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The reconfigurable intelligent surfaces (RISs) are expected to be a cheap way to extend service areas of base stations. This is especially promising in the millimeter wave and THz bands (from 30 GHz to +300 GHz) where base station coverage is expected to be modest and suffer greatly from blockages. As the RISs can potentially be large (physically and via number of sub-elements), there is a good change that a user is in the near field of the RIS. This paper considers RIS near field propagation and achievable power levels close to these surfaces. Ideal energy levels are looked into among with the impact of beamforming and beam squinting. Human safety issues close to these surfaces are also analyzed from the energy density point of view. It is shown that the achievable received power in the near field are very good, but the beam squinting may have a significant impact on the received power and frequency response. We also conclude that RISs are safe for humans even at close proximity due to relatively large channel losses in the reflected channels and hence low power densities in the air.

The D-band among all sub-THz and THz radio frequencies is foreseen as the first one to be utilized for communications. Such high frequency communication links are particularly susceptible to shadowing events, e.g., caused by the human body. In this paper we present results of a D-band channel measurement campaign, which was conducted to characterize the impact of human blockage, with a focus on the excess attenuation and temporal evolution of human body shadowing. The attenuation caused by single-person blockers with different physical characteristics was measured with human frontal and lateral crossing the line-of-sight link. Predicting results of two knife-edge diffraction models are compared with the measurement curves, which both underestimate the attenuation levels especially for the volunteer with larger size. Meanwhile, we quantify the deep fading duration varying with the fading depth, which helps to optimize the beam alignment strategy in order to maintain the sufficient signal-to-noise ratio when the dominant path is heavily obstructed.

Future 6G and beyond wireless systems aim at ultra-high data rates from very large bandwidths in millimeter-wave and terahertz bands. Large bandwidth signals in antenna arrays lead to beam squint or frequency-selective array response causing losses in the beamforming gain. Recently, several hybrid analog-digital beamforming (HBF) schemes, such as the delayphase precoding employing time delay networks or wider-beam codebook design, have been proposed to tackle the beam squint. However, the severity of beam squint has not been thoroughly investigated in the literature. In this paper, by analyzing the impacts of the beam squint on the uniform planar array (UPA) and uniform linear array (ULA) structures, we analytically and numerically show that the effect of beam squint is less severe in the former. Based on this fact, we propose a simplified HBF design in which the analog beamformer is designed for the center frequency, while digital beamformers for all subcarrier frequencies are obtained based on water-filling and minimum mean square error solutions. Simulation results show that the proposed scheme for UPAs performs very close to the optimal digital beamforming scheme.

A need for measurement data for frequencies above 100 GHz has increased. In this paper, we present the measurement system for millimeter wave frequencies (30–300 GHz). It is based on Keysight PNA-X vector network analyser with frequency extenders mounted on custom built 3-axis rotation stages (both the transmitter and receiver sides). We present some initial results from measurements conducted in an office corridor. Both line-of-sight (LOS) and non-LOS (NLOS) cases are included in the measurement set-ups. The results show that in all cases the channels are sparse with clear clusters from large reflecting surfaces, such as walls and floors. The measured power levels are rather strong indicating relatively low reflection losses. While the channels were sparse, there were still typically relatively large numbers of distinguishable paths. The numbers of observable paths as well as the delay spread of the channel increase with increasing beamwidth.

Upcoming beyond fifth generation (5G) communications systems aim at further enhancing key performance indicators and fully supporting brand-new use cases by embracing emerging techniques, e.g., reconfigurable intelligent surface (RIS), integrated communication, localization, and sensing, and mmWave/THz communications. The wireless intelligence empowered by state-of-the-art artificial intelligence techniques has been widely considered at the transceivers, and now the paradigm is deemed to be shifted to the smart control of radio propagation environment by virtue of RISs. In this paper, we argue that to harness the full potential of RISs, localization and communication must be tightly coupled. This is in sharp contrast to 5G and earlier generations, where localization was a minor additional service. To support this, we first introduce the fundamentals of RIS mmWave channel modeling, followed by RIS channel state information acquisition and link establishment. Then, we deal with the connection between localization and communications, from a separate and joint perspective.

This paper presents and overview, challenges and some recent results on channel modeling of the phased array type reconfigurable intelligent surfaces (RISs) in the millimeter Wave band (mmWave, 30‐300 GHz) networks. The RISs have been under intense investigation lately for their ability to provide control on the mmWave sparse channels. This has been shown to improve the coverage and signal power levels in environments where line-of-sight (LoS) paths cannot be guaranteed. The gain properties of RISs in LoS channels are analyzed with respect to carrier frequency and the size of the RIS. It is shown that the RISs provide the best gain compared to LoS links at lower frequencies due to the larger RIS sizes for a fixed number of antenna elements. On the other hand, high gain RISs tend to push the near field far away from the array in the low frequencies. These observations mean that different parts of the mmWave band have advantages and disadvantages when utilizing RIS assisted channels.

The THz frequencies (0.1–10 THz) have shown great potential for long distance satellite communications in the recent research papers. This band offers superior bandwidths compared to those enabled by the lower frequencies, with relaxed pointing requirements compared to optical systems, making the the THz band very appealing for providing backhaul connectivity on vast numbers of airplanes. At the same time, the path losses are very high and extremely directional antennas are required to operate on the links exceeding thousands of kilometers in the satellite applications. This paper derives a framework for stochastic geometry based estimation of average signal levels in THz satellite networks. Specific focus herein is on the airplane-satellite links. The produced stochastic models are shown to be exact with a simulation model. The analysis shows that while the average path losses are very high, there are antenna solutions that can overcome the losses and give respectable signal-to-noise ratios. The derived models are very useful in satellite network analysis, as well as in the link budget calculations for future high frequency satellite systems.

In this paper, we utilize tools from stochastic geometry to estimate the interference propagation via reconfigurable intelligent surface (RIS) in the millimeter wave (mmWave, 30–300 GHz) band and specifically on the D band (110–170 GHz). The RISs have been of great interest lately to maximize the channel gains in non-line-of-sight (NLOS) communication situations. We derive expressions for stochastic interference level in RIS powered systems and validate those with simulations. It will be shown that the interference levels via RIS link are rather small compared to the designed RIS link or the LOS interference as the random interference loses significant part of the RIS gain. We also analyse the validity of far field channel and antenna gains in the near field of a large array. It is shown that, while the high frequency systems require large arrays that push the far field far away from the antenna, the far field equations are very accurate up to about half way of the near field.

The future high frequency systems require very high antenna gains to cope with the large channel losses. In order to obtain large gain and flexible beamforming, large antenna arrays are often considered. With hundreds or thousands of antenna elements, transmit energy is highly concentrated on the target. However, large numbers of antenna elements make the antennas very large compared with the wavelength. Therefore, it is likely for the user to be in the near field of the array. In this region, the maximum antenna gain is obtained by focusing the energy on the desired locations. The downside is a need to know the exact locations of the radios. This paper focuses on analysis of the traditional linear phase beam steering and near field focusing beam steering and the impact of the user location uncertainty on the achievable antenna gain. The uncertainty in the user location can arise, for instance, due to user mobility. The results show that the near field focusing gives superior gain in the near field of the antenna array, but is more sensitive to the user location information than the linear phase beam steering.

Reconfigurable intelligent surfaces (RISs) are considered as potential technologies for the upcoming sixth-generation (6G) wireless communication system. Various benefits brought by deploying one or multiple RISs include increased spectrum and energy efficiency, enhanced connectivity, extended communication coverage, reduced complexity at transceivers, and even improved localization accuracy. However, to unleash their full potential, fundamentals related to RISs, ranging from physical-layer (PHY) modelling to RIS phase control, need to be addressed thoroughly. In this paper, we provide an overview of some timely research problems related to the RIS technology, i.e., PHY modelling (including also physics), channel estimation, potential RIS architectures, and RIS phase control (via both model-based and data-driven approaches), along with recent numerical results. We envision that more efforts will be devoted towards intelligent wireless environments, enabled by RISs.

Wireless connectivity in airplanes is becoming more important, demanded, and common. One of the largest bottlenecks with the in-flight Internet is that the airplane is far away from both the satellites and the ground base stations during most of the flight time. Maintaining a reliable and high-rate wireless connection with the airplane over such a long-range link thus becomes a challenge. Microwave frequencies allow for long link distances but lack the data rate to serve up to several hundreds of potential onboard customers. Higher bands in the millimeter-wave spectrum (30 GHz–300 GHz) have, therefore, been utilized to overcome the bandwidth limitations. Still, the per-user throughput with state-of-the-art millimeter-wave systems is an order of magnitude lower than the one available with terrestrial wireless networks. In this paper, we take a step further and study the channel characteristics for the terahertz band (THz, 0.3 THz–10 THz) in order to map the feasibility of this band for aviation. We first propose a detailed channel model for aerial THz communications taking into account both the non-flat Earth geometry and the main features of the frequency-selective THz channel. We then apply this model to estimate the characteristics of aerial THz links in different conditions. We finally determine the altitudes where the use of airplane-to-satellite THz connection becomes preferable over the airplane-to-ground THz link. Our results reveal that the capacity of the airborne THz link may reach speeds ranging from 50–150 Gbps, thus enabling cellular-equivalent data rates to the passengers and staff during the entire flight.

This paper documents a simple parametric polynomial line-of-sight channel model for 100—450 GHz band. The band comprises two popular beyond fifth generation (B5G) frequency bands, namely, the D band (110—170 GHz) and the low-THz band (around 275—325 GHz). The main focus herein is to derive a simple, compact, and accurate molecular absorption loss model for the 100—450 GHz band. The derived model relies on simple absorption line shape functions that are fitted to the actual response given by complex but exact database approach. The model is also reducible for particular sub-bands within the full range of 100—450 GHz, further simplifying the absorption loss estimate. The proposed model is shown to be very accurate by benchmarking it against the exact response and the similar models given by International Telecommunication Union Radio Communication Sector. The loss is shown to be within ±2 dBs from the exact response for one kilometer link in highly humid environment. Therefore, its accuracy is even much better in the case of usually considered shorter range future B5G wireless systems.

Providing high-bandwidth and fast-speed links, wireless local area networks (WLANs) in the Terahertz (THz) band have huge potential for various bandwidth-intensive indoor applications. However, due to the specific phenomena in the THz band, including severe reflection loss, indoor blockage effects, multi-path fading, the analysis on the interference and coverage probability at a downlink is challenging. In this paper, indoor blockage effects caused by the walls and human bodies are analyzed. Next, a statistical THz channel model is proposed to characterize the THz indoor propagation. In light of these, the moment generating functions of the aggregated interference and theoretical expressions for the mean interference power are derived. As a result, the approximated coverage probability and average network throughput are derived. Extensive numerical results show that for the nearest access point (nearest-AP) user association scheme, the optimal AP density is 0.15/m 2, which results in the coverage probability reaches 93% and the average network throughput is 30 Gbps/m 2. In addition, by adopting a novel line-of-sight access point (LoS-AP) user association mechanism, the coverage probability and the average network throughput can be further improved by 3 percent and 2 Gbps/m 2, respectively.

This paper focuses on deriving expected values for the transmit (TX) and receive (RX) antenna gains in terahertz (THz) wireless fronthaul and backhaul links under stochastic beam misalignment, which is created by antenna movement coming from the building or antenna mast swaying. In particular, four different antenna movement models are considered: (i) Gaussian motion of a single antenna; (ii) Gaussian motion of both the TX and RX antennas; (iii) 2-dimensional (2D) Gaussian motion of a single antenna; and (iv) 2D Gaussian motion of the one antenna and one-dimensional Gaussian motion of the other. Models (i) and (iii) depict fronthaul scenarios, in which the access point is usually installed in high buildings or in road-sides. Models (ii) and (iv) may model backhaul applications. To verify the analysis and quantify the impact of beam misalignment, analytic and simulations results are provided that reveal that the antenna motion can cause a significant degradation on the expected value of the TX and RX antenna gains. Moreover, the derived models are used in a link budget assessment and insightful results for a number of realistic scenarios are given. These result clearly highlight the impact of beam misalignment in the received signal quality as well as the importance of taking into account the beam misalignment and using the correct antenna movement model, when evaluating its impact.

The paper studies the joint impact of phase noise (PN) and co-channel interference (CCI) in indoor terahertz (THz) uplink. We formulate the theoretical framework that quantifies the impact of PN on the transceiver antenna directivity by extracting exact closed-form and low-complexity tight approximations for the expected gains. Additionally, by employing stochastic geometry, we model the propagation environment of indoor THz wireless systems and provide the analytical characterization of the CCI in the presence of PN, in terms of its expected value. The analysis is verified through computer simulations that reveal the accuracy of the presented theory with moderate numbers of users. The paper provides readily available tools for analyzing and designing indoor THz networks.

This paper derives a simplified polynomial molecular absorption loss model for 200—450 GHz band. This band has a high potential for near future short range, high data rate applications due to large spectral resources and reasonable path loss. The frequencies around 300 GHz are among the first ones where the molecular absorption loss becomes a significant factor. This loss increases exponentially with the distance and is not a major issue at short distances (few meters), but entirely blocks signals over large distances. Modeling of the molecular absorption loss is relatively straightforward, but requires large numbers of parameters from spectroscopic databases. This paper derives a simplified polynomial absorption loss model for the major absorption lines. This extends and gives more accurate absorption loss values in comparison to the existing ITU-R absorption loss models. The simplified polynomial expressions by ITU-R are mostly limited to about 350 GHz frequency, although being rather accurate up to about 400 GHz. This paper gives a simpler and more accurate absorption loss model to those bands. A part of the considered band, 275—450 GHz band, is subject to World Radiocommunication Conference 2019 (WRC-19) for allocation and operational characterization for the future communication applications.

This paper studies the impact of the line-of-sight (LOS) and non-LOS (NLOS) path loss at 300 GHz frequency band in a multipath propagation setting. The aim is to model the impact of the multipath signal components in stochastic indoor environment as a function of antenna patterns, and the number of the multipath signal components. The outcome of simulation model is that the general free space path loss is an accurate measure of the propagation loss in the LOS case. In the NLOS cases, the free space path loss model remains accurate with slight increase in the path loss exponent from 2 to approximately 2.2 to 2.7 depending on the material properties. Such low path loss exponents are caused by highly directional antennas omitting the most the interference from the propagation environment. As a consequence, the channel appears as a free space channel with some additional loss caused by reflections in NLOS directions.

Time domain channel model based on impulse response is introduced for wireless nanosensor networks which is used in the terahertz band (THz band: 0.1—10 THz) and short range (1—100 cm). We assume two-ray ground-reflection model. In THz band, rough surface on the reflector has effect on the impulse response due to very short wavelength. This paper focuses on frequency domain and time domain channel models and, for wireless communication analysis the impulse response is very important. Both models represent molecular absorption and rough surface effect which are unique aspects in THz band. The time domain channel model has multiple delayed taps even in LoS path. Reflected path has significantly strong effect to received signal from LoS path at long distance between transmitter and receiver than at short distance, relatively. These channel models are essential for a development of THz band communication techniques.

Future smart vehicles will incorporate high-data-rate communications and high-resolution radar sensing capabilities operating in the millimeter- wave and higher frequencies. These two systems are preparing to share and reuse many common functionalities, such as steerable millimeter- wave antenna arrays. Motivated by this growing overlap, which is advanced further by space and cost constraints, the vehicular community is pursuing a vision of unified vehicular communications and radar sensing that represents a major paradigm shift for next-generation connected and self-driving cars. This article outlines a path to materialize this decisive transformation. We begin by reviewing the latest developments in hybrid vehicular communications and radar systems, and then propose a concept of unified channel access over millimeter-wave and higher frequencies. Our supporting system-level performance characterization relies upon real-life measurements and extensive ray-based modeling to confirm the significant improvements brought by our proposal to mitigating the interference and deafness effects. Since our results aim to open the door to unified vehicular communications and radar sensing, we conclude by outlining the potential research directions in this rapidly developing field.

Results at mm wave frequencies have shown that it is important to evaluate co-channel interference between directional beams that cross each other. Terahertz frequency bands, especially around 300 GHz, has been considered for future wireless communications. As far as we know, no work has considered the rain induced interference at THz frequencies. In this work, we evaluate the rain induced interference at 300 GHz. Results are calculated also at 60 GHz in order to make a comparison. We combined bistatic radar equation, first order multiple scattering approximation and full Mie scattering calculations to existing drop size distribution models to estimate interference due to rain. Considered effective path lengths between transmitter and receiver are 100 m and 500 m and the effect of selected drop size distribution is studied. Overall interference levels was observed to be approximately 20 dBm smaller at 300 GHz than at 60 GHz and angular dependency of interfering power was much more forward oriented for 300 GHz frequency. The results show that rain induces interference has significantly different behaviour at THz as compared to lower frequencies.

Impulse response based channel model is vital for wireless communication analysis and modeling. This paper considers the impulse response of the terahertz band (THz band: 0.1–10 THz) for reflected path in case of short range (1–100 cm) wireless communication. In indoor application, it is necessary to consider multipath channel. In analysis of reflected path, rough surface of reflector is considered with Rayleigh roughness factor. The validity of the model is investigated with experimental THz band measurements (up to 2THz).

The terahertz band (0.1–10 THz) point-to-point links offer very large spectral resources for extremely high data rate links or an ability to share the resources among large numbers of users or devices. The latter case causes a need to consider interference in the THz band networks. The stochastic geometry is a powerful tool to estimate the network level interference and its moments in the case of random networks. This paper extends the previous works by considering bandwidth limited networks, i.e., instead of generic modeling of the interference, we apply bandwidth limited transmissions to better model realistic networks that utilize limited resources. Furthermore, we assume heterogenous network with some of the nodes utilizing directional antennas, and others using isotropic antennas. Network is further assumed to be sparse in a sense that the noise causes partial limitations in the achievable signal-to-noise-plus-interference (SINR). The mean interference and the SINR are derived and their validity is verified with computer simulations.

This paper focuses on giving a simplified molecular absorption loss model for a 275–400 GHz frequency band, which has significant potential for variety of future short and medium range communications. The band offers large theoretical data rates with reasonable path loss to theoretically allow even up to kilometer long link distances when sufficiently high gain antennas are used. The molecular absorption loss in the band requires a large number of parameters from spectroscopic databases, and, thus, the exact modeling of its propagation characteristics is demanding. In this paper, we provide a simple, yet accurate absorption model, which can be utilized to predict the absorption loss at the above frequency band. The model is valid at a regular atmospheric pressure, it depends on the distance, the relative humidity, and the frequency. The existing simplified model by ITU does not cover frequencies above 350 GHz and has more complexity than our proposed model. The molecular absorption loss increases exponentially with the distance, decreasing the utilizable bandwidth in the vicinity of the absorption lines. We provide a model to approximate the window widths at the above frequency band. This model depends on the distance, the relative humidity, the frequency, and the maximum tolerable loss. It is shown to be very accurate below one kilometer link distances.

In this paper, we present a novel two-path channel model for wireless terahertz (THz) systems operating in the range of 275 to 400 GHz, which accommodates both the channel particularities and the transceivers parameters. The channel particularities include the frequency selectivity, path-loss, as well as the atmospheric conditions, namely temperature, relative humidity and pressure, while the transceiver parameters, which are taken into account, are the antenna gains as well as the power of the transmitted signal. Finally, we evaluated the THz system performance in terms of average signal to noise ratio and ergodic capacity.

In this paper, we establish an appropriate system model for the terahertz (THz) wireless links in the range of 275 to 400 GHz, which accommodates the channel particularities and transceivers parameters. The former includes the frequency selectivity, pathloss, as well as the atmospheric conditions, namely temperature and pressure, whereas the latter are assumed to be consisted of the antenna gains as well as the power allocation of the transmitted signal. Moreover, we present analytical expressions of low computational complexity, for the evaluation of the average SNR, and capacity of the line of sight wireless THz links. These expressions are expected to be the key tools for the design of the THz link.

Communications systems operating in the millimeter-wave (mmWave) and terahertz (THz) band have been recently suggested to enable high data-rate vehicle-to-vehicle (V2V) communications in 5G and beyond wireless networks. However, massive deployment of such systems may lead to significant interference, affecting the performance of information transmission. While the multipath interference caused by the signal reflections from the road has been extensively discussed in the literature, the interference caused by the vehicles on the side lanes has been insufficiently studied so far. In this paper, using a combination of measurement, simulation, and analytical methods, we comprehensively characterize the interference from the side lanes in two typical deployments including highway and urban road environments for millimeter-wave and low terahertz bands. Both the multipath interference and direct interference from the transmitting vehicles on the side lanes are taken into account. As a result of our study, we reveal that: the interference from the side lanes can be well approximated using two-dimensional stochastic models without any significant loss of accuracy; and even when highly directional antennas are used there are special spatial configurations, where the interference may greatly affect the performance of the communication systems. We lately apply the developed models to estimate the signal-to-interference ratio and link capacity of mmWave/THz band V2V communications.

This article discusses the basic system architecture for THz wireless links with bandwidths of more than 50 GHz into optical networks. New design principles and breakthrough technologies are required in order to demonstrate terabit- per-second data rates at near zero latency using the proposed system concept. Specifically, we present the concept of designing the baseband signal processing for both the optical and wireless links and using an E2E error correction approach for the combined link. We provide two possible electro-optical baseband interface architectures, namely transparent optical-link and digital- link architectures, which are currently under investigation. THz wireless link requirements are given as well as the main principles and research directions for the development of a new generation of transceiver front-ends that will be capable of operating at ultra-high spectral efficiency by employing higher-order modulation schemes. Moreover, we discuss the need for developing a novel THz network information theory framework, which will take into account the channel characteristics and the nature of interference in the THz band. Finally, we highlight the role of PBF, which is required in order to overcome the propagation losses, as well as the physical layer and medium access control challenges.

Impulse response is vital for wireless communication analysis and modeling. This paper considers the impulse response of the terahertz band (THz band: 0.1–10 THz) for short range (1–100 cm) wireless communication. Earlier works derived the impulse response from transmittance by assuming a linear phase, which corresponds to a line-of-sight (LoS) propagation delay to a receiver. However, the linear phase leads to a symmetric impulse response before and after the LoS propagation delay. Physically, it is impossible for a signal to arrive before the LoS propagation delay since this violates causality. To address this issue, this study derives a phase function leading to an impulse response that satisfies causality. The validity of the derived model is verified with experimental THz band measurements (up to 2 THz), which show excellent agreement with the results predicted by the theory. From the impulse response, coherence bandwidth is found for both the entire THz band and its subbands. The results show significant variations in the coherence bandwidth as a function of the center frequency. Knowledge of these variations supports selection of the proper center frequency for wireless communications in the THz band.

A system enabling simultaneous cooling and board-to-board communications is proposed and analyzed. It is shown that hollow pipes used in computer cooling systems can be applied for communications with extreme data rates at distances up to tens of centimeters. This is done by using wireless communications in the terahertz frequency band, 0.1–10 THz. The experiments were performed in order to observe how straight and curved pipes of different diameters and lengths affect THz signals propagating inside the pipes. The measured pulses were recorded and used in numerical evaluation of bit error rate and throughput taking into account the effect of all possible combinations of N previous symbols. The numerical results show the dependency of the intersymbol interference on the delay profile of the channel and on the symbol period. The results demonstrate that even with simple on–off keying modulation the throughput reaches few terabits per second with qualitatively low bit error rates. This enables communications between rate-hungry electronics inside computers such as central and graphical processing units while simultaneously providing the cooling functionality.

Mean interference power and probability of outage in the THz band (0.1–10 THz) networks are studied. The frequency band has potential for enabling future short range communication systems because of the large available spectrum resources. This can enable huge data rates, or on the other hand, large numbers of users sharing the resources. The latter case is closely related to the subject of this paper on interference modeling for dense THz networks with stochastic geometry. We use it to estimate the average behavior of random networks. The literature has shown convenient closed form solutions for the mean interference power in ultrahigh frequency band (UHF, 300 MHz - 3 GHz). Those are not always readily applicable for the THz band. This is especially the case when THz band is modeled with the molecular absorption and free space path loss. Still, the mean interference power does have closed form solutions in all cases, but in some, numerical approximations have to be used. We provide the derivation and analysis of the mean interference power and the outage probability. The results are verified with computer simulations.

Future nanonetworks are formed by large numbers of autonomous, nano-sized sensors. These are often envisioned to be paired with one or more layers of higher complexity devices, providing access to the external networks. The number of devices sharing the same frequency resources can theoretically be very high, up to several hundreds per square meter. This causes the overall interference of the network to increase with the complexity of the network. In this work, stochastic geometry is utilized to derive the moments of the summed interference in the case of multi-tier nanonetworks in the terahertz frequency band (0.1--10 THz). All the devices in all the tiers of the network are assumed to be Poisson distributed. Based on this assumption, models for the moments of interference are derived and they are shown by computer simulations to predict the mean interference and its higher moments exactly.

This thesis focuses on THz band channel modeling and characterization. This vast frequency band spans from 100 GHz to 10 THz. The approximately 10,000 GHz bandwidth together with advances in THz capable electronics have made this band highly potential for many future applications, e.g., imaging and nanodevice-to-nanodevice communications. The latter is the reference application of this thesis and it focuses on the communication among very small and simple devices. The main focus of the thesis is on the THz channel characterization. Therefore, the channel models presented herein are also suitable for communications at macroscopic scale. The THz band offers opportunities, but has many problems as well. One of these is molecular absorption, which causes frequency selective fading to signals. The fading is caused by the signals’ energy absorption in the resonance frequencies of the molecules in the communication medium. Based on the conservation of energy, the absorption is understood to cause a new type of noise in the THz links: transmission induced noise. This noise component is analyzed from multiple physical viewpoints. The THz signals have short enough wavelengths to theoretically allow scattering on aerosols in the atmosphere. Scattering causes frequency dependent loss of the signals, but also a signal spread in time over multiple scattering events. It is shown here that in some specific atmospheric conditions the scattering causes signal loss and time spread. In addition to the theoretical channel models, measurements on a variety of propagation phenomena are conducted and analyzed. These include penetration losses, rough surface reflections and scattering, and diffraction. Through the measurements, it can be shown that the THz band communications is feasible in non-line-of-sight (NLOS) conditions in spite of the above phenomena. In the last part of this thesis, stochastic geometry is applied to the THz band in order to estimate the mean interference powe, Tiivistelmä Tämä väitöskirja paneutuu THz-taajuisien kanavien mallintamiseen. Tämä valtavan laaja kaista ulottuu sadasta gigahertsistä aina kymmeneen terahertsiin asti. Noin 10000 GHz:n kaistanleveys, yhdistettynä THz-taajuudet mahdollistavien elektroniikan komponenttien kehitykseen, tekee tästä kaistasta erittäin houkuttelevan vaihtoehdon moniin tulevaisuuden sovelluksiin. Näitä ovat mm. kuvantaminen ja nanolaitteiden välinen tietoliikenne. Viimeisin on tämän väitöskirjan viitekehys ja keskittyy hyvin pienien ja yksinkertaisien laitteiden väliseen viestintään. Työn keskittyy pääosin THz-kanavamallinnukseen, joten esitettyjä tuloksia voidaan hyödyntää myös nanoskaalaa suuremmissa verkoissa. THz-taajuudet avaavat mahdollisuuksia, mutta tuovat myös ongelmia. Yksi näistä on molekulaariabsorptio, joka aiheuttaa taajuusselektiivistä häipymää signaaleihin. Tämä ilmiö johtuu sähkömagneetisen energian absorbotumisesta ilman molekyylien resonanssitaajuuksilla. Sen on myös arveltu johtavan uudenlaisen kohinan syntyyn, lähetysten indusoimaan kohinaan, perustuen energian säilymislakiin. Lähetysten indusoimaa kohinaa tutkitaan tässä työssä erilaisista fysikaalisista näkökulmista. THz-taajuisen säteilyn aallonpituus on riittävän lyhyt mahdollistamaan sironta ilmassa olevista aerosoleista. Sironta aerosoleista johtaa taajusriippuvaan signaalitehon häviöön, mutta myös signaalitehon leviämiseen ajassa monisironnan kautta. Työssä todennetaan, että sopivissa olosuhteissa sironta lisää häviöitä ja viivehajetta kanavassa. Teoreettisten kanavamallien lisäksi analysoidaan mittauksin alemmilta taajuusalueilta tuttuja etenemisilmiöitä, kuten signaalin läpäisyä, heijastuksia ja sirontaa pinnoilta, sekä diffraktioita. Mittausten kautta voidaan näyttää, että THz-taajuinen tiedonsiirtolinkki voidaan luoda myös ilman näköyhteyttä yllä mainittujen ilmiöiden kautta. Työn viimeisessä osassa sovelletaan stokastista geometriaa THz-taajuuksille keskimääräisen häiriötehon ja toimintakyvyttömyystodennäk

Impulse response of the terahertz band (0.1–10 THz) for wireless nanosensor networks is considered. For wireless communication analysis and modeling, the impulse response is very important. In the earlier works, the impulse response has been derived from the transmittance by assuming a linear phase shift. However, the linear phase shift only leads to a symmetric impulse response before and after the LoS propagation delay. Physically, it is impossible for a signal to arrive before the LoS propagation delay since this violates causality. In this paper, a phase shift function leading to an impulse response satisfying causality is derived. The validity of the derived model is shown by comparison between measurements and results predicted by the theory.