Page 47 - 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

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Frequency [THz] Frequency [THz] Frequency [THz]
(a) Altitude = 0 km. (b) Altitude =16km. (c) Altitude =30km.
Fig. 4 – Zenith transmittance of clear Mars atmosphere for various altitudes.
3.2.2 Re lect‑arrays 10 -14

R lect‑array antennas are being used by the systems
10 -12
such as satellite communications, radars and deep‑space
communication links [24]. Traditional aperture antennas
-10
can provide high gain but they are not as electronically 10
lexible as phased arrays. However, the implementation
cost of phased arrays is high. Re lect‑arrays offering high Pressure [bar] 10 -8
gain, low cost, ease of manufacturing as well as electronic
lexibility are a compromise between aperture antennas -6
10
and phased arrays. To illustrate, the MarCO spacecraft
communicated with Earth at a distance of 160 million km
-4
10
on X‑band via a high‑gain re lect‑array antenna with small
volume [25]. Operating frequencies of r lect‑array an‑
tennas are now shifting towards THz frequencies [26]. 10 -2 80 100 120 140 160 180 200 220 240 260
Thus, they offer the potential to be employed in THz space
Temperature [K]
links. On the other hand, enabling technologies need to be
studied because RF and MEMS technologies such as semi‑ Fig. 5 – Pressure vs temperature.
conductor diodes and MEMS lumped elements do not ap‑
ply to the THz band due to loss and size constraints [27].
3.2.3 Ultra‑massive MIMO 3.3 Spectrum sharing

The concept of Ultra‑Massive Multiple Input Multiple Out‑
Spectrum beyond 275 GHz is not largely regulated in
put (UM‑MIMO) has been introduced in [28] for comba‑
the Radio Regulations (RR). Footnote 5.565 of RR
ting the distance problem in THz communications.
identi ies numerous frequency bands in the range
According to this concept, using novel materials such
from 275 GHz to 1000 GHz that are used by
as graphene to build antennas with a number of
passive services, namely Radio Astronomy Services
antenna elements in a small footprint is possible.
Utilizing both space and frequency, the coverage range (RAS), Earth Exploration Satellite Services (EESS),
can be increased. However, the realization of this and Space Research Service, and states that the
concept poses several challenges [28]. The activity of these services must be protected from
performance of UM‑MIMO depends on the THz channel; harmful interference of active services until the
thus, accurate THz channel mod‑ els are required. frequency allocation is established [30]. Active and
Moreover, to control arrays, dynamic beam‑forming passive services will coexist on the spectrum.
algorithms are needed [20]. Moreover, the performance
Current spectrum sharing studies, which aim to
of THz MIMO links can also be affected by the
identify the bands where the coexistence of active
frequency‑dependent diffraction of THz waves, which
and passive services is possible, mainly focus on
arises from the divergence of THz beams from their mo-
the interference to EESS [31] because RAS
dulation side‑bands. This results in degraded
telescopes are located in high dry mountains.
bit‑error‑ rate performance due to the detection of
unwanted spec‑ trum information. Thus, novel However, coexistence studies should cover
detection and demodula‑ tion methods are required in both RAS and EESS because both can be affected
this direction [29]. by THz ground/satellite and inter‑satellite links.
© International Telecommunication Union, 2021 35