A method and device for determining available communication sub-channels in an OFDM communication system is disclosed. The method comprises the steps of transmitting, on at least one first sub-channel (207), information (210) regarding sub-channels available for a first transmission of at least one first data packet (245), receiving, on at least one second sub-channel (250), information (225) regarding sub-channels available for a second transmission, determining at least one set of available sub-channels based on the information regarding the first and second transmissions (430). In one aspect of the invention, at least one set of the determined available sub-channels is further provided to a receiving system. The information is being provided in a separate transmission, or within a data packet, or within each subsequent data packet or in selected data packets.