WO2015065276A1

WO2015065276A1 - Methods and apparatuses related to device-to-device communication

Info

Publication number: WO2015065276A1
Application number: PCT/SE2014/051266
Authority: WO
Inventors: Gino Masini; Mojgan FADAKI; Stefan WÄNSTEDT; Mats Folke
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2013-11-01
Filing date: 2014-10-27
Publication date: 2015-05-07

Abstract

In an example method according to the disclosed techniques, a target network node receives (330, 470), from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communication between the first user equipment and one or more other user equipments. The target network node then sets aside (360, 495) the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

Description

Methods and apparatuses related to Device-to-Device

Communication

TECHNICAL FIELD

The present disclosure relates generally to device-to-device (D2D) wireless communication, and more specifically relates to cellular network support for D2D communication.

BACKGROUND

The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and the fourth-generation wireless system commonly known as Long Term Evolution (LTE), which is referred to more formally by 3GPP as the Evolved Universal Terrestrial Access Network (E-UTRAN). (The

abbreviation Έ-UTRAN" is sometimes understood to stand for Evolved UMTS Terrestrial Access Network.) LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system, relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes.

Device-to-device communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite, such as WiFi Direct. These systems generally operate in unlicensed spectrum.

Recently, the use of device-to-device (D2D) communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of

communicating devices and at the same time to allow devices to operate in a controlled interference environment. Some of these proposals for so-called "Proximity Services" are discussed in the 3GPP report "3^rd Generation Partnership Project; Technical Specification Group Services and Systems Aspects; Feasibility study for Proximity Services (ProSe) (Release 12)," 3GPP TR 22.803, v.12.2.0 (June 2013), available at www.3gpp.org. Typically, it is suggested that such device-to-device communication should share the same spectrum as the cellular system, for example through the reservation of some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative, as spectrum is a scarce resource and because (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.

LTE D2D scenarios currently studied in 3GPP can be grouped in three categories, namely in-network, out-of-network, and partial-network scenarios. In in-network scenarios, all user equipments (UEs) participating in a given D2D communication session are within network coverage. In out-of-network scenarios, on the other hand, all participating UEs are outside network coverage. In partial-network scenarios, some participating UEs are within coverage and some are out.

In the event that a D2D UE is under network coverage in an LTE network (e.g., in an in- network or a partial-network scenario), the radio resources it uses for D2D may be set aside by the eNodeB (eNB, 3GPP terminology for an LTE base station), which avoids scheduling legacy traffic on the same resources, so as to avoid interference.

In-network or partial-network scenarios may also involve mobility of one or more D2D UEs (i.e., UEs involved in D2D communications with other UEs) between eNBs. Existing solutions for handing over UEs may be inadequate.

SUMMARY

Information about radio resources allocated to a user equipment (UE) for device-to-device (D2D) communication can be signaled from a source network node to a target network node, using existing handover signaling. In the event that other involved D2D UEs also require handover, a handover decision by the source network node can be taken jointly, where the decision takes into account that there are direct communications between the UEs and, in some embodiments, taking into account whether other involved D2D UE(s) are close to the cell border.

Embodiments of the present techniques include methods implemented by source and target network nodes, which may be eNBs, for example, as well as methods implemented by D2D- capable UEs. Other embodiments include corresponding network node and UE apparatus.

In an example embodiment, a source network node of a wireless communication system implements a method that includes determining that handover is required for a first UE that is using a set of radio resources set aside by the source network node for device-to-device (D2D) communications between the first UE and one or more other UEs, and sending, to a target network node, one or more handover messages for the first UE. The one or more handover messages indicate the set of radio resources set aside by the source network node for D2D communications between the first UE and the one or more other UEs. In another example embodiment corresponding to the above, a target network node of a wireless communication system implements a method that includes receiving, from a source network node, one or more handover messages for a first UE served by the source network node. The one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device (D2D) communication between the first UE and one or more other UEs. The method implemented in the target network node in this example further includes setting aside the same set of radio resources for use by the first UE for D2D communications, in response to the one or more handover messages.

In another example embodiment, a method implemented in a source network node serving both a first UE and a second UE involved in D2D communication with one another includes determining that a handover of the first UE is required. The source network node requests the second UE to measure a target network node, and subsequently receives a

measurement from the second UE. In response to determining that the measurement indicates that the target network node is adequate for the second UE, the source network node initiates handover of both the first and second UEs to the target network node.

In some embodiments, the handover of the first and second UEs may comprise sending one or more handover messages to the target network node indicating a set of radio resources set aside by the source network node for D2D communications between the first and second UEs. In still another example embodiment, a first UE served by a source network node in a communications network and involved in D2D communication with a second UE implements a method according to which it receives, from the source network node, a command to handover to a target network node. In response to receiving the command to handover, the first UE sends, to the second UE, a message identifying the target network node and indicating that the first UE is being handed over.

In an example embodiment corresponding directly to the above, a second UE served by a source network node in a communications network and involved in D2D communications with a first UE receives, from the first UE and via the D2D communications, a message identifying a target network node and indicating that the first UE is being handed over to the target network node. In response to receiving this message, the second UE performs a measurement of the target network node and reports the measurement to the source network node.

Variations of these methods will be detailed below. Other embodiments of the techniques include network node apparatus (e.g., eNB and/or HeNB apparatus) adapted to carry out one or several of the techniques summarized above, as well as UE apparatus adapted to carry out one or more of the techniques disclosed herein.

Of course, the techniques, systems, and apparatus described herein are not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates components of an E-UTRAN system.

Figure 2 illustrates in-network, partial-network, and out-of-network D2D scenarios. Figure 3 is a signal flow diagram illustrating a handover procedure in accordance with some of the techniques disclosed herein.

Figure 4 is a signal flow diagram illustrating another example procedure in accordance with some of the techniques disclosed herein.

Figure 5 is a process flow diagram illustrating an example method as implemented in a network node.

Figure 6 is a signal flow diagram illustrating yet another example procedure.

Figure 7 is a block diagram illustrating components of an example network node. Figure 8 is a block diagram illustrating components of an example wireless terminal. Figure 9 is a block diagram of an example network node having functional modules. Figure 10 is another block diagram illustrating an example network node having functional modules.

Figure 11 is another block diagram illustrating an example network node having functional modules.

Figure 12 is a block diagram illustrating an example wireless terminal having functional modules.

DETAILED DESCRIPTION

Within the context of this disclosure, the terms "wireless terminal" or "wireless device" encompass any terminal which is able to communicate wirelessly with another device (as well as, optionally, with an access node of a wireless network) by transmitting and/or receiving wireless signals. Thus, the term "wireless terminal" encompasses, but is not limited to: a user equipment (e.g., an LTE UE), a mobile terminal, a stationary or mobile wireless device for machine-to-machine communication, an integrated or embedded wireless card, an externally plugged in wireless card, a dongle etc. Throughout this disclosure, the terms "user equipment" and "UE" are sometimes used to exemplify various embodiments. However, this should not be construed as limiting, as the concepts illustrated herein are equally applicable to other wireless terminals. Hence, whenever a "user equipment" or "UE" is referred to in this disclosure, this should be understood as encompassing any wireless terminal as defined

In the discussion that follows, specific details of particular embodiments of the presently disclosed techniques and apparatus are set forth for purposes of explanation and not limitation. It will be appreciated by those skilled in the art that other embodiments may be employed apart from these specific details. Furthermore, in some instances detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not to obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or in several nodes. Some or all of the functionality described herein may be implemented using hardware circuitry, such as analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc. Likewise, some or all of the functionality may be implemented using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Where nodes that communicate using the air interface are described, it will be appreciated that those nodes also have suitable radio communications circuitry. Moreover, at least certain aspects of the technology can additionally be considered to be embodied entirely within any form of computer-readable memory, including non- transitory embodiments such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementations of the technology described herein may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analog) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understood to comprise one or more processors or one or more controllers, and the terms computer, processor, and controller may be employed interchangeably. When provided by a computer, processor, or controller, some or all of the functionality described herein may be provided by a single dedicated computer or processor or controller, by a single shared computer or processor or controller, or by a plurality of individual computers or processors or controllers, some of which may be shared or distributed. Moreover, the term "processor" or "controller" also refers to other hardware capable of performing such functions and/or executing software, such as the example hardware recited above. References throughout the specification to "one embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification are not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

While the following examples are described in the context of handover in LTE systems, the principles described in the following disclosure may be equally applied to other cellular networks.

The LTE network is also known as the Evolved Universal Terrestrial Radio Network or Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN is made up of eNBs, which are connected to each other via the X2 interface, and which are connected to either a Mobility Management Entity (MME) or Serving Gateway (S-GW) node by the S1 interface. The E-UTRAN may be supplemented by home eNBs (HeNBs) or other low-power nodes, and HeNB GW entities, which are also linked to the other nodes of the network via the S1 and X2 interfaces.

Figure 1 shows an example diagram of an E-UTRAN architecture, as part of an LTE-based communications system 2. Nodes in the core network 4 include one or more Mobility Management Entities (MMEs) 6, which are control nodes for the LTE access network, and one or more Serving Gateways (SGWs) 8, which route and forward user data packets while acting as mobility anchors. The MMEs 6 and SGWs 8 communicate with base stations 10 referred to in LTE as eNBs, over the S1 interface. The eNBs 10 can include two or more of the same or different categories of eNBs, e.g., macro eNBs, and/or micro/pico/femto eNBs. The eNBs 10 communicate with one another over the X2 interface. The S1 interface and X2 interfaces are defined in the LTE standard. A user equipment (UE) 12 can receive downlink data from and send uplink data to one of the base stations 10 with that base station 10 being referred to as the serving base station of the UE 12.

As noted above, D2D scenarios currently studied in 3GPP can be grouped in the following categories namely in-network, out-of-network, and partial-network scenarios. These scenarios are illustrated in Figure 2. In in-network scenarios, all UEs 12 participating in a given D2D communication session are within the network coverage provided by eNBs 10. In out-of-network scenarios, on the other hand, all participating UEs 12 are outside network coverage. In partial-network scenarios, some participating UEs 12 are within coverage of an eNB 10 and some are out. In the event that a D2D UE is under network coverage (i.e., in an in-network scenarios), the radio resources it uses for D2D may be set aside by the serving eNB, which avoids scheduling legacy traffic on the same resources to avoid interference.

In-network or partial-network scenarios may also involve mobility of one or more D2D UEs (i.e., UEs involved in D2D communications with other UEs) between eNBs. These UEs may include at least one UE that is under network coverage, and which may be using radio resources for D2D for which it had previously negotiated with its serving eNB. In the event that one or more of these D2D UEs needs to be handed over to another eNB, the issue of how to treat the radio resources allocated to it for D2D use must be considered. More particularly, in the event that a D2D UE must be handed over to another cell, i.e., from a serving base station, which may be referred to as a "source network node," to another base station, which may be referred to as a "target network node," there are two

requirements that should be met. These two requirements may conflict with each other. First, the D2D UE needs to reconfigure its wireless transmission and reception according to the instructions it receives from the source network node and/or target network node, as in any handover. In the LTE context, the source and target network nodes are eNBs, which communicate these instructions using Radio Resource Control (RRC) signaling. Second, the D2D UE needs to retain the set of radio resources it is currently using for D2D, in order not to lose communication with other D2D UEs. Standards released prior to this disclosure do not include any mechanism to signal, to the target network node/cell, information regarding the D2D radio resources allocated to the UE. These standards likewise do not include any way to efficiently manage the situation where one or more other D2D UEs, connected to the D2D UE being handed over, are also in- coverage and require handover to another cell. As a result, using previously available techniques for handover can cause interruption of D2D service after handover. This can be a critical problem in some circumstances, since one important application for D2D UEs is for public safety services.

Information about radio resources allocated to a UE for D2D communication can be signaled from a source eNB to a target eNB, using existing handover signaling. In the event that other involved D2D UEs also require handover, a handover decision by the eNB can be taken jointly, where the decision takes into account that there are direct communications between the UEs and, in some embodiments, taking into account whether other involved D2D UE(s) are close to the cell border. Embodiments of the present techniques include methods implemented by source and target eNBs, as well as methods implemented by D2D-capable UEs. Other embodiments include corresponding eNB and UE apparatus.

In an example embodiment, an eNB serving at least a first UE involved in D2D

communication and that has set aside (i.e., allocated) radio resources for the first UE to use in D2D communication determines that a handover of the first UE is required and sends one or more handover messages to a target eNB, the one or more handover messages comprising information identifying or otherwise referring to the set-aside radio resources. In response to receiving an acknowledgement of the handover messages, the eNB completes handover of the first UE to the target eNB.

In another example embodiment corresponding to the above, a target eNB receives one or more handover messages for a first UE from a source eNB, the one or more handover messages comprising information identifying or otherwise referring to radio resources set aside by the source eNB for D2D communications by the first UE. The target eNB then sets aside the same radio resources for use by the first UE in D2D communications.

In another example embodiment, an eNB is serving both a first UE and a second UE involved in D2D communication with one another, and determines that a handover of the first UE is required. In response to determining that handover of the first UE is required, the eNB requests the second UE to measure a target eNB, and subsequently receives a

measurement from the second UE. In response to determining that the measurement indicates that the target eNB is adequate for the second UE, the eNB initiates handover of both the first and second UEs to the target eNB. In some embodiments, the handover of the first and second UEs may comprise sending one or more handover messages comprising information identifying or otherwise referring to radio resources set aside by the eNB for D2D communications between the first and second UEs.

In still another example embodiment, a first UE involved in D2D communication with a second UE determines that it is being handed over from a source eNB to a target eNB. In response to this determining, the first UE sends information identifying or otherwise referring to the target eNB to the second UE, and then continues with a handover procedure. In an example embodiment corresponding directly to the above, a second UE served by a source eNB and involved in D2D communication with a first UE receives, from the first UE, information identifying or otherwise referring to the target eNB. In response to receiving this information, the second UE initiates a measurement of the target eNB and reports the measurement to the source eNB. The second UE subsequently receives a handover command directing the second UE to hand over to the target eNB.

Variations of these methods will be detailed below. Other embodiments of the techniques include base station apparatus (e.g., eNB and/or HeNB apparatus) adapted to carry out one or several of the techniques summarized above, as well as UE apparatus adapted to carry out one or more of the techniques disclosed herein.

According to the techniques detailed herein, information about radio resources allocated to a UE for D2D communication can be signaled from a source eNB to the target eNB, using existing handover signaling. Furthermore, in some embodiments of the presently disclosed techniques, a handover decision by the eNB can be taken jointly, for several UEs at a time, in the event that other involved D2D UEs also require handover. In other words, the eNB takes a decision to handover multiple UEs at once, based at least partly on the fact that the multiple UEs are engaged with each other in D2D communications. This joint handover decision can take into account that there are direct communications between the UEs and, in some embodiments, may further take into account whether other involved D2D UE(s) are close to the cell border.

After receiving information about allocated D2D radio resources, a target eNB can pre- allocate the same resources, if possible, to the same UE, with no need to change the UE D2D configuration. This can result in significantly reduced RRC signaling, which may in turn lead to increased UE battery life, with little or no D2D service interruption.

Jointly handing over D2D UEs that have connections among them can result in better quality of service for D2D communications, by avoiding or reducing D2D across cell boundaries, and can further result in more efficient radio resource usage, since reducing D2D across cells reduces the need to allocate radio resources for D2D in more than one cell. One approach to handover according to the presently disclosed techniques is illustrated in the signaling flow diagram of Figure 3, and involves an X2 handover between the source and the target eNBs. It will be appreciated that while this particular signal flow applies to an LTE network and identifies specific LTE messages, the principles outlined in the figure and detailed below may be applied to other networks, using similar messages. The following details apply to the several operations shown in the figure, from the top down.

As seen at block 310 of Figure 3, the D2D UE is served by a source eNB and is using a set of radio resources for D2D communication with other UEs. The D2D UE may have previously negotiated with the source eNB for these resources. At some point, handover of the D2D UE to a target eNB is required. The determination that handover is needed may be triggered by conventional mechanisms, such as in response to the D2D UE measuring the target cell and reporting the measurement or measurements to the source eNB. As shown at block 320, the X2 handover procedure begins.

The handover procedure illustrated generally in Figure 3 may follow the detailed steps specified in the 3GPP specifications, for example, as specified in 3GPP TS 36.300, v.11.7.0 (Sept. 2013) and 3GPP TS 36.423, v.1 1.6.0 (Sept. 2013), both of which are available at www.3gpp.org. However, in some embodiments of the currently disclosed techniques, the messages sent to the target eNB by the source eNB include an information element (IE) that comprises information about (i.e., identifying or otherwise referring to) configured radio resources for D2D for the UE that is being handed over. In the example procedure shown in Figure 3, this IE is included in HANDOVER REQUEST message 330.

As seen in Figure 3, the target eNB receives the message and, in some embodiments, replies with a X2AP HANDOVER REQUEST ACKNOWLEDGE message 340. The handover procedure continues, as shown at block 350, e.g., as specified in the 3GPP specifications. When reconfiguring the UE for D2D in the target cell , the target eNB takes the information regarding D2D radio resources into account. Depending on configuration, load and radio resource availability in the target cell, the target eNB may be able to configure the D2D UE with the same set of radio resources that it was using in the source cell. This is shown at block 360 of Figure 3. Note that a variation of the above can also be envisaged, where the source eNB sends to the target eNB an IE with the information about configured radio resources for D2D for the UE handed over through the handover signaling (as in the approach described above), but an S1 handover is performed instead. This enables handing over a D2D UE between two eNBs that do not have an X2 interface set up between them. Figure 4 illustrates another, related, approach. In the scenario addressed by this approach, two UEs (D2D UE1 and D2D UE2) connected to a serving eNB are using a set of radio resources for D2D communication between them, and a measurement report indicates that one of them should be handed over to another cell. This handover could be done according to the approach illustrated in Figure 3. However, if nothing more is done, the other UE is left behind to be served by the original eNB. This could lead to D2D communication across cells, resulting in that radio resources are set aside for D2D communication in both cells. Thus, it can be beneficial for the serving eNB to consider a joint handover of both UEs, to avoid this situation. Figure 4 is a signal flow diagram illustrating one possible approach to this joint handover. As shown at block 410, D2D UE1 and D2D UE2 are both served by a source eNB, and they are using a set of radio resources for D2D communications. One or both of the D2D UEs may have previously negotiated with the source eNB for these resources. At some point, D2D UE2 moves out of the reliable coverage area of the source eNB and needs to be handed over to a target eNB. Again, the determination handover is needed may be triggered by conventional mechanisms, such as in response to the D2D UE2 measuring the target cell and reporting the measurement to the source eNB, as shown at 420.

In response to determining that handover is needed for D2D UE2, the source eNB, which knows that D2D UE1 and D2D UE2 are involved in D2D communication, decides that UE1 should be requested to measure the target eNB, as shown at block 430. As shown at 440, which illustrates a target cell measurement request that identifies the target eNB, the source eNB thus requests UE1 to measure the target eNB, before requesting the handover for UE2. The source eNB can determine that the UEs are in D2D communication with one another by, for example, determining that radio resources have been set aside for D2D communication between the UEs, or by evaluating ProSe group identifiers associated with the UEs. UE1 makes a measurement of the target eNB, in response to the measurement request, and reports it to the source eNB. This is shown in Figure 4 as measurement report 450, which includes the measurement and an identifier for the target eNB. If the measurement report indicates that the target eNB is adequate also for UE1 (i.e., provides adequate coverage for UE1), then the source eNB decides to handover both UE1 and UE2, as shown at block 460. The source eNB then prepares HANDOVER REQUEST messages for both UEs and sends them toward the target eNB, as shown at 470. As shown at 475, the target eNB replies to the source eNB with HANDOVER REQUEST

ACKNOWLEDGE message(s) for both UEs. The source eNB then sends handover commands to both UEs, as shown at 480 and 485, and the handover procedures continue for both UEs, as shown at 490.

Information about radio resources allocated for D2D for the UEs may be exchanged between source and target eNBs with handover signaling, e.g., according to the techniques discussed above in connection with Figure 3. Thus, for example, one or both of the HANDOVER REQUEST messages sent from the source eNB to the target eNB may include an IE that indicates a set of radio resources set aside by the source network node for D2D

communication between D2D UE1 and D2D UE2. The target eNB may then pre-allocate and configure the same resources for the UEs as were used in the source eNB, as shown at block 495. A variant of the process illustrated in the signal flow diagram of Figure 4 is shown in the process flow diagram of Figure 5. As in the scenario described above, an eNB is serving both a first UE and a second UE involved in D2D communication with one another, and thus can be thought of as a "source network node." The illustrated process is shown from the perspective of this source network node.

As shown at block 510, the source eNB determines that a handover of the first UE is required, e.g., based on an evaluation of a measurement report received from the first UE. In response to determining that handover of the first UE is required, the eNB requests the second UE to measure a target eNB (i.e., a target network node) as shown at block 520. After subsequently receiving a measurement from the second UE, as shown at block 530, the source eNB determines that the measurement indicates that the target eNB is adequate for the second UE, as shown at block 540. As shown at block 550, the source eNB then initiates handover of both the first and second UEs to the target eNB. As discussed above, in some embodiments the handover of the first and second UEs may comprise sending one or more handover messages comprising information identifying or otherwise referring to radio resources set aside by the eNB for D2D communications between the first and second UEs.

An example signal flow illustrating another related approach is shown in Figure 6. Once again, the scenario addressed by this approach, two (or more) UEs (D2D UE1 and D2D UE2) connected to a serving eNB are using a set of radio resources for D2D communication between them, and one of them needs to be handed over to another cell. In the approach shown in Figure 6, UE2 signals to UE1 information about its target eNB immediately after a handover of UE2 has been triggered by the source eNB. UE1 then stores this information, and, in response to learning that UE2 is being handed over, measures the target eNB for UE2's handover and reports the measurement to the source eNB. This allows the source eNB to then take a handover decision for UE1 , as appropriate.

Figure 6 illustrates details of an example of this approach. As shown at block 610, D2D UE1 and D2D UE2 are initially served by a source eNB, and are using a set of radio resources for D2D communications. At some point, D2D UE2 moves out of the coverage of the source eNB and needs to be handed over to a target eNB. More particularly, the source eNB receives a measurement report 615 from UE2, and makes a handover decision for UE2, based on the measurement report, as shown at block 620. The source eNB triggers handover for D2D UE2 toward the target eNB by sending a Handover Request message 625 to the target eNB, which responds with a Handover Request Reply 630. The source eNB then sends a Handover Command 635 to D2D UE2, which subsequently sends a Handover Confirm message 640 to the target eNB. In response to the Handover Command, D2D UE2 communicates to D2D UE1 the information about its target eNB over the D2D interface, as shown at 645, and then continues with its handover procedure. Note that this information serves to inform UE1 that UE2 is being handed over. UE1 stores this information, as shown at block 650, and, in response to learning that UE2 is being handed over, measures the target cell and reports the measurement to the source eNB, as shown at 655.

In response to UEVs measurement of the target eNB, the source eNB decides to hand over UE1 to the target eNB, as shown at block 660. A handover procedure then ensues, as shown at 665, 670, 675, and 680. Information about the radio resources allocated for D2D for the UEs may be exchanged between the source and target eNBs with handover signaling, e.g., according to the techniques described in connection with Figure 3. In this case, the target eNB may pre- allocate and configure the same resources for the UEs as in the source eNB.

Several of the techniques and processes described above can be implemented in a network node, such as an eNodeB (eNB) or HeNB. Figure 7 is a schematic illustration of a node 1 in which a method embodying any of the presently described network-based techniques can be implemented. For any given mobility transaction, the node illustrated in Figure 7 may correspond to a source eNB or a target eNB, for example.

A computer program for controlling the node 1 to carry out a method embodying any of the presently disclosed techniques is stored in a program storage 30, which comprises one or several memory devices. Data used during the performance of a method embodying the present invention is stored in a data storage 20, which also comprises one or more memory devices. During performance of a method embodying the present invention, program steps are fetched from the program storage 30 and executed by a Central Processing Unit (CPU) 10, retrieving data as required from the data storage 20. Output information resulting from performance of a method embodying the present invention can be stored back in the data storage 20, or sent to an Input/Output (I/O) interface 40, which includes a network interface for sending and receiving data to and from other network nodes, e.g., via an X2 and/or S1 interface, and which may also include a radio transceiver for communicating with one or more terminals. The CPU 10 and its associated data storage 20 and program storage 30 may collectively be referred to as a "processing circuit." It will be appreciated that variations of this processing circuit are possible, including circuits that comprise one or more of various types of programmable circuit elements, e.g., microprocessors, microcontrollers, digital signal processors, field-programmable application-specific integrated circuits, and the like, as well as processing circuits where all or part of the processing functionality described herein is performed using dedicated digital logic.

Accordingly, in various embodiments of the invention, processing circuits, such as the CPU 10, data storage 20, and program storage 30 in Figure 7, are configured to carry out one or more of the techniques described in detail above. While some embodiments may comprise an eNB or HeNB in an LTE network, for example, other embodiments may include base station nodes and/or radio network controllers designed for operation in other types of networks and including one or more such processing circuits. In some cases, these processing circuits are configured with appropriate program code, stored in one or more suitable memory devices, to implement one or more of the techniques described herein. Of course, it will be appreciated that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module.

Figure 8 illustrates features of an example wireless device 800 that can be used in one or more of the non-limiting example embodiments described. The UE 800 may in some embodiments be a wireless device that is configured for device-to-device (D2D)

communication with one or several other UEs. UE 800 comprises a transceiver circuit 820 configured to communicate with one or more base stations as well as a processing circuit 810 for processing the signals transmitted and received by the transceiver unit 820.

Transceiver circuit 820 includes a transmitter 825 coupled to one or more transmit antennas 828 and receiver 830 coupled to one or more receiver antennas 833. The same antenna(s) 828 and 833 may be used for both transmission and reception. Receiver 830 and transmitter 825 use known radio processing and signal processing components and techniques, typically according to a particular telecommunications standard such as the 3GPP standards for LTE. Note also that transceiver circuit 820 may comprise separate radio and/or baseband circuitry for each of two or more different types of radio

communication, such as radio/baseband circuitry adapted for E-UTRAN access and separate radio/baseband circuitry adapted for D2D communication; thus, transceiver circuit 820 may also be configured to carry out D2D communications with one or more other UEs, likewise using standardized protocols. The same applies to the antennas - while in some cases one or more antennas may be used for accessing multiple types of networks or for multiple modes of communication, in other cases one or more antennas may be specifically adapted to a particular radio access network or communication mode. Because the various details and engineering tradeoffs associated with the design and implementation of such circuitry are well known and are unnecessary to a full understanding of the invention, additional details are not shown here. Processing circuit 810 comprises one or more processors 840 coupled to one or more memory devices 850 that make up a data storage memory 855 and a program storage memory 860. Processor 840, identified as CPU 840 in Figure 8, may be a microprocessor, microcontroller, or digital signal processor, in some embodiments. More generally, processing circuit 810 may comprise a processor/firmware combination, or specialized digital hardware, or a combination thereof. Memory 850 may comprise one or several types of memory such as read-only memory (ROM), random-access memory, cache memory, flash memory devices, optical storage devices, etc. Because terminal 800 supports a cellular radio access network in addition to D2D communications, processing circuit 810 may include separate processing resources dedicated to one or several radio access technologies or radio communications modes, in some embodiments. Again, because the various details and engineering tradeoffs associated with the design of baseband processing circuitry for mobile devices are well known and are unnecessary to a full understanding of the invention, additional details are not shown here. Typical functions of the processing circuit 810 include modulation and coding of transmitted signals and the demodulation and decoding of received signals. In several embodiments of the present invention, processing circuit 810 is adapted, using suitable program code stored in program storage memory 860, for example, to carry out one of the techniques described above for facilitating mobility of D2D-capable UEs in a cellular network. Of course, it will be appreciated that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module.

As discussed above, the various techniques illustrated in Figures 3-6, and variants thereof, may be implemented using processing circuits like those in Figures 7 and 8, where the processing circuits are configured, e.g., with appropriate program code stored in memory circuits, to carry out the operations described above. While some of these embodiments are based on a programmed microprocessor or other programmed processing element, it will be appreciated, as noted above, that not all of the steps of these techniques are necessarily performed in a single microprocessor or even in a single module. It will be further appreciated that embodiments of the presently disclosed techniques further include computer program products for application in a user terminal as well as corresponding computer program products for application in a base station apparatus.

Various aspects of the above-described embodiments can also be understood as being carried out by functional "modules," which may be program instructions executing on an appropriate processor circuit, hard-coded digital circuitry and/or analog circuitry, or appropriate combinations thereof. Thus, for example, a target network node 900 adapted to carry out one or more of the techniques disclosed herein is shown in Figure 9 and can be understood to include a receiving module 910 for receiving, from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communication between the first user equipment and one or more other user equipments. The example target network node 900 may also be understood to include a resource allocation module 920 for setting aside the same set of radio resources for use by the first user equipment for D2D communications. It should be appreciated that the target network node 900 shown in Figure 9 may have a structure like that shown in Figure 7, in some embodiments. Similarly, an example source network node 1000 adapted to carry out one or more of the techniques described above is illustrated in Figure 10 and includes a handover module 1010 for determining that handover to a target network node is required for a first user equipment that is using a set of radio resources set aside by the source network node for D2D communications between the first user equipment and one or more other user equipments. The source network node 1000 also includes a sending module 1020 for sending, to the target network node, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the source network node for D2D communications between the first user equipment and the one or more other user equipments. In some embodiments, sending module 1020 may be further adapted to request a second user equipment, involved in D2D communications with the first user equipment, to report measurements of the target network node, in response to determination by handover module 1010 that handover of the first UE is required, and to send, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment. Like the target network node 900 shown in Figure 9, the source network node 1000 shown in Figure 10 may have a structure like that shown in Figure 7, in some embodiments.

Figure 11 is another illustration of an example source network node that is adapted to serve both a first user equipment and a second user equipment involved in D2D communication with one another. Once again, the source network node may have a structure like that shown in Figure 7, in some embodiments. This source network node includes a handover module 1 110 for determining that handover to a target network node is required for the first user equipment, a sending module 1 120 for requesting the second user equipment to measure a target network node, and a receiving module 1 130 for subsequently receiving a measurement from the second user equipment. The handover module 1 110 in this example is further adapted to initiate handover of both the first and second user equipments to the target network node, in response to determining that the received measurement indicates that the target network node is adequate for the second user equipment. In some

embodiments, the handover module 11 10 is adapted to do this by sending, to the target network node, one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communications between the first user equipment and the second user equipment.

Likewise, Figure 12 illustrates an example UE 1200, where the functionality described above is carried out by functional modules. The illustrated UE 1200 includes a receiving module 1210 and a sending module 1220, as well as a measurement module 1230. In some embodiments or instances, the receiving module 1210 is adapted to receive, from a source network node, a command to handover to a target network node in the wireless

communications network, and the sending module 1220 is adapted to send to a second UE, using D2D communications, a message identifying the target network node and indicating that the UE 1200 is being handed over, in response to the received command to handover. Correspondingly, in some embodiments or instances, the receiving module 1210 is adapted to receive, from another UE, via the D2D communications, a message identifying a target network node in the wireless communications network and indicating that the other UE is being handed over to the target network node, while the measurement module 1230 is adapted to perform a measurement of the target network node and report the measurement to a source network node, in response to receiving the message. It will be appreciated that a single UE may be configured so that it can operate in both manners, at appropriate times.

In view of the detailed examples described above, it will be appreciated that embodiments of the presently disclosed methods and apparatus include, but are not limited to, the example embodiments in the following list. Example Embodiments

1. A method in a target network node of a wireless communication system, the method comprising:

receiving, from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device, D2D, communication between the first user equipment and one or more other user equipments; and setting aside the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages. 2. The method of example embodiment 1 , wherein setting aside the same set of radio resources for use by the first user equipment is further in response to determining that the same set of radio resources is available for use in a cell served by the target network node. 3. The method of example embodiment 1 or 2, further comprising receiving, from the source network node, a handover message for a second user equipment involved in D2D communication with the first user equipment.

4. The method of example embodiment 3, wherein the handover message for the second user equipment indicates that the same set of radio resources is set aside by the source node for D2D communications between the second user equipment and one or more other user equipments.

5. A method in a source network node of a wireless communication system, the method comprising:

determining that handover is required for a first user equipment that is using a set of radio resources set aside by the source network node for device-to-device, D2D, communications between the first user equipment and one or more other user equipments; and

sending, to a target network node, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the source network node for D2D communications between the first user equipment and the one or more other user equipments. 6. The method of example embodiment 5, wherein the source network node is serving a second user equipment that is involved in D2D communications with the first user equipment, using the set-aside set of radio resources, the method further comprising:

in response to determining that handover of the first UE is required, requesting the second user equipment to report measurements of the target network node; and

sending, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment. 7. The method of example embodiment 6, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the source network node for D2D communications between the second user equipment and one or more other user equipments. 8. The method of example embodiment 5, further comprising:

requesting the second user equipment to report measurements of the target network node, prior to determining that handover to the target network node is required for the first user equipment;

jointly determining that handover is required for the first and second user

equipments, based on the reported measurements of the target network node from the second user equipment and further based on measurements of the target network node reported from the first user equipment; and sending, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

9. A method in a source network node of a wireless communication system, wherein the source network node is serving both a first user equipment and a second user equipment involved in device-to-device, D2D, communication with one another, the method comprising: determining that handover is required for the first user equipment;

requesting the second user equipment to measure a target network node;

subsequently receiving a measurement from the second user equipment; and, in response to determining that the measurement indicates that the target network node is adequate for the second user equipment, initiating handover of both the first and second user equipments to the target network node.

10. The method of example embodiment 9, wherein initiating handover of both the first and second user equipments to the target network node comprises sending, to the target network node, one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communications between the first user equipment and the second user equipment. 11. The method of any of example embodiments 1-10, wherein said indicating a set of radio resources comprises identifying or otherwise referring to the set-aside resources.

12. The method of any of example embodiments 1-1 1 , wherein said source network node and said target network nodes are each eNodeBs.

13. A method in a first user equipment served by a source network node in a

communications network and involved in device-to-device, D2D, communications with a second user equipment, the method comprising: receiving, from the source network node, a command to handover to a target network node; and

sending to the second user equipment, via the D2D communications, a message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover.

14. A method in a second user equipment served by a source network node in a

communications network and involved in device-to-device, D2D, communications with a first user equipment, the method comprising:

receiving from the first user equipment, via the D2D communications, a message identifying a target network node and indicating that the first user equipment is being handed over to the target network node; and,

in response to receiving the message, performing a measurement of the target

network node and reporting the measurement to the source network node.

15. The method of example embodiment 14, wherein the method further comprises receiving from the source network node, in response to the reported measurement, a command to handover to the target network node.

16. The method of any of example embodiments 13-15, wherein said source network node and said target network nodes are each eNodeBs.

17. A network node apparatus for use in a wireless communication system, the network node apparatus comprising:

a network interface configured to receive, from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device, D2D, communications between the first user equipment and one or more other user equipments; and

a processing circuit operatively connected to the network interface and configured to set aside the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

18. The network node apparatus of example embodiment 17, wherein the processing circuit is configured to determine that that the same set of radio resources is available for use in a cell served by the network node apparatus and to set aside the same set of radio resources for use by the first user equipment further in response to said determining.

19. The network node apparatus of example embodiment 17 or 18, wherein the network interface is further configured to receive, from the source network node, a handover message for a second user equipment involved in D2D communication with the first user equipment.

20. The network node apparatus of example embodiment 19, wherein the handover message for the second user equipment indicates that the same set of radio resources is set aside by the source network node for D2D communications between the second user equipment and one or more other user equipments.

21. The network node apparatus of any of example embodiments 17-20, wherein the one or more handover messages for the first user equipment indicate the set-aside resources by identifying or otherwise referring to the set-aside resources.

22. The network node apparatus of any of example embodiments 17-21 , wherein the network node apparatus and the source network node are eNodeBs.

23. A network node apparatus for use in a wireless communication system, the network node apparatus comprising a radio transceiver, a network interface and a processing circuit operatively coupled to the radio transceiver and network interface, wherein the processing circuit is configured to:

determine that handover is required for a first user equipment that is using a set of radio resources set aside by the network node apparatus for device-to-device, D2D, communication between the first user equipment and one or more other user equipments; and

send to a target network node, using the network interface, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the network node apparatus for D2D communication between the first user equipment and one or more other user equipments. 24. The network node apparatus of example embodiment 23, wherein the processing circuit is further configured to:

in response to determining that handover of the first UE is required, use the radio transceiver to request a second user equipment, which is involved in D2D communications with the first user equipment, using the set-aside set of radio resources, to report measurements of the target network node; and send to the target network node, via the network interface, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

25. The network node apparatus of example embodiment 24, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the network node apparatus for D2D communications between the second user equipment and one or more other user equipments.

26. The network node apparatus of example embodiment 23, wherein the processing circuit is further configured to:

request the second user equipment to report measurements of the target network node, prior to determining that handover is required for the first user equipment;

jointly determine that handover is required for the first and second user equipments, based on the reported measurements of the target network node from the second user equipment and further based on measurements of the target network node reported from the first user equipment; and

send, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment. 27. A network node apparatus for use in a wireless communication system and adapted to: receive, from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device, D2D, communications between the first user equipment and one or more other user equipments; and set aside the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

28. The network node apparatus of example embodiment 27, wherein the network node apparatus is further adapted to determine that that the same set of radio resources is available for use in a cell served by the network node apparatus and to set aside the same set of radio resources for use by the first user equipment further in response to said determining. 29. The network node apparatus of example embodiment 27 or 28, wherein the network node apparatus is further adapted to receive, from the source network node, a handover message for a second user equipment, wherein the second user equipment is involved in D2D communications with the first user equipment.

30. The network node apparatus of any of example embodiments 27-29, wherein the network node apparatus and the source network node are eNodeBs. 31. A network node apparatus for use in a wireless communication system and adapted to: determine that handover is required for a first user equipment that is using a set of radio resources set aside by the network node apparatus for device-to-device, D2D, communication between the first user equipment and one or more other user equipments; and

send, to a target network node, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the network node apparatus for D2D communication between the first user equipment and one or more other user equipments. 32. The network node apparatus of example embodiment 31 , wherein the network node apparatus is further adapted to:

in response to determining that handover of the first user equipment is required, request a second user equipment, which is involved in D2D communications with the first user equipment, using the set-aside set of radio resources, to report measurements of the target network node; and

send, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment. 33. The network node apparatus of example embodiment 32, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the network node apparatus for D2D communications between the second user equipment and one or more other user equipments. 34. The network node apparatus of example embodiment 31 , wherein the network node apparatus is further adapted to: request the second user equipment to report measurements of the target network node, prior to determining that handover is required for the first user equipment;

send, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

35. A network node apparatus for use in a wireless communication system and adapted to: determine that handover is required for a first user equipment that is involved in

device-to-device, D2D, communication with a second user equipment, wherein both the first and second user equipments are served by the network node apparatus;

request the second user equipment to measure a target network node;

subsequently receive a measurement from the second user equipment; and, initiate handover of both the first and second user equipments to the target network node, in response to determining that the measurement indicates that the target network node is adequate for the second user equipment,.

36. The network node apparatus of example embodiment 35, wherein the network node apparatus is adapted to send, to the target network node, one or more handover messages indicating a set of radio resources set aside by the source network node for D2D

communications between the first user equipment and the second user equipment.

37. The network node apparatus of any of example embodiments 31-36, wherein the network node apparatus and the target network node are eNodeBs.

38. The network node apparatus of any of example embodiments 17-37, wherein indicating the set of radio resources comprises identifying or otherwise referring to the set-aside resources. 39. A user equipment apparatus configured for device-to-device, D2D, communications, the user equipment apparatus comprising:

a radio transceiver configured to communicate with a source network node in a

wireless communications network and to communicate with one or more other user equipments via D2D communications and further configured to receive, from the source network node, a command to handover to a target network node in the wireless communications network; and

a processing circuit operatively connected to the radio transceiver and configured to send to a second user equipment, using D2D communications, a message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover.

40. A user equipment apparatus configured for device-to-device, D2D, communications, the user equipment apparatus comprising:

a radio transceiver configured to communicate with a source network node in a

wireless communications network and to communicate with one or more other user equipments via D2D communications and further configured to receive from a first user equipment, via the D2D communications, a message identifying a target network node in the wireless communications network and indicating that the first user equipment is being handed over to the target network node; and,

a processing circuit operatively connected to the radio transceiver and configured to perform a measurement of the target network node and report the measurement to the source network node, in response to receiving the message.

41. The user equipment apparatus of example embodiment 40, wherein the processing circuit is further configured to receive, via the radio transceiver and in response to the reported measurement, a command from the source network node to handover to the target network node.

42. A user equipment apparatus configured for device-to-device, D2D, communications and for operation in a wireless communications network, wherein the user equipment apparatus is adapted to:

receive, from a source network node, a command to handover to a target network node in the wireless communications network; and

send to a second user equipment, using D2D communications, a message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover. 43. A user equipment apparatus configured for device-to-device, D2D, communications and for operation in a wireless communications network, wherein the user equipment apparatus is adapted to:

receive, from another user equipment, via the D2D communications, a message identifying a target network node in the wireless communications network and indicating that the other user equipment is being handed over to the target network node; and,

perform a measurement of the target network node and report the measurement to a source network node, in response to receiving the message.

44. The user equipment apparatus of example embodiment 43, wherein the user equipment apparatus is further adapted to receive, in response to the reported measurement, a command from the source network node to handover to the target network node. 45. The user equipment apparatus of any of example embodiments 39-44, wherein the source network node and the target network node are eNodeBs.

Examples of several embodiments of the present techniques have been described in detail above, with reference to the attached illustrations of specific embodiments. Because it is not possible, of course, to describe every conceivable combination of components or techniques, those skilled in the art will appreciate that the present invention can be implemented in other ways than those specifically set forth herein, without departing from essential characteristics of the invention. The present embodiments are thus to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS What is claimed is:

receiving (330, 470), from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device, D2D, communication between the first user equipment and one or more other user equipments; and setting aside (360, 495) the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

2. The method of claim 1 , wherein setting aside (360, 495) the same set of radio resources for use by the first user equipment is further in response to determining that the same set of radio resources is available for use in a cell served by the target network node.

3. The method of claim 1 or 2, further comprising receiving (470), from the source network node, a handover message for a second user equipment involved in D2D communication with the first user equipment.

4. The method of claim 3, wherein the handover message for the second user equipment indicates that the same set of radio resources is set aside by the source node for D2D communications between the second user equipment and one or more other user equipments.

determining (320) that handover is required for a first user equipment that is using a set of radio resources set aside by the source network node for device-to- device, D2D, communications between the first user equipment and one or more other user equipments; and

sending (330), to a target network node, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the source network node for D2D communications between the first user equipment and the one or more other user equipments.

6. The method of claim 5, wherein the source network node is serving a second user equipment that is involved in D2D communications with the first user equipment, using the set-aside set of radio resources, the method further comprising:

in response to determining that handover of the first UE is required, requesting (440) the second user equipment to report measurements of the target network node; and

sending (470), to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

7. The method of claim 6, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the source network node for D2D communications between the second user equipment and one or more other user equipments.

8. The method of claim 5, further comprising:

requesting (440) the second user equipment to report measurements of the target network node, prior to determining that handover to the target network node is required for the first user equipment;

jointly determining (460) that handover is required for the first and second user

equipments, based on the reported measurements of the target network node from the second user equipment and further based on measurements of the target network node reported from the first user equipment; and sending (470), to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

9. A method in a source network node of a wireless communication system, wherein the source network node is serving both a first user equipment and a second user equipment involved in device-to-device, D2D, communication with one another, the method comprising: determining (510) that handover is required for the first user equipment;

requesting (520) the second user equipment to measure a target network node; subsequently receiving (530) a measurement from the second user equipment; and, in response to determining (540) that the measurement indicates that the target network node is adequate for the second user equipment, initiating (550) handover of both the first and second user equipments to the target network node.

10. The method of claim 9, wherein initiating (550) handover of both the first and second user equipments to the target network node comprises sending, to the target network node, one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communications between the first user equipment and the second user equipment.

1 1. The method of any of claims 1-10, wherein said indicating a set of radio resources comprises identifying or otherwise referring to the set-aside resources.

12. The method of any of claims 1-1 1 , wherein said source network node and said target network nodes are each eNodeBs.

13. A method in a first user equipment served by a source network node in a

communications network and involved in device-to-device, D2D, communications with a second user equipment, the method comprising:

receiving (635), from the source network node, a command to handover to a target network node; and

sending (645) to the second user equipment, via the D2D communications, a

message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover.

14. A method in a second user equipment served by a source network node in a

receiving (645) from the first user equipment, via the D2D communications, a

message identifying a target network node and indicating that the first user equipment is being handed over to the target network node; and,

in response to receiving the message, performing a measurement of the target network node and reporting (655) the measurement to the source network node.

15. The method of claim 14, wherein the method further comprises receiving (675) from the source network node, in response to the reported measurement, a command to handover to the target network node.

16. The method of any of claims 13-15, wherein said source network node and said target network nodes are each eNodeBs.

17. A network node apparatus (1) for use in a wireless communication system, the network node apparatus comprising:

a processing circuit (10, 20, 30) operatively connected to the network interface and configured to set aside the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

18. The network node apparatus (1) of claim 17, wherein the processing circuit (10, 20, 30) is configured to determine that that the same set of radio resources is available for use in a cell served by the network node apparatus (1) and to set aside the same set of radio resources for use by the first user equipment further in response to said determining.

19. The network node apparatus (1) of claim 17 or 18, wherein the network interface is further configured to receive, from the source network node, a handover message for a second user equipment involved in D2D communication with the first user equipment.

20. The network node apparatus (1) of claim 19, wherein the handover message for the second user equipment indicates that the same set of radio resources is set aside by the source network node for D2D communications between the second user equipment and one or more other user equipments.

21. The network node apparatus (1) of any of claims 17-20, wherein the one or more handover messages for the first user equipment indicate the set-aside resources by identifying or otherwise referring to the set-aside resources.

22. The network node apparatus (1) of any of claims 17-21 , wherein the network node apparatus (1) and the source network node are eNodeBs.

23. A network node apparatus (1) for use in a wireless communication system, the network node apparatus comprising a radio transceiver, a network interface and a processing circuit (10, 20, 30) operatively coupled to the radio transceiver and network interface, wherein the processing circuit (10, 20, 30) is configured to:

determine that handover is required for a first user equipment that is using a set of radio resources set aside by the network node apparatus (1) for device-to- device, D2D, communication between the first user equipment and one or more other user equipments; and

send to a target network node, using the network interface, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the network node apparatus (1) for D2D communication between the first user equipment and one or more other user equipments.

24. The network node apparatus (1) of claim 23, wherein the processing circuit (10, 20, 30) is further configured to:

in response to determining that handover of the first UE is required, use the radio transceiver to request a second user equipment, which is involved in D2D communications with the first user equipment, using the set-aside set of radio resources, to report measurements of the target network node; and send to the target network node, via the network interface, one or more handover messages for the second user equipment, in response to reported

measurements of the target network node from the second user equipment.

25. The network node apparatus (1) of claim 24, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the network node apparatus (1) for D2D communications between the second user equipment and one or more other user equipments.

26. The network node apparatus (1) of claim 23, wherein the processing circuit (10, 20, 30) is further configured to:

jointly determine that handover is required for the first and second user equipments, based on the reported measurements of the target network node from the second user equipment and further based on measurements of the target network node reported from the first user equipment; and send, to the target network node, one or more handover messages for the second user equipment, in response to reported measurements of the target network node from the second user equipment.

27. A network node apparatus (1) for use in a wireless communication system and adapted to:

receive, from a source network node, one or more handover messages for a first user equipment served by the source network node, the one or more handover messages indicating a set of radio resources set aside by the source network node for device-to-device, D2D, communications between the first user equipment and one or more other user equipments; and set aside the same set of radio resources for use by the first user equipment for D2D communications, in response to the one or more handover messages.

28. The network node apparatus (1) of claim 27, wherein the network node apparatus (1) is further adapted to determine that that the same set of radio resources is available for use in a cell served by the network node apparatus (1) and to set aside the same set of radio resources for use by the first user equipment further in response to said determining.

29. The network node apparatus (1) of claim 27 or 28, wherein the network node apparatus (1) is further adapted to receive, from the source network node, a handover message for a second user equipment, wherein the second user equipment is involved in D2D

communications with the first user equipment.

30. The network node apparatus (1) of any of claims 27-29, wherein the network node apparatus (1) and the source network node are eNodeBs.

31. A network node apparatus (1) for use in a wireless communication system and adapted to:

send, to a target network node, one or more handover messages for the first user equipment, the one or more handover messages indicating the set of radio resources set aside by the network node apparatus (1) for D2D communication between the first user equipment and one or more other user equipments.

32. The network node apparatus (1) of claim 31 , wherein the network node apparatus (1) is further adapted to:

33. The network node apparatus (1) of claim 32, wherein the one or more handover messages for the second user equipment indicate that the same set of radio resources is set aside by the network node apparatus (1) for D2D communications between the second user equipment and one or more other user equipments.

34. The network node apparatus (1) of claim 31 , wherein the network node apparatus (1) is further adapted to:

35. A network node apparatus (1) for use in a wireless communication system and adapted to:

determine that handover is required for a first user equipment that is involved in

device-to-device, D2D, communication with a second user equipment, wherein both the first and second user equipments are served by the network node apparatus (1);

request the second user equipment to measure a target network node;

36. The network node apparatus (1) of claim 35, wherein the network node apparatus (1) is adapted to send, to the target network node, one or more handover messages indicating a set of radio resources set aside by the source network node for D2D communications between the first user equipment and the second user equipment.

37. The network node apparatus (1) of any of claims 31-36, wherein the network node apparatus (1) and the target network node are eNodeBs.

38. The network node apparatus (1) of any of claims 17-37, wherein indicating the set of radio resources comprises identifying or otherwise referring to the set-aside resources.

39. A user equipment apparatus (800) configured for device-to-device, D2D,

communications, the user equipment apparatus (800) comprising:

a radio transceiver (820) configured to communicate with a source network node in a wireless communications network and to communicate with one or more other user equipments via D2D communications and further configured to receive, from the source network node, a command to handover to a target network node in the wireless communications network; and

a processing circuit (810) operatively connected to the radio transceiver (820) and configured to send to a second user equipment, using D2D communications, a message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover.

40. A user equipment apparatus (800) configured for device-to-device, D2D,

communications, the user equipment apparatus comprising:

a radio transceiver (820) configured to communicate with a source network node in a wireless communications network and to communicate with one or more other user equipments via D2D communications and further configured to receive from a first user equipment, via the D2D communications, a message identifying a target network node in the wireless communications network and indicating that the first user equipment is being handed over to the target network node; and, a processing circuit (810) operatively connected to the radio transceiver (820) and configured to perform a measurement of the target network node and report the measurement to the source network node, in response to receiving the message.

41. The user equipment apparatus (800) of claim 40, wherein the processing circuit (810) is further configured to receive, via the radio transceiver (820) and in response to the reported measurement, a command from the source network node to handover to the target network node.

42. A user equipment apparatus (800) configured for device-to-device, D2D,

communications and for operation in a wireless communications network, wherein the user equipment apparatus (800) is adapted to:

send to a second user equipment, using D2D communications, a message identifying the target network node and indicating that the first user equipment is being handed over, in response to receiving the command to handover.

43. A user equipment apparatus (800) configured for device-to-device, D2D,

44. The user equipment apparatus (800) of claim 43, wherein the user equipment apparatus (800) is further adapted to receive, in response to the reported measurement, a command from the source network node to handover to the target network node.

45. The user equipment apparatus (800) of any of claims 39-44, wherein the source network node and the target network node are eNodeBs.