There has been considerable effort to let more wireless devices operate in white space spectrum, that is within frequency bands and geographic areas where no wireless devices are active. Making white space available is certainly useful, but there are other sharing opportunities as well, some of which have been obscured by dangerous misconceptions about the concept of unused spectrum. This paper discusses allowing more devices to operate safely in gray space spectrum, that is spectrum that is actively being used in that transmissions are underway—something many economic models assume is impossible. The paper focuses on primary–secondary sharing, so devices gaining access to spectrum operate on a secondary basis in a way that never causes harmful interference to primary systems. Examples of primary–secondary gray space sharing mechanisms are described in which devices are allowed to share spectrum with broadcasting, radar, and cellular systems. Quantitative analysis shows that it is technically possible to support significant communications among secondary devices in spectrum that is already heavily used by cellular or radar. However, gray space sharing generally causes primary and secondary systems to be more technically interdependent than white space sharing, so different policy and governance structures are needed. Secondary market rules can support gray space sharing in cases where there is a single primary spectrum user, such as a cellular carrier. In cases where technology is static, the regulator may be able to control access for secondary devices. However, in cases with multiple primary users and multiple secondary users of spectrum, as might be seen in bands with radar for example, a new kind of governance body will be needed to facilitate spectrum sharing.
