In this paper, we investigate the secrecy performance of multiple-inputmultiple-output wiretap channels with outdated channel state information (CSI) at the transmitter. The joint effect of multiple jamming signals and noise at the eavesdropper is studied assuming that the transmitter adopts a transmit antenna selection technique, while both the legitimate receiver and the eavesdropper use a maximal-ratio combining scheme to achieve spatial diversity in reception. Exact closed-form expressions for the nonzero secrecy rate probability and the secrecy outage probability for an arbitrary number of power distributed jamming signals are derived, by assuming both perfect and imperfect CSI. The general secrecy outage probability expressions are simplified for 2 special cases, ie, different and equal power distributed interferers. Furthermore, we conduct an asymptotic secrecy outage analysis at high signal-to-noise ratio, which reveals that the expected diversity gain cannot be realized for imperfect CSI and full diversity order can only be achieved under the perfect CSI condition. In addition, our results reveal that the number of antennas at the eavesdropper as well as the number of jamming signals do not affect the system diversity order. Simulation results are performed to illustrate the effects of imperfect feedback and other key system parameters on the secrecy performance. Finally, our analytical results are validated through Monte Carlo simulations.
The use of efficient beam tracking mechanisms becomes necessary in highly directional communication of the fifth‐generation systems. In this context, this paper analyzes the tracking problem and proposes a framework that exploits the samples in the user equipment (UE) dataset (historical UE dataset) to efficiently estimate and predict the channel state at the base station. The framework is composed of two steps. First, a supervised learning algorithm, namely, K‐nearest neighbors (K‐NN), is evaluated as a mean of (i) finding the most similar historical samples to some beam measurements reported by the UE and (ii) predicting the corresponding channel state. As a second step, a sampling and reconstruction strategy based on graph signal processing (GSP) called K‐nearest neighbors with reconstruction (K‐NN‐R) is introduced in order to reduce the beam search space during the beam measurement stage, which allows a more efficient usage of the feedback channel. Simulation results illustrate the performance of the proposal in terms of normalized mean square error in comparison with three traditional/baseline prediction techniques. The K‐NN technique provides a better performance than the baseline approaches for any length of the observed temporal window and with full beam sweep. Meanwhile, the K‐NN‐R framework outperforms the baseline approaches with only half of the beam pairs and throughout a significantly low length of observed temporal window.
This paper investigates the secrecy outage performance of multiple-input multiple-output (MIMO) wiretap channels, where a generalized selection combining (GSC) scheme is assumed at the legitimate receiver, while the transmitter employs a transmit antenna selection (TAS) technique and the eavesdropper adopts a maximal-ratio combining (MRC) scheme. Assuming that the eavesdropper is subject to noise and jamming, a closed-form expression for the secrecy outage probability is derived, based on which the diversity and array gains are determined after performing an asymptotic analysis. The derived expression allows for arbitrary power distributed jamming signals, and are simplified to two special cases, i.e., distinct and equal power distributed jamming signals. Some representative numerical results are depicted to show the effects of the key system parameters on the secrecy performance. Finally, the proposed analysis is corroborated through Monte Carlo simulations.
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