Spectral occupancy at VHF: implications for frequency-agile cognitive radios

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Abstract
Frequency-agile cognitive radio is a potential solu- tion to the problem of inefficient use of radio spectrum in the 30- 300 MHz (VHF) range. This is especially attractive if networks based on this technology can operate in spectrum left unused by existing users, as opposed to being allocated new spectrum through refarming. This paper presents a preliminary survey of this band in a large U.S. city, with the goal of quantifying spectral occupancy and thereby gaining some insight into the feasibility of this approach. A comparable measurement of the 25-90 MHz band in a rural environment is also presented, for comparison. In the urban measurement, it is found that sufficient spectrum is probably available: for example, it is estimated that approximately 80% of the 30-60 MHz band is essentially free of signals to within about 7 dB of the environmental noise limit at 30 kHz spectral resolution. However, these measurements are limited in duration, spatial sampling, temporal density, and spectral resolution. Requirements for a more comprehensive measurement campaign are proposed.

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