A transmitting apparatus comprising: a storage unit which stores a first known signal defined in a first wireless communication system and a second known signal defined in a second wireless communication system, wherein the second wireless communication system is a Multiple-Input-Multiple-Output MIMO system, and the first wireless communication system is a system that is not compatible with a MIMO system; a selector which selects either a first packet format in which the second known signal is placed in a front portion thereof or a second packet format in which the first known signal is further placed before the second known signal; a transmitter which transmits signals in the packet format selected by said selector; and a monitoring unit configured for monitoring the presence of a communication apparatus which is not compatible with the second wireless communication system but is compatible with the first wireless communication system, wherein said selector is configured for selecting either said first packet format or said second packet format based on a monitoring result obtained by said monitoring unit; wherein: - if the monitoring unit has not detected any communication apparatus compatible with the first wireless communication system, the selector selects the first packet format; - if the monitoring unit has detected a communication apparatus compatible with the first wireless communication system, the selector selects the second packet format. a storage unit which stores a first known signal defined in a first wireless communication system and a second known signal defined in a second wireless communication system, wherein the second wireless communication system is a Multiple-Input-Multiple-Output MIMO system, and the first wireless communication system is a system that is not compatible with a MIMO system; a selector which selects either a first packet format in which the second known signal is placed in a front portion thereof or a second packet format in which the first known signal is further placed before the second known signal; a transmitter which transmits signals in the packet format selected by said selector; and a monitoring unit configured for monitoring the presence of a communication apparatus which is not compatible with the second wireless communication system but is compatible with the first wireless communication system, wherein said selector is configured for selecting either said first packet format or said second packet format based on a monitoring result obtained by said monitoring unit; wherein: - if the monitoring unit has not detected any communication apparatus compatible with the first wireless communication system, the selector selects the first packet format; - if the monitoring unit has detected a communication apparatus compatible with the first wireless communication system, the selector selects the second packet format. - if the monitoring unit has not detected any communication apparatus compatible with the first wireless communication system, the selector selects the first packet format; - if the monitoring unit has detected a communication apparatus compatible with the first wireless communication system, the selector selects the second packet format.