Systems and methods relating to a transmission timing difference between cells in a multi-carrier system in which at least one of the cells is subject to Clear Channel Assessment (CCA) before transmitting are disclosed. In some embodiments, a method of operation of a User Equipment (UE) comprises determining a transmission timing difference between a first cell that operates on a first carrier and a second cell that operates on a second carrier, wherein CCA is required to be performed on at least one of a first channel in which the first carrier is located and a second channel in which the second carrier is located before the UE is permitted to transmit. The method further comprises using the transmission timing difference for one or more operational tasks of the UE. The transmission timing difference is determined when the first channel and the second channel are available at the UE.