Nanoscale sensor networks:the THz band as a communication channel

Abstract 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 con- ducted 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 power and outage probabilities in dense networks formed from nanodevices. Because of the large losses in the channel, large interference levels require large numbers of devices. Stochastic geometry offers perfect tools to estimate the mean interference, and also in the case of directional antennas, which are most likely implemented in all the THz band devices due to large power losses in the channel.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 toimintakyvyttömyystodennäköisyyden selvittämiseksi tiheissä nanolaitteiden muodostamissa verkoissa. Isojen kanavahäviöiden takia suuri häiriötaso vaatii suuren määrän laitteita. Stokastinen geometria antaa täydelliset työkalut häiriötason estimointiin. Tätä voidaan myös hyödyntää suuntaavien antennien tapauksessa, joita tullaan suurella todennäköisyydellä käyttämään kaikissa THz-laitteissa johtuen suurista signaalihäviöistä kanavassa

Analysis of received power via RIS in near field LOS channels

Abstract 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

Stochastic geometry based interference analysis of multiuser mmWave networks with RIS

Abstract 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

Channel modeling for reflective phased array type RISs in mmWave networks

Abstract 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

Simplified molecular absorption loss model for 275–400 gigahertz frequency band

Abstract 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

Stochastic analysis of multi-tier nanonetworks in THz band

Abstract 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

Simple molecular absorption loss model for 200—450 gigahertz frequency band

Abstract 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

Stochastic geometry analysis for mean interference power and outage probability in THz networks

Abstract 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

A line-of-sight channel model for the 100–450 gigahertz frequency band

Abstract 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

LOS and NLOS channel models for indoor 300 GHz communications

Abstract 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