an appropriate weighting). With reference to this figure, "OCn" corresponds to the CIO towards neighbor cell "n", "maxDeltaOCn" and "minDeltaOCn" correspond to the above described ClOlower and ClOupper respectively, "mroOCn" is the best "CIO" value, i.e. ClOdefault, configured by MRO towards neighbor cell "n". Applications 410 require more restricted ranges for the CIO values than the relaxed applications 420.

In particular for the applications 410, when both "too slow" and "too late" handover events are above pre-configured thresholds, MRO might not change the current CIO value but it restricts (or put to "0") the range of values allowed to MLB. When the number of too slow handover events is above the pre-configured threshold whereas the number of too fast handover events is below the respective threshold, MRO may reduce the current CIO val- ue as well as it restricts (or put to "0") the range for lower CIO values whereas it increases the range for upper CIO values.

In case 41 1 , the range between the values for ClOlower and ClOupper may be decreased due to a high number of "too slow" and "too late" handover events but does not change ClOdefault.

In case 412, ClOupper may be decreased 413 due to a high number of "too slow" handover events. Also ClOdefault may be changed 414 as the number of too fast handover events is below the respective threshold. As ClOlower is dependent on ClOdefault, it may be changed automatically.

In case 415, ClOlower may be changed, i.e. may be raised, due to a high number of "too slow" handover events. Also ClOdefault may be changed 416.

In case 417, ClOlower may be raised 418 due to a high number of "too slow" handover events. ClOdefault may be kept.

Changing ClOlower and/or ClOupper may always result in a change of the range. Vice versa, if the range needs to be changed, it may be sufficient to change one of these values.

In the relaxed applications 420, the ranges may be increased. For instance, in case 421 , ClOupper 422 and ClOlower 423 may be changed to provide a greater range of values. In case 424, both values ClOupper and ClOlower may be changed to keep the same range, but considering the number of "too slow" and "too late" handover events (425).

Figure 5 shows a cellular network system 500 according to an exemplary embodiment of the invention. The cellular network system comprises a network element 101 and a user equipment 102. The user equipment 102 is served by a source cell and can be handed over to a target cell being neighboured to the source cell.

The network element 101 configures a handover procedure to be carried out by the user equipment 102 from the source cell to the target cell within the cellular network system 500. The handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset. A different or identical cell individual offset is assigned to each neighboring cell, i.e., also to the target cell, in relation to the source cell. The cell individual offset can be configured and adapted by a mobility robustness optimization algorithm and a mobility load balancing algorithm. The network element comprises a computation unit 502, a setting unit 503, an adaptation unit 504 and a configuration unit 505. The computation unit 502 is adapted to compute a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm, and is adapted to compute a second value lower than the first value and a third value higher than the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm. The setting unit 503 is adapted to set the cell individual offset to the first value by the mobility robustness optimization algorithm. The adaption unit 504 is adapted to adapt the cell individual offset between the second value and the third value by the mobility load balancing algorithm. The configuration unit 505 is adapted to configure a handover procedure for the user equipment from the source cell to a target cell based on measurements being based on the adapted cell individual offset.

The network element may comprise a receiving unit, for example a receiver as known by a skilled person. The network element may also comprise a transmitting or sending unit, for example a transmitter. The receiver and the transmitter may be implemented as one single unit, for example as a transceiver 501 . The transceiver or the receiving unit and the sending unit may be adapted to communicate with the user equipment or another network element via an antenna.

The computation unit 502, the setting unit 503, the adaptation unit 504 and the configuration unit 505 may be implemented as single units or may be implemented for example as part of a standard control unit, like a CPU or a microcontroller.

In one embodiment, the network element is a centralized entity, like an OAM unit, which can communicate with a base station. In another embodiment, the network element is a combination of a centralized entity and a base station, wherein parts of the features are provided by the centralized entity and part of the features are provided by the base sta- tion. In a further embodiment, the network element is a base station. The base station may be any type of access point or point of attachment, which is capable of providing a wireless access to a cellular network system. Thereby, the wireless access may be provided for the user equipment, or for any other network element, which is capable of communicating in a wireless manner. The base station may be a NodeB, eNB, home NodeB or HeNB, or any other kind of access point.

The user equipment (UE) may be any type of communication end device, which is capable of connecting with the described base station. The UE may be in particular a cellular mobile phone, a Personal Digital Assistant (PDA), a notebook computer, a printer and/or any other movable communication device.

The user equipment may comprise a receiving unit or receiver which is adapted for receiving signals from the network element, directly or indirectly via another network element. The user equipment may comprise a transmitting unit for transmitting signals. The trans- mitting unit may be a transmitter as known by a skilled person. The receiver and the transmitting unit may be implemented as one single unit, for example as a transceiver 506. The transceiver or the receiver and the transmitting unit may be adapted to communicate with the network element via an antenna. The user equipment may further comprise a control unit 507 for controlling and configuring handovers based on information received from the network element. The control unit may be implemented as a single unit or may be implemented for example as part of a standard control unit, like a CPU or a microcontroller. Having regard to the subject matter disclosed herein, it should be mentioned that, although some embodiments refer to a "base station", "eNB", etc., it should be understood that each of these references is considered to implicitly disclose a respective reference to the general term "network component", "network element" or, in still other embodiments, to the term "network access node". Also other terms which relate to specific standards or specific communication techniques are considered to implicitly disclose the respective general term with the desired functionality.

It should further be noted that a base station as disclosed herein is not limited to dedicated entities as described in some embodiments. Rather, the herein disclosed subject matter may be implemented in various ways in various locations in the communication network while still providing the desired functionality. According to embodiments of the invention, any suitable entity (e.g. components, units and devices) disclosed herein, e.g. the configuration unit, are at least in part provided in the form of respective computer programs which enable a processor device to provide the functionality of the respective entities as disclosed herein. According to other embodi- ments, any suitable entity disclosed herein may be provided in hardware. According to other - hybrid - embodiments, some entities may be provided in software while other entities are provided in hardware.

It should be noted that any entity disclosed herein (e.g. components, units and devices) are not limited to a dedicated entity as described in some embodiments. Rather, the herein disclosed subject matter may be implemented in various ways and with various granularities on device level while still providing the desired functionality. Further, it should be noted that according to embodiments a separate entity (e.g. a software module, a hardware module or a hybrid module) may be provided for each of the functions disclosed herein. According to other embodiments, an entity (e.g. a software module, a hardware module or a hybrid module (combined software/hardware module)) is configured for providing two or more functions as disclosed herein.

It should be noted that the term "comprising" does not exclude other elements or steps. It may also be possible in further refinements of the invention to combine features from different embodiments described herein above. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

List of reference signs:

100 Cellular network system

101 Network element

102 User equipment

103 Source cell

104 Target cell

200 Cellular network system

200 Base station

201 Base station

300 Cellular network system

301 Cell with fast UEs

302 Cell with slow UEs

303 Cell with medium UEs

400 Application example

410 First group of applications

41 1 First case

412 Second case

413 Third value needs to be changed

414 First value needs to be changed

415 Third case

416 First value can be changed

417 Fourth case

418 No change required

420 Second group of applications

421 First case

422 Changing third value

423 Changing second value

424 Second case

425 Allow relaxation 500 Cellular network system

501 Transceiver of the network element

502 Computation unit of the network element Setting unit of the network element Adaptation unit of the network element Configuration unit of the network element Transceiver of the user equipment Control unit of the user equipment

Claims

CLAIMS:

1 . A method for configuring a handover procedure of a user equipment from a source cell to a target cell within a cellular network system (100), the cellular network system comprising the source cell and at least one neighboring target cell, wherein the handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset, wherein a cell individual offset is assigned to each of the at least one neighboring target cell in relation to the source cell, wherein the cell individual offset is adaptable by a mobility robustness optimization algorithm and a mobility load balancing algorithm, the method comprising computing a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm,

computing a second value lower than or equal to the first value and a third value higher than or equal to the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm, setting the cell individual offset to the first value by the mobility robustness optimi- zation algorithm,

adapting the cell individual offset between the second value and the third value by the mobility load balancing algorithm, and

configuring a handover procedure for the user equipment from the source cell to a target cell based on the user equipment measurements being based on the adapted cell individual offset.

2. The method as set forth in claim 1 , further comprising initiating the handover procedure for the user equipment from the source cell to the target cell triggered by the user equipment measurements being based on the adapted cell individual offset.

3. The method as set forth in any one of the preceding claims, wherein computing the first value is based on an evaluation of the handover performance.

4. The method as set forth in claim 3, further comprising

if the handover performance changes, re-computing the first value, the second value and the third for the cell individual offset for each of the at least one neighboring target cell, re-setting the cell individual offset to the re-computed first value by the mobility robustness optimization algorithm, and

adapting the cell individual offset between the re-computed second value and the re-computed third value by the mobility load balancing algorithm.

5. The method as set forth in any one of the claims 3 or 4, wherein the handover performance is evaluated by determining a success rate of handovers from the source cell towards each of the at least one neighboring target cell.

6. The method as set forth in any one of the preceding claims, wherein the second value and the third value are computed as absolute cell individual offset limits.

7. The method as set forth in any one of the preceding claims, wherein the second value and the third value are computed as a deviation with respect to the first value.

8. The method as set forth in any one of the preceding claims, further comprising computing different second values and different third values for different groups of user equipments.

9. The method as set forth in any one of the preceding claims, wherein computing the second value and the third value comprises considering information with respect to actual and/or previous values of the cell individual offset, the information being provided by the mobility load balancing algorithm.

10. The method as set forth in claim 9, the information comprising statistic values for cell individual offsets.

1 1 . A network element being adapted to configure a handover procedure of a user equipment from a source cell to a target cell within a cellular network system (100), the cellular network system comprising the source cell and at least one neighboring target cell, wherein the handover procedure is based on measurements being performed by the user equipment prior to a handover, wherein the measurements are based on a cell individual offset, wherein a cell individual offset is assigned to each of the at least one neighboring target cell in relation to the source cell, wherein the cell individual offset is adapta- ble by a mobility robustness optimization algorithm and a mobility load balancing algorithm, the network element comprising a computation unit being adapted to compute a first value for the cell individual offset for each of the at least one neighboring target cell representing an optimum cell individual offset for the mobility robustness optimization algorithm, and being adapted to compute a second value lower than or equal to the first value and a third value higher than or equal to the first value for the cell individual offset for each of the at least one neighboring target cell, the second value representing the lowest value being acceptable for the mobility robustness optimization algorithm and the third value representing the highest value being acceptable for the mobility robustness optimization algorithm,

a setting unit being adapted to set the cell individual offset to the first value by the mobility robustness optimization algorithm,

an adaption unit being adapted to adapt the cell individual offset between the second value and the third value by the mobility load balancing algorithm, and

a configuration unit being adapted to configure a handover procedure for the user equipment from the source cell to a target cell based on the user equipment measure- ments being based on the adapted cell individual offset.

12. The network element as set forth in claim 1 1 , wherein the network element is a centralized entity and/or a base station.

13. A user equipment (102), the user equipment (102) being adapted to communicate with a network element as set forth in claim 1 1 .

14. A cellular network system (100), the cellular network system (100) comprising a network element as set forth in claim 1 1 .