THE Fire Services Department's radio communication system suffers serious interference, occupying 40 per cent of the airtime of the already congested channels. In a paper to legislators, the Government said all 15 channels of the radio system had experienced interference in recent years, mainly from mainland cities and from local sources generated by illegal radio operators in the transport industry. Although the Telecommunications Authority had tried to negotiate with the mainland telecommunications authority, little improvement had been achieved. It said any interference would inevitably delay the response to emergency calls. The efficiency of the Fire Services Communication Centre was also hit because radio messages became unintelligible, resulting in errors and delay in receiving crucial information. Interference in the form of crackling noises and irrelevant conversations also distracted the communication centre's operators from discerning the content of the incoming message. ''The situation is highly unsatisfactory as accurate data on which instant decisions have to be made by operational personnel at the scene of incidents are vital when life and property at stake,'' said the administration. It said, given the rapid pace of development in the territory, the number of fire and ambulance calls had doubled in 13 years to 468,848 in 1992. There was keen competition for free channel time among radio users. The Director of Fire Services, Lam Chek-yuen, proposed to legislators to replace the existing network with a trunked radio system, which would cost $69.5 million. He hoped the new system would be installed by September 1995.