In this technique, disjoint sub bands of frequencies are allocated to different users on a continuous-time basis. Each user is allocated a unique frequency band in which to transmit and receive on. During a call, no other user can use the same frequency band. In order to reduce interference between users allocated different channel bands, guard bands are used to act as buffer zones. These guard bands are necessary because of the impossibility of achieving ideal filtering for separating the different users. The bandwidths of FDMA channels are generally low (30kHz) as each channel only supports one user. FDMA is used as the primary breakup of large allocated frequency bands and is used as part of most multi-channel systems.
The figure below shows the allocation of available bandwidth into several channels.
FDMA showing that the each narrow band channel is allocated to a single user
FDMA spectrum, where the available bandwidth is subdivided into narrower
band channels 