Opportunistic Interference Alignment for Cognitive MIMO-NOMA Downlink Networks

Opportunistic interference alignment (OIA) is an extensively known precoding technique in a multiple-input multiple-output (MIMO) system that uses IA over spatial dimensions to enhance the rate of cognitive users. On the other hand, the power-domain non-orthogonal multiple access (NOMA) achieves more spectral efficiency over orthogonal multiple access (OMA). This work investigates underlay opportunistic communication over a cognitive radio system using an effective amalgamation of OIA and NOMA. The cognitive users utilize the opportunities for superposition coding (SC) signal transmission, which further improves spectral efficiency. The system's performance has been demonstrated using an achievable sum rate and available transmission opportunities performance metrics for cognitive users. Numerical results indicate that the proposed OIA-based MIMO-NOMA system achieves an improved sum rate for cognitive users compared to the conventional OIA-based MIMO-OMA system.