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Cellular Radio

The basic concept of cellular radio dates back to 1947, including numerous, low-power transmit/receive antennae. Scattered throughout a metropolitan area, such an architecture served to increase the effective subscriber capacity of SMR/TMR radio systems, by breaking the area of coverage into small cells. Thereby, each frequency could be reused in non-adjacent cells. Additionally, the cells could be split, or subdivided, further as the traffic demands of the system increase; in other words, the network was highly scaleable. Traditional cellular radio, it should be noted, involves a circuit-switching mode.

This original concept is the basis of cellular radio, which first appeared in Chicago on October 13, 1983. AT&T operated that network for exactly 79 days, when the MFJ took effect. At that point a subsidiary of Ameritech assumed ownership [12-2]. By 1984, the Chicago network already was saturated in some cells and cellular telephones were in the hands of 91,600 people, growing to 19 million by 1994 [12-3] and [12-2]. Multiple low-power transmit/receive antennae are distributed throughout a geographic area, with each cell site having a relatively small, circular area of coverage (Figure 12.8). The coverage of the individual cells overlap with those of neighboring cells, with a minimum cell diameter of one mile and a maximum of about five miles [12-7]. As the terminal device moves from out of the effective range of one cell, the call is switched from cell antenna to another through a process known as hand-off, in order to maintain connectivity at acceptable signal strength. The hand-off is controlled through a MTSO (Mobile Traffic Switching Office) or MTSX (Mobile Traffic Switching Exchange), which is the functional equivalent of the PSTN Central Office. The MTSO also provides access to the PSTN, usually via leased lines. Further, the MTSOs are interconnected, either through the PSTN or through private microwave or leased-line facilities.


Figure 12.8  Cellular network with Mobile Traffic Switching Office (MTSO) connected to PSTN.

The process of hand-off uses a break and make approach. The connection with one cell site antenna is broken before connection is reestablished with another preselected cell site. As the duration of the break is very short, it is not noticeable in a voice communication. It, however, renders data communications difficult, at best.

Each cell site supports a limited number of frequency channels in order to take advantage of frequency reuse. For instance, U.S. analog AMPS networks divide 333 frequencies among cell sites, with the average cell site supporting 57 channels. In order to improve the performance of the network by reducing crosstalk, the original omnidirectional antennae were replace with a vectored version. Vectoring involves three vectors, each with coverage of 120 degrees. Splitting the frequencies into three vectors had the unfortunate effect of reducing by two-thirds the number of channels available to any given user in the cell site. As a result, more cell sites of smaller size were required, which entailed additional cost in the range of $500,00 to $1 million per site. Additionally, and for aesthetic reasons, many cities and towns are placing moratoria on the construction of new antennae sites. Metawave Communications and others are developing a solution in the form of antenna which divide the area of coverage into multibeam antennae to support all 333 channels on the basis of 12 beams, each with a 360 degree sweep. The smart antennae communicate continuously with the MTSO in order that channel allocation is managed cell-by-cell and on the basis of the network as a whole, based on shifts in traffic patterns [12-17].

Cellular Standards

Cellular standards are numerous and, as they vary along national lines, are incompatible. Standards include both analog and digital solutions with the clear trend toward digital. Digital systems offer the advantages of improved error performance and improved bandwidth utilization through compression. Digital systems also support data communications much more effectively.

Analog Cellular

Analog cellular was the first approach and still is the most prevalent, given its widespread implementation in the early years. Analog standards include AMPS, N-AMPS, TACS and NMT [12-15].

AMPS (Advanced Mobile Phone System)
was developed by Motorola and AT&T and is used in the United States. It is an analog technology operating on 50 MHz in the 800 MHz band and supports 666 channels split into 30-kHz voice channels, one each for forward and reverse communications. In the United States, 25 MHz and 333 channels each are provided to the wireline carrier (telephone company or telephone company consortium) and nonwireline carrier. Because it is based on FDMA, and FDD transmission, AMPS does not handle data well, with transmission generally being limited to 6,800 bps. Although widely deployed in the U.S., Australia, The Philippines and other countries, AMPS is rapidly giving way to digital technology.
N-AMPS (Narrowband AMPS)
also developed by Motorola, enhances the performance of an analog AMPS system. System capacity is improved by splitting the 30-kHz channels into three 10-kHz channels, thereby tripling AMPS capacity. While N-AMPS has not been widely deployed, it is used by some U.S. carriers. Motorola equipment is required.
TACS (Total Access Communications System)
is a derivative of AMPS developed for use in the U.K. in the 900 MHz band. At this higher frequency, more native bandwidth is available; therefore, TACS supports up to 1,000 channels, compared with the 666 supported by AMPS. TACS has found acceptance in very few nations, and it is not considered to be a long-term technology solution. JTAC (Japanese Total Access Communications System) is a TACS variation, operating in the 800 to 900 MHz ranges.
NMT (Nordic Mobile Telephone)
was developed and placed into service in the early 1980s in Scandinavian countries including, Denmark, Finland, Norway, and Sweden. NMT 450 operates in the 450 MHz range, which yields excellent signal propagation. Therefore, it is especially appropriate for sparsely populated areas supported by few cell sites. NMT 450 has found little acceptance outside of the Scandinavian countries. NMT 900 operates in the 900 MHz range, and is appropriate for more densely populated areas. NMT 900 has found acceptance in certain countries in Asia, as well as the Nordic countries, although it is not considered to be a long-term technology.


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