Saving energy with censored sensors

	Rick Blum

Since when has censorship become a green activity?
Since Rick Blum and other experts in signal processing began seeking to make sensor networks and radar arrays more energy-efficient.
Sensor networks, says Blum, the Robert W. Wieseman Chair in electrical engineering, are playing a growing role in everyday life.
On a bridge, for example, sensors detect and process the signals caused by the stresses from passing vehicles and then transmit data to a central decision point. This helps engineers determine more quickly and accurately the likelihood that a bridge will fail.
Sensors also detect the presence of radioactive or other toxic waste in the environment. They can determine whether or not a cell is malignant. And they are used in military surveillance, air traffic control, home automation and other applications.
Most sensors transmit data wirelessly, says Blum, who recently received four major research grants in the area. They are powered by batteries, which use large amounts of energy.
Blum creates mathematical formulas that allow sensor networks to cut energy consumption without loss in performance. Two of his new grants are enabling him to apply his energy-saving techniques to a new type of radar system called MIMO radar.
Smart sensors
Energy in a sensor network is saved, Blum says, by designing smart sensors. Because not all the data detected by sensors is equally important, engineers seek to design sensors that “censor” unnecessary data by not transmitting it.
In a network of smart sensors, says Blum, “Each sensor looks at its own data and judges whether or not that data is informative. If not, then the sensor does not transmit, thereby saving energy.”
Blum discussed this concept in July in the journal IEEE Transactions on Signal Processing. The article detailed advances made to date and described how Blum’s new work could lead to further energy savings.
“Researchers have made a convincing case for a mode of operation called censoring, where sensors transmit only highly informative observations,” Blum wrote. “These sensors transmit only for very large or small likelihood ratios. We refer to this approach as per-sensor censoring.”
Blum’s article introduced a new concept—the ordering of sensor transmissions, in which the sensors with the most important data transmit their information first.
“Each sensor node can, on its own, determine when to transmit based only on the transmissions of the other sensors. The process stops when sufficient evidence is accumulated to decide for one hypothesis or the other.
“Our analytical proofs,” Blum said in an interview, “have shown that we can cut energy consumption by more than half with no loss of performance.”
Blum’s article, co-written with Brian Sadler of the Army Research Laboratory, was titled “Energy Efficient Signal Detection in Sensor Networks Using Ordered Transmissions.”
Grants from four federal agencies
Blum recently received major awards from four federal agencies. Grants from the National Science Foundation (NSF) and the Army Research Office grants will support work in sensor networks. Grants from the Office of Naval Research and the Air Force Office of Scientific Research will support work in MIMO radar systems.
MIMO—multiple-input and multiple-output—communications systems improve performance by using multiple antennas to transmit and receive radio signals.
MIMO principles have been applied only recently to radar, says Blum, but they have shown promise in overcoming the shortcomings of traditional radar systems. The older systems determine the distance and location of a target by sending signals from one point out into space and measuring the time required for the signal to bounce off the target and return.
Several factors can reduce such a system’s reliability, Blum says. The angle of the target might shift, weakening the returning signal. The outgoing signal might hit several objects and diminish, or it might miss the object being tracked.
“With regular radar, you often have to ‘see’ the target from just the right direction in order to get an accurate picture of the target,” says Blum. “This is due to the fact that the magnitude of the return signal can change drastically because of a small change in the angle of the target.”
MIMO overcomes these obstacles by transmitting radio signals from several locations.
“If you see from multiple viewpoints,” says Blum, “you are going to get a much better picture. One or two illuminations might come back weak, but not all of them. And the antennas in a MIMO radar system do not need to be very far apart in order to get a usable return signal.”
The grants from the Office of Naval Research and the Air Force Office of Scientific Research will enable Blum to investigate, among other things, how sensor censoring in networks can be adapted to achieve energy savings in MIMO radar systems.
Blum taught a tutorial on MIMO radar earlier this year at the 2008 IEEE International Conference on Acoustics, Speech, and Signal Processing in Las Vegas, Nevada. IEEE, the Institute of Electrical and Electronics Engineers, is the largest organization of its kind in the world.
Blum also co-authored an article “MIMO Radar with Widely Separated Antennas,” which was published by IEEE Signal Processing Magazine in January.
In his 25-year research career, Blum has received numerous awards from NSF, the Army Research Office, the Office of Naval Research and the Air Force Office of Scientific Research, as well as the Army Research Laboratory and the Pennsylvania Infrastructure Technology Alliance.
--Kurt Pfitzer