Page 22 - 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

the problem of the co‑channel interference, also utilizing the THz band. However, RIS built from the metasurfaces
the freedom of angle diversity by moving the and discrete element semiconductors also enables user‑
razor‑sharp THz beam to ic or targeted direc- customized settings. A similar RIS can suf iciently in‑
tions. Using UM‑MIMO, a certain group of arrays/antenna crease the THz signal power by re lecting the THz signals
elements can be designated for communicating to a towards a speci ic direction. This can be achieved by in‑
speci ic user. This special mode utilizes various data troducing required phase shifts of the discrete elements
streams onto a single carrier frequency, thereby in the RIS. In addition, a suf iciently large RIS supporting
increasing the per user capacity, which is also bene icial the aforementioned features can be acquired within
when the communication links are operating in a limited miniature footprints at high frequencies, such as the THz
bandwidth and a high SNR scheme. This special mode band [22, 70, 71]. RISs have already been considered for
enhances the rate by the virtue of Spatial Multiplexing improving the coverage performance of THz indoor com‑
(SM), provided that the channel matrix of the UM‑MIMO munications at the sea level, as in [72], where the
has suf icient rank and diversity. Conclusively, any authors have proposed a suboptimal search scheme for
amalgamation across UM‑SM and UM‑BF is realizable. the RIS phase shifts. Additionally, RISs have also been
Additionally, for maximizing the utilization of the THz consi-dered for THz drone communications [73]. For the
band, promising Tbps links, multiple transmission drone networks at the low altitude scenarios, e.g., ℎ =

windows can be employed simul‑ taneously. For this 100 m and below, up to sea level, THz communication
purpose, multiband UM‑MIMO utilizes different carrier ranges can be substantially extended with the aid of RISs
frequencies by tuning electrically the frequency deployed on top of buildings, roofs etc. This can also be
response of the plasmonic nano‑antennas. One of the achieved for drone‑to‑ground and ground‑to‑drone links
major pros of this multiband UM‑MIMO technique is by placing RISs near the intended user access points
that the data can be processed within a substantially [14], Thus, RIS can considerably increase the coverage of
smaller bandwidth, hence, lowering the complexity of a drone network e.g., drone base station, agricultural
the system design with an improved lexibility of the monitoring in a rural area with a limited or no terrestrial
spectrum. In this research arena, novel frequency, communication infrastructure. etc.
space and time modulation and coding methods are re‑
quired to be proposed for such UM‑MIMO communication 4.2 Spectrum and interference management
systems. UM‑MIMO can also be leveraged for DSNs due
With the progression towards 6G, exploitation of the
to very large beam‑forming gains to overcome the huge
higher frequency bands above existing sub 6 GHz spec‑
path loss over the THz band. Moreover, as a byproduct,
trum has become more appealing than ever. This will
razor‑sharp THz beams would substantially mitigate the
raise the need of sharing the spectrum using cognitive
interference among communicating drones in the DSNs.
radio sensing with lexibility [74]. THz spectrum has
Nevertheless, the design of UM‑MIMO systems for DSNs
been identi ied as a prime communication band for mo‑
and drone networks should also incorporate the effect
bile communications within 6G research [28, 75]. Also, 2G
mobility of the drones to avoid Tx‑Rx antenna beam mis‑
to 5G networks across the globe have been utilizing lower
alignment and maximize the beam‑forming. For the case
frequency bands, which will also be available in 6G net‑
of DBS, implementation of UM‑MIMO will be essential to
works. Therefore, various spectrum management tech‑
provide aerial coverage to several users at the same time
niques will be needed for managing the lower, mid and
[64]. However, this will be a challenging task, as the lying
higher frequency bands intelligently. Here, the massive
drones with a limited battery support and single anten‑
THz band will suf ice the spectrum scarcity by assigning
nas will need to be replaced with UM‑MIMO, which will be
different frequency sub‑bands to different users subject
an important research avenue for the upcoming 6G sys‑
to different scenarios, mitigating the conventional issue of
tems. For instance, recently, THz UM‑MIMO communi‑
interference. For instance, in [76, 77], Long‑User‑Central‑
cations has been considered for a Space‑Air‑Ground In‑
Window (LUCW) is considered, where the farther users
tegrated Network (SAGIN) comprising of terrestrial, air‑
are allocated to the central sub‑bands of a THz transmi-
borne and spaceborne networks [65]. With the plasmonic
ssion window, while the the users at shorter
antenna arrays having nano‑antenna spacings, it will be
transmission ranges are provisioned with the edge
possible to practically implement THz systems onto the
sub‑bands in a window. Such an interference management
lying drones within miniature footprints [52].
can also be employed within a drone network, where a
Recon igurable Intelligent Surfaces drone (e.g., drone base station) serving different users can
utilize dif‑ ferent sub‑bands each several GHz wide [12],
Recently, novel tunable metasurfaces are referred to as supporting capacity values in the order of several 100s
igurable Intelligent Surfaces (RIS), which can be of Gbps even under BM fading and MP fading as
used for controlling and optimizing the wireless channel discussed earlier in Section 3. Nevertheless, interference
environment [66, 67, 68, 69]. Generally, strong NLOS sig‑ in THz band com‑ munications usually occurs in dense
nals having specular re lections take the surfaces of the scenarios [78, 79]. Thus, drone networks can be deployed
existing building infrastructures as electric mirrors, par‑ strategically, e.g., via sharp pencil beam‑forming, to mitigate
ticularly at considerably miniature wavelengths across the issue of the interference [6].
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