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Returning to passive radar

Rick Blum and his team are using advanced signal processing techniques to produce models for radar that can be tools for the design of real-world products.

What do you envision when you think of radar? Massive, rotating antennas at airports tracking your last flight? Colorful precipitation maps presented by your local TV meteorologist? Beams sweeping the sky in search of incoming missiles?

Get used to experiencing radar much closer to home, says Rick S. Blum, the Robert W. Wieseman Professor of electrical and computer engineering. Thanks to a concept known as multiple input multiple output (MIMO) radar, which Blum is credited with inventing with a team of academics and Bell Labs collaborators, sensors are getting better at identifying smaller objects at close range—perhaps that pedestrian crossing the street in front of you, or the burglar entering your side window.

"I'd like to see radar work its way into commercial devices, and this seems to be happening, maybe first with automobiles," says Blum. He currently has National Science Foundation funding to do the math to quantify the feasibility of passive radar, which uses advanced signal processing techniques to measure reflections from objects illuminated by ambient radio signals in the environment—cellular phone transmissions, AM, FM, and TV broadcasts, for example.

The idea of passive radar is nearly as old as radar itself. Before and during World War II, the British deployed transmitters and receivers to create an active radar net around the south of England, which could detect Nazi aircraft forming over France. For their part, the Germans figured out how to detect RAF fighters over the North Sea by measuring beams from the British transmitters reflecting off of them.

With conventional active radar, a central transmitter sends high power pulses of radio waves in many directions. If a target, such as an aircraft, is in range, the signal will bounce back to the originating antenna (the detection part of radar). The time it takes to receive this return signal indicates how far away the object is, and the Doppler effect uses small shifts in that signal's frequency to determine whether the target is moving away or towards the radar station.

MIMO radar is derived from the principles of MIMO communications techniques that give you faster Wi-Fi with multiple antennas and allow stronger cellular coverage. Blum worked with researchers at Bell Labs who made the surprising discovery that adding transmit and receive antennas "significantly increased the rate that data could be passed," he says. "Nobody expected that."

"The idea is simple: If you want to understand a situation better you look at it from different points of view," he says. "Radars located far apart provide quite different views of an object. Combining these streams of data gives more reliable object detection." The technique also makes it easier to identify and reject spurious emissions and clutter in the signals.

Their highly cited work on MIMO radar led many to credit Blum's team with inventing the concept. A series of three articles introducing MIMO radar have been cited nearly 4,000 times. A 2016 analysis by the Canadian government found that Blum is in the top 1 percent of researchers cited in the radar field.

Read the full story in the 2017 Vol. 2 edition of the Lehigh Research Review.

-Bob Fisher is a freelance writer with Lehigh University's Office of Communications and Public Affairs.

April 11, 2017

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