Mobile phone Frequencies

In half-duplex radio, both transmitters use the same frequency.

Only one party can talk at a time.

Mobile-phone Frequencies

In the dark ages before Mobile phones, people who really needed mobile-communications ability installed radio telephones in their cars. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter -- big enough to transmit 40 or 50 miles (about 70 km). It also meant that not many people could use radio telephones -- there just were not enough channels.

The genius of the cellular system is the division of a city into small Mobiles. This allows extensive frequency reuse across a city, so that millions of people can use Mobile phones simultaneously.

A good way to understand the sophistication of a Mobile phone is to compare it to a CB radio or a walkie-talkie.

Full-duplex vs. half-duplex - Both walkie-talkies and CB radios are half-duplex devices. That is, two people communicating on a CB radio use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the Mobile can talk at once.

Channels - A walkie-talkie typically has one channel, and a CB radio has 40 channels. A typical Mobile phone can communicate on 1,664 channels or more!

Range - A walkie-talkie can transmit about 1 mile (1.6 km) using a 0.25-watt transmitter. A CB radio, because it has much higher power, can transmit about 5 miles (8 km) using a 5-watt transmitter. Mobile phones operate within Mobiles, and they can switch Mobiles as they move around. Mobiles give Mobile phones incredible range. Someone using a Mobile phone can drive hundreds of miles and maintain a conversation the entire time because of the cellular approach.

In full-duplex radio, the twotransmitters use different frequencies,

so both parties can talk at the same time.
Mobile phones are full-duplex.

In a typical analog Mobile-phone system in the United States, the Mobile-phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into Mobiles. Each Mobile is typically sized at about 10 square miles (26 square kilometers). Mobiles are normally thought of as hexagons on a big hexagonal grid, like this:

Because Mobile phones and base stations use low-power transmitters, the same frequencies can be reused in non-adjacent Mobile. The two purple Mobile can reuse the same frequencies.

Each Mobile has a base station that consists of a tower and a small building containing the radio equipment. We'll get into base stations later. First, let's examine the "Mobile" that make up a cellular system.