Patent Publication Number: US-2023136589-A1

Title: Techniques for direct secondary cell activation using temporary reference signals

Description:
CROSS REFERENCE 
     The present application for patent claims the benefit of U.S. Provisional Patent Application No. 63/272,977 by TAKEDA et al., entitled “TECHNIQUES FOR DIRECT SECONDARY CELL ACTIVATION USING TEMPORARY REFERENCE SIGNALS,” filed Oct. 28, 2021, assigned to the assignee hereof, and expressly incorporated by reference herein. 
    
    
     FIELD OF TECHNOLOGY 
     The following relates to wireless communications, including techniques for direct secondary cell (SCell) activation using temporary reference signals. 
     BACKGROUND 
     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). Examples of such multiple-access systems include fourth generation (4G) systems such as Long Term Evolution (LTE) systems, LTE-Advanced (LTE-A) systems, or LTE-A Pro systems, and fifth generation (5G) systems which may be referred to as New Radio (NR) systems. These systems may employ technologies such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), or discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM). A wireless multiple-access communications system may include one or more base stations or one or more network access nodes, each simultaneously supporting communication for multiple communication devices, which may be otherwise known as user equipment (UE). 
     In some wireless communications systems, a UE may communicate with the network via one or more serving cells (e.g., a primary cell (PCell) and a secondary cell (SCell)). In some wireless communications systems, synchronization signal blocks (SSBs) may be used to signal time and frequency tracking information of the SCell. However, these conventional SSB techniques may suffer from increased latency when activating SCells. 
     SUMMARY 
     The described techniques relate to improved methods, systems, devices, and apparatuses that support techniques for direct secondary cell (SCell) activation using temporary reference signals. Generally, the described techniques provide for direct SCell activation using temporary reference signals. In particular, aspects of the present disclosure support techniques for using layer three (L3) signaling (e.g., radio resource control (RRC) signaling) on a primary cell (PCell) to configure and activate an SCell at a user equipment (UE) using temporary reference signals. For example, a UE may receive an RRC message via a PCell which instructs the UE to activate an SCell. Subsequently, the UE may transmit an RRC complete message acknowledging the instruction to activate the SCell, and monitor resources on the SCell. The UE may then receive a temporary reference signal via the SCell that the UE may use to perform time and/or frequency tracking of the SCell, which may enable the UE to begin monitoring channel state information (CSI) reference signals (CSI-RS) on the SCell. 
     A method for wireless communication at a UE is described. The method may include receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and monitoring a second resource of the second serving cell for the reference signal based on the control message. 
     An apparatus for wireless communication at a UE is described. The apparatus may include a processor, memory coupled with the processor, and instructions stored in the memory. The instructions may be executable by the processor to cause the apparatus to receive, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, receive, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, transmit, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and monitor a second resource of the second serving cell for the reference signal based on the control message. 
     Another apparatus for wireless communication at a UE is described. The apparatus may include means for receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, means for receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, means for transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and means for monitoring a second resource of the second serving cell for the reference signal based on the control message. 
     A non-transitory computer-readable medium storing code for wireless communication at a UE is described. The code may include instructions executable by a processor to receive, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, receive, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, transmit, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and monitor a second resource of the second serving cell for the reference signal based on the control message. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, via the message, an indication of a set of multiple candidate reference signal resources and receiving, via the control message and based on the message, an indication of the second resource from the set of multiple candidate reference signal resources, where monitoring the second resource of the second serving cell for the reference signal may be based on the indication of the second resource. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, via the control message, an indication of a time offset associated with the second resource for the reference signal, where monitoring the second resource may be based on the time offset. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the time offset indicates a period of time between the first resource and the second resource. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, via the control message, an indication of the second resource for the reference signal, where monitoring the second resource may be based on receiving the indication of the second resource. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the control message includes a downlink control information (DCI) message. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the control message includes an aperiodic channel state information (A-CSI) request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, where monitoring the second resource may be based on the one or more parameters. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the one or more parameters include a structure of the reference signal, a component carrier for the reference signal, a bandwidth part (BWP) for the reference signal, or any combination thereof. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving the reference signal via the second serving cell based on the monitoring and communicating with the second serving cell based on time and frequency tracking information determined using the reference signal. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for receiving, based on the reference signal, automatic gain control (AGC) information associated with the second serving cell, where communicating with the second serving cell may be based on the AGC information. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the reference signal may be received prior to an earliest synchronization signal block (SSB) which the UE may be capable of receiving via the second serving cell. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the reference signal includes a temporary reference signal, a tracking reference signal, a non-zero power channel state information reference signal (NZP-CSI-RS), or any combination thereof. 
     A method for wireless communication at a base station is described. The method may include transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     An apparatus for wireless communication at a base station is described. The apparatus may include a processor, memory coupled with the processor, and instructions stored in the memory. The instructions may be executable by the processor to cause the apparatus to transmit, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, transmit, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, receive, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and transmit the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     Another apparatus for wireless communication at a network entity is described. The apparatus may include means for transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, means for transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, means for receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and means for transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     A non-transitory computer-readable medium storing code for wireless communication at a network entity is described. The code may include instructions executable by a processor to transmit, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell, transmit, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell, receive, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message, and transmit the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for transmitting, via the message, an indication of a set of multiple candidate reference signal resources and transmitting, via the control message and based on the message, an indication of the second resource from the set of multiple candidate reference signal resources, where transmitting the reference signal within the second resource of the second serving cell may be based on the indication of the second resource. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for transmitting, via the control message, an indication of a time offset associated with the second resource for the reference signal, where transmitting the reference signal within the second resource may be based on the time offset. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the time offset indicates a period of time between the first resource and the second resource. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for transmitting, via the control message, an indication of the second resource for the reference signal, where transmitting the reference signal within the second resource may be based on transmitting the indication of the second resource. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the control message includes a DCI message. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the control message includes an A-CSI request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for transmitting, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, where transmitting the reference signal within the second resource may be based on the one or more parameters. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the one or more parameters include a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, or any combination thereof. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for communicating with the UE via the second serving cell based on time and frequency tracking information that may be determined based on the reference signal. 
     Some examples of the method, apparatuses, and non-transitory computer-readable medium described herein may further include operations, features, means, or instructions for transmitting, based on the reference signal, AGC information associated with the second serving cell, where communicating with the UE via the second serving cell may be based on the AGC information. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the reference signal may be transmitted prior to an earliest SSB which the UE may be capable of receiving via the second serving cell. 
     In some examples of the method, apparatuses, and non-transitory computer-readable medium described herein, the reference signal includes a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, or any combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates an example of a wireless communications system that supports techniques for direct secondary cell (SCell) activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  2    illustrates an example of a wireless communications system that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  3    illustrates an example of a resource configuration that supports techniques for direct SCell activation in accordance with aspects of the present disclosure. 
         FIG.  4    illustrates an example of a resource configuration that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  5    illustrates an example of a process flow that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  6    illustrates an example of a resource configuration that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIGS.  7  and  8    show block diagrams of devices that support techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  9    shows a block diagram of a communications manager that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  10    shows a diagram of a system including a device that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIGS.  11  and  12    show block diagrams of devices that support techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  13    shows a block diagram of a communications manager that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  14    shows a diagram of a system including a device that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIGS.  15  through  18    show flowcharts illustrating methods that support techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
         FIG.  19    illustrates an example of a network architecture that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In some wireless communications systems, a user equipment (UE) may communicate with the network via one or more serving cells (e.g., a primary cell (PCell) and a secondary cell (SCell)). The network may activate additional serving cells at the UE to increase data throughput, to alleviate network congestion, or both. Some wireless communications systems utilize synchronization signal blocks (SSBs) to indicate time and frequency tracking information (e.g., automatic gain control (AGC) information) for an SCell during SCell activation. However, these conventional SSB techniques may suffer from increased latency when activating SCells. 
     Temporary reference signals may enable UEs to quickly perform AGC for an SCell where the UE adjusts a receive amplifier gain and performs time/frequency tuning with the SCell. According to some wireless communications systems, a PCell in wireless communications with a UE may configure the SCell that is to be activated at the UE to transmit temporary reference signals, and subsequent medium access control-control element (MAC-CE) signaling on the PCell may trigger the activation of the SCell. However, in some cases, the use of separate control signaling (e.g., radio resource control (RRC) and MAC-CE signaling) to configure and activate the SCell may lead to increased control signaling overhead, and may delay the activation of the SCell. Other wireless communications systems have implemented direct SCell activation via RRC signaling which does not require MAC-CE signaling. However, some conventional direct SCell activation techniques do not provide signaling or other configurations which enable the UE to efficiently identify and monitor resources used for temporary reference signals on the SCell. 
     Accordingly, aspects of the present disclosure provide techniques for direct SCell activation using temporary reference signals. In particular, aspects of the present disclosure provide techniques for using layer three (L3) signaling (e.g., RRC signaling) on a PCell to configure and activate an SCell at a UE using temporary reference signals. For example, a UE may receive an RRC message from a PCell which instructs the UE to activate an SCell. Subsequently, the UE may transmit an RRC complete message acknowledging the instruction to activate the SCell, and monitor resources on the SCell. The UE may then receive a temporary reference signal from the SCell that the UE uses to perform time and/or frequency tracking of the SCell, which may enable the UE to begin monitoring channel state information (CSI) reference signals (CSI-RSs) on the SCell. 
     In some aspects, the resources for the temporary reference signal and/or other parameters for the temporary reference signal (e.g., structure, format, component carrier, bandwidth part (BWP)) may be indicated via the L3 message (or a physical downlink control channel (PDCCH) transmission carrying the L3 message). Additionally, or alternatively, the resources and/or parameters for the temporary reference signal may be received via a MAC-CE or layer one (L1) message (e.g., downlink control information (DCI) message) from the PCell following reception of the L3 message. In some cases, the L3 message (e.g., RRC message) may indicate a set of active transmission configuration indicator (TCI) states (e.g., active quasi co-location (QCL) configurations/assumptions) for the SCell, where the temporary reference signal is transmitted via one of the active TCI states. 
     Aspects of the disclosure are initially described in the context of wireless communications systems. Additional aspects of the disclosure are described in the context of example resource configurations and an example process flow. Aspects of the disclosure are further illustrated by and described with reference to apparatus diagrams, system diagrams, and flowcharts that relate to techniques for direct SCell activation using temporary reference signals. 
       FIG.  1    illustrates an example of a wireless communications system  100  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The wireless communications system  100  may include one or more network entities  105  (e.g., base stations, network nodes), one or more UEs  115 , and a core network  130 . In some examples, the wireless communications system  100  may be a Long Term Evolution (LTE) network, an LTE-Advanced (LTE-A) network, an LTE-A Pro network, or a New Radio (NR) network. In some examples, the wireless communications system  100  may support enhanced broadband communications, ultra-reliable communications, low latency communications, communications with low-cost and low-complexity devices, or any combination thereof. 
     The network entities  105  may be dispersed throughout a geographic area to form the wireless communications system  100  and may be devices in different forms or having different capabilities. The network entities  105  and the UEs  115  may wirelessly communicate via one or more communication links  125 . Each network entity  105  may provide a coverage area  110  over which the UEs  115  and the network entity  105  may establish one or more communication links  125 . The coverage area  110  may be an example of a geographic area over which a network entity  105  and a UE  115  may support the communication of signals according to one or more radio access technologies. 
     The UEs  115  may be dispersed throughout a coverage area  110  of the wireless communications system  100 , and each UE  115  may be stationary, or mobile, or both at different times. The UEs  115  may be devices in different forms or having different capabilities. Some example UEs  115  are illustrated in  FIG.  1   . The UEs  115  described herein may be able to communicate with various types of devices, such as other UEs  115 , the network entities  105 , or network equipment (e.g., core network nodes, relay devices, integrated access and backhaul (IAB) nodes, or other network equipment), as shown in  FIG.  1   . 
     The network entities  105  (e.g., base stations, network nodes) may communicate with the core network  130 , or with one another, or both. For example, the network entities  105  may interface with the core network  130  through one or more backhaul links  120  (e.g., via an S1, N2, N3, or other interface). The network entities  105  may communicate with one another over the backhaul links  120  (e.g., via an X2, Xn, or other interface) either directly (e.g., directly between network entities  105 ), or indirectly (e.g., via core network  130 ), or both. In some examples, the backhaul links  120  may be or include one or more wireless links. 
     One or more of the network entities  105  described herein may include or may be referred to by a person having ordinary skill in the art as a base transceiver station, a radio base station, an access point, a radio transceiver, a NodeB, an eNodeB (eNB), a next-generation NodeB or a giga-NodeB (either of which may be referred to as a gNB), a Home NodeB, a Home eNodeB, or other suitable terminology. 
     A UE  115  may include or may be referred to as a mobile device, a wireless device, a remote device, a handheld device, or a subscriber device, or some other suitable terminology, where the “device” may also be referred to as a unit, a station, a terminal, or a client, among other examples. A UE  115  may also include or may be referred to as a personal electronic device such as a cellular phone, a personal digital assistant (PDA), a tablet computer, a laptop computer, or a personal computer. In some examples, a UE  115  may include or be referred to as a wireless local loop (WLL) station, an Internet of Things (IoT) device, an Internet of Everything (IoE) device, or a machine type communications (MTC) device, among other examples, which may be implemented in various objects such as appliances, or vehicles, meters, among other examples. 
     The UEs  115  described herein may be able to communicate with various types of devices, such as other UEs  115  that may sometimes act as relays as well as the network entities  105  and the network equipment including macro eNBs or gNBs, small cell eNBs or gNBs, or relay base stations, among other examples, as shown in  FIG.  1   . 
     The UEs  115  and the network entities  105  may wirelessly communicate with one another via one or more communication links  125  over one or more carriers. The term “carrier” may refer to a set of radio frequency spectrum resources having a defined physical layer structure for supporting the communication links  125 . For example, a carrier used for a communication link  125  may include a portion of a radio frequency spectrum band (e.g., a BWP) that is operated according to one or more physical layer channels for a given radio access technology (e.g., LTE, LTE-A, LTE-A Pro, NR). Each physical layer channel may carry acquisition signaling (e.g., synchronization signals, system information), control signaling that coordinates operation for the carrier, user data, or other signaling. The wireless communications system  100  may support communication with a UE  115  using carrier aggregation or multi-carrier operation. A UE  115  may be configured with multiple downlink component carriers and one or more uplink component carriers according to a carrier aggregation configuration. Carrier aggregation may be used with both frequency division duplexing (FDD) and time division duplexing (TDD) component carriers. 
     In some examples (e.g., in a carrier aggregation configuration), a carrier may also have acquisition signaling or control signaling that coordinates operations for other carriers. A carrier may be associated with a frequency channel (e.g., an evolved universal mobile telecommunication system terrestrial radio access (E-UTRA) absolute radio frequency channel number (EARFCN)) and may be positioned according to a channel raster for discovery by the UEs  115 . A carrier may be operated in a standalone mode where initial acquisition and connection may be conducted by the UEs  115  via the carrier, or the carrier may be operated in a non-standalone mode where a connection is anchored using a different carrier (e.g., of the same or a different radio access technology). 
     The communication links  125  shown in the wireless communications system  100  may include uplink transmissions from a UE  115  to a network entity  105 , or downlink transmissions from a network entity  105  to a UE  115 . Carriers may carry downlink or uplink communications (e.g., in an FDD mode) or may be configured to carry downlink and uplink communications (e.g., in a TDD mode). 
     A carrier may be associated with a particular bandwidth of the radio frequency spectrum, and in some examples the carrier bandwidth may be referred to as a “system bandwidth” of the carrier or the wireless communications system  100 . For example, the carrier bandwidth may be one of a number of determined bandwidths for carriers of a particular radio access technology (e.g., 1.4, 3, 5, 10, 15, 20, 40, or 80 megahertz (MHz)). Devices of the wireless communications system  100  (e.g., the network entities  105 , the UEs  115 , or both) may have hardware configurations that support communications over a particular carrier bandwidth or may be configurable to support communications over one of a set of carrier bandwidths. In some examples, the wireless communications system  100  may include network entities  105  or UEs  115  that support simultaneous communications via carriers associated with multiple carrier bandwidths. In some examples, each served UE  115  may be configured for operating over portions (e.g., a sub-band, a BWP) or all of a carrier bandwidth. 
     Signal waveforms transmitted over a carrier may be made up of multiple subcarriers (e.g., using multi-carrier modulation (MCM) techniques such as orthogonal frequency division multiplexing (OFDM) or discrete Fourier transform spread OFDM (DFT-S-OFDM)). In a system employing MCM techniques, a resource element may consist of one symbol period (e.g., a duration of one modulation symbol) and one subcarrier, where the symbol period and subcarrier spacing are inversely related. The number of bits carried by each resource element may depend on the modulation scheme (e.g., the order of the modulation scheme, the coding rate of the modulation scheme, or both). Thus, the more resource elements that a UE  115  receives and the higher the order of the modulation scheme, the higher the data rate may be for the UE  115 . A wireless communications resource may refer to a combination of a radio frequency spectrum resource, a time resource, and a spatial resource (e.g., spatial layers or beams), and the use of multiple spatial layers may further increase the data rate or data integrity for communications with a UE  115 . 
     One or more numerologies for a carrier may be supported, where a numerology may include a subcarrier spacing (Δf) and a cyclic prefix. A carrier may be divided into one or more BWPs having the same or different numerologies. In some examples, a UE  115  may be configured with multiple BWPs. In some examples, a single BWP for a carrier may be active at a given time and communications for the UE  115  may be restricted to one or more active BWPs. 
     The time intervals for the network entities  105  or the UEs  115  may be expressed in multiples of a basic time unit which may, for example, refer to a sampling period of T s =1/(Δf max ·N f ) seconds, where Δf max  may represent the maximum supported subcarrier spacing, and N f  may represent the maximum supported discrete Fourier transform (DFT) size. Time intervals of a communications resource may be organized according to radio frames each having a specified duration (e.g., 10 milliseconds (ms)). Each radio frame may be identified by a system frame number (SFN) (e.g., ranging from 0 to 1023). 
     Each frame may include multiple consecutively numbered subframes or slots, and each subframe or slot may have the same duration. In some examples, a frame may be divided (e.g., in the time domain) into subframes, and each subframe may be further divided into a number of slots. Alternatively, each frame may include a variable number of slots, and the number of slots may depend on subcarrier spacing. Each slot may include a number of symbol periods (e.g., depending on the length of the cyclic prefix prepended to each symbol period). In some wireless communications systems  100 , a slot may further be divided into multiple mini-slots containing one or more symbols. Excluding the cyclic prefix, each symbol period may contain one or more (e.g., N f ) sampling periods. The duration of a symbol period may depend on the subcarrier spacing or frequency band of operation. 
     A subframe, a slot, a mini-slot, or a symbol may be the smallest scheduling unit (e.g., in the time domain) of the wireless communications system  100  and may be referred to as a transmission time interval (TTI). In some examples, the TTI duration (e.g., the number of symbol periods in a TTI) may be variable. Additionally, or alternatively, the smallest scheduling unit of the wireless communications system  100  may be dynamically selected (e.g., in bursts of shortened TTIs (sTTIs)). 
     Physical channels may be multiplexed on a carrier according to various techniques. A physical control channel and a physical data channel may be multiplexed on a downlink carrier, for example, using one or more of time division multiplexing (TDM) techniques, frequency division multiplexing (FDM) techniques, or hybrid TDM-FDM techniques. A control region (e.g., a control resource set (CORESET)) for a physical control channel may be defined by a number of symbol periods and may extend across the system bandwidth or a subset of the system bandwidth of the carrier. One or more control regions (e.g., CORESETs) may be configured for a set of the UEs  115 . For example, one or more of the UEs  115  may monitor or search control regions for control information according to one or more search space sets, and each search space set may include one or multiple control channel candidates in one or more aggregation levels arranged in a cascaded manner. An aggregation level for a control channel candidate may refer to a number of control channel resources (e.g., control channel elements (CCEs)) associated with encoded information for a control information format having a given payload size. Search space sets may include common search space sets configured for sending control information to multiple UEs  115  and UE-specific search space sets for sending control information to a specific UE  115 . 
     Each network entity  105  may provide communication coverage via one or more cells, for example a macro cell, a small cell, a hot spot, or other types of cells, or any combination thereof. The term “cell” may refer to a logical communication entity used for communication with a network entity  105  (e.g., over a carrier) and may be associated with an identifier for distinguishing neighboring cells (e.g., a physical cell identifier (PCID), a virtual cell identifier (VCID), or others). In some examples, a cell may also refer to a geographic coverage area  110  or a portion of a geographic coverage area  110  (e.g., a sector) over which the logical communication entity operates. Such cells may range from smaller areas (e.g., a structure, a subset of structure) to larger areas depending on various factors such as the capabilities of the network entity  105 . For example, a cell may be or include a building, a subset of a building, or exterior spaces between or overlapping with geographic coverage areas  110 , among other examples. 
     A macro cell generally covers a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by the UEs  115  with service subscriptions with the network provider supporting the macro cell. A small cell may be associated with a lower-powered network entity  105 , as compared with a macro cell, and a small cell may operate in the same or different (e.g., licensed, unlicensed) frequency bands as macro cells. Small cells may provide unrestricted access to the UEs  115  with service subscriptions with the network provider or may provide restricted access to the UEs  115  having an association with the small cell (e.g., the UEs  115  in a closed subscriber group (CSG), the UEs  115  associated with users in a home or office). A network entity  105  may support one or multiple cells and may also support communications over the one or more cells using one or multiple component carriers. 
     In some examples, a carrier may support multiple cells, and different cells may be configured according to different protocol types (e.g., MTC, narrowband IoT (NB-IoT), enhanced mobile broadband (eMBB)) that may provide access for different types of devices. 
     In some examples, a network entity  105  may be movable and therefore provide communication coverage for a moving geographic coverage area  110 . In some examples, different geographic coverage areas  110  associated with different technologies may overlap, but the different geographic coverage areas  110  may be supported by the same network entity  105 . In other examples, the overlapping geographic coverage areas  110  associated with different technologies may be supported by different network entities  105 . The wireless communications system  100  may include, for example, a heterogeneous network in which different types of the network entities  105  provide coverage for various geographic coverage areas  110  using the same or different radio access technologies. 
     The wireless communications system  100  may be configured to support ultra-reliable communications or low-latency communications, or various combinations thereof. For example, the wireless communications system  100  may be configured to support ultra-reliable low-latency communications (URLLC). The UEs  115  may be designed to support ultra-reliable, low-latency, or critical functions. Ultra-reliable communications may include private communication or group communication and may be supported by one or more services such as push-to-talk, video, or data. Support for ultra-reliable, low-latency functions may include prioritization of services, and such services may be used for public safety or general commercial applications. The terms ultra-reliable, low-latency, and ultra-reliable low-latency may be used interchangeably herein. 
     In some examples, a UE  115  may also be able to communicate directly with other UEs  115  over a device-to-device (D2D) communication link  135  (e.g., using a peer-to-peer (P2P) or D2D protocol). One or more UEs  115  utilizing D2D communications may be within the geographic coverage area  110  of a network entity  105 . Other UEs  115  in such a group may be outside the geographic coverage area  110  of a network entity  105  or be otherwise unable to receive transmissions from a network entity  105 . In some examples, groups of the UEs  115  communicating via D2D communications may utilize a one-to-many (1:M) system in which each UE  115  transmits to every other UE  115  in the group. In some examples, a network entity  105  facilitates the scheduling of resources for D2D communications. In other cases, D2D communications are carried out between the UEs  115  without the involvement of a network entity  105 . 
     The core network  130  may provide user authentication, access authorization, tracking, Internet Protocol (IP) connectivity, and other access, routing, or mobility functions. The core network  130  may be an evolved packet core (EPC) or 5G core (5GC), which may include at least one control plane entity that manages access and mobility (e.g., a mobility management entity (MME), an access and mobility management function (AMF)) and at least one user plane entity that routes packets or interconnects to external networks (e.g., a serving gateway (S-GW), a Packet Data Network (PDN) gateway (P-GW), or a user plane function (UPF)). The control plane entity may manage non-access stratum (NAS) functions such as mobility, authentication, and bearer management for the UEs  115  served by the network entities  105  associated with the core network  130 . User IP packets may be transferred through the user plane entity, which may provide IP address allocation as well as other functions. The user plane entity may be connected to IP services  150  for one or more network operators. The IP services  150  may include access to the Internet, Intranet(s), an IP Multimedia Subsystem (IMS), or a Packet-Switched Streaming Service. 
     Some of the network devices, such as a network entity  105 , may include subcomponents such as an access network entity  140 , which may be an example of an access node controller (ANC). Each access network entity  140  may communicate with the UEs  115  through one or more other access network transmission entities  145 , which may be referred to as radio heads, smart radio heads, or transmission/reception points (TRPs). Each access network transmission entity  145  may include one or more antenna panels. In some configurations, various functions of each access network entity  140  or network entity  105  may be distributed across various network devices (e.g., radio heads and ANCs) or consolidated into a single network device (e.g., a network entity  105 ). 
     The wireless communications system  100  may operate using one or more frequency bands, typically in the range of 300 megahertz (MHz) to 300 gigahertz (GHz). Generally, the region from 300 MHz to 3 GHz is known as the ultra-high frequency (UHF) region or decimeter band because the wavelengths range from approximately one decimeter to one meter in length. The UHF waves may be blocked or redirected by buildings and environmental features, but the waves may penetrate structures sufficiently for a macro cell to provide service to the UEs  115  located indoors. The transmission of UHF waves may be associated with smaller antennas and shorter ranges (e.g., less than 100 kilometers) compared to transmission using the smaller frequencies and longer waves of the high frequency (HF) or very high frequency (VHF) portion of the spectrum below 300 MHz. 
     The wireless communications system  100  may utilize both licensed and unlicensed radio frequency spectrum bands. For example, the wireless communications system  100  may employ License Assisted Access (LAA), LTE-Unlicensed (LTE-U) radio access technology, or NR technology in an unlicensed band such as the 5 GHz industrial, scientific, and medical (ISM) band. When operating in unlicensed radio frequency spectrum bands, devices such as the network entities  105  and the UEs  115  may employ carrier sensing for collision detection and avoidance. In some examples, operations in unlicensed bands may be based on a carrier aggregation configuration in conjunction with component carriers operating in a licensed band (e.g., LAA). Operations in unlicensed spectrum may include downlink transmissions, uplink transmissions, P2P transmissions, or D2D transmissions, among other examples. 
     A network entity  105  or a UE  115  may be equipped with multiple antennas, which may be used to employ techniques such as transmit diversity, receive diversity, multiple-input multiple-output (MIMO) communications, or beamforming. The antennas of a network entity  105  or a UE  115  may be located within one or more antenna arrays or antenna panels, which may support MIMO operations or transmit or receive beamforming. For example, one or more network entity antennas or antenna arrays may be co-located at an antenna assembly, such as an antenna tower. In some examples, antennas or antenna arrays associated with a network entity  105  may be located in diverse geographic locations. A network entity  105  may have an antenna array with a number of rows and columns of antenna ports that the network entity  105  may use to support beamforming of communications with a UE  115 . Likewise, a UE  115  may have one or more antenna arrays that may support various MIMO or beamforming operations. Additionally, or alternatively, an antenna panel may support radio frequency beamforming for a signal transmitted via an antenna port. 
     The network entities  105  or the UEs  115  may use MIMO communications to exploit multipath signal propagation and increase the spectral efficiency by transmitting or receiving multiple signals via different spatial layers. Such techniques may be referred to as spatial multiplexing. The multiple signals may, for example, be transmitted by the transmitting device via different antennas or different combinations of antennas. Likewise, the multiple signals may be received by the receiving device via different antennas or different combinations of antennas. Each of the multiple signals may be referred to as a separate spatial stream and may carry bits associated with the same data stream (e.g., the same codeword) or different data streams (e.g., different codewords). Different spatial layers may be associated with different antenna ports used for channel measurement and reporting. MIMO techniques include single-user MIMO (SU-MIMO), where multiple spatial layers are transmitted to the same receiving device, and multiple-user MIMO (MU-MIMO), where multiple spatial layers are transmitted to multiple devices. 
     Beamforming, which may also be referred to as spatial filtering, directional transmission, or directional reception, is a signal processing technique that may be used at a transmitting device or a receiving device (e.g., a network entity  105 , a UE  115 ) to shape or steer an antenna beam (e.g., a transmit beam, a receive beam) along a spatial path between the transmitting device and the receiving device. Beamforming may be achieved by combining the signals communicated via antenna elements of an antenna array such that some signals propagating at particular orientations with respect to an antenna array experience constructive interference while others experience destructive interference. The adjustment of signals communicated via the antenna elements may include a transmitting device or a receiving device applying amplitude offsets, phase offsets, or both to signals carried via the antenna elements associated with the device. The adjustments associated with each of the antenna elements may be defined by a beamforming weight set associated with a particular orientation (e.g., with respect to the antenna array of the transmitting device or receiving device, or with respect to some other orientation). 
     A network entity  105  or a UE  115  may use beam sweeping techniques as part of beam forming operations. For example, a network entity  105  may use multiple antennas or antenna arrays (e.g., antenna panels) to conduct beamforming operations for directional communications with a UE  115 . Some signals (e.g., synchronization signals, reference signals, beam selection signals, or other control signals) may be transmitted by a network entity  105  multiple times in different directions. For example, the network entity  105  may transmit a signal according to different beamforming weight sets associated with different directions of transmission. Transmissions in different beam directions may be used to identify (e.g., by a transmitting device, such as a network entity  105 , or by a receiving device, such as a UE  115 ) a beam direction for later transmission or reception by the network entity  105 . 
     Some signals, such as data signals associated with a particular receiving device, may be transmitted by a network entity  105  in a single beam direction (e.g., a direction associated with the receiving device, such as a UE  115 ). In some examples, the beam direction associated with transmissions along a single beam direction may be determined based on a signal that was transmitted in one or more beam directions. For example, a UE  115  may receive one or more of the signals transmitted by the network entity  105  in different directions and may report to the network entity  105  an indication of the signal that the UE  115  received with a highest signal quality or an otherwise acceptable signal quality. 
     In some examples, transmissions by a device (e.g., by a network entity  105  or a UE  115 ) may be performed using multiple beam directions, and the device may use a combination of digital precoding or radio frequency beamforming to generate a combined beam for transmission (e.g., from a network entity  105  to a UE  115 ). The UE  115  may report feedback that indicates precoding weights for one or more beam directions, and the feedback may correspond to a configured number of beams across a system bandwidth or one or more sub-bands. The network entity  105  may transmit a reference signal (e.g., a cell-specific reference signal (CRS), a CSI-RS), which may be precoded or unprecoded. The UE  115  may provide feedback for beam selection, which may be a precoding matrix indicator (PMI) or codebook-based feedback (e.g., a multi-panel type codebook, a linear combination type codebook, a port selection type codebook). Although these techniques are described with reference to signals transmitted in one or more directions by a network entity  105 , a UE  115  may employ similar techniques for transmitting signals multiple times in different directions (e.g., for identifying a beam direction for subsequent transmission or reception by the UE  115 ) or for transmitting a signal in a single direction (e.g., for transmitting data to a receiving device). 
     A receiving device (e.g., a UE  115 ) may try multiple receive configurations (e.g., directional listening) when receiving various signals from the network entity  105 , such as synchronization signals, reference signals, beam selection signals, or other control signals. For example, a receiving device may try multiple receive directions by receiving via different antenna subarrays, by processing received signals according to different antenna subarrays, by receiving according to different receive beamforming weight sets (e.g., different directional listening weight sets) applied to signals received at multiple antenna elements of an antenna array, or by processing received signals according to different receive beamforming weight sets applied to signals received at multiple antenna elements of an antenna array, any of which may be referred to as “listening” according to different receive configurations or receive directions. In some examples, a receiving device may use a single receive configuration to receive along a single beam direction (e.g., when receiving a data signal). The single receive configuration may be aligned in a beam direction determined based on listening according to different receive configuration directions (e.g., a beam direction determined to have a highest signal strength, highest signal-to-noise ratio (SNR), or otherwise acceptable signal quality based on listening according to multiple beam directions). 
     The wireless communications system  100  may be a packet-based network that operates according to a layered protocol stack. In the user plane, communications at the bearer or Packet Data Convergence Protocol (PDCP) layer may be IP-based. A Radio Link Control (RLC) layer may perform packet segmentation and reassembly to communicate over logical channels. A MAC layer may perform priority handling and multiplexing of logical channels into transport channels. The MAC layer may also use error detection techniques, error correction techniques, or both to support retransmissions at the MAC layer to improve link efficiency. In the control plane, the RRC protocol layer may provide establishment, configuration, and maintenance of an RRC connection between a UE  115  and a network entity  105  or a core network  130  supporting radio bearers for user plane data. At the physical layer, transport channels may be mapped to physical channels. 
     The UEs  115  and the network entities  105  may support retransmissions of data to increase the likelihood that data is received successfully. Hybrid automatic repeat request (HARQ) feedback is one technique for increasing the likelihood that data is received correctly over a communication link  125 . HARQ may include a combination of error detection (e.g., using a cyclic redundancy check (CRC)), forward error correction (FEC), and retransmission (e.g., automatic repeat request (ARQ)). HARQ may improve throughput at the MAC layer in poor radio conditions (e.g., low signal-to-noise conditions). In some examples, a device may support same-slot HARQ feedback, where the device may provide HARQ feedback in a specific slot for data received in a previous symbol in the slot. In other cases, the device may provide HARQ feedback in a subsequent slot, or according to some other time interval. 
     The UEs  115  and the network entities  105  of the wireless communications system  100  may support techniques for direct SCell activation using temporary reference signals. In particular, aspects of the present disclosure provide techniques for using L3 signaling (e.g., RRC signaling) on a PCell to configure and activate an SCell at a UE  115  using temporary reference signals. For example, a UE  115  may receive an L3 message (e.g., RRC message) from a PCell which instructs the UE  115  to activate an SCell. In some aspects, the PCell and the SCell may be associated with (e.g., supported by) the same network entity  105  or different network entities  105 . Subsequently, the UE  115  may transmit an L3 response message (e.g., RRC complete message) acknowledging the L3 message (e.g., acknowledging the instruction to activate the SCell), and monitor resources on the SCell. The UE  115  may then receive a temporary reference signal from the SCell that the UE uses to perform time and/or frequency tracking of the SCell, which may enable the UE  15  to begin monitoring CSI-RSs on the SCell. 
     In some aspects, the resources for the temporary reference signal and/or other parameters for the temporary reference signal (e.g., structure, format, component carrier, BWP) may be indicated via the L3 message (or a PDCCH transmission carrying the L3 message). Additionally, or alternatively, the resources and/or parameters for the temporary reference signal may be received via a MAC-CE or L1 message (e.g., DCI message, PDCCH transmission) from the PCell following reception of the L3 message. In some cases, the L3 message (e.g., RRC message) may indicate a set of active TCI states (e.g., active QCL assumptions) for the SCell, where the temporary reference signal is transmitted via one of the active TCI states. 
     Techniques described herein may provide for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation relative to SCell activation schemes that do not use temporary reference signals. Additionally, techniques described herein may re-use (or re-purpose) fields within existing control signaling used for the SCell activation (e.g., reuse fields within uplink DCI messages which schedule RRC response messages for SCell activation), which may enable direct SCell activation without increasing control signaling used for SCell activation. 
       FIG.  2    illustrates an example of a wireless communications system  200  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The wireless communications system  200  may implement, or be implemented by, aspects of wireless communications system  100 . The wireless communications system  200  may include a UE  115 - a  and a network entity  105 - a , which may be examples of UEs  115  and network entities  105 , as described with reference to  FIG.  1   . 
     The wireless communications system  200  may support wireless communications with wireless devices (e.g., UE  115 - a ) via one or more serving cells  205  of the wireless communications system  200 . In particular, each serving cell  205  may be supported by one or more network entities  105  of the wireless communications system  200 . For example, as shown in  FIG.  2   , the wireless communications system  200  may include a first serving cell  205 - a  supported by the network entity  105 - a , and a second serving cell  205 - b  supported by the network entity  105 - a . The serving cells  205  may include PCells, SCells, primary-secondary cells (PSCells) of a secondary cell group (SCG), or any combination thereof. For example, the first serving cell  205 - a  may include a PCell, and the second serving cell  205 - b  may include an SCell. The wireless communications system  200  may include any quantity of serving cells  205  supported by any quantity of network entities  105 . For example, in additional or alternative cases, the first serving cell  205 - a  may be supported by the network entity  105 - a , and the second serving cell  205 - b  may be supported by a second network entity  105  (not shown) which is different from the network entity  105 - a.    
     In some cases, the first serving cell  205 - a , the second serving cell  205 - b , or both, may be associated with a given radio access technology, such as a 5G radio access technology, an NR access technology, a 4G radio access technology, an LTE radio access technology, or any combination thereof. In some cases, techniques described herein may be implemented in the context of dual connectivity scenarios. In this regard, the second serving cell  205 - b  may be associated with the same or different radio access technology as the radio access technology associated with the first serving cell  205 - a . For example, in cases where the first serving cell  205 - a  is associated with a 5G or NR access technology, the second serving cell  205 - b  may be associated with a 4G radio access technology, an LTE radio access technology, or both. Moreover, in some cases, the first serving cell  205 - a  and the second serving cell  205 - b  may be associated with different frequency bands associated with a common radio access technology. For example, in some cases, both the first and second serving cells  205 - a  and  205 - b  may be associated with an NR access technology, where the first serving cell  205 - a  is associated a frequency range one (FR1) frequency band of the NR access technology and the second serving cell  205 - b  is associated a frequency range two (FR2) frequency band of the NR access technology. 
     In some aspects, the UE  115 - a  may communicate with the network entity  105 - a  using one or more beams, one or more carriers, one or more communications links, or any combination thereof. For example, each serving cell  205  may be associated with a same or different frequency range, separate beams, separate component carriers, and/or communications links to facilitate wireless communications between the UE  115 - a  and the respective serving cells  205 . In some cases, the communication link  210  may include an example of an access link (e.g., a Uu link). The communication link  210  may include a bi-directional link that can include both uplink and downlink communication. For example, the UE  115 - a  may transmit uplink transmissions, such as uplink control signals or uplink data signals, to the network entity  105 - a  using the communication link  210 , and the network entity  105 - a  may transmit downlink transmissions, such as downlink control signals or downlink data signals, to the UE  115 - a  using the communication link  210 . 
     The wireless communications system  200  may support temporary reference signals to expedite the activation process during an SCell activation to improve efficiency. The temporary reference signal may be supported for SCell activation in, for example, FR1, FR2, and/or some other frequency range(s). Broadly, the temporary reference signal may support functionalities related to AGC settling, time and/or frequency tracking/tuning during SCell activation, and the like. 
     In some aspects, a temporary reference signal may also be referred to as an aperiodic reference signal, which may be an example of a tracking reference signal, an aperiodic CSI-RS, a persistent CSI-RS, a semi-persistent CSI-RS, a sounding reference signal (SRS), a reference signal based on primary sync signal (PSS) and/or secondary sync signal (SSS), or any combination thereof. Other examples of reference signal types that may be configured as an aperiodic reference signal include, but are not limited to, a phase tracking reference signal, a beam tracking/management reference signal, and the like. Accordingly, the terms tracking reference signal, aperiodic reference signal, new temporary reference signal, and the like, may be used interchangeably herein. 
     For example, in some cases, a set of multiple tracking reference signals may form a temporary reference signal. The set of multiple tracking reference signals may be transmitted/broadcast by an SCell for time and frequency tracking (and/or AGC) by a UE  115  during SCell activation. In such cases, each tracking reference signal of the set of multiple tracking reference signals may span one or more OFDM symbols in one or more slots. The tracking reference signals may be consecutive in the time domain, or may be separated in the time domain by one or more OFDM symbols. For example, an SCell may transmit/broadcast a first pair of tracking reference signals and a second pair of tracking reference signals, where each of the first pair and second pair of tracking reference signals include tracking reference signals in consecutive slots. In this example, the first pair of tracking reference signals and the second pair of tracking reference signals may be separated in the time domain by one or more slots (e.g., two slot gap between the pairs of tracking reference signals). 
     In some cases, a tracking reference signal waveform may be selected as the temporary reference signal (e.g., as an aperiodic reference signal) for SCell activation. In some examples, the temporary reference signal may be triggered by DCI, MAC CE, and the like. Some wireless communications systems utilize temporary reference signals to improve SCell activation. Temporary reference signals may enable UEs  115  to quickly perform AGC for an SCell where the UE  115  adjusts a receive amplifier gain and performs time/frequency tuning with the SCell. A UE  115  may measure the triggered temporary reference during the SCell activation procedure no earlier than within a configured time threshold (e.g., no earlier than a slot m). Depending on a periodicity of SSBs on the SCell, the temporary reference signal may be received by the UE  115  via the SCell prior to an earliest receivable SSB, which may expedite the SCell activation process. 
     In some aspects, the UE  115 - a  and the network entity  105 - a  of the wireless communications system  200  may support techniques for direct SCell activation using temporary reference signals. In particular, the UE  115 - a  and the network entity  105 - a  (e.g., serving cells  205 - a ,  205 - b ) may support techniques for using RRC signaling (e.g., L3 signaling) on a PCell to configure and activate an SCell at the UE  115 - a  using temporary reference signals. 
     For example, as illustrated in  FIG.  2   , the UE  115 - a  may establish wireless communications with the first serving cell  205 - a . In some aspects, the UE  115 - a  may establish the wireless communications with the first serving cell  205 - a  by initiating or otherwise performing a setup procedure with the first serving cell  205 - a . In some aspects, the first serving cell  205 - a  and the second serving cell  205 - b  may be associated with the same frequency band (e.g., intra-band carrier aggregation). In some aspects, the first serving cell  205 - a , the second serving cell  205 - b , or both, may include a PCell, an SCell, a PSCell of an SCG, or any combination thereof. For example, in cases where the first serving cell  205 - a  includes a PCell, the second serving cell  205 - b  may include an SCell. The first serving cell  205 - a  and the second serving cell  205 - b  may be supported by the same network entity  105 - a  or different network entities  105 . 
     In some aspects, the UE  115 - a  may receive, via the first serving cell  205 - a , an RRC message  215  (e.g., L3 message) including an indication to activate the second serving cell  205 - b . In this regard, the RRC message  215  may initiate an RRC configuration and/or reconfiguration procedure for activating the second serving cell  205 - b  at the UE  115 - a . In some aspects, the RRC message  215  may trigger a reference signal  230  on the second serving cell  205 - b  for SCell activation. In this regard, the RRC message  215  may directly activate the second serving cell  205 - b , and may activate the reference signal  230  on the second serving cell  205 - b . For example, the RRC message  215  may include an indication of a resource (or set of resources) for a reference signal  230  on the second serving cell  205 - b  which will be used to facilitate SCell activation. 
     As noted previously herein, the UE  115 - a  may use the reference signal  230  on the second serving cell  205 - b  to perform time and frequency tracking, AGC, or both during activation of the second serving cell  205 - b . The reference signal  230  may include a temporary reference signal, a tracking reference signal, a non-zero power channel state information reference signal (NZP-CSI-RS), an SSB, or any combination thereof. For example, the temporary reference signal  230  triggered by the RRC message  215  may include one or multiple NZP-CSI-RS resource sets, where each NZP-CSI-RS resource set includes one or multiple NZP-CSI-RS resources labeled as trs-info. 
     The RRC message  215  may indicate one or more parameters associated with the reference signal  230  on the second serving cell  205 - b . Parameters associated with the reference signal  230  which may be indicated via the RRC message  215  may include a structure of the reference signal  230 , a component carrier for the reference signal  230 , a BWP for the reference signal  230 , TCI states for the reference signal  230 , QCL configurations (e.g., QCL assumptions) for the reference signal  230 , or any combination thereof. For example, the RRC message  215  may indicate a structure/type of reference signal  230 , which component carrier(s) are associated with the triggered reference signal  230 , and/or which BWP(s) are associated with the triggered reference signal  230  within the indicated component carriers. 
     By way of another example, the RRC message  215  may indicate one or more TCI states (e.g., one or more active QCL configurations/assumptions) associated with the second serving cell  205 - b  which may be used to transmit the triggered reference signal  230 . For instance, the RRC message  215  may indicate TCI state information and/or QCL information which indicates which reference signal  230  the NZP-CSI-RS resources are QCLed with, and whether the QCL source reference signal  230  may include an SSB or other NZP-CSI-RS resource. For the purposes of the present disclosure, the terms “QCL configuration” and “QCL assumption” may be used interchangeably. 
     In some aspects, the UE  115 - a  may receive an additional control message  225 - a  (e.g., additional control signaling) via the first serving cell  205 - a . For example, the UE  115 - a  may receive a MAC-CE or other L2 message (e.g., control message  225 - a ) via the first serving cell  205 - a  in addition to the RRC message  215 . In some cases, the control message  225 - b  may be transmitted/received together with the RRC message  215  (e.g., within a same physical downlink shared channel (PDSCH) message). In additional or alternative cases, the control message  225 - b  and the RRC message  215  may be transmitted/received in separate control messages (e.g., within separate PDSCH messages). In some aspects, the control message  225 - a  (e.g., MAC-CE, L2 message) may include an indication of the resource for the reference signal  230  on the second serving cell  205 - b . Additionally, or alternatively, the control message  225 - a  may indicate one or more parameters associated with the reference signal  230  on the second serving cell  205 - b  (e.g., structure, component carrier, BWP, TCI states, QCL configurations). In this regard, the resource(s) and/or other parameters for the reference signal  230  on the second serving cell  205 - b  may be indicated via the RRC message  215  (e.g., L3 message), via the control message  225 - a  (e.g., MAC-CE, L2 message), or both. 
     The UE  115 - a  may transmit, via the first serving cell  205 - a , an RRC response message  220  (e.g., RRC complete message, L3 response message). In some aspects, the UE  115 - a  may transmit the RRC response message  220  in response to the RRC message  215 . The RRC response message  220  may indicate a completion of the RRC configuration/reconfiguration procedure which was triggered by the RRC message  215 . As such, the transmission of the RRC response message  220  may indicate an end of T RRC_Process  time interval, and a beginning of an activation time interval T ActivationTime  for activating the second serving cell  205 - b . Additionally, or alternatively, the UE  115 - a  may transmit the RRC response message  220  based on receiving the control message  225 - a  (e.g., MAC-CE, L2 message). 
     In some implementations, the UE  115 - a  may receive an additional control message  225 - b  (e.g., additional control signaling) via the first serving cell  205 - a . For example, the UE  115 - a  may receive the additional control message  225 - b  including a DCI message or other L1 message via the first serving cell  205 - a . In some aspects, the UE  115 - a  may receive the control message  225 - b  (e.g., DCI message) following transmission of the RRC response message  220 . In particular, the UE  115 - a  may receive the control message  225 - b  (e.g., DCI message) within the activation time interval (T ActivationTime ) following the transmission of the RRC response message  220 . In this regard, the UE  115 - a  may receive the control message  225 - b  (e.g., DCI message, L1 message) based on receiving the RRC message  215 , receiving the control message  225 - a  (e.g., MAC-CE, L2 message), transmitting the RRC response message  220 , or any combination thereof. 
     In some aspects, the control message  225 - b  (e.g., DCI message, L1 message) may include an indication of the resource for the reference signal  230  on the second serving cell  205 - b . Additionally, or alternatively, the control message  225 - b  (e.g., DCI message, L1 message) may indicate one or more parameters associated with the reference signal  230  on the second serving cell  205 - b  (e.g., structure, component carrier, BWP, TCI states, QCL configurations). In this regard, the resource(s) and/or other parameters for the reference signal  230  on the second serving cell  205 - b  may be indicated via the RRC message  215  (e.g., L3 message), via the control message  225 - a  (e.g., MAC-CE, L2 message), via the control message  225 - b  (e.g., DCI message, L1 message), or any combination thereof. 
     The UE  115 - a  may identify the resource for the reference signal  230  which is to be received via the second serving cell  205 - b . The UE  115 - a  may identify the resource for the reference signal  230  within the activation time interval (T ActivationTime ) following the transmission of the RRC response message  220  (e.g., RRC complete message, L3 response message). In this regard, the UE  115 - a  may identify the resource for the reference signal  230  associated with time and frequency tracking for the second serving cell  205 - b  based on receiving the RRC message  215 , receiving the control message  225 - a  (e.g., MAC-CE, L2 message), transmitting the RRC response message  220 , receiving the control message  225 - b  (e.g., DCI message, L1 message), or any combination thereof. 
     Additionally, or alternatively, the UE  115 - a  may identify one or more parameters (e.g., structure, component carrier, BWP, TCI states, QCL configurations) associated with the reference signal  230 . The UE  115 - a  may identify the resource(s) and/or other parameters for the reference signal  230  based on the higher-layer configuration and trigger signaling (e.g., RRC message  215 , MAC-CE, DCI message) received via the first serving cell  205 - a . In other words, the resource and other parameters for the reference signal  230  for the second serving cell  205 - b  may be provided to the UE  115 - b  by the trigger signaling, preliminarily provided to the UE  115 - a  via earlier RRC configurations or signaling, via signaling which triggers the reference signal  230 , or any combination thereof. 
     Subsequently, the UE  115 - a  may monitor the resource for the reference signal  230  on the second serving cell  205 - b . In this regard, the UE  115 - a  may monitor the resource for the reference signal  230  based on identifying the resource and/or other parameters (e.g., structure, component carrier, BWP, TCI states, QCL configurations) associated with the reference signal  230 . Moreover, the UE  115 - b  may monitor the resource for the reference signal  230  based on receiving the RRC message  215 , receiving the control message  225 - a  (e.g., MAC-CE, L2 message), transmitting the RRC response message  220 , receiving the control message  225 - b  (e.g., DCI message, L1 message), or any combination thereof. 
     In some cases, the UE  115 - a  may assume that a TCI state (e.g., QCL configuration or QCL assumption) of the reference signal  230  to be transmitted by the second serving cell  205 - b  may be selected from a set of active TCI states (or active QCL configurations), if sets of active TCI states/QCL configurations have been configured. In other words, the second serving cell  205 - b  may not transmit the reference signal  230  with a TCI state (or QCL configuration) which is not in a configured active set of TCI states/QCL configurations. 
     For example, as noted previously herein, the UE  115 - a  may receive an indication of a set of active TCI states and/or active QCL configurations via the RRC message  215 , via the control message  225 - a  (e.g., MAC-CE), via the control message  225 - b  (e.g., DCI message), or any combination thereof. In other words, the set of active TCI states may be activated by the RRC message  215  which directly activates the SCell (e.g., second serving cell  205 - b ), via the MAC-CE of a PDSCH transmission carrying the RRC message  215  which directly activates the SCell, via a DCI message which directly activates the SCell, or any combination thereof. In this example, the UE  115 - a  may assume that the reference signal  230  will be transmitted in accordance with one of the active TCI states/active QCL configurations, and may monitor the resource for the reference signal  230  based on (e.g., in accordance with) one or more of the active TCI states and/or active QCL configurations. 
     The UE  115 - a  may receive the reference signal  230  (e.g., temporary reference signal, tracking reference signal, NZP-CSI-RS, SSB) via the second serving cell  205 - b . The UE  115 - a  may receive the reference signal  230  within the resource for the reference signal  230 , and based on monitoring the resource for the reference signal  230 . Additionally, the UE  115 - a  may receive the reference signal  230  in accordance with the one or more parameters (e.g., structure, component carrier, BWP, active TCI state, active QCL configuration) for the reference signal  230 . In some aspects, the UE  115 - a  may receive the reference signal  230  prior to an earliest SSB which the UE  115 - a  is capable of receiving via the second serving cell  205 - b.    
     In some aspects, the UE  115 - a  may perform time and frequency tracking during activation of the second serving cell  205 - b  based on receiving the reference signal  230  via the second serving cell  205 - b . For example, the reference signal  230  may be used by the UE  115 - a  to perform time and frequency tracking and/or AGC during activation of the second serving cell  205 - b . In this regard, the UE  115 - a  may be configured to perform measurements and/or adjust time tracking and/or frequency tracking for the second serving cell  205 - b  based on receiving the reference signal  230  via the second serving cell  205 - b.    
     Upon performing time/frequency tracking, AGC, or both, during activation of the second serving cell  205 - b , the UE  115 - b  may be able to perform CSI reporting procedures with the second serving cell  205 - b . Accordingly, in some aspects, the UE  115 - a  may receive a CSI-RS  235  via the second serving cell  205 - b . The UE  115 - a  may receive the CSI-RS  235  based on performing the time and frequency tracking (e.g., AGC) for the second serving cell  205 - b . The UE  115 - b  may be configured to perform measurements on the received CSI-RS  235  for CSI reporting. Subsequently, the UE  115 - a  may transmit a CSI report  240  via the second serving cell  205 - b . In particular, the UE  115 - a  may transmit the CSI report  240  based on performing measurements on the CSI-RS  235 . In this regard, the CSI report  240  transmitted to the second serving cell  205 - b  may include an indication of the measurements performed on the CSI-RS  235 . 
     In some aspects, the UE  115 - a  may communicate with the second serving cell  205 - b  based on performing the time and frequency tracking (e.g., AGC) during activation of the SCell based on the reference signal  230 . Additionally, or alternatively, the UE  115 - a  may communicate with the second serving cell  205 - b  based on receiving the CSI-RS  235 , transmitting the CSI report  240 , or both. 
     Techniques described herein may provide for improved wireless communications by improving direct SCell activation using RRC signaling. In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals  230  used for SCell activation via RRC signaling. By enabling the UE  115 - a  to identify resources for temporary reference signals  230  using RRC signaling, techniques described herein may reduce a time required for SCell activation relative to SCell activation schemes that do not use temporary reference signals. Additionally, techniques described herein may re-use (or re-purpose) fields within existing control signaling used for the SCell activation (e.g., reuse fields within uplink DCI messages which schedule RRC response messages for SCell activation), which may enable direct SCell activation without increasing control signaling used for SCell activation. 
       FIG.  3    illustrates an example of a resource configuration  300  that supports techniques for direct SCell activation in accordance with aspects of the present disclosure. The resource configuration  300  may implement, or be implemented by, aspects of wireless communications system  100 , wireless communications system  200 , or both. The resource configuration  300  illustrates a first SCell activation scheme  305 - a  and a second SCell activation scheme  305 - b . In particular, the first SCell activation scheme  305 - a  illustrates an SCell activation procedure which utilizes MAC-CE signaling, and the second SCell activation scheme  305 - b  illustrates a direct SCell activation procedure which utilizes RRC signaling. 
     According to some conventional techniques, upon receiving the SCell activation command in a slot, a UE  115  may support transmitting a valid CSI report and applying the actions related to the SCell activation command for the SCell being activated no later than in slot 
     
       
         
           
             n 
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                     T 
                     HARQ 
                   
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                     T 
                     
                       activation 
                       ⁢ 
                       _ 
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                       time 
                     
                   
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                       CSI 
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                       Reporting 
                     
                   
                 
                 
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     T HARQ  may refer to the timing (in ms) between the downlink data transmission and the acknowledgment of the downlink data transmission (e.g., HARQ-ACK feedback). T activation_time  may refer to the SCell activation delay in ms. If the SCell being activated is known and belongs to FR1, T activation_time  may be T FirstSSB +5 ms if the SCell measurement cycle is equal to or smaller than 160 ms (e.g., to support fine tracking) or T FirstSSB_Max +T rs +5 ms if the SCell measurement cycle is larger than 160 ms (e.g., to support AGC plus fine time/frequency tracking). If the SCell is unknown and belongs to FR1, provided that certain conditions are satisfied, T activation_time  may be T FirstSSB_Max +T SMTC_Max +2*T rs +5 ms (e.g., to support AGC, fine time/frequency tracking, and SSB detection). T rs  may generally refer to the SSB-based measurement and timing configuration (SMTC) periodicity of the SCell being activated if the UE  115  has been provided with an SMTC configuration for the SCell in the SCell addition message. Otherwise, T rs  may refer to the SMTC configured in the measObjectNR having the same SSB frequency and subcarrier spacing. 
     If the UE  115  is not provided an SMTC configuration or measurement object on this frequency, the requirement which involves T rs  may be applied with T rs  being equal to 5 ms assuming the SSB transmission periodicity is 5 ms. T FirstSSB  may refer to the time to the end of the first complete SSB burst indicated by the SMTC after slot 
     
       
         
           
             n 
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               T 
               HARQ 
             
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                     T 
                     HARQ 
                   
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                     3 
                     ⁢ 
                         
                     ms 
                   
                 
                 
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     T FirstSSB_Max  may refer to the time to the end of the first complete SSB burst indicated by the SMTC after slot 
     
       
         
           
             n 
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               HARQ 
             
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                     ms 
                   
                 
                 
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     This may fulfill the requirement that, in FR1 and in the case of intra-band SCell activation, the occasion when all active serving cells and SCells being activated or released are transmitting SSB burst in the same slot. In the case of inter-band SCell activation, this may refer to the first occasion when the SCell being activated is transmitting an SSB burst. In FR2, this may refer to the occasion when all active serving cells and SCells being activated or released are transmitting SSB burst in the same slot. 
     For example, referring to the first SCell activation scheme  305 - a , a UE  115  may be in wireless communications with a PCell  310 - a  at a time in which an SCell  310 - b  is deactivated at the UE  115 . The SCell  310 - b  may transmit/broadcast SSBs  315  (e.g., SSBs  315 - a ,  315 - b ,  315 - c ,  315 - d ) at a periodicity  335 , where the SSBs  315  may be used for time/frequency tracking, AGC, or both, during activation of the SCell  310 - b . The UE  115  may receive, via the PCell  310 - a , a control message  320  (e.g., RRC message) which configures or initiates activation of the SCell  310 - b  at the UE  115 . Subsequently, the UE  115  may receive, via the PCell  310 - a , an activation command  325  (e.g., MAC-CE) which activates the SCell  310 - b . The UE  115  may transmit a feedback message  330  (e.g., ACK) in response to the activation command  325  after a time interval T HARQ . 
     Continuing with reference to the first SCell activation scheme  305 - a , an activation time T ActivationTime  may be initiated following transmission of the feedback message  330 . Following transmission of the feedback message  330 , there may be some delay (e.g., 3 ms delay) before the UE  115  may begin communicating with the SCell  310 - b . Depending on the periodicity  335  of the SSBs  315  and the relative timing of the SSBs  315  on the SCell  310 - b , the delay following the feedback message  330  may cause the UE  115  to miss SSB  315 - b  on the SCell  310 - b . As a result, the UE  115  may have to wait until the following SSB  315 - c  to perform time/frequency tracking and AGC for the SCell  310 - b . Upon receiving the SSB  315 - c , there may be some delay (e.g., 2 ms) until an end of T ActivationTime  and a beginning of T CSI_Reporting . Following an end of T CSI_Reporting  the UE  115  may begin receiving CSI-RSs via the SCell  310 - b , and transmit CSI reports in order to communicate via the activated SCell  310 - b . Accordingly, the SCell activation delay (SCell ActivationDelay , or N Direct ) for the first SCell activation scheme  305 - a  may be represented as 
     
       
         
           
             
               
                 
                   T 
                   HARQ 
                 
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                     activation 
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     Additionally, or alternatively, direct SCell activation techniques (e.g., fast SCell activation) using L3 signaling (e.g., RRC signaling) may be used to activate an SCell  310  at a UE  115 . As noted previously herein in the context of MAC-CE SCell activation techniques, direct SCell activation techniques may exhibit an activation delay requirement, which may be a function of T ActivationTime  and a beginning of T CSI_Reporting  In some cases, a duration of T ActivationTime  may be based on an assumption that SSBs  315  are used for SCell  310  activation. 
     For example, referring to the second SCell activation scheme  305 - b , a UE  115 - b  may be in wireless communications with a PCell  310 - c  at a time in which an SCell  310 - d  is deactivated at the UE  115 - b . As noted previously herein, the SCell  310 - b  may transmit/broadcast SSBs  315  (e.g., SSB  315 - e ,  315 - f ), where the SSBs  315  may be used for time/frequency tracking and/or AGC during activation of the SCell  310 - d . The UE  115  may receive, via the PCell  310 - a , an RRC message  340  which configures or initiates activation of the SCell  310 - d  at the UE  115 - b . Transmission/reception of the RRC message  340  may mark the beginning of a time interval (T RRC_Process ) for RRC configuration/reconfiguration. 
     Continuing with reference to the second SCell activation scheme  305 - b , the UE  115 - b  may transmit an RRC complete message  345  in response to the RRC message  340 . The RRC complete message  345  may indicate an acknowledgment of the RRC message  340  and/or the configuration/activation of the SCell  310 - d . The RRC complete message  345  may be transmitted following a time interval T 1 , which defines a delay from slot 
     
       
         
           
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     A duration of time interval T 1  may be based on a complexity and capability of the UE  115 - b , and may therefore be UE-implementation dependent. An activation time T ActivationTime  may be initiated following transmission of the RRC complete message  345 . Moreover, T FirstSSB  may define a duration from the RRC complete message  345  and a first SSB  315 - e  which may be received and/or processed by the UE  115 - b . Upon receiving the SSB  315 - e , there may be some delay (e.g., 2-3 ms) until an end of T ActivationTime  and a beginning of T CSI_Reporting . Following an end of T CSI_Reporting , the UE  115 - b  may begin receiving CSI-RSs via the SCell  310 - d , and transmit CSI reports in order to communicate via the activated SCell  310 - d . Accordingly, the SCell activation delay (SCell ActivationDelay , or N Direct ) for the second SCell activation scheme  305 - b  may be represented as T RRC_Process +T 1 +T activation_time +T CSI_Reporting −3 ms. 
     Attendant advantages of the aspects of the present disclosure may be further shown and described with reference to  FIGS.  4  and  5   . 
       FIG.  4    illustrates an example of a resource configuration  400  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The resource configuration  400  may implement, or be implemented by, aspects of wireless communications system  100 , wireless communications system  200 , resource configuration  300 , or any combination thereof. 
     The resource configuration  400  includes an SCell activation scheme  405  which illustrates a direct SCell activation procedure which utilizes RRC (e.g., L3) signaling. As noted previously herein, techniques for direct SCell activation using RRC signaling and temporary reference signals may reduce how long it takes for a UE  115  to perform SCell activation. 
     For example, as shown in  FIG.  4   , a UE  115 - c  may be communicatively coupled to a PCell  410 - a  (e.g., first serving cell), where the PCell  410 - a  initiates activation of an SCell  410 - b  (e.g., second serving cell) at the UE  115 - c . As noted previously herein, the SCell activation delay (S CellActivationDelay , or N Direct ) for activation of the SCell  410 - b  at the UE  115 - c  may be represented as T RRC_Process +T 1 +T activation_time +T CSI_Reporting −3 ms. 
     The SCell  410 - b  may transmit SSBs  450  (e.g., SSBs  450 - a ,  450 - b ,  450 - c ) at an SSB periodicity. According to some conventional techniques, SSBs  450  may be used by the UE  115 - c  for time/frequency tracking, AGC, or both, during activation of the SCell  410 - b . However, use of the SSBs  450  for time/frequency tracking may result in increased latency for SCell  410 - b  activation. 
     Accordingly, as described herein, the SCell activation scheme  405  illustrated in  FIG.  4    may utilize a reference signal  435  to facilitate direct SCell activation (e.g., fast SCell activation) at the UE  115 - c . The reference signal  435  may be used for time and frequency tracking and/or AGC during activation of the SCell  410 - b  to expedite SCell activation at the UE  115 - c . The reference signal  435  may include a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, a temporary/aperiodic SSB, or any combination thereof. In particular, the UE  115 - c  may receive the reference signal  435  prior to a first SSB  450  (e.g., SSB  450 - b ) which may be received/processed by the UE  115 - c  via the SCell  410 - b . In this regard, the use of the reference signal  435  may reduce a time it takes for the UE  115 - c  to perform time/frequency tracking for the SCell  410 - b , which may expedite CSI reporting and communications between the UE  115 - c  and the SCell  410 - b . In some aspects, the reference signal  435  may be transmitted/repeated multiple times (e.g., multiple reference signal bursts). For example, the UE  115 - c  may receive a first reference signal  435  (e.g., first reference signal burst) for AGC, and a second reference signal  435  (e.g., second reference signal burst) for time/frequency tracking. 
     In cases where the reference signal  435  is triggered/activated via the RRC message  415 , the MAC-CE message  420 , and/or a MAC-CE message of a PDSCH carrying the RRC message  415  that directly activates the SCell  410 - b , the reference signal  435  may be triggered after T RRC_Process +T 1 +x, where x may be 0 ms, 3 ms, and the like. Comparatively, in cases where the DCI message  430  (e.g., UL DCI format with aperiodic CSI (A-CSI) request) triggers the reference signal  435 , the DCI message triggering the reference signal  435  may be received after T RRC_Process +T 1 +x, where x may be 0 ms, 3 ms. While the DCI message  430  is shown as occurring after the RRC message  415  in the time domain, this is provided solely for illustrative purposes. For example, in some cases, the DCI message  430  may schedule a PDSCH transmission including the RRC message  415  and/or the MAC-CE  420 , and may therefore come before the RRC message  415  and/or the MAC-CE  420  in the time domain. 
     In some aspects, a resource(s) and/or other parameters (e.g., structure, component carrier, BWP, TCI state, QCL configuration) for the reference signal  435  on the SCell  410 - b  may be indicated to the UE  115 - c  via higher-layer configuration and trigger signaling received via the PCell  410 - a . For example, the resource for the reference signal  435  may be indicated via an RRC message  415  (e.g., L3 message), a MAC-CE  420  (e.g., L2 message), a DCI message  430  (e.g., L1 message), or any combination thereof. As noted previously herein, in some cases, the MAC-CE  420  and the RRC message  415  may be received via a same PDSCH message and/or via different PDSCH messages. 
     The RRC message  415  transmitted via the PCell  410 - a  may initiate configuration and activation of the SCell  410 - b  at the UE  115 - c . In this regard, the RRC message  415  may initiate an RRC configuration and/or reconfiguration procedure for activating the second serving cell (e.g., SCell  410 - b ) at the UE  115 - c  during a time interval T RRC_Process  The UE  115 - c  may transmit an RRC response message  425  (e.g., RRC complete message) in response to the RRC message  415 . The RRC response message  425  may indicate an acknowledgment of the RRC message  415  and/or the configuration/activation of the SCell  410 - b.    
     In some aspects, the RRC response message  425  may be transmitted following a time interval T 1 , which defines a delay from slot 
     
       
         
           
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     A duration of time interval T 1  may be based on a complexity and capability of the UE  115 - c , and may therefore be UE-implementation dependent. In cases where the UE  115 - c  performs a handover (e.g., handover between PCells  410  during the SCell  410 - b  activation), the time interval T 1  may be replaced by time interval T interrupt +T 2 +T 3 . In such cases, T interrupt +T 2 +T 3  may define delays or interruptions of the SCell  410 - b  activation which are attributable to the handover. In some aspects, the MAC-CE  420  may be received within T RRC_Process , T 1 , or both. 
     In some cases, depending on the periodicity of SSBs  450  on the SCell  410 - b  and a relative timing of the SSBs  450 , the SCell  410 - b  may transmit/broadcast an SSB  450  (e.g., SSB  450 - b ) prior to transmission of the RRC response message  425  (e.g., before T RRC_Process , within T RRC_Process , within T 1 ). However, the UE  115 - c  may be unable to receive/process the SSB  450 - a  received prior to an activation time interval T ActivationTime  which follows the RRC response message  425 . As such, for wireless communications systems which do not utilize the reference signal  435  described herein, the UE  115 - c  may have to wait until the next SSB  450 - b  to perform time/frequency tacking and AGC for activation of the SCell  410 - b . Accordingly, techniques described herein which configure the reference signal  435  may reduce a latency of time/frequency tracking and/or AGC at the UE  115 - c , which may reduce a latency of SCell  410 - b  activation. The activation time interval T ActivationTime  may be occur following transmission of the RRC response message  425 . In some aspects, the reference signal  435  may be received within the activation time interval T ActivationTime  in the time domain. The term T TempRS  may define a duration from the transmission of the RRC response message  425  and the reception of the reference signal  435 . As such, T FirstSSB &lt;T ActivationTime . In some aspects, the UE  115 - c  may receive the DCI message  430  (e.g., L1 message) within T ActivationTime . As noted previously herein, the activation time interval T ActivationTime  may end after some delay (e.g., 2-3 ms) following reception of the reference signal  435 . 
     Upon receiving the reference signal  435 , the UE  115 - c  may perform time and frequency tracking and/or AGC for activation of the SCell  410 - b  based on the reference signal  435 . For example, the reference signal  435  may be used for time and frequency tracking and/or AGC during activation of the SCell  410 - b . In this regard, the UE  115 - c  may be configured to perform measurements and/or adjust time tracking and/or frequency tracking for the second serving cell (e.g., SCell  410 - b ) based on receiving the reference signal  435  via the SCell  410 - b.    
     Upon performing time/frequency tracking and/or AGC for the SCell  410 - b  based on the reference signal, and after an end of the activation time interval T ActivationTime , the UE  115 - c  may be able to perform CSI reporting procedures with the SCell  410 - b  during a CSI reporting time interval T CSI_Reporting  In other words, T CSI_Reporting  may define a delay until the first available CSI report including uncertainties of CSI-RS resources and CSI reports. 
     For example, as shown in  FIG.  4   , the UE  115 - c  may receive a CSI-RS  440  via the SCell  410 - b  following an end of T CSI_Reporting  The UE  115 - c  may receive the CSI-RS  440  based on performing the time and frequency tracking (e.g., AGC) for activation of the SCell  410 - b  based on the reference signal  435 . The UE  115 - c  may be configured to perform measurements on the received CSI-RS  440  for CSI reporting. Subsequently, the UE  115 - c  may transmit a CSI report  445  via the SCell  410 - b . In particular, the UE  115 - c  may transmit the CSI report  445  based on performing measurements on the CSI-RS  440 . In this regard, the CSI report  240  transmitted to the SCell  410 - b  may include an indication of the measurements performed on the CSI-RS  440 . 
     As shown in  FIG.  4   , in some cases, the CSI reporting (e.g., reception of CSI-RS  440 , transmission of CSI report  445 ) may be performed prior to the first SSB  450 - b  which is capable of being received/processed by the first UE  115 - c . In this regard, techniques described herein may enable the UE  115 - c  to perform CSI reporting earlier as compared to some conventional SCell activation techniques, which may expedite activation of the SCell  410 - b  and improve user experience at the UE  115 - c.    
       FIG.  5    illustrates an example of a process flow  500  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. In some examples, process flow  500  may implement, or be implemented by, aspects of wireless communications system  100 , wireless communications system  200 , resource configuration  300 , resource configuration  400 , or any combination thereof. 
     The process flow  500  may include a UE  115 - d , a first serving cell  505 - a , and a second serving cell  505 - b , which may be examples of UEs  115  and serving cells  205  as described with reference to  FIGS.  1  and  2   . In particular, the first serving cell  505 - a  and the second serving cell  505 - b  illustrated in  FIG.  5    may include examples the first serving cell  205 - a  and the second serving cell  205 - b  illustrated in  FIG.  2   , respectively. In this regard, the first serving cell  505 - a  may be an example of an PCell, and the second serving cell  505 - b  may be an example of an SCell which is activated at the UE  115 - d  by the PCell. In some aspects, the first serving cell  505 - a  and the second serving cell  505 - b  may be associated with (e.g., supported by) a single network entity  105  of a wireless communications system (e.g., network entity  105 - a  illustrated in  FIG.  2   ). Additionally, or alternatively, the first serving cell  505 - a  and the second serving cell  505 - b  may be associated with (e.g., supported by) different network entities  105 . 
     In some examples, the operations illustrated in process flow  500  may be performed by hardware (e.g., including circuitry, processing blocks, logic components, and other components), code (e.g., software or firmware) executed by a processor, or any combination thereof. Alternative examples of the following may be implemented, where some steps are performed in a different order than described or are not performed at all. In some cases, steps may include additional features not mentioned below, or further steps may be added. 
     At  510 , the UE  115 - d  may establish wireless communications with the first serving cell  505 - a . In some aspects, the UE  115 - d  may establish the wireless communications with the first serving cell  505 - a  by initiating or otherwise performing a setup procedure with the first serving cell  505 - a . In some aspects, the first serving cell  505 - a  and the second serving cell  505 - b  may be associated with the same frequency band (e.g., intra-band carrier aggregation). In some aspects, the first serving cell  505 - a , the second serving cell  505 - b , or both, may include a PCell, an SCell, a PSCell of an SCG, or any combination thereof. For example, in cases where the first serving cell  505 - a  includes a PCell, the second serving cell  505 - b  may include an SCell. The first serving cell  505 - a  and the second serving cell  505 - b  may be supported by the same network entity  105  or different network entities  105 . 
     At  515 , the UE  115 - d  may receive, via the first serving cell  505 - a , an RRC message (e.g., L3 message) including an indication to activate the second serving cell  505 - b . In this regard, the RRC message may initiate an RRC configuration and/or reconfiguration procedure for activating the second serving cell  505 - b  at the UE  115 - d.    
     In some aspects, the RRC message may trigger a reference signal on the second serving cell  505 - b  for SCell activation. In this regard, the RRC message may directly activate the second serving cell  505 - b , and may activate the reference signal on the second serving cell  505 - b . For example, the RRC message may include an indication of a resource (or set of resources) for a reference signal on the second serving cell  505 - b  which will be used to facilitate SCell activation. 
     As noted previously herein, the reference signal on the second serving cell  505 - b  may be used to perform time and frequency tracking and/or AGC during activation of the second serving cell  505 - b . The reference signal may include a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, an SSB, or any combination thereof. For example, the temporary reference signal triggered by the RRC message may include one or multiple NZP-CSI-RS resource sets, where each NZP-CSI-RS resource set includes one or multiple NZP-CSI-RS resources labeled as trs-info. 
     The RRC message may indicate one or more parameters associated with the reference signal on the second serving cell  505 - b . Parameters associated with the reference signal which may be indicated via the RRC message may include a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, TCI states for the reference signal, QCL configurations (e.g., QCL assumptions) for the reference signal, or any combination thereof. For example, the RRC message may indicate a structure/type of reference signal, which component carrier(s) are associated with the triggered reference signal, and/or which BWP(s) are associated with the triggered reference signal within the indicated component carriers. By way of another example, the RRC message may indicate one or more TCI states (e.g., one or more active TCI states) associated with the second serving cell  505 - b  which may be used to transmit the triggered reference signal. For instance, the RRC message may indicate TCI information and/or QCL information which indicates which reference signal the NZP-CSI-RS resources are QCLed with, and whether the QCL source reference signal may include an SSB or other NZP-CSI-RS resource. 
     At  520 , the UE  115 - d  may receive an additional control message (e.g., additional control signaling) via the first serving cell  505 - a . For example, the UE  115 - d  may receive a MAC-CE or other L2 message via the first serving cell  505 - a  following reception of the RRC message at  515 . 
     In some aspects, the control message (e.g., MAC-CE, L2 message) received at  520  may include an indication of the resource for the reference signal on the second serving cell  505 - b . Additionally, or alternatively, the control message may indicate one or more parameters associated with the reference signal on the second serving cell  505 - b  (e.g., structure, component carrier, BWP, TCI states, QCL configurations). In this regard, the resource(s) and/or other parameters for the reference signal on the second serving cell  505 - b  may be indicated via the RRC message (e.g., L3 message) at  515 , via the control message (e.g., MAC-CE, L2 message) at  520 , or both. 
     At  525 , the UE  115 - d  may transmit, via the first serving cell  505 - a , an RRC response message (e.g., RRC complete message, L3 response message). In some aspects, the UE  115 - d  may transmit the RRC response message in response to the RRC message at  515 . The RRC response message may indicate a completion of the RRC configuration/reconfiguration procedure which was triggered by the RRC message at  515 . As such, the transmission of the RRC response message may indicate an end of T RRC_Process  time interval, and a beginning of an activation time interval T ActivationTime  for activating the second serving cell  505 - b . Additionally, or alternatively, the UE  115 - d  may transmit the RRC response message at  525  based on receiving the control message (e.g., MAC-CE, L2 message) at  520 . 
     At  530 , the UE  115 - d  may receive an additional control message (e.g., additional control signaling) via the first serving cell  505 - a . For example, the UE  115 - d  may receive a DCI message or other L1 message via the first serving cell  505 - a . In some aspects, the UE  115 - d  may receive the control message (e.g., DCI message) at  530  following transmission of the RRC response message at  525 . In particular, the UE  115 - d  may receive the DCI message at  530  within the activation time interval (T ActivationTime ) following the transmission of the RRC response message. In this regard, the UE  115 - d  may receive the control message (e.g., DCI message, L1 message) at  530  based on receiving the RRC message at  515 , receiving the control message (e.g., MAC-CE, L2 message) at  520 , transmitting the RRC response message at  525 , or any combination thereof. 
     In some aspects, the control message (e.g., DCI message, L1 message) received at  530  may include an indication of the resource for the reference signal on the second serving cell  505 - b . Additionally, or alternatively, the control message (e.g., DCI message, L1 message) received at  530  may indicate one or more parameters associated with the reference signal on the second serving cell  505 - b  (e.g., structure, component carrier, BWP, TCI states, QCL configurations). In this regard, the resource(s) and/or other parameters for the reference signal on the second serving cell  505 - b  may be indicated via the RRC message (e.g., L3 message) received at  515 , via the control message (e.g., MAC-CE, L2 message) received at  520 , via the control message (e.g., DCI message, L1 message) received at  530 , or any combination thereof. 
     At  535 , the UE  115 - d  may identify the resource for the reference signal which is to be received via the second serving cell  505 - b . The UE  115 - d  may identify the resource for the reference signal within the activation time interval following the transmission of the RRC response message (e.g., RRC complete message, L3 response message) transmitted at  525 . In this regard, the UE  115 - d  may identify the resource for the reference signal associated with time and frequency tracking for the second serving cell  505 - b  at  535  based on receiving the RRC message at  515 , receiving the control message (e.g., MAC-CE, L2 message) at  520 , transmitting the RRC response message at  525 , receiving the control message at  530 , or any combination thereof. 
     Additionally, or alternatively, the UE  115 - d  may identify one or more parameters (e.g., structure, component carrier, BWP, TCI states, QCL configurations) associated with the reference signal. The UE  115 - d  may identify the resource(s) and/or other parameters for the reference signal based on the higher-layer configuration and trigger signaling (e.g., RRC message, MAC-CE, DCI message) received via the first serving cell  505 - a . In other words, the resource and other parameters for the reference signal for the second serving cell  505 - b  may be provided to the UE  115 - b  by the trigger signaling, preliminarily provided to the UE  115 - d  via earlier RRC configurations or signaling, via signaling which triggers the reference signal, or any combination thereof. 
     At  540 , the UE  115 - d  may monitor the resource for the reference signal on the second serving cell  505 - b . In this regard, the UE  115 - d  may monitor the resource at  540  based on identifying the resource and/or other parameters (e.g., structure, component carrier, BWP, TCI states, QCL configurations) associated with the reference signal at  535 . Moreover, the UE  115 - b  may monitor the resource for the reference signal at  540  based on receiving the RRC message at  515 , receiving the control message (e.g., MAC-CE, L2 message) at  520 , transmitting the RRC response message at  525 , receiving the control message at  530 , or any combination thereof. 
     In some cases, the UE  115 - d  may assume that a TCI state (e.g., QCL configuration or QCL assumption) of the reference signal to be transmitted by the second serving cell  505 - b  may be selected from a set of active TCI states (or active QCL configurations), if sets of active TCI states/QCL configurations have been configured. In other words, the second serving cell  505 - b  may not transmit the reference signal with a TCI state (or QCL configuration) which is not in a configured active set of TCI states/QCL configurations. 
     For example, as noted previously herein, the UE  115 - d  may receive an indication of a set of active TCI states and/or active QCL configurations via the RRC message received at  515 , via the control message (e.g., MAC-CE) received at  520 , via the control message (e.g., DCI message) received at  530 , or any combination thereof. In other words, the set of active TCI states may be activated by the RRC message which directly activates the SCell (e.g., second serving cell  505 - b ), via the MAC-CE of the PDSCH carrying the RRC message which directly activates the SCell, via a DCI message which directly activates the SCell, or any combination thereof. In this example, the UE  115 - d  may assume that the reference signal will be transmitted in accordance with one of the active TCI states/active QCL configurations, and may monitor the resource for the reference signal based on (e.g., in accordance with) one or more of the active TCI states and/or active QCL configurations. 
     At  545 , the UE  115 - d  may receive the reference signal (e.g., temporary reference signal, tracking reference signal, NZP-CSI-RS, SSB) via the second serving cell  505 - b . The UE  115 - d  may receive the reference signal within the resource for the reference signal identified at  535 , and based on monitoring the resource at  540 . Additionally, the UE  115 - d  may receive the reference signal in accordance with the one or more parameters (e.g., structure, component carrier, BWP, active TCI state, active QCL configuration) for the reference signal which were determined at  535 . In some aspects, the UE  115 - d  may receive the reference signal at  545  prior to an earliest SSB which the UE  115 - d  is capable of receiving via the second serving cell  505 - b.    
     At  550 , the UE  115 - d  may perform time and frequency tracking for the second serving cell  505 - b . The UE  115 - d  may perform time/frequency tracking during activation of the second serving cell  505 - b  based on receiving the reference signal via the second serving cell  505 - b  at  534 . 
     For example, the reference signal may be used by the UE  115 - d  to perform time and frequency tracking (e.g., AGC) during activation of the second serving cell  505 - b . In this regard, the UE  115 - d  may be configured to perform measurements and/or adjust time tracking and/or frequency tracking for the second serving cell  505 - b  based on receiving the reference signal at  545 . 
     At  555 , the UE  115 - d  may receive a CSI-RS via the second serving cell  505 - b . The UE  115 - d  may receive the CSI-RS at  555  based on performing the time and frequency tracking (e.g., AGC) for activation of the second serving cell  505 - b  at  550 . The UE  115 - b  may be configured to perform measurements on the received CSI-RS for CSI reporting. 
     At  560 , the UE  115 - d  may transmit a CSI report via the second serving cell  505 - b . In particular, the UE  115 - d  may transmit the CSI report at  555  based on performing measurements on the CSI-RS received at  555 . In this regard, the CSI report transmitted to the second serving cell  505 - b  at  560  may include an indication of the measurements performed on the CSI-RS received at  555 . 
     At  565 , the UE  115 - d  may communicate with the second serving cell  505 - b . The UE  115 - d  may communicate with the second serving cell  505 - b  based on performing the time and frequency tracking at  550  based on the reference signal. Additionally, or alternatively, the UE  115 - d  may communicate with the second serving cell  505 - b  based on receiving the CSI-RS at  555 , transmitting the CSI report at  560 , or both. 
     Techniques described herein may provide for improved wireless communications by improving direct SCell activation using (e.g., RRC signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling the UE  115 - d  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation relative to SCell activation schemes that do not use temporary reference signals. Additionally, techniques described herein may re-use (or re-purpose) fields within existing control signaling used for the SCell activation (e.g., reuse fields within uplink DCI messages which schedule RRC response messages for SCell activation), which may enable direct SCell activation without increasing control signaling used for SCell activation. 
       FIG.  6    illustrates an example of a resource configuration  600  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The resource configuration  600  may implement, or be implemented by, aspects of wireless communications system  100 , wireless communications system  200 , resource configuration  300 , resource configuration  400 , process flow  500 , or any combination thereof. 
     The resource configuration  600  includes an SCell activation scheme  605  which illustrates a direct SCell activation procedure which utilizes RRC (e.g., L3) signaling. As noted previously herein, techniques for direct SCell activation using RRC signaling and temporary reference signals may reduce how long it takes for a UE  115  to perform SCell activation relative to SCell activation schemes that do not use temporary reference signals. 
     For example, as shown in  FIG.  6   , a UE  115 - e  may be communicatively coupled to a PCell  610 - a  (e.g., first serving cell), where the PCell  610 - a  initiates activation of an SCell  610 - b  (e.g., second serving cell) at the UE  115 - e . As noted previously herein, the SCell activation delay (SCell ActivationDelay , or N Direct ) for activation of the SCell  610 - b  at the UE  115 - e  may be represented as T RRC_Process +T 1 +T activation_time +T CSI_Reporting −3 ms. 
     The SCell  610 - b  may transmit SSBs  650  (e.g., SSBs  650 - a ,  650 - b ,  650 - c ) at an SSB periodicity. According to some conventional techniques, SSBs  650  may be used by the UE  115 - e  for time/frequency tracking, AGC, or both, during activation of the SCell  610 - b . However, use of the SSBs  650  for time/frequency tracking may result in increased latency for SCell  610 - b  activation. 
     Accordingly, as described herein, the SCell activation scheme  605  illustrated in  FIG.  6    may utilize a reference signal  635  to facilitate direct SCell activation (e.g., fast SCell activation) at the UE  115 - e . The reference signal  635  may be used for time and frequency tracking and/or AGC during activation of the SCell  610 - b  to expedite SCell activation at the UE  115 - e . The reference signal  635  may include a temporary reference signal, a tracking reference signal, one or a set of NZP-CSI-RS resources, a temporary/aperiodic SSB, or any combination thereof. In particular, the UE  115 - e  may receive the reference signal  635  prior to a first SSB  650  (e.g., SSB  650 - b ) which may be received/processed by the UE  115 - e  via the SCell  610 - b . In this regard, the use of the reference signal  635  may reduce a time it takes for the UE  115 - e  to perform time/frequency tracking for the SCell  610 - b , which may expedite CSI reporting and communications between the UE  115 - e  and the SCell  610 - b.    
     In some aspects, the UE  115 - e  may receive an RRC message  615  (e.g., L3 message) via the PCell  610 - a , where the RRC message  615  includes an indication to activate the SCell  610 - b . In this regard, the RRC message  615  may initiate an RRC configuration and/or reconfiguration procedure for activating the SCell  610 - b  at the UE  115 - e  during a time interval T RRC_Process . 
     Additionally, the UE  115 - e  may receive a DCI message  620  in an uplink DCI format that schedules a physical uplink shared channel (PUSCH) message carrying an RRC reconfiguration complete message (and/or RRC reconfiguration complete message) responsive to the RRC message  615 . For example, as shown in  FIG.  6   , the UE  115 - e  may receive an uplink DCI message  620  which indicates a resource for transmitting an RRC response message  625  (e.g., RRC configuration/reconfiguration complete message) responsive to the RRC message  615 . In this regard, the DCI message  620  may indicate time/frequency resources which are usable by the UE  115 - e  for transmitting the RRC response message  625  in response to the RRC message  615 . 
     In some aspects, the DCI message  620  may additionally or alternatively indicate an activation of a reference signal associated with time and frequency tracking for the SCell  610 - b . In other words, the DCI message  620  may trigger or activate the reference signal  635  which will be used for time and frequency tracking for the SCell  610 - b . For example, the reference signal  635  associated with the SCell  610 - b  may be triggered/activated via one or more A-CSI request fields within the DCI message  620 . Accordingly, because the DCI message  620 , which is used to indicate time/frequency resources which are usable by the UE  115 - e  for transmitting the RRC response message  625 , is also used to indicate an activation of a reference signal (e.g., reference signal  635 ) associated with time and frequency tracking for SCell  610 - b , additional control signaling messages to indicate the activation of the reference signal can be avoided and control signaling overhead is not increased. In other words, by re-using (or re-purposing) existing signaling used for SCell activation (namely, the DCI message  620 ), techniques described herein may enable direct SCell activation using temporary reference signals without increasing control signaling overhead. 
     In some cases, the DCI message  620  may indicate or select the activated reference signal  635  and/or the activated reference signal resource from a set of candidate reference signals/resources which may be used for SCell activation at the UE  115 - e . In particular, configurations, resources, and timing offsets of candidate reference signals on the to-be-activated SCell  610 - b  may be configured by higher-layer signaling, where the DCI message  620  selects or otherwise indicates which configuration/resource/timing offset is to be used. For example, the RRC message  615  (or other control signaling) may indicate a set of candidate reference signal resources (or set of candidate reference signal configurations/offsets) which may be used for activating serving cells at the UE  115 - e . In this example, the DCI message  620  may indicate which reference signal resource (e.g., which reference signal configuration/offset) of the set of candidate reference signal resources on the SCell  610 - b  is to be used. For instance, the A-CSI request field of the DCI message  620  may indicate which temporary reference signal resource from the set of candidate reference signal resources is triggered/activated on SCell  610 - b.    
     In some aspects, the DCI message  620  may indicate a resource or relative timing for the reference signal  635 . In some cases, the DCI message  620  may expressly indicate time/frequency resources which are to be used for transmitting/receiving the reference signal  635 . In additional or alternative implementations, the exact timing of the reference signal  635  may be determined based on a number of factors or parameters including, but not limited to, a timing of the PUSCH message carrying the RRC reconfiguration complete message (e.g., timing of RRC response message  625 ), and a time offset  655  indicated via the DCI message  620  (e.g., indicated via the A-CSI request field of the DCI message  620 ). 
     For example, in some implementations, the DCI message  620  may indicate a time offset  655  associated with a resource allocated for the reference signal  635  on SCell  610 - b . In particular, the DCI message  620  may indicate a time offset  655  which defines a quantity of slots/TTIs between transmission of the RRC response message  625  and the resource for the reference signal  635 . In this example, the reference signal  635  may be triggered on/within slot n+k+x+m, where slot n is the slot (or TTI) in which the UE  115 - e  receives the DCI message  620  scheduling the RRC response message  625 , k defines a number of slots/TTIs between slot n and the slot for transmitting/receiving the RRC response message  625 , x is a pre-defined number of slots/TTIs (e.g., x=3 ms), and m (e.g., time offset  655 ) defines a number of slots/TTIs between slot n+k+x and the slot/TTI in which the reference signal  635  is triggered. In some cases, the value of x may be pre-configured, signaled by the network (e.g., via RRC message  625 ), or both. Thus, the timing of the reference signal  635  relative to the timing of the RRC response message  625  may be based on the value of x and the value of m (e.g., time offset  655 ), where the value of x may be pre-configured or signaled via the network, and the value of m (e.g., time offset  655 ) is indicated via the DCI message  620 . 
     In some aspects, the slots/TTIs shown and described in  FIG.  6    may be based on the SCS of the respective cells (e.g., SCS of PCell  610 - a , SCS of SCell  610 - b ). The slots/TTIs may (or may not) be aligned across the PCell  610 - a  and the SCell  610 - b  depending on the SCS for each of the respective serving cells. In some aspects, the DCI message  620  may indicate the time offset  655  (e.g., indicate the value of m) based on the SCS of the PCell  610 - a , the SCS of the SCell  610 - b , or both. 
     In some aspects, a resource(s) and/or other parameters (e.g., structure, component carrier, BWP, TCI state, QCL configuration) for the reference signal  635  on the SCell  610 - b  may be indicated to the UE  115 - e  via higher-layer configuration and trigger signaling received via the PCell  610 - a . For example, the resource for the reference signal  635  may be indicated via the RRC message  615  (e.g., L3 message), the DCI message  620  (e.g., L1 message), or both. 
     The UE  115 - e  may transmit the RRC response message  625  (e.g., RRC complete message) in response to the RRC message  615 . The RRC response message  625  may indicate an acknowledgment of the RRC message  615  and/or the configuration/activation of the SCell  610 - b . In particular, the UE  115 - e  may transmit the RRC response message  625  via the resource which was indicated via the DCI message  620 . In other words, the UE  115 - e  may transmit the RRC response message  625  in accordance with the scheduling of the RRC response message  625  signaled via the DCI message  620 . 
     In some aspects, the RRC response message  625  may be transmitted following a time interval T 1  (e.g., following k slots/TTIs), which defines a delay from slot 
     
       
         
           
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     A duration of time interval T 1  may be based on a complexity and capability of the UE  115 - e , and may therefore be UE-implementation dependent. In cases where the UE  115 - e  performs a handover (e.g., handover between PCells  610  during the SCell  610 - b  activation), the time interval T 1  may be replaced by time interval T interrupt +T 2 +T 3 . In such cases, T interrupt +T 2 +T 3  may define delays or interruptions of the SCell  610 - b  activation which are attributable to the handover. 
     In some cases, depending on the periodicity of SSBs  650  on the SCell  610 - b  and a relative timing of the SSBs  650 , the SCell  610 - b  may transmit/broadcast an SSB  650  (e.g., SSB  650 - b ) prior to transmission of the RRC response message  625  (e.g., before T RRC_Process , within T RRC_Process , within T 1 ). However, the UE  115 - e  may be unable to receive/process the SSB  650 - a  received prior to an activation time interval T ActivationTime  which follows the RRC response message  625 . As such, for wireless communications systems which do not utilize the reference signal  635  described herein, the UE  115 - e  may have to wait until the next SSB  650 - b  to perform time/frequency tacking and AGC for activation of the SCell  610 - b.    
     Accordingly, techniques described herein which configure the reference signal  635  may reduce a latency of time/frequency tracking and/or AGC at the UE  115 - e , which may reduce a latency of SCell  610 - b  activation. The activation time interval T ActivationTime  may be occur following transmission of the RRC response message  625 . In some aspects, the reference signal  635  may be received within the activation time interval T ActivationTime  in the time domain. The term T TempRS  may define a duration from the transmission of the RRC response message  625  and the reception of the reference signal  635 . As such, T FirstSSB &lt;T ActivationTime . In some aspects, the reference signal  635  may be transmitted/repeated multiple times (e.g., multiple reference signal bursts). For example, the UE  115 - e  may receive a first reference signal  635  (e.g., first reference signal burst) for AGC, and a second reference signal  635  (e.g., second reference signal burst) for time/frequency tracking. 
     Upon receiving the reference signal  635  on the SCell  610 - b , the UE  115 - e  may perform time and frequency tracking and/or AGC for activation of the SCell  610 - b  based on the reference signal  635 . For example, the reference signal  635  may be used for time and frequency tracking and/or AGC during activation of the SCell  610 - b . In this regard, the UE  115 - e  may be configured to perform measurements and/or adjust time tracking and/or frequency tracking for the second serving cell (e.g., SCell  610 - b ) based on receiving the reference signal  635  via the SCell  610 - b.    
     Upon performing time/frequency tracking and/or AGC for the SCell  610 - b  based on the reference signal, and after an end of the activation time interval T ActivationTime , the UE  115 - e  may be able to perform CSI reporting procedures with the SCell  610 - b  during a CSI reporting time interval T CSI_Reporting  In other words, T CSI_Reporting may define a delay until the first available CSI report including uncertainties of CSI-RS resources and CSI reports. 
     For example, as shown in  FIG.  6   , the UE  115 - e  may receive a CSI-RS  640  via the SCell  610 - b  following an end of T CSI_Reporting  The UE  115 - e  may receive the CSI-RS  640  based on performing the time and frequency tracking (e.g., AGC) for activation of the SCell  610 - b  based on the reference signal  635 . The UE  115 - e  may be configured to perform measurements on the received CSI-RS  640  for CSI reporting. Subsequently, the UE  115 - e  may transmit a CSI report  645  via the SCell  610 - b . In particular, the UE  115 - e  may transmit the CSI report  645  based on performing measurements on the CSI-RS  640 . In this regard, the CSI report  240  transmitted to the SCell  610 - b  may include an indication of the measurements performed on the CSI-RS  640 . 
     As shown in  FIG.  6   , in some cases, the CSI reporting (e.g., reception of CSI-RS  640 , transmission of CSI report  645 ) may be performed prior to the first SSB  650 - b  which is capable of being received/processed by the first UE  115 - e . In this regard, techniques described herein may enable the UE  115 - e  to perform CSI reporting earlier as compared to some conventional SCell activation techniques, which may expedite activation of the SCell  610 - b  and improve user experience at the UE  115 - e.    
     Techniques described herein may provide for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation relative to SCell activation schemes that do not use temporary reference signals. Additionally, techniques described herein may re-use (or re-purpose) fields within existing control signaling used for the SCell activation (e.g., reuse fields within uplink DCI messages which schedule RRC response messages for SCell activation), which may enable direct SCell activation without increasing control signaling used for SCell activation. 
       FIG.  7    shows a block diagram  700  of a device  705  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  705  may be an example of aspects of a UE  115  as described herein. The device  705  may include a receiver  710 , a transmitter  715 , and a communications manager  720 . The device  705  may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses). 
     The receiver  710  may provide a means for receiving information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). Information may be passed on to other components of the device  705 . The receiver  710  may utilize a single antenna or a set of multiple antennas. 
     The transmitter  715  may provide a means for transmitting signals generated by other components of the device  705 . For example, the transmitter  715  may transmit information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). In some examples, the transmitter  715  may be co-located with a receiver  710  in a transceiver module. The transmitter  715  may utilize a single antenna or a set of multiple antennas. 
     The communications manager  720 , the receiver  710 , the transmitter  715 , or various combinations thereof or various components thereof may be examples of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  720 , the receiver  710 , the transmitter  715 , or various combinations or components thereof may support a method for performing one or more of the functions described herein. 
     In some examples, the communications manager  720 , the receiver  710 , the transmitter  715 , or various combinations or components thereof may be implemented in hardware (e.g., in communications management circuitry). The hardware may include a processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic device, a discrete gate or transistor logic, discrete hardware components, or any combination thereof configured as or otherwise supporting a means for performing the functions described in the present disclosure. In some examples, a processor and memory coupled with the processor may be configured to perform one or more of the functions described herein (e.g., by executing, by the processor, instructions stored in the memory). 
     Additionally, or alternatively, in some examples, the communications manager  720 , the receiver  710 , the transmitter  715 , or various combinations or components thereof may be implemented in code (e.g., as communications management software or firmware) executed by a processor. If implemented in code executed by a processor, the functions of the communications manager  720 , the receiver  710 , the transmitter  715 , or various combinations or components thereof may be performed by a general-purpose processor, a DSP, a central processing unit (CPU), an ASIC, an FPGA, or any combination of these or other programmable logic devices (e.g., configured as or otherwise supporting a means for performing the functions described in the present disclosure). 
     In some examples, the communications manager  720  may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the receiver  710 , the transmitter  715 , or both. For example, the communications manager  720  may receive information from the receiver  710 , send information to the transmitter  715 , or be integrated in combination with the receiver  710 , the transmitter  715 , or both to receive information, transmit information, or perform various other operations as described herein. 
     The communications manager  720  may support wireless communication at a UE in accordance with examples as disclosed herein. For example, the communications manager  720  may be configured as or otherwise support a means for receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The communications manager  720  may be configured as or otherwise support a means for receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The communications manager  720  may be configured as or otherwise support a means for transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The communications manager  720  may be configured as or otherwise support a means for monitoring a second resource of the second serving cell for the reference signal based on the control message. 
     By including or configuring the communications manager  720  in accordance with examples as described herein, the device  705  (e.g., a processor controlling or otherwise coupled to the receiver  710 , the transmitter  715 , the communications manager  720 , or a combination thereof) may support techniques for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation while reducing control signaling used for the SCell activation. 
       FIG.  8    shows a block diagram  800  of a device  805  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  805  may be an example of aspects of a device  705  or a UE  115  as described herein. The device  805  may include a receiver  810 , a transmitter  815 , and a communications manager  820 . The device  805  may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses). 
     The receiver  810  may provide a means for receiving information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). Information may be passed on to other components of the device  805 . The receiver  810  may utilize a single antenna or a set of multiple antennas. 
     The transmitter  815  may provide a means for transmitting signals generated by other components of the device  805 . For example, the transmitter  815  may transmit information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). In some examples, the transmitter  815  may be co-located with a receiver  810  in a transceiver module. The transmitter  815  may utilize a single antenna or a set of multiple antennas. 
     The device  805 , or various components thereof, may be an example of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  820  may include a downlink message receiving manager  825 , a control message receiving manager  830 , a response message transmitting manager  835 , a monitoring manager  840 , or any combination thereof. The communications manager  820  may be an example of aspects of a communications manager  720  as described herein. In some examples, the communications manager  820 , or various components thereof, may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the receiver  810 , the transmitter  815 , or both. For example, the communications manager  820  may receive information from the receiver  810 , send information to the transmitter  815 , or be integrated in combination with the receiver  810 , the transmitter  815 , or both to receive information, transmit information, or perform various other operations as described herein. 
     The communications manager  820  may support wireless communication at a UE in accordance with examples as disclosed herein. The downlink message receiving manager  825  may be configured as or otherwise support a means for receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The control message receiving manager  830  may be configured as or otherwise support a means for receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The response message transmitting manager  835  may be configured as or otherwise support a means for transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The monitoring manager  840  may be configured as or otherwise support a means for monitoring a second resource of the second serving cell for the reference signal based on the control message. 
       FIG.  9    shows a block diagram  900  of a communications manager  920  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The communications manager  920  may be an example of aspects of a communications manager  720 , a communications manager  820 , or both, as described herein. The communications manager  920 , or various components thereof, may be an example of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  920  may include a downlink message receiving manager  925 , a control message receiving manager  930 , a response message transmitting manager  935 , a monitoring manager  940 , a reference signal receiving manager  945 , a network entity communicating manager  950 , or any combination thereof. Each of these components may communicate, directly or indirectly, with one another (e.g., via one or more buses). 
     The communications manager  920  may support wireless communication at a UE in accordance with examples as disclosed herein. The downlink message receiving manager  925  may be configured as or otherwise support a means for receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The control message receiving manager  930  may be configured as or otherwise support a means for receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The response message transmitting manager  935  may be configured as or otherwise support a means for transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The monitoring manager  940  may be configured as or otherwise support a means for monitoring a second resource of the second serving cell for the reference signal based on the control message. 
     In some examples, the downlink message receiving manager  925  may be configured as or otherwise support a means for receiving, via the message, an indication of a set of multiple candidate reference signal resources. In some examples, the control message receiving manager  930  may be configured as or otherwise support a means for receiving, via the control message and based on the message, an indication of the second resource from the set of multiple candidate reference signal resources, where monitoring the second resource of the second serving cell for the reference signal is based on the indication of the second resource. 
     In some examples, the control message receiving manager  930  may be configured as or otherwise support a means for receiving, via the control message, an indication of a time offset associated with the second resource for the reference signal, where monitoring the second resource is based on the time offset. In some examples, the time offset indicates a period of time between the first resource and the second resource. 
     In some examples, the control message receiving manager  930  may be configured as or otherwise support a means for receiving, via the control message, an indication of the second resource for the reference signal, where monitoring the second resource is based on receiving the indication of the second resource. In some examples, the control message includes a DCI message. In some examples, the control message includes an A-CSI request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     In some examples, the downlink message receiving manager  925  may be configured as or otherwise support a means for receiving, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, where monitoring the second resource is based on the one or more parameters. In some examples, the one or more parameters include a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, or any combination thereof. 
     In some examples, the reference signal receiving manager  945  may be configured as or otherwise support a means for receiving the reference signal via the second serving cell based on the monitoring. In some examples, the network entity communicating manager  950  may be configured as or otherwise support a means for communicating with the second serving cell based on time and frequency tracking information determined using the reference signal. In some examples, the reference signal receiving manager  945  may be configured as or otherwise support a means for receiving, based on the reference signal, AGC information associated with the second serving cell, where communicating with the second serving cell is based on the AGC information. 
     In some examples, the reference signal is received prior to an earliest SSB which the UE is capable of receiving via the second serving cell. In some examples, the reference signal includes a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, or any combination thereof. 
       FIG.  10    shows a diagram of a system  1000  including a device  1005  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  1005  may be an example of or include the components of a device  705 , a device  805 , or a UE  115  as described herein. The device  1005  may communicate wirelessly with one or more network entities  105 , UEs  115 , or any combination thereof. The device  1005  may include components for bi-directional voice and data communications including components for transmitting and receiving communications, such as a communications manager  1020 , an input/output (I/O) controller  1010 , a transceiver  1015 , an antenna  1025 , a memory  1030 , code  1035 , and a processor  1040 . These components may be in electronic communication or otherwise coupled (e.g., operatively, communicatively, functionally, electronically, electrically) via one or more buses (e.g., a bus  1045 ). 
     The I/O controller  1010  may manage input and output signals for the device  1005 . The I/O controller  1010  may also manage peripherals not integrated into the device  1005 . In some cases, the I/O controller  1010  may represent a physical connection or port to an external peripheral. In some cases, the I/O controller  1010  may utilize an operating system such as iOS®, ANDROID®, MS-DOS®, MS-WINDOWS®, OS/2®, UNIX®, LINUX®, or another known operating system. Additionally, or alternatively, the I/O controller  1010  may represent or interact with a modem, a keyboard, a mouse, a touchscreen, or a similar device. In some cases, the I/O controller  1010  may be implemented as part of a processor, such as the processor  1040 . In some cases, a user may interact with the device  1005  via the I/O controller  1010  or via hardware components controlled by the I/O controller  1010 . 
     In some cases, the device  1005  may include a single antenna  1025 . However, in some other cases, the device  1005  may have more than one antenna  1025 , which may be capable of concurrently transmitting or receiving multiple wireless transmissions. The transceiver  1015  may communicate bi-directionally, via the one or more antennas  1025 , wired, or wireless links as described herein. For example, the transceiver  1015  may represent a wireless transceiver and may communicate bi-directionally with another wireless transceiver. The transceiver  1015  may also include a modem to modulate the packets, to provide the modulated packets to one or more antennas  1025  for transmission, and to demodulate packets received from the one or more antennas  1025 . The transceiver  1015 , or the transceiver  1015  and one or more antennas  1025 , may be an example of a transmitter  715 , a transmitter  815 , a receiver  710 , a receiver  810 , or any combination thereof or component thereof, as described herein. 
     The memory  1030  may include random access memory (RAM) and read-only memory (ROM). The memory  1030  may store computer-readable, computer-executable code  1035  including instructions that, when executed by the processor  1040 , cause the device  1005  to perform various functions described herein. The code  1035  may be stored in a non-transitory computer-readable medium such as system memory or another type of memory. In some cases, the code  1035  may not be directly executable by the processor  1040  but may cause a computer (e.g., when compiled and executed) to perform functions described herein. In some cases, the memory  1030  may contain, among other things, a basic I/O system (BIOS) which may control basic hardware or software operation such as the interaction with peripheral components or devices. 
     The processor  1040  may include an intelligent hardware device (e.g., a general-purpose processor, a DSP, a CPU, a microcontroller, an ASIC, an FPGA, a programmable logic device, a discrete gate or transistor logic component, a discrete hardware component, or any combination thereof). In some cases, the processor  1040  may be configured to operate a memory array using a memory controller. In some other cases, a memory controller may be integrated into the processor  1040 . The processor  1040  may be configured to execute computer-readable instructions stored in a memory (e.g., the memory  1030 ) to cause the device  1005  to perform various functions (e.g., functions or tasks supporting techniques for direct SCell activation using temporary reference signals). For example, the device  1005  or a component of the device  1005  may include a processor  1040  and memory  1030  coupled to the processor  1040 , the processor  1040  and memory  1030  configured to perform various functions described herein. 
     The communications manager  1020  may support wireless communication at a UE in accordance with examples as disclosed herein. For example, the communications manager  1020  may be configured as or otherwise support a means for receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The communications manager  1020  may be configured as or otherwise support a means for receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The communications manager  1020  may be configured as or otherwise support a means for transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The communications manager  1020  may be configured as or otherwise support a means for monitoring a second resource of the second serving cell for the reference signal based on the control message. 
     By including or configuring the communications manager  1020  in accordance with examples as described herein, the device  1005  may support techniques for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation while reducing control signaling used for the SCell activation. 
     In some examples, the communications manager  1020  may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the transceiver  1015 , the one or more antennas  1025 , or any combination thereof. Although the communications manager  1020  is illustrated as a separate component, in some examples, one or more functions described with reference to the communications manager  1020  may be supported by or performed by the processor  1040 , the memory  1030 , the code  1035 , or any combination thereof. For example, the code  1035  may include instructions executable by the processor  1040  to cause the device  1005  to perform various aspects of techniques for direct SCell activation using temporary reference signals as described herein, or the processor  1040  and the memory  1030  may be otherwise configured to perform or support such operations. 
       FIG.  11    shows a block diagram  1100  of a device  1105  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  1105  may be an example of aspects of a network entity  105  as described herein. The device  1105  may include a receiver  1110 , a transmitter  1115 , and a communications manager  1120 . The device  1105  may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses). 
     The receiver  1110  may provide a means for receiving information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). Information may be passed on to other components of the device  1105 . The receiver  1110  may utilize a single antenna or a set of multiple antennas. 
     The transmitter  1115  may provide a means for transmitting signals generated by other components of the device  1105 . For example, the transmitter  1115  may transmit information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). In some examples, the transmitter  1115  may be co-located with a receiver  1110  in a transceiver module. The transmitter  1115  may utilize a single antenna or a set of multiple antennas. 
     The communications manager  1120 , the receiver  1110 , the transmitter  1115 , or various combinations thereof or various components thereof may be examples of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  1120 , the receiver  1110 , the transmitter  1115 , or various combinations or components thereof may support a method for performing one or more of the functions described herein. 
     In some examples, the communications manager  1120 , the receiver  1110 , the transmitter  1115 , or various combinations or components thereof may be implemented in hardware (e.g., in communications management circuitry). The hardware may include a processor, a DSP, an ASIC, an FPGA or other programmable logic device, a discrete gate or transistor logic, discrete hardware components, or any combination thereof configured as or otherwise supporting a means for performing the functions described in the present disclosure. In some examples, a processor and memory coupled with the processor may be configured to perform one or more of the functions described herein (e.g., by executing, by the processor, instructions stored in the memory). 
     Additionally, or alternatively, in some examples, the communications manager  1120 , the receiver  1110 , the transmitter  1115 , or various combinations or components thereof may be implemented in code (e.g., as communications management software or firmware) executed by a processor. If implemented in code executed by a processor, the functions of the communications manager  1120 , the receiver  1110 , the transmitter  1115 , or various combinations or components thereof may be performed by a general-purpose processor, a DSP, a CPU, an ASIC, an FPGA, or any combination of these or other programmable logic devices (e.g., configured as or otherwise supporting a means for performing the functions described in the present disclosure). 
     In some examples, the communications manager  1120  may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the receiver  1110 , the transmitter  1115 , or both. For example, the communications manager  1120  may receive information from the receiver  1110 , send information to the transmitter  1115 , or be integrated in combination with the receiver  1110 , the transmitter  1115 , or both to receive information, transmit information, or perform various other operations as described herein. 
     The communications manager  1120  may support wireless communication at a network entity in accordance with examples as disclosed herein. For example, the communications manager  1120  may be configured as or otherwise support a means for transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The communications manager  1120  may be configured as or otherwise support a means for transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The communications manager  1120  may be configured as or otherwise support a means for receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The communications manager  1120  may be configured as or otherwise support a means for transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     By including or configuring the communications manager  1120  in accordance with examples as described herein, the device  1105  (e.g., a processor controlling or otherwise coupled to the receiver  1110 , the transmitter  1115 , the communications manager  1120 , or a combination thereof) may support techniques for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation while reducing control signaling used for the SCell activation. 
       FIG.  12    shows a block diagram  1200  of a device  1205  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  1205  may be an example of aspects of a device  1105  or a network entity  105  as described herein. The device  1205  may include a receiver  1210 , a transmitter  1215 , and a communications manager  1220 . The device  1205  may also include a processor. Each of these components may be in communication with one another (e.g., via one or more buses). 
     The receiver  1210  may provide a means for receiving information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). Information may be passed on to other components of the device  1205 . The receiver  1210  may utilize a single antenna or a set of multiple antennas. 
     The transmitter  1215  may provide a means for transmitting signals generated by other components of the device  1205 . For example, the transmitter  1215  may transmit information such as packets, user data, control information, or any combination thereof associated with various information channels (e.g., control channels, data channels, information channels related to techniques for direct SCell activation using temporary reference signals). In some examples, the transmitter  1215  may be co-located with a receiver  1210  in a transceiver module. The transmitter  1215  may utilize a single antenna or a set of multiple antennas. 
     The device  1205 , or various components thereof, may be an example of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  1220  may include a downlink message transmitting manager  1225 , a control message transmitting manager  1230 , a response message receiving manager  1235 , a reference signal transmitting manager  1240 , or any combination thereof. The communications manager  1220  may be an example of aspects of a communications manager  1120  as described herein. In some examples, the communications manager  1220 , or various components thereof, may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the receiver  1210 , the transmitter  1215 , or both. For example, the communications manager  1220  may receive information from the receiver  1210 , send information to the transmitter  1215 , or be integrated in combination with the receiver  1210 , the transmitter  1215 , or both to receive information, transmit information, or perform various other operations as described herein. 
     The communications manager  1220  may support wireless communication at a network entity in accordance with examples as disclosed herein. The downlink message transmitting manager  1225  may be configured as or otherwise support a means for transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The control message transmitting manager  1230  may be configured as or otherwise support a means for transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The response message receiving manager  1235  may be configured as or otherwise support a means for receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The reference signal transmitting manager  1240  may be configured as or otherwise support a means for transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
       FIG.  13    shows a block diagram  1300  of a communications manager  1320  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The communications manager  1320  may be an example of aspects of a communications manager  1120 , a communications manager  1220 , or both, as described herein. The communications manager  1320 , or various components thereof, may be an example of means for performing various aspects of techniques for direct SCell activation using temporary reference signals as described herein. For example, the communications manager  1320  may include a downlink message transmitting manager  1325 , a control message transmitting manager  1330 , a response message receiving manager  1335 , a reference signal transmitting manager  1340 , a UE communicating manager  1345 , or any combination thereof. Each of these components may communicate, directly or indirectly, with one another (e.g., via one or more buses). 
     The communications manager  1320  may support wireless communication at a network entity in accordance with examples as disclosed herein. The downlink message transmitting manager  1325  may be configured as or otherwise support a means for transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The control message transmitting manager  1330  may be configured as or otherwise support a means for transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The response message receiving manager  1335  may be configured as or otherwise support a means for receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The reference signal transmitting manager  1340  may be configured as or otherwise support a means for transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     In some examples, the downlink message transmitting manager  1325  may be configured as or otherwise support a means for transmitting, via the message, an indication of a set of multiple candidate reference signal resources. In some examples, the control message transmitting manager  1330  may be configured as or otherwise support a means for transmitting, via the control message and based on the message, an indication of the second resource from the set of multiple candidate reference signal resources, where transmitting the reference signal within the second resource of the second serving cell is based on the indication of the second resource. 
     In some examples, the control message transmitting manager  1330  may be configured as or otherwise support a means for transmitting, via the control message, an indication of a time offset associated with the second resource for the reference signal, where transmitting the reference signal within the second resource is based on the time offset. In some examples, the time offset indicates a period of time between the first resource and the second resource. 
     In some examples, the control message transmitting manager  1330  may be configured as or otherwise support a means for transmitting, via the control message, an indication of the second resource for the reference signal, where transmitting the reference signal within the second resource is based on transmitting the indication of the second resource. In some examples, the control message includes a DCI message. In some examples, the control message includes an A-CSI request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     In some examples, the downlink message transmitting manager  1325  may be configured as or otherwise support a means for transmitting, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, where transmitting the reference signal within the second resource is based on the one or more parameters. In some examples, the one or more parameters include a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, or any combination thereof. 
     In some examples, the UE communicating manager  1345  may be configured as or otherwise support a means for communicating with the UE via the second serving cell based on time and frequency tracking information that is determined based on the reference signal. 
     In some examples, the reference signal transmitting manager  1340  may be configured as or otherwise support a means for transmitting, based on the reference signal, AGC information associated with the second serving cell, where communicating with the UE via the second serving cell is based on the AGC information. 
     In some examples, the reference signal is transmitted prior to an earliest SSB which the UE is capable of receiving via the second serving cell. In some examples, the reference signal includes a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, or any combination thereof. 
       FIG.  14    shows a diagram of a system  1400  including a device  1405  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The device  1405  may be an example of or include the components of a device  1105 , a device  1205 , or a network entity  105  as described herein. The device  1405  may communicate wirelessly with one or more network entities  105 , UEs  115 , or any combination thereof. The device  1405  may include components for bi-directional voice and data communications including components for transmitting and receiving communications, such as a communications manager  1420 , a network communications manager  1410 , a transceiver  1415 , an antenna  1425 , a memory  1430 , code  1435 , a processor  1440 , and an inter-station communications manager  1445 . These components may be in electronic communication or otherwise coupled (e.g., operatively, communicatively, functionally, electronically, electrically) via one or more buses (e.g., a bus  1450 ). 
     The network communications manager  1410  may manage communications with a core network  130  (e.g., via one or more wired backhaul links). For example, the network communications manager  1410  may manage the transfer of data communications for client devices, such as one or more UEs  115 . 
     In some cases, the device  1405  may include a single antenna  1425 . However, in some other cases the device  1405  may have more than one antenna  1425 , which may be capable of concurrently transmitting or receiving multiple wireless transmissions. The transceiver  1415  may communicate bi-directionally, via the one or more antennas  1425 , wired, or wireless links as described herein. For example, the transceiver  1415  may represent a wireless transceiver and may communicate bi-directionally with another wireless transceiver. The transceiver  1415  may also include a modem to modulate the packets, to provide the modulated packets to one or more antennas  1425  for transmission, and to demodulate packets received from the one or more antennas  1425 . The transceiver  1415 , or the transceiver  1415  and one or more antennas  1425 , may be an example of a transmitter  1115 , a transmitter  1215 , a receiver  1110 , a receiver  1210 , or any combination thereof or component thereof, as described herein. 
     The memory  1430  may include RAM and ROM. The memory  1430  may store computer-readable, computer-executable code  1435  including instructions that, when executed by the processor  1440 , cause the device  1405  to perform various functions described herein. The code  1435  may be stored in a non-transitory computer-readable medium such as system memory or another type of memory. In some cases, the code  1435  may not be directly executable by the processor  1440  but may cause a computer (e.g., when compiled and executed) to perform functions described herein. In some cases, the memory  1430  may contain, among other things, a BIOS which may control basic hardware or software operation such as the interaction with peripheral components or devices. 
     The processor  1440  may include an intelligent hardware device (e.g., a general-purpose processor, a DSP, a CPU, a microcontroller, an ASIC, an FPGA, a programmable logic device, a discrete gate or transistor logic component, a discrete hardware component, or any combination thereof). In some cases, the processor  1440  may be configured to operate a memory array using a memory controller. In some other cases, a memory controller may be integrated into the processor  1440 . The processor  1440  may be configured to execute computer-readable instructions stored in a memory (e.g., the memory  1430 ) to cause the device  1405  to perform various functions (e.g., functions or tasks supporting techniques for direct SCell activation using temporary reference signals). For example, the device  1405  or a component of the device  1405  may include a processor  1440  and memory  1430  coupled to the processor  1440 , the processor  1440  and memory  1430  configured to perform various functions described herein. 
     The inter-station communications manager  1445  may manage communications with other network entities  105 , and may include a controller or scheduler for controlling communications with UEs  115  in cooperation with other network entities  105 . For example, the inter-station communications manager  1445  may coordinate scheduling for transmissions to UEs  115  for various interference mitigation techniques such as beamforming or joint transmission. In some examples, the inter-station communications manager  1445  may provide an X2 interface within an LTE/LTE-A wireless communications network technology to provide communication between network entities  105 . 
     The communications manager  1420  may support wireless communication at a network entity in accordance with examples as disclosed herein. For example, the communications manager  1420  may be configured as or otherwise support a means for transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The communications manager  1420  may be configured as or otherwise support a means for transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The communications manager  1420  may be configured as or otherwise support a means for receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The communications manager  1420  may be configured as or otherwise support a means for transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. 
     By including or configuring the communications manager  1420  in accordance with examples as described herein, the device  1405  may support techniques for improved wireless communications by improving direct SCell activation using RRC signaling (e.g., L3 signaling). In particular, techniques described herein may provide signaling and other configurations which enable the network to indicate sets of resources and other parameters for temporary reference signals used for SCell activation via RRC signaling. By enabling UEs  115  to identify resources for temporary reference signals using RRC signaling, techniques described herein may reduce a time required for SCell activation while reducing control signaling used for the SCell activation. 
     In some examples, the communications manager  1420  may be configured to perform various operations (e.g., receiving, monitoring, transmitting) using or otherwise in cooperation with the transceiver  1415 , the one or more antennas  1425 , or any combination thereof. Although the communications manager  1420  is illustrated as a separate component, in some examples, one or more functions described with reference to the communications manager  1420  may be supported by or performed by the processor  1440 , the memory  1430 , the code  1435 , or any combination thereof. For example, the code  1435  may include instructions executable by the processor  1440  to cause the device  1405  to perform various aspects of techniques for direct SCell activation using temporary reference signals as described herein, or the processor  1440  and the memory  1430  may be otherwise configured to perform or support such operations. 
       FIG.  15    shows a flowchart illustrating a method  1500  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The operations of the method  1500  may be implemented by a UE or its components as described herein. For example, the operations of the method  1500  may be performed by a UE  115  as described with reference to  FIGS.  1  through  10   . In some examples, a UE may execute a set of instructions to control the functional elements of the UE to perform the described functions. Additionally, or alternatively, the UE may perform aspects of the described functions using special-purpose hardware. 
     At  1505 , the method may include receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The operations of  1505  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1505  may be performed by a downlink message receiving manager  925  as described with reference to  FIG.  9   . 
     At  1510 , the method may include receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The operations of  1510  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1510  may be performed by a control message receiving manager  930  as described with reference to  FIG.  9   . 
     At  1515 , the method may include transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The operations of  1515  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1515  may be performed by a response message transmitting manager  935  as described with reference to  FIG.  9   . 
     At  1520 , the method may include monitoring a second resource of the second serving cell for the reference signal based on the control message. The operations of  1520  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1520  may be performed by a monitoring manager  940  as described with reference to  FIG.  9   . 
       FIG.  16    shows a flowchart illustrating a method  1600  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The operations of the method  1600  may be implemented by a UE or its components as described herein. For example, the operations of the method  1600  may be performed by a UE  115  as described with reference to  FIGS.  1  through  10   . In some examples, a UE may execute a set of instructions to control the functional elements of the UE to perform the described functions. Additionally, or alternatively, the UE may perform aspects of the described functions using special-purpose hardware. 
     At  1605 , the method may include receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The operations of  1605  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1605  may be performed by a downlink message receiving manager  925  as described with reference to  FIG.  9   . 
     At  1610 , the method may include receiving, via the message, an indication of a set of multiple candidate reference signal resources. The operations of  1610  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1610  may be performed by a downlink message receiving manager  925  as described with reference to  FIG.  9   . 
     At  1615 , the method may include receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The operations of  1615  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1615  may be performed by a control message receiving manager  930  as described with reference to  FIG.  9   . 
     At  1620 , the method may include receiving, via the control message and based on the message, an indication of the second resource from the set of multiple candidate reference signal resources. The operations of  1620  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1620  may be performed by a control message receiving manager  930  as described with reference to  FIG.  9   . 
     At  1625 , the method may include transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The operations of  1625  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1625  may be performed by a response message transmitting manager  935  as described with reference to  FIG.  9   . 
     At  1630 , the method may include monitoring a second resource of the second serving cell for the reference signal based on the control message, where monitoring the second resource of the second serving cell for the reference signal is based on the indication of the second resource. The operations of  1630  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1630  may be performed by a monitoring manager  940  as described with reference to  FIG.  9   . 
       FIG.  17    shows a flowchart illustrating a method  1700  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The operations of the method  1700  may be implemented by a UE or its components as described herein. For example, the operations of the method  1700  may be performed by a UE  115  as described with reference to  FIGS.  1  through  10   . In some examples, a UE may execute a set of instructions to control the functional elements of the UE to perform the described functions. Additionally, or alternatively, the UE may perform aspects of the described functions using special-purpose hardware. 
     At  1705 , the method may include receiving, via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The operations of  1705  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1705  may be performed by a downlink message receiving manager  925  as described with reference to  FIG.  9   . 
     At  1710 , the method may include receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The operations of  1710  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1710  may be performed by a control message receiving manager  930  as described with reference to  FIG.  9   . 
     At  1715 , the method may include receiving, via the control message, an indication of a time offset associated with the second resource for the reference signal. The operations of  1715  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1715  may be performed by a control message receiving manager  930  as described with reference to  FIG.  9   . 
     At  1720 , the method may include transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The operations of  1720  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1720  may be performed by a response message transmitting manager  935  as described with reference to  FIG.  9   . 
     At  1725 , the method may include monitoring a second resource of the second serving cell for the reference signal based on the control message, where monitoring the second resource is based on the time offset. The operations of  1725  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1725  may be performed by a monitoring manager  940  as described with reference to  FIG.  9   . 
       FIG.  18    shows a flowchart illustrating a method  1800  that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The operations of the method  1800  may be implemented by a network entity or its components as described herein. For example, the operations of the method  1800  may be performed by a network entity  105  as described with reference to  FIGS.  1  through  6  and  11  through  14   . In some examples, a network entity may execute a set of instructions to control the functional elements of the network entity to perform the described functions. Additionally, or alternatively, the network entity may perform aspects of the described functions using special-purpose hardware. 
     At  1805 , the method may include transmitting, to a UE via a first serving cell, a message including an indication to activate a second serving cell different from the first serving cell. The operations of  1805  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1805  may be performed by a downlink message transmitting manager  1325  as described with reference to  FIG.  13   . 
     At  1810 , the method may include transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell. The operations of  1810  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1810  may be performed by a control message transmitting manager  1330  as described with reference to  FIG.  13   . 
     At  1815 , the method may include receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message. The operations of  1815  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1815  may be performed by a response message receiving manager  1335  as described with reference to  FIG.  13   . 
     At  1820 , the method may include transmitting the reference signal to the UE within a second resource of the second serving cell based on the control message. The operations of  1820  may be performed in accordance with examples as disclosed herein. In some examples, aspects of the operations of  1820  may be performed by a reference signal transmitting manager  1340  as described with reference to  FIG.  13   . 
       FIG.  19    illustrates an example of a network architecture  1900  (e.g., a disaggregated base station architecture, a disaggregated RAN architecture) that supports techniques for direct SCell activation using temporary reference signals in accordance with aspects of the present disclosure. The network architecture  1900  may illustrate an example for implementing one or more aspects of the wireless communications system  100 . The network architecture  1900  may include one or more CUs  160 - a  that may communicate directly with a core network  130 - a  via a backhaul communication link  120 - a , or indirectly with the core network  130 - a  through one or more disaggregated network entities  105  (e.g., a Near-RT RIC  175 - b  via an E2 link, or a Non-RT RIC  175 - a  associated with an SMO  180 - a  (e.g., an SMO Framework), or both). A CU  160 - a  may communicate with one or more DUs  165 - a  via respective midhaul communication links  162 - a  (e.g., an F1 interface). The DUs  165 - a  may communicate with one or more RUs  170 - a  via respective fronthaul communication links  168 - a . The RUs  170 - a  may be associated with respective coverage areas  110 - a  and may communicate with UEs  115 - a  via one or more communication links  125 - a . In some implementations, a UE  115 - a  may be simultaneously served by multiple RUs  170 - a.    
     Each of the network entities  105  of the network architecture  1900  (e.g., CUs  160 - a , DUs  165 - a , RUs  170 - a , Non-RT RICs  175 - a , Near-RT RICs  175 - b , SMOs  180 - a , Open Clouds (O-Clouds)  1905 , Open eNBs (O-eNBs)  1910 ) may include one or more interfaces or may be coupled with one or more interfaces configured to receive or transmit signals (e.g., data, information) via a wired or wireless transmission medium. Each network entity  105 , or an associated processor (e.g., controller) providing instructions to an interface of the network entity  105 , may be configured to communicate with one or more of the other network entities  105  via the transmission medium. For example, the network entities  105  may include a wired interface configured to receive or transmit signals over a wired transmission medium to one or more of the other network entities  105 . Additionally, or alternatively, the network entities  105  may include a wireless interface, which may include a receiver, a transmitter, or transceiver (e.g., an RF transceiver) configured to receive or transmit signals, or both, over a wireless transmission medium to one or more of the other network entities  105 . 
     In some examples, a CU  160 - a  may host one or more higher layer control functions. Such control functions may include RRC, PDCP, SDAP, or the like. Each control function may be implemented with an interface configured to communicate signals with other control functions hosted by the CU  160 - a . A CU  160 - a  may be configured to handle user plane functionality (e.g., CU-UP), control plane functionality (e.g., CU-CP), or a combination thereof. In some examples, a CU  160 - a  may be logically split into one or more CU-UP units and one or more CU-CP units. A CU-UP unit may communicate bidirectionally with the CU-CP unit via an interface, such as an E1 interface when implemented in an O-RAN configuration. A CU  160 - a  may be implemented to communicate with a DU  165 - a , as necessary, for network control and signaling. 
     A DU  165 - a  may correspond to a logical unit that includes one or more functions (e.g., base station functions, RAN functions) to control the operation of one or more RUs  170 - a . In some examples, a DU  165 - a  may host, at least partially, one or more of an RLC layer, a MAC layer, and one or more aspects of a PHY layer (e.g., a high PHY layer, such as modules for FEC encoding and decoding, scrambling, modulation and demodulation, or the like) depending, at least in part, on a functional split, such as those defined by the 3rd Generation Partnership Project (3GPP). In some examples, a DU  165 - a  may further host one or more low PHY layers. Each layer may be implemented with an interface configured to communicate signals with other layers hosted by the DU  165 - a , or with control functions hosted by a CU  160 - a.    
     In some examples, lower-layer functionality may be implemented by one or more RUs  170 - a . For example, an RU  170 - a , controlled by a DU  165 - a , may correspond to a logical node that hosts RF processing functions, or low-PHY layer functions (e.g., performing fast Fourier transform (FFT), inverse FFT (iFFT), digital beamforming, physical random access channel (PRACH) extraction and filtering, or the like), or both, based at least in part on the functional split, such as a lower-layer functional split. In such an architecture, an RU  170 - a  may be implemented to handle over the air (OTA) communication with one or more UEs  115 - a . In some implementations, real-time and non-real-time aspects of control and user plane communication with the RU(s)  170 - a  may be controlled by the corresponding DU  165 - a . In some examples, such a configuration may enable a DU  165 - a  and a CU  160 - a  to be implemented in a cloud-based RAN architecture, such as a vRAN architecture. 
     The SMO  180 - a  may be configured to support RAN deployment and provisioning of non-virtualized and virtualized network entities  105 . For non-virtualized network entities  105 , the SMO  180 - a  may be configured to support the deployment of dedicated physical resources for RAN coverage requirements which may be managed via an operations and maintenance interface (e.g., an O1 interface). For virtualized network entities  105 , the SMO  180 - a  may be configured to interact with a cloud computing platform (e.g., an O-Cloud  1905 ) to perform network entity life cycle management (e.g., to instantiate virtualized network entities  105 ) via a cloud computing platform interface (e.g., an O2 interface). Such virtualized network entities  105  can include, but are not limited to, CUs  160 - a , DUs  165 - a , RUs  170 - a , and Near-RT RICs  175 - b . In some implementations, the SMO  180 - a  may communicate with components configured in accordance with a 4G RAN (e.g., via an O1 interface). Additionally, or alternatively, in some implementations, the SMO  180 - a  may communicate directly with one or more RUs  170 - a  via an O1 interface. The SMO  180 - a  also may include a Non-RT RIC  175 - a  configured to support functionality of the SMO  180 - a.    
     The Non-RT RIC  175 - a  may be configured to include a logical function that enables non-real-time control and optimization of RAN elements and resources, Artificial Intelligence (AI) or Machine Learning (ML) workflows including model training and updates, or policy-based guidance of applications/features in the Near-RT RIC  175 - b . The Non-RT RIC  175 - a  may be coupled to or communicate with (e.g., via an A1 interface) the Near-RT RIC  175 - b . The Near-RT RIC  175 - b  may be configured to include a logical function that enables near-real-time control and optimization of RAN elements and resources via data collection and actions over an interface (e.g., via an E2 interface) connecting one or more CUs  160 - a , one or more DUs  165 - a , or both, as well as an O-eNB  1910 , with the Near-RT RIC  175 - b.    
     In some examples, to generate AI/ML models to be deployed in the Near-RT RIC  175 - b , the Non-RT RIC  175 - a  may receive parameters or external enrichment information from external servers. Such information may be utilized by the Near-RT RIC  175 - b  and may be received at the SMO  180 - a  or the Non-RT RIC  175 - a  from non-network data sources or from network functions. In some examples, the Non-RT RIC  175 - a  or the Near-RT RIC  175 - b  may be configured to tune RAN behavior or performance. For example, the Non-RT RIC  175 - a  may monitor long-term trends and patterns for performance and employ AI or ML models to perform corrective actions through the SMO  180 - a  (e.g., reconfiguration via O1) or via generation of RAN management policies (e.g., A1 policies). 
     The following provides an overview of aspects of the present disclosure: 
     Aspect 1: A method for wireless communication at a UE, comprising: receiving, via a first serving cell, a message comprising an indication to activate a second serving cell different from the first serving cell; receiving, via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell; transmitting, via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message; and monitoring a second resource of the second serving cell for the reference signal based at least in part on the control message. 
     Aspect 2: The method of aspect 1, further comprising: receiving, via the message, an indication of a plurality of candidate reference signal resources; and receiving, via the control message and based at least in part on the message, an indication of the second resource from the plurality of candidate reference signal resources, wherein monitoring the second resource of the second serving cell for the reference signal is based at least in part on the indication of the second resource. 
     Aspect 3: The method of any of aspects 1 through 2, further comprising: receiving, via the control message, an indication of a time offset associated with the second resource for the reference signal, wherein monitoring the second resource is based at least in part on the time offset. 
     Aspect 4: The method of aspect 3, wherein the time offset indicates a period of time between the first resource and the second resource. 
     Aspect 5: The method of any of aspects 1 through 4, further comprising: receiving, via the control message, an indication of the second resource for the reference signal, wherein monitoring the second resource is based at least in part on receiving the indication of the second resource. 
     Aspect 6: The method of any of aspects 1 through 5, wherein the control message comprises a DCI message. 
     Aspect 7: The method of any of aspects 1 through 6, wherein the control message comprises an A-CSI request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     Aspect 8: The method of any of aspects 1 through 7, further comprising: receiving, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, wherein monitoring the second resource is based at least in part on the one or more parameters. 
     Aspect 9: The method of aspect 8, wherein the one or more parameters comprise a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, or any combination thereof. 
     Aspect 10: The method of any of aspects 1 through 9, further comprising: receiving the reference signal via the second serving cell based at least in part on the monitoring; and communicating with the second serving cell based at least in part on time and frequency tracking information determined using the reference signal. 
     Aspect 11: The method of aspect 10, further comprising: receiving, based at least in part on the reference signal, automatic gain control information associated with the second serving cell, wherein communicating with the second serving cell is based at least in part on the automatic gain control information. 
     Aspect 12: The method of any of aspects 10 through 11, wherein the reference signal is received prior to an earliest SSB which the UE is capable of receiving via the second serving cell. 
     Aspect 13: The method of any of aspects 1 through 12, wherein the reference signal comprises a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, or any combination thereof. 
     Aspect 14: A method for wireless communication at a network entity, comprising: transmitting, to a UE via a first serving cell, a message comprising an indication to activate a second serving cell different from the first serving cell; transmitting, to the UE via the first serving cell, a control message indicating a first resource for transmitting a response message responsive to the message, and triggering a reference signal associated with time and frequency tracking for the second serving cell; receiving, from the UE via the first serving cell and within the first resource indicated via the control message, the response message acknowledging the message; and transmitting the reference signal to the UE within a second resource of the second serving cell based at least in part on the control message. 
     Aspect 15: The method of aspect 14, further comprising: transmitting, via the message, an indication of a plurality of candidate reference signal resources; and transmitting, via the control message and based at least in part on the message, an indication of the second resource from the plurality of candidate reference signal resources, wherein transmitting the reference signal within the second resource of the second serving cell is based at least in part on the indication of the second resource. 
     Aspect 16: The method of any of aspects 14 through 15, further comprising: transmitting, via the control message, an indication of a time offset associated with the second resource for the reference signal, wherein transmitting the reference signal within the second resource is based at least in part on the time offset. 
     Aspect 17: The method of aspect 16, wherein the time offset indicates a period of time between the first resource and the second resource. 
     Aspect 18: The method of any of aspects 14 through 17, further comprising: transmitting, via the control message, an indication of the second resource for the reference signal, wherein transmitting the reference signal within the second resource is based at least in part on transmitting the indication of the second resource. 
     Aspect 19: The method of any of aspects 14 through 18, wherein the control message comprises a DCI message. 
     Aspect 20: The method of any of aspects 14 through 19, wherein the control message comprises an A-CSI request field that triggers the reference signal associated with time and frequency tracking for the second serving cell. 
     Aspect 21: The method of any of aspects 14 through 20, further comprising: transmitting, via the message, the control message, or both, an indication of one or more parameters associated with the reference signal, wherein transmitting the reference signal within the second resource is based at least in part on the one or more parameters. 
     Aspect 22: The method of aspect 21, wherein the one or more parameters comprise a structure of the reference signal, a component carrier for the reference signal, a BWP for the reference signal, or any combination thereof. 
     Aspect 23: The method of any of aspects 14 through 22, further comprising: communicating with the UE via the second serving cell based at least in part on time and frequency tracking information that is determined based at least in part on the reference signal. 
     Aspect 24: The method of aspect 23, further comprising: transmitting, based at least in part on the reference signal, automatic gain control information associated with the second serving cell, wherein communicating with the UE via the second serving cell is based at least in part on the automatic gain control information. 
     Aspect 25: The method of any of aspects 23 through 24, wherein the reference signal is transmitted prior to an earliest SSB which the UE is capable of receiving via the second serving cell. 
     Aspect 26: The method of any of aspects 14 through 25, wherein the reference signal comprises a temporary reference signal, a tracking reference signal, an NZP-CSI-RS, or any combination thereof. 
     Aspect 27: An apparatus for wireless communication at a UE, comprising a processor; memory coupled with the processor; and instructions stored in the memory and executable by the processor to cause the apparatus to perform a method of any of aspects 1 through 13. 
     Aspect 28: An apparatus for wireless communication at a UE, comprising at least one means for performing a method of any of aspects 1 through 13. 
     Aspect 29: A non-transitory computer-readable medium storing code for wireless communication at a UE, the code comprising instructions executable by a processor to perform a method of any of aspects 1 through 13. 
     Aspect 30: An apparatus for wireless communication at a network entity, comprising a processor; memory coupled with the processor; and instructions stored in the memory and executable by the processor to cause the apparatus to perform a method of any of aspects 14 through 26. 
     Aspect 31: An apparatus for wireless communication at a network entity, comprising at least one means for performing a method of any of aspects 14 through 26. 
     Aspect 32: A non-transitory computer-readable medium storing code for wireless communication at a network entity, the code comprising instructions executable by a processor to perform a method of any of aspects 14 through 26. 
     It should be noted that the methods described herein describe possible implementations, and that the operations and the steps may be rearranged or otherwise modified and that other implementations are possible. Further, aspects from two or more of the methods may be combined. 
     Although aspects of an LTE, LTE-A, LTE-A Pro, or NR system may be described for purposes of example, and LTE, LTE-A, LTE-A Pro, or NR terminology may be used in much of the description, the techniques described herein are applicable beyond LTE, LTE-A, LTE-A Pro, or NR networks. For example, the described techniques may be applicable to various other wireless communications systems such as Ultra Mobile Broadband (UMB), Institute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Flash-OFDM, as well as other systems and radio technologies not explicitly mentioned herein. 
     Information and signals described herein 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 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 components described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a DSP, an ASIC, a CPU, 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 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 herein may 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. 
     Computer-readable media includes both non-transitory computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A non-transitory storage medium may be any available medium that may be accessed by a general-purpose or special-purpose computer. By way of example, and not limitation, non-transitory computer-readable media may include RAM, ROM, electrically erasable programmable ROM (EEPROM), flash memory, compact disk (CD) ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other non-transitory medium that may be used to carry or store desired program code means in the form of instructions or data structures and that may be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of computer-readable medium. Disk and disc, as used herein, include CD, laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above are also included within the scope of computer-readable media. 
     As used herein, including in the claims, “or” as used in a list of items (e.g., 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). Also, as used herein, the phrase “based on” shall not be construed as a reference to a closed set of conditions. For example, an example step that is described as “based on condition A” may be based on both a condition A and a condition B without departing from the scope of the present disclosure. In other words, as used herein, the phrase “based on” shall be construed in the same manner as the phrase “based at least in part on.” 
     The term “determine” or “determining” encompasses a wide variety of actions and, therefore, “determining” can include calculating, computing, processing, deriving, investigating, looking up (such as via looking up in a table, a database or another data structure), ascertaining and the like. Also, “determining” can include receiving (such as receiving information), accessing (such as accessing data in a memory) and the like. Also, “determining” can include resolving, selecting, choosing, establishing and other such similar actions. 
     In the appended figures, similar components or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If just the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label, or other subsequent reference label. 
     The description set forth herein, in connection with the appended drawings, describes example configurations and does not represent all the examples that may be implemented or that are within the scope of the claims. The term “example” used herein means “serving as an example, instance, or illustration,” and not “preferred” or “advantageous over other examples.” The detailed description includes specific details for the purpose of providing an understanding of the described techniques. These techniques, however, may be practiced without these specific details. In some instances, known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the described examples. 
     The description herein is provided to enable a person having ordinary skill in the art to make or use the disclosure. Various modifications to the disclosure will be apparent to a person having ordinary skill in the art, and the generic principles defined herein may be applied to other variations without departing from the scope of the disclosure. Thus, the disclosure is not limited to the examples and designs described herein but is to be accorded the broadest scope consistent with the principles and novel features disclosed herein.