Cellular Telephones - How They Work --------------------------------- (December, 1986) By Bruce Alston This is a non-technical explanation of the newest in mobile telephone communications, the cellular telephone. For some background let's review the mobile telephone as we knew it prior to late 1983 when cellular systems began operating in Chicago and Washington/Baltimore. Improved Mobile Telephone Service (IMTS) allows calls to be made from a car to a land telephone or vice-versa. Car-to-car service is also available. Based on radio transmission characteristics any city or town can have a maximum of 12 radio channels in the 150 Mhz band for mobile telephone service. The transmitter power for the base station (telephone company) can go as high as 200 watts Effective Radiated Power (ERP). This may cover an area of 20 to 25 miles depending upon terrain. The mobile radio is limited to 15, 25, possibly 50 watts ERP, keeping in mind the power consumption from the automobile battery. To receive the signal from the mobile radio the telephone company encircles the transmitter with receivers, so wherever the mobile unit might be, it can be heard, as it also must hear the base station transmitter. With IMTS in New York City, Los Angeles, or Madison, Wisconsin, or any city, only 12 mobile telephone conversations can work at one time, assuming the FCC allocated these cities all 12 channels. The FCC has allocated 666 channels in the 800 Mhz band for cellular telephone service. The maximum power for the base station is 100 watts ERP, for the mobile radio 7 watts ERP (that is not a misprint 7 watts!). Based on transmission characteristics, a cellular radio system can have up to 333 channels in a given geographic area. Each area can have two cellular systems, each with its own 333 channels in a given geographic area. Each area can have two cellular systems, each with its own 333 channels for the total 666. Picture the IMTS system with its receivers encircling one powerful transmitter. Change the receivers to combined transmitter/receiver/control equipment located throughout the geographic area. These are called cell sites. Where the one powerful transmitter base station was located, cellular has an MTSO (Mobile Telephone Switching Office) that channels telephone calls from the land lines to the cell site nearest the mobile radio. The MTSO can also switch mobile-to-mobile calls. As the mobile unit travels from one cell site toward another, where a more powerful signal can be transmitted between mobile radio and cell site, the MTSO switches the connection to the best cell site. It now looks as if a maximum of 333 calls could go on in any one cellular system at any given time. This is not so. Based on topography and radio interference patterns, the same radio channel might be used in two or more cell sites in the same system. These cell sites are probably 10 to 15 miles apart, unless a mountain or hill is in the way. In the United States, various manufacturers are claiming that a properly engineered cellular system can handle up to 75,000 calls at a given time. (The telephone term is 75,000 BHCA - Busy Hour Call Attempts.) No system has been installed that approaches this figure. Notice, though, that this beats the 12 BHCA of IMTS with a heavy stick if cellular is only capable of half its proposed capacity. Let's suppose your cellular telephone (it can be in a car, on a boat, or carried with you) has the number (516) 555-2600. I'm in Red Lodge, Montana and want to call you. Using my friend s telephone, of course, I dial 5165552600 and wait while the call goes through the regular telephone system. It will end up at the (516) 555 MTSO where it is sent to all the (516) 555 cell sites and transmitted. If your mobile telephone is turned on it will recognize the call, inform the MTSO that it is in service, and the MTSO will assign its most powerful cell site a voice channel for the conversation. The MTSO will also transmit information to your radio advising of the channel number on which you will be talking to me. Your radio will ring. I'll hear ringing, when you answer we talk. You push no buttons, turn no knobs. When the call is over, we both hang up. Should you wish to call me, pick up your handset, dial my number, push the SEND button, and wait until you get a busy, I answer, or you have a "ring-don't-answer" condition. Yes, you can use your modem... But cellular telephony is in its infancy; results may not always be all that you hoped for. Right now, voice communication is the principal commitment of cellular systems. In review, cellular telephones have opened a whole new area of usage availability. Having an older mobile telephone means that you might receive a call if one of twelve circuits were open, and you might be able to make a call under the same conditions. With cellular systems, when you are in the coverage area and your telephone is turned on, you will receive calls and you can make calls and expect to have the ability to talk until you are finished. The city of Sacramento, California, has seven cell sites. Anywhere you drive in that area you have cellular service. If you drive toward San Francisco, as soon as you get within range of cell sites, service is again available. The mobile radio has a "no service" light that is on when you are not in cellular range. If you have a "transportable" cellular radio, pack it with you into the dentist s office, or bank, or whatever, and use your telephone, both to send and receive calls. Cellular telephones can be equipped with every type of regular telephone feature: speed dialing, last number redial, call forwarding, three-way calling, call waiting, and eventually cellular service will be available in every community and along the highway between towns. Prior to deregulation and divestiture, IMTS service was provided only by the local telephone company, called "wireline" companies. Now, each city or town with cellular service can have two companies, the "wireline" (local telephone company) and "non-wireline," a Radio Common Carrier (RCC). Each company has a total of 333 radio channels in the 800 Mhz range devoted to cellular telephones. Actually, 312 channels in each group are for the voice communications and 21 are used for control data transmission (the information that tells the mobile radio which voice channel to use, for example). Cellular service is already so popular that the FCC is allocating additional channels for the service. Since cellular radio in the rest of the world uses up to 1000 channels, most cellular telephones are designed to cover these channels. For detailed information on cellular radio, consult "EIA Interim Standards, Mobile Station to Land Station, CIS-3-A," available from the Electronic Industries Association. Cellular communications derives its name from the radiotelephone signal being transmitted by a series of low-powered microwave antennas or cells. History First proposed by Bell Laboratories' creative thinkers in the late 1940s, the advanced computer technology to actually make cellular work was developed in the 1960s. The FCC, after a 13-year discussion, formulated its "final" rules on implementing the technology in 1981. (Other countries, such as Japan, Saudi Arabia, and Scandinavia acted more quickly and began operating cellular systems in 1979 to 1981.) Chicago was chosen as the city for an experimental system in 1979, and a second experiment was built in Washington/Baltimore, going on air in late 1981. Both experiments proved that the cellular systems functioned perfectly and that cellular communications is a valuable service. The FCC then issued an order licensing cellular systems for the country's 305 largest population centers; to date, the 100 largest markets are either online or soon will be. Each market is served by two cellular companies: a "wireline company," a subsidiary of the local existing phone company after the historic breakup, and a "non-wireline company," one that is not associated with the phone company. Two providers of service, according to the FCC, would prevent a monopolistic marketplace and foster competition. How a Cellular System Works The FCC designated the 800 Mhz band for cellular communications. Of the total 999 thirty-Khz-wide channels in the band, 333 channels are reserved for the wireline cellular company, 333 are reserved for the non-wireline company, and the last 333 are held in reserve for future cellular (or other mobile) service. When a cellular call is initiated, it is received by the closest low-power microwave antenna in the cellular area. From there, the call is routed completely over the microwave system if it is going to another cellular phone, or if it is going to a landline (regular phone), the call is then routed through a highly sophisticated computer switch and connected through to regular landline phones. As a vehicle moves throughout the cellular area (the geographic area in which the cellular company operates), the signal is automatically "handed off" from one cell to the next, so that the signal stays strong and clear. Just as an FM broadcast channel can be used in many cities across the country, a cellular channel can be used in different parts of the coverage area. This geographic sharing permits a cellular system to use radio channels more efficiently than existing mobile phone systems. A number of phone conversations can take place throughout a cellular area, at the same time, on the same channel without interfering with each other. Cost Cellular hardware varies according to the area of the country, and features of the model. Generally speaking, perhaps $995 to $1,800 or so for a vehicle-mounted unit, and $2,000 to $3,000 for portable and transportable units. Leasing and rentals are available in some areas. For the usage of the unit, the phone company charges a monthly fee, and a small charge per call.