  Physical layer security has been considered as an important security approach
in wireless communications to protect legitimate transmission from passive
eavesdroppers. This paper investigates the physical layer security of a
wireless multiple-input multiple-output (MIMO) orthogonal frequency division
multiplexing (OFDM) communication system in the presence of a multiple-antenna
eavesdropper. We first propose a transmit-filter-assisted secure MIMO-OFDM
system which can destroy the orthogonality of eavesdropper's signals. Our
proposed transmit filter can disturb the reception of eavesdropper while
maintaining the quality of legitimate transmission. Then, we propose another
artificial noise (AN)-assisted secure MIMO-OFDM system to further improve the
security of the legitimate transmission. The time-domain AN signal is designed
to disturb the reception of eavesdropper while the legitimate transmission will
not be affected. Simulation results are presented to demonstrate the security
performance of the proposed transmit filter design and AN-assisted scheme in
the MIMO-OFDM system.
