As mentioned above, in FDMA non-overlapping frequency bands are allocated to different users on a continuous time basis. Hence, signals assigned to different users are clearly orthogonal, at least ideally. In practice, out-of-band spectral components can not be completely suppressed leaving signals not quite orthogonal. This necessitates the introduction of guard bands between frequency bands to reduce adjacent channel interference, i.e., inference from signals transmitted in adjacent frequency bands (see also Figure 1(a)).
It is advantageous to combine FDMA with time-division duplexing (TDD) to avoid simultaneous reception and transmission which would require insulation between receive and transmit antennas. In this scenario, the base station and portable take turns using the same frequency band for transmission. Nevertheless, combining FDMA and frequency division duplex is possible in principle as is evident from the analog FM-based systems deployed throughout the world since the early 1980's.
