Simplex Vs. Duplex

When people use walkie-talkie radios to communicate, only one person can talk at a time (the person doing the talking has to press a button). This is because walkie-talkie radios only use one communication frequency - a form of communication known as simplex:



Simplex: Using a walkie-talkie you have to push a button to talk one-way.
Of course, this is not how mobile phones work. Mobile phones allow simultaneous two-way transfer of data - a situation known as duplex (if more than two data streams can be transmitted, it is called multiplex):



Duplex: Allows simultaneous two-way data transfers.
The communication channel from the base station to the mobile device is called the downlink, and the communication from the mobile device back to the base station is called the uplink. How can duplex communication be achieved? Well, there are two possible methods which we will now consider: TDD and FDD.