In other embodiments for unlicensed carrier operation, the eNB and the activated UE can maintain a DL time synchronization timer. The DL time synchronization timer can be set to a value Xms (e.g., X=10 or 20 ms, the value of X depends on time for which UE can autonomously maintain time-frequency sync without receiving any eNB transmissions). The subframe in which the DL time synchronization timer is set to Xms can be any subframe in which the eNB transmits PDSCH to the UE (from UE perspective, it will be any subframe where PDSCH is received from the eNB). Alternately, the subframe can be any subframe where the UE transmits a positive acknowledgement (HARQ-ACK) in response to a PDSCH received from the eNB (in this case, from eNB perspective, it will be any subframe in which a positive acknowledgement is received from the UE). The DL time synchronization timer is then counted down in subsequent subframes. When the DL time synchronization timer expires (i.e., when it becomes 0), the eNB determines that the UE may not have fine time frequency synchronization. Although, the UE may not have fine time frequency synchronization, the UE can continue to monitor Scell subframes for control signaling, as it may be able to decode some PDCCH/EPDCCH and/or PDSCH messages (albeit with a higher bit error rate) When the eNB determines the UE may not have fine time frequency synchronization, the eNB can schedule PDSCH transmissions with a conservative PDSCH assignment (e.g., using MCS corresponding to QPSK and one spatial layer instead of 64QAM and multiple spatial layers), for a few subframes (e.g., 1 or 2 subframes) until the UE regains fine time/frequency sync to decode more aggressive PDSCH assignments. This alternative avoids the overhead of re-activation command transmission on the PCell (described below). In the alternative, the eNB can send an activation command (in this example, a re-activation command) to the UE on the PCell. Upon reception of the activation command on the Pcell in subframe n, the UE determines that reference signal transmissions are present in at least subframes (n) to n+p (e.g., p=1 or 2) and regains fine time-frequency sync using the signals in those subframes.