Figure 1: We have different options for spectrum sharing. We use Inc as an abbreviation for the incumbent of the system. (a) The PS owns sufficient number of spectra to support all of its requirements. (b) The incumbent PS has only the critical number of spectra and CS has a wide spectrum. (c) The PS is LSA licensee of CS. After the overview, we concentrate more specifically on this setting where CS allows spectrum use to PS. (d) The incumbent is a roaming user at the CS network. (1) CS allows spectrum use. (2) PS allows spectrum use. (3) CS is allowed to use the spectrum given that CS is obligated to give spectrum when needed.