Secrecy outage of TAS/GSC in Nakagami-m fading channels

This paper considers transmit antenna selection (TAS) and receive generalized selection combining (GSC) for secure communication in MIMO wiretap channel, where confidential messages transmitted from an N_A-antenna transmitter to an N_B-antenna legitimate receiver are overheard by an N_E-antenna eavesdropper. We assume that the main channel and the eavesdropper´s channel undergo Nakagami-m fading with fading parameters m_B and m_E, respectively. In an effort to assess the secrecy performance, we present a closed-form expression for the secrecy outage probability. We then derive closed-form expressions for the secrecy outage probability at high signal-to-noise ratio (SNR) for two realistic scenarios: 1) the legitimate receiver is located close to the transmitter, and 2) the legitimate receiver and the eavesdropper are located close to the transmitter. In the first scenario, we confirm that the secrecy diversity order is m_BN_BN_A. In the second scenario, we confirm that the secrecy diversity order collapses to zero. While this may appear discouraging at a first glance, we show that low secrecy outage probability can still be achieved.