Transmit Antenna Selection with Alamouti Coding and Power Allocation in MIMO Wiretap Channels

In this work, we propose a new transmit antenna selection (TAS) scheme which examines the trade-off between feedback overhead and secrecy performance in multiple-input multiple-output wiretap channels. Our new scheme is carried out in two steps. First, the transmitter selects the first two strongest antennas to maximize the instantaneous signal-to-noise ratio (SNR) of the transmitter-receiver channel. Second, Alamouti coding is employed at the selected antennas in order to perform secure data transmission. When equal power is applied to the selected antennas, we refer to our new scheme as TAS-Alamouti. To provide valuable insights into TAS-Alamouti, we derive new closed-form expressions for the secrecy performance metrics. In terms of these metrics, we show how in a Rayleigh fading channel that our TAS-Alamouti scheme outperforms the traditional single TAS scheme conditioned on the SNR of the transmitter-receiver channel being larger than a specific value. We show how in some antenna configurations no additional feedback, relative to single TAS, is required in order to realize such performance enhancements. Furthermore, we show how optimal power allocation (OPA) across the selected antennas at the transmitter leads to a new scheme, which we refer to as TAS-Alamouti-OPA, that outperforms single TAS unconditionally. Relative to TAS-Alamouti, TAS-Alamouti-OPA requires only one additional feedback bit.