Methods of operation of a wireless device are provided. In particular, Media Access Control (MAC) protocol aspects are disclosed relating to Licensed Assisted Access (LAA) cells and, more generally, to cells of a cellular communications network operating in an unlicensed frequency spectrum. According to one aspect, a method of operation of a wireless device comprises transmitting an Uplink (UL) transmission on a cell for a corresponding UL Hybrid Automatic Repeat Request (HARQ) process, the cell operating in an unlicensed frequency spectrum, and setting a locally maintained status for the UL HARQ process to Acknowledgement (ACK) based on an assumption that the UL transmission was successful.