| When U.S. armed forces move
into hostile territory, establishing communications takes top priority.
Though we haven't seen the last of the traditional radio operator,
today's mounted soldier is more likely to use a "mobile ad hoc network,"
or MANET, to share and receive information with others in the field
and at the command post.
These rapidly deployable, wireless networks exchange vital information
in the form of digital packets—without the need for miles
of cable or bulky central base stations. Yet, while MANETs have
already proven their value in the field, researchers at MITRE want
to improve them to meet the warfighter's future job requirements.
MANETs offer several advantages over traditional radio networks.
They are harder to disrupt because of the way they're designed:
every part of the network is equally important. Each unit, most
often placed in a mobile platform such as a tank or communications
vehicle, acts as a router for the next unit. Losing a piece of the
network doesn't collapse the whole.
Nevertheless, there are several areas open to improvement. MANETS
use a common radio frequency (RF) channel throughout any given network,
which sometimes results in "packet collisions" between sets of digital
data. These collisions reduce the amount of information moving through
the common channel, slowing information flow and sometimes halting
it altogether. Other potential problems include hostile jamming
and signal fading that occurs as the transmissions bounce off surrounding
objects.
MITRE believes we have devised a method for increasing data throughput
and communications range—a method that will also increase
the flexibility of the system and reduce enemy interference. The
secret lies in finding the right mathematical algorithms to enhance
and direct RF signals transmitted and received by antenna arrays
—without physically pointing the antennas.
"The Army is already doing ad hoc networks among tanks, Humvees,
and so on, using omnidirectional antennas," says MITRE's John Fite,
a principal digital signal processing engineer. "But it would greatly
benefit from better antenna designs and algorithms that provide
anti-jamming capability and greater capacity. We are in the process
of developing improved algorithms."
Antennas: From One Wire to Many
The simplest antennas are single antenna elements (often just a
wire) that broadcast in and receive from all directions. The physical
nature of these omnidirectional antennas limits their effectiveness—they're
prone to interference, require power amplification to increase their
range, and reach only a portion of their intended audience. That
potential unreliability isn't really a problem for a radio station
blasting 50,000 watts of rock and roll, but for battlefield personnel
it might mean the difference between life and death.
Over the years, engineers have devised more efficient and sophisticated
antennas. A flexible and effective solution is the use of antenna
arrays. Typically, an antenna array consists of a circle or grid
of antennas instead of just one. By combining the outputs from each
antenna in different ways, the designer can form a composite antenna
beam that points in desired directions. An array becomes a "smart
adaptive antenna" when you attach the right kind of digital signal
processing system to automatically determine the proper combination
of signals from each antenna, constantly adapting the combination's
weights to keep pointing the antenna in the right direction.
Adaptive arrays aren't new. But at MITRE, scientists are working
on new and better signal processing algorithms for the arrays—algorithms
that can cope with the warfighter's increasingly complex need for
more data in a rapidly changing environment.
"Arrays give you higher data communication capacity than single
antennas," Fite says. "But in typical adaptive array processing,
only one thing is moving fast—either the antenna or the thing
you're tracking. We want to have two antennas able to point to each
other on the fly, constantly adjusting."
Much of MITRE's research focuses on what's called "space time adaptive
array processing," or STAP. More specifically, our scientists are
attempting to achieve their goals through "blind STAP," an approach
that adapts the antenna without requiring specific knowledge of
the antenna array geometry, the propagation channel, or the direction
of arrival of the signal.
In the typical antenna array, operators have both a fully calibrated
array and the knowledge of what direction the signal is coming from.
(A calibrated array has been painstakingly measured and tuned.)
Each antenna element of the array is connected to a digital radio
that outputs phase-shifted versions of the received signal. If you
take all the outputs and feed them into a digital signal processor—"do
the math"—you create a combined signal that looks like the
signal from a directional antenna.
Unfortunately, most of the algorithms for electronically aiming
antennas that achieve high performance are computationally intensive.
Blind STAP's characteristics make it a promising alternative to
current adaptive array algorithms.
"Blind STAP is unique. Regular algorithms rely on a calibrated
array, but in war, part of the antenna could get blown off or a
tank might get a new piece of hardware that makes your original
calibration measurements obsolete," Fite says. "To achieve optimum
radio communications, you need to know three things: the channel,
the antenna, and the signal. The signal is what we really know the
best. With blind algorithms, we exploit knowledge about the signals
to learn about the antenna and the channel."
"Using blind STAP algorithms, we only need to know qualities about
the signal itself," adds Larry Thomson, a MITRE communications engineer.
"No calibration table is necessary, so changes in the field are
no problem. We don't even need to know what direction the signal
is coming from. Instead, we use knowledge of characteristics of
the signal to adapt the array so that it's pointing in the right
direction. We feed information into the computer that gives the
same output as if we used a directional antenna."
"Our algorithms are also computationally efficient—they don't
eat up much computing power when compared to conventional adaptive
array algorithms," Fite adds. "It's the equivalent of getting a
faster modem on your computer. With the reduced computational requirements,
you can simplify the radio system or deploy the extra resources
to another task."
"Some of this has been done before," Thomson says. "But we think
we can do it better. At MITRE, we want to know, what is the best
way to accomplish this in the context of the military's needs? What
is best for the warfighter? We're trying to build algorithms for
use in the battlefield—for more people, less jamming, and
less interference."
Rules to Live By
MANETs need more than better signal processing to work properly.
Radio networks also require sets of rules—protocols—that
provide guidelines, such as "whose turn is it to talk next?" Unfortunately,
most protocols govern omnidirectional antennas and don't take advantage
of newer technology. MITRE hopes our work in blind STAP will bring
the creation of new protocols a step closer. (A separate team of
our employees is currently pursuing the protocol issue.)
"Avoiding collisions are what protocols are about, for wired or
wireless communications," Thomson explains. He envisions a time
when a MANET user in the field could actually receive and interpret
two simultaneous signals. Instead of experiencing a packet collision—the
likely outcome now—the receiver could store one set of signals
to listen to later. Though this would require advanced hardware,
software, and a set of protocols to match, the scenario is well
within the realm of possibility.
"In the information age, there is a constant demand for faster
and more reliable communications," Fite says. "Especially in the
warfighting environment, this is a tall order. These are robust
algorithms. They work under difficult conditions, even with damaged
antennas or in an environment with lots of interference. We believe
the blind STAP algorithms we're developing at MITRE are a critical
next-step for the continuing success of mobile ad hoc networks." |
