A radio communication system includes a communication channel for the transmission of data packets from a primary station to a secondary station. In operation, on detection of a data packet, the secondary station transmits an acknowledgement signal to the primary station to indicate the status of the received data packet, and resets a timer. While the timer is running, the secondary station modifies a characteristic of uplink transmissions. In one embodiment the modification includes the secondary station transmitting a negative acknowledgement for each time at which a data packet could have been transmitted if no indication of a data packet is received. In another embodiment the modification includes the secondary station altering a parameter relating to transmission of channel quality information to the primary station.