Patent Publication Number: US-11647483-B2

Title: Devices and methods for device to device (D2D) communication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application PCT/EP2018/056690, filed on Mar. 16, 2018, the disclosure of which is hereby referenced in its entirety. 
    
    
     TECHNICAL FIELD 
     In general, the present disclosure relates to the field of wireless communications. More specifically, the present disclosure relates to devices and methods for device-to-device (D2D) communication in a wireless communication network, and in particular, to a user equipment (UE) for D2D communication, a network entity for allocating D2D communication resources, and corresponding methods. 
     BACKGROUND 
     A user equipment (UE) supporting device-to-device (D2D), in particular vehicle-to-vehicle (V2V) communication can operate in two modes for sidelink radio resource allocation. In a first mode, known as “scheduled resource allocation”, a UE requests transmission radio resources from a base station, and the base station allocates dedicated transmission radio resources to the UE. In a second mode, known as “UE autonomous resource selection”, the UE on its own selects radio resources from preconfigured resource pools. The scheduled resource allocation mode can be, for instance, a 3GPP LTE-A mode 3, and the autonomous resource selection mode can be, for instance, a 3GPP LTE-A mode 4, which are described in 3GPP standard TS 36.213 V15.0.0 (2017-12). 
     The situations in which users are out of coverage (OOC) of the cellular network arise in early stages of network deployments or may exist due to obstacles (e.g., a road tunnel or a topology condition creating severe fading) in any stage of network deployments. The OOC situations may occur over space, time, and frequency and may be static or dynamic. Furthermore, the OOC may be expected (known) or unexpected (unknown) and may be limited by the network infrastructure from different sides. 
     Out-of-coverage V2V communications are supported by the 3GPP standard LTE-A Release 14 (see 3GPP TS 36.300, 3GPP TS 36.331, 3GPP TS 36.32, 3GPP TS 36.212 and 3GPP TS 36.101) by allowing users to autonomously select radio resources for transmitting V2V data and control information based on a sensing mechanism when they are out-of-coverage, from a resource pool pre-configured by the network. This mode of operation is referred to as “Mode 4” in the standard. Pre-configured pools can be optionally mapped to geographical zones by the network, where the users select the pool according to its location, i.e., its zone. 
     However, the OOC case is not exhaustively tackled in the 3GPP standard: users are only allowed to make autonomous resource selection, which is not collision-free as opposed to in-coverage mode of operation for V2V. This has a negative impact on the reliability of the transmissions. 
     Moreover, the pools can only be semi-statically pre-configured by the network, which may easily result in resource usage inefficiency, i.e., under- or over-utilization of the radio resources. As OOC resource pools are not allowed to overlap with the in-coverage pools, this limits the size of the OOC resources. 
     Mapping of the OOC resource pools to zones is done very coarsely, allowing the zones to be defined only as equal-sized, rectangular/square-shaped geographical areas, which may again easily result in radio resource usage inefficiency, as the road and communication traffic of the vehicles are heterogeneously distributed over space and time. 
     Furthermore, characteristics of OOC areas (e.g., boundaries, durations) cannot be well reflected on the pre-configuration of the resource pools. Therefore, the OOC mode of operation defined by the state-of-art standard is not the optimum solution for resource allocation and can be improved in many ways. 
     In light of the above, there is a need for improved devices and methods for D2D communication, allowing allocating radio resources and transmitting data for D2D communication in a reliable and efficient manner. 
     SUMMARY 
     The present disclosure describes improved devices and methods for D2D communication that allow for allocating radio resources and transmitting data for D2D communication in a reliable and efficient manner. 
     Generally, embodiments of the disclosure relate to a user equipment (UE) for a wireless communication network and to a network entity such as a base station, as well as to corresponding methods for allocating radio resources and transmitting data for D2D communication in a reliable and efficient manner. More specifically, embodiments of the present disclosure can exploit radio resource management control by the network infrastructure in coverage, to increase the reliability and efficiency of radio resource utilization for out-of-coverage (OOC) events, i.e. when the users are out of coverage of the cellular network (e.g., a base station). 
     According to embodiments of the disclosure, the users, i.e. user equipments, can keep using their latest resource allocations in coverage of the cellular network also when the users are out of coverage in a way configured and signaled by a base station, which is herein also referred to as “Extended Mode 3”. Furthermore, embodiments of the disclosure can operate the Extended Mode 3 in parallel and/or through interaction with Mode 4 operation in OOC situations, and improve the OOC Mode 4 resource allocation and selection procedures. Finally, user equipments can provide information about OOC events or situations to the base station when they are back in coverage, so as to assist them for better resource allocation during OOC events, i.e. when the users are out of coverage. 
     Embodiments of the disclosure provide a key advantage of increasing the reliability efficiency of resource utilization performance of D2D, in particular V2V, communications under OOC situations by exploiting in-coverage radio resource management control provided via the network entity and better performance than exclusive OOC mode of operation of users, defined by current D2D communication standards. 
     More specifically, according to a first aspect, the disclosure relates to a user equipment (UE) which comprises: a communication interface configured to perform communication with one or more nearby UEs using one or more of a plurality of resources in a cellular communication network; and a processing unit configured to operate the UE in a first mode and in a second mode, wherein the first mode is a scheduled resource allocation mode and the second mode is an autonomous resource selection mode or an idle mode and the processing unit is configured to switch from the first mode to the second mode, if a first condition relating to an out of coverage (OOC) event (also referred to as OOC status, condition or situation) of the UE is met; and a second condition related to a further parameter is met. 
     The UE can be configured to determine the first and/or the second condition on its own or it can be configured to receive the results of respective determinations of these conditions by another network entity, in particular a base station. If the first condition relating to an out of coverage event of the UE is met then the UE is out of coverage. 
     An OOC or OOC event can be defined, for instance, by one or more of the following conditions: (a) the UE has no serving cellular network, (b) the power received by the UE from the cellular network is less than a threshold value, and/or (c) the UE is not interested in performing communication on a frequency provided by the cellular network. 
     The scheduled resource allocation mode can comprise, for instance, a 3GPP LTE-A mode 3 and the autonomous resource selection mode can comprise, for instance, a 3GPP LTE-A mode 4, wherein the transmission modes 3 and 4 are as described in 3GPP standard TS 36.213 V15.0.0 (2017-12). 
     In a further possible implementation form of the first aspect, the UE further comprises a memory configured to store a plurality of operation profiles, and the processing unit is configured to, until the second condition is met, operate the UE in the first mode according to one of the plurality of operation profiles. An operation profile can define communication resources and/or a communication behaviour to be used by the UE. 
     In a further possible implementation form of the first aspect, the plurality of operation profiles comprises at least one operation profile used in case of an unexpected OOC event. In case of an unexpected OOC even the UE does not know of this event in advance, e.g. because it was not provided with any information, e.g. from a network entity or another UE, that the OOC will occur. 
     In a further possible implementation form of the first aspect, according to the operation profile, the processing unit is configured to operate the UE in the first mode by using active semi-persistent schedule (SPS) configurations and allocated resources for periodic, aperiodic and/or single transmissions, until the second condition is met. 
     In a further possible implementation form of the first aspect, the plurality of operation profiles comprise at least one operation profile associated with an expected OOC event. An expected OOC event is an event that the UE knows in advance, either by information from the network entity or another UE and/or by inferring this from available information, e.g. map data. 
     In a further possible implementation form of the first aspect, according to the operation profile associated with an expected OOC event, the processing unit is configured to operate the UE in the first mode, until the second condition is met, by using active semi-persistent schedule (SPS) configurations and/or allocated resources for periodic transmissions, by selecting one or more of a plurality of resources assigned by a network entity of the cellular communication network to be used during the expected OOC event and/or by using a one-shot schedule provided by a network entity of the cellular communication network. 
     In an implementation form, a one-shot schedule is a single, complete resource assignment to the UE, consisting of, e.g., different time and frequency slots, to be exclusively used starting from the time the OOC condition is met until the time the OOC condition is over. 
     In a further possible implementation form of the first aspect, the second mode is an autonomous resource selection mode, and the processing unit is further configured to operate the first mode in parallel with the second mode, in response to an OOC event until a second condition is met, in particular by using the first mode for periodic transmissions and the second mode for aperiodic transmissions. 
     In a further possible implementation form of the first aspect, the second mode is an autonomous resource selection mode, and the communication interface is configured to sense a load of the resources for the second mode and the processing unit is configured to replace a transmission on resources allocated for the first mode with a high priority transmission of the second mode, in particular in case the load of the resources for the second mode is larger than a load threshold for the second mode. 
     In a further possible implementation form of the first aspect, the second condition is met if one or more of the following is met: a time elapsed since the start of the OOC event is larger than a time threshold; a distance travelled by the UE since the start of the OOC event is larger than a distance threshold; the UE does not operate in the first mode with a probability defined by a network entity of the communication network; a load of the resources allocated for the first mode is larger than a predefined threshold. The UE can be configured to sense the load of the resources allocated for the first mode. 
     In a further possible implementation form of the first aspect, in response to an OOC event, the communication interface is configured to sense a load of the resources, in particular a channel busy ratio (CBR) value, and the processing unit is configured to record information about times and/or locations, at which the sensed load is larger than a load threshold, and/or context information of the UE, e.g., a motion state, a vehicle velocity, road conditions, et cetera, and to provide the information to a network entity of the cellular communication network. This advantageously allows the network to gather information about where OOC events occur and provide other UEs with information thereon, i.e. to optimize resource allocation for other UEs and to avoid unexpected OOC events. 
     In a further possible implementation form of the first aspect, the second mode is an autonomous resource selection mode, wherein, in response to an OOC event, the communication interface is configured to use downlink resources in the second mode. 
     In a further possible implementation form of the first aspect, the second mode is an autonomous resource selection mode, wherein in the second mode the communication interface is configured to transmit information about the number of next transmissions of the same type, e.g., media access control (MAC) protocol data units (PDUs) of the same size, belonging to the same sidelink communication process, in particular to reserve the same selected resources for that number of times on the communication interface, to other UEs. 
     In a further possible implementation form of the first aspect, the second mode is an autonomous resource selection mode, wherein in the second mode the communication interface is configured to: transmit the information of the number of next transmissions to other UEs, in case the number is smaller than a threshold value, in particular a threshold value provided by a network entity of the communication network; and/or transmit a random number for the number of transmissions, in case the number is larger than the threshold value. 
     According to a second aspect, the disclosure relates to a corresponding method of operating a UE, wherein the UE comprises a communication interface configured to perform communication with one or more nearby UEs using one or more of a plurality of resources in a cellular communication network and a processing unit configured to operate the UE in a first mode and a second mode, wherein the first mode is a scheduled resource allocation mode and the second mode is an autonomous resource selection mode or an idle mode. The method comprises the steps of: switching from the first mode to the second mode, if a first condition relating to an out of coverage event of the UE is met; and a second condition related to a further parameter is met. 
     According to a third aspect, the disclosure relates to a network entity, in particular a base station, for managing communication resources in a cellular communication network, wherein the network entity is configured to communicate with a UE, wherein the UE can be operated in a first mode and in a second mode, wherein the first mode is a scheduled resource allocation mode and the second mode is an autonomous resource selection mode or an idle mode, and to provide the UE information about when to switch to the second mode, in particular based on one or more of the following conditions: a time elapsed since the start of an OOC event is larger than a time threshold; a distance traveled by the UE since the start of an OOC event is larger than a distance threshold; the UE does not operate in the first mode with a probability defined by the network entity; and/or a load of the resources allocated for the first mode is larger than a predefined threshold. 
     In a further possible implementation form of the third aspect, the network entity is configured to provide a plurality of operation profiles to the UE, wherein a processing unit of the UE is configured to, until the second condition is met, operate the UE in the first mode according to one of the plurality of operation profiles. 
     In a further possible implementation form of the third aspect, the plurality of operation profiles comprise one or more operation profiles to be used in case of an unexpected OOC event of the UE. 
     In a further possible implementation form of the third aspect, according to the operation profile the UE is configured to operate in the first mode by using active semi-persistent schedule (SPS) configurations and allocated resources for periodic, aperiodic and/or single transmissions, until the second condition is met. 
     In a further possible implementation form of the third aspect, the plurality of operation profiles comprise at least one operation profile associated with an expected OOC event of the UE. 
     In a further possible implementation form of the third aspect, according to the operation profile associated with an expected OOC event, the UE is configured to operate in the first mode, until the second condition is met, by using active semi-persistent schedule (SPS) configurations and/or allocated resources for periodic transmissions, by selecting one or more of a plurality of resources assigned by the network entity to be used during the expected OOC event and/or by using a one-shot schedule provided by the network entity. 
     In a further possible implementation form of the third aspect, the network entity is configured to generate the operation profile associated with an expected OOC event on the basis of information provided by another UE having experienced the expected OOC event. 
     In a further possible implementation form of the third aspect, the network entity is configured to provide a time threshold, a distance threshold and/or a probability to the UE and the second condition is met if one or more of the following conditions is met: a time elapsed since the start of the OOC event is larger than the time threshold; a distance travelled by the UE since the start of the OOC event is larger than the distance threshold; the UE does not operate in the first mode with the probability provided by the network entity; a load of the resources allocated for the first mode is larger than a predefined threshold. 
     In a further possible implementation form of the third aspect, the network entity is configured to determine, on the basis of information provided by another UE having experienced the expected OOC event, a size of resource pools to be used by the UE in the first mode and/or second mode. 
     In a further possible implementation form of the third aspect, the network entity is configured to map the resource pools to be used by the UE in the first mode and/or second mode to moving zones, such as a moving group of vehicles. In an implementation form, moving zones can be further configured, e.g., by a timer or START/STOP times. 
     In a further possible implementation form of the third aspect, the network entity is configured to allocate downlink resources to be used by the UE in the first mode and/or the second mode. 
     According to a fourth aspect, the disclosure relates to a method for operating a network entity, in particular a base station, for managing communication resources in a cellular communication network, wherein the network entity is configured to communicate with a UE, wherein the UE can be operated in a first mode and in a second mode, wherein the first mode is a scheduled resource allocation mode and the second mode is an autonomous resource selection mode or an idle mode, wherein the method comprises the step of providing the UE information about when to switch to the second mode, in particular based on one or more of the following conditions: a time elapsed since the start of an OOC event is larger than a time threshold; a distance traveled by the UE since the start of an OOC event is larger than a distance threshold; the UE does not operate in the first mode with a probability defined by the network entity; and/or a load of the resources allocated for the first mode is larger than a predefined threshold. 
     According to a fifth aspect, the disclosure relates to a computer program comprising program code for performing the method of the second or fourth aspect when executed on a computer. 
     The invention can be implemented in hardware and/or software. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further embodiments of the disclosure will be described with respect to the following figures, wherein: 
         FIG.  1    shows a schematic diagram illustrating a user equipment (UE) in communication with a network entity; 
         FIG.  2    shows a schematic diagram of a communication network comprising a plurality of network entities and a plurality of UEs; 
         FIG.  3    shows a schematic diagram of a communication network comprising a network entity and a plurality of UEs; 
         FIG.  4    shows a schematic diagram of exemplary resource utilization for a UE in extended mode 3; 
         FIG.  5    shows a schematic diagram of a communication network comprising a plurality of network entities and a plurality of UEs, 
         FIG.  6    shows a schematic diagram of exemplary resource utilization for providing a semi-persistent-schedule; 
         FIG.  7    shows a schematic diagram of exemplary resource utilization for providing a one-shot schedule; 
         FIG.  8    shows resource allocation and utilization based on a sensing result by a UE; 
         FIG.  9    shows a schematic diagram of pre-empting extended mode 3 transmissions by a UE; 
         FIG.  10    shows a schematic diagram summarizing a procedure for V2V communication in a communication network; 
         FIG.  11    shows a schematic diagram summarizing a procedure for V2V communication in a communication network; 
         FIG.  12    shows a schematic diagram of a SPS (semi-persistent schedule) configuration by a network entity; 
         FIG.  13    shows resource allocation and utilization based on a SPS configuration; 
         FIG.  14    shows a diagram illustrating a method of operating a user entity; and 
         FIG.  15    shows a diagram illustrating a method of allocating resources for D2D communication. 
     
    
    
     In the various figures, identical reference signs will be used for identical or at least functionally equivalent features. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present disclosure may be placed. It will be appreciated that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present invention is defined by the appended claims. 
     For instance, it will be appreciated that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. 
     Moreover, in the following detailed description as well as in the claims, embodiments with different functional blocks or processing units are described, which are connected with each other and/or which exchange signals. It will be appreciated that the present disclosure covers embodiments as well, which include additional functional blocks or processing units that are arranged between the functional blocks or processing units of the embodiments described below. 
     Finally, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise. 
     As will be described in more detail in the following, embodiments of the disclosure relate to a D2D or V2V communication network comprising at least a user entity (also referred to as a UE hereafter) and a network entity (also referred to as a base station hereafter), which allow for a significant increase in reliability and efficiency of radio resource utilization. 
     D2D (device-to-device) or V2V (vehicle-to-vehicle) communication can take place when user equipments (UEs), in particular vehicular user equipments, are both in coverage and out of coverage (OOC) of the cellular network. V2V communication can consist of two different types of data traffic: first, periodic messages, e.g., cooperative awareness messages (CAMs), carrying information such as vehicle position and velocity; second, aperiodic messages, transmitted upon a triggering unexpected event, e.g., decentralized environmental notification messages (DENMs) for safety warning purposes. 
     A centralized network infrastructure entity, such as a base station, can manage radio resource allocation for in-coverage and OOC V2V communications. Modes of operation and resource pools for in-coverage and OOC communications are configured by the network infrastructure entity. The configuration information is periodically broadcast to users via system information messages, e.g., System Information Block 21 (SIB 21). 
     To determine if a user is out of coverage, a network entity (hereafter also referred to as a base station) can measure received powers of the users fading in/out over time around a certain location at a certain frequency; collect Radio Resource Control (RRC) signaling (e.g., connection or state changes) or measurement reports of users over time at a certain location; or use a map information, e.g., installed by the network operator. The network infrastructure elements, e.g., base stations (BSs), performing the above functions can store information centrally or in a cooperating manner. 
     When the users are in coverage, resources can be allocated via a scheduled mode of operation (hereafter referred as “Mode 3” as in the 3GPP standard), wherein the users can request transmission resources from the network entity, e.g., the base station. The base station then schedules resources for V2V data and control information transmission. V2V resources are bounded by the resource pools (configured by the network) as follows: normal transmission pools for in-coverage; pre-configured transmission pools for out-of-coverage; exceptional transmission pools for handover; or reception pools covering all above. 
     The schedule in coverage can be a dynamic or semi-persistent schedule (SPS). The semi-persistent schedule (SPS) is preferred for periodic V2V traffic, e.g., transmission of CAMs. SPS is configured, and activated/deactivated by the network entity. The users report their V2V traffic pattern information (message size, periodicity, priority, etc.) to the network entity, in order to assist this configuration. Activating multiple SPS configurations for single user is possible. When the users are in coverage, the network entity can collect location information and channel busy ratio (CBR) measurement reports from the users to make better resource allocations. Both in coverage and out of coverage of the cellular network, users can adapt their transmissions (e.g., modulation and coding scheme) based on CBR measurements. 
       FIG.  1    shows a schematic diagram illustrating a communication network  100  comprising a user equipment (UE)  101  according to an embodiment and a network entity  131  according to an embodiment. The UE  101  is configured to communicate with one or more nearby UEs (not shown in  FIG.  1   ) via a sidelink (i.e. D2D or V2V) communication channel and communicate with the network entity  131  via an uplink/downlink communication channel. 
     In the embodiment shown in  FIG.  1   , the UE  101  could be implemented in the form of a user equipment such as a mobile phone or a vehicle or a communication module of a vehicle. However, it will be appreciated that embodiments of the disclosure apply to user equipments other than mobile phones or vehicles as well. In an exemplary embodiment, the network entity  131  can be for instance a base station and is configured to manage communication resources in the cellular communication network  100 . 
     As illustrated in  FIG.  1   , the UE  101  comprises a communication interface  103  configured to perform communication with one or more nearby UEs (not shown in  FIG.  1   ) using one or more of a plurality of resources allocated by the network entity  131  in the cellular communication network  100 . Furthermore, the UE  101  comprises a processing unit  105  configured to operate the UE  101  in a first mode and in a second mode, wherein the first mode is a scheduled resource allocation mode and the second mode is an autonomous resource selection mode or an idle mode, as already mentioned above. By way of example, the scheduled resource allocation mode can be a 3GPP LTE-A mode 3 and the autonomous resource selection mode can be a 3GPP LTE-A mode 4. 
     As will be described in more detail in the context of  FIG.  2   , the processing unit  105  is configured to switch from the first mode to the second mode, if a first condition relating to an out of coverage (OOC) event or status of the UE  101  is met and a second condition related to a further parameter is met. 
     In an embodiment, the second condition is met if one or more of the following conditions is met: a time elapsed since the start of the OOC event is larger than a time threshold; a distance travelled since the start of the OOC event is larger than a distance threshold; the UE  101  does not operate in the first mode with a probability, wherein the UE  101  stays in the first mode randomly with that probability; and/or a load of the resources allocated for the first mode is larger than a predefined threshold. The network entity  131  of the cellular communication network can be configured to provide the time threshold, the distance threshold and/or the probability to the UE  101 . 
     In an embodiment, the network entity  131  is configured to generate a plurality of operation profiles associated with an expected OOC event on the basis of information provided by another UE having experienced the expected OOC event. Moreover, the network entity  131  is configured to provide the plurality of operation profiles to the UE  101 . The UE can comprise a memory  107  (as shown in  FIG.  1   ) configured to store the plurality of operation profiles, wherein the plurality of operation profiles can comprise at least one operation profile (a kind of default operation profile), wherein the processing unit can be configured to, until the second condition is met, operate the UE in the first mode according to the at least one default operation profile. An operation profile can define communication resources and/or a communication behaviour to be used by the UE  101 . 
     In an embodiment, the network entity  131  is configured to determine a size of resource pools to be used by the UE  101  in the first mode and/or second mode, on the basis of information provided by another UE having experienced the expected OOC event. Furthermore, the network entity  131  can map the resource pools to be used by the UE  101  in the first mode and/or second mode to moving zones such as a moving group of vehicles, wherein zones can be further configured, e.g., by a timer or START/STOP times. The network entity  131  can also allocate downlink resources to be used by the UE  101  in the first mode and/or second mode. 
     If an unexpected OOC event occurs, the processing unit  105  can operate the UE  101  in the first mode according to another default operation profile in response to the unexpected OOC event. Furthermore, the processing unit  105  of the UE  101  can be configured to operate the UE  101  in the first mode by using active semi-persistent schedule (SPS) configurations and allocated resources for periodic, aperiodic and/or single transmissions, until the second condition is met. 
     If an expected OOC event occurs, on the other hand, the processing unit  105  of the UE  202  can be configured to operate the UE  101  in the first mode according to the operation profile associated with the expected OOC event, in response to the expected OOC event. Furthermore, the processing unit  105  can operate the UE  101  in the first mode by using active semi-persistent schedule (SPS) configurations and allocated resources for periodic transmissions, by selecting one or more of a plurality of resources assigned by the network entity  131  to be used during the expected OOC event and/or by using a one-shot schedule provided by the network entity  131 . The one-shot schedule is a single, complete resource assignment to the UE  101  and can comprise, for instance, different time and frequency slots to be exclusively used starting from the time the OOC condition is met until the time the OOC condition is over. 
     According to a further embodiment, in case the second mode is an autonomous resource selection mode, the processing unit  105  of the UE  101  is further configured to operate the first mode in parallel with the second mode, in response to an OOC event until a second condition is met, in particular by using the first mode for periodic transmissions and the second mode for aperiodic transmissions. 
     According to a further embodiment, in case the second mode is an autonomous resource selection mode, the communication interface  103  of the UE  101  is configured to sense a load of the resources for the second mode and the processing unit  105  of the UE  101  is configured to replace a transmission on resources allocated for the first mode with a high priority transmission of the second mode, in particular in case the load of the resources for the second mode is larger than a load threshold for the second mode. 
     In response to an OOC event, the communication interface can also sense a load of the resources, in particular a CBR value, allocated for the first mode and the second condition is met if the sensed load is larger than a load threshold for the first mode. 
     Thus, in an embodiment, the communication interface  103  of the UE  101  is configured to sense a load of the resources, in particular a CBR value, allocated for the first mode and the processing unit  105  of the UE  101  is configured to record information about times and/or locations, at which the sensed load is larger than a load threshold, and/or context information of the UE  101 , e.g. motion state, road conditions, and to provide the information to the network entity  131 . 
     According to a further embodiment, the communication interface  103  of the UE  101  is configured to use downlink resources in the second mode, i.e. the autonomous resource selection mode. Furthermore, the communication interface  103  of the UE  101  is configured to transmit information (e.g. a counter) about the number of next transmissions of the same type, e.g., MAC PDUs of same size, belonging to the same sidelink communication process, in particular to reserve the same selected resources for that number of times on the communication interface  103  to other UEs. 
     In case the second mode is an autonomous resource selection mode, the communication interface  103  of the UE  101  can also transmit information about the number of upcoming transmissions of the communication interface  103  to other UEs, in case the number of transmissions is smaller than a threshold value (provided, for instance, by the network entity  131 ) and to transmit a random number for the number of transmissions, in case the number of transmissions is larger than the threshold value. 
     As discussed above, the embodiments of the present disclosure can exploit in-coverage radio resource management control for out-of-coverage (OOC) events, which is shown in  FIG.  2   . It is to be appreciated that the out-of-coverage (OOC) events refer to the situations in which the user equipments (UEs)  101  are out of coverage of the network entity, i.e. base station  103 .  FIG.  2    show a schematic diagram of the communication network  100  comprising a plurality of network entities  131   a - b  according to an embodiment and a plurality of UEs  101   a - h  according to an embodiment, wherein an OOC event occurs as the UEs  101   a - h  enter a tunnel, which is limited by the network infrastructure (base stations) at the two ends of the tunnel. The base stations  131   a - b  can cooperate to control the radio resources for V2V communications for the OOC event. According to an embodiment radio resource can be managed in response to the OOC event on the basis of one or more of the following steps. 
     In step  1 , the UEs  101   a - h  can keep their allocated or “pre-scheduled” resources in coverage especially for periodic or semi-persistent transmissions for OOC events, hence extending resource allocations in the first mode, i.e. Mode 3, to the OOC situations. This solution is also referred to as “Extended Mode 3” hereafter. 
     In step  2 , embodiments of the disclosure allow coexistence of the Extended Mode 3 and the Mode 4 operation (i.e. the second mode or UE autonomous resource selection) as well as interaction between the Extended Mode 3 and the Mode 4 operation in OOC situations. 
     In step  3 , embodiments of the disclosure can improve the Mode 4 resource allocation and selection procedures for OOC events. In step  4 , the UEs  101   a - h  can also provide information about the OOC situations to the network infrastructure entities  131   a - b , i.e. the base stations, when they are back in coverage of the cellular network  100 . 
     According to the current standard, the UEs  101  generally switch completely to the autonomous resource selection (i.e. mode 4) if they are out of coverage of the base station. On the other hand, according to an embodiment, the mode 3 can be extended into the area out of coverage in certain OOC scenarios in order to achieve higher reliability and resource usage efficiency. The OOC events comprise the following two types: a Type I OOC event, being unexpected or unknown by the network entity  131 ; or a Type II OOC event, being expected or known by the network entity, i.e. base station  131 . 
       FIG.  3    shows a schematic diagram of the communication network  100  comprising the network entity  131  according to an embodiment and a plurality of UEs  101   a - b  according to an embodiment, wherein the Type I OOC event may occur, e.g., because of extreme fading conditions for the UE  101   a , as the UE  101   a  enters an area in which the communication with the base station  131  is hindered by a building or other obstacles. 
     In such case, as shown in  FIG.  4   , the UEs  101  can operate in Extended Mode 3 and keep using their latest scheduled resources or SPS configurations/resources for periodic transmissions, according to a default operation profile configured by the network entity  131  (stage  403 ). The default operation profile of UEs  101  in case of an unexpected OOC event, can be broadcast to all UEs  101  by the base station(s)  131  via SIB21 message. The default user operation profile indicates the users to keep their latest allocated resources according to a threshold, e.g., a timer, a probability, or a distance from the network coverage boundary, after they enter the OOC area. After the threshold is exceeded, the user releases the Extended Mode 3 resources, namely completely switches to Mode 4, while the user is still out of coverage (stage  405 ). 
       FIG.  5    show a schematic diagram of the communication network  100  comprising a plurality of network entities  131   a - b  according to an embodiment and a plurality of UEs  101   a - g  according to an embodiment, wherein a type II OOC event occurs, which is known and expected by the network entities  131   a - b  (i.e. base stations), e.g., as a road tunnel bounded by the network coverage at two ends. 
     In such a case, the base stations  131   a - b  can allocate or “pre-schedule” the OOC resources specific to request of each UE  101   a - g  and/or provide additional configuration/information for the OOC event when allocating the resources. When the UEs  101   a - g  enter the out of coverage area, the UEs  101   a - g  can use this resource allocation and behave according to the specific configuration. 
     The resource allocation can be in form of providing a semi-persistent-schedule (SPS) or a one-shot schedule for the entire OOC area. Multiple resources can be provided to the UEs  101 , among which the UEs  101  can select one resource autonomously for the OOC event, based on, e.g., sensing results. The specific configuration can be providing the schedule (SPS or one-shot) with a threshold, e.g., a timer and/or a probability, for each user to keep using the allocated resources when they enter the out of coverage area. 
     After the threshold is exceeded, the UEs  101  can release the Extended Mode 3 resources, namely completely switch to Mode 4, while they are still out of coverage of the base stations  131 . The specific OOC configuration can be sent to the UEs  101  by RRC signaling, e.g., SPS activation or DCI  5 A messages. With a known/expected OOC event, the network entity  131  will have a chance to make predictions in order to make better radio resource management for V2V communication under an OOC event. 
     Exemplary resource utilization for providing a semi-persistent-schedule (SPS) and a one-shot schedule by the UE  101  according to an embodiment are illustrated in  FIGS.  6  and  7    respectively. 
     As shown in  FIG.  6   , the network entity or base station  131  can activate a semi-persistent-schedule (SPS) in response to an OOC event with a timer or probability and the UE  101  can use the specific SPS and operate in extended mode 3 according to an operation profile configured by the base station  131  upon entry into an OOC area expected by the base station  131  (stage  603 ). Afterwards, the UE  101  can release resources from extended mode 3 when the SPS timer expires or with the indicated probability and switch to mode 4 completely (stage  605 ). 
     Similar to  FIG.  6   ,  FIG.  7    shows resource utilization and allocation for providing a one-shot schedule to the UE  101  according to an embodiment, wherein the base station  131 , implemented, for instance, as an eNB, can signal a complete one-shot schedule as soon as the UE  101  enters an out of coverage area and the UE  101  can use a one-shot schedule throughout an OOC area expected by the network entity  131  (stage  703 ). 
     In response to an OOC event, the UEs  101  can make autonomous resource selection (Mode 4) for aperiodic transmissions (e.g., decentralized environmental notification messages or DENMs). This mode can be operated in parallel or/and through interaction with the Extended Mode 3 for an OOC event, as will be further described in more detail further below. 
     According to an embodiment, the UEs  101  make sensing on the allocated resources in Extended Mode 3 when they are out of coverage, and the UEs  101  may release Extended Mode 3 resources based on the sensing result, e.g., release if the sensed receive power is above a threshold. The threshold to release Extended Mode 3, e.g., a CBR value, can be further configured by the network entity  131  and signaled to the UEs  101  before they enter the area out of coverage. 
     The UEs  101  may release Extended Mode 3 and switch to Mode 4 while they are still out of coverage, which can occur even before the timer or the distance indicated by the base station  131 , if the sensing threshold is exceeded, e.g., due to large number of collisions. 
       FIG.  8    shows resource allocation and utilization based on a sensing result by the UE  101  according to an embodiment, wherein the UE  101  can operate in Extended Mode 3 and keep using its latest scheduled resources (stage  803 ) and later the UE  101  can release resources, i.e. stop transmitting, in Extended Mode 3 based on a sensing result and switch to mode 4 completely (stage  805 ). 
     In case of congestion in aperiodic traffic, which is operated by Mode 4, the UE  101  can preempt, i.e., skip, the pre-scheduled Extended Mode 3 transmissions with lower priority (e.g., cooperative awareness messages, CAMs) to transmit high priority messages (e.g., crash notification). 
     The SPSs allowed to be preempted for certain priorities can be further configured by the network entity  131  i.e. the base station and signaled to the UE  101  before the UE  101  enters the OOC area.  FIG.  9    shows a schematic diagram of preempting Extended Mode 3 transmissions by the UE  101  according to an embodiment for high priority aperiodic messages, wherein the UE  101  first operates in extended mode 3 (stage  901 ), then senses congestion in mode 4 traffic and has a high-priority transmission such as crash notification DENM. The UE  101  can thus preempt or skip an Extended Mode 3 transmission with a low priority (stage  903 ), for instance, the UE  101  can replace a cooperative awareness message (CAM) by a decentralized environmental notification messages (DENMs) providing information about a car crash. 
     In an embodiment, the network entity  131  can better allocate the resources for an OOC event (both in Extended Mode 3 and Mode 4) via the following advantageous embodiments. 
     In a first embodiment, the network entity  131  can improve the configuration of OOC resource pools in case of a Type II OOC event, i.e. an expected/known OOC event, by determining the size of Mode 4 OOC resource pools based on the information about the OOC event provided by the UEs  101 . Instead of geographical zones, moving zones can be mapped to the OOC resources. Each zone can be allocated to a group, i.e., a cluster of road or data traffic users. Moving zones can be configured by a timer to be used by the UEs  101  to keep using the corresponding resources for an OOC event. This is also useful when the UEs  101  cannot determine their location (e.g., inside a tunnel) to find the geographical zone according to the current standard. 
     In a further embodiment, the network entity  131  can include downlink (DL) spectrum into the resource pool for V2V communication under an OOC event, while resources for V2V communications are allocated from the uplink spectrum according to the conventional standard. 
     In case of a Type II OOC event (i.e. an expected/known OOC event), the network entity  131  according to an embodiment may allocate resource pools for the OOC usage, also from the downlink (DL) spectrum. The OOC pools are configured to be used after a time or distance away from the in-coverage—OOC boundary so as to not interfere with in-coverage DL traffic. The pools belonging to the DL spectrum could be further configured by the network entity  131  and signaled to the UEs  101  before they enter an OOC area. Included DL frequency bands for V2X operation can be provided as “inter-band carrier aggregation”. 
     Additionally, the autonomous resource selection procedure in Mode 4 under an OOC event can also be improved by the UE  101  according to a further embodiment. In the current standard, the UEs  101  keep (reserve) each resource they selected based on sensing for a random number of transmission times, and announce this value (the resource reselection counter) to the other UEs via Sidelink Control Information (SCI). In turn, other UEs exclude that resource when making sensing for that number of times. Therefore, if the UE  101  transmits less than the announced number, the reserved resources will not be used at all. 
     Instead, the UE  101  according to an embodiment in OOC Mode 4 can reserve and announce their “actual” number of transmissions in Sidelink Control Information (SCI) instead of a random value. Announcing the actual number can especially increase the resource efficiency for one-shot or small number of transmissions, so as not to reserve or occupy the resources more than the required number of times. The actual number of transmissions could be announced, if the number is below a threshold value determined by the network entity  131 . For numbers above that threshold, the UEs  101  may again choose a random number. 
     Furthermore, the UE  101  can provide information about an OOC event to the network entity  131  according to an embodiment: in case of a Type I OOC event (unknown), any information about the OOC event provided by the UEs  101  to the network entity  131  can enable the network entity  131  to be aware of the OOC event. In case of a Type II OOC event (known), any information about the OOC event provided by the UEs  101  to the network entity  131  can improve the resource allocation performance by the network entity  131 . 
     In a further embodiment, the UEs  101  can perform additional measurements when they are out of coverage of the cellular network. The network entity, i.e. base station  131  can determine which UE  101  to collect which data and signal this information to the UEs  101  before they enter an out of coverage area. As a minimal set of measurements, the UEs  101  can measure and save when and where the CBR was larger than a threshold value, wherein the threshold value can be provided by the network entity  131 . 
     Moreover, the UEs  101  can additionally collect data, e.g., Probe Vehicle Data (as in intelligent transportation system (ITS)) or data regarding: road traffic such as vehicle position, velocity, density, etc.; communication traffic such as probability of aperiodic services (e.g., emergency brake notification); or environment data such as propagation conditions, the weather in an OOC area, etc. 
     In a further embodiment, the UEs  101  can report information about OOC events when they are back in coverage: the UEs  101  can report the data they collected regarding the OOC events to the base stations  131  when they are back in coverage, wherein reporting can be performed via extra fields to be added to the CBR report or UE assistance information messages. 
     In a further embodiment, the network entities  131  can exchange information regarding OOC events: the network infrastructure nodes, such as base stations  131 , can cooperate with each other by means of exchanging information regarding the OOC events in order to improve resource allocations for the OOC events. Exchanged information may comprise measurements or data reported by the UEs  101  regarding the OOC events. 
       FIG.  10    shows a schematic diagram summarizing a procedure  1000  for V2V communication in a communication network according to an embodiment when an OOC event occurs. The procedure  1000  shown in  FIG.  10    comprises the following steps: 
     First, the base station  131  measures the UE power or collect RRC (Radio Resource Control) signaling or measurement reports of UEs over time at a certain location; an operator installs map information; the UEs signal information about OOC events to the base station(s)  131 ; the base stations  131  can cooperate (step  1001 ). 
     The base station  131  is checked if it is aware of the OOC event (step  1003 ). 
     If yes, the base station  131  can signal detailed information for extended mode 3 and make predictions; the base station  131  can further configure the OOC pools with extra information such as zones associated with a timer and the use of downlink spectrum for an OOC event (step  1005 ). 
     The UE  101  can switch to extended mode 3 according to the provided UE-specific information in case of an expected OOC event (step  1007 ). 
     If no, the base station  1311  configures a default operation profile for the UE  101  in case of an unexpected OOC event and broadcasts the configuration via RRC SIB21 message (step  1009 ). 
     The UE  101  can switch to extended mode 3 according to the default operation profile in case of an unexpected OOC event (step  1011 ). 
     The UE  101  can operate mode 4 in parallel with the extended mode 3 (step  1013 ). 
     The UE  101  can perform sensing on extended mode 3 resources (in addition to mode 4 resources) for an OOC event (step  1015 ). 
     The UE  101  can reserve the exact number of transmissions instead of a random number and announce to other UEs via sidelink control information (step  1017 ). 
     The UE  101  checks if there are collisions on the extended mode 3 transmissions (step  1019 ). 
     If yes, the UE  101  can release the pre-scheduled extended mode 3 and switch completely to mode 4 (step  1021 ). 
     The UE  101  checks if there are collisions or congestions on the mode 4 transmissions (step  1023 ). 
     The UE  101  can preempt or skip its extended mode 3 transmission having lower priority e.g. CAM for higher priority mode 4 transmissions, e.g. DENM (step  1025 ). 
       FIG.  11    shows a schematic diagram summarizing a procedure  1100  for V2V communication in the communication network  100  according to an embodiment, wherein the communication network  100  comprises a first base station  131   a , a first UE  101   a , a second base station  131   b  and a second UE  101   b  and wherein the first UE  101   a  is entering an out of coverage area of the cellular network defined by a tunnel and the second UE  101   b  is leaving the tunnel. The procedure  1100  shown in  FIG.  11    comprises the following steps: 
     The network operator can install map information and transmit the map information to the first base station  131   a  (step  1101 ). 
     The first UE  101   a  transmits the channel and position information to the first base station  131   a  (step  1102 ). 
     The second UE  101   b  transmits the information about the OOC event, e.g. channel information, collisions, position information and velocity history to the second base station  131   b  as the second UE  101   b  is leaving the out of coverage area (step  1103 ). 
     The second base station  131   b  shares the information about the OOC event with the first base station  131   a  (step  1104 ). 
     The first base station  131   a  configures a UE default operation profile based on the received information from the second base station  131   b  (step  1105 ). 
     The default operation profile can be broadcast to the first UE  101   a  via SIB21 message (step  1106 ). 
     The first UE  101   a  sends a request of SPS (semi-persistent schedule) or resource, e.g. for periodic traffic to the first base station  131   a  (step  1107 ). 
     If the first base station  131   a  realizes that the first UE  101   a  will enter the out of coverage area defined by the tunnel, the first base station  131   a  can configure SPS (semi-persistent schedule) for the first UE  101   a  specifically for the OOC event (step  1108 ). 
     The first base station  131   a  transmits the SPS configuration to the first UE  101   a  for the OOC event (step  1109 ). 
     After the first UE  101   a  enters the out of coverage area, the first UE  101   a  can use Extended Mode 3 based on the received configuration from the first base station  131   a  in parallel to mode 4 transmissions, or the first UE  101   a  can perform sensing on resources and then release or preempt resources (step  1110 ). 
     After the first UE  101   a  leaves the out of coverage area, the first UE  101   a  can transmit the information about the OOC event to the second base station  101   b  (step  1111 ). 
     According to another embodiment, the communication network supports the 3GPP standard LTE-A Release 14. The base station  131 , hence the network infrastructure, is aware of an OOC event (in particular a type II OOC event) and configures an SPS (semi-persistent schedule) associated with a timer, according to which the UE  101  requesting the SPS will keep using the allocated resources also for the OOC event. The configured timer can be signaled by the base station  131  as an extra field in the RRC message “sps-ConfigSL-14”  1200  as shown in  FIG.  12   , before the UE  101  enters an area out of coverage of the base station. The extra field to be added can be:
         sps-TimeKeep-r16 INTEGER (0 . . . 10000),
 
wherein INTEGER (0 . . . 10000) represents 0, 1, . . . 10000 msec.
       

     The UE  101  receiving the above SPS configuration can keep on using the associated resources for the indicated amount of time upon entering an out of coverage area. Upon the expiration of the timer, the UE  101  can switch to Mode 4 operation, i.e., release Extended Mode 3, if the UE  101  is still out of coverage, which is illustrated in  FIG.  13   . 
       FIG.  13    shows resource allocation and utilization based on a SPS configuration according to an embodiment, wherein the base station  131  activates a SPS (semi-persistent schedule) with a timer for an OOC event. When the UE  101  enters an out of coverage area, the UE  101  can operate in extended mode 3 for transmissions (stage  1303 ). After the SPS timer expires, the UE  101  can release extended mode 3 and switch to mode 4 completely (stage  1305 ). 
       FIG.  14    shows a diagram illustrating a method  1400  of operating the UE  101 . The method  1400  comprises the step  1401  of switching from the first mode to the second mode, if a first condition relating to an out of coverage event/status of the UE  101  is met; and a second condition related to a further parameter is met. 
       FIG.  15    shows a diagram illustrating a method  1500  for operating the network entity  131 . The method  1500  comprises the step  1501  of providing the UE  101  with information about when to switch to the second mode, in particular based on one or more of the following conditions: a time elapsed since the start of an OOC event is larger than a time threshold; a distance travelled by the UE  101  since the start of an OOC event is larger than a distance threshold; the UE  101  does not operate in the first mode with a probability defined by the network entity  131 ; and/or a load of the resources allocated for the first mode is larger than a predefined threshold. 
     While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations or embodiments, such feature or aspect may be combined with one or more other features or aspects of the other implementations or embodiments as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “include”, “have”, “with”, or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprise”. Also, the terms “exemplary”, “for example” and “e.g.” are merely meant as an example, rather than the best or optimal. The terms “coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other. 
     Although specific aspects have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein. 
     Although the elements in the following claims are recited in a particular sequence with corresponding labelling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
     Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.