Pushing the Ultrawideband Envelope
Many questions about the technology remain unanswered.
Researchers are investigating the use of alternative radio transmission methods for military information systems. A recently launched program examines the use of ultrawideband technology in robust, scalable communications devices and networks, in radar and in collision avoidance sensors.
As the U.S. military moves to a fully networked presence on the battlefield, rapid data transfer across hostile radio environments is needed. Small units and individual soldiers become nodes in a web reaching from tactical combat zones to unmanned and manned aerial platforms, ground vehicles, ships, satellites, regional commands, theater and national headquarters. Connecting these various points while avoiding jamming and spectrum issues is a major consideration for system designers.
Spectrum allocation and interference issues become more evident at the tactical level, where local atmospheric phenomenon, dense foliage or urban terrain may make communications difficult. Ultrawideband (UWB) radio technology presents one possible solution because of its unique capabilities. Instead of transmitting across a single band of spectrum, UWB devices broadcast very short pulses of data across multiple frequencies. This makes UWB transmissions very difficult to intercept, and because the pulses are across several gigahertz of bandwidth, they may avoid frequency conflicts common to conventional radios.
Although new to the commercial sector (SIGNAL, April, page 21), the U.S. military has used UWB technology since the late 1970s. Much of the technology was classified and developed for use in low-power, covert communications devices, ground-penetrating radar, collision avoidance and telemetry systems for unmanned aerial vehicles (UAVs).
The goal of the Arlington, Virginia-based Defense Advanced Research Projects Agency’s (DARPA’s) networking in the extreme (NETEX) program, Arlington, Virginia, is to create new wireless technologies that weave robust networks in complex and hostile environments while coordinating the assignment of available spectrum. The effort focuses on discovering additional capabilities for UWB radios and exploiting the technology’s unique properties to form scalable communications systems.
The program seeks to advance the development of UWB radios and a complementary suite of networking and medium access control (MAC) layer protocols and algorithms that can maintain communications networks under heavy jamming, adverse atmospheric conditions or terrain interference. Designers envision the development of applications such as rapidly deployable distributed unattended sensors for urban monitoring and surveillance and network-based precision geolocalization systems to track personnel, vehicles and sensors in areas where the global positioning system (GPS) is ineffective.
According to NETEX Program Manager Stephen Griggs, despite years of research much still is not known about UWB. Although it behaves differently than conventional radios in a number of ways, until recently there has been no coordinated effort to study its interaction with other systems. “Every single claim about the technology has a huge number of issues behind it. For example, how does it behave differently from a narrowband system?” Griggs says. A key issue to be addressed is how to ensure that UWB does not jam or interfere with other UWB devices. Other factors the program will explore are the best ways to build multiple access systems and what happens to routing layers in UWB systems, he says.
While the goal of NETEX is to research new networking approaches based on UWB, Griggs believes that much of the work actually will center on applying the technology to meet specific types of military needs. UWB can be used for a variety of related functions such as geolocation or personal networking. All of these applications have different solutions, he says, adding that despite years of study and development, the military has only deployed UWB systems in small numbers. Because so few devices are operational, little is known about how they interact with each other. “There’s all kinds of ways you can make six or 12 devices work together. But what about 50, 500 or 1,000?” he offers.
Griggs declines to speculate about the project’s outcome, noting that the first phase of the program has just begun and that it will be more than a year before concrete data is available. However, he is sanguine about the potential to develop different kinds of operations and functions for networking systems. “There are a great number of caveats,” he cautions.
NETEX has three phases across a five-year timeline. If the program is successful, two 18-month phases will be followed by a one-year trial phase. Launched early this year, the preliminary phase seeks to understand the effects of UWB systems and their operation on existing military spectrum users. “Hopefully, by doing that we can figure out what you don’t want to do with UWB. This will let us understand where UWB can be operated in a manner we believe will provide military utility,” he says.
This first phase is vital to the program’s future development. Researchers will gather data allowing them to understand the best operational frequency bands, bandwidth, modulation schemes, and power levels for UWB networking systems. This also will determine the technology’s ultimate utility for military networking. “It’s the whole ball of information that specifies what you can do. We hope to learn what the limitations are from phase one. Right now, we’re not going to pre-vision or pre-specify where we expect to end up,” Griggs warns.
If the first phase is successful, system development will begin in the second part of the program. This will include improving existing transmitter and receiver design and generating new types of similar devices. Work also will focus on MAC and networking protocols to support UWB-specific systems. Griggs believes that these applications will differ, although not vastly, from the way current radio networks are put together. The third phase will involve a physical prototype for evaluation.
In addition, phase two will feature additional efforts beginning with the launch of an interference study to understand the effects of UWB devices on military systems better. A modeling and simulation program also will begin to verify test results and analytically predict UWB performance in regions not directly covered by experiments.
The experiments will take place in a controlled laboratory environment. Griggs notes that at this phase of the program, the only fieldwork done will be some open field measurements to support modeling and simulation efforts. The third and final prototype stage will involve field studies, he adds.
Although DARPA officials are hesitant to speculate about potential benefits of UWB technology, industry proponents are hopeful about its future. According to Dr. Robert J. Fontana, president of Multispectral Solutions Incorporated, Germantown, Maryland, DARPA has been involved with UWB systems for many years, particularly in applications for UAVs. He notes that from his firm’s perspective, the first real uses of UWB systems by the government were for UAV datalinks, collision avoidance sensors and radar altimeters.
Fontana notes that although UWB is inherently hard to detect, part of that quality comes from operating at low power levels. However, some commercial proponents of the technology are pushing for higher data rates, which will increase power requirements and reduce its military utility. “The problem is the principles by which UWB is good for low probability of detection—and conversely, low probability of interference—go down the drain because you’re putting out a tremendous amount of energy,” he says.
Military applications typically operate at much lower data rates. This is not because it is the best the system can do, but to strike a balance between low detection and moving as much data as possible. Fontana notes that his firm built a UWB wireless local area network system called Draco that could communicate up to two kilometers between nodes at data rates of 1.544 megabits per second. While he believes this is a very good example of a future tactical UWB system, he does not recommend attempting to move data at rates above 100 megabits because it would negate the technology’s ability to operate covertly.
A recent ruling by the Federal Communications Commission (FCC) restricted the outdoor use of unlicensed commercial UWB systems. Fontana observes that many commercial vendors claim their devices only operate at distances up to 10 meters. However, the actual range that these devices can operate under the FCC’s Part 15 rule is more like 70 to 80 meters. He attributes this discrepancy between what commercial firms are achieving and what can be done to receiver design. “Typically, if you only have to go to 10 meters or even 3 meters for some applications, there is no need to build a super-sensitive receiver to do that if you are already doing just fine. You build something simple, make it cheap, and it will work fine over range,” he says.
But there is a complication to this because even though a device may only receive UWB transmissions at 3 meters, another different piece of equipment may be able to detect it much farther away, Fontana observes. This creates a dilemma with GPS devices because their receivers are orders of magnitude more sensitive than commercial UWB devices, he says.
Because GPS receivers are so sensitive, Fontana suggests using a GPS receiver for UWB devices because it can detect emissions from hundreds of meters away. These reception issues also are at the heart of the FCC’s decision to severely restrict commercial UWB use until more data is collected.
When used in government applications, UWB systems typically operate in bands that are disjointed from other frequency bands or by filtering emissions to avoid interference. However, many commercial UWB operators claim that filtering would destroy their waveforms, Fontana says.
Erwin Rosenbury, program leader of the Minipower Impulse Radio Program at Lawrence Livermore Laboratories, Livermore, California, notes that UWB has advantages in urban areas where cellular telephones and other communications systems run into multipath difficulties. These dead spots in the radio environment occur when two microwaves from a transmitter cross paths and cancel each other out before the signal reaches the receiving device. This does not occur with UWB because the signal pulses are dispersed over a swath of frequencies. Although spread spectrum radios achieve some of this protection by operating over 1 or 2 megabits, UWB devices spread their spectrum over gigabits. “It’s like spread spectrum on steroids,” he opines.
UWB also is effective in radar. Commercial proponents of the technology cite its ground-penetrating capabilities and its ability to pass through walls and other structures. Although these are useful approaches, Fontana notes that UWB-based radar is very useful as radar altimeters and collision detection sensors in UAVs. The Naval Surface Weapons Center, Dahlgren, Virginia, also is developing a radar system that will detect power lines and other cables that are a threat to low-flying helicopters. Fontana explains that UWB was the only technique that was able to detect a wire in sufficient time to allow a pilot to make evasive maneuvers.