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Unmanned aerial vehicles (UAVs) are widely used in wireless communication networks due to their rapid deployment and high mobility. However, in practical scenarios, the existence of obstacles and eavesdroppers will seriously interfere with the communication quality of the UAV network and produce a security risk. Thus, this paper combines reconfigurable intelligent surface (RIS) technology with UAVs to build a secure UAV communication network. Normally, a rotary‐wing UAV (labeled as UAV‐S) acting as a base station sends information signals to a legitimate user on the ground with RIS equipment. However, there is a passive eavesdropper on the ground who can steal the information. Therefore, a friendly UAV jammer (labeled as UAV‐J) with a fixed location is introduced to send jamming signals to confuse the eavesdropper. The goal of this paper is to maximize the average secrecy rate of the communication network by jointly optimizing the flight trajectory, transmit power of the UAV‐S and UAV‐J, and phase shifter of the RIS. Since the constructed problem is highly nonconvex, an alternating optimization algorithm based on successive convex approximation techniques is proposed to solve the problem. Simulation results show that the proposed algorithm can achieve a higher secrecy rate in comparison with other schemes.

Reconfigurable intelligent surface (RIS)‐empowered communications is on the rise and is a promising technology envisioned to aid in 6G and beyond wireless communication networks. RISs can manipulate impinging waves through their electromagnetic elements enabling some sort of control over the wireless channel. The potential of RIS technology is explored to perform a sort of virtual equalization over‐the‐air for frequency‐selective channels, whereas equalization is generally conducted at either the transmitter or receiver in conventional communication systems. Specifically, using an RIS, the frequency‐selective channel from the transmitter to the RIS is transformed to a frequency‐flat channel through elimination of inter‐symbol interference (ISI) components at the receiver. ISI is eliminated by adjusting the phases of impinging signals particularly to maximize the incoming signal of the strongest tap. First, a general end‐to‐end system model is provided and a continuous to discrete‐time signal model is presented. Subsequently, a probabilistic analysis for elimination of ISI terms is conducted and reinforced with computer simulations. Furthermore, a theoretical error probability analysis is performed along with computer simulations. It is analysed and demonstrated that conventional RIS phase alignment methods can successfully eliminate ISI, and the RIS‐aided communication channel can be converted from frequency‐selective to frequency‐flat.

Practical methods to mitigate level and variation of passive intermodulation with respect to time and temperature of coaxial cavity type duplexer are proposed through a variety of experimental results. From the findings derived in this study, a stable passive intermodulation response with less than 1 dB variation was shown in the environment temperature range of −10–85 °C. The duplexer with time‐stable PIM response at room temperature had a stable PIM value even in the environment temperature range.

This letter compares the 3 dB axial ratio (AR) bandwidth (BW) of transmission mode single‐screen linear‐to‐circular polarisation convertors based on three different slot topologies. Using a conventional centre‐connected cross‐slot arrangement as a baseline, we show that a significant performance improvement can be achieved by creating a more elaborate design with unit cells composed of multiple unequal length offset rectangular‐shaped apertures. The additional flexibility to tailor the shape of the Transverse Electric (TE) and Transverse Magnetic (TM) spectral amplitude and phase responses is exploited to achieve an 84% increase in the BW relative to the baseline for a freestanding structure, which is designed to work at a centre frequency of 10.0 GHz. Experimental AR results are used to verify the new design methodology.

UWB suspended stripline (SSL) bandpass filter with an adjustable notched band and four transmission zeros (Tzs) is proposed here. To prevent interference signals from the wireless local‐area network (WLAN) entering UWB system, a short‐stub resonant network is introduced to this UWB filter. In addition, utilizing different resonant modes of the two short–stubs, four Tzs are generated to improve the skirt selectivity. Both the centre frequency of the notched band and the positions of Tzs can be controlled by tuning the structural parameters. The UWB SSL bandpass filter is designed, fabricated and tested. The measured results are in good agreement with the simulated results.

This letter presents a new method of focused energy delivery (FED) with low grating lobes for precision electronic warfare (PREW). The transmitted signal model is modified by equipping each platform with a linear uniform antenna array, enabling the same performance with lower power and providing a greater degree of freedom in the design of the transmitted signals than traditional methods. Moreover, the L∞‐norm is considered as a regularization term in the model to simultaneously realize FED and alleviate the grating lobes. To solve the nonconvex NP‐hard multiobjective optimization problem (MOP), block coordinate descent (BCD) is employed to separate the problem into two subproblems with respect to the amplitude and phase components of the signals. Then, these two subproblems are solved using Taylor expansion (TE) and the alternating direction penalty method (ADPM), respectively. Simulation results demonstrate that the proposed method exhibits superior performance compared to previous methods.

An efficient modes analyses technique for isotropic or anisotropic material filled 2D metallic waveguides with an arbitrary contour using the multifilament current method (MFCM) is presented. The ideal PEC boundary of a 2D waveguide is replaced by a shell with a high conductivity and electrical small thickness. The thin lossy shell not only can well approximate the boundary condition of PEC waveguide wall therefore without altering the initial waveguide modes, but also can let the external excitation penetrate through to excite the inside modes, resulting in a high internal field intensity at the frequency of each mode. In this case, the modes are revealed by the peaks of field intensity responses, and the spurious modes which existed in traditional source‐free modes determination techniques can be completely avoided. Based on this idea, a generalized impedance boundary condition (GIBC) is formulated to represent the lossy waveguide wall and further utilized in the MFCM for simulating the internal field intensity over frequency. Three different configurations of a 2D waveguide are considered. The computed modes are compared with that obtained from commercial software, and an excellent agreement is achieved, yet an competitive advantage on simulation performances is observed by using the proposed technique.

This paper proposes drone assisted device‐to‐device cooperative communication (DA‐DDCC) for critical situations during post‐disaster management. The proposed network utilizes the autonomous mode of D2D communication for setting up the link in the absence of a central node. This network incorporates cooperative communication using drone in D2D session for improving reliability of the overall system. A probability‐based statistical channel model for such networks is proposed by taking the statistical independence of links into consideration. Unlike the existing air‐to‐ground (A2G) channel models that use either Rayleigh or Rician distribution for uplink (UL) and downlink (DL) channel modelling, our approach takes the probability of occurrence of line of sight (LoS) into account while predicting the appropriate channel distribution for UL and DL separately. For performance evaluation of the proposed network, average outage probability and average capacity are derived using the proposed channel model. Monte Carlo simulations are conducted to verify our analysis. Moreover, a multi‐cluster DA‐DDCC scenario is also being analyzed through simulations from an interference perspective to justify the usefulness of the proposed channel model. Results obtained through this investigation can be utilized in selecting various crucial system parameters judiciously for enhanced performance during post‐disaster scenario.

Synchronisation of radar systems can be used to suppress interference and enable communication between radars. In this paper, a generalised Kuramoto model is proposed, which can be used to synchronise pulse repetition frequencies of two radars over‐the‐air. The generalised approach allows the adjustment of a sampling factor which influences the convergence of the two pulse repetition frequencies. Furthermore, sampling criteria are derived, which can be used to estimate whether convergence occurs. Finally, it is shown by means of simulation that the model together with the criteria allows to reliably predict convergence and to perform synchronisation.

This paper investigates the joint relay and channel selection problem using a deep reinforcement learning (DRL) algorithm for cooperative communications in a dynamic jamming environment. The latest types of jammers include the mobile and smart jammer that contains multiple jamming patterns. This new type of jammer poses serious challenges to reliable communications such as huge environment states, tightly coupled joint action selections and real‐time decision requirements. To cope with these challenges, a DRL‐based relay‐assisted cooperative communication scheme is proposed. In this scheme, the joint selection problem is constructed as a Markov decision process (MDP) and a double deep Q network (DDQN) based anti‐jamming scheme is proposed to address the unknown and dynamic jamming behaviors. Concretely, a joint decision‐making network composed of three sub‐networks is designed and the independent learning method of each sub‐network is proposed. The simulation results show that the user agent is able to anticipate the jammer behaviors and elude the jamming in advance. Furthermore, compared with the sensing‐based algorithm, the Q learning‐based algorithm and the existing DRL‐based anti‐jamming approaches, the proposed algorithm maintains a higher average normalized throughput.