CR [1] is a promising solution to the problem of spectrum

scarcity in wireless communications. The basic idea of CR is

to allow secondary (unlicensed) users (SUs) to opportunistically

utilize the spectrum of PUs (licensed), provided that the

SU transmission do not harmfully affect the PUs. In this paper,

we focus on the underlay CR approach, where the SUs access

the spectrum occupied by the PUs, given that the interference

they cause to the PUs is less than a certain threshold such that

the Quality-of-Service (QoS) of the PUs can be ensured