Page 44 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

2. APPLICATIONS
Terahertz Band
Communication Links Utilizing THz space links can pave the way for novel appli‑
cations, some of which are discussed as follows.

Terrestrial Space 2.1 Earth observation
Links Links
Earth observation using arti icial satellites began with the
launch of Sputnik 1 by the former Soviet Union in 1957.
Inte
Dee
Nano-scaleo-scale
Macro Nan Inter-satelliter-satellite Deep Spacep Space
Macroscalescale
Since then, many Earth observation satellites have been
launched. Most of these satellites occupy Low Earth Or‑
Fig. 1 – Classi ication of THz links [8]. bits (LEO) and transmit a large amount of data to Earth
daily. Recently, an increasing number of LEO satellites
solutions such as large
are being launched so that the bandwidth used is get
THz antenna arrays enabling ex‑ tremely high gains.
ting congested. To reduce the transmission delays and
Moreover, THz waves are not affected by
support the transmission of a large amount of sensing
turbulence‑induced scintillation, which is observed as
data to Earth, technologies supporting high data rates are
intensity luctuations at the signal, as much as FSO
required. FSO communications, providing connectivity
links [7, 3].
within a few kilometers using laser beams, have been pro‑
posed as a viable solution [9]. For instance, EDRS employs
THz band communication links can be classi ied into
FSO communications between LEO satellites collecting
two as terrestrial and non‑terrestrial networks as shown
Earth observation data and GEO satellite relaying data to
in Fig. 1. Terrestrial networks comprise macroscale
Earth. However, with the start of a New Space Era, satel‑
and nano‑scale links. In this study, we consider non‑
terrestrial network components ground/space and space lites are getting miniaturized, and deploying many small
links consisting of inter‑satellite and deep space commu‑ satellites, e.g., CubeSats, is preferred [10]. The power and
nication links. We aim to identify the challenges related size requirements of FSO systems far exceed the limita‑
to THz space links and discuss the possible solutions. tions of cube/micro/nano‑satellites. On the other hand,
The realization of THz space links poses several building THz transceivers with a large number of antenna
challenges. The spreading loss due to the expansion of arrays, i.e., phased Multiple‑Input and Multiple‑Output
propagating electromagnetic waves is increasing (MIMO) arrays, in a small footprint, is possible thanks to
drastically with the frequency. This limits the novel materials such as graphene [11]. Thus, for long‑
communication distance to few meters on Earth due to distance and high‑data‑rate near‑Earth transmission, THz
immature THz source technology, which can enable communications can be leveraged in the future. Large
transmit power on the order of milliwatts. THz arrays are also advantages over FSO links in terms of
beam‑alignment, e.g., they can provide automatic align‑
Moreover, strong molecular absorption results in high
ment by their scanning ability [12]. However, there exist
atmospheric attenuation in Earth‑to‑space links; thus,
issues to realize such THz transceivers. The main impedi‑
limits the utilization of the high THz band. Arti icial
ments include the lack of practical THz signal sources and
satellites occupy higher atmospheric layers of Earth or
detectors, implementation and optimization of antenna
deep space where the air molecules are scarce or none.
arrays [12].
Therefore, THz inter‑satellite links do not experience sig‑
ni icant atmospheric attenuation. Regarding another ter‑
2.2 Interplanetary communications by hybrid
restrial planet Mars, atmospheric attenuation is expected
to be low compared to Earth because water molecules, THz/FSO links
which are the primary source of atmospheric
attenuation, are scarce in the Mars atmosphere. These
Current state‑of‑the‑art technologies
create an opportunity of utilizing the high THz band,
used are not able to support high data rate interplanetary
consequently providing high data rates. In line with this,
communications as a part of space information networks,
later we simu‑ late the transmittance of Mars’s
numerous applications including
atmosphere in clear and dusty atmospheric conditions
space observation, Internet of Things (IoT), and maritime
using an accurate radiative transfer tool called Planetary
T illustrate, Reconnaissance Orbiter
Spectrum Generator (PSG) to show the availability of a
large bandwidth for Mars communication. employs X‑band (8‑12 GHz) and Ka‑band (26.5‑40 GHz)
Deep Network,
comprises deep facilities
The rest of the paper is organized as follows. In for commanding and tracking The data rate
Section 2, we describe the applications of THz space between megabits per second The
links. In Section 3, we discuss the challenges THz band services such as live video feeding, high‑resolution scien‑
communications encounter, and then in Section 4, we ti ic data streaming, virtual reality for controlling rovers
simulate zenith transmittance of Mars atmosphere. In other real‑time data transmission
Section 5, conclusions are stated. will require higher data Although latency
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