Hybrid Overlay/Underlay Cognitive Radio Network with MC-CDMA

Cognitive radio (CR) allows secondary users (SUs) to exploit the under-utilized radio spectrum of the primary networks. To fully utilize the primary spectrum and maximize the spectral efficiency, overlay and underlay transmissions, which exploit the white and grey spaces respectively, should be used together. However for underlay, the SUs need to transmit at low power to avoid causing harmful interference to the PUs, whereas the PUs will cause high interference to the SUs. Thus interference mitigation is a major issue in underlay spectrum utilization. In this paper, a hybrid transmission system that exploits both overlay and underlay is proposed using MC-CDMA due to its interference rejection and diversity exploitation capabilities. Unlike existing approaches, which separate the use of overlay and underlay spectrum, the proposed schemes utilize the entire spectrum for underlay transmission to minimize the PU interference while using the spectrum holes for overlay transmission to maximize data rate. Two techniques that operates at full-load and overload scenarios are proposed. The overload system is then extended to the multi-user underlay case to further improve the spectrum utilization.