Page 19 - 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

altitudes, ℎ , are considered i.e., ℎ = 100 m, 500 m, and we emphasize that THz band communication in drone

1 km, whereas ℎ i.e., the Rx drone altitude for each networks can promise massive rate links even under

setting is obtained using ℎ , (angle in degrees between realistic BM fading and MP fading con‑ ditions. We refer the

Tx and Rx drones), and . readers to [12] for an in‑depth ca‑ pacity analysis of THz
It can be seen for a given ℎ , changing , i.e., the communications for drones and the other three aerial

∘
direction of communication from 0 (vertically‑up) vehicles, where both standard narrowband and variable
∘
(Fig. 4(a)) to 90 (horizontal) (Fig. 4(b)) down to 180 ∘ bandwidth capacity computa‑ tions are considered for
(vertically‑down)(Fig. 4(c)) do not incur considerable various altitudes, distances, posi‑ tions/orientations of the
∘ ∘
variations in the ca‑ pacity. This is due to the dense and vehicles (i.e., the entire range of from 0 to 180 ) by
homogeneous atmosphere across lower atmospheric leveraging LBLRTM for THz absorp‑ tion gains, evaluating
altitudes. Nevertheless, increasing ℎ shows promising no fading, BM fading and MP fading conditions.

∘
capacity improvements. For instance, at ℎ = 100 m, 0 ,

and = 100 m, capacity values correspond to 505.8 Gbps 4. OPEN ISSUES AND RESEARCH DIREC‑
TIONS
and 34.52 Gbps with WF and EP allocation schemes,
respectively. For the identical and settings but at a
Design and implementation of THz‑enabled drone net‑
higher Tx drone altitude, ℎ = 1 km, the capacity values

works and DSNs require novel communication schemes
stand at 652.6 Gbps and 62.89 Gbps with WF and EP
and networking protocols, including but not limited to
allocations, respectively. This is because traversing up
modulation and waveform design, ultra‑massive Multi‑
across the atmosphere from 100 m to 1 km observes
ple Input Multiple Output (MIMO), spectrum and inter‑
substantial decrements in the water vapor concentration
ference management, Medium Access Control (MAC) and
levels [12], which can be highly leveraged in drone
higher network layers, security and privacy issues.
networks communicating over the THz band.
Additionally, for overcoming the distance issue observed
across lower atmospheric levels, for instance, for ℎ 4.1 Physical layer
lower than 100 m, multiple drones can be deployed
THz band drone communications primarily requires en‑
suf iciently close to each other, in a networked fashion,
hanced THz band channel models. For this purpose,
where they can be treated as relays. These results
measurement‑based studies need to be pursued at drone
showcase the massive capacity potential of the THz band
altitudes in various propagation environments and under
for drone networks, promising links in the order of up to
drone mobility scenarios, so that the existing line‑of‑sight
several 10s of Gbps using EP allocation, and up to many
and non line‑of‑sight models with beam misalignment
100s of Gbps with WF power allocation for transmission
and generic multipath fading (as considered in this work)
ranges up to 100 m. Fig. 5 depicts the ergodic capacity
can be improved with speci ic stochastic channel models
trend for short range, i.e., = 10 m, under BM fading and
for THz links among drones. A recent work on active and
MP fading parameters [12]. Interestingly, it can be seen
passive THz systems is presented in [50], where measure‑
in Fig. 5(a) that for MP fading parameter, = 1, which
ment results at 140 GHz (0.14 THz) have been provided
corresponds to pure NLOS, Rayleigh fading, increasing
for rooftop surrogate satellite systems and terrestrial net‑
the normalized jitter standard deviation, / does not works. Based on the enhanced channel models, modula‑

cause substantial ergodic capacity degradation. Due to
tion and waveform design should be tailored for THz band
nearby re lections the NLOS MP fading components are communications in drone networks or DSNs.
more dominating than the BM fading components for
short range; hence severeness of BM does not affect the
Modulation
ergodic capacity. Meanwhile, Fig. 5(b) shows that for
ixed BM fading parameter, / = 5, MP fading The state‑of‑the‑art modulation schemes that can be po‑

degrades the ergodic capacity by 26 % for EP allocation, tentially employed for THz band communications in‑
and 16.5 % for WF allocation, as the MP effect is varied clude Single‑Carrier (SC) modulation, multi‑carrier mo-
from = 10, indicating a strong LOS along with NLOS dulation, Orthogonal Frequency Division Multiplexing
components to =1, i.e., pure NLOS Rayleigh fading. (OFDM), Cyclic Pre ix Orthogonal Frequency Division
Next, we present the achievable ergodic capacity at Multiplexing (CP‑OFDM) and even Non‑Orthogonal
= 50 m, under variable BM fading with Rayleigh MP Multiple Access (NOMA). In what follows, we discuss
fading ( = 1) in Fig. 6(a), and variable MP fading with each of the aforementioned modulation schemes in the
/ = 5 Fig. 6(b). For this range, increasing / from 1 perspective of THz band communications for drone

to 10 decreases the ergodic capacity substantially, e.g., by networks. Non‑overlapping transmission windows are
an order of magnitude for EP allocation, as / is termed as Single Carrier (SC) modulation, having some

increased from 1 to 10. On the other hand, decreasing , provision of the carrier aggregation [51]. However, due to
from 10 to 1 with given / = 5 shows no considerable the intrinsic frequency‑selective nature of THz channel,

change in ergodic capacity. The ergodic capacity results multi‑carrier modulations would also help in some form
in Fig. 5 and Fig. 6 depicting that at short ranges, it is the of carrier aggregation with multiple individual/
MP fading that mainly affects ergodic capacity, while at non‑overlapping single carriers [52]. The implementation
long range, it is mainly the BM fading. With this analysis, of practical THz another task,
© International Telecommunication Union, 2021 7