ata or PDCCH messages are received on a SCell for a certain period of time. This is the only case in which deactivation can be executed autonomously by the UE. Serving cell activation/deactivation is performed independently for each SCell, allowing the UE to be activated only on a particular set of SCells. Activation/deactivation is not applica- ble to the PCell because it is required to always remain activated when the UE has an RRC connection to the network [8]. New Radio Access Layer 2/3 Aspects and System Operation 283 PCell Data transmission Data transmission MAC activation SCell 1 command Data transmission Data transmission deactivation control ellement command control element Short DRX Long DRX SCell 2 Data transmission Deactivation timer Figure 2.32 Illustration of SCell activation/deactivation procedure [9]. As already mentioned, if the UE is configured with one or more SCells, the network may activate and deactivate the configured SCells. The PCell is always activated. The network activates and deactivates the SCell(s) by sending an activation/deactivation MAC CE described in Section 2.2.1. Furthermore, the UE maintains a sCellDeactivationTim timer per configured SCell (except the SCell configured with PUCCH) and deactivates the associ- ated SCell upon its expiration. The same initial timer value is applied to each instance of the sCellDeactivationTimer and it is configured by RRC signaling. The configured SCells are initially deactivated upon addition and after a handover. The HARQ feedback for the MAC PDU containing SCell activation/deactivation MAC CE is not impacted by PCell, PSCell, and PUCCH SCell interruptions due to SCell activation/deactivation [9]. References 3GPP Specifications12 [1] 3GPP TS 23.501, System Architecture for the 5G System (Release 15), December 2018. [2] 3GPP TS 33.501, Security Architecture and Procedures for 5G System (Release 15), December 2018. [3] 3GPP TS 37.324, NR, Service Data Adaptation Protocol (SDAP) Specification (Release 15), September 2018. [4] 3GPP TS 37.340, Mul