ch the eNB may transmit. To transmit on a carrier Ci # CiVciEC, the eNB performs carrier sensing for a minimum sensing interval Tmc 25 us prior to trans- mitting on carrier Cj, and it may transmit on carrier Ci immediately after sensing the car- rier Ci to be idle for at least the sensing interval Tmc. The carrier Ci is considered to be idle for Tmc if the channel is sensed to be idle during all time intervals in which such idle sensing is performed on carrier Cj in a given time interval Tmc. The eNB will not continuously transmit on a carrier Ci # CiTCiEC for a period exceeding T'MCOT' where the value of the latter parameter is determined using the channel access class used for carrier Cj. In Type B1, a single CWp value is maintained for the set of carriers C. For determining CW for performing channel access on carrier Cj, if at least 80% of HARQ feedback corresponding to PDSCH transmission(s) in reference subframe k of all car- riers CiEC are determined as NACK, then CW is increased for each priority class pe {1, 2, 4} to the next higher permissible value. In Type B2, a CW value is main- tained independently for each carrier C. For determining Ninit for the carrier Cj, CW value of carrier Cj1 EC is used, where Cj1 is the carrier with largest CW among all car- riers in set C. Discontinuous transmission on an unlicensed carrier with limited maximum transmis- sion duration has an impact on some LTE/NR essential functionalities that support AGC setting, time and frequency synchronization of the UEs, and channel reservation. Channel reservation refers to the transmission of signals by an LAA node after gaining channel access via a successful LBT operation, SO that other nodes that receive the transmitted signal with energy above a certain threshold sense the channel to be occupied. 902 Chapter 8 8.4 Layer 2/3 Aspects In this section, we discuss the L2/L3 aspects and impacts of unlicensed band operation in LTE and NR-based systems. The uncertainty of the cell availability for HARQ retransmis- sions on unlicensed carr