Terahertz Communication Systems Research Articles

Indoor Terahertz Communications: How Many Antenna Arrays Are Needed?

Terahertz (THz) communications promise to be the next frontier for wireless networks. Novel solutions should be explored to overcome the hardware constraints and the severe path loss. In this paper, we study a low-complexity indoor THz communication system with antenna subarrays. The Saleh-Valenzuela (S-V) channel model is modified to characterize the THz channel. By exploiting the hybrid beamforming with multiple subarrays, we analyze the ergodic capacity of the system and obtain an upper bound. Furthermore, with the analysis of performance degradation for the uncertainty in THz phase shifters, we provide a guidance on the design of antenna subarray size and number for certain long-term data rate requirements with different distances. Simulation results validate the effectiveness of the ergodic capacity upper bound, and show that the proposed THz system and antenna array structure can efficiently achieve capacity gains and support THz communications.

Gbps terahertz external modulator based on a composite metamaterial with a double-channel heterostructure.

The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nanoscale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this Letter, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultrafast, and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1 GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19 rad phase shift was realized. A wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2 Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultrafast dynamic devices for THz wireless communication and imaging systems.

Graphene-enabled electrically controlled terahertz spatial light modulators.

In this Letter, we demonstrate a broadband terahertz (THz) spatial light modulator using 5×5 arrays of large area graphene supercapacitors. Our approach relies on controlling spatial charge distribution on a passive matrix array of patterned graphene electrodes. By changing the voltage bias applied to the rows and columns, we were able to pattern the THz transmittance through the device with high modulation depth and low operation voltage. We anticipate that the simplicity of the device architecture with high contrast THz modulation over a broad spectral range could provide new tools for THz imaging and communication systems.

Active graphene plasmonic grating for terahertz beam scanning device

We have proposed an electrically controllable terahertz (THz) beam scanning device based on asymmetric graphene plasmonic grating. It is proved that the dispersion relation of surface plasmon waves can be efficiently manipulated on the periodic metal–dielectric–graphene plasmonic structure by an external bias voltage, and thus the derivation angle of the THz beam can be actively tuned. The numerical results show that the radiation angle of THz waves through this device can be tuned from −18° to 18° with the bias changing from 53.5mV into 368.8mV at the operating frequency of 0.5THz. This electrically active beam scanning device has important application in the THz imaging, radar and communication systems.

Modeling of Terabit Geostationary Terahertz Satellite Links From Globally Dry Locations

While terahertz (THz) communication systems, operating from 100 GHz to 1 THz, have the potential to exploit wide swaths of unused spectrum for ultra-high bitrate communication, there are significant challenges. Particularly, the strong absorption of water vapor can result in very high atmospheric attenuation. We modeled a ground to geostationary satellite link and found that using large aperture THz stations, patterned after the 12.5 m Atacama Large Microwave Array dish and the 3.5 m Herschel Space Observatory optics, worst 10th percentile data rates in excess of one terabit per second in the THz bands are possible. The key is to site ground stations in dry regions. We locate these by coupling our link model, which selects optimum modulation and carrier bandwidth, with global, high-resolution satellite water vapor measurements. We present detailed maps showing modeled link performance over the surface of the Earth. Smaller apertures on aircraft and balloons are also able to exceed 1 terabit/second due to their location above nearly all water vapor. Compared to free-space optical links, evidence suggests THz systems are superior where fog, cloud cover and clear-air turbulence are of concern.

Atmospheric Attenuation in Wireless Communication Systems at Millimeter and THz Frequencies [Wireless Corner

This paper intends to give an overview about atmospheric propagation effects affecting millimeter and terahertz (THz) communication systems. The main focus is on attenuation caused by atmospheric gases and liquid water droplets, either in the form of suspended particles into clouds or rain falling hydrometeors. Theoretical aspects about each of them are presented, emphasizing on those that deserve special attention as frequency increases. Statistics of attenuation estimated from meteorological data and some experimental results, as in the case of rain attenuation, obtained in Madrid, Spain, are presented throughout the paper, thus providing further insights about the phenomena discussed.

Performance Analysis of Submillimeter-Wave Indoor Communications Using Blocking Probability

For multi-gigabit wireless transmission in fiber-wireless (FiWi) networks, one of the good promising candidates is terahertz (THz) communications. In order to analyze the performance of THz communication systems, where there exist multiple paths caused by THz signal’s reflections, a proper channel model for describing the THz frequency characteristics and an adequate path selection algorithm is important. By bringing the blocking probability to describe the blocking effect of the propagation signals, this paper provides a modified THz channel model and proposes a path selection algorithm for finding the dominant signal. This paper also analyzes the correlation coefficient between propagation signals. Numerical results reveal that the modified channel model is suitable for describing the blocking effect of the propagation signals in the FiWi indoor environment and the proposed algorithm makes use of path diversity well to achieve the highest data transmission rate.

Design and Performance Analysis of a Multi Wavelength Terahertz Modulator Based on Triple-Lattice Photonic Crystals

Terahertz (THz) communication has important applications in high-speed and ultra broadband wireless access networks. The THz modulator is one of the key devices in a THz communications system. Wavelength division multiplexing (WDM) can expand the capacity of THz communications systems, so research on multi wavelength THz modulators has significant value. By combining photonic-crystal and THz technology, a novel type of multi wavelength THz modulator based on a triple-lattice photonic crystal is proposed in this paper. Compared to a compound-lattice photonic crystal, a triple-lattice photonic crystal has a larger gap width of 0.196. Simulation results show that six beams of THz waves can be modulated simultaneously with high performance. This modulator's extinction ratio is as large as 34.25 dB, its insertion loss is as low as 0.147 dB, and its modulation rate is 2.35 GHz.

Graphene nanoribbon based terahertz antenna on polyimide substrate

Graphene nano ribbon based terahertz patch antenna on polyimide substrate is designed and its radiation characteristics are investigated in the 725–775GHz band. The terahertz communication system consists of higher data rate transmission, low transmit power with secured wireless communication. The proposed antenna consists of graphene nano ribbon as radiating patch and also the ground plane separated by a 20μm thin polyimide substrate. The antenna has achieved the broad impedance bandwidth (>5%) in the band of operation. The design has yielded a peak gain of 5.71dB at 750GHz. The antenna is simulated by using the finite element method based simulator Ansys - HFSS.

Dynamic trapping of terahertz waves by silicon-filled metallic grating structure

We investigate the feasibility of dynamic trapping of terahertz waves using a silicon-filled metallic grating structure. Using the dispersion relation analysis and the two-dimensional finite element method simulations, we reveal that, if a graded refractive index distribution in the grooves is optical induced, the device has the ability to dynamic trap terahertz waves of different frequencies at different positions (so-called trapping rainbow). Moreover, we demonstrate that the trapped position of a certain frequency of the terahertz waves can be moved continuously along the grooves in subwavelength scale by ingenious control of the distributions of the refractive indices of silicon filled in the grooves. Our design has the potential for the construction of active plasmonic terahertz devices, such as optical controlled terahertz filter, router and demultiplexer in a broadband terahertz communication system.

Radiation characteristics of terahertz waveguide‐fed circularly polarised antipodal exponentially tapered slot antenna

A terahertz (THz) waveguide-fed circularly polarised (CP) antipodal exponentially tapered slot antenna (AETSA) is designed, fabricated and measured. This antenna is fed by a WR2.2 rectangular waveguide. The integrated AETSA is fabricated by deep reactive-ion etching on silicon wafers, gold-sputtering and gold–gold thermo-compression bonding. Optimised exponential curves are introduced to the profiles of tapered slots to realise the CP performance. The measured radiation patterns at 490 GHz show beamwidths of 42° and 39° in the 0°-plane and 90°-plane, with low sidelobes. Furthermore, the measured axial ratios (ARs) from 480 to 500 GHz are lower than 2 dB. The THz waveguide-fed CP AETSA has potential to be applied in compact THz imaging, detection and communication systems.

A novel photonic crystal band-pass filter using degenerate modes of a point-defect microcavity for terahertz communication systems

Compact devices are important for the realization of terahertz communications systems. This article proposes a novel photonic crystal-based device for realizing microminiature, high-selectivity high Q band-pass filters (BPF) and the design of a dual-mode square lattice photonic crystal BPF that utilizes the degenerated modes of a point defect microcavity is presented. To design a high Q microcavity, the photonic band-gap is initially calculated using the plane wave expansion method. Second, the eigenfrequencies and modal fields of a point defect microcavity that generates localized states in the band-gap are calculated by a supercell method. Finally, the characteristics of mode splitting and the proposed dual-mode BPFs are numerically studied by a full-wave time-domain method. © 2014 The Authors. Microwave and Optical Technology Letters Published by Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:792–797, 2014

Electrically tunable terahertz wave modulator based on complementary metamaterial and graphene

In this paper, we design and numerically demonstrate an electrically controllable light-matter interaction in a hybrid material/metamaterial system consisting of an artificially constructed cross cut-wire complementary metamaterial and an atomically thin graphene layer to realize terahertz (THz) wave modulator. By applying a bias voltage between the metamaterial and the graphene layer, this modulator can dynamically control the amplitude and phase of the transmitted wave near 1.43 THz. Moreover, the distributions of current density show that this large modulation depth can be attributed to the resonant electric field parallel to the graphene sheet. Therefore, the modulator performance indicates the enormous potential of graphene for developing sophisticated THz communication systems.

Research on indoor channel measurement and simulation at 340 GHz

Ultra-broadband terahertz communication systems are expected to help satisfy the ever-growing need for unoccupied bandwidth. Due to high attenuation of terahertz wave, it can be widely used in indoor WLAN data communication. Future THz WLANs will rely on not only the line-of-sight (LOS) but also the nonline-of-sight (NLOS) channels to perform data communication. Hence, both kinds of channels have to be characterized. In this paper, we present the measures of ultra-broadband channel at 340 GHz for an indoor scenario. The measured channel transfer function is compared with a ray tracing simulation performed with the indoor scenario. Additionally, we show the reflection losses of some building and plastic materials which could be required as input data for the ray tracing algorithm.

An ultrathin terahertz lens with axial long focal depth based on metasurfaces

The plasmonic resonance effect on metasurfaces generates an abrupt phase change. We employ this phase modulation mechanism to design the longitudinal field distribution of an ultrathin terahertz (THz) lens for achieving the axial long-focal-depth (LFD) property. Phase distributions of the designed lens are obtained by the Yang-Gu iterative amplitude-phase retrieval algorithm. By depositing a 100 nm gold film on a 500 μm silicon substrate and etching arrayed V-shaped air holes through the gold film, the designed ultrathin THz lens is fabricated by the micro photolithography technology. Experimental measurements have demonstrated its LFD property, which basically agree with the theoretical simulations. In addition, the designed THz lens possesses a good LFD property with a bandwidth of 200 GHz. It is expected that the designed ultrathin LFD THz lens should have wide potential applications in broadband THz imaging and THz communication systems.

Spatial Terahertz Modulator

Terahertz (THz) technology is a developing and promising candidate for biological imaging, security inspection and communications, due to the low photon energy, the high transparency and the broad band properties of the THz radiation1,2,3. However, a major encountered bottleneck is lack of efficient devices to manipulate the THz wave, especially to modulate the THz wave front. A wave front modulator should allow the optical or electrical control of the spatial transmission (or reflection) of an input THz wave and hence the ability to encode the information in a wave front4. Here we propose a spatial THz modulator (STM) to dynamically control the THz wave front with photo-generated carriers. A computer generated THz hologram is projected onto a silicon wafer by a conventional spatial light modulator (SLM). The corresponding photo-generated carrier spatial distribution will be induced, which forms an amplitude hologram to modulate the wave front of the input THz beam. Some special intensity patterns and vortex beams are generated by using this method. This all-optical controllable STM is structure free, high resolution and broadband. It is expected to be widely used in future THz imaging and communication systems.

Design of terahertz band-stop filter based on a metallic resonator on high-resistivity silicon wafer

In this paper, we present a terahertz (THz) band-stop filter realized by fabricating a metallic T-shaped resonator pattern on the high-resistivity silicon wafer. The filter exhibits two typical band-stop response characteristics depending on the incident direction of electric field with respect to the T-shaped resonator. When the long and the short arms of the T-shaped resonator were electrically polarized by changing the incident THz wave transmission directions, the corresponding central frequencies of the band-stop filter were found to be 0.436 THz at −42dB and 0.610 THz at −28 dB, respectively. Using three-dimensional (3D) finite-integral time-domain simulations, the band-stop filter was designed, which can operate in the wavelength between 0.2 and 0.8 THz. Experimental verification was also performed using a free space THz time-domain spectroscopy system. The band-stop response characteristics are in good agreement with the simulation results. The interesting THz band-stop filtering properties suggest a promising application in the modern THz communication systems, THz time-domain spectroscopic imaging and THz continuous wave imaging.

10-Gbit/s Wireless Communication System at 300 GHz

A 10-Gbit/s wireless communication system operating at a carrier frequency of 300 GHz is presented. The modulation scheme is amplitude shift keying in incoherent mode with a high intermediate frequency (IF) of 30 GHz and a bandwidth of 20 GHz for transmitting a 10-Gbit/s baseband (BB) data signal. A single sideband transmission is implemented using a waveguide-tapered 270-GHz highpass filter with a lower sideband rejection of around 60 dB. This paper presents an all-electronic design of a terahertz communication system, including the major modules of the BB and IF band as well as the RF modules. The wireless link shows that, aided by a clock and data recovery circuit, it can receive 2 7 –1 pseudorandom binary sequence data without error at up to 10 Gbit/s for over 1.2 m using collimating lenses, where the transmitted power is 10 μW.

Terahertz Antenna Phase Shifters Using Integrally-Gated Graphene Transmission-Lines

We propose the concept and design of terahertz (THz) phase shifters for phased antenna arrays based on integrally-gated graphene parallel-plate waveguides (GPPWGs). We show that an active transmission-line may be realized by combining GPPWGs with double-gate electrodes, in which the applied gate voltage can control the guiding properties of the gated sections. This may enable the realization of THz electronic switches and tunable loaded-lines for sub mm-wave antenna systems. Based on these active components, we theoretically and numerically demonstrate several digital and analog phase shifter designs for THz frequencies, with a wide range of phase shifts and small return loss, insertion loss and phase error. The proposed graphene-based phase shifters show significant advantages over other available technology in this frequency range, as they combine the low-loss and compact-size features of GPPWGs with electrically-programmable phase tuning. We envision that these electronic phase shifters may pave the way to viable phased-arrays and beamforming networks for THz communications systems, as well as for high-speed, low-RC-delay, inter/intra-chip communications.

Modeling and Simulation of Terahertz Devices

Research and development in the exploitation of millimeter and submillimeter waves has undergone impressive growth during the last decade due to the potential technological uses of the terahertz (THz) portion of the spectrum with its unique location between the microwave (i.e., electronic) and the optical domains, which nominally corresponds to the frequency range from 300 GHz (submillimeter wave) to 10 THz. There are a number of important applications in the THz band, including atmospheric science, biodetection and imaging, THz spectroscopy, threat detection, broadband communications, and the detection of bioagents relevant to battlefield defense. Due to the sensitivity of THz radiation to the configuration of large organic molecules, THz spectroscopy will support and sustain new technologies for the remote detection of potentially harmful bioagents [1], [2]. Furthermore, the possibility to fabricate solid-state sources and detectors will allow the production of fully integrated biosensors [3]. Wireless communication is another promising application of THz technology. In particular, mobile videophones and highspeed mobile Internet access would be greatly facilitated if THz carriers would be used at least in some links of the cellular network. All solid-state THz communication systems can be envisioned [4] that supply a valid alternative to the expensive and less-developed all-optical communication technology.