For future sixth generation (6G) space-air-ground-sea integrated networks, leveraging troposcatter for beyond line-of-sight (BLoS) propagation offers a promising solution to achieving long-range unmanned aerial vehicle (UAV) communications. In this paper, a three-dimensional (3D) multiple-input multiple-output (MIMO) geometry based stochastic model (GBSM) is proposed for long-range UAV-to-ground (U2G) communications. The proposed channel model incorporates UAV 3D mobility and troposcatter characteristics, enabling a distance-dependent transition from visual-range to BLoS communications. A path loss model that accounts for stochastic obstacle deployments and troposcatter characteristics is developed, and the corresponding link budget is calculated to evaluate the feasibility of BLoS communications. Furthermore, troposcatter clusters are modeled and their space-time evolution is characterized to capture channel non-stationarity. Statistical properties, including space-time-frequency correlation function (STF-CF), delay/Doppler power spectral densities (PSDs), root mean square (RMS) delay/Doppler spreads, and coherence bandwidth/time, are also derived and analyzed. The proposed channel model provides valuable guidance for the design of long-range U2G communications.

With over 80% of mobile traffic occurring indoors, optimizing indoor wireless communication performance is crucial. For indoor communication systems, interior walls play a significant role in shaping system performance, yet their impacts on wireless communication performance at the link level remain insufficiently explored. In this paper, we propose an evaluation method for indoor wireless performance, considering wall materials and locations. Based on the evaluation method, the relationships between interior walls and signal propagation are investigated, considering line-of-sight (LOS) probability, blockage height, blockage density, and wall materials. The signal propagations are divided into four cases, including the direct path, the reflection path and/or penetration path through walls, and the multipath from scatterers. Moreover, spatial cross-correlation function (SCCF), spectrum efficiency (SE), and energy efficiency (EE) are analyzed to unveil the impacts of walls on channel characteristics and system performance. Our findings reveal that strategically adjusting the placement and materials of walls, while keeping network settings unchanged, can significantly enhance system performance.

Massive multiple-input multiple-output (MIMO) is considered as a key technology in the sixth generation (6 G) communication systems, bringing unique channel characteristics such as spherical wavefront and spatial non-stationarity effects. In this paper, we propose a super-resolution ray launching (SRRL) method to efficiently and accurately model the channel characteristics of massive MIMO scenarios. SR-RL simplifies simulations by performing RL only on the selected antenna elements based on the coherence distance and ignoring other elements. Ray paths traced from the selected elements are distributed to the ignored elements using the reflecting planes and possible obstacles along the ray trajectory. The obtained ray paths of both selected and ignored elements are recorded in a table, revealing a pattern of changes in multipath components across the antenna array. Compared to traditional brute-force methods, SR-RL can reduce computational time significantly, and captures spherical wavefront and spatial non-stationarity effects without sacrificing accuracy.

Reconfigurable intelligent surface (RIS) technology has emerged as a key direction for 6 G mobile communication systems due to its revolutionary potential in channel control, coverage enhancement, and energy efficiency optimization. In this paper, a novel ray tracing (RT) channel model for RIS-based beamforming at the transmitter (Tx) side is proposed. The model considers line-of-sight (LoS) conditions for both the Tx and receiver (Rx). At the Tx side, the feed antenna directs the beam toward the RIS, ensuring that at least one reflected ray from the center of the RIS reaches the Rx. The Rx captures the reflected rays arriving from the direction of the RIS. The statistical properties of the channel model for RIS-based beamforming in indoor scenarios are analyzed including path loss, delay power spectral density (PSD), and angular PSD. The statistical properties of the proposed channel model can well matched the channel measurements.

The explosive growth of the sixth-generation ( 6 G ) wireless system necessitates efficient base station (BS) deployment to balance coverage, data rates, and economic costs. In this paper, we propose a novel BS deployment scheme that jointly optimizes the number and locations of the deployed BSs, considering limited BS throughputs and practical non-uniform user distributions. A variant multi-dimensional knapsack problem is formulated and then solved by proposing a novel depth-limited backtracking dynamic programming (DLB-DP) algorithm with a BS-user association (BSUA) algorithm. We compare the proposed DLB-DP algorithm with three state-of-the-art benchmark algorithms in a hotspot scenario. The results demonstrate that the proposed algorithm outperforms the considered alternatives in terms of coverage, data rates, and robustness under varying user densities.

The sixth generation (6 G) communications will achieve broader coverage and support more communication scenarios. Satellite communications, which are important complements to ground communications, are expected to realize global coverage. This paper aims to propose a novel three-dimensional (3D) geometry-based stochastic model (GBSM) for satellite-to-maritime channel. The proposed model consists of line-of-sight (LoS) component, single-bounce (SB) components caused by the sea surface and ship body, and multi-bounce (MB) components due to ducting effect. The satellite trajectory and arbitrary movement of ship are also taken into consideration. The model considers atmosphere effect and rain effect, which are not negligible in satellite communication systems. Finally, statistical properties such as space-time-frequency (STF) correlation function, root mean square (RMS) delay spread, and Doppler power spectral density (PSD) are derived and analyzed.

The Industrial Internet of Things (IIoT) is a promising scenario for Industry 4.0, where the existence of rich scatterers can affect the quality of communication in the factory. In this paper, we investigate the multiple base stations (BSs) deployment problem based on the ray tracing (RT) method in IIoT scenarios considering coverage. We simplify the construction of IIoT scenarios by considering the geometric and channel characteristics to improve computational efficiency in RT simulation. Then, the BS deployment problem is formulated subject to the path loss and energy efficiency. The particle swarm optimization (PSO) algorithm with the surrogate model from RT simulation is proposed to solve the formulated problem. Moreover, the surrogate model is used to replace the all-time RT simulation in the PSO algorithm. The simulation results demonstrate that the proposed method effectively addresses BS deployment challenges in IIoT scenarios.

The emerging drones enabled air-to-ground (A2G) communications in millimeter-wave (mm-wave) bands are facing the Doppler effect that arises from the inevitable wobbling of the drones. The fast time-varying channel for drones A2G communications can make the channel state information (CSI) from channel estimation outdated, i.e., imperfect CSI. In this article, we introduce two detectors to demodulate the received signal and get the instantaneous bit error probability (BEP) of a mm-wave drones A2G link under imperfect CSI. Based on the designed detectors, we propose an adaptive modulation scheme to maximize the average transmission rate under imperfect CSI by optimizing the signal transmission time subject to the maximum tolerable BEP. A power control policy is proposed in conjunction with the adaptive modulation to minimize the transmission power while maintaining both the BEP under the threshold and the maximized average transmission rate. Numerical results show that the proposed adaptive modulation scheme in conjunction with the power control policy can maximize the temporally averaged transmission rate while reducing the power consumption by up to 50% for wobbling mm-wave drones A2G links.

In this paper, we study the impacts of user mobility on the average bit error probability (ABEP) and average achievable data rate (ADR) of ultra-reliable and low-latency communication (uRLLC) systems. Specifically, based on the time autocorrelation function (ACF) of the sixth generation pervasive channel model (6GPCM), we derive the expression of the pairwise error probability (PEP) of uRLLC systems adopting the M-ary phase shift keying (MPSK) modulation. In addition, we propose a new framework for the calculation of the average ADR considering the channel estimation period and cyclic prefix (CP) length. Moreover, the calculation methods of maximal data transmission period and transmission latency are given. Numerical results show that the increasing user mobility degrades the ABEP and average ADR. Impacts of user mobility on the ABEP surpass its influences on the average ADR. Furthermore, the tightness of analytical results is validated by Monte Carlo simulations. The performance analysis will guide the flexible designs of transmission blocklength under different reliability constraints.

Base station (BS) deployment is not a one-time endeavor, as when transitioning to higher frequency bands, coverage holes may arise, and the initial deployment may be unsatisfactory. In such cases, the deployment of additional BSs may be necessary. In this paper, we formulate an optimization problem to deploy additional BSs, attempting to minimize overlap coverage rate while satisfying the limitations of coverage rate and spectral efficiency. An improved genetic algorithm is adopted to solve the optimization problem. Numerical results show that the improved genetic algorithm can effectively minimize the overlap coverage rate and outperforms the original genetic algorithm and the particle swarm optimization (PSO) algorithm in terms of the minimum overlap coverage, spectral efficiency, the number additional BSs, and the convergence speed.

Ray tracing (RT) is an effective deterministic channel modeling method, but the computational burden for dynamic scenarios is high. To address this issue, this study categorizes objects involved in various types of motion within dynamic scenarios based on their motion characteristics, distinguishing between static and dynamic objects. Subsequently, the subinterval of the optimal split plane suitable for bounding volume hierarchies (BVH) construction is calculated, and the simplified interval BVH algorithm (SIBVH) is applied to the vehicle-to-vehicle (V2V) communication scenario. The simulation results indicate that, SIBVH based RT algorithm can trace rays much faster than traditional RT method, and improve computational efficiency significantly. The proposed algorithm can be effectively used in complex high-mobility communication scenarios.

In massive multiple-input multiple-output (MIMO) systems, the channel estimation scheme is subject to the spatial non-stationarity and inevitably power leakage in the beam domain. In this paper, a beam domain channel estimation scheme is investigated for spatial non-stationary (SNS) massive MIMO systems considering power leakage. Specifically, a realistic mas-sive MIMO beam domain channel model (BDCM) is introduced to capture the spatial non-stationarity considering power leakage by introducing the illustration of visibility region (VR). Then, a beam domain structure-based sparsity adaptive matching pursuit (BDS-SAMP) scheme is proposed based on the cross-block sparse structure and power ratio threshold of beam domain channel. Finally, the simulation results validate the accuracy of proposed BDS-SAMP scheme with low pilot overhead and reasonable complexity by comparing with conventional schemes.

The power consumption is a critical bottleneck of the unmanned aerial vehicle (UAV) air-to-ground (A2G) link due to the limited on-board battery. Meanwhile, the fast time-varying channel for millimeter-wave (mm-wave) A2G links may lead to the aged channel state information (CSI) under inevitable UAV wobbling. In this paper, we propose two optimization problems to evaluate the energy efficiency performance of the mm-wave UAV A2G link under wobbling. Then, we design two algorithms to maximize the energy efficiency of a mm-wave UAV A2G link by optimizing the transmission time and the UAV hover height subject to bit error probability while considering aged CSI. The numerical results present the spectral-energy efficiency trade-off and the optimum hover height of the mm-wave UAV A2G link under aged CSI.

The authors regret the errors in[1, eqs. (13), (27), (34), and (35)]. The corrections for these equations are given as follows:

Three-dimensional (3-D) massive multiple-input-and-multiple-output (MIMO) systems, which explore degrees of freedom in both the vertical and the horizontal dimensions, are a promising technology to enhance spectral efficiency in the next-generation communication systems. As an emerging modulation technology with millimetre wave (mm-wave) communication in massive MIMO systems with limited radio-frequency (RF) chains, the spatial scattering modulation (SSM) makes use of beamspace domain resources to further improve the spectral efficiency. However, most published works on the SSM only focus on two-dimensional (2-D) MIMO systems. In this paper, we generalize the SSM system to 3-D space. First, we design a novel generalized 3-D SSM system by considering both vertical and horizontal angles to determine scattering paths, which are selected to convey information bits. Then, we propose a whitening filter based optimal detection algorithm to detect the received symbols with correlated noise, where the correlation is generated by the combination of the received signals from the large-scale planar receiving antenna array. Next, we design a 2-D fast Fourier transform (FFT) based transceiving approach to improve the hardware friendliness. After that, we propose a low-complexity detector based on the linear minimum mean square error (MMSE) detection algorithm. Moreover, we derive the union upper bound on average bit error probability (ABEP) in a closed form and the asymptotic performance expression for the generalized 3-D SSM system. Finally, we analyse the impact of transmission environment on the ABEP performance. Numerical results show that the generalized 3-D SSM system outperforms the conventional 2-D SSM system, which reduces the ABEP by$10\times $with the same signal-to-noise ratio (SNR) level under the typical indoor environment.

The tremendous popularity of Internet-of-Things (IoT) applications and wireless devices has prompted a massive increase of indoor wireless traffic. To further explore the potential of indoor IoT wireless networks, creating constructive interactions between indoor wireless transmissions and the built environments becomes necessary. The electromagnetic (EM) wave propagation indoors would be affected by the EM and physical properties of the building material, e.g., its relative permittivity and thickness. In this article, we construct a new multipath channel model by characterizing wall reflection (WR) for an indoor Line-of-Sight (LOS) single-user multiple-input-multiple-output (MIMO) system and derive its ergodic capacity in the closed-form. Based on the analytical results, we define the wireless friendliness of building material based on the spatially averaged indoor capacity and propose a scheme for evaluating the wireless friendliness of building materials. Monte Carlo simulations validate our analytical results and manifest the significant impact of the relative permittivity and thickness of building material on indoor capacity, indicating that the wireless friendliness of building materials should be considered in the planning and optimization of indoor wireless networks. The outcomes of this article would enable appropriate selection of wall materials during building design, thus enhancing the capacity of indoor LOS MIMO communications.