Duplex transmission schemes for cellular telecommunications systems

an overview or tutorial explaining the forms of duplex transmission used by cellular telecommunications systems including FDD and TDD

Basic radio communications systems use a single channel and what is known as a press to talk system, where the user presses a button or "pressel" on the microphone to talk, and then releases the pressel to listen on the same frequency. This system is known as simplex as it uses a single channel. For a phone system a full duplex system is required where it is possible to speak in both directions at the same time. There are two main ways in which this can be achieved. The first is to transmit in one direction on one frequency, and simultaneously transmit in the other direction on another. To achieve this there must be sufficient frequency separation and filtering to ensure that the transmitter does not interfere with the receiver. A scheme that uses one frequency for transmitting traffic in one direction and another frequency for traffic in the other is known as Frequency Division Duplex (FDD).