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4
Conclusions and Recommendations
The history and challenges of wireless communications, as
outlined in previous chapters, suggest a variety of strategies that
could be pursued to fulfill the vision for untethered military
communications systems. This chapter summarizes and integrates key
points made in the preceding chapters to provide a set of 12
recommendations directed to the DOD and DARPA. Organizational
changes are recommended that would provide an environment conducive
to the development and military application of state-of-the-art
commercial technology. To meet defense-unique needs, specialized
R&D and demonstration efforts are recommended that focus on
various aspects of wireless technology, from the highest network
level down to individual components.
4.1 History And Challenges Of
Wireless
Communications
Voracious consumer demand is stimulating many advances in
commercial wireless communications technology, particularly
cellular and cordless telephones. The portfolio of wireless
services now available in the commercial marketplace includes a
wide range of telephony, paging, and data applications delivered
over a variety of service offerings ranging from land mobile radio
to cellular to satellite communications. Each service offers a
unique combination of coverage region, bandwidth, subscriber
equipment properties, and connectivity. In the aggregate,
commercial wireless capabilities are considerable, yet many
technical challenges
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remain. The cost of wireless voice systems needs to be reduced
and their quality improved. Specialized wireless data networks have
not taken off as yet, perhaps because they are not powerful enough
or because mass market applications have yet to emerge.
Considerable research to address these and other issues is under
way, both in the United States and overseas. Industry road maps
suggest that, by early in the twenty-first century, commercial
wireless communications will meet the long-term goal of enabling
users to communicate ''anytime, anywhere."
The DOD uses a variety of wireless systems that are based on
1970s and 1980s technology and designed to serve specific needs.
The DOD no longer drives the evolution of state-of-the-art
communications technology but still needs access to it, perhaps
more than ever. As threats to peace change from global to regional
conflict, a transformation is taking place in military roles,
missions, and communications needs. The vision for military
communications stresses C4I and
the protection of the lives of U.S. personnel, who will be based
principally in the United States but will need to be prepared to
move quickly throughout the world to carry out a variety of
missions, including noncombat roles such as peacekeeping and
humanitarian response. Such missions are nontraditional in the
sense that coordination with foreign partners may be essential,
whereas national survival will not be at risk as was anticipated
during the Cold War. In addition, the need for U.S.-based
logistical support will grow, and new systems will be required to
counter terrorism. Thus the accurate, timely transmission of
information will be perhaps more essential than ever in meeting
military objectives. Effective global communications systems will
be critical.
The civilian and military sectors have a long history of
interaction in the design and deployment of wireless communications
technology. In the Gulf War, DOD used commercial wireless equipment
such as GPS receivers and found that the performance was comparable
to that of equipment designed explicitly to meet military needs.
Yet current commercial technologies and practices cannot meet all
military needs. For example, the military cannot tolerate the long
lead times—on the order of months to years—that are
typical in the building of commercial communications
infrastructures. Commercial wireless companies carry out elaborate
advance planning and measurement operations, whereas the military
requires networks that can be organized quickly and can adapt
rapidly to changing operating conditions (including spectrum
availability). These networks will also have to be compatible with
other military communications systems, both new and old.
Differences between military and commercial needs also have
implications for network architecture. Commercial research on
integrated (i.e., multimedia) systems is oriented toward network
architectures based on the base-station-oriented model. It is not
clear whether that approach or
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the peer-to-peer design will be more appropriate in future
military settings. Previous DARPA research on packet radio networks
has encompassed base-station-oriented, peer-to-peer, and multihop
networks.
The evolution of technology is also influenced by organizational
differences between the two sectors that encompass market drivers,
acquisition practices, and maintenance and repair arrangements.
Commercial R&D and manufacturing are oriented toward mass
markets of tens of millions of units. Adding functionality to
designs has to be justified by consumer demand. By contrast,
military equipment is designed to provide the functions required to
fulfill missions. The number of units produced is tiny relative to
commercial markets. Commercial customers acquire equipment and
subscribe to services as they are ready to use them, expect
effective function within minutes, and rely on equipment
manufacturers for repairs. The military acquires equipment on a
contingency basis, operates its own repair facilities, and is
prepared to train its personnel to use communications systems.
Many research efforts are under way to realize the commercial
and military visions for wireless communications. Fueled by the
success of cellular communications and projections of
ever-expanding markets for wireless services, the commercial sector
is pushing ahead in various areas. One objective is to enable
portable devices to communicate at the high bit rates needed for
advanced information services. Another objective is to advance the
state of the art for software radios as a means of fostering
economies of scale in R&D and manufacturing in a world of
diverse and changing technical standards. By using multiple types
of operating software, such radios can serve as single hardware
platforms capable of transmitting and receiving signals that
conform to a variety of standards. Meanwhile, DOD is taking a dual
approach to wireless technology development by both conducting its
own research, focusing primarily on components, while also relying
increasingly on commercial technologies to ensure interoperability
and systems integration. The DARPA GloMo program has initiated a
broad range of coordinated R&D efforts that will provide
enabling technologies for future military systems. Among the
commercial technologies that the military expects to use are
Internet protocols and ATM.
These observations lead to the following general
conclusions:
•
A large gap remains between public expectations for mobile
communications ("anytime, anywhere") and the available
technology.
•
Over the next 10 years or so, market forces will fill this gap
by developing new technologies for commercial wireless
communications.
•
The military thus has much to gain from positioning itself to
use COTS communications equipment to the greatest extent
possible.
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•
Some military needs for wireless communications technologies
will exceed or differ significantly from anticipated commercial
developments. For example, the military has unique concerns with
respect to network design, security, interoperability, and
multimode/multiband systems.
•
The commercial sector has its own incentives to produce advanced
communications devices, components, and subsystems as well as
complete systems. To use commercial technologies effectively, the
DOD will have to take special measures to promote the development
and acquisition of COTS products that can be integrated into
systems that meet specialized military requirements.
These conclusions provide the basis for the recommendations
presented in the remainder of this chapter. The 12 recommendations
are organized in a hierarchical order from the general to the
specific. The first three focus on organizational changes, or
meta-issues, that need to be addressed by DOD to help align
military requirements with commercial products and services and
provide an environment conducive to the absorption of
state-of-the-art technologies. The other nine recommendations
identify R&D project that should be carried out by DARPA to
advance the synergy between military and commercial systems while
also meeting specialized military needs that will exceed
anticipated commercial developments. The R&D recommendations
are presented in order of priority, reflecting the committee's view
that high-level systems issues are of paramount importance. The
recommended research excludes subjects that will be adequately
covered by the commercial sector.
4.2 Standards Development
1. The DOD should participate in standards-setting activities
for wireless communications technologies and systems.
With commercial demand for wireless technology growing worldwide
and DOD budgets flat or falling, incentives for future
commercial-military synergy need to be provided by the military
side. Consequently the defense community needs to gain a deep
understanding of technology trends so as to obtain advance notice
of new concepts and influence the development of cost-effective
equipment that meets military needs. Although new technologies can
originate in diverse settings that include industry, academia, and
nonmilitary government laboratories, the features of available
equipment are determined to a large extent in the process of
standards setting.
Current R&D and standards activities are focused on
enhancing wireless communications technology to make it possible to
move many types
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of information (including data, video, and images) to and from
portable wireless devices. Although this work is certain to produce
new technology, the commercial deployment of these innovations is
not assured. The availability of innovative technology in the
marketplace depends on business, social, and government policy
factors. Military planners need to maintain a continuing awareness
of the difference between what is possible technically and what is
available in the market to meet military needs.
Because military requirements often exceed those of the
commercial market, opportunities for the DOD to use commercial
products depend on equipment details, which are reflected in
standards. Such details might be of little initial interest to
consumers. Yet the defense community is often ahead of industry in
recognizing features that will eventually be important to all
users; if these needs are not addressed in the standards-setting
processes then they might have to be met later in commercial
settings at great cost, both financially and in terms of network
performance. An example is the poor state of network security in
analog cellular systems, an issue not considered in the initial
design process. Newer digital systems help alleviate this problem,
but adding security features to older equipment remains an
expensive and difficult challenge. The military, by contrast,
always plans for system security in advance.
Standards also influence whether "hooks" or interfaces are
designed into commercial technologies to enable modifications that
would meet specialized military needs. By participating in the
standards process, government agencies will make it possible to
embed standard devices in military-specific system architectures
and generally promote a capability for cost-effective systems
integration.
The DOD could influence standards setting by participating in
activities such as the ATM Forum, the IETF, and the Multimode
Multiband Information Transfer System Forum. Effective
participation will be constrained by the rules governing standards
organizations and by the abilities of DOD management and
participating individuals to influence technical decisions and
political processes. They will need to understand how standards for
wireless communications are established, a complex process that has
influenced the very different evolutionary paths of wireless
technologies in the United States, Europe, and Japan. In addition,
the DOD could benefit from the analysis, simulation, and laboratory
testing of candidate technologies to support defense interests in
the establishment of specific standards. These activities could
reveal the limits of some technologies that would not be apparent
in more benign commercial settings but are likely to be of
long-term significance to consumers. By conducting such tests and
sharing the results with the commercial sector, defense agencies
will improve the chances that adopted standards
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will serve immediate military needs and also benefit the
commercial sector in the long run. Participation in standards
creation could ultimately prove to be more cost-effective than
commissioning equipment explicitly designed to meet military
needs.
4.3 Demonstration And Testing
Rather
Than Development
2. The DOD should pursue a vigorous process of technology
demonstration and testing prior to development and procurement. In
particular, the focus should be on system concepts based on
commercial technologies and specialized military enhancements.
As the defense budget is reduced, less money will be available
for major procurements. Nevertheless, an inventory of advanced
technologies can be maintained and deployed as needed if military
equipment is adapted whenever possible from commercial
technologies. Examples of this strategy include the Condor project,
in which DOD is supporting the development of a cryptography module
on top of the cellular telephony system, and the Army's plans for
the digitized battlefield. The Condor project is focusing on core
noncommercial technologies such as on-the-move, high-bandwidth,
phased-array antenna technology. The Army's digitized battlefield
network will include switches, routers, and hubs based on
commercially available technology. The Army is not building its own
ATM switches, which are widely available in the private sector, but
instead is developing high-speed encryptors and decryptors that are
compatible with commercial switches.
Military planners need to understand the thrust of commercial
developments and identify critical technologies that are unlikely
to emerge—soon enough or ever—from the commercial sector.
These technologies need to be developed by the DOD. For all other
elements of wireless systems, new commercial developments need to
be tested and evaluated on a continuing basis in military exercises
to determine and verify their suitability for military use.
Resources available to support this approach include the federal
defense laboratories, which can provide a critical bridge between
the military's operational needs and private technology
development, and the defense industry, which is well equipped to
collaborate in the development of prototype military-specific
equipment. In addition, both the DOD and industry will continue to
rely heavily on U.S. colleges, universities, and technical schools
to provide competent engineers, technicians, programmers,
operations staff, and scientists as well as key technological
breakthroughs and innovative ideas vital to U.S. military and
commercial competitiveness.
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The DOD's technology demonstration and testing efforts need to
take into account the unique communications needs of each military
service. A mechanism is needed to help integrate the requirements
of these varying systems and find a common architecture that would
cost-effectively support the majority of those requirements. This
task might be carried out by an R&D subunit such as DARPA or an
existing Pentagon-level joint program office. In the past the
military has focused on one radio subsystem at a time, with the
result that interoperability has been minimal. The vision of the
future calls for the development of an overall system concept with
a standard set of multimedia protocols and waveforms to assure
interoperability at all echelons, with individual radios specified
and procured to work within this architecture.
4.4 Procurement
3. The DOD should plan a new approach to procurement that will
identify now commercial infrastructure systems and subscriber
equipment can best be used for military purposes and how to
purchase commercial equipment in the most productive way.
The DOD needs to develop models to analyze how best to use
commercial systems and equipment. For example, a planning system
could be created to keep track of commercial communications
infrastructure and service access points within a given
geographical region. Such a system needs to be comprehensive,
monitoring telephony, data, microwave, satellite, and fiber-optic
services. The analysis could suggest the most effective means of
information delivery for each source-destination pair, identifying
the gateways and networking software required for interoperability
and backup paths. This information could be used for planning or
real-time robust network management, performance optimization, and
repair.
A system is also needed to evaluate the potential for defense
equipment based on commercial technology by comparing the
cost-effectiveness and performance of legacy hardware to those of
COTS products. In many cases COTS components may represent
improvements for the military in terms of cost-effectiveness, power
effectiveness, or other important features. The planning system
needs to be capable of determining how to apply these internal
components to anticipated defense applications. As part of this
process the DOD needs to develop the expertise necessary to
translate its operational needs into requirements for commercially
available equipment.
Once the DOD identifies how best to use commercial technologies,
cost-efficient acquisition strategies need to be pursued.
Governmentwide
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purchases might achieve greater economies of scale than would
purchases by individual military services. The DOD could use this
opportunity to become a more effective customer. Government
acquisitions often add specialized requirements that prohibit
commercial suppliers from offering bulk rates. As an alternative,
DOD could explore how to purchase bulk quantities of standardized
equipment and then separately acquire enhancements that align a
system with military requirements. It might also be possible for
designated agencies to acquire equipment and then distribute it to
other government users. For example, the Army Signal Corps could
acquire all DOD communications equipment and provide for all
maintenance and upgrades on the basis of annual agreements.
Consolidated purchasing might enable cost-effective reliance on
commercial providers for maintenance, logistics, and training.
The DOD can also foster commercial-defense synergy by allowing
multiple vendors to build subsystems that meet open hardware and
software interface standards and by selecting, from the field of
possible vendors, those most capable of creating an ongoing
competitive production over a long, sustained product life. When
procurement programs keep at least three vendors competitive over
the course of the system, the competition encourages the evolution
of advanced features and improvements in cost-effectiveness. The
STU III is an example of a fully competitive acquisition in which
three vendors competed for production of several hundred thousand
secure telephones. In this controlled market, each vendor was
required to meet open interoperability standards, but each was also
allowed to implement unique features and functions to attract
market share and compete on price. Each competitor's model updates
kept the other competitors busy matching features and prices, a
process that benefited both the users and the government while also
motivating ongoing technology insertion. The SpeakEASY radio is
another example of open system architecture. An "open system forum"
allows contractors to participate in setting hardware and software
standards that specify open interfaces between system
components.
4.5 Modeling And Simulation
4. DARPA should build on current research in modeling and
simulation to incorporate the communications traffic, mobility of
network elements, and radio propagation encountered in mobile
military information networks.
The performance of wireless communications systems depends on
three phenomena: communications traffic, mobility of network
elements, and radio propagation. Accurate assumptions about these
phenomena
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need to be made if the DOD is to design, deploy, and operate
effective systems. Modeling and simulation are the best available
tools for optimizing system design and predicting performance.
Various commercial packages are available for simulating
communications systems consisting of established components and
subsystems. The quality of the results they produce depends on the
accuracy of the models of traffic, mobility, and radio propagation
that they incorporate. Good models of narrowband radio propagation
are available. However, models that incorporate site-specific
characteristics of buildings, terrain, and foliage are not
commercially mature, and there are no radio propagation models that
incorporate adaptive antennas. Furthermore, available "teletraffic"
models apply to only a few simplified conditions, and mobility
models are confined to abstract formulations (e.g., fluid flow,
Brownian motion) that have not been validated with reference to
practical conditions. As a consequence, existing tools cannot
provide realistic analyses of complex DOD systems. Much more is
needed, especially in the site-specific channel modeling, traffic,
and mobility areas, to provide military planners with the tools
necessary to rapidly deploy wireless systems in battlefield
scenarios in a wide range of modern theaters (e.g., built-up urban
areas). DARPA should stimulate research to derive and validate
models of wireless channel effects using modern modem and antenna
technology so that appropriate protocols, applications program
interfaces, and optimization algorithms can be developed.
Research leading to techniques for accelerating the run-time of
complex wireless system simulations would benefit the military and
commercial sectors alike. Simulation of communications systems is
often performed by signal-processing workstation tools, which
enable programmers to define a complex communication system by
connecting icons representing predefined building blocks. This
approach enables programming to be automated, but the resulting
simulation code is often inefficient to run. DARPA research in this
area could build on the impressive progress already made in the S3
program using parallel processors to simulate communications
systems. This program includes a wireless component that could be
enhanced to simulate the traffic, mobility, and radio propagation
conditions of military communications.
DARPA could also establish libraries of code representing
communications waveforms, protocols, source coding techniques, and
network interfaces. This research would accelerate system design
while also reducing the expense of simulation software maintenance,
which typically costs 10 times more than software creation because
of the longer time frame involved and rapid advances in technology.
Research on system development and maintenance methods will help
identify the most effective strategy.
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Finally, integrated tools are needed to assess the performance
of the subsystem elements of a software radio operating within a
large-scale network in motion over a broad geographic area. These
tools would enable researchers to explore network quality,
connectivity, stability with respect to multiple performance
criteria, and unexpected problems. Some research groups are
developing their own tools to investigate specific aspects of
performance. DARPA could attempt to provide an integrated
capability built on a common application interface that would
reveal the effects of each system component on the performance of
the entire system. This capability would enable the optimization of
an entire system rather than just the individual components.
4.6 Network Architecture
5. DARPA should initiate research to produce network
architectures that incorporate commercial products in a manner that
meets military requirements.
The performance of a wireless communications system depends in
large part on the coordination of network elements. The
architecture of a network defines these elements, identifies pairs
of network elements that communicate directly, and specifies the
protocols for that communication. Architectures for wireless
systems differ according to how terminal modems are connected
(peer-to-peer versus base-station-oriented design), how
infrastructure elements are connected (hierarchical versus
distributed), and the nature of communications with other networks.
Given the special needs of military wireless communications
networks, designers need to adopt a system architecture that takes
maximum advantage of the capabilities of commercial devices and
subsystems yet also provides unique interfaces and protocols as
dictated by military needs. DARPA research in this area is likely
to reveal new network architectures that use commercial products
and services in an innovative way to serve military aims and make
it possible to gracefully absorb new technology at the subsystem
and component level.
Within a new network architecture, many key issues remain to be
resolved with respect to protocols for wideband data services.
Current commercial systems treat each application (e.g., Internet,
telephony, video dial tone) separately instead of taking an
integrated approach. Recent discussions of nomadicity underscore
the importance of considering wireless and mobile components and
conditions as part of heterogeneous and interoperating network
contexts. Military wideband packet radios, which are designed from
the outset for integrated information services, could provide a
useful model for future commercial developments. However,
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research is needed to determine network topologies and protocols
that make full use of the capabilities of advanced radios.
In creating a new network architecture, protocols and algorithms
need to be optimized to meet the objectives of military operations.
This is a complex task. As an example of the complex
interdependence of algorithms and architecture, the design of
effective routing techniques needs to aim for the following
objectives:
•
Routing to maximize data rate, with minimum transport delay and
minimum delay variance;
•
LPD/AJ routing around vulnerable spots;
•
Routing for priority assignment and allocation of resources;
•
Routing for battery power conservation;
•
Routing for congestion avoidance;
•
Routing for access to servers for specific real-time
applications; and
•
Routing to avoid anticipated propagation anomalies.
The difficulty of achieving all these objectives is compounded
by the need to run several different applications on mobile
terminals at any given time. Therefore, the routing protocol needs
to process each packet individually to meet the QoS objective of
the transmitting application.
In a mobile wireless network it should not be necessary for
every terminal to assume the size, weight, power, or cost burdens
associated with critical network services such as database servers,
image servers, speech recognition and synthesis, transcoding,
transcrypting, position location, and health and safety reporting.
Research on network architecture can identify the best way to
distribute services among network elements with the aim of
minimizing the weight and power consumption of devices carried by
personnel.
4.7 Network Security
6. DARPA should conduct research aimed at understanding and
bridging the differences between security needs in commercial and
military networks.
The commercial sector is developing a variety of data networking
products and services that are likely to be integrated into
single-hop base-station-oriented architectures. However, as noted
in Chapter 3, the requirements for security in commercial networks
are likely to be different from those in defense applications. For
example, troops on the ground or ships at sea that do not wish to
disclose their location require high
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LPD/I performance levels, a requirement not faced by most
commercial users. These differences need to be understood and
accommodated if the military is to make use of commercial wireless
technologies.
Research is needed to improve the information-processing
features of encryption techniques. For example, the end-to-end
encryption of all traffic would obviate the need for interfaces or
bridges between interacting systems. However, current end-to-end
encryption algorithms require considerable network overhead to
establish the cryptographic synchronization. DARPA could seek
improved methods of end-to-end cryptographic synchronization
through multiple networks with lower overhead than is currently
possible. With such methods, the time required to establish
cryptographic synchronization between active network members would
be reduced significantly.
DARPA also needs to design network protocols that allow for
commonality of hardware and software interfaces as well as security
differences that meet the needs of all applications. These
protocols need to provide multilevel security, long identified as
important for defense communications. The concept entails the
shared use of a network by individuals with differing authorization
to access information of differing levels of sensitivity.
Multilevel security implies the management of access to computing
and communications systems and to information transmitted or stored
in those systems commensurate with individuals' authorization. In
general, the design and implementation of multilevel security
systems have been imperfect, and wireless and mobile applications
compound the challenge.
4.8 High-Density Communications
Platforms
7. DARPA should conduct research aimed at reducing co-site
interference.
Military ships, combat aircraft, UAVs, and mobile battlefield
systems all operate a variety of communications systems in close
proximity to one another. The placement of numerous radios in the
same general location typically results in interference and many
compromises. Current technology designed to reduce the effects of
co-site interference on radio performance is quite limited. For
example, power combiners can connect up to five transmitters to a
single antenna, but only if the frequencies are sufficiently
separated. Receive co-site filters can suppress the carrier of
colocated transmitters, but broadband signals are not suppressed
adequately.
DARPA needs to catalog the co-site enviromental of military
platforms, identify both common and unusual problems, and design
more effective
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solutions that are useful over a broad spectrum, such as 2 MHz
to 2 GHz. Specific co-site problems that require attention include
the following:
•
The performance of an antenna changes when it is used near
another antenna or metallic structure, and it no longer provides
the expected beam shape.
•
Electronic equipment emits low levels of RF radiation from
internal local oscillators and data buses—signals that
represent interference to other radio receivers.
•
Radio receivers transmit a small amount of the local oscillator
frequency used to tune the radio, causing interference that is
particularly problematic if the local oscillator is dithered,
hopped, or modulated.
•
Radios transmit signals not only on the intended carrier
frequency and modulation but also (in an attenuated fashion) on
carrier harmonics, intermodulation distortion products,
intermediate frequencies, up-conversion local oscillator
frequencies, and the broadband noise of each power amplifier
stage.
This R&D would be timely because the full deployment of
software radios operating in broad frequency ranges will depend on
the development of techniques to overcome co-site interference and
high-power in-band interference. When serving several subscribers
in the same band and mode at the same time, SINCGARS radios would
experience significant interference. Significant technical
challenges are associated with co-site interference generated
within the software radio itself. Additional issues arise in the
effort to enhance mobility of forces. For example, consolidating
multiple mechanized vehicles will produce a single command track,
which now has six SINCGARS radios and the usual problems associated
with multimode radios, regardless of whether the radio architecture
is analog, digital, or software based.
Research is also needed to minimize the interference caused by
the spurious emissions (spurs) and harmonics of co-site
transmitters and receivers. When many radios are co-located or
networked, it might be possible for each transmitter-receiver pair
to agree on a common frequency plan to minimize spurs and harmonics
or on common filtering plans to minimize co-site degradation.
Additional research topics could include time and frequency
management (to prioritize transmissions) and reception time slots
(to minimize the impact of transmissions likely to cause
interference to other receive channels). Such research would need
to take into account the interleaving and coding present in each
channel and the likelihood of interference. Finally, a new
generation of highly linear filters is needed with improved agility
and a wider range of operating frequency than is currently
possible.
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Representative terms from entire chapter:
software radios
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4.9 Software Radios
8. DARPA should carry out research and demonstration projects
designed to field software radio technology for military
applications.
Software radio is a far more versatile technology than the term
''radio" implies. These radios can operate as part of a network and
perform a vast array of electronic and computational functions
(e.g., database management, transcryption) through the downloading
of software. A networked radio can function in ways not envisioned
when the component is manufactured. In a military scenario, some
radios can function as active interrogators while others act in a
passive manner, undetected but coordinated by active units.
Similarly, software radios can perform services that until now were
unique to each application.
A software radio could be a leading part of the C4I infrastructure: With appropriate
software it could be applied to signal intelligence, electronic
intelligence, communications navigation and identification,
electronic warfare, information warfare, electronic
countermeasures, missile tracking, guidance, or commercial paging
or telephony. The cost-effectiveness of software radios increases
with the number of available software functions, the ease of
performing new tasks, and the ease of cooperating with other
network systems to accomplish larger tasks. The multiple roles that
can be played by software radios could have an impact on DOD's
organizational structure, as services provided by individual
organizations and procurements are combined in one system.
Commercial software radios are likely to have more limited
capabilities (e.g., changing signals and bandwidth on a single
frequency) than are military versions, which are being designed to
span large frequency ranges and implement many legacy waveforms.
However, the commercial sector is achieving rapid advances in many
software-radio components, such as A/D converters, DSP chips, RF
amplifiers, displays, batteries, and data-storage devices. The DOD
can use these COTS products to good advantage. At the same time,
DARPA needs to undertake specialized R&D focusing on antennas
(Sections 4.10) and filters (Section 4.12) for military
applications. Exploratory research on novel components and designs
could also be beneficial (Section 4.13). In addition, to identify
any necessary improvements and make optimal use of this promising
technology, DARPA needs to demonstrate software radio technology on
defense platforms where density, power, and weight are
critical.
4.10 Smart Antennas
9. DARPA should conduct the research needed to adapt smart
antennas for mobile military applications.
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Commercial applications for smart, adaptive antennas are limited
to relatively low-cost and unsophisticated single-band units with
limited flexibility in beam pattern. Moreover, virtually all
existing adaptive antennas for mobile radio applications are
designed for use at base stations rather than mobile units.
Military applications require antenna functionality for several
orders of magnitude of frequency coverage as well as electronic
tuning, the coupling of more than one transmitted and received
signal, and a diversity of beam shapes ranging from omnidirectional
to pencil beams. Several technical challenges need to be overcome
before such antennas can be produced. DARPA research needs to
pursue the following objectives, among others:
•
Achievement of useful antenna performance over a large frequency
range;
•
Development of low-cost techniques for implementing beam shaping
with affordable companion electronics that implement the phase
shifting and weighting (i.e., the electrical process of feeding
antenna elements); and
•
Improvements in the cost-effectiveness of frequency-agile
couplers with built-in co-site filtering.
4.11 Smart Waveforms
10. DARPA should conduct research to produce transmission
techniques that adapt to a wide range of operating conditions.
Commercial communications systems tend to operate under
predicable, stable conditions. Therefore, commercial
waveforms—characterized by frequency band, bit rate,
modulation method, and source coding and channel coding
techniques—are designed for operation over a limited range of
conditions. For example, high-tier systems are designed for one set
of conditions, whereas low-tier systems are designed for a
different set of operating conditions. The operating environments
and transmission conditions encountered by military communications
systems are more unpredictable and subject to change. Military
systems would therefore benefit from transmission technologies that
adapt to changing conditions.
Recent research has produced theoretical results on the
optimization of bit rate, modulation method, source coding, and
channel coding techniques. New DARPA research should be aimed at
uniting the theory of adaptive modulation and coding with the
emerging technologies of advanced software radios (Section 4.9),
which promise a practical means of implementing adaptive schemes.
The results of the recommended modeling and simulation research
(Section 4.5) would provide valuable tools for performing this
work. In addition to meeting military needs for adaptable
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systems, this work could serve the long-term needs of the
commercial sector by providing core technologies for integrated
personal communications systems that combine the advantages of
several of today's separate systems.
One objective should be to develop modulation and coding
strategies that allow the demodulator to acquire the properties of
interference in the time domain, the frequency domain, or the
constellation domain so that the interference can be avoided or
canceled. These techniques would concentrate energy in the regions
of frequency, time, or code space that provide the greatest link
margin. Another objective should be to produce transmission
techniques that establish a data rate consistent with interference
and fading conditions. These techniques should allow for selection
of a data rate over two or more decades of bit rate. They should
also be designed for rapid carrier synchronization time, rapid
timing synchronization, and minimum training time for learning
channel conditions. The choice of data rate should be consistent
with adaptive source coding, which gracefully degrades the
perceived quality of the end-to-end application as the channel
quality declines.
In addition, for defense applications new waveforms are needed
that allow for very high spread-spectrum processing gain. Typical
spread-spectrum techniques require considerable computational power
to be detected by the desired receiver. The high signal-processing
gain required to jointly discover frequency offset, baud boundary
(or baud boundary offset), and spreading code alignment at these
high processing gains is inconsistent with traditional portable
radio design. In addition, new spread-spectrum waveforms are needed
that allow for further reductions in the detectability of
transmissions, direction, and properties of the spread signal.
4.12 Filter Technology
11. DARPA should conduct research to overcome the limitations of
current filter technology for use in military software radios and
high-density platforms.
Both software radios and high-density platforms require advanced
filters. Filters currently constitute 25 percent of the volume of a
typical software radio and are used in receive preselectors, power
amplifier output filters, local oscillators, and mixers. To a great
extent the radio receiver's sensitivity and dynamic range are
determined by the selectivity and the losses of the preselectors,
and the radio's co-site performance is determined by the
selectivity of output filters and the filters in the local
oscillators and upconverters. Active and digital filters are not
appropriate for these functions because they introduce noise that
degrades performance. Handheld
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software radios would be improved by the miniaturization of
filtering functions and improvements in frequency tuning range and
selectivity.
There is also a need for filters that cover wide frequency
ranges. Commercial radio equipment spans either a small frequency
band or a few selectable bands. Military radios that span wide
frequency ranges (such as 2 MHz to 2 GHz) require lumped filters
built of inductors and capacitors to handle the low end of the
frequency range and transmission-line techniques for filtering at
the high end. Research is required to identify new circuit and
materials technologies that allow for tunable, highly selective
filters that span this entire range, operate at higher power
levels, and take up less physical volume. In addition, new
processing techniques are needed to enable the monolithic
integration of highly selective filters with semiconductor devices
and to produce multisection filters in more sophisticated
shapes.
4.13 Novel Components
12. DARPA should develop novel components to enhance the
flexibility of software radios.
Software radios have been significantly enabled by novel
components, notably DSPs and FPGAs, which allow new waveforms to be
added to fielded systems through the installation of new software
and hardware. Additional novel components could further enhance the
flexibility of software radio architectures. For example,
components could be designed to reduce the equipment size, weight,
and power needed to accommodate military designs that incorporate a
wide range of potential future waveforms and large numbers of
legacy waveforms. Radio performance could be improved through
research into new DSP architectures that have adaptive resolution,
clock speed, instruction sets, memory architectures, and arithmetic
functions designed to the specific signal processing of
communications systems.
Similarly, novel FPGAs that either interconnect analog circuit
elements or integrate analog and digital operation could be of
great value. A chip containing analog circuit elements and FPGAs
could be applied to the analog front-end functions of many
communications systems while also providing a broad range of
interfaces to other systems. Research is required to identify the
semiconductor processes and circuit topologies that provide
sufficient isolation between the various analog functions,
interconnection with minimal loss, and circuits with sufficient
versatility to be configured for many applications.
Finally, the monolithic integration of nearly the entire
software radio function could provide experience in combining
analog and digital signal
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functions on a common substrate. It is currently difficult to
keep digital noise from coupling into analog circuitry and
degrading performance. Research on how to implement
wide-dynamic-range analog circuitry monolithically with digital
circuitry would provide the basis for implementing an entire
software radio on a single component. Such an effort needs to
encompass the monolithic implementation of high-performance filters
that are tunable over large frequency ranges.
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