Patent Publication Number: US-11044645-B2

Title: Sidelink-assisted handover in cellular networks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP2016/073342, filed on Sep. 29, 2016, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure, in some embodiments thereof, relates to assisting a handover of a user equipment (UE) between networks infrastructure apparatuses and, more specifically, but not exclusively, to assisting a handover of one or more UEs between networks apparatuses using a sidelink between the UE(s) and one or more other UEs. 
     One of the key technologies introduced by future wireless networks, and in particular cellular networks such as, for example, 5G (5 th  generation) cellular networks, is a capability of the UEs to simultaneously maintain a cellular connection and one or more direct device-to-device (D2D) connections for communicating with other UE(s). 
     The D2D communication may be utilized by a sidelink channel through which the UEs may communicate with one another. The sidelink may be in-band, i.e. share the same frequency band(s) and/or carrier(s) as the cellular network and/or out-of-band, i.e. assigned with frequency band(s) and/or carrier(s) that are not used by the cellular link. 
     The D2D communication may serve a plurality of existing and/or future applications and may address the need to support connectivity and access for the growing multitude of high mobility UEs, for example, vehicular users and/or network enabled vehicles such as V2V (vehicle to vehicle communication) and/or V2X vehicle to anything communication) capable vehicles. 
     SUMMARY 
     According to an aspect of some embodiments of the present invention there is provided a first user equipment, UE, apparatus comprising a processor adapted to execute the following operations to assist a handover of a second UE from a source network apparatus to a target network apparatus:
         Maintain a sidelink connection with the second UE.   Maintain and/or establish a cellular connection, in particular with the target network apparatus.   Report to the target network apparatus a link state information, based on the link state information received from the second UE through the sidelink connection;   Receive from the target network apparatus a first control information calculated based on the link state information.   Send to the second UE through the sidelink connection a second control information, based on the first control information, to assist the handover of the second UE to the target network apparatus.       

     The processor is further adapted to configure the first UE to serve as a relay to provide an uplink and/or a downlink to the second UE through the sidelink connection while the second UE is detached from any of the network apparatuses. 
     The first control information and/or the second control information include one or more of: a timing synchronization information and a connectivity information. Wherein the link state information indicates a status of a cellular link of the UE and includes one or more of: a channel measurements, a connectivity information and/or a timing synchronization information. 
     Optionally, in case the first UE is attached to the source network apparatus, the processor is adapted to detach from the source network apparatus prior to attaching to the target network apparatus. 
     In case the second UE is attached to the source network apparatus, the processor is adapted to maintain the sidelink connection with the second UE while the second UE detaches from the source network apparatus in order to attach to the target network apparatus. 
     Optionally, the processor is adapted to maintain the sidelink connection by disconnecting and reconnecting the sidelink connection with the second UE using the first control information and/or the second control information. 
     Optionally, the processor is adapted to maintain the connection with the second UE through the sidelink connection with at least a third UE. The at least third UE is connected to the second UE through the sidelink connection. 
     According to an aspect of some embodiments of the present invention there is provided a UE attached to a cellular network through a source network apparatus comprising a processor adapted to execute the following operations to assist a first UE to attach to a target network apparatus:
         Maintain a sidelink connection with the first UE.   Maintain and/or establish a cellular connection, in particular with the source network apparatus.   Report to the source network apparatus a link state information, based on the link state information received from the first UE through the sidelink connection.   Receive from the source network apparatus a first control information calculated based on the link state information.   Send to the first UE through the sidelink connection a second control information, based on the first control information, to assist an attachment of the first UE to the target network apparatus.       

     Optionally, the processor is adapted to configure the second UE to serve as a relay to provide an uplink and/or a downlink to the first UE through the sidelink connection while the first UE is detached from any of the network apparatuses. 
     In case the first UE is attached to a source network apparatus, the first UE detaches from the source network apparatus in order to attach to the target network apparatus. 
     Optionally, the processor is adapted to assist the attachment of the first UE to the source network apparatus after the first UE temporarily detaches from the source network apparatus. 
     Optionally, the processor is adapted to maintain the connection with the first UE through the sidelink connection with at least a third UE. The at least third UE is connected to the second UE through the sidelink connection. 
     According to an aspect of some embodiments of the present invention there is provided a network apparatus of a cellular network, comprising a processor adapted to:
         Provide a cellular service to an attached assisting UE connected through a sidelink connection to an assisted UE.   Receive, from the assisting UE, a link state information, based on the link state information of the assisted UE transmitted by the assisted UE to the assisting UE through the sidelink connection.   Transmit to the assisting UE a first control information calculated based on the link state information.   Receive an attach request from the assisted UE to attach using a second control information received by the assisted UE from the assisting UE through the sidelink connection, the second control information is based on the first control information.   Accept the attach request of the assisted UE to provide the cellular service to the assisted UE.       

     Optionally, in case the assisted UE is attached to another network apparatus, the processor is further adapted to negotiate with the other network apparatus a handover of the assisted UE in order to instruct the assisted UE to detach from the other network apparatus while maintaining the sidelink connection with the assisting UE. 
     According to an aspect of some embodiments of the present invention there is provided a method of assisting a handover of an assisted UE from one network apparatus to another network apparatus, comprising:
         Providing, by a network apparatus, a cellular service to an attached assisting UE, the assisting UE is connected through a sidelink connection to an assisted UE;   Receiving from the assisting UE, a link state information, based on the link state information of the assisted UE transmitted by the assisted UE to the assisting UE through the sidelink connection.   Transmitting to the assisting UE a first control information calculated based on the link state information.   Receiving an attach request from the assisted UE to attach to the network apparatus using a second control information received by the assisted UE from the assisting UE, the attach request is forwarded by the assisting UE from the assisted UE to the network apparatus, the second control information is based on the first control information.   Attaching the assisted UE to the network apparatus to provide the cellular service to the assisted UE.       

    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of embodiments of the invention. In this regard, the description taken with the drawings makes apparent to those skilled in the art how embodiments of the invention may be practiced. 
       In the drawings: 
         FIG. 1  is a flowchart of an exemplary process for assisting a handover of a UE between network apparatuses using a sidelink between the UE and at least one other UE, according to some embodiments of the present invention; 
         FIG. 2A  through  FIG. 2F  are schematic illustrations of an exemplary sequence of a sidelink-assisted handover of a UE between network apparatuses, according to some embodiments of the present invention; 
         FIG. 3A  and  FIG. 3B  are schematic illustrations of an exemplary sidelink-assisted handover, in particular during an attach phase for an out of coverage assisting UE, according to some embodiments of the present invention; 
         FIG. 4A  and  FIG. 4B  are schematic illustrations of an exemplary sidelink-assisted handover, in particular during an attach phase for an out of coverage assisted UE, according to some embodiments of the present invention; 
         FIG. 4C  is a schematic illustration of an exemplary sidelink-assisted handover, in particular following an attach phase for an out of coverage assisted UE, according to some embodiments of the present invention; 
         FIG. 5  is a flowchart of an exemplary process for assisting a handover of a UE between network apparatuses, in particular following a detach phase of the UE from a network apparatuses using a sidelink between the UE and at least one other UE, according to some embodiments of the present invention; 
         FIG. 6A  and  FIG. 6B  are schematic illustrations of an exemplary sidelink-assisted handover, in particular following a detach phase for an out of coverage assisted UE, according to some embodiments of the present invention; 
         FIG. 7A  and  FIG. 7B  are schematic illustrations of an exemplary sidelink-assisted attachment, in particular following a temporary detachment of the assisted UE, according to some embodiments of the present invention; 
         FIG. 8  is a flowchart of an exemplary process of a network apparatus handing over one or more UEs connected by a sidelink, according to some embodiments of the present invention; 
         FIG. 9  is a schematic illustration of an exemplary interaction during a sequence of an exemplary sidelink-assisted handover initiated by a UE, according to some embodiments of the present invention; 
         FIG. 10  is a schematic illustration of an exemplary interaction during a sequence of an exemplary sidelink-assisted handover initiated by a network apparatus, according to some embodiments of the present invention; 
         FIG. 11  is a schematic illustration of an exemplary system for assisting a handover, in particular during an attach phase, of a plurality of UEs between network infrastructure apparatuses, according to some embodiments of the present invention; 
         FIG. 12  is a schematic illustration of an exemplary system for assisting a handover, in particular during following a detach phase, of a plurality of UEs between network infrastructure apparatuses, according to some embodiments of the present invention; 
         FIG. 13  is a schematic illustration of an exemplary UE, according to some embodiments of the present invention; and 
         FIG. 14  is a schematic illustration of an exemplary network infrastructure apparatus, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure, in some embodiments thereof, relates to assisting a handover of a user equipment (UE) between networks infrastructure apparatuses and, more specifically, but not exclusively, to assisting a handover of one or more UEs between networks apparatuses using a sidelink between the UE(s) and one or more other UEs. 
     The present disclosure presents systems and methods for handing over one or more UEs, for example, a cellular phone, a cellular apparatus, a mobile device, a vehicular cellular user, an endpoint and/or the like from a source network apparatus to a target network apparatus with assistance of one or more UEs communicating with the handed over UE(s) over a sidelink channel. The assisted handover is directed towards providing an ultra-reliable low-latency communication (URLLC) for the UEs connected to a wireless network, in particular a cellular network, for example, a 5G (5 th  generation) cellular network and/or the like. The network apparatus may be, for example, a node B, an evolved Node B (eNB), a base station, a radio network controller (RNC) and/or the like. The assisting UE that is attached to an attached network apparatus, for example, the source network apparatus or the target network apparatus assists the handover by relaying information between the attached network apparatus and the (assisted) handed over UE(s) through the sidelink channel, for example, D2D communication supported by, for example, the 5G cellular network. The assisting UE may receive link state information from the assisted UE(s), for example, channel measurements information and/or connectivity state information and may relay it to the attached network apparatus. The link state information may indicate and/or relate to among other indications, a quality and/or strength of the cellular signal and may include, for example, a signal arrival/departure time and/or angle, timing difference between two signals, a reference signal received power (RSRP), a reference signal received quality (RSRQ) and other measurement related to the cellular channel. The link state information of the assisted UE may assist the attached network apparatus to identify connectivity condition(s) of the assisted UE(s) to determine if and/or when the assisted UE(s) may be handed over. The assisting UE may further relay timing and/or control information from the attached network apparatus to the assisted UE. The timing information may include, for example, timing advance (TA), an offset from a time base provided by for example, a global positioning system (GPS), a global navigation satellite system (GNSS), a coordinated universal time (UTC) and/or the like. This may allow the assisted UE(s) to synchronize to the timing provided by the target network apparatus even before the assisted UE(s) are actually attached to the target network apparatus. The assisting UE may be further adapted to relay uplink and/or downlink data from the attached network apparatus to one or more of the assisted UE(s) while the assisted UE(s) are not attached to any network apparatus, for example, the assisted UE(s) are out of coverage. 
     The assisted handover may present significant advantages. One of the major problems inherent to the cellular network(s) is a latency problem during the handover of the UE(s) from the source network apparatus to the target network apparatus. Prior to the handover process, the migrating UE(s) that need to be handed over are synchronized according to timing synchronization information provided by the source network apparatus. However, during the handover the handed over UE(s) may need to synchronize according to the timing synchronization information provided by the target network apparatus. This may be required since typically the network apparatuses, for example, the source and the target network apparatuses are not synchronized with each other in a cellular networks using frequency division duplex (FDD), even for network apparatuses serving adjacent cells (coverage areas). Timing synchronization may be essential to avoid timing synchronization and coexistence problems that may result from the fact that the cellular connection for the uplink/downlink and the sidelink may need to coexist while sharing the same frequency band(s) and/or carrier(s) (in-band). For example, a cellular network using orthogonal frequency division multiplexing (OFDM) waveforms may suffer from inter-symbol interference (ISI) if time offsets of signals transmitted from the UE to the receiving network infrastructure apparatus exceed a predefined threshold level that may typically be duration of the cyclic prefix (CP). This may result from signals overlapping “over the air” and, even if the individual times of arrival may be detected, the signals cannot be processed separately. At the same time, these misalignments cause inter-carrier interference (ICI), as the circular convolution properties between signal and channel are affected in such way that subcarriers&#39; orthogonality and/or other alignment of the subcarriers&#39; is violated. 
     During the handover sequence, a significant amount of control, timing and/or signaling information may need to be exchanged between the source network apparatus and the target network apparatus in order to provide cellular service to UE, for example, assign a TA index, assign cellular resources and/or the like. The information exchange may introduce significant delays. Moreover, during the handover, the timing information that needs to be provided to the handed over UE(s) that may lead to additional delays. This may result in unacceptable latency, interruptions and/or failures of the cellular service provided to the handed over UE(s). This may significantly affect the serviceability of the cellular network and make it inappropriate to serve URLLC applications, for example, safety applications, emergency applications and/or the like. 
     The assisted handover using the sidelink may expedite the handover process by allowing the handed over UE(s) to adapt early to the timing of the target network apparatus. As the assisting UE(s) is attached to the network apparatus, either the target or the source network apparatus, the assisting UE(s) may forward the timing synchronization information from the attached network apparatus to the handed over UE(s). The handed over UE(s) may therefore adopt the timing of the target network apparatus even before they request to attach and once instructed to attach and/or sensing a significant cellular signal from the target network apparatus may rapidly attach to it. Through the early adoption of the timing provided by the target network apparatus by the handed over UE(s), the cellular sidelink D2D communication may become highly preferable for URLLC applications. With the handed over UE synchronizing to the timing of the target network apparatus prior to the actual handover and/or attachment, the latency involved with the handover may be significantly reduced to meet the quality of Service (QoS) and/or URLLC requirements of the URLLC applications. 
     Additionally, the assisting UE(s) may convey the link state information of the out of coverage assisted UE(s) to the attached network apparatus. The attached network apparatus may then decide, based on the conveyed link state information, when and whether to initiate the detachment, the handover and/or the attachment of the handed over UE(s). 
     Moreover, while the handed over UE(s) are out of coverage and/or are not served by any of the network apparatuses, they may remain synchronized with the timing information originating from the attached network apparatus that is received through the assisting UE(s). This may allow the out of coverage handed over UE(s) to rapidly attach and/or reconnect to the attached network apparatus. 
     Another problem of the cellular network(s), in particular during the handover, is the loss of service after the migrating UE(s) detach from the source network apparatus and before they attach to the target network apparatus. 
     The assisted handover may significantly reduce the impact of this problem and possibly overcome it all together since while the handed over UE(s) are not attached to any of the network apparatuses they may still be served with the uplink and/or downlink connection relayed through the assisting UE(s). 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and/or methods set forth in the following description and/or illustrated in the drawings and/or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways. 
     Embodiments of the present invention may be a system, a method, and/or a computer program product. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention. 
     The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. 
     Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. 
     The computer readable program instructions may execute entirely on the user&#39;s computer, partly on the user&#39;s computer such as the user equipment (UE), as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer such as the network apparatus or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, field-programmable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instructions by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention. 
     Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions. 
     According to some embodiments of the present invention, there are presented systems and methods for assisting a handover of one or more UEs between a source network apparatus and a target network apparatus using a sidelink connecting the UEs. The assisted handover may typically be directed towards assisting the handover of a plurality of UEs migrating from a source coverage area serviced by the source network apparatus to a target coverage area serviced by the target network apparatus. Furthermore, the UEs may be migrating gradually from one coverage area to the other, for example, one or more of the UEs (a first UEs) may leave the source coverage area earlier than a one or more other UEs (a second UEs), the first UE(s) may enter the target coverage area earlier than the second UE(s) and/or the like. 
     The assisted handover may be applied for a plurality of UEs and/or network apparatuses deployment scenarios, some of which are described hereinafter. However, the presented embodiments should not be construed as limiting since the assisted handover may apply for a plurality of other deployment scenarios. Moreover, the assisted handover procedure presented herein may be executed for assisting the assisted UE(s) during only part of the handover process, for example, the attach phase and/or the detach phase. 
     In some embodiments of the present invention, during the assisted handover, the UEs may switch roles. One or more of the UEs may perform as an assisting UE(s) to assist the assisted UE(s) during one or more phases of the handover, for example, the detach phase and/or the detach phase and may become the assisted UE(s) during another phase of the handover. The same may happen also in the other direction, in which one or more UEs that are assisted UE(s) during one or more phases of the handover phases may become the assisting UE(s) during other phase(s) of the handover. 
     According to some embodiments there are presented two main modes of assisted handover of one or more UEs between a source network apparatus and a target network apparatus. In the first mode, the handover may be initiated by one or more of the UEs while in the second mode the handover may be initiated by a network apparatus. 
     Reference is now made to  FIG. 1 , which is a flowchart of an exemplary process for assisting a handover of a UE between network apparatuses using a sidelink between the UE and at least one other UE, according to some embodiments of the present invention. A process  100  may be executed by one or more assisting UEs, for example, the first UE to assist a handover of one or more assisted UEs, for example, the second UE from a source network apparatus to a target network apparatus, both network apparatuses providing connectivity to a wireless network, in particular a cellular network. In particular, the process  100  may be applied by the first UE(s) once attached to the target network apparatus, to assist the second UE(s) to attach to the target network apparatus while the second UE migrates from the source coverage area to the target coverage area. For some deployment scenarios of the UEs  204  all steps of the process  100  while for other deployment scenarios some steps may not be applicable as described herein after. 
     For brevity, the process  100  is described for one assisting UE assisting the handover of one assisted UE. The process  100  however may be extended to assist the handover of a plurality of additional assisted UEs. 
     The first UE performing as the assisting UE may attach to the target network apparatus while maintaining a sidelink communication channel with the second UE that is the assisted UE throughout the entire handover process. The sidelink is a communication channel through which the UEs communicate with each other such as, for example, D2D communication. The first UE assists the second UE during the handover by forwarding information from the target network apparatus to the second UE and vice versa. The first UE may forward, for example, timing synchronization information and/or control information transmitted from the target network apparatus to the second UE. In the other direction, the first UE may collect, for example, link state information from the second UE and report the link state information to the target network apparatus. The timing synchronization information may include, for example, TA, an offset from a time base provided by, for example, a GPS, a GNSS, a UTC and/or the like. The control information may include, for example, instructions issued by the network apparatus to the UE(s), for example, sidelink maintain/discard, initiate attachment, initiate detachment and/or the like. The link state information may include, for example, channel measurement(s) that may indicate among other indications, a quality and/or strength of the cellular signal and may include, for example, a signal arrival/departure time and/or angle, a timing difference between two signals, an RSRP, an RSRQ and other measurement related to the cellular channel. The link state information may further include connectivity indications of the respective UE, for example, attach/detach information with respect to a respective network apparatus, quality of service (QoS) and/or the like. The first UE that periodically reports its link state information to the target network apparatuses may include the link state information received from the second UE to the reported information. 
     Through the information forwarded back and forth between the target network apparatus and the second UE, the target network apparatus may be aware of the link state of the second UE and the second UE may be exposed to the timing synchronization and/or control information provided by the target network apparatus. The availability of the link state information from the second UE combined with the early adoption of the target network apparatus timing by the second UE prior to the attachment may lead to a rapid, smooth and/or efficient handover. 
     Reference is also made to  FIG. 2A  through  FIG. 2F , which are schematic illustrations of an exemplary sequence of a sidelink-assisted handover of a UE between network apparatuses, according to some embodiments of the present invention. Depending on the various deployment scenarios, some phases of the presented sequence may occur during the assisted handover while other phase may not take place as is described herein after. The wireless network is provided by a universal terrestrial radio access network (UTRAN) comprising a plurality of network apparatuses, for example, a node B, an evolved Node B (eNB), a base station, a radio network controller (RNC) and/or the like connecting to a core of the wireless network. The UTRAN may further include one or more of a plurality of other network infrastructure elements, for example, Mobility Management Entities (MME), Serving Gateways (SGW) and/or the like. Typically, the network infrastructure apparatuses are not synchronized with each other, i.e. they may have different timing with respect to each other. However, there may be scenarios when two or more of the network infrastructure apparatuses have a common time base. 
     Two network apparatuses  202 A and  202 B each provide coverage for the wireless network in coverage areas  212 A and  212 B respectively. The network apparatus  202 A is designated herein after as a source network apparatus  202 A while the network apparatus  202 B is designated herein after as a target network apparatus  202 B. A plurality of UEs  204 , in particular a first UE, UE  204 A, and a second UE, UE  204 B, may connect to the cellular network through an uplink and/or downlink cellular connections to the network apparatuses  202 A or  202 B when located within the coverage area  212 A and  212 B respectively. Each of the UEs  204  may be, for example, a cellular phone, a cellular apparatus, a mobile device, an endpoint, a vehicular cellular user, a network enabled vehicle (V2X and/or V2V capable vehicle) and/or the like. 
     The UEs  204  are D2D enabled allowing each of the UEs  204  to communicate directly with one or more of the other UEs  204  over the sidelink channel (referred to as sidelink herein after). The sidelink may be utilized in-band with respect to the cellular connection, i.e. the sidelink communication occupies a frequency band(s) and/or carrier(s) used by the cellular connection, typically the uplink connection. Optionally, the sidelink is out-of-band with respect to the cellular connection, i.e. the sidelink communication is conducted in a frequency band(s) and/or carrier(s) not used by the cellular connection. 
     As shown at  102 , the UE  204 A (the first UE) that is the assisting UE maintains a sidelink with the UE  204 B (the second UE) that is the assisted UE. Maintaining the sidelink between the UE  204 A and the UE  204 B may require setting up the sidelink by connecting and/or reconnecting the sidelink during the assisted handover process  100 . However, through the entire process  100  the UE  204 A continuously maintains the sidelink with the UE  204 B such that the UE  204 A and UE  204 B may communicate with each other throughout the process  100  as shown in  FIG. 2A  through  FIG. 2E . 
     In some embodiment of the present invention, the UE  204 A is initially attached to the source network apparatus  202 A as shown in  FIG. 2A . In the deployment scenario shown in  FIG. 2A , the UE  204 A and the UE  204 B may maintain the sidelink since they are both connected to the source network apparatus  202 A, and receive their timing information, for example, the TA from the source network apparatus  202 A. The UE  204 A and the UE  204 B are therefore timing synchronized with each other and may maintain the sidelink (in particular the in-band sidelink) with each other simultaneously with their cellular connection to the source network apparatus  202 A as shown in  FIG. 2A . 
     In some embodiments of the present invention, the UE  204 A is not attached to any network apparatus  202 , for example, the UE  204 A is out of coverage. However, while the UE  204 A is out of coverage, the  204 A maintains the sidelink with the UE  204 B either directly or through one or more intermediate UEs  204 . The intermediate UEs  204  may form a chain for connecting the UE  240 A with the UE  204 B. 
     Reference is now made to  FIG. 3A  and  FIG. 3B , which are schematic illustrations of an exemplary sidelink-assisted handover, in particular during an attach phase for an out of coverage assisting UE, according to some embodiments of the present invention. An assisting UE such as the UE  204 A is out of coverage, i.e. not connected to any network apparatus such as the network apparatus  202 , in particular, the UE  204 A is not connected to either one of the source network apparatus  202 A and the network apparatus  202 B. The UE  204 A maintains a sidelink with the UE  204 B that may be connected to the source network apparatus  202 A as shown in  FIG. 3A . Optionally, the UE  204 A may maintain the sidelink with the UE  204 B through one or more intermediate UEs  204 , for example, a UE  204 C that maintains the sidelink with both the UE  204 A and the UE  204 B as shown in  FIG. 3B . The UE  204 C may forward messages such as, for example, link state information, uplink data and/or the like from the UE  204 A to the UE  204 B and vice versa, i.e. timing/control information, downlink data and/or the like originating from the source network apparatus  202 A and relayed through the UE  204 B. 
     The UE  204 A and the UE  204 B may maintain the sidelink since the UE  204 A is out of coverage and receives its timing synchronization information from the UE  204 B that in turn receives the timing synchronization information from the source network apparatus  202 A. Since both the UE  204 A and the UE  204 B use timing synchronization information, for example, the TA originating from the source network apparatus  202 A, the UE  204 A and the UE  204 B are timing synchronized with each other. This allows the UE  204 B to maintain the sidelink (in particular the in-band sidelink) with the UE  204 A simultaneously with its cellular connection to the source network apparatus  202 A as shown in  FIG. 3A . The same applies to the deployment scenario shown in  FIG. 3B  where both the UE  204 A and the UE  204 C are out of coverage and use timing synchronization information originating from the source network apparatus  202 A received through the UE  204 B. 
     As the UE  204 B is still attached to the source network apparatus  202 A as shown in  FIG. 3A  and/or  FIG. 3B , based on instructions received from the source network apparatus  202 A, the UE  204 B may serve as a relay for the UE  204 A while the UE  204 A is out of coverage. As a relay the UE  204 B may convey the link state information from the UE  204 A to the source network apparatus  202 A. In the other direction, as discussed before, the UE  204 B may forward the timing synchronization and/or control information from the source network apparatus  202 A to the UE  204 A. Assuming the source network apparatus  202 A has sufficient cellular resources to serve the UE  204 A, the UE  204 B may be instructed by the source network apparatus  202 A to further perform as the relay to transfer uplink and/or downlink data between the source network apparatus  202 A and the UE  204 A. The same applies in case the UE  204 A is connected to the UE  204 B through the intermediate UE(s)  204 , for example, the UE  204 C that performs as a relay between the UE  204 A and the UE  204 B. 
     Reference is made once again to  FIG. 1 . 
     As shown at  104 , which is a decision point, the UE  204 A may take different steps depending on its cellular connectivity state. In case the UE  204 A is attached to the source network apparatus  202 A as shown in  FIG. 2A , the process  100  branches to  106 . In case the UE  204 A is not attached to the source network apparatus  202 A, for example, the UE  204 A is out of coverage as shown in  FIG. 3A  and/or  FIG. 3B , the process  100  branches to  108 . 
     As shown at  106 , the UE  204 A detaches from the source network apparatus  202 A i.e. disconnects the cellular connection with the source network apparatus  202 A as shown in  FIG. 2B . The detachment of the UE  204 A from the source network apparatus  202 A may be initiated by the UE 204 A and/or by the source network apparatus  202 A. The UE  204 A may request to detach due to, for example, a low signal of the cellular connection, for example, in case a signal interference plus noise ratio (SINR) is below a predefined threshold level. Alternatively and/or additionally, the UE  204 A may detach automatically without requesting the source network apparatus  202 A to detach, for example, in case the SINR is below the predefined threshold level. The UE  204 A may further initiate the detachment in case it detects one or more other network apparatuses, in particular the target network apparatus  202 B presenting a better cellular connection signal, for example, a better SINR and/or the like. In case the detachment is initiated by the source network apparatus  202 A the detachment instruction to the UE  204 A may result from, for example, a poor cellular connection to the UE  204 A and/or cellular resource(s) are unavailable at the source network apparatus  202 A for serving the UE  204 A. The source network apparatus  202 A may also initiate the detachment following negotiations with one or more other network apparatuses, in particular the target network apparatus  202 B that picks up the cellular signal from the UE  204 A. The poor cellular signal may result from, for example, the UE 204 A moves out of the coverage area  212 A as shown in  FIG. 2B . Complementary, the detection of a cellular signal from the target network apparatus  202 B may result from, for example, the UE  204 A advancing into the coverage area  212 B as shown in  FIG. 2B . 
     The UE  204 A maintains the sidelink such that the UE  204 A remains connected to the UE  204 B during and following the detachment from the source network apparatus  202 A. 
     The UE  204 B is still attached (connected) to the source network apparatus  202 A as shown in  FIG. 2B . Based on instructions received from the source network apparatus  202 A, the UE  204 B may serve as a relay for the UE  204 A while the UE  204 A is detached from any network apparatus  202 , in particular, the source network apparatus  202 A and the target network apparatus  202 B. As a relay the UE  204 B may convey the link state information from the UE  204 A to the source network apparatus  202 A. In the other direction, the UE  204 B may forward the timing synchronization and/or control information from the source network infrastructure apparatus  202 A to the UE  204 A. Assuming the source network apparatus  202 A has sufficient cellular resources to serve the UE  204 A, the UE  204 B may be instructed by the network apparatus  202 A to further perform as the relay UE  204 B to transfer uplink and/or downlink data between the source network apparatus  202 A and the UE  204 A. Naturally, the UE  204 B may serve as a relay only while the UE  204 B is in the coverage area  212 A and connected to the source network apparatus  202 A. 
     A shown at  108 , the UE  204 A attaches, i.e. establishes a cellular connection to the target network apparatus  202 B by requesting to attach, to the target network apparatus  202 B when detecting a sufficient cellular signal from the target network apparatus  202 B, for example, when entering the coverage area  212 B as shown in  FIG. 2C . 
     As shown at  110 , the UE  204 A reports to the target network apparatus  202 B the link state information received from the UE  204 B through the sidelink such that the target network apparatus is made aware of the presence of the UE  204 B and its link state information. The UE  204 A may forward the link state information of the UE  204 B to the target network apparatus  202 B while reporting its own link state information, e.g. periodically, as part of the cellular communication protocol. 
     Based on the reported link state information of the UE  204 B, the target network apparatus  202 B may evaluate one or more actions that may be taken with respect to the UE  204 B, for example, evaluating a handover of the UE  204 B to the target network apparatus  202 B and/or the like. Optionally, in case the UE  204 B is attached to the source network apparatus  202 A, the target network apparatus  202 B may initiate a negotiation session with the source network apparatus  202 A as part of the evaluation. The evaluation made by the target network apparatus  202 B may consider availability of cellular resources of the target network apparatus  202 B required to serve the UE  204 B, a timing synchronization information available from the UE  204 B and/or the like. The target network apparatus  202 B may further negotiate with the source network apparatus  202 A as part of the evaluation. 
     As shown at  112 , the UE  204 A may receive a first control information from the target network apparatus  202 B. The first control information calculated by the target network apparatus  202 B may be based on the evaluation conducted by the target network apparatus  202 B based on the reported link state information of the UE  204 B. The first control information may include timing synchronization information, for example, the TA, for the UE  204 A and/or for the UE  204 B. The first control information may further include control information for the UE  204 A and/or for the UE  204 B, for example, instructing the UE  204 A to provide an uplink/downlink connection for the UE  204 B, instructing the UE  204 A to maintain the sidelink, instructing the UE  204 A to terminate the sidelink, instructing the UE  204 B to attach to the target network apparatus  202 B, instructing the UE  204 B to detach from the source network apparatus  202 A and/or the like. 
     The first control information may be derived from an evaluation of one or more of a plurality of decision conditions and/or criteria. The decision conditions and/or criteria may include, for example, value(s) of the link state information provided by the UE  204 B, cellular and/or sidelink communication requirement(s), availability of cellular resources at the source network apparatus  202 A and/or the target network apparatus  202 B and/or the like. Furthermore, the target network apparatus  202 B may negotiate with one or more of the other network apparatus, in particular, the source network apparatus  202 A in order to explore the decision conditions and/or criteria. 
     As shown at  114 , the UE  204 A transmits a second control information to the UE  204 B. The second control information may be based on the first control information received from the target network apparatus  202 B. The second control information may include timing synchronization information, for example, the TA, for the use by the UE  204 B. The second control information may further include control information for the UE  204 B, for example, instructing the UE  204 B to attach to the target network apparatus  202 B, instructing the UE  204 B to detach from the source network apparatus  202 A and/or the like. 
     In some embodiment of the present invention, the UE  204 B is not attached (disconnected) to any network apparatus, for example, the UE  204 B is out of coverage. The out of coverage UE  204 B maintains the sidelink with the UE  204 A either directly and/or or through one or more intermediate UEs  204 . The intermediate UEs  204  may form a chain for connecting the UE  240 B with the UE  204 A. 
     Reference is now made to  FIG. 4A  and  FIG. 4B , which are schematic illustrations of an exemplary sidelink-assisted handover, in particular during an attach phase for an out of coverage assisted UE, according to some embodiments of the present invention. An assisted UE such as the UE  204 B is out of coverage, i.e. not attached (connected) to any network apparatus such as the network apparatus  202 , in particular, the UE  204 B is not connected to either one of the source network apparatus  202 A and the network apparatus  202 B. The UE  204 B maintains the sidelink with the UE  204 A attached to the target network apparatus  202 B as shown in  FIG. 4A . Optionally, the UE  204 B may maintain the sidelink with the UE  204 A through one or more intermediate UEs  204 , for example, a UE  204 C that maintains the sidelink with both the UE  204 A and the UE  204 B as shown in  FIG. 4B . The UE  204 C may forward messages from the UE  204 A to the UE  204 B and vice versa. 
     The UE  204 A and the UE  204 B may maintain the sidelink since the UE  204 B is out of coverage and receives its timing synchronization information from the UE  204 A that in turn receives the timing synchronization information from the target network apparatus  202 B. Since both the UE  204 A and the UE  204 B use timing synchronization information, for example, the TA originating from the target network apparatus  202 B, the UE  204 A and the UE  204 B are timing synchronized with each other. This allows the UE  204 A to maintain the sidelink (in particular the in-band sidelink) with the UE  204 B simultaneously with its cellular connection to the target network apparatus  202 B as shown in  FIG. 4A . The same applies to the deployment scenario shown in  FIG. 4B  where both the UE  204 B and the UE  204 C are out of coverage and use timing synchronization information originating from the target network apparatus  202 B received through the UE  204 A. 
     At this stage the UE  204 A is already attached (connected) to the target network apparatus  202 B as shown in  FIG. 4A  and/or  FIG. 4B . Based on instructions received from the target network apparatus  202 B, the UE  204 A may serve as a relay for the UE  204 B while the UE  204 B is out of coverage. As a relay the UE  204 A may convey the link state information from the UE  204 B to the target network apparatus  202 B. In the other direction, as discussed before, the UE  204 A may forward the timing synchronization and/or control information from the target network apparatus  202 B to the UE  204 B. Assuming the target network apparatus  202 BA has sufficient cellular resources to serve the UE  204 B, the UE  204 A may be instructed by the target network apparatus  202 B to further perform as the relay to transfer uplink and/or downlink data between the target network apparatus  202 B and the UE  204 B. The same applies in case the UE  204 B is connected to the UE  204 A through the intermediate UE(s)  204 , for example, the UE  204 C that performs as a relay between the UE  204 B and the UE  204 A. 
     Reference is made once again to  FIG. 1 . 
     As shown at  116 , which is a decision point, the process  100  may branch to different paths depending on the cellular connectivity state of the UE  204 B. In case the UE  204 B is attached to a network apparatus  202 , in particular the source network apparatus  202 A as shown in  FIG. 2C , the process  100  branches to  118 . In case the UE  204 B is not attached to any source network apparatus  202 , for example, the UE  204 A is out of coverage as shown in  FIG. 4A  and/or  FIG. 4B , the process  100  branches to  124 . 
     As shown at  118 , which is another decision point, the UE  204 B connected to the source network apparatus  202 A as shown in  FIG. 2C , forwards to the source network apparatus  202 A the second instruction information received through the UE  204 A. In particular, the instruction of significance is the instruction (originating from the target network apparatus  202 B) to maintain the sidelink with the UE  204 A and attach to the target network apparatus  202 B. 
     The source network apparatus  202 A may instruct the UE  204 B to take one of several actions with respect to reconnecting and/or maintaining the sidelink with the UE  204 A and/or a handover of the UE  204 B to the network apparatus  202 B. The decision on the type of action may be based on one or more of a plurality of decision conditions and/or criteria. Furthermore, the source network apparatus  202 A may negotiate with one or more of the other network apparatus, in particular, the target network apparatus  202 B in order to explore the decision conditions and/or criteria. 
     A first action taken by the source network infrastructure apparatus  202 A may be instructing the UE  204 B to detach, i.e. terminate the cellular connection with the source network apparatus  202 A and maintain the sidelink with the UE  204 A. In this case, the process  100  branches to  124 . 
     A second action taken by the source network infrastructure apparatus  202 A may be performing a direct handover of the UE  204 B to the target network infrastructure apparatus  202 B. Naturally, the direct handover based on the negotiation made between the source network apparatus  202 A and the target network apparatus  202 B. The direct handover may take place, for example, in case the cellular signal detected by the UE  204 B from the target network apparatus  202 B is sufficiently strong and the target network apparatus  202 B has the cellular resources to support the UE  204 B. In this case, the process  100  branches to  122 . 
     A third action taken by the source network apparatus  202 A may be instructing the UE  204 B to maintain the cellular connection with the source network apparatus  202 A (remain attached) and terminate the sidelink with the UE  204 A. In this case, the process  100  branches to  120 . 
     As shown at  120 , following the instruction from the source network infrastructure apparatus  202 A, the UE  204 B terminates the sidelink with the UE  204 A and remains attached to the source network infrastructure apparatus  202 A as shown in  FIG. 2F . Naturally, the UE  204 B is not handed over. 
     As shown at  122 , the UE  204 B is directly handed over from the source network apparatus  202 A to the target network apparatus  202 B. During the direct handover the UE  204 B may maintain the sidelink with the UE  204 A. Maintaining the sidelink may assist the UE  204 B to quickly adapt to the timing synchronization information originating from the target network apparatus  202 B. 
     As shown at  124 , following the instruction from the source network apparatus  202 A, the UE  204 B detaches from the source network apparatus  202 A, i.e. disconnects the cellular connection with the source network apparatus  202 A as shown in  FIG. 2D . 
     As shown at  126 , the UE  204 B may attach to the target network apparatus  202 B when the UE  204 B is in the coverage area  212 B of the target network apparatus  202 B as shown in  FIG. 2E . The UE  204 A may use the second control information, for example, the timing synchronization information originating from the target network apparatus  202 B and forwarded by the UE  204 A for attaching to the target network apparatus  202 B. The UE  204 B is synchronized to the timing of the target network apparatus  202 B provided through the second control information. As result, the UE  204 B attachment process to the target network apparatus  202 B may be rapid and/or smooth since the UE  204 B is already adapted to the timing of the target network apparatus  202 B. The attachment process is made efficient as the UE  204 B may reduce and/or avoid time-consuming synchronization sequences that are typically required for the UE  204 B to adapt to the timing of the target network apparatus  202 B. 
     The UE  204 B may attach to the target network apparatus  202 B immediately after detaching from the source network apparatus  202 A as shown in  FIG. 2E  in case the UE  204 B is already in the coverage area  212 B when detaching from the source network apparatus  202 A. However, the UE  204 B may not necessarily attach to the target network apparatus  202 B after detaching from the source network apparatus  202 A, for example, the UE  204 B is out of coverage as shown in  FIG. 4A  and/or  FIG. 4B . While the UE  204 B is out of coverage the UE  204 B may maintain the sidelink with the UE  240 A (directly and/or through the UE  204 C). While the UE  204 B is out of coverage, the UE  204 A may serve as the relay for transferring timing, control, and/or data between the target network infrastructure apparatus  202 B and the UE  204 B. When the UE  204 B enters the coverage area  212 B, the UE  204 B may attach to the target network apparatus  202 B as presented herein above. 
     In case the UE  204 B maintained the sidelink with the UE  204 A through the intermediate UE(s)  204  before attaching to the target network apparatus  202 B, the UE  204 B may maintain the sidelink with the intermediate UE(s)  204  also after attaching to the target network apparatus  202 B. 
     Reference is now made to  FIG. 4C , which is a schematic illustration of an exemplary sidelink-assisted handover, in particular following an attach phase for an out of coverage assisted UE, according to some embodiments of the present invention. An assisting UE such as the assisting UE  204 A assists the attachment of an assisted UE such as the assisted UE  204 B to a network apparatus such as the target network apparatus  202 B. During the assisted handover process  100 , the UE  204 B maintains the sidelink with the UE  204 A through one or more intermediate UEs, for example, the UE  204 C. After the UE  204 B attaches to the target network apparatus  202 B, the UE  204 B may still maintain the sidelink with the UE  204 C. Optionally, the UE  204 A may also maintain the sidelink with the UE  204 C. The sidelink may be maintained since the UE  204 C is out of coverage and may use timing information synchronization originating from the target network apparatus  202 B that may be received from the UE  204 A and/or the UE  204 B as a control information such as the second control information. 
     During the process  100  there may be one or more sessions in which the UE  204   a  and the UE  204 B may need to perform a sidelink disconnect followed by an immediate sidelink reconnect in order to maintain the sidelink due to timing synchronization problems between the UE  204 A and the UE  204 B. The disconnect/reconnect session may be required, for example, after the UE  204 A attaches to the target network apparatus  202 B and while the UE  204 B may still be attached to the source network apparatus  202 A. Another disconnect/reconnect session may be required after the UE  204 B detaches from the source network apparatus  202 A and the UE  204 A is already attached to the target network apparatus  202 B. 
     The timing synchronization problems may result from the fact that typically, the source network apparatus  202 A and the target network apparatus  202 B may not be synchronized. As result, a timing offset may develop between the UE  204 A using timing information originating from the target network apparatus  202 B and the UE  204 B using timing information originating from the source network apparatus  202 A. Therefore, in order to maintain the sidelink, the sidelink may need to be disconnected and immediately reconnected using different timing synchronization information that may be shared between the UE  204 A and the UE  204 B. For example, the sidelink between the UE  204 A and the UE  204 B may be disconnected and immediately reconnected using timing synchronization information originating from the target network apparatus  202 B and relayed by the UE  204 A to the UE  204 B. 
     However, throughout the entire handover process  100 , the sidelink between the UE  204 A and the UE  204 B is maintained with some very short disconnect/reconnect session(s) such that from a functional point of view, the sidelink is maintained during the entire handover process  100 . The synchronization problems may be of lesser impact in case the sidelink is out-of-band, as the sidelink may be maintained for higher levels of the timing offset. In case the source network apparatus  202 A and the target network apparatus  202 B are synchronized, the synchronization problem may be significantly reduced and typically removed such that the sidelink may be maintained during the entire handover process  100  with no synchronization offsets. 
     Reference is now made to  FIG. 5 , which is a flowchart of an exemplary process for assisting a handover of a UE between network apparatuses, in particular following a detach phase of the UE from a network apparatuses using a sidelink between the UE and at least one other UE, according to some embodiments of the present invention. 
     Reference is also made once again to  FIG. 2A ,  FIG. 2B ,  FIG. 2C ,  FIG. 3A  and  FIG. 3B . 
     A process  500  may be executed by one or more assisting UEs, for example, the second UE, UE  204 B, to assist a handover of one or more assisted UEs, for example, the first UE, UE  204 A, from a source network apparatus such as the source network apparatus  202 A to a target network apparatus such as the source network apparatus  202 A. In particular, the process  500  is executed by the UE  204 B that is attached to the source network apparatus  202 A to assist the UE  204 A after detaching from the source network apparatus  202 A. The process  500  may be applied by the UE  204 B during one or more scenarios, for example, the UE  204 A migrates out of a source coverage area such as the coverage area  212 A towards a target coverage area such as the coverage area  212 B as shown in  FIG. 2A . In another one or more scenarios, the UE  204 B may execute the process  500  to assist the UE  204 A to reconnect to the source network apparatus  202 A after detaching from the source network apparatus  202 A, for example, as result of the UE  204 A leaving temporarily the coverage area  212 A. For some deployment scenarios of the UEs  204  all steps of the process  500  while for other deployment scenarios some steps may not be applicable as described herein after. 
     For brevity, the process  500  is described for assisting the handover of one assisted UE  204 . However, the process  500  may be extended to assist the attachment of a plurality of additional assisted UEs  204 . 
     The UE  204 B is attached (connected) to the source network apparatus  202 A while maintaining the sidelink with the UE  204 A during the handover process. The UE  204 B assists the UE  204 A by forwarding information from the source network apparatus  202 A to the UE  204 A and vice versa. The UE  204 B may forward, for example, the timing synchronization information and/or the control information transmitted from the source network apparatus  202 A to the UE  204 A. In the other direction, the UE  204 B may collect, for example, the link state information from the UE  204 A and report the link state information to the source network apparatus  202 A. The UE  204 B that periodically reports its link state information to the source network apparatuses  202 A may add the link state information received from the UE  204 A to the reported information. 
     As shown at  502 , the UE  204 B (the second UE) now acting as the assisting UE maintains the sidelink with the UE  204 A (the first UE) that is now the assisted UE. Maintaining the sidelink between the UE  204 A and the UE  204 B may require setting up the sidelink by connecting and/or reconnecting the sidelink during the assisted handover process  500 . However, throughout the entire process  500  the UE  204 B continuously maintains the sidelink with the UE  204 B such that the UE  204 A and UE  204 A may communicate with each other as shown in  FIG. 2A  through  FIG. 2C . 
     As shown at  504 , simultaneously to maintaining the connection to the source network apparatus  202 A, i.e. (remains attached), the UE  204 B maintains the sidelink with the UE  204  that may not be attached to any network apparatus  202 , in particular, the UE  204 A is not attached to the source network apparatus  202 A. 
     In some embodiment of the present invention, the UE  204 A is initially attached to the source network apparatus  202 A as shown in  FIG. 2A . The detachment of the UE  204 A from the source network apparatus  202 A may be initiated by the UE 204 A and/or by the source network apparatus  202 A as result, for example, the UE  204 A may leaving the coverage area  212 A. The UE  204 A may request to detach due to, for example, a low signal of the cellular connection, for example, in case the SINR is below a predefined threshold level. Alternatively and/or additionally, the UE  204 A may detach automatically without requesting the source network apparatus  202 A to detach, for example, in case the SINR is below the predefined threshold level. The UE  204 A may further initiate the detachment in case it detects one or more other network apparatuses, in particular the target network apparatus  202 B presenting a better cellular connection signal, for example, a better SINR and/or the like. In case the detachment is initiated by the source network apparatus  202 A the detachment instruction to the UE  204 A may result from, for example, a poor cellular connection to the UE  204 A and/or cellular resource(s) are unavailable at the source network apparatus  202 A for serving the UE  204 A. The source network apparatus  202 A may also initiate the detachment following negotiations with one or more other network apparatuses, in particular the target network apparatus  202 B that picks up the cellular signal from the UE  204 A. The UE  204 A may also initiate the detachment when picking up the cellular signal from the target network apparatus  202 B that may result from, for example, the UE  204 A advancing into the coverage area  212 B as shown in  FIG. 2B . 
     While the UE  204 A is attached to the source network apparatus  202 A the UE  204 B and the UE  204 A may maintain the sidelink as they are both synchronized with other since they are both attached to the source network apparatus and use timing information, for example, the TA provided by the source network apparatus  202 A. 
     In some embodiments of the present invention, the UE  204 A is not attached to any network apparatus  202 , for example, the UE  204 A is out of coverage as shown in  FIG. 3A  and  FIG. 3B . 
     While the UE  204 A is out of coverage and the UE  204 B is attached to the source network apparatus  202 A, the sidelink is maintained between the UE  204 B and the UE  204 A using the timing information originating from the source network apparatus  202 A. The sidelink may be maintained either directly as shown in  FIG. 3A  or through one or more intermediate UEs  204 , for example, the UE  204 C as shown in  FIG. 3B . The intermediate UEs  204  may form a chain for connecting the UE  240 A with the UE  204 B. The UE  204 A and the UE  204 B may maintain the sidelink since the UE  204 A is out of coverage and receives its timing synchronization information from the UE  204 B that in turn receives the timing synchronization information from the source network apparatus  202 A. Since both the UE  204 A and the UE  204 B use timing synchronization information, for example, the TA originating from the source network apparatus  202 A, the UE  204 A and the UE  204 B are timing synchronized with each other. This allows the UE  204 B to maintain the sidelink (in particular the in-band sidelink) with the UE  204 A simultaneously with its cellular connection to the source network apparatus  202 A as shown in  FIG. 3A . The same applies to the deployment scenario shown in  FIG. 3B  where both the UE  204 A and the UE  204 C are out of coverage and use timing synchronization information originating from the source network apparatus  202 A received through the UE  204 B. 
     As shown at  506 , the UE  204 B reports to the source network apparatus  202 A the link state information received from the UE  204 A through the sidelink such that the source network apparatus is made aware of the presence of the UE  204 A and its link state information. The UE  204 B may forward the link state information of the UE  204 B to the source network apparatus  202 A while reporting its own link state information, e.g. periodically, as part of the cellular communication protocol. Based on the reported link state information of the UE  204 B, the source network apparatus  202 A may evaluate one or more operations that may be taken with respect to the UE  204 A, for example, evaluating a handover of the UE  204 A to the target network apparatus  202 B and/or the like. The source network apparatus  202 A may also initiate a negotiation session with the target network apparatus  202 B as part of the evaluation. As part of the evaluation the source network apparatus  202 A may consider its availability of cellular resources to serve the UE  204 A, a timing synchronization information available from the UE  204 A and/or the like. 
     Assuming the source network apparatus  202 A has sufficient cellular resources to serve the UE  204 A, the UE  204 B may be instructed by the source network apparatus  202 A to further perform as the relay to transfer uplink and/or downlink data between the source network apparatus  202 A and the UE  204 A. When connected through the intermediate UE  204 C the UE  204 C performs as the relay between the UE  204 A and the UE  204 B for transferring the uplink and/or downlink data between the UE  204 B and the UE  204 A. 
     As shown at  508 , the UE  204 A may receive a first control information from the source network apparatus  202 A. The control information calculated by the source network apparatus  202 A may be based on the evaluation conducted by the source network apparatus  202 A based on the reported link state information of the UE  204 A. The first control information may include timing synchronization information, for example, the TA, for the UE  204 B and/or for the UE  204 A. The first control information may further include control information for the UE  204 B and/or for the UE  204 A, for example, instructing the UE  204 B to provide an uplink/downlink connection for the UE  204 A, instructing the UE  204 B to maintain the sidelink, instructing the UE  204 B to terminate the sidelink, instructing the UE  204 A to attach to the target network apparatus  202 B, instructing the UE  204 B to detach from the source network apparatus  202 A and/or the like. 
     As shown at  510 , the UE  204 B transmits a second control information to the UE  204 A. The second control information may be based on the first control information received from the source network apparatus  202 A. The second control information may include timing synchronization information, for example, the TA, for use by the UE  204 A. The second control information may further include control information for the UE  204 A, for example, instructing the UE  204 A to attach to the target network apparatus  202 B, instructing the UE  204 A to detach from the source network apparatus  202 A and/or the like. 
     Based on the link state information received from the UE  204 A, on the decision criteria and/or the negotiation with the target network apparatus  202 B, the source network infrastructure apparatus  202 A may further perform a direct handover of the UE  204 A to the target network infrastructure apparatus  202 B. This may take place in case, for example, the UE  204 A is within the coverage area  212 B as shown in  FIG. 2B  and the UE  204 A detects a sufficiently strong cellular signal from the target network apparatus  202 B. This decision may be taken after evaluating the target network apparatus  202 B has sufficient cellular resources to serve the UE  204 A. 
     As discussed before, while the UE  204 B may apply the process  500  for assisting the UE  204 A that is migrates directly from the coverage area  212 A to the coverage area  212 B as shown in  FIG. 2B  and  FIG. 2C , the UE  204 B may also apply the process  500  for assisting the UE  204 A in other deployment scenarios. For example, in a scenario where the UE  204 A goes through a no coverage area as shown in  FIG. 3A  and  FIG. 3B  before migrating into the coverage area  212 B. 
     Reference is now made to  FIG. 6A  and  FIG. 6B , which are schematic illustrations of an exemplary sidelink-assisted handover, in particular following a detach phase for an out of coverage assisted UE, according to some embodiments of the present invention. An assisting UE such as the second UE, UE  204 B simultaneously maintains a cellular connection with a network apparatus such as the source network apparatus  202 A and an assisted UE such as the first UE, UE  204 A. The UE  204 A that was previously in a no coverage area as shown in  FIG. 3A , now enters a coverage area such as the coverage area  212 B of a network apparatus such as the network apparatus  202 B. As shown in  FIG. 6A , the UE  204 B maintains the sidelink directly with the UE  204 A while the UE  204 A travels into the coverage area  212 B. The UE  204 B may maintain the sidelink with the UE  204 A traveling into the coverage area  212 B through one or more intermediate UEs, for example, the UE  204 C as shown in  FIG. 6B . 
     Through the sidelink the UE  204 B that is attached to the source network apparatus may assist the UE  204 A to attach to the target network apparatus  202 B by forwarding information from the source network apparatus  202 A to the UE  204 A and vice versa. 
     In case the source network apparatus  202 A and the target network apparatus  202 B are synchronized, the UE  204 A that uses the timing synchronization received from the source network apparatus  202 A through the UE  204 B is already adapted to the timing of the target network apparatus  202 B thus the attach phase may be significantly more rapid, smooth and/or efficient. However even in case the source network apparatus  202 A and the target network apparatus  202 B are not synchronized, still the timing offset may typically not be extremely large thus the UE  204 A may need to search for the cellular signal of the target network apparatus  202 B in a significantly narrower frequency band. 
     Moreover, during the entire period in which the UE  204 A detached from any network apparatus  212 , the UE  204  may still have an uplink and/or a downlink data connection through the source network apparatus  202 A with the UE  204 B serving as a relay. 
     The process  100  and the process  500  and/or parts thereof may be executed sequentially with the first UE, UE  204 A and the second UE, UE  204 B switching roles. The UE  204 A applies the process  100  to assist the handover of the UE  204 B while the UE  204 B may apply the process  500  to assist the attachment of the UE  204 A. This may typically take place while both the UE  204 A and the UE  204 B are travelling (migrating) in the same direction from the coverage area  212 A towards the coverage area  212 B. 
     However the process  100  and the process  500  and/or parts thereof may also be executed independently from each other to support only one or role phases of the handover process, for example, the detach phase and/or the attach phase. 
     As discussed before the UE  204 B may also execute the process  500  to assist the UE  204 A to reconnect to the source network apparatus  202 A after temporarily detaching from the source network apparatus  202 A. 
     Reference is made one again to  FIG. 5 ,  FIG. 2A ,  FIG. 3A  and  FIG. 3B . 
     Reference is also made to  FIG. 7A  and  FIG. 7B , which are schematic illustrations of an exemplary sidelink-assisted attachment, in particular following a temporary detachment of the assisted UE, according to some embodiments of the present invention. An assisting UE such as the second UE, UE  204 B simultaneously maintains a cellular connection with a network apparatus such as the source network apparatus  202 A and an assisted UE such as the first UE, UE  204 A. 
     The UE  204 A that was initially attached to the source network apparatus  202 A as shown in  FIG. 2A  may travel into a no coverage area. For example, the UE  204 A may travel outside the coverage area  212 A as shown in  FIG. 3A  and/or  FIG. 3B . As another example, as shown in  FIG. 7A  and/or  FIG. 7B , the UE  204 A may travel into a no coverage area  710  that is within the coverage area  212 A. The no coverage area  710  may be, for example, a tunnel, a valley, a mountain terrain and/or the like in which the cellular signal from the source network apparatus  202 A may be insufficient for the UE  204 A to maintain the cellular connection. The UE  204 A may thus detach from the source network apparatus  202 A either by instruction from the source network apparatus  202 A and/or automatically in case, for example, the SINR is below the predefined threshold level. 
     The UE  204 B may maintain the sidelink with the UE  204 A either directly as shown in  FIG. 7A  and/or through one or more intermediate UEs, for example, the UE  204 C as shown in  FIG. 7B . 
     While the UE  204 A is out of coverage the UE  204 B may apply the process  500 , in particular, the steps  502 ,  504 ,  506 ,  508  and  510  to assist the UE  204 A by forwarding information from the UE  204 A to the source network apparatus  202 A and vice versa. 
     In case the UE  204 A that is travelling reaches an area in which the cellular signal from the source network apparatus  202 A is strong enough to attach, the UE  204 A that is already using the timing synchronization originating from the source network apparatus  202 A may easily attach to the source network apparatus  202 A. It should be noted that for such a scenario, the source network apparatus  202 A is considered as the target network apparatus to which the UE  204 A attempts to attach. 
     Such a scenario may take place, for example, when the UE  204 A was initially attached to the source network apparatus  202 A as shown in  FIG. 2A , travelled outside the coverage area  212 A as shown in  FIGS. 3A and/or 3B  and returns into the coverage area  212 A. As another example, the UE  204 A may exit the no coverage area  710  and re-enter the coverage area  212 A. 
     Reference is now made to  FIG. 8 , which is a flowchart of an exemplary process of a network apparatus handing over one or more UEs connected by a sidelink, according to some embodiments of the present invention. 
     Reference is also made once again to  FIG. 2A  through  FIG. 2F . 
     A process  800  may be executed by one or more network apparatus for handing over one or more UEs, for example, a first UE such as, for example, the UE  204 A and/or a second UE such as, for example, the UE  204 B. The process  800  may be a complementary process for process  100  and/or the process  500  in which one or more of the UE  204 A and/or the UE  204 B may assist each other UE during the attach phase and/or following the detach phase as presented in the processes  100  and/or  500 . While the processes  100  and  500  are presented from the point of view of the first UE 204 A and the second UE  204 B executing the processes  100  and  500  respectively, the process  800  is presented from the point of view of the respective network apparatus, the source network apparatus  202 A and the target network apparatus  202 B. 
     For some deployment scenarios of the UEs  204 , all steps of the process  800  while for other deployment scenarios some steps may not be applicable as described herein after. The same applies for the sidelink-assisted handover phases as presented in  FIG. 2A  through  FIG. 2F . For brevity, the process  800  is described for handing over one UE  204 . However, the process  800  may be extended to hand over a plurality of additional UEs  204 . 
     As described before, during the assisted handover the UE  204 A and the UE  204 B may switch their roles, such that the UE  204 A may assist the UE  204 B during some handover scenarios and/or phases while the UE  204 B may assist the UE  204 A during other handover scenarios and/or phases. In order to maintain consistency the UEs  204  are therefore designated the “assisting UE  204 ” and the “assisted UE  204 ”. The assisting UE  204  is in coverage and attached to one of the network apparatuses designated the “attached network apparatus  202 ”, for example, a source network apparatus such as the source network apparatus  202 A or a target network apparatus such as the target network apparatus  202 B. The assisted UE  204  however may be in and out of coverage and may therefore not be attached to the attached network apparatus  202  during one or more of the handover phases. 
     During the assisted handover the assisting UE  204  and the assisted UE  204  maintain a sidelink between them such that the assisting UE  204  assists the handover by forwarding information from the network apparatus(s)  202  to the assisted UE  204  and vice versa. The assisting UE  204  may forward, for example, the timing synchronization information and/or the control information transmitted from the network apparatus  202  to which the assisting UE  204  is attached. In the other direction, the assisting UE  204  may collect, for example, the link state information from the assisted UE  204  and report the link state information to the source network apparatus  202 A. The assisting UE  204  that periodically reports its link state information to the attached network apparatuses  202  may add the link state information received from the assisted UE  204  to the reported information. 
     As shown at  802 , the attached network apparatus  202 , for example, the source network apparatus  202 A and/or the target network apparatus  202 B provides connectivity to a cellular network for one or more of a plurality of UEs  204 , for example, the UE  204 A and/or the UE  204 B. 
     As shown at  804 , the attached network apparatus  202  receives link state information reported by the attached assisting UE  204 . The reported link state information may include link state information of the assisted UE  204  received by the assisting UE  204  through the sidelink with the assisted UE  204 . The assisting UE  204  may include forward the link state information of the assisted UE  204  as part of its own reported link state information. 
     As shown at  806 , the attached network apparatus  202  calculates a first control information. The first control information may be derived from the evaluation conducted by the attached network apparatus  202  based on the reported link state information of the assisted UE  204 . The first control information may include timing synchronization information, for example, the TA, for the assisting UE  204  and/or for the assisted UE  204 . 
     The first control information may further include control information for the assisting UE  204  and/or for the assisted UE  204 , for example, instructing the assisting UE  204  to provide an uplink/downlink connection for the assisted UE  204 , instructing the assisting UE  204  to maintain the sidelink, instructing the assisting UE  204  to terminate the sidelink, instructing the assisted UE  204  to attach, instructing the assisted UE  204  to detach and/or the like. The first control information may be based on one or more of the plurality of decision conditions and/or criteria. Furthermore, the attached network apparatus  202  may negotiate with one or more of the other network apparatus. For example, in case the assisted UE  204  is migrating from the coverage area  212 A towards the coverage area  212 B as shown in  FIG. 2D , the source network apparatus  202 A may negotiate with the target network apparatus  202 B the instruction(s) (that may be part of the first control information) to the assisting UE  204  and/or the assisted UE  204 . 
     As shown at  808 , the attached network apparatus  202  transmits the first control information to the assisting UE  204 . 
     As shown at  810 , the attached network apparatus  202  may receive an attach request from the assisted UE  204 . For example, as shown in  FIG. 2A , the UE  204 A being the assisted UE  204  migrates into the coverage area  212 B and may request to attach to the target network apparatus  202 B. In another example, as shown in  FIG. 2D , the UE  204 B being the assisted UE  204  migrates into the coverage area  212 B and may request to attach to the target network apparatus  202 B. The attach request received from the assisted UE  204  may be based on the second control information received from the assisting UE  204  that in turn is based on the first control information received from the attached network apparatus. For example, the attached network apparatus  202  may instruct (through the first control information) the assisted UE  204  to attach to the attached network apparatus  202 . In such case, the assisted UE  204  may adapt the timing synchronization information, for example the TA provided by the attached network apparatus  202  and forwarded by the assisting UE  204 . 
     In other deployment scenarios, for example, as described for the process  500  and shown in  FIG. 2C , the attached network apparatus  202  being the source network apparatus  202 A is not receiving the attach request that is transmitted by the assisted UE  204 , being the UE  204 A to the target network apparatus  202 B. However, the assisted UE  204 A may be assisted by the UE  204 B attached to the source network apparatus  202 A as described in the process  500 . 
     In yet other deployment scenarios, for example, as described for the process  500  and shown in  FIG. 7A  and/or  FIG. 7B , the attached network apparatus  202  being the source network apparatus  202 A receives the attach request from the UE  204 A being the assisted UE  204 . The UE  204 A is temporarily detached from the source network apparatus  202 A but maintains the sidelink with the assisting UE  204 B performing as the assisting UE  204 . 
     As shown at  912 , the attached network apparatus  202  receives accepts the attach request and attaches the assisted UE  204  to the attached network apparatus  202  to connect the assisted UE  204  to the cellular network. By adapting to the timing synchronization information originating from the attached network apparatus  202 , the assisted UE  204  may quickly attach to the attached network apparatus  202  avoiding a prolonged synchronization sequence that may typically be involved with the attachment. 
     As discussed before the processes  100  and/or  500  and the complementary process  800  may be applied for the first handover mode and/or for the second handover mode. Where in the first mode, the handover may be initiated by one or more of the UEs  204  and in the second mode, the handover may be initiated by one or more of the network apparatuses  202 . 
     The UE(s)  204  may initiate the handover (the first mode) in one or more situations, for example, on detecting an insufficiently high cellular signal from its serving network apparatus  202 , on detecting a sufficiently high cellular signal from another network apparatus  202  and/or the like. As described before, the handover, in particular the terminating the cellular connection during the detach phase, may be initiated automatically by the UE(s)  204  and/or it may be instructed by the network apparatus  202  serving the UE(s)  204 . 
     Under some conditions, the network apparatus  202  may be the one to initiate the handover (the second mode). Such conditions may include, for example, detecting the UE(s)  204  is out of coverage of a serving network apparatus  202 , detecting the UE  204 A enters a coverage area  212  of another network apparatus  202 , detecting the UE  204 A is migrating (travelling) toward a coverage area  212  of the other network apparatus  202  and/or the like. These conditions may be identified by the network apparatus  202  by analyzing and and/or evaluating the link state information received from the UE (s)  204 . The network apparatus  202  may further evaluate information received from one or more other network apparatuses  202  relating to the UE(s)  204 , for example, a network apparatus  202 A the UE(s)  204  is currently/previously attached to and/or the like. The network apparatus  202  may also evaluate availability of its own cellular resources needed to serve the UE(s)  204  such that the UE(s)  204  may be properly served with the cellular service after handed over. 
     Reference is now made to  FIG. 9 , which is a schematic illustration of an interaction during a sequence of an exemplary sidelink-assisted handover initiated by a UE, according to some embodiments of the present invention. An interaction sequence  900  presents an exemplary sequence of a UE initiated assisted handover (the first mode). The interaction sequence  900  may be a combination of assisted handover processes such as the process  100  and the process  500  with a complementary process such as the process  800  in an exemplary network deployment as described in  FIG. 2A  through  FIG. 2E . The interaction sequence  900  is sequentially arranged from the top towards the bottom of the interaction sequence  900 . For completeness, the MME and SGW are also presented, however the interaction with these network elements is out of scope of the present invention. Initially two UEs such as the first UE, UE  204 A, and the second UE, UE  204 B, are connected to a cellular network through a network apparatus such as the source network infrastructure apparatus  202 A as shown in  FIG. 2A . Both the UE  204 A and the UE  204 B have a cellular connection to the source network apparatus  202 A for uplink/downlink data transfer. The UE  204 A and the UE  204 B are connected to each other through the sidelink. During the first part of the interaction sequence  900  during which the UE  204 A is handed over from the source network apparatus  202 A to a network apparatus such as the target network apparatus  202 B, the UE  204 B acts as the assisting UE to assist the UE  204 A acting as the assisted UE. At some point in time the UE  204 A detaches from the source network apparatus  202 A as shown in  FIG. 2B  and described in step  106  of the process  100 . At this stage, the UE  204 B maintains a cellular connection to the source network apparatus  202 A and the sidelink with the UE  204 A. When the UE  204 A enters a coverage area such as the coverage area  212 B of the target network apparatus  202 B, the UE  204 A tries to attach to it as shown in  FIG. 2C  and described in step  108  of the process  100 . Prior and during the attachment to the target network apparatus, the UE  204 A may be assisted by the UE  204 B as described in step  108  of the process  500 . The UE  204 A receives timing synchronization information from the target network apparatus  202 B and may therefore terminate the sidelink with the UE  204 B since the offset in the timing may be above the predefined threshold level (since typically the target network apparatus  202 B and the source network apparatus  202 A are not synchronized). After the UE  204 A attaches to the target network apparatus  202 B and establishes the cellular connection, the UE  204 A tries to reconnect to the UE  204 B. At this time the UE  204 A and the UE  204 B may switch their roles such that the UE  204 A may now perform as the assisting UE  204  while the UE  204 B may now become the assisted UE  204 . After conveying the reconnect request to the source network apparatus  202 A, the source network apparatus  202 A may instruct the UE  204 B to detach. The sidelink is reconnected between the UE  204 A and the UE  204 B and the UE  204 A may serve as a relay for the UE  204 B as shown in  FIG. 2D  and described in step  114  of the process  100 . Finally, after receiving the timing synchronization information from the target network apparatus  202 B, the UE  204 B tries to attach to the target network apparatus  202 B as shown in  FIG. 2E  and described in step  126  of the process  100 . 
     Reference is now made to  FIG. 10 , which is a schematic illustration of an interaction during a sequence of an exemplary sidelink-assisted handover initiated by a network apparatus, according to some embodiments of the present invention. An interaction sequence  1000  presents an exemplary sequence of a network apparatus initiated assisted handover (the second mode). The interaction sequence  900  may be a combination of assisted handover processes such as the process  100  and the process  500  with a complementary process such as the process  800  in an exemplary network deployment as described in  FIG. 2A  through  FIG. 2E . The interaction sequence  1000  is sequentially arranged from the top towards the bottom of the interaction sequence  1000 . For completeness, the MME and SGW are also presented, however the interaction with these network elements is out of scope of the present invention. Initially two UEs such as the UE  204 A and the UE  204 B are connected to a cellular network through a network apparatus such as the source network apparatus  202 A as shown in  FIG. 2A . Both the UE  204 A and the UE  204 B have a cellular connection to the source network apparatus  202 A for uplink/downlink data transfer. The UE  204 A and the UE  204 B are connected to each other through a sidelink. At some point in time the UE  204 A detaches from the source network apparatus  202 A as shown in  FIG. 2B  and described in step  106  of the process  100 . At this time the UE  204 B acts as the assisting UE  204  and the UE  204 A is the assisted UE  204 . Later on, the UE  204 B also detaches from the source network apparatus  202 A while maintaining the sidelink with the UE  204 A as shown in  FIG. 2D  and described in step  124  of the process  100 . When the UE  204 A enters the coverage area  212 B of the target network apparatus  202 B, the UE  204 A tries to attach to it as shown in  FIG. 2C  and described in step  108  of the process  100 . After the UE  204 A attaches to the target network apparatus  202 B and establishes the cellular connection, the UE  204 A may need to reconnect to the UE  204 B as shown in  FIG. 2C  to maintain the sidelink. At this time the UE  204 A and the UE  204 B may switch their roles such that the UE  204 A may now perform as the assisting UE  204  while the UE  204 B may now become the assisted UE  204 . The UE  204 A receives timing synchronization information from the target network infrastructure apparatus  202 B and may therefore need to reconnect the sidelink with the UE  204 B in case the sidelink timing was based on timing provided by the UE  204 B. If however the sidelink timing was provided by the UE  204 A, the sidelink may be maintained as before avoiding the need to reconnect. The sidelink is reconnected between the UE  204 A and the UE  204 B and the UE  204 A may serve as a relay for the UE  204 B as shown in  FIG. 2D  and described in step  110  of the process  100 . Finally, after receiving the timing synchronization information from the target network infrastructure apparatus  202 B, the UE  204 B tries to attach to the target network infrastructure apparatus  202 B as shown in  FIG. 3E  and described in step  126  of the process  100 . 
     Reference is now made to  FIG. 11 , which is a schematic illustration of system for assisting a handover, in particular during an attach phase, of a plurality of UEs between network infrastructure apparatuses, according to some embodiments of the present invention. A system  1100  comprises two network apparatuses such as the source network apparatus  202 A and the target network apparatus  202 B each having a coverage area  212 A and  212 B respectively. A process such as the process  100  may be carried out by an assisting UE such as the UE  204 A to assist a handover of a plurality of assisted UEs  204 , for example, a UE 204 B 1 , a UE  204 B 2  and/or a UE  204 B 3 . Such a scenario may typically occur when the UE  204 B 1 , the UE  204 B 2  and/or the UE  204 B 3 , for example, mobile phone(s) used by user(s) travelling in one or more vehicles, V2X and/or V2V vehicle(s) and/or the like are migrating from the coverage area  212 A towards the coverage area  212 B. The assisted UEs may be attached (connected via a cellular link) to the source network apparatus  202 A such as, for example, the UE  204 B 2  or they may not be attached such as, for example, the UE  204 B 1  and/or the UE  204 B 3 . The UE  204 B 3  may not have a cellular connection since it may be out of the coverage areas,  212 A and/or  212 B thus unable to attach to the source network apparatus  202 A or the target network apparatus  202 B respectively. 
     Reference is now made to  FIG. 12 , which is a schematic illustration of system for assisting a handover, in particular during following a detach phase, of a plurality of UEs between network infrastructure apparatuses, according to some embodiments of the present invention. A system  1200  comprises two network apparatuses such as the source network apparatus  202 A and the target network apparatus  202 B each having a coverage area  212 A and  212 B respectively. A process such as the process  500  may be carried out by an assisting UE such as the UE  204 B to assist a handover of a plurality of assisted UEs  204 , for example, a UE 204 A 1 , a UE  204 A 2 , a UE  204 A 3  and/or a UE  204 A 4 . Such a scenario may typically occur when, for example, the UE  204 A 1 , the UE  204 A 2  and/or the UE  204 A 3 , for example, mobile phone(s) used by user(s) travelling in one or more vehicles, V2X and/or V2V vehicle(s) and/or the like are migrating from the coverage area  212 A towards the coverage area  212 B. In another scenario the UE  204 B may assist the UE  204 A 4  that is gone into a temporary no coverage area such as the no-coverage area  710 , to reconnect to the source network apparatus  202 A. 
     Reference is now made to  FIG. 13 , which is a schematic illustration of an exemplary UE, according to some embodiments of the present invention. A UE such as the UE  204 , for example, the UE  204 A, the UE  204 B and/or the UE  204 C may be adapted to execute a process such as the process  100  and/or the process  500  for assisting a handover of one or more other UEs between network infrastructure apparatuses such as the network apparatuses  202 . The network infrastructure apparatuses  202  such as, for example, the source network apparatus  202 A and the target network apparatus  202 B are connected to a network core  1310 . 
     The UE  204  comprises a receiver  1302  and a transmitter  1306  for communicating over a cellular channel with the source network apparatus  202 A and/or the target network apparatus  202 B and a processor(s)  1304  for controlling the process  100  and/or the process  500 . The receiver  1302  and a transmitter  1306  also provide communication capabilities for communicating with one or more other UEs  204  over a sidelink channel that may be in-band and/or out-of-band with respect to the cellular channel. 
     The processor(s)  1304  may include one or more processors (homogenous or heterogeneous), which may be arranged for parallel processing, as clusters and/or as one or more multi core processing units. The processor(s)  1304  may execute program instructions from one or more storage devices, for example, a volatile memory, a non-volatile memory, a memory card and/or the like. 
     Reference is also made to  FIG. 14 , which is a schematic illustration of an exemplary network infrastructure apparatus, according to some embodiments of the present invention. A network apparatus such as the network apparatuses  202  executing the process  800  is connected to a network core such as the network core  1310 . The network apparatuses  202 , for example, the source network apparatus  202 A and/or the target network apparatus  202 B may provide connectivity to a cellular network for a plurality of UEs such as the UE  204 . The network apparatuses  202  comprises a receiver  1402  and a transmitter  1406  for communicating over a cellular channel with the plurality of UEs  204  and a processor(s)  1404  for controlling a handover of one or more of the UEs  204  to another network apparatus  202 . The processor(s)  1404  may include one or more processors (homogenous or heterogeneous), which may be arranged for parallel processing, as clusters and/or as one or more multi core processing units. The processor(s)  1404  may execute program instructions from one or more storage devices, for example, a volatile memory, a non-volatile memory, a hard drive and/or the like. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 
     It is expected that during the life of a patent maturing from this application many relevant D2D connectivity interface will be developed and the scope of the term D2D connectivity interface respectively are intended to include all such new technologies a priori. 
     As used herein the term “about” refers to ±10%. 
     The terms “comprises”, “comprising”, “includes”, “including”, “having” and their conjugates mean “including but not limited to”. This term encompasses the terms “consisting of” and “consisting essentially of”. 
     The phrase “consisting essentially of” means that the composition or method may include additional ingredients and/or steps, but only if the additional ingredients and/or steps do not materially alter the basic and novel characteristics of the claimed composition or method. 
     As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof. 
     The word “exemplary” is used herein to mean “serving as an example, instance or illustration”. Any embodiment described as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments and/or to exclude the incorporation of features from other embodiments. 
     The word “optionally” is used herein to mean “is provided in some embodiments and not provided in other embodiments”. Any particular embodiment of the invention may include a plurality of “optional” features unless such features conflict. 
     Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. 
     Whenever a numerical range is indicated herein, it is meant to include any cited numeral (fractional or integral) within the indicated range. The phrases “ranging/ranges between” a first indicate number and a second indicate number and “ranging/ranges from” a first indicate number “to” a second indicate number are used herein interchangeably and are meant to include the first and second indicated numbers and all the fractional and integral numerals there between. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. 
     All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention. To the extent that section headings are used, they should not be construed as necessarily limiting.