Patent Publication Number: US-11653413-B2

Title: Secondary cell activation in new radio system

Description:
INCORPORATION BY REFERENCE 
     This present application claims the benefit of Chinese Application No. 202110142753.6, filed on Feb. 2, 2021, which claims the benefit of International Application No. PCT/CN2020/074874, “Methods and Apparatus of SCell Activation in New Radio System” filed on Feb. 12, 2020. The disclosures of both the prior applications are incorporated herein by reference in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to wireless communications, and specifically relates to new radio (NR) communication. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     A device capable of carrier aggregation may receive or transmit simultaneously on multiple component carriers in order to increase an overall data rate. The device can thus operate in multiple cells transmitted from a same base station. The multiple cells can include a primary cell and one or more secondary cells. The secondary cells can be activated or deactivated dynamically to adapt to data bursts between the device and the base station. In this way, a high data throughput can be achieved while a low power consumption can be maintained for the device. 
     SUMMARY 
     Aspects of the disclosure provide a method of activating multiple secondary cells. The method can include receiving on a primary cell (PCell) at a user equipment (UE) a first medium access (MAC) control element (CE) for activating a first secondary cell (SCell) and a second SCell for the UE in a wireless communication system. The first and second SCells can operate in a same band. No active serving cell operates on the same band for the UE. In response to that the first SCell is a known SCell, the second SCell is an unknown SCell, and both the first and second SCells operate in the same band that is a frequency range 2 (FR2) band, the first and second SCells can be activated in parallel without performing cell search and reference signal received power (RSRP) measurement and reporting over the first and second SCells. 
     In an embodiment, the activating the first and second SCells in parallel includes receiving on the PCell a second MAC CE for indicating a first transmission configuration indication (TCI) state for receiving one of a physical downlink control channel (PDCCH) or a physical downlink shared channel (PDSCH) of the second SCell, performing a time-frequency tracking process on the second SCell based on an SSB indicated in the first TCI state, receiving a third MAC CE on the PCell for activating a semi-persistent (SP) channel state information reference signal (CSI-RS) resource set of the second SCell, and performing a channel state information (CSI) reporting process based on the SP CSI-RS resource set activated by the third MAC CE on the second SCell. 
     In an embodiment, the first MAC CE for activating the first and second SCells is received in slot n, and a CSI report of the CSI reporting process is transmitted no later than in slot 
               n   +         T   HARQ     +     T   activation_time     +     T   CSI_reporting         New   ⁢         Radio   ⁢           (   NR   )     ⁢         slot   ⁢         length         ,         
where:
 
     T HARQ  denotes a delay between a downlink data transmission associated with the first MAC CE and a corresponding hybrid automatic repeat request (HARQ) acknowledgement, 
     T CSI_reporting  denotes a delay including an uncertainty period in acquiring a first available downlink CSI resource during the CSI reporting process, processing time for the CSI report, and an uncertainty period in acquiring a first available CSI reporting resource, and 
     T activation_time  denotes an activation delay that is
 
 T   MAC_CE +max( T   uncertainty_MAC   +T   FineTime   +T   SSB   ,T   uncertainty_SP ),
 
     where,
         T MAC_CE  denotes a delay including MAC CE parsing and applying time,   T uncertainty_MAC  denotes a period between a reception of the first MAC CE and a reception of a last one of the second MAC CE and a fourth MAC CE for indicating a TCI state for receiving the other one of the PDCCH and the PDSCH of the SCell,   T FineTime  denotes a period between the UE completing processing of the last one of the second MAC CE and the fourth MAC CE, and a timing of the SSB indicated in the first TCI state,   T SSB  denotes a period for processing a received signal of the SSB indicated in the first TCI state, and   T uncertainty_SP  denotes a period between the reception of the first MAC CE and a reception of the third MAC CE.       

     In an embodiment, wherein T activation_time  is 3 ms+max(T uncertainty_MAC +T FineTime +2 ms, T uncertainty_SP ). 
     In an embodiment, the activating the first and second SCells in parallel includes receiving on the PCell a second MAC CE for indicating a first TCI state for receiving one of a PDCCH or a PDSCH of the second SCell, performing a time-frequency tracking process on the second SCell based on an SSB indicated in the first TCI state, receiving on the PCell a radio resource control (RRC) message for configuring periodic CSI-RS of the second SCell, and performing a CSI reporting process based on the periodic SP CSI-RS configured by the RRC message on the second SCell. 
     In an embodiment, the first MAC CE for activating the SCell is received in slot n, and a CSI report of the CSI reporting process is transmitted no later than in slot 
               n   +         T   HARQ     +     T   activation_time     +     T   CSI_reporting         New   ⁢         Radio   ⁢           (   NR   )     ⁢         slot   ⁢         length         ,         
where:
 
     T HARQ  denotes a delay between a downlink data transmission associated with the first MAC CE and a corresponding HARQ acknowledgement, 
     T CSI_reporting  denotes a delay including an uncertainty period in acquiring a first available downlink CSI resource during the CSI reporting process, processing time for the CSI report, and an uncertainty period in acquiring a first available CSI reporting resource, and 
     T activation_time  denotes an activation delay that is
 
max( T   MAC_CE   +T   uncertainty_MAC   +T   FineTime   +T   SSB   ,T   uncertainty_RRC   +T   RRC_delay   −T   HARQ ),
 
     where,
         T MAC_CE  denotes a delay including MAC CE parsing and applying time,   T uncertainty_MAC  denotes a period between a reception of the first MAC CE and a reception of a last one of the second MAC CE and a fourth MAC CE for indicating a TCI state for receiving the other one of the PDCCH and the PDSCH of the SCell,   T FineTime  denotes a period between the UE completing processing of the last one of the second MAC CE and the fourth MAC CE, and a timing of the SSB indicated in the first TCI state,   T SSB  denotes a period for processing a received signal of the SSB indicated in the first TCI state,   T uncertainty_RRC  denotes a period between the reception of the first MAC CE and a reception of the RRC message, and   T RRC_delay  denotes a period for processing the RRC message.       

     In an example, for the RRC signaling, T activation_time  is max(T uncertainty_MAC +5 ms+T FineTime , T uncertainty_RRC +T RCC_delay −T HARQ ). 
     In an embodiment, the method can further include in response to that both the first and second SCells are unknown SCells and operate in the same band that is an FR2 band, performing a cell search on the first SCell, transmitting an RSRP measurement report indicating RSRP measurements associated with an SSB of the first SCell, and activating the second SCell without performing cell search and RSRP measurement and reporting over the second SCell. 
     In an embodiment, the method can further include in response to that the first SCell is a known SCell, the second SCell is an unknown SCell, and both the first and second SCells operate in the same band that is a frequency range 1 (FR1) band, activating the first and second SCells in parallel without performing automatic gain control (AGC) gain settling and cell search over the first and second SCells. In an embodiment, the PCell is a primary cell of a primary cell group (PCG), or a primary cell of a secondary cell group (SCG). 
     Aspects of the disclosure provide an apparatus. The apparatus can include circuitry configured to receive on a PCell at a UE a MAC CE for activating a first SCell and a second SCell for the UE in a wireless communication system. The first and second SCells operating in a same band, and no active serving cell operates on the same band for the UE. The circuitry can further be configured to, in response to that the first SCell is a known SCell, the second SCell is an unknown SCell, and both the first and second SCells operate in the same band that is a frequency range 2 (FR2) band, activate the first and second SCells in parallel without performing cell search and reference signal received power (RSRP) measurement and reporting over the first and second SCells. 
     Aspects of the disclosure provide a non-transitory computer-readable medium storing instructions that, when executed by a processor, cause the processor to perform the method of activating multiple secondary cells. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein: 
         FIG.  1    shows a wireless communication system  100  according to an embodiment of the disclosure. 
         FIG.  2    shows another wireless communication system  200  according to an embodiment of the disclosure. 
         FIG.  3    shows a secondary cell (SCell) activation process  300  according to an embodiment of the disclosure. 
         FIG.  4    shows another SCell activation process  400  according to an embodiment of the disclosure. 
         FIG.  5    shows another SCell activation process  500  according to an embodiment of the disclosure. 
         FIG.  6    shows a multi-SCell activation process  600  according to an embodiment of the disclosure. 
         FIG.  7    shows another multi-SCell activation process  700  according to an embodiment of the disclosure. 
         FIG.  8    shows a single SCell activation process  800  according to an embodiment of the disclosure. 
         FIG.  9    shows a multi-SCell activation process  900  according to an embodiment of the disclosure. 
         FIG.  10    shows a multi-SCell activation process  1000  according to embodiments of the disclosure. 
         FIG.  11    shows an apparatus  1100  according to embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG.  1    shows a wireless communication system  100  according to an embodiment of the disclosure. The system  100  can include a user equipment (UE)  101  and a base station  105 . The wireless communication system  100  can be a cellular network. The UE  101  can be a mobile phone, a laptop computer, a tablet computer, and the like. The base station  105  can be an implementation of a gNB in New Radio (NR) of a fifth generation (5G) system. The 5G NR is a radio interface specified in communication standards developed by the 3rd Generation Partnership Project (3GPP). Accordingly, the UE  101  can communicate with the base station  105  according to 3GPP NR communication protocols specified in respective communication standards. For example, the system  100  can be a standalone 5G system. The UE  101  interacts with a 5G core network (not shown) via the base station  105 . In other examples, the system  100  may operate according to communication standards other than the 5G NR standards. 
     In one example, the UE  101  and the base station  105  are configured to employ carrier aggregation techniques to communicate with each other. Accordingly, multiple cells  110  and  120   a - 120   n  can be configured between the UE  101  and the base station  105 . Depending on capability of the UE  101 , different number of serving cells can be configured. Each of the multiple cells can correspond to a downlink component carrier, and an uplink component carrier. Alternatively, a cell can be configured asymmetrically, and only an uplink component carrier or a downlink component carrier is transmitted over the respective serving cell. 
     The downlink component carriers can be transmitted in parallel allowing for an overall wider downlink bandwidth and correspondingly higher downlink data rates. Similarly, the uplink component carriers can be transmitted in parallel allowing for an overall wider uplink bandwidth and correspondingly higher uplink data rates. Different cells can operate on frequency division duplex (FDD) mode or time division duplex (TDD) mode. For cells configured with TDD mode, different uplink-downlink configurations can be used for different component carriers. 
     The multiple cells include a primary cell (PCell)  110  and one or more secondary cells (SCells)  120   a - 120   n . The PCell  110  can be established to be a first serving cell, for example, after an initial access procedure. A radio resource control (RRC) connection can be established in the PCell  110 . The SCells  120   a - 120   n  can be subsequently configured through RRC signaling on the PCell  110 . 
     In an embodiment, the UE  101  can dynamically activate or deactivate one or more of the SCells  120   a - 120   n  under the control of the base station to adapt to data traffic bursts from the base station  105  to the UE  101 . For example, when a downlink traffic volume is low, the SCells  120   a - 120   n  can be in a deactivated status. When the base station  105  detects an arrival of a high volume downlink traffic, the base station  105  can signal an activation command to the UE  101 . The activation command can be, for example, carried in a medium access control (MAC) control element (CE), a downlink control information (DCI), or the like. The activation command may specify one or more SCell indices corresponding to a set of SCells to be activated. 
     In response to receiving the activation command from the base station  105 , the UE  101  may perform an SCell activation process to activate the SCells indicated in the activation command. The SCell activation process may include a sequence of operations, and thus may incur an SCell activation delay. For example, the sequence of operations may include parsing the MAC CE to obtain the activation command, preparation (or configuration) of hardware and software for reception and transmission on the SCell (e.g., protocol stack software application, radio frequency (RF) module tuning), automatic gain control (AGC) tuning and time-frequency synchronization on the SCell, and the like. 
     As a result of the SCell activation process, the UE  101  can become ready to perform normal operations on the SCells being activated. For example, the operations can include sounding reference signal (SRS) transmission, channel state information (CSI) reporting, physical downlink control channel (PDCCH) monitoring, physical downlink shared channel (PDSCH) monitoring, physical uplink control channel (PUCCH) transmission, and the like. 
     On the other hand, when the base station  105  detects a light downlink traffic locally, the base station  105  can signal a deactivation command to the UE  101 . The deactivation command may indicate which activated SCells are to be deactivated. In response, the UE  101  can deactivate those indicated SCells and terminate operations on those deactivated SCells. Alternatively, other mechanisms (e.g., a timer) can be employed for deactivating an SCell. 
     As described above, SCells can be activated to increase a data rate for the UE  101  when data traffic from the base station  105  towards the UE  101  is high, while SCells can be deactivated to save power for the UE  101  when data traffic towards the UE  101  is low. 
     To take advantage of the above SCell activation/deactivation mechanism, the SCell activation delay is desired to be short to avoid causing latency to the bursty downlink traffics from the base station  105  to the UE  101 . 
     In an embodiment, the system  100  can apply carrier aggregation over frequency bands separated into two different frequency ranges: frequency range 1 (FR 1) and frequency range 2 (FR2). For example, as specified in 3GPP standards, FR1 can include sub-6 GHz frequency bands, while FR2 can include frequency bands from 24.25 GHz to 52.6 GHz. For example, the PCell  110  may operate in FR1 while the SCells  120   a - 120   n  may operate in FR2. When new spectra are available, the specification of FR1 or FR2 may be expanded. 
     Typically, for SCells operating in FR1, beamforming techniques are not used. For SCells operating in FR2, beamforming techniques can be employed and directional transmission (e.g., beam sweeping operation) may be performed at both the UE  101  and the base station  105 . Synchronization signal blocks (SSBs) can be transmitted for purpose of time-frequency synchronization and broadcast of system information. For example, the base station  105  can perform a beam sweeping to transmit a sequence of SSBs (referred to as an SSB burst set) towards different directions to cover a cell. Each SSB among the SSB burst set is transmitted with a different transmission (Tx) beam. Such an SSB burst set can be periodically transmitted with a period of 5 ms, 10 ms, 20 ms, and the like. 
     The UE  101  may also perform a beam sweeping process for an SCell operating in FR2. During this process, the UE  101  can use a different reception (Rx) beam to receive the different SSB burst set. For each link corresponding to a pair of Tx beam and Rx beam, the UE  101  can measure reference signal received power (RSRP) of each beam pair link at physical layer (L1). This measurement is referred to as L1-RSRP measurement. Results of the L1-RSRP measurement can indicate link qualities corresponding to each beam pair link, and therefore indicate which Tx beams (each indicated by an SSB index) are best choices for downlink transmission, and which Rx beams are best choices for receiving a signal from a Tx beam. The L1-RSRP measurement results can be reported to the base station  105  in some examples. 
     The L1-RSRP measurement results can be reported from physical layer to RRC layer at the UE  101  in some examples. A layer three (L3) RSRP measurement results can be derived from the L1-RSRP measurement results at the RRC layer. A L3-RSRP measurement report including beam level measurement results of a specific SCell can be transmitted from the UE  101  to the base station  105 . When multiple SCells are configured, a L3-RSRP measurement report can include beam level and/or cell level measurement results. For example, based on the beam level information, the base station  105  can determine a Tx beam for a respective SCell, while based on the cell level information, the base station  105  can select best SCells for activation or deactivation. 
     For example, at the base station, when transmitting a signal, such as a PDCCH, a PDSCH, a CSI reference signal (CSI RS) and the like, a best choice of Tx beam can be selected based on beam pair link qualities indicated by the L3-RSRP report or the L1-RSRP report. The base station  105  may indicate an SSB (e.g., using an SSB index) corresponding to the selected Tx beam to the UE  101 . The indication can be in the form of a transmission configuration indication (TCI) state signaled from the base station  105  to the UE  101 . The TCI state can provide an SSB (using an SSB index) and a quasi-co-location (QCL) type (e.g., Type D corresponding to spatial receiver parameters). Such a TCI state can indicate to the UE  101  that the SSB is quasi-co-located (QCLed) with the to-be-transmitted signal in terms of the QCL type. When Type-D TCI state is indicated, based on the SSB index indicated TCI state and the previously acquired L1-RSRP (or L3-RSRP) measurement results, the UE  101  can use a best Rx beam corresponding to the indicated SSB index for reception of the signal. 
     When beamforming is employed and the TCI mechanism is utilized, during the SCell activation process to activate an SCell as described herein, the SCell activation delay may further include periods for waiting for TCI state indications useful for some operations related with the SCell activation process. For example, a TCI state may be signaled to the UE  101  in a MAC CE (or a MAC CE activation command) to indicate an SSB (in form of an SSB index) for PDCCH or PDSCH reception. Accordingly, the time-frequency synchronization operation can be performed based on this SSB, and a Rx beam corresponding to this SSB can be used for the time-frequency synchronization operation. For another example, a MAC CE (or a MAC CE activation command) may be signaled to the UE  101  to activate a set of CSI-RS resources. This MAC CE can also indicate an SSB index for CSI-RS reception. Accordingly, a Rx beam corresponding to the indicated SSB index can be determined at the UE  101 , and used for measuring the respective CSI RS. In the above examples, receiving those MAC CEs may also cause delays for the SCell activation process. 
       FIG.  2    shows another wireless communication system  200  according to an embodiment of the disclosure. The system  200  can employ dual connectivity (DC) mechanisms to increase data throughput at a UE  201 . The system  200  can include the UE  201  and two base stations  202 - 203 . The base station  202  can serve as a master node, while the base station  203  can serve as a secondary node. A first cell group  211 , referred to as a master cell group (MCG), is established between the master node  202  and the UE  201 , while a second cell group  231 , referred to as a secondary cell group (SCG), is established between the secondary node  203  and the UE  201 . The MCG  211  can include a PCell  210  and SCells  220   a - 220   n , while the SCG  231  can include a primary SCG cell (PSCell)  230  and SCells  240   a - 240   n.    
     In an embodiment, the nodes  202  and  203  can independently perform resource scheduling for the MCG  211  and the SCG  231 , respectively. In some examples, an RRC connection can be provided on the PCell  210 . However, in some examples, no RRC connection is provided on the PSCell  230  or the SCells  220   a - 220   n  and  240   a - 240   n . In addition, configurations between the MCG  211  and the SCG  23 ) can be independent. The configurations can include frequency bands, bandwidth, number of component carriers, frame structures of component carriers (e.g., frequency division duplex (FDD) or time division duplex (TDD)), and the like. 
     For each cell group  211  or  231 , the PCell  210  or the PSCell  230  can first be established, and the SCells can be configured, for example, by RRC signaling on the PCell  210 . The respective SCells can then be activated or deactivated to adapt to status of data traffic. For example, an SCell activation or deactivation command MAC CE can be received on the PCell  210  to add or remove one or more SCells belonging to the MCG  211 . Similarly, an SCell activation or deactivation command MAC CE can be received on the PSCell  230  to add or remove one or more SCells belonging to the SCG  231 . 
     In various embodiments, the base stations  202  and  203  may utilize different or a same radio access technology (RAT). For example, both the base stations  202  and  203  can employ 5G NR RAT. Such a configuration is referred to as NR-DC mode. Or, the base stations  202  and  203  may employ different RAT. Such a configuration is referred to as MR-DC, a short for muti-RAT DC. For example, the master node  202  employs a Long Term Evolution (LTE) air interface (e.g., Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)), while the secondary node  203  employs the 5G NR air interface. Such a configuration is referred to as EN-DC. In contrast to EN-DC, there can be another DC mode referred to as NE-DC. In NE-DC, a master node can be a 5G gNB employing the NR air interface, while a secondary node can be an LTE eNB employing the E-UTRAN air interface. 
     The SCell activation delay reduction techniques disclosed herein can be applied to various scenarios where a UE is operating on a PCell or a PSCell in standalone mode ( FIG.  1    example), or NR-DC or MR-DC (e.g. EN-DC, or NE-DC) mode ( FIG.  2    example), and one or more SCells are being activated based on TCI states signaled on the PCell or the PSCell. 
       FIG.  3    shows a single SCell activation process  300  according to an embodiment of the disclosure. The UE  101 , the base station  105 , and the PCell  110  and the SCells  120   a - 120   n  in the  FIG.  1    example are used for explanation of the process  300 . By performing the process  300 , an SCell (e.g., the SCell  120   a ) can be activated. 
     In the  FIG.  3    example, the PCell  110  has been established. The SCells  120   a - 120   n  has been configured by the base station  105  to the UE  101 . Accordingly, control signaling (e.g., MAC CEs) can be received on the PCell  110  for activating or deactivating one of the SCells  120   a - 120   n.    
     In addition, the PCell  110  can operate on FR1 or FR2 (e.g., millimeter wave region). The SCells  120   a - 120   n  can be configured to operate on FR2. Accordingly, beamforming can be employed in the SCells  120   a - 120   n , and the TCI scheme can be employed for indicating Rx beams at the UE  101 . Further, semi-persistent (SP) CSI-RS is used for CSI reporting on the respective SCells  120   a - 120   n . Accordingly, a MAC CE activation command can be used for activating an SP CSI-RS resource set during the process  300 . 
     Additionally, the SCell  120   a  can be the first SCell to be activated on an FR2 band during the process  300 . Assuming another SCell, for example, the SCell  120   n , has already been established on FR2 before the SCell  120   a , and operates in a frequency band neighboring that of the SCell  120   a . As the SCells  120   a  and  120   n  can be co-located on a same site, beamformed radio channels of the two SCells  120   a  and  120   n  can have a similar property. Accordingly, when activating the SCell  120   a , the UE  101  can take advantage of the known channel property of the SCell  120   n  to simplify the process  300  of activating the SCell  120   a . For example, signaling of TCI states and/or CSI reporting may become unnecessary under certain conditions. Accordingly, when the SCell  120   a  is not the first SCell to be activated on an FR2 band, a process for activating the SCell  120   a  can be different from the process  300 . 
     Furthermore, the process  300  can be performed with an assumption that the SCell  120   a  is known to the UE  101 . For example, before the UE  101  receives an activation command for activating the SCell  120   a , the UE  101  has sent a valid L3-RSRP measurement report (or a L1-RSRP measurement report) of the SCell  120   a  with SSB indices (beam level information). In addition, an interval between the L3-RSRP measurement report and the activation command is short enough (e.g. within several SCell measurement cycles or 5 DRXs) such that the measurement results can be used as the basis for determining the TCI states useful for the activation of the SCell  120   a . When the above conditions are satisfied, the SCell  120   a  operating on FR2 is said to be known to the UE  101 . 
     For example, for a first SCell activation in FR2 bands, the SCell is known if it has been meeting the following conditions:
         (i) During the period equal to 4s for UE supporting power class 1 and 3s for UE supporting power class 2/3/4 before UE receives the last activation command for PDCCH TCI, PDSCH TCI (when applicable) and semi-persistent CSI-RS for CQI reporting (when applicable): the UE has sent a valid L3-RSRP measurement report with SSB index, and SCell activation command is received after L3-RSRP reporting and no later than the time when UE receives MAC-CE command for TCI activation.   (ii) During the period from L3-RSRP reporting to the valid CQI reporting, the reported SSBs with indexes remain detectable according to certain cell identification conditions, and the TCI state is selected based on one of the latest reported SSB indexes.       

     In contrast, when the above conditions are invalid, the SCell  120   a  operating on FR2 is said to be unknown to the UE  101 . For example, for unknown SCell, the activation commands for PDCCH TCI, PDSCH TCI (when applicable), semi-persistent CSI-RS for CQI reporting (when applicable), and configuration message for TCI of periodic CSI-RS for CQI reporting (when applicable) can be based on a latest valid L1-RSRP reporting. 
     In such a scenario, after receiving an activation command for activating the SCell  120   a , the UE  101  can perform an L1-RSRP measurement process to obtain beam pair link qualities, accordingly derive an L3-RSRP measurement report with beam level information, and transmit the L3-RSRP measurement report to the base station  105 . Thereafter, based on the latest L3-RSRP measurement report, the base station  105  can transmit activation command(s) for PDCCH TCI, PDSCH TCI (when applicable), and SP CSI-RS for CQI reporting. Such an SCell activation process for activating an unknown SCell would incur a longer delay than the process  300 . 
     In  FIG.  3   , the process  300  can start with reception of an MAC CE  341  for single SCell activation, and end with transmitting a valid CSI report at the end of a CSI reporting process  325 . The process  300  can include five phases  351 - 355 . 
     At phase  351 , the MAC CE  341  for single SCell activation can be received. The MAC CE  341  can be carried in a PDSCH  331  transmitted on the PCell  110 . The UE  101  may then decode a transport block (TB) from the PDSCH  331  followed by a cyclic redundancy check (CRC) verification. When the CRC verification is successful, the UE  101  may transmit a hybrid automatic repeat request (HARQ) ACK feedback. A delay between the PDSCH  331  and the ACK  332  is denoted T HARQ    301 . 
     The MAC CE  341  may specify an SCell index corresponding to the SCell  120   a , such that the UE  101  can know the SCell  120   a  is to be activated based on a configuration of SCells previously received on the PCell  110 . Additionally, the MAC CE  341  may indicate more than one SCell for activation. 
     At phase  352 , one or multiple MAC CEs  342  for channels and/or SP CSI-RS on the SCell  120   a  can be received on the PCell  110 . As an example, three PDSCHs  333 - 335  of the PCell  110  are shown in  FIG.  3    that carry those MAC CEs  342 . A first MAC CE, denoted MAC CE  342 - 1 , can be a MAC CE for indicating a TCI state for PDCCH reception on the SCell  120   a . A second MAC CE, denoted MAC CE  342 - 2 , can be a MAC CE for activating TCI states for PDSCH reception on the SCell  120   a . A third MAC CE, denoted MAC CE  342 - 3 , can be a MAC CE for activating an SP CSI-RS resource sent on the SCell  120   a . The MAC CE  342 - 3  can indicate a TCI state for receiving SP CSI-RS over respective SP CSI-RS resource. 
     An SSB indicated by one of the TCI states identified by the MAC CE  342 - 1  or  342 - 2  can later be used for time-frequency tracking on the SCell  120   a . The SSB indicated by one of the TCI states identified by the MAC CE  342 - 1  or  342 - 2  can also be used for PDCCH or PDSCH reception on the SCell  120   a . An SSB indicated by the TCI state identified by the MAC CE  342 - 3  can later be used for CS measurement and reporting of the SCell  120   a.    
     The order of arrivals of the PDSCHs  333 - 335  can vary depending on transmission decisions of the base station  105 . In an example, the SSB of the earlier one of the MAC CE  342 - 1  or  342 - 2  is used for the time-frequency tracking on the SCell  120   a . Thus, once the SSB is available, the UE  101  can proceed to enter a next operation without waiting for the late-arrived MAC CE in order to avoid delay. In an example, the SSB of one of the MAC CEs  342 - 1  and  342 - 2  is designated for usage of time-frequency tracking. Accordingly, the UE  101  may wait for this MAC CE before initiating the next operation. In addition, under some scenarios, one of the MAC CEs  342 - 1  or  342 - 2  may not be transmitted. 
     As shown in  FIG.  3   , a delay  311 , denoted T uncertainty_MAC , incurs between reception of the PDSCH  331  and reception of the last one of the PDSCHs  333 - 335 . In some examples, the MAC CEs  342  may be carried in the PDSCH  331 . Accordingly, the delay T uncertainty_MAC  can be reduced to zero. 
     Corresponding to each of the PDSCHs  333 - 335 , an HARQ ACK feedback can be transmitted from the UE  101  to the base station  105 . In  FIG.  3   , only the ACK of the last one of the PDSCHs  333 - 335  is shown. A delay, denoted T HARQ    315 , occurs between reception of the last of the PDSCHs  333 - 335  and the transmission of the ACK  336 . 
     While three PDSCHs  333 - 335  are shown in  FIG.  3    for separately carrying the MAC CEs  342 - 1 ,  342 - 2 , and  342 - 3 , it is possible that the MAC CEs  342 - 1 ,  342 - 2 , and  342 - 3  are carried in fewer (one or two) PDSCHs. 
     At phase  353 , a MAC CE processing and applying process (e.g., processes  321  and  322 ) can be performed. In the process  321 , a TB of the last one of the PDSCHs  333 - 335  can be received from a physical layer at a MAC layer of the UE  101 . Fields of the MAC CE are then parsed. Parsing processes similar to the process  321  can be performed for each of the MAC CEs  342 . The process  322  may include software application and RF warmup operations. Parameters (e.g., spatial receiver parameters, carrier frequency of the SCell  120   a , and the like) can be applied at the UE  101  to make the UE  101  ready for receiving synchronization signals from the SCell  120   a.    
     The operations during the phase  353  can incur a delay  312 , denoted T MAC_CE . Considering capability of the UE  101 , the delay T MAC_CE    312  can be bounded to be smaller than or equal to 3 ms in order to control a total SCell activation delay. 
     At phase  354 , a fine time-frequency synchronization process (e.g., processes  323  and  324 ) can be performed. In the process  323 , the UE  101  may wait for arrival of a first complete SSB, and perform reception of the SSB. For example, the SSB captured by the UE  101  can be that indicated by the TCI state of the MAC CE  342 - 1  or  342 - 2 . The Rx beam used by the UE  101  for reception of the SSB can be determined based on the TCI state of the MAC CE  342 - 1  or  342 - 2 . 
     A delay  313 , denoted T FineTime , can occur during the process  323 . T FineTime  can be the time period between the UE  101  completes processing the last activation command of the MAC CEs  342  for PDCCH TCI, PDSCH TCI (when applicable) and SP CSI-RS (when applicable) and the timing of the first complete available SSB corresponding to the TCI state of the MAC CE  342 - 1  or  342 - 2 . 
     In the process  324 , the UE  101  can process a received signal of the SSB, and conduct fine time-frequency tuning accordingly. As a result, the UE  101  becomes ready for monitoring PDCCH and PDSCH of the SCell  120   a , and performing the CSI reporting process  325 . A delay  314 , denoted T SSB , occurs during the process  324 . Considering capability of the UE  101 , the delay T SSB    314  can be bounded to be smaller than or equal to 2 ms in order to control the total SCell activation delay. 
     At a final phase  355 , the CSI reporting process  325  can be performed. The process  325  can include operations of acquiring a first available downlink CSI reference resource, processing for CSI reporting, and reporting CSI measurement results with a first available CSI reporting resources. Accordingly, a delay  303 , denoted T CSI_reporting , can occur that includes a delay including uncertainty in acquiring the first available downlink CSI reference resource, UE processing time for the CSI reporting, and uncertainty in acquiring the first available CSI reporting resources. 
     As can be seen, the consumed time of the process  300  between reception of the SCell activation command in the MAC CE  341  and transmission of the CSI report at the end of the CSI reporting process  325  is
 
 T   HARQ   +T   activation_time   +T   CSI_reporting .  (I)
 
In the above expression, T HARQ  is the delay T HARQ    301 . T CSI_reporting  is the delay T CSI_reporting    303 . T activation_time    302  can be a sum of T uncertainty_MAC , T MAC_CE , T FineTime , and T SSB  
 
 T   activation_time   =T   uncertainty_MAC   +T   MAC_CE   +T   FineTime   +T   SSB .
 
When T MAC_CE  and T SSB  take the maximum values 3 ms and 2 ms, respectively, set for bounding the SCell activation delay, a maximum value of the delay T activation_time  can be estimated to be
 
 T   uncertainty_MAC   +T   FineTime +5 ms.
 
Accordingly, if the SCell activation command is received in slot n, the UE  101  can transmit the CSI report and apply actions (e.g., PDCCH monitoring) related to the activation command for the SCell  120   a  being activated no later than in slot
 
             n   +           T   HARQ     +     T   activation_time     +     T   CSI_ceporting         NR   ⁢         slot   ⁢         length       .           
The NR slot length depends on a subcarrier spacing used in the SCell  120   a . For example, corresponding to the subcarrier spacing of 15, 30, 60, 120, and 240 KHz, the slot length can be 1, ½, ¼, ⅛, and 1/16 ms, respectively.
 
       FIG.  4    shows another single SCell activation process  400  according to an embodiment of the disclosure. The process  400  has been improved to reduce SCell activation time compared with the process  300 . In the process  300  in  FIG.  3   , phase  353  does not start until MAC CE(s) for channels (PDCCH TCI indication MAC CE  342 - 1  and PDSCH TCI activation MAC CE  342  (when applicable)) and MAC CE for SP CSI-RS (SP CSI-RS resource set activation MAC CE  342 - 3 ) are all available. However, to reduce SCell activation delay, in the process  400 , the MAC CE for SP CSI-RS is decoupled from the start of phase  353 . In other words, phase  353  can start even when a MAC CE for SP CSI-RS is not received yet. This decoupling is feasible because the fine time-frequency tracking process ( 323  and  324 ) of phase  354  does not rely on information provided by the MAC CE for SP CSI-RS. 
     Accordingly,  FIG.  4    shows two separate timelines  401  and  402 . Operations on the timeline  401  is similar to that in  FIG.  3    except that the PDSCH (e.g., PDSCH  335 ) carrying the MAC CE  342 - 3  for SP CSI-RS is excluded from the MAC CEs  342 . Now on timeline  401 , MAC CEs  442  are shown which can correspond to the MAC CEs  342 - 1  and  342 - 2 . 
     On the timeline  402 , reception of a MAC CE  472  for SP CSI-RS is shown in parallel with reception of MAC CEs  442  and the processes  321 - 324 . Specifically, a PDSCH  481  carrying the MAC CE  472  can be received on the PCell  110  after a delay T uncertainty_SP    461 . Then, an ACK  482  is transmitted from the UE  101  on the PCell  110  after a delay T HARQ    462 . Thereafter, during a delay T MAC_CE    463 , operations similar to that of the processes  321  and  322  are performed. The MAC CE  472  is parsed and applied during the delay T MAC_CE    463 . 
     Based on the operations of the two timelines  401402 , the CSI reporting process  325  can start after the timing of the end of the SSB processing process  324  or the timing of the end of the MAC CE process during T MAC_CE    463 , whichever comes late. In this way, the SCell activation delay of the SCell  120   a  can be reduced when the delay T uncertainty_SP    461  is longer than the delay T uncertainty_MAC  and the MAC CE process during T MAC_CE    463  ends earlier than the SSB processing process  324 . 
     Accordingly, the delay T activation_time  in the expression (1) can be estimated as
 
 T   MAC_CE +max( T   uncertainty_MAC   +T   FineTime   +T   SSB   ,T   uncertainty_SP ).
 
When T MAC_CE  and T SSB  take the maximum values 3 ms and 2 ms, respectively, set for bounding the SCell activation delay, a maximum value of the delay T activation_time  can be estimated to be
 
3 ms+max( T   uncertainty_MAC   +T   FineTime +2 ms ,T   uncertainty_SP ).
 
       FIG.  5    shows another single SCell activation process  500  according to an embodiment of the disclosure. Different from the process  400  where the SCell  120   a  is known to the UE  101 , the process  500  assumes that the SCell  120   a  is unknown to the UE  101 . Accordingly, compared with the process  400 , the process  500  includes an additional phase  551  between the phases  351  and  352  as shown in  FIG.  5   . During the phase  551 , an L1-RSRP measurement process is performed on the SCell  120   a  being activated, and measurement results are reported to the base station  105  on the PCell  110 . 
     Specifically, the process  500  starts with phase  351  where the PDSCH  331  carrying the SCell activation MAC CE  341  is decoded, and the ACK  332  is transmitted on the PCell  110 . Following phase  351 , a MAC CE processing process ( 521  and  522 ) is performed and incurs a delay T MAC_CE    511 . Operations  521  and  522  are similar to that of the processes  321  and  322  in  FIG.  4   . Then, an AGC tuning process  523  and a cell search process  524  are successively performed. 
     In an embodiment, the processes  523  and  524  incur a delay  512 , denoted T cell identification with Rx training . T cell identification with Rx training  is a period for cell identification with Rx beam training. T cell identification with Rx training  can includes time for AGC tuning and cell search with Rx beam training. In an example, T cell identification with Rx training  can be a period of 24*T rs . T rs  can n SSB measurement timing configuration (SMTC) periodicity of the SCell  120   a  being activated when the UE  101  has been provided with an SMTC for the SCell  120   a  in an SCell addition message. Alternatively, T rs  can be a periodicity of an SMTC of a measurement object (e.g., another SCell) having a same SSB frequency and subcarrier spacing as the SCell  120   a . In an example, if the UE  101  is not provided the SMTC configuration or the measurement object, the requirement which involves T rs  can be applied with T rs =5 ms assuming an SSB transmission periodicity is 5 ms. 
     After the cell search process  524 , an L1-RSRP measurement process  525  and an L1-RSRP reporting process  526  can be successively performed. In the process  525 , beam sweepings with Tx beams of the base station  105  and beam sweepings with Rx Beams of the UE  101  can be performed on the SCell  120   a . Beam pair link qualities (e.g., RSRP) can be measured based on SSBs. In the process  526 , L1-RSRP measurement results of the SCell  120   a  can be reported to the base station  105  on the PCell  110 . In another example, the L1-RSRP measurement results of the SCell  120   a  can be provide from the physical layer to the RRC layer of the UE  101 , and L3-RSRP measurement results can accordingly be derived and reported to the base station  105  on the PCell  110 . At this point, the base station  105  becomes aware of the beam level link qualities, and can use the beam level link qualities as a basis for determining the TCI states carried in the MAC CEs  442  and MAC CE  472 . 
     In total, the phase  551  incurs a delay of
 
 T   MAC_CE (511)+ T   cell identification with Rx training   +T   L1-RSRP,measure   +T   L1-RSRP,report ,
 
where T L1-RSRP,measure    513  and T L1-RSRP,report    514  correspond to the processes  525  and  526 , respectively. T L1-RSRP,report    514  can include time for acquiring CSI reporting resources.
 
     After the end of the L1-RSRP reporting process, operations similar to that of the process  400  are performed. For example, MAC CEs  442  for PDCCH TCI, and PDSCH TC, and MAC CE  472  for SP CS-RS activation are received. The fine time-frequency synchronization ( 323  and  324  in  FIG.  4   ) is performed followed by the CSI reporting process ( 325  in  FIG.  4   ). 
     As shown, both the delays T uncertainty_MAC    311  and T uncertainty_SP    461  are measured relative to the end of the L1-RSRP reporting process  526 , which is different from the process  400 . 
     Accordingly, with consideration of the SCell  120   a  being unknown, in the process  500 , the delay T activation_time  in the expression (1) can be
 
 T   MAC-CE (511)+ T   cell identification with Rx training   +T   L1-RSRP,measure   +T   L1-RSRP,report   +T   HARQ   +T   MAC_CE (312 or 463)+max( T   uncertainty_MAC   +T   FineTime   +T   SSB   ,T   uncertainty_SP ).
 
When T MAC_CE  and T SSB  take the maximum values 3 ms and 2 ms, respectively, that are set for bounding the SCell activation delay, a maximum value of the delay T activation_time  of the process  500  can be estimated to be
 
6 ms+ T   cell identification with Rx training   +T   L1-RSRP,measure   +T   L1-RSRP,report   +T   HARQ +max( T   uncertainty_MAC   +T   FineTime +2 ms ,T   uncertainty_SP ).
 
       FIG.  6    shows a multi-SCell activation process  600  according to an embodiment of the disclosure. During the process  600 , one MAC CE command is received for activating two SCells (one known, one unknown) operating in a same band in FR2. The  FIG.  1    example is used to explain the process  600 . The two SCells can be a first SCell  120   n  that is known and a second SCell  120   a  that is unknown as shown in  FIG.  1   . 
     Assuming the unknown SCell  120   a  is to be activated solely without other active serving cell or known cell operating in the same FR2 band, the process  500  in  FIG.  5    example would be performed to activate the SCell  120   a . However, with the known SCell  120   n  in the same FR2 band as the SCell  120   a , the process  500  can be simplified and transformed into the process  600 , and the activation delay of the unknown SCell  120   a  can be reduced. 
     In some examples, when the two SCells  120   n  and  120   a  operate in a same FR2 band (intra-band FR2 SCells), the UE  101  can assume a deployment that the two SCells are co-located at a same site, and the transmitted signals from the two serving cells  120   n  and  120   a  can have the same downlink spatial domain transmission filter on one OFDM symbol in the same FR2 band. Thus, Tx beams from the SCells  120   n  and  120   a  can have a same beam direction at a time. Channels transmitted on the SCells  120   n  and  120   a  may have similar properties. 
     For the process  600 , the SCell  120   n  is known to the base station  105 . The base station  105  thus has the knowledge of the Tx beam qualities for the SCell  120   n . Because the SCells  120   a  and  120   n  are intra-band FR2 cells, the base station  105  can use the knowledge of the Tx beam qualities in the SCell  120   n  for selection Tx beams in the SCell  120   a , and signal TC states accordingly. As a result, there is no need for the to-be-activated unknown SCell  120   a  to perform L1-RSRP measurement and report process (e.g., the operations  525  and  526  in the  FIG.  5    example are not performed) during the multi-SCell activation process  600 . 
     In addition, in some examples, when the two SCells  120   n  and  120   a  operate in a same FR2 band (intra-band FR2 SCells), the UE  101  can further assume a deployment that the two SCells have a similar frame timing. For example, a maximum receiving timing difference (MRTD) for intra-band non-contiguous carrier aggregation in FR2 can be 260 ns in an example. 
     For the process  600 , the SCell  120   n  is known to the UE  101 . The UE  101  thus knows the frame timing on the SCell  120   n . Because the SCells  120   a  and  120   n  are intra-band FR2 cells, frame timing on the SCell  120   a  can be established, for example, within an error of an MRTD. As a result, the cell search (or cell detection) (e.g., operation  524  in the  FIG.  5    example) can be avoided during the multi-SCell activation process  600 . 
     Further, in some examples, for the two SCells  120   n  and  120   a  operate in a same FR2 band (intra-band FR2 SCells), the UE  101  may be configured with a same set of RF circuitry for reception of the two SCells  120   n  and  120   a . Accordingly, RF module adjustment (e.g., spatial filter application for Rx beam forming, or AGC settling) for the two SCells  120   n  and  120   a  can be performed at the same time. When the SCell  120   n  is known to the UE  101  (AGC related parameters are known), the corresponding AGC settling process (e.g., the operation  523  in  FIG.  5   ) can be skipped for both the SCells  120   n  and  120   a  during the multi-SCell activation process  600 . 
     Specifically, in the process  600 , a MAC command  641  for activating both the known SCell  120   n  and the unknown SCell  120   a  can be received. The MAC command  641  can be carried in a MAC CE transmitted on a PDSCH. The UE  101  may feedback a HARQ ACK in response to successfully decoding the PDSCH within a delay T HARQ    601 . The UE  101  may parse the MAC CE to obtain content of the MAC command  641 . At this moment, the UE  101  can be aware of that the SCells  120   n  and  120   a  are to be activated. 
     In an example, the UE  101  may determine that the SCell  120   n  is known while the SCell  120   a  is unknown and the SCells  120   n  and  120   a  are intra-band FR2 SCells based on configurations and operation history. Accordingly, the UE  101  can make decision that no AGC settling, cell search (cell detection), or L1-RSRP measurement and reporting is to be performed for both the SCells  120   n  and  120   a.    
     Subsequently, the UE  101  may carry out activation processes  600 N and  600 A in parallel corresponding to the SCells  120   n  and  120   a , respectively. As shown in  FIG.  6   , the processes  600 N and  600 A can each be similar to the steps performed during the phases  352 - 355  for both the timelines  401 - 402  in the process  400  shown in  FIG.  4   . 
     For example, in the process  600 N, MAC commands  642 - 1   n  and  642 - 2   n  indicating TCI states for transmissions of PDCCH and PDSCH, respectively, can be received after a period of T uncertainty_MAC    611   n . A HARQ ACK corresponding to the last of the MAC commands  642 - 1   n  and  642 - 2   n  can be transmitted incurring a delay of T HARQ    615   n . Thereafter, a MAC CE processing and application process can take a time of T MAC_CE    612   n  followed by a fine frequency and timing synchronization process lasting for a period of T FineTime    613   n  plus T SSB    614   n.    
     For CSI-RS activation and corresponding TCI state indication, a MAC command  672   n  can be received after a period of T uncertainty_MAC . In response, a HARQ ACK can be transmitted after a delay of T HARQ    662   n . Then, a MAC CE processing and application process can last for a period of T MAC_CE    663   n . After the above fine timing synchronization and CSI-RS MAC CE application are both completed, a CSI reporting process can be performed which may cause a delay of T CSI_reporting    603   n.    
     For the process  600 A, the steps as shown in  FIG.  6    can be performed in a way similar to those of the process  600 N. For example, the delays  611   a - 615   a ,  661   a - 663   a , and  603   a  in the process  600 A can take place similarly as the counter parts  611   n - 615   n ,  661   n - 663   n , and  603   n  in the process  600 N. 
     In addition, for the process  600 A, in some examples, the TCI configurations for the unknown SCell  120   a  can be different from that for the known SCell  120   n  because each serving cell&#39;s timing/Doppler configuration RS (e.g., SSB or CSI-RS) can be configured independently. Accordingly, the TCI and CSI-RS configurations (e.g., the corresponding MAC commands) for each of the SCell  120   n  and  120   a  may not come at the same time. 
     Accordingly, for the process  600 A for activating the unknown SCell  120   a , the delay T activation_time  in the expression (1) can be estimated as
 
 T   MAC_CE (612 a or 663 a )+max( T   uncertainty_MAC (611 a )+ T   FineTime (613 a )+ T   SSB (614 a ),  T   uncertainty_SP (661 a )).
 
When T MAC_CE  and T SSB  take the maximum values of 3 ms and 2 ms, respectively, set for bounding the SCell activation delay, a maximum value of the delay e a can be estimated to be
 
3 ms+max( T   uncertainty_MAC (611 a )+ T   FineTime (613 a )+2 ms ,T   uncertainty_SP (661 a )).
 
     In some examples, instead of SP CSI-RS, periodic CSI-RS can be used for CSI measurement and reporting on to-be-activated SCells. Corresponding to the periodic CSI-RS, RRC signaling can be used to indicate the corresponding CSI-RS resource and TCI state in place of the CSI-RS activation MAC commands  672   n  or  672   a  in the  FIG.  6    example. The process  600 A (similarly for the process  600 N) can be adjusted in the following way. 
     An RRC message, for example, carried in a PDSCH, can be received at the place of the MAC command  672   a  in the corresponding timeline  604  after a delay of T uncertainty_RRC . Then, a process of processing and applying the configuration of the RRC message can be performed which can cause a delay T RRC_delay . The delay can correspond to the periods  662   a - 663   a . Accordingly, when periodic CSI-RS is employed in the to-be-activated unknown SCell  120   a , the delay T activation_time  in the expression (1) can be estimated as
 
max( T   uncertainty_MAC (611 a )+ T   MAC_CE (612 a )++ T   FineTime (613 a )+ T   SSB (614 a ),  T   uncertainty_MAC (661 a )+ T   RRC_delay (662 a/ 663 a )− T   HARQ (601 or 662 a )).
 
When T MAC_CE  and T SSB  take the maximum values 3 ms and 2 ms, respectively, set for bounding the SCell activation delay, a maximum value of the delay T activation_time  can be estimated to be
 
max( T   uncertainty_MAC (611 a )+ T   FineTime (613 a )+5 ms ,T   uncertainty_RRC (661 a )+ T   RRC_delay (662 a/ 663 a )− T   HARQ (601 or 662 a )).
 
       FIG.  7    shows another multi-SCell activation process  700  according to an embodiment of the disclosure. During the process  700 , similar to the process  600 , one MAC CE command is received for activating two SCells operating in a same band in FR2. However, different from the process  600 , both the two to-be-activated SCells are unknown. In addition, no active serving cells operate in the same FR2 band. The  FIG.  1    example is used to explain the process  700 . The two SCells can be a first SCell  120   n  and a second SCell  120   a  that are both unknown SCells. 
     As no active serving cell or known SCell in the same FR2 band is available, the base station  105  has no knowledge about qualities of Tx beams from the base station  105 , and thus has no basis for determining TCI states for channels and RSs transmitted from the base station  105 . In addition, the UE  101  cannot determine its frame timing based on a known SCell or active serving cell. Accordingly, the UE  101  can determine to perform AGC settling, cell search, and L1-RSRP measurement and report on one (e.g., SCell  120   n ) of the two intra-band FR2 SCells  120   n  and  120   a . For the other SCell  120   a , the UE  101  can hold the activation procedure until the L1-RSRP report process on the SCell  120   n  has been completed. 
     Thereafter, the SCell  120   n  becomes a known SCell. The SCells  120   n  and  120   a  can continue the activation process  700  in a way similar to the process  600  in the  FIG.  6    example. 
     Specifically, as shown in  FIG.  7   , a MAC command  741  for activating both the SCells  120   n  and  120   a  is received and acknowledged during T HARQ    701 , and processed and applied during T MAC_CE    771   n . Then, ACG settling and cell search are performed during a period  772   n . As a result, in an example, AGC gain is adjusted for both the SCells  120   n  and  120   a , and the RF module of SCells  120   a  is time/frequency synchronized to the SCell  120   n . Thereafter, an L1-RSRP measurement and report process can be performed during the periods  773   n - 774   n , and completed by the time T 1 . 
     After the time T 1 , activation processes for the SCells  120   n  and  120   a  can be carried out in parallel. The corresponding time periods  711   n - 715   n ,  761   n - 763   n ,  703   n ,  711   a - 715   a ,  761   a - 763   a , and  703   a  are shown in  FIG.  7   . 
     In some examples, one MAC CE command is used to activate multiple inter-band FR2 SCells. The inter-band FR2 SCells may or may not be co-located. Timing or beam quality information of a known SCell may or may not be used for activating an unknown SCell. 
       FIG.  8    shows a single SCell activation process  800  according to an embodiment of the disclosure. The  FIG.  1    example is used for explanation of the process  800 . During the process  800 , the SCell  120   a  is an unknown SCell operating on an FR1 band, and is to be activated. In addition, no known cell or active serving cell is available on the FR1 band. Thus, the SCell  120   a  has no other cells to rely on while being activated. 
     For example, whether an SCell in FR1 is known or unknown is defined as follows. An SCell in FR1 is known if the SCell meets the following conditions:
         (i) During a certain period (e.g., equal to max(5*measCycleSCell, 5*DRX cycles) for FR1) before the reception of an SCell activation command, the UE has sent a valid measurement report for the SCell being activated and the SSB measured remains detectable according to certain cell identification conditions.   (ii) the SSB measured during the period equal to max(5*measCycleSCell, 5*DRX cycles) also remains detectable during the SCell activation delay according to the certain cell identification conditions.
 
Otherwise the SCell in FR1 is unknown.
       

     As the to-be-activate SCell  120   a  operates in FR1, in some examples, beamforming is not employed. Under such a configuration, L-RSRP measurement and report for indicating beam qualities are not performed in the process  800 . In addition, the TCI state indication scheme is not utilized, and thus there is no need to provide MAC CE commands for indicating TCI states for downlink control or data channels, or CSI-RS reception in the process  800 . However, as the SCell  120   a  is an unknown SCell, AGC tuning, cell search, and time-frequency fine tuning are still performed. 
     Specifically, during the process  800 , a PDSCH  831  carrying a MAC command  841  for activating the SCell  120   a  can be received at the U E  101  in the PCell  110 . A HARQ ACK  832  can be transmitted from the UE  101  after T HARQ    801 . During the period T MAC_CE    811 , a MAC CE parsing process can be performed to obtain the MAC command  841 ; and, accordingly, a software application and RF warm up process can be performed. 
     Thereafter, during the period T CellSearch    812 , the UE  101  can wait for one or more SSB on the SCell  120   a  to perform AGC adjustment, and wait for another SSB to perform a time/frequency synchronization (cell detection). Subsequently, a fine time/frequency retuning can be performed during the period T SSB    814  after a first SSB is available in the SCell  120   a  at the end of the period T FineTime    813 . After the fine time retuning operation, a CSI measurement and report process can be performed during the period T CSI_Reporting    803 . The CSI measurement can be based on SSB or CSI-RS. 
     As shown, a delay period incurred during the process  800 , denoted T activation_time    802 , can be a sum of the periods  811 - 814 . The process  800  can last for a period that is a sum of the periods  801 - 803 . 
       FIG.  9    shows a multi-SCell activation process  900  according to an embodiment of the disclosure. Using the  FIG.  1    example for explanation of the process  900 , the multiple SCells to be activated in the process  900  can be a first SCell  120   n  that is known, and a second SCell  120   a  that is unknown. In addition, the two SCells  120   a  and  120   n  can be FR1 intra-band SCells operating in a same FR1 band. Further, no active serving cells are available in the same FR1 band. 
     As the SCell  120   n  is a known SCell, an AGC tuning and a cell search can have been performed recently. Accordingly, during the process  900 , no AGC tuning or cell search is performed. Previously obtained parameters for AGC tuning or time/frequency synchronization (e.g., frame timing) can be reused. 
     For the SCell  120   a  that is unknown, assuming the SCell  120   n  is not available, AGC tuning and cell search would be performed during an activation process for activating the SCell  120   a  (similar to the process  800 ). As the known SCell operates in the same FR1 band as the unknown SCell  120   a , the SCell  120   a  can rely on previously obtained information of the known SCell  120   n  to expedite its activation process. 
     For example, a same RF circuit (e.g., an amplifier) can be shared by the SCells  120   n  and  120   a , and AGC tuning can be done for the SCells  120   n  and  120   a  at the same time. For another example, as the SCells  120   n  and  120   a  are in the same FR1 band, an MRTD can be configured to be within a predefined range (e.g., 260 ns). The unknown SCell  120   a  can thus determine its frame timing based on frame timing of the known SCell  120   n , and use this timing as basis to perform a fine time/frequency tuning. 
     Specifically, during the process  900 , a PDSCH  931  can be received at the UE  101  which carries a MAC CE command  941 . After a delay of T HARQ    910 , a HARQ ACK  932  can be fed back to the base station  105 . Then, during a period of T MAC_CE    911 , a MAC CE carried in the PDSCH  931  can be parsed at MAC layer to obtain the MAC CE command  941 . The MAC CE command  941  may indicate cell IDs of the SCells  120   n  and  120   a . Also during the period of T MAC_CE    911 , an RF module warming up process can be performed. The respective RF module can be adjusted for reception on both the SCells  120   n  and  120   a.    
     Thereafter, similar activation processes on the SCells  120   n  and  120   a  can be performed in parallel on the timelines  901  and  902 , respectively, as shown in  FIG.  9   . For example, for the SCell  120   n , the UE  101  can wait for a first available SSB during the period T FineTime    913   n , and perform fine time tuning during the period T SSB    914   n . After the fine time tuning, a CSI measurement and reporting process can be performed during the period T CSI_reporting    903   n.    
     In a similar way, the UE  101  may perform respective operations during the periods  913   a ,  914   a , and  903   a  to complete the activation process of the SCell  120   a . As shown, an activation delay, T activation_time ,  902 , can be a sum of the periods  911 ,  913   n , and  914   n  for the SCell  120   n , or a sum of the periods  911 ,  913   a , and  914   a  for the SCell  120   a.    
       FIG.  10    shows a multi-SCell activation process  1000  according to embodiments of the disclosure. The process  1000  can start from S 1001 , and proceed to S 1010 . 
     At S 1010 , a first MAC CE can be received on a PCell at a UE for activating a first SCell and a second SCell for the UE in a wireless communication system. For example, the MAC CE can included a MAC CE command indicating identities of the first and second SCells. The first and second SCells can operate in a same band (e.g., a FR2 band or a FR1 band). No active serving cell operates on the same band for the UE. 
     At S 1020 , in response to that the first SCell is a known SCell, the second SCell is an unknown SCell, and both the first and second SCells operate in the same band that is a FR2 band, the first and second SCells can be activated in parallel without performing cell search and RSRP measurement and reporting over the first and second SCells. 
     For example, the UE can determine whether the first SCell is a known SCell, or the second SCell is an unknown SCell based on historic operations performed on the first or second SCells. The UE can determine whether the first and second SCells operate in the same FR2 band based on related configurations. Accordingly, in response to the first SCell being known, the UE can determine to activate the unknown second SCell without performing the cell search or RSRP measurement and reporting operations. In this way, activation of the unknown second SCell can be expedited. The process  1000  can proceed to S 1099 , and terminate at S 1099 . 
       FIG.  11    shows an apparatus  1100  according to embodiments of the disclosure. The apparatus  1100  can be configured to perform various functions in accordance with one or more embodiments or examples described herein. Thus, the apparatus  1100  can provide means for implementation of mechanisms, techniques, processes, functions, components, systems described herein. For example, the apparatus  1100  can be used to implement functions of UEs or base stations in various embodiments and examples described herein. The apparatus  1100  can include a general purpose processor or specially designed circuits to implement various functions, components, or processes described herein in various embodiments. The apparatus  1100  can include processing circuitry  1110 , a memory  1120 , and a radio frequency (RF) module  1130 . 
     In various examples, the processing circuitry  1110  can include circuitry configured to perform the functions and processes described herein in combination with software or without software. In various examples, the processing circuitry  1110  can be a digital signal processor (DSP), an application specific integrated circuit (ASIC), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), digitally enhanced circuits, or comparable device or a combination thereof. 
     In some other examples, the processing circuitry  1110  can be a central processing unit (CPU) configured to execute program instructions to perform various functions and processes described herein. Accordingly, the memory  1120  can be configured to store program instructions. The processing circuitry  1110 , when executing the program instructions, can perform the functions and processes. The memory  1120  can further store other programs or data, such as operating systems, application programs, and the like. The memory  1120  can include non-transitory storage media, such as a read only memory (ROM), a random access memory (RAM), a flash memory, a solid state memory, a hard disk drive, an optical disk drive, and the like. 
     In an embodiment, the RF module  1130  receives a processed data signal from the processing circuitry  1110  and converts the data signal to beamforming wireless signals that are then transmitted via antenna arrays  1140 , or vice versa. The RF module  1130  can include a digital to analog converter (DAC), an analog to digital converter (ADC), a frequency up converter, a frequency down converter, filters and amplifiers for reception and transmission operations. The RF module  1130  can include multi-antenna circuitry for beamforming operations. For example, the multi-antenna circuitry can include an uplink spatial filter circuit, and a downlink spatial filter circuit for shifting analog signal phases or scaling analog signal amplitudes. The antenna arrays  1140  can include one or more antenna arrays. 
     The apparatus  1100  can optionally include other components, such as input and output devices, additional or signal processing circuitry, and the like. Accordingly, the apparatus  1100  may be capable of performing other additional functions, such as executing application programs, and processing alternative communication protocols. 
     The processes and functions described herein can be implemented as a computer program which, when executed by one or more processors, can cause the one or more processors to perform the respective processes and functions. The computer program may be stored or distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with, or as part of, other hardware. The computer program may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. For example, the computer program can be obtained and loaded into an apparatus, including obtaining the computer program through physical medium or distributed system, including, for example, from a server connected to the Internet. 
     The computer program may be accessible from a computer-readable medium providing program instructions for use by or in connection with a computer or any instruction execution system. The computer readable medium may include any apparatus that stores, communicates, propagates, or transports the computer program for use by or in connection with an instruction execution system, apparatus, or device. The computer-readable medium can be magnetic, optical, electronic, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. The computer-readable medium may include a computer-readable non-transitory storage medium such as a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a magnetic disk and an optical disk, and the like. The computer-readable non-transitory storage medium can include all types of computer readable medium, including magnetic storage medium, optical storage medium, flash medium, and solid state storage medium. 
     While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.