Cell Phones

A typical cell phone with a stub antenna. Courtesy: Motorola, Inc.

You may use one every day, and they seem to work like regular telephones, but how do cellular phones connect to other phones, cellular and regular (wired)? Maybe you have noticed “cell phone towers” appearing in more and more locations. When you make a call on your cell phone, it is wirelessly linked to the telephone network via these towers so your call can be connected. This leads to a number of questions: What do these towers do? Why are these towers located where they are? How do engineers decide how many are needed so that you can make your call when and where you need to?

In the mid-1940s, the first mobile wireless phone services appeared in the United States. These services used one tower in each metropolitan area. Since the technology was very expensive and the market small, one tower could handle all the phone calls. However, demand for mobile phone services began to grow, and technology improved so that phones could be smaller and less expensive. Engineers anticipated these trends, and in the 1960s began researching and developing what is now today’s cellular phone service.

At first glance, providing such a service using a single centrally-located antenna—pretty much like AM or FM broadcast radio—seems reasonable. However, radio frequency (RF) spectrum is a limited commodity, and regulatory agencies (in the United States, the Federal Communications Commission or FCC), limit the amount of RF spectrum available for cell phone service in order to meet the needs of many different services, such as satellite television. In effect, these regulations would have limited the maximum number of simultaneous phone calls to mere thousands in each city—far below the expected demand. This reality forced engineers to adopt the cellular approach, where multiple cell phone towers are sited to cover a large geographic area.

In the cellular system each tower—or base station—covers a roughly circular area called a cell. A large region can be split into a number of cells. This allows different base stations to use the same frequencies, or channels, for communication links as long as a sufficient distance separates them. This is known as frequency re-use, and allows thousands or even hundreds of thousands of mobile telephone users in a metropolitan area to share far fewer channels. When you make a call, the system may assign a channel to you that is already in use by dozens of other people. However, the other subscribers are assigned to other cells, and you are the only one in your cell to get that channel.

A second important aspect to the cellular concept is that with each base station covering a smaller area, phones need less transmit power to reach any base station. This is a major advantage, since reducing the required transmit power reduces the size of the battery and the weight of the phone. Progress in miniaturization has been astonishing—few people using today’s palm-sized phones are aware of the “bag” or “briefcase phones” of the early and mid-1980s. This miniaturization would not have been possible without the cellular architecture.

But what happens when you make a call and then keep talking as you move about a large area and leave your original cell? The cell phone network automatically keeps track of the strength of the signal from your phone at multiple antennas. As you move, the base station receiving the strongest signal changes, and the network “hands off” your call from one base station to another. Handoffs are normally unnoticed by the user and help the battery last longer between charges.

Although it might seem attractive to make the cells smaller and smaller, there are technological challenges. First, smaller cell sizes increase the need for management of mobile users as they move about. In other words, smaller cell sizes require more frequent handoffs. Another constraint is antenna location, which is often limited by available space and aesthetics. Fortunately, both problems can be overcome. Greater handoff rates can be handled by increases in network processing speed, and creative antenna placement techniques (such as on lamp posts or sides of buildings) are allowing more base stations to be inconspicuous.