nly exposed to the MAC sublayer for which one HARQ entity is required per serving cell. In both uplink and downlink, there is one independent HARQ entity per serving cell and one TB is generated per TTI in the absence of spatial multiplexing. Each TB and its associated HARQ retransmissions are mapped to a single serving cell [11]. Fig. 4.56 depicts HARQ-ACK timing requirements in a cross-carrier scheduling scenario when a UE receives and transmits to from/to two gNBs. 544 Chapter 4 Cross-carrier scheduling Control CC1 (30 kHz SCS) Channel 30.26 us UE processing time (10 symbols + TA) UL CC1 DL CC1 CC2 (30 kHz SCS) Slot (500 us) DL CC2 Same-carrier scheduling CC1 (30 kHz SCS) 30.26 us UE processing time (10 symbols + TA) Control CC2 (30 kHz SCS) channel Slot (500 us) Figure 4.56 UE processing time considering timing difference between different cells [65]. In the NR downlink, retransmissions are scheduled in the same way as new data, that is, they may occur at any time and at an arbitrary frequency location within the downlink cell bandwidth. The scheduling assignment contains the necessary HARQ-related control signal- ing such as process number, new-data indicator, CBGTI, and CBGFI in case of CBG-based retransmission as well as information to handle the transmission of the acknowledgment in the uplink such as timing and resource indication. Upon receiving a scheduling assignment in the DCI, the receiver would attempt to decode the TB after soft combining with previous retransmissions. Since transmissions and retransmissions are scheduled using the same framework, the UE needs to know whether the transmission is a new transmission, in which case the soft buffer should be flushed, or a retransmission, in which case soft combining should be performed. Therefore, an explicit new-data indicator is included for the scheduled TB as part of the scheduling information transmitted in the downlink. The new-data indica- tor is toggled for a new TB. Upon reception of a downlink scheduling assignment, the UE checks the new-dat