Any communication requires a medium which is commonly termed as channel. These communication channels provide pathways to convey information. A communication channel can be either a physical transmission medium or a logical connection in terms of resources (Frequency, Time) over a multiplexed medium.

In wireless communication the physical transmission medium is air, here channel refers to a frequency band needed for communication. In this section, the types of channels that can be used by the wireless transmitter and receiver are described.

Simplex Channels: Channels on which, a station can transmit only in one direction are simplex channels. The receivers do not have to communicate back even to send reply or acknowledgements. This is called Simplex transmission. These types of channels are used in communication systems in which there is a single transmitter and multiple receivers. Simplex is like a one-way road in which the traffic travels only in one direction, no vehicle from the opposite direction is allowed to enter. Examples of systems using simplex channels are T.V. and AM/FM radios.

Duplex Channels: To enable two-way communication (called full-duplex communication), duplex channels are used by the Transmitter(Tx)/Receiver(Rx). Communication systems in which both stations have to transmit and receive simultaneously, use these types of channels. The full duplex transmission mode is like a two-way road in which traffic can flow in both directions at the same time. Two simplex channels together form a duplex channel. There are two ways to provide this type of duplexing. They are Frequency Division Duplexing (FDD) and Time Division Duplexing(TDD).

1.3.1) Frequency Division Duplexing (FDD)

As shown in Figure 1, in FDD two separate channels are allotted between the transmitter and receiver. One channel is used to transmit and the other to receive.

1.3.2) Time Division Duplexing (TDD)

In TDD, If two systems want to communicate with each other, only one frequency channel is allotted as shown in Figure 2. The stations will use the same channel to transmit and to receive on a time sharing basis alternatively.

All the technologies used to build a Wide Area network use either FDD or TDD. Most of the modern Mobile Radio telephony use FDD.