How do Mobile Phones Work?

A mobile phone is really a radio - an extremely sophisticated radio, but a radio nonetheless.

A good way to understand the sophistication of a mobile phone is to compare it to a CB Radio.

A CB is a Simplex Device. This means that two people are communicating using the same frequency, so only one person can talk at a time. By comparison, a mobile phone is a Duplex Device.

Duplex Communication

A mobile phone is a duplex device - it uses one frequency for talking and a second, separate frequency for listening. A CB radio has 40 channels. A mobile phone can communicate on 1,664 channels. Mobile phones also operate within cells and they can switch from cell to cell as they move around.

What's a Cell?

Cells give mobile phones incredible range. A walkie-talkie can transmit perhaps a mile. A CB radio, because it has much higher power, can transmit perhaps 5 miles. Someone using a mobile phone can, on the other hand, drive across an city and maintain a conversation the entire time. It does this by using several 'Cells'.

How a Cell works

The key to the cellular system is the realization that a city can be divided up into small cells, and that the cells allow extensive frequency reuse across a city. Frequency reuse is what lets millions of people own mobile phones without problems.

Meteor divides up an area (such as a city) into cells. Each cell is typically sized at about 10 square miles (perhaps 3 miles by 3 miles). Cells are normally thought of as hexagons on a big hexagonal grid (see picture). Because mobile phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells.

Each cell has a base station that consists of a tower and a small building containing the radio equipment. For example, a typical site has 3 x antennae, which has 48 voice channels of which 6 are used for control. Therefore a maximum of 42 calls can be made simultaneously.

What are the advantages of cellular technology?

Mobile phones have low-power transmitters in them. Low-power transmitters have two advantages:

v      The power consumption of the mobile phone, which is normally battery-operated, is relatively low. Low power means small batteries, and this is what has made hand-held mobile phones possible.

v      The transmissions of a base station and the phones within its cell do not make it very far outside the cell.

Therefore, in the figure above, both of the orange cells can use the same frequencies. The same frequencies can be reused extensively in non-adjacent cells across the city.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of sites. Each carrier also runs a central office called the MSC (Mobile Switching Centre). The MSC can handle many thousands of calls and subscribers each day