Tools of the Trade
From left, Niyanta Mogre, an undergraduate electrical and computer engineering major, Alex Wyglinski, and PhD candidate Srikanth Pagadarai sample the airwaves in Worcester.
by Alexander Wyglinski, Assistant Professor of Electrical and Computer Engineering
It truly is a wireless world. In the United States alone, there were 255 million cell phone users in 2007, or close to 85 percent of the population. As mobile phones become ubiquitous and the demand for mobile applications and services (web surfing, tweeting, online gaming) increases, the need for bandwidth to carry all that traffic grows apace.
That's a problem, since virtually every bit of prime radio frequency (RF) spectrum (100 MHz to 3 GHz) is already spoken for. Regulatory agencies such as the Federal Communications Commission (FCC) allocate finite frequency bands to various entities. These licenses grant those entities exclusive rights to use a piece of the spectrum. The government also establishes rules that prevent unlicensed transmissions from occurring in these bands.
However, numerous studies have shown that most RF spectrum is underutilized and that the spectrum scarcity problem is artificially generated by the traditional licensing framework, which is simply not designed to ensure the spectrum’s optimum use.
Engineers, economists, regulators, lawyers, and even the FCC are rethinking the way the spectrum is utilized. WPI's Wireless Innovation Laboratory (WI Lab) is one of only a few labs in the nation investigating a promising solution to the apparent spectrum scarcity problem. Called dynamic spectrum access (DSA), it would enable unlicensed devices to hunt for and temporarily "borrow" an unoccupied band of spectrum, as long as they can instantly get out of the way should the license holders need to transmit.
One of the key questions confronting proponents of the DSA model is just how much unoccupied spectrum there really is. With a grant from the National Science Foundation,the WI Lab has been conducting an exhaustive study of spectrum usage in several mid-sized cities in the Northeast— including Buffalo, Pittsburgh, Providence, Rochester, and Worcester. This kind of study has never been attempted. In each city, I work with both graduate and undergraduate students to measure the RF spectrum using a specially designed wireless sampling system.
The centerpiece is a custom-designed antenna that can "see" a 60-degree swath of the horizon and pull in signals between 88 MHz and 3 GHz (a band that includes television, FM radio, WiFi, WiMax, pagers, public safety radio, maritime radio, military radio and GPS). Samples are taken at 25 time intervals at five sites, for a total of 3.5 to 4 hours of data per city. Through triangulation, the source of each broadcast can be determined and—through a tedious search of public records—matched to a specific license holder. A spectrum analyzer controlled by a software package called Spectrum Quality Utility Interface for Radio Electromagnetics (SQUIREL), developed by Alex Camilo '09, automates the data gathering and feeds the data into a model that portrays the occupancy of each portion of the spectrum over time.
To better understand how spectrum usage varies by season and how it may be changing over time, we are taking samples in Worcester every three months. Ultimately, all of the data will be available to other researchers on the WI Lab website.
Preliminary analysis shows that there are, in fact, substantial portions of frequently unoccupied spectrum in every city. Our future work will focus on developing options for sharing this spectrum to promote the continued growth of the many wireless applications that we are increasingly dependent upon. In addition to DSA, these options may include cognitive radios and software-defined radios, which can rapidly adapt and configure themselves to prevailing conditions by implementing most of their radio functions in digital logic and software.
The growing demand on the radio spectrum is one of the most critical infrastructure issues facing the United States and many other industrialized nations. I am hopeful that the work we are doing at WPI will play an important role in helping shape the future of wireless communications.