A method and apparatus can be configured to determine that a sub-symbol slot is an unoccupied sub-symbol slot. The unoccupied sub-symbol slot corresponds to an unoccupied operating channel. The unoccupied sub-symbol slot is located within the duration of a first symbol. The method may also comprise transmitting a reservation signal in at least one sub-symbol slot between the unoccupied sub-symbol slot and the beginning of a second symbol. The number of sub-symbol slots within the duration of the first symbol is larger than one.