According to Wikipedia, FDMA aka “Frequency Division Multiple Access” is an access method that is used by radio systems to share the radio spectrum. The terminology “multiple access” implies the sharing of the resource amongst users, and the “frequency division” describes how the sharing is done: by allocating users with different carrier frequencies of the radio spectrum.
With FDMA, the given Radio Frequency (RF) bandwidth is divided into smaller frequency bands called subdivisions. Each subdivision has its own carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same subdivision at the same time. Essentially, the control mechanism designates a receive station for each of the subdivisions.
This source also explains TDMA or “Time Division Multiple Access.” This is an access method that is used by radio systems to share the radio spectrum. The terminology, “multiple access,” implies the sharing of the resource amongst users while the “time division” describes how the sharing is done: by dividing and allocating users into different timeslots in a carrier frequency of the radio spectrum.
TDMA is used in the Global System for Mobile Communications (GSM) and Personal Digital Cellular (PDC). It is also used extensively in satellite systems, local area networks, physical security systems, and combat-net radio systems. This means that two way radios and cell phones utilize this technology as well.
TDMA is a type of time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to abase station this becomes particularly difficult because the mobile phone can move around and vary thetiming advance required to make its transmission match the gap in transmission from its peers.
Both of these technologies are utilized in the two-way radio industry, so it is important to note the ways in which these influence communications. With the knowledge and understanding of how these methods work, new products are able to evolve.