The Effectiveness of Opportunistic Spectrum Access: A Measurement Study
Ben Y. Zhao
ACM/IEEE Transactions on Networking (TON 2012)
[Full Text in PDF Format, 1.41MB]
Dynamic spectrum access networks are designed to allow today's bandwidth-hungry "secondary devices" to share spectrum allocated to legacy devices, or "primary users." The success of this wireless communication model relies on the availability of unused spectrum and the ability of secondary devices to utilize spectrum without disrupting transmissions of primary users. While recent measurement studies have shown that there is sufficient underutilized spectrum available, little is known about whether secondary devices can efficiently make use of available spectrum while minimizing disruptions to primary users.
In this paper, we present the first comprehensive study on the presence of
"usable" spectrum in opportunistic spectrum access systems, and whether
sufficient spectrum can be extracted by secondary devices to support
traditional networking applications. We use for our study fine-grain usage
traces of a wide spectrum range (20 MHz-6GHz) taken at four locations in
Germany, the Netherlands, and Santa Barbara, CA. Our study shows that on
average, 54% of spectrum is never used and 26% is only partially used.
Surprisingly, in this 26% of partially used spectrum, secondary devices can
utilize very little spectrum using conservative access policies to minimize
interference with primary users. Even assuming an optimal access scheme and
extensive statistical knowledge of primary-user access patterns, a user can
only extract between 20%-30% of the total available spectrum. To provide
better spectrum availability, we propose frequency bundling, where
secondary devices build reliable channels by combining multiple unreliable
frequencies into virtual frequency bundles. Analyzing our traces, we find that
there is little correlation of spectrum availability across channels, and that
bundling random channels together can provide sustained periods of reliable
transmission with only short interruptions.