Hybrid Overlay/Underlay MC-CDMA for Cognitive Radio Networks with MMSE Channel Equalization

Cognitive Radio (CR) aims to access the spectrum opportunistically mainly by two access strategies, overlay and underlay. Overlay can utilize the spectrum whenever the primary system is off. On the other hand, underlay can utilize the spectrum at any time with considering the interference limit of the primary system. However, by using hybrid methods and performing proper equalization technique, spectrum can be utilized more efficiently. In this paper, we propose a hybrid MC-CDMA system which utilizes the whole spectrum for underlay transmission rather than solely the occupied parts. Due to the interference rejection capability, MC-CDMA can minimize the interference from the Primary User (PU) and exploit more diversity. Furthermore, overlay utilizes the spectrum holes while maintaining orthogonality to the underlay by Orthogonal Variable Spreading Factor (OVSF) codes. Chip and symbol- level MMSE-based modified equalizers are proposed for the system and the underlay performance of the system is evaluated. Simulation results show that BER performance improves significantly with the modified MMSE equalizers and for all PU occupancy levels.