Crossband operation
Crossband (cross-band, cross band) operation is a method of telecommunication in which a radio station receives signals on one frequency and simultaneously transmits on another for the purpose of full duplex communication or signal relay. To avoid interference within the equipment at the station, the two frequencies used need to be separated, and ideally on different 'bands'. An unattended station working in this way is a radio repeater. It re-transmits the same information that it receives. This principle is used by telecommunications satellites and terrestrial mobile radio systems. Uses. Crossband operation is sometimes used by amateur  radio operators. Rather than taking it in turns to transmit on the same frequency, both operators can transmit at the same time but on different bands, each one listening to the frequency that the other is using to transmit. A variation on this procedure includes establishing contact on one frequency and then changing to a pair of other frequencies to exchange messages. Crossband operation is also used in communication between ships (intership) with a HF installation. Frequencies that may be used can be found in the 'Manual for use by the Maritime Mobile and Maritime Mobile-Satellite Services'. Usually intership  communication is simplex only (VHF or MF), HF gives the possibility to work duplex but usually the transmitter and receiver are so close to each other that this may cause problems. The solution is to work on frequencies that are far apart e.g.: sending on 8MHz and receiving on 12MHz. Further reading. Additional articles on crossband repeat usage and setup: