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Why is a cell phone called a cell phone? One of the most interesting things about a cell phone is that it's really a radio. It's really an advanced walkie talkie, if you want to think about it. Before cell phones, people who needed mobile communications installed radio telephones in their car. In the radio telephone system, there was one big central antenna per city and maybe a handful of channels that were available on that antenna. You had to put this big,
high powered radio in your car. And when I say big, I mean like it filled half your trunk. And it had to be big because if you were twenty miles away from that central tower, you had to transmit at thirty watts or something. You had a radio station in your car to use this system. The idea of a cell phone changed all that completely. The cellular phone system divides this city into a bunch of small cells. This allows extensive frequency reuse across the city, so that millions
of people can use cell phones simultaneously. Here's how it works. The carrier chops up an area such as the city, into these cells. Each cell is typically sized at about ten square miles perhaps three miles by three miles. Cells are normally thought of as hexagons on a big hexagonal grid. Each cell has a base station that consists of a tower and a small building containing the radio equipment. Cell phones have low power transmitters in m and the base
station is also transmitting at low power. Low power transmitters have two advantages. First, the power consumption of the cell phone, which is normally battery operated, is relatively low. Low power means small batteries, and this is what has made handheld cellular phones possible. Second, the transmissions of the base station and the phones within its cell do not make it very far outside that cell. Therefore, the same frequencies can
be reused extensively 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 towers, but because so many people are using cell phones, costs remain fairly low per user. Each carrier in each city also runs one central office called the Mobile Telephone Swishing Office or MTSO. This office handles all the phone connections to the normal land base phone system and controls all
the base stations in the region. As you move towards the edge of your cell, so you're in a car and you're driving away from the tower for your cell, your cells base station will note that your signal strength is diminishing. Meanwhile, the base station in the cell you're moving toward, which is listening and measuring the signal strength on all frequencies, not just its own one seventh of the frequency spread, we'll be able to see that your
phone signal strength is increasing. The two base stations coordinate themselves through the mt s O, and at some point your phone gets a signal on a control channel telling it to change frequencies. This handoff switches your phone to the new cell, and you don't even know it's happening. In most cases, as you're driving down Interstate Highway, for example, your phone might switch towers every five minutes, and that's completely invisible to someone who's making an hour long call.
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