Secrecy performance of MIMO Nakagami-m wiretap channels with optimal TAS and different antenna schemes

Transmit antenna selection (TAS) in wiretap channels has arisen as a simple promising scheme to improve physical layer security. However, up to now, TAS schemes have aimed at maximising the instantaneous signal-to-noise ratio of the legitimate receiver, which by its essence is a sub-optimal strategy from a secrecy point of view. In this paper, unlike previous works, we propose a novel TAS scheme for multiple-input multiple-output wiretap channels by exploiting full channel state information. The proposed scheme achieves higher secrecy performance by selecting an antenna that maximises the secrecy rate. In our analysis, we assume that the receiver (Rx) and the eavesdropper employ either selection combining (SC) or maximal-ratio combining (MRC) to combine the received signals, and this results in four different receiver combining arrangements. By considering Nakagami-m fading, secrecy performance is analysed. Specifically, closed-form expressions for the secrecy outage probability are derived, which can be used as a quality of service metric. Moreover, an asymptotic analysis is carried out and the respective diversity/array gains are obtained. The ergodic secrecy rate is also investigated for the case of multiple-input single-output wiretap channels. Insightful discussions are attained from our analysis. For instance, it is shown that the performance gain of MRC over SC at the Rx does not depend on the number of antennas and combining scheme at the eavesdropper. More interestingly, we observe that such a performance gain is independent of the TAS scheme used at the transmitter (Tx). On the other hand, it is shown that the secrecy gap between SC and MRC at the eavesdropper is independent of the number of antennas at the Tx, but it depends on the number of antennas at the Rx. Finally, it is demonstrated that the diversity gain is the same for all antenna configurations.