Terahertz band: Next frontier for wireless communications

Abstract

This paper provides an in-depth view of Terahertz Band (0.1–10 THz) communication, which is envisioned as a key technology to satisfy the increasing demand for higher speed wireless communication. THz Band communication will alleviate the spectrum scarcity and capacity limitations of current wireless systems, and enable new applications both in classical networking domains as well as in novel nanoscale communication paradigms. In this paper, the device design and development challenges for THz Band are surveyed first. The limitations and possible solutions for high-speed transceiver architectures are highlighted. The challenges for the development of new ultra-broadband antennas and very large antenna arrays are explained. When the devices are finally developed, then they need to communicate in the THz band. There exist many novel communication challenges such as propagation modeling, capacity analysis, modulation schemes, and other physical and link layer solutions, in the THz band which can be seen as a new frontier in the communication research. These challenges are treated in depth in this paper explaining the existing plethora of work and what still needs to be tackled.

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:

• 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.

• 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.