HK1248963B - Signaling for inter-cell d2d discovery in an lte network - Google Patents

Description

Signaling for inter-cell D2D discovery in LTE networks

This application is a divisional application of the application with national application number 201480053377.1, application date October 21, 2014, and invention name “Signaling for Inter-Cell D2D Discovery in LTE Networks”

Priority claim

This application claims the benefit of U.S. Provisional Patent Application Serial No. 61/898,425, filed on October 31, 2013, the entire contents of which are incorporated herein by reference.

Technical Field

Embodiments are applicable to wireless communications. Some embodiments relate to 3GPP LTE (Long Term Evolution) networks. Some embodiments relate to direct device-to-device (D2D) communication. Some embodiments relate to device discovery in LTE networks.

Background Art

Proximity-based applications and services represent a rapidly evolving social and technological trend, significantly impacting the evolution of cellular wireless/mobile broadband technologies. These services rely on mutual awareness between two devices or users in close proximity and can include applications such as public safety operations, social networking, mobile commerce, advertising, and gaming. Device-to-device (D2D) discovery is the first step in enabling D2D services. However, many unresolved issues remain regarding device discovery for D2D communications, particularly inter-cell proximity-based services (ProSe).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example operating environment in which some embodiments may be implemented.

FIG2 illustrates a portion of a wireless channel according to some embodiments.

FIG3 is a flowchart of a method for supporting inter-cell D2D discovery according to some embodiments.

4 is a block diagram of basic components of a communication device in accordance with some embodiments.

5 is a block diagram of a machine that executes various embodiments.

DETAILED DESCRIPTION

The following description and accompanying drawings sufficiently illustrate specific embodiments to enable those skilled in the art to implement the present invention. Other embodiments may incorporate structural, logical, circuit, program, and other formal changes. Portions and features of some embodiments may be incorporated into or substituted for portions and features of other embodiments. The aforementioned embodiments in the claims include all available equivalents of those claims.

FIG1 illustrates an example operating environment 100 in which some embodiments may be implemented. In operating environment 100, an evolved Node B (eNB) 102 broadcasts synchronization signals 103 within a serving cell 106. In some embodiments, a mobile device, such as a UE, operating as a synchronization source or peer radio head (PRH) (not shown in FIG1 ) can broadcast these synchronization signals 103. The PRH can also serve as a synchronization reference point. In some embodiments, the synchronization signals 103 can include information defining a common timing reference.

In various embodiments, one or more UEs 108 within a serving cell 106 may receive synchronization signals 103 from the eNB 102 and utilize these synchronization signals 103 to enter a synchronous mode of operation based on a common timing reference defined by the synchronization signals 103. One or more UEs 108 may support LTE Proximity-Based Services (ProSe). In some embodiments, UEs 110 outside of the serving cell 106 may not be able to receive the synchronization signals 103 from the eNB 102 and may operate in an asynchronous mode. These UEs 110 may operate in one or more neighboring cells 112 relative to the serving cell 106.

UEs 108 in synchronous mode utilize a synchronous discovery protocol to discover each other using discovery signals 104. According to the synchronous discovery protocol, during predetermined periodic time intervals, UEs 108 in synchronous mode may only need to monitor the air interface and/or send discovery signals 104 over the air interface. The synchronous discovery protocol may include a low duty cycle to allow UEs 108 in synchronous mode to enter a sleep state between periodic time intervals, thereby saving energy on the part of UEs 108 in synchronous mode. However, UEs 110 in asynchronous mode may not be able to use the synchronous discovery protocol, but may need to use an asynchronous discovery protocol. The asynchronous discovery protocol may require UEs 110 in asynchronous mode to continuously send discovery signals 104 and/or continuously monitor the air interface for discovery signals 104 sent by other UEs 108, resulting in a significant increase in power consumption levels. Therefore, inter-cell device-to-device (D2D) discovery may become difficult or impossible to achieve in other operations.

FIG2 illustrates a portion of a wireless channel 200 according to some embodiments. As shown in FIG2 , a portion of the time resources of wireless channel 200 are allocated to implement a discovery resource pool 202, while other time resources of wireless channel 200 are included in a non-discovery time interval 204. Wireless channel 200 also includes a discovery announcement region 206.

To conserve power associated with discovery operations, synchronous mode UEs, such as UE 108 ( FIG. 1 ), may utilize and/or monitor wireless channel 200 only during discovery resource pools 202 and not during non-discovery time intervals 204. However, depending on which discovery resource pool 202 is demarcated from non-discovery time intervals 204, asynchronous mode UEs, such as UE 110 ( FIG. 1 ), may not be aware of a common timing reference. Consequently, to ensure that they transmit discovery signals during periods when synchronous mode UE 108 is monitoring wireless channel 200, asynchronous mode UEs 110 in existing systems may be forced to continuously transmit such discovery signals. This can result in interference with non-discovery communications on wireless channel 200 during non-discovery time intervals 204 and increase power consumption on the part of asynchronous mode UE 110, further complicating inter-cell D2D discovery operations.

To guide these and other relationships, embodiments provide an apparatus and method for supporting inter-cell D2D discovery that can be applied to asynchronous network deployments. Depending on the synchronization characteristics of the deployment (e.g., a synchronous deployment in a time-division duplexing (TDD) system or a synchronous deployment in a typical frequency-division duplexing (FDD) system), inter-cell D2D discovery can be supported in a variety of different ways. Inter-cell D2D discovery can also be supported differently based on the level of coordination between neighboring cells, the level of network assistance available at the UE terminal for inter-cell D2D discovery, and so on.

For synchronous deployments, a network-wide configuration of D2D discovery resource pools can be achieved, thus considerably simplifying the inter-cell D2D discovery process. For asynchronous deployments, D2D discovery resource pools can generally be configured in a cell-specific manner, and embodiments can provide methods to avoid overlap of D2D discovery resources of adjacent cells, thereby avoiding asynchronous interference, which can be more difficult to control than synchronous interference. At least some inter-eNB coordination can help minimize overlapping D2D discovery resource pools between asynchronous cells. Thus, some embodiments provide a coarse inter-eNB time resolution of at most a few radio frames.

In addition to the configuration information of the neighboring cell's D2D discovery resource pool, participating UEs supporting inter-cell D2D discovery also use the synchronization reference time and frequency synchronization source information for the cell to discover other UEs. Embodiments provide a method for making the configuration information of the neighboring cell's D2D discovery resource pool available to UEs that wish to participate in inter-cell D2D discovery.

Regarding reference time and frequency synchronization source information, in some current systems, based on the primary synchronization signal (PSS), secondary synchronization signal (SSS) or positioning reference signal (PRS) of the neighboring cell, the UE can obtain this information directly from the neighboring cell. However, not all UEs can obtain the PSS/SSS/CRS of the neighboring cell. According to the current long-term evolution (LTE) specification, the UE requires the broadband signal to have a signal-to-noise ratio (SINR) of at least -6dB to detect the PSS/SSS, but the near-far effect may prevent some UEs from detecting the PSS/SSS of the neighboring cell. Therefore, at least for these UEs, the D2D discovery performance will be reduced. As an alternative to using PSS/SSS/CRS to obtain such information, an embodiment provides a method for the UE to relay the D2D discovery resource pool configuration information, reference time information and frequency synchronization information of the serving cell to other UEs that can be served by the neighboring cell.

In some embodiments, the eNB 102 or PRH can select a UE 108 or a group of UEs to relay the transmission timing of the corresponding serving cell, so that UEs belonging to other cells and neighboring cells can use this "two-hop" synchronization reference to obtain time and frequency synchronization of the neighboring cell for inter-cell discovery. Therefore, according to an embodiment, the eNB 102 includes hardware processing circuitry to send configuration information of the D2D discovery resource pool configured by the neighboring cell 112 to the selected UE 108 or group of UEs. The configuration information will include the timing offset between the UE 108's serving cell 106 and the neighboring cell 112. Although only one neighboring cell 112 is described, the embodiments are not limited thereto, and the configuration information may include configuration information for multiple D2D discovery resource pools configured by multiple neighboring cells 112. The selected UE 108 can then relay some or all of this information to other UEs outside the serving cell 106.

To enable UE 108 to perform the relay, eNB 102 allocates resources to UE 108, which UE 108 uses to periodically transmit synchronization information of serving cell 106. At the beginning of each occurrence of D2D discovery resource pool 202 ( FIG. 2 ), these resources may also include time resources for a discovery notification signal, which includes data corresponding to discovery notification field 206 ( FIG. 2 ).

In some embodiments, the discovery notification signal can be the same signal as the synchronization signal. However, in some embodiments, the relay UE 108 may need to transmit the synchronization signal more frequently than the discovery notification signal (i.e., more frequently than the D2D discovery resource pool 202 appears). Although the discovery notification signal can carry D2D discovery resource pool configuration information in addition to synchronization information for the serving cell 106, the UE 108 may need to relay the synchronization information for the serving cell more frequently than the resource pool configuration information indicated by the discovery notification signal. This is because, depending on the level of synchronization/asynchronous nature between neighboring cells, if these relayed synchronization signals are only transmitted as part of the discovery notification signal just before the D2D discovery resource pool of the corresponding cell, the UE receiving the synchronization and configuration information may not be able to achieve time/frequency synchronization with the cell. This will raise at least two issues. First, because in addition to transmitting the discovery notification signal at the beginning of each periodic occurrence of the discovery resource pool, the UE will also experience increased power consumption because of the additional periodic transmission of the synchronization signal with a period shorter than the traditional discovery period. Second, resource allocation is required for the transmission of this "relayed" synchronization signal.

To solve the second problem, eNB 102 may allocate the above resources so that UE 108 sends synchronization information to other UEs in neighboring cell 112, such as UE 110, more frequently than UE 108 sends discovery notification signals (ie, more frequently than the D2D discovery resource pool appears).

Regarding the first issue, for example, if information about the coarse timing offset between the serving and neighboring cells is signaled by each respective serving cell to the associated UE, the increase in power consumption can be minimized. If this coarse timing offset information is transmitted, the discovery notification signal itself is sufficient for the UE to achieve synchronization for inter-cell discovery operations, thereby avoiding the need to separately relay the serving cell's synchronization information.

However, in some circumstances, the discovery notification signal itself may not be sufficient to provide the synchronization information. At least in these cases, the eNB 102 will allocate resources to the selected UE to relay the synchronization information. The eNB 102 may allocate these resources so that the UE 108 can send synchronization information more frequently than if the D2D discovery resource pool were present.

The eNB 102 may also allocate resources to avoid overlapping transmissions of synchronization information relayed from UEs served by different cells. In one embodiment, the eNB 102 may reserve subframes for relaying synchronization information for the serving cell 106 to reduce or eliminate overlap between subframes for relaying synchronization information for the serving cell 106 and subframes for relaying synchronization information for one or more serving cells in the neighboring cells 112. This is particularly important when the corresponding D2D discovery resource pools 202 do not overlap. In particular, in some embodiments, the eNB 102 may reserve time-frequency resources on every Kth subframe (e.g., a "synchronization relay subframe") within the D2D discovery resource pool 202, where K is greater than 1, for this purpose. In other embodiments, the eNB 102 may reserve time-frequency resources on every Kth subframe within a set of available D2D subframes of the serving cell 106 for this purpose.

In embodiments where the synchronization signal is a narrowband signal, the UE 108 may use unused physical resource block (PRB) pairs of these synchronization relay subframes to transmit the discovery signal 104. However, the UE 108 should ensure adequate protection of the synchronization signal transmission from in-band emissions. For example, a listening UE 110 in cell 112 ( FIG. 1 ) may not be able to receive the synchronization signal relayed by the UE 108 in cell 106 due to high interference from in-band emissions caused by other discovery signals 104 transmitted in cell 106 at maximum transmission power in adjacent PRB pairs of the synchronization relay subframe. Therefore, in an embodiment, the eNB 102 may limit the transmission power of D2D transmissions on PRBs to be less than a maximum transmission power, except for the transmission power of relayed synchronization signals in subframes allocated for transmission of discovery notification signals or relayed synchronization signals. The maximum transmission power value in the synchronization relay subframe may be predefined or configured by the network 100 via higher layers.

It should be noted that the actual measured impact of in-band emissions may depend on the number of UEs selected to relay the serving cell synchronization signal. Typically, it is beneficial to have only some selected UEs relay this information to minimize the impact on UE power consumption.

In addition to the synchronization relay subframes scattered within the D2D discovery resource pool, the eNB 102 may also configure additional subframes between the D2D discovery resource pools as synchronization relay subframes to increase the speed and reliability of acquiring synchronization information of the neighboring cell 112. For both types of synchronization relay subframes, in the frequency dimension, the actual synchronization signal sent by the UE may be limited to a center PRB pair, so that the above-mentioned set of PRB pairs is located at the center of the system uplink (UL) bandwidth. Alternatively, the eNB 102 may allocate a set of PRB pairs according to a cell-specific offset relative to the center of the system UL bandwidth. This type of cell-specific mapping in the frequency dimension is more beneficial for the synchronization relay subframes appearing between the two D2D discovery resource pools to avoid overlapping of synchronization signals sent by UEs belonging to different neighboring cells.

The eNB 102 may also allocate resources to meet additional criteria. For example, UEs selected to relay synchronization information for a specific cell may transmit their relayed synchronization signals on the same physical resources to benefit from single frequency network (SFN) gains, while trading off an increase in effective delay spread. However, some embodiments may reduce the available delay spread by configuring an extended cyclic prefix (CP) for the D2D discovery resource pool.

FIG3 is a flow chart of a method 300 for supporting inter-cell D2D discovery according to some embodiments. Example method 300 is described with reference to elements of FIG1-2. eNB 102 ( FIG1 ) can perform at least some operations of method 300 to enable UEs 108 and 110 to obtain information required for inter-cell D2D discovery. By way of non-limiting example, and as previously described herein, such information can include reference time and frequency synchronization source information for neighboring cells and configuration information for D2D discovery resource pools for neighboring cells.

In operation 302, the eNB 102 sends signaling to the UE 108 indicating configuration information for at least a device-to-device (D2D) discovery resource pool 202. The configuration information includes a timing offset between the serving cell 106 of the UE 108 and one or more neighboring cells 112. Each D2D discovery resource pool 202 includes D2D resources configured by a corresponding neighboring cell 112. Although one neighboring cell 112 has been described with respect to various embodiments, it will be appreciated that information may be provided for inter-cell D2D discovery between multiple neighboring cells.

In operation 304, the eNB 102 allocates resources to the UE 108 for periodically transmitting synchronization information of the serving cell 106 by the UE 108. As previously described herein, the eNB 102 can allocate resources based on different criteria to achieve different effects. For example, the eNB 102 can allocate resources so that the UE 108 can send synchronization information to UEs 108 in neighboring cells more frequently than the UE 108 sends discovery notification signals.

The eNB 102 can perform other operations to support inter-cell D2D discovery as part of the example method 300. For example, the eNB 102 can limit the transmission power of the D2D transmission, and the eNB 102 can allocate resources to PRB pairs to avoid overlap between subframes relaying synchronization information of the serving cell and subframes relaying synchronization information of one or more neighboring cells.

FIG4 is a block diagram of basic components of a communication device 400 according to some embodiments. Communication device 400 may be adapted to function as UE 108 or 110 ( FIG1 ) or eNB 102 ( FIG1 ). According to the embodiments described above with reference to FIG1-3 , communication device 400 may support methods for inter-cell D2D discovery. It should be noted that when communication device 400 functions as eNB 102, it may be stationary and non-mobile.

In some embodiments, the communication device 400 may include one or more processors and may be configured with instructions stored on a computer-readable storage device. When the communication device 400 acts as a UE 108, the instructions may cause the communication device 400 to receive signaling indicating configuration information for a D2D discovery resource pool of at least one neighboring cell 112 ( FIG. 1 ). As previously described herein, the signaling may also include a timing offset between the serving cell 106 ( FIG. 1 ) and the at least one neighboring cell 112, enabling the communication device 400 to receive and interpret a discovery signal 104 from a UE outside the serving cell 106. The communication device 400 may then transmit a transmission notification signal including the timing offset and synchronization information of the serving cell 106 to a second communication device outside the serving cell 106.

When the communication device 400 acts as an eNB 102 ( FIG. 1 ), the instructions cause the communication device 400 to send signaling to the UE 108 ( FIG. 1 ) indicating configuration information for the D2D discovery resource pool. As previously described herein, the configuration information includes the timing offset between the serving cell 106 ( FIG. 1 ) and the neighboring cell 112 for the UE 108. While one neighboring cell 112 is described, it will be understood that the embodiments are not limited thereto, and the timing offset may be provided to any number of neighboring cells relative to the serving cell.

The communication device 400 may include physical layer circuitry 402 for transmitting and receiving signals to and from other communication devices using one or more antennas 401. The physical layer circuitry 402 may also include medium access control (MAC) circuitry 404 for controlling access to the wireless medium. The communication device 400 may also include processing circuitry 406 and memory 408 configured to perform the operations described herein. In some embodiments, the physical layer circuitry 402 and processing circuitry 406 may be configured to perform the operations described in Figures 1-3.

According to some embodiments, the MAC circuit 404 may be configured to contend for the wireless medium and configure subframes or packets for communication on the wireless medium, and the physical layer circuit 402 may be configured to transmit and receive signals. The physical layer circuit 402 may include modulation/demodulation circuits, up-conversion/down-conversion circuits, filtering circuits, and amplification circuits, among others.

In some embodiments, the processing circuitry 406 of the communication device 400 may include one or more processors. In some embodiments, two or more antennas 401 may be coupled to a physical layer circuitry 402 configured to transmit and receive signals. Memory 408 may store information for configuring the processing circuitry 406 to perform operations for configuring and transmitting message frames and performing various operations described herein. Memory 408 may include any type of memory, including non-transitory memory, for storing information in a machine-readable form (e.g., a computer). For example, memory 408 may include a computer-readable storage device, a read-only memory (ROM), a random access memory (RAM), a magnetic disk storage medium, an optical storage medium, a flash memory device, and other storage devices and media.

Antenna 401 may include one or more directional or omnidirectional antennas, such as dipole antennas, monopole antennas, patch antennas, loop antennas, microstrip antennas, or other antennas suitable for transmitting RF signals. In some embodiments, a single antenna with multiple apertures may be used instead of two or more antennas. In these embodiments, each aperture may be considered a separate antenna. In some multiple-input multiple-output (MIMO) embodiments, these antennas may be effectively separated according to the spatial diversity and different channel characteristics that may be caused between each of the above antennas and the antenna of the transmitting station.

In some embodiments, the communication device 400 may include one or more of a keyboard, a display, a non-volatile memory port, multiple antennas, a graphics processor, an application processor, a speaker, and other mobile device components. The display may be an LCD screen including a touch screen.

In some embodiments, the communication device 400 may be part of a portable wireless communication device, such as a personal digital assistant (PDA), a laptop or portable computer with wireless communication capabilities, a web tablet, a wireless phone, a smart phone, a wireless headset, a pager, an instant messaging device, a digital camera, an access point, a television, a medical device (such as a heart rate monitor, a blood pressure monitor, etc.), or other device that can wirelessly receive and/or send information.

Although the communication device 400 is shown as having multiple separate functional elements, two or more of these functional elements can be combined or implemented as a combination of software-configured elements, such as a processing element including a digital signal processor (DSP), and/or other hardware elements. For example, some elements may include one or more microprocessors, DSPs, field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), radio frequency integrated circuits (RFICs), and combinations of various hardware and logic circuits for performing at least one function described herein. In some embodiments, the functional elements of the communication device 400 may refer to one or more processors operating on one or more processing elements.

Embodiments may be implemented in hardware, firmware, and software, alone or in combination. Embodiments may also be implemented as instructions stored on a computer-readable device, which instructions may be read and executed by at least one processor to perform the operations described herein. Computer storage devices may include any non-volatile storage mechanism that stores information in a machine (computer) readable form. For example, computer-readable storage devices may include read-only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, and other storage devices and media.

[0064] Fig. 5 is a block diagram of a machine 500 for performing various embodiments. In some embodiments, the machine 500 may operate as a standalone device or may be connected (e.g., via a network) to other machines.

Machine (e.g., computer system) 500 may include a hardware processor 502 (e.g., a central processing unit (CPU), a graphics processing unit (GPU), a hardware processor core, or any combination thereof), main memory 504, and static memory 506, some or all of which may communicate with each other via an interconnect 508 (e.g., a bus). Machine 500 may also include a power management device 532, a graphics display device 510, an alphanumeric input device 512 (e.g., a keyboard), and a user interface (UI) navigation device 514 (e.g., a mouse). In some embodiments, graphics display device 510, alphanumeric input device 512, and UI navigation device 514 may be touch screen displays. Machine 500 may also include a storage device 516 (i.e., a drive unit), a signal generating device 518 (e.g., a speaker), a network interface device/transceiver 520 coupled to an antenna 530, and one or more sensors 528, such as a global positioning system (GPS) sensor, a compass, an accelerometer, and other sensors. The machine 500 may include an output controller 534, such as a serial (e.g., Universal Serial Bus (USB), parallel, or other wired or wireless connection (e.g., infrared (IR), near field communication (NFC), etc.) for communicating with or controlling one or more peripheral devices (e.g., printers, card readers, etc.).

The storage device 516 may include a machine-readable medium 522 on which is stored one or more data structures or instructions 524 (e.g., software) that implement or utilize any one or more of the techniques or functionality described herein. The instructions 524 may also reside, in whole or in part, within the main memory 504, within the static storage 506, or within the hardware processor 502 during execution by the machine 500. In one example, one or any combination of the hardware processor 502, the main memory 504, the static storage 506, and the storage device 516 may constitute a machine-readable medium.

Although machine-readable medium 522 is illustrated as a single medium, the term “machine-readable medium” may include a single medium or multiple media (such as a centralized or distributed database, and/or a combined cache and server) configured to store one or more instructions 524 .

The term "machine-readable medium" may include any medium capable of storing, encoding, or carrying instructions 524 to be executed by the machine 500 and enabling the machine 500 to perform any one or more of the techniques of the present invention, or capable of storing, encoding, or carrying data structures used by or related to the instructions 524. Non-limiting examples of machine-readable media may include solid-state memory and optical and magnetic media. In one example, large-scale machine-readable media may include machine-readable media having particles with multiple rest masses. Specific examples of large-scale machine-readable media may include: non-volatile memory, such as semiconductor memory devices (such as electrically programmable read-only memory (EPROM) or electrically erasable programmable read-only memory (EEPROM)) and flash memory devices; magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

The instructions 524 may also be sent or received using a transmission medium via the network interface device/transceiver 520 using any of a number of transmission protocols, such as frame relay, Internet Protocol (IP), Transmission Control Protocol (TCP), User Datagram Protocol (UDP), Hypertext Transfer Protocol (HTTP), etc.

Although the subject matter of the present invention has been described with respect to some embodiments, it is not intended to be limited to the specific forms described herein. Those skilled in the art will recognize that the various technical features of the described embodiments may be combined in accordance with this disclosure. Furthermore, it will be appreciated that various modifications and substitutions may be made by those skilled in the art without departing from the scope of the present invention.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b), which requires that the abstract ascertain the nature and gist of the technical disclosure. It is to be understood that the Abstract is not to be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.

Claims (25)

1. An apparatus for a user equipment (UE), comprising: Processing circuit, the processing circuit being configured to: Decode the first synchronization signal of the serving cell; Decode signaling including discovery configuration information for secondary link communication, the signaling originating from the serving cell, the discovery configuration information indicating: A discovery resource pool with a set of subframes; and Synchronization offset; Encoding the transmission of secondary link synchronization signals based on the indicated synchronization offset provides a synchronization reference for the discovery resource pool to support inter-cell device-to-device D2D discovery, wherein the transmission of the secondary link synchronization signals is more frequent than the discovery cycle used for the transmission of discovery signals; and A memory, coupled to the processing circuitry, is configured to store the synchronization offset. 2. The apparatus of claim 1, wherein the serving cell is provided by an evolved Node B, i.e., an eNB. 3. The apparatus of claim 2, wherein the UE is configured to use a peer radio head (PRH). 4. The apparatus of claim 1, wherein the UE operates in accordance with a synchronization discovery protocol. 5. The apparatus of claim 1, wherein the processing circuit is configured to: The discovery signal is encoded for periodic transmission to another UE's cell. 6. The apparatus of claim 1, wherein the UE enters a sleep state between periodic transmissions of the discovery signal. 7. The apparatus of claim 1, further comprising: One or more antennas; and A transceiver circuit, coupled to the processing circuit and the one or more antennas, is configured to transmit discovery configuration information. 8. The apparatus of claim 1, wherein the processing circuitry is configured to: For transmission to another UE, a discovery notification signal is encoded, the discovery notification signal including the secondary link synchronization signal and the discovery resource pool. 9. The apparatus of claim 8, wherein the processing circuitry is configured to: For transmission to the other UE, the secondary link synchronization signal is encoded. 10. The apparatus of claim 9, wherein the processing circuitry is further configured to: The synchronization signal is encoded for periodic transmission to the other UE. 11. The apparatus of claim 10, wherein the secondary link synchronization signal is periodically transmitted according to a period defined by the serving cell. 12. A non-transitory computer-readable storage device, comprising instructions stored thereon, which, when executed by one or more processors of a user equipment (UE), cause the UE to perform the following operations: Decode the first synchronization signal of the serving cell; Decode the signaling including the discovery configuration information of the serving cell, the signaling originating from the serving cell, the discovery configuration information indicating: A discovery resource pool with a set of subframes; and Synchronization offset; The discovery resource pool is provided with a synchronization reference to support inter-cell device-to-device D2D discovery by encoding the transmission of secondary link synchronization signals based on the indicated synchronization offset, wherein the transmission of the secondary link synchronization signals is more frequent than the discovery cycle used for the transmission of discovery signals. 13. The non-transitory computer-readable storage device of claim 12, wherein the serving cell is provided by an evolved Node B. 14. The non-transitory computer-readable storage device of claim 13, wherein the UE is configured to have a peer radio head (PRH). 15. The non-transitory computer-readable storage device of claim 12, wherein the UE is configured to operate in accordance with a synchronization discovery protocol. 16. The non-transitory computer-readable storage device of claim 12, wherein the instructions, when executed by the one or more processors of the UE, cause the UE to enter a sleep state between periodic transmissions of the discovery signal. 17. The non-transitory computer-readable storage device of claim 12, wherein the instructions, when executed by the one or more processors of the UE, further cause the UE to: The discovery signal is encoded for periodic transmission to the second cell. 18. The non-transitory computer-readable storage device of claim 12, wherein the secondary link synchronization signal is sent to another UE in a cell adjacent to the serving cell. 19. An evolved node B, or eNB, includes: The hardware processing circuit is configured as follows: Encode the transmission of the first synchronization signal of the first cell associated with the eNB; By encoding the signaling used for transmission to the first user equipment (UE) within the first cell, secondary link discovery is supported to indicate configuration information for a discovery resource pool already configured by the second cell, wherein the configuration information indicates: The discovery resource pool has a set of subframes; and Synchronization offset, wherein the first UE is configured to transmit the secondary link synchronization signal more frequently than the discovery period used for the transmission of the discovery signal; and A memory, coupled to the hardware processing circuitry, is configured to store the synchronization offset. 20. The eNB of claim 19, wherein the hardware processing circuitry is further configured to: Resources are allocated to the first UE for periodic transmissions performed by the first UE based on the synchronization offset, wherein the periodic transmissions are performed by periodically transmitting the synchronization information of the first cell to at least a second UE in the second cell. 21. The eNB of claim 20, wherein the resources include time resources for transmitting a discovery signal at the beginning of each discovery resource pool occurrence. 22. The eNB of claim 21, wherein the synchronization offset is a time offset between the first cell and the second cell. 23. The eNB of claim 20, wherein the resources are defined in the frequency dimension to a set of Physical Resource Block (PRB) pairs, wherein the set of PRB pairs is located at the center of the system uplink bandwidth. 24. The eNB of claim 20, wherein the resources are defined in the frequency dimension to a set of Physical Resource Block (PRB) pairs, wherein the set of PRB pairs is allocated according to a cell-specific offset relative to the center of the system uplink bandwidth. 25. The eNB of claim 20, wherein the hardware processing circuitry is further configured to: The transmission power of D2D transmissions on physical resource blocks (PRBs), excluding D2D transmissions carrying synchronization information on subframes allocated for transmitting discovery signals or relayed synchronization signals, is limited such that the transmission power is less than the maximum transmission power.