The Evolution of Untethered Communications (1997)

Page 10

1
Past, Present, and Future

Humans have long dreamed of possessing the capability to communicate with each other anytime, anywhere. Kings, nation-states, military forces, and business cartels have sought more and better ways to acquire timely information of strategic or economic value from across the globe. Travelers have often been willing to pay premiums to communicate with family and friends back home. As the twenty-first century approaches, technical capabilities have become so sophisticated that stationary telephones, facsimile (fax) machines, computers, and other communications devices—connected by wires to power sources and telecommunications networks—are almost ubiquitous in many industrialized countries. The dream is close to becoming reality. The last major challenge is to develop affordable, reliable, widespread capabilities for "untethered" communications, a term coined by the U.S. military and referring to the union of wireless and mobile technologies. Because "untethered" is not a widely used term, this report concentrates on "wireless" communications systems that use the radio frequency (RF) part of the electromagnetic spectrum. These systems and their component technologies are widely deployed to serve mobile users.

Mobile wireless communications is a shared goal of both the U.S. military and civilian sectors, which traditionally have enjoyed a synergistic relationship in the development and deployment of communications technology. The balance of that long-standing interdependence is changing now as a result of trends in the marketplace and defense operations and budgets. These trends suggest that market forces will propel advances

Page 11

in technology to meet rising consumer expectations. However, the military may need to take special measures to field cost-effective, state-of-the-art untethered communications systems that meet defense requirements.

This chapter lays the foundation for an analysis of military needs in this area by chronicling the evolution of military and civilian applications of communications technology, from ancient times leading up to the horizon of 2010. Section 1.1 is an overview of the challenge facing the U.S. military. Section 1.2 provides an historical perspective on the development of communications infrastructures. Section 1.3 outlines the wireless systems currently used by the U.S. military and the related research and development (R&D) activities. Sections 1.4 through 1.7 recount the evolution and current status of commercial wireless systems. Section 1.8 compares the development paths for wireless technologies in the United States, Europe, and Japan.

1.1 Overview

In the final years of the twentieth century, all aspects of wireless communications are subject to rapid change throughout the world. Dimensions of change include the following:

These changes are fueled by opportunities for profit and public benefit as perceived by executives, investors, and governments. Although the patterns are global, the details differ significantly from country to country. Each dimension of change is complex and all of them interact. Overall, the dynamic nature of wireless communications creates a mixture of confusion and opportunity for stakeholders throughout the world.

A principal attraction of wireless communications is its capability to serve mobile users. Because mobility is an important feature of military operations, the U.S. armed forces have always played a leading role in the development and deployment of wireless communications technology.

Page 12

In the coming years, however, it appears that the commercial sector will have sufficient incentives and momentum to push the technical envelope on its own. At the same time, flat or declining defense budgets are motivating the military to adopt commercial products and services to an increasing extent. Yet there are significant differences between military and commercial requirements. Thus, it is important to examine carefully the opportunities for, and limitations to, military use of commercial wireless communications products and services.

In contrast to other areas of information technology, wireless communications has yet to converge toward a single technical standard or even a very small number of them. Instead it appears that diversity will endure for the foreseeable future. In this environment, the management and coordination of complex, diverse systems will be an ongoing challenge, particularly for the U.S. military, which coincidentally has to adapt to new threats and responsibilities after more than half a century of following the paradigm set by World War II and the Cold War. Information is now assuming greater strategic importance than ever before in warfare and other military operations, and so the wide deployment of cost-effective, state-of-the-art wireless communications systems has become particularly critical.

The present situation recalls previous epochs in which breakthroughs in hardware—aircraft carriers, jet aircraft, tactical missiles, nuclear weapons—have led to radical revisions of military doctrine. The next great revolution in military affairs could be shaped by information technology: global communications, ubiquitous sensors, precision location, and pervasive information processing. Advanced command, control, communications, computing, and intelligence (C4I) systems could make it possible to monitor an adversary, target specific threats, and neutralize them with the best available weapon. Admiral William Owens, former vice chairman of the Joint Chiefs of Staff, has called such an integrated capability a ''system of systems." Using such a system, a commander could observe the battle from a computer screen, select the most threatening targets, and destroy them with the press of a button. Battles would be won by the side with the best information, not necessarily the one with the largest battalions.

But unlike the military hardware of the past, information technology is advancing at a breakneck pace in a worldwide marketplace, driven not by military requirements but by the industrial and consumer sectors. Increasingly these technologies are available worldwide, and the best technology is no longer limited to U.S. manufacture and control. Highly accurate position data transmitted by satellite are now available to any yachtsman. High-resolution satellite photographs are for sale around the

Page 13

world. Any nation can purchase the latest communications gadgets from the electronics stores of Tokyo.

Therein lies the challenge for the U.S. military: how to exploit the advances in affordable technology fueled by worldwide consumer demand while also maintaining technical capabilities that significantly exceed those of any potential adversary.

1.2 Historical Perspective

Throughout most of history, the evolution of communications technologies has been intimately intertwined with military needs and applications. Some of the earliest government-sponsored R&D projects focused on communications technologies that enabled command and control. A synergistic relationship then evolved between the military and commercial sectors that accelerated the technology development process. Now large corporations develop the latest communications technologies for international industrial and consumer markets shaped by government regulation and international agreements. World trade in telecommunications equipment and services was valued at $115 billion in 1996 (The Economist, 1997).

Modern wireless communication systems are rooted in telephony and radio technologies dating back to the end of the nineteenth century and the older telegraphy systems dating back to the eighteenth century. Wireless systems are also influenced by and increasingly linked to much newer communications capabilities, such as the Internet, which originated in the 1960s. All wireless systems transmit signals over the air using different frequency transmission bands designated by government regulation. Table 1-1 provides an overview of wireless RF communications systems and services and the frequency bands they use.¹Each frequency band has both advantages and disadvantages. At low frequencies the signal propagates along the ground; attenuation is low but atmospheric noise levels are high. Low frequencies cannot carry enough information for video services. At higher frequencies there is less atmospheric noise but more attenuation, and a clear line of sight is needed between the transmitter and receiver because the signals cannot penetrate objects. These frequencies offer greater bandwidth, or channel capacity.

1.2.1 Communications Before the Industrial Age

The annals of antiquity offer examples of muscle-powered communications: human runners, homing pigeons, and horse relays. Perhaps the earliest communications infrastructure was the road network of Rome, which carried not only the legions needed to enforce the emperor's will

Page 14

Page 15

but also messengers to direct forces far from the capital. Ancient societies also developed systems that obviated the need for physical delivery of information. These systems operated within line-of-sight distances (later extended by telescope): smoke signals, torch signaling, flashing mirrors, signal flares, and semaphore flags (Holzman and Pehrson, 1995). Observation stations were established along hilltops or roads to relay messages across great distances.

1.2.2 Telegraphy

The first comprehensive infrastructure for transmitting messages faster than the fastest form of transportation was the optical telegraph, developed in 1793. Napoleon considered this his secret weapon because it brought him news in Paris and allowed him to control his armies beyond the borders of France. The optical telegraph consisted of a set of articulated arms that encoded hundreds of symbols in defined positions. Under a military contract, the signaling stations were deployed on strategic hilltops throughout France, linking Paris to its frontiers. By the mid-1800s, 556 stations enabled transmissions across more than 5,000 kilometers (km).

The optical telegraph was superseded by the electrical telegraph in 1838, when Samuel Morse developed his dot-and-dash code. Now information could be transmitted beyond visible distances without significant delay. In an 1844 demonstration on a government-funded research testbed, Morse sent the message "What Hath God Wrought?" from Baltimore to the U.S. Capitol (Bray, 1995).

The rapid deployment of telegraphic lines around the world was driven by the need of nineteenth-century European powers to communicate with their colonial possessions. High-risk technology investments were required. After the use of rubber coating was demonstrated on cables deployed across the Rhine River, the first transatlantic cable was laid in 1858, but it failed within months. A new cable designed by Lord Kelvin was laid in 1866 and operated successfully on a continuous basis.

The result was a rapidly expanding telegraphic network that reached every corner of the globe. By 1870, Great Britain communicated directly with North America, Europe, the Middle East, and India. Other nations scrambled to duplicate that system's global reach, for no nation could trust its critical command messages to the telegraphic lines of a foreign power.

1.2.3 Early Wireless

Within a few decades of its widespread deployment, telegraphy began to lose customers to a new technology—radio. In 1895 Guglielmo

Page 16

Marconi demonstrated that electromagnetic radiation could be detected at a distance. Great Britain's Royal Navy was an early and enthusiastic customer of the company that Marconi created to develop radio communications. In 1901 Marconi bridged the Atlantic Ocean by radio, and regular commercial service was initiated in 1907 (Masini, 1996).

The importance of this new technology became evident with the onset of World War I. Soon after hostilities began, the British cut Germany's overseas telegraphic cables and destroyed its radio stations. Then Germany cut Britain's overland cables to India and those crossing the Baltic to Russia. Britain enlisted Marconi to put together a string of radio stations quickly to reestablish communications with its overseas possessions.

The original Marconi radios were soon replaced by more advanced equipment that exploited the vacuum tube's capability to amplify signals and operate at higher frequencies than did older systems. In 1915 the first wireless voice transmission between New York and San Francisco signaled the beginning of the convergence of radio and telephony. The first commercial radio broadcast followed in 1920 (Lewis, 1993). The use of higher frequencies (called shortwaves) exploited the ionosphere as a reflector, greatly increasing the range of communications. By World War II, shortwave radio had developed to the point where small radio sets could be installed in trucks or jeeps or carried by a single soldier. The first portable two-way radio, the Handie-Talkie, appeared in 1940. Two-way mobile communications on a large scale revolutionized warfare, allowing for mobile operations coordinated over large areas.

1.2.4 Telephony

The telephone was first demonstrated in 1876. A telephone network based on mechanical switches and copper wires then grew rapidly. The high cost of the cables limited the number of conversations possible at any one time; as demand increased, multiplexing techniques, such as time division and frequency division, were developed.

A mix of independent operators ran telephone services in the early days. Subscribers to different services could not call each other even when in the same town. In 1913 the U.S. government allowed American Telephone and Telegraph (AT&T) to assume control of the national telephone network in return for becoming a regulated monopoly delivering "universal" service. Yet it was not until the 1950s that unified network signaling was offered to subscribers, allowing them to make direct-dial long-distance telephone calls (Calhoun, 1992). Since then, the rapid extension of the long-distance telephone network has been made possible by advances in photonic communications and network control technologies.

Page 17

1.2.5 Communications Satellites

The concept of using geosynchronous satellites for communications purposes was first suggested in 1945 by the science fiction writer Arthur C. Clarke, then employed at Britain's Royal Aircraft Establishment, part of the Ministry of Defence. Satellites of this type are positioned above the equator and move in synch with Earth's rotation. In 1954 J.R. Pierce at AT&T's Bell Telephone Laboratories developed the concept of orbital radio relays and identified the key design issues for satellites: passive versus active transmission, station keeping, attitude control, and remote vehicle control (Bray, 1995). Pierce advocated an approach of reaching geostationary orbit in successive stages of technology development, starting with nonsynchronous, low-orbit satellites. Hughes Aircraft Company advocated a geostationary concept based on the company's patented station-keeping techniques.

In 1957 the Soviet Union launched Sputnik, the first satellite to be placed in orbit. Amateur radio operators were able to pick up its low-power transmissions all over the world. In 1960 the National Aeronautics and Space Administration (NASA) and Bell Laboratories launched the first U.S. communications satellite, Echo-1, in a low Earth orbit. The first satellite-based voice message was sent by President Dwight Eisenhower using passive transmission techniques. The next advance in satellite technology was the successful launch of the TELSTAR system by NASA and Bell Laboratories. Using active transmission technology TELSTAR delivered the first television transmission across the Atlantic in 1962. Because it was placed in an elliptical orbit that varied from low to medium altitudes, the satellite was visible contemporaneously to Earth stations on both sides of the Atlantic for only about 30 minutes at a time. Clearly geostationary orbits were desirable if satellites were to be used for continuous telephone and television communications across long distances.

In 1963 Hughes Aircraft and NASA achieved geosynchronous orbit (known as GEO today) with the successful launch of the SYNCOM satellite. The satellite was placed in an orbit of approximately 36,210 km, a distance that allowed it to remain stationary over a given point on Earth's surface. SYNCOM led the way for the next several decades of satellite systems by demonstrating that synchronous orbit was achievable, and that station keeping and attitude control were feasible. Today most satellites, both military and commercial, are of the GEO variety.

COMSAT was formed by an act of Congress in 1962 and represented U.S. commercial interests in satellite technology development at Intelsat, established in 1964 as an international, government-chartered organization to coordinate worldwide satellite communications issues. INTELSAT-II (Early Bird) was launched into a geosynchronous orbit in

Page 18

1965 and supported 240 telephone links or one television channel. Channel capacities are now measured in the tens of thousands of voice channels (the INTELSAT-VI, launched in 1987, supports 80,000 voice channels).

The first military satellites, the DSCS-I group, were launched by the U.S. Air Force in 1966. Three launches placed 26 lightweight (100-pound) satellites in near-geosynchronous orbit. These systems supported digital voice and data communications using spread-spectrum technology (an important signal-processing approach discussed extensively in Chapter 2). The satellites were replaced in the 1970s by the DSCS-II group, which increased channel capacity by using spot-beam antennas with high gain to boost the received power. The first cross-linked military satellites, the LES 8/9, were launched in 1976. This demonstration fostered a vision of space-based architectures—without vulnerable ground relays—for communication, navigation, surveillance, and reconnaissance.

Satellites offer several advantages over land-based communications systems. Rapid, two-way communications can be established over wide areas with only a single relay in space, and global coverage with only a few relay hops. Earth stations can now be set up and moved quickly. Furthermore, satellite systems are virtually immune to impairments such as multipath fading (channel impairments are discussed in Chapter 2). But with the rapid deployment of undersea fiber-optic links, the use of satellite channels for telephony has been on the decline. The high capacity of fiber provides for competitive costs, which, combined with low latency, have attracted consumers. The future of the satellite industry depends on the emergence of applications other than fixed telephony channels. A new generation of satellite systems is being deployed to provide mobile telephone services (see Section 1.5).

1.2.6 Mobile Radio and the Origins of Cellular Telephony

The early development of mobile radio was driven by public safety needs. In 1921 Detroit became the first city to experiment with radio-dispatched police cars. However, transmission from vehicles was limited by the difficulty of producing small, low-power transmitters suitable for use in automobiles. Two-way systems were first deployed in Bayonne, New Jersey, in the 1930s. The system operated in "push-to-talk" (i.e., half-duplex) mode; simultaneous transmission and reception, or full-duplex mode, was not possible at the time (Calhoun, 1988).

Frequency modulation (FM), invented in 1935, virtually eliminated background static while reducing the need for high transmission power, thus enabling the development of low-power transmitters and receivers for use in vehicles. World War II stimulated commercial FM manufacturing capacity and the rapid development of mobile radio technology. The

Page 19

need for thousands of portable communicators accelerated advances in system packaging and reliability and reduced costs. In 1946 public mobile telephone service was introduced in 25 cities across the United States. The initial systems used a central transmitter to cover a metropolitan area. The inefficient use of spectrum and the coarseness of the electronic filters severely limited capacity: Thirty years after the introduction of mobile telephone service the New York system could support only 543 users.

A solution to this problem emerged in the 1970s when researchers at Bell Laboratories developed the concept of the cellular telephone system, in which a geographical area is divided into adjacent, non-overlapping, hexagonal-shaped "cells." Each cell has its own transmitter and receiver (called a base station) to communicate with the mobile units in that cell; a mobile switching station coordinates the handoff of mobile units crossing cell boundaries. Throughout the geographical area, portions of the radio spectrum are reused, greatly expanding system capacity but also increasing infrastructure complexity and cost.

In the years following the establishment of the mobile telephone service, AT&T submitted numerous proposals to the Federal Communications Commission (FCC) for a dedicated block of spectrum for mobile communications. Other than allowing experimental systems in Chicago and Washington, D.C., the FCC made no allocations for mobile systems until 1983, when the first commercial cellular system—the advanced mobile phone system (AMPS)—was established in Chicago. Cellular technology became highly successful commercially with the miniaturization of subscriber handsets.

1.2.7 The Internet and Packet Radio

The original concepts underlying the Internet were developed in the mid-1960s at what is now the Defense Advanced Research Projects Agency (DARPA), then known as ARPA. The original application was the ARPANET, which was established in 1969 to provide survivable computer communications networks. The ARPANET relied heavily on packet switching concepts developed in the 1960s at the Massachusetts Institute of Technology, the RAND Corporation, and Great Britain's National Physical Laboratory (Kahn et al., 1978; Hafner and Lyon, 1996; Leiner et al., 1997). This approach was a departure from the circuit-switching systems used in telephone networks (see Box 1-1).

The first ARPANET node was located at the University of California at Los Angeles. Additional nodes were soon established at Stanford Research Institute (now SRI International), the University of California at Santa Barbara, and the University of Utah. The development of a host-to-host protocol,²the network control protocol (NCP), followed in 1970,

Page 20

enabling network users to develop applications. At the same time, the ALOHA Project at the University of Hawaii was investigating packet-switched networks over fixed-site radio links. The ALOHANET began operating in 1970, providing the first demonstration of packet radio access in a data network (Abramson, 1985). The contention protocols used in ALOHANET served as the basis for the "carrier-sense multiple access with collision detection" (CSMA/CD) protocols used in the Ethernet local area network (LAN) developed at Xerox Palo Alto Research Center in 1973. The widespread use of Ethernet LANs to connect personal computers (PCs) and workstations allowed broad access to the Internet, a term that emerged in the late 1970s with the design of the Internet protocol (IP). The need to link wired, packet radio, and satellite networks led to the specifications for the transmission control protocol (TCP), which replaced NCP and shifted the responsibility for transmission from the network to the end hosts, thereby enabling the protocol to operate no matter how unreliable the underlying links.³

The development of microprocessors, surface acoustic wave filters, and communications protocols for intelligent management of the shared radio channel contributed to the advancement of packet radio technology in the 1970s. In 1972 ARPA launched the Packet Radio Program, aimed at developing techniques for the mobile battlefield, and SATNet, an experimental satellite network. In 1983 ARPA launched a second-generation packet radio program, Survivable Adaptive Networks, to demonstrate how packet radio networks could be scaled up to encompass much larger numbers of nodes and operate in the harsh environment likely to be encountered on the mobile battlefield.

Page 21

FIGURE 1-1 Military radios are designed for different uses. Combat net radios,
for example, are designed for communications within a battle group.

1.3 Military Wireless Systems And Research

1.3.1 Terrestrial Systems

Radio communications technology is widely used by U.S. military units at all levels. The many different types of military radios and applications cause a variety of communication problems. The military environment magnifies common difficulties such as the failure of one radio type to communicate with another type (interoperability), failure of one user to communicate with another (connectivity), incompatibility of new radios with old radios (legacy systems), and one radio at a location interfering with another radio at the same location (co-site interference).

In general, U.S. military radio systems can be categorized by the location of users and the information they broadcast and receive (see Figure 1-1). Multiple radios are often gathered together in an aircraft, shipboard radio room, or communications van to form tactical radio complexes and command-and-control centers. The radios operate simultaneously using many different waveforms across several frequency bands (e.g., high frequency [HF], very high frequency [VHF], and ultrahigh frequency [UHF]).

Combat net radios take the form of either a single radio in a vehicle (much like a car radio) or a device like a "walkie-talkie" carried around by a soldier. Most of the information broadcast on combat net radios consists of voice communications, often to share position information. Many of today's combat net radios have been enhanced to carry data in addition to voice. In general, combat net radios have fewer capabilities and cost less than do tactical radios (see Table 1-2). Military radios generally cost much more than commercial systems supporting similar applications.

Deployed military radios have various shortcomings. For example, the mobile subscriber equipment (MSE), the U.S. Army's mobile telephone system for the battlefield, was designed to be like a cellular telephone but is outdated compared to current technology. The single-channel

Page 22

ground and airborne radio system (SINCGARS) has been updated with recent technology, including programmable microprocessors, application-specific integrated circuits (ASICs), and surface-mount technology, but it implements a series of outdated waveform standards for single-channel digital voice. Furthermore, SINCGARS has experienced severe co-site interference problems because it hops transmission frequencies within the VHF band, a design capability that helps prevent jamming by adversaries but results in hops onto channels already in use for other communications traffic. The mobile subscriber radio terminal (MSRT) costs $70,000 and is about the size of a microwave oven; an updated version, introduced in 1994, is no less expensive and no smaller. Numerous HF radios have been built by the Army, but most are in storage because these radios are not simple push-to-talk designs and user training for the difficult HF channel has not been widespread.

The problems posed by individual radios are exacerbated by the difficulties encountered in linking communications systems of varying sophistication together (see Box 1-2). Special interfaces can be designed; SINCGARS, for example, can be interfaced into the MSRT. Inherent interoperability is among the features sought in sophisticated future systems. But in the near term, front-line troops will continue to use both existing and evolving radios, such as SINCGARS, mobile tactical satellite (TACSAT)

Page 23

terminals, MSE, MSRT, and packet radios. The Army is struggling with how to upgrade the MSE, a proprietary system. The SINCGARS is expected to be replaced and upgraded with a tri-service joint tactical radio in 1999.

The U.S. Department of Defense established IP as the underlying ''building code" for the Army, making a commitment to migrate all communications networks to the same basic structure as the Internet to position the military to integrate and leverage the advances in commercial information technologies. The Army's Task Force XXI "Tactical Internet" (Booz-Allen & Hamilton, 1995) was the first major experimental fielding of this new architecture (Sass and Eldridge, 1994; Sass, 1996).

Page 24

1.3.2 Satellite Systems

Satellite systems play a major role in military communications. They are attractive alternatives to land-based systems because they provide mobile and tactical communications to a large number of users over a wide geographical area. In addition, communication links can be added or deleted quickly, and satellites are less vulnerable to destruction or enemy exploitation than are land-based systems.

The DOD uses both military and commercial satellites to meet its communications needs. Fleet communications are supported by the government-owned FLTSAT and contractor-owned LEASAT systems, both of which are geosynchronous. The U.S. Air Force uses FLTSAT, the elliptical-orbit Satellite Data System, and the DSCS-III satellites to support the AFSATCOM satellite system. The DSCS, a vital component of the global defense communications system, is the DOD's primary system for long-haul, high-volume trunk traffic. The operational DSCS space segment consists of a mix of DSCS-II and DSCS-III satellites.

In 1982 the military began developing new satellite and terminal technology for MILSTAR, a millimeter-wave system operating in the 30–60 gigahertz (GHz) range. This new system consists of both geosynchronous and inclined-orbit satellites. The system provides enhanced antijam (AJ) capabilities as well as hardening against nuclear attack. Only a few of the planned eight MILSTAR satellites have been deployed so far. The complete system would provide two satellites per coverage area over the continental United States and the Atlantic, Pacific, and Indian oceans.

In general, existing tactical-satellite ground terminals incorporate new technology (e.g., microprocessors, ASICs, surface-mount technology) but are still forced to implement legacy waveforms. As a result, they have generally not kept pace with innovations in commercial communications waveforms and standards. In the case of MILSTAR, the military uses a noncommercial frequency band and is therefore unable to use—or take advantage of the price reductions in—commercial hardware. The new Joint Tactical Terminal (one of the systems listed in Table 1-2) is designed using modern radio technology, perhaps even including software-defined radios (see Section 1.3.3.2). High data rates sufficient for multimedia transmissions can be achieved only with the most advanced technology. For example, the global broadcast system (GBS), part of the U.S. Navy's UHF Follow-On satellites 8, 9, and 10, has bandwidth exceeding 100 megabits per second (Mbps) and worldwide coverage.

The most widely used military satellite system is the global positioning system (GPS), which encompasses 18 to 24 satellites in inclined orbits transmitting spread-spectrum signals. The GPS receivers extract precise time and frequency information from these signals to determine with

Page 25

great accuracy the receiver location, velocity, and acceleration. The system can be used by anyone with a receiver.⁴Commercial GPS receivers are used for numerous applications, including surveying, aircraft and ship navigation, and even recreational activities on land. Although launching and upkeep of the entire fleet of satellites are paid for by the United States, commercial GPS receivers were used by both sides in the Gulf War.

1.3.3 Research Initiatives in Untethered Communications

The DOD's vision for future communications systems is typically expressed in general terms, such as "multimedia to the foxhole" (see Box 1-3). For example, the Army's architecture for the digitized battlefield of the twenty-first century consists of fixed high-bandwidth infrastructure at the Army, theater, and corps levels, integrated with the DOD's global grid (a concept for spanning the world with high-bandwidth computing and communications systems) and based on asynchronous transfer mode (ATM) wide-area networking technology (Sass and Gorr, 1995). Bandwidth is allocated not only up and down the command hierarchy but also horizontally to cooperating formations. At the division level and below, wireless extensions provided by mobile radio access points (RAPs) will link the front-line combat communications systems to the infrastructure in the rear areas. The RAP is a wheeled or tracked vehicle with an on-the-move antenna system. The RAPs carry extensive communications systems and are interconnected by high-capacity trunk radios capable of

Page 26

communicating at up to 45 Mbps over a range of 30 km. Satellites or other systems may provide back-up communications.

To the committee's knowledge, the operational requirements for future untethered communications have not been translated into technical specifications. In the future, technical specifications will need to be formulated in a way that will make it possible to determine which commercial technologies are capable of meeting military needs. As an alternative, some general DOD requirements can be inferred from military plans and the known technical capabilities of existing and emerging communications technologies. For example, future military wireless systems will require high data rates—the long-range goal is at least 10 Mbps—and the capability to transmit over broad and variable frequency bands (some experimental radios are designed to span frequencies from 2 MHz to 2 GHz). The systems will need to be rapidly deployable and the infrastructure will need to be mobile. Multilevel communications security that encompasses the most secure levels possible will be needed. Furthermore, to enable worldwide strategic communications, the new equipment will need to be interoperable with older military systems as well as those used by foreign allies and international forces. There are more than 17 different U.S. defense communications networks, and none are readily interoperable at present. New concepts and technologies will clearly be needed to meet all these requirements.

To meet its future communications requirements, the DOD is funding a number of research and demonstration projects, typically pursuing high-risk ventures with potentially high payoff. The most comprehensive DOD-funded initiative dealing with untethered communications is the Global Mobile Information Systems (GloMo) program initiated by DARPA in 1994. Other relevant research initiatives deal with software-defined radios, communications systems, and radio technology (Leiner et al., 1996).

1.3.3.1 Global Mobile Information Systems Program

The overarching goal of GloMo is to develop technology for robust end-to-end information systems in a global mobile environment by exploiting commercial products and generating new technologies with applications in both commercial and military domains. The program supports a wide range of research projects, which are identified based on the priorities of GloMo managers rather than on a systems approach to the development of top-down solutions. Notably missing from the program, for example, is a comprehensive assessment of the suitability of various network architectures, even though all other component needs are dictated by the system design. (Network architecture issues are discussed in detail in Chapters 2 and 3.) The GloMo program currently focuses on

Page 27

developing innovative technologies that span the following research thrusts.

Design Infrastructure. This effort spans tools, languages, and environments for designing and deploying wireless systems. Research areas include computer-aided design tools for estimating power and designing low-power systems, design libraries and models for mixed-signal integrated circuits (ICs) suitable for implementing highly integrated RF chip sets, and simulation tools for modeling the propagation of radio waves and higher-level protocols.

Untethered Nodes. This effort focuses on high-performance, modular, low-cost, and low-power wireless nodes. Research activities are aimed at developing the next generation of agile, highly integrated radio technology. Radio control points are exposed to higher software layers to make radios and applications more adaptable to changing needs and conditions. Complementary metal oxide semiconductor (CMOS) technology (an inexpensive, low-power technology) is being pushed to its limits to achieve high-speed RF circuitry coupled to high levels of integration. Several activities are combining custom signal processing for audio and video with the radio circuitry. In these efforts radios are viewed as modular building blocks that can be combined to yield systems with different cost-performance-function attributes. Some projects are investigating the architectures of software radios, in which many of the radio functions are performed by software combined with very-high-performance processing architectures.

Network Protocols and Algorithms. This effort deals with the development of robust network architectures and techniques for rapid deployment of wireless networks. Research efforts include the development of new packet-radio routing schemes such as dynamic routing protocols for ad hoc networking. The concepts being studied are not limited to end-node mobility: Other possibilities include base-station mobility and network reconfiguration as base stations are repositioned in a battlefield scenario.

End-to-End Networking. This effort addresses how best to operate across a heterogeneous mix of underlying networks, both wireless and wired. Research areas include extensions to TCP/IP that will enable mobile users to access the Internet, satellite extensions to the Internet, and overlay wireless networking that supports mobility across diverse wireless subnetworks inside buildings and in the wider area.

Mobile Applications Support. This effort deals with the development of distributed computing techniques that will enable applications to adapt

Page 28

to varying network connectivity and quality of service (QoS) needs. The techniques being studied include software agents (sometimes called mediators or proxies) that adapt data representations to the capabilities of bandwidth-constrained wireless links; methods of performing computations in the wireline infrastructure on behalf of power- and display-limited portable devices such as personal digital assistants (PDAs); capabilities to move code between wired and portable nodes to provide location-dependent or new functionality when the node is poorly connected; file system structures that operate whether well connected, disconnected, or poorly connected to a wired infrastructure; event-notification protocols that enable applications to learn of changes to the underlying network connectivity and QoS; and techniques for structuring applications to exploit information about their current location.

1.3.3.2 Software-Defined Radio Research

The DOD is devoting considerable attention to designing and demonstrating software-defined radios, none of which is in production as yet. The most prominent of these initiatives is the SpeakEASY program sponsored by DARPA, the Air Force Rome Laboratory, and the Army Communication Electronics Command. The key objective of SpeakEASY is to change the paradigm for military radios. In the past, radios were based on "point designs" with negligible capabilities for functional upgrades or waveform changes—capabilities that define SpeakEASY. In phase 1 of the program, analog-to-digital (A/D) converters were used to complete the radio signal path and high-speed digital signal processors (DSPs) were used for filtering and demodulation. The key technologies demonstrated in phase 1 include digital frequency conversion and wideband signal processing.

In SpeakEASY phase 2, modular radio elements (separate modules for the analog elements, A/D converter, and DSPs) will be integrated on an open-architecture bus. The key objective of phase 2 is to demonstrate a software-defined networking radio with support for legacy and future waveform evolution using a single architecture. This approach increases production volume, reduces costs, and enhances logistical support. The open-architecture design implies that competitive bids would be sought for commercial boards, modules, and software. Other goals include the use of commercial modules in the radio and the commercialization of any functions developed specifically for the radio.

The Naval Research Laboratory has an ongoing research program focusing on a software-defined radio known as the Joint C⁴I Terminal (JCIT). The JCIT grew out of an Army requirement for an advanced, helicopter-based command-and-control system. The JCIT will incorporate

Page 29

multiple software-defined radios for combat net, intelligence communications, and military data links on a single platform.

Also under development is the advanced communications engine (ACE), which evolved from a project sponsored by DARPA. The ACE is a software-defined digital radio with capabilities for multiple simultaneous band and channel transmissions (it has six receiving and transmitting channels). The initial prototypes demonstrate "dual-use" (i.e., both military and commercial) capabilities including those of combat net radios SINCGARS and Have Quick (a UHF system designed to provide secure air-to-air and air-to-ground communications with AJ capabilities) and commercial avionics radios such as GPS, VHF air to ground, and the aircraft communications addressing and reporting system.

A very ambitious program, Millennium, was initiated to design an ultra-wideband radio. One objective was to demonstrate extremely high speed (approximately 1 billion samples per second) A/D data converters for both military and commercial communications. After the data conversion process, all tuning, filtering, demodulation, and decoding functions are performed by software (these processes and the associated technologies are discussed in Chapter 2).

1.3.3.3 Communications Systems Research

Several important research programs focus on complete communication systems. The DARPA Battlefield Awareness and Data Dissemination (BADD) program combines radios, ATM routers, and various communications networks and airborne relays from the Army's digital battlefield technology development effort for the deployment of high-speed data and large-file image transfer to the forward area. The Bosnia Command and Control Augmentation program, which is phase 1 of the GBS and focuses on satellite communications, grew out of BADD testing. Phase 2 of the GBS involves the incorporation of DirecTV transponders into Navy UHF satellites. Phase 3 will provide the means for stand-alone satellite transfer of high-speed data and large-file images.

1.3.3.4 Radio Component Research

The DOD's Extremely Lightweight Antenna program produced a compact, lightweight (under 2 pounds), and wideband (85 MHz to 2.2 GHz) antenna. The antenna incorporates a directional wideband satellite beam as well as low-gain omnidirectional radiation patterns. The DARPA Advanced Digital Receiver Technology program was initiated to demonstrate technology elements for software-defined receivers in communications,

Page 30

radar, and electronic warfare. Several of these functions might be merged into one digital receiver unit.

1.3.3.5 Small Unit Operations

The Small Unit Operations Situational Awareness System includes a significant wireless communications component. One goal of the research is to create a radio system for exchanging information among groups of up to 12 foot soldiers operating in an area of approximately 4 km2.

1.3.3.6 Modeling and Simulation

The Scalable Self-Organizing Simulations (S3) Program, supported by DARPA and the National Science Foundation, uses parallel computers to simulate communications networks. This program includes projects that create models and a library of computer programs for simulating mobility, radio propagation, and teletraffic patterns in large-scale wireless networks.

1.4 Commercial Terrestrial Mobile Telephone
Systems And Services

Commercial wireless communications systems have exhibited remarkable growth over the past decade (see Figure 1-2). There are currently more than 50 million U.S. cellular subscribers (Hill, 1997) and more than 34 million U.S. paging subscribers (Mooney, 1997). An estimated 17 percent of the U.S. population now has cellular service, compared to 95 percent with wireline telephone service (Hill, 1997). There are also 50 million subscribers to systems based on the global system for mobile communications (GSM) standard, the European cellular technology. Worldwide, the total number of subscribers to cellular systems is projected at just under 200 million (Hill, 1997). It should be noted that these figures, as market research estimates, are fundamentally imprecise and, moreover, tend to be volatile because of the dynamic nature of the wireless industry.

Throughout the world, wireless communication systems are enabling developing countries to provide instant telephone service to new subscribers who otherwise would have to wait years for wireline access. Although wireless users are still far outnumbered by the approximately 700 million wireline telephone users worldwide, the number of new wireless subscribers is growing 15 times faster than the wireline subscriber base, and this pace is expected to accelerate in the coming years. Analysts predict that, by the year 2010, there will be equal numbers of wireless and wireline connections throughout the world.

Wireless mobile telephone systems can be divided into three generations.

Page 31

FIGURE 1-2 The number of U.S. cellular subscribers and cell sites soared between
1984 and 1996. Note that 1984 figures are for January 1985. Source: Reproduced
with the Cellular Telephone Industry Association's permission from the CTIA's
Semi-Annual Data Survey.

The first generation, introduced in the 1980s and early 1990s, uses analog cellular and cordless telephone technology. Second-generation systems transmit speech in digital format. They provide advanced calling features and some nonvoice services. There are two categories of second-generation systems. High-tier systems feature high-power transmitters, base stations with coverage ranges on the order of kilometers, and subscribers moving at vehicular speeds. Low-tier systems, serving subscribers moving at pedestrian speeds, have low-power transmitters with a range on the order of 100 meters (m). Some of these systems are designed primarily for indoor use. Third-generation systems, planned for introduction after 2002, are expected to integrate disparate services, including broadband information services that cannot be delivered with second-generation technology. Many users are looking forward to the increased convenience promised by the integration or compatibility of systems (see Box 1-4). In addition to terrestrial mobile telephone systems, other commercial wireless systems include satellite communications, mobile data systems, and wireless local area networks (LANs).

1.4.1 First-Generation Systems

Of the original wireless communications systems deployed in the 1980s, the most popular was the analog cordless telephone, which uses

Page 32

radio to connect a portable handset to a unit that is wired to the public switched telephone network. Hundreds of millions of such devices have been produced, and the technology has been standardized in Europe under the cordless telephone first-generation (CT0, CT1, and CT1+) standards. There is no single U.S. standard. Analog cordless telephones have ranges limited to tens of meters and require a dedicated telephone line. Cellular systems have enabled much greater mobility.

In establishing cellular service in 1983 the FCC divided the United States into 734 cellular markets (called metropolitan statistical areas and rural service areas), each with an "A-side" and "B-side" cellular service provider. Historically, the designation of A or B indicated the origins of the cellular provider: An A-side provider did not originate in the traditional telephone business and was called a nonwireline carrier, whereas a B-side provider had roots in traditional services and was called a wireline carrier. Each cellular carrier is licensed to use 25 MHz of radio spectrum in the 800-MHz band to provide two-way telephone and data communications for its particular market. Because the U.S. analog cellular system is standardized with AMPS, any cellular telephone is capable of working in any part of the country.

The AMPS cellular standard uses analog FM and full-duplex radio channels. The frequency division multiple access (FDMA) technique enables multiple users to share the same region of spectrum. This standard supports clear communication and inexpensive mobile telephones, but the transmissions are easy to intercept on a standard radio receiver and therefore are susceptible to eavesdropping. As of late 1996, 88 percent of all cellular telephones in the United States used the AMPS standard (digital

Page 33

cellular standards have only recently become available). Outside of the United States and Canada, a wide variety of incompatible analog cellular systems have been deployed (see Table 1-3). The European cellular service, which predated the AMPS system, used the Nordic mobile telephone (NMT) standard beginning in 1982. Other European nations and Japan also developed analog standards.

1.4.2 Second-Generation Systems

Spurred by growing consumer demand for wireless services, standards organizations in North America, Europe, and Japan have specified new technologies to meet consumer expectations and make efficient use of allocated spectrum bands. These second-generation systems use advanced digital signal processing, compression, coding, and network-control techniques to conserve radio bandwidth, prevent eavesdropping and unauthorized use of networks, and also support additional services (e.g., voice mail, three-way calling, and text transmission retrieval).

In the United States, second-generation technologies have been deployed in the original 800-MHz cellular bands and in personal communications bands around 1900 MHz that were allocated by the FCC between 1995 and 1997. In Europe and most other parts of the world, second-generation technologies are deployed in the 900-MHz cellular bands and in 1800-MHz personal communications bands. Japan operates digital cellular systems in various bands between 800 MHz and 1500 MHz as well as a personal communications band near 1900 MHz.

The most widespread second-generation techniques include three high-tier standards: the European standard, GSM; and two North American standards, IS-136, a time division multiple access (TDMA) technique, and IS-95, a code division multiple access (CDMA) technique.⁵The GSM standard, which has been adopted in more than 100 countries, specifies a complete wide-area communications system. The other two standards specify only the communications between mobile telephones and base stations. A separate standard, IS-41, governs communications between mobile switching centers and other infrastructure elements in the United States. Table 1-4 summarizes the properties of the principal high-tier second-generation systems.

Among low-tier standards, the personal handyphone system (PHS) provides mobile telephone services to several million Japanese subscribers. Two other standards, digital European cordless telecommunications (DECT) and cordless telephone second generation (CT2), from the basis of several wireless business telephone (i.e., private branch exchange, or PBX) products. A fourth low-tier system is the personal access communications system (PACS), a U.S. standard. Although PACS has attracted considerable industry interest, it has not been widely deployed to date. Table 1-5 summarizes the properties of low-tier systems.

Page 34

Page 35

Page 36

In addition to the 1900-MHz licensed personal communications bands (see Table 1-5, the fifth column), the FCC has allocated the 1910–1930 MHz band for unlicensed low-tier systems. Commercial products based on DECT, PHS, and a modified version of PACS (designated PACS-UB, for unlicensed band) are under consideration for deployment in the 1910–1930 MHz band.

Each of the second-generation systems has distinct features and limitations, but none was designed specifically with the problems of large, complex organizations such as the military in mind. Nevertheless, it is possible to combine disparate approaches in a customized network built to meet the unique voice and data communications needs of an organization with national reach (see Box 1-5).

Page 37

Page 38

The commercial success of second-generation wireless telephone systems has stimulated widespread interest in enhancing their capabilities to meet public expectations for advanced information services. For example, new speech-coding techniques offering improved voice quality have been introduced to all three high-tier systems. Efforts are also under way to make these systems more attractive for data services. Accordingly, standards for fax-signal transmission have been established, and standards for circuit-switched data transmission at rates of up to 64 kilobits per second (kbps) are under development for GSM and CDMA. In addition, technology for packet-switched data transmission, suitable for providing wireless Internet access, is being developed for all second-generation systems. The technology base will continue to grow as R&D organizations worldwide design innovations for a third generation of wireless communications systems.⁶

1.4.3 Third-Generation Systems

The original concept for third-generation wireless systems emerged from an International Telecommunications Union (ITU) initiative known as the future public land mobile telecommunication system (FPLMTS).⁷Over the past decade the ITU advanced the concept of a wireless system that would encompass technical capabilities a clear step above those of second-generation cellular systems. The current name for the third-generation system is International Mobile Telecommunications-2000 (IMT-2000). The number refers to an early target date for implementing the new technology and also the frequency band (around 2000 MHz) in which it would be deployed.

As envisioned in the IMT-2000 project, the third-generation wireless system would have a worldwide common radio interface and network. It would support higher data rates than do second-generation systems yet be less expensive. It would also advance other aspects of wireless communications by reducing equipment size, extending battery life, and improving ease of operation. In addition, the system would support the services required in developing as well as developed nations. Box 1-6 lists the complete set of goals established in 1990 for FPLMTS.

Since 1990 IMT-2000 recommendations have been approved that elaborate on the initial goals, establish security principles, prescribe a network architecture, present a plan for developing nations, establish radio interface requirements, and specify a framework for a satellite component. The ITU anticipated an international competition leading to a radio interface that could be developed and deployed by the year 2000. The competing radio interfaces would provide minimum outdoor data rates of 384 kpbs and an indoor rate of 2 Mbps. Other than providing a forum for discussion of

Page 39

standards proposals, the ITU has not adopted clear plans of how to proceed beyond the point of reviewing the proposals.

The 1995 World Radio Conference set aside spectrum for nations to consider for the deployment of IMT-2000. The bands are 1920-1980 MHz and 2110-2170 MHz for terrestrial communications and 1980-2010 MHz and 2170-2200 MHz for satellites. As noted in Table 1-4 and Table 1-5, the United States has already allocated spectrum bands to personal communications that include part of the lower IMT-2000 band, making it unlikely

Page 40

that U.S. service providers could deploy IMT-2000 at all. Early on, attention to the ITU work was limited in both Europe and the United States, where growth in second-generation digital cellular and personal communications markets has been strong. It was the Japanese, virtually alone among all nations, who insisted that the ITU program proceed as fast as possible because they were running out of spectrum for their cellular and personal communications systems.⁸The Japanese were able to keep the IMT-2000 program on schedule, resulting in an ITU call for radio-interface proposals, now due in mid-1998. In support of this effort, the Japanese radio standards group is developing one or more Japanese standards for use in the ITU-2000 spectrum. Presumably the standard(s) will be submitted to the ITU for possible worldwide use.

Meanwhile, the European telecommunications industry established a framework for developing third-generation mobile wireless technology. The universal mobile telephone system (UMTS) is intended to replicate the commercial success achieved a decade earlier with GSM. The UMTS schedule calls for establishing the technology base by December 1997, deploying a minimum system in 2002, and achieving a full system in 2005. The technical goals of UMTS closely resemble many of the IMT-2000 goals. The Europeans plan to propose the technologies adopted for UMTS as candidates for IMT-2000.

In the United States, action on this issue did not take place until mid-1997, when the four U.S. CDMA cellular infrastructure manufacturers—Lucent Technologies, Motorola, Nortel, and QUALCOMM, Inc.—announced a third-generation program called Wideband cdmaOne. Like many candidate systems under consideration in Europe and Japan, the U.S. system uses a 5-MHz CDMA signal, although the operating parameters and design features differ from those of foreign counterparts. Additional U.S. proposals for IMT-2000 could emerge from other communities of companies supporting other digital radio interface standards.⁹

Among related developments, interest in "nomadicity" is growing within the Internet community in the United States. As originally conceived, the national information infrastructure (NII) placed little emphasis on the wireless delivery of information to mobile users (Computer Science and Telecommunications Board, 1994). But with the growth in demand for Internet services, reflected by the transition to private suppliers, providers are seeking to leverage Internet technology either directly or as part of heterogeneous networks. Plans are being made to accommodate nomads (i.e., mobile users) who draw on a variety of communications, computing, and information systems simultaneously, a concept that will require attention by multiple industries to issues such as security, interoperability, and synchronization within and between systems (Cross-Industry Working Team, 1995).

Page 41

Other ITU activities are addressing network aspects of IMT-2000.¹⁰Here again the Japanese have made major contributions toward the establishment of a single worldwide network to support wireless systems. Only in mid-1997 did the U.S. and European delegations begin to make significant contributions, concerned about their current investments in cellular and personal communications networks and the possible effects of establishing a worldwide network that was incompatible with their systems. The latest U.S. and European proposals emphasize the idea of a family of networks supporting a family of radio interfaces through the use of appropriate gateways to achieve worldwide roaming and interoperability.

Although it is clear that many new wireless communications technologies will emerge in the 2002-2005 time frame, it is not clear when and how they will be commercialized. The robust evolution of second-generation systems will limit commercial incentives to introduce a new generation of systems. It is possible that advances in second-generation systems will meet future demand for mobile telephone services and that a demonstrated demand for high-bit-rate data services will be necessary to stimulate the commercial deployment of third-generation technology.

1.5 Commercial Satellite Systems

Satellite systems can be classified by frequency and orbit. Above 1 GHz a satellite signal easily penetrates the ionosphere. Transmission at higher frequencies is desirable because additional bandwidth is available there, but then expensive components are needed to overcome signal attenuation, absorption, and path loss (see Chapter 2 for a discussion of channel impairments). Most satellite systems are of the GEO variety, offering configuration simplicity, wide footprint (i.e., one satellite covers an entire geographical region), and fixed satellite-to-ground-terminal characteristics. But GEO systems also have a number of disadvantages, including long propagation delays (a round-trip takes approximately half a second), high transmitter-power requirements, and poor coverage at the far northern and southern latitudes. Moreover, GEO satellites are expensive to launch, and, because only a handful of satellites are typically used to achieve global coverage, they are vulnerable to single points of failure.

The International Maritime Satellite (INMARSAT) Organization, formed in 1979, is now backed by the governments of 75 member countries. Its first satellites (INMARSAT-A) became operational in 1982, supporting voice and low-rate data applications with analog FM technology. By the end of 1993, 30,000 ground terminals were in operation. The next generation of INMARSAT satellites (INMARSAT-B and C) used digital technology, but data rates remained low (600 bps). With the introduction of INMARSAT-M in 1996 it is now possible to use laptop computer-sized satellite terminals

Page 42

for voice and low-rate (2.4 kbps) data transmission. However, the voice quality of this system remains poor due to propagation delay, and data transmission rates are 10 times slower than those of a standard modem.

In the late 1980s QUALCOMM deployed the OMNITracs vehicle-tracking and communications system for both North America (using GSTAR satellites) and Europe (using EUTELSAT satellites). The service provides two-way messaging and automatic position reporting. By 1997 more than 200,000 trucks, most of them in the United States, were equipped with the system. The use of such systems in Europe has been restricted by high equipment costs and expectations for less-costly alternatives with the next generation of systems.

Recently introduced GEO systems for data communications include Mobilesat in Australia and MSAT in North America (see Table 1-6). Innovations in GEO systems include spot beams for custom broadcast coverage and improved on-board processing. Although GEO satellite communications systems are not fully mobile (i.e., the terminals are not handheld), innovations in terminal design have enabled the development of private networks and rapidly reconfigurable systems. Very small aperture terminals (VSATs) use small Earth-station antennas to form private networks through links to GEO satellites. The VSAT is the result of more than 20 years of advances in digital Earth-station technology. The applications have evolved from point-to-point transmission links to networking terminals that leverage the broadcasting capability of satellites.

Page 43

The VSAT terminals offer various types of access. Fast-response protocols are used for time-sensitive transactions such as credit card purchases and hotel or airline reservations, throughput-efficient access is used for file transfers, and circuit-switched access is used for speech and digital video. (Throughput is the fraction of time during which a channel can be used.) An important feature of VSAT technology is ease of deployment: Installation takes approximately 2 hours. Companies are now installing VSATs at the rate of more than 1,500 per month. There are more than 200,000 VSATs worldwide, operating in nearly every country; individual networks range in size from as few as 20 nodes operating in a shared-hub environment to nearly 10,000 in the General Motors Corporation network.

In 1994 direct-broadcast satellites (DBSs) became operational, some two decades after the first experiments were performed with this technology. These systems broadcast a signal from a GEO satellite with sufficient power to allow direct reception in a home, office, or vehicle with an inexpensive receiver. The two primary applications for DBS systems are television and radio; emerging applications include DirecPC and GBS. Systems for direct-broadcast television are operational in Europe, Japan, and the United States. By the end of 1996 these systems had more than 2.5 million U.S. subscribers. Digital audio broadcasting (DAB) has the potential to provide every radio within a service area with continuous transmissions of a sound quality comparable to that of a compact disc. Systems are being tested around the world that deliver DAB from satellites as well as from terrestrial antennas.

Communications systems using non-GEO satellites are emerging as major players in commercial wireless applications. These satellites are characterized as either medium Earth orbit (MEO) or low Earth orbit (LEO). The LEOs, deployed in either circular or elliptical orbits of 500 to 2,000 km, offer several advantages including reduced propagation delay and low transmit-power requirements, allowing the use of handheld terminals. But at these altitudes a system requires many satellites to achieve global coverage. Furthermore, satellite movement relative to the ground terminal introduces Doppler shift in the received signal, and each satellite is visible from a ground terminal for only a few minutes at a time so that handoffs between satellites are frequent. The MEO satellites offer features that represent a compromise between LEOs and GEOs. The MEOs are deployed in circular orbits at an altitude of about 10,000 km. Approximately 10 to 15 satellites (more than GEOs but fewer than LEOs) are required for global coverage, and average visibility is one to two hours per satellite (less than for GEOs but more than for LEOs). The Doppler shift in MEOs is also considerably less than that in LEOs, but higher transmit power is required.

Page 44

The majority of new satellite systems that will become operational by the year 2000 are LEO or MEO systems. These satellites can be categorized further by size. Big LEO/MEOs (see Table 1-7) support voice and data communications with large satellites (weighing 400–2,000 kilograms [kg]) and operate at frequencies above 1 GHz. Little LEOs use much smaller satellites (weighing 40–100 kg) and operate in the UHF and VHF bands, thereby enabling the use of inexpensive transmission hardware for both the satellite and ground terminal. The 36-satellite Orbcomm system is an example.

Most of these systems provide voice and low-rate data to mobile users with handheld terminals. The link rates for little LEOs are asymmetric, with lower rates on the uplink (ground to satellite) than on the downlink (satellite to ground) because of power limitations in the handheld unit. Teledesic is unusual because it is intended primarily for broadband wireless data communications with stationary terminals at integrated services digital network (ISDN) rates. Teledesic and Iridium have direct intersatellite communication links independent of the ground segment, enabling the provision of services to countries lacking a communications infrastructure. Iridium is designed to consist of 66 satellites arranged in six planes, all in a nearly polar orbit. Each satellite is expected to serve as a "switchboard in the sky," routing each channel of voice traffic through various other satellites in the system; communications are eventually delivered to an appropriate ground-based gateway to terrestrial telecommunications.

Globalstar is a LEO digital telecommunications system that will begin offering wireless telephone, data, paging, fax, and position location services worldwide beginning in 1998. The 48-satellite constellation operating 1,410 km from the planet surface serves as a "bent-pipe" relay to local ground-based infrastructure.

1.6 Mobile Data Services

Commercial packet-switched mobile data services emerged after the success of short-message, alphanumeric one-way paging systems. Mobile data networks provide two-way, low-speed, packet-switched data communication links with some restrictions on the size of the message (10 to 20 kilobytes) in early systems. Services provided by mobile data networks include the following:

Page 45

Page 46

The first commercial mobile data network was Ardis, a private network developed in 1983 by IBM Corporation and Motorola to enable IBM to provide computing facilities in the field. By 1990 Ardis was deployed in more than 400 metropolitan areas and 10,700 cities and towns using 1,300 base stations. By 1994 Ardis (since then owned by Motorola) provided nationwide roaming for approximately 35,000 users, at a rate of 45 million messages per month, and a data rate of 19.2 kbps.

In 1986, Swedish Telecomm and Ericsson Radio Systems AB introduced Mobitex and deployed it in Sweden. This system is available in the United States, Norway, Finland, Great Britain, the Netherlands, and France. The system supports a data rate of 8 Mbps and nationwide roaming (international roaming is planned). This service is distributed by RAM Mobile Data in the United States, where by 1994 it had 12,000 subscribers. A total of 840 base stations are connected to 40 switching centers to cover 100 metropolitan areas and 6,300 cities and towns.

Cellular digital packet data (CDPD) technology was developed by IBM, which together with nine operating companies formed the CDPD Forum to develop an open standard and multivendor environment for a packet-switched network using the physical infrastructure and frequency bands of the AMPS systems. The CDPD specification was completed in 1993 with key contributions from IBM, McCaw Cellular Communications, Inc., and Pacific Communications Sciences, Inc. Deployment of the 19.2-kbps CDPD infrastructure, designed to make use of idle channels in analog cellular systems, commenced in 1995.

In the 1990s Metricom, Inc., developed a metropolitan-area network that was deployed first in the San Francisco Bay area and then in Washington, D.C. The signaling rate of this system is advertised at 100 kbps but the actual data rate is substantially slower. The Metricom system uses ''frequency hopping" spread-spectrum (FHSS) technology in the lower frequencies (around 900 MHz) of the unlicensed industrial, scientific, and medical (ISM) bands.¹¹

In 1996 the European Telecommunications Standards Institute (ETSI) standard for mobile data services, trans-European trunked radio (TETRA), was completed. It is currently being used primarily for public safety purposes. Work is in progress to enhance the digital cellular and personal communications technologies. More recently, the digital cellular standards (GSM, IS-95, PHS, PACS, and IS-136) have been updated to support packet-switched mobile data services at a variety of data rates. Key features

Page 47

of existing mobile data services are shown in Table 1-8. Although many services are available, the mobile data market has grown more slowly than have voice services.

1.7 Wireless Local Area Networks

Wireless LANs provide data rates exceeding 1 Mbps in coverage areas with dimensions on the order of tens of meters. They are used for a variety of applications, including the following:

In 1990 the Institute of Electrical and Electronics Engineers (IEEE) formed a committee to develop a standard for wireless LANs operating at 1 and 2 Mbps. In 1992 the ETSI chartered a committee to develop a standard for high-performance radio LANs (HIPERLAN) operating at 20 Mbps.

Table 1-9 indicates the technical features of various LAN products (including some that use the infrared portion of the spectrum and are therefore not examined in detail in this report). The market for wireless LAN products is growing rapidly but not nearly as fast as the market for wireless voice applications. The $200 million market for wireless LANs is tiny compared to the cellular industry, which is worth billions (Wickelgren, 1996).

1.8 Comparison Of International Research,
Development, And Deployment Strategies

Commercial wireless technologies have followed divergent evolutionary paths in different parts of the world. For example, strong contrasts are evident in the transition from first-generation cellular systems to second-generation systems in the United States and Europe. At first a single U.S. system was used for analog cellular communications, AMPS, and every cellular telephone in the United States and Canada could communicate

Page 48

Page 49

Page 50

with every base station. By contrast, European users were faced with a complex mixture of incompatible analog systems. To maintain mobile telephone service, an international traveler in Europe needed up to five different telephones. The situation was reversed by second-generation systems. Now there is a single digital technology, GSM, deployed throughout Europe (and in more than 100 countries worldwide), whereas the United States has become a technology battleground for three competitors: GSM (DSC-1900), TDMA (IS-136), and CDMA (IS-95).

The differences in technology evolution are due in large measure to different government policies in Europe, the United States, and Japan, the world's principal sources of wireless technologies. Three types of government policies influence developments in wireless systems: policies on radio spectrum regulation, approaches to R&D, and telecommunications industry structure. The reasons for the shifts in the above example can be found primarily in changes in spectrum regulation policies adopted in the 1980s. In establishing first-generation systems in the United States in the late 1970s, the FCC regulated four properties of a radio system: noninterference, quality, efficiency, and interoperability. In the 1980s, deregulation was in vogue and the scope of the FCC's authority was restricted to noninterference; the other properties were deemed commercial issues to be settled in the marketplace. Although this policy stimulated innovation in the U.S. manufacturing industry, it also meant that operating companies had to choose among various competing technologies.

In Europe, the main trend in government regulation in the 1980s was a move from national authority to multinational regulation under the aegis of the European Community (EC; now the European Union [EU]). The EC had a strong interest in establishing continental standards for common products and services, including electric plugs and telephone dialing conventions. In this context the notion of a telephone that could be used throughout Europe had a strong appeal. To advance this notion, the EC offered new spectrum for cellular service on the condition that the operating industries of participating countries agree on a single standard. Attracted by the availability of free spectrum, operating companies (many of them government-owned) in 15 countries put aside national rivalries and adopted the GSM standard.

Thus, a new pattern of technical cooperation was established in Europe. This cooperation was reinforced by the European Commission (the administrative unit of the EU), which funded cooperative precompetitive research focusing on advanced communications systems, first in the Research for Advanced Communications in Europe (RACE) program and then in the Advanced Communications Technologies and Services (ACTS) program. In both programs a consortium of companies and universities

Page 51

performs the research. Spectrum management rules continue to prescribe a single standard for each service, meaning that an industry consensus is required before a standard is introduced. Once a technology is established, companies enter the competitive phase of product development and marketing. This process promotes a thorough investigation of technologies prior to standardization and assures economies of scale when commercial service begins. In preparation for UMTS, scheduled for initial deployment in 2002, extensive R&D and evaluation of competing prototypes have been under way since 1994. All of this activity will provide European industry with a strong technical base for realizing the goals for mobile communications in the first decade of the next century.

The U.S. approach to communications technology R&D is much more competitive. Individual companies perform much of this research in the context of their product marketing plans. Coordination takes place within diverse standards organizations such as the Telecommunications Industry Association, IEEE, and American National Standards Institute. Some interaction also takes place in the GloMo program, which brings together universities and industry to fill specific technology gaps identified by DARPA program managers. But for the most part standards setting is a competitive rather than cooperative process, with each company or group of companies striving to protect commercial interests. The FCC rules for spectrum management allow license holders to transmit any signals, subject only to constraints on interference with the signals of other license holders. Similar flexibility is extended to unlicensed transmissions. As a consequence, there are multiple competing standards (seven in the case of wideband personal communications) for wireless service in the United States.

Government policies on industry structure also strongly influence technology development. After the FCC issued cellular operating licenses, most of the companies that began offering cellular service had limited technical resources and relied almost entirely on vendors and consultants for technical expertise. Even the cellular subsidiaries of the regional Bell operating companies had to build a new base of expertise: Under the terms of the consent decree that broke up AT&T in 1984, these cellular companies had no access to the abundant technical resources of Bellcore, the research unit of the regional Bell companies. In this environment, much of the new wireless communications technology in the United States has come from the manufacturing industry, with the result that proprietary rather than open network-interface standards have proliferated. The published technical standards for wireless communications were at first confined to the air interface between terminals and base stations. Eventually the industry adopted a standard for intersystem operation to facilitate roaming. Many other interfaces, especially those between switching

Page 52

centers and base stations, remain proprietary but the situation is changing to allow fully open systems.

By contrast, the European cellular operating industry has been dominated by national telephone monopolies. These companies have strong research laboratories that participate fully in technology creation and standards setting. To gain the advantage of flexibility in equipment procurement, operating companies favor mandatory open interfaces, a preference reflected in the GSM standard.

Little has been published concerning the factors that influence the evolution of wireless communications technology in Japan. In recent years NTT, the dominant telecommunications operating company, has provided a strong coordinating mechanism for creating and standardizing new technology. The biggest success has been PHS, which entered commercial service in 1995 and attracted 4 million subscribers in its first year of operation. The initial R&D for PHS was conducted by NTT, but it licenses many manufacturers to offer PHS equipment. Now many Japanese companies are cooperating in a study of wideband CDMA technology for third-generation systems. A joint experimental trial of one system is scheduled for the end of 1997. In addition to corporate R&D, a government organization, Research and Development Center for Radio Systems, is a significant source of wireless communications technology in Japan.

Worldwide efforts to guide the evolution of wireless communications technology come together in the IMT-2000 project. National delegations to IMT-2000 reflect their country's policies: The U.S. delegation pushes for diversity,¹²the Europeans advocate a structure favorable to UMTS and its descendants, and the Japanese delegation favors convergence to a small number of worldwide standards. Other countries assert their own service needs, which in some cases can be met by mobile communications satellites and in other cases by wireless local loops.

1.9 Summary And Report Organization

The history of wireless communications suggests a number of key points to be considered in evaluating potential future strategies for the DOD and DARPA. Wireless technology has now evolved to a point where the goal of "anytime, anywhere" communications is within reach. Since 1980 consumer demand for cordless and cellular telephones has driven rapid growth in wireless services, especially for voice communications. Wireless data services have not taken off as yet although expectations are high, given the growth of Internet applications. Extensive research is under way to develop third-generation commercial wireless systems, which are expected to be in place before 2010. These trends suggest that

Page 53

the DOD will continue to have an ample selection of advanced commercial wireless technologies from which to choose.

The DOD, which currently uses a variety of wireless systems based on 1970s and 1980s technology, is relying increasingly on commercial wireless products to cope with reductions in defense budgets and the growing need for flexible systems that can be deployed rapidly. In the Gulf War, the DOD used commercial equipment such as GPS receivers and INMARSAT links and found that performance was comparable to that of technologies designed explicitly to meet military needs. However, the DOD will continue to have unique needs for security, interoperability, and other features that might not be met by commercial products. The gaps between commercial technologies and military needs are difficult to identify precisely because, although the DOD has defined its vision for future untethered systems in general terms, projected operational needs have apparently not been translated into technical specifications that conform to the capabilities of commercial products.

The GloMo program and other military R&D efforts are attempting to meet DOD's future communications needs and have produced some useful results. However, none of these programs has adopted a systems approach to the problem, most notably with respect to the design of a network architecture. There may be other unmet needs as well; however, the committee based its work on first principles rather than an assessment of GloMo. A new strategy may be needed to identify the needs more specifically as a basis for determining where to focus DARPA's R&D efforts and where commercial products will suffice.

The effort to evaluate commercial technologies in light of defense needs will be complicated by the characteristics of the U.S. marketplace. In Europe there is a single standard (GSM) for digital wireless communications, and precompetitive research on new wireless technologies is carried out in cooperative, government-funded programs. The U.S. wireless market features a mixture of competing standards, and most technology R&D is conducted by individual companies. This environment forces operators to choose from an assortment of competing technologies.

The remainder of this report is an attempt to help the DOD devise strategies for making those choices. Chapter 2 provides technical background on the many issues that need to be addressed in designing wireless communications systems, which are extremely complex. The highly technical discussion may not interest all readers but is fundamental to any informed analysis of wireless systems. Chapter 3 explores the opportunities for and barriers to synergy between the military and commercial sectors in the development of wireless technologies. Chapter 4 integrates all the information presented in this report to provide a set of recommendations for the DOD and DARPA.

Page 54

Notes

1. This report does not address unguided optical communications systems, which use the 103–107 gigahertz frequency band (infrared, visible, and ultraviolet light), because the commercial products that operate in these bands are designed for indoor applications and therefore would not be of great use in military applications.

2. A protocol is a set of rules, encoded in software, for performing specific functions.

3. The developments since the mid-1970s, when the use of computer networks moved beyond the ARPA research community, paved the way for commercial services. The CSNet project, funded by the National Science Foundation (NSF) for the computer science community, eventually led to the NSFNET and a dramatic increase in the number of interconnected nodes. The commercialization of Internet service was symbolized by the decommissioning of the ARPANET in 1990 and privatization of the NSFNET in 1995.

4. Two types of codes are used to spread the signal. A long code is reserved for use by the military to obtain location information within a few meters of accuracy and timing information within 100 nanoseconds. A shorter code is used by commercial systems to obtain location information accurate to within 100 meters.

5. A fourth digital modulation technique, based on Motorola's iDEN technology, is used by some specialized U.S. mobile radio services in the lower 800-MHz band to provide cellular-like voice, trunked radio, paging, and messaging services.

6. One integrated solution not addressed in detail in this report is the new generation of public safety radio networks. These systems are used in both the military and commercial sectors for applications such as law enforcement and fire fighting. Until recently these systems were characterized simply as 25-kilohertz FM voice radios and 9.6-kbps modems. In the past a municipal law enforcement radio system typically was deployed as a redundant overlay of towers and repeaters separate from the radio systems operated by fire, health, highway, and other municipal departments. Today's tight budgets often force municipalities to pool departmental funds to upgrade public safety radios and establish a single system with enough capacity to meet every user's needs. To assist in this process the Association of Public Safety Communication Officers (APCO), which includes law enforcement, highway, forestry, health, and many other municipal and federal users, recently initiated an ambitious program called Project-25 to reduce the cost of next-generation radios. APCO Project-25 seeks to reduce user dependence on proprietary radios from a single manufacturer (generally the system installer) and introduce cost competition in the upgrading and replacement market at the municipality level. The strategy is to standardize a digital-modulation radio, which would be described as APCO Project-25 compliant, thus opening up public radio purchasing to a variety of competing manufacturers. Some radios that are APCO Project-25 compliant are now available and are being adopted by the Federal Law Enforcement Radio Users Group (representing radio users in the Federal Bureau of Investigation, Drug Enforcement Agency, Secret

Page 55

Service, Department of the Treasury, and other civilian agencies). The APCO Project-25 process has encouraged an unprecedented level of cooperation among municipal radio users.

7. These activities are carried out by the ITU Radiocommunication Sector (ITU-R) Working Party 8/13, later renamed ITU-R Task Group 8/1.

8. The implementation of standards based on IMT-2000 in Japan clearly would give Japanese companies early experience with the technology and perhaps position them to dominate future world markets for IMT-2000 products.

9. Although optical communications systems are not addressed in detail in this report, in large part because the commercial research focuses on indoor applications, the advantages of laser systems need to be mentioned. A laser produces optical radiation by stimulating emissions from an electronic or chemical material. Unlike light produced by incandescent or fluorescent sources, the resultant beam is coherent and exhibits extremely low angular divergence, properties that enable transmissions spanning great distances (i.e., thousands of miles). The data, voice, images, or other signals are modulated on a beam of light, which is detected by an optical receiver and decoded. The transmitter and receiver need to be in direct visual contact, and so the laser beam is steered in the appropriate direction using mirrors or other optical elements. Laser communications systems offer several advantages over RF systems. The main advantage is high capacity: Systems now under development will support transmissions in the range of hundreds of megabits per second, with systems under consideration attaining the gigabits-per-second range. Another advantage is the low power requirement for point-to-point communications (orders of magnitude lower than RF systems). All the energy is focused into a very narrow beam because the physical dispersion of a laser beam in space is minimal. Furthermore, laser communications systems offer security benefits because almost no energy is diffused outside the laser beam, which is therefore not easily detected by an adversary. This combination of features makes laser communications systems attractive for secure transmissions between hub points in mobile, dynamically changing environments (e.g., between base stations on vehicle-mounted switching facilities). However, laser systems are sensitive to interference from other light sources, such as the sun, and any obstructions of the visual link by dust, rain, or fog. There is also a risk of damage to the eyes of unprotected observers. Finally, components for laser-based systems are much more expensive than those for RF systems and therefore are unlikely to penetrate the commercial market for some time.

10. These activities are carried out by the ITU Telecommunications Sector, Study Group 11.

11. The ISM bands (at 902–928 MHz, 2400–2483 MHz, and 5700–5850 MHz) are available for any wireless device that uses less than 1 watt of transmit power.

12. The United States participates in the IMT-2000 process in Task Group 8/1 through a delegation led by the FCC.