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Beyond the commercial roll-out of fifth-generation (5G) wireless networks at frequencies spanning up to the millimeter-wave (mmWave) spectrum, the quest for increasing data rates persists. To meet this demand, higher frequencies over the sub-terahertz and terahertz (THz) bands (roughly from 100 GHz to 10 THz) will be central to ubiquitous wireless communications in the sixth-generation (6G) networks. In particular, THz frequencies promise to support ample spectrum, above hundred Giga-bit-per-second (Gbps) data rates, massive connectivity, denser networks, and highly secure transmissions. Moreover, this spectrum also opens the door to wireless sensing with unprecedented resolution and accuracy. However, there are several major challenges that impact the transceiver and antenna system design, propagation characteristics and channel modeling, physical layer solutions, medium access layer protocols, network spectrum access, and resource management techniques. In addition, THz communications will be complemented by enablers at both the infrastructure and algorithmic levels. At the infrastructure level, emerging beyond-5G technologies such as reconfigurable intelligent surfaces, ultra-massive MIMO configurations, and integrated access and backhaul, can boost the gains of THz communications. At the algorithmic level, novel signal processing techniques and networking protocols can get around the THz quasi-optical propagation characteristics and mitigate the microwave characteristics to enable seamless connectivity. These selected Best Readings provide a current look at the state of the art in the field of Terahertz Communications.

Issued: April 2022


Chong Han, Shanghai Jiao Tong University, China
Josep Miquel Jornet, Northeastern University, USA

Hina Tabassum, York University, Canada
Gianni Pasolini, University of Bologna, Italy

Editorial Staff

Matthew C. Valenti
Editor-in-Chief, ComSoc Best Readings
West Virginia University
Morgantown, WV, USA

Xianbin Wang
Associate Editor-in-Chief, ComSoc Best Readings
Western University
London, ON, Canada

Overview and Tutorials

I. F. Akyildiz, J. M. Jornet, and C. Han, “TeraNets: Ultra-broadband Communication Networks in the Terahertz Band,” IEEE Wireless Communications Magazine, vol. 21, no. 4, pp. 130-135, August 2014.
This paper provides one of the very first holistic looks at the field of terahertz communications. It encompasses both macro and nano scales applications, goes beyond the traditional electronic and photonic approaches to terahertz device technologies while highlighting the role of graphene plasmonics, and follows a bottom-up approach to describe new physical, link, network, and transport layer solutions.

C. Lin and G. Y. Li, “Terahertz Communications: An Array-of-Subarrays Solution,” IEEE Communications Magazine, vol. 54, no. 12, pp. 124-131, December 2016. 
This article advocates the use of array-of-subarray structure for indoor multiuser terahertz communication systems from both the circuit and communication perspectives. The article highlights the distinction of terahertz and mmWave communications considering channel characteristics and hardware limitations. Compared to a fully connected structure, an array-of-subarrays structure was shown to be spectral and energy efficient.

T. S. Rappaport, Y. Xing, O. Kanhere, S. Ju, A. Madanayake, S. Mandal, A. Alkhateeb, and G. C. Trichopoulos, “Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond,” IEEE Access, vol. 7, pp 78729-57, June 2019.
This paper highlights the recent regulatory and standard body rulings for beyond 100 GHz systems. To combat distance-based propagation losses, this paper discusses various approaches to enhance antenna gains and minimize the computational complexity of signal processing used in adaptive antenna arrays. New insights related to power-efficient beam steering algorithms are presented. 

S. Ghafoor, N. Boujnah, M. H. Rehmani, and A. Davy, “MAC Protocols for Terahertz Communication: A Comprehensive Survey,” IEEE Communications Surveys & Tutorials, vol. 22, no. 4, pp. 2236-2282, Fourth Quarter 2020.
This paper provides a comprehensive survey of existing medium access (MAC) protocols in the context of network topology, channel access mechanisms, and link establishment strategies. This survey advocates the use of novel MAC protocols including efficient channel access, control message exchange, link establishment, mobility management, and line-of-sight blockage mitigation protocols. Challenges and open research directions on Terahertz MAC protocols are also presented.

H. Do, S. Cho, J. Park, H.-J. Song, N. Lee, and A. Lozano, “Terahertz Line-of-Sight MIMO Communication: Theory and Practical Challenges,” IEEE Communications Magazine, vol. 59, no. 3, pp. 104-109, March 2021.
This article points out practical challenges in MIMO-terahertz transmissions, including misalignments, frequency variations, inter-symbol interference, low resolution analog-to-digital converters, and array apertures. Experimental evidence is provided to demonstrate the non-incremental benefits over mmwave-based 5G communications.

Z. Chen, C. Han, Y. Wu, L. Li, C. Huang, Z. Zhang, G. Wang, and W. Tong, “Terahertz Wireless Communications for 2030 and Beyond: A Cutting-Edge Frontier,” IEEE Communications Magazine, vol. 59, no. 11, pp. 66-72, November 2021.
This paper presents four interdisciplinary directions to unleash the full potential of THz communications, namely, integrated sensing and communication, ultra-massive MIMO and dynamic hybrid beamforming, intelligent surfaces, and machine/deep learning. Furthermore, open problems as well as integration of these solutions are elaborated.

H. Sarieddeen, M.-S. Alouini, and T. Y. Al-Naffouri, “An Overview of Signal Processing Techniques for Terahertz Communications,” Proceedings of the IEEE, vol. 109, no. 10, pp. 1628-1665, October 2021.
This paper provides a tutorial on THz-specific signal processing techniques for classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. The paper highlights the significance of ultra-massive multiple-input multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces in overcoming the distance problem at very high frequencies.

Special Issues

Special Issue on “THz Communication for Mobile Heterogeneous Networks,” IEEE Communications Magazine, vol. 56, no. 6, June 2018.

Special Issue on “THz Communications: A Catalyst for the Wireless Future,” IEEE Communications Magazine, vol. 58, no. 11, November 2020.

Special Issue on “THz Communications and Networking,” IEEE Journal on Selected Areas in Communications, vol. 39, no. 6, June 2021.

Special Issue on “Terahertz RF Electronics and System Integration,” Proceedings of the IEEE, vol. 105, no. 6, June 2017.

Topic: Standardization and Policy

V. Petrov, T. Kurner, and I. Hosako, “IEEE 802.15.3d: First Standardization Efforts for Sub-Terahertz Band Communications toward 6G,” IEEE Communications Magazine, vol. 58, no. 11, pp. 28-33, November 2020.
The article introduces the IEEE 802.15.3d standard for sub-Terahertz transmission along with its target applications and usage scenarios (e.g., integrated access and backhaul, kiosks download, intra-device communication), while pin-pointing the specifics of the IEEE 802.15.3d at physical and medium access layers.

M. J. Marcus, “6G Spectrum Policy Issues Above 100 GHz,” Editorial, IEEE Wireless Communications, vol. 28, no. 6, pp. 7-8, December 2021.
This editorial presents the key policy issues related to the use of the spectrum above 100 GHz, which are mostly related to coexistence and spectrum sharing between commercial (communications) and scientific (sensing) users.

IEEE Standard for High Data Rate Wireless Multi-Media Networks-Amendment 2: 100 Gb/s Wireless Switched Point-to-Point Physical Layer,” in IEEE Std 802.15.3d-2017 (Amendment to IEEE Std 802.15.3-2016 as amended by IEEE Std 802.15.3e-2017), pp.1-55, 18 Oct. 2017.
This standard defines physical (PHY) and medium access (MAC) layer specifications for high data rate wireless connectivity (typically over 200 Mbps) at the lower THz frequency range between 252 GHz and 325 GHz for switched point-to-point links.

Topic: Channel and Propagation Modeling

R. Piesiewicz, C. Jansen, D. Mittleman, T. Kleine-Ostmann, M. Koch, and T. Kurner, “Scattering Analysis for the Modeling of THz Communication Systems,” IEEE Transactions on Antennas and Propagation, vol. 55, no. 11, pp. 3002-3009, November 2007.
This paper studies and verifies the analytical model for the reflection coefficient to acquire the knowledge of the reflective properties of building materials. Furthermore, by means of ray-tracing simulations, the effect of wall and ceiling roughness on propagation in indoor scenarios is illustrated.

J. M. Jornet and I. F. Akyildiz, “Channel Modeling and Capacity Analysis for Electromagnetic Wireless Nanonetworks in the Terahertz Band,” IEEE Transactions on Wireless Communications, vol. 10, no. 10, pp. 3211-3221, October 2011.
This paper presents the first complete channel model and capacity analysis for the entire THz band (from 100 GHz to 10 THz). Highly parameterizable equations derived from radiative transfer theory and that leverage the HITRAN database are provided to analyze the losses, noise and, ultimately, capacity of the THz channel. Besides the models themselves, the results highlight the opportunity for pulse-based communications in short-range THz links.

S. Priebe and T. Kurner, “Stochastic Modeling of THz Indoor Radio Channels,” IEEE Transactions on Wireless Communications, vol. 12, no. 9, pp. 4445-4455, September 2013.
Allowing for the fast generation of channel realizations, this paper introduces a universal stochastic spatial-temporal model for ultra-broadband THz indoor radio channels from 275 to 325 GHz. This model comprises complete channel characteristics such as the fully polarimetric, complex ray amplitudes, times of arrival as well as angles of arrival and departures in both azimuth and elevation.

C. Han, A. O. Bicen, and I. F. Akyildiz, “Multi-Ray Channel Modeling and Wideband Characterization for Wireless Communications in the Terahertz Band,” IEEE Transactions on Wireless Communications, vol. 14, no. 5, pp. 2402-2412, May 2015.
This paper develops a unified multipath channel model in the Terahertz band based on ray tracing techniques. Moreover, the distance-varying and frequency-selective nature, the coherence bandwidth, the delay spread, the wideband channel capacity, and 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.

N. A. Abbasi, J. Gomez-Ponce, S. Shaikbepari, S. Rao, R. Kondaveti, S. Abu-Surra, G. Xu, C. Zhang, and A. Molisch, “Ultra-Wideband Double Directional Channel Measurements for THz Communications in Urban Environments,” in Proc., IEEE International Conference on Communications (ICC), June 2021.
This paper presents results from one of the first measurement campaigns for medium-distance (up to 35 m) outdoor channels in urban environments, where both the directions and delays of multipath are measured with good resolution.

Y.-H. Chen, L. Yan, and C. Han, “Hybrid Spherical- and Planar-Wave Modeling and DCNN-powered Estimation of Terahertz Ultra-massive MIMO Channels,” IEEE Transactions on Communications, vol. 69, no. 10, pp. 7063-7076, October 2021.
To accurately and efficiently capture the propagation in both near-field and far field regions, this paper investigates a hybrid spherical- and planar-wave channel model (HSPM), by adopting planar-wave model within subarray and spherical-wave channel model among subarrays. Furthermore, a two-phase HSPM channel estimation mechanism based on a deep convolutional-neural-network (DCNN) is developed.

Y. Chen, Y. Li, C. Han, Z. Yu, and G. Wang, “Channel Measurement and Ray-Tracing-Statistical Hybrid Modeling for Low-Terahertz Indoor Communications,” IEEE Transactions on Wireless Communications, vol. 20, no. 12, pp. 8163-8176, December 2021.
This paper investigates a wideband channel measurement campaign between 130 GHz and 143 GHz in a typical meeting room, based on which a realistic power delay profile and a combined multipath clustering and matching procedure is proposed. Furthermore, the line-of-sight path loss, power distributions, temporal and spatial features, and correlations among THz multipath characteristics are comprehensively analyzed.

Topic: Devices for THz Communications

J. M. Jornet and I. F. Akyildiz, “Graphene-based Plasmonic Nano-antenna for Terahertz Band Communication in Nanonetworks,” IEEE Journal on Selected Areas in Communications, vol. 31, no. 12, pp.685-694, December 2013.
This paper presents for the first time the concept and the corresponding analytical model for innovative terahertz antennas built with graphene nanoribbons. These antennas are up to two orders of magnitude smaller than their metallic counterparts and can efficiently operate in the THz band by leveraging the properties of surface plasmon polariton waves.

A. Tredicucci and M. S. Vitiello, “Device Concepts for Graphene-Based Terahertz Photonics,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 20, no. 1, pp. 130-138, January-February 2014.
This paper provides a survey on the latest achievements in graphene-based THz photonics and discusses the future perspectives of this research field. In particular, graphene-based modulators and plasmonic devices, detectors, lasers and other signal sources are here thoroughly discussed.

J. V. Siles, K. B. Cooper, C. Lee, R. H. Lin, G. Chattopadhyay, and I. Mehdi, “A New Generation of Room-Temperature Frequency-Multiplied Sources With up to 10× Higher Output Power in the 160-GHz–1.6-THz Range,” IEEE Transactions on Terahertz Science and Technology, vol. 8, no. 6, pp. 596-604, October 2018.
This paper addresses the design, fabrication, and testing of a new generation of all-solid-state terahertz sources with frequency multiplication that provide up to ten times more output power at room temperature than the previous state-of-the-art. The new sources are based on a novel JPL-patented circuit topology called “on-chip power combining”. The measured performance of prototypes operating at 180 GHz, 240 GHz, 340 GHz, 530 GHz, 1 THz, and 1.6 THz are presented as well.

A. Singh, M. Andrello, N. Thawdar, and J. M. Jornet, “Design and Operation of a Graphene-Based Plasmonic Nano-Antenna Array for Communication in the Terahertz Band,” IEEE Journal on Selected Areas in Communications, vol. 38, no. 9, pp. 2104-2117, September 2020.
This paper presents an innovative array architecture for THz communications that leverages the unique properties of graphene-based plasmonic devices. Basically, each array element consists of an on-chip plasmonic source, an on-chip plasmonic modulator and an on-chip antenna, allowing unprecedented phase and amplitude control per element.

H.-J. Song and N. Lee, “Terahertz Communications: Challenges in the Next Decade,” IEEE Transactions on Terahertz Science and Technology, vol. 12, no. 2, March 2022.
Mainly from device and system perspectives, this paper briefly reviews the progress of THz communications made in the last decade and discusses some of the most challenging issues that need to be overcome for future wireless systems at THz frequencies, including not only the 6G network but also short-range connectivity and fixed wireless links.

Topic: PHY Layer Design and Resource Allocation Strategies

R. Piesiewicz, T. Kleine-Ostmann, N. Krumbholz, D. Mittleman, M. Koch, J. Schoebel, and T. Kurner, “Short-Range Ultra-Broadband Terahertz Communications: Concepts and Perspectives,” IEEE Antennas and Propagation Magazine, vol. 49, no. 4, pp 24-39, December 2007.
This paper was published in 2007. Although it is a bit dated, it provides a system-level analysis of a THz communication link, discussing the antenna gain, the front-end radio technology, the propagation phenomena and the frequency window choice. It also proposes and studies the adoption of dielectric mirrors to be adopted in scenarios where the line-of-sight link is not present.

J. M. Jornet and I. F. Akyildiz, “Femtosecond-long Pulse-based Modulation for Terahertz Band Communication in Nanonetworks,” IEEE Transactions on Communications, vol. 62, no. 5, pp. 1742-1754, May 2014.
This paper presents for the first time a simple yet-efficient communication scheme for short-range THz communications based on the transmission of femtosecond-long pulses following an on-off keying modulation spread in time. The results show that, due to the nature of noise and multi-user interference at THz frequencies, the maximum data-rate is achieved when more zeros than ones are transmitted, motivating the use of low-weight coding schemes.

C. Lin and G. Y. Li, “Indoor Terahertz Communications: How Many Antenna Arrays Are Needed?,” IEEE Transactions on Wireless Communications, vol. 14, no. 6, pp. 3097-3107, June 2015.
This paper investigates a low-complexity indoor THz communication system with antenna subarrays. By exploiting hybrid beamforming with multiple subarrays, the ergodic capacity of the system is analyzed and an upper bound is obtained. Furthermore, a guidance on the design of the antenna subarray size is provided, taking into account the performance degradation due to the uncertainty in THz phase shifters.

L. Yan, C. Han, and J. Yuan, “A Dynamic Array-of-Subarrays Architecture and Hybrid Precoding Algorithms for Terahertz Wireless Communications,” IEEE Journal on Selected Areas in Communications (JSAC), vol. 38, no. 9, pp. 2041-2056, September 2020.
This paper proposes a dynamic array-of-subarrays (DAoSA) hybrid precoding architecture to reduce the power consumption while meeting the data rate requirement in Terahertz ultra-massive MIMO systems. The connections between RF chains and subarrays are intelligently adjusted through a network of switches.

C. Han and I. F. Akyildiz, “Distance-Aware Bandwidth-Adaptive Resource Allocation in the Terahertz Band,” IEEE Transactions on Terahertz Science and Technology, vol. 6, no. 4, pp. 541- 553, July 2016.
This paper develops a distance-aware bandwidth-adaptive resource allocation scheme for THz band communication networks. The proposed scheme captures the unique channel peculiarities including the relationship between the distance and the bandwidth, and strategically utilizes the spectrum to enable multiple ultrahigh-speed links. Based on the developed scheme, the sub-windows of the THz spectrum, the modulations, and the transmit power are adaptively allocated, for both single-user and multiuser communications.

C. Lin, G. Y. Li, and L. Wang, “Subarray-Based Coordinated Beamforming Training for mmWave and Sub-THz Communications,” IEEE Journal on Selected Areas in Communications, vol. 35, no. 9, pp. 2115-2126, September 2017.
This paper studies mmWave and sub-THz systems with array-of-subarray architecture. To accommodate the ultrabroad bandwidth in the mmWave and sub-THz bands, time-delay phase shifters are introduced in the system design. Moreover, beamforming training with hybrid processing is investigated to extract the dominant channel information, which would fully exploit channel characteristics while respecting the nature of circuit hardware.

A. Shafie, N. Yang, S. A. Alvi, C. Han, S. Durrani, and J. M. Jornet, “Spectrum Allocation with Adaptive Sub-band Bandwidth for Terahertz Communication Systems,” IEEE Transactions on Communications, vol. 70, no. 2, February 2022.
This paper investigates the impacts of adaptive sub-band bandwidth and sub-band assignment in multi-band-based spectrum allocation on multi-connectivity-enabled multiuser THz communication systems. The optimization problem with the primary focus on spectrum allocation is formulated and solved by reasonable approximations and transformations and iterative algorithms based on the successive convex approximation technique.

Topic: Network Architecture and Protocols

Q. Xia, Z. Hossain, M. Medley, and J. M. Jornet, “A Link-Layer Synchronization and Medium Access Control Protocol for Terahertz-Band Communication Networks,” IEEE Transactions on Mobile Computing, vol. 20, no. 1, pp.2-18, September 2019.
This paper presents an innovative medium access control (MAC) protocol designed for ultra-directional THz communication networks. In contrast to traditional MAC protocols in which the transmitter initiates the data-transfer through a handshake (e.g., CSMA/CA) or directly with the data (e.g., ALOHA), it is the receiver who sweeps the space through beam steering and periodically interrogates the nodes for data. Extensive numerical results with the first ns-3 extension for THz networks are provided to show the performance of the proposed technique.

J. Sayehvand and H. Tabassum, “Interference and Coverage Analysis in Coexisting RF and Dense TeraHertz Wireless Networks,” IEEE Wireless Communications Letters, vol. 9, no. 10, pp. 1738-1742, October 2020.
This paper highlights the benefit of hybrid sub-6GHz and THz communications in a large-scale network. In particular, the statistics of aggregate interference, association probability of users to THz network, and the coverage probability are derived. Numerical results demonstrate the impact of molecular absorption on the association of users to the THz network.

C. Bosso, P. Sen, X. Cantos-Roman, C. Parisi, N. Thawder, and J. M. Jornet, “Ultrabroadband Spread Spectrum Techniques for Secure Dynamic Spectrum Sharing Above 100 GHz Between Active and Passive Users,” in Proc., IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), December 2021.
This paper proposes the utilization of spread spectrum techniques to facilitate the coexistence of active (e.g., communications and radar) and passive (e.g., atmospheric sensing) users of the spectrum above 100 GHz. After designing such a system, analytically and experimentally demonstrating its feasibility, a study case is conducted to understand the requirements for on-the-ground THz users to avoid interference in orbiting sensing satellites. This study informs current ITU efforts aimed at facilitating coexistence between active and passive users.

M. T. Hossan and H. Tabassum, “Mobility-Aware Performance in Hybrid RF and Terahertz Wireless Networks,” IEEE Transactions on Communications, vol. 70, no. 2, pp. 1376-1390, February 2022.
This paper highlights the benefit of hybrid sub-6GHz and THz communications in a large-scale network where users are moving with a predefined velocity. In particular, the statistics of the horizontal and vertical handoff probability, aggregate interference along with molecular absorption noise, association probability of users to THz network, and the mobility-aware coverage probability are characterized. Numerical results demonstrate that high molecular absorption can be beneficial (in terms of minimizing interference) for specific deployment intensity of terahertz base-stations and the benefits can outweigh the drawbacks of signal degradation it introduces.

Q. Xia and J. M. Jornet, “Multi-Hop Relaying Distribution Strategies for Terahertz-Band Communication Networks: A Cross-Layer Analysis,” IEEE Transactions on Wireless Communications, early access, 2022.
This paper derives for the first time optimal relaying strategies for multi-hop THz links. The cross-layer analysis captures all the key peculiarities of THz communications, including the trade-off between antenna gain and beamwidth in directional THz systems, the distance-dependent bandwidth of the THz channel, the delay associated to neighbor discovery and MAC at the link layer, and the buffer overflow risk when transmitting at tremendously high data-rates.

Topic: Emerging Applications

J. Kokkoniemi, J. M. Jornet, V. Petrov, Y. Koucheryavy, and M. Juntti, “Channel Modeling and Performance Analysis of Airplane-Satellite Terahertz Band Communications,” IEEE Transactions on Vehicular Technology, vol. 70, no. 3, pp. 2047-2061, March 2021.
This paper explores yet a different scenario for THz networks: airplane connectivity through plane to satellite communication. A channel model that takes into account the non-uniform molecular absorption through the atmosphere due to changes in pressure, temperature and molecular composition of the atmosphere at different altitudes as well as the curvature of the Earth is derived. Extensive numerical results are provided to study the feasibility and trade-offs of this technology.

Y. Xing and T. S. Rappaport, “Terahertz Wireless Communications: Co-sharing for Terrestrial and Satellite Systems above 100 GHz,” IEEE Communications Letters, vol. 25, no. 10, pp. 3156-3160, October 2021.
This letter presents recent global spectrum regulations as well as fundamental atmospheric and rain attenuation considerations at frequencies above 100 GHz. Moreover, rooftop surrogate satellite measurements at 140 GHz are presented, showing the isolation between the terrestrial networks and surrogate satellite systems as well as the isolation between terrestrial mobile users and co-channel fixed backhaul links.

S. Nie and I. F. Akyildiz, “Channel Modeling and Analysis of Inter-Small-Satellite Links in Terahertz Band Space Networks,” IEEE Transactions on Communications, vol. 69, no. 12, pp. 8585-8599, December 2021.
This paper characterizes the inter-satellite channels at THz band and analyzes their link capacity in both Earth’s upper atmosphere and deep space. Specifically, an orbital perturbation model for satellite formation flying is proposed and analyzed with actual ephemeris information. Moreover, a channel model is developed to investigate the diversity in both polarization and frequency domains for THz band inter-small-satellite communications.

H. Elayan, P. Johari, R. M. Shubair, and J. M. Jornet, “Photothermal Modeling and Analysis of Intra-body Terahertz Nanoscale Communication,” IEEE Transactions on NanoBioscience, vol. 16, no. 8, pp. 755 – 763, December 2017.
In this paper, the feasibility of intra-body THz communications is studied. More specifically, besides the propagation properties of THz signals in biological tissues, a photothermal model is developed to understand how THz radiation is absorbed and converted into heat. The extensive numerical results in this work can guide future derivations of safety thresholds for THz irradiation.

H. Sarieddeen, N. Saeed, T. Y. Al-Naffouri, and M.-S. Alouini, “Next Generation Terahertz Communications: A Rendezvous of Sensing, Imaging, and Localization,” IEEE Communications Magazine, vol. 58, no. 5, pp. 69-75, May 2020.
This paper details the peculiarities of sensing, imaging, and localization applications in the THz band. Then, how their coalescence results in enhanced environment-aware system performance in beyond-5G use cases is illustrated. This paper further discusses the implementation aspects of this merging of applications in the context of shared and dedicated resource allocation, highlighting the role of machine learning.

S. Fan, Y. Wu, C. Han, and X. Wang, “SIABR: A Structured Intra-Attention Bidirectional Recurrent Deep Learning Method for Ultra-Accurate Terahertz Indoor Localization,” IEEE Journal on Selected Areas in Communications (JSAC), vol. 39, no. 7, pp. 2226-2240, July 2021.
This paper proposes a Structured Intra-Attention Bidirectional Recurrent (SIABR) deep learning method to solve the CSI-based three-dimensional (3D) THz indoor localization problem. As a two-level structure, the features of individual multi-path rays are first analyzed in the recurrent neural network with the attention mechanism at the lower level.

A. Singh, M. Andrello, E. Einarsson, N. Thawdar, and J. M. Jornet, “A Hybrid Intelligent Reflecting Surface with Graphene-based Control Elements for THz Communications,”  in Proc.,  IEEE International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), May 2020.
In this paper, an innovative hybrid reflect-array design to serve as an enabler of an intelligent reflecting surface is presented, modeled, and numerically analyzed. This design proposes the utilization of graphene as the tunable element (thus, overcoming the main challenge for IRS above 100 GHz, namely, the lack of a tunable unit per element) and a metal as the reflective element (with reflection efficiency much larger than graphene-only structures).

C. Liaskos, S. Nie, A. Tsioliaridou, A. Pitsillides, S. Ioannidis, and I. Akyildiz, “End-to-End Wireless Path Deployment With Intelligent Surfaces Using Interpretable Neural Networks,” IEEE Transactions on Communications, vol. 68, no. 11, pp. 6792-6806, November 2020.
This paper proposes a novel intelligent surface networking algorithm based on interpretable neural networks, where surface units are represented as neural network nodes, and connectivity as neural links. The neural network is required to yield ideal output, such as total signal delivery to the receiver, and back-propagation rules optimize the link weights accordingly.

W. Gao, Y. Chen, C. Han, and Z. Chen, “Distance-Adaptive Absorption Peak Modulation (DA-APM) for Terahertz Covert Communications,” IEEE Transactions on Wireless Communications, vol. 20, no. 3, pp. 2064-2077, March 2021.
This paper proposes a distance-adaptive absorption peak modulation (DA-APM) for THz covert communications to enhance covertness of the wireless link, which dynamically modulates signals under the molecular absorption peaks in the THz spectrum. Furthermore, an optimization framework is proposed to optimize the covertness of the system.

H. Guerboukha, R. Shrestha, Z. Fang, E. Knightly, and D. M. Mittleman,  “Jamming at Terahertz Frequencies: A Theoretical And Numerical Study,” in Proc., IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), September 2021.
This paper studies the feasibility and impact of jamming at THz frequencies, as opposed to the more broadly studied risk of eavesdropping. Different carrier frequencies and modulations are considered in the study.