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Digital Cellular

Digital cellular standards clearly will dominate the cellular radio world into the future. Current digital standards include JDC, USDC and GSM, with GSM clearly being the dominant. As one might expect, most digital standards, including GSM, are based on TDMA and TDD.

JDC (Japanese Digital Cellular)
operates in various frequency bands in the 800 MHz, 900 MHz, 1400 MHz, and 1500 MHz ranges. It has not found acceptance outside Japan, with the exception of a few Asian countries under Japanese economic influence.
USDC (United States Digital Cellular)
was developed as a standard by the TIA (Telecommunications Industry Association) subcommittee TR 45.3. Operating in the 800 MHz band, USDC has not found broad acceptance and likely will not survive.
GSM (Global System for Mobile Communications)
was adopted by the CEPT in 1987 as the standard for European cellular. GSM operates in the 800 MHz and 900 MHz frequency ranges and is ISDN-compatible. GSM offers additional security in the form of a Subscriber Identification Module (SIM), which plugs into a card slot in the handset, much as a PCMCIA card fits into a laptop computer. The SIM contains user-profile data, a description of access privileges and features, and identification of the cellular carrier in the area of home registry. The SIM can be used with any GSM set, providing complete mobility across nations and carriers supporting GSM (assuming cross-billing relationships). Clearly, GSM will emerge as the international standard of choice. GSM is in place, and is predominant throughout Europe and much of Asia, supporting full roaming privileges from country-to-country. While some U.S. cellular carriers are trialing GSM, it is not being deployed aggressively in this country—that is to the disadvantage of U.S. users.

The Future of Cellular Radio

According to Global Telecoms Business there are now over 35 million cellular subscribers in more than 150 countries. Cumulative growth rates are more that 30% per year. The United States market grew by almost 50% in 1993 and by 325% in Pakistan. They go on to say that wireless now accounts for nearly 50% of Ericsson’s sales, compared to a mere 6% in 1984. Although worldwide cellular penetration currently stands at about 0.8%, estimates are that it will grow to incredible levels. Lehman Brothers, for example, estimates cellular penetration of 9.4% by the year 2000; that equates to 105 million users of a total accessible population of 1.1 billion[12-18].

In order to achieve that level of penetration, cellular rates for airtime must be reduced. With pressure from PCS/PCN and other technologies during the next decade, that is likely to be the case. Merrill Lynch estimates that rates may drop as much as two-thirds during the next ten years, from the current average of $0.45 to $0.15 by 1994. Nonetheless, GSM licenses in Europe fetch prices in the range of $100 million; AirTouch Communications CFO Chris Christensen is quoted as forecasting that its European GSM investments will hit cash flow break-even in three to four years and will hit income break-even in five years.

Cellular radio is big business, to say the least. AirTouch spun off from Pacific Telesis (Pacific Bell and Nevada Bell) in 1993, with the IPO (Initial Public Offering) raising $1.25 billion. In 1993, pre-tax profits were estimated at $108 million on revenues of $1.2 billion. Subsequently, AirTouch formed a joint venture with US West, effectively merging the two networks in order to reduce overheads, increase the footprint and combat the competitive threat of PCS. The spin-off of Airtouch yielded multiple benefits to PacTel. First, it relieved pressure from the California regulators, who became increasingly concerned about the same holding company owning such a profitable nonregulated subsidiary—PacTel was concerned about the PUC making demands for the cellular business to help subsidize the regulated business. Second, it allowed the public offering, which provided sorely needed cash to PacTel. Third, it allowed the merger of AirTouch with US West. Finally, it cleared the way for PacTel to bid for PCS licenses in its franchised serving areas, which was not allowed for telephone companies holding cellular licenses in the same geographic area.

Cellular Data Communications

Cellular data communications is difficult at best, as the analog FM networks are not designed to support data, certainly not at speeds above 9.6 Kbps. Given the high error rates typical in cellular networks, the data protocol must allow for repeated transmissions of errored and lost data packets, thereby lowering throughput to the range of 1200 bps. Additionally, radio modems must be used in order to interface the computer to the cellular CPE and, in turn, to the network. While a number of manufacturers have developed PCMCIA cellular modems which operate at speeds up to 28.8 Kbps (cost ???@ $500) [12-19], throughput is much less. Digital cellular systems improve on the analog approach, offering all of data communications advantages enjoyed in the wired world. Those advantages include more usable bandwidth, improved error performance and, therefore, enhanced throughput.

In either event, data over cellular networks suffers from the same issues that affect cellular voice, only much more so. For instance, the break and make connection technique used in cellular networks has a very deleterious impact on data communications. Additionally, signal fade affects communications as the terminal moves farther from the cell site, especially at high speed. To compensate for these factors, cellular modems must employ the most robust error correction protocols.

In support of data communications over AMPS cellular networks, several manufacturers recently (January 1995) unveiled data-ready cellular phones. Some of the devices have their own keypads, while others connect to a laptop PC for data entry. Display screens vary in size from 16 to 160 characters. In addition to support for voice and short message service, they will support Internet access, facsimile, and email [12-20].


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