WO2016064450A1

WO2016064450A1 - Device-to-device synchronization sequences

Info

Publication number: WO2016064450A1
Application number: PCT/US2015/028638
Authority: WO
Inventors: Chenxi Zhu
Original assignee: Fujitsu Limited
Priority date: 2014-10-23
Filing date: 2015-04-30
Publication date: 2016-04-28

Abstract

A method of generating a device-to-device synchronization signal may include determining a device-to-device identity group. The method may further include determining a device-to-device identity. The device-to-device identity group and the device-to-device identity may together indicate a synchronization source identity. The method may further include generating a secondary device-to-device synchronization signal sequence based on, at least, the device-to-device identity group and a modulo of the device-to-device identity divided by 3. The method may further include transmitting the secondary device-to-device synchronization signal sequence.

Description

DEVICE-TO-DEVICE SYNCHRONIZATION SEQUENCES

FIELD

The embodiments discussed herein are related to device-to-device (D2D) synchronization sequences.

BACKGROUND

Device-to-device (D2D) communication may allow data transmissions to be made directly between two or more devices or terminals of a telecommunication system. The D2D communication may overlay regular cellular communications, and may be performed with or without cellular network coverage.

The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described herein may be practiced.

SUMMARY

According to an aspect of an embodiment, a method of generating a device-to- device synchronization signal may include determining a device-to-device identity group. The method may further include determining a device-to-device identity. The device-to-device identity group and the device-to-device identity may together indicate a synchronization source identity. The method may further include generating a secondary device-to-device synchronization signal sequence based on, at least, the device-to-device identity group and a modulo of the device-to-device identity divided by 3. The method may further include transmitting the secondary device-to-device synchronization signal sequence.

The object and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1 is a diagram of an example wireless communication network;

Figure 2 is a diagram of an example wireless device that may be included in the example wireless communication network of Figure 1 ; Figure 3 is a flowchart of an example method of generating a secondary device- to-device synchronization signal (SD2DSS) that may be performed by the wireless device of Figure 2;

Figure 4A is a flowchart of an example method of generating a device-to-device synchronization signal (D2DSS) that may be performed by the wireless device of Figure 2; and

Figure 4B is a flowchart of an example method of determining a synchronization source identity from a D2DSS that may be performed by the wireless device of Figure 2. DESCRIPTION OF EMBODIMENTS

Some embodiments as herein described may relate to a communication system based on the 3rd Generation Partnership Project's (3GPP) Long Term Evolution (LTE) radio access network. Descriptions involving LTE may also apply to 3GPP's Long Term Evolution Advanced (LTE-A) radio access network. However, the embodiments described herein are not limited to the example communication systems described. Rather, the embodiments described herein may be applicable to other communication systems as well.

Future releases of LTE/LTE-A standards may support device-to-device (D2D) communication. For example, LTE-A releases 12 and 13 may support D2D communication allowing wireless devices, such as user equipment (UE), to transmit directly to other wireless devices. The D2D communication may support discovery and/or synchronization between wireless devices.

In some instances, using D2D communication may increase network capacity. For example, D2D communication may permit spatial multiplexing, which may allow for higher relative spectrum usage. Employing D2D communication may also permit throughput between devices to be increased if a D2D link experiences better channel quality than a cellular link. Using D2D communication may reduce resource usage when data is transmitted once between two devices during a D2D transmission, as compared to transmitting the same data twice between the two devices over a cellular link, i.e., once through an uplink (UL) transmission from a transmitting terminal to a base station and once through a downlink (DL) transmission to a receiving terminal from the base station.

Device-to-device communication may reduce communication latency of a telecommunication network. For example, D2D communication may not relay data through a base station and/or a core network, thus potentially reducing the transit time of the data and/or the load on the base station and/or the core network. A wireless device may transmit a primary D2D synchronization signal (PD2DSS), which may provide an initial, coarse time and frequency estimation to a wireless device receiving the PD2DSS, and a secondary D2D synchronization signal (SD2DSS), which may provide a fine time and frequency estimation to the wireless device receiving the SD2DSS. In some embodiments, the SD2DSS sequences may reuse secondary synchronization signal (SSS) sequences as may be transmitted by a base station according to, at least, LTE release 8.

Embodiments of the present invention will be explained with reference to the accompanying drawings.

Figure 1 is a diagram of an example wireless communication network 100. The network 100 may be configured to provide wireless communication services to one or more wireless devices 104 via one or more access points, such as an access point 102. The wireless communication services may be voice services, data services, messaging services, and/or any suitable combination thereof. The network 100 may include a Frequency Division Multiple Access (FDMA) network, an Orthogonal FDMA (OFDMA) network, a Code Division Multiple Access (CDMA) network, a Time Division Multiple Access (TDMA) network, and/or any other suitable wireless communication network. In some embodiments, the network 100 may be configured as a third generation (3G) wireless communication network and/or a fourth generation (4G) wireless communication network. In these or other embodiments, the network 100 may be configured as a long-term evolution (LTE) or LTE advanced (LTE-A) wireless communication network.

The access point 102 may be any suitable wireless communication network communication point and may include, by way of example, a base station, an evolved node B (eNB) base station, a remote radio head (RRH), or any other suitable communication point. The wireless devices 104 may include any devices that may use the network 100 for obtaining wireless communication services and may include, by way of example, a cellular phone, a smartphone, a personal data assistant (PDA), a laptop computer, a personal computer, and a tablet computer, or any other similar device.

The wireless devices 104 may be configured to perform device-to-device (D2D) communication. In some embodiments, the wireless devices 104 may be configured to perform D2D communication both with assistance from the access point 102 and without assistance form the access point 102. Performing D2D communication with assistance from the access point 102 may be described herein as "in-network" D2D communication. Performing D2D communication without assistance from the access point 102 may be described herein as "out-of-network" D2D communication. In some embodiments, in- network D2D communication may be performed while the wireless devices 104 are connected to the access point 102 and out-of-network D2D communication may be performed while the wireless devices 104 are not connected to the access point 102. For example, the wireless devices 104 may perform out-of-network D2D communication while the wireless devices 104 are outside of a communication range of the access point 102.

The access point 102 may transmit a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) according to LTE release 8, such as described in section 6.1 1 of the 3 GPP technical specification (TS) 36.21 1. The PSS and the SSS may be transmitted by the access point 102 such that the wireless devices 104 may determine, in part, a frame timing of a cell associated with the access point 102 and a physical-layer cell identity of the cell associated with the access point 102. In some embodiments, 168 different SSS sequences may be employed.

In some embodiments, there may be 504 physical-layer cell identities, represented by N p ¹¹ , which may include an integer in the range of 0 to 503. The physical-layer cell identities may be grouped into 168 physical-layer cell-identity groups,

(1)

represented by N_/D and each group may include 3 physical-layer identities, represented by . The SSS may carry which may include an integer in the range of 0 to 167.

(2)

The PSS may carry N , which may include an integer in the range of 0 to 2. By way of example, the physical layer cell identity may be associated with and according to the following expression.

Cell (2)

3N^^J + N ID

The PSS sequence may be generated according to a function of the N_/D . The SSS sequence may be generated according to a function of both the and the Thus, for example, generating the PSS sequences and the SSS sequences may be represented by the following expressions.

P55( ^})

The wireless devices 104 may be equipped for D2D communication. In some embodiments, one or more of the wireless devices 104 may transmit a physical D2D synchronization channel (PD2DSCH) and/or a D2D synchronization signal (D2DSS), which may provide synchronization to other wireless devices 104 within transmission range. The D2DSS may allow time and/or frequency synchronization for the reception of D2D transmissions from the wireless devices 104.

The D2DSS and/or PD2DSCH may be transmitted by wireless devices 104 within network coverage, within partial network coverage, or outside of network coverage. For example, the D2DSS may be transmitted by wireless devices 104 to participate in in-network D2D communication and/or out-of-network D2D communication.

The D2DSS may include a PD2DSS and/or an SD2DSS. The PD2DSS may provide an initial, coarse time and frequency estimation to a wireless device receiving the PD2DSS. The SD2DSS may provide a fine time and frequency estimation to the wireless device receiving the SD2DSS. In some embodiments, the PD2DSS and the SD2DSS may function in a manner similar to the PSS and the SSS, respectively, which were discussed above.

In some embodiments, an identity of a synchronization source, such as an identity of a first wireless device 104a transmitting the D2DSS, may be associated with a D2D identity group, represented herein by ¾ and a D2D identity within the group, represented herein by s ^ . The may include an integer in the range of 0 to 167. Alternately or additionally, the may be carried by the SD2DSS. The s ^ may include an integer in the range of 0 to M, where ¥ may be equal to or greater than 3. In some embodiments, each integer of the ¾ may be mapped to a root index that may be employed in generating the PD2DSS sequence. Alternately or additionally the ¾ may be carried by the PD2DSS.

In some embodiments, the PD2DSS sequences may be similar to the PSS sequences, but may include relatively more PD2DSS sequences. For example, the PD2DSS sequences may include M sequences, where the PSS sequences may include 3 sequences. By way of example, employing more PD2DSS sequences may reduce collision probabilities in D2D communication. In some embodiments, the waveform of the PD2DSS may include single-carrier frequency-division multiplexing (SC-FDM) without discrete Fourier transform precoding (DFT-precoding). Alternately or additionally, 2 symbols may be included in a subframe for the PD2DSS. In some embodiments, the SD2DSS may employ the same sequences as the SSS. By way of example, the 168 SSS sequences may be employed as SD2DSS sequences. In some embodiments, the waveform of the SD2DSS may include SC-FDM without DFT- precoding. Alternately or additionally, the SD2DSS may be transmitted with reduced power relative to the PD2DSS. Alternately or additionally, 2 symbols may be included in a subframe for the SD2DSS.

In some embodiments, the PD2DSS sequence may be generated according to a

(2)

function of S% . The SD2DSS sequence may be generated according to a function of both 5¾^} and

As indicated above, the SD2DSS may reuse the sequences of the SSS. A mapping of SSS sequences to SD2DSS sequences may be defined to account for the

(2) (2)

domain of 5_/D being greater than the domain of N_/D .

In some embodiments, the SD2DSS sequences may be generated according to the following expression.

Put another way, the SD2DSS sequence may be generated in the same manner as the SSS sequences, but with in place of and the modulo of divided by 3, which may also be expressed as

3), in place of N_/D . Thus, for example, the entire set of 168 SSS sequences may be reused without ambiguity.

One or more of the wireless devices 104 such as a second wireless device 104b may receive the PD2DSS and the SD2DSS. The second wireless device 104b may detect the PD2DSS before the SD2DSS. Thus, for example, the second wireless device 104b may have knowledge of ¾ , and by extension, 5_/D mod 3, at the time the SD2DSS is detected. The second wireless device 104b may not be prompted to perform additional computations to map the SD2DSS sequences to the SSS sequences.

Figure 2 is a diagram of an example wireless device 202. The wireless device 202 may generally correspond to the wireless devices 104 of Figure 1. The wireless device 202 may include an antenna 210, a transceiver 220, and hardware 230. The hardware 230 may include an application-specific integrated circuit (ASIC), a Field-Programmable Gate Array (FPGA), or any other digital or analog circuitry configured to perform operations, such as the operations described as performed by the wireless devices 104 of Figure 1. As illustrated in Figure 2, the hardware 230 may include a processor 232, a memory 234, and data storage 236. In these and other embodiments, the processor 232, the memory 234, and the data storage 236 may be configured to perform some or all of the operations performed by the hardware 230. In other embodiments, the hardware 230 may not include one or more of the processor 232, the memory 234, and the data storage 236.

Generally, the processor 232 may include any suitable special-purpose or general-purpose computer, computing entity, or processing device including various computer hardware or software modules and may be configured to execute instructions stored on any applicable computer-readable storage media. For example, the processor 232 may include a microprocessor, a microcontroller, a digital signal processor (DSP), an ASIC, an FPGA, or any other digital or analog circuitry configured to interpret and/or to execute program instructions and/or to process data. Although illustrated as a single processor in Fig 2, the processor 232 may include any number of processors configured to perform individually or collectively any number of operations described herein. Additionally, one or more of the processors may be present on one or more different electronic devices. In some embodiments, the processor 232 may interpret and/or execute program instructions and/or process data stored in the memory 234, the data storage 236, or the memory 234 and the data storage 236. In some embodiments, the processor 232 may fetch program instructions from the data storage 236 and load the program instructions in the memory 234. After the program instructions are loaded into the memory 234, the processor 232 may execute the program instructions.

The memory 234 and data storage 236 may include computer-readable storage media or one or more computer-readable storage mediums for carrying or having computer-executable instructions or data structures stored thereon. Such computer- readable storage media may be any available media that may be accessed by a general- purpose or special-purpose computer, such as the processor 232. By way of example, and not limitation, such computer-readable storage media may include non-transitory computer-readable storage media including Random Access Memory (RAM), Readonly Memory (ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), Compact Disc Read-Only Memory (CD-ROM) or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory devices (e.g., solid state memory devices), or any other storage medium which may be used to carry or store desired program code in the form of computer-executable instructions or data structures and which may be accessed by a general-purpose or special-purpose computer. Combinations of the above may also be included within the scope of computer-readable storage media. Computer-executable instructions may include, for example, instructions and data configured to cause the processor 232 to perform a certain operation or group of operations.

The antenna 210 may be coupled to the transceiver 220. The antenna 210 may have any number of configurations. The antenna 210 may also be configured to transmit and receive wireless communication signals in a wireless communication network. In particular, the antenna 210 may be configured to transmit wireless communications between the wireless device 202 and an access point, and to transmit and receive the D2DSS, the PD2DSS, and/or the SD2DSS. The antenna 210 may send the received wireless communication signals to the transceiver 220.

The antenna 210 may be further configured to receive wireless communication signals for transmission from the transceiver 220. The antenna 210 may transmit the wireless communication signals to other wireless devices.

The hardware 230 may be configured to perform operations based on the wireless communication signals. For example, in some embodiments, the hardware 230 may be configured to receive wireless communication signals from the transceiver 220 and to decode the wireless communication signals to extract data from the wireless communications signals. In some embodiments, the wireless communication signal may include the PD2DSS and the SD2DSS. Alternately or additionally, the hardware 230 may also determine the 5_/D from the PD2DSS. Alternately or additionally, using the value of S^mod 3, the hardware 230 may determine the S^from the SD2DSS. Alternately or additionally, the hardware 230 may determine an identity of a synchronization source that transmitted the PD2DSS and the SD2DSS from the values of the S$ and the

In some embodiments, the PD2DSS and/or the SD2DSS may be included in the D2DSS.

The hardware 230 may be configured to perform other operations that are described herein as performed by wireless devices. For example, the hardware 230 may be configured to perform the operations described as performed by the wireless devices 104 of Figure 1 , and/or the methods as described below with reference to Figures 3-4B.

Figure 3 is a flowchart of an example method 300 of generating an SD2DSS. The method 300 may be performed by the wireless devices 104 of Figure 1 and/or by the wireless device 202 of Figure 2. The method 300 may begin at block 302 by determining a D2D identity group. For example, the method 300 may include determining an 5_/D corresponding to the 5_/D described with reference to Figures 1 and 2. In some embodiments, the D2D identity group may include an integer between 0 and 167, inclusive.

The method 300 may continue at block 304 by determining a D2D identity. For

Γ2) (2) example, the method 300 may include determining an 5_/D corresponding to the 5_/D described with reference to Figures 1 and 2. The D2D identity group and the D2D identity may together indicate a synchronization source identity. The synchronization source identity may correspond to the synchronization source identity described with reference to Figures 1 and 2. In some embodiments, the D2D identity may include an integer between 0 and M, inclusive, where M may be equal to or greater than 3.

The method 300 may continue at block 306 by generating an SD2DSS sequence based on, at least, the device-to-device identity group and a modulo of the device-to- device identity divided by 3. In some embodiments, the SD2DSS may correspond to an SSS sequence according to the value of the D2D identity group in place of a physical- layer cell-identity group and a modulo of the D2D identity divided by 3 in place of a physical-layer identity. For example, the method 300 may include generating the SD2DSS sequence according to the generation of the SD2DSS sequence described with reference to Figures 1 and 2.

The method 300 may continue at block 308 by transmitting the SD2DSS sequence. In some embodiments, the SD2DSS may be transmitted as part of a transmitted D2DSS. For example, the D2DSS corresponding to the D2DSS described with reference to Figures 1 and 2 may be transmitted. The D2DSS may further include a PD2DSS. The PD2DSS may be based on the D2D identity. For example, the D2DSS

Γ2)

may include a PD2DSS generated based on as described with reference to Figures 1 and 2.

For this and other processes and methods disclosed herein, the functions performed in the processes and methods may be implemented in differing order. Furthermore, the outlined operations are provided only as examples, and some of the operations may be optional, combined into fewer operations, or expanded into additional operations without detracting from the essence of the embodiments. Figure 4A is a flowchart of an example method 400 of generating a D2DSS. The method 400 may be performed by the wireless devices 104 of Figure 1 and/or by the wireless device 202 of Figure 2.

The method 400 may begin at block 402 by determining a D2D identity group. For example, the method 400 may include determining an 5_/D corresponding to the 5_/D described with reference to Figures 1-3. In some embodiments, the D2D identity group may include an integer between 0 and 167, inclusive.

The method may continue at block 404 by determining a D2D identity. For

(2) (2) example, the method 400 may include determining an 5_/D corresponding to the 5_/D described with reference to Figures 1-3. The D2D identity group and the D2D identity may together indicate a synchronization source identity. The synchronization source identity may correspond to the synchronization source identity described with reference to Figures 1-3. In some embodiments, the D2D identity may include an integer between 0 and M, inclusive, where M may be equal to or greater than 3.

The method may continue at block 406 by generating a PD2DSS sequence based on the device-to-device identity. For example, the method 400 may include generating a

PD2DSS generated based on ¾ as described with reference to Figures 1-3.

The method may continue at block 408 by generating an SD2DSS sequence base on, at least, the device-to-device identity group and a modulo of the device-to-device identity divided by 3. In some embodiments, the SD2DSS sequence may correspond to an SSS sequence according to the value of the D2D identity group in place of a physical- layer cell-identity group and a modulo of the D2D identity divided by 3 in place of a physical-layer identity. For example, the method 300 may include generating the SD2DSS sequence according to the generation of the SD2DSS sequence described with reference to Figures 1-3.

The method 400 may continue at block 410 by transmitting a D2DSS including the PD2DSS sequence and the SD2DSS sequence. For example, the method 400 may include transmitting a D2DSS corresponding to the D2DSS described with reference to Figures 1-3.

For this and other processes and methods disclosed herein, the functions performed in the processes and methods may be implemented in differing order. Furthermore, the outlined operations are provided only as examples, and some of the operations may be optional, combined into fewer operations, or expanded into additional operations without detracting from the essence of the embodiments.

Figure 4B is a flowchart of an example method 450 of determining a synchronization source identity from a D2DSS. The D2DSS may be generated by the method 400 of Figure 4A. In some embodiments, the method 450 may be performed by the wireless devices 104 of Figure 1 and/or by the wireless device 202 of Figure 2. Optionally, the method 450 may be performed in conjunction with the method 400 of Figure 4A. For example, the method 400 of Figure 4A may be performed by a first wireless device of the wireless devices 104 of Figure 1 and the method 450 of Figure 4B may be performed by a second wireless device of the wireless devices 104.

The method 450 may begin at block 452 by receiving a D2DSS. For example, the method 450 may include receiving a D2DSS corresponding to the D2DSS described with reference to Figures 1-4A. The D2DSS may include a PD2DSS sequence and an SD2DSS sequence. For example, the D2DSS may include a PD2DSS sequence and an SD2DSS sequence corresponding to the PD2DSS sequences and the SD2DSS sequences described with reference to Figures 1-4A.

The method 450 may continue at block 454 by deriving a D2D identity from the

PD2DSS sequence. For example, the method 450 may include deriving the ¾ from the PD2DSS sequence as described with reference to Figures 1 and 2.

The method 450 may continue at block 456 by deriving a D2D identity group

(1) from the SD2DSS sequence. For example, the method 450 may include deriving the 5_/D from the SD2DSS sequence as described with reference to Figures 1 and 2.

The method 450 may continue at block 458 by determining a synchronization source identity from the D2D identity group and the D2D identity. For example, the method 450 may include determining a synchronization source identity from the ¾ and the as described with reference to Figures 1 and 2.

For example, in some embodiments, the method 450 may further include establishing coarse time synchronization with the first wireless device based at least in part on the PD2DSS. Alternately or additionally, the method may include establishing coarse frequency synchronization with the first wireless device based at least in part on the PD2DSS.

In some embodiments, the method 450 may further include establishing fine time synchronization with the first wireless device based at least in part on the SD2DSS. Alternately or additionally, the method may include establishing fine frequency synchronization with the first wireless device based at least in part on the SD2DSS.

As indicated above, some embodiments described herein may include the use of a special purpose or general purpose computer (e.g., the processor 232 of Figure 2A) including various computer hardware or software modules, as discussed in greater detail below. Further, as indicated above, embodiments described herein may be implemented using computer-readable media (e.g., the memory 234 of Figure 2A) for carrying or having computer-executable instructions or data structures stored thereon.

As used herein, the terms "module" or "component" may refer to specific hardware implementations configured to perform the actions of the module or component and/or software objects or software routines that may be stored on and/or executed by general purpose hardware (e.g., computer-readable media, processing devices, etc.) of the computing system. In some embodiments, the different components, modules, engines, and services described herein may be implemented as objects or processes that execute on the computing system (e.g., as separate threads). While some of the system and methods described herein are generally described as being implemented in software (stored on and/or executed by general purpose hardware), specific hardware implementations or a combination of software and specific hardware implementations are also possible and contemplated. In this description, a "computing entity" may be any computing system as previously defined herein, or any module or combination of modulates running on a computing system.

Terms used herein and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including, but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes, but is not limited to," etc.).

Additionally, if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations.

In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." or "one or more of A, B, and C, etc." is used, in general such a construction is intended to include A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B, and C together, etc. For example, the use of the term "and/or" is intended to be construed in this manner.

Further, any disjunctive word or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" should be understood to include the possibilities of "A" or "B" or "A and B."

All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present disclosure have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the present disclosure.

Claims

CLAIMS What is claimed is:

1. A method of generating a device-to-device synchronization signal, the method comprising:

determining a device-to-device identity group;

determining a device-to-device identity, where the device-to-device identity group and the device-to-device identity together indicate a synchronization source identity;

generating a secondary device-to-device synchronization signal sequence based on, at least, the device-to-device identity group and a modulo of the device- to-device identity divided by 3; and

transmitting the secondary device-to-device synchronization signal sequence.

2. The method of claim 1, wherein the device-to-device identity group is an integer between 0 and 167, inclusive.

3. The method of claim 1, wherein the device-to-device identity is an integer between 0 and M, inclusive, where M is equal to or greater than 3.

4. The method of claim 1, wherein the secondary device-to-device synchronization signal sequence is transmitted as part of a transmitted device-to-device synchronization signal.

5. The method of claim 4, wherein the device-to-device synchronization signal includes a primary device-to-device synchronization sequence.

6. The method of claim 5, wherein the primary device-to-device synchronization sequence is generated based on the device-to-device identity.

7. The method of claim 1, wherein:

the device-to-device identity group is an integer between 0 and 167, inclusive; the device-to-device identity is an integer between 0 and M, inclusive, where M is equal to or greater than 3; and

the secondary device-to-device synchronization signal sequence is transmitted as part of a transmitted device-to-device synchronization signal including a primary device-to-device synchronization sequence generated based on the device-to-device identity.

8. A wireless device comprising:

hardware to:

determine a device-to-device identity group, determine a device-to-device identity, where the device-to-device identity group and the device-to-device identity together indicate a synchronization source identity and

generate a secondary device-to-device synchronization signal sequence based on the device-to-device identity group and a modulo of the device-to-device identity divided by 3; and

an antenna to transmit the secondary device-to-device synchronization signal sequence.

9. The wireless device of claim 8, wherein the device-to-device identity group is an integer between 0 and 167, inclusive.

10. The wireless device of claim 8, wherein the device-to-device identity is an integer between 0 and M, inclusive, where M is equal to or greater than 3.

11. The wireless device of claim 8, wherein the secondary device-to-device synchronization signal sequence is transmitted as part of a transmitted device-to-device synchronization signal.

12. The wireless device of claim 11, wherein the device-to-device synchronization signal includes a primary device-to-device synchronization sequence.

13. The wireless device of claim 12, wherein the primary device-to-device synchronization sequence is generated based on the device-to-device identity.

14. A system comprising:

a first wireless device to:

determine a device-to-device identity group, determine a device-to-device identity, where the device-to-device identity group and the device-to-device identity together indicate a synchronization source identity,

generate a primary device-to-device synchronization signal sequence based on the device-to-device identity,

generate a secondary device-to-device synchronization signal sequence based on, at least, the device-to-device identity group and a modulo of the device-to-device identity divided by 3, and

transmit a device-to-device synchronization signal including the primary device-to-device synchronization signal sequence and the secondary device-to-device synchronization signal sequence; and a second wireless device to:

receive the device-to-device synchronization signal,

derive the device-to-device identity from the primary device-to- device synchronization signal sequence,

derive the device-to-device identity group from the secondary device-to-device synchronization signal sequence, and

determine a synchronization source identity of the first wireless device based on the device-to-device identity and the device-to-device identity group.

15. The system of claim 14, wherein the second wireless device is further to establish coarse time synchronization with the first wireless device based at least in part on the primary device-to-device synchronization signal.

16. The system of claim 15, wherein the second wireless device is further to establish coarse frequency synchronization with the first wireless device based at least in part on the primary device-to-device synchronization signal.

17. The system of claim 14, wherein the second wireless device is further to establish fine time synchronization with the first wireless device based at least in part on the secondary device-to-device synchronization signal.

18. The system of claim 17, wherein the second wireless device is further to establish fine frequency synchronization with the first wireless device based at least in part on the secondary device-to-device synchronization signal.

19. The system of claim 14, wherein the device-to-device identity group is an integer between 0 and 167, inclusive.

20. The system of claim 14, wherein the device-to-device identity is an integer between 0 and M, inclusive, where M is equal to or greater than 3.