There is provided mechanisms for managing uplink transmission of user data. A method is performed by a baseband unit. The baseband unit is operatively connected to at least two radio units. The method comprises sending a control message to the at least two radio units. The control message comprises parameters pertaining to transmission of uplink user data from the at least two radio units to the baseband unit. The parameters specify a transmission window per each of the at least two radio units for the transmission of uplink user data to the baseband unit. The method comprises receiving uplink user data from the at least two radio units in accordance with the parameters.