Radio link monitoring for sidelink communication

Abstract

Methods, systems, and devices for wireless communication are described. A user equipment (UE) communicates with one or more other UEs via a sidelink communication link and performs radio link monitoring (RLM) of the sidelink. A first UE transmits (1005) a first message to a second UE via the sidelink communication link in a first transmission time interval (TTI). The first UE monitors (1010) the sidelink communication link during a second TTI after the first TTI for a feedback message from the second UE in response to the first message transmitted by the first UE. The monitoring is part of an RLM procedure for the sidelink communication link. Based on an unsuccessful reception of the feedback message in the second TTI, the first UE increments (1015) a counter.

Description

[0001] Cross-reference to related applications

[0002] This patent application claims the benefit of priority to U.S. Patent Application No. 16 / 994,116, titled "RADIO LINK MONITORING FOR SIDELINK COMMUNICATIONS," filed on August 14, 2020, by Wu et al., which claims the benefit of U.S. Provisional Patent Application No. 62 / 893,680, titled "RADIO LINK MONITORING FOR SIDELINK COMMUNICATIONS," filed on August 29, 2019, by Wu et al., and the above U.S. patent applications have been assigned to the assignee hereof. Technical field

[0003] The following generally relates to wireless communication and, more specifically, to radio link monitoring for sidelink communications. Background art

[0004] Wireless communication systems are widely deployed to provide various types of communication content, such as voice, video, packet data, messaging, broadcasting, etc. These systems can support communication with multiple users by sharing available system resources (e.g., time, frequency, and power). Examples of these multi-access systems include fourth-generation (4G) systems (e.g., Long-Term Evolution (LTE) systems, LTE-Advanced (LTE-A) systems, or LTE-A Pro systems) and fifth-generation (5G) systems (which may be referred to as New Radio (NR) systems). These systems may employ techniques such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDMA), or discrete Fourier transform - spread - orthogonal frequency division multiplexing (DFT-S-OFDM). A wireless multi-access communication system may include one or more base stations or one or more network access nodes, and each base station or network access node simultaneously supports communication for multiple communication devices, which may otherwise be referred to as user equipment (UE).

[0005] In some cases, UEs may communicate directly with each other without transmitting through a base station or through a relay point. Such communication may be referred to as sidelink, device-to-device (D2D), vehicle-to-vehicle (V2V) communication, or another term for sidelink communication. In sidelink communication, a synchronization signal block (SSB) or a channel state information reference signal (CSI-RS) from a transmitter (e.g., a UE) may not be available and may thus not be used by a UE for radio link monitoring (RLM) in a sidelink configuration. Summary of the invention

[0006] The described technology relates to improved methods, systems, devices, and apparatuses for supporting radio link monitoring for sidelink communication. Generally, the described technology provides a user equipment (UE) for monitoring radio link failure (RLF) in a sidelink communication link. The UE can communicate directly with one or more other UEs in the sidelink communication link. A first UE can send data transmissions to one or more other UEs, and the first UE can monitor for feedback (e.g., hybrid automatic repeat request (HARQ) feedback) from one or more other UEs. In the case where the HARQ feedback includes a failure, the first UE can increment a counter that the UE can use to determine RLF.

[0007] A method for wireless communication at a first UE is described. The method can include: sending a first message to a second UE via a sidelink communication link in a first transmission time interval, and during a second transmission time interval after the first transmission time interval, monitoring the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE, where the monitoring is part of a radio link monitoring process for the sidelink communication link, and as part of the radio link monitoring process, incrementing a counter based on an unsuccessful reception of the feedback message in the second transmission time interval.

[0008] An apparatus for wireless communication at a first UE is described. The apparatus can include a processor, a memory coupled to the processor, and instructions stored in the memory. The instructions can be executed by the processor to cause the apparatus to: send a first message to a second UE via a sidelink communication link in a first transmission time interval, and during a second transmission time interval after the first transmission time interval, monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE, where the monitoring is part of a radio link monitoring process for the sidelink communication link, and as part of the radio link monitoring process, increment a counter based on an unsuccessful reception of the feedback message in the second transmission time interval.

[0009] Describes another apparatus for wireless communication at a first UE. The apparatus may include units for the following operations: sending a first message to a second UE via a sidelink communication link in a first transmission time interval, and during a second transmission time interval after the first transmission time interval, monitoring the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE, wherein the monitoring is part of a radio link monitoring process for the sidelink communication link, and as part of the radio link monitoring process, incrementing a counter based on an unsuccessful reception of the feedback message in the second transmission time interval.

[0010] Describes a non-transitory computer-readable medium storing code for wireless communication at a first UE. The code may include instructions executable by a processor to perform the following operations: sending a first message to a second UE via a sidelink communication link in a first transmission time interval, and during a second transmission time interval after the first transmission time interval, monitoring the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE, wherein the monitoring is part of a radio link monitoring process for the sidelink communication link, and as part of the radio link monitoring process, incrementing a counter based on an unsuccessful reception of the feedback message in the second transmission time interval.

[0011] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for the following: receiving a plurality of negative feedback messages from the second UE, failing to decode a plurality of feedback messages from the second UE, incrementing a counter based on the total number of negative feedback messages and the number of feedback messages that failed to be decoded, and determining a radio link failure for the sidelink communication link based on the counter exceeding a threshold.

[0012] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for the following: receiving a plurality of negative feedback messages from the second UE, incrementing a counter based on the number of negative feedback messages, and determining a radio link failure for the sidelink communication link based on the counter exceeding a threshold.

[0013] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for the following: failing to decode a plurality of feedback messages from the second UE, incrementing a counter based on the number of feedback messages that failed to be decoded, and determining a radio link failure for the sidelink communication link based on the counter exceeding a threshold.

[0014] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: resetting a counter based on successful decoding of a Hybrid Automatic Repeat reQuest (HARQ) for a feedback message from a second UE.

[0015] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: receiving a HARQ feedback message before a threshold of a counter, and resetting the counter based on the HARQ feedback message.

[0016] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: monitoring one or more feedback messages from a second UE after incrementing the counter and before the threshold of the counter, and determining a radio link failure for a sidelink communication link based on not successfully receiving any of the one or more feedback messages from the second UE after the threshold of the counter is met.

[0017] In some examples of the methods, apparatuses, and non-transitory computer-readable media described herein, the threshold of the counter may be determined during establishment of the sidelink communication link based on the speed of the first UE or the second UE, the congestion level of the sidelink communication link, the priority level associated with data of a first message transmitted via the sidelink communication link, the application scenario in which the sidelink communication link can be used, or a combination thereof.

[0018] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: determining an upper limit and a lower limit for the threshold of the counter, where the upper limit and the lower limit may be based on pre-configured values, the priority level associated with data of a first message transmitted via the sidelink communication link, the application scenario in which the sidelink communication link can be used, or a combination thereof.

[0019] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: reconfiguring the threshold of the counter based on a change in the speed of the first UE or the second UE, the congestion level of the sidelink communication link, a change in the priority level associated with data transmitted via the sidelink communication link, or a combination thereof.

[0020] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: determining a threshold for a counter based on a preconfigured threshold, a value determined during establishment of a sidelink communication link, a priority level associated with data of a first message transmitted via the sidelink communication link, an indication from a base station, or a combination thereof.

[0021] In some examples of the methods, apparatuses, and non-transitory computer-readable media described herein, a value for a counter may be determined during establishment of a sidelink communication channel via radio resource control (RRC) signaling.

[0022] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: determining a radio link failure for a sidelink communication link based on the counter exceeding a threshold for the counter.

[0023] Some examples of the methods, apparatuses, and non-transitory computer-readable media described herein may also include operations, features, units, or instructions for: determining an unsuccessful reception of a feedback message based on a failure to successfully decode the feedback message in a second transmission time interval or in one or more subsequent feedback opportunities. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Examples of wireless communication systems in accordance with aspects of the present disclosure are shown.

[0025] Figure 2 Examples of wireless communication systems in accordance with aspects of the present disclosure are shown.

[0026] Figure 3A and Figure 3B Examples of slot diagrams in accordance with aspects of the present disclosure are shown.

[0027] Figure 4A and Figure 4B Examples of slot diagrams in accordance with aspects of the present disclosure are shown.

[0028] Figure 5 Examples of processing flows in accordance with aspects of the present disclosure are shown.

[0029] Figure 6 and Figure 7 Examples of block diagrams of devices in accordance with aspects of the present disclosure are shown.

[0030] Figure 8 Examples of block diagrams of communication managers in accordance with aspects of the present disclosure are shown.

[0031] Figure 9 A diagram showing a system including a device according to aspects of the present disclosure.

[0032] Figures 10 to 13 A flowchart depicting a method according to aspects of the present disclosure. Detailed Description

[0033] In some cases of a wireless communication system, a user equipment (UE) and a base station may communicate via a radio link. The UE or the base station may monitor the radio link to check for radio link failure (RLF). The UE or the base station may monitor for RLF by monitoring specific signals in the channel. For example, in the case of a 5G NR downlink, the UE may measure a synchronization signal block (SSB) or a channel state information reference signal (CSI-RS) for radio link monitoring (RLM).

[0034] In other cases, one or more UEs may communicate directly with each other in a sidelink communication channel. In some cases, this communication configuration may not include a UE relaying a message to other UEs via a base station. The sidelink communication configuration may be an example of device-to-device (D2D) communication, vehicle-to-vehicle (V2V) communication, or another example of sidelink communication in an Internet of Everything (IoE) communication system.

[0035] In the case of sidelink communication, performing RLM by measuring the SSB and CSI-RS of the channel (as in cellular links or communication from a base station to a UE) may not be reliable because multiple UEs may use the same SSB. Thus, the measurement of the SSB may not correctly correspond to the expected sidelink channel or UE. Additionally, the SSB transmission may be decoupled from data transmission, which may mean that the UE sending data may also not send the SSB. The CSI-RS may also not be present in the sidelink communication channel, and thus, the UE may not rely on the CSI-RS in sidelink communication for RLM.

[0036] RLM may be performed at a UE transmitting data in a sidelink communication configuration by monitoring and detecting hybrid automatic repeat request (HARQ) feedback (e.g., an acknowledgement (ACK) or a negative acknowledgement (NACK)). For example, a first UE may send a data packet to a second UE, and then the first UE may monitor for a HARQ response from the second UE, the HARQ response including an ACK / NACK indicating the status of the second UE's reception of the message.

[0037] RLM can be performed in both unicast and multicast sidelink scenarios. In a unicast configuration, RLM can be performed to determine whether the connection between two UEs communicating via the sidelink has failed. RLM can also be performed in a multicast sidelink scenario, where one UE can send the same data transmission to more than one other UE over the sidelink. In multicast, RLM can be performed to determine whether any of the one-to-many connections between the UE sending the data and the UEs receiving the data have failed. A failure in either the unicast configuration or the multicast configuration can be an RLF.

[0038] In a sidelink communication system, a first UE can send a message (e.g., a data packet) to one or more other UEs over a sidelink channel. The UE can monitor for ACK / NACK from the receiving UE in a particular transmission time interval (TTI) (e.g., a time slot). In some cases, the first UE can determine that a HARQ feedback failure has occurred in a particular TTI (e.g., a time slot). To perform RLM, the first UE can start a counter (e.g., a timer) in the TTI corresponding to the expected transmission of ACK / NACK from the receiving UE. When the counter counts up to a threshold number of failures, or if a successful HARQ feedback is not received when the timer counts to zero, the first UE can declare an RLF. The expiration of the counter can correspond to consecutive failures of HARQ feedback from the receiving UE. The failure of HARQ feedback can be due to the failure of the HARQ feedback transmission of the receiving UE, the failure of the sending UE to decode the HARQ feedback, or if the sending UE receives the HARQ feedback, successfully decodes it, and determines that the HARQ feedback is a NACK.

[0039] Thus, the UE can perform RLM based on HARQ feedback detection to achieve RLF in a sidelink communication system without SSB or CSI-RS. The UE can declare an RLF and then can take actions to restore the connection to resolve the RLF. Then, the UE can re-establish the connection between the UE and the receiving UE.

[0040] Aspects of the present disclosure are first described in the context of a wireless communication system. Aspects of the present disclosure are further described in the context of time slot diagrams and processing flows. Aspects of the present disclosure are further illustrated and described by reference to apparatus diagrams, system diagrams, and flowcharts related to radio link monitoring for sidelink communication.

[0041] Figure 1FIG. 0 illustrates an example of a wireless communication system 100 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. The wireless communication system 100 may include one or more base stations 105, one or more UEs 115, and a core network 130. In some examples, the wireless communication system 100 may 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 certain cases, the wireless communication system 100 may support enhanced broadband communication, ultra-reliable (e.g., mission-critical) communication, low-latency communication, communication with low-cost and low-complexity devices, or any combination thereof.

[0042] The base stations 105 may be dispersed throughout a geographic area to form the wireless communication system 100, and may be devices of different forms or having different capabilities. The base stations 105 and the UEs 115 may communicate wirelessly via one or more communication links 125. Each base station 105 may provide a coverage area 110, and the UEs 115 and the base stations 105 may establish one or more communication links 125 over the coverage area 110. The coverage area 110 may be an example of a geographic area over which the base stations 105 and the UEs 115 may support communication of signals according to one or more radio access technologies.

[0043] The UEs 115 may be dispersed throughout the coverage area 110 of the wireless communication system 100, and each UE 115 may be stationary, or mobile, or stationary or mobile at different times. The UEs 115 may be devices of different forms or having different capabilities. In Figure 1 FIG. 8 illustrates some example UEs 115. The UEs 115 described herein may be capable of communicating with various types of devices, such as other UEs 115, base stations 105, or network devices (e.g., core network nodes, relay devices, integrated access and backhaul (IAB) nodes, or other network devices), as Figure 1 FIG. 10 shows.

[0044] The base stations 105 may communicate with the core network 130, or with each other, or both. For example, the base stations 105 may interface with the core network 130 via one or more backhaul links 120 (e.g., via S1, N2, N3, or other interfaces). The base stations 105 may communicate with each other directly (e.g., directly between the base stations 105) or indirectly (e.g., via the core network 130) via the backhaul links 120 (e.g., via X2, Xn, or other interfaces), or both. In some examples, the backhaul links 120 may be or include one or more wireless links.

[0045] One or more base stations 105 described herein may include or may be referred to by those of ordinary skill in the art as base station transceivers, radio base stations, access points, radio transceivers, Node B, eNodeB (eNB), next-generation Node B or giga-Node B (both of which may be referred to as gNB), home Node B, home eNodeB, or other suitable terms.

[0046] UE 115 may include or may be referred to as a mobile device, wireless device, remote device, handheld device, or subscription device, or some other suitable term, where the "device" may also be referred to as a unit, station, terminal, or client, among other examples. UE 115 may also include or may be referred to as a personal electronic device such as a cellular phone, personal digital assistant (PDA), tablet computer, laptop computer, or personal computer. In some examples, UE 115 may include or be referred to as a wireless local loop (WLL) station, Internet of Things (IoT) device, Internet of Everything (IoE) device, or machine type communication (MTC) device, and in other examples, it may be implemented in various objects such as household appliances, or vehicles, meters, and other examples.

[0047] UE 115 described herein may be capable of communicating with various types of devices, such as other UE 115s (which may sometimes act as relays), as well as base stations 105 and network devices including macro eNBs or gNBs, small cell eNBs or gNBs, or relay base stations, and other examples, as Figure 1 shown.

[0048] UE 115 and base station 105 may communicate wirelessly with each other via one or more communication links 125 over one or more carriers. The term "carrier" may refer to a set of radio frequency spectrum resources having a defined physical layer structure for supporting communication link 125. For example, a carrier for communication link 125 may include a portion (e.g., bandwidth part (BWP)) of a radio frequency spectrum band that operates according to one or more physical layer channels for a given radio access technology (e.g., LTE, LTE-A, LTE-A Pro, NR). Each physical layer channel may carry acquisition signaling (e.g., synchronization signals, system information), control signaling for coordinating the operation of the carrier, user data, or other signaling. The wireless communication system 100 may use carrier aggregation or multi-carrier operation to support communication with UE 115. According to a carrier aggregation configuration, UE 115 may be configured with multiple downlink component carriers and one or more uplink component carriers. Carrier aggregation may be used with frequency division duplex (FDD) and time division duplex (TDD) component carriers.

[0049] In some examples (e.g., in a carrier aggregation configuration), a carrier may also have acquisition signaling or control signaling that coordinates operations for other carriers. A carrier may be associated with a frequency channel (e.g., an evolved universal mobile telecommunications system terrestrial radio access (E-UTRA) absolute radio frequency channel number (EARFCN)) and may be positioned according to a channel raster for discovery by UE 115. A carrier may operate in stand-alone mode, in which UE 115 may perform initial acquisition and connection via the carrier, or the carrier may operate in non-stand-alone mode, in which a different carrier (e.g., of the same or a different radio access technology) is used to anchor the connection.

[0050] The communication link 125 shown in the wireless communication system 100 may include an uplink transmission from UE 115 to base station 105 or a downlink transmission from base station 105 to UE 115. A carrier may carry downlink or uplink communication (e.g., in FDD mode) or may be configured to carry both downlink and uplink communication (e.g., in TDD mode).

[0051] A carrier may be associated with a particular bandwidth of the radio frequency spectrum, and in some examples, the carrier bandwidth may be referred to as the "system bandwidth" of the carrier or the wireless communication system 100. For example, the carrier bandwidth may be one of several defined bandwidths for a carrier of a particular radio access technology (e.g., 1.4, 3, 5, 10, 15, 20, 40, or 80 megahertz (MHz)). Devices of the wireless communication system 100 (e.g., base station 105, UE 115, or both) may have a hardware configuration that supports communication on a particular carrier bandwidth or may be configured to support communication on one of a set of carrier bandwidths. In some examples, the wireless communication system 100 may include a base station 105 or UE 115 that supports simultaneous communication via carriers associated with multiple carrier bandwidths. In some examples, each served UE 115 may be configured to operate on a portion (e.g., a sub-band, a BWP) or all of the carrier bandwidth.

[0052] The signal waveform transmitted via a carrier can be composed of multiple subcarriers (e.g., using multi-carrier modulation (MCM) techniques such as orthogonal frequency division multiplexing (OFDM) or discrete Fourier transform spread OFDM (DFT-S-OFDM)). In a system adopting MCM techniques, a resource element can be composed of a symbol period (e.g., the duration of a modulated symbol) and a subcarrier, where the symbol period and the subcarrier spacing are inversely proportional. The number of bits carried by each resource element can depend on the modulation scheme (e.g., the order of the modulation scheme, the coding rate of the modulation scheme, or both). Therefore, the more resource elements received by UE 115 and the higher the order of the modulation scheme, the higher the data rate for UE 115. Wireless communication resources can refer to a combination of radio frequency spectrum resources, time resources, and spatial resources (e.g., spatial layers or beams), and the use of multiple spatial layers can further improve the data rate or data integrity for communicating with UE 115.

[0053] One or more digital schemes for a carrier can be supported, where the digital scheme can include subcarrier spacing (Δf) and cyclic prefix. A carrier can be divided into one or more BWPs with the same or different digital schemes. In some examples, UE 115 can be configured with multiple BWPs. In some examples, a single BWP for a carrier can be active at a given time, and the communication for UE 115 can be restricted to one or more active BWPs.

[0054] The time intervals for base station 105 or UE 115 can be expressed as multiples of a basic time unit. For example, the basic time unit can refer to T s = 1 / (Δf max ·N f ) seconds of sampling period, where Δf max can represent the maximum supported subcarrier spacing, and N f can represent the maximum supported discrete Fourier transform (DFT) size. The time intervals of communication resources can be organized according to radio frames each having a specified duration (e.g., 10 milliseconds (ms)). Each radio frame can be identified by a system frame number (SFN) (e.g., ranging from 0 to 1023).

[0055] Each frame may include a plurality of consecutively numbered sub - frames or time slots, and each sub - frame or time slot may have the same duration. In some examples, a frame may be divided into (e.g., in the time domain) a plurality of sub - frames, and each sub - frame may be further divided into a number of time slots. Alternatively, each frame may include a variable number of time slots, and the number of time slots may depend on the sub - carrier spacing. Each time slot may include a number of symbol periods (e.g., depending on the length of the cyclic prefix before each symbol period). In some wireless communication systems 100, a time slot may be further divided into a plurality of mini - slots containing one or more symbols. In addition to the cyclic prefix, each symbol period may contain one or more (e.g., N f f

[0056] sub - frames, time slots, mini - slots, or symbols may be the smallest scheduling units (e.g., in the time domain) of the wireless communication system 100 and may be referred to as transmission time intervals (TTIs). In some examples, the TTI duration (e.g., the number of symbol periods in a TTI) may be variable. Additionally or alternatively, the smallest scheduling unit of the wireless communication system 100 may be dynamically selected (e.g., in a burst of shortened TTIs (sTTIs)).

[0057] Physical channels may be multiplexed on a carrier according to various techniques. For example, one or more of time - division multiplexing (TDM) techniques, frequency - division multiplexing (FDM) techniques, or hybrid TDM - FDM techniques may be used to multiplex physical control channels and physical data channels on a downlink carrier. A control region for a physical control channel (e.g., a control resource set (CORESET)) may be defined by a number of symbol periods and may extend across the system bandwidth of the carrier or a subset of the system bandwidth. One or more control regions (e.g., CORESETs) may be configured for a set of UEs 115. For example, one or more of the UEs 115 may monitor or search a control region for control information according to one or more search space sets, and each search space set may include one or more control channel candidates in one or more aggregation levels arranged in a cascaded manner. The aggregation level for a control channel candidate may refer to a number of control channel resources (e.g., control channel elements (CCEs)) associated with the encoded information for a control information format having a given payload size. The search space set may include a common search space set configured to send control information to a plurality of UEs 115 and a UE - specific search space set for sending control information to a specific UE 115.

[0058] Each base station 105 can provide communication coverage via one or more cells (e.g., macro cells, small cells, hotspots, or other types of cells, or any combination thereof). The term "cell" can refer to a logical communication entity for communicating with the base station 105 (e.g., via a carrier), and can be associated with an identifier for distinguishing neighbor cells (e.g., a physical cell identifier (PCID), a virtual cell identifier (VCID), or others). In some examples, a cell can also refer to a geographic coverage area 110 or a portion of the geographic coverage area 110 (e.g., a sector) on which the logical communication entity operates. Depending on various factors, such as the capabilities of the base station 105, such a cell can cover a smaller area (e.g., a structure, a subset of a structure) to a larger area. For example, a cell can be or include a building, a subset of a building, or an external space between or overlapping with the geographic coverage areas 110, and other examples.

[0059] Macro cells typically cover a relatively large geographic area (e.g., with a radius of several kilometers) and can allow UEs 115 with service subscriptions to have unrestricted access to the network provider supporting the macro cell. Compared with macro cells, small cells can be associated with base stations 105 with lower power, and small cells can operate in the same or different (e.g., licensed, unlicensed) frequency bands as macro cells. A small cell can provide unrestricted access to UEs 115 through a service subscription with the network provider, or can provide restricted access to UEs 115 associated with the small cell (e.g., UEs 115 in a closed subscriber group (CSG), UEs associated with users in a home or office). The base station 105 can support one or more cells and can also use one or more component carriers to support communication through one or more cells.

[0060] In some examples, a carrier can support multiple cells and can be configured with different cell types according to different protocol types that can provide access for different types of devices (e.g., MTC, narrowband IoT (NB-IoT), enhanced mobile broadband (eMBB)).

[0061] In some examples, the base station 105 can be movable and thus provide communication coverage for a mobile geographic coverage area 110. In some examples, different geographic coverage areas 110 associated with different technologies can overlap, but different geographic coverage areas 110 can be supported by the same base station 105. In other examples, overlapping geographic coverage areas 110 associated with different technologies can be supported by different base stations 105. The wireless communication system 100 can include, for example, a heterogeneous network where different types of base stations 105 provide coverage for various geographic coverage areas 110 using the same or different radio access technologies.

[0062] The wireless communication system 100 may support synchronous or asynchronous operations. For synchronous operations, the base stations 105 may have similar frame timings, and transmissions from different base stations 105 may be approximately aligned in time. For asynchronous operations, the base stations 105 may have different frame timings, and in some examples, transmissions from different base stations 105 may not be aligned in time. The techniques described herein may be used for synchronous or asynchronous operations.

[0063] Some UEs 115, such as MTC or IoT devices, may be low-cost or low-complexity devices and may provide automated communication between machines (e.g., via machine-to-machine (M2M) communication). M2M communication or MTC may refer to data communication technologies that allow devices to communicate with each other or with the base station 105 without human intervention. In some examples, M2M communication or MTC may include communication from a device that integrates sensors or meters to measure or capture information and relay such information to a central server or application that utilizes the information or presents the information to a person interacting with the application. Some UEs 115 may be designed to collect information or implement automated behavior of machines or other devices. Example applications of MTC devices include smart metering, inventory monitoring, water level monitoring, equipment monitoring, medical monitoring, wildlife monitoring, weather and geological event monitoring, fleet management and tracking, remote security sensing, physical access control, and transaction-based commercial charging.

[0064] Some UEs 115 may be configured to operate in power-saving modes, such as half-duplex communication (e.g., a mode that supports one-way communication via transmission or reception rather than simultaneous transmission and reception). In some examples, half-duplex communication may be performed at a reduced peak rate. Other power-saving techniques for UEs 115 include entering a power-saving deep sleep mode when not participating in active communication and operating on a limited bandwidth (e.g., according to narrowband communication). For example, some UEs 115 may be configured for operation using a narrowband protocol type associated with a defined portion or range within a carrier, within a guard band of the carrier, or outside the carrier (e.g., a set of subcarriers or resource blocks (RBs)).

[0065] The wireless communication system 100 can be configured to support ultra-reliable communication or low-latency communication or various combinations thereof. For example, the wireless communication system 100 can be configured to support ultra-reliable low-latency communication (URLLC) or mission-critical communication. The UE 115 can be designed to support ultra-reliability, low latency, or critical functions (e.g., mission-critical functions). Ultra-reliable communication can include private communication or group communication and can be supported by one or more mission-critical services (e.g., mission-critical push-to-talk (MCPTT), mission-critical video (MCVideo), or mission-critical data (MCData)). Support for mission-critical functions can include prioritization of services, and mission-critical services can be used for public safety or general commercial applications. The terms ultra-reliable, low-latency, mission-critical, and ultra-reliable low-latency can be used interchangeably herein.

[0066] In some examples, the UE 115 is also capable of directly communicating with other UEs 115 via a device-to-device (D2D) communication link 135 (e.g., using a peer-to-peer (P2P) or D2D protocol). One or more UEs 115 utilizing D2D communication can be located within the geographical coverage area 110 of the base station 105. Other UEs 115 in such a group of UEs can be located outside the geographical coverage area 110 of the base station 105, or conversely, cannot receive transmissions from the base station 105. In some examples, a group of UEs 115 communicating via D2D communication can utilize a one-to-many (1:M) system, where each UE 115 transmits to each other UE 115 in the group. In some examples, the base station 105 facilitates the scheduling of resources for D2D communication. In other cases, D2D communication is performed between UEs 115 without involving the base station 105.

[0067] In some systems, the D2D communication link 135 can be an example of a communication channel (e.g., a sidelink communication channel) between vehicles (e.g., the UE 115). In some examples, vehicles can communicate using vehicle-to-everything (V2X) communication, vehicle-to-vehicle (V2V) communication, or some combination thereof. Vehicles can signal information related to traffic conditions, signal scheduling, weather, safety, emergencies, or any other information related to the V2X system. In some examples, vehicles in a V2X system can communicate with roadside infrastructure such as roadside units via vehicle-to-network (V2N) communication through one or more network nodes (e.g., the base station 105), or communicate with the network, or communicate with both the roadside infrastructure and the network.

[0068] The core network 130 may provide user authentication, access authorization, tracking, Internet Protocol (IP) connectivity, and other access, routing, or mobility functions. The core network 130 may be an evolved packet core (EPC) or a 5G core (5GC), which may include at least one control plane entity that manages access and mobility (e.g., a mobility management entity (MME), an access and mobility management function (AMF)) and at least one user plane entity (UPF) that routes packets or interconnects to an external network (e.g., a serving gateway (S-GW), a packet data network (PDN) gateway (P-GW), or a user plane function (UPF)). The control plane entity may manage non-access stratum (NAS) functions, such as mobility, authentication, and bearer management of the UE 115 served by the base station 105 associated with the core network 130. User IP packets may be transmitted through the user plane entity. The user plane entity may provide IP address allocation and other functions. The user plane entity may be connected to the network operator IP services 150. The operator IP services 150 may include access to the Internet, an intranet, an IP multimedia subsystem (IMS), or packet-switched streaming services.

[0069] Some network devices (such as the base station 105) may include subcomponents such as the access network entity 140, which may be an example of an access node controller (ANC). Each access network entity 140 may communicate with the UE 115 through one or more other access network transmission entities 145, which may be referred to as radio heads, intelligent radio heads, or transmit / receive points (TRPs). Each access network transmission entity 145 may include one or more antenna panels. In some configurations, the various functions of each access network entity 140 or the base station 105 may be distributed across various network devices (e.g., radio heads and ANCs) or consolidated into a single network device (e.g., the base station 105).

[0070] The wireless communication system 100 may operate using one or more frequency bands, typically in the range of 300 megahertz (MHz) to 300 gigahertz (GHz). Generally speaking, the region from 300 MHz to 3 GHz is referred to as the ultra-high frequency (UHF) region or the decimeter band because the length of the wavelength range is from approximately one decimeter to one meter. UHF waves may be blocked or redirected by buildings and environmental features. However, the UHF waves may penetrate structures sufficiently for macrocells to provide service to the UE 115 located indoors. Compared to transmissions using smaller frequencies and longer waves in the high frequency (HF) or very high frequency (VHF) portions with a spectrum below 300 MHz, UHF wave transmissions may be associated with smaller antennas and shorter ranges (e.g., less than 100 km).

[0071] The wireless communication system 100 may also operate in the super high frequency (SHF) region using a frequency band from 3 GHz to 30 GHz (also known as the centimeter band), or in the extremely high frequency (EHF) region of the spectrum (e.g., from 30 GHz to 300 GHz) (also known as the millimeter band). In some examples, the wireless communication system 100 may support millimeter wave (mmW) communication between the UE 115 and the base station 105, and the EHF antennas of the corresponding devices may be smaller and closer together than UHF antennas. In some examples, this may facilitate the use of antenna arrays within the device. However, the propagation of EHF transmissions may suffer greater atmospheric attenuation and shorter ranges than SHF or UHF transmissions. The techniques disclosed in this application may be used in transmissions using one or more different frequency regions, and the specified use of frequency bands across these frequency regions may vary according to the country or regulatory body.

[0072] The wireless communication system 100 may use both licensed and unlicensed radio frequency spectrum bands. For example, the wireless communication system 100 may employ licensed-assisted access (LAA), LTE-unlicensed (LTE-U) radio access technology, or NR technology in an unlicensed frequency band such as the 5 GHz industrial, scientific, and medical (ISM) band. When operating in an unlicensed radio frequency spectrum band, devices such as the base station 105 and the UE 115 may employ carrier sensing for collision detection and avoidance. In some examples, operation in the unlicensed frequency band may be based on a carrier aggregation configuration in combination with a component carrier operating in a licensed frequency band (e.g., LAA). Operations in the unlicensed spectrum may include downlink transmissions, uplink transmissions, peer-to-peer (P2P) transmissions, or device-to-device (D2D) transmissions, among other examples.

[0073] The base station 105 or the UE 115 may be equipped with multiple antennas, which may be used to employ techniques such as transmit diversity, receive diversity, multiple-input multiple-output (MIMO) communication, or beamforming. The antennas of the base station 105 or the UE 115 may be located within one or more antenna arrays or antenna panels, which may support MIMO operation or transmit or receive beamforming. For example, one or more base station antennas or antenna arrays may be co-located at an antenna assembly (e.g., an antenna tower). In some examples, the antennas or antenna arrays associated with the base station 105 may be located in a variety of geographical locations. The base station 105 may have an antenna array with multiple rows and columns with antenna ports, and the base station 105 may use the antenna array to support beamforming for communication with the UE 115. Similarly, the UE 115 may have one or more antenna arrays, which may support various MIMO or beamforming operations. Additionally or alternatively, the antenna panel may support radio frequency beamforming for signals transmitted via the antenna ports.

[0074] Base station 105 or UE 115 may use MIMO communication to utilize multipath signal propagation and increase spectral efficiency by transmitting or receiving multiple signals via different spatial layers. These techniques may be referred to as spatial multiplexing. For example, multiple signals may be transmitted by a transmitting device via different antennas or different combinations of antennas. Similarly, multiple signals may be received by a receiving device via different antennas or different combinations of antennas. Each of the multiple signals may be referred to as a separate spatial stream and may carry bits associated with the same data stream (e.g., the same codeword) or different data streams (e.g., different codewords). Different spatial layers may be associated with different antenna ports for channel measurement and reporting. MIMO techniques include single-user MIMO (SU-MIMO) (where multiple spatial layers are transmitted to the same receiving device) and multi-user MIMO (MU-MIMO) (where multiple spatial layers are transmitted to multiple devices).

[0075] Beamforming, which may also be referred to as spatial filtering, directional transmission, or directional reception, is a signal processing technique that may be used at a transmitting device or a receiving device (e.g., base station 105, UE 115) to form or direct an antenna beam (e.g., a transmit beam, a receive beam) along a spatial path between the transmitting device and the receiving device. Beamforming may be achieved by combining signals transmitted via the antenna elements of an antenna array such that some signals propagating in a particular direction with respect to the antenna array experience constructive interference while other signals experience destructive interference. The adjustment of the signals transmitted via the antenna elements may include the transmitting device or the receiving device applying an amplitude offset, a phase offset, or both to the signals carried via the antenna elements associated with the device. The adjustment associated with each antenna element may be defined by a set of beamforming weights associated with a particular direction (e.g., with respect to the antenna array of the transmitting device or the receiving device, or with respect to some other direction).

[0076] Base station 105 or UE 115 may use beam scanning techniques as part of a beamforming operation. For example, base station 105 may use multiple antennas or antenna arrays (e.g., antenna panels) to perform beamforming operations for directional communication with UE 115. Some signals (e.g., synchronization signals, reference signals, beam selection signals, or other control signals) may be transmitted by base station 105 multiple times in different directions. For example, base station 105 may transmit signals according to different sets of beamforming weights associated with different transmission directions. Transmissions in different beam directions may be used (e.g., by a transmitting device such as base station 105, or by a receiving device such as UE 115) to identify a beam direction for later transmission or reception by base station 105.

[0077] Some signals, such as data signals associated with a particular receiving device, can be transmitted by base station 105 in a single beam direction (e.g., the direction associated with a receiving device such as UE 115). In some examples, the beam direction associated with a transmission along a single beam direction can be determined based on signals transmitted in one or more beam directions. For example, UE 115 can receive one or more signals transmitted by base station 105 in different directions and can report to base station 105 an indication of the signal that UE 115 received with the highest signal quality or in other respects an acceptable signal quality.

[0078] In some examples, multiple beam directions can be used to perform transmissions by a device (e.g., by base station 105 or UE 115), and the device can use a combination of digital precoding or radio frequency beamforming to generate a combined beam for transmission (e.g., from base station 105 to UE 115). UE 115 can report feedback indicating precoding weights for one or more beam directions, and the feedback can correspond to a configured number of beams across the system bandwidth or one or more subbands. Base station 105 can transmit reference signals (e.g., cell-specific reference signals (CRS), channel state information reference signals (CSI-RS)), which can be precoded or not precoded. UE 115 can provide feedback for beam selection, which can be a precoding matrix indicator (PMI) or codebook-based feedback (e.g., multi-panel type codebook, linear combination type codebook, port selection type codebook). Although these techniques are described with reference to signals transmitted by base station 105 in one or more directions, UE 115 can employ similar techniques to transmit signals multiple times in different directions (e.g., for identifying beam directions for subsequent transmissions or receptions by UE 115), or for transmitting signals in a single direction (e.g., for transmitting data to a receiving device).

[0079] A receiving device (e.g., UE 115) may attempt multiple receive configurations (e.g., directional listening) when receiving various signals (such as synchronization signals, reference signals, beam selection signals, or other control signals) from a base station 105. For example, the receiving device may attempt multiple receive directions by receiving via different antenna subarrays, by processing received signals according to different antenna subarrays, by receiving according to different sets of receive beamforming weights (e.g., different sets of directional listening weights) applied to signals received at multiple antenna elements of an antenna array, or by processing received signals according to different sets of receive beamforming weights applied to signals received at multiple antenna elements of an antenna array. Any of the above ways may be referred to as “listening” according to different receive configurations or receive directions. In some examples, the receiving device may use a single receive configuration to receive along a single beam direction (e.g., when receiving a data signal). The single receive configuration may be aligned in a beam direction determined based on listening according to different receive configuration directions (e.g., a beam direction determined to have the highest signal strength, the highest signal-to-noise ratio (SNR), or otherwise acceptable signal quality based on listening according to multiple beam directions).

[0080] The wireless communication system 100 may be a packet-based network operating according to a layered protocol stack. In the user plane, the communication of the packet data convergence protocol (PDCP) layer may be IP-based. The radio link control (RLC) layer may perform packet segmentation and reassembly for communication over logical channels. The media access control (MAC) layer may perform prioritization and multiplexing of logical channels to transport channels. The MAC layer may also use error detection techniques, error correction techniques, or both to support retransmission at the MAC layer to improve link efficiency. In the control plane, the radio resource control (RRC) protocol layer may provide the establishment, configuration, and maintenance of an RRC connection between the UE 115 and the base station 105 or the core network 130 that supports radio bearers for user plane data. At the physical layer, transport channels may be mapped to physical channels.

[0081] UE 115 and base station 105 may support retransmission of data to increase the likelihood of successful data reception. Hybrid Automatic Repeat reQuest (HARQ) feedback is a technique used to increase the likelihood of correctly receiving data on communication link 125. HARQ may include a combination of error detection (e.g., using Cyclic Redundancy Check (CRC)), Forward Error Correction (FEC), and retransmission (e.g., Automatic Repeat reQuest (ARQ)). HARQ can improve throughput at the MAC layer under poor radio conditions (e.g., low signal-to-noise ratio conditions). In some examples, a device may support same-slot HARQ feedback, where the device may provide HARQ feedback in a specific slot for data received in previous symbols in that slot. In other cases, the device may provide HARQ feedback in a subsequent slot or according to some other time interval.

[0082] One or more UEs 115 may communicate directly with each other via a communication link in a sidelink communication configuration. UE115 may perform RLM on a communication link with one or more other UEs 115. RLM on the cellular link between base station 105 and UE 115 generally may include: monitoring SSBs transmitted by UE 115 or base station 105, or measuring the radio link using CSI-RS transmitted by UE 115 or base station 105. In sidelink communication between one or more UEs 115, SSBs and CSI-RS may not be available or reliable.

[0083] Accordingly, based on the data transmission of UE 115, UE 115 may perform RLM in a sidelink configuration to monitor for HARQ feedback from other UEs 115. In one case, based on a first HARQ feedback failure, UE 115 may start a counter to quantify the number of HARQ feedback failures corresponding to several HARQ feedback failures. For each received HARQ feedback failure, UE 115 may increment the counter. When the counter reaches a threshold, UE 115 may declare an RLF in the sidelink channel. In one example, UE 115 may initiate an action to reconnect with one or more other UEs 115.

[0084] In another case, based on a first HARQ feedback failure, UE 115 may start a timer to quantify the amount of time corresponding to several HARQ feedback failures. The timer may continue to count throughout each received HARQ feedback failure period. When the timer reaches a threshold, UE 115 may declare an RLF in the sidelink channel. In some cases, UE 115 may initiate an action to reconnect with one or more other UEs 115.

[0085] Figure 2FIG. 0 shows an example of a wireless communication system 200 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. In some examples, wireless communication system 200 may implement aspects of wireless communication system 100. Wireless communication system 200 may include base station 105-a and UEs 115-a and 115-b. Base station 105-a may be an example of base station 105 as described with reference to Figure 1 UEs 115-a and 115-b may be examples of UEs 115 as described with reference to Figure 1 Base station 105-a may serve one or more UEs 115 within coverage area 110-a. Base station 105-a may communicate with one or more UEs 115 via communication link 215. UEs 115-a and 115-b may be connected in a sidelink communication configuration and may communicate via sidelink channel 205. Wireless communication system 200 may show an example of a unicast sidelink configuration, but the RLM techniques described herein may also be applied to multicast or broadcast sidelink configurations.

[0086] UE 115-a may send first message 210 via sidelink channel 205. Message 210 may be sent by UE 115-a in a first TTI (e.g., in slot N). Based on the transmission of first message 210, UE 115-a may expect to receive HARQ feedback, e.g., an ACK / NACK message, from UE 115-b. UE 115-a may monitor for HARQ feedback in a second TTI (e.g., slot N+K). Based on monitoring for HARQ feedback in the second TTI, UE 115-a may start a counter (e.g., a timer) that may count the number of consecutive HARQ feedback failures. When the counter counts to a threshold number of HARQ failures or the timer counts down to zero, UE 115-a may declare RLF.

[0087] Sidelink channel 205 may be an example of a sidelink communication link. A sidelink communication link may include one or more channels. For example, UE 115-a may send message 210 in a physical sidelink shared channel (PSSCH). UE 115-b may send HARQ feedback in a physical sidelink feedback channel (PSFCH).

[0088] In some cases, UE 115-a may receive an ACK from UE 115-b via the sidelink channel 205 in the expected TTI and correctly decode the ACK. In other cases, the HARQ feedback from UE 115-b may fail. The HARQ feedback failure may be due to one or more reasons. In one case, UE 115-b may send a NACK to UE 115-a. In some cases, UE 115-a may expect HARQ feedback from UE 115-b, but UE 115-a may be unable to decode the feedback. This can be regarded as a HARQ feedback failure. In other cases, UE 115-a may correctly decode the feedback, but the feedback may be a NACK. This NACK can also be regarded as a HARQ feedback failure.

[0089] If the HARQ feedback is not received in slot N+K, UE 115-a may start a counter or a timer in slot N+K. If the HARQ feedback is not received in slot N+K or another later slot before the counter meets the threshold or the timer counts down to zero, UE 115-a may declare RLF. If the HARQ feedback is received before the counter reaches the threshold (or before the timer expires), UE 115-a may reset the counter or the timer. UE 115-a may restart the counter or the timer at the next failed HARQ feedback instance in the slot.

[0090] In the case where UE 115-a declares RLF of the sidelink 205 based on the counter or the timer reaching the threshold, UE 115-a may perform actions to re-establish a connection with UE 115-b. This may involve communicating with the base station 105-a via the link 215-a to re-establish contact with UE 115-b via the communication link 215-b.

[0091] The counter (or the timer associated with the counter) started by UE 115-a based on monitoring for HARQ feedback may have a specified threshold determined based on one or more parameters. In some cases, the counter may have a common pre-configured threshold (e.g., common to one or more UEs 115, cells, or base stations 105). The pre-configured threshold may be based on wireless communication standards. For example, the pre-configured threshold may be determined based on information elements (IEs) (e.g., SL-preconfigruationNRIEs, including sl-MaxNumConsecutiveDTX-r16).

[0092] In other cases, the threshold of the counter can be negotiated between UE 115-a and UE 115-b using IEs during the establishment of a sidelink connection (e.g., via RRC signaling). The counter value can be negotiated based on one or more UE communication parameters, such as the speed of UE 115 (the speed of UE 115-a, the speed of UE 115-b, or the relative speed between UE 115-a and UE 115-b), congestion level, data priority, application type, or other communication parameters.

[0093] In some cases, the threshold can be configured at UE 115-a based on signaling from base station 105-a or other devices (e.g., UE 115-b or other UEs 115). For example, base station 105-a can send a system information block (SIB) message (e.g., sl-maxNumConsecutiveDTX) indicating the threshold. The SIB message can include RRC signaling. In other cases, the threshold can be indicated to UE 115-a in an IE (e.g., SL-ConfigDedicatedNR IE).

[0094] For example, in cases of low congestion level, the counter or timer can have a larger value. The congestion level can be measured based on channel quality measurements, such as reference signal received power (RSRP), reference signal received quality (RSRQ), channel busy rate (CBR), or other congestion measurements. The congestion measurements can be determined by UE 115-a, UE 115-b, and negotiated between UE 115-a and UE 115-b. In other cases, based on measurements performed by base station 105-a or measurements signaled to base station 105-a, base station 105-a can signal the congestion measurement to one or both of UE 115-a and 115-b. Then, the congestion measurement can be mapped to a timer value.

[0095] The counter or timer can also be mapped based on data priority. For example, if the data priority is low, the counter can have a larger value. The data priority can be based on the relative priority of a message sent from the transmitting UE to the receiving UE compared to other data transmissions of the transmitting UE or other communications within the system. If the data transmission is for secure message transmission, the counter can also have a smaller value. Secure message transmission can be an example of a high-priority message.

[0096] A counter or timer may also have a value within a specified range and not have different values. The counter value range may have an upper bound and a lower bound. The bounds may be predefined or preconfigured (e.g., based on a wireless communication standard or control signaling from a base station, or both). The range may also be negotiated between UE 115-a and UE 115-b in a sidelink system. The range of the counter value may depend on the data priority or the type of application for data transmission. For example, if the data type is a high-priority security message, the counter may still have a range of values, but the upper bound of the range of values for the timer may be smaller, so that the counter has a smaller value.

[0097] The value of the counter or timer may be indicated by a network node (e.g., a base station). This may apply to cases where sidelink communication is scheduled by the base station. For example, the base station may select a counter value based on UE speed or congestion level. The UE speed or congestion level may be reported to the base station by one or more UEs.

[0098] A specific TTI may be monitored based on HARQ feedback from UE 115-a for a transmission from UE 115-b to activate the counter or timer. Each time UE 115-a detects a HARQ feedback failure, the counter may be incremented. The counter may be incremented until a threshold number of HARQ feedback failures occur, at which point UE 115-b may declare RLF. In other cases, the counter may be or correspond to a timer, and the counter may count down based on the value of the timer. The value of the timer may be determined based on the same parameters as the value of the counter (e.g., preconfiguration, UE speed, congestion level, application type, data priority, etc.).

[0099] Figure 3A An example of a time slot diagram 301 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure is shown. In some examples, the time slot diagram 301 may implement aspects of wireless communication systems 100 and 200. A UE 115 operating in a sidelink configuration may receive and send messages based on the time slot diagram 301. A first transmitting UE 115 (e.g., UE 115-a as described with reference to Figure 2 may send a data transmission 310 according to configuration 305-a. Configuration 305-a may be an example of a PSSCH. A second receiving UE 115 (e.g., UE 115-b) may send a HARQ feedback transmission 315 based on configuration 305-b. Configuration 305-b may be an example of a PSFCH. Based on monitoring for the HARQ feedback transmission 315 received from the receiving UE 115 (e.g., UE 115-b), the transmitting UE 115 (e.g., UE 115-a) may start a timer 325-a.

[0100] The first UE 115 (e.g., UE 115-a) may send a data transmission 310 in a particular time slot according to transmission configuration 305-a. For example, UE 115 may send a first data transmission 310-a. The data transmission 310-a may correspond to a particular time slot or TTI N. The data transmission 310-a may be sent to the second UE 115 in a sidelink communication channel (e.g., sidelink channel 205). Based on sending the data transmission 310-a, the first UE 115 may monitor for a HARQ feedback transmission 315-a from the second UE 115 in time slot 320-a.

[0101] In some cases, the HARQ feedback transmission 315-a may correspond to a HARQ feedback failure. The HARQ feedback failure may be because the HARQ feedback transmission 315-a is decoded by the first UE and is a NACK transmission. In other cases, the HARQ feedback failure may be that the HARQ feedback transmission 315-a is not correctly decoded at the first UE 115. In any case of a HARQ feedback failure, based on the HARQ feedback failure, the first UE 115 may start a timer 325.

[0102] For example, the HARQ feedback failure may occur in time slot 320-a. Based on this failure, the first UE 115 may start a timer 325-a. The timer 325-a may have a time length based on pre-configuration or based on one or more parameters (e.g., data priority, speed of the first UE or the second UE, congestion level, or another UE parameter). The timer 325-a may count down for the configured time length until a threshold time period (counting down to zero, or counting up to the configured time) or until a HARQ feedback is successfully received.

[0103] If the timer 325-a counts down to zero before the first UE 115 successfully receives a HARQ feedback, the first UE 115 may declare that an RLF has occurred.

[0104] Optionally, a HARQ feedback may be received before the timer expires. Figure 3B An example of a time slot diagram 302 that supports radio link monitoring for sidelink communication according to aspects of the present disclosure is shown. In some examples, the time slot diagram 301 may implement aspects of wireless communication systems 100 and 200. A UE 115 operating in a sidelink configuration may receive and send messages based on the time slot diagram 302. The first transmitting UE 115 (e.g., as referred to Figure 2The described UE 115 - a) may send a data transmission 310 according to configuration 305 - c. Configuration 305 - c may be an example of a PSSCH. A second receiving UE 115 (e.g., UE 115 - b) may send a HARQ feedback transmission 315 based on configuration 305 - d. Configuration 305 - c may be an example of a PSFCH. Based on monitoring for the HARQ feedback transmission 315 received from the receiving UE 115 (e.g., UE 115 - b), the sending UE 115 (e.g., UE 115 - a) may start a timer 325 - b.

[0105] According to transmission configuration 305 - c, the first UE 115 (e.g., UE 115 - a) may send a data transmission 310 in a particular time slot. For example, UE 115 may send a first data transmission 310 - b. The data transmission 310 - b may correspond to a particular time slot N. The data transmission 310 - b may be sent to the second UE 115 in a sidelink communication channel (e.g., sidelink channel 205). Based on sending the data transmission 310 - b, UE 115 may monitor for a HARQ feedback transmission 315 - b from the second UE 115 in time slot 320 - b (e.g., time slot N + K).

[0106] Due to a HARQ feedback failure, the first UE 115 may not receive the expected HARQ feedback in time slot 320 - c. Based on this failure, the first UE 115 may start a timer 325 - b, which may count down to zero until interrupted by a successful HARQ feedback. The length of the timer 325 - b may be determined by one or more parameters. As the timer counts down, the first UE 115 may also send another data transmission 310 - c, and based on the HARQ feedback transmission 315 - c from the second UE 115, may expect a HARQ feedback in time slot 320 - d. The first UE 115 may also not correctly receive the HARQ feedback transmission 315 - c in time slot 320 - d (or the HARQ feedback transmission 315 - c is a NACK), and the timer 325 - b may continue to count down. The first UE 115 may send another data transmission 310 - d to the second UE 115 via the sidelink communication channel. The second UE 115 may send a HARQ feedback transmission 315 - d to the first UE 115 via the sidelink communication channel. The first UE 115 may receive a HARQ feedback in time slot 320 - e and may successfully decode the HARQ feedback (which may be an ACK). Thus, the HARQ feedback transmission 315 - d may be a successful HARQ feedback transmission. Based on this success, the first UE 115 may reset the timer 325 - b and may not declare an RLF.

[0107] In the event of a later HARQ feedback failure, the first UE 115 may restart timer 325. For example, the first UE 115 may send a data transmission 310-e at a later point in time. Based on this data transmission, the first UE may monitor time slot 320-f for an expected HARQ feedback transmission 315-e. In some cases, the first UE may receive the HARQ feedback transmission 315-e, successfully decode it, and determine that the HARQ feedback transmission 315-e contains an ACK. In other cases, the first UE 115 may not receive the HARQ feedback transmission 315-e, or may receive the HARQ feedback transmission 315-e and be unable to decode it, or may be able to decode it and determine that it contains a NACK. In any of these cases, the first UE 115 may determine that the HARQ feedback transmission 315-e contains a failure, and the first UE 115 may thus start timer 325-c, which may count up to a threshold time length unless the HARQ feedback transmission is a successful transmission.

[0108] Timer 325-c may have the same timer value as timer 325-b, or may have a different value. In cases where timer reconfiguration occurs, timer 325-c may have a different value. Renegotiation or reconfiguration of timer 325 may occur in cases where one or more UE parameters have changed since the previous renegotiation of the timer. Renegotiation of the timer may correspond to an increase or decrease in the timer value.

[0109] For example, one or both of the first UE 115 and the second UE 115 may change speed. The UE speed or the relative speed between UEs may change from a high-speed category to a medium-speed category, which may require a change in the timer value. Or, the relative UE speed may exceed a higher or lower threshold boundary, which may also result in renegotiation of the timer value.

[0110] The congestion level may also change within the communication system, which may affect renegotiation of the timer. For example, the first or second UE 115 may monitor the RSRQ of the communication channel and may determine that the RSRQ is satisfied (e.g., exceeds a threshold). This may result in renegotiation of the timer.

[0111] In another case, the data priority of data transmission 310 may change. In these cases, timer 325 may have a value determined based on a data priority of the first data transmission 310. When the data priority changes, the value of the timer may increase (e.g., in cases where the priority decreases) or the value of the timer may decrease (e.g., in cases where the priority increases).

[0112] In other cases, serving base station 105 may instruct one or more UEs 115 to change the timer value. The instruction may be based on a changed parameter detected or signaled to the base station by one of the UEs 115 communicating over the sidelink or from another UE 115.

[0113] Any reason for timer negotiation may initiate timer renegotiation at the first UE. In some cases, the base station may indicate the value of the timer to the UE. In other cases, the UE may determine to change timer 325 without input from the base station.

[0114] In other cases, the first UE 115 may monitor HARQ feedback failures based on a counter that counts the number of failures rather than a timer that counts down until successful HARQ feedback occurs.

[0115] Figure 4A An example of a time slot diagram 401 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure is shown. In some examples, the time slot diagram 401 may implement aspects of wireless communication systems 100 and 200. The time slot diagram 401 may be an example of communication of one or more UEs 115 communicating in a sidelink configuration. A UE 115 that transmits a data transmission to another UE 115 may count several (e.g., one or more) consecutive HARQ feedback failures according to counter 425. A UE 115 operating in a sidelink configuration may receive and transmit messages based on the time slot diagram 401. According to example time slot configuration 405-a, a transmitting UE 115 (e.g., UE 115-a described with reference to Figure 2 or the first UE described with reference to Figure 3A and Figure 3B ) may transmit a data transmission 410. Configuration 405-a may be an example of a PSSCH. A receiving UE 115 (e.g., UE 115-b described with reference to Figure 2 or the second UE 115 described with reference to Figure 3A and Figure 3B ) may transmit a HARQ feedback transmission 415 based on configuration 405-b. Configuration 405-b may be an example of a PSFCH. Based on monitoring for a HARQ feedback transmission 415 received from a second receiving UE 115 (e.g., UE 115-b), a first transmitting UE 115 (e.g., UE 115-a) may start counter 425-a.

[0116] According to transmission configuration 405-a, a first UE 115 (e.g., UE 115-a) may send a data transmission 410 in a specific time slot. For example, UE 115 may send a first data transmission 410-a. The data transmission 410-a may correspond to a specific time slot N. The data transmission 410-a may be sent to a second UE 115 in a sidelink communication channel (e.g., sidelink channel 205). Based on sending the data transmission 410-a, UE 115 may monitor for a HARQ feedback transmission 415-a from the second UE 115 in time slot 420-a (e.g., time slot N+K).

[0117] In some cases, the HARQ feedback transmission 415-a may correspond to a HARQ feedback failure. The HARQ feedback failure may be because the HARQ feedback transmission 415-a is a NACK transmission. In other cases, the HARQ feedback failure may be that the HARQ feedback transmission 415-a may not be correctly decoded at the first UE 115 (regardless of whether it is an ACK or a NACK). In any case of HARQ feedback failure, the first UE 115 may start or increment a counter 425 based on the HARQ feedback failure.

[0118] For example, the HARQ feedback failure may occur in time slot 420-a. Based on this failure, the first UE 115 may start or increment a counter 425-a. Based on one or more parameters (e.g., data priority, speed of the first or second UE, congestion level, or another UE parameter), the counter 425-a may have a pre-configured number of failures to count up. Each time a HARQ feedback failure occurs, the counter 425-a may be incremented until a HARQ feedback success occurs or until a pre-configured threshold number of counts is reached.

[0119] If the counter 425-a counts to the threshold number of failures before the first UE 115 successfully receives a HARQ feedback, the first UE 115 may declare that an RLF has occurred. For example, the counter 425-a may be configured with five counts. Based on the data transmission 410-b, the first UE 115 may monitor for a HARQ feedback transmission 415-b in time slot 420-b. The time slot 420-b may be the fifth time slot in which UE 115 has monitored for a HARQ feedback transmission 415 from the second UE 115. A fifth HARQ failure may occur in time slot 420-b. At this time, the counter 425-a may have reached the threshold number of counts of a specific counter value. Therefore, the first UE 115 may declare an RLF and may continue to perform steps to resolve the RLF and resume sidelink communication with the second UE.

[0120] Optionally, HARQ feedback may be received before the counter 425 reaches the threshold. Figure 4B FIG. 402 illustrates an example of a time slot diagram that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. In some examples, the time slot diagram 402 may implement aspects of the wireless communication systems 100 and 200. A UE 115 operating in a sidelink configuration may receive and transmit messages based on the time slot diagram 402. A first transmitting UE 115 may transmit a data transmission 410 according to a configuration 405-c. The configuration 405-c may be an example of a PSSCH. A second receiving UE 115 may transmit a HARQ feedback transmission 415 based on a configuration 405-d. The configuration 305-c may be an example of a PSFCH. Based on monitoring for the HARQ feedback transmission 415 received from the second receiving UE 115, the first transmitting UE 115 (e.g., UE 115-a) may start a counter 425-b.

[0121] According to the transmission configuration 405-c, a first UE 115 (e.g., UE 115-a) may transmit a data transmission 410 in a particular time slot. For example, the UE 115 may transmit a first data transmission 410-c. The data transmission 410-c may correspond to a particular time slot N. The data transmission 410-c may be transmitted to a second UE 115 in a sidelink communication channel (e.g., sidelink channel 205). Based on transmitting the data transmission 410-c, the UE 115 may monitor for a HARQ feedback transmission 415-c from the second UE 115 in a time slot 420-c (e.g., time slot N+K).

[0122] Due to HARQ feedback failure, the first UE may not be able to detect the expected HARQ feedback in slot 420-c. Based on this failure, the first UE 115 can start counter 425-b. For example, the first UE 115 can start counting the counter from an initial value (e.g., the initial value is 0). Whenever a HARQ feedback failure occurs, counter 425-b can be incremented, unless interrupted by an instance of successful HARQ feedback. The first UE 115 can also send another data transmission 410-c, and based on the HARQ feedback transmission 415-d from the second UE 115, it can expect HARQ feedback in slot 420-d. The first UE 115 may also not detect the HARQ feedback transmission 415-d in slot 420-d, and based on this failure, counter 425-b can be incremented. The first UE 115 can send another data transmission 410-d to the second UE 115 via the sidelink communication channel. The second UE 115 can send a HARQ feedback transmission 415-e to the first UE 115 via the sidelink channel. The first UE 115 can receive the HARQ feedback in slot 420-e and can successfully decode the HARQ feedback (which can be an ACK). Therefore, the HARQ feedback transmission 415-e can be a successful HARQ feedback transmission. Based on this success, the first UE 115 can reset counter 425-b and may not declare RLF.

[0123] In the case of a later HARQ feedback failure, the first UE 115 can restart counter 425. For example, the first UE 115 can reset the counter to the initial value and start counting the counter from the initial value. The initial value can be zero. The first UE 115 can send a data transmission 410-f at a later time point. Based on this data transmission, the first UE can monitor slot 420-f for the expected HARQ feedback transmission 415-f. In some cases, the first UE can receive the HARQ feedback transmission 415-f, successfully decode it, and determine that the HARQ feedback transmission 415-f contains an ACK. In this case, the first UE 115 can not start or increment counter 425. In other cases, the first UE 115 may not receive the HARQ feedback transmission 415-f, or may receive the HARQ feedback transmission 415-f and be unable to decode it, or may be able to decode it and determine that it contains a NACK. In any of these cases, the first UE 115 can determine that the HARQ feedback transmission 415-f includes a HARQ feedback failure, and therefore the first UE 115 can start or increment counter 425-c.

[0124] Counter 425-c may have the same timer value as counter 425-b, or may have a different value. In the event of timer reconfiguration, counter 425-c may have a different value. Renegotiation or reconfiguration of counter 425 may occur where one or more UE parameters have changed since the previous renegotiation of the timer. Renegotiation of the counter may correspond to an increase or decrease in the timer value.

[0125] Figure 5 An example of process flow 500 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure is shown. In some examples, process flow 500 may implement aspects of wireless communication systems 100 and 200, and time slot diagrams 301, 302, 401, and 402. Process flow 500 may include UE 115-c and UE 115-d, which may be examples of UE 115 as described with reference Figure 1 to FIG. 4. UE 115-c may be an example of a first transmitting UE 115, and UE 115-d may be an example of a second receiving UE 115. UE 115-c and 115-d may be configured in a sidelink communication system and may communicate via a sidelink communication link. UE 115-c may perform RLM by detecting a HARQ feedback failure from UE 115-d.

[0126] At 505, a first UE 115 (e.g., UE 115-c) may transmit a first message to a second UE 115 (e.g., UE 115-d) via a sidelink communication link in a first TTI. The first message may be an example of a first data transmission.

[0127] At 510, UE 115-c may monitor the sidelink communication link for a feedback message 515 from UE 115-d in response to the first message transmitted by UE 115-c during a second TTI after the first TTI. The monitoring may be part of an RLM process for the sidelink communication link.

[0128] Before or after UE 115-c increments a counter at 520, UE 115-d may send an additional message 515 to UE 115-c. Before and after UE 115-c increments the counter at 520, UE 115-d may also send an additional message 515. UE 115-c may increment the counter based on the number of negative feedback messages.

[0129] At 520, as part of the RLM process, based on the reception of a negative feedback message for the unsuccessful reception of a feedback message in the second TTI, UE 115-c may increment a counter. Based on a pre-configured threshold, a value determined during the establishment of the sidelink communication link, a priority level associated with the data of the first message transmitted via the sidelink communication link, an indication from the base station, or a combination thereof, UE 115-d may determine a threshold for the counter or a timer duration for a timer. The value for the counter or the timer duration for the timer may be determined during the establishment of the sidelink communication channel via RRC signaling.

[0130] In some cases, UE 115-c may receive several negative feedback messages from UE 115-d (e.g., one or more additional messages transmitted by UE 115-d at 515). UE 115-c may determine RLF for the sidelink communication link based on the counter exceeding the threshold.

[0131] In other cases, UE 115-c may also fail to decode several feedback messages from UE 115-d. UE 115-c may increment the counter based on the number of feedback messages that decoding fails. UE 115-c may determine RLF for the sidelink communication link based on the counter exceeding the threshold.

[0132] In some cases, UE 115-c may receive a HARQ feedback message (e.g., ACK) before the timer expires. UE 115-c may reset the timer based on the HARQ feedback message (e.g., ACK).

[0133] After starting the timer, UE 115-c may monitor for one or more feedback messages from UE 115-d within the timer duration associated with the timer. Based on the unsuccessful reception of any of the one or more feedback messages from UE 115-d after the timer duration expires, UE 115-c may determine RLF for the sidelink communication link.

[0134] UE 115-c may determine RLF for the sidelink communication link based on the counter exceeding the threshold or when the timer expires. The threshold for the counter or the timer duration for the timer determined during the establishment of the sidelink communication link may be based on the speed of UE 115-c or UE 115-d, the congestion level of the sidelink communication link, the priority level associated with the data of the first message transmitted via the sidelink communication link, the application scenario using the sidelink communication link, or a combination thereof.

[0135] UE 115-c can determine upper and lower bounds for a threshold of a counter or a timer duration of a timer. The upper and lower bounds can be based on pre-configured values, a priority level associated with data of a first message transmitted via a sidelink communication link, an application scenario using the sidelink communication link, or a combination thereof. Based on a change in speed of UE 115-c or UE 115-d, a congestion level of the sidelink communication link, a change in a priority level associated with data transmitted via the sidelink communication link, or a combination thereof, UE 115-c can reconfigure the threshold of the counter or the timer duration of the timer.

[0136] UE 115-c can determine an unsuccessful reception of a feedback message based on a failure to successfully decode the feedback message in a second TTI or in one or more subsequent feedback opportunities.

[0137] Figure 6 Block diagram 600 illustrates a device 605 that supports radio link monitoring for sidelink communication, in accordance with aspects of the present disclosure. Device 605 can be an example of aspects of UE 115 as described herein. Device 605 can include a receiver 610, a communication manager 615, and a transmitter 620. Device 605 can also include a processor. Each of these components can communicate with each other (e.g., via one or more buses).

[0138] The receiver 610 can receive information such as packets, user data, or control information associated with various information channels (e.g., control channels, data channels, and information related to radio link monitoring for sidelink communication, etc.). The information can be passed to other components of device 605. The receiver 610 can be an example of aspects of the transceiver 920 described with reference to Figure 9 The receiver 610 can employ a single antenna or a set of antennas.

[0139] The communication manager 615 can transmit a first message to a second UE via a sidelink communication link in a first transmission time interval, and monitor the sidelink communication link for a feedback message in response to the first message transmitted by the first UE during a second transmission time interval after the first transmission time interval, where the monitoring is part of a radio link monitoring process for the sidelink communication link and, as part of the radio link monitoring process, increment a counter based on an unsuccessful reception of the feedback message in the second transmission time interval. The communication manager 615 can be an example of aspects of the communication manager 910 described herein.

[0140] The communication manager 615 or its sub-components can be implemented in hardware, code executed by a processor (e.g., software or firmware), or any combination thereof. If implemented in code executed by a processor, the functions of the communication manager 615 or its sub-components can be performed by a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof, which are designed to perform the functions described in this disclosure.

[0141] The communication manager 615 or its sub-components can be physically located in various positions, including being distributed such that portions of the functionality are implemented by one or more physical components at different physical locations. In some examples, aspects of the communication manager 615 or its sub-components can be separate and distinct components according to aspects of this disclosure. In some examples, the communication manager 615 or its sub-components can be combined with one or more other hardware components, including but not limited to input / output (I / O) components, transceivers, network servers, another computing device, one or more other components described in this disclosure, or combinations thereof, according to aspects of this disclosure.

[0142] The transmitter 620 can send signals generated by other components of the device 605. In some examples, the transmitter 620 can be co-located with the receiver 610 in a transceiver module. For example, the transmitter 620 can be an example of aspects of the transceiver 920 described with reference to Figure 9 The transmitter 620 can employ a single antenna or a group of antennas.

[0143] In some examples, the communication manager 615 described herein can be implemented as a chipset of a wireless modem, and the receiver 610 and the transmitter 620 can be implemented as a collection of analog components (e.g., amplifiers, filters, phase shifters, antennas, etc.). The wireless modem can obtain and decode signals from the receiver 610 through a receiving interface, and can output signals through a transmitting interface for transmission to the transmitter 620.

[0144] Actions performed by the UE communication manager 615 as described herein can be implemented to achieve one or more advantages. One implementation can allow the device 605 to effectively detect RLF in a sidelink communication link. If the device 605 has not detected RLF, this can reduce the number of transmissions and retransmissions that may have occurred. This can further allow the device 605 to save power and extend battery life by more effectively performing communication and faster recovering the connection in the sidelink channel. This may also result in more efficient use of resources, as the number of retransmissions can be reduced based on declaring RLF and earlier recovering sidelink communication.

[0145] Figure 7 FIG. 700 is a block diagram showing a device 705 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. The device 705 may be an example of aspects of the device 605 or UE 115 as described herein. The device 705 may include a receiver 710, a communication manager 715, and a transmitter 735. The device 705 may also include a processor. Each of these components may communicate with each other (e.g., via one or more buses).

[0146] The receiver 710 may receive information such as packets, user data, or control information associated with various information channels (e.g., control channels, data channels, and information related to radio link monitoring for sidelink communication, etc.). The information may be passed to other components of the device 705. The receiver 710 may be an example of aspects of the transceiver 920 described with reference to Figure 9 The receiver 710 may employ a single antenna or a set of antennas.

[0147] The communication manager 715 may be an example of aspects of the communication manager 615 as described herein. The communication manager 715 may include a sidelink component 720, a feedback component 725, and a failure component 730. The communication manager 715 may be an example of aspects of the communication manager 910 described herein.

[0148] The sidelink component 720 may send a first message to a second UE in a first transmission time interval via a sidelink communication link.

[0149] The feedback component 725 may monitor the sidelink communication link during a second transmission time interval after the first transmission time interval for a feedback message in response to the first message sent by the first UE, where the monitoring is part of a radio link monitoring process for the sidelink communication link.

[0150] As part of the radio link monitoring process, based on the unsuccessful reception of the feedback message in the second transmission time interval, the failure component 730 may increment a counter.

[0151] The transmitter 735 may send signals generated by other components of the device 705. In some examples, the transmitter 735 may be co-located with the receiver 710 in a transceiver module. For example, the transmitter 735 may be an example of aspects of the transceiver 920 described with reference to Figure 9 The transmitter 735 may employ a single antenna or a set of antennas.

[0152] The processor of UE 115 can increment a counter to effectively determine when RLF occurs in the sidelink communication channel. Based on configuring UE 115 to detect RLF based on the counter, the processor of UE 115 can effectively determine that RLF has occurred and can take actions to restore the sidelink communication link with one or more other UEs. The processor of UE 115 can declare RLF in some cases, and then the processor can initiate a command to re-establish communication through the failed link or in some cases through a new link.

[0153] Figure 8 FIG. 800 is a block diagram of a communication manager 805 that supports radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. The communication manager 805 can be an example of aspects of the communication manager 615, communication manager 715, or communication manager 910 described herein. The communication manager 805 can include a sidelink component 810, a feedback component 815, a failure component 820, a counter controller 825, a declaration component 830, and a timer controller 835. Each of these modules can communicate directly or indirectly with each other (e.g., via one or more buses).

[0154] The sidelink component 810 can send a first message to a second UE in a first transmission time interval via a sidelink communication link.

[0155] The feedback component 815 can monitor the sidelink communication link during a second transmission time interval after the first transmission time interval for a feedback message in response to the first message sent by the first UE, where the monitoring is part of a radio link monitoring process for the sidelink communication link.

[0156] In some examples, the feedback component 815 can receive several negative feedback messages from the second UE. In some examples, the feedback component 815 may fail to decode several feedback messages from the second UE. In some examples, the feedback component 815 can receive a HARQ feedback message before a timer expires. In some examples, after incrementing the counter, the feedback component 815 can monitor one or more feedback messages from the second UE before a threshold of the counter. In some examples, the feedback component 815 can determine an unsuccessful reception of a feedback message based on a failure to successfully decode the feedback message in the second transmission time interval or in one or more subsequent feedback opportunities.

[0157] As part of a radio link monitoring process, the failure component 820 can increment a counter based on an unsuccessful reception of a feedback message in the second transmission time interval.

[0158] The counter controller 825 may increment a counter based on the number of negative feedback messages. In some examples, the counter controller 825 may increment the counter based on the number of feedback messages with decoding failures.

[0159] In some examples, the counter controller 825 may determine a threshold for the counter based on a preconfigured threshold, a value determined during the establishment of the sidelink communication link, a priority level associated with data of a first message transmitted via the sidelink communication link, an indication from a base station, or a combination thereof.

[0160] In some examples, the counter controller 825 may determine a threshold of the counter based on a preconfigured threshold, a value determined during the establishment of the sidelink communication link, a priority level associated with data of a first message transmitted via the sidelink communication link, an indication from a base station, or a combination thereof.

[0161] In some examples, the counter controller 825 may determine an upper limit and a lower limit for the threshold of the counter, where the upper limit and the lower limit are based on a preconfigured value, a priority level associated with data of a first message transmitted via the sidelink communication link, an application scenario using the sidelink communication link, or a combination thereof.

[0162] In some examples, the counter controller 825 may reconfigure the threshold of the counter based on a speed change of the first UE or the second UE, a congestion level of the sidelink communication link, a change in the priority level associated with data transmitted via the sidelink communication link, or a combination thereof. In some cases, a value for the counter is determined during the establishment of the sidelink communication channel via RRC signaling.

[0163] In some cases, the threshold of the counter determined during the establishment of the sidelink communication link is based on the speed of the first UE or the second UE, a congestion level of the sidelink communication link, a priority level associated with data of a first message transmitted via the sidelink communication link, an application scenario using the sidelink communication link, or a combination thereof.

[0164] The assertion component 830 may determine a radio link failure for the sidelink communication link based on the counter exceeding the threshold. In some examples, the assertion component 830 may determine a radio link failure for the sidelink communication link based on not successfully receiving any one of one or more feedback messages from the second UE after a timer duration expires.

[0165] In some examples, the assertion component 830 may determine a radio link failure for the sidelink communication link based on the counter exceeding the threshold.

[0166] The timer controller 835 may reset the counter based on a HARQ feedback message (e.g., an ACK feedback message).

[0167] Figure 9 FIG. 900 illustrates a system 900 in accordance with aspects of the present disclosure, the system 900 including a device 905 that supports radio link monitoring for sidelink communication. The device 905 may be an example of or include components of the device 605, the device 705, or the UE 115 as described herein. The device 905 may include components for two-way voice and data communication, including components for sending and receiving communications, including a communication manager 910, an I / O controller 915, a transceiver 920, an antenna 925, a memory 930, and a processor 940. These components may communicate electronically via one or more buses (e.g., bus 945).

[0168] The communication manager 910 may send a first message to a second UE via a sidelink communication link in a first transmission time interval and monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE during a second transmission time interval after the first transmission time interval, wherein the monitoring is part of a radio link monitoring process for the sidelink communication link and, as part of the radio link monitoring process, increment a counter based on an unsuccessful reception of the feedback message in the second transmission time interval.

[0169] The I / O controller 915 may manage input and output signals for the device 905. The I / O controller 915 may also manage peripheral devices not integrated into the device 905. In some cases, the I / O controller 915 may represent a physical connection or port to an external peripheral device. In some cases, the I / O controller 915 may implement an operating system such as or other known operating systems. In other cases, the I / O controller 915 may represent or interact with a modem, a keyboard, a mouse, a touch screen, or similar device. In some cases, the I / O controller 915 may be implemented as part of a processor. In some cases, a user may interact with the device 905 via the I / O controller 915 or via hardware components controlled by the I / O controller 915.

[0170] The transceiver 920 may communicate bidirectionally via one or more antennas, wired or wireless links as described herein. For example, the transceiver 920 may represent a wireless transceiver and may communicate bidirectionally with another wireless transceiver. The transceiver 920 may also include a modem to modulate packets and provide the modulated packets to the antenna for transmission and to demodulate packets received from the antenna.

[0171] In some cases, the wireless device may include a single antenna 925. However, in certain cases, the device may have more than one antenna 925, which may be capable of simultaneously transmitting or receiving multiple wireless transmissions.

[0172] The memory 930 may include random access memory (RAM) and read only memory (ROM). The memory 930 may store computer-readable, computer-executable code 935 that includes instructions that, when executed, cause the processor to perform the various functions described herein. In certain cases, the memory 930 may contain a basic input / output system (BIOS) and others, which may control basic hardware or software operations, such as interactions with peripheral components or devices.

[0173] The processor 940 may include intelligent hardware devices (e.g., general-purpose processors, DSPs, central processing units (CPUs), microcontrollers, ASICs, FPGAs, programmable logic devices, discrete gate or transistor logic components, discrete hardware components, or any combination thereof). In some cases, the processor 940 may be configured to operate a memory array using a memory controller. In other cases, the memory controller may be integrated into the processor 940. The processor 940 may be configured to execute computer-readable instructions stored in a memory (e.g., memory 930) to cause the device 905 to perform various functions (e.g., functions or tasks supporting radio link monitoring for sidelink communication).

[0174] The code 935 may include instructions for implementing aspects of the present disclosure, including instructions for supporting wireless communication. The code 935 may be stored in a non-transitory computer-readable medium such as system memory or other types of memory. In certain cases, the code 935 may not be directly executable by the processor 940, but may (e.g., when compiled and executed) cause a computer to perform the functions described herein.

[0175] Figure 10 A flowchart depicting a method 1000 for supporting radio link monitoring for sidelink communication in accordance with aspects of the present disclosure is shown. The operations of method 1000 may be implemented by a UE 115 or its components as described herein. For example, the operations of method 1000 may be performed by a communication manager as described with reference to Figures 6 to 9 In some examples, the UE may execute an instruction set to control functional elements of the UE to perform the functions described herein. Additionally or alternatively, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0176] At 1005, the UE may send a first message to a second UE via a sidelink communication link in a first transmission time interval. The operation of 1005 may be performed according to the methods described herein. In some examples, aspects of the operation of 1005 may be performed by a sidelink component as described with reference to Figures 6 to 9 The aspects of the operation of 1005 may be performed by a sidelink component as described with reference to

[0177] At 1010, the UE may monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE during a second transmission time interval after the first transmission time interval, wherein the monitoring is part of a radio link monitoring process for the sidelink communication link. The operation of 1010 may be performed according to the methods described herein. In some examples, aspects of the operation of 1010 may be performed by a feedback component as described with reference to Figures 6 to 9 The aspects of the operation of 1010 may be performed by a feedback component as described with reference to

[0178] At 1015, as part of the radio link monitoring process, the UE may increment a counter based on an unsuccessful reception of the feedback message in the second transmission time interval. The operation of 1015 may be performed according to the methods described herein. In some examples, aspects of the operation of 1015 may be performed by a failure component as described with reference to Figures 6 to 9 The aspects of the operation of 1015 may be performed by a failure component as described with reference to

[0179] Figure 11 A flowchart illustrating a method 1100 for supporting radio link monitoring for sidelink communication in accordance with aspects of the present disclosure is shown. The operations of method 1100 may be implemented by the UE 115 or its components as described herein. For example, the operations of method 1100 may be performed by a communication manager as described with reference to Figures 6 to 9 The aspects of the operation of method 1100 may be performed by a communication manager as described with reference to

[0180] At 1105, the UE may send a first message to a second UE via a sidelink communication link in a first transmission time interval. The operation of 1105 may be performed according to the methods described herein. In some examples, aspects of the operation of 1105 may be performed by a sidelink component as described with reference to Figures 6 to 9 The aspects of the operation of 1105 may be performed by a sidelink component as described with reference to

[0181] At 1110, the UE may monitor a sidelink communication link during a second transmission time interval after a first transmission time interval for a feedback message from a second UE in response to a first message sent by the first UE, where the monitoring is part of a radio link monitoring process for the sidelink communication link. The operations at 1110 may be performed according to the methods described herein. In some examples, aspects of the operations at 1110 may be performed by a feedback component as described with reference to Figures 6 to 9 as described.

[0182] At 1115, the UE may receive a plurality of negative feedback messages from the second UE. The operations at 1115 may be performed according to the methods described herein. In some examples, aspects of the operations at 1115 may be performed by a feedback component as described with reference to Figures 6 to 9 as described.

[0183] At 1120, as part of a radio link monitoring process, the UE may increment a counter based on an unsuccessful reception of a feedback message during the second transmission time interval. The operations at 1120 may be performed according to the methods described herein. In some examples, aspects of the operations at 1120 may be performed by a failure component as described with reference to Figures 6 to 9 as described.

[0184] At 1125, the UE may increment a counter based on the number of negative feedback messages. The operations at 1125 may be performed according to the methods described herein. In some examples, aspects of the operations at 1125 may be performed by a counter controller as described with reference to Figures 6 to 9 as described.

[0185] At 1130, the UE may determine a radio link failure for the sidelink communication link based on the counter exceeding a threshold. The operations at 1130 may be performed according to the methods described herein. In some examples, aspects of the operations at 1130 may be performed by a declaration component as described with reference to Figures 6 to 9 as described.

[0186] Figure 12 FIG. shows a flowchart of a method 1200 depicting radio link monitoring in support of sidelink communication according to aspects of the present disclosure. The operations of method 1200 may be implemented by the UE 115 or its components as described herein. For example, the operations of method 1200 may be performed by a communication manager as described with reference to Figures 6 to 9 as described. In some examples, the UE may execute an instruction set to control functional elements of the UE to perform the functions described herein. Additionally or alternatively, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0187] At 1205, the UE may send a first message to a second UE via a sidelink communication link in a first transmission time interval. The operation of 1205 may be performed according to the methods described herein. In some examples, aspects of the operation of 1205 may be performed by a sidelink component as described with reference to Figures 6 to 9 The aspects of the operation of 1205 may be performed by a sidelink component as described with reference to

[0188] At 1210, the UE may monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE during a second transmission time interval after the first transmission time interval, where the monitoring is part of a radio link monitoring process for the sidelink communication link. The operation of 1210 may be performed according to the methods described herein. In some examples, aspects of the operation of 1210 may be performed by a feedback component as described with reference to Figures 6 to 9 The aspects of the operation of 1210 may be performed by a feedback component as described with reference to

[0189] At 1215, the UE may fail to decode several feedback messages from the second UE. The operation of 1215 may be performed according to the methods described herein. In some examples, aspects of the operation of 1215 may be performed by a feedback component as described with reference to Figures 6 to 9 The aspects of the operation of 1215 may be performed by a feedback component as described with reference to

[0190] At 1220, as part of a radio link monitoring process, the UE may increment a counter based on the unsuccessful reception of the feedback message in the second transmission time interval. The operation of 1220 may be performed according to the methods described herein. In some examples, aspects of the operation of 1220 may be performed by a failure component as described with reference to Figures 6 to 9 The aspects of the operation of 1220 may be performed by a failure component as described with reference to

[0191] At 1225, the UE may increment a counter based on the number of feedback messages that failed to be decoded. The operation of 1225 may be performed according to the methods described herein. In some examples, aspects of the operation of 1225 may be performed by a counter controller as described with reference to Figures 6 to 9 The aspects of the operation of 1225 may be performed by a counter controller as described with reference to

[0192] At 1230, the UE may determine a radio link failure for the sidelink communication link based on the counter exceeding a threshold. The operation of 1230 may be performed according to the methods described herein. In some examples, aspects of the operation of 1230 may be performed by a declaration component as described with reference to Figures 6 to 9 The aspects of the operation of 1230 may be performed by a declaration component as described with reference to

[0193] Figure 13FIG. 1300 is a flow chart depicting a method for supporting radio link monitoring for sidelink communication in accordance with aspects of the present disclosure. Operations of method 1300 may be implemented by UE 115 or its components as described herein. For example, operations of method 1300 may be performed by a communication manager as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0194] At 1305, the UE may send a first message to a second UE via a sidelink communication link in a first transmission time interval. The operation at 1305 may be performed according to the methods described herein. In some examples, aspects of the operation at 1305 may be performed by a sidelink component as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0195] At 1310, the UE may monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE during a second transmission time interval after the first transmission time interval, wherein the monitoring is part of a radio link monitoring procedure for the sidelink communication link. The operation at 1310 may be performed according to the methods described herein. In some examples, aspects of the operation at 1310 may be performed by a feedback component as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0196] At 1315, as part of the radio link monitoring procedure, the UE may start a counter based on the unsuccessful reception of the feedback message in the second transmission time interval. The operation at 1315 may be performed according to the methods described herein. In some examples, aspects of the operation at 1315 may be performed by a failure component as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0197] At 1320, the UE may receive a HARQ feedback message before the timer expires. The operation at 1320 may be performed according to the methods described herein. In some examples, aspects of the operation at 1320 may be performed by a feedback component as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0198] At 1325, the UE may reset the counter based on the HARQ feedback message. The operation at 1325 may be performed according to the methods described herein. In some examples, aspects of the operation at 1325 may be performed by a timer controller as described with reference to Figures 6 to 9 Additional or alternative, the UE may use dedicated hardware to perform aspects of the functions described herein.

[0199] It should be noted that the methods described in this application describe possible implementations, and the operations and steps can be rearranged or otherwise modified, and other implementations are possible. In addition, aspects from two or more methods can be combined.

[0200] Although aspects of LTE, LTE-A, LTE-A Pro, or NR systems are described for purposes of illustration and the terms LTE, LTE-A, LTE-A Pro, or NR are used in most of the description, the techniques described herein are applicable beyond LTE, LTE-A, LTE-A Pro, or NR networks. For example, the described techniques can be applicable to a variety of other wireless communication systems such as Ultra Mobile Broadband (UMB), Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDM, and other systems and radio technologies not explicitly mentioned herein.

[0201] The information and signals described herein can be represented using any of a variety of different technologies and methods. For example, data, instructions, commands, information, signals, bits, symbols, and chips mentioned throughout the specification can be represented by voltage, current, electromagnetic waves, magnetic fields or magnetic particles, optical fields or optical particles, or any combination thereof.

[0202] A general-purpose processor, DSP, ASIC, CPU, FPGA, or other programmable logic device, discrete gate or transistor logic, discrete hardware component, or any combination thereof that can be designed to perform the functions described herein can be used to implement or execute the various exemplary blocks and components described in connection with the present disclosure. The general-purpose processor can be a microprocessor, or alternatively, the processor can be any processor, controller, microcontroller, or state machine. The processor can also be implemented as a combination of computing devices (e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in combination with a DSP core, or any other such configuration).

[0203] The functions described herein can be implemented in hardware, software executed by a processor, firmware, or any combination thereof. If implemented in software executed by a processor, the functions can be stored or transmitted on a computer-readable medium as one or more instructions or code. Other examples and implementations also fall within the scope of the present disclosure and the appended claims. For example, due to the nature of software, the functions described herein can be implemented using software, hardware, firmware, hardwired, or any combination thereof executed by a processor. The features implementing the functions can also be physically located in various positions, including being distributed such that portions of the functions are implemented at different physical locations.

[0204] A computer-readable medium includes both a non-transitory computer storage medium and a communication medium, where the communication medium includes any medium that facilitates the transfer of a computer program from one location to another. The non-transitory storage medium can be any available medium accessible by a general-purpose or special-purpose computer. By way of example, and not limitation, non-transitory computer-readable media can include RAM, ROM, electrically erasable programmable ROM (EEPROM), flash memory, compact disc read-only memory (CD-ROM) or other optical disc storage, magnetic disk storage or other magnetic storage devices, or any other non-transitory medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if software is transmitted using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of computer-readable medium. As used herein, the disk and optical disc include CD, laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc, where the disk generally magnetically reproduces data, and the optical disc optically reproduces data with a laser. Combinations of the above can also be included within the scope of computer-readable medium.

[0205] As used herein, including in the claims, the "or" used in a list of items (e.g., a list of items beginning with a phrase such as "at least one of" or "one or more of") means an inclusive list, such that for example, a list of at least one of A, B, or C means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). Also, as used herein, the phrase "based on" should not be construed as referring to a closed set of conditions. For example, an exemplary step described as "based on condition A" can be based on both condition A and condition B without departing from the scope of the present disclosure. In other words, as used herein, the phrase "based on" should be interpreted in the same manner as the phrase "at least partially based on". Additionally, "a number of" can mean "one or more".

[0206] In the figures, similar components or features may have the same reference numeral. Additionally, each of the same type of components can be distinguished by a dash following the reference numeral and a second label that differentiates between similar components. If only the first reference numeral is used in the specification, the specification can apply to any one of the similar components having the same first reference numeral, regardless of the second reference numeral or any other subsequent reference numerals.

[0207] The specification presented herein describes exemplary configurations in conjunction with the accompanying drawings and does not represent all examples that can be implemented or fall within the scope of the claims. The term "example" as used herein means "serving as an example, instance, or illustration" rather than "preferred" or "superior to other examples". The detailed description includes specific details for the purpose of providing an understanding of the technology. However, these technologies can be implemented without these specific details. In some instances, well-known structures and devices are shown in block diagram form to avoid obscuring the concept of the described examples.

[0208] A specification is provided herein to enable a person of ordinary skill in the art to make or use the present disclosure. Various modifications to the present disclosure will be apparent to a person of ordinary skill in the art, and the general principles defined herein can also be applied to other variations without departing from the scope of the present disclosure. Therefore, the present disclosure is not limited to the examples and designs described herein, but is consistent with the broadest scope of the principles and novel features disclosed herein.

Claims

1. A method for wireless communication at a first user equipment (UE), comprising: Receiving signaling indicating a threshold number of unsuccessfully received feedback messages associated with a radio link monitoring process for a sidelink communication link; Sending a first message to a second UE via the sidelink communication link in a first transmission time interval; Monitoring the sidelink communication link for a feedback message in response to the first message sent by the first UE from the second UE during a second transmission time interval after the first transmission time interval, wherein the monitoring is part of the radio link monitoring process for the sidelink communication link; Incrementing a counter at least in part based on an unsuccessful reception of the feedback message in the second transmission time interval as part of the radio link monitoring process; Monitoring whether the sidelink communication link corresponds to a radio link failure at least in part based on whether the counter exceeds the threshold number at least in part based on the incremented counter; Receiving a hybrid automatic repeat request (HARQ) feedback message including a negative acknowledgment before the counter exceeds the threshold number of unsuccessfully received feedback messages; and Resetting the counter at least in part based on the HARQ feedback message including the negative acknowledgment.

2. The method according to claim 1, further comprising: Decoding failures for several feedback messages from the second UE; Incrementing the counter at least in part based on the number of feedback messages with decoding failures; And Determining a radio link failure for the sidelink communication link at least in part based on the counter exceeding the threshold number.

3. The method according to claim 2, further comprising: Resetting the counter at least in part based on successful decoding of the HARQ feedback message from the second UE.

4. The method according to claim 1, further comprising: Monitoring for one or more feedback messages from the second UE after incrementing the counter and before the counter exceeds the threshold number; And Determining a radio link failure for the sidelink communication link at least in part based on no successful reception of any of the one or more feedback messages from the second UE after the counter exceeds the threshold number.

5. The method according to claim 4, wherein, The threshold number is determined at least in part based on the speed of the first UE or the second UE, the congestion level of the sidelink communication link, a priority level associated with data of the first message sent via the sidelink communication link, an application scenario using the sidelink communication link, or a combination thereof during establishment of the sidelink communication link.

6. The method according to claim 4, further comprising: Determine an upper limit and a lower limit for the threshold quantity, wherein the upper limit and the lower limit are at least partially based on a preconfigured value, a priority level associated with data of the first message transmitted via the sidelink communication link, an application scenario using the sidelink communication link, or a combination thereof.

7. The method according to claim 4, further comprising: Reconfigure the threshold quantity at least partially based on a change in speed of the first UE or the second UE, a congestion level of the sidelink communication link, a change in a priority level associated with data transmitted via the sidelink communication link, or a combination thereof.

8. The method according to claim 1, further comprising: Determine the threshold quantity at least partially based on a preconfigured threshold, a value determined during establishment of the sidelink communication link, a priority level associated with data of the first message transmitted via the sidelink communication link, signaling received from a network entity indicating the threshold quantity, or a combination thereof.

9. The method according to claim 1, wherein The signaling indicating the threshold quantity of the feedback message that was not successfully received is received during establishment of the sidelink communication link via radio resource control (RRC) signaling.

10. The method according to claim 1, further comprising: Determine the radio link failure for the sidelink communication link at least partially based on the counter exceeding the threshold quantity.

11. The method according to claim 1, further comprising: Determine the unsuccessful reception of the feedback message at least partially based on the feedback message not being successfully decoded in the second transmission time interval or in one or more subsequent feedback opportunities.

12. An apparatus for wireless communication at a first user equipment (UE), comprising: A processor, A memory coupled to the processor, the memory including instructions executable by the processor to cause the apparatus to perform the following operations: Receive signaling indicating a threshold quantity of feedback messages that were not successfully received and are associated with a radio link monitoring procedure for a sidelink communication link; Transmit a first message to a second UE via the sidelink communication link in a first transmission time interval; During a second transmission time interval after the first transmission time interval, monitor the sidelink communication link for a feedback message in response to the first message transmitted by the first UE from the second UE, wherein the monitoring is part of the radio link monitoring procedure for the sidelink communication link; As part of the radio link monitoring procedure, increment a counter at least partially based on the feedback message not being successfully received in the second transmission time interval; Monitor whether the sidelink communication link corresponds to a radio link failure at least partially based on incrementing the counter, based on whether the counter exceeds the threshold quantity; Receive a Hybrid Automatic Repeat reQuest (HARQ) feedback message including a negative acknowledgment before the counter exceeds a threshold number of the feedback messages that were not successfully received; and Reset the counter at least in part based on the HARQ feedback message including the negative acknowledgment.

13. The device according to claim 12, wherein The instructions may further be executed by the processor to cause the device to: Fail to decode a number of feedback messages from the second UE; Increment the counter at least in part based on the number of feedback messages for which decoding has failed; And Determine a radio link failure for the sidelink communication link at least in part based on the counter exceeding the threshold number.

14. The device according to claim 13, wherein, The instructions may further be executed by the processor to cause the device to: Reset the counter at least in part based on successful decoding of a Hybrid Automatic Repeat reQuest (HARQ) feedback message from the second UE.

15. The apparatus according to claim 12, wherein The instructions may further be executed by the processor to cause the device to: After incrementing the counter, monitor for one or more feedback messages from the second UE before the counter exceeds the threshold number; And Determine a radio link failure for the sidelink communication link at least in part based on not successfully receiving any of the one or more feedback messages from the second UE after the counter exceeds the threshold number.

16. The device according to claim 15, wherein The threshold number is determined at least in part during establishment of the sidelink communication link based on the speed of the first UE or the second UE, the congestion level of the sidelink communication link, a priority level associated with data of the first message transmitted via the sidelink communication link, an application scenario in which the sidelink communication link is used, or a combination thereof.

17. The device according to claim 15, wherein, The instructions may further be executed by the processor to cause the device to: Determine an upper limit and a lower limit for the threshold number, where the upper limit and the lower limit are at least in part based on a preconfigured value, a priority level associated with data of the first message transmitted via the sidelink communication link, an application scenario in which the sidelink communication link is used, or a combination thereof.

18. The apparatus according to claim 15, wherein The instructions may further be executed by the processor to cause the device to: Reconfigure the threshold number at least in part based on a change in the speed of the first UE or the second UE, the congestion level of the sidelink communication link, a change in a priority level associated with data transmitted via the sidelink communication link, or a combination thereof.

19. The apparatus according to claim 12, wherein, The instructions may further be executed by the processor to cause the device to: Determine the threshold number at least in part based on a preconfigured threshold, a value determined during establishment of the sidelink communication link, a priority level associated with data of the first message transmitted via the sidelink communication link, receiving signaling from a network entity indicating the threshold number, or a combination thereof.

20. The device according to claim 12, wherein The signaling indicating the threshold number is received during establishment of the sidelink communication link via radio resource control (RRC) signaling.

21. The apparatus according to claim 12, wherein, The instructions may also be executed by the processor to cause the device to: Determine the radio link failure for the sidelink communication link based at least in part on the counter exceeding the threshold number.

22. The apparatus according to claim 12, wherein, The instructions may also be executed by the processor to cause the device to: Determine the unsuccessful reception of the feedback message based at least in part on the failure to successfully decode the feedback message in the second transmission time interval or in one or more subsequent feedback opportunities.

23. An apparatus for wireless communication at a first user equipment (UE) includes: A unit for receiving signaling indicating a threshold number of unsuccessfully received feedback messages associated with a radio link monitoring procedure for a sidelink communication link; A unit for transmitting a first message to a second UE in a first transmission time interval via the sidelink communication link; A unit for monitoring the sidelink communication link for a feedback message in response to the first message transmitted by the first UE during a second transmission time interval after the first transmission time interval, where the monitoring is part of the radio link monitoring procedure for the sidelink communication link; A unit for incrementing a counter based at least in part on the unsuccessful reception of the feedback message in the second transmission time interval as part of the radio link monitoring procedure; A unit for monitoring whether the sidelink communication link corresponds to a radio link failure based at least in part on whether the counter exceeds the threshold number based on incrementing the counter; A unit for receiving a hybrid automatic repeat request (HARQ) feedback message including a negative acknowledgment before the counter exceeds the threshold number of unsuccessfully received feedback messages; and A unit for resetting the counter based at least in part on the HARQ feedback message including the negative acknowledgment.

24. A non-transitory computer-readable medium storing code for wireless communication at a first user equipment (UE), the code including instructions executable by a processor to: Receive signaling indicating a threshold number of unsuccessfully received feedback messages associated with a radio link monitoring procedure for a sidelink communication link; Transmit a first message to a second UE in a first transmission time interval via the sidelink communication link; During a second transmission time interval after the first transmission time interval, monitor the sidelink communication link for a feedback message from the second UE in response to the first message sent by the first UE, where, The monitoring is part of the radio link monitoring procedure for the sidelink communication link; Increment a counter based at least in part on the unsuccessful reception of the feedback message in the second transmission time interval as part of the radio link monitoring procedure; Monitor whether the sidelink communication link corresponds to a radio link failure based at least in part on whether the counter exceeds the threshold number based on incrementing the counter; Receive a Hybrid Automatic Repeat reQuest (HARQ) feedback message including a negative acknowledgment before the counter exceeds a threshold number of the unsuccessfully received feedback messages; And Reset the counter at least in part based on the HARQ feedback message including the negative acknowledgment.

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