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(Page Fourteen) Appendix: Cellular Telephone Basics continued. . .

 

 

From A History of Engineering and Science in the Bell System: Communications Sciences (1925 -- 1980)

The latter is responsible for the high voice quality and high signaling reliability of the Advanced Mobile Phone Service.

In any given area, both the size of the cells and the distance between cells using the same group of channels determine the efficiency with which frequencies can be reused. When a system is newly installed in an area (when large cells are serving only a few customers), frequency reuse is unnecessary. Later, as the service grows, a dense system will have many small cells and many customers), a given channel in a large city could be serving customers in twenty or more nonadjacent cells simultaneously. The cellular plan permits staged growth. To progress from the early to the more mature configuration over a period of years, new cell sites can be added halfway between existing cell sites in stages. Such a combination of newer, smaller cells and original, larger cells is shown in Figure 11-36.

One cellular system is the Western Electric AUTOPLEX-100. In this system, a mobile or portable unit in a given cell transmits to and receives from a cell site, or base station, on a channel assigned to that cell. In a mature system, these cell sites are located at alternate corners of each of the hexagonal cells as shown in Figure 11-36. Directional antennas at each cell site point toward the centers of the cells, and each site is connected by standard land transmission facilities to a 1AESS switching system and system controller equipped for Advanced Mobile Phone Service operation (called a mobile telecommunications switching office, or MTSO). Start-up and small-city systems use a somewhat more conventional configuration with a single cell site at the center of each cell.

The efficient use of frequencies that results from the cellular approach permits Advanced Mobile Phone Service customers to enjoy a level of service almost unknown with present mobile telephone service. Grades of service of P(0.02) are anticipated,compared to today's all-too-common P(0.5) or worse. At the same time, the number of customers in a large city can be increased from a maximum of about one thousand for a conventional system to several hundred thousand. Also, because of the stored-program control capability of MTSOs equipped with the lAESS system, Custom Calling Services and man other features can be offered, some unique to mobile service. Other, smaller, switches provided by Western Electric or other vendors are also available to serve smaller cities and towns.

System Operation: Unlike the MJ and MK systems, Advanced Mobile hone Service dedicates a special subset of the 333 allocated channels solely to signaling and control. Each mobile or portable unit is equipped with a frequency synthesizer (to generate any one of the 333 channels) and a high speed modem (10 kbps). When idle, a mobile unit chooses the "best control channel to listen to (by measuring signal strength) and reads the high-speed messages coming over this channel. The messages include the identities of called mobiles, local general control information, channel assignments for active mobiles and "filler" words to maintain synchronism. These data are made highly redundant to combat multi-path interference. A user is alerted to an incoming call when the mobile unit recognizes its identity code in the data message. From the user's standpoint, calls are initiated and received as they would be from any business or residence telephone.

As a mobile unit engaged in a call moves away from a cell site and its signal weakens, the MTSO will automatically instruct it to tune to a different frequency, one assigned to the newly entered cell. This is called handoff. The MTSO determines when handoff should occur by analyzing measurements of radio signal strength made by the present controlling cell site and by its neighbors. The returning instructions for handoff sent during a call must use the voice channel. The data regarding the new channel are sent rapidly (in about 50 milliseconds), and the entire retuning process takes only about 300 milliseconds. In addition to channel assignment, other MTSO functions include maintaining a list of busy (that is, off-hook) mobile units and paging mobile units for which incoming calls are intended.

Regulatory Picture. The FCC intends cellular service to be regulated by competition, with two competing system providers in each large city: a wire-line carrier and a radio common carrier. To prevent any possible cross-subsidization or favoritism, the Bell operating companies must offer their cellular service through separate subsidiaries. These subsidiaries will be chiefly providers of service and, in fact, are currently barred from leasing or selling mobile or portable equipment. Such equipment will be sold by nonaffiliated enterprises or by American Bell Inc.

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