In TDMA systems users share the same frequency band by accessing the channel in
non-overlapping time intervals in a round robin fashion
[Falconer, Adachi, and Gudmundson, 1995].
Since the signals do not overlap they are clearly orthogonal and the signal of
interest is easily extracted by switching the receiver on only during the
transmission of the desired signal.
Hence, the receiver ``filters'' are simply windows instead of the bandpass
filters required in FDMA.
As a consequence, the guard time between transmissions can be made as small as
the synchronization of the network permits.
Guard times of 30-50 
s between time slots are commonly used in TDMA based
systems.
As a consequence, all users must be synchronized with the base station to within
a fraction of the guard time
which is achievable by distributing a master clock signal on one of the base
station's broadcast channels.
TDMA can be combined with time-division duplexing (TDD) or frequency-division duplexing (FDD). The former duplexing scheme is used for example in the Digital European Cordless Telephone (DECT) standard and is well suited for systems in which base-to-base and mobile-to-base propagation paths are similar, i.e., systems without extremely high base station antennas. Since both the portable and the base station transmit on the same frequency, some signal processing functions for the down-link can be implemented in the base station, as discussed above for FDMA/TDD systems.
In the cellular application the high base station antennas make FDD the more appropriate choice. In these systems, separate frequency bands are provided for up-link and down-link communication. Notice, that it is still possible and advisable to stagger the up-link and down-link transmission intervals such that they don't overlap to avoid that the portable must transmit and receive at the same time. With FDD the up-link and down-link channel are not identical and hence signal processing functions can not be implemented in the base-station; antenna diversity and equalization have to be realized in the portable.
