找到一个 3G的简介.
3G Technology
Here is a simple introduction to some aspects of 3G radio transmission technologies (RTTs). You will find the subjects covered in this section useful if you later consider the more detailed discussions in the sections on 3G Standards and 3G Spectrum.
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:
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):
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.
