Full length articleTerahertz band: Next frontier for wireless communications
Introduction
Over the last few years, wireless data traffic has drastically increased due to a change in the way today’s society creates, shares and consumes information. This change has been accompanied by an increasing demand for much higher speed wireless communication anywhere, anytime. In particular, wireless data rates have doubled every eighteen months over the last three decades and are quickly approaching the capacity of wired communication systems [1]. Following this trend, 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.
In this context, Terahertz Band communication [2], [3], [4], [5], [6], [7], [8] is envisioned as a key wireless technology to satisfy this demand, by alleviating the spectrum scarcity and capacity limitations of current wireless systems, and enabling a plethora of long-awaited applications in diverse fields (Section 2). The THz Band is the spectral band that spans the frequencies between 0.1 THz and 10 THz. While the frequency regions immediately below and above this band (the microwaves and the far infrared, respectively) have been extensively investigated, this is still one of the least-explored frequency bands for communication.
There are several reasons that motivate the use of the THz Band for ultra-broadband communication networks:
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Wireless technologies below 0.1 THz are not able to support Tbps links. On the one hand, advanced digital modulations, e.g., Orthogonal Frequency Division Multiplexing (OFDM), and sophisticated communication schemes, e.g., very large scale Multiple Input Multiple Output (MIMO) systems, are being used to achieve a very high spectral efficiency at frequencies below 5 GHz. However, the scarcity of the available bandwidth limits the achievable data rates. For example, in Long-Term Evolution Advanced (LTE-A) networks, peak data rates in the order of 1 Gbps are possible when using a four-by-four MIMO scheme over a 100 MHz aggregated bandwidth [9]. These data rates are three orders of magnitude below the targeted 1 Tbps. On the other hand, millimeter wave (mm-Wave) communication systems, such as those at 60 GHz, can support data rates in the order of 10 Gbps within one meter [10]. While this is definitely the path to follow, this data rate is still two orders of magnitude below the expected demand. The path to improve the data rate involves the development of more complex transceiver architectures able to implement physical layer solutions with much higher spectral efficiency. However, the usable bandwidth is limited to less than 7 GHz, which effectively imposes an upper bound on the data rates.
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Compact wireless technologies above 10 THz are not able to support Tbps links. Despite the very large available bandwidth in Free Space Optical (FSO) communication systems, which operate at infrared (IR) frequencies and above, there are several issues that limit the practicality of these schemes for personal wireless communications. Low transmission power budget due to eye-safety limits, the impact of several atmospheric effects on the signal propagation (e.g., fog, rain, dust or pollution), high diffuse reflection losses, and the impact of misalignment between transmitter and receiver, limit both the achievable data rates and transmission range of FSO communication systems [2]. For example, a IR FSO communication system able to support 10 Gbps wireless links in case of Line-of-Sight (LOS) propagation has been proposed for Wireless Local Area Networks (WLANs) in [11]. However, only much lower data rates are supported in the case of diffused Non-Line-of-Sight (NLOS) communication, as analytically demonstrated in [12]. Similarly, in [13], an indoor FSO communication system able to support a 1-Gbps link at visible light frequencies was reported. By following a totally different approach, a long-distance FSO system able to support 1.28 Tbps link was demonstrated in [14]. Typical fiber communication equipment was used to generate and detect high capacity optical signals, which are then injected into an optical front-end. Only the optical front-end is 12 cm×12 cm×20 cm and almost 1 kg in weight, and does not include the signal generation/detection and modulation/demodulation blocks. All these constraints limit the feasibility of this large-scale optical approach for personal and mobile wireless communications.
On its turn, the THz Band offers a much larger bandwidth, which ranges from tens of GHz up to several THz depending on the transmission distance. The available bandwidth is more than one order of magnitude above state-of-art mm-Wave systems, while the frequency of operation is at least one order of magnitude below that of FSO systems. In addition, the technology required to make THz Band communication a reality is rapidly advancing, and the development of new transceiver architectures and antennas built upon novel materials with unprecedented properties are finally overcoming one of the major challenges in the so-called THz gap (Section 3).
However, there still exist several research challenges both from the device and the communication perspectives that require innovative solutions and even the revision of well-established concepts in wireless communications. One of the main challenges is imposed by the very high path loss at THz Band frequencies, which poses a major constraint on the communication distance. Additional challenges range from the implementation of compact high-power THz Band transceivers, the development of efficient ultra-broadband antennas at THz Band frequencies, and the characterization of the frequency-selective path loss of the THz Band channel, to the development of novel modulations, transmission schemes and communication protocols tailored to the peculiarities of this paradigm. Many of these challenges are common to mm-Wave communication systems and, as a result, the solutions proposed in this paper can also benefit those systems.
In addition to all these challenges, the THz Band is not yet regulated. Therefore, it is the right time for the telecommunications community to jointly define and pave the way for the future of this novel communication paradigm. In this direction, the IEEE 802.15 Wireless Personal Area Networks (WPAN) Study Group 100 Gbit/s Wireless (SG100G) [15], formerly known as the IEEE 802.15 WPAN Terahertz Interest Group (IGThz), has been recently established. The ultimate goal of the SC100G is to work towards the first standard for THz Band communication able to support multi-Gbps and Tbps links.
In this paper, we review the state of the art in THz Band communications networks and provide an in-depth view of this novel networking paradigm both from the device perspective as well as from the communication and information theoretic point of view. In Section 2, we describe many potential applications of ultra-broadband communications in the THz Band. In Section 3, we state the challenges in device technologies, which include transceiver and antenna designs in the THz Band. In Section 4, we outline the communication challenges in terms of channel modeling, physical, link, network and transport layers functionalities for THz Band communication networks. In Section 5, we review the state of the art in terms of experimental and simulation platforms and identify the main challenges in their realization. Finally, we conclude the paper in Section 6.
Section snippets
Applications of terahertz band communication
The very large bandwidth provided by the THz Band opens the door to a variety of applications which demand ultra-high data rates and allows the development of a plethora of novel applications in classical networking scenarios as well as in new nanoscale communication paradigms. Some of these applications can already be foreseen and others will undoubtedly emerge as technology progresses.
Challenges in terahertz band device technologies
In this section, the device design and development challenges for THz Band are surveyed. The limitations and possible solutions for ultra-high-speed transceiver architectures are highlighted, and the challenges in the development of ultra-broadband antennas and antenna arrays are explained.
Challenges in terahertz band communication networks
There are many challenges in the realization of efficient and practical THz Band communication networks, which require the development of innovative solutions at the different layers of the protocol stack. In the following, the main challenges are discussed in a bottom-up approach, by starting from the THz Band channel modeling, up to the development of transport layers solutions for ultra-broadband communication networks in the THz Band. As we mentioned in the introduction of this paper, many
Experimental and simulation testbeds
The validation of the developed solutions requires the development of experimental platforms. Ideally, these platforms should be integrated by at least one transceiver and one receiver, and should be able to support stable THz Band links. For the time being, several platforms at frequencies below 1 THz have been built and successfully utilized for data transmission, channel measurements and propagation studies. In [105], a setup based on a Schottky diode sub-harmonic mixer combined with a
Conclusions
The Terahertz Band (0.1–10 THz) is envisioned to satisfy the need for Tbps wireless links in the near future. THz Band communication will address the spectrum scarcity and capacity limitations of current wireless systems, and enable a plethora of applications, such as ultra-fast massive data transfers among nearby devices, or high-definition videoconferencing among mobile personal devices in small cells. In addition, the THz Band will also enable novel networking paradigms at the nanoscale,
Acknowledgments
The authors would like to thank Dr. Ozgur B. Akan, Dr. Kaushik Chowdhury, Dr. Eylem Ekici, and Dr. Xudong Wang, for their constructive criticism which helped to improve the quality of the paper.
This work was supported by the U.S. National Science Foundation (NSF) under Grant No. CCF-1349828.
Ian F. Akyildiz received the B.S., M.S., and Ph.D. degrees in Computer Engineering from the University of Erlangen–Nürnberg, Germany, in 1978, 1981 and 1984, respectively. Currently, he is the Ken Byers Chair Professor in Telecommunications with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, the Director of the Broadband Wireless Networking (BWN) Laboratory and the Chair of the Telecommunication Group at Georgia Tech. Since 2013, he is a FiDiPro
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Ian F. Akyildiz received the B.S., M.S., and Ph.D. degrees in Computer Engineering from the University of Erlangen–Nürnberg, Germany, in 1978, 1981 and 1984, respectively. Currently, he is the Ken Byers Chair Professor in Telecommunications with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, the Director of the Broadband Wireless Networking (BWN) Laboratory and the Chair of the Telecommunication Group at Georgia Tech. Since 2013, he is a FiDiPro Professor (Finland Distinguished Professor Program (FiDiPro) supported by the Academy of Finland) in the Department of Electronics and Communications Engineering, at Tampere University of Technology, Finland, and the founding director of NCC (Nano Communications Center). Since 2008, he is also an honorary professor with the School of Electrical Engineering at Universitat Politècnica de Catalunya (UPC) in Barcelona, Catalunya, Spain and the founding director of N3Cat (NaNoNetworking Center in Catalunya). Since 2011, he is a Consulting Chair Professor at the Department of Information Technology, King Abdulaziz University (KAU) in Jeddah, Saudi Arabia. He is the Editor-in-Chief of Computer Networks (Elsevier) Journal, and the founding Editor-in-Chief of the Ad Hoc Networks (Elsevier) Journal, the Physical Communication (Elsevier) Journal and the Nano Communication Networks (Elsevier) Journal. He is an IEEE Fellow (1996) and an ACM Fellow (1997). He received numerous awards from IEEE and ACM. His current research interests are in nanonetworks, Terahertz Band communication networks, Long Term Evolution Advanced (LTE-A) networks, cognitive radio networks and wireless sensor networks.
Josep Miquel Jornet received the Engineering Degree in Telecommunication and the Master of Science in Information and Communication Technologies from the Universitat Politècnica de Catalunya, Barcelona, Spain, in 2008. He received the Ph.D. degree in Electrical and Computer Engineering from the Georgia Institute of Technology, Atlanta, GA, in 2013, with a fellowship from “la Caixa” (2009–2010) and Fundación Caja Madrid (2011–2012). He is currently an Assistant Professor with the Department of Electrical Engineering at the University at Buffalo, The State University of New York. From September 2007 to December 2008, he was a visiting researcher at the Massachusetts Institute of Technology, Cambridge, under the MIT Sea Grant program. He was the recipient of the Oscar P. Cleaver Award for outstanding graduate students in the School of Electrical and Computer Engineering, at the Georgia Institute of Technology in 2009. He also received the Broadband Wireless Networking Lab Researcher of the Year Award at the Georgia Institute of Technology in 2010. He is a member of the IEEE and the ACM. His current research interests are in electromagnetic nanonetworks, graphene-enabled wireless communication, Terahertz Band communication networks and the Internet of Nano-Things.
Chong Han received the Bachelor of Engineering degree in Electrical Engineering and Telecommunications from The University of New South Wales, Sydney, Australia, in 2011, and received the Master of Science degree in Electrical and Computer Engineering from Georgia Institute of Technology, Atlanta, USA, in 2012. Currently, he is a graduate research assistant in the Broadband Wireless Networking Laboratory (BWN Lab), School of Electrical and Computer Engineering, Georgia Institute of Technology. He is pursuing his Ph.D. degree under the supervision of Prof. Ian F. Akyildiz. He is a student member of the IEEE. His current research interests are in Terahertz Band communication networks, Internet of Things, Internet of Nano-Things, Electromagnetic Nanonetworks, and Graphene-enabled Wireless Communication.