A Review on Evolution of Terahertz Communication

A Review on Evolution of Terahertz Communication Dhriti H. Rudrapal KGCE, Karjat Raigad, Maharashtra, India [email protected]

Ashish L. Lohana KGCE, Karjat Raigad, Maharashtra, India [email protected]

Abstract— over the last few years, wireless data traffic has drastically increased due to the change in a way today’s society creates, shares and consumes information. Along with this change there is an increasing demand for much increase in data rate of wireless communication anywhere and anytime. Wireless Terabit-per-second (Tbps) links are expected to become a reality within the next five to ten years. Advanced physical layer solutions and, more importantly, new spectral bands will be required to support these extremely high data rates. Terahertz Band communication is the wireless technology to satisfy this demand, by alleviating the spectrum scarcity and capacity limitations of current wireless systems. Terahertz (THz) radiation is the electromagnetic radiation in a frequency interval from 0.3 to 10 THz (1 mm–30 μm wavelength), is the next frontier in science and technology. This band occupies a large portion of the electromagnetic spectrum between the far infrared and microwave bands. This paper provides an overview of terahertz wireless communication and the technical challenges for emerging application of terabit wireless systems. The main issue for terahertz wave propagation is generation of such ultrahigh frequencies and the high atmospheric attenuation, which is dominated by water vapour absorption in the terahertz frequency band. The technical challenges in designing such a system and the techniques to overcome the challenges will be discussed. A review of current technological trends and potential applications of terahertz band communication are also discussed here. Keywords— Terahertz band, Wireless communication, Terabit-per-second (Tbps) links

I. INTRODUCTION

Biru S. Bhanvase

KGCE, Karjat Raigad, Maharashtra, India [email protected]

interest for use in wireless communications systems, especially for short-distance applications such as wireless local area networks (WLANs) or wireless personal area networks (WPANs), because of its inherent large bandwidth. Compared to conventional cellular systems operating at frequencies below 2 GHz, we can simply expect that THz waves in the 100 GHz – 1 THz range can provide at least more than 50 to 500 times larger bandwidth [1,2]. In addition, it is technically challenging to produce and detect coherent terahertz radiation, though inexpensive commercial sources now exist in the 0.3–1.0 THz range, which include gyrotrons, backward wave oscillators, and resonant tunneling diodes. II. GENERATION OF TERAHERTZ CARRIER FREQUENCY One of the possible sources of Terahertz radiation is Far Infrared Laser (FIR Laser). While the output from the FIR laser is fairly narrowband for a chosen operating wavelength, but it is a best source for high power (nominal 100mW) requirements with high stability. The design of FIR laser needs ease of use and controls, neutral cavity thermal expansion, and extremely low vacuum leak rate. The FIR lasers can be potentially operated in the frequency range of 50μm to 5000μm, but for that design should allow the use of dielectric waveguide resonator tube bores over a wide range of diameters. These lasers can generate very stable output power after only a few hours of warm up time, and in a 24 hour period the phase stability is better than 10° in absolute phase.

Terahertz radiation begins at a wavelength of one millimetre and proceeds into shorter wavelengths, it is sometimes known as the sub-millimeter band and its radiation as sub-millimeter waves. At these ultra-high frequencies the key challenges are the design of small integrated transceivers with highly directive, steerable antenna and also the high path loss. In this frequency range, for the generation and modulation of coherent electromagnetic signals the conventional electronic devices and methods are not useful, rather requires new devices and techniques.

For more frequency stability a CO2 laser is developed by Pacific Research (Topanga, CA). They have designed a custom CO2 laser that proved to be extremely stable in power and frequency drift. With a typical output power of close to 200 watts continuous wave (CW) in the CO2 laser, the resulting power levels produced by the far-infrared lasers are impressive, usually around 100 milliwatts. However, the amplification of an optical system needs optical electron pumping by an external laser. As a result the performance is limited at room temperature, and also the size might limit the use in some of the typical applications [3].

The earth's atmosphere is a strong absorber of terahertz radiation in specific water vapor absorption bands, so the range of terahertz radiation is limited enough to affect its usefulness in long-distance communications. However, at distances of ~10 meters the band may still allow many useful applications in imaging and construction of high bandwidth wireless networking systems. In spite of the poor output power and sensitivity of emitters and detectors operating at terahertz wave frequencies, the THz wave signal is attracting great

Photo-mixing: Generating continuous wave Terahertz radiation efficiently is a difficult task, since the Terahertz range is at the extremely low-frequency end of optical technologies and at the extreme high-frequency limit of today’s electronic technologies. One way to fill this THz gap is to use Photo-mixers, which combine both optical and electronic technology. The basic principle of photo-mixing is shown in figure 1. Two laser sources are combined and focused on the photo-mixer. Due to the superposition of two

laser-beams, a beat signal (vTHz = v1–v2) of THz frequency is generated. Photo-mixing means the periodic generation of carriers in a photoconductor by a modulated laser beam. The modulated laser-beam is absorbed in the semiconductor material and generates periodically electrons and holes, which are separated in an applied electric field. The resulting THzmodulated current is typically fed into an antenna, which emits CW THz radiation into a dielectric as for example air. Other wave guiding techniques appears too lossy at these frequencies.

necessary baseband processing. Figure 4 shows the experimental setup of 300-GHzwireless link based on ASK modulation and direct detection. A photonic transmitter comprising a UTC-PD and optical modulator can connect THz communications to fiber-optic networks easily using the radioon-fiber configuration. In this configuration the transmitted power of less than 200 mW can limit the bit error rate (BER) of less than 1×10-9.

Figure 1: Basic principle of photo-mixing [4] III. TERAHERTZ TRANSMITTER AND RECEIVER Optical Carrier (λ1)

Information Signal

Figure 2:

Photo Mixer

Optical Modulator

Optical Amplifier (EDFA, SOA)

Optical Carrier (λ2)

Optic/Electric Converter

IV. TERAHERTZ RADIO INTERFACE

To Optical Fiber Link

THz Transmitting Antenna

Power Amplifier

Figure 2: Photonic based Terahertz Transmitter THz Carrier Signal Generator

Information Signal

IF/Baseband Signal Processing

De-modulator (Mixer & Filter)

Figure 4: 300 GHz Band Experiment setup [5]

THz Receiving Antenna

Low Noise Amplifier (Pre Amplifier)

Figure 3: Terahertz Receiver The detailed blocks of Terahertz Communication Transmitter and Receiver are shown in figure 2 & 3. The proposed transmitter system is based on photonics. The electronics systems process the baseband signal in electrical domain, modulate it by terahertz carrier, amplify it and then transmit by the antenna in the form of electromagnetic waves. Photonic systems process the signal in optical domain; even amplify it using optical amplifiers, then convert the signal from optical to electrical form and radiate by the antenna in the form of electromagnetic waves of Terahertz frequencies. The receiver system captures the Terahertz signal; amplify it using Low Noise Amplifier (LNA), down-convert and demodulate using single or multiple stages and do the

Classical modulation schemes can be used for THz Band communication, but they will not be able to fully benefit from THz Band channel properties. The ultra-broadband bandwidth associated to each transmission window in the THz Band drastically changes with small variations in the distance. For example, for distances much below like 1 m, THz Band channel behaves as almost a 10-THz-wide window. However, the bandwidth of several transmission windows is reduced by more than 10% when increasing the transmission distance from 1 to 10 m. This requires the development of different modulations for different applications, based on the targeted distance [6]. The large available bandwidth enables wireless communication of very high throughput in the range of 100 Gbit/s, even with a simple modulation scheme, such as on-off keying (OOK) or amplitude shift keying (ASK). In recent works, Uni-travelling carrier photodiodes (UTC-PD) and Schottky barrier diode (SBD) detectors are used in transmitter and receiver. These transceivers are designed and fabricated for larger bandwidth in the 300 GHz band. With these devices it is possible to achieve 24 Gbit/s error-free data transmission at 300 GHz over a 50 cm distance. The modulation is based on the asynchronous exchange of one hundred- femto secondlong pulses by following an on–off modulation spread in time (TSOOK). The main components of the power spectral density of these pulses are within the THz Band. Despite their very short duration, these pulses can be detected and analogically demodulated with the plasma wave transceivers and hybrid plasmonic. This modulation is mainly valid for short distances, in which molecular absorption does not drastically impact the channel [7]. In a new scheme the nodes intelligently share the channel by adapting the modulation scheme according to the transmission distance. In particular, a

node can adaptively choose modulations based on the transmission distance in order to occupy either (i) the entire transmission bandwidth, (ii) the central part of the transmission window (this information reaches both close and far nodes), or (iii) the sides of the transmission window (the information only reaches nearby devices).

In this section, the device design and development challenges for THz Band are listed. The limitations and possible solutions for ultra-high-speed transceiver architectures are highlighted. In spite of various challenges in THz development and communication, a wide area of applications are suggested here [8].

models cannot be utilized for Terahertz Band frequencies, as they are not characterized by the peculiarities of THz frequency range. However, the limited transmission power of nano-devices and the broadband nature of the generated signals, there is a need to model the entire Terahertz Band for distances below one meter. The main difference of THz band with other frequency bands comes from the molecular absorption loss. Due to this absorption loss propagating wave suffers from signal attenuation. Terahertz waves interact strongly with polar molecules, for example like water. To avoid the need for excessive power at the transmitter, the THz band needs highly directional, line-of-sight wireless systems. In addition, besides LOS communication, NLOS propagation needs to be considered.

A. Limitations

B. Applications

a) Microwave Antennas at THz Frequency: Antenna design and fabrication is a technological challenge in the applications of Terahertz band. The designing of antennas would be done by scaling the antenna structures to the THz band. THz frequencies require very small structures capable of high gain and large bandwidth, so alternative fabrication methods should be employed. An array of antennas can be developed using silicon micro-fabrication method, in which thin gold plated silicon wafers are formed by photolithographic process.

a) 5G Cellular Networks: In the next generation small cells the THz Band communication can be used as a part of hierarchical cellular networks or heterogeneous networks. The THz Band will provide small cells with ultra-high-speed data communication within coverage areas of up to 10 m. The major difference from a user point of view between 4G and 5G techniques must be something else than increased peak bit rate; for example higher number of simultaneously connected devices, higher system spectral efficiency (data volume per area unit), lower battery consumption, lower outage probability (better coverage), high bit rates in larger portions of the coverage area, lower latencies, higher number of supported devices, lower infrastructure deployment costs, higher versatility and scalability or higher reliability of communications.

V. LIMITATIONS AND APPLICATIONS

b) Multiband and Ultra-broadband Antennas: For THz communication high data rates like multi-Gbps and Tbps links are needed. For these high speed links Ultrabroadband and multi-band antennas are necessary. Sinuous antennas can support much larger bandwidths than other classical antenna designs. The sinuous antennas have also proven a good choice for THz Band detectors. Performance characteristics of broad-band antennas have not yet been systematically performed in the THz frequencies for bandwidth, impedance and polarization. Besides some of the THz band applications need very large omni-directional bandwidth, whereas some others need directional antennas. All these requirements result in various challenges in the design of THz Band antennas. c) Very large antenna arrays: The main challenge with a THz Band antenna is very small gain and effective area. As a result it is necessary to design a very large antenna arrays. The small size of THz Band antenna makes it necessary to integrate a very large number of antennas with very small footprint. But this again creates another research challenges. The interaction and coupling effects among nearby antenna elements become unavoidable. Also the large Bandwidth requirements cannot be availed by the classical phase array antennas to support multi-Gbps or Tbps links. d) Challenges with communication Channel: The THz band radiation is highly dependent on channel behavior because of the impact of molecular absorption on the signal propagation. The existing lower frequency channel

b) Wireless local area networks (WLAN): The THz wireless link for future wireless local area networks (WLANs) is based on higher order Quadrature amplitude modulation (QAM) techniques. The system adopts super heterodyne transceivers and parallel digital signalprocessing techniques. THz Band communication enables the seamless interconnection between ultra-high-speed wired networks, e.g., fiber optical links, and personal wireless devices such as laptops and tablet-like devices (no speed difference between wireless and wired links). This will facilitate the use of bandwidth- intensive applications across static and mobile users, mainly in indoor scenarios. Some specific applications are high-definition holographic video conferencing or ultrahigh- speed wireless data distribution in data centers. c) Wireless personal area networks (WPAN): This application means ultra-high speed ad-hoc connections between devices over short distances, for example between a camera and a notebook or between an external hard disk and a laptop. The typical deployment will be indoors, for example on a desktop. The alignment of high gain antennas should be ideally done by automatic beam steering, but rough manual alignment may be thinkable in case of less directive antennas also.

d) Kiosk downloading: This is a special type of a WPAN, where one device is connected to a fixed kiosk download station offering ultrahigh downloads of multimedia-content, e.g. a movie, to a mobile device. Typical transmission ranges are in the order of a few cm under rather known channel conditions, facilitating less directive antennas with manual antenna alignment. Multiple reflections between the transmitter and receiver may still limit the achievable data rates. e) Wireless connections in data centers: The capacity of the data centers can be optimized by dynamically configuring the architecture, which is extremely difficult with fully wired system. The introduction of ultrahigh data rate wireless hops is possible with Terahertz communication. For dynamic reconfiguration the antenna system with beam steering and beam switching capabilities are implemented. Computers can be equipped with highly directive steerable antennas on top of the racks. Connections between different computers are realized by aligning the beams of the corresponding antennas. f) Wireless backhauling: Wireless backhaul is the wireless communication and network infrastructure responsible for transporting communication data from end users or nodes to the central network or infrastructure and vice versa. As an intermediate wireless communication infrastructure it connects smaller networks with the backbone or the primary network. Fixed links with ultra high data rates can be utilized for this wireless extension of backbone networks. The deployment will be outdoors with transmission ranges of several 100 m up to a few kilometers using very directive antennas. g) Secure Terabit Wireless Communication: For military and defense applications the ultra broadband secure link can be established by Terahertz communication. Due to high atmospheric attenuation at THz frequencies, large antenna arrays are used to generate very narrow, almost razorsharp beams, which drastically limit the eavesdropping probability. The spread spectrum techniques can also be implemented over ultra broadband bandwidth to prevent and overcome common jamming attacks. h) Health Monitoring Systems: Nano-scale sensors can be used to monitor patient’s blood contents like sodium, glucose and other ions, cholesterol, cancer biomarkers or the presence of different infections. These nano-sensors are distributed around the body to collect relevant data about the patient’s health. A wireless interface between these nano-sensors and a micro-device such as a cell phone or specialized medical equipment could be used to collect all these data and forward them to the healthcare provider. i) The Internet of Nano-things: Nanotechnology is enabling the development of novel devices which are able to generate process and transmit multimedia content at the nano-scale. The interconnection of

pervasively deployed multimedia nano-devices with existing communication networks and ultimately the Internet defines a novel communication paradigm that is further referred to as the Internet of Multimedia Nano-Things (IoMNT). The IoMNT is a truly cyber-physical system with a plethora of applications in the biomedical, security and defense, environmental and industrial fields, amongst others. j) Ultra high speed chip-to-chip communications: This application means wireless links inside computers or any other electronic devices. It is of high relevance because wired connectors and micro strip lines on printed circuit boards potentially become a bottleneck of upcoming bus systems and inter chip connections. Transmission ranges are a few cm with both LOS and NLOS situations. VI. CONCLUSION According to Shannon theory, the broad bandwidth of the terahertz frequency bands can be used for terabit-per-second (Tbps) wireless communication systems. This enables several new applications, such as optic-fiber replacement, and wireless Tbps file transferring. Although terahertz technology could satisfy the demand for an extremely high data rate, a number of technical challenges need to be overcome before its development. Radiologists find this area of study so attractive because t-rays (terahertz waves) are non-ionizing, results no harm to human tissues. In this paper, the review of terahertz band technology is presented from the device point of view, by investigating the transceiver architectures of different technologies. Also various macro and micro scale terahertz band applications are discussed here. Moreover, we have highlighted the challenges in terms of channel modeling and transceiver design. We believe that this report has defined a roadmap for the development of this new frontier terahertz wireless communication.

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