A Survey on Terahertz Communications

With the exponential growth of the data traffic in wireless communication systems, terahertz(THz) frequency band is envisioned as a promising candidate to support ultra-broadband for future beyond fifth generation(5G), bridging the gap between millimeter wave(mmWave) and optical frequency ranges. The purpose of this paper is to provide a comprehensive literature review on the development towards THz communications and presents some key technologies faced in THz wireless communication systems. Firstly, despite the substantial hardware problems that have to be developed in terms of the THz solid state superheterodyne receiver, high speed THz modulators and THz antennas, the practical THz channel model and the efficient THz beamforming are also described to compensate for the severe path attenuation. Moreover, two different kinds of lab-level THz communication systems are introduced minutely, named a solid state THz communication system and a spatial direct modulation THz communication system, respectively. The solid state THz system converts intermediate frequency(IF) modulated signal to THz frequency while the direct modulation THz system allows the high power THz sources to input for approving the relatively long distance communications. Finally, we discuss several potential application scenarios as well as some vital technical challenges that will be encountered in the future THz communications.

Towards Intelligent Reflecting Surface Empowered 6G Terahertz Communications: A Survey

Terahertz(THz)communications have been widely envisioned as a promising enabler to provide adequate bandwidth and achieve ultra-high data rates for sixth generation(6G)wireless networks.In order to mitigate blockage vulnerability caused by serious propagation attenuation and poor diffraction of THz waves,an intelligent reflecting surface(IRS),which manipulates the propagation of incident electromagnetic waves in a programmable manner by adjusting the phase shifts of passive reflecting elements,is proposed to create smart radio environments,improve spectrum efficiency and enhance coverage capability.Firstly,some prospective application scenarios driven by the IRS empowered THz communications are introduced,including wireless mobile communications,secure communications,unmanned aerial vehicle(UAV)scenario,mobile edge computing(MEC)scenario and THz localization scenario.Then,we discuss the enabling technologies employed by the IRS empowered THz system,involving hardware design,channel estimation,capacity optimization,beam control,resource allocation and robustness design.Moreover,the arising challenges and open problems encountered in the future IRS empowered THz communications are also highlighted.Concretely,these emerging problems possibly originate from channel modeling,new material exploration,experimental IRS testbeds and intensive deployment.Ultimately,the combination of THz communications and IRS is capable of accelerating the development of 6G wireless networks.

MIMO-Terahertz in 6G Nano-Communications:Channel Modeling and Analysis

With the development of wireless mobile communication technology,the demand for wireless communication rate and frequency increases year by year.Existing wireless mobile communication frequency tends to be saturated,which demands for new solutions.Terahertz(THz)communication has great potential for the future mobile communications(Beyond 5G),and is also an important technique for the high data rate transmission in spatial information network.THz communication has great application prospects in military-civilian integration and coordinated development.In China,important breakthroughs have been achieved for the key techniques of THz high data rate communications,which is practically keeping up with the most advanced technological level in the world.Therefore,further intensifying efforts on the development of THz communication have the strategic importance for China in leading the development of future wireless communication techniques and the standardization process of Beyond 5G.This paper analyzes the performance of the MIMO channel in the Terahertz(THz)band and a discrete mathematical method is used to propose a novel channel model.Then,a channel capacity model is proposed by the combination of path loss and molecular absorption in the THz band based on the CSI at the receiver.Simulation results show that the integration of MIMO in the THz band gives better data rate and channel capacity as compared with a single channel.

Terahertz Band: Lighting up Next-Generation Wireless Communications

With the explosion of wireless data rates,the terahertz(THz)band(0.1–10 THz)is envisioned as a promising candidate to break the existing bandwidth bottleneck and satisfy the ever-increasing capacity demand.The THz wireless communications feature a number of attractive properties,such as potential terabit-per-second capacity and high energy efficiency.In this paper,an overview on the state-of-the-art THz communications is studied,with a special focus on key technologies of THz transceivers and THz communication systems.The recent progress on both electronic and photonic THz transmitters are presented,and then the THz receivers operating in direct-and heterodyne reception modes are individually surveyed.Based on the THz transceiver schemes,three kinds of THz wireless communication systems are reviewed,including solid-state electronic systems,photonics-assisted systems and all-photonics systems.The prospective key enabling technologies,corresponding challenges and research directions for lighting up high-speed THz communication systems are discussed as well.

Toward 6G Communication Networks:Terahertz Frequency Challenges and Open Research Issues

Future networks communication scenarios by the 2030s will include notable applications are three-dimensional(3D)calls,haptics communications,unmanned mobility,tele-operated driving,bio-internet of things,and the Nanointernet of things.Unlike the current scenario in which megahertz bandwidth are sufficient to drive the audio and video components of user applications,the future networks of the 2030s will require bandwidths in several gigahertzes(GHz)(from tens of gigahertz to 1 terahertz[THz])to perform optimally.Based on the current radio frequency allocation chart,it is not possible to obtain such a wide contiguous radio spectrum below 90 GHz(0.09 THz).Interestingly,these contiguous blocks of radio spectrum are readily available in the higher electromagnetic spectrum,specifically in the Terahertz(THz)frequency band.The major contribution of this study is discussing the substantial issues and key features of THz waves,which include(i)key features and significance of THz frequency;(ii)recent regulatory;(iii)the most promising applications;and(iv)possible open research issues.These research topics were deeply investigated with the aim of providing a specific,synopsis,and encompassing conclusion.Thus,this article will be as a catalyst towards exploring new frontiers for future networks of the 2030s.

Hybrid Precoding for Cluster-Based Multi-Carrier Beam Division Multiple Access in Terahertz Wireless Communications

Terahertz(THz)wireless communication has the capability to connect massive devices using its ultra-large spectrum resource.We propose a hybrid precoding scheme for the cluster-based multi-carrier beam division multiple access(MC-BDMA)to enable THz massive connections.Both the inter-beam interference and inter-band power leakage in this system are considered.A mathematical model is established to analyze and reduce their effects on the THz signal transmission.By considering the peculiarities of THz channels and characteristics of THz hardware components,we further propose a three-step hybrid precoding algorithm with low complexity,where the received signal power enhancement,the inter-beam interference elimination,and the inter-band power leakage suppression are conducted in succession.Simulation results are presented to demonstrate the high spectrum efficiency and high energy efficiency of our proposed algorithm,especially in the massive-connection scenarios.

Autoencoder with Fitting Network for Terahertz Wireless Communications:A Deep Learning Approach

Terahertz wireless communication has been regarded as an emerging technology to satisfy the ever-increasing demand of ultra-high-speed wireless communications.However,affected by the imperfections of cheap and energy-efficient Terahertz devices,Terahertz signals suffer from serve hybrid distortions,including in-phase/quadrature imbalance,phase noise and nonlinearity,which degrade the demodulation performance significantly.To improve the robustness against these hybrid distortions,an improved autoencoder is proposed,which includes coding the transmitted symbols at the transmitter and decoding the corresponding signals at the receiver.Moreover,due to the lack of information of Terahertz channel during the training of the autoencoder,a fitting network is proposed to approximate the characteristics of Terahertz channel,which provides an approximation of the gradients of loss.Simulation results show that our proposed autoencoder with fitting network can recover the transmitted symbols under serious hybrid distortions,and improves the demodulation performance significantly.

Milestones of Wireless Communication Networks and Technology Prospect of Next Generation(6G)

Since around 1980,a new generation of wireless technology has arisen approximately every 10 years.First-generation(1G)and secondgeneration(2G)began with voice and eventually introduced more and more data in third-generation(3G)and became highly popular in the fourthgeneration(4G).To increase the data rate along with low latency and mass connectivity the fifth-generation(5G)networks are being installed from 2020.However,the 5G technology will not be able to fulfill the data demand at the end of this decade.Therefore,it is expected that 6G communication networks will rise,providing better services through the implementation of new enabling technologies and allowing users to connect everywhere.6G technology would not be confined to cellular communications networks,but would also comply with non-terrestrial communication system requirements,such as satellite communication.The ultimate objectives of this work are to address the major challenges of the evolution of cellular communication networks and to discourse the recent growth of the industry based on the key scopes of application and challenges.The main areas of research topics are summarized into(i)major 6G wireless networkmilestones;(ii)key performance indicators;(iii)future new applications;and(iv)potential fields of research,challenges,and open issues.

Design of a Novel THz Modulator for B5G Communication

Wireless data traffic has expanded at a rate that reminds us of Moore’s prediction for integrated circuits in recent years,necessitating ongoing attempts to supply wireless systems with ever-larger data rates in the near future,despite the under-deployment of 5G networks.Terahertz(THz)communication has been considered a viable response to communication blackout due to the rapid development of THz technology and sensors.THz communication has a high frequency,which allows for better penetration.It is a fast expanding and evolving industry,driven by an increase in wireless traffic volume and data transfer speeds.A THz modulator based on a hybrid metasurface was devised and built in this work.The device’s modulation capabilities were modelled and proved experimentally.The electrolyte is an ion-gel medium implanted between graphene and metasurface,and the active material is graphene.On the metasurface,the interaction between the THz wave and graphene is improved.Additionally,an external bias voltage was employed to actively regulate the THz waves by tuning the electrical conductivity of graphene.The results show that with a minimal bias voltage,the device can achieve a modulation depth of up to 73%at the resonant frequency.Furthermore,during the modulation process,the resonance frequency remains almost constant.As a result,the proposed gadget offers a unique tool for substantial THz amplitude modulation at low voltages.

A 3-D Hybrid Dynamic Channel Model for Indoor THz Communications

To meet the demands for the explosive growth of mobile data rates and scarcity of spectrum resources in the near future,the terahertz(THz)band has widely been regarded as a key enabler for the upcoming beyond fifth-generation(B5G)wireless communications.An accurate THz channel model is crucial for the design and deployment of the THz wireless communication systems.In this paper,a three-dimensional(3-D)dynamic indoor THz channel model is proposed by means of combining deterministic and stochastic modeling approaches.Clusters are randomly distributed in the indoor environment and each ray is characterized with consideration of molecular absorption and diffuse scattering.Moreover,we present the dynamic generation procedure of the channel impulse responses(CIRs).Statistical properties are investigated to indicate the non-stationarity and feasibility of the proposed model.Finally,by comparing with delay spread and K-factor results from the measurements,the utility of the proposed channel model is verified.

Reliable and Energy-Aware Job Offloading at Terahertz Frequencies for Mobile Edge Computing

In this paper,we co-design the transmission power and the offloading strategy for job offloading to a mobile edge computing(MEC)server at Terahertz(THz)frequencies.The goal is to minimize the communication energy consumption while providing ultra-reliable low end-to-end latency(URLLC)services.To that end,we first establish a novel reliability framework,where the end-to-end(E2E)delay equals a weighted sum of the local computing delay,the communication delay and the edge computing delay,and the reliability is defined as the probability that the E2E delay remains below a certain pre-defined threshold.This reliability gives a full view of the statistics of the E2E delay,thus constituting advancement over prior works that have considered only average delays.Based on this framework,we establish the communication energy consumption minimization problem under URLLC constraints.This optimization problem is non-convex.To handle that issue,we first consider the special single-user case,where we derive the optimal solution by analyzing the structure of the optimization problem.Further,based on the analytical result for the single-user case,we decouple the optimization problem for multi-user scenarios into several sub-optimization problems and propose a sub-optimal algorithm to solve it.Numerical results verify the performance of the proposed algorithm.

A 220 GHz Frequency-Division Multiplexing Wireless Link with High Data Rate

With the development of wireless communication,the 6G mobile communication technology has received wide attention.As one of the key technologies of 6G,terahertz(THz)communication technology has the characteristics of ultra-high bandwidth,high security and low environmental noise.In this paper,a THz duplexer with a half-wavelength coupling structure and a sub-harmonic mixer operating at 216 GHz and 204 GHz are designed and measured.Based on these key devices,a 220 GHz frequency-division multiplexing communication system is proposed,with a real-time data rate of 10.4 Gbit/s for one channel and a transmission distance of 15 m.The measured constellation diagram of two receivers is clearly visible,the signal-to-noise ratio(SNR)is higher than 22 dB,and the bit error ratio(BER)is less than 10^(−8).Furthermore,the high definition(HD)4K video can also be transmitted in real time without stutter.

A Semi-Blind Method to Estimate the I/Q Imbalanc of THz Orthogonal Modulator

In terahertz communication,the direct frequency conversion structure in which orthogonal mixer is the main frequency conversion unit,makes engineers get into trouble of in-phase(I)branch and quadrature(Q)branch imbalance,carrier wave leakage,etc.These damages result in system performance tremendous degrades.We proposed a semiblind method to estimate the I/Q imbalance of THz orthogonal modulator,based on predefined preamble and pilot symbols for quadrature amplitude modulation(QAM).In this paper,a transmitter with Y band quadrature mixer and 20Gbps base-band signal has been tested.The bandwidth of the baseband signal was 7GHz,and the modulation type was 16QAM.By this method,7dB improvement of the system’s symbol Mean Square Error(MSE)has been got.That means the proposed method can be used to eliminate the I/Q imbalance effectively.

Multi-User Semantic Communications System with Spectrum Scarcity

Nowadays,the emerging paradigm of semantic communications seems to offer an attractive opportunity to improve the transmission reliability and efficiency in new generation communication systems.In particular,focusing on spectrum scarcity,expected to afflict the upcoming sixth generation(6G)networks,this paper analyses the semantic communications behavior in the context of a cell-dense scenario,in which users belonging to different small base station areas may be allocated on a same channel giving rise to a non-negligible interference that severely affects the communications reliability.In such a context,artificial intelligence methodologies are of paramount importance in order to speed up the switch from traditional communication to the novel semantic communication paradigm.As a consequence,a deep-convolution neural networks based encoder-decoder architecture has been exploited here in the definition of the proposed semantic communications framework.Finally,extensive numerical simulations have been performed to test the advantages of the proposed framework in different interfering scenarios and in comparison with different traditional or semantic alternatives.

A high-precision terahertz retrodirective antenna array with navigation signal at a different frequency

Future communications will provide higher transmission rates and higher operating frequencies.In addition,agile beam tracking will be an inevitable trend in technology development.The terahertz retrodirective antenna array proposed and discussed in this paper can be a better solution for agile beam tracking.The array receives a 40-GHz navigation signal and accurately retransmits a 120-GHz beam in the direction of the arrival wave.Simulation results indicate that the proposed array with a stacked sandwich structure has realized the tracking of the received wave.The scanning radiation pattern shows that the array gain is 23.87 d B at 19.9°when the incident angle is 20°with a relative error of only 0.5%,meaning that there is a lateral error of only 8.7 m at a transmission distance of 5 km.

Single-lane 200 Gbit/s photonic wireless transmission of multicarrier 64-QAM signals at 300 GHz over 30 m

Recently,wireless communication capacity has been witnessing unprecedented growth.Benefits from the optoelectronic components with large bandwidth,photonics-assisted terahertz(THz)communication links have been extensively developed to accommodate the upcoming wireless transmission with a high data rate.However,limited by the available signalto-noise ratio and THz component bandwidth,single-lane transmission of beyond 100 Gbit/s data rate using a single pair of THz transceivers is still very challenging.In this study,a multicarrier THz photonic wireless communication link in the 300 GHz band is proposed and experimentally demonstrated.Enabled by subcarrier multiplexing,spectrally efficient modulation format,well-tailored digital signal processing routine,and broadband THz transceivers,a line rate of 72 Gbit/s over a wireless distance of 30 m is successfully demonstrated,resulting in a total net transmission capacity of up to 202.5 Gbit/s.The single-lane transmission of beyond 200 Gbit/s overall data rate with a single pair of transceivers at 300 GHz is considered a significant step toward a viable photonics-assisted solution for the next-generation information and communication technology (ICT) infrastructure.