A method in a wireless device for performing an uplink transmission to a base station in a wireless access system, wherein the wireless access system comprises one or more mini-slot configurations, the method comprising performing a clear channel assessment for identifying a transmission opportunity and determining the start of a mini-slots. The wireless device performing an uplink transmission within the determined mini-slot wherein the transmission is performed according to a predetermined configuration for mini-slot communication between the base station and the wireless device, such that the base station can identify the start of the uplink transmission.