Once considered as only a luxury item, mobile phones may soon become commonplace, at least in rental cars, thanks to an innovation in telephone transmission tech­nology.

That innovation comes in the guise of the cellular mobile telephone system, one which works by transmitting messages through cells located throughout a certain area, such as a city. As opposed to conventional mobile phones, cellular mobile phones are claimed to pro­vide greater privacy, improved transmission quality, and greater availability of phone lines. In a pilot program in Chicago, three of the largest daily car rental companies, Hertz, Budget Rent A Car, and National Car Rental, have all begun equipping cars with cellular phones.

Budget was first out of the chute, renting Lincoln Town Cars with cellular phones at both Chicago air­ports during the early part of Oc­tober 1983. Later that same month, Hertz, which had placed orders for the phones in August, made 100 cars, including Thunderbirds, Cougars, Lincolns, and other luxury models, available to renters in the Chicago area. And, at about the same time, National announced that they would soon begin offering mobile phones in luxury-class cars at O'Hare International Airport.

The cellular phone service is now available only in Chicago and is operated by Ameritech Mobile Com­munications, formerly the Advance Mobile Phone Service. All three rental companies, however, have stated plans to expand the availability of the cellular service, con­tingent on FCC licensing proce­dures, to New York, Los Angeles, Philadelphia, Washington D.C., and other major cities across the country.

Similar cellular telephone service is expected to be available in 30 major markets within two years.

Meanwhile at the vanguard operation in Chicago, Hertz says the rate for the rental of their vehicles with mobile phones will be $10 a day over the local rental rate for the car. Long distance or trans-Atlantic calls made from mobile phones must be done on the renter's credit card or collect. Local calls can be made from the car without operator assistance, and the customer will be charged 45 cents per minute. A tape in the unit will record the length of calls. Budget's Lincolns with mobile phones will rent for about $47.95 per day, plus a usage charge, about $8 per day more than the $39.95 charged for Lincolns without phones. Reservations are required for phone-equipped Lincolns from Budget in Chicago. At National, cars equipped with cellular mobile phones can be reserved 24 hours in advance through the company's toll- free number. Phones will be rented for a flat daily charge plus a per minute charge for airtime used for local calls. Long-distance calls will be charged to the renter's telephone credit card.

Currently a cellular system is under construction in Los Angeles and is expected to be in operation in time for the 1984 Olympics. Pending Public Utilities Commission approval, the service will be marketed for PacTel Mobile Access, an AT&T subsidiary, by Advanced Mobile Phone Service, Inc., in Van Nuys, CA.

Cellular technology was developed by Bell Laboratories to make more efficient use of the limited radio frequency spectrum. It is based on a series of low-power antennas, or cell sites, arranged in a network of cells and connected to a central, computerized switching office. As the caller moves from one area of the city to another, the computer hands off the call to the appropriate cell site. This system greatly increases the number of mobile telephone customers who can be served, and reduces one of the problems closely associated with car-phone use, that of frequent busy signals.

The cellular service will offer an increased capacity of 666 channels compared with the current 25 channels available to the conventional mobile telephone users, meaning a car caller won't have to wait for an open channel, as frequently hap- pens. The communication quality is equal to a typical home or office phone and, cellular is not subject to static.

Both Budget and National will equip their cars with Dyna T.A.C. Cellular Mobile Radio telephones manufactured by Motorola, Inc. They are compatible with all U.S. and Canadian cellular systems and have features such as on-hook call processing, which allows the driver to leave the handset cradled while placing a call, lifting it only when the called party answers. Furthermore, they offer touch-tone signaling from the phone's keypad, providing the ability to access corporate phone networks as well as long-distance carriers such as MCI and GTE Sprint. Hertz' phones are made by Advanced Mobile Phone Service, Inc., a subsidiary of AT&T, and offer features similar to those of the Motorola phones.