n the downlink than in the uplink. Furthermore, the RF implementation complexity and cost of operating multiple simultaneously active uplink carriers are often higher than the cor- responding complexity/cost of the downlink. The scheduling grants and radio resource assign- ments can be transmitted on either the same cell as the corresponding data, referred to as self- scheduling, or on a different cell than the corresponding data, referred to as cross-carrier scheduling. The NR carrier aggregation uses L1/L2 control signaling for scheduling the UE in the downlink, and uplink control signaling to transmit HARQ-ACKs. The uplink feedback is typically transmitted on the PCell to allow asymmetric carrier aggregation. In certain use cases where there are a large number of downlink component carriers and a single uplink component carrier, the uplink carrier would be overloaded with a large number of feedback information. To avoid overloading a single carrier, it is possible to configure two PUCCH groups where the feedback corresponding to the first group is transmitted in the uplink of the PCell and the feedback corresponding to the other group of carriers is transmitted on the PSCell. If carrier aggregation is enabled, the UE may receive and transmit on multiple 282 Chapter 2 carriers, but operating multiple carriers is only needed for high data rates, thus is advanta- geous to deactivate unused carriers. Activation and deactivation of component carriers can be done through MAC CEs (see Section 2.2.1), where a bitmap is used to indicate whether a configured SCell should be activated or deactivated. As we mentioned earlier, to ensure reasonable UE power consumption when carrier aggre- gation is configured, an activation/deactivation mechanism of cells is supported. When an SCell is deactivated, the UE no longer needs to receive the corresponding PDCCH or PDSCH, it cannot transmit in the corresponding uplink, it is not required to perform CQI measurements on that cell. On the other hand, when an SCell is activated, the U