A half-duplex radio device (100) generates a repetitive transmission of an uplink message to a base station (150) of the cellular network. The radio device (100) splits the repetitive transmission of the uplink message into a sequence of multiple transmit periods and configures at least one measurement gap between the transmit periods. In the transmit periods, the radio device (100) sends the repetitive transmission to the base station. In the at least one measurement gap configured between the transmit periods, the radio device (100) temporarily switches to receiving at least one downlink signal from the base station (150). Based on the received at least one downlink signal, the radio device (100) estimates a frequency error of a reference frequency source of the radio device (100).