In recent years, line-of-sight multiple-input multiple-output (LoS-MIMO) systems have attracted significant attention for their potential in ultra-high throughput space communications. This work addresses E-band LEO satellite transmission with three advances: A dynamic spatio-temporal coupling model for non-orthogonal propagation and non-homologous clocks LoS-MIMO channels. A dual-feedback symbol timing recovery (STR) architecture integrating a frequency-domain timing offset detection (TOD) algorithm that combines an optimized Barton estimator and time-domain resampling, enabling continuous tracking and compensation for thousands of parts-per-million timing frequency deviations (TFDs). Closed-form expressions for TOD estimation error, loop convergence constraints, and residual timing jitter. Simulations under various transmitter delay differences and signal-to-noise ratios verify the resistance to TFD and low residual jitter of the algorithm, showing excellent agreement with theoretical predictions. To our knowledge, this is the first work to address STR architectures across multi-station with non-homologous clocks in satellite-to-ground LoS-MIMO systems, providing a foundational framework for future ultra-high-throughput space communications.

Satellite Internet of Things (IoT) network plays a crucial role in providing global coverage. To improve the efficient utilization of spectral and hardware resources in the satellite communications, integrated sensing and communication (ISAC) has attracted considerable attention. In addition, the greater broadcast nature of the satellite-terrestrial integrated network makes it extremely vulnerable to illegal eavesdropper (Eve). In this paper, we adopt non-orthogonal multiple access (NOMA) to support more users for the ISAC of a low earth orbit satellite system, with well-designed precoding according to whether the channel state information (CSI) of the Eve is perfect to ensure security. First, a joint precoding optimization problem is proposed to maximize the sum secrecy rate (SSR) of multiple users by considering the perfect CSI of the Eve. Then, we formulate a secure precoding optimization problem based on the imperfect CSI of the Eve, aiming to maximize the SSR of multiple users via artificial jamming. To address the non-convexity of the optimization problems, we transform them into the convex ones based on successive convex approximation, which includes Taylor’s approximation, arithmetic-geometric mean inequality, and linear matrix inequality. In addition, semi-definite relaxation and iterative penalty function methods are respectively used to optimize the secure precoding problems in the two cases: perfect CSI and imperfect CSI. Simulation results show that the proposed NOMA-ISAC scheme improves the SSR compared to the traditional time division multiple access while ensuring the sensing performance.

In this paper, we investigate the physical layer security for multi-beam satellite communications in the presence of multiple eavesdroppers (Eves). In particular, a worst-case eavesdropping scheme is considered. To achieve a positive secrecy rate, we consider a non-orthogonal multiple access (NOMA) scheme with imperfect channel state information on Eves. Fur-thermore, we design a robust precoding algorithm to maximize the achievable secrecy rate of legitimate ground user equipments, which satisfies the quality of service for each user, secure outage probability constraint and the NOMA decoding order between legitimate users. In contrast to the conventional total transmit power constraint, the algorithm is designed under joint total and per-beam transmit power constraints. We first combine the decomposition-based large deviation inequality, the arithmetic-geometric mean inequality with a penalty function iterative algorithm to solve the non-convex precoding problem, and further analyze the computational complexity of the algorithm. Simulation results verify the robustness and superiority of the proposed algorithm.

We investigate multi-user physical layer security assisted by an unmanned aerial vehicle (UAV) for multi-beam satellite communications in the presence of an eavesdropper (Eve) within the same beam. In particular, the UAV is exploited as a jammer that deliberately generates artificial noise to confuse Eve. To achieve a positive secrecy rate, we consider a non-orthogonal multiple access (NOMA) scheme with imperfect channel state information at the user and Eve. Specifically, we design a robust precoding algorithm to maximize the minimum achievable secrecy rate that satisfies the quality of service for each user and the NOMA decoding order between legitimate users in each beam. In addition, the algorithm is designed under joint total and per-beam transmit power constraints. We first combine the Bernstein-type inequality, the arithmetic-geometric mean inequality with a semi-definite relaxation iterative algorithm to solve the non-convex problem, and further analyze the complexity of the proposed precoding design. Simulation results show that the algorithm significantly improves the security performance.

With the rapid development of various services in wireless communications, spectrum resource has become increasingly valuable. Faster than Nyquist (FTN) signaling, proposed in the 1970s, is a promising paradigm for improving spectrum utilization. This paper proposes the variable-packing-ratio (VPR)-based transmissions for high spectrum efficiency (SE) and security, respectively. Aided by deep learning (DL)-based estimation, the proposed scheme for high SE can achieve a higher capacity than the conventional Nyquist-criterion transmission with negligible modification to existing communication paradigms (e.g., spectrum allocation or frame structure). More importantly, for VPR-based secure transmission, a dynamic generation scheme is proposed to produce randomly distributed positions to switch the packing ratio, which can effectively avoid detections and attacks. In addition, we propose a simplified DL-based packing ratio estimation for both of these two scenarios so that the receiver can estimate the packing ratio without any in-band or out-band control messages. Simulation results show that the proposed simplified estimation achieves nearly the same accuracy and convergence speed as the original multi-branch fully-connected structure with a complexity reduction of 20 folds. Finally, we derive the SE of the proposed VPR transmission under different channels. The numerical results validate the correctness of the derivation and demonstrate the SE gains of the VPR scheme beyond conventional Nyquist transmission.

Faster‐than‐Nyquist (FTN) signalling is regarded as a promising technology in satellite communications for the past few years, however, existing inter‐symbol interference (ISI) elimination algorithms for FTN signalling cannot eliminate the linear distortion caused by the input‐multiplexing and output‐multiplexing filters, which are very common in satellite communications. On the strength of the fractionally‐spaced equaliser (FSE), the authors propose a precoding‐based frequency‐domain linear distortion equaliser (PFLDE) to eliminate the above‐mentioned linear distortion in FTN systems. On the one hand, by virtue of fast Fourier transform (FFT) and inverse FFT, the proposed PFLDE lends itself naturally to high‐throughput parallel architectures, which are quite essential in high throughput satellite communications. On the other hand, the mean square error performance of the proposed PFLDE is superior to that of the existing time‐domain FSEs. Furthermore, combining PFLDE with the existing ISI elimination algorithms for FTN signalling, the satisfactory bit error ratio performance can be obtained.

A downlink user scheduling algorithm based on the pricing scheduling factor is proposed for the established relay‐assisted massive multiple input multiple output (MIMO) model, which represents the typical remote cell with multiple relays and a massive MIMO base station. Different from the traditional scheduling algorithms, the authors' proposed scheduling algorithm simultaneously takes account of the strength of the desired signal and interference, in which the proposed scheduling factor takes interference strength as the price to select appropriate users. An analysis on the optimal precoding vector is given by assuming each relay adopts equal gain transmission (EGT). According to the analysis, a non‐cooperative solution based on the phase difference of EGT is proposed to search the optimal transmission direction of EGT with low complexity. An analytical upper bound of system sum rate is also obtained. Simulation results indicate that the proposed scheduling algorithm has excellent sum rate performance and slight rate degradation compared with the brute‐force search algorithm.

Based on the field-programmable gate array (FP-GA) structure, a versatile transmission loss tester for wireless communications, especially satellite communications, is developed to obtain the bit error rate (BER) and transmission loss efficiently. The hardware of the tester can be treated as an integrated radio frequency (RF) transmission system with an embedded additive white Gaussian noise (AWGN) module. The FPGA-based RF system can generate at most 10 independent channels (ICs) with multiple rolling factors. Multiple symbol rates from 2 Ksps to 34 Msps can be supported with an appropriate multistage up-sampling processing. Furthermore, at most 640 sub-channels can be generated with the help of poly-phase filter banks and a well-designed FPGA realization of quadrature phase shift keying (QPSK) modem. The precomputation and test results show that a high dynamic range of transmitting and receiving signal levels can be supported. In addition, the expected transmission loss at one specific BER can be easily observed and acquired with the help of an integrated automated monitoring software developed by Qt development tool.

Without channel state information at the users and relay node, a non‐coherent transmission scheme for the two‐way relaying systems is investigated in this study. At the relay node, the authors first detect the uplink data with non‐coherent maximum likelihood receiver by fully utilising the large receiving freedom of relay's large‐scale antennas. Channel estimations can also be obtained by treating the detected uplink data as pilot symbols. Then, the relay node forwards the detected data by using beamforming. To assure the reliable detection of the users' transmitted symbols non‐coherently, the authors propose a transmitting constellation design with the idea of searching an optimal angle for rotating one user's constellation. Specially, for M ‐PSK constellations, they derive the optimal rotation angle, which is highly consistent with the exhaustive searching results. In addition, they also analyse the lower bound of channel estimations in terms of normalised mean square error (NMSE). From simulation results, the proposed non‐coherent transmission scheme achieves considerable error performance, and the NMSE lower bounds of channel estimations are also tight when the number of relay's antennas approaches to be large.

Based on the criterion of maximising the end‐to‐end receiving signal‐to‐noise power ratio, a dual‐antenna selection scheme combining with distributed space–time block code is proposed for two‐way relaying networks with amplify‐and‐forward protocol in Rayleigh fading channels. To facilitate the performance analysis, the authors derive the lower bound and asymptotic expressions for the network outage probability performance. Moreover, a concise solution for optimal power allocation is also determined to minimise the outage probability under a total power constraint. From the asymptotic outage probability analysis, the authors demonstrate that the proposed dual‐antenna selection scheme can achieve full outage diversity. Finally, numerical results verify the accuracy of the authors’ analysis. Notably, the proposed scheme achieves competitive performance improvement compared with the beamforming scheme and the existing antenna selection schemes in the state of the arts.

A general distributed multi-antenna two-way relaying network with multiple relays is considered in this paper. We first obtain a tight lower bound of pairwise error probability (PEP) of a maximum likelihood detector for the general distributed linear dispersion code for a half-duplex amplify-and-forward two-way relaying network (TWRN) consisting of two sources with each having single antenna and N relays with each having two antennas. Furthermore, by jointly considering signal precoding at the sources and signal processing at the relays, a general distributed concatenated recursive Alamouti-circulant space-time block code is proposed for the considered TWRNs. Our design ensures that the equivalent channel matrices at both source nodes are the so-called the recursive Alamouti-circulant matrices, with each block being a product of the two Alamouti channel matrices. Based on a lower-upper bound strategy and an induction method, asymptotic PEP formula is attained to show that given the optimal angle rotation matrix and the precoding matrix, the code can meet the lower bound of the diversity gain, as well as the maximum coding gain. In addition, the proposed rate one code turns to be effectively decodable.

The multi-way relaying networks (MWRNs) are investigated, in which multiple users exchange their information with the help of a relay node. We constraint that all users form a single group to share information with each other by using half-duplex communication mode. For such networks, an universal linear dispersion space-time block code (STBC) is introduced based on amplify-and-forward (AF) relaying protocol. Furthermore, a lower bound of pairwise error probability (PEP) of maximum likelihood (ML) detector is first derived, which reveals that the best-achievable diversity gain function is determined by the signal to noise ratio ρ, the user number K and the antenna number of the relay M, i.e., In ρ/ρM(K-1). Specifically, we design a distributed quasi-orthogonal STBC for the MWRN with K = 3 and demonstrate that the design can obtain a larger diversity gain than those in the one-way and two-way relaying networks by Monte Carlo simulations.