Blind Opportunistic Interference Alignment in MIMO Cognitive Radio Systems
Opportunistic interference alignment (OIA) is a technique that enables the multiple-input multiple-output (MIMO) secondary user (SU) to exploit the inherent spatial spectrum holes of the MIMO primary user's (PU) communication link while the PU transmissions remain interference free, thus generalizing the concept of interweave cognitive radio (CR) systems. A typical characteristic of an interweave technique is that, in general, no interaction between the PU and the SU systems is assumed, thus rendering infeasible, in many cases, the channel state information (CSI) requirements of the existing OIA techniques. The goal of the present paper is to propose a MIMO SU transceiver system that applies an OIA technique in a manner completely oblivious to the PU system so that both the alignment technique and the corresponding CSI estimation scheme require no interaction with the PU system. To this end, at first, a new improved OIA technique is proposed. The resulting technique requires CSI that can be estimated blindly at the SU ends and achieves better performance in several cases as it aligns the PU and SU signals to orthogonal sub-spaces at both the PU and the SU receivers. Then, a novel blind CSI estimation scheme is developed for the new OIA technique. The exact theoretical ergodic capacity of the new OIA technique is derived for the perfect CSI case. Some insightful theoretical results are also derived for the imperfect CSI case which, to the best of our knowledge, are some of the first ones in the OIA/IA literature. The performance of the proposed schemes is verified via extensive simulations.