Technology coming closer.

3G TECHNOLOGY:
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):

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).

Macro Cells, Micro Cells, and Pico Cells

The 3G network might be divided up in hierarchical fashion:

Macro cell - the area of largest coverage, e.g., an entire city.
Micro cell - the area of intermediate coverage, e.g., a city centre.
Pico cell - the area of smallest coverage, e.g., a "hot spot" in a hotel or airport.
3G Spectrum:


When you read about radio spectrum this means a range of radio frequencies. The bandwidth of a radio signal is defined as being the difference between the upper and lower frequencies of the signal. For example, in the case of a voice signal having a minimum frequency of 300 hertz (Hz) and a maximum frequency of 3,300 Hz, the bandwidth is 3,000 Hz (3 KHz).

The amount of bandwidth needed for 3G services could be as much as 15-20 MHz. Compare this with the bandwidth of 30-200 KHz used for current 2G communication and you can see that there is as much as a 500-fold increase in the amount of bandwidth required. Now you can appreciate why radio spectrum has become such a precious and scarce resource in the information age - everybody from television broadcasters to the military wants spectrum, and it is in short supply. Michael Powell, the chairman of the U.S. Federal Communications Commission (FCC), has suggested that spectrum demand "is going to forever outstrip supply". The telecoms operators have had to buy 3G spectrum from governments around the world, and those governments - realising that they own a precious, valuable resource - have sought to sell that spectrum at the highest possible price.

Radio spectrum is often organised (and sold) as paired spectrum - a bit of spectrum in a lower frequency band, and a bit of spectrum in an upper frequency band (see the section on 3G Technology for an explanation of paired spectrum). Paired spectrum is often specified in a form like "2x15MHz" meaning 15MHz in a lower band and 15MHz in an upper band. This technique of two users talking to each other on two separate frequencies is called Frequency Division Duplex, or FDD (see the section on 3G Technology for an explanation of FDD). W-CDMA is an FDD technique (i.e., it requires paired spectrum) whereas TD-CDMA is a TDD technique (i.e., it can use unpaired spectrum).