Methods, systems, and devices for wireless communications are described. The method includes communicating control signaling scheduling a sidelink transmission via a physical sidelink channel, receiving, by a first user equipment (UE), the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel based on the control signaling, and monitoring the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

BACKGROUND

The following relates generally to wireless communications, and more specifically to physical sidelink channel packet-based synchronization.

In some examples, a UE may lose a connection to a synchronization source (e.g., synchronization source outage, etc.). In some examples, parameters for transmissions configured based on the synchronization source may be relatively strict. In some examples, a transmission sent after loss of the synchronization source may detrimentally impact communication throughput.

SUMMARY

The described techniques relate to improved methods, systems, devices, or apparatuses that support physical sidelink channel packet-based synchronization. Generally, the described techniques provide for improvements to signal synchronization based on a device losing a connection to a synchronization source (e.g., synchronization source outage, etc.). In some examples, a first UE may lose synchronization with a synchronization source. In some examples, the described techniques include the first UE determining a time synchronization or frequency synchronization, or both, based on one or more packet transmissions from a second UE that has maintained its synchronization. For example, the first UE may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel. The first UE may receive the sidelink transmission comprising one or more reference signals from a second UE via the physical sidelink channel in accordance with the control signaling. The first UE may use the one or more reference signals for determining time synchronization or frequency synchronization, or both, for communicating with the second UE.

A method of wireless communication is described. The method may include communicating control signaling scheduling a sidelink transmission via a physical sidelink channel, receiving, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel based on the control signaling, and monitoring the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

An apparatus for wireless communication is described. The apparatus may include a processor, memory coupled with the processor, and instructions stored in the memory. The instructions may be executable by the processor to cause the apparatus to communicate control signaling scheduling a sidelink transmission via a physical sidelink channel, receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel based on the control signaling, and monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

Another apparatus for wireless communication is described. The apparatus may include means for communicating control signaling scheduling a sidelink transmission via a physical sidelink channel, receiving, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel based on the control signaling, and monitoring the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

A non-transitory computer-readable medium storing code for wireless communication is described. The code may include instructions executable by a processor to communicate control signaling scheduling a sidelink transmission via a physical sidelink channel, receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel based on the control signaling, and monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, receiving the sidelink transmission may include operations, features, means, or instructions for receiving, based on the control signaling, the one or more reference signals in one or more reference signal symbol periods allocated within a resource of the physical sidelink channel for the sidelink transmission.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving synchronization confidence information from the second UE, where the time synchronization may be determined using the one or more reference signals based on a synchronization confidence level satisfying a confidence threshold.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, receiving the synchronization confidence information may include operations, features, means, or instructions for receiving the synchronization confidence information in an application layer message from the second UE.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, by the first UE, a second sidelink transmission including one or more reference signals from a third UE, and receiving second synchronization confidence information from a third UE, where the time synchronization may be determined without using the one or more reference signals of the second sidelink transmission based on a second synchronization confidence level not satisfying a confidence threshold.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving location information of the second UE, where the time synchronization may be determined based on the location information.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a distance between the second UE and the first UE based on the location information, where the time synchronization may be determined based on the estimated distance.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the location information includes a zone identification of the second UE.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, receiving the location information may include operations, features, means, or instructions for receiving a safety message including the location information.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving location information from a set of UEs including the second UE, where the time synchronization may be determined based on the location information for the set of UEs.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a statistical distance metric based on the location information for the set of UEs, where the time synchronization may be determined based on the statistical distance metric.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a frequency offset based on the one or more reference signals, where the frequency synchronization may be determined based on the estimated frequency offset.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving a set of sidelink transmissions from a set of UEs including the second UE, and determining a set of frequency offsets, where each frequency offset may be determined for a respective sidelink transmission of the set of sidelink transmissions, and where the frequency synchronization may be determined based on the set of frequency offsets.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a distance between the second UE and the first UE based on a transmission power associated with the sidelink transmission, where the time synchronization may be determined based on the estimated distance.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a pathloss based on the sidelink transmission.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a distance between the first UE and the second UE based on the estimated pathloss, where the time synchronization may be determined based on the estimated distance.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for determining a received signal strength associated with the sidelink transmission, and estimating the pathloss based on the determined received signal strength.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the received signal strength includes a received signal strength indicator (RSSI), a reference signal received power (RSRP) associated with the sidelink transmission, or both.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for determining, by the first UE, a loss of synchronization with a synchronization source, where the time synchronization, the frequency synchronization, or both, may be determined using the one or more reference signals of the sidelink transmission based on the loss of synchronization.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for determining the time synchronization based on, descrambling a symbol of a first reference signal of the one or more reference signals, extracting the descrambled symbol of the first reference signal in frequency domain, determining a channel impulse response based on the extracted descrambled symbol, and determining a time delay estimate based on the determined channel impulse response.

Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for estimating a timing drift based on the one or more reference signals, where the time synchronization may be determined based on the estimated timing drift.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the physical sidelink channel includes a physical sidelink control channel or a physical sidelink shared channel.

In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the one or more reference signals include at least one demodulation reference signal (DMRS).

DETAILED DESCRIPTION

The present techniques provide improvements to signal synchronization based on a device losing a connection to a synchronization source (e.g., synchronization source outage, etc.). The present techniques include physical sidelink channel packet-based synchronization. In some examples, the physical sidelink channel may include physical sidelink control channel (PSCCH) or physical sidelink shared channel (PSSCH)). In some examples, time synchronization of a first UE may be determined based on the first UE receiving one or more packet transmissions from at least one other UE (e.g., at least a second UE) that has maintained its synchronization. In some examples, the first UE may compensate for propagation delay based on a distance to a second UE that transmitting the one or more packet transmissions (e.g., a transmitter to receiver (Tx-Rx) distance between the first UE and the second UE). The Tx-Rx distance may be determined based on location information (e.g., zone identification of the first UE and zone identification of the second UE, etc.). In some examples, zone identification information may be included in sidelink control information (SCI). In some examples, frequency error or frequency drift of the first UE caused by loss of synchronization may be compensated based on frequency error estimation determined by a packet transmission of the second UE that is received by the first UE. In some examples, frequency error may be compensated based on wireless wide area network (WWAN) concurrency and frequency tracking loop (FTL) usage.

Aspects of the disclosure are initially described in the context of a wireless communications system. Aspects of the disclosure are further illustrated by and described with reference to a wireless communications system, an environment, a timing diagram, and a process that relate to physical sidelink channel packet-based synchronization. Aspects of the disclosure are further illustrated by and described with reference to apparatus diagrams, system diagrams, and flowcharts that relate to physical sidelink channel packet-based synchronization.

FIG.1illustrates an example of a wireless communications system100that supports avoiding packet data convergence protocol holes for bearer in dual connectivity mode across multiple radio access technologies in accordance with aspects of the present disclosure. The wireless communications system100may include one or more base stations105, one or more UEs115, and a core network130. In some examples, the wireless communications system100may be a Long Term Evolution (LTE) network, an LTE-Advanced (LTE-A) network, an LTE-A Pro network, or a New Radio (NR) network. In some examples, the wireless communications system100may support enhanced broadband communications, ultra-reliable (e.g., mission critical) communications, low latency communications, communications with low-cost and low-complexity devices, or any combination thereof.

In some examples, a first UE (e.g., a first UE115ofFIG.1) may communicate with a second UE (e.g., a second UE115ofFIG.1) based on a configuration (e.g., cellular vehicle to everything (C-V2X) configuration). In some examples, the first UE may lose synchronization with a synchronization source (e.g., a C-V2X synchronization source, a GNSS synchronization source, a serving cell, a primary cell (PCell), eNB, gNB, synchronization reference (SyncRef) UE, sidelink synchronization signal, (SLSS), physical sidelink broadcast channel (PSBCH), etc.). In some examples, C-V2X synchronization conditions may be relatively stringent. In some examples, the first UE may synchronize based on one or more packet transmissions from the second UE. In some instances, control signaling scheduling may be communicated between the first UE (e.g., an unsynchronized UE) and the second UE (e.g., a synchronized UE) over a physical sidelink channel. In some examples, the control signaling scheduling may include control signaling, or a control channel, or a shared data channel, or a shared data transmission, or any combination thereof.

In some examples, the first UE may determine a time synchronization, a frequency synchronization, or both, based on one or more sidelink transmissions the first UE receives from the second UE over the physical sidelink channel. In some examples, the first UE may receive control sidelink transmissions or data sidelink transmissions, or both, via a control sidelink channel or data sidelink channel, or both. In some instances, the first UE may monitor the physical sidelink channel based on the time synchronization or the frequency synchronization, or both. In some examples, after losing communication with a synchronization source the first UE may regain or maintain synchronization based on the monitoring of the physical sidelink channel. Thus, the present techniques improve synchronization (e.g., C-V2X synchronization) by enabling a UE to regain and maintain synchronization after losing communication with a synchronization source.

It is noted that the techniques described herein are not limited to a particular format or configuration (e.g., V2X, C-V2X, etc.), but may be used for any sidelink transmission that includes reference signals that may be used for time synchronization or frequency synchronization, or both.

FIG.2illustrates an example of a wireless communications system200that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. In some examples, wireless communications system200may implement aspects of wireless communications system100.

As illustrated, wireless communications system200may include UE115-a, UE115-b, UE115-c, UE115-d, UE115-e, UE115-f, and base station105-a, any of which may be an example of a UE115or a base station105, respectively, as described herein with reference toFIG.1. As shown, UE115-aand UE115-bremain within coverage of base station105-a, UE115-cand UE115-dare in partial coverage (via UE115-a), and UE115-e, and UE115-fare out of coverage. Wireless communications system200may also include downlink205and uplink210. Base station105-amay use downlink205to convey control or data information, or both, to UE115-a. And UE115-amay use uplink210to convey control or data information, or both, to base station105-a. In some cases, downlink205may use different time or frequency resources, or both, than uplink210.

In some examples, UE115-aand UE115-bmay maintain synchronization with one or more synchronization sources. Examples of synchronization sources include one or more base stations (e.g., base station105-a), one or more satellites (e.g., global navigation satellite system (GNSS), etc.), or any combination thereof. After UE115-aor UE115-bis synchronized to a synchronization source, UE115-aor UE115-bmay verify that the physical sidelink channel transmission timing offset is within a preset range (e.g., ±391 nanosecond) of a reference timing of the synchronization source. In some examples, UE115-aor UE115-bmay verify that a modulated carrier frequency for sidelink transmissions are accurate to within a preset range (e.g., ±0.1 parts per million) observed over a period of one time slot (e.g., 0.5 milliseconds) as compared to a reference frequency of the synchronization source. However, when loss of synchronization occurs (e.g., outage of a synchronization source, GNSS outage, etc.), UE transmissions may be suspended because a transmission by an unsynchronized UE may impact other devices in the same network (e.g., a same C-V2X network). In some examples, C-V2X transmission may specify accurate timing (e.g., very accurate) to operate physical layer procedures, as a wrong transmission may impact overall C-V2X networks.

In some examples, UE115-bmay lose synchronization (e.g., synchronization source outage, etc.). In some examples, UE115-bmay communicate (e.g., transmit, receive, or both) control signaling with UE115-a, where the UE115-buses the control signaling to schedule a sidelink transmission via a physical sidelink channel215between UE115-band UE115-a. In some examples, the control signaling may include control signaling, or a control channel, or a shared data channel, or a shared data transmission, or any combination thereof. Examples of physical sidelink channel215may include physical sidelink control channel (PSCCH) or physical sidelink shared channel (PSSCH), or both. Based on the control signaling, UE115-bmay receive from UE115-aa sidelink transmission over physical sidelink channel215that includes one or more reference signals. In some examples, UE115-bmay receive from UE115-acontrol sidelink transmissions or data sidelink transmissions, or both, via a control sidelink channel or data sidelink channel, or both. The sidelink transmission may be one or more packets communicated via a control channel (e.g., a PSCCH), a data channel (e.g., a PSSCH), or both. In some examples, one or more aspects of the described techniques may be implemented based on 5G NR C-V2X. In some examples, the UE115-amay implement 5G NR C-V2X physical sidelink control channel (PSCCH) and/or physical sidelink shared channel (PSSCH) packets-based synchronization.

In some examples, UE115-bmay determine a time synchronization, a frequency synchronization, or both, based at least in part on the one or more reference signals. In some examples, UE115-bmay monitor the physical sidelink channel215based on the time synchronization, the frequency synchronization, or both. Thus, UE115-bmay compensate for loss of a synchronization source by physical sidelink channel packet-based synchronization. In some examples, UE115-bmay determine the time synchronization based on packet transmissions from at least UE115-a(e.g., and a synchronization maintained by UE115-a). In some examples, propagation delay caused by UE115-blosing synchronization may be compensated by UE115-bestimating a distance between UE115-band UE115-a(e.g., Tx-Rx distance).

In some examples, UE115-bmay use location information to estimate the distance between UE115-band UE115-a. In some examples, UE115-amay transmit sidelink control information to UE115-bover physical sidelink channel215. In some examples, the sidelink control information may include location information (e.g., location of UE115-a, a zone identifier (ID) of UE115-a, etc.). In some examples, the location information may include 5G NR C-V2X location information (e.g., 5G NR C-V2X zone ID). A transmitting UE's location may be indicated in sidelink control information (e.g., stage2) associated with a PSSCH. In some examples, UE115-bmay determine location information for itself (e.g., zone ID of UE115-b, etc.). In some examples, one or more of the UEs may be configured by the network (e.g., base station), or may be preconfigured, with zones with respect to a geographical area for each zone, and a zone identifier be associated with a current geographical area in which a UE is located. Accordingly, UE115-bmay estimate a distance between UE115-band UE115-abased on location information of UE115-aand location information of UE115-b. For example, UE115-bmay estimate a distance between UE115-band UE115-abased on UE115-bdetermining UE115-bis in zone 2 and UE115-ais in zone 4, and then estimating a distance between zone 4 and zone 2.

In some examples, UE115-bmay estimate the Tx-Rx distance based on an absolute value of a difference between the zone of UE115-band the zone of UE115-a, {circumflex over (d)}Tx-Rx=|ZoneTx−ZoneRx|, where ZoneRxis a known location information of a receiving (Rx) UE (e.g., UE115-b) (e.g., even under GNSS out-of-coverage, some sensors can also be used), and ZoneTxis a known location information of a transmitting (Tx) UE (e.g., UE115-a). In some examples, UE115-bmay compensate for the packet reception time based on the following equation:

τ^c=τ^-d^Tx·Rxvlight
where vlightis the speed of light (e.g., 3×10{circumflex over ( )}8 meters per second). In some examples, UE115-bmay adjust a slot timing by {circumflex over (τ)}c(e.g., for adjusting a GNSS-lost NR V2X slot timing).

In some examples, UE115-bmay estimate a distance between UE115-band UE115-abased on a received power such as received signal strength indicator (RSSI) or reference signal received power (RSRP). In some examples, UE115-bmay estimate a pathloss based on the received power (e.g., RSSI, RSRP, or other received signal power). In some examples, UE115-bmay estimate a distance between UE115-band UE115-abased on the estimated pathloss. In some examples, UE115-bmay estimate a distance between UE115-band UE115-abased on a transmission power. In some examples, UE115-bmay estimate a transmission power of UE115-abased on a configured transmission power of UE115-b, or a preconfigured transmission power.

In some examples, pathloss may be impacted detrimentally by radio channel environments (e.g., in area with multiple buildings or other objects, traveling in a car or other vehicle, interfering radio signals, etc.). In some examples, UE115-bmay approximate a pathloss estimation based on the radio channel environment. In some examples, UE115-bmay estimate a relatively coarse distance between UE115-band UE115-abased on the approximated pathloss estimation.

In some examples, UE115-bmay compensate for frequency error (e.g., frequency drift caused by loss of synchronization source) based on one or more packet transmissions transmitted to UE115-bover the physical sidelink channel215by UE115-a. In some examples, UE115-bmay perform frequency error estimation based on the one or more packet transmissions. In some examples, UE115-bmay perform wireless wide area network (WWAN) concurrency and frequency tracking loop (FTL) usage based on the one or more packet transmissions. In some examples, UE115-bmay compensate for the frequency error based on the frequency error estimation, WWAN concurrency, or FTL usage, or any combination thereof.

In some examples, UE115-cmay experience a loss of a synchronization source. In some examples, UE115-cmay compensate for loss of synchronization by physical sidelink channel packet-based synchronization with UE115-a. In some examples, UE115-cmay determine a time synchronization or a frequency synchronization, or both, based on packet transmissions from at least UE115-a. In some examples, UE115-cmay use location information to estimate the distance between UE115-cand UE115-a. In some examples, UE115-amay transmit sidelink control information to UE115-cover physical sidelink channel220. In some examples, UE115-dmay compensate for frequency error (e.g., frequency drift caused by loss of synchronization source) based on one or more packet transmissions transmitted to UE115-dover the physical sidelink channel220by UE115-c.

In some examples, UE115-dmay experience a loss of a synchronization source. In some examples, UE115-dmay compensate for loss of synchronization by physical sidelink channel packet-based synchronization with UE115-c, where the synchronization of UE115-cmay depend the synchronization UE115-a. In some examples, UE115-dmay determine a time synchronization or a frequency synchronization, or both, based on packet transmissions from at least UE115-c. In some examples, UE115-dmay use location information to estimate the distance between UE115-dand UE115-c. In some examples, UE115-cmay transmit sidelink control information to UE115-dover physical sidelink channel225. In some examples, UE115-dmay compensate for frequency error (e.g., frequency drift caused by loss of synchronization source) based on one or more packet transmissions transmitted to UE115-dover the physical sidelink channel225by UE115-c.

In some examples, UE115-fmay experience a loss of a synchronization source. In some examples, UE115-fmay compensate for loss of synchronization by physical sidelink channel packet-based synchronization with UE115-e. In some examples, UE115-fmay determine a time synchronization or a frequency synchronization, or both, based on packet transmissions from at least UE115-e. In some examples, UE115-fmay use location information to estimate the distance between UE115-fand UE115-e. In some examples, UE115-emay transmit sidelink control information to UE115-fover physical sidelink channel230. In some examples, UE115-fmay compensate for frequency error (e.g., frequency drift caused by loss of synchronization source) based on one or more packet transmissions transmitted to UE115-fover the physical sidelink channel230by UE115-e.

FIG.3illustrates an example of an environment300that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. In some examples, environment300may implement aspects of wireless communications system100or wireless communications system200. As shown, environment300illustrates a physical sidelink channel transmission305(e.g., physical sidelink control channel (PSCCH) transmission or physical sidelink shared channel (PSSCH) transmission). In the illustrated example, physical sidelink channel transmission305may depict a physical sidelink channel packet transmission.

In the illustrated example, physical sidelink channel transmission305may include symbols of a physical sidelink channel slot340. As shown, physical sidelink channel transmission305may include subchannels345of a physical sidelink channel, wherein subchannels345includes a set of subchannels or a set of physical resource blocks for the physical sidelink channel. In the illustrated example, physical sidelink channel transmission305may include reference signal310, reference signal315, and reference signal320. At least one of reference signals310,315, and320may be an example of a demodulation reference signal.

In some examples, a synchronized device (e.g., UE115-a) may transmit physical sidelink channel transmission305. In some examples, a synchronization-lost device (e.g., UE115-b) may receive physical sidelink channel transmission305and the synchronization-lost device may determine a time synchronization, a frequency synchronization, or both, based on the received physical sidelink channel transmission305. In some examples, the synchronization-lost device may determine the time synchronization, the frequency synchronization, or both, based on at least one of reference signals310,315, and320.

In some examples, the synchronization-lost device may perform symbol delay estimation (e.g., orthogonal frequency-division multiplexing (OFDM) symbol delay estimation). Based on its configuration, the synchronization-lost device may expect to receive a symbol of reference signal310at some time or location, expect to receive a symbol of reference signal315at some time or location, or expect to receive a symbol of reference signal310at some time or location, or any combination thereof. In some examples, the synchronization-lost device may determine a difference between the actual time or location of the received symbol of reference signal310and the expected time or location of the symbol of reference signal310. In addition or alternatively, the synchronization-lost device may determine a similar difference for a symbol of reference signal315or reference signal320, or both. In some examples, the synchronization-lost device may determine a time drift or frequency drift, or both, based on the determined difference (e.g., offset) between the actual time or location of a received symbol of a reference signal (e.g., reference signal310, reference signal315, or reference signal320) and the expected time or location of the symbol of the reference signal.

In some examples, the synchronization-lost device may descramble a symbol of a reference signal (e.g., reference signal310, reference signal315, or reference signal320) and extract the descrambled symbol of the reference signal in the frequency domain. In some examples, the synchronization-lost device may determine a channel impulse response based at least in part on the extracted descrambled symbol and determine a time delay estimate based at least in part on the determined channel impulse response. In some examples, the synchronization-lost device may determine a time synchronization based at least in part on a determined time delay estimate. In some examples, the synchronization-lost device may determine a time delay based at least in part on a determined first path arrival time associated with the one or more references signals.

As illustrated, the synchronization-lost device may determine a timing drift (e.g., drifted slot time325), determine a correction factor (e.g., slot timing correction330) to correct the timing drift, and determine delay estimation335based on the drifted slot time325and the slot timing correction330.

In some examples, the synchronization-lost device may estimate and compensate for a timing drift based on a physical side channel packet reception (e.g., physical sidelink channel transmission305). In some examples, the synchronization-lost device may receive a sidelink transmission (e.g., a PSSCH packet), and perform OFDM symbol delay estimation through reference signal channel estimation (e.g., DMRS channel estimation). In some examples, the synchronization-lost device may extract de-scrambled DMRS symbol in frequency domain using a fast Fourier transform (FFT), Hjk, where j and k denote the jthRx antenna and the kthreference signal orthogonal frequency-division multiplexing symbol. In some examples, the synchronization-lost device may drive a time domain channel impulse response, hjk=IFFT{Hjk}, to determine the OFDM symbol delay. In some examples, the synchronization-lost device may determine the index of peak or center of mass, {circumflex over (τ)}=arg max ΣjΣk|hjk(i)|2, to compensate the timing drift of the synchronization-lost device by the estimated OFDM symbol delay estimation {circumflex over (τ)}. When the slot timing of the synchronization-lost device is {tilde over (τ)}, then the corrected timing may be determined by {tilde over (t)}+{circumflex over (τ)}. In some examples, OFDM symbol delay estimation may be performed in other manners. The above example considers a time domain channel impulse response and its peak index. However, there can be many other methods including frequency domain and time domain that may be used for OFDM symbol delay estimation. In addition, for the delay estimation, there can be many other methods such as first path arrival and other non-linear methods. Drifting timing correction may also be implemented using other methods. Some averaging, median, minimum, or other linear/non-linear methods, which consider multiple delay estimation, may be also used

In some examples, the synchronization-lost device may receive at least one application layer message. In some examples, one or more application layer message may be received in the physical sidelink channel transmission305. At least one of the application layer messages may include synchronization confidence information from a synchronized device (e.g., UE115-a). In some examples, the synchronization-lost device may determine a time synchronization, a frequency synchronization, or both, based at least in part on the synchronization confidence information of the synchronized device. The synchronization confidence information may indicate a quality of a synchronization of a synchronized device or a degree to which the synchronized device is accurately synchronized. In an example, from Application-assisted information, drifting NR SL UEs can use NR SL PSSCHs from synchronized NR SL UEs.

In some examples, the synchronization-lost device may receive a first synchronization confidence information from a first synchronized device in a first transmission and receive a second synchronization confidence information from a second synchronized device (e.g., UE115-a) in a second transmission (e.g., physical sidelink channel transmission305). In some examples, the synchronization-lost device may determine that the second synchronization confidence information indicates the synchronization of the second synchronized device is more accurate or a higher quality than the synchronization of the first synchronized device. Accordingly, the synchronization-lost device may select to use a transmission from the second synchronized device to determine a time synchronization, a frequency synchronization, or both, based on the second synchronization confidence information being more accurate or of a higher quality than the first synchronization confidence information. In some examples, the synchronization-lost device may select to use a transmission from any synchronized device with a synchronization confidence information that satisfies or exceeds a synchronization confidence threshold, or the first synchronized device that the synchronization-lost device determines to have a synchronization confidence information that satisfies or exceeds the synchronization confidence threshold.

In some examples, the synchronization-lost device may compensate for frequency error (e.g., frequency drift of the synchronization-lost device due to the loss of synchronization, crystal oscillator drift, etc.). In some examples, a GNSS-lost UE's frequency error can keep increasing. For instance, due to crystal oscillator drift (XO) drift, NR V2X's frequency error can be increasing when there is no GNSS sync to correct the frequency error. Frequency offset may be estimated by one or more received PSCCH/PSSCH packets. In some examples, the synchronization-lost device may determine a frequency offset to correct for the frequency error based on a reference signal of physical sidelink channel transmission305(e.g., reference signal310, reference signal315, or reference signal320). In some examples, the synchronization-lost device may compensate for frequency error based on synchronization confidence information of a synchronized device.

In some examples, the synchronization-lost device may estimate a frequency offset by comparing a phase difference between adjacent reference signal symbols (e.g., phase difference between reference signal310symbol and reference signal315symbol, or phase difference between reference signal315symbol and reference signal320symbol, or both). In some examples, the synchronization-lost device may compare the phase difference between adjacent reference signal symbols across all antennas of the synchronization-lost device. In some examples, the synchronization-lost device may estimate a frequency offset based on the following equation:

f^f⁢c⁢o=Δ⁢f2⁢π·NFFTNFFT+⁢NCP·1dRS_space⁢ang⁡(∑i⁢∑j⁢(Hj+Δi⁢Hji*))
where Δf is subcarrier space (e.g., sub6 30 kHz), Hjiis a de-scrambled reference signal symbol in the frequency domain, dRS_spaceis an OFDM symbol distance between adjacent reference signal OFDM symbols, and Δ is the distance between two reference signal symbols (e.g., between two DMRS OFDM symbols). In some examples, the synchronization-lost device may perform intra-RS OFDM symbol processing to determine the frequency offset. In some examples, the frequency offset determined by the synchronization-lost device may include compensation for both Doppler and frequency error.

In some examples, an application executing on the synchronization-lost device may estimate both speed and trajectory information of devices (e.g., synchronized devices) that send messages to the synchronization-lost device. In some examples, an application may track the speed and trajectory of each received message based on message information and tracking. In some examples, even under loss of a synchronization source, some enhanced sensors may be configured to still provide the synchronization-lost device's speed. In some examples, an application may provide the Doppler impacts to a modem (e.g., a modem of the synchronization-lost device). In some examples, the modem may adjust the Doppler impacts based on the application information.

In some examples, the synchronization-lost device may determine its own speed or trajectory, or both, and determine the frequency offset based at least in part on its own determined speed (e.g., compensate for Doppler impact to modem of the synchronization-lost device). In some examples, the synchronization-lost device may receive multiple packets. In some examples, the synchronization-lost device may receive multiple frequency offset estimations based on the multiple received packets. In some examples, the synchronization-lost device may use some linear (e.g., simple average) or non-linear methods, or both, to determine a multiple frequency offset estimation.

In some examples, the synchronization-lost device may receive packets from multiple synchronized devices (e.g., UE115-aand one or more additional UE devices, etc.). In some examples, the synchronization-lost device may determine a frequency offset for each of the multiple synchronized devices. In some examples, the synchronization-lost device may determine an overall frequency offset based on each of the multiple determined frequency offsets. In some examples, the synchronization-lost device may receive a zone identifier in each packet received from at least two of the multiple synchronized devices. In some examples, the synchronization-lost device may determine a propagation delay based on location information associated with the zone identifiers. In some examples, the synchronization-lost device may determine a propagation delay compensation based on the multiple zone identifiers received in the multiple packets. In some examples, linear (e.g., average) or non-linear methods, or both, may be used to determine a propagation delay compensation. In some examples, the synchronization-lost device may determine a propagation delay compensation based on the zone identifiers, the linear methods, or the non-linear methods, or any combination thereof.

In some examples, the synchronization-lost device may receive safety messages in applications (e.g., V2X safety messages received in applications executing on the synchronization-lost device). In some examples, a received safety message may include location information. In some examples, the application (e.g., V2X application) may receive a safety message and estimate a propagation delay based on the location information included in the safety message and the known location of the synchronization-lost device.

In some examples, the synchronization-lost device may receive an indication of a received Rx power (e.g., received signal strength indicator (RSSI) or reference signal received power (RSRP)) from a synchronized device (e.g., in a received Rx power measurement report). In some examples, the synchronization-lost device may estimate pathloss based on the received Rx power. In some examples, the synchronization-lost device may know its own configured transmission Tx power. In some examples, the synchronization-lost device may estimate pathloss based on the its own configured transmission Tx power. In some examples, pathloss may be impacted detrimentally by radio channel environments (e.g., synchronization-lost device is in area with multiple buildings or other objects, is traveling in a car or other vehicle or is a vehicle that is traveling, other radio signals interfere with a transmission from a synchronized device, etc.). In some examples, the synchronization-lost device may approximate a pathloss estimation based on the radio channel environment. In some examples, the synchronization-lost device may estimate a relatively coarse distance between the synchronization-lost device and a synchronized device based on the approximated pathloss estimation.

In some examples, the synchronization-lost device may track frequency error associated with a wireless network connection (e.g., wireless wide area network (WWAN). In some examples, the synchronization-lost device may track the frequency error through frequency tracking loop (FTL). In some examples, the synchronization-lost device may compensate for a frequency error in a first network (e.g., vehicle to everything (V2X) network) based on the tracked frequency error of the wireless network connection. In some examples, the synchronization-lost device may use a unit conversion to convert the frequency error of the wireless network to the first network. In one example, (frequency error of the first network)=(wireless network frequency error)×(unit conversion), where the unit conversion is a predetermined value.

FIG.4illustrates an example of a timing diagram400that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. In some examples, timing diagram400may implement aspects of wireless communications system100. In some cases, packet reception time is impacted by propagation time delay. For example, under free space condition, a Tx-Rx distance of 300 meters can cause 1 microsecond of propagation delay, which may impact how slot timing is determined (e.g., GNSS-synced NR V2X slot timing).

In some examples, timing diagram400may depict a timeline405of a synchronized device (e.g., UE115-a) and a timeline410of a synchronization-lost device (e.g., UE115-b). At415the synchronized device may transmit a packet via a physical sidelink channel (e.g., physical sidelink channel transmission305). The drifted slot timing of the synchronization-lost device may be represented at420.

In the illustrated example, the synchronization-lost device may receive the packet at425. As shown, propagation delay430may indicate a time delay between the synchronized device transmitting the packet at415and the synchronization-lost device receiving the packet at425. As depicted, the drifted slot timing420of the synchronization-lost device may be determined by the synchronization-lost device. Accordingly, the synchronization-lost device may determine the slot timing adjustment435based on the packet arriving at425and the drifted slot timing420.

FIG.5illustrates an example of a process500that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. In some examples, process500may implement aspects of wireless communications system100. As illustrated, process500may include UE115-g, UE115-h, and base station105-b, which may be examples of a UE115or a base station105, as described herein with reference toFIG.1.

At505, UE115-gmay lose synchronization with base station105-b. At505, UE115-gmay determine that it has lost a synchronization source.

At510, base station105-bmay optionally transmit control signaling to UE115-h. In some examples, UE115-hmay maintain communication with a synchronization source (e.g., base station105-b, or a satellite connection, or both).

At515, UE115-hmay transmit control signaling to UE115-g. In some examples, the control signaling of515may schedule a sidelink transmission via a physical sidelink channel.

At520, UE115-hmay transmit a sidelink transmission to UE115-gvia the physical sidelink channel. In some examples, the sidelink transmission of520may include one or more reference signals based on the control signaling.

At525, UE115-gmay determine the time synchronization, frequency synchronization, or both, based on the one or more reference signals from the sidelink transmission of520.

At530, UE115-gmay monitor the physical sidelink channel based on the time synchronization, the frequency synchronization, or both. In some examples, UE115-gmay maintain the time synchronization, the frequency synchronization, or both, based on the monitoring of the physical sidelink channel.

FIG.6shows a block diagram600of a device605that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The device605may be an example of aspects of a UE115as described herein. The device605may include a receiver610, a communications manager615, and a transmitter620. The device605may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses).

The communications manager615may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel, receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.), and monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals. The communications manager615may be an example of aspects of the communications manager910described herein.

FIG.7shows a block diagram700of a device705that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The device705may be an example of aspects of a device605, or a UE115as described herein. The device705may include a receiver710, a communications manager715, and a transmitter735. The device705may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses).

The communications manager715may be an example of aspects of the communications manager615as described herein. The communications manager715may include a control manager720, a synchronization manager725, and a monitoring manager730. The communications manager715may be an example of aspects of the communications manager910described herein.

The control manager720may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel. The synchronization manager725may receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.). The monitoring manager730may monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

The transmitter735may transmit signals generated by other components of the device705. In some examples, the transmitter735may be collocated with a receiver710in a transceiver module. For example, the transmitter735may be an example of aspects of the transceiver920described with reference toFIG.9. The transmitter735may utilize a single antenna or a set of antennas.

FIG.8shows a block diagram800of a communications manager805that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The communications manager805may be an example of aspects of a communications manager615, a communications manager715, or a communications manager910described herein. The communications manager805may include a control manager810, a synchronization manager815, a monitoring manager820, a location manager825, an estimation manager830, and a frequency manager835. Each of these modules may communicate, directly or indirectly, with one another (e.g., via one or more buses).

The control manager810may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel. In some examples, the control manager810may receive, based on the control signaling, the one or more reference signals in one or more reference signal symbol periods allocated within a resource of the physical sidelink channel for the sidelink transmission. In some cases, the physical sidelink channel includes a physical sidelink control channel or a physical sidelink shared channel. In some cases, the one or more reference signals include at least one demodulation reference signal (DMRS).

The synchronization manager815may receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.). In some examples, the synchronization manager815may receive synchronization confidence information from the second UE, where the time synchronization is determined using the one or more reference signals based on a synchronization confidence level satisfying a confidence threshold.

In some examples, the synchronization manager815may receive the synchronization confidence information in an application layer message from the second UE. In some examples, the synchronization manager815may receive, by the first UE, a second sidelink transmission including one or more reference signals from a third UE. In some examples, the synchronization manager815may receive second synchronization confidence information from a third UE, where the time synchronization is determined without using the one or more reference signals of the second sidelink transmission based on a second synchronization confidence level not satisfying a confidence threshold.

In some examples, the synchronization manager815may determine, by the first UE, a loss of synchronization with a synchronization source, where the time synchronization, the frequency synchronization, or both, are determined using the one or more reference signals of the sidelink transmission based on the loss of synchronization. In some examples, the synchronization manager815may determine the time synchronization based on. In some examples, the synchronization manager815may descramble a symbol of a first reference signal of the one or more reference signals. In some examples, the synchronization manager815may extract the descrambled symbol of the first reference signal in frequency domain. In some examples, the synchronization manager815may determine a channel impulse response based on the extracted descrambled symbol. In some examples, the synchronization manager815may determine a time delay estimate based on the determined channel impulse response.

The monitoring manager820may monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

The location manager825may receive location information of the second UE, where the time synchronization is determined based on the location information. In some examples, the location manager825may receive location information from a set of UEs including the second UE, where the time synchronization is determined based on the location information for the set of UEs.

The estimation manager830may estimate a distance between the second UE and the first UE based on the location information, where the time synchronization is determined based on the estimated distance. In some examples, the estimation manager830may receive a safety message including the location information. In some examples, the estimation manager830may estimate a statistical distance metric (e.g., average distance, mean distance, median distance, etc.) based on the location information for the set of UEs, where the time synchronization is determined based on the statistical distance metric.

In some examples, the estimation manager830may estimate a distance between the second UE and the first UE based on a transmission power associated with the sidelink transmission, where the time synchronization is determined based on the estimated distance. In some examples, the estimation manager830may estimate a pathloss based on the sidelink transmission. In some examples, the estimation manager830may estimate a distance between the first UE and the second UE based on the estimated pathloss, where the time synchronization is determined based on the estimated distance.

In some examples, the estimation manager830may determine a received signal strength associated with the sidelink transmission. In some examples, the estimation manager830may estimate the pathloss based on the determined received signal strength. In some examples, the estimation manager830may estimate a timing drift based on the one or more reference signals, where the time synchronization is determined based on the estimated timing drift.

In some cases, the location information includes a zone identification of the second UE. In some cases, the received signal strength includes a received signal strength indicator (RSSI), a reference signal received power (RSRP) associated with the sidelink transmission, or both.

The frequency manager835may estimate a frequency offset based on the one or more reference signals, where the frequency synchronization is determined based on the estimated frequency offset. In some examples, the frequency manager835may receive a set of sidelink transmissions from a set of UEs including the second UE. In some examples, the frequency manager835may determine a set of frequency offsets, where each frequency offset is determined for a respective sidelink transmission of the set of sidelink transmissions, and where the frequency synchronization is determined based on the set of frequency offsets.

FIG.9shows a diagram of a system900including a device905that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The device905may be an example of or include the components of device605, device705, or a UE115as described herein. The device905may include components for bi-directional voice and data communications including components for transmitting and receiving communications, including a communications manager910, an I/O controller915, a transceiver920, an antenna925, memory930, and a processor940. These components may be in electronic communication via one or more buses (e.g., bus945).

The communications manager910may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel, receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.), and monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

FIG.10shows a flowchart illustrating a method1000that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The operations of method1000may be implemented by a UE115or its components as described herein. For example, the operations of method1000may be performed by a communications manager as described with reference toFIGS.6through9. In some examples, a UE may execute a set of instructions to control the functional elements of the UE to perform the functions described herein. Additionally or alternatively, a UE may perform aspects of the functions described herein using special-purpose hardware.

At1005, the UE may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel. The operations of1005may be performed according to the methods described herein. In some examples, aspects of the operations of1005may be performed by a control manager as described with reference toFIGS.6through9.

At1010, the UE may receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.). The operations of1010may be performed according to the methods described herein. In some examples, aspects of the operations of1010may be performed by a synchronization manager as described with reference toFIGS.6through9.

At1015, the UE may monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals. The operations of1015may be performed according to the methods described herein. In some examples, aspects of the operations of1015may be performed by a monitoring manager as described with reference toFIGS.6through9.

FIG.11shows a flowchart illustrating a method1100that supports physical sidelink channel packet-based synchronization in accordance with aspects of the present disclosure. The operations of method1100may be implemented by a UE115or its components as described herein. For example, the operations of method1100may be performed by a communications manager as described with reference toFIGS.6through9. In some examples, a UE may execute a set of instructions to control the functional elements of the UE to perform the functions described herein. Additionally or alternatively, a UE may perform aspects of the functions described herein using special-purpose hardware.

At1105, the UE may communicate control signaling scheduling a sidelink transmission via a physical sidelink channel. The operations of1105may be performed according to the methods described herein. In some examples, aspects of the operations of1105may be performed by a control manager as described with reference toFIGS.6through9.

At1110, the UE may receive, by a first UE, the sidelink transmission including one or more reference signals from a second UE via the physical sidelink channel (e.g., based on the control signaling, based on data signaling, etc.). The operations of1110may be performed according to the methods described herein. In some examples, aspects of the operations of1110may be performed by a synchronization manager as described with reference toFIGS.6through9.

At1115, the UE may monitor the physical sidelink channel based on time synchronization, frequency synchronization, or both, determined using the one or more reference signals. The operations of1115may be performed according to the methods described herein. In some examples, aspects of the operations of1115may be performed by a monitoring manager as described with reference toFIGS.6through9.

At1120, the UE may receive, based on the control signaling, the one or more reference signals in one or more reference signal symbol periods allocated within a resource of the physical sidelink channel for the sidelink transmission. The operations of1120may be performed according to the methods described herein. In some examples, aspects of the operations of1120may be performed by a control manager as described with reference toFIGS.6through9.

At1125, the UE may receive synchronization confidence information from the second UE, where the time synchronization is determined using the one or more reference signals based on the synchronization confidence level satisfying a confidence threshold. The operations of1125may be performed according to the methods described herein. In some examples, aspects of the operations of1125may be performed by a synchronization manager as described with reference toFIGS.6through9.

At1130, the UE may receive location information of the second UE, where the time synchronization is determined based on the location information. The operations of1130may be performed according to the methods described herein. In some examples, aspects of the operations of1130may be performed by a location manager as described with reference toFIGS.6through9.

Aspect 1: A method for wireless communication, comprising: communicating control signaling scheduling a sidelink transmission via a physical sidelink channel; receiving, by a first UE, the sidelink transmission comprising one or more reference signals from a second UE via the physical sidelink channel; and monitoring the physical sidelink channel based at least in part on time synchronization, frequency synchronization, or both, determined using the one or more reference signals.

Aspect 2: The method of aspect 1, wherein receiving the sidelink transmission comprises: receiving the one or more reference signals in one or more reference signal symbol periods allocated within a resource of the physical sidelink channel for the sidelink transmission.

Aspect 3: The method of any of aspects 1 through 2, further comprising: receiving synchronization confidence information from the second UE, wherein the time synchronization is determined using the one or more reference signals based at least in part on a synchronization confidence level satisfying a confidence threshold.

Aspect 4: The method of aspect 3, wherein receiving the synchronization confidence information comprises: receiving the synchronization confidence information in an application layer message from the second UE.

Aspect 5: The method of any of aspects 3 through 4, further comprising: receiving, by the first UE, a second sidelink transmission comprising one or more reference signals from a third UE; and receiving second synchronization confidence information from the third UE, wherein the time synchronization is determined without using the one or more reference signals of the second sidelink transmission based at least in part on a second synchronization confidence level not satisfying the confidence threshold.

Aspect 6: The method of any of aspects 1 through 5, further comprising: receiving location information of the second UE, wherein the time synchronization is determined based at least in part on the location information.

Aspect 7: The method of aspect 6, further comprising: estimating a distance between the second UE and the first UE based at least in part on the location information, wherein the time synchronization is determined based at least in part on the estimated distance.

Aspect 8: The method of any of aspects 6 through 7, wherein the location information comprises a zone identification of the second UE.

Aspect 9: The method of any of aspects 6 through 8, wherein receiving the location information comprises: receiving a safety message comprising the location information.

Aspect 10: The method of any of aspects 1 through 9, further comprising: receiving location information from a plurality of UEs including the second UE, wherein the time synchronization is determined based at least in part on the location information for the plurality of UEs.

Aspect 11: The method of aspect 10, further comprising: estimating a statistical distance metric based at least in part on the location information for the plurality of UEs, wherein the time synchronization is determined based at least in part on the statistical distance metric.

Aspect 12: The method of any of aspects 1 through 11, further comprising: estimating a frequency offset based at least in part on the one or more reference signals, wherein the frequency synchronization is determined based at least in part on the estimated frequency offset.

Aspect 13: The method of any of aspects 1 through 12, further comprising: receiving a plurality of sidelink transmissions from a plurality of UEs including the second UE; and determining a plurality of frequency offsets, wherein each frequency offset is determined for a respective sidelink transmission of the plurality of sidelink transmissions, and wherein the frequency synchronization is determined based at least in part on the plurality of frequency offsets.

Aspect 14: The method of any of aspects 1 through 13, further comprising: estimating a distance between the second UE and the first UE based at least in part on a transmission power associated with the sidelink transmission, wherein the time synchronization is determined based at least in part on the estimated distance.

Aspect 15: The method of any of aspects 1 through 14, further comprising: estimating a pathloss based at least in part on the sidelink transmission.

Aspect 16: The method of aspect 15, further comprising: estimating a distance between the first UE and the second UE based at least in part on the estimated pathloss, wherein the time synchronization is determined based at least in part on the estimated distance.

Aspect 17: The method of any of aspects 15 through 16, further comprising: determining a received signal strength associated with the sidelink transmission; and estimating the pathloss based at least in part on the determined received signal strength.

Aspect 18: The method of aspect 17, wherein the received signal strength comprises a received signal strength indicator (RSSI), a reference signal received power (RSRP) associated with the sidelink transmission, or both.

Aspect 19: The method of any of aspects 1 through 18, further comprising: determining, by the first UE, a loss of synchronization with a synchronization source, wherein the time synchronization, the frequency synchronization, or both, are determined using the one or more reference signals of the sidelink transmission based at least in part on the loss of synchronization.

Aspect 20: The method of any of aspects 1 through 19, further comprising: determining the time synchronization based at least in part on; descrambling a symbol of a first reference signal of the one or more reference signals; extracting the descrambled symbol of the first reference signal in frequency domain; determining a channel impulse response based at least in part on the extracted descrambled symbol; and determining a time delay estimate based at least in part on the determined channel impulse response.

Aspect 21: The method of any of aspects 1 through 20, further comprising: estimating a timing drift based at least in part on the one or more reference signals, wherein the time synchronization is determined based at least in part on the estimated timing drift.

Aspect 22: The method of any of aspects 1 through 21, wherein the physical sidelink channel comprises a physical sidelink control channel or a physical sidelink shared channel.

Aspect 23: The method of any of aspects 1 through 22, wherein the one or more reference signals include at least one DMRS.

Aspect 25: An apparatus for wireless communication, comprising at least one means for performing a method of any of aspects 1 through 23.

Techniques described herein may be used for various wireless communications systems such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), single carrier frequency division multiple access (SC-FDMA), and other systems. The terms “system” and “network” are often used interchangeably. A code division multiple access (CDMA) system may implement a radio technology such as CDMA2000, Universal Terrestrial Radio Access (UTRA), etc. CDMA2000 covers IS-2000, IS-95, and IS-856 standards. IS-2000 Releases may be commonly referred to as CDMA2000 1×, 1×, etc. IS-856 (TIA-856) is commonly referred to as CDMA2000 1×EV-DO, High Rate Packet Data (HRPD), etc. UTRA includes Wideband CDMA (WCDMA) and other variants of CDMA. A time division multiple access (TDMA) system may implement a radio technology such as Global System for Mobile Communications (GSM). An orthogonal frequency division multiple access (OFDMA) system may implement a radio technology such as Ultra Mobile Broadband (UMB), Evolved UTRA (E-UTRA), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDM, etc.

The wireless communications system or systems described herein may support synchronous or asynchronous operation. For synchronous operation, the stations may have similar frame timing, and transmissions from different stations may be approximately aligned in time. For asynchronous operation, the stations may have different frame timing, and transmissions from different stations may not be aligned in time. The techniques described herein may be used for either synchronous or asynchronous operations.