Each cellphone uses a separate temporary radio channel to talk to the cell site. The site of the cell speaks with many mobiles at the same time, using a channel by mobile. The channels use a pair of frequencies for communication. One to transmit from the cell site, the direct link, and a frequency for the cell site to receive calls from the users, the reverse link.
Communication between mobile units can be half-duplex or full-duplex. In the case of half-duplex, the transmission and reception communications between the mobile units are not at the same time, ie talking and listening can not be done at the same time. In case of full-duplex communication, the transmission and reception communication is at the same time, that is, you can speak and listen at the same time.
When communications between mobile units are within a cell, and if the cell is half-duplex, then it will require only one frequency pair. If it is full-duplex, then the frequency pair requirement will be two.
When a mobile unit communicates with a mobile unit outside the cell, then the frequency pair requirement will be one per cell for both half-duplex communication and full-duplex communication. Therefore, system resources are used more if mobile units communicate with each other in full duplex mode.