The recent progress in terahertz channel characterization and system design

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As the standardization of 5G New Radio (NR) systems operating in micro- and millimeter-wave frequency bands is over, scientific and industrial communities have begun to address the question of what 6G communications systems might or should be. While technological specifics are still in their early development phase, there is a common agreement that these systems will utilize the lower part of the terahertz band, namely, 100-300 GHz. This band poses a number of specific challenges for system designers, including the effects related to channel characteristics and the conceptually new requirements for electronics. This paper aims to report the current state-of-the-art channel characterization and communications system design. With respect to the former, we consider dynamic human body blockages and micromobility impairments. For the latter, we mainly concentrate on the physical layer devices for direct conversion schemes and the design of the so-called reconfigurable intelligent surfaces that will potentially serve as a cost-efficient blockage mitigation technique.

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1. Introduction Expanding the available bandwidth is the main tool for increasing rates at the access interface in cellular systems. This trend has been in place since 3G systems with 3G UMTS, 4G LTE, and 5G NR utilizing 2 MHz, 20 MHz, and 50 MHz wide basic channels, respectively, thus offering higher bandwidth. With 5G New Radio systems operating in the millimeter wave (mmWave, 24-100 GHz) band, the next logical step is the sub-terahertz/terahertz (sub-THz/THz, 100-300 GHz) band and even higher, where enormous yet unregulated bandwidth is available. Such bandwidth may result in data rates potentially supporting a plethora of upcoming applications, including virtual/augmented reality (VR/AR), holographic communications, etc. The use of the THz band poses several challenges to systems designers. The first set is related to still unsolved propagation effects that were already evident in the mmWave band, including dynamic blockage by human bodies and the micromobility phenomenon. The former makes mmWave/THz systems unusable under the conditions they were invented for large dense crowds generating enormous traffic demands. The solution to this problem requires fast and accurate blockage detection mechanisms and methods to decrease the frequency of blockage events. Micromobility is closely related to the utilization of directional antennas and is expected to become more relevant at THz frequencies, where the directivity of antenna radiation patterns will further increase. The development of large-scale antenna devices for the front ends of ultra-directional THz transmitters and receivers, as well as the smart 6G antenna environment, must comply with the criteria of cost and power efficiency. Moreover, supporting extremely large bandwidths is another challenge. This solution requires the use of advanced circuits forTHz signal generation and processing that are naturally compatible with ultrafast and spectrally effective communication schemes. This, in turn, can be accomplished via partial replacement of intrinsically limited Si-based electronics with more capable A3B5 electronic and radiophotonic devices. This paper aims to report the recent progress in THz channel characterization and system design. In the former topic, we concentrate on two effects currently affecting the public deployment of mmWave/THz communication systems: dynamic human body blockage and micromobility. In the context of system design, we consider state-of-the-art coherent communications with direct conversion and reconfigurable intelligent surfaces (RISs) for reflection-aided channels. The latter can be utilized as a simple and efficient block avoidance technique. The remainder of this paper is organized as follows. We begin with the effect of dynamic human body blockage in Section 2. In Section 3, we discuss recent progress in micromobility characterization. Section 4 describes the design of state-of-the-art THz communication systems. Finally, in Section 4.2 we consider a potential blockage avoidance solution for 5G and 6G systems - RISs. Finally, the conclusions are presented in the last section. 2. Dynamic blockage 2.1. Motivation and impact Dynamic blockage of the propagation path by human bodies is one of the critical factors affecting the performance of THz communications systems. This property was first observed in the mmWave domain, where it was demonstrated to result in 15-25 dB attenuation [1] depending on the propagation conditions and environmental characteristics. The eventual impact of this phenomenon is a drastic drop in the received signal strength resulting in either the downgrading of the utilized modulation and coding scheme (MCS) or outage conditions. Due to an even Shorter wavelength attenuation in the THz band is expected to be even more drastic. Several methods have been proposed for human body blockage avoidance. First, because of the inherent multi-path propagation at mmWave/THz frequencies, a non-line-of-sight (nLoS) path can be utilized for continuing service in the case of blockage. When no outage is experienced bandwidth reservation techniques may still support the active session as proposed in [2, 3]. However, as indicated in the 3GPP TR 38.901 standard, even at mmWave frequencies these paths generally generate less than 10% of the LoS path power. When blockage leads to an outage, multi-connectivity functionality standardized by 3GPP can be utilized [4, 5]. However, this approach requires dense deployment of mmWave/THz base stations (BS), which may not be available during the first deployment phases. In addition, even when both approaches are utilized together including the implementation of multiconnectivity between mmWave and THz radio access technologies (RAT), there is still a non-negligible probability that a session initially accepted for service will eventually be lost as a result of blockage. Finally, an alternative method is to embed blockage avoidance techniques in the physical layer by utilizing beam focusing [6] and/or non-diffractive beams [7]. However, these studies are still in their infancy. Blockage detection techniques are required to enforce blockage avoidance. These techniques can be classified as proactive and reactive techniques [8]. However, for both one needs to understand not only the duration of the blockage periods but also the duration of the signal fall and rise times. The signal fall time is of critical importance as it is not allowed to benchmark the development test but can serve as an indicator of the type of impairment allowing to distinguish blockage from other events including fast fading and/or micromobility discussed in Section 3. 2.2. Related work In this section, we outline related studies that characterize the impact of dynamic human body blockages in the THz band. We start by briefly reviewing the results of the LoS path propagation and blockage. First, we highlight a notable deficiency in the current body of research on propagation phenomena beyond the 100 GHz. Studies on the attenuation and time dynamics of the blockage process remain scarce. For instance, [9] provides an overview of propagation loss measurements at 140 GHz for various building materials: however, the characteristics of the blockage process have not been reported. The authors of [10, 11] reported attenuation values for 300 GHz for vehicular blockage in different configurations, but these results cannot be extrapolated to human body blockage owing to the different materials and geometries. Numerous authors have investigated the transparency characteristics of materials, as evidenced in [9, 12, 13]. For example, [14] explored the attenuation induced by various materials in the sub-THz range, whereas [12] reported measurements at four different frequencies, with two considered to belong to the sub-THz range. The study in [15] presented an analytical model based on the average blockage probability estimated empirically as follows a function of the relative positions of the receiver (Rx) and transmitter (Tx), average Tx/Rx heights, antenna parameters, and room size. In general, despite the existing body of data regarding the penetration of sub-THz signals through diverse materials and measurement campaigns related to vehicular communications, a comprehensive characterization of the human body attenuation process within the sub-THz frequency spectrum is lacking. This is especially concerning for the signal fall and rise times. 2.3. Recent results The first step towards this direction was taken in [8], where the authors investigated the signal blockage by the human body at a frequency of 156 GHz in the indoor environment, as shown in Fig. 2a. An example of the signal behavior under blockage is shown in Fig. 1, where it is marked by a ”direct trace”. The authors focused on blockage characteristics, including average signal attenuation and timing metrics such as signal fall and rise times, and blockage duration. For transmission over a distance of 3-7 m, the average attenuation was found to range from 8 to 15 dB depending on the LoS height and the distance between the Tx and Rx. The blockage duration varied from 5 to 10% for different Tx-Rx distances (with corresponding nominal values of 360-390 ms), while the rise and fall times of the signal gradually increased from 60 to 100 ms as the Tx-Rx distance increased and remained unchanged for different LoS altitudes. Figure 1. Signal behavior under blockage for straight and reflected propagation Figure 2. Illustrating considerations in LoS and reflective propagation In [16], the authors investigated the effect of dynamic blockage by the human body on the signal strength in a highly directive channel in the sub-THz frequency range under typical indoor conditions with reflection from different materials, as shown in Fig. 2b. An illustration of the blocked signal under the reflected propagation of different materials is shown in Fig. 1. Through an extensive measurement campaign at the 156 GHz carrier frequency, reflection and blocking losses along various nLoS paths were reported, as well as the corresponding blockage duration, fall, and rise times. Empirical studies were validated using material-dependent 156 GHz radio propagation models for 24 different measurement configurations. Upon visual examination of Fig. 1, it is clear that signal levels received from paths involving reflection in unobstructed conditions are notably lower than those of direct line-of-sight paths. This discrepancy is primarily attributed to the losses incurred during reflection. Additionally, distinct variations were observed in both obstruction patterns and signal strengths both preceding and following the occurrence of an obstruction event. It is also visible that the plots representing the incidence of radio signals are closely clustered together, almost overlapping, particularly in the case of concrete and drywall. Any slight disparities observed, especially in the case of glass, can likely be Table 1 Mean signal attenuation, fall, and rise times LoS propagation Tx-RX distance,
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About the authors

Alexander S. Shurakov

Moscow Pedagogical State University; HSE University

Author for correspondence.
Email: alexander@rplab.ru
ORCID iD: 0000-0002-4671-7731
Scopus Author ID: 55266061300
ResearcherId: E-4118-2014

PhD, Associate Professor, Department of General and Experimental Physics, Moscow Pedagogical State University

1 M. Pirogovskaya St, bldg. 1, Moscow, 119991, Russian Federation; 20 Myasnitskaya St, Moscow, 101000, Russian Federation

Evgeny V. Mokrov

RUDN University

Email: mokrov-ev@rudn.ru
ORCID iD: 0000-0003-3290-4541
Scopus Author ID: 56512031300
ResearcherId: AAK-6348-2021

Candidate of Physical and Mathematical Sciences, Senior lecturer of Department of Probability Theory and Cyber Security of Peoples’ Friendship University of Russia

6 Miklukho-Maklaya St, Moscow, 117198, Russian Federation

Anatoliy N. Prikhodko

Moscow Pedagogical State University; HSE University; Russian Quantum Center

Email: anprihodko@hse.ru
ORCID iD: 0000-0002-4859-8975
Scopus Author ID: 57207500541
ResearcherId: ADC-0507-2022

Junior Researcher, Specialized Department of Quantum Optics and Telecommunications of Scontel CJSC, HSE University

1 M. Pirogovskaya St, bldg. 1, Moscow, 119991, Russian Federation; 20 Myasnitskaya St, Moscow, 101000, Russian Federation; Skolkovo, 143025, Russian Federation

Margarita I. Ershova

Moscow Pedagogical State University

Email: mi.ershova@mpgu.su
ORCID iD: 0009-0009-6785-4389
Scopus Author ID: 58298409900
ResearcherId: JNB-5214-2023

Junior Researcher, Laboratory of quantum detectors, Moscow Pedagogical State University

1 M. Pirogovskaya St, bldg. 1, Moscow, 119991, Russian Federation

Vyacheslav O. Begishev

RUDN University

Email: begishev-vo@rudn.ru
ORCID iD: 0000-0002-7232-4157
Scopus Author ID: 56562837400
ResearcherId: AAF-6491-2019

Candidate of Physical and Mathematical Sciences, Assistant professor of the Department of Probability Theory and Cyber Security of Peoples’ Friendship University of Russia

6 Miklukho-Maklaya St, Moscow, 117198, Russian Federation

Abdukodir A. Khakimov

RUDN University

Email: khakimov-aa@rudn.ru
ORCID iD: 0000-0003-2362-3270
Scopus Author ID: 57194233776
ResearcherId: AAD-1134-2019

Candidate of Technical Sciences, Junior researcher of the Department of Probability Theory and Cyber Security of Peoples’ Friendship University of Russia

6 Miklukho-Maklaya St, Moscow, 117198, Russian Federation

Yevgeny A. Koucheryavy

RUDN University

Email: kucheryavyy-ea@rudn.ru
ORCID iD: 0000-0003-3976-297X
Scopus Author ID: 6507253900
ResearcherId: D-7976-2018

Doctor of Technical Sciences, Professor of the Department of Probability Theory and Cyber Security of Peoples’ Friendship University of Russia

6 Miklukho-Maklaya St, Moscow, 117198, Russian Federation

Gregory N. Gol’tsman

Moscow Pedagogical State University; HSE University; Russian Quantum Center

Email: goltsman@rplab.ru
ORCID iD: 0000-0002-1960-9161
Scopus Author ID: 7006771637
ResearcherId: A-4189-2014

Doctor of Physical and Mathematical Sciences, Head of Department, Department of General and Experimental Physics, Moscow Pedagogical State University

1 M. Pirogovskaya St, bldg. 1, Moscow, 119991, Russian Federation; 20 Myasnitskaya St, Moscow, 101000, Russian Federation; Skolkovo, 143025, Russian Federation

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Copyright (c) 2024 Shurakov A.S., Mokrov E.V., Prikhodko A.N., Ershova M.I., Begishev V.O., Khakimov A.A., Koucheryavy Y.A., Gol’tsman G.N.

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