Communication Systems: Simplex, Half Duplex, Full Duplex (FDX), Time Division Duplex (TDD)

Communication systems transfer information between 2 or more users. Communications systems may transfer information in one direction at a time on the same channel (simplex), in two directions on different time at different times (half duplex), or simultaneously on two different channels (full duplex). There are various approaches including TDD that allow the appearance of full duplex operation although the actual transmission system uses simplex or half duplex operation.

Simplex
Simplex communication allows the transmission of information between users, but only one direction at a time on the same channel or frequency. The common use of Simplex systems is traditional television or audio broadcast radio systems that transmit a signal from a single transmitter to many receivers.

Half Duplex
Half duplex communication provides the ability to transfer voice or data information in either direction between communications devices but not at the same time. The information may be transmitted on the same frequency or divided into different channels. When divided into different channels, one channel of frequency is used for transmitting and the other channel or frequency is used for receiving.

The use of different frequencies is common in half duplex radio transmission because the transmitter and receiver are commonly connected to the same antenna. If the same transmitter and receiver frequency were used, the high transmitter power would probably destroy the receiver circuitry.

Full Duplex (FDX)
Full Duplex communication is the process of transferring of voice or data signals in both directions at the same time. Full duplex operation normally assigns the transmitter and receiver to different communication channels. When the communications system uses two different frequencies for simultaneous communication, it is called frequency division duplex (FDD). One frequency is used to communicate in one direction and the other frequency is required to communicate in the opposite direction.

The definition of full duplex becomes confusing when it is applied to the end result of simultaneous voice and data communication. This is because it is possible to provide information at the input and output of a communication system while not actually sending the information simultaneously in a communication system. When a communication system provides for simultaneous two-way communication by time sharing, it is called time division duplex (TDD).

Time Division Duplex (TDD)
Time division duplex (TDD) communication uses a single channel or frequency to provide simultaneous two-way communications between devices by time-sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period of time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously. Figure below shows the basic operation of FDD and TDD system.