Spectrum Sharing Without Licenses: Opportunities and Dangers1

The current spectrum management policy typically gives exclusive and unlimited access to license-holders within their domain, and offers meager transmission opportunities for non-license-holders. This paper addresses spectrum management techniques in which no licensing is required and individual devices have real-time access to shared spectrum. An example is the 30 MHz of unlicensed spectrum allocated by the Federal Communications Commission in the new Personal Communications Services band, and industry is already requesting hundreds of MHz more of the same. Such spectrum has several advantages. It eliminates the delays of the licensing process. It facilitates mobility, as a license is not required wherever a system may operate. It also promotes spectrum sharing, as one device may transmit while others in the area are idle. This paper discusses some of the challenges to be overcome. Foremost among these is an inherent Tragedy of the Commons resulting from the fact that device designers lack an incentive to conserve the shared spectrum resource. This phenomenon is quantitatively demonstrated in a practical scenario. Some options for this problem are also discussed.