How It Works

What most people don't know about a mobile phone is that it is actually a radio. The telephone was invented by Alexander Graham Bell in 1876, and wireless communication can trace its roots to the invention of the radio by Nikolai Tesla in the 1880s. The best features of both worlds combine in a small package in the palm of your hand.

The genius of the mobile phone system is the division of a city into small cells, hence the term 'cellular phone'. This allows extensive frequency reuse across an expansive area, so that millions of people can use mobile phones simultaneously.
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A Cell Apart

In a typical mobile phone network, the mobile-phone carrier receives about 800 frequencies to use across an area. The carrier chops up the area into cells. Each cell is typically sized at about 26 square kilometers. Cells are normally thought of as hexagons on a big hexagonal grid.

Because cell phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent cells. Two adjacent cells can reuse the same frequencies.Each cell has a base station that consists of a tower and a small building containing the radio equipment.

A single cell in an mobile phone system uses one-seventh of the available duplex voice channels. That is, each cell is using one-seventh of the available channels so it has a unique set of frequencies and there are no collisions:

• A mobile-phone carrier typically gets 832 radio frequencies to use in an area.
• Each mobile phone uses two frequencies per call -- a duplex channel -- so there are typically 395 voice channels per carrier.
• Therefore, each cell has about 56 voice channels available.

In other words, in any cell, 56 people can be talking on their mobile phone at one time. With digital transmission methods, the number of available channels increases. For example, a GSM-based digital system, like the ones in Europe and the UK, can carry three times as many calls as an American analog system, so each cell has about 168 channels available.

Making The Call

Cell phones have low-power transmitters in them. Many mobile phones have two signal strengths: 0.6 watts and 3 watts (for comparison, most CB radios transmit at 4 watts). The base station is also transmitting at low power. Low-power transmitters have two advantages:

• The transmissions of a base station and the phones within its cell do not make it very far outside that cell. The same frequencies can be reused extensively across the city.
• 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 handheld mobile phones possible.

The cellular approach requires a large number of base stations in a city of any size. A typical large city can have hundreds of towers. But because so many people are using mobile phones, costs remain low per user. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system, and controls all of the base stations in the region.

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