Frequency and Time Division Duplex.

The channel used to transmit data in 2G, 4G and 5G needs to be shared between users. This sharing is done by either spltting on time or frequency. In time division a user will have the whole of the channel for a timeslice, or a short time. In frequency division a user will have a sub channe or narrower channel for the whole time. But this sub-channel will be narrower, in terms of frequency, than the whole channel. Many communication systems will use one or the other, and many will use a mix of both time and frequency division communications.

Communications between a user and the base station needs to be both ways. In a full duplex system the communication between the user and the base station can occur at the same time, the user (and base station) can receive data at the same time as sending data. This is duplex communication.

MIMO [Multiple Input Multiple Output] ia a technology where the data is sent in parallel, rather than in sequence. So if the data is sent on (for example) 3 channels then the speed is increased by 3-fold. This is used extensively in 5G. But in 6G, to achieve the proposed speeds then the companies are looking at FDD-MINO, which uses frequency division, multiple channels to achieve the speeds. FDD-MINMO will become a key technology in 6G and is being used in enhancing 5G.