Page 45 - 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

VENUS

Rover

Relay
Orbiter Lander
MARS

EARTH

CubeSats

Fig. 2 – Interplanetary communications: Black arrows (THz links), red arrows (FSO links).

is still an issue, these services can be enabled by hybrid 3.1 Molecular absorption loss
THz/FSO links as a part of space information networks.
In case of an outage due to beam pointing errors or Molecular absorption loss, which occurs when the part of
strong atmospheric attenuation, THz links can be used as wave energy is transformed into molecular energy due
a backup [14] since THz links are more robust to weather to the vibration of molecules, is one of the main im‑
conditions and pointing errors. pediments affecting Earth‑space THz links. Water vapor
molecules, which are scarce in the atmosphere of Mars,
are the primary sources of molecular absorption on Earth
THz communication capability can be integrated into FSO
in THz frequencies [15]. Inter‑satellite THz links among
communication architecture to support high data rates
Low Earth Orbits (LEO), Medium Earth Orbits (MEO), and
and can lead to novel applications such as space explo‑
Geosynchronous Orbit (GEO) satellites are not affected by
ration. For instance, Mars rovers communicate with Earth
molecular absorption loss because they operate at the al‑
through relay orbiters since the availability of orbiters
titudes where water vapor is almost none. To combat
is much longer than that of rovers. A Mars rover can
the high atmospheric attenuation on Earth, several ap‑
communicate with a relay orbiter through THz links and
proaches have been proposed. In the following section we
the relay orbiter relays messages using FSO links because
give an overview of these approaches.
THz communications are more robust compared to FSO
in ground/space links. CubeSats are being used in inter‑
planetary missions during mission‑critical events, which 3.1.1 Dry ground sites
are exempli ied by Mars Cube One (MarCO). CubeSats can
be equipped with THz transceivers in the future so that Atmospheric Precipitable Water Vapor (PWV) is the pri‑
they can communicate with the landing spacecraft using mary cause of strong atmospheric attenuation at THz
low latency THz links and, to relay information to Earth frequencies. Considering ground‑based telescopes of
using FSO links. THz radio astronomy such as Atacama Large Millime‑
ter/Submillimeter Array (ALMA) located at a high and
dry plateau of Chile and Combined Array for Research in
3. CHALLENGES AND SOLUTIONS Millimeter‑Wave Astronomy (CARMA) which was opera‑
ting in the United States, Suen et al. identi ied the
loca‑ tions on Earth with lowest water vapor [15]. Dry
In this section, we discuss the several challenges THz sites for radio astronomy as well as satellite
space links encounter. These challenges include molecu‑ communications in‑ clude Antarctica, Greenland, the
lar absorption and spreading loss, interference to passive Atacama Desert, and the Tibetan Plateau. Numerous dry
services. We also discuss the potential directions to sites that can provide acceptable performance have been
address these challenges. identi ied in the United States and Europe as well.

© International Telecommunication Union, 2021 33