In recent years, the use of massive multiple-input multiple-output (MIMO) systems and higher frequency bands for next-generation urban rail transportation systems has emerged as an intriguing research topic due to its potential to significantly increase network capacity by utilizing available narrowband and broadband spectrums. In metro and mining applications, the high-reliability wireless sensor network (WSN) plays a vital role in providing personal safety, channel optimization, and improving operational performance. Through the duration of 1921–2023, this paper provides the survey on the progress of fifth-generation (5G) and beyond-fifth-generation (B5G) wireless communication systems in underground environments such as tunnels and mines, the evolution of the earliest technologies, development in channel modeling for vehicle-to-vehicle (V2V) communications, and realization of different wireless propagation channels in high-speed train (HST) environments. In addition, the most recent advanced channel modeling methods are examined, including the development of new algorithms and their use in intelligent transportation systems (ITS); mathematical, analytical, and experimental techniques for propagation design; and the significance of the radiation characteristics, antenna placing, and physical environment effect on wireless communications. Leaky coaxial cable (LCX) and distributed antenna system (DAS) designs are introduced in the demonstrated systems for improving the channel capacity of narrowband and wideband channels as well as the spatial characteristics of various MIMO systems. The review article concludes by figuring out open research directions for future technologies.

In future hybrid fiber and radio access networks, wavelength division multiplexing passive optical networks (WDM-PON) based fifth-generation (5G) fronthaul systems are anticipated to coexist with current protocols, potentially leading to non-linearity impairment due to stimulated Raman scattering (SRS). To meet the loss budget requirements of 5G fronthaul networks, this paper investigates the power changes induced by SRS in WDM-PON based 5G fronthaul systems. The study examines wavelength allocation schemes utilizing both the C-band and O-band, with modulation formats including non-return-to-zero (NRZ), optical double-binary (ODB), and four-level pulse amplitude modulation (PAM4). Simulation results indicate that SRS non-linearity impairment causes a power depletion of 1.3 dB in the 20 km C-band link scenario, regardless of whether the modulation formats are 25 Gb/s or 50 Gb/s NRZ, ODB, and PAM4, indicating that the SRS-induced power changes are largely independent of both modulation formats and modulation rates. This effect occurs when only the upstream and downstream wavelengths of the 5G fronthaul are broadcast. However, when the 5G fronthaul wavelengths coexist with previous protocols, the maximum power depletion increases significantly to 10.1 dB. In the O-band scenario, the SRS-induced maximum power depletion reaches 1.5 dB with NRZ, ODB, and PAM4 modulation formats at both 25 Gb/s and 50 Gb/s. Based on these analyses, the SRS non-linearity impairment shall be fully considered when planning the wavelengths for 5G fronthaul transmission.

In order to meet the growing traffic demands, the formulation of the NG-EPON protocol is under heated discussion. Therefore, for the wavelength allocation scheme of NG-EPON, adopting O-band wavelengths is considered as the potentially feasible solution. While the low dispersion property of O-band would induce four-wave mixing (FWM) to NG-EPON networks. In this paper, polarization control methods are used to mitigate the FWM effect in NG-EPON networks, including orthogonal linear polarization, circular polarization and left to right circular polarization alternation methods. For the 4-channel NG-EPON networks, the FWM-induced sensitivity penalty is achieved as 0.3 dB by using the orthogonal linear polarization method, compared with the FWM-induced sensitivity penalty of 4.1 dB, which is the worst-case scenario. By adopting the circular polarization method, the FWM-induced sensitivity penalty is measured as 0.2 dB, which is 3.9 dB better than that of the worst-case scenario. By employing the left to right circular polarization alternation method, the FWM-induced sensitivity penalty is 0.2 dB, compared with the worst-case scenario, whose sensitivity penalty is 4.1 dB induced by the FWM. In addition, for the 8-channel NG-EPON networks, the FWM-induced sensitivity penalty is decreased to 0.4 dB for minimum, compared with the worst-case scenario, whose sensitivity penalty is unpredictable.

The rapid expansion of terahertz (THz) communication, nondestructive testing and biosensing puts forward urgent demands for absorbers with switchable working bandwidth, yet parts of existing graphene-based absorbers adopt costly noble-metal backplanes and can hardly realize reversible narrow and broadband absorption conversion. In this work, a three-layer metamaterial absorber is designed, where low-cost tungsten replaces precious metals as reflective substrate, polyimide serves as intermediate dielectric and patterned graphene composes the top absorbing layer. The finite element method (FEM) is employed to investigate the synergistic modulation of THz absorption characteristics by graphene’s Fermi level (E_f) and relaxation time (τ) across the 0–6 THz frequency range. Simulation results reveal that increasing E_f from 0.1 eV to 0.9 eV effectively broadens the effective absorption range. At fixed E_f = 0.9 eV, dual discrete absorption peaks with peak absorptivity up to 99.8% emerge at τ = 0.1 ps, while reducing τ to 0.05 ps enables an ultrawide 2.5 THz high-efficiency absorption band (absorptivity ≥ 90%) including a 1.4 THz near-perfect absorption (absorptivity ≥ 99%) region. Benefiting from high geometric symmetry, the proposed structure exhibits polarization-insensitive absorption and stable performance for incident angles up to 60°. This numerical work provides design references for low-cost switchable THz absorbers.