This study introduces a new design for an ultra-compact shared-aperture antenna utilizing a quarter-mode substrate integrated waveguide (QMSIW) cavity. The proposed antenna operates as a 4 × 4 multi-input multi-output (MIMO) system in three 5G/6G millimeter-wave (MMw) bands, while functioning as a single element antenna for a 5.5 GHz wireless fidelity Microwave (Mw) band. The antenna comprises four QMSIW cavity resonators; each QMSIW is loaded with dual slots to produce tri-band MMw operation at 28 GHz, 38 GHz, and 0.13 THz. The four cavities are arranged to reuse the entire aperture by creating a conventional open-loop antenna that operates at a frequency of 5.5 GHz. Simulation, measurement, and co-simulation results show that the proposed antenna has a quad-band operation and exhibits favorable characteristics. The measured scattering parameters validate the simulated ones over the four bands under consideration. The lowest values of the measured total radiation efficiencies are 80%, 73%, 62%, and 72% (co-simulation) within the four covered bands, respectively. The antenna peak gains are 1.8 to 1.85 dBi, 4.0 to 4.5 dBi, 4.3 to 4.5 dBi, and 6.5 to 6.6 dBi within the covered bands. Furthermore, the design satisfies MIMO and diversity conditions (envelope correlation coefficient and branch power ratio) over frequency bands of operation. All excellent results are achieved from an ultra-compact size in terms of footprint area (0.018${\lambda }_0^2$), where λ₀ represents the free space wavelength at 5.5 GHz. The antenna boasts an excellent reuse aperture utilization efficiency (RAU) of 92% and a large ratio frequency of 23, making it an ideal candidate for compact devices. With its superior performance, the proposed design is well-suited for a range ofs wireless communication systems, including mobile devices and the Internet of Things.

Underwater optical wireless communication (UOWC) is a recent advancement in optical wireless communication (OWC) that offers a higher capacity and bandwidth for underwater applications than traditional acoustic systems. However, UOWC links are vulnerable to the complex influences of aquatic environmental conditions involving concentrations of contaminant factors and sources of noise that degrade their performance, thus complicating their design and practical implementation. To compensate for the deterioration caused by these issues, techniques such as low-density parity-check (LDPC) coding have been proposed to enhance error correction and system performance. Therefore, in this study, we evaluate the performance of LDPC-coded line-of-sight UOWC channels under different aquatic environmental impacts, noise factors, and optical property parameters. The signal-to-noise ratio (SNR), bit error rate (BER), and Q-factor were among the most important metrics studied via simulations using OptiSystem v.21 and MATLAB. The results show that for a 6G-UOWC system operating at 60 GHz and 10 Gbps, the maximum transmission distances are 25 m in pure water and 1.96 m in harbor water at a depth of 7 m. Solar noise reduces these distances, whereas LDPC coding significantly improves performance by extending the maximum distance to 29 m in clear water. Moreover, a comparative analysis of different field-of-view (FOV) settings revealed that a lower FOV reduces the impact of solar noise across depths. In addition, the maximum achievable data rate with LDPC coding reached 18 Gbps, with distances varying from 21 m under optimal conditions to 1.65 m under less favorable conditions.

Despite their multitude of benefits, free-space optical (FSO) communication systems experience severe performance degradation due to the diverse atmospheric conditions commonly present in their deployment areas. Optical signals transmitted through the atmosphere are subject to substantial scintillation losses caused by fluctuations in the refractive index even under clear weather conditions. To mitigate these challenges, in this study, the gamma-gamma distribution model was employed to statistically analyze the channel characteristics and irradiance fluctuations. The impact of pointing errors in a strong turbulence regime under both clear and light–fog weather conditions was also considered. The proposed Coded-Optical Improved Quadrature Spatial Modulation over Gamma-Gamma Atmospheric Turbulence Channels (COIQSM-GG) offers an effective solution for enhancing the reliability and resilience of FSO systems against such atmospheric turbulences. By integrating low-density parity check (LDPC) coding and the OIQSM technique and adapting them to the gamma-gamma channel model, COIQSM-GG improves spectral efficiency, data rates, link distance extension, and communication reliability. In addition, a comprehensive performance assessment that primarily focuses on the bit error rate (BER) pertaining to FSO links using an intensity modulation and direct detection (IM/DD) scheme is presented and discussed in detail. The results indicate that the proposed solution achieves an approximately 20.4 dB reduction in optical transmission power at a slight error rate (BER) of 10 ^-5 for clear weather and a 20.53 dB reduction for light fog weather at a distance of 2 km with ½ LDPC.

In this study, a performance evaluation of radio over multimode fibre system using coded orthogonal frequency division multiplexing (COFDM) is presented. The performance analyses based on bit-error ratio (BER) are extensively investigated. In addition, the effect of fibre length on BER performance is presented. Moreover, the effect of the exponent refractive index profile of the multimode fibre on the BER of COFDM is studied considering maximum likelihood ratio and sub-optimal simplified log likelihood ratio as a soft-bit metrics for soft-decision viterbi decoding.

This study introduces an accurate non-linear distortion analysis for vertical cavity surface emitting lasers (VCSELs) based on Volterra series approach in directly modulated scheme. Closed-form expressions of the optical response of VCSELs in terms of modulation response, harmonic distortions, and second- and third-order intermodulation distortions are derived. The impact of external optical feedback on the VCSEL's response and intermodulation distortion is investigated. Moreover, the suggested non-linear model avoids the numerical solution of laser rate equations and provides in detail a non-linear distortion analysis of VCSELs.

In this letter, an accurate nonlinear Volterra series analysis for vertical-cavity surface-emitting lasers (VCSELs) is presented. Closed-form expressions for the modulation response, harmonic distortions, and second- and third-order intermodulation distortions are derived. Moreover, the suggested nonlinear model avoids the numerical solution of laser rate equations and provides in-depth insight of a distortion analysis of VCSEL.