A new trade-off scheme for MIMO OFDM-based cognitive radio systems over correlated fading channels

A joint pre- and post-coder (JPROC) is proposed for MIMO OFDM-based cognitive radio systems over correlated fading channels. The main aim is to have a trade-off between licensed networks and un-licensed networks in the underlay scenario. To handle the un-desired interferences on the main networks, the eigen-values of this JPROC should be small in each licensed network, as much as possible, whereas un-licensed users have higher rates. Frequency selective MIMO fading channels are coded with orthogonal space-time-frequency block codes (OSTFBCs) to improve the bad effects of these small eigen-values on the signal to noise ratio (SNR) in the main network. On the other hand, the system is operated in both transmit- and receivecorrelation scenarios. We have the correlation matrices at all the transmitters imperfectly. This is because there is any error in the estimation of the transmit- and the receive- angle spreads. The matrix of the JPROC is obtained by solving a robust convex optimization problem in the multi-user scenario. The optimization problem is represented under some constraints such as the outage probability of eavesdropping as a criterion for secrecy. An upper-band of the pair-wise error probability is minimized, while controlling the produced interferences. The effective performance of this JPROC is finally shown using simulations.