Page 17 - 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

(SLAM), where the overall accuracy is enhanced by
obtaining very high resolution images of the
surroundings (environment). SLAM techniques
comprise of three main stages: 1) image capturing of
the surroundings, 2) range estima‑ tion from the user,
and 3) aggregation of the images at the approximated
ranges. For example, an accuracy of the sub‑centimeter
levels can be acquired by making 3‑ dimensional images
of the surroundings with the aid of THz signals and
projecting the time and angle of arrival details from the
user for estimating the locations. Instead of ground‑based
localization, THz‑enabled DSNs can assist localization
applications with an improved aerial ield of view over a
Fig. 3 – System model of a THz‑enabled drone network at various atmo‑
larger coverage area [22]. spheric altitudes consisting of a transmitter drone at altitude ℎ , and a
receiver drone at altitude, ℎ . For a given ℎ , ℎ is found according to
2.4 Drone base stations and .

Fig. 3 illustrates a simple system model, where a
The concept of Drone Base Stations (DBS) has recently
transmit‑ ter drone, Tx, and a receiver drone, Rx are
emerged in the literature, e.g., [29, 30, 31, 32], where

a hovering/mobile drone with base station‑like capabi- hovering at alti‑ tudes ℎ and ℎ , respectively. is the
phase from the verti‑ cal axis, determining the direction
lities can serve the ground users as an alternative ∘
back‑up to the terrestrial network. DBS has already of communication. For instance, = 0 corresponds to the
∘
been investigated in [33] as an extension of 5G. vertically‑up communi‑ cation, 90 denotes horizontal
∘
direction, while 180 is the vertically‑down direction.
However, there is some recent work on DBS for 6G
As discussed in Section 1, THz waves experience a
networks and tech‑ nologies. For instance in [7], DBSs
dominant water vapor absorption gain in addition to the
have been considered as a subset of Wireless
free space spread gain. Besides, the Beam Misalignment
Infrastructure Devices (WIDs), promising coverage and
(BM) fading and Multipath (MP) fading also contribute
capacity improvements for the prospective 6G
to the overall gain obtained as follows [12, 38]:
networks. In [34], DBSs have been recognized as a major
challenge related to the intelligent handover of drones
= , (1)
in multiple DBS networks for 6G technologies. By
deploying DBS, aerial wireless coverage can be provided where, is the path gain coef icient, and and refer

to the ground users as well as to the hot spots, to BM and MP fading, respectively. includes the free

especially in the areas/regions with scarce or no space spread gain coef icient, , and the absorption gain

communication infrastructure [35]. Also, it will be pos‑ coef icient, given as:

sible to provide seamless high rate connectivity for real‑
time multimedia streaming and/or for on‑demand appli‑
cations, such as in concerts [36, 37], reducing the overall ( , ℎ , ℎ , ) = ( , ℎ , , ) = ( , ) ( , ℎ , , ) , (2)

communication load of a nearby terrestrial base station.
where denotes the carrier frequency of the THz wave in
Moreover, in case of a natural disaster, multiple DBSs can
hertz, ℎ , ℎ are the transmitter (Tx) and receiver (Rx) al‑
be deployed over the entire disaster‑struck area with the
titudes in meters, respectively. is the transmission dis‑
damaged communication infrastructure, e.g., macro‑hot
tance between Tx and Rx, and denotes the relative posi‑
spots [29], thereby providing high rate communication
tion of Rx with respect to Tx as depicted in Fig. 3. ( , )
backup from the air. Here, for DBS scenarios, bandwidth
is the free space spread gain coef icient due to the THz
should be adequately provisioned with an ultra‑low la‑
wave attenuation as it propagates across the atmosphere
tency service to provide uninterrupted coverage to the
via an isotropic antenna, obtained as:
ground users. Thanks to a large THz bandwidth, THz‑
enabled DBSs can support each of the above use cases ef‑
( , ) = , (3)
fectively. Still, THz‑enabled DBSs will also take into ac‑ 4
count frequent handovers between adjacent DBSs for pro‑
viding seamless coverage to the ground users [34]. where is the speed of the THz wave in free space, i.e.,
299,792,458 / .
3. CAPACITY OF THZ COMMUNICATION
AMONG DRONES ( , ℎ , , ), the absorption gain coef icient is mainly

Having discussed the insights into the possible applica‑ induced by the water vapor molecules present in the at‑
tions of the THz‑enabled drone networks, in this section, mosphere, mathematically expressed as,
we present THz channel capacity analysis and some re‑
sults for practical drone scenarios as our motivation for 1/2
considering THz communications in DSNs. ( , ℎ , , ) = ( ( , (ℎ , , ), (ℎ , , ))) , (4)

© International Telecommunication Union, 2021 5