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TDD is a duplex method which means that time is divided into time slots and te user sends data in some time slots and receive data in the rest of time slots
TDMA is a multiple access method which means that time is divided into time slots and each user can access the medium -either transmit or receive- in certain time slots and other users use the medium in the rest time slots.
in order to deep understand the differences you should understand he differences between Multiple Access methods -which allow many users o share the medium - and Duplex methods wich identify how Tx and Rx relate to each other in time or frequency or .....
TDD:
to establish a commmunication (a call for example in GSM) you will need from your mobile to send information to the network (Base station in GSM): this is the uplink direction. Conversely you will receive information from the network to the mobile: this is the downlink direction. Then you need two directions (duplex) to make a communication.
The TDD duplex mode tells you that you will use of these directions (downlink or uplink) alternatively to make your communication (i.e;: downlink then uplink then downlink then uplink and so on...). The duration of each direction transmission depends on the technology.
TDMA:
TDMA is multiple access method, meaning that it describe HOW you will share the radio resource (i.e. the frequencies in GSM or the codes in UMTS).
TDMA tells you that you will use the radio resource only during a given time. Usually this is represented by the use of slots that you can use at a certain time. That is way there are called "timeslots".
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