Massive machine type communications or internet of things (IoT) over wireless systems have invoked attention in terms of security problems, especially for multi-input multi-output (MIMO) schemes. In this paper, we propose efficient physical layer security (PLS) enhancing methods for space-time block coding (STBC), as well as spatial multiplexing (SM) schemes. The proposed methods pre-distort the transmit signals by using the phase information of the legitimate MIMO channel, and, as a result, the illegal eavesdropper cannot extract any information. The proposed predistortion for the STBC schemes is done so that the channel matrix at the receiver is a real-valued one, which results in full-rate and full-diversity gain for more than two transmit antennae. Therefore, compared to the conventional schemes, the proposed scheme eventually leads to the higher performance gain and lower detection complexity in addition to the high security protection. By extending the principle of the proposed method for STBC, a predistortion scheme is also proposed for SM by using the phase information of the column space of the channel matrix. The simulation results investigated in this paper reveal that the proposed methods can achieve enhanced error rate performances, as well as high security protection.

In this paper, the security performance of artificial noise (AN)-aided space time block coding (STBC) systems in wiretap channel is evaluated. The security performance is estimated in terms of the secrecy rate, which is defined as the difference of mutual information of the legitimate receiver and illegal eavesdropper. We derive a generalized secrecy rate formula for the system utilizing AN-aided STBC, and also provide an expression for the approximated value with which the estimation time can be highly reduced. The simulation results reveal that the approximated formula well matches to the full estimation with a few orders of less complexity. In addition, it is proved that the maximum achievable secrecy rate for M -ary modulation scheme approaches to log 2 M bps/Hz when AN power is extremely high, showing perfect security protection at the expense of power efficiency.