Terahertz Channel Research Articles

Precoding and Detection for Broadband Single Carrier Terahertz Massive MIMO Systems Using LSQR Algorithm

The terahertz (THz) communication utilizes the frequency spectrum above 300 GHz and is widely considered as a promising solution to the future high-speed short-range wireless communication beyond millimeter wave communication. While providing tens of gigahertz bandwidth, it is subjected to high-propagation path loss, inter-symbol, and inter-user interferences. The massive multiple-input-multiple-output (MIMO) can be applied to address these problems by cooperation between many access point antennas. However, the THz channel characteristics, including high-propagation path loss, frequency selectivity, and a big number of samples per channel impulse response, require carefully tailored algorithm for massive MIMO signal processing. In this paper, we propose a single carrier minimum mean square error precoding and detection algorithm for frequency selective THz channels. The MIMO signal transmission is described with the block matrices. A gain control heuristic is introduced to reduce the complexity. The sparsity property of the channel is utilized to construct sparse channel matrices and the least square QR algorithm is applied to efficiently solve the problems. Besides the uniform antenna array, the hybrid array consisting of several subarrays is considered as well. The simulation results show that the massive MIMO array can provide a satisfactory performance in terms of bit error rate.

TeraSim: An ns-3 extension to simulate Terahertz-band communication networks

In the quest of higher wireless data-rates, Terahertz (THz)-band (0.1-10 THz) communication is envisioned as a key wireless technology of the next decade. In parallel to the development of THz transceivers and antennas, simulation tools are needed to expedite the development of communication and networking protocols tailored to this novel networking paradigm, at a fraction of the cost. The few simulation platforms developed to date for THz communication networks do not capture the peculiarities of the THz channel or the capabilities of THz devices. In this paper, TeraSim, i.e., an open source network simulation platform for THz communication networks is presented. TeraSim is built as an extension for ns-3, which is one of the most widely used teaching and education network simulation software. The simulator has been developed considering two major types of application scenarios, namely, nanoscale communication networks (average transmission range usually below one meter) and macroscale communication networks (distances larger than one meter). The simulator consists of a common channel module, separate physical and link layers for each scenario, and two assisting modules, namely, THz antenna module and energy harvesting module, originally designed for the macroscale and nanoscale scenario, respectively. The structure, relations and content of each module are presented in detail. Extensive simulation and test results are provided to validate the functionalities of the implemented modules. TeraSim is expected to enable the networking community to test THz networking protocols without having to delve into the channel and physical layers.

Study of atmospheric attenuation characteristics of terahertz wave based on line‐by‐line integration

SummaryTerahertz band communication promises new solution for satisfying the increasing demand for ultrahigh‐speed wireless communication. Channel models capturing the unique peculiarities of the terahertz (THz) band are required for communication systems designing. Extreme high molecular absorption is a distinctive phenomenon that has to be involved in terahertz communication models. Research in this field has mainly focused on the characteristics along the horizontal propagation path. In this paper, we developed a unified molecular absorption model along the slant propagation path of the THz wave, based on the line‐by‐line integration method developed by Van‐Vleck and Weisskopf and combining the molecular spectral line in the HITRAN database. Then, an in‐depth analysis on the THz channel characteristics is carried out by the developed propagation models. The attenuation characteristics of terahertz waves with frequencies in the range of 0.1 to 1 THz are analyzed by theoretical and mathematical modeling. The results show that the terahertz communication channel has a strong dependence on both the molecular composition of the medium and the transmission distance. The experimental results also indicate the strong absorption frequency points, weak absorption frequency points, and spectral windows.

Pulse-level beam-switching for terahertz networks

Communication in Terahertz (THz) band is envisioned as a promising technology to meet the ever-growing data rate demand, and to enable new applications in both nano-scale and macro-scale wireless paradigms. In this study, we propose the first system-level design that is suitable for THz communication in macro-scale range with 100+ Gbps data rate. The design is based on the proposed terahertz pulse-level beam-switching with energy control (TRPLE), and motivated by the rise in Graphene-based electronics, which include not only compact generator and detector for pulse communication, but also the capability of beam scanning aided with nano-antenna-arrays. The very high path loss seen in THz wireless channel requires the use of narrow beam to reach longer transmission ranges. On the other hand, impulse radio that emits femtosecond-long pulses allows the beam direction to steer at pulse-level, rather than at packet-level. For TRPLE, we mathematically analyze the data rate for an arbitrary wireless link under the THz channel characteristics and the energy modulation scheme. Then, a novel optimization model is formulated to solve the parameters of the inter-pulse separation and the inter-symbol separation, in order to maximize the data rate while meeting the interference requirement. With the optimization, the data rate of 167 Gbps is shown achievable for most users in 20-m range. A MAC protocol framework is then presented to harness the benefits of the pulse separation optimization.

Scenario modules, ray‐tracing simulations and analysis of millimetre wave and terahertz channels for smart rail mobility

Millimeter wave (mmWave) and Terahertz (THz) technologies are the enabler of providing high-data rate communications for `smart rail mobility'. In this paper, six scenario modules for mmWave and THz train-to-infrastructure channels are defined and constructed. All the main objects, such as tracks, stations, crossing bridges, tunnels, cuttings, barriers, pylons, buildings, vegetation, traffic signs, billboards, trains, etc., are modeled according to the typical geometries and materials in reality. The three-dimensional (3D) models of these six modules are publicly available and freely downloadable. Furthermore, extensive ray-tracing simulations in the 60 GHz band with 8 GHz bandwidth are made in all the six modules with two antenna setups. Path loss exponent, shadow factor, Ricean K -factor, root-mean-square (RMS) delay spread, and coherence bandwidth are extracted and analyzed. These channel characteristics show that the objects that might not be so impacting on lower frequency channels indeed influence mmWave channel properties, and therefore, they can even play a more important role in the channels at higher frequency bands - THz. The channel characteristics analyzed in this study provide the foundation for future work that aims to streamline the design, simulation, and development of mmWave and THz communications enabling smart rail mobility.

Performance Assessment of Balloon-Borne Trace Gas Sounding with the Terahertz Channel of TELIS

Short-term variations in the atmospheric environment over polar regions are attracting increasing attention with respect to the reliable analysis of ozone loss. Balloon-borne remote sensing instruments with good vertical resolution and flexible sampling density can act as a prototype to overcome the potential technical challenges in the design of new spaceborne atmospheric sensors and represent a valuable tool for validating spaceborne observations. A multi-channel cryogenic heterodyne spectrometer known as the TErahertz and submillimeter LImb Sounder (TELIS) has been developed. It allows limb sounding of the upper troposphere and stratosphere (10–40 km) within the far infrared (FIR) and submillimeter spectral regimes. This paper describes and assesses the performance of the profile retrieval scheme for TELIS with a focus on the ozone (O3), hydrogen chloride (HCl), carbon monoxide (CO), and hydroxyl radical (OH) measured during three northern polar campaigns in 2009, 2010, and 2011, respectively. The corresponding inversion diagnostics reveal that some forward/instrument model parameters play important roles in the total retrieval error. The accuracy of the radiometric calibration and the spectroscopic knowledge has a significant impact on retrieval at higher altitudes, whereas the pointing accuracy dominates the total error at lower altitudes. The TELIS retrievals achieve a vertical resolution of ∼2–3 km through most of the stratosphere below the balloon height. Dominant water vapor (H2O) contamination and low abundances of the target species reduce the retrieval sensitivity at the lowermost altitudes measured by TELIS. An extensive comparison shows that the TELIS profiles are consistent with profiles obtained by other limb sounders. The comparison appears to be very promising, except for discrepancies in the upper troposphere due to numerical regularization. This study not only consolidates the validity of balloon-borne TELIS FIR measurements, but also demonstrates the scientific relevance and technical feasibility of terahertz limb sounding of the stratosphere.

Link Budget Analysis for Massive-Antenna-Array-Enabled Terahertz Satellite Communications

Broadband satellite communications can enable a plethora of applications in customer services, global nomadic coverage and disaster prediction and recovery. Terahertz (THz) band is envisioned as a key satellite communication technology due to its very broad bandwidth, astrophysical observation advantages and device maturing in recent years. In this paper, a massive-antenna-array-enabled THz satellite communication system is proposed to be established in Tanggula, Tibet, where the average altitude is 5.068 km and the mean-clear-sky precipitable water vapor (PWV) is as low as 1.31 mm. In particular, a link budget analysis (LBA) framework is developed for THz space communications, considering unique THz channel properties and massive antenna array techniques. Moreover, practical siting conditions are taken into account, including the altitude, PWV, THz spectral windows, rain and cloud factors. On the basis of the developed link budget model, the massive antenna array model, and the practical parameters in Tanggula, the performances of signal-to-noise ratio (SNR) and capacity are evaluated. The results illustrate that 1Tbit/s is attainable in the 0.275—0.37 THz spectral window in Tanggula, by using an antenna array of the size 64.

Compressed Detection for Pulse‐Based Communications in the Terahertz Band

Terahertz (THz) band (0.1‐10 THz)‐based electromagnetic communications are envisioned as a key technology to enable future high‐data‐rate short‐range ultrabroadband communications. However, one of the fundamental bottlenecks is the efficiency of the analog‐to‐digital converters (ADCs) considering the formidable challenge of sampling the signal at the Nyquist rate eventually increasing transceiver design complexity. The Compressed sensing (CS) framework enables the successful reconstruction of sparse signals from a small set of projections onto a random vector which would lead to sub‐Nyquist rate sampling. In this paper, THz band channel estimation based on the theory of CS is developed. The proposed approach exploits the fact that transmitting an ultrashort pulse through a multipath THz channel leads to a received THz signal that can be approximated by a linear combination of a few atoms from a predefined dictionary, yielding thus a sparse representation of the received signal. The fundamental idea is in the design of the dictionary of atoms that closely matches the transmitted pulse leading thus to a higher probability of CS reconstruction. The Orthogonal Matching Pursuit (OMP) algorithm is used to identify the strongest atoms in the projected signal. This reconstructed signal is subsequently used as a reference template in a correlator‐based detector. The bit error rate (BER) performance of the proposed detector is analyzed and compared with the conventional CS‐based channel estimation and reconstruction approach. Extensive simulations show that, for different design parameters, our proposed detector outperforms the traditional CS‐based correlator receiver for the same sampling rate leading thus to a much‐reduced use of analog‐to‐digital resources. Moreover, the proposed detector has been shown to reduce the hardware complexity of the receiver by significantly reducing the number of parallel mixer‐integration branches.

A Non-cooperative Game-based Power Control for Electromagnetic Wireless Nanosensor Networks

Owing to the limited energy-storage capacity of a nanosensor in electromagnetic wireless nanosensor networks (EM-WNSNs), energy efficiency has become an issue in EM-WNSNs. In this study, a non-cooperative power control game (NPCG) is applied to EM-WNSNs for the emission power control of nanosensors, its use generates energy savings. First, a system model is established based on the terahertz channel properties. Second, an NPCG for EM-WNSNs is presented, along with a novel utility function, Nash equilibrium (NE) and its existence and uniqueness, and a power control algorithm based on game theory. Third, the point-to-point communication between nanosensors and a nanocontroller is simulated. Simulation results show that the NE of the NPCG is constrained by the profit and cost parameters of the utility function.

On-Demand Probabilistic Polling for Nanonetworks Under Dynamic IoT Backhaul Network Conditions

Driven by the high sensing resolution provided by advanced nano technologies, electromagnetic-based wireless nano sensor networks (EM-WNSNs) operating in the terahertz (THz) band is becoming an integral part of the Internet of Things (IoT). Data acquisition for EM-WNSNs faces two challenges: 1) the dynamic IoT backhaul bandwidth and 2) THz channel conditions that jointly impact resource utilization efficiency. Specifically, the mismatch between the demands of the EM-WNSNs and the available bandwidth of the backhaul link reduces the bandwidth efficiency of IoT backhaul or the energy efficiency of EM-WNSNs. To address the new constraints that emerge from the EM-WNSN, an on-demand probabilistic (OP) polling scheme is proposed for the backhaul tier of EM-WNSNs which is composed of nano sinks that aggregate and transport data from nano sensors to the IoT gateway. OP polling adjusts the packet aggregation process of nano sinks according to the up-to-date network conditions. From performance evaluation, OP polling achieves an 18% higher bandwidth efficiency compared to the previously proposed on-demand efficient polling and 69% less energy consumption compared to the greedy polling.

0.4 THz Photonic-Wireless Link With 106 Gb/s Single Channel Bitrate

To accommodate the demand of exponentially increased global wireless data traffic, the prospective data rates for wireless communication in the market place will soon reach 100 Gb/s and beyond. In the lab environment, wireless transmission throughput has been elevated to the level of over 100 Gb/s attributed to the development of photonic-assisted millimeter wave and terahertz (THz) technologies. However, most of recent demonstrations with over 100 Gb/s data rates are based on spatial or frequency division multiplexing techniques, resulting in increased system's complexity and energy consumption. Here, we experimentally demonstrate a single channel 0.4 THz photonic-wireless link achieving a net data rate of beyond 100 Gb/s by using a single pair of THz emitter and receiver, without employing any spatial/frequency division multiplexing techniques. The high throughput up to 106 Gb/s within a single THz channel is enabled by combining spectrally efficient modulation format, ultrabroadband THz transceiver and advanced digital signal processing routine. Besides that, our demonstration from system-wide implementation viewpoint also features high transmission stability, and hence shows its great potential to not only decrease the system's complexity, but also meet the requirements of prospective data rates for bandwidth-hungry short-range wireless applications.

Stochastic geometry analysis of interference and coverage in Terahertz networks

Terahertz (THz) band communication has been envisioned as a key wireless technology for providing unprecedented high data rates. However, due to the severe path loss, the transmission distance in THz band is very limited. Therefore, the beamforming techniques are explored to enlarge the communication range in THz networks, particularly employed at Access Points (APs). Interference is a critical factor affecting the performance of THz networks. In this paper, the interference and the coverage probability are investigated for the THz band communications with beamforming APs. First, based on the Line-of-Sight (LoS) and Non-Line-of-Sight (NLoS) ray propagation models of THz signals, the interferences from neighboring users and beamforming APs are modeled in closed forms based on stochastic geometry methods. Then, the Signal-to-Interference-plus-Noise-Ratio (SINR) and the coverage probabilities are further derived based on the THz channel and interference model. Extensive simulations are carried out to evaluate the interference and coverage for the THz band communications with beamforming APs, for the different system parameters, which include SINR threshold, density of APs, transmission frequency, gain of beamforming AP, density of users and radius of interference area. Learning from the simulation results, high density of APs with a small beam-width and the transmission at THz frequencies with low absorption efficient, such as 0.67 THz, are recommended to mitigate the interference and achieve a better coverage performance.

Terahertz Channel Model and Link Budget Analysis for Intrabody Nanoscale Communication.

Nanosized devices operating inside the human body open up new prospects in the healthcare domain. Invivo wireless nanosensor networks (iWNSNs) will result in a plethora of applications ranging from intrabody health-monitoring to drug-delivery systems. With the development of miniature plasmonic signal sources, antennas, and detectors, wireless communications among intrabody nanodevices will expectedly be enabled at both the terahertz band (0.1-10 THz) as well as optical frequencies (400-750 THz). This result motivates the analysis of the phenomena affecting the propagation of electromagnetic signals inside the human body. In this paper, a rigorous channel model for intrabody communication in iWNSNs is developed. The total path loss is computed by taking into account the combined effect of the spreading of the propagating wave, molecular absorption from human tissues, as well as scattering from both small and large body particles. The analytical results are validated by means of electromagnetic wave propagation simulations. Moreover, this paper provides the first framework necessitated for conducting link budget analysis between nanodevices operating within the human body. This analysis is performed by taking into account the transmitter power, medium path loss, and receiver sensitivity, where both the THz and photonic devices are considered. The overall attenuation model of intrabody THz and optical frequency propagation facilitates the accurate design and practical deployment of iWNSNs.

Relay‐assisted nanoscale communication in the THz band

The bit error rate (BER) performance of a cooperative relaying transmission scheme for wireless nanosensor networks in the terahertz (THz) band is investigated. Nanosensor networks comprising several graphene-based devices deployed at the nanoscale are considered. Both amplify-and-forward (AF) and decode-and-forward (DF) relaying modes are studied. A line-of-sight (LOS) channel model in the THz band is considered which takes into account both spreading loss and molecular absorption loss. Given the high path loss and level of noise from significant random fluctuations through the THz channel, relay-assisted schemes offer advantages in terms of significant performance improvements. To quantify the likely benefits, the predicted BER of the proposed scheme is derived. A simulation of the proposed relay schemes based on the THz LOS channel model utilising a Monte Carlo method is conducted. The results obtained show that a performance improvement of 2.2 dB for AF and of 5 dB for DF is achievable at a target BER of 10−5.

Analytical Characterisation of the Terahertz In-Vivo Nano-Network in the Presence of Interference Based on TS-OOK Communication Scheme

The envisioned dense nano-network inside the human body at terahertz (THz) frequency suffers a communication performance degradation among nano-devices. The reason for this performance limitation is not only the path loss and molecular absorption noise, but also the presence of multi-user interference and the interference caused by utilising any communication scheme, such as time spread ON—OFF keying (TS-OOK). In this paper, an interference model utilising TS-OOK as a communication scheme of the THz communication channel inside the human body has been developed and the probability distribution of signal-to-interference-plus-noise ratio (SINR) for THz communication within different human tissues, such as blood, skin, and fat, has been analyzed and presented. In addition, this paper evaluates the performance degradation by investigating the mean values of SINR under different node densities in the area and the probabilities of transmitting pulses. It results in the conclusion that the interference restrains the achievable communication distance to approximate 1 mm, and more specific range depends on the particular transmission circumstance. Results presented in this paper also show that by controlling the pulse transmission probability and node density, the system performance can be ameliorated. In particular, SINR of in vivo THz communication between the deterministic targeted transmitter and the receiver with random interfering nodes in the medium improves about 10 dB, when the node density decreases one order. The SINR increases approximate 5 and 2 dB, when the pulse transmitting probability drops from 0.5 to 0.1 and 0.9 to 0.5.

Distributed Timely Throughput Optimal Scheduling for the Internet of Nano-Things

Nanotechnology is enabling the development of miniature devices able to perform simple tasks at the nanoscale. The interconnection of such nano-devices with traditional wireless networks and ultimately the Internet enables a new networking paradigm known as the Internet of Nano-Things (IoNT). Despite their promising applications, nano-devices have constrained power, energy, and computation capabilities along with very limited memory on board, which may only be able to hold one packet at once and, thus, requires packets to be delivered before certain hard deadlines. Toward this goal, a fully-distributed computation-light provably-correct scheduling/MAC protocol is introduced for bufferless nano-devices, which can maximize the network throughput, while achieving perpetual operation. More specifically, the proposed scheduling algorithm allows every nano-device to make optimal transmission decisions locally based on its incoming traffic rate, virtual debts, and channel sensing results. It is proven that the proposed algorithm is timely throughput optimal in the sense that it can guarantee reliable data delivery before deadlines as long as the incoming traffic rates are within the derived maximum network capacity region. This feature not only can lead to high network throughput for the IoNT, but also guarantees that the memory of each device is empty before the next packet arrives, thus addressing the fundamental challenge imposed by the extremely limited memory of nano-devices. In addition, the optimal deadline is derived, which guarantees that all the nano-devices can achieve perpetual communications by jointly considering the energy consumption of communications over the terahertz channel and energy harvesting based on piezoelectric nano-generators.

Review of weather impact on outdoor terahertz wireless communication links

As the demand for higher wireless data rates continues to increase, there has been more interest in extending the carrier frequency of wireless links from the millimeter wave range into the terahertz (THz) range. For long distance wireless communications, THz links can suffer significant signal loss due to atmospheric weather effects. Obviously, for evaluating wireless system link budgets, it will be important to estimate the weather impact on high capacity data links and compare the performance degradation of THz links to that of the competing approach of free-space optical wireless. This paper reviews the impact of weather on THz wireless links and emphasizes THz attenuation and channel impairments caused by atmospheric gases (in particular water vapor), airborne particulates (such as dust, fog, clouds, and rain), refractive index inhomogeneities which are driven by air turbulence, and their associated scintillations.

Modulation and rate adaptation algorithms for terahertz channels

The ever increasing demand for higher wireless data rates has led to the development of new techniques to increase spectrum efficiency. However, the limited bandwidth of the frequency bands that are currently used for wireless communication bounds the maximum data rate possible. In the past few years, researchers have developed new devices that work as Terahertz (THz) transmitters and receivers. The development of these devices and the large available bandwidth of the THz band is a possible solution to this ever increasing demand. However, THz communication is still in its infancy and more research needs to be done to bring THz technology into every day life.This paper considers two specific problems — modulation of terahertz pulses and rate adaptation for terahertz communication systems. We describe a pulse modulation technique where multiple symbols are transmitted in each pulse. The technique is shown to achieve terabit/sec datarates. Subsequently, we examine the problem of rate adaptation which is significant at these frequencies due to the high distance based attenuation. Algorithms are presented to fairly share the channel among multiple users located at different distances from the access point. The research is based on simulations in Matlab and the channel model used is based on our own measurements using a time-domain terahertz system. The channel model is also presented in this paper.

Characterizing terahertz channels for monitoring human lungs with wireless nanosensor networks

We characterize terahertz wireless channels for extracting data from nanoscale sensors deployed within human lungs. We discover that the inhalation and exhalation of oxygen and carbon dioxide causes periodic variation of the absorption coefficient of the terahertz channel. Channel absorption drops to its minimum near the end of inhalation, providing a window of opportunity to extract data with minimum transmission power. We propose an algorithm for nanosensors to estimate the periodic channel by observing signal-to-noise ratio of the beacons transmitted from the data sink. Using real respiration data from multiple subjects, we demonstrate that the proposed algorithm can estimate the minimum absorption interval of the periodic channel with 98.5% accuracy. Our analysis shows that by confining all data collections during the estimated low-absorption window of the periodic channel, nanosensors can reduce power consumption by six orders of magnitude. Finally, we demonstrate that for wireless communications within human lungs, 0.1–0.12 THz is the least absorbing spectrum within the terahertz band.

TAB-MAC: Assisted beamforming MAC protocol for Terahertz communication networks

Terahertz (THz) communication is envisioned as one of the key technologies to satisfy the increasing demand for higher-speed wireless communication networks. The very high path loss at THz frequencies and the power limitations of THz transceivers limit the communication distance in THz networks. Beamforming directional antennas are needed simultaneously in transmission and in reception to communicate over distances beyond a few meters. This results in many challenges at the link layer, which cannot be easily addressed with existing Medium Access Control (MAC) protocols. In this paper, an Assisted Beamforming MAC protocol for THz communication networks (TAB-MAC) is presented. The protocol exploits two different wireless technologies, namely, WiFi at 2.4 GHz and THz-band communication. In particular, nodes rely on the omnidirectional 2.4 GHz channel to exchange control information and coordinate their data transmissions (Phase 1), whereas the actual data transfer occurs at THz frequencies only after the nodes have aligned their beams (Phase 2). A mathematical framework is developed to analyze the performance of the TAB-MAC protocol in terms of packet delay and throughput, and theoretical upper bounds are derived as functions of total data size, data frame size, node density and data rate in THz band. Numerical results are provided to evaluate the performance of the proposed protocol under different scenarios and define the protocol design guidelines. The results show that the proposed protocol maximizes the THz channel utilization and achievable throughput.