Opportunistic Interference Alignment in Cognitive Radio Networks with Space-Time Coding

For a multiuser multiple-input–multiple-output (MIMO) overlay cognitive radio (CR) network, an opportunistic interference alignment (IA) technique has been proposed that allows spectrum sharing between primary users (PU) and secondary users (SU) while ensuring zero interference to the PU. The CR system consists of one PU and K SUs where the PU uses space-time wa-ter-filling (ST-WF) algorithm to optimize its transmission and in the process, frees up unused eigen-modes that can be exploited by the SU. The SUs make use of an optimal power allocation algorithm to align their transmitted signals in such a way their interference impairs only the PUs unused eigen-modes. Since the SUs optimal power allocation algorithm turns out to be an optimal beamformer with multiple eigen-beams, this work initially proposes combining the diversity gain property of space-time block codes, the zero-forcing function of IA and beam-forming to optimize the SUs transmission rates. This proposed solution requires availability of channel state information (CSI), and to eliminate the need for CSI, this work then combines dif-ferential STBC (D-STBC) scheme with optimal IA precoders (consisting of beamforming and ze-ro-forcing) to maximize the SUs data rates. Simulation results confirm the accuracy of the pro-posed solution.