In wireless communications systems, it is often important that users be allowed to transmit information to the base station
and receive information from the base station simultaneously. This effect is called duplexing. Duplexing may be done in the
frequency domain or in the time domain.
In the frequency division duplex (FDD) systems two distinct bands of frequencies are provided for every user (Fig. 2.1a).
The forward channel (downlink) provides traffic from the base station to the mobile and the reverse channel (uplink) provides
traffic from the mobile to the base station. In FDD any duplex channel actually consists of two simplex channels. For the given
system, the frequency split between the forward and reverse channel is constant. To allow simultaneous radio transmission
and reception on the duplex channel pair using the same antenna, the device called duplexer is used in each mobile unit and
In time division duplex (TDD) systems, the time instead of frequency is used to separate the forward and reverse channel
(Fig. 2.1b). Because the time split between the forward and reverse channel is usually very small, then the transmission and
reception of data appears simultaneous to the user. TDD allows communication on a single frequency channel and simplifies
the user equipment since a duplexer is not required.
Fig. 2.1. Duplexing techniques: a) FDD, b) TDD.