Duplex communication is the transmission of voice and/or data signals that allows simultaneous 2-way communication. To provide duplex communication on analog systems, each voice path was assigned to a different transmitter and frequency. This process of using two frequencies for duplex communication is called frequency division duplex (FDD). Another method that can be used for duplex communication is time division duplex (TDD). TDD provides two way communications between two devices by time sharing.

When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously.

The GSM system uses a combination of FDD and TDD communication. One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction. However, the GSM system also uses TDD as the transmitter and receiver communicate at different times. The time offset of transmission and reception simplifies the design of the mobile device (less radio filter parts).

The radio frequency separation between the forward (downlink) and reverse (uplink) frequencies varies based on the frequency band. In general, the higher the frequency, the larger the frequency separation between the forward and reverse channels. For GSM 900 MHz, the frequency separation is 45 MHz, for PCN the frequency separation is 95 MHz...