The Crowded Sky

By Marlis Manley Broadhead

Channels and packets and pausing, oh my!

The Crowded Sky

If it’s been a while since your last physics class, it helps to think of wireless messages as airborne planes, crisscrossing along narrowly delineated courses. The first step of air traffic control is to know where the planes are every minute at every elevation of every longitude and latitude. The second step is to make sure that no two of them occupy the same air space at the exact same moment, for obvious reasons.

It’s the same for wireless transmissions, radio waves traveling in prescribed channels. And like a parade of airborne planes, they don’t occupy the complete channel, but travel in packets of information—messages that are received as complete but are really chunked to reduce the risk that whole messages will be lost. This chunking creates gaps, like the spaces between planes, and these gaps allow for other transmissions to be sent through the same general space.

The Crowded Sky

So how serious is it when radio waves enter the same channels at the same time—in other words, when interference happens? If you’re working on a computer, you may experience a slowdown; on a cell phone, you may find your reception cutting out or get disconnected. Irritating, but not earth-shaking. But it’s imaginable that in the not-too-distant future—when the world is even more dependent upon wireless transmissions—there could be total network outages. In industry think tanks, this is referred to as the ‘tragedy of the commons’—spectrum overload. Catastrophic failure.

The National Science Foundation (NSF) is providing four years of funding to help WIL researchers learn how to keep this from happening—on both small and grand scales.

“The name of the game is how not to interfere. This has become the fundamental issue.” —Dennis Roberson

Channeling interference

The Crowded Sky
Dennis Roberson with wireless devices used in measuring interference in the Wireless Interference Lab

WIL Principal Investigator Dennis Roberson, who was formerly the chief technology officer at Motorola, says that the current wireless environment is not only crowded, it’s extremely complicated because the transmissions work on various combinations of time, space, and power. Roberson likens this mix to signal soup and says that the problems begin back at the design stage. Engineers figure out how to build a product by solving a specific problem, and once the product works, the engineer’s job is finished. In most cases, Roberson points out, the designers don’t have access to extensive information about the larger environment in which the product will be launched, information that if shared would help them design products that would perform even better in that larger environment and, importantly, would not impact other devices as significantly as they do today.

“The name of the game,” says Roberson about the environment of wireless communication, “is how not to interfere. This has become the fundamental issue.” Joining him on the project are professors Cindy Hood of the Computer Science (CS) department, Francis (Wu-Hon) Leung, Joseph LoCicero, and Don Ucci of the Electrical and Computer Engineering (ECE) department, and more than a dozen students drawn from CS and ECE at both undergraduate and graduate levels. This IIT team is partnering with Stevens Institute of Technology, Motorola Labs, and Shared Spectrum Company.

The Crowded Sky

With a common goal of discovering how to prevent wireless interference from impacting network performance, WIL researchers and their partners employ a three-pronged research methodology developing and comparing (1) the analytical models, (2) simulation models, and (3) laboratory-based experimental measurements. Using this approach, models for the interference generated by various devices are being developed and refined.

IIT’s efforts have been focused on the transmitter side. This involves characterizing the radio wave outputs of a large variety of mostly commonplace devices (microwave ovens, wireless local area networks, cordless phones, various Bluetooth devices, etc.).

Stevens Institute of Technology, in New Jersey, works on the receiver side, checking the effects of wireless transmissions on the networks.

Motorola Labs, in Chicago, supports experiments using their extensive array of specialized equipment and facilities, such as their shielded rooms where no external interference can penetrate the environment.

Shared Spectrum Company, in Virginia, performs a variety of external environmental measurements of real channels using precision spectrum analyzers to scan power usage to see what the actual everyday environment looks like—sort of like a morning traffic-news reporter scanning the highways for congestion.