Page 46 - 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

1 1 1
Total Total Total
0.9 H O 0.9 H O 0.9 H O
2 2 2
0.8 0.8 0.8
0.7 0.7 0.7
Transmittance 0.5 Transmittance 0.5 Transmittance 0.5
0.6
0.6
0.6
0.4
0.4
0.4
0.3 0.3 0.3
0.2 0.2 0.2
0.1 0.1 0.1
0 0 0
0 1 0 1 0 1
10 10 10 10 10 10
Frequency [THz] Frequency [THz] Frequency [THz]
(a) Altitude = 0 km. (b) Altitude =16km. (c) Altitude =35km.
Fig. 3 – The transmittance of Earth‑space links at several altitudes from one of the driest locations of the Earth, i.e., ALMA, denoting the fraction of EM
∘
radiation after experiencing molecular absorption. The zenith angle is 35 .

Suen et al. further investigated the performance of a THz obtain the availability of the link according to some
ground to geostationary satellite links [16]. They have criteria (e.g., Line of Sight (LOS) and weather
shown that utilizing radio astronomy platforms with conditions). The analysis in [20] shows that even with a
large aperture antenna arrays, which are located at dry relatively high water vapor density, data rates on the
sites of Earth, for satellite communications 1 terabit/ order of tens of Gbps can be achieved.
second link performance can be exceeded in clear
atmosphere conditions [16]. In [17], the authors show 3.2 Spreading loss
that 1 terabit/second is attainable in the low THz band
for ground/satellite links utilizing massive antenna As an Electromagnetic (EM) wave propagates through a
arrays and establishing the ground stations at Tanggula, medium it expands and this leads to a loss called sprea-
Tibet, where PWV is very low. ding loss. Spreading loss is one of the
signi icant challenges limiting communication to short
3.1.2 High and low altitude platforms distances at THz frequencies because free space path
loss increases with the frequency in a quadratic
To combat high atmospheric attenuation in relation according to Friis’ law. Regarding space
ground/satellite THz links, placing transceivers on links, we consider the distances at least on the
airborne platforms has been proposed in [16, 18]. High order of thousands of kilometers. On the other
and low altitude platforms such as aerostats, aircraft, and hand, transmit power in THz frequencies is on the
high altitude balloons can be employed for transceiver order of milliwatts due to immature THz source
placement. These platforms need a lower aperture technology, which is also called the THz gap. Thus,
diameter compared to ground‑based transceivers and high gain antennas with high directivity are
can also offer performances comparable to ground‑based required for THz space links. Several approaches
platforms with large apertures since they operate at the for combating the problem of high propagation
altitudes where water vapor density is low [16]. In loss have been proposed in the literature
Fig. 3, Earth‑space link transmittances for various [21]. Some of these solutions apply to indoor
altitude levels in a dry location of Chile, which are and nano‑scale communications such as intelligent
simulated in the Planetary Spectrum Generator surfaces controlling the behavior of an EM wave [22]
(PSG) [19], is depicted. According to this, numerous and graphene plasmonic nano‑antennas [23]. In the
bands, which are not feasible to use at sea level, are following section, we discuss the potential
available for use at high altitudes. solutions for THz space links.

3.1.3 Hybrid ground/satellite links 3.2.1 Radio astronomy optics

Placing ground stations only at dry locations can limit Large aperture THz optics, which are exempli ied by
the potential of THz communication. Akyildiz et al. pro‑ ALMA comprising 54 re lector antennas with
pose ground‑satellite links enabled by microwave 12 meter diameter and 12 smaller antennas with a
(e.g., X band (8‑12 GHz), Ku band (12‑18 GHz, Ka‑band 7 meter dish diameter, are already being used by radio
(26.5‑ 40 GHz)), and mm‑Wave/THz bands for small astronomy. In line with this, one approach is
employing large aperture THz ground stations and
satellites called CubeSats in [20]. If a ground/satellite
airborne stations with smaller apertures [16]. Large
link is not suitable for transmission at THz
apertures can provide high gain; however, one
frequencies, mmWave and microwave bands can be
drawback is that the construction cost increases with
utilized. The idea is based on sending a pilot signal to the diameter [18].

34 © International Telecommunication Union, 2021