Patent Publication Number: US-2023156834-A1

Title: Discovery and measurement timing configurations for new radio sidelink communications

Description:
TECHNICAL FIELD 
     This application relates to wireless communication systems, and more particularly, to discovery and measurement timing configurations for new radio sidelink communications. 
     INTRODUCTION 
     Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). A wireless multiple-access communications system may include a number of base stations (BSs), each simultaneously supporting communications for multiple communication devices, which may be otherwise known as user equipment (UE). 
     To meet the growing demands for expanded mobile broadband connectivity, wireless communication technologies are advancing from the LTE technology to a next generation new radio (NR) technology. For example, NR is designed to provide a lower latency, a higher bandwidth or throughput, and a higher reliability than LTE. NR is designed to operate over a wide array of spectrum bands, for example, from low-frequency bands below about 1 gigahertz (GHz) and mid-frequency bands from about 1 GHz to about 6 GHz, to high-frequency bands such as millimeter wave (mmWave) bands. NR is also designed to operate across different spectrum types, from licensed spectrum to unlicensed and shared spectrum. Spectrum sharing enables operators to opportunistically aggregate spectrums to dynamically support high-bandwidth services. Spectrum sharing can extend the benefit of NR technologies to operating entities that may not have access to a licensed spectrum. 
     NR may support various deployment scenarios to benefit from the various spectrums in different frequency ranges, licensed and/or unlicensed, and/or coexistence of the LTE and NR technologies. For example, NR can be deployed in a standalone NR mode over a licensed and/or an unlicensed band or in a dual connectivity mode with various combinations of NR and LTE over licensed and/or unlicensed bands. 
     In a wireless communication network, a BS may communicate with a UE in an uplink direction and a downlink direction. Sidelink was introduced in LTE to allow a UE to send data to another UE (e.g., from one vehicle to another vehicle) without tunneling through the BS and/or an associated core network. The LTE sidelink technology has been extended to provision for device-to-device (D2D) communications, vehicle-to-everything (V2X) communications, and/or cellular vehicle-to-everything (C-V2X) communications. Similarly, NR may be extended to support sidelink communications, D2D communications, V2X communications, and/or C-V2X over licensed bands and/or unlicensed bands. 
     BRIEF SUMMARY OF SOME EXAMPLES 
     The following summarizes some aspects of the present disclosure to provide a basic understanding of the discussed technology. This summary is not an extensive overview of all contemplated features of the disclosure and is intended neither to identify key or critical elements of all aspects of the disclosure nor to delineate the scope of any or all aspects of the disclosure. Its sole purpose is to present some concepts of one or more aspects of the disclosure in summary form as a prelude to the more detailed description that is presented later. 
     In an aspect of the disclosure, a method of wireless communication performed by a first sidelink user equipment (UE) may include establishing, with a second sidelink UE, a beamformed link; transmitting, to the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link; and transmitting, to the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     In an additional aspect of the disclosure, a method of wireless communication performed by a first sidelink user equipment (UE) may include establishing, with a second sidelink UE, a beamformed link; receiving, from the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link; and receiving, from the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     In an additional aspect of the disclosure, a first sidelink user equipment (UE) may include a memory; a transceiver; and at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to establish, with a second sidelink UE, a beamformed link; transmit, to the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link, and transmit, to the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     In an additional aspect of the disclosure, a first sidelink user equipment (UE) may include a memory; a transceiver; and at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to establish, with a second sidelink UE, a beamformed link; receive, from the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link; and receive, from the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     Other aspects, features, and instances of the present invention will become apparent to those of ordinary skill in the art, upon reviewing the following description of specific, exemplary instances of the present invention in conjunction with the accompanying figures. While features of the present invention may be discussed relative to certain aspects and figures below, all instances of the present invention can include one or more of the advantageous features discussed herein. In other words, while one or more instances may be discussed as having certain advantageous features, one or more of such features may also be used in accordance with the various instances of the invention discussed herein. In similar fashion, while exemplary aspects may be discussed below as device, system, or method instances it should be understood that such exemplary instances can be implemented in various devices, systems, and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a wireless communication network according to some aspects of the present disclosure. 
         FIG.  2    illustrates a sidelink wireless communication network according to some aspects of the present disclosure. 
         FIG.  3 A  illustrates an established beamformed link according to some aspects of the present disclosure. 
         FIG.  3 B  illustrates resources associated with a beacon signal according to some aspects of the present disclosure. 
         FIG.  4 A  illustrates multiple established beamformed links according to some aspects of the present disclosure. 
         FIG.  4 B  illustrates a signaling diagram of a communication method according to some aspects of the present disclosure. 
         FIG.  5 A  illustrates an established beamformed link according to some aspects of the present disclosure. 
         FIG.  5 B  illustrates resources associated with a beacon signal according to some aspects of the present disclosure. 
         FIG.  6    is a signaling diagram of a communication method according to some aspects of the present disclosure. 
         FIG.  7    is a block diagram of an exemplary user equipment (UE) according to some aspects of the present disclosure. 
         FIG.  8    is a block diagram of an exemplary base station (BS) according to some aspects of the present disclosure. 
         FIG.  9    is a flow diagram of a communication method according to some aspects of the present disclosure. 
         FIG.  10    is a flow diagram of a communication method according to some aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts. 
     This disclosure relates generally to wireless communications systems, also referred to as wireless communications networks. In various instances, the techniques and apparatus may be used for wireless communication networks such as code division multiple access (CDMA) networks, time division multiple access (TDMA) networks, frequency division multiple access (FDMA) networks, orthogonal FDMA (OFDMA) networks, single-carrier FDMA (SC-FDMA) networks, LTE networks, GSM networks, 5 th  Generation (5G) or new radio (NR) networks, as well as other communications networks. As described herein, the terms “networks” and “systems” may be used interchangeably. 
     An OFDMA network may implement a radio technology such as evolved UTRA (E-UTRA), Institute of Electrical and Electronic Engineers (IEEE) 802.11, IEEE 802.16, IEEE 802.20, flash-OFDM and the like. UTRA, E-UTRA, and Global System for Mobile Communications (GSM) are part of universal mobile telecommunication system (UMTS). In particular, long term evolution (LTE) is a release of UMTS that uses E-UTRA. UTRA, E-UTRA, GSM, UMTS and LTE are described in documents provided from an organization named “3rd Generation Partnership Project” (3GPP), and cdma2000 is described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). These various radio technologies and standards are known or are being developed. For example, the 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications associations that aims to define a globally applicable third generation (3G) mobile phone specification. 3GPP long term evolution (LTE) is a 3GPP project which was aimed at improving the universal mobile telecommunications system (UMTS) mobile phone standard. The 3GPP may define specifications for the next generation of mobile networks, mobile systems, and mobile devices. The present disclosure is concerned with the evolution of wireless technologies from LTE, 4G, 5G, NR, and beyond with shared access to wireless spectrum between networks using a collection of new and different radio access technologies or radio air interfaces. 
     In particular, 5G networks contemplate diverse deployments, diverse spectrum, and diverse services and devices that may be implemented using an OFDM-based unified, air interface. In order to achieve these goals, further enhancements to LTE and LTE-A are considered in addition to development of the new radio technology for 5G NR networks. The 5G NR will be capable of scaling to provide coverage (1) to a massive Internet of things (IoTs) with an ultra-high density (e.g., ˜1M nodes/km 2 ), ultra-low complexity (e.g., ˜10 s of bits/sec), ultra-low energy (e.g., −10+ years of battery life), and deep coverage with the capability to reach challenging locations; (2) including mission-critical control with strong security to safeguard sensitive personal, financial, or classified information, ultra-high reliability (e.g., ˜99.9999% reliability), ultra-low latency (e.g., ˜1 ms), and users with wide ranges of mobility or lack thereof; and (3) with enhanced mobile broadband including extreme high capacity (e.g., ˜10 Tbps/km 2 ), extreme data rates (e.g., multi-Gbps rate, 100+ Mbps user experienced rates), and deep awareness with advanced discovery and optimizations. 
     The 5G NR may be implemented to use optimized OFDM-based waveforms with scalable numerology and transmission time interval (TTI); having a common, flexible framework to efficiently multiplex services and features with a dynamic, low-latency time division duplex (TDD)/frequency division duplex (FDD) design; and with advanced wireless technologies, such as massive multiple input, multiple output (MIMO), robust millimeter wave (mmWave) transmissions, advanced channel coding, and device-centric mobility. Scalability of the numerology in 5G NR, with scaling of subcarrier spacing, may efficiently address operating diverse services across diverse spectrum and diverse deployments. For example, in various outdoor and macro coverage deployments of less than 3 GHz FDD/TDD implementations, subcarrier spacing may occur with 15 kHz, for example over 5, 10, 20 MHz, and the like bandwidth (BW). For other various outdoor and small cell coverage deployments of TDD greater than 3 GHz, subcarrier spacing may occur with 30 kHz over 80/100 MHz BW. For other various indoor wideband implementations, using a TDD over the unlicensed portion of the 5 GHz band, the subcarrier spacing may occur with 60 kHz over a 160 MHz BW. Finally, for various deployments transmitting with mmWave components at a TDD of 28 GHz, subcarrier spacing may occur with 120 kHz over a 500 MHz BW. 
     The scalable numerology of the 5G NR facilitates scalable TTI for diverse latency and quality of service (QoS) requirements. For example, shorter TTI may be used for low latency and high reliability, while longer TTI may be used for higher spectral efficiency. The efficient multiplexing of long and short TTIs to allow transmissions to start on symbol boundaries. 5G NR also contemplates a self-contained integrated subframe design with uplink/downlink scheduling information, data, and acknowledgement in the same subframe. The self-contained integrated subframe supports communications in unlicensed or contention-based shared spectrum, adaptive uplink/downlink that may be flexibly configured on a per-cell basis to dynamically switch between uplink and downlink to meet the current traffic needs. 
     Various other aspects and features of the disclosure are further described below. It should be apparent that the teachings herein may be embodied in a wide variety of forms and that any specific structure, function, or both being disclosed herein is merely representative and not limiting. Based on the teachings herein one of an ordinary level of skill in the art should appreciate that an aspect disclosed herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, such an apparatus may be implemented or such a method may be practiced using other structure, functionality, or structure and functionality in addition to or other than one or more of the aspects set forth herein. For example, a method may be implemented as part of a system, device, apparatus, and/or as instructions stored on a computer readable medium for execution on a processor or computer. Furthermore, an aspect may comprise at least one element of a claim. 
     Aspects of the present disclosure may provide several benefits. Sidelink UEs may form communication links between one another. The established link may be a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the UEs. The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the UEs, the beamformed link between the UEs may allow for spatial reuse of available resources due to reduced interference among the UEs. 
       FIG.  1    illustrates a wireless communication network  100  according to some aspects of the present disclosure. The network  100  includes a number of base stations (BSs)  105  and other network entities. A BS  105  may be a station that communicates with UEs  115  and may also be referred to as an evolved node B (eNB), a next generation eNB (gNB), an access point, and the like. Each BS  105  may provide communication coverage for a particular geographic area. In 3GPP, the term “cell” can refer to this particular geographic coverage area of a BS  105  and/or a BS subsystem serving the coverage area, depending on the context in which the term is used. 
     A BS  105  may provide communication coverage for a macro cell or a small cell, such as a pico cell or a femto cell, and/or other types of cell. A macro cell generally covers a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscriptions with the network provider. A small cell, such as a pico cell, would generally cover a relatively smaller geographic area and may allow unrestricted access by UEs with service subscriptions with the network provider. A small cell, such as a femto cell, would also generally cover a relatively small geographic area (e.g., a home) and, in addition to unrestricted access, may also provide restricted access by UEs having an association with the femto cell (e.g., UEs in a closed subscriber group (CSG), UEs for users in the home, and the like). A BS for a macro cell may be referred to as a macro BS. A BS for a small cell may be referred to as a small cell BS, a pico BS, a femto BS or a home BS. In the example shown in  FIG.  1   , the BSs  105   d  and  105   e  may be regular macro BSs, while the BSs  105   a - 105   c  may be macro BSs enabled with one of three dimension (3D), full dimension (FD), or massive MIMO. The BSs  105   a - 105   c  may take advantage of their higher dimension MIMO capabilities to exploit 3D beamforming in both elevation and azimuth beamforming to increase coverage and capacity. The BS  105   f  may be a small cell BS which may be a home node or portable access point. A BS  105  may support one or multiple (e.g., two, three, four, and the like) cells. 
     The network  100  may support synchronous or asynchronous operation. For synchronous operation, the BSs may have similar frame timing, and transmissions from different BSs may be approximately aligned in time. For asynchronous operation, the BSs may have different frame timing, and transmissions from different BSs may not be aligned in time. 
     The UEs  115  are dispersed throughout the wireless network  100 , and each UE  115  may be stationary or mobile. A UE  115  may also be referred to as a terminal, a mobile station, a subscriber unit, a station, or the like. A UE  115  may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a tablet computer, a laptop computer, a cordless phone, a wireless local loop (WLL) station, or the like. In one aspect, a UE  115  may be a device that includes a Universal Integrated Circuit Card (UICC). In another aspect, a UE may be a device that does not include a UICC. In some aspects, the UEs  115  that do not include UICCs may also be referred to as IoT devices or internet of everything (IoE) devices. The UEs  115   a - 115   d  are examples of mobile smart phone-type devices accessing network  100 . A UE  115  may also be a machine specifically configured for connected communication, including machine type communication (MTC), enhanced MTC (eMTC), narrowband IoT (NB-IoT) and the like. The UEs  115   e - 115   h  are examples of various machines configured for communication that access the network  100 . The UEs  115   i - 115   k  are examples of vehicles equipped with wireless communication devices configured for communication that access the network  100 . A UE  115  may be able to communicate with any type of the BSs, whether macro BS, small cell, or the like. In  FIG.  1   , a lightning bolt (e.g., communication links) indicates wireless transmissions between a UE  115  and a serving BS  105 , which is a BS designated to serve the UE  115  on the downlink (DL) and/or uplink (UL), desired transmission between BSs  105 , backhaul transmissions between BSs, or sidelink transmissions between UEs  115 . 
     In operation, the BSs  105   a - 105   c  may serve the UEs  115   a  and  115   b  using 3D beamforming and coordinated spatial techniques, such as coordinated multipoint (CoMP) or multi-connectivity. The macro BS  105   d  may perform backhaul communications with the BSs  105   a - 105   c , as well as small cell, the BS  105   f . The macro BS  105   d  may also transmits multicast services which are subscribed to and received by the UEs  115   c  and  115   d . Such multicast services may include mobile television or stream video, or may include other services for providing community information, such as weather emergencies or alerts, such as Amber alerts or gray alerts. 
     The BSs  105  may also communicate with a core network. The core network may provide user authentication, access authorization, tracking, Internet Protocol (IP) connectivity, and other access, routing, or mobility functions. At least some of the BSs  105  (e.g., which may be an example of an evolved NodeB (eNB) or an access node controller (ANC)) may interface with the core network  130  through backhaul links (e.g., S1, S2, etc.) and may perform radio configuration and scheduling for communication with the UEs  115 . In various examples, the BSs  105  may communicate, either directly or indirectly (e.g., through core network), with each other over backhaul links (e.g., X1, X2, etc.), which may be wired or wireless communication links. 
     The network  100  may also support mission critical communications with ultra-reliable and redundant links for mission critical devices, such as the UE  115   e , which may be a vehicle (e.g., a car, a truck, a bus, an autonomous vehicle, an aircraft, a boat, etc.). Redundant communication links with the UE  115   e  may include links from the macro BSs  105   d  and  105   e , as well as links from the small cell BS  105   f . Other machine type devices, such as the UE  115   f  (e.g., a thermometer), the UE  115   g  (e.g., smart meter), and UE  115   h  (e.g., wearable device) may communicate through the network  100  either directly with BSs, such as the small cell BS  105   f , and the macro BS  105   e , or in multi-hop configurations by communicating with another user device which relays its information to the network, such as the UE  115   f  communicating temperature measurement information to the smart meter, the UE  115   g , which is then reported to the network through the small cell BS  105   f . The network  100  may also provide additional network efficiency through dynamic, low-latency TDD/FDD communications, such as vehicle-to-vehicle (V2V), vehicle-to-everything (V2X), cellular-vehicle-to-everything (C-V2X) communications between a UE  115   i ,  115   j , or  115   k  and other UEs  115 , and/or vehicle-to-infrastructure (V2I) communications between a UE  115   i ,  115   j , or  115   k  and a BS  105 . 
     In some implementations, the network  100  utilizes OFDM-based waveforms for communications. An OFDM-based system may partition the system BW into multiple (K) orthogonal subcarriers, which are also commonly referred to as subcarriers, tones, bins, or the like. Each subcarrier may be modulated with data. In some instances, the subcarrier spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system BW. The system BW may also be partitioned into subbands. In other instances, the subcarrier spacing and/or the duration of TTIs may be scalable. 
     In some instances, the BSs  105  can assign or schedule transmission resources (e.g., in the form of time-frequency resource blocks (RB)) for downlink (DL) and uplink (UL) transmissions in the network  100 . DL refers to the transmission direction from a BS  105  to a UE  115 , whereas UL refers to the transmission direction from a UE  115  to a BS  105 . The communication can be in the form of radio frames. A radio frame may be divided into a plurality of subframes, for example, about 10. Each subframe can be divided into slots, for example, about 2. Each slot may be further divided into mini-slots. In a FDD mode, simultaneous UL and DL transmissions may occur in different frequency bands. For example, each subframe includes a UL subframe in a UL frequency band and a DL subframe in a DL frequency band. In a TDD mode, UL and DL transmissions occur at different time periods using the same frequency band. For example, a subset of the subframes (e.g., DL subframes) in a radio frame may be used for DL transmissions and another subset of the subframes (e.g., UL subframes) in the radio frame may be used for UL transmissions. 
     The DL subframes and the UL subframes can be further divided into several regions. For example, each DL or UL subframe may have pre-defined regions for transmissions of reference signals, control information, and data. Reference signals are predetermined signals that facilitate the communications between the BSs  105  and the UEs  115 . For example, a reference signal can have a particular pilot pattern or structure, where pilot tones may span across an operational BW or frequency band, each positioned at a pre-defined time and a pre-defined frequency. For example, a BS  105  may transmit cell specific reference signals (CRSs) and/or channel state information-reference signals (CSI-RSs) to enable a UE  115  to estimate a DL channel. Similarly, a UE  115  may transmit sounding reference signals (SRSs) to enable a BS  105  to estimate a UL channel. Control information may include resource assignments and protocol controls. Data may include protocol data and/or operational data. In some instances, the BSs  105  and the UEs  115  may communicate using self-contained subframes. A self-contained subframe may include a portion for DL communication and a portion for UL communication. A self-contained subframe can be DL-centric or UL-centric. A DL-centric subframe may include a longer duration for DL communication than for UL communication. A UL-centric subframe may include a longer duration for UL communication than for UL communication. 
     In some instances, the network  100  may be an NR network deployed over a licensed spectrum. The BSs  105  can transmit synchronization signals (e.g., including a primary synchronization signal (PSS) and a secondary synchronization signal (SSS)) in the network  100  to facilitate synchronization. The BSs  105  can broadcast system information associated with the network  100  (e.g., including a master information block (MLB), remaining minimum system information (RMSI), and other system information (OSI)) to facilitate initial network access. In some instances, the BSs  105  may broadcast the PSS, the SSS, and/or the MIB in the form of synchronization signal blocks (SSBs) over a physical broadcast channel (PBCH) and may broadcast the RMSI and/or the OSI over a physical downlink shared channel (PDSCH). 
     In some instances, a UE  115  attempting to access the network  100  may perform an initial cell search by detecting a PSS from a BS  105 . The PSS may enable synchronization of period timing and may indicate a physical layer identity value. The UE  115  may then receive an SSS. The SSS may enable radio frame synchronization, and may provide a cell identity value, which may be combined with the physical layer identity value to identify the cell. The SSS may also enable detection of a duplexing mode and a cyclic prefix length. The PSS and the SSS may be located in a central portion of a carrier or any suitable frequencies within the carrier. 
     After receiving the PSS and SSS, the UE  115  may receive a MIB. The MIB may include system information for initial network access and scheduling information for RMSI and/or OSI. After decoding the MIB, the UE  115  may receive RMSI and/or OSI. The RMSI and/or OSI may include radio resource control (RRC) information related to random access channel (RACH) procedures, paging, control resource set (CORESET) for physical downlink control channel (PDCCH) monitoring, physical uplink control channel (PUCCH), physical uplink shared channel (PUSCH), power control, SRS, and cell barring. 
     After obtaining the MIB, the RMSI and/or the OSI, the UE  115  can perform a random access procedure to establish a connection with the BS  105 . For the random access procedure, the UE  115  may transmit a random access preamble and the BS  105  may respond with a random access response. Upon receiving the random access response, the UE  115  may transmit a connection request to the BS  105  and the BS  105  may respond with a connection response (e.g., contention resolution message). 
     After establishing a connection, the UE  115  and the BS  105  can enter a normal operation stage, where operational data may be exchanged. For example, the BS  105  may schedule the UE  115  for UL and/or DL communications. The BS  105  may transmit UL and/or DL scheduling grants to the UE  115  via a PDCCH. The BS  105  may transmit a DL communication signal to the UE  115  via a PDSCH according to a DL scheduling grant. The UE  115  may transmit a UL communication signal to the BS  105  via a PUSCH and/or PUCCH according to a UL scheduling grant. 
     In some aspects, the UEs  115   c  and UE  115   d  may be sidelink UEs. The sidelink UEs  115   c  and  115   d  may form a communication link between one another. The established link may be a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the UE  115   c  and the UE  115   d . The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the UE  115   c  and the UE  115   d , the beamformed link between the UE  115   c  and the UE  115   d  may allow for spatial reuse of available resources in the network  100  due to reduced interference among the UEs. 
       FIG.  2    illustrates sidelink resources associated with a wireless communication network  200  according to some aspects of the present disclosure. The wireless communications network  200  may include a base station  105   a  and UEs  115   a ,  115   b , and  115   c , which may be examples of a BS  105  and a UE  115  as described with reference to  FIG.  1   . Base station  105   a  and UEs  115   a  and  115   c  may communicate within geographic coverage area  110   a  and via communication links  205   a  and  205   b , respectively. UE  115   c  may communicate with UEs  115   a  and  115   b  via sidelink communication links  210   a  and  210   b , respectively. In some examples, UE  115   c  may transmit SCI to UEs  115   a  and  115   b  via the sidelink control resources  220 . The SCI may include an indication of resources reserved for retransmissions by UE  115   c  (e.g., the reserved resources  225 ). In some examples, UEs  115   a  and  115   b  may determine to reuse one or more of the reserved resources  225 . 
     In some aspects, a device in the wireless communication network  200  (e.g., a UE  115 , a BS  105 , or some other node) may convey SCI to another device (e.g., another UE  115 , a BS  105 , sidelink device or vehicle-to-everything (V2X) device, or other node). The SCI may be conveyed in one or more stages. The first stage SCI may be carried on the PSCCH while the second stage SCI may be carried on the corresponding PSSCH. For example, UE  115   c  may transmit a PSCCH/first stage SCI  235  (e.g., SCI-1) to each sidelink UE  115  in the network (e.g., UEs  115   a  and  115   b ) via the sidelink communication links  210 . The PSCCH/first stage SCI-1  235  may indicate resources that are reserved by UE  115   c  for retransmissions (e.g., the SCI-1 may indicate the reserved resources  225  for retransmissions). Each sidelink UE  115  may decode the first stage SCI-1 to determine where the reserved resources  225  are located (e.g., to refrain from using resources that are reserved for another sidelink transmission and/or to reduce resource collision within the wireless communications network  200 ). Sidelink communication may include a mode 1 operation in which the UEs  115  are in a coverage area of BS  105   a . In mode 1, the UEs  115  may receive a configured grant from the BS  105   a  that defines parameters for the UEs  115  to access the channel. Sidelink communication may also include a mode 2 operation in which the UEs  115  operate autonomously from the BS  105   a  and perform sensing of the channel to gain access to the channel. In some aspects, during mode 2 sidelink operations, the sidelink UEs  115  may perform channel sensing to locate resources reserved by other sidelink transmissions. The first stage SCI-1 may reduce the need for sensing each channel. For example, the first stage SCI-1 may include an explicit indication such that the UEs  115  may refrain from blindly decoding each channel. The first stage SCI-1 may be transmitted via the sidelink control resources  220 . The sidelink control resources  220  may be configured resources (e.g., time resources or frequency resources) transmitted via a PSCCH  235 . In some examples, the PSCCH  235  may be configured to occupy a number of physical resource blocks (PRBs) within a selected frequency. The frequency may include a single subchannel  250  (e.g., 10, 12, 15, 20, 25, or some other number of RBs within the subchannel  250 ). The time duration of the PSCCH  235  may be configured by the BS  105   a  (e.g., the PSCCH  235  may span 1, 2, 3, or some other number of symbols  255 ). 
     The first stage SCI-1 may include one or more fields to indicate a location of the reserved resources  225 . For example, the first stage SCI-1 may include, without limitation, one or more fields to convey a frequency domain resource allocation (FDRA), a time domain resource allocation (TDRA), a resource reservation period  245  (e.g., a period for repeating the SCI transmission and the corresponding reserved resources  225 ), a modulation and coding scheme (MCS) for a second stage SCI-2  240 , a beta offset value for the second stage SCI-2  240 , a DMRS port (e.g., one bit indicating a number of data layers), a physical sidelink feedback channel (PSFCH) overhead indicator, a priority, one or more additional reserved bits, or a combination thereof. The beta offset may indicate the coding rate for transmitting the second stage SCI-2  240 . The beta offset may indicate an offset to the MCS index. The MCS may be indicated by an index ranging from 0 to 31. For example, if the MCS is set at index 16 indicating a modulation order of 4 and a coding rate of 378, the beta offset may indicate a value of 2 thereby setting the coding rate to 490 based on an MCS index of 18. In some examples, the FDRA may be a number of bits in the first stage SCI-1 that may indicate a number of slots  238  and a number of subchannels reserved for the reserved resources  225  (e.g., a receiving UE  115  may determine a location of the reserved resources  225  based on the FDRA by using the subchannel  250  including the PSCCH  235  and first stage SCI-1 as a reference). The TDRA may be a number of bits in the first stage SCI-1 (e.g., 5 bits, 9 bits, or some other number of bits) that may indicate a number of time resources reserved for the reserved resources  225 . In this regard, the first stage SCI-1 may indicate the reserved resources  225  to the one or more sidelink UEs  115  in the wireless communication network  200 . 
     In some aspects, the UEs  115   a  and UE  115   c  may be sidelink UEs. The sidelink UEs  115   a  and  115   c  may form a communication link between one another. The established link may be a beamformed link. The beamformed link may be a (bi-)directional link established by one more directional antennas in each of the UE  115   a  and the UE  115   c . The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the UE  115   a  and the UE  115   c , the beamformed link between the UE  115   a  and the UE  115   c  may allow for spatial reuse of available resources in the network  200  due to reduced interference among the UEs. 
       FIG.  3 A  illustrates a beamformed link between the UE  115   a  and the UE  115   c  according to some aspects of the present disclosure. The UE  115   a  and the UE  115   b  may be sidelink UEs. In this regard, the UE  115   a  may establish a radio resource control (RRC) connection with the UE  115   c . In some instances, the UE  115   a  may establish a PC5-RRC connected mode state with the UE  115   c . The PC5-RRC connected state may enable exchanging of access-stratum level information for alignment between the UE  115   a  (e.g., a transmitting UE) and the UE  115   c  (e.g., a receiving UE) to support SL unicast communications. The unicast communications may be one-to-one communications between the UE  115   a  and the UE  115   c . In some aspects, the UE  115   a  may have multiple PC5-RRC connections with multiple UEs for unicast communications between the UE  115   a  and the multiple UEs. For example, referring to  FIGS.  1 ,  2 ,  3 A,  4 A , and/or  5 A, the UE  115   c  may have a PC5-RRC connection with the UE  115   a  and the UE  115   c.    
     The established link may be a beamformed link. For example, the beam  310  associated with the UE  115   c  and the beam  312  associated with the UE  115   a  may create a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the UE  115   a  and the UE  115   c . The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the UE  115   a  and the UE  115   c , the beamformed link between the UE  115   a  and the UE  115   c  may allow for spatial reuse of available resources due to reduced interference among the UEs. 
       FIG.  3 B  illustrates resources associated with a beacon signal  320  according to some aspects of the present disclosure. In  FIG.  3 B , the x-axis represents time in some arbitrary units and the Y-axis represents frequency in some arbitrary units. The UE  115   a  may transmit a periodic discovery and measurement timing (DMTC) configuration via the established link to the UE  115   c . The UE  115   a  may transmit the periodic DMTC configuration via a beamformed link with the UE  115   c . In this regard, the UE  115   a  may transmit the DMTC configuration to the UE  115   c  via a PSSCH, a physical sidelink control channel (PSCCH), or other suitable channel. The DMTC configuration may include parameters to enable the UE  115   a  and/or the UE  115   c  to measure and report the status of the beamformed links corresponding to beams  310  and  312 . The DMTC configuration may specify values for DMTC parameters defining beacon signal  320  transmission timing and resources (e.g., slot(i), slot (i+k), etc.). The specified values may define periodic DMTC occasions that include periodic beacon signal transmission windows for beacon signal  320  transmissions from the UE  115   a  to the UE  115   c  and from the UE  115   c  to the UE  115   a . The DMTC configuration may include UE  115   a  beacon signal period  342 , UE  115   c  beacon signal period  344 , and/or the time/frequency resources associated with a DMTC resource window  330 . When operating in sidelink mode 1, the UE  115   a  may receive the DMTC configuration from a BS (e.g., the BS  105  or the BS  800 ). In this regard the UE  115   a  may receive the DMTC configuration from the BS in a configured grant. The UE  115   a  may transmit (e.g., forward) the DMTC configuration received from the BS to the UE  115   c . When operating in sidelink mode 2, the UE  115   a  may determine the DMTC configuration. The UE  115   a  may transmit the DMTC configuration to the UE  115   c  in a configured grant. The DMTC configuration may enable beamformed radio link failure detection by the UE  115   c  (e.g., the receiving UE). 
     In some aspects, the UE  115   a  may transmit a periodic beacon signal  320  to the UE  115   c  as indicated by arrows  350  and  354  based on the DMTC configuration. In this regard, the UE  115   a  may transmit the beacon signal  320  to the UE  115   c  via a PSSCH. Additionally or alternatively, the UE  115   c  may transmit a periodic beacon signal  320  to the UE  115   a  as indicated by arrow  352  based on the DMTC configuration. In this regard, the UE  115   c  may transmit the beacon signal  320  to the UE  115   a  via a PSSCH. The UE  115   c  may measure aspects of the beacon signal  320  to determine the status (e.g., quality status) of the beamformed links corresponding to beams  310 ,  312 . The status of the beacon signal  320  may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). In some aspects, the beacon signal  320  may be used by other UEs  115  to discover neighboring UEs  115 . For example, the beacon signal  320  carried by the PSSCH may include information to assist RF discovery by a third UE  115  (e.g., the UE  115   b ). The beacon signal  320  may be received by the UE  115   b  and indicate the DMTC of the UE  115   c  or any other UE  115  that the UE  115   a  has a unicast connection with. For example, the beacon signal  320  may include the layer 2 ID of all UEs  115  that the UE  115   a  has a unicast connection with and their corresponding DMTCs. The UE  115   b  may determine that the UE  115   a  is within the vicinity of the UE  115   c  based on receiving the DMTC of the UE  115   c.    
     The UE  115   a  may transmit the beacon signal  320  within the DMTC resource window  330 . The DMTC window  330  may include a set of time and frequency resources in which the beacon signal  320  can be transmitted. For example, the DMTC resource window  330  may include a set of resource elements (REs). The set of REs may include time resources (e.g., symbols, slots (slot (i), slot (i+k)), sub-slots) and frequency resources (e.g., frequency subcarriers, frequency bands, frequency ranges). The UE  115   a  may transmit the beacon signal  320  in a subset of REs in the DMTC resource window  330 . The DMTC configuration received by the UE  115   c  (e.g., from the UE  115   a  and/or the BS  105 ) may indicate the REs defining the DMTC resource window  330  and/or the subset of REs carrying the beacon signal  320 . In some aspects, the UE  115   c  may search and/or monitor the entire DMTC resource window  330  (e.g., all REs, symbols, slots, and/or frequency subcarriers within the DMTC window) for the beacon signal  320  carried by the PSSCH in a subset of REs of the DMTC resource window  330 . 
     The UE  115   a  may transmit the beacon signal  320  according to the UE  115   a  beacon signal period  342 , for example, at about 10 ms, 20 ms, 40 ms, 80 ms, 160 ms, or any suitable periodicity. In some aspects, the beacon signal  320  may include a sidelink channel state information-reference signal (CSI-RS). The UE  115   c  may measure aspects of the received CSI-RS to determine the status (e.g., the quality) of the beamformed link. The status of the beacon signal  320  may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     The UE  115   c  may generate a CSI report describing the quality of the beamformed link. The CSI report may include information related to the channel conditions in the beamformed link between the UE  115   a  and the UE  115   c . For example, the CSI report may include a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     In some aspects, the beacon signal  320  may include second-stage sidelink control information  340  (SCI-2). Additionally or alternatively, the UE  115   c  may determine the status of the beamformed link based upon decoding the SCI-2  340 . For example, if the UE  115   c  successfully decodes the SCI-2  340 , the UE  115   c  may determine the quality of the beamformed link to be acceptable. However, if the UE  115   c  is unable to successfully decode the SCI-2  340 , the UE  115   c  may determine the quality of the beamformed link to be unacceptable. 
     If the quality of the beacon signal  320  fails to satisfy a threshold (e.g., based on CSI-RS measurement(s), SCI-2  340  decoding, etc.), then the UE  115   c  may generate a beam failure indication (BFI) to the medium access layer (MAC) layer (e.g. layer 2) of the UE  115   c . The UE  115   c  may transmit the BFI to the UE  115   a  over a signal transmitted from the UE  115   c  to the UE  115   a . For example, the UE  115   c  may transmit the BFI to the UE  115   a  over a beacon signal  320 , a PSSCH, a PSCCH, a PBSCH, or a combination thereof. 
     In some aspects, the UE  115   a  may transmit a plurality of periodic beacon signals  320  as indicated by arrows  350  and  352 . The UE  115   a  may transmit the plurality of periodic beacon signals  320  in a plurality of beam directions in addition to beam  312 . In some instances, each of the beacon signals  320  may be transmitted in a different beam direction. For example, and without limitation, the UE  115   a  may transmit four beacon signals  320 . Each of the four beacon signals may be transmitted about ninety degrees from the adjacent beam directions. The DMTC configuration may include time/frequency resource information (e.g., pointers to resource blocks) associated with each of the beacon signals  320 . The DMTC configuration may further include beam direction information (e.g., a beam direction index, a beam direction codebook) associated with each of the different beacon signals  320 . The UE  115   c  may perform beam sweeping to monitor for each of the different beacon signals  320 . The UE  115   c  may measure the quality of the different received beacon signals  320 . For example, the UE  115   c  may have an established beamformed link over a first directional beam  312  from the UE  115   a . However, due to channel conditions and/or relative positions of the UE  115   a  and the UE  115   c  (e.g., due to movement of the UE  115   a  and/or the UE  115   c  and/or movement of interfering structure(s) and/or device(s) between the UE  115   a  and the UE  115   c ), the UE  115   c  may measure a higher quality channel over a different directional beam (e.g., a second directional beam different than the first directional beam  312 ). The UE  115   c  may transmit a CSI report to the UE  115   a  indicating a higher channel quality over the different directional beam. The UE  115   a  and the UE  115   c  may reestablish an RRC connection via the different directional beam based on the CSI report. 
       FIG.  4 A  illustrates beamformed links between the UE  115   c  and the UEs  115   a  and  115   b  according to some aspects of the present disclosure. The UEs  115   a ,  115   b , and  115   c  may be sidelink UEs. In this regard, the UEs  115   a  and  115   b  may establish a radio resource control (RRC) connection with the UE  115   c . In some instances, the UEs  115   a  and  115   b  may establish a PC5-RRC connected mode state with the UE  115   c . The PC5-RRC connected state may enable exchanging of access-stratum level information for alignment between the UE  115   c  and the UEs  115   a  and  115   b  to support SL unicast communications. The unicast communications may be one-to-one communications between the UE  115   c  and each of the UEs  115   a  and  115   b . In some aspects, the UE  115   c  may have multiple PC5-RRC connections with multiple UEs for unicast communications between the UE  115   c  and the multiple UEs. For example, referring to  FIGS.  1 ,  2 ,  3 A,  4 A , and/or  5 A, the UE  115   c  may have a PC5-RRC connection with the UE  115   a  and the UE  115   b.    
     The established link may be a beamformed link. For example, the beam  310  associated with the UE  115   c  and the beam  312  associated with the UE  115   a  may create a beamformed link. The beam  310  associated with the UE  115   c  and the beam  414  associated with the UE  115   b  may create a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the UE  115   a , the UE  115   b , and the UE  115   c . The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the UE  115   c  and the UEs  115   a  and  115   b , the beamformed link between the UE  115   c  and the UEs  115   a  and  115   b  may allow for spatial reuse of available resources due to reduced interference among the links between the UEs  115   a ,  115   b , and the UE  115   c  according to some aspects of the present disclosure. 
       FIG.  4 B  illustrates a signaling diagram of a communication method according to some aspects of the present disclosure. In some aspects, the UE  115   c  may have multiple established links with a group of UEs (e.g., the UEs  115   a  and  115   b ) over beamformed links. In this case, the UE  115   c  may not transmit a separate beacon signal to each of the UEs in the group. Instead, the UE  115   c  may transmit the beacon signal in a single groupcast transmission to the group of UEs. For example, the destination ID in the SCI-2 associated with the beacon signal may be a groupcast destination ID that identifies the group of UEs. The groupcast beacon signal may reduce communication overhead as compared to multiple unicast transmissions to each UE in the group. 
     At action  402 , the UE  115   a  may transmit a beacon signal to the UE  115   c . The UE  115   a  may transmit the beacon signal via a PSSCH over beam  312 . 
     At action  404 , the UE  115   a  may transmit the status of beam  310  to the UE  115   c . The UE  115   a  may transmit the status as a CSI report associated with a beacon signal received over beam  310 . The UE  115   a  may transmit the status via a PSSCH over beam  312 . 
     At action  406 , the UE  115   b  may transmit a beacon signal to the UE  115   c . The UE  115   b  may transmit the beacon signal via a PSSCH over beam  414 . 
     At action  408 , the UE  115   b  may transmit the status of beam  310  to the UE  115   c . The UE  115   b  may transmit the status as a CSI report associated with a beacon signal received over beam  310 . The UE  115   b  may transmit the status via a PSSCH over beam  414 . 
     At action  410 , the UE  115   c  may transmit a groupcast beacon signal over beam  310 . In some aspects, the UE  115   c  may have multiple established links with the UEs  115   a  and  115   b . In this case, the UE  115   c  may not transmit a separate beacon signal to each of the UEs  115   a  and  115   b . Instead, the UE  115   c  may transmit the beacon signal in a single groupcast transmission to the UEs  115   a  and  115   b . For example, the destination ID in the SCI-2 associated with the beacon signal may be a groupcast destination ID that identifies a group of UEs (e.g., the UEs  115   a  and  115   b ). The groupcast beacon signal may reduce communication overhead as compared to multiple unicast transmissions to each UE in the group (e.g., the UEs  115   a  and  115   b ). 
     At action  412 , the UE  115   c  may transmit the status of the beam  312  to the UE  115   a  and transmit the status of the beam  414  to the UE  115   b . The UE  115   c  may transmit the status of the beam  312  as a CSI report associated with a beacon signal received at action  402 . The UE  115   c  may transmit the status of the beam  414  as a CSI report associated with a beacon signal received at action  406 . The UE  115   c  may transmit the status of the beams  312  and  414  via a PSSCH. 
       FIG.  5 A  illustrates a beamformed link between the UE  115   a  and the UE  115   c  according to some aspects of the present disclosure. The UE  115   a  and the UE  115   b  may be sidelink UEs. In this regard, the UE  115   a  may establish a radio resource control (RRC) connection with the UE  115   c . In some instances, the UE  115   a  may establish a PC5-RRC connected mode state with the UE  115   c . The PC5-RRC connected state may enable exchanging of access-stratum level information for alignment between the UE  115   a  (e.g., a transmitting UE) and the UE  115   c  (e.g., a receiving UE) to support SL unicast communications. The unicast communications may be one-to-one communications between the UE  115   a  and the UE  115   c . In some aspects, the UE  115   a  may have multiple PC5-RRC connections with multiple UEs for unicast communications between the UE  115   a  and the multiple UEs. For example, referring to  FIGS.  1 ,  2 ,  3 A,  4 A , and/or  5 A, the UE  115   c  may have a PC5-RRC connection with the UE  115   a  and the UE  115   c.    
       FIG.  5 B  illustrates resources associated with a beacon signal according to some aspects of the present disclosure. In  FIG.  5 B , the x-axis represents time in some arbitrary units and the Y-axis represents frequency in some arbitrary units. The UE  115   a  may transmit a periodic discovery and measurement timing (DMTC) configuration via the established link to the UE  115   c . The UE  115   a  may transmit the periodic DMTC configuration via a beamformed link with the UE  115   c . In this regard, the UE  115   a  may transmit the DMTC configuration to the UE  115   c  via a PSSCH, a physical sidelink control channel (PSCCH), or other suitable channel. The DMTC configuration may include parameters to enable the UE  115   a  and/or the UE  115   c  to measure and report the status of the beamformed links corresponding to beams  310  and  312 . 
     In some aspects, the UE  115   c  may refrain from transmitting the periodic beacon signal  320  within a time period after a number of successful communications between the UE  115   c  and the UE  115   a  satisfies a threshold. The beacon signal  320  may be used to determine a status of the beamformed link between the UE  115   c  and the UE  115   a . Additionally or alternatively, the status of the beamformed link may be determined based on a sequence of successful transmissions between the UE  115   c  and the UE  115   a . For example, the UE  115   c  may transmit a sequence of transport blocks (TBs) to the UE  115   a  via the beamformed link. If the TBs are successfully received by the UE  115   a , as indicated by an ACK  512  being transmitted from the UE  115   a  to the UE  115   c  in response to each of the TBs, then the UE  115   c  may refrain from transmitting the beacon signal(s) for a time period (e.g., a DMTC transmission period, 10 ms, 20 ms, 40 ms, 80 ms, or 160 ms) as the successful transmission of the TBs is an indication of the quality of the beamformed link. In some aspects, the UE  115   c  may be configured to transmit the beacon signal  320  when the numConsecutiveDTX satisfies a threshold (e.g., number of NACKs  510  is greater than a threshold). In some aspects, the UE  115   c  may be configured to transmit the beacon signal  320  when the sl-maxnumConsecutiveDTXForDMTC is greater than a threshold. The UE  115   c  may transmit an indicator indicating to the UE  115   a  that the next scheduled beacon signal  320  will be skipped. For example, the UE  115   c  may transmit a code point in the SCI-1 indicating to the UE  115   a  that the next scheduled beacon signal  320  will be skipped, for example, when the numConsecutiveDTX is less than a threshold. 
     As shown in  FIG.  5 B , the numConsecutiveDTX may be incremented by one when a HARQ NACK  510  indicates an unsuccessful reception of a TB by the UE  115   a . Conversely, the numConsecutiveDTX may be decremented by one when a HARQ ACK  512  indicates a successful reception of a TB by the UE  115   a . When the numConsecutiveDTX is greater than a threshold of zero, the UE  115   c  may transmit the beacon signal  320  as indicated by arrows  514 . When the numConsecutiveDTX satisfies the threshold of zero, the UE  115   c  may skip transmitting the beacon signal  320  as indicated by arrow  516 . 
       FIG.  6    illustrates a signaling diagram of a communication method according to some aspects of the present disclosure. At action  602 , the BS  105  may determine a DMTC configuration for the UEs  115   c  and  115   a . In this regard, the BS  105  may determine the DMTC configuration to enable the UE  115   a  and the UE  115   c  to measure and report the status of a beamformed link between the UE  115   a  and the UE  115   c . The DMTC configuration may specify values for DMTC parameters defining beacon signal transmission timing and resources. The specified values may define periodic DMTC occasions that include periodic beacon signal transmission windows for beacon signal transmissions from the UE  115   c  to the UE  115   a . The DMTC configuration may include the beacon signal periodicity and/or the time/frequency resources associated with a DMTC resource window. 
     At action  604 , the BS  105  may transmit the DMTC configuration to the UE  115   c . In this regard, the BS  105  may transmit the DMTC configuration to the UE  115   c  via a configured grant and/or downlink control information (DCI). 
     At action  605 , the UE  115   c  may transmit the DMTC configuration to the UE  115   a . In this regard, the UE  115   c  may transmit the DMTC configuration to the UE  115   a  via a physical sidelink shared channel (PSSCH), a physical sidelink control channel (PSCCH), or other suitable channel. 
     At action  606 , the UE  115   c  may transmit a beacon signal to the UE  115   a . In this regard, the UE  115   c  may transmit the beacon signal to the UE  115   a  via a physical sidelink shared channel (PSSCH), a physical sidelink control channel (PSCCH), or other suitable channel. 
     At action  608 , the UE  115   a  may decode the PSSCH carrying the beacon signal in a DMTC window. The DMTC window may include a set of time and frequency resources in which the beacon signal can be transmitted. For example, the DMTC resource window may include a set of resource elements (REs). The set of REs may include time resources (e.g., symbols, slots, sub-slots) and frequency resources (e.g., frequency subcarriers, frequency bands, frequency ranges). The UE  115   c  may transmit the beacon signal in a subset of REs in the DMTC resource window at action  606 . The DMTC configuration received by the UE  115   a  at action  605  may indicate the REs defining the DMTC resource window and/or the subset of REs carrying the beacon signal. In some aspects, the UE  115   a  may search and/or monitor the entire DMTC resource window (e.g., all REs, symbols, slots, and/or frequency subcarriers within the DMTC window) for the beacon signal. 
     At action  610 , the UE  115   a  may transmit the status of the beamformed link to the UE  115   c . In this regard, the status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     At action  612 , the UE  115   a  may transmit a beacon signal to the UE  115   c . In this regard, the UE  115   a  may transmit the beacon signal to the UE  115   c  via a physical sidelink shared channel (PSSCH), a physical sidelink control channel (PSCCH), or other suitable channel. In some aspects, the UE  115   a  may perform any or all of the actions executed by the UE  115   c . In other words, the UE  115   a  may transmit a DMTC configuration and/or a beacon signal over the beamformed link to the UE  115   c . The UE  115   c  may measure the quality of the beacon signal and transmit a CSI report to the UE  115   a . In this fashion, the UE  115   a  and the UE  115   c  may determine the quality of the bidirectional communication link. The UE  115   c  may transmit a beacon signal to the UE  115   a  and the UE  115   a  may transmit a different beacon signal to the UE  115   c  in a bidirectional fashion. The bidirectional beacon signals between the UE  115   c  and the UE  115   a  may further include control elements for managing the beamformed link. 
     At action  614 , the UE  115   c  may decode the PSSCH carrying the beacon signal in a DMTC window. The DMTC configuration received by the UE  115   c  at action  604  may indicate the REs defining the DMTC resource window and/or the subset of REs carrying the beacon signal. In some aspects, the UE  115   c  may search and/or monitor the entire DMTC resource window (e.g., all REs, symbols, slots, and/or frequency subcarriers within the DMTC window) for the beacon signal. 
     At action  616 , the UE  115   c  may transmit the status of the beamformed link to the UE  115   a . In this regard, the status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     At action  618 , the UE  115   c  may transmit one or more transport blocks to the UE  115   a . In this regard, the UE  115   c  may transmit the one or more transport blocks to the UE  115   a  in one or more PSSCHs. 
     At action  620 , the UE  115   a  may successfully decode the one or more transport blocks carried by the one or more PSSCHs. 
     At action  622 , the UE  115   a  may transmit one or more HARQ ACK messages to the UE  115   c . In this regard, the UE  115   a  may transmit a HARQ ACK message to the UE  115   c  via a physical sidelink feedback channel (PSFCH). 
     At action  624 , the UE  115   c  may transmit a beacon signal skip indicator to the UE  115   a . In this regard, the UE  115   c  may transmit an indicator indicating to the UE  115   a  that the next scheduled beacon signal will be skipped. For example, the UE  115   c  may transmit a code point in an SCI-1 indicating to the UE  115   a  that the next scheduled beacon signal will be skipped. The beacon signal skip indicator may be transmitted based on the UE  115   c  receiving one or more HARQ ACK messages at action  622  indicating the quality of the beamformed link between the UE  115   c  and the UE  115   a  to be acceptable and a beacon signal is not required for a time period. 
       FIG.  7    is a block diagram of an exemplary UE  700  according to some aspects of the present disclosure. The UE  700  may be the UE  115  in the network  100  or  200  as discussed above. As shown, the UE  700  may include a processor  702 , a memory  704 , a beacon signal module  708 , a transceiver  710  including a modem subsystem  712  and a radio frequency (RF) unit  714 , and one or more antennas  716 . These elements may be coupled with each other and in direct or indirect communication with each other, for example via one or more buses. 
     The processor  702  may include a central processing unit (CPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a controller, a field programmable gate array (FPGA) device, another hardware device, a firmware device, or any combination thereof configured to perform the operations described herein. The processor  702  may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The memory  704  may include a cache memory (e.g., a cache memory of the processor  702 ), random access memory (RAM), magnetoresistive RAM (MRAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), flash memory, solid state memory device, hard disk drives, other forms of volatile and non-volatile memory, or a combination of different types of memory. In some instances, the memory  704  includes a non-transitory computer-readable medium. The memory  704  may store instructions  706 . The instructions  706  may include instructions that, when executed by the processor  702 , cause the processor  702  to perform the operations described herein with reference to the UEs  115  in connection with aspects of the present disclosure, for example, aspects of  FIGS.  2 - 6  and  9 - 10   . Instructions  706  may also be referred to as code. The terms “instructions” and “code” should be interpreted broadly to include any type of computer-readable statement(s). For example, the terms “instructions” and “code” may refer to one or more programs, routines, sub-routines, functions, procedures, etc. “Instructions” and “code” may include a single computer-readable statement or many computer-readable statements. 
     The beacon signal module  708  may be implemented via hardware, software, or combinations thereof. For example, the beacon signal module  708  may be implemented as a processor, circuit, and/or instructions  706  stored in the memory  704  and executed by the processor  702 . 
     In some aspects, the beacon signal module  708  is configured to transmit and/or receive a beacon signal via a physical sidelink shared channel (PSSCH). The UE receiving the beacon signal may measure aspects of the beacon signal to determine the status (e.g., quality status) of the beamformed link. The status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). In some aspects, the beacon signal may be used by other UEs to discover neighboring UEs. The beacon signal carried by the PSSCH may include information to assist RF discovery by a third UE. 
     As shown, the transceiver  710  may include the modem subsystem  712  and the RF unit  714 . The transceiver  710  can be configured to communicate bi-directionally with other devices, such as the BSs  105  and/or the UEs  115 . The modem subsystem  712  may be configured to modulate and/or encode the data from the memory  704  and the beacon signal module  708  according to a modulation and coding scheme (MCS), e.g., a low-density parity check (LDPC) coding scheme, a turbo coding scheme, a convolutional coding scheme, a digital beamforming scheme, etc. The RF unit  714  may be configured to process (e.g., perform analog to digital conversion or digital to analog conversion, etc.) modulated/encoded data from the modem subsystem  712  (on outbound transmissions) or of transmissions originating from another source such as a UE  115  or a BS  105 . The RF unit  714  may be further configured to perform analog beamforming in conjunction with the digital beamforming. Although shown as integrated together in transceiver  710 , the modem subsystem  712  and the RF unit  714  may be separate devices that are coupled together to enable the UE  700  to communicate with other devices. 
     The RF unit  714  may provide the modulated and/or processed data, e.g. data packets (or, more generally, data messages that may contain one or more data packets and other information), to the antennas  716  for transmission to one or more other devices. The antennas  716  may further receive data messages transmitted from other devices. The antennas  716  may provide the received data messages for processing and/or demodulation at the transceiver  710 . The antennas  716  may include multiple antennas of similar or different designs in order to sustain multiple transmission links. The RF unit  714  may configure the antennas  716 . 
     In some instances, the UE  700  can include multiple transceivers  710  implementing different RATs (e.g., NR and LTE). In some instances, the UE  700  can include a single transceiver  710  implementing multiple RATs (e.g., NR and LTE). In some instances, the transceiver  710  can include various components, where different combinations of components can implement RATs. 
     In some aspects, the processor  702  may be coupled to the memory  704 , the beacon signal module  708 , and/or the transceiver  710 . The processor  702  and may execute operating system (OS) code stored in the memory  704  in order to control and/or coordinate operations of the beacon signal module  708  and/or the transceiver  710 . In some aspects, the processor  702  may be implemented as part of the beacon signal module  708 . 
       FIG.  8    is a block diagram of an exemplary BS  800  according to some aspects of the present disclosure. The BS  800  may be a BS  105  as discussed above. As shown, the BS  800  may include a processor  802 , a memory  804 , a beacon signal module  808 , a transceiver  810  including a modem subsystem  812  and a RF unit  814 , and one or more antennas  816 . These elements may be coupled with each other and in direct or indirect communication with each other, for example via one or more buses. 
     The processor  802  may have various features as a specific-type processor. For example, these may include a CPU, a DSP, an ASIC, a controller, a FPGA device, another hardware device, a firmware device, or any combination thereof configured to perform the operations described herein. The processor  802  may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The memory  804  may include a cache memory (e.g., a cache memory of the processor  802 ), RAM, MRAM, ROM, PROM, EPROM, EEPROM, flash memory, a solid state memory device, one or more hard disk drives, memristor-based arrays, other forms of volatile and non-volatile memory, or a combination of different types of memory. In some instances, the memory  804  may include a non-transitory computer-readable medium. The memory  804  may store instructions  806 . The instructions  806  may include instructions that, when executed by the processor  802 , cause the processor  802  to perform operations described herein, for example, aspects of  FIGS.  2 - 6  and  9 - 10   . Instructions  806  may also be referred to as code, which may be interpreted broadly to include any type of computer-readable statement(s). 
     The beacon signal module  808  may be implemented via hardware, software, or combinations thereof. For example, the beacon signal module  808  may be implemented as a processor, circuit, and/or instructions  806  stored in the memory  804  and executed by the processor  802 . 
     The beacon signal module  808  may be used for various aspects of the present disclosure, for example, aspects of  FIGS.  2 - 6  and  9 - 10   . In some aspects, the beacon signal module  708  is configured to determine a DMTC configuration and transmit the DMTC configuration to a UE (e.g., the UE  115  or  700 ). 
     Additionally or alternatively, the beacon signal module  808  can be implemented in any combination of hardware and software, and may, in some implementations, involve, for example, processor  802 , memory  804 , instructions  806 , transceiver  810 , and/or modem  812 . 
     As shown, the transceiver  810  may include the modem subsystem  812  and the RF unit  814 . The transceiver  810  can be configured to communicate bi-directionally with other devices, such as the UEs  115  and/or  800 . The modem subsystem  812  may be configured to modulate and/or encode data according to a MCS, e.g., a LDPC coding scheme, a turbo coding scheme, a convolutional coding scheme, a digital beamforming scheme, etc. The RF unit  814  may be configured to process (e.g., perform analog to digital conversion or digital to analog conversion, etc.) modulated/encoded data from the modem subsystem  812  (on outbound transmissions) or of transmissions originating from another source such as a UE  115  or UE  700 . The RF unit  814  may be further configured to perform analog beamforming in conjunction with the digital beamforming. Although shown as integrated together in transceiver  810 , the modem subsystem  812  and/or the RF unit  814  may be separate devices that are coupled together at the BS  800  to enable the BS  800  to communicate with other devices. 
     The RF unit  814  may provide the modulated and/or processed data, e.g. data packets (or, more generally, data messages that may contain one or more data packets and other information), to the antennas  816  for transmission to one or more other devices. This may include, for example, a configuration indicating a plurality of sub-slots within a slot according to aspects of the present disclosure. The antennas  816  may further receive data messages transmitted from other devices and provide the received data messages for processing and/or demodulation at the transceiver  810 . The antennas  816  may include multiple antennas of similar or different designs in order to sustain multiple transmission links. 
     In some instances, the BS  800  can include multiple transceivers  810  implementing different RATs (e.g., NR and LTE). In some instances, the BS  800  can include a single transceiver  810  implementing multiple RATs (e.g., NR and LTE). In some instances, the transceiver  810  can include various components, where different combinations of components can implement RATs. 
     In some aspects, the processor  802  may be coupled to the memory  804 , the beacon signal module  808 , and/or the transceiver  810 . The processor  802  may execute OS code stored in the memory  804  to control and/or coordinate operations of the beacon signal module  808 , and/or the transceiver  810 . In some aspects, the processor  802  may be implemented as part of the beacon signal module  808 . In some aspects, the processor  802  is configured to transmit via the transceiver  810 , to a UE, an indicator indicating a configuration of sub-slots within a slot. 
       FIG.  9    is a flow diagram of a communication method  900  according to some aspects of the present disclosure. Aspects of the method  900  can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a wireless communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE  115  or UE  700 , may utilize one or more components, such as the processor  702 , the memory  704 , the beacon signal module  708 , the transceiver  710 , the modem  712 , and the one or more antennas  716 , to execute aspects of method  900 . The method  900  may employ similar mechanisms as in the networks  100  and  200  and the aspects and actions described with respect to  FIGS.  2 - 6   . As illustrated, the method  900  includes a number of enumerated actions, but the method  900  may include additional actions before, after, and in between the enumerated actions. In some aspects, one or more of the enumerated actions may be omitted or performed in a different order. 
     At action  910 , the method  900  includes a first sidelink UE (e.g., the UE  115  or the UE  700 ) establishing a link with a second sidelink UE. In this regard, the first UE may establish a radio resource control (RRC) connection with the second sidelink UE. In some instances, the first UE may establish a PC5-RRC connected mode state with the second UE. The PC5-RRC connected state may enable exchanging of access-stratum level information for alignment between the first UE (e.g., a transmitting UE) and the second UE (e.g., a receiving UE) to support SL unicast communications. The unicast communications may be one-to-one communications between the first UE and the second UE. In some aspects, the first UE may have multiple PC5-RRC connections with multiple UEs for unicast communications between the first UE and the multiple UEs. For example, referring to  FIGS.  1 ,  2   , and/or  4   a , the UE  115   c  may have a PC5-RRC connection with the UE  115   a  and the UE  115   c.    
     The established link may be a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the first UE and the second UE. The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the first UE and the second UE, the beamformed link between the first UE and the second UE may allow for spatial reuse of available resources due to reduced interference among the UEs. 
     At action  920 , the method  900  includes the first UE transmitting a periodic discovery and measurement timing (DMTC) configuration via the established link to the second sidelink UE. The first UE may transmit the periodic DMTC configuration via a beamformed link with the second sidelink UE. In this regard, the first UE may transmit the DMTC configuration to the second UE via a physical sidelink shared channel (PSSCH), a physical sidelink control channel (PSCCH) or other suitable channel. The DMTC configuration may include parameters to enable the first and/or second UEs to measure and report the status of the beamformed link. The DMTC configuration may specify values for DMTC parameters defining beacon signal transmission timing and resources. The specified values may define periodic DMTC occasions that include periodic beacon signal transmission windows for beacon signal transmissions from the first UE to the second UE. The DMTC configuration may include the beacon signal periodicity and/or the time/frequency resources associated with a DMTC resource window. When operating in sidelink mode 1, the first UE may receive the DMTC configuration from a BS (e.g., the BS  105  or the BS  800 ). In this regard the first UE may receive the DMTC configuration from the BS in a configured grant. The first UE may transmit (e.g., forward) the DMTC configuration received from the BS to the second UE. When operating in sidelink mode 2, the first UE may determine the DMTC configuration. The first UE may transmit the DMTC configuration to the second UE in a configured grant. The DMTC configuration may enable beamformed radio link failure detection by the receiving UE. 
     At action  930 , the method  900  includes the first UE transmitting a periodic beacon signal to the second sidelink UE based on the DMTC configuration. In this regard, the first UE may transmit the beacon signal to the second UE via a physical sidelink shared channel (PSSCH). The second UE may measure aspects of the beacon signal to determine the status (e.g., quality status) of the beamformed link. The status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). In some aspects, the beacon signal may be used by other UEs to discover neighboring UEs. The beacon signal carried by the PSSCH may include information to assist RF discovery by a third UE. The beacon signal may be received by the third UE and indicate the DMTC of the second UE or any other UE that the first UE has a unicast connection with. For example, the beacon signal may include the layer 2 ID of all UEs that the first UE has a unicast connection with and their corresponding DMTCs. A third UE may determine that the first UE is within the vicinity of the second UE based on receiving the DMTC of the second UE. 
     The first UE may transmit the beacon signal within the DMTC resource window. The DMTC window may include a set of time and frequency resources in which the beacon signal can be transmitted. For example, the DMTC resource window may include a set of resource elements (REs). The set of REs may include time resources (e.g., symbols, slots, sub-slots) and frequency resources (e.g., frequency subcarriers, frequency bands, frequency ranges). The first UE may transmit the beacon signal in a subset of REs in the DMTC resource window. The DMTC configuration received by the second UE (e.g., from the first UE and/or a BS) may indicate the REs defining the DMTC resource window and/or the subset of REs carrying the beacon signal. In some aspects, the second UE may search and/or monitor the entire DMTC resource window (e.g., all REs, symbols, slots, and/or frequency subcarriers within the DMTC window) for the beacon signal carried by the PSSCH in a subset of REs of the DMTC resource window. 
     The first UE may transmit the beacon signal according to a DMTC transmission periodicity, for example, at about 10 ms, 20 ms, 40 ms, 80 ms, 160 ms, or any suitable periodicity. In some aspects, the beacon signal may include a sidelink channel state information-reference signal (CSI-RS). The second UE may use the received CSI-RS to determine the status (e.g., the quality) of the beamformed link. The status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     The second UE may generate a CSI report describing the quality of the beamformed link. The CSI report may include information related to the channel conditions in the beamformed link between the first UE and the second UE. For example, the CSI report may include a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     In some aspects, the beacon signal may include second-stage sidelink control information (SCI-2). Additionally or alternatively, the second UE may determine the status of the beamformed link based upon decoding the SCI-2. For example, if the second UE successfully decodes the SCI-2, the second UE may determine the quality of the beamformed link to be acceptable. However, if the second UE is unable to successfully decode the SCI-2, the second UE may determine the quality of the beamformed link to be unacceptable. 
     If the quality of the beacon signal fails to satisfy a threshold (e.g., based on CSI-RS measurement(s), SCI-2 decoding, etc.), then the second UE may generate a beam failure indication (BFI) to the medium access layer (MAC) layer (e.g. layer 2) of the second UE. The second UE may transmit the BFI to the first UE over a signal transmitted from the second UE to the first UE. For example, the second UE may transmit the BFI to the first UE over a beacon signal, a PSSCH, a PSCCH, a PBSCH, or a combination thereof. 
     In some aspects, the first UE may transmit a plurality of periodic beacon signals. The first UE may transmit the plurality of periodic beacon signals in a plurality of beam directions. In some instances, each of the beacon signals may be transmitted in a different beam direction. For example, and without limitation, the first UE may transmit four beacon signals. Each of the four beacon signals may be transmitted about ninety degrees from the adjacent beam directions. The DMTC configuration may include time/frequency resource information (e.g., pointers to resource blocks) associated with each of the beacon signals. The DMTC configuration may further include beam direction information (e.g., a beam direction index, a beam direction codebook) associated with each of the different beacon signals. The second UE may perform beam sweeping to monitor for each of the different beacon signals. The second UE may measure the quality of the different received beacon signals. For example, the second UE may have an established beamformed link over a first directional beam from the first UE. However, due to channel conditions and/or relative positions of the first and second UE (e.g., due to movement of the first and/or second UEs and/or movement of interfering structure(s) and/or device(s) between the first and second UEs), the second UE may measure a higher quality channel over a different directional beam (e.g., a second directional beam different than the first directional beam). The second UE may transmit a CSI report to the first UE indicating a higher channel quality over the different directional beam. The first and second UEs may reestablish an RRC connection via the different directional beam based on the CSI report. 
     In some aspects, the second UE may perform any or all of the actions  910 ,  920 , or  930 . In other words, the second UE may transmit a DMTC configuration and/or a beacon signal over the beamformed link to the first UE. The first UE may measure the quality of the beacon signal and transmit a CSI report to the second UE. In this fashion, the first and second UEs may determine the quality of the bidirectional communication link. The first UE may transmit a beacon signal to the second UE and the second UE may transmit a different beacon signal to the first UE in a bidirectional fashion. The bidirectional beacon signals between the first UE and the second UE may further include control elements for managing the beamformed link. For example, the beacon signal carried by the PSSCH may include a number of consecutive discontinuous transmissions (numConsecutiveDTX). The sidelink HARQ process may (re-)initialize numConsecutiveDTX to zero for each PC5-RRC connection that has been established between the first and second UEs. If the HARQ process of the first UE fails to receive a physical sidelink feedback channel (PSFCH) communication (e.g., an ACK or a NACK) associated with a PSSCH transmission, the numConsecutiveDTX may be incremented by 1. If the numConsecutiveDTX reaches a threshold (sl-maxnumConsecutiveDTX) the HARQ process may report a radio link failure (RLF) to the RRC. 
     In some aspects, the first UE may transmit multiple beacon signals to multiple UEs. The first UE may establish multiple unicast connections to multiple UEs over different beamformed links. The first UE may transmit beacon signals to each of the multiple UEs over the different beamformed links. The DMTC configuration may include pointers to the other beacon signal resources (e.g., the DMTC resource windows indicating the time/frequency resources associated with different directions for different UEs). In this manner, the second UE may measure the quality of multiple beacon signals associated with the different beamformed links intended for other UEs and transmit (e.g., report) the quality of the multiple beacon signals to the first UE in a CSI report. In this way, the second UE may switch to the beamformed link with the highest quality. 
     In some aspects, the first UE may have multiple established links with a group of UEs over the same beamformed link. In this case, the first UE may not transmit a separate beacon signal to each of the UEs in the group. Instead, the first UE may transmit the beacon signal in a single groupcast transmission to the group of UEs. For example, the destination ID in the SCI-2 associated with the beacon signal may be a groupcast destination ID that identifies the group of UEs. The groupcast beacon signal may reduce communication overhead as compared to multiple unicast transmissions to each UE in the group. 
     In some aspects, when the beamformed link between the first UE and the second UE fails, the first UE may broadcast beacon signals over multiple beam directions. In this regard, the first UE may transmit an SCI-2 with SCI format 2-A indicating the cast type as broadcast. The first UE may include a broadcast ID in the destination ID of the SCI-2. The first UE may transmit broadcast beacons in addition to the unicast beacon signals. The second UE may search for the broadcasted beacon signals by decoding the broadcast ID in the SCI-2 within the DMTC resource window. The second UE may detect one or more of the broadcasted beacons signals from the first UE. The second UE may determine the highest quality beacon signal of the one or more detected broadcast beacons signals. The second UE attempt to re-establish a beamformed link with the first UE via the beam direction associated with the highest quality beacon signal. 
     In some aspects, the first UE may refrain from transmitting the periodic beacon signal within a time period after a number of successful communications between the first sidelink UE and the second sidelink UE satisfies a threshold. The beacon signal may be used to determine a status of the beamformed link between the first and second UE. Additionally or alternatively, the status of the beamformed link may be determined based on a sequence of successful transmissions between the first UE and the second UE. For example, the first UE may transmit a sequence of transport blocks (TBs) to the second UE via the beamformed link. If the TBs are successfully received, as indicated by an ACK being transmitted to the first UE in response to each of the TBs, then the first UE may refrain from transmitting the beacon signal(s) for a time period (e.g., a DMTC transmission period, 10 ms, 20 ms, 40 ms, 80 ms, or 160 ms) as the successful transmission of the TBs is an indication of the quality of the beamformed link. In some aspects, the first UE may be configured to transmit the beacon signal when the numConsecutiveDTX satisfies a threshold (e.g., number of NACKs is greater than a threshold). In some aspects, the first UE may be configured to transmit the beacon signal when the sl-maxnumConsecutiveDTXForDMTC is greater than a threshold. The first UE may transmit an indicator indicating to the second UE that the next scheduled beacon signal will be skipped. For example, the first UE may transmit a code point in the SCI-1 indicating to the second UE that the next scheduled beacon signal will be skipped, for example, when the numConsecutiveDTX is less than a threshold. 
       FIG.  10    is a flow diagram of a communication method  1000  according to some aspects of the present disclosure. Aspects of the method  1000  can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a wireless communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE  115  or UE  700 , may utilize one or more components, such as the processor  702 , the memory  704 , the beacon signal module  708 , the transceiver  710 , the modem  712 , and the one or more antennas  716 , to execute aspects of method  1000 . The method  900  may employ similar mechanisms as in the networks  100  and  200  and the aspects and actions described with respect to  FIGS.  2 - 6   . As illustrated, the method  1000  includes a number of enumerated actions, but the method  1000  may include additional actions before, after, and in between the enumerated actions. In some aspects, one or more of the enumerated actions may be omitted or performed in a different order. 
     At action  1010 , the method  1000  includes a first sidelink UE (e.g., the UE  115  or the UE  700 ) establishing a link with a second sidelink UE. In this regard, the first UE may establish a radio resource control (RRC) connection with the second sidelink UE. In some instances, the first UE may establish a PC5-RRC connected mode state with the second UE. The PC5-RRC connected state may enable exchanging of access-stratum level information for alignment between the first UE (e.g., a receiving UE) and the second UE (e.g., a transmitting UE) to support SL unicast communications. The unicast communications may be one-to-one communications between the first UE and the second UE. In some aspects, the first UE may have multiple PC5-RRC connections with multiple UEs for unicast communications between the first UE and the multiple UEs. For example, referring to  FIGS.  1 ,  2   , and/or  4   a , the UE  115   c  may have a PC5-RRC connection with the UE  115   a  and the UE  115   c.    
     The established link may be a beamformed link. The beamformed link may be a directional link established by one more directional antennas in each of the first UE and the second UE. The beamformed link may enable higher data rates for longer distances compared to non-beamformed links using omni-directional antennas. The beamformed link may compensate for pathloss at higher frequencies (e.g., FR2, FR2x frequencies). As beamforming enables directional transmission between the first UE and the second UE, the beamformed link between the first UE and the second UE may allow for spatial reuse of available resources due to reduced interference among the UEs. 
     At action  1020 , the method  1000  includes the first UE receiving a periodic discovery and measurement timing (DMTC) configuration via the established link from the second sidelink UE. The first UE may receive the periodic DMTC configuration via a beamformed link with the second sidelink UE. In this regard, the first UE may receive the DMTC configuration from the second UE via a physical sidelink shared channel (PSSCH), a physical sidelink control channel (PSCCH) or other suitable channel. The DMTC configuration may include parameters to enable the first and/or second UEs to measure and report the status of the beamformed link. The DMTC configuration may specify values for DMTC parameters defining beacon signal transmission timing and resources. The specified values may define periodic DMTC occasions that include periodic beacon signal transmission windows for beacon signal transmissions from the second UE to the first UE. The DMTC configuration may include the beacon signal periodicity and/or the time/frequency resources associated with a DMTC resource window. When operating in sidelink mode 1, the second UE may receive the DMTC configuration from a BS (e.g., the BS  105  or the BS  800 ). In this regard the second UE may receive the DMTC configuration from the BS in a configured grant. The first UE may receive the DMTC configuration from the second UE. When operating in sidelink mode 2, the second UE may determine the DMTC configuration. The second UE may transmit the DMTC configuration to the first UE in a configured grant. The DMTC configuration may enable beamformed radio link failure detection by the second UE. 
     At action  1030 , the method  1000  includes the first UE receiving a periodic beacon signal to the second sidelink UE based on the DMTC configuration. In this regard, the first UE may receive the beacon signal from the second UE via a physical sidelink shared channel (PSSCH). The first UE may measure aspects of the beacon signal to determine the status (e.g., quality status) of the beamformed link. The status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). In some aspects, the beacon signal may be used by other UEs to discover neighboring UEs. The beacon signal carried by the PSSCH may include information to assist RF discovery by a third UE. The beacon signal may be received by the third UE and indicate the DMTC of the first UE or any other UE that the second UE has a unicast connection with. For example, the beacon signal may include the layer 2 ID of all UEs that the second UE has a unicast connection with and their corresponding DMTCs. A third UE may determine that the second UE is within the vicinity of the first UE based on receiving the DMTC of the first UE. 
     The first UE may receive the beacon signal within the DMTC resource window. The DMTC window may include a set of time and frequency resources in which the beacon signal can be transmitted. For example, the DMTC resource window may include a set of resource elements (REs). The set of REs may include time resources (e.g., symbols, slots, sub-slots) and frequency resources (e.g., frequency subcarriers, frequency bands, frequency ranges). The first UE may receive the beacon signal in a subset of REs in the DMTC resource window. The DMTC configuration received by the first UE (e.g., from the second UE and/or a BS) may indicate the REs defining the DMTC resource window and/or the subset of REs carrying the beacon signal. In some aspects, the first UE may search and/or monitor the entire DMTC resource window (e.g., all REs, symbols, slots, and/or frequency subcarriers within the DMTC window) for the beacon signal carried by the PSSCH in a subset of REs of the DMTC resource window. 
     The first UE may receive the beacon signal according to a DMTC transmission periodicity, for example, at about 10 ms, 20 ms, 40 ms, 80 ms, 160 ms, or any suitable periodicity. In some aspects, the beacon signal may include a sidelink channel state information-reference signal (CSI-RS). The first UE may use the received CSI-RS to determine the status (e.g., the quality) of the beamformed link. The status of the beacon signal may include, without limitation, a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     The first UE may generate a CSI report describing the quality of the beamformed link. The CSI report may include information related to the channel conditions in the beamformed link between the first UE and the second UE. For example, the CSI report may include a channel quality indicator (CQI), a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal to interference and noise ratio (SINR), and/or a rank indicator (RI). 
     In some aspects, the beacon signal may include second-stage sidelink control information (SCI-2). Additionally or alternatively, the first UE may determine the status of the beamformed link based upon decoding the SCI-2. For example, if the first UE successfully decodes the SCI-2, the first UE may determine the quality of the beamformed link to be acceptable. However, if the first UE is unable to successfully decode the SCI-2, the first UE may determine the quality of the beamformed link to be unacceptable. 
     If the quality of the beacon signal fails to satisfy a threshold (e.g., based on CSI-RS measurement(s), SCI-2 decoding, etc.), then the first UE may generate a beam failure indication (BFI) to the medium access layer (MAC) layer (e.g. layer 2) of the first UE. The second UE may receive the BFI from the first UE over a signal transmitted from the first UE to the second UE. For example, the first UE may transmit the BFI to the second UE over a beacon signal, a PSSCH, a PSCCH, a PBSCH, or a combination thereof. 
     In some aspects, the first UE may receive a plurality of periodic beacon signals. The first UE may receive the plurality of periodic beacon signals in a plurality of beam directions. In some instances, each of the beacon signals may be received from a different beam direction. For example, and without limitation, the second UE may transmit four beacon signals. Each of the four beacon signals may be transmitted by the second UE about ninety degrees from the adjacent beam directions. The DMTC configuration may include time/frequency resource information (e.g., pointers to resource blocks) associated with each of the beacon signals. The DMTC configuration may further include beam direction information (e.g., a beam direction index, a beam direction codebook) associated with each of the different beacon signals. The first UE may perform beam sweeping to monitor for each of the different beacon signals. The first UE may measure the quality of the different received beacon signals. For example, the first UE may have an established beamformed link over a first directional beam from the second UE. However, due to channel conditions and/or relative positions of the first and second UE (e.g., due to movement of the first and/or second UEs and/or movement of interfering structure(s) and/or device(s) between the first and second UEs), the first UE may measure a higher quality channel over a different directional beam (e.g., a second directional beam different than the first directional beam). The first UE may transmit a CSI report to the second UE indicating a higher channel quality over the different directional beam. The first and second UEs may reestablish an RRC connection via the different directional beam based on the CSI report. 
     In some aspects, the first UE may perform any or all of the actions  910 ,  920 , or  930 . In other words, the first UE may transmit a DMTC configuration and/or a beacon signal over the beamformed link to the second UE. The second UE may measure the quality of the beacon signal and transmit a CSI report to the first UE. In this fashion, the first and second UEs may determine the quality of the bidirectional communication link. The first UE may transmit a beacon signal to the second UE and the second UE may transmit a different beacon signal to the first UE in a bidirectional fashion. The bidirectional beacon signals between the first UE and the second UE may further include control elements for managing the beamformed link. For example, the beacon signal carried by the PSSCH may include a number of consecutive discontinuous transmissions (numConsecutiveDTX). The sidelink HARQ process may (re-)initialize numConsecutiveDTX to zero for each PC5-RRC connection that has been established between the first and second UEs. If the HARQ process of the second UE fails to receive a physical sidelink feedback channel (PSFCH) communication (e.g., an ACK or a NACK) associated with a PSSCH transmission, the numConsecutiveDTX may be incremented by 1. If the numConsecutiveDTX reaches a threshold (sl-maxnumConsecutiveDTX) the HARQ process may report a radio link failure (RLF) to the RRC. 
     In some aspects, the first UE may receive multiple beacon signals from multiple UEs. The second UE may establish multiple unicast connections to multiple UEs over different beamformed links. The second UE may transmit beacon signals to each of the multiple UEs over the different beamformed links. The DMTC configuration may include pointers to the other beacon signal resources (e.g., the DMTC resource windows indicating the time/frequency resources associated with different directions for different UEs). In this manner, the first UE may measure the quality of multiple beacon signals associated with the different beamformed links intended for other UEs and transmit (e.g., report) the quality of the multiple beacon signals to the second UE in a CSI report. In this way, the first UE may switch to the beamformed link with the highest quality. 
     In some aspects, the second UE may have multiple established links with a group of UEs over the same beamformed link. In this case, the second UE may not transmit a separate beacon signal to each of the UEs in the group. Instead, the second UE may transmit the beacon signal in a single groupcast transmission to the group of UEs. For example, the destination ID in the SCI-2 associated with the beacon signal may be a groupcast destination ID that identifies the group of UEs. The groupcast beacon signal may reduce communication overhead as compared to multiple unicast transmissions to each UE in the group. 
     In some aspects, when the beamformed link between the first UE and the second UE fails, the second UE may broadcast beacon signals over multiple beam directions. In this regard, the second UE may transmit an SCI-2 with SCI format 2-A indicating the cast type as broadcast. The second UE may include a broadcast ID in the destination ID of the SCI-2. The second UE may transmit broadcast beacons in addition to the unicast beacon signals. The first UE may search for the broadcasted beacon signals by decoding the broadcast ID in the SCI-2 within the DMTC resource window. The first UE may detect one or more of the broadcasted beacons signals from the second UE. The first UE may determine the highest quality beacon signal of the one or more detected broadcast beacons signals. The first UE attempt to re-establish a beamformed link with the second UE via the beam direction associated with the highest quality beacon signal. 
     In some aspects, the second UE may refrain from transmitting the periodic beacon signal within a time period after a number of successful communications between the first sidelink UE and the second sidelink UE satisfies a threshold. The beacon signal may be used to determine a status of the beamformed link between the first and second UE. Additionally or alternatively, the status of the beamformed link may be determined based on a sequence of successful transmissions between the first UE and the second UE. For example, the first UE may receive a sequence of transport blocks (TBs) from the second UE via the beamformed link. If the TBs are successfully received, as indicated by an ACK being transmitted by the first UE to the second UE in response to each of the TBs, then the second UE may refrain from transmitting the beacon signal(s) for a time period (e.g., a DMTC transmission period, 10 ms, 20 ms, 40 ms, 80 ms, or 160 ms) as the successful transmission of the TBs is an indication of the quality of the beamformed link. In some aspects, the second UE may be configured to transmit the beacon signal when the numConsecutiveDTX satisfies a threshold (e.g., number of NACKs is greater than a threshold). In some aspects, the second UE may be configured to transmit the beacon signal when the sl-maxnumConsecutiveDTXForDMTC is greater than a threshold. The first UE may receive an indicator from the second UE indicating that the next scheduled beacon signal will be skipped. For example, the first UE may receive a code point from the second UE in the SCI-1 indicating that the next scheduled beacon signal will be skipped, for example, when the numConsecutiveDTX is less than a threshold. 
     Further aspects of the present disclosure include the following: 
     Aspect 1 includes a method of wireless communication performed by a first sidelink user equipment (UE), the method comprising establishing, with a second sidelink UE, a beamformed link; transmitting, to the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link; and transmitting, to the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     Aspect 2 includes the method of aspect 1, wherein the transmitting the periodic beacon signal comprises transmitting the periodic beacon signal via a physical sidelink shared channel (PSSCH). 
     Aspect 3 includes the method of any of aspects 1-2, further comprising refraining from transmitting the periodic beacon signal within a time period after a number of successful communications between the first sidelink UE and the second sidelink UE satisfies a threshold. 
     Aspect 4 includes the method of any of aspects 1-3, wherein the transmitting the periodic beacon signal comprises transmitting the periodic beacon signal within a DMTC resource window. 
     Aspect 5 includes the method of any of aspects 1-4, wherein the periodic beacon signal comprises at least one of second-stage sidelink control information (SCI-2); a sidelink channel state information reference signal (SL CSI-RS); a pointer to one or more additional DMTC configurations; or a beam status indicator associated with the beamformed link. 
     Aspect 6 includes the method of any of aspects 1-5, further comprising receiving, from the second sidelink UE, a status associated with the beamformed link based on the periodic beacon signal. 
     Aspect 7 includes the method of any of aspects 1-6, further comprising transmitting, to the second sidelink UE, a plurality of periodic beacon signals, wherein the plurality of periodic beacon signals includes the periodic beacon signal; and each of the plurality of periodic beacon signals is transmitted in a different beam direction. 
     Aspect 8 includes the method of any of aspects 1-7, further comprising receiving, from a base station (BS), the DMTC configuration via a configured grant. 
     Aspect 9 includes a method of wireless communication performed by a first sidelink user equipment (UE), the method comprising establishing, with a second sidelink UE, a beamformed link; receiving, from the second sidelink UE, a periodic discovery and measurement timing (DMTC) configuration via the beamformed link; and receiving, from the second sidelink UE, a periodic beacon signal based on the DMTC configuration. 
     Aspect 10 includes the method of aspect 9, wherein the receiving the periodic beacon signal comprises receiving the periodic beacon signal via a physical sidelink shared channel (PSSCH). 
     Aspect 11 includes the method of any of aspects 9 or 10, wherein the receiving the periodic beacon signal comprises receiving the periodic beacon signal within a DMTC resource window. 
     Aspect 12 includes method of any of aspects 9-11, wherein the periodic beacon signal comprises at least one of second-stage sidelink control information (SCI-2); a sidelink channel state information reference signal (SL CSI-RS); a pointer to one or more additional DMTC configurations; or a beam status indicator associated with the beamformed link. 
     Aspect 13 includes the method of any of aspects 9-12, further comprising transmitting, to the second sidelink UE, a status associated with the beamformed link based on the periodic beacon signal. 
     Aspect 14 includes the method of any of aspects 9-13, further comprising receiving, from the second sidelink UE, a plurality of periodic beacon signals, wherein the plurality of periodic beacon signals includes the periodic beacon signal; and each of the plurality of periodic beacon signals is received from a different beam direction. 
     Aspect 15 includes the method of any of aspects 9-14, further comprising determining a quality level associated with the periodic beacon signal; generating a beam failure indicator (BFI) based on the quality level not satisfying a threshold; and transmitting, to the second sidelink UE, the BFI via a physical sidelink shared channel (PSSCH). 
     Aspect 16 includes a non-transitory computer-readable medium storing one or more instructions for wireless communication, the one or more instructions comprising one or more instructions that, when executed by one or more processors of a first sidelink user equipment, cause the one or more processors to perform any one of aspects 1-8. 
     Aspect 17 includes a non-transitory computer-readable medium storing one or more instructions for wireless communication, the one or more instructions comprising one or more instructions that, when executed by one or more processors of a first sidelink user equipment, cause the one or more processors to perform any one of aspects 9-15. 
     Aspect 18 includes a first sidelink user equipment (UE) comprising one or more means to perform any one or more of aspects 1-8. 
     Aspect 19 includes a first sidelink user equipment (UE) comprising one or more means to perform any one or more of aspects 9-15. 
     Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     The various illustrative blocks and modules described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a DSP, an ASIC, an FPGA or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may 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 conjunction with a DSP core, or any other such configuration). 
     The functions described herein may be implemented in hardware, software executed by a processor, firmware, or any combination thereof. If implemented in software executed by a processor, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Other examples and implementations are within the scope of the disclosure and appended claims. For example, due to the nature of software, functions described above can be implemented using software executed by a processor, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items (for example, a list of items prefaced by a phrase such as “at least one of” or “one or more of”) indicates 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). 
     As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the spirit and scope thereof. In light of this, the scope of the present disclosure should not be limited to that of the particular instances illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.