Low power consumption and high spectrum efficiency as the great challenges for multi-device access to Internet-of-Things (IoT) have put forward stringent requirements on the future intelligent network. Ambient backscatter communication (ABcom) is regarded as a promising technology to cope with the two challenges, where backscatter device (BD) can reflect ambient radio frequency (RF) signals without additional bandwidth. However, minimalist structural design of BD makes ABcom security vulnerable in wireless propagation environments. By virtue of this fact, this paper considers the physical layer security (PLS) of a wireless-powered ambient backscatter cooperative communication network threatened by an eavesdropper, where the BD with nonlinear energy harvesting model cooperates with decode-and-forward (DF) relay for secure communication. The PLS performance is investigated by deriving the secrecy outage probability (SOP) and secrecy energy efficiency (SEE). Specifically, the closed-form and asymptotic expressions of SOP are derived as well as the secrecy diversity order for the first time. As an energy-constrained device, balancing power consumption and security is major concern for BD, thus the SEE of the proposed network is studied. The results from numerical analysis show that the performance improvement of SOP and SEE is impacted by system parameters, including transmit power, secrecy rate threshold, reflection efficiency and distance between the source and BD, which provide guidance on balancing security and energy efficiency in ambient backscatter cooperative relay networks.

In this paper, a tensor-based joint channel parameter estimation and information symbol detection scheme is developed for millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) communication systems. At the base station (BS), the information symbols are encoded according to the Khatri-Rao space-time (KRST) method and transmitted through time-varying channels. The received signals at the mobile station (MS) are constructed into a nested complex-valued parallel factor (PARAFAC) model, which contains an outer model and an inner model, respectively. With outer model, we estimate the compound channel matrix and detect the information symbols considering the sparse scattering nature of mmWave channels. With inner model, we extract physical parameters, including angles of arrival/departure (AoAs/AoDs), Doppler shifts and complex path gains from the estimated compound channel matrix. These physical parameters can be used to significantly reduce feedback overhead. A tricky way here is that we convert complex inner-submodel into a real one such that the computational complexity is reduced. Compared with existing schemes, the proposed one improves the estimation accuracy with low computational complexity, and is applicable for both uniform linear arrays (ULAs) and uniform planar arrays (UPAs). Simulation results demonstrate the effectiveness of the proposed joint channel estimation and information symbol detection scheme.

In this paper, we investigate the reliability and security performance of cooperative multirelay systems, where both source and relay nodes are energy-constrained nonlinear energy harvesters, scavenging energy from a power beacon nearby. Our analysis is based on practical model since residual hardware impairments (RHIs) and channel estimation errors (CEEs) are considered. Aiming at improving the system efficiency, three representative relay selection strategies are considered: i) random relay selection (RRS); ii) suboptimal relay selection (SRS) and iii) optimal relay selection (ORS). To characterize the security performance of the considered strategies, we derive closed-form analytical expressions of the reliability and security in terms of outage probability (OP) and intercept probability (IP).We further discuss the asymptotic expressions and scaling laws of OP with the number of relays. The IP is analyzed for non-colluding and colluding scenarios. The numerical results illustrate that: i) There is a tradeoff between reliability and security, that is when the outage constraint is relaxed, the IP can be enhanced, and vice versa; ii) The outage performance of the ORS and SRS schemes outperform RRS, indicating that relay selection can enhance reliability performance; iii) There are error floors for the OP due to the CEEs; iv) Colluding eavesdroppers can enhance eavesdropping attacks by sharing their intercepted information; and v) Although RHIs and CEEs have deleterious effects on the OP, they can protect the information transmission against eavesdropping attacks.

As a promising technology in the next generation mobile network, millimeter-wave (mmWave) communication can mitigate the spectrum crunch of improving the network capacity by exploiting the large underutilized spectrum bands of the mmWave frequencies. The hybrid (analog/digital) beamforming of multi-data streams are widely used to further the spectrum efficiency of mmWave relay system when faced with the complex environment or long distance communication. This paper investigates the hybrid beamforming scheme for the decode-and-forward (DF) mmWave massive multiple-input multiple-output (MIMO) relay system with mixed structure and full-connected structure. We optimize hybrid beamforming of relay system by maximizing the sum rate of the overall system as an objective function. To reduce the computational complexity, we reformulate the original problem as two single-hop mmWave MIMO sum-rate maximization subproblems. Then, the piecewise successive approximation method is proposed based on the criterion which jointly designs the analog and digital beamforming stages by trying to avoid the loss of information at each stage. The hybrid beamforming of the two subproblems can be solved by the proposed scheme united with the idea of successive interference cancelation (SIC), the baseband block diagonalization (BD) scheme, and waterfilling power allocation method. Finally, simulation results confirm that the proposed optimal method can achieve good performance in hybrid beamforming design of relay system with both mixed and full-connected structures.

Millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems can obtain sufficient beamforming gains to combat severe path loss in signal propagation. The hybrid (analog/digital) beamforming with multiple data streams can be utilized to further improve mmWave spectral efficiency. In this paper, we focus on the hybrid beamforming design of a downlink mmWave massive multi-user MIMO (MU-MIMO) system based on full-connected structure, and aim to maximize the sum rate of the overall system as an objective function. In the analog beamforming stage, a piecewise successive iterative approximation (PSIA) algorithm is proposed to design the analog beamformer and combiner. This algorithm not only has a linear property, but also can obtain closed-form solutions. In the digital beamforming stage, the piecewise successive approximation method is utilized to design the digital beamforming based on the criterion to avoid the loss of information, which can help reduce the computational complexity and is also implemented simply. The results show that the proposed scheme achieves good sum-rate performance in the mmWave massive MU-MIMO system, and outperforms the state-of-the art MIMO hybrid beamforming design schemes, even when the number of base station antennas is not very large.