Current signal processing algorithms excel at impairment compensation when the parameters of optical fiber systems are precisely defined. However, their effectiveness diminishes considerably in the presence of imprecise parameters. By integrating ‘knowledge application from physics to neural networks (NN)’ with ‘information feedback from NNs to physics’, this paper proposes a physics-regulated digital backpropagation (PR-DBP) algorithm, which shows great promise for impairment compensation with imprecise fiber parameters. The PR-DBP employs an optimization-estimation-initialization loop structure. The optimization process provides the network’s adaptability by minimizing the loss value as in conventional NNs. The estimation process dynamically tracks physical parameters by extracting information from NNs to the physical domain. The initialization process offers a physics-based global control over the neural network, thereby mitigating the risk of overfitting by preventing an excessive focus on loss minimization. Moreover, a phase-shift weight function is applied to further improve algorithmic efficiency. Numerical analyses indicate that the PR-DBP significantly outperforms conventional methods in optical fiber systems with imprecise parameters, achieving a bit error rate reduction by an order of magnitude. As a mutually reinforcing part of impairment compensation, a high-accuracy and adaptive fiber parameter estimation is also demonstrated.

The Industrial Internet of Things (IIoT) is a typical future application for the mobile networks. Unmanned aerial vehicle (UAV) has attracted a great deal of interest in relay assisted wireless communication systems due to its benefits of high mobility, rapid deployment and high probability of line-of-sight (LoS) transmissions. This paper proposes the three-dimensional (3-D) geometry-based non-stationary channel models for wireless communication underlaying UAV-based relay assisted industrial internet of things (IIoT) networks in the sub-terahertz (sub-THz) band. In order to accurately describe the propagation characteristics of the UAV-based wireless channels in the sub-THz band, the propagation gain and atmospheric absorption gain in free space, LoS path, single UAV-based relay and double UAVbased relay propagation paths are considered in the proposed channel models. The channel impulse response (CIR) expressions of different propagation paths are derived respectively. The statistical properties of the channel models including path loss, channel capacity, temporal auto-correlation function (T-ACF), and Doppler power spectral density (DPSD) at 140 GHz band are investigated and analysed, with a performance comparison at 60 GHz band.

In this paper, we investigate a novel physical layer secure (PLS) potential technique which is summarized into a category of index modulation (IM) scheme, namely the spatial field digital modulation (SFDM). In an SFDM communication system, bit streams are modulated in the spatial distribution of electromagnetic (EM) field. Orbital angular momentum (OAM) mode groups can be utilized for beamforming and thus realize an OAM based SFDM (OAM-SFDM) system. This kind of IM systems that utilize the distribution of EM field to transmit information can radiate different signals to different directions and possess inherent anti-eavesdropping capability. Since an eavesdropper lacks prior knowledge, it is difficult to construct appropriate and effective estimators for signal demodulation. However, such system is vulnerable when potential eavesdroppers adopt clustering algorithm, a type of non-realtime demodulation. We propose two PLS strategies with different computational complexity, the power hopping (PH) method and the symbol variation (SV) method, to confront single-antenna eavesdroppers and multi-antenna eavesdroppers, respectively. Both approaches are key-free, and do not require any prior channel state information (CSI) about eavesdroppers. The mentioned methods could constantly disrupt the statistical properties of eavesdroppers’ channels and even their received signals. Theoretical analysis and numerical simulations have been conducted to validate the feasibility of these two schemes. Furthermore, a prototype of OAM-SFDM based PLS communication system is built in a realistic scenario. The experimental results demonstrate that both PH method and SV method can effectively resist clustering algorithm in their corresponding application scenarios.

The integration of unmanned aerial vehicles (UAVs) and terahertz (THz) technology can provide high data rate for air-to-ground (A2G) communications. In this paper, a non-stationary geometric sub-THz multiple-input multiple-output (MIMO) channel model is proposed for UAV A2G communication based on a three-dimensional (3D) semi-spherical model. The detailed channel modeling of reflection fading and scattering fading on the rough surface in sub-THz band is considered. According to the proposed geometry-based stochastic model (GBSM), the space-time correlation function (STCF) has been derived and the impact of several important UAV-related parameters on the STCF has been investigated and compared to the millimeter-wave (mm-wave) bands. The results indicate that the stationary time separation of channel and the dependency of spatial cross-correlation function (S-CCF) on antenna element spacing in sub-THz band of 140GHz are both smaller than those in mm-wave band of 28GHz.

The performance of radio-over-fiber optical transmission employing vertical-cavity surface-emitting lasers (VCSELs), and further wireless transmission, of the two major ultra-wideband (UWB) implementations is reported when operating in the 60-GHz radio band. Performance is evaluated at 1.44 Gbit/s bitrate. The two UWB implementations considered employ dual-carrier modulation orthogonal frequency-division multiplexing (DCM-OFDM) and binary phase-shift keying impulse radio (BPSK-IR) modulation respectively. Optical transmission distances up to 40 km in standard single-mode fiber and up to 500 m in bend-insensitive single-mode fiber with wireless transmission up to 5 m in both cases is demonstrated with no penalty. A simulation analysis has also been performed in order to investigate the operational limits. The analysis results are in excellent agreement with the experimental work and indicate good tolerance to chromatic dispersion due to the chirp characteristics of electro-optical conversion when a directly-modulated VCSEL is employed. The performance comparison indicates that BPSK-IR UWB exhibits better tolerance to optical transmission impairments requiring lower received optical power than its DCM-OFDM UWB counterpart when operating in the 60-GHz band.