Due to their flexible deployment and on-demand mobility, small-scale unmanned aerial vehicles (UAVs) are anticipated to be involved in widespread communication applications in the forthcoming fifthgeneration (5G) networks. However, the confidentiality of UAV communication applications is vulnerable to security threats due to the broadcast nature and dominant line-of-sight (LoS) channel conditions, and physical-layer security (PLS) technique can be applied for secrecy performance enhancement in such a context. On the other hand, it is also promising to exploit UAVs to cooperatively protect secure communications. This article provides an overview of the recent research efforts on UAV-involved secure communications at the physical layer. We focus on the design of secure transmission schemes according to different roles of UAVs and the optimization of introduced degrees of freedom (DoFs) by the unique characteristics of UAVs. We also propose some future research directions on this topic.
We propose a spatially spread quint (SS-quint) of only dipoles or loops, for direction of arrival (DOA) and polarisation estimation. The proposed SS-quint is spatially centrosymmetric. Based on this centrosymmetry, the authors develop a computationally low-cost DOA and polarisation estimator via vector-cross-product. Compared with the spatially spread electromagnetic vector-sensor, the proposed SS-quint consists of only dipoles or loops, and thus, its components have more consistent responses. Compared with a previously proposed SS-quint configuration, which is required to be strictly uniformly Lshaped, the proposed SS-quint has a more flexible array configuration in the sense that it is not restricted to any particular shape. The Cramér-Rao bounds are derived and simulation results are provided, to demonstrate the performance of the proposed SS-quint array configuration. (a) Results with the first source, (b) Results with the second source, (c) Average MSE of both sources
This paper considers joint active user detection (AUD) and channel estimation (CE) for massive connectivity scenarios with sporadic traffic. The state-of-art method under a Bayesian framework to perform joint AUD and CE in such scenarios is approximate message passing (AMP). However, the existing theoretical analysis of AMP-based joint AUD and CE can only be performed with a given fixed point of the AMP state evolution function, lacking the analysis of AMP phase transition and Bayes-optimality. In this paper, we propose a novel theoretical framework to analyze the performance of the joint AUD and CE problem by adopting the replica method in the Bayes-optimal condition. Specifically, our analysis is based on a general channel model, which reduces to particular channel models in multiple typical MIMO communication scenarios. Our theoretical framework allows ones to measure the optimality and phase transition of AMP-based joint AUD and CE as well as to predict the corresponding performance metrics under our model. To reify our proposed theoretical framework, we analyze two typical scenarios from the massive random access literature, i.e., the isotropic channel scenario and the spatially correlated channel scenario. Accordingly, our performance analysis produces some novel results for both the isotropic Raleigh channel and spatially correlated channel case.
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