Duplex operation is process of communication simultaneously in both directions of a telecommunication channel. Mobile radio systems typically require two-way duplex communication. To allow two-way communications, mobile systems are either frequency division duplex (FDD), time division duplex (TDD) or combined FDD and TDD. TDD systems can use FDMA, TDMA or CDMA access technologies.

Frequency division duplex (FDD) systems use two frequencies to allow simultaneous communication. One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction.

Time division duplex (TDD) communication uses a single frequency to provide two-way communications between devices by time-sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time (usually within 100 ms). 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.

Figure 1.13 shows the different types of duplex systems. Frequency division duplex is a communications channel that allows the transmission of information in both directions (not necessarily at the same time) via separate bands (frequency division). Time division duplex (TDD) is a process of allowing 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...