This book address the issue of spectrum sensing as crucial routine for the unlicensed CRN to co-allocate with licensed primary networks, and share the same spectrum bands in an opportunistic manner. So far, the primary networks didn't deal with the CRNs and regards its signals as noise or intruders; this facilitate the fast emerge of the CRN, whereas there is no annoying modification needed to the existing networks. However, in the near future when the concept gets matured, primary networks should share some information with the CRN in order to have better understanding of the surrounding...