Deadlock prevention techniques for RSU synchronous chains

Abstract

A deadlock prevention technique for a roadside unit (RSU) synchronization chain is disclosed that can include detecting a first reference signal associated with a first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device, transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal, and, in response to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as the synchronization reference source for the wireless communication device, initiating a waiting interval, and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference, and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

Description

[Technical Field]

[0001] 1. Field of Disclosure The present disclosure relates generally to the field of wireless communications, and more particularly to synchronizing timing of devices in a wireless communications system.

[0002] 2. Description of Related Art Vehicle-to-everything (V2X) communication systems can be deployed along roadways to provide wireless data connectivity to vehicles traveling on the roadways. A V2X communication system can include V2X roadside units (RSUs) that can communicate with V2X on-board units (OBUs) of passing vehicles. Successful communication between the V2X RSUs and the V2X OBUs can depend on establishing timing synchronization between these various devices. Summary of the Invention

[0003] An example method for wireless communication by a wireless communication device according to the present disclosure may include detecting a first reference signal associated with a first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; and, responsive to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as the synchronization reference source for the wireless communication device, initiating a waiting interval, and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; and refraining from transmitting SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0004] An example wireless communication device according to the present disclosure may comprise a transceiver; a memory; and one or more processors communicatively coupled to the transceiver and the memory, the one or more processors configured to: detect a first reference signal associated with the first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmit sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; and, in response to a subsequent determination that the first synchronization reference has become unavailable, adopt a second synchronization reference as the synchronization reference source for the wireless communication device; initiate a waiting interval; and, following expiration of the waiting interval, monitor the wireless carrier for a second reference signal associated with the second synchronization reference; and refrain from transmitting a SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0005] An example apparatus for a wireless communication device according to the present disclosure may comprise means for detecting a first reference signal associated with a first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; means for transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; and means for, in response to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as a synchronization reference source for the wireless communication device, initiating a waiting interval, and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference, and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0006] 11. An exemplary non-transitory computer-readable medium that may store instructions for wireless communication by a wireless communication device according to the present disclosure, the instructions including code for: detecting a first reference signal associated with the first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; and, responsive to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as the synchronization reference source for the wireless communication device; initiating a waiting interval; and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0007] This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used alone to determine the scope of the claimed subject matter. The subject matter should be understood by reference to the entire specification, any or all drawings, and appropriate portions of each claim of this disclosure. The above, together with other features and examples, are described in more detail below in the following specification, claims, and accompanying drawings. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram of a positioning system, according to one embodiment. [Figure 2] FIG. 1 is a diagram of a fifth generation (5G) New Radio (NR) positioning system illustrating one embodiment of a positioning system (e.g., the positioning system of FIG. 1) implemented within a 5G NR communication network. [Figure 3] FIG. 1 illustrates an example of a frame structure and associated terminology for NR. [Figure 4] FIG. 1 illustrates an example of a wireless communication system. [Figure 5]FIG. 1 is a diagram illustrating an example of an operating environment. [Figure 6] FIG. 10 is a diagram illustrating an example of a first event flow. [Figure 7] FIG. 10 is a diagram illustrating an example of a second event flow. [Figure 8] FIG. 1 is a flow diagram of a method of wireless communication by a wireless communication device, according to one embodiment. [Figure 9] FIG. 1 is a block diagram of an embodiment of a UE that can be utilized in the embodiments described herein. [Figure 10] FIG. 2 is a block diagram of an embodiment of a base station that can be utilized in the embodiments described herein.

[0009] According to some example implementations, like reference numerals in various figures refer to like elements. Additionally, multiple instances of an element may be indicated by the first numeral of that element followed by a letter or hyphen and a second numeral. For example, multiple instances of element 110 may be indicated as 110-1, 110-2, 110-3, etc., or as 110a, 110b, 110c, etc. When referring to such an element using only the first numeral, it should be understood to refer to any instance of that element (e.g., element 110 in the previous example refers to elements 110-1, 110-2, and 110-3, or elements 110a, 110b, and 110c). DETAILED DESCRIPTION OF THE INVENTION

[0010] The following description is directed to several implementations for the purposes of illustrating the inventive aspects of various embodiments, however, those skilled in the art will readily recognize that the teachings herein can be applied in many different ways. Described implementations include those based on the Institute of Electrical and Electronics Engineers (IEEE) 802.15.4 standard for ultra-wideband (UWB), the IEEE 802.11 standard (including that identified as Wi-Fi® technology), the Bluetooth® standard, code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), Global System for Mobile communications (GSM), GSM / General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), Terrestrial Trunked Radio (TETRA), Wideband-CDMA (W-CDMA), Evolution Data Optimized (EV-DO), 1xEV-DO, EV-DO Rev. A, EV-DO Rev B, High Rate Packet Data (HRPD), High Speed ​​Packet Access (HSPA), High Speed ​​Downlink Packet Access (HSDPA), High Speed ​​Uplink Packet Access (HSUPA), Evolved High Speed ​​Packet Access (HSPA+), Long Term EvolutionThe present invention may be implemented in any device, system, or network capable of transmitting and receiving radio frequency (RF) signals according to any communications standard, such as any of the LTE (LTE Evolution), Advanced Mobile Phone System (AMPS), or other known signals used to communicate within wireless, cellular, or internet of things (IoT) networks, such as systems utilizing 3G, 4G, 5G, 6G technologies, or further implementations thereof.

[0011] As used herein, an "RF signal" includes electromagnetic waves that transport information through space between a transmitter (or transmitting device) and a receiver (or receiving device). As used herein, a transmitter may transmit a single "RF signal" or multiple "RF signals" to a receiver. However, a receiver may receive multiple "RF signals" corresponding to each transmitted RF signal due to the propagation characteristics of RF signals through multiple channels or paths.

[0012] Additionally, unless otherwise specified, references to "reference signals," "positioning reference signals," "reference signals for positioning," etc. may be used to refer to signals used for positioning of user equipment (UE). As described in more detail herein, such signals may include any of a variety of signal types, but may not necessarily be limited to positioning reference signals (PRS) as defined in the relevant wireless standards.

[0013] 1 is a simplified diagram of a positioning system 100 in which a UE 105, a location server 160, and / or other components of the positioning system 100 can use techniques provided herein for deadlock prevention of an RSU synchronization chain, according to one embodiment. The techniques described herein may be implemented by one or more components of the positioning system 100. The positioning system 100 can include a UE 105, one or more satellites 110 (also referred to as space vehicles (SVs)), which may include Global Navigation Satellite System (GNSS) satellites (e.g., Global Positioning System (GPS), GLONASS, Galileo, Beidou, etc.) and / or Non-Terrestrial Network (NTN) satellites, base stations 120, access points (APs) 130, a location server 160, a network 170, and external clients 180. In general, the positioning system 100 can estimate the location of the UE 105 based on RF signals received by and / or transmitted from the UE 105, as well as known locations of other components (e.g., GNSS satellites 110, base stations 120, APs 130) that transmit and / or receive the RF signals. Further details regarding specific location estimation techniques are discussed in more detail with respect to FIG. 2.

[0014] It should be noted that FIG. 1 provides only a generalized illustration of the various components, and that any or all of the components may be utilized as appropriate, and that each of the components may be replicated as needed. Specifically, while only one UE 105 is shown, it will be understood that many UEs (e.g., hundreds, thousands, millions, etc.) may utilize the positioning system 100. Similarly, the positioning system 100 may include more or fewer base stations 120 and / or APs 130 than those shown in FIG. 1. The illustrated connections connecting the various components in the positioning system 100 include data and signaling connections, which may include additional (intermediate) components, direct or indirect physical and / or wireless connections, and / or additional networks. Furthermore, components may be rearranged, combined, separated, substituted, and / or omitted depending on the desired functionality. In some embodiments, for example, the external client 180 may be directly connected to the location server 160. Those skilled in the art will recognize many modifications to the illustrated components.

[0015] Depending on the desired functionality, network 170 may include any of a variety of wireless and / or wired networks. Network 170 may include any combination of, for example, public and / or private networks, local area networks and / or wide area networks, etc. Furthermore, network 170 may utilize one or more wired and / or wireless communication technologies. In some embodiments, network 170 may include, for example, a cellular or other mobile network, a wireless local area network (WLAN), a wireless wide-area network (WWAN), and / or the Internet. Examples of network 170 include a Long Term Evolution (LTE) wireless network, a fifth-generation (5G) wireless network (also referred to as a New Radio (NR) wireless network or a 5G NR wireless network), a Wi-Fi WLAN, and the Internet. LTE, 5G, and NR are wireless technologies defined or being defined by the 3rd Generation Partnership Project (3GPP®). Network 170 may also include more than one network and / or more than one type of network.

[0016] Base stations 120 and access points (APs) 130 may be communicatively coupled to network 170. In some embodiments, base stations 120s may be owned, maintained, and / or operated by a cellular network provider and may utilize any of a variety of wireless technologies, as described herein below. Depending on the technology of network 170, base stations 120 may include a node B, an evolved node B (eNodeB or eNB), a base transceiver station (BTS), a radio base station (RBS), an NR node B (gNB), a next generation eNB (ng-eNB), etc. A base station 120 that is a gNB or ng-eNB may be part of a next generation radio access network (NG-RAN), which may connect to a 5G core network (5G Core Network, 5GC) if network 170 is a 5G network. The functionality performed by base station 120 in previous generation networks (e.g., 3G and 4G) may be separated into different functional components (e.g., radio units (RUs), distributed units (DUs), and central units (CUs)) and layers (e.g., L1 / L2 / L3) to account for Open Radio Access Networks (O-RAN) and / or Virtualized Radio Access Networks (V-RAN or vRAN) in 5G or later networks, which may be executed on different devices in different locations connected, for example, via fronthaul, midhaul, and backhaul connections. As referred to herein, a "base station" (or ng-eNB, gNB, etc.) may include any or all of these functional components.The AP 130 may include, for example, a Wi-Fi AP or a Bluetooth AP or an AP with cellular capabilities (e.g., 4G LTE and / or 5G NR). Thus, the UE 105 can send and receive information to and from network connectivity devices, such as the location server 160, by accessing the network 170 via the base station 120 using the first communication link 133. Additionally or alternatively, the AP 130 may also be communicatively coupled to the network 170 such that the UE 105 can communicate with network connectivity devices, including the location server 160, and internet connectivity devices using the second communication link 135 or via one or more other mobile devices 145.

[0017] As used herein, the term "base station" may generally refer to a single physical transmission point or multiple co-located physical transmission points, which may be located at a base station 120. A Transmission Reception Point (TRP) (also known as a transmit / receive point) corresponds to this type of transmission point, and the term "TRP" may be used interchangeably with the terms "gNB," "ng-eNB," and "base station" herein. In some cases, a base station 120 may include multiple TRPs, e.g., each TRP associated with a different antenna or different antenna array for the base station 120. As used herein, the transmit functionality of a TRP may be performed using a transmission point (TP), and / or the receive functionality of a TRP may be performed by a reception point (RP), which may be physically separate or distinct from the TP. However, a TRP may include both a TP and an RP. A physical transmission point may include an array of antennas at a base station 120 (e.g., as in a Multiple Input-Multiple Output (MIMO) system and / or if the base station utilizes beamforming). The term "base station" may additionally refer to multiple non-co-located physical transmission points, which may be a Distributed Antenna System (DAS) (a network of spatially separated antennas connected to a common source via a transport medium) or a Remote Radio Head (RRH) (a remote base station connected to a serving base station).

[0018] As used herein, the term “cell” may generally refer to a logical communication entity used for communication with base station 120 and may be associated with an identifier (e.g., a Physical Cell Identifier (PCID), a Virtual Cell Identifier (VCID)) that distinguishes neighboring cells operating over the same or different carriers. In some examples, a carrier may support multiple cells, and different cells may be configured according to different protocol types (e.g., Machine-Type Communication (MTC), Narrowband Internet-of-Things (NB-IoT), Enhanced Mobile Broadband (eMBB), etc.) that may provide access to different types of devices. In some cases, the term “cell” may refer to a portion (e.g., a sector) of a geographic coverage area in which the logical entity operates.

[0019] The satellites 110 may be utilized for positioning the UE 105 in one or more ways. For example, the satellites 110 (also referred to as space vehicles (SVs)) may be part of a global navigation satellite system (GNSS), such as the Global Positioning System (GPS), GLONASS, Galileo, or Beidou. Positioning using RF signals from GNSS satellites may include measuring multiple GNSS signals at a GNSS receiver of the UE 105 to perform code-based and / or carrier-based positioning, which may be highly accurate. Additionally or alternatively, the satellites 110 may be utilized for NTN-based positioning, in which the satellites 110 may functionally operate as TRPs (or TPs) of a network (e.g., an LTE and / or NR network) and may be communicatively coupled to the network 170. In particular, the reference signals (e.g., PRS) transmitted by the satellite 110 NTN-based positioning may be similar to the reference signals transmitted by the base station 120 and may be coordinated by the location server 160. In some embodiments, the satellites 110 used for NTN-based positioning may be different from the satellites used for GNSS-based positioning. In some embodiments, NTN nodes may include non-terrestrial vehicles, such as airplanes, balloons, drones, etc., in addition to or in place of NTN satellites.

[0020] Location server 160 may comprise a server and / or other computing device configured to determine an estimated location of UE 105 and / or provide data (e.g., “assistance data”) to UE 105 to facilitate location measurement and / or location determination by UE 105. According to some embodiments, location server 160 may comprise a Home Secure User Plane Location (SUPL) Location Platform (H-SLP), which may support the SUPL User Plane (UP) location solution defined by the Open Mobile Alliance (OMA) and may support location services for UE 105 based on subscription information for UE 105 stored in location server 160. In some embodiments, location server 160 may include a Discovered SLP (D-SLP) or an Emergency SLP (E-SLP). The location server 160 may also comprise an Enhanced Serving Mobile Location Center (E-SMLC) that supports the location of the UE 105 using a control plane (CP) location solution for LTE radio access by the UE 105. The location server 160 may further comprise a Location Management Function (LMF) that supports the location of the UE 105 using a control plane (CP) location solution for NR or LTE radio access by the UE 105.

[0021] In a CP location solution, signaling for controlling and managing the location of the UE 105 may be exchanged between elements of the network 170 and the UE 105 as signaling from the perspective of the network 170 using existing network interfaces and protocols. In a UP location solution, signaling for controlling and managing the location of the UE 105 may be exchanged between the location server 160 and the UE 105 as data from the perspective of the network 170 (e.g., data transported using Internet Protocol (IP) and / or Transmission Control Protocol (TCP)).

[0022] As mentioned previously (and described in more detail below), the estimated location of the UE 105 may be based on measurements of RF signals sent from and / or received by the UE 105. In particular, these measurements may provide information regarding the relative distance and / or angle of the UE 105 from one or more components in the positioning system 100 (e.g., GNSS satellites 110, AP 130, base station 120). The estimated location of the UE 105 may be estimated geometrically (e.g., using multi-angulation and / or multilateration) based on the distance and / or angle measurements along with the known positions of the one or more components.

[0023] Terrestrial components such as APs 130 and base stations 120 may be fixed, although embodiments are not so limited. Mobile components may also be used. For example, in some embodiments, the location of the UE 105 may be estimated based at least in part on measurements of RF signals 140 communicated between the UE 105 and one or more other mobile devices 145, which may be mobile or fixed. As shown, the other mobile devices may include, for example, a mobile phone 145-1, a vehicle 145-2, a static communication / positioning device 145-3, or other static and / or mobile devices capable of providing wireless signals used to position the UE 105, or a combination thereof. Wireless signals from the mobile devices 145 used for positioning the UE 105 may include, for example, RF signals using Bluetooth (including Bluetooth Low Energy (BLE)), IEEE 802.11x (e.g., Wi-Fi), Ultra-Wideband (UWB), IEEE 802.15x, or a combination thereof. The mobile device 145 may additionally or alternatively use non-RF wireless signals for positioning of the UE 105, such as infrared signals or other optical techniques.

[0024] The mobile device 145 may include other UEs communicatively coupled to a cellular or other mobile network (e.g., network 170). When one or more other mobile devices 145, including a UE, are used to determine the location of a particular UE 105, the UE 105 whose location is being determined may be referred to as a “target UE,” and each of the other mobile devices 145 used may be referred to as an “anchor UE.” For purposes of determining the location of the target UE, the location of each of the one or more anchor UEs may be known and / or determined together with the target UE. Direct communication between the one or more other mobile devices 145 and the UE 105 may include sidelink and / or similar device-to-device (D2D) communication technologies. Sidelink, defined by 3GPP, is a form of D2D communication under the cellular-based LTE and NR standards. UWB may be one such technology in which positioning of a target device (e.g., UE 105) may be facilitated using measurements from one or more anchor devices (e.g., mobile devices 145).

[0025] According to some embodiments, one form of D2D communication used by the mobile device 105, such as when the UE 105 includes and / or is integrated into a vehicle, may include vehicle-to-everything (V2X) communication. V2X is a communication standard for vehicles and associated entities to exchange information about the traffic environment. V2X can include vehicle-to-vehicle (V2V) communication between V2X-enabled vehicles, vehicle-to-infrastructure (V2I) communication between vehicles and infrastructure-based devices (commonly referred to as roadside units (RSUs)), vehicle-to-person (V2P) communication between vehicles and nearby people (pedestrians, cyclists, and other road users), etc. Furthermore, V2X can use any of a variety of wireless RF communication technologies. Cellular V2X (CV2X) is a form of V2X that uses cellular-based communications, such as LTE (4G), NR (5G), and / or other cellular technologies in a direct communication mode defined by 3GPP. The UE 105 shown in FIG. 1 may correspond to a vehicle, an RSU, or a component or device on another V2X entity used to communicate V2X messages. Thus, in embodiments in which V2X is used, the static communication / positioning device 145-3 (which may correspond to an RSU) and / or the vehicle 145-2 may communicate with the UE 105 and be used to determine the location of the UE 105 using techniques similar to those used by the base station 120 and / or the AP 130 (e.g., using multi-angulation and / or multilateration). It should be further noted that the mobile device 145 (which may include a V2X device), the base station 120, and / or the AP 130 may be used together (e.g., in a WWAN positioning solution) to determine the location of the UE 105, according to some embodiments.

[0026] The estimated location of the UE 105 can be used for various purposes, such as to assist in direction finding or navigation for a user of the UE 105 or to assist another user (e.g., associated with the external client 180) in determining the location of the UE 105. “Location” may also be referred to herein as a “location estimate,” “estimated location,” “location,” “position,” “position estimate,” “position fix,” “estimated position,” “location fix,” or “fix.” The process of determining a location may be referred to as a “positioning,” “position determination,” “location determination,” or the like. The location of the UE 105 may comprise the absolute location of the UE 105 (e.g., latitude and longitude and possibly altitude) or the relative location of the UE 105 (e.g., location expressed as a distance north-south, east-west, and possibly up-down from some other known fixed location (e.g., including the location of a base station 120 or AP 130), or from some other location, such as the location of the UE 105 at some known prior time, or the location of a mobile device 145 (e.g., another UE) at some known prior time). A location may be specified as a geodetic location, including coordinates that may be absolute (e.g., latitude, longitude, and optionally altitude), relative (e.g., relative to some known absolute location), or local (e.g., X, Y, and optionally Z coordinates according to a coordinate system defined for a local area such as a factory, warehouse, university campus, shopping mall, sports stadium, or convention center). The location may alternatively be a civic location, and may then comprise one or more of a street address (e.g., including the name and sign of a country, state, county, city, road and / or street, and / or road or street number), and / or a sign or name of a point, building, part of a building, floor of a building, and / or room within a building, etc.The location may also include an indication of uncertainty or error, such as the horizontal and possibly vertical distance within which the location error is expected to lie, or an indication of an area or volume (e.g., a circle or ellipse) within which the UE 105 is expected to be located with some level of confidence (e.g., 95% confidence).

[0027] The external client 180 may be a web server or remote application that may have some association with the UE 105 (e.g., that may be accessed by a user of the UE 105), or may be a server, application, or computer system that provides location services to some other user, which may include obtaining and providing the location of the UE 105 (e.g., to enable services such as finding friends or relatives or locating children or pets). Additionally or alternatively, the external client 180 may obtain and provide the location of the UE 105 to emergency service providers, government agencies, etc.

[0028] As previously mentioned, the exemplary positioning system 100 may be implemented using a wireless communication network, such as an LTE-based network or a 5G NR-based network. FIG. 2 shows a diagram of a 5G NR positioning system 200 illustrating an embodiment of a positioning system (e.g., positioning system 100) that implements 5G NR. The 5G NR positioning system 200 may be configured to determine the location of the UE 105 by using access nodes, which may include NR NodeBs (gNBs) 210-1 and 210-2 (collectively and inclusively referred to herein as gNBs 210), an ng-eNB 214, and / or a WLAN 216, to implement one or more positioning methods. The gNBs 210 and / or ng-eNBs 214 may correspond to the base stations 120 of FIG. 1, and the WLANs 216 may correspond to one or more access points 130 of FIG. 1. Optionally, the 5G NR positioning system 200 may additionally be configured to determine the location of the UE 105 by using the LMF 220 (which may correspond to the location server 160) to implement one or more positioning methods. Here, the 5G NR positioning system 200 comprises the UE 105 and components of a 5G NR network, including a Next Generation (NG) Radio Access Network (RAN) (NG-RAN) 235 and a 5G Core Network (5G CN) 240. The 5G network may also be referred to as an NR network. The NG-RAN 235 may also be referred to as a 5G RAN or an NR RAN, and the 5G CN 240 may also be referred to as an NG Core Network.

[0029] The 5G NR positioning system 200 can further utilize information from satellites 110. As previously indicated, the satellites 110 can include GNSS satellites of a GNSS system such as the Global Positioning System (GPS) or similar systems (e.g., GLONASS, Galileo, Beidou, Indian Regional Navigational Satellite System (IRNSS)). Additionally or alternatively, the satellites 110 can comprise NTN satellites that can be communicatively coupled to the LMF 220 and operatively function as a TRP (or TP) within the NG-RAN 235. Thus, the satellites 110 can communicate with one or more gNBs 210.

[0030] 2 provides only a generalized illustration of the various components, and any or all of the components may be utilized as appropriate, and each of the components may be duplicated or omitted as needed. Specifically, while only one UE 105 is shown, it will be understood that many UEs (e.g., hundreds, thousands, millions, etc.) may utilize the 5G NR positioning system 200. Similarly, the 5G NR positioning system 200 may include a greater (or lesser) number of satellites 110, gNBs 210, ng-eNBs 214, wireless local area networks (WLANs) 216, access and mobility management functions (AMFs) 215, external clients 230, and / or other components. The illustrated connections connecting the various components in the 5G NR positioning system 200 include data and signaling connections, which may include additional (intermediate) components, direct or indirect physical and / or wireless connections, and / or additional networks. Additionally, components may be rearranged, combined, separated, substituted, and / or omitted depending on the functionality desired.

[0031] The UE 105 may include and / or be referred to as a device, a mobile device, a wireless device, a mobile terminal, a terminal, a mobile station (MS), a Secure User Plane Location (SUPL)-enabled terminal (SET), or some other name. Moreover, the UE 105 may correspond to a mobile phone, a smartphone, a laptop, a tablet, a personal digital assistant (PDA), a navigation device, an Internet of Things (IoT) device, or some other portable or mobile device. Typically, but not necessarily, the UE 105 may support wireless communications using one or more radio access technologies (RATs), such as using GSM, CDMA, W-CDMA, LTE, High Rate Packet Data (HRPD), IEEE 802.11 Wi-Fi®, Bluetooth, Worldwide Interoperability for Microwave Access (WiMAX™), 5G NR (e.g., using NG-RAN 235 and 5G CN 240), etc. The UE 105 may also support wireless communications using a WLAN 216 (such as one or more RATs as previously described with respect to FIG. 1), which may connect to other networks, such as the Internet. Use of one or more of these RATs may enable the UE 105 to communicate with and / or receive location information regarding the UE 105 (e.g., via elements of the 5G CN 240 not shown in FIG. 2 or possibly via a Gateway Mobile Location Center (GMLC) 225) external clients 230. The external clients 230 of FIG. 2 may correspond to the external clients 180 of FIG. 1 implemented in or communicatively coupled to a 5G NR network.

[0032] The UE 105 may comprise a single entity or multiple entities, such as in a personal area network where the user may utilize audio, video, and / or data I / O devices and / or body sensors and a separate wired or wireless modem. An estimate of the UE 105's location may be referred to as a location, location estimate, location fix, fix, position, position estimate, or position fix, and may be geodetic, thus providing the UE 105's location coordinates (e.g., latitude and longitude), which may or may not include an altitude component (e.g., elevation, height or depth above ground, floor, or basement). Alternatively, the UE 105's location may be expressed as a civic location (e.g., as a postal address or designation of some point or small area within a building, such as a particular room or floor). The UE 105's location may also be expressed as an area or volume (defined either geodetically or in civic form) within which the UE 105 is expected to be located with some probability or confidence (e.g., 67%, 95%, etc.). The location of the UE 105 may also be a relative location comprising distance and direction or relative X, Y (and Z) coordinates defined with respect to some origin at a known location, which may be defined, for example, geodetically, civic-wise, or with reference to a point, area, or volume shown on a map, floor plan, or building plan. In the description contained herein, use of the term location may include any of these variations unless otherwise indicated. When calculating the location of a UE, it is common to determine the values ​​of the local X, Y, and possibly Z coordinates and then, if necessary, convert the local coordinates to absolute coordinates (e.g., for latitude, longitude, and altitude above or below mean sea level).

[0033] The base stations in the NG-RAN 235 shown in FIG. 2 may correspond to the base stations 120 in FIG. 1 and may include gNBs 210. Pairs of gNBs 210 in the NG-RAN 235 may be connected to each other (e.g., directly as shown in FIG. 2 or indirectly via other gNBs 210). The communication interface between the base stations (gNBs 210 and / or ng-eNBs 214) may be referred to as the Xn interface 237. Access to the 5G network is provided to the UE 105 via wireless communication between the UE 105 and one or more of the gNBs 210, which may provide wireless communication access to the 5G CN 240 on behalf of the UE 105 using 5G NR. The wireless interface between the base stations (gNBs 210 and / or ng-eNBs 214) and the UE 105 may be referred to as the Uu interface 239. 5G NR radio access may also be referred to as NR radio access or 5G radio access. In Figure 2, the serving gNB for UE 105 is assumed to be gNB 210-1, but other gNBs (e.g., gNB 210-2) can act as serving gNBs when UE 105 moves to another location, or can act as secondary gNBs to provide additional throughput and bandwidth to UE 105.

[0034] The base stations in the NG-RAN 235 shown in FIG. 2 may also or instead include next-generation evolved node Bs, also referred to as ng-eNBs 214. The ng-eNBs 214 may be connected to one or more gNBs 210 in the NG-RAN 235, e.g., directly or indirectly via other gNBs 210 and / or other ng-eNBs. The ng-eNBs 214 may provide LTE and / or evolved LTE (eLTE) wireless access to the UE 105. Some gNBs 210 (e.g., gNB 210-2) and / or ng-eNBs 214 in FIG. 2 may be configured to function as positioning-only beacons, which can transmit signals (e.g., positioning reference signals (PRS)) and / or broadcast assistance data to assist in positioning the UE 105, but cannot receive signals from the UE 105 or from other UEs. Some gNBs 210 (e.g., gNB 210-2 and / or another gNB not shown) and / or ng-eNB 214, which may be configured to function as detection-only nodes, may scan for signals including, for example, PRS data, assistance data, or other location data. Such detection-only nodes may not transmit signals or data to UEs, but may transmit signals or data (e.g., related to PRS, assistance data, or other location data) to other network entities (e.g., 5G CN 240, external client 230, or one or more components of the controller) that may receive and store or use the data for positioning of at least the UE 105. Note that while only one ng-eNB 214 is shown in FIG. 2, some embodiments may include multiple ng-eNBs 214. Base stations (e.g., gNBs 210 and / or ng-eNBs 214) may communicate directly with each other via the Xn communication interface. Additionally or alternatively, the base station may communicate directly or indirectly with other components of the 5G NR positioning system 200, such as the LMF 220 and the AMF 215.

[0035] The 5G NR positioning system 200 may also include one or more WLANs 216 that may connect to a Non-3GPP Interworking Function (N3IWF) 250 in the 5G CN 240 (e.g., in the case of an untrusted WLAN 216). For example, the WLAN 216 may support IEEE 802.11 Wi-Fi access for the UE 105 and may comprise one or more Wi-Fi APs (e.g., AP 130 of FIG. 1). Here, the N3IWF 250 may connect to other elements in the 5G CN 240, such as the AMF 215. In some embodiments, the WLAN 216 may support another RAT, such as Bluetooth. The N3IWF 250 may provide support for secure access by the UE 105 to other elements in the 5G CN 240 and / or may support interworking of one or more protocols used by the WLAN 216 and the UE 105 to one or more protocols used by other elements of the 5G CN 240, such as the AMF 215. For example, the N3IWF 250 may support IPSec tunnel establishment with the UE 105, termination of IKEv2 / IPSec protocols with the UE 105, termination of the N2 and N3 interfaces to the 5G CN 240 for the control plane and user plane, respectively, and relay of uplink (UL) and downlink (DL) control plane Non-Access Stratum (NAS) signaling between the UE 105 and the AMF 215 across the N1 interface. In some other embodiments, the WLAN 216 may connect directly to elements in the 5G CN 240 (e.g., the AMF 215, shown by the dashed line in FIG. 2 ) without going through the N3IWF 250. For example, a direct connection of WLAN 216 to 5GCN 240 may occur if WLAN 216 is a trusted WLAN to 5GCN 240 and may be enabled using a Trusted WLAN Interworking Function (TWIF) (not shown in FIG. 2 ), which may be an element within WLAN 216. Note that while only one WLAN 216 is shown in FIG. 2 , some embodiments may include multiple WLANs 216.

[0036] An access node may comprise any of a variety of network entities that facilitate communication between the UE 105 and the AMF 215. As mentioned, this may include the gNB 210, the ng-eNB 214, the WLAN 216, and / or other types of cellular base stations. However, an access node providing the functionality described herein may additionally or alternatively include entities that facilitate communication for any of a variety of RATs not shown in FIG. 2, which may include non-cellular technologies. Thus, the term “access node” as used in the embodiments described herein below may include, but is not necessarily limited to, the gNB 210, the ng-eNB 214, or the WLAN 216.

[0037] In some embodiments, an access node such as gNB210, ng-eNB214, and / or WLAN216 (alone or in combination with other components of 5G NR positioning system 200) may be configured to obtain location measurements of uplink (UL) signals received from UE105 and / or obtain downlink (DL) location measurements from UE105 obtained by UE105 for DL ​​signals received by UE105 from one or more access nodes in response to receiving a request for location information from LMF220. As mentioned, Figure 2 shows access nodes (gNB 210, ng-eNB 214, and WLAN 216) configured to communicate according to 5G NR, LTE, and Wi-Fi communication protocols, respectively, although access nodes configured to communicate according to other communication protocols may be used, such as, for example, a Node B using a Wideband Code Division Multiple Access (WCDMA) protocol for a Universal Mobile Telecommunications Service (UMTS) Terrestrial Radio Access Network (UTRAN), an eNB using an LTE protocol for an Evolved UTRAN (E-UTRAN), or Bluetooth Beacon using a Bluetooth protocol for a WLAN. For example, in a 4G Evolved Packet System (EPS) providing LTE wireless access to a UE 105, the RAN may comprise an Evolved Packet System (EPS), which may include base stations with eNBs supporting LTE wireless access. The core network for the EPS may comprise an Evolved Packet Core (EPC). In that case, the EPS may include the E-UTRAN plus the EPC, which in Figure 2 corresponds to the NG-RAN 235 and the EPC corresponds to the 5GCN 240. The methods and techniques described herein with respect to obtaining the civic location of the UE 105 may also be applicable to such other networks.

[0038] The gNB 210 and ng-eNB 214 may communicate with the AMF 215, which communicates with the LMF 220 for positioning functionality. The AMF 215 may support mobility of the UE 105, including cell changes and handovers of the UE 105 from an access node of a first RAT (e.g., the gNB 210, the ng-eNB 214, or the WLAN 216) to an access node of a second RAT. The AMF 215 may also be responsible for supporting signaling connections to the UE 105 and possibly data and voice bearers for the UE 105. The LMF 220 may support positioning of the UE 105 using CP location solutions when the UE 105 accesses the NG-RAN 235 or WLAN 216, and may include Assisted GNSS (A-GNSS), Observed Time Difference Of Arrival (OTDOA) (sometimes called Time Difference Of Arrival (TDOA) in NR), Frequency Difference Of Arrival (FDOA), Real Time Kinematic (RTK), Precise Point Positioning (PPP), Differential GNSS (DGNSS), Enhanced Cell ID (ECID), Angle of Arrival (AoA), and Angle of Launch (AoA). The LMF 220 may support positioning procedures and methods, including UE-assisted / UE-based and / or network-based procedures / methods, such as Area of ​​Departure (AoD), WLAN positioning, round trip delay (RTT), multi-cell RTT, and / or other positioning procedures and methods. The LMF 220 may also process location service requests for the UE 105, for example, received from the AMF 215 or from the GMLC 225. The LMF 220 may be connected to the AMF 215 and / or the GMLC 225.In some embodiments, a network such as the 5GCN 240 may additionally or alternatively implement other types of location support modules, such as an Evolved Serving Mobile Location Center (E-SMLC) or a SUPL Location Platform (SLP). It should be noted that in some embodiments, at least a portion of the positioning functionality (including determining the location of the UE 105) may be performed at the UE 105 (e.g., by measuring downlink PRS (DL-PRS) signals transmitted by wireless nodes such as the gNB 210, the ng-eNB 214, and / or the WLAN 216, and / or by using assistance data provided to the UE 105 by, for example, the LMF 220).

[0039] The Gateway Mobile Location Center (GMLC) 225 may support location determination requests for the UE 105 received from the external client 230 and may forward such location determination requests to the AMF 215 for forwarding by the AMF 215 to the LMF 220. A location response from the LMF 220 (e.g., including a location estimate for the UE 105) may likewise be returned to the GMLC 225 either directly or via the AMF 215, which may then return the location response (e.g., including the location estimate) to the external client 230.

[0040] A Network Exposure Function (NEF) 245 may be included in the 5GCN 240. The NEF 245 may support secure exposure of capabilities and events related to the 5GCN 240 and the UE 105 to the external client 230, in which case it may be referred to as an Access Function (AF) and may enable secure provision of information from the external client 230 to the 5GCN 240. The NEF 245 may be connected to the AMF 215 and / or to the GMLC 225 for the purposes of obtaining the location (e.g., civic location) of the UE 105 and providing that location to the external client 230.

[0041] As further shown in Figure 2, the LMF 220 may communicate with the gNB 210 and / or the ng-eNB 214 using the NR Positioning Protocol annex (NRPPa) as defined in 3GPP Technical Specification (TS) 38.455. NRPPa messages may be transferred between the gNB 210 and the LMF 220 and / or between the ng-eNB 214 and the LMF 220 via the AMF 215. As further shown in Figure 2, the LMF 220 and the UE 105 may communicate using the LTE Positioning Protocol (LPP) as defined in 3GPP TS 37.355. Here, LPP messages may be transferred between the UE 105 and the LMF 220 via the AMF 215 and the serving gNB 210-1 or serving ng-eNB 214 for the UE 105. For example, LPP messages may be transferred between the LMF 220 and the AMF 215 using messages for service-based operations (e.g., based on the Hypertext Transfer Protocol (HTTP)) and between the AMF 215 and the UE 105 using a 5G NAS protocol. The LPP protocol may be used to support positioning of the UE 105 using UE-assisted and / or UE-based positioning methods, such as A-GNSS, RTK, TDOA, multi-cell RTT, AoD, and / or ECID. The NRPPa protocol may be used to support positioning of the UE 105 using network-based positioning methods, such as ECID, AoA, uplink TDOA (UL-TDOA), and / or may be used by the LMF 220 to obtain location-related information from the gNB 210 and / or ng-eNB 214, such as parameters defining DL-PRS transmissions from the gNB 210 and / or ng-eNB 214.

[0042] In the case of UE 105 access to WLAN 216, LMF 220 may use NRPPa and / or LPP to obtain the location of UE 105 in a manner similar to that just described for UE 105 access to gNB 210 or ng-eNB 214. Accordingly, NRPPa messages may be forwarded between WLAN 216 and LMF 220 via AMF 215 and N3IWF 250 to support network-based positioning of UE 105 and / or forwarding of other location information from WLAN 216 to LMF 220. Alternatively, NRPPa messages may be forwarded between N3IWF 250 and LMF 220 via AMF 215 to support network-based positioning of UE 105 based on location-related information and / or location measurements known to or accessible to N3IWF 250 and forwarded from N3IWF 250 to LMF 220 using NRPPa. Similarly, LPP and / or LPP messages may be transferred between UE 105 and LMF 220 via AMF 215, N3IWF 250, and serving WLAN 216 for UE 105 to support UE-assisted or UE-based positioning of UE 105 by LMF 220.

[0043] In a 5G NR positioning system 200, positioning methods may be categorized as “UE-assisted” or “UE-based.” This may depend on where the request to determine the location of the UE 105 originates. For example, if the request originates at the UE (e.g., from an application or “app” executed by the UE), the positioning method may be categorized as UE-based. On the other hand, if the request originates from an external client 230, an LMF 220, or another device or service within the 5G network, the positioning method may be categorized as UE-assisted (or “network-based”).

[0044] Using UE-assisted positioning methods, the UE 105 can obtain location measurements and send the measurements to a location server (eg, the LMF 220) for calculation of a location estimate for the UE 105. Location measurements of the RAT-dependent positioning method may include one or more of the following for one or more access points for the gNB210, the ng-eNB214, and / or the WLAN216: Received Signal Strength Indicator (RSSI), Round Trip signal propagation Time (RTT), Reference Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), Reference Signal Time Difference (RSTD), Time of Arrival (TOA), AoA, Receive Time-Transmission Time Difference (Rx-Tx), Differential AoA (DAoA), AoD, or Timing Advance (TA). Additionally or alternatively, similar measurements may be made from sidelink signals transmitted by other UEs, which, if their locations are known, may serve as anchor points for positioning of the UE 105. Location measurements may also or instead include measurements of RAT-independent positioning methods such as GNSS (e.g., GNSS pseudoranges, GNSS code phase, and / or GNSS carrier phase for satellites 110), WLAN, etc.

[0045] In a UE-based positioning method, the UE 105 may obtain location measurements (which may, for example, be the same as or similar to location measurements for a UE-assisted positioning method) and may further calculate the location of the UE 105 (e.g., with the help of assistance data received from a location server such as the LMF 220, SLP, or broadcast by the gNB 210, ng-eNB 214, or WLAN 216).

[0046] In a network-based positioning method, one or more base stations (e.g., gNB210 and / or ng-eNB214), one or more APs (e.g., in WLAN216), or N3IWF250 may obtain location measurements (e.g., RSSI, RTT, RSRP, RSRQ, AoA, or TOA measurements) for signals transmitted by UE105, and / or, in the case of N3IWF250, may receive measurements obtained by UE105 or APs in WLAN216 and send the measurements to a location server (e.g., LMF220) for calculation of a location estimate for UE105.

[0047] Positioning of the UE 105 may also be classified as UL-based, DL-based, or DL-UL-based depending on the type of signal used for positioning. For example, if the positioning is based solely on signals received at the UE 105 (e.g., from a base station or other UEs), the positioning may be classified as DL-based. Conversely, if the positioning is based solely on signals transmitted by the UE 105 (e.g., which may be received by a base station or other UEs), the positioning may be classified as UL-based. DL-UL-based positioning includes positioning based on signals both transmitted and received by the UE 105, such as RTT-based positioning. Sidelink (SL)-assisted positioning includes signals communicated between the UE 105 and one or more other UEs. According to some embodiments, the UL, DL, or DL-UL positioning described herein may be capable of using SL signaling as a supplement or replacement for SL, DL, or DL-UL signaling.

[0048] Depending on the type of positioning (e.g., UL-based, DL-based, or DL-UL-based), the type of reference signal used may vary. For example, for DL-based positioning, these signals may include PRS (e.g., DL-PRS transmitted by the base station or SL-PRS transmitted by other UEs) that may be used for measuring TDOA, AoD, and RTT. Other reference signals that may be used for positioning (UL, DL, or DL-UL) may include a Sounding Reference Signal (SRS), a Channel State Information Reference Signal (CSI-RS), a synchronization signal (e.g., a synchronization signal block (SSB), a Synchronization Signal (SS)), a Physical Uplink Control Channel (PUCCH), a Physical Uplink Shared Channel (PUSCH), a Physical Sidelink Shared Channel (PSSCH), a Demodulation Reference Signal (DMRS), etc. Furthermore, the reference signal may be transmitted in a Tx beam and / or received in an Rx beam (e.g., using beamforming techniques), which may affect angle measurements such as AoD and / or AoA.

[0049] FIG. 3 illustrates an example of a frame structure and associated terminology for NR, which can serve as the basis for physical layer communication between a UE 105 and a base station / TRP. The transmission timeline for each of the downlink and uplink may be partitioned into radio frames. Each radio frame may have a predetermined time length (e.g., 10 ms) and may be partitioned into 10 subframes, each 1 ms long and indexed 0 through 9. Each subframe may include a variable number of slots depending on the subcarrier spacing. Each slot may include a variable number of symbol periods (e.g., 7 or 14 symbols) depending on the subcarrier spacing. The symbol periods within each slot may be assigned an index. A minislot may include a subslot structure (e.g., 2, 3, or 4 symbols). Additionally, a complete Orthogonal Frequency-Division Multiplexing (OFDM) of a subframe is illustrated in FIG. 3, showing how a subframe can be partitioned into multiple resource blocks (RBs) across both time and frequency. A single RB can contain a grid of resource elements (REs) spanning 14 symbols and 12 subcarriers.

[0050] Each symbol in a slot may indicate a link direction (e.g., downlink (DL), uplink (UL), or flexible) or data transmission, and the link direction for each subframe may be dynamically switched. The link direction may be based on the slot format. Each slot may contain DL / UL data and DL / UL control information. In NR, synchronization signal blocks (SSBs) are transmitted. The SSBs include a primary synchronization signal (PSS), a secondary synchronization signal (SSS), and a two-symbol physical broadcast channel (PBCH). The SSBs may be transmitted within fixed slot locations, such as symbols 0 to 3 as shown in Figure 3. The PSS and SSS may be used by the UE for cell search and acquisition. The PSS may provide half-frame timing, and the SS may provide cyclic prefix (CP) length and frame timing. The PSS and SSS may provide cell identity information. The PBCH carries some basic system information, such as downlink system bandwidth, timing information within a radio frame, SS burst set period, and system frame number.

[0051] 4 illustrates an exemplary wireless communication system 400. The wireless communication system 400 consists of multiple roadside units (RSUs) 404-1, 404-2, 404-3, and 404-4 disposed within an enclosed space 401. A road 402 extends along the bottom of the enclosed space 401, allowing vehicles to pass through the enclosed space 401. According to aspects of the present disclosure, the RSUs 404-1, 404-2, 404-3, and 404-4 may be capable of communicating with on-board units (OBUs) of vehicles passing through the enclosed space 401 on the road 402. In some implementations, the RSUs 404-1, 404-2, 404-3, and 404-4 can communicate with the OBUs of passing vehicles along with providing wireless data connectivity to such vehicles.

[0052] According to aspects of the present disclosure, RSUs 404-1, 404-2, 404-3, and 404-4 may be vehicle-to-everything (V2X) RSUs configured to communicate with V2X OBUs of passing vehicles in accordance with one or more V2X communication standards and / or protocols. For example, in some implementations, RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to communicate in accordance with V2X communication standards and / or protocols defined in one or more Technical Standards (TSs) and / or Technical Reports (TRs) published by the Third Generation Partnership Project (3GPP) and / or the European Telecommunications Standards Institute (ETSI). According to aspects of the disclosure, one or more of the RSUs 404-1, 404-2, 404-3, and 404-4 may be implemented as UEs, any or all of which may be the same as or similar to the UE 105 of Figure 1. In some implementations, one or more of the RSUs 404-1, 404-2, 404-3, and 404-4 may be implemented as base stations, any or all of which may be the same as or similar to the base station 120 of Figure 1 or the gNB 210 of Figure 2.

[0053] According to aspects of the present disclosure, the enclosed space 401 may be a tunnel, such as a vehicular tunnel, that provides a passage for the road 402 underground or through an obstacle (e.g., a mountain or a building). In some implementations, as shown in FIG. 4 , the RSUs 404-1, 404-2, 404-3, and 404-4 may be suspended from an upper surface (e.g., a ceiling) of the enclosed space 401. The RSU 404-1 is located closest to the open end 403 of the enclosed space 401, and the RSUs 404-2, 404-3, and 404-4 are located successively farther away from the open end 403. In some implementations, the distance between consecutive RSUs among the RSUs 404-1, 404-2, 404-3, and 404-4 may be in a range of approximately 100 to 300 meters. As reflected by the dashed arrows in FIG. 4, the enclosed space 401 may extend further than shown in FIG. 4, and the plurality of RSUs in the wireless communication system 400 may include additional RSUs disposed along portions of the enclosed space 401 that are not shown in FIG. 4.

[0054] According to aspects of the present disclosure, to support wireless communication with OBUs of vehicles passing through the enclosed space 401, RSUs 404-1, 404-2, 404-3, and 404-4 may perform synchronization operations to establish synchronization between their timing and between their timing and the timing of the OBUs of the passing vehicles. In some implementations, RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to synchronize their timing with timing indicated by a received synchronization signal and transmit a synchronization signal indicative of their timing. According to aspects of the present disclosure, each of RSUs 404-1, 404-2, 404-3, and 404-4 may select a synchronization reference source and synchronize its timing with the timing indicated by the received synchronization signal corresponding to that synchronization reference source. According to aspects of the present disclosure, possible synchronization references that a given one of RSUs 404-1, 404-2, 404-3, and 404-4 may be able to use as a synchronization reference source may include Global Navigation Satellite Systems (GNSS) as well as other RSUs. In this specification, the term SyncRef RSU is used to refer to an RSU that is used by another RSU as a synchronization reference source.

[0055] According to aspects of the present disclosure, the RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to follow a priority scheme when selecting a synchronization reference that they use as a synchronization reference source. In some implementations, the priority scheme may give a GNSS reference a higher priority than an RSU reference, such that if a GNSS reference is available to the RSU, the RSU selects the GNSS reference over any RSU reference that may be available to the RSU for use as a synchronization reference source. Among the RSU references, RSU references corresponding to RSUs that use the GNSS reference as a synchronization reference source may be given a higher priority than RSU references corresponding to RSUs that use other RSU references as synchronization reference sources.

[0056] According to aspects of the present disclosure, RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to use an alternate synchronization reference as their synchronization reference source when their currently used reference becomes unavailable. In some cases, this may include using a reference of lower priority than the priority of the lost reference. For example, if a given one of RSUs 404-1, 404-2, 404-3, and 404-4 is using a GNSS reference as its synchronization reference source and the GNSS reference becomes unavailable, the RSU may adopt an RSU reference corresponding to another RSU as its synchronization reference source.

[0057] 5 illustrates an example operating environment 500. In the operating environment 500, the RSUs 404-1, 404-2, 404-3, and 404-4 implement a synchronization chain. The synchronization chain implementation may enable some RSUs in the communication system 400 of FIG. 4 to synchronize their timing with the timing of a GNSS reference that is not directly available to them by propagating GNSS-reference timing from RSUs for which the GNSS reference is available.

[0058] 5, RSU 404-1 is located near the open end 403 of the enclosed space 401 of FIG. 4 and, thereby, can receive a reference signal 507 transmitted over a wireless carrier by a GNSS device 506 (which may be, for example, a satellite transmitter). Thus, a GNSS reference is available for use by RSU 404-1 as a synchronization reference source, and RSU 404-1 can synchronize with that GNSS reference according to the timing indicated by the reference signal 507. RSUs 404-2, 404-3, and 404-4 are located far from the open end 403 of the enclosed space 401 and may not be able to receive reference signal 507 or any other GNSS reference signal. Thus, there may not be a GNSS reference available for use as a synchronization reference source by RSUs 404-2, 404-3, or 404-4.

[0059] To enable other RSUs to synchronize with the GNSS timing indicated by the reference signal 507, the RSU 404-1 may transmit sidelink synchronization signals (SLSSs) 508-1 over a wireless carrier. The RSU 404-2, which is the closest RSU to the RSU 404-1, may be able to receive the SLSS 508-1 and thus may be able to use the RSU 404-1 as a synchronization reference source. The RSU 404-2 may synchronize with the RSU 404-1, and therefore with the GNSS reference corresponding to the reference signal 507, according to the timing indicated by the SLSS 508-1.

[0060] Similarly, RSU 404-2 may transmit SLSS 508-2 over a wireless carrier, and RSU 404-3 may receive SLSS 508-2 and use RSU 404-2 as a synchronization reference source by synchronizing with the timing indicated by SLSS 508-2. RSU 404-3 may then transmit SLSS 508-3 over a wireless carrier, and RSU 404-4 may receive SLSS 508-3 and use RSU 404-3 as a synchronization reference source by synchronizing with the timing indicated by SLSS 508-3. RSU 404-4 may transmit SLSS 508-4 to enable the next RSU in the synchronization chain (not shown) to synchronize with the preceding RSU and therefore with the GNSS source.

[0061] According to aspects of the present disclosure, transmission of SLSSs 508-1, 508-2, 508-3, and 508-4 by RSUs 404-1, 404-2, 404-3, and 404-4, respectively, may include transmission of sidelink SSBs (S-SSBs), similar to the SSBs described above with reference to the NR frame structure of Figure 3. Similar to the SSBs of Figure 3, which include a PSS, SSS, and a two-symbol PBCH, the S-SSB corresponding to SLSS 508-1, 508-2, 508-3, or 508-4 may consist of a sidelink PSS (S-PSS), a sidelink SSS (S-SSS), and a multi-symbol physical sidelink broadcast channel (PSBCH). The S-SSB may occupy an entire slot, and the S-PSS and S-SSS may each occupy two symbols within that slot.

[0062] It is worth noting that some RSUs in a synchronization chain in the operating environment 500 may be capable of receiving reference signals (such as GNSS reference signals and / or SLSSs) associated with multiple synchronization references. For example, the RSU 404-1 may be capable of receiving both the reference signal 507 transmitted by the GNSS device 506 and the SLSS 508-2 transmitted by the RSU 404-2. Similarly, the RSU 404-2 may be capable of receiving both the SLSS 508-1 transmitted by the RSU 404-1 and the SLSS 508-3 transmitted by the RSU 404-3, and the RSU 404-3 may be capable of receiving both the SLSS 508-2 transmitted by the RSU 404-2 and the SLSS 508-4 transmitted by the RSU 404-4.

[0063] According to aspects of the present disclosure, an RSU in operating environment 500 that can receive reference signals (e.g., GNSS reference signals and / or SLSS) associated with multiple synchronization references can choose among the multiple synchronization references according to a priority scheme such as that described above with reference to FIG. 4. For example, the priority scheme can give higher priority to GNSS references over RSU references, such that RSU 404-1 can use the GNSS reference as a synchronization reference source rather than using RSU 404-2 as a SyncRef RSU. In another example, the priority scheme can give higher priority to SyncRef RSUs that use the GNSS reference as a synchronization reference source rather than a SyncRef RSU that uses the SyncRef RSU as a synchronization reference source, such that RSU 404-2 can use RSU 404-1 as a SyncRef RSU rather than using RSU 404-3 as a SyncRef RSU.

[0064] According to aspects of the present disclosure, an RSU in operating environment 500 may be configured to use an alternate synchronization reference as a synchronization reference source when the currently used reference becomes unavailable. If multiple alternate synchronization references are available for a given RSU, the RSU may select from among the multiple alternate sources based on factors that may include the priority of each of the multiple alternate sources according to a priority scheme. An RSU that has only one available alternate synchronization reference may adopt that alternate reference when the currently used reference becomes unavailable.

[0065] In some implementations, each of RSUs 404-1, 404-2, 404-3, and 404-4 may have one respective synchronization reference available for potential adoption in the event of losing the synchronization reference used in connection with propagating GNSS-based timing along the synchronization chain in operating environment 500. For example, RSU 404-2 may be available for use as a SyncRef RSU by RSU 404-1 as an alternative to the GNSS reference, RSU 404-3 may be available for use as a SyncRef RSU by RSU 404-2 as an alternative to RSU 404-1's use, and RSU 404-4 may be available for use as a SyncRef RSU by RSU 404-3 as an alternative to RSU 404-2's use. According to aspects of the present disclosure, RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to adopt an alternate synchronization reference as a synchronization reference source when a currently used synchronization reference is lost, monitor a wireless carrier for a reference signal (e.g., a GNSS reference signal and / or an SLSS) associated with the alternate synchronization reference, and transmit the SLSS on the wireless carrier according to the timing indicated by the reference signal.

[0066] In the operating environment 500, if the GNSS reference becomes unavailable to the RSU 404-1 (e.g., because the GNSS device 506 stops transmitting the reference signal 507 or because weather conditions or other obstacle(s) prevent the reference signal 507 from reaching the RSU 404-1), there may be a possibility that a timing deadlock may form among the RSUs 404-1, 404-2, 404-3, and 404-4. Figure 6 is a block diagram of an example event flow 600 illustrating a situation in which loss of the GNSS reference in the operating environment 500 of Figure 5 may lead to a timing deadlock.

[0067] When the event flow 600 starts, the synchronization references available to the RSU 404-1 may be the GNSS reference and the SyncRef RSU 404-2, the synchronization references available to the RSU 404-2 may be the SyncRef RSUs 404-1 and 404-3, and the synchronization references available to the RSU 404-3 may be the SyncRef RSUs 404-2 and 404-4. The RSU 404-1 may be using the GNSS reference as a synchronization reference source due to the higher priority of the GNSS reference relative to the priority of the SyncRef RSU 404-2. The RSU 404-2 may be using the SyncRef RSU 404-1 as a synchronization reference source due to the higher priority of the SyncRef RSU 404-1 relative to the priority of the SyncRef RSU 404-3. The RSU 404-3 may be using the SyncRef RSU 404-2 as a synchronization reference source due to the higher priority of the SyncRef RSU 404-2 relative to the priority of the SyncRef RSU 404-4.

[0068] According to the event flow 600, at block 605, a GNSS reference associated with a reference signal 507 transmitted by the GNSS device 506 may be lost. As described above, this loss of the GNSS reference may occur, for example, when the GNSS device 506 stops transmitting the reference signal 507 or when weather conditions or other obstacle(s) prevent the reference signal 507 from reaching the RSU 404-1. At block 610, the RSU 404-1 may detect that the GNSS reference is unavailable. In response to detecting that the GNSS reference is unavailable, the RSU 404-1 may stop transmitting the SLSS 508-1 at block 615.

[0069] At block 620, the RSU 404-2 may detect that the RSU 404-1 is unavailable as a SyncRef RSU (based on the absence of an SLSS 508-1 on the wireless carrier). In response to detecting that the RSU 404-1 is unavailable as a SyncRef RSU, at block 625, the RSU 404-2 may adopt the RSU 404-3 as a SyncRef RSU. Upon adopting the RSU 404-3 as the SyncRef RSU, at block 630, the RSU 404-2 may start transmitting SLSSs 508-2 according to the timing indicated by the SLSS 508-3 transmitted by the RSU 404-3. These SLSSs 508-2 may be the same as the SLSSs 508-4 transmitted by the RSU 404-4, which also uses the RSU 404-3 as a SyncRef RSU.

[0070] In block 635, RSU 404-3 may switch from using RSU 404-2 to using RSU 404-4 as its SyncRef RSU and may identify RSU 404-2 as an alternative, lower priority SyncRef RSU based on the SLSS 508-2 transmitted by RSU 404-2 starting in block 630. This switch may not cause a change in the SLSS 508-3 transmitted by RSU 404-3.

[0071] At block 640, RSU 404-1 may receive only the SLSS 508-2 transmitted by RSU 404-2 and may adopt RSU 404-2 as its SyncRef RSU. At block 645, RSU 404-1 may start transmitting SLSS 508-1 according to the timing indicated by SLSS 508-2. This timing may correspond to the timing indicated by SLSS 508-3, which may correspond to the timing indicated by SLSS 508-4. Thus, at block 650, a timing deadlock may exist between RSUs 404-1, 404-2, 404-3, and 404-4.

[0072] Disclosed herein are deadlock prevention techniques that can be implemented to prevent timing deadlock in an RSU synchronization chain, such as that of the operating environment 500 of FIG. 5 , when a GNSS reference is unavailable. According to such deadlock prevention techniques, RSUs of the RSU synchronization chain (e.g., RSUs 404-1, 404-2, 404-3, and 404-4) can be configured to initiate a waiting interval when they switch their synchronization reference source. In some implementations, an RSU can be configured to initiate a waiting interval when it switches from a higher-priority synchronization reference to a lower-priority synchronization reference. An RSU that initiates a waiting interval upon switching to a different synchronization reference can pause its SLSS transmission for the duration of the waiting interval. Following expiration of the waiting interval, the RSU can check for reference signals associated with the newly adopted synchronization reference. If reference signals associated with the newly adopted synchronization reference are found, the RSU can resume SLSS transmission (according to the timing indicated by those reference signals). If the reference signal associated with the newly adopted synchronization reference is not found, the RSU may continue to refrain from transmitting the SLSS.

[0073] 7 is a block diagram of an example event flow 700 that may represent an implementation of the disclosed deadlock prevention techniques in the operating environment 500 of FIG. 5 , according to aspects of the present disclosure. Prior to the start of the event flow 700, each of the RSUs 404-1, 404-2, 404-3, and 404-4 may be configured to initiate a wait interval upon switching synchronization references. In some implementations, implementation of such a wait interval may be limited to synchronization reference source changes that involve switching from a higher-priority reference to a lower-priority reference. In other implementations, such a wait interval may also be implemented in situations involving switching between synchronization sources of similar priority. In yet other implementations, such a wait interval may be implemented with all synchronization reference source changes, regardless of the priority of the newly adopted synchronization reference relative to the previously utilized synchronization reference.

[0074] When the event flow 700 starts, the synchronization references available to the RSU 404-1 may be the GNSS reference and the SyncRef RSU 404-2, the synchronization references available to the RSU 404-2 may be the SyncRef RSUs 404-1 and 404-3, and the synchronization references available to the RSU 404-3 may be the SyncRef RSUs 404-2 and 404-4. The RSU 404-1 may be using the GNSS reference as a synchronization reference source due to the higher priority of the GNSS reference relative to the priority of the SyncRef RSU 404-2. The RSU 404-2 may be using the SyncRef RSU 404-1 as a synchronization reference source due to the higher priority of the SyncRef RSU 404-1 relative to the priority of the SyncRef RSU 404-3. The RSU 404-3 may be using the SyncRef RSU 404-2 as a synchronization reference source due to the higher priority of the SyncRef RSU 404-2 relative to the priority of the SyncRef RSU 404-4.

[0075] According to the event flow 700, a GNSS reference associated with a reference signal 507 transmitted by the GNSS device 506 may be lost at block 705. As mentioned above, this loss of the GNSS reference may occur, for example, when the GNSS device 506 stops transmitting the reference signal 507 or when weather conditions or other obstacle(s) prevent the reference signal 507 from reaching the RSU 404-1. At block 710, the RSU 404-1 may detect that the GNSS reference is unavailable. In response to detecting that the GNSS reference is unavailable, the RSU 404-1 may stop transmitting the SLSS 508-1 at block 715.

[0076] At block 720, the RSU 404-2 may detect that the RSU 404-1 is unavailable as a SyncRef RSU (based on the absence of the SLSS 508-1 on the wireless carrier). In response to detecting that the RSU 404-1 is unavailable as a SyncRef RSU, at block 725, the RSU 404-2 may adopt the RSU 404-3 as the SyncRef RSU. Due to switching from using the RSU 404-1 to using the RSU 404-3 as the SyncRef RSU, at block 730, the RSU 404-2 may begin a waiting interval and may refrain from transmitting the SLSS 508-2 during the waiting interval.

[0077] At block 735, the RSU 404-3 may detect that the RSU 404-2 is unavailable as a SyncRef RSU (based on the absence of the SLSS 508-2 on the wireless carrier). In response to detecting that the RSU 404-2 is unavailable as a SyncRef RSU, the RSU 404-3 may adopt the RSU 404-4 as the SyncRef RSU at block 740. Due to switching from using the RSU 404-2 to using the RSU 404-4 as the SyncRef RSU, the RSU 404-3 may begin a waiting interval at block 745 and may refrain from transmitting the SLSS 508-3 during the waiting interval.

[0078] In block 750, the RSU 404-1 may receive only the SLSS 508-2 transmitted by the RSU 404-2 and may adopt the RSU 404-2 as the SyncRef RSU. Due to the RSU 404-1 switching from using the GNSS reference to using the RSU 404-2 as the SyncRef RSU, in block 755, the RSU 404-1 may begin a waiting interval and may refrain from transmitting the SLSS 508-1 during the waiting interval.

[0079] At block 760, RSU 404-4 may refrain from transmitting SLSS 508-4 on the wireless carrier due to the absence of SLSS 508-3 of RSU 404-3 on the wireless carrier. At block 765, RSU 404-3 may refrain from transmitting SLSS 508-3 on the wireless carrier due to the absence of SLSS 508-4 of RSU 404-4 on the wireless carrier. At block 770, RSU 404-2 may refrain from transmitting SLSS 508-2 on the wireless carrier due to the absence of SLSS 508-3 of RSU 404-3 on the wireless carrier. At block 775, RSU 404-1 may refrain from transmitting SLSS 508-1 on the wireless carrier due to the absence of SLSS 508-2 of RSU 404-2 on the wireless carrier. At block 780, RSUs 404-1, 404-2, 404-3, and 404-4 may all be silent (refrain from SLSS transmission), so that timing deadlock may be avoided.

[0080] Figure 8 is a flow diagram of a method 800 of wireless communication by a wireless communication device, according to one embodiment. Means for performing the functionality illustrated in one or more of the blocks illustrated in Figure 8 may be performed by hardware and / or software components of an RSU, such as any of RSUs 404-1, 404-2, 404-3, and 404-4 of Figures 4 and 5. In some implementations, such an RSU may be implemented as a UE or a base station (e.g., a gNB), example components of which are shown in Figures 9 and 10, respectively, and described in further detail below.

[0081] In block 810, the functionality includes detecting, by the wireless communication device, a first reference signal associated with the first synchronization reference over a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device. For example, in the operating environment 500 of FIG. 5, the RSU 404-2 may detect the SLSS 508-1 over the wireless carrier while the RSU 404-1 is the SyncRef RSU that the RSU 404-2 uses as its synchronization reference source. According to some implementations, the means for performing the functionality in block 810 may include a bus 905, a processor 910, a digital signal processor (DSP) 920, a wireless communication interface 930, a memory 960, and / or other components of the UE, as shown in FIG. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of the base station, as shown in FIG. 10.

[0082] In block 820, the functionality includes transmitting an SLSS on a wireless carrier according to the timing indicated by the first reference signal. For example, in the operating environment 500 of FIG. 5, the RSU 404-2 may transmit an SLSS 508-2 on a wireless carrier according to the timing indicated by the SLSS 508-1. According to some implementations, the means for performing the functionality in block 820 may include a bus 905, a processor 910, a DSP 920, a wireless communication interface 930, a memory 960, and / or other components of a UE, as shown in FIG. 9. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of a base station, as shown in FIG. 10.

[0083] In block 830, the functionality includes determining that the first synchronization reference has become unavailable. For example, in the operating environment 500 of FIG. 5, the RSU 404-2 may determine that the RSU 404-1 is no longer available as a SyncRef RSU (e.g., based on an inability to detect and / or receive the SLSS 508-1 over the wireless carrier). According to some implementations, the means for performing the functionality in block 830 may include a bus 905, a processor 910, a DSP 920, a wireless communication interface 930, a memory 960, and / or other components of the UE, as shown in FIG. 9. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of the base station, as shown in FIG. 10.

[0084] In block 840, the functionality includes employing a second synchronization reference as a synchronization reference source for the wireless communication device in response to determining in block 830 that the first synchronization reference has become unavailable. For example, in the operating environment 500 of FIG. 5, in response to determining that the RSU 404-1 is no longer available as a SyncRef RSU, the RSU 404-2 may employ the RSU 404-3 as the SyncRef RSU to serve as the synchronization reference source for the RSU 404-2. According to some implementations, the means for performing the functionality in block 840 may include a bus 905, a processor 910, a DSP 920, a wireless communication interface 930, a memory 960, and / or other components of the UE, as shown in FIG. 9. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of the base station, as shown in FIG. 10.

[0085] In block 850, the functionality includes initiating a waiting interval due to the switch to a different synchronization reference in block 840. For example, in the operating environment 500 of FIG. 5, the RSU 404-2 may initiate a waiting interval due to the switch from the RSU 404-1 to the RSU 404-3 as the SyncRef RSU that serves as the synchronization reference source for the RSU 404-2. According to some implementations, the means for performing the functionality in block 850 may include a bus 905, a processor 910, a DSP 920, a wireless communication interface 930, a memory 960, and / or other components of a UE, as shown in FIG. 9. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of a base station, as shown in FIG. 10.

[0086] In block 860, the functionality includes monitoring the wireless carrier for a second reference signal associated with a second synchronization reference following expiration of the waiting interval initiated in block 850, and refraining from transmitting an SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier. For example, in the operating environment 500 of FIG. 5 , following expiration of the waiting interval initiated due to a switch from RSU 404-1 to RSU 404-3 as the SyncRef RSU functioning as the synchronization reference source for RSU 404-2, RSU 404-2 may monitor the wireless carrier for an SLSS 508-3 associated with RSU 404-3, and may refrain from transmitting an SLSS 508-2 on the wireless carrier while the SLSS 508-3 is not detected on the wireless carrier. According to some implementations, the means for performing the functionality in block 860 may include a bus 905, a processor 910, a DSP 920, a wireless communication interface 930, a memory 960, and / or other components of a UE, as shown in Figure 9. According to some other implementations, the means for performing the functionality in block 810 may include a bus 1005, a processor 1010, a DSP 1020, a wireless communication interface 1030, a memory 1060, and / or other components of a base station, as shown in Figure 10.

[0087] FIG. 9 is a block diagram of an embodiment of a UE 105, which may be utilized as described herein above (e.g., to implement one of the RSUs 404-1, 404-2, 404-3, and 404-4 of FIGS. 4 and 5). For example, the UE 105 may perform one or more of the functions of the method illustrated in FIG. 8. Note that FIG. 9 is intended only to provide a generalized illustration of various components, any or all of which may be utilized as desired. Note that in some instances, the components illustrated in FIG. 9 may be localized in a single physical device and / or distributed among various networked devices that may be located in different physical locations. Furthermore, as previously mentioned, the UE functionality described in the foregoing embodiments may be performed by one or more of the hardware and / or software components illustrated in FIG. 9.

[0088] UE 105 is shown comprising hardware elements that may be electrically coupled (or in communication as needed) via bus 905. The hardware elements may include processor(s) 910, which may include, but are not limited to, one or more general-purpose processors (e.g., application processors), one or more special-purpose processors (such as digital signal processor (DSP) chips, graphics acceleration processors, application-specific integrated circuits (ASICs)), and / or other processing structures or means. Processor(s) 910 may comprise one or more processing units that may be housed in a single integrated circuit (IC) or multiple ICs. As shown in FIG. 9, some embodiments may have a separate DSP 920 depending on desired functionality. Location determination and / or other determinations based on wireless communication may be performed in processor(s) 910 and / or in wireless communication interface 930 (described below). The UE 105 may also include one or more input devices 970, which may include, but are not limited to, one or more keyboards, touchscreens, touchpads, microphones, buttons, dials, switches, etc., and one or more output devices 915, which may include, but are not limited to, one or more displays (e.g., touchscreens), light emitting diodes (LEDs), speakers, etc.

[0089] The UE 105 may also include a wireless communication interface 930, which may comprise, but is not limited to, a modem, a network card, an infrared communication device, a wireless communication device, and / or a chipset (such as a Bluetooth device, an IEEE 802.11 device, an IEEE 802.15.4 device, a Wi-Fi device, a WiMAX device, a WAN device, and / or various cellular devices), which may enable the UE 105 to communicate with other devices as described in the above embodiments. The wireless communication interface 930 may enable data and signaling to be communicated (e.g., transmitted and received) with the TRP of the network, as described herein, for example, via an eNB, a gNB, an ng-eNB, an access point, various base stations and / or other access node types, and / or other network components, computer systems, and / or any other electronic devices communicatively coupled to the TRP. Communication may be performed via one or more wireless communication antenna(s) 932 that transmit and / or receive wireless signals 934. According to some embodiments, the wireless communication antenna(s) 932 may include multiple individual antennas, an antenna array, or any combination thereof. The antenna(s) 932 may be capable of transmitting and receiving wireless signals using beams (e.g., Tx beams and Rx beams). Beamforming may be performed using digital and / or analog beamforming techniques using respective digital and / or analog circuitry. The wireless communication interface 930 may include such circuitry.

[0090] Depending on the desired functionality, the wireless communication interface 930 may include separate receivers and transmitters, or any combination of transceivers, transmitters, and / or receivers, for communicating with base stations (e.g., ng-eNBs and gNBs) and other terrestrial transceivers, such as wireless devices and access points. The UE 105 may communicate with different data networks, which may comprise a variety of network types. For example, a WWAN may be a CDMA network, a time division multiple access (TDMA) network, a frequency division multiple access (FDMA) network, an orthogonal frequency division multiple access (OFDMA) network, a single-carrier frequency division multiple access (SC-FDMA) network, WiMax (IEEE 802.16), etc. A CDMA network may implement one or more RATs, such as CDMA2000®, WCDMA, etc. CDMA2000® includes IS-95, IS-2000, and / or IS-856 standards. A TDMA network may implement GSM, a Digital Advanced Mobile Phone System (D-AMPS), or some other RAT. An OFDMA network may utilize LTE, LTE Advanced, 5G NR, etc. 5G NR, LTE, LTE Advanced, GSM, and WCDMA are described in documents from 3GPP. CDMA2000® is described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). 3GPP and 3GPP2 documents are publicly available. A wireless local area network (WLAN) may also be an IEEE 802.11x network, and a wireless personal area network (WPAN) may be a Bluetooth network, an IEEE 802.15x network, or some other type of network. The techniques described herein may also be used for any combination of WWAN, WLAN, and / or WPAN.

[0091] The UE 105 may further include sensor(s) 940. The sensor(s) 940 may comprise, but are not limited to, one or more inertial sensors and / or other sensors (e.g., accelerometer(s), gyroscope(s), camera(s), magnetometer(s), altimeter(s), microphone(s), proximity sensor(s), light sensor(s), barometer(s), etc.), some of which may be used to obtain location-related measurements and / or other information.

[0092] Embodiments of the UE 105 may also include a Global Navigation Satellite System (GNSS) receiver 980 capable of receiving signals 984 from one or more GNSS satellites using an antenna 982 (which may be the same as the antenna 932). Positioning based on GNSS signal measurements may be utilized to complement and / or incorporate the techniques described herein. The GNSS receiver 980 may use conventional techniques to extract the position of the UE 105 from GNSS satellites of GNSS systems such as the Global Positioning System (GPS), Galileo, GLONASS, the Quasi-Zenith Satellite System (QZSS) over Japan, the IRNSS over India, and the Beidou Navigation Satellite System (BDS) over China. Furthermore, the GNSS receiver 980 can be used with various augmentation systems (e.g., Satellite Based Augmentation System (SBAS)) associated with or adapted for use with one or more global and / or regional navigation satellite systems, such as, for example, the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), the Multi-functional Satellite Augmentation System (MSAS), and the Geo Augmented Navigation system (GAGAN).

[0093] It should be noted that while the GNSS receiver 980 is illustrated in FIG. 9 as a separate component, embodiments are not limited to this. As used herein, the term “GNSS receiver” may include hardware and / or software components configured to acquire GNSS measurements (measurements from GNSS satellites). In some embodiments, the GNSS receiver may therefore comprise a measurement engine executed (as software) by one or more processors, such as the processor(s) 910, the DSP 920, and / or a processor in the wireless communication interface 930 (e.g., in a modem). The GNSS receiver may also optionally include a positioning engine, which can use the GNSS measurements from the measurement engine to determine the position of the GNSS receiver using an Extended Kalman Filter (EKF), Weighted Least Squares (WLS), a particle filter, or the like. The positioning engine may also be executed by one or more processors, such as the processor(s) 910 or the DSP 920.

[0094] The UE 105 may further include and / or be in communication with memory 960. Memory 960 may include, but is not limited to, local and / or network-accessible storage devices, disk drives, drive arrays, optical storage devices, solid-state storage devices such as random access memory (RAM) and / or read-only memory (ROM), which may be programmable, flash-updateable, etc. Such storage devices may be configured to implement any suitable data store, including, but not limited to, various file systems, database structures, etc.

[0095] The memory 960 of the UE 105 may also comprise software elements (not shown in FIG. 9 ) including other code, such as an operating system, device drivers, executable libraries, and / or one or more application programs, which may include computer programs provided by various embodiments and / or may be designed to implement methods and / or configure systems provided by other embodiments, as described herein. By way of example only, one or more procedures described with respect to the method(s) described above may be implemented as code and / or instructions in the memory 960 executable by the UE 105 (and / or the processor(s) 910 or DSP 920 within the UE 105). In some embodiments, such code and / or instructions may then be used to configure and / or adapt a general-purpose computer (or other device) to perform one or more operations in accordance with the described methods.

[0096] 10 is a block diagram of an embodiment of a base station 120 that may be utilized as described herein above (e.g., to implement one of the RSUs 404-1, 404-2, 404-3, and 404-4 of FIGS. 4 and 5). For example, the UE 105 may perform one or more of the functions of the method shown in FIG. 8. Note that FIG. 10 is intended only to provide a generalized illustration of various components, any or all of which may be utilized as desired. In some embodiments, the base station 120 may serve as a gNB, an ng-eNB, and / or (more generally) a TRP.

[0097] The base station 120 is shown comprising hardware elements that may be electrically coupled (or may be in communication as needed) via a bus 1005. The hardware elements may include processor(s) 1010, which may include, but are not limited to, one or more general-purpose processors, one or more special-purpose processors (such as DSP chips, graphics acceleration processors, ASICs, etc.), and / or other processing structures or means. As shown in FIG. 10, some embodiments may have a separate DSP 1020 depending on the desired functionality. According to some embodiments, location determination and / or other determinations based on wireless communications may be provided in the processor(s) 1010 and / or in a wireless communication interface 1030 (described below). The base station 120 may also include one or more input devices, which may include, but are not limited to, a keyboard, a display, a mouse, a microphone, button(s), dial(s), switch(es), etc., and one or more output devices, which may include, but are not limited to, a display, a light-emitting diode (LED), a speaker, etc.

[0098] The base station 120 may also include a wireless communication interface 1030, which may include, but is not limited to, a modem, a network card, an infrared communication device, a wireless communication device, and / or a chipset (such as a Bluetooth device, an IEEE 802.11 device, an IEEE 802.15.4 device, a Wi-Fi device, a WiMAX device, a cellular communication facility, etc.), which may enable the base station 120 to communicate as described herein. The wireless communication interface 1030 may enable data and signaling to be communicated (e.g., transmitted and received) to UEs, other base stations / TRPs (e.g., eNBs, gNBs, and ng-eNBs), and / or other network components, computer systems, and / or any other electronic devices described herein. Communication may be performed via one or more wireless communication antenna(s) 1032 that transmit and / or receive wireless signals 1034.

[0099] The base station 120 may also include a network interface 1080, which may include support for wired communication technologies. The network interface 1080 may include a modem, a network card, a chipset, etc. The network interface 1080 may include one or more input and / or output communication interfaces to allow data to be exchanged with a network, a communication network server, a computer system, and / or any other electronic device described herein.

[0100] In many embodiments, base station 120 may further comprise memory 1060. Memory 1060 may include, but is not limited to, local and / or network-accessible storage devices, disk drives, drive arrays, optical storage devices, solid-state storage devices such as RAM and / or ROM, which may be programmable, flash-updateable, etc. Such storage devices may be configured to implement any suitable data store, including, but not limited to, various file systems, database structures, etc.

[0101] The memory 1060 of the base station 120 may also comprise software elements (not shown in FIG. 10 ), including other code, such as an operating system, device drivers, executable libraries, and / or one or more application programs, which may include computer programs provided by various embodiments as described herein and / or may be designed to implement methods and / or configure systems provided by other embodiments. By way of example only, one or more procedures described with respect to the method(s) discussed above may be implemented as code and / or instructions in the memory 1060 executable by the base station 120 (and / or the processor(s) 1010 or DSP 1020 within the base station 120). In some embodiments, such code and / or instructions may then be used to configure and / or adapt a general-purpose computer (or other device) to perform one or more operations in accordance with the described methods.

[0102] It will be apparent to those skilled in the art that substantial variations may be made according to particular requirements. For example, customized hardware might also be used and / or particular elements might be implemented in hardware, software (including portable software such as applets), or both. Furthermore, connectivity to other computing devices, such as network input / output devices, might be utilized.

[0103] With reference to the accompanying figures, components that may include memory may also include non-transitory machine-readable media. As used herein, the terms "machine-readable medium" and "computer-readable medium" refer to any storage medium that participates in providing data that causes a machine to operate in a specific manner. In the embodiments provided above, various machine-readable media may participate in providing instructions / code to a processor and / or other device(s) for execution. Additionally or alternatively, machine-readable media may be used to store and / or transport such instructions / code. In many implementations, computer-readable media are physical and / or tangible storage media. Such media may take many forms, including, but not limited to, non-volatile media and volatile media. Common forms of computer-readable media include, for example, magnetic and / or optical media, any other physical media with a pattern of holes, RAM, programmable ROM (PROM), erasable PROM (EPROM), FLASH-EPROM, any other memory chip or cartridge, or any other medium from which a computer can read instructions and / or code.

[0104] The methods, systems, and devices described herein are examples. Various embodiments may omit, substitute, or add various procedures or components, as appropriate. For example, features described with respect to some embodiments may be combined in various other embodiments. Different aspects and elements of the embodiments may be similarly combined. Various components of the diagrams provided herein may be embodied in hardware and / or software. Also, technology evolves, and therefore, many of the elements are examples that do not limit the scope of the disclosure to those specific examples.

[0105] It has proven convenient at times, principally for reasons of common usage, to refer to such signals as bits, information, values, elements, symbols, characters, variables, terms, numbers, numerical values, or the like. It should be understood, however, that all of these or similar terms are merely convenient labels and are to be associated with the appropriate physical quantities. Unless otherwise expressly stated, and as is clear from the above description, throughout this specification, descriptions utilizing terms such as "processing," "calculating," "calculating," "determining," "ascertaining," "identifying," "associating," "measuring," "performing," and the like, should be understood to refer to actions or processes of a particular apparatus, such as a special purpose computer or similar special purpose electronic computing device. Thus, in the context of this specification, a special purpose computer or similar special purpose electronic computing device is capable of manipulating or transforming signals that are commonly represented as physical electronic, electrical, or magnetic quantities within the memories, registers, or other information storage, transmission, or display devices of the special purpose computer or similar special purpose electronic computing device.

[0106] The terms "and" and "or" as used herein may include a variety of meanings that are expected to depend, at least in part, on the context in which such terms are used. Generally, when "or" is used to associate a list, such as A, B, or C, it is intended to mean A, B, and C, which are used herein in an inclusive sense, as well as A, B, or C, which are used herein in an exclusive sense. Additionally, as used herein, the term "one or more" may be used to refer to any feature, structure, or characteristic in the singular, or may be used to refer to any combination of features, structures, or characteristics. However, it should be noted that this is merely an illustrative example, and claimed subject matter is not limited to this example. Furthermore, the term "at least one of," when used to associate a list, such as A, B, or C, may be interpreted to mean any combination of A, B, and / or C, such as A, AB, AA, AAB, AABBCCC, etc.

[0107] While several embodiments have been described, various modifications, alternative configurations, and equivalents may be used without departing from the scope of the present disclosure. For example, the above elements may merely be components of a larger system in which other rules may take precedence over or otherwise modify the application of various embodiments. Also, some steps may be taken before, during, or after the above elements are considered. Therefore, the above description does not limit the scope of the present disclosure.

[0108] In view of this description, embodiments may include different combinations of features. Example implementations are described in the following numbered clauses.

[0109] Clause 1. A method for wireless communication by a wireless communication device, the method comprising: detecting a first reference signal associated with a first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier in accordance with timing indicated by the first reference signal; and, responsive to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as a synchronization reference source for the wireless communication device, initiating a waiting interval; and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0110] Clause 2. The method of clause 1, including, in response to detecting a second reference signal on the wireless carrier while the second synchronization reference is a synchronization reference source for the wireless communication device, transmitting an SLSS on the wireless carrier in accordance with timing indicated by the second reference signal.

[0111] Clause 3. The method of any of clauses 1 to 2, wherein the second synchronization reference is a reference of lower priority than the first synchronization reference.

[0112] Clause 4. The method of any of clauses 1 to 3, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

[0113] Clause 5. The method of clause 4, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

[0114] Clause 6. The method of clause 5, wherein the second synchronization reference corresponds to a second V2X RSU in a V2X RSU synchronization chain.

[0115] Clause 7. The method of any of clauses 1 to 6, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

[0116] Clause 8. The method of any of clauses 1 to 6, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

[0117] Clause 9. A wireless communications device comprising: a transceiver; a memory; and one or more processors communicatively coupled to the transceiver and the memory, wherein the one or more processors are configured to: detect a first reference signal associated with the first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communications device; transmit sidelink synchronization signals (SLSSs) on the wireless carrier in accordance with timing indicated by the first reference signal; and, in response to a subsequent determination that the first synchronization reference has become unavailable, adopt a second synchronization reference as a synchronization reference source for the wireless communications device, initiate a waiting interval; and, following expiration of the waiting interval, monitor the wireless carrier for a second reference signal associated with the second synchronization reference; and refrain from transmitting SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0118] Clause 10. The wireless communication device of Clause 9, wherein the one or more processors are configured to, in response to detecting a second reference signal on the wireless carrier while the second synchronization reference is a synchronization reference source for the wireless communication device, transmit an SLSS on the wireless carrier in accordance with timing indicated by the second reference signal.

[0119] Clause 11. A wireless communication device according to any of clauses 9 to 10, wherein the second synchronization reference is of lower priority than the first synchronization reference.

[0120] Clause 12. The wireless communication device of any of clauses 9 to 11, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

[0121] Clause 13. The wireless communications device of clause 12, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

[0122] Clause 14. The wireless communications device of clause 13, wherein the second synchronization reference corresponds to a second V2X RSU in a V2X RSU synchronization chain.

[0123] Clause 15. A wireless communication device according to any of clauses 9 to 14, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

[0124] Clause 16. A wireless communication device according to any of clauses 9 to 14, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

[0125] Clause 17. An apparatus for a wireless communication device, comprising: means for detecting a first reference signal associated with a first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; means for transmitting sidelink synchronization signals (SLSSs) on the wireless carrier in accordance with timing indicated by the first reference signal; and means for, in response to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as a synchronization reference source for the wireless communication device and initiating a waiting interval; and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0126] Clause 18. The apparatus of clause 17, comprising means for transmitting an SLSS on the wireless carrier in accordance with timing indicated by the second reference signal in response to detecting a second reference signal on the wireless carrier while the second synchronization reference is a synchronization reference source for the wireless communication device.

[0127] Clause 19. The apparatus of any of clauses 17 to 18, wherein the second synchronization reference is a reference of lower priority than the first synchronization reference.

[0128] Clause 20. The apparatus of any of clauses 17 to 19, wherein the apparatus is one of a plurality of Vehicle-to-Everything (V2X) Roadside Units (RSUs) in a V2X RSU synchronization chain, and the second synchronization reference corresponds to a second V2X RSU of the plurality of V2X RSUs in the V2X RSU synchronization chain.

[0129] Clause 21. The apparatus of any of clauses 17 to 20, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

[0130] Clause 22. The apparatus of any of clauses 17 to 20, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

[0131] Clause 23. A non-transitory computer-readable medium storing instructions for wireless communication by a wireless communication device, the instructions including code for: detecting a first reference signal associated with the first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier in accordance with timing indicated by the first reference signal; and, responsive to a subsequent determination that the first synchronization reference has become unavailable, adopting a second synchronization reference as the synchronization reference source for the wireless communication device; initiating a waiting interval; and, following expiration of the waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; and refraining from transmitting the SLSSs on the wireless carrier while the second reference signal is not detected on the wireless carrier.

[0132] Clause 24. The non-transitory computer-readable medium of clause 23, the instructions including code for: in response to detecting a second reference signal on the wireless carrier while the second synchronization reference is a synchronization reference source for the wireless communication device, transmitting an SLSS on the wireless carrier according to timing indicated by the second reference signal.

[0133] Clause 25. The non-transitory computer-readable medium of any of clauses 23 to 24, wherein the second synchronization reference is a reference of lower priority than the first synchronization reference.

[0134] Clause 26. The non-transitory computer-readable medium of any of clauses 23 to 25, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

[0135] Clause 27. The non-transitory computer-readable medium of clause 26, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

[0136] Clause 28. The non-transitory computer-readable medium of clause 27, wherein the second synchronization reference corresponds to a second V2X RSU in a V2X RSU synchronization chain.

[0137] Clause 29. The non-transitory computer-readable medium of any of clauses 23 to 28, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

[0138] Clause 30. The non-transitory computer-readable medium of any of clauses 23 to 28, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

Claims

1. 1. A method for wireless communication by a wireless communication device, comprising: detecting, on a wireless carrier, a first reference signal associated with a first synchronization reference while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; In response to a subsequent determination that the first synchronization reference has become unavailable, employing a second synchronization reference as the synchronization reference source for the wireless communication device; Starts a waiting interval, Following expiration of said waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; refraining from transmitting an SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier; A method comprising:

2. 10. The method of claim 1, comprising, in response to detecting the second reference signal on the wireless carrier while the second synchronization reference is the synchronization reference source for the wireless communication device, transmitting a SLSS on the wireless carrier in accordance with timing indicated by the second reference signal.

3. The method of claim 1 , wherein the second synchronization reference is a lower priority reference than the first synchronization reference.

4. 10. The method of claim 1, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

5. 5. The method of claim 4, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

6. 6. The method of claim 5, wherein the second synchronization reference corresponds to a second V2X RSU in the V2X RSU synchronization chain.

7. 2. The method of claim 1, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

8. 2. The method of claim 1, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

9. 1. A wireless communication device, comprising: A transmitter / receiver, Memory and one or more processors communicatively coupled to the transceiver and the memory, wherein the one or more processors: detecting, on a wireless carrier, a first reference signal associated with a first synchronization reference while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; In response to a subsequent determination that the first synchronization reference has become unavailable, employing a second synchronization reference as the synchronization reference source for the wireless communication device; Starts a waiting interval, Following expiration of said waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; refraining from transmitting an SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier; 10. A wireless communication device configured to:

10. 10. The wireless communication device of claim 9, wherein the one or more processors are configured to, in response to detecting the second reference signal on the wireless carrier while the second synchronization reference is the synchronization reference source for the wireless communication device, transmit a SLSS on the wireless carrier according to timing indicated by the second reference signal.

11. 10. The wireless communication device of claim 9, wherein the second synchronization reference is a lower priority reference than the first synchronization reference.

12. 10. The wireless communication device of claim 9, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

13. 13. The wireless communication device of claim 12, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

14. 14. The wireless communication device of claim 13, wherein the second synchronization reference corresponds to a second V2X RSU in the V2X RSU synchronization chain.

15. 10. The wireless communication device of claim 9, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

16. 10. The wireless communication device of claim 9, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

17. 1. An apparatus for a wireless communication device, comprising: means for detecting, on a wireless carrier, a first reference signal associated with the first synchronization reference while the first synchronization reference is a synchronization reference source for the wireless communication device; means for transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; In response to a subsequent determination that the first synchronization reference has become unavailable, employing a second synchronization reference as the synchronization reference source for the wireless communication device; Starts a waiting interval, Following expiration of said waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; means for refraining from transmitting an SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier; An apparatus comprising:

18. 20. The apparatus of claim 17, comprising: means for transmitting a SLSS on the wireless carrier in accordance with timing indicated by the second reference signal in response to detecting the second reference signal on the wireless carrier while the second synchronization reference is the synchronization reference source for the wireless communication device.

19. 20. The apparatus of claim 17, wherein the second synchronization reference is a lower priority reference than the first synchronization reference.

20. 18. The apparatus of claim 17, wherein the apparatus is one of a plurality of vehicle-to-everything (V2X) roadside units (RSUs) in a V2X RSU synchronization chain, and the second synchronization reference corresponds to a second V2X RSU of the plurality of V2X RSUs in the V2X RSU synchronization chain.

21. 18. The apparatus of claim 17, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

22. 20. The apparatus of claim 17, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

23. 1. A non-transitory computer-readable medium storing instructions for wireless communication by a wireless communication device, the instructions comprising: detecting a first reference signal associated with the first synchronization reference on a wireless carrier while the first synchronization reference is a synchronization reference source for the wireless communication device; transmitting sidelink synchronization signals (SLSSs) on the wireless carrier according to timing indicated by the first reference signal; In response to a subsequent determination that the first synchronization reference has become unavailable, employing a second synchronization reference as the synchronization reference source for the wireless communication device; Starts a waiting interval, Following expiration of said waiting interval, monitoring the wireless carrier for a second reference signal associated with the second synchronization reference; refraining from transmitting an SLSS on the wireless carrier while the second reference signal is not detected on the wireless carrier; Contains code for Non-transitory computer-readable medium.

24. 24. The non-transitory computer-readable medium of claim 23, wherein the instructions include code for, in response to detecting the second reference signal on the wireless carrier while the second synchronization reference is the synchronization reference source for the wireless communication device, transmitting a SLSS on the wireless carrier according to timing indicated by the second reference signal.

25. 24. The non-transitory computer-readable medium of claim 23, wherein the second synchronization reference is a lower priority reference than the first synchronization reference.

26. 24. The non-transitory computer-readable medium of claim 23, wherein the wireless communication device is a Vehicle-to-Everything (V2X) Roadside Unit (RSU).

27. 27. The non-transitory computer-readable medium of claim 26, wherein the V2X RSU is one of a plurality of V2X RSUs in a V2X RSU synchronization chain.

28. 28. The non-transitory computer-readable medium of claim 27, wherein the second synchronization reference corresponds to a second V2X RSU in the V2X RSU synchronization chain.

29. 24. The non-transitory computer-readable medium of claim 23, wherein the first synchronization reference is a Global Navigation Satellite System (GNSS) reference and the second synchronization reference is a Synchronization Reference (SyncRef) User Equipment (UE).

30. 24. The non-transitory computer-readable medium of claim 23, wherein the first synchronization reference is a first synchronization reference (SyncRef) user equipment (UE) and the second synchronization reference is a second SyncRef UE.

Images