This paper investigates secure cooperative transmissions in a dual-hop MIMO relay system using a combined transmit antenna selection (TAS) and maximal-ratio combining (MRC) scheme over the Nakagami-m fading channels, where an adaptive decode-and-forward relaying protocol and an multi-antenna eavesdropper are considered. Due to the feedback delay, channel state information (CSI) for TAS might be outdated at both the source and the relay. To evaluate the secrecy performance of the TAS/MRC scheme and the impacts of outdated CSI, the closed-form expressions for the metrics of exact ergodic secrecy rate and exact secrecy outage probability are derived under both perfect and outdated CSI conditions in a channel feedback error model. In order to explicitly reveal the behaviors of the secrecy performance in high signal-to-noise ratio regime, asymptotic expressions for both the metrics are further derived. As validated by simulation results, analytically numerical results demonstrate that outdated the CSI always results in a loss in the secrecy performance, but the loss can be recovered by increasing the number of antennas at legitimate receivers. Furthermore, the outdated CSI yields a reduced secrecy diversity order, whereas only the perfect CSI leads to the full secrecy diversity order.

This paper addresses the target localization problems based on received signal strength (RSS) measurements in underwater acoustic wireless sensor network (UWSN). Firstly, the problems based on the maximum likelihood (ML) criterion for estimating target localization in cases of both known and unknown transmit power are respectively derived, and fast implementation algorithms are proposed by transforming the non-convex problems into a generalized trust region subproblem (GTRS) frameworks. A three-step procedure is also provided to enhance the estimation accuracy in the unknown target transmit power case. Furthermore, the Cramer–Rao lower bounds (CRLBs) in both cases are derived. Computer simulation results show the superior performance of the proposed methods in the underwater environment.

This study investigates the application of artificial fish swarm algorithm (AFSA) in the power allocation for multiple‐input and multiple‐output orthogonal frequency‐division multiplexing (MIMO‐OFDM) relay underwater acoustic (UWA) communication systems. First, by using the singular value decomposition technique, the two‐hop transmission links are converted into the virtual direct links in an single‐input and single‐output OFDM (SISO‐OFDM) system. Then, a power allocation optimisation problem, together with the assignment of subcarriers and relay nodes, are formulated for the virtual SISO‐OFDM system. Finally, the problem‐solving algorithms are proposed in two parts. Computer simulation results show that the proposed AFSA scheme improves in both power consumptions and diversity gains compared with two existing schemes for UWA communication systems.

The received signal strength (RSS) based target localization problem in underwater acoustic wireless sensor networks (UWSNs) is considered. Two cases with respect to target transmit power are considered. For the first case, under the assumption that the reference of the target transmit power is known, we derive a novel weighted least squares (WLS) estimator by using an approximation to the RSS expressions, and then transform the originally non-convex problem into a mixed semi-definite programming/second-order cone programming (SD/SOCP) problem for reaching an efficient solution. For the second case, there is no knowledge on the target transmit power, and we treat the reference power as an additional unknown parameter. In this case, we formulate a WLS estimator by using a further approximation, and present an iterative ML and mixed SD/SOCP algorithm for solving the derived WLS problem. For both cases, we also derive the closed form expressions of the Cramer–Rao Lower Bounds (CRLBs) on root mean square error (RMSE). Computer simulation results show the superior performance of the proposed methods over the existing ones in the underwater acoustic environment.

To improve the secrecy performance of the untrusted relay system with a multiple-antenna destination, a novel full-duplex destination jamming scheme with optimal antenna selection (FDJ-OAS) is proposed in this paper. The exact closed-form expressions of the ergodic achievable secrecy rate are derived for the proposed scheme. To attain more insightful results, the approximated closed-form expressions for the ergodic achievable secrecy rate and the optimal power allocation factor for FDJ-OAS are derived in the case with a large-scale antenna array, and simple power allocation solutions are also provided in the high and low SNR regimes, respectively. Furthermore, we derive the asymptotic expressions for the secrecy outage probability (SOP) under different asymptotic conditions for the proposed scheme, and the analysis results indicate that, the secrecy diversity order of FDJ-OAS is significantly superior to that of the comparable schemes. Simulation results show that, the analytical curves match well with the simulation results, and the advantage of FDJ-OAS is also clearly demonstrated through performance comparison.

In this article, sensitized TiO₂ photocatalysts were prepared by combining a hydrothermal process with a post-thermaltreatment. Results indicate that the post-thermal treatment is an effective and facile method to promote the photocatalytic activity of the sensitized TiO₂. The sensitized TiO₂ photocatalysts calcined at an optimum temperature of 400 °C exhibit an enhanced photocatalytic activity, which is mainly related to an appropriate sensitization and the dangling bonds. The dangling bonds can absorb methyl orange (MO) molecules via van der Waals' forces and prevent the recombination of the hole–electron pair by releasing unpaired electrons or capturing other electrons. The released electron is delocalized and promoted into the conduction band by thermal energy and further transferred to O₂, thus generating a superoxide radical anion (˙O²⁻) that is responsible for the degradation of MO in the dark.