Combined approach of zero forcing precoding and cooperative jamming: A secrecy tradeoff

We consider the problem of secret communication through cognitive relay assisted interference channels in which secondary user (SU) is allowed to transmit simultaneously with the primary user (PU) over the same channel instead of waiting for an idle channel which is traditional for a cognitive radio. We propose two cooperative jamming (CJ) schemes to improve the secrecy rate of cognitive interference channels, where the multiple antenna cognitive relay transmit weighted jamming signals to create additional interferences in the direction of eavesdropper with the purpose of confusing it. In the first CJ scheme, transmitter has the knowledge of all the channel state information (CSI) including eavesdropper channel while in the second CJ scheme transmitter has no CSI of the eavesdropper channel. The proposed CJ schemes are designed to cancel out jamming interference at both the primary receiver (PR) and the secondary receiver (SR) maintaining interference at the PR under a certain threshold. CJ with full CSI enhances secrecy rate by employing zero forcing (ZF) precoding while CJ with partial CSI also combining with ZF precoding improves the secrecy rate by degrading the signal-to-interference plus noise ratio (SINR) at the eavesdropper. Our results show that a combined approach of ZF precoding with CJ scheme is more significant in improving secrecy rate of cognitive interference channels for both full CSI and partial CSI scenarios.