A survey of interference mitigation in underlay cognitive radio networks

In this chapter we survey the three primary approaches in cognitive radio; namely the interweave, overlay, and underlay approaches. We then focus on underlay secondary networks which have several advantages over the other two. For example, underlay networks avoid the error-prone sensing problem and the need for dynamic re-allocation of spectrum associated with the interweave approach. They also do not require the large amount of side information about the primary users needed in the overlay approach. However, conventional underlay networks have to adopt a conservative approach in transmit power levels and, therefore, interference suppression is needed. After an overview of interference mitigation in underlay networks for ultra-wide band (UWB) systems we present a novel interference-minimizing code assignment (IMCA) scheme for code-division multiple-access (CDMA) secondary networks. While technically an underlay approach the IMCA scheme opportunistically assigns the secondary user codes available in the code subspace to minimize the mean square crosscorrelations between codes of existing and arriving users. Furthermore, the scheme does not require knowledge of the codes or symbol epoch of the primary CDMA network. It thus attains the advantages of opportunistic and underlay networks and avoids the large side information requirements of overlay systems. Significant performance gains are achieved over random code assignment. We then briefly discuss relaying as another technique for extending the range of underlay networks. Finally, we present our conclusions and assessment of underlay networks.