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:
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.
TDD vs. FDDWireless duplexing has been traditionally implemented by dedicating two separate frequency bands: one band for the uplink and one band for the downlink (this arrangement of frequency bands is called paired spectrum). This technique is called Frequency Division Duplex, or FDD. The two bands are separated by a "guard band" which provides isolation of the two signals:
FDD: Uses paired spectrum - one frequency band for the uplink, one frequency band for the downlink.
Duplex communications can also be achieved in time rather than by frequency. In this approach, the uplink and the downlink operate on the same frequency, but they are switched very rapidly: one moment the channel is sending the uplink signal, the next moment the channel is sending the downlink signal. Because this switching is performed very rapidly, it does appear that one channel is acting as both an uplink and a downlink at the same time. This is called Time Division Duplex, or TDD. TDD requires a guard time instead of a guard band between transmit and receive streams.
Symmetric Transmission vs. Asymmetric Transmission
Data transmission is symmetric if the data in the downlink and the data in the uplink is transmitted at the same data rate. This will probably be the case for voice transmission - the same amount of data is sent both ways. However, for internet connections or broadcast data (e.g., streaming video), it is likely that more data will be sent from the server to the mobile device (the downlink).
FDD transmission is not so well suited for asymmetric applications as it uses equal frequency bands for the uplink and the downlink (a waste of valuable spectrum). On the other hand, TDD does not have this fixed structure, and its flexible bandwidth allocation is well-suited to asymmetric applications, e.g., the internet (see this PDF file for more details). For example, TDD can be configured to provide 384kbps for the downlink (the direction of the major data transfer), and 64kbps for the uplink (where the traffic largely comprises requests for information and acknowledgements). See this PDF file for more details. (three-g.net)
