Free-space optical (FSO) links are contemplated as a potential paradigm to yield efficient point-to-point communication in wireless systems. Notably, these links are used to provide the proficient wireless connectivity between the radio frequency (RF) wireless network and the fiber optic-based network. To this end, more attention has been paid to mixed FSO-RF systems where single-hop FSO transmission and single-hop RF transmission configuration is used. In this paper, we propose a mixed FSO-RF dual-hop simultaneous wireless information and power transfer (SWIPT) relaying system in the presence of power amplifier (PA). It is assumed that FSO links suffer from Málaga (M)-turbulence with pointing errors and RF links experience the double shadowed Rician fading. Particularly, the physical layer security is analyzed by deriving the closed-form expressions for secrecy metrics such as secure outage probability, strictly positive secrecy capacity, and secrecy throughput. We consider the three scenarios: 1) FSO-side eavesdropping attack 2) RF-side eavesdropping attack 3) Simultaneous FSO- and RF-side eavesdropping attacks. The asymptotic approximations for the final results are also derived to obtain secrecy diversity order. The effect of SWIPT parameters and PA efficiency on the secrecy performance are then further investigated in detail. In summary, the results show that the reliability and security of the proposed system can be enhanced by utilizing efficient PAs at the RF front end depending on the eavesdropper's location and can also be controlled by the SWIPT parameters.

Sparse code multiple access (SCMA) is a promising code-domain non-orthogonal multiple access (NOMA) technique that supports high-level connectivity. Through the combination of SCMA and multiple-input multiple-output (MIMO) techniques, MIMO-SCMA schemes are capable of enhancing the overall spectral efficiency. This paper proposes novel MIMO-SCMA designs for both uplink and downlink Rayleigh fading channels. Rather than directly combining the SCMA and spatial multiplexing techniques presented in the previous literature, the proposed designs consider not only the frequency or time diversity, but also simultaneously include space diversity. Codebook design criteria for different transmission scenarios are respectively investigated, and a low-complexity codebook design algorithm is proposed based on the cross-entropy method. In addition, since the new designs suggest a dense factor graph, conventional message-passing algorithm (MPA)-based detectors are less suitable. Consequently, low-complexity MIMO-SCMA codeword detectors based on depth-first and breadth-first tree-search algorithms are respectively investigated. It is shown that the overall designs are able to simultaneously achieve a higher diversity order, enhanced BER results, and lower levels of detection complexity for both uplink and downlink scenarios, when compared to conventional MIMO-SCMA schemes.

In this article, we consider secure communication over asymmetric dual-hop simultaneous wireless information and power transfer system having one free-space optical (FSO) link followed by one radio frequency (RF) link. We assume that the optical transmitter conveys secure information to the RF receiver through an intermediate relay, which exploits the decode-and-forward relaying technique. The FSO link encounters Málaga ( M ) distributed turbulence with pointing error, and RF link suffers double shadowed Rician fading. We investigate the consequences of pointing error, atmospheric turbulence, and energy harvesting on the performance of the proposed mixed FSO/RF SWIPT framework. Specifically, we derive the analytical expressions for statistical characteristics such as probability density function, amount of fading, average bit error rate (BER), outage probability (OP), and ergodic capacity. The asymptotic expression for OP is also derived to obtain the diversity order of the system. Based on these results, the physical layer security analysis is carried out, and the analytical expressions for secure OP and strictly positive secrecy capacity are derived. Herein, the two eavesdroppers are considered. In the first scenario, it is assumed that only the FSO link is overheard, while in the second scenario, both FSO and RF links are under simultaneous eavesdropping attack.

Reconfigurable antenna systems are a key technology for sixth-generation (6G) networks. They enable dynamic adaptation in frequency, spatial, and polarization domains. This paper presents a unified framework for fluid antennas, pinching antennas, electromagnetically reconfigurable antennas, graphene-based terahertz (THz) antennas, and stretchable or conformal arrays. Physical principles, architectural integration, and control mechanisms are discussed in detail. Representative 6G use cases are examined, including vehicular ultra-reliable low-latency communication (URLLC), THz backhaul in cell-free massive multiple-input multiple-output (CF-mMIMO) networks, satellite–terrestrial integration, and joint communication–sensing (JCAS). Architectural interactions with reconfigurable intelligent surfaces, holographic MIMO, and integrated sensing and communication platforms are outlined, along with their roles in multi-band operation, energy harvesting, and backhaul enhancement. Key implementation challenges are identified in channel modeling, hardware fabrication, control protocols, scalability, and standardization. The paper concludes by identifying potential research directions for advancing reconfigurable antenna technologies within the broader 6G ecosystem.