Patent Publication Number: US-2021185628-A1

Title: Method and apparatus for synchronization in sidelink communications

Description:
TECHNICAL FIELD 
     This disclosure relates generally to wireless communications and, more particularly, to a method and apparatus for synchronization in sidelink communications. 
     BACKGROUND 
     Sidelink (SL) communication is a wireless radio communication directly between two or more user equipment devices (hereinafter “UE”). In this type of communication, two or more UEs that are geographically proximate to each other can directly communicate without going through an eNB in an long-term evolution (LTE) system or a gNB in New Radio, or a core network. Data transmission in sidelink communications is thus different from typical cellular network communications, in which a UE transmits data to an eNB or a gNB (i.e., uplink transmissions) or receives data from an eNB or a gNB (i.e., downlink transmissions). In sidelink communications, data is transmitted directly from a source UE to a target UE through the Unified Air Interface, e.g., PC5 interface. 
     SUMMARY OF THE INVENTION 
     The exemplary embodiments disclosed herein are directed to solving the issues related to one or more problems presented in the prior art, as well as providing additional features that will become readily apparent by reference to the following detailed description when taken in conjunction with the accompany drawings. In accordance with various embodiments, exemplary systems, methods, and computer program products are disclosed herein. It is understood, however, that these embodiments are presented by way of example and not limitation, and it will be apparent to those of ordinary skill in the art who read the present disclosure that various modifications to the disclosed embodiments can be made while remaining within the scope of the invention. 
     Synchronization is an essential prerequisite for all mobile networks to operate. It is fundamental to data integrity, and without it, data errors and networks outages can occur, resulting in additional operational costs. A BS relies on having access to reliable and accurate synchronization signals from a synchronization source (e.g., a core network or a satellite) in order to generate radio signals based on synchronization and maintain frame alignment for data transmission. Effective synchronization also permits seamless handover of subscriber or UE connections between adjacent BSs. Routers and switches in the transport network may therefore be required to provide synchronization to BSs in order for them to handle and transport data properly. Additionally, the mobile network evolution to LTE and future planning for 5G networks and services has generated an increasing need for the delivery of accurate synchronization to the UEs in the network. Apart from the need for these networks to provide ever increasing data rates and lower network latencies, more sophisticated synchronization schemes are needed to support new features. 
     Sidelink (SL) communication, a new feature introduced in LTE, is supported and typically used in communications such as Device-to-Device (D2D), or Vehicle-to-Everything (V2X) communications. SL communications include broadcasts, group communications, and direct unicast communications. Each UE in the sidelink communication may separately obtain a synchronization reference for sidelink transmission from a different synchronization source, e.g., a base station (e.g., an eNB in a LTE communication system and a gNB in NR), or a satellite, or another UE. A first UE with a first synchronization reference from a first synchronization source, for example, an eNB, can determine a first transmission timing according to a first sidelink synchronization signal and/or physical sidelink broadcast channel (SLSS/PSBCH) received from an adjacent second UE with a second synchronization reference from a second different synchronization source, for example, a gNB, and vice versa, if the first synchronization reference is aligned with the second synchronization reference, the first UE can receive a sidelink service from the second UE based on the first synchronization reference, otherwise, based on the first transmission timing. This is especially important when frames are misaligned between the synchronization references on the two UEs received from different synchronization sources. However, when a SLSS/PSBCH is not transmitted from the second UE to the first UE since the transmission condition is not satisfied, for example, when an indication parameter from an eNB, networkControlledSyncTx, is off or a RSRP (reference signal received power) is smaller than a threshold value, e.g., when the first UE is close to the corresponding eNB, the transmission timing for V2X communication of the second UE is not available for the first UE. Since the first UE cannot identify the data transmitted from the second UE without the transmission timing alignment the data transmission from the second UE. Thus, there exists a need to develop a method and apparatus for synchronization in sidelink communications. 
     In one embodiment, a method for performing sidelink communications, the method includes: receiving, by a first wireless communication device, a first message from a first wireless communication node; determining, by the first wireless communication device, at least one first synchronization reference according to the first message; selecting, by the first wireless communication device, one of the at least one first synchronization reference as a second synchronization reference for receiving a sidelink communication signal; and performing the sidelink communications with the second wireless communication device according to the second synchronization reference, wherein the first message comprises configuration information for the sidelink communication. 
     In a further embodiment, a method for performing sidelink communications, the method includes: detecting, by a first communication device, at least one first synchronization signal on at least one corresponding carrier from a first wireless communication node according to first information; determining, by the first wireless communication device, a second synchronization reference for the sidelink communications with a second wireless communication device; and performing the sidelink communications with the second wireless communication device according to the second synchronization reference, wherein the sidelink communications comprise at least one of the following: transmitting and receiving sidelink communication signals. 
     In another embodiments, a method for performing sidelink communications, the method includes: transmitting a first sidelink synchronization signal for the sidelink communications, by a first wireless communication device, when a first condition is satisfied; determining, by the second wireless communication device, a first synchronization reference; and performing the sidelink communications between the first wireless communication device and the second wireless communication device according to the first synchronization reference, wherein the first condition comprises at least one of the following: the first wireless communication device receives a first message from a first wireless communication node, wherein the first message further comprises at least one of the following: at least one parameter of a resource pool and at least one synchronization timing, wherein the resource pool comprises at least one resource in the time and frequency domain and the synchronization timing comprises at least one time offset value; and the first synchronization reference is determined from at least one of the following: one of at least one first synchronization signal on at least one corresponding first carrier detected from the first wireless communication node and one of at least one second synchronization signal on at least one corresponding second carrier detected from a second wireless communication node. 
     In another embodiment, a computing device includes at least one processor and a memory coupled to the processor, the at least one processor configured to carry out the method. 
     Yet, in another embodiment, a non-transitory computer-readable medium has stored thereon computer-executable instructions for carrying out the method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that various features are not necessarily drawn to scale. In fact, the dimensions and geometries of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1A  illustrates an exemplary hybrid wireless communication network, in accordance with some embodiments of the present disclosure. 
         FIG. 1B  illustrates a block diagram of an exemplary wireless communication system for transmitting and receiving downlink, uplink and sidelink communication signals, in accordance with some embodiments of the present disclosure. 
         FIG. 2  illustrates an exemplary hybrid wireless communication network, in accordance with some embodiments of the present disclosure. 
         FIG. 3  illustrates a method for performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
         FIG. 4  illustrates a method for performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
         FIG. 5  illustrates a method for performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
         FIG. 6  illustrates a method performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
         FIG. 7  illustrates an exemplary hybrid wireless communication network, in accordance with some embodiments of the present disclosure. 
         FIG. 8  illustrates a method for performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
         FIG. 9  illustrates a method or performing a sidelink synchronization in the exemplary hybrid wireless communication system, in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Various exemplary embodiments of the invention are described below with reference to the accompanying figures to enable a person of ordinary skill in the art to make and use the invention. As would be apparent to those of ordinary skill in the art, after reading the present disclosure, various changes or modifications to the examples described herein can be made without departing from the scope of the invention. Thus, the present invention is not limited to the exemplary embodiments and applications described or illustrated herein. Additionally, the specific order or hierarchy of steps in the methods disclosed herein are merely exemplary approaches. Based upon design preferences, the specific order or hierarchy of steps of the disclosed methods or processes can be re-arranged while remaining within the scope of the present invention. Thus, those of ordinary skill in the art will understand that the methods and techniques disclosed herein present various steps or acts in a sample order, and the invention is not limited to the specific order or hierarchy presented unless expressly stated otherwise. 
     Embodiments of the present invention are described in detail with reference to the accompanying drawings. The same or similar components may be designated by the same or similar reference numerals although they are illustrated in different drawings. Detailed descriptions of constructions or processes well-known in the art may be omitted to avoid obscuring the subject matter of the present invention. Further, the terms are defined in consideration of their functionality in embodiment of the present invention, and may vary according to the intention of a user or an operator, usage, etc. Therefore, the definition should be made on the basis of the overall content of the present specification. 
       FIG. 1A  illustrates an exemplary hybrid wireless communication network  100 , in accordance with some embodiments of the present disclosure. In a wireless communication system, a network side communication node or a base station (BS) can be a node B, an E-utran Node B (also known as Evolved Node B, eNodeB or eNB), a gNodeB (also known as gNB) in new radio (NR) technology, a pico station, a femto station, or the like. A terminal side node or a user equipment (UE) can be a long range communication system like a mobile phone, a smart phone, a personal digital assistant (PDA), tablet, laptop computer, or a short range communication system such as, for example a wearable device, a vehicle with a vehicular communication system and the like, and are generally referred to as “communication devices” herein. A network and a terminal side communication node are represented by a BS  102  and a UE  104 , respectively, and in all the embodiments in this disclosure hereafter, and are generally referred to as “communication nodes” herein. Such communication nodes may be capable of wireless and/or wired communications, in accordance with various embodiments of the invention. It is noted that all the embodiments are merely preferred examples, and are not intended to limit the present disclosure. Accordingly, it is understood that the system may include any desired combination of UEs and BSs, while remaining within the scope of the present disclosure. 
     Referring to  FIG. 1A , the wireless communication network  100  includes a first BS  102 A (e.g., gNB), a second BS  102 B (e.g., eNB), a first UE  104 A (e.g., NR-UE), and a second UE  104 B (e.g., LTE-UE). The UE  104 A each can be a vehicle that is moving in a first cell  101  covered by the BS  102 A and a second cell  110  covered by the BS  102 B. In some embodiments, the first cell  101  is in the second cell  110 . In some embodiments, the UE  104 A has direct communication channels  103 - 1 A and  103 - 1 B with the BS  102 A as well as the BS  102 B, respectively. Similarly, the UE  104 B can be also a vehicle that is moving in the same cell  110  covered by the BS  102 B, but may not have a direct communication channel with the BS  102 A or is out of coverage of the cell  101 . Although the UE  104   b  does not have a direct communication channel with the BS  102 A, it forms a direct communication channel  105  with its neighbor UEs, e.g., UE  104 A within a sidelink (SL) communication group  112 . The direct communication channels between the UE  104  and the BS  102  can be through interfaces such as an Uu interface, which is also known as UMTS (Universal Mobile Telecommunication System (UMTS) air interface. The direct communication channels  105  between the UEs can be through a PC5 interface, which is introduced to address high moving speed and high density applications such as Vehicle-to-everything (V2X) and Vehicle-to-Vehicle (V2V) communications. The BS  102  is connected to a core network (CN)  108  through an external interface  107 , e.g., an Iu interface. 
     The UE  104 A obtains its synchronization reference from the corresponding BS  102 A, which obtains its own synchronization reference from the core network  108  through an internet time service, such as a public time NTP (Network Time Protocol) server or a RNC (Radio Frequency Simulation System Network Controller) server. This is known as network-based synchronization. Alternatively, the BS  102 A can also obtain synchronization reference from a Global Navigation Satellite System (GNSS)  109  through a satellite signal  106 , especially for a large BS in a large cell which has a direct line of sight to the sky, which is known as satellite-based synchronization. The main advantage of the satellite-based synchronization is full independency providing a reliable synchronization signal as long as the station remains locked to a minimum number of GPS (Global Positioning System) satellites. Each GPS satellite contains multiple atomic clocks that contribute very precise time data to the GPS signals. GPS receivers on the BS  102 A decode these signals, effectively synchronizing the corresponding BS  102 A to the atomic clocks. This enables corresponding BS  102 A to determine the time within 100 billionths of a second (i.e., 100 nanoseconds), without the cost of owning and operating atomic clocks. 
     Similarly, the UE  104 B can obtain a synchronization reference from the corresponding BS  102 B which further obtains its own synchronization reference from the core network  108  or from a GNSS  109 , as discussed in detail above. The UEs  104 A can also obtain a synchronization reference through the UE  104 B in sidelink communications, wherein the synchronization reference of the UE  104 B can be either network-based or satellite-based, as described above. 
       FIG. 1B  illustrates a block diagram of an exemplary wireless communication system  150  for transmitting and receiving downlink, uplink and sidelink communication signals, in accordance with some embodiments of the present disclosure. The system  150  may include components and elements configured to support known or conventional operating features that need not be described in detail herein. In one exemplary embodiment, system  150  can be used to transmit and receive data symbols in a wireless communication environment such as the wireless communication network  100  of  FIG. 1A , as described above. 
     System  150  generally includes a first BS  102 A, a second  102 B, a first UE  104 A and a second UE  104 B, collectively referred to as BS  102  and UE  104  below for ease of discussion. The BSs  102  each includes a BS transceiver module  152 , a BS antenna array  154 , a BS memory module  156 , a BS processor module  158 , and a network interface  160 , each module being coupled and interconnected with one another as necessary via a data communication bus  180 . The UE  104  includes a UE transceiver module  162 , a UE antenna  164 , a UE memory module  166 , a UE processor module  168 , and an I/O interface  169 , each module being coupled and interconnected with one another as necessary via a date communication bus  190 . The BS  102  communicates with the UE  104  via a communication channel  192 , which can be any wireless channel or other medium known in the art suitable for transmission of data as described herein. 
     As would be understood by persons of ordinary skill in the art, system  150  may further include any number of modules other than the modules shown in  FIG. 1B . Those skilled in the art will understand that the various illustrative blocks, modules, circuits, and processing logic described in connection with the embodiments disclosed herein may be implemented in hardware, computer-readable software, firmware, or any practical combination thereof. To clearly illustrate this interchangeability and compatibility of hardware, firmware, and software, various illustrative components, blocks, modules, circuits, and steps are described generally in terms of their functionality. Whether such functionality is implemented as hardware, firmware, or software depends upon the particular application and design constraints imposed on the overall system. Those familiar with the concepts described herein may implement such functionality in a suitable manner for each particular application, but such implementation decisions should not be interpreted as limiting the scope of the present invention. 
     A wireless transmission from a transmitting antenna of the UE  104  to a receiving antenna of the BS  102  is known as an uplink transmission, and a wireless transmission from a transmitting antenna of the BS  102  to a receiving antenna of the UE  104  is known as a downlink transmission. In accordance with some embodiments, a UE transceiver  162  may be referred to herein as an “uplink” transceiver  162  that includes a RF transmitter and receiver circuitry that are each coupled to the UE antenna  164 . A duplex switch (not shown) may alternatively couple the uplink transmitter or receiver to the uplink antenna in time duplex fashion. Similarly, in accordance with some embodiments, the BS transceiver  152  may be referred to herein as a “downlink” transceiver  152  that includes RF transmitter and receiver circuitry that are each coupled to the antenna array  154 . A downlink duplex switch may alternatively couple the downlink transmitter or receiver to the downlink antenna array  154  in time duplex fashion. The operations of the two transceivers  152  and  162  are coordinated in time such that the uplink receiver is coupled to the uplink UE antenna  164  for reception of transmissions over the wireless communication channel  192  at the same time that the downlink transmitter is coupled to the downlink antenna array  154 . The UE transceiver  162  communicates through the UE antenna  164  with the BS  102  via the wireless communication channel  192  or with other UEs via the wireless communication channel  193 . The wireless communication channel  193  can be any wireless channel or other medium known in the art suitable for sidelink transmission of data as described herein. 
     The UE transceiver  162  and the BS transceiver  152  are configured to communicate via the wireless data communication channel  192 , and cooperate with a suitably configured RF antenna arrangement  154 / 164  that can support a particular wireless communication protocol and modulation scheme. In some exemplary embodiments, the UE transceiver  162  and the BS transceiver  152  are configured to support industry standards such as the Long Term Evolution (LTE) and emerging 5G standards (e.g., NR), and the like. It is understood, however, that the invention is not necessarily limited in application to a particular standard and associated protocols. Rather, the UE transceiver  162  and the BS transceiver  152  may be configured to support alternate, or additional, wireless data communication protocols, including future standards or variations thereof. 
     The processor modules  158  and  168  may be implemented, or realized, with a general purpose processor, a content addressable memory, a digital signal processor, an application specific integrated circuit, a field programmable gate array, any suitable programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof, designed to perform the functions described herein. In this manner, a processor may be realized as a microprocessor, a controller, a microcontroller, a state machine, or the like. A processor may also be implemented as a combination of computing devices, e.g., a combination of a digital signal processor and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a digital signal processor core, or any other such configuration. 
     Furthermore, the steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in firmware, in a software module executed by processor modules  158  and  168 , respectively, or in any practical combination thereof. The memory modules  156  and  166  may be realized as RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. In this regard, the memory modules  156  and  166  may be coupled to the processor modules  158  and  168 , respectively, such that the processors modules  158  and  168  can read information from, and write information to, memory modules  156  and  166 , respectively. The memory modules  156  and  166  may also be integrated into their respective processor modules  158  and  168 . In some embodiments, the memory modules  156  and  166  may each include a cache memory for storing temporary variables or other intermediate information during execution of instructions to be executed by processor modules  158  and  168 , respectively. Memory modules  156  and  166  may also each include non-volatile memory for storing instructions to be executed by the processor modules  158  and  168 , respectively. 
     The network interface  160  generally represents the hardware, software, firmware, processing logic, and/or other components of the base station  102  that enable bi-directional communication between BS transceiver  152  and other network components and communication nodes configured to communication with the BS  102 . For example, network interface  160  may be configured to support internet or WiMAX traffic. In a typical deployment, without limitation, network interface  160  provides an 802.3 Ethernet interface such that BS transceiver  152  can communicate with a conventional Ethernet based computer network. In this manner, the network interface  160  may include a physical interface for connection to the computer network (e.g., Mobile Switching Center (MSC)). The terms “configured for” or “configured to” as used herein with respect to a specified operation or function refers to a device, component, circuit, structure, machine, signal, etc. that is physically constructed, programmed, formatted and/or arranged to perform the specified operation or function. The network interface  160  could allow the BS  102  to communicate with other BSs or core network over a wired or wireless connection. 
     Referring again to  FIG. 1A , as mentioned above, the BS  102  repeatedly broadcasts system information associated with the BS  102  to one or more UEs (e.g.,  104 ) so as to allow the UE  104  to access the network within the cell (e.g.,  101  for BS  102 A and  110  for BS  102 B) where the BS  102  is located, and in general, to operate properly within the cell. Plural information such as, for example, downlink and uplink cell bandwidths, downlink and uplink configuration, configuration for random access, etc., can be included in the system information, which will be discussed in further detail below. Typically, the BS  102  broadcasts a first signal carrying some major system information, for example, configuration of the cell  101  through a PBCH (Physical Broadcast Channel). For purposes of clarity of illustration, such a broadcasted first signal is herein referred to as “first broadcast signal.” It is noted that the BS  102  may subsequently broadcast one or more signals carrying some other system information through respective channels (e.g., a Physical Downlink Shared Channel (PDSCH)), which are herein referred to as “second broadcast signal,” “third broadcast signal,” and so on. 
     Referring again to  FIG. 1B , in some embodiments, the major system information carried by the first broadcast signal may be transmitted by the BS  102  in a symbol format via the communication channel  192  (e.g., a PBCH). In accordance with some embodiments, an original form of the major system information may be presented as one or more sequences of digital bits and the one or more sequences of digital bits may be processed through plural steps (e.g., coding, scrambling, modulation, mapping steps, etc.), all of which can be processed by the BS processor module  158 , to become the first broadcast signal. Similarly, when the UE  104  receives the first broadcast signal (in the symbol format) using the UE transceiver  162 , in accordance with some embodiments, the UE processor module  168  may perform plural steps (de-mapping, demodulation, decoding steps, etc.) to estimate the major system information such as, for example, bit locations, bit numbers, etc., of the bits of the major system information. The UE processor module  168  is also coupled to the I/O interface  169 , which provides the UE  104  with the ability to connect to other devices such as computers. The I/O interface  169  is the communication path between these accessories and the UE processor module  168 . 
     In some embodiments, the UE  104  can operate in a hybrid/heterogeneous communication network in which the UE communicates with the BS  102 , and with other UEs, e.g., between  104 A and  104 B. As described in further detail below, the UE  104  supports sidelink communications with other UE&#39;s as well as downlink/uplink communications between the BS  102  and the UE  104 . As discussed above, sidelink communication allows the UEs  104 A and  104 B within a sidelink communication group  112  to establish a direct communication link with each other, or with other UEs from different cells, without requiring the BS  102  to relay data between UE&#39;s. 
       FIG. 2  illustrates an exemplary hybrid wireless communication network  200 , in accordance with some embodiments of the present disclosure. In the illustrated embodiments, the network  200  comprises 2 BSs  102 A/ 102 B and 2 UEs  104 A/ 104 B. In the illustrated embodiments, a first BS  102 A and a second BS  102 B are different, for example, the first BS  102 A is a gNB and the second BS  102 B is an eNB. A first UE  102 A and a second UE  102 B are both in a first cell  101  covered by the first BS  102 A. Further, the first cell  101  is also within the coverage of the second cell  110  by the second BS  102 B. The first UE  104 A and the second UE  104 B can directly communicate with both the first BS  102 A and the second BS  102 B to obtain synchronization references. In some embodiments, the network  200  may comprise a plurality of heterogeneous sub-cells, for example the plurality of heterogeneous sub-cells can comprise first-level IAB (integrated access and backhaul) nodes, second level IAB nodes, etc. It is noted that the illustrated embodiment is merely an example, and are not intended to limit the present disclosure. Accordingly, it is understood that the network  200  may include any desired combination of UEs  104 , and BSs  102 , while remaining within the scope of the present disclosure. 
     In the illustrated embodiments, UE  104 A is a NR-UE which can determine a first synchronization reference by detecting a first synchronization signal received from the BS  102 A (e.g., gNB) and UE  104 B is a LTE-UE which can determine a second synchronization reference by detecting a second synchronization signal received from the BS  102 B (e.g., eNB). In some embodiments, the first synchronization reference from the first synchronization signal on the UE  104 A and the second synchronization signal received from the BS  102 B on the UE  104 B are misaligned. Further, the UE  104 B is located near a center region  202  of the LTE cell  110  and therefore, the UE  104 B does not required to transmit a sidelink synchronization signal (SLSS) and physical sidelink broadcast channel (PSBCH) if it is not configured or pre-configured to transmit. The synchronization reference used for transmitting a sidelink communication signal by the UE  104 B should be obtained by the UE  104 A such that the UE  104 A can successfully receive the sidelink communication signal transmitted from the UE  104 B. 
       FIG. 3  illustrates a method  300  for performing a sidelink synchronization in the exemplary hybrid wireless communication system  200 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  300  of  FIG. 3 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 3  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. 
     The method  300  starts with operation  302  in which UE  104 A obtains a configuration signaling for sidelink (SL) communications from the BS  102 A according to some embodiments. In some embodiments, the configuration signaling comprises at least one time offset value. In some embodiments, the at least one time offset value in the configuration signaling can be used by the UE  102 A to obtain at least one corresponding synchronization reference for sidelink communications. For example, there are n time offset values, i.e., offset(0), offset(1), offset(2), . . . and offset(n−1), wherein n is an positive integer and n is equal to or greater than 1. In some embodiments, the time offset value is determined by the BS  102 A or pre-configured by the system. 
     In some embodiments, the configuration signaling received by the UE  104 A from the BS  102 A further comprises information of a plurality of carriers of the BS  102 B and a plurality of time offset values corresponding to the plurality of carriers. In some embodiments, the configuration signaling received by the UE  104 A from the BS  102 A may also comprise information of a plurality of resource pools. In some embodiments, the plurality of resource pools each can be one of the following: a transmitting resource pool and a receiving resource pool. In some embodiments, the plurality of resource pools each corresponds to at least one of the plurality of time offset values and the plurality of carriers each corresponds to at least one of the plurality of time offset values. In some embodiments, a mapping relationship between the time offset value and the resource pool can be one of the following: one time offset value corresponds to one resource pool, one time offset value corresponds to multiple resource pools, and multiple time offset value corresponds to one resource pool. 
     In some embodiments, operation  302  also comprises determining at least one synchronization reference by detecting at least one synchronization signal from the corresponding BSs  102 . For example, the UE  104 A can determine a first synchronization reference by detecting a corresponding synchronization signal received from the BS  102 A and the UE  104 B can determine a second synchronization reference by detecting a corresponding synchronization signal received from the BS  102 B. In some embodiments, the first synchronization reference determined by the UE  104 A can be used as a first synchronization reference for transmitting a first SL communication signal by the UE  104 A and similarly, the second synchronization reference determined by the UE  104 B can be used as a second synchronization reference for transmitting a second SL communication signal by the UE  104 B. 
     The method  300  continues with operation  304  in which the UE  104 A determines at least one synchronization reference for receiving at least one SL communication signal from the UE  104 B in the SL communications according to some embodiments. The synchronization reference for receiving can be determined according to the first synchronization reference by detecting the first synchronization signal received from the BS  102 A and the at least one time offset value. In some embodiments, the synchronization reference for receiving in SL communications on the corresponding carriers and/or on the corresponding resource pool is equal to a summation of the first synchronization reference and the time offset value. In some embodiments, the at least one carrier is a carrier corresponding to a frequency range in the SL communications. In some other embodiments, the at least one carrier is a carrier in a serving cell of the corresponding UE  104 B. 
     When the configuration signaling comprises a plurality of time offset values, for example, there are n time offset values, i.e., offset(0), offset(1), offset(2), . . . and offset(n−1), a plurality of synchronization reference for SL communication can be determined, i.e., the first synchronization reference+offset(0), the first synchronization reference+offset(1), the first synchronization reference+offset(2), . . . and the first synchronization reference+offset(n−1), respectively. In some embodiments, the UE  104 A can receive SL communication signals in SL communications from the UE  104 B on n corresponding resource pools according to the corresponding one of the n synchronization references. In some other embodiments, one of the n synchronization references can be selected as the transmitting synchronization reference and indicated by the BS  102 A to the UE  102 A and the n synchronization references can be used as the receiving synchronization references for receiving SL communication signals. In some other embodiments, the UE  102 A can randomly select one of the n synchronization references for transmitting SL communication signals to the UE  104 B. 
     In some embodiments, the UE  104 A can obtain a first synchronization reference determined from the BS  102 A and a second synchronization reference determined from the BS  102 B. When the frequency ranges for a first sidelink communication, e.g., a NR-V2X signal with another NR-UE for advanced V2X business and for a second sidelink communication, e.g., an LTE-V2X signal with anther LTE-UE, the first synchronization reference is used as the synchronization reference for the first sidelink communication and the second synchronization reference is used as the synchronization reference for the second sidelink communication. 
     The method  300  continues with operation  306  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on the at least one corresponding carrier and/or the at least one corresponding resource pool. 
       FIG. 4  illustrates a method  400  for performing a sidelink synchronization in the exemplary hybrid wireless communication system  200 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  400  of  FIG. 4 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 4  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. 
     The method  400  starts with operation  402  in which UE  104 A obtains carrier information from the BS  102 A according to some embodiments. In some embodiments, the carrier information can be transmitted through RRC signaling. In some embodiments, the carrier information received from the BS  102 A is used to indicate information of at least one carrier on which the UE  104 A can detect at least one synchronization signal to determine at least one first synchronization reference. In some embodiments, operation  402  also comprises determining at least one synchronization reference by detecting at least one synchronization signal received by the UEs  104  from the corresponding BSs  102 . Specifically, the UE  104 A can determine a second synchronization reference by detecting a corresponding synchronization signal received from the BS  102 A and the UE  104 B can determine a third synchronization reference by detecting a corresponding synchronization signal received from the BS  102 B. In some embodiments, the second synchronization reference determined by the UE  104 A can be used as a first synchronization reference for transmitting a first SL communication signal by the UE  104 A and similarly, the third synchronization reference received by the UE  104 B can be used as a second synchronization reference for transmitting a second SL communication signal by the UE  104 B. 
     The method  400  continues with operation  404  in which the UE  104 A detects the at least one synchronization signal from the BS  102 B on the at least one corresponding carrier indicated by the BS  102 A. 
     The method  400  continues with operation  406  in which the UE  104 A determines at least one synchronization reference for the SL communications with the UE  104 B in according to some embodiments. The at least one first synchronization reference obtained by detect the at least one synchronization signal can be then used as the at least one synchronization reference for SL communications on the at least one carrier. In some embodiments, n synchronization signals from the BS  102 B are detected by the UE  104 A, wherein n is a positive integer and n is equal to or greater than 1. In some embodiments, when the at least one carrier indicated by the BS  102 A does not comprise a carrier on which the SL communications are performed, the UE  104 A may select one of the at least one first synchronization reference of the at least one corresponding carrier from the BS  102 B as the synchronization reference for SL communications. In some embodiments, the BS  102 A can also select one of the at least one first synchronization reference as the synchronization reference for SL communications. In some embodiments, the first synchronization reference of the synchronization signal with the highest received power is selected as the synchronization reference for the SL communications. The at least one synchronization reference for SL communication is for one of the following: transmitting and receiving SL communication signals. 
     In some embodiments, the UE  104 A can obtain a first synchronization reference determined from the BS  102 A and a second synchronization reference determined from the BS  102 B. When the frequency ranges for a first sidelink communication, e.g., an NR-V2X signal with another NR-UE for advanced V2X business and for a second sidelink communication, e.g., an LTE-V2X signal with anther LTE-UE, the first synchronization reference is used as the synchronization reference for the first sidelink communication and the second synchronization reference is used as the synchronization reference for the second sidelink communication. 
     The method  400  continues with operation  408  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on the at least one corresponding carrier and/or the at least one corresponding resource pool. 
       FIG. 5  illustrates a method  500  for performing a sidelink synchronization in the exemplary hybrid wireless communication system  200 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  500  of  FIG. 5 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 5  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. In the illustrated embodiments, the UE  104 A is in a first cell  101  covered by the BS  102 A and a second cell  110  covered by the BS  102 B. The BS  102 A and the BS  102 B are gNB and eNB in an NR and LTE system, respectively. The frequency range for the sidelink communications is covered by the BS  102 B and/or the BS  102 A or is within a list of frequencies configured by the BS  102 B and/or the BS  102 A, respectively. It should be noted that the coverage configurations of the UEs  104  by the BSs  102  s can vary and are within the scope of this invention. 
     The method  500  starts with operation  502  in which a first message to trigger a transmission of a sidelink synchronization signal and/or physical sidelink broadcast channel (SLSS/PSBCH) by the UE  104 A is transmitted from the BS  102 A to the UE  104 A according to some embodiments. In some embodiments, when there is SL data to be transmitted from the UE  104 A, the first message can be an RRC broadcast signaling in which an indication parameter for SLSS/PSBCH transmission, networkContrlledSyncTx, is ON. In some embodiments, when the indication parameter for SLSS/PSBCH transmission, networkContrlledSyncTx, is OFF, the first message can be a synchronization signal from the BS  102 A. The UE  104 A can determine a synchronization reference by detecting the synchronization signal from the BS  102 A, which is further used as the synchronization reference for the sidelink communications. In some embodiments, when the indication parameter for SLSS/PSBCH transmission, networkContrlledSyncTx, is OFF, the first message can be a configuration signaling, in which the BS  102 A configures at least one time offset value. In some embodiments, the first message can be at least one synchronization signal from the BS  102 A in which the UE  104 A can determine at least one synchronization reference. In some embodiments, the first message can be at least one synchronization signal from the BS  102 A in which the UE  104 A can determine at least one synchronization reference which is further used for the SL communication. 
     In some embodiments, when the UE  104 A is not covered by the BS  102 B, the first message to trigger the transmission of SLSS/PSBCH by the UE  102 A can be an RRC broadcast signaling in which an indication parameter for SLSS transmission, networkContrlledSyncTx, is ON. In some embodiments, when the indication parameter for SLSS transmission, networkContrlledSyncTx, is OFF or not configured, the first message can be at least one synchronization signal from the BS  102 A in which the UE  104 A can determine at least one synchronization reference which is further used for the SL communications. 
     In some embodiments, operation  502  also comprises obtaining at least one synchronization reference according to at least one synchronization signal received by the UEs  104  from the corresponding BSs  102 . Specifically, the UE  104 A can determine a first synchronization reference by detecting a corresponding synchronization signal received from the BS  102 A and the UE  104 B can determine a second synchronization reference by detecting a corresponding synchronization signal received from the BS  102 B. In some embodiments, the first synchronization reference determined by the UE  104 A can be used as a first synchronization reference for transmitting a first SL communication signal by the UE  104 A during the SL communications and similarly, the second synchronization reference determined by the UE  104 B can be used as a second synchronization reference for transmitting a second SL communication signal by the UE  104 B during the SL communications. 
     The method  500  continues with operation  504  in which the UE  104 A transmits a SLSS/PSBCH according to some embodiments. In some embodiments, the SLSS/PSBCH is transmitted according to the first synchronization reference determined by the UE  104 A. 
     The method  500  continues with operation  506  in which a synchronization reference for the SL communications is determined by the UE  104 B by detecting the SLSS/PSBCH signal from the UE  104 A according to some embodiments of the present disclosure. In some embodiments, when the UE  104 B initially does not have the knowledge of an exact timing of the transmission of the SL communication signal from the UE  104 A, a detection of the SLSS/PSBCH signal by the UE  104 B is performed continuously in the time domain until the SLSS/PSBCH signal is detected and the corresponding synchronization reference is determined by the UE  104 B. In some embodiments, the detection of the SLSS/PSBCH signal can be triggered by a direct message from the BS  102 B, e.g., a RRC message or a system message. In some embodiments, the synchronization reference can be further used by the UE  104 B to track the UE  104 A for accurate synchronization during subsequent SL communications. 
     In some embodiments, the UE  104 A can obtain a first synchronization reference determined from the BS  102 A and a second synchronization reference determined from the BS  102 B. When a first frequency range for a first sidelink communication, e.g., an NR-V2X signal with another NR-UE for advanced V2X business, and a second frequency range for a second sidelink communication, e.g., an LTE-V2X signal with anther LTE-UE, the first synchronization reference is used as the synchronization reference for the first sidelink communication and the second synchronization reference is used as the synchronization reference for the second sidelink communication. In some embodiments, the first frequency range is different from the second frequency range. 
     The method continues with operation  508  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on at least one corresponding carrier and/or at least one corresponding resource. 
     In some embodiments, at least two of the methods  300 ,  400 ,  500  and  600  presented in  FIGS. 3-6  can be combined to determine the at least one synchronization reference for the sidelink communication. In some embodiments, the at least two of the methods  300 ,  400 ,  500 , and  600  are combined according to their priorities. For example, the priority of the methods from high to low is ordered as the following: methods  500  or  600 &gt;method  400 &gt;method  300 . 
       FIG. 6  illustrates a method  600  performing a sidelink synchronization in the exemplary hybrid wireless communication system  200 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  600  of  FIG. 6 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 6  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. In the illustrated embodiments, the UE  104 A is in a first cell  110  covered by the BS  102 B and not in a second cell  101  covered by the BS  102 A. The BS  102 A and the BS  102 B are gNB and eNB in an NR and LTE system, respectively. The frequency range for the sidelink communications is covered by the BS  102 B or is within a list of frequencies configured by the BS  102 B. It should be noted that the coverage configurations of the UEs  104  by the BSs  102  s can vary and are within the scope of this invention. 
     The method  600  starts with operation  602  in which a first message to trigger a transmission of a first sidelink synchronization signal and/or physical sidelink broadcast channel (SLSS/PSBCH) by the UE  104 A is transmitted from the BS  102 B to the UE  104 A according to some embodiments. In some embodiments, when there is SL communication data to be transmitted from the UE  104 A, the first message can be an RRC signaling in which an indication parameter for SLSS transmission, e.g., networkContrlledSyncTx, is ON. In some embodiments, the first message also comprises at least one synchronization signal from the BS  102 B in which the UE  104 A can determine at least one synchronization reference which is further used for the SL communication. In some embodiments, when the indication parameter for SLSS/PSBCH transmission, e.g., networkContrlledSyncTx, is OFF, the first message can be a synchronization signal from the BS  102 B. The UE  104 A can determine a synchronization reference by detecting the synchronization signal from the BS  102 B, which is further used as the synchronization reference for the sidelink communications. In some embodiments, when the indication parameter for SLSS transmission, e.g., networkContrlledSyncTx, is OFF or is not configured, the first message can be at least one synchronization signal from the BS  102 B in which the UE  104 A can determine at least one synchronization reference. 
     The method  600  continues with operation  604  in which the UE  104 A transmits a SLSS/PSBCH according to some embodiments. In some embodiments, the UE  104 A transmits a SLSS/PSBCH according to the first synchronization reference. In some embodiments, the UE  104 B can determine a SL synchronization reference by detecting the first synchronization reference for receiving the SL communication data transmitted from the UE  104 A. 
     In some embodiments, referring back to  FIG. 5 , a second message to trigger a transmission of a second sidelink synchronization signal and/or physical sidelink broadcast channel (e.g., NR-based SLSS/PSBCH, hereinafter NR-SLSS and/or NR-PSBCH) by the UE  104 A is transmitted from the BS  102 A to the UE  104 A according to some embodiments. In some embodiments, the first and the second SLSS/PSBCH are transmitted according to the corresponding synchronization references determined by the UE  104 A on the corresponding synchronization signals from the BSs  102 B and  102 A, respectively. The synchronization references can be aligned or misaligned. In some embodiments, a first resource for transmitting the first SLSS/PSBCH and a second resource for transmitting the second SLSS/PSBCH are determined by the corresponding BSs  102 A and  102 B. In some other embodiments, the first and second resources for transmitting the first and second SLSS/PSBCH are pre-configured. In some embodiments, when the first and the second resources overlap in the time domain and/or when the UE  104 A has limited capability to transmit the first and the second SLSS/PSBCH simultaneously, only the first SLSS/PSBCH is transmitted on the first resource or on the overlapped subframes, time slots, OFDM (orthogonal frequency division multiplexing) symbols, SC (single carrier)-OFDM symbols. In some other embodiments, when the first and the second resources overlap in the time domain and/or when the UE  104 A has limited capability to transmit the first and the second SLSS/PSBCH simultaneously, the first and the second SLSS/PSBCH are transmitted alternatively on the first and the second resources. 
     In some other embodiments, when the first and the second resources overlap in the time domain and/or when the UE  104 A has limited capability to transmit the first and the second SLSS/PSBCH simultaneously, the transmission of the first and the second SLSS/PSBCH is configured by the corresponding BSs  102  or pre-configured according to the subframe index of the first and the second resources. For example, if the subframe index mod N equals to 1, the UE  104 A transmits the first SLSS/PSBCH on the first resource and if the subframe index mod N equals to 0, the UE  104 A transmits the second SLSS/PSBCH on the second resource, wherein N is a positive integer. 
     In some embodiments, when a synchronization reference is needed for transmitting and/or receiving a SL communication signal, a UE  104 A when not covered by the BS  102 A can determine the synchronization reference according to the priority of the synchronization reference it can obtain. When the pre-configuration indicates a gNB has the highest priority on a frequency and when the UE  104 A can determine a synchronization reference directly from the BS  102 A (i.e., gNB), BS  102 B (i.e., eNB), GNSS  109  and through SLSS/PSBCH from other UE which can determine its synchronization reference directly or indirectly from the corresponding BS (e.g., BS  102 A and BS  102 B), the priority order from high to low on the concerned frequency is listed below: 
     (1) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 A, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated in the coverage of a gNB network. 
     (2) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference the BS  102 A through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated out of the coverage of a gNB network. 
     (3) GNSS  109 , wherein GNSS is reliable. 
     (4) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the GNSS  109 , wherein the SLSSID of NR-SLSS is a value dedicated for GNSS or NR-SLSS/PSBCH is transmitted in the resource(s) dedicated for GNSS. 
     (5) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference from the GNSS  109  through a different UE, wherein the SLSSID of NR-SLSS is not the value dedicated for GNSS or NR-SLSS/PSBCH is not transmitted in the resource(s) dedicated for GNSS. 
     (6) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 B, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated in the coverage of an eNB network. 
     (7) a NR-SLSS/PSBCH of a UE which indirectly obtains a synchronization reference from the BS  102 B through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated out of the coverage of an eNB network. 
     (8) a NR-SLSS/PSBCH of a UE which cannot obtains a synchronization reference from a BS or a GNSS or a different UE that obtains its synchronization reference directly from a BS or a GNSS. In some other embodiments, when the pre-configuration indicates gNB has the highest priority on a frequency and when the UE  104 A can determine a synchronization reference directly from the BS  102 A (i.e., gNB), BS  102 B (i.e., eNB), GNSS  109  and through SLSS/PSBCH from other UE which can determine its synchronization reference directly or indirectly from the corresponding BS (e.g., BS  102 A and BS  102 B), the priority order from high to low on the concerned frequency is listed below: 
     (1) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 A, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated in the coverage of a gNB network. 
     (2) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference the BS  102 A through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated out of the coverage of a gNB network. 
     (3) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 B, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated in the coverage of an eNB network. 
     (4) a NR-SLSS/PSBCH of a UE which indirectly obtains a synchronization reference from the BS  102 B through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated out of the coverage of an eNB network. 
     (5) GNSS  109 , wherein GNSS is reliable. 
     (6) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the GNSS  109 , wherein the SLSSID of NR-SLSS is a value dedicated for GNSS or NR-SLSS/PSBCH is transmitted in the resource(s) dedicated for GNSS. 
     (7) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference from the GNSS  109  through a different UE, wherein the SLSSID of NR-SLSS is not the value dedicated for GNSS or NR-SLSS/PSBCH is not transmitted in the resource(s) dedicated for GNSS. 
     (8) a NR-SLSS/PSBCH of a UE which cannot obtains a synchronization reference from a BS or a GNSS or a different UE that obtains its synchronization reference directly from a BS or a GNSS. 
     In some embodiments, when a synchronization reference is needed for transmitting and receiving a SL communication signal, a UE  104 A when not covered by the BS  102 A can determine the synchronization reference according to the priority of the SLSS/PSBCH it can detect. When the pre-configuration indicates GNSS has the highest priority on a frequency and when the UE  104 A can determine a synchronization reference directly from the BS  102 A (i.e., gNB), BS  102 B (i.e., eNB), GNSS  109  and through SLSS/PSBCH from other UE which can determine its synchronization reference directly or indirectly from the corresponding BS (e.g., BS  102 A and BS  102 B), the priority order from high to low on the concerned frequency is listed below: 
     (1) GNSS  109 , wherein GNSS is reliable. 
     (2) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the GNSS  109 , wherein the SLSSID of NR-SLSS is a value dedicated for GNSS or NR-SLSS/PSBCH is transmitted in the resource(s) dedicated for GNSS. 
     (3) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference from the GNSS  109  through a different UE, wherein the SLSSID of NR-SLSS is not the value dedicated for GNSS or NR-SLSS/PSBCH is not transmitted in the resource(s) dedicated for GNSS. 
     (4) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 A, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated in the coverage of a gNB network. 
     (5) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference the BS  102 A through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated out of the coverage of a gNB network. 
     (6) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 B, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated in the coverage of an eNB network. 
     (7) a NR-SLSS/PSBCH of a UE which indirectly obtains a synchronization reference from the BS  102 B through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated out of the coverage of an eNB network. 
     (8) a NR-SLSS/PSBCH of a UE which cannot obtains a synchronization reference from a BS or a GNSS or a different UE that obtains its synchronization reference directly from a BS or a GNSS. 
     In some other embodiments, when the pre-configuration indicates GNSS has the highest priority on a frequency and when the UE  104 A can determine a synchronization reference directly from the BS  102 A (i.e., gNB), BS  102 B (i.e., eNB), GNSS  109  and through SLSS/PSBCH from other UE which can determine its synchronization reference directly or indirectly from the corresponding BS (e.g., BS  102 A and BS  102 B), the priority order from high to low on the concerned frequency is listed below: 
     (1) GNSS  109 , wherein GNSS is reliable. 
     (2) a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the GNSS  109 , wherein the SLSSID of NR-SLSS is a value dedicated for GNSS or NR-SLSS/PSBCH is transmitted in the resource(s) dedicated for GNSS; a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 A, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated in the coverage of a gNB network; or a NR-SLSS/PSBCH from a UE which directly obtains a synchronization reference from the BS  102 B, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated in the coverage of an eNB network. 
     (3) a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference from the GNSS  109  through a different UE, wherein the SLSSID of NR-SLSS is not the value dedicated for GNSS or NR-SLSS/PSBCH is not transmitted in the resource(s) dedicated for GNSS; a NR-SLSS/PSBCH from a UE which indirectly obtains a synchronization reference the BS  102 A through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for a gNB in coverage and the UE is indicated out of the coverage of a gNB network; or a NR-SLSS/PSBCH of a UE which indirectly obtains a synchronization reference from the BS  102 B through a different UE, wherein the SLSSID of NR-SLSS is part of a set defined for an eNB in coverage and the UE is indicated out of the coverage of an eNB network. 
     (4) a NR-SLSS/PSBCH of a UE which cannot obtains a synchronization reference from a BS or a GNSS or a different UE that obtains its synchronization reference directly from a BS or a GNSS. 
     In some embodiments, the UE  104 A transmits NR-SLSS/PSBCH on the second resource and detects SLSS/PSBCH on a third resource. When the second and the third resources overlap on the time domain, the NR-SLSS/PSBCH will not be transmitted on the second resource by the UE  104 A. In some embodiments, when the second and the third resources overlap in the time domain, the transmission of the NR-SLSS/PSBCH is configured by the BS  102 A or pre-configured. For example, the NR-SLSS/PSBCH can be transmitted at least once in a period of L in the time domain or in N sidelink synchronization resources, wherein L and N are positive integers. 
     The method  600  continues with operation  606  in which a synchronization reference for the SL communications is determined by detecting the SLSS/PSBCH signal from the UE  104 A according to some embodiments of the present disclosure. In some embodiments, when the UE  104 B initially does not have the knowledge of an exact timing of the transmission of the SLSS/PSBCH signal from the UE  104 A, a detection of the SLSS and/or a PSBCH signal by the UE  104 B is performed continuously in the time domain, and the SLSS/PSBCH signal is detected by the UE  104 B. In some embodiments, the detection of the SLSS/PSBCH signal can be triggered by a direct message from the BS  102 B, e.g., a RRC signaling or a system message. In some embodiments, the synchronization reference can be further used by the UE  104 B to track the UE  104 A for accurate synchronization during subsequent SL communications. 
     In some embodiments, the UE  104 A can obtain a first synchronization reference determined from the BS  102 A and a second synchronization reference determined from the BS  102 B. When the frequency ranges for a first sidelink communication, e.g., an NR-V2X signal with another NR-UE for advanced V2X business and for a second sidelink communication, e.g., an LTE-V2X signal with anther LTE-UE, the first synchronization reference is used as the synchronization reference for the first sidelink communication and the second synchronization reference is used as the synchronization reference for the second sidelink communication. 
     The method  600  continues with operation  608  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on at least one corresponding carrier and/or at least one corresponding resource. 
       FIG. 7  illustrates an exemplary hybrid wireless communication network  700 , in accordance with some embodiments of the present disclosure. In the illustrated embodiments, the network  700  comprises 2 BSs  102 A/ 102 B and 2 UEs  104 A/ 104 B. In the illustrated embodiments, the 2 BSs  102 A/ 102 B are different, for example, the first BS  102 A is a gNB and the second BS  102 B is an eNB. The first UE  102 A and the second UE  102 B are both in a first cell  101  covered by the first BS  102 A. Further, the first cell  101  is also within the coverage of the second cell  110 . The first UE  104 A and the second UE  104 B can directly communicate with both the first BS  102 A and the second BS  102 B to obtain synchronization references. In some embodiments, the network  200  may comprise a plurality of heterogeneous sub-cells, for example the plurality of heterogeneous sub-cells can comprise first-level IAB (integrated access and backhaul) nodes, second level IAB nodes, etc. It is noted that the illustrated embodiment is merely an example, and are not intended to limit the present disclosure. Accordingly, it is understood that the network  200  may include any desired combination of UEs  104 , and BSs  102 , while remaining within the scope of the present disclosure. 
     In the illustrated embodiments, UE  104 A is a NR-UE which can obtain a first synchronization reference from a first synchronization signal received from the BS  102 A (e.g., gNB) and UE  104 B is a LTE-UE which can obtain a second synchronization reference from a second synchronization signal received from the BS  102 B (e.g., eNB). In some embodiments, the first synchronization reference and the second synchronization reference are misaligned. Further, the UE  104 B is also located near a center region  202  of the LTE cell  110  and therefore, the UE  104 B does not required to transmit a sidelink synchronization signal (SLSS) to its neighbor UEs  104 . A synchronization reference for transmitting used by the UE  104 B is required such that the UE  104 A can use this synchronization reference to successfully receive data transmitted from the UE  104 B during sidelink communications. 
       FIG. 8  illustrates a method  800  for performing a sidelink synchronization in the exemplary hybrid wireless communication system  700 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  800  of  FIG. 8 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 8  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. 
     The method  800  continues with operation  802  in which the UE  104 A detects the at least one synchronization signal from the BS  102 B on at least one corresponding first carrier indicated by the BS  102 A according to carrier information. In some embodiments, the carrier information is pre-configured by the manufacture. The carrier information indicates information of the at least one first carrier on which the UE  104 A can detect at least one synchronization signal from the BS  102 B and further determine at least one first corresponding synchronization reference. 
     The method  800  continues with operation  804  in which the UE  104 A determines the at least one synchronization reference for the SL communications with the UE  104 B in according to some embodiments. In some embodiments, the at least one first synchronization reference determined by detecting the at least one synchronization signal on the at least one first carrier from the BS  102 B can be then used as the at least one synchronization reference for SL communications on at least one second carrier according a mapping relationship between the first and the second carriers. In some embodiments, the mapping relationship can be configured by BS  102 B or pre-configured. The at least one synchronization reference for SL communication comprises at least one of the following: a synchronization reference for transmitting and a synchronization reference for receiving SL communication signals. For example, the UE  104 A detects a synchronization signal on carrier  1  and carrier  2  and determines a synchronization reference  1  and  2 , respectively. When the carrier  1  and carrier  2  for detecting synchronization signals correspond to carrier  3  and  4  for SL communications according to the mapping relationship, the synchronization reference  1  from the carrier  1  is used as the synchronization reference for SL communications on the carrier  3  and the synchronization reference  2  from the carrier  2  is used as the synchronization reference for SL communication on the carrier  4  by the UE  104 A. 
     In some other embodiments, the at least one first synchronization reference determined by detecting the at least one synchronization signal on the at least one first carrier from the BS  102 B can be then used as the at least one synchronization reference for SL communications on the at least one second carrier according to corresponding system information. Specifically, the UE  104 A detects the at least one synchronization signal from the BS  102 B and determine the at least one first synchronization reference and by decoding the system information (i.e., SIB1 from the BS  102 B to determine the at least one second synchronization reference. In some embodiments, when the SIB1 from the BS  102 B comprises scheduling information of SIB21 or system information for indicating scheduling SL communications, the UE  104 A uses the at least one first synchronization reference as the at least one second synchronization reference. In some embodiments, the UE  104 A can combine the aforementioned methods based on the mapping relationship and system information to determine the at least one second synchronization reference for the SL communications. 
     The method  800  continues with operation  806  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on the at least one corresponding carrier and/or the at least one corresponding resource pool. 
       FIG. 9  illustrates a method  900  for performing a sidelink synchronization in the exemplary hybrid wireless communication system  700 , in accordance with some embodiments of the present disclosure. It is understood that additional operations may be provided before, during, and after the method  900  of  FIG. 9 , and that some operations may be omitted or reordered. The communication system comprises 2 BSs  102  and 2 UEs  104 . It should be noted that  FIG. 9  is an example and a communication system comprising any combination of BSs  102  and UEs  104  are within the scope of this disclosure. 
     The method  900  starts with operation  902  in which UE  104 A obtains a configuration signaling for sidelink (SL) communications from the BS  102 B according to some embodiments. In some embodiments, the configuration signaling is for Vehicle-to-everything (V2X) communications. In some embodiments, the configuration signaling comprises at least one time offset value. In some embodiments, the at least one time offset value in the configuration signaling can be used by the UE  102 A to determine at least one corresponding synchronization reference for sidelink communications (e.g., V2X). For example, there are n time offset values, i.e., offset(0), offset(1), offset(2), . . . and offset(n−1), wherein n is an positive integer and n is equal to or greater than 1. In some embodiments, the time offset value is determined by the BS  102 B or pre-configured by the system. 
     In some embodiments, the configuration signaling received by the UE  104 A from the BS  102 B further comprises information of a plurality of carriers of the BS  102 B and a plurality of time offset values corresponding to the plurality of carriers. In some embodiments, the configuration signaling received by the UE  104 A from the BS  102 B may also comprise information of a plurality of resource pools. In some embodiments, the plurality of resource pools comprises at least one of the following: an uplink resource pool and a downlink resource pool. In some embodiments, the plurality of resource pools each corresponds to at least one of the plurality of time offset values. In some embodiments, a mapping relationship between the time offset value and the resource pool can be one of the following: 1 time offset value corresponds to 1 resource pool, 1 time offset corresponds to multiple resource pools, and multiple time offset corresponds to 1 resource pool. 
     The method  900  continues with operation  904  in which the UE  104 A determines at least one synchronization reference for transmitting or receiving at least one SL communication signal from the UE  104 B in the SL communications according to some embodiments. The synchronization reference for transmitting or receiving the at least one SL communication signal can be determined according to the first synchronization reference determined on the first synchronization signal received from the BS  102 B and the at least one time offset value. In some embodiments, the synchronization reference for receiving in SL communications on the corresponding carriers and/or on the corresponding resource pool is equal to a summation of the first synchronization reference and the time offset. In some embodiments, the at least one carrier is a carrier corresponding to a frequency range in the SL communications. In some other embodiments, the at least one carrier is a carrier in a serving cell of the corresponding UE  104 B. 
     When the synchronization timing comprises a plurality of time offset values, for example, there are n time offset values, i.e., offset(0), offset(1), offset(2), . . . and offset(n−1), a plurality of synchronization reference for SL communication can be determined, i.e., the first synchronization reference+offset(0), the first synchronization reference+offset(1), the first synchronization reference+offset(2), . . . and the first synchronization reference+offset(n−1), respectively. In some embodiments, the UE  104 A can receive SL communication signals in SL communications from the UE  104 B on n corresponding resource pools according to the corresponding one of the n synchronization references. In some other embodiments, one of the n synchronization references can be selected as the transmitting synchronization reference and indicated by the BS  102 B to the UE  102 A and the n synchronization references can be used as the receiving synchronization references for receiving SL communication signals. In some other embodiments, the UE  102 A can randomly select one of the n synchronization references for transmitting SL communication signals to the UE  104 B. 
     The method  900  continues with operation  906  in which the UEs  104 A and  104 B perform SL communications, i.e., transmitting and receiving SL communication signals, according to the determined at least one corresponding synchronization reference. In some embodiments, the UE  102 A can transmit and receive SL communication signals according the determined at least one synchronization reference on the at least one corresponding carrier and/or the at least one corresponding resource pool. 
     While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not by way of limitation. Likewise, the various diagrams may depict an example architectural or configuration, which are provided to enable persons of ordinary skill in the art to understand exemplary features and functions of the invention. Such persons would understand, however, that the invention is not restricted to the illustrated example architectures or configurations, but can be implemented using a variety of alternative architectures and configurations. Additionally, as would be understood by persons of ordinary skill in the art, one or more features of one embodiment can be combined with one or more features of another embodiment described herein. Thus, the breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments. 
     It is also understood that any reference to an element herein using a designation such as “first,” “second,” and so forth does not generally limit the quantity or order of those elements. Rather, these designations can be used herein as a convenient means of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not mean that only two elements can be employed, or that the first element must precede the second element in some manner. 
     Additionally, a person having ordinary skill in the art would understand that information and signals can be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits and symbols, for example, which may be referenced in the above description can be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     A person of ordinary skill in the art would further appreciate that any of the various illustrative logical blocks, modules, processors, means, circuits, methods and functions described in connection with the aspects disclosed herein can be implemented by electronic hardware (e.g., a digital implementation, an analog implementation, or a combination of the two, which can be designed using source coding or some other technique), various forms of program or design code incorporating instructions (which can be referred to herein, for convenience, as “software” or a “software module), or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware, firmware or software, or a combination of these technique, depends upon the particular application and design constraints imposed on the overall system. Skilled artisans can implement the described functionality in various ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. 
     Furthermore, a person of ordinary skill in the art would understand that various illustrative logical blocks, modules, devices, components and circuits described herein can be implemented within or performed by an integrated circuit (IC) that can include a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, or any combination thereof. The logical blocks, modules, and circuits can further include antennas and/or transceivers to communicate with various components within the network or within the device. A general purpose processor can be a microprocessor, but in the alternative, the processor can be any conventional processor, controller, or state machine. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other suitable configuration to perform the functions described herein. 
     If implemented in software, the functions can be stored as one or more instructions or code on a computer-readable medium. Thus, the steps of a method or algorithm disclosed herein can be implemented as software stored on a computer-readable medium. Computer-readable media includes both computer storage media and communication media including any medium that can be enabled to transfer a computer program or code from one place to another. A storage media can be any available media that can be accessed by a computer. By way of example, and not limitation, such computer-readable media can include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. 
     In this document, the term “module” as used herein, refers to software, firmware, hardware, and any combination of these elements for performing the associated functions described herein. Additionally, for purpose of discussion, the various modules are described as discrete modules; however, as would be apparent to one of ordinary skill in the art, two or more modules may be combined to form a single module that performs the associated functions according embodiments of the invention. 
     Additionally, memory or other storage, as well as communication components, may be employed in embodiments of the invention. It will be appreciated that, for clarity purposes, the above description has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units, processing logic elements or domains may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processing logic elements, or controllers, may be performed by the same processing logic element, or controller. Hence, references to specific functional units are only references to a suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organization. 
     Various modifications to the implementations described in this disclosure will be readily apparent to those skilled in the art, and the general principles defined herein can be applied to other implementations without departing from the scope of this disclosure. Thus, the disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the novel features and principles disclosed herein, as recited in the claims below.