Physical layer security of MIMO–OFDM systems by beamforming and artificial noise generation

In this paper we address physical layer security in multiple-input-multiple-output (MIMO) frequency selective wireless channels in the presence of a passive eavesdropper, i.e., the associated channel is unknown to the transmitter. Signalling is based on orthogonal frequency division multiplexing (OFDM). Spatial beamforming and artificial noise broadcasting are chosen as the strategy for secure transmission. The contribution of channel frequency selectivity to improve secrecy is presented by performance and probabilistic analysis. Moreover, we investigate the capability of the eavesdropper to jeopardize the security of the system (defined as the SNR difference between the intended receiver and the eavesdropper) by mitigating the interfering effect of the artificial noise using zero forcing as a receive beamforming strategy. The results show that although zero forcing is not the optimal strategy to maximize the SNR, it offers (from the eavesdropper’s perspective) a better performance than MMSE for MIMO frequency selective channels and thus threatens the overall security of the system.