WO2012103937A1

WO2012103937A1 - Apparatus and method for determing if a user equipment is to be handed over

Info

Publication number: WO2012103937A1
Application number: PCT/EP2011/051404
Authority: WO
Inventors: Peter Szilagyi
Original assignee: Nokia Siemens Networks Oy
Priority date: 2011-02-01
Filing date: 2011-02-01
Publication date: 2012-08-09

Abstract

A method comprises determining if a user equipment is to be handed over to a target cell from a source cell in dependence on channel quality information. The channel quality information comprises information on a quality of a channel between said user equipment and said source cell.

Description

APPARATUS AND METHOD FOR DETERMING IF A USER EQUIPMENT IS

TO BE HANDED OVER

Some embodiments of the present invention relate to a method and apparatus for determining if a user equipment is to be handed over.

A communication system can be seen as a facility that enables communication sessions between two or more entities such as user terminals, base stations and/or other nodes by providing carriers between the various entities involved in the commu^¬ nications path. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communications may com- prise, for example, communication of data for carrying commu^¬ nications such as voice, electronic mail (email) , text mes^¬ sage, multimedia and/or content data and so on. Non-limiting examples of services provided include two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless communication system at least a part of commu^¬ nications between at least two stations occurs over a wire^¬ less link. Examples of wireless systems include public land mobile networks (PLMN) , satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN) . The wireless systems can typi^¬ cally be divided into cells, and are therefore often referred to as cellular systems. A user can access the communication system by means of an appropriate communication device or terminal. A communication device of a user is often referred to as user equipment (UE) . A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling access to a communication network or communications directly with other users. The communica^¬ tion device may access a carrier provided by a station, for example a base station of a cell, and transmit and/or receive communications on the carrier.

The communication system and associated devices typically op^¬ erate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, it can be defined if carrier aggregation is used. Communication protocols and/or parameters which shall be used for the connection are also typically defined. An ex- ample of attempts to solve the problems associated with the increased demands for capacity is an architecture that is known as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technol^¬ ogy. The LTE is being standardized by the 3^rd Generation Partnership Project (3GPP) . The various development stages of the 3GPP LTE specifications are referred to as releases. A further development of the LTE is referred to as LTE-Advanced (LTE-A) . The LTE-Advanced aims to provide further enhanced services by means of even higher data rates and lower latency with reduced cost. HSPA (high speed packet access) is another example of a communication standard.

Handover in cellular networks such as GSM, 3G HSPA and LTE is a concept of switching the cell which serves a user equipment (UE) . The cell serving a user before the handover takes place is called the source cell and the cell serving the user equipment after the handover is called the target cell. The handover decision is based on radio measurements applied by the user equipment but eventually made by the source cell. If the source cell decides that a handover should be per^¬ formed and the target cell also accepts the user equipment, the user equipment is handed over from the source cell to the target cell. If the target cell does not accept the user equipment, the handover is not performed.

According to an embodiment there is provided a method com^¬ prising determining if a user equipment is to be handed over to a target cell from a source cell in dependence on channel quality information, said channel quality information com^¬ prising information on a quality of a channel between said user equipment and said source cell.

The determining may comprise comparing signal strength infor- mation of said source and target cells and determining said user equipment is to be handed over to the target cell if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount, said threshold amount comprising a handover threshold which is dependent on the channel quality information.

The handover threshold may comprise a cell handover value which is modifiable in dependence on the channel quality in^¬ formation .

The handover threshold may comprise a cell handover value which is decreased if said channel quality information is in^¬ dicative a radio link failure. The handover threshold may comprise a cell handover value which is decreased if said channel quality information is at or below a given quality. The handover threshold may comprise a cell handover value which is unchanged if said channel quality information is at or exceeds a given quality.

The method may comprise mapping said channel quality informa- tion to a scale where a predetermined value is indicative of a radio link failure.

The method may comprise mapping said channel quality informa^¬ tion to a scale where a first value of a sequence is indica- tive of a lowest channel quality and a last value of the se^¬ quence is indicative of a highest channel quality.

The method may comprise determining said user equipment is to be handed over if the signal strength of the target cell mi- nus the signal strength of the source cell is greater than a cell handover value minus max (0, func (channel quality indi^¬ cator) ) where func (channel quality indicator) is a monotone decreasing function of the channel quality indicator value. func (channel quality indicator) may be equal to f . (N - chan^¬ nel quality indicator) where f is a scaling factor and N is a integer parameter indicating a lowest channel quality indica^¬ tor value above and including which the user equipment re^¬ ceives no compensation of the cell handover value.

According to another embodiment, there is provided a method comprising comparing signal strength information of a source cell with signal strength information of a target cell; and determining if a user equipment is to be handed over to the target cell from the source cell in dependence on said com^¬ parison and channel quality information.

According to another embodiment, there is provided a method comprising determining if a user equipment is to be handed over to a target cell from a source cell in dependence on channel quality information.

The channel quality information may comprise information on a quality of a channel between said user equipment and said source cell.

The determining may comprise taking into account signal strength information of at least one of said source and tar- get cells.

The determining may comprise comparing said signal strength information of said source and target cells. The determining may comprise determining said user equipment is to be handed over to the target cell if a difference be^¬ tween said signal strength of said source and target cells satisfies a threshold requirement. The determining may comprise determining if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount.

The threshold amount may comprise a handover threshold which is dependent on the channel quality information.

A computer program may comprise computer executable instruc^¬ tions which when run on a processor perform the method described above. According to another embodiment, there is provided an appara^¬ tus comprising at least one processor, and at least one mem^¬ ory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to: determine if a user equipment is to be handed over to a target cell from a source cell in de^¬ pendence on channel quality information, said channel quality information comprising information on a quality of a channel between said user equipment and said source cell.

The at least one memory and the computer program code may be configured, with the at least one processor, to compare sig^¬ nal strength information of said source and target cells and to determine said user equipment is to be handed over to the target cell if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount, said threshold amount comprising a handover threshold which is dependent on the channel quality information.

The handover threshold may comprise a cell handover value which is decreased if said channel quality information is in^¬ dicative a radio link failure. The handover threshold may comprise a cell handover value which is decreased if said channel quality information is at or below a given quality. The handover threshold may comprise a cell handover value which is unchanged if said channel quality information is at or exceeds a given quality. The at least one memory and the computer program code may be configured, with the at least one processor, to map said channel quality information to a scale where a predetermined value is indicative of a radio link failure. The at least one memory and the computer program code may be configured, with the at least one processor, to map said channel quality information to a scale where a first value of a sequence is indicative of a lowest channel quality and a last value of the sequence is indicative of a highest channel quality.

The determining may comprise determining said user equipment is to be handed over if the signal strength of the target cell minus the signal strength of the source cell is greater than a cell handover value minus max (0, func (channel qual^¬ ity indicator) ) where func (channel quality indicator) is a monotone decreasing function of the channel quality indicator value . func (channel quality indicator) may be equal to f . (N - chan^¬ nel quality indicator) where f is a scaling factor and N is a integer parameter indicating a lowest channel quality indica^¬ tor value above and including which the user equipment re^¬ ceives no compensation of the cell handover value.

According to another embodiment, there is provided an appara^¬ tus comprising at least one processor, and at least one mem^¬ ory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to: compare signal strength informa^¬ tion of a source cell with signal strength information of a target cell; and determine if a user equipment is to be handed over to the target cell from the source cell in de- pendence on said comparison and channel quality information.

According to another embodiment, there is provided an appara^¬ tus comprising at least one processor, and at least one mem^¬ ory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to: determine if a user equipment is to be handed over to a target cell from a source cell in de^¬ pendence on channel quality information. The channel quality information may comprise information on a quality of a channel between said user equipment and said source cell.

The determining may comprise taking into account signal strength information of at least one of said source and tar^¬ get cells.

The determining may comprise comparing said signal strength information of said source and target cells.

The at least one memory and the computer program code may be configured, with the at least one processor, to determine said user equipment is to be handed over to the target cell if a difference between said signal strength of said source and target cells satisfies a threshold requirement.

The at least one memory and the computer program code may be configured, with the at least one processor, to determine if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount .

A user equipment may comprise an apparatus as described above . A base station may comprise an apparatus as described above.

It should be appreciated that features of one embodiment may be used with any other of the embodiments. Some embodiments will now be described, by way of example only, with reference to the following examples and accompanying drawings in which:

Figure 1 schematically shows part of a communications net- work;

Figure 2 shows an example of a communication device;

Figure 3 shows an example of controller apparatus for a base station;

Figure 4 shows a situation where embodiments may be applied; Figure 5 shows method steps performed which may be performed in a user equipment; and

Figure 6 shows method steps performed in a base station, a user equipment, or partially in a user equipment and par^¬ tially in a base station.

In the following certain exemplifying embodiments are explained with reference to wireless or mobile communication systems serving mobile communication devices. Before explaining in detail the certain exemplifying embodiments, certain general principles of a wireless communication system and the nodes thereof are briefly explained with reference to Figures 1 to 3 to assist in understanding of the herein described em^¬ bodiments .

In a mobile system a user can be provided with a mobile com^¬ munication device 1 that can be used for accessing various services and/or applications. The access can be provided via an access interface between the mobile user device 1 and an appropriate wireless access system, for example an access node. An access node can be provided by a base station. Fig^¬ ure 1 shows part of a radio access network (RAN) , including a first base station 2 and a second base station 2. The term base station will be used in the following and is intended to include the use of any of these access nodes or any other suitable access node. The base stations each have a cell as^¬ sociated therewith. The access system also comprises a mobil^¬ ity management entity (MME) 12. The mobile management entity 12 and the base stations can be connected, for example, by means of a SI interface.

Although not shown, a gateway function between the access systems, a core network 22 and/or another network such as the packet data network may also be provided by means of appro- priate gateway nodes. Regardless of the gateway arrangement, a communication device can be connected to an external data network, for example the internet via the access nodes and the base station. The mobile communication devices can access the communication system based on various access techniques, such as code divi^¬ sion multiple access (CDMA) , or wideband CDMA (WCDMA) , the latter technique being used by some communication systems based on the third Generation Partnership Project (3GPP) specifications. For LTE (long term evolution) and LTE-A (long term evolution - advanced) , OFDMA (Orthogonal Frequency Divi^¬ sion Multiplexing) in the DL (down link) and single-carrier FDMA in the UL (uplink) can be used. Other examples include time division multiple access (TDMA) , frequency division mul^¬ tiple access (FDMA) , space division multiple access (SDMA) and so on. In a wireless system a network entity such as a base station provides an access node for communication de^¬ vices.

A non-limiting example of mobile architectures where the herein described principles may be applied is known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) . Non-limiting examples of appropriate access nodes are a base station of such system, for example what is known as NodeB (NB) or enhanced NodeB (eNB) in the vocabulary of the 3GPP specifications. Other examples include base stations of sys^¬ tems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access) . Access nodes can provide cellular sys^¬ tem level base stations providing E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical layer protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards mobile communica- tion devices.

Regardless of the underlying standard, a mobile communication device can be provided wireless access via at least one base station or similar wireless transceiver node of an access system. An access system may be provided by a cell of a cel^¬ lular system or another radio service area enabling a communication device to access a communication system. Therefore an access system is hereinafter referred to as a radio ser^¬ vice area or cell. Typically a cell is provided by a base station site. A base station site can provide a plurality of sectors, for example three radio sectors, each sector provid^¬ ing a cell or a sub radio service area of a cell. Figure 2 shows a schematic, partially sectioned view of a communication device 1 that a user can use for communication. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile communica^¬ tion device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) such as a mobile phone or what is known as a 'smart phone', a portable computer provided with a wire^¬ less interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communi- cation capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email) , text message, multimedia and so on. Users may thus be offered and provided numerous ser- vices via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. User may also be provided broadcast or multicast data. Non- limiting examples of the content include downloads, televi^¬ sion and radio programs, videos, advertisements, various alerts and other information.

The mobile communication device 1 may receive and transmit signals over an air interface 28 via appropriate apparatus for receiving and transmitting signals. In Figure 2 transceiver apparatus is designated schematically by block 27. The transceiver may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrange- ment may be arranged internally or externally to the mobile device .

A mobile communication device is also typically provided with at least one data processing entity 23, at least one memory 24 and other possible components 29 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with base stations and other communication devices. The data process- ing, storage and other relevant control apparatus can be pro^¬ vided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 26. Possible control functions in view of configuring the mobile communication device for reception and/or transmission of signalling informa- tion and data by means of the data processing facility in ac^¬ cordance with certain embodiments of the present invention will be described later in this description.

The user may control the operation of a communication device by means of a suitable user interface such as keypad 22, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 25, a speaker and a microphone are also typically provided. Furthermore, a mobile communica^¬ tion device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

Figure 3 shows an example of a control apparatus 30, for ex^¬ ample to be coupled to a base station and/or part of the base station itself. The control apparatus 30 can be arranged to provide control on use of resources for communications by mo^¬ bile communication devices that are in the service area. The control apparatus 30 can be configured to provide control functions in association with generation and communication of resource allocation information and other related information and for coordination of resource allocation for signalling and data communications by means of the data processing fa^¬ cility in accordance with certain embodiments described be- low. For this purpose the control apparatus 30 comprises at least one memory 31, at least one data processing unit 32, 33 and an input/output interface 34. Via the interface the con^¬ trol apparatus can be coupled to receiver and transmitter ap^¬ paratus of a base station. The control apparatus 30 can be configured to execute an appropriate software code to provide the control functions.

In one embodiment, a user equipment 1 is configured to be handed over from one of the base stations 2 (source cell) to another of the base stations 2 (target cell) . It should be appreciated that in some embodiments, handing over from one cell to another may be handing over from one cell to another cell where both of the cells are served by the same base sta^¬ tion or base station site.

Currently, the radio parameter on which the initial decision is based is the signal strength of the source and target cells, as measured and reported by the user equipment. Radio link failure (RLF) may occur around handovers with increased probability as compared to other parts of the network, for example places closer to the base station with better coverage and signal reception. RLF is one of the most common causes of dropped calls and in the case of data streaming ap^¬ plications, the decreased download rate due to the mechanism that the underlying TCP (transmission control protocol) han^¬ dles the packet loss. The dropped calls or decreased

download rate reduces the quality of service experienced by the users of the network. Embodiments may effectively reduce the probability of RLF around handovers in cellular networks while retaining the values of mobility parameters. Examples of mobility parame^¬ ters are cell specific handover thresholds. Embodiments may be such that they do not interfere with the network dimen^¬ sioning, coverage planning and cell structure. Embodiments may enhance the handover decision process to take into ac^¬ count the channel quality of the user equipment that reports the radio measurements, and not only the signal strength of the source and target cells. Embodiments may be used with any suitable network, for example HSDPA and LTE networks. Of course, embodiments can be used with any other suitable net^¬ works . Both HSDPA and LTE use a channel quality indicator (CQI) which is reported by the user equipment. This may be re^¬ ported periodically, in response to a request from a base station or the like, or may be triggered by one or more events. In HSDPA and LTE, CQI is an integer describing the downlink channel quality of the user equipment. The CQI is reported periodically, in some embodiments, once in every X milliseconds. X is configurable and may for example be 1, 2, 4, etc. The CQI may be reported periodically by a user equipment to its current serving cell. In LTE, the currently defined 16 possible CQI values are CQI = 0, 1, 15 with 0 indicating RLF, 1 meaning the lowest but still usable channel quality and 15 meaning the best channel quality. In HSDPA, there is twice the number of possible CQI values. Addition^¬ ally, RLF is indicated by 31. The CQI values are from 0 to 31. 0 is the lowest but still usable channel quality and 30 means the best channel quality.

Handover of user equipment is controlled by events based on radio measurements reported by the user equipment. Cur- rently, in LTE, the event that is used to trigger the hand^¬ over is called the A3 event which means that the signal of the target cell becomes better than the source cell by a threshold. This threshold is called the A3 threshold or handover threshold. HSDPA and GSM use the same principle.

The handover threshold is usually set to 3dB by default which means that a user equipment will generally be handed over to a target cell if the target cell's signal is better than that of the current serving cell of the user equipment by at least 3dB.

Using this method, the target cell is required to have a bet^¬ ter signal than that of the source cell. However, it may happen that the source cell is not been able to provide an acceptable service to a user equipment while at the same time the target cell will be able to better serve the user. How^¬ ever, handover may not be performed because the signal of the target cell is not yet better than that of the source cell by the defined handover threshold. This situation may lead to a short radio link failure for a user equipment until the hand^¬ over condition, i.e. the required 3dB difference is fulfilled and the user equipment is finally handed over to the target cell. A handover such as this, which leads to a brief RLF, is sometimes referred to as a "too late" handoff expressing that there would have been no RLF if the handover had been performed earlier. Handover is usually performed by cell edge users and it is the cell edge users who are mostly ex^¬ posed to increased interference that may further deteriorate their radio channel quality compared to users which are closer to the base station. Embodiments may use a mechanism which predicts a user equipment having a high risk of running into an RLF situation shortly before the handoff is completed and prevents this from occurring. This may contribute to the robustness of the radio link of user equipment and thus im^¬ prove the quality of its service experienced by the users.

Embodiments provide a method that adjusts the handover threshold for those users who have poor channel quality and are about to perform handover.

In one embodiment, the CQI values are remapped. In HSDPA, the highest CQI value, that is 31, means RLF. In contrast, in LTE, the CQI value 0 means RLF. Accordingly, in one embodiment, the HSDPA values for CQI are remapped. The remap^¬ ped HSDPA CQI values are denoted CQI' . Thus, CQI' = CQI for LTE. CQI' = (CQI +1) mod 32 for HSDPA. The intention behind the remapping is to uniformly make CQI' = 0 meaning RLF in both LTE and HSDPA and to ensure that increasing CQI' means increasing downlink channel quality. In LTE, it means identity mapping since CQI and LTE is already designed that way. In HSDPA, it means shifting up and moving the original CQI = 31 to CQI' = 0.

Of course in alternative embodiments, the HSDPA values for CQI may be unchanged whilst the LTE values are mapped to the HSDPA value. In alternative embodiments, there may be no re^¬ mapping and the existing CQI values are used without modifi- cation.

The handover condition of the user equipment is fulfilled if the following inequality continuously holds for a certain time interval called time to trigger, usually in the order of seconds:

Starget ^~~ S _{S OU}rce > HO_THRESHOLD .

Starget and S _{S 0U}rce are the signal strength of the target and source cells respectively. This may be as reported by the user equipment. The HO_THRESHOLD is the cell specific hand^¬ over threshold which may be, for example +3dB. The cell spe^¬ cific handover threshold may be set by the network operator or automatically by OAM policy (operations administration and maintenance) . Of course HO_THRESHOLD may be replaced by any other cell handover value.

In embodiments, the cell specific handover threshold is modi^¬ fied by taking into account also the channel quality of the user equipment by the following formula:

Starget - S_source > HO_THRESHOLD - F (CQI') .

In one embodiment, F (CQI') = max (0, func (CQI'). CQI' is the remapped CQI and func (CQI') is a monotone decreasing function of this CQI' value. For example,

func (CQI') = f (N - CQI) . N is an integer parameter denoting the lowest CQI' above and including which the UE does not receive any compensation in the handover threshold and f is a scaling factor denoting how much compensation (in dB) a user equipment gets per one CQI' value unit. In one LTE embodi^¬ ment, N may be equal to 2 and f equal to 1.0. This provides a +2dB compensation for UEs with CQI' equal to 0 and a +ldB compensation for UEs with CQI' equal to 1. User equipment having higher CQI' values may not be affected. For HSDPA, since there is a finer CQI resolution, the relevant parame^¬ ters may, by way of example only be N = 4 and f = 0.5.

In some embodiments, the N and f parameters are cell spe^¬ cific. Additionally or alternatively, the N and f parameters may be network wide parameters. max (0, func (CQI')) means the maximum of 0 and the result of the function func (CQI') . Accordingly, the value which is subtracted from the threshold will be 0 unless func (CQI') is greater than 0. Accordingly, for the LTE scenario, this will occur when CQI' is 0 or 1. Similarly, for HSDPA, the value subtracted from the hand^¬ over threshold will be 2.5 for a CQI' of 3, 2 for a CQI' of 2, 1.5 for a CQI' of 1 and 1 for a CQI' of 0. Reference is now made to Figure 4 which illustrates an em^¬ bodiment of the present invention. In Figure 4, the source cell is referenced 50 whilst the target cell is referenced 52. The user equipment 54 is shown moving away from the source cell towards the target cell. Four different posi- tions of the mobile station are shown and are respectively referenced a) to d) . Consider a scenario where the user equipment is moving from the source cell to the target cell without using one of the embodiments. In this arrangement, the user equipment will suffer from a too late handover.

When the mobile station is at position a) , the signal of the source cell is better than that of the target cell and the user equipment receives a good service. The user equipment continues to move to position b) . Between position a) and position b) is a threshold where S_{S 0U}rce = S_target - By position b) , the signal of the source cell is worse than the target cell but the user equipment still receives an acceptable ser^¬ vice. Thus, when the mobile station is in position a) or b, the source cell is able to provide a service without any RLF. The user equipment moves to position c) . The source cell is no longer able to provide an acceptable service but the sig^¬ nal of the target cell is not good enough by, for example 3dB, to trigger the handover. Thus with the current imple^¬ mentation, the user equipment would remain attached to the source cell and would suffer a RLF.

In position d) , the mobile station is now such that the sig^¬ nal of the target cell is better than the required threshold by, for example 3dB. The mobile station would thus be handed over to the target cell.

In contrast, in embodiments, when in position c) the user equipment would have received a threshold compensation of 2dB resulting in a +ldB effective handover threshold. On that basis, the handover to the target cell would have been per^¬ formed thus preventing the RLF. Thus, in embodiments of the present invention, handover is made earlier than would be the case with the current propos^¬ als .

Simulations using embodiments show that there may be a sig- nificant reduction in the overall call drop ratio.

Some embodiments may decrease the number of RLFs in some simulations by at least 30%. The number of RLFs in a radio access network due to too late handoffs, may be decreased.

Embodiments may not jeopardise mobility robustness, network planning and cell coverage planning as the implementation does not require the modification of the cells specific hand^¬ over threshold that would be applied to all user equipment. Only those user equipment that have a high risk of RLF before they reach the target cell are assisted. Accordingly, some embodiments would not drive extra traffic to cells in which that traffic would not appear anyway, even without the em^¬ bodiments .

Embodiments of the present invention have been described in relation to HSDPA and LTE networks which have CQI support. However, there are some proposals to bring channel estimation and channel quality indicators into, for example GSM/GPRS/EDGE making possible implementation of embodiments also in these radio access technologies.

Reference is now made to Figure 5 which shows a method per- formed by the user equipment. In step SI, the user equipment is configured to determine the CQI . In step S2, the user equipment is configured to make measurements for S_target - In step S3, the user equipment is configured to make measure^¬ ments for S source - In step S4, the user equipment is config- ured to transmit the information to the eNodeB. It should be appreciated that steps SI, S2 and S3 may take place together or in a different order. Steps SI, S2 and S3 will be re^¬ peated from time-to-time. The information which is transmitted in step S4, may include all of the information obtained in steps SI, S2 and S3. Al^¬ ternatively, one or more of the pieces of the information may be separately transmitted. It should be appreciated that in some embodiments the user equipment may make the measurements in order for the CQI to be determined but that the eNodeB actually determines from the measurement information the CQI. Likewise, the user equipment may make the measurements and/or determine S_target and/or S_{S 0U}rce - Where the user equipment only determines the measurements for S_target and/or S_{S 0U}rce , the eNodeB is configured to determine the S_target and/or S_{S 0U}rce from the provided infor^¬ mation . It should be appreciated that in practice, some or all of the processing may be carried out by one or more processors of the user equipment. Reference is now made to Figure 6 which shows a method. In one embodiment, this method may be performed in the eNodeB. In step Tl, the eNodeB receives the CQI, S_target and S _{S 0U}rce in^¬ formation. In step T2, if necessary, the eNodeB determines CQI, Starget and/or S_{S 0U}rcce - It should be appreciated that one or more of these parameters may not need to be determined as they may be supplied by the user equipment.

In step T3, CQI is mapped, if necessary to CQI' . In step T4, Starget _/ S_{S 0U}rce and CQI' are input to the equation mentioned previously. Depending on the results of that equation, it is determined if handover is required. If handover is required, then the eNodeB will take the necessary steps in order to cause the user equipment to be handed over to the target cell. It should be appreciated that steps Tl to T5 may take place in one or more processors of the eNodeB.

In one modification, a user equipment may be configured to carry out at least one of steps T4 and T5. Steps Tl, T2 and/or T3 may optionally be performed in the user equipment. Optionally, one or more of these steps may be performed in the eNodeB and the results provided to the user equipment.

In one embodiment, CQI reports are created by the UE and are transmitted in UL to the eNB . Similar, the condition for the A3 event is evaluated in the UE before it is reported to the eNB where the report may trigger a handover procedure.

Thus, in some embodiments the A3 event in the UE remains un- changed but the eNB sets the threshold (per UE) according to the currently reported CQI value of the UE . The threshold for handover is configured by the eNB for a UE handover to be: HO_THRESHOLD - max ( 0 , func (CQI ' (UE) ) ) , where HO_THRESHOLD denotes some default value which applies to the UEs most of the time and only a UE experiencing poor channel conditions will apply a lower threshold. This embodiment may be transparent to the UE and may not require any changes in the UE stan^¬ dards .

In other embodiments, the UE itself considers the current CQI value when evaluating the trigger condition for the A3 event. The threshold for handover configured by the eNB can be the same for all UEs in a cell, but the UE will internally apply an adapted threshold HO_THRESHOLD - max (0, func (CQI' (UE) ) . This embodiment may be transparent to the eNB and may not re^¬ quire frequent updates of the measurement configuration for the UE . This may require changes in the UE standard. In one embodiment, the channel quality indicators are mes^¬ sages that are sent to, for example the base station and pro^¬ vide channel quality information. The channel quality infor^¬ mation may include one or more of the carrier level received signal strength indication (RSSI), block error rate (BLER) or the highest modulation and coding scheme (MCS) which can be decoded in the UE with a predefined maximum block error rate.

It should be appreciated that at least some of the method steps may be implemented by one or more computer programs. Such computer program (s) may comprise one or more computer instructions which when run on one or more processors cause the associated method step to be performed.

Whilst embodiments of the present invention have been de- scribed in relation to the LTE, GSM and HSPA systems, it should be appreciated that embodiments of the present inven^¬ tion can be used in conjunction with any other suitable standard . It is noted that whilst embodiments may have been described in relation to user equipment or mobile devices such as mo^¬ bile terminals, embodiments of the present invention may be applicable to any other suitable type of apparatus suitable for communication via access systems. A mobile device may be configured to enable use of different access technologies, for example, based on an appropriate multi-radio implementa^¬ tion. Reference has been made to an eNodeB. It should be appreci^¬ ated, that embodiments may be implemented in any other suit^¬ able base station or radio access node. In some embodiments, one or more of the method steps may be performed in a control node which may be associated with the base station or sepa- rate therefrom.

It is also noted that although certain embodiments may have been described above by way of example with reference to the exemplifying architectures of certain mobile networks, em- bodiments may be applied to any other suitable forms of com^¬ munication systems than those illustrated and described herein. It is also noted that the term access system may be understood to refer to any access system configured for ena^¬ bling wireless communication for user accessing applications.

The above described operations may require data processing in the various entities. The data processing may be provided by means of one or more data processors. Similarly various enti^¬ ties described in the above embodiments may be implemented within a single or a plurality of data processing entities and/or data processors. The data processing entities may be controlled by one or more computer programs which may be stored in one or more memories of the apparatus. Alternatively or additionally appropriately adapted computer program code product may be used for implementing the embodi^¬ ments, when loaded to a computer or a processor. The program code product for providing the operation may be stored on and provided by means of a carrier medium such as a carrier disc, card or tape. A possibility may be to download the program code product via a data network.

For example the embodiments of the invention may be imple- mented as a chipset, in other words a series of integrated circuits communicating among each other. The chipset may comprise microprocessors arranged to run code, application spe^¬ cific integrated circuits (ASICs) , or programmable digital signal processors for performing the operations described above.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits can be by and large a highly automated process. Complex and powerful software tools may be available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate . Programs, such as those provided by Synopsys, Inc. of Moun^¬ tain View, California and Cadence Design, of San Jose, Cali^¬ fornia may automatically route conductors and locate compo^¬ nents on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit may have been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication. The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, vari- ous modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims.

Claims

CLAIMS :

1. A method comprising:

determining if a user equipment is to be handed over to a target cell from a source cell in dependence on channel quality information, said channel quality information comprising information on a quality of a channel between said user equipment and said source cell.

2. A method as claimed in claim 1, wherein said determining comprises comparing signal strength information of said source and target cells and determining said user equip^¬ ment is to be handed over to the target cell if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount, said threshold amount comprising a handover threshold which is dependent on the channel quality information.

3. A method as claimed in claim 2, wherein said hand- over threshold comprises a cell handover value which is modi^¬ fiable in dependence on the channel quality information.

4. A method as claimed in claim 2 or 3, wherein said handover threshold comprises a cell handover value which is decreased if said channel quality information is indicative a radio link failure.

5. A method as claimed in claim 2, 3 or 4, wherein said handover threshold comprises a cell handover value which is decreased if said channel quality information is at or be^¬ low a given quality.

6. A method as claimed in claim 2, 3, 4 or 5, wherein said handover threshold comprises a cell handover value which is unchanged if said channel quality information is at or ex^¬ ceeds a given quality.

7. A method as claimed in any preceding claim, com- prising mapping said channel quality information to a scale where a predetermined value is indicative of a radio link failure .

8. A method as claimed in any preceding claim, com- prising mapping said channel quality information to a scale where a first value of a sequence is indicative of a lowest channel quality and a last value of the sequence is indica^¬ tive of a highest channel quality.

9. A method as claimed in any preceding claim, wherein said determining comprises determining said user equipment is to be handed over if the signal strength of the target cell minus the signal strength of the source cell is greater than a cell handover value minus max (0, func (channel quality in- dicator) ) where func (channel quality indicator) is a mono^¬ tone decreasing function of the channel quality indicator value .

10. A method as claimed in claim 9, wherein func

(channel quality indicator) = f . (N - channel quality indica^¬ tor) where f is a scaling factor and N is a integer parameter indicating a lowest channel quality indicator value above and including which the user equipment receives no compensation of the cell handover value.

11. A method comprising:

comparing signal strength information of a source cell with signal strength information of a target cell; and determining if a user equipment is to be handed over to the target cell from the source cell in dependence on said comparison and channel quality information.

12. A computer program comprising computer executable instructions which when run on a processor perform the method of any of claims 1 to 11.

13. An apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to:

determine if a user equipment is to be handed over to a target cell from a source cell in dependence on channel quality information, said channel quality information com^¬ prising information on a quality of a channel between said user equipment and said source cell.

14. An apparatus as claimed in claim 13, wherein the at least one memory and the computer program code are configured, with the at least one processor, to compare signal strength information of said source and target cells and to determine said user equipment is to be handed over to the target cell if said signal strength of said target cell is greater than said signal strength of said source cell by at least a threshold amount, said threshold amount comprising a handover threshold which is dependent on the channel quality information .

15. An apparatus as claimed in claim 14, wherein said handover threshold comprises a cell handover value which is modifiable in dependence on the channel quality informa^¬ tion.

16. An apparatus as claimed in claim 14 or 15, wherein said handover threshold comprises a cell handover value which is decreased if said channel quality information is indica^¬ tive a radio link failure.

17. An apparatus as claimed in claim 14, 15 or 16, wherein said handover threshold comprises a cell handover value which is decreased if said channel quality information is at or below a given quality.

18. An apparatus as claimed in any of claims 14 to 17, wherein said handover threshold comprises a cell handover value which is unchanged if said channel quality information is at or exceeds a given quality.

19. An apparatus as claimed in any of claims 13 to 18, wherein the at least one memory and the computer program code are configured, with the at least one processor, to map said channel quality information to a scale where a predetermined value is indicative of a radio link failure.

20. An apparatus as claimed in any of claims 13 to 19, wherein the at least one memory and the computer program code are configured, with the at least one processor, to map said channel quality information to a scale where a first value of a sequence is indicative of a lowest channel quality and a last value of the sequence is indicative of a highest channel quality .

21. An apparatus as claimed in any of claims 13 to 20, wherein said determining comprises determining said user equipment is to be handed over if the signal strength of the target cell minus the signal strength of the source cell is greater than a cell handover value minus max (0, func (chan- nel quality indicator) ) where func (channel quality indica^¬ tor) is a monotone decreasing function of the channel quality indicator value.

22. An apparatus as claimed in claim 21 wherein func

23. An apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to:

compare signal strength information of a source cell with signal strength information of a target cell; and

determine if a user equipment is to be handed over to the target cell from the source cell in dependence on said comparison and channel quality information.

24. A user equipment comprising an apparatus as claimed in any of claims 13 to 23.

25. A base station comprising an apparatus as claimed in any of claims 13 to 23.