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A Trail of Bread Crumbs: Improving Radio Communications with Wireless Relays
Two U.S. Coast Guard officers step cautiously down a flight of stairs into the deepest hold of a cargo ship. The foreign vessel had been stopped by the Coast Guard shortly after it entered American waters. The nine-man Coast Guard team boarded the ship to search it for contraband: weapons, drugs, and illegal immigrants. Three two-man teams fan out to search the holds while two officers monitor the crew and the commander maintains radio communications from the bridge.
The beams of the officers' flashlights sweep across the passageway. Suddenly, a figure is illuminated crouched in the shadow of a doorway. There is a gleam of metal in the figure's hand. The officers command the man to raise his hands and step into the light, but instead he bolts down the corridor. Is the man armed? Is this an ambush? One of the officers shouts into his radio for back-up, but the radio spits out only static. Their commander is eight decks above and 600 feet aft of their location. The radio can not penetrate such a vast amount of bulkhead.
The Solution Is Just Around the Corner
Whether in the service of a squad of Army Rangers exploring a cave complex in Afghanistan or a National Guard convoy wending through the tight alleys of an Iraqi city, radio communications can be hampered by surrounding materials and structures. In these cases, "around the corner" often means "out of range." As the scenario above shows, this is an ongoing problem on board ships.
The instinctive solution to this problem is to increase the strength of the transmission. "The Coast Guard did some tests with different frequencies and different radios," explains Thomas Bronez, a principal signal processing engineer at MITRE. "They were hoping that with just the right frequency and just enough power they'd be able to establish radio links even through the thick metal of a ship. The tests proved that that was not a viable solution. When radio waves can't get through, they can't get through at all."
Bronez, however, has developed a solution that avoids the "brute force" approach. His solution employs small wireless relays to establish a path of multiple, short-range communication links. As a radio operator travels deeper into an environment that blocks or reflects radio transmissions, he leaves behind him a trail of the wireless relays. When the operator communicates, the radio transmission doesn't have to penetrate all the way through to the receiving unit. It merely has to reach the nearest relay unit. The relay then rebroadcasts the transmission, giving it enough boost to reach the next relay. By skipping down the trail of relays, the radio transmission can reach its destination loudly and clearly.
"Some people don't understand how in this day and age we can still have difficulties with effective communication," says Bronez. "'I've got a cell phone and I can talk to anybody,' they think. What people forget to take into account is that when they talk on their cell phone, there is a high-powered cell tower nearby to transmit the call. The wireless relays are like carrying your own little cell towers with you. Every unit is a tower."
The idea for wireless relays arose from Bronez's work on MITRE's Netted Sensors Initiative, which is exploring the concept of obtaining information from inhospitable locations by scattering a network of small, battery-powered sensors. "Both netted sensors and wireless relays rely on multi-hop communications," says Bronez. "That is, each node isn't asked to transmit its data all the way to its destination. Each node just needs to transmit the data to its neighbor, and that neighbor will transmit it to its own neighbor. And eventually the data will get where it needs to go."
When MITRE's Service Intelligence Department identified interior communications as a problem for several sponsors, researchers from MITRE's Signal Processing Center saw how the concept of battery-powered relays could serve as a solution if they were small enough and could support the strenuous network requirements of digital voice communications. Together, the two departments obtained funding from the MITRE Technology Program and successfully turned the concept into reality.
With a backpack full of wireless relays, a soldier could leave behind the equivalent of a bread crumb trail of devices as he pushed his way into hostile territory. The current prototype of the wireless relay measures 4" x 8" x 3" and with its package of electronics and batteries weighs about four pounds. Commercial production would easily reduce both the size and weight of the units, perhaps to the size of a deck of cards with current technologies. "Ideally, you'd like to get to the point where they are small enough and cheap enough so that you can leave them behind," says Bronez. MITRE is concentrating on further developing the capabilities of the relays, enabling them to retransmit not only voice broadcasts, but also support point-to-point transmission of sensor data and video.
To test the prototype, MITRE conducted a trial that mimicked the Coast Guard's board and search procedure. By trailing out seven relays in passageways and stairwells of the Coast Guard test vessel State of Maine, plus two more in the engine room (whose bulk of equipment blocks direct radio transmissions), the MITRE team succeeded in creating radio links from every corner of the ship to the bridge. Encouraged by this success, the Coast Guard ordered a supply of prototypes to conduct its own tests.
Under the Sun and Beneath the Ground
In a series of experiments conducted in a cave system, MITRE successfully demonstrated the effectiveness of wireless relays in subterranean environments. News of this demonstration reached the ears of city officials in Washington, D.C. who had been dismayed to discover that the radio systems of their emergency responders would not operate in the underground tunnels of the Metro system. The officials have contacted MITRE about installing wireless relays along the subway lines to ensure that in case of a disaster first responders could effectively communicate.
MITRE is also seeking to provide a simpler and more affordable method for monitoring our borders by combining its research in wireless relays and netted sensors. Hundreds of small sensor arrays, each equipped with a wireless relay, could be placed along portions of the border that are difficult to patrol. As the sensors transmitted their data along the relay, their power requirements would be minimal—small enough to be supplied by a solar panel and battery. MITRE has just completed development and testing of a prototype of such a solar-powered relay node with surveillance sensors. "We're trying to provide a communication infrastructure for border monitoring sensors," says Bronez.
By taking new approaches to old problems—the "bread crumb trail" solution to radio transmissions as opposed to the failed "brute strength" solution—and exploiting its vast and varied resources—exploration of netted sensors inspiring the development of wireless relays—MITRE continues to transform the way its sponsors communicate.
—by Christopher Lockheardt
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Page last updated: May 17, 2006 | Top of page
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