I. Introduction
In spectrum sharing a secondary service is able to access the white spaces in a frequency band which is formally allocated to the primary service [1]. White spaces are those parts of the spectrum allocated to the primary user which are under-utilized in some particular times and specific locations. Various schemes are proposed in the literature for spectrum sharing (see e.g., [2] and references therein). Our focus is on the underlay spectrum sharing [3]. in underlay spectrum sharing, the secondary service can always access to the spectrum subject to an interference threshold constraints at the primary service receiver [2], [3].