Hybrid overload MC-CDMA for Cognitive Radio networks

This paper proposes the use of an overloaded hybrid MC-CDMA system to improve the spectral efficiency of Cognitive Radio (CR) network. The overlay transmission in the unused parts of the spectrum is full-loaded MC-CDMA signals with scrambling. The underlay signal, using a separate scrambling code from the overlay signals, utilizes the whole spectrum taking into account of the interference threshold of the Primary User (PU). On the receiver side, chip-level MMSE is used for overlay detection, interference reconstruction and cancellation. Symbollevel MMSE is then used to detect the underlay. Since overlay is performing chip-level equalization and data detection, there is no need for other users´ signatures. Therefore, the proposed system does not add complexity to the overlay system in comparison with the conventional MMSE receivers. Simulation results show that the underlay transmission does not affect the overlay performance while achieving good performance for different PU occupancy levels. It is also shown that it is less sensitive to PU interference level and significantly outperforms previous methods.