Patent Description:
<NPL>, discuss the D2D synchronization procedure for synchronized transmission of D2D discovery and communication signal. R1-<NUM> notes that eNB should manage some of the UEs within coverage, especially cell edge UEs, to transmit D2DSS/PD2DSCH in the above scenarios. For example, eNB can configure a threshold, and only the UEs whose RSRP or PSS/SSS receiving power is lower than the threshold may transmit D2DSS/PD2DSCH.

A detailed description of systems and methods consistent with embodiments of the present disclosure is provided below. While numerous specific details are set forth in the following description in order to provide a thorough understanding of the embodiments disclosed herein, some embodiments may be practiced without some or all of these details. Moreover, for the purpose of clarity, certain technical material that is known in the related art has not been described in detail in order to avoid unnecessarily obscuring the disclosure.

Wireless mobile communication technology uses various standards and protocols to transmit data between a node (e.g., a transmission station or a transceiver node) and a wireless device (e.g., a mobile communication device). Some wireless devices communicate using orthogonal frequency division multiple access (OFDMA) in a downlink (DL) transmission and single carrier frequency division multiple access (SC-FDMA) in an uplink (UL) transmission. Standards and protocols that use orthogonal frequency division multiplexing (OFDM) for signal transmission include the 3rd Generation Partnership Project (3GPP) long term evolution (LTE) Rel. <NUM>, <NUM> and <NUM>; the Institute of Electrical and Electronics Engineers (IEEE) <NUM> standard (e.g., <NUM>. 16e, <NUM>), which is commonly known to industry groups as WiMAX (Worldwide interoperability for Microwave Access); and the IEEE <NUM>-<NUM> standard, which is commonly known to industry groups as WiFi.

In a 3GPP radio access network (RAN) LTE system, the node may be a combination of Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Node Bs (also commonly denoted as evolved Node Bs, enhanced Node Bs, eNodeBs, or eNBs) and Radio Network Controllers (RNCs), which communicate with the wireless device, known as a user equipment (UE). The DL transmission may be a communication from the node (e.g., eNB) to the wireless device (e.g., UE), and the UL transmission may be a communication from the wireless device to the node.

Proximity-based applications and proximity services (ProSe) represent an emerging social-technological trend. Proximity-based communication, which are also referred to herein as device-to-device (D2D) or peer-to-peer services or communication, is a powerful technique for increasing network throughput by enabling direct communications between mobile stations rather than using network infrastructure, and has a wide variety of applications. For example, D2D has been proposed for local social networks, content sharing, location-based marketing, service advertisements, public safety networks, mobile-to-mobile applications, and other services. D2D communications are of interest due to their ability to reduce load on a core network or a RAN, increase data rates due to direct and short communication paths, provide public safety communication paths, and provide other functionality. The introduction of a ProSe capability in LTE would allow the 3GPP industry to serve this developing market, and, at the same time, serve the urgent needs of several public safety services. This combined use may enable economy of scale advantages because the resulting system may be used for both public safety and non-public-safety services, where possible.

There are various alternatives to realize such a direct communication path between mobile devices. In one embodiment, the D2D air interface PCS (i.e., interface for D2D communication) could be realized by some type of short-range technology, such as Bluetooth or Wi-Fi, or by reusing licensed LTE spectrum, such as a UL spectrum in frequency division duplex (FDD) system and UL subframes in time division duplex (TDD) system. Furthermore, D2D communications can be generally divided into two parts. The first part is device discovery, whereby UEs are able to determine that they are within range and/or available for D2D communication. Proximity detection may be assisted by network infrastructure, may be performed at least partially by the, and/or may be performed largely independent of the network infrastructure. The second part is direct communication, or D2D data communication, between UEs, which includes a process to establish a D2D session between UEs as well as the actual communication of user or application data. D2D communication may or may not be under continuous control of a mobile network operator (MNO). For example, the UEs may not need to have an active connection with an eNB in order to take part in D2D communications. It should be noted that D2D communication (i.e., the second part) can be implemented and operated by D2D capable UEs independently without support of D2D discovery (i.e., the first part).

In general, three different deployment scenarios need to be supported for D2D communication, including in-network coverage, partial network coverage, and out-of-network coverage. <FIG> illustrates an out-of-network coverage scenario where UE <NUM> is in direction communication with UE <NUM>. The UEs <NUM>-<NUM> are out-of-coverage of an eNB or are not capable of direct communication with an eNB. <FIG> illustrates a partial network coverage scenario where UE <NUM> is within coverage of an eNB <NUM> and in communication with UE <NUM> that is not within coverage of any eNB <NUM>. <FIG> illustrates an in-network coverage scenario with a plurality of UEs <NUM>-<NUM> which are all in network coverage and in D2D communication with each other. <FIG> illustrates a UE <NUM> that is within coverage of a first eNB <NUM> that is in D2D communication with UE <NUM> that is within coverage of a second eNB <NUM>.

One of common essential issues applicable to D2D communication in each of the above scenarios is how to design a synchronization protocol in order to enable D2D UE to synchronize to other UEs to achieve time and frequency synchronization for D2D communication. For in-network coverage scenarios, the synchronization procedure is reasonably straightforward. According to agreement by the LTE RAN1 working group, synchronization sources that are eNBs have higher priority over synchronization sources which are D2D UEs, and hence all D2D enabled UEs in coverage will connect to the serving cell and derive the synchronization information based on a detected primary synchronization signal (PSS) or secondary synchronization source (SSS). However, the situation becomes a bit more complicated for partial network coverage and out-of-network coverage scenarios, and how to design the synchronization procedure for these cases to achieve the design target of synchronous D2D communication is still an open problem.

In some embodiments, the present disclosure addresses synchronization procedures for scenarios when UEs are within different coverage areas, such as the scenarios of <FIG>. For example, when UEs are within different coverage areas (i.e., different cells or one UE is out of coverage) the UEs may be out of synchronization. In one embodiment, an eNB may designate UEs at coverage border area to transmit D2D synchronization signals (D2DSSs) according to a reference signal received power (RSRP) report. In some embodiment, this may be undesirable due to unnecessary transmission of D2DSS(s) which may thereby increasing UE power consumption as well as reducing usage efficiency or wireless spectrum resources.

The present application discloses synchronization procedures with details on related network and UE behaviors such as how to select synchronization sources that are within in-coverage. <FIG> illustrate example operation according to example embodiments. <FIG> illustrates a synchronization procedure for a partial network scenario with a plurality of in-coverage UEs <NUM>-<NUM> and a plurality of out-of-coverage UEs <NUM>-<NUM>. UEs <NUM>, <NUM>, <NUM>, and <NUM> are shown as part of a D2D cluster <NUM>. In one embodiment, UE <NUM> is selected as a synchronization source and transmits D2DSS and physical device-to-device shared channel (PD2DSCH) messages with synchronization information. UEs <NUM> and <NUM> receive the synchronization information to fall in synch with the UEs <NUM> and <NUM> as well as the eNB <NUM>. <FIG> illustrates a synchronization procedure for an inter-cell D2D communication scenario with UEs <NUM>, <NUM> within a cell <NUM> coverage area and UEs <NUM>, <NUM> within a cell <NUM> coverage area. Specifically, UE <NUM> may be selected as a synchronization source and propagates timing information of cell <NUM> using D2DSS or PD2DSCH. Similarly, UE <NUM> may be selected as a synchronization source in cell <NUM> and propagates timing information of cell <NUM> using D2DSS or PD2DSCH.

In one embodiment, a synchronization source (e.g., UE <NUM> of <FIG> and UE <NUM> of <FIG>) might also forward the eNB-originated resource pool configuration via PD2DSCH to UEs out of network coverage (e.g., UE2 and UE3) or that of neighbor cells after decoding a System Information Block Type <NUM> (SIB1) message. Furthermore, one design objective may include minimizing the number of synchronization nodes without sacrificing D2D communication performance. For example, having a minimal number of SS nodes is desirable to reduce the signaling overhead as well as to minimize the UE power consumption. Each eNB <NUM> may explicitly configure a UE as a synchronization source via dedicated RRC signaling to limit a number of UEs that are sending synchronization information. In an embodiment, UEs may autonomously decide when to act as a synchronization source based on preconfigured criteria, such as criteria defined in a 3GPP standard or by an eNB <NUM>.

This application presents two general approaches: a non-claimed reactive scheme and a claimed proactive scheme. In the reactive scheme, the network controls the selection and reselection of UEs as synchronization sources. For example, the network (i.e., an eNB <NUM>) decides which D2D UEs, which are in-coverage, will transmit synchronization signals in a periodical manner. In an example, one UE in network coverage conditionally requests that the network configure it to act as a synchronization source for D2DSS transmission. For example, the UE may request configuration as a synchronization source in response to detecting a D2DSS with timing information not originating from a UE-camped eNB. As another example, the UE may request configuration as a synchronization source for UEs that do not have an eNB-UE interface and can communicate only through a direct link. Note that in future LTE releases a new UE category can be introduced that may not support eNB-UE air interface but supports a UE-UE interface, such as D2D interfaces via an unlicensed spectrum, such as WiFi, or a licensed spectrum, such as a spectrum of a 3GPP LTE network. This new category of devices may benefit from reduced complexity, low power consumption, and low cost.

One example of when a reactive scheme may be used is detection of independent synchronization sources. The scheme may include either muting of their operation or alignment of their timing with the network. The presence of independent synchronization sources (I-SS) may first be detected by other D2D capable UEs (e.g., UEs in a RRC_IDLE mode) that do periodic scanning for synchronization sources. In an example, a single scanning interval is larger than or equal to a D2DSS period plus scanning switching time. The D2D capable UEs propagating eNB timing can force the I-SS to cease its asynchronous operation, if an appropriate rule is defined. For instance, there may be a rule that I-SS shall cease D2DSS transmission if a gateway synchronization source (G-SS) with a predefined stratum level is detected. In this case, the in-coverage UE (that detected the I-SS) may become G-SS and start to periodically transmit D2DSS using the eNB timing and thereby trigger a synchronization source re-selection procedure at the I-SS. This reactive approach may require allocation of UE or cell-specific time scanning intervals. Depending on the network settings, UEs may autonomously take the G-SS role and start periodical transmission of D2DSS signals on the pre-allocated synchronization resource. Alternatively, UEs may report I-SS detection to the eNB and follow eNB instructions to initiate D2DSS periodical transmission. It should be noted that reporting of I-SS detection may trigger multiple UEs to report the same I-SS to the same eNB, for example, if multiple UEs detect I-SS simultaneously. In order to avoid such situation, the UE-specific time scanning intervals may be assigned to reduce probability of simultaneous I-SS detection by different UEs. An eNB may configure one or more specific UEs (or other transmitters) to transmit D2DSS once the one or more specific UEs report detection of the I-SS and/or provide a scheduling request for direct communication.

<FIG> illustrates an example communication flow of a method <NUM> for an eNB-controlled reactive synchronization source enabling scheme. An in-coverage UE <NUM> and corresponding eNB <NUM> may communicate and/or perform operations in the described manner to either mute or synchronize with out-of-coverage UEs <NUM>. The out-of-coverage UEs <NUM> may include UEs that are within coverage of a different eNB, are not connected to another eNB or cell, or are not capable of communicating with an eNB using an eNB-UE interface.

The method <NUM> begins and, at time period <NUM>, the in-coverage UE <NUM> synchronizes with the eNB <NUM> and an RRC connection is set up. For example, the in-coverage UE <NUM> may acquire synchronization to the eNB <NUM> in response to a cell search procedure and may establish a radio resource control (RRC) connection with D2D capable eNB by performing a random access procedure. The in-coverage UE <NUM> is camped on the chosen cell in response to time period <NUM>.

At time period <NUM>, the eNB provides scanning configuration information to the in-coverage UE <NUM>. This may occur, for example, in response to D2D communication capability information for the in-coverage UE <NUM> being transferred to the eNB <NUM>. The scanning configuration information may indicate when the in-coverage UE <NUM> should scan for synchronization sources and/or when a report should be send to the eNB <NUM>. For example, the scanning configuration information may be applicable for the in-coverage UE <NUM> when the in-coverage UE <NUM> is in RRC_Connected mode. Although scanning configuration information is shown as explicitly indicated by the eNB <NUM>, a portion or all of the scanning configuration information may be built into a standard such that it is not necessary to explicit all or a portion of the scanning configuration information.

The scanning configuration information may include reporting criterion that triggers the in-coverage UE <NUM> to send a scanning report. Sending a scanning report may be either periodical or a single event. A plurality of example implementations for event-triggered reporting criteria is provided herein. In the first alternative, a plurality of sub-conditions may need to be met in order for the in-coverage UE <NUM> to send a scanning report. The first condition may include that at least one D2DSS is detected. Specifically, the detected D2DSS may need to be transmitted by either a UE that is outside coverage of the eNB <NUM> (e.g., see UEs <NUM>, <NUM> of <FIG>) or by a UE relaying synchronization information from another eNB (e.g., see UEs <NUM>, <NUM> of <FIG> that are within a different cell coverage area than UEs <NUM>, <NUM>). The second condition may include that at least one of the detected D2DSS(s) is not synchronized to the UE-camped network (e.g., timing difference between detected DSDSS and eNB <NUM> is larger than a predefined threshold). The idea is to avoid the situation that an in-coverage UE <NUM> becomes a synchronization source when the detected D2DSS transmitted by out-of-coverage UE <NUM> is already synchronized to the camped eNB <NUM> due to other in-coverage synchronization signal multi-hop forwarding. The third condition is that the in-coverage UE <NUM>, which is in an RRC_Connected mode, is not already configured as a synchronization source.

In a second alternative, one or more of the above sub-conditions of the first alternative may be required and an additional requirement may also be included. In the second alternative, the additional requirement may include that a measurement result for the serving cell (i.e., eNB <NUM>) is less than or equal to a threshold parameter, such as a serving cell signal strength threshold. For example, an RSRP value of a cell specific reference signal (CRS) or signal strength of a PSS or SSS may need to be less than the parameter. The threshold value may be configured to limit selection of the in-coverage UE <NUM> as a synchronization source when the in-coverage UE <NUM> is near a network border but still within network coverage. The threshold parameter may include an RSRP threshold that may be configured by SIB. For example, the threshold parameter may be configured with of a plurality of available values. Example values may include values from the set {-infinity, -<NUM>, -<NUM>,. , -<NUM>, +infinity} Decibel-milliwatts (dBm), wherein the values between -<NUM> and -<NUM> dBm are incremented by <NUM> dBm.

According to one example implementation, for an in-coverage UE <NUM> instructed by eNB as a D2D synchronization source through dedicated RRC message, in each subframe in the D2DSS resource, the UE shall transmit D2DSS if the subframe does not conflict with cellular transmission from the in-coverage UE <NUM> perspective regardless of transmitting scheduling assignment or D2D data. Alternatively, for an in-coverage UE <NUM> who becomes a D2D synchronization source, if the eNB has not instructed it by dedicated signaling to act as synchronization source, the UE shall transmit D2DSS if the subframe does not conflict with cellular transmission from the in-coverage UE <NUM> perspective and UE is transmitting scheduling assignment or D2D data within the scheduling assignment or D2D data period. Other conditions may also be required, such as whether the in-coverage UE <NUM> has a proper capability. Additional requirements may include that the subframe is within the scheduling assignment or D2D data period in which scheduling assignment or data is transmitted. An additional requirement may include that the in-coverage UE <NUM> is in RRC_Connected mode and/or that the UE is not transmitting scheduling assignment or D2D data within the same time period as the D2D SS.

In a third alternative, one or more of the above sub-conditions of the first and second alternatives may be required, in addition to yet another additional requirement. Specifically, the second alternative may require that a signal strength of the detected D2DSS be at least greater than or equal to a threshold value. For example, the RSRP or reference signal received quality (RSRQ) for at least one detected D2DSS from UEs in/outside network coverage or inter-cell UEs must be required to exceed a peer signal quality or peer signal strength threshold.

In an example, the scanning configuration information may include one or more sub-conditions for when the in-coverage UE <NUM> should scan for synchronization sources. For example, depending on network settings, the in-coverage UE <NUM> may trigger a scanning procedure when all or some of the predefined or eNB <NUM> configured sub-conditions are satisfied. For example, one or more of the sub-conditions of the above alternatives may need to be satisfied before scanning, such as that a measured RSRP value for a signal from the eNB <NUM> exceeds a cell strength threshold. In an example, an entering condition must be met for a duration corresponding to a "timeToTrigger" parameter configured by eNB <NUM> in order for the event to be triggered According to a further example, the in-coverage UE <NUM> scales the timeToTrigger parameter depending on its speed.

The scanning configuration information may indicate a reporting format for reports to be sent to the eNB <NUM>. The reporting formation information may indicate quantities or parameters that the in-coverage UE <NUM> should include in the scanning report. In an example, the in-coverage UE <NUM> reports one or more of the following such as synchronization source identity, stratum level, and strength of detected D2DSS signal. According to another example, the reporting format (list of reported parameters and related metrics) may be pre-configured (specified) instead of being configured as part of the scanning configuration.

Time period <NUM> includes various operations and methods for enabling a synchronization source. At time period <NUM>, the in-coverage UE <NUM> performs a synchronization source scanning operation to detect potential D2DSS(s) in the proximity and applies the scanning configuration information from E-UTRAN, as discussed in relation to time period <NUM> above. For example, the in-coverage UE <NUM> may detect the out-of-coverage UEs <NUM>. The scanning configuration information may be preconfigured based on a communication standard, or in any other manner. If the in-coverage UE <NUM> detects the presence of a synchronization source, it may acquire several information items from the D2DSS including detected synchronization source identity, stratum level, and strength of detected D2DSS signal. The eNB <NUM> may provide the in-coverage UE <NUM> with the parameters to simplify scanning performance such as the resources that the in-coverage UE <NUM> should scan for the presence of D2DSS signals or time intervals during which the scanning should be conducted.

At time period <NUM> the in-coverage UE <NUM> transfers scanning results to the E-UTRAN (i.e., eNB <NUM>), including information indicated by the reporting format discussed above in relation to the scanning configuration information. The report includes a plurality of detected D2DSS arranged in order of decreasing receiving power order, for example, the synchronization cell with strongest D2DSS receiving power may be included first. Moreover, the in-coverage UE <NUM> may be configured to provide a number of periodic reports after having triggered a scanning report event. For example, this event-triggered periodic report can be achieved by means of parameters "reportAmount" and "reportInterval", which may be preconfigured or configured by the eNB <NUM> and which specify respectively the number of periodic reports and the time period between them. If event-triggered periodic reporting is configured, the in-coverage UE's <NUM> count of the number of scanning reports may be reset to zero whenever a new D2DSS meets the entry condition. If the in-coverage UE <NUM> is configured to perform periodic measurement reporting, the in-coverage UE <NUM> may start reports immediately when the periodic reporting timer is expired.

At time <NUM>, the eNB <NUM> enables the in-coverage UE <NUM> as a synchronization source. The eNB <NUM> decides whether the reporting in-coverage UE <NUM> is selected as a synchronization source according to the scanning result reporting at time <NUM>. For example, the eNB <NUM> may compare information in the report from the in-coverage UE <NUM> to other UEs which may have detected one or more of the same synchronization sources and select the UE that is closest or otherwise best situated to communicate with the out-of-synch UEThe eNB <NUM> may send dedicated RRC signaling to UE <NUM> which enables the in-coverage UE <NUM> as the synchronization source and includes parameters such as transmission power for D2DSS or PD2DSCH. The parameters may indicate that the in-coverage UE <NUM> shall transmit D2DSS if the UE is in RRC Connected mode. The parameters indicate that the in-coverage UE <NUM> will have a stratum level higher than a detected out-of-coverage UE <NUM>.

At time <NUM>, the in-coverage UE <NUM> transmits D2DSS/PD2DSCH periodically to provide a synchronization reference to the out-of-coverage UEs <NUM> and/or other UEs within a D2D cluster. The in-coverage UE <NUM> may derive the timing information for the synchronization reference from the eNB <NUM> and propagate it further. After the out-of-coverage UEs <NUM> detect D2DSS from the in-coverage UE <NUM>, the out-of-coverage UEs <NUM> will synchronize to the in-coverage UE <NUM> and thereby be synchronized with the eNB <NUM>. The in-coverage UE <NUM> may only send D2DSS/PD2DSCH as a synchronization reference when D2D communication is triggered by the upper layer of the in-coverage UE <NUM>. For example, an application layer, RRC layer, or other layer may indicate that D2D communication should take place and then the D2DSS/PD2DSCH may be sent. Triggering transmission of the D2DSS/PD2DSCH based on an upper layer may help to minimize power consumption at the in-coverage UE <NUM>. The in-coverage UE <NUM> may send D2DSS/PD2DSCH as a synchronization reference always in each subframe in the D2DSS/PD2DSCH resource that does not conflict with cellular communication from the UE <NUM> perspective, after it is instructed as a D2D synchronization source by the eNB through dedicated RRC signaling.

Another approach for D2D synchronization is a proactive scheme, which is subject to the claimed invention. In the proactive approach, the network may configure D2D capable UEs to periodically transmit D2DSS signals in order to prevent appearance of the I-SS in the network proximity or coverage holes. For instance, the eNB <NUM> may pre-allocate periodic synchronization resources and configure all D2D capable UEs to periodically transmit D2DSS signals. Alternatively, the eNB <NUM> configures specific conditions that should be met to start autonomous transmission of D2DSS signals. An RSRP threshold is pre-configured or signaled by the eNB <NUM>, so that if the received power of the D2DSS (or cell-specific reference signal (CRS) in case of eNB <NUM>) is below threshold, in-coverage UE <NUM> autonomously starts D2DSS transmission. It should be noted that not only threshold relative to eNB <NUM> but also relative to other UE synchronization sources can be configured and signaled by the eNB <NUM>.

The E-UTRAN (eNB <NUM>) may also provide additional parameters for D2DSS configuration, such as a transmission power, wireless resource zones, or the like. In one embodiment, the D2DSS configuration is broadcast by the eNB <NUM> on system information (e.g., SIB) so that in-coverage UEs <NUM> in an RRC Idle state still conditionally act as synchronization sources, at least in some circumstances.

In one embodiment, the two following conditions may be considered or evaluated at the in-coverage UE <NUM> side: in-coverage UE <NUM> detects any D2DSS originated from out-of-coverage UE <NUM> (see, e.g., <FIG>) or other eNBs (see, e.g., <FIG>), and the measured RSRP or PSS/SSS signal quality from E-UTRAN serving cell (e.g., eNB <NUM>) falls below a configured threshold. In response to detecting these conditions, the in-coverage UE may autonomously become a synchronization source and start forwarding eNB-originated timing by transmitting D2DSS on eNB <NUM> configured time/frequency resources. Similarly, any of the other conditions defined in the reactive approach illustrated in <FIG> may also be considered by the incoverage UE <NUM> to decide whether and/or when to autonomously forward timing information. For example, the in-coverage UE <NUM> may consider information regarding the timing difference between detected D2DSS and a camped eNB <NUM> and the detected D2DSS signal strength, or the like. In one embodiment, the proactive approach allows UEs that are in an RRC Idle mode to act as synchronization sources while the reactive approach may not allow the UEs to act as synchronization sources in RRC Idle mode due to the required communication with the eNB <NUM>.

<FIG> is a schematic block diagram of a UE <NUM> configured to operate according to one or more of the reactive approach and proactive approach discussed above. For example, the UE <NUM> may implement the functionality of the in-coverage UE <NUM> and/or the out-of-coverage UEs <NUM> discussed in relation to <FIG>. The UE <NUM> includes a communication component <NUM>, a synchronization component <NUM>, a settings component <NUM>, a scan component <NUM>, a report component <NUM>, an activation component <NUM>, and a timing transmission component <NUM>. The components <NUM>-<NUM> are given by way of example only and may not all be included in all embodiments and exampled implementations discussed herein. Each of the components <NUM>-<NUM> may be included in or may be implemented by a UE <NUM>.

The communication component <NUM> is configured to communicate with a base station and/or one or more peer UEs. For example, the communication component <NUM> may communicate with an eNB to obtain network services, such as voice services and data services; receive configuration information such as scanning configuration information; or the like. The communication component <NUM> may also be configured to communicate with one or more D2D enabled UEs for D2D communications and proximity services.

The synchronization component <NUM> is configured to synchronize with a synchronization source. For example, if the UE <NUM> is within range of an eNB, the synchronization component <NUM> may synchronize with the eNB based on timing information received from the eNB. Similarly, if the UE <NUM> is outside of network coverage, the UE <NUM> may synchronize with a synchronization source that has a highest detected stratum level, such as another UE or an in-coverage UE.

The settings component <NUM> is configured to receive scanning configuration information from an eNB <NUM>. The scanning configuration information may include any of the information or settings discussed herein, such as the settings discussed in relation to the reactive or proactive approaches. In one embodiment, the settings component <NUM> receives scanning configuration information that indicates when the UE <NUM> should scan for synchronization sources. For example, the settings component <NUM> may receive a cell signal strength threshold that indicates that the UE <NUM> is to scan for D2D synchronization sources in response to a signal from the eNB <NUM> falling below the cell signal strength threshold while the UE <NUM> remains within coverage of the eNB <NUM>. In one embodiment, the scanning configuration information may indicate one or more resource zones (frequency bands and/or timing) in which to scan for synchronization sources, such as scan for D2DSS or PD2DSCH, that include timing information.

In one embodiment, the scanning configuration information may include one or more reporting requirements. For example, the reporting requirements may indicate when the UE <NUM> will send a report to the eNB <NUM> and what information it will include. In one embodiment, the reporting requirements may include one or more reporting triggers. In one embodiment, a reporting requirement may include that a detected D2D synchronization source is not synchronized with the eNB <NUM>. In one embodiment, the reporting requirement may indicate a threshold value for a base station signal strength. In one embodiment, the reporting requirements may indicate that a report should include one or more of an identity of the D2D synchronization source, a stratum level of the D2D synchronization source, and a signal strength of a signal received from the D2D synchronization source. These details may be derived, for example, from a D2DSS during scanning.

The scanning configuration information may also include information on when the UE <NUM> should autonomously activate itself as a synchronization source. For example, one or more of the above scanning or reporting settings may be used to trigger the transmission of timing information or other synchronization information for receipt for one or more nearby D2D UE. In one embodiment, the scanning configuration information includes D2D zones including periodic time and frequency resources for transmitting peer synchronization signals comprising the timing information.

The scan component <NUM> is configured to scan for D2D synchronization sources. For example, the scan component <NUM> may scan for peer synchronization sources that are out of synch with the UE <NUM> based on scanning configuration information which has been preconfigured and/or received from an eNB <NUM>. In one embodiment, the scan component <NUM> is configured to scan for D2D synchronization sources when in response to a signal from the eNB <NUM> (e.g., based on an RSRP or RSRQ) falling below a cell signal strength threshold or cell quality threshold.

The report component <NUM> is configured to report detection of out-of-synch synchronization sources to an eNB <NUM>. In one embodiment, the report component <NUM> reports to the eNB <NUM> based on reporting requirements received in scanning configuration information received from the eNB <NUM>. For example, the report component <NUM> may evaluate one or more report triggering conditions and report to the eNB <NUM> when those conditions are met. In one embodiment, the report component <NUM> may include details about a detected D2D synchronization source which may be derived from a detected D2DSS or PD2DSCH. For example, the report may include one or more of an identity of the synchronization source, a stratum level of the synchronization source, and a signal strength or signal quality of a signal received from the synchronization source.

The activation component <NUM> is configured to activate the UE <NUM> as a D2D synchronization source. In one embodiment, the activation component <NUM> activates the UE <NUM> as a synchronization source in response to receiving a signal from the eNB <NUM> enabling the UE <NUM> as a synchronization source. In one embodiment, the signal from the eNB <NUM> may include one or more details for the UE <NUM> including a stratum level (such as a stratum level higher than a detected synchronization source), a transmission power for timing information, a timing for sending timing information, and/or frequency resources on which timing information should be sent.

In one embodiment, the activation component <NUM> activates the UE <NUM> as a synchronization source autonomously. For example, the activation component <NUM> may evaluate one or more trigger conditions to determine when the UE <NUM> should begin forwarding timing information in a D2DSS or PD2DSCH. In one embodiment, the trigger conditions may be received from the eNB <NUM> or may be stored by the UE <NUM> based on a communication standard, such as 3GPP LTE. Example trigger conditions may include that a signal from the eNB <NUM> is below a cell signal strength threshold, that an out-of-synch UE has been detected, or the like. In one embodiment, the activation component <NUM> is configured to autonomously activate the UE <NUM> as a synchronization source while the UE is in an idle mode, such as RRC_Idle.

The timing transmission component <NUM> is configured to transmit signals to provide a synchronization reference. In one embodiment, the timing transmission component <NUM> causes the UE <NUM> to transmit D2DSS or a physical device-to-device shared channel signal (PDSCH) that includes timing information derived from the eNB <NUM>. The transmitted signals may service as a synchronization reference to one or more UEs that are within range of the UE <NUM>. In one embodiment, the timing transmission component <NUM> forwards timing information originating from the eNB <NUM> using a peer synchronization signal in response to the activation component autonomously activating the wireless communication device as a synchronization source. In one embodiment, the timing transmission component <NUM> may forward timing information when the UE <NUM> is in an RRC_Idle mode. In one embodiment, the timing transmission component <NUM> transmits timing information in response to a trigger by an upper layer of the UE <NUM>.

<FIG> is a schematic block diagram of an eNB <NUM> configured to operate according to one or more of the reactive approach and proactive approach discussed above. For example, the eNB <NUM> may implement the functionality of the eNB <NUM> of <FIG>. The eNB <NUM> includes a communication session component <NUM>, a scan configuration component <NUM>, a report receipt component <NUM>, a selection component <NUM>, and an enablement component <NUM>. The components <NUM>-<NUM> are given by way of example only and may not all be included in all embodiments. Each of the components <NUM>-<NUM> may be included in or may be implemented by an eNB <NUM>.

The communication session component <NUM> is configured to communicate with one or more UEs and establish communication sessions with the one or more UEs. For example, the communication session component <NUM> may be able to communicate with UEs that are within range of the eNB <NUM> to establish and maintain a communication session. In one embodiment, the communication session component <NUM> may provide reference signals or other timing information to synchronize with a UE while establishing a connection or communication session with the UE.

The scan configuration component <NUM> may provide scanning configuration information to one or more UEs that are within range of the eNB <NUM>. For example, scan configuration component <NUM> may provide any of the scanning configuration information discussed herein, such as scanning configuration information discussed above in relation to the settings component <NUM>.

The report receipt component <NUM> is configured to receive a scanning report from the UE <NUM>, such as a report provided by the report component <NUM>. In one embodiment, the report may include any of the report information discussed herein such as an identity, a signal strength, or a stratum level of a synchronization source detected by the UE <NUM>. In one embodiment, the report receipt component <NUM> is configured to receive a plurality of reports from a plurality of different UEs.

The selection component <NUM> determines whether to select a UE as a synchronization source. For example, the selection component <NUM> may determine whether to select the UE <NUM> after a report is received from the UE <NUM>. The selection component may select one of a plurality of UEs which have sent a scanning report that identifies the same one or more out-of-synch synchronization sources. For example, the selection component <NUM> may select a UE that has the best signal strength for the particular synchronization source(s).

The enablement component <NUM> sends a signal to a UE to enable it as a synchronization source. For example, the enablement component <NUM> may send a message that enables the UE <NUM> as a synchronization source. The enablement component <NUM> may provide one or more synchronization details to the selected UE including one or more of a stratum level, transmission power, or other information in order to provide synchronization information to nearby D2D devices.

<FIG> is a schematic flow chart diagram illustrating an example method <NUM> for D2D synchronization. The method <NUM> may be performed by a wireless communication device, such as the UE <NUM> of <FIG>.

The method <NUM> begins and a synchronization component <NUM> is configured to synchronize <NUM> with a base station, such as the eNB <NUM>. For example, the wireless communication device may synchronize <NUM> upon connection to the base station and/or establishing a communication session with the base station.

An activation component <NUM> autonomously activates <NUM> the wireless communication device as a synchronization source based on one or more trigger conditions. The trigger conditions may include any of the conditions discussed above in relation to the scanning configuration information or the proactive approach. In one embodiment, the trigger conditions are received from the base station. In another embodiment, the trigger conditions are preconfigured at the wireless communication device based on a communication standard.

The timing transmission component <NUM> forwards <NUM> timing information originating from the base station using a peer synchronization signal. In one embodiment, the timing transmission component <NUM> forwards <NUM> the timing information in response to the activation component <NUM> autonomously activating <NUM> the wireless communication device as a synchronization source. The timing transmission component <NUM> may send a D2DSS or PD2DSCH that includes the timing information. In one embodiment, one or more of autonomously activating <NUM> and forwarding <NUM> may occur while the wireless communication device remains in an idle mode.

<FIG> is a schematic flow chart diagram illustrating an example method <NUM> for D2D synchronization. The method <NUM> may be performed by a base station, such as the eNB <NUM> of <FIG>.

The method <NUM> begins and a communication session component <NUM> communicates <NUM> with one or more UEs within a coverage area of the eNB <NUM> and the communication session component <NUM> synchronizes <NUM> with one or more UEs. For example, the communication session component <NUM> may send one or more signals with timing information as a synchronization reference for the UEs.

A scan configuration component <NUM> provides <NUM> signaling to the one or more UEs to configure synchronization transmission conditions for synchronization signal transmission. For example, the scan configuration information may include any of the scanning requirements, reporting requirements, requirements for activation, or any other of the scanning configuration information discussed herein. In one embodiment, the synchronization transmission conditions specify when the one or more UEs within the coverage area of the eNB should autonomously send synchronization signals. The UEs may then autonomously evaluate the synchronization transmission conditions to determine when to activate themselves as synchronization sources and forward timing information.

<FIG> provides an example illustration of a mobile device, such as a UE, a mobile station (MS), a mobile wireless device, a mobile communication device, a tablet, a handset, or another type of mobile wireless device. The mobile device may include one or more antennas configured to communicate with a node, macro node, low power node (LPN), or transmission station, such as a base station (BS), an eNB, a base band unit (BBU), a remote radio head (RRH), a remote radio equipment (RRE), a relay station (RS), radio equipment (RE), or another type of wireless wide area network (WWAN) AP. The mobile device may be configured to communicate using at least one wireless communication standard, including 3GPP LTE, WiMAX, High Speed Packet Access (HSPA), Bluetooth, and WiFi. The mobile device may communicate using separate antennas for each wireless communication standard or shared antennas for multiple wireless communication standards. The mobile device may communicate in a WLAN, a wireless personal area network (WPAN), and/or a WWAN.

<FIG> also provides an illustration of a microphone and one or more speakers that may be used for audio input and output from the mobile device. The display screen may be a liquid crystal display (LCD) screen or other type of display screen, such as an organic light emitting diode (OLED) display. The display screen may be configured as a touch screen. The touch screen may use capacitive, resistive, or another type of touch screen technology. An application processor and a graphics processor may be coupled to internal memory to provide processing and display capabilities. A non-volatile memory port may also be used to provide data input/output options to a user. The non-volatile memory port may also be used to expand the memory capabilities of the mobile device. A keyboard may be integrated with the mobile device or wirelessly connected to the mobile device to provide additional user input. A virtual keyboard may also be provided using the touch screen.

Various techniques, or certain aspects or portions thereof, may take the form of program code (i.e., instructions) embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, a non-transitory computer readable storage medium, or any other machine readable storage medium wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the various techniques. In the case of program code execution on programmable computers, the computing device may include a processor, a storage medium readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. The volatile and non-volatile memory and/or storage elements may be a RAM, an EPROM, a flash drive, an optical drive, a magnetic hard drive, or another medium for storing electronic data. The eNB (or other base station) and UE (or other mobile station) may also include a transceiver component, a counter component, a processing component, and/or a clock component or timer component. One or more programs that may implement or utilize the various techniques described herein may use an application programming interface (API), reusable controls, and the like. Such programs may be implemented in a high-level procedural or an object-oriented programming language to communicate with a computer system. However, the program(s) may be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations.

It should be understood that many of the functional units described in this specification may be implemented as one or more components, which is a term used to more particularly emphasize their implementation independence. For example, a component may be implemented as a hardware circuit comprising custom very large scale integration (VLSI) circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A component may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like.

Components may also be implemented in software for execution by various types of processors. An identified component of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions, which may, for instance, be organized as an object, a procedure, or a function. Nevertheless, the executables of an identified component need not be physically located together, but may comprise disparate instructions stored in different locations that, when joined logically together, comprise the component and achieve the stated purpose for the component.

Indeed, a component of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within components, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network. The components may be passive or active, including agents operable to perform desired functions.

Reference throughout this specification to "an example" means that a particular feature, structure, or characteristic described in connection with the example is included in at least one embodiment or example implementation of the present disclosure. Thus, appearances of the phrase "in an example" in various places throughout this specification are not necessarily all referring to the same embodiment or example implementation.

Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on its presentation in a common group without indications to the contrary. In addition, various embodiments and examples of the present disclosure may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present disclosure.

Claim 1:
A user equipment, UE (<NUM>, <NUM>), the UE (<NUM>, <NUM>) being in coverage of a cell of an evolved universal terrestrial radio access network, E-UTRAN, wherein the UE (<NUM>, <NUM>) is configured to:
determine a reference signal received power, RSRP, measurement of a synchronization signal received from the cell of the E-UTRAN, wherein the synchronization signal is a cell-specific reference signal, CRS;
compare the RSRP measurement to a signal strength threshold that has been signaled to the UE (<NUM>, <NUM>) from an evolved Node B, eNB (<NUM>); and
transmit, in response to a determination that the RSRP measurement is below the signal strength threshold, a direct link synchronization signal for discovery or communication and autonomously activate the UE (<NUM>, <NUM>) as a synchronization source,
wherein the UE (<NUM>, <NUM>) is in Radio Resource Control, RRC, idle mode; and the UE (<NUM>, <NUM>) is configured to receive direct link synchronization signal configuration broadcasted by the eNB on a system information block, and wherein the transmitted direct link synchronization signal for discovery or communication is configured based on the received direct link synchronization signal configuration.