Science Interviews

How a Mobile Phone Works

Diana - I've come to speak to Chris Cox here at the Science Festival to talk about the intimate workings of the mobile phone. Chris, when I make a call on my shiny new phone what's happening?

Chris C - There's quite a lot of steps it has to go through. Basically it all starts off when we speak into the microphone of a mobile phone and that turns our speech into a nice electrical signal representing the sound. That all then gets digitised, converted into a stream of binary 1s and 0s that represents the signal. The next step is it all gets compressed so that we all have to send fewer 1s and 0s. Basically that makes the whole process more efficient, slight loss of sound quality but it does mean that with lower data rates form each mobile phone the base station can accommodate more mobile phones in each cell.

Diana - How much is it compressed by?

Chris C - Basically it's typically down by a factor of ten or so. The original signal is 64,000 bits every second and it gets squashed to somewhere between 5000 and 13,000.

Diana - Here's an example of me talking at 64,000 bits per second...

Diana - And here's an example of me talking at 5,000 bits per second...

Diana - So you can hear how the audio degrades, especially when you're not getting a very good signal and that gives you that 'in the shower effect.' How is it possible to locate the last known position of a mobile phone? We hear about that a lot on the telly – maybe on Crimewatch, something like that – but how is it actually done?

Chris C - That's quite an important technique, often for emergency calls, tracing crimes and stuff like that. The most common technique is basically by triangulation. If you've got a mobile phone sitting in the middle of, say a triangle of three base stations then the mobile phone can measure the time at which the different signals from the different base stations all arrive. By comparing those times it can work out exactly where it is and report the result back, basically the same technique that seismologists use for locating earthquakes.

Diana - There've been plans floating around, lots of rumours saying that they're going to put mobile phones on planes. I don't know if I want to listen to someone saying 'I'm on the plane' but if they do that then how's it going to work?

Chris C - Well I must confess I'm not sure I do either. Let's see how that all works. First of all, there's been a bit of a delay because they want to make certain there's no interference with the electrical systems on board the aircraft. Once they have made absolutely sure of that there's going to be a tiny base station in the aeroplane itself which will just be picking up signals from the hundred or so passengers on board. Then after picking up the signals the aeroplane will send that down to a dedicated receiver on the ground, just by a point-to-point link that can handle a really high data rate.

Diana - We've had the eighties brick and we have lovely touch-screen phone that can do what seems like just about everything. What could possibly be next?

Chris C- There's a few different things. First of all, fairly similar to little receivers in aeroplanes we're likely to be getting little base station receivers for our homes as well. That is going to allow us to have all the capacity of a mobile phone base station that would normally be over a whole town for our own home. That will allow us to have really high data rates for really fast applications on our mobile devices in our home.

Diana - Would it be possible to share these base stations even if it's not your home base station? If you're out and about you could get this high data rate from someone else's house.

Chris - Er – you would need to sort the billing arrangements for that! Try to sort out if it's you paying the bill or the other person owning the other thing paying the bill. That is probably the next step down the road but we'll probably get there eventually.

March 2009