WO2015096845A1

WO2015096845A1 - A method for managing communications between a first mobile terminal and a second mobile terminal

Info

Publication number: WO2015096845A1
Application number: PCT/EP2013/077875
Authority: WO
Inventors: Oemer Bulakci; Jian Luo
Original assignee: Huawei Technologies Duesseldorf Gmbh
Priority date: 2013-12-23
Filing date: 2013-12-23
Publication date: 2015-07-02
Also published as: CN105794302B; CN105794302A

Abstract

A method for managing communications between a first mobile terminal and a second mobile terminal The invention relates to a method (100) for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal communicates with the second mobile terminal over a device-to-device, D2D, communication link, the method (100) comprising determining (101) a relocation of the second mobile terminal towards the target cell, determining (103) a common communication resource of the serving cell and of the target cell for the D2D communication link,and allocating (105) the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications.

Description

DESCRIPTION

A method for managing communications between a first mobile terminal and a second mobile terminal

TECHNICAL FIELD

The invention relates to the field of wireless communication systems, in particular to wireless network-controlled device-to-device communication systems.

BACKGROUND

Direct device-to-device (D2D) communications is a communication technology, which can be implemented, e.g., in fifth generation (5G) mobile and wireless networks. In network- controlled D2D communications, resource allocation for a D2D communication link is managed by a network infrastructure to attain a certain level of quality of service. A typical example of mobile D2D communication systems relates to Car-to-Car (Car2Car)

communications, which can have challenging latency requirements. One challenge of realizing D2D communications is the D2D handover, HO, in mobile D2D communication systems. Current schemes for a D2D HO can result in high experienced interference on a D2D communication link, and thus, can yield in D2D communication failure or interruption, since e.g. the D2D common resource allocation has been performed too late, and/or can introduce high latency to the D2D communications when, e.g., the D2D communication link is broken and the data are transferred over, e.g., the X2 interface.

In the document WO 201 1/109027, a HO scheme for direct peer-to-peer communication systems is described. SUMMARY

It is the object of the invention to provide an efficient scheme for performing a D2D handover in mobile D2D communication systems.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures. The invention is based on the finding that prior to a D2D handover from a serving cell to a target cell, a common communication resource of the serving cell and of the target cell can be determined and allocated to the D2D communication link. Therefore, the D2D handover can be performed while maintaining the D2D communication link, which allows for low- latency D2D communications. Furthermore, interference to the communication signals of the D2D communication link can be reduced. The determining of the common communication resource can be performed in a centralized or distributed manner by the network

infrastructure. According to a first aspect, the invention relates to a method for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal communicates with the second mobile terminal over a device-to-device, D2D, communication link, the method comprising determining a relocation of the second mobile terminal towards the target cell, determining a common communication resource of the serving cell and of the target cell for the D2D communication link, and allocating the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications. Thus, an efficient scheme for performing a D2D handover can be provided.

The communications can relate to multi-cell direct D2D communications within the communications network. The D2D communication link can be a radio link oran optical link between the first mobile terminal and the second mobile terminal. The first mobile terminal or the second mobile terminal can be a cellular phone, a

smartphone, a mobile computer, or a user equipment. The first mobile terminal or the second mobile terminal can be a handheld device or can be integrated in a vehicle, e.g., a car. The first mobile terminal or the second mobile terminal can be configured to communicate wirelessly. The first mobile terminal or the second mobile terminal can be members of a D2D communication group.

The communication resource can define the physical resource for the D2D communication link. The communication resource can, e.g., be a physical resource block (P B) in long term evolution (LTE) communication technology. The communication resource can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator. A communication resource of the serving cell can initially be allocated to the D2D communication link. The determining of the relocation of the second mobile terminal towards the target cell can be performed by detecting and/or predicting the relocation. Detecting the relocation can comprise comparing a transmission characteristic of the D2D communication link, e.g., a transmission power, a received power and a received signal quality, respectively, with a predetermined threshold, wherein the relocation is detected if the transmission characteristic falls below and increases above, respectively, the predetermined threshold. Detecting the relocation can further comprise comparing a transmission characteristic of the mobile terminal to network infrastructure link of at least one mobile terminal. Predicting the relocation can comprise determining a movement trajectory of the second mobile terminal and determining a location of the second mobile terminal at a predetermined future time instant. Predicting the relocation can further comprise determining a movement trajectory of both first and second mobile terminals. Predicting the relocation can further comprise evaluating a handover statistic of the serving cell with regard to the target cell. The prediction can therefore imply an a-priori detection.

The determining of the common communication resource of the serving cell and of the target cell for the D2D communication link can relate to determining a commonly available communication resource of the serving cell and of the target cell or a communication resource of the serving cell and of the target cell that can be made commonly available.

The allocating of the common communication resource of the serving cell and of the target cell to the D2D communication link can comprise a reserving of the common communication resource within the serving cell and within the target cell. In a first implementation form of the method according to the first aspect, the determining of the common communication resource of the serving cell and of the target cell is performed by a central entity, by a serving base station or by a target base station. Thus, the

determining of the common communication resource can be performed efficiently by the network infrastructure. By using a central entity, a centralized approach for the determining of the common communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed by a dedicated component of the communications network. By using a serving base station or a target base station, a distributed approach for the determining of the common communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed directly between involved components of the communications network. The serving base station can be configured to manage the communication resources of the serving cell. The target base station can be configured to manage the communication resources of the target cell. The central entity can be connected to the serving base station and to the target base station.

The determining of the relocation of the second mobile terminal can be performed by the central entity or the serving base station. The relocation of the second mobile can be determined by the central entity based on context information. The determining of the common communication resource can be performed in a centralized manner by the central entity. The determining of the common communication resource can be performed in a distributed manner by the serving base station or the target base station.

The allocating of the common communication resource can be performed by the serving base station and/or the target base station during the communications.

In a second implementation form of the method according to the first aspect as such or the first implementation form of the first aspect, the determining of the common communication resource of the serving cell and of the target cell is performed by a central entity, and the determining of the common communication resource of the serving cell and of the target cell comprises sending, by a serving base station, a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the central entity, sending, by a target base station, a second message comprising second context information, the second context information comprising communication resources of the target cell, to the central entity, determining, by the central entity, a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the central entity, a message comprising the common communication resource of the serving cell and of the target cell to the serving base station and to the target base station. Thus, the common communication resource can efficiently be determined. By using a central entity, a centralized approach for the determining of the common

communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed by a dedicated component of the communications network. The sending of the second message can be performed by the target base station upon reception of a request message from the central entity upon the determination of the relocation.

The first message or the second message can be an electronic message. The first message or the second message can be formatted, e.g., according to the X2 interface standard or the interface between base stations and the central entity. The first context information or second context information can comprise communication resources of the serving cell or of the target cell, a priority of the D2D communication link, an identification of the D2D communication link, a modulation parameter of the D2D

communication link, e.g., a filter bank multi carrier (FBMC) pulse shape, or a load of the serving cell or of the target cell.

The message comprising the common communication resource can be an electronic message. The message comprising the common communication resource can be formatted, e.g., according to the X2 interface standard or the interface between base stations and the central entity.

The communication resources of the serving cell can be bound to a licensed spectrum of a communications network operator. The communication resources of the target cell can be bound to a licensed spectrum of a communications network operator or a virtual

communications network operator.

In a third implementation form of the method according to the second implementation form of the first aspect, the method further comprises determining, by the central entity, a

communication resource from alternative communication resources if common

communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. Thus, a fallback solution for the determining of the common communication resource can be provided. The alternative communication resources can be bound to an unlicensed or a light-licensed spectrum of a communications network operator. The alternative communication resource can be related to an alternative mode of operation of the D2D communication link. The alternative mode of operation can be an alternative communbation technology, e.g., free- space optical communications.

The message comprising the common communication resource can indicate an alternative mode of operation of the D2D communication link. Consequently, the central entity can inform the serving base station and/or the target base station about the alternative mode of operation.

In a fourth implementation form of the method according to the first aspect as such or the first implementation form of the first aspect, the determining of the common communication resource of the serving cell and of the target cell is performed by a serving base station or by a target base station, and the determining of the common communication resource of the serving cell and of the target cell comprises sending, by the serving base station, a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the target base station, determining, by the target base station, a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the target base station, an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the serving base station if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available. Thus, the common communication resource can efficiently be determined. By using a serving base station or a target base station, a distributed approach for the determining of the common communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed directly between involved components of the communications network.

The first message can be an electronic message. The first message can be formatted, e.g., according to the X2 interface standard.

The first context information can comprise communication resources of the serving cell, a priority of the D2D communication link, an identification of the D2D communication link, a modulation parameter of the D2D communication link, e.g., a filter bank multi carrier (FBMC) pulse shape, or a load of the serving cell. The acknowledgement message comprising the common communication resource can be an electronic message. The acknowledgement message comprising the common

communication resource can be formatted, e.g., according to the X2 interface standard. The communication resources of the serving cell can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator. The communication resources of the target cell can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator. In a fifth implementation form of the method according to the fourth implementation form of the first aspect, the method further comprises sending, by the target base station, a negative- acknowledgement message comprising second context information, the second context information comprising further communication resources of the target cell, to the serving base station if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available,

determining, by the serving base station, a communication resource from further common communication resources of further communication resources of the serving cell and the further communication resources of the target cell if further common communication resources of further communication resources of the serving cell and the further

communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the serving base station, an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the target base station. Thus, the common communication resource can be determined upon the basis of further communication resources of the serving cell and further communication resources of the target cell.

The negative-acknowledgement message can be an electronic message. The negative- acknowledgement message can be formatted, e.g., according to the X2 interface standard.

The second context information can comprise communication resources of the target cell, a priority of the D2D communication link, an identification of the D2D communication link, a modulation parameter of the D2D communication link, e.g., a filter bank multi carrier (FBMC) pulse shape, or a load of the target cell.

The acknowledgement message comprising the common communication resource can be an electronic message. The acknowledgement message comprising the common

communication resource can be formatted, e.g., according to the X2 interface standard. The further communication resources of the serving cell can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator. The further communication resources of the target cell can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator.

In a sixth implementation form of the method according to the fifth implementation form of the first aspect, the method further comprises determining, by the serving base station, a communication resource from alternative communication resources if further common communication resources of the further communication resources of the serving cell and the further communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. Thus, a fallback solution for the determining of the common

communication resource can be provided.

The alternative communication resources can be bound to an unlicensed or a light-licensed spectrum of a communications network operator or a virtual communications network operator. The alternative communication resource can be related to an alternative mode of operation of the D2D communication link. The alternative mode of operation can be an alternative communication technology, e.g., free-space optical communications.

The acknowledgement message comprising the common communication resource can indicate an alternative mode of operation of the D2D communication link. Consequently, the serving base station can inform the target base station about the alternative mode of operation.

Determining, by the serving base station, the communication resource from alternative communication resources can alternatively be performed upon reception of a negative- acknowledgement, NACK, message from the target base station. The target base station can send a NACK message for performing the fallback solution.

In a seventh implementation form of the method according to one of the second

implementation form of the first aspect to the sixth implementation form of the first aspect, the first context information further comprises a priority among a plurality of priorities of the D2D communication link, and the common communication resource of the serving cell and of the target cell is further determined upon the basis of the priority of the D2D communication link. Thus, managing communications in the communications network can be realized more efficiently.

The priority of the D2D communication link can be a real value, e.g., 0.3 or 0.8, wherein a high value can indicate a high priority and a low value can indicate a low priority. The priority of the D2D communication link can further be a quantized value, wherein the quantized value can indicate, e.g., a high priority, a medium priority or a low priority.

The priority of the D2D communication link can be determined upon the basis of a movement speed of the first mobile terminal and/or the second mobile terminal. The priority of the D2D communication link can further be determined upon the basis of a desired quality of service of the D2D communication link.

In an eighth implementation form of the method according to one of the second

implementation form of the first aspect to the seventh implementation form of the first aspect, the first context information further comprises an identification of the D2D communication link, and the identification of the D2D communication link is associated with the determined common communication resource of the serving cell and of the target cell. Thus, the D2D communication link and the determined common communication resource can be identified uniquely. The identification of the D2D communication link can be globally unique or locally unique, e.g., within a cluster of cells.

The identification of the D2D communication link can be a unique number, e.g., 128, or a unique sequence of characters, e.g., "ABC". The identification of the D2D communication link can be stored in a database of the serving base station, the target base station and/or the central entity.

In a ninth implementation form of the method according to the first aspect as such or one of the preceding implementation forms of the first aspect, the communication resource of the D2D communication link is at least one of a transmission time, a transmission frequency, a transmission space, a transmission code, a transmission modulation, or a transmission power of the D2D communication link. Thus, the method can be applied using different communication technologies. The transmission time of the D2D communication link can relate to a time division multiple access (TDMA) scheme. The transmission frequency of the D2D communication link can relate to a frequency division multiple access (FDMA) scheme. The transmission space of the D2D communication link can relate to a space division multiple access (SDMA) scheme. The transmission code of the D2D communication link can relate to a code division multiple access (CDMA) scheme. The transmission modulation of the D2D communication link can comprise a modulation format, a modulation pulse shape, e.g., a filter bank multi carrier (FBMC) pulse shape, a carrier frequency spacing, and/or a symbol time spacing. The transmission power of the D2D communication link can relate to the power of the

electromagnetic waves of the D2D communication link.

According to a second aspect, the invention relates to a communication system for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal communicates with the second mobile terminal over a device-to-device, D2D, communication link, the communication system comprising a serving base station being configured to determine a relocation of the second mobile terminal towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications, and a central entity being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link. Thus, an efficient scheme for performing a D2D handover can be employed. By using a central entity, a centralized approach for the determining of the common communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed by a dedicated component of the communications network.

The communication system can further comprise a target base station. The target base station can also be configured to allocate the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications. The method according to the first aspect of the invention can be performed by the

communication system according to the second aspect of the invention. Further features of the communication system according to the second aspect of the invention can directly result from the functionality of the method according to the first aspect of the invention. Based on the various implementation forms described above for the method according to the first aspect of the invention, the communication system according to the second aspect of the invention can also be provided with at least one of the respective implementation forms of the method according to the first aspect of the invention. According to a third aspect, the invention relates to a communication system for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal communicates with the second mobile terminal over a device-to-device, D2D, communication link, the communication system comprising a serving base station being configured to determine a relocation of the second mobile terminal towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications, and a target base station being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link. Thus, an efficient scheme for performing a D2D handover can be employed. By using a serving base station or a target base station, a distributed approach for the determining of the common communication resource of the serving cell and of the target cell can be realized. Therefore, the determining of the common communication resource can be performed directly between involved components of the communications network.

The target base station can also be configured to allocate the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications.

The method according to the first aspect of the invention can be performed by the

communication system according to the third aspect of the invention. Further features of the communication system according to the third aspect of the invention can directly result from the functionality of the method according to the first aspect of the invention. Based on the various implementation forms described above for the method according to the first aspect of the invention, the communication system according to the third aspect of the invention can also be provided with at least one of the respective implementation forms of the method according to the first aspect of the invention.

According to a fourth aspect, the invention relates to a central entity for determining a common communication resource of a serving cell and of a target cell for a device-to-device, D2D, communication link between a first mobile terminal and a second mobile terminal in a communications network, wherein the first mobile terminal communicates with the second mobile terminal over the D2D communication link, the central entity comprising a

communication interface being configured to receive, from a serving base station, a first message comprising first context information, the first context information comprising communication resources of the serving cell, to receive, from a target base station, a second message comprising second context information, the second context information comprising communication resources of the target cell, and to send a message comprising a common communication resource of the serving cell and of the target cell to the serving base station and to the target base station, and a processor being configured to determine a

communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. Thus, the common communication resource can efficiently be determined.

The central entity can be a stand-alone station within the communications network. The central entity can further be integrated in the serving base station or the target base station. The central entity can further be realized as a function or functionality within a virtualized communications network. The central entity may coordinate a single base station or a cluster of base stations where the size of the cluster can be updated, e.g., based on the cell load and load on the interface between the central entity and the base stations. The communication interface can be connected to the serving base station and/or the target base station. The communication interface can be configured to transmit and receive electronic messages. The communication interface can further be configured to transmit and receive messages formatted, e.g., according to the X2 interface standard. The above-mentioned second message can be received by the communication interface from the target base station upon sending a request message by the communication interface to the target base station, upon the determination of the relocation.

The processor can be configured to execute a program code of a computer program product.

The method according to the first aspect of the invention can be performed by the central entity according to the fourth aspect of the invention. Further features of the central entity according to the fourth aspect of the invention can directly result from the functionality of the method according to the first aspect of the invention.

In a first implementation form of the central entity according to the fourth aspect, the processor is further configured to determine a communication resource from alternative communication resources if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. Thus, a fallback solution for the determining of the common communication resource can be provided.

According to a fifth aspect, the invention relates to a computer program product comprising a program code for performing the method according to the first aspect as such or one of the implementation forms of the first aspect when executed on a computer. Thus, the method can be applied in an automatic and repeatable manner.

The computer program product can be provided in form of a machine-readable program code. The program code can comprise a series of commands for a processor of the computer. The processor of the computer can be configured to execute the program code. The invention can be implemented in hardware and/or software.

BRIEF DESCRIPTION OF DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a diagram of a method for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention;

Fig. 2 shows a diagram of a communication system for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention;

Fig. 3 shows a diagram of a communication system for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention;

Fig. 4 shows a diagram of a central entity for determining a common communication

resource of a serving cell and of a target cell for a device-to-device, D2D, communication link between a first mobile terminal and a second mobile terminal in a communications network according to an embodiment of the invention;

Fig. 5 shows a diagram of a mobile D2D scenario in a cellular mobile network according to an embodiment of the invention;

Fig. 6 shows a diagram of a centralized coordination approach for a mobile D2D scenario according to an embodiment of the invention; Fig. 7 shows a diagram of a distributed coordination approach for a mobile D2D scenario according to an embodiment of the invention;

Fig. 8 shows a diagram of a mobile D2D scenario for a triggered handover of a mobile

terminal;

Fig. 9 shows a signal flow diagram of a centralized coordination approach according to an embodiment of the invention;

Fig. 10 shows a signal flow diagram of a distributed coordination approach in a first case according to an embodiment of the invention;

Fig. 1 1 shows a signal flow diagram of a distributed coordination approach in a second case according to an embodiment of the invention; Fig. 12 shows a signal flow diagram of a distributed coordination approach in a second case using a fallback solution; and

Fig. 13 shows a diagram of a mobile D2D scenario with simultaneous D2D pair handovers from different serving base stations.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 shows a diagram of a method 100 for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention.

The first mobile terminal communicates with the second mobile terminal over a device-to- device, D2D, communication link. The method 100 comprises determining 101 a relocation of the second mobile terminal towards the target cell, determining 103 a common communication resource of the serving cell and of the target cell for the D2D communication link, and allocating 105 the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications.

The communications can relate to multi-cell direct D2D communications within the communications network. The D2D communication link can be a radio link oran optical link between the first mobile terminal and the second mobile terminal.

The first mobile terminal or the second mobile terminal can be a cellular phone, a smartphone,a mobile computer, or a user equipment. The first mobile terminal or the second mobile terminal can be a handheld device or can be integrated in a vehicle, e.g., a car. The first mobile terminal or the second mobile terminal can be configured to communicate wirelessly. The first mobile terminal or the second mobile terminal can be members of a D2D communication group.

The communication resource can define the physical resource for the D2D communication link. The communication resource can, e.g., be a physical resource block (P B) in long term evolution (LTE) communication technology. The communication resource can be bound to a licensed spectrum of a communications network operator or a virtual communications network operator. A communication resource of the serving cell can initially be allocated to the D2D communication link. The determining 101 of the relocation of the second mobile terminal towards the target cell can be performed by detecting and/or predicting the relocation. Detecting the relocation can comprise comparing a transmission characteristic of the D2D communication link, e.g., a transmission power and/or a received power, with a predetermined threshold, wherein the relocation is detected if the transmission characteristic falls below the predetermined threshold. Detecting the relocation can further comprise comparing a transmission characteristic of the mobile terminal to network infrastructure link of at least one mobile terminal. Predicting the relocation can comprise determining a movement trajectory of the second mobile terminal and determining a location of the second mobile terminal at a predetermined future time instant. Predicting the relocation can further comprise determining a movement trajectory of both first and second mobile terminals. Predicting the relocation can further comprise evaluating a handover statistic of the serving cell with regard to the target cell. The prediction can therefore imply an a-priori detection. The determining 103 of the common communication resource of the serving cell and of the target cell for the D2D communication link can relate to determining a commonly available communication resource of the serving cell and of the target cell or a communication resource of the serving cell and target cell that can be made commonly available

The allocating 105 of the common communication resource of the serving cell and of the target cell to the D2D communication link can comprise a reserving of the common communication resource within the serving cell and within the target cell. Fig. 2 shows a diagram of a communication system 200 for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention.

The first mobile terminal communicates with the second mobile terminal over a D2D communication link. The communication system 200 comprises a serving base station 201 being configured to determine a relocation of the second mobile terminal towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications, and a central entity 205 being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link.

The communication system 200 further comprises a target base station 203. The target base station 203 can also be configured to allocate the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications.

The determining of the common communication resource of the serving cell and of the target cell can be performed by the central entity 205. The determining of the common

communication resource of the serving cell and of the target cell can comprise sending, by the serving base station 201 , a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the central entity 205, sending, by the target base station 203, a second message comprising second context information, the second context information comprising communication resources of the target cell, to the central entity 205, determining, by the central entity 205, a

communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the central entity 205, a message comprising the common

communication resource of the serving cell and of the target cell to the serving base station 201 and to the target base station 203.

The determining of the common communication resource of the serving cell and of the target cell can further comprise determining, by the central entity 205, a communication resource from alternative communication resources if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. The first context information can further comprise a priority among a plurality of priorities of the D2D communication link, wherein the common communication resource of the serving cell and of the target cell can further be determined upon the basis of the priority of the D2D communication link. The first context information can further comprise an identification of the D2D communication link, wherein the identification of the D2D communication link can be associated with the determined common communication resource of the serving cell and of the target cell.

The communication resource of the D2D communication link can be at least one of a transmission time, a transmission frequency, a transmission space, a transmission code, a transmission modulation, or a transmission power of the D2D communication link.

The above-mentioned second message can be received by the central entity 205 from the target base station upon sending a request message by the central entity 205 to the target base station, upon the determination of the relocation.

Fig. 3 shows a diagram of a communication system 300 for managing communications between a first mobile terminal and a second mobile terminal in a communications network comprising a serving cell and a target cell according to an embodiment of the invention.

The first mobile terminal communicates with the second mobile terminal over a D2D communication link. The communication system 300 comprises a serving base station 301 being configured to determine a relocation of the second mobile terminal towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications, and a target base station 303 being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link.

The target base station 303 can also be configured to allocate the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal and the second mobile terminal to maintain the D2D communication link for the communications.

The determining of the common communication resource of the serving cell and of the target cell can be performed by the serving base station 301 or by the target base station 303. The determining of the common communication resource of the serving cell and of the target cell can comprise sending, by the serving base station 301 , a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the target base station 303, determining, by the target base station 303, a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the target base station 303, an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the serving base station 301 if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available.

The determining of the common communication resource of the serving cell and of the target cell can further comprise sending, by the target base station 303, a negative- acknowledgement message comprising second context information, the second context information comprising further communication resources of the target cell, to the serving base station 301 if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available,

determining, by the serving base station 301 , a communication resource from further common communication resources of further communication resources of the serving cell and the further communication resources of the target cell if further common communication resources of further communication resources of the serving cell and the further

communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell, and sending, by the serving base station 301 , an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the target base station 303.

The determining of the common communication resource of the serving cell and of the target cell can further comprise determining, by the serving base station 301 , a communication resource from alternative communication resources if further common communication resources of the further communication resources of the serving cell and the further communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell.

Determining, by the serving base station 301 , the communication resource from alternative communication resources can alternatively be performed upon reception of the negative- acknowledgement message from the target base station 303. The target base station 303 can send a negative-acknowledgement, NACK, message for performing the fallback solution.

The first context information can further comprise a priority among a plurality of priorities of the D2D communication link, wherein the common communication resource of the serving cell and of the target cell can further be determined upon the basis of the priority of the D2D communication link.

The first context information can further comprise an identification of the D2D communication link, wherein the identification of the D2D communication link can be associated with the determined common communication resource of the serving cell and of the target cell.

Fig. 4 shows a diagram of a central entity 205 for determining a common communication resource of a serving cell and of a target cell for a device-to-device, D2D, communication link between a first mobile terminal and a second mobile terminal in a communications network according to an embodiment of the invention.

The first mobile terminal communicates with the second mobile terminal over the D2D communication link. The central entity 205 comprises a communication interface 401 being configured to receive, from a serving base station, a first message comprising first context information, the first context information comprising communication resources of the serving cell, to receive, from a target base station, a second message comprising second context information, the second context information comprising communication resources of the target cell, and to send a message comprising a common communication resource of the serving cell and of the target cell to the serving base station and to the target base station, and a processor 403 being configured to determine a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common

communication resource of the serving cell and of the target cell.

The central entity 205 can be a stand-alone station within the communications network. The central entity 205 can further be integrated in the serving base station or the target base station. The central entity 205 can further be realized as a function or functionality within a virtualized communications network.

The communication interface 401 can be connected to the serving base station and/or the target base station. The communication interface 401 can be configured to transmit and receive electronic messages. The communication interface 401 can further be configured to transmit and receive messages formatted, e.g., according to the X2 interface standard.

The communication interface 401 can be configured to receive the second message from the target base station upon sending a request message by the communication interface 401 to the target base station, upon the determination of the relocation.

The processor 403 can be configured to execute a program code of a computer program product. The processor 403 can further be configured to determine a communication resource from alternative communication resources if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell. The first context information can further comprise a priority among a plurality of priorities of the D2D communication link, wherein the common communication resource of the serving cell and of the target cell can be further determined upon the basis of the priority of the D2D communication link. The first context information can further comprise an identification of the D2D communication link, wherein the identification of the D2D communication link can be associated with the determined common communication resource of the serving cell and of the target cell.

The communication resource of the D2D communication link can be at least one of a transmission time, a transmission frequency, a transmission space, a transmission code, a transmission modulation, or a transmission power of the D2D communication link. Fig. 5 shows a diagram of a mobile D2D scenario in a cellular mobile network according to an embodiment of the invention. The scenario comprises a first mobile terminal 501 as MT1 , a second mobile terminal 503 as MT2, a serving cell as Cell #1 , a target cell as Cell #2, and a D2D link between the first mobile terminal 501 and the second mobile terminal 503. Herein, the term D2D link used in this figure and used in the following figures has the same meaning as the term D2D communication link used in the present application.

The invention is related to a handover process of D2D devices in wireless communication systems. The invention can enable a D2D paired handover without a connection interruption. Direct D2D communications is a promising technology, which can be implemented, e.g., in fifth generation (5G) mobile and wireless networks. In network controlled D2D

communications, resource allocation is managed by the serving base station (BS) or, generally, the network infrastructure, to attain a certain level of quality of service (QoS). Furthermore, D2D communications can be integrated into cellular networks, e.g., the

Proximity Services (ProSe) in LTE Release 12.

In this regard, the following benefits can be envisaged for D2D communications. User benefits relate to a reduced power consumption, to an increased throughput, to a discovery of geographically close activities, to a support for a large number of connected devices, and to low-latency communications. Operator benefits relate to a high network spectrum efficiency and traffic offloading, and to a coverage extension.

One challenge of realizing D2D communications is the D2D handover in mobile D2D scenarios. One typical example of mobile D2D is Car-to-Car (Car2Car) communications, which has challenging latency requirements. Other examples are related to D2D

communications within crowd movements, such as the movement of participants of a large event toward public transportation, after ending of the large event. One typical case in the mobile D2D scenarios is the movement of a D2D pair from one cell to another. Another typical case is that at least one of the D2D devices is moving from one cell to another when multi-cell D2D communications is allowed. For applications like the Car2Car communications and crowd movement, a proper handover, HO, procedure is desirable to ensure that the D2D communication link is kept functional during the HO procedure.

An example of a mobile D2D scenario, where both mobile terminals or devices of a D2D pair move in roughly the same direction, can be considered. Furthermore, this D2D pair can move from one cell to another. This scenario is illustrated in Figure 5. When one mobile terminal or device of the D2D pair is handed over from the serving cell, Cell #1 , to the target cell, Cell #2, while the other mobile terminal or device is still in the serving cell, Cell #1 , the D2D transmission can be interrupted due to interference caused by other transmissions in the target cell, Cell #2. For certain applications, such interruption is not tolerable. As examples, the interruption of the D2D communication link can cause problems in Car2Car communications, while in crowd movement scenarios, such interruption can cause capacity decrease due to the capacity limitation of the direct links, i.e., the MT-BS-MT links.

Therefore, the HO mechanism should be able to avoid such interruption, i.e., to keep the D2D communication link functional during HO. A major issue of the D2D HO mechanism is to manage the resource allocation for the D2D communication link across the serving and target cells to avoid interference and to abide system specifications. In the following, this issue is explained using the example of Figure 5. Before the HO, a dedicated resource is allocated to the D2D communication link in the serving cell, Cell #1 , to ensure QoS and to avoid interference. Such resource can include time, frequency, space, code, etc. One example of such resource is a Physical Resource Block (PRB) defined in LTE. After a certain time period of At the HO of the second mobile terminal MT2, 503, is triggered, and before the HO of the first mobile terminal MT1 , 501 , the D2D communication link can be present across both cells, i.e., it becomes a multi-cell D2D communication link. The multi-cell D2D communication link can be affected by the interference caused by transmissions in the target cell, Cell #2, that use the same resource of the D2D communication link. Therefore, a proper resource allocation mechanism is designed for the HO so that the interference can be avoided or kept below a required level, and the D2D communication link may not be interrupted. Fig. 6 shows a diagram of a centralized coordination approach for a mobile D2D scenario according to an embodiment of the invention. The scenario comprises a serving base station 201 as BS1 , a target base station 203 as BS2, a central entity 205 as central unit, CU, a first mobile terminal 501 as MT1 , a second mobile terminal 503 as MT2, a serving cell as Cell #1 , a target cell as Cell #2, and a D2D communicatbn link between the first mobile terminal 501 and the second mobile terminal 503. The first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, are located within the serving cell, Cell #1 , during a certain time period before a handover of the second mobile terminal MT2, 503, to the target cell, Cell #2.

A common communication resource of the D2D communication link is determined by the central entity 205 or central unit CU. The serving base station BS1 , 201 , is configured to send a first message comprising a priority of the D2D communication link, a load of the serving cell, Cell #1 , available communication resources of the serving cell, Cell #1 , and/or a trajectory prediction of the second mobile terminal MT2, 503, to the central entity 205 or central unit CU. The target base station BS2, 203, is configured to send a second message comprising a load of the target cell, Cell #2, and/or available communication resources of the target cell, Cell #2, to the central entity 205 or central unit CU. The allocation of the common communication resource to the D2D communication link can be performed by the serving base station BS1 , 201 , or by the target base station BS2, 203.

The above-mentioned second message can be received by the central entity 205 or central unit CU from the target base station upon sending a request message by the central entity 205 or central unit CU to the target base station, upon the determination of the relocation.

Fig. 7 shows a diagram of a distributed coordination approach for a mobile D2D scenario according to an embodiment of the invention. The scenario comprises a serving base station 301 as BS1 , a target base station 303 as BS2, a first mobile terminal 501 as MT1 , a second mobile terminal 503 as MT2, a serving cell as Cell #1 , a target cell as Cell #2, and a D2D communication link between the first mobile terminal 501 and the second mobile terminal 503. A common communication resource of the D2D communication link is arranged between the serving base station BS1 , 301 , and the target base station BS2. The first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, are located within the serving cell, Cell #1 , during a certain time period before a handover of the second mobile terminal MT2, 503, to the target cell, Cell #2.

For applications like Car2Car communications and crowd movement, a proper HO procedure is desirable to ensure that the D2D communication link is kept functional during the HO procedure. The invention can provide techniques and mechanisms to address this issue. An idea of the invention is to arrange a common resource of both cells for the D2D

communication link before the HO is triggered. Firstly, early detection of the HO can be performed, which can be attained, e.g., by lowering a HO threshold and/or predicting a trajectory of the D2D pair. Secondly, a common resource of both cells can be arranged, i.e., reserved for and allocated to the D2D communication link, to avoid interference to the D2D communication link. For the arrangement of the common resource, the BSs of both cells can coordinate. Two approaches of BS coordination are described, a centralized coordination depicted in Fig. 6, and a distributed coordination depicted in Fig. 7.

The centralized coordination can be performed as follows: A central entity 205 or central unit CU can coordinate the common resource allocation of the D2D communication link. The central entity 205 or CU can be an operation and maintenance (O&M) system, a macro BS, e.g., in the case of heterogeneous networks (HetNet) or one of the BSs 201 , 203. The central entity 205 or CU may be a function or functionality, e.g., in a virtualized network. The central entity 205 or CU or the central entity or CU role may also be dynamically assigned to respective BSs 201 , 203 or access nodes depending on, e.g., the number of neighboring cells, cell size, and cell location. The central entity 205 or CU can collect D2D relevant information elements from the corresponding BSs 201 , 203 and can decide on the common resource allocation. When the common resource allocation is not possible and/or a fall back solution is assumed or estimated to provide the necessary QoS, the central entity 205 or CU can employ the fallback solution. The fallback solution can be a switching to an unlicensed band, i.e., a frequency band without a dedicated license to an operator, e.g., the industrial, scientific and medical (ISM) band at 2.4 GHz or 5.7 GHz, or a Dedicated Short-Range Communications (DSRC) band at 5.9 GHz, provided that the mobile terminals 501 , 503 have access capabilities to operate in unlicensed and/or license-exempt bands, e.g., WiFi and DSRC. The fallback solution can be switching to another communication technology, e.g., free-space optical communications. The distributed coordination can be performed as follows. The base station 1 , BS1 , 301 , and base station 2, BS 2, 303 can communicate with each other via, e.g., via an X2 interface. The BS 1 , 301 , and BS 2, 303, can follow a protocol to exchange resource proposals and achieve a joint decision on the common resource. The BS 1 , 301 , and BS 2, 303, can decide to employ the fallback solution as described above if the common resource allocation is not possible and/or the fallback solution is assumed or estimated to provide the desired QoS. The distributed coordination can also be referred to as decentralized coordination and can comprise the cases where central entities or CUs, e.g., at two BSs, coordinate.

Fig. 8 shows a diagram of a mobile D2D scenario for a triggered handover of a mobile terminal. The scenario comprises a first mobile terminal 501 as MT1 , a second mobile terminal 503 as MT2, a serving cell as Cell #1 , a target cell as Cell #2, and a D2D

communication link between the first mobile terminal 501 and the second mobile terminal 503. The first mobile terminal MT1 , 501 , and/or the second mobile terminal MT2, 503, can move towards the target cell, Cell #2. Therefore, a handover to the target cell or Cell #2 can be desired.

A specific way of handling mobile D2D communications is to stop the D2D communication link during the HO of the mobile terminals 501 , 503 or devices and to switch back to the conventional MT-BS-MT manner. In other words, the data to be transferred via the D2D communication link can be rerouted to be transferred via the links between MTs and BSs. However, this scheme may not be applicable to all applications. Firstly, keeping the D2D communication link functional during the HO can be desirable for certain applications.

Secondly, in the non-D2D mode, the serving base station or source base station can send the data to the target base station, e.g., via X2 interface. This can introduce additional delay for the data transfer, compared to a direct D2D data transfer. Such additional delay can become an issue for latency-sensitive communications, e.g., Car2Car communications. Thirdly, using a non-D2D mode at a cell edge, where a HO can take place, can be challenging both for uplink (UL) and downlink (DL) communications due to weak received signal levels.

Another specific way to tackle mobile D2D communications is a handshake-like procedure for the D2D HO between two BSs. A major idea can be that after the HO of the second mobile terminal MT2, 503, is triggered, the HO of the first mobile terminal MT1 , 501 , can be triggered automatically, either by the serving BS or by the second mobile terminal MT2, 503 Specifically, the HO request message can comprise a proposed common identifier (ID) and a proposed resource allocation for the D2D communication link. This idea can have several drawbacks, which are described as follows: Firstly, for latency-sensitive D2D applications, it can be too late to arrange a common resource allocation for the D2D communication link during the HO. This can mean that the probability that the D2D communication link will suffer from interference can be high. Secondly, there may be no indication of priority for the D2D pair to facilitate latency-sensitive D2D applications. Thirdly, the case that the proposed resource allocation, e.g., by the serving BS, during the HO is not available in the target cell may not be resolved. Fourthly, there may be no fallback solution when the proposed resource allocation is not agreed by both serving and target BSs.

In the following, it is firstly assumed that the frequency reuse factor is 1 , i.e., neighboring cells reuse the same frequency band. An extension to cases with frequency reuse factor larger than 1 is also described.

Fig. 9 shows a signal flow diagram of a centralized coordination approach according to an embodiment of the invention. The signal flow diagram comprises a serving base station 201 as BS1 , a target base station 203 as BS2, a central entity 205 as central unit CU, a first mobile terminal 501 as MT1 , and a second mobile terminal 503 as MT2. Between the first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, a D2D communication link can exist. The serving base station BS1 , 201 , can predict that MT2 or BS2 is on the trajectory and can send a D2D context, comprising a priority, a trajectory prediction, a speed, a D2D unique ID, and/or a Tx power, as well as a load and available D2D resources to the central entity 205 or CU. The central entity 205 or CU can send a request for load and available D2D resources from BS2 to the target base station BS2, 203. The target base station BS2, 203, can send a load and available D2D resources to the central entity 205 or CU. The central entity 205 or CU can decide a common radio resource allocation for this D2D ID or fallback to an unlicensed band, and can send this decision to the serving base station BS1 , 201 , and the target base station BS2, 203. The first mobile terminal MT1 , 501 , the second mobile terminal MT2, 503, and the serving base station BS1 , 201 , can perform a HO procedure comprising a measurement and/or a HO decision. The serving base station BS1 , 201 , can send a HO request for this D2D ID to the target base station BS2, 203. The target base station BS2, 203, can assign a common radio resource or fallback to an unlicensed band. The first mobile terminal MT1. 501 , the second mobile terminal MT2, 503, the serving base station BS1 , 201 , the target base station BS2, 203, and the central entity 205 or CU can finalize the HO of MT2 and MT1 consecutively or jointly.

In the approach with centralized coordination, the central entity 205 or CU can gather all relevant information for the determination or decision of the common resource allocation between the corresponding BSs 201 , 203. The relevant pieces of information are described as follows. Depending on the scenario, not all of the following information may be sent to the central entity 205 or CU. The D2D-related information can be encapsulated as D2D context.

The D2D context can comprise the following: firstly, a priority for the D2D pair, e.g., a high priority for latency-sensitive applications such as Car2Car and a medium priority for offloading, such as crowd movement; secondly, a predicted trajectory for the D2D pair;

thirdly, velocity or speed information, which can also be quantized, e.g., low, medium, and high speed for the D2D pair; fourthly, a D2D unique ID, wherein the ID can be unique globally or for a given set of cells; fifthly, D2D resources for the D2D pair, e.g., frequency- time resources, transmitting or Tx power, and/or FBMC pulse shape; sixthly, available D2D resources in the serving cell, i.e., the resources that are available or can be made available for the D2D communication link; seventhly, available D2D resources in the target cell, wherein two types of available resources can exist: generally available resources or resources that can be available for this specific D2D pair. For the second type, the resource can be flagged with the unique D2D ID. Further relevant pieces of information can comprise a load of the target cell, or a load of the serving cell.

Two types of available resources are described as follows: Generally available resources can relate to resources that can be used by any D2D pair being handed over from the serving cell. Resources that can be available for a specific D2D pair can relate to resources that can be used by a specific D2D pair based on, e.g., a D2D context and/or a resource

management strategy in the target cell. For example, for other D2D pairs, there can be strong interference on such resources.

A signal flow diagram of this approach is depicted in Fig. 9 in association with Fig. 6 referring to centralized coordination.

Firstly, the HO of a D2D pair and the target BS 203 are predicted. Such prediction can be based on a trajectory of the D2D pair and/or history information. There can be two options for such trajectory prediction or estimation. The trajectory can be estimated by the serving base station 201 or the serving cell and then sent to the central entity 205 or CU, or the

information for the trajectory prediction can be sent to the central entity 205 or CU, and, afterwards, the central entity 205 or CU can perform the estimation.

The first option can be efficient in terms of signaling overhead, whereas the second option can yield better estimations based on the knowledge and information available at the central entity 205 or CU. The history information can comprise HO statistics between the serving cell and neighboring cells and associated measurement profiles. Such history information can, e.g., reveal a road between two cells.

After the target BS 203 has been predicted, the serving base station 201 or serving cell can send D2D context information to the central entity 205 or CU, which can comprise the pieces of information as described above. If there are multiple D2D pairs in the serving cell, the D2D context can be sent for each D2D pair, when at least one mobile terminal 501 , 503 of such pair is on the move and, thus, a HO may be desired. Furthermore, the serving BS1 201 can be an access node serving the cell where the mobile terminals MT1 , 501 , and MT2, 503, are currently in, and the target BS 203 can be an access node serving the cell where the mobile terminals MT1 and MT2, 501 , 503, are to be handed over. Accordingly, the mechanisms and the techniques described herein can be applied to cases where the respective cells mentioned belong to the same BS. Moreover, a serving cell or a target cell can comprise a cluster of BSs, e.g., in a cloud-based radio access network, also known as centralized radio access network, (C-RAN) implementation. In the next step, the central entity 205 or CU can send a request to the target BS 203 and inform this BS 203 to send the information that is relevant to the D2D HO. For example, such information can comprise load and available D2D resources for the D2D pair. Depending on the radio resource management ( M) strategy undertaken in the target cell and the D2D context, e.g., speed information and trajectory prediction, each D2D pair can be assigned to a specific set of resources. An RRM strategy can be, e.g., assigning the same resource for sufficiently separated D2D pairs or non-D2D users. In such a case, either the relevant D2D context can be sent to the target base station 203 or target cell by the central entity 205 or CU so that the specific set of resources can be determined or the central entity 205 or CU can be informed about the RRM strategy and, hence, the central entity 205 or CU can decide on the set of specific resources. Based on the information from both serving BS 201 and target BS 203, the central entity 205 or CU can decide on the common resource allocation for the D2D pair. If no common resource can be allocated, the central entity 205 or CU can also decide to execute the fallback solution. In case of the fallback solution, the D2D communication link can be requested to utilize e.g., unlicensed band instead of licensed band. Depending on the common radio access capabilities of the mobile terminals MT1 , 501 , and MT2, 503, a specific unlicensed frequency band and/or a corresponding technology can be assigned to the D2D communication link. The parameters for the fallback solution can be selected based on the available

measurements in both serving and target cells, as well as those in the central entity 205 or CU. For the fallback solution, additional measurements can also be performed, e.g., via spectrum sensing schemes. The use of such schemes can be requested by the central entity 205 or CU. The set of the parameters can be selected such that a target QoS can be achieved on the D2D communication link. Furthermore, the mobile terminals MT1 , 501 , and MT2, 503, may be requested to conduct measurements that can be used for the decision of the fallback solution. After the above-mentioned decision is made by the central entity 205 or CU, information about the decided common radio resource or the fallback solution can be sent to both serving and target BSs 201 , 203. Once the HO of either of the mobile terminals MT1 and MT2, 501 , 503, in the D2D pair is triggered, the decision of the central entity 205 or CU can be applied by both serving and target BSs 201 , 203. The central entity 205 or CU can also request to set the common resource prior to the HO. In the case that the common resource for the D2D communication link is different from the original one, the D2D communication link resource can be switched to the common resource prior to the HO. Furthermore, the HOs of the mobile terminals MT1 and MT2, 501 , 503, can be performed consecutively or jointly. In the case of a consecutive HO, the HO of the second mobile terminal MT2, 503, can be carried out before that of the first mobile terminal MT1 , 501. In the case of a joint HO, both mobile terminals MT1 and MT2. 501 , 503, can be handed over simultaneously. After handing over both mobile terminals MT1 and MT2, 501 , 503, the serving cell can utilize the corresponding D2D resources for other purposes.

Fig. 10 shows a signal flow diagram of a distributed coordination approach in a first case according to an embodiment of the invention. The signal flow diagram comprises a serving base station 301 as BS1 , a target base station 303 as BS2, a first mobile terminal 501 as MT1 , and a second mobile terminal 503 as MT2. Between the first mobile terminal MT1. 501 , and the second mobile terminal MT2, 503, a D2D communication link can exist. The serving base station BS1 , 301 , can perform a trajectory estimate, and/or predict that BS2 is on the trajectory. The serving base station BS1 , 301 , can send a request for allocating the same resource for D2D, other candidate resources, a priority of D2D, and/or a D2D ID to the target base station BS2, 303. The target base station BS2, 303, can send an acknowledgement, ACK, message for the proposed radio resources and an acknowledged, ACKed, resource ID to the serving base station BS1 , 301. The first mobile terminal MT1 , 501 , the second mobile terminal MT2, 503, the serving base station BS1 , 301 , and the target base station BS2, 303, can perform a HO procedure and an assignment of the common radio resources.

Fig. 1 1 shows a signal flow diagram of a distributed coordination approach in a second case according to an embodiment of the invention. The signal flow diagram comprises a serving base station 301 as BS1 , a target base station 303 as BS2, a first mobile terminal 501 as MT1 , and a second mobile terminal 503 as MT2. Between the first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, a D2D communication link can exist. The serving base station BS1 , 301 , can perform a trajectory estimate and/or predict that MT2 or BS2 is on the trajectory. The serving base station BS1 , 301 , can send a request for allocating the same resource for D2D, other candidate resources, a priority of D2D, and/or a D2D ID, to the target base station BS2, 303. The target base station BS2, 303, can send a negative- acknowledgement, NACK, message for the proposed radio resources, and/or a new proposal from BS2, to the serving base station BS1 , 301 . The serving base station BS1 , 301 , can send an acknowledgement, ACK, message for the proposed radio resources to the target base station BS2, 303. The first mobile terminal MT1 , 501 , the second mobile terminal MT2, 503, the serving base station BS1 , 301 , and the target base station BS2, 303, can perform a HO procedure and an assignment of the common radio resources. Fig. 12 shows a signal flow diagram of a distributed coordination approach in a second case using a fallback solution. The signal flow diagram comprises a serving base station 301 as BS1 , a target base station 303 as BS2, a first mobile terminal 501 as MT1 , and a second mobile terminal 503 as MT2. Between the first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, a D2D communication link can exist. The serving base station BS1 , 301 , can perform a trajectory estimate and/or predict MT2 or BS2 is on the trajectory. The serving base station BS1 , 301 , can send a request for allocating the same resource for D2D, other favorable candidate resources, a priority, and/or a D2D ID, to the target base station BS2, 303. The target base station BS2, 303, can send a negative-acknowledgement, NACK, message for the proposed radio resources, and/or a new proposal from BS2, to the serving base station BS1 , 301. The serving base station BS1 , 301 , can send a negative- acknowledgement, NACK, message for the proposed radio resources and/or a D2D operation mode, i.e. fallback to unlicensed mode, to the target base station BS2, 303. The first mobile terminal MT1 , 501 , the second mobile terminal MT2, 503, the serving base station BS1 , 301 , and the target base station BS2, 303, can perform a HO procedure and/or a change of D2D mode of operation. Thus, a fallback solution can be applied.

In the approach with decentralized or distributed coordination, the common resource allocation or the fallback solution can be decided by the serving base station 301 and target base station 303 or the serving and target cells in a distributed manner as shown in Fig. 7, referring to distributed coordination.

Firstly, the HO of a D2D pair and the target BS 303 can be predicted by the serving BS 301 , based on, e.g., an estimated trajectory and/or history as explained above. After predicting that the target base station BS2, 303, comprises the target cell, the serving base station BS1 , 301 , can send a request to the target base station BS2, 303, to ask the target base station BS2, 303, to allocate the same resource for the D2D pair as in the serving cell. The aforementioned resource can comprise time, frequency, space, code, etc. and/or a range of such resources. This request can be sent within a certain time period before the HO of the second mobile terminal MT2, 503. The time of sending this request can be decided based on the reliability of the trajectory estimation and ground speed of the mobile terminals MT1 and MT2, 501 , 503, in the D2D pair. The reliability of the trajectory estimation can be determined based on history and/or measurement trends. Along with this request, the D2D context information can also be sent to the target base station BS2, 303. Like in the centralized coordination approach, the D2D context information can comprise a D2D ID and a priority level of the D2D pair. It can also comprise a list of candidate resources that the serving cell is able to make available for the D2D pair, e.g., in case that the same resource is not available in the target cell. After receiving the request and the D2D context information, the target BS 303 can check its resource availability and send back an acknowledgement, ACK, to allocate the same resource or deny it, i.e., negative-ACK, NACK. Depending on whether the target BS 303 sends back an ACK or a NACK, the following two cases can be distinguished: In a first case as shown in Fig. 10, the target base station BS2, 303, can send an ACK, i.e., the target base station BS2, 303, can be able to allocate the same resource which is currently used by the serving base station BS1 , 301 , for the D2D pair or one of the candidate resources proposed by the serving base station BS1 , 301 . For the latter, the ACK can be sent together with an index of the accepted resource for the D2D communication link.

Accordingly, the D2D communication link can be switched to this resource prior enough to the HO procedure.

In a second case as shown in Fig. 1 1 , the target base station BS2, 303, can send an NACK, i.e., the target base station BS2, 303, can be unable to allocate any of the proposed common resources. In this case, the target base station BS2, 303, can send a list of its further proposed resources to the serving base station BS1 , 301 , along with the NACK. The further proposed resources can also be either of the resource types mentioned above. After receiving this proposal, the serving base station BS1 , 301 , can decide whether to make one of such resources available for the D2D communication link. Such a decision can be based on the current resource allocation in the serving base station BS1 , 301 , and the priority of the D2D communication link.

If one of such proposed resources by the target base station BS2, 303, can be made available at the serving base station BS1 , 301 , the serving base station BS1 , 301 , can send back an ACK to the target base station BS2, 303, with a resource index, referring to the signal flow diagram in Figure 1 1 . Afterwards, the serving base station BS1 , 301 , can initiate or inform the D2D communication link to switch to the selected resource.

If none of such proposed resources is suitable, the serving base station BS1 , 301 , can decide on the fallback solution as depicted in Figure 12. After deciding on the fallback solution, the serving base station BS1 , 301 , can assist the D2D pair to switch to a proper unlicensed frequency band temporarily and inform the target base station BS2, 303, about this D2D operation mode in an unlicensed band. After the D2D communication link is fully in the coverage area of the target base station BS2, 303, the D2D communication link can switch back to licensed bands with a resource allocated by the target base station BS2, 303.

Before the serving base station BS1 , 301 , decides on the fallback solution, both BSs 301 , 303 can negotiate iteratively until a common resource can be found or a maximum iteration number is exceeded. Furthermore, in case of the fallback solution, the serving base station BS1 , 301 , can send respective measurements, e.g., an estimated QoS for a given

unlicensed band, to the target base station BS2, 303. With the measurements in the target cell, the fallback solution can be performed more efficiently.

Like in the centralized coordination approach, the HOs of the mobile terminals MT1 and MT2, 501 , 503, can be performed consecutively or jointly. After handing over both mobile terminals MT1 and MT2, 501 , 503, the serving cell can utilize the corresponding D2D resources for other purposes.

The described approaches can further be extended to a case of a D2D group with more than two mobile terminals. When there are more than two mobile terminals or devices in a mobile D2D group, a set of common resources can be assigned to this group before the HO. The set of common resource allocation can be decided by the above-mentioned centralized or distributed coordination approaches, once the HO of the group is predicted. The common resources can be flagged by a unique D2D group ID.

Within a group, a group head or group heads can be selected from the mobile terminals MT1 and MT2, 501 , 503, or devices in this mobile D2D group. The group head can be responsible for managing the resource allocation within its group. That is, the group head can receive the set of resources to be allocated for this group via the centralized or distributed coordination approaches and, then, the group head can coordinate the internal resource allocation. The set of resources can be allocated to different pairs via D2D group internal coordination. The group head can decide on the fallback solution for particular D2D communication link pairs. Due to such internal coordination, less signaling can be employed, compared to the case that resources are allocated directly from BSs 301 , 303 to each D2D pair.

Fig. 13 shows a diagram of a mobile D2D scenario with simultaneous D2D pair handovers from different serving base stations. The scenario comprises a first mobile terminal 501 as MT1 , a second mobile terminal 503 as MT2, a third mobile terminal 1301 as MT3, a fourth mobile terminal 1303 as MT4, a first serving cell as Cell #1 , a second serving cell as Cell #3, a target cell as Cell #2, a first D2D communication link between the first mobile terminal 501 and the second mobile terminal 503, and a second D2D communication link between the third mobile terminal 1301 and the fourth mobile terminal 1303. The handovers can be performed, e.g., from the first serving cell, Cell #1 and from the second serving cell, Cell #3, to the target cell, Cell #2. The first mobile terminal MT1 , 501 , and the second mobile terminal MT2, 503, are located within the first serving cell, Cell #1 , during a certain time period before a handover of the second mobile terminal MT2, 503, to the target cell, Cell #2. The third mobile terminal MT3, 1301 , and the fourth mobile terminal MT4, 1303, are located within the second serving cell, Cell #3, during a certain time period before a handover of the fourth mobile terminal MT4, 1303, to the target cell, Cell #2. The described approaches can further be extended to a case with multiple simultaneous D2D pair HOs from different serving cells to the same target cell.

When there are multiple simultaneous D2D pair HOs from different serving cells to the same target cell, as depicted in Fig. 13, the technique can be applied with the following extensions.

Relating to the centralized coordination approach, if all serving cells as well as the target cell are connected to one central entity or CU, the technique can directly be applied. If different central entities or CUs are involved, communications between all involved central entities or CUs can be desirable to achieve a joint decision on the common resources for all related D2D pairs.

Relating to the distributed coordination approach, firstly, the target BS can receive resource proposals from all serving BSs. Afterwards, for each serving BS, the target BS can either acknowledge the resource proposals directly, or send back another proposal of resources. For the proposed resources for different serving BSs, a D2D context of different D2D pairs can be taken into account. This can mean that either different serving BSs can get different resources, or they can get the same resource provided that the separation between the different D2D pairs is large enough to allow the use of the same resource. In each iteration, the proposed resources for each serving BS can be changed. For example, in the first iteration, resources 1 and 2 can be proposed to serving BSs 1 and 2, respectively. In the second iteration, if none of the serving BSs has accepted the resource proposal, the target BS can propose resources 2 and 1 for the serving BSs 1 and 2, respectively. In the case that the fallback solution is applied, the target BS can ensure that the different D2D

communication links use different resources in the unlicensed band.

In the described approaches, the frequency reuse factor is 1 , i.e., neighboring cells reuse the same frequency band. The described approaches can further be extended to a case of a frequency reuse factor larger than 1. When the frequency reuse factor is larger than 1 , the serving and target cells can use different, e.g., orthogonal, frequency bands. When the serving and target cells use different frequency bands, the currently used D2D resource may not be used at all in the target cell. In this scenario, there can be two cases. Firstly, the frequency reuse can be so large that all the neighboring cells of a target cell can use different frequencies, e.g., a GSM-like frequency reuse. In this case, the original D2D resource can be used during the HO process. Accordingly, the serving cell can only inform the target base station or target cell about the D2D pair and the used resource.

Secondly, at least two of the neighboring cells of the target cell can use the same frequency. Thus, in this case, there can be D2D pairs being handed over from other serving cells to this target cell, e.g., as illustrated in Fig. 13. Such D2D pairs can use the same resource.

Therefore, the HO procedure can still be desirable for such scenarios, which can either confirm that the original D2D resource can be used during the HO or the D2D communication link can switch to another D2D resource before the HO. After the HO, a different resource can be assigned to the D2D pairs by the target base station or target cell.

In the following, further applications and/or scenarios are described. Mobile D2D

communication can also apply to scenarios where either of the D2D mobile terminals or devices is moving to a neighboring cell. The neighboring cell can be any access point, e.g., a macrocell, a picocell, a femtocell, a microcell, and/or a relay cell. The D2D resource can be reused in the related cells, as long as interference can be mitigated, e.g., via antenna directivity or power control management. In this case, the common resource can be chosen so that the interference level of the D2D communication link can be below a given level to ensure the fulfillment of the corresponding QoS specification.

The technique can apply to both cases of frequency division duplex (FDD) and or time division duplex (TDD) of the BS. Furthermore, the D2D communication link can be either in a TDD band, an FDD-UL and/or an FDD-DL band. When a filter bank multiple access (FBMC) technique is utilized, the pulse shape utilized for the D2D pair can be exchanged between the involved entities, e.g, the serving base station BS1 , the target base station BS2, and the central entity or CU. In case of a fallback solution, the information regarding radio access capabilities of the mobile terminals in the D2D pair, e.g., WiFi and/or DSRC, can be exchanged between the involved entities, e.g., the serving base station BS1 , the target base station BS2, and the central entity or CU. In addition, simultaneous operation of available radio access technologies can be requested to attain the desired QoS by exploiting diversity.

The invention can prevent a D2D communication link from being interrupted during a HO process and can allow a smooth HO of the D2D communication link. This can be particularly beneficial for time-sensitive applications, e.g., Car2Car communications. Further benefits can be observed in case of crowd movement, e.g., in terms of network load relaxation. The respective communication protocols can be modified according to the mechanisms and techniques presented herein.

The invention relates to a centralized coordination method in a mobile communication system comprising a serving BS, a target BS, and a central entity or central unit CU, the centralized coordination method being adapted to arrange a common resource for the D2D communication link in advance to a handover process, and to keep the D2D communication link functional during the handover. The method can comprise the following steps of predicting a HO and a target cell, sending D2D context information from the serving BS to the central entity or CU, requesting, by the central entity or CU, resource allocation information of the target BS, deciding on the common resource for the D2D communication link or the usage of a fallback solution by the central entity or CU, sending the decision from the central entity or CU to both serving and target BSs, switching a D2D communication link resource within a certain time period before the HO, if the decided common resource differs from the originally used resource, and switching the D2D communication link to an unlicensed band prior enough to the HO if the fallback solution is decided to be used.

The invention relates to a distributed coordination method in a mobile communication system comprising a serving BS and a target BS, the distributed coordination method being adapted to arrange a common resource for the D2D communication link in advance to a handover process, and to keep the D2D communication link functional during the handover. The method can comprise the following steps of predicting a HO and a target cell, sending a request from the serving BS to the target BS to allocate the same resource for the D2D communication link, wherein the request is along with a list of candidates of different or alternative resources proposed by the serving BS, sending an ACK/NACK message from the target BS to the serving BS, wherein the ACK message can be along with an identity of the acknowledged resource, wherein the NACK message can be along with a list of proposed resources by the target BS, deciding on the common resource or the usage of a fallback solution, by the serving BS, switching a D2D communication link resource within a certain time period before the HO, if the decided common resource differs from the originally used resource, switching a D2D communication link to an unlicensed band prior enough to the HO if the fallback solution is decided to be used.

The D2D context information used in the centralized coordination method or the distributed coordination method comprises the proposed resource allocation forthe D2D communication link, a D2D priority level, a D2D unique ID, available D2D resources, and, when available, an FBMC pulse shape information. According to the centralized coordination method or the distributed coordination method, when there are more than two mobile terminals in a D2D group, a set of resources is assigned to the group, which can be flagged by the D2D group ID and the set of resources can be assigned within the group either in a distributed manner or by a group head.

In the distributed coordination approach, if there are multiple simultaneous D2D pair HOs from different serving cells to the same target cell, the target BS sends different proposals to different serving base stations or serving cells to avoid a possible interference between the D2D communication links of the different serving cells. The resources being proposed for each serving cell can comprise resources that are not proposed for other serving cells at all, i.e., different resources, and/or resources that are also proposed for at least one other serving cell, i.e. the same resource, provided that the D2D pairs that can use such resource do not generate interference to each other, wherein the target BS can decide on this issue based on the D2D context of different D2D pairs, e.g. according to the locations of the D2D pairs and their transmission power.

In case that the frequency reuse factor is larger than 1 and that all neighboring cells of a target cell use different frequencies, the HO procedure is simplified as follows. The serving base station or serving cell just informs the target base station or target cell, or the central entity or CU about the resource that the D2D communication link is using. The D2D communication link uses the original resource during the HO procedure.

Claims

1 . A method (100) for managing communications between a first mobile terminal (501 ) and a second mobile terminal (503) in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal (501 ) communicates with the second mobile terminal (503) over a device-to-device, D2D, communication link, the method (100) comprising: determining (101 ) a relocation of the second mobile terminal (503) towards the target cell; determining (103) a common communication resource of the serving cell and of the target cell for the D2D communication link; and allocating (105) the common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal (501 ) and the second mobile terminal (503) to maintain the D2D communication link for the communications.

2. The method (100) of claim 1 , wherein the determining (103) of the common communication resource of the serving cell and of the target cell is performed by a central entity (205), by a serving base station (301 ) or by a target base station (303).

3. The method (100) of claim 1 or 2, wherein the determining (103) of the common communication resource of the serving cell and of the target cell is performed by a central entity (205), and the determining (103) of the common communication resource of the serving cell and of the target cell comprises: sending, by a serving base station (201 ), a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the central entity (205); sending, by a target base station (203), a second message comprising second context information, the second context information comprising communication resources of the target cell, to the central entity (205); determining, by the central entity (205), a communication resource from common

communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell; and sending, by the central entity (205), a message comprising the common communication resource of the serving cell and of the target cell to the serving base station (201 ) and to the target base station (203).

4. The method (100) of claim 3, further comprising: determining, by the central entity (205), a communication resource from alternative communication resources if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell.

5. The method (100) of claim 1 or 2, wherein the determining (103) of the common communication resource of the serving cell and of the target cell is performed by a serving base station (301 ) or by a target base station (303), and the determining (103) of the common communication resource of the serving cell and of the target cell comprises: sending, by the serving base station (301 ), a first message comprising first context information, the first context information comprising communication resources of the serving cell, to the target base station (303); determining, by the target base station (303), a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common

communication resource of the serving cell and of the target cell; and sending, by the target base station (303), an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the serving base station (301 ) if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available.

6. The method (100) of claim 5, further comprising: sending, by the target base station (303), a negative-acknowledgement message comprising second context information, the second context information comprising further

communication resources of the target cell, to the serving base station (301 ) if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available; determining, by the serving base station (301 ), a communication resource from further common communication resources of further communication resources of the serving cell and the further communication resources of the target cell if further common communication resources of further communication resources of the serving cell and the further

communication resources of the target cell are available, the determined communication resource forming the common communication resource of the serving cell and of the target cell; and sending, by the serving base station (301 ), an acknowledgement message comprising the common communication resource of the serving cell and of the target cell to the target base station (303).

7. The method (100) of claim 6, further comprising: determining, by the serving base station (301 ), a communication resource from alternative communication resources if further common communication resources of the further communication resources of the serving cell and the further communication resources of the target cell are not available, the determined communication resource forming the common communication resource of the serving cell and of the target cell.

8. The method (100) of one of the claims 3 to 7, wherein the first context information further comprises a priority among a plurality of priorities of the D2D communication link, and wherein the common communication resource of the serving cell and of the target cell is further determined upon the basis of the priority of the D2D communication link.

9. The method (100) of one of the claims 3 to 8, wherein the first context information further comprises an identification of the D2D communication link, and wherein the identification of the D2D communication link is associated with the determined common communication resource of the serving cell and of the target cell.

10. The method (100) of one of the preceding claims, wherein the communication resource of the D2D communication link is at least one of: a transmission time, a transmission frequency, a transmission space, a transmission code, a transmission modulation, or a transmission power of the D2D communication link.

1 1 . A communication system (200) for managing communications between a first mobile terminal (501 ) and a second mobile terminal (503) in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal (501 ) communicates with the second mobile terminal (503) over a device-to-device, D2D, communication link, the communication system (200) comprising: a serving base station (201 ) being configured to determine a relocation of the second mobile terminal (503) towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal (501 ) and the second mobile terminal (503) to maintain the D2D communication link for the communications; and a central entity (205) being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link.

12. A communication system (300) for managing communications between a first mobile terminal (501 ) and a second mobile terminal (503) in a communications network comprising a serving cell and a target cell, wherein the first mobile terminal (501 ) communicates with the second mobile terminal (503) over a device-to-device, D2D, communication link, the communication system (300) comprising: a serving base station (301 ) being configured to determine a relocation of the second mobile terminal (503) towards the target cell, and to allocate a common communication resource of the serving cell and of the target cell to the D2D communication link between the first mobile terminal (501 ) and the second mobile terminal (503) to maintain the D2D communication link for the communications; and a target base station (303) being configured to determine the common communication resource of the serving cell and of the target cell for the D2D communication link.

13. A central entity (205) for determining a common communication resource of a serving cell and of a target cell for a device-to-device, D2D, communication link between a first mobile terminal (501 ) and a second mobile terminal (503) in a communications network, wherein the first mobile terminal (501 ) communicates with the second mobile terminal (503) over the D2D communication link, the central entity (205) comprising: a communication interface (401 ) being configured to receive, from a serving base station (201 ), a first message comprising first context information, the first context information comprising communication resources of the serving cell; to receive, from a target base station (203), a second message comprising second context information, the second context information comprising communication resources of the target cell; and to send a message comprising a common communication resource of the serving cell and of the target cell to the serving base station (201 ) and to the target base station (203); and a processor (403) being configured to determine a communication resource from common communication resources of the communication resources of the serving cell and the communication resources of the target cell if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are available, the determined communication resource forming the common

communication resource of the serving cell and of the target cell.

14. The central entity (205) of claim 13, wherein the processor (403) is further configured to determine a communication resource from alternative communication resources if common communication resources of the communication resources of the serving cell and the communication resources of the target cell are not available, the determined

communication resource forming the common communication resource of the serving cell and of the target cell.

15. A computer program product comprising a program code for performing the method (100) of one of the claims 1 to 10 when executed on a computer.