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Abstract

Surpassing 100 Gbps data throughput is a key objective and an active area of research for sixth-generation (6G) wireless networks that can only be met by exploiting the TeraHertz (THz) frequency band (0.3 - 10 THz). THz channel modeling faces new challenges given the emerging relevance of scattering and molecular absorption in this frequency range as well as the lack of a reliable library of material properties. In this work, we address these challenges by measuring systematically the dielectric properties of 27 common building and office materials and reporting an in-house three-dimensional ray-launching (3D-RL) algorithm that uses the created material library and accounts for rough surface scattering and atmospheric attenuation. In order to validate the proposed algorithm, a channel sounder measurement campaign has been performed in a typical indoor environment at 300 GHz. Simulations and measurements show good agreement, demonstrating the need for modelling scattering and atmospheric absorption in the THz band. The proposed channel model approach enables scenarios at THz frequencies to be investigated by simulation, providing a relevant knowledge for the development of ultra-high-speed wireless communication systems.

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... In this framework, we have been implementing a raylaunching algorithm that includes surface scattering through a Rayleigh-Rice theory (following the path set by others [3]) and atmospheric absorption using ITU recommendation [4] that has been validated against channel soundings measurements at 0.3 THz [5], [6]. We have been also carrying an extensive campaign of time-domain measurement covering the frequency range up to 1 THz to construct an extensive database of common materials found in indoor scenarios [7] that we have crosscorrelated against existing scattered data [6]. ...
... In this framework, we have been implementing a raylaunching algorithm that includes surface scattering through a Rayleigh-Rice theory (following the path set by others [3]) and atmospheric absorption using ITU recommendation [4] that has been validated against channel soundings measurements at 0.3 THz [5], [6]. We have been also carrying an extensive campaign of time-domain measurement covering the frequency range up to 1 THz to construct an extensive database of common materials found in indoor scenarios [7] that we have crosscorrelated against existing scattered data [6]. This combined approach enables us to assess here the impact that ignoring surface scattering and atmospheric absorption, as well as using microwaves material properties rather than the actual mmw and THz material properties have on the propagation planning at 0.45 THz. ...
... Simulations are based on a three-dimensional (3D) raylaunching algorithm based on geometrical optics and the uniform theory of diffraction [6]. The simulation box is gridded to map accurately enough the objects' geometries in the scenario. ...
... To this extent, a number of investigations on the scattering properties of various materials at sub-THz frequencies have been carried out [15,16]. The work in reference [17] presents reflection coefficient measurements of 50 different building materials that were modelled based on the Fresnel-Rayleigh equations and Kirchhoff scattering theory with limitations to resolve the scattering coefficients of materials with a complex structure. ...
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... Using higher frequency terahertz (THz) communications can effectively alleviate the increasingly tight spectrum resources and the capacity constraints of current wireless systems. The THz wave with a wavelength range of 30 µm to 1 mm is an electromagnetic spectrum between the millimeter wave and infrared ray [1][2][3][4]. Compared with microwave communication, THz communication has a larger transmission capacity, better confidentiality, and stronger anti-interference ability. ...
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... This, in turn, increases the likelihood of blockage. Although most studies do only focus on LoS signals, there have been few recent papers (e.g., [199]- [201]) on diffusionbased channels for THz communications that explore how nonspecular scattering can play a valuable role in THz wireless systems, especially when the LoS link is temporarily blocked. Additionally, rough surfaces on normal objects hinder specular reflections at the scale of THz signals' wavelengths. ...
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Key advances in Semiconductor Terahertz (THz) Technology now promises important new applications enabling scientists and engineers to overcome the challenges of accessing the so-called "terahertz gap". This pioneering reference explains the fundamental methods and surveys innovative techniques in the generation, detection and processing of THz waves with solid-state devices, as well as illustrating their potential applications in security and telecommunications, among other fields. With contributions from leading experts, Semiconductor Terahertz Technology: Devices and Systems at Room Temperature Operation comprehensively and systematically covers semiconductor-based room temperature operating sources such as photomixers, THz antennas, radiation concepts and THz propagation as well as room-temperature operating THz detectors. The second part of the book focuses on applications such as the latest photonic and electronic THz systems as well as emerging THz technologies including: whispering gallery resonators, liquid crystals, metamaterials and graphene-based devices. This book will provide support for practicing researchers and professionals and will be an indispensable reference to graduate students in the field of THz technology. Key features: Includes crucial theoretical background sections to photomixers, photoconductive switches and electronic THz generation & detection. Provides an extensive overview of semiconductor-based THz sources and applications. Discusses vital technologies for affordable THz applications. Supports teaching and studying increasingly popular courses on semiconductor THz technology.
Article
Terahertz (THz) Kiosk application offers ultrahigh downloads of digital information to users’ handheld devices. System configuration and multiple paths between the transmitter and receiver have important impact on the achievable data rates. In this paper, the propagation channel of THz Kiosk downloading application is investigated and a stochastic channel model is proposed. Based on channel measurements using a vector network analyzer, a 3-D ray-tracing simulator is calibrated to conduct simulations for an in-depth analysis of the different channel characteristics. Successively, the observed propagation paths are classified and the key channel parameters in time, frequency, and spatial domains are modeled for each type of ray. The resulting stochastic model is evaluated in terms of Rician K-factor and root mean square delay spread. Compared to the reference data, the mean absolute errors of these two metrics of the three investigated scenarios are less than 1.48 dB and 0.07 ns, respectively. The results show that the target 100 gigabits per second data rate is achievable at tens of gigahertz system bandwidth at proper communication distance and higher order modulation schemes. The developed channel model allows system design engineers to generate realizations of propagation channel efficiently for designing Kiosk-fashion close-proximity communication systems in the THz band.
Article
In this paper, intra-wagon wireless communication performance is analyzed, in order to account for inherent scenario complexity in the deployment phase of wireless systems toward the implementation of a context-aware environment. A real commercial passenger wagon has been simulated by means of an in-house-developed 3-D ray launching code, accounting for embedded wagon elements as well as variable user densities within the passenger wagon. Onboard measurements of a designed and deployed wireless sensor network are obtained, showing good agreement with wireless channel estimations for two different frequencies of operation. Energy consumption behavior and user density impact have also been analyzed and estimated as a function of network topology and the operational mode. These results can aid in wireless transceivers deployment configurations, in order to minimize power consumption, optimize interference levels, and increase overall service performance.
Article
Simulation techniques based on deterministic methods such as Ray Tracing and Ray Launching, are widely used to perform radioplanning tasks. However, the quality of the simulations depends on the number of rays and the angular resolution. The computational cost of these simulations in High Definition prevents their use in complex environments and their Low Definition counterparts are used instead. In this article we propose a technique based on collaborative filtering to lessen the poor quality problems of Low Definition simulations. We show that our approach obtains results very similar to those of High Definition in much less time. Also, we compare our approach with other well-known techniques and we show that it performs better in terms of accuracy and precision. The use of combined deterministic/collaborative filtering techniques allows the estimation of radioplanning tasks in large, complex scenarios with a potentially large amount of transceivers.
Article
Intelligent transportation systems (ITSs) are currently under intense research and development for making transportation safer and more efficient. The development of such vehicular communication systems requires accurate models for the propagation channel. A key characteristic of these channels is their temporal variability and inherent time-changing statistics, which have a major impact on electromagnetic propagation prediction.
Article
A novel and efficient deterministic approach to model radio wave propagation channels in complex indoor environments improving prediction accuracy is proposed. This technique combines a 3-D Ray Launching algorithm based on Geometrical Optics with a Diffusion Equation method based on the equation of transfer. A comparison between the Geometrical Optics only approach and the new method considering the Diffusion Equation has been presented for studying indoor radio wave propagation. The Geometrical Optics-Diffusion Equation method achieves better agreement with measurements, while resulting in high computational efficiency, with approximately 40% savings in simulation time.
Article
A tabulation of relative dielectric constant and loss tangent is presented for a number of common materials. Measurements were made by using an interference spectrometer to measure insertion loss and phase shift of dielectric samples.
Article
Wireless communications systems have been rapidly growing during the last two decades, and they are gaining a significant role for multiple communication tasks within public transportation buses. In this paper, the impact of topology and morphology of different types of urban buses is analyzed with the aid of an in-house developed 3D ray launching code and compared with onboard measurements of a deployed wireless sensor network (WSN). The presence of human beings has been taken into account, showing a significant influence in the signal attenuation in the case of considering persons. In addition, the statistical analysis of simulation results considering both large- and small-scale fading has been performed, providing good agreement with statistics for typical indoor environments. In addition, a WSN has been programmed and deployed within the buses in order to analyze topological impact with overall system performance, with the aim of minimizing the energy consumption as well as nondesired interference levels. The use of deterministic techniques destined to consider the inherent complexity of the buses can aid in wireless system planning in order to minimize power consumption and increase overall system capacity.
Article
This paper presents measurements and statistical characterization of 300- to 320-GHz desktop channels. The measurements are performed in line-of-sight (LoS) and non-LoS (NLoS) environments. From the large set of LoS measured data, the parameters for the single-slope path-loss model with shadowing are devised. The results show that the path-loss exponent is around 1.9 and that the variations due to shadowing are similar across different frequencies and different bandwidths. Furthermore, the impact of different materials on the path loss is studied in the NLoS environment. The results show that metal objects in the propagation path cause multiple strong reflections, leading to a higher path loss. Furthermore, the statistical analysis of multipath propagation is performed. The root-mean-square (RMS) delay spread, the mean excess delay, the maximum excess delay, and the coherence bandwidth for LoS and NLoS environments are calculated. The results show that the mean excess delay and RMS delay spread increase with distance and that the RMS delay spread in the desktop terahertz channel is much smaller than that in typical indoor ultrawideband channels. In addition, the power delay profiles for LoS and NLoS environments are analyzed. The results show that strong reflections from the transmitter and receiver electronics are present in both LoS and NLoS environments. Finally, the statistical analysis of the measured signal amplitude in LoS and NLoS environments is performed. For both LoS and NLoS propagation environments, it is found that the lognormal distribution provides the best fit.
Book
Electromagnetic wave scattering from random rough surfaces is an active, interdisciplinary area of research with myriad practical applications in fields such as optics, acoustics, geoscience and remote sensing. Focusing on the case of random rough surfaces, this book presents classical asymptotic models used to describe electromagnetic wave scattering. The authors begin by outlining the basic concepts relevant to the topic before moving on to look at the derivation of the scattered field under asymptotic models, based on the Kirchhoff-tangent plane, in order to calculate both the scattered field and the statistical average intensity. More elaborated asymptotic models are also described for dealing with specific cases, and numerical results are presented to illustrate these models. Comparisons with a reference numerical method are made to confirm and refine the theoretical validity domains. The final chapter derives the expressions of the scattering intensities of random rough surfaces under the asymptotic models. Its expressions are given for their incoherent contributions, from statistical calculations. These results are then compared with numerical computations using a Monte-Carlo process, as well as with experimental models, for sea surface backscattering.
Article
Terahertz (0.06–10 THz) Band communication is envisioned as a key technology for satisfying the increasing demand for ultra-high-speed wireless links. In this paper, first, a unified multi-ray channel model in the THz Band is developed based on ray tracing techniques, which incorporates the propagation models for the line-of-sight, reflected, scattered, and diffracted paths. The developed theoretical model is validated with the experimental measurements (0.06–1 THz) from the literature. Then, using the developed propagation models, an in-depth analysis on the THz channel characteristics is carried out. In particular, the distance-varying and frequency-selective nature of the Terahertz channel is analyzed. Moreover, the coherence bandwidth and the significance of the delay spread are studied. Furthermore, the wideband channel capacity using flat and water-filling power allocation strategies is characterized. Additionally, the temporal broadening effects of the Terahertz channel are studied. Finally, distance-adaptive and multi-carrier transmissions are suggested to best benefit from the unique relationship between distance and bandwidth. The provided analysis lays out the foundation for reliable and efficient ultra-high-speed wireless communications in the (0.06–10) THz Band.
Conference Paper
Using terahertz time-domain spectroscopy (THz-TDS), we measured optical constants of nine different types of common natural stones: slate, gneiss, marble, mudstone, sandstone, dolomite, granite, tuff, and diorite. The result shows that most natural stones are fairly transparent in THz frequency range, and dolomite, in particular, exhibits a nearly uniform refractive index of 2.7 over frequency range from 0.2 to 1 THz. The high refractive index of dolomite is comparable to silicon, and a fabricated dolomite THz lens shows an applicable performance.
Article
A novel deterministic approach to model the radio wave propagation channels in complex indoor environments reducing computational complexity is proposed. This technique combines a neural network and a 3-D ray launching algorithm in order to compute wireless channel performance in indoor scenarios. An example of applying the method for studying indoor radio wave propagation is presented and the results are compared with a very high resolution fully 3-D ray launching simulation as the reference solution. The new method allows the use of a lower number of launched rays in the simulation scenario whereas intermediate points can be predicted using neural network. Therefore a high gain in terms of computational efficiency (approximately 80% saving in simulation time) is achieved.
Conference Paper
At Terahertz frequencies, commonly used indoor building materials like plaster must be considered as rough, so that rough surface scattering exerts a significant impact on ultra broadband THz propagation channels. Allowing for the modeling of scattering, the characteristics of scattered multipath components like angle and time of arrival as well as phase distributions are investigated in a small office scenario. A double directional channel model is derived in the AoA, AoD and ToA domain. The dependence of the multipath characteristics on the surface roughness is demonstrated. Exemplary channel realizations are generated according to the proposed approach and are contrasted to ray tracing simulations for the evaluation of the applicability of the model.
Article
Ultra broadband communication systems operated at THz frequencies will require the thorough knowledge of the propagation channel. Therefore, an extensive measurement campaign of 50 GHz wide indoor radio channels is presented for the frequencies between 275 and 325 GHz. Individual ray paths are resolved spatially according to angle of arrival and departure. A MIMO channel is recorded in a 2??2 configuration. An advanced frequency domain ray tracing approach is used to deterministically simulate the THz indoor propagation channel. The ray tracing results are validated with the measurement data. Moreover, the measurements are utilized for the calibration of the ray tracing algorithm. Resulting ray tracing accuracies are discussed.
Article
Providing the basis for fast system simulations and the adequate design of upcoming THz communication systems, a novel stochastic 300 GHz indoor channel model is introduced. It combines both the modeling in time as well as in frequency domain in order to account for the significant frequency dispersion of ultra broadband THz channels. Not only amplitude, phase and temporal, but also spatial channel information is considered. That way, MIMO systems as well as novel antenna concepts can be simulated. Verified and calibrated frequency domain ray tracing simulations in an office scenario provide the data basis for the derivation of model parameters. Model channel realizations are tested against ray tracing predictions and channel measurements. A complete scenario-specific parameter set is given for the considered environment, so that the model can be implemented for further use and future THz communication links can be designed under consideration of realistic propagation conditions.
Article
* Chapter 1: Background and introduction to mobile communications * Chapter 2: Multipath propagation in mobile communications * Chapter 3: Basic multipath mechanisms * Chapter 4: Propagation modelling * Chapter 5: Short-term channel behaviour from the two-path model * Chapter 6: Short-term behaviour of many-path models and scenarios * Chapter 7: Aspects of simulation and measuremen * Chapter 8: Antenna principles * Chapter 9: Array antennas in a multipath environment * Appendices
Article
Recent years have seen a tremendous increase in the demand for wireless bandwidth. To support this demand by innovative and resourceful use of technology, future communication systems will have to shift towards higher carrier frequencies. Due to the tight regulatory situation, frequencies in the atmospheric attenuation window around 300 GHz appear very attractive to facilitate an indoor, short range, ultra high speed THz communication system. In this paper, we investigate the influence of diffuse scattering at such high frequencies on the characteristics of the communication channel and its implications on the non-line-of-sight propagation path. The Kirchhoff approach is verified by an experimental study of diffuse scattering from randomly rough surfaces commonly encountered in indoor environments using a fiber-coupled terahertz time-domain spectroscopy system to perform angle- and frequency-dependent measurements. Furthermore, we integrate the Kirchhoff approach into a self-developed ray tracing algorithm to model the signal coverage of a typical office scenario.