Cognitive MIMO quadrature spatial modulation systems with mutual primary-secondary co-channel interference

Quadrature spatial modulation (QSM) in cognitive radio (CR) is a new spectral efficient multiple-input multiple-output (MIMO) digital modulation technique. In QSM, the spatial constellation diagram of conventional spatial modulation (SM) system is extended to include both in-phase and quadrature components of the data symbol. As such, QSM combined with CR techniques achieve significant improvement in the overall spectral efficiency while the advantages of SM are retained. In this paper, we study the performance of QSM-MIMO-CR systems in the presence of mutual primary-secondary co-channel interference. A closed-form expression for the average pair-wise error probability (PEP) of the secondary system is derived and used to calculate a tight upper bound of the average bit error rate (ABER) over Rayleigh fading. In addition, a simple asymptotic expression is derived and analyzed. Simulation results, which corroborate the numerical ones, show the importance of QSM in improving the overall secondary performance.