# Transcript

ALLAN JONES
Hello, I’m Allan Jones, and I produced the material here in Section 4. Now for several years, I’ve benefited from being able to talk to colleagues about topics that relate to Section 4. Helen Donelan, whom you met in Section 1. Hello, Helen.
HELEN DONELAN
Hello.
ALLAN JONES
Widespread mobile communications began with mobile analogue voice telephony, which is now referred to as first generation. And that was followed by digital telephony, referred to as 2G. Mobile data didn’t really take off, though, until the third generation, or 3G. This uses a wireless technology called Wideband Code Division Multiple Access, or WCDMA. What's distinctive about WCDMA?
HELEN DONELAN
Well, the trick with WCDMA is to replace each one and zero of data with a burst of very short binary signals that we call chips. Like digital data, chips are binary in the sense that they only have two possible states. But instead of calling these states one or zero, we call them one or minus one.
So a one in the user’s data might be replaced by tens or hundreds of very brief chips, each being either a one or a minus one. The sequence of ones and minus ones used to represent each data bit is called a code. So in Wideband CDMA, each user has their own code or chip sequence for representing the ones and zeros of their data. If you get the codes right, you can transmit several people’s data simultaneously and on the same frequency. And there’s no interference between them.
ALLAN JONES
OK. In fourth generation, a very different procedure is used. What’s the basic idea here?
HELEN DONELAN
4G isn’t based on CDMA. It’s based on Orthogonal Frequency Division Multiplexing, or OFDM. The idea of OFDM is to divide the radio channel into many very narrow slices which we call subchannels. You could have around 100 subchannels or more in a single radio channel. Each of these slices can be treated independently of the others, with its own carrier wave and is modulated to carry data just within that subchannel.
You can allocate some subchannels to one user and others to another user and so on. And that’s how multiple access is achieved in 4G. You get other benefits too, the main one being improved resistance to multipath interference.
ALLAN JONES
OK, thanks.