Regarding Multiplexing and Multiple Access
TDM is a general concept of partitioning the communication media into non-overlapping time periods (slots). These slots may be assigned, independently of each other, to various communication agents (channels). At any given instant of time, there is only one channel transmitting over the media, but due to the fast switching between the channels the media is seen as though carrying few channels simultaneously.

TDMA is a subset of TDM. TDMA means using TDM concept while the channels are physically separate. Why give a separate name to (essentially) the same concept? Because TDMA is (one of) the most commonly employed subsets of TDM which has additional engineering complications which are special to it (and a wide set of techniques to address these complications).



FDM is a general concept of partitioning the communication media into non-overlapping frequency ranges (bands). These bands are assigned, independently of each other, to various communication agents (channels). All channels are transmitting in their frequency band (or several bands) over the media, therefore FDM allows for a true simultaneous communication over multiple channels.

FDMA is a subset of FDM. When FDM protocol is employed between physically separate channels it is called FDMA.



TDM and FDM may be used together.

For example: partitioning an available frequency spectrum into bands (FDM), and transmitting multiple channels in each band alternatively (TDM).